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Fig. 1 Composition of lignocellulosic biomass and the structural roles of cellulose, hemicellulose, and lignin. 46
Fig. 2 Monolignol structure and positions indicated by blue numbers and letters.
so woods and hardwoods, respectively. As this is the most common linkage, many deligni cation processes target this speci c linkage. For so woods, the 5 – 5 linkage is also impor- tant, and has an abundance of 18 – 25% per 100 C 9 units, while this linkage occurs only around 3 – 9% in hardwoods. 52 In the process of isolating technical lignins, both the labile aryl-alkyl and b -O-4 bonds are most prone to cleavage. 53 This results in technical lignins having more condensed and variable structures than native lignin, and a wide variety in molecular weight ( M w ). Mass average values ( M w ) of 1000 – 15000 gmol − 1 for soda lignin, 1500 – 25000gmol − 1 forKra lignin, and 1000 – 150 000 g mol − 1 for lignosulfonates have been reported, depending on botanical origin and process conditions. 54 Native lignin is a virtually in nite macromer that is both randomly- and poly-branched. 55 The bonds between the lignin and surrounding hemicellulose and cellulose found in LCC have recently been reviewed. 56 All so wood lignin, and 47 – 66% of hardwood lignin, is reportedly bound covalently to carbohy- drates, and mainly to hemicellulose. The most common types of linkages found in LCCs are benzyl ether-, benzyl ester-, ferulate ester-, phenyl glycosidic- and diferulate ester bonds. 57 Note that due to the high degree of variability in inter-unit and LCC
radical cross-coupling reactions which results in the complex, and varied, lignin network. The ratios of the three monolignols in lignin from di ff erent sources can vary quite signi cantly, hardwood lignins contain G- (25 – 50%) and S-units (50 – 70%), so wood lignins contain mostly G-units (80 – 90%), while grass lignin contains mixtures of S- (25 – 50%), G- (25 – 50%) and H- units (10 – 25%). 49 The monolignol composition (H : G : S ratio) can also vary between tissue types in the same organism, which has been illustrated in the cork oak, Quercus suber . Lignin from the xylem (1 : 45 : 55) and phloem (1 : 58 : 41) di ff er less in composition than the two compared to the phellem (cork-part, 2 : 85 : 13). These di ff erences a ff ect the occurrence of speci c interunit linkages, where an increase in S-units lead to an increase in alkyl – aryl ether ( b -O-4) bonds: 68% in cork, 71% in phloem, 77% in xylem. 50 The di ff erence in abundance of the three monolignols lead tomany di ff erent types of interunit linkages in lignin, speci - cally between angiosperm (hardwood and grass) and gymno- sperm(so wood) lignin. 51 The most common interunit linkage is the b -O-4alkyl – aryl ether bond (Fig. 3), which occurs between 45 – 50% or 60 – 62% of phenyl propylene unit (C 9 units) in
RSCAdv. , 2023, 13 , 12529 – 12553 | 12531
© 2023 The Author(s). Published by the Royal Society of Chemistry
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