(33f) Influence of pH and Sulphate on the Morphology of Boehmite

The synthesis of nano-crystals with different shapes as rods, plates and cubes, etc. has been paid much attention in recent years owing to their specific physic-chemistry properties. Boehmite is a crucial precursor of alumina (a material widely used in ceramics, composites and catalysts because of its high stability, varying structure (g,d,a,) and controllable mesoporous textures) and was usually prepared by hydrothermal treatment of aluminum hydroxide or aluminum salts. The former work showed that the shape of boehmite is connected closely with the pH of the slurry. The boehmite nano-rods with an aspect ratio lower than 10 were obtained at acidic condition and the boehmite plates was formed at alkaline condition. But how to get well-crystallized boehmite nanorods with high aspect ratio is still a challenge.

In this study, the effect of the pH and sulphate on the shape of boehmite was investigated and a facile sulphate-assisted hydrothermal method was developed to produce boehmite nanorods with an aspect ratio greater than 20. Amorphous Al(OH)₃ precipitate was formed by mixing AlCl₃ and NH₄OH solutions at room temperature, the treating of the precipitate at 240¡æ in solutions with different initial pH and sulphate conc. led to the formation of boehmite with different shapes. In the absence of sulphate, plate-like particles were formed at the initial pH >7 (Fig.1a), whereas the nanorods composed of the small particles with a diameter of about 5 nm were obtained at the initial pH <7 (Fig.1b). The aspect ratio increased from 6 to 10.5 as the decrease of the initial pH from 6.5 to 3.9. Well-crystallized boehmite nanorods (Fig.1c) were formed in the presence of 0.05 mol/L sulphate (using H₂SO₄ at pH<7.0 and Na₂SO₄ at pH>7.0) in the pH range of 4.3-9.5. The aspect ratio of boehmite nanorods increased with the increase of pH or sulphate conc. The relationship between the boehmite shape and the solution composition was investigated by the thermodynamic equilibrium calculation of the Al(OH)₃-sulphate aqueous system at elevated temperatures.

Fig.1. Morphology of boehmite formed at different hydrothermal solutions

Initial pH: a-9.5, b-5.0, c-5; initial conc. of sulphate ( mol/L): a-0, b-0, c-0.05; temperature(^oC) : 240, time(h): 16.0.