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LIBRARY Michigan State Unlversity This is to certify that the thesis entitled Mercaptopropyl Functionalized HMS Silicas with Wormhole Framework Structures: Direct Synthesis and PrOperties as Hg+2 Trapping Agents presented by Emily Jane McKimmy has been accepted towards fulfillment of the requirements for M°S° degree in 2001 Date Q6. ”1' 0905/ 0-7 639 MS U is an Affirmative Action/Equal Opportunity Institution PLACE IN RETURN BOX to remove this checkout from your record. To AVOID FINES return on or before date due. MAY BE RECALLED with earlier due date if requested. DATE DUE DATE DUE DATE DUE 6/01 cJCIRCIDatoDuepss-ms MERCAPTOPROPYL FUNCTIONALIZED HMS SILICAS WITH WORMHOLE FRAMEWORK STRUCTURES: DIRECT SYNTHESIS AND PROPERTIES AS Hg2+ TRAPPING AGENTS By Emily Jane McKimmy A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Chemistry 200 1 ABSTRACT MERCAPTOPROPYL FUNCTIONALIZED HMS SILICAS WITH WORMHOLE FRAMEWORK STRUCTURES: DIRECT SYNTHESIS AND PROPERTIES AS Hg2+ TRAPPING AGENTS By Emily Jane McKimmy Toxic heavy metals, especially mercury, are of global environmental concern. In order to improve upon current remediation techniques, organic-functionalized mesoporous silicas, denoted MP-HMS, have been tailored as a mercury-trapping agent. By the direct assembly of 3-mercaptopropyltrimethoxysilane MP-HMS mesostructures, in the presence of an amine surfactant, from 3-mercaptopropyltrimethoxysilane (MPTMS) and tetraethylortosilicate (TEOS) under electronically neutral synthesis conditions, over half of the silicon sites in the wormhole mesostructures contain mercaptan moieties, the highest level of organic loading reported thus far for an organofunctional mesostructure. In addition the wormhole framework allows for unhindered accessibility to the mercaptan, at least at Hg2+ levels typical of contaminated waste streams. With these ultra-highly functionalized mesostructures, unparalleled levels of Hg2+ binding have been achieved. Because of the short-order range of the wormhole motif and the high level of functionalization every mercaptan can be accessed by the mercury. Hg2+ binding capacity of 7.3 mmole Hg2+lg or 1.46 g Hg2+lg have been achieved at Hg”: SH ratios of 1.4. MP-HMS has been successfully shown to reduce mercury concentrations to below the EPA hazardous waste limit of 0.200 ppm and even to below the EPA drinking water limit of 0.002 ppm. In addition to its remarkable mercury remediation abilities, it also is easily synthesized in a one-step procedure. The MP-HMS trapping agents are stable, recyclable, low cost material with a very high affinity for mercury. ACKNOWLEDGEMENTS I want to express my appreciation to Dr. Pinnavaia for his support and guidance through out this endeavor. I would also like to convey my gratitude for his having such stimulating research projects and allowing me to explore different paths. I am grateful for his shared appreciation of environmental chemistry. Additionally, I would like thank all my group members for their friendship and rewarding discussions. I want to especially thank Yutaka Mori for his assistance and patience and Jainisha Shah for her support. My family has played an important role in getting me to this place in time. My parents, Mark and Pam, showed me the importance of an education and helped to instill a desire in me to go to graduate school. Most importantly, I would like to recognize the support I have gotten from my husband, Matt. He has made these two and half years of work manageable and has given the encouragement necessary to attain my goals. iii TABLE OF CONTENTS LIST OF TABLES ................................................................................ _...vii LIST OF FIGURES ................................................................................. viii ABBREVATIONS .................................................................................... x 1. Chapter I: Introduction ......................................................................... 1 1.1 Definition of Porous Materials ......................................................... l 1.2 Synthesis of Mesoporous Molecular Sieves ......................................... 1 1.3 Functionalization ......................................................................... 3 1.3.1 Methods of Functionalization ................................................ 4 1.4 Heavy Metal Trapping .................................................................. 9 1.5 Research Objectives .................................................................... 10 1.6 References .............................................................................. '. 12 2. Chapter II. Synthesis of Mercaptopropyl Functionalized HMS Mesostructure for Mercury Trapping ................................................................................. 15 2.1 Introduction .............................................................................. 15 2.2 Experimental Section .................................................................. 16 2.2.1 Direct Assembly of MP-HMS ............................................ 16 2.2.2 Reagents ..................................................................... 16 2.2.3 Physical Measurements .................................................... 17 2.3 Results and Discussion ................................................................ 17 2.4 Comparison of MP-functionalized Mesostructures ................................ 32 2.5 Conclusion ........... A ................................................................... 32 iv 2.6 References. ............................................................................. 34 3. Chapter III. Mercury Trapping by MP-HMS Mesostructures ....................... 36 3.1 Introduction ............................................................................ 36 3.2 Experimental ........................................................................... 37 3.2.1 Uptake of Mercury by MP-HMS ......................................... 37 3.2.2 Leaching of Mercury ........................................................ 38 3.2.3 Regeneration ............................................................... 38 3.2.4 Reagents ...................................................................... 39 3.2.5 Physical Measurements ..................................................... 39 3.3 Results and Discussion ................................................................ 40 3.3.1 Time Dependence Uptake of MP-HMS .................................. 40 3.3.2 Mercury Uptake Isotherm .................................................. 41 3.3.3 Mercury Concentration and MP-HMS Solid to Solution Ratio. . . ....44 3.3.4 MP-HMS Stability Towards Leaching Adsorbed Mercury ........... 47 3.3.5 Regeneration ................................................................. 48 3.4 Comparison of Mercaptan Functionalized Mesostructures ....................... 51 3.5 Conclusion .............................................................................. 52 3.6 References ............................................................................... 53 4. Chapter IV. Future Goals ..................................................................... 54 4.1 Extended X-ray Absorbance Fine Structure ....................................... 54 4.2 Acid Extraction of Surfactant ........................................................ 55 4.3 Adsorption of Mercury in a Column Bed .......................................... 55 4.4 Anion Traps ............................................................................ 56 4.5 References ............................................................................. 59 vi Table 2.1 Table 2.2 Table 3.1 Table 3.2 Table 3.3 Table 3.4 LIST OF TABLES Cross-linking parameters and molecular weight of MP- HMS with x = 0.1, 0.3, and 0.5 where x represents the molar fraction of total silicon present as mercaptopropyl groups in the initial reaction mixture. The data was calculated from the de-convoluted solid state 29Si NMR. .. Comparison of Mercaptan Functionalized Materials. . . . . . Effect of mercury concentration on the adsorption by MP- HMS. The ratio of HgZVS was held constant while the mercury concentration and total volume changed x represents the molar fraction of total silicon present as mercaptopropyl groups in the initial reaction mixture ...... Mercury concentrations in leachates of TCLP experiments ..................................................... ‘. .. Results for regeneration of MP-HMS as compared to FMMS ............................................................... Comparison of Mercaptan Functionalized Mesoporous Materials ............................................................ vii 26 33 46 48 49 51 Figure 1.1 Figure 2.1 Figure 2.2 Figure 2.3 Figure 2.4 Figure 2.5 Figure 2.6 Figure 3.1 LIST OF FIGURES Grafting versus Direct Assembly Mechanisms for functionalization of mesoporous materials, where R is an organo group and R' is an alkoxide ................................... Powder X-ray diffraction patterns of HMS silica and mercaptopropyl-functionalized silica MP-HMS; x represents the molar fraction of total silicon present as mercaptopropyl groups in the initial reaction mixture ................................. N2 adsorption-desorption isotherms for HMS and MP-HMS where x represents the molar fraction of total silicon present as mercaptopropyl groups in the initial reaction mixture ............ 298i MAS NMR spectra for MP-HMS x = 0.3 and 0.5 where x represents the molar fraction of total silicon present as mercaptopropyl groups in the initial reaction mixture ............. 13C MAS NMR spectra for MP-HMS x = 0.3 and x = 0.5 where x represents the molar fraction of total silicon present as mercaptopropyl groups in the initial reaction mixture ............. Infrared spectra of MP-HMS x = 0.3 and non-functionalized HMS, where x represents the molar fraction of total silicon present as mercaptopropyl groups in the initial reaction mixture .................................................................... TEM image of MP-HMS where x represents the molar fraction of total silicon present as mercaptopropyl groups in the initial reaction mixture.A. x = 0.5, B. x = 0.3, and C. x = 0.1 ............ Change of mercury concentration as a function of time in the adsorption of MP-HMS x: 0.5, the initial Hg concentration was 65 ppm. The overall SH: Hg2+ was 0.6; the same depletion was obtained at room temperature and at 65 °C .......................... viii 8 20 22 25 28 30 31 40 Figure 3.2 Figure 3.3 Figure 3.4 Figure 3.5 Figure 4.1 Figure 4.2 Mercury adsorption isotherm at room temperature for MP- HMS with x = 0.3 and a mercaptan content of 3.6 SH mmol/g... Mercury adsorption isotherm at room temperature for MP- HMS with x = 0.5 and a mercaptan content of 5.2 mmol/g ...... N2 isotherm of pristine MP-HMS with x = 0.3 and mercury- laden MP-HMS x = 0.3 after saturation at an initial ng”: SH ratio of 1.0 ............................................................... N2 adsorption-desorption isotherms for MP-HMS with x = 0.5 where x represents the molar fraction of total silicon present as mercaptopropyl groups in the initial reaction mixture. The mesostructure was and loaded with mercury and then acid washed to remove the mercury ........................................ TEM image of spherical MP-HMS x = 0.5 where x represents the molar fraction of total silicon present as mercaptopropyl groups in the initial reaction mixture ................................. N2 Isotherm of EN-HMS with x = 0.1 and x = 0.2 where x represents the molar fraction of total silicon present as ' ethylenediamine groups in the initial reaction mixture. . . . . . . ix 42 42 43 50 56 58 BET B] H cm CP MAS NMR DRIFTS EPA EtOH FMMS 8 H-bonding HCl HK HMS mmol MPTMS MP-HMS MP-MCM-41 LIST OF ABBREVIATIONS Brunauer-Emmett-Teller Barrett-Joyner-Halenda Centimeters Cross Polarized Magic Angle Spinning Nuclear Magnetic Resonance Diffuse Reflectance Infrared Fourier Transform Spectrometer Environmental Protection Agency Ethanol Functionalized Monolayers on Mesoporous Supports Grams Hydrogen bonding Hydrochloric acid Horvath and Kawazoe pore size distribution model Wormhole mesoporous silica synthesized with neutral amine surfactant under 8010 conditions Anionic inorganic precursor Cationic inorganic precursor Neutral inorganic precursor International Union of Pure and Applied Chemistry Metal cation Magic Angle Spinning Mobil Composition of Matter 41 hexagonal Mobil Composition. of Matter 48 cubic Mobil Composition of Matter50 lamellar Meter Milliliter Millimoles 3-Mercaptopropyltrimethoxysilane 3-Mercaptopropyltrimethoxysilane functionalized HMS 3—Mercaptopropyltrimethoxysilane functionalized MCM—41 MP-MSU-X MSU-X nm NMR N010 P/Po PEO ppb PPm PXRD N oqo A U) U) (I) + + ' S“X'I+ SBA-15 SH-SBA-IS SBET SO Mercaptopropyl functionalized MSU-X mesostructured silicas Wormhole mesostructured silicas synthesized with PEO based surfactants and TEOS under neutral (N010) assembly conditions Normal (equalivance/L) Nanometer (10’9 m) Nuclear Magnetic Resonance Non-ionic amphiphilic PEO based surfactant Neutral assembly pathway utilizing H-bonding between PEO based surfactant and inorganic precursor Relative pressure P = Pressure Po = Saturation pressure Polyethylene oxide Parts per billion (ug/L) Parts per million (mg/L) Powder X-ray diffraction Incompletely condensed silica sites Si(OSi)2(OH)2 Incompletely condensed silica sites Si(OSi)3(OH) Completely condensed silica sites Si(OSi)4 Anionic amphiphilic surfactant Cationic amphiphilic surfactant Pathway l electrostatic assembly between cationic surfactant and anionic silica precursor Electrostatic assembly between cationic surfactant and cationic silica precursors halogen ions as mediating counter ions Large pore hexagonal mesostructured silica assembled under high acid low pH conditions with TEOS as the inorganic precursor and triblock copolymer PEO based surfactant 3-Mercaptopropyltrimethoxysi1ane functionalized SBA-15 Specific surface area in m2/g obtained from the linear part of the adsorption isotherm using Brunauer Emmett Teller equation Neutral amphiphilic amine surfactant xi Neutral assembly pathway between neutral amine surfactant and TEOS Tetrethylorthosilicate Functionalized Q2 site RSi(OSi)3OH Functionalized Q3 site RSi(OSi)3 Halogen or anionic counter ion Molar ratio of 3-Mercaptopropyltrimethoxysilane added to MP—HMS Molar ratio of TEOS added to MP—HMS xii Chapter I. Introduction 1.1 Definition of Porous Materials The International Union of Pure and Applied Chemistry, IUPAC, has classified porous materials as microporous if the pore diameter is less than 2.0 nm, mesoporous for pore diameters between 2-50 nm, and macroporous for pore diameters greater than 50 nm. Porous materials are of interest to chemists due to their expansive applicability. Prominent functions of porous materials include ion exchangers, heterogeneous catalysts, gas adsorbents, and molecular sieves. One of the most common porous materials is zeolites. Zeolites are comer sharing tetrahedron aluminosilicates with uniform pores in the range of 0.2-1.4 nm. Additionally, they are very stable and rigid compounds that can be found in homes as water softeners as well as in industry as heterogeneous catalysts for their use in the process of fluidized catalytic cracking of hydrocarbons to gasoline. 1.2 Synthesis of Mesoporous Molecular Sieves In an attempt to expand the utility of zeolites by enlarging the pore diameter, Mobil Corporation researchers synthesized a new class of materials known as, Mobil 3 Composition of Matter 1 and denoted MCM-41 (hexagonal), MCM-48 (cubic) and MCM- 50 (lamellar). These mesostructures are products of an electrostatic, hydrothermal reaction of aluminosilicate gel in the presence of quaternary ammonium surfactant. Electrostatic assembly utilizes charge matching of a cationic surfactant (S+) and an anionic inorganic precursor (T) to produce long range ordered mesoporous materials with uniform pore size distribution. Surfactants are molecules which have a hydrophobic alkyl chain “tail” and a hydrophilic quaternary ammonium cation “head” (CnH2n+1(CH3)3N)+. Individual surfactant molecules aggregate to form a micelle in order to increase solvent entropy. These micelles are the template around which the inorganic precursor assembles to yield a mesoporous silicate. Brunauer-Emmett-Teller (BET) surface areas are estimated to be 1000 m2/g for the MCM-41. The pore diameter can be tailored, by use of swelling agents such as mesitylene or by varying the akyl chain length, to range in size from 3.0 to ~10.0 nm 2. Building upon MCM-41, Tanev and Pinnavaia synthesized another mesoporous silica material 3. This material was assembled via non-electrostatic hydrogen-bonding interactions between neutral surfactant micelles (S0) and an uncharged silica species (10) as the inorganic precursor. Hydrogen bonding between the silica species, formed by hydrolysis of tetraethylorthosilicate (T E08), and the head group of the neutral amine surfactant micelle yielded a mesostructured material termed HMS. This material has a large number of disordered channels. Moreover, the pores are regular in diameter yet they lack long-range packing order. The unique nature of the HMS morphology has been termed wormhole as opposed to the long range ordered hexagonal MCM-41. HMS has a surface area of about 1000 mzlg and pore diameters in the range of 3-5 nm 4. Furthermore, Bagshaw and Pinnavaia synthesized MSU-X materials. MSU-X materials also utilize the hydrogen-bonding interaction between an uncharged silica species, TEOS, and a neutral surfactant. However, for this synthesis the neutral triblock copolymer polyethylene oxide-polypropylene oxide- polyethylene oxide (EOx-POy-EOX) was used 5. Concomitantly, this material too has a wormhole motif, a surface area of up to 1200 m2/g, and a pore diameter of up to 5.8 nm 5. In a similar manner to MSU-X, Stucky synthesized SBA-15 6’7. SBA-15 also uses a triblock copolymer, specifically EOxPOyEOx as the organic structure—directing agent. However, SBA-15 is assembled in the presence of strong acids, under electrostatic conditions denoted, S+X'l*. The charged silicate precursor (F) is obtained under acidic reaction conditions and interacts with protonated surfactant molecules via anions (X'), specifically Cl' from HCl. At pH ~1, the positively charged protonated silicate species interacts preferentially with the more hydrophilic EO block(s) to form the mesoporous silicate material. The pore diameter of SBA-15 can be tailored to range from 4.6 to beyond 10.0 nm and the material has a surface area above 800 mZ/g. Furthermore, SBA- 15 has a hexagonal morphology, not a wormhole as compared to MSU-X. Additionally, SBA-15 has a regular structure and much thicker silica walls than MCM-41, which impart a greater hydrothermal stability to the material 6. 1.3 Functionalization The aforementioned porous inorganic materials can be synthesized with purely silicate precursors or with a combination of silicate and organosilicate such as RSi(OR’)3, where R and R’ can be the same but most commonly R will have a hetero atom and R’ will not. This combination of an inorganic-organic hybrid material has vast potential in a plethora of applications. One such application is that of a heavy metal adsorbent. When R is a chelating ligand such as mercaptans or amines, and is attached to silica support matrices the once inert silica becomes functionalized and has the ability to trap toxic metals. In addition to organic ligands used as functional groups, there are two other prominent categories. The first is that of transition metal or p—block metals, which can act as Lewis acids"’9 and are used as heterogeneous catalysts. The second are transition metal complexes, such as Mn(Salen)l°, Mn(bipyridine)ll and Pd(C2H5)12, which are primarily utilized as homogenous catalysts. Organic functionalization will be the focus of further discussion. Organic modification of the silicates permits precise control over the surface properties and pore sizes of the mesoporous sieves for specific applications, while simultaneously stabilizing the materials towards hydrolysis 13. Inorganic-organic units are typically bound through covalent Si-C bonds. TEOS has four alkoxide groups covalently bonded to silicon, which can be hydrolyzed. If all four are cross-linked during a reaction the silicon center is termed Q4 (completely condensed silica sites, Si-(OSi)4) as opposed to Q3 and Q2 (incompletely condensed silica sites Si-(OSi)3-OH and Si-(OSi)2-(OH)2, respectively). If an organic group is attached to the Si center then there are no longer four hydrolyzable sites but rather three, thus given the new terminology of T sites. A completely condensed T3 silica site is RSi-(OSi)3 as opposed to an incompletely condensed site T2, RSi-(OSi)2-OH. 1.3.1 Methods of Functionalization There are three main methods by which organic functionalization can be achieved. The first is that of impregnation. Impregnation is a chemical mixing, wherein the addition of an organic group is incorporated by its infusion into the inorganic structure. Binding occurs by physical adsorption of the organic moiety to the surface. This is rarely used and a rather imprecise method. Secondly, there is grafting. The process of grafting refers to the post-synthesis treatment of a previously prepared material, usually after surfactant removal. Mesoporous silicates possess surface silanol (Si-OH) groups that can be present in high concentration and have the ability to be anchor points for organic functionalization 13. Thus, by utilizing these hydroxyl groups as specific anchor points for the organic group there is more control of the location and quantity of the R groups in comparison to impregnation. Nevertheless, the material typically needs to have open pores in which there is space for this incorporation of organic groups. So the surfactant must be removed in order to allow for grafting. For many materials the surfactant is removed via calcination. The process of calcination causes partial dehydroxylation thus limiting the number of sites to which the organic group can attach. This limited number of sites for anchoring also causes inhomogeneous distribution of the organic species. However, by using a neutral surfactant for the templating of the pores, as is done with HMS and MSU- X, the surfactant can be removed by solvent extraction, as opposed to calcination, which dehydroxylates the inorganic precursor leaving it less reactive for further functionalization. Also, the process of synthesizing the material, removing the surfactant and lastly grafting on the organic species is tedious. The third method is that of direct assembly also referred to as co-condensation. In order to use this approach it is important to ensure that there is no phase separation of the reactants so that one is able to obtain a uniform distribution of functional groups. The organic species is added in the initial synthesizing step with the inorganic precursor and surfactant. Thus the organic moiety is straightforwardly incorporated into the framework. Both grafting and direct assembly have certain advantages. Organo- functionalized mesostructures can be reliably prepared through grafting methods. One only needs to ensure the presence of SiOH groups on the surface and a pore size capable of accommodating the organo siloxane reagent. The hydrothermal stability of grafted vinyl—functionalized MCM-41 was determined to be greater than that of directly assembled vinyl—functionalized MCM-41 14. There is also more pore size control afforded by grafting. Nonetheless, direct assembly allows for a greater ease of material preparation, and a more homogeneous distribution of organic moieties. Furthermore, direct assembly allows for a greater degree of covalent bond formation to the framework. About 80 percent T3 sites can be achieved for direct assembly versus less than 60 percent with grafting. Figure 1 is an illustration of the grafting process versus the direct assembly process for functionalization. Figure 1.1. 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I00 I00 00.35.002.000”. 0....on 1.4 Heavy Metal Trapping Focusing on the ability of organo-functionalized materials to trap metals ions, the background and remediation history of mercury is examined. Toxic heavy metals, especially mercury, are of global environmental concern. Mercury in water from mineral sources is present in Japan, the Philippines, and parts of India 15. Mercury’s prevalence in water makes its removal pertinent to establishing potable drinking water and healthy ecosystems. In addition to dissolution from the underlying rock, mercury can be present from water that has been contaminated by sewage or industrial effluents, or by leaching from unsecured dump sites and the tailings ponds of active and abandoned mines “5. The usual form of mercury in aqueous solution is the Hg2+ ion ‘7 . Mercury has two oxidation states, Hg (I) and Hg (II), bUt the first of these- that contains the unusual ion +Hg--Hg+ - is stable only as insoluble salts such as ngClg. It disproportionates in solution. Hg.“ (aq) —> Hg“ (aq) + Hg (I) Traditional methods for remediation of mercury ions from solutions include 18,19 20,21 activated carbon , zeolites , silicates (e.g. silica gel)”, ion exchange”, sulfide precipitation”, and clays”. Nonetheless, the drawbacks of these materials include that of low loading capacity, low selectivity, and relatively small metal ion binding constants. In order to improve on these techniques and develop new ones, the well-established chemistry of mercury must be exhorted. Mercury (II) is a very “soft” Lewis acid, which forms stable complexes preferentially with soft Lewis bases such as mercaptan ligands. Therefore, the selectivity and metal ion binding constants of remediation materials can be vastly improved by trapping it with chelating ions coupled with a support matrix. 27,28 29,30 Using this information, silica gel 26, clays , carbon , and organic polymers (polystyrene, cellulose, or polymethylmethacrylate)31'33 were used as support matrices functionalized with mercaptan, most commonly in the form of 3- mercaptopropyltrimethoxysilane (MP). This addition of an organo group improved upon the selectivity and metal binding constants. Nevertheless, a better support matrix was needed to significantly improve metal loading. Among the most promising recently investigated adsorbent materials are mercaptan-functionalized mesoporous molecular sieves specifically mercaptan- functionalized MCM-4l, SBA-15, MSU-X, and HMS materials denoted as MP-MCM-4land FMMS, SH-SBA-IS, MP-MSU-X, and MP-HMS, respectively. 1.5 Research Objectives The focus of this research is to optimize the mercury adsorption capacity of MP- HMS. The first step in obtaining higher mercury loading is to examine the material’s properties. In the current literature Liu at Pacific Northwest Laboratories has grafted 3.2- mmol S/ g MGM-4134. Another researcher in this field, Mercier at Laurentian University has directly assembled 2.3 mmol S/g MSU-X35. In order to improve upon these results this research focused on achieving higher functionality of mercaptopropyl groups in MP- HMS by direct assembly synthesis. By increasing the mercaptopropyl loading, the mercury loading capacity should also be increased. Another aspect of optimizing the mercury adsorption capacity is to determine mercury’s molecular conformations when trapped by MP-HMS. By knowing the bonds formed by the complexed mercury, MP- HMS can be properly tailored or the mercury can be trapped under conditions, which facilitate these bonds. In addition to the functionality of the material, the accessibility to 10 the functional groups will also be investigated, i.e., the difference between the wormhole morphology of the MP—HMS and the hexagonal morphology of MP-MCM-4l. Additionally, the durability of the material will be probed. From a commercial standpoint the capacity of the material to be reused is important. Therefore, the regeneration of mercury loaded MPvHMS was examined. The durability is not only indicated by the ability to be regenerated but also by the long-term storage capacity, or lack there of. The potential of mercury loaded MP—HMS as a permanent storage material will be examined. 11 1.6 References: (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) Beck, J. S.; Vartuli, J. C.; Roth, W. J .; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T. W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834-10843. Beck, J. S.; Vartuli, J. C.; Kennedy, G. J.; Kresge, C. T.; Roth, W. J .; Schramm, S. E. Chem. Mat. 1994, 6, 1816-1821. Tanev, P. T.; Pinnavaia, T. J. Science 1995, 267, 865-867. Pauly, T. R.; Pinnavaia, T. J. Chem. Mat. 2001, 13, 987-993. Bagshaw, S. A.; Prouzet, E.; Pinnavaia, T. J. Science 1995, 269, 1242-1244. Zhao, D. Y.; Feng, J. L.; Huo, Q. S.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548-552. Zhao, D. Y.; Sun, J. Y.; Li, Q. Z.; Stucky, G. D. Chem. Mat. 2000, 12, 275-+. Hammond, W.; Prouzet, E.; Mahanti, S. D.; Pinnavaia, T. J. Microporous Mesoporous Mat. 1999, 27, 19-25. Tanev, P. T.; Chibwe, M.; Pinnavaia, T. J. Nature 1994, 368, 321-323. Kim, G. J. a. K., S. H. Catal. Lett. 1999, 139-143. Kim, S. S.; Zhang, W. Z.; Pinnavaia, T. J. Catal. Lett. 1997, 43, 149-154. Mehnert, C. P. a. Y. J. Y. Chem. Commun. 1997, 2215-2216. Stein, A.; Melde, B. J .; Schroden, R. C. Adv. Mater. 2000, 12, 1403-1419. Lim, M. H.; Stein, A. Chem. Mat. 1999, 11 , 3285-3295. Fryxell, G. E.; Liu, J.; Mattigod, S. Mater. Technol. 1999, 14, 188-191. Bunce, N. Environmental Chemistry; Second ed.; Wuerz Publishing Ltd: Winnipeg Canada, 1998. 12 (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30) (31) NN Greenwood, A. E. Chemistry of the Elements; Second ed.; Butterworth Heinemann: Oxford, 1998. Faust, S. D.; Schultz, C. M. J. Environ. Sci. Health PartA-Environ. Sci. Eng. Toxic Hazard. Subst. Control 1983, 18, 95-102. Otani, Y.; Emi, H.; Kanaoka, C.; Uchijima, 1.; Nishino, H. Environ. Sci. Technol. 1988, 22, 708-711. Huang, C. P.; Hao, O. .1. Environmental Technology Letters 1989, 10, 863-874. Zamzow, M. J .; Murphy, J. B. Sep. Sci. Technol. 1992, 27, 1969-1984. Mercier, L.; Pinnavaia, T. J. Adv. Mater. 1997, 9, 500-&. Ghazy, S. E. Sep. Sci. Technol. 1995, 30, 933-947. Wagner-Dobler, 1.; von Canstein, H.; Li, Y.; Timmis, K. N.; Deckwer, W. D. - Environ. Sci. Technol. 2000, 34, 4628-4634. Sikalidis, C. A.; Alexiades, C.; Misaelides, P. Toxicological and Environmental Chemistry 1989, 20-1, 175-180. Howard, A. G.; Volkan, M.; Ataman, D. Y. Analyst 1987, 112, 159-162. Mercier, L.; Pinnavaia, T. J. Microporous Mesoporous Mat. 1998, 20, 101- 106. Lagadic, I. L.; Mitchell, M. K.; Payne, B. D. Environ. Sci. Technol. 2001, 35, 984-990. Mohan, D.; Gupta, V. K.; Srivastava, S. K.; Chander, S. Colloid Surf. A- Physicochem. Eng. Asp. 2001, 177, 169-181. Hsi, H. C.; Rood, M. J .; Rostam-Abadi, M.; Chen, S. G.; Chang, R. Environ. Sci. Technol. 2001, 35, 2785-2791. Phillips, R. J.; Fritz, J. S. Anal. Chem. 1978, 50, 1504—1508. 13 (32) Sugii, A.; Ogawa, N.; Hashizume, H. Talanta 1980, 27, 627-631. (33) Deratani, A.; Sebille, B. Anal. Chem. 1981, 53, 1742-1746. (34) Feng, X.; Fryxell, G. B; Wang, L. Q.; Kim, A. Y.; Liu, J.; Kemner, K. M. Science 1997, 276, 923—926. (35) Brown, J .; Richer, R.; Mercier, L. Microporous Mesoporous Mat. 2000, 37, 41- 48. 14 Chapter 11. Synthesis of Mercaptopropyl Functionalized HMS Mesostructure for Mercury Trapping 2.1 Introduction A great deal of attention has been focused on organically functionalized mesoporous materials due to their application in the field of adsorption, specifically for heavy metal remediation. Current research 1'5 has focused on the synthesis of mercaptan functionalized mesoporous materials, because they were expected to exhibit a high affinity for the toxic Hg2+ through a coordinative soft Lewis acid- soft Lewis base interaction. The potential use of these materials as mercury adsorbents depends upon the loading and accessibility of the mercaptan groups. The functionalization by 3-mercaptopropyltrimethoxysilane (MPTMS) of surfactant-templated mesoporous materials has been reported. Different synthesis conditions and reagents yield different materials. Electrostatic interactions between an ionic surfactant and charged silica have been utilized to prepare hexagonal MCM—41 6 and SBA-15 7, under basic and acidic reaction conditions, respectively. Hydrogen bonding between a neutral surfactant and uncharged silica precursor has been used to assemble wormhole MSU-X 8 and HMS 9 mesostructures. The functionalization of each of these materials by MPT MS has been reported in the literature. These organo- functionalized mesoporous materials are denoted FMMS 10 or MP-MCM —41 4, SH-SBA- 15 3, MP-MSU-X '1 and, MP-HMS 1, respectively. 15 2.2 Experimental Section 2.2.1 Direct Assembly of MP-HMS Mercaptopropyl-functionalized wormhole silicon, denoted MP-HMS was prepared by the direct assembly of a reaction mixture with molar composition: 0.22 dodecylamine surfactant: x MPTMS: (1-x)TEOS: 5 ethanol: 160 water The synthesis of MP-HMS was accomplished by dissolving 4.07 g of dodecylamine in 23.00 g of ethanol at 65 °C. This solution was then added to 288.00 g of water heated to 65 °C. This solution was then mixed for 30 minutes at 65 °C. The Io/So ratio remained constant for each x value, i.e., the same amount of surfactant was consistently added to the same amount of total silicon. However, the total silicon source was varied by the ratio: x:(l-x) MPTMS: TEOS mixture where x equals the mole fraction of silicon in the reaction mixture that is organo-functionalized. Additionally, x, the mercaptopropyl content, was varied from 0.0 to 0.5 with retention of the mesostructure. The TEOS and MPTMS were mixed together and then added to the surfactant solution. The reaction mixture was sealed and allowed to age with stirring for 72 h at 65 °C. The resulting product was vacuum filtered and air-dried for 24 h. The surfactant was removed using Soxhlet extraction over ethanol for 24 hours. The ethanol-extracted product was dried in air for 24 h before use. 2.2.2 Reagents Dodecylamine, 3-mercaptopropyltrimethoxysilane (MPT MS) and tetraethylorthosilicate (TEOS) were obtained from Aldrich Chemical Company. These reagents were used as received without further purification. Deionized water and 16 absolute ethanol were used in the synthesis and the latter was also used for the surfactant extraction. 2.2.3 Physical Measurements 29Si MAS-NMR spectra were collected on a Varian 400 solid state NMR spectrometer with a field strength of 400 MHz, under single-pulse mode with a zirconia rotor at a spinning frequency of 4 kHz. A pulse delay of 400 seconds was employed so that there was sufficient time for the nuclei to relax before application of another pulse. Talcum powder was used as a reference. The cross-polarized technique was used for the 13C MAS NMR. The chemical shifts were externally referenced to adamantane. Infrared spectra were recorded in the 4000-400 cm'1 range using a Nicolet FI‘ IR Prote’gé 460 spectrometer equipped with a diffuse reflectance accessory (DRIFT S). TEM micrographs were obtained on a JEOL 100 CX microscope with a CeB5 filament and an accelerating voltage of 120 KV. Powdered samples were dispersed by sonicating in the presence of ethanol for 20-40 minutes. One drop of the sonicated sample was pipetted onto a carbon- coated copper grid with a holey film and allowed to evaporate before analysis. 2.3 Results and Discussion The Solo direct-assembly of MP-HMS molecular sieve silicas with wormhole framework structures was accomplished using dodecylamine as the structure directing surfactant, SO, and tetraethylorthosilicate (TEOS) plus 3-mercaptopropyltrimthoxysilane (MPT MS) as the inorganic precursors, 10. Three different molar fractions of MPT MS were investigated, namely x = 0.1, 0.3, and 0.5. 17 The main advantages of the direct assembly method are its ability to provide a uniform distribution of mercaptan groups, and the capacity to incorporate a greater amount of mercaptan than is possible by the grafting approach. Dodecylamine, a neutral surfactant, was used as a structure-directing agent. Amine surfactants with shorter molecular chains i.e., octylamine, pack less efficiently due to reduced van der Walls interactions between the pendant chains '2. Longer amine chains typically yield materials with higher surface areas and larger pore diameters l but the ratio of textural pores to framework pores is lower than for dodecylamine. This ratio affects the mercury adsorption capacity of the material. 3-mercaptopropyltrimethoxysilane was used as the functionalization reagent for three reasons. Firstly, it is miscible with tetraethylorthosilicate (TEOS), the silica source. Direct assembly functionalization can only be accomplished if there is no phase separation between the reagents. Secondly, the short alkyl chain functionality (three carbon atoms) of the 3-mercaptopropyltrimethoxysilane enabled it to fit into the void spaces between the surfactant chains without disturbing the structure of the micelle assembly 13 thus easily incorporating the organic moieties. Thirdly, the mercaptan moieties have a high affinity for mercury, which is necessary for efficient mercury trapping. The ratio of ethanol to water was an important factor to consider when synthesizing MP-HMS. In addition to framework mesoporosity, a mesostructure can exhibit complementary textural porosity. The textural mesoporosity of HMS silicas is determined by the polarity of the medium in which the framework is assembled M. Highly polar solvents, such as 90:10 (vzv) water-ethanol, characteristically afford 18 wormhole frameworks with fundamental particle sizes < 200 nm and textural mesopores that are comparable to the framework pores in overall volume '5. Textural porosity might allow for more efficient transport of mercury to framework mercaptan. Mesostructures assembled through hydrogen-bonded pathways have a wormhole motif that is less ordered than the hexagonal frameworks formed under electrostatic conditions. The wormhole channel motif is a potentially important structural feature for adsorption. The channel branching within the framework can facilitate access to reactive sites i.e., the mercaptan, on the framework walls '4 allowing for greater adsorption. The powder X-ray diffraction patterns for MP-HMS mesostructures exhibit a single peak at a low angle, as illustrated in Figure 2.1. The peak correlates to the pore-to- pore separation in the framework, i.e., the pore diameter plus the pore wall thickness. The functionalized material has a much lower PXRD intensity, which continues to decrease with increasing mercaptan loading. This loss of intensity was due to contrast matching between the inorganic framework and the organic ligand. 19 Intensity (au) fi 20 A.x=0.0 B.x=0.l C.x=0.3 D.x=0.5 Figure 2.]. Powder X-ray diffraction patterns of HMS silica and mercaptopropyl- functionalized silica MP-HMS; x represents the molar fraction of total silicon present as mercaptopropyl groups in the initial reaction mixture. 20 Figure 2.2 provides the N2 adsorption isotherms for the mesostructured MP-HMS assembled from dodecylamine with x = 0.0 — 0.5. For the samples prepared with x = 0.0 - 0.3 the isotherms had a type IV 16 shape as expected for mesoporous silica. However, with the higher functionalized material, prepared at x = 0.5, a type I shape or microporous, isotherm was observed. The surface area was calculated using the adsorption isotherm and the BET model. A framework pore by definition is the space that was occupied by the surfactant during the mesostructure assembly process; the distance across this open space is the pore diameter. The smaller framework pores are filled by nitrogen at a lower partial pressure, below a value of 0.5 P/Po. In addition to the pore void space there can also be an interparticle or intraparticle void space, which is termed the textural porosity. The presence of textural porosity was displayed by a significant uptake of N2 at high relative pressures (P/Po > 0.90). This textural porosity was the dominant porosity in the MP-HMS with x = 0.3. Textural porosity was also observed in the isotherms for the MP-HMS products prepared with x = 0.0 and x = 0.1. 21 Volume adsorbed (cmstg) 200 Figure 2.2. N2 adsorption-desorption isotherms for HMS and MP-HMS where x represents the molar fraction of total silicon present as mercaptopropyl groups in the initial reaction mixture. The isotherm of x = 0.3 is offset vertically by 300 cm3/g and the isotherm of x = 0.1 is offset vertically by 200 cm3/g for clarity. 22 In order to characterize the structural properties of the MP-HMS, 29Si and 13C solid state, magic angle spinning NMR (MAS NMR) were performed. The 29Si spectra allowed the quantification of the cross-linking and the calculation of the millimole (mmol) of sulfur per gram of material. The analysis of the spectra was done by de- convoluting the peaks and calculating the integral of the de-convoluted peaks. The degree of cross-linking was determined from the ratio of fully cross-linked Si centers (Q4 and T3) to the partially cross-linked Si centers (Q3 and T2), and is indicative of the framework stability. The greater the cross-linking, the greater the stability. Furthermore, the ratio of functionalized silicon center to the non-functionalized silicon centers (T3+T2/Q4+Q3+T3+T2) allowed for the calculation of the ratio of the functional silicon centers to the non-functionalized silicon centers. To assign the propyl chains of the MP-HMS, l3C MAS NMR was performed. Because of the low abundance of 13C, the cross-polarization (CP) technique was employed. In this technique the hydrogen attached to the carbon is pulsed, to transfer the magnetization carbon, resulting in a higher population of nuclei in the lower spin state and enhance l3C signal. Figure 2.3 shows the 2(”Si spectra of MP—HMS mesostructures with varying degrees of MPTMS functionalization. In each spectrum the Q4- Si(OSi)4 resonance occurred near -110 ppm, and the Q3 - Si(OSi)3OH was near -100 ppm. The T3 — SHCH2CH2CH2-Si(OSi)3 moiety resonated approximately at -69 ppm and the T2 - SHCH2CH2CH2-Si(OSi)3OH was located around -59 ppm. The increase in functionalization of the material is illustrated by the increase in the intensity of the T 23 bands; MP-HMS with x = 0.1 has a weak resonance at -69 ppm whereas the resonance at -69 ppm of the MP-HMS with x = 0.5 is of equal intensity to the Q bands at -110 ppm. 24 29 Si NMR MP-HMS , f g 1'2 1a 1 2’ 5). ,r' 1., If ix Ni . 1'; \\ i :" \ 1 ,. I. g 1, x = 0.3 . WWVMWN’ ~.Mr’ \M. k‘NVmVA‘W/HVVNAJKV‘N; Figure 2.3. 29Si MAS NMR spectra for MP-HMS x = 0.3 and 0.5 where x represents the molar fraction of total framework silicon center present as mercaptopropyl groups in the initial reaction mixture. 25 Table 2.1 presents a summary of the cross-linking parameters Q4/Q3 and T3/T2 calculated from the de-convoluted 29Si MAS NMR spectra. The increase in functionalization is further evidenced by the calculation of the molecular weight for each different x value of MP-HMS. An increase of molecular weight is observed due to the higher mass of sulfur as opposed to silicon as well as a decrease in hydroxyl groups because of greater cross-linking. The amount of MP moieties incorporated within the mesostructures calculated varied slightly from what was added. Table 2.1. Cross-linking parameters and the equivalent weight of MP-HMS with x = 0.1, 0.3, and 0.5 where x represents the molar fraction of total silicon present as mercaptopropyl groups in the initial reaction mixture. The data was calculated from the de-convoluted solid state 29Si NMR. Calculated EW mmol xexp xcalc Q4/Q3 '1‘3/1‘2 (Q4+T3)/(Q3/T2) of MP-HMS SH/g g/mol 0.1 0.08 2.78 - 2.18 68.73 1.43 0.3 0.28 2.90 - 2.64 81.68 3.63 0.5 0.46 3.28 2.55 2.89 94.63 5.28 26 A representative l3C CP MAS NMR spectrum is shown in Figure 2.4. The carbon attached to the Si center (C3) resonated near 8 ppm. The other carbons of the propyl chain (C1 and C2) were further downfield at 25 ppm and 19 due to the electron- withdrawing character of the SH group”. Besides the resonances from the carbon of the propyl chain, two additional weak resonances were observed at the S7 and 40 ppm. These peaks were assigned to ethanol used in the synthesis and extraction procedure. The surface silanol groups most likely become ethoxylated during synthesis or the extraction procedure. This hypothesis was further supported by 13C CP MAS NMR of HMS (not shown). The spectrum of the HMS, which should have no carbon resonances, does exhibit two resonances. The first resonates was at 56 ppm and the other at 9 ppm. 27 l3C CP NMR C1 C3 -Si-CH2-CH2-CH2-SH C2 3 2 1 EtOH EtOH l x = 0.3 l 1 1 111/!me x = 0.5 1 00 50 0 -50 1313'“ Figure 2.4. 13C MAS NMR spectra for MP-HMS x = 0.3 and x = 0.5 where x represents the molar fraction of total silicon present as mercaptopropyl groups in the initial reaction mixture. 28 A diffuse reflectance (DRIFTS) accessory was used to obtain the IR spectra of the MP-HMS and HMS. The accessory consists of flat mirrors that direct the incoming radiation onto a spherical focusing mirror”. The radiation is focused on the KBr diluted sample and diffusely reflected into a 360° circle. Diffusely reflected radiation is made up of light that is reflected, transmitted, absorbed, and scattered by the sample. Having interacted with the sample, this light contains pertinent spectral information. A second spherical mirror collects the diffusely reflected light, and reflects it off mirrors and focuses the light on the detector. A total of 200 scans were used to obtain the spectra. Figure 2.5 illustrates the IR spectrum for the non-functionalized HMS and the functionalized MP-HMS. The presence of the S-H stretching vibration was evidenced by the weak band observed at 2564 cm". Non-functionalized HMS had no bands present in the S-H or C-H stretching regions. The alkyl C-H stretch is observed in the range of 2853-2962 cm". Both the functionalized and non-functionalized materials produced a strong band in the C-H stretch region. The functionalized material was expected to have these bands due to the carbons on the propyl chain. However, the presence of these C-H bands in the spectrum of the non-functionalized material was unexpected, but was attributed to residual ethanol as previously discussed for the 13C NMR. The strong bands around 1100 cm'1 were due to the Si-O stretch, and were observed in both the HMS and MP-HMS, as expected. 29 % Transm ntance Wavenumbers(cm1) Figure 2.5. Infrared spectra of MP-HMS x = 0.3 and non-functionalized HMS, where x represents the molar fraction of total silicon present as mercaptopropyl groups in the initial reaction mixture. 30 The preservation of the wormhole morphology of the material with increased functionalization is shown by the transmission electron microscopy micrographs (Figure 2.6). Figure 2.6. TEM image of MP-HMS where x represents the molar fraction of total silicon present as mercaptopropyl groups in the initial reaction mixture. A. x = 0.5, B. x =0.3, and C. x = 0.1. 31 2.4 Comparison of MP Functionalized Mesostructures MP-HMS is not the only mercaptan-functionalized mesoporous material that has been prepared. As previously mentioned earlier MCM-41 6, SBA-15 7, and MSU-X 8 mesostructures have all been functionalized by MPTMS and denoted FMMS '0 or MP- MCM —41 4, SH-SBA-lS 3, and MP-MSU-X ”, respectively. The properties of MP- HMS in comparison of the aforementioned MP-functionalized mesoporous structures are summarized in Table 2.2. 2.5 Conclusion Mercaptopropyl functionalized HMS mesostructures have been synthesized by direct assembly via a Sol0 mechanism. The mercaptan content of the products can be easily tailored so as to include high or low loadings. The surface area and pore volume decrease with increasing the degree of functionalization. The wormhole motif is retained at all levels of mercaptan loading. The MP-HMS with x = 0.3 has a higher surface area than previously reported MP-functionalized mesostructures. A high surface area is important for metal ion trapping. The MP-HMS with x = 0.5 has a higher content of mercaptan moieties incorporated into the material. This is also an important feature for metal ion trapping. 32 6:63 any... .me Soc BESDO .0 9mm 43 Bum—330 d .vi 3 vague—«U .o dad u 0&3 8 ©82330 .n .522 mg “ma 3 beanie—AND d was m; and mmd 0m 80:0 no u x .mSEiE N6 5: Cd 98 ad wwd 9m 2085 md u x .mEIdE in can Wm mwd v._ “REG _.c u x .mSEiE 3 N2 3 :3 3“ 625 mszozdz 3v cod < Z w=ESO em _ -oEE 3388 Ema 3:: Sam BEE U. _E ux 2258 m: s eoeeaee m: eoeee m: am: am: mess mm 6&2? m2: 130,—. d: 46 3.5.4 Stability Towards Leaching The United Stated Environmental Protection Agency (US EPA) has developed a Toxicity Characteristic Leaching Procedure (TCLP). The procedure was designed to study the mobility of hazardous substances and thus must be considered in the development of a remediation technology. This test is done in order to determine whether or not a material may be safely disposed in a landfill without further treatment. The leaching stability of MP-HMS under pseudo landfill conditions was evaluated. The TCLP is widely used in the United States to determine if a waste is hazardous or if a treated waste meets the treatment standards for land disposal. Table 3.2 shows the analytical results of the leaching test of mercury-laden MP-HMS. The value reported by the Pacific Northwest Laboratories group for Hg2+ leaching from their FMMSS mesostructure is also given in Table 3.2. The test conditions were similar for the two different materials. The durable chemical stability of the mercury-mercaptan covalent bond, as well as the bonds between the MPTMS groups and the framework silica, is evident by the fact that less than 0.1 percent of the trapped mercury was released. In addition to the chemical stability of the trapped mercury, there is also the stability from bacterial degradation to consider. Inorganic mercury may be methylated by bacteria to give methyl mercury, the deadliest form of mercury 2. The small pore size of the MP-HMS should prevent bacteria (at least 2000 nm in size) from solubilizing the mercury. Thus mercury-laden MP-HMS can be expected to have long-term durability as a permanent waste for disposal. 47 Table 3.2 Mercury concentrations in leachates of TCLP experiments. Material Initial ppm Hg” in Final ppm Hg2+ in % Hg2+ in leachate material filtrate MP—HMS x = 0.1 2140 2.08 0.10 MP—HMS x = 0.5 2124 0.81 0.04 FMMSS 335 10.5 3.13 3.5.5 Regeneration Table 3.3 shows the results of the regeneration experiment. The MP-HMS remained effective even after regeneration in 6 M HCl. The FMMS material was regenerated by washing the mercury-laden mesostructure in 12 M HCl 5. Such a high acid concentration might hydrolyze the mercaptan groups from the silica surface and result in the lower loading capacity for the regenerated mesostructure. Thus the use of lower concentration HCl is one reason MP-HMS has better regeneration capacity than FMMS. These results illustrate the durability of the MP-HMS mesostructures towards acid washing. After the regeneration cycle, MP—HMS returns 78-87% of its binding capacity depending on the degree of functionalization. This is in comparison to FMMS material prepared by grafting of MCM-41, which returned only 41% of its binding capacity upon regeneration. As shown by the nitrogen isotherm in Figure 3.5, the MP-HMS framework is almost completely restored to its initial volume when the Hg2+ is removed by acid extraction. 48 Table 3.3. Results for regeneration of MP-HMS as compared to FMMS. MP-HMS Initial Hg2+ adsorbed Second Hg2+ loading Hg2+ binding capacity (mmol/g) camacity (mmol/g) returned (%) x = 0.3 3.87 3.37 87 x = 0.5 7.21 5.66 78 FMMST 2.51 1.04 41 49 250 _ . MP-HMS J regenerated MP-HMS 200 l A -1 2’ 4 (O E _ 3 _ '0 150 a) . .0 _ 8 I < - ng" laden m _J E 100 2 o > 50 j .1 O T l T I l l l I T T T I T 1 T I T T T O 0 2 0.4 O 6 0 8 1 P/Po Figure 3.5. N2 adsorption-desorption isotherms for MP-HMS with x = 0.5 where x represents the molar fraction of total silicon present as mercaptopropyl groups in the initial reaction mixture, and then having been loaded with mercury, and acid washed to remove the mercury. 50 3.6 Comparison of Materials MP-HMS is not the only mercaptan-functionalized mesoporous material that has been tested for mercury remediation. As previously mentioned FMMS, MP-MCM-41, SH-SBA-IS, and MP-MSU-X, mercaptopropyl functionalized mesoporous structures, have been synthesized and used to adsorb mercury. A comparison of each material’s adsorption properties is given in Table 3.4. Table 3.4. Comparison of Mercaptan Functionalized Mesostructures. Mesostructure Method of Framework SH Hg2“ Initialb Finalb SH Morphology content binding ppm ppm incorporation mmol/g capacity FMMS5 Grafting Hexagonal 3.2 2.5 10 0.0012 MP—Msu-x6 Direct Wormhole 2.3 2.3 30 ~0.01 SH-SBA-IS7 Grafting Hexagonal N/A 2.3 10.2 0.00 MP—MCM-418 Direct Hexagonal 3.4 2.1 MP-HMS Direct Wormhole 1 .4 1 .2 x = 0.11 MP-HMS Direct Wormhole 3.6 5 .0 15 0.02 x=0§ MP-HMS Direct Wormhole 5.3 7.3 65 < 0.002 x=0§ * This work. a. The binding capacity was obtained by adding an excess of mercury to mercapto groups. b. The initial ppm and final ppm values were obtained under conditions where the mercapto groups were present in large excess over mercury. 51 3.7 Conclusion An organo-functionalized silica mesostructure, MP-HMS, with quite favorable mercury adsorption properties has been synthesized. MP-HMS is extremely selective, and has a high binding capacity for mercury, up to7.3 mmol Hg2+lg or 1.46 g Hg2+lg at an Hg”: S ratio of 1.4 With the synthesis of MP-HMS, a new and promising method has been developed for remediation of mercury. .MP-HMS has been successfully shown to reduce mercury concentrations to below the EPA hazardous waste limit of 0.200 ppm and even to below the EPA drinking water limit of 0.002 ppm. In addition to its remarkable mercury remediation abilities, it also is easily synthesized in a one-step procedure. The MP-HMS is a stable, recyclable, and low cost material with a high affinity for mercury. The wormhole motif morphology of MP-HMS is one reason for its efficiency as a mercury-trapping agent. As opposed to the more monolithic hexagonal analogs, the wormhole motif allows greater access to the binding sites. In addition to better access to the mercaptan, MP-HMS also incorporates a larger amount of mercaptan per gram of material than other functionalized mesoporous materials. With more binding sites and greater accessibility to these sites, MP-HMS is an optimal material with which to remediate mercury. Furthermore, MP—HMS can be efficiently regenerated, which should further enhance the usefulness of MP-HMS and decrease the cost of its application. 52 3.8 References: (1) (2) (3) (4) (5) (6) (7) (8) Wagner-Dobler, 1.; von Canstein, H.; Li, Y.; Timmis, K. N.; Deckwer, W. D. Environ. Sci. Technol. 2000, 34, 4628-4634. Mitra, S. Mercury in the Ecosystem: Its Dispersion and Pollution Today; Trans Tech Publications: New York, 1986. McKay, G.; Bino, M. J .; Altamemi, A. R. Water Res. 1985, 19, 491-495. Marczenko, A. Spectrophotometric Determination of Elements; John Wiley & Sons Inc: New York, 1976. Feng, X.; Fryxell, G. B; Wang, L. Q.; Kim, A. Y.; Liu, J .; Kemner, K. M. Science 997, 276, 923-926. Brown, J .; Richer, R.; Mercier, L. Microporous Mesoporous Mat. 2000, 37, 41- 48. Liu, A. M.; Hidajat, K.; Kawi, S.; Zhao, D. Y. Chem. Commun. 2000, 1145-1146. Lim, M. H.; Blanford, C. F.; Stein, A. Chem. Mat. 1998, 10, 467-471. 53 Chapter IV. Future Goals 4.1 Extended X-ray Absorbance Fine Structure On the basis of the ionic charge of Hg2+ and RS’, one would expect a mercury to sulfur ratio of 0.5 for the full complexation of mercury by sulfur. This, however, is not the observed ratio. Instead, a ratio of ~ 1.4 is observed at the maximum mercury capacity. This means that the bonding involved is more than just Hg-S formation. One possibility is that there is a bridging oxygen, i.e., -S-Hg-O-Hg-S-, which would yield an Hg/S ratio of ~1, as is observed with some other materials 1'3. But an explanation for the greater than one ratio needs to be formed. One explanation is that some of the mercury might bind to oxygen from the incompletely condensed silicon centers. Another possibility is the formation of -S-Hg -O-Hg-OH linkages. A technique that could help elucidate the high Hg2+:S ratio and binding mechanism would be extended X-ray absorbance fine structure (EXAFS) analysis. The EXAFS technique is a structural probe based upon X-ray spectroscopy from which local structure (arrangement and spacing of atoms) can be deduced due to a diffraction-like phenomenon“. Structural information consisting of bond distances, structural disorder, coordination number and chemical identity of coordinating atoms can be elucidated by this method. This method will be utilized to determine the coordination of the mercury in the material and the local structure of the mercury. Samples of x = 0.1, 0.3, and 0.5 will be prepared at different mercury loading temperatures with an Hg2+/S ratio greater than one. This should allow us to determine whether or not mercury adsorbed at higher temperatures binds differently. These results may help elucidate the reason why MP-HMS can bind mercury in excess of the 54 mercaptan concentration. EXAFS spectra will be obtained in collaboration with the group of Professor Kim Hayes at the University of Michigan who has beam time at the National Synchrotron Beam in California. A second set of samples will also be submitted for analysis. These will be the regenerated MP—HMS with x = 0.3 and x = 0.5. The samples will be loaded with the same amount of mercury as the first set of samples submitted. After adsorbing mercury the samples, the material was air-dried, washed in 6 M HCl, and relOaded with the same amount of mercury. Once the data have been analyzed for the samples submitted, the chemical bonding between the mercury and the MP-HMS structure may be elucidated. 4.2 Acid Extraction of Surfactant The inclusion of ethanol in the material during synthesis and surfactant extraction was discussed in Chapter II. In order to minimize the incorporation of ethanol, acid extraction of the surfactant was preformed. One alternative method to Soxhlet extraction is acid extraction 5 . A solution of one mole of HCl to every one mole of dodecylamine was prepared in 100 mL deionized water. This solution was added to 500 mg of MP- HMS x = 0.3. This suspension was refluxed for 1 hr and then filtered with a second 100 mL of water and the same amount of HCl. 4.3 Adsorption of Mercury in a Column Bed Future experiments for mercury adsorption will include a column bed test. Thus far all mercaptan functionalized mesoporous materials have only been tested for mercury binding under batch scale conditions. The next step is to fill a column with MP-HMS and run mercury contaminated water through the bed and analyze the effluent. This can be done with the material in its powder form as well as in resin bead form. Figure 4.1 55 shows a transmission electron microscopy (TEM) image of spherical MP-HMS, which might be used to fill a column for mercury remediation. Figure 4.1. TEM image of spherical MP—HMS x = 0.5 where x represents the molar fraction of total silicon present as mercaptopropyl groups in the initial reaction mixture. 4.4 Anion Traps In addition to remediation of the mercury, remediation of other toxic compounds utilizing HMS will be examined. The prevalence of arsenite/arsenate in groundwater is problematic in many regions of the world, including the United States. In November of 2001 President Bush lowered the acceptable arsenic level in drinking to 10 ppb from the previous limit of 50 ppb. This new limit presents a need for cost effective adsorbents, and HMS could provide the properties needed to address this need. Fryxell et a1 6 utilized metal-chelated ligands immobilized on mesoporous silica as selective mesoporous anion traps. The mesoporous silica was functionalized by grafting of an ethylenediamine- terminated silane. I propose to functionalize HMS with 3- aminopropyltrimethoxysilane by direct assembly and use it to trap toxic anions. 56 Mesoporous silica was functionalized with an ethylenediamine (en) terminated silane, by grafting 6. This functionalized material was then treated with copper chloride in order to immobilize the copper on the mesoporous support. The copper was incorporated by stirring copper (II) chloride with the functionalized mesoporous material for a few hours. The Cu-en-Si functionalized silica was shown to bond oxoanions through an ion pairing mechanism. Chromate levels were reduced from 100 ppm to < 0.01 ppm and arsenate levels were reduced from 100 ppm to 1.4 ppm. The binding of the anions to the Cu-en-Si support was explained by the authors using an analogy of a lock and key mechanism. The Cu-en octahedral complex contains an electrophilic basket with C3 symmetry that forms an ideal host for a tetrahedral anion. Once the anion is coordinated within the C3 “basket”, it “unlocks” the complex, releasing an en ligand, and binding it directly to the cupric ion. In my future work a functionalized HMS will be prepared from TEOS and N-[(3- trimethoxysilyl)propyl}-ethylenediamine by direct assembly methods analogous to those used to prepare MP-HMS. The initial ‘x’ values will be 0.1 and 0.2. The N2 isotherms for the en-HMS are shown in Figure 4.2. The hysteresis loop for x = 0.] indicates that the pore sizes are not uniform. IUPAC classifies this shape of hysteresis loop as being that of type H3. The hysteresis loop for the X = 0.2 is more similar to that of MP-HMS or an H2 type loop. The hysteresis loop has an almost flat plateau before the desorption begins. One future goal is to obtain higher functionality of the en-HMS, i.e., x = 0.4 - 0.5. Another future goal is to immobilize copper with en-HMS and examine its anion trapping ability following a similar procedure to that of Fryxell. 57 700 7 _ / ’ 600 // / / / / / / 500 ‘ / ’ Volume Adsorbed (ems/g) 8 O 1 300 t 200 t 100 - / en-HMS x = 0.2 o l l l i 0 0 2 0.4 0 6 0 8 1 PlPo Figure 4.2. N2 Isotherm of en-HMS with x = 0.1 and x = 0.2 where x represents the molar fraction of total silicon present as ethylenediamine groups in the initial reaction mixture. 58 4.5 References: (1) (2) (3) (4) (5) (6) Feng, X.; Fryxell, G. E.; Wang, L. Q.; Kim, A. Y.; Liu, J .; Kemner, K. M. Science 1997, 276, 923—926. Brown, J .; Mercier, L.; Pinnavaia, T. J. Chem. Commun. 1999, 69-70. Liu, A. M.; Hidajat, K.; Kawi, S.; Zhao, D. Y. Chem. Commun. 2000, 1145-1146. Mande, C. B. a. C. Advances in X-Ray Spectroscopy; First ed.; Pargamon Press Inc: New York, 1982. Cassiers K., V. D. V. P., Vassant E. F. Chem. Commun. 2000, 2489-2490. Fryxell, G. E.; Liu, J.; Hauser, T. A.; Nie, Z. M.; Ferris, K. F.; Mattigod, S.; Gong, M. L.; Hallen, R. T. Chem. Mat. 1999, 11, 2148-2154. 59