The chemical vapour deposition of Tetraethoxysilane on Zeolite ZSM-5

Doctoral Thesis


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University of Cape Town

This thesis reports on the investigation of a cyclic chemical vapour deposition (CVD) method eliminating unwanted surface reactions and enhancing shape selective properties by depositing inert silica onto the external surface of zeolite ZSM-5. A CVD treatment consists out of a deposition, flushing and heating step followed by calcination in flowing air. Modifications are carried out in up to 33 cycles on one and the same sample employing tetraethoxysilane (TEOS) as silica precursor at deposition temperatures ranging from 50 to 400°C. After each CVD treatment the catalyst is tested with appropriate test reactions in order to quantify the extent of modification. The gradual inertisation of the external surface is followed with the catalytic cracking of 1,3,5-triisopropylbenzene (1,3,5-TiPB) which is too bulky to enter the micro-pore structure of ZSM-5. The disproportination of toluene (TDP) is employed to monitor changes of the internal activity and alterations in the shape selective properties of the catalyst. The analysis of reactor effluent during the deposition, flushing and heating step of the modification cycle allows investingating the reaction pathway. The deposition reaction proceeds with the reaction of the alkoxy groups of TEOS with the terminal hydroxyls of the zeolite under the formation of ethanol.

Bibliography: leaves 119-124.