(676b) Contrasting Methylating Agents (CH3XHn, X={N, P, O, S, F, Cl}) for Solid-Acid Catalyzed Reactions to Re-Examine Acid Strength and Confinement Effects

Methanol dehydration to dimethyl ether (DME) is a probe reaction previously used to access and deconvolute acid strength and confinement effects for various solid Brønsted acid catalysts. Among them are zeolites with varying confinement and topology, and polyoxometalates with varying acid strength. 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 (< 450 K, >0.3 kPa CH₃OH). 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 molecules. Specifically, we revisit the role of zeolite topology and confinement effects by contrasting CHA and MFI zeolite frameworks to a 2D surface model of a zeolite, and we revisit the role of acid strength in Keggin type WO₃ polyoxometalates (POMs) by varying the metal heteroatom identity (Al, Si, P, and S). While we have examined all three steps involved in methanol dehydration (surface methylation, methanol methylation, and concerted DME formation), here we present exemplar data on surface methylation by the CH₃XH_n series that indicate the most facile surface methylations occur with CH₃NH₂ donors, followed by CH₃OH, CH₃SH, CH₃PH₂, CH₃Cl, and the weakest methyl donor being CH₃F. While proton affinities describe differences between functional groups, reaction energies suggest that confinement and acid strength alters probe molecule behavior to different extents. This, in turn, motivates revisiting further conclusions derived from probe reaction chemistry.