SELECTIVE TOLUENE-METHANOL ALKYLATION OVER MODIFIED ZSM-5 ZEOLITE CATALYSTS

Abstract

ABSTRACT H-ZSM-5 zeolite was modified with divalent elements of groups 11 and VIII of periodic system. The modifications were achieved by impregnating H-2SM-5, containing 35 wt% of alumina as a binder, with metal precursors of Mg, Sr, Ba, Co and Ni. Structure of the modified catalysts was characterized by means of XRD and FTIR techniques. The distribution and strength of different acid sites of zeolites were investigated using temperature-programmed desorption of ammonia. The active Bronsted acid sites were confirmed by measuring the rate constants for cracking of n-hexane, as a probe molecule. Changes resulted in reduction of pore opening size of the modified ZSM-5 were monitored by a standard test reaction of probe molecules of m-xylene and ethylbenzene. Selective alkylation of toluene with methanol to p-xylene was studied over modified ZSM-5 catalysts. The effect of reaction parameters such as temperature, weight hourly space velocity (WHSV), and methanol/toluene molar ratio has been also considered. It was found that the p-selectivity decreased according to the following order: Sr/ZSM-5 ≥ Mg/ZSM-5 > Ba /ZSM-5 > Ni/ZSM-5 ≥ Co/ZSM-5 >H-ZSM-5. Performance of the catalysts was correlated with alterations found in distribution and strength of different acidic sites, and regulation of pore opening size due to the adopted modifications. The p-selectivity of Sr/ZSM-5 and Mg/ZSM-5 catalysts is due to removal of stronger Bronsted acid sites located on the external surface and reduction of the pore opening size of zeolite. Sr/ZSM-5 catalyst exhibited higher conversion levels overall the studied range of the reaction temperatures. It was suggested that the better activity of Sr modified zeolite catalyst could be attributed to the presence of partially non-transformed Sr2+ ions as Sr (NO2 which promotes formation of carbonium ions as an intermediate complex of the alkylation. The deactivation of H-ZSM-5, and Sr/ZSM-5 zeolite catalysts, due to coke deposition during run, was also considered. The catalysts were slightly affected by coke deposition, in terms of toluene conversion, during the run of 2 h on stream. The stability and activity of Sr/ZSM-5 catalyst towards the selective toluenemethanol alkylation to produce p-xylene indicated that the modification with strontium is an effective and convenient route for modification of ZSM-5 zeolite type.