Toluene methylation over modified HZSM-5 catalysts: effect of combined silylation and dealumination

Abstract

p-Xylene is valuable raw material for the chemical and petrochemical industries, e.g., for producing polyesters and plasticizers. Currently, p-xylene is produced by methylation of toluene and toluene disproportionation. The shape-selective alkylation of toluene with methanol is also a promising way of producing p-xylene in the absence of undesired by-product as compared with toluene disproportionation. The difficulty of achieving a high p-xylene selectivity arises from several major side reactions during toluene methylation. In order to improve the selectivity to p-xylene, the acidity and pore structure of zeolite need to be modified properly. In this work, HZSM-5 catalyst was synthesized at a desired SiO2/Al2O3 molar ratio of 200 via hydrothermal synthesis. It was modified by various methods including chemical liquid deposition with tetraethyl orthosilicate (CLD), dealumination with oxalic acid (DeAl), and the combination of CLD and DeAl. The modified catalysts were tested for toluene methylation with methanol at atmospheric pressure, 400 °C, WHSVs of 24 and 40 h-1, and toluene-to-methanol (T/M) molar ratios of 4:1 and 8:1. The as-synthesized HZSM-5 catalyst possesses irregular hexagonal prisms with rectangular insertion in morphology. The highest p-xylene selectivity around 80 % was obtained using the catalyst modified by silylationdealumination sequence (DeAl-CLD-HZ5) under the following reaction conditions; a constant temperature of 400 °c, WHSV of 40 h-1, and T/M molar ratio of 8:1.