The catalytic dehydration of methanol to dimethyl ether (DME) by Brønsted acid sites in solid acids proceeds through a variety of reactive intermediates and mechanisms. Under typical reaction conditions (>400 K, >1 kPa CH₃OH) in aluminosilicate zeolites, methanol dehydration occurs via concerted dehydration mechanisms, with increasing methanol pressures that lead to systematic increases in the coverage of methanol per acid site (1-5 CH₃OH/H⁺). Methanol clusters of different molecularity are able to eliminate DME with different Gibbs free energy barriers, resulting in turnover rates (per H⁺) that show a complex dependence on methanol pressure. The relative stabilities of both DME formation transition states and the prevalent methanol cluster precursors at an acid site depend sensitively both on the positioning and orientation of co-adsorbates at proximal acid sites in the lattice, because such co-adsorbates provide H-bonding interactions, and on the size of the confining secondary environment, which influence the stability and structure of molecular clusters and networks that are present during reaction. Turnover rates of methanol dehydration to DME become inhibited at high methanol pressures when acid sites are confined within small-pore, cage-window zeolites (CHA, AEI, LTA, LEV), but not within medium- or large-pore zeolites (MFI, BEA) or mesoporous acids (MCM-41, SiO₂); in small-pore topologies, such conditions favor the formation of methanol clusters of higher molecularity (tetramers, pentamers) than the cluster size (trimer) with the lowest barrier to eliminate DME. We will discuss these and other kinetic and mechanistic insights that are revealed by a combination of experimental kinetic and spectroscopic studies on well-defined zeolite materials, and theoretical approaches that combine density functional theory calculations with microkinetic modeling approaches.

John Di Iorio

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