Study Of Deep Eutectic Solvents Pretreatment And Enzymatic Saccharification Process Of Lignocellulosic Biomass

The production of fuel ethanol from lignocellulosic feedstock,based on the idea of enzyme-based biomass refining,not only contributes to the replacement of petrochemical energy but also to the national“double carbon target”,which is a key strategic requirement for the realization of biomass resource utilization.However,the economics of producing fuel ethanol today are dismal.The basic causes of this are that downstream biochemical conversion is constrained by the complicated physicochemical composition and dense multistage structure of biomass.Additionally,the pretreatment substrate has porous particle characteristics,and this can make it difficult for cellulase to mass transfer,which lowers the effectiveness of the substrate’s saccharification process.Therefore,in this study,choline chloride and glycerol-based deep eutectic solvents（ChCl/Gly-DESs）were coordinated with different metal salts to investigate the relationship between their physicochemical properties and the dissolution and structural evolution of biomass components,as well as to reveal the component separation mechanism of DESs pretreatment.Subsequently,the porous particle properties of the pretreated substrate were analyzed,and the intrinsic properties of particle liquefaction during high-solid enzymatic hydrolysis and its influence on mass transfer and enzymatic conversion were elucidated.The following are the key conclusions:（1）The ChCl/Gly-DESs with different metal salt coordination had significant differences in the component selectivity of the biomass pretreatment process,and the Lewis acid/base strength and hydrogen bond interaction of the solvent system were the key factors affecting the fractionation of biomass components.In general,pretreatment efficiency was lower for solvents with fewer acidic sites or active protons（e.g.,ChCl/Gly-KCl and ChCl/Gly-Mg Cl₂）and for solvents with near hydrogen bond donor and acceptor capabilities（e.g.,ChCl/Gly-Zn Cl₂）.In contrast,alkaline ChCl/Gly-K₂CO₃ could effectively remove 72%of the lignin and retain 71%–81%of the cellulosic sugars,with a glucose yield of up to 80.6%of the pretreated substrate.In addition,acidic ChCl/Gly-Fe Cl₃ had 48%delignification and cellulose retention of 75%,with a glucose yield of up to 97.3%of the pretreated substrate.（2）Acidic and basic ChCl/Gly-DESs had completely different component solubility patterns.Among them,the delignification process of basic ChCl/Gly-K₂CO₃ exhibited independent solvent diffusion and surface reaction rate control with heat-absorbing properties(ΔH of 17.1 k J·mol^-1 and surface activation energy of 20.3 k J·mol^-1),and the increase in reaction temperature was more favorable to alleviate the diffusion constraints at the initial stage of pretreatment,resulting in a maximum 7.7-fold increase in reaction rate.However,excessive degradation of hemicellulose during ChCl/Gly-Fe Cl₃ pretreatment(yielding a maximum of 91g·kg^-1 xylose and 4 g·kg^-1 furan derivatives)resulted in the dissolution separation of biomass components that no longer followed independent diffusion/reaction rate control.Based on the structural characterization of the lignin,the structure of the lignin changed to varying degrees depending on the nature of the solvents.In particular,the overall core structure of the basic ChCl/Gly-K₂CO₃ lignin remained intact,while the A_α,A_γ,A_β（S）,G₂ and FA signals of the ChCl/Gly-Fe Cl₃ lignin spectra all disappeared,indicating that lignin underwent more chemical and structural changes in an acidic environment.（3）The porous particle characteristics of lignocellulosic biomass and the distribution of water states around it affected the transfer of enzymes and solutes,and were also associated with enzymatic sugar production.Compared to the feedstock,the average particle size of the ChCl/Gly-K₂CO₃ pretreated biomass particles decreased by approximately 15%（from 359μm to 305μm）,and the total pore volume(4.31 m L·g^-1)and pore area(2.26 m²·g^-1)increased by7.38%and 48.49%,respectively;their pore volume and pore area at the tissue and cellular scales continued to be high at approximately 96%,with the internal pore area being 1063 times greater than the surface area of the external particles.On the other hand,at 10 wt.%and 15 wt.%solid loadings,the average particle size of the substrate rapidly decreased from 305μm to 101μm and 140μm within 3 h of hydrolysis,respectively,and remained stable thereafter（～150μm）;the porosity of the particles increased to～85%after 12 h of hydrolysis,transforming them from large surface pores to internal micropores.Thus,the reduction in particle size of biomass particles in the sub-millimeter range was not the main reason for the increase in enzymatic hydrolysis yield;the enzymatic hydrolysis performance was more dependent on the pore area inside the biomass particles.Furthermore,the main transfer carriers during enzymatic hydrolysis were free water and capillary water,with the latter contributing most to mass transfer during the initial stages of high-solid hydrolysis.With the degradation of the holocellulose component,liquefaction of the biomass particles took place prior to sugar production.At solids loadings of no more than 15 wt.%,the free water content of the slurry could reach approximately 90%after 0.5–1 h of hydrolysis.Further,compared to the biomass particle structure,the increase in free water activity with high availability had a strong positive correlation with the enzymatic hydrolysis yield.Finally,inspired by the rapid liquefaction of particles,this study proposed a short interval fed batch strategy to reconstruct the high-solid enzymatic hydrolysis process,which resulted in a comparable or even higher sugar yield.