Pd/C-catalyzed dechlorination of polychlorinated biphenyls under hydrogen gas-free conditions

Experimental study of catalytic hydro-dechlorination (HDC) of Aroclor 1232

Polychlorinated biphenyls (PCBs) is a group of persistent organic pollutants that still requires remediation and reduction long after the production is discontinued. Aroclor 1232 is a commercial PCB mixture that has been much less studied. To address this void, this paper presented catalytic hydro-dechlorination (HDC) of Aroclor 1232 using palladium on activated carbon (Pd/AC) at atmospheric pressure. Experimental variables studied including three mild temperatures, 22.5 °C, 50 °C, and 80 °C, and four different co-catalyst loadings. This batch of Aroclor 1232 constituted of more than 83% of mono-, di-, and tri-, chlorinated biphenyls (CB), in addition to biphenyl, tetra- and penta CBs. HDC efficiency increased with temperature, and reached 99.9% within 4 h of reaction at 80 °C. HDC efficiency also increased with co-catalyst loading. HDC efficiencies followed the trend of para > meta > ortho positions among isomers. The increase of temperature and Et3N dosage are especially effective in dechlorination at ortho and meta positions. The apparent activation energy of Aroclor 1232 HDC was estimated as 25.57 kJ/mol based on pseudo-first order assumption, indicating that the reaction may be diffusion limited. Given the modest reaction conditions used, the HDC of Aroclor 1232 can potentially be a low-cost process.

A catalyzed method to remove polychlorinated biphenyls from contaminated transformer oil

The disposal of polychlorinated biphenyls (PCBs) as persistent organic pollutants from the environment has been normally performed by isolation from soil or water because of their biological activity and toxic potential. In the present investigation, catalytic hydrodehalogenation was used to detoxify PCBs-contaminated transformer oil. All reactions were directed on an oil containing 11.09 wt% of PCBs utilizing palladium supported on multi-walled carbon nanotubes (Pd/MWCNTs). The amount of hexa-chlorine homologues reduced considerably from 5.07% to less than 800 ppm utilizing HDC at the atmosphere of argon. Moreover, the amounts of long half-lives and bioaccumulative congener of PCB 153 decreased considerably from 3.2% to less than 200 ppm. Besides, the quantity of some environmental pollutants like PCB 105 as a mono-ortho-substituted congener decreased considerably. The significant effects of reaction time, reaction temperature, and catalyst concentration on the efficiency were confirmed and modeled through Box-Behnken design. The optimal reaction condition with an efficiency of 96.67% was 70°C, with catalyst loading of 8 wt% and reaction time of 3.23 h. Furthermore, the quantity of turnover frequency of Pd/MWCNTs showed that it has more activity than palladium-carbon active supported in the ambient pressure without utilizing hydrogen gas in transformer oil complex. The study of the kinetic model revealed that the required activation energy (of 12.99 kJ/mol) to remove PCBs from transformer oil utilizing the present catalyst was lower than other catalyzed hydrodechlorination methods.

Metal catalyzed defunctionalization reactions

The chronological development of metal assisted defunctionalization reactions is discussed from the stoichiometric to the catalytic stage with their application in synthetic organic chemistry.

Practical remediation of the PCB-contaminated soils

A practical method for the elimination of PCBs from PCB-contaminated soil has been developed by the combination of Soxhlet extraction using a newly-developed modified Soxhlet extractor possessing an outlet valve on the extraction chamber with the chemical degradation. Various types of PCBs contaminated in soils could be completely extracted in refluxing hexane, and the subsequent hydrodechlorination could also be completed within 1 h in a hexane-MeOH (1 : 5) solution in the presence of Pd/C and Et3N under ordinary hydrogen pressure and temperature without the transfer of the extracted PCBs to other reaction container (a complete one-pot procedure). The present system is quite useful as a simple, safe, mild and reliable remediation method of PCB-contaminated soil.

Reactivity of polychlorinated biphenyls in nucleophilic and electrophilic substitutions

To explain the chemical reactivity of polychlorinated biphenyls in nucleophilic (S(N)) and electrophilic (S(E)) substitutions, quantum chemical calculations were carried out at the B3LYP/6-31G(d) level of the Density Functional Theory in gas phase. Carbon atomic charges in biphenyl structure were calculated by the Atoms-in-Molecules method. Chemical hardness and global electrophilicity index parameters were determined for congeners. A comparison of calculated descriptors and experimental data for congener reactivity in the S(N) and S(E) reactions was made. It is shown that interactions in the S(N) mechanism are reactions of the hard acid-hard base type, these are the most effective in case of highly chlorinated substrates. To explain the congener reactivity in the SE reactions, correct descriptors were not established. The obtained results can be used to carry out chemical transformations of the polychlorinated biphenyls in order to prepare them for microbiological destruction or preservation.