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

Abstract

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.

Heterolytic Splitting of H2 and CH4 on γ-Alumina as a Structural Probe for Defect Sites

“…Thus, the migration of the oxygen atom toward the Al III center is not fully achieved and the resulting bond is 2.0 Å (vs 1.92 Å in the case of H 2 dissociation), resulting in a smaller stabilization of the Al III . It should be underlined that the exoenergetic C-H bond activation of methane with Al atoms as found in this study is in sharp contrast with what has been obtained on small clusters, 31 which predicted an endoenergetic dissociative adsorption of C-H bond of propane on tricoordinated aluminum atoms (132 kJ‚mol -1 ).…”

Low-Temperature Activation of Branched Octane Isomers over Lanthanum-Exchanged Zeolite X Catalysts

“…This is in good agreement with theoretical calculations for cracking ethane . It has also been suggested that strong Lewis acid sites in MFI and FAU catalyze alkane dehydrogenation, , including even activation of the CH bond by oxidative addition to extraframework aluminum …”

“…On the basis of density functional calculations, Fǎrcaşiu et al. suggested the insertion of Al 3+ into a CH bond for the activation of propane . La 3+ could act in a similar way.…”

A Density Functional Theory Based Approach to Extraframework Aluminum Species in Zeolites

“…Catlow and collaborators conducted a theoretical study on the defect formation upon zeolite dealumination . Farcasiu and Lukinskas studied, by density functional theory (DFT) methods, the structure of dialuminum oxide clusters, as models for EFAL species, and their role in H−H and C−H bond dissociation. Zhidomirov et al also studied the dialuminum oxide clusters, as models of EFAL and Lewis acid sites on zeolites.…”