Study On Direct Synthesis Of Aromatic Hydrocarbons From Syngas Via Oxygen-containing Intermediate Route Over Zr/H-ZSM5 Bifunctional Catalysts

Aromatic hydrocarbons are important basic raw materials in chemical industry,which are widely used in the synthesis of plastics,nylon,spices and other chemicals.At present,aromatics are mainly prepared by petroleum reforming and cracking.The syngas obtained from coal,natural gas and biomass can be converted to aromatics by catalysis,which can effectively alleviate the problem of energy pressure in our country as a new preparation route of non-petroleum aromatics.The route of direct synthesis of aromatics from syngas via oxygen-containing intermediates means that syngas is firstly converted to oxygen-containing intermediates such as methanol,dimethyl ether or ketene on bifunctional catalysts composed of metal oxide catalysts such as Cu,Zn and Zr,and then the intermediates were further converted to olefins by C-C coupling on zeolites,and the olefins were converted to aromatics by oligomerization,cyclization and isomerization at last.Zr-based catalyst has strong CO adsorption capacity and high aromatics selectivity（about 80%）,but its hydrogen dissociation ability is weak,the conversion of CO is low（about 5%）and the by-product CO₂ selectivity is high.In order to solve the above problems,this thesis was aim to improve the defects of easy carbon deposition,sintering and weak H₂ activation of Zr-based catalysts by doping different metal components in Zr-based catalyst system,and modifie molecular sieves to improve the conversion of CO,the selectivity of light aromatics and the stability of catalytic reaction.Finally,the reaction mechanism of direct synthesis of aromatics from syngas is explored.The optimum catalytic reaction conditions for direct synthesis of aromatics from syngas via oxygen-containing intermediates wasinvestigated.The contents and conclusions of the study mainly include the following three aspects:（1）Based on the Gibbs free energy minimization method,the thermodynamic factors and reaction conditions of the main reactions in the route of direct production of aromatics from syngas via oxygen-containing intermediates were simulated and analyzed.The main reactions are exothermic reactions in the range of 100℃-500℃.From the point of view of thermodynamics,it is considered that there are more light aromatic products at lower temperature.The thermodynamic simulation of the reaction process shows that the theoretical value of the yield of BTX component can reach more than 89%under the conditions of reaction temperature 370℃,pressure 3 MPa and H₂/CO=2.（2）Based on the fact that the transition metal Mg can reduce the activity of water-gas shift reaction and improve the distribution of liquid phase products,Mg-doped MgZrO_x catalysts and Zr O₂ catalysts were prepared by the co-precipitation method and combined with zeolite molecular sieve H-ZSM5 to form bifunctional catalysts.The experimental results show that when MgZrO_x-doped metal oxide catalysts are coupled with molecular sieves,the CO conversion is 15.8%.The selectivity of CO₂ decreased from 21%to 17%.The selectivity of C₅₊products increased（63%-67%）.The proportion of BTX components has increased from 21.8%to 27.4%Adjusting the reaction conditions to optimize the performance of the catalyst,the experimental results show that at 400℃,3 MPa,syngas（H₂/CO=2）gas-phase rate 1200 m Lg^-1h^-1,and the mass ratio of metal oxide catalyst to molecular sieve 1:2,the combined catalysts of MgZrO_x catalyst and H-ZSM5-117 molecular sieve has the best performance in the direct synthesis of aromatics from syngas,the conversion of CO is 15.3%and the selectivity of C₅₊products is 66.4%,while the proportion of BTX components is 32.2%.（3）The MgZrO_x/H-ZSM5 catalysts can reduce the selectivity of CO₂ and improve the selectivity of aromatics products,but there are still some problems such as low conversion of CO.By adjusting the catalyst preparation method and Zr/Mg mass ratio,the MgZrO_x/H-ZSM5 catalyst with Zr/Mg mass ratio of 8:2 was prepared by coprecipitation method and applied to the direct synthesis of aromatics from syngas,the CO conversion was 17.2%,the CO₂ selectivity was 17.6%,the selectivity of C₅₊products was 62.9%,and the proportion of BTX components is 33.3%.The transition metal Zn and Fe as promoters were introduced into the modified metal oxide catalyst by the impregnation method.The experimental results show that the excessive transition metal can not effectively improve the performance of the catalyst.The introduction of 3wt%Fe promoter can increase the conversion of CO to 25.6%and the selectivity of C₅₊products to 61.6%and the proportion of BTX components is 31.2%without affecting the product distribution of the catalyst.The molecular sieves modified by metal elements Ce and Mn were introduced by the impregnation method,and the experimental results showed that the introduction of5wt%Mn displayed the best catalytic performance with the conversion of CO was 26.1%,the selectivity of CO₂ products was 17.3%,and the selectivity of C₅₊products was as high as 68.6%.Then 20wt%Si O₂ was introduced by liquid phase deposition method and the catalyst was modified twice to increase the proportion of BTX to 42.6%.Finally,the stability experiment of the modified catalyst was carried out,and it was found that the catalyst could maintain high catalytic activity within 80 hours,and the CO conversion was stable at 26.9%.The selectivity of C₅₊products was 69%,and the proportion of BTX components in liquid phase products was 29.7%after reaction.