Spectroscopy of Hydrothermal Reactions. 1. The CO₂−H₂O System and Kinetics of Urea Decomposition in an FTIR Spectroscopy Flow Reactor Cell Operable to 725 K and 335 bar

A combined microflow reactor and short-path-length spectroscopy cell along with the accompanying process controls are described to obtain real-time, in situ transmission IR spectra of reaction components of aqueous solutions up to 725 K and 335 bar. Quantitation of the spectra was required to obtain kinetics and equilibrium constants. The extinction coefficient of CO2 in H2O at 275 bar was found to increase monotonically from 1.52 × 106 at 298 K to 2.26 × 106 cm2 mol-1 at 573 K. Also, CO2 dissolved in H2O was rotationally quenched on the IR time scale below 375 K but progressed into rotational diffusion around 625 K and finally essentially free rotation above 700 K. The kinetics and pathway of hydrothermolysis of urea to CO2 and NH3 were determined directly from spectral data at 473−573 K. Good agreement was obtained between experimental and calculated concentration−time data by using a reaction model consisting of (NH2)2CO → NH4+ + OCN- and NH4+ + OCN- + H2O → CO2 + 2NH3. The Arrhenius parameters for the first-order reaction are Ea = 84.2 kJ mol-1 and ln A (s-1) = 17.5, and for latter pseudo-second-order reaction are Ea = 58.5 kJ mol-1 and ln A (L mol-1 s-1) = 17.1. The global rate of formation of CO2 without the kinetic model is first-order and has different Arrhenius parameters. As part of this study, the species of 0.1 m (NH4)2CO3 equilibrium were determined at 298−650 K and 275 bar. The equilibrium shifted from the hydrolyzed ionic components at lower temperature to the neutral CO2, NH3, and H2O components at higher temperature. Therefore, the (NH4)2CO3 equilibrium does not influence the kinetic model of urea above about 475 K.