Preparation And Characterization Of Molybdenum Carbide-based Catalysts And Their Catalytic Performance For Higher Alcohols Synthesis From Syngas

Under the increasing pressure of energy security and environmental protection,the vigorous development of coal liquefaction technology is not only a strategic choice to promote the clean and efficient utilization and transformation of coal,but also a strategic choice to enhance national energy security.Currently,coal liquefaction technology is moving towards high-efficient and high-end.And highly efficient catalytic conversion of syngas into higher mixed alcohols is one of the important ways to cleanly use coal resources.Higher mixed alcohols can be used as an oil additive with high octane number,explosion-proof,anti-vibration and low-pollution,which can be used to blend and produce high-efficiency and environmentally friendly alcohol-oil hybrid fuel;it can also be further separated and refined to obtain high value-added products.With its large market capacity and broad prospects,it is expected to become an alternative energy source to reduce its dependence on petroleum.The design and development of high-efficiency catalysts are the key to achieving syngas conversion reactions.In this paper,the molybdenum carbide-based catalysts with Pt-like structure were selected as the targets,the low temperature thermal plasma technology with ultra-high reaction temperature,high reaction energy and rapid quenching was used to prepare the MoC-based catalysts and their catalytic performance were studied.The preparation conditions and catalytic performance differences between Mo₂C and MoC catalysts were explored.A series of heterojunction MoC-based composite catalysts were constructed and their catalytic properties were studied.The research results provide references for the development high-efficiency catalysts for higher alcohols synthesis from syngas.The main findings were summarized as follows:（1）The pure phase MoC was prepared by thermal plasma technology for the first time,which provided a new method for the synthesis of nano-MoC.The MoC catalyst（Pla-Ht-MoC）prepared by thermal plasma had high dispersion,larger BET surface area and good thermal stability.The catalyst surface was rich in disordered and multi-defective graphite carbon.At 300℃,the Pla-Ht-MoC catalyst showed excellent catalytic performance for syngas to higher alcohols,with CO conversion,total alcohol selectivity,C₂₊OH selectivity and STY_ROH reaching 21.5%,51.7%,69.7%,and 109.3mg/g_cat/h,respectively.The Pla-Ht-MoC catalyst prepared by thermal plasma technology combined with the precursor method is significantly better than the CH₄-MoC catalyst prepared by the common method.The evaluation of the stability of the catalysts at 300 h were found that the Pla-Ht-MoC catalyst promoted the conversion of the dissociative adsorption sites of CO into the active sites of CO insertion under the action of high-energy free radicals and alkali promoter,thus showing that the CO conversion was stable and the alcohol products selectivity increased.In order to further improve the catalytic performance,Co-MoC bimetallic catalyst with uniform particle size,high dispersion and large specific surface area was prepared by in-situ addition of Co promoter by thermal plasma technology.The Co（C）MoC catalyst with Co（CH₃COO）₂ as cobalt source,at 300℃,CO conversion,total alcohol selectivity,C₂₊OH selectivity and STY_ROH reached 33.0%,55.6%,81.4%and 86.5 mg/g_cat/h,respectively.Compared with the Pla-Ht-MoC catalyst without Co promoter,the catalytic performance of the Co（C）MoC catalyst added with Co promoter was significantly improved.（2）Mo₂C catalysts were controllable synthesized on the basis of MoC catalysts,the differences in catalytic performance for syngas to higher alcohols between Mo₂C and MoC catalysts was studied.The Mo₂C catalysts were rich in CO non-dissociative adsorption sites and show high selectivity for alcohol products,while the MoC catalysts prepared by the same method had larger BET surface area,and the surface were rich in Mo²⁺species,which were conducive to the CO dissociative adsorption to form long-chain C₂₊OH products,and the low electron density Mo⁴⁺species promoted CO non-dissociative adsorption and further improved the selectivity of C₂₊OH.（3）A series of MoC-based heterojunction composite catalysts were prepared,the differences in catalytic performance of between different heterojunction composite catalysts and between heterojunction catalysts and single component catalysts were studied.Heterojunction composite catalysts were generally superior to single-component catalysts in the selectivity of alcohol products.The synergistic effect of heterojunction composite catalysts between different phases enhanced the catalytic performance.Among them,MoS₂/MoC catalyst had the highest selectivity for alcohols,and the Mo species with lower binding energy on its surface greatly promoted the activity and selectivity of MoS₂/MoC catalyst for synthesising higher alcohols,thus showed excellent catalytic performance.The MoS₂/MoC catalyst total alcohol selectivity at 300℃was as high as 64.6%and the CO conversion was 12.6%.At 330℃,the CO conversion reached 28.9%,the selectivity of total alcohol reached 55.8%,the selectivity of C₂₊OH was as high as 75.9%,and the STY_ROH reached 192.2 mg/g_cat/h.The heterophase interfaces of heterojunction composite catalysts were more sensitive to temperature.Therefore,the development of high-performance heterojunction composite structure catalysts requires clarifying the specific relationship between reaction temperature and catalytic performance,then determine the best reaction temperature.In addition,the sheet structure MoC catalyst（S-MoC）made from MoS₂showed more excellent catalytic performance for syngas to higher alcohols,the CO conversion,total alcohol selectivity,C₂₊OH selectivity and STY_ROH of the S-MoC catalyst at 300℃were 19.7%,65.9%,67.4%and 149.0 mg/g_cat/h,respectively.