Production Of Small Molecular Aromatic Compounds By Hydrocracking Of Pyrolytic Lignin

To obtain high quality bio-oil is the ultimate goal of bio-oil upgrading.However,the water-insoluble phase in bio-oil(also called pyrolytic lignin)is difficult to be upgraded because of its high degree of polymerization and the easy formation of coke during depolymerization.Therefore,in this paper investigates thehydrocracking of pyrolytic lignin into small-molecular aromatic compounds,especially aromatic hydrocarbons.Firstly,Ni is loaded on five supports including HZSM-5,Al-MCM-41,mUSY,Al2O3 and SiO2,and the effects of these different Ni-based catalysts on direct cracking and hydrocracking of pyrolytic lignin are studied.It is found that the hydrocracking process can produce more oil phase products and reduce the yield of carbon deposition.The comparison of hydrocracking with different catalysts shows Ni/HZSM-5 and Ni/mUSY with more acid sites are more conducive to promoting the monocyclic and bicyclic aromatic hydrocarbons,with the total yields of 24.42wt%and 20.07wt%,respectively.It is proposed that the better catalytic performances of the above two cases are attributed to the synergistic effect of Ni-catalyzed hydrogenation reaction and cracking reaction promoted by the acid sites on the supports.Moreover,the microporous structure of Ni/HZSM-5 favors the production of small molecular aromatic hydrocarbons;the yield of single-ring aromatic hydrocarbons is 16.61 wt%and the total yield of benzene,toluene and xylenes(BTX)is 11.61 wt%.According to the comparison of different Ni-based catalysts,Ni/HZSM-5 is most superior in generating small-molecular aromatic compounds.Based on this catalyts,the effects of different reaction conditions on the hydrocracking of pyrolytic lignin are further investigated,including reaction temperature,pressure and the amount of metal Ni loading on HZSM-5.It is found that the increasing pressure facilitates the dissociation of H2 to form active H,which reacts with the small molecular fragments formed by the depolymerization of pyrolytic lignin to generate stable small molecular compounds.Higher reaction temperature promotes the depolymerization of pyrolytic lignin,but a excessive high temperature will cause the secondary cracking to produce more gas.The amount of Ni loading has two effects.A low Ni loading will weaken the hydrogenation reaction,while a high Ni loading will affect the acidity of the catalyst,both of which lower the yields of small aromatic hydrocarbons.Comprehensive consideration,the case with 10Ni/HZSM-5,the reaction temperature of 500℃ and the pressure of 4MPa is the most favorable to the hydrocracking of pyrolytic lignin,in which the yields of oil phase products,small molecular compounds and single-ring aromatic hydrocarbons are 51.3wt%,,35.63wt%and 16.61wt%,respectively.In order to enhance the hydrocracking of pyrolytic lignin,the active metals on HZSM-5 are changed,including Ru,Pd and the bimetals Ru-Ni and Pd-Ni.It is found that the increase of Ru or Pd loading is beneficial to the hydrogenation and cracking reactions,and thereby increases the yield of aromatic hydrocarbons.Adding the second metal Ni to Ru or Pd can significantly improve the catalytic performances,due to the hydrogen spillover between Ru and Ni or Pd and Ni which significantly strengthens the hydrogenation reaction.Compared with 2Ru-5Ni/HZSM-5,2Pd-5Ni/HZSM-5 has less acid sites,which weakens its catalytic activity for cracking.Therefore,2Ru-5Ni/HZSM-5 achieves the highest monocyclic aromatic compounds yield of 21.08 wt%,and the total BTX yield reaches 14.95wt%.