Low-temperature methanol dehydration to dimethyl ether (DME) and water is a probe reaction used to assess the chemical characteristics of Brønsted acid sites, such as acid strength and confinement effects in zeolite catalysts. Prior experimental work has studied methanol dehydration in many zeolites with varying connectivities, void dimensions, and fractions of proximal Al sites detected by divalent cation titrations. Theoretical studies of zeolite catalysis must overcome the large configurational space in which reactions may occur. For example, the MFI framework has 26 distinct O atoms and Al can be substituted at either T-site attached to each O atom (12 T-sites total), leading to 48 total T-O ‘site-pairs’. Here, we attempted to overcome these configurational complexities while studying methanol dehydration via the concerted and sequential mechanisms at all T-O site-pairs in CHA (4 T-O pairs), MFI (48), BEA (32), AEI (12), MOR (14), ERI (7), FER (12), STI (11), AFX (8), PHI (8), RTH (16), LEV (6), LTA (3), MEL (24), TON (10), MTT (19), and MTW (21). These calculations quantify confinement through structure-function relationships for each T-site and are more specific than those broadly examining the properties of each zeolite; this information can be leveraged to inform catalyst design by identifying the structural motifs surrounding the most active T-sites within a framework. Additionally, within the CHA and MFI frameworks, the most extensively studied for these reactions, we explored the role of methanol clusters—protonated complexes of methanol molecules—that form at Brønsted acid sites at conditions relevant to kinetic studies. These clusters lead to inhibition in CHA because methanol must desorb from (CH3OH)4H+ clusters to form concerted transition states, (CH3OH)2–CH3+–H2O‡, that control turnover rates. No such inhibition is observed in MFI despite calculations showing that methanol clusters also form. CHA zeolites can also be synthesized such that two Al share six-member rings (6-MR) without changing the Si/Al of the resulting crystals. Samples with higher fractions of these ‘paired’ sites have higher rates of methanol dehydration per H+ than those with only isolated Al not sharing 6-MR. DFT calculations indicate that nearby sites lower dehydration barriers by H-bonding to the conjugate base of a paired site on which dehydration occurs and that these H-bonds are mediated by coadsorbed methanol, shortening H-bonding distances.

Comprehensive theoretical studies on the effects of zeolite framework, Al location, and Al proximity on methanol dehydration to dimethyl ether

ACS Fall 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.