Study On Direct Synthesis Of Aromatic Hydrocarbons From Syngas Via Fischer-Tropsch Route Over Fe-HZSM-5 Bifunctional Catalysts

Aromatics,especially benzene,toluene and xylene,are often used as chemical raw materials for the synthesis of fine chemicals,organic solvents and other products,which are currently mainly derived from catalytic reforming and thermal cracking of fossil resources.In recent years,aromatics can be directly produced from synthesis gas by coupling olefin aromatization catalysts with Fischer-Tropsch catalysts.The direct synthesis gas to aromatics route（STA）has the advantages of high raw material conversion,short process and easy product separation.In this thesis,based on F-T Fe based catalysts and olefin aromatization HZSM-5 molecular sieves,30Fe K&Hie Mo ZSM-5（25）and30Fe K&Hie Mo ZSM-5（25）were preparedby doping the F-T component with metal additives and modifying the pore structure of HZSM-5 molecular sieves with acid modulation and pore structure,aiming to solve the problems of high selectivity of hydrocarbon by-products in the gas phase and low percentage of aromatics in the liquid phase products in the STA reaction.Furthermore,the effects of the reaction conditions were also investigated.The research content and conclusions are divided into the following parts:（1）To explore the reaction performance of synthesis gas to olefin based on Fischer-Tropsch Fe based catalysts,the effects of different additives on the CO conversion and selectivity of olefin products were discussed by adding K,Na,Mn,Mg and Ni.It was found that the addition of K additives decreased the CO₂ selectivity by 5.24%,increased the selectivity of olefin to hydrocarbon products by 19.73%and the selectivity of C₅₊products to 51.9%.The addition of K significantly reduced the CO₂ selectivity and increased the selectivity of olefins and C₅₊products in the hydrocarbon group.The performance of the bifunctional catalyst to aromatics reaction was investigated by coupling commercial HZSM-5 molecular sieves with modified Fe-based catalysts（Fe K,Fe Na,Fe Mn,Fe Mg,Fe Ni）.Olefin was not detected after coupling the molecular sieves,and the percentage of aromatics in the liquid phase product increased to 10-42%;the selectivity of aromatic products in the liquid phase product increased to 95.17%after the addition of K additives.The results indicated that olefins were consumed as aromatic intermediates in the active site of the HZSM-5 catalyst.（2）Based on the catalytic system of Fe K&HZSM-5,the effects of different coupling methods and the percentage of F-T components of the bifunctional catalyst on the reaction performance of synthesis gas to aromatics were investigated.Four types of bifunctional catalysts with different contact distances were prepared by using impregnation,sticky coating,pellet mixing and layered mixing.The highest CO conversion of 96.41%was found for the pellet blend,which was 2.13%and 2.97%higher than that of the impregnation and sticky-coat methods,respectively,and 3.21%higher than that of the layered blend,while the highest C₅₊selectivity of 47.14%was found for the pellet blend.The effect of the bifunctional catalyst on the STA reaction was investigated by varying the proportion of each component of the bifunctional catalyst,and bifunctional catalysts were designed with 15%,30%and 50%of the Fischer-Tropsch component,respectively.It was found that the CO conversion and CO₂ selectivity remained basically unchanged at different ratios,whilethe product moved towards the formation of C₅₊oil with the increase of the Proportion of Fischer Tropsch components,and the proportion of aromatics in the liquid phase product gradually decreased.The highest aromatic yield was achieved with 97.12%of the liquid phase product at 30%of the Fischer-Tropsch additive.The olefin selectivity was 1.25%at 50%of the F-T additive,which was attributed to the reduced acidity of the molecular sieve in the catalyst,resulting in incomplete conversion to the aromatic product.（3）The effect of acidity and pore structure on product distribution was investigated based on the 30Fe K&Hie HZSM-5 catalytic system.By preparing Fe K&Hie HZSM-5with different silica-aluminium ratios（Si O₂/Al₂O₃=25,50,85）,the CO conversion and CO₂ selectivity remained basically unchanged after 12 h of reaction.It was found that as the silica-aluminium ratio increased,the C₅₊content of hydrocarbon productivity increased and the content of aromatic in the liquid phase product decreased,from 96.71%to 64.55%,indicating that a lower amount of acid could still promote the aromatization reaction,but the lower acidity is more favorable for the formation of iso-alkanes.Co ZSM-5,Zn ZSM-5 and Mo ZSM-5 were prepared by the impregnation method,and the characterization of catalysts revealed that the acidity of the molecular sieves decreased after the metal loading,and the addition of Zn metal helped to increase the selectivity of C₅₊in the hydrocarbon products（from 57.91%to 61.02%）and the proportion of BTX components in the liquid phase products（from 22%to 31%）,and the Mo metal modification could increase the proportion of aromatic hydrocarbons in the liquid phase products,with the percentage of non-aromatic components was less than 1%.（4）In order to improve the content of molecular sieve intermediate pore,the graded molecular sieve Hie HZSM-5 was prepared by alkali treatment method,and the hollow molecular sieve Hol HZSM-5 was prepared by hydrothermal treatment method to prepare.After the Fischer-Tropsch catalyst coupled with the molecular sieve,the gas phase product component content is basically the same,indicating that the gas phase component selectivity is independent of molecular sieve pore structure.The liquid phase product aromatic selectivity increased from 8%to12%after modification.It is attributed to the fact that the graded pores are more conducive to the removal of light aromatics from the pores of the molecular sieve and avoid excessive alkylation reactions.Both Fe K&Hie Mo ZSM-5 and Fe K&Hie Zn ZSM-5 catalysts were prepared by double modification of HZSM-5 with the preparation of molecular pore channels and the loading of metals to modulate the surface acidity of the molecular sieves.The results showed that the selectivity of C₅₊to hydrocarbon product was increased with the highest selectivity of69.45%for the Fe K&Hie Zn ZSM-5 catalytic system.The highest percentage of monocyclic aromatic hydrocarbons and BTX fractions in the liquid phase products by Fe K&Hie Mo ZSM-5 were 80.69%and 49.35%,respectively.（5）The effects of reaction conditions（temperature,pressure,GHSV,hydrogen to carbon ratio）on the reaction performance of the catalysts were investigated based on the30Fe K&Hie Mo ZSM-5 and 30Fe K&Hie Zn ZSM-5 catalytic systems.The experimental results show that increasing the temperature,pressure,GHSV and hydrogen to carbon ratio can help to improve the CO conversion and C₅₊selectivity of hydrocarbon products,while high temperature can lead to catalyst coking and deactivation,and high pressure can reduce the proportion of aromatic hydrocarbons in the liquid phase products.Low GHSV can lead to a lower monocyclic aromatic hydrocarbon fraction and high hydrogen to carbon ratio can reduce the syngas conversion.Compared to the Mo additive,the Zn additive is more resistant to coking at high temperatures and pressures,maintaining96.03%CO conversion at 4500 m L/h/g GHSV.Under the standard conditions,the percentage of BTX in the liquid phase of the Mo modified product was 15.70%,which was higher than that of the Zn modified products.Under the conditions of 350°C,3 MPa,1800 m L/h/g,H₂/CO of 2 and 36 h reaction time,the CO conversion of both catalytic systems reached 97.37%,the CO₂ selectivity was 25.70%,and the percentage of aromatics in the Mo-modified liquid phase product was 90.55%and the percentage of BTX was 47.52%.