Studies on Physico-Chemical Properties of Silica Aerogels and Related Materials Synthesized at an Ambient Pressure

Abstract

Silica aerogels are transparent, porous and lightest synthetic solids. Aerogels posses more than 95% porosity and consist of less than 5% of solids material. Silica aerogels, prepared by sol-gel method followed by supercritical drying, have various extraordinary properties viz. low density (~20 mg/cm3) and thermal conductivity (0.05 W/mK), low sound velocity (~100ms) and large surface area (> 1500 m2/g). Due to this unique combination of properties, aerogels are considered as a new class solid state porous material having various scientific and technological applications like superinsulating transparent fillers in double walled windows, catalytic supports, gellifying agents, Cerenkov detectors, filters and containers for capturing of micrometeorites in space. But the structure of the aerogels gets deteriorated with the exposure to moisture. Hence, in order to use the aerogels in humid surroundings, it is necessary to convert the hydrophilic nature of the aerogels to hydrophobic nature. The present thesis describes the results of detailed studies made on the synthesis and characterization of silica aerogels and related nanoporous materials by various methods and precursors at ambient pressure which has been divided in three part literature survey and review of sol gel chemistry, synthesis of silica aerogels and related porous materials via salt routes and alkoxy routes. As the thesis deals with sol-gel process that involves understanding of colloids, brief introductions to various aspects of colloidal states are given in Chapter 1. This chapter includes definition and classification of colloids, properties colloidal dispersions, types of sols, charge on the colloidal particles, aggregation and stability of colloidal particles, forces between colloidal particles, emulsion and gels. Introduction to use of sol-gel for the preparation of porous nanoparticles has been described in Chapter 2. Basic aspects underlying the sol-gel transition and preparation of silica gels, effect of various catalysts on the sol-gel kinetics, dependence of particle growth on the pH of the sol, sources of the silica and synthesis of porous materials like aerogels using ambient pressure drying technique have been also discussed in Chapter 2. As the thesis deals with the synthesis of silica aerogel and related porous materials by various methods and precursors at ambient pressure. The different precursors for sol-gel processing have been discussed in Chapter 3. The synthesis of silica aerogels at ambient pressure based on the concept of “spring back effect” which is related to various solid surface interactions have been described in the Chapter 4. In addition, the different instruments and methods used in the present work to characterize the solid surface like the contact angle measurements, Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), Transmission electron microscopy (TEM), Thermogravimetric analyses (TGA) and BET surface area analyzer technique etc. are described with the relevant principles of their operation and working, in this Chapter. Chapter 5 describes the synthesis of silica aerogels by using sodium silicate as a precursor at ambient pressure. The effects of two different solvents (i.e., hexane and toluene) used for exchange on hydrophobicity and its physical properties of sodium-silica-based aerogels are discussed. Trimethylchlorolsilane (TMCS) was used as a hydrophobic reagent in the surface modification process. The molar ratio of sodium silicate/TMCS (A) was varied by volume from 2 to 12 ml. Also investigated were the effects pH and temperature on the gelation of hydrosol and the physical properties of the nanoporous silica aerogels (particle size, pore volume, surface area, porosity, and pore diameter). The thermal stability of the nanoporous silica aerogel was studied in the temperature range 25 to 800oC. The optical transmittance of the aerogel was found to decrease from 20 to 80% with an increasing A value. The pore size distribution (PSD) of the aerogels obtained by toluene solvent exchange was narrower and more uniform, whereas a broad and non-uniform PSD was observed for aerogels obtained by hexane solvent exchange. The results have been discussed by taking into consideration the PSD, steric effects and vapour pressures of various solvents and sol-gel processing parameters. The surface chemical modification was confirmed by the FTIR spectroscopy. The modified aerogels were thermally stable up to 350oC. Chapter 6 describes Preparation of hydrophobic mesoporous silica powder with a high specific surface area by surface modification of a wet-gel slurry and spray-drying. A hydrophobic mesoporous silica powder was prepared by surface modification of a sodium silicate-based wet-gel slurry. The effects of the volume percentage (%V) of trimethylchlorosilane (TMCS), used as surface modifying agent, on the physicochemical properties of the silica powder were investigated. It was observed that as the %V of TMCS in the simultaneous solvent exchange and surface modification process increased, so did the specific surface area and cumulative pore volume of the resulting silica powder. Hydrophobic silica powder with low tapping density (0.27 g/cm3), high specific surface area (870 m2/g), and a large cumulative pore volume (2.2 cm3/g) was obtained at 10%V TMCS. Surface silanol groups of the wet gel slurry were replaced by non-hydrolysable methyl groups (-CH3), resulting in a hydrophobic silica powder as confirmed by FT-IR spectroscopy and contact angle measurements. The different characterization techniques such as FE-SEM, EDS, TG-DTA, and nitrogen physisorption were employed to characterize the silica powders produced and to compare the properties of modified and unmodified silica powders. Moreover, a spray-dying technique were used in the present study, which significantly reduced the overall processing time, making our method suitable for economic and large-scale industrial production of silica powder. In Chapter 7, production of low-density sodium silicate-based hydrophobic silica aerogel beads by a novel fast gelation process and ambient pressure drying process, reported. The beads were prepared by acid-base sol-gel polymerization of sodium silicate in aqueous ammonia solution via the ball dropping method (BDM). To minimize shrinkage during drying, wet silica beads were initially prepared; their surfaces were then modified using trimethylchlorosilane (TMCS) via simultaneous solvent exchange and surface modification. The effects of the volume percentage (%V) of TMCS on the physical and textural properties of the beads were investigated. The specific surface area and cumulative pore volume of the silica aerogel beads increased with an increase in the %V of TMCS. Silica aerogel beads with low packing bed density (0.081 g/cm3), high surface area (917 m2/g), and large cumulative pore volume (2.8 cm3/g) was obtained when 10%V TMCS was used. Properties of the final product were examined by FE-SEM, TEM, BET, and TG-DT analyses. Surface chemical modifications were confirmed by FTIR spectroscopy. The hydrophobic silica aerogel beads were thermally stable up to 411oC. The obtained results for modified silica beads are compared with unmodified silica beads. In Chapter 8, the experimental results on the synthesis of tetraethoxysilane (TEOS) based silica aerogel with high specific surface area and large pore volume, via ambient pressure drying (APD) route, are reported. The silica aerogels were prepared by the acid-base sol-gel polymerization of TEOS precursor followed by the drying of the alcogels at an ambient pressure. The solvent present in the alcogel (i.e. ethanol) was replaced by a non-polar solvent such as hexane prior to the surface modification step. In order to minimize the drying shrinkage, the surface of the gels was modified using trimethylchlorosilane (TMCS) before the APD. The FTIR spectra of the surface modified aerogels showed Si-CH3 peaks at 2965, 1656 and 850 cm-1. The effect of the base catalyst (NH4OH) addition to the sol, at different time intervals (T), on the physical and textural properties of the resulting aerogels has been investigated. It has been observed that the surface area and the cumulative pore volume of the aerogels enhanced considerably from 819 to 1108 m2/g and 2.65 to 4.7 cm3/g, respectively with an increase in the T value from 6 to 48 h. Silica aerogels with very low bulk density (0.06 g cm-3), extremely high specific surface area (1108 m2 g-1) and large cumulative pore volume (4.7 cm3/g) could be synthesized by drying the alcogels at the ambient pressure. The aerogels were mesoporous solids with the average pore size ranging from 12 to 17 nm. The results have been discussed by taking in to consideration the hydrolysis and condensation reactions during the sol-gel polymerization of the TEOS precursor. In continuation with these studies, Monolithic silica aerogels with extremely low-density and high surface area were successfully prepared using iso-propanol as the preparative solvent by a two step (acid-base) sol-gel polymerization of TEOS precursor followed by the drying of the alcogels at a room temperature are reported in the Chapter 9. The effects of various sol-gel parameters on the monolithicity of the aerogels were investigated. Tetraethoxysilane was hydrolyzed and condensed in different solvents (methanol, ethanol, butanol, and isopropanol) using oxalic acid and NH4OH as the catalysts. To minimize shrinkage due to drying, the surfaces of the gels were modified using trimethylchlorosilane (TMCS) before the APD via a one-step solvent exchange/surface modification process. The effects of different solvents on the physical and textural properties of the resulting aerogels were investigated. It was observed that solvents containing longer chains of alkyl groups (-CH2-CH3) formed high silica polymerization in the alcogels which enhanced a distinct “spring-back effect” during APD, and consequently preserved the highly porous silica network by preventing collapse. Silica aerogels with very low bulk densities (0.041 g/cm3), extremely high specific surface areas (1150 m2/g), and large cumulative pore volumes (5.2 cm3/g) were successfully synthesized using isopropanol as a preparative solvent. Experimental results on the recovery of high-surface area mesoporous silica from waste hexaflurosilicic acid (H2SiF6) of fertilizer industry are reported in the Chapter 10. The process involves the reaction of hexaflurosilicic acid (50 ml, 24 wt% H2SiF6) and 100 ml, 0.297 M Na2CO3 to generate the alkaline aqueous slurry. Silica was separated from the slurry by filtration and the sodium fluoride was extracted from the aqueous solution by evaporation method. The obtained mesoporous silica was characterized by N2 absorption/desorption (BET), thermogravimetric analysis (TGA), x-ray diffraction (XRD), scanning electron microscope (SEM), and EDS. The results confirm that the separation of silica and NaF was successful and the final products have high purity. The silica product was found to have an average pore diameter of 4.14 nm and a high surface area (up to 800 m2/g). The process reported in this study may significantly reduce the release of hazardous materials into the environment and it might confer economic benefits to the responsible industries. The general conclusions drawn from these studies and the scope for the future work in the field of aerogels, in general, and hydrophobicity, in particular, have been presented in the Chapter 11.