Interlayer grafting of kaolinite using trimethylphosphate

Abstract

Interlayer grafting of kaolinite using trimethylphosphate (TMP), a phosphoric acid triester, was achieved using a methoxy-modified kaolinite (MeO-Kaol) as an intermediate. First, TMP was intercalated between the layers of MeO-Kaol upon dispersing MeO-Kaol to TMP at room temperature (TMP/MeO-Kaol_RT). The X-ray diffraction (XRD) pattern of TMP/MeO-Kaol_RT revealed that the basal spacing of MeO-Kaol was increased from 0.86 to 1.28 nm. The 1.28 nm diffraction line disappeared and the 0.86 nm diffraction line appeared upon washing of TMP/MeO-Kaol_RT with an excess of ethanol. The thermogravimetry (TG) curve of TMP/MeO-Kaol_RT showed mass loss at 50–180 °C. Solid-state ¹³C nuclear magnetic resonance spectroscopy with cross polarization and magic angle spinning techniques (¹³C CP/MAS NMR) and Fourier transform infrared spectroscopy (FT-IR) spectra showed the presence of methoxy groups in TMP/MeO-Kaol_RT. The C/P molar ratio of TMP/MeO-Kaol_RT was 3.0. The liquid-state ³¹P NMR spectrum of the guest species extracted with CDCl₃ showed one signal at 3.0 ppm, which was the same as the chemical shift of TMP in CDCl₃. These results clearly indicate intercalation of TMP between the layers of MeO-Kaol. Next, the dispersion of MeO-Kaol in TMP was heated at 150 °C under a nitrogen atmosphere (TMP/MeO-Kaol_150). The XRD pattern of TMP/MeO-Kaol_150 revealed that the basal spacing was increased from 0.86 to 1.12 nm. The 1.12 nm diffraction line was scarcely changed upon washing of TMP/MeO-Kaol_150 with an excess of ethanol and water. The TG curve of TMP/MeO-Kaol_150 showed mass loss at 300–400 °C. The ¹³C CP/MAS NMR and IR spectra showed the presence of TMP moieties in TMP/MeO-Kaol_150. The C/P molar ratio of TMP/MeO-Kaol_150 was 1.7. The solid-state ¹H MAS NMR spectrum of TMP/MeO-Kaol_150 revealed the presence of POH groups. SEM images, ²⁷Al MAS NMR spectra and Al/Si molar ratios observed for kaolinite and TMP/MeO-Kaol_150 indicate preservation of the kaolinite structure. These results clearly indicate the proceeding of interlayer grafting of kaolinite using TMP with hydrolysis of a limited amount of P-OMe groups.