Studies on finely divided wood. Part 1. Extraction of lignin with neutral solvents

Previous work on extraction of lignin and cellulose from wood, ground in various ways, and other attempts to isolate 'native lignins' are reviewed. Present opinions about the state of lignin in wood are summarized. Possible methods of disintegration and suitable neutral solvents for extraction of lignin are discussed in connection with experience gathered in the present investigation. It was found that by grinding wood, dispersed in a non-swelling liquid, in a vibrational ball mill it is possible to extract 50per thousand or more of the lignin from spruce ( Picea excelsa ), providing the charge to the mill is small enough. A 'standard method' is presented: Wood is reduced to pass a 20 mesh screen, pre-extracted to remove resins etc., ground 48 hrs. in a Lampen mill (12 g. charge) and 48 hrs. in a vibrational ball mill (6 g. charge), using toluene for dispersion, and extracted with aqueous dioxane. After evaporation of the solvent, the lignin is dissolved in aqueous acetic acid, precipitated into water, dried, dissolved in C 2 H 4 Cl 2 -EtOH (2 + 1 by volume), precipitated into ethyl ether, washed, and dried. A faintly cream-colored, ash-free powder, 'milled wood lignin' (M.W.L.), is obtained. The molecular weight of M.W.L. from spruce is about 11000 (weight average) or about 60 phenyl-propane units. Lignincarbohydrate complexes, extractable from milled wood with dimethylformamide, are slightly soluble in dioxane and contaminate M. W.L., which contains a small amount of sugars. These sugars are the same as in hemicellulose and occur in the same proportions. Determinations of the contents of phenolic hydroxyl groups and ultraviolet light absorption studies indicate similar structures of M.W.L., and lignin derivatives isolated under mild conditions. Determination of p -hydroxy-benzyl alcohol groups gave a value half of that of Brauns' native lignin but double that of wood, and it is concluded that the value of M.W.L. is equal, or close to, that of proto-lignin. It is suggested that M.W.L. is a very useful material for lignin chemists.