Phosphoric Acid-ultrasonic Separation Eucalyptus Lignin And Characterization Its Structure

In this article,phosphoric acid-ultrasonic method is used to pretreat eucalyptus raw materials,and lignin?EL1 and EL2?are extracted from the pretreatment liquid and pretreatment residues,respectively.Through the analysis and characterization of raw materials and pretreatment residue,the composition,structure and morphology changes of samples before and after pretreatment were investigated.Using fourier transformed infrared?FT-IR?,nuclear magnetic resonance?NMR?and other characterization methods,the chemical structure of EL1 and EL2 is compared with the chemical structure of eucalyptus milled wood lignin?EMWL?,the changes of lignin chemical structure during phosphoric acid-ultrasonic pretreatment were discussed.The main research results are as follows:The eucalyptus lignin is separated by phosphoric acid-ultrasonic pretreatment,the purity of EL2 is 88.25%,and the yield is 31.45%.The results of FT-IR,X-ray diffraction?XRD?and scanning electron microscopy?SEM?showed that the crystalline form of cellulose changed from cellulose?structure to cellulose?structure,and the crystallinity increased from 53.8%to 62.3%after pretreatment,indicating that phosphoric acid-ultrasonic pretreatment can effectively degrade part of hemicellulose and lignin.According to the analysis of molecular weight results,the number average molecular weight of EL2 is 2804 g·mol^-1,the weight average molecular weight is10894 g·mol^-1,and its polydispersity index is 3.885;the number average molecular weight of sulfate lignin?EKL?is 1995 g·mol^-1,weight average molecular weight is 6193 g·mol^-1,polydispersity index is 3.104;EMWL's number average molecular weight is 3698 g·mol^-1,weight average molecular weight is20627 g·mol^-1,polydispersity index is 5.578.It shows that this method can obtain lignin with larger molecular weight.The chemical structures of EL1 and EL2 were characterized by ¹³C NMR,2D HSQC and ¹H NMR,and compared with EMWL.The results showed that the fracture?-O-4?was the main reason for the degradation of lignin macromolecules,and spironidone was unstable and easy to degrade.The quantitative ¹³C NMR analysis of lignin showed that the content of type G and type S spirodienone in EMWL was 0.07/Ar,which was higher than that reported previously.Calculated by quantitative 2D HSQC,the?-O-4?content of EMWL and EL2 are 55/100Ar and 22.8/100Ar,respectively.Compared with EMWL,the content of?-O-4?of EL2 is significantly reduced.The content of?-?and?-5 structures is not much different from the content of EMWL,indicating that in the process of phosphoric acid-ultrasonic pretreatment,the main degradation is?-O-4?structure,while?-?and?-5 The carbon-carbon structure is relatively stable and not easily degraded.¹H NMR and ³¹P NMR were used to quantitatively calculate the content of phenolic hydroxyl groups in lignin.The quantitative results of ¹H NMR were:29.9%of phenolic hydroxyl groups in EMWL and 53.3%of phenolic hydroxyl groups in EL2;quantification of ³¹P NMR the results are:the phenolic hydroxyl content in EMWL is 28.17%,and the phenolic hydroxyl content in EL2 is 53.74%.Compared with EMWL,the increased phenolic hydroxyl content of EL2 is mainly due to the cleavage of?-O-4?.