Synthesis and Characterization of Nano-Aerogels

SYNTHESIS AND CHARACTERIZATION OF OXIDE NANOSTRUCTURES

VIA A SOL-GEL ROUTE IN SUPERCRITICAL CO₂

(Spine Title: Synthesis and Characterization of Oxide Nanostructures in ScCO₂)

(Thesis format: Monograph)

by

Ruohong Sui

Graduate Program

in

Engineering Science

Department of Chemical and Biochemical Engineering

A thesis submitted in partial fulfillment

of the requirements for the degree of

Doctor of Philosophy

Faculty of Graduate Studies

The University of Western Ontario

London, Ontario, Canada

 Ruohong Sui 2007
THE UNIVERSITY OF WESTERN ONTARIO

FACULTY OF GRADUATE STUDIES

CERTIFICTE OF EXAMINATIONS

Joint Supervisor ______________________________ Prof. Paul Charpentier Joint-Supervisor ______________________________ Prof. Amin Rizkalla Supervisory Committee ______________________________ Prof. Wankei Wan ______________________________ Prof. Hugo deLasa

Examiners ______________________________ Dr. Keith P. Johnston ______________________________ Dr. Zhifeng Ding ______________________________ Dr. Mita Ray ______________________________ Dr. Argyrios Margaritis

The thesis by

Ruohong Sui

Entitled:

Synthesis and Characterization of Oxide Nanostructures

Via a Sol-Gel Route in Supercritical CO₂
is accepted in partial fulfillment of the

requirements for the degree of

Doctor of Philosophy

Date____________________________ _______________________________

Chair of Thesis Examination Board

Abstract and Key Words

Fundamental and applied research on synthesizing oxide nanomaterials in supercritical CO₂ (scCO₂) is of potential importance to many industrial and scientific areas, such as the environmental, chemistry, energy, and electronics industries. This research has focused on synthesizing silica (SiO₂), titania (TiO₂) and zirconia (ZrO₂) nanostructures via a direct sol-gel process in CO₂, although it can be extended to other metal oxides and hybrid materials. The synthesis process was studied by in situ FTIR spectrometry. The resulting nanomaterials were characterized using electron microscopy, N₂ physisorption, FTIR, X-ray diffraction and thermal analysis.

Silicon, titanium and zirconium alkoxides were used as precursors due to their relatively high solubility in scCO₂. Acetic acid was used as the primary polycondensation agent for polymerization of the alkoxides, not only because it is miscible in scCO₂, but also as it is a mild polycondensation agent for forming well-defined nanostructures. The synthesis was carried out in a batch reactor, either in an autoclave connected with the in situ FTIR, or in a view cell with sapphire windows for observation of phase changes. The process was controlled by means of a LabView software through a FieldPoint^® interface. The nano spherical particles of SiO₂ aerogel with a diameter of ca. 100 nm were obtained by means of rapid expansion of supercritical solution (RESS). TiO₂ nanofibers with diameters of ca. 10 ~ 80 nm and nanospheres with a diameter of 20 nm were prepared in the high-pressure vessel by means of controlling the synthesis conditions followed by extraction of organic components using scCO₂. The mechanism of fibrous and spherical nanostructure formation was studied using in-situ ATR-FTIR spectrometry. It was found that the nanofibers were formed through polycondensation of a Ti acetate complex that leads to 1-dimensional condensation, while the nanospheres were formed through polycondensation of a Ti acetate complex that leads to 3-dimensional condensation. Using the same method, ZrO₂ nanospherical particles with a diameter of ca. 20 nm and mesoporous monoliths were also produced. The reaction rate and product properties were found to be a function of initial concentration of the precursor, the ratio of acid/alkoxides, the temperature, and the pressure. In order to obtain crystalline phases, the as-prepared aerogels were calcined under several different temperatures in the range of 300-600 °C. Anatase and rutile TiO₂ nanocrystallites, as well as tetragonal and monoclinic ZrO₂ were obtained depending on calcination temperature. The resulting materials exhibited a mesoporous structure and a high surface area.

The in situ ATR-FTIR spectra during the sol-gel process were studied using a chemometrics method. The pure-component spectra of the various reaction ingredients and products were extracted from the overlapped spectra, and the pure-component concentration profiles as well as the precursor conversion curves were subsequently obtained. The concentration-time curves allowed an assessment of the reaction kinetics. The silica alkoxide was found to react with acetic acid gradually to form SiO₂, and the reaction was favored by a higher temperature and a lower pressure. The titanium and zirconium alkoxides, however, reacted with acetic acid quickly to form metal acetate complexes, which subsequently grew into metal oxide particles.

ScCO₂ was found to be a superior solvent for synthesizing oxide nanomaterials, because of its zero surface tension that maintains the nanostructures and the high surface areas of the nanomaterials. Acetic acid was an excellent polycondensation agent for synthesizing SiO₂, TiO₂ and ZrO₂ nanomaterials, because of the formation of Si, Ti and Zr acetate coordination compounds that can be stabilized in CO₂. Stabilization of the colloidal particles in CO₂ was studied using both the ATR-FTIR and solubility parameter approaches. According to the ATR-FTIR study, the interaction between CO₂ molecules and the metal-bridging acetate is featured by Lewis-acid and Lewis-base interactions, which facilitates solubility. The solubility parameter calculation results showed that the acetate group decreases the solubility parameters of the macromolecules, improving solubility in CO₂.

This research showed that the direct sol-gel process in CO₂ is a promising technique for synthesizing SiO₂, TiO₂ and ZrO₂ materials with high surface areas and various nanoarchitectures.


Key Words: SiO₂ nanospheres, TiO₂ nanofibers, TiO₂ nanospheres, ZrO₂ nanospheres, mesoporous ZrO₂ monolith, aerogel, sol-gel, supercritical CO₂, metal alkoxides, carboxylic acid, acetic acid, ATR-FTIR, chemometrics, SIMPLISMA modeling, self-assembly, LA-LB interaction.
Dedication

To my dearest wife and daughters:

Jingyan, Gayle and Florrie.

Acknowledgements

First and foremost, I would like to thank my mother, Qingjie, and my father, Qinglu Sui, for the guidance when I got lost and the freedom when we have different opinions. To my wife, Jingyan, and daughters, Gayle and Florrie, for their sacrifice of my presence at home for five years and their support of my study thousands of miles away. I cannot thank my wife enough for my whole life for all her understanding and hard work and taking care of my whole family.

I am grateful to my supervisors, Professor Paul Charpentier and Amin Rizkalla, for their continuous guidance throughout my PhD study at Western. I would like to thank Professor Wankei Wan and Hugo DeLasa, for their kind help during my research.

I would like to express my sincere gratitude to Mr. Fred Pearson of the Brockhouse Institute for Materials Research, McMaster University, and Mr. Ron Smith of the Biology Department, UWO, for training me on the HRTEM and TEM, many thanks go to Dr. Todd Simpson of the Nanotech Laboratory, and Mr. Kobe Brad of the Surface Science Western for their help on SEM, and to Ms. Tatiana Karamaneva for her help in XRD analysis.

I would like to express my passion to all the colleagues in my group: to Ming Jia for his kind orientation of the campus and his contribution to the high-pressure appliances, to Yousef Bakhbakhi and Xinsheng Li for their sharing their knowledge of ATR-FTIR, to SM Zahangir Khaled and Ms. Behnez Hojjati for their cooperation on synthesis of the nanocomposites, to William Xu for helping me with Matlab, and to Ms. Rahima Lucky, Jeff Wood, Shawn Dodds, Kevin Burgess, Muhammad Chowdhury, Niraj Pancha and Colin Ho for their friendship and help.

I would like to thank Professor Keith Johnston of University of Texas (Austin) for his time of reading of my thesis and the corrections he made, although he could not attend the tele-examination as the external examiner due to the flood in Austin, Texas.

I would like to thank OGS scholarship and the financial funding from the Canadian Natural Science and Engineering Research Council (NSERC) and the Materials and Manufacturing Ontario EMK program.

Table of Contents

Abstract and Key Words iii

Dedication vi

Acknowledgements vii

Table of Contents ix

List of Tables xi

List of Figures xii

List of Appendices xxii

List of Abbreviations, Symbols, Nomenclature xxiii

Chapter 1. Introduction 1

Chapter 2. Direct Sol-Gel Process in SCF: A Review 19

Chapter 3. Materials and Methods 31

Chapter 4. Synthesis and Characterization of Silica Aerogel Particles 54

Chapter 5. Synthesis and Characterization of Titania Nanofibers and Nanospheres 70

Chapter 6. Synthesis and Characterization of ZrO2 Nanoarchitectures 112

Chapter 7. Kinetics Study on Direct Sol-Gel Reactions in CO2 by Using In Situ ATR-FTIR Spectrometry 131

Chapter 8. Stabilization of the Colloidal Particles in CO2 159

Chapter 9. Summary and Conclusions 179

Bibliography 185

Appendices 205

Curriculum Vitae 227

List of Tables

Table 1.1. The selected viscosities and densities of CO2 in vapor, liquid and supercritical phases.59, 60 8

Table 1.2. Critical properties for selected supercritical fluids in chemical reactions57 10

Table 3.3. Crystallographic data for rutile and anatase. 46

Table 5.4. IR absorption peaks corresponding to COO-1 stretching vibration in various acetates. 75

Table 5.5. Results of the reaction of TBO with acetic acid in CO2. 81

Table 6.6. Synthesis conditions of ZrO2 structures in CO2 and characterization results. 119

Table 8.7. Solubility parameters of scCO2 under selected temperatures and pressures. 300, 303 173

Table 8.8. The thermodynamic properties and the average solubility parameters of the alkoxides. 174

Table 8.9. The related atomic and group contributions to the energy of vaporization and mole volume at 25 °C.305 175

Table 8.10. Solubility parameters of the macromolecules calculated using the group contribution method. 176

List of Figures

Figure 1.1. Schematic: the phase diagram of a typical material. 7

Figure 1.2. Surface tension of saturation liquid CO2 vs. pressure. The data points were labeled with the corresponding saturation temperatures at the specific pressures. In the supercritical region, the surface tension is zero.60 9

Figure 1.3. Flow chart of conventional sol-gel route. 17

Figure 1.4. Mechanisms of hydrolysis and condensation of metal alkoxides.105 18

Figure 3.5. The view cell. 33

Figure 3.6. Schematic of experimental setup: (A) computer with LabView Virtual Instrument, (B) FieldPoint by National Instruments, (C) temperature controller, (D) thermocouple, (E) pressure transducer (F) stainless steel view cell equipped with sapphire windows, (G) pneumatic control valve, (H) needle valve, (I) check valve, (J) syringe pump, (K) CO2 cylinder. 35

Figure 3.7. The Front Panel of Pressure Control. A: switch for automatic or manual control; B: pressure cylinder indicator; C: pneumatic valve open-close indicator; D: pneumatic valve; E: read error out; F: write error out; G: pressure setpoint; H: setpoint and real pressure indicator; I: PID tuning parameters; J: stop button. 36

Figure 3.8. Schematic of experimental setup: autoclave with online FTIR and GC-MS. (A) computer; (B) FTIR; (C) temperature and RPM controller with pressure display; (D) 100 ml autoclave equipped with diamond IR probe; (E) needle valves; (F) check valves; (G) syringe pump; (H) container for carboxylic acid; (I) CO2 cylinder. 38

Figure 3.9. Schematic of particle collection vessel for the RESS process. 39

Figure 3.10. Schematic of the reactor with the ATR-FTIR probe assembly. 41

Figure 3.11. Schematic of the DicompTM probe. Infrared radiation is reflected into the chemically resistant ATR disk that is in contact with the reaction mixture. The drawing is adapted from ASI. 42

Figure 3.12. Schematic: interactions of a specimen with incident electrons (redrawn from Ref. 154). 43

Figure 3.13. Schematic of a unit cell of crystals.158 45

Figure 3.14. Schematic of X-ray reflection on the crystal planes. 47

Figure 3.15. BET plot of N2 adsorption on silica gel at 91 K. The data was obtained from Ref. 161 49

Figure 3.16. Classification of hysteresis loops as recommended by the IUPAC.164 51

Figure 3.17. A schematic DSC curve demonstrating the appearance of glass transition, crystallization and melting. 53

Figure 4.18. ATR-FTIR plot of the progress of an aerogel reaction in CO2, where the lines are experimental data: 4.4 mmol TEOS + 22 mmol HOAc + 4.4 mmol H2O at 3000 psig and 60 C from 1 to 7 h at 1-h intervals. 60

Figure 4.19. FTIR spectrum of the SiO2 aerogel powder, there are three obvious absorptions (1065, 928 and 797cm-1) in the range between 4000 and 600 cm-1 (The small absorptions at 3294 and 1709 cm-1 are due to water and acetic acid residue respectively). 61

Figure 4.20. Effect of acid type on TEOS condensation activity in CO2. The points are experimental data from the ATR-FTIR absorbance at 1066 cm-1 (n = 3). The experimental conditions are 60 °C and 3000 psig, 0.044 M TEOS, 0.176 M acid, and 2.75 M acetone. 62

Figure 4.21. Effect of water and acid concentration on TEOS condensation activity in CO2. The points are experimental data from the ATR-FTIR absorbance at 1066 cm-1 (n = 3). Series 1 (♦) : 0.088 M TEOS + 0.352 M HOAc; Series 2 (■): 0.088 M TEOS + 0.352 M HOAc + 0.088 M H2O; Series 3 (▲): 0.088 M TEOS + 0.704 M HOAc. The experimental conditions are 50 °C and 3000 psig. 63

Figure 4.22. Effect of temperature on TEOS condensation activity in CO2. The points are experimental data from the ATR-FTIR absorbance at 1066 cm-1 (n = 3). The experimental conditions are 3000 psig, 0.088 M TEOS, and 0.352 M acetic acid. 66

Figure 4.23. Effect of pressure on TEOS condensation activity in CO2. The points are experimental data from the ATR-FTIR absorbance at 1066 cm-1 (n = 3). The experimental conditions are 50 °C, 0.088 M TEOS, and 0.352 M acetic acid. 66

Figure 4.24. SEM of SiO2 aerogel powder. The experimental conditions are: (a) 1.1 mmol TEOS + 7.7 mmol 96% HCOOH in the 25-mL view cell, at 40 °C and 2000 psig; (b) 0.176 mmol TEOS + 2.64 mmol 96% HCOOH in the 25-mL view cell, at 40 °C and 2000 psig. 67

Figure 4.25. SEM of SiO2 aerogel powder: (a) collected from the high pressure mixer upon decompression. The experimental conditions are 0.044 M TEOS + 0.176 M HOAc, at 60 °C and 3000 psig; (b) collected using the Rapid Expansion of Supercritical Solutions (RESS) process. The experimental conditions are 0.044 M TEOS + 0.176 M HOAc, at 60 °C and 6000 psig. 68

Figure 5.26. Reported structures of TA oligomers with acetate ligands: (a) chelating bidentate; (b) bridging bidentate; (c) monodentate; (d) Ti6O4(OBun)8(OAc)8, rutilane shape; (e) Ti6O6(OPri)6(OAc)6, hexaprismane shape. (a) ~ (c), Ref.229; (d) and (e) images were modified from Ref.231. The black balls stand for Carbon, the red ones stand for Oxygen, Titanium is in the middle of the octahedrons. 75

Figure 5.27. Photographs following the reaction in the view cell: (a) start of the reaction; (b) the moment directly proceeding gel-formation. 77

Figure 5.28. Photographs: (a) the low-density monolith, and (b) the cracked high-density monoliths. 79

Figure 5.29. Isotherms of TiO2-1 high-density monolith: (a) as-prepared and calcined at (b) 380 °C, (c) 500 °C, and (d) 600 °C. 84

Figure 5.30. Isotherm: TiO2-13 low-density monolith. (a) as-prepared and calcined at (b) 380 °C, (c) 500 °C, and (d) 600 °C. 85

Figure 5.31. SEM: (a) TiO2-1 monolith at low magnification, and (b) at high magnification. The sample was calcined at 500 °C. The arrows indicate where the mesopores are present. 88

Figure 5.32. SEM of as-prepared TiO2-11 monolith at increasing magnification: (a) 50 times; (b) 100 times; (c) 1,000 times and (d) 10,000 times. The arrow indicates where the macropores are present. 89

Figure 5.33. SEM: (a) as-prepared TiO2-11; (b) TiO2-11 calcined at 380 °C (the sample was coated with gold to prevent charging); (c) TiO2-13 nanofiber clusters under a low magnification; and (d) TiO2-13 nanofiber clusters under a high magnification. 90

Figure 5.34. SEM: (a) TiO2-18 nanofibers with a diameter of 40 nm, calcined at 380 °C; (b) TiO2-17 curled nanofibers, calcined at 380 °C. 91

Figure 5.35. SEM. The TiO2-10 rods calcined at 380 °C: (a) at a low magnification, and (b) at a high magnification. 91

Figure 5.36. SEM. The result of scCO2 drying before the complete condensation. 92

Figure 5.37. TiO2 synthesized in isopropanol. The synthesis condition: the initial concentration of TIP = 1.1 mol/L, HOAc/alkoxide molar ratio = 5.5, 60 C. Scale bar: 200 nm. 92

Figure 5.38. TiO2-1 nano spherical particles calcined at 380 °C: (a) TEM and (b) HRTEM. 93

Figure 5.39. TEM and HRTEM images: (a) TiO2-11 nanofibers calcined at 380 °C; (b) TEM of TiO2-13 nanofibers calcined at 380 °C; (c) HRTEM of TiO2-13 nanofibers calcined at 380 °C; and (d) TEM of TiO2-18 nanofibers calcined at 380 °C. 94

Figure 5.40. DSC of TiO2-1, -2, -3, -4 aerogels synthesized at 6000 psig and different temperature (a) 40 °C; (b) 50 °C; (c) 60 °C; (d) 70 °C. The initial concentration of TBO: 1.48 mol/L, acetic acid/HOAc: 4.15. 96

Figure 5.41. TGA of as-prepared TiO2-11. 97

Figure 5.42. XRD: amorphous, anatase (a) and rutile (r) phases of the TiO2 mono spherical particles calcined at different temperatures. (A). TiO2-4 synthesized at 70 °C calcined at 300 °C; (B) TiO2-3 synthesized at 60 °C and calcined at 300 °C (B), 380 °C (C), 500 °C (D) and 600 °C (E). The initial concentration of TBO: 1.5 mol/L, acetic acid/HOAc: 4.12, pressure: 6000 psig. 98

Figure 5.43. XRD: nanocrystalline TiO2-11, r: rutile, a: anatase (A) as-prepared TiO2-a aerogel, and calcined at (B) 380 °C, (C) 500 °C (D) 600 °C (E) 800 °C. 100

Figure 5.44. XRD: nanocrystalline TiO2-13, r: rutile, a: anatase (A) calcined at 380 °C, (B) 500 °C (C) 600 °C. 101

Figure 5.45. Powder ATR-FTIR spectra of TiO2-1 nanospherical particles. (A) as-prepared, (B) calcined at 200 °C, (C) calcined at 300 °C, (D) calcined at 400 °C of TiO2-1. 102

Figure 5.46. (a) IR spectrum of TIP. (b) ~ (e) in situ FTIR spectra of polymerization of TIP with acetic acid, at 60 °C and 4500 psig, initial concentration: TIP=1.1 mol/L, HOAc/TIP=5.5 (mol/mol). Reaction time: (b) 10 min; (c) 230 min; (d) 250 min; (e) 4320 min. 104

Figure 5.47. (a) FTIR spectrum of complex 2 synthesized in CO2. The infrared spectrum was recorded on a Bruker Vector 22 FTIR instrument. The crystals were dispersed in a KBr tablet. (b). In situ FTIR spectrum of polymerization of TIP with acetic acid, at 60 °C and 4500 psig, initial concentration: TIP=1.1 mol/L, HOAc/TIP=3.5 (mol/mol). 105

Figure 5.48. (a) IR spectrum of TBO. (b) to (e) in situ FTIR spectra of polymerization of TBO with acetic acid, at 60 °C and 4500 psig, initial concentration: TBO=1.5 mol/L, HOAc/TBO=5.0 (mol/mol). Reaction time: (b) 10 min; (c) 100 min; (d) 1280 min; (e) 5840 min. 106

Figure 5.49. In situ FTIR spectra of polymerization of TBO with acetic acid, at 60 °C and 4500 psig, initial concentration: TBO=1.1 mol/L, HOAc/TBO=5.5 (mol/mol). Reaction time: (a) 10 min; (b) 100 min; (c) 300 min; (d) 330 min; (e) 5800 min. 107

Figure 5.50. Schematic of the skeletal arrangements of complex 2 (R= Pri) (a) and complex 1 (R= Bun) and 3 (R= Pri) (b). In the scheme, all acetate groups, some of the oxo bonds, and two bridging OR groups are omitted to make the structure simpler. 108

Figure 5.51. Schematic of nanostructure formation. Polycondensation of complex 2 or 1 led to formation of either straight or irregular-shape macromolecules. Coacervates and tactoids were formed to lower the energy of the macromolecules in CO2, which in turn resulted in the formation of either fibers or spheres.105 109

Figure 6.52. (a) The homogeneous transparent phase at the initial stage of the reaction; (b) the formation of the opaque white phase; (c) the formation of the transparent gel in the view cell. 117

Figure 6.53. SEM and TEM images. The translucent monolith of ZrO2-2-400: (a) SEM and (b) TEM. The translucent monolith of ZrO2-2-400: (a) SEM and (b) HRTEM. 121

Figure 6.54. SEM. Precipitate chunks of ZrO2-7. 122

Figure 6.55. Isotherms of the ZrO2 as-prepared and calcined samples at 500 °C: (a) ZiO2-2; (b) ZrO2-4. See Table 5.1 for the synthesis conditions for ZrO2-2 and -4. 124

Figure 6.56. BJH desorption pore-size-distributions of the as-prepared and calcined at 500 °C: (a) ZrO2-2; (b) ZrO2-4. See Table 5.1 for the synthesis conditions for ZrO2-2 and -4. 125

Figure 7.57. Three-dimensional in situ FTIR spectra: 0.088 M TEOS reacting with 0.362M HOAc in CO2 at reaction times of 0-360 minutes and at 50 ºC and 3000 psig. 139

Figure 7.58. In situ FTIR spectra of 0.088 M TEOS reacting with 0.362M HOAc in CO2 at reaction time of 0-360 minutes and at 50 ºC and 3000 psig. 140

Figure 7.59. Comparison of the self-modeling resolved spectra with (a) TEOS and (b) SiO2 aerogel spectra. The spectra of TEOS in (a) and SiO2 in (b) were experimentally collected by means of ATR-FTIR. 142

Figure 7.60. The relative concentration profiles obtained from the self-modeling method. The source IR data was obtained at 50 °C and 3000 psig, and the initial concentrations of TEOS and acetic acid was 0.088 and 0.362 M, respectively. 143

Figure 7.61. Temperature’s effect on the precursor conversion within a reaction time of 0-360 minutes. The initial concentration of TEOS and acetic acid was 0.088 and 0.362 M, respectively, and the pressure was 3000 psig. 144

Figure 7.62. Pressure’s effect on the precursor conversion within a reaction time of 0-360 minutes. The initial concentration of TEOS and acetic acid was 0.088 and 0.362 M, respectively, and the temperature was 50 °C. 144

Figure 7.63. The three-dimensional in-situ FTIR spectra of 1.10 M TIP reacting with 3.85 M HOAc within a reaction time from 10 to 3720 minutes in CO2. The reaction temperature and pressure were 60 ºC and 4500 psig, respectively. 146

Figure 7.64. In situ FTIR spectra of 1.10 M TIP reacting with 3.85 M HOAc in CO2 within a reaction time from 10 to 3720 minutes, at 60 ºC and 4500 psig. The arrows show the absorbance change trend. 147

Figure 7.65. FTIR. (a) The black curve is one of the resolved spectra from in situ IR and the curve-fitting results, the residual sum of squares: 4.02E-8; (b) and (c): the spectra of single crystals of 2 and 3, respectively. Note: the black curves = experimental or resolved IR spectra; the pink curve = the fitting curve; blue curves = the individual Gaussian function curves; and the red curve = the residual curve. The number 2 and 3 denote Ti6O6(OPri)6(OAc)6 and Ti6O4(OPri)8(OAc)8, respectively. 148

Figure 7.66. (a) The other resolved spectrum from the in situ IR spectra, which is attributed to the aerogel product, and (b) TiO2 aerogel spectrum. 149

Figure 7.67. The relative concentration profiles during TIP reacting with HOAc in CO2 within a reaction time from 10 to 3720 minutes, at 4500 psig and 60 ºC; the initial concentration of TIP and HOAc was 1.10 and 3.85 M, respectively. 150

Figure 7.68. The relative concentration profiles during TIP reacting with HOAc in CO2 within a reaction time from 10 to 610 minutes, at 4500 psig and 60 ºC; the initial concentration of TIP and HOAc was 1.10 and 6.05 M, respectively. 151

Figure 7.69. Three-dimensional in situ FTIR spectra during the direct sol-gel process of ZBO with acetic acid within a reaction time of 10-1330 minutes, at 40 °C and 4500 psig. The initial concentration of ZBO and acetic acid was 0.547 and 1.76 M, respectively. 154

Figure 7.70. Two-dimensional in situ FTIR spectra during the direct sol-gel processing of ZBO with acetic acid within a reaction time of 10-1330 minutes, at 40 °C and 4500 psig. The initial concentration of ZBO and acetic acid was 0.547 and 1.76 M, respectively. The arrows indicate the absorbance change trend. 155

Figure 7.71. Two resolved spectra from the in situ IR spectra by means of SIMPLISMA modeling. One spectrum is attributed to the Zr-acetate complex and the other to the ZrO2 aerogel. 156

Figure 7.72. The relative concentration profiles of Zr-acetate complex and the condensation product during the direct sol-gel processing of ZBO with acetic acid in scCO2 with a reaction time of 10-1330 minutes, at 40 °C and 4500 psig. The initial concentration of ZBO and acetic acid was 0.547 and 1.76 M, respectively. 156

Figure 8.73. In situ IR spectra: (a) 1.3 M TMOS reacting with 5.3 M HOAc in CO2 at a reaction time of 360 minutes, at 50 °C and 4000 psig; (b) 1.10 M TIP reacting with 3.85 M HOAc in CO2 at a reaction time 40 minutes, at 60 °C and 4500 psig; and (c) 0.547 ZBO reacting with 1.76 M HOAc in CO2 at a reaction time of 40 minutes, at 40 °C and 4500 psig. 161

Figure 8.74. Schematic drawing of possible LA-LB interactions between CO2 and (a) the bridging acetate bidentate, (b) the chelating acetate bidentate, and (c) the acetate monodentate. M: Si, Ti or Zr. 162

Figure 8.75. Schematic drawing of ATR-FTIR analysis. The aerogel powder was pressed against the diamond mirror to obtain higher absorbance peaks. 164

Figure 8.76. The ATR-FTIR spectra of (a) a-TiO2-A, (b) a-TiO2-B and (c) a-ZrO2 before addition of CO2 (black) and at one minute after venting of CO2 (red). 166

Figure 8.77. The IR spectra and the curve-fitting results of CO2 interacting with (a) a-TiO2-A, the residual sum of squares: 3.86E-3; (b) a-TiO2-B, the residual sum of squares: 4.01E-4; and (c) a-ZrO2, the residual sum of squares: 1.81E-3. Note: the black curves = experimental spectra; the pink curve = the fitting curve; blue curves = the individual Gaussian function curves; and the red curve = the residual curve. 168

Figure 8.78. Schematic of possible association modes of LA-LB interaction between CO2 and metal bridging acetate. (a) d-p orbital overlap between metal (M) and acetate, and CO2 associates with the π bond either in a vertical or a parallel mode; (b) the partial positive carbon CO2 associates with the lone pairs of oxygen either in T-shape or bended T-shape. 170

Figure 8.79. The bending and stretching vibrations of a free CO2 molecule. The two bending vibrations absorb at the same frequency of IR beam due to the symmetry of the CO2 molecule. 170

Figure 8.80. IR spectra of CO2 impregnated into (a) a-TiO2-A, (b) a-TiO2-B and (c) a-ZrO2 in the ν3 stretching mode region, at one minute after venting of CO2; (d) CO2 impregnated into a-TiO2-A at five minutes after venting of CO2. 171

List of Appendices
Appendix 1. American Chemical Society’s Policy on Theses and Dissertations….…..207

Appendix 2. Copyright Permissions of Acta Crystallography Journals…………….….209

Appendix 3. Physical Properties of the Raw Materials Used in This Thesis…….…….210

Appendix 4. Powder XRD Analysis Conditions……………………………………..…211

Appendix 5. N₂ Physisorption Analysis Conditions……………………………………212

Appendix 6. Block Panel of LabView Vis……………………………………………...218

Appendix 7. Instrumentation and Configuration of the Temperature and Pressure Control

………………………………………………………………………………………….219

Appendix 8. Synthesis and Characterization of the Ti-Acetate Single Crystals……….220

Appendix 9. Description of the SIMPLISMA Modeling……………………………….222

Appendix 10. Statistical Results of the SIMPLISMA Modeling……………………….225

Appendix 11. Calculation of Solubility Parameters Using Group Contribution Method

…………………………………………………………………………………………..227
List of Abbreviations, Symbols, Nomenclature

Abbreviations:

Abs: absorbance

ATR-FTIR: attenuate total reflection Fourier transform infrared spectroscopy

BET: Brunauer-Emmett-Teller

DSC: differential scanning calorimeter

FTIR: Fourier transform infrared spectroscopy

HOAc: acetic acid

LA-LB: Lewis acid and Lewis base

NMR: nuclear magnetic resonance

ScCO₂: supercritical CO₂

ScH₂O: supercritical H₂O

SEM: scanning electron microscopy

SIMPLISMA: Simple-to-use interactive self-modeling mixture analysis

SMCR: self-modeling curve resolution

TA: titanium alkoxide

TBO: titanium(IV) n-butoxide

TEM: transmission electron microscopy

TEOS: tetraethyl orthosilicate

TMOS: tetramethyl orthosilicate

TGA: thermogravimetric analysis

TIP: titanium(IV) iso-propoxide

TMOS: tetramethyl orthosilicate

XRD: X-ray diffraction

ZBO: zirconium(IV) butoxide

ZPO: zirconium(IV) propoxide
Symbols:

1: Ti₆O₄(OBuⁿ)₈(OAc)₈

2: Ti₆O₆(OPrⁱ)₆(OAc)₆

3: Ti₆O₄(OPrⁱ)₈(OAc)₈

: integrated intensity of rutile

: integrated intensity of anatase

B: pure component matrix

b_ij: element of pure component matrix

C: concentration coefficient matrix

C₀: initial concentration

C_t: concentration at reaction time t

c_ij: element of concentration coefficient matrix

D: in situ FTIR matrix

d_ij: element of in situ FTIR matrix

D_pore: adsorption average pore diameter

D_scher: crystallite size

E_coh: cohesive energy density

E_a: activation energy

k: reaction rate constant

P_c: critical pressure

P_re: reaction pressure

R: universal gas constant

S_bet: Brunauer-Emmett-Teller (BET) surface area

T_c: critical temperature

T_re: reaction temperature

V: molar volume

V_c: critical volume

V_pore: pore volume per gram

: weight fraction of rutile

α: offset for SIMPLISM modeling

β: half-width at half-height of the diffraction peak

δ: solubility parameter

: solubility parameter of supercritical fluid

Δe_i: energy of vaporization

Δe_ir: energy of vaporization of repeating unit

: reaction standard enthalpy

E_V: energy change upon isothermal vaporization

H_V: enthalpy change during vaporization

Δυ^‡: activation volume

Δυ_i: mole volume

Δυ_ir: mole volume of repeating unit

θ: half the angle of diffraction

λ: X-ray wavelength

: reduced density of supercritical fluid

: reduced density of liquid at its normal boiling point