| Lignocellulosic biomass is an important renewable resource,it is an effective way to alleviate the current problems of fossil resource shortage and environmental pollution through biorefining to produce bio-based energy,bulk chemicals and functional materials.As an important matrix polysaccharide directly involved in cell wall construction,hemicellulose seriously affects the anti-biodegradability and separation and conversion efficiency of lignocellulose by regulating the arrangement and supramolecular structure of cellulose microfibrils.Therefore,it is important to explore the deposition pattern of hemicellulose and its influence mechanism on the supramolecular structure of cellulose to decipher the anti-degradation barrier of plant cell wall and realize the comprehensive utilization of whole components of biomass.In this study,the spatial deposition of hemicellulose and its association with cellulose and lignin are firstly revealed by in situ,non-destructive immunofluorescence labeling and laser confocal Raman techniques;secondly,the hemicellulose from different growth years is separated and extracted by alkali method,and the molecular structure evolution of hemicellulose during the growth process is elucidated;finally,the deep eutectic solvents(DES)is used to gradient remove mannans and xylans for investigating the influence of mannans and xylans on the supramolecular structure of cellulose.The main findings are as follows:(1)The spatial deposition of the main components during lignification of Pinus massoniana showed regional selectivity and heterogeneity.In the unlignified cell wall,cellulose was first deposited to form the cell wall skeleton structure,xylan and mannans were distributed in small amounts,while lignin was hardly deposited.During lignification,with the formation of S1 layer of the secondary wall(SW),lignin was first deposited in the cell corner(CC)and the compound middle lamella(CML),and gradually spread to the SW with the thickening of the cell wall.During the formation and lignification of the sublayers of the SW,mannans were gradually deposited and evenly dispersed throughout the SW,indicating that mannans were closely associated with cellulose,and influence the arrangement of cellulose microfibrils and supramolecular structure;while xylans were only present in the lignified cell wall,deposited first in S1 and S3 layers,and then filled in S2 layer,CC and middle lamella(ML),indicating that xylans were closely related to lignin.(2)Using alkali separation-gradient ethanol precipitation method to get hemicellulose from Pinus massoniana at different growth years,it was found that xylose(49.76～52.78%)was mainly in H40 sample,and mannose(43.50～45.24%)was mainly in H60 and H90 samples.The results of molecular structure characterization of hemicellulose showed that,with the increase of growth time,the structure of hemicellulose samples was basically the same,and the relative contents of monosaccharide components was stable.The polydispersity of hemicellulose showed a tendency to gradually increase(especially for H90 sample),indicating that its molecular chain length gradually increases.At the same time,the crystallinity and crystallite size of the cellulose also showed a trend of gradual increase,while the d-spacings remained basically the same,which was beneficial to the orientation of cellulose microfibrils and improved the biodegradability of the wood.It showed that the hemicellulose structure of Pinus massoniana has little effect on the crystal structure of cellulose as the growth year increases,but the cellulose crystallinity and crystallite size gradually increased.(3)Choline chloride/glycol/water system was used for selective gradient removal of mannan from delignified Pinus massoniana(DL).The results showed that 57.01% of mannan could be selectively removed by 10 min of treatment;the cellulose crystallinity increased uniformly with the increase of mannans removal rate,indicating a positive correlation between mannans removal rate and cellulose crystallinity.The crystallite size of(1-10)crystalline planes of cellulose Iβ gradually increased,while the crystallite size and d-spacings of(110)and(200)crystalline planes remained basically the same,indicating that the structure of(1-10)crystalline planes of mannans and cellulose Iβ were closely related.The removal of mannan and the change of the supramolecular structure of cellulose could contribute to the enzymatic hydrolysis efficiency of cellulose up to95.47%.(4)Choline chloride/monoethanolamine system was used for selective gradient removal of xylan from DL materials.The results showed that a large amount of xylan was removed(70.51%)after 180 min of treatment,accompanied by degradation of lignin(75.31%)and arabinose(89.50%),which was mainly due to the existence of a tight and stable LCC structure between xylan and lignin.With the increase of xylan removal rate,the crystallinity of cellulose increased,and when the xylan removal rate exceeded 70%,the crystallinity increased rapidly,and the crystallite size of(1-10),(110)and(200)crystalline planes of cellulose Iβ increased gradually,and the d-spacings remained basically the same.It was found that the correlation between xylan removal rate and cellulose crystallinity was consistent with Boltzmann model.The simultaneous removal of xylan and lignin,synergistic with the change of cellulose supramolecular structure prompted the enzymatic hydrolysis efficiency of cellulose to 100%. |
PDF Full Text Request