Keggin-Al30: An intercalant for Keggin-Al30 pillared montmorillonite

Abstract

Clay minerals intercalated with inorganic species, known as pillared interlayered clays, have received extensive attention in recent decades. In this work, by comparison with those of Keggin-Al13, the formation processes of Keggin-Al30 are discussed based on different hydrolysis reactions. X-ray diffraction (XRD), field-emission scanning electron microscope, and thermogravimetric analyses were applied to compare differences in crystal structure, morphology, and thermal stability between Keggin-Al13 and Keggin-Al30 (sulfates). High temperature and excess monomeric Al species are key factors controlling the transformation from ε-Keggin-Al13 to δ-Keggin-Al13 and finally to Keggin-Al30. A decrease of the crystal symmetry from Keggin-Al13 to Keggin-Al30 was confirmed by shifts of reflections (and decreases of intensities) on XRD patterns. Keggin-Al30 has a more compact structure, which leads to higher thermal stability. The distributions of functional groups and positive charges on the surface of Keggin-Al30 result in a “lying flat” arrangement configuration in the interlayer region of montmorillonite. Exploring the behaviors of polycations as intercalants from their formation, transformation, and properties during the pillaring process may offer better understandings of the enhanced properties and wide applications of pillared interlayered clays.