Alcohothermal Treatments of Gibbsite: Mechanisms for the Formation of Boehmite

Abstract

Abstract--Gibbsite samples of various particle sizes (0.2-80 #m) were heated at 250~ in a series of straight-chain primary alcohols under the autogenous vapor pressure of the alcohol (alcohothermal treatment of gibbsite). The treatment in ethanol yielded pure boehmite, the morphology of which was similar to that of the boehmite obtained by hydrothermal treatment of gibbsite. In middle-range alcohols, the boehmite yields were low (50% if 80 t~m gibbsite was used), and the products were contaminated by a poorly crystallized phase, having a x-alumina-like structure. The products preserved the morphology of the originating gibbsite, this feature being similar to the thermal dehydration of gibbsite. Complete conversion to boehmite was also attained in mineral oil (a hydrocarbon mixture, which was used as a limit of higher alcohol. The morphology of the boehmite formed in this medium was identical to that of the product prepared by thermal dehydration of gibbsite in a sealed bomb without a medium. If fine particle-size gibbsite was used, the yield of boehmite decreased and the yield of the poorly crystallized phase increased in all the media.The reaction mechanisms may be discussed in terms of the reported mechanisms for the thermal and hydrothermal formations of boehmite from gibbsite. Thus, in lower alcohols boehmite formed by a dissolution-recrystallization mechanism, whereas in middle-range or higher alcohols it formed by intraparticle hydrothermal reaction mechanism proposed by de Boer and coworkers for the thermal dehydration of gibbsite. The difference in behavior in middle-range and higher alcohols can be explained in terms of the solubility of water in the medium: In the middle-range alcohols, water molecules formed by partial dehydration of gibbsite were removed from the gibbsite particles into the medium so that dehydration proceeded in a manner similiar to that of thermal dehydration, whereas in the higher alcohols, the low solubility of water in the medium allowed the water molecules to remain on the surface of the particles, thereby promoting the complete hydrothermal formation of boehmite.

Formation of Organic Derivatives of Boehmite by the Reaction of Gibbsite with Glycols and Aminoalcohols

“…On the other hand, water molecules were not stabilized in aprotic solvents (hydrocarbons and related compounds) and the high fugacity (activity) of water facilitated the hydrothermal reaction of gibbsite resulting in complete formation of boehmite. Note that if gibbsire alone was heated in an autoclave at 250~ it transformed completely to boehmite (Inoue et aL, 1989). Apparently (ostensibly), the gibbsite structure was stabilized by the presence ofglycolic media; however, this stabilization effect was not due to the direct interaction between glycol molecules and gibbsite, but was due to lowering the fugacity (activity) of water by the glycol Figure 6.…”

“…In aprotic solvents gibbsite transformed completely into boehmite, whereas in protic solvents, unreacted gibbsite remained. Inoue et al (1989) examined the boehmite formation from gibbsite in alcoholic media and found that in an alcohol having a carbon number > 5 (higher alcohols), the boehmite yield increased with an increase in the carbon number of the alcohol. They attributed this increase in the boehmite yield to the increase in fugacity of water formed by partial dehydration ofgibbsite and concluded that in these higher alcohols, boehmite formed by means of an intraparticle hydrothermal reaction mechanism, originally proposed by de Boer et al (1954ade Boer et al ( , 1954bde Boer et al ( , 1964 for the formation of boehmite during the thermal dehydration of gibbsite.…”

“…Kubo and Uchida (1970) also reported the formation of a methyl derivative of boehmite by the reaction of gibbsite with methanol at high temperatures. Inoue et al (1989), however, examined the reaction of gibbsite in a series of straight-chain primary alcohols (higher than ethanol) at 250"C and found that boehmite was the predominant product and that a transition alumina was also formed if alcohols having carbon numbers 5-8 were used. Organic derivation of boehmite was not formed in these experiments.…”

Microwave assisted hydrolysis of aluminium metal and preparation of high surface area γ Al2O3 powder

“…In many chemical routes, boehmite and bayerite have been used as the starting materials for the preparation of alumina. Boehmite is prepared by various methods such as hydrothermal treatment of gibbsite (Inoue et al 1989), hydrolysis of alkoxides (Yoldas 1975) or aluminium nitrate solutions (Mani et al 1991), etc. Aluminium metal has also been used as a precursor for the synthesis of aluminium hydroxides and these hydroxides are used to prepare alumina particles (Kannan et al 1997;Mo and Yuan 1993;Thiruchitrambalam et al 2004).…”

Solvothermal crystallization of nanocrystals of metal oxides

“…In 1989, one of the present authors showed that α-alumina was synthesized from microcrystalline gibbsite (Al(OH) 3 ) in 1,4-butanediol at 285 °C [17], much lower temperature than that required by the hydrothermal reaction. He also examined the reaction of gibbsite in a series of alcohols and found that two different reactions took place depending on the carbon number of the alcohol [18]. When higher alcohols were used as the reaction media, hydrothermal reaction by the water formed by thermal dehydration of the starting material took place [16].…”