Catalytic ozonation of acetone and toluene on alumina-supported manganese oxide

Abstract

Volatile organic compounds (VOCs) are important air pollutants that can have adverse health effects. Ozone-assisted catalytic oxidation (catalytic ozonation) is an effective technique for removal of VOCs from air. In this thesis, catalytic ozonation was used to remove two VOCs (acetone and toluene) from air by using γ-alumina-supported manganese oxide catalyst. This study addresses the nature and role of reaction intermediates, role of support, reaction kinetic and pathway, and removal of a binary mixture of VOCs. A combination of in situ diffuse reflectance infrared Fourier transform spectroscopy, X-ray spectroscopy techniques, and a number of temperature programmed analyses were used to achieve objectives of this investigation. Catalyst characterization showed that Mn2O3 was the dominant manganese phase of the catalyst. It was found that surface carboxylate intermediates were essential for an effective oxidation process, and they did not directly cause catalyst deactivation. Despite different chemical properties of acetone and toluene, surface carboxylates formed on alumina sites of the catalyst during catalytic ozonation of both VOCs. The presence of manganese sites was necessary for further oxidation of the surface carboxylates. At low reaction temperatures (e.g. 25 °C), undesired products such as acetic acid and formic acid accumulated on the surface of the catalyst and reduced the activity of the catalyst. Deactivation caused by these compounds could be reversed by heating the spent catalyst to 425 °C (under nitrogen flow) and desorbing the undesired products. Apparent activation energies of 33 and 40 kJ mole-1 were obtained for catalytic ozonation of toluene and acetone, respectively. Catalytic ozonation of the binary mixture of acetone and toluene was favourable for toluene conversion, and repressive for acetone conversion. Increase of reaction temperature (up to 90 °C) improved catalyst stability, increased removal of both VOCs, enhanced COx yield, and decreased the gap between toluene and acetone conversions. Findings from this work were used to propose possible reaction pathways for catalytic ozonation of acetone and toluene.