We report a ZSM-5 based catalyst with surface modification of SiO 2 to increase the selectivity of para-xylene (PX) in xylene (X) in the methanol-to-aromatics process. The effect of acid strength and acid amount in HZSM-5, Zn/P/ZSM-5, and Zn/ P/Si/ZSM-5 on the catalytic performance, including methanol conversion, aromatic yield, and PX selectivity, were studied. The total acid strength and acid amount of the catalyst were crucial for high methanol conversion (around 100%) and high yield of aromatics (>60%), whereas weak external acid sites present in a small amount played an important role in increasing the PX selectivity (in the X isomers) from the usual 23−24% to 89.6%. The results validated the use of a catalyst having a core with strong acid sites in a large amount and an external shell with weak acid sites in a small amount. The contribution of the external surface reaction, including alkylation, isomerization, and dealkylation, to the PX selectivity was evaluated by using PX or ortho-X separately as feedstock. A Zn/P/Si/ZSM-5 catalyst worked well in continuous reaction/catalyst-regeneration cycles, and it also converted recycled toluene into PX by an alkylation route.

Relationship between aromatics distribution, in the process of methanol to aromatics (MTA), and the conversion of methanol and the catalyst acidity was investigated over a series of Zn/P/ZSM-5 catalysts with different Si/Al molar ratios and zinc loading. To understand the contribution of aromatization, isomerization, dealkylation and alkylation reactivity of the catalyst to the aromatics distribution, coke deposition degree of Zn/P/ZSM-5 catalyst was tailored as using different feedstocks including methanol, xylene or the mixture of methanol and toluene. With the coke deposition, the amount of different types of acidic sites of catalyst varied significantly, characterized by NH3-temperature programmed desorption (NH3-TPD) and pyridine-infrared methods. Aromatization, dealkylation, alkylation, and isomerization showed sensitivity to a reduction in the density of strongly acidic sites. Dealkylation reaction was preferentially inhibited just by slightly decreasing the density of strong acid sites. However, aromatization and isomerization reaction were inhibited only when the density of strong acid sites was significantly decreased. In all cases, alkylation was found to be uninfluenced by acidic site density. A Zn/P/ZSM-5 catalyst with Si/Al molar ratio of 14 and 3% (w) Zn loading exhibited aromatics yields of 75% and xylene yields of about 35%, indicating potential for industrial application.