Modification of the hydroxyl surface in potassium-acetate-intercalated kaolinite between 25 and 300 °C

Changes in the hydroxyl surfaces of potassium-acetate-intercalated kaolinite have been studied over the ambient to predehydroxylation temperature range using a combination of X-ray diffraction (XRD) and Raman spectroscopy. Upon intercalation, the kaolinite expanded along the c-axis direction to 13.88 Angstrom When the intercalation complex is heated over the 50 to 300 degrees C range, X-ray diffraction shows the existence of three additional intercalation phases, with d spacings of 9.09, 9.60, and 11.47 Angstrom. The amount of each phase is temperature-dependent. Thermal analysis shows three phase changes, at 88, 293, and 364 degrees C. These expansions are reversible and upon cooling, the intercalation complex returned to its original spacing. The 13.88 Angstrom phase existed only in the presence of water. It is proposed that the expanded kaolinite intercalation phases result from the orientation of the acetate within the intercalation complex. The completely intercalated kaolinite showed a single Raman band at 3607 cm(-1) attributed to the inner-surface hydroxyl hydrogen bonded to the acetate ion. This band corresponds to the 13.88 Angstrom phase. Mild heating of the intercalated complex at 50 degrees C caused a rearrangement of the surface structure, with Raman hydroxyl-stretching bands being observed at 3594, 3604, and 3624 cm(-1). Further thermal treatment at 100 degrees C caused these bands to shift to 3599, 3605, and 3624 cm(-1). At the predehydroxylation temperature for potassium-acetate-intercalated kaolinite (250 degrees C), two bands were observed, at 3601 and 3633 cm(-1) At this temperature, only the 11.62 Angstrom phase existed, suggesting that the 3601 cm(-1) band is related to the 11.62 Angstrom phase. Above this temperature, no hydroxyls are spectroscopically evident. When the complex is cooled to room temperature, the Raman spectra of the hydroxyl surfaces are identical to those of the initial intercalation complex, showing that the thermal modification of the kaolinite surfaces is reversible.