Iridium Clusters in NaX Zeolite Cages: Synthesis, Characterization, and Selective Catalysis of CO Hydrogenation

Abstract

[Ir(CO)[sub 2](acac)] in the pores of NaX zeolite was treated in CO at 1 atm and various temperatures and characterized by infrared and extended X-ray absorption fine structure (EXAFS) spectroscopies. The iridium precursor was converted predominantly into [HIr[sub 4](CO)[sub 11]][sup [minus]] at 70[degrees]C, and predominantly into [Ir[sub 6](CO)[sub 15]][sup 2[minus]] at 175[degrees]C, as indicated by infrared spectra. The infrared spectra indicate that both of these clusters interact through ion pairing with the zeolite support. The chemistry of their synthesis in the zeolite cages parallels that on the basic MgO surface and in basic solutions. The zeolite-supported samples catalyzed CO hydrogenation at 20 atom and 250[degrees]C, having a relatively low activity in comparison with those of typical Fischer-Tropsch catalysts. The catalyst gave a non-Schulz-Flory distribution of hydrocarbon products with a maximum at C[sub 1] and a relative maximum at C[sub 4]. Infrared and EXAFS spectra of the catalyst used with equimolar CO + H[sub 2] feed showed that the predominant iridium-containing species was [Ir[sub 6](CO)[sub 15]][sup 2[minus]]. 46 refs., 5 figs., 3 tabs.