(100b) Origins of Changes in Methanol Dehydration Turnover Rates on Brønsted Acid Sites in Zeolites with Different Al Distributions

Methanol dehydration to dimethyl ether (DME) is a selective probe reaction used to study heterogeneous Brønsted acids. Methanol dehydration occurs when one methanol directly methylates another and this mechanism dominates over others based on kinetic data, infrared spectroscopy, and density functional theory (DFT) calculations [1,2]. Previous work found that DME formation turnover rates (per H⁺) depend on both the strength of the acid [1]—as determined by deprotonation energy (DPE)—and the size of the pore surrounding the active site, with turnover rates increasing as the void size approaches that of the concerted transition state (433 K, 0.1–20 kPa CH₃OH) [2]. More recently, kinetic studies have shown that turnover rates also increase systematically with higher fractions of Al sharing 6-member rings in CHA—so-called ‘paired’ configurations—with first- and zero-order rate constants indicating a decrease in dehydration free energy barriers of 5–7 kJ mol^−1 on paired sites compared to isolated sites [3]. Subsequent density functional theory (DFT) calculations showed that these barriers decrease because of H-bonding between paired sites facilitated by co-adsorbed CH₃OH [4]. These calculations revealed certain arrangements of Al sites that yield alternating cationic and anionic charges, reducing DME formation barriers (Fig. 1). Finally, different arrangements of Al, at constant Si:Al ratios, showed that activation barriers can be higher or lower around a surface methylation transition state. Considering these different Al arrangements is crucial for understanding Brønsted acid catalysis in zeolites.

References

[1] J. Catal. 2011, 278, 78–93.

[2] J. Phys. Chem. C 2014, 118, 17787–17800.

[3] ACS Catal. 2017, 7, 6663–6674.

[4] Angew. Chem. Int. Ed. 2020, 59, 18686–18694.