Ionic liquid driven esterification of aqueous fermentation products

Short-chain carboxylic acids are appealing targets for resource recovery because they are platform chemicals that can be produced from biomass, bio-waste or CO2 through anaerobic microbial conversion. Side streams that contain these feedstocks can be valorised as renewable carbon sources to added value products instead of being disposed. The microbial conversion and primary recovery through membrane electrolysis results in a clean acid stream with carboxylic acid concentrations below 100 g/L in the current state of the art. A further concentration and separation from this aqueous stream is required to use the acids as building blocks. This is however challenging due to the hydrophilicity, low titres and relatively low value of these compounds, which render conventional separation methods unattractive. The aim of this work is to address this separation bottleneck through biphasic esterification, i.e. a combined extraction and conversion to the higher value esters in a hydrophobic layer. Ionic liquids are appropriate compounds as hydrophobic layer because of their good solvating properties, high stability and their non-volatility, which facilitates separation of the higher volatile esters. The non-volatility results in a substantially reduced energy input compared to distillation, by avoiding energy loss as latent heat of the extractant. Ionic liquids furthermore have the ability to be tuned towards a specific application by combination of the anion and the cation; a compelling feature for extraction of relatively polar compounds into a hydrophobic layer. A proof of concept was successfully delivered in this exploratory study for the model compound acetic acid with a phosphonium ionic liquid, demonstrating it is possible to produce ethyl acetate from 100% renewable resources (biomass/CO2 and bio-ethanol) at reasonable rates and mild reaction conditions (atmospheric pressure, T ≤ 75°C). The development of a low-cost separation method for an integrated pipeline for the transformation of organic side streams to valuable bioproducts can provide competitive edge to the biosector