Novel Mo-based Catalysts Prepared By Thermal Plasma And Their Catalytic Performance For Higher Alcohols Synthesis From Syngas

Modern society is facing with the petrochemical energy depletion and the increasing environmental pollution caused by the burning of petrochemicals.So,looking for an efficient and clean alternative energy source become very necessary.Higher alcohols could be used as an excellent gasoline substitute because of its high calorific value,high octane number and low pollution.Also,higher alcohols are a chemical intermediate in many industries,and its potential economic value is extremely huge.The synthesis of higher alcohols from syngas is a significant route in terms of economy and safeguarding the energy security of our country.Catalyst is the key issue of this route,and how to improve the selectivity of target products while ensuring the conversion efficiency of catalysts has always been the goal pursued by researchers in this field.Molybdenum-based catalysts exhibit excellent catalytic properties due to the special properties of their noble metalloid.However,the molybdenum-based catalysts prepared by the conventional methods still have various problems,and the conversion efficiency,selectivity and stability of the catalysts have not yet reached the standards of industrial application.Catalysts with different preparation methods have great differences in morphology and structure.It is necessary to find a new method to prepare catalysts with high activity and high stability.As a new technology in recent years,low-temperature thermal plasma has ultra-high reaction temperature,high reaction energy and rapid quenching process.These special properties enable it to rapidly prepare ultra-fine nano-particles catalysts with high stability and defects.In this dissertation,by focuing on the surface structure control of catalysts by plasma method,a series of molybdenum-based catalysts was prepared by this new method.The influence of plasma method on the morphology of the catalysts was studied.The catalytic performance of the catalysts for the synthesis of higher alcohols were investigated.The reaction conditions were further optimized,and the catalytic reaction pathway and the structure-activity relationship of the catalysts were also explored.The three main findings were summarized as follows:?1?Nano-sized MoO₂ spherical particles were prepared by low temperature thermal plasma.The particle size was around 20 nm and the dispersion were excellent.MoO₂ particles were a special core-shell structure with amorphous species on the surface and crystalline phase inside.This special structure reduced the electron density on the surface of the catalyst,which was beneficial to the adsorption and activation of CO molecules.The catalytic activity with time on stream of the MoO₂-Pla catalyst was carried out for 300 h,and the activity was not reduced at all.This was because the armor-type catalyst was formed in the ultra-high temperature environment of the thermal plasma,and the surface of the catalyst was physically passivated,and the anti-sintering and anti-carbon deposition abilities were obviously improved.The optimized regulation of Co promoter was carried out on the catalyst.The results showed that the specific surface area and active sites of the catalysts increased after the addition of cobalt promoter.When Co/Mo ratio is 0.5,there was a significant interaction between Mo and Co species.The cobalt promoter regulated the ratio of the dissociative adsorption site of CO to the non-dissociation adsorption site,which enhanced the growth of carbon chain and improved the catalytic activity of the catalyst greatly.?2?The bi-active phase MoO₂-MoC catalyst was prepared by thermal plasma method.The particle size of MoO₂ was about 20 nm and the particle size of MoC was about 5 nm.The catalyst particle size was small,and the particle distribution was extremely uniform.At the same time,it was found that the low-valent Mo species on the surface of the catalyst was in an electron-deficient state,and the MoC species had stronger adsorption and activation ability for CO.The MoO₂-MoC-Pla catalyst prepared by plasma method could expose more active sites.Moreover,the catalytic performance for higher alcohols synthesis from syngas was greatly improved by regulating the ratio of different kinds of active sites.The ratio of the raw material ammonium molybdate to hexamethylenetetramine was optimized to improve the performance,and it was found that the content of MoC phase in the catalyst gradually increased with the increase of the content of hexamethylenetetramine.However,the MoO₂-MoC-Pla-1/20 catalyst generated a large amount of carbon films,which was formed by the decomposition of excessive amounts of hexamethylenetetramine.The characterization of BET and CO-TPD revealed that these carbon films were not conducive to the exposure of the active sites of the catalyst.The MoO₂-MoC-Pla-1/15catalyst had more active species and adsorption types,which made the performance for higher alcohols synthesis from syngas to be the best.The CO conversion,alcohols selectivity,C₂₊OH selectivity and space-time yield of MoO₂-MoC-Pla-1/15 catalyst at300°C reached 21.7%,62.2%,74.4%and 132.7 mg·g^-1·h^-1,respectively.?3?MoS_x catalyst was prepared by thermal plasma method using ammonium tetrathiomolybdate as raw material.The particle size of the catalyst particles was greatly reduced compared to the conventional method and the molybdenum sulfide stack had only 5-10 layers.The morphology with low crystallinity,high dispersion and high disorderness could greatly increase the exposure amount of active surface of MoS_x species.The surface of the catalyst was a special electronic structure in the state of electron deficient,which could increase the adsorption and activation ability of the catalyst for CO.Compared with that of MoS₂-TPR,the CO conversion of MoS_x-Pla and MoS_x-S-Pla catalysts did not change significantly,which could be due to the formation of metal molybdenum under the severe reaction conditions of plasma.The alcohols selectivity and C₂₊OH selectivity of MoS_x-Pla and MoS_x-S-Pla were significantly improved,indicating that the plasma method could optimize the surface species of the catalysts and improve the selectivity of higher alcohols.The addition of sulfur powder to the raw materials had no significant effect on the catalyst phase and catalytic performance.The performance of MoS_x-Pla catalyst reached the best at300°C,and the CO conversion,alcohols selectivity,C₂₊OH selectivity and the space-time yield were 23.7%,60.7%,71.3%and 194.7 mg·g^-1·h^-1,respectively.