Methanol dehydration to form dimethyl ether (DME) occurs with increased turnover rates in H-SSZ-13 zeolites (CHA framework) with larger fractions of ‘paired’ Brønsted acid sites (Al–O–(Si–O)n–Al, n &lt; 3). Periodic density functional theory (DFT) calculations can give insights into why higher rates prevail on paired vs. isolated sites and the interaction between paired sites during reaction. On isolated sites, CH3OH dimer complexes form favorably, but further adsorption of CH3OH is unfavorable because of entropic losses. CH3OH reacts preferably via concerted associative routes, where two CH3OH react directly to form DME. Eight-member ring (8-MR) structures stabilize transition states on isolated sites, which form H-bonds to framework O atoms, in both pathways. On paired sites, adsorbed CH3OH and CH3OH complexes can interact between sites and across 6-MR structures, stabilizing reactive intermediates and leading to increased turnover rates. Our previous work has shown that deprotonation energy (DPE)—a theoretical probe of acid strength—decreases for some configurations of paired Brønsted acid sites in CHA; the increased acid strength is a result of charge-dipole interactions between the anionic conjugate base and the proton of a nearby paired site. At reaction conditions, however, protons are covered by CH3OH or complexes of 2 or more CH3OH molecules. Interactions between sites with differing coverage lead to changes in barrier heights for the two pathways; CH3OH and dimer complexes can interact with deprotonated sites by stabilizing sites involved in acid-catalyzed CH3OH dehydration through charge-dipole interactions, similar to bare proton stabilization of deprotonated sites. With CH3OH on a second site sharing the 6-MR of CHA, surface methylation barriers in the dissociative path can decrease from 122 on an isolated site to 114 kJ mol−1 on paired sites and remain unchanged at 123 kJ mol−1 for both isolated and paired sites. Experimental results indicate that surface methoxy (CH3–Z) coverage increases during reactions with increasing fractions of paired sites, formed via the sequential dissociative pathway; DFT calculations corroborate a change in mechanism, from the associative to dissociative pathway, as the barrier for surface methylation in the dissociative route decreases in the presence of another nearby site and the barrier for associative DME formation increases.

Methanol dehydration over H-SSZ-13 with controlled site proximity: Effects of site proximity and coverage

2019 Spring National Meeting

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