Journal of Colloid and Interface Science 247, 132–137 (2002) doi:10.1006/jcis.2001.8079, available online at http://www.idealibrary.com on A Crystallite Packing Model for Pseudoboehmite Formed during the Hydrolysis of Trisecbutoxyaluminium to Explain the Peptizability Wayde N. Martens, ∗, 1 J. Theo Kloprogge, ∗ Ray L. Frost, ∗ and John R. Bartlett† ∗ Centre for Instrumental and Developmental Chemistry, Queensland University of Technology, 2 George Street, GPO Box 2434, Brisbane, Queensland 4001, Australia; and †Materials Division, Australian Nuclear Science and Technology Organization, Private Mail Bag 1, Menai, New South Wales 2234, Australia Received July 20, 2001; accepted October 29, 2001 A model for pseudoboehmite crystallite packing formed dur- ing the hydrolysis of trisecbutoxyaluminium is postulated. The model describes platelike crystallites of pseudoboehmite stacked in a sharing edges only configuration. With this type of stacking, the pore sizes detected are approximately equal to the crystallite sizes of the hydrolysates. The hydrolysates age via a dissolution re- precipitation reaction. This increases the size of the crystallite size of the pseudoboehmite formed, speeding peptization by allowing nitrate ions to enter pores and access the surfaces of the crystallites. This type of model also allows an explanation for the peptization kinetics of systems containing sec-butanol formed during the hy- drolysis of trisecbutoxyaluminium. C  2002 Elsevier Science (USA) Key Words: psudeoboehmite; boehmite; alumina; peptization. INTRODUCTION The hydrolysis of aluminium alkoxides and subsequently pep- tization of the resultant alumina has been documented by authors such as Yoldas (1, 2). Hydrolysis of aluminium alkoxides such as trisecbutoxyaluminium yields an alumina precipitate of alu- minium oxyhydroxide (AlOOH) via Eq. [A]: Al(O R) 3 + 2H 2 O → AlOOH + 3 ROH. [A] The resultant hydrolysate formed from the hydrolysis pro- cess is psudeoboehmite in water at 90 ◦ C or amorphous AlOOH in room-temperature water hydrolysis (1, 2). The resultant hy- drolysate can then be broken down to colloidal-size particles in a process known as peptization. Peptization involves the formation of a electrostatic or steric barriers to stabilize the colloidal-size particles. The use of agitation and heat also breaks down the particle size. Yoldas found that the peptization of alumina was extremely slow at low peptization temperature (1, 2). It was also 1 To whom correspondence should be addressed. E-mail: w.martens@qut. edu.au. found from his work that the aging of alumina is extremely im- portant to the phase of alumina formed. If the hydrolysate was left in contact with hydrolysis solution in cold water hydroly- sis systems, it was found that the amorphous aluminium oxy- hydroxide would form pseudoboehmite and then bayerite via dissolution re-precipitation. The aging of the high-temperature hydrolysates was found not to affect the properties of the alu- mina present. Many investigations have described the aging and phase trans- formations of alumina hydrolysates (3–8) and the influences of electrolytes and nonelectrolytes on the aging (3, 4, 9). Little work, however, has been conducted to study the effects of aging on the peptization of alumina hydrolysates (7). Dunning stated that the structures of hydrous oxides/oxides are influenced by many kinetic factors that interact and compete with each other (5). Dunning defined the structure of hydrous oxides/oxides as comprising particle size, lattice structure, shape and morphology distribution, structure of the surfaces, imper- fections, impurities, and inclusions in the crystals. Dunning also studied the interaction of particles resulting from aggregation, agglomeration, and flocculation. Three kinetic processes where identified that lead to the structural evolution of precipitated crystals (5). a) Nucleation of particles, where new phases are generated in the mother solution and the first minute particles are formed. b) The growth of these nuclei to form primary particles, which are supported by the mother liquor solution by acting as a dissolution media. c) The aging of the primary particles, which can facilitate changes in the shape, structure, perfection/crystallinity, size, ag- gregation, agglomeration, and flocculation of the particles. Bye et al. (3, 4) identified three main aging mecha- nisms, which are similar to Dunning’s (5) observations. These three stages are (a) condensation–polymerization; (b) aggregation–cementation; (c) re-crystallization. Condensation– polymerization involves the internal ordering of the alumina via a condensation or polymerization reaction (oxalation) as stated by Fricke and Meyring (9) and Feitknecht (10), which can be 132 0021-9797/02 $35.00 C  2002 Elsevier Science (USA) All rights reserved.