Catalytic conversion of sodium lignosulfonate to vanillin: Engineering aspects. part 1. effects of processing conditions on vanillin yield and selectivity

The commercial production of vanillin from sodium lignosulfonate under highly alkaline conditions, catalyzed by Cu²⁺ at elevated temperature and pressures up to ∼10 bar, has been simulated in a 3-L stirred reactor. Initially, the process was operated in the presence of nitrogen in dead-end mode, and it was shown that vanillin and vanillic acid were formed by hydrolysis at temperatures of 120, 140, and 160 C. At the two higher temperatures, the amount of vanillin produced was the same. Subsequently, experiments were conducted at the same elevated pressures and temperatures with addition of air or oxygen-enriched air once the temperature in the reactor had reached temperatures similar to those used when only hydrolysis occurred. In this case, the concentration of vanillin at 140 and 160 C was equal to that due to hydrolysis, and the subsequent 2-fold increase was due to oxidation. In addition, both vanillic acid and acetovanillone (which has rarely been reported) were produced, as was hydrogen. Thus, for the first time, it has been shown that the production of vanillin (and other compounds) from sodium lignosulfonate at elevated temperatures involves hydrolysis and oxidation, with hydrolysis starting at just above 100 C, that is, much lower than has previously been reported. Approximately 50% is produced by each mechanism. In addition, the orders of the reactions of the different steps were estimated, and the reaction mechanisms are discussed.