Ethene Dimerization and Hydrogenation over a Zeolite-Supported Rh(I)-Carbonyl Complex: Mechanistic Insights from DFT Modeling

Using DFT calculations within a quantum mechanical/molecular mechanical scheme, we present a model study on a zeolite-supported Rh(I) complex, [Rh(CO)(C₂H₄)]⁺, to rationalize the experimentally observed ethene hydrogenation and dimerization. Our computational results show that the coordination of an ethene to the Rh center of a [Rh(CO)(C₂H₄)]⁺ complex is thermodynamically favorable over H₂ coordination. The diethyl complex [Rh(CO)(C₂H₅)₂]⁺ resulting from hydrogenation acts as a branching point of two catalytic cycles of ethene conversion, to hydrogenation or dimerization. The Rh-acyl complex [Rh(COCH₂CH₃)(C₂H₅)(C₂H₄)]⁺ is the in situ-generated active species initiating the dimerization, as it entails a tremendous lowering of the C-C coupling barrier, by more than 100 kJ mol^-1. Overall, free energy barriers of ethene hydrogenation (89-92 kJ mol^-1) are calculated 4-7 kJ mol^-1 lower than the barrier for dimerization, 96 kJ mol^-1, in qualitative agreement with the experimentally observed selectivity. Finally, a side reaction of the Rh-acyl complex yields a qualitative explanation of the experimentally observed steady increase in butene selectivity.