Effect Of Residual Lignin From Acidic Pretreated Woody Biomass On Enzymatic Hydrolysis

In this paper,focusing on the inhibition effect of lignin on enzymatic hydrolysis,the difference of enzymatic hydrolysis between hardwood and softwood in acidic pretreatment was analyzed from the perspective of residual lignin,as well as the physicochemical properties and structural changes of the residual lignin under different acidic pretreatment intensities and the structure-activity relationship of its inhibitory effect on enzymatic hydrolysis were further studied.In addition,an acid-alkali combined pretreatment process based on in-situ modification of lignin was used to effectively alleviate the inhibitory effect of lignin in acidic pretreated softwood on cellulase hydrolysis.Results will give insights towards improvement of enzymatic hydrolysis on acid-pretreated woody biomass.Poplar and larch were subjected to hydrothermal and dilute acid pretreatment respectively.Under the same pretreatment conditions,the enzymatic hydrolysis hydrolysis yield of pretreated larch(4.4-14.0%)was significantly lower than that of pretreated poplar(31.5-50.9%).Two lignin fractions(extractable lignin and residual bulk lignin)from acidic-pretreated poplar and larch were fractionally separated to systematically study their differential effects on enzymatic hydrolysis.The results showed that the inhibitory effect of bulk lignin on enzymatic hydrolysis was significantly higher than that of extractable lignin in two kinds of woody biomass,and both extractable lignin and bulk lignin from acidic-pretreated larch exhibited the stronger inhibition on enzymatic hydrolysis than those from acidic-pretreated poplar.Non-productive binding and physical blocking of residual bulk lignin in acidic-pretreated poplar reduced enzymatic hydrolysis by 21.9%and 25.2%respectively,while two mechanisms of lignin in acidic-pretreated larch reduced enzymatic hydrolysis by 4.5%and 56.1%,respectively.More importantly,the isolated lignin in acidic-pretreated larch had a significantly higher inhibitory effect on enzymatic hydrolysis than that of acidic-pretreated poplar,indicating the non-productive adsorption of lignin was related to the distribution of lignin on the surface of pretreated materials.The chemical structure changes of acidic-pretreatment lignin were studied under different pretreatment intensities,and the results showed that with the increase of pretreatment intensity,the content ofβ-O-4 aryl ether bond of acidic-pretreatment lignin decreased,while the content of total condensed aromatic units and Phenolic OH increased.With the addition of lignin in acidic-pretreated poplar and larch,enzymatic hydrolysis yields of Avicel decreased from 68.4%to 29.9-59.3%and 25.9-41.7%,respectively.Total condensed aromatic units(r~2=0.81/0.92),condensed Phenolic OH(r~2=0.92/0.93),and total Phenolic OH(r~2=0.96/0.86)in residual bulk lignin of acidic-pretreated poplar and larch were negatively correlated with the Avicel 72 h yield of enzymatic hydrolysis.The higher the degree of polycondensation of lignin in hydrothermal/dilute acid pretreatment poplar and larch,the stronger the inhibitory effect on enzymatic hydrolysis.To overcome the recalcitrance of residual lignins in acid-pretreated larch(AL),a combined acid and alkali pretreatment with in-situ lignin modification was developed.The results showed that introducing in-situ lignin modification with 2-naphthol to acid pretreatment improved the enzymatic digestibility of AL by 12.7-14.4%,through suppressing lignin repolymerization.The obviously higher improvement(57.8-88.3%)was achieved by applying alkali post-treatment with poly(ethylene glycol)diglycidyl ether(PEGDE)on AL,mainly due to the function of in-situ lignin modification with PEGDE for reducing enzyme non-productive binding on lignins.More importantly,the synergism of 2-naphthol and PEGDE modification facilitated the enzymatic hydrolysis of AL more significantly.Its beneficial mechanism was explored by investigating the effects of in-situ lignin modification on lignin properties,including extraction yields,functional groups,and enzyme affinity of lignins.