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.