Structural and Morphological Features of Disperse Alumina Synthesized Using Aluminum Nitrate Nonahydrate

Transformation of Al(NO3)3∙9H2O (upon heating in the range of 20-1200 °C) into blends of amorphous and crystalline boehmite (210-525 °C), amorphous alumina and crystalline γ-Al2O3 (850 °C), and crystalline α-Al2O3 (1100 °C) was analyzed using X-ray diffraction (XRD), high-resolution transmission electron microscope (HRTEM), infrared (IR) spectroscopy, thermogravimetry, and low-temperature nitrogen adsorption. Boehmite consists of nanoparticles of 6-10 nm in diameter, and part of them has crystalline structure observed in HRTEM images, despite they are XRD amorphous. The nanoglobules are surrounded by amorphous aluminum hydroxide with chains of -AlO(H)-O-AlO(H)- of 1-5 nm in length. Heating of samples at 350-525 °C gives mesoporous aluminum hydroxide with a relatively narrow pore size distribution. An increase in calcination temperature to 850 °C decreases the porosity of alumina composed of amorphous and crystalline (γ-Al2O3) phases. Calcination at 1100 °C gives α-Al2O3 with strongly decreased porosity of aggregates.