Standard transition aluminas. Electron microscopy studies

Abstract

The aim of this paper is to present the results of characterization of the particle shapes of six standard transition aluminas samples using transmission and scanning electron microscopies; selected area electron diffraction, in parallel with X-ray powder diffraction were used for confirmation of the different transition aluminas types. The transition aluminas -chi; kappa; gamma; theta; delta; and eta were supplied by ALCOA Central Laboratory. The chi-; kappa-;gamma-and delta-Al203 microcrystals are pseudomorphs from their respective precursors gibbsite and boehmite. However, theta-Al203 microcrystals are not pseudomorphs after the standard delta-Al203 sample. Also, eta-Al203 are not pseudomorphs after bayerite somatoids.

Surface acid–base properties and hydration/dehydration mechanisms of aluminum (hydr)oxides

“…In sample A, the predominant phase was found to be gibbsite with a little bayerite phase. Sample B was a sort of transition alumina [33], mainly semicrystalline γ -alumina phase. Sample C was wellcrystallized α-alumina structure.…”

“…Apart from α-Al 2 O 3 , another six transition crystal structures occur: γ -, κ-, θ -, η-, δ-, and χ -Al 2 O 3 [33]. All these structures are based on a more or less close-packed oxygen lattice with aluminum ions in the octahedral and/or tetrahedral interstices.…”

Effect of precursor on the performance of alumina for the dehydration of methanol to dimethyl ether

“…Bayerite is formed at room temperature and transforms to Gibbsite with high crystallinity. The high and sharp diffraction peak in diffractogram (a) indicates that the obtained mixture phases have a large crystalline size (see Table ( [25,36], the appearance of the new δ -Al 2 O 3 is known to affect the catalytic performance of the catalyst during the MTD reaction as will be discussed later. It is apparent from this data that the bulk structures of γ-and η-Al 2 O 3 are very similar to each other, and in some instances are considered to be identical [34,37,38].…”

Influence of annealing temperature on the phase transformation of Al2O3

“…The temperature is mainly responsible for the phase transitions. At temperature range of 500 °C and 700 °C, the -Al2O3 material was formed [5,6], the annealing at 750 °C exhibited the Al2O3 [7], and the Al2O3 were formed between 900 °C and 1000 °C [6,8]. The stable -Al2O3 can be formed at 1100 °C and above [1,5,9].…”