Formation of dioxins in the catalytic combustion of chlorobenzene and a micropollutant-like mixture on Pt/gamma-Al2O3.
Catalytic combustion over a 2 wt % Pt/gamma-Al2O3 catalyst of chlorobenzene (PhCl) and of a micropollutant-like mixture representative for a primary combustion offgas has been investigated. Typical conditions were 1000-1500 ppm of organics in the inflow, contact times approximately 0.3 s, 16% O2 in nitrogen at approximately 1 bar, and temperature range 200-550 degrees C. PhCl reacts considerably slower than when processing Cl-free compounds such as heptane. At intermediate temperatures--and incomplete conversion--byproducts are formed, especially polychlorobenzenes (PhClx). These are accompanied by polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) at levels of about 10(-6) relative to PhClx. 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 PCDD/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 therefrom 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.
De Jong
Vincent
V
Center for Chemistry and the Environment, Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, NL-2300 RA Leiden, The Netherlands.
Cieplik
Mariusz K
MK
Louw
Robert
R
D000537:Aluminum Oxide / Q000737:chemistry; D002384:Catalysis; D002722:Chlorobenzenes; D004147:Dioxins / Q000737:chemistry*; D005740:Gases; D017745:Incineration; D010984:Platinum / Q000737:chemistry
D002722:Chlorobenzenes; D004147:Dioxins; D005740:Gases; D010984:Platinum; C031294:chlorobenzene; D000537:Aluminum Oxide