Abstract: A series of acidic ionic liquids, including N-methyl-2-pyrrolidonium dihydrogen phosphate ([NMPH]H₂PO₄), caprolactam dihydrogen phosphate ([NHCH]H₂PO₄), N,N′-dimethylformamide dihydrogen phosphate ([DMFH]H₂PO₄), and N,N′-dimethylacetamide dihydrogen phosphate ([DMEH]H₂PO₄), were synthesized and characterized. Oxa-Michael addition of β-phenylethanol to methyl vinyl ketone (MVK) was used as a model reaction. The effects of cation structure, ionic liquid amount, β-phenylethanol/MVK ratio, reaction temperature, and reaction time were measured. Under the optimized conditions of n(β-phenylethanol):n(MVK) = 1:2, 25 °C, 24 h and using [NMPH]H₂PO₄ as catalyst, the β-phenylethanol conversion was 95%. The ionic liquid was stable and could be reused at least 5 times with a slight loss of activity. All the amide-based acidic ionic liquids gave higher conversion compared with imidazolium acidic ionic liquid, which showed that the effect of cation structure was important. A possible reaction mechanism was proposed and the evidence for the role of amide-based acidic ionic liquids in Oxa-Michael addition was given.

Key words: amide-based acidic ionic liquid, catalysis, Oxa-Michael addition, β-phenylethanol, methyl vinyl ketone