Platinum-mordenite catalysts for n-Hexane isomerization: Characterization by X-ray absorption spectroscopy and chemical probes
- North Carolina State Univ., Raleigh, NC (United States)
Platinum-mordenite (Pt-MOR) catalysts were prepared from NH{sub 4}-MOR by ion exchange with (Pt{sup II}(NH{sub 3}){sub 4})(OH){sub 2}, calcination in O{sub 2} at 350{degrees}C, and reduction in H{sub 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{sub 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{sub 3} and re-reduction at 350{degrees}C; elemental analysis evidenced 90% exchange of protons for K{sup +} ions. The product distribution and observed activation energies for C{sub 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{sub 2} temperature-programmed desorption, we conclude that the Pt-MOR catalysts consist of small Pt clusters hosted within the mordenite crystals. The PtL{sub 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. The infrared spectrum of CO adsorbed on Pt-H-MOR contains an intense band at 2084 cm{sup -1}, which is assigned to linear CO moieties on Pt clusters. The infrared spectrum of CO adsorbed on Pt-KH-MOR evidences a red shift of the linear CO band, which the authors suggest is due to electrostatic interactions between carbonyl O atoms and nearby K{sup +} ions. 45 refs., 9 figs., 6 tabs.
- OSTI ID:
- 110990
- Journal Information:
- Journal of Catalysis, Journal Name: Journal of Catalysis Journal Issue: 1 Vol. 149; ISSN 0021-9517; ISSN JCTLA5
- Country of Publication:
- United States
- Language:
- English
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