Protic ionic liquids from di- or triamines: even cheaper Brønsted acidic catalysts

Dicationic and tricationic ionic liquids, synthesised by proton transfer from sulfuric acid and corresponding di- and triamines, were characterised and used as Brønsted acidic catalysts in biodiesel production. It has been demonstrated that the new ionic liquids are less expensive than those based on monoamines (even on triethylamine) and are highly acidic. Their performance in a model catalytic esterification reaction surpassed that of sulfuric acid and other literature reports, due to the capability of the ionic liquids to absorb water while phase-separating the ester, thereby shifting the reaction equilibrium towards products. The overall catalytic performance was an interplay between acidity (quantified by Gutmann AN) and physical properties: density and viscosity, which enabled easy phase separation, thereby separating water from the ester. The substrate scope and recycling studies have demonstrated that the new ionic liquids are robust and recyclable catalysts.