Characterisation of templated xerogels for molecular sieve application

This paper presents the results of the characterisation of templated silica xerogels as precursor material for molecular sieve silica membranes for ga s separation. The template agent integrated in the xerogel matrix is a meth yl ligand covalently bended to the siloxane network in the form of methyltr iethoxysilane (MTES). Several surface and microstructural characterisation techniques such as TGA, FTIR, NMR, and nitrogen adsorption have been employ ed to obtain information on the reaction mechanisms involved in the sol-gel processing of such molecular sieves. The characterisation results show the effects of processing parameters such as heat treatment temperature, and t he concentration of the covalently bonded template on the development of th e pore structure. It was found that calcination temperature significantly e nhanced the condensation reactions thus resulted in more Si-O-Si groups bei ng formed. This was also confirmed with the data of FTIR characterisation s howing enhanced silicon bands at higher heat treatment temperatures. As a r esult of the promoted densification and shrinkable pore network the micropo re volume also reduced with increasing methyl ligand molar ratio. However, the mean pore diameter does not change significantly with calcination tempe rature. While the contribution of the templates towards controlling pore si ze is less precise, increasing the methyl ligand molar ratio results in the broadening of the pore size distribution and lower pore volume. Higher tem plate concentration induces the collapse of the xerogel matrix due to capil lary stress promoting dense xerogels with low pore volume (C) 2001 Elsevier Science B.V. All rights reserved.