The selective oxidation of ethylbenzene to acetophenone with tert-butyl hydroperoxide in the presence of an SiO2/Al2O3-supported manganese catalyst occurs with a conversion of 91% and a selectivity of 98%, in the absence of an organic solvent, in supercritical carbon dioxide. The catalysts could be reused at least eight times without significant loss of activity. This procedure simplifies the isolation of the reaction products and has the advantage of using only carbon dioxide as a solvent under mild conditions. The electrochemical data for Mn nanocatalysts immobilized at the surface of multi-walled carbon nanotubes were also studied. The results indicate that the Mn catalysts anchored on the modified SiO2/Al2O3 mixed-oxide via 2-aminoethyl-3-aminopropyltrimethoxysilane (2-AE-3-APTMS) have an easily oxidizable environment, lower oxidation potential and lower charge-transfer resistance, thus leading to a higher catalytic activity and selectivity than when 3-aminopropyltrimethoxysilane (3-APTES) is used. (C) 2012 Elsevier B.V. All rights reserved.