Infrared emission study of the thermal transformation mechanism of Al13-pillared clay

Abstract

Pillaring of smectites with Al₁₃ was studied in situ by infrared emission spectroscopy. Exchange of montmorillonite with Al₁₃ resulted in Al–OH and Al–H₂O OH-stretching modes of Al₁₃ at 3682 and 3538 cm^–1. Upon transformation of Al₁₃ the Al–OH stretching band disappeared above 400 °C. The Al–H₂O OH-stretching band was replaced by bands around 3574 and 3505 cm^–1, indicating structural rearrangement within the Al₁₃. Although the band intensities strongly diminished, they were still observed at 800 °C, suggesting that the pillar still contained OH groups and was incompletely converted to aluminum oxide. Below 1750 cm^–1 the Al₁₃-exchanged montmorillonite displayed bands at 642, 1008, 1321, 1402 and 1512 cm^–1. The 1512 cm^–1 band disappeared around 500 °C, followed by the 1402, 1321 and 1008 cm^–1 bands above 600 °C. The 642 cm^–1 band diminished in intensity but was still observed at 800 °C. At 700 °C a new band was observed at 722 cm^–1, indicating the formation of an Al–O bond. Expansion of saponite with Al₁₃ resulted in bands at 3626 and 3244 cm^–1. At 700 °C both bands disappeared, indicating that the transition to aluminum oxide was complete. The Mg–OH band was hardly influenced by calcination, indicating the pillaring mechanism in saponites does not involve the octahedral clay layer. Below 1750 cm^–1 Al₁₃-exchanged saponite showed bands at 1316, 1424 and 1521 cm^–1. Calcination resulted in disappearance of the 1521 and 1316 cm^–1 bands around 600 °C, while the 1424 cm^–1 band disappeared around 400 °C. The 1316 cm^–1 band was replaced by bands around 1269, 1314 and 1388 cm^–1 above 600 °C, indicating the formation of M^IV–O(H)–Al^VI linkages between pillars and saponite tetrahedral layers.