The thermal stability of mixed phenylphosphonic acid/water intercalates of kaolin and halloysite. A TG-EGA and VT-DRIFTS study

Phenylphosphonic acid undergoes a topotactic reation with the available hydroxyls on the gibbsite sheet of kaolin and halloysite during reflux in a water-acteone mixture at 70degreesC. The reaction occurs much more rapidly with halloysite than with kaolin and the number of hydroxyls consumed is much higher for the halloysite than the kaolin. Variable temperature X-ray diffraction confirmed that the d-spacing for both kaolin-phenylphosphonic acid (KPPA) and halloysite-phenylphosphonic acid (HPPA) remained stable at 15.4 Angstrom to temperatures greater than 450 degreesC. Variable temperature diffuse reflectance infrared Fourier transform spectroscopy (VT-DRIFTS) together with TG-MS analysis of the evolved gases confirmed that the phenyl ring was not thermally desorbed until after dehydroxylation began to take place. Moreover, these techniques showed that the complexes contained water which was desorbed near 150 degreesC. In addition VT-DRIFTS showed that this water was hydrogen bonded to the remaining surface hydroxyls on the gibbsite sheet resulting in a shift of the AlAlOH band from 934 to 885 cm(-1) in both HPPA and KPPA. This band shifted back to 934 cm(-1) when the hydrogen bonded water (characterised by an OH bending mode at 1643 cm(-1) and an OH stretching mode at 3547 cm(-1)) was thermally desorbed.