ARTICLE pubs.acs.org/JPCC
Mechanism and Regioselectivity of the Electrochemical Reduction in Polychlorobiphenyls (PCBs): Kinetic Analysis for the Successive Reduction of Chlorines from Dichlorobiphenyls A. Muthukrishnan,†,§ Vadim Boyarskiy,‡ M. V. Sangaranarayanan,*,† and Irina Boyarskaya‡ † ‡
Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India Department of Chemistry, St. Petersburg State University, Universitetskii pr. 26, St. Petersburg, 198504 Russia
bS Supporting Information ABSTRACT: The regioselective electrochemical reduction of three different dichlorobiphenyls is analyzed using quantum chemical calculations and convolution potential sweep voltammetry. The heterogeneous rate constants of CCl bond reductions of each of the dichlorobiphenyls are estimated. The mechanism of the electrochemical reduction is confirmed by estimating the intrinsic barrier from the experimental data on transfer coefficients as well as theoretical calculations involving bond length and potential energy diagrams. The reductions follow the Marcus Hush quadratic activation driving force relation barring the meta-Cl of the 3,4-dichlorobiphenyl which obeys ButlerVolmer kinetics. The first electron transfer step is rapid in comparison with the bondbreaking, implying the stepwise reduction for all the dichlorobiphenyls studied here. High performance liquid chromotagraphy analysis of the products of the bulk electrolysis confirms the order of the reduction in dichlorobiphenyls viz. the ease of reduction follows the order ortho-Cl > para-Cl > meta-Cl.
1. INTRODUCTION Polychlorobiphenyls (PCBs) constitute a major environmental pollutant. PCBs are employed for diverse purposes such as coolants for transformers, capacitors, pesticide extenders, cutting oils, reactive flame retardants, lubricating oils, hydraulic fluids, adhesives, wood floor finishes, paints, dedusting agents, waterproofing compounds, casting agents, vacuum pump fluids, fixatives in microscopy, surgical implants, etc. The decomposition of PCBs is mandatory since PCBs are highly toxic for humans and animals. Hence diverse techniques have been developed for the degradation of PCBs and among them, the following deserve mention: (a) thermal oxidative degradation at high temperatures;1 (b) hydrogenation of PCBs using bimetallic surfaces;2 (c) cobalt-catalyzed methoxycarbonylation;3 (d) photochemical decomposition;4,5 (e) biochemical degradation of PCBs using bacteria;6,7 and (f) decomposition of polychlorobiphenyls using supercritical water containing NaOH into phenol, biphenyl, and CO2.8 The conventional chemical methods of decomposition of polychlorobiphenyls such as reduction, hydrogenation, and dechlorination have also been known.2,5,912 It must be emphasized that among diverse methods of removal of pollutants, electrochemical decomposition constitutes the safest and most effective method. Furthermore, selective electrochemical dechlorinations can be achieved. One of the earliest analyses for the electrochemical reduction of the polychlorobiphenyls and polychloronapthalenes r 2011 American Chemical Society
consists of employing the technique of interrupted sweep voltammogram.13 Alternately, the presence of aromatic radicalanion mediators such as biphenyl and naphthalene enhance the rate of the reduction of polychlorobiphenyls.14 The electrochemical decomposition of 1-chloronapthalene using naphthalene radical anion mediator provides complete dechlorination.15 The regioselective electrochemical decomposition of 2,4-dichlorobiphenyls and a preliminary kinetic and mechanistic analysis using convolution potential sweep voltammtery (CPSV) have been recently carried out.16 The electrochemical reduction of polychlorobiphenyls involves the dissociation of the CCl bonds. The CPSV method is a simple and effective technique for the elucidation of the reaction mechanism.17 The decomposition of each CCl bond in polychlorobiphenyls follows one of the two different mechanisms viz (i) stepwise and (ii) concerted (Scheme 1). The relative rates of the two steps of the reduction (electron transfer vs the bond breaking step) determine the mechanism of the reduction. If the bond breaking step is slow relative to the electron transfer, the reaction follows the stepwise mechanism, and analogously, if the bond breaking step is faster, the concerted mechanism is inferred. The neutral radical formed after dissociation of chloride Received: July 13, 2011 Revised: November 30, 2011 Published: December 05, 2011 655
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Scheme 1. Schematic Representation of Dechlorination of Polychlorobiphenyls
anion undergoes reduction at a lower potential than the parent compound and leads to the formation of the anion. The anion abstracts a proton predominantly from solvent18 thereby yielding the completely dechlorinated compounds (Scheme 1): The theoretical analysis of the electrochemical systems can be accomplished either using multiscale dynamical simulations as is customary in the study of energy storage devices or employing quantum chemical calculations in order to obtain insights regarding the reaction mechanisms. Each of these strategies has inherent merits. In the field of reaction mechanism for electrochemical processes, it is customary to employ CPSV in conjunction with quantum chemical calculations. For example, the CX bond length before and after the electron transfer and the spin density of the carbon atom attached to the halogen are employed as diagnostic criteria for the mechanistic distinction. The potential energy diagram also reveals the stability of the anion radicals which is used for further confirmation. Here, we report the kinetic and mechanistic aspects of the successive decomposition of chlorines from dichlorobiphenyls using CPSV and density functional calculations. The three dichlorobiphenyls are considered as illustrative examples. The rate and mechanism of the each chlorine reduction is discussed separately with the help of various diagnostic criteria.
Figure 1. Structure of three dichlorobiphenyls used for the present investigation.
employing the Becke three parameter hybrid exchange in conjunction with the correlation functional developed by Lee, Yang, and Parr (B3LYP). The 6-31+G(d) basis set was employed for the structural optimization both in vacuum and solvent (acetonitile). Further, hybrid functionals such as B3LYP are often superior than pure functionals and are customarily employed in investigating the reaction mechanism of the dissociative electron transfer reactions. The open shell model (unrestricted) and the closed shell model (restricted) was used for the anionic radicals and neutral molecules respectively. The energy minimization was carried out until no imaginary frequencies were obtained. The solvent was treated using the polarizable continuum model. The relaxed potential energy scanning (based on full energy optimization at each step) was performed for the radical anions as a function of the CCl bond distance in steps of 0.05 Å.
2. METHODOLOGY 2.1. Experiment. Acetonitrile (ACN) (HPLC grade, SRL, India,) was distilled over anhydrous CaH2 protected by 3 Å molecular sieves in argon atmosphere. The supporting electrolyte was tetrabutylammonium perchlorate (TBAP; SigmaAldrich, electrochemical grade) and used without further purification. The reactants 2,3-dichlorobiphenyl (PCB-5), 3,4dichlorobiphenyl (PCB-12), and 3,5-dichlorobiphenyl (PCB-14) from OEKANAL were used as received. The cyclic voltammetric experiments were carried out using an electrochemical workstation (CH Instruments 660A), in a single-compartment electrochemical cell thermostatted at 298 K. The working electrode was a glassy carbon with a diameter of 3 mm (CH Instruments, USA) and polished with the alumina slurry and sonicated before use. The Ag/Ag+ (10 mM) electrode was the quasi-reference electrode and Pt wire as the counter electrode. The uncompensated solution resistance was measured before each experiment and the ohmic drop was compensated leaving the residual resistance of ∼35 Ω. 2.2. Quantum Chemical Calculations. The Quantum chemical calculations were performed using Gaussian 03 software19
3. RESULTS AND DISCUSSION Figure 1 depicts the PCB congeners investigated here. 3.1. Peak Potential and Peak Width Analysis. All three dichlorobiphenyls employed for the analysis exhibit two irreversible peaks corresponding to the dissociation of the two carbonchlorine bonds as shown in Figure 2. The corresponding peak potential values are provided in Table 1. The peaks remain irreversible even at 100 V s1 indicating the fast decomposition of the radical anions into the corresponding neutral radical and the chlorine anion. The peak separation plays a crucial role in the kinetic analysis of the electrochemical reduction. For example, the two irreversible peaks are close to each other in PCB-5 (162 mV at 0.1 V s1) and this becomes even closer at higher scan rates. For PCB-12 and PCB-14, the peaks are separated by more than 200 mV (235 and 214 mV, respectively) and even at larger scan rates, distinct peaks are observed. The variation of the peak 656
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the mechanism of the electrochemical reduction.20 The slopes of the plot between Ep and log (v) and the peak widths are given in the Table 1. The dEp/d log(v) values for the dichlorobiphenyls occur in the range of 3060 mV (except for PCB-14) and hence electron transfer and bond breaking may together control the rate of the reaction. In PCB-14, dEp/d log(v) is >60 mV which implies the initial electron transfer to be the rate determining step. The peak width is calculated from the difference between peak potential and half peak potential for the two individual peaks. The peak width pertaining to the second peak has been calculated employing the standard procedures21 and the calculated peak widths lie in the range of 5497 mV which imply the reaction rate being controlled by both the processes. Another method for distinguishing between stepwise and concerted mechanism lies in deducing the transfer coefficient values using22
potential with scan rate and the peak width measurements are the two important parameters regarding the preliminary inference on
α¼
1:856RT FðEp=2 Ep Þ
ð1Þ
where Ep and Ep/2 denote the peak potential and half peak potential respectively. The transfer coefficients arising from this procedure are provided in Table 1. Since the deduced values are >0.5, stepwise mechanism is inferred. 3.2. Controlled Potential Electrolysis. Bulk electrolysis has been carried out to investigate the regioselectivity in the reduction of CCl bonds. For this purpose, glassy carbon (6 mm, Bioanalytical Systems U.S.A.), Pt spiral electrode, and the Ag/Ag+ (10 mM) are used as the working, counter, and reference electrodes, respectively. A total of 15 mL of 2,3-dichlorobiphenyl (5.9 mM) with 0.1 M tetraethylammonium bromide in ACN is taken in a four-necked bulk electrolysis cell, and the solution was completely flushed with high purity argon gas. A constant potential of 2.6 V vs Ag/Ag+ (10 mM) is applied to the working electrode with constant stirring and the slow streaming of argon gas in the solution. The completion of the electrolysis is inferred from the current value which drops to the background current. The electrolysis product is poured into the 30 mL of water and the resulting compound is extracted using ethyl acetate. The residue which results after evaporating ethyl acetate is analyzed using HPLC. The same procedure is applied for the electrolysis of 3,4-dichlorobiphenyl also, at the potential of 2.6 V vs Ag/Ag+ (10 mM). Since 3,5-dichlorobiphenyl has two metachlorines, no selectivity in reduction is possible. As mentioned above, the residue after dissolving in ACN is subjected to high performance liquid chromatography (HPLC) analysis (SHIMADZU LC-2010C). In the reverse phase chromatogram, the reverse phase column (INERTSIL ODS-3 V
Figure 2. Background subtracted cyclic voltammogram of the reduction of (a) 2,3-dichlorobiphenyl (PCB-5), (b) 3,4-dichlorobiphenyl (PCB-12), and (c) 3,5-dichlorobiphenyl (PCB-14) in ACN/0.1 M TBAP at glassy carbon electrode, v = 0.1 V s1.
Table 1. Parameters Extracted from the Cyclic Voltammogram and Bulk Electrolysis 2,3-dichlorobiphenyl
a
3,4-dichlorobiphenyl
parameters from cyclic voltammetric analysis
a
a
1st Cl
2nd Cl
Peak potential(V) vs Ag/Ag+ at 0.1 V s1
2.476
Ep/2-Ep (mV)
71 to 90
dEp/dlog(v)
64 mV
transfer coefficient from peak widths
0.61
standard reduction potential (V) hetrogeneous rate constants (cm s1) 103
2.412 2.3
2.661 13.7
selectivity in reduction (bulk electrolysis)
ortho
meta
3,5-dichlorobiphenyl
a
a
1st Cl
2nd Cl
1st Cla
2nd Cla
2.638
2.417
2.652
2.438
2.652
45 to 48
71 to 85
48 to 59
54 to 97
49 to 81
36 mV
59 mV
45 mV
73 mV
83 mV
0.62
0.88
0.69
0.80
2.522 12.7
34.4
2.535 14.7
2.701 42.2
para
meta
meta
meta
See Scheme 2. 657
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given in Table 1. At large negative potentials due to the reduction of biphenyl moiety, the plateau region is difficult to obtain. Hence a fitting procedure is employed using28 the empirical equation I = a0 + {a1/(1 + exp[(E a2)/a3])}, where a0, a1, a2, and a3 are fitting constants. The availability of the convoluted current enables the estimation of the heterogeneous electron transfer rate constants (kET) using pffiffiffiffi Il IðtÞ lnðkET Þ ¼ lnð DÞ ln ð4Þ iðtÞ
Scheme 2. Regioselectivity in the Reduction of Dichlorobiphenyls
If the dependence of ln(kET) on E is nonlinear, it indicates the validity of the MarcusHush theory of quadratic activation driving force relationship.29 The transfer coefficient is estimated from ln(kET) vs E data viz. (150*4.6 mm) 5 μ) and the 80% methanol + 20% water mixture served as the stationary and mobile phase respectively. The retention time of the authentic sample of the compounds are as follows: ortho-chlorobiphenyl (8.93 min), meta-chlorobiphenyl (11.68 min), and para-chlorobiphenyl (11.00 min); 2,3-dichlorobiphenyl (12.46 min); and 3,4-dichlorobiphenyl (17.3 min). From the analysis of the authentic samples, the peak at 11.65 min refers to the presence of meta-chlorobiphenyl, whereas that at 12.42 min suggests the presence of 2,3-dichlorobiphenyl. Hence, the elimination of ortho-chlorine is confirmed. For 3,4-dichlorobiphenyl, the peaks appear at 11.67 and 16.62 min which indicate the presence of meta-chlorobiphenyl after the electrolysis. This inter alia indicates the elimination of parachlorine from the 3,4-dichlorobihenyl. Hence, the chlorines are eliminated in the order of ortho-Cl, para-Cl, and then meta-Cl (Scheme 2): The same order of elimination has also been inferred from the chemical reduction of PCBs using naphthalenesodium complex23 and is in a good agreement with other selectivity results pertaining to electrochemical reduction of chlorinated benzoic esters.24 3.3. Convolution Potential Sweep Voltammetry. The CPSV is a very valuable technique in electrode kinetics17,25,26 since it provides comprehensive information on kinetic parameters and reaction mechanism. On account of employing a wide range of scan rates and the convolution algorithm, the diffusion coefficients, symmetry factors, and heterogeneous rate constants are deduced accurately. Furthermore, an a priori assumption of the ButlerVolmer mechanism is not required. The convolution current (I) is related to the experimentally observed current (i) through the convolution integral 1 Z t iðuÞ du IðtÞ ¼ pffiffiffi π 0 ðt uÞ1=2
α¼
RT d lnðkET Þ F dE
ð5Þ
In the above equation, R and T denote respectively the universal gas constant and absolute temperature. In order to obtain α, the procedure advocated by Maran et al.28 wherein successive linear fits for each 15 points with 1 mV per point were constructed. The variation of the transfer coefficient with potential provides an insight into the mechanism of the electrochemical reduction. 2,3-Dichlorobipheyl (PCB-5). The potential-dependent convolution current for the reduction of 2,3-dichlorobiphenyl is shown in Figure 3a, and the diffusion coefficient of 2,3-dichlorobiphenyl calculated using eq 2 is given in Table 1. Thus, this procedure yields the electron transfer rate constant as a function of the potential (Figure 4a). The subsequent numerical differentiation of ln(kET) values with respect to the potential (E) yields the transfer coefficients (Figure 5, panels a and b). The nonlinear variation of individual reduction of chlorines indicates that the reduction follows the Marcus-hush quadratic activation energydriving force relationship. The depicted variation of α with E indicates the reduction of the two different chlorines: the first peak refers to the ortho-Cl reduction, confirmed from the bulk electrolysis (cf. section 3.2), and the second peak corresponds to the meta-Cl in 3-chlorobiphenyl formed as a products of the first stage of reduction. The standard reduction potentials pertaining to each reduction can be estimated from α vs E variation since α = 0.5 when E = E0.16 The heterogeneous rate constants for the reduction of CCl bonds in 2,3-dichlorobiphenyl is provided in Table 1. The magnitude of the transfer coefficient plays a central role in inferring the reaction mechanism.30 The fast electron transfer processes lead to α > 0.5 while α < 0.5 imply the electron transfer step to be rate determining. Figure 5, panels a and b, depicts the variation of the transfer coefficient with potential. The slope of these lines provides an estimate of the intrinsic barrier which in this case is
