A Mechanistic Study on the Catalytic Combustion of Benzene and Chlorobenzene

Abstract

The catalytic combustion of benzene (C 6H 6), hexadeuterobenzene (C 6D 6), and chlorobenzene (PhCl) was investigated under various conditions on a 2 wt% Pt/γ–Al 2O 3 catalyst. Typical conditions were 1000 ppm of organics in the inflow, contact times of ∼0.3 s, and 16% O 2 in nitrogen at ∼1 bar. Benzene as such reacted very easily, much faster than PhCl per se, with T 50% only ∼145°C. With C 6H 6/C 6D 6 the kinetic isotope effect ranged from 2.5 to 1.5 between 130°C and 160°C. Cocombustion of C 6H 6/C 6D 6/PhCl led to lower rates for the benzenes but higher rates for PhCl, to give comparable T 50% values of around 250°C. Between 200°C and 300°C k H/ k D was ∼1.6. Comparable results were obtained with C 6H 6/C 6D 6/C 2Cl 4. In this case the side reaction, chlorination, is visible from formed C 6H 5Cl and C 6D 5Cl; it appears to occur without H/D isotope effect. If the O 2 concentration were increased from 8 to 14% combustion rates for C 6H 6 were increased to a limited extent; between 153°C and 213°C the order in O 2 is ∼0.2. Also the conversion of PhCl was measured at 328°C with O 2 partial pressures ranging from 1 to 16%; above 4% the conversion decreased, while the level of polychlorinated benzenes (PhCl x ) increased almost fivefold, from 0.55 to 2.5% of the PhCl input, when [O 2] was raised from 4 to 16%. Cocombustion of PhCl and heptane gave much higher rates for the former, while the output of PhCl x was greatly reduced; at 16% O 2 from 2.5% for combustion of PhCl per se, to 0.25% with 2.3 mol of heptane per mole PhCl in the feed. Water had a much less beneficial effect. The mechanism(s) are discussed on the basis of the operation of (at least) two different types of active sites. In the absence of chlorine a CH(D) bond in sorbed benzene is split, and the surface-bound H and phenyl moieties are oxidized, most likely via phenoxyl entities which are subject to rapid breakdown. Chlorine—e.g., formed from added PhCl upon its combustion—acts as a poison, the more so when using PhCl alone. Then, a slow CCl bond activation occurs on another type of site. Added heptane, through its hydrogen, can remove Cl from the metal surface and regenerate the sites for sorption and CH bond activation. The side reaction, (oxy)chlorination, is best described as recombination of a surface-bound phenyl entity with—electrophilic—chlorine, presumably at an oxidized Pt site.