Research On Producing Aromatic Hydrocarbons By Catalytic Thermal Cracking Of Lignin Depolymerization Products

Lignin,the second largest organic material on earth after cellulose,is the only plant resource that contains a benzene ring structure.High-value monocyclic aromatic compounds（e.g.benzene,toluene,xylene）can be obtained by selectively breaking the linkage bonds of the benzene rings in lignin by fast pyrolysis,which has promising applications.Research on lignin pyrolysis conversion has attracted much attention,but the phenolic hydroxyl group on the benzene ring of the intermediate produced by fast pyrolysis of lignin is difficult to remove,resulting in low selectivity and yield of aromatic hydrocarbons.In this work,guaiacol was selected as the main model compound of lignin to carry out catalytic thermal cracking for the production of aromatic compounds.Firstly,experimental studies on pyrolysis were carried out with lignin as raw material by means of thermogravimetric analyzer and Py-GC/MS.Thermogravimetric analysis showed that lignin pyrolysis mainly occurred between 150-700°C,and the thermal weight loss rate accelerated with the increase of the heating rate.The effect of reaction temperature on the distribution of lignin pyrolysis products was investigated by Py-GC/MS.It was found that guaiacyl group substances were the main components of lignin pyrolysis products,and the amount produced increased first and then decreased with the increase of temperature,and the most guaiacylates were produced at 500°C,reaching 42.27%.In response to the hydrogen-deficient problem of biomass pyrolysis gas,this paper introduced methanol into the pyrolysis gas catalytic cracking process for co-cracking reaction using guaiacol as a model compound for lignin pyrolysis.Among the five Si O₂-Al₂O₃ catalysts with different Si O₂ contents,the highest feedstock conversion and aromatic yield were obtained when the co-cracking reaction of guaiacol with methanol was catalysed by Si O₂-Al₂O₃ with 33%Si O₂ content.When the molar ratio of guaiacol to methanol was 1:25,the reaction temperature was 400°C,the WHSV was 8/3 h^-1,the N₂ flow rate was 25 m L/min and the reaction time was60 min,the conversion of guaiacol reached 100%and the yield of aromatic hydrocarbons reached57.93%.In the co-cracking reaction,methanol provided atmosphere to the reaction system and converted phenols to aromatics by hydrogen transfer and methylation.The results of co-cracking of other lignin-derived phenols with methanol showed that methanol addition was also effective in promoting the conversion of phenols,anisole and mixed phenols to aromatics.The metal（Pt,Ru,Ni,Co）loading modification was applied to the Si O₂-Al₂O₃ catalyst with33%Si O₂ content by isovolume impregnation method,and the effect of the loading modified 33-SA type catalyst on the co-cracking of guaiacol/mixed phenols with methanol for the preparation of aromatic compounds was investigated under the optimum working conditions screened.It was found that the metal Pt loaded modified 33-SA catalyst was the best catalyst to promote the production of aromatic compounds by co-cracking of lignin-derived phenols and methanol,and the metal Pt loading was beneficial for prolonging the catalyst stability and improving the anti-carbon build-up performance.Finally,the 33-SA catalyst loaded with Pt was applied to co-catalytic cracking reaction of lignin with methanol with a relative content of aromatic hydrocarbons as high as 81.71%.This indicates that the addition of methanol is suitable for complex mixed phenolic systems as well as lignin.