HZSM-5 modified by silica CVD for shape-selective production of p-xylene: Influence of in situ and ex situ preparation conditions of the zeolite

HZSM-5 prepared in situ or ex situ from the NH4-type one was modified by chemical vapor deposition (CVD) of silica for the shape-selective production of p-xylene from methylation of toluene, and the influence of preparation conditions upon the obtained selectivity was studied. The external surface of the ex situ prepared HZSM-5 deteriorated partially, maybe, due to the dealumination caused by moisture contained in the atmosphere. Because of the deteriorated external surface, the CVD could not sufficiently produce a uniform structure of silica to finely control the pore-opening size. On the other hand, the in situ prepared HZSM-5 which was not exposed to the atmosphere kept a fine structure of the external surface on which the silica could be deposited with a regulated structure. Fine control of the pore-opening size was thereby realized more uniformly on the in situ prepared HZSM-5. In situ preparation of H-form zeolite was thus found to be preferential for the CVD modification.