Liquid–Liquid Equilibria of Formic Acid and Furfural in a Biphasic Aqueous–Organic System: Optimization of Solvent and Amine Extractant

Liquid-liquid equilibria of formic acid (FA) and furfural in a biphasic aqueous-organic system were studied. Fourteen kinds of samples with a high boiling point and a large density difference with water were first screened to measure the distribution coefficient of furfural. Eight solvents with a high furfural distribution coefficient were further screened for FA extraction. Six solvents (1,2-dimethoxybenzene, 2,4-dimethyl-1-nitrobenzene, diethyl phthalate, tributyl phosphate, dibutyl phthalate, and dibutyl sebacate) with a low FA distribution coefficient were then selected to measure the mutual solubilities at 298.15 K. Furthermore, two amine extractants (tri-n-octylamine and amine N235) were diluted in six solvents for the extraction of FA and furfural from aqueous solutions to optimize the solvent and extractant. 2,4-Dimethyl-1-nitrobenzene or diethyl phthalate was proven to be more suitable for this system. Effects of the amine extractant concentration in solvents were evaluated to balance the furfural and FA distribution coefficients with a smaller loss of furfural selectivity and significantly increasing FA selectivity, and the thermal decomposition of FA and furfural in selected solvent + amine extractant systems was then studied. In general, 2,4-dimethyl-1-nitrobenzene (15 wt % amine N235) favored the extraction equilibrium of FA with acceptable furfural selectivity loss. Typically, when the mass concentration of amine extractant in the solvent is increased from 0 to 15 wt %, the distribution coefficient for FA increases from 0.027 to 6.676 and the distribution coefficient for furfural decreased from 14.491 to 8.290. The 2,4-dimethyl-1-nitrobenzene-containing product was thermally stable enough at 473.15 K.