Structural Analysis Of Typical Lignin And Study On The Preparation Of Aromatics By Catalytic Pyrolysis

With the gradual exhaustion of fossil energy and the increasingly serious problems of environmental pollution,we urgently need to find a new energy source that can replace fossil energy.Biomass has received extensive attention from researchers due to its advantages of being renewable,environmentally friendly and abundant.Papermaking black liquor is a high-quality biomass resource which contains a large amount of lignin,but it has not yet been efficiently used.Lignin is considered to be a good source for the production of phenols or aromatic hydrocarbons.Therefore,the use of lignin in the papermaking black liquor to prepare aromatic hydrocarbons by catalytic pyrolysis is an effective method to achieve high value utilization of the papermaking black liquor,so the following studies were carried out:Firstly,the bamboo pulp papermaking black liquor was selected as the raw material,which was directly evaporated and purified to obtain black liquor solids and black liquor lignin,respectively.The ultimate analysis,proximate analysis,FT-IR analysis and Thermogravimetric Analysis of black liquor solids and black liquor lignin were carried out.It was found that the black liquor solids contained a large amount of inorganic elements left over from the papermaking process compared with black liquor lignin,in which the content of Na and K elements reached 30.57%and 7.67%,respectively.Besides,black liquor solids also contained fewer types of functional groups compared with black liquor lignin,and their structures were relatively simple.The results of Thermogravimetric Analysis showed that compared with black liquor lignin,the addition of HZSM-5 reduced the maximum weight loss peak temperature of the black liquor solids and increased the maximum weight loss rate of the black liquor solids,which meaned that the formation of volatiles was promoted.Secondly,the feasibility of black liquid solids and black liquid lignin catalyzed pyrolysis to prepare aromatic hydrocarbons was explored,and the effect of basic impurities in black liquid solids on lignin catalyzed pyrolysis to prepare aromatics was investigated.The experimental results showed that the relative contents of monocyclic aromatic hydrocarbons prepared by catalytic pyrolysis of black liquor lignin and black liquor solids reached 53.21%and 66.26%,respectively,which indicated that black liquid solids was more suitable than pure black liquid lignin for catalytic pyrolysis to prepare monocyclic aromatic hydrocarbons.This was due to the synergy between the alkali（Na₂CO₃ and NaOH）in black liquor solids and HZSM-5,thereby promoting the generation of volatiles and the removal of O during catalytic pyrolysis of lignin.Finally,the mechanism of NaOH promoting lignin catalytic pyrolysis to prepare aromatic hydrocarbons was further studied.It was found that Na OH could affect the reaction path of lignin/HZSM-5 catalytic pyrolysis to promote the generation of monocyclic aromatic hydrocarbons.This was mainly reflected in two aspects:the first point was that Na OH could promote H-transfer before passing through the catalyst layer;thus,more oxygenates are transformed into aromatic hydrocarbons by deoxygenation over HZSM-5.Besides,NaOH promoted the deoxygenation of phenols to prepare aromatics over HZSM-5,resulting in suppression of the pathway for aromatics formation by the phenolic pool mechanism which was more easily form coke,therefore,the deactivation of the HZSM-5 catalyst was alleviated;the second point was that NaOH promoted the removal of methoxy groups connected to benzene ring,which reduced the formation of methoxyphenol compounds and increased the production of phenol and alkylphenols,and phenol and alkylphenols were more likely to enter the internal pores of HZSM-5 and reacted to form aromatic hydrocarbons.