(25e) Supported Molecular Metal Complex and Metal Cluster Catalysts: Design, Synthesis, Characterization, and Performance

Essentially molecular rhodium catalysts were synthesized from the organometallic precursor Rh(C₂H₄)₂(acetylacetonate) on zeolite HY and on MgO and characterized by infrared and X-ray absorption spectroscopies. The supported rhodium complexes anchored to the zeolite, initially in the form of Rh(C₂H₄)₂, selectively catalyzed ethylene dimerization, typically at 298 K and 1 bar, but when the catalyst was poisoned by CO; or the support was changed to MgO or zeolite NaY; or the rhodium was converted into small clusters, the ethylene underwent predominantly hydrogenation instead of dimerization. The preciseness of the synthesis of the supported rhodium species facilitated determination of structure-catalyst performance relationships that led to a mechanistic picture according to which the dimerization proceeds by a mechanism involving both the rhodium complexes and zeolite surface OH groups; the reaction is facilitated by H₂ and proceeds as one ethylene molecule is activated by an isolated rhodium complex and another by a weakly acidic Al–OH group.