(224g) Esterification of Dilute Aqueous Carboxylic Acids with Metallosurfactant Type Catalysts

Treatment of effluents with low concentration of valuable constituents is an uprising task for various industries. Pulping serves as representative example, producing high amounts of aqueous effluents with low (hydroxy-) carboxylic acid load. For the low molecular weight carboxylic acids in this aqueous environment, an emulsion based process concept was developed and proven [1]. This process concept combines physical liquid-liquid extraction with chemical conversion of the carboxylic acids. Core of this concept is to provide a quasi-homogeneous regime through emulsification. Induced by the strong surface-active catalyst 4-dodecylbenzenesulfonic acid (4‑DBSA), the emulsification highly enhances the process by boosting the rate of reaction and the mass transfer. Nevertheless, emulsions cannot be processed in standard equipment. This problem is addressed by modification of the catalyst properties to limit the surfactant potential while maintaining the catalytic activity. Combination with various transition metals as described by de la Iglesia et al. [2] was chosen for this modification. A main benefit of modifying 4-DBSA with transition metals is the formation of stable metallosurfactants, nearly insoluble in water.

Various transition metals e.g., nickel and iron were chosen for modification. The catalysts were characterized via IR spectroscopy and elemental analysis. Various stoichiometric ratios of surfactant to metal ion were tested for applicability in enhancing biphasic esterification via interfacial catalysis. Various solvents and modifiers were investigated for their potential of increasing the solubility and catalyst stability. Phase separation behavior and suppression of the surfactant properties was evaluated via interfacial tension measurements of catalyst-containing solvent and water. Concentration and temperature effects on interfacial tension were investigated. A nickel-based metallosurfactant with a stoichiometric ratio of 1:2 (metal to surfactant), dissolved in the solvent 1-octanol performed best. 1‑octanol further serves as a reactant in biphasic esterification. Catalytic performance of the metallosurfactants was tested in a batch setup consisting of a 500 ml three-neck flask, phase ratio (solvent to aqueous phase) of 1, feed concentration of 1 mol l^-1 acetic acid and a reaction temperature of 60°C.

The modification of 4-DBSA with transition metals increased the interfacial tension to a magnitude suitable for gravitational phase separation. The nickel-based metallosurfactant revealed a phase separation behavior equivalent to a surfactant free reference experiment. Although the interfacial tension decreased by 75 % for the maximum catalyst load in pure 1‑octanol, no emulsification was observed. Regarding the catalytic performance, the metallosurfactants yielded 25.7 % of acetic acid conversion and a separation efficiency of over 53%. In contrast, the uncatalyzed conversion is at 4.6 % for the same experimental parameters.

Based on this outcome, the modification of 4-DBSA with transition metals and the application of the thus generated metallosurfactants is a promising approach for interfacial catalysis of biphasic esterification. Emulsification can be prevented, while maintaining catalytic activity and high mass transfer area.

[1] A. Toth, S. Lux, D. Painer, M. Siebenhofer, React. Chem. Eng. 2018, 3 (6), 905 – 911. DOI: 10.1039/C8RE00194D.

[2] P. de La Iglesia, V. W. Jaeger, Y. Xi, J. Pfaendtner, L. D. Pozzo, Langmuir 2015, 31 (33), 9006 – 9016. DOI: 10.1021/acs.langmuir.5b02071.