Preparation of lignin microspheres and digestible substrates by deep eutectic solvent pretreatment using moso bamboo
The current widely used preparation methods for lignin microspheres have some drawbacks including low yield,high cost,and complex procedures.To solve the drawbacks of the conventional preparation methods,this study proposed a polyol-based deep eutectic solvent(DES)to pretreat bamboo under mild condition(110 ℃,1 h).Most reported DES pretreatments applied were at a low solid loading of 5％-10％,which greatly enhanced the processing cost.A high solid bamboo feedstock loading of 20％was innovatively established in this study.After the pretreatment,the lignin removal was 69.7％,and the removed lignin could be recovered by a simple method with a high recovery yield of 97.1％.Scanning electron microscopy(SEM),gel permeation chromatography(GPC),two-dimensional heteronuclear single quantum coherence nuclear magnetic resonance(2D HSQC NMR),X-ray diffraction(XRD)and Zeta-sizer analysis were applied for comprehensive characterization.The results showed that the structure of lignin obtained by this system was intact and existed in the form of microspheres.The particle size of lignin microspheres tailored from 1 102 to 6 187 nm with the enhancement of pretreatment temperature,indicating that the size of the lignin microsphere could be adjusted.After the characterization for the lignin microspheres,the scanning electron microscopy analysis showed that the lignin microspheres recovered by this DES all had a regular sphere-like shape with a smooth surface.The recovered lignin microspheres also had a great dispersity,which was different from the lumpy or flake lignin obtained from previous DES system.The gel permeation chromatography analysis showed that the structure of lignin was intact under this condition,and the particle size of lignin microspheres was negatively correlated with the molecular weight of lignin.2D HSQC NMR result implied that the recovered lignin had an intact lignin structure with less condensed structure and significant grafted diols tails,which might be the main promoter for the lignin microsphere formation process.In addition,under different pretreatment conditions,more than 90％of cellulose was recovered,which possessed significant cracks on the fibers'surface,implying the strong swelling effect of the DES.The degree of polymerization(DP)decreased from 782 to 240.The enhanced fibers'cracks and decreased DP increased the contact between cellulose and cellulase,thus enhancing the contact area between cellulase and substrate,and greatly improving the digestibility of cellulose.Notably,the glucan enzymatic hydrolysis yield could reach nearly 100％,eight times higher than that of the raw bamboo feedstock,suggesting the excellent promotion for the cellulose accessibility.Overall,the pretreatment of bamboo with choline chloride/1,4-butanediol(BDO)/aluminum chloride DES can separate lignin,cellulose,and hemicellulose,and obtain microscope-level spherical lignin,realized one-pot preparation of lignin microspheres,and at the same time co-produced fermentable sugar.
deep eutectic solventmoso bamboodelignificationenzymatic hydrolysislignin microspheres
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