Preparation And Application Of Lignin Nanoparticles And Their Magnetic Composites

With the gradual deterioration of the global environment and the increasing exhaustion of fossil energy,the development and utilization of new renewable and clean energy has become an inevitable trend.Lignin is a natural macromolecular polymer in nature that is second only to cellulose in content,and is the most abundant aromatic compound in nature.However,lignin has a complex molecule structure and broad molecular weight distribution,which greatly restricts the high-value application of lignin.In this study,the homogeneity of lignin and the controllable preparation of lignin nanoparticles（LNPs）were achieved by combining physical and chemical means,and magnetic nanocomposites were prepared using LNPs as carriers and reducing agents in the field of dye catalysis.In this paper,ball milling mechanical pretreatment and ethanol solution fractionation methods were used to reduce the heterogeneity of original lignin,and L-fractions and S-fractions with narrow molecular weight distribution and different hydrophilic group content were obtained.The controllable preparation of LNPs was achieved by solvent shifting methods usingγ-valerolactone as the solvent.Compared with L-fractions,S-fractions had higher molecular weight and lower hydrophilic group content and lower polydispersity index.At the same time,with the decrease of ethanol concentration,the molecular weight of the lignin fractions showed an increasing trend,while the content of hydrophilic groups showed a decreasing trend.LNPs prepared from different lignin fractions exhibited different morphologies,in which LNPs of L-fractions were porous spherical nanoparticles and LNPs of S-fractions were solid spherical nanoparticles.Compared with the original lignin,the particle size distribution of LNPs prepared from L-fractions and S-fractions was narrowed.At the same time,the particle size and Zeta potential of LNPs decreased with the decrease of ethanol concentration,and the particle size distribution（30～120 nm）and Zeta potential（-50.36 m V）of LNPs of the S60 fraction were the lowest.The LNPs prepared from the S60fraction exhibited excellent water dispersion stability and which still had nearly 100%retention after 36 days in water.In addition,the pretreatment of ball milling made the prepared lignin nanoparticles tend to shift to small size.Using lignin nanoparticles as the excellent carrier for magnetic Fe₃O₄ NPs and Pd NPs,the dispersed magnetic Fe₃O₄ NPs were first adsorbed on the surface of LNPs,and then the Pd NPs were loaded on the surface of the LNPs by the in-situ reduction synthesis method,and finally the LNPs@Fe₃O₄@Pd NPs magnetic nanocomposites were obtained.When Fe₃O₄/Pd NPs=0.588,the LNPs@Fe₃O₄@Pd NPs magnetic nanocomposites exhibited excellent catalytic efficiency for the degradation of dyes,which could achieve nearly 100%removal of methylene blue within a reaction time of only 30 s,and its apparent rate constant was k=5.89 min^-1.The introduction of magnetic Fe₃O₄ NPs achieved the improvement of catalytic efficiency,in which the apparent rate constant of LNPs@Fe₃O₄@Pd NPs-3 is 61.6times higher than that of LNPs@Pd NPs.The introduction of magnetic Fe₃O₄ NPs also improved the cycling performance of the magnetic nanocomposites,that is,the LNPs@Fe₃O₄@Pd NPs magnetic nanocomposites can still maintain nearly 100%catalytic degradation efficiency to methylene blue after 10 cycles of recycling.In addition,the LNPs@Fe₃O₄@Pd NPs magnetic nanocomposites also exhibited excellent photothermal conversion performance,that is,the temperature of the filter paper loaded magnetic nanocomposites increased to 116.5°C with the average heating rate of 1.54℃/s when the near-infrared laser was irradiated for 120 s.Based on the study of photothermal conversion performance,the preparation of magnetic nanocomposite liquid marbles realized the controllable catalysis of methylene blue.