PULPING WITH THE HYDROTROPIC SOLUTIONS(II)

RELATION BETWEEN THE CHEMICAL STRUCTURES OF HYDROTROPIC SALTS AND THEIR DELIGNIFICATION EFFECTS

Abstract

For extraction of lignin from plant materials hydrotropic solutions were suggested by R. H. McKee (1946). In this paper pulping effects of nine kinds of hydrotropic salt were compared on beech and spruce wood. The salts employed in this study were as follows: sodium benzenesulfonate, sodium toluenesulfonate, sodium xylenesulfonate, sodium mesitylenesulfonate, sodium ethylbenzensulfonate, sodium butylbenzenesulfonate, sodium cymenesulfonate, sodium butylsulfonate and sodium octylsulfonate (Fig. 1). These were adopted from the consideration of their chemical structures. All the salts used were synthesized. The elemental analyses of the salts proved them to he very pure (Table 1).
Wood flour samples extracted with alcohol-benzene 1:2 were cooked with aqueous solutions of the hydrotropic salts of various concentrations (35, 25 and 15% in weight) at 160°C for 6 hours (Table 2, 3 and 4. Fig. 2 and 3).
Of the nine salts sodium cymenesulfonate, sodium butylbenzenesulfonate, sodium ethylbenzenesulfonate and sodium xylenesulfonate were more effective than the others. The order of delignification effect of the inferior salts were as follows: sodium toluenesulfonate, sodium octylsulfonate, sodium benzenesulfonate and sodium butylsulfonate. The effect of sodium mesitylenesulfonate seemed to be abnormal. Softwood lignin can not be removed with a hydrotropic solvent so easily as hardwood lignin. Sodium cymenesulfonate, however, showed a considerable hydrotropic effect even to spruce lignin.
From the results given above, the following conclusion may be drawn. Of the homologous series of seven derivatives of sodium benzenesulfonate, the hydrotropic effect to lignin increases with increasing in number and in size of alkyl groups attached to the benzene nucleus. The delignification effect of alkylarylsulfonate is larger than that of alkylsulfonate. No difference was recognized practically among the hydrotropic effects of eight salts to carbohydrates.
The authors reported that the dissolution of lignin with a hydrotropic solution may take place in two steps. In the first the lignin is changed by the action of hot water at cooking, and in the second the lignin so altered is dissolved in the hydrotropic solution (J. Japanese Forestry Soc., 36, 343, 1954). In order to confirm this conclusion, a series of cooking experiments was carried out. Namely wood flour samples were cooked by a 35% sodium xylenesulfonate solution from one to three times (at 100°C, 2 hours), with or without water pre-cookings (160°C, 1 hour). The result (Table 5) indicates that the water pre-cooking promotes considerably the effect of hydrotropic solution to lignin. This may offer an evidence to the conclusion in the preceding paper.