An enhanced nonpolarity effect of silica-supported perfluoroalkyl sulfonylimide on catalytic fructose dehydration

Abstract

–SO₂NHSO₂C₄F₉/CF₃-functionalized silica was prepared. The preparation experiments introduced different amounts of –CH₃ moieties to regulate the surface properties. Introduction of more –CH₃ in the –SO₂NHSO₂C₄F₉ series brought about the materials' macroscopic hydrophobicity as expected, shown by the increased contact angle; but it was not so in the –SO₂NHSO₂CF₃ series, which kept their hydrophilicity. In the application of fructose dehydration, the yields of HMF catalysed by the –C₄F₉-series catalysts were 10% higher than those by the –CF₃-series catalysts at the same level of –CH₃ introduced in 20% aqueous ethanol. This was due to their macroscopic hydrophobicity. However, the maximum amount of –CH₃ introduced in both the –C₄F₉– and –CF₃-series catalysts caused ca. 6% higher HMF yields, suggesting that the –CH₃ moiety always promoted the dehydration. It was proposed that –CH₃ changed the surface properties, even in the hydrophilic catalysts. This was confirmed by fluorescence spectrometry with pyrene as a probe molecule. An obvious enhancement of the surface nonpolarity in the introduction of a high amount of –CH₃ was shown by the band III/I ratio of the spectra (0.60–0.83). The improved nonpolarity helped to repel water, both for the –C₄F₉– and –CF₃-series catalysts. In particular, for the –C₄F₉ series, their water-repellent ability was strengthened, and the –C₄F₉ group preferably repelled the produced water away from the adjacent NH site. This could weaken HMF rehydration at the acid site. The regulation effect of –CH₃ along with the enhancement caused by –C₄F₉ thus raised the total yield of HMF to ca. 15%. The enhanced nonpolarity effect by perfluoroalkyl around the acid site should be interesting to design new solid acids with hydrophilicity–hydrophobicity balance.