Structure and properties of cobalt-exchanged H-ZSM5 catalysts for dehydrogenation and dehydrocyclization of alkanes

Citation
W. Li et al., Structure and properties of cobalt-exchanged H-ZSM5 catalysts for dehydrogenation and dehydrocyclization of alkanes, J PHYS CH B, 105(6), 2001, pp. 1176-1184
Citations number
31
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Physical Chemistry/Chemical Physics
Journal title
JOURNAL OF PHYSICAL CHEMISTRY B
ISSN journal
1520-6106 → ACNP
Volume
105
Issue
6
Year of publication
2001
Pages
1176 - 1184
Database
ISI
SICI code
1520-6106(20010215)105:6<1176:SAPOCH>2.0.ZU;2-6
Abstract
Co/H-ZSM5 catalysts with Co/Al ratios of 0.09-0.22 were prepared by aqueous exchange. Turnover rates for propane conversion to propene and to C-6-C-8 aromatics on these catalysts are about 10-fold higher than on H-ZSM5. The s electivities to propene, aromatics, and H-2 are also higher on Co/H-ZSM5 th an on H-ZSM5. The rate of D-2 exchange with OH groups increases with increa sing Co/Al ratio, suggesting that Co cations catalyze D-2 dissociative chem isorption steps that limit the rate of isotopic exchange. Co cations also c atalyze hydrogen recombinative desorption steps, which limit the rate of pr opane dehydrogenation and aromatization reactions. The density of residual zeolitic hydroxyls was measured by D-2-OB isotopic exchange and by changes in the intensity of OH infrared bands as a function of Co content. D-2-OH a nd infrared measurements showed that Co2+ cations replace 1.1-1.3 zeolitic protons, suggesting the predominant presence of Co2+- O-Co2+ dimers, with s ome Co2+ monomers, each bridging two next-nearest neighbor Al sites. The lo cation and structure of exchanged Co cations were probed using X-ray absorp tion spectroscopy (XAS) and temperature-programmed reduction (TPR). No Hz c onsumption was detected up to 1273 K during TPR in any of the Co/H-ZSM5 sam ples, consistent with the absence of CoOx crystallites, which reduce at sim ilar to 800 K. In situ near-edge X-ray absorption studies confirmed that Co species remain as divalent cations during exposure to H-2 or C3H8 at 773 K . Near-edge and fine structure analysis detected Co2+ cations with similar structure in all Co/H-ZSM5 samples (Co/Al < 0.22), and Co coordination chan ges from octahedral to tetrahedral upon sample dehydration at 773 K in He. Radial structure functions showed weak contributions from the first and sec ond shells around Co. This reflects the nonunifomm nature of the distance a nd orientation in Al-Al next-nearest neighbor sites in ZSM5.