The samples were prepared as follows to minimize the use of NaOH in the oxidation of lignosulfonate to produce vanillin; the milk of lime was added to the S. P. waste liquor to pH 10.5 and the precipitate formed was removed. The milk of lime was further added to the mother liquor to pH 11.5 and the precipitate was separated, and the centrifuged was used as samples.
Forty grams of the sample (containing 3.0g of lignosulfonic acid, ca. 30g of water and 70g of lime and other substances) was oxidized with various amounts of solid NaOH, in an autoclave with a magnetic stirrer. The effect of NaOH concentration was investigated without catalyst under the initial pressure of 6.5kg/cm2 at 160°C. The highest yield was obtained when the NaOH concentration was approx. 2N. Therefore this concentration of NaOH was used throughout the following experiments. The maximum yield was obtained at 3 hrs. of the reaction. Under the same condition the samples were also oxidized in sodium peroxide solution. In the experiments optimum time was shortened but no higher yields were obtained in sodium peroxide solution.
The rate of air oxidation of vanillin in NaOH solution was then investigated. It was found that the reaction is of the first order, with rate constant 1.42×10-2. The rate constant for the formation of vanillin by the oxidation of lignosulfonate was estimated to be 1.47×10-1 on the assumption that the oxidation reaction of lignosulfonate is of the first order. The catalytic effects by a little addition of some inorganic oxides, salts, and other related organic compounds were also investigated, No higher yields were obtained than that in the case of the addition of nitrobenzene, with which the maximum yield of vanillin was 17.29% of lignosulfonic acid.