THE CHEMICAL-VAPOR AND LIQUID DEPOSITION OF TETRAETHOXYSILANE ON THE EXTERNAL SURFACE OF ZSM-5

The external acidity of ZSM-5 was modified by chemical vapour depositi on (CVD) and chemical liquid deposition (CLD) of tetraethoxysilane [Si (OC2H5)(4)] using a static vacuum system, a vapour phase flow system a nd liquid phase deposition. Temperature programmed desorption (TPD) te chniques were used to characterise the acidity changes arising from th ese modifications. Pyridine was used as a probe for the total acidity and 4-methyl quinoline (MQ) was used to probe the external acidity. Th e adsorption capacities of the samples were measured using n-hexane, p -xylene, o-xylene and 1,2,4-trimethyl benzene. The extent of Si(OC2H5) (4) deposition was strongly dependent on temperature in both vapour ph ase flow and static vacuum systems. Continuous Si(OC2H5)(4) deposition was observed in the presence of H2O at relatively high temperatures w hen decomposition products were removed from the sample. It is propose d that physisorbed species need to be removed by evacuation or calcina tion to re-expose active sites, thereby enabling complete inertisation of the external surface acidity to occur, and that a more uniform cov ering can be obtained when a gradual deposition process is used. Such a process may be achieved by the use of diluents or by preventing over exposure of Si(OC2H5)(4) to the sample under conditions where continuo us deposition may occur. It was shown that it is possible to reduce th e relative external surface acidity by 97% without significant changes in acidity as measured by Py-TPD or adsorption capacity. (C) 1998 Els evier Science B.V. All rights reserved.