Impact of hierarchical pore structure on the catalytic performances of MFI zeolites modified by ZnO for the conversion of methanol to aromatics

ZnO-containing MFI zeolite catalysts with bimodal and trimodal hierarchical pore structures were prepared, characterized and studied for the conversion of methanol to aromatics. Treatments of H-ZSM-5 with NH4F and NaOH generated bigger micropores with a mean size of around 0.8 nm and mesopores with mean sizes of 5-20 nm, respectively. The combination of alkaline and fluoride treatments resulted in a trimodal pore structure. The method for H-ZSM-5 treatments affected the dispersion of ZnO. The fluoride treatment favoured the dispersion of ZnO, whereas the alkaline treatment led to large ZnO particles. We clarified that the hierarchical pore structure, acidity and dispersion of ZnO played crucial roles in the formation of aromatics. Benzene, toluene and xylenes (BTX) mainly constituted the aromatics over our catalysts, and the yield of BTX decreased with increasing reaction time. A larger density of Bronsted acidity favoured the yield of BTX at the initial stage but was unbeneficial to the stability for BTX formation. The increase in pore hierarchy suppressed the coke deposition inside the micropores and increased the coke tolerance, thus enhancing the catalyst stability for BTX formation. The catalyst with a larger pore hierarchy also showed higher selectivities for aromatics and BTX. Aromatics can be formed via lower olefin intermediates by hydrogen-transfer or dehydrogenation pathways. We propose that ZnO, in particular the highly dispersed ZnO clusters, enhances the selectivity for aromatics by catalysing the dehydrogenation pathway, whereas the hierarchical pore structure facilitates the transfer of reaction intermediates and thus accelerates the formation of aromatics.