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Production of volatile fatty acids (VFAs) by fermentation is a potential sustainable alternative for conventional petrochemical routes to VFAs. Due to the low VFA content of fermentation broths, robust and economical separation technology has to be devised to recover the VFA. Liquid-liquid extraction of VFAs with the phosphonium phosphinate ionic liquid (IL) [P 666,14 ][Phos] allows good VFA extractability. For an extraction process using [P 666,14 ][Phos] to be green, it is essential to efficiently regenerate the solvent and recover the VFA. To obtain insight into the (strong) intermolecular interactions between [P 666,14 ][Phos] and acetic acid, selected as a model VFA, 1 H NMR, 31 P NMR, FT-IR and isothermal titration calorimetry (ITC) were applied. The observations were used to interpret operations to recover acetic acid from the IL, which included evaporation at elevated temperature under vacuum, possibly assisted by nitrogen stripping, in situ esterification and back-extraction with volatile bases. Through evaporative regeneration with nitrogen stripping, HAc could be removed, but only down to an HAc/IL molar ratio of 1. The remaining molar equivalent of HAc-IL interacts tightly with the IL by partial proton transfer and strong hydrogen bonding interactions with the phosphinate anion. Back-extraction of HAc with trimethylamine (TMA) and subsequent decomposition of the HAc-TMA complexes allowed for successful IL regeneration. This process uses ten times less amine (TMA) than conventional amine-based extraction processes (e.g. tri-n-octyl amine), and provides a sustainable process route to obtain pure carboxylic acids from highly diluted aqueous solutions without generating large streams of byproducts. Further valorization via in-line vaporization/catalytic ketonization or via in-line thermal decomposition and ketonization of the TMA-HAc salt was also demonstrated, showing the potential of the VFAs as a green platform for bio-based chemicals.

Extractive distillation is a distillation technique that can be applied for separation of azeotropic mixtures and mixtures of close-boiling compounds. In extractive distillation a solvent is applied to increase the relative volatility by interacting with the components in the system, typically with a difference in affinity for the components. To study the relation between the affinity strength and the effect of the solvent on the relative volatility, the research here presented focused on a solvent selection for the separation of two close-boiling, highly polar binary systems. 1) The acids valeric acid and its isomer 2-methylbutyric acid (ΔTboil = 10°C, ΔpKa = 0.1), and 2) the bases diethyl methylamine and diisopropylether (ΔTboil = 3-6°C, ΔpKb ≈ 14). For both the acids and the bases the binary VLE-data and ternary data with solvents were measured using an ebulliometer. The solvents resulted in negative deviations of Raoult's law. The solvent selection was performed based on literature, followed by an experimental evaluation of the possible solvent types. For the valeric acid mixture the solvent 2-methylbutyrate, which is similar in structure to the components in the mixture, appeared to be most promising. In the ether-amine mixture a series of hydrogen bonding solvents with a hydroxyl group were found most promising, and also the regeneration of the solvent was an important factor in the study.