Modification of HZSM-5 by CVD of various silicon compounds and generation of para-selectivity

The modification of HZSM-5 zeolite was carried out by a method of chemical vapor deposition of various silicon alkoxides. We thus obtained the HZSM-5 zeolites with the different pore-opening sizes and with the varied extents of inactivation of acid sites on the external surface. Among the various silicon compounds used in this study, the SiOCH3(C3H7)(3) was effective in selective inactivation of acid sites on the external surface of HZSM-5 zeolite without affecting its intracrystalline void space. From the characterization of the modified HZSM-5 zeolites, and its relationship with the shape selectivity, we concluded that the enhancement of para-selectivity in the methylation of toluene was ascribable more closely to controlling the pore-opening size than to the inactivation of acid sites on the external surface. (C) 1996 Academic Press, Inc.