GB695301A - Improvements in or relating to manufacture of vanillin from lignin - Google Patents

Abstract

<PICT:0695301/IV (b)/1> Lignin and its derivatives are oxidized in aqueous alkaline solution under pressure at an elevated temperature by passing through the solution a mixed gas in fine bubbles containing air diluted with inert gases, the partial pressure of oxygen being less than 6.6 p.s.i. When using sulphite waste liquors as a source of lignin (preferably after fermentation to remove carbohydrates), these are concentrated to 8-12 per cent solids and 7-12 per cent (e.g. 8 per cent) by weight of caustic soda (or potash) added; a copper catalyst, e.g. 2-4 per cent of the sulphate, may also be included. The operating temperature is up to 185 DEG C., e.g. 140-170 DEG C. The oxygen partial pressure may be up to 20 p.s.i. before the temperature reaches 100 DEG C. and is then reduced progressively as the temperature rises. Preferred conditions are between 0.5 and 6.6 p.s.i. at 100-140 DEG C. and 0.075-2.5 (or better 1.5) p.s.i. at 140-170 DEG C.; thus at 165 DEG C. and 0.5 p.s.i. of oxygen the optimum cooking time is 3 hours. Fig. 1 shows a suitable autoclave 1 for the reaction, air being passed in through pipe 3. The air issues through holes 6 and is mixed by propeller 19 with steam arising from the liquid. The mixed gases are sucked in through 20, pass down the hollow shaft 16 and are forced up through the liquid via the hollow cross-piece 18. In an alternative form, Fig. 4 (not shown), the air and steam are mixed outside the autoclave. The actual partial pressure of oxygen at any instant depends on the temperature of the liquid and the pressure at which the air is supplied; these may be controlled manually or automatically. The successive stages of the process at different temperatures may be conducted in different autoclaves. Examples are given of the process using the apparatus of Fig. 1. The product is mainly vanillin (as sodium salt), together with vanillic acid, guaiacol, acetovanillone and oxalic acid. When using sulphite waste liquor obtained from hardwoods, syringic acid and syringaldehyde are also produced. Specification 607,978 is referred to.