Porous hybrid organic-inorganic silica materials: preparation, structural and transport properties

Abstract

The aim of this project was to prepare a series of silica materials based on sol-gel processing of alkoxysilanes using glucose and glycerol as templates for potential applications in membrane design for pervaporation. The materials were characterized using structural and dynamic techniques to gain information about the effect of the templates on the formation of micro- and mesoporous silicates. The interaction between templates and silica matrices were investigated using FTIR, Raman Spectroscopy, Solid State NMR Spectroscopy, Physisorption and SEM. Close contact between templates and silica networks was observed by NMR cross polarization studies. The chemistry was then extended to prepare hybrid organic-inorganic silica materials by introducing organic ligands, with glycerol as a template to control the porosity of the hybrid materials. By varying the ligand as well as the template, the physical properties of the gel can be controlled. Composites of hydroxypropylcellulose, HPC, and silica were also prepared and characterized. There was no phase separation during sol-gel processing suggesting HPC was dispersed homogenously in the silica matrices. This was also confirmed by solid state NMR. Temperature dependence showed some indications of conformational change in the HPC within the silicate, above 308K. The transport properties of the hybrid materials were observed by monitoring the diffusion behaviour of water and several selected solvents using Pulsed Field Gradient NMR. The self-diffusion of water and the organic solvents in the hybrid silica materials were two to three orders of magnitude smaller than in the liquid bulk suggesting restricted diffusion at the pore surface. The effect of surface polarity also contributed to water and solvents diffusivities. The temperature dependence of diffusion was useful to derive the activation energy whereas the dependence on NMR observation time provided information on both tortuosity and pore connectivity of the hybrid silica materials. The hybrid silica membranes were prepared by spin coating of polymeric silica sol on top of a macroporous alumina support after being occluded by colloidal silica. It was then used for pervaporation of water ethanol mixtures. The results implied that separation factor increased as the temperature increased. However permeate fluxes were less affected.