Platinum-Mordenite Catalysts for n-Hexane Isomerization: Characterization by X-Ray Absorption Spectroscopy and Chemical Probes

Abstract

Abstract Platinum-mordenite (Pt-MOR) catalysts were prepared from NH 4 -MOR by ion exchange with [Pt II (NH 3 ) 4 ][OH] 2 , calcination in O 2 at 350°C, and reduction in H 2 at 350°C. The resultant Pt-H-MOR was active for n -hexane isomerization and hydrocracking via bifunctional catalysis at 240-300°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 KNO 3 and re-reduction at 350°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-MOR 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-MOR 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.