(461a) Influence of Clusters of Brønsted Acid Sites on Methanol Dehydration Rates in Zeolite Catalysts

Dimethyl ether (DME) and water are selectively formed from methanol at low temperatures over Brønsted acid catalysts, which serves as a simple probe reaction to evaluate confinement and acid strength of zeolites. CHA zeolites can be selectively synthesized with controlled fractions of Al sharing six-member rings (6-MR) without altering the bulk Si:Al. These ‘paired’ sites can be titrated using divalent Co²⁺ cations and distinguished from ‘isolated’ Al not sharing 6-MR. CHA samples with higher fractions of paired Al form DME at higher rates per H⁺ than those with predominantly isolated Al during kinetic studies, and first- and zero-order rate constants derived from CHA samples with varying paired-Al content indicate paired sites have free energy barriers 5–7 kJ mol^−1 lower than isolated sites.

Density functional theory (DFT) calculations indicate that these higher turnover rates occur because, when a cationic transition state forms at an acid site, its conjugate base is stabilized by H-bonding with the methanol species co-adsorbed to the neighboring site. Periodicity in these DFT calculations reveals that repeating cation-anion chains reduce barriers more than estimated experimentally, by up to 15 kJ mol^−1. Not all Al pairs increase rates, however, as DFT-calculated surface methylation barriers vary from 106–128 kJ mol^−1 across 23 distinct two-Al pairs (compared to 122 kJ mol^−1 for isolated sites). Beyond pairs, clusters of Brønsted acid sites (>2 total sites spaced 1–3 T-sites apart) can more significantly change surface methylation barriers, which range from 89–147 kJ mol^−1 near clusters of 3 acid sites, 67–163 kJ mol^−1 near clusters of 4, and 56–170 kJ mol^−1 near clusters of 5 (Fig. 1). These data show that clusters sites can have even greater effects on the rates of zeolite-catalyzed reactions than paired sites and give new insights into the behavior of low Si:Al zeolites.