The Fractionation Of Acid Lignin From Black Liquor And The Lignin Structure Research Of Low Molecular Weight

Research and utilization of biomass energy has become popular research intoday’s world. As a part of the biomass resources, lignin has rich aromatic structurewhich makes it become a good alternative to chemical-based materials and polymers.Lignin is extracted from black liquor and plant materials by researchers,Substitutability of products was developed, greatly reducing the consumption of the oilindustry. It not only produces good economic returns, but has a very large reduction inenergy consumption.However，lignin research based on high-purity and high-valuehas much further to rise.In this thesis, acid precipitation and ultrafiltration method is used to extract ligninfrom black liquor. Under different pH values and different membrane molecular, thequalities of lignin which were intercepted is explored, comparing them. The qualitydissolution law of lignin is studied preliminarily. Analysis the structure of ligninsamples in order to provide theoretical basis of lignin mass production and high valueutilization.Different separation methods have different influences on acid precipitation oflignin dissolution. And the quality of lignin gained by adjusting the pH to thepredetermined value is more than gained by fractionation step by step. Also the qualityof lignin is maximum when the pH reduce to4in the fractionation process. When thepH reduce to2, most of the lignin has been precipitated, then the solution showtransparent state.When the same black liquor is subjected for ultrafiltration, the black liquor passedthe membrane of10K has the highest flux, following black liquor passed themembrane of1K, and the black liquor passed the membrane of5K has the lowestflux. After ultrafiltration, the content of acid soluble lignin in the permeated liquid isslightly higher than original black liquor, but the content of the acid insoluble lignin inthe permeated liquid sharply decline relative to the original black liquor, and the totallignin content is lower than the original black liquor; the content of acid soluble ligninwhich is in interception solution is slightly lower relative to the original black liquor,but the acid insoluble lignin content increases significantly, and the total content oflignin is also somewhat increased. The total quality of the lignin which was extractedby the method combined ultrafiltration with acid precipitation is less than the quality of lignin which was isolated directly by acid precipitation process.Fourier transform infrared spectroscopic is used to analyze of each componentlignin sample, to understand the influences of different separation methods on ligninstructure and to determine the variations of its structure depending on the strength ofthe absorption peak of groups.Hydroxyl groups and carboxyl functional groups in lignin are quantificationallyanalyzed by31P-NMR, finding that the content of syringyl OH has the highestpercentage of all the phenolic hydroxyl group, following by guaiacyl and demethylatedOH, and the proportion of the p-hydroxy-phenyl is the lowest. In the sequentialseparation processes, the content of aliphatic OH, total phenolic hydroxyl and COOHdecrease significantly, showing that lignin reaction has taken place fully in thecontinuous process of reducing the pH, and producing other small molecules to makethe hydroxyl content become smaller. Phenolic hydroxyl group contents of ligninwhich is precipitated from the permeated liquid of ultrafiltration are also different,indicating that the functional group content is also related to the molecular weight ofthe lignin.The molecular weight distributions and changes of lignin which is gained by acidprecipitation and ultrafiltration are characterized by GPC, finding that in the acidprecipitation fractionation process, with the continuous decreasing of pH, weightaverage molecular weight and number average molecular weight of lignin which wasprecipitated decrease continuously, and the dispersion also show a decreasing trend,indicating that acid precipitation process has important implications for the ligninwhich can isolate team points.