Process And Mechanism On The Coproduction Of Xylo-Oligosaccharides,Glucose/Bioethanol And Lignin By Non-Enzymatic Catalysis

Sugarcane bagasse（SCB）is a typical kind of industrial waste,mainly composed of cellulose,hemicellulose,and lignin.SCB is a potential raw material for replacing petroleum resources to produce clean fuel,high value-added chemicals,and functional materials.In this study,the principle of"preferential utilization of hemicellulose by non-enzymatic catalysis"was proposed,and the efficient utilization of SCB has been studied systematically.Firstly,a series of green and efficient non-enzymatic catalysis pretreatments were adopted to produce xylo-oligosaccharide（XOS）,and the optimal production process of XOS with high yield and low DP was explored.Then,the pretreated residue was used for enzymatic hydrolysis to convert glucose or fermentation to produce bioethanol,which could achieve the coproduction of XOS and glucose/bioethanol.Finally,dioxane was used to extract the lignin from the enzymatic hydrolysis residue of SCB and Camellia oleifera shell（COS）,and its chemical structure and composition were comprehensively analyzed.In addition,the cellulolytic enzymatic hydrolysis lignin（CEL）was modified by esterification,and its feasibility in the preparation of bio-based resins was evaluated.In conclusion,this paper was aimed to achieve comprehensive utilization of cellulose,hemicellulose,and lignin through green non-enzymatic catalysis technologies,which could provide a theoretical basis for biomass refining.The main conclusions were as follows:The acetyl-assisted autohydrolysis pretreatment was proposed to coproduce XOS and glucose for the first time.The yield of XOS（DP 2-5）obtained at 200°C-10 min from the acetyl-assisted autohydrolysis（65%SCB+35%WH）was 52.99%.The yields of Xylobiose（X₂）,Xylotriose（X₃）,and Xyloteraose（X₄）increased by approximately 17.87%,2.56%,and 11.01%,respectively,compared to the reaction of SCB alone.Moreover,96.87%glucose was obtained from enzymatic hydrolysis by（acetyl-assisted）autohydrolysis pretreatment.In the process of acetyl-assisted autohydrolysis,the hydrolysis of the acetyl groups in the white birch cloud provide a stronger acidic environment for the system,thereby promoting the degradation of hemicellulose into XOS and reducing the average DP.Meanwhile,this pretreatment could destroy the dense structure of lignocellulosic materials（LCM）,further improve the accessibility of cellulose,and increase the efficiency of enzymatic hydrolysis.Co-catalysis of FeCl₂+MgCl₂ was used for the coproduction of XOS and glucose.In the pretreatment of Fe Cl₂ or Mg Cl₂,the order of xylan degradation ability was influenced as follows:Fe Cl₂>Mg Cl₂>inorganic salt concentration>pretreatment temperature>pretreatment time.Results revealed that 48.14%XOS（DP 2-5）was obtained at the condition of 0.05 M Fe Cl₂+0.05 M Mg Cl₂-140°C-30 min,and the concentration of X₂-X₄ was 7.93 g/L,accounting for 74.7%of XOS（DP 2-5）.In the co-catalysis of Fe Cl₂+Mg Cl₂,Fe Cl₂ played a leading role in xylan degradation,while Mg Cl₂ played assisted to enhance the degradation of high-DP XOS into low-DP XOS.In addition,the glucose yield after Mg Cl₂pretreatment was 71.62%,and Mg²⁺has the ability to adsorb or chelate the solid residues,thereby accelerating enzymatic hydrolysis,while high concentration of Fe²⁺would inhibit the enzyme activity.Moreover,the pretreatment solution rich in XOS could be directly used as prebiotics for the proliferation of Bifidobacterium in vitro,and its maximum concentration of metabolized short-chain fatty acid was9.46 g/L.Non-isothermal subcritical CO₂-assisted seawater autohydrolysis was an efficient method for the coproduction of low-DP XOS and glucose.The maximum yield of XOS was 51.44%with a low xylose concentration（3.09 g/L）at 205°C-5 MPa,and the concentration low-DP（2-4）XOS was 9.10 g/L,accounting for 79.13%of the total XOS.Both ion catalysis of seawater and the chemical action of CO₂forming H₂CO₃ efficiently improved the total XOS yield and reduced the average DP of XOS.The pretreated SCB had loosely structured organization,porosity and high specific surface area,making it easier for cellulase to penetrate and hydrolyze cellulose and achieve the improved glucose production（up to 91.05%）.Additionally,a comprehensive evaluation of three non-enzymatic methods for XOS and glucose coproduction was carried out from various aspects.The effects of pretreatment intensity,fermentation systems and surfactants on simultaneous saccharification and fermentation（SSF）of high lignin content substrates for bioethanol production were investigated.The autohydrolysis pretreatment could improve the ethanol yield（45.82%）to a certain extent,and the ethanol yield was increased with the rise of pretreatment temperature.Besides,the ethanol yields in sodium acetate buffer（SAAB）and sodium citrate buffer（CASCB）system were higher than that in deionized water（DW）system after SSF for 120 h.In SAAB system,the highest ethanol yield was obtained at p H=5.5（66.67%,11.96 g/L）.And the byproducts in SAAB system were lower than that in DW and CASCB system.Moreover,the surfactants could greatly increase ethanol production（up to82.28%）when using high-lignin content LCM as the substrate.It was found that the buffer system could maintain the filter paper enzyme activity（FPA）,while the surfactant could increase the FPA.In a word,the buffer maintained the enzyme activity and yeast growth by keeping the dynamic stability of p H,and the surfactant could reduce the non-productive adsorption of enzymes on residual lignin,thus promoting the SSF process.The effects of different pretreatments on the chemical structure of CEL from SCB and COS before/after pretreatments were comparatively studied,and the CEL was modified by esterification.Results revealed that the pretreatment could significantly improve the CEL yield with the purity of 58.3%and the sugar content was as low as 1.21%under the condition of autohydrolysis（200°C-10 min）.The molecular weight of CEL was decreased with the increase of pretreatment intensity.During the pretreatment process,the degradation of lignin was dominated by the cleavage ofβ-O-4 bond,and the original structural units of lignin was less damaged.Parallelly,the increased phenolic hydroxyl of CEL obtained from the pretreatment had a higher reactivity,which was beneficial to the subsequent modification and utilization.In addition,succinic anhydride was used to esterify the CEL,the content of carboxyl groups in the modified CEL（C-CEL）was significantly increased.Finally,the potential application of C-CEL in UV-resistant and antibacterial bio-based resins was explained through feasibility analysis.