Final Report: Catalytic Dehalogenation of PCBs and Pesticides in Supercritical FluidsEPA Grant Number: R823526
Title: Catalytic Dehalogenation of PCBs and Pesticides in Supercritical Fluids
Investigators: King, R. Bruce , Bhattacharyya, Nripendra K. , King, Charles M.
Institution: University of Georgia Research Foundation
EPA Project Officer: Lasat, Mitch
Project Period: October 1, 1995 through September 30, 1997
Project Amount: $189,104
RFA: Exploratory Research - Engineering (1995) Recipients Lists
Research Category: Engineering and Environmental Chemistry , Land and Waste Management
Objective:The scientific and practical objective of this research project was to develop a new process to displace soil extraction/high temperature incineration as a technology for PCB destruction. A new technology for PCB and chlorinated pesticide detoxification by low-temperature homogeneous catalytic chlorine displacement was proposed. It was envisioned that a one-step PCB soil extraction/soil detoxification process would be conceivable in conventional PCB extraction solvents. If proven, this new process could be demonstrated in environmentally benign supercritical fluid carbon dioxide. This new concept should be applicable to real-world soil and sediment contamination sites. An estimated 900 million metric tons of soil are contaminated with PCBs in the United States.
The scientific approach was based on the industrial use of zero valent nickel/organophosphorus (ligand) complexes as established industrial catalysts for synthesis of useful organic intermediates. These compounds also are known to activate or insert into the carbon chlorine bonds of aliphatic and aromatic halocarbon compounds. Alkoxyborohydride reagents are established compounds for reducing nickel to the zero-valent state and also are a source of hydride (H) or hydrogen for hydrogenolysis of carbon-metal-chlorine bonds. By coupling these known independent scientific facts, a catalytic process for reductive dechlorination of PCBs was proposed. Based on industrial precedents, soluble low-valent nickel/ligand complexes also are extractable in conventional solvents used for PCB extraction from soils. Hence, a one-step soil extraction/PCB detoxification process was proposed as part of the scientific approach.
This approach to PCB detoxification would expect to result in lower molecular weight dechlorinated PCB congeners with mainly ortho substitution patterns and, in theory, 100 percent biphenyl from commercial Aroclor compositions. Sodium chloride (salt) would be a by-product and final chlorine sink. Biphenyl is nontoxic and completely biodegradable. Chlorine could be recovered and recycled by the well-established industrial process for caustic-chlorine production by Nafion membrane/electrolysis of brine. Ortho-substituted low molecular weight PCB congeners are known to be much less toxic. It was demonstrated in the first year of research that five homogeneous nickel/ligand complexes are all active for reductive dechlorination of PCB congeners, using nontoxic decachlorobiphenyl as a model compound. In particular, catalysts for selective meta-para chlorine displacement were found. A distinct correlation exists for percent chlorine displacement versus the molecular size and stereochemistry of the nickel/ligand fragment. Nickel/triethylphosphine complexes are, to date, the most active for the RD process giving a lower molecular weight product distribution of mostly ortho products and a 15 percent yield of biphenyl (complete dechlorination product). The RD process has been demonstrated to be active even at room temperature, owing to the high reactivity of the soluble nickel complexes. In addition, we have been able to extract and simultaneously dechlorinate the model congener in synthetic soil (silica), using a conventional Soxhlet apparatus typically used for PCB extraction of small environmental samples. Aroclors were tested in one-step Soil Extraction/ PCB Detoxification (the UGASEPD process).
This new approach would displace conventional extraction/high-temperature incineration as a method of destroying PCBs. High-temperature incineration is controversial because it is costly and leads to incompletely oxidized products as well as the highly toxic chlorinated dibenzofurans and chlorinated dibenzodioxins. This new technology should be applicable to contaminated soils from the controlled state-operated PCB landfills and United States Army, Navy, and Air Force PCB landfills at military bases. The established chemistry should be applicable to the highly toxic dioxin-like congeners, which are only meta-para substituted derivatives, the tetrachlorodibenzofurans (only meta-para substituted) and the tetrachlorodibenzodioxins (only meta-para substituted).
Summary/Accomplishments (Outputs/Outcomes):Our demonstration of PCB extraction/PCB detoxification supports the concept that PCBs can be decontaminated from soil columns in a one-step process by admixture of the nickel catalyst and hydride reagent to a soil matrix. In the presence of environmental concentrations of model PCB congeners or Aroclors, the heterogeneous mixture is subjected to subcritical fluid extraction with tetrahydrofuran at reflux. Nickel complex and hydride reagent co-extract with the PCBs and mediate chlorine displacement during extraction. Aroclor 1254 and 1232, in neat form, have been obtained from the Supelco Corporation for evaluation in the Sohxlet equipment. Aroclor 1221, mostly ortho substituted PCB congeners, continued to show partial chlorine displacement at a lower molecular weight product distribution.
The complete detoxification of 3,3'4,4'-tetrachlorobiphenyl, a co-planar PCB congener and precursor to the most toxic furans and dioxins, to a 99+ percent yield of nontoxic, biodegradable biphenyl has been confirmed. This was accomplished by use of a nickel catalyst that was predicted to be active for meta-para chlorine displacement from PCB congeners from experimental observations on decachlorobiphenyl (DCBP) reductive dechlorination and molecular mechanics computer modeling of the organometallic transformations.
The importance of the "ligand effect" on nickel promoted reductive dechlorination of model PCB congeners was demonstrated. It was confirmed that by control of the steric size of the ligand-metal complex, the magnitude of dechlorination from a PCB congener will vary: (a) sterically small ligands promote attack at the meta-para positions, in addition to the hindered ortho position; (b) sterically large ligands induce only para-chlorine displacement from model congeners or no chlorine displacement. This trend is somewhat analogous to microbial reductive dechlorination, promoted by naturally occurring enzymes. Table 1 summarizes the "Ligand Cone-Angle Effect." These data represent completely new observations on methods for PCB detoxification and correlate PCB magnitude and rate of detoxification by chlorine removal with the molecular architecture of the PCB congener and the interacting metal-ligand complex.
Table 1. Percent Chlorine Displacement and Product Distribution From DCBP as a Function of the Phosphine and Phosphite Ligand Cone Angle for NiL2Cl2 Complexes.
|% Cl Displaced||70%||65%||30%||0%||40%||30%||15%|
|Biphenyl (Yield %)||Yes||Yes||No||No||No||No||No|
|Congener Produced||Di, Tri, Tetra||Di, Tri, Tetra||Penta, Hexa||No Reaction||Tetra, Penta||Penta, Hexa||Octa, Nona|
|CL Position||o||o||o/m/p||No Reaction||o/m/p||o/m/p||o/m|
The magnitude of chlorine displacement is related to the nickel-ligand cone angle parameter, particularly for phosphine ligands. These results imply a stereochemical effect on the course of reductive dechlorination related to the steric bulk of the ligand. Most importantly, our observations predicted that several complexes should be highly active for meta-para chlorine displacement from PCB congeners that are in the class of the coplanar PCBs (toxicologically the most potent and precursors to the most toxic furans and dioxins). In addition, the observation of biphenyl as a reaction product implied that attack at the ortho position of the biphenyl ring also was occurring. These nickel complexes also should be active for chlorine displacement from the pure orthogonal, conformationally restricted PCBs, some of which are precursors to the "endocrine disruptors."
The smallest cone-angle complex tested to date, Ni[PMe3]2Cl2, provided a five component product from decachlorobiphenyl. All PCB congeners were ortho-substituted indicating that near quantitative meta-para chlorine displacement had occurred. An estimated 50 percent yield of biphenyl, the complete dechlorination product, implied that extensive ortho chlorine displacement also was occurring. The order of meta-para >> ortho for chlorine displacement is analogous to enzyme-catalyzed microbial reductive dechlorination.
An oxidative addition mechanism has been partially confirmed by single crystal x-ray diffraction and mass spectrometry of intermediates of the transition metal/ligand fragment added to the para position of decachlorobiphenyl, a model congener. A meta C12Cl9NiL2Cl isomer, where L is triethylphosphine, also has been structurally characterized. A C12Cl8NiL2 intermediate also has been isolated and characterized by mass spectrometry. This corresponds to metal insertion into the ortho-ortho' positions of decachlorobiphenyl. Hence, some of the catalysts are active for ortho chlorine displacement, leading to nontoxic biphenyl, due to the smaller steric bulk of the ligand. This latter intermedidate is a transition metal analog of perchlorodibenzofuran with the metal in a pseudo-aromatic five-membered ring. Molecular mechanics computer modeling is being used to optimize the design of several homogeneous transition metal catalysts. This computational tool predicted that small cone angle metal/ligand fragments are the most stable for oxidative insertion into the hindered ortho positions and is being used for experimental design. This is consistent with the stereochemistry of this organometallic reaction. Our experiments have partially confirmed this computational prediction by the use of small cone angle Ni(II)[PMe3]2CL2 for extensive hydrogenolysis of the PCBs under reducing conditions.
Journal Articles on this Report : 1 Displayed | Download in RIS Format
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||King, CM, King RB, Bhattacharyya NK, and Newton MG. Organonickel chemistry in the catalytic hydrodechlorination of polychlorobiphenyls (PCBs): Ligand steric effects and molecular structure of reaction intermediates. Journal of Organometallic Chemistry, April 2000;600(1-2):63-70.||