Effects of zeolite structure and aluminum content on thiophene adsorption, desorption, and surface reactions

The adsorption and desorption of thiophene and the reactions of thiophene-derived adsorbed species in He, H₂, and O₂ were examined on H-ZSM5, H-Beta, and H-Y with varying Si/Al ratios. Thiophene adsorption uptakes (per Al) were independent of Al content, but were above unity and influenced by zeolite structure (1.7, 2.2, and 2.9 on H-ZSM5, H-Beta, and H-Y). These data indicate that thiophene oligomers form during adsorption and that their size depends on spatial constraints within zeolite channels. Adsorption and oligomerization occur on Brønsted acid sites at 363 K. Thiophene/toluene adsorption from their mixtures show significant thiophene selectivity ratios (10.3, 7.9, and 6.4, for H-ZSM5, H-Beta, and H-Y zeolites), which exceed those expected from van der Waals interactions and reflect specific interactions with Brønsted acid sites and formation of toluene–thiophene reaction products. Treatment of thiophene-derived adsorbed species above 363 K in He or H₂ led to depolymerization of thiophene oligomers and to the formation of unsaturated adsorbed species with a 1:1 thiophene/Al stoichiometry on all zeolites and at all Si/Al ratios. These unsaturated species desorb as stable molecules, such as H₂S, hydrocarbons, and larger organosulfur compounds, formed via ring opening and hydrogen transfer from H₂ or co-adsorbed species, and also form stranded unsaturated organic deposits. Smaller channels and higher Al contents preferentially formed H₂S, benzotiophenes, and arene products during treatment in He or H₂, as a result of diffusion-enhanced of secondary reactions of desorbed thiophene molecules with adsorbed thiophene-derived species. Only oxidative regeneration treatments led to full recovery of thiophene uptake capacities. A preceding treatment in H₂, however, led to the partial recovery of thiophene-derived carbon atoms as useful hydrocarbons and decreased the amount of CO₂ and SO₂ formed during subsequent oxidative treatments required for regeneration.