Vibrational spectroscopy and dehydroxylation of aluminum (oxo)hydroxides: Gibbsite

The infrared absorption, infrared emission, and Fourier transform (FT)-Raman spectra of a series of gibbsites well defined by X-ray diffraction have been obtained. Hydroxyl stretching frequencies were found at similar to 3670, 3620, 3524, 3452, 3395, 3375, and similar to 3300 cm(-1) Hydroxyl deformation vibrations were observed at 1059, 1023, 969, 938, and 915 cm(-1). Hydroxyl stretching bands were observed in the Raman spectra at 3524, 3436, and 3365 cm(-1) and correspond well with the three infrared hands. These bands are both Raman and infrared active. The bands at similar to 3670, 3620, and 3395 cm(-1) are infrared active only. The hydroxyl stretching frequencies show a pronounced blue shift, while the hydroxyl deformation modes show a pronounced red shift. Infrared absorption hands were observed at 3413, 3283, and 3096 cm(-1) for the hydroxyl stretching frequencies and at 1024, 969, and 914 cm(-1) for the hydroxyl deformation frequencies. Low-frequency infrared absorption vibrations were found at similar to 860, 838, 800, 747, 666, similar to 625, 585, 560, 522, 452, and 423 cm(-1) and infrared emission bands at 834, 778, 728, 652, 640, 609, 580, 512, and 489 cm(-1). The infrared emission low- frequency bands moved to higher frequencies upon thermal treatment. The dehydroxylation of gibbsite was followed by the combination of infrared emission spectroscopy and differential thermal analysis over the 200 to 750 degrees C temperature ranges. Dehydroxylation is followed by the loss of intensity of the hydroxyl stretching frequencies observed at 3620 and 3351 cm(-1) and by the loss of intensity of the hydroxyl deformation modes at 1024 cm(-1). Dehydroxylation starts at 220 degrees C and is complete by 350 degrees C. Some variation in the gibbsite endotherms was found between the synthetic and natural gibbsite dehydroxylations. Spectral changes in the low-frequency bands confirm that dehydroxylation commenced at 220 degrees C.