Effect of synthesis parameters on precipitation of nanocrystalline boehmite from aluminate solutions

Abstract

Nanocrystalline boehmite (γ-AlOOH) is a cost-effective material for the production of γ-Al 2O 3 finding many industrial applications as catalyst or catalyst support, membranes and adsorbents. The preparation conditions applied in the production step of nanocrystalline boehmite strongly affect its morphology, which in turn is reflected to the final transition alumina. In this work, a precipitation method for the production of nanocrystalline boehmite is described studying the effect of pH, temperature and ageing time on the morphology of the final precipitate. The experiments were performed at temperatures 30, 60 and 90 °C, under moderate pH conditions (5, 6, 7) and 1 week of ageing in the mother liquor. What are noteworthy in the performed experiments are the starting solution used and the mixing procedure. The starting solution was a supersaturated sodium aluminate solution (SSA) with concentration equal to the Bayer liquor, which is not usually used in such synthesis experiments. On the other hand, the mixing procedure did not follow the usual route of addition of the neutralization agent (acid) to the SSA solution; on the contrary the SSA solution was added to the neutralization agent. Amorphous boehmite was prepared at temperatures as low as 30 °C and pH 7 under prolonged ageing conditions. At 60 °C the formation of pure nanocrystalline boehmite with crystallites 3–8 nm was facilitated at pH 6 and pH 7 after ageing in the mother liquor, while at the higher temperature of 90 °C the formation of pure nanocrystalline boehmite with crystallite size between 3 and 13 nm was achieved at pH 5, pH 6 and pH 7. Ageing and temperature influenced the crystallinity of the precipitated phases, with prolonged ageing and high temperatures inducing high crystallinity. The pH conditions also had a strong effect on the crystallite size of precipitates. Actually, for the same temperature and ageing time the higher the pH the larger the crystallites of the precipitates.