Study On The Generation Of Pseudo Lignin And Its Mechanism On The Saccharification Of Cellulose

In this paper,the effects of dilute acid pretreatment on the enzymatic hydrolysis of bamboo residues(bamboo chips)were studied.During dilute acid pretreatment,the existence of pseudo-lignin was found and confirmed.Under dilute acid pretreatment,the intermediate products and structural changes of pseudo-lignin were revealed,and its inhibition on enzymatic hydrolysis was studied.In addition,the effect of lignin degradation products on the formation of pseudo-lignin was studied,and the strategy of inhibiting the formation of pseudo-lignin to improve the enzymatic hydrolysis performance of dilute acid pretreated materials was proposed.The main conclusions are as follows:(1)Relations between moso bamboo surface properties pretreated by kraft cooking and dilute acid with enzymatic digestibility.The delignification of dilute acid pretreated bamboo resiudes was linear with the efficiency of enzymatic hydrolysis(R~2=0.89).The enzyme hydrolysis efficiency of the pretreated materials was increased to 51.7%at 160 ~oC,10%effective alkali concentration and 1 h holding time.After dilute acid pretreatment,most of hemicellulose were removed,and the degree of removal has a good linear relationship with the enzymatic hydrolysis efficiency(R~2=0.99)of pretreated materials.When bamboo residues were pretreated at 160 ~oC and acid concentration of 1.5%for 1 h,the enzymatic hydrolysis efficiency was only25.4%,which indicated that the inhibition of lignin on cellulase was stronger than that of hemicellulose.The surface hydrophobicity of bamboo residues pretreated with dilute sulfuric acid is 0.7-2.0 L/g,which is higher than that of kraft cooked materials.Meanwhile,the accessibility of bamboo residues pretreated with dilute sulfuric acid is 412.6-472.1 mg/g dried substrate,which is lower than that of kraft cooked materials(662.2-1346.2 mg/g dried substrate).It's indicated that the removal of lignin is more conducive to reduce the surface hydrophobicity of the material and improve its accessibility of cellulose,so as to improve the efficiency of enzymatic hydrolysis.In addition,in the process of dilute acid pretreatment,it was found that with the increasing of acid concentration,the lignin content in the recovered bamboo resiudes increased gradually,which is speculated the generation of"pseudo lignin".(2)Effect of dilute acid pretreatment with different carbohydrates on the structure and immediate products of pseudo lignin.To further confirm the existence of pseudo-lignin,bamboo holocellulose was pretreated with dilute acid under different conditions,and the components of pretreated materials were analyzed.Since the temperature of pretreatment increased from 150 ~oC to 190 ~oC,the yield of pseudo-lignin increased from 7.25%to 13.45%,and the number of small spherical particles on the surface of the pre-treated lignocellulose increased gradually.It's indicated that the increasing intensity of pretreatment could intensify the degradation of polysaccharide in the holocellulose and form pseudo-lignin.In adiition,xylose was more likely to form pseudo-lignin pellets than glucose when pretreated under acidic conditions,and the amount of pseudo-lignin tended to be flat with the increase of pretreated time(1-120 min).The analysis of the intermediate products suggested that the key factor of CGPL(pseudo-lignin generated from acid pretreated glucose)and CXPL(pseudo-lignin generated from acid pretreated xylose)is 5-HMF and furfural,respectively.When the pretreatment time increased from 1 to 120min,the the value of D(0.5)of CGPL and CXPL increased from 1.94?m and 18.08?m to 36.31?m and 44.99?m,respectively.(3)The effect of structural difference between pseudo-lignin(CGPL and CXPL)and lignin and their effect on enzymatic hydrolysis efficiency were studied.The results showed that the contents of condensed phenol hydroxyl(1.30 mmol/g),non-condensed phenol hydroxyl(1.74mmol/g)and total hydroxyl(3.05 mmol/g)of GPL were higher than XPL(0.85 mmol/g,0.85mmol/g and 1.70 mmol/g),indicating that the structures of these two kinds of pseudo-lignin were significantly different.Compared with the structure of pseudo-lignin,ball mailed lignin from pretreated bamboo residues(PMWL)has higher hydroxyl content(3.51 mmol/g),slightly lower carboxylic acid content(0.22 mmol/g),stronger hydrophobicity(1665.80 mL/g)and more surface charge(-7.00 mmol/g).The results of enzyme adsorption experiments showed that lignin had a stronger inhibition effect on cellulase than that of pseudo-lignin;QCM-D analysis suggested that the adsorption of XPL on cellulase was weaker,and the inhibition of lignin on cellulase hydrolysis was higher than that of pseudo-lignin.(4)The model compounds(glucose and xylose)and lignin degradation products(syringaldehyde,vanillin,4-hydroxybenzaldehyde,Syringate,vanillic acid,4-hydroxybenzoic acid,ferulic acid and p-coumaric acid)were acid pretreated to study the effect of lignin degradation products on the formation of pseudo-lignin.When 4-hydroxybenzoic acid and p-coumaric acid added in pretreatment,the yield of glucose-based pseudo-lignin increased from21.6%to 22.3%and 22.4%,respectively.While only 4-hydroxybenzaldehyde inhibit the yield of xylose-based pseudo-lignin from 32.2%to 31.9%,indicating that there were differences between the formation paths of two different sources of pseudo-lignin,and the model degradation products of lignin play different role in their formation.Lignin model compounds can change the content of total phenols and intermediate products in pre-hydrolyzate.It was found that the presence of 4-hydroxybenzoic acid can change the aggregation structure and hydroxyl functional group content of pseudo-lignin,and increase the weight average molecular weight of the pseudo-lignin from 2680 and 2710 g/mol to 2950 and 7070 g/mol.Based on the these,it is inferred that 4-hydroxybenzoic acid mainly takes part in the formation of pseudo-lignin by substituting 5-HMF and its derivatives or aromatic ring structure formed by glucose degradation,while in the formation of xylosyl pseudo-lignin,it takes part in the main structure of pseudo-lignin by esterification reaction form.(5)The inhibitory effect of additives on lignin and pseudo-lignin in dilute acid pretreatment.The addition of 2-naphthol and 2-naphthol-7-sulfonic acid in the process of acid pretreatment can reduce the polycondensation of lignin and inhibit the formation of pseudo-lignin.Under the condition of low strength dilute acid pretreatment,2-naphthol-7-sulfonic acid can reduce the formation of microsphere deposit on the surface of pretreated materials.Under the condition of high strength dilute acid pretreatment,a small amount of 2-naphthol(0-4%)can inhibit the formation of pseudolignin,while 2-naphthol-7-sulfonic acid sodium always has an inhibitory effect on the formation of pseudo-lignin,of which the best addition is 4%.