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SF L: THESIS lllllllllllllllllllllllllllllllllll 3 1293 010461 This is to certify that the dissertation entitled INTERCALATION CHEMISTRY OF LAYERED DOUBLE HYDROXIDES: MATERIALS ENGINEERING AT A MOLECULAR LEVEL presented by Sang Kyeong Yun has been accepted towards fulfillment of the requirements for Ph.D. degree in Chemistry Thomas J. Pinnavaia (WK WW jor professor Date ?"//" ill/I MS U is an Affirmative Action/Equal Opportunity Institution 0-12771 _- _ _fi #,—5‘ ‘ — ~ 4 ._4_,_ ‘%fi44 LIBRARY Michigan State University PLACE ll RETURN aoxwmmmmummm TO AVOID FINES Mum on or Moro mm. DATE DUE DATE DUE DATE DUE MSU I. An mm Action/Emu! Opponuniiy Intuition W1 INTERCALATION CHEMISTRY OF LAYERED DOUBLE HYDROXIDES: MATERIALS ENGINEERING AT A MOLECULAR LEVEL By Sang Kyeong Yun A DISSERTATION Submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Chemistry 1994 ABSTRACT INTERCALATION CHEMISTRY OF LAYERED DOUBLE HY DROXIDES: MATERIALS ENGINEERING AT A MOLECULAR LEVEL Sang Kyeong Yun The pillaring chemistry of Mg3Al(OH)2 layered double hydroxide (LDH) was studied with several types of robust polyoxometalates (POMS) with high molecular dimension and charge to prepare new supergallery LDH intercalates potentially useful for shape-selective adsorption and catalysis. A new pillaring method of Mg-Al LDH carbonate with POM has been developed. Reaction of Mg1-xAlx LDH hydroxides ((1-x)/x z 2, 3, or 4) with tetraethylorthosilicate was studied in order to understand the intercalation chemistry of Mg-Al LDHs. The structural, thermal and textural properties of the LDH intercalates were elucidated based on XRD, FI‘ IR, TGA, DTA, TEM, SAED, EDS, 29Si MAS NMR and N2 adsorption-desorption studies. The acid/base functionality of the prepared Mg1-xAlx LDH-carbonates,-silicates and -polyoxometalate intercalates was examined using 2-methyl-3-butyn-2-ol (MBOH) as a reactive probe. The pillaring reactions of Mg3Al layered double hydroxides (LDHs) were studied for polyoxometalates (POMS) with Keggin (a-H2W1204o6'), Dawson (a-P2W130626'), and Finke (C04(HzO)2(PW9034)210') structures. When pre-swelled with adipate anions, LDH hydroxide precursors afforded highly crystalline pillared products at high reaction temperature (100°C). Depending on pillaring condition different gallery orientations were observed for the Dawson- and Finke-structure POMS. The differences in orientations were rationalized in terms of different electrostatic and hydrogen-bonding interactions between the POM pillars and the LDH layers. A gallery height of ~10 A was observed for the spherical Keggin- POM intercalated LDH, whereas two different gallery heights of 14.5 A and 12.8 A were found for the cylindrical Dawson-POM intercalated LDHs prepared at room temperature and boiling temperature, respectively. The cylindrically shaped Finke-POM also exhibited two different gallery heights of 13.3 A and 12.6 A, depending on reaction temperature. However, upon thermal treatment at 2 100°C, the Dawson and Finke ion-intercalated LDHs exhibited only one type of gallery POM orientation, regardless of preparation conditions. The gallery microporosity and crystalline layered structure of each LDH-POM intercalate were retained up to 200°C. Proposed is a new way of interpreting the textural properties of LDH-POM intercalates by the layer unit charge of e+. Another series of XRD, EDS and N2 adsorption-desorption studies indicate that the broad XRD reflections centered at ~11 A are most probably responsible for the interphase POM salts precipitated on the external surfaces of the LDH crystals. Polyoxometalates (POMS) of lacunary Dawson (a-P2W1706110‘), Finke (Zn4(HzO)2(A8W9O34)210‘ and WZn3(HzO)2(ZnW9034)212'), doubled Dawson (P4W3oZn4(I-120)2011215) and macro inorganic polyoxocryptates (NaSb9W2103613', NaP5W30011014’, NaAS4W40014027') structures also have been successfully pillared into Mg3Al(OH)2 LDH. Synthetic meixnerite precursor, [Mg3Al(OH)2][OH'], derived by the thermal decomposition and reconstitution procedure of the Mg3Al LDH carbonate, showed.a promising route for the preparation of crystalline LDH-POM intercalates at mild reaction conditions without any swelling agent. Incorporation of the macromolecular oxide POMS with oxygen numbers of 61 to 140 and charge from 10' to 27' into the LDH gallery resulted in crystalline LDH-POM intercalates stable up to 250°C. Topotactic pillaring reactions of Mg3Al-LDH with POMS exhibited considerable increase in gallery height from ~4.0 A of the LDH carbonate up to ~17 A with retention of the hexagonal crystal morphology. Gallery orientation of the POM intercalants were mainly determined by H-bonding interactions between the oxygen framework of the POM pillar and LDH layers. Nitrogen adsorption/desorption study on the LDH intercalates showed that access to the gallery micropores was achieved upon POM pillaring. A new synthetic route to polyoxometalate pillared layered double hydroxides of the type [Mg1-x2+Alx3+(OH)2][POMn']x/n, where POMn' is the Keggin ion H2W1204o6', is presented based on the atopotactic reactions of the corresponding LDH carbonate with glycerol or triethyleneglycol at l 120°C to 130°C and subsequent ion exchange reaction of the polyol- solvated LDH intermediates with the pillaring Keggin anion. This new synthesis strategy offers important advantages over all previously reported synthesis methods, as it allows the direct conversion of readily obtainable LDH carbonates to POM pillared derivatives. Equally important, the surface areas for the products obtained via the polyol route (~l60 m2/ g) are appreciably larger than the value (83 m2/ g) obtained for the same LDH intercalate prepared by ion exchange of a LDH-hydroxide intermediate (i.e., synthetic meixnerite) with H2W1204o6-. These differences in surface areas underscore the importance of synthetic methodology in mediating the micropore accessibility of pillared LDH derivatives. The acid/base functionality of the LDH-Keg gin ion intercalates were examined using 2- methyl-3-butyn—2-ol (MBOH) as a reactive probe. The LDH-H2W1204o6' intercalate obtained from the triethyleneglycolate intermediate exhibited high reactivity for the base-catalyzed disproportionation of MBOH, whereas the products derived from the LDH-glycolate and -hydroxide were relatively inactive. These differences in reactivity towards an organic substrate also emphasize the importance of synthesis methodology in regulating access in pillared LDH intercalates. Layered double hydroxides (LDHS) interlayered with Silicate anions were prepared by reaction of tetraethylorthosilicate (TEOS) with synthetic meixnerite-like precursors of the type [Mg1-xAi;,(OI-I)2][OH']x-ZH20, where (1-x)/x z 2, 3, or 4. TEOS hydrolysis occurred readily at ambient temperature in the galleries of the hydroxide precursors with (1-x)/x 2: 3 or 4, but a temperature of ~100°C was required to achieve Silicate intercalation for the LDH composition with (l-x)/x z 2. On the basis of the observed gallery heights (~7.0 - ~7.2 A) and 29Si MAS NMR Spectra that indicated the presence of Q2, Q3, and Q4 SiO4 sites, the intercalated silicate anions are assigned short chain structures in which some O38iOH groups have been grafted to the LDH layers by condensation with MOH groups on the gallery surfaces. The LDH-silicates exhibited comparable non-microporous N2 BET surface areas in the range 59 - 85 m2/ g, but they differed substantially in acid/base reactivities, as judged by their relative activities for the catalytic dehydration / disproportionation of 2-methyl-3- butyn-2-ol (MBOH). Under reaction conditions where the LDH Structure is retained (150°C), all the silicate intercalates showed mainly basic reactivities for the disproportionation of MBOH to acetone and acetylene. However, all the LDH silicates were less reactive than the corresponding LDH carbonates. Conversion of the LDH silicates to metal oxides at 450°C introduced acidic activity for MBOH dehydration, whereas the metal oxides formed by LDH carbonate decomposition were exclusivity basic under analogous conditions. An effort also has been made to understand the water behavior and textural properties of hydrotalcite-like layered double hydroxides (LDHS) with compositions corresponding to Mg1-xAlx(OI-I)2](CO3)x/2-nH2O where (1-x)/x z 2, 3, or 4, as a function of different charge densities and preparation methods. In the commonly used variable pH process, LDH precipitation was initiated at the elevated pH value of the starting carbonate solution and terminated at pH z 10. In contrast, the constant pH method allowed the entire precipitation process to be carried out at pH 10. Both synthesis methods yielded air-dried LDH carbonates containing pore water condensed between aggregated crystal platelets and adsorbed water bound to intragallery and external surfaces. Pore water was readily removed by heating to 60°C, but the temperature for complete removal of surfaces bound water increased from 240°C to 280°C (5°C/min) with increasing Al3+ content. A bimodal loss of surface water was consistent with the presence of "intrinsic" water bound to the intracrystal gallery surfaces and "extrinsic" water held at external surfaces. The assignment of the two types of water was further verified by temperature-dependent XRD and FI'IR studies. The textures of the LDH reaction products, as reflected in crystal morphologies, surface areas, and pore size distributions, were highly dependent on the preparation method and the layer charge density. fine grained crystals with rough surfaces and relatively high surface areas were obtained by the variable pH method, whereas the constant pH method afforded larger, well-formed hexagonal crystals. All of the products prepared by the variable pH method exhibited mesopores with radii in the range 50 - 300 A. In contrast, the constant pH method gave Mg3Al- and Mg4AI—LDH carbonates crystals with narrow mesopore distributions near 20 A radius. TEM images provided evidence for the accommodation of pore water in voids formed by edge-face crystal aggregation and for cofacial stacking disorders that contribute both to mesopores and to the binding of extrinsic surface water. ACKNOWLEDGMENTS My sincere thanks are given to Professor Thomas J. Pinnavaia for his invaluable encouragement and guidance throughout the course of this research. His impressive dedication and insight to science made my stay in his laboratory a lot enjoyable and fruitful. I also thank the other members of my committee: Professor Jeffrey S. Ledford, Professor Subhendra D. Mahanti and especially Professor Mercouri G. Kanatzidis for his helpful comments as a second reader. Appreciation is also extended to Dr. Stuart A. Solin at NBC for sharing a common research interest in the magnetic properties of some layered systems and Professor Karen Klomparens for precious discussions and assistance in Electron Microscopy experiments. I also owes special thanks to many friends in Pinnavaia research group for their friendship and encouragement. I have been so lucky to meet and get along with such great friends from all over the world who made my graduate years much more enjoyable and pleasant and helped me to widen my scientific career: Tie Lan, Danyun Li, Peter Tanev, Anis Barodawalla, Gary Succaw, Dr. Padmananda D. Kaviratna, Dr. Jean-Rémi Butruille, Astrid Baviere, Professor Jialiang Wang, Professor M. Elena Perez-Bemal and Professor Ricardo Ruano—Casero. It also was a great time for me to share a research interest in catalysis with my friends, Professor Vera R. L. Constantino and Dr. Malama Chibwe. I would like to viii give my sincere appreciation to my friend, Dr. Malama Chibwe for sharing all my difficult times in many ways. I would also like to thank the Honor Society for International Scholars (Phi Beta Delta) at Michigan State University for honorably presenting me the Phi Beta Delta Medallion. I would like to give all my best wishes to the fellows of the society. I am very pleased to give my special gratitude to Professor Moo-Jin Jun, Professor Sung Rack Choi, Professor Chang Hwan Kim and all the other professors at Yonsei University in Korea for their advice and encouragement throughout my graduate years. I also thank the friendship from many friends over the Pacific that has been always in my heart. Most importantly I would like to give my deepest gratitude to my family for their encouragement and support over the years. Special appreciation goes to my mother for her priceless love. Financial support given by a NIH fellowship (1991-1994), Center for Fundamental Materials Research and Department of Chemistry at Michigan State University is gratefully acknowledged. A deep appreciation also is extended to the Max-Planck-Gesellschaft for providing me a postdoctoral research fellowship, thanks to which I can further a wonderful research opportunity at the Max-Planck-Institut in Germany. All the concerns shown by Professor Dr. Wilheim F. Maier in the Institute also are gratefully appreciated. ix TABLE OF CONTENTS LIST OF TABLES .................................................... xiii LIST OF FIGURES ................................................... xv GENERAL INTRODUCTION ..................................... 1 CHAPTER ONE REVIEWS ON LAYERED DOUBLE HYDROXIDES AND POLYOXOMETALATES .............................................. 13 I. A Review on Layered Double Hydroxides .................. 14 A. Introduction ........................................................ 14 B. Synthesis and Structural Features ............................ 17 C. Physicochemical Properties and Related Applications 27 D. Intercalation and Pillaring Chemistry ...................... 30 E. Characterization ................................................... 32 II. A Review on Polyoxometalates ............................... 38 A Introduction ........................................................ 38 B. Structure and Reactivity ........................................ 39 C. Catalytic Applications ........................................... 50 References .............................................................. 56 CHAPTER TWO LAYERED DOUBLE HYDROXIDES INTERLAYERED BY POLYOXOMETALATE ANIONS WITH KEGGIN (a-H2W120406‘), DAWSON (a-P2W130626‘) AND FINKE (C04(HzO)2(PW9034)210') STRUCTURES ........................................................... 65 Abstract ................................................................. 66 A. Introduction ........................................................ 67 B. Experimental ....................................................... 70 1. Materials ........................................................... 70 2. Pillaring of Mg3Al LDH by POMS .............................. 71 3. Physical measurements ........................................... 73 C. Results and Discussion ........................................... 76 D. Conclusion .......................................................... 109 References .............................................................. 111 CHAPTER THREE MACROPOLYOXOMETALATES PILLARED LAYERED DOUBLE HYDROXIDES ........................................................... 114 Abstract ................................................................. 115 A. Introduction ........................................................ 116 B. Experimental ....................................................... 118 1. Preparation of LDHs and POMs ................................. 118 2. Pillaring reactions ................................................. 122 3. Physical measurements ........................................... 123 C. Results and Discussion ........................................... 125 D. Conclusion .......................................................... 152 References .............................................................. 153 CHAPTER FOUR A NEW POLYOL ROUTE TO KEGGIN-ION PILLARED LAYERED DOUBLE HYDROXIDES ................................. 156 Abstract ................................................................. 157 A. Introduction ........................................................ 158 B. Experimental ....................................................... 160 1. Materials preparation ............................................. 160 2. Catalytic acid/base studies ........................................ 162 3. Physical measurements ........................................... 163 C. Results and Discussion ........................................... 165 D. Conclusion .......................................................... 179 References .............................................................. 180 CHAPTER FIVE SYNTHESIS AND CATALYTIC PROPERTIES OF SILICATE- INTERCALATED LAYERED DOUBLE HYDROXIDES FORMED BY INTRAGALLERY HYDROLYSIS OF TETRAETHYL- ORTHOSILICATE ....................................................... 182 Abstract ................................................................. 183 A. Introduction ........................................................ 184 B. Experimental ....................................................... 186 1. Materials preparation ............................................. 186 2. Catalytic studies ................................................... 187 3. Physical measurements ........................................... 188 C. Results and Discussion ........................................... 191 D. Conclusion .......................................................... 208 References .............................................................. 209 CHAPTER SIX WATER CONTENT AND PARTICLE TEXTURE OF SYNTHETIC HYDROTALCITE—LIKE LAYERED DOUBLE HY DROXIDES .. 212 Abstract ................................................................. 212 A. Introduction ........................................................ 215 B. Experimental ....................................................... 217 l. LDH synthesis ..................................................... 217 2. Characterization methods ......................................... 218 C. Results and Discussion ........................................... 220 D. Conclusion . ............... . ............. . ........................... 244 References .............................................................. 246 xii 1.1. 1.2. 1.3. 1.4. 1.5. 2.1 2.2 2.3 3.1 3.2 LIST OF TABLES Ionic radii of some divalent and trivalent cations, in A. ............ 18 M111-XMIIIX LDH carbonates reported at various metal substitutions ......................................................................... 19 Crystallographic parameters of pyroaurite, sjogrenite and hydrotalcite. ........................................................................ 24 Acid catalysis by heteropolyacids of H3.x[XMle4o]. ............... 51 Some reported oxidation reactions catalyzed by the heteropolyacids of H3+n[PM012-nVnO4o] (HPA-n, where n = 2-8) + PdII system (PdII in the form of PdSO4, Pd(OAc)2, or PdClz). ........................................................................... 55 Gallery height (A) of pillared LDH-POM intercalates prepared from a LDH-hydroxide precursor and a LDH-adipate precursor at 100°C, where POM is a-H2W1204o6', a-P2W130626', or C04(HzO)2(PW9034)210‘. ................................................. 89 Summary of chemical compositions and textural properties of the LDH-[Am] intercalates, where An° is C032: OH', a-H2W120406', a-P2W130626‘, or C04(HzO)2(PW9O34)210'. ..................................................... 97 BET/N2, total and microporous surface areas of the prepared LDH products in A2 per the layer unit charge of e+. .............. 100 Observed gallery heights (A) for the macropolyoxometalate (MPOM) pillared Mg1-xAlx LDH intercalates after being heated at the temperature shown for 1h under N2, where MPOM is a-P2W1706110'. Zn4(H20)2(A8W9034)210‘, WZn30120)2(ZnW9034)217-'. P4W3oZn4(H20)2011216. NaSb9W2103518', NaP5W30011014' or NaAS4W4oOl4027‘. 139 Summary of chemical compositions and textural properties of the macropolyoxometalate(MPOM) pillared Mg1-xAlx LDH xiii 4.1 4.2 5.1. 5.2 5.3. 5.4. 6.1 6.2 6.3 intercalates, where MPOM is a-P2W17O6110-, 2040120)2(A8W9034)210', WZn3O‘120)2(ZnW9034)212‘. P4W302D4(H20)201121(*, NaSb9W2103613-, NaP5w30011014- or NaA54W40014027'. ......................................................... 151 Compositions and textural properties of the M g1-xAlx LDH carbonate and a-H2W1204o6‘ intercalates prepared via polyol route. ................................................................. 173 MBOH conversion over Mg1-xAlx- [H2W120406'] LDH catalysts. ............................................................................ 178 2S’Si MAS NMR data for the prepared Mgl-xAlx LDH silicates. ............................................................................ 200 Chemical compositions and textural properties of the Mg 1-xAlx LDH carbonates and silicates. .............................................. 202 Reactivity of LDH carbonates and silicates for MBOH conversion. ........................................................................ 206 MBOH reactivity of amorphous metal oxides derived from LDH carbonates and silicates. ...................................................... 207 Properties of air-dried Mg1-xAlx LDH carbonates prepared by the variable and constant pH methods. .................................. 222 Surface water content of the M g1-xAlx LDH carbonates prepared by the variable pH method. .................................... 227 The BET IN2 surface areas and mesopore volumes over several ranges of pore radii for Mg1-xAlx LDH carbonates. ............... 242 xiv 0.1. 1.1. 1.2. 1.3. 1.4. 1.5. 1.6. 1.7. 1.8. 1.9. 1.10. 2.1. LIST OF FIGURES Access to the interlayer pores by intercalation/pillaring approaches in inorganic layered materials. ............................... 3 Polyhedral representation of the layered double hydroxide (LDH) intercalates. ............................................................... 15 Layered double hydroxides (LDHS) of the [Mu1-mex(OH)2](An')-71-120 where M11 = Mg, Ni, Zn, Co, Cu, etc.; MIII = A1, Cr, Fe, etc.; An' = Halide, C032: 8042', NO3‘, etc. ............................................................................ 16 Some superstructures obtained by cation ordering in the LDH layers. ................................................................................. 22 (110) projections of the layered structures of pyroaurite and sjogrenite, [Mg6Fe2(OH)16](CO3)-~4.5H20. ........................... 23 (A) The arrangement of interlayer water and carbonate species perpendicular to the (001) plane. (B) Oxygen site distribution in the interlayer region along the (011 ) plane. ......................... 26 Thermal decomposition and layer reconstitution modes of the LDH carbonates. .................................................................. 28 Two different types of M06 octahedra: (A)Type l and (B) Type II. ......................................................................... 41 Polyoxometalate structures of the five common types. ............. 47 Keggin structure of the [XM1204o]n' type and related structures of capped, lacunary and isomeric forms. ................................ 48 Reactivities of the lacunary Keggin anion ([XM11039]H‘) and the reaction products. ................................................................ 49 Schematic representation of the formation of LDH derivatives pillared by Keggin POMn' ions with nearly spherical symmetry and by Dawson and Finke POMn- ions with cylindrical XV 2.2. 2.3. 2.4. 2.5. 2.6. 2.7. 2.8. 2.9. 2.10. 2.11. symmetry. ........................................................................... 69 Summary of the synthetic methods used to prepare LDH intercalates pillared by POM“: ions. ....................................... 78 XRD patterns of Mg3Al LDH-[a-H2W1204o6'] reaction products prepared by (A) mixed metal oxide precursor, (B) LDH hydroxide precursor at pH 4.5, and (CF) LDH hydroxide precursor with variations in reaction conditions. .................................... 80 XRD patterns of the oriented film of the LDH-[a-H2W1204o6'] intercalate preheated at the temperature indicated. ................... 82 XRD patterns of the oriented film of the LDH-[a- P2W130626'] intercalate preheated at the temperature indicated. ................... 87 XRD patterns of the oriented film of the LDH- [C04(I-120)2(PW9O34)210'] intercalate preheated at the temperature indicated. ..................................................... 88 Proposed POMn' gallery orientations for the LDH intercalates during drying (2 100°C) under N2: (A) a-H2W1204o6‘. (B) a-P2W130626‘ and (C) CO4(HzO)2(PW9O34)210‘. ............. 90 FTIR spectra of the authentic a-P2W130626' POM salt and the LDH-[a- P2W130626'] intercalates prepared at room temperature via LDH-hydroxide precursor and at 100°C via LDH-adipate precursor. ........................................................ 92 FT IR spectra of the room temperature LDH-[a-P2W130626] product preheated at different temperatures for 1h under N2; (a) room temp.; (b) 100°C; (c) 200°C. .................................... 93 FTIR spectra of the authentic C04(HzO)2(PW9O34)210‘ POM salt and the LDH-[C04(HzO)2(PW9O34)210'] intercalates prepared at room temperature via LDH-hydroxide precursor and at 100°C via LDH-adipate precursor. ................................................... 94 Representative TEM images of the LDH-POM intercalates, (A) LDH-[a-H2W1204o6'] and (B) LDH-[C04(HzO)2(PW9034)210‘]. ..................................... 95 xvi 2.12. 2.13. 2.14. 3.1. 3.2. 3.3. 3.4. 3.5. Structural models proposed for the impurity by-product formation upon polyoxometalate pillaring. ............................ 105 Powder XRD patterns showing impurity phase reflections as well as (001) reflections. The solid product (B) was obtained by suspending solid Mg3Al-CO3 (A) in aqueous a-H2W12O4o6' (3 times excess amount of the LDH AEC) solution at room temp. for 1h. This solid was then x-rayed at room temp. after heating at (C) 100°C and (D) 200°C for 1h under N2. XRD pattern of this solid after rehydration in water for 30h was then taken (E). XRD of (F) is for the product obtained by the solid state reaction of Mg3Al-CO3 and a-H2W12O4o6'. ...................................... 106 A schematic illustration of EDS compositional scanning on the hexagonal crystal (~2000 A in diameter) of the product prepared by room temperature aqueous reaction of Mg3Al LDH carbonate with (NH4)6[a-H2W 1204061. ................................ 108 Structures of the polyoxometalates (POMs) employed for the pillaring reactions:(A) a lacunary Dawson (a-P2W17O6110'), (B) Finke (2040120)2(A8W9034)210° and WZn3(H2O)2(ZnW9034)212‘), (C) a doubled Dawson (P4W3oZn4(H2O)2011216'), and macro inorganic polyoxocryptates of (D) NaSb9W2103513‘, (E) NaP5W30011014‘ and (F) NaAS4W40014027' structures. .................................. 128 Room temperature XRD patterns of the oriented film sample of the LDH-[a-P2W17O6110'] intercalate pre-heated at different temperatures; Room Temp.; 100°C; 200°C. ........................... 130 Room temperature XRD patterns of the oriented film sample of the LDH-[WZn3(H2O)2(ZnW9034)212‘] intercalate pre-heated at different temperatures; Room Temp.; 100°C; 200°C. ............. 131 Room temperature XRD patterns of the oriented film sample of the LDH-[Zn4(H2O)2(AsW9034)210'] intercalate pre-heated at different temperatures; Room Temp.; 100°C; 200°C. ............. 132 Room temperature XRD patterns of the oriented film sample of the LDH-[P4W3oZn4(H2O)2011216'] intercalate pre-heated at different temperatures; Room Temp.; 100°C; 200°C. ............. 133 xvii 3.6. 3.7. 3.8. 3.9. 3.10. 3.11. 3.12. 3.13. 3.14. 3.15. 3.16. 4.1. 4.2. Room temperature XRD patterns of the oriented film sample of the LDH-[NaSb9W3108618-] intercalate pre-heated at different temperatures; Room Temp; 100°C; 200°C; 250°C. ................ 136 Room temperature XRD patterns of the oriented film sample of the LDH-[NaP5W30011014'] intercalate pre-heated at different temperatures; Room Temp.; 100°C; 200°C; 250°C. ................ 137 Room temperature XRD patterns of the oriented film sample of the LDH-[NaAs4W4001 4037'] intercalate pre-heated at different temperatures; Room Temp; 100°C; 200°C; 250°C. ................ 138 FTIR spectra of the LDH-[a-P2W17O6110-] intercalate. .......... 141 FTIR spectra of the LDH-[Zn4(H2O)2(AsW9O34)210‘] intercalate. ......................................................................... 142 FTIR spectra of the LDH- [WZn3(H2O)2(ZnW9034)212‘] intercalate. ......................................................................... 143 FTIR spectra of the LDH-[P4W3oZn4(I-I2O)2011216'] intercalate. ......................................................................... 144 FTIR spectra of the LDH-[NaSb9W2103613‘] intercalate. ........ 145 FTIR spectra of the LDH-[NaP5W3oOllol4‘] intercalate. ........ 146 FTIR spectra of the LDH-[NaAS4W4oOl4027‘] intercalate. ...... 147 Representative TEM images of a LDH-[NaP5W30011014r] intercalate prepared by ion exchange reaction of a Mg3Al LDH hydroxide precursor. .................................................. 148 XRD patterns (Cu-K0) of oriented film samples of Mg/Al LDH metatungstate reaction products obtained from (A) meixnerite, (B) LDH-glycerolate and (C) LDH-triethyleneglycolate precursors. ........................................................................ 168 FTIR spectrum for the Mg/Al LDH-[a-H2W12O4o6'] prepared from a LDH-glycerolate precursor. ...................................... 169 xviii 4.3. 4.4. 5.1. 5.2. 5.3. 5.4. 6.1. 6.2. 6.3. 6.4. 6.5. 6.6. 6.7. Observed metal-oxygen FTIR vibrations for the Mg/Al LDH-[a-H2W12O4o6‘] prepared from a LDH-glycerolate precursor. ......................................................................... 170 Representative TEM images of the LDH metatungstate prepared using (A) meixnerite and (B) LDH-glycerolate precursors. ..... 172 Powder XRD patterns of (A) Mg2Al-, (B) Mg3Al- and (C) Mg4Al-LDH silicate intercalates. .................................... 195 TEM images (A and B) and selected area electron diffraction (SAED) pattern (C) of the Mg3Al-silicate compound. ......................................................................... 196 FTIR spectra for (A) Mg2Al-, (B) Mg3Al- and (C) Mg4Al-silicate intercalates. ............................................ 198 29Si MAS NMR spectra of (A) Mg2Al-, (B) Mg3Al- and (C) Mg4Al-silicate intercalates. ............................................ 199 TGA curves (5°C/min) for Mg3Al LDH carbonates: (A) air- dried sample and (B) sample pre-dried at 60°C for 10 h. ........ 223 DTA curves for (A) Mg2Al-, (B) Mg3Al-, and (C) Mg4Al- LDH carbonates prepared by the variable pH method and dried at 60°C in air. ............................................................ 225 Temperature dependence of the thin film XRD patterns for an air-dried Mg2Al LDH carbonate prepared by the variable pH method. ........................................................................ 229 Temperature dependence of the p0wder XRD patterns for an air-dried Mg2Al LDH carbonate prepared by the variable pH method. ........................................................................ 230 XRD patterns of a Mg2Al LDH carbonate (the variable pH method) upon thermal dehydration and rehydration. .............. 231 FTIR spectra of the (A) Mg2Al, (B) Mg3Al and (C) Mg4AI LDH carbonates prepared by the variable pH method. ............ 236 TEM images of M g3Al LDH carbonates prepared by (A) the xix 6.8. 6.9. 6.10. 6.11. variable pH method and (B-D) the constant pH method. .......... 237 TEM images of a Mg3Al LDH carbonate prepared by the variable pH method showing interparticle textures: (A) cofacial stacking and (B) edge-face aggregation of thin crystals. .......... 238 N2 adsorption/desorption isotherms (at -196°C) for Mg3Al LDH carbonates prepared by (A) the variable pH method and (B) the constant pH method. ................................................ 239 Pore size distributions obtained from the N2 desorption isotherms of M g3Al LDH carbonates prepared by (A) the variable pH method and (B) the constant pH method. ............. 240 A possible structural model for the interfacial regions of LDH derivatives exhibiting textural porosity in the pore radius range near 20 A. ......................................................................... 243 XX GENERAL INTRODUCTION In recent years, porous inorganic layered materials with a structural periodicity have been extensively used in many areas of applications, especially in molecular sieving and shape-selective adsorption and catalysis}:3 Many of such layered compounds with interlayer pores (free gallery spaces) have been prepared using intercalants of organics, organometallics, metal chelates, molecular oxoclusters and transition metal halide clusters. The unique structural and physicochemical properties of these layered intercalation compounds have made this class of materials one of the most important research subjects in solid state chemistry. Compounds with two-dimensional layered structures possess basal planes of atoms that define a gallery region between the stacked structural units. In addition to such layered structures with electrically neutral layers as graphite or FeOCl where the galleries are empty with the basal planes of adjacent layers in van der Waal's contact, some ionic lamellar solids with a fixed charge can be intercalated with varieties of ionic guests with the charge separation between the gallery species and the host layers being an intrinsic feature of their structure. Pillared lamellar solids are 1 2 intercalation compounds in which two special criteria are met4: (i) the interlayer guest species are sufficiently robust to function as a molecular props or pillars between the host layers, causing expansion of the interlayer gallery region to a height comparable to the van der Waal’s dimension of the guest; and (ii) the pillaring species are laterally spaced at a distance substantially larger than the van der Waal's diameter of the pillar in order to compensate the electrical charge balance (Figure 0.1). These structural properties result in the formation of a two-dimensional, intracrystalline microporous network of potential utility for shape-selective adsorption and catalysis. In most of the pillared lamellar solids reported to date, the gallery height is comparable to the van der Waal's thickness of the host layers. The gallery free space, which can be accessible for many materials applications, especially in catalysis, of the layered intercalate compounds is accessible only to water and other small polar molecules capable of solvating the gallery counter ions and the charged layer surfaces. Removal of the solvating molecules by outgassing at elevated temperatures causes the galleries to be inaccessible, especially if the intercalated counter ions are small relative to interstices in the gallery region. However, it should be possible, through a judicious choice of pillaring agent, to molecularly engineer pillared derivatives in which the gallery height is substantially larger than the thickness of the host layers. By mediating the height and lateral separation of the pillars, one can vary over a wide range the height and width of the slot-shaped pores contained within the interlayer galleries. In general, ionic lamellar solids are preferred hosts for the synthesis of pillared materials because gallery ions of different sizes and shapes may be readily introduced as pillars by simple ion exchange reactions. Intercalation/ Pillaring d1 — . Pillar or Intercalant Interlayer Pore (: Free Gallery Space) r O ; small interlayer molecular species intercalated in the layered compounds ; new intercalants of molecular pillars introduced d1 : Interlayer or Gallery Height \ d2 : Interpillar Distance Figure 0.1. Access to the interlayer pores by intercalation/pillaring approaches in inorganic layered materials such as layered metal phosph(on)ates, layered silicates, layered metal oxides or layered double hydroxides. Gallery (interlayer) dimension of the pillared derivatives can be expressed as d1*d2. 4 Several classes of materials have been considered as the host layers of ionic lamellar solids. These include layered silicate clays (LSCs), layered double hydroxides (LDHS) and layered metal phosphates (LMPs), as well as layered silicic acids (LSAs) and layered metal oxides (LMOs).1'3 Recently, considerable attention on the pillared derivatives of LSCs and LDHs has been paid for their catalytic applications. 1:5‘10 Layered double hydroxides (LDHs) represent a potentially useful class of ionic solids for forming microporous pillared derivatives. In general, LDHs are two-dimensional solid bases while zeolites and clays function as crystalline solid acids. Many different oxo anions have been intercalated into LDH layers by topotactic ion exchange reactions,8 but the oxo ions have been small (CO32',SO 2', Cr042', etc.) and the interlayer gallery heights have been limited to values corresponding to one or two layers of space-filling oxygen. Therefore, pillared derivatives with larger gallery heights and pore sizes are desired for catalytic reactions. There has been considerable interest in the design and application of microporous pillared solids with "supergallery" structures}-3 Supergallery derivatives, a new class of materials, are those pillared lamellar intercalates in which the interlayer thickness is substantially larger, at least twice, than that of the host layers.4 Among several kinds of candidates, polyoxycations and polyoxoanions have been proven the most potential interlayer guest species for the preparation of supergallery derivatives of LSCs2:5’11 and LDHs12'19, respectively. Opposite from LSCs, positively charged layers of LDHs can incorporate many different species of organic and inorganic anions between the layers.8 Pillared 5 layered double hydroxides (LDHS) with polyoxoanions have recently attracted an increasing interest as a new class of pillared materials, due to the diversity in the design of these nanoscaled materials. ”'19 Polyoxometalates (POMS) present ideal pillaring agents for LDHs because of their wide range of thermal stability and catalytic activity.20,21 POMS generally posses structures consisting of multiple layers of space- filling oxygens as well as a wide range of charge densities. Robust POMS Should impart large gallery heights, and those with suitably high charge densities should give rise to large lateral anion spacings, thereby providing access to the intracrystalline gallery surfaces. In addition to the broad spectrum of compositional and charge variation of the host layers, most importantly, a reasonable choice of the guest polyoxometalate (POM) anions would result in new chemical and physical properties upon intercalation. Many choices of POMS with high thermal stabilities and different catalytic activities are available?“23 New micropores can be also accessible, by varying the charge density of the host layers (lateral control) and the size and charge of the guest POMS (longitudinal and lateral control). Up to now, several strategies have been tried to intercalate POMS such as V100286‘ 12:13:15 and Keggin types14,16-19 of H2W120406‘, SiV3W90407', SiW110393‘, and PW9034 '. However, those POMS have relatively limited moleCular heights (5 10 A) and charge densities (s 9 6 /equiv.). Pillared LDH derivatives of POMS with bigger molecular dimension than Keggin-structure are strongly demanded for more extensive applications of these materials. Molecular engineering of this 6 class of materials at the molecular level is expected to give rise to new aspects of application, especially in shape-selective adsorption and catalysis. One of the objectives of this research is to synthesize "supergallery" LDH intercalates in which the gallery height is two or more times as large as the host layers. Molecular engineering of the LDH compounds via intercalation of POMS at the molecular level was therefore studied in terms of gallery pore regulation and introduction of new chemical application. In order to achieve supergallery LDHS materials, it is essential to choose and intercalate POMS with relatively large molecular dimension (high gallery height) and high ionic charge (large lateral spacing). We have focused on the intercalation of LDHS with POMS of Keggin, Dawson, Double Dawson, and Macro Inorganic Polyoxocryptates which may provide new supergallery dimensions which could not be achieved by any other LDH intercalates. Systematic approaches for the preparation of crystalline LDH-POM derivatives were first studied using Mg3Al(OH)2 LDH and POMS of Keggin, Dawson and Finke structures (Chapter II). Variations in pillaring method, reaction temperature and time, and use of preswelling agents were considered for the preparation of better crystalline derivatives. Details on the structural, thermal and textural properties of those derivatives are elucidated based on several experimental techniques. Interlayer orientation of the gallery POMS was determined and rationalized in terms of electrostatic and hydrogen-bonding interactions between gallery POMS and LDH layers. Intracrystalline microporous Mg-Al LDH-POM intercalates were also prepared by pillaring some Macro-POMS, using [Mg3Al(OH)2](OH')-flhO precursor (Chapter III). Significant variations 7 of POM Size and charge were considered in order to obtain supergallery LDHS. A new pillaring method of Mg-Al-CO3 LDHS with Keggin POM has been also developed (Chapter IV). Carbonate form of the LDH was directly pillared with the POM in the presence of polyols such as glycerol and triethyleneglycol via one-step swelling/decarbonation mechanism. Crystalline LDH-POM intercalates obtained by this polyol route exhibited remarkably improved textural properties of microporosity, compared to the intercalate obtained by the conventional method. A second general objective of this work is to understand the chemistry of LDH-hydroxide and -carbonate precursors used to form the pillared derivatives. Interlamellar silication reactions of the Mg1-xAlx LDH hydroxides ((1-x)/x z 2, 3, or 4) with Si(OC2H5)4 were also studied in order to understand the base catalyzed sol- gel process by the interlayer hydroxyl anions in a confined LDH system (Chapter V). Gallery hydrolysis and condensation reactions of the intercalated TEOS molecules, catalyzed by the interlayer OH' ion in the meixnerite precursors, afforded a two-dimensional mixed bilayers of LDH-Silicate compounds where the gallery silicates have a chain-like two dimensional structure. Different approaches for the M g-Al LDHS preparation as well as the effect of layer composition on the physicochemical properties such as water content, thermal, and textural properties of the LDH carbonates was of particular interest (VI). Important information on water behavior and layered structural features have been obtained for the [Mg1- 8 xAlx(OH)2](CO3)x/2-szO ((1-x)/x z: 2, 3 or 4) prepared by two different methods. The use of the prepared LDH intercalates as catalysts for selected organic chemical conversions was also investigated. Variations in acid/base properties of LDH-carbonate, -silicate and -Keggin POM intercalates were observed due to the differences in their physicochemical properties (Chapters IV and V). Incorporation and immobilization of the Dawson POM anions between the LDH layers resulted in an interesting shape- selectivity toward an oxidation reaction due to the spatial restriction in the gallery region of the pillared LDH-POM intercalates. Magnetic properties of Ni1-xAlx LDHS and Cu(OH)3(NO3) were also studied as a function of different Ni amount. The magnetism and oxidation catalysis works are not presented in this thesis but can be shown elsewhere soon, together with all the works. presented in this dissertation.2434 References 1. Mitchell, 1. V. (Ed.) Pillared Layered Structures; Elsevier: London, 1990 and references therein. 2. Burch, R. (Ed.) Pillared Clays - Catalysis Today; Elsevier: Amsterdam, 1991, 2, 185 and references therein. 3. Occelli, M. L.; Robson H. E. (Eds.) Expanded Clays and Other Microporous Solids; Van Nostrand Reinhold: New York, 1992 and references therein. 4. a) Pinnavaia, T. J.; Kim, H. In Zeolite Microporous Solids: Synthesis, Structure, and Reactivity; Kluwer Academic Publishers: Netherlands, 1992; pp 79-90. b) Pinnavaia, T. J.; Kwon, T.; Yun, S. K. ibid, 1992; pp 91-104. 5. Pinnavaia, T. J. Science 1983, 220, 365 and references therein. 6- Figueras, F. Catal. Rev. Sci. Eng. 1988, 30, 457. 7. Bruce, L.; Tumey, T. Chem. Aust. 1988, 277. 8. Cavani, F.; Trifiro, F.; Vaccari, A. In Catalysis Today; Elsevier: Amsterdam, 1991; Vol. 11, pp 173 and references therein. 9. Pinnavaia, T. J. In Expanded Clays and Other Microporous Solids; Van Nostrand Reinhold: New York, 1992; pp 1. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 10 Clearfield, A.; Kuchenmeister, M.; Wang, J .; Wade, K. In Zeolite Chemistry and Catalysis; Elsevier: Amsterdam, 1991; pp 485. Yamanaka, S.; Hattori, M. In Chemistry of Microporous Crystals; Elsevier: Amsterdam, 1991; Vol. 60, pp 89 and references therein. Kwon, T.; Tsigdinos, G. A.; Pinnavaia, T. J. J. Am. Chem. Soc. 1988, 110, 3653. Drezdzon, M. A. Inorg. Chem. 1988, 27, 4628. Kwon, T.; Pinnavaia, T. J. Chem. Mater. 1989, 1, 381. Chibwe, K.; Jones, W. Chem. Mater. 1989, 1, 489. Dimotakis, E. D.; Pinnavaia, T. J. Inorg. Chem. 1990, 29, 2393. Wang, J.; Tian, Y.; Wang, R.-C.; Clearfield, A. Chem. Mater. 1992, 4, 1276. Kwon, T.; Pinnavaia, T. J. J. Mol. Catal. 1992, 74, 23. Narita, E.; Kaviratna, P. D.; Pinnavaia, T. J. J. Chem. Soc., Chem. Commun. 1993, 60. Pope, M. T.; Muller, A. Angew. Chem. Int. Ed. Engl. 1991, 30, 34. Pope, M. T. Heteropoly and Isopoly 0xometalates; Springer-Verlag: New York; 1983. Garcia-Gariby, M. A.; Zhang, Z.; Turro, N. J. J. Am. Chem. Soc., 1991, 113, 6212. 11 23. Fox, M. A.; Cardona, R.; Gaillard, E. J. Am. Chem. Soc. 1987, 109, 6347. 24. ”Molecular Engineering of Layered Structures 11. Synthetic Approaches to Some New Pillared Derivatives" T. J. Pinnavaia, T. Kwon and S. K. Yun, NATO ASI Ser., Ser. C, 1992, 352, 91. 25. "Organic Chemical Conversions Catalyzed by Intercalated Layered Double Hydroxides (LDHS)" T. J. Pinnavaia, M. Chibwe, V. R. L. Constantino and S. K. Yun, App. Clay Sci. 1994, Accepted. 26. "Layer Rigidity in 2D Disordered Ni-Al Layer Double Hydroxides" S. A. Solin, D. Hines, S. K. Yun, T. J. Pinnavaia and M. F.. Thorpe, J. Non-Cryst. Solids, 1994, Submitted. 27. "Water Content and Particle Texture of Synthetic Hydrotalcite-Like Layered Double Hydroxides" S. K. Yun and T. J. Pinnavaia, Chem. Mater. 1994, Submitted. 28. "A New Polyol Route to Keggin-Ion Pillared Layered Double Hydroxides" S. K. Yun, V. R. L. Constantino and T. J. Pinnavaia, Microporous Mater. 1994, Submitted. 29. "Silicate-Intercalated Layered Double Hydroxides Formed by Intragallery Hydrolysis of Tetraethylorthosilicate" S. K. Yun, V. R. L. Constantino and T. J. Pinnavaia, Clays Clay Miner. 1994, Submitted. 30. "Intercalation of Polyoxometalate Anions with Keg gin (a-H2W12O4o6' ), Dawson (a-P2W130626') and Finke (C04(H2O)2(PW9034)210') 12 Structures in Layered Double Hydroxides" S. K. Yun and T. J. Pinnavaia, Inorg. Chem. 1994, Submitted. 31. "Novel Macropolyoxometalate Pillared Layered Double Hydroxides" S. K. Yun and T. J. Pinnavaia, Inorg. Chem. 1994, Submitted. 32. "Dawson (a-P2W130626') Polyoxometalate Intercalated Layered Double Hydroxides as Shape-Selective Oxidation Catalysts" S. K. Yun, M. Chibwe and T. J. Pinnavaia, 1994, Manuscript in Preparation. 33. "Magnetic Properties of [Ni1-xAlx(OH)2](CO3)x/2-fi120 (x = 0.469 ~ 0.102) Layered Double Hydroxides (LDHS)" S. A. Solin, D, Hines, S.K. Yun and T. J. Pinnavaia, 1994, Manuscript in Preparation. 34. "Magnetic Properties of Cu2(OH)3(NO3) and Cu2(OH)3(OCOCH3) Layered Compounds" S. A. Solin, D, Hines, S.K. Yun and T. J. Pinnavaia, 1994, Manuscript in Preparation. CHAPTER ONE REVIEWS ON LAYERED DOUBLE HYDROXIDES AND POLYOXOMETALATES 13 14 I. A Review on Layered Double Hydroxides A. Introduction Layered double hydroxides (LDHS) are an important class of materials currently receiving considerable attention. Many studies have been carried out on these mixed metal hydroxides,l which are often referred to as hydrotalcite (HT)-like compounds. Many aspects of LDH chemistry from the synthetic and structural details to materials applications of these materialsz‘5 have been recently explored and reviewed Therefore, several important results from previous studies, which are the basis of present work, on the LDHS will be briefly discussed in this chapter. LDHS are represented by the general formula: [M111_ mex(OI-I)2J(An')x,n ~d-IZO where M11 2 Mg, Ni, Co, Zn, Cu, etc.; Mm: Al, Cr, Fe, etc. LDH compounds have positively charged brucite-like (Cdlz-type) mixed metal hydroxide layers (Figure 1.1). In terms of charge, LDHS are mirror images of the much studied family of cationic clay minerals such as smectites. That is, the layers are positively charged in LDHS, while negatively charged in smectite clays. In that respect, LDHS are often called as anionic clays. The layer charge created by isomorphous 15 substitutions of some M11 by MIII is balanced by the interanionic species, A“: (Figure 1.2). Figure 1.1. Polyhedral representation of the layered double hydroxide (LDH) intercalates where C032' and water molecules occupy gallery regions between the edge-shared mixed metal hydroxide layers. 16 .283 wfioam .83 of 88m 3 wézv 805.35 8.3. 05 wnzognsm 3 3.2528 on 58 Emma: arc—BO .8» 5.02 .-~vOm .-~mOU .023.— " -=< was .on_ .5 ._< n ES ”.88 .50 .60 5N ._z a: n =2 22.3 o~§€$ Emovgzizé 2: ac 25.: 82662 03:8 823 .2 28E :0 . 9:3: .2qu :0 Cal. ..=< =0 :52 A52 :0 17 Several approaches have been tried to synthesize LDH compounds with MHIMIII ratios of between 2 and 5.6-11 The nature of the metal cations, the type of anions, the Size and morphology of clay particles can be varied. Significant efforts also have been given to intercalation of LDHS with varieties of organic and inorganic anions. This has led to a search for uses of LDH compounds in three main areas: heterogeneous catalysts and catalyst precursors, ion exchangers and adsorbers, and new materials applications. B. Synthesis and Structural Features There are a wide range of synthetic variables for the preparation of LDH with the type of M111_XMIIIX(OI-I)2](An-)x,n 41120: nature of MII and MI“, MHIMIII ratio, variation of interstitial anions, amount of water, crystal morphology and Size, reaction conditions.5'll These variables lead to an almost infinite range of synthetic materials. However, nature places a number of restriction which narrows the field considerably. Among several synthetic methods such as precipitation5»3.11, induced hydrolysis7’10, and salt-oxide methods9, the aqueous precipitation method‘5:8 is usually preferred due to its convenience. This method allows considerable variability with respect to the nature of MII and Mm, as well as the MH/MIII ratio. Good control over the particle morphology and Stu-face area can be also obtained. 18 Usually metal cations with Similar ionic radii are preferred. 12 Table 1.1 shows the ionic radius of some cations which have been used for LDH preparations. Table 1.1. Ionic radii of some divalent and trivalent cations, in A. Be Mg Cu Ni Co Zn Fe Mn Cd Ca 030 0.65 0.69 0.72 0.74 0.74 0.76 0.80 0.97 0.98 M11 M” Al Ga Ni Co Fe Mn Cr v Ti In 0.50 0.62 0.62 0.63 0.64 0.66 0.69 0.74 0.76 0.81 A basic pH is generally required for LDHS formation by precipitation: the final pH usually is between 8-11. The time and temperature of the crystallization step determines the crystal morphology and, in particular, the catalytically important surface area. Usually the higher the temperature and the longer the digestion time, the larger the resulting the thin hexagonal crystals and the smaller the surface area.3-6.3 LDH structures are reported to exist for values of x (divalent metal substitution) in the range 01-05, which is significantly dependent on the nature of metal atoms. Table 1.2 Shows the reported optimal values of x for obtaining some pure LDH carbonates. Beyond optimal values of x for 19 Table 1.2. MULXMHIX LDH carbonates reported at various metal substitutions LDH carbonate x Reference Mg1.xAlx-CO3 0.17-0.33 13 0.20-0.34 6 010-034 14 0.15-0.33 15 M1,xAl x-CO3 0.17-0.33 16 025-034 17 0.20-0.41 15 ZnMCrx-Cog. 0.25-0.35 18 a LDH, either the pure metal hydroxides or other compounds with different Structures were obtained. The unit cell parameter a can be taken as an index of the non- stoichiometry with respect to the formation of pure LDHS. 19 For example, in Mg1-xAlx LDHS the parameter a decreases with increasing x within the range of pure LDH formation, since the ionic radius of Al3+ is smaller than that of Mg2+. For an ideal octahedron the unit cell a value 20 corresponding to the metal-metal bond length can be calculated as following.16 a = J2— (M-O) where a 2 unit cell a parameter (M-O) = the metal ion-oxy gen ion distance But, taking account of the mean metal ion radius 2 of the metal ions constituting the LDH layers, 0: J2— z = J2— (1-x)*R(MIl) + xaR(MII1) where z = (1-x):R(M11) + xxRCMm), the mean metal ion radius x = the stoichiometric amount of metal ion substitution For Mg1_xAlx(OI-I)2](CO3)XQ d120, the values of x ranges usually from 0.2 to 0.33 between which pure single phase of HT-like LDH compounds have been observed. 13,20 The unit cell a dimension was found to decrease with increasing isomorphous metal substitution of smaller Al for Mgzl9 ionic radii for A13+ and Mg2+ are 0.53 A and 0.72 A, respectively (cf., Table 1.1). The decrease in basal spacing with increasing value of x is observed due to the greater coulombic attractive force 21 between the positively charged brucite-like layer and the negatively charged interlayer species.l6 As can be expected, the unit layer charge and charge density decrease in proportion to the Al content of the unit cell. Depending on the ratio of divalent to trivalent metal, in the host LDH layers, a certain type of superstructure can be available. For example, substitution of an Al3+ ion for a Mg2+ ion creates a resultant charge of +1. Therefore, Al substitutions are likely to be as far apart as possible because of mutual repulsions; substitutions in adjacent sites will be the least likely to occur unless accompanied by vacant cation sites, i.e., a gibbsite unit within a brucite layer. 15 Based on this Simple consideration several types of superstructural metal distribution were proposed5-16 and a few observed”:21 (Figure 1.3). In LDH compounds containing simple anions, adjacent brucite-like layers are stacked so that the hydroxyl groups on the lower surface of one layer are directly above those on the upper surface of the one below, as in gibbsite (Al(OH)3), rather than Staggered as in brucite (Mg(OH)2).22-23 Layers can be stacked in the form of a 3-layer polytype -(BC)(CA)(AB)(BC)- (rhombohedral symmetry) or in a less symmetrical two layer (-(BC)(CB)(BC)-) arrangement (Figure 1.4).22,23 The mineral hydrotalcite, [Mg6A12(OH)16](CO3) ~4H20, crystallizes in rhombohedral symmetry (pyroaurite group). Manasseite has the same chemical composition but hexagonal symmetry (sjogrenite group) with 2-layer polytype with a -(BC)(CB)(BC)- OH layer sequence. Table 1.3 summarizes the structural characteristics of the three minerals, pyroaurite, Sjogrenite and hydrotalcite.23 22 IIIIIIIIIIII iggI'I/n, Q'I'IIMII, r “r ‘II,III .I,I.II.I. .I.I.I.I,II, I/anIIIIIIII .‘I’ III 'I'I'IagIII', .§.1 . . .1111111 .I. IIIIIIII, Figure 1.3. Some superstructures obtained by cation ordering in the LDH layers. Related MIIIMIII ran'os and superstructure parameters are (A) 1/1, a=aab=3015.(B)2/l.a=b=8015.(C)3/l.a=b=280. (D) 6/1.a= b = 2101/7 , and (E) 8/1, a = b = 330. Open and closed circles represent MII and Mm, respectively, and layer hydroxyl groups are omitted for clarity. j 23 Pyroaurite S jt'igrenite Figure 1.4. (110) projections of the layered structures of pyroaurite and sjagrenite. ngeFe2-~4.5H20. 24 Table 1.3. Crystallographic parameters of pyroaurite, sjogrenite and hydrotalcite. Pyroaurite Sjegtenité" " Hydrotalcite Spacial group 3R 2H 3R a (A) 3.11 3.11 ' 3.05 c (A) 23.4l=30' 15.61=2c' 22.81a3c' c'(A) 7.803 7.805 7.603 2(m61/ee11) 3/8=3M 2/8=2M . l/2=3M Density (g/cm3) 2.13 2.11 2.09 Interatomic Distances, A M-OH (6x) 2.065 2.06 2.03 011-1120 2.93 2.92 2.84 H20-C03 in gallery 2.76-3.11 2.76-3.11 2.71-3.05 Angle OH-H20-OH, degrees 158 160 160 OH-OH in brucite-like sheet 2.04 2,04 2,00 OH-Z, sheet-interlayer 2.33 2.88 2.80 25 There are two kinds of water in LDH compounds: one type is at interstitial positions between the basic layers which constitute, together with the interlayer CO32' anion, the gallery species; the other type is physically adsorbed on the external surface of the layer:s‘.'5‘ The importance of water content and location, interstitial or surface, in LDH compounds has been found to govern many materials properties such as proton9-24 and ionic2~S conductivities. Acidic and basic properties, as well as the ion exchange properties, of LDH compounds also are expected to be closely related to water content and type, as in smectite clay826‘23. For all the phases of the pyroaurite-sjogrenite group, the general formula may be written as [M111-mex(OI-I)2J(CO3)X,2(HZO)H3X,Z)_A.29 Carbonate anion and interstitial water, the interlayer constituents with high degree of mobility and hydrogen bonding,22-23-30 occupy the interlayer positions. The main features of the interlayers are groups of six oxygen sites distributed closely around those symmetry axes that pass through the hydroxyl groups of adjacent brucite-like layers (Figure 1.5) .2931 Preferred loci for interlayer oxygen atoms could be found at about 0.56 A radially distant from the three-fold axes. Each of the oxygen sites would thus have a statistical occupancy of 0.167 oxygen. Within each group, these sites are so near together that, at the most, only one can be occupied by an oxygen atom of water or carbonate anion. Some of the groups of oxygen sites occupied by H20 or C032: can be empty, which is usually the ease,29 creating oxygen site vacancy, A. However, considering the existence of surface water, the general formula for HT -like LDHS should be rewritten as [M111_mex(OH)2][(CO3)x,2(I-IZO)1 43x,” A] thzO. The amount of interstitial water, in equilibrium due to their solution-like 26 behavior,22»23-30 seems to depend on the oxygen site vacancy (A), as well as on the degree of metal substitution for a LDH compound. The surface water content might be dependent on ambient conditions such as humidity or sample drying condition. (A) (B) Figure 1.5. (A) The arrangement of interlayer water and carbonate species perpendicular to the (001) plane. Hydroxyl groups above (closed circle) and below (open circle) the ab plane are represented and metal cations centered at each octahedron are omitted for clarity. (B) Oxygen Site distribution in the interlayer region along the (011) plane: hydrogen- bonding interactions of interest are shown as dashed lines. 27 C. Physicochemical Properties and Related Applications LDHS are anion exchangers. A widespread application of layered double hydroxides is anticipated by the pronounced anion-exchange capacity toward inorganic32‘38 and organic39'46 anions. They strongly prefer multiply charged anions, such as carbonates or phosphates. The order of selectivity for anions is approximately CO32- > $04 - >> OH' >F' > C1' > Br- > N03- > 1'. Their most important use may well be as anion exchangers and adsorbents of ecologically undesirable anions from dilute ‘ and possibly radioactive aqueous waste steams2 and as halogen scavengers in polyolefin production42. In general, LDHS are two-dimensional solid bases while zeolites and clays function as crystalline solid acids. Many catalysts are formed from LDH precursors. LDH compounds show an unique thermal decomposition behavior. According to Sato et al.47-48, hydrotalcite-like compounds have several distinct regions of metastable phases as a function of heat treatment as Shown in Figure 1.6. The nature of the thermal decomposition of LDHS is particularly interesting and important because this thermal process leads to catalytically active metal oxides with high surface area and homogeneous compositions. Generally speaking, the thermal decomposition of M(II)- M(III) carbonate LDHS leads to metal oxide-hydroxide mixtures that are mildly basic and have exceptionally well-dispersed metal atoms.2 Impregnation procedures for the preparation of transition metal catalysts normally can not achieve such high and homogeneous metal dispersions. 28 [Mai-meAOH) 21(C0 902 2320 A ~(100-250°C) Y Mnr-mex(0H)2](C03)x/2 4' z H2O 320,003?- A ~(400-600°C) Ev Mn2(1-x)/(2+x)Mm2x/(2+x) Dz/(Z-I-x) 0 + X/2 C02 ‘1' H20 A~(>800°C) Y (1-3x/2) M"O + x/2 ManzO4 Figure 1.6. Thermal decomposition and layer reconstruction modes of the LDH carbonates. 29 Special interests have been placed on the third and fourth stages of the thermally formed metal oxide systems for their potential uses in the catalytic applications.1 The mixed metal oxide obtained after heating at 400 - 600°C has a defect NaCI-like structure which cafi be reconverted to the original LDH structure upon hydration. And the final stage of thermal treatment results in basic metal oxide and spinel oxides. Several reports have already shown their catalytic activities as catalysts and catalyst precursors in many reactions1 such as Ziegler-Nana and Friedel-Craft catalyses,34t49 ~Aldol condensations,50 and synthesis gas—to-methanol production-51. In addition, LDHS have recently found increasing interest as fast proton9-24 and ionic25 conductors. Medical applications for LDHS are a promising area as well. For example, the capability of Mg-Al LDH carbonates to act as a long-term buffer makes them potential antiacids.52 Application as molecular sieving materials for gas separation was also reported.33:53 A further practical application may be found in the production iof ceramic aluminum nitride from hydrotalcite poly(acrylonit1ide) complexes.54 30 D. lntercalation and Pillaring Chemistry There have been significant efforts to prepare'LDHs containing anionic species other than carbonate anion as a step for the materials applications of this class of compounds. Some of the molecular species intercalated into LDH layers to date, mostly by Simple inorganic anion exchange rcactions, are presented below. Other preparation methods of LDH intercalates also have been developed: calcined LDH precursor method, synthetic meixnerite method and direct coprecipitation method i) inorganic anions: halide ions (F', Cl', Br, and 1'); small oxo anions (C0323 NO3', ClO4‘, 8042‘, S2032”, and CrO42-, etc.); inorganic coordination compounds (NiCl42', CoCl42', Fe(CN)53', Fe(CN)64—, etc.); silicate anion (SiO(OH)3‘) ii) polyoxometalates: 111/10702416. [V 1002816”, [H2W1204016. [PV3W904ol‘h [SiV3W904ol7'. [BVW11040173 [SiW1103918h BIC- 31 iii) organic anions: (di)carboxylates (adipate, oxalate, succinate, benzoate, phthalate, terephthalate, etc.); anionic surfactants (alkyl sulfonates, alkyl sulfates, etc.); metallomacrocycles (Co(II) or Cu(II) phthalocyanine tetrasulfonate, Mn(III)-meso-tetrakis-(2,6—dichloro—3-sulfonato- phenyl)porphyrin, etc. ). The number and orientation of the interlayer anions are determined mainly by the electrostatic and H-bonding interactions between the intercalants and the LDH layers. Interlayer distance of the LDH intercalates can be calculated by subtracting the thickness of the host layer (~48 A) from an observed basal spacing. I Considerable progress in the intercalation of macro organic41t43-45-46 and inorganic55'65 molecular anions has been made mainly for their potential uses as molecular sieving materials and shape selective catalysis. The importance of pore regulation as well as variation of the intercalant chemical activity in catalytic reactions has been claimed for the pillared derivatives of the LDHS with polyoxometalates (POMs)37-55-59»53-54 and metallomacrocycles45. 32 E. Characterization In this section a brief overview on some general characterization methods used for LDH derivatives is described Much of the details on the theoretical and experimental aspects of the following materials characterization methods can be found and were extensively reviewed elsewhere. Scanning electron micrograph (SEM) and transmission electron micrograph (T EM) provide general pictures of textural and crystal morphologies of the LDH intercalates. TEM study usually provides more informations on the LDH structures; for example, interlayer distance can be calculated and compared with the X-ray data, when it is used at high magnification. Electron diffraction patterns due to the layer atomic constituents as well as interlayer species also can be obtained by the selected area electron diffraction (SAED) study at TEM mode. Based on the obtained SAED pattern, one can estimate the lattice dimension by the Bragg's relation and the small angle approximation: d-— “do where dhkz = the lattice dimension in A L = the camera length in cm A = the electron wavelength, e.g., 0.037 A for 100 kV D = the measured length on the diffractogram in cm 33 Energy Dispersive Spectroscopy (EDS) also is available for a semiquantitative elemental analysis of the products, in addition to the more quantitative one of Inductively Coupled Plasma (ICP) elemental analysis. X-ray diffraction (XRD) study provide a quick and convenient way of characterizing LDH intercalates. XRDS of oriented film samples, prepared by drying a water suspension of a LDH intercalate on a microscope glass slides, or of powder samples provide a direct estimation of the basal Spacing and (001) Bragg reflections of that intercalate as well as some non-( 001) reflections. Degree of the turbostratic layer stacking, a layer stacking with a very poor alignment along the ab planes, and layer rigidity after intercalation partly affected by the interpillar distance are the main factors of the line broadening in the x-ray reflections of this class of materials.56 Crystalline domain of LDH intercalates also can be calculated from the line width at half maximum and intensity of x-ray diffraction peaks according to the Scherrer equation: where L is the mean crystalline domain in A along the c-axis, )t is the wavelength of the x-ray source, K is a constant very near to unity and B is the width of a 20 reflection at half-height in radians. Adsorption-desorption isotherms of a probe molecule can provide quantitative information on the textural properties such as surface area and 34 pore structure of the LDH intercalates. The surface area can be obtained from the adsorption branch by applying the BET equation. 67 l C-1_P_+ l Vads(P0-P) VmC P0 VmC where Vm = monolayer capacity in cm3 (adsorbate)/ g (adsorbent) P 2 pressure of the adsorbate P0 = saturated vapor pressure of the adsorbate C = an energetical constant dependent on the adsorbate's hcats of adsorption and condensation Most adsorption isotherms fit the BET equation in the range of P/Po = 0.05 to 0.25 where the correlation coefficient is near one. Once the Vm value is calculated from the plot of the above equation, the BET surface area (SBEI‘. m2/ g) can then be calculated from the following equation. ‘ vaAe. * SEEP I W = 4.37 Vm where V838 = the volume of one mole of gas, 22414 cm3/mol N = Avogadro's number, 6.02291023 Acs = the cross sectional area of adsorbate molecule in the completed monolayer, 16.281020 m2 for N2 35 Comparing the adsorption isotherm of a porous solid with a standard adsorption isotherm for the same adsorbate on a non-porous solid can give the so called t-plot.68 The t-plot is a plot of the adsorbed volume on a sample versus the statistical thickness of the adsorbed. layer on the non- porous reference. The values of the Standard adsorbate layer statistical thickness, t, are obtained with the help of the BEI‘ theory. The t-plot is valid up to P/Po = 0.4. This method provides the total surface area (St), the non-microporous surface area (Snomp), and the microporous volume (V11) for a solid sample. For a non-microporous solid, a plot of the adsorbed volume versus t, which is a function of NH), will be a straight line passing through. the origin. The slope of this straight line is proportional to the surface area as following. St = 0*K*bto = 15.47.10 where St = the total surface area in m2/ g o = the thickness of a Single molecular layer of N2 molecules, 3.54 A K = constant, 4.37 m2/cm3 bu) = the slope of the line passing through the origin of the t-plot For microporous solids the t-plot exhibits two distinct regions with different slopes. The first region of the t-plot can be used to calculate the specific surface area of the solid as above. The second domain of the two regions (t s 6.0) can be fitted to another straight line. The intercept of this 36 second line gives the gaseous microporous STP volume (Vs'rp) which can be converted to the microporous liquid volume (Vuaiq) = 0.00154 Vs'rp) or the equivalent microporous surface area (8,, = KVgrp). The slope of this second line (bu) provides the non-microporous, mesopore plus macropore surface area (Snow. 2 oszbu = 15.47 8 bd). While the adsorption isotherm can be usedfor the BET equation and t-plot, the desorption isotherm is useful for determining pore size distribution of the solid. Pores in solids are classified as following, based on their diameter, d: Micropores: d < 20 A Mesopores: 20 A < d < 500 A Macropores: d > 500 A Pillared layered materials usually exhibit a broad range of their pore sizes coming from gallery micropore as well as textural meso and macropores, while zeolites usually Show a narrow distribution of intracavity microporesfi9 A recent study on the importance and application of the mesopores in catalysis showed the benefit of the presence of mesopores as well as micropores, which is usually the case in pillared lamellar solids.70 Mesopore distribution of a solid is usually determined from the desorption branch, while the micropore Size distribution can be determined from the Horvath-Kawazoe plot using the adsorption branch. 37 Fourier transformed infrared (FTIR) and nuclear magnetic resonance (NMR) studies on the LDH compounds often provide useful information on the interlayer molecular configuration and molecular interaction associated with the intercalants. Most frequently used was the FTIR technique for molecular oxo compounds from" C032“, S042“, and Cr042' to polyoxometalates. NMR was also used to confirm the molecular identity after a intercalation reaction as well as to look at the metal coordination environment of the host layer. Thermal analysis of thermogravimetric analysis (TGA) and differential thermal analysis (DTA) is also valuable in characterizing the thermal decomposition mode of LDH intercalates. A stepwise loss of weight, starting with the removal of water molecules, was usually observed in TGA on continuous heating. 38 II. A Review on Polyoxometalates A. Introduction Inorganic metal-oxy gen cluster anions of polyoxometalates (POMS) ' form an unique class of compounds with topological and electronic versatilities.71:72 These clusters of POMS, so called isopoly- and heteropolyanions, contain highly symmetrical core assemblies of M0,, units (M = M0, W, V, and etc.). POMS are important models for elucidating the biological and catalytic reactivity of metal-chalcogenide clusters, because of the wide range of metal-metal interactions in the oxo—clusters from very weak to as much strong as metal-metal bonding which are controlled by choice of different metals (3d, 4d or 5d), degree of reduction and extent of protonation.71'74 POMS have found many applications in analytical and clinical chemistry, catalysis, biochemistry (i.e., electron transport inhibition), medicine (i.e., antitumoral, antiviral and anti-HIV activities) and solid state devices.71'74 39 B. Structure and Reactivity Isopolyanions are POMS of high nuclearity formed mainly by the addenda atoms of Group V and VI metals such as V, Nb, Ta, Mo and W. They are normally made by acidification of the anions [MO4]D' (M = V, Mo and W) or, with Nb or Ta by dissolving M205 molecular species in alkaline solution. Heteropolyanions are also usually highly symmetric and are closely related structurally to the isopolyanions, with the hetero atoms often being enclosed by a cavity in the parent isopoly species. Isopoly- and heteropolyoxoanions of the early transition elements are represented by the general formulas [NImOy]P' Isopolyanions [XmeOqu' (X S m) Heteropolyanions where M is usually molybdenum (VI) or tungsten (VI), less frequently vanadium, niobium or tantalum, or mixture of these elements, in their highest oxidation states. Such polyoxometalate (POM) anions form a variety of structurally distinct metal oxide complexes based on quasi- octahedrally-coordinated metal atoms.71-72 Many POMS of isopoly- and heteropolyanions have been prepared and isolated from both aqueous and nonaqueous solutions. And the 4O structures of POMS are governed by the electrostatic and ionic radius-ratio principles. POMS therefore can be described as assemblages of metal- centered M0,. polyhedra that are linked by shared comers, edges and faces where no addenda M06 octahedra can exceed more. than two unshared oxygen atoms. There are two types of different M06 units found in POM structures, as shown in Figure 1.7.75 Type I has only one terminal oxygen atom, while Type II has two terminal oxygen atoms. Many of the W and Mo POMS are known to be on larger and neutral Mn03n cages that encapsulate anionic subunits and are linked to them only by weak (> 2.2 A) bonds. For example, [W5019]2- of the Type I POM contains a W5013 cage encapsulating 02'. [W5019]2- therefore can be expressed as [(W5013)(02' )l. [PM012040P' as [(M012036)(P043')l and [P2M01806216‘ as [(M013054)(PO43')2]. In the limit an infinite large cage is formed by the linking of the Type I M06 units, as in M003-2.I'I2O.76 Consecutive linkages of Type II octahedra were found in the POMS as [MO4016]4' where the (022-)2 was encapsulated in the M04012 ring and thus expressed as [M04012)(022‘)2l- POMS of [82M0502414' as [(M05015)(3032')l and [P4W3O4o]12- as [(W3024)(PO43')4] belong to the same category. In the limit of a large ring of Type II polyanion an infinite chain structure of the composition M03 was obtained, as in the M003 compound.77 41 .= 25. av eee _ 8E5 8.8.38 52 a 8.5 228% 6.5... .5 Beam Amv A Cr(OI-I)3 + 6M0042- + 6H20 although the pH at which such reactions are rapid and/or complete may vary widely depending on the POM involved. POMS are often much more stable towards H3O+, and numerous crystalline heteropoly acids are known. Crystalline heteropoly acids and salts are frequently highly hydrated forms with up to 50 molecules of water per anion. Much of this water is zeolitic in nature. A broad spectrum of heteroatoms can be incorporated in heteropolyanions and the heteroatoms are occasionally stabilized in usual oxidation state or coordination geometries. Organic, organometallic- and metal-metal bonded derivatives of POMS also have been prepared. Many POMS are 46 strong oxidizing agents and undergo multiple reversible one- or two- electron reductions leading to intensely colored mixed valence species known as heteropoly blues. POMS are known which can accept as many as 32 electrons without major structural changes. Many materials applications of POMS such as ion exchangers, ion-selective membranes and catalysts have been focused on their redox and ion-exchange properties, and their high charges and molecular weights. [M6019ln' (A) 47 [XM120421n' (E) [X2M60181n' (b) 8 “a 83 a Figure 1.8. Polyoxometalate structures of the five common types. 48 3H ‘ ’lthO‘O‘ (rd) \ ‘ 1: (Id) B-lXMgO34l (C310 Isomers Locuncry species Figure 1.9. Keggin structure of the [XM12O4o]n' type and related structures of capped, lacunary and isomeric forms. 49 .Sozeoa .8:er 05 can $.36" :20": :25 Ewqu .9258. 05 .«o moat—88¢ .34 2am.”— mem 111'. Ham 50 C. Catalytic Applications Polyoxometalates (POMS) have been widely used as homogeneous and heterogeneous acid and oxidative catalysts by virtue of their unique physicochemical properties.71-72t33r85 They are also of great interest as model systems for the study of fundamental problems of catalysis.35-37 That is, polyoxometalate surfaces are models for metal oxide surfaces and the oxidation activity can be understood in terms of the loss of surface oxygen atoms and delocalization of the remaining electrons over several metal atoms under relatively mild conditions. Acid catalysis by POMS can be achieved by, for example, heteropolyacid (HPA) compounds of the type Hg-x[XXMV112O4o] with the Keggin structure. The high Bronsted acidity of HPAS, greatly surpasses the strength of the traditional acid catalysts such as aluminosilicates, . 1'13P04/Si02, etc., is of fundamental importance for catalysis.83 They are even much more active than the zeolite HY in the dehydration of 2- propanol.88 POMS usually have the advantages of being nonvolatile, odorless and thermally stable, compared with other conventional acids. Since HPA are readily soluble in water and oxygen-containing solvents and their stability in the solid state is fairly high, they are used as both homogeneous and heterogeneous catalysts. Table 1.4 shows some of the POM-catalyzed acid reactions. 51 Table 1.4. Acid catalysis by heteropolyacids of 1134470412040] abbreviated as XM12. Reaction Catalyst . Reference CH3CH=CH2 '1' H2O —’ PW 12, Sinz, PMO 12 89 CHBCH(OH)CH3 ' C6I'15C-CH 1+ H2O "" PW l2, PM012 ”,91 C6H5COCH3 CH3CH(0H)CH3- W12 92 CH2=CHCH3 '1' H20 (eighteen2 + CH3OH —- ' PW 12/Si02, PW12/C, SiW12/C 93,94 (CH3)2COCH3 C2H50H + HOAC -’ PW12/C, SinC 95 C2H50AC + H2O ‘ CH3CH(CH2)COOH _’ PW12, Sinz 96 CH2=CHCH3 + C0 4’ H2O 52 In the crystal lattice there are nonlocalized protons rapidly exchanging with the protons of the crystal water molecules of the so called secondary structure and with the protons localized. on the 112-bridging oxygen atoms of the POMS. When the water of crystallization is removed, dehydration takes place and all the protons become localized. Thus water plays an important role in the formation of the proton structure of crystalline HPAS.97 The hydrated protons in HPA hydrates with a high water content exist in the form of dioxonium ions H502+ and more complex hydrates H+(I-'I20)n.97»98 The H502+ ion appears as a result of the formation of the strong quasi-symmetrical hydrogen bond, Hzo...H+...OHZ. In aqueous solution the compounds are strong acids which are sometimes completely dissociated. Upon reduction the anion basicity increases and, consequently, the anion has a tendency to become protonated. Since the acid centers are considered to be at the bridging oxygens on the POM surface, surface electronic structure and electron density of a POM are considered as important factors governing the acidic properties of POMS.71 In heterogeneous catalysis HPA and their metal salts are used as massive and deposited catalysts. The best carriers are neutral supports such as silica gel and activated charcoal. The properties of catalysts based on the metal salts depend significantly on the nature of the metal cations, the degree of substitution of hydrogen in the acid by the metal, and the solubility of the salt. By virtue of the rapid absorption of polar molecules, the catalytic reactions can occur not only on the surface but also ‘on the S3 bulk of the massive HPA. With regard to polar substances, solid HPAS behave similarly to highly concentrated solutions, so-called the ”pseudo- liquid phase".99-100 In contrast to polar molecules, non-polar ones are incapable of being absorbed in the bulk of the HPA; they interact only with the surfaces of the catalyst. . The use of POMS as oxidation catalysts is based on the reversibility of their redox reactions and their ability to enter into many-electron oxidation-reduction reactions. Many POMS are fairly strong and many- electron oxidants. They are reduced by various reductants both electrochemically and photochemically. On reduction the HPAS are converted into heteropoly blues, mixed valence complexes which are colored intense blue as a result of intervalence electronic transitions, generally with the retention of the initial Structure of the HPA. The addition of electrons increases the basicity of the POM and may be accompanied by its protonation. 101 The oxidative properties of POMS are closely related to their acid properties. The degree of protonation of the POM increases with increasing basicity of the initial POM anion and the degree of reduction with decrease of the pH of the medium. The heteropolyanions are protonated preferentially on reduction at the bridging oxygen atoms probably in the angular M-O-M linkages. The basic reversible redox reactions involved in substrate oxidations are shown as following, where [HPA]ox and [HPA]red signify the oxidized and reduced forms of a heteropolyacid (HPA) anion: 5 4 [HPA]ox + Substrate + xH+ «- qunwmd + Oxidized Substrate Hxlm’Alted + x/4 02 9 [HPAon + x/2 H20 The oxidative properties of HPA, widely used as catalysts of liquid phase oxidation, depend on the nature of both the ligands and the central atom. Some of the heterogeneous and homogeneous oxidation reactions catalyzed by POMS are listed in Table 1.5. In principle, with HPAS it is possible to achieve bifunctional oxidative and acid catalysis. It also has been reported that, together with the oxidative properties of the HPAS, their acidity plays a significant role in the oxidation of methacroleine.106 By altering the composition of the HPAS, it is also possible to vary independently and within wide limits their oxidizing and acidic properties. 107 55 Table 1.5. Some reported oxidation reactions catalyzed by the heteropolyacids of H3+n [PM012-nVnO4o] (HPA-n, where n = 2-8) + PdII system (PdII in the form of PdSO4, Pd(0Ac)2, or PdCl2. Reaction Substrate “' '1 A Reference (1,114 + HOAc -+ ethylene 102 AcOCHz-CHZOH R1R2CHOH -' R1 = H’ R2 = Me, Pr; R1R2G=O R1 =3 MC, R2 = Ht; 103 ' R1 = R2 = M6 ArH+ HOAc-' ArH = benzene, toluene 104 ArOAc + H20 ‘ ArH + 02114 -’ ArH = benzene, furan 105 ATCH=CH2 '1' H20 56 References 1. Cavani, F.; Trifiro, F.; Vaccari, A. In Catalysis Today; Elsevier: Amsterdam, 1991; Vol. 11, pp 173 and references therein. 2. Reichle, W. T. Chemtech 1986, 58. 3. Reichle, W. T. Solid State Ionics 1986, 22, 135. 4. Carrado, K. A.; Kostapapas, A.; Suib, S. L. Solid State Ionics 1988, 26, 77. 5. De Roy, A.; Forano, C.; E1 Malki, K.; Besse, J.-P. In Expanded Clays and Other Microporous Solids; Van Nostrand Reinhold: New York, 1992, 1992; pp 108. 6. Miyata, S. Clays Clay Miner. 1980, 28, 50. 7. Taylor, R. M. Clay Miner. 1984. 19, 591. 8. Reichle, W. T. J. Catal. 1985, 94, 547. 9. De Roy, A.; Besse, J.-P.; Bondot, P. Mater. Res. Bull. 1985, 20, 1091. 10. Hansen, H. C. B.; Taylor, R. M. Clay Miner. 1990, 25, 161. 11. Park, I. Y.; Kuroda, K.; Kato, C. Solid State Ionics 1990, 42, 197. 12. Miyata, 8.; Kumura, T. Chem. Lett. 1973, 843. 13. Gastuche, M. C.; Brown, G.; Mortland, M. Clay Miner. 1967, 7, 177. Air—4 14. 15. 16. 17. 18. 19. 20. 21. 24. 26. 27. S7 Miyata, S.; Kumura, T.; Hattori, H.; Tanabe, K. Nippon Kagaku Zasshi, 1971, 92, 514. Sato, T.; Fujita, H.; Endo, T.; Shimada, M. React. Solids 1988, 5, 219. Brindley, G. W.; Kikkawa, S. Am. Miner. 197 9, 64, 836. Kruissink, E. C.; van Reijen, L. L.; Ross, J. R. H. J. Chem. Soc., Faraday Trans. 1, 1981, 77, 649. Del Piero, G.; Di Conca, M.; Trifiro, F.; Vaccari, A. In Reactivity of Solids; Elsevier: Amsterdam, 1985; pp 1029. Pausch, 1.; Lohse, H. H.; Schiirmann, K.; Allmann, R. Clays Clay Miner. 1986, 34, 507. Mascolo, G.; Marino, 0. Miner. Mag. 1980, 43, 619. Taylor, H. F. W. Miner. Mag. 1969, 37, 338. . Allmann, R. Acta. Cryst. 1968, B24, 972. . Allmann, R. Chimia 1970, 24, 99. Lal, M.; Howe, A. T. J. Chem. Soc., Chem. Commun. 1980, 737. . Lal, M.; Howe, A. T.; J. Solid State Chem. 1981, 39, 377. Vicente, M. A.; Sanchez-Camazano, M.; Sanchez-Martin, M. J .; Del. Arco, M.; Martin, C.; Rives, V.; Vicente-Hemandez, J. Clays Clay Miner. 1989, 37, 157. Ogawa, M.; Kuroda, K.; Kato, C. Chem. Lett. 1989, 1659. 28. 29. S 8 Pinnavaia, T. J. Science 1 983, 220, 365 and references therein. Taylor, H. F. W. Miner. Mag. 1973, 39, 376. 30. Marcelin, G.; Stockhausen, N. J.; Post, J. F.; Schutz, A. J. Phys. Chem. 31. 32. 33. 35. 36. 37. 38. 39. 41. 42. 1989, 93, 4646. Allmann, R. Neues Jahrb. Min., Monatsh 1969, 552. Boehm, H. P.; ,Steinle, 1.; Vieweger, C. Angew. Chem. Int. Ed. Engl. 1977, 16, 265. ‘ Miyata, S.; Hirose, T. Clays Clay Miner. 1978, 26, 441. . Miyata, S. Clays Clay Miner. 1983, 31, 305. Giannelis, E. P.; Nocera, D. G.; Pinnavaia, T. J. Inorg. Chem. 1987, 26, 203. Suzuki, E.; Idemura, S.; Ono, Y. Clays Clay Miner. 1989, 37, 173. Bhattacharyya, A.; Hall, D. B. Inorg. Chem. 1992, 31, 3869.‘ Salinas-Lopez, E.; Ono, Y. Micro. Mater. 1993, 1, 33. Kopka, H.; Beneke, K.; Lagaly, G. J. Coll. Interface Sci. 1988, 123, 427. . Chibwe, K.; Jones, W. J. Chem. Soc., Chem. Commun. 1989, 926. Park, I. Y.; Kuroda, K.; Kato, C. Chem. Lett. 1989, 2057. Meyn, M.; Beneke, K.; Lagaly, G. Inorg. Chem. 1990, 29, 5201. 43. 45. 47. 49. 51. 52. 53. 55. 59 Park, I. Y.; Kuroda, K.; Kato, C. J. Chem. Soc. Dalton Trans. 1990, 3071. . Sato, T.; Okuwaki, A. Solid State Ionics 1991, 45, ‘43. Chibwe, M.; Pinnavaia, T. J. J. Chem. Soc., Chem. Commun. 1993, 278. . Carrado, K. A.; Forman, J. E.; Botto, R. E.; Winans, R. E. Chem. Mater. 1993, 5, 472. Sato, T.; Kato, K.; Endo, T.; Shimada, M. React. Solids 1986, 2, 253. . Sato, T.; Fujita, H.; Endo, T.; Shimada, M. React. Solids 1988, 5, 219. Brindley, G. W.; Kikkawa, s. Clays Clay Miner. 1980, 28, 87. . Reichle, W. T. J. Catal. 1980, 28, 50. Herman, R. G.; Klier, K.; Simmons, G. W.; Finn, B. P.; Bulko, J. B.; Kobylinski, T. P. J. Catal. 1979, 56, 407. a) Miyata, S. Kagaku Gijutsuhi 1977, MOL 15(10), 32. b) White, J. L.; Hene, S. L. Ind. Eng. Prod. Res. Dev. 1983, 22, 665. Pinnavaia, T. J.; Rameswaran, M.; Dimotakis, E. D.; Giannelis, E. P.; Rightor, E. G. Faraday Discussion Chem. Soc. London 1989, 87, 227. . Sugahara, Y.; Yokoyamd N.; Kuroda, K.; Kato, C. Ceram. Int. 1988, 14, 163. Kwon, T.; Tsigdinos, G. A.; Pinnavaia, T. J. J. Am. Chem. Soc. 1988, 110, 3653. 56. 57. 58. 59. 60. 61. 62. 63. 60 Drezdzon, M. A. Inorg. Chem. 1988, 27, 4628. Kwon, T.; Pinnavaia, T. J. Chem. Mater. 1989, 1, 381. Chibwe, K.; Jones, W. Chem. Mater. 1989, 1, 489: Twu, J.; Dutta, P. K. J. Phys. Chem. 1989, 93, 7863. Dimotakis, E. D.; Pinnavaia, T. J. Inorg. Chem. 1990, 29, 2393. Twu, J.; Dutta, P. K. Chem. Mater. 1992, 4, 398. Wang, J.; Tian, Y.; Wang, R.-C.; Clearfield, A. Chem. Mater. 1992, 4, 1276. ' Kwon, T.; Pinnavaia, T. J. J. Mol. Catal. 1992, 74, 23. 64. Tatsumi, T.; Yamamoto, K.; Tajima, H.; Tominaga, H.-O. Chem. Lett. 65. 67. 1992, 815. Narita, E.; Kaviratna, P. D.; Pinnavaia, T. J. J. Chem. Soc., Chem. Commun. 1993, 60. . Moore, D. M.; Reynolds Jr., R. C. X-Ray Diffraction and the Identification and Analysis of Clay Minerals; Oxford: New York, 1989. Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309. . De Boer, J. H.; Lippens, B. C.; Linsen, B. G.; Broekhoff, J. C. P.; Van den Heuvel, A.; Osinga, Th. J. J. Coll. Interface Sci. 1966, 21, 405. 69. 70. 71. 72. 74. 75. 76. 77. 78. 79. 80. 81. 82. 61 Behrens, P. Adv. Mater. 1993, 5, 127. Butruille, J.-R.; Pinnavaia, T. J. Catal. Today 1992, 14, 141. Pope, M. T.; Muller, A. Angew. Chem. Int. Ed. ‘Engl. 1991, 30, 34 and references therein. 3 Pope, M. T. Heteropoly and Isopoly Oxometalates; Springer-Verlag: New York; 1 983 and references therein. . Day, V. W.; Klemperer, W. G. Science 1985, 228, 533. Hagen, K. S. Angew. Chem. Int. Ed. Engl. 1992, 31, 1010. Pope, M. T. Inorg. Chem. 1972, 11, 1973. Krebs, B. Acta Cryst. 1972, B28, 2222. Kihlborg, L. Ark. Kemi. 1963, 21,3. ' Pettersson, L.; Andersson, 1.; ohman, L.-O. Inorg. Chem. 1986, 25, 4726. Contant, R.; Tézé, A. Inorg. Chem. 1985, 24, 4610 Alizadeh, M. H.; Harmalker, S. P.; Jeannin, Y.; Martin-Frere, J.; Pope, M. T. J. Am. Chem. Soc. 1985, 107, 2662. Jorris, T. L.; Kozik, M.; Baker, L. C. W. Inorg. Chem. 1990, 29, 4584. Knoth, W. H.; Domaille, P. J.; Harlow, R. L. Inorg. Chem. 1986, 25, 1577. 85. 89. 91. 93. 94. 95 62 Misono, M. Catal. Rev. Sci. Eng. 1987, 29, 269. Garcia-Gariby, M. A.; Zhang, 2.; Turro, N. J. J. Am. Chem. Soc., 1991, 113, 6212. Fox, M. A.; Cardona, R.; Gaillard, E. J. Am. Chem. Soc. 1987, 109, 6347. Fournier, M.; Louis, C.; Che, M.; Chaquin, P.; Masure, D. J. Catal. 1989, 119, 400. Masure, D.; Chaquin,.P.; Louis, C.; Che, M.; Fournier, M. J. Catal. 1989, 119, 415. . Otake, M.; Onoda, T. Shokubai 1976, 18, 169. Onoue, T.; Mizutani, Y.; Akiyama, S.; Izumi, Y. Chemtech 1978, 8, 432. . Otake, M. J. Synth. Org. Chem. Japan 1981, 39, 385. Matsuo, K.; Urabe, K.; Izumi, Y. Chem. Lett. 1981, 1315. . Otake, M.; Onoda, T. Shokubai 1975, 17, 13. Igarashi, A.; Matsuda, T.; Ogino, V. J. Japan Petrol. Inst. 197 9, 22, 331. Ono, Y.; Baba, T. In VIIIth International Congress on Catalysis; Verlag Chemie: Berlin, 1984; Vol. 5, pp 405. Izumi, Y.; Hasebe, R.; Urabe, K. J. Catal. 1983, 84, 402. 63 96. Otake, M.; Onoda, T. J. Catal. 1975, 38, 494. 97. Spitsyn, V. I.; Torchenkova, E. A.; Kazanskii, L. P. In Advances in Science and Engineering, Inorganic Chemistry; Moscow, 1984; Vol. 10, pp 65. ' 98. Brown, G. M.; Noe-Spirlet, M. R.; Busing, W. R.; Levy, L. A. Acta Cryst. 1977, 33, 1038. 99. Misono, M.; Sakata, K.; Yoneda, Y.; Lees, W. Y. In VIIth International Congress on Catalysis, Preprints; Tokyo, 1 9 80; Vol. 27. 100. Misono, M. In Catalysis by Acids and Bases; Elsevier: Amsterdam; 1985, pp 147. 101. a) Varga, G. M.; Papaconstantinou, E.; Pope, M. T. Inorg. Chem. 197 0, 9, 662. b) Papaconstantinou, E.; Pope, M. T. Inorg. Chem. 197 0, 9, 667. 102. Likholobov, V. A.; Volkhonskii, M. G.; Ermakov, Yu. 1.; Matveev, K. 1.; Kuznetsova, L. I. USSR Patent 504754, 197 6. 103. Kozhevnikov, I. V. Dokl. Akad. Nauk SSSR, 197 7, 235, 1347. 104. Taraban'ko, V. E. React. Kinet. Catal. Latt. 197 8, 8, 77. 105. Taraban'ko, V. E. Kinetika i Kataliz 197 8, 19, 1160. 106. Misono, M. In VIIth International Congress on Catalysis, Preprints; Tokyo, 1980; B27. 64 107. Niyama, H.; Saito, Y.; Echigova, E. In VIIth International Congress on Catalysis, Preprints; Tokyo, 1 980; C13. CHAPTER TWO LAYERED DOUBLE HYDROXIDES INTERLAYERED BY POLYOXOMETALATE ANIONS WITH. KEGGIN (a-H2W120406'), DAWSON (a-P2W130525') AND FINKE (C04(H20)2(PW9O34)210') STRUCTURES 65 Abstract The pillaring reactions of Mg3Al layered double hydroxides (LDHS) were studied for polyoxometalates (POMS) with Keggin (a-H2W12O406), Dawson (a-P2W130625), and Finke (C04(I-I2O) 2(PW9034)210-) structures. When pre-Swelled with adipate anions, LDH hydroxide precursors afforded highly crystalline pillared products at high reaction temperature (100°C). The structural, thermal and textural properties of the LDH-POM intercalates were elucidated based on XRD, FTIR, TEM, EDS and N2 adsorption- desorption studies. Depending on pillaring condition different gallery orientations were observed for the Dawson- and Finke-structure POMS. The differences in orientations were rationalized in terms of different electrostatic and hydrogen-bonding interactions between the POM pillars and the LDH layers. A gallery height of ~10 A was observed for the spherical Keggin- POM intercalated LDH, whereas two different gallery heights of 14.5 A and 12.8 A were found for the cylindrical Dawson-POM intercalated LDHS prepared at room temperature and boiling temperature, respectively. The cylindrically shaped Finke-POM also exhibited two different gallery heights of 13.3 A and 12.6 A, depending on reaction temperature. However, upon thermal treatment at 2 100°C, the Dawson and Finke ion-intercalated LDHS exhibited only one type of gallery POM orientation, regardless of preparation conditions. The gallery microporosity and crystalline layered structure of each LDH-POM intercalate were retained up to 200 °C. Another series of XRD, EDS and N2 adsorption-desorption studies indicate that the broad XRD reflections centered at ~11 A are most probably responsible for the interphase POM salts precipitated on the external surfaces of the LDH crystals. 67 A. Introduction There has been considerable interest in the design and applications of microporous pillared solids with "supergallery" structures 1'3 wherein the interlayer thickness or gallery height is substantially larger than that of the host layers.2 This structural characteristic results in a two-dimensional, microporous intercalate in which most of the unit cell volume is contained in the chemically accessible gallery region rather than in the dense host layers. Among the various classes of ionic lamellar solids, layered double hydroxides (LDHS) are uniquely suited for pillaring by polyoxometalate (POM) anions, because they are the only known family of layered materials with permanent positive charge on the layers. Owing to the diversity of approaches to the design of these nanoscaled materials,4'll a broad spectrum of physicochemical properties are possible. Most importantly, pillared microporous derivatives are accessible, by mediating the charge density of the host layers (lateral poresize control) and the size and charge of the pillaring POMS (longitudinal and lateral poresize control). Approaches to the preparation of pillared LDH-POM intercalates have been limited to date to the Size of XM12O4on' Keggin-structures,4'11 which have a diameter (~10 A) approximately two times as large as the LDH layer thickness (~4.8 A). LDH derivatives pillared with larger POMS than Keggin ions are potentially more interesting for achieving increased access to the gallery space. 68 In the present work, we have undertaken a systematic approach to the synthesis of crystalline LDH intercalates interlayered by Dawson (01- P2W130625) and Finke (C04(1-I2O)2(PW9034)210') structures with molecular diameters much larger than the 4.8 A, as well as the Keggin ion (a—H2W12O406) structure. AS illustrated in Figure 2.1, each class of the POM can impart a different gallery pore structures upon pillaring reaction, resulting in a new members of LDH-POM derivatives. a-H2W120406' with the T 0 symmetry of the (Jr-Keggin ion structure has a nearly spherical Shape, which will limit the possible orientations and gallery heights upon intercalation in LDH galleries. On the other hand, the other two POMS with lower molecular symmetries can provide gallery micropores of different sizes, depending on their interlayer orientations. For example, the parallel and perpendicular gallery orientations of a-P2W130626, with the D31, symmetry of the Dawson structure, and of C04(H2O) 2(PW9034)210', with the C21, symmetry of the Finke structure, can result in 2.1 A and 5.0 A differences, respectively, in the pore size along the c direction of a layered LDH-POM intercalate. Differences in pillaring method, reaction temperature and time, and the use of preswelling agent were considered in the present work as factors governing the intercalation of POM. The structural, thermal and textural properties of the pillared products were examined based on XRD, FT IR, TEM, EDS and N2 adsorption-desorption studies. The interlayer orientation of the gallery POMS was determined and rationalized in terms of electrostatic and hydrogen-bonding interactions between the gallery POMS and the LDH layers. Also, the nature of an impurity co-product formed in the intercalation reactions, which has been a subject of differing interpretations9-1042, was clearly demonstrated in the present work. 69 DEBS? 3050330 53 2.2 .528 SEE new c8389 3 use .3088? 305%? 530: .33 22 eEOm £33. ,3 eons—E 33833.0 In: .3 532:8 05 mo sou—3:80am.— otmEonom .mm 0.53m ill. . D D D 8230;522:0460 .5 83023".— oU D .620.— .85510 a $808- ??N: . .622 auteeem a .Nnoo e6 .-noz.._oa . | a 8 U s M A4 _ coo-00000000000000 70 B. Experimental 1. Materials Rcagent grade chemicals were used as received. Na2WO4-2H2O was a gift from GTE Sylvania Company. Adipic acid was purchased from Fisher. H2WO4, Na2HPO4-7H2O and other reagents were purchased from Aldrich. Deionized water was used for the preparation of all aqueous solutions. The pH of reaction mixtures was monitored by 3 Fisher Model 750 selective ion analyzer. The ammonium salt of 01-H2W1204o6' and the potassium salts of a-P2W180626’ and C04(H2O) 2(PW9O34)210- were prepared by literature methodsl3'15. Mg3Al LDH Carbonate. An M g3Al LDH carbonate was prepared by a coprecipitation method at constant pH15. A 250-mL quantity of 1.0 M mixed metal nitrate solution of Mg2+ and Al3+ (Mg2+/Al3+ = 3.0) was added with vigorous stirring into 500 mL of water pre-adj usted with NaOH to pH 10.0 at 40°C. The reaction pH was kept at 10.0 (:0.1) by the co- addition of a mixed solution of 1 M Na2CO3 and 2 M NaOH (CO32'/Al3+ = 1.5). Upon the completed addition of the mixed base solution the reaction pH was controlled by the addition of 2 M NaOH solution. The resultant white suspension was further stirred for 3h at 40°C and then the temperature was increased to 70°C for 40 h. The white product was then filtered, washed with deionized water and dried in air at room temperature. ICP elemental analysis and thermogravimetric analysis (TGA) gave the composition [Mg3.11Al(OH)8.221(C03)0.5 -98 H20. 71 Mg3Al LDH Hydroxide (meixnerite). An aqueous suspension of synthetic meixnerite (1.0 wt %) was prepared by the thermal decomposition of the corresponding Mg3Al LDH carbonate and the subsequent reconstitution of the resulting mixed metal oxide solid solution into an LDH hydroxide 17. Mg3Al LDH carbonate (1.0 g) was calcined in a quartz tube furnace for 5 h at 500°C under a N2 flow to form a solid oxide. The oxide then was transferred to a sealed flask containing 100 mL of degassed H2O to form the LDH hydroxide after a reaction time of 5 days. ICP Mg and Al analyses was indicative of the composition W8308AI(OI'D8.16](OH)1.O'XH20- Magnesium Hydroxide. Into 100 mL of vigorously stirred water preadjusted at pH 10.0 with 2 M NaOH was added slowly a 1.0 M Mg2+ nitrate solution (100 mL) at 40°C. The reaction pH was kept at 10.0 (10.1) by the addition of 2 M NaOH throughout the delivery of the nitrate solution and the subsequent 4 h reaction period at 40°C. The white precipitate was allowed to age for 40 h at 70°C. After cooling to room temperature the product was filtered, washed with water and dried in air at room temperature. 2. Pillaring of Mg3Al LDHS by POMS. Pillaring reactions were carried out under N2 to avoid contact with atmospheric CO2 during the reaction. Deionized and decarbonated water was used to prepare all aqueous solutions. POM solutions (0.01 M - 0.02 M) in three-fold excess of the LDH anion exchange capacity (AEC) were used for the pillaring reactions. 72 Figure 2 summarizes the six different synthesis methods that have been reported in the literature‘l'lo for the preparation of LDH-POM intercalates. In the present work, three of these pillaring methods were examined to compare the crystallinity of products pillared by a- H2W12O405 Ke g gin ions. Method 111. A mixed metal oxide, freshly formed by calcining a Mg3Al LDH carbonate at 500°C under N 2 for 5 h, was added all at once to the POM solution at room temperature, as described by Chibwe and Jones 7. The reaction mixture was then stirred for 30 h at room temperature under N2. Upon being centrifuged and washed, the product was dried in air at room temperature. Method IV. An aqueous meixnerite suspension was added to an adipic acid solution in two-fold excess of the AEC, in order to form the corresponding Mg3Al LDH adipate. After'being allowed to stir for 1 h at 50°C, the suspension was allowed to settle and then decanted, leaving a slurry of the LDH-adipate precursor. A 100 mL portion of degassed boiling water was added into the freshLDH-adipate slurry, and then the hot suspension was added slowly to the POM solution at 100°C under N2. After being allowed to stir for an additional 0.5 h at reflux temperature, the product was washed with water and dried in air. Method V. An aqueous suspension of synthetic meixnerite was added dropwise to a POM solution under N2 at room temperature, while the reaction pH was maintained at 4.5 by the co-addition of 0.01 M HNO3. After being allowed to stir for an additional 0.5 h, the product was washed with water and dried in air. In an alternative procedure an aqueous 73 meixnerite suspension was added dr0pwise to the POM solution at room temperature under N2. After the completed delivery of the meixnerite, the reaction mixture was stirred for an additional period (0.5 h or 5 h) at the same temperature without adjusting the pH. The solid product was then washed with water and dried in air. To examine the impact of reaction temperature on the product crystallinity, another set of reaction was carried at an elevated reaction temperature where the aqueous meixnerite suspension was added to the POM solution at 70°C under N 2. The reaction mixture was then allowed to stir for 5 h before being washed and dried in air. For 01-P2W130626 and C04(H2O)2(PW9O34)210' as the pillaring anions, two different pillaring reactions were carried out according to method IV using the LDH adipate as the precursor at a reaction temperature of 100°C and method V using a meixnerite precursor and ambient pH conditions at room temperature. 3. Physical measurements. X-ray diffraction patterns were obtained with a Rigaku x-ray diffractometer equipped with DMAXB software and a curved graphite monochromator attachment for Cu K; X-ray radiation. XRD patterns were recorded at room temperature for oriented film specimens (about 70 mg) supported on glass slides. X-ray diffraction patterns of samples preheated at 100°C and 200°C for 1h under N2 flow in a quartz furnace were obtained at room temperature. 74 Fourier transformed infrared (FTIR) spectra of samples dispersed in KBr pellets (1-2 wt %) were recorded with an IBM model IR 408 spectrometer. FTIR Spectra for some LDH-POM samples heated for 1 h at 100°C or 200°C under N2, were recorded on a IR transparent silicon cell at room temperature where droplets of the aqueous suspension of a sample (~5 mg/~1 mL) were first dried. Thermogravimetric analyses (T GA) were carried out under N2 for solid samples weighing ~70 mg on a Cahn 121TG System at a heating rate of 5°C/min. Nitrogen adsorption/desorption isotherms at liquid nitrogen temperature were obtained on a Coulter Omnisorb (TM) 360CX sorptometer using ultrahigh purity nitrogen as the adsorbate and helium as the carrier gas. Samples of about 100 mg were outgassed overnight at 130°C under a vacuum of 10'5 torr. Surface areas (Sm-5r) were obtained by the BET . method13. For pillared and non-pillared LDH intercalates, the BET equation was applied over the partial pressure ranges of 0.05 < P/Po < 0.1 and 0.05 < P/Po < 0.2, respectively, where correlation coefficients were near one, in accord with previous surface area/pore analyses of pillared clays. 19 The t- plot method20 was used to determine the total (St) and microporous (Sm) surface areas. Transmission electron microscopy (TEM) was carried out with a JEOL 100 CX instrument at the Center for Electron Optics at MSU. Specimens for TEM imaging were prepared by dipping copper grids coated with holey carbon films into an aqueous suspension of the solid and allowing the water to evaporate. Energy dispersive spectroscopy (EDS) was 75 performed using the same instrument in STEM mode (44 A beam size) in order to obtain compositional information. Elemental analyses were performed by ICP emission spectroscopy at the Toxicology Laboratory at MSU. About 15 mg of solid sample was dissolved in 50 mL of 1.5 vol % HNOg solution for each analysis. 76 C. Results and Discussion As is summarized in Figure 2.2, at least six different approaches, designated Methods 1 - VI, have been reported for the pillaring of layered double hydroxides by polyoxometalate anions with Keggin ion structures4'10. Among all of these approaches, Method 1 represents the direct most efficient pathway as it affords the desired product by direct co-precipitation reaction of M“, M111 and POMn'. However, this method has been shown to be useful10 only for acidic LDH derivatives, such as those with layer compositions of the type [Zn 1-xAlx(OH)2x+]. Efforts to prepare pillared forms of typically basic LDH derivatives, e.g., layer compositions. of the type [Mg1.xAlx(OH)2X+], afford instead poorly ordered salts of the POM (see below) or base-hydrolyzed derivatives of the POM. Method 11 involves an ion exchange reaction between a simple LDH-chloride or -nitrate and the pillaring POM. This was the method used to prepare the first examples of pillared LDH derivatives based on V 100236 and XM 120401“ Keggin ions as the pillaring anion.4.6 The key to successfully implementing ion exchange method 11 is to utilize a LDH-chloride or nitrate precursor that has been prepared by coprecipitation, but which has not been isolated from suspension and dried. Drying the precursor even at ambient temperature removes water from the metastable hydrate formed in the coprecipitation reaction and this renders the gallery surfaces less accessible for ion exchange. Consequently, the ion exchange reactions of desired LDH precursor with POM anions are difficult to complete, and prolonged exchange reactions typically are accompanied by hydrolysis reactions that afford undesirable by-products. 77 The importance of fully hydrated LDH precursors for pillaring ion exchange reactions has been emphasized recently by Clearfield and his co-workers9. Methods 111, IV, and V, which are the pathways of interest in the present work, all utilize a readily synthesized LDH carbonate21 as the starting material. Pathway III, as originally described by Chibwe and Jones7, converts the LDH carbonate to a mixed metal oxide solid solution, 17 which is then allowed to undergo a hydrolytic reconstitution reaction in the presence of the POM to form the LDH-POM. In contrast, pathways IV and V allow the mixed metal oxide to undergo reconstitution reaction in the absence of POM to form first a synthetic meixnerite intermediate, wherein the gallery anion iS hydroxide. The LDH hydroxide can then be allowed to react with an organic acid according to pathway IV to form an organo anion precursor3, which subsequently is converted by ion exchange reaction to the pillared LDH polyoxometalate.5t8 Alternatively, the LDH hydroxide can be converted directly to the LDH-POM intercalate by direct ion exchange reaction.22 This latter pathway is somewhat surprising, because POMS are readily hydrolyzed in basic solution.23 However, the hydroxide ion is readily displaced from the gallery surfaces by POMn', and the immobilized POMn' is stabilized toward basic hydrolysis. 78 (1-x)M" + mm x.- iM"i..M"‘.(0H)zJ(X">./. 41120 I l l 1 l (XE: POM-j (XE: Cl‘, Ol' N037 (XE: C0323 . (XE: 0A..) ; A (~soo °C) Mun-symmmmlemxpmemo mo 1 1M"i..M‘".(0H)21(om. 311,0 ..... = it!) an i011) (IV) new (V1 EM) : : y 5 i rM"i..M‘".(omzi(0A"9,/.. -Y'H20 [Mu1-mex(OH)211P0M]x/n .z'Hzo Figure 2.2. Summary of the synthetic methods used to prepare LDH intercalates pillared by POMD' ions: (1) direct coprecipitation method: (11) simple anion exchange method: (111) calcined metal oxide method: (IV) and (VI) organic precursor method: (V) synthetic meixnerite precursor method. Method (IV) uses the synthetic meixnerite to prepare LDH-Organic precursor, but the organic precursor in method (V1) is prepared by a coprecipitation during the synthesis of LDH. 79 The preparation of LDH-POM intercalates as represented by methods IV and VI in Figure 2.2 are chemically equivalent insofar as they both utilize an anion exchange reaction between an organo anion precursor and the pillaring POMn'. This can be exceptionally efficient conversion as evidenced by the original work of Drezdzon5. However, method VI utilizes a coprecipitation reaction to prepare the organo anion precursor, whereas method IV makes use of the reaction of an organic acid with an LDH hydroxide. The latter acid-base reaction is less susceptible to competing Side reactions (e.g ., organo anion salt formation) than the coprecipitation reaction. Consequently, method IV generally is preferred over method VI. Also, we should note that the recently reported reaction of LDH carbonate with glycerol at elevated temperatures (160-180°C) most likely involve the formation of LDH glycerolates.24 The glycerolates and related polyol derivatives Show considerable promise as precursor to POMW-pillared products. 25 We now consider the relative merits of pathways III, IV and V, for the synthesis of LDH intercalates pillared by 01-H2W120406 Keggin ions. As shown by the XRD patterns in Figure 2.3, all three methods afford reaction products containing a 001 reflection corresponding to a pillared Mg3Al-LDH phase with a basal spacing of ~14.7 A. In addition, each method produces a by-product characterized by a broad reflection near 20 = ~8° (11 A). The same by-product also is observed for derivatives prepared by reaction pathways 1, II and VI. All efforts to circumvent by-product formation or to remove the by-product by selective dissolution have been unsuccessful. The nature of the undesired by-product will be discussed more fully below. (001) (003) Relative Intensity W LB 1141) TlllTllllldllllllllllll 2 10 18 26 34 42 50 2 Theta (Degrees) Figure 2.3. XRD patterns of Mg3Al LDH-[a—H2W120406] reaction products prepared by (A) mixed metal oxide precursor, method 111 at 25°C; (B) LDH-hydroxide precursor, method V at pH 4.5, 25°C; (C) method V at ambient pH, 25°C, 0.5 h; (D) method V at ambient pH, 25°C, 5 h; (E) method V at ambient pH, 70°C, 5 h; (F) LDH-adipate precursor, method IV at ambient pH, 70°C, 5 h. 81 Returning to the XRD patterns in Figure 2.3, we see from part A that the reconstitution reaction of a mixed metal oxide in the presence of the Keggin ion according to method 111 affords a very poor yield of pillared product, the major product being the undesired 11 A phase. The yield of pillared product. obtained by ion exchange reaction of the LDH hydroxide with the Keggin ion (method V) is improved by conducting the reaction at ambient pH and at an elevated reaction temperature (cf., Figure 2.3, parts B- E). The best yield of the pillared phase, however, is provided by method V, which utilizes an LDH-adipate precursor for ion exchange reaction with the Keggin ion (cf., Figure 2.3, part F). The product obtained from this latter method exhibited seven orders of (001) reflection corresponding to a basal spacing of 14.5 A. As Shown by the XRD patterns in Figure 2.4, the by- product formation can be further reduced by increasing the reaction temperature to 100°C. Taking the layer thickness of the Mg3Al-LDH as 4.8 A, one obtain a gallery height of 9.7 A, which is consistent with the van der Waal's diameter (10.2 A) estimated from crystallographic data for a Keggin ion salt26. The interlayer a-H2W12O406 ion most likely has the C2 axis of the oxygen framework orthogonal 'to the LDH layers to optimize H bonding to the gallery hydroxyl groups, as proposed previouslyé. Also, we note that the pillared Mg3Al-[a-H2W120405] intercalate was stable to 200°C under N2, as judged by the XRD patterns shown in Figure 2.4. 82 (001) E) a (003) In" H (002) E (004) (005) (006) (007) ‘6 o E 2 25C 0: k 100°C 2200°c lllllljllllllllllllllll 2 10 18 26 34 42 50 2 Theta (Degrees) Figure 2.4. XRD patterns of the oriented film of the LDH-[a-H2W1204o6‘] intercalate prepared by method IV using Mg3Al-adipate precursor. Diffraction patterns were taken at room temperature after drying under N2 flow for 1 h at the temperature indicated. 83 Based on the above results for the pillaring of LDH by Keggin 0i- H2W12O4o5‘ ion, we investigated the effectiveness of methods IV and V for LDH pillaring by Dawson and Finke POMn' ions. The reaction condition utilized for the two methods are summarized in the following scheme: Method V LDH-c032: 1' “50° C),- LDH-OH' POM '25 Ce LDH-[POMn'] 2. H20, 25°C Method IV was we LDH-[Adipate] 100°C Reaction temperature (25°C vs. 100°C) and use of preswelling agent (adipic acid) are considered as potential factors for obtaining different gallery orientations of the cylindrical DawSon and Finke POMn' ions. The crystalline product obtained by the pillaring reaction of the Mg3Al-adipate with the Dawson a—P2W180626' ion at 100°C (method IV) exhibited eight orders of (001) reflections (Figure 2.5). The observed basal spacing of 17.6 A is in good accord with the sum of the crystallographic diameter of the POM along its C2 axis (9.5 A)27, the van der Waal's radius of oxygen atom (1.4 A) and the LDH layer thickness (4.8 A). Gallery 01- P2W130625 anions, therefore, have the C2 axis orthogonal to the LDH layers in order to optimize the H-bonding interactions between the LDH layer 84 hydroxyls and the 14 apical oxygen atoms on the POM oxygen framework. Obtained LDH-[a-P2W130626] intercalate also was thermally stable up to 200°C under N2. Almost no difference in basal spacing is observed for the pillared product dried over the temperature range 25-200°C. On the other hand, the pillaring reaction carried out using Mg3Al- hydroxide at room temperature (method V) gave a product with a basal spacing of 19.3 A, implying that the C3 axis of the a-P2W130626 oxygen framework most likely is orthogonal to the LDH layer stacking direction. The observed gallery height of 14.5 A is in consonant with the sum of the crystallographic diameter of the POM (11.6 A) along the C3 axis27 and the van der Waal's radius of oxygen (1.4 A), assuming that there is no water molecule between the POM and the LDH layers due to the electrostatic and H-bonding interactions between them. Upon hcating (2 100°C), this room temperature product showed a decreased basal spacing (17.4 A) from 19.3 A, which was not recovered upon rehydration. This observation suggests that the gallery a-P2W130626 ion in the kinetic product obtained at room temperature seems to change its orientation, upon the thermal dehydration of the gallery water, to a more thermodynamically favored one with more H- bonding interactions. With the C3 axis of the a—P2W180626' oxygen framework orthogonal to the LDH layers, the room temperature product has total 6 apical oxygen atoms participating in H-bonding with the LDH hydroxyl groups, whereas 14 oxygen atoms are available for the H-bonding with the C2 axis orthogonal. The air-dried pillared derivatives of LDH-[CO4(H2O) 2(PW9O34)210-], prepared at 100°C using the adipate precursor (method IV), exhibited a basal spacing of 17.4 A. This LDH-POM intercalate was thermally stable up to 85 200°C under N2 (Figure 2.6). The air-dried product formed from the reaction of LDH hydroxide and the POM at 25°C (method V) showed a basal spacing of 18.1 A. Based on the crystallographic dimension of this Finke type POM15, the room temperature product has a gallery C04(I-120) 2(PW9O34)210' orientation with its C2 axis orthogonal to the LDH layers. And once heated, the gallery POM comes in closer contact with the LDH layers to have more favorable H-bonding interactions, which is the case for the POM orientation in the pillared product prepared by method IV. Table 2.1 summarizes the observed gallery heights for the LDH-POM intercalates at different drying temperatures. Each intercalate showed different gallery contractions upon thermal treatment. Air-dried LDH-POM intercalates exhibited gallery heights in very good agreement with the expected values based on the proposed gallery POM orientations. Once heated, all the intercalates showed a decreased gallery height. Removal of interlayer water will occur upon heating, resulting in stronger electrostatic and H-bonding interactions between POMS and LDH layer hydroxyl groups. This will cause the POM to adopt a thermodynamically stable gallery orientation. Pillar reorientation upon heating (2 100°C), therefore, looks responsible for the observed gallery contractions of 2.5 A and 3.2 A for the LDH-[a-P2W130626] and - [C04(H2O)2(PW9034)210“] products, respectively, prepared by method V. Upon drying at 200°C the a- H2W120406 intercalated LDH showed only a 0.2-0.4 A decrease in the gallery height, whereas the LDH-[a-P2W130626] and -[C04(I-120) 2(PW9O34)210‘] products prepared by method IV showed 1.7 A and 2.5 A decreases, respectively. The pillared products obtained by methods IV and V Showed essentially the same gallery height after heating at 86 200°C, indicating the same gallery orientation of the POM pillars at this Stage. Figure 2.7 illustrates the observed gallery orientation of the three POM pillars in the obtained products after drying (2 100°C). Based on the observed gallery heights of the LDH-POM intercalates dried at 100—200°C, one can propose that the POMS are strongly bounded to the LDH layers by electrostatic and hydrogen bonding forces, since the observed gallery heights are somewhat smaller than the minimum van der Waal's dimension of the POM. Hydrogen bonding interaction therefore seems an important factor for determining the gallery orientation of the POM. Charge density matching between the POM and the LDH layers controls the lateral separation of POM pillars in the gallery space. (001) (003) (002) (004) (005) (006) (007) (008) W WM lmoc W 200°C T l l l l l l r l l T I l l l I I l l I l l l 10 18 26 34 42 50 2 Theta (Degrees) Relative Intensity 2 Figure 2.5. XRD patterns of the oriented film of the LDH-[a-P2W180526'] intercalate prepared by method IV using Mg3Al-adipate precursor. Diffraction patterns were taken at room temperature after drying under N2 flow for 1 h at the temperature indicated. (001) (002) (003) (004) CM (005) (006) (007) 25°C J 100°C J 2 200°C lllllllllllllll'lllllll 2 10 18 26 34 42 50 2 Theta (Degrees) Relative Intensity Figure 2.6. XRD patterns of the oriented film of the LDH- [C04(H20) 2(PW9034)210'] intercalate prepared by method IV using Mg3Al- adipate precursor. Diffraction patterns were taken at room temperature after drying under N2 flow for 1 h at the temperature indicated. $33688.— .< v; v5 < we :83 203 83a combs Co 6.29... 9133 Be 5; 23 SO..— .8: 05 .0 6.3502. 3.34 .22 :03 Lo 33 329.08% 330.08 338.8 085 03 28555 E 82a> 2: 2: a: 92 one owe 0. 8m 2: m 2: a: m 92 2nd m one o. 8_ ed m mi we m 3; 03 m 2: .E 0.82 E v.82 E v.82 E 958: 38.2 x 8.: {was 36.3 x <3: 22: x «No: .O—NAVMOmEvNAOvavOU .eNoOE >29th 0090233115 .cEOnm . .069: an “8585 83:54.54 a can 92 a seeded 6366223 a See needed 888.85 20.354 83:3 06 9e 232. team .3 636,—. 1A ( 2 100°C) 1A“: 100°C) Figure 2.7. Proposed POM“ gallery orientations for the LDH intercalates during drying (2 100°C) under N 2: (A) a-H2W120406. (B) G'P2w180626' and (C) C04(H20)2(PW9034)21°‘. 91 The retention of the Dawson a-P2W130525' structure in the intercalated state also was verified by the FTIR spectra (Figure 2.8). The frequencies of the PO (at 1092 and 1020 cm'l) and W-O linkages (at 957, 912 and 756 cm'l) are very similar for the authentic potassium salt of the POMl4 and the LDH intercalate prepared by method IV. But, the product prepared by method V showed v(W-O) modes slightly shifted by 38 cm"1 to lower frequency, probably due to a different POM orientation and a local change in the chemical environment around the oxygen framework. Molecular reorientation of the gallery Dawson ion prepared by method V did not change the v(P-O) vibrations upon heating to 200°C, but resulted in a slight high wavenumber shift, by ~2 cm'l, for the oxygen framework vibrations of the POM, as shown in Figure 2.9. The two Finke-ion pillared products prepared by methods IV and V showed basically the same vibration modes of the oxygen framework of the gallery C04(HzO)2(PW9O34)210', as shown in Figure 2.10. The prominent stretching vibration bands due to the P- O (1038 cm‘l), terminal W-O (961 and 941 cm'l), comer-sharing W-O-W (884 cm'l), and edge-sharing W-O-W (772 and 739 cm'l) groups for the authentic POM salt143-8 were observed at very similar positions for both reaction products. TEM images of the original LDH carbonate and the POM intercalates are consistent with the topotactic nature of the pillaring reactions. Hexagonal crystals ranging from 600 A to 2500 A in diameter were observed for the intercalates before and after the pillaring reactions (Figure 2.11). § (C) E g (B) F. g (A) 3’: llljlllllllllllllllllllllllllllllllllll 400500600700800900100011001200 Wavenumber(cm'l) Figure 2.8. FTIR spectra of Mg3Al LDH-[a-P2W130626] intercalates: (A) prepared at 100°C from a LDH-adipate precursor and dried at 25°C. (B) prepared at 25°C from a LDH-OH- precursor and dried at 25°C. Spectrum C is for the authentic salt K5[a-P2W130625]. Relative Transmission (96) lllllllllllllllllllIllllllllllllllillll 400 500 600 700 800 9001100011001200 Wavenumber(cm' ) Figure 2.9. FT IR spectra of Mg3Al LDH-[a-P2W130626] intercalate prepared at 25°C from a LDH-hydroxide precursor and dried under nitrogen for 1 h at (a) 25°C, (b) 100°C and (c) 200°C on a IR transparent silicon cell. (C (B) V \ (A) Relative Transmission (90) lTlllllFllllllIlllllIFITTIIIIIIIIIl[III 400 500 600 .700 800 9001100011001200 Wavenumber(cm' ) Figure 2.10. FTIR spectra of Mg3Al LDH-[C04(I-le)2(PW9034)210'] intercalates: (A) Prepared at 100°C from a LDH-adipate precursor and dried at 25°C. (B) Prepared at 25°C from a LDH-OH' precursor and dried at 25°C. Spectrum C is for the authentic salt K10[C04(HzO) 2(PW9O34)210‘]. 95 Amy can TwovOQBNE- 2 $04 3c mean—SHOH—Gm 20m 4 82 2 :5 28m .2e.gvmomaeflomzvauvzoq :01— 2: co .8me SEE 033.5858. 2N ousmi 96 Table 2.2 summarizes the elemental analyses and the textural properties of the original Mg3Al-CO3 used to prepare the meixnerite precursor and the LDH-POM intercalates obtained at 100°C from the adipate precursor (method IV). Some of the layer Mg 2+ ions were depleted upon the pillaring reactions, as was also observed in recent works9:1 1. In our study, a basic suspension of the meixnerite (pH ~11.3) was introduced either directly to the POM solution (method V) or to the adipic acid (method IV). The reaction pH measured at room temperature after reaction with the POMs was in the range of 7.0 - 8.0 in absence of adipate (method V) and about 6.5 in the presence of adipic acid (method IV). A decrease in the reaction pH upon pillaring is indicative of the depletion of the more basic Mg2+ ions, as was previously observed in a recent study of the physico-chemical properties of Mg-Al LDHS.l7 The products obtained from the LDH-adipate precursor (method IV) showed more Mg2+ depletion (than the product obtained using meixnerite (method V). For example, the starting Mg3Al-CO3 gave a Mg2+ to Al3+ ratio of 3.11, but the LDH-[a-H2W120406] product derived from LDH-adipate showed a Mg2+ to Al3+ ratio of 2.23, and the product derived from meixnerite gave 2.84. Also, the Dawson and Finke POMn' pillared derivatives prepared from meixnerite at 25°C exhibited Mgz’r/Al3'+ ratios of 2.82 and 3.07, respectively. But, the Mg2+/Al3+ ratios for the LDH intercalates prepared at 100°C from the LDH adipate were 1.88 and 2.46, respectively. On the basis of changes in chemical composition, therefore, the high temperature pillaring reactions are not topotactic reactions. A59 35 8&5“ 32808:: e5 3 83 80.3 .39 832.. 852. .23 B. ” ._.ovfav8.o a As e8 3vade a E 8:38 83 E59 0 ”Ema—8a 3580:. m2 Eoc . 33a? 5 means. 295.». 05 .3 33530 .260:— =am 22.33 .a n a: 33.—2.— 3268; 6.. 8S 2 839:2 AME 2: .8 02:83 29$ 83 .333225 2253 Bu «0.8 can 3. nzpmm . .5338 2a6<$qw mam an an $2 an: a; ”co.— uovd .o_~AvnOm>EV£O~=Vv00 an an Q: .8 a: ”8; ”a: ssoeanma o o m: E: a; n 8; u an .A 08023N:-3 ow N. N 1 .m on ea was 92 H8.. ”mum s 0 3 z s - - - - - ”8; ”3m :0 o o a: an: - ”8; u :.m $00 .2 awe. 3 E. 3 E. :5 H _< H wEV Aamv .m. m mm B m £838 595 eoaaoaencu ._.~m002-_t§=omoaom .26 ouswE 149 Table 3.2 provides the compositions of the LDH-POM intercalates prepared from Mg3Al LDH-hydroxide. The LDH hydroxide and carbonate have Mg/Al z 3.3. Our pillaring reaction products exhibited some depletion of the layer Mg2+ cations, as evidenced by Mg/Al ratios of 2.5-3.0, but the depletion is small compared to the reported reaction products obtained at 100°C using the LDH-adipate precursor“. Table 3.2 also provides the textural informations for the LDH intercalates, which was studied by the N2 adsorption-desorption measurements. Since polyoxometalates have large equivalent weight in comparison to carbonate, the surface areas per unit positive charge are presented in AZ/e+, in addition to the common m2/ g unit of the surface area. Polyoxometalate pillared LDH products exhibited a wide range of BET/N2 surface areas from 30 to 120 m2/g. Pillaring reactions afforded access to the gallery micropores, which was indicated by the type I N2 adsorption isotherms at lower P/Po region. The Dawson (a-P2W17061)10- and the doubly condensed Dawson (P4W3oZn4(l-IzO)20112)16' ions afforded higher total surface areas as well as higher microporous surface areas than the Finke ions of (Zn4(H20)2(A8W9034)2)10' or (WZn3(H20)2(ZnW9034)2)12' intercalated LDH compound, which is consistent with that the higher charge densities of the Dawson and double-Dawson ions. The LDH interlayered by (WZn3(I-le)2(ZnW9O34)2)12' exhibited a higher surface area (in Azle‘t) than the Zn4(I-120)2(ASW9034)210' ion intercalate. Among the three oxocryptate-intercalated products, the (NaP5W300110)14* LDH intercalate exhibited the highest gallery microporous surface area, which also is in agreement with the observed gallery height (cf. Table 3.1). However, all the observed microporous surface areas are not as high as one 150 may expect from the gallery heights and charge densities of the POM ions employed in the present study. Access to the micropores of the LDH-POM intercalates is significantly dependent on the pillaring method, as well as on the LDH precursor.24,26 Even for the same POM pillaring agent, substantial differences in microporous surface areas can be observed depending on the precursor LDHs. To date, no conclusive interpretation has been provided for the differences in micropore accessibility for the LDH-POM intercalates prepared by different pillaring methods. Limited access to the microporous galleries may be caused by the deposition of the POM salt impurity phase at the LDH edges or by structural defects that result in a higher concentration of POM pillars near the gallery edges. Further work is needed to test these possibilities. 151 .0330 cobw— 304 .3: 59 3.3528 203 A+o\~$s£=omoaom 6.4 can—mi 173 Table 4.1. Compositions and textural properties of the prepared Mg 1.xAlx LDH intercalates. M814“): Intercalated Anion Mg/Al Sggr 85 S? q, (Sm) LDH Precursor molar ratio lg .111 ls .111 Is b co} 4.12 73 30 o o Hydroxide a-H2W120m° 3.54 33 97 59 62 Glycerolate a-H2W120406' 3-62 159 189 122 65 Triethyleneglycolate a-H2W120406' 3.30 168 196 121 62 ‘ BET/N2 surface areas (SEE-p) were calculated from N2 adsorption in the range 0.05 < P/Po < 0.2 (for LDH-CO3) and 0.05 < P/Po < 0.1 (for LDH-POM‘s) where their correlation coefficients were near one; total surface area (S0 and microporous surface area (Sm) were obtained by the t-plot method. b This LDH carbonate was prepared by direct coprecipitation. 174 The data in Table 4.1 also show that the polyol route provides pillared LDH-POM derivatives with N2 BET surface areas of ~160 m2/g, a value substantially larger than the 83 m2/ g surface area for the analogous derivative obtained from the meixnerite precursor. Approximately 65 % of the total surface area falls in the micropore size range below 20 A for all three pillared products, thereby confirming a pillared structure with laterally spaced POM ions in the gallery. In contrast, the starting LDH carbonate is completely non-microporous. The substantial difference in microporous surface areas for the pillared products prepared from the polyol and meixnerite precursors (122 and 59 m2/ g, respectively) underscores the importance of the synthesis method in determining the micropore accessibility of the pillared products. Although meixnerite and the LDH- glycerolate afford products with comparable amounts of pillared LDH-POM and impurity phases, as judged by XRD (see Figure 4.1), the surface areas differ dramatically. Thus, the method of synthesis most likely influences the ' pillar distribution controlling access to the gallery micropores. Also, it is noteworthy that products obtained from the glycerolate and triethyleneglycolate precursors have essentially identical microporous surface areas, even though the relative amounts of pillared and impurity phases contained in the two products differ substantially (cf., Figure 4.1). It is unlikely that the impurity phase is microporous and capable of contributing substantially to the overall surface area of the reaction product. For instance, the salt-like 11 A product formed by reaction of a Mg2+/Al3+ LDH carbonate and metatungstate in the absence of a polyol is non- microporous. Thus, gallery access to the pillared phase, even by N2 molecule, can be significantly dependent on the pillaring method. 175 Polyoxometalate anions intercalated into LDH hosts have been shown to promote redox reactions analogous to those for the POM in homogeneous solution”. 20: 21. However, POM anions also can function as Bronsted acids when protonated22 and the acidity function can influence redox activity23. Thus, the intercalation of a POM into an LDH structure opens the possibility of incorporating acid functionality into an intrinsically basic solid. In the present work we have examined the acid/base function of the LDH reaction products obtained from polyol and meixnerite precursors, using 2—methyl-3-butyn-2-ol (MBOH) as the probe molecule. This substrate is known to undergo disproportionation over a basic catalyst and dehydration over an acidic catalyst ,as shown in the following scheme 1 1: ICH3 Basic Pathway r H3C C—O + HCECH 9‘3 H3C-(ll-CECH —~~ ' OH MBOH CH Acidic I 3 > HzCr-C-CECH Pathway 176 The results shown in Table 4.2 indicate that the reaction product prepared from the LDH glycerolate exhibits appreciable reactivity toward MBOH conversion, but the products derived from the LDH triethyleneglycolate and meixnerite show little or no catalytic activity. These differences in chemical reactivity again emphasize the importance of synthetic method in regulating substrate access to the micropores. Essentially all of the reactivity observed for the LDH-metatungstate derived from the glycerolate precursor occurs along the basic pathway. That is, the LDH-POM intercalate is primarily a basic solid, with essentially no acid/base duality. Some reduction in activity occurs with increasing time on stream, perhaps due to partial structural collapse at the reaction temperature of 150°C, but the nearly exclusive basic reactivity is retained. Owing to the low MBOH conversions observed for the products derived from meixnerite and the triethyleneglycolate, the acid functionality reported in Table 4.2 for these materials is not chemically significant. Many factors can influence chemical access to the micropores of POM-pillared LDH materials. We know from these and related studies currently under investigation that the synthesis method is important in determining micropore access by nitrogen and organic substrates. Two reaction products with essentially identical XRD and spectral properties can exhibit markedly different porosities and catalytic properties. Such differences imply that synthesis-dependent pillar distributions and/or structural defects are important in controlling micropore access. The existence of such defects are only now becoming apparent and additional work is needed for their characterization. We have recently provided indirect evidence for the existence of textural mesopores in LDH intercalates 177 resulting, perhaps, from the presence of incomplete (discontinuous) brucite layers in the LDH stacking sequence“. Other possible defects include non- uniform layer charge distributions that give rise to a higher concentration of pillaring POM ions at gallery edges than at gallery interiors, hence, blocking out substrate access through the crystallite edges. The deposition of the salt impurity phase at the LDH edges, as suggested by our on- going complementary studies of LDH intercalates may also inhibit gallery access. Thus, future studies of pillared LDH intercalates need to consider synthesis- dependent effects on the structure, adsorption and catalytic performance of these materials. 178 .8289, egg—c 895 Ema—88 Doom: 8 088mm 2: $5838 coca 38538 33 noun—8.3 5:58 05 E 382280 SH 338m 05 EN 8:5 .85an 8.. 5.8% fl 33534 a {CH 05 5 Bo: NZ 895 erfl 3 8 es Nd! «mo— 8888 05 £38 ooméo— 5 08m 23:8 53» 8888 ..o manage wéomd .052 3 8303.. £932 ouch—.88 05 3 Bo: o: 8.8: AN 88 erfl 8 852.88 882980 a WN Wb Vu m N on we mg N on m? 2 _ o.a_OQ>_wc=o_>5oth-_24wo< 8 8 c8 Ev 0:5. . 88:85 :04 88388 In: 3308 3NI-3-x— 08882 a 0 880—00825 In: .8 8:888 282x00 8:0 meoEmcan0 2280.5 .NW 033. 203 gallery space accessible to guest molecules with a kinetic diameter greater than nitrogen ( ~4.6 A ), despite the 7.0 - 7.2 A gallery heights. The inaccessibility of the gallery surfaces may be due to the blocking effects caused by the deposition of silica at the crystallite edges. Some indirect evidence for such blocking is provided by the adsorption data. Note that the LDH-silicate intercalates exhibit subatantially smaller mesopore volumes than the LDH carbonates This suggests some changes in interparticle texture occurred during the TEOS reaction, probably due to the deposition of the extragallery silica. The catalytic reactivities of our Mg1-xAlx LDH silicates were examined using the dehydration / disproportionation of 2-methyl-3-butyn- 2-ol (MBOH) as a probe reaction. This latter substrate is very efficient in characterizing the surface acidic and basic properties of solid catalysts”. MBOH undergoes disproportionation over a basic catalyst and dehydration over an acidic catalyst, as shown in the following scheme: 9H3 Basic Pathway r H3C C—O + HCECH (51.13 OH MBOH CH Acidic I 3 T H2C=C-CECH Pathway 204 For comparison with the catalytic performance of LDH silicates, the MBOH test also was carried out for the corresponding LDH carbonates. Both the LDH carbonate and silicate intercalates after activation at 150°C retained their layered structures as judged by XRD and TGA studies. As shown by the results in Table 5.3 the catalytic activities for MBOH conversion decreased in the order of Mg4Al > Mg3Al > Mngl for both classes of LDH intercalates. The LDH carbonates showed exclusively basic selectivities, producing acetylene and acetone disproportionation products. The higher magnesium content of the layers seemed to improve the basic reactivity of the LDH carbonates. The reactivity pattern of the carbonate derivatives also could be related to their surface areas that increase in the same order (see Table 5.2). For the LDH-silicates, the Mngl intercalate with the highest silicate content showed relatively low overall reactivity. Owing to the low conversion observed for the Mngl silicate, the relative acid-base selectivity in this case is not chemically significant. However, both the Mg3Al and Mg4Al silicates showed high reactivities and high basic selectivities. On the basis of the relative MBOH reactivities, the LDH carbonate compounds are more active than the LDH silicates, indicating that the intragallery silicate anions are not as basic as the carbonate anions. Table 5.4 summarizes the MBOH test results obtained for mixed metal oxide catalysts formed by thermal decomposition of LDH intercalates after activation at 450°C. TGA and XRD studies showed that both the silicate and carbonate intercalates decomposed and became X-ray amorphous after thermal treatment at 450°C. The oxides derived from LDH carbonates show higher MBOH activity than those derived from the LDH silicate compounds. The mixed metal oxides obtained from the 205 carbonate precursors decreased in MBOH reactivity in the order Mg3Al > Mngl > Mg4Al. The reactivities of the oxides derived from LDH-silicates paralleled the silicate content of the initial intercalate with the catalytic activity decreasing in the order Mngl > Mg4Al > Mg3Al silicate. Acidic as well as basic activity was observed for the oxides obtained from LDH- silicate precursors, while only basic selectivity was observed for the oxides derived from LDH carbonate precursors. Condensation reaction of the interlayer Si-OH groups and the Al-OH groups of the LDH layer during the thermal treatment at 450°C might afford the acidic sites. 206 .3533: 835a 5:82 .o 250:8 8g anEcwRE n h 2 .EN ..o-c 0°65 M 228382 82934.2 a 5N b 87. New C: Swazi-Z32 N.mA b 87. ada 9 _ 2 a 87 Q? _ ow 289232 Wm N mm 6.3 c: 28_=m-_ao< Eo< a» 0.95 ca o5 cocoaom 82.8.25 IDA dew-"32.00 :09)— com mew—«285 In: no 5:583— .m.m vim-- 207 823—26 8285 883-. go 3:882 882 Enocawfii n b a .EN 8: Doomv H EBEKEB 88381». a 26 2 we 02 ofl 38.2222 cod 2 8? QB 2 _ $8-222 who 2 2 2.0 om: 28.2222 22 a 87. 0.2 o: $8-222 E 8 9 W2 02 282-222 2.2 i. 87. 32 2 2 $8-222 E: h .255 2352““ 223522 82.—3:00 Gov 9:3. 33:32 Eo< ow 072m 2 2 888% .ofiaooi IQ..— .822885 In: Eoc 83.88 $228 288 gong-5:2 .«o 5388. $092 .vfi 03mg- 208 D. Conclusion The base-catalyzed hydrolytic condensation reactions of TEOS by gallery OH‘ ions of [Mg1-xAlx(OH)2] LDH hydroxides ((1-x)/x z 2, 3, or 4) were studied. The reactions resulted in the formation of two- dimensional silicate layers between the LDH layers with gallery height ~7.0 A. A chain-like hexagonal arrangement of the interlayer silicate is proposed analogous to 6-(sti205)x. Mixed metal oxides formed from LDH-silicate precursors calcined at 450°C showed bifunctional acid/base catalytic activity, whereas the LDH-silicate structures (150°C) showed only basic activities. 209 References H . Cavani, F.; Trifiro, P.; Vaccari, A. In Catalysis Today; Elsevier: Amsterdam, 1991; Vol. 11, pp 173 and references therein. 2. Kwon, T.; Tsigdinos, G. A.; Pinnavaia, T. J. J. Am. Chem. Soc. 1988, 110. 3653. 3. Drezdzon, M. A. Inorg. Chem. 1988, 27, 4628. 4. Chibwe, K.; Jones, W. Chem. Mater. 1989, 1, 489. 5. Dimotakis, E. D.; Pinnavaia, T. J. Inorg. Chem. 1990, 29, 2393. 6. Wang, J.; Tian, Y.; Wang, R.-C.; Clearfield, A. Chem. Mater. 1992, 4, 1276. 7. Narita, E.; Kaviratna, P. D.; Pinnavaia, T. J. J. Chem. Soc., Chem. Commun. 1993, 60. 8. Tatsumi, T.; Yamamoto, K.; Tajima, H.; Tominaga, H. Chem. Lett. 1992, 815. 9. Schutz, A.; Biloen, P. J. Solid State Chem. 1987, 68, 360. 10. Fyfe, C. A.; Fu, G.; Grondey, H. Abstracts of Papers, Spring Meeting of the materials Research Society, 1994, April 4-8. 11. Sato, T.; Fujita, H.; Endo, T.; Shimada, M. React. Solids 1988, 5, 219. 12. Lauron-Pemot, H.; Luck, F.; Popa, J. M. Appl. Catal. 1991, 78, 213. 210 13. Conner, A. 2.; Elving, P. J.; Benischeck, J .; Tobias, P. E.; Steingiser, S. Ind. Eng. Chem. 1950, 42, 106. 14. Brunauer, 8.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309. 15. Yamanaka, S.;Hattori, M. In Chemistry of Microporous Crystals; Elsevier: Amsterdam, 1991; Vol. 60, pp 89 and references therein. 16. a) Hou, W.; peng, B.; Yan, Q.; Fu, X.; Shi, G. J. Chem. Soc., Chem. Commun. 1993, 253. b) Hou, W.; Ma, J.; Yan, Q.; Fu, X. J. Chem. Soc., Chem. Commun. 1993, 1144. 17. Meyn, M.; Beneke, K.; Lagaly, G. Inorg. Chem. 1990, 29, 5201 and references therein. 18. Chibwe, M.; Pinnavaia, T. J. J. Chem. Soc., Chem. Commun. 1993, 279. 19. Carrado, K. A.; Forman, J. E.; Botto, R. E.; Winans, R. E. Chem. Mater. 1993, 5, 472. 20. Ohtsuka. K.; Suda, M.; Tsunoda, M.; Ono, M. Chem. Mater. 1990, 2, 511. 21. Harris, M. T.; Brunson, R. R.; Byers, C. H. J. Non-Cryst. Solids 1990, 121, 397. 22. Groenen, E. J. J.; Emeis, C. A.; van der Berg, J. P.; de Jong-Versloot, P. C. Zeolite 1987, 7, 474. 211 23. Heidemann, D.; Grimmer, A.-R.; Hilbert, C.; Starke, P.; Magi, M. Z. Anorg. Allg. Chem. 1985, 528, 22. 24. De Boer, J. H.; Lippens, B. C.; Linsen, B. G.; Broekhoff, J. C. P.; Van den Heuvel, A.; Osinga, Th. J. J. Coll. Interface Sci. 1966, 21, 405. CHAPTER SIX WATER CONTENT AND PARTICLE TEXTURE OF SYNTHETIC HYDROTALCITE-LIKE LAYERED DOUBLE HYDROXIDES 212 213 Abstract Hydrotalcite-like layered double hydroxides (LDHS) with compositions corresponding to [Mg1-xAlx(OI-I)2](CO3)x/2-nH20, where (1- x)/x z 2, 3, and 4, were prepared by complementary variable- and constant-pH c0precipitation methods. In the commonly used variable pH process, LDH precipitation was initiated at the elevated pH value of the starting carbonate solution and terminated at pH z 10. In contrast, the constant pH method allowed the entire precipitation process to be carried out at pH 10. Both synthesis methods yielded air-dried LDH carbonates containing pore water condensed between aggregated crystal platelets and adsorbed water bound to intragallery and external surfaces. Pore water was readily removed by heating to 60°C, but the temperature for complete removal of surfaces bound water increased from 240°C to 280°C (5°C/min) with increasing Al3+ content. A bimodal loss of surface water was consistent with the presence of "intrinsic" water bound to the intracrystal gallery surfaces and "extrinsic" water held at external surfaces. The textures of the LDH reaction products, as reflected in crystal morphologies, surface areas, and pore size distributions, were highly dependent on the preparation method and the layer charge density. Fine grained crystals with rough surfaces and relatively high surface areas were obtained by the variable pH method, whereas the constant pH method afforded larger, well-formed hexagonal crystals. All of the products prepared by the variable pH method exhibited mesopores with radii in the range 50-300 A. In contrast, the constant pH method gave, Mg3Al- and 2 14 Mg4Al-LDH carbonates crystals with narrow meSOpore distributions near 20 A radius. TEM images provided evidence for the accommodation of pore water in voids formed by edge-face crystal aggregation and for cofacial stacking disorders that contribute both to mesopores and to the binding of extrinsic surface water. 215 A. Introduction Hydrotalcite-like layered double hydroxides, henceforth abbreviated LDHS, consist of positively charged brucite-like (Mg(OI-I)2 - type) layers separated by counter anions and water molecules. The chemical composition of this class of intercalation compounds can be expressed in general as [M111-xM111x(OI-I)2]X+[An']x/n-fl-IzO, where MII and MIII are the divalent and trivalent cations in the octahedral interstices of the hydroxide layer and AD' is the charge-balancing interlayer gallery anion. Owing in part to their anion exchange propertiesl'3, certain LDH derivatives are useful materials for pollution prevention and waste clean-up. They also are valued as precursors for the preparation of a number of industrially important catalysts.4 More recently, LDH intercalates have been investigated as catalyst supports, particularly for the immobilization of biomimetic catalysts with properties suitable for the in situ remediation of contaminated soils.5 Another important feature of LDH compositions is their ionic/protonic conductivity,6 which makes them potentially useful for sensor and other device applications. Many of the materials properties of LDH intercalates are related in part to the exceptional mobilities of the gallery anions. Ionic mobilities depend on the water content of the gallery region occupied by the anions, as well as on the particle texture "and water contained in interparticle pores.7 The acid/base properties and ion-exchange behavior of LDH compounds also should depend on water content and particle texture. Therefore, it is important to understand the partitioning of water between 216 structural sites in the gallery and non-crystallographic sites in LDH compounds. However, relatively few studies have been reported for LDH intercalates, as compared to the more extensive knowledge available for water in silica gels3, clays9, and zeolites“). With few exceptions1 1’12, the distinction between different types of water in LDH compounds has not been generally recognized. De Roy12 has identified two types of surface- bound water in LDH materials, namely, "intrinsic" water structurally intercalated between the brucite-like layers and ”extrinsic" water bound to external surfaces. However, more detailed insights into the relationship between water content, particle texture and method of LDH synthesis are lacking. The present work investigates the water content for a series of LDH carbonates of the type [M g1-xAlx(OH)2](CO3)x/2-fl-IzO with compositions corresponding to (l-x)/x values of approximately 2.0, 3.0 and 4.0. In addition to describing the relationship bean water content and the layer composition (charge density), we also report the surface area and mesoporosity of LDH carbonates obtained by two different coprecipitation methods. In the first method, which is commonly used for LDH synthesis, the pH is allowed to vary at the initial stages of particle nucleation and precipitation. In the second, newly developed method of the present work, the pH is held constant at all stages of particle formation. These different synthesis methods result in substantial differences in textural properties for LDH products of equivalent chemical composition. 217 B. Experimental 1. LDH synthesis Variable pH Method. A mixed Mg(NO3)2-6HzO and AI(NO3)3-9HZO solution (2.0 M, 250 mL) with a Mg2+lAl3+ ratio of 2.0, 3.0 or 4.0, was added at a rate of ~20 mL/min to enough 1.0 M Na2C03 solution at 40°C so that the overall CO32'IA13+ ratio was 1.5. Regardless of the M g2+lAl3+ ratio, a white precipitate immediately formed upon addition of the first drop of the mixed metal nitrate solution to the basic Na2CO3 solution. Once the pH of the reaction mixture neared a value of 10.0, a solution of 2.0 M NaOH was added along with the mixed Mg2+lAl3+ solution to hold the reaction pH at 10.0 (:01). After complete delivery of the mixed nitrate solution, the reaction. mixture was stirred vigorously for 4h at 40°C and then aged for 40h at 70°C with good stirring. Upon completion of the digestion period, the product suspension was cooled to room temperature and centrifuged. The resulting white product was then washed free of carbonate ion (as observed by A gN 03 test) by repeatingly forming a. slurry in deionized water and then centrifuging. All products were air-dried on glass plates. Constant pH Method. In this method a 500-mL quantity of H20 was first heated at 40°C and the pH was adjusted to 10.0 by the addition of several drops of a solution formed by mixing equal volumes of 1.0 M NazCO3 and 2.0 M NaOH. A mixed metal nitrate solution with a Mg2+/Al3+ ratio of 2.0, 3.0 or 4.0 and the mixed base solution were then 218 added to the reaction vessel at rates that maintained the reaction pH at 10.0 $0.1. In this way the reaction pH was kept constant from the very beginning of the precipitation process. Once all of the mixed base solution was consumed, additional 2.0 M NaOH was added to keep the reaction pH at 10.0 for the rest of precipitation reaction. The reaction mixture was stirred vigorously for 4h at 40°C and then aged for 40h at 70°C with good stirring. The final products were washed and dried as described in the variable pH method. 2. Characterization methods. Elemental analyses were performed by ICP emission spectroscopy using solutions prepared by dissolving approximately 40 mg of solid sample in 100 mL of 20 % (v/v) HNO3. X-ray diffraction patterns of LDH powders were recorded on a Rigaku diffractometer equipped with DMAXB software, and a goniometer fitted with a variable temperature sample stage. The temperature dependence of the LDH samples was investigated using two complementary methods that gave equivalent results. One series of samples was heated for 10 min under N2 at six temperatures in the range 50-250°C and then cooled to room temperature to record the diffraction patterns. In a second series of eXperiments the samples were heated to the same six temperatures at a rate of 5°C/min, and the powder patterns were recorded at these temperatures. The heating rates and amounts of sample used in these latter experiments approximated the conditions used to record the thermogravimetric data. Dehydrated samples were allowed to rehydrate in 219 air until no further changes were observed in the diffraction patterns. Differential thermal analysis (DTA) was carried out under N2 on a DuPont 990 thermal analysis system. Zn metal was used as a calibration standard. Thermogravimetric analysis (TGA) was carried out on a Cahn 121 TG analyzer. Approximately 50 mg (TGA) and 15 mg (DTA) quantities of each LDH sample, either air-dried or pre-dried at 60°C for 10h in an oven, were heated at rates of 5°C/min (TGA) and 20°C/min (DTA). Transmission electron micrographs (TEM) were obtained with a JEOL 100 CX TEM at the Center for Electron Optics at MSU. Samples for TEM were prepared by dipping copper grids coated with holey carbon films into LDH powder or into sonicated LDH suspensions in ethanol or water. Nitrogen adsorption/desorption isotherms at liquid nitrogen temperature were obtained on a Coulter Omnisorb (TM) 360 CX sorptometer using ultrahigh purity nitrogen as the adsorbate and helium as the carrier gas. For surface area measurements and pore size distribution measurements, about 70 mg of each LDH carbonate was outgassed overnight at 100°C or 200°C under vacuum (10‘5 torr). For each outgassing temperature, the BET surface area was determined from the low pressure region (0.05 < P/Po < 0.25) of the adsorption isotherm. MeSOpore volumes and pore size distributions were obtained from the desorption isotherm using the BJH cylindrical pore model.13 220 C. Results and Discussion In general, LDH intercalation compounds are formed by co- precipitation reaction of M2+, M3+ and An' ions from aqueous solution. The reaction stoichiometry, temperature, time and pH are important synthesis variables that can influence the particle size and texture of the final products4. In addition, the method of mixing the reagents can be important in mediating the nucleation and precipitation process”. Hydrotalcite-type LDH carbonates typically are prepared by simply adding a solution of Mg2+ and Al3+ cations to a solution of Na2C03 until the pH of the reaction mixture reaches a pH of ~10.0 and then a solution of NaOH is used to maintain the pH value until the precipitation process is completed. In one synthetic method of the present work we use this "variable pH" methodology to obtain representative [Mg1- xAlx(OI-I)2](CO3)x/2 compositions with (1-x)/x (Mg2+/Al3+) ratios of ~ 2.0, 3.0 and 4.0. In addition, we utilized a second synthetic method, wherein the metal ion, carbonate, and hydroxide solutions are mixed under conditions of "constant pH” (10.0 10.1) at all stages of the nucleation/precipitation process. All other synthesis parameters were held constant for the two synthesis methods. Since pH can greatly influence initial particle nucleation we expected the variable pH and constant pH synthesis methods to afford products with different textural properties. Table 6.1 summarizes the compositions and structural parameters for the products prepared by both synthetic methods. It is seen that the Mg2+/Al3+ compositions of the final products are close to the values used 221 in the reaction stoichiometries, though the constant pH method tends to give somewhat higher levels of Al3+ substitution than the variable pH method. Notably, the in-plane unit cell a dimension decreases with increasing isomorphous substitution of Mg2+ by Al3+, reflecting the fact that the ionic radii for Al3+ and Mg2+ are 0.53 and 0.72 A, respectively15. The increase in basal spacing with decreasing Al3+ substitution is consistent with the decreased coulombic attractive force between the positively charged brucite-like layers and the negatively charged interlayer anion516. As expected, the unit layer charge and charge density increase in proportion to the Al content of the layers. The water desorption behavior of Mg1-xAlx LDH carbonates was examined in part by thermogravimetric analysis (T GA). Regardless of the synthesis method, all air-dried samples exhibited two general types of water, namely, pore water formed by capillary condensation between LDH particles and adsorbed surface water bound to gallery and external surfaces. As shown in Figure 6.1, curve A, an air dried Mg3Al LDH carbonate loses about 30 wt % H20, equivalent to ~63 moles of water per mole of Al, below 110°C, and then an additional ~11 wt % H20 or ~1.8 moles per Al between 110-250°C. However, if the sample is pre-dried at 60°C for 10h, then all the pore water is removed and only surface water is desorbed below 250°C as shown in Figure 6.1, curve B. Above ~250°C the LDH carbonate begins to undergo dehydroxylation of the brucite-like layers and loss of carbonate. 222 339 u :9. E838 «£032-52 a ..o 82 . .2 :8 u a 2 .2223 32820 E 8:23.... 82.858 . v86 wood 305 806 Sod God «4?» 56:8 omega 8qu «Nd mg .w now 3 .w mow 5w N4. .03.< .ED .934 mod mod 36 8.m cod 3d _ 4. .ae :00 3.3 cow ems. nnfi «mg. 2.6 $6 4. .36an 33m 09 .o and N~ md 8N6 vmmd Nmmd x ..cczazmnsm $2 232 2&2 232 :32 2&2 232 8&2; 8.82 mg .588 8502 me 222:; .3052: In 5838 can 05mg? 2: .3 E29 8.33:3 IQ:— x_ 8 P we 8. a _: 8.8.9.5 o.8~ m. no G 8 no 8 8.39.5 0.8. ENE ..Bmm .3: 2%: 2%: :32 2&2 232 seeded 85o: ma .525 85c: ma scat; 882880 In: £41-52 on. do.“ EVE 20a .«0 mowafl wagon ~26 $38 6.83 0529» 2258.: 95 RE: .Emmv v.38 cont—a ~Z\._.mm 2E. .00 2an 243 [7 J Continuous layers ; r : é r r :2: l l = :2: Interrupted layers J E 1:: 1 =1 ' l :2 = E: at the lutcrface :21 A 1:2: :2: = 1:1 1:1 k I J L 1; :I . r If I j r A Continuous layers ' r {I n r I J Defect sites contributing to mesopores Figure 6.11. A possible model for the interfacial regions of LDH derivatives exhibiting textural porosity in the pore radius range near 20 A. Water and gallery anions are not represented. 244 D. Conclusion Air-dried layered double hydroxides of general composition [Mg1- xAlx(OI-I)2](CO3)-nH2O, whether prepared by the variable pH or constant pH method, contain two general types of water, namely, interparticle pore water and surface-bound (extrinsic and intrinsic) water. Pore water is easily lost upon heating at 60°C, whereas intrinsic surface water bound at positions in the gallery region is lost above 150°C. Owing to increasing electrostatic interactions between the LDH layers and interlayer carbonate anions, the desorption temperature for intrinsic water increases with increasing Al3+ substitution. Extrinsic water adsorbed in multilayers at interfaces between cofacially stacked crystals is lost at temperatures between the temperature for loss of pore water (60°C) and the onset temperature for loss of intrinsic water (150°C). In general, increasing the layer charge by increasing the Al3+ substitution results in larger amounts of surface water, as well as in higher desorption temperatures. The textural properties of LDH carbonates, as reflected in crystal morphologies, surface areas and mesopore distributions, depend on the method of synthesis. Products prepared by the variable pH method give fine-grained, higher surface area crystals with rough surfaces. In contrast, the constant pH method affords larger hexagonal crystals with smooth surfaces. Depending on Al3+ substitution and preparation method, two distinctive pore distributions are observed, namely broad distributions with pore radii in the range50-300 A and narrow distributions with mesopore maxima near 20 A radius. Regardless of the layer charge density, the 245 variable pH method affords broad textural pore distributions. The constant pH method affords broad mesopore distributions for Mg2Al-LDH carbonates, but sharp distributions near 20 A radius for Mg3Al- and Mg4Al- LDH carbonates. The dependence of particle texture on synthesis method may be useful in mediating the materials performance properties of LDH compositions for potential applications as ionic conductors, anion exchangers, and selective heterogeneous catalysts. 246 References l. 2. Meyn, M.; Beneke, K.; Lagaly, G. Inorg. Chem. 1990, 29, 5201. Carrado, K. A.; Kostapapas, A.; Suib, S. L. Solid State Ionics 1988, 26, 77 and references therein. . Reichle, W. T. Solid State Ionics 1986, 22, 135. . Cavani, F.; Trifiro, F.; Vaccari, A. In Catalisis Today; Elsevier: Amsterdam, 1991; Vol. 11, pp 183 and references therein. 5. Chibwe, M.; Pinnavaia, T. J. J. Chem. Soc., Chem. Commun. 1993, 278 6. a) Lal, M.; Howe, A. T. J. Chem. Soc., Chem. Commun. 1980, 737. b) de Roy, A. ;Besse, J. P.; Bondot, P. Mater. Res. Bull. 1985, 20, 1091. c) Lal, M.; Howe, A. T. J. Solid State Chem. 1981, 39, 377. d) Moneyron, J. E.; de Roy, A.; Besse, J. P. Solid State Ionics 1991, 46, 175. . a) Allmann, R. Chimia 1970, 24, 99b) Allmann, R. Acta Cryst. 1968, B24, 972. d) de Roy, A.; Besse, J. P. Solid State Ionics 1989, 35, 35. e) de Roy, A.; Besse, J. P. Solid State Ionics 1991, 46, 95. . Etzler, F. M.; Conners, J. J. Langmuir 1991, 7, 2293 and references therein. Sposito, G.; Prost, R. Chem. Reviews 1982, 82, 553 and references therein. 10. Pfeifer, H. Surf. Sci. 1975, 52, 434 and references therein. 11. Lal, M.; Howe, A. T. J. Solid State Chem. 1981, 39, 368. 12. de Roy, A.; Forano, C.; Malki, K. E.; Besse, J. P. In Expanded Clays and Other Microporous Solids; Van Nostrand Reinhold: New York, 1992; Vol. 2, pp 108. 247 13. Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity, 2nd Ed.; Academic Press: London, 1982. 14. Courty, P.; Marcilly, C. In Preparation of Catalysts III; Elsevier: Amsterdam, 1983; pp. 485. 15. Shannon, R. D.; Prewitt, C. T. Acta Cryst. 1969, B25, 925. 16. Brindley, G. W.; Kikkawa, S. Am. Miner. 1979, 64, 836. 17. Marcelin, G.; Stockhausen, N. J.; Post, J. F. M.; Schutz, A. J. Phys. Chem. 1989, 93, 4646. 18. Hansen, H. C. B.; Taylor, R. M. Clay Miner. 1990, 25, 161. 19. Sato, T.; Fujita, H.; Endo, T.; Shimada, M.; Tsunashima, A. React. Solids 1988, 5, 219. 20. Sato, T.; Kato, K.; Endo, T.; Shimada, M. React. Solids 1986, 2, 253. 21. Miyata, S.; Kumura, T. Chem. Lett. 1973, 843. 22. Brindley, G. W.; Kikkawa, S. Clays Clay Miner. 1980, 28, 87. 23. Rey, F.; Fornés, V.; Rojo, J. M. J. Chem. Soc., Faraday Trans. 1992, 88, 2233. ' 24. Ferraro, J. R. (ed.) The Sadtler infrared spectra handbook of minerals and clays, Sadtler Res. Lab., Div. of Bio-Rad Lab. Inc., Philadelphia, 1982. 25. Hemandez—Moreno, M. J.; Ulibarri, M. A.; Rendon, J. L.; Sema, C. J. Phys. Chem. Minerals 1985, 12, 34. 26. Lippencott, E. R.; Schroeder, R. J. Chem. Phys. 1955, 23, 1099. 27. Nakamoto, K.; Margoshes, M.; Rundle, R. E. J. Am. Chem. Soc. 1955, 248 77, 6480. 28. Miyata, S. Clays Clay Miner. 1975, 23, 369. 29. Kruissink, E. C.; Reijen, L. L. J. Chem. Soc., Faraday Trans] 1981, 77, 649. 30. Bish, D. L.; Brindley, G. W. Am. Miner. 197 7, 62, 458. 31 Grey, 1. E.; Ragozzini, R. J. Solid State Chem. 1991, 94, 244. 32. Phy. Chem. Div. 1985 IUPAC. Pure & Appl. Chem. 1985, 57, 603.