Effects of Solvent Composition and Hydrogen Pressure on the Catalytic Conversion of 1,2,4,5-Tetrachlorobenzene to Cyclohexane

Toward the development of a “green” technology for cleaning soil contaminated by halogenated hydrophobic organic contaminants, here we demonstrate that combined use of palladium (Pd) and rhodium (Rh) catalysts enables the conversion of 1,2,4,5-tetrachlorobenzene (TeCB) to cyclohexane in mixtures of water and ethanol. We tested the hypotheses that, in batch reactors, (1) an increased ratio of water to ethanol in water/ethanol solvents would increase the reaction rates of both Pd-catalyzed hydrodehalogenation (HDH) and Rh-catalyzed hydrogenation, and (2) catalytic reaction rate coefficients would be constant above a hydrogen (H2) pressure threshold, but would decrease with decreasing H2 pressure below that threshold. These hypotheses were derived from a Langmuir–Hinshelwood model for the heterogeneous catalytic reactions. Complete conversion of TeCB to cyclohexane was achieved at all experimental conditions tested, suggesting that the proposed technology may be technically viable. Concentration data were consistent with an apparent first-order kinetic model in which Pd-catalyzed HDH and Rh-catalyzed hydrogenation occur in series. As expected, HDH and hydrogenation rate coefficients increased as the fraction of water in the solvent increased. However, contrary to expectations, HDH rate coefficients decreased when H2 pressure increased from 69 to 207 to 345 kPa. We attributed this to the displacement of TeCB by H2 on the catalyst surface at higher H2 pressures. No statistically significant effect of H2 pressure on hydrogenation rate coefficients was observed. The findings suggest that the proposed technology should be operated with at least 50% water in the solvent and a H2 pressure as low as 30–70 kPa

Effects of Solvent Composition and Hydrogen Pressure on the Catalytic Conversion of 1,2,4,5-Tetrachlorobenzene to Cyclohexane

Toward the development of a “green” technology for cleaning soil contaminated by halogenated hydrophobic organic contaminants, here we demonstrate that combined use of palladium (Pd) and rhodium (Rh) catalysts enables the conversion of 1,2,4,5-tetrachlorobenzene (TeCB) to cyclohexane in mixtures of water and ethanol. We tested the hypotheses that, in batch reactors, (1) an increased ratio of water to ethanol in water/ethanol solvents would increase the reaction rates of both Pd-catalyzed hydrodehalogenation (HDH) and Rh-catalyzed hydrogenation, and (2) catalytic reaction rate coefficients would be constant above a hydrogen (H2) pressure threshold, but would decrease with decreasing H2 pressure below that threshold. These hypotheses were derived from a Langmuir–Hinshelwood model for the heterogeneous catalytic reactions. Complete conversion of TeCB to cyclohexane was achieved at all experimental conditions tested, suggesting that the proposed technology may be technically viable. Concentration data were consistent with an apparent first-order kinetic model in which Pd-catalyzed HDH and Rh-catalyzed hydrogenation occur in series. As expected, HDH and hydrogenation rate coefficients increased as the fraction of water in the solvent increased. However, contrary to expectations, HDH rate coefficients decreased when H2 pressure increased from 69 to 207 to 345 kPa. We attributed this to the displacement of TeCB by H2 on the catalyst surface at higher H2 pressures. No statistically significant effect of H2 pressure on hydrogenation rate coefficients was observed. The findings suggest that the proposed technology should be operated with at least 50% water in the solvent and a H2 pressure as low as 30–70 kPa

Simplified Analysis of Chlorinated Ethenes in Water Samples by Gas Chromatography with Flame Ionization Detection

Chlorinated ethenes are among the most common environmental contaminants and are known or suspected carcinogens. This class of compounds includes perchloroethene (PCE), trichloroethene (TCE), and their breakdown products, including dichloroethene (DCE) isomers and vinyl chloride (VC). Engineers and scientists must be able to measure concentrations of these chemicals in water samples to assess site contamination, monitor clean-up progress, and test possible remediation technologies. Gas chromatography with flame ionization detection (GC/FID) is a common method for measuring these contaminants in environmental samples. In this study, we tested the hypothesis that FID response factors are equal for all chlorinated ethene compounds. The rationale for the investigation was that if the hypothesis is correct, a single calibration curve can be used for GC/FID analysis of all chlorinated ethene compounds, saving time and money during sample analysis. Based on our measurements, a single calibration curve (FID response versus mass of analyte injected) is applicable to analysis of PCE, TCE, and all three DCE isomers (r2=0.990, n=50 measurements), allowing for simplified quantification of those chemicals. However, the apparent FID response factor for VC was lower by ∼40%, indicating that a separate calibration curve would need to be used to accurately estimate the VC concentration in water samples. The difference in the apparent VC response factor is caused predominantly by losses of VC to volatilization during the analysis