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

Abstract

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.

The Initial Stages of Solid Acid-Catalyzed Reactions of Adsorbed Propane. A Mechanistic Study by in Situ MAS NMR

“…Despite several decades of efforts to understand the initial step of alkane activation on solid acids such as zeolites, heteropolyacids, or sulfated zirconias, it is still not clear how the initial adsorbed carbenium ions are generated from saturated hydrocarbon: σ-bond protolysis by superacidic sites, hydride abstraction by strong Lewis acid sites, and oxidative dehydrogenation followed by protonation have all been suggested but not univocally demonstrated. − However, the recent literature seems to indicate that the activation mechanism may be quite different depending on the solid acid, one-electron oxidation for sulfated zirconias , and the Haag−Dessau (protolysis) mechanism on zeolites. , Once generated, the adsorbed carbenium ion may undergo all well-documented competitive reactions such as deprotonation, β-scission, alkylation, and oligomerization.…”

Diffusion of Molecular Hydrogen through Porous Materials: The Importance of Framework Flexibility

“…The effects of basis set superposition error (BSSE) on the hydrogen interaction energies is likely to be about 1 kcal/ mol, on the basis of our own calculations and other studies of H 2 physisorption over zeolite clusters using a similar level of theory. 43,48 The overall tendency of these offsets would be to shift the calculated barrier as a whole, having relatively less effect on the energy differences which are used to calculate the barrier heights. As we were primarily interested in comparing energy barrier heights and not absolute interaction energies (and further because of the large number of energies sampled), we did not attempt to correct for BSSE in our calculations.…”

“…Within the explicitly calculated DFT region, the hydrogen molecule has a direct interaction with all other atoms. The combination of the basis set and the functional used here has been used numerous times successfully for calculations regarding other silica cluster systems − and also has been employed for the modeling of molecular adsorption in cluster models of zeolites. , The embedding region of the cluster does not directly interact with the hydrogen molecule but provides a representation of the bulk environmental forces on the central six-ring. The use of MNDO has proven to provide a good representation of the mechanically responsive silica environment − in other embedded ONIOM silica calculations.…”

Theoretical Studies of Carbocations in Ion Pairs. 8. Search for Anchimeric Assistance in the Ionization of 2-Butyl Cation Precursors

“…The carbocationic mechanism has long been established for reactions of hydrocarbons under acid catalysis, the intermediates being the same as in carbocationic solvolyses . It turns out that most if not all carbocationic reactions in solution are controlled by ion pairing, whereas on solid acids, carbocations can intervene only tightly paired with anionic sites on the surface, or the reaction may not involve fully formed carbocations, but cationoidic species, in which the cation is still partially bonded to the anion . The reaction features (product distribution, kinetics, structure−reactivity relationships) in the latter case are similar to those of conversions involving ion pairs in solution …”