(7c) Kinetic Assessments of the Influence of Active Site Distribution in Brønsted Acid Zeolites on Toluene Methylation Catalysis

Toluene methylation to para-xylene (p-X), a key plastics precursor, involves a complex reaction network. Zeolites contain micropores that confine reactive intermediates and transition states and contain a diversity of Brønsted acid sites associated with distinct Al locations and arrangements within confining voids or at unconfined external surfaces. Here, we assess the influence of acid site distribution in MFI on xylenes formation rates (per H⁺) and isomer selectivity during toluene methylation at low temperatures (<433 K) and conversions (<1%). MFI with varied crystallite sizes and Al proximity at fixed Al content (Si/Al~50) were obtained from synthetic methods and commercial sources. Rates were zero-order in DME pressures (>25 kPa) and transition from a first- to zero-order dependence in toluene pressures (0.2-8.8 kPa), which reflect increasing coverages of co-adsorbed toluene on surfaces covered with DME-derived intermediates reacting in a kinetically-relevant C-C formation step. Rates and selectivity are insensitive to intracrystalline residence times and external acid site content. However, rates increase with the fraction of proximal Al sites, with initial p-X selectivities that were higher on MFI with predominantly isolated Al than on MFI containing proximal Al. We further assess how rates and selectivities vary at acid sites located within different void sizes (0.53-3.0 nm) in aluminosilicates (TON, MFI, BEA, MCM-41). Among zeolites, rates (per H⁺) increase with increasing micropore diameter but are lower on mesoporous Al-MCM-41. Taken together with selectivity trends, these results suggest that differences in rates and selectivity reflect intrinsic kinetic differences due to variations in active site distribution within confining micropores. Overall, these findings highlight the importance of combining synthesis and characterization with detailed kinetic assessments to elucidate synthesis-structure-function relationships that describe how active site distributions influence rates and selectivity in complex networks on microporous catalysts.