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 activation on Al and La oxides has been performed by means of ab initio quantum mechanical calculations. In a first approach, transition states and products for the heterolytic chemisorption of CH4 on Al(OH)3 and La(OH)3 molecules have been determined at the RHF-SCF level of theory. Transition-state structures correspond to a four-center interaction with an energy barrier of about 40 kcal/mol. This high activation barrier supports the heterolytic mechanism proposed for this reaction. The products are methyl-metal compounds, CH3-Me (Me = Al, La), with the carbon atom negatively charged. Catalytic surfaces have been modeled by introducing the effects due to the lattice through both point charges and ab initio model potentials. The selectivity of these catalysts towards C2 is discussed on the grounds of the differences found for the relative stability of the methyl-metal intermediates.