Oxidacao de o-Xileno a anidrido ftalico sobre catalisadores de V_2O_5 suportados em TiO_2 (Anatase)

Abstract

The oxidation of o-xylene to phthalic anhydride, over V_2O_5/TiO_2 (anatase) catalysts, was studied. Catalysts were prepared by wet impregnation with NH_4VO_3-oxalic acid solutions and grafting method employing VO(O"1Bu)_3. Catalysts characterization was carried out by chemical and thermogravimetric analysis, LRS, FTIR, XRD, XPS and TPR. New vanadia species coordinated to the titania support surface were formed. Crystalline V_2O_5 was detected for loadings higher than the required for the theoretical monolayer coverage of the anatase support surface. For low V_2O_5 loadings (<1% wt) uncoated titania was detected. For o-xylene oxidation the influence of the organic reactant partial pressure and the contact time was investigated in the temperature range 533-633 k. Besides phthalic anhydride, o-tolualdehyde, phthalide and carbon dioxide were found as reaction products. Higher phthalic anhydride selectivities were obtained for catalysts containing the monolayer loading of vanadia. The oxidation reactions of o-tolualdehyde and phthalide, which are intermediates in the catalytic oxidation of o-xylene, were studied for different reactant concentrations, contact times and reaction temperatures. At low temperatures and low contact times part of the reactant fed was not obtained as a product detected by chromatography; this can be attributed to the formation of polymeric compounds that can either desorb or stay over the catalyst surface as a carbonaceous deposit (coke). o-Xylene and o-tolualdehyde adsorbed species are important coke sources. Low coke levels were obtained for phthalide oxidation..Available from Fundacao para a Ciencia e a Tecnologia, Servico de Informacao e Documentacao, Av. D. Carlos I, 126, 1200 Lisboa / FCT - Fundação para o Ciência e a TecnologiaSIGLEPTPortuga