Studies on the synthesis of nano-alumina in air and water stable ionic liquids

Abstract

In this paper we report on the synthesis of aluminium oxide nanoparticles in two different air and water stable ionic liquids namely, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl) amide [BMP]TFSA and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) amide [EMIm]TFSA. The ionic liquids [BMP]TFSA and [EMIm]TFSA exhibit biphasic behaviour beginning with an AlCl3 concentration of 1.6 mol L–1 and 2.5 mol L–1, respectively. Different aluminium oxides were prepared by the hydrolysis of AlCl3 dissolved in the employed ionic liquids, followed by calcinations. AlCl3·6H2O was found to be the main hydrolysis product of the AlCl3 containing ionic liquids. The as-prepared AlCl3·6H2O is subject to thermal decomposition producing different forms of transition oxides, depending on the original liquid composition. Alumina was also obtained by hydrolysis of as-prepared AlCl3·6H2O by NH4OH solution. Hydrolysis of the upper phase of the biphasic mixture of 3 M AlCl3–[EMIm]TFSA with NH4OH produces transparent alumina which is converted into α-Al2O3 with a low amount of δ-Al2O3 after calcinations at 1000 °C. The formation of pseudoboehmite and γ-Al2O3 as intermediates are observed at 200 °C and 550 °C, respectively. The obtained pseudoboehmite is mesoporous with an average pore diameter of 3.8 nm and a high surface area of 226 m2 g–1. The chloroaluminite obtained by hydrolysis of the homogenous mixture of 1 M AlCl3–[BMP]TFSA undergoes thermal decomposition yielding amorphous alumina at 200 °C, which transforms completely to well crystallized α-Al2O3 at 1000 °C. The obtained α-alumina is porous with an average pore size of about 10 nm. This suggests that the ionic liquid [BMP]TFSA acts as a template and that it decomposes thermally at elevated temperature producing pores.