In Situ Study On The Mechanism Of Methanol Thiolation Reaction Catalyzed By ZSM-5@t-ZrO₂

The methanol thiolation process is the mainstream production process of synthetic methanethiol at home and abroad.Modified acidic zeolites and metal oxides with both acid and base properties are widely investigated as the catalysts for methanethiol production through the methanol-H₂S process.Based on the acid-base catalytic mechanism,the author’s research group has successfully synthesized the catalyst with excellent catalytic performance and good resistance to carbon and sulfur deposition by hydrothermal method through enriching the acid-base active sites of the catalyst and constructing a special mesoporous-microporous hierarchical structure bifunctional ZSM-5@t-ZrO₂catalyst.In view of the excellent catalytic performance of bifunctional ZSM-5@t-ZrO₂catalyst,the paper proposes a comparison of ZSM-5/t-ZrO₂physical mixed catalysts,ZSM-5 molecular sieves and t-ZrO₂catalysts,revealing the thiolation reaction mechanism catalyzed by bifunctional ZSM-5@t-ZrO₂catalyst.The materialization properties of bifunctional ZSM-5@t-ZrO₂catalyst was revealed by means of XRD,IR,N₂-adsorption/desorption,XPS,XRF,NH₃/CO₂-TPD.The results showed that the hydrothermal coating process has changed part of the framework structure of the composite catalyst and formed a special mesoporous-microporous hierarchical structure with larger specific surface area;The surface of bifunctional ZSM-5@t-ZrO₂catalyst shows obvious zirconium-rich structure,indicating the formation of coating structure.The hydrothermal coating process modified the acid-base distribution of bifunctional ZSM-5@t-ZrO₂catalyst to a certain extent,and made it form a suitable acid-base synergetic catalytic center.In situ diffuse reflectance Fourier transform infrared spectroscopy（Drifts）was used to investigate the adsorption and conversion characteristics of single methanol and single hydrogen sulfide molecules at different reaction temperatures and pressures.The results showed that both methanol and hydrogen sulfide molecules exhibited high reactivity at 370℃and 0.7 MPa reaction pressure.Compared with ZSM-5 zeolite,t-ZrO₂catalyst and ZSM-5/t-ZrO₂catalyst,the vibration peak of methoxy group on ZSM-5@t-ZrO₂bifunctional catalyst was more obvious,which indicated that the synergistic effect between the acid sites of ZSM-5 and those of t-ZrO₂on the bifunctional catalyst enhanced the adsorption and dissociation of methanol molecules.Compared with t-ZrO₂and ZSM-5/t-ZrO₂catalysts,the vibration peaks of sulfhydryl groups on ZSM-5@t-ZrO₂bifunctional catalysts are more obvious,and there is no infrared absorption peak of sulfur species,indicating that the micro-mesoporous structure of bifunctional catalysts can alleviate the formation of sulfur species on the catalyst surface to a certain extent.Furthermore,it shows excellent catalytic performance.By means of in situ diffuse reflectance Fourier transform infrared spectroscopy（Drifts）and differential scanning calorimetry（DSC）,the effects of pre-adsorption mode,type of catalyst,pre-sulfurization time,molar ratio of hydrogen sulfide to methanol,reaction temperature and pressure on the mechanism of thiolation reaction were investigated.The results indicate that the adsorption mode of H₂S followed by methanol is conducive to the formation of sulfhydryl groups in the reaction process.The dissociation of hydrogen sulfide molecules into sulfhydryl groups at the base site of the catalyst is the rate-determining step of the thiolation reaction.Compared with the t-ZrO₂and ZSM-5/t-ZrO₂physical mixed catalysts,there was no infrared absorption peaks of species for carbon deposion and sulfur deposion on the surface of bifunctional ZSM-5@t-ZrO₂catalyst,and the results of DSC characterization showed smaller heat release,ZSM-5@t-ZrO₂catalyst showed good resistance to carbon and sulfur deposition,which is closely related to its larger specific surface area and mesoporous-microporous hierarchical structure.Under the conditions of pre-sulfurization for 1 h,reaction pressure of 0.7 MPa,molar ratio of hydrogen sulfide to methanol of 2:1 and reaction temperature of 370℃,the matched formation rate of methoxy group and sulfhydryl group on the bifunctional catalyst was constructed to reduce the formation of species for carbon deposion and sulfur deposion on the surface of the catalyst,thus achieving the best catalytic performance.Methanol molecule is adsorbed and dissociated into methoxy group at the acid site of the bifunctional catalyst,and hydrogen sulfide molecule is adsorbed and dissociated into sulfhydryl group at the base site of the bifunctional catalyst.The sulfhydryl group with higher electronegative ability attacks the methoxy group and occurs the nucleophilic substitution reaction of SN₂,and finally generates the target product methanethiol.The two-phase composite design enriches the active sites of the catalyst,and the special coating structure improves the affinity of the two phases,thus enhancing the bifunctional synergistic effect between acid sites and base sites.