Catalytic dehydrogenation of ethane over mononuclear Cr(III) surface sites on silica. part I. C - H activation by σ-bond metathesis

Abstract

Mononuclear Cr(III)-silica models have been studied by quantum chemical methods with respect to catalytic activity toward dehydrogenation of ethane. Both cluster and slab models have been developed and used to explore the conceptual model of mononuclear Cr(III) with three covalent ligands that coordinate through oxygen. The study focuses on a reaction mechanism consisting of three main reaction steps: (1) C - H activation of ethane according to -bond metathesis and accompanied by the formation of O - H and Cr - C bonds, (2) -H transfer to chromium with subsequent loss of ethene and (3) regeneration of the chromium site under evolution of H2. An alternative mechanism is also explored, in which C - H activation takes place at a reactive hydridochromium complex. Stationary points pertaining to these reactions have been optimized, and free energy calculations are used to identify the rate-determining steps. The influence of the local structure of the chromium surface sites is explored by means of a number of idealized surface models and electronic energy profiles for the reactions.