Synthesis and application of a deep eutectic solvent as a green catalyst for use in oil esterification: DOE optimization and method greenness analysis

Abstract

Deep eutectic solvents (DESs), as a new generation of green solvents, have attracted the attention of researchers because they meet all the requirements of green chemistry. DESs are made by combining a hydrogen-bond acceptor (HBA) and a hydrogen-bond donor (HBD) in a solid-phase reaction and are liquids at room temperature with melting points lower than those of the individual components. In the current research, new DESs containing tetrabutylammonium bromide as hydrogen-bond acceptor (HBA) and oxalic acid as a hydrogen-bond donor (HBD) in mole ratios of 1 : 1 to 1 : 5 have been synthesized and characterized by FT-IR and NMR spectroscopy. The physicochemical properties and electrochemical behavior of the DES have also been investigated. Cytotoxic studies on 3T3 cell lines showed no toxic effects of the synthesized DES. It showed no inhibition of 5 different microorganisms, confirming its eco-friendly, non-toxic and biocompatible nature. The synthesized DES was used as a green catalyst in place of sulphuric acid for the esterification of used oil. The esterification efficiency was optimized using the design of experiment (DoE) approach by varying the amounts of methanol and DES and the reflux time. An esterification efficiency of 96.3% was achieved at optimum variable values of 8.5 mL methanol, 0.5 g DES and 10 min reflux time at 60 °C. Analysis of variance showed that the amount of DES was the most significant variable. The effects of interaction variables, i.e., DES amount with reflux time and methanol volume with reflux time, were also found to be statistically significant. The greenness indices of the classical esterification reaction and the DES-based esterification were compared using three different greenness methods, namely, the National Environmental Methods Index (NEMI), the Analytical Procedure Index (GAPI) and the Analytical Eco-Scale method. A comparative method greenness analysis of the catalysts reported for the esterification reaction was conducted using the Analytical Eco-Scale method.