Fractionation Of Lignocellulose Into Non-Condensed Lignin And Carbohydrates With Aryl Sulfonic Acid And Alcohol

The high-value utilization of lignocellulosic biomass consisting of lignin,cellulose and hemicellulose is one of the effective ways to facilitate the implementation of the"dual carbon"strategy.Breaking the dense structure of lignocellulose with efficient pretreatment methods is the precondition of full-component conversion and utilization of lignocellulose.However,the conventional fractionation methods generally involve harsh reaction conditions such as high temperature and high acid concentration,which facilitates the formation of stable C-C bonds and results in a condensed lignin structure which enhances the difficulties in depolymerization.It also leads to the degradation of cellulose and hemicellulose,which limits the comprehensive utilization of lignocellulosic biomass.To address this issue,this work focus on developing a mild fractionation method to achieve high-efficiency separation of non-condensed lignin and carbohydrates（hemicellulose and cellulose）.P-toluenesulfonic acid was used to break the lignin-carbohydrate complex bonds（LCC bonds）among lignin and carbohydrates,so as to enhance the lignin extraction efficiency.The critical issues which affect lignin condensation were identified by studying the physicochemical properties of the separated lignin.Furthermore,microwave irradiation was used in selective cleavage of LCC bonds based on its nonthermal mechanism,and the alcohol-involved alkoxylation was beneficial for aryl ether bonds preservation and condensation inhibition,which addressed the difficulty in limited extraction efficiency of non-condensed lignin.Based on those results,the effect of different substituted groups on the benzene ring of aryl sulfonic acid on lignocellulose separation efficiency was studied,and the synergistic mechanism of aryl sulfonic acid and alcohol solvent to promote lignin dissolution was discussed in detail combining with molecular simulation calculation,which provided theoretical and technical support for the development of efficient fractionation method of lignocellulosic components.Firstly,using poplar wood as lignocellulosic material and ethanol as solvent,the separation ability and action mechanism of different acid catalysts（hydrochloric acid,sulfuric acid,formic acid,and p-toluenesulfonic acid）were evaluated.The results showed that under the same acid concentration（1 mol/L）,the extraction efficiency of lignin was the highest（47.15%）with p-toluenesulfonic acid,and the yield of hemicellulose conversion and cellulose retention was89.73%and 97.88%,respectively.The calculations of Molecular Dynamics and Density Functional Theory（DFT）demonstrated that the hydrogen bonds between p-toluenesulfonic acid and the lignin-carbohydrate complex were responsible for the efficient cleavage of LCC bond.Theπ-πstacking interaction between benzene rings on lignin and p-toluenesulfonic acid was beneficial to the dissolution of lignin in ethanol.The influence of key control factors on the condensation structure of lignin during p-toluenesulfonic/ethanol pretreatment was investigated.The cleavage of LCC bonds and extraction efficiency of lignin were enhanced with the increase of acid concentration.However,benzyl carbocation was easier to form at high acid concentration,resulting in a condensed lignin structure,therefore,the extraction efficiency of non-condensed lignin needs to be further improved.Secondly,the effects of different heating methods（microwave heating and conventional heating）and alcohol solvents（methanol,ethanol,and isopropanol）on the fractionation of poplar into non-condensed lignin were investigated.It was found that methanol with higher polarity showed excellent thermal response under microwave irradiation to promote the separation of lignin and carbohydrate.At the same time,methanol with higher reactivity attacked the C_αsite of lignin as a nucleophile to form a methoxyl group,which occupied the carbocation sites to prevent the formation of the C-C bonds.The results of the kinetic study showed that microwave heating can significantly reduce the activation energy of LCC bond decomposition compared with conventional heating,demonstrating the involvement of microwave irradiation could not only produce a thermal effect but also reduce the energy barrier of LCC linkages cracking via nonthermal effect,and thereby facilitated lignin exfoliation from carbohydrates.In this way,cellulose,hemicellulose,and non-condensed lignin can be separated efficiently at low temperature and acid concentration.Furthermore,a fractionation method using p-toluenesulfonic acid in methanol under microwave irradiation was optimized.Under the optimal reaction conditions（p-toluenesulfonic acid concentration of 0.5 mol/L,reaction temperature of 85℃,reaction time of 30 min,and microwave power of 1000 W）,the extraction efficiency of lignin reached 87.91%,the yield of hydrogenolysis aromatic monomer from isolated lignin reached 41.99%.Besides,94.29%of cellulose was preserved after fractionation,which can be efficiently converted into glucose with a yield of 98.47%.The conversion of hemicellulose was 63.65%,and the remaining hemicellulose（36.35%）was retained in the solid part in the form of xylan.The results of lignin model compounds and DFT theory calculations showed that p-toluenesulfonic acid combined with methanol could selectively break LCC bonds as well as preserveβ-O-4 bonds in lignin under microwave irradiation.From the thermodynamic point,it proved that the C_α-methoxylated lignin was more difficult to shed the methoxy group and form benzyl carbocation intermediates,which ensured the non-condensed structure of lignin.Combined with the applicability of different lignocellulosic biomass,the microwave-assisted p-toluenesulfonic acid/methanol pretreatment system was an effective way to obtain high yield of non-condensed lignin and carbohydrates.Based on those results,the effects of different aryl sulfonic acid（p-toluenesulfonic acid,p-hydroxybenzenesulfonic acid,benzenesulfonic acid and 4-chlorobenzene sulfonic acid）with methanol solvent on the fractionation of lignocellulosic biomass were studied.It was found that the dissolution of lignin in aryl sulfonic acid/methanol system was a weak interaction transformation process.The formation of hydrogen bonds among the lignin,aryl sulfonic acid and methanol,as well asπ-πstacking interaction between benzene rings on aryl sulfonic acid and lignin,which weakened the intermolecular and intramolecular hydrogen bonds of lignin and thus promoted the separation and dissolution of non-condensed lignin.As the substituent on the benzene ring of aryl sulfonic acid changed from the electron-donating group（-CH₃ and-OH）to non-substituent group and electron-absorbing group（-Cl）,the electron cloud density on sulfonic acid group and benzene ring decreased,which weakened the constraint to H⁺and thus enhanced the ion conduction rate,leading to cellulose leaching.The hydroxyl substituent on benzene ring of p-hydroxybenzenesulfonic acid could form stronger hydrogen bonding andπ-πstacking interaction with the hydroxyl groups and benzene ring of lignin,respectively,which largely improved the extraction efficiency of non-condensed lignin（84.64%）and hemicellulose conversion（80.65%）at a relatively low acid concentration（0.25 mol/L）.Besides,p-hydroxybenzenesulfonic acid had a moderate conduction ability of H⁺without causing excessive damage to cellulose,obtaining the yield of cellulose retention over 90%,showing potential applications in lignocellulosic fractionation.