| Lignocellulosic biomass can be bio-transformed into energy and chemicals,substituting non-renewable fossil resources.However,the lignin in raw materials hinders the enzymatic hydrolysis of cellulose through physical barriers and non-productive adsorption.While delignification can enhance cellulose conversion rates,different delignification methods not only have significant cost differences but also markedly vary in their impact on enzymatic hydrolysis efficiency.To investigate the influence of delignification methods on enzymatic hydrolysis,this study employs different delignification approaches based on the chemical composition and fiber structure of lignocellulosic raw materials.The process includes preliminary pretreatment of softwood and hardwood chips using the sulfate method and hydrothermal treatment method respectively.This is followed by an oxidative treatment for additional delignification of the sulfate-treated softwood and organosolv extraction for the hydrothermal-treated hardwood samples.Enzymatic hydrolysis is then performed on these pretreated samples to study the impact of the aforementioned delignification methods on the enzymatic conversion of the samples.The main content and results are as follows:（1）Alkali-oxygen and chlorine dioxide were employed for supplementary delignification of sulfate-pretreated Pinus massoniana chips（KP）samples,and the enzymatic saccharification rates of these two oxidatively delignified specimens were compared.Findings indicate that when the lignin content was comparable（～17%）,the polysaccharide enzymatic conversion rate for the chlorine dioxide delignified sample（KP-D）significantly outperformed that of the alkali-oxygen delignified sample（KP-O）.With an enzyme dosage of 10 FPU/g substrate and an enzymolysis duration of 48 hours,the KP-D sample exhibited an enzymatic monosaccharide yield of 81%and a total sugar extraction rate of 62%.In contrast,the KP-O sample revealed an enzymatic monosaccharide yield of 47%and a total sugar extraction rate of 36%.Upon augmenting the enzyme dosage to 20 FPU/g substrate,the enzymatic monosaccharide yield for the KP-D sample rose to 99%and the total sugar extraction rate reached 76%;concurrently,the enzymatic monosaccharide yield for the KP-O sample was 65%,and the total sugar extraction rate was 51%.（2）The primary factors influencing enzymatic efficiency due to the two different oxidative delignification methods were examined.This study investigated the changes in enzyme adsorption and hydrolysis capacities caused by cellulose component oxidation and alterations in enzyme adsorption ability following lignin component oxidation.It was found that the oxidation of cellulose components slightly enhanced enzyme adsorption and hydrolysis capabilities,with the alkali-oxygen treatment exhibiting a higher degree of improvement than chlorine dioxide treatment.Conversely,the enzyme adsorption capacity for lignin oxidized with chlorine dioxide was lower than that of alkali-oxygen-treated samples.Additionally,a comparison of enzymatic saccharification rates and lignin release patterns during enzymolysis of the two types of oxidatively delignified samples at p H 5.0 and 5.5 revealed a strong correlation between enzymatic saccharification rate and lignin release rate.A positive correlation was also observed between the lignin release rate and the concentration of free proteins in the enzymolysis solution.The analysis suggests that lignin release during enzymolysis facilitates cellulose hydrolysis and promotes enzymatic conversion by increasing the concentration of free proteins in the enzymolysis solution.Examination of lignin distribution on the surface of substrate residues before and after enzymolysis indicated that lignin located on the surface of enzymolysis substrate fibers is preferentially released during the process.（3）To elevate enzymatic conversion rates,an alkaline extraction was performed on oxidatively delignified pine samples,both before enzymatic hydrolysis and intermediately between two phases of hydrolysis.The study reveals that such alkaline extraction procedures consistently ameliorate the enzymatic hydrolysis efficiency.When alkaline extraction precedes enzymatic hydrolysis,the monosaccharide yields for samples KP-O*and KP-D*escalated from the initial figures of 17.0%and 35.4%to 27.4%and 42.7%respectively.Conversely,when implementing alkaline extraction amid two hydrolysis phases,the monosaccharide yields for KP-O*and KP-D*surged to 31.2%and 41.6%,respectively.Nevertheless,upon the reutilization of residual enzymatic activity from the initial hydrolysis phase,the aggregate monosaccharide yields for both KP-O*and KP-D*after two hydrolysis stages receded to 21.9%and 37.1%,respectively.（4）Supplemental delignification of autohydrolyzed Liquidambar samples was conducted using acetone,methanol,and their mixtures,with subsequent comparison of their respective impacts on enzymatic polysaccharide conversion rates.The respective lignin removal rates after extraction with acetone,methanol,and their mixture were 26.9%,30.0%,and 33.2%.However,the enzymatic monosaccharide yield from the organosolv-extracted samples did not improve when the enzyme loading was 3 and 5 FPU/g substra glycan,but rather exhibited a slight decrease compared to the control autohydrolyzed Liquidambar samples.Particularly at an enzyme loading of 5 FPU/g substra glycan,the methanol-extracted samples displayed a marked decline in enzymatic monosaccharide yield from 66.9%in the control group to 57.9%.Assessment of the pore size distribution,cumulative pore volume,and specific surface area of the fiber cell walls pre-and post-organosolv extraction revealed that the extracted samples had reduced cumulative pore volume and specific surface area in relation to smaller pore sizes compared to the unextracted samples,especially within the 40 to 60 nm range.This suggests that the abundance of mesopores in the enzymolysis substrate’s cellulose cell wall plays a crucial role in cellulase diffusion,and a decrease in their number may account for the reduced enzymolysis efficiency.Moreover,to investigate the effect of organosolv-extractable lignin on enzymolysis,it was introduced into a lignin-free cellulose enzymolysis system,where no influence on cellulose enzymolysis was detected.NMR analysis of this subset of organosolv-extractable lignin indicated high phenolic hydroxyl content,low molecular weight,and a reduced content of natural unit bonds.These structural features align with the observed lack of impact on cellulose enzymolysis,suggesting that the decline in enzymatic monosaccharide yield following organosolv delignification is likely due to alterations in the cellulose cell wall structure induced by delignification,rather than the intrinsic properties of the lignin itself. |
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