New Boric Acid-based Ternary Deep Eutectic Solvents For Effective Lignocellulose Components Deconstruction Into Its Highvalue Utilization

A novel near-neutral deep eutectic solvent(DES)was developed to depolymerise wheat straw to extract lignin and promote subsequent enzymatic saccharification of the crude cellulose residue.The DES consisted of choline chloride(Ch Cl),boric acid(BA)and polyethylene glycol-200(PEG-200),where BA and PEG-200 were complexed in situ to promote lignin depolymerisation.The results showed that this DES was able to dissolve large amounts of lignin and hemicellulose,and retained cellulose well.In subsequent enzymatic saccharification for a limited time(5 days)and at a low loading of enzyme(30FPU/g),the enzymatic hydrolysis yielded reached 593 mg/g.The removal of lignin was positively correlated with the amount of BA in the DES.2D-HSQC NMR spectroscopy,IR spectroscopy and thermogravimetric analysis showed that the regenerated lignin was a typical G-S-H type,retaining its intact structure(β-O-4,β-β,β-5,etc.),and its carbon chain skeleton was stable.Gel permeation chromatography analysis showed that the extracted lignin had a low molecular weight and a narrow distribution range.Thermal cracking tests indicated that the pyrocarbon content at the endpoint of thermal cracking for D-Lignin was higher than that for both A-Lignin and E-Lignin,and from the thermal cracking products,there were also a greater variety of aromatic compounds from the H-unit for D-Lignin.Superior to A-Lignin and E-Lignin,D-Lignin was more potential to be thermal cracked to biomass oil and converted to aromatic platform chemicals.Quantitative calculations showed that BA and-OH can form hydrogen bonds with the same treatment effect as Cl-,and that BA has more effective sites than Cl~-to break the hydrogen bonding network between lignin and cellulose.Importantly,DES can be used for multiple times without significant decay of depolymerisation properties and its structure had no change after multiple cycles virtually.To further investigate the optimal conditions lignin depolymerisation in wheat straw by DES,a double response value response surface test was designed using the BBD model with temperature,time and liquid to solid ratio(L/S)as the influencing factors.The mathematical model and interaction between the three factors onlignin removal rate and cellulose recovery were obtained,and the optimal conditions for lignin depolymerisation in wheat straw by PBDES was determined.Two highly significant(p<0.01)quadratic polynomial mathematical models(the BBD model and the mathematical model)with correlation coefficients of R1=0.9784 and R2=0.9996 were developed based on the experiments,respectively.The optimum process conditions were determined that the temperature was 125.382°C,the time was 4 h and the liquid-to-solid ratio(L/S)was19.880.Under these conditions,the difference between the predicted and measured values was less than 1%,that can be concluded that the response surface-based optimisation of process parameters for lignin depolymerisation in wheat straw was accurate and reliable,and had a certain reference value.