Comparative Analysis of Kinetic Parameters of Sustainable Branched Esters Obtained from Lauric Acid

Abstract

A comparison between four esterification reaction systems to obtain new sustainable branched esters using Novozym 435 as a biocatalyst, the same acid (lauric acid), and four alcohols with different chain lengths and side chains (2-hexyl-1-decanol, 2-ethyl-1-hexanol, 2-butyl-1-octanol, and 3,7-dimethyl-1-octanol) has been carried out. The parameters of the reaction have been optimized in 0.5 g of biocatalyst, temperature of 70 °C, and the stoichiometric molar ratio (1:1). Under these conditions, conversion values of >90% are obtained in the four reactions. Using a kinetic model developed by the authors and based on a Bisubstrate Ping-Pong mechanism, where internal diffusional limitations are considered, the kinetic parameters for each reaction system were determined and the theoretical conversion values closely matched the experimental results, validating the model for this wide range of substrates. Attending at the conversion values obtained, where both the reaction rate and transport rate are considered, the esterification with 3,7-dimethyl-1-octanol leads to the highest average rate, followed by the reactions with 2-ethyl-1-hexanol, 2-butyl-1-octanol, and, finally, 2-hexyl-1-decanol. In the first two systems, the ones with alcohols of shorter side chain and chain length, respectively, the kcat values are very high (49.526 and 90.13 Mh–1 g–1, respectively) and so is the reaction rate, leading to a high average rate. However, when 3,7-dimethyl-1-octanol is used, the conversion values decrease at long reaction times, due to the high volatility of this alcohol. In the reaction system with 2-butyl-1-octanol, there is mixed control of the reaction and transport stages with higher values of the effectiveness factor (above 0.5 in most cases). Finally, in the reaction with 2-hexyl-1-decanol, the alcohol with the longest chain length and side chain, and the highest molecular weight and viscosity, internal diffusional limitations are very high (with low values of the effectiveness factors as expected, around 0.2 for all conditions tested), and the reaction rate is quite low as well, which explains the low average rates obtained. The obtained branched esters are of interest in the biolubricant sector, and the kinetic parameters calculated in this study can be useful to allow simulation, further optimization, and scale up of the esterification process.