Oxidation of o-xylene to phthalic anhydride over V2O5/TiO2 catalysts .3. Study of organic residue formed on the catalyst surface

An organic residue is formed on V2O5/TiO2 (anatase) catalysts during the oxidation of o-xylene and o-tolualdehyde. V2O5 catalysts supported on TiO2 (0.6, 1, and 5% V2O5/TiO2), prepared by wet impregnation, were exposed to o-xylene/air and o-tolualdehyde/air mixtures under different operating conditions, and, after different times of exposure to reaction conditions, the catalysts were characterized by FTIR spectroscopy and temperature programmed oxidation. The compounds formed were also extracted and analyzed by mass spectrometry. Organic molecules containing no more than two aromatic rings, formed by dimerization of adsorbed molecules, were detected and their characteristics were found to vary with features of the catalytic surface and with operating conditions. Larger contents of residue were obtained at lower temperatures and lower contact times and, for some experimental conditions, a constant amount of such compounds was observed after 1 h of exposure under reaction conditions. The analysis of samples used with different reactant mixtures showed that adsorbed o-xylene can lead to the formation of a ''residue'' on the surface. (C) 1996 Academic Press, Inc.