Contrasting Methanol Dehydration Analogs across Brønsted-Acid Catalysts

Methanol dehydration to dimethyl ether (DME) is a probe reaction previously used to access and deconvolute acid strength and confinement effects for various Brønsted acid catalysts. Among them are zeolites with varying acid strength and topology, and polyoxometalates. Methanol dehydration can occur via two competing mechanisms, a concerted (associative) or sequential (dissociative), where the former forms DME in a single bimolecular reaction, while the latter forms DME via a surface-bound methyl (Z–CH₃) intermediate. Density functional theory (DFT) calculations suggest that methanol dehydrates via the concerted route at typical reaction conditions (415 K, >0.3 kPa CH₃OH) in CHA. Here, we perform DFT calculations to revisit the methanol dehydration reaction by systematically varying functional (electronic) groups from –OH in methanol to –NH₂, –PH₂, –SH, –F, and –Cl in methylamine, methyl phosphine, methanethiol, methyl fluoride, and methyl chloride, respectively, to fundamentally address if the conclusions from methanol dehydration about Brønsted acid catalysts are extendible to other probe reactions. Specifically, we revisit the role of zeolite topology and confinement effects by contrasting 3-dimensional zeolites (CHA and MFI) to 2-dimensional surface analogs. Moreover, we revisit the role of acid strength in Keggin type WO₃ polyoxometalates (POMs) by varying the metal heteroatom identity (Al, Si, P, and S). Preliminary data indicates that the basicity and proton affinity from each functional group influences intrinsic activation barriers, suggesting that the probe molecule choice is sensitive to different Brønsted-acid catalyst morphologies. In MFI, methylamine and methyl phosphine bind too strongly, consistent with their basicity and high proton affinities; the methanethiol energy profile resembles that of methanol, consistent with both having similar proton affinities. Methyl fluoride and methyl chloride, in contrast, bind too weakly. We aim to clarify if the probe molecule of choice influences our current understanding of Brønsted acid catalysts. Recent findings from DFT and experimental kinetic and spectroscopic data suggest that methanol dehydration barriers are influenced by coadsorbate effects that resemble alternating cationic and anionic charges present in zwitterionic polymers. These interactions, in turn, are due to H–bonding interactions between adsorbates, which may be theoretically decoupled from methanol activity by exploring methanol dehydration reaction analogs with varying functional groups.