Structures, energies, and electrostatics for methane complexed with alumina clusters

Ab initio calculations were used to investigate properties of complexes for med from the association of CH4 with Al2O3, Al4O6, and Al8O12 alumina clust ers. Methane attaches to a surface Al atom of the cluster to form a complex with an Al-C separation that varies between 2.2 and 2.5 Angstrom. The rota tional motion for methane in these complexes is highly fluxional. Extrapola ted G2MP2 well depths fbr the CH4- - -Al2O3, CH4- - -Al4O6. and CH4- - -Al8 O12 complexes are 21, 14, and 17 kcal/mol, respectively. These different we ll depths ale determined by the accessibility of the Al atom to which CH4 b inds and the size of the alumina cluster. The electrostatics of the three a lumina clusters are very similar, with a charge on the surface Al atom of 2.2 to 2.3. The potential energy surface for a CH4- - -Al2nO3n cluster is represented semiquantitatively by an analytic Function consisting of two-bo dy potentials. The results of this study suggest that the adsorption energy for alkane molecules binding to alumina materials depends very strongly on the structure of the binding site.