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

Abstract

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.