Mechanism of reaction of CeO2–CaO–Pd/HZSM5 catalyst in the Syngas process in the presence of sulfur-containing impurities

CeO2–CaO–Pd/HZSM5 catalyst, prepared using HZSM5 supports, with CeO2–CaO as the additive components, HZSM5 as the acid component and Pd as the hydrogenation component, have been prepared by the impregnation method, and the catalytic and sulfur-tolerant performance in the Syngas-To-Dimethyl ether (STD) reaction with thiophene and H2S as probe sulfur poisons were examined. The prepared catalyst exhibited higher catalytic performance and better sulfur resistance than a conventional catalyst based on Cu/ZnO/Al2O3 and HZSM5. The sulfur content, Pd sulfidity and acid concentration were determined in order to ascertain the adsorption sites of sulfides on the surface of the catalyst, and the products of the STD reaction from sulfur-containing syngas in an autoclave were analysed by GC-MS at different reaction times in order to determine the sulfur-tolerance mechanism of the catalyst. The study confirms that almost no H2S is absorbed on the surface of CeO2–CaO–Pd/HZSM5 catalyst, while thiophene is absorbed and activated on the acid sites of HZSM5, and hydrogen is adsorbed dissociatively on Pd active sites, followed by active hydrogen spillover to the acid sites to promote hydrogenation and thus convert the adsorbed thiophene to hydrocarbons and hydrogen sulfide.

A two-layer ONIOM study of thiophene cracking catalyzed by proton- and cation-exchanged FAU zeolite

A two-layer ONIOM study on the hydrodesulfurization mechanism of thiophene in H-FAU and M-FAU (M = Li(+), Na(+), and K(+)) has been carried out. The calculated results reveal that in H-FAU, for a unimolecular mechanism, the rate-determining step is hydrogenation of alkoxide intermediate. The assistance of H2O and H2S molecules does not reduce the difficulty of the C-S bond cracking step more effectively. A bimolecular hydrodesulfurization mechanism is more favorable due to the lower activation barriers. The rate-determining step is the formation of 2-methylthiophene, not the C-S bond cracking of thiophene. Moreover, the ring opening of thiophene is much easier to occur than the desulfurization step. A careful analysis of energetics indicates that H2S, propene, and methyl thiophene are the major products for the hydrodesulfurization process of thiophene over H-FAU zeolite, in good agreement with experimental findings. In M-FAU zeolites, both unimolecular and bimolecular cracking processes are difficult to occur because of the high energy barriers. Compared to the case on H-FAU, the metal cations on M-FAU increase the difficulty of occurrence of bimolecular polymerization and subsequent C-S bond cracking steps. Graphical abstract Hydrodesulfurization process of thiophene can take place in H-FAU zeolite. Two different mechanisms, unimolecular and bimolecular ones, have been proposed and evaluated in detail. The bimolecular mechanism is more favorable due to lower activation barrier as described in the picture above. Our calculated data indicate that H2S, propene, and methylthiophene are the major products, in good agreement with experimental observations. The effect of metal cations on the reaction mechanism is also investigated in this work.

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 C(3)H(8) 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 ( approximately 10) 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 approximately 534 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, H(2), and C(3)H(8), residual thiophene-derived species desorb as stable fragments, such as H(2)S, 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. H(2) and C(3)H(8) remove larger amounts of adsorbed species as unreacted thiophene than He, suggesting that dehydrogenation reactions are inhibited or reversed by a hydrogen source. C(3)H(8) removes a larger fraction of thiophene-derived intermediates as hydrocarbons and organosulfur compounds than H(2) or He; thus, hydrogen atoms formed during C(3)H(8) dehydrogenation are more effective in the removal of unsaturated deposits than those formed from H(2). 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.