Effect of synthesis parameters on precipitation of nanocrystalline boehmite from aluminate solutions D. Panias, A. Krestou ⁎ National Technical University of Athens – School of Mining and Metallurgical Engineering, 9 Heroon Politechniou Str, 15780, Zografos, Greece Received 29 August 2006; received in revised form 16 January 2007; accepted 16 January 2007 Available online 2 February 2007 Abstract Nanocrystalline boehmite (γ-AlOOH) is a cost-effective material for the production of γ-Al 2 O 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. © 2007 Elsevier B.V. All rights reserved. Keywords: Boehmite; Nanocrystalline; Neutralization; Precipitation 1. Introduction Alumina is a low-cost material finding many applications in catalysis, manufacturing of ceramics, mechanical ceramics [1,2], refractory, electrotechnology, electronics and biotechnology [3]. The wide variety of these applications comes from the fact that alumina occurs in two forms, corundum or α-alumina and transition aluminas. Due to its hexagonal close packing of oxygen ions microstructure, α-alumina presents excellent mechanical, electrical, thermal and optical properties. On the other hand, transition aluminas, have a cubic close packing of oxygen microstructure resulting in high surface area, mesoporosity and surface acidity and as a result they are extensively used as adsorbents, catalysts, catalyst supports and membranes. Among all transition aluminas, γ-Al 2 O 3 is applied in the field of heterogeneous catalysis [4] while boehmite is a direct parent for highly pure γ-Al 2 O 3 of reasonably high surface area. Upon controlled calcination under air flow, boehmite undergoes a topotactic transformation into γ-Al 2 O 3 so that the conserved morphology and size of boehmite particles are reflected to the final transition alumina [5]. Moreover, boehmite is an energy- saving precursor material for the production of alumina, although it dehydrates at a higher temperature than gibbsite (490 °C and 310 °C, respectively). This is attributed to the low enthalpy of the dehydration reaction of boehmite (72 kJ/mol Al 2 O 3 ) compared to the corresponding one of gibbsite (187 kJ/mol Al 2 O 3 ) [6]. The aforementioned advantages of boehmite have forced many researchers to develop various procedures for the preparation of boehmite, and especially of boehmite with low crystallinity [1,2,4,7], since crystalline boehmite usually gives lower-surface area γ-Al 2 O 3 [4]. The most popular method is the Powder Technology 175 (2007) 163 – 173 www.elsevier.com/locate/powtec ⁎ Corresponding author. Tel.: +30 210 7722177; fax: +30 210 7722168. E-mail address: akrestou@metal.ntua.gr (A. Krestou). 0032-5910/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2007.01.028