Study On The Dissolution Behaviors Of Alkali Lignin In Ionic Liquid And Lignin Functional Micro/nanospheres

Lignin is a kind of biomass resource with rich benzene ring structure.As one of the main industrial lignin,alkali lignin(AL)has complex molecular structure and is difficult to form homogeneous system with common solvents,which seriously restricts the research progress and high-value utilization of AL.The development of lignin-based nano-functional materials according to the structural characteristics of AL and suitable solvent system is of great significance for solving the above problems.In this dissertation,industrial alkali lignin was used as the raw material,and the"green solvent"ionic liquids(ILs)were used as the solvent system.The dissolution behavior of lignin in basic ILs was analyzed.The microstructure changes and aggregation behavior of AL and the preparation of lignin micro/nanospheres based on ILs were investigated.The application properties of alkali lignin-based micro/nanospheres in adsorption of heavy metal ions and electrochemical field were discussed through different functional design.The detailed results were as follows:Firstly,the basic strength of the series of imidazole-based ILs was quantitatively characterized by UV-vis and Hammett value method.The solubility of AL in different imidazolium ILs([Emim][Ac],[Bmim][Ac],[C5mim][Ac]and[Mmim]DMP)under mild conditions was investigated.The structural changes and oxygen-containing groups of lignin in the dissolution process were discussed.The results showed that the basicity of the imidazole-based ILs was not only related to the length of the alkyl chain in the imidazole unit,but also related to the dissociation strength of anions.Compared with weakly basic[Mmim]DMP ionic liquid,imidazole acetate ionic liquid with stronger basicity had higher lignin solubility,and the solubility reached 33%w/w.During the dissolution process,ILs mainly attacked the oxygen-containing groups and aryl ether bonds on the side chain of lignin,in which the content of methoxy groups decreased,the?-O-4'bond was broken obviously,the?-?'bond was partially degraded,the?-5'bond was relatively stable,and unsaturated bonds were formed.These results indicated that demethylation and dehydration reactions occured during the dissolution process of lignin.The lignin macromolecules were degraded into smaller and uniform structural units,resulting in the decrease of relative molecular weight.However,the main structure of the lignin aromatic ring remained stable.Secondly,using grass alkali lignin as raw materials,the aggregation behavior of AL was investigated based on the weakly basic[Mmim]DMP solvent system.By dropping anti-solvent into the[Mmim]DMP-lignin solution,spherical lignin micro-nano particles(LM/NPs)with a particle size between 200 nm and 1.5?m were obtained;Then,based on the solvent system of[Emin][Ac]with better lignin solubility,the self-assembly of the alkali lignin nanospheres(ALNPs)with controllable structure was realized.The AL formed uniform nanospheres with a particle size range of 51.6-210.8 nm.The particle size of the ALNPs could be controlled by controlling the initial lignin concentration,stirring speed and anti-solvent drop speed.When the concentration of lignin was low,the yield of ALNPs increased with the increase of the initial concentration of lignin.During the preparation process of the ALNPs,the main structure of the aromatic ring of the lignin was stable,and the nanospheres had high stability in the p H range of 4-10.Thirdly,using different sources of AL(grass,hardwood and softwood lignin)and acetylated alkali lignin(ACAL)as raw materials,the effect of hydroxyl on the aggregation behavior of AL in[Emim][Ac]was investigated.The results showed that the hydroxyl group had a great influence on the aggregation of lignin in the solvent system.AL from different sources had been acetylated to remove more than 85%of the phenolic hydroxyl groups,and the hydroxyl groups of grass alkali lignin were more likely to be acetylated.In the process of the aggregation of AL,due to the large intermolecular and intramolecular forces,AL was easier to aggregate into larger aggregates,but the particle size of single molecule spheres was smaller.However,due to the removal of hydroxyl groups,the intermolecular and intramolecular forces of ACAL were reduced,making the single-molecule conformation of ACAL more extended.Furthermore,the effects of different hydroxyl groups on the aggregation behavior of lignin were as follows:condensed phenolic hydroxyl>guaiacyl and catechol hydroxyl>aliphatic hydroxyl.Fourthly,based on the aggregation behavior of lignin in basic ILs solvent system,carboxyl modified alkali lignin nanosphere(CLNPs)was designed by microwave-assisted modification and anti-solvent dropping in[Emim][Ac]solvent system,which could adsorb Pb(?)ions in solution.The results showed that the phenolic hydroxyl groups of CLNPs decreased significantly.The CLNPs had a uniform spherical morphology with a small particle size and an average particle size of 73.9 nm,which was not easy to aggregate.The carboxyl content(1.8 mmol/g)of CLNPs adsorbent was higher than that of original alkali lignin(0.95mmol/g).The specific surface area of CLNPs was 8.63 m²/g,which was about 3-fold higher than that of the original lignin.It was found that the maximum adsorption capacity of CLNPs for Pb(II)was 333.26 mg/g,which was higher than that of most lignin-based adsorbents and traditional commercial adsorbents.The adsorption kinetics and adsorption isotherm models showed that the adsorption of Pb(?)in solution by CLNPs conformed to the quasi-second-order kinetic model,and the adsorption process was single-layer chemical adsorption.In addition,CLNPs showed an excellent recycling performance,with only 27.0%adsorption capacity loss after 10 consecutive adsorption-desorption cycles.Finally,nitrogen and phosphorus dual-doped alkali lignin-based carbon microspheres(MLCM)were prepared by pre-oxidation and carbonization of[Mmim]DMP-lignin solution,which was used as a supercapacitor electrode material.The results showed that compared with the directly carbonized alkali lignin carbon(LC),MLCM had a spherical structure with higher specific surface area(938.1 m²/g)and pore volume(0.64 cm³/g).Nitrogen and phosphorus were successfully doped into MLCM,with a content of 2.28 at.%and 1.23 at.%respectively.The doping of heteroatoms effectively improved the Faraday effect and the conductivity of carbon materials.Compared with LC materials,MLCM materials showed superior electrochemical performance.In the 1mol/L H₂SO₄ electrolyte system,MLCM showed the highest specific capacitance of 338.2 F/g at a current density of 0.8 A/g.The capacitance retention rate reached 70.2%at a current density of 10 A/g.Furthermore,MLCM was used as a positive and negative electrode material to assemble a symmetrical supercapacitor to test the electrochemical performance of its two electrodes.The device showed a specific capacitance of 194 F/g at a current density of 1A/g.In addition,the device maintained excellent cycle stability after 5000 times of charging and discharging at a current density of 2 A/g,and the capacitance retention rate reached 97.3%.Lignin-based carbon microspheres have good application prospects in the field of energy storage.