An Effective Heterogeneous Catalyst of [BMIM]₃ PMo₁₂ O₄₀ for Selective Sugar Epimerization

The development of heterogeneous catalysts for the epimerization of sugars has received much less attention than that for the isomerization of sugars. To date, molybdates are the most effective catalysts for the epimerization of sugars, although they lack stability toward hydrolysis of their active sites in water. To solve the issue of the formation of a highly water-soluble heteropolyblue (PMo_red ) for phosphomolybdates (PMos) in aqueous reaction systems, herein, a 1-butyl-3-methylimidazolium phosphomolybdate ([BMIM]₃ PMo₁₂ O₄₀ ) was synthesized through an ion-exchange method. This catalyst was effective and selective for the C2-epimerization of sugars under mild reaction conditions (<100 °C; 1-2 h) with good water-tolerant properties. The reaction was confirmed to occur in a heterogeneous manner and no leaching of PMo_red was detected by means of UV/Vis spectroscopy. Moreover, the catalyst can be simply separated by filtration and reused for at least eight cycles without a drop in catalytic activity. XRD, FTIR, and X-ray photoelectron spectroscopy measurements indicate that the catalyst is stable under the reaction conditions. In a comparison of the catalytic activity and surface wettability with those of other PMo salts, that is, 1-ethyl-3-methylimidazolium phosphomolybdate ([EMIM]₃ PMo₁₂ O₄₀ ), 1-hexyl-3-methylimidazolium ([HexMIM]₃ PMo₁₂ O₄₀ ), [choline]₃ PMo₁₂ O₄₀ , and cetyltrimethylammonium phosphomolybdate ([CTA]₃ PMo₁₂ O₄₀ ), it is found that [BMIM]₃ PMo₁₂ O₄₀ has more appropriate hydrophobic-hydrophilic balance, which should be responsible for better catalytic activity and stability.