Study On The Bonding Form Of H-type Monomer In Pinus Massoniana Compression Wood Lignin

To reduce environmental pollution and realize the high-value utilization of lignin,people need to further understand the structural composition and corresponding properties of lignin.As one of the three main units constituting the structure of lignin,the p-hydroxyphenyl(H)unit has a vital influence on the formation of lignin.However,the linkage type,reactivity,and existence mode of the H unit in lignin are still unclear and require further research.Studying how the H unit participates in the construction of lignin macromolecules and their linkage with other units will help people understand the formation mechanism and degradation process of lignin.In addition,the H unit has higher activity than the guaiacyl(G)and syringyl(S)units.If lignin is combined with bioengineering and the content of the H unit is increased,this will increase the utilization value of lignin.In this paper,Pinus massoniana compression wood was used as raw material,and opposite wood milled wood lignin(OW-MWL),side wood milled wood lignin(SW-MWL),compression wood milled wood lignin(CW-MWL)were extracted,and normal Pinus massoniana and Sugarcane bagasse were used as raw materials to extract normal wood milled wood lignin(NW-MWL)and Sugarcane bagasse milled wood lignin(SCB-MWL).The ¹H,¹³C,2D-HSQC,and ³¹P NMR techniques were used to analyze all lignin samples to explore the influence of the H unit on the structural composition of lignin and the linkage between the units.To further analyze and identify the linkage type of the H unit in lignin and the corresponding characteristic peak signal,this paper uses coniferyl and p-coumarol alcohol as raw materials to prepare acid dehydrogenation polymers(A-G-DHP,A-H-DHP,A-GH-DHP)and neutral dehydrogenation polymers(N-G-DHP,N-H-DHP,N-GH-DHP).Using ¹H,¹³C,and 2D-HSQC NMR techniques to analyze all dehydrogenated polymers,identify the main linkage type of the H unit and the corresponding peaks,and apply it to the structure study of lignin.The main results were presented as follows:(1)Explore the influence of the H unit on the structural composition of lignin and the way of linkage between units.Five kinds of milled wood lignins(CW-MWL,SW-MWL,OW-MWL,NW-MWL,SCB-MWL)were extracted from Pinus massoniana compression wood,normal Pinus massoniana,and Sugarcane bagasse,and characterized by ¹H,¹³C,2D-HSQC,and ³¹P NMR.The analysis results showed that,compared with SW-MWL,OW-MWL,NW-MWL,and SCB-MWL,CW-MWL contained more H units,and these H units were not present in lignin as special phenolic compounds such as p-coumaric acid.In addition,the characteristic peak signals at 9.20 and 9.34 ppm of the CW-MWL ¹H spectrum and 167 ppm of the ¹³C spectrum might be related to the H unit.2D-HSQC NMR results indicated that more H units might lead to a decrease in the content of?-O-4,?-5,and 5-5-O-4 linkages.³¹P NMR results showed that the H-type phenolic hydroxyl content of CW-MWL was the highest,which was 1.09 mmol/g.Although the content of H-type phenolic hydroxyl groups in SCB-MWL was 1.61 mmol/g,these phenolic hydroxyl groups were mainly provided by the p-coumaric acid structure.(2)Analysis and identification of the main linkage types and characteristic signal of the H unit in acid dehydrogenated polymer and neutral dehydrogenated polymer.In acid dehydrogenation polymers,H units were mainly coupled to each other through?-O-4,?-?,and?-5 linkages,and the content of?-O-4 linkages was much lower than that of?-?and?-5linkages.In neutral dehydrogenation polymers,H units were mainly coupled to each other through?-O-4,?-?,and?-5 linkages.In addition,in acid dehydrogenated polymers and neutral dehydrogenated polymers,the characteristic signal at 9.06-9.55 ppm of the ¹H spectrum was attributed to the phenolic hydroxyl group of the H unit,while the characteristic peaks at 153.92,156.69,and 158.82 ppm of the ¹³C spectrum were attributed to the carbon signal related to the structure of the H unit.(3)In summary,H units were mainly present in lignin as?-O-4,?-O-4,?-?,and?-5linkages.In CW-MWL,the H unit was mainly connected by?-O-4,?-?,and?-5 linkages.The characteristic peaks at 9.20 and 9.34 ppm in the ¹H NMR spectrum were caused by the phenolic hydroxyl group of the H unit,and the characteristic peak at 167 ppm in the ¹³C NMR spectrum was caused by the carbon signal of the H unit structure.