PLATINUM-MORDENITE CATALYSTS FOR N-HEXANE ISOMERIZATION - CHARACTERIZATION BY X-RAY-ABSORPTION SPECTROSCOPY AND CHEMICAL PROBES

Platinum-mordenite (Pt-MOR) catalysts were prepared from NH4-MOR by ion exchange with [Pt-II(NH3)(4)][OH](2), calcination in O-2 at 350 degrees C, and reduction in H-2 at 350 degrees C. The resultant Pt-H-MOR was active for n-hexane isomerization and hydrocracking via bifunctional catalysis at 240-300 degrees C and 1 atm. The observed activation energies for C-6 branched-isomer formation are unusually low, suggesting that the isomerization rates were controlled by pore diffusion. A Pt-KH-MOR catalyst was prepared by ion exchange with aqueous KNO3 and re-reduction at 350 degrees C; elemental analysis evidenced 90% exchange of protons for K+ ions. The product distribution and observed activation energies for C-6 branched-isomer formation over Pt-KH-MOR are consistent with n-hexane isomerization via bifunctional catalysis. Hydrocracking was strongly suppressed, and light hydrocarbons were formed primarily by Pt-catalyzed hydrogenolysis. From in situ extended X-ray absorption fine structure spectroscopy and H-2 temperature-programmed desorption, we conclude that the Pt-MOP catalysts consist of small Pt clusters hosted within the mordenite crystals. The Pt L(III) X-ray absorption near-edge structure (XANES) spectra of Pt-H-MOR and Pt-KH-MOR are closely similar, suggesting that the electronic structure of the Pt clusters is unaffected by mordenite acid-base chemistry. For the freshly reduced catalysts, a XANES feature at 10 eV relative energy is assigned to Pt-H antibonding states. The infrared spectrum of CO adsorbed on Pt-H-MOP contains an intense band at 2084 cm(-1). which is assigned to linear CO moieties on Pt clusters. A small peak at 2124 cm(-1) is assigned to isolated Pt-I-CO species, which we infer are formed by oxidative fragmentation of Pt clusters. The infrared spectrum of CO adsorbed on Pt-KH-MOR evidences a red shift of the linear CO band, which we suggest is due to electrostatic interactions between carbonyl O atoms and nearby K+ ions. (C) 1994 Academic Press, Inc.