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

Abstract

Fatty 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 SO 3 . However, intense coloration occurs during the sulfonation process. The sulfonation of ME was performed through the addition of SO 3 (SO 3 /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 SO 2 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 SO 2 were formed, and the rate profiles of the formation of MES and SO 2 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 SO 3 during the liberation of SO 3 from the 1:2 adduct of ME and SO 3 , a key intermediate in the sulfonation process.Keywords Sulfonation Á Fatty acid methyl ester Á Coloration Á Sulfur trioxide Á Sulfur dioxide Á Olefinic unit Á Bleach Á Palm oil Abbreviations MES Fatty acid methyl ester sulfonate ME Fatty acid methyl ester IV Iodine value Fig. 11 Relation between the rates of formation of MES (black triangles) and SO 2 (black squares). Sulfonation of ME was carried out through addition of SO 3 for 2 h and subsequent aging for 1 h at 80°C with a SO 3 /ME molar ratio of 1.2 Scheme 2 Formation process of olefinic units J Am Oil Chem Soc (2013) 90:903-909 909

Synthesis, Characterization and Exploratory Application of Anionic Surfactant Fatty Acid Methyl Ester Sulfonate from Waste Cooking Oil

“…A bleaching process was therefore needed. 30 % hydrogen peroxide (with 10 mL methanol) was added to the aged product and stirred in a flat flask for 1 h at 60°C to neutralize the reaction mixture [25]. Finally, 30 and 5 wt% NaOH solutions were added drop-wise while stirring to increase the pH of the bleached product up to 6-7 [26].…”