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

摘要

Temperature programmed desorption and infrared and X-ray absorption near-edge spectroscopies were usedduring adsorption and reactions of thiophene in order to probe adsorbed intermediates and catalytic structuresresponsible for thiophene reactions with propane or H2 on H-ZSM5 and Co/H-ZSM5.Infrared spectra showedthat thiophene interacts with acidic OH groups in H-ZSM5 via hydrogen bonding at ambient temperature.Noadditional 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 C=C bonds or Satomsand to S–H species were detected and H-ZSM5 and Co/H-ZSM5 acquired colors typical of thiopheneoligomers.Slightly above ambient temperatures, benzene and H2S formed from pre-adsorbed thiophene.Theseresults indicate that hydrogen-bonded thiophene undergoes ring opening or oligomerization near ambienttemperature on acidic OH groups in H-ZSM5.Some of the adsorbed thiophene (20–50%) interacts weakly withchannel walls or with residual Na cations and desorbs unreacted.The remaining adsorbed thiophene desorbs asH2S, aromatic hydrocarbons, and organosulfur compounds, such as methylthiophene and benzothiophene, orforms irreversibly adsorbed unsaturated organic deposits. In situ infrared studies during thiophene andthiophene–propane reactions at 773 K on H-ZSM5 and Co/H-ZSM5 showed that surface coverages ofthiophene-derived intermediates were low on acidic OH groups and Co cations.Co K-edge X-ray absorptionnear-edge spectra measured during these reactions confirmed that Co2+ cations do not reduce or sulfide; theirlocal environment, however, changes slightly, apparently because of interactions of strongly adsorbed specieswith Co cations.Sulfur K-edge X-ray absorption spectra detected small amounts of organosulfur species, butno inorganic sulfides, after thiophene, thiophene–H2 , and thiophene–propane reactions, consistent with theobserved stability of exchanged cations against reduction and sulfidation.S : Al ratios were less than 0.04 at. onall samples; these amounts represent less than 1% of the S-atoms removed from thiophene as H2S duringcatalytic propane–thiophene reactions.