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

ABSTRACT: 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-OCH3 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-COH2 + (which can be considered as formaldehyde (HCHO) adsorbed on the Al3+ Lewis acid sites). The strong electrophilic character of the Al-COH2 + intermediate can strongly accelerate the C-C bond formation with CH4, 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 via the Al-COH2 + intermediate.

SUBMITTER: Chu Y

PROVIDER: S-EPMC6115684 | biostudies-other | 2018 Aug

REPOSITORIES: biostudies-other

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

Chu Yueying Y Yi Xianfeng X Li Chengbin C Sun Xianyong X Zheng Anmin A

Chemical science 20180627 31

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<sub>3</sub> on the Lewis acid site to the methyl group of the protonated methan ...[more]

PMID: 30310577

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Project description:A systematic molecular level and spectroscopic investigation is presented to show the cooperative role of Brønsted acid and Lewis acid sites in zeolites for the conversion of methanol. Extra-framework alkaline-earth metal containing species and aluminum species decrease the number of Brønsted acid sites, as protonated metal clusters are formed. A combined experimental and theoretical effort shows that postsynthetically modified ZSM-5 zeolites, by incorporation of extra-framework alkaline-earth metals or by demetalation with dealuminating agents, contain both mononuclear [MOH]+ and double protonated binuclear metal clusters [M(?-OH)2M]2+ (M = Mg, Ca, Sr, Ba, and HOAl). The metal in the extra-framework clusters has a Lewis acid character, which is confirmed experimentally and theoretically by IR spectra of adsorbed pyridine. The strength of the Lewis acid sites (Mg > Ca > Sr > Ba) was characterized by a blue shift of characteristic IR peaks, thus offering a tool to sample Lewis acidity experimentally. The incorporation of extra-framework Lewis acid sites has a substantial influence on the reactivity of propene and benzene methylations. Alkaline-earth Lewis acid sites yield increased benzene methylation barriers and destabilization of typical aromatic intermediates, whereas propene methylation routes are less affected. The effect on the catalytic function is especially induced by the double protonated binuclear species. Overall, the extra-framework metal clusters have a dual effect on the catalytic function. By reducing the number of Brønsted acid sites and suppressing typical catalytic reactions in which aromatics are involved, an optimal propene selectivity and increased lifetime for methanol conversion over zeolites is obtained. The combined experimental and theoretical approach gives a unique insight into the nature of the supramolecular zeolite catalyst for methanol conversion which can be meticulously tuned by subtle interplay of Brønsted and Lewis acid sites.

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

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

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