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DDT dechlorination electrocatalyzed by a synthetic iron porphyrin in pyridine and dimethyl sulfoxide.

Abstract.--An iron porphyrin, (TPP)FeCl, where TPP is the dianion of tetraphenylporphyrin, was examined as the catalyst for the reductive dechlorination of 1.1- bis(4-chlorophenyl)-2,2,2-trichloroethane (p,p'-DDT) in pyridine and dimethyl sulfoxide (DMSO). Cyclic voltammetry indicated that (TPP)FeCl undergoes three reversible reductions and can be reduced electrochemically into (TPP)[Fe.sup.II], [(TPP)[Fe.sup.I]]- and [[(TPP)[Fe.sup.I]].sup.2-] in both solvents. The doubly reduced species, [[(TPP)[Fe.sup.I]].sup.-], is able to electrocatalyze the dechlorination of p,p'-DDT through the formation of a [sigma]-bonded iron porphyrin intermediate, followed by a cleavage of the [sigma]-bond upon electroreduction, to generate three dechlorinated products, 1,1-bis(4-chlorophenyl)-2,2-dichloroethane (p,p'-DDD), 1,1-bis(4-chlorophenyl)-2,2-dichloroethylene (p,p'-DDE) and 1,1-bis(4-chlorophenyl)-2-chloroethylene (p,p'-DDMU). Controlled potential bulk electrolysis combined with GC-MS analysis confirmed that (TPP)FeCl exhibits a better catalytic performance towards DDT dechlorination in pyridine than in DMSO. An overall electrocatalytic mechanism was proposed.

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DDT (1,1-bis(4-chlorophenyl)-2,2,2-trichloroethane), one of the 12 key persistent organic pollutants (EPA website), was utilized worldwide in the 1940s as an effective insecticide in agriculture and its use has been banned since the 1970s as concerns arise about its severe environmental consequences. DDT has been known as an endocrine disruptor and a potential carcinogen for both human beings and other animals (Turusov 2002; Sharma 2003; Beard 2006; Mariussen & Fonnum 2006; Sikka & Wang 2008; Eskenazi et al. 2009; Roy et al. 2009). Its biodegradation in the environment is very slow which leads to its accumulation in soil, water and animal tissues (Sarkar 2008). More recently, development of new techniques for dechlorination of DDT pollution has received much worldwide attention (Quensen III et al. 1988; Sayles et al. 1997; Quensen III et al. 1998; Alonso et al. 2002; Huang & Zhao 2008; Shimakoshi et al. 2008; Hisaeda & Shimakoshi 2010; Tahara et al. 2010; Shao et al. 2010; Tahara & Hisaeda 2011; Zhu et al. 2011).

Macrocyclic complexes containing various transition metal ions were examined extensively for the dechlorination of DDT and other organochlorides. For example, cobalamin derivatives (e.g. vitamin B12; Hisaeda & Shimakoshi 2010), cobalt porphyrins (Alonso et al. 2002; Zhu et al. 2011), and cobalt phthalocyanines (Shao et al. 2010) have been investigated for their catalytic properties in DDT reductive dechlorination. A common mechanism has been proposed which involves the electroreduction of the cobalt(II) center in each macrocyclic compound to form a cobalt(I) species (Shao et al. 2010; Zhu et al. 2011). This species is a supemucleophile and capable of reacting with DDT to generate a Co-C [sigma]-bonded intermediate (Hisaeda & Shimakoshi 2010). Further electroreduction of this intermediate leads to cleavage of the Co-C [sigma]-bond and generates dechlorinated products of DDT. Formation of the [sigma]-bonded intermediate is therefore the key step in the electrocatalytic process. Similarly, many metalloporphyrins containing transition metal ions (e.g. Ru, Fe, Os, Mo, Ir, Ni) are able to form different types of metal-carbon bonded complexes with organochlorides (Guilard & Kadish 1988). However, research into using these metalloporphyrins as catalysts for the dechlorination of DDT and other organochlorides is not as extensive as for cobalt macrocyclic complexes.

We investigate a synthetic iron porphyrin, (TPP)FeCl (Figure 1) (where TPP = the dianion of tetraphenylporphyrin), for its catalytic properties toward DDT dechlorination in pyridine and DMSO. Cyclic voltammetry confirmed that (TPP)FeCl undergoes three reversible reductions. The doubly reduced species is an electrocatalyst which is able to react with DDT to form a a-bonded intermediate. Upon electroreduction, this intermediate decomposes to generate dechlorinated products. Conversion of DDT and distribution of dechlorinated products in pyridine and DMSO were examined by means of controlled potential electrolysis combined with GC-MS analysis.

MATERIALS AND METHODS

(TPP)FeCl, anhydrous pyridine (99.8%), dimethyl sulfoxide (DMSO, 99.9%), 1,1-bis(4-chlorophenyl)-2,2,2-trichloroethane (p,p'-DDT, 98%), 1,1 -bis(4-chlorophenyl)-2,2-dichloroethane (p,p' - DDD, 97%), 1,1-bis(4-chlorophenyl)-2,2-dichloroethylene (p,p' - DDE, 99%) and 1,1-bis(4-chlorophenyl)-2-chloroethylene (p,p' - DDMU, 99%) were obtained from Sigma-Aldrich and used as received (structures of p,p'-DDT, p,p'-DDE, p,p'-DDD and p,p' - DDMU are presented in Figure 1). Tetra-n-butylammonium perchlorate (TBAP) was purchased from Fluka Chemical Company and used as the supporting electrolyte without further purification.

Cyclic voltammetry was performed in a three-electrode cell using a Pine Instrument Company AFCBP1 potentiostat. A glassy carbon disk electrode was utilized as the working electrode while a platinum wire and a saturated calomel electrode (SCE) were employed as the counter and the reference electrodes, respectively. All electrochemical measurements were carried out under a nitrogen atmosphere.

The catalytic dechlorination of DDT was investigated using controlled potential bulk electrolysis. A home-made "H" type cell with a fritted glass layer to separate the cathode and anode chambers was used for bulk electrolysis. The working and counter electrodes were made from platinum mesh, and the reference electrode was an SCE. Both the working electrode (area [approximately equal to] 2.5 [cm.sup.2]) and the reference electrode, which was put in a bridge containing the same solvent and supporting electrolytes as the solution used, were placed in one compartment while the counter electrode was put in the other compartment. The potential used for the bulk electrolysis in the catalytic reduction was set at -1.60 V in pyridine and at -1.40 V in DMSO.

A GC-MS system (HP6890-GC and HP5973-MSD) was utilized to monitor catalytic reductive dechlorination reactions. The GC was equipped with a Phenomen 79973 ZB-5 column (length 30 m; ID 250 pm, film 0.25 [micro]m). The area-normalized method was applied for the analysis of GC peaks and the relative molar response factor of DDT was defined as 1.0 while the values for DDE, DDD and DDMU were measured as 0.827 ([+ or -]0.011), 0.948 ([+ or -]0.010) and 0.979 ([+ or -]0.012), respectively. Conversion (%) of DDT and yield (%) of each dechlorination product (DDD, DDE or DDMU) were defined as follows:

Conversion (%) = (mole of DDT converted) / (mole of initial DDT) x 100;

Product yield (%) = (mole of a specific product) / (mole of initial DDT) x 100.

RESULTS AND DISCUSSION

Cyclic voltammetry of (TPP)FeCl in pyridine and DMSO in the absence and presence of DDT.--Reductions of (TPP)FeCl in pyridine and DMSO, respectively, containing 0.1 M TBAP are summarized in Figures 2-3. In the absence of DDT, three reversible one-electron reductions were observed in both solvents with [E.sub.1/2] = 0.20, -1.47 and -1.70 V (processes i - iii, Figure 2b) in pyridine and -0.04, -1.11 and -1.60 V (processes i - iii, Figure 3b) in DMSO. These results are consistent with previous values obtained from (TPP)FeCl and other iron porphyrins under similar solvent conditions (Kadish et al. 1976, 1999, 2000). The first two reversible reductions were assigned to be metal-centered, corresponding to the [Fe.sup.III]/[Fe.sup.II] and [Fe.sup.II]/[Fe.sup.I] processes, respectively. Both [Fe.sup.III] and [Fe.sup.II] centers in the porphyrins are axially bound with either one or two solvent molecules to form five- or six-coordinated complexes in a coordinating solvent such as pyridine and DMSO (Kadish et al. 2000). Meanwhile, the third reduction takes place on the macrocyclic ring of the Fe porphyrin. Electrode reactions of three reductions are presented in Figure 4a.

Upon addition of DDT to both (TPP)FeCl solutions (Figures 2b-e, 3b-e), the first and third reductions (processes i and iii) do not exhibit a big change in the cathodic peak current ([i.sub.pc]), indicating that there is no clear interaction between DDT and the neutral (TPP)[Fe.sup.III]Cl or the triply reduced [[(TPP)[Fe.sup.I]].sup.2-]. This is not the case for the second reduction, whose cathodic peak current ([i.sub.pc]) is enhanced remarkably and becomes irreversible as the concentration of DDT goes up in both the pyridine and DMSO solutions. This increase in reduction current is not simply from the direct electroreduction of DDT, because in the absence of (TPP)FeCl, DDT alone cannot be reduced at a potential around the second reduction process of (TPP)FeCl (Figures 2a and 3a). It implies the existence of an electrocatalytic process involving DDT and the electrogenerated [[(TPP)[Fe.sup.I]].sup.-].

Guilard & Kadish (1988) observed that various low-valent [Fe.sup.I] porphyrins are able to react with alkyl or aryl halides (RX) to form [sigma]-bonded [Fe.sup.III] porphyrins which can undergo further electroreductions and/or electrooxidations. The doubly reduced [[(TPP)[Fe.sup.I]].sup.-] in this study is similarly able to react with DDT, an alkyl chloride, to form a [sigma]-bonded [Fe.sup.III] porphyrin complex as shown in Figure 4b. Upon electroreduction, this complex is decomposed through the cleavage of its Fe-C [sigma]-bond, ultimately converting into DDT dechlorination products. Meanwhile, [[(TPP)[Fe.sup.I]].sup.-] is recovered and continues to serve as the catalyst for further DDT dechlorination at an applied reduction potential (Figures 4c and d).

It should be pointed out that an additional irreversible reduction peak, process ii' was observed in the DMSO solution at various DDT concentrations (Figures 3c-e). This phenomenon was previously seen when cobalt porphyrins were utilized for DDT dechlorination in N,N'-dimethylformamide solution (Zhu et al. 2011) and has been attributed to the electroreduction of [sigma]-bonded cobalt porphyrin intermediates generated between DDT and the reduced cobalt porphyrins. Similarly, process ii' in Figure 3 can also be attributed to the electroreduction of the a-bonded iron complex generated between DDT and [[(TPP)[Fe.sup.I]].sup.-] in DMSO.

The increase in cathodic peak current ([i.sub.p]) for the [Fe.sup.II]/[Fe.sup.I] process of (TPP)FeCl as a function of increasing DDT concentration is shown in Figure 5, where [i.sub.p] and [i.sub.p0] are the measured cathodic peak currents obtained from cyclic voltammograms of the iron porphyrin in the presence and absence of DDT, respectively. Measured [i.sub.p]/[i.sub.p0] ratios are very close to each other in pyridine and DMSO when the [DDT]/[Fe] molar ratio is lower than 4, but the [i.sub.p]/[i.sub.p0] becomes larger in pyridine as the [DDT]/[Fe] ratio increases above 4. This observation suggests that at a higher DDT concentration, the turnover rate to regenerate the corresponding catalyst, [[(TPP)[Fe.sup.I]].sup.-], should be faster in pyridine than in DMSO and that pyridine is a better solvent for DDT catalytic reductive dechlorination. This conclusion is also supported by the results of controlled potential bulk electrolysis in both solution conditions, as discussed below.

Controlled-potential electrolysis and the GC-MS analysis of DDT dechlorination products.--Determination of catalytic properties of (TPP)FeCl was accomplished by two separate bulk electrolysis with identical (TPP)FeCl concentrations in both pyridine and DMSO containing 0.2 M TBAP and 10 eq. DDT. Reduction potentials applied for the electrolysis were slightly more negative than the cathodic peak potentials ([E.sub.pc]) of the second reduction, such that the doubly reduced species [[(TPP)[Fe.sup.I]].sup.-] can be generated electrochemically in both solvents. After electrolysis, the dechlorination products of DDT were analyzed by GC-MS and the corresponding results were illustrated in Figures 6-8.

After 60 min electrolysis (Figure 6), 93.7% DDT was catalytically dechlorinated in pyridine while only 67.1% DDT was converted in DMSO. After 90 min, no DDT was detected in the pyridine solution, but 23% DDT remained even after 120 min electrolysis in the DMSO solution (Figure 6). This observation clearly indicated that the turnover rate of the catalyst in pyridine is much faster than in DMSO and that pyridine is a more suitable solvent for DDT dechlorination than DMSO. This result is consistent with what was observed in the cyclic voltammograms shown in Figures 2 and 3.

The product analysis revealed that three DDT dechlorinated products, DDD, DDE and DDMU were obtained under both solution conditions after bulk electrolysis (Figures 7 and 8). In the pyridine solution, the yields of DDE and DDD were much higher than that of DDMU after 30 minute electrolysis and these two yields continued to increase to their maximum values around 60 min. After that, both yields started to decrease with electrolysis time. DDMU, on the other hand, had a very low yield at the beginning, but its concentration increased almost linearly with reaction time. After 90 min, DDMU became the predominate product in the pyridine solution whose yield reached 55.5% (Figure 7). These results clearly indicate that DDD and DDE are firstly generated in DDT dechlorination, both of which further convert into DDMU with reaction time. DDMU is very stable and becomes the end product of DDT dechlorination under our experimental conditions.

The product distribution profile shown in Figure 8 disclosed that, in the DMSO solution, the yield of DDE was over 40% during the entire electrolysis period while the yield of DDMU was always lower than 10%. This observation clearly differs from the results in pyridine where DDMU was the major product of DDT dechlorination after 90 min bulk electrolysis (Figure 7). Because DDMU is a more extensive dechlorinated product than DDE, pyridine is a better solvent than DMSO for DDT dechlorination.

The lower catalytic performance of (TPP)FeCl in DMSO might be explained from the electrochemical processes shown in Figure 3 where an extra reduction (process ii') was observed. This process is associated with the electroreduction of the a-bonded [Fe.sup.III] porphyrin complex formed between Fe(I) porphyrin and DDT or perhaps one of DDT dechlorination products (e.g. DDD, DDE). During bulk electrolysis in DMSO, the reduction potential was applied at -1.40 V which is slightly negative than the peak potential of process ii, but is not negative enough for the reduction in process ii' to take place to produce more extensively dechlorinated products.

CONCLUSIONS

Cyclic voltammetry and controlled potential bulk electrolysis revealed that (TPP)FeCl can be reduced electrochemically into (TPP)[Fe.sup.II], [[(TPP)[Fe.sup.I]].sup.-] and [[(TPP)[Fe.sup.I]].sup.2-] in pyridine and DMSO. The doubly reduced species [[(TPP)[Fe.sup.I]].sup.-] can electrocatalyze the degradation of DDT through the formation of an iron porphyrin intermediate containing a Fe-C c-bond, which is decomposed upon electroreduction to generate three dechlorinated products, DDD, DDE and DDMU. Compared with DMSO, pyridine is a more suitable solvent for DDT dechlorination which is reflected by a much higher yield of DDMU in the products. An overall electrocatalytic mechanism for DDT reductive dechlorination is given in Figure 4.

ACKNOWLEDGEMENTS

The financial support of the Robert Welch Foundation (A0-0001) is gratefully acknowledged. Dr. Frederick B. Stangl is appreciated for reviewing this paper.

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JS at: jianguo.shao@mwsu.edu

Jianguo Shao (1) and Rina Kuwahara (1)

(1) Department of Chemistry, Midwestern State University, Wichita Falls, Texas 76308
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Date:Feb 1, 2012
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