Reductive dehalogenation of chlorinated aliphatic compounds in electrolytic systems

Persistent Link:
http://hdl.handle.net/10150/283929
Title:
Reductive dehalogenation of chlorinated aliphatic compounds in electrolytic systems
Author:
Liu, Zhijie
Issue Date:
1999
Publisher:
The University of Arizona.
Rights:
Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.
Abstract:
A series of chlorinated low-molecular-weight alkanes and alkenes was transformed electrolytically at metal cathodes at potentials from -0.3 to -1.4V (vs. SHE). Products included nonchlorinated hydrocarbons and less chlorinated intermediates. Product distributions are highly dependent on cathode material and applied cathode potential. Kinetics was first-order in the concentration of the halogenated targets. The specific first-order rate constants are function of cathode potential, cathode material, solution characteristics, and reactant identify. When transformation kinetics was governed by polarization resistance, rate constants were correlated with degree of halogenation and standard reduction potential for the predominant transformation reaction (as indicated by product analysis). Log-transformed reaction rate constants for reduction of chlorinated alkanes, derived via experiments at the same cathode potential (E(c) = -1.0 or -1.2V vs. SHE), were linearly related to carbon-halogen bond dissociation energies. A physical model for the observed correlation was developed from transition-state theory. The chlorinated ethenes reacted much faster than predicted from bond enthalpy calculations, suggesting that alkenes are not transformed via the same mechanism as the chlorinated alkanes. Polarographic study demonstrated that the shift of E₁/₂ of CCl₄ reduction was correlated with water concentration in solvent-predominated mixtures. Successful interpretation of these findings with a physical model suggested that solvents involved the rate-determining step of CCl₄ electrolysis both kinetically and mechanistically. The capture of trichloromethyl radicals with a spin trap (PBN) in an electrochemical system provided direct evidence supporting the free radical mechanism in electrolytic reduction of CCl₄. Gas-phase reductions of chlorinated alkanes and alkenes were studied in a modified fuel cell. Reactor performance was a function of the metal catalyst amended to the reactor cathode, the reactor potential, cathode temperature, the target compound identity, the partial pressure of O₂(g) in the cathode chamber and the condition (time in service) of the cathode. Single-pass CCl₄ conversions could achieve 90 percent with a mean residence time for gases in the porous cathode much less than a second. Reactor performance deteriorated with the presence of oxygen and time in service. Conversion efficiency was restored, however, by temporarily eliminating the halogenated target(s) from the influent stream or by briefly reversing reactor polarity.
Type:
text; Dissertation-Reproduction (electronic)
Keywords:
Chemistry, Analytical.; Engineering, Environmental.
Degree Name:
Ph.D.
Degree Level:
doctoral
Degree Program:
Graduate College; Chemical and Environmental Engineering
Degree Grantor:
University of Arizona
Advisor:
Arnold, Robert G.

Full metadata record

DC FieldValue Language
dc.language.isoen_USen_US
dc.titleReductive dehalogenation of chlorinated aliphatic compounds in electrolytic systemsen_US
dc.creatorLiu, Zhijieen_US
dc.contributor.authorLiu, Zhijieen_US
dc.date.issued1999en_US
dc.publisherThe University of Arizona.en_US
dc.rightsCopyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.en_US
dc.description.abstractA series of chlorinated low-molecular-weight alkanes and alkenes was transformed electrolytically at metal cathodes at potentials from -0.3 to -1.4V (vs. SHE). Products included nonchlorinated hydrocarbons and less chlorinated intermediates. Product distributions are highly dependent on cathode material and applied cathode potential. Kinetics was first-order in the concentration of the halogenated targets. The specific first-order rate constants are function of cathode potential, cathode material, solution characteristics, and reactant identify. When transformation kinetics was governed by polarization resistance, rate constants were correlated with degree of halogenation and standard reduction potential for the predominant transformation reaction (as indicated by product analysis). Log-transformed reaction rate constants for reduction of chlorinated alkanes, derived via experiments at the same cathode potential (E(c) = -1.0 or -1.2V vs. SHE), were linearly related to carbon-halogen bond dissociation energies. A physical model for the observed correlation was developed from transition-state theory. The chlorinated ethenes reacted much faster than predicted from bond enthalpy calculations, suggesting that alkenes are not transformed via the same mechanism as the chlorinated alkanes. Polarographic study demonstrated that the shift of E₁/₂ of CCl₄ reduction was correlated with water concentration in solvent-predominated mixtures. Successful interpretation of these findings with a physical model suggested that solvents involved the rate-determining step of CCl₄ electrolysis both kinetically and mechanistically. The capture of trichloromethyl radicals with a spin trap (PBN) in an electrochemical system provided direct evidence supporting the free radical mechanism in electrolytic reduction of CCl₄. Gas-phase reductions of chlorinated alkanes and alkenes were studied in a modified fuel cell. Reactor performance was a function of the metal catalyst amended to the reactor cathode, the reactor potential, cathode temperature, the target compound identity, the partial pressure of O₂(g) in the cathode chamber and the condition (time in service) of the cathode. Single-pass CCl₄ conversions could achieve 90 percent with a mean residence time for gases in the porous cathode much less than a second. Reactor performance deteriorated with the presence of oxygen and time in service. Conversion efficiency was restored, however, by temporarily eliminating the halogenated target(s) from the influent stream or by briefly reversing reactor polarity.en_US
dc.typetexten_US
dc.typeDissertation-Reproduction (electronic)en_US
dc.subjectChemistry, Analytical.en_US
dc.subjectEngineering, Environmental.en_US
thesis.degree.namePh.D.en_US
thesis.degree.leveldoctoralen_US
thesis.degree.disciplineGraduate Collegeen_US
thesis.degree.disciplineChemical and Environmental Engineeringen_US
thesis.degree.grantorUniversity of Arizonaen_US
dc.contributor.advisorArnold, Robert G.en_US
dc.identifier.proquest9946847en_US
dc.identifier.bibrecord.b39918002en_US
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