The chemical vapour and liquid deposition of tetraethoxysilane on ZSM-5, mordenite and beta

Abstract

The effect of using different silanisation procedures as well as different exposure times and deposition–calcination cycles on the silanisation of three different zeolites, i.e. ZSM-5, mordenite and beta, has been investigated. Tetraethoxysilane (TEOS) was used as a silanisation agent. The three procedures used were chemical vapour deposition (CVD) in a static vacuum system and a vapour phase flow system, respectively, and chemical liquid deposition (CLD). Experiments were carried out with a view to characterising the extent to which the external sites were inertised and the pore openings narrowed. Unidimensional channel structures (e.g. mordenite) or aluminium-rich zeolites (mordenite, beta) were more affected with respect to pore mouth narrowing than the siliceous three-dimensional zeolite ZSM-5. At high temperatures, the presence of water increases the rate of TEOS deposition for zeolites with higher aluminium contents. The adsorbed species resulting from the TEOS deposition reaction or diluents may occupy the pore openings, reducing the extent to which pore mouth blocking occurs. Continuous removal of the adsorbed products resulting from TEOS decomposition results in a faster, less selective deposition. Cyclic TEOS deposition can be used to obtain a more complete coverage by periodic removal of competitively adsorbed species. When agglomerates were crushed after modification, it appeared that acid sites on external surfaces not previously inertised during CVD were exposed, thus partly restoring the external surface acidity.