| Aromatic hydrocarbons are one of the important basic chemical raw materials,which are mainly derived from the catalytic reforming of naphtha.At present,the market demand for aromatic hydrocarbons is enormous,and the demand growth rate is fast.Due to China’s abundant coal resources,it is particularly important to develop technologies for producing chemical products using coal-based syngas as raw materials.Among them,Fischer-Tropsch synthesis technology is the most important and most studied process technology in the syngas conversion.The content of N,S,and other elements in the raw material gas(syngas)of Fischer-Tropsch synthesis is very low,therefore aromatic hydrocarbons and other products produced by the Fischer-Tropsch synthesis process have the advantages of clean and high efficiency,and the source of syngas is wide and cheap.However,the currently developed catalysts exhibit high CO2 selectivity,carbon deposition,and severe reaction conditions.In this regard,based on the idea of metal-encapsulated zeolites bifunctional catalysis for aromatization reaction,this subject explored a highly efficient and stable synthesis method of metal embedded zeolites Co@HZSM-5,so that the aromatization reaction successfully proceeds under mild reaction conditions and exhibits low activity of water gas shift reaction.The physical and chemical properties of catalysts,including sample morphology,crystal structure,acidity,reduction performance,and coke deposition,were characterized by XRD,N2 physical adsorption,SEM,NH3-TPD,H2-TPR,and TG-DTA.Then on this basis,Mo was introduced as the second aromatization active center,and the effects of the Mo content,W/F,and Si/Al ratio in the Mo/Co@HZSM-5 catalytic system on the direct synthesis of aromatics from syngas were investigated.Experimental results show that:The optimal conditions for the synthesis of Co@HZSM-5 catalyst by solvent-free method,hydrothermal synthesis method,and two-step crystallization method were investigated by changing the ratio of synthetic raw materials and crystallization time,and the synthesized catalyst was used for direct synthesis of aromatics by syngas reaction.We found that the Co@HZSM-5 catalyst synthesized by two-step crystallization method(Co loading is 10 wt%,Si/Al=40)under the reaction conditions with T=260℃,P=1.0 MPa,W/F=5 gcat·h·mol-1 have the best performance.The CO conversion rate reached 62.6%,the selectivity of aromatics in the liquid phase exceeded 20.3%,and the aromatics products were mainly light aromatics with a selectivity of up to 72.7%.Through characterization analysis,it can be seen that the embedded catalyst Co@HZSM-5 has more acidic sites and suitable acid strength,which effectively promotes the secondary aromatization reaction of the low-carbon product of the Fischer-Tropsch synthesis reaction.Due to the unique embedded structure,it not only guarantees the stability of the metal active phase and zeolite structure but also has certain hierarchical pores.The shape-selective confinement effect of the zeolites and the existence of hierarchical pores effectively inhibit the formation of long-chain hydrocarbons and reduce the formation of carbon deposits,thereby increasing the stability of the catalyst against carbon deposits.In order to further improve the selectivity of aromatic hydrocarbons,the second active metals Zn,W,and Mo were introduced as aromatization active centers.Although the CO conversion rate of catalysts all decreased,the introduction of Mo increased the selectivity of aromatic hydrocarbons.It was found that when the Mo loading was 6 wt%and the Si/Al=40,the reaction performance reached the best under the conditions with T=260℃,P=1.0 MPa,W/F=3 gcat·h·mol-1.The CO conversion rate is 37.0%,and the selectivity of aromatics in the liquid phase exceeds 33.5%.This is due to the presence of Mo species,which promotes the methane product to continue to aromatize based on a large amount of heat released by the Fischer-Tropsch synthesis reaction,thereby improving aromatic selectivity. |
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