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

The reactivity of coordinatively unsaturated centers of aluminum oxide clusters, found for example in extraframework aluminum species of steamed zeolites, was examined theoretically on the example of the reaction of a hydrogen molecule with dialuminum hydroxide clusters (HO)(2)(H2O)(n)Al-O-Al(OH)(2)(H2O) (1). In the cluster, one aluminum atom is tricoordinated and the other is tetracoordinated (2, n = 0), or both aluminum atoms are tetracoordinated (3, n = 1). The system studied is also a model for transitional aluminas, in which two or three tetracoordinated aluminum centers can occur next to each other. Density functional theory calculations with electron correlation at the B3LYP/6-31G** level have identified a complex with physisorbed hydrogen and a complex with chemisorbed hydrogen in each case. Each of them was more stable than the corresponding complex formed by the corresponding one-aluminum cluster. The transition structures for chemisorption were identified. The reaction coordinate for chemisorption revealed that the reaction is a case of metal ion catalysis, rather than an acid-base reaction with heterolytic dissociation of hydrogen. The potential energy barrier (PEB) for hydrogen chemisorption was lower for the two-Al clusters than for the one-Al clusters. The chemisorption on a silicon-aluminum cluster, (HO)(2)(H2O)Al-O-Si(OH)(3) was also found to occur, but it had a higher FEB than for the corresponding two-aluminum cluster. Thus, zeolites can also exchange hydrogen, albeit less effectively than alumina, whereas the extraframework aluminum species in steamed zeolites should exchange hydrogen easier than the intact zeolite. (C) 2001 Elsevier Science B.V. All rights reserved.