Self-Solidifying Quaternary Phosphonium-Containing Ionic Liquids as Efficient and Reusable Catalysts for Biodiesel Production

Unusual self-solidifying tris(2,4,6-trimethoxyphenyl)phosphonium-containing ionic liquids were synthesized for biodiesel production via the catalytic esterification of a variety of free fatty acids. These ionic liquids were synthesized by a two-step method involving the reaction between tris(2,4,6-trimethoxyphenyl)phosphine and 1,3-propanesulfonate to form a TTMPP-PS zwitterion followed by acidification with different acids. The obtained solid-state ionic liquids were soluble during the reaction to conduct homogeneous catalysis and precipitated after the reaction, which was followed by alcohol evaporation, indicating the great ease of heterogeneous recovery; therefore, the ionic liquids had the combined advantages of both homogeneous and heterogeneous catalysts. As a result, such ionic liquids show great potential for biodiesel production because of their pronounced activity, excellent stability, and recyclability.