(224g) Esterification of Dilute Aqueous Carboxylic Acids with Metallosurfactant Type Catalysts
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. 1âoctanol 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 60Â°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 1âoctanol, 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.
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