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Introduction Hydrotalcites consist of stacked layers of metal cations (M 2+ and M 3+ ) similar to brucite-like structures. Substitution of divalent cations for trivalent ones, of similar radii, gives rise to positively charged
Abstract
Hydrotalcites based upon gallium as a replacement for aluminium in hydrotalcite over a Mg/Al ratio of 2:1 to 4:1 were synthesised. The d(003) spacing varied from 7.83 Å for the 2:1 hydrotalcite to 8.15 Å for the 3:1 gallium containing hydrotalcite. A comparison is made with the Mg/Al hydrotalcite in which the d(003) spacing for the Mg/Al hydrotalcite varied from 7.62 Å for the 2:1 Mg hydrotalcite to 7.98 Å for the 4:1 hydrotalcite. The thermal stability of the gallium containing hydrotalcite was determined using thermogravimetric analysis. Four mass loss steps at 77, 263–280, 485 and 828 °C with mass losses of 10.23, 21.55, 5.20 and 7.58% are attributed to dehydration, dehydroxylation and decarbonation. The thermal stability of the gallium containing hydrotalcite is slightly less than the aluminium hydrotalcite.
Abstract
Hydrotalcite was synthesised by co-precipitation method, calcined and characterized by XRD, BET, IR and TG/DTA/DTG analyses and tested as solid base catalyst in the transesterification of soybean oil with methanol, achieving a methyl ester content of 99.5%. The thermal decomposition of hydrotalcite calcined occurred in four mass loss steps at 28, 105, 203 and 400 °C. The hydrotalcite was recovered and through a simple evaluation by TG/DTA/DTG techniques it was found that at 500 °C is the temperature, where the organic matter should be eliminated from the catalyst. This study shows the importance of thermal analysis in the evaluation of the recovery temperature of hydrotalcite.
Abstract
The hydrotalcite based upon manganese known as charmarite Mn4Al2(OH)12CO3·3H2O has been synthesised with different Mn/Al ratios from 4:1 to 2:1. Impurities of manganese oxide, rhodochrosite and bayerite at low concentrations were also produced during the synthesis. The thermal stability of charmarite was investigated using thermogravimetry. The manganese hydrotalcite decomposed in stages with mass loss steps at 211, 305 and 793 °C. The product of the thermal decomposition was amorphous material mixed with manganese oxide. A comparison is made with the thermal decomposition of the Mg/Al hydrotalcite. It is concluded that the synthetic charmarite is slightly less stable than hydrotalcite.
Abstract
Abstract
The effect of Cu/Al molar ratio on the high-temperature adsorption characteristics of CO2 on the mixed oxides of Cu–Al hydrotalcite skeletal structure has been studied by thermogravimetry. The Cu/Al molar ratio of the hydrotalcites synthesized was varied between 1.0 and 3.0, and the adsorption temperature ranged from ambient to 600 °C. The hydrotalcite with Cu/Al molar ratio of 2.0 was found to be the most suitable adsorbent for high-temperature CO2 adsorption, in both the capacity and the rate of adsorption. The activation energy values suggested that the physical adsorption dominates at low temperatures (<400 °C) and the chemisorption dominates at high temperatures (>400 °C).
Introduction Layered double hydroxides (LHDs) are also known as hydrotalcite like materials or anionic clays. Many LDHs such as hydrotalcite, takovite, carrboydite, reevesite, honessite, pyroaurite, and iowaite occur in nature
Introduction Hydrotalcites, or layered double hydroxides, which general formula may be represented as follows: [M 1− x II M x III (OH) 2 ] A x / n n − · n H 2 O, (M II , M III —metal cations, A n − —interlayer anion), are
Thermal decomposition of the hydrotalcite
Thermogravimetric analysis and hot stage Raman spectroscopic study
Summary
A combination of thermogravimetry and hot stage Raman spectroscopy has been used to study the thermal decomposition of the synthesised zinc substituted takovite Zn6Al2CO3(OH)164H2O. Thermogravimetry reveals seven mass loss steps at 52, 135, 174, 237, 265, 590 and ~780C. MS shows that the first two mass loss steps are due to dehydration, the next two to dehydroxylation and the mass loss step at 265C to combined dehydroxylation and decarbonation. The two higher mass loss steps are attributed to decarbonation. Raman spectra of the hydroxyl stretching region over the 25 to 200C temperature range, enable identification of bands attributed to water stretching vibrations, MOH stretching modes and strongly hydrogen bonded CO3 2--water bands. CO3 2- symmetric stretching modes are observed at 1077 and 1060 cm-1. One possible model is that the band at 1077 cm-1is ascribed to the CO3 2- units bonded to one OH unit and the band at 1092 cm-1is due to the CO3 2- units bonded to two OH units from the Zn-takovite surface. Thermogravimetric analysis when combined with hot stage Raman spectroscopy forms a very powerful technique for the study of the thermal decomposition of minerals such as hydrotalcites.</o:p>
Introduction Hydrotalcites are layered double hydroxides (LDHs) which show the brucite-like network characteristic of [Mg(OH) 2 ]. In brucite, Mg 2+ ions are octahedrally coordinated by hydroxyl anions, giving rise to the edge
