The catalytic oxidation of chlorinated aromatics over vanadium pentoxide/titanium dioxide catalysts

Small amounts of toxic polychlorinated aromatic compounds (i.e. polychlorinated dibenzodioxins [PCDDs] and dibenzofurans [PCDFs]) are formed during the thermal incineration of municipal and medical waste. V2O5/TiO 2-based catalysts are known to be active for the catalytic oxidation of PCDDs/PCDFs and are used successfully in commercial applications. Nevertheless, the mechanistic aspects of the oxidative destruction of chlorinated aromatics over V2O5/TiO2-based catalysts are still not clearly understood.; In our efforts to advance further this understanding, we have conducted kinetic and in situ infrared spectroscopic studies with several model compounds over V2O5/TiO2 catalysts. In particular, the oxidation of six-carbon chlorinated aromatic (i.e., chlorobenzene, 1,2-, 1,3-, and 1,4-dichlorobenzene, and 2-chlorophenol) and aliphatic (i.e., cyclohexyl-chloride) compounds, as well as the oxidation of benzene were examined. Observed differences in the reaction rates and activation energies can be correlated to structural differences of these compounds, in light of a common reaction mechanism.; The first step of this reaction mechanism is the formation of a surface phenolate species as observed in the in situ FTIR studies. The formation of this phenolate species proceeds either through a nucleophilic substitution on a chlorine atom on a transition metal oxide site in the case of the chlorinated benzenes or through the elimination of water on a surface hydroxyl or an oxide site in the case of phenols. Finally, benzene adsorbs through the abstraction of a hydrogen atom. The adsorbed phenolate then undergoes an electrophilic substitution on the aromatic ring forming several partial oxidation intermediates observed by in situ FTIR, which are similar for the various aromatic substrates studied. These species are formed on the catalyst surface even in the absence of gas phase oxygen, indicating that surface oxygen is involved in their formation.; Results of field studies suggest that carbon deposition is a possible reason for the deactivation of commercial vanadia/titania catalysts used for PCDD/PCDF control. This deactivation behavior was simulated in the lab with the use of 2-methylnaphthalene (mNpht) a known coke-precursor, which was added to the reacting gas mixture during the oxidation of m-DCB over V2O5/TiO2. Depending on the reaction temperature, catalysts deactivated by mNpht could be regenerated upon removal of mNpht from the reaction gas mixture. FTIR results indicate the presence of different oxygenated (e.g., phthalic anhydride, quinone and lactone-type species) and carboxylate (e.g., phthaleates) species on the catalyst surface under such conditions.