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.

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