Nature and Properties of Pseudoboehmites Formed in the Presence of Organic and Inorganic Ligands

Abstract

AbstractPoorly crystalline aluminum oxyhydroxides (pseudoboehmites), with similar x‐ray diffractograms, were synthesized at room temperature at the initial Al concentration of 2 × 10−3 mol L−1 and pH 8.2 in the presence of selected inorganic or organic ligands (1.4 mol L−1 chloride; 0.14 mol L−1 sulfate; 2 × 10−5 mol L−1 citric, tartaric, or tannic acid; 4 × 10−5 mol L−1 tannic acid). Electron micrographs show that these poorly crystalline materials range from fibrous shape to shapeless fine colloids (< 10 nm) or their aggregates before air drying. However, after air drying, the pseudoboehmites consist of fibrillar particles as well as many large aggregates (> 5 µm) even after ultrasonification. The amount of ligands in the precipitates varies from 0.1% for chloride (28 mmol kg−1) to 29.8% for tannate (175 mmol kg−1). The specific surface of the pseudoboehmites ranges from 30.0 hm2 kg−1 to 58.0 hm2 kg−1 due to the presence of foreign ligands which promote structural distortion in the precipitation products and/or aggregation of the particles. The PZC values of the pseudoboehmites formed in the presence of inorganic and organic ligands vary from 8.0 to 9.3. The net positive charge of the poorly crystalline aluminum oxyhydroxides varies with the kinds of ligands present during their precipitation. The data obtained in this study show that pseudoboehmite samples formed under the varying ionic environments differ significantly in their chemical composition and surface properties.