CO₂-Switchable Solvents as Entrainer in Fluid Separations

CO2-switchable solvents, typically neutral solvents that switch with CO2 into ionic species, were investigated for use as entrainer in fluid separations such as extractive distillation. Their switchable nature was investigated, which may facilitate liquid–liquid extraction or extractive distillation as ionic liquid (IL), whereas during regeneration their decarboxylation into the amine form prevents temperature shoot-up. Studied elements included a property screening and detailed mechanistic and kinetic studies on the switching of 2-ethylhexylamine and N,N-benzyl methylamine. Decarboxylation of a 50 vol % switchable solvent, 25 vol % heptane, and 25 vol % toluene mixture at 1.00 × 104 Pa showed a 40% CO2 release before reaching the operational pressure, and total decarboxylation took over 2 h. An effective increase of the relative volatility of heptane/toluene was found, showing that, indeed, CO2-switchable solvents can be applied for extractive distillation. However, low pressure in combination with elevated temperature will lead to quick decarboxylation, limiting the operational window of this class of solvents. Their use in low temperature application such as C4-distillations or liquid–liquid extraction appears more suited.