The Thermal Decomposition of Mg-Al Hydrotalcites: Effects of Interlayer Anions and Characteristics of the Final Structure

Abstract

The thermal decomposition of hydrotalcites (HTs) with different interlayer anions in the 298–523 K region has been investigated by using transmission electron microscopy (TEM), thermogravimetric analysis (TGA), X‐ray diffraction (XRD), and IR, 27Al MAS‐NMR and X‐ray absorption near‐edge structure (XANES) spectroscopy. The thermal stability of the HT with interlayer oxalate was remarkably higher than that of HTs with interlayer carbonate; the onset temperatures for decomposition were 523 K and 473 K, respectively. It is proposed that the basicity of the interlayer anion is the key factor in the onset of dehydroxylation of the brucite‐like layers: the lower the basicity, the more thermally stable the HT compound. After heat treatment at 723 K, small Mg(Al)O domains (≈5 nm) within the HT crystallites cause broadening of the XRD reflections. The electron diffraction pattern consists of spots and rings, due to nonrandomly oriented crystalline material. Formation of disordered bonds, caused by nonideal packing between the decomposing adjacent cation layers in the (1 1 1) direction, could explain the expanded d value in the resulting MgO‐like phase observed with XRD and electron diffraction. The orientation of the Mg(Al)O domains in the heat‐treated material may be crucial for the so‐called “memory effect” of HTs.

Preparation of supported metal catalysts starting from hydrotalcites as the precursors and their improvements by adopting “memory effect”

“…The intensity of the reflection by basal plane (003) of the HT formed on the catalysts by the reconstitution was plotted against the calcination temperature (figure 4a). Both peak intensity and first order rate constant showed the maximum value at 600°C; a good correlation was [16]. A shoulder may be present around 3000 cm )1 ; this can be assigned to hydrogen bonding between interlayer water and interlayer carbonate [4].…”

Hydrotalcites supported on carbon nanofibers as solid base catalysts for the synthesis of MIBK

“…In general, our TGA-MS results obtained with unsupported HT with interlayer CO 3 2− reveal two distinct weight losses with the evolution of water and CO 2 with a total weight loss of 45%. First, the removal of interlayer water can be observed at temperatures up to 473-483 K; second, dehydroxylation and decarboxylation is observed above 573 K [1,45]. This pattern of decomposition is rather specific for HT.…”

First Generation Biodiesel

“…Immersed CaO into ammonium carbonate solution and calcined the catalyst at high temperature of 900 °C. With this catalyst, a FAME yield of 94% was obtained for the transesterification of jatropha curcas oil with a relatively lower methanol/oil ratio (9:1) and catalyst amount (1.5 wt%) at a reaction temperature of 70 °C (Roelofs et al, 2002). The general formula of LDHs is [M 2+ (1-x) M 3+ x (OH) 2 ] x+ (A n-) x/n · yH 2 O, and its idealized layered structure is shown in Fig.…”