Study On The Preparation Of Regenerated Cellulose From Lignocellulose Deconstruction Via Molten Salt Hydrate And The Promotion Of Enzymatic Saccharification

The demand for fossil energy sources is ever increasing in modern society,and the extensive use of petroleum energy has caused a series of severe environmental problems.Under the dual pressure of limited fossil energy and environmental protection,it is of crucial importance to find renewable resources to replace fossil energy sources.Lignocellulose derived from plant photosynthesis has the potential to be sustainably converted into various biofuels or high-value-added chemicals within the concept of biorefinery.However,natural cellulose is a highly crystalline biopolymer with an extensive hydrogen bonding network,which can stabilize its ordered supramolecular structure.This potential is limited by the fact that cellulose is present in nature with a high proportion of the “cellulose I” crystalline allomorphs.Therefore,breaking the hydrogen bond between the cellulose molecular chains and defibrillating lignocellulose is important prerequisites for realizing its efficient conversion and utilization.In recent years,with the increasing awareness of environmental protection,the research of green solvent pretreatment system of cellulose has received attention.In this work,lithium bromide trihydrate(LBTH)was used as the solvent pretreatment system to study the dissolution and non-dissolution pretreatment of cellulose in LBTH.In addition,further use of LBTH for non-dissolving pretreatment of wheat straw.The research will provides a theoretical foundation for the efficient pretreatment of lignocellulose.First,the dissolution and regeneration of cellulose in LBTH were investigated.Microcrystalline cellulose(MCC)was dissolved in LBTH,and then three regeneration solvents of ethanol,water,and dimethylacetamide(DMAc)were used to regenerate cellulose from the cellulose solution,respectively.The structure and properties of different regenerated cellulose were studied.The results showed that the regeneration solvent could affect the restructuring of the cellulose molecular chain during the regeneration process and thus change the supramolecular structure of cellulose.The ethanol was beneficial to the recrystallization of cellulose in the LBTH.The crystal structure of the regenerated cellulose obtained from ethanol,water,and DMAc was changed from cellulose I to cellulose II,poorly crystallized cellulose II,and amorphous structure,respectively.In addition,the crystallinity of regenerated cellulose was reduced.The crystallinity of the regenerated cellulose obtained by ethanol,water,and DMAc regeneration was 70.2%,42.3% and 33.2%,respectively.Regenerated nanocellulose with better dispersion could be obtained through water regeneration,and the regenerated cellulose obtained by ethanol and DMAc regeneration was a cellulose gel with nanostructures.In addition,cellulose regenerated by ethanol had the highest thermal stability,while cellulose regenerated by DMAc had the worst thermal stability.Secondly,LBTH was used for non-dissolving pretreatment of cellulose.MCC was placed in LBTH for non-dissolution pretreatment at room temperature,and the effect of different pretreatment time on the structure and properties of MCC was studied.The results showed that MCC could be quickly decrystallized in LBTH at room temperature.The cellulose pretreated by LBTH could be easily separated,and the cellulose substrate after pretreatment was significantly swelled in water.MCC could be completely deconstructed within 30 min,and its natural cellulose I was transformed into an amorphous structure.Based on the kinetic study of cellulose enzymatic saccharification,it was found that the pretreated cellulose could be efficiently saccharified with low enzyme dosage.After LBTH pretreatment for 30 min,the cellulose substrate was subjected to saccharification with an enzyme dosage of 2.5 mg-protein/g-glucan and substrate concentration of 1%.The enzymatic conversion after 24 h was 98.3%,while the saccharification conversion of raw MCC under the same enzymatic hydrolysis conditions was only 16.7%.In addition,cycling experiments showed that LBTH could be reused at least five times and still maintained excellent decrystallization and enzymatic saccharification effects,and the loss of lithium salt during the cycling experiment was less than 1%.Finally,the non-dissolving pretreatment effect of wheat straw with LBTH was studied.Simultaneously,a two-step pretreatment of wheat straw was conducted for comparison.In the first step,wheat straw was pretreated with dilute acid,and then the non-dissolving pretreatment of the dilute acid-pretreated wheat straw with LBTH was carried out.The results showed that the direct treatment of wheat straw with LBTH could reduce its crystallinity,but the crystal structure of cellulose in wheat straw was not changed.Yet,the microstructure of wheat straw was destroyed after the two-step pretreatment,and its crystal structure was changed from cellulose I to amorphous cellulose.The results of enzymatic saccharification showed that the conversion of enzymatic saccharification of wheat straw pretreated by dilute acid and the followed LBTH was greatly improved.After saccharification at the enzyme dosage of 2.5 mg-protein/g-glucan for 48 h,the conversion of glucan has increased from 12.9 % Increased to 58.6%.In summary,lithium bromide trihydrate molten salt hydrate is a green solvent that can efficiently deconstruct lignocellulose,and has great application potential in the field of the lignocellulose pretreatment.