Methanol dehydration to dimethyl ether occurs on Brønsted acid zeolites via the associative mechanism under most relevant reaction conditions (&gt;0.3 kPa CH3OH, 403–473 K). In the case of CHA zeolites, turnover rates (per H+) increase (first-order), reach a maximum (zero-order), and then decrease (negative-order) with increasing methanol pressure. This apparent high-pressure kinetic inhibition reflects a progressive change in the surface coverage of methanol n-mer clusters (n = 2–5) as evidenced by experiment and theory, and the preferential formation of DME through the methanol trimer cluster. Additionally, rates (per H+) increase systematically with the fraction of 6-MR acid site pairs in CHA zeolites, reflecting the preferential stabilization of DME transition states via hydrogen bonding with a co-adsorbed methanol at a spectating acid site. In contrast, methanol dehydration rates on MFI zeolites do not show high-pressure kinetic inhibition or differences in turnover rates (per H+) with changes in acid site proximity. This appears to reflect the greater flexibility conferred to intermediate and transition state clusters of increasing size when they are confined within larger 10-membered ring (10-MR) MFI channels than the more constrained 8-MR window-cage topology of the CHA framework, as evidenced by DFT calculations. As a result, larger methanol clusters form more readily in MFI than in CHA, but are also able to access a variety of DME-formation transition states with similar Gibbs free energy. This study highlights the influence of confining void environment and spatial constraints on the structure of confined alkanol molecular clusters and their propensity to undergo dehydration catalysis.

Effects of molecular clustering in zeolites on high-pressure kinetic inhibition and sensitivity to Brønsted acid site proximity during methanol dehydration catalysis

ACS Fall 2021

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