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A Supramolecular View on the Cooperative Role of Brønsted and Lewis Acid Sites in Zeolites for Methanol Conversion

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Organization
Abstract
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(μ-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.
Keywords
methanol-to-olefins, Lewis acidity, Brønsted acidity, ZSM-5, Density functional theory, infrared spectroscopy, calcium NMR

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Chicago
Bailleul, Simon, Irina Yarulina, Alexander Hoffman, Abhay Dokania, Edy Abou-Hamad, Abhishek Dutta Chowdhury, Giovanni Pieters, et al. 2019. “A Supramolecular View on the Cooperative Role of Brønsted and Lewis Acid Sites in Zeolites for Methanol Conversion.” Journal of the American Chemical Society.
APA
Bailleul, S., Yarulina, I., Hoffman, A., Dokania, A., Abou-Hamad, E., Chowdhury, A. D., Pieters, G., et al. (2019). A Supramolecular View on the Cooperative Role of Brønsted and Lewis Acid Sites in Zeolites for Methanol Conversion. Journal of the American Chemical Society.
Vancouver
1.
Bailleul S, Yarulina I, Hoffman A, Dokania A, Abou-Hamad E, Chowdhury AD, et al. A Supramolecular View on the Cooperative Role of Brønsted and Lewis Acid Sites in Zeolites for Methanol Conversion. Journal of the American Chemical Society. 2019;
MLA
Bailleul, Simon et al. “A Supramolecular View on the Cooperative Role of Brønsted and Lewis Acid Sites in Zeolites for Methanol Conversion.” Journal of the American Chemical Society (2019): n. pag. Print.
@article{8627777,
  abstract     = {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(μ-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.},
  author       = {Bailleul, Simon and Yarulina, Irina and Hoffman, Alexander and Dokania, Abhay and Abou-Hamad, Edy and Chowdhury, Abhishek Dutta and Pieters, Giovanni and Hajek, Julianna and De Wispelaere, Kristof and Waroquier, Michel and Gascon, Jorge and Van Speybroeck, Veronique},
  issn         = {0002-7863},
  journal      = {Journal of the American Chemical Society},
  keywords     = {methanol-to-olefins,Lewis acidity,Brønsted acidity,ZSM-5,Density functional theory,infrared spectroscopy,calcium NMR},
  language     = {eng},
  title        = {A Supramolecular View on the Cooperative Role of Brønsted and Lewis Acid Sites in Zeolites for Methanol Conversion},
  url          = {http://dx.doi.org/10.1021/jacs.9b07484},
  year         = {2019},
}

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