Computational Investigation into the Origins of Lewis Acidity in Zeolites

Sokol, Alexey A.; Catlow, C. Richard A.; Garcés, Juan M.; Kuperman, Alex

doi:10.1002/1521-4095(200012)12:23<1801::aid-adma1801>3.0.co;2-m

Cited by 28 publications

(22 citation statements)

References 23 publications

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“…The activation barriers, however, appeared to be prohibitive (close to 200 kJ/mol) due to a very unstable vicinal disilanol species, where two pentahedral silicon atoms are part of a two-membered ring. Similar disilanol intermediates were previously invoked as possible defects in siliceous zeolite models, [232][233][234] and were also invoked for the desilication of SAPO-34. 240 Silaghi et al identified much easier initiation steps for the dealumination mechanisms upon water attack, 223,241 leading to the formation of a Al(OH) 3 (H 2 O) EFAl, and a silanol nest (Figure 5-b and 4-c).…”

Section: Simulation Of the Dealumination / Desilication Mechanismssupporting

confidence: 60%

Toward the Atomic Scale Simulation of Intricate Acidic Aluminosilicate Catalysts

Chizallet

2020

ACS Catal.

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Zeolites are nanoporous aluminosilicates with well-defined crystalline structures, considered key assets in heterogeneous catalysis, with a broad range of industrial applications. Computational investigations dealing with zeolite catalysts have been undertaken for decades with simple models of the bulk sites, known to be bridging Si IV −OH−Al IV groups, in the case where the compensation cation is a proton. Real zeolite catalysts used in practice are however finite size and intricate objects, with external surface sites and defects, among other sources of complexity. Amorphous silica−alumina may also be obtained as a consequence of zeolite post-treatments or synthesized on purpose to obtain acid sites that are milder than those of zeolites. In the present Perspective, some of the achievements in the field of the atomic-scale simulation of intricate aluminosilicate catalysts (zeolites, amorphous silica−aluminas) of industrial relevance are reviewed. Emphasis is put on the simulation of the mechanisms of post-treatments of zeolites, and on the special structure and reactivity of acid sites at external surfaces of zeolites and on amorphous silica−alumina, that is shown to differ from the properties of the bulk bridging sites. Moreover, directions for future investigations are proposed.

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Section: Simulation Of the Dealumination / Desilication Mechanismssupporting

confidence: 60%

Toward the Atomic Scale Simulation of Intricate Acidic Aluminosilicate Catalysts

Chizallet

2020

ACS Catal.

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“…On the basis of 1 H DQ-MAS NMR and the 13 C MAS NMR of adsorbed 2-13 C-acetone, we have obtained the spatial details of the Brønsted and Lewis acid sites in a dealuminated H-Y zeolite and demonstrated that spatial proximities can result in a synergy effect that remarkably increases the acid strength of the zeolite [27,28]. In zeolites, Brønsted acid sites are due to bridging hydroxyl groups (SiOHAl) and a 4-coordinate framework aluminum, while Lewis acid sites are due to either EFAL [29,30] or three-coordinate framework aluminum species [31,32]. Recently, we have used 27 Al DQ-MAS NMR spectroscopy to reveal the detailed spatial proximities of the various aluminum species in dealuminated H-Y zeolites [33].…”

Section: English Textmentioning

confidence: 99%

Brønsted/Lewis Acid Sites Synergy in H-MCM-22 Zeolite Studied by 1H and 27Al DQ-MAS NMR Spectroscopy

Wang

Chen

et al. 2012

Chinese Journal of Catalysis

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“…5 Experimental evidence for a catalytic activity of Lewis sites can be found, e.g., in the work of Wichterlová et al 5 Investigation of possible defect sites created by thermal treatment is therefore necessary to improve our understanding of the catalytic properties of zeolites. Sokol et al 6 have discussed the formation of the defects in sodalite and chabazite structures; however, only bulk structures have been taken into account.…”

Section: Introductionmentioning

confidence: 99%

Defect sites at the (001) surface of mordenite: An ab initio study

Benco

Häfner

2003

The Journal of Chemical Physics

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The mechanisms and energetics of the formation of various defects upon dehydration of the surface of pure-siliceous and Al-monosubstituted mordenite are investigated using a periodic ab initio density functional theory technique. An energetically favorable defect at the pure-siliceous surface is a strained two-membered Si–O ring (2MR) formed via elimination of a water molecule from a pair of neighboring terminal silanol groups. Assuming the formation of two-membered rings, the dehydration-energy of the (001) surface of pure-silica mordenite is 133 kJ/mol. A relatively high reaction barrier of 179 kJ/mol coincides with the experimental observation that these defects are formed at high temperatures >700 K. Despite a short Si–Si distance of 2.35 Å across the 2MR which is comparable to the bond length between Si atoms in silicon in diamond structure, the electron-localization function reveals no bonding interaction between Si atoms on the 2MR. In the Al-substituted surfaces, the dehydration proceeds via proton transfer from the Brønsted-acid site (BA) to a neighboring terminal hydroxyl group. The low values of two subsequent energetic barriers of dehydration of 13 and 10 kJ/mol suggest that the surface BA sites are likely to be destroyed at even modest temperatures. The most stable defects formed in this mechanism are ones containing a threefold-coordinated Al atom and a defect with both an Al atom and a bridging OH group located on a two-membered ring. The heat of reaction of only 9 kJ/mol and the activation energy of the transformation between these two configurations of 26 kJ/mol suggest that both defects occur with similar probability.

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Computational Investigation into the Origins of Lewis Acidity in Zeolites

Cited by 28 publications

References 23 publications

Toward the Atomic Scale Simulation of Intricate Acidic Aluminosilicate Catalysts

Toward the Atomic Scale Simulation of Intricate Acidic Aluminosilicate Catalysts

Brønsted/Lewis Acid Sites Synergy in H-MCM-22 Zeolite Studied by 1H and 27Al DQ-MAS NMR Spectroscopy

Defect sites at the (001) surface of mordenite: An ab initio study

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