Study On Reaction Mechanism And Selective Regulation Of Aromatics From The Syngas Via Fischer-Tropsh Route

The efficient use of syngas opens up new ways for the production of clean fuels and chemical products.It not only reduces the consumption of fossil energy,but also creates higher economic benefits through waste utilization.As an important basic organic chemical,aromatics are produced from coal,natural gas and biomass and other non-petroleum carbon resources through synthesis gas.It has broad prospects.The Fischer-Tropsch synthesis technology is one of the best ways and development directions to replace the petroleum route to convert coal,natural gas,biomass and other carbon-containing resources into clean liquid fuels and high value-added chemicals through synthesis gas.Therefore,The direct conversion of syngas(CO/H₂)to aromatics by the Fischer-Tropsch synthesis route has received much attention.Compared with the methanol route,the complex Fischer-Tropsch product composition makes it more difficult to clarify the process of aromatics formation and product regulation.Therefore,this paper proposes a series of strategies to explore the reaction mechanism and related factors affecting aromatics selectivity.For example,using a silicon layer to coat the acid sites on the outer surface of HZSM-5,Fe Mn Fischer-Tropsch catalyst and molecular sieve single-stage and two-stage reaction,under simulated Fischer-Tropsch conditions,using ethylene and propylene as model molecules to explore its aromatization behavior on HZSM-5molecular sieves.In addition,a Na-adjusted Fe Ni bimetallic Fischer-Tropsch catalyst with excellent performance was developed,and the effects of its electronic properties,carbonization behavior,and interface effects on the product were explored,and the Na-adjusted Fe-Ni bimetallic catalyst was creatively constructed.The coupling of Fischer-Tropsch catalyst and HZSM-5 molecular sieve is used for the one-step production of light aromatics from syngas,and simultaneously solves the problems of stability and selectivity of two types of catalysts(Fisher-Tropsch catalyst and molecular sieve).The relevant research results are as follows:1.Coating HZSM-5 with an inert silicon layer can significantly improve the selectivity of para-xylene(PX),mainly because it effectively prevents para-xylene(PX)from isomerizing into M-xylene and O-xylene at the acidic site of the outer surface of HZSM-5.Increasing the coating degree of HZSM-5 molecular sieve,The PX selectivity is greatly improved,but it affects the total selectivity of aromatic products in the liquid phase,It is mainly due to the excessive silica coating,which prevents the FTS product from entering the zeolite channel and/or on the outer surface to the aromatization reaction.2.The research results of the single-stage and two-stage reaction of Fe Mn Fischer-Tropsch catalyst and HZSM-5 molecular sieve show that,unlike the methanol route,in the synthesis of aromatics by the syngas via Fischer-Tropsch route,it is mainly the conversion of C₅₊Fischer-Tropsch intermediates to aromatics rather than light Olefins,light olefins are mainly hydrogenated and isomerized into alkanes.In a single-stage reactor,comparing the particle mixing mode of the coupled catalyst,through the research on the Fischer-Tropsch catalyst and the molecular sieve layered packing experiment,it is found that the farther they are layed,the ethylene concentration around the molecular sieve can be effectively reduced,and benzene and toluene can be further suppressed.The alkylation reaction with ethylene significantly increases the selectivity of BTX(benzene,toluene and xylene)in the total aromatics from 48.3%to 68.9%.3.Under the simulated local Fischer-Tropsch synthesis reaction environment,the model reaction results of ethylene,propylene and heptane show that the H₂O and CO₂ generated in situ in the Fischer-Tropsch synthesis reaction play a key role in promoting the formation of aromatics,the reasons were as followed,the existence of H₂O can increased the acid strength of the molecular sieve and CO₂ help drive the H transfer process,promoting the cyclization dehydrogenation reaction to generate aromatics.4.The DFT calculation further proves the negative effect of protons(H⁺)on the dissociation of CO.Compared with the pure Fe₅C₂(510)surface,the d-band center of the H⁺-Fe₅C₂(510)surface moves to low energy,away from the Fermi level.This result shows that the H⁺-Fe₅C₂(510)surface has relatively low dissociation activity for CO,which indicates that the introduction of protons(H⁺)is not conducive to the dissociation of CO.In addition,comparing the CO bond distance(1.196?)of CO adsorption on the surface of Fe₅C₂(510),the smaller CO bond distance of H⁺-Fe₅C₂(510)(1.174?)indicates that CO dissociation becomes difficult,which is consistent with the result of the d-band center.Therefore,it can be inferred that the protons on the acidic HZSM-5 molecular sieve are close to the Fe or Fe₅C₂ phase,and the dissociation of CO can be completely suppressed.5.Compared with?-Fe₂O₃-0.75Na,Fe Ni Ox(5:1)-0.41Na has stable catalytic activity and high selectivity of?-olefin(C₂⁼-C₄⁼).Appropriate addition of Ni can effectively inhibit Fe-based Fischer-Tropsch catalyst carbon deposition,thereby significantly improving the stability of the Fischer-Tropsch catalyst.Due to the weak adsorption of CO on Ni and the electron transfer from Fe to Ni,the adsorption and dissociation of CO on Fe is reduced,thereby changing the carbonization of Fe Behavior,more Fe₃C active centers are formed during the reaction,which is beneficial to reduce methane and improve the selectivity of light olefins,which is consistent with the experimental and DFT calculation results.In addition,due to the electron transfer between Fe and Ni,the intrinsic catalytic properties of Ni are changed and the methanation reaction is weakened.6.Interestingly,the alkali promoter inhibits hydrogenation in the Fischer-Tropsch reaction,but here,extra Na addition will significantly increase the selectivity of C₁^o-C₄^o.Combination of characterization and DFT calculations confirms that extra Na addition will weaken the Fe-Ni interaction,reduce the electron transfer from Fe to Ni,and make Ni show its intrinsic catalytic properties.Due to the strong hydrogenation ability on Ni sites,it will greatly increase the selectivity of undesirable light alkanes.7.With the help of DFT calculations,through the study of the C-C coupling behavior of the Fe-Ni interface,it is found that the CH₂ species adsorbed on the Fe-Ni interface tends to form C₂H₄ through C-C coupling,and C₂H₄ can be spontaneously desorbed from the surface,Indicating that the formation of light olefins is advantageous,and further chain growth is inhibited.This due to the electron repulsion effect caused by the accumulation of electrons on the Fe-Ni interface,which weakens the bonding strength of CH₂ and C₂H₄species.8.The Fischer-Tropsch product distribution significantly affects the stability of HZSM-5molecular sieve and the selectivity of aromatic products.For Fe Ni Ox(5:1)-0.41Na,C₅₊products are mainly concentrated in low carbon numbers.These products are converted on HZSM-5 to high value-added aromatics(the total aromatics selectivity in the liquid phase reaches 98.6%,of which toluene,Ethylbenzene and xylene light aromatics selectivity is65.7%),and HZSM-5 molecular sieve has good catalytic stability,on the contrary,Fe Mn Ox(5:1)-0.40Na,because the oxygen vacancies in Mn Ox promote CO dissociation,C₅₊high carbon number products increase in selectivity,these Fischer-Tropsch products are converted to polycyclic branched aromatic hydrocarbons on HZSM-5,and the stability of HZSM-5molecular sieve is significantly reduced.At the same time,the study found that in the coupling reaction of the Fischer-Tropsch catalyst and the molecular sieve,the Na on the Fischer-Tropsch catalyst can migrate to the outer surface of the HZSM-5 molecular sieve to poison acid sites,thereby inhibiting the H transfer reaction and avoiding light olefin hydrogenation,greatly increasing the selectivity of high value-added light olefins in the gas phase.