AB-INITIO SCF-MO STUDY OF THE CHEMISORPTION OF METHANE ON AL AND LA OXIDE SURFACES

A theoretical approach to the rate-determining step for methane activa tion on Al and La oxides has been performed by means of ab initio quan tum mechanical calculations. In a first approach, transition states an d products for the heterolytic chemisorption of CH4 on Al(OH)(3) and L a(OH)3 molecules have been determined at the RHF-SCF level of theory, Transition-state structures correspond to a four-center interaction wi th an energy barrier of about 40 kcal/mol, This high activation barrie r supports the heterolytic mechanism proposed for this reaction. The p roducts are methyl-metal compounds, CH3-Me (Me Al, La), with the carbo n atom negatively charged. Catalytic surfaces have been modeled by int roducing the effects due to the lattice through both point charges and ab initio model potentials. The selectivity of these catalysts toward s C2 is discussed on the grounds of the differences found for the rela tive stability of the methyl-metal intermediates. (C) Academic Press, Inc.