Adsorption, desorption, and conversion of thiophene on H-ZSM5

The dynamics and stoichiometry of thiophene adsorption and of rearrangements of thiophene-derived adsorbed species in O-2, He, H-2, and C3H8 carriers were measured using chromatographic methods and mass spectrometry on H-ZSM5 and H-Y zeolites. Thiophene adsorption obeyed Langmuir isotherms on both zeolites. Adsorption uptakes were 1.7 and 2.8 thiophene/Al at 363 K on H-ZSM5 and H-Y zeolites, respectively, after removal of physisorbed thiophene. These stoichiometries differed for these two zeolite structures but did not depend on their Al content (Si/Al = 13-85). Adsorption from a thiophene-toluene mixture showed thiophene selectivities (similar to10) greater than expected from van der Waals interactions. These adsorption stoichiometries, without contributions from physisorption, and the color changes detected indicate that thiophene adsorption occurs concurrently with oligomerization on acidic OH groups and that oligomer size depends on spatial constraints within channels. Thiophene oligomers decompose at similar to534 K during subsequent thermal treatment to form molecular thiophene with all carriers, leaving behind unsaturated thiophene-derived species with a 0.9-1.1 thiophene/Al stoichiometry, confirming the specificity of OH groups and the oligomeric nature of bound thiophene during adsorption at 363 K. With He, H2, and C3H8, residual thiophene-derived species desorb as stable fragments, such as H2S, ethene, propene, arenes, and heavier organosulfur compounds (methylthiophene and benzothiophene) during thermal treatment; they also form unsaturated organic deposits that cannot desorb without hydrogenation events. H2 and C3H8 remove larger amounts of adsorbed species as unreacted thiophene than He, suggesting that dehydrogenation reactions are inhibited or reversed by a hydrogen source. C3H8 removes a larger fraction of thiophene-derived intermediates as hydrocarbons and organosulfur compounds than H2 or He; thus, hydrogen atoms formed during C3H8 dehydrogenation are more effective in the removal of unsaturated deposits than those formed from H2. Thiophene-derived adsorbed species are completely removed only with O-2-containing streams at 873 K, a process that fully recovers initial adsorption capacities. This study provides a rigorous assessment of the nature and specificity of thiophene adsorption processes on acidic OH groups and of the identity and removal pathways of adsorbed species in various reactive environments.