Study On Catalytic Pyrolysis Of Lignin With Alkali Modified HZSM-5 Zeolites

As one of the three main components of lignocellulosic biomass,with its wide sources,large reserves,renewable and many other advantages,lignin is given more and more attention by people.Among kinds of lignin conversion technologies,lignin can be directly converted into benzene,methylbenzene and xylene and other high value-added chemicals during catalytic fast pyrolysis?CFP?,which therefore is regarded as a promising technology.For the CFP of lignin,the HZSM-5 zeolite was an effective option of catalyst and had unique advantage,as the characteristic acidities and the MFI type of pore structure.However,the commercial HZSM-5 zeolite was microporous materials with few mesoporous,the bulky oxygenous intermediates?such as guaiacol,syringol and their derivatives?of lignin pyrolysis were not diffuse into the pore of the zeolite to take part in the catalytic reactions.Hences,the HZSM-5 should be undergone modification for preferable performance,and the alkali treatment was an effective method to introduce mesopores into the crystals of zeolites.A series of modified HZSM-5 zeolites were prepared by post-treated desilication of a commercial HZSM-5 zeolite.The HZSM-5 zeolites were treated with different concentrations?0.1⁰.5 M?NaOH solutions and with a kinds of NaAlO2,Na2CO3,tetrapropylammonium hydroxide?TPAOH?solutions.The catalyst samples were characterized by XRF,XRD,TEM,N₂ adsorption-desorption and NH₃-TPD techniques.The alkaline treatment changed the morphology,SiO2/Al2O3,pore properties and the density of acid sites.The modified HZSM-5 zeolite catalysts used in catalytic fast pyrolysis?CFP?of lignin was investigated by Py-GC/MS.The alkaline treatment improved the catalytic activity of modified HZSM-5 for cracking bulky oxygenates released from lignin.The highest hydrocarbons selectivity?80%?on the Total Ion Chromatogram peak area percentage?TIC,%?was obtained in CFP of lignin with the zeolite treated by 0.3 M NaOH solution.For CFP of lignin with the NaAlO2,Na2CO3 and TPAOH solutions treated HZSM-5,the hydrocarbons selectivity were also increased compared to the CFP of lignin with the parent HZSM-5 zeolite.The circulation experiments and regeneration experiments were employed to evaluate the lifetime of the two different catalyst?HZSM-5 and 0.3M NaOH modified HZSM-5?.The spent and regenerated catalysts were characterized by N₂ adsorption-desorption and NH₃-TPD techniques to analyse the reason of deactivation,and the spent catalysts were characterized by SEM,TGA,FTIR and UV-Vis techniques to analyse the coke deposited on the catalyst.The deactivation of the two different zeolites was become more and more serious along with the increase of the number of cycles.The difference was that the performance of 0.3M NaOH modified HZSM-5 was still better than the one of unmodified HZSM-5 after four cycles.The coke of the spent catalysts was removed through calcination in air at high temperature and the catalytic activity was recovered to a large extent.The performance of 0.3M NaOH modified HZSM-5 was still better than the one of unmodified HZSM-5 after regeneration.The amount of coke deposited on the modified HZSM-5 was less than the one of unmodified HZSM-5,and the combustion activation energy of the coke deposited on the modified HZSM-5 was lower than the one of unmodified HZSM-5.The coke of the modified HZSM-5 contains more condensed aromatics relatively.