Efficient Lignocellulosic Resources Fractionation And Valorization In Deep Eutectic Solvents

Wood resource is a kind of natural carbon-neutral green sustainable resource from plant,the key to the transformation and utilization of wood resources is effectively breaking through its own "biomass recalcitrance",in recent years,the use of physical,thermochemical,biological and several combination of processing techniques to convert wood resources into high value-added fuels,chemicals and materials are widely studied.However,how to break through the "biomass reclacitrance" of wood resources efficiently under mild conditions and make use of its component is still challenging.Efficient removing of the "biomass recalcitrance" of wood resources under mild conditions and continuing transformation and utilization of the separation components were achieved by screening and regulating the new green deep eutectic solvent.The model compound study reveals the mechanism of bonds cleavage and conversion during wood resources processing.According to the difference of deep eutectic solvent system,the isolated lignin components can be converted into lignin microspheres or aromatic-based chemicals.Cellulose components can be converted to nanocellulose by high-intensity ultrasound or enzyme hydrolysis to high yield monosaccharides.A new strategy for efficient conversion and utilization of wood resources under the green deep eutectic solvent medium is provided.The main research is divided into the following five parts:(1)The preparation,screening and physical chemical performance characterization of deep eutectic solvents,and the selection of suitable deep eutectic solvents to effectively break through the "biomass recalcitrance" of wood resources.The results show that:the deep eutectic solvent of choline chloride/oxalic acid shows a higher hydrogen bond acidity(1.31),which can effectively break the ether bonds in the "biomass recalcitrance" model compound of β-D-phenyl glucopyranoside.At the same time,the deep eutectic solvent of choline chloride/oxalic acid showed good hemicellulose and lignin dissolution effect,but the effect of cellulose dissolution was poor,indicating that the deep eutectic solvent of choline chloride/oxalic acid has the potential to break through the "biomass recalcitrance" of wood raw materials and selective component separation.(2)The "biomass recalcitrance" of poplar wood raw materials is effectively broken by microwave heating choline chloride/oxalic acid deep eutectic solvent,and all the separated components are converted and utilized.The results show that microwave heating can significantly improve the processing efficiency,ultrafast removing of the poplar wood"biomass recalcitrance" can be achieved at 80℃ for 3 min.By comparing with the 2D HSQC NMR of milled wood lignin,it was found that both in the oil bath or microwave heating deep eutectic solvent system,the "biomass recalcitrance" and the aromatic ether bonds in the poplar wood raw materials were both broken,resulting in high purity(more than 90%)and yield(more than 90%)of low molecular weight condensed lignin.The lignin micro-nanosphere can be prepared by simple ethanol/water solvent,nanocellulose can be prepared through a simple ultrasonic treatment of the resulting cellulose residue,part cellulose and hemicellulose components were converted into 5-HMF and sugars dissolved in deep eutectic solvents,whole component of wood material utilization was achieved.(3)Three components of deep eutectic solvent which can effectively inhibit lignin condensation was carried out,and the mechanism of effective suppression of lignin condensation was revealed by the study of lignin model compound in the three components of deep eutectic solvent and the regulation of the reaction pathway.The results showed that the deep eutectic solvent system of three components of choline chloride/oxalic acid/ethylene glycol can well inhibit the condensation of lignin,when oxalic acid is the main hydrogen bond donor in the deep eutectic solvent system,the ether bonds of the lignin model are broken,and ethylene glycol can protect the unstable aldehydes to form acetals.When ethylene glycol is the main hydrogen bond donor in the deep eutectic solvent system,oxalic acid can catalyze the formation of carbon cation intermediates,and ethylene glycol can selectively protect the carbon cation intermediates and obtain a ethylene glycol protected lignin model compound.Different choline chloride/oxalic acid/diols deep eutectic solvents system have a protective effect on lignin model compounds,diols can selectively protect carbon positive ion intermediates,and protection of deep eutectic solvents can also be applied to more complex lignin model compounds.(4)The component separation of wood raw materials was carried out in three components of deep eutectic solvent which can inhibit condensation,and the separation components were selectively converted into high-yielding monosaccharide or aromatic-based chemicals,and the deep eutectic solvent was recovered and reused.The results showed that after the treatment in deep eutectic solvent,protected lignin with high aromatic ether bond content was separated by tetrahydrofuran/water extraction.Ethylene glycol protected lignin was converted to 22%yield aromatic chemicals by Ru/C catalyzed hydrogenolysis or selectively acidolysis to 12.5%yield of acetal-based aromatic chemicals by Fe(OTf)3 acidolysis.Cellulose residue components can be enzymatic hydrolyzed to high yield glucose and xylose.Deep eutectic solvents can be recycled at least twice with high yield,and NMR analysis shows that the recovered deep eutectic solvent was with high purity.(5)A microwave-heated deep eutectic solvent combined with high-intensity ultrasonication to rapidly prepare nanocellulose strategy is proposed.Compared with the traditional method,the use of strong acid is avoided,and the preparation efficiency is greatly improved.The results show that:choline chloride/oxalic acid deep eutectic solvent can be used for the treatment of different cellulosic raw materials to produce cellulose nanocrystals or nanofibers,however,for ultra-high cellulose content with ultrastrong hydrogen bond interaction cotton,only microwave assisted deep eutectic solvent treatment combined with high intensity ultrasonication can produce cellulose nanocrystals.The possible mechanism is:microwave-assisted deep eutectic solvent treatment has a certain degradation effect on the amorphous region of cellulose,while the hydrogen bond competition in deep eutectic solvent system can reduce the strong hydrogen bond interaction between cellulose molecular chains,so after simple ultrasound treatment,high yield cellulose nanocrystals can be obtained.At the same time,the amorphous part of cellulose is converted into the corresponding 5-HMF.