The effect of digestion on the surface area and porosity of alumina

The stability of gamma alumina with respect to phase transitions and loss of surface area can be greatly improved by digestion of the aluminium hydroxide precursor. Pseudoboehmite (aluminium hydroxide) was formed by precipitating aluminium nitrate with ammonium hydroxide at pH 6. The freshly precipitated gel has a very low surface area, but the surface area increases after digestion of the hydroxide in the mother liqueur. Digestion leads to a 'drier' hydroxide as shown by thermogravimetry. After calcination to 500 degrees C, the resulting alumina has a surface area of 230 to 310 m(2) g(-1), depending on the length of digestion of the hydroxide. Aluminas prepared from aged precursors have better thermal stability than those prepared without the digestion step. The digested alumina was able to withstand calcination to 1200 degrees C for 12 h, and maintained a surface area of similar to 68 m(2) g(-1). The transformation to the alpha phase is delayed in the digested samples. The effect of digestion on the surface area and improved thermal stability are explained on the basis of a reduction in the number of defect sites responsible for surface diffusion. (C) 2000 Elsevier Science B.V. All rights reserved.