Isostructural Zeolite-Supported Rhodium and Iridium
Complexes: Tuning Catalytic Activity and Selectivity by Ligand Modification
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Abstract
A family
of isostructural, essentially molecular complexes of rhodium
and of iridium anchored to HY zeolite was synthesized from M(C<sub>2</sub>H<sub>4</sub>)<sub>2</sub>(acac) and M(CO)<sub>2</sub>(acac)
(M = Rh, Ir; acac is acetylacetonate), with the initial supported
species being M(C<sub>2</sub>H<sub>4</sub>)<sub>2</sub> and M(CO)<sub>2</sub>, 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<sub>2</sub> (and
sometimes D<sub>2</sub>), giving ethane and, when the metal was rhodium,
butenes, and, when D<sub>2</sub> 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
