Ir/KLTL zeolites: Structural characterization and catalysis of n-hexane reforming

Ir/KLTL zeolite catalysts were prepared by incipient wetness impregnation of LTL zeolites with [Ir(NH3)(5)Cl]Cl-2. The catalysts were characterized by extended X-ray absorption fine structure (EXAFS) spectroscopy, infrared spectroscopy, and H-2 chemisorption. EXAFS data show that the average Ir cluster size (after treatment at 300 degrees C in H-2) increased from about 7 to 20 Angstrom as the zeolite K:Al atomic ratio increased from 0.34 to 1.56. Infrared spectra of adsorbed CO show that the electron donation to the Ir increased as the K:Al ratio increased. In contrast to the performance reported for Pt/KLTL zeolites with metal clusters as small as those observed in the present experiments, the Ir/KLTL catalyst has a low selectivity for dehydrocyclization of n-hexane at 440-480 degrees C and 1 atm with a H-2:n-hexane feed molar ratio of 6. Instead, the catalysts are selective for hydrogenolysis. The selectivity is insensitive to the K:Al ratio, but the activity for dehydrocyclization is a maximum at a K:Al atomic ratio of about 1. The results show that even the smallest Ir clusters to which electron donation is significant still behave essentially like metallic Ir in the catalytic reactions. (C) 1996 Academic Press, Inc.