From Biobased Pelargonic Acid to Polyol Esters: Zinc Oxide as a Recyclable Catalyst for Controlled Esterification and Kinetic Analysis

Abstract

This work reports zinc(II) oxide as an effective catalyst precursor for the esterification of pelargonic acid with polyalcohols, operating as a homogeneous system coupled to a reactive-distillation setup for the continuous removal of water. The catalytic system was initially evaluated using benchmark polyols, including the diols 1,2-propanediol and 1,2-butanediol, as well as the triol trimethylolpropane. Subsequently, a comprehensive investigation of glycerol esterification was performed to establish how the main operating parameters (temperature, catalyst loading, reactant molar ratio, and gas stripping) affect the final glyceride composition ranges, determined by 1H NMR spectroscopy. At the end of the reaction, the catalyst was recovered predominantly in the form of zinc(II) carboxylates, while under specific conditions, zinc alkoxide species, such as zinc(II) monoglycerolate, were also detected by ATR-FTIR spectroscopy. The residual zinc concentration in the reaction mixtures was quantified by ICP–MS spectrometry, revealing average low levels of metal contamination (3–30 ppm). The recyclability of the recovered catalyst was assessed through a dedicated series of catalytic tests, demonstrating effective reuse over five consecutive reaction cycles. In addition, kinetic data for each esterification step were obtained, providing insights into the reaction pathway. Overall, the proposed system merges the high activity and tunable selectivity typical of homogeneous catalysis with the practical advantages of heterogeneous catalysts, namely, catalyst recovery and reuse. This approach therefore represents a valuable and sustainable strategy for the synthesis of polyol esters with a controlled degree of substitution.