2022 年 26 巻 1-2 号 p. 1-7
Negatively charged groups are immobilized on the neutral inorganic surfaces of kaolinite by a reaction between interlayer hydroxyls of kaolinite and 1,3-dimethylimidazolium dimethylphosphate (DMI+DMP−), an ionic liquid. This interlayer grafting gives the product possessing covalently-bonded DMP− moieties with DMI+. The successful grafting is confined by a lack of differences in the 1.26-nm diffraction line due to expansion of the kaolinite interlayers between the reaction products before and after washing with an excess amount of methanol at 60°C for 2 days, and by the presence of upfield signal at −10.0 ppm in the solid-state 31P nuclear magnetic resonance signal with magic angle spinning (31P MAS NMR) spectrum compared to the 31P liquid NMR signal of DMI−DMP+ in CDCl3 at 0.57 ppm. When the product obtained by the interlayer grafting is treated with 1 mol/L hydrochloric acid, the Fourier-transform infrared spectrum and thermogravimetric curve of the acid-treated product reveal complete removal of DMI+. Additionally, the XRD pattern of the product displays a decrease in the basal spacing from 1.26 to 1.03 nm accompanied by the 31P MAS NMR signal at −8.0 ppm which are present upfield from the 31P NMR signal at 0 ppm of phosphoric acid. Acid treatment using hydrochloric acid thus removed DMI+ from the interlayer space, but the DMP− moieties were converted into dihydrogen phosphate moieties that remained grafted to the interlayer surfaces of kaolinite. These results demonstrate the formation of a proton-exchangeable and asymmetrical layered inorganic solid.
