In situ FT-IR and FT-RAMAN studies of reactions in the hydrothermal environment

摘要

Hydrothermal processing is an alternative destruction method for hazardous waste; however, the details of reactions in the high/temperature pressure aqueous environment are generally unknown. Micro-scale flow reactors equipped for in situ vibrational spectroscopy were used to probe the reaction pathways and kinetics of several compounds at hydrothermal conditions.;FT-IR spectroscopy permitted the identification and quantitation of species involved in the decompositions of urea, carbohydrazide, and guanidinium nitrate. Urea (1.05 m) at P = 275 bar, T = 473-573 K decomposed to CO$sb2$ and NH$sb3$ in two steps through the intermediate ammonium isocyanate. Carbohydrazide (1.07 m) reacting at 275 bar and 503-543 K followed a two-step pathway involving the insertion of water to form the hydrazinium salt, (NH$sb2$NH)C(=O)O$cdot$NH$sb3$NH$sb2$, which then decomposed into hydrazine and CO$sb2$. The decomposition of guanidinium nitrate (l.09 m) at 275 bar and 513-573 K was an extension of the urea pathway, preceded by the initial, autocatalytic neutralization of the guanidinium cation to neutral guanidine, followed by hydrolysis of guanidine to urea. Kinetic constants and Arrhenius parameters were determined, with the aid of computer modeling, for most of the individual steps.;FT-Raman spectroscopy was used to quantify the decomposition products of hydroxylammonium nitrate at T = 425-500 K and P = 275 bar indicating the global stoichiometry:$$rmlbrack NHsb3OHsp+rbrack lbrack NOsbsp{3}{-}rbrack to 0.8 Nsb2O + 0.4 HNOsb3 + 1.8 Hsb2 O + 0.1 Osb2cr$$The high exothermicity of the reaction necessitated the employment of a non-isothermal kinetic analysis technique. The apparent activation energies of the induction process leading to the exothermic event are consistent with the formation of a critical concentration of a species (probably HNO$sb2$) which catalyzed the process.