| Abstract: | The surface properties of a range of aluminas, obtained by calcination of crystalline and gelatinous aluminium hydroxides at temperatures from 300° to 1000° and characterised by X-ray powder diffractometry, have been examined by nitrogen adsorption. On calcination the hydroxides exhibited two distinct types of behaviour which can be related to their texture. Crystalline hydroxides underwent a pronounced activation, the increased surface area being almost wholly confined to the small pores (micro and transitional pores) formed on dehydration. Gelatinous hydroxides, on the other hand, had high surface areas before calcination and these underwent a progressive diminution in the temperature range studied. The structures obtained on calcination depended to a marked degree on the crystallinity of the starting materials. Between 500° and 800°, the amorphous hydroxide and poorly organised pseudoboehmites gave ill-defined γ-aluminas whereas the crystalline trihydroxides gave well-defined η- and θ-aluminas. At 1000°, the product from the amorphous hydroxide still retained a surface area of 132 m2/g, this high residual area being in character with the slowness of the conversion to δ-and α-alumina. An empirical approach is used to analyse the adsorption data in terms of three mechanisms of adsorption: monolayer-multilayer coverage, capillary condensation, and micropore filling. |
