Selective conversion of chitosan to levulinic acid catalysed by acidic ionic liquid: Intriguing NH2 effect in comparison with cellulose

摘要

A highly selective route to produce levulinic acid (LA), one of the top twelve platform chemicals, from chitosan has been provided by catalysis of acidic ionic liquids (ILs). The effect of NH2 group on the conversion efficiency, IL recyclability and catalytic mechanism has been elucidated thoroughly in comparison with cellulose. Dilution effect was displayed significantly at lower chitosan intake, whereby the yield of LA was dramatically improved up to 64%. The relationship between IL structure and LA yield was established, disclosing that acidity of IL dictates the conversion efficiency of chitosan to LA predominantly. Moreover, methyl isobutyl ketone (MIBK) proved to be efficient extractant for the separation of LA product, and IL can be recycled without loss of activity by supplementing sulfuric acid. In addition, the chemical structures and morphologies of the solid residues were investigated by SEM and IR, indicating two-stage mechanism that chitosan first quaternizes and depolymerizes to glucosammonium salt, followed by transformation into LA through NH3 elimination.