Simplified waste-free process for synthesis of nanoporous compact alumina under technologically advantageous conditions

Precipitated ammonium aluminium carbonate hydroxide (NH₄Al(OH)₂CO₃) is a promising precursor for preparation of nanostructured Al₂O₃. However, the experimental conditions, such as the low concentration of Al³⁺ salt solution, high temperature and/or pressure, long reaction time, and excessive amount of the (NH₄)₂CO₃ precipitating agent, make this process expensive for large-scale production. Here, we report a simpler and cheaper route to prepare nanostructured alumina by partial neutralisation of a nearly saturated aqueous solution of Al(NO₃)₃ with (NH₄)₂CO₃ as a base at pH < 4. Synthesis in the acidic region led to formation of a polynuclear aluminium cluster (Al₁₃), which is an important “green” solution precursor for large-area preparation of Al₂O₃ thin films and nanoparticles. Control of the textural properties of the final alumina product during calcination of the prepared aluminium (oxy)hydroxide gel was accomplished by adding low-solubility aluminium acetate hydroxide (Al(OH)(CH₃COO)₂) as a seed to the Al(NO₃)₃ solution before neutralisation. The large Brunauer–Emmett–Teller specific surface area (376 m² g⁻¹) and narrow pore size distribution (2–20 nm) of the prepared compact alumina suggest that the chelating effect of the acetate ions affects the structures of the forming transition aluminas, and the evolved gases produced by decomposition of Al(OH)(CH₃COO)₂ and NH₄NO₃ as a by-product of the reaction during calcination prevent particle agglomeration. Other advantages of the proposed process are its versatility and the ability to obtain high purity materials without producing large amounts of by-products without the need for washing and energy saving by using a low processing temperature, and the possibility of recycling the generated CO₂ and NH₃ gases as the (NH₄)₂CO₃ reagent.

Flow chart of the proposed process for production of nanoporous alumina monoliths.