EFFECT OF PRESSURE ON THE OXIDATIVE COUPLING OF METHANE IN THE ABSENCE OF CATALYST

The oxidative coupling of methane was carried out in the absence of catalyst in a continuous flow setup at total pressures up to 1,000 kPa, temperatures from 950 to 1,230 K, and inlet molar ratios of CH4/O2 down to 2.5. At constant temperature and residence time, the conversions of methane and oxygen increase drastically with increasing pressure. At oxygen conversions higher than 80%, product selectivities are comparable at different pressures. The space-time yield of the C2 products reaches a level comparable to that required for industrial operations from 400 kPa on. A radical-reaction network consisting of 38 elementary reactions allows to describe the experimental data. To describe adequately the effect of total pressure, the pressure fall-off behavior of the rate coefficients for the unimolecular reactions in the network has to be taken into account explicitly. General features of the reaction mechanism do not depend on the total pressure. Methyl and hydrogen peroxy radicals are the most abundant radicals. The total pressure increase results in a drastic increase of the concentrations of the chain carriers, particularly the hydrogen peroxy radical. Higher pressures favor the oxidative route from ethane to ethylene compared to the pyrolytic route. Increasing the total pressure leads to an increase of the primary and a decrease of the consecutive Co formation relative to the coupling. The balance between these nonselective routes determines the effect of the total pressure on the integral selectivity to C2 products at different conversions. The major contribution to the integral CO selectivity comes from the oxidation of methyl radicals.