Synthetic Methods to Tune Active Site Distribution in Zeolite Catalysts for Regioselective Methylation of Toluene to para-Xylene
Sopuruchukwu A. Ezenwa, Rajamani Gounder
Toluene methylation to para-xylene involves a complex reaction network on Brønsted acid zeolites. Here, we assess the influence of acid site location and distribution within MFI zeolites on xylene formation rates and isomer selectivity during toluene methylation at low temperatures (<433 K). MFI samples of fixed Al content (Si/Al = 50), but with varied crystallite sizes and Al siting and distribution, were synthesized using different organic structure-directing agents and obtained from different commercial sources. We show kinetic evidence that rates and selectivity on MFI zeolites are insensitive to intracrystalline and bed-scale transport phenomena and to the presence of acid sites unconfined at external surfaces, but vary significantly among MFI zeolites with different Al site distributions. We also show that rates and selectivities vary systematically as acid sites are confined within different void sizes among other aluminosilicates (TON, MFI, BEA, MCM-41), indicating that the catalytic diversity among MFI samples for toluene methylation arises from differences in Al siting within different channel and intersection locations. Overall, these findings highlight the importance of combining synthesis, characterization, and kinetic assessments to elucidate synthesis-structure-function relationships that describe how active site distributions influence rates and selectivity in complex networks in microporous catalysts.