Dechlorination of pentachlorophenol in supercritical carbon dioxide with a zero-valent silver–iron bimetallic mixture

Abstract

A continuous stream of pentachlorophenol (PCP, 10–20 mg min⁻¹) in supercritical carbon dioxide (scCO₂) was dechlorinated efficiently by a heated column (25 × 1 cm diameter) of a zero-valent silver–iron (Ag⁰/Fe⁰) bimetallic mixture. Dechlorination efficiencies in successive 10 min fractions of reactor eluate were influenced appreciably by the temperature and pressure within the reactor column(s), the flow rate of the mobile phase and especially by the composition of the feedstock solvent. During 1 h of operation at 450 °C, organically-bound chlorine was liberated, virtually quantitatively, from a 20% (w/v) feedstock stream (0.1 ml min⁻¹ merged with 4 ml min⁻¹ scCO₂), and deposited as chloride ion on the surfaces of the metal particles. Sea sand, maintained under identical conditions, was capable of dechlorinating the substrate only partially (50% loss of the GC peak area for substrate; only partially dechlorinated products). By contrast, the dechlorination was virtually quantitative with the Ag⁰/Fe⁰ support. There was no evidence of PCP substrate or chlorinated aromatics among the products. In addition to approximately equal quantities of phenol and methylated phenols [o-and m-cresol; dimethyl- (2,4-, 2,6-, 3,4-, 3,5-, 3,5-dimethyl)phenol and trimethyl-(2,3,6-, 2,4,6-trimethyl)phenol] the remaining 5% of the product mixture consisted of methylated benzenes (m- and p-xylene, 1,2,4-trimethyl-, pentamethyl- and hexamethyl-benzene). If the feedstock solvent was changed from methanol to propan-2-ol, the product distribution was changed to ca. 50% phenol, 36% 1,2,4-trimethylbenzene and the remainder methylated phenols. In extended operation the reactor was run continuously for 14 h without apparent loss of activity provided that the chloride was washed from the Ag⁰/Fe⁰ metal surfaces at ca. 3 h intervals.

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Green Context

While the destruction of toxic compounds per se does not fall under the green chemistry umbrella, we do recognise that chemical detoxification processes are currently necessary and should be subject to the principle of green chemistry. In this paper, the authors describe the use of an environmentally benign solvent and a very efficient bimetallic mixture to optimise the efficiency and minimise the environmental impact of a dechlorination reaction. It is also interesting to read this article alongside the authoritative review on organochlorine compounds and their role in the environment published in a previous issue of this journal (Green Chem., 2000, 2, 173). We cannot and should not consider all organochlorine compounds as being harmful to the environment but we must also recognise that some of these compounds are harmful and must be dealt with by using our greenest technologies.

JHC

Introduction

The abiotic dehalogenation of aromatic organochlorine (OC) toxicants continues to attract a great deal of attention among researchers. A broad array of different techniques have been proposed to accelerate transformation(s) that reduce the number of substituent chlorine atoms on aromatic rings. Many of the methods for the treatment of chlorophenol-containing waters and waste streams are oxidative and include near-critical water oxidation^1,2 wet air oxidation^3,4 and various oxidation processes that use UV photolysis^5–7 and/or Fenton’s reagent.^8,9 Yet other approaches have involved the use of ultrasound^10,11 γ-radiolysis,¹² electrochemical oxidation¹³ or microwaves.¹⁴ In addition, separation based on catalytic adsorption has also been used.¹⁵

Catalytic hydroprocessing over hydrogen that mediate the reduction of aryl-chlorine substituents to chloride at relatively low temperatures has been reported with Ni, Pd-, Pt- and Rh-based catalysts.^16–20 The liquid phase catalytic hydrodechlorination of chlorophenols over Pd/C²¹ and in the gas phase over Ni⁰/Mo⁰/Al₂O₃,²² Ni⁰/ Al₂O₃¹⁶ and Ni⁰/SiO₂^23,24 has been assessed. Similarly, steam at high temperature has been used to efficiently dechlorinate aromatic chlorocompounds²⁵ and in an earlier report,²⁶ the hydrogenation/hydrogenolysis of phenol was described. Static water-based systems have also been studied with a bimetallic Pd⁰/Fe⁰ catalyst.^27,28

Hydrogen treatment of HOArCl_x isomers in the range 423–573 K, yielded phenol as the only appreciable product. Hoke et al., using commercial Pd/C in the presence of base, reported HOAr as the only organic product²¹ whereas Chon and Allen²² recovered benzene and chlorobenzenes from hydrodechlorinations of chlorophenols over NiMo/Al₂O₃. The benzene ring however, remained intact. Under these conditions, phenol was hydrogenated to cyclohexanol and cyclohexanone with benzene being formed at T > 523 K.

Whereas extraction methods can transfer and concentrate target toxicants from one medium to another, they do not detoxify the toxicants per se. It would be more efficient to combine the extraction/mobilization of aryl-organochlorine compounds with an on line dechlorination step. Such a dechlorination stage that could be combined with an extraction into supercritical carbon dioxide (scCO₂) has been reported²⁹ for polychlorinated biphenyl (Aroclor 1242 and 1248) mixtures. With the current report, the dechlorination method is extended to pentachlorophenol (PCP).

Experimental

Chemicals

Sodium dispersion [40% (w/v) in oil] was purchased from Alfa Aesar, Ward Hill, MA, acetic anhydride (Ac₂O), ethylene glycol dimethyl ether (1,2-DME), hexane, and silver nitrate were purchased from Fisher Scientific, Ottawa, ON and were used as received. Ag/Fe⁰ was prepared from ∼40-mesh iron that had been washed copiously with 6 M HCl and rinsed with distilled water. Sufficient aqueous AgNO₃ to result in a 2% (w/w) surface coverage of the iron, was added to the aqueous iron suspension that was gently mixed on a rotary evaporator for 12 h. Ag⁰/Ni⁰ was prepared in similar fashion by reaction of aqueous AgNO₃ [sufficient to provide a 2% (w/w) surface coverage] with an aqueous suspension of pre-washed 50–100 mesh nickel granules during 12 h.

Reactor

The dechlorination assembly²⁹ consisted of source of pressurized scCO₂, (Prepmaster, Isco Corporation, Lincoln, NE), a mixing tee and a reactor unit. Target substrate chemical in a suitable solvent, was delivered at 0.1 ml min⁻¹ to the mixing tee (1/16″ i.d.), merged with a scCO₂ stream (ca. 4 ml min⁻¹) and fed to one or two stainless steel (ss) HPLC column assemblies [10 mm inner diameter (i.d.) × 25 cm] connected in series, that were filled with test zero-valent metal (or metal mixture) encased in an insulating alumina jacket (fashioned from a thin walled alumina tube (Alfa Aesar, Danvers, MA), that had been cut lengthwise to provide two semi-cylinders). The ss column-alumina jacketed assemblies were heated separately with 80-turn coils of high resistance heating wire that were energized from variable transformers. Pressure within the reactor was maintained with a terminal restrictor made of capillary quartz (25 cm × 0.050 mm) tubing (Chromatographic Specialties, Brockville, ON).

Reactor operation

After a short delay to purge residues of air from the system (during which time only scCO₂ was fed to the reactor), feedstock, 0.1 ml min⁻¹, was added continuously via the HPLC pump to the scCO₂ stream and transported to the reactor. Measurements at the exit of the capillary restrictor indicated a flow rate corresponding to 200–850 ml min⁻¹ of decompressed gas. The exit tip of the capillary restrictor was immersed in hexane (25 ml) to trap products from the reactor eluate. Each experiment was continued until five or six consecutive traps had been collected, each corresponding to 30 or 10 min of cumulative trapping of reactor eluate. The course of the dechlorination was monitored by gas chromatography–mass spectrometry (GC–MS) and by titration to measure levels of residual organically bound chlorine in the eluate.

Organically bound chlorine

The general procedure outlined in ref. 30 was followed. Residual organically bound chlorine in the organic trapping solution was determined by titration with standardized AgNO₃ (ca. 0.01 M). Sodium dispersion, 2 ml, was added to a vigorously stirred 10 ml aliquot of hexane trapping solution that had been further diluted 5-fold with fresh hexane. Methanol, 2 ml, was added dropwise; the reaction was continued for 5 min then quenched by the addition of propan-2-ol (15 ml) followed by 90 ml water. The aqueous phase was diluted to 100 ml in a volumetric flask. A 20 ml aliquot of the reaction mixture was acidified to pH 6–7 with nitric acid (7 M), supplemented with 5 drops of potassium chromate indicator solution then titrated with AgNO₃. Blank determinations for chloride in reactor eluate consisted of an identical procedure in the absence of added sodium dispersion.

Gas chromatography–mass spectrometry

GC–MS was performed on an Agilent Technologies model 5890 series II gas chromatograph fitted with a model 5970 mass selective detector. The HP1 capillary column (30 m × 0.25 mm i.d.) was eluted with helium at 0.5 ml min⁻¹. After an initial hold for 2.5 min at 40 °C, the column was ramped, at 25 °C min⁻¹, to 100 °C, held for 1 min, followed by heating to 210 °C at 10 °C min⁻¹ and held for a further 1 min prior to cooling. The temperature of the injector and detector were maintained at 250 and 280 °C, respectively. Eluting components were identified tentatively by comparison of experimental mass spectra with spectra catalogued in the National Institute of Standards and Technology (NIST) or Wiley 318 spectral libraries and corroborated by co-chromatography and spectral matching with authentic standards.

Results and discussion

The addition of 0.1 ml methanolic solution to the scCO₂ mobile phase is considered to result in a single supercritical phase³¹ that possesses attractive features including increased solvent strength while the viscosities and fluidities approach those of supercritical carbon dioxide (scCO₂).³²

The extent of loss of the gas chromatography (GC) peak for pentachlorophenol (PCP) in the reactor eluate was influenced appreciably by the identity of the zero-valent (ZV) metal, by the temperature and pressure within the reactor column(s) and by the composition of the feedstock solvent. As had been observed previously,²⁹ higher reactor column operating temperatures favoured the dechlorination of PCP. With a single column of zero-valent silver–iron (Ag⁰/Fe⁰) bimetallic mixture and moderate operating temperature, increasing quantities of PCP were detected in successive trapping solutions. However, at higher operating temperatures, the reaction was more complete and more persistent (shorter range of PCP recoveries) over the six successive traps that constituted each trial (Table 1). At 450 °C, the disappearance of PCP from a 20% (w/v) PCP solution in methanol (fed to the reactor at 0.1 ml min⁻¹) was virtually complete over the course of the 1 h trial.

Table 1 Percentage losses of pentachlorophenol (PCP) with the concomitant release of chloride in six sequential trapping solutions collected at the exit during 10 min for a single reactor column containing zero-valent silver–iron bimetallic mixture

Feedstock solvent^a	PCP in feedstock (%)	Reactor temp/°C	Loss^b of PCP (%)	Yield of Cl^−^c (%)
a MeOH, methanol; DGDE, diethyleneglycol diethyl ether; Ac2O, acetic anhydride; DME, ethylene glycol dimethyl ether; IBMK, isobutyl methyl ketone.b Percentage loss of PCP in the reactor eluate (as determined by GC) relative to 1 ml (0.1 ml min^−1 × 10 min) of feedstock.c [1 − (ratio of organically bound chlorine in the trapping solution/organically bound chlorine in an equivalent quantity of feedstock)] × 100.
MeOH	5	350	75–96
MeOH	5	350	77–95
DGDE	5	400	95–99	88–91
MeOH	10	400	95–100	89–95
Ac2O–DME (1/8)	10	400	97–100	76–81
IBMK	10	450	89–96	72–85
MeOH	20	450	88–99	96–97
MeOH	20	450	100	91–94
MeOH	20	450	100	93–95

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The dechlorination efficiency (percentage yield), defined as (1 − the ratio of the quantity of organically bound chlorine in the reactor eluate divided by the content of organically bound chlorine in an equivalent quantity of feedstock) × 100, was monitored by argentiometric titration. The dechlorination yields among consecutive 10 min trapping solutions and the range of dechlorination efficiencies (for the six traps) are also recorded in Table 1. Whereas PCP was reacted virtually completely and the reactor could be run for 1 h, residual organically bound chlorine remained in each trap. By connecting two reactor columns in series, the dechlorination became more efficient (Table 2). The flow rate of mobile phase scCO₂ also influenced the dechlorination yield (Table 3). Increased mobile phase flow rates that resulted in a decreased contact time of substrate with the metal surface, decreased the dechlorination efficiency somewhat. With 52.4 g of ca. 40 mesh Ag⁰/Fe⁰ bimetallic mixture required to fill the column completely, the void volume was estimated to be <1 cm³.

Table 2 Observations with two sequential Fe⁰ reactor columns connected in series or a single reactor column for a 20% (w/v) methanolic feedstock delivered, at 0.1 ml min⁻¹, to the reactor maintained at 450 °C under 250 atm of scCO₂

Trap
1	2	3	4	5	6
a Percentage reduction in the gas chromatography peak for PCP.b Percentage reduction in the quantity of organically bound chlorine.
Two columns
PCP loss^a (%)	100	100	100	100	100	100
Loss org Cl^b (%)	98.0	98.0	98.3	98.5	98.5	98.5
One column
PCP loss^a (%)	96.9	98.9	99.6	99.3	99.8	99.8
Loss org Cl^b (%)	92.9	94.3	94.8	95.3	95.3	95.3

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Table 3 Influence of the scCO₂ flow rate at constant pressure (250 atm) on the percentage reduction in the peak area of pentachlorophenol for a single reactor column of Fe⁰ operated at 450 °C

Trial number	Decompressed CO2 flow rate/ ml min^−1	Loss of PCP peak^a (%)
a Range observed for the six sequential traps of reactor eluate that had been collected during 10 min.
1	220	96.5–97.8
2	300	94.5–95.8
3	350	92.0–94.5
4	450	89.6–92.0
5	600	87.1–91.0

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The course of the reaction was probed further by measuring the quantity of chloride that could be extracted from the Ag/Fe bimetallic mixture contained in one column. After continuous reaction of a 20% (w/v) methanolic PCP solution during 1 h (corresponding to the addition of 799.0 mg Cl to the column), the packing material was washed copiously with water. The percentage loss of the PCP peak (GC) and the dechlorination yield (titration) are recorded in Table 4. The combined water washes contained 737 mg of chloride in close agreement with the mean percent dechlorination in the eluate of 94.7% of the organically bound chlorine (corresponding to the release of 757 mg of chloride). The chlorine content of PCP was converted efficiently to chloride and was deposited on the surfaces of the metal particles.

Table 4 Substrate losses and dechlorination yields observed in six sequential traps for a 20% (w/v) methanolic PCP feedstock delivered, at 0.1 ml min⁻¹, to a single Ag⁰/Fe⁰ column maintained at 450 °C/250 atm

Trap
1	2	3	4	5	6
PCP peak loss (GC) (%)	96.9	98.9	99.6	99.3	99.8	99.8
Dechlorination (titration) (%)	92.9	94.3	94.8	95.3	95.3	95.3

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To evaluate the magnitude of thermally induced dechlorinations, a companion experiment was conducted using silica (sea sand) to fill the reactor column. With the optimized reaction conditions (450 °C/250 atm, 1.5 ml min⁻¹ scCO₂), a feedstock of 10% (w/v) PCP in 1,2-DME delivered at 0.1 ml min⁻¹ was used to collect six traps of eluate. The resulting chromatograms indicated the loss of ca. 50% of the PCP peak area and the formation of 4-chlorophenol, 3,5-dichlorophenol, 2,3,5-trichlorophenol, 2,3,4-trichlorophenol and 2,3,4,5-tetrachlorophenol that comprised the remainder of the products. Only a trace of phenol was detected in any of the eluate solutions. Thus, thermally induced dechlorination is inefficient relative to the action of the Ag⁰/Fe⁰ particles.

A variety of products were detected by GC–MS (Table 5) from trials using a 15% (w/v) methanolic PCP feedstock. Conditions were as before (0.1 ml min⁻¹ feedstock delivered to the reactor at 450 °C/250 atm) except that successive traps were collected during 30 min. In addition to the anticipated phenol (46.9 ± 1.0%), methylated products, principally methylated phenols (47.5 ± 1.5) and methylated benzenes (5.1 ± 0.5%) dominated the remaining products. Table 5 also provides a measure of the levels of repeatability that was achieved in these studies and indicates that 74.6 ± 2.7% of the substrate (mean mass balance) was accounted for among the products. Presumably other products were not detected because they co-eluted from the GC with the solvent.

Table 5 Percentage distributions of products in successive 30-min traps collected at the exit of a single reactor column of silver–iron (2% w/w) bimetallic mixture from a methanolic PCP [15% (w/v)] feedstock delivered to the reactor (450 °C/250 atm) at 0.1 ml min⁻¹

Trap
Product	1	2	3	4	5
p-Xylene	1.2	1.3	1.2	1.3	0.9
m-Xylene	1.8	1.7	1.8	1.8	1.6
1,2,4-Trimethylbenzene	N.D.	1.7	1.5	1.8	1.6
Pentamethylbenzene	0.2	N.D.	N.D.	N.D.	N.D.
Hexamethylbenzene	1.3	0.8	0.7	0.7	0.5
Phenol	45.2	46.9	47.5	47.4	47.6
o-Cresol	33.6	31.3	30.7	29.9	30.3
m-Cresol	12.0	12.2	12.3	11.9	12.6
2,6-Dimethylphenol	1.7	1.6	1.6	1.7	1.8
2,4-Dimethylphenol	1.3	1.4	1.4	1.4	1.4
3,4-Dimethylphenol	N.D.	N.D.	N.D.	N.D.	N.D.
3,5-Dimethylphenol	N.D.	0.2	0.3	0.3	0.3
2,4,6-Trimethylphenol	0.8	0.7	0.7	0.5	0.4
2,3,6-Trimethylphenol	0.5	0.4	0.3	0.2	0.2
4-Chlorophenol	N.D.	N.D.	0.5	0.8	1.0
Mass balance	0.706	0.751	0.777	0.761	0.736

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Trap
Product	1	2	3	4	5	Mean
Methylated phenols	49.9	47.8	47.4	45.9	47.0	47.5 ± 1.5
Phenol	45.2	46.9	47.5	47.4	47.6	46.9 ± 1.0
Methylated benzenes	4.5	5.5	5.2	5.6	4.6	5.1 ± 0.5
Chlorinated phenols	N.D.	N.D.	0.5	0.8	1.0

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It was of interest to evaluate the reactor under conditions of extended operation. A further 23 traps were collected during the total of 14 h of continued operation. At selected intervals (2.5 or 3 h), the flow of feedstock was interrupted and the column was washed with 30 ml of water–methanol (1/1) to remove accumulated chloride from the particle surfaces. The reactor column was subsequently dried with scCO₂ for 30 min prior to continuing the addition of substrate. Chlorinated product was detected (albeit in low yield) in traps 3–5 of this experiment (Table 5). The results of monitoring the course of the reaction by GC–MS (28 successive traps) are summarized in Table 6. For clarity, the different products have been grouped into methylated phenols, methylated benzenes and chlorinated phenols. Over the course of the extended trial, there were no major differences in the distributions of products [mean percentage phenol in traps 6–28, 54.6 ± 3.9; mean percentage methylated phenols, 42.3 ± 4.4; mean percentage methylated benzenes, 3.1 ± 0.8]. Substrate PCP was not detected in any of the traps and chlorinated phenols (4-chlorophenol and 2,4-dichlorophenol that collectively accounted for a maximum of 0.36–1.26% yield) were detected only intermittently (in traps 3, 4, 5, 16, 17, 20, 21 and 22). Chlorinated products were first detected in trap 3 and increased in concentration in traps 4 and 5 (Table 5). Similar concentrations were detected in traps 15 and 16 and in traps 20, 21 and 22. However, as summarized in Table 6, the appearance of chlorinated phenols among the products was eliminated by the 30 ml methanol–water wash. It is postulated that chloride partially blocked access of the substrate to active sites on the catalyst. The activity of the reactor column apparently was restored completely by this washing procedure. There was no reason why the experiment could not have been prolonged appreciably.

Table 6 Percentage distributions of recovered products in 30 min cumulative fractions of reactor eluate either prior to (traps 5, 11, 17, 22 or 28) or post (traps 6, 12, 18 or 23) an on-column wash with 30 ml methanol–water (1∶1)

Trap
Product	5	6	11	12	17	18	22	23	28
Methylated benzenes	4.8	2.6	3.4	3.7	3.8	3.8	3.2	3.0	1.8
Phenol	48.4	47.8	50.2	52.5	58.4	57.0	53.4	40.2	58.8
Methylated phenols	45.9	49.6	46.3	43.8	36.9	39.3	42.2	56.5	39.3
Chlorinated phenols	0.9	N.D.	N.D.	N.D.	1.8	N.D.	1.2	N.D.	N.D.
Mass balance	0.737	0.703	0.761	0.722	0.737	0.738	0.687	0.711	0.702

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It was also of interest to gain insight into the source of the methyl substituents in the dechlorinated products. Several patents have described the ortho/para methylation of phenolic substrates by methanol over metal oxide catalysts at elevated temperature.^33–35 An alternate source of the methyl groups in the products might be the scCO₂ mobile phase. Carbon dioxide can be reduced at iron surfaces in the presence of water to form short chain alkanes. Approximately 90% of the products consisted of methane.³⁶ A possible probe might be to use a different solvent to dissolve the substrate PCP. Table 7 records the product distribution when substrate PCP, in propan-2-ol, was delivered to the reactor (450 °C/250 atm) at 0.1 ml min⁻¹. Although there were no differences in the product identities and the mean mass balance over the six traps accounted for 87.7 ± 2.1% of the PCP substrate, the distribution of products were appreciably different from those observed in methanol (Table 5). Whereas phenol accounted for 50.4 ± 1.0% of the products in propan-2-ol, (not appreciably different from the 46.9 ± 1.0% observed in methanol), methylated phenols accounted for only 13.3 ± 1.7% vs. 47.5 ± 1.5% in the methanol carrier. The remainder of the products in the propan-2-ol carrier was 1,2,4-trimethylbenzene (36.5 ± 1.0%). Propan-2-ol solvent has been used a source of thermally induced hydrogen radicals and was anticipated to lower the contribution of methyl groups from the solvent. This unanticipated reaction is under investigation.

Table 7 Dechlorination of PCP, 10% (w/v) in propan-2-ol, delivered at 0.1 ml min⁻¹, to a single reactor column of silver–iron (2% w/w) bimetallic mixture maintained at 450 °C/250 atm

Trap
1	2	3	4	5	6
Loss PCP peak (%)	100	100	100	100	100	100
Dechlorination (%)	98.0	98.7	98.7	99.3	99.3	99.3
Phenol/μmol	162	160	168	170	168	167
1,2,4-Trimethylbenzene/μmol	118	120	120	122	120	121
o-Cresol	23	15	22	22	20	22
m-Cresol	17	9	10	14	14	17
2,4-Dimethylphenol	13	9	9	8	8	8
Total mass/μmol	333	313	335	332	330	332
Mass balance	0.887	0.833	0.892	0.884	0.878	0.887

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Trap
Distribution of products (%)	1	2	3	4	5	6	Mean
Methylated phenols	15.9	10.6	12.3	13.2	13.3	14.1	13.3 ± 1.7
Phenol	48.6	51.1	50.1	51.2	50.9	50.3	50.4 ± 1.0
Methylated benzenes	35.4	38.3	35.8	36.7	36.4	36.4	36.5 ± 1.0
Chlorophenols	N.D.	N.D.	N.D.	N.D.	N.D.	1.6

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The ability to influence the selectivity/direction of the reaction would be helpful. The loss of the hydroxyl group from the substrate is considered undesirable as a detoxification route since the lipophilicity of the product is increased and a functional group, that can facilitate metabolic transformations, is lost. Thus, methanol is favoured as a solvent because of the formation of phenol and methylated phenols at the expense of substituted benzenes.

Acknowledgements

Financial support from the Natural Science and Engineering Research Council of Canada is gratefully acknowledged.

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