Study On The Synthesis Of Methanethiol From H₂S-rich Syngas Over Mo-based Catalysts

Methanethiol (CH₃SH) is an important chemical raw material and common organic intermediate. Industrially, methanethiol is synthesized in the gas phase from methanol and hydrogen sulfide over tungsten-based catalysts. Recently, there is an increasing interest in the route of one-step synthesis of methanethiol from H₂S-rich syngas (CO/H₂/H₂S). Compared to the CH₃OH-H₂S route for production of CH₃SH, this method is attractive and promising since it skips the step of the synthesis of CH₃OH from syngas, furthermore, the feedstock composition is simple and easily available.This dissertation focuses on the preparations of Mo-based catalysts, the analysis of structure-activity relationships and the studies of the pathway of the CH₃SH synthesis from H₂S-rich syngas. Mo-based catalysts were prepared by impregnation method and the promoters, supports and preparation conditions were investigated in detail. Thus optimized catalysts were tested in the scale-up experiments. To analyze the structure-activity relationship, we performed BET, XRD, LRS, XPS, TG, TPR, TPD and ESR characterizations for the selected catalysts. Moreover, to investigate the reaction network and the mechanism of the methanethiol formation from H₂S-rich syngas, we performed detailed analysis of the product distribution and characterizations of the sulfided catalysts. The results of the dissertation were concluded as follows:(1) It is essential to deposit a basic additive on the Mo-based catalysts for the synthesis of CH₃SH. The potassium-doped Mo-based catalysts exhibit the highest activity than that of the catalysts doped with the otherâ… A andâ…¡A group basic promoters. H₂-TPR and LRS characterizations suggest that the addition of a base leads to the transformation of octahedrally coordinated Mo to tetrahedrally coordinated Mo, thus leads to the split of low temperature reduction peaks. The interaction between the basic components (B) and Mo lead to the formation of B-Mo interface phase, which are closely correlated with the formation of CH₃SH.(2) It has been found that Fe, Co, Ni and Te have evident promoting effects for the K-Mo/SiO₂ catalysts. Interestingly, nonmetallic tellurium was found to have an effective promoting effect on the K₂MoO₄/SiO₂ catalyst for the CH₃SH synthesis from H₂S-rich syngas and the promoting effects of the promoters were in the order of Co (Ni) > Te > Fe. The K-Mo-Co/SiO₂ catalysts prepared by the co-impregnation method exhibited highest activity when the Co/K₂MoO₄ molar ratio was 0.33-0.35. However, the K-Mo-Te/SiO₂ catalysts prepared in the order that Te impregnated first and followed by K₂MoO₄ exhibited highest activity when the Te/K₂MoO₄ molar ratio was 0.5. The studies of structure-activity relationship for the Co- and Te-promoted K-Mo catalysts indicate that the promoting effect of Te can be interpreted in terms of "electronic effect promoter", while that of Co is explained as that Co combines with Mo-S species to form the so-called "Co-Mo-S" phase which favors the hydrogenation reactions in the formation of CH₃SH.(3) For the supported tri-component catalysts for CH₃SH synthesis, the preparation conditions are as follows: SiO₂ was chosen as the support, and K : Mo : Co molar ratio is 2 : 1 : 0.35, and MoO₃ loading is 25%(wt%), and citric acid is added as a chelating agent. The K₂MoCo_0.35O/SiO₂(CA) catalyst prepared in those cases without calcinations exhibits the highest activity. The studies of the effects of the preparation conditions on activity were conducted. The results show that Mo species in the K-Mo-Co catalyst prepared with weak acidic supports exist in the form of tetrahedrally configuration, which is hard to be reduced and sulfided, and this effect results in K₂O_xMoS_4-x becoming dominant species after sulfidation, which are closely related with the formation of methanethiol. The addition of citric acid is favorable to improve the dispersion of active component and to form the "Co-Mo-S" species. The catalysts calcined at 400℃in inert atmosphere exhibit similar activity as the catalysts without calcinations. The decomposition of citric acid takes place in inert atmosphere above 212℃, which gives rise to partially reduction of Mo and Co species. While the decomposition of citric acid takes place in air above 400℃, which leads to the enhancement of the interaction between the active components and supports, and affects the dispersion of the Mo and Co species, thus results in the loss of the active components, leading to the decrease of the activity.(4) At CO/H₂/H₂S=1/1/2 (v/v), 0.2 MPa, 3000 h^-1 and 300℃, mainly CH₃SH, COS, CO₂ and H₂O were formed, along with small amounts of CS₂, C_1-2 hydrocarbons (CH₄, C₂H₄ and C₂H₆) and thioethers (CH₃SCH₃, CH₃SSCH₃ and CH₃SSSCH₃) over potassium-promoted Mo-based catalysts. We firstly report that CH₃SSCH₃ and CH₃SSSCH₃ are produced from the H₂S-rich syngas under these reaction conditions. Studies of the reaction pathway show that COS is a primary product, which is then hydrogenated to CH₃SH and H₂O. The disproportionation of COS leads to the formation of CS₂. Most of CO₂ originates from water-gas shift reaction. The hydrocarbons and thioethers originate from the secondary reactions of CH₃SH. This dissertation gives a more clear illustration for the pathway of the CH₃SH synthesis from H₂S-rich syngas(5) The results of mechanism studies show that the base-doped Mo-based catalysts are the bifunctional catalysts. A possible mechanism for the CH₃SH synthesis over the K-Mo-(Co)-S and/or K-Mo-(Co)-S-O active phases was proposed as follows: H₂, CO and H₂S are adsorbed firstly, wherein H₂S is thought to be supply enough S^2- and/or SH^- groups, the function of potassium thus be suggested to furnish K-S and/or K-SH bonds, into which the non-dissociative adsorption of CO can insert. The formed carbonyl sulfide (COS_ads) and/or thioformate (HOCS_ads) can then be hydrogenated to methylthiolate (CH₃S_ads) by the spillover of active hydrogen atoms on the sulfided Mo or Co-Mo components (Mo⁴⁺-S^2-, Co-Mo-S, S₂^2-, or S_x species), or they are hydrogenated after migration from the potassium component, the subsequent hydrogenation of methylthiolate (CH₃S_ads) produces CH₃SH.(6) The most optimized K-Mo-Co catalyst shows a good repeatability and stability. The results of the scale-up experiments show that the catalyst has the prospects in industrial applications.(7) This work is sponsored by Evonik Degussa GmbH (Germany). The related achievements have been granted by some European, Korean and Chinese patents and published in several papers. Both parties are satisfied with the progress of the cooperation.