Coloration Process in the Sulfonation of Fatty Acid Methyl Ester with Sulfur Trioxide

Abstract

AbstractFatty acid methyl ester sulfonate (MES), an anionic surfactant suitable as a laundry detergent, is produced commercially by the direct sulfonation of fatty acid methyl ester (ME) with SO3. However, intense coloration occurs during the sulfonation process. The sulfonation of ME was performed through the addition of SO3 (SO3/ME molar ratio = 0.8–1.2) for 1–4 h and subsequent aging for 1 h at 80 °C. Even when the amount of unsaturated bonds in ME was reduced to give an iodine value (IV) of less than 0.01, coloration occurred during the sulfonation, making the unbleached product unsuitable for commercial use. The formation of olefinic units and SO2 was observed during the reaction, and the color of the reaction mixture was strongly correlated with their amounts. Further analysis confirmed that equimolar amounts of the olefinic unit and SO2 were formed, and the rate profiles of the formation of MES and SO2 agreed well with each other. From these observations, we concluded that the coloration of the reaction mixture was caused by oxidation of the alkyl chain with SO3 during the liberation of SO3 from the 1:2 adduct of ME and SO3, a key intermediate in the sulfonation process.