IRIDIUM CLUSTERS IN KLTL ZEOLITE - SYNTHESIS, STRUCTURAL CHARACTERIZATION, AND CATALYSIS OF TOLUENE HYDROGENATION AND N-HEXANE DEHYDROCYCLIZATION

摘要

Iridium clusters incorporating about six atoms each, on average, were prepared in KLTL zeolite by decarbonylation (in H-2 at 400 degrees C) of iridium carbonyl clusters formed by treatment of adsorbed [Ir(CO)(2)(acac)] in CO at 1 atm and 175 degrees C. The supported species were characterized by infrared and extended X-ray absorption fine structure (EXAFS) spectroscopies. The iridium carbonyls formed from [Ir(CO)(2)(acac)] were predominantly [HIr4(CO)(11)](-) with a small amount of [Zr(CO)(4)](-). The synthesis chemistry of iridium carbonyls in the basic KLTL zeolite parallels that in basic solutions. Shifts of the nu(co) bands of the iridium carbonyl clusters relative to those of the same clusters in solution indicate strong interactions between the clusters and zeolite cations. The decarbonylated sample, approximated as Ir-6/KLTL zeolite, is catalytically active for toluene hydrogenation at 60-100 degrees C, with the activity being approximately the same as those of Ir-4 and Ir-6 clusters supported on metal oxides, but an order of magnitude less than that of a conventional supported iridium catalyst consisting of aggregates of about 50 atoms each, on average. The catalyst is also active for conversion of n-hexane + H-2 at 340-420 degrees C, but the selectivity for aromatization is low and that for hydrogenolysis is high, consistent with earlier results for conventionally prepared (salt-derived) iridium clusters of about the same size supported in KLTL zeolite. The zeolite-supported iridium clusters are the first prepared from both salt and organometallic precursors; the results indicate that the organometallic and conventional preparation routes lead to supported iridium clusters having similar structures and catalytic properties. (C) 1997 Academic Press. [References: 59]