Influence of Catalyst Acidity on Dealkylation, Isomerization and Alkylation in MTA Process

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.