Impact of medium-pore zeolite topology on para-xylene production from toluene alkylation with methanol

Abstract

There are few studies in zeolite catalysis literature that have developed structure–performance relationships for a particular reaction by considering a broad class of zeolite framework structures possessing identical pore size and similar Si/Al ratios. Here we have synthesized seven different medium-pore zeolites with distinct topologies that span all possible pore dimensionalities: a 1-dimensional zeolite ZSM-23; 2-dimensional zeolites TNU-10 and MCM-22; and 3-dimensional zeolites ZSM-11, ZSM-5, TNU-9, and IM-5. The performance of each catalyst was tested using the toluene alkylation with methanol (TAM) reaction as a benchmark to assess their ability to generate high p-xylene selectivity while maintaining long-term stability. Among the catalysts tested, zeolite MCM-22 (MWW type) emerged as a standout in terms of its high selectivity (60% p-xylene) and exceptionally long lifetime. We show that the total turnover number for MCM-22 is 4-times larger than the second-best catalyst (zeolite IM-5) and 180-times larger than the least effective catalyst (zeolite TNU-10). It was also demonstrated that catalytic tests at high pressure (41 atm) and atmospheric pressure resulted in markedly different performance for MCM-22 compared to other medium-pore zeolites. Our findings revealed that coke formation in MCM-22 at high pressure is beneficial for its overall performance; and that the nature of coke formation, which is presumably dependent upon reaction pressure, is critical to achieving high p-xylene selectivity and enhanced catalyst stability. Collectively, these findings identify an optimal catalyst and set of reaction conditions for the TAM reaction. In addition, the structure–performance relationships uncovered in this study defy conventional expectations in that the zeolites with the lowest mass transport limitations and the least coke accumulation are not the most effective catalysts for this reaction.