Low temperature decomposition of PCB by TiO2-based V2O5/WO3 catalyst: evaluation of the relevance of PCDF formation and insights into the first step of oxidative destruction of chlorinated

Applied Catalysis B: Environmental 34 (2001) 113–127

Low temperature decomposition of PCB by TiO2-basedV2O5/WO3 catalyst: evaluation of the relevance of PCDF

formation and insights into the first step of oxidativedestruction of chlorinated aromatics

Roland Weber∗, Takeshi SakuraiResearch Institute, Ishikawajima-Harima Heavy Industries Co. Ltd., 1 Shin-Nakahara-Cho, Isogo-ku, Yokohama 235-8501, Japan

Received 16 April 2001; received in revised form 22 May 2001; accepted 25 May 2001

Abstract

The reduction of polychlorinated biphenyls (PCB) emissions to the environment are contemporary issues of global efforts.Within this goal, the destruction of PCB is a challenge also for applied catalysis, in particular with respect to the destructionof PCB waste and the off-gas cleaning of incineration facilities.

In this study, PCB were destroyed on a V2O5/WO3 supported titanium catalyst at low temperature in the range of 150–300◦C.At a space velocity of 5000 h−1 more than 98% could be removed. Below 250◦C, the higher chlorinated PCB remained partlyunchanged on the catalyst for several minutes. In contrast, the oxidation process lasted up to hours at a temperature of 150◦C.

At around 200◦C and below a significant part of the PCB were oxidised to the more toxic polychlorinated dibenzofurans(PCDF). The PCDF remained mainly adsorbed on the catalyst. At 250◦C, no significant amount of PCDF were detected andat 300◦C no byproducts were found.

The oxidation of selected individual PCB isomers and the isomer specific analysis of PCDF formed give an insight intothe initial step of degradation of chlorinated aromatics on V2O5/WO3-TiO2 catalysts. The results indicated that hydrogenabstraction is the preferred step and an initial abstraction of chlorine is only a minor pathway. This suggests an electrophilicoxygen transfer during the destruction of PCB.

During the destruction experiments, a slight chlorination of the non-degraded PCB occurred. On the other hand, no mea-surable dechlorination/hydrogenation was observed. © 2001 Elsevier Science B.V. All rights reserved.

Keywords:PCB; Destruction; V2O5/WO3-TiO2; PCDF formation; TEQ; Chlorine shift; Hydrogen abstraction; Chlorination

1. Introduction

The release of polychlorinated biphenyls (PCB)poses a serious threat to public health and the envi-ronment [1,2]. Although their production has been

∗ Corresponding author. Tel.:+81-45-759-2164;fax: +81-45-759-2149.E-mail address:[email protected] (R. Weber).

discontinued, PCB are still remaining in large quan-tities in, e.g. capacitors, electrical transformers andtransformer oils or as hydraulic fluids in coal min-ing. Therefore, PCB are targeted by governments aspart of a global treaty on persistent organic pollutants[3].

The base line remediation technology for PCB isincineration. However, for a safe destruction, tem-peratures of more than 1200◦C are required which

0926-3373/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved.PII: S0926-3373(01)00211-9

114 R. Weber, T. Sakurai / Applied Catalysis B: Environmental 34 (2001) 113–127

demands expensive hazardous waste incinerationfacilities [4].

Therefore, several alternative PCB destructiontechnologies have been proposed during the last twodecades or are already commercially applied includ-ing based catalysed decomposition (BCD) process[5], KOH/PEG method [6], UV-irradiation [7], sub-and supercritical water treatment [8,9] or biodegra-dation [10]. However, to date there is no state of theart technology for a destruction method alternative tohazardous waste incineration. Since PCB are includedin a variety of matrices, different technologies mayprovide an optimum solution for different waste.

The possibility of catalytic oxidation of PCB wasinvestigated by Subbanna et al. [11] and Hagenmaier[12]. Hagenmaier investigated different oxidation cat-alysts. At temperature of 400◦C, a destruction degreeof more than 99.99% could be achieved [12]. Sab-bana et al. tested various catalysts (including Cr2O3,CuCr2O4, Pt/Pd, Co3O4, or CuO) in the temperaturerange of 500–600◦C and reported a destruction effi-ciency ranging from 69.3 to 97.3%.

Recently, some PCB congeners have been assignedtoxic equivalent factors (TEF) by WHO/ICPS [1].Thus, the PCB contribute, together with polychlori-nated dibenzodioxins (PCDD) and polychlorinateddibenzofurans (PCDF), to the toxic equivalency(TEQ) value in off-gases of regulated facilities (e.g.waste incinerators or hazardous waste incinerators).Due to the presence of PCB in the flue gas of wasteincinerators [13–15] and other thermal processes, theyare of interest especially in facilities that are regulatedby law with respect to off-gas TEQ values. Therefore,PCB are an environmental key subject in two respects:off-gas cleaning and destruction of PCB waste.

The temperature range applied for catalytic fluegas cleaning for PCDD/PCDF in, e.g. waste inciner-ation is generally between 180 and 300◦C. Recently,it was proposed that SCR catalysts may be used forPCDD/PCDF removal at operation temperatures aslow as 150◦C [16] or even lower temperatures.1 It hasbeen shown that the TiO2-based V2O5/WO3 catalysts,which were originally designed for the removal of ni-trogen oxides (NOx) by selective catalytic reduction

1 During a real application trial in a sintering plant at 110–140◦C,the catalytic removal decreased within 6 months from 60 to 0%efficiency [17].

(SCR) [18–22], are very effective for PCDD/PCDFremoval in the same temperature range used for thedeNOx-reaction [23–25]. With the addition of ammo-nia these catalysts can, therefore, be used for the com-bined destruction of NOx , and enable PCDD/PCDFstack gas values below 0.1 ng TEQ/N m3.

In this study, we investigated the temperature depen-dency of PCB destruction in the temperature rangesrelevant to or proposed for flue gas cleaning. Addition-ally, we evaluated the possibilities and the limits of thedestruction of PCB waste in this temperature range.Further, PCDF and PCDD formation during catalyticoxidation of PCB was assessed and an insight intothe initial steps of oxidative destruction of chlorinatedaromatic compounds was provided.

2. Materials and methods

2.1. Catalyst

For this study, a commercial catalyst (V2O5/WO3on TiO2 basis) especially developed for the simulta-neous destruction of PCDD/PCDF and nitrogen ox-ides was used. According to the manufacturer, the spe-cific surface area of the catalyst was between 60 and75 m2/g and the pore volume was above 25 ccm/g.

The catalyst was subjected to elemental analysisaccording to the Japanese Industrial Standard Method(JISM) by using a sequential plasma spectrometerICPS-7500 (Shimadzu Co. Ltd., Kyoto, Japan). Theresults are shown in Table 1. According to X-raydiffraction analysis by a MXP-3 (Mac Science Co.Ltd., Yokohama, Japan) the structure of the TiO2phase is of anatase type.

2.2. Chemicals

PCB mixtures used in this study included a ClophenA 30 sample and transformer oil corresponding to

Table 1Composition and structural data of the catalyst

Detected phase(XRD)

Catalyst composition (wt.%)

TiO2 V2O5 WO3

Anatase 78.2 7.1 5.9

R. Weber, T. Sakurai / Applied Catalysis B: Environmental 34 (2001) 113–127 115

Clophen A 60. For the experiments, these PCB mix-tures were combined to get a homogeneous homologuedistribution of diCB to heptaCB. The mixture alsocontained measurable amount of octaCB and nonaCB(Fig. 4).

The PCDF, generally present in the ppm levels incommercial mixtures [26–28], were separated fromthe PCB on an alumina column before application ofthe mixtures to avoid their interference with PCDFthat may form during the experiments.

The individual hexaCB isomers (2,2′,3,4,4′,5′-(#138), 2,3,3′,4′,5,5′- (#162), 3,3′,4,4′,5,5′- (#169))were purchased from Wellington (Ont., Canada). Fornumbering of the PCB substitution see Fig. 1.

2.3. Flow-stream experiments

The honeycomb catalyst were carefully crushed andsieved. The particle size of the resulting flakes wereabout 0.6 mm× 1 mm× 1 mm. Approximately 5 g ofthe catalyst was placed in the quartz tube (13 mmi.d.) of the flow reactor (Fig. 2). Before closing thereactor, the PCB mixture (1000–10,000�g) was ap-plied to silica and placed in the evaporating zone.After the reactor reached a set temperature, the gasflow was started. The composition of the gas mix-ture was chosen to correspond to that in municipalwaste incineration (10% O2, 70% N2, 20% H2O).The volumetric flow through the catalyst bed corre-

Fig. 2. Schematic of the reactor. (1) O2/N2 source; (2) flow controller; (3) pump; (4) water; (5) preheating system; (6) evaporation portfor compounds; (7) furnace; (8) pyrex reactor; (9) catalyst bed; (10) washing bottles for sampling (empty/toluene); (11) active carbon trap;(12) gas meter.

Fig. 1. Molecular structure and positions of substituents in PCDD,PCDF and PCB.

sponded to a space velocity (SV) of 5000 h−1 andwas in the order of SCR catalysts used in municipalwaste incinerators. The flow rate was regulated us-ing mass flow controllers (Shinagawa Seiki, Japan).When equilibrium was reached, the experiment wasstarted by heating the evaporation zone for 10 minand continuing the flow for 30 or 120 min, respec-tively.


2.4. Stationary experiments

The PCB mixture or individual isomers were ap-plied in toluene directly on the catalyst (100 mg) andthe toluene was subsequently evaporated at room tem-perature. The catalyst was heated to the respectivetemperature and, after the reaction time, was rapidlycooled to room temperature.

Measurements with a thermocouple (Cr–Ni–Cr,Sakaguchi Corp. Tokyo, Japan) showed that the cata-lyst reached the desired temperature after about 120 s.The cooling process lasted about 20 s.

2.5. Analysis and quantification

2.5.1. ExtractionThe catalysts were extracted for 12 h by Soxhlet ex-

traction with toluene. The glass tubes after the catalystwere rinsed with toluene. These rinses were combinedwith the toluene in the washing bottle (impinger). Thetoluene in the washing bottle and the extracts of thecatalysts were analysed separately.

2.5.2. Clean-up, analysis and quantificationThe clean-up procedures are described elsewhere

[29,30]. Analysis was carried out by high resolutiongas chromatography on a HP 6890 gas chromato-graph coupled to a HP 5973 mass selective detector(low resolution mass spectrometry) or a MicromassAutospec Ultima (high resolution mass spectrome-try). The quantification for PCB and PCDD/PCDFwas carried out by isotope dilution mass spectrome-try with 13C-labelled standards. The GC columns usedfor the isomer specific analysis of PCB and analysisof hydroxylated PCB was a DB-5 fused silica column(30 m, 0.32 mm i.d., 0.25�m film thickness, J&W Sci-entific, Folsom, USA). For the isomer specific analysisof PCDD/PCDF a SP-2331 column (60 m, 0.25 mmi.d., 0.2�m film thickness, Supelco, Bellefonte, USA)was used.

2.5.3. Calculation of removal efficiency anddestruction efficiency

Removal efficiency (RE) describes the ability of thecatalyst to remove PCB from the gas stream and wascalculated as: ((PCB)inlet − (PCB)outlet)/(PCB)inlet.

Destruction efficiency (DE) describes the ability ofthe catalyst to destroy PCB during the experimental

time and was calculated as:((PCB)inlet−[(PCB)outlet+(PCB)on cat])/(PCB)inlet.

The %PCB recovery in the impinger was calculatedas: recoveryimpinger = 100− RE.

The %PCB recovery on the catalyst was calculatedas: recoverycatalyst= (PCB)on catalyst/(PCB)inlet.

3. Results and discussion

The catalyst was tested for the destruction effi-ciency of PCB during flow experiments using a tem-perature range of 150–300◦C. The high activity of thiscommercial V2O5/WO3-TiO2 catalyst was alreadydemonstrated during our former investigation of thedestruction efficiency of PCDD/PCDF, chlorinatedbenzenes and polyaromatic hydrocarbons [31].

3.1. Removal efficiency and destruction efficiency ofPCB during flow

The catalyst showed a removal efficiency of morethan 98% in the entire temperature range tested(Table 2). The removal efficiency showed an unusualtemperature dependency during the 30 min of the re-action time. Removal efficiency decreased from 300to 200◦C and surprisingly increased below 200◦C. At150◦C, we found a higher RE compared to 250◦C(Table 2).

The reason for this behaviour became obvious whenthe remaining PCB on the catalyst were analysed. At150◦C, 77% of the PCB were recovered unchangedon the catalyst (Table 2). Increasing the temperatureto 200◦C only 12% of the initial concentration wererecovered and at 250◦C only 0.05% of total PCBsremained unoxidized on the catalyst. At 300◦C, noPCB were detected2 on the catalyst. Therefore, at300◦C we found a destruction efficiency of more than99.9%.

An additional experiment was performed at 150◦Cwith an extended reaction time (120 min). During thisexperiment, the removal efficiency decreased com-pared to the 30 min run and was lower compared tothe experiment at 250 and 300◦C (30 min; Table 2).This demonstrates that at low temperatures the initially

2 Detection limit for individual PCB congeners was between 0.02and 0.05 ng.


Table 2Removal efficiency and recovery of PCB and PCDF formation during catalytic destruction of the PCB mixture (10 mg PCB, 5000 h−1,10% O2, 70% N2, 20% H2O)

150◦C(30 min)

150◦C(120 min)

200◦C(30 min)

200◦C(120 min)

250(30 min)

300(30 min)

Removal efficiency (%) 99.95 99.58 98.78 99.04 99.82 99.92Recovery on catalyst 77.4 20.5 13 0.4 0.05 n.d.b

PCDF after catalyst (�g/g PCB) n.m.a 0.862 1.949 2.425 0.002 n.d.b

PCDF on catalyst (�g/g PCB) 715.3 843.1 382.9 24.2 0.0003 n.d.b

a Not measured.b Not detected (detection limit for single congeres: PCB 0.02–0.05 ng, for PCDF 0.02 ng).

adsorbed PCB can partly desorb and finally pass thecatalyst bed. However, more than 20% of the startingconcentration of PCB were found to be still adsorbedon the catalyst (Table 2).

A closer examination of the homologue pattern ofthe PCB adsorbed on the catalyst showed a shift tohigher chlorinated homologues compared to the ho-mologue pattern in the starting PCB mixture (Figs. 3and 4). Therefore, the lower chlorinated PCB were de-graded faster. For example, at 150◦C more than 70%of diCB were destroyed after 30 min while more than90% of hexaCB to nonaCB were recovered on thecatalyst (Fig. 3). Increasing the temperature to 250◦C

Fig. 3. Recovery of PCB on the catalyst in dependence of the degree of chlorination.

(30 min) only a small fraction of PCBs with a degreeof chlorination of more than hexaCB were recoveredon the catalyst (Fig. 3). An analogous trend was ob-served for PCDD and PCDF during a former study[31].

The main cause of the differences in destructionrate is the change of “redox potential” with changingdegree of chlorination. With an increase of the degreeof chlorination the PCB show an increasing resistanceagainst oxidation. According to this effect, the higherchlorinated PCB are less susceptible to destructioncompared to lower chlorinated PCB when exposed forthe same residence time to the catalyst surface.


Fig. 4. Homologue distribution of the initial PCB mixture in comparison to the pattern of PCB adsorbed on the catalyst and PCB recoveredin the off gas (200◦C, 30 min, 5000 h−1).

In contrast to the homologue pattern-shift on thecatalyst towards higher chlorinated homologues, thehomologue pattern of PCB recovered in the impinger(off-gas) was shifted to the lower chlorinated con-geners when compared to the starting PCB mixture.This is shown in Fig. 4 for an experiment at 200◦C.This trend is due to the changing volatility of the PCBwith chlorination degree. The volatility of PCB de-creases with increasing degree of chlorination. Higherchlorinated PCB are retained on the catalyst surfacelonger, therefore, increasing the chance of oxidation.The effect of decreasing volatility with increasingchlorine substitution overcompensates the effect ofan increasing “redox potential” during PCB removal.This resulted in a decreasing removal efficiency ofPCB homologues with decreasing chlorine degree.These competing effects and a decreasing removal ef-ficiency with decreasing chlorination degree was alsofound for chlorobenzenes during a previous study[31].

3.2. PCDF formation during catalytic oxidation ofPCB

One key question for assessment of a PCB de-struction technology is whether the more toxicPCDD/PCDF are formed and under which conditionand application their potential conversion to PCDF has

no relevance. The conversion of PCB to PCDF onlydemands the insertion of an oxygen (ether-bridge;compare Fig. 1), which is generally observed dur-ing thermal oxidation of PCB in the presence of air[32–34]. During a long-term study (2 weeks) it wasshown that the oxidation of PCB to PCDF occurs inthe presence of air already at temperature as low as300◦C without any catalyst [32]. Under short-termpyrolysis conditions at 550◦C, up to 25% of thedegraded PCB were converted to PCDF [33].

In the catalytic destruction study reported by Sub-banna et al. [11], the possibility of PCDD/PCDFformation was surprisingly pre-excluded, and hence,PCDF and PCDD were not measured. Hagenmaiershowed that at 400◦C during PCB destruction noPCDF are formed with certain oxidation catalysts.However, no information was available for the be-haviour of PCB on catalysts at lower temperatures.

The commercial PCB mixtures themselves gener-ally contain PCDF at the ppm level [26–28]. Prior tothe tests of the present study, these PCDF originallypresent were separated from the PCB to avoid inter-ference with PCDF possibly formed during the ex-periments. After this separation step, no PCDF weredetected in the PCB mixture used for the experiments.

For the experiments, we chose an initial concen-tration of 10 mg PCB to enable the detection ofPCDD/PCDF formation one order of magnitude lower

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than the PCDF generally present in the commercialPCB mixtures. Lower conversion rates are irrelevantfor any practical applications with respect to toxicityconsiderations of potentially formed PCDF in relationto those of removed PCB.

3.2.1. PCDF detected in the catalyst off-gasIn the low temperature runs at 150 and 200◦C,

we detected 800–2000 ng PCDF (calculated per gramPCB used) in the catalyst off-gas (impinger) (Table 3).At 250◦C, less than 2 ng were detected in the impingerand at 300◦C no PCDF could be detected. With respectto TEQ values, the amount of PCDF in the off-gaswere insignificant in all runs compared to the amountof PCB removed (Table 3). However, the TEQ valuesof the PCDF in the off gas are in the same range com-pared to the TEQ from non removed PCB, and there-fore, have an impact on the total removal efficiencywith respect to TEQ (Table 3).

3.2.2. PCDF detected on the catalystThe catalysts were also subjected to PCDD/PCDF

analysis after the experiments. At 300 and 250◦C, noPCDF were detected on the catalyst. However, the

Fig. 5. PCDF formed during catalytic destruction of PCB at 150◦C (120 min); 200◦C (30 min); 250◦C (30 min) (5000 h−1); and 100◦C(static, air, 24 h).

amount of PCDF formed at 200 and 150◦C on the cat-alyst was relatively high (Fig. 5; Table 3). The TEQ ofthe starting PCB mixture was 4.60�g/g. In the exper-iments at 150◦C after 30 min reaction time, the TEQvalue increased by 144% and after 2 h even by 410%.At 200◦C, the TEQ also increased after a reactiontime of 30 min (by 165%), however, had already de-creased after 2 h by 88.6% (0.52�g/g PCB used) com-pared to the initial TEQ of the starting PCB (4.6�g/g)(Table 3).

The resulting PCDF isomer patterns (distributionof congeners within the same degree of chlorina-tion) formed were similar among the different ex-periments. However, the homologue distribution ofPCDF formed showed a dependence on temperatureand time (Fig. 5). The homologue distribution ofPCDF formed at 200◦C (30 min) was dominated bypentaCDF. When prolonging the residence time to2 h, the homologue pattern shifted to higher chlo-rinated PCDF dominated by hexaCDF. Comparingthese findings to the experiment at 150◦C (2 h), ashift to lower chlorinated congeners was observed,with tetraCDF dominating (Fig. 5). We additionallyperformed a stationary experiment at 100◦C for 24 h.


In this experiment, the homologue pattern shifted toa dominance of triCDF and a considerable amount ofdiCDF were formed (Fig. 5).

This homologue shift of PCDF can be explained bytwo effects.

• With decreasing temperature the higher chlorinatedPCB are less oxidised (Fig. 3), reducing the proba-bility to form higher chlorinated PCDF. For exam-ple, at 150◦C (2 h) still less than 5% of heptaCB areoxidised.

• The second impact on the resulting PCDF homo-logue pattern is the destruction of PCDF takingplace in parallel to their formation. This can be de-rived, e.g. from the comparison of PCDF detectedat 200◦C after 30 min (383 ug PCDF/g PCB) andafter 2 h (31 ug PCDF/g PCB) (Fig. 5). Similar toPCB, the lower chlorinated PCDF are destroyedfaster [31], shifting the isomer pattern to the higherchlorinated homologues.

3.3. GC/MS screening for further byproducts

No PCDD were detected during any PCB destruc-tion experiments of the present study. This is under-standable when considering the respective molecu-lar structure: a conversion of PCB to PCDD wouldrequire a disruption of the biphenyl-bridge and asecond insertion of an oxygen (compare molecularstructures in Fig. 1). On the other hand, the completeabsence of PCDD indicates that no mono-aromaticintermediates (chlorinated benzenes or phenols) playa role in the observed PCDF formation but that thePCDF formation on V2O5/WO3-TiO2 is a pure in-tramolecular cyclisation in the temperature rangetested.

An exclusive formation of PCDF as key byproductsis also reported for high temperature pyrolysis of PCB[33] or accidental PCB fires [34].

Additionally we performed GC/MS-analysis inSCAN mode to detect other degradation products.In the experiments at 150 and 200◦C hydroxy-lated biphenyls were detected. The concentration ofOH-PCB were lower compared to the formed PCDF.However, due to the lack of OH-PCB standards, asound quantification of these compounds was notpossible. At 250 and 300◦C, no further byproductsduring PCB destruction were detected.

3.4. Investigation of catalytic oxidation of individualPCB isomers

Using a complex PCB mixture does not allow adetailed investigation of the oxygen insertion sinceit is not possible to correlate the formed complexPCDF mixture with individual PCB. Further, with-out the knowledge of the main elimination step (HH,HCl or ClCl elimination) during oxygen insertion it isalso not possible to correlate the homologue pattern ofPCDF formed with the destruction degree of PCB ho-mologues. Therefore, we conducted experiments us-ing individual PCB isomers to get an insight into themechanism of PCDF formation during PCB degrada-tion and to obtain a baseline knowledge for these cor-relations.

Theortho-positions (2,2′,6,6′ positions; see Fig. 1)are the key positions during the formation of PCDFfrom PCB. Therefore, we selected single hexaCB iso-mers with no-, mono-, and di-ortho-chlorine substitu-tion (to get an insight into the elimination mechanism— HH, HCl and ClCl — during the oxygen inser-tion). We chose 180◦C as testing temperature since atthis temperature we found a measurable destructionof hexaCB within 15 min. Therefore, PCDF were ex-pected to be formed as an initial oxidation product.The destruction degree of the tested hexaCB was be-tween 10 and 30%.

3.4.1. Catalytic oxidation of 3,4,5,3′,4′,5′-hexaCB(no ortho-chlorine substitution)

Up to 5% of the degraded 3,4,5,3′,4′,5′-hexaCBwere converted to 2,3,4,6,7,8-hexaCDF. Sincethe 3,4,5,3′,4′,5′-hexaCB is substituted with onlyhydrogen in ortho-position, no HCl eliminationwas observed during the transformation to PCDF(Fig. 6A).

3.4.2. Catalytic oxidation of 2,3,5,3′,4′,5′-hexaCB(mono-ortho-chlorine substitution)

For 2,3,5,3′,4′,5′-hexaCB there are options of H-and Cl-elimination during oxygen insertion for PCDFformation. Around 3% of the degraded hexaCB wereconverted to 1,2,4,6,7,8-hexaCDF by abstraction oftwo hydrogens. This was the major PCDF isomerformed. Additionally, 2,3,4,6,8-pentaCDF was formedby abstraction of HCl as a minor product. The ratioof HH- to HCl-abstraction was about 20:1. Therefore,


Fig. 6. Isomer specific formation of PCDF during degradation of selected hexaCB on V2O5/WO3-TiO2 catalyst (180◦C, 15 min).

abstraction of HH was highly prefered over HCl elim-ination (Fig. 6B).

3.4.3. Catalytic oxidation of 2,3,4,2′,4′,5′-hexaCB(di-ortho-chlorine substitution)

1,2,3,6,7,8-hexaCDF (about 1% of degradedhexaCB) was the main PCDF formed. Besides,1,3,4,6,7,8-hexaCDF was found as a minor product.

However, the expected 1,2,3,6,7,9-hexaCDF was notformed by “simple” HH abstraction during degrada-tion of 2,3,4,2′,4′,5′-hexaCB (Fig. 6C). The forma-tion of 1,2,3,6,7,8-hexaCDF can be explained by a2,3-shift of the chlorine in the 2′,4′,5′-fragment dur-ing the oxygen insertion (Fig. 6). This 2,3-shift of thechlorine is also observed during PCDF formation fromPCB in air at elevated temperatures (600◦C) [35].


Additionally, three pentaCDF were formed byHCl-elimination during oxygen insertion. They weredetected at concentrations one order of magnitudelower than for the main hexaCDF. 1,2,3,7,8- and1,3,4,6,7-pentaCDF were formed by simple HCl elim-ination and to a lesser extent the 2,3,4,6,7-pentaCDFby HCl elimination including a 2,3-shift of chlorine.

Only an insignificant amount of 2,3,6,7-tetraCDFwas formed. Therefore, the elimination of ClCl dur-ing oxygen insertion was a minor pathway on theV2O5/WO3-TiO2 catalyst at low temperature. On theother hand, the ClCl abstraction is a main pathway ofPCDF formation from PCB during high temperaturepyrolyis (550–700◦C) [33,35].

3.5. Mechanistic aspects of PCB degradation —insight into the initial steps of oxidative destruction

The final products of complete oxidation of chlo-rinated organic compounds are CO2, H2O and HCl.In the studies of Hagenmaier [12] and Jones andRoss [36] only the formation of these final productswas observed. Krishnamoorthy et al. additionally de-tected a considerable amount CO with a selectivity ofapproximately 40% during oxidation of dichloroben-zene on a V2O5/TiO2 based catalyst [37]. Further,Green et al. [38] detected a significant formation ofCO and Cl2 during destruction of tetrachloroethyleneand dichloromethane on Cr-ZSM-5 and Cr-Y. How-ever, these final products do not give any insight intothe mechanism involved during catalytic degradation,nor into the initial steps of deep oxidation.

Recently, Krishnamoorthy et al. reported the vi-bration bonds of carboxylates, maleates and pheno-lates as potential intermediates during oxidation ofdichlorobenzene on V2O5/Al2O3 [39] and in a cat-alyst screening study including V2O5/TiO2 [40] byFT–IR spectroscopic studies. The authors proposed anCl-abstraction as the initial step of degradation of theinvestigated chlorinated benzene.

On the other hand, the detailed investigation ofPCDF formation during degradation of PCB in ourstudy propose that as initial step of catalytic oxi-dation of chlorinated aromatics at low temperaturespreferably the positions of hydrogen-substitution areattacked: in the observed formation of PCDF fromPCB, the main product results from elimination oftwo hydrogen substituents. The elimination of HCl

is only a minor pathway and the abstraction of twochlorine was merely found (see above; Fig. 6).

One may ask if the oxygen transfer is the major ini-tial degradation step of deep oxidation. In our opinionan intermediate (PCDF) of up to 5% of degraded PCBas it is found in the early stage of degradation (Section3.3) is a strong indicator that it is indeed the case. Es-pecially since it has to be taken into account that thePCDF formation requires a hydrogen- (or chlorine-)abstraction on each ring and is, therefore, the productof two oxidation steps.

But even if we assume that other main initial degra-dation steps may exist that do not involve oxygentransfer (which seems unlikely to us), a preferred ini-tial degradation of the C–Cl bond can be excludedin our study. The dependence of PCB degradation ontheir degree of chlorination give here evidence forthe stability of the C–Cl bond of chlorinated aro-matics during catalytic destruction: with increasingdegree of chlorination, the degradation velocity de-creases (Fig. 3). This is in agreement with an initialattack on the hydrogen positions. If the initial degra-dation step would be governed by a weakness of theC–Cl bonds, the opposite trend should be observed.Therefore, in our opinion the disruption of C–Cl bondsplay only a minor role as an initial step in low temper-ature destruction of chlorinated aromatics on oxida-tion catalysts. On the other hand, the detection of HCland ClCl abstraction during PCDF formation suggeststhat the abstraction of Cl is possible to a certain extentduring an early stage of the degradation.

An understanding of the preferred attack on the po-sitions of hydrogen substituents can be derived frombasic chemistry considerations. The reaction is, chem-ically, an oxidation. The insertion of the metal oxidecatalyst into the C–Cl bond would, however, be a re-duction. This reduction reaction seems not possiblefrom considerations concerning the oxidation state.V2O5 (V5+) is already in its highest oxidation stateand is, therefore, not capable to participate in a reduc-tion.

Furthermore, with respect to the theory on oxy-gen transfer on heterogeneous oxidation catalysts, anattack on the positions of the hydrogen substituentsin the target PCB seems more probable. The oxy-gen species on heterogeneous catalysts are generallyclassified into two types: electrophilic (O− (oxide),O2− (superoxide), O22−(peroxide)) and nucleophilic


(O2− (oxide ion)) oxygen [41–43]. For catalysts thatfavour deep oxidation, the type of oxygen detected waselectrophilic [41]. For V2O5–TiO2, O− is proposedto be responsible for deep oxidation at temperaturesbelow 380◦C [44]. Our finding of an oxygen attack onmainly the carbons that are substituted with hydrogen(electron-rich positions) support this finding for thedestruction of chlorinated aromatics. The positions ofchlorine substitution are electron-poor, and therefore,do not support an electrophilic attack. This explainsthe priority of HH→ HCl → ClCl elimination duringthe PCDF formation from PCB on a molecular basis.

Since aromatics are unsaturated hydrocarbons,epoxidation has to be considered as one pathway ofoxygen transfer during electrophilic attack [43]. Theobserved 2,3-shift of chlorine during the formation ofPCDF from single PCB isomers (Fig. 6C) is in ouropinion an indication that epoxidised intermediatesplay a role during the degradation.

3.6. Partial chlorination of PCB

During the destruction of the individual hexaCBisomers we observed a small formation of heptaCB(<1%). Therefore, a minor chlorination took placeduring degradation of the hexaCB. This can also bederived from the recovery of significantly more than100% of octaCB and nonaCB in the experiments atprolonged retention times at 150◦C (Fig. 3). Whenrecalculating the additional formed octaCB from theamount of the heptaCB degraded,3 the amount oflower chlorinated isomers that were subject to fur-ther chlorination was estimated to be less than 2%.Therefore, on V2O5/WO3-TiO2 catalysts, the chlorinespecies formed during degradation of the chlorinatedaromatics can chlorinate other chlorinated aromaticspresent.

A chlorination of chlorinated aromatics during de-struction of monochlorobenzene was also reported fora Pt/Al2O3 catalyst in the study of Van den Brink etal. [45]. However, on the Pt/Al2O3 catalyst and underthe applied conditions this chlorination reaction wasmore pronounced.

3 The heptaCB were present in the initial PCB mix in considerablehigher concentration than octaCB; and octaCB were present inhigher concentration than nonaCB (Fig. 4).

3.7. Steric aspects of oxygen insertion during PCDFformation

1,9-Substituted PCDF were hardly formed duringthe oxidation of the PCB-mixture. This is surprisingsince a large fraction of the PCB in the commer-cial mixture are substituted in the 2,2′-position,4

which would result in 1,9-substituted PCDF duringa simple abstraction of two hydrogen (without chlo-rine shift or rearrangement). However, already in thedegradation of 2,3,4,2′,4′,5′-hexaCB (substituted in2,2-position) we did not observe the formation ofthe expected 1,2,3,6,7,9-hexaCDF (1, 9-substituted),instead, 1,2,3,6,7,8-hexaCDF was formed as themain resulting PCDF, including a 2,3-chlorineshift.

One explanation for the discrimination of 1,9-substituted PCDF could be made based on ther-modynamic considerations: the substitution of the1,9-position in PCDF causes steric crowding ofthe two chlorine atoms in 1- and 9-position [47](Fig. 7) and are, therefore, thermodynamically lessfavoured congeners. On the other hand, the PCDFpattern formed during pyrolysis of commercial PCBin the presence of air [27] or in accidental PCB fires[27,34] at high temperature (generally associatedwith thermodynamic control) contain 1,9-substitutedPCDF.

Therefore, the observation that 1,9-substitutedPCDF are hardly formed in low temperature oxidationof PCB on V2O5/WO3-TiO2 is, in our opinion, betterexplained by a kinetic control: for the PCDF forma-tion, a coplanar conformation of the PCB is a neces-sary requirement for insertion of the oxygen bridgeto form the coplanar PCDF. For di-ortho-substitutedPCB a coplanar anti-conformation is possible withoutsteric hindrance (Fig. 7), however, in this coplanaranti-conformation the ether-bridge cannot be formedby HH abstraction. The co-planarsyn-conformationwhich would allow the abstraction of HH is hinderedby steric crowding of the chlorine substituents (Fig. 7).Due to these steric requirements, the 2,2′-substitutedPCB form PCDF preferably after a 2,3-chlorine shiftor by HCl elimination (Fig. 6C) under the appliedconditions.

4 Commercial PCB-pattern, see e.g. in [46].


Fig. 7. Coplanarsyn-conformation (A) and anti-conformation(B) of 2,2′-chlorine substituted PCB and steric crowding of1,9-substituted PCDF (C).

3.8. Relevance of the observed PCDF formation fortechnical application

The two potential applications of catalytic PCB de-struction — off-gas cleaning in waste incinerators anddestruction of PCB waste — have to be evaluated sep-arately. The main difference between these two pro-cesses is the required removal efficiency. An emis-sion value of 0.1 ng TEQ/N m3 may be required asthe most stringent regulation for waste incinerators[48,49]. Catalysts in MWI are mostly operated around250◦C. During our tests even the lowest removal effi-ciency of 99% of TEQ (including the PCDF formed)

from the experiments at 200◦C would be sufficient tomeet this stringent regulation limit, even for inciner-ators containing 10 ng TEQ/N m3 in the raw gas. Fornewer incinerators with raw gas values of around 2 ngTEQ/N m3, a considerable lower removal efficiency isrequired. Therefore, catalysts may be operated, for ex-ample, at a higher space velocity. However, operationparameters have to be confirmed during actual plantoperation.

For destruction of hazardous PCB waste, new tech-nologies have to be evaluated by comparison withhigh temperature incineration as the state of the arttechnology. In a recent survey of the United NationsEnvironmental Programme (UNEP) [3] a destructionefficiency of 99.9–99.99999% was reported for 16high-temperature incinerators and a destruction effi-ciency of 99.9999% was recommended. From the lab-oratory test, temperatures below 250◦C seem not suf-ficient to reach this aim. However, the catalytic testat 300◦C with a destruction efficiency of more than99.9% and the absence of PCDF formation seems apromising starting point. Hagenmaier already pointedout that the removal efficiency for a catalytic systemcan be adjusted to the required destruction efficiencyby selection of the appropriate space velocity. Thishas to be evaluated by taking into account economicalconsiderations of the size of the catalytic reactor.

4. Conclusion

The onset of catalytic destruction of PCB (andtherefore, polyaromatic compounds) on highly ac-tive V2O5/WO3-TiO2 catalysts was found already attemperatures of around 100◦C.

Below 200◦C the PCB were decomposed onlyslowly. The majority of PCB remain adsorbed on thecatalyst and are destroyed at prolonged reaction time.Therefore, a destruction efficiency of more than 98%can also be achieved at temperatures around 200◦C.

At 300◦C, deep oxidation occurred without detec-tion of by-products. At 150 and 200◦C considerableconversion of PCB to PCDF was observed. The PCDFformed mainly remained adsorbed on the catalyst andare destroyed during extended reaction times. Lessthan 1% of the PCDF formed were detected in theoff-gas of the catalyst. Consequently, the observedconversion of PCB to PCDF has no relevance for cat-


alytic off-gas cleaning in, e.g. municipal waste incin-eration facilities. In these plants, the decrease of totalTEQ will correlate with the destruction efficiency ofPCDD/PCDF and PCB present in the raw gas. There-fore, the catalyst may be operated for off-gas cleaningeven below 200◦C. Field studies are necessary to con-firm the feasibility of low temperature applications.For a temperature as low as 150◦C the long-term effi-ciency has to be evaluated.

For the destruction of PCB residues by low-temperaturecatalytic destruction aiming for destruction of>99.99%, the conversion of PCB to PCDF has to beconsidered and the catalysts have to be operated wellabove 250◦C.

The PCDF formation during catalytic PCB destruc-tion gives an insight into the first degradation step ofchlorinated aromatic compounds on V2O5/WO3-TiO2at low temperatures. The major initial step is anoxygen transfer. In this oxygen transfer mainly thecarbons carrying hydrogen are attacked while theabstraction of chlorine is only a minor step duringlow-temperature destruction. This finding of preferredoxygen attack on the carbons substituted with hy-drogen (i.e. electron-rich positions) proposes that theresponsible mechanism involved is an electrophilicoxygen transfer.

Acknowledgements

The authors would like to express their thanks toTakaaki Ohno for his great care in the analysis of thelaboratory experiments and Caroline Gaus for criticalreading of the manuscript.

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Documents

Low temperature decomposition of PCB by TiO2-based V2O5/WO3 catalyst: evaluation of the relevance of PCDF formation and insights into the first step of oxidative destruction of chlorinated