THE ISOLATION AND OXIDATION OF ASPEN LIGNINS

Abstract

A study has been made of the conditions affecting the isolation of a lignin fraction by a procedure involving a moderate temperature catalytic hydrogenation of pre-extracted aspen wood meal. The effect of varying the initial hydrogen pressure, the catalyst, the time and maximum temperature of reaction, and the nature and acidity of the suspending medium were studied. The weight of the residual pulp, the effectiveness of lignin removal, the weight and methoxyl content of the resulting chloroform-soluble fraction containing the lignin, and the yields of vanillin and syringaldehyde obtained by the alkaline nitrobenzene oxidation of this same fraction were the factors serving as the bases for comparison.Initial hydrogen pressures greater than 500 p.s.i.g. were not required, although below this value there was some loss of efficiency. Neither the use of water or dioxane alone was satisfactory as a suspending medium, but the mixed aqueous-organic medium was required to remove the released lignin. Considering the resultant slight acidity of the reaction product, it is suggested that in such an isolation two separate stages are involved: that of the water functioning to wet and permeate the wood and serve as the acidic medium and that of the organic solvent phase functioning to remove the liberated lignin after the hydrolytic cleavage.Data are presented and interpreted to indicate the existence of a lignin-carbohydrate complex which is cleaved under the conditions of catalytic hydrogenation. Under the conditions of these experiments practical temperatures for lignin removal are in the range 170 – 180 °C. for an aqueous-organic solvent medium, but some thirty degrees lower for an 85% acetic acid medium. Although it has been possible to isolate all the lignin, with good methoxyl recovery, consistently no more than approximately fifty per cent of the required aldehydes is obtainable on oxidation. The increasing value of the syringaldehyde – vanillin ratio with increasing temperature is due to a decreased vanillin yield.Indications are that the use of copper chromium oxide may be preferred over Raney nickel as a catalyst for this isolation procedure.