Jianli Yu

KINETICS AND MECHANISM OF METHANE OXIDATION IN SUPERCRITICAL WATER

We oxidized methane in supercritical water at 250 atm and at temperatures between 525 and 587 °C. The methane conversions ranged from 3 to 70%. CO was the product present in the highest yields at low conversions (< 10%), but CO2 became the most abundant product at higher conversions. These experimental results were used to test the predictions of a detailed chemical kinetics model, which is based on gas-phase oxidation mechanisms and kinetics and comprised 150 elementary reaction steps. The model predicted methane disappearance rates that were about 30–50% faster than those observed experimentally. This behavior led to consistently high predictions of the methane conversion and the CO2 yield. However, the model accurately predicted the yields of CO and CO2 as a function of the methane conversion. The predicted activation energy for the pseudo-first-order rate constants of 36 ± 3 kcal/mol is similar to the experimental value of 44 ± 6 kcal/mol. Overall, the ability of the model to predict several of the experimental observations demonstrates that the analogy between gas-phase oxidation and oxidation in supercritical water is a good one. A sensitivity analysis revealed that the calculated methane concentration is most sensitive to the kinetics of OH + H2O2 = HO2 + H2O, OH + HO2 = H2O + O2, H2O2 = OH + OH and HO2 + HO2 = O2 + H2O2. These reactions control the concentration of OH radical, which is the main oxidant under SCWO conditions.

RECENT ADVANCES IN CATALYTIC OXIDATION IN SUPERCRITICAL WATER

ABSTRACT This article summarizes recent research in homogeneous and heterogeneous catalytic oxidation in supercritical water. We consider both selective partial oxidation for chemical synthesis and complete oxidation for waste destruction. Recent advances in selective catalytic oxidation in supercritical water center around the conversion of p-xylene to terephthalic acid catalyzed homogeneously by MnBr2. Terephthalic acid yields of >90 mol% can be achieved from reactions at 400°C. Using water as the reaction medium provides genuine opportunities for both a more economical and more environmentally benign terephthalic acid production process. Recent advances in complete oxidation via catalysis in supercritical water include the demonstration of heteropolyacids as effective homogeneous oxidation catalysts, and alkali carbonates and carbons as effective heterogeneous catalysts. Additionally, progress has been made in screening transition metal oxide catalysts and determining the reaction-induced chemical and physical changes that take place in the hydrothermal environment. Bulk MnO2 is a good catalyst for complete oxidation because it combines high activity, hydrothermal stability, activity maintenance, and resistance to metal leaching under reaction conditions.