Experimental and Modeling Study of the Effects of NaOH on Propane Oxidation Kinetics in Supercritical Water

Catalytic oxidation in supercritical water represents an important potential pathway to cogenerate valuable chemicals and energy from biomass and waste. The influence of NaOH catalyst addition on propane oxidation kinetics in supercritical water was investigated in this work, and potential reaction rate changes to explain the observed effects were identified. This was achieved first through an experimental study in the Eastern Michigan University Supercritical Batch Reactor system and then through a modeling study based on perturbations of an existing detailed chemical kinetic model. Test mixtures were 0.014% propane by volume with a propane-to-oxygen equivalence ratio of 0.8 and the remainder water with NaOH at 0.1 M (pH approx.13.5) for a total mixture density of 222 mg/mL. Reaction times ranged from 8 to 30 min for a temperature of 375 °C and pressure of 220 bar. Experimental observations revealed that while the rate of propane consumption and the type of reaction products were unaffected by the addition of NaOH, the yield of oxygenates increased significantly. The peak yield of methanol and ethanol increased by two times, while the peak yield of acetone increased from near zero to 20%. The yields of propene, ethene, and methane were not significantly affected. Subsequent modeling analysis indicated that the effect of NaOH addition is likely on the R–O2 kinetics, which are the primary pathways to oxygenate formation at these conditions. A consistent predicted increase in R–O + H2O2 = R–OH + HO2 rates for all oxygenates was found, which could be related to deprotonation of R–O2H to R–O2-, HO2 to O2-, or H2O2 to HO2- in the strongly basic mixture. Results specific to acetone suggest that NaOH affects the product branching from the i-C3H7O radical, which is possibly related to catalysis of ionic isomerization reactions. The present results motivate and guide both broader investigation of catalytic oxidation for other compounds and more focused investigation into fundamental ionic reaction kinetics at these conditions.