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.