| Lignocellulosic biomass is the most abundant renewable organic carbon resource in nature.After component separation,fractionation and conversion,treated lignocellulose can be converted into various chemicals,fuels and materials with high value.It possesses important significance in replacing traditional non-renewable fossil resources,alleviating the pressure of energy shortage and reducing environmental pollution.Nevertheless,the complex chemical structure greatly limits its value-added utilization.Therefore,pretreatment of lignocellulosic biomass to break the antidegradation barrier and deconstruct the plant cell wall plays a critical role in high-value utilization of lignocellulosic biomass.Based on this,this work developed a new physical-chemical combined pretreatment method,which is capable of separating lignin preferentially,obtaining high removal rate,ensuring high solid yield and polysaccharide retention,and significantly improving the enzymatic hydrolysis of cellulose.Subsequently,the mechanism of cellulase non-productive adsorption onto pretreated lignin was explored,and a series of N and S dual-doped porous carbon nanomaterials were successfully prepared through modification and carbonization of lignosulfonate.Main results of this article are stated as follows:(1)Mechanical pulping-deep eutectic solvent/p-toluenesulfonic acid combined pretreatmentThis work developed mechanical pulping-deep eutectic solvent(DES)/ptoluenesulfonic acid(p-TsOH)combined pretreatment which achieved high delignification rate(52.9-89.9%),high delignification selectivity(1.0-1.34),solid yield and polysaccharide retention rate.Meanwhile,enzymatic hydrolysis of cellulose(52.985.2%)was significantly enhanced.The lignin network structure was broken into lowmolecular weight fragments,and its molecular weight decreased from 8100-8810 g/mol to 3360-3650 g/mol.In addition,cleavage of aryl-ether and ester bonds between lignin substructures released more phenolic hydroxyl groups which increased from 1.8-2.2 mmol/g to 2.6-4.5 mmol/g.(2)Organic solvent fractionation of DES/p-TsOH-extracted ligninFour types of organic solvents(ethyl acetate,ethanol,acetone,1,4-dioxane/water)were used for fractionation of lignin extracted by p-toluenesulfonic acid and deep eutectic solvent in the pretreatment process according to the difference of hydrogen bond strength and solubility between solvents and lignin molecules.Compared with the parent lignin,the polydispersity of each fraction decreased from 2.26-3.31 to 1.35-1.44,respectively.The thermal stability,content of hydroxyl groups and bonding sites of fractionated lignin were different,and the purity of lignin was improved.The improvement of these structural characteristics is conducive to the dissolution and depolymerization of lignin and the high value utilization of lignocellulosic biomass.(3)Preparation of gallic acid(GA)-based DES for pretreatment of hybrid poplarGA-DES was successfully prepared by mixing GA monohydrate with choline chloride(ChCl),the most commonly used as hydrogen bond acceptor.It was found that GA-DES had excellent lignin dissolution and depolymerization performance and the preliminarily optimized molar ratio of ChCl to GA was 2:1.When heated,released H+from GA hydrolyzed aryl-ether bonds,broke LCCs and reconstructed the hydrogen bond network.Formation of hydrogen bonds between GA-lignin hydroxyl groups,and?-? conjugation/hydrophobic interactions between lignin segments and GA aromatic rings inhibited the stacking and repolymerization behavior of separated lignin fragments.After pretreatment,the cellulase hydrolysis rate was significantly improved.Besides,when compared with ChCI:LA(1:2,mol)-DES,the lignin removal rate of GADES treatment was slightly lower than that of LA-DES while the solid yield was higher than that of the former.(4)Mechanism of non-productive cellulase adsorption onto DES-extracted ligninIn the process of DES pretreatment,aromatic rings and side chains of condensed lignin species formed hydrophobic/stacking interaction with non-polar structures of cellulase,inducing a competitive relationship with the cellulose substrate and reducing its hydrolysis rate.The released free phenolic compounds contained a large number of phenolic-OHs,which could induce hydrogen bonds formation with the polar moieties of cellulase,resulting in cellulase non-productive adsorption and inhibition of enzymatic hydrolysis.Although the content of carboxyl groups increased,the electrostatic repulsion force was insufficient to balance the formation of hydrophobic interaction and hydrogen bonds.The final result was that some cellulases adsorbed onto lignin surface,and the number of enzymes involved in hydrolysis process of cellulose decreased.(5)Preparation of N-S co-doped porous carbon using modified lignin derivative as the precursorA series of N-S co-doped porous carbon materials(N-S/C)were successfully prepared through ammoxidation and carbonization of lignosulfonate with H2O2NH3·H2O.Characterization including SEM,TEM and N2 adsorption/desorption results showed the structure of N-S/C is honeycomb-like,and there are a lot of nanopore structures on the surface and inside.XPS results confirmed the successful doping of N and S.The doping amount,structure,graphitization degree,and the oxygen reduction reaction(ORR)performance could be adjusted by controlling the carbonization temperature.The optimized temperature was 700?,meanwhile the content of doped N and the proportion of pyridine N reached the highest.The main existing forms of S were-C-S-C-and-C-SOx-C-(x=2,3,4),and the content of S was lower than that of N,indicating that this method was more suitable for the selective doping of pyridine N.Under alkaline condition,the initial potential of N-S/C was significantly higher than that of untreated lignosulfonate.The limiting diffusion current density(Jlim)of NS/C 700 was better than that of 5%Pt/C and close to that of 20%Pt/C.In addition,the chronoamperometric measurement showed that the N-S/C700 catalyst remained stable for at least 12 h. |
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