Highly Efficient Synthesis of Alkyl Levulinates from alpha-Angelica Lactone, Catalyzed with Lewis Acidic Trifloaluminate Ionic Liquids Supported on Carbon Nanotubes

摘要

Levulinic acid esters (LAEs) were synthesized from alpha-angelica lactone and alcohols, in a reaction catalyzed by a new family of chloride-free Lewis acidic ionic liquids, containing trifloaluminate anions, [Al(POTf)(3+n)](n-). Changing the catalyst from poorly soluble Al(OTf)(3) (used as suspension) to fully homogeneous trifloaluminate ionic liquids resulted in shorter reaction times required for full alpha-AL conversion (60 min at 60 degrees C for 0.1 mol % catalyst loading) and unprecedented selectivities to LAEs, reaching >99%. Supporting the trifloaluminate ionic liquid on multiwalled carbon nanotubes gave an easily recyclable system, with no leaching observed over six cycles. Mechanistic considerations suggest that the propensity of Al(OTf)(3) to undergo very slow hydrolysis results in the correct balance of Bronsted and Lewis acidic sites in the system, which inhibit byproduct formation.