Reactivity of Brönsted acid ionic liquids as dual solvent and catalyst for Fischer esterifications

Abstract

Several water-stable ionic liquids with different acidity and affinity were synthesized and applied as both solvents and acid catalysts for Fischer esterification of ethanol reacting with four aliphatic carboxylic acids (acetic acid, n-hexanoic acid, lauric acid, and stearic acid). Among the studied ionic liquids, [(n-bu-SO 3 H) MIM][HSO 4 ] (3-butyl-1-(butyl-4-sulfonyl) imidazolium sulfate) and [(n-bu-SO 3 H) MIM][p-TSO] (3-butyl-1-(butyl-4-sulfonyl) imidazolium toluenesulfonate) show higher reactivity for the production of ethyl esters. The catalytic activities of these ionic liquids are strongly dependent on the acidity of their anions and cations, as well as their hydrophilicity and affinity with the reactants. Water refluxing through the condenser may be another important reason for obtaining high conversion of esterification, indicating a water-sequester process is still needed in order to obtain a higher yield of ester in the ionic liquid catalyzed esterification system. Kinetics studies show the conversions of the acids increase with reaction temperature and time, and reach equilibrium within about two hours. The apparent activation energies are 39.1±2.0, 49.7± 2.5, 51.4±2.5 and 59.3±3.0 kJ·mol −1 for the formation of ethyl acetate, ethyl n-hexanoate, ethyl laurate and ethyl stearate, respectively.

A New Approach Towards Acid Catalysts with High Reactivity Based on Graphene Nanosheets

“…Second, the side chains of an ionic liquid may also be “soft” and able to form a microzone to fix the substrate. This phenomenon has been described in our earlier work as well as in other published reports,7c, e, f, 10 in which the catalytic activity of an ionic liquid was found to depend not only on the acidity of its acid functional groups but also on its hydrophilicity and its affinity with reactants. However, because they are liquids with high boiling points, the use of ionic liquids as dual solvents and homogeneous catalysts always faces the problems of product separation and solvent recycling.…”

“…Generally, the higher the acid strength and density of the acid sites an acid catalyst has, the higher its reactivity is 7e. f Therefore, to design a catalyst with a higher acid reactivity, the strength and density of an acid are primarily considered. For example, a SO 3 H‐based ion‐exchange resin has a higher acid reactivity than a COOH‐based resin because the acid strength of SO 3 H is “stronger” than that of COOH.…”

Structural Effects on the Physico‐Chemical and Catalytic Properties of Acidic Ionic Liquids: An Overview

“…Unfortunately, the preparation of Brønsted acidic ILs bearing aromatic sulfonic acid groups is not simple; it requires initial treatment of the appropriate base with benzyl chloride, transformation of the resulting quaternary halide into the hydrogen sulfate salt by use of concentrated sulfuric acid, and then sulfonation of the aromatic ring with chlorosulfonic acid in dichloromethane (Scheme 30). Scheme [135] in a comparative study in which several water-stable ILs with different acidities were used for esterifications of four aliphatic acids (acetic, hexanoic, lauric and stearic) with ethanol. Brønsted acidic ILs bearing alkane sulfonic acid groups on acyclic trialkanylammonium cations, however, were also synthesized and used as dual solvent-catalysts for Fisher esterification reactions of acetic acid, metacetonic acid, and benzoic acid with ethanol, butanol, and benzyl alcohol by Liu et al [136] The reactions went to completion smoothly at room temperature, or at 110°C, after 1.5-3.0 h in good yields of 88-95 %.…”

Kinetics of the reaction of ethanol and lauric acid catalyzed by deep eutectic solvent based on benzyltrimethylammonium chloride

“…During past decades, as relatively clean catalysts and excellent solvents, many ionic liquids (ILs) have been employed to replace solid heterogeneous catalysts for esterification due to their properties, such as strong acidity, low vapour pressure, high thermal stability, and reusability . In particular, many ILs with −SO 3 H functional group exhibit adequate catalyst performance and have been used extensively for esterification . For instance, Wei et al have synthesized several ionic liquids and used them as acid catalysts for the esterification of lauric acid and ethanol.…”

“…In particular, many ILs with −SO 3 H functional group exhibit adequate catalyst performance and have been used extensively for esterification . For instance, Wei et al have synthesized several ionic liquids and used them as acid catalysts for the esterification of lauric acid and ethanol. Nevertheless, the large‐scale commercial application of ionic liquids is still challenging, which can be attributed to their poor biodegradability, the requirement of expensive raw materials, and complicated preparation procedures .…”