Brønsted acid sites in H-form zeolites promote the formation of multi-molecular clusters of H₂O and alcohols at partial pressures far below those required for intrapore condensation. While bimolecular alcohol and H₂O dimers have been studied extensively, recent kinetic and theoretical data suggest that larger cationic clusters of up to 8 molecules can form in gas-phase conditions relevant to catalysis. Lercher and co-workers showed that these H₂O clusters can deprotonate Brønsted acid sites, blocking regions near Al to force reactions to occur in different environments and accelerating reactions by altering the chemical potential of adsorbates and transition states. DFT calculations indicate that clusters of 3–5 CH₃OH can form at >20 kPa and 415 K in CHA, while intrapore condensation does not occur until >900 kPa and bulk condensation occurs near 1150 kPa. Within MFI, the preferred cluster size depends weakly on the Al position, but clusters of 3–5 CH₃OH are still predicted to form above 10 kPa and 415 K. These cationic clusters resemble chains that are most enthalpically favorable when interacting with the conjugate base at two points, forming a ring consisting of H-bonds between CH₃OH (one of which is protonated) and two O-atoms of the AlO₄⁻ conjugate base. Termolecular transition states for the dehydration of CH₃OH to dimethyl ether dominate in CHA zeolites instead of commonly depicted bimolecular transition states. This preference for termolecular transition states prevails at nearly all conditions relevant to kinetic studies (> 0.3 kPa CH₃OH) and lead to CH₃OH pressure dependence that varies from first-order to negative-order as the dominant surface species transitions from cationic CH₃OH dimers to CH₃OH tetramers. This inhibition also occurs on CHA samples with different spatial Al arrangements, specifically those with higher fractions of ‘paired’ Al sharing six-membered rings (6-MR) with the same Si/Al ratio. Notably, the spatial arrangement of paired Al leads to interactions between sites, altering the structure and behavior of clusters and transition states. DFT calculations on these paired sites indicate two cationic CH₃OH clusters form—one for each Brønsted acid site—and that these chains can interact simultaneously with both conjugate bases. These data show that liquid-like clusters of protic species form readily on Brønsted acid sites and can alter kinetic behavior and transition state solvation at conditions relevant to catalysis.

Ryoh-suke Sekiya

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