Hydrothermal reaction of nitriles: Reaction pathways, mechanisms and kinetics.

摘要

Nitriles or compounds containing the cyano functional group comprise an important class of industrial reagents and solvents, finding applications in a wide range of chemical industries. Unfortunately, they also represent a potential environmental challenge owing to their water solubility and toxicity. Several problems inherent to nitrile-containing aqueous wastes severely limit the application of conventional remediation technologies and have, in turn, catalyzed several new research areas. In particular, Wet Air Oxidation (WAO) and Supercritical Water Oxidation (SCWO) have emerged as promising alternatives for the safe and effective remediation of these wastes. Preliminary investigations in these areas have, however, mainly focused on overall destruction efficiencies while placing little emphasis on the underlying reaction products, pathways and controlling kinetics required for optimizing a viable process.; This research has identified the hydrothermal reaction products for both saturated and unsaturated nitriles. The product spectra were assembled into reaction networks consisting of pathways that account for the formation of the observed products. The reaction network of an unsaturated nitrile, acrylonitrile, was complex owing to its dual chemical functionality. In contrast, the reaction network of saturated nitriles consisted solely of sequential hydrolysis steps leading ultimately to refractory carboxylic acids and ammonia.; Autocatalytic kinetics were observed for hydrothermal reaction of saturated nitriles. Supplemental experiments and modeling of the hydrothermal physical chemistry assisted in development of a combined kinetics and solution thermodynamics model which accurately captured the kinetic behavior. Autocatalysis also introduced a strong initial reactant concentration dependence as well as kinetic coupling in simulations of multicomponent systems.; The kinetic effects of varying solvent electrostatics near the critical point of water were also investigated. Analysis using the Kirkwood formalism suggested the formation of a polar transition state for nitrile hydrolysis. Finally, the effect of moderate concentrations of nitric acid in nitrile hydrolysis was investigated and modeled. Rapid conversion of the nitriles was observed under these conditions with the reaction products acting as internal neutralizing agents. Overall, these studies may offer guidance in determining optimal WAO and SCWO reaction conditions to facilitate remediation of nitrile-containing waste streams.