Selective enzymatic synthesis of amide surfactants from diethanolamine

Abstract

Kinetic strategies for selective preparation of amides from diethanolamine have been designed using Novozym 435 lipase as a biocatalyst. Two different routes, direct acylation and transacylation, have been optimized. In n-hexane, the reaction is more selective for formation of the amide by direct acylation, while in dioxane, the O-acylation reaction is highly favored. However, the initial rates of direct acylation and transacylation are both higher in dioxane than in n-hexane because of the low solubilities of both diethanolamine and its corresponding ion-pair complex with the fatty acid. At 30 °C, the high viscosities of mixtures of diethanolamine with fatty acids limit the extent of reaction and the corresponding yields of the amide. This effect is greater in n-hexane in which the viscosity (5.52 cSt) was four times greater than dioxane. An increase in temperature to 60 °C increases the conversion and decreases the viscosity of the n-hexane solution to 0.8–0.9 cSt. At 60 °C when equimolar amounts of reactants are employed, the transacylation route produces both higher conversions (71–77 mol%) and greater selectivities to the amide (74–94%) than the direct acylation reaction (69–74 mol% conversion and 76–86% selectivity). For both synthesis routes, the volumetric productivity of the reactor is restricted for reactant concentrations above 0.8 M. At 60 °C, conversions via the direct acylation reaction can be increased to 92 and 80 mol% in dioxane and n-hexane, respectively. These conversions require using a two-fold excess of diethanolamine. The resultant selectivities are 98 and 100%, respectively.