Kinetic, infrared, and X-ray absorption studies of adsorption, desorption, and reactions of thiophene on H-ZSM5 and Co/H-ZSM5

Abstract

Temperature programmed desorption and infrared and X-ray absorption near-edge spectroscopies were used during adsorption and reactions of thiophene in order to probe adsorbed intermediates and catalytic structures responsible for thiophene reactions with propane or H2 on H-ZSM5 and Co/H-ZSM5. Infrared spectra showed that thiophene interacts with acidic OH groups in H-ZSM5 via hydrogen bonding at ambient temperature. No additional bands were detected on Co/H-ZSM5, suggesting the absence of specific interactions with Co cations. During adsorption at ambient temperatures, infrared bands assigned to CH2 groups near CC bonds or S-atoms and to S–H species were detected and H-ZSM5 and Co/H-ZSM5 acquired colors typical of thiophene oligomers. Slightly above ambient temperatures, benzene and H2S formed from pre-adsorbed thiophene. These results indicate that hydrogen-bonded thiophene undergoes ring opening or oligomerization near ambient temperature on acidic OH groups in H-ZSM5. Some of the adsorbed thiophene (20–50%) interacts weakly with channel walls or with residual Na cations and desorbs unreacted. The remaining adsorbed thiophene desorbs as H2S, aromatic hydrocarbons, and organosulfur compounds, such as methylthiophene and benzothiophene, or forms irreversibly adsorbed unsaturated organic deposits. In situ infrared studies during thiophene and thiophene–propane reactions at 773 K on H-ZSM5 and Co/H-ZSM5 showed that surface coverages of thiophene-derived intermediates were low on acidic OH groups and Co cations. Co K-edge X-ray absorption near-edge spectra measured during these reactions confirmed that Co2+ cations do not reduce or sulfide; their local environment, however, changes slightly, apparently because of interactions of strongly adsorbed species with Co cations. Sulfur K-edge X-ray absorption spectra detected small amounts of organosulfur species, but no inorganic sulfides, after thiophene, thiophene–H2, and thiophene–propane reactions, consistent with the observed stability of exchanged cations against reduction and sulfidation. S∶Al ratios were less than 0.04 at. on all samples; these amounts represent less than 1% of the S-atoms removed from thiophene as H2S during catalytic propane–thiophene reactions.