Dehydroxylation sequences of gibbsite and boehmite: study of differences between soak and flash calcination and of particle-size effects

The thermal transformation sequences of boehmite (γ-AlOOH) and two grades of gibbsite [γ-Al(OH)3] upon soak and flash calcination are reported. The techniques used were X-ray diffraction (XRD), differential thermal analysis (DTA), Fourier-transform IR (FTIR) and 27Al magic-angle spinning (MAS) NMR spectroscopies. Boehmite undergoes the dehydroxylation sequence boehmite, γ, δ, θ, α-Al2O3 under both soak and flash calcination. The dehydroxylation sequence of gibbsite, however, depends on the calcination method and the particle size of the feed material. Soak calcination of a fine gibbsite (ca. 0.5 µm) gave the dehydroxylation sequence gibbsite, χ, κ, α-Al2O3; with flash calcination the sequence gibbsite, χ, γ, δ, θ, α-Al2O3 was observed. Soak calcination of coarse gibbsite (ca. 14 µm) gave both the dehydroxylation pathways (a): gibbsite, boehmite, γ, δ, θ, α-Al2O3 and (b): gibbsite, χ, κ, α-Al2O3, and pathway (a) was predominant. Flash-calcined coarse gibbsite experiences a crossover between these routes (χ–γ) without formation of κ-Al2O3. Flash calcines of gibbsite undergo this χ–γ phase change at ca. 800°C.