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.