(79a) Quantum Chemical and Detailed Chemical Kinetic Modeling of Methylamine Oxidation: Applications to Atmospheric and Supercritical Water Chemistries

Previous attempts to model both
low-temperature atmospheric and supercritical water oxidation of methylamine
have failed due to a lack of elementary kinetics for peroxy radical reactions
involving methylamine and its free-radical derivatives [1].  We have explored the potential energy
surface (PES) for the reaction of CH₂NH₂ + O₂
using ab initio quantum chemical calculations with the CBS-QB3 method [2].  While the PES is similar to its
hydrocarbon analogue, CH₃CH₂ + O₂, there are
discernable differences both in kinetics and mechanisms due to the presence of
the nitrogen atom.  Using the
information from the PES, we have computed reaction rate constants from
transition state theory.  These
rate constants are then used as input to improve an existing detailed chemical
kinetic model (DCKM) [1] to describe the homogeneous oxidation of methylamine
under both atmospheric and supercritical water conditions.  In the case of atmospheric methylamine
oxidation, we probe the effect of the low-temperature peroxy-radical reactions
and water concentration on the fundamental kinetics and mechanisms at 350°C,
with particular attention given to the yet unexplained large experimental
ammonia yields [3].  For
methylamine supercritical water oxidation, we model reactivity at 683 K and 249
atm.  In both cases, we use our
mechanistic simulations to determine global reaction orders and activation
energies.  Reaction pathway and
sensitivity analyses are used to identify the main routes of chemical
transformations and most important elementary reactions during both types of
homogeneous methylamine oxidation.

(1) Benjamin, K.M.; Savage, P.E. Ind. Eng. Chem. Res. 2005,
44, 9785.

(2) Montgomery, J.A., Jr.; Frisch, M.J.; Ochterski, J.W.;
Petersson, G.A. J. Chem. Phys. 1999, 110, 2822.

(3) Cullis, C.F.; Willsher,
J.P. Proc. R. Soc. London Ser. A 1951, 209, 218.