Influence of Genesis on Mechanochemical Activation and Reactivity of Boehmite Prepared by Thermal and Hydrothermal Transformation of Gibbsite

The focus of this paper is on the mechanical activation and reactivity of boehmite (γ-AlOOH) synthesized by thermal transformation (B-TH) and hydrothermal transformation (B-HT) of the same gibbsite (Al₂O₃·3H₂O) precursor. The central idea is to emphasize the role of sample genesis. The samples used had similar size distribution and largely differed in their BET surface area, 264 and 2.98 m²/g for B-TH and B-HT, respectively. Mechanical activation was carried out for different durations, up to 240 minutes, using a planetary mill. On milling, the span of change in physicochemical properties, namely geometric specific surface area, BET specific surface area, degree of amorphization, microcrystalline dimension, and macro strain (in 〈020〉/stacking direction of AlO₄(OH)₂ octahedra layers) was more for B-TH vis-à-vis B-HT. The anomalous decrease in the BET surface area reported in the literature for B-TH is not observed in B-HT. The nature of alteration in pore size distributions with milling time is used to present a plausible explanation for the contrasting change in BET surface area. The samples also showed the opposite sign for macro strain; a negative sign for B-TH which was characterized by a relatively weaker interlayer bonding. The reactivity of the samples was evaluated in terms of leachability in alkali and the lowering of boehmite to γ-Al₂O₃ transformation temperature. In general, higher reactivity was observed for B-TH vis-à-vis B-HT. The reactivity was correlated with physicochemical changes. Despite wide differences in reactivity, the change in reactivity could be expressed in terms of simple multivariate equations with high correlation coefficients.