Swift hopping gallium over [AlO<sub>4</sub>]<sup>−</sup> tetrahedra in Ga/ZSM‐5: A DFT study

Kusmin, Ilya V.; Zhidomirov, G. M.; Solkan, V. N.

doi:10.1002/qua.21339

Cited by 5 publications

(2 citation statements)

References 21 publications

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“…High-silica zeolites modified with gallium are known as effective catalysts for aromatization of light alkanes. , It is generally believed that these catalysts have a bifunctional nature: − the role of the modifying Ga cations in these processes is usually associated with the dehydrogenation function, whereas the residual Brønsted acid sites in the zeolite are thought to promote secondary reactions such as oligomerization, cracking, and ring-closure steps. Despite numerous experimental − and computational studies ,− reporting on the chemistry and reactivity of the intrazeolite gallium species, there is still considerable debate on the structure of the active site and the relevance of Lewis or Brønsted acid sites in the various constituent reactions of alkane aromatization …”

Section: Introductionmentioning

confidence: 99%

Anionic Oligomerization of Ethylene over Ga/ZSM-5 Zeolite: A Theoretical Study

2008

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Reaction mechanisms for the oligomerization of ethylene on extraframework gallium sites in Ga/ZSM-5 zeolite are studied by density functional theory cluster calculations. The nature of the transition states and intermediates is analyzed by topological analysis of the electron density distribution functions in the framework of the quantum theory of atoms in molecules. Interaction of olefins with the stable intermediate [Ga 3+ (H -)(C 2 H 5 -)] + of the catalytic ethane dehydrogenation and with the initial extraframework Ga + site are considered. For the [Ga 3+ (H -)(C 2 H 5 -)] + site, interaction of ethylene with the hydride ion is a facile process. Addition of ethylene to the Ga-alkyl fragment and oligomer chain growth are rather difficult and compete with ethylene desorption via the β-H transfer reaction. A novel, alternative, and more facile reaction path for the oligomerization of olefins is proposed. This mechanism proceeds via interaction of two C 2 H 4 molecules with the extraframework Ga + cation in ZSM-5 zeolite, resulting in formation of vinyl and ethyl groups coordinating to gallium. Interaction of a further C 2 H 4 molecule with the vinyl group leads to dimerization. Such an anionic oligomerization mechanism for light olefins is argued to be at least as favorable as the conventional process catalyzed by Brønsted acid sites. The anionic nature of the transition states and intermediates may result in selectivity patterns during hydrocarbon conversion over Ga/ZSM-5 zeolites completely different from the case when zeolitic Brønsted acid sites are the catalytic sites via the conventional carbenium ion mechanism.

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Section: Introductionmentioning

confidence: 99%

Anionic Oligomerization of Ethylene over Ga/ZSM-5 Zeolite: A Theoretical Study

2008

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“…Ga-containing zeolites are known as efficient catalyst for light alkane dehydrogenation and aromatization . Recent experimental − and theoretical studies ,− have contributed significantly to our understanding of the mechanisms underlying alkane activation over these catalysts. It has been concluded that a rather low but stable catalytic activity of reduced Ga/ZSM-5 zeolites associates with the presence of extra-framework Ga + cations in the microporous matrix. , The initial C−H bond activation proceeds over the Lewis acid−base pair formed by Ga + ion and an adjacent basic framework oxygen of the zeolite ( I + RH → II , Scheme )…”

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confidence: 99%

Structure−Reactivity Relationship for Catalytic Activity of Gallium Oxide and Sulfide Clusters in Zeolite

Pidko

Santen

2009

J. Phys. Chem. C

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The structure-reactivity relationship for the catalytic performance of cationic oxygen-and sulfur-containing Ga clusters in high-silica zeolite in ethane dehydrogenation is derived from the results of DFT calculations. Computed Gibbs free energy changes for the active site regeneration step via H 2 desorption are used as a reactivity descriptor that reflects strength of the active Lewis acid-base pairs. It is shown that the activation free energies as well as the free energy changes of the initial C-H bond activation and final H 2 recombination elementary steps of the catalytic cycle scale linearly with the values of free energy change of the active site regeneration. The energetics of the intermediate step involving alkene desorption via β-C-H cleavage does not apparently depend on the properties of the active site. The relationship obtained points to the optimum composition and structure of the intrazeolite Ga cluster for the catalytic dehydrogenation of light alkanes.

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