Text provided by Philip A. Marrone [ChemE]
Supercritical water oxidation is an innovative and promising technology that can be used for the destruction of hazardous organic wastes. Above its critical point (374oC, 221 bar), water behaves as a nonpolar rather than polar solvent, due primarily to the loss of hydrogen bonding that occurs under these conditions and indicated by a decrease in the dielectric constant from 80 at ambient conditions to <5 above the critical point. Thus nonpolar organics are completely soluble in supercritical water along with O2, and can be rapidly and efficiently oxidized to CO2 and H2O. Our research is focused on determining the reaction kinetics and overall mechanisms for both oxidation and hydrolysis (no oxygen) of model organic compounds in supercritical water.
Methylene chloride (CH2Cl2) is one model compound we have studied that undergoes significant hydrolysis under supercritical conditions. The reaction is an ionic based nucleophilic substitution reaction between CH2Cl2 and H2O with a polar transition state complex intermediate. In experiments we conducted, it was observed that most of the CH2Cl2 breakdown occurred in a non-isothermal, subcritical section of preheater tubing rather than the isothermal, supercritical main reactor. This result is not that surprising, given that the loss of polarity of the water solvent as it is heated from sub- to supercritical conditions makes it harder and harder to stabilize the polar intermediate, thereby slowing the reaction. However, this phenomenon cannot be captured in the standard Arrhenius form for the reaction rate constant alone. Instead, a modification must be made to the rate constant form to account for the changing nature of the water solvent environment. This required modification is given by Kirkwood theory as a correction term to the rate constant and depends mainly on the dielectric constant of the solvent medium, but also on temperature and the dipole moment and radius of the reactant molecules and their transition state complex.
Although tabulated values of the dipole moment do exist for some stable molecules, there are no such tabulations for unstable intermediate species. One can attempt to calculate the dipole moment using group contribution methods and vector addition, but this procedure can be very tedious for anything but very simple species and suffers from inaccuracies due to the ignoring of many compound specific second order effects. The most straightforward method to calculate the dipole moment of any species (stable or unstable) is through ab initio calculations to first determine the charge density surrounding a species, and then integrating over volume to arrive at the dipole moment according to its definition as the first moment of charge. These calculations are currently being performed via FORTRAN code on the xolas system.