An enhanced nonpolarity effect of silicasupported perfluoroalkyl sulfonylimide on catalytic fructose dehydration

-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-C(4)F(9)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.