The Formation and Structural and Phase Transformations of Aluminum Hydroxy Species in Hydrothermal Synthesis under Conditions of Homogeneous Precipitation from Sulfate Solution

Abstract

By X-ray powder diffraction analysis, Raman spectroscopy, scanning and transmission electron microscopy, and low-temperature nitrogen adsorption method, it was studied how the temperature of hydrothermal synthesis affects the phase composition of precursor species and the structure and morphology of alumina obtained under conditions of low-temperature hydrothermal synthesis at 90–130°C in the system 0.5Al₂(SO₄)₃ · 18H₂O∥0.2C₁₆H₃₃(CH₃)₃NBr∥5i-PrOH∥85H₂O∥2CO(NH₂)₂. It was found that, at T_HTS = 90–110°C, alumina microspheres with globular structure form from X-ray amorphous hydrated alumina with an admixture of NH₄Al₃(SO₄)₂(OH)₆. It was shown that needle-like alumina particles with slitlike pores form from NH₄Al(OH)₂CO₃ obtained at T_HTS = 110–120°C. At T_HTS = 130°C, under conditions of the formation of γ-AlOOH, alumina spherulites constituted by needle-like particles with block–layer structure form. It was determined that the temperature range of the formation of metastable crystalline modifications of alumina is controlled by the phase composition of hydrothermal synthesis products.