Ammonium Aluminum Carbonate Hydroxide Precursor Research Articles

Effects of chlorides on the phase transformation of alumina

The discovery that γ-alumina can be prepared at lower calcination temperature from an ammonium aluminum carbonate hydroxide (AACH) precursor templated with rosinyl trimethyl ammonium chloride has led to the assumption that Cl- is capable of lowering the temperature of the phase transformation of AACH to γ-alumina. To verify this hypothesis, several chlorides were introduced into the calcination process of the AACH precursor. The samples were characterized using X-ray diffraction, thermogravimetry and differential scanning calorimetry as well as X-ray photoelectron spectroscopy. The results showed that Cl- has great effects on the crystallinity of γ-alumina obtained by the calcination of AACH. The effects varied with the type, dosage and introduction method of the particular chloride. It was also found that Cl- began to take effect only after the AACH had been decomposed to form amorphous alumina. Therefore, the true function of Cl- is to decrease the temperature of the phase transformation from the amorphous phase to γ-alumina: the phase transformation temperature decreases by as much as 115 K.

Hydrodesulfurization of 4,6-Dimethyldibenzothiophene over CoMo Catalysts Supported on γ-Alumina with Different Morphology

Nanostructured γ-alumina with two different morphologies (rod-like and cube-like) was used as support for CoMo hydrodesulfurization catalyst. Both γ-aluminas were prepared by thermal decomposition of ammonium aluminum carbonate hydroxide precursor, which was synthesized by a convenient hydrothermal method at two pH values. Fourier transform infrared spectroscopy of prydine adsorption, thermogravimetric analysis, and 27Al magic angle spinning (MAS) NMR showed that the rod-like γ-alumina exhibited a lower acidity than the cube-like γ-alumina. The result of X-ray diffraction and temperature-programmed reduction indicated that CoMo oxidic catalysts supported on the rod-like γ-alumina presented higher reducibility compared to those of cube-like γ-alumina, because more β-CoMoO4 was formed on the surface of the rod-like γ-alumina than that of the cube-like γ-alumina. After sulfidation, a large stack with slightly longer MoS2 slabs was formed on the rod-like γ-alumina supports, thereby creating a catalyst with hi...

A Decomposition Route for Synthesis of Al2O3 Crystal and its Applications as Thermal Conductivity Filler

Platelike α‐Al2O3 powder was obtained at 850°C by thermal decomposition of ammonium aluminum carbonate hydroxide precursor, which was treated by hydrofluoric acid. The results showed the decomposition of the as‐prepared precursors experienced via three steps, adsorption of physical water, dehydration of crystalline water, and decomposition of the precursor, and the activation energies of the above three steps were calculated using the Coats–Redfern method. Meanwhile, the intermediate of (NH4)3AlF6 obtained played an important role in decreasing activation energy of the decomposition and phase transformation. Moreover, the thermal conductivity of the epoxy resin filled with platelike alumina was improved obviously.

Fabrication of high-surface-area γ-alumina by thermal decomposition of AACH precursor using low-temperature solid-state reaction

In this paper, systematical investigation has been made for the new simple synthesis pathway of catalytic alumina with high surface area through thermal decomposition of ammonium aluminum carbonate hydroxide (AACH), which has been prepared by a low-temperature solid-state reaction in non-aqueous system. The results indicate that the molar ratios of NH4HCO3/Al influence crystallinities and the morphologies of AACH. Well-crystallized AACH with nanorod morphology and low-crystallized AACH with cluster structure can be obtained with different molar ratios NH4HCO3/Al. The γ-Al2O3 product with high surface area (511m2/g) can be obtained from the calcination of AACH at 500°C. Moreover, the results also indicate that crystallinities and morphologies of AACH affect their thermal decomposition behaviors. In addition, the formation of high-surface-area is possibly attributed to intracrystalline porosity of as-prepared γ-Al2O3.

Rapid Transformation of α-Alumina Nanocomposite Powders by Microwave Assisted Combustion

The microwave-assisted combustion synthesis as a route to obtain ultrafine α-alumina and magnesium aluminate composite powders starting from the ammonium aluminum carbonate hydroxide precursor was investigated. The synthetic temperature and the crystallite size of the α- alumina nanocomposite powder were significantly affected by the environmental atmosphere in the microwave assisted combustion. The α-alumina and spinel composite powder was obtained by the microwave-assisted combustion at temperature of 1000°C under H2/N2, but at 1150°C by the normal heat process under air. The rapid transformation from γ- to α-phase was achieved by microwave assisted nucleation at low temperature of 900°C under H2/N2 atmosphere. The least crystallite size of 26.2 nm was obtained under H2/N2 atmosphere at 1000°C for 10 min.

Fabrication and synthesis of α-alumina nanopowders by thermal decomposition of ammonium aluminum carbonate hydroxide (AACH)

An α-Al2O3 nanopowder was prepared via the thermal decomposition of ammonium aluminum carbonate hydroxide (AACH). The AACH precursor, which had an average crystallite size of 5–8nm, was fabricated at a reaction temperature of 8°C with the pH of the aqueous ammonium hydrogen carbonate (AHC) precursor solution set to 10, which afforded the highest [NH4+][AlO(OH)n(SO4)−3−n/2]][HCO2−] ionic concentration. The AACH precursor was transformed to NH4Al(SO4)2, rhombohedral (Al2(SO4)3), amorphous-, θ-, and α-Al2O3 according to the heat-treatment temperature. A time-temperature-transformation (TTT) diagram for thermal decomposition in air was determined. The critical particle size of θ-Al2O3 in the θ→α phase transformation was ∼30nm. Homogeneous, spherical α-Al2O3 nanopowders with an average particle size of 70nm were prepared by firing the precursor crystallites at 1150°C for 3h in air.