A Mechanistic Study on the Catalytic Combustion of Benzene and Chlorobenzene

The catalytic combustion of benzene (C6H6), hexadeuterobenzene (C6D6), and chlorobenzene (PhCl) was investigated under various conditions on a 2 wt% Pt/γ–Al2O3 catalyst. Typical conditions were 1000 ppm of organics in the inflow, contact times of ∼0.3 s, and 16% O2 in nitrogen at ∼1 bar. Benzene as such reacted very easily, much faster than PhCl per se, with T50% only ∼145°C. With C6H6/C6D6 the kinetic isotope effect ranged from 2.5 to 1.5 between 130°C and 160°C. Cocombustion of C6H6/C6D6/PhCl led to lower rates for the benzenes but higher rates for PhCl, to give comparable T50% values of around 250°C. Between 200°C and 300°C kH/kD was ∼1.6. Comparable results were obtained with C6H6/C6D6/C2Cl4. In this case the side reaction, chlorination, is visible from formed C6H5Cl and C6D5Cl; it appears to occur without H/D isotope effect. If the O2 concentration were increased from 8 to 14% combustion rates for C6H6 were increased to a limited extent; between 153°C and 213°C the order in O2 is ∼0.2. Also the conversion of PhCl was measured at 328°C with O2 partial pressures ranging from 1 to 16%; above 4% the conversion decreased, while the level of polychlorinated benzenes (PhClx) increased almost fivefold, from 0.55 to 2.5% of the PhCl input, when [O2] was raised from 4 to 16%. Cocombustion of PhCl and heptane gave much higher rates for the former, while the output of PhClx was greatly reduced; at 16% O2 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.