Brønsted/Lewis acid sites synergistically promote the initial C-C bond formation in the MTO reaction

The methanol-to-olefin (MTO) reaction is an active field of research due to conflicting mechanistic proposals for the initial carbon-carbon (C-C) bond formation. Herein, a new methane-formaldehyde pathway, a Lewis acid site combined with a Brønsted acid site in zeolite catalysts can readily activate dimethyl ether (DME) to form ethene, is identified theoretically. The mechanism involves a hydride transfer from Al-OCH on the Lewis acid site to the methyl group of the protonated methanol molecule on the adjacent Brønsted acid site leading to synchronous formation of methane and Al-COH (which can be considered as formaldehyde (HCHO) adsorbed on the Al Lewis acid sites). The strong electrophilic character of the Al-COH intermediate can strongly accelerate the C-C bond formation with CH , as indicated by the significant decrease of activation barriers in the rate-determining-step of the catalytic processes. These results highlight a synergy of extra-framework aluminum (EFAl) Lewis and Brønsted sites in zeolite catalysts that facilitates initial C-C bond formation in the initiation step of the MTO reaction the Al-COH intermediate.