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TO AVOID FINES return on or MAY BE RECAUED with earlier due date if requested. 4 DATE DUE DATE DUE DATE DUE l CATALYTIC UPGRADING OF SUCCINATES TO ITACONIC ACID By Dushyant Shekhawat A DISSERTATION Submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Chemical Engineering 2000 "‘ flI-Q ‘r LI“ Vt in. ABSTRACT CATALYTIC UPGRADING OF SUCCINATES TO ITACONIC ACID By Dushyant Shekhawat acid or its derivatives and formaldehyde were comprehensively studied. A novel process and catalysts to produce itaconic acid via condensation of succinic The nature of sites on the support played an important role in the formation of citraconates in the vapor phase reaction. Neither highly acidic nor basic sites on the support favored the reaction. The highly acidic supports were found to be very active for cracking succinates into carbon dioxide, but not for condensation to citraconic anhydride. The basic supports gave little citraconic anhydride but catalyzed the Cannizzaro reaction of formaldehyde to methanol and formic acid, thus preventing formaldehyde from participating in the desired condensation. In contrast, y-alumina, a mildly acidic support, without any salt added showed significant activity for the formation of citraconates from succinates. Results from different feedstocks over y-alumina at the base case (standard) conditions are summarized in Table below. Summary of Results from Difl‘erent Feedstocks over y-Alumina S.N. Feedstock Yield of Conv of Selectivity citraconates (%) succinates (‘VQ (%) 1 DMS + trioxane 35 48 73 2 SAN + trioxane 44 67 66 3 MMS + trioxane 26 40 78 4 DMS + Formalin 29 42 7O 5 DMS + Formcel 34 56 61 The decay in catalyst activity was much slower with Fonnalin. Yields of citraconic anhydride and conversion of DMS dropped ofl~ very slowly over time for reaction times out to five hours. Coking on the catalyst was also much less with Formalin, likely because the water present steam-cleaned the catalyst during the reaction. Coking involves both the succinate species and formaldehyde. Citraconic anhydride yield stabilizes following acid site deactivation. Upon deactivation, the alumina catalyst was regenerated by exposure to air at 500 °C for five hours. The yields of citraconic anhydride were identical before and after the regeneration process, which demonstrated the robust nature of the oxide catalysts and their ability to be regenerated. Yield of citraconic anhydride increased with increasing reaction temperature, but selectivity lowered due to more cracking of DMS at elevated temperatures. Citraconic anhydride yields were highest at 380 °C. Higher formaldehyde to DMS molar ratios gave better yields and selectivities of citraconic anhydride. The yield of citraconic anhydride decreased with increasing liquid feed flow rate, but selectivity remained unchanged. Citraconic anhydride yields were increased with increasing catalyst bed length, but the selectivities decreased. The condensation reaction of DMS with Forrnalin over intermediate surface area alumina is not mass transfer limited. The subsequent process steps for separation of citraconic acid from unreacted succinates and isomerization of citraconic acid to itaconic acid have also been studied. A maximum 99.5% of purity of itaconic acid was observed. Finally, a process concept for itaconic acid production from succinates and formaldehyde is proposed. The calculated feedcost is $0.50/lb itaconic acid produced (base case results) for a 20-MM lb itaconic acid/yr capacity plant. I dedicate this work to: lily parents, Ichwaj Kanwar and Inder Singh Shekhawat & va aunt and uncle, Sarala mid Fateh Singh Bika iv 4,}.‘1- ‘r .A'.“ ACKNOWLEDGMENTS I would like to express my thanks to Dr. Dennis Miller, my major professor, and Dr. James Jackson for their guidance and suggestions throughout the course of this work. Thanks to Dr. Kris Berglund, Dr. Martin Hawley, and Dr. Thomas Pinnavaia for their service as my doctoral committee members. I would like to thank Dr. N Kirthivasan and Bryan Hogle for their valuable contributions to this work. Thanks to Mat Peabody, CEO Applied CarboChemicals, for helpful feedback in numerous group meetings. I also acknowledge Dr. V Mirca and Ancuta Cernat for their contribution. Thanks to Sriraman Varadarajan, Dr. Man Tarn, Dr. Shubham Chopade, Dr. Zhigang Zhang, Rajesh Baskaran, Frank Jere, Mike Shafer and Dr. Atul Dhale for their excellent suggestions and enjoyable company. A special thanks goes to my wife, Archana, for helping me to realize my dreams. I would like to express my thanks and appreciation to the Bika family in Minneapolis. Their support has been very important in making me and my children, Suveer and Pragya, feel at home in Lansing. My sincere thanks and appreciation also go to my sister, Shail, brother-in-law, Jeevraj Rathore, and brother, Basant, for their continued support and encouragement in my endeavors. Thanks to the Chemical Engineering Department at Michigan State University, Applied CarboChemicals, and the Crop and Food Bioprocessing Center, State of Michigan Research Excellence Fund for financial support during the development of this work. italiabl hit of Flgu (later 1. l l 12 List of Tables TABLE OF CONTENTS List of Figures Chapter 1. Introduction 1.1. 1.2. 1.3. 1.4. 1.5. 1.6. Background Succinic Acid and Succinic Anhydride Alkyl Esters of Succinic Acid 1,4-Butanediol, 'y-Butyrolactone, and Tetrahydrofuran Stobbe Condensation 1.5.1. Introduction 1.5.2. Mechanism of Stobbe Condensation 1.5.3. Applications of Stobbe Condensation 1.5.3.1. Lactonic Acids 1.5.3.2. The Naphthol Synthesis 1.5.3.3. The Indone Synthesis 1.5.3.4. The Tetrahydroindanone Synthesis 1.5.3.5. The Tetralone Synthesis 1.5.3.6. The Equilenone Synthesis Itaconic Acid and Its Isomers 1.6.1. Current Manufacturing Process (Fermentation) 1.6.2. Catalytic Route 1.6.3. Uses of Itaconic Acid and Its Isomers xvi xviii 11 ll 11 11 12 12 12 14 15 1.7. Formaldehyde and Its Sources 1.7.1. 1.7.2. 1.7.3. 1.7.4. 1.7.5. 1.7.6. 1.7.7. 1.7.8. Formalin Properties of Formaldehyde Manufacturing Processes Forrncel Trioxane Paraformaldehyde Methylal Safety Factors 1.8. Catalyst 1.8.1. 1.8.2. 1.8.3. Surface Area Acidic and Basic Properties on Solid Surfaces 1.8.2.1. Acid Strength and Hammett Acidity Function 1.8.2.2. Temperature Programmed Desorption method 1.8.2.3. DRH’TS Study Difi‘erent Catalyst Supports 1.8.3.1. Alumina 1.8.3.2. Zirconia and Titania 1.8.3.3. Silica 1.8.3.4. Zeolites 1.8.3.5. Aluminum Phosphate 1.8.3.6. Hydrotalcites/Mg-Al Mixed Oxides 1.9. Research Objectives vii 18 18 18 20 21 21 22 23 24 24 24 26 27 29 29 30 30 31 31 32 33 33 34 D) g.) '1' O ’ 3: st ' imp ‘ 4.: k... Chapter 2. Experimentation and Analysis 2.1. 2.2. 2.3. 2.4. 2.5. 2.6. 2.7. 2.8. 2.9. Reactor Vessel Reactor Furnace Feed System 2.3.1. Succinic Acid Esters and 1,3,5-Trioxane Feed 2.3.2. Succinic Acid Esters and Formalin Feed 2.3.3. Succinic Anhydride and 1,3,5-Trioxane Feed 2.3.4. Monomethyl Succinate and 1,3,5-Trioxane Gas Flow System Product Collection System ANALYSIS 2.6.1. High-Performance Liquid Chromatography 2.6.2. Gas Chromatography 2.6.3. Formaldehyde Analysis Product Identification Hydrolysis of Products Product Yield and Selectivity Calculations Chapter 3. Experimental Methods 3.1. Catalyst Materials 3.1.1. Aluminum Phosphates (A1P04) 3.1.2. Hydrotalcites viii 37 37 38 41 42 42 42 44 44 45 46 48 52 54 55 56 56 59 59 60 60 \‘v 14 .\. - . l A a ‘3 I.“ 'V 4‘ 1.“ “V1 to“. n .. 31.. ii A‘- ‘1. 3.1.3. Aluminum Oxide 62 3.1.4. Zirconia 63 3.1.5. Iron Oxide 63 3.1.6. Titania 63 3.2. Supported Salt Catalysts 64 3.3. Catalyst Characterization 64 3.3.1. BET Surface Area 64 3.3.2. Acid Strength by Hammett Indicators 66 3.3.3. Temperature Programmed Desorption (TPD) Using Probe Molecules 67 3.3.4. DRIFTS Study of Pyridine Adsorption 67 3.4. Feed Preparation 69 3.5. Reactor Operation 70 3.5.1. Catalyst Loading and Unloading in Reactor 70 3.5.2. Operation of Reactor 71 3.5.3. Reactor Shutdown 72 Chapter 4. Catalyst Screening for the Reaction of Dimethyl Succinate and Trioxane 73 4. 1. Introduction ' 73 4.2. Products fi’om Succinates and Formaldehyde 74 4.3. Reaction Conditions 77 4.4. Catalyst Characterization 78 4.4.1. Surface Area 4.4.2. Acid-Base Measurements 78 78 4.4.2.1. Temperature Prograrmned Desorption (TPD) Studies 4.4.2.2. Acid Strength by Hammett Indicators 4.4.2.3. DRIFTS Studies 4.5. Control Experiments 4.5.1. Dirnethyl Succinate 4.5.2. Diethyl Succinate 4.5.3. Trioxane 4.5.4. Citraconic Anhydride 4.5.5. Citraconic Anhydride and Diethyl Succinate 4.5.6. Itaconic Acid 4.6. Catalyst Screening Studies 4.6.1. Silica 4.6.2. Zeolites 4.6.3. Other Supports 4.6.3.1. IronOxide 4.6.3.2. Titania 4.6.3.3. Zirconia 4.6.3.4. Hydrotalcites 4.7. Alumina Supports 4.7.1. Low Surface Area Aluminas 80 83 83 85 85 86 87 87 88 89 89 89 9O 91 91 92 93 93 94 94 48 4.8. 4.9. 4.10. 4.11. 4.12. 4.13. Chapter 5. Condensation of Succinic Anhydride and Trioxane to Citraconic Anhydride 5. 1 . 5.2. 4.7.2. Intermediate Surface Area Aluminas 4.7.2.1. SA3177 Alumina 4.7.2.2. Base Supported on SA3177 4.7.3. High Surface Area Aluminas 4.7.4. Aluminum Phosphates Parametric Studies 4.8.1. Efl‘ect of Pressure 4.8.2. Effect of Temperature 4.8.3. Feed Molar Ratio Hydrolysis of Products Extended Run Catalyst Deactivation Catalyst Regeneration Summary Introduction Succinic Anhydride and Trioxane in Molten Phase 5.2.1. Reaction Conditions 5.2.2. Base Case Results over y-Alumina (SA3177) 5.2.3. Parametric Studies over y—Alumina (SA3177) 5.2.3.1. Temperature 101 101 102 104 105 105 105 106 113 118 118 122 124 125 127 127 127 128 129 131 131 5.3. 5.2.3.2. Feed Molar Ratio 5.2.3.3. Liquid Feed Flow Rate 5.2.3.4. Carrier Gas Flow Rate 5.2.3.5. Longer Reactor Catalyst Bed 5.2.3.6. Hydrolysis 5.2.3.7. Deactivation Studies 5.2.4. Other Catalysts Used 5.2.4.1. Empty Reactor 5.2.4.2. Glass Beads 5.2.4.3. Iron Oxide Succinic Anhydride and Trioxane in a Solution 5.3.1. Reaction Conditions 5.3.2. Base Case Results 5.3.2.1. Efiea of Temperature 5.3.2.2. Effect of Carrier Gas 5.3.2.3. Hydrolysis 5.3.2.4. Deactivation Studies 5.3.3. Other Catalyst Used 5.3.3.1. KHzPO4/SA3177 5.3.3.2. CPG-75 5.3.3.3. SA6175 5.4. Summary xii 135 136 142 143 144 145 146 146 147 148 148 149 149 151 152 153 153 154 154 155 156 156 Chapter 6. Salaam u 6-1 Chapter 6. Condensation of Dimethyl Succinate and Formaldehyde in Aqueous Solutions to Citraconic Anhydride 6.1. 6.2. 6.3. 6.4. 6.5. 6.6. Introduction Reaction Conditions Control Experiments of Formalin Base Case Results Other Catalysts Used 6.5.1. 6.5.2. 6.5.3. 6.5.4. 6.5.5. 6.5.6. 6.5.7. Intermediate Surface Area Alumina 6.5.1.1. Salts Supported on SA3177 Alumina 6.5.1.2. Acid Treated SA3177 6.5.1.3. Alumina-In-House High Surface Area Alumina Low Surface Area Alumina (SA3132) Hydrotalcites/ Magnesium-Aluminum Mixed Oxides Aluminum Phosphates Carbon Beads Process Optimization over SA3177 6.6.1. 6.6.2. 6.6.3. 6.6.4. 6.6.5. F orrncel as a Source of Formaldehyde Carbon Dioxide as a Carrier Gas Helium Gas Flow Rate Liquid Feed Flow Rate Feed Molar Ratio 159 159 160 160 162 168 169 169 173 173 174 175 175 182 184 184 185 186 188 188 189 195 6.7. 6.8. 6.9. 6.6.6. Efl‘ect of Temperature 6.6.7. Particle size 6.6.8. Longer Reactor Catalyst Bed 6.6.9. Hydrolysis Catalyst Deactivation Studies Catalyst Regeneration Summary Chapter 7. Reactor Modeling 7.1. 7.2. 7.3. 7.4. Pressure Drop calculation Mass Transfer Calculations V 7.2.1. Calculation of Observable Rate 7.2.2. Diffusivity Estimation 7.2.3. Calculation for Observable Modulus Residence Time and WSHV Calculation Reaction Kinetics 7.4.1. Calculation of Rate Constants from Control Experiments 7.4.1.1. Citraconic Anhydride Cracking Reaction 7.4.1.2. Dirnethyl Succinate Cracking Reaction 7.4.1.3. Formaldehyde Reactions 7.4.2. Equilibrium Calculations 7.4.3. Presentation of Equations 7.4.4. Results xiv 201 203 203 204 209 210 213 216 216 218 218 220 221 222 223 223 223 225 226 227 229 230 (11pm 8. is} -. Mites 131‘ 7.5. Molecular Modeling Chapter 8. Process Development 8.1. 8.2. 8.3. 8.4. Introduction Process Specifications Feed costs Calculation Process Concept for Itaconic Acid Production 8.4.1. 8.4.2. 8.4.3. 8.4.4. 8.4.5. 8.4.6. 8.4.7. 8.4.8. Esterification Reactor Reactor Flash Drum Distillation Column 1 Reactive Distillation Column Succinic Acid Crystallizer Isomerization Reactor Itaconic Acid Crystallizer Chapter 9. Summary and Recommendations 9.1. 9.2. Appendices Summary List of References Recommendations 233 237 237 237 238 240 240 242 242 242 243 243 244 245 247 247 250 255 264 .144. R 13"" 2:13 P . II ‘1 real R 1‘ . 3'5 .41 r l E ,C. ash .cl 8E . is“ ‘n...‘ Table 1.1. Table 1.2. Table 1.3. Table 2.1. Table 2.2. Table 4.1. Table 4.2. Table 4.3. Table 4.4. Table 4.5. Table 4.6. Table 4.7. Table 4.8. Table 4.9. Table 5.1. Table 5.2. Table 5.3. Table 5.4. Table 5.5. LIST OF TABLES Physical properties of succinates under study 4 Physical properties of itaconic and its isomers 13 Properties of monomeric formaldehyde 19 Retention time and response factors of different compounds in HPLC 49 Retention time and response factors of different compounds in GC 54 Properties of different catalyst material used in this study 79 Results from control runs with dirnethyl succinate 86 Results from the control run of citraconic anhydride 88 Results from difi‘erent metal oxides 92 Results from salts supported on SA3132 99 Efl‘ects of loading on SA3177 103 Results from extended run (17 hrs) 12] Efi‘ect of coking on catalyst weight gain and surface area of the catalyst 123 Yield of citraconic anhydride before and after the regeneration of catalyst 124 Reaction conditions 129 Results fiom succinic anhydride and trioxane using SA3177 130 Results at different feed molar ratios 137 Efi‘ect of outlet helium flow rate on results 143 Comparison of results from longer reactor with regular reactor 144 . | all. . i . t l A o «.1. 4 .\ . rhl» q ., y 8 a‘ . ,0 falls. (Hal. [Kry {file IKE ,§ ~ ~ . was.“ .14 ..w 3». arm in...“ .5 . .5 .1 ML ..... . 3... 4.... 4. ... u. .4.» by u. r 9 . n or r o . o l 5 . o l T . \- Ta 3"“ \ - “Nil Table 5.6. Results from succinic anhydride and trioxane after and before hydrolysis 145 Table 5.7. Reaction conditions 150 Table 5.8. Results from MMS and trioxane feed in methanol 151 Table 5.9. Efl‘ect of carrier gas on results (Before hydrolysis) 152 Table 5.10. Results before and after hydrolysis of product 153 Table 6.1. Reactor operating conditions 160 Table 6.2. Comparison of results from Alumina-In-House and SA3177 174 Table 6.3. Results from AlPOs with P/Al molar ratio of 0.5 and 1.0. 183 Table 6.4. Efi‘ect of temperature on results 202 Table 6.5. Summary of experiments conducted to see WHSV effects 204 Table 6.6.1 Results before and after hydrolysis of product 209 Table 6.7. Catalyst weight gain fi'om difi‘erent catalyst materials 211 Table 6.8. Yield of citraconic anhydride before and after the regeneration of catalyst 2 12 Table 7.1. Flow rates at the reactor inlet 216 Table 7.2. Comparison of predicted and experimental values of concentrations 231 Table7.3. Ab initio and PM3 data for reactants, intermediates, and products involved in the mechanism of the formation of citraconic anhydride 234 Table 7 .4. Relative energies calculated in gas phase from PM3 and ab initio methods at each step of the mechanism, normalized to the energy of the starting materials 236 Table 9.1. Summary of results from difi‘erent feedstocks over y-alumina 248 bii'l'fill . is." » ‘ t' q ’ l a. ab. :15”: I 1 “'"‘ib‘._. p .4 .F0 \.\. mm LIST OF FIGURES Figure l. 1. Reaction Pathways from Succinic Acid 3 Figure 1.2. Mechanism of Stobbe Condensation 10 Figure 2.1. Cross-section of Vapor Phase Reactor 39 Figure 2.2. Schematic Flow Diagram of Reactor System 40 Figure 2.3. Product Collection System 47 Figure 2.4. A typical chromatogram fiom HPLC 50 Figure 2.5. A typical chromatogram from GC 53 Figure 3.1. The flow diagram of a TPD setup 70 Figure 4.1. Transesterification products fi'om succinates 75 Figure 4.2. Transesterification reactions of citraconic acid ' 76 Figure 4.3. Ammonia TPD profiles of the catalyst used 81 Figure 4.4. C02 TPD profiles of catalyst used 82 Figure 4.5. DRIFTS spectra from pyridine adsorption 84 Figure 4.6. Yield of citraconic anhydride from different alumina supports 95 Figure 4.7 . Conversion of DMS from different alumina supports 96 Figure 4.8. Yield of C02 from difi‘erent alumina supports 97 Figure 4.9. Selectivity of citraconic anhydride from difi‘erent alumina supports 98 Figure 4.10. Comparison of results at low pressures vs high pressures (at same WHSV) 107 Figure 4.11. Efi‘ect of hydrolysis on the yield of citraconic acid at high pressures (at same WHSV) 108 'NI '1 v ’\ - r’. 'h l I. III- .a j n w' b" r0; ‘ ‘. .3..- s M. "as: j . u 3“» Figure 4.12. Yield of citraconic anhydride at various temperatures (at same WHSV) 109 Figure 4.13. Conversion of DMS at various temperatures (at same WHSV) 110 Figure 4.14. Yield of C02 at various temperatures (at same WHSV) 111 Figure 4.15. Selectivity of citraconic anhydride fi'om DMS at various temperatures 1 12 Figure 4.16. Efi‘ect of feed molar ratio on the yield of citraconic anhydride 114 Figure 4.17. Effect of feed molar ratio on the conversion of dimethyl succinate 115 Figure 4.18. Efl‘ect of fwd molar ratio on the yield of C02 116 Figure 4.19. Efl‘ect of feed molar ratio on the selectivity 117 Figure 4.20. Yield of citraconic acid after hydrolysis (SA3177) 119 Figure 4.21. Results fiom extended time experiment before hydrolysis 120 Figure 5.1. Efl‘ect of temperature on the yield of citraconic anhydride 132 Figure 5.2. Efi‘ect of temperature on the conversion of succinic anhydride 133 Figure 5.3. Efl‘ect of temperature on the selectivity to citraconic anhydride 134 Figure 5.4. Efi‘ect of temperature on the yield of C02 135 Figure 5.5. Efl‘ect of liquid feed flow rate on the yield of citraconic anhydride 138 Figure 5.6. Efi‘ect of liquid feed flow rate on the conversion of succinic anhydride 139 Figure 5.7. Efi‘ect of liquid feed flow rate on the selectivity to citraconic anhydride 140 Figure 5.8. Efi‘ect of liquid feed flow rate on the yield of C02 141 Figure 6.1. Citraconic anhydride yield from different formaldehyde sources 163 Figure 6.2. Conversion of DMS fiom difl‘erent formaldehyde sources 164 rzeii ', . :48 by . ,uhv at: O 1 (by s. whim. 0“ .1. n"... 3“ -‘-s..“§' I o‘- rhh .!5 A.» .rW Illl in v. a. \ .u: x: 1‘ a n b: . Figure 6.3. Figure 6.4. Figure 6.5. Figure 6.6. Figure 6.7. Figure 6.8. Figure 6.9. Figure 6.10 Figure 6.11 Figure 6.12 Figure 6.13 Figure 6.14 Figure 6.15 Figure 6.16 Figure 6.17 Figure 6.18 Figure 6.19 Figure 6.20 Figure 6.21 Figure 6.22 Figure 6.23 Figure 6.24 Selectivity from difi‘erent formaldehyde sources Yield of MMS from different formaldehyde sources Yield of C02 from difi’erent formaldehyde sources Comparison of results from supported and unsupported SA3 17 7 Yield of CO2 from KH2PO4/SA3177 and SA3177 only Yield of citraconic anhydride from different hydrotalcites Conversion of DMS fi'om different hydrotalcites . Selectivity from different hydrotalcites . Yield of MS from different hydrotalcites . Yield of CO2 from difi‘erent hydrotalcites 165 166 167 171 172 177 178 179 180 181 . Yield of citraconic anhydride using Fonncel at difierent feed flow rate . Efl‘ect of liquid feed flow rate on yield of C02 . Conversion of DMS using Fonncel at different feed flow rate . Yield of citraconic anhydride using Formalin at different flow rate . Conversion of DMS using F ormalin at diflerent feed flow rate . Efl'ect of feed molar ratio on yield of citraconic anhydride . Efi‘ect of feed molar ratio on conversion of dimethyl succinate . Efi‘ect of feed molar ratio on selectivity . Efi‘ect of feed molar ratio on MS yield . Effect of feed molar ratio on yield of carbon dioxide . Yield of citraconic anhydride from different size catalyst beds . Conversion of dimethyl succinate from different size catalyst beds 190 191 192 193 194 196 197 198 199 200 205 206 Figure 6.25. Selectivity fi'om difl‘erent size catalyst beds 207 Figure 6.26. Yield of carbon dioxide fi'om difi‘erent size catalyst beds 208 Figure 7.1. List of reactions included in the kinetic model 224 Figure 7.2. Concentration vs residence time 232 Figure 7.3. Reaction scheme for the formation of citraconic anhydride from dimethyl succinate and formaldehyde including the possible intermediates 23 5 Figure 8.1. A schematic of a process concept for conversion of succinate to itaconic acid 241 I It CHAPTER 1 INTRODUCTION 1.1. Background A number of chemicals are obtained from petroleum, natural gas, coal, and other fossil carbonaceous materials. But, biomass-derived feedstocks offer a potential . alternative for producing variety of chemicals which are produced traditionally from fossils. There are several reasons for increased interest in biomass derived feedstocks. Nature puts a limit on traditional resources, so it is necessary to find alternate carbon sources for the production of basic feedstocks, shifting from a fossil to a renewable carbon base. The escalation of the cost of petroleum and natural gas and uncertainties in the long-range supply have triggered our attention towards biomass feedstocks. The use of biomass would reduce residues and wastes of the agriculture and forestry industries and will also boost our agriculture industry. It is also known that biomass conversion is more environmentally benign then petrochemical conversion. The complicated biomass refining is not complicated anymore because of recent advances in separation methods. Finally, the rapid growth in biogenetic engineering has made easier the commercial production of specific and complex materials from biomass. The l xiii: ma: 1:- ‘ncur z gazing a i 1 't. ‘ 9'11 ..:. tub“ ¢(' mi the Mt cord: laziest re; iii-game: it igh cos The focus of this research project is on the production of industrial commodities and specialty chemicals from biomass-derived (renewable) succinates. Some of the possible reaction pathways from succinic acid are illustrated in Figure 1.1. Succinic acid, also lmown as 1,4-butanedioic acid, and its alkyl esters are very valuable compounds in producing a variety of specialty chemicals. Succinic acid, containing two carboxylic acid groups and two reactive methylene groups, undergoes many chemical transformations to specialty chemicals and commodities. Among some of the known reactions are the Stobbe condensation (condensation of alkyl esters of succinic acid with aldehydes and ketones), reaction with amino compounds (making succinimides), esterification, hydrogenation (y—butyrolactone, tetrahydrofuran, and 1,4obutanediol) and many more. i The high cost of producing succinic acid ($2.72/lb)(1) via petroleum-based routes has prevented its commercial application. The fermentation process of producing succinic acid from biomass seems promising over the present petroleum process. Some pundits in this industry are projecting its market price as low as $0.20/1b. So, it is desirable to reconsider the above described chemical transformations as potential chemical processes. 1.2. Succinic Acid and Succinic Anhydride Succinic acid (1,4-butanedioic acid), Cid-1604, is a constituent of almost all plants and animal tissues. It was first prepared as the distillate from amber (Latin, succinum) for which it was named. Succinic anhydride (3,4—dihydro-2,5-furandione), C4H403, was first obtained by dehydration of succinic acid. Succinic acid is used in the food, pharmaceutical, cosmetics, agriculture, textile, and polymer industries. Physical properties of succinic acid and its anhydride are summarized in Table 1.1. 32m 20m iHHfi 41.4-br Ester; . I Cal. 64) 3311?; C 2 ' Hilde l D r (IIHz-COOH CH2-COOH (succinic acid) (1) Stobbe condensation (4) CHz-COOCH3 products *— I CHz-COOCH3 (dimethyl succinate) (y (2) l (2;\ (3) HO—CHz-CHz-CHz-CHz-OH U a0 (5) E10 (1 ,4—butanediol) (tetrahydrofuran) (Y-butyrolactone) (N -a1kyl-2-pyrrolidinone) (1) Esterification process with methanol (2) Catalytic hydrogenation over copper chromite, copper, or copper-chromium- manganese (3) Hydrogenation of solution of succinic acid and R’NH2 where, R’ = H (ammonia) or an alkyl group (alkyl amine) (4) Condensation with aldehydes or ketones and metal alkoxide (5) Condensation of ‘y-butyrolactone with R’NH2, an alkyl amine Figure 1.1. Reaction Pathways from Succinic Acid (2) Table 1.1. Physical properties of succinates under study1 Properties Succinic acid Succinic Dimethyl Diethyl Anhydride succinate succinate M01. wt. 118 100 146 174 MP (°C) 188 119 19 -21 BP (°C) 235 (dehy) 261 197 216 Density (g/ml) 1.572 1.234 1.120 1.010 Solubility in water 7.7 Insoluble 6.8 1.8 __(g/100 g solvent) 121 @ 100 °C Solubility in ethanol 8.0 Insoluble 23.2 Miscible (g/ 100 g solvent) Enthalpy of formation -822.9 -524.1 -851.0 (gas)(1rJ/mol) rProperties at 25 °C, unless otherwise indicated. Succinic anhydride is currently manufactured by catalytic hydrogenation of maleic anhydride (2). Maleic anhydride is produced from the vapor phase oxidation of hydrocarbons or benzene over a solid catalyst. Maleic anhydride is very inexpensive ($0.44/1b) (1). Raney nickel, nickel, or palladium on different carriers are used as the catalyst for hydrogenation process (2). The reaction is carried out in liquid phase at a temperature of 120—180 °C and at moderate pressures (72-580 psi). The yield of the hydrogenation reaction is virtually theoretical. Succinic anhydride is dissolved in hot water to yield succinic acid, which is separated as crystals upon cooling, filtered and dried. The preparation of succinic acid by fermentation has been studied extensively. Yields of succinic acid from glucose as high as >100% (CO2 is incorporated) based on glucose charged have been reported (3). Historically, acetic acid as a co-product reduced yields of succinic acid, but recent advances have nearly eliminated acetate formation. But, the fermentation processes are not commercially acceptable yet, because of an inexpensive petroleum feedstock available. Process economics and hence the market '11» b1 i «”5 ”11‘5“ ., . t4!“- ‘.’ ”‘I Lit n13” ......,.;. My“. 3 33;: 1011‘ 4.,» a“ fi NJ! i. .» .‘9 price of succinic acid is based mostly on separation methods employed to remove the acid from the product mixture. The fermentation technology for the production of succinic acid is gaining momentum with new innovations in process technology (4-8). This would lead to a definite drop in the price of the acid as a raw material. Succinic acid is also obtained in large amounts as a by-product in the production of adipic acid (2). Worldwide consumption of succinic acid and succinic anhydride is the range of 18,000-20,000 tons/year (2). Mostly it is consumed and produced in Japan (Kawasaki Kasei, Nippon Shokubai, Takedo Chemical, Kyowa Hakko, and New Japan Chemicals). Buffalo Color is main producer in the US (500 tons/year) (2). Succinic acid currently is priced at $2.72/lb (l) and succinic anhydride at $1.71/1b (1). Heating of succinic anhydride above 200 °C causes decarboxylation and the formation of the dilactone of gamma ketopimelic acid (1,6—dioxaspiro[4.4]nonane-2,7- dione) (2, 9-10): 0 O 2 0 Heat ——-> O O '1' C02 Succinic anhydride Dilactone of gamma kitopimelic acid The above reaction takes place at lower temperatures in the presence of alkali. Succinic anhydride undergoes thermal decomposition in the gas phase to give carbon monoxide and carbon dioxide (1 1-13). Succinic anhydride can be decomposed even in the liquid phase at low temperature (13). Succinic acid is stabilized against the I , v9)" .“1' pub. A. L W 7“} H.“ has: deteriorative effects of heat by the addition of small amounts (0.5 wt%) of boric acid (12). 1.3. Alkyl Esters of Succinic Acid Esterification is a very common, well-understood process (14-15). Succinic acid readily undergoes esterification to alkyl esters upon mixing and heating with the appropriate alcohol in the presence of small amounts of sulfuric acid (2). R-COOH + R’OH = R-COO-R’ + H20 Esterification generally proceeds rapidly, but in many cases the yield of ester is limited by equilibrium constraints. Because of this, most commercial processes either use a large excess of alcohol or constantly remove one of the reaction products to drive the reaction to completion. Excess amount of alcohol is used to minimize the yield of monoester of succinic acid. In a laboratory study, yields of 85% and 95% were reported for methyl and ethyl succinates, respectively, from succinic acid (16) via this pathway. Several patents describe the esterification of succinic acid as part of its recovery from the aqueous waste streams of adipic acid formation processes (17-22). Esters of succinic acid can be made by routes other than direct succinic acid esterification. According to the Davey-McKee technology (23-24), maleic anhydride is first hydrolyzed and esterified to dialkyl maleate and then hydrogenated over a metal catalyst to the esters of succinic acid. Physical properties of dimethyl succinate and diethyl succinate are given in Table 1.1. 1,1 LLBu The mm: L . ,. n n ' w“. W: flannel“. t b t l ' I .rzmei 1.4. 1,4-Butanediol, y—Butyrolactone, and Tetrahydrofuran The catalytic reduction of succinic acid or ester yields 1,4-butanediol (BDO), 7- butyrolactone (GBL), tetrahydrofuran (THF), or a mixture of these compounds, depending upon the catalyst and reaction conditions (2526). The chemical structures of 1,4.butanediol, y-butyrolactone, and tetrahydrofuran are given in Figure 1.1. Demand for 1,4—butanediol and 'y-butyrolactone has been steadily increasing these past years. The largest uses of 1,4—butanediol are internal consumption in manufacturing of tetrahydrofuran and y—butyrolactone (26). But demand for 1,4-butanediol has steadily increased owing to its use in the manufacture of polytetrarnethylene glycol (PTMEG) and polybutylene terephthalate (PBT) resin, along with a steady increase in the use of polyurethane (26). Butyrolactone is principally consumed by the manufactures by reaction with methylamine or ammonia to produce N-methyl-Z-pyrrolidinone and 2- pyrrolidinone, respectively. A review of current technologies used by the industry shows that the Reppe process is still the method being followed by the majority of manufacturing companies for production of 1,4-butanediol, but maleic anhydride based routes are the only ones in new plants being built. The low cost of acetylene as a raw material offsets any other alternative routes such as succinic acid or esters. Gas-phase catalytic hydrogenation of succinate to form 1,4-butanediol, 'y-butyrolactone, or tetrahydrofuran is gaining momentum as the recent development of efficient fermentation technologies promises to lower succinic acid costs. Catalysts mentioned in the literature for the hydrogenation of succinates include copper chromites with various additives, copper-zinc oxides with promoters, silica-supported metal catalysts, and many others (25-26). 15. 5101 15.1. In . ~5013q '- st.“ .5\-' V ”PM" ‘ Huh“. ’1‘? im‘ u rhrt ”an. \ “1" ’ - alik‘ . I v ‘11:“!- ._ . . 0“}k‘ 5 A 1.5. Stobbe Condensation 1.5.1. Introduction One important reaction that is specific to esters of succinic acid is the Stobbe condensation. Ethyl, methyl, or t-butyl esters of succinic acid are used in Stobbe condensation with aldehyde or ketone compounds. This condensation reaction has led to the preparation of a variety of unsaturated and saturated substituted derivatives of succinic acid. The reaction of carbonyl group (aldehydes or ketones) with esters of succinic acids to form alkylidenesuccinic acid (substituted itaconic acids) or a tautomer is called the Stobbe condensation. Hans Stobbe performed this reaction first in 1893 using a mixture of acetone and diethyl succinate in sodium ethoxide (27). One mole of a metal , alkoxide per mole of succinate and aldehyde or ketone is required for the Stobbe condensation. The salt of the half ester is the primary product of the reaction. OOC2H5 + NaOR‘ cooczn5 é + C2H50H + R‘OH Hzcnzcooczn5 R2C= CHZCOONa R2C=O + E Sometimes dialkylidenesuccinic acid is also formed from the condensation of two molecules of aldehyde with one molecule of ester (28). The yield of mono- or di- substituted products depends to a considerable extent upon the reaction conditions with low temperatures favoring the formation of di-substituted product (28). A variety of carbonyl groups e. g. aliphatic, aromatic, and a, B-unsaturated aldehydes or alicyclic, aliphatic, and aromatic ketones or diketones or keto esters or cyano ketones can undergo the Stobbe condensation (28-31). Dimethyl, diethyl, di-t-butyl succinate and also a- substituted alkyl-, aryl-, aralkyl, and alkylidene-succinates have been widely studied (28). “fig“. 5 “hula-x“. V 151 Ma The I . "‘|“afl;:o 3 c man.-.- s . \."‘~v-~ 5; «3...; w H Ian 1:. . c I ‘5'.)1‘1": H 4,, "Had“ w""“OIJ ! 33L; 5.1: ““Jhn "M4 \ . a. ru‘ .‘1 ‘fl' .“.‘~) Sodium ethoxide, potassium-t-butoxide, and sodium hydride are the most commonly used condensing agents (28). 1.5.2. Mechanism of Stobbe Condensation The mechanism of Stobbe condensation is depicted in Figure 1.2 (28). The condensing agent (a base) catalyzes the loss of a proton from dimethyl succinate. The resulting carbanion (I) facilitates nucleophilic attack on the positively polarized carbon of a ketone molecule (11). The resulting intermediate (III) is stabilized by a lactone ring formation (IV). Deprotonation from the lactone derivative (IV) proceeds via formation of the half ester of substituted itaconic acid (V). 1.5.3. Applications of Stobbe Condensation The Stobbe condensation is very useful in synthesis of unsaturated and saturated (by hydrogenation) succinic acids, and also has wide applications in preparing some other substances, i.e., substituted lactones, naphthols, indones, tetrahydroindanones, and tetralones. The general applications of the Stobbe condensation are discussed below. 1.5.3.1. Lactonic Acids Alkylidenesuccinic acids (or half-esters) give bromoparaconic acids (or esters) by treatment with bromine. Bromoparaconic acids on treatment with boiling water give a, B-unsaturated lactonic acids or dilactones. y-Lactones are formed when the product of Stobbe condensation with a ketone RCOR’ is heated with halogen acid, water, and acetic acid (28). R COOR R’ R' J ‘— 9° 0) /-\« 0 o 0—R (H) C, O—R (I) (111) R. R. 4’60R O O R. {i Icl—OR OR R. a 4“— C; 0 . (V) 0 (IV) Figure 1.2. Reaction mechanism of Stobbe condensation 10 1.5.3.2. The Naphthol Synthesis Alkylidenesuccinic acids (or half-esters) may undergo cyclodehydration and enolization to give substituted l-naphthol-3-carboxylic acid if acids (or esters) have the appropriate stereochemical configuration (an aryl group cis to the CHzCOOH group). Sodium acetate and acetic anhydride are commonly used for ring closure (28). 1.5.3.3. The Indone Synthesis Alkylidenesuccinic acid having an aryl group cis to the carboxyl group may undergo cyclodehydration to form a substituted indoneacetic acid and some isomeric lactone. Sulfuric acid, hydrogen fluoride, zinc chloride-acetic acid-acetic anhydride, sodium acetate—acetic acid-acetic anhydride and aluminum chloride have been used for above cyclization (28). 1.5.3.4. The Tetrahydroindanone Synthesis When the Stobbe condensation product with a cyclic ketone is heated with a mixture of halogen acid, water, and acetic acid, the resulting half-ester is hydrolyzed and the acid loses carbon dioxide to form a y-lactone (as discussed in Lactonic acid). Either the 'y-lactones or the unsaturated dicarboxylic acid thus produced may undergo ring closure with zinc chloride in acetic acid-acetic anhydride to give tetrahydroindanone (28). 1.5.3.5. The Tetralone Synthesis ‘y-Arylbutyrolactones produced via the Stobbe condensation according to the lactonic acid synthesis may be reduced to substituted y—arylbutyric acids, which on 11 l V ' 51“)“.‘s' .iL‘."vhe c Magi ‘ 6...“. firm hiss-Amt- . v.doq~‘ t .9 Lt 1131 325:0 'Vp 5:(:( "H. cyclodehydration give substituted l-tetralones. The reduction and cyclization may be carried out by the conventional methods (28). 1.5.2.6. The Equilenone Synthesis The Stobbe condensation product with 7-methoxy keto nitrile gives an unsaturated ketone upon hydrolysis and decarboxylation, which on catalytic hydrogenation yields an Equilenone (28). 1.6. Itaconic Acid and Its Isomers Citraconic acid and mesaconic acid are isomers of itaconic acid. Their structures are as follows: H2C=C—COOH H3C—C—COOH Hsc—fi-COOH H2 —COOH H —COOH HOOC—CH Itaconic acid Citraconic acid Mesaconic acid Itaconic acid is also known as methylene succinic acid or methylene butanedioic acid. It is a very valuable monomer for polymerization because of conjugation between one of the two carboxylic acid groups and the methylene group. The methylene group is able to take part in addition polymerization, giving polymers with many free carbonyl groups that confer advantageous properties in resulting polymers. . Physical properties of the itaconates and its isomers are given in Table 1.2. 1.6.1. Current Manufacturing Process (Fermentation) Itaconic acid is currently manufactured by the process of fermentation of sugars. The fermentation is carried out in stainless-steel fermenters utilizing molasses (widely 12 1 l 1.}. A y.‘g €3~§¢ ‘3 “a... in s; . | i; ‘05,“ ‘ u. ' ' . ~>t a ,~‘ Table 1.2. Physical properties of itaconic acid and its isomers‘ Properties Itaconic Acid Citraconic acid Citraconic anhydride Mesaconic acid Moi. wt. 130 130 112 130 MP (°C) 175 93-4 7-8 204-5 BP (°C) Dec Dec 213-4 Sublime Density (jlml) 1.632 1.617 1.247 1.466 Solubility in water 9.5 360 380 2.7 @ 18 °C (g/100 g solvent) 72.6 @70 °C 117.5 @100 °C Solubility in ethanol 19.8 30.6 @ 18°C (g/ 100 g solvent) Enthalpy (gas) of -729.0 -740.0 formation (kJ/mol) . 1Properties at 25 °C, unless otherwise indicated used because of its cost) or other sugars as the raw material. Initial carbohydrate in the fermenter charge is the order of 15-25%. This mixture is sterilized and incubated with a culture of Aspergillus terreus (32). Sterile air is passed into the broth at 35-40 °C for 3-7 days under a pressure of 10-15 psig. The reaction mixture is kept at constant pH of about 5.0 by adding lime to prevent the formation of any undesired products or contamination. Ammonium sulfates as a nitrogen source and some metals (Mg and Zn) are also added for proper culture growth. Itaconic acid is produced from citric acid present in the broth. Citric acid is dehydrated by the enzyme, aconitate hydratase, to icis-aconitic acid. Decarboxylation of cis-acotinic by enzyme, aconitate decarboxylase, gives itaconic acid. Itaconic acid is separated from the broth by acidification, concentration and crystallization when fermentation process is complete. Iwata Kagaku Kogyo Kabushiki Kaisha (Japan) is the largest producer of itaconic acid with 20 million lb per year (32, 37). Cargill is the only US manufacturer of itaconic acid. 13 1.6.2. Catalytic Route Tate and Berg in Pfizer first prepared citraconic anhydride and subsequently itaconic acid catalytically (1974) (33). They made these isomers by the vapor phase catalytic condensation of succinic anhydride with trioxane. A 60-80% yield of citraconic anhydride with 90-98% conversion of succinic anhydride was claimed using thorium sulfate, potassium diacid phosphate, lithium carbonate or lithium phosphate on an alumina support, Alundum (Norton, Inc.). The yield of citraconic anhydride was observed to be maximum when the molar ratio was 5 to 1 of formaldehyde to succinic anhydride. The reaction was carried out in a micro reactor at 340-410 °C. Catalyst deactivation was also reported upon prolonged exposure of the catalyst, but details of deactivation were not provided. Citraconic anhydride was hydrolyzed to get citraconic acid and subsequently isomerized at temperatures around 200 °C to form itaconic acid. The Denki Kagaku Kogyo, Inc. (Japan) also claimed some processes for synthesis methods of itaconic acid, citraconic acid and citraconic anhydride in their patents (1974- 75) (34-36). They prepared these compounds from the reaction of succinic acid or its derivatives with formalin (37% solution of formaldehyde in water) using ion exchanged zeolites-13X or silica-alumina containing group I3 or IIB metal salts. A maximum 30% yield of citraconic acid, citraconic anhydride, and itaconic anhydride at 92% conversion of formaldehyde and 31% conversion of DMS (molar ratio 2:1 DMS to formaldehyde) is reported with SiOz—A1203 catalyst at 370 °C. Contrary to Tate and Berg, the molar ratio of 2:1 ~ 5:1 of succinate to formaldehyde was preferred for better results. Citraconic acid is also prepared by the pyrolysis of citric acid (37). The pyrolysis of citric acid gives citraconic anhydride along with its isomer itaconic anhydride. 14 Citraconic acid is obtained by the hydrolysis of citraconic anhydride. Citraconic anhydride can be obtained from pyrolysis of itaconic acid under vacuum at about 170 °C (37). 1.6.3. Uses of Itaconic Acid and Its Isomers Itaconic acid is a very valuable monomer for polymerization because of the conjunction of its two carboxyl groups and its methylene group. The methylene group is able to take part in addition polymerization giving polymers with many free carboxyl groups that confer advantageous properties on the resulting polymer. Sodium polyitaconate or other alkali salts may be used in detergents to improve clarity and color, used in bleaches as a stabilizer and used in metal cleaner for rust removal (38). The calcium salts of grafts on nylon in itaconic acid exhibit excellent resistance to hole melting (39). Itaconic acid itself polymerizes very slowly to give low molecular weight products, so it is widely used in copolymerization. Itaconic acid is a specialty monomer that affords performance advantages to certain polymers when the acid is incorporated in small amount as a polymer. Itaconic acid is primarily used in polymerization for improved fiber toughness, improvement of the emulsion stabilization, super absorbing polymers (SAP), and performance characteristics such as adhesion to substrates. Styrene-butadiene latexes containing low levels of itaconic acid (below 10%) are widely used in carpet backing (40-42) and paper coating (43-44). Emulsion stability, clarity, water resistance of the coatings, and adhesion to substrates are improved by the itaconic acid. Masking tapes impregnated with the copolymer of butadiene, styrene, and 15 Via A“ ;“\n ,9 R5515» vrfi'\‘ «All. . We: is i " 3:4 “ M51 itaconic acid shows higher strength and greater resistance to delamination (45). Resistance to picking of coating from paper in high speed printing with tacky inks is improved in tetrapolymers of butadiene, styrene, acrylonitrile, and itaconic acid (46). The blend of copolymers of styrene, butadiene, and itaconic acid with dispersions of polyethylene and glycerols rosin esters affords excellent heat-sealing and laminating adhesives for paperboards (47). Acrylic and methylacrylic ester copolymers with itaconic acid exhibit a wide range of applications, such as coating, binders, adhesives, polishes, and textiles finishes. The acrylic-itaconic acid coatings show high hardness, excellent adhesion, and resistance to detergent solutions and staining by foodstuffs (48). Copolymers of butyl methylacrylate, itaconic acid, and higher (hexyl, octyl, decyl) methacrylates may be used in anchor coats for polyester photographic stripping films used in intaglio printing (49- 50). Emulsion copolymers of 2-ethylhexyl acrylate, acrylonitrile, and itaconic acid with propyleneimine gives binders for nonwoven rayon webs (51). The resulting fabrics possessed soft hand, good drape, and good resistance to alkaline detergents and cleaning solvents. The aqueous solution of c0polymers of acrylamide and itaconic acid is an effective dispersant and adhesive (52). The copolymer of acrylic acid with itaconic acid is a very good etching agent for lithographic plates (53). Copolymers of vinylidene chloride, acrylic comonomers, and itaconic acid have considerable utility as coating for films, particularly in packaging and photography. The incorporation of itaconic acid provides strongly bonded heat seals that retain their adhesion after immersion in boiling water. Vinylidene chloride coatings containing the monomer exhibit improved adhesion to paper, cellophane, and poly(ethylene l6 .3n‘h' (5,...1. I v I" 5 '1“: h 90-," Visual A I x .. nuke stunt. terephthalate) films (54). The copolymers of vinylidene fluoride, tetrafluoroethylene, vinyl butyrate, and itaconic acid give rust-resistant coatings for galvanized steel (55). Copolymers made of vinyl acetate, acrylate, and itaconic acid are also used for coatings (56). Such coatings on chipboard were smooth and receptive to ink and displayed excellent resistance to pick by inks. Dental cements made of acrylic-itaconic acid c0polymers that are cured with polyvalent metal compounds such as aluminosilicates or oxides of zinc and magnesium possess good compressive and adhesive strength and physiological compatibility (57). The dimethyl, diethyl and di-n-butyl esters of itaconic acid can also be used in copolymers, e. g., for adhesives and esters with long chain alcohol have been proposed as plasticizers. Itaconic acid produces N substituted pyrrolidinones withamines that can be used as thickeners for greases (58). Some other pyrrolidinones made from itaconic acid and a wide range of amines have potential uses in detergents, shampoos, pharmaceuticals and herbicides. A condensate of lauric acid and aminoethylethanolamine reacts with itaconic acid to give an imidazoline derivative useful as an active ingredient in shampoos (59). The cyclic adducts of itaconic acid with long-chain alkyl amines, oil soluble adducts, confer antirust properties on gasolines and fuel oils (60). Citraconates are not as useful as their counterpart itaconates. The a, B—substituted double bond of the citraconates is less reactive in homopolymerization than its isomer itaconates. Copolymers of citraconic anhydride are useful in molding, adhesives, and coating (61). 17 1.3.1. l ,. r’-o;-‘v I" u. I». M I 1 r M...- q. . . . "numb. ‘ . “Flaw I ~ A 535 «ti 399?.“ '.I «m. .t A ‘5”- .J.“ t ’62:“ z t: ,7 DEE ; ..‘|"‘-t .. .‘ ‘_4 :‘V‘VIH . -7 9 ' “Wait 5 .1,‘ arm.“ "1. ;,,-.-_ .H x.“ “h ‘Oté ‘1’1' A F “MW?“E: 1.7. Formaldehyde and Its Sources 1.7.1. Formalin Formaldehyde, CH20, is the first in the series of aliphatic aldehydes. Because of its relatively low cost, and high order of chemical reactivity, formaldehyde has become one of the world’s most important industrial chemicals (62-64). The carbonyl group of formaldehyde, which carries two hydrogen atoms and no alkyl group, provides it higher chemical reactivity. Annual worldwide production capacity of formaldehyde is 12 x 10‘5 metric ton of equivalent 37 wt% aqueous solution (62). All formaldehyde is produced from methanol and it is usually marketed in the form of aqueous solution about 37 percent by weight dissolved formaldehyde. The standard 37 wt% U.S.P. solution, also known as Formalin, contains sufficient methanol (7 to 15% by weight) to prevent precipitation of polymer under ordinary conditions of transportation and storage. The current selling price of 37 wt% formaldehyde solution, Formalin, is about $0.09/lb (1), although because of the relatively small volume of merchant material, price assessment is difficult. Mostly, formaldehyde production facilities are located by user facilities; it is not economical to transport the water in the solution for any significant distance. 1.7.2. Properties of Formaldehyde The properties of monomeric formaldehyde are listed in Table 1.3 (62). Formaldehyde is highly soluble in water. The dissolved formaldehyde is principally in the form of monohydrate, methylene glycol, CH2(OH)2, which itself tends to polymerize to polyoxymethylene glycols, HO.(CH20),..H (63-64). A small concentration of monomeric formaldehyde is also present but its concentration is well under 0.1 % even in 18 Table 1.3. Properties of monomeric formaldehyde (62-63) Molecular weight 30.026 Boiling point (°C) -19 Melting point (°C) -117 Density at -80 C (g/cm3) 0.9151 Density at —20 c (g/cm3) ' 0.8153 Heat of formation at 25 °C (kJ/mol) -1 15 .9 Free energy of formation (gas phase) at 25 °C -109.9 (kJ/mol) Heat of combustion (kJ/mol) 561 Heat of vaporization at -19 °C (kJ/mol) 23.3 Heat gpacity (J/mol-K) 35.4 Flammability in air (vol%) 7-73 ignition temperature (°C) 430 concentrated solution at 60 °C. Low formaldehyde concentrations favor methylene glycol and high concentrations favor the polyoxymethylene glycols. The action of methanol in preventing polymer precipitation in formaldehyde solution is due to formation of herniacetals, which exist in a state of chemical equilibrium with the hydrated formaldehyde (methylene glycols) in solutions to which it has been added (64). HO-CHz-OH + CH30H Q HO—CHz-OCHg + H20 Methylene glycol Methanol Formaldehyde hemiformal Water The dissolved formaldehyde is completely available for chemical reaction, the solutions do not give up dissolved formaldehyde even on being warmed. The largest end-use market for formaldehyde is construction (65). Urea- formaldehyde resins, phenol-formaldehyde resins, and polyacetal resins are the largest commercial derivatives of formaldehyde which are widely used in the housing and building industry (63). Dyes, tanning agents, dispersants, vitamins, flavorings, and pharmaceuticals make up its smaller end-use markets. Formaldehyde is also used as a corrosion inhibitor of metals and as a preservative and disinfectant in cosmetics and soap 19 (65). Borden Packaging and Chemicals (1600 m.tJyr). Georgia-Pacific (1100 m.t./yr), and Celanese (800 m.t.lyr) are some of the large US producers of formaldehyde (65). 1.7.3. Manufacturing Processes Essentially all of the world’s formaldehyde production is derived from methanol. There are two existing processes that compete with each other for the manufacture of formaldehyde from methanol (62-63). First, known as the “silver process”, is based on the air oxidation of methanol over a silver catalyst and under conditions of excess methanol to avoid the explosive range (62-63). In this process a portion of the methanol is dehydrogenated to formaldehyde. The reactions occur at essentially atmospheric pressure and 600 to 650 °C. Methanol conversion is 65-75% per pass. The other process, known as the “oxide process”, is based on air oxidation of methanol under conditions of lean methanol concentration to avoid the explosive range (62-63). The reaction occurs over a mixed oxide catalyst containing iron oxide and molybdenum oxide in a ratio of 1.5 to 3. In contrast to the silver process, all of the formaldehyde is made by the exothermic reaction at atmospheric pressure and at 300-400 °C. At high temperatures, formaldehyde decomposes almost exclusively to carbon monoxide and hydrogen as indicated by the equation (64): CH20 (g) ¢> C0 + H2 Catalysts have significant influence on formaldehyde. decomposition. In the presence of finely divided platinum, decomposition is stated to occur at 150 °C. Various inorganic materials, such as sodium carbonate, alumina, and chromium oxide etc., also accelerate formaldehyde decomposition at lower temperature. 20 an“: '5 > .wusi '. N. a ti La" 17.4. g \ 4. '- c~. Formaldehyde is a reducing agent. The Cannizzaro reaction involves the reduction of one molecule of formaldehyde with the oxidation of other (64). 2CH20 (aq) + H20 -> CH30H + HCOOH Although it is normally alkali catalyzed, the Cannizzaro reaction also takes place when formaldehyde is heated in the presence of acids. Formic acid can be decomposed further to carbon monoxide and hydrogen. 1.7.4. Formcel There are some other formaldehyde solutions available commercially. Forrncel (66), a product of Celanese (a member of the Hoechst group), contains 55 wt% formaldehyde, 35 wt% methanol, and 10-wt% water. It is an equilibrium mixture of hemiacetals of the methanol and formaldehyde. Forrncel is less subject to side reactions than aqueous formaldehyde solution, is stable at room temperature, and offers readily available formaldehyde and methanol for chemical reactions. The boiling point of Forrncel is 102 °C and its specific gravity 1.071. Georgia-Pacific provides a 60 wt% formaldehyde solution in water, but this formaldehyde solution is not stable, precipitates, below 60 °C. 1.7 .5. Trioxane Trioxane, (CH2O)3. the cyclic trimer of formaldehyde, is a very good source of water-free pure formaldehyde. It melts at 62 to 64 °C and boils without decomposition at 115 °C. Trioxane forms an azeotrope with water, which distills at 91.3 °C and contains 21 70 wt% trioxane. Trioxane is soluble in water, dimethyl succinate, diethyl succinate, alcohol, ketones, organic acids, ethers, phenols etc. Trioxane preparation primarily involves distilling a 60 to 65 wt% formaldehyde solution in the presence of 2 wt% sulfuric acid and extracting trioxane from the distillate by means of a water-immiscible solvent such as methylene chloride (64). A crystallization process is used to isolate trioxane from the mixture. The thermal decomposition of trioxane in the gaseous state occurs at 270 to 345 °C and it is a homogeneous reaction of first order (64). Decomposition of trioxane vapors to formaldehyde gas can be achieved at 200 to 240 °C in fixed bed reactor using a suitable catalyst like potassium acid sulfate on activated carbon or silicon carbide, phosphoric acid on silicon carbide or “Amberlite” IR-120 ion exchange resins. Trioxane readily depolymerizes to monomeric formaldehyde at comparatively low temperature in the presence of strong acids, such as sulfuric acid, hydrochloric acid, and phosphoric acid, or acidic material such as ferric chloride and zinc chloride (64). The monomeric formaldehyde produced by this method is very reactive and polymerizes to a high molecular weight polyoxymethylene in the absence of a formaldehyde acceptor (64). By use of this depolymerization reaction, trioxane may be employed as a special form of anhydrous formaldehyde. 1.7.6. Paraformaldehyde Paraformaldehyde, H0.(CH20),..H (n = 8 to 100), is a polymer of formaldehyde. It is a mixture of polyoxymethylene glycols containing about 95 wt% formaldehyde and a balance of free and combined water. The melting point of paraformaldehyde ranges from 22 ”it ,. 151) s l ‘ Aflu' light. A . . ..,.,. J" i .\r . e "‘l‘av he... ””l‘u I‘m. at q. ‘. ' C‘- 120 to 170° C and it depends on degree of polymerization. Paraformaldehyde gradually vaporizes to formaldehyde at ambient conditions Upon prolonged exposure (64). The depolymerization is very fast at elevated temperature. Paraformaldehyde completely depolymerizes to monomeric formaldehyde and some water. Paraformaldehyde dissolves slowly in cold water, but it dissolves rapidly in hot water, hydrolyzing and depolymerization as it dissolves. Formaldehyde solutions are obtained by dissolving paraformaldehyde in hot water. Dilute acids or alkalies considerably accelerate the rate of depolymerization of paraformaldehyde in hot water. Paraformaldehyde is not soluble in other common solvents. Dissolving gaseous formaldehyde excessively in water produces paraformaldehyde (64). When the concentration of formaldehyde in aqueous solution is increased by evaporation or distillation, the concentration and average molecular weight of dissolved polyoxymethylene glycols increases and precipitation takes place. The lower polyoxymethylene glycols primarily formed undergo further reaction upon standing and paraformaldehyde is formed. 1.7.7. Methylal Methylal, CH2(0CH3)2, also known as formaldehyde dimethyl acetal, formal, and dimethoxymethane, is the dimethyl ether of methylene glycol. It is a colorless, flammable, ether-like liquid that boils at 42.3 °C and freezes at —105 °C (64). It dissolves in approximately three times its volume of water and is infinitely miscible with alcohol, ether, and other organic solvents. It is converted to formaldehyde by vapor phase oxidation in the presence of a methanol oxidation catalyst. Methylal is prepared by 23 distillation from the equilibrium mixture obtained by addition of an acid catalyst to an aqueous solution of formaldehyde and methanol (64). 1.7.8. Safety Factors Formaldehyde is poisonous by inhalation and by swallowing (62). The vapors of formaldehyde are irritating to the eyes, nose, and throat. The formaldehyde solutions are irritating to the skin and can cause severe eye burns. Ingestion of the solution irritates and inflames the mouth, throat, and stomach, causing nausea and vomiting. The carcinogenic potential of formaldehyde has not been determined. Results from various studies differ. 1.8. Catalyst Most of the largest-scale catalytic processes take place with gaseous reactants in the presence of solid catalysts, which are mostly porous inorganic materials. Catalysis takes place as one or more of the reactants is chemisorbed on the surface and reacts there. The activity and selectivity of the catalyst depends strongly on the surface composition, texture, and structure. The surface area, porosity, pore shape, pore size distribution, mean pore size, particle size distribution, and the shapes and sizes of particle are some of important parameters which describe the catalyst texture of porous solid. 1.8.1. Surface Area The surface area of a solid catalyst involves the internal surface area associated With pores and of the external surface area developed by the outer boundary of the 24 Mk 1» Do: an l 1 ba- 'N. 'ui particles. The BET method, developed in 1938 by Brunauer, Emmett, and Teller, is widely used for surface area determination of a solid catalyst (67). The basis of BET procedure is physisorption. Physical adsorption is equilibrium coverage similar to surface liquefaction. Produced by van der Waals forces originating in surface atoms, it is approximately the same for all materials. Coverage proceeds first with adsorption on surface atoms but is quickly followed by the generation of additional layers even before complete monolayer forms. Since the process is exothermic and at equilibrium, the amount decreases as temperature increases. Easily measurable quantities are found close to the normal boiling point of the adsorbate. At low pressures (p/po = 0.1), monolayer formation follows the Langmuir equation Vaar = (Kp/ p0) V... (1+ Kp/ po) (1) with Vm the monolayer volume, p the pressure, p0 the saturation pressure at measurement temperature, V“. the volume of gas adsorbed at pressure p, and K a constant. The BET equation is given by p = 1 (c-l) Vaar(po-p) Vmc+ VnC (plpo) (ID The parameter c is the BET constant that is related to the heat of adsorption. By plotting mes (po-p) against p/po a straight line results in which Slope=S = “-1) (1H) Intercept = = —1- (IV) VMC 1 = V (S + I) ( ) 25 A ¥r Surface Area = V». * N * A». (VI) Where N is Avogadro’s number and Am is the cross sectional area of adsorbate molecule, which, for nitrogen, is 16.2. A2. The BET surface area of the sample is expressed in 2lg. In this technique a weighed sample of the catalyst material to be analyzed is placed in a tube and heated under vacuum to be degassed. The sample tube is then cooled in liquid nitrogen in a flowing stream of nitrogen in helium at a fixed nitrogen partial pressure. After equilibrium, the sample is heated and the amount of nitrogen desorbed is measured. The sequence is then repeated with successively higher partial pressures. 1.8.2. Acidic and Basic Properties on Solid Surfaces Solid acids have been widely used as catalysts or catalyst supports in the industry for many years. Use of solid acid catalysts provides several advantages over liquid acid catalysts, e.g., high catalytic activity and selectivity are observed, repeated use of solid acid catalysts is possible, separation of a solid acid catalyst from a reaction mixture is easy, solid acid catalysts do not corrode reactors, and there is no disposal problem. A solid acid shows a tendency to donate a proton or to accept an electron pair, whereas a solid base tends to accept a proton or to donate an electron pair. Acidic and basic sites on the surface of y—alumina can be visualized according to the scheme shown below. Alumina dehydrates at high temperature: 26 lid :4, «a. OH OH O heat + ——O-—Al O A: -—-> -—O—Al—O—Al -H20 Lewis acid Basic site site (I) (11) However, there is always sufficient water present on the catalyst surface to give H + H \O/ 0‘ (m —’ | +H20 ——O——Al——O—Al Bronsted Basic site acid Site (In) The Lewis acid site is visualized as an incompletely coordinated aluminum atom formed by dehydration, and the weak Bronsted site as a Lewis site which has adsorbed moisture, while the basic site is considered to be negatively charged oxygen atom. The electronegativity of the Lewis site is weakened by adsorption of water, since an electron pair from the oxygen atom of the water molecule is donated to the Lewis site. Thus, the negative charge of oxygen at a basic site becomes higher when water is adsorbed on a Lewis site due to a weaker inductive effect of the aluminum atom. 1.8.2.1. Acid Strength and Hammett Acidity Function The acid strength of a solid is defined as the ability of the surface to convert an adsorbed neutral base into its conjugate acid. If the reaction proceeds by means of proton transfer from the surface to the adsorbate, the acid strength is given by the Hammett acidity function Ho, 27 Ho = pKa + log[B]/[BH +], (VII) Where [B] and [BH+] are respectively the concentrations of the neutral base (basic indicator) and its conjugate acid and pKa is pK3H+. If the reaction takes place by means of electron pair transfer from the adsorbate to the surface, then the Hammett function, H0, is given by Ho = pKa + log[B]/[AB], (VIII) Where [AB] is the concentration of the neutral base which reacted with the Lewis acid or electron pair acceptor, A. The acidity of a solid is expressed as the number or mmol of acid sites per unit weight or per unit surface area. For the determination of strength of a solid acid, the amine titration method using Hammett indicators is used (68-69). In the amine titration method, pretreated catalyst samples in a nonpolar solvent like dry benzene are titrated using a base, n-butyl amine, in presence of suitable Hammett indicators. Hammett indicators, a series of arylalcohols, which react with acid to form carboniurn ion, have a known pKa value for the acid-base color change. Catalyst samples are distributed to several sample glasses, and two drops of one of the Hammett indicator solutions are added into it. The end point of titration is determined visually from the resultant color changes and quantity of added titrant gives the number of acid sites with strength less than pKa’s of the Hammett indicator used. Using a range of Hammett indicators with different pKa, the distribution of acid strength is determined. Similarly, benzoic acid titration is used to determine the basicity of the support. 28 1.8.2.2. Temperature Programmed Desorption (TPD) method The gas chemisorption method using Temperature Programmed Desorption (TPD) is used to determine the acid-base properties of the catalyst (68-69). In TPD, the solid sample is kept in quartz spring balance, evacuated, and a base is introduced for adsorption. Now, evacuation is carried. out at high temperature for desorption of adsorbed base on acid sites. No further decrease in sample weight on evacuation indicates the base is chemically adsorbed on the surface and gives the acid strength. Strong gaseous bases such as ammonia, pyridine, and triethyl amine adsorb on acid sites with a strength proportional to the acid strength and are difficult to desorb on elevated temperature evacuation of the adsorbed bases from acid sites. The acid strength distribution of the catalyst surface is studied by temperature-programmed desorption of _ ammonia or pyridine or triethyl amine, whereas surface basicity of the catalyst is ‘ measured by stepwise thermal desorption of CO2, phenol or nitric oxide. In our work, gases (NH3, CO2, etc.) are adsorbed onto the catalyst surface in controlled quantities and then desorbed by programmed heating in a Micromeritics Pulse-Chemisorb 2700. The amine titration method and TPD method give the sum of the amount of both Bronsted and Lewis acid sites. So, another method is needed to distinguish the Lewis and Bronsted acidities on the surface. 1.8.2.3. DRIFTS Study The infrared spectra of bases (ammonia or pyridine) adsorbed onto the catalyst surface show different IR peaks for Lewis and Bronsted acidities. A Perkin Elmer 2000 FTIR spectrophotometer is used for DRIFTS studies in our work. The peaks at 1,450, 29 1,490, and 1,610 cm'1 which are observed on all the mixed oxides are characteristic peaks of pyridine coordinately bonded to Lewis acid sites (68). The peak at 1,540 cm’1 is due to pyridinium ion formed by the adsorption on Bronsted acid sites (68). 1.8.3. Different Catalyst Supports 1.8.3.1.Alumina Alumina has been extensively used as a catalyst or catalyst support since it has all of the interesting features of a satisfactory support and also represents many of the problems encountered in the selection of a support. Alumina is amphoteric in nature, which means that can act as an acid in a basic medium or as a base in acidic medium. Alumina has a high melting point, ~2000 °C, which is also a desirable characteristic for a support. Alumina in the hydroxide form can also be a voluminous gel. This makes possible the production of alumina in the form of high surface area, highly porous, comparatively low-density support material. The most important properties of alumina are its transition phases that exist over a very large temperature range (70). Aluminas are the trihydroxides, A1(OH)3, of which the two crystalline forms, Gibbsite and Bayerite, are the most common. Loss of a water molecule leads to the oxyhydroxide, A10(0H), Boehmite. Further dehydration leads to the transition aluminas that have the generic formula, A1203.xH20 with 010w 83.8 N tea 0 83> 3.5 88813 ESwfiQ 30E ogfionom .NN 23mm". :8 .2.. AI 2282.8 “once:— 4 U A>>V see, ....... ................ .533-‘01-(«4 4’ 4'?.'.\’~ V 5 NV. ( ‘Y‘ ‘Jn . -.:.-.:.-.-.-.;. “M. -: s' . . \ . Mg. -:g.. “A“ \‘»‘V’ 'r \ '- . e.\o_ Ile‘ :_- . .e a. e . n v A :5 8c .258 :3- “-:-,x-;c-:-:<~'-;-~‘fi :-:-;-:-:' :. fi'Ne'u'r‘u'n‘ 'A'fi'u f“. I .. )3: 89:3 888M IV '. .‘.‘::r‘fi&'- -:;:-.’-:::3.’-.’-.<:1:= \'.‘.'-'.‘.N~‘J‘-V .. a: E NM m\ coo... \ at a 40 without any gaps to give good insulation. Two more 6 x 11.5 x 12.5 mm brick pieces were added at the top and bottom for insulation of the connecting assembly and a part of the nipple on either side. The whole assembly, including the bricks enclosing the furnace which encloses the reactor, is mounted on an aluminum stand for stability and access. The furnace is controlled by an Omega series CN-2010 programmable temperature controller with the temperature of the surface of the reactor being read with a thermocouple. The set point is then adjusted to achieve the desired catalyst bed reaction temperature and the difference is always accounted by keeping a higher set point. 2.3. Feed System The feed was fed into the reactor using Bio-Rad Soft-Start Pumps (Model No. 125-1250), a dual piston and positive displacement pump. The pump is capable of pumping fluids up to 6000 psi and has a flow rate range of 0.02 ml/min to 9.9 ml/min in increments of 0.01 ml/min and also measures the pressure it is pumping against. The pump sounds an alarm if pressure of the reactor system exceeds the defined pressure limit. A 1/8 inch Teflon tubing was used from the burette to the pump and 1/ 16 inch stainless steel tubing was used from pump outlet to the reactor inlet. The feed was kept in a burette well above the pump heads. The choice of feed dictates the feed configuration of the system used and so, the liquid feed system was designed accordingly. 41 2.3.1. Succinic Acid Esters and 1,3,5-Trioxane Feed If trioxane is used as a formaldehyde source, then the feeds are combined and only a single pump is used. The solubility of 1,3,5-trioxane in diethyl succinate and dimethyl succinate is 1 mol/mol of ester. The feed line from the burette to 60 cm before the reactor inlet, including pumps, is heated at a temperature of 70 °C using heating tapes and variable autotransforrner if the feed ratio is above 1 mol of trioxane to 1 mol of succinate. In all cases, the 60 cm.of feed tubing just before the reactor inlet is heated at 250 °C using 0.5 inch wide heating tape. 2.3.2. Succinic Acid Esters and Formalin Feed Dimethyl succinate is not miscible with Formalin when the molar ratio of formaldehyde to DMS is less than one. Diethyl succinate is immiscible with Formalin at ‘ any molar ratio. In these cases, the two reactants are pumped by separate Bio-Rad pumps; otherwise one pump is enough. 2.3.3. Succinic Anhydride and 1,3,5-Trioxane Feed Succinic anhydride (mp 118 °C) and trioxane (mp 64 °C) both are solids at room temperature. The Bio-Rad pumps are not advised to be used at temperature above 50 °C. A Simplex syringe pump (Model No. MSP-5 Series 1) from PDC Machines was therefore used to feed succinic anhydride and trioxane in molten phase. The syringe barrel, 500 ml capacity, is made of stainless steel and has packing of Graphite Filled Teflon (GRTEF), which allows its use at temperature up to 200 °C. The syringe of the syringe pump was wrapped with 1 inch wide heating tapes. The barrel outlet on the top was connected to a 42 three-way Autoclave Engineers Valve that can be also used at high temperature. One port of valve was connected to the top of the reactor through 1/16 inch stainless steel tubing. The 1/16 inch was kept inside 1/4 inch copper tubing and steam was flowed through the copper tubing to keep the 1/16 inch feed line above 100 °C. The whole copper tubing was insulated with glass wool and polyethylene pipe insulation. The valve and copper tubing between the purrip and reactor were also wrapped with 0.5 inch wide heating tape to superheat the steam around to 115 °C. The mixture of feed melts completely around 105 °C and trioxane vaporizes at 118 °C, so it was desired to keep everything between those temperature limits. Four different surface thermocouples were used for better temperature control of feed. The first thermocouple was cemented on the body of the barrel, the second was on the top of the barrel, the third was installed on the body of the valve, and the fourth one was on the outer surface of the copper tubing. The third port of the valve was used for refilling the pump. The pump is huge, but very capable of pumping the liquid accurately from flow rates of 2.5 ml/hr to 66.63 ml/hr at a wide range of pressures. But the dead volume inside the barrel after the plunger reaches the top caused a lot of problems for us. The syringe barrel has to be disassembled to clean inside the barrel. An Eldex pump (Model No. A-30-S) was also tried to feed trioxane and succinic anhydride in the molten state. The feed line including the pump head and burette was kept at 120 °C and the feed line length was also minimized to avoid any clogging, but the Eldex pump was not efficient for feeding in molten phase. 43 It was very tedious and cumbersome to feed succinic anhydride and trioxane in the molten state, as the feed lines and reactor inlet often plugged and the reactor had to be disassembled and cleaned before it could be used. 2.3.4. Monomethyl Succinate and 1,3,5-Trioxane It was decided to feed succinic anhydride and trioxane in methanol. Succinic anhydride and trioxane were dissolved in methanol at 60 °C. Succinic anhydride was converted completely into monomethyl succinate (MMS); it was confirmed by GC and HPLC that the conversion of succinic anhydride was complete and that no dimethyl succinate was formed. The mixture of monomethyl succinate, trioxane, and methanol was pumped into the reactor using a Bio-Rad pump at desired flow rates. 2.4. Gas Flow System Helium was used as a carrier gas in most experiments to aid in vaporization of feeds and to sweep the vaporized feed into the reactor. A 1/8 inch tubing connects the helium cylinder to the top of the reactor. The helium line was also heated at 200 °C using 0.5 inch wide heating tape. The pressure delivered to the reactor is controlled by the regulator on the cylinder, and the reactor pressure is monitored using the digital readings from the pump as well as the analog readings from the regulator. No carrier gas was used in some experiments when monomethyl succinate and trioxane were taken in methanol, because an excess amount of methanol acted as a carrier gas in the reactor system. Carbon dioxide was also used as a canier gas in one experiment. A backpressure regulator was used downstream to control any pressure drop around the fixed-bed reactor and for better pressure control in the reactor system. The exiting gas flow rate is controlled by using Omega Rotameter (Part N o. FL-3445-C) and outlet gas flow rate is calculated using simple soap bubble meter. Two Riken Infrared Gas Analyzers (Model No. RI-550A) were used to analyze carbon dioxide and carbon monoxide directly from the exiting gas. 2.5. Product Collection System The product collection system also evolved over time. In earlier studies, a three- way switching valve (Part No. SS-41X82) from Whitey was used to switch the product collection after each 30 minutes. The three-way valve was not operable at higher temperature, because the system used to plug all the time and the' valve started leaking because of its high temperature incompatibility. The three-way valve was replaced by two two-way high temperature compatible valves (Part No. SS-2H) from Nupro. The whole reactor outlet system from reactor outlet to the product trap was kept at 200 °C using 0.5 wide heating system. Now, the problem arose from the dead volume between the tee and the closed two-way valve. The performance of these two-way valves was not satisfactory either, as they started leaking after some time. Next, a 6-port two position Valco valve (Part No. 26UWTY) with small internal port size (0.030 inch) was used to switch the product collection. The carbonaceous material resulting from cracking in the reactor plugged the small diameter internal ports, and it was very difficult to clear those ports. Finally, a large internal port (0.067 inch) Valco valve (Part No. 2L6UWTY) was used for the latter part of these studies. The Valco valve is kept in a heating chamber at 45 200 °C, also supplied by Valco (Part No. HVEB). Tubing from the reactor outlet to the valve and tubing from the valve to the sample cylinders is heated at 200 °C using 0.5 inch wide heating tape to keep the products in the vapor phase as long as possible. Products are trapped in 25 ml stainless steel Whitey sample cylinders (Part No. SS-4CS-TW-25). The outlet from the 25 ml cylinders was connected to a 10 ml Whitey sample cylinders (Part No. SS-4CS-TW-10) to collect the remainder of liquid which went with the gas stream. The schematics of the product collection system and whole reactor system are shown in Figure 2.3 and Figure 2.2, respectively. The temperature of product traps was varied with the feed used for the reaction. The traps were kept in cold or icy water if the feed was dimethyl succinate and formalin, in order to collect the highly volatile methanol and formaldehyde in the stream. Boiling water was used to surround to the product trap if succinic anhydride and trioxane were fed in molten phase, because they solidify at temperature below 100 °C. In some experiments, the product collection traps contained dimethyl sulfoxide (DMSO) and were immersed in boiling water to avert solidification of the unreacted succinic anhydride and paraformaldehyde resulting from polymerization of formaldehyde in the collection trap 2.6. ANALYSIS ‘ Products like succinic acid and succinic anhydride are not volatile enough to elute in the gas chromatography (GC) column used for analysis. It is also not possible to detect quantitatively formaldehyde dissolved in the product mixture in the GC. Some of our product mixtures from each run were hydrolyzed to obtain the total yield of 46 503% 8:00:00 8:85 .m.~ 0.5me 6 82.. 3.632 .0 8e: 3:28.. 2%: be .8 ES 2%: be as 35 :8 Es 838:8 888m L. \4 NO + + 88888 e... 93 2.. 82.8 N ten o 83> 1' a 888.. 88m 47 citraconates. Hydrolyzed products cannot be injected in GC, because of the nonvolatile sulfuric acid added for hydrolysis. Because the response of dimethyl succinate in high- perforrnance liquid chromatography (HPLC) was very poor and methanol does not appear in the column used for HPLC analysis, both HPLC and GC were used to identify and separate the reaction products. 2.6.1 High-Performance Liquid Chromatography An Isco Model 2350 dual piston HPLC pump was used for flow of the mobile 2 phase. A Bio-Rad HPX-87H organic acid column is used for the product identification. This 300 x 7.8 mm ID Aminex HPLC column uses crosslinked styrene divinylbenzene resin as a packing material. The HPX-87H column separates organic acids using ion exclusion and reverse phase mechanisms. Organic acids elute from the column in order of increasing pK.. Anions are eluted near the void volume, and acids which have been ionized in the acidic eluent elute according to the fraction of the acid ionized. The column separates neutral species, such as esters and alcohols, by reverse phase partitioning. The eluant is polar while the resin matrix is nonpolar, so the nonpolar compounds are adsorbed by the resin and are eluted after charged molecules. A guard column, 30 x 3.6 mm, (Part No. 125-0129) from Bio-Rad was also employed just before the column to remove anything that interferes with the separation or shortens the lifetime of the primary column. A cartridge holder (Part No. 125-0131) from Bio-Rad was used to hold the guard column. The guard column and cation H refill cartridge are specifically prepared of divinyl benzene matrix for the acidic mobile phase used in HPLC analysis. 48 In earlier studies, only a UV detector was used in HPLC analysis at a km of 225 nm. A UV detector gives a very large peak for small amounts of a highly conjugated compound like citraconic acid, whereas it does not respond much for dimethyl succinate even at high concentrations. To overcome this problem, an R1 detector (Waters 410 Differential Refractometer) was also used with the UV detector. The maximum wavelength in the UV detector was increased to 270 nm to avoid offscale peaks from highly UV sensitive citraconic acid and its isomers, because concentrated samples were injected in the HPLC for good response in the RI detector. Chromatograms were recorded using a Waters 745 Data Module. A typical chromatogram from the HPLC is shown in Figure 2.4. Values of retention times and response factors of products and reactants in HPLC (RI detector) are given in Table 2.1. The HPLC calibration was conducted several times during this work, so the most recent values of retention times and response factors are listed here. The method for calculation of response factors, the actual product concentrations, Data Module parameters, and RI detector parameters are given in Appendices. Table 2.1. Retention time and response factors of different compounds in HPLC Compound name Retention Time (min) Response Factor Oxalic acid 11.0 1.00 Citraconic anhydride/acid 13.6 0.52 Succinic anhydride/acid 15.2 0.52 Monomethyl succinate 18.0 0.44 Formaldehyde 20.5 0.80 Eimethyl succinate 22.7 3.00 49 CHANNEL A INJECT 08/27/99 15:56:08 A1402 50% ofRun 140.2 + 3.9 ml of CH3CN 50% 15.86 g/l oxalic acid INPUT OVERRANGE AR RT= 19.84 DSS 8/27/99 15:56:68 FILE 1 METHOD 2 RUN 162 NAME CONC RT AREA RF 1 0 10.08 309736 OXALIC INT STD 10.38 15704640 1 CITRACONIC 0.226 13.88 6019402 0.52 SUCCINIC 0.114 14.86 3443745 0.52 MMS 0.25 17.79 10305180 0.38 FORMAL ' 0.243 20.32 7645473 0.8 DMS 0.27 22.96 1572312 2.7 Figure 2.4. A typical chromatogram from HPLC 50 An acidic solvent is required as a mobile phase for HPX-87H columns. But, our products and reactants are not soluble in water. So organic modifiers like acetonitrile are needed not only to improve the column resolution but also to solublize compounds to be separated in the mobile phase. Adding acetonitrile to the eluent reduces resin/compound interactions and subsequently the strongly bound compounds elute more rapidly. Acetonitrile is the most suitable organic modifier because it has intermediate polarity, low viscosity, and low UV adsorption. 40% acetonitrile is an upper limit for the HPX- 87H column used in analysis. The high volatility of acetonitrile introduces analytical complication. The composition of the mobile phase in the sample loop of the RI detector, the mobile phase reservoir, and the sample injected cannot be the same, because highly volatile acetonitrile tends to escape during handling. This difference in the mobile phase composition results in an additional signal in the HPLC chromatogram. A UV detector is not affected by small changes in the mobile phase compositions, because the mobile phase solvent does not absorb at low UV wavelengths. In earlier studies, 32% acetonitrile in 0.005 M sulfuric acid solution was taken as a mobile phase. But, the solvent peak from the mobile phase of 32% acetonitrile solution interferes with the monomethyl succinate peak. So 20% acetonitrile in 0.005 M sulfuric acid was used as a mobile phase in the analysis after Run 91. Oxalic acid was used as an internal standard. All samples and oxalic acid solutions were prepared in the mobile phase. The HPLC column temperature was kept at 40 °C using a column heater for a better resolution. A six p‘ort Valco valve with a manual standard electric actuator (Part No. EHC6W) was used for the sample injection. With the valve in “load” position, sample was injected into a twenty microliter sample loop 51 through the injection port. The valve was switched to “inject” position to carry the sample contained in the sample loop into the column. The sample 100p was being flushed several times just before the injection by the mobile phase. 2.6.2 Gas Chromatography Products were also analyzed in a Varian 3300 Gas Chromatograph (GC) with a Flame Ionization Detector (FID) and helium as a carrier gas. The column used for analysis was a Supelco fused silica intermediate capillary column, SPBl, of 0.53 mm ID and 30 m long. The output signal generated from the GC was collected using a Perkin Elmer Single Channel Interface and the data were analyzed using Omega-2 software from Perkin Elmer. The separation in this intermediate column depends on the volatility of injected compounds; the higher volatile compounds elute earlier in the column and the low volatile compound elutes later. The temperature program for the column was developed using trial and error method to obtain maximum separation of the peaks in the product distribution. A typical chromatogram from the GC is shown in Figure 2.5. The GC was initially run with a series of known concentrations to determine response factors. Methyl lactate was used as an internal standard since its retention time does not interfere with that of expected products and reactants. The response factor of methyl lactate was taken as one and response factors of the various compounds in the mixture sample were calculated relative to methyl lactate. Sample injection size was 0.2 micro liter. Values of retention times and response factors (only the most recent values) of products and reactants are given in Table 2.2. The method for calculation of response 52 File B140_02 Dushyant Collection: 15:45:34 Aug 27 1999 Method: KIRTHI pk# RT Area . 1 1.157 50494304 2 6.019 108268536 3 10.139 21401244 4 10.237 25995980 2 {3:}: 23%: 0.2189 g ofRun 140.2 - + 0.0436 g methyl lactate 7 10.553 11080857 8 11.960 16711015 9 12.320 31338521 10 14.128 20906634 millivolts 0.0 2000.0 4000. 0 60go 0 ,. 8999-0 2.. 0.0- __.. 2._—. 1 * tr“ - g . r - 1.157 Methanol 5-0 _2‘ _ --—- 6.019 Methyl } ; lactate 10 0 ,;,__..... .._. -. a 7. 535 mix: 3: " ' Dimethyl 8110611112816 15.0 1"". H - “ 12.75 Monomethyl 81106111813 20.0 1 Figure 2.5. A typical chromatogram from GC 53 factors, the actual product concentrations and GC conditions and parameters are given in Appendices. Outlet gases from reactor were analyzed directly using CO and CO2 meter. Table 2.2. Retention time and response factors of different compounds in GC Compound name Retention Time (min) Response Factor Methanol 1.1 0.65 Formaldehyde ' 1.8 - Methyl lactate 5.9 1.00 Citraconic anhydride 9.6 - 11.0 0.59 Dimethyl succinate 11.9 — 12.3 0.65 Monomethyl succinate 13.5 — 15.0 0.92 2.6.3. Formaldehyde Analysis The HPLC was used for quantifying formaldehyde in the liquid products, but the sodium sulfite method was used to quantify the formaldehyde present in the gaseous products. In some experiments, outlet gases from the reactor were flowed through a test tube containing sodium sulfite solution to trap the unreacted formaldehyde gas. The quantitative liberation of sodium hydroxide occurs when formaldehyde reacts with sodium sulfate to form the formaldehyde-bisulfate addition product: CH2O + Na2S02, + H2O => NaOH + CH2(NaSO3)OH In the sodium sulfite method, fifty ml of a 1.0 M solution of pure sodium sulfite (prepared by dissolving 126 g of the anhydrous salt in sufficient distilled water to make one liter of solution) and three drops of thymolphthalein indicator solution (0.1% in 54 alcohol) are placed in a 500 m1 flask and carefully neutralized by titration with acid solution until the blue color of the indicator has disappeared. An accurately measured formaldehyde sample is then added to the sodium sulfite and the resulting mixture titrated slowly with the hydrochloric acid solution to complete discoloration. One millimole of normal acid is equivalent to 0.03003 g formaldehyde and the per cent formaldehyde in the sample is determined by the following equation: Acid titer x Normality of acid x 3.003 ‘ Weight of sample %Formaldehyde = The sodium sulfite method was also used to analyze the formaldehyde in control runs of formaldehyde. 2.7 Product Identification A majority of the reaction products were identified using gas chromatography by comparing their relative retention times with those of the mixtures of neat samples. Gas Chromatography coupled with Mass Spectrometry (GC-MS) was also used to verify the identification of the reaction products. The various compounds in the sample collected were identified by matching the mass spectra and retention time of each compound with reference spectra collected by analyzing a standard sample. The database in the GC-MS facility enables easy comparison, and a near-perfect match was made for almost all compounds. Other analytical methods like Thin Layer Chromatography and NMR Spectroscopy were also tried to determine unknowns. 55 2.8. Hydrolysis of Products The raw product exiting the reactor was a mixture of citraconates and succinates as discussed in detail in Chapter 4.1. Analysis of all of these products was difficult in HPLC or GC, as several of the citraconates co-elute with their analog succinates. To clearly evaluate product yield and selectivity, it was necessary to hydrolyze the product mixture in aqueous sulfuric acid solution to recover all species as the free acid. Usually, 20% of the citraconate was in the form of monomethyl or dimethyl ester, so reported yields for unhydrolyzed mixtures were lower than the actual values. The hydrolysis of product mixture was carried out by refluxing the sample in the mobile phase used in HPLC analysis plus 5-7 drops of concentrate sulfuric acid for 24 hours. 2.9 Product Yield and Selectivity Calculations GC and HPLC results were placed in an Excel spreadsheet to facilitate ready calculation of product yield, conversion, selectivity and an overall carbon balance on the system was performed. The percentage yield of a product based on feed was calculated by the relation, Moles of A in product Yield of product A ”l = ( o) Moles of succinate in feed x100 (I) The carbon dioxide yields are also based on succinate in feed, but divided by two to account for fact that each mole of succinate ester gives two mole of carbon dioxide upon cracking. The percentage conversion of dimethyl succinate (DMS) is given by 56 Moles of DMS in feed - Moles of DMS in product x Conversion ‘7 = ( °) Moles of DMS in feed 100 (II) The conversion of dimethyl succinate to the monomethyl ester of succinic acid and succinic acid is not considered part of “succinate conversion”, because these species can be recycled along with unreacted dimethyl succinate in the process. Conversion of succinates is calculated by the relation Conversion of succinates (%) = (Conversion of DMS - Yield of MMS - Yield of SA) (III) The percentage selectivity of citraconic acid is defined by, Yields of citraconic anhydride x Selectivity = Conversron of succrnates 100 (IV) Finally, a balance on total succinate carbon is done for the experiment as a measure of the quality of the experiment. Results are reported as the percentage of initial succinate carbon recovered in the product mixture. Carbon of succinate recovered is given by (V) Conversion of DMS - Yield of CA - Carbon recovered (%) = 100 - Yield of MS - Yield of SA - Yield of C02 For some experiments, an overall carbon balance was also performed. Overall carbon recovery is given by 57 Overall carbon recovery (%) = [100 _ [moles of carbon in - moles of carbon out x 100 DWI) moles of carbon in 58 CHAPTER 3 EXPERIMENTAL METHODS 3.1. Catalyst Materials Several types of commercial aluminum oxide (A1203) were obtained from Norton Chemical Process Product Corporation for use as catalysts. These include the Norton materials specified as SA3132, SA3177, SA6173, and SA6175. These alumina materials were ground to the 30/60 mesh size particles using sieve plates and calcined for six hours at 500 °C before loading into the reactor. Zirconia supports manufactured by MEI, Inc. were also evaluated; this zirconia has about 15% alumina as a binder. Magnesium oxide and iron oxide (Aldrich Chemical Co.) were also tested. A number of metal oxides were prepared in the laboratory and evaluated for the condensation reaction. These include alumina, titania, iron oxide, aluminum phosphates (AlPOa), and hydrotalcites (MgO/Al203 compounds). The catalyst preparation and characterization were done by Dr. N. Kirthivasan. Typical preparation procedures for these materials are given in the following sections. 59 3.1.1. Aluminum Phosphates (AIPOJ Aluminum phosphates were prepared from aluminum chloride (A1C13.6H20, FW = 241.43) or aluminum nitrate (Al(N03)3.9H2O, F. W. = 375.13) and ammonium hydrogen phosphate ((NH4)2HPO4, FW '= 132.06) by the method given in the literature (71). Samples with P/Al molar ratios of 0.5, 1, and 1.5 were prepared. Aluminum chloride or aluminum nitrate and ammonium hydrogen phosphate were dissolved in water, acidified with nitric acid and the hydrogel was formed by adding 30% ammonia solution to achieve a pH of 5. The resultant residue was filtered and washed with deionized water. The sample was dried at 120 °C overnight and then calcined at 400 °C for three hours. As an example, an AlPOx with P/Al ratio of 0.5 was prepared by dissolving 75 g of aluminum nitrate nonahydrate and 11.5 g of ammonium hydrogen phosphate in 800 ml of water. Acidifying the solution with nitric acid dissolved any insoluble white residue that was formed during the process. About 22 ml nitric acid was required for dissolving the precipitate. The pH of this solution was 0.5. This solution was precipitated with 49 ml ammonia (25%) to a pH of 5. The final residue was washed with 4 liters of water, dried in an oven at 110 °C overnight, and calcined at 400 °C for three hours. 3.1.2. Hydrotalcites . Hydrotalcite catalyst samples with Mg/[Al + Mg] molar ratios of 0.005, 0.01, 0.02, 0.04, 0.06, 0.12, 0.25, and 0.33 were prepared from gels produced by mixing two solutions to the procedure described by Corma et al (74). In each batch, MgCl2 (FW 95.22) and AlC13.6H2O (FW 241.43) were mixed to obtain the desired Mg/[Al + Mg] 60 ratio, and then the mixture was dissolved in deionized water to prepare an aqueous solution with a concentration of 1.5 M. A second aqueous solution was prepared by dissolving NH40H and (NI-14)2CO3 in deionized water with appropriate amounts calculated according to the relations NILOH = 2.2 Mg2+ + 3.2 A1?” and (2032‘ = 0.5 A1” (73). Na2C03 and NaOH salts were also used instead of ammonium salts for coprecipitation. To prepare hydrotalcite, the second solution was added slowly into the first solution over a period of 2-3 hours to maintain the pH of the slurry at 13 and stirred, during which time a white precipitate formed. The precipitate was then filtered and washed with deionized water at room temperature. The catalyst sample was dried at 100 °C overnight and then calcined at 450 °C. For example, a catalyst preparation with Mg/(Al + Mg) = 0.25 involved dissolution of 1.78 g of MgCl2 and 14.30 g of A1C13 in 60 ml water. In solution B, 5.6 g of Na2CO3 and 3.5 g of N aOH were dissolved in 50 ml water to make a solution that was 0.1 M in Na2C03 at pH 13. The latter solution was mixed into the former over a period of two hours, wherein a gel was obtained and the final pH was 10. The gel was aged overnight and washed with deionized water until the residual filtrate was free of Cl’ ions. The residue was dried overnight and calcined at 450 °C for six hours. The weight of the final catalyst was 3.6 g. A similar procedure was followed for preparing catalysts with Mg/(Al + Mg) atomic ratios of 0.5 and 0.75. A different approach was followed for the preparation of magnesia-alumina co- catalysts with low quantities of magnesia. Mg-Al mixed oxides with Mg/(Mg + A1) ratios of 0.005, 0.01, 0.02, 0.04, 0.06, 0.1, and 0.12 were prepared using aqueous solutions of aluminum nitrate and magnesium chloride and coprecipitated with ammonia 61 solution. The residue was filtrated and washed free from chloride and later calcined in the air at 350 °C. As an example, a magnesia-alumina co-catalyst with a Mg/(Mg + Al) ratio of 0.02 was prepared by dissolving 110.3 g of aluminum nitrate and 0.63 g of magnesium chloride in 800 ml water. The initial pH of the solution was 1.8. The solution was precipitated wit 25% ammonia solution over a period of 2 hours until a pH of 9.0 was attained. This required about 85 m1 of ammonia. The gel was aged overnight, washed free from chloride, dried at 100 °C overnight, and calcined at 350 °C for three hours. The weight of the final catalyst after calcination was 15.50 g. A similar procedure was followed while preparing catalysts with Mg/(Mg + Al) ratios of 0.005, 0.01, 0.04, 0.06, 0.1, and 0.12. The molarity of the precipitating solution was maintained constant when catalysts with varying Mg content were prepared. 3.1.3. Aluminum Oxide A sample of pure alumina (also known as Alumina-In-House) was also prepared by using method described above starting with A1C13.6H20 or A1(N03)3.9H20. As an example, alumina was prepared by dissolving 60 g of aluminum chloride in 800 ml of water. The pH of initial solution was about 2.2. The solution was filtered to remove any suspended impurities and then precipitated with 100 m1 of 25% ammonia solution to a pH of 9.0. The precipitate was aged overnight, filtered, and washed free of chloride ions. The residue was dried overnight in an oven at 110 °C and then calcined at 400 °C for three hours. The weight of calcined catalyst was 16.75 g. 62 3.1.4. Zirconia Zirconium hydroxide or zirconia (ZrO2, F. W. = 132.22) was obtained by hydrolysis of zirconium oxychloride (ZrOC12.8H20, F. W. = 322.25) with 28% aqueous ammonia solution at a pH of 8, followed by washing with deionized water until no chloride ion was detected in the filtrate. - Resulting Zr(OH)2 was dried at 110 °C for twenty four hours and then calcined at 500 °C for six hours. Alumina - zirconia (80 : 20) was also prepared starting from ZrOC12.8H2O and A1(NO3)3.9H20. 3.1.5. Iron Oxide Fifty grams of ferric chloride (FeCl3.6H20, F. W. = 270.3) was dissolved in 350 ml of water to get 15 g of ferric oxide (Fe203, F. W. = 159.69). A 30 % ammonia solution was added into ferric chloride solution to attain pH of solution to 8.0. The residue was washed with deionized water, dried at 100 °C overnight, and calcined at 500 °C for six hours. 3.1.6. Titania Titania (Ti02, F. W. = 79.86) catalyst was prepared from titanium (IV) butoxide (T i[O(CH2)3CH3]4, F. W. = 340.36) by dissolving it in water and acidifying it with nitric acid until a clear solution was obtained. Thus, 63 g of titanium butoxide was dissolved in 500 ml water and acidified with 60 m1 nitric acid. The initial pH of the solution was 0.7. This solution was precipitated with aqueous ammonia followed by washing the precipitates with deionized water, drying at 110 °C overnight, and calcining at 400 °C for three hours. The final weight of the catalyst after calcination was 15.95 g. 63 3.2. Supported Salt Catalysts The incipient wetness method was used to load catalyst salt on support. A solution with the desired quantity of salt, in an amount just sufficient to fill the pores and wet the outside of the particle, was introduced into the support in incipient wetness method. The wetted support oxide was dried slowly to properly crystallize the salt on the pore surface. The dry mixture was then calcined in a furnace at 500 °C for 4-5 hours. The supported salts include KH2P04, Ce2(S04)3, Ce(S04)2, ZnCl2, LaCl3, Li2CO3, K2C03, and KOH. The quantities of these salts on the support oxides varied; specific loadings are given in the Results. 3.3. Catalyst Characterization In this Section, methods for measuring the catalyst properties are discussed. The catalyst properties studied are: BET surface area, acidic strength, surface acidity and basicity, and type of acidic sites (Lewis or Bronsted). 3.3.1. BET Surface Area The Pulse-Chemisorb 2700 from Micromeritics was used to determine the BET surface area of solid material used in experiments. The Pulse-Chemisorb and the N2/He flow controller was turned on for 20-30 minutes before testing begins. A Dewar flask filled with liquid nitrogen was placed around the glass 100p located on the right hand side of the Pulse-Chemisorb 2700 apparatus to trap any impurities in the test gases. A known amount of a sample was taken into a sample tube. The mass of the sample was ideally taken such that the total area of the sample being tested was near 25 m2. The sample tube containing solid catalyst was attached to the Pulse-Chemisorb 2900. The nitrogen and helium tanks valves were opened and the pressure was kept at least 15 psig. The set points on the flow controller were adjusted to 15.2 (ml/min) for the helium and 0.8 (ml/min) for the nitrogen (95% He, 5% N2). The two valve on the flow controllers and the He/N 2 valve on the Pulse Chemisorb 2700 were opened. The sample was outgassed by placing a heating mantel around the sample tube and heating to 250 °C for about an hour. Then, the sample was cooled to room temperature. Once the detector output was stable, the signal was adjusted to zero and the area counter was cleared. The chart recorder was turned on at 0.5 cm/min and 50 mV span. The relative conductivity was kept negative. One ml of pure nitrogen gas was injected into the Pulse-Chemisorb 2700 via the septum located at the center of the apparatus. A peak appearing a few seconds after injection represents the excess nitrogen in the stream. When the detector had re-stabilized, the calibration knob was adjusted such that the area displayed on the LED display was 0.93. This represents the volume of nitrogen (in ml) injected at STP. The relative conductivity was changed to positive, and the area counter was cleared. A Dewar flask filled with liquid nitrogen was placed around the sample tube. A peak appeared due to nitrogen adsorption on the sample. The peak area was recorded and the area counter was cleared when the detector returned to almost zero. The area of this peak was not used in the final analysis. The relative conductivity was changed back to negative and the Dewar flask around the sample tube was removed. A beaker of water was placed around the sample 65 tube in order to bring the sample back to room temperature quickly. A peak evolved as the nitrogen desorbed from the material’s surface. This peak area is the volume of nitrogen desorbed from the sample and was used to determine the total surface area The set points on the flow controller were now adjusted to 14.4 (ml/min) for the helium and 1.6 (ml/min) for the nitrogen (90% He, 10% N2). The desorption peak area was measured using method discussed above. The above method was again repeated for the flow rates of 13 (ml/min) for the helium and 3 (ml/min) for the nitrogen (81.25% He, 18.75% N2). The sample was removed form the Pulse-Chemisorb 2700. The total surface area can be obtained using the BET program found on the computer in the laboratory. 3.3.2. Acid Strength by Hammett Indicators The determination is made by placing the catalyst sample in powder form into a test tube, adding dry benzene (a non-polar solvent), and shaking briefly. The indicators used for the acid strength determination are described in several publications (74). Catalyst samples are distributed to several sample glasses, and two drops of one of the Hammett indicator solutions are added to each. Now, catalyst samples are titrated with n- butyl amine. The end point of titration is determined visually from the resultant color changes and quantity of added titrant gives the number of acid sites with strength less than pK.’s of the Hammett indicator used. Using a range of Hammett indicators with different pK., the distribution of acid strength is determined. Similarly, benzoic acid titration is used to determine the basicity of the support. 66 3.3.3. Temperature Programmed Desorption (TPD) Using Probe Molecules The Pulse-Chemisorb 2700 machine was also used in acid-base characterization of the catalyst using temperature pro grarnmed desorption method. The machine was turned on for 20-30 minutes before testing began. The known amount of catalyst was placed into a sample tube. The argon was passed through the catalyst sample at 500 °C for two hours. The catalyst was then saturated with the probe molecule (ammonia for acidic sites and carbon dioxide for basic sites) at room temperature for one hour at a flow rate of 10 mllmin. The catalyst was then flushed with helium for one hour at room temperature to remove loosely adsorbed probe molecules, then was heated at a steady rate of 20 °C/min in helium up to 500 °C. The profile was recorded as chemisorbed probe molecules left the surface of the catalyst with increasing temperature. The flow diagram ofa TPD setup is shown in Figure 3.1. The titration and TPD methods do not distinguish between Lewis sites and Bronsted sites. The acid amount, which is measured, is the sum of the amounts of Bronsted and Lewis sites at a certain acid strength. It order to elucidate the catalytic action of solid catalysts, it is necessary to distinguish between Bronsted and Lewis sites. 3.3.4. DRIFTS Study of Pyridine Adsorption DRIFTS spectra were collected in a Perkin Elmer 2000 FTIR equipped with a DRIFTS cell (Harrick Scientific, HVC-DR2) in conjunction with a praying mantis mirror assembly (Harrick Scientific, DRA-2C0). The DRIFT S cell had a provision for heating the catalyst with the aid of a heating tape that was fixed inside the reactor. A thermocouple underneath the cup measured the temperature of the catalyst. The cell also 67 macaw 9:. 2: mo EEmEQ 32... ._.m Semi 380 23 32¢. .coecflyoom 820m 298m T... 0.1 @1 H015 D QUE 882 32m Alll £236 0: 0: 8=ob=e0 32... o>3> 8% 68 had a provision for gas flow and cooling. For DRIFTS studies, the catalysts were crushed and mixed with KBr to give a 33 wt % mixture. The KBr-catalyst mixtures were calcined at 400 °C for three hours and cooled back to room temperature prior to contact with pyridine. The vapors of pyridine were passed over the catalyst in a steady stream of helium (BOC, 99.995 %) at a rate of 50 mllmin. The reactor was heated to the desired temperature and the spectra were recorded by taking 10 scans at that particular temperature. A subtraction of spectra before and after pyridine adsorption was done to minimize background peaks. 3.4. Feed Preparation There were two reactants, succinates and formaldehyde, in our reaction. Three different forms of succinates were evaluated: dimethyl succinate, diethyl succinate, and succinic anhydride. Diethyl succinate was used in earlier experiments, but the transesterification reaction of methanol from formaldehyde with diethyl succinate led to formation of dimethyl succinate and ethyl methyl succinate. The other byproducts (monomethyl succinate, monoethyl succinate, and succinic anhydride) were also observed, complicating analysis of the product mixture. Diethyl succinate was thus replaced by dimethyl succinate to avoid the transesterification and to minimize undesired byproducts. Three sources of formaldehyde were evaluated: 1,3,5-trioxane (a cyclic trimer of formaldehyde), Formalin (a mix of 37 wt% formaldehyde, 12 wt% methanol, and 51 wt% water), and Formcel (blends of 55 wt% formaldehyde, 10 wt% water, and 35 wt% methanol). Trioxane decomposes to gaseous formaldehyde on heating above 350 °C. 69 1,3,5-Trioxane’s solubility in diethyl succinate and dimethyl succinate is 1 mol/mol of ester. If the desired molar ratio was above this value, then the whole feed line from burette to 60 cm before to inlet of reactor including pumps was heated at temperature of around 70 °C using heating tapes and variable autotransforrner; otherwise only 60 cm of tubing just before the reactor inlet was heated at 250 °C using 0.5 inch wide heating tape. Succinic acid esters and Formalin were pumped separately using two separate Bio-Rad pumps. 3.5. Reactor Operation The following sections describe the procedure for conducting a reaction. The symbols referred to are shown in Figure 2.2 in Chapter 2. 3.5.1. Catalyst Loading and Unloading in Reactor The reactor tube was accessed by opening three Swagelok nuts N1, N2, and N3 and moving the reactor up and clamping it at the top. The connection at the top of the reactor was then opened and the reactor was pulled down and removed. This procedure was followed to avoid the removal of the furnace element and the insulating firebricks in each experiment. The known amount of catalyst material was then added through the top of the reactor and was held in place at the bottom by a glass tube fitted with a quartz frit as shown in Figure 2.1. A 'similar procedure was followed for catalyst unloading after completion of an experiment. The catalyst after completion of the run was weighed for possible weight gain and stored. 70 In selected experiments, the reactor system was tested for any leakage. The helium inlet valve (V1) was turned on to allow helium to be fed into the reactor and the reactor was pressurized to 60 psi. Now, the backpressure regulator (R1) and the helium gas cylinder regulator were closed all the way to hold the helium in the reactor system. If there were no leakage in the reactor system then the reading on helium gas cylinder downstream pressure gauge would not change over time, otherwise the leak was traced by using soap and then fixed it. 3.5.2. Operation of Reactor After loading the calcined catalyst into the reactor tube, the thermocouple (T1) was inserted (from the bottom) in the gap between the furnace and the reactor tube and the tip of the thermocouple was placed against the outer surface of the reactor body. The set point was then adjusted to achieve the desired catalyst bed reaction temperature and the difference was always accounted by keeping a higher set point. The glass wool and duct-tape were used to close the gaps between the insulation bricks and the reactor tubing at the top and bottom, respectively. Now, heating tapes were wrapped at the top and bottom of the reactor. The thermocouple (T2) was used to measure the temperature at the reactor top (feed preheat). The helium inlet valve (V1) was then turned on to allow helium to be fed into the reactor and the reactor was pressurized to the desired value using the helium gas cylinder regulator. Helium flow rate was set using the flow controller (F 1), and the actual flow rate was measured with the soap bubble meter (M1). The CO, (M2) and CO2 (M3) meters were also turned on. The reactor tubing was heated to the desired temperature using the furnace; the temperature was measured by the 71 thermocouple touching the outer wall of the surface and allowed to stabilize for 50-60 minutes. With the reactor heated up and pressurized to the desired conditions, the feed was pumped into the reactor using the Bio-Rad pump(s) (P1 (and P2) at the flow rate of 0.10 mllmin to 0.45 mllmin. liquid flow rate was set with the digital meter in the pump, but actual flow rate was calculated by measuring the differential volume of liquid fed from the burette over time. Outlet helium flow rate was set using the rotameter and the actual flow rate was measured with the bubble soap meter (M1). The product was collected in the 25 ml collection vessel (C1) for 30 minutes and after 30 minutes the product line was switched to the other collection vessel (C2) using the Valco valve (V V). Most experiments were carried out for three to five hours and samples were taken every 30 minutes. The product was weighed and also the reading from the burette, CO, and C02 meters was taken. 3.5.3. Reactor Shutdown For experiment shutdown, first the feed pump was stopped. Heaters, CO, and CO2 meters were turned off one hour after stopping the pump. Helium flow was also stopped by closing the regulator on the gas cylinder and by turned off the valve (V1). The reactor system was cleaned thoroughly. The catalyst after completion of the experiment was weighed for possible weight gain and stored. 72 CHAPTER 4 CATALYST SCREENING FOR THE REACTION OF DINIETHYL SUCCINATE AND TRIOXANE 4.1. Introduction A series of catalyst and supporting materials were tested to achieve an optimal yield of citraconic anhydride from succinates and formaldehyde. The state of feed and catalyst evolved along the course of time. Several different forms of succinates and formaldehyde were used as feedstocks for the reaction. Pros and cons of each source of formaldehyde and succinate were evaluated. The condensation of succinates with formaldehyde leads to the formation of citraconic anhydride: 0 H20 HO ""'""O / on + —> O + Formaldehyde O O o O R/ \Ft Succrnate Citraconic anhydride Ether Where R = CH3, C2H5. 73 Citraconic anhydride easily hydrolyzes to citraconic acid and subsequent isomerization of citraconic acid gives itaconic acid: 0 Citraconic anhydride Citraconic, acid Itaconic acid 4.2. Products from Succinates and Formaldehyde Diethyl succinate (DES) was the first succinate employed in the reaction with 1,3,5-trioxane, a source of formaldehyde. The basic idea behind this work was to utilize a cheap fermentation-derived product, succinic acid. Diethyl succinate is an ideal green chemistry product because it can be produced from fermentation-derived succinic acid and ethanol. But the use of diethyl succinate as a reactant led to difficulty in product analysis and was thus discontinued. Formaldehyde in the presence of a basic catalyst gives methanol via the Cannizzaro reaction: OH + HO/\ =0 + H20 ——-> O Formaldehyde Methanol Formic acid The resulting methanol forms a series of transesterification products with diethyl succinate, as shown in Figure 4.1 (R = C2H5). The desired product, citraconic anhydride, also gives a series of esters in presence of methanol and ethanol as depicted in Figure 4.2 (R = C2115). To avoid this, diethyl succinate was replaced by dimethyl succinate to avoid transesterification and thus minimize undesired byproducts. 74 O HO O H + ROH + 21120 AV.— 0 o Succinic anhydride Succinic acid 1 H20 0 + 0 ”ON + H20 JOMR + ROH OR T— O O Monoalkyl ester of succinic acid Dialkyl ester of succinic acid + ——_OH ‘ Methanol O o __ I OH + ROH ‘— H20 +0 R 0 MMS O + H Alkyl methyl succinate Methanol I ii 0 O NO + H2O , l Dimethyl succinate Figure 4.1. Transesterification products from succinates 75 / Dimethyl citraconate + H20 0 Citraconic anhydride HO \ OR-l- O Dialkyl citraconate H20 OH + HO—<—;~O __h-_ ‘ Methanol O O \ Monomethyl ester of citraconic acid + H20 OH Methanol + O OH 0 H2O v‘ "l \ H Citraconic acid .1. ll ‘— ROH + HO OH O O Monoalkyl citraconate Figure 4.2. Transestrification reactions of citraconic acid 76 Carbon dioxide, carbon monoxide, and lower alkanes were also noticed in the products, in addition to citraconic anhydride and hydrolyzed products of succinates and citraconates. The source of carbon dioxide and lower alkanes was the cracking of succinates and citraconates. Formic acid, a Cannizzaro product, also contributes in the formation of carbon dioxide at high temperature: /\ 0=C=O + O __> Formjc acid Carbon dioxide HO H 2 Carbon monoxide mainly comes from the decomposition of formaldehyde: _"'O ——> 0:0 4» H2 Formaldehyde Carbon monoxide _ 4.3. Reaction Conditions The reaction studies involve primarily succinic acid esters and trioxane as the feed with helium as the carrier gas. The reaction conditions that were used included temperatures from 320 to 450 °C with a preferred range of 350-380 °C, pressure from 40 to 400 psi, with a preferred value of 60 psig, liquid flow rate of 0.1 to 0.5 mllmin, and a succinate to formaldehyde ratio of 1:0.5 to 1:5. The amount of catalyst material used in each experiment depended upon the support used. The weight of the catalyst taken, and hence the height of the catalyst bed, along with the reactor dimensions, reaction conditions, and reactant flow rates were used to calculate the residence times. The residence time and the Weight Hourly Space Velocity (WHSV) calculations are presented in the Section 7.3. 77 The “base case” conditions for the experiments conducted are 60 psi g (0.5 Mpa absolute), a total. liquid flow rate of 0.12 mllmin, helium flow rate of 25 ml(STP)/min, a catalyst quantity of 5.0 g, a succinate to formaldehyde ratio of 1:2, and a temperature of 380 °C. Most experiments were carried out for five hours at steady state, with samples taken in 30 minute intervals. 4.4. Catalyst Characterization The properties of the catalysts obtained from commercial sources or prepared by the methods described in Section 3.3 have been characterized for their surface area and acid/base properties. Results of these characterization measurements are given in Table 4.1 for the catalysts used in this study. 4.4.1. Surface Area Total surface area measurements were conducted by nitrogen adsorption according to the standard BET method. Table 4.1 gives the surface area of different catalyst material used in this study. Surface area was also measured for some used and regenerated catalysts. The loading of salts on catalyst supports did not make any difference on surface area of catalyst supports. 4.4.2. Acid-Base Measurements Acid site concentration and strength on the catalyst surface was measured by temperature programmed desorption (TPD) of ammonia using a Micromeritics 78 Table 4.1. Properties of Different Catalyst Material Used in This Study S.N. Catalyst S. A. Acid Acid site Basic site (mZ/g) Strength (k) density(mmol/g) density(mmol/g) 1 AlPO, P/Al ratio A 0.5 150 -0.2 to -3.2 0.582 - B 0.8 137 -0.2 to —3.2 1.220 - C 1.0 156 -0.2 to -3.2 2.345 - D 1.5 76 -0.2 to -3.2 1.810 - 2 Alumina, MSU 173 +1.1 to —0.2 0.671 0.150 3 Alumina, Norton A SA3132 32 - - - B SA3177 107 +1.1 to —0.2 0.250 0.050 C SA6173 220 - - - D SA6175 236 -0.2 to —3.2 - 0.091 4 Hydrotalcites, % Mg A 0.5 163 +2.4 to -—1.2 0.608 0.157 B 1 178 +2.4 to -1.2 0.646 0.161 C 2 150 +2.4 to —1.2 0.782 0.220 D 4 174 +2.4 to -1.2 1.020 0.195 E 6 176 +2.4 to -1.2 0.676 0.141 F 10 - - - - G 12 171 +2.4 to —1.2 0.704 0.260 H 25 - - - I 75 143 +2.4 to -1.2 - - 5 Iron oxide - - - - 6 Magnesia 0.77 >+4.8 0 0.196 7 Titania 45 +2.4 0.122 0.007 8 Zirconia, MEI Inc 62 +2.4 0.066 0.068 9 Zirconia, MSU - - - - Chemisorb 2700. Base site concentration was determined by TPD of carbon dioxide in the same instrument. Acid site strength and base site strength were further characterized by adsorption of Hammett indicators in dry benzene. DRIFTS study with pyridine and CO2 gave the nature of the sites, e. g. whether they were Bronsted or Lewis in character. 79 4.4.2.1. Temperature Programmed Desorption (TPD) Studies The ammonia TPD profiles of the catalysts used in this study are given in Figure 4.3. The acid site density of AlP04 was much greater than the other catalysts and had a peak maximum at 240 °C. Table 4.1 shows acid site densities of the catalysts calculated by calibrating the peak areas obtained with known volumes of ammonia. When a quantity of NaOH amounting to 0.7 mmollg of catalyst was loaded on A1P04, the total number of acid sites reduced considerably, as can been seen in the TPD profile. The peak maxima shifted to 210 °C, indicating that the base neutralized the stronger acid sites. The TPD profile of NaOH/AlPO4 was very close to that of SA 3177 and CPG-75. CPG-3000 did not adsorb ammonia, indicating it did not have any acid sites. Alumina magnesia co- catalyst with Mg/[Mg + A1] = 0.36 (AM-36) had an acid site density similar to SA3177 but the peak maxima was at 290 °C. TPD experiments showed no presence of acid and base sites on magnesia. Although magnesia is known to have a strong basic character, the low surface area did not provide enough sites to show any acid-base characteristics. Any contribution towards the acid or base character for such low surface area supports is only due to the surface sites. CPG-3000 did not adsorb any NH3 or C02 because it also has a low surface area. CPG-75, alumina-magnesia co-catalyst (36% Mg) and NaOI-I/AlPOa had surface areas from 150- 170 mzlg and had comparable values of acid sites. Even though A1P04 had a surface area of 171 mzlg, its acid site density was the largest of any material studied. Figure 4.4 shows the carbon dioxide TPD profiles of the catalysts used in the study. A1P04 did not adsorb any carbon dioxide, while NaOH/A1P04 had enough basic sites to adsorb carbon dioxide. Table 4.1 shows basic site densities of the catalyst used in 80 8m: 3338 2: do 8505 an; «308:2. .3. 23mm. 3.825821%: 253-081.? 308+ E 9% 33:21.1 913.: 5.5.161 3-321.: COM ADV 233.2.th 0mm 3N fv 10109er 111 slunog 81 8m: 3.330 05 me 8505 GAE. N00 .3» 2:3... 00.. On m 05.25032 IXI c 0.32 1‘1 E.— m\ ,5. . - . .. .r a... 3.82 (.(< . ., . / I a a... _ 2 .w _ : .,.. 3%: _._. 7. ,. ....<‘ 2.3: 8.9.: coo— Ong co: on: cow. on». odoo. -‘lll L, lllllf. rlinvl. L! - 3-1.1- -_ \p--. . -. I 8N :d _N.N U 2 . am ON 3. _ _ 3. v2.2 . . hm E”. __ 3 dN :82 _.Eo coo—-82 6523a Becca Swear 9m 18 22-82 2; 7:5 82-9.: “25:3 83:68-8 3 18 3: Ba 15 $2 HE 52533. N n n6 . v6 0 on a 8:983 052% 3% 2.. 2m 2:5,: co 2.8% was a: 5:983 052.3 283 2.— m /°\ + H / \H Methanol Dimethyl ether . Water One mole of water is expected to form from two moles of methanol fed into the reactor. The Cannizzaro reaction utilized resulting water and, hence, left less water in the trap than stoichiometrically possible from the above reaction. In one other run, only methanol was fed into the reactor at the base case conditions. The stoichiometrically expected amount of water was collected in the product trap and dimethyl ether was observed as a gas-phase product. 4.5.4. Citraconic Anhydride Conversion of citraconic anhydride over alumina, SA3177, was studied at 350 °C and 80 psi. Carbon dioxide was the only product formed from citraconic anhydride. A significant amount of coking (1.8 g of catalyst weight gain after 2.5 hours of the reaction) 87 was observed on the surface of the catalyst. Citraconic anhydride recovery was 78% after the reaction. Decomposition of anhydrides is anticipated at elevated temperatures (1 1-12). Detailed results from one of the control runs of citraconic anhydride are given in Table 4.3. Table 4.3. Results from the control run of citraconic anhydride Total time of the reaction 2.5 hr Total citraconic anhydride fed ' 28.43 g Total citraconic anhydride recovered 21.96 g Total carbon dioxide out 1.82 g Expected carbon dioxide from 6.33 g lost citraconic anhydrideI 2.50 g Total catalyst weight gain 1.84 g Expected catalyst weight gain from 6.33 g lost citraconic anhydride? 2.10 g IAssume 1 mole of carbon dioxide per mole citraconic anhydride lost 73 moles of coke (mol wt. ~ 13) are expected from each mole of citraconic anhydride lost 4.5.5. Citraconic Anhydride and Diethyl Succinate Citraconic anhydride and diethyl succinate were fed together into the reactor to check any reactivity of reactant, diethyl succinate, towards the desired product, citraconic anhydride. This reaction was carried out with SA3132 alumina at the temperature range of 350 to 410 °C (single experiment) and the reactor pressure of 350 psig. The molar ratio of 4:1 of diethyl succinate to citraconic anhydride was taken in the feed because this molar ratio was anticipated in the product. Conversion of citraconic anhydride was not significant (2%) at low temperatures (350 to 380 °C), but 27% conversion of citraconic 88 anhydride was observed at 410 °C. Similarly, diethyl succinate conversion was very low (11 - 18%) at low temperatures (350 - 380 °C) and higher (21%) at 410 °C. In this experiment, the temperature was ramped without changing the catalyst or stopping the run. 4.5.6. Itaconic Acid Itaconic acid is a solid at room temperature and its solubility in water is also very low, so very dilute solution of itaconic acid in water (70 g/l) was fed into the reactor. Complete conversion of itaconic acid was achieved with alumina, SA3177, at the base case conditions. The products of itaconic acid conversion included citraconic anhydride, carbon dioxide, and carbon monoxide. Yields of citraconic anhydride, carbon dioxide, and carbon. monoxide from itaconic acid were 45%, 20%, and 5%, respectively. This indicates, at reaction conditions, citraconic anhydride is the preferred product and itaconic acid is not expected to be present. 4.6. Catalyst Screening Studies 4.6.1. Silica In early studies, three different surface area CPGs (Controlled Pore Glass) containing KH2P04 and KOH were tested for the reaction of dimethyl/diethyl succinate and trioxane at the temperature range of 350-470 °C, but were not found to be promising. The reactor pressure was kept higher, ~300-400 psi, to obtain higher residence time and to push the products through the reactor. The yield of citraconic anhydride never exceeded one percent based on the diethyl succinate feed. The conversion of diethyl 89 succinate was highest with the higher surface area CPG-75 (surface area = 240 mzlg). The conversion of diethyl succinate over CPGs increased with increasing temperature. Catalyst deactivation was also observed along with the catalyst coking. The other two CPGs employed for this reaction were CPG-3000 (surface area = 7 mzlg) and CPG-500 (35 mzlg). This work was started with the loading of KH2P04 on CPG because the Stobbe condensation reaction is base-catalyzed and requires strong bases like alkali alkoxides or hydrides. Most runs with CPGs could not be completed because of reactor plugging. 1,3,5- Trioxane was used in an excess molar quantity (5:1) with succinates, and conversion of trioxane into formaldehyde was limited by the low activity of silica supports. The unconverted trioxane always caused plugging of the reactor outlet. and forced the run to be stopped before completion several times. These preliminary experiments were not successful as far as product formation was concerned, but gave a valuable insight into designing and operating the reactor system to handle all possible reactants, products, and operating conditions. 4.6.2. Zeolites The zeolite-13X, zeolite-13X with Ce2(SO4)3, and ion—exchanged zeolite-13X with NdC13 and 1203 were found to be very active for the cracking the reactants into carbon dioxide and carbon monoxide. As discussed earlier, zeolites have strong Bronsted and Lewis acid sites which were responsible for coking formation on zeolite; subsequently deactivation of zeolites was quite a bit faster than other catalysts utilized. Carbon dioxide, carbon monoxide, and water were the only products in the first sixty 90 minutes of the reaction of dimethyl succinate and trioxane when zeolite was used as a catalyst. After 60 minutes of cracking, catalyst activity was not sufficient for the desired reaction. At low temperature (300 °C), 100% conversion of dimethyl succinate via cracking was observed without forming any citraconic anhydride. A maximum citraconic acid of 7% yield at 80% conversion of diethyl succinate was obtained with zeolites at 380 °C. 4.6.3. Other Supports A series of other metal oxide supports like iron oxide, zirconia, titania, and hydrotalcites have been also screened as catalysts to obtain citraconic anhydride from dimethyl/diethyl succinate and trioxane. Runs were carried out with both commercial metal oxides, if available, and in-house prepared metal oxides to confirm results. Results ‘ from different metal oxides are given in Table 4.4 and are discussed in the following sections in detail. 4.6.3.1. Iron Oxide Iron oxide was screened for the formation of citraconic anhydride from dimethyl succinate and trioxane at the base case conditions. Iron oxide was completely inactive for the desired reaction. Formaldehyde was entirely converted into methanol and carbon dioxide via the Cannizzaro reaction. This suggests that formaldehyde was not available for the desired reaction. Conversion of dimethyl succinate was not significant (30%) over iron oxide catalyst. The products of dimethyl succinate conversion included monomethyl ester of succinic acid, succinic anhydride, and carbon dioxide. 91 Table 4.4. Results from different metal oxides Support Surface DMS CA yield MS (:02 yield CHzO area (mzlg) conv (%) (%) yield (%) (%) conv (%) Iron oxide2 - 33 o o 52‘LS 100 Iron oxide3 - 72 o 14 40“~s 100 Titania2 - 30 0 2o 2 21 Titania3 45 80 o 20 28 9o Zirconia2 — 31 3 9 10 93 Zirconia3 62 33 7 9 8 86 Hydrotalcite” 170 38 4 12 11 93 1Reaction conditions. Temperature: 380° C; pressure: 60 psi; feed: DMS + Trioxane; liquid feed flow rate: 0.10 mllmin; molar ratio: 2 to 1 (formaldehyde to DMS) :In-house metal oxide 3Commercial metal oxide “Yield of C02 is based on DMS fed, but here the main source of C02 is the Cannizzaro reaction 5C0; meter was out of range all the time, so the maximum range of the meter was used in the calculation 685% Zirconia + 15% Alumina 790% Magnesia + 10% Alumina 4.6.3.2. Titania Reactions of dimethyl succinate with trioxane were also performed over titania prepared in laboratory from titanium (l'V) butoxide and titania obtained from Degussa. No citraconic anhydride was obtained from either of the titanias used for dimethyl succinate conversion. Conversions of dimethyl succinate and formaldehyde were different over each titania. Detailed results are shown in Table 4.4. In-house titania was prepared fiom titanium (IV) butoxide, so it might be possible that the organic ligand was 92 not removed from titania crystals completely. Residue of the organic substance could cause blocking of pores and hence curtail activity of the catalyst. 4.6.3.3. Zirconia A maximum of 3% yield of citraconic anhydride at 20% conversion of dimethyl succinate was obtained at the base case conditions using zirconia as a catalyst material. Zirconia support was prepared in laboratory from zirconium oxychloride using the sol-gel method described in Section 3.1.4. Monomethyl ester of succinic acid and carbon dioxide were other products obtained from the reaction over zirconia. A formaldehyde conversion of 80% was obtained with zirconia. Commercial zirconia, which contains 15% alumina, was supplied by MEI Corporation and was also employed for succinate conversion to citraconates. This MEI zirconia (15% alumina + 85% zirconia) yielded 6% citraconic anhydride at 32% conversion of dimethyl succinate. The product distribution from MEI zirconia was similar to zirconia prepared in the laboratory. 4.6.3.4. Hydrotalcites Results from hydrotalcite support are also summarized in Table 4.4. Contrary to strongly acidic zeolites, basic catalysts facilitate the Cannizzaro reaction of formaldehyde. Hydrotalcite catalysts (90: 10 2: Aleg) gave little citraconic anhydride but catalyzed the Cannizzaro reaction of formaldehyde to form methanol and formic acid. Hydrotalcites supports were studied extensively in the later part of work with Formalin and dimethyl succinate as a feed. 93 4.7. Alumina Supports Several different 'y-aluminas were evaluated with surface area ranging from 32 mzlg to 225 mzlg. Most of the aluminas used in this work were supplied by Norton, Inc with the trade name Alundum. Intermediate surface area alumina and aluminum phosphate were also prepared in the laboratory by methods described in Section 3.3. Commercial aluminas are described in this work by the stock number used by Norton, e.g., SA3132 for low surface area alumina, SA3177 for intermediate surface area alumina, and SA6173 and SA6175 for high surface area alumina. Intermediate surface area alumina and aluminum phosphate prepared by Dr. N. Kirthivasan in lab are known here by Alumina-In-House and AlPOa, respectively. Figure 4.6 summarizes the citraconic anhydride yields obtained over time from various alumina catalyst supports for the conversion of dimethyl succinate. Conversion of dimethyl Succinate, yield of the cracking product carbon dioxide, and selectivity over time from different alumina catalyst supports are depicted in Figure 4.7, Figure 4.8, and Figure 4.9, respectively. Results from different types of aluminas are discussed in the following sections in detail. ' 4.7.1. Low Surface Area Aluminas The low surface area Alundum, SA3132, was found to be less active than other aluminas used. Several different salts supported on SA3132 were surveyed for the formation of citraconic anhydride from diethyl succinate and trioxane. Results from different salts surveyed are listed in Table 4.5. A series of experiments were carried out with SA3132 impregnated with ceric sulfate. There were two reasons behind the selection of ceric sulfate as a catalyst for the 94 com 3.253 3258 «58:3 2.80:6 Eoc net—SEE“ 2.8856 .8 Eu; 6% nun—mi 3:5 as? 832m 03 com ova ova Eu 2: G2 o2 om so on o — p b — _ p — _ n p h - o VOA? 1*: 32:58:34 :s..... r m E _ macaw ii... Qt _ o .3» 053m 2:5 as: 882m 9mm com ohm ova 2m 2: o2 o2 co co om o _ — _ n — p — — . _ p o 4!; I .311sz ,2 I S _ 5.433 ...... ..X/ / ON if: - %;,,,,,,,,., . r em v0fi ax; £52 an 22> g win we 8.23:8 Beaum— _E .26 8 323 .6 38$ .3. 22: 103 As discussed above, the objective behind using base supported on SA3177 was to enhance the activity of SA3177 by selectively poisoning the coking sites on it. However, no citraconate formation was observed, and lower conversions of dimethyl succinate from base supported on SA3177 suggest that the base also killed the weakly acidic sites which are active for the desired reaction. However, the presence of a base on SA3177 facilitated the Cannizzaro reaction and, hence, the formation of methanol and formic acid. Formic acid results in carbon dioxide at elevated temperatures employed for the reaction. 4.7.3. High Surface Area Aluminas Yields of citraconic anhydride from the reaction of dimethyl succinate and trioxane over high surface area alumina SA6173 (surface area = 220 mzlg), SA6175 (surface area = 236 mzlg), alumina-in-house (prepared in lab from AlCl3), and AlPOa (prepared in lab) were 16 to 19%; these values were close to alumina SA3177 (surface area = 100 mzlg). Yield of citraconic anhydride was low in the first sample before reaching a maximum and then stabilizing after 120 minutes with trioxane and dimethyl succinate (Figure 4.6). High surface area aluminas were very active in the beginning of the reaction but deactivated by coking upon prolonged exposure to reactants. For high surface area aluminas, high conversion of succinates (Figure 4.7) was accompanied by lower yield of citraconic acid in the beginning of the reaction due to heavy cracking of succinates into carbon monoxide and carbon dioxide (Figure 4.8). The selectivity for citraconic anhydride of these higher surface area materials started low and increased over time, finally approaching that of the SA3177. The production of carbon monoxide and 104 carbon dioxide was very high in the reaction, and then declined to a small value after about 240 minutes (Figure 4.8). 4.7 .4. Aluminum Phosphates Aluminum phosphate (AlPOa) employed in this study were prepared from two different set of precursors as described in Section 3.1.1 to determine the effect of precursor on surface characteristics and, hence, the effect on citraconic anhydride yields. A maximum citraconic anhydride yield of 18% at 60% conversion of dimethyl succinate was observed over AlPOa as a support at the base case conditions. Yield of citraconic anhydride, conversion of dimethyl succinate, yield of carbon dioxide, and selectivities are given and also compared with alumina supports in Figure 4.6, Figure 4.7, Figure 4.8, and . Figure 4.9, respectively. AlPOa results also follow alumina trends. A higher activity in ‘ the beginning of the run resulted in high conversion of dimethyl succinate; later the citraconic anhydride yield (Figure 4.6) and dimethyl succinate conversion (Figure 4.7) stabilize. Significantly lower yield of carbon dioxide was observed over AlPOa compared to SA3177 (Figure 4.8). Yields of monomethyl succinate and methanol over mm were 25% and 31%, respectively. 4.8. Parametric Studios 4.8.1. Effect of Pressure Most experiments were run at a reactor pressure of 60 psi and temperature of 350 to 410 9C. Experiments in the beginning of this work using CPGs were carried out at high pressure of 400 psi. The reason behind selecting high pressure was to avoid 105 clogging by pushing products through reactor using high pressures. In the latter part of this study, it was confirmed that high reactor pressure did not make any difference on the total yield of citraconates. The comparison of results (unhydrolyzed) at low and high pressures is presented in Figure 4.10. The yield of citraconic anhydride before hydrolysis at high pressure was slightly lower than the yield at low pressure. But, overall yields of citraconates after hydrolysis at both pressures were the same at high and low pressures. Yield of citraconates before and after hydrolysis at high (400 psi) and low (60 psi) pressures are shown in Figure 4.11. At high pressures, the equilibrium favors the esterification reaction and, thus, more mono ester and diethyl esters of citraconic acid are formed. The conversion of dimethyl succinate was higher at 400 psi than at 60 psi and so selectivity was lower at high pressure. Surprisingly, the amount of carbon dioxide evolved was significantly lower at high pressure. But the catalyst deactivation rate and catalyst weight gain after the completion of the reaction were the same at low and high pressures. One other run was duplicated to confirm the results at 400 psi. 4.8.2. Effect of Temperature The effect of temperature on citraconic anhydride yield (unhydrolyzed results) and dimethyl succinate conversion is given in Figure 4.12 and Figure 4.13, respectively. The conversion of dimethyl succinate or diethyl succinate increased with increase in temperature, but selectivity decreased with temperature. The yield of citraconic anhydride went through a maximum at 380 °C with the reactor temperature (Figure 4.12). 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E 8226 N00 mo 22> .36 82mE 9:5 25 832m OS of of oi ca ow om — - p _ .— d— U Sunk. tar. U Snub in: U ommnh IT 111 (%) 70:) JG mam com 8522882 32?» 3 mEQ So: out???“ 088.38 ‘8 53823 .26 “:sz 2:5 can 832m of of 9: GE 99 3 oo ow on c u u w v u “ fl “ u o - om S P m . - on m“ fig!!! WM) % U Svub if... I cc 0 cwmnh :52. T J» U ommuh I9! I on co 112 (Figure 4.14) resulting in high conversion of succinates (Figure 4.13) and lower selectivity (Figure 4.15). 4.8.3. Feed Molar Ratio Formaldehyde was used in molar excess over the alkyl ester of succinic acid. Several different molar ratios of formaldehyde to succinate (l : 2, 2 : 1, and 4 : l) were examined for our reaction. The effect of molar ratio of formaldehyde to dimethyl succinate in feed was studied in detail in the later part of this work and discussed in Section 6.6.5. The effect of feed molar ratio on the yield of citraconic anhydride, conversion of dimethyl succinate, yield of carbon dioxide, and selectivity to citraconic anhydride using SA3177 are shown in Figure 4.16, Figure 4.17, Figure 4.18, and Figure 4.19, respectively. Yield of citraconic anhydride is based on the dimethyl succinate fed into the reactor. Yields of citraconic anhydride from the 2 to 1 molar ratio (dimethyl succinate to formaldehyde) feed were almost half of those observed from the feed molar ratio of l to 2 molar ratio of dimethyl succinate to formaldehyde. Increasing the feed molar ratio (DMS to formaldehyde) produced a decrease in the conversion of dimethyl succinate. Selectivity of citraconic anhydride formation from dimethyl succinate increased with increasing the feed molar ratio of dimethyl succinate to formaldehyde. 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Tmnesmnmznlnl m” _ H;_E...7£.,5..+u ow lg! om 0 V3 —1 O N V) (\l O (”'3 mm ov (%) humans 117 4.9. Hydrolysis of Products The raw product exiting the reactor was a mixture of citraconic anhydride, citraconic acid, monomethyl ester of citraconic acid, and dimethyl citraconate. To clearly evaluate product yields and selectivity, it was necessary to hydrolyze the product mix in aqueous H2804 solution to recover all species as the free acid. Usually, about 20% of the citraconate was in the form of monomethyl or dimethyl ester, so reported yields for unhydrolyzed mixtures were lower than the actual values. In Run 65 (DMS + T0, SA3177), 35% yield of citraconic acid was observed following hydrolysis of the product. Figure 4.20 shows how the yields of citraconic anhydride formation increased after hydrolyzing the product. The first column in Figure 4.20 shows the unhydrolyzed yield of citraconic anhydride. The second column represents the yield of citraconates after hydrolysis but not severe hydrolysis; the sample was prepared in the HPLC mobile phase on the experiment day but was injected after one month. The acidic mobile phase hydrolyzes the reaction product to some extent over time. The yield of citraconates after the severe hydrolysis of the product mixture (refluxing the sample in the mobile phase used in HPLC analysis plus 5-7 drops of concentrate sulfuric acid for twenty-four hours) is represented by the third column of the Figure 4.20. 4.10. Extended Run The full time scale of catalyst deactivation is shown in an extended time experiment in Figure 4.21 (unhydrolyzed complete results) and Table 4.7 (hydrolyzed results). Trioxane and dimethyl succinate were feed materials and SA3177 alumina was used as the catalyst. 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Feed Molar Ratio The molar ratio of 5:1 of formaldehyde to succinic anhydride was also examined for the reaction besides the base case molar ratio of 2:1 of formaldehyde to succinic anhydride. Results from the reaction of succinic anhydride and trioxane at different feed molar ratio are given in Table 5.3. There was no significant trend found in results of the condensation reaction of succinic anhydride and formaldehyde from different molar ratios. Succinic anhydride conversion was 87% at 5 :1 molar ratio compare to 80% at 2:1 molar ratio after one hour of the reaction. Higher conversion of succinic anhydride at 5:1 resulted in higher yield (26%) of carbon dioxide compare to 23% yield of carbon dioxide at 2:1 molar ratio. However, yield of citraconic anhydride was lower, 32%, at 5:1 molar ratio, whereas the yield was 43% at 2:1 molar ratio. Thus, higher selectivity was observed at 2:1 molar ratio than 5:1. The molar ratio study could not be conducted in detail due to cumbersome process of feeding succinic anhydride and trioxane in the molten phase. 5.2.3.3. Liquid Feed Flow Rate The condensation reaction of succinic anhydride and trioxane was carried out at liquid feed flow rates of 0.10 mllmin (6ml/hr) and 0.17 mllmin (10 ml/hr), keeping other parameters unchanged. Figure 5.5, Figure 5.6, Figure 5.7, and Figure 5.8 present yield of citraconic anhydride, conversion of succinic anhydride, selectivity of citraconic anhydride, and the yieldof carbon dioxide evolved from the reaction, respectively, at the two different liquid feed flow rates. 136 Table 5.3. Results at different feed molar ratios1 Elapsed Yield of citraconic Conversion of succinic Yield of C02 (%)3 time (min) anhydride (%) anhydride (%) 2:12 5:12 2:12 5:12 2:1T 5:12 30 25 32 84 83 29 34 6O 43 26 8O 87 23 26 9O 43 34 76 8O 16 3 l 120 28 41 16 150 18 62 14 1Reaction conditions: Temperature = 380 °C; pressure = 60 psi; feed = succinic anhydride and trioxane; liquid feed flow rate = 0.10 mllmin; outlet gas flow rate = 27 ml(STP)/min; pre-heat temperature = 200 °C. Feed molar ratio: succinic anhydride to formaldehyde 3'Yield of carbon dioxide (%) = moles of C02 evolved * 100/ (2 * moles of succinic anhydride fed) Conversion of succinic anhydride was significantly affected by increasing liquid feed flow rate, which is expected according to simple reaction kinetics. The conversion of succinic anhydride at higher liquid feed flow rate (10 ml/hr) was always ~20% less than the conversion at lower liquid feed flow rate (6 ml/hr). The lower conversion of succinic anhydride at 10 ml/hr was reflected in lower yield of citraconic anhydride and carbon dioxide at that flow rate. Yields of carbon dioxide evolved from the reaction also follows succinic anhydride conversion trends. There was always a 6-percentage points lower yield of carbon dioxide at higher liquid feed flow rate than at lower liquid feed flow rate. Selectivities for citraconic anhydride formation from succinic anhydride were higher at the higher liquid feed flow rate of 10 ml/hr. 137 03 8853 02828 cc 2% 8 3.: soc Be 2%: co spam .3 23mm o2 cm— 35 2E. caesm 8 L 00 On :SE 2 u 38 32m III :58 O H 03.. 305+ l 2 l 2 you rmm rem 1mm -ov -. we row (%) apppflqu'e oguoomigo 30 pp“ 138 02835 3533 .«o eommeo>=oo no 83 >6: Bow Baa: mo “comm .06 223» E25 25 825m 2: o: 02 8 8 cm _ — — F _ _ :55 cm H van.— 3OEIII. .3}:— 0 H 03.. 3OEI§I ov me. 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In the other set of experiments, the helium flow rate was varied from 55 ml(STP)/min to 82 ml(STP)/min, whereas the reaction temperature was kept at 350 °C and the liquid feed flow rate was fixed at 0.167 mllmin (10 mllhr). Table 5.4 gives the results at different helium flow rates. Although the first set of experiments did not give any conclusive results, it is clear from the second set of data that conversion of succinic anhydride at higher helium flow rate (82 ml(STP)/min) was significantly lower than the conversion at lower helium flow rate (55 ml(STP)/min). The yield of citraconic anhydride was also lower at higher helium flow rate in the same proportion as the conversion of succinic anhydride and, thus, the selectivity remained the same at both helium flow rates employed for the reaction. 142 Table 5.4. Effect of outlet helium flow rate on results1 Elapsed Yield of citraconic anhydride (%) Conversion of succinic anhydride (%) Time T = 350 °c T = 380 °c T = 350 °C T = 380 °c (min) 55 82 27 55 55 82 27 55 ml/min mllmin mllmin 611/an mllmin mllmin mllmin mllmin 3o 30 27 25 17 95 80 84 95 6O 38 30 A 43 42 77 61 80 74 9o 33 27 43 65 54 76 120 30 28 58 41 150 19 18 57 62 1Reaction conditions: Pressure = 60 psi; feed = succinic anhydride and trioxane (l to 2/3 molar ratio); liquid feed flow rate = 0.10 mllmin; pre-heat temperature = 200 °C. 5.2.3.5. Longer Reactor Catalyst Bed The condensation reaction of succinic anhydride with trioxane was also carried out with a longer reactor catalyst bed (11 g catalyst weight vs. 5.2 g catalyst weight) to see the effects of increasing weight hourly space velocity (WHSV) on the desired results. The diameter of the reactor was the same. The experiment with the longer reactor was conducted at the base case conditions and results from the longer reactor are compared with the regular reactor in Table 5.5. A higher conversion of succinic anhydride and a significantly lower yield of citraconic anhydride, coupled with the almost doubled yield of carbon dioxide using the longer reactor, suggests that the extra length of the reactor bed contributed to cracking of citraconates and succinates. 143 Table 5.5. Comparison of results from longer reactor with regular reactorl Elapsed Yield of citraconic Conversion of succinic Yield of C02 (%) Time anhydride (%) anhydride (%) (min) 11.0 g2 5.2 g2 11.0 g2 5.2 g’ 11.0 g7 5.2 g2 30 5 3O 99 95 54 27 60 21 38 86 77 43 29 9O 20 33 85 65 47 18 120 30 58 10 150 19 57 9 1Reaction conditions: Temperature = 350 °C; pressure = 60 psi; feed = succinic anhydride and trioxane (1 to 2/3 molar ratio); liquid feed flow rate = 0.10 mllmin; outlet gas flow rate = 27 ml(STP)/min; pre-heat temperature = 200 °C. zCatalyst weight taken into the reactor bed 5.2.3.6. Hydrolysis Hydrolysis of the reaction products from the reaction of succinic anhydride and trioxane was carried out to convert all citraconates to citraconic acid and succinates to succinic acid. Results from succinic anhydride and trioxane after and before hydrolysis are shown in Table 5.6. Yield of citraconates and the conversion of succinates were not changed much after hydrolysis of the reaction products from the reaction of succinic anhydride and trioxane over SA3177. Again, this is because no methanol is produced in the reaction to facilitate ester formation. 144 Table 5.6. Results1 from succinic anhydride and trioxane after and before hydrolysis Elapsed Time Yield of Citraconates (%) Conv of succinates (%) (min) Before After Before After 60 43 43 73 74 9O 43 -44 67 67 1Reaction conditions: Temperature = 380 °C; pressure = 60 psi; feed = succinic anhydride and trioxane (l to 2/3 molar ratio); liquid feed flow rate = 0.10 mllmin; outlet gas flow rate = 27 ml(STP)/min; pre-heat temperature = 200 °C. 5.2.3.7 . Deactivation Studies The rate of catalyst deactivation was very high with succinic anhydride and trioxane feed compared to dimethyl succinate and trioxane feed. The deactivation of catalyst was caused by coking on the surface of the catalyst resulting from cracking of ‘ reactants and products at the reaction conditions employed. Higher yields of carbon dioxide evolved from the reaction were observed due to excessive cracking. The yield of citraconic anhydride dropped drastically to lower values after peaking in the 2'id and 3rd product samples. The conversion of succinic anhydride also decreased sharply over time. This is in contrast to dimethyl succinate as the feed, where the yield of citraconic anhydride and the conversion of dimethyl succinate almost stabilized after two hours of reaction. The lower conversion and thermal decomposition of succinic anhydride also caused the plugging problem in the reactor downstream and difficulties in product analysis. The larger portion of unreacted succinic anhydride, coupled with paraformaldehyde formation resulting from polymerization of formaldehyde, solidified inside the tiny ports of the switching valve or inside the tubing at any cold spot and caused the reactor-plugging problem. The catalyst weight gain was 2.1 g after three 145 hours of reaction of succinic anhydride and trioxane, whereas the catalyst gain was 1.5 g after five hours of the reaction of dimethyl succinate and trioxane. As discussed earlier, coking was observed from succinic anhydride feed even with an inactive material, glass beads. It was obvious from control experiments that coking involved the reactants, succinate and formaldehyde, and the product, citraconic anhydride. 5.2.4. Other Catalysts Used Intermediate surface area alumina, SA3177, was found to be the best catalyst material for this condensation reaction, so it was primarily used for studies with succinic anhydride and trioxane. But, some experiments were also carried out with catalyst material having different catalyst characteristics than SA3177 to confirm our postulates about particular characteristics. For example, a reaction was conducted with a basic support, iron oxide (Fe203), to confirm the postulate that a base catalyst without having any acidic sites does not work for the desired reaction. Similarly, the empty reactor and glass beads were used to test the thermal stability of succinic anhydride and trioxane over a material that has very low surface area and acidic/basic site concentration. 5.2.4.1. Empty Reactor Succinic anhydride and trioxane were passed through the empty reactor to see any reactivity of reactants at the reaction conditions in 'the absence of any catalyst material. Although no yield of citraconic anhydride was observed, 15% conversion of succinic anhydride was noticed. It was inferred from the empty reactor reaction that the condensation reaction of succinic anhydride and trioxane to form citraconic anhydride is 146 not a simple thermal reaction and succinic anhydride is not thermally stable at elevated temperatures. 5.2.4.2. Glass Beads Glass beads, 0.5 mm diameter (Quackenbush Company, Inc.), discussed in Section 4.5.1 and Section 6.5.7, were placed in the reactor to characterize the thermal stability of succinic anhydride and trioxane over an inert material. A 13% yield of citraconic anhydride at 78% conversion of succinic anhydride over glass beads was observed with 60% product recovery. High yield of carbon dioxide, 54%, was also noticed due to excessive cracking of reactants. The run with succinic anhydride and trioxane over glass beads was repeated to verify reproducibility. Succinic anhydride decomposes. like other acid anhydrides at elevated temperature (<320 °C) (9-13). The decomposition of succinic anhydride produces a coating of activated carbon on the glass bead surface. We postulate that the activated carbon produced by cracking of succinic anhydride provided a platform for the condensation reaction of succinic anhydride and trioxane to form citraconic anhydride and this is why we observed citraconic anhydride in the product. Interestingly, no monomethyl ester of succinic acid was seen in the product. Monomethyl ester of succinic acid is formed from the reaction of succinic anhydride with methanol, where methanol is generated from the Cannizzaro reaction of formaldehyde in the presence of basic sites on the catalyst. But, the Cannizzaro reaction does not occur over glass beads. First, the basic or acidic sites required for the CanniZzaro reaction are not present on inert glass bead surface. The occurrence of the Cannizzaro reaction has 147 been seen over activated carbon which does contain acidic and basic sites, from the reaction of dimethyl succinate and Formalin (see Section 6.5.6). Second, not enough water, which is also required for the Cannizzaro reaction, is available, because no water was fed to the reactor. Some water maybe produced from the formation of citraconic anhydride from succinic anhydride and formaldehyde, but that is likely consumed by formaldehyde to polymerize into paraformaldehyde instead of facilitating the Cannizzaro reaction. 5.2.4.3. Iron Oxide Reactions of succinic anhydride with trioxane were also performed over iron oxide (Fe203) (Aldrich). Because glass beads gave 13% citraconic anhydride from succinic anhydride, it was decided to test iron oxide, a very basic catalyst without any acidic characteristics, for the desired reaction. Complete conversion of succinic anhydride was observed over iron oxide (compared to 30% conversion of dimethyl succinate at similar conditions). Complete conversion of succinic anhydride over ferric oxide suggests that succinic anhydride is not stable at elevated temperature in the presence of basic material. The carbon dioxide meter was out of range in the reaction of succinic anhydride with trioxane. Formaldehyde was entirely converted into methanol and carbon dioxide via the Cannizzaro reaction. 5.3. Succinic Anhydride and Trioxane in Solution Succinic anhydride is a good feed material for the reaction, as the highest yields of citraconates were achieved with very good selectivities. There were several problems 148 associated with the use of succinic anhydride as a feed material, e.g., faster catalyst deactivation, reactor tube plugging, and difficulties in product and reactant handling. To attempt to get rid of these problems caused by succinic anhydride as the feed, succinic anhydride and trioxane were mixed with methanol to make a feed solution of monomethyl succinate and trioxane. 5.3.1. Reaction Conditions Reaction conditions employed for monomethyl succinate and trioxane in methanol were also readjusted due to the nature of the feed. Reactants became very dilute because of the excessive amount of methanol needed to solubilize them in methanol. The liquid feed flow rate was therefore increased to maintain the same succinate feed rate as with other feeds. Product samples were taken every 15 minutes and only every other sample was analyzed. Helium was used as a carrier gas, but some experiments were carried out without any helium. The product collection traps were immersed in cold water to collect methanol. The standard reaction conditions are summarized in Table 5.7 . 5.3.2. Base Case Results Results from the condensation reaction of monomethyl succinate (MMS) and formaldehyde in methanol before hydrolysis are given in Table 5.8 along with those over KHzPO4/SA3177 discussed in the next section; succinate conversion was lower with this feed mixture and conditions but selectivity (citraconates formed/succinate converted) was better at 75-80%. After hydrolysis, a maximum of 31% .yield of citraconates after was observed at 40% conversion of succinates. Dimethyl succinate and succinic acid were 149 Table 5.7. Reaction conditions S.N. Condition Value 1 Reaction temperature (°C) 350 2 Reactor pressure (psi) 60 3 Pre-heat temperature (°C) 200 4 Feed molar composition (mol%) Monomethyl succinate 9.7 Trioxane 18.8 (formaldehyde equivalent) Methanol 60.0 Helium l 1.5 5 Liquid feed flow rate (ml/min) 0.3 6 Carrier gas flow rate (ml(STP)/min) 20 also found in products. The dimethyl succinate was formed from the esterification of monomethyl succinate. The catalyst performance was very consistent after the first 30 minutes of the reaction and the product composition in each sample collected was reproducible at that time. 70% conversion of monomethyl succinate, 23% yield of citraconic anhydride, 30% yield of dimethyl succinate, and 11% yield of succinic acid were observed consistently after the first 30 minutes of the reaction. 150 Table 5.8. Results from ms and trioxane feed in methanol' Elapsed Yield of CA Conv of Yield of Yield of SA Yield of C02 Time (%) MMS (%) DMS (%) (%) (%) (min) A2 B? A B A B A A 30 20 6 72 94 23 55 5 16 6O 26 5 70 86 29 78 10 10 9O 23 3 7O 93 3O 74 ll 6 120 22 7O 25 11 8 1Reaction conditions: Temperature = 350 °C; pressure = 60 psi; feed = monomethyl succinate and trioxane (1 to 2/3 molar ratio) in methanol; liquid feed flow rate = 0.30 mllmin; outlet gas flow rate = 20 ml(STP)/min; pre-heat temperature = 200 °C. 2A = Catalyst is unsupported SA3177 ' 3B = Catalyst is 0.15 mmol IGIzP04/g SA3177 In one another separate experiment, only monomethyl succinate in methanol was fed into the reactor at the reaction conditions described above. Yields of dimethyl succinate and succinic acid were 54% and 6% respectively at 64% conversion of monomethyl succinate. The yield of carbon dioxide was always less than 1% and, hence, very little coking was found on the catalyst after the reaction. 90% of feed by mass was recovered in the product after the reaction. Some methanol lost during the reaction in the form of dimethyl ether, which passed through the product trap. 5.3.2.1. Effect of Temperature The effect of temperature on the formation of citraconic anhydride from monomethyl succinate and trioxane over SA3177 was also studied. Reactions were performed at 350 °C and 380 °C keeping all other reaction conditions unchanged. Yield 151 of citra tempera 5.3.2.2. C out wl [ht reactor in Table 5.9 succinates gas, com; SUCCinates of citraconic anhydride and conversion of monomethyl succinate were insensitive to temperature. 5.3.2.2. Effect of Carrier Gas Conversion of monomethyl succinate and trioxane in methanol was also carried out without using helium as a carrier gas to increase concentrations of reactants at the reactor inlet. Complete results with and without helium as a carrier gas are presented in Table 5.9. After hydrolysis, a maximum 22% yield of citrconates at 30% conversion of succinates after hydrolysis was achieved at the base case conditions without any carrier gas, compared to a maximum of 31% yield of citraconates at 40% conversion of succinates with helium as a carrier gas. Higher yields of dimethyl succinate and succinic Table 5.9. Effect of carrier gas on results (before hydrolysis)l Elapsed Yield of CA (%) Conv of MMS Yield of DMS Yield of SA (%) Time (%) (%) (min) He@20 No He He@20 No He He@20 No He He@20 No He mllmin carrier mllmin carrier mllmin carrier mllmin carrier 30 20 16 72 78 23 47 5 l 1 6O 26 18 70 78 29 45 10 13 90 23 16 7O 83 3O 54 1 1 15 120 22 18 70 84 25 52 1 1 l3 chaction conditions: Temperature = 350 °C; pressure = 60 psi; feed = monomethyl succinate and trioxane (1 to 2/3 molar ratio) in methanol; liquid feed flow rate = 0.30 mllmin; pre-heat temperature = 200 °C. 152 acid were observed with no carrier gas, compared to yields of dimethyl succinate and succinic acid with carrier gas; this is likely because of longer residence time. 5.3.2.3. Hydrolysis The raw product coming from the reactor was also hydrolyzed to quantify all citraconates present in the product." Succinates were converted into succinic acid in the hydrolysis. Table 5.10 shows results where all yields, conversions, and selectivities are based on hydrolyzed results. The yield of citraconates after hydrolysis was usually increased by 20% of unhydrolyzed yields. Table 5.10. Results before and after hydrolysis of product1 Elapsed Yield of citraconates (%) Conv of Succinates (%) Selectivity? Time (min) Before After Before After (%) 30 20 25 45 46 54 60 26 31 31 40 78 90 23 25 30 33 76 1Reaction conditions: Temperature = 350 °C; pressure = 60 psi; feed = monomethyl succinate and trioxane (1 to 2/3 molar ratio) in methanol; liquid feed flow rate = 0.30 mllmin; outlet gas flow rate = 20 rnl(STP)/min; pre-heat temperature = 200 °C. 2Selectivity is based on hydrolyzed products. 5.3.2.4. Deactivation Studies Coking of the catalyst was observed from monomethyl succinate and trioxane feed in methanol, but not at a significant level as with succinic anhydride feed. The catalyst weight gain was only 0.66 g after three hours of reaction from monomethyl succinate and 153 trioxane feed, whereas the catalyst weight gain was 2.1 g after three hours of reaction of succinic anhydride and trioxane feed. First, monomethyl succinate does not have thermal stability problems as does succinic anhydride at elevated temperatures. Second, a very dilute solution of monomethyl succinate in methanol was used. Catalyst performance was very steady with monomethyl succinate as a feed. The conversion of monomethyl succinate and yield of citraconic anhydride stabilized after fifteen minutes of reaction for the rest of the reaction time 5.3.3. Other Catalyst Used Most of the experiments with monomethyl succinate feed were carried out with intermediate surface area alumina, SA3177, but high surface area alumina, SA6173, high surface area silica, CPG-75, and KH2P04 supported on SA3177 were also employed for the reaction. Results from different catalysts are discussed in the following sections. 5.3.3.1. KHzPOJSA3177 Detailed results before hydrolysis from KHzPOa supported on SA3177 are given in Table 5.8. Conversion of monomethyl succinate and trioxane was also performed over KHzPOtt supported on SA3177. KH2P04 was used here to poison strong acidic sites on alumina, which are responsible for coking on the catalyst. Loading of 0.15 mmol KHzPOJg SA3177 was taken for the reaction because it corresponds to the density of strong acidic sites on SA3177. A maximum 5% yield of citraconic anhydride was obtained over KH2P04 supported on SA3177 from the reaction of monomethyl succinate and formaldehyde in methanol. Higher conversion of monomethyl succinate (86%) was 154 observed over KH2P04 supported on SA3177, compared to 70% conversion with SA3177 alone. Interestingly, KHzPoa supported on SA3177 overwhelmingly catalyzed the esterification of monomethyl succinate to dimethyl succinate. A higher yield (78%) of dimethyl succinate was obtained, compared to 30% yield of dimethyl succinate from SA3177 alone. The absence of succinic acid in the product suggests that de-esterification of monomethyl succinate was completely halted due to the absence of acidic sites on the catalyst. Amount of carbon dioxide evolved from the reaction due to cracking was also very low over KHzPoa supported on SA3 l7 7. 5.3.3.2. CPG-75 In the early part of this study, silica (CPG) was comprehensively studied for the reaction of diethyl succinate and trioxane, but no experiment ”was carried out with unsupported high surface area silica (CPG-75). High loading of KH2P04 (2 mmollg) over CPG-75 was employed for the reaction of diethyl succinate and trioxane and the yield of citraconic anhydride was less than one percent at higher conversion of dimethyl succinate. It was well established that a base loading on any active catalyst kills the desired reaction, so it was decided to conduct one experiment of monomethyl succinate and trioxane without loading any bases on a high surface area silica, CPG-75. The reaction of monomethyl succinate and trioxane in methanol solvent over unsupported CPG-75 was conducted at 350 °C and 400 °C. No citraconic anhydride was formed at 350 °C, and 3% yield of citraconic anhydride was achieved at 400 °C. Esterification of monomethyl succinate was dominant over CPG-75 and yields of dimethyl succinate were 79% and 60% at 350 °C and 400 °C, respectively. CPG-75 gave 155 monomethyl succinate conversions of 91% at 350 °C and 92% at 400 °C. No succinic acid was formed at any temperature used for the reaction. 5.3.3.3. SA6175 High surface area alumina, SA6175, (235 m2/g) was also employed for the reaction of monomethyl succinate and trioxane at the base case conditions. A maximum citraconate yield of 18% after hydrolysis was obtained at 30% conversion of succinates. Coking was also observed on the catalyst surface. 5.4. Summary The use of succinic anhydride and trioxane as a feed for the formation of citraconic anhydride over various catalysts was studied. Succinic anhydride and trioxane were fed into the reactor in the molten phase or as a feed solution in methanol. Optimization of reaction conditions was also conducted to maximize citraconic anhydride yields. These optimization studies involved temperature, feed compositions, liquid feed and gas flow rate, and catalyst bed length. The yield of citraconic anhydride from the reaction of succinic anhydride and trioxane over SA3177 was as high as 43% of theoretical at selectivity ranging from 50- 70%. The rate of catalyst deactivation was very high with succinic anhydride and trioxane feed and thus the yield of citraconic anhydride and conversion of succinic anhydride decreased sharply over time; this is likely caused by coking on the surface of the catalyst resulting from the thermal decomposition of highly unstable succinic anhydride at elevated temperatures. Citraconic anhydride formation was even noticed 156 with the glass beads (an inactive material), likely because of the activated carbon produced from the decomposition of succinic anhydride. The hydrolysis of reaction products from the reaction of succinic anhydride and trioxane did not affect yield of citraconates, because no methanol is produced in the reaction to facilitate ester formation. The yield of citraconic anhydride from succinic anhydride and trioxane increased with increasing temperature, but decreased with increasing feed molar ratio of formaldehyde to succinic anhydride. The conversion of succinic anhydride sharply decreased with decreasing liquid feed flow rate, but yield of citraconic anhydride did not decrease with decreasing liquid feed flow rate in the same proportion as conversion of dimethyl succinate. Thus, the reaction is highly selective at the higher liquid feed flow rate. A higher conversion of succinic anhydride and a significantly lower yield of citraconic anhydride were observed from the longer catalyst bed. After hydrolysis of raw products from the reactor, a maximum 31% yield of citraconates at 40% conversion of succinates from the reaction of monomethyl succinate and trioxane over SA3177 was achieved at the base case conditions. The reaction of monomethyl succinate and trioxane was not affected much by the reaction temperature. With no carrier gas used for the reaction of monomethyl succinate and trioxane, the yield of citraconic anhydride decreased, while the yield of dimethyl succinate significantly increased. Succinic anhydride is a good feed material for the reaction, as the highest yields of citraconate were achieved with very good selectivities. In the laboratory, reactions are somewhat more difficult to conduct with succinic anhydride, because it has a very high 157 boiling point (269 °C) and solidifies on any cold surface. This led to frequent plugging of reactor tubes and collection traps during experiment. On a commercial scale, the pr0perties of succinic anhydride should pose less of a problem. Coking on the catalyst was not significant from the reaction of monomethyl succinate and trioxane and, hence, results were stabilized over time. Monomethyl succinate was also proved to be a good feed material for the reaction, as the highest selectivities of citraconates were achieved and the catalyst deactivation was also minimal. Dilution of monomethyl succinate feed in methanol may make it an economically unfavorable process, because excessive methanol would impart an increase in the reactor, heating, and the separation costs of the process. A small amount of methanol is also lost in the reactor at reaction temperatures as dimethyl ether. Dimethyl succinate is produced by esterification of succinic acid and succinic anhydride can be produced by heatin g succinic acid. Production of monomethyl succinate from succinic acid requires one additional step, first forming succinic anhydride from succinic acid, and then its esterification. Monomethyl succinate does not have any significant advantage over dimethyl succinate. 158 CHAPTER 6 CONDENSATION OF DIMETI-IYL SUCCINATE AND FORMALDEHYDE IN AQUEOUS SOLUTIONS TO CITRACONIC ANHYDRIDE 6.1. Introduction This chapter presents the studies of the condensation reaction of dimethyl ‘ succinate with formaldehyde in the vapor phase over various catalysts where the source of formaldehyde is a commercially available formaldehyde solution. Earlier in this work, 1,3,5 trioxane, the cyclic trimer of formaldehyde, was used as a source of formaldehyde for the reaction because of its ease in handling. But trioxane is not commercially available, so it is necessary to look to other sources of formaldehyde for the reaction which are commercially available. Formalin, a mix of 37 wt% formaldehyde, 10 wt% methanol, and 53 wt% water, is a readily available formaldehyde solution and is widely used in this study. One another commercially available source of formaldehyde, Forrncel, produced by Celanese, was also used in this study. Forrncel solutions are blends of 55 wt% formaldehyde, 35 wt% methanol, and 10 wt% water. 159 6.2. Reaction Conditions The reaction studies involve primarily dimethyl succinate and formaldehyde solution as the feed with helium as a carrier gas. Most experiments were canied out for five hours at steady state, with samples taken in 30-minute intervals. Typical reactor operating conditions are given in Table 6.1. Table 6.1. Reactor Operating Conditions S.N. Condition Range Base case condition 1 Reaction temperature (°C) 380 - 380 380 2 Reactor pressure (psi) 60 60 3 Pro-heat temperature (°C) 200 - 250 250 4 Feed molar composition (mol%) Dimethyl succinate 7.4 — 14 9.5 Formaldehyde 4.4 - 34.6 18.8 Methanol 4.2 - 40.6 4.8 Water 8.9 - 42.0 45.1 Helium 21.9-31.5 21.9 5 Liquid feed flow rate (ml/min) 0.14 - 0.42 0.15 6 Outlet gas flow rate (ml(STP)/rnin) 27 - 82 27 6.3. Control Experiments of Formalin Formalin was passed through the empty reactor to characterize its stability at the normal reaction temperature. The recovery of formaldehyde solution after the reaction 160 was complete, which suggests that formaldehyde does not escape from the solution even at higher temperatures. The sodium sulfite method (discussed in Section 2.6.3) was used for the determination of formaldehyde concentration in control experiments of Formalin. Complete (100%) conversion of formaldehyde was obtained when Formalin itself was passed over 'y-alumina (SA3177) at the base case conditions. The catalyst activity was not changed over the five-hour reaction time because the water present in Formalin oxidized any coke formed from the cracking of formaldehyde. A significant amount of carbon monoxide was observed due to the decomposition of formaldehyde at the reaction temperature. A 16% yield of carbon dioxide and methanol, Cannizzaro products, was observed. Reactions with Formalin were also done at 200 °C using different catalysts such as y-alumina, SA3177, AlPOa, and hydrotalcites (36% Al + 64% Mg, also identified as AM-36). The formation of carbon dioxide, carbon monoxide, and methanol was monitored. Methanol is a product of the Cannizzaro reaction of formaldehyde, and formic acid formed decomposes to give carbon dioxide and hydrogen at the reaction conditions. Base catalysts are responsible for the Cannizzaro reaction. The acid sites on the catalysts are responsible for the cracking reaction. With SA3177 alumina, the initial 30 minutes reaction gave only cracking products and most of the formaldehyde was destroyed. Analysis of gas samples revealed C1, C2, and C3 hydrocarbons. In another experiment, the acidic sites on SA3177 were poisoned by base addition to block the initial activity. In this case, the products were substantially methanol. Carbon dioxide was also prominent. Substantial methanol was formed when AM-36 was used as catalyst. Carbon dioxide generation was very high. In AlPO4, there was hardly any 161 methanol produced, carbon monoxide was detected in the reactor outlet gas, and much of the formaldehyde was recovered. The basic sites in AlPOa were responsible for the conversion of formaldehyde to methanol, and the acidic sites tended to preserve the formaldehyde for the necessary reaction. 6.4. Base Case Results Intermediate surface area alumina being identified as an optimal catalyst material for the desired reaction, was extensively used in this study. Commercial available 7- alurnina, SA3177 (Alundum), and alumina-in-house, prepared in the laboratory, both were used as intermediate surface area aluminas. Results from alumina-in-house were very similar to the results from SA3177 and, hence, it was decided to use readily available SA3177 to avoid the tedious preparation of alumina-in-house. Most experiments with SA3177 were carried out without any salt impregnated. Supported SA3177 was also used in some experiments to accomplish some specific task; this is discussed in later sections. Yields of citraconic anhydride, conversions of dimethyl succinate, selectivities of citraconic anhydride from dimethyl succinate, yields of monomethyl succinate, and yields of carbon dioxide evolved from the reaction of dimethyl succinate and Formalin over SA3177 are given and compared with other sources of formaldehyde in Figure 6.1, Figure 6.2, Figure 6.3, Figure 6.4, and Figure 6.5 respectively. Section 4.7.2.1 discusses results of trioxane and dimethyl succinate feed. Results from Forrncel are discussed in - Section 6.6.1. 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I V) O V) O N N F" '— (%) swmo P19! I O ('3 .mm 166 Omm 88:8 evict—«8.8 €80.93 89c NOD mo 22> .00 223m 03 3,5 05:. 832m cmm com cm_ oo_ L h _ L /\ Ba 35 Ba 8385+ Ba £8 23 annual-I Bow mza 2a .ooéomln cm 7 T T I o— on on ow ow ow (%) z0310 Pm 167 The condensation reaction of dimethyl succinate with Formalin over SA3177 was conducted at the base case conditions. A maximum 26% yield of citraconic anhydride at 82% conversion of dimethyl succinate was observed. After hydrolysis of product samples, 42% conversion of succinates was achieved with a citraconates yield of 29%, hence, a selectivity of about 70%. Conversions of dimethyl succinate were higher with Formalin as compared to other formaldehyde sources (see Figure 6.2). Higher conversion of dimethyl succinate was a result of hydrolysis of dimethyl succinate during the reaction over acidic alumina in the presence of the excess water from Formalin. Consequently, higher yields (see Figure 6.5) of monomethyl succinate and succinic anhydride were obtained. Yields of monomethyl succinate and succinic acid from Formalin and dimethyl succinate feed were about 30% and 10%, respectively at 80% conversion of dimethyl succinate. Two advantages of using Formalin are apparent: first, the decay in catalyst activity was much slower with Formalin. Yields of citraconic anhydride and conversion of dimethyl succinate dropped off very slowly over time for reaction times out to five hours. Yields of monomethyl succinate remained almost constant at 30% throughout the reaction time of five hours. Similarly, yields of carbon dioxide evolved from the cracking of reactants stabilized after starting at a maximum in the first sample (Figure 6.5). Second, the gain in the catalyst weight due to coking was much less with Formalin, likely because the water present steam-cleaned the catalyst during the reaction. 168 6.5. Other Catalysts Used Intermediate Surface area alumina has been already established as an optimal catalyst material for citraconic anhydride production, so it was used for the process optimization study. However, several additional catalyst materials such as high surface area aluminas, aluminum phosphates, hydrotalcites, glass beads, and activated carbon were also used for the reaction of dimethyl succinate and Formalin. The most significant effort of this study was to tailor the catalyst acidic-basic sites by changing magnesia content in alumina-magnesia mixed catalyst and then to correlate the support characteristics with the yield of citraconic anhydride and the conversion of dimethyl succinate. 6.5.1. Intermediate Surface Area Alumina 6.5.1.1. Salts Supported on SA3177 Alumina Conversion of dimethyl succinate and Formalin was also performed over ceric sulfate (Ce2(SO4)3) and KH2P04 supported on SA3177. Reactions with ceric sulfate supported on SA3177 were conducted in the early part of this study. There were two reasons behind the selection of ceric sulfate as a catalyst for the formation of citraconic anhydride. One, initially, thorium sulfate was selected as a salt additive because thorium sulfate supported alumina was successfully used, in a prior art, for this reaction (33). But, the radioactive nature of thorium sulfate made its intended uses difficult, so ceric sulfate, similar compound to thorium sulfate, became our choice. Two, cerium salts are widely used as coking reducers in industry. A maximum 16% yield of citraconic anhydride at 75% conversion of dimethyl succinate was obtained over 0.5 mmng ceric 169 sulfate supported on SA3177 at 410 °C and the feed molar ratio of 1 to 1.1 of dimethyl succinate to formaldehyde as Formalin. The condensation of succinates and formaldehyde was also conducted over SA3177 with ceric sulfate using diethyl succinate, different loading of ceric sulfate, and temperature of 380 °C. But the results were not significantly different from those described here. Reaction of dimethyl succinate and Formalin was also performed over KH2P04 supported on SA3177 at the base case conditions. KHzPO4 was impregnated onto SA3177 to selectively deactivate the strongly acidic coking sites on alumina. It has been already shown that the catalyst activity is eliminated by a base loading corresponded to the density of strong acid sites on SA3177 (0.15 mmollg catalyst). A loading of 0.015 mmol KHzPO4/ g SA3177 was used for the reaction of dimethyl succinate and Formalin, which is 10% of the acidic site density calculated from ammonia TPD. Figure 6.6 gives the comparison of conversion of dimethyl succinate and yield of citraconic anhydride over KHZPO4 supported on SA3 17 7 and SA3177 alone. Conversion of dimethyl succinate over KHzPO4 supported on SA3177 was not much different than over SA3177 alone. However, the yield of citraconic anhydride over 101sz supported on SA3177 was always 3 to 4% more than the yield of citraconic anhydride over SA3177 alone. After hydrolysis of the reaction products, a maximum 33% yield of citraconates at 43% conversion of succinates was observed over KH2P04 supported on SA317, compared to 29% yield of citraconates at 43% conversion of succinates over SA3177 alone. The comparison of yield of carbon dioxide from KHzPO4 supported on SA3177 and SA3177 alone is shown in Figure 6.7. Yields of carbon dioxide evolved from the reaction over KH2P04 supported on SA3177 were slightly less than the yields over 170 ntm. nu Fm. 0:20 mm PmB . cow 171 3:0 5233 23 h: mc0mmmv~ 80¢ NOD :0 22> so 2&5 Es: 25 832m 02 com on com o: 2: om o F b F _ b L _ o.o T on - 2: m p E 230%:2 52: ~00 .6 2021... mm. 25:w b: 2m :5: ~00? 205+ w . on W - com - 0.8 172 SA3177. But catalyst weight gain with ICE-12PO4/SA3177 was 1.2 g after five hours of the reaction, compare to 1.0 g catalyst weight gain with SA3177 alone. Summary of results from 0.015 mmol KH2P04/g SA3177 compared to SA3177: conversion of dimethyl stayed the same, yields of citraconic anhydride were 3-4% points higher, yields of carbon dioxide were slightly lower, and more catalyst weight gain was observed. Results from 0.015 mmol KH2P04/ g SA3177 were slightly better than the SA3177 alone, but not as advantageous as anticipated. 6.5.1.2. Acid Treated SA3177 SA3177 alumina was treated with sulfuric acid to poison basic sites, which are likely responsible for the consumption of formaldehyde during the reaction via the Cannizzaro reaction. Loading of 0.15 mmol HZSO4/g SA3177 was taken for the reaction‘ because it corresponds to the density of the basic sites on SA3177. The method used for loading the base was also employed for impregnation of sulfuric acid on SA3177. The acid treated SA3177 was not active for the reaction as anticipated. The yield of citraconic anhydride over acid treated was less than 5% at low conversions of dimethyl succinate (47% in 2"d sample and 25% in 3rd sample). The conversion of formaldehyde over acid treated SA3177 was low (25%) as compared to 80% with untreated SA3177, so we were successful in reducing formaldehyde consumption. Low conversion of formaldehyde was also evident from the lower yield of methanol. The yield of succinic acid was abnormally high (18%) in the first sample because hydrolysis of dimethyl succinate was facilitated by extra acid present on the surface of the catalyst. 173 6.5.1.3. Alumina-In-House y-Alumina (alumina-in-house or AIH) prepared in our laboratory was also tested for the condensation reaction of dimethyl succinate and Formalin. Results obtained from AIH are given in Table 6.2 along with results for SA3177. Properties of the AIH may be seen in Table 4.1. AIH was found to be a more active catalyst for the cracking reaction because of its higher surface area and surface acidity. After the first sample, yields of citraconic anhydride and conversions of dimethyl succinate from AIH were comparable with yields and conversions from SA3177. Higher extent of cracking of reactants over AIH was reflected in higher yields of carbon dioxide and, therefore, lower selectivities of citraconic anhydride. The catalyst weight gain of 1.5 g was observed after 2.5 hours of the reaction. Yields of citraconates increased after hydrolysis of product samples. Table 6.2. Comparison of results from Alumina-ln-House and 315.3177l Elapsed Yield of citraconic Conversion of Yield of C02 Selectivity (%) Time anhydride (%) Succinates (%) (%) (min) AIH SA3177 AIH SA3177 AIH SA3177 AIH SA3177 30 5 18 80 46 22 22 7 61 60 23 26 63 42 24 8 4O 61 90 23 26 59 43 24 10 41 57 120 23 22 43 37 ' 18 6 46 59 150 19 21 54 4O 19 6 38 52 1Reaction conditions: Temperature = 380 °C; pressure = 60 psi; feed = dimethyl succinate + Formalin (1 to 2 molar ratio of DMS to formaldehyde); liquid feed flow rate = 0.15 mllmin; outlet gas flow rate = 27 ml(STP)/min; pre-heat temperature = 200 °C. 174 6.5.2. High Surface Area Alumina The condensation reaction of dimethyl succinate and Formalin was also carried out over high surface area aluminas, SA6173 (surface area = 220 m2/g) and SA6175 (surface area = 236 mzlg), from Norton. Reactions over high surface area aluminas were carried out at the base case conditions except the dimethyl succinate to formaldehyde molar ratio of 1 to 1.1 was taken. A maximum 15% yield of citraconic anhydride at 92% conversion of dimethyl succinate was observed over SA6173. Results obtained from SA6175 were very similar to the results obtained from SA6173. Higher yields of carbon dioxide from the reaction of dimethyl succinate and Formalin over high surface alumina were indicative of excessive cracking of reactants. It is concluded from these reactions that the high surface area with high acid site density of aluminas facilitate the cracking reaction rather than the desired reaction. 6.5.3. Low Surface Area Alumina (SA3132) In an earlier study, two experiments were carried out with diethyl succinate and Formalin over 0.5 mmng ceric sulfate supported on a low surface area alumina, SA3132 (surface area 32 m2/g). A maximum 7% yield of citraconic anhydride at 52% conversion of diethyl succinate was observed over supported SA3132 at the base case conditions. A small amount of carbon dioxide and coking on the catalyst were also observed. The low surface area Alundum, SA3132, was found to be less active than the other aluminas used. 175 6.5.4. Hydrotalcites] Magnesium-Aluminum Mixed Oxides Hydrotalcites, mixed oxide of magnesia and alumina, are interesting catalysts for ‘ study because they can be prepared in different Mngl molar ratios to give different surface acidities. After calcination, these compounds lose interlayer anions and water to form mixed oxides that can be used as solid base catalysts (e.g., Mg-Al-O). The measured acidity and basicity of the hydrotalcites prepared are given in Table 4.1. As discussed in Section 1.8.3.5, the number and strength of the acid sites for calcined hydrotalcites decline as the Mg content of the hydrotalcite increases and vice versa. Hydrotalcite catalyst samples with Mg/[Al + Mg] molar ratios of 0.005, 0.01, 0.02, 0.04, 0.06, 0.12, 0.25, and 0.33 were prepared in our laboratory by the method described in Section 3.1.2 and studied comprehensively for the formation of citraconic anhydride from dimethyl succinate and Formalin. Reactions were conducted at the'base case conditions. Yields of citraconic anhydride, conversions of dimethyl succinate, selectivities of citraconic anhydride from dimethyl succinate, yields of monomethyl succinate, and yields of carbon dioxide evolved from the reaction over different hydrotalcites are given and compared with SA3177 in Figure 6.8, Figure 6.9, Figure 6.10, Figure 6.11, and Figure 6.12, respectively. As the Mg content of the hydrotalcite increases, the conversion of dimethyl succinate and the yield and selectivity for citraconic anhydride formation decline. Yield of monomethyl succinate also decreases as the Mg content of the mixed catalysts increases because the basic catalyst does not favor the de-esterification reaction. In contrast, yields of carbon dioxide and methanol were higher for the high Mg content hydrotalcites, because the basic catalysts facilitate the Cannizzaro reaction. 176 82280.2»; E8020 :5: 03.235 .0200qu :0 22 > we 05mm..— Aees as: enmesm 8m EN 2a SN 2: e: on oo 8 3 p _ _ p L p L F _ 0.0 E??? OmExNTeT . 0m2§+ - o m 03,224+ Ow2$mdlll . - o S . Q: ‘7I7\y - one. 92. (%) 9199me oluoomgo Jo plolA 177 3228823 32220 :5: mEO :0 22.20250 .00 Sawm— Eé as: 88% SM EN 98 2m 2: or on on 8 on otm ._ _ .e 2&5 25 as: 8m EN oem SN om. a: 0.: cm on em — _ h L F l? I..- P _ h _ 0 E§ .N _ e enema EEC ea; 832m 8m 2N ex ea 2: on. OS on r p P F p P _ Ll E.— m‘ I I 10- U] 5- EDDDDFeed flow rate 0.15 ml/min lllllFeed flow rate 0.30 ml/min iLittlt'Feed flow rate 0.45 ml/min 0 I I I T I I I I 30 60 90 120 150 180 210 240 270 300 Elapsed Time (min) Figure 6.13. Yield of citraconic anhydride using Formcel at different feed flow rate 190 Conversion of DMS (X) 100 40-' DDEJDDFeed flow rate 0.15 ml/min lllll Feed flow rate 0.30 ml/min may: Feed flow rate 0.45 ml/min 20-‘ 0 I I I I I I I I 30 60 90 120 150 180 210 240 270 300 Elapsed Time (min) Figure 6.14. Conversion of DMS using Forrncel at different feed flow rate 191 50 If U] 40-* 3'? v “'30-: O 0 Hut 0 'U E 2°11 >4 DECIDE] Feed flow rate 0.15 ml/min IIIII Feed flow rate 0.30 ml/min ***** Feed flow rate 0.45 ml/min lO-‘ D I I D I I 'I' I C] * t I D a t t I a 0 I I I I I I I 30 90 120 150 180 210 240 270 Elapsed Time (min) Figure 6.15. Effect of liquid flow rate on yield of C02 (Forrncel) 192 300 3O 254 Cl C] A I D 3320- I I u a D .2 I g c +3 I '6‘ 15— . I “5 I 2 £10— ElflElDDFeed flow rate 0.15 mI/min “ll-Feed flow rate 0.30 mI/min 3 0 I I I u I I I I 30 60 90 120 150 180 210 240 270 Elapsed Time (min) Figure 6.16. Yield of citraconic anhydride using Formalin at different flow rate 193 300 Conversion of DMS (X) 90 70-‘ t * DECIDE! Feed flow rate 0.15 ml/min an: Feed flow rate 0.30 ml/min 60 I I I I I I I I 30 60 90 120 150 180 210 240 270 300 Elapsed Time (min) Figure 6.17. Effect of feed flow rate on conversion of DMS (Formalin) 194 ~7% higher than the yields at the flow rates of 18 ml/hr and 27 mllhr. respectively. Yields of monomethyl succinate and succinic anhydride were not affected by changing the liquid flow rates. The selectivity of citraconic anhydride formation from dimethyl succinate was not affected much by different liquid feed flow rates because yields of citraconic anhydride and conversions of dimethyl succinate were changed in same proportion by changing the liquid fl0w rate. 6.6.5. Feed Molar Ratio The effect of molar ratio of formaldehyde to dimethyl succinate in the feed was also comprehensively studied for the formation of citraconic anhydride from dimethyl succinate and formaldehyde. Several different molar ratios of formaldehyde to dimethyl succinate (4:1, 2:1, 1:1, and 0.5:1) were studied for the reaction. Reaction conditions were same as the base case except a liquid feed flow rate of 0.30 mllmin and a gas flow rate of 55 ml(STP)/min was used. The molar concentration of dimethyl succinate, hence WSHV (kg DMS/kg*hr), was kept constant for each run with different feed molar ratio but the same liquid and gas flow rates; and this was done by diluting the feed mixture as needed with a mixture of methanol and water. ' The relative molar concentration of methanol and water was kept the same as their relative molar concentration in Forrncel. Effect of feed molar ratio on yields of citraconic anhydride, conversions of dimethyl succinate, selectivities of citraconic anhydride from dimethyl succinate, yields of monomethyl succinate, and yields of carbon dioxide are given in Figure 6.18, Figure 6.19, Figure 6.20, Figure 6.21, and Figure 6.22 respectively. 195 3O D D I U E20-I 3 :‘i 0 o J M n o 3 2’. 5* 10‘ I I I u I —I I ElElDDD Molar ratio 4:1 $231! Molar ratio 2:1 00.0.0.0 Molar ratio 1:1 Illll Molar ratio 0.5:1) 0 I I I I I I m I 30 60 90 120 150 180 210 240 270 300 . Elapsed Time (min) . Figure 6.18. Effect of feed molar ratio on yield of citraconic anhydride 196 90* 80‘ DECIDE! Molar ratio (29.0.0.0 Molar ratio W3 Molar ratio 97319.4" ”Add 33: Ill” Molar ratio 1) g 70 '1 a I “-1 o E o 60 " U I a: U 50 a II I 40 I I i I I I I I 30 60 90 120 150 180 210 240 270 300 Elapsed Time (min) Figure 6.19. Effect of feed molar ratio on conversion of dimethyl succinate 197 Selectivity (%) 100 E] El I:I . 80— I Cl . A 60-1 a A . A D J 40— A 20 ’ " DEIDRE! Molar ratio 4:1; MAMMolar ratio 2:1 Ill” Molar ratio 0.521) 0 I I I I I I I I I 30 60 90 120 l 50 180 210 240 270 300 Elapsed Time (min) Figure 6.20. Effect of feed molar ratio on selectivity 198 35 30- 25m 3 V" 2 20 .0 S. g 15“ I :1 S A 10-* UDDDD Molar ratio 4:1 r M Molar ratio 2:1 5.1 may: Molar ratio 1:1 Illll Molar ratio 0.5:1) 0 I I I I I I I I 30 60 90 120 150 180 210 240 270 300 Elapsed Time (min) Figure 6.21. Effect of feed MMS yield 199 molar ratio on 40 35 -l 30 -I 33, 25 fl 3 IIIID Molar ratio 4:1; L) dommslbhu’nuhr an 20 3- mar Molar ratio 1:1 '8 —Holar ratio 0.5:1) IE! 0 $1 15 10 -‘ l U 5- 0 I I I I I I I I I 30 60 90 120 150 180 210 240 270 300 Elapsed Time (min) Figure 6.22. Effect of feed molar ratio on yield of C02 200 Yield of citraconic anhydride was increased with increasing feed molar ratio of formaldehyde to dimethyl succinate. A maximum 26% yield of citraconic anhydride at 66% conversion of dimethyl succinate was achieved from 4 to 1 molar ratio of formaldehyde to dimethyl succinate, whereas feed with lean in formaldehyde gave 11% yield of citraconic anhydride at 57% conversion of dimethyl succinate. The difference in conversion of dimethyl succinate from different feed molar ratio was not as pronounced as in the yield of citraconic anhydride. Thus, the selectivity of citraconic anhydride from dimethyl succinate increased with increasing the feed molar ratio of formaldehyde to dimethyl succinate. Yield of carbon dioxide from the reaction increased slightly with increasing the feed molar ratio of formaldehyde to dimethyl succinate. Yield of monomethyl succinate decreased with increasing the feed molar ratio, because the water content of feed, responsible for de-esterification of dimethyl succinate, decreased with‘ increasing the feed molar ratio. Product samples from each feed molar ratio studied were hydrolyzed to check the overall yields of citraconates. The catalyst weight gain after the completion of the reaction decreased with decreasing the molar ratio, because the feed lean in formaldehyde contained more water, which tends to clean the coke deposited on the catalyst surface. 6.6.6. Effect of Temperature The effect of temperature on the formation of citraconic anhydride from dimethyl succinate and Formalin over SA3177 was also studied. Reactions were performed at 350 °C and 380 °C, keeping all other reaction conditions unchanged. The effect of 201 temperature on citraconic anhydride yield (unhydrolyzed results), dimethyl succinate conversion, carbon dioxide yield, and selectivity is shown in Table 6.4. Table 6.4. Effect of temperature on resultsI Elapsed Yield of citraconic Conversion of Yield of C02 Selectivity (%) Time anhydride (%) ' DMS (%) (%) (min) 350 °C 380 °C 350 °C 380 °C 350 °C 380 °C 350 °C 380 °C 30 12 18 50 46 24 22 23 61 60 15 26 30 42 10 8 52 61 9O 17 26 21 43 8 10 78 5 l 120 16 22 21 37 8 6 77 59 150 15 21 21 4O 6 6 75 52 180 15 23 19 35 7 4 76 63 210 13 20 20 40 5 5 67 50 240 12 20 15 33 5 6 79 59 270 13 20 15 33 5 5 87 58 300 12 18 15 27 4 5 82 58 IReaction conditions: Pressure = 60 psi; feed = dimethyl succinate + Formalin (l to 2 molar ratio of DMS to formaldehyde); liquid feed flow rate = 0.15 mllmin; outlet gas flow rate = 27 ml(STP)/min; pre-heat temperature = 200 °C. Higher yields of citraconic anhydride were achieved at 380 °C. The conversion of dimethyl succinate increased with increasing temperature. It is noteworthy that selectivity to citraconic anhydride was highest at 350 °C. At higher temperature, 202 succinates cracked into carbon monoxide and carbon dioxide, resulting in high conversion of succinates and lower selectivity. 6.6.7. Particle size A smaller size (60/100 mesh) of catalyst particles was also utilized for the condensation of dimethyl succinate and formaldehyde to observe the mass transfer effects. Identical results were observed from 60/ 100 mesh size and 30/60 mesh size catalyst particles, suggesting that the condensation reaction is not mass transfer limited. 6.6.8. Longer Reactor Catalyst Bed The condensation reaction of dimethyl succinate and Formalin was also conducted with a longer reactor catalyst bed to see the effects of catalyst bed length or weight hourly space velocity (WHSV) on the desired results. Table 6.5 gives the summary of a series of experiments, which were carried out to see the effects of catalyst bed length on the results. Effects of catalyst bed length on yields of citraconic anhydride, conversions of dimethyl succinate, selectivities of citraconic anhydride from dimethyl succinate, and . yields of carbon dioxide from the reaction of dimethyl succinate and Formalin over SA3177 are given in Figure 6.23, Figure 6.24, Figure 6.25, and Figure 6.26, respectively. Yield of citraconic anhydride, yield of carbon dioxide evolved from the reaction, and conversion of dimethyl succinate increased with increasing the catalyst bed length, keeping other reaction conditions unchanged. The conversion of dimethyl succinate was not changed with changing catalyst bed length if the WSHV was kept constant by 203 Table 6.5. Summary of experiments conducted to see WHSV effectsl Experiment Weight of catalyst Liquid flow WHSV2 used (g) rate (ml/min) Shorter reactor (FR = 0.15 mllmin) 5.2 0.15 0.90 Longer reactor (FR = 0.15 mllmin) 11.0 0.15 0.45 Longer reactor (FR = 0.30 mllmin) 11.2 0.30 0.90 Longer reactor (FR = 0.30 mllmin): 11.3 0.30 0.90 1Reaction conditions: Temperature = 350 °C; Pressure = 60 psi; feed = dimethyl succinate + Formalin (1 to 2 molar ratio of DMS to formaldehyde); outlet gas flow rate = 27 ml(STP)/min; pro-heat temperature = 200 °C. 2Weight Hourly Space Velocity (WHSV) = kg succinate/(kg catalyst * hr). '5 was a repeat of previous experiment to check reproducibility. adjusting the liquid flow rate, which shows that the reactor exhibits predictable behavior regarding WHSV. However, the yield of citraconic anhydride decreased with increasing bed length at constant WSHV, which suggests that the extra length of catalyst bed led to cracking of citraconic anhydride. Better selectivity for citraconic anhydride formation was observed with a shorter reactor catalyst bed or at lower WSHV. Yields of monomethyl succinate followed the dimethyl succinate conversion trends with. the catalyst bed length and WSHV. 6.6.9. Hydrolysis Hydrolysis of the reaction products from the reaction of dimethyl succinate and Formalin was carried out to convert all citraconates to citraconic acid and succinates to succinic acid. Four product samples out of total ten samples collected from the each run were usually hydrolyzed using the method described in Section 2.8. Table 6.6 shows 204 20 I l ' ng- _ Cl . s " a E a: >3 = Cl .C * G 120 P G E D .9. * C , * 8 :1: a In :3 8" 0 “-0 0 IE’. 0 s: 43 QDDDDShorter reactor (FR=O.15 ml/min) Ill-l Longer reactor EFR=O.15 ml/ming um Longer reactor FR=O.30 ml/min 0 I T l T l T T l 30 60 90 1 20 1 50 l 80 2 10 240 270 300 l s (1 Tim ' Figure 6.23. Yield of gégcgmc aghyglrWe from different size catalyst beds l\) 0 VI Conversion of DMS (X) 100 I 800 60* 40— DDDDDShorter reactor (FR=O.15 ml/min) Illll Longer reactor (FR=O.15 ml/ming m Longer reactor FR=O.3O ml/min 20 I I I I I I I I 30 60 90 120 150 180 210 240 270 300 Elapsed Time (min) Figure 6.24. Conversion of dimethyl succinate from different size of catalyst beds 206 Selectivity (X) 100 80-1 60-‘ 40-' 20 QDDDDShorter reactor (FR=O.15 ml/min) MLonger reactor EFR=O.15 ml/min Illll Longer reactor FR=O.3O mI/min 0 I I I I I I I I I 30 60 90 120 150 180 210 240 270 Elapsed Time (min) Figure 6.25. Selectivit from different size of catalyst eds 207 300 70 60-* Yield of 002(2) 50— UflDflDShorter reactor (FR=O.15 ml/min) 40~ AMMLonger reactor (FR=O.15 ml/ming Ill” Longer reactor FR=O.30 ml/min so—l C N O l UD~ 0 I I I I I F I I 30 60 90 120 150 180 210 240 270 300 Elapsed Time (min) Figure 6.26. Yield of carbon dioxide from different size of catalyst beds 208 results where all yields, conversions, and selectivities are based on hydrolyzed results. The yield of citraconates after hydrolysis usually increased by 20% over the unhydrolyzed yields. Table 6.6. Results before and after hydrolysis of product1 Elapsed Yield of citraconates (%) Conv of Succinates (%) Selectivityz Time (min) Before After Before After (%) 60 26 26 42 55 48 90 26 29 45 42 70 180 23 26 37 46 57 240 20 25 3O 42 60 1Reaction conditions: Temperature = 380 °C; pressure = 60 psi; feed = dimethyl succinate and Formalin (1 to 2 molar ratio of DMS to formaldehyde); liquid feed flow rate = 0.15 mllmin; outlet gas flow rate = 27 ml(STP)/min; pre-heat temperature = 200 °C. 2Selectivity is based on hydrolyzed products. 6.7. Catalyst Deactivation Studios Catalyst deactivation was much slower with dimethyl succinate and Formalin and, hence, the conversion of dimethyl succinate and yield of citraconic anhydride decreased very little over time. The catalyst weight gain was much less pronounced from dimethyl succinate and Formalin feed, because the water present reacts with the coke deposited to clean the catalyst during the reaction. The catalyst weight gain of 0.9 g was observed after five hours of reaction from dimethyl succinate and Formalin feed, whereas the catalyst gain was 1.5 g after five hours of the reaction from dimethyl succinate and trioxane feed. In early stages of the reaction, catalyst activity was not very extensive for 209 the reaction of Formalin and dimethyl succinate, unlike the reaction of dimethyl succinate and trioxane where excessive cracking of succinates and very low yield of citraconic anhydride were observed in the beginning of the reaction. Coking was observed from dimethyl succinate and Formalin feed over all catalyst materials used except glass beads. The rate of catalyst activity decay depends upon the coke deposited on the catalyst material during reaction from the cracking of reactants or products. Coking results in the catalyst weight gain over time. The rate of the cracking reaction depends upon surface properties such as surface area, surface acidic site concentration, and type of acidity and reaction conditions such as temperature, feed flow rate, and feed material. Thus, the catalyst weight gain is directly related to the surface properties of the catalyst material if reaction conditions are unchanged. Table 6.7 gives the catalyst weight gain over different catalyst materials used for the reaction of Formalin and dimethyl succinate. It was obvious from control experiments that coking involved the reactants, succinate and formaldehyde, and the product, citraconic anhydride. 6.8. Catalyst Regeneration The catalyst loses its activity upon its prolonged exposure to the reactants for the formation of citraconic anhydride. To prove that catalyst activity can be recovered by catalyst regeneration is very crucial for this process to be commercially viable. It has already been shown that the regenerated catalyst was as good as the fresh catalyst for the reaction (see Section 4.12). It was decided to confirm these results described in Section 4.12 with Formalin and dimethyl succinate at optimal reaction 210 Table 6.7. Catalyst weight gain from different catalyst materialsl Catalyst Wt. before expt Wt. after expt Wt. gain Wt. gain Reaction (g) (g) (g) (%)2 Time (hr) Beads 13.00 13.02 0.02 0 2 SA3177 5.26 6.20 0.94 18 5 SA31773a 5.32 6.20 0.88 17 5 SA31773b 5.30 6.50 1.20 22 5 SA3177“ 5.24 6.45 1.21 23 5 SA31773d 5.50 7.08 1.58 29 5 SA3177‘c 5.20 5.95 0.75 14 5 SA3177“ 11.00 13.54 2.54 23 5 98 A1 + 2 Mg 6.74 7.89 1.15 ' 17 5 96 A1 + 4 Mg 5.03 6.34 1.31 26 5 94 A1 + 6 Mg 4.50 5.66 1.16 26 5 88 A1 + 12 Mg 4.85 7.00 2.15 44 5 Alumina(AIH) 6.30 7.88 1.58 25 2.5 AlPO (PlAl=1) 1.90 3.40 1.50 79 5 AlPO(P/Al=0.5) 2.15 3.28 1.13 53 5 ICatalyst weight gains are given at the base case conditions, otherwise specified. 2Wt. gain (%) = (wt. after expt - wt. before expt)* lOO/wt. before ex t. 3‘coz as a carrier gas; 3')0.15 mmol KHzPO4/g supported SA3177; °Liquid feed flow rate = 0.30 mllmin; 3dForrncel is used as a formaldehyde source; 3‘,'1‘ = 350 °C; 3fLonger catalyst bed used. 211 conditions for the process. The catalyst regeneration process illustrated in Section 4.12. was followed here to regenerate the catalyst upon deactivation. Reactions of Formalin and dimethyl succinate over fresh and regenerated catalyst were conducted at identical conditions. Yields of citraconic anhydride and conversion of succinates were identical for both fresh and regenerated catalysts (Table 6.8). This suggests that coke residual on catalyst surface decreases the activity of catalysts and also indicates that other means of deactivation are less probable. Table 6.8. Yield of citraconic anhydride before and after the regeneration of catalyst1 Elapsed Yield of Citraconic Yield of Conversion of DMS Time Anhydride (%) C02 (%) (%) (min) Before After Before After Before After 30 8 8 l 1 12 77 74 60 12 13 5 5 57 62 90 11 ll 8 8 59 63 120 12 l l 4 3 54 60 150 10 9 6 6 58 59 180 9 10 3 2 55 53 210 12 10 5 5 47 50 1Reaction conditions: Temperature = 380 °C; pressure = 60 psi; feed = dimethyl succinate + Formalin (l to 2 molar ratio of DMS to formaldehyde); liquid feed flow rate = 0.20 mllmin; outlet gas flow rate = 27 ml(STP)/min; pre-heat temperature = 200 °C. 212 6.9. Summary The reaction of dimethyl succinate and formaldehyde in the vapor phase over various catalysts was performed at the base case conditions. Commercially available formaldehyde solutions such as Formalin and Forrncel were used as a source of formaldehyde for the reaction. Optimization of reaction conditions for the reaction of dimethyl succinate and Formalin over SA3177 was conducted to maximize citraconic anhydride yields. These studies involved changing feed compositions, temperature, liquid feed and gas flow rate, catalyst particle size, and reactor catalyst bed size to determine the region where both desired product yields were high and the catalyst was stable over time. ‘ Dimethyl succinate and Formalin were proven to be a good feed material combination for the reaction, as decent yields of citraconate were obtained with very good selectivities of citraconates and the catalyst activity was also stable for a longer time. After hydrolysis of raw products, a maximum 29% yield of citraconates at 42% conversion of succinates were observed from the reaction of dimethyl succinate and Formalin over SA3177 at the base case conditions. Dimethyl succinate is not prone to cracking at elevated temperature as is succinic anhydride and, hence, is a more stable compound at the reaction conditions. Formalin is also proven to be advantageous for the desired reaction. First, the decay in catalyst activity was much slower with Formalin as yields of citraconic anhydride and conversion of dimethyl succinate only dropped off very slowly over time .for reaction times out to five hours. Second, the gain in the catalyst weight due to coking was much less with Formalin, likely because the water present cleans the catalyst during the reaction. Excessive water present in Formalin also 213 causes problem: the excessive water makes the feed dilute and also facilitates de— esterification reactions of dimethyl succinate. Forrncel with dimethyl succinate gave higher yield of citraconates (34% after hydrolysis) compared to Formalin and dimethyl succinate feed. But the rate of catalyst decay was faster with Forrncel than Formalin because Forrncel contains less water to react with the coke formed. Mass balances from Forrncel and dimethyl succinate were also poor due to loss of excessive methanol present in Forrncel as dimethyl ether. Several other catalyst materials were also tested for the reaction of dimethyl succinate and trioxane. It was confirmed that a mildly acidic support in nature like SA3177, without any salts added, showed significant activity for the formation of citraconates from succinates. Higher surface area or highly acidic supports were found to be very active for cracking reaction of dimethyl succinate. Also, the coke-forming tendency is directly related to the acidity of supports. The results observed with hydrotalcites are in accord with those of other catalysts regarding the surface acidity or basicity properties required for an active catalyst. The condensation reaction of dimethyl succinate with Formalin over intermediate surface area alumina is not mass transfer limited. Higher formaldehyde to dimethyl succinate molar ratio gave better yields and selectivities of citraconic anhydride. Carbon dioxide as a carrier gas did not make any significant difference in yields of in the catalyst activities. Yield of citraconic anhydride decreased with increasing liquid feed flow rate, but selectivity remained unchanged. Yield of citraconic anhydride increased with increasing the reaction temperature, but selectivity lowered due to more cracking of dimethyl succinate at elevated temperatures. Citraconic anhydride yields were increased 214 with increasing the catalyst bed length, but selectivities decreased. The regenerated catalyst upon deactivation reproduced the fresh catalyst results. 215 CHAPTER 7 REACTOR MODELING 7.1. Pressure Drop calculation The feed flow rates from a typical experiment are given in Table 7.1. The total liquid flow rate to the reactor is 9.0 mllhr. Table 7.1. Flow rates at the reactor inlet Species Volumetric flow Mass flow rate Molar flow rate rate (ml/hr) (g/hr) (“101/hf) DMS 4.0 4.47 0.0306 Formalin 5.0 i 5.40 Formaldehyde 2.00 0.0666 Water ' 2.81 0.1560 Methanol 0.59 0.0186 Helium 3300 @ 298°C 0.27 0.0663 Total 9.0 (liquid) 10.13 0.3380 216 Total vapor flow rate = 0.3380 mol/hr = 3622 cm3/hr @ reaction conditionsCT=380°C and P=5 atm) Reactor tube diameter (D) = 1.12 cm Superficial vapor velocity at the inlet of the reactor (v) 3 v=[m]x[ 1" )-:-(3.14x(1.12 cm/2)2) hr 36008ec v = 1.03 cm/sec Density of feed at reaction conditions (p) = (4.47 g/hr + 5.4 g/hr + 4*0.0663 g/hr)/(3622 cm3/hr) p = 0.0028 g/cm3 The viscosity of the feed at reaction conditions (1.1) is taken as an average of water and helium, 11 = 0.026 cp (air’s viscosity @ 380 °C and 1 atm) p. = 2.6 x 10'3 g/(cm.sec) Bed porosity (e) = 0.7 Particle size (DP) = 0.038 cm (30/60 mesh size) Ergun’s pressure drop equation (77); 9.1:: pv 1.38 M +1.75pv (1) AL pr 8 DP Substituting values from above in equation (I), we have AP/AL = 7 g/cmzsec2 217 AP/AL = 70 Palm Catalyst bed length = 8 em = 0.08 m, then AP = 6 Pa = 5.8 x10'5 atm = 0.0009 psi This is an extremely small number; thus for typical WSHV we do not expect pressure drop to be a limiting factor. Reynolds number for the packed bed (Rep) Dppv = II C? (1 _ 8);! ( ) Rep = 0.42 Similarly, Reynolds number for the reactor tube (Remix) Rembc = 12 7.2. Mass Transfer Calculations 7.2.1. Calculation of Observable Rate Assume flow through reactor is steady state and constant volume. The rate equation for DMS is given by dFDMs an =-(l—as )(Rmmus) (111) Where, FDMS = Cums x V (IV) where, CDMS = DMS concentration at any position in the reactor V = volumetric flow rate @ reaction conditions (5 atrn and 380 °C) 218 is give XDM: Cara] = 3622 cm3/hr = 1.01 cm3/sec Vp = reactor catalyst bed volume 86 = bed porosity = 0.7 Assume the reaction is first order in DMS, then the rate of equation (observable) is given by Robs. DMS = nkCDMs (V) T] = effectiveness factor (~constant) k = first order rate constant Combining equation (I11), (IV), and (V) and then rearranging, dCDMs : ‘(1-8)de[%] (VI) C DMS Integrating equation (VI) from the reactor inlet to outlet, CDMs(0ut) __ _ 1’: ln[ CDM$(in) J" (I 3W“) (VII) substituting (CDMs(out)/CDMs(in)) = (1 - Xmas). where XnMs is conversion of XDMS, and k’ =kn in equation (VII), ln(1— XDMS) = —(1-as)k[%'-] (VIII) Solving equation (V111) for k. when XnMs = 0.8 (80% conversion of DMS) and catalyst bed volume is 8 cm3, k. = 0.675 cm3/(cm3 of cat.sec) (IX) DMS inlet concentration CDMSUD) = (moms/Drom)P/RT 219 (mums and mom from Table 7.1, P = 5 atm, T = 653 K, and R = 82.06 cm3.atml(mol.K)) amen) = 8.5 x1043 mol/cm3 (X) Solving Roesnms at inlet of the reactor using values equation (IX), (X), and (V), Rmmsan) = 5.7 x 10*5 mol/(cm3 of cat.sec) (x1) 7.2.2. Diffusivity Estimation ' The diffusivity of a binary gas pair of A (DMS) and B (H20) molecules is given by(771 DAB =1.85832x10‘3T3’2 [_1_ +1)” (x11) Pa 711190.713 MA Ma DAB = Diffusivity in cmzlsec T = 653 K P = 5 atm MA = 146 and M3 = 18 6A3 = average collision diameter based on the Lennard-J ones potential (20.13 = collision integral based on the Lennard-J ones potential = 1.0 (assume not much interactions between DMS and water) 6A3 = (0,. + 0'3)/2 (X111) 63 =. 3.6 A (from (78)) (XIV) GA is estimated from molar volumes, oA = 1.18V,,"3 (from (78)) (XV) 220 Vp = (# of C atoms in A)*Vp(C) + (# of 02 in A)*Vp(0) + (# of H2 atoms in A)*Vp(H) (XVI) Substituting molar volumes from (78) in equation (XVI) Vp = 6* 14.8 + 4*9.1 + 10*3.7 Vp = 162.2 cm3/mol (XVII) From equations (XV) and (XVII), as = 6.4 A (XVIII) From equation (XIII), (XIV), and (XVIII), GAB = 5.0 A Now, substituting values in equation (XII) DAB = 5.7 x 10’2 cm2/sec Effective diffusivity (Degas), Dale = (4,?) Dis (XIX) 4),, = particle porosity ~ 0.4 (low) DenMs = 9.1 x 10'3 cmZ/sec 7.2.3. Calculation for Observable Modulus Observable modulus is give by _ Ra... DMs(in)L2 — De. DMsCDMs(in) 7792 L = particle size = 0.038 cm (30/60 mesh size) Substituting values from previous sections into equation (XX), we obtain 11th ~ = 0.107 221 The small value of the observable modulus suggests that the reaction is not significantly mass transfer limited. Uncertainties in this calculation arise from Dams, and Robspms. The calculated modulus is calculated at the extreme conditions at the inlet of the reactor; the value given above is a upper limit for modulus. 7 .3. Residence Time and WSHV Calculation Residence time (1, sec) for fixed-bed reactor is given by, r=(VR--Wi]+v (XXIII) pea: VR = Reactor volume = 8 cm3 W3 = weight of the catalyst bed = 5.2 g pea. = density of the catalyst without pores ,= 2.31 g/cm3 V = Total vapor flow to the reactor (fi'om Section 7.1) = 3622 cm3/hr = 1.01 cm3/sec Substituting values of VR, W3, pm, and V in equation (XXIII), t = 5 .7 sec The Weight Hourly Space Velocity (WHSV) is the ratio of the mass flow rate of DMS to the mass of the catalyst used (W B). The mass flow rate of DMS to the reactor is 4.47 g/hr (Table 7.1), hence WSHV = (4.47 g of DMS/hr)/(5.2 g of catalyst) WSHV = 0.86 g of DMs/(g of cat*hr) 222 7.4. Kinetic Modeling A kinetic model for the formation of citraconic anhydride from dimethyl succinate and formaldehyde over y-alumina (SA3177) has been developed. An analysis gives the observable modulus n¢2 (Section 7.2.3) of less than 0.10 for y—alumina (SA3177) at all reaction conditions with the diffusitivity of 9.1 x 1045 cmzlsec (Section 7.2.2), indicating negligible mass transport limitations within the support material. Thus, the intrinsic kinetic rate constants can be obtained from the experimental data. The reactions included in the kinetic model are listed in Figure 7.1. 7.4.1. Calculation of Rate Constants from Control Experiments 7.4.1.1. Citraconic Anhydride Cracking Reaction Carbon dioxide was the only product formed from citraconic anhydride at the base case conditions. It is assumed that the cracking reaction of citraconic anhydride (Reaction 2 in Figure 7.1) is a first order reaction; the rate expression is given by: dCc dr =—k7C.- (1) Solving differential equation (I) will give: Cc = Ccoe— [(71 (II) Or X.=1-e""" (III) where, Cc (moles/l) = concentration of citraconic anhydride at the time t, Cco (moles/l) = concentration of citraconic anhydride at t = 0, 223 DMS + HCHO —-> CAN + 2MeOH (1) 1(7 CAN —-> C02 7‘ H2 (2) 1<3 DMS + H20 =—= MMS + MeOH (3) K3 K; Mg + H20 SA + MeOH (4) k4 k5 k2 2HCHO + H20 —> MeOH + HCOOH (6) FAST HCOOH —> C02 + H2 (7) k6 HCHO _"> C0 + H2 (8) Figure 7.1. List of reactions included in the kinetic model (DMS = dimethyl succinate, MMS = monomethyl succinate, SA = succinic acid, and CAN = citraconic anhydride) 224 Xc == fractional conversion of citraconic anhydride at the 1 k7 (sec°') = first order rate constant for the cracking reaction 1 (sec) = residence time At the steady state, 1: = 10.7 sec and Xc = 0.20; solving equation (III) for k7, k7 = 0.021 sec'I (IV) 7.4.1.2. Dimethyl Succinate Cracking Reaction The rate expression for the cracking reaction of dimethyl succinate (Reaction 5 in Figure 7.1) is given by: $9- : —k5Cd (V) dc Solving differential equation (V) will give: C4 = Cdoe- "57 (VI) Or Xd=l-e-kST (VII) where, Cd (moles/1) = concentration of dimethyl succinate at the time t, Cdo (moles/l) = concentration of dimethyl succinate at 1: = 0, X, = fractional conversion of dimethyl succinate at the 1 k5 (sec'1) = first order rate constant for the cracking reaction At the steady state, I = 18 sec and X. = 0.07; solving equation (VII) for k5, k5 = 0.004 sec’l (VIII) 225 7.4.1.3. Formaldehyde Reactions Formaldehyde undergoes the Cannizzaro reaction in the presence of basic sites and forms methanol and formic acid; formic acid further decomposes to carbon dioxide at elevated temperatures (Reaction 6 and 7 in Figure 7.1). Formaldehyde itself also decomposes at elevated temperatures to give carbon monoxide (Reaction 8 in Figure 7.1). It was assumed that the Cannizzaro reaction and the decomposition reaction are second and first order reactions with respect to formaldehyde, respectively. The rate equation is given by: _iCl = 2k2C/2Cw‘l'k6Cf (IX) dr where, Cf (moles/l) = concentration of formaldehyde at time 1, CW (moles/l) = concentration of water, which is assumed constant because it is present in excess k2 (lzmoles'zsec'l) = third order rate constant for the Cannizzaro reaction k6 (sec'l) = first order rate constant for the decomposition reaction Complete conversion (100%) of formaldehyde was obtained when Formalin itself was passed over “II-alumina (SA3177). The catalyst activity was not changed over the five-hour reaction time because the Water present in Formalin oxidized any coke formed from the cracking of formaldehyde. However, a significant amount of the unreacted formaldehyde is recovered in the product from the reaction of dimethyl succinate and formaldehyde. Data from the reaction of dimethyl succinate and formaldehyde were therefore used to calculate the rate constants for the formaldehyde reactions, instead of the control run of the formaldehyde. It was assumed that the only source of carbon 226 monoxide is the decomposition of formaldehyde. The concentration of formaldehyde was also adjusted by subtracting the concentration of citraconic anhydride formed in the reaction to account for formaldehyde consumption in the primary reaction. The rate constants, kg and kg, were adjusted simultaneously until the difference in the predicted and experimental values of concentrations of formaldehyde and carbon monoxide were minimized. This was accomplished using the software Polymath. The calculated values of rate constants are k2 = 4012mol’2sec‘I (X) k6 = 0.014 sec“1 (XI) 7.4.2. Equilibrium Calculations The rate equations for succinates (Reaction 3 and 4 in Figure 7.1) are given by: dCd DMS: - — = k3Cde — k — 3CmCoh (XII) d1 (16.. NEWS: d = k3Cde — k - 3CmCoh - k4Cma + k - 4CsaCoh (XIII) 1' SA: E'- = k4Cma — k - 4CsaCoh (XIV) d1 Kc] = CWCO’I‘ = £- (XV) Cd¢Cw¢ k - 3 CmeCohc [(4 K: = = . . X 2 Cma k — 4 ( VI) We also know that CdO = Cd + Cu: + Csa (XVII) Cob = Coho + Cm + 2Csa (XVIII) 227 Cw: Cwo-Cm-ZCsa (XIX) where, Cd (moles/1) = concentration of DMS at the time T, Cm (moles/l) = concentration of MS at the time 1:, Cu (moles/l) == Concentration of succinic acid at the time 1, CW (moles/l) = Concentration of water at the time t, Cob (moles/l) = Concentration of methanol at the time 1, k3, k3, k4, and k4 (lmol'lsec'l) = second order rate constants, K.) = equilibrium constant for the Reaction 3 in Figure 7.1, Kg = equilibrium constant for the Reaction 4 in Figure 7.1. Subscript ‘e’ represents the concentration of the particular species at the equilibrium and subscript ‘0’ denotes the concentration at 't = 0. By simplifying above equations we get - gd—Cf = k3[Cd(Cw0 - Cdo + Cd - C30) - (Cdo - Cd — Csa)(Coho + CdO - Cd + Ca) / Ke1)] (XX) 11;;- = k4[Cm(Cw0 - C40 + Cd - Cso) — Csa(Coh0 + Cdo — Cd + Csa)/Ke2)] (XXI) Cm=Cd0—Cd—Csa (XXII) It was observed from several different runs of dimethyl succinate and formaldehyde that Reaction 3 and Reaction 4 in Figure 7.1 are equilibrium reactions. The equilibrium constants, K] and K2, were calculated by using the reactor outlet concentrations; the values are: Kc! = 0.192 (XXIII) 228 Kel = 0.075 (XXIV) The rate constants, kg, and k4, were adjusted simultaneously in equations XXII, XXIII, and XXIV until the difference in the predicted and experimental values of concentrations of dimethyl succinate, monomethyl succinate, and succinic acid are minimized. This was accomplished using the software Polymath. The calculated values of rate constants are k3 = 3.98 lmol"sec" . (XXV) k4 = 3.05 lmol'lsec'l (XXVI) 7 .4.3. Presentation of Equations The one-dimensional molar balances in an integral, tubular reactor are given below for all species involved in the reactor system: Citraconic anhydride: Edy: = kICdCf — k7Cc (XXVII) T dCd DMS: --d—; = kICdCf + k3Cde — k - 3CmColr + kst (XXVIII) de MMS: —d = k3Cde - k — 3CmCoh - k4Cma + k - 4CsaCoh (XXIX) 1' SA: dc“ = k4Cma — k - 4CsaCoh (XXX) ’1' dCf 2 Formaldehyde: — -d— = kICdCf + 2k 2Cf Cw + keC/ (XXXI) r MeOH: dCoh 2 T = k1CdCf/2 + k3Cde — k - 3CmCoh - k4Cma + k - 4CsaCoh + kZCf Cw (XXXII) 1' 229 Water: ‘— ddCW = kZCf ZCW + k3Cde -' k -— 3CmCoh ' k4Cma + k - 4CsaCob (W) 1 co: dc” = kscf (XXXIV) dz' coz: d?“ = kst + KoC; + kic. (XXXV) T where k; (lmol'lsec‘l) = first order rate constant for the desired reactions Other variables are defined and calculated in the previous sections. The rate constant k1 was adjusted until the difference in the predicted and experimental values of concentrations of all species in the reaction system were minimized. This was accomplished using the software Polymath. The calculated value of rate constant k; is k1 = 4.11mol"sec'l (XXXVI) 7.4.4. Results The predicted values of concentrations of species present in the product stream of the reactor are compared with the experimental concentrations at different conditions in Table 7.2. The concentrations of each species in the last 2.5 hours of the reaction are averaged and then used for the model. The concentrations of product species predicted from kinetic model developed here are plotted against the residence time in Figure 7 .2. The concentration of citraconic anhydride reaches maximum at t = 18 see before it starts declining because of the secondary cracking reaction. It is obvious from Figure 7.2 that the concentrations of 230 m; Rd 86 who ”2 a: on. no.“ A565 a. o: 80 :3 3.0 2.0 so 8.. mod N: 8.. Ass: n. o: oo as 3.2 5.2 2 .w 8.2 Em :5 8.2 Asee n. e: moo: one 9: So OS 2.. 3o 2: 85 €68 w o: 5:838:00 .NH 83E..— Aoomv 08F cocoEmox 2 E N_ o_ w _ b F F k «QUIT OUIeI 99.4%. Raw materials: succinic acid ($0.25/1b), methanol ($0.04/1b), and Formalin (37.5 wt% formaldehyde) (30.09/1b). 237 Feed to the reactor: 2 to 1 molar ratio of formaldehyde (Formalin) to dimethyl succinate. Reactions: DMS + Formaldehyde => Citraconic Anhydride Citraconic anhydride + Water => Citraconic acid Citraconic acid => Itaconic acid. Reaction conditions: reactor temperature = 380 °C and pressure = 1 atm. Catalyst: 'y-alumina. Design parameters: (1) in reactor, 30% yield of citraconic anhydride at 43% per pass conversion of succinates and thus 70% selectivity to citraconic anhydride from succinate. 70% overall conversion of DMS. (2) 80% recovery of itaconic acid from citraconic anhydride isomerization/ purification. (3) 50% conversion of formaldehyde. (4) WHSV = 0.90 kg succinate/(kg catalyst * hr) Constraints: (1) Unreacted succinate and formaldehyde are recycled. (2) MeOH should be mostly recovered and then recycled. 8.3. Feed costs Calculation => Itaconic acid (IA) production rate =20x1061bIAlyr = 1.54 x 1051bmolIA/yr => Citraconic anhydride (CAN) production rate = (1.54 x 105/0.8)1bmol CAN/yr 238 (80% recovery of IA from CAN isomerization/purification) = 1.92 x 105 lbmol CAN/yr => DMS feed rate = (1.92 x 105/0.7) lbmol DMS/yr (Overall reactor yield with complete succinate conversion is 70%) = 2.75 x 1051bmolDMS/yr = 4.01 x 107 1b DMS/yr = Succinic acid feed rate = 2.75 x 1061bmol SA/yr = 3.24 x 1071b SA/yr =9 Methanol feed rate = 2 x 2.75 x 1061bmolMeOH/yr = 5.50 x 1061bmol MeOH/yr = 1.76 x 107 1b MeOH/yr =9 Formaldehyde feed rate = 2 x 2.75 x 106 x 0501me] formaldehyde/yr (Formaldehyde to DMS is 2: l , but 50% conversion of formaldehyde) = 2.75 x 106 lbmol formaldehyde/yr => Formalin feed rate = (2.75 x 106 x 30/0.375) 1b Formalin/yr = 2.12 x 107 lb Formalin/yr (Formalin contains 37.5 wt% formaldehyde) => Succinic acid cost ($llb itaconic acid produced) = $0.25lleA x [(3.24 x 107 1b SA/yr)/(2.0 x 107 lb IA/yr)] 239 = $0.40/lb IA :3 Formaldehyde cost ($llb itaconic acid produced) = $0.09/lb Formalin x [(2.12 x 107 lb Formalin/yr)/(2.0 x 107 lb IA/yr)] = $0.10/lb IA => Feedstock costs (Sllb itaconic acid produced) = Succinic acid cost + Formaldehyde cost (Methanol required for esterification of succinic acid is mostly recovered) = $0.50/lb IA 8.4. Process Concept for Itaconic Acid Production A schematic of a process concept for conversion of succinate to itaconic acid is given in Figure 8.1. The primary components of the proposed process are discussed individually in the following sections in detail. 8.4.1. Esterification Reactor The esterification of succinic acid with methanol takes place in the esterification reactor. Section 1.3 discusses about the esterification of succinic acid in detail. We have also studied ester formation of succinic acid in our laboratory via standard esterification techniques. A reactive distillation column can be used for the esterification reaction (77). By using the distillation column, it is possible to get high reaction conversion and separation simultaneously. 240 S 8 $55 oo comm—0280 no.“ Eco—So 889:. a mo 23528 < ...w Samoa » 3.0m €83.30 3.9....» 03... . 0.38%.. Emma... 8.50 .083”. 2.8.0.0... ..0 8:32. .3385: “comma... 3.50 5.2.3. 0.5: ... 8.3.. 565...: 03.. .5... .8332.» .2.... 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