Analysis of Mechanically Induced Reactivity of Boehmite Using Kinetics of Boehmite to γ-Al2O3 Transformation

Abstract

This article focuses on the mechanically induced reactivity of boehmite prepared by thermal decomposition of gibbsite. Boehmite, which retained the morphology of gibbsite, was characterized by a specific surface area of 264 m2/g. Mechanical activation (MA) was carried out in a planetary mill up to 240 minutes. The samples were characterized in terms of morphology, characteristic particle diameters, Brunauer Emmett Teller (BET) specific surface area (SSABET), microcrystallite dimension (MCD), microstrain (e) and Fourier transform infrared spectroscopy. The reactivity was construed from the kinetics of thermal transformation of boehmite into γ-Al2O3. The transformation observed between 600 K and 900 K (327 °C and 627 °C), manifested itself as two overlapping peaks in the differential thermogravimetric plot. These peaks correspond to two stages of dehydroxylation involving Al2OH and AlOH groups in succession. The peaks were resolved using Gaussian deconvolution. The reactivity was assessed separately for the two stages by comparing the fraction reacted in MA samples (α) with that of nonactivated sample (α ref). During both stages, enhanced kinetics, as revealed by α-α ref plots, indicated an increase in reactivity with MA. The transformation mechanism conformed to n th order reaction (f[α] = [1 – α] n with n = 1.3–1.5 in both stages). Values of n remained similar for the activated and reference samples. Activation energies (E a) for the first and second dehydroxylation stages were respectively 115 and 300 kJ/mol for the nonactivated sample. E a for the second stage decreased exponentially to a value of 222 kJ/mol after 240 minutes of milling. An anomalous negative correlation between reactivity and SSABET was observed. Reactivity parameters were strongly correlated with MCD and e. A plausible explanation for the observed correlations is presented.