One-step Hydrogenation Of CO_x To Aromatics Over Bifunctional Catalyst

As one of the most important basic chemicals,aromatics account for nearly one-third of the global petrochemical market.Nearly 70%of aromatics in the world are produced through petrochemical routes.In order to reduce the dependence on petroleum resources,direct synthesis of aromatics through syngas or CO2hydrogenation has attracted many researchers'attention.Based on the idea of reaction coupling,aerogel ZnO-ZrO2/H-ZSM-5 was developed for the process of CO2 hydrogenation to aromatics process and syngas to aromatics process.The catalytic performance and the relationship between structure and activity were studied in this paper.Abifunctional catalyst obtained by grind-mixing ZnO-ZrO2 aerogel?Zn:Zr=1:8?with H-ZSM-5?Si/Al=68?.Under the condition of 340?,4 MPa,7200 g h-1gcat-1,,H2/CO2 = 3/1,the catalyst has achieved 15.9%CO2 conversion rate,76.0%aromatics selectivity,with only 34.0%CO selectivity and 0.4%methane selectivity.The space-time yield?STY?of aromatics reaches 0.25 g goxiade-1 h-1,which is 7 times higher than the highest STY reported.By comparing the catalytic performance of the bifunctional catalysts with ZnO-ZrO2 prepared by different synthesis methods,we found that the aerogel ZnO-ZrO2 synthesized by sol-gel method has the best performance compared with impregnation method,coprecipitation method and hard template method.After the further characterization?XRD,EPR,TEM ect.?,it is found that the ultral high acticity is because the high O vacancy concerntration.The super small partical size and uniform dispersion of Zn2+ in the t-ZrO2 lattice greatly increase the oxygen vacancyconcentration for ZnO-ZrO2 areogel.The catalytic performances of bifunctional catalysts with different Zn contents,indicate that highly dispersed Zn is the active site for H2,and the proper amount of Zn?Zn:Zr=1:8?can greatly increase the oxygen vacancy concentration of ZrO2.This study has also found that an appropriate strong acid density for H-ZSM-5 is beneficial to the formation of aromatics,and has a stronger balance driving force to CO2 activation.By changing the mixing types of ZnO-ZrO2 and H-ZSM-5,we have found that in the bifunctional catalyst system,where ZnO-ZrO2 and H-ZSM-5 are closely and uniformly mixed,has a higher aromatics selectivity and higher CO2 conversion rate.By changing the contact time,we came up with the reaction route:?1?CO2 is first hydrogenated on ZnO-ZrO2 to form methanol.?2?The methanol finally transfers to H-ZSM-5 to form light olefins.?3?The light olefins are further aromatized on the strong acid sites of H-ZSM-5 and get aromatics products.In view of the low STY in syngas to aromatics process,this paper has doubled the highest STY of aromatics to 0.24 g goxide-1 h-1 using ZnO-ZrO₂ aerogel/H-ZSM-5 bifunctional catalyst.For the problem of high CO2 selectivity,it was found by probe reactions that CO2 mainly came from the water-gas shift reaction.By co-feeding 8%CO2 with syngas,the CO2 selectivity has been successfully suppressed to O,without affecting the yield of aromatics.