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

The effect of using different silanisation procedures as well as different exposure times and deposition-calcination cycles on the silanisation of thr ee different zeolites, i.e. ZSM-5, mordenite and beta, has been investigate d. Tetraethoxysilane (TEOS) was used as a silanisation agent. The three pro cedures used were chemical vapour deposition (CVD) in a static vacuum syste m and a vapour phase flow system, respectively, and chemical liquid deposit ion (CLD). Experiments were carried out with a view to characterising the e xtent to which the external sites were inertised and the pore openings narr owed. Unidimensional channel structures (e.g. mordenite) or aluminium-rich zeolites (mordenite, beta) were more affected with respect to pore mouth na rrowing than the siliceous three-dimensional zeolite ZSM-5. At high tempera tures, the presence of water increases the rate of TEOS deposition for zeol ites with higher aluminium contents. The adsorbed species resulting from th e TEOS deposition reaction or diluents may occupy the pore openings, reduci ng the extent to which pore mouth blocking occurs. Continuous removal of th e adsorbed products resulting from TEOS decomposition results in a faster, less selective deposition. Cyclic TEOS deposition can be used to obtain a m ore 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 Pestering the external surface acidity. (C) 2000 Elsevier Sci ence B.V. All rights reserved.