Isostructural Zeolite-Supported Rhodium and Iridium Complexes: Tuning Catalytic Activity and Selectivity by Ligand Modification

A family of isostructural, essentially molecular complexes of rhodium and of iridium anchored to HY zeolite was synthesized from M(C 2 H 4 ) 2 (acac) and M(CO) 2 (acac) (M = Rh, Ir; acac is acetylacetonate), with the initial supported species being M(C 2 H 4 ) 2 and M(CO) 2, each bonded to the zeolite through two M–O bonds. Each was used as a catalyst at 300 and 373 K and atmospheric pressure for the conversion of ethylene in the presence of H 2 (and sometimes D 2 ), giving ethane and, when the metal was rhodium, butenes, and, when D 2 was present, HD. The high degree of uniformity of the metal complexes allowed a precise spectroscopic elucidation of the predominant species present during catalysis. The CO ligands were inhibitors of the catalytic reactions, with the metal dicarbonyl complexes lacking measurable activity under our conditions. The CO ligands also served as probes helping to characterize the structures and electronic properties of the catalytic metal complexes. The data show that subtle changes in the bonding of the ligands markedly affect the catalytic performance.