Decomposition Mechanism Of Mixed Mercaptan By ZSM-5 Zeolite Modified With Rare Earth (Ce/Pr)

Volatile sulfur-containing organic compounds（S-VOCs）,such as methyl mercaptan（CH₃SH）and ethyl mercaptan（C₂H₅SH）,have extremely low olfactory thresholds and are prone to emit malodorous odors,which can generate sulfur oxides after oxidation and promote the generation of acid rain and acid fog,and can also react with free radicals to cause photochemical smog,which is currently the primary target of air pollution prevention and control.The current methods of thiol removal include adsorption,absorption,catalytic oxidation and catalytic decomposition,among which catalytic decomposition of CH₃SH mainly generates CH₄ and H₂S and other products that can be used resourcefully,which is currently a safer and more environmentally friendly method,and therefore has attracted the attention of scholars.However,in the past decades,scholars have mainly focused on the catalysts for high performance decomposition of CH₃SH,but less attention has been paid to the coexistence of C₂H₅SH,especially under the conditions of coexistence of the two,how the removal efficiency and reaction behavior change,and whether the reaction mechanism is the same for both low carbon mercaptans.After the mechanism of the reaction is clearly studied,high performance catalysts can be designed according to the active site of the reactant decomposition,which can break the bottleneck of CH₃SH decomposition that cannot balance high activity and stability at low temperature.In this paper,the RE/ZSM-5 catalysts were synthesized by modifying ZSM-5molecular sieve with Ce and Pr,two rare earth elements with similar properties,and used to decompose methyl ethyl mixed mercaptan,and the changes of catalytic reaction behavior after the mixture of two gases and the reaction mechanism of CH₃SH and C₂H₅SH decomposition were investigated by combining various characterization,kinetics and density flooding theory calculations,and the CH₃SH decomposition mechanism was synthesized.The main conclusions are summarized as follows.（1）Ce and Pr modified ZSM-5 molecular sieves were used for the decomposition of mixed methanethiols,and the results showed that the removal efficiency of mixed thiols was improved after the modification,and the activity of Ce/ZSM-5 was stronger than that of Pr/ZSM-5;the catalyst characterization,the carbon accumulation of sulfur on the surface of both catalysts after the reaction and density flooding theory（DFT）calculations revealed that the introduction of Ce and Pr into ZSM-5 molecular sieves lead to different active sites of the catalysts,and the Ce modified molecular sieve becomes a two-site catalyst with coupled acid and oxygen sites,while the Pr modified molecular sieve mainly exhibits strong acidity.（2）CH₃SH and C₂H₅SH exhibit opposite stability on Ce/ZSM-5 and Pr/ZSM-5catalysts with different active sites,so it is speculated that the reaction mechanism is different between them.By comparing the CO₂ content in the products of the decomposition of the two gases and the different consumption of reactive oxygen on the catalysts,as well as the activity of a series of designed catalysts with different redox properties for the decomposition of CH₃SH and C₂H₅SH,we found that the reactive oxygen is mainly involved in the decomposition reaction of CH3SH,while it has less effect on C₂H₅SH;the changes of gas-phase products were analyzed by RSH-TPD,in situ DRIFTS analysis of gas-solid interface product changes,and combined with DFT calculations to simulate the reaction path of CH₃SH and C₂H₅SH decomposition,the results show that CH₃SH decomposition in two steps,the first generation of CH₃SCH₃ and H₂S at low temperature,high temperature by the oxygen site attack CH₃SCH₃ to generate CO₂,CH₄ and H₂S requires more energy than the acid site break CH₃SCH₃ requires less energy.In contrast,the decomposition of C₂H₅SH requires only one reaction step,and the energy required for direct cleavage of C₂H₅SH by the acid site to generate C₂H₄ and H₂S is much lower than that of the oxygen site;by comparing the activity and kinetic changes of CH₃SH and C₂H₅SH before and after mixing,it was found that C₂H₅SH is easier to decompose than CH₃SH,and there is mutual inhibition after mixing the two,and using RSH-TPD and in The results showed that the Ce/ZSM-5 catalyst prepared in this thesis could adapt to a variety of reaction conditions and maintain the reaction at high air velocity and low concentration for 160 h without loss.The results showed that the Ce/ZSM-5catalyst prepared in this thesis can adapt to a variety of reaction conditions and maintain a continuous reaction at high air velocity and low concentration for 160 h without signs of deactivation.