Synthesis of Rh6(CO)16 in Supercages of Zeolite HY: Reaction Network and Kinetics of Formation from Mononuclear Rhodium Precursors via Rh4(CO)12 Facilitated by the Water Gas Shift Half-Reaction

Rh(I)(CO)(2) complexes anchored to zeolite HY were converted into Rh-4(CO)(12) in the zeolite supercages upon exposure to flowing CO + H2O at 35 degrees C, and the chemistry and kinetics were characterized with infrared spectroscopy. Rh-6(CO)(16) formed along with Rh-4(CO)(12), but only in low yield, although it is more stable than Rh-4(CO)(12). The formation of Rh-6(CO)(16) was hindered by trapping of Rh-4(CO)(12) in the supercages and by the low rate of transport of the mononuclear rhodium species. However, exposure of the sample to wet helium at 80 degrees C caused the Rh-4(CO)(12) to fragment, generating anchored Rh(I)(CO)(2) and also Rh-6(CO)(16). IR spectra recorded under various conditions led to elucidation of the reaction network for cluster formation and breakup and a strategy of repetitive treatments that boosted the yield of Rh-6(CO)(16) to >90%. The reversible formation and breakup of the rhodium carbonyl clusters were facilitated by the half-reactions of the water gas shift reaction, with gas-phase products identified by mass spectrometry. The results show how understanding of the reactions within a zeolite allows control of the nuclearity of encaged metal clusters, an important class of catalyst.