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Surface enhanced dynamic nuclear polarization solid-state NMR spectroscopy sheds light on Bronsted-Lewis acid synergy during the zeolite catalyzed methanol-to-hydrocarbon process.

ABSTRACT: After a prolonged effort over two decades, the reaction mechanism of the zeolite-catalyzed methanol-to-hydrocarbon (MTH) process is now well-understood: the so-called 'direct mechanism' (via direct coupling of two methanol molecules) is responsible for the formation of the initial carbon-carbon bonds, while the hydrocarbon pool (HCP)-based dual cycle mechanism is responsible for the formation of reaction products. While most of the reaction events occur at zeolite Brønsted acid sites, the addition of Lewis acid sites (i.e., via the introduction of alkaline earth cations like calcium) has been shown to inhibit the formation of deactivating coke species and hence increase the catalyst lifetime. With the aim to have an in-depth mechanistic understanding, herein, we employ magic angle spinning surface-enhanced dynamic nuclear polarization solid-state NMR spectroscopy to illustrate that the inclusion of Lewis acidity prevents the formation of carbene/ylide species on the zeolite, directly affecting the equilibrium between arene and olefin cycles of the HCP mechanism and hence regulating the ultimate product selectivity and catalyst lifetime.

SUBMITTER: Dutta Chowdhury A

PROVIDER: S-EPMC7068724 | biostudies-literature | 2019 Oct

REPOSITORIES: biostudies-literature

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Surface enhanced dynamic nuclear polarization solid-state NMR spectroscopy sheds light on Brønsted-Lewis acid synergy during the zeolite catalyzed methanol-to-hydrocarbon process.

Dutta Chowdhury Abhishek A Yarulina Irina I Abou-Hamad Edy E Gurinov Andrei A Gascon Jorge J

Chemical science 20190821 39

After a prolonged effort over two decades, the reaction mechanism of the zeolite-catalyzed methanol-to-hydrocarbon (MTH) process is now well-understood: the so-called 'direct mechanism' (<i>via</i> direct coupling of two methanol molecules) is responsible for the formation of the initial carbon-carbon bonds, while the hydrocarbon pool (HCP)-based dual cycle mechanism is responsible for the formation of reaction products. While most of the reaction events occur at zeolite Brønsted acid sites, the ...[more]

PMID: 32190235

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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.

Dataset Information

Surface enhanced dynamic nuclear polarization solid-state NMR spectroscopy sheds light on Bronsted-Lewis acid synergy during the zeolite catalyzed methanol-to-hydrocarbon process.

Publications

Surface enhanced dynamic nuclear polarization solid-state NMR spectroscopy sheds light on Brønsted-Lewis acid synergy during the zeolite catalyzed methanol-to-hydrocarbon process.

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