Catalytic Performance of Modified ZSM-5 Designed with Selectively Passivated External Surface Acidity by Phosphorus

A phosphorus modified HZSM-5 sample with excellent p-xylene selectivity was successfully obtained by a method of reversing the external surface ZSM-5 crystals to positive charged by HDTMA, which could selectively deposit the phosphorus on the external surface and pore entrance. The texture properties were characterized by XRD, XRF and N-2 adsorption-desorption. The accessibility of acid sites was determined by NH3-TPD and Py/Coll-IR. The solid-state NMR measurement indicated that more phosphorus did not enter the pores of the MZS-1.5P sample and existed as H4P2O7 or polyphosphates not connected to aluminum. Applied to alkylation of toluene with methanol, MZS-1.5P sample all exhibited excellent para-selectivity at reaction temperatures of 360-420 degrees C and WHSV of 1.0-4.0 h(-1). The TPD of xylene analysis showed that the interactions between diffusing molecule and surface of ZSM-5 as well as the geometric factors play important roles in the diffusion process.