Solid-State ³¹P Nuclear Magnetic Resonance Study of Interlayer Hydroxide Surfaces of Kaolinite Probed with an Interlayer Triethylphosphine Oxide Monolayer

The solid acidity of the interlayer aluminol surfaces of kaolinite was explored by solid-state ³¹P nuclear magnetic resonance with magic angle spinning (MAS) using triethylphosphine oxide (TEPO), which formed a monolayer with a uniform orientation between the layers of kaolinite as a probe molecule. Intercalation of TEPO between the layers of kaolinite was achieved using methoxy-modified kaolinite as an intermediate. The presence of TEPO in the reaction products was revealed by the two signals at 21 and 7 ppm, which were assignable to ethyl groups in TEPO, in the solid-state ¹³C nuclear magnetic resonance with cross polarization and magic angle spinning techniques (¹³C CP/MAS NMR). The presence of TEPO between the layers of kaolinite was demonstrated by the expansion of basal spacing from 0.86 nm, the interlayer distance of methoxy-modified kaolinite to 1.16 nm, as shown by the X-ray diffraction patterns, suggesting the formation of a TEPO monolayer between the layers of kaolinite. The formation of hydrogen bonds between the P=O groups of TEPO and the aluminol groups on the interlayer surfaces of kaolinite was also revealed by the appearance of an additional OH stretching band at 3598 cm^-1 in the Fourier-transform infrared spectrum and narrow solid-state ³¹P MAS NMR signals observed at 55-53 ppm which were shifted from the position of the physisorbed TEPO (50 ppm). These results clearly indicate that the solid acidity of interlayer aluminol groups of methoxy-modified kaolinite was probed using an interacted TEPO monolayer.