Dehydroxylation of Aluminum (Oxo)Hydroxides Using Infrared Emission Spectroscopy. Part II: Boehmite

Abstract

The dehydroxylation of boehmite has been studied by the application of infrared emission spectroscopy over the 200 to 750 °C temperature range. The dehydroxylation is followed by the loss of intensity of the hydroxyl stretching frequencies observed at 3478, 3319, and 3129 cm⁻¹ and by the loss of intensity of the hydroxyl deformation modes at 1140 and 1057 cm⁻¹. Dehydroxylation starts at 250 °C and is complete by 450 °C. No difference was found between the synthetic and natural boehmite dehydroxylation. The hydroxyl stretching frequencies show a pronounced blue shift, while the hydroxyl deformation modes show a pronounced red shift. Infrared absorption bands were observed at 3413, 3283, and 3096 cm⁻¹ for the hydroxyl stretching frequencies and at 1161 and 1071 cm⁻¹ for the hydroxyl deformation frequencies. Low-frequency infrared absorption bands are observed at 749, 635, and 542 cm⁻¹ and infrared emission bands at 811, 716, 611, and 456 cm⁻¹. The infrared emission low-frequency bands moved to higher frequencies upon thermal treatment. Spectral changes in the low-frequency bands confirm that dehydroxylation commenced at 250°C. Infrared emission spectroscopy allows the phase changes of the Al₂O₃–H₂O alumina system to be studied in situ at the elevated temperatures.