We discuss how different arrangements (Al−O(−Si−O)x−Al) of framework Al atoms between isolated (x ≥ 3) or paired configurations (x = 1, 2), the latter defined functionally by their ability to exchange extraframework divalent cations, can generate Brønsted acid site ensembles of different reactivity. Methanol dehydration to dimethyl ether is used as a catalytic probe reaction to show that paired acid sites accelerate turnover rates (per H+, 415 K) by an order-of-magnitude over isolated acid sites in CHA zeolites (eight-membered-ring; 8-MR, 1 T-site), but not in MFI zeolites (10-MR, 12 T-sites) under equivalent reaction conditions. DFT calculations are combined with kinetic data and in situ infrared spectra to provide evidence that methanol dehydration proceeds via different mechanisms on paired and isolated acid sites in CHA zeolites. These results demonstrate how synthetic control of active site arrangements at the atomic level can impart catalytic diversity, even among single T-site zeolites of fixed composition.

Acid site proximity effects on methanol dehydration catalysis by zeolites

255th National Meeting (2018)

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