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 clus ters, found for example in extraframework aluminum species of steamed zeoli tes, was examined theoretically on the example of the reaction of a hydroge n 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 othe r is tetracoordinated (2, n = 0), or both aluminum atoms are tetracoordinat ed (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 correlati on at the B3LYP/6-31G** level have identified a complex with physisorbed hy drogen and a complex with chemisorbed hydrogen in each case. Each of them w as more stable than the corresponding complex formed by the corresponding o ne-aluminum cluster. The transition structures for chemisorption were ident ified. The reaction coordinate for chemisorption revealed that the reaction is a case of metal ion catalysis, rather than an acid-base reaction with h eterolytic 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)A l-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 hydro gen, albeit less effectively than alumina, whereas the extraframework alumi num species in steamed zeolites should exchange hydrogen easier than the in tact zeolite. (C) 2001 Elsevier Science B.V. All rights reserved.