Hydrogen isotopic exchange between D<sub>2</sub> and H<sub>2</sub>O on alumina prepared by sputtering

DFT calculations at the B3LYP/6-31G** level were conducted on the reaction of the propane molecule with the aluminum hydroxide clusters (HO) 3 Al(OH 2 ) x (x ) 0,1). Weak, physisorbed (van der Waals) complexes were identified. Chemisorption does not involve the Brønsted acidity of the catalyst, as no hydron transfer occurs. Instead, the reaction involves insertion of the aluminum atom into a C-H bond, followed by the migration of the hydrogen atom from aluminum to oxygen, to form the chemisorbed intermediate, (H 2 O) x+1 (HO) 2 Al-CH 2 Et or (H 2 O) x+1 (HO) 2 Al-CHMe 2 , with the latter having a higher energy barrier. The elimination of hydrogen from Cβ and oxygen gives then H 2 and propene, which forms a strong π complex with the aluminum cluster for x ) 0. The first step, chemisorption, has a lower energy barrier than the second, elimination, but still higher than the hydrogen dissociation on the same clusters. Thus, the rate relationship H 2 /D 2 exchange > H 2 /RH exchange > RH dehydrogenation is predicted, as was experimentally observed. The tetracoordinated aluminum cluster (x ) 1) reacts with the hydrocarbon by the same pathway as the tricoordinated aluminum cluster (x ) 0) but with higher barriers for both steps; the barriers are reduced for the larger cluster (HO) 2 (H 2 O)Al-O-Al(OH) 2 (H 2 O). The alternative pathway, forming the alkyl-oxygen adduct (HO) 2 Al(OH 2 ) x (H)-O(R)H is too high in energy to compete. Examination of butane and isobutane establishes the reactivity order: prim C-H > sec-C-H > tert-C-H. For isobutane, essentially only methyl C-H cleavage should occur in the common first step for hydrogen exchange and dehydrogenation. In the second step, i.e., the β C-H cleavage in the Al-alkyl intermediate, the reactivity order is tert-C-H > sec-C-H > prim C-H. "Broken lattice" zeolites and especially extraframework aluminum species present in steamed zeolites should be more reactive than the intact zeolite lattices. Thus, the mechanism is relevant for the activation of alkanes for acid-catalyzed conversions on these catalysts, which have insufficient acid strength to cleave C-H and C-C bonds by hydron transfer.

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Theoretical Calculation of the Interaction of Hydrogen with Models of Coordinatively Unsaturated Centers on Alumina

Fǎrcaşiu

Lukinskas

1999

J. Phys. Chem. A

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Ab initio calculations with large basis sets and electron correlation were conducted on the reaction of hydrogen molecules with (HO) 3 Al(OH 2 ) x clusters, where x ) 0, 1, and 2. In this way, the reactivity of Al(III) species was studied as a function of their level of coordinative unsaturation. For the tricoordinated species, the geometry of the starting cluster was obtained by the optimization of the species (HO) 3 Al(OH 2 ) x+1 and removal of the extra water molecule, on the basis of idea that aluminum oxide surfaces are formed by calcination of hydrated forms. Two models, one assuming rigidity and the other allowing for flexibility of the tricoordinated aluminum center, were examined. The complexes with physisorbed and chemisorbed hydrogen were optimized in the same way. The reactions of tetra-and pentacoordinated aluminum clusters were studied without any constraints on the geometry. The calculations predicted the hydrogen chemisorption to be endothermic in all cases, the order being E(x ) 0) < E(x ) 1) < E(x ) 2). The chemisorption pathway was investigated and its transition structure and energy barrier were established. The energy barriers for chemisorption, determined as the relative energies of the transition structures E TS , varied with the coordination number of the aluminum atom as E TS (x ) 0) < E TS (x ) 1) < E TS (x ) 2). The barriers were similar for the rigid and for the flexible tricoordinated aluminum clusters. A significant conclusion is that tetracoordinated sites on alumina must be thought of as reactive (if not the reactive) sites. The literature description (on the basis of ab initio calculations with small basis sets at the HF level) of hydrogen chemisorption as an acid-base reaction, involving hydrogen heterolysis concerted with the attachment of the proton to oxygen (basic site) and the hydride to aluminum (acid site), is not substantiated by our calculations. Instead, the chemisorption occurs through the interaction of H 2 with the aluminum (metal ion catalysis) until both hydrogen atoms are bonded to Al, after which one of the hydrogens migrates to an adjacent oxygen atom. B3LYP calculations give results in reasonable agreement with the MP2 calculations, attesting to the appropriateness of the density functional theory method for these types of structures.

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Hydrogen isotopic exchange between D₂ and H₂O on alumina prepared by sputtering

Cited by 5 publications

References 15 publications

Theoretical study of hydrogen dissociation on dialuminum oxide clusters and aluminum–silicon–oxygen clusters, as models of extraframework aluminum species and zeolite lattice

Theoretical study of hydrogen dissociation on dialuminum oxide clusters and aluminum–silicon–oxygen clusters, as models of extraframework aluminum species and zeolite lattice

Mechanism and Reactivity of Alkane C−H Bond Dissociation on Coordinatively Unsaturated Aluminum Ions, Determined by Theoretical Calculations

Theoretical Calculation of the Interaction of Hydrogen with Models of Coordinatively Unsaturated Centers on Alumina

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Hydrogen isotopic exchange between D2 and H2O on alumina prepared by sputtering

Cited by 5 publications

References 15 publications

Theoretical study of hydrogen dissociation on dialuminum oxide clusters and aluminum–silicon–oxygen clusters, as models of extraframework aluminum species and zeolite lattice

Theoretical study of hydrogen dissociation on dialuminum oxide clusters and aluminum–silicon–oxygen clusters, as models of extraframework aluminum species and zeolite lattice

Mechanism and Reactivity of Alkane C−H Bond Dissociation on Coordinatively Unsaturated Aluminum Ions, Determined by Theoretical Calculations

Theoretical Calculation of the Interaction of Hydrogen with Models of Coordinatively Unsaturated Centers on Alumina

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Product

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Hydrogen isotopic exchange between D₂ and H₂O on alumina prepared by sputtering