Surface and porosity of nanocrystalline boehmite xerogels

Abstract

Boehmite xerogels are prepared by hydrolysis of Al(OC4H9)3 followed by peptization with HNO3 (H+/Al = 0, 0.07, 0.2). XRD and TEM show that these gels are made of nanosized crystals (5-9 nm in width and 3 nm thick). According to the amount of acid, no significant differences are found in size and shape, but only in the spatial arrangement of the crystallites. Nitrogen adsorption-desorption isotherms of nonpeptized gels are of type IV, whereas isotherms of peptized gels are of type I. These isotherms are analyzed by the t-plot method. The majority of pore volume results from intercrystalline mesopores, but the peptized gels also contain intercrystalline micropores. The particle packing is very dense for the gel peptized with H+/Al = 0.2 (porosity = 0.26), but it is less dense in non-peptized gel (porosity = 0.44). Heating these gels under vacuum creates, from 250 degrees C onwards, an intracrystalline microporosity resulting from the conversion of boehmite into transition alumina. But heating also causes intercrystalline micropores collapsing. The specific surface area increases up to a limit temperature (300 degrees C for nonpeptized gels and 400 degrees C for peptized) beyond which sintering of the particles begins and the surface decreases. The PSD are calculated assuming a cylindrical pore geometry and using the corrected Kelvin equation proposed by Kruk et al. Peptized xerogels give a monomodal distribution with a maximum near 2 nm and no pores are larger than 6 nm. Nonpeptized gels have a bimodal distribution with a narrow peak near to 2 nm and a broad unsymmetrical peak with a maximum at 4 nm. Heating in air above 400 degrees C has a strong effect on the porosity. As the temperature increases, there is a broadening of the distribution and a marked decrease of small pores (below 3 nm). However, even after treatment at 800 degrees C, micropores are still present.

Sol–gel derived mesoporous and microporous alumina membranes

“…The decomposition of organic groups and the elimination of adsorbed water in the membrane pores most likely occur in the 170-300°C range. The slower gradual weight loss (wt *12%) in the 300-450°C range may correspond to the boehmite phase transformation to cAl 2 O 3 by the elimination of OH -groups [43]. The weight loss beyond 600°C was negligible.…”

Aluminum hydroxide microstructural units in gelled media aged, or nonaged, with alcohol and water

“…This behavior is typical of porous solids with cavities in the range of mesopores radius, i.e., 1.0 to 7.5 nm. Alphonse and Courty [6] observed, for nonpeptized xerogels obtained by aluminum tri-sec-butoxide hydrolysis, that adsorption-desorption isotherms form an E-type hysteresis loop. The hysteresis loops of the WS xerogels (Fig.…”

“…Boehmite, pseudoboehmite, bayerite, and norstrandite were identified according to pH, temperature, and other process parameters [3]. More recently, it has been demonstrated that pseudo-boehmite is made up of micro-or nanosized crystallites of boehmite [6], and it has been argued that the large loss of ignition is due to water adsorption on them. Nanometric dried powders can be obtained from gel media through vaporization of the liquid phase.…”

The effect of iron phosphate, alumina and silica coatings on the morphology of carbonyl iron particles

“…• C to 350 • C. [10] Thermal stability of aluminum oxides prepared from gel describes [29] the following Eqn (6): …”

“…On the other hand, gelling in polymerized systems occurs as a result of the polymerization by chemical reaction. [9] Boehmite, aluminum oxyhydroxide (γ -AlOOH), is a precursor of alumina, and exists under two distinct forms, wellcrystallized boehmite and pseudoboehmite (also called gelatinous boehmite), [10,11] with significantly different morphologies, porosities and surfaces areas (nanocrystallized particles 8-9 nm). Boehmite nanoparticles are prepared from alumina sol that consists of aluminum tri-sec-butoxide Al(OC 4 H 9 ) 3 , water and nitric acid HNO 3 .…”