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

Precipitated ammonium aluminium carbonate hydroxide (NH 4 Al(OH) 2 CO 3 ) is a promising precursor for preparation of nanostructured Al 2 O 3 . However, the experimental conditions, such as the low concentration of Al 3+ salt solution, high temperature and/or pressure, long reaction time, and excessive amount of the (NH 4 ) 2 CO 3 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(NO 3 ) 3 with (NH 4 ) 2 CO 3 as a base at pH < 4. Synthesis in the acidic region led to formation of a polynuclear aluminium cluster (Al 13 ), which is an important "green" solution precursor for large-area preparation of Al 2 O 3 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)(CH 3 COO) 2 ) as a seed to the Al(NO 3 ) 3 solution before neutralisation. The large Brunauer-Emmett-Teller specific surface area (376 m 2 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)(CH 3 COO) 2 and NH 4 NO 3 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 CO 2 and NH 3 gases as the (NH 4 ) 2 CO 3 reagent.