On the Enhanced Selectivity of HZSM-5 Modified by Chemical Liquid Deposition

The external surfaces and pore mouth regions of HZSM-5 samples with different crystal sizes were modified by chemical liquid deposition (CLD) with tetraethoxysilane (TEOS), which led to the passivation of unselective acid sites. The modification was found to be more effective for zeolite samples of larger crystal size. The diffusivity of o-xylene was substantially reduced after silylation, while the diffusivity of toluene hardly changed. Dealumination of the external surface of the zeolite crystals enhanced the silylation effects; this was related primarily to the removal of acid sites associated to extra-framework alumina. For the modified catalysts, a significant increase in the selectivity to p-xylene in the disproportionation of toluene was achieved.