Reactivity of Surface Lewis and Brønsted Acid Sites in Zeolite Catalysis: A Computational Case Study of DME Synthesis Using H‑SSZ-13

The stability and reactivity of Lewis and Brønsted acid sites at the H-SSZ-13 surface are investigated for the (101) and (001) surfaces. We focus on the conversion of methanol to dimethyl ether (DME) as a probe reaction that is prototypical for the reactivity of acidic zeolites, for example, in the methanol-to-olefins process. We use periodic density functional theory (DFT) calculations in combination with highly accurate DLPNO-CCSD(T) calculations on cluster models. At Brønsted acid sites, DME can be formed via concerted and stepwise mechanisms. The barriers for acid sites located at the surface are comparable to those located in the bulk. DME formation on a Lewis acid site is similar to the concerted mechanism since two adsorbed methanol molecules react with each other directly. However, the oxygen of the adsorbed methanol is bound to the Al atom and an analogy can therefore also be drawn with a methoxy group and thus the second step of the stepwise mechanism on Brønsted acid sites. The barriers for DME formation on a Lewis acid site are more similar to the concerted mechanism of the Brønsted acid sites and are therefore at 400 °C significantly higher than the stepwise mechanism at Brønsted acid sites.