Formation of the pseudoboehmite structure in the system Al_x(An)₃–NH₄OH–H₂O

Hydrated aluminum oxide is widely used in various fields – from catalyst production to medicine. Several modifications of hydrated aluminum oxide are known, including gibbsite, bayerite, nordstrandite, pseudoboehmite, and boehmite. Of particular interest is the synthesis of Al₂O₃ · nH₂O with a pseudoboehmite structure, which represents a highly dispersed aluminum hydroxide with a developed specific surface area. This study investigates the influence of aluminum salt concentration and type, aging temperature, and duration on the phase composition, properties of the resulting hydrated aluminum oxide precipitates, and their structural and adsorption characteristics. It is shown that the precipitates obtained from sulfate, nitrate, and chloride solutions by ammonia neutralization exhibit poor filterability, with filtration coefficients in the range of 6.7 · 10⁻⁵ to 7.8 · 10⁻⁷ m/s. It was found that slightly crystallized pseudoboehmite forms from chloride- and nitrate-containing solutions with anion concentrations of 1.8 mol/L, whereas X-ray amorphous products form from sulfate-containing solutions of the same concentration. Dilution of the sulfate system to 0.9 mol/L SO₄²⁻ leads to the formation of crystalline pseudoboehmite, and further dilution to 0.3 mol/L SO₄²⁻ results in gibbsite formation. Lowering the aging temperature of the precipitates to 40 °C leads to the formation of poorly crystallized pseudoboehmite. This product is a powder composed of non-isometric particles sized 20.0–50.0 µm. The specific surface area of pseudoboehmite synthesized from aluminum sulfate was found to be higher than that of samples obtained from aluminum nitrate and chloride solutions, reaching 407.41 m²/g. This is attributed to the presence of pores with diameters of 3.826–4.544 nm – approximately 1.5 times smaller than those in the nitrate- and chloride-derived samples.

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