| Lignin is the third highest natural material in the worldwide production,it is significant to research the high valued-added utilization of lignin originated from the paper making or the biomass refining.This study focused on alkali-lignin effectively hydrothermal conversion and aimed at the yield of liquid fuel and phenols.It mainly discussed different factors(like reaction media,reacting conditions and the catalyst species)on the liquid products yield and the characteristics of the liquid products and also the yield of phenols.To improve the yield of the liquid products and phenols,the reaction system with formic acid salt as hydrogen donor and Pd/C as catalyst were investigated.The addition of nano-oxidants,and the mixture of water and three different solvents as reaction media were also investigated.The relationship between the liquid products yield and the components of the alkali-lignin,and the influence about the characteristics of the lignin structure on the products yield were also analyzed.The effects of different catalysts on the properties of alkali lignin depolymerization products were deeply discussed.The differences in yield and composition of lignin depolymerization products with different sources and physicochemical properties from 7 kinds of lignins were clarified,and the effects of lignin structure on the product were also analyzed.On this basis,the tubular continuous hydrothermal catalytic conversion reaction device was designed.The amplification experiment under the optimal process was conducted,which would provide support for the technical engineering applications.The results suggested that the optimal liquid production from hydrothermal was 56.26%,phenols was 12.14% when reaction temperature was 325?,reaction time was 30 min,the rotating speed was 500r/min,and the dose of nickel formate and Pd/C was 10%.The content of carbon and hydrogen of the liquid products increased,the oxygen decreased rapidly,the ratio of O/C decreased from the 0.25 to 0.05,and the H/C promoted from the 1.00 to 1.02.With Pd/C,the de-oxygen efficiency could be improved obviously when nickel formate added,which benefit the optimization and the utilization of the liquid products.The hydrothermal liquid products from the fir alkali-lignin contained dimers and oligomers which were incompletely degradated products or condensed after degradation.From the results of LC-HRMS analysis,the addition of nickel formate and Pd/C resulted in the relative increase of monomer and dimer in the hydrothermal desorption liquid product of lignin and the relative decrease of trimers,which indicated that nickel formate and Pd/C promoted the depolymerization of the lignin,and inhabit formation of the oligomer.The industrial lignin from different fiber raw materials has different chemical structure which lead different liquid products and total phenol yield.The yield of liquid products from the hardwood lignin hydrothermal was lower than that of softwood lignin.To improve the conversion rate of hydrothermal liquid products from hardwood lignin,we investigated the effects of nano-titania,nano-copper oxide,manganese dioxide,nano iron tetrachloride,nanoferric ferrite,tungsten oxide,nano-manganese oxide,nano-nickel oxide.The results showed that the yield of the liquid products was the highest about 43.46% when manganese dioxide added.For the total phenol yield,the highest was 10.30% when adding nano-copper oxide.The upgrading of the oligomers from liquid products by pyrolysis was studied.The liquid products from the hydrothermal conversion were flashed at different temperatures and analyzed by GC/MS.With the temperature rising from 160? to 200?,the total phenol content increased from 18.56% to 34.17%,which was significantly higher than the total phenol content in the liquid phase product after hydrothermal conversion.When the temperature increased,the total phenol product decreased with the temperature increasing,it indicated that there were lots of oligomers in the hydrothermal conversion liquid phase products.These oligomers could be converted into monophenols by further low temperature pyrolysis to improve its yield,which also improved the total phenolic compounds content of alkali-lignin hydrothermal conversion products,and pointed out the direction to enhance the liquid product quality.In this study,the yield of liquid product derived from alkali lignin depolymerization increased in different extent after addition 10 wt.% nickel formate under three kinds of mixed solvent medium(ethanol-water,ethyl acetate-water,and ethylene glycol-water)at 300? for 30 min.The highest yield of liquid product was recorded under the ethanol-water mixed solvent medium.Under the optimized reaction condition(40/60,V/V),the yield of liquid product and phenolic compounds was 74.34% and 15.62%,respectively.The liquid product obtained from alkali lignin hydrothermal depolymerization under ethanol-water mixed solvent medium was flashed and recovered.When the residue,mainly contained oligomers,was pyrolyzed at 500 °C,the phenols and other compounds derived from lignin were the most abundant and the yield of phenolic compounds was increased significantly.Moreover,the methyl aryl ether bond of the distillation residue would break down during the pyrolysis process,resulting in a certain amount of catechol generation.Those results provided guidelines for the utilization of liquid products.In order to increase the yield of liquid product and phenol compounds derived from hardwood lignin,the hydrothermal depolymerization experiments were carried out under the pure water medium.Six kinds of commercial catalyst(boron powder,tungsten carbide,ferrocene,HZSM-5,ZSM-5,and SBA-15)and two kinds of self-made catalyst(zinc chromate and LaNiO3)were added and the addition amount was 5%.Moreover,10% nickel formate was used for catalytic experiment as well.The yield of liquid product and phenol compounds derived from alkali lignin hydrothermal depolymerization was increased in different extent at 325 °C for 30 min.The highest yield of liquid product and phenol compounds was 70.23% and 8.35%,respectively with the addition of tungsten carbide.In this study,the adaptability of hydrothermal conversion technology for different lignin was investigated.Seven kinds of lignin with different sources and characteristics were researched.The results indicated that the yield of liquid product obtained from softwood alkali lignin(Rizhao Huatai paper mill)was highest(89.13%).The maximum phenol compounds were obtained from milled wood lignin of fir wood and the yield reached to 16.91%.The dioxane fir lignin modified by formaldehyde(FAL)appeared more secondary reactions during the hydrothermal conversion process,the yield of liquid product and phenolic compounds was lower than fir alkali lignin.The results suggested that the low content of oligomer in lignin was not favorable to the hydrothermal products formation.In contrast,the hydrothermal depolymerization process would produce more oligomer than alkali lignin itself via chain breaking and complex secondary reaction.The yield of liquid product and phenol compounds obtained from the fir alkali lignin modified by mild Fenton oxidation was equivalent to that of raw alkali lignin.It indicated that proper oxidation modification of alkali lignin has no significant effect on the yield of liquid product and phenol compounds increase during the hydrothermal depolymerization process.The alkali lignin hydrothermal depolymerization was verified by the self-design continuous reactor under the optimized conditions.Various indexes including the yield of depolymerization product,the composition of liquid product,and the yield of phenol compounds were detected.Moreover,the feasibility of catalysts on alkali lignin hydrothermal regulation was evaluated.All of these efforts established a good foundation for the subsequent engineering application and liquid product utilization. |
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