Research On The Reaction Mechanisms And Products Regulation During The Conversion Of Lignin And Its Derivatives Into Liquid Fuels

Among all the lignocellulosic biomass components,lignin has the natural aromatic ring structures,which makes it a potential resource for production of aromatic hydrocarbons or cycloalkane as drop-in liquid fuel.Besides,making good use of lignin can largely promote the efficiency of total biomass utilization.To solve the scientific problems existed in the area of lignin conversion,enzymatic hydrolysis lignin(EHL)obtained from bio-ethanol production and pyrolytic lignin(PL)from biomass pyrolysis procedure were used to explore the potential conversion routes.The knowledge about the heavy fraction of crude pyrolysis bio-oil(named phenolic oligomers or pyrolytic lignin)is still deficient.For instance,the information of its structures and formation mechanisms are unclear.To solve the problems,the novel techniques containing HPLC/Qtof-MS,GPC and ¹H-¹³C HSQC NMR were used to characterize the compounds in real pyrolytic lignin in detail.The pyrolytic lignin from model compounds was tested as well and then the results were compared with the real PL.The analysis results showed that pyrolytic lignin from bio-oil mainly consisted of lignin monomers,dimers/polymers with complex sidechains and connected through C-C linkages,so that they had the potential to further convert into high-value products.However,the pyrolytic lignin from pyrolysis of simple lignin monomers had high amount of char/coke,indicating that char/coke should primarily come from secondery reactions of aromatic free radicals.The DFT calculation was further performed to reveal the possible formation mechanisms of pyrolytic lignin,and provided the possible strategies for suppressing its formation from biomass pyrolysis procedure.As for the enzymatic hydrolysis lignin(EHL),¹H-¹³C HSQC NMR technique was also performed to characterize its structures.Compared with pyrolytic lignin,EHL retained more natural C-O ether linkages,and had higher oxygen content.Under the final purpose for production of liquid fuels,these two kinds of lignin resources both have a huge potential of further conversion.Therefore,different catalysts systems were designed and applied in the catalytic fast pyrolysis conversion route and two-step liquid phase conversion route.As for the catalytic fast pyrolysis conversion route,Lewis-acid catalyst niobium penoxide(Nb₂O₅)was induced in the system.The results suggested that Nb₂O₅ catalyst has excellent abilities of oxygen removal as well as suppression of polycyclic aromatic hydrocarbons(PAHs)coke formation.Under the optimized condition(650?,C/L 9:1),EHL had the highest yield of total aromatic hydrocarbons,and the selectivity toward monocyclic aromatic hydrocarbons(MAHs)could reach 90%.Catalytic pyrolysis of PL had the similar aromatic yields as EHL,but gave higher yield of solid chars.Combined with theoretic calculation,the possible formation sequence of molecules and the mechanisms of deoxygenation and PAHs/coke suppression were proposed.According to the calculation results,the surface acid sites of Nb₂O₅ could stretch and weaken the C_Ar-O bonds,which made the bonds cleavage easily under high temperature.Besides,along the reaction routes,the molecules with lower oxygen content had weaker adsorption energy accordingly,so the products without oxygen would readily leave the catalysts surface once formed,thus avoiding the further reaction to form PAHs or cokes.As far as the two-step liquid phase conversion route,Nb-Mo mesoporous metal oxide catalysts were used to realize the production of alkyl phenols as the main phenolic platform compounds from lignin.Under the condition that the molar ratio of Nb and Mo was 5:5,the catalyst gave a better lignin depolymerization results,and the loss of Mo during the reaction could be avoided as well.Under the optimized condition,444.4mg/g Lignin and 386.5 mg/g Lignin of aromatic monomers could be obtained from EHL and PL,repectively.The alkyl phenols from EHL were primarily C8 derivatives,however,PL gave C14?C16 alkyl phenols as main products,which resulted from deeper degree of alkylation reactions.Aiming at hydrodeoxygenation of the phenolic platform compounds,the model compound ethylphenol and active carbon based metal catalyst were used to study the HDO reaction under mild condition(1 bar N₂,water solvent and methanol as hydrogen source),and the production of cyclohexanols were obtained as the main product.Based on this results,further products regulation methods were applied to push the HDO of phenols to the deoxygenated products.Here,more complex lignin model compound,dihydroeugenol(DHE),was used,and low pressure H₂ and solid acidic catalyst Nb₂O₅were induced to the reaction system,100%conversion and selectivity to C9hydrocarbons were finally achieved.In this study,deuterium agents(D₂O,D₂ and methanol-d₄)reactions and theoretic calculations were carried out,and together clarified the separated roles of different catalysts and different hydrogen sources in this physically-mixed-catalyst,water-phase,methanol-assisted mild hydrodeoxygenation system.From the researches of detailed techniques above,the two possible lignin conversion routes,i.e.catalytic fast pyrolysis conversion route and two-step liquid phase conversion route,were proved to be feasible.Thus,Aspen Plus software was used here to design and simulate the different conversion process systems starting from lignin sources.The energy conversion efficiencies of EHL and PL under the two systems were calculated,then analyzed and compared with each other.This study may provide a valuable theoretic guideline for the choices of conversion routes of lignins with different structure properties in the future.