Detoxification of aryl-organochlorine compounds by catalytic reduction in supercritical carbon dioxide

Abstract

Pentachlorophenol, octachloronaphthalene and decachlorobiphenyl were smoothly converted to cyclohexanol, decalin and dicyclohexyl, respectively, by reaction, during 0.5-2 h, with excess hydrogen over alumina-supported palladium (5% w/w) in the presence/absence of supercritical carbon dioxide (50-90 °C). With these conditions, dechlorinations and dearomatization to their cyclic analogs were complete but no carbon-carbon bond scission was observed and only traces of partial deoxygenation/dimerization of the pentachlorophenol substrate (to form 1,1A-oxybiscyclohexane) was seen. The scCO 2 medium functioned as an inert support for the reactions; differences in rates of reaction between chlorinated compounds and their aromatic hydrocarbon homologs were not observed. These results suggest that environmentally recalcitrant chlorinated aromatic compounds can be detoxified facilely by catalytic reduction with H 2 under mild conditions.

Catalytic hydrogenation of polycyclic aromatic hydrocarbons over palladium/γ-Al2O3 under mild conditions

“…Thus, the presence of added CO 2 had no perceptible influence on the rate/course of the hydrogenation. In previous studies of the hydrodechlorination (HDC) of pentachlorophenol with this catalyst, it had been observed that rates of overall reaction, HDC plus dearomatization, were reduced somewhat by the presence of supercritical carbon dioxide [35]. It is considered that the active surface sites for hydrodechlorination and for hydrogenation/dearomatization are different.…”

“…Despite the fact that PAHs can be biodegraded, their persistence in soil, as measured in terms of half-life, can exceed eight years [33]. Our previous research [34,35] on catalytic hydrodechlorination of pentachlorophenol (PCP), octachloronaphthalene, decachlorobiphenyl and subsequent reduction of phenol, naphthalene or biphenyl by hydrogenation over selected transition metals had demonstrated that palladium on gamma-alumina was the most active of the catalysts evaluated.…”

“…The process would be more attractive if the toxicants were to be detoxified during the extraction process. We have optimized procedures for the detoxification of chlorinated aromatics (PCBs and chlorinated phenols) in scCO 2 by hydrodechlorination over bimetallic mixtures (Ag 0 /Fe 0 , Pd 0 /Fe 0 , Pd 0 /Mg 0 ) [34] or noble metal catalyst (Pd 0 /␥-Al 2 O 3 , Pt 0 /␥-Al 2 O 3 or Ni 0 /SiO 2 /Al 2 O 3 ) [35]. Yet extractions do not mobilise only chlorinated aromatics -other classes of non-polar toxicants would be equally susceptible to extraction.…”

A Washing Procedure to Mobilize Mixed Contaminants from Soil

“…The hexane back‐extract could have been detoxified by catalytic hydrodechlorination over palladium sorbed to alumina. In this technique, the hexane is merged with a H 2 –supercritical CO 2 mixture and transported to a heated reactor (Yuan et al, 2003). The reactor effluent containing bicyclohexyl is judged to be toxicologically innocuous relative to the influent PCBs.…”

Estimates of total polychlorinated biphenyl (PCB) compounds in soils/sediments by hydrogenolysis to dicyclohexyl

“…A broad array of techniques has been proposed to accelerate hydrodechlorination that includes electrochemical, 15 photochemical, 16 ultrasonic, 17 microbial, 18 radiolytic/ thermal 19 and chemical methods. 20,21 . In general, chemical methods remain the more widely exploited methods to accomplish the dechlorination of organochlorine (OC) compounds.…”