Study On Biomass Catalytic Pyrolysis With Modified ZSM-5 For The Production Of Monocyclic Aromatic Hydrocarbons

Biomass is the only renewable energy that can be converted into liquid fuels.It has the advantages of large reserves,wide distribution,and"near zero"carbon dioxide emissions during utilization.In recent years,the technology for producing high quality bio-oil by catalytic fast pyrolysis（CFP）of biomass has received more and more attention.Among them,the use of ZSM-5 catalyst can enrich the bio-oil with monocyclic aromatic hydrocarbons,such as benzene,toluene and xylene（BTX）.BTX is an important fuel additive and bulk chemical raw material,which currently mainly derived from the catalytic cracking of naphtha.Therefore,the production of aromatics by catalytic pyrolysis of biomass not only provides an effective way for the utilization of biomass more efficient,but also meet the urgent needs for the production of green energy and chemical products in the future.However,the disadvantages of biomass pyrolysis products,such as high oxygen content,complex composition,and low activity of the commercial ZSM-5 catalyst,make the output of monocyclic aromatic hydrocarbons low and the market competitiveness not strong.The thesis studied the product generation mechanism and the structure-activity relationship of the catalyst by changing the acidity and pore structure of the ZSM-5 catalyst.It also explored ways and methods to improve the conversion efficiency of biomass to monocyclic aromatic hydrocarbons by regulating the oxygen-containing intermediate products.First,the effect of metal loading on the acidity and catalytic performance of ZSM-5catalyst was studied.The ZSM-5 catalyst was loaded with 2 wt.%Zn,Fe,Ca,Ce,and La by the impregnation method.The characterization results showed that the 2 wt.%metal loading has significant effect on the acid content and acid distribution of the catalyst,but It has little effect on its crystal structure and pore structure.ZSM-5 modified by different metals showed great differences in the catalytic pyrolysis of biomass.Among them,the BTX yield of Zn/ZSM-5 was increased by 19.3%compared with ZSM-5,while Ca,Ce and La modified ZSM-5 catalyst reduced the output of BTX.Further analysis found that the BTX production is positively correlated with the strong acid content of the catalyst when the metal loading amount is low.In particular,the loading of 2 wt.%Zn increased the strong acid content of the catalyst,and promoted the production of aromatics.The loading of Ca,Ce and La reduced the strong acid content of the catalyst,resulting in a decrease in BTX output.When the Zn loading increased to 10 wt.%,the mesopore volume of the catalyst reduced greatly,resulting in the limited mass transfer in pores,and the low activity of the catalyst.Then,the influence of the mesoporous structure of the catalyst on the catalytic pyrolysis of biomass was explored.A series of ZSM-5 catalysts with micro-mesoporous structure were hydrothermally synthesized using green templates（sucrose,starch and microcrystalline cellulose）.The micro mesoporous ZSM-5 has the characteristics of low strong acid content and high mesoporous content when compared with the microporous ZSM-5 catalyst.Both the organics yield and aromatic hydrocarbon content are increased in catalytic pyrolysis of biomass.The yield of BTX is positively correlated with the mesopore volume of ZSM-5catalyst.The addition of 10%starch increased the mesopore volume of the catalyst in the range of 16-45 nm,and the mesopore volume and acidity of the catalyst achieved an optimal balance,which significantly increased the organic products,and the yield of BTX reached up to 91.84 mg/g.Alkaline treatment of the synthesized micro-mesoporous ZSM-5 catalyst will generate new mesopores in the range of 4-8 nm,and the yield of BTX reaches 94.53 mg/g.However,loading it with Zn and increasing the content of strong acid can greatly increase the content of xylene products.In order to further reveal the mechanism of the mesoporous structure on the performance of the ZSM-5 catalyst,four typical bio-oil model compounds（acetic acid,furfural,guaiacol and eugenol）were used as raw materials for catalytic conversion experiments.It is found that the conversion efficiency of the model compound with micro-mesoporous catalyst was higher than that of the microporous catalyst,but the carbon yield of BTX decreased with the increase of the molecular weight of the reactants.The conversion efficiency of acetic acid and furfural under the action of the two catalysts is close,but the micro-mesoporous catalyst reduced the coke deposition of acetic acid and furfural in the micropores.Unlike the microporous catalysts,the mesopores in the micro-mesoporous catalyst provided channels for the transmission of phenolic reactants,enhanced the debranching and ring-opening reactions of phenolic substances in the catalyst system,generating more aromatic hydrocarbons and coke deposits.The above comprehensive analysis shows that the introduction of large mesoporous channels on the surface of the catalyst can significantly improve the mass transfer capacity and promote the depolymerization of macromolecular substances.While the introduction of small mesopores by alkali treatment can promote the decomposition of phenolic substances,thereby increasing the yield of BTX products.In order to promote the formation of aromatics by regulating the pyrolysis intermediate products,three metal oxides（Ca O,Al₂O₃ and Zn O）and ZSM-5 were selected to form a dual catalyst system.The research showed that the addition of Ca O effectively reduced the content of carboxylic acid and methoxyphenol in the pyrolysis volatiles,and increased the hydrogen to carbon ratio of the volatiles.The addition of Al₂O₃ significantly promoted the cracking of oxygen-containing compounds with a molecular weight greater than 109 g/mol.The volatiles catalyzed by metal oxides can be converted to aromatics more easily with ZSM-5 catalyst.Among them,the Ca O-ZSM-5 dual catalyst can produe a higher yield of BTX.Besides,the effect of pretreatment on biomass pyrolysis volatiles was further explored.The research showed that the wet torrefaction resulting in the content increase of sugars that be more easily converted into BTX.The yield of BTX from catalytic pyrolysis of 150℃wet torrefied biomass increased by 22.9%.Finally,the three methods of biomass torrefaction pretreatment,ZSM-5 catalyst modification,and dual catalysts were combined to apply to the catalytic pyrolysis of biomass.The mass yield and carbon yield of BTX reached up to 105.0 mg/g and 18.16%when the150℃wet torrefied biomass was used,accompanied by the Zn/ZSM-5 micro-mesoporous catalyst and CaO.