Structural Elucidation Of Lignin From Fast-growing Eucalyptus/poplar And The Fabrication Of Lignin/PBAT Composites

Lignocellulosic biomass is a renewable carbon-based resource,and the realization of full conversion and utilization of its components is of great practical significance for the effective replacement of petroleum-based energy.As the most abundant renewable aromatic resource,lignin can be converted into high value-added chemicals and bio-based materials.However,lignin has complex structures and high heterogeneity,making it difficult to achieve effective upgrading and transformation.Therefore,in this paper,two typical fast-growing materials of Eucalyptus and Poplar were selected to achieve high-yield separation of native lignin via green and efficient deconstruction methods,and the molecular structure characteristics of lignin in different morphological regions and growth stages were explored.Considering the emerging biorefinery model,the evolution of the lignin molecular structures in biorefinery were monitored,which provided a theoretical reference for the conversion and utilization of lignin.Benefitting from the functional and biodegradable properties of lignin,the targeted chemical modification of lignin was firstly performed,and then eco-friendly functional composites have been fabricated,which would offer a paradigm for the high-value conversion of lignin in the biorefinery model.Inspired by the traditional isolation method of native lignin,a method based on double ball-milling and enzymatic hydrolysis processes to achieve native lignin(DEL)with high yield(94?98%)and structural integrity.Parallelly,MWL and CEL were sequentially isolated from the three Eucalyptus woods to compare the structural characteristics of different lignin.It was found that the different lignin samples revealed the structural characteristics of Eucalyptus lignin from different morphological regions.The prepared DEL presented similar structures to MWL and CEL,contained high?-O-4 linkages and abundant S-type units,and could be used as an ideal native lignin sample for structure characterization.Based on the obtained quantitative structural information,the potential molecular structure models of different Eucalyptus lignins were constructed.Based on the above-mentioned high-yield isolation method of native lignin,DELs were isolated from Poplar wood at different growth stages(3,6,18 months)to explore the dynamic structural changes of lignin.Confocal Raman microscopy and solid-state NMR were used to monitor the microscopic distribution and structural characteristics of components of the Poplar cell wall during the growth.It was confirmed that the lignin macromolecules gradually polymerized with the growth of the Poplar.The revealed dynamic evolution of lignin in the cell wall and the potential molecular structural model provided a visual theoretical reference for the structural transformation of lignin at the early growth stage of Poplar.A mechanochemical method combined dilute acid impregnation and short-time ball milling was developed,which effectively broke the recalcitrance of Eucalyptus wood.The enzymatic hydrolysis of treated substrates could simultaneously acquire highly concentrated fermentable sugars(glucose and xylose)and residual lignin.It was found that mechanochemical pretreatment significantly reduced both degree of polymerization and crystallinity of cellulose within materials and improved the enzymatic hydrolysis efficiency(up to 95%).The residual lignin exhibited high structural integrity and purity,which was expected to be used for catalytic depolymerization of lignin.This study provides a green and efficient feasible way for the conversion and utilization of lignocelluloses.An integrated process based on hydrothermal pretreatment and sulfate delignification was proposed to achieve the decomposition of Eucalyptus and Poplar.DELs isolated from the Eucalyptus and Poplar woods were selected as"lignin model"to explore the structural transformation of lignin during the bleaching processes of H₂O₂,Cl O₂ and O₃.Results revealed that the depolymerization and condensation of lignin occurred during the integrated process.The structures of lignin were significantly changed during the Cl O₂ bleaching,mainly including the degradation of benzene rings,the increase of H-type units and carboxyl groups.And,it was found that hydrothermal pretreatment facilitated the subsequent bleaching process.In order to improve the interfacial compatibility between lignin and poly(butylene adipate-co-terephthalate)(PBAT),hydrothermal lignin was quickly esterified under microwave-assisted solvent-free conditions,which notably changed the physical and chemical properties of lignin,and successfully fabricated the lignin-based(30-50 wt%)biodegradable composites.The mechanical properties of the fabricated modified lignin/PBAT composites are remarkably enhanced,and the elongation at break still increased by 500%even if lignin loading up to 40 wt%,and the composites presented good UV shielding performance.This was mainly ascribed to the ideal dispersibility and compatibility of esterified lignin in the PBAT matrix.Molecular dynamics simulation calculations proved unprecedentedly that the esterification modification of lignin can significantly enhance the molecular interaction of the composite system,thereby improving the compatibility of matrixes and the mechanical properties of the composites.This study proposed a feasible pathway for fabricating biodegradable composites,and the obtained composites possessed turntable mechanical properties and excellent UV-shielding performance,which was promising to be used in packaging fields.