Ethene hydrogenation: Vs. dimerization over a faujasite-supported [Rh(C₂H₄)₂] complex. A computational study of mechanism

Single-site catalysts offer great chances for unraveling the mechanism and the selectivity of catalytic mechanism, in particular when the system is experimentally well characterized. A particular interesting system of this type is the hydrogenation and the dimerization of ethene by the faujasite-supported complex [Rh(C₂H₄)₂]⁺. We have examined this system computationally, treating periodic models with a density functional method. The complex [Rh(C₂H₄)₂]⁺ binds in a bidentate fashion, as previously suggested, inside the faujasite supercage at the oxygen atoms of a 12-member ring. The calculations on this model complex showed ethene hydrogenation to be preferred over dimerization. The highest free energy barrier for forming a C-H bond was calculated at 33 kJ mol^-1 at room temperature. This value is significantly lower than the lowest activation free energy, 97 kJ mol^-1, calculated for C-C bond formation. The results of this mechanistic study allow one to rationalize the experimental observation that the faujasite-supported [Rh(C₂H₄)₂]⁺ complex in the presence of H₂ is active for hydrogenation, producing ethane as the main product.