Brønsted acid sites of zeolitic strength in amorphous silica-alumina

Poduval, Dilip G.; Veen, J.A.R. van; Rigutto, Marcello; Hensen, Emiel J. M.

doi:10.1039/c000019a

Cited by 65 publications

(74 citation statements)

References 28 publications

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“…The temperature required for a conversion of 40% (T40) is used to measure the acidity. The T40 values for FCC(micro) and FCC(meso) were 592 and 609 K. These values are typical for amorphous silica-alumina samples, which contain only few zeolite-like BAS [43,56]. Thus, we speculate that only very few zeolite-type acid sites remain due to the extensive steamdeactivation procedure.…”

Section: Composite Catalyst Characterizationmentioning

confidence: 90%

Texture, acidity and fluid catalytic cracking performance of hierarchical faujasite zeolite prepared by an amphiphilic organosilane

Tempelman

Zhu

Gudun

et al. 2015

Fuel Processing Technology

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Section: Composite Catalyst Characterizationmentioning

confidence: 90%

Texture, acidity and fluid catalytic cracking performance of hierarchical faujasite zeolite prepared by an amphiphilic organosilane

Tempelman

Zhu

Gudun

et al. 2015

Fuel Processing Technology

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“…However, at a molecular scale, it remains crucial to understand the precise chemical behavior of the ASA Brønsted acid sites, how they catalyze cracking reactions and the reasons of their milder acidity of ASA as compared to zeolites. In particular, it is proposed that ASA surface would exhibit very few BAS with an acidic character very similar to BAS found in zeolite [21,30]. However, this interpretation remains questionable because it is also well-known that the strength depends on both the local BAS atomic structure and the confinement effect depending on the pores' size surrounding the BAS [31,32].…”

Section: Introductionmentioning

confidence: 96%

“…Generally, the catalytic activity of ASA in acid-catalyzed reactions is lower than that of zeolites [18,81]. This could be either due to intrinsically weaker sites [82], or to a lower amount of the same kind of sites (bridging SiA(OH)AAl groups) than in zeolites [21,30]. There is, however, a lack of detailed data both on the stability of carbenium species on these solids, and on their implication in the conversion of hydrocarbons, in particular cracking of alkenes.…”

Section: Introductionmentioning

confidence: 98%

Revisiting carbenium chemistry on amorphous silica-alumina: Unraveling their milder acidity as compared to zeolites

Leydier

Chizallet

Costa

et al. 2015

Journal of Catalysis

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a b s t r a c tAmorphous silica-aluminas (ASA) are prominent solids for their acidic properties. They are of first interest in catalysis, in particular for cracking reactions. Their relative acidity as compared to zeolites is a long-lasting general issue, driven by the limited knowledge of the surface structure of ASA, due to their amorphous nature. In the present contribution, thanks to a first principles approach based on our original ASA surface model, and on the study of a model mordenite zeolite (MOR), we propose a rational explanation for this feature. We compare by density functional theory calculations the ability of ASA and MOR to generate carbenium species from isobutene, versus p-complexes and alkoxide species. On ASA, carbenium species can be formed on pseudo-bridging silanols (PBS) only and are much less stable than in MOR. Then, we investigate the cracking pathway of a model alkene, 2,4,4-trimethyl-2-pentene (DIB), by quantifying the stability of relevant intermediates of carbocationic and alkoxide natures. The carbocationic pathway is favored in MOR, whereas on ASA, this pathway is strongly activated. By contrast, the PBS on ASA initiate preferentially a tertiary alkoxide route, or possibly a combined carbocationic/alkoxy route. Finally, the i-butene desorption step is also limiting in MOR due to the confinement effect induced by the zeolite pores. As a result, the higher cracking reactivity of zeolites as compared to ASA is mainly attributed to the favored nature of the carbenium route, stabilized by higher electrostatic confinement effect.

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“…56,[96][97][98][99]120,126 Common sense suggests that the more positively charged the proton, the more acidic a surface OHgroup. CO is able to probe charges along the surface because of its dipolar nature.…”

Section: Acidity Of Oh Groups On Asa: On the Dominant Role Of The Stamentioning

confidence: 99%

“…Structural hypotheses are not unanimous between research groups. [96][97][98][99][100][101] New insights have recently been acquired through DFT calculations, 87,[102][103][104] leading to a surface model for silicated alumina, which accounts for the presence of original surface sites at the origin of mild acidity. Note that, to the best of our knowledge, despite significant effort on the simulation of bulk aluminosilicate glasses for geology, 105,106 simulation studies of the surface of ASA have not been undertaken before, except for some earlier attempts to choose "local" models, as aluminosilsesquioxanes.…”

Section: 81mentioning

confidence: 99%

Density functional theory simulations of complex catalytic materials in reactive environments: beyond the ideal surface at low coverage

Chizallet

Raybaud

2014

Catal. Sci. Technol.

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Most efficient heterogeneous catalysts used industrially are generally very complex systems. Far away from perfect crystallinity and well-defined oriented surfaces at low coverage, they involve structural disorder, heterogeneous site distribution with variable coordination and structural dependence upon the chemical environment. Unravelling their atomic-scale structures and understanding their roles in the catalytic reaction are not easy tasks, as the respective contributions of each type of site to the spectroscopic or catalytic responses are generally convoluted. Computational chemistry is of great help to address these issues. In the present perspective review, we highlight two relevant systems involved in numerous industrial applications: the amorphous silica alumina support; and subnanometric platinum clusters, possibly doped with tin and/or indium, supported on γ-Al 2 O 3 . The structural complexity is inherent to the amorphous nature of an oxide support on the one hand, and to the ultra-dispersed form of a mono-and multi-metallic catalyst on the other hand. In each case, Density Functional Theory (DFT) calculation was used to provide an original structure for active sites and to reveal how the corresponding multi-step reactions are influenced. Moreover, we highlight how theoretical studies including coverage effects on complex systems, induced by (T, P) reaction conditions, offer enriched perspectives with respect to studies on ideal surfaces at low coverage.

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Brønsted acid sites of zeolitic strength in amorphous silica-alumina

Cited by 65 publications

References 28 publications

Texture, acidity and fluid catalytic cracking performance of hierarchical faujasite zeolite prepared by an amphiphilic organosilane

Texture, acidity and fluid catalytic cracking performance of hierarchical faujasite zeolite prepared by an amphiphilic organosilane

Revisiting carbenium chemistry on amorphous silica-alumina: Unraveling their milder acidity as compared to zeolites

Density functional theory simulations of complex catalytic materials in reactive environments: beyond the ideal surface at low coverage

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