| Lignin,the second largest natural polymer next to cellulose,accounts for about 30%of all organic carbon in the biological field and is widely considered as an excellent renewable raw material for the production of chemicals.To date,a tremendous amount of efforts have been made to develop effective methods for the utilization of lignin,including pyrolysis,hydrothermal depolymerization,hydrogenolysis and oxidation.Among them,pyrolysis technology is an effective mean to convert lignin directly into liquid fuels and high value-added chemicals.However,the poor quality of bio-oil and the presence of oligomers in bio-oil are the main obstacles to the commercial application of the technology.Therefore,how to improve the quality of bio-oil and explore the formation mechanism of oligomers in bio-oil are of important theoretical and practical significance.In this dissertation,industrial lignin was mainly used as research object,and a qualitative and quantitative method for the analysis of oligomers in bio-oil was established.Based on this,the effects of pyrolysis conditions and lignin structure on the composition and properties of oligomers and the pathways to regulate its content were discussed.The detailed research contents are listed as follows:?1?The chemical characterization of alkali lignin?AL?and corn cob hydrolyzed residue lignin?CHRL?was conducted in detail.The results showed that the H/C and O/C atomic ratios of these two lignins were basically the same,with little difference in high calorific value?HHV?,and the average weight molecular weight of AL?Mw=6553 g/mol?was much higher than that of CHRL?Mw=3258 g/mol?.In addition,AL has more content of total phenolic hydroxyl group,carboxyl group and methoxy group than CHRL.The 2D HSQC NMR results showed that the main linkages of these two lignins are?-O-4??-???-5??-1??-O-4.Compared with CHRL,AL has more aliphatic branched-chain structure,?-O-4,?-1 and?–?linkages,while CHRL has more flavonoid structure,?-O-4 linkage and aromatic ring structural unit.The results of thermogravimetric?TG?analysis showed that the Mw of lignin was positively correlated with the initial degradation temperature,the temperature of maximum weight loss was positively correlated with the aryl ether bond content in lignin,and the condensation degree of lignin was also positively correlated with the content of"carbon residue".?2?The effects of water-oil ratio and water temperature on the distribution of polar and non-polar components in different fractions were investigated.The results showed that water was suitable as an extraction solvent for the fractionation of liquid phase products.When the water-to-oil ratio was 20:1 and the extraction temperature was 20°C,polar and non-polar components were enriched in the water-soluble phase?WS,36.56 wt%?and water-insoluble phase?WI,63.44 wt%?fractions,respectively.Although extraction temperature had no significant effect on the properties of WS and WI,the distribution of polar and non-polar components was markedly affected?due to the dispersion and polarity of the compounds?.In addition,further analysis by 2D HSQC-NMR showed that side-chain structures of the phenolic derivatives in the fractions were more sensitive than aromatic ring structures to the temperature of the separation operation.?3?Solvent fractionation method was used to obtain oligomer-rich fractions from the pyrolytic liquid phase products of AL and CHRL.The results showed that four oligomer-rich fractions,namely,AL-DI,AL-EEI,CHRL-DI and CHRL-EEI,were obtained by sequentially fractionation of the two liquid phase products via extraction with a combination of water,dichloromethane and diethyl ether.The yields were 12.16%,10.88%,10.70%and 3.61%,respectively.The results of FTIR and quantitative 31P NMR showed that the chemical composition of each fraction depends on the chemical nature of the extraction solvent and the hydroxyl content is closely related to the hydrogen bonding ability of the extraction solvent.In addition,the thermal stability of the oligomer-rich fraction is closely related to its chemical nature,and the chemical composition exerts a greater influence on the thermal degradation behavior than the molecular weight.?4?The effects of different pyrolysis temperatures and pyrolysis atmospheres on the content,composition and characteristics of the oligomer-rich fractions obtained after fractionation were carried out.The results showed that the pyrolysis temperature exhibits little effect on the functional group strength of oligomer-rich fractions,and the atmosphere exhibits a greater influence on the content of the different structural linkages?such as?-O-4,?-?and stilbene structure?in the oligomer-rich fractions and its composition and characteristics.Additionally,the presence of a certain amount of hydrogen promotes the preferential hydrogenation of unsaturated bonds during lignin pyrolysis and is beneficial to reduce the content of oligomers.?5?The formation mechanism and regulatory pathway of oligomers during lignin pyrolysis were investigated.Studies have shown that the formation pathway of polycyclic aromatic hydrocarbons?PAHs?and its derivatives was from the secondary reaction of the primary products during lignin conversion,and the apparent activation energy increases with the size of benzene ring,viz.,it is beneficial to the formation of PAHs and its derivatives at high temperatures.The ESI+-HRMS results showed that a large amount of?-O-4 and C?-C?bonds were broken during lignin pyrolysis process,and the oligomers mainly composed of dimers and trimers were formed.Besides,pyrolysis under hydrogen atmosphere promotes the conversion of oligomers with higher molecular weight into dimers and trimers with lower molecular weight.?6?Studies showed that starting from the chemical structural properties of the raw materials,the lignin with a relatively small molecular weight is selected,and the chemical structure of the lignin is regulated,for example,controlling the proportion of the guaiacol-type?G type?structural unit in the lignin structure and conducting pyrolysis at600°C is an effective pathway to reduce the content of oligomers in the pyrolysis liquid phase products. |
PDF Full Text Request