Catalytic properties of carbon xerogels for wet oxidation of aniline

Wet air oxidation (WAO) is an old process developed in the middle of the XX century, but still the centre of very active and contemporary research especially in what concerns catalyst development. In ideal conditions soluble hydrocarbons can be oxidized in aqueous solution to carbon dioxide and water. Organic pollutants with heteroatoms are more challenging, because they require harsh conditions for complete oxidation and pose the risk of increased toxicity in case of incomplete degradation. Among known critical pollutants there is aniline, which presents carcinogenicity and toxicity (reproductive, developmental, neural and acute) to humans and to aquatic organisms. It is also a model compound for pesticide like molecules. Our previous interest in the catalytic WAO (CWAO) of aniline using metal supported catalysts (Gomes et al, 2004) prompted us to study the development of highly mesoporous materials which can be very efficient not only as catalytic supports, but as catalysts themselves. Using the conventional sol-gel approach (formaldehyde to resorcinol ratio of 2:1) we have fabricated different carbon xerogels with various degrees of mesoporous surface area. The materials were characterized by N2-adsorption, TG analysis and SEM. We synthesized four different base materials. They were further activated in order to explore the effect of the surface oxidation on the activity and stability of the materials. They in the CWAO of aniline aqueous solutions and the results quantified in terms of substrate conversion and TOC removal. The experimental apparatus for WAO consisted of batch high pressure reactor described elsewhere (Gomes et al, 2004). A recent review concerning carbon materials for CWAO (Stuber et al, 2005), shows that the studies on the stability of these materials has been considerably neglected. This is mainly due the fact that experiments of this nature are time consuming. We made repetition trials adding fresh substrate to the reactor in order to test the continued material performance. We compared the activity of the synthesized materials with that of different commercial activated carbons. As representative example, a carbon xerogel with a mesoporous surface area of 199 m/g (SMES=776 m/g) led to a complete aniline conversion after one hour oxidation, with a corresponding TOC removal of 83% (measured after 3 hours), under conditions of 200oC and 6,9 bar oxygen partial pressure (air used oxidant, total pressure 50 bar). The selectivity towards CO2 was calculated indirectly, from the TOC measurement as being 84%. We concluded that the conversion is directly related with the mesoporous character of the material. However the performance does not depend only on the texture, but also on the degree of surface oxidation of the carbon xerogel, which we found to be determinant in the stability of the material under the operating conditions. Acknowledgments. This research was carried out under the projects POCI/58252/EQU/2004, POCTI/1181/2003 and SFRH/BD/16565/2004, approved by the Fundação para a Ciência e a Tecnologia (FCT), Programa Operacional (POCTI/POCI) and co-supported by FEDER. References Stüber F., Font J., Fortuny A., Bengoa C., Eftaxias A. and Fabregat A. (2005). Carbon materials and catalytic wet air oxidation of organic pollutants in wastewater. Topics in Catalysis, 33, 3-50. Gomes H.T., Samant P.V., Serp P., Kalck P., Figueiredo J.L. and Faria J.L. (2004). Carbon Nanotubes and Xerogels as Supports of Well Dispersed Pt Catalysts for Environmental Applications. Applied Catalysis B: Environmental,;54, 175-182.