Formation of dioxins in the catalytic combustion of chlorobenzene and a micropollutant-like mixture on Pt/γ-Al2O3

Catalytic combustion over a 2 wt % Pt/gamma-Al2O3 catalyst of chlorobenzene (PhCl) and of a micro pollutant-like mixture representative for a primary combustion offgas has been investigated. Typical conditions were 1000-1500 ppm of organics in the inflow, contact times similar to0.3 s, 16% O-2 in nitrogen at similar to1 bar, and temperature range 200-550 degreesC. PhCl reacts considerably slower than when processing Cl-free compounds such as heptane. At intermediate temperatures-and incomplete conversion-byproducts are formed, especially polychlorobenzenes (PhCl,). These are accompanied by polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) at levels of about 10(-6) relative to PhCl,. Additional HCI-made by co-reacting PhCl with tert-butylchloride-leads to much higher levels of PhClx and PCDD/Fs. Using the micropollutant-like mixture, the total chlorine input is reduced almost 20-fold, but it nevertheless leads to a 30-fold higher PCDD/F output. This is ascribed to reaction of the small amounts of (chloro)phenols in the mixture. The congener/isomer patterns of the PCDC/Fs for the mixture and with PhCl per se are quite comparable with those found in emissions from incinerators. As carbon is not present nor formed on the catalyst surface, de-novo formation there from cannot be involved. Rather condensation of phenolic entities or like precursors must have occurred. Consequences and options to ensure safe application are briefly discussed as well.