GAS PHASE OXIDATION OF THE XYLENES. GENERAL KINETICS

The kinetics of the slow combustion of the three xylene isomers have been studied manometrically in a quartz vessel, under static conditions at subatmospheric pressures over the temperature range from 410 to 550 degrees, employing 1:1 to 1:20 hydrocarbon: oxygen mixtures. It was shown that W-max = k.P-n. where P is the total initial pressure, W-max the maximum rate developed and n is 2.8, 1.9 and 1.5 for m-, o- and p-xylene, respectively. Although W-max is affected by changes in mixture composition and temperature, the value of n is independent of these parameters. Arrhenius plots were linear between 410 and 550 degrees and the activation energies for the over-all oxidation process were calculated to be 38, 39 and 40 kcal./mole for o-, m- and p-xylene, respectively. The greater ease of oxidation of o-xylene was ascribed to the greater reactivity of the chain branching intermediate. The lifetime of this intermediate was calculated to be 2 min, for o-xylene as contrasted to 20 and 17 min, for m- and p-xylene, respectively. Addition of inert gases (He, A, N-2 and SF6) increased W-max thus suggesting that this rate is governed by a diffusion controlled process at the walla of the reaction vessel. Studies of the competitive oxidation of binary mixtures of the xylenes indicated that the chain propagation reactions proceed at essentially equal rates in all three oxidations and have nearly equal activation energies.