Selective toluene-methanol alkylation over modified ZSM-5 zeolite catalysts

H-ZSM-5 zeolite was modified with divalent elements of groups II and VIII o f periodic system. The modifications were achieved by impregnating H-ZSM-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 me ans of XRD and FTIR techniques. The distribution and strength of different acid sites of zeolites were investigated using temperature-programmed desor ption of ammonia. The active Bronsted acid sites were confirmed by measurin g 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 moni tored by a standard test reaction of probe molecules of m-xylene and ethylb enzene. Selective alkylation of toluene with methanol to p-xylene was studi ed over modified ZSM-5 catalysts. The effect of reaction parameters such as temperature, weight hourly space velocity (WHSV), and methanol/toluene mol ar ratio has been also considered. It was found that the p-selectivity decr eased according to the following order: Sr/ZSM-5 greater than or equal to Mg/ZSM-5 > Ba /ZSM-5 > Ni/ZSM-5 greater t han or equal to Co/ZSM-5 > H-ZSM-5. Performance of the catalysts was correl ated with alterations found in distribution and strength of different acidi c sites, and regulation of pore opening size due to the adopted modificatio ns. 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 reducti on of the pore opening size of zeolite. Sr/ZSM-5 catalyst exhibited higher conversion levels overall the studied range of the reaction temperatures. I t was suggested that the better activity of Sr modified zeolite catalyst co uld be attributed to the presence of partially non-transformed Sr2+ ions as Sr (NO3)(2) which promotes formation of carbonium ions as an intermediate complex of the alkylation. The deactivation of H-ZSM-5, and Sr/ZSM-5 zeolit e catalysts, due to coke deposition during run, was also considered. The ca talysts were slightly affected by coke deposition, in terms of toluene conv ersion, during the run of 2 h on stream. The stability and activity of Sr/Z SM-5 catalyst towards the selective toluene-methanol alkylation to produce p-xylene indicated that the modification with strontium is an effective and convenient route for modification of ZSM-5 zeolite type.