NMR Characterization Of Alkaline Lignin Structures Based On The Lignin Model Compounds

Lignin is the most abundant aromatic biopolymer on earth.It is a promising material that can be widely used in many fields.Although plenty of technical lignins can be produced from pulp and paper industry,most of which are underutilized.The structure of technical lignin is quite different from that of native lignin,which is reflected in a serious destruction of native linkages and simultaneously formation of new complexed structures during pulping process.However,present understanding on the technical lignin structure is largely dependent on the prediction from the reaction of model compounds and the characterization of many native structures.Still,there is lack of work concentrated on the clarification of new derivatized structures to provide new and deep insights into the technical lignins.This prompted us to perform a series of experiments.In this work,the popular alkaline lignin was chosen as a technical lignin material.The reaction of the model compounds with lignin native structures was performed for drawing the reaction pathways.Besides,the product models were synthetized or isolated as references.On this basis,the new structures and small-molecular-weight component in alkaline lignin were authenticated by 2D HSQC NMR and GC-MS.Firstly,prompted by the reaction of ?-O-4 models during alkaline pulping treatment,we synthetized G/S arylglycerol compounds and different-unit connected enol ether compounds(G-G,S-G,G-S and S-S).The arylglycerol and enol ether structures were assigned by 2D HSQC NMR with the aids of synthetized reference compounds.According to the 2D NMR information,the laws for formation of derivatization of arylglyceorl and enol ether structures were investigated.It was found that the formation and stable existing of arylglycerol depends largely on the etherified form of phenol hydroxyl group(also named “nonphenolic form”).Although the arylglyceorl structures are produced by cleavage non-phenolic ?-aryl ether bonds,they can be destroyed as the most of aryl ether bonds were cleaved to release free phenolic hydroxyl groups.Despite the stable formation of enol ether linkages under alkaline pulping condition,they are easy to degrade as the lignin is separated out from alkaline solution,even stored in air under room temperature.Then,the differences between alkaline lignin and kraft lignin were investigated.Quantitative analysis of the products from ?-O-4 model compounds revealed that the yields of enol ether and arylglycerol products for alkaline pulping treatment were 39% and 40% respectively,much higher than the yields for kraft pulp treatment for which the enol ether yield was 23% and the arylglycerol yield was below 5%.Whereas,the stilbene products produced from ?-5 model compound after alkaline treatment were less than that after kraft treatment.These results were also agreed by the quantitative analysis of alkaline and kraft lignins from softwood and hardwood materials by 2D HSQC NMR.In alkaline lignins,the content of enol ether structure was about 10%,and the content of arylglycerol structure was in range of 8-13%.These are much higher than the enol ether content(1-7%)and arylglycerol content(1-3%)in kraft lignins.Furthermore,the C?-condensed structure in alkaline lignin was authenticated,and the formation and derivatization of this structure were investigated.The product with C?-condensed structure was identified as we performed the reaction of ?-aryl ether compound composed by S units,which prompted us to synthetize three similar compounds with C?-condensed structure to be used as reference compounds,and authenticate this structure in hardwood alkaline lignins.It was found that the C?-condensed structure in hardwood alkaline lignin is in form that S unit(2,6-dimethoxyphenol)replaces the ?-OH of ?-aryl ether side chain at its para-position.This condensed structure mainly exists in the lignin from low-temperature alkaline treatment(145 °C).However,high temperature treatment(170 °C)results in the degradation of C-C bonds and ether bond in this structure,and formation of enol ether,1,1-stilbene and 1,2-stilbene structures.Finally,the lignin monomers and dimers produced from alkaline pulping treatment of hardwood material were reported and identified.It was incidentally found that plenty of small lignin molecules can be produced from pulping treatment of hardwood material during purification of various technical lignins.With the calibration of standard polystyrene,it was found that the small molecules composed by lots of lignin monomers and dimers.Afterwards,the monomers and dimers were authenticated by using 2D HSQC NMR and GC-MS,with aids of synthetized model compounds and the derivatization pathways of lignin native structures concluded from our former work.Totally,10 monomers and 20 dimers were identified from the small molecules.The monomer 4-hydroxy-3,5-dimethoxy-phenylglycerol was firstly identified from alkaline lignin except for other common lignin monomers.20 kinds of dimers with various chemical structures,including biphenyl,diarylmethane,?-1 or ?-5 linked stilbene,?-1 linked diarylethane,(enol ether)-linked arylglycerol and ?-? structures,were all firstly identified in the form of dimer in the hardwood alkaline lignin.This work provides insights into the authentication of new structures in alkaline lignin and identification of small molecules generated from alkaline pulping treatment of the hardwood material.This not only offers a deep fundamental understanding on the depolymerization,derivatization and condensation reactions of lignin during alkaline pulping process,as well as the lignin structural characteristics,but also will accelerate further exploration on some more suitable and efficient approaches for application of this type of technical lignin.