ZSM-5 extrudates modified with phosphorus as a super effective MTP catalyst: Impact of the acidity on binder

Abstract

ZSM-5 extrudates catalyst was modified by wet-impregnation using phosphoric acid with various mass concentration (5%, 10% and 15%) and their performances were respectively evaluated in the methanol to propylene (MTP) reaction. This work focused on the changes in property of the extrudates and the interplay of P species with zeolite ZSM-5 and γ-Al2O3 binder before and after steam treatment. After introducing phosphorous acid, the surface P species was formed on the binder through P hydroxy bonding with Al hydroxy, thus allowed the decrease in number of Lewis acid sites (LAS) of binder. A small amount of P species interacted with TFAl in ZSM-5 powder, due to high silica ZSM-5 zeolite, leading to the reduction of Brønsted acid sites (BAS). For the P-modified extrudates, propylene selectivity and catalytic lifetime was improved compared to its pristine one, due to the increase of micropore area and the decrease of acid density with low ratio of strong acid sites/weak acid sites (S/W). After steaming at 500°C, the ratio of S/W on the P-modified extrudates further decreased. For the unmodified extrudates, the decrease was mainly enabled by the reduction in acid density and strong BAS, while for P-modified extrudates, the change of strong LAS of the binder to weak acid was more significant and its total LAS amount is nearly unchanged. The steamed P-modified catalyst (10wt%) showed high propylene selectivity of 47.5% and the longest lifetime of 218h at WHSV of 6h−1. The mechanism of regulating the acidity of extrudates by phosphorus modification has been proposed.