Crystallinity of boehmite and its effect on the phase transition temperature of alumina

Abstract

The effect of the crystallinity of boehmite powders on the temperatures of c-Al 2 O 3 formation and the h-to a-Al 2 O 3 transformation was investigated using boehmite powders of varying crystallite size prepared under various hydrothermal conditions. With increasing crystallite size of the boehmite powders, the specific surface area decreased and the expanded (020) d-spacing approached the reported value. Thermogravimetric ( TG) profiles of more poorly crystalline boehmite indicated the presence of excess water molecules of different binding energy located on the surface or in the interlayer. The crystallite size of boehmite also showed a strong correlation with the formation temperature of c-Al 2 O 3 and the phase transition temperature of h-to a-Al 2 O 3 . Since both these temperatures are increased with increasing the crystallite size of boehmite, this is an important factor in determining the conditions for obtaining c-, h-and a-Al 2 O 3 from boehmite. By relating the TG weight loss to the water and OH contents in boehmite and c-Al 2 O 3 , respectively, the crystallite size of boehmite can be related to the water content. This indicates that the c-Al 2 O 3 transformed from boehmite of smaller crystallite size contains larger amounts of OH groups, implying more poorly crystalline c-Al 2 O 3 . The reported topotactic transformation of boehmite via c-to h-Al 2 O 3 indicates a relationship between the crystallite size of the boehmite and that of the resulting transition alumina, which could explain the change of the phase transition temperature from h-to a-Al 2 O 3 .been performed to clarify the relation between the boehmite

Diffuse Reflection Infrared Spectroscopy (Drifts): Application to the in Situ Analysis of Catalysts

“…Contrary to the results reported in the literature [10,27,28], treatment at 723 K for 45 min does not appear to be sufficient to complete this conversion. It is in fact observed that the wide bands around 3400 and 3200 cm −1 are still present and that the OH stretching bands above 3650 cm −1 , typical of alumina, are not yet clearly defined (Fig.…”

The Powder Preparation of Blue Cobalt Aluminate at 210°C Using the Malonate Method

“…The α-Al 2 O 3 phase of polymorphic aluminum oxides is known to form at temperatures above 1000 o C. 15 The blue CAO crystallite contents in the samples prepared at 210 o C could be determined using the maximum value of the (311) peak. The sample concentrations obtained from the 31-, 35-and 39-precursors were 66, 80 and 71%, respectively.…”

Thermally Induced Formation of Transition Aluminas from Boehmite

“…According to Refs., [1,21,36] it can be assumed that the strong endothermal peak and the large weight loss of approximately 13.7% appearing near 540 C correspond to the transformation of boehmite to g-Al 2 O 3 . Tettenhorst [37] and Tsukada et al [38] have shown that the transition temperature decreases with decreasing size of the boehmite crystallites. Thus, the weak endothermal peak and the weight loss of nearly 2% starting behind 350 C might be attributed to the transformation of small boehmite crystallites.…”

“…Alternatively, the first endothermal peak and the weight loss of nearly 2% can be attributed to the release of water adsorbed on the surface of boehmite crystallites or intercalated in the interlayers of the crystal structure. [1,38] In order to verify this hypothesis, additional information about the location of the water molecules and about the water content in annealed boehmite was obtained from the Raman investigations, which were performed in the wavenumber range 2900-3800 cm À1 on samples calcined for 20 h at the temperatures between 300 and 600 C (Figure 3). The modes appearing between RT and 400 C can be related to different ÀOH stretching vibrations.…”