Electrochemical Degradation And Modification Of Softwood Kraft Lignin

With the depletion of petrochemical resources,more and more researchers have focused their attention on renewable resources.Lignin is the second largest renewable biomass resource after cellulose,and its high value application has received extensive attention.However,due to its difficulty in modification and utilization,it is burned after concentration in the form of black liquor,resulting in great environmental pollution and waste of resources.Separating lignin from black liquor and decomposing it into small-molecule aromatic compounds by electrochemical methods,producing a series of fine chemical products,not only can realize the efficient utilization of lignin,but also have a positive contribution on environmental protection.In this thesis,softwood kraft lignin was used as raw material,and was degraded by electrochemical catalytic oxidation.The degradation products were separated and purified,and conducted to further qualitative and quantitative analysis in order to explore the effect of different conditions on the yield of small molecule substances.The crosslinked film material was prepared from the degraded lignin and polyvinyl alcohol,and the effect of electrochemical modification on the properties of lignin/polyvinyl alcohol film was investigated.In the morphology analysis and electrochemical performance test of the electrode materials,the following rules were found:In KOH solution the order of oxygen evolution potential of the three electrodes is:Ti/Pt>Ti/RuO2-IrO2>Ti/Ta2O5-IrO2;the order of impedance is:Ti/Ta2O5-IrO2<Ti/RuO2-IrO2<Ti/Pt.The order of oxygen evolution potential of the three electrodes in[BMIM]Cl solution is:Ti/Ta2O5-IrO2>Ti/RuO2-IrO2>Ti/Pt;the order of impedance is:Ti/Ta2O5-IrO2<Ti/Pt<Ti/RuO2-IrO2.Considering the working efficiency and energy consumption of the electrode,in the electrochemical degradation system of lignin,the electrode is selected as follows:In KOH solution medium,a Ti/RuO2-IrO2 electrode was used;in the[BMIM]Cl solution medium,a Ti/Ta2O5-IrO2 electrode was used.Ti/RuO2-IrO2 was used as the anode and stainless steel was used as the cathode.Under KOH homogeneous alkaline system,the lignin was electrochemically degraded,and the degradation products were separated and purified,and then qualitatively and quantitatively analyzed by Gas chromatography-mass spectrometry(GC-MS).The result shows that after degradation,lignin will produce small molecules such as aromatic ketones,aromatic aldehydes,aromatic acids,benzenes and so on.The optimum process conditions for electrochemical oxidation of lignin under this system are:lignin concentration 40 g/L,current density 60 mA/cm~2,reaction temperature 80℃,reaction time 3 h.Under the optimum conditions,the total yield of lignin degradation products was 281.589(g/kg lignin),wherein the yield of aromatic aldehydes was 34.421(g/kg lignin),and the yield of aromatic acids was 239.128(g/kg lignin),the yield of benzene and phenol is 8.040(g/kg lignin).It can be seen from Gel-permeation chromatography(GPC)analysis that in the electrochemical catalytic oxidation process,not only the degradation of lignin macromolecules occurs,but also the degraded small molecule fragments are further polymerized.Ti/Ta2O5-IrO2 was used as the anode and stainless steel was used as the cathode.The lignin was electrochemically degraded in[BMIM]Cl system.After the degradation products were separated and purified,the qualitative and quantitative analysis was carried out by GC-MS.Studies have found that the degradation products of lignin are the same as those of electrochemical degradation in KOH system.And various degradation conditions have different effects on the type of small molecules.The optimal conditions for electrochemical catalytic oxidative degradation of lignin under[BMIM]Cl system were obtained by single factor experiment,that is the mass fraction of[BMIM]Cl is 60%,the mass ratio of lignin to[BMIM]Cl was 1:6,current density50 mA/cm~2,pH 5,50℃,and 4 h.Under the optimal conditions,the lignin was electrochemically degraded,and result shows that the electrochemical degradation of lignin in[BMIM]Cl system is a first-order reaction,and its kinetic equation is ln C=-0.0963t+10.0939 R~2=0.9984.The composite films were prepared by cross-linking of kraft lignin,the degraded lignin with polyvinyl alcohol,respectively.And the tensile properties,optical properties,gas permeability,hydrophilicity,surface morphology,thermostability and dynamic thermomechanical properties of the composite film were characterized.The results show that in the lignin/polyvinyl alcohol composite film,it is not just a simple physical blending of lignin and PVA,but under the crosslinking of formaldehyde,a chemical reaction occurs to form new ether bonds.The investigation also found that the electrochemically modified lignin has higher compatibility with PVA molecules in the composite membrane,it makes the tensile strength of the composite membrane decreases,however,the elongation at break and the water resistance are increases.