Catalytic hydrodechlorination of dioxins over palladium nanoparticles in supercritical CO2 swollen microcellular polymers

Journal of Hazardous Materials 227–228 (2012) 18–24

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Catalytic hydrodechlorination of dioxins over palladium nanoparticles in supercritical CO2 swollen microcellular polymers Ben-Zen Wu a , Hsiang-Yu Chen b , Joanna S. Wang c , Chung-Sung Tan d , Chien M. Wai c , Weisheng Liao a,∗ , KongHwa Chiu a,∗ a

Department of Chemistry, National Dong Hwa University, Hua-Lien 970, Taiwan, ROC Department of Chemistry, Chung Yuan Christian University, Chung-Li, Tao-Yuan 320, Taiwan, ROC c Department of Chemistry, University of Idaho, Moscow, ID 83844m, USA d Department of Chemical Engineering, National Tsing Hua University, HsinChu 300, Taiwan, ROC b

h i g h l i g h t s     

Pd nanoparticles are embedded in microcellular high density polyethylene (Pd/m-HDPE). Pd/m-HDPE is used as heterogeneous catalysts in supercritical carbon dioxide (sc-CO2 ). Dioxins are remedied via hydrodechlorination and hydrogenation over Pd/m-HDPE in sc-CO2 . The final products are dechlorinated and benzene-ring-saturated dioxins. Pd/m-HDPE can be recyclable and reusable without complicated cleaning procedures.

a r t i c l e

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Article history: Received 6 January 2012 Received in revised form 26 April 2012 Accepted 27 April 2012 Available online 6 May 2012 Keywords: Heterogeneous catalysts Palladium nanoparticles Supercritical carbon dioxide Microcellular polymers Polychlorinated dibenzo-p-dioxins Polychlorinated dibenzofurans

a b s t r a c t In this study, palladium nanoparticles embedded in monolithic microcellular high density polyethylene supports are synthesized as heterogeneous catalysts for remediation of 1,6-dichlorodibenzo-p-dioxin and 2,8-dichlorodibenzofuran in 200 atm of supercritical carbon dioxide containing 10 atm of hydrogen gas and at 50–90 ◦ C. Stepwise removal of chlorine atoms takes place first, followed by saturation of two benzene rings with slower reaction rates. The pseudo first order rate constant of initial hydrodechlorination for 2,8-dichlorodibenzofuran is 4.3 times greater than that for 1,6-dichlorodibenzo-p-dioxin at 78 ◦ C. The catalysts are easily separated from products and can be recyclable and reusable without complicated recovery and cleaning procedures. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) are a class of persistent organic pollutants (POPs) unintentionally generated from biochemical processes, combustion, and anthropogenic activities as the major source (e.g., incineration or industrial manufacturing) [1–3]. Their chemical structures are shown in Scheme 1. The number of chlorine atoms attached to the dibenzo-p-dioxin and dibenzofuran backbones is between 1 and 8 resulting in 75 PCDD and 135 PCDF congeners, respectively. PCDD/Fs can cause carcinogenic, neurological, and other

∗ Corresponding authors. Tel.: +886 3 8235175; fax: +886 3 8633570. E-mail addresses: [email protected] (W. Liao), [email protected] (K. Chiu). 0304-3894/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhazmat.2012.04.075

adverse health effects in human beings, and the extent of their toxicity is graded relative to the most toxic congener, 2,3,7,8tetrachlorodibenzo-p-dioxin. These toxic compounds enter the environment mainly through flue gas or fly ash, contaminate soil/sediment, and bio-accumulate in organisms. The high chemical and thermal stabilities of PCDD/Fs make their destruction difficult through natural processes (e.g., photodegradation). Consequently, development of a suitable remediation method for the destruction of PCDD/Fs is urgent and important. Catalytic reductive dechlorination over noble metals has been exclusively studied in recent years for remediation of PCDD/Fs and seems to be a more promising method than oxidative, photolytic, biochemical, and other reductive methods because of its simplicity, high efficiency, mild operation conditions, and minimal possibility of generating new PCDD/Fs [1,2]. Current developed catalytic reductive dechlorination methods for remediation of PCDD/Fs over

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Cl

1

2 3 4

10

O O 5

9

6

Cl

Cl

8

3

7

2

4

5

O

6

Cl

7 8

1

9

Scheme 1. Chemical structures of PCDDs and PCDFs.

noble metals can be classified into two major catalytic systems. One catalytic system uses H2 as the hydrogen source [4–7] while the other catalytic system applies 2-propanol instead [8–14]. Both catalytic systems are carried out in aqueous/organic solvents over noble metals on powdered porous supports (e.g., Pt/C or Pd/Al2 O3 ). The reason for using powdered forms of catalysts is to lessen the internal mass transfer resistance in the support structures. The usual products of both catalytic systems are partially and totally dechlorinated dibenzo-p-dioxin and dibenzofuran. The chlorine atoms are removed one by one following first-order kinetics. The residual PCDD/Fs and products are recovered by extraction with solvents, and the catalysts are separated by filtration, washed, sonicated, and dried for reuse. In some cases, a suitable amount of NaOH is used to neutralize generated HCl which may cause catalyst deactivation and metal leaching. Although current catalytic reductive dechlorination methods over noble metals for remediation of PCDD/Fs are efficient, several drawbacks are observed including slow external/internal mass transfer, generation of secondary liquid waste, and complicated recovery/cleaning of the powdered catalysts. In the present work, supercritical carbon dioxide (sc-CO2 ) and microcellular high density polyethylene stabilized palladium nanoparticles (Pd/m-HDPE) are used as the solvent medium and heterogeneous catalysts for H2 -based catalytic reductive dechlorination of PCDD/Fs (hydrodechlorination) to address the above-mentioned disadvantages. Mild critical conditions, nontoxicity, high diffusivity, low viscosity, and total H2 miscibility are some advantages of sc-CO2 over conventional solvents. PCDD/Fs are sc-CO2 -soluble and can be extracted from fish oil, soil, and sediments with high extraction efficiency [15,16]. By substituting aqueous/organic solvents with sc-CO2 for catalytic hydrodechlorination of PCDD/Fs, remediation efficiency would be enhanced while generation of liquid waste would be minimized making the remediation method more environmentally friendly. Using Pd/m-HDPE as a new heterogeneous catalyst for hydrodechlorination of PCDD/Fs in sc-CO2 is based on the following considerations: (1) Pd is good for H2 dissociation and has high resistance to HCl attack; (2) high density polyethylene, containing only C, H, and single bonds, has no strong interaction with reactants and products, thus avoiding significant sorption problems which may retard reaction, product recovery, and catalyst cleaning; (3) polymers usually swell in sc-CO2 and this phenomenon alleviates the internal mass transfer resistance in the polymer matrix; and (4) the microcellular structure of HDPE can further facilitate the diffusion of molecules inside the polymer. Since the internal mass transfer resistance is reduced by sc-CO2 -swollen microcellular HDPE, larger sizes of catalyst supports can be used instead of powdered form, which makes the handling and recovery of catalysts much easier. The main objective of this paper is to determine the catalytic activity of Pd/m-HDPE for hydrodechlorination of PCDD/Fs in sc-CO2. Two congeners, 1,6-dichlorodibenzo-p-dioxin and 2,8dichlorodibenzofuran are used as examples for demonstration. The existence of external/internal mass transfer resistance, the time/procedure for product collection under different temperatures and concentrations, and the possible sorption of PCDD/Fs to Pd/m-HDPE are first tested and determined since these factors

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influence the calculated mass balance and the observed reaction rate. The process of catalytic hydrodechlorination of PCDDs in sc-CO2 over Pd/m-HDPE is in situ monitored by UV/Vis spectroscopy, and the trap solution is analyzed by GC/MS to obtain product distribution information under different temperatures and reaction times. A reaction mechanism is proposed based on experimental results. The same procedure is then applied to catalytic hydrodechlorination of PCDFs over Pd/m-HDPE in sc-CO2 . In the final section, the rate constants of hydrodechlorination and hydrogenation of PCDD/Fs are compared with each other. 2. Experimental 2.1. Chemicals and reagents Palladium hexafluoroacetylacetonate [Pd(hfa)2 ] (97%), high density polyethylene beads (HDPE, diameter = 4 mm; height = 2 mm), hexane, 2-propanol, and n-dodecane as internal standard were obtained from Aldrich (Milwaukee, WI, USA). 1,6Dichlorodibenzo-p-dioxin (1,6-DCDD), 1-monochlorodibenzo-pdioxin (1-MCDD), dibenzo-p-dioxin (DD), 2,8-dichlorodibenzofuran (2,8-DCDF), 2-monochlorodibenzofuran (2-MCDF), and dibenzofuran (DF) were purchased from AccuStandard (New Haven, CT, USA). CO2 and H2 were provided by local suppliers in Taiwan. 2.2. Monolithic catalyst preparation The experimental setups for Pd/m-HDPE synthesis and catalytic reactions of PCDD/Fs in sc-CO2 as shown in Fig. 1 are similar to literature reports [17–19]. For preparing Pd/m-HDPE, three steps are involved: supercritical foaming, supercritical impregnation, and hydrogen reduction. A beaker filled with 2.5 g of white HDPE beads (Tm = 130 ◦ C) was placed in a 20 mL high-pressure reactor maintained at 140 ◦ C and pressurized with 200 atm of CO2 for the supercritical foaming step. After 3 h, the reactor was depressurized in 10 s. A monolithic m-HDPE cylinder (height = 2.0 cm; diameter = 1.8 cm) was formed. Subsequently, 100 mg of Pd(hfa)2 along with the m-HDPE cylinder were placed in the high-pressure reactor maintained at 90 ◦ C and pressurized with 100 atm of CO2 for the supercritical impregnation step. After 3 h, the reactor was depressurized. The Pd2+ in the precursor was reduced to a zero-valence state with 10 atm of H2 under sc-CO2 and the resulting Pd/m-HDPE catalyst was cleaned with sc-CO2 to remove impurities. 2.3. Hydrodechlorination The PCDD/F stock solution was prepared by dissolving 5–25 mg of PCDD/Fs in 50 mL of 2-propanol. For each PCDD/F hydrodechlorination experiment, 100–300 ␮L of the stock solution were added into a small beaker. After the solvent evaporated, the beaker was placed on top of the Pd/m-HDPE catalyst in a reactor (Cell 2) maintained at a temperature range of 50–90 ◦ C. The reactor was then sealed and pressurized with 100 atm of CO2 for pre-dissolution of PCDD/Fs. CO2 (200 atm) containing 10 atm of H2 in a storage cell (Cell 1) was introduced into the reactor. The CO2 phase in the reactor was monitored by in situ UV/Vis spectrometer (Model 440, Spectral Instruments, Inc., Tucson, AZ, USA) [19]. After a variable reaction time, the outlet valve was opened and the system was flushed with 200 atm of CO2 at a flow rate of 1 mL/min for 30 min for product collection and catalyst cleaning. The effluent was trapped in 10 mL of hexane. After depressurization, the end tubing was cleaned using 2 mL of hexane injected via a 0.5 mL syringe. The collection and cleaning hexane solutions were analyzed by UV/Vis spectroscopy (GENESYS 10S, Thermo Scientific) and GC/MS (Varian CP 3800 with Saturn 2000) with a VF-5ms column

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Fig. 1. Experimental setup for synthesis of Pd/m-HDPE and catalytic hydrodechlorination in sc-CO2 .

(L = 60 m, Id = 0.32 mm, thickness = 0.25 ␮m). The GC/MS conditions were as follows: injector = 250 ◦ C, oven was held at 70 ◦ C for 2 min, increased from 70 to 230 ◦ C at a rate of 10 ◦ C/min, increased from 230 to 300 ◦ C at a rate of 20 ◦ C/min and held at 300 ◦ C for 3.5 min [20]. 3. Results and discussion 3.1. Catalytic hydrodechlorination of 1,6-dichlorodibenzo-pdioxin over Pd/m-HDPE in sc-CO2 The CO2 pressure in all experiments is set to be 200 atm. Higher pressure usually yields better solvation but safety is a concern. Lower pressure may result in two phases as the reaction proceeds. The amount of hydrogen gas is 1000 times greater than that of the reactants to ensure enough hydrogen gas through the whole reaction process. The volume of Pd/m-HDPE is 5.09 cm3 while the Pd nanoparticles and the pore of m-HDPE are 10.5 ± 3.5 nm and 20–60 ␮m determined by TEM and SEM, respectively, as shown in Fig. 2. The diffusion of molecules in the porous matrix is highly dependent on the pore size [21]. The diffusion coefficient of gas molecules in micro-sized pores is over 100 times higher than in nano-sized pores, such as pores of activated carbon or alumina supports. Therefore, the creation of micro-sized pores of HDPE would enhance molecule diffusion in its structure. The existence of external and internal mass transfer resistance in Pd/m-HDPE for catalytic hydrodechlorination of PCDD/Fs in sc-CO2 was checked by varying the stirring speed (0–1200 rpm) and catalyst size (5.09–0.08 cm3 ). Increasing stirring speed can increase the fluid velocity in the stagnant boundary layer surrounding a catalyst while reducing the catalyst size can decrease mean path or diffusion time of reactants from catalyst surface to metal particle surface inside the catalyst. If the observed reaction rate increases when increasing stirring speed or reducing catalyst size, significant external and internal mass transfer resistances exist. In our case, the conversion does not increase with increased stirring speed or reduced catalyst size, indicating that both mass transfer resistances are insignificant. Before performing catalytic hydrodechlorination over Pd/m-HDPE in scCO2 , the collection procedure and time after reaction were checked and determined because a low mass balance would result in misinterpretation of experimental data [4,9]. 1,6-DCDD stock solution (0.2 mL) was placed in the reactor and the system was flushed with 200 atm of CO2 at a flow rate of 1 mL/min and at 50 ◦ C in the absence

of Pd/m-HDPE and H2 . The CO2 flow rate was set to be 1 mL/min because higher flow rates consume more CO2 while lower flow rates result in longer collection times. Effluent was trapped in a 10 mL hexane trap solution which was analyzed by UV/Vis spectroscopy every 5 min. The results are shown in curve (1), Fig. 3. The absorbance increased rapidly in the first 20 min and started to level off after 30 min indicating that most of the 1,6-DCDD was flushed out of the reactor. By combining the trap solution and the endtubing cleaning solution, the mass balance at 30 min was 95.1%. In subsequent trials, the amount of 1,6-DCDD was changed to 0.1 mL (curve 2) and 0.04 mL (curve 3) while the temperature of curve 4 was set at 70 ◦ C instead of 50 ◦ C. The absorbance of the three curves was lower than that of curve 1, but the trends of increasing absorbance are the same as for curve 1. For curve 5, Pd/m-HDPE was added to the reactor along with the 1,6-DCDD to see if the catalyst retained significant 1,6-DCDD and changed the trend of increasing absorbance. Static time (30 min) before the collection process was applied to allow the catalyst and 1,6-DCDD to contact with each other. The trend of increasing absorbance, again, did not change compared the other curves, indicating that significant sorption and interaction between Pd/m-HDPE and 1,6-DCDD does not occur in sc-CO2 , which may cause low mass balance problems and catalyst deactivation. The mass balances of curves 1–5 are all over 95% in 30 min collection time suggesting that the product collection procedure and a 30 min collection time are suitable and sufficient to obtain a satisfying mass balance value under the tested experimental conditions. Further increases in collection time gained little improvement in mass balance indicated by the level-off of each curve. Other reactants and products used in this work basically showed similar trends. Therefore, after each reaction, 200 atm of CO2 at a flow rate of 1 mL/min was used to flush the system for 30 min to collect reaction products and, at the same time, to clean the catalyst. The possible hydrodechlorinated products of 1,6-DCDD are 1MCDD and DD. Fig. 4a shows the UV/Vis spectra of these three compounds at different concentrations in hexane (7.90 × 10−6 , 1.53 × 10−5 , and 7.33 × 10−5 M). 1,6-DCDD has a peak maxima at 233 nm while 1-MCDD and DD have peak maxima at 226 nm. The absorbance coefficient ratio for these three peak maxima is 81:23:1. Fig. 4b shows the in situ UV/Vis spectra in sc-CO2 for catalytic hydrodechlorination of 1,6-DCDD over Pd/m-HDPE at 78 ◦ C. Each curve represents a UV/Vis spectrum at a different reaction time. The top absorption curve recorded at 20 s reaction time had a

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Fig. 2. (a) SEM images of the pores and (b) TEM images of Pd nanoparticles within Pd/m-HDPE. Scale bars are 30 ␮m and 200 nm, respectively.

Fig. 3. UV/Vis absorbance of 1,6-DCDD in the hexane trap solution vs time plot monitored at 233 nm during the collection process. Conditions: 200 atm of CO2 at a flow rate of 1 mL/min with variable reactant amounts and at different temperatures.

pattern similar to that recorded in hexane (Fig. 4a.) but its peak maxima was a little blue-shifted. As the reaction proceeded, 1, 6-DCDD absorption decreased rapidly and the peak maxima gradually shifted from 230 nm to 223 nm. Compared with Fig. 4a, it is clear that 1,6-DCDD is mainly converted to 1-MCDD and then to DD. After a while, the peak maxima around 223 nm disappeared and a new peak maximum appeared around 200 nm. This probably indicates

the formation of hexahydro-DD with one benzene ring saturated. After a few hours, no UV/Vis absorption was observed, indicating the formation of dodecahydro-DD with two saturated benzene rings. GC/MS analysis further confirms the existence of all products with standard solutions or with library data if not commercially available (e.g., Hexahydro-DD at 16.89 min and dodecahydro-DD at 16.49 min in specific GC/MS conditions). The details of product distribution are discussed in the following paragraph. According to the in situ UV/Vis spectra, Pd/m-HDPE can convert 1,6-DCDD to DD via hydrodechlorination and then to dodecahydro-DD via hydrogenation in sc-CO2 under mild conditions. Tundo et al. studied catalytic hydrodechlorination of PCDD/Fs collected from the fly ash of a municipal solid waste incinerator over Pt/C and Pd/C in isooctanewater with a phase transfer agent (Aliquat 336) as a promoter at 50 ◦ C and 1 atm of H2 with stirring speed of 1000 rpm [4]. Pd/C performed much better than Pt/C in this catalytic system. Zhang et al. reported catalytic hydrodechlorination of PCDD/Fs over Pd/C under a flow of H2 in an ethanol-water solution (v/v = 1:1) containing NaOH at 30–80 ◦ C with a magnetic stirrer [5,6]. The final product of both cases was chlorine-free DD. No benzene ring saturated product was observed. Obviously, Pd/m-HDPE in sc-CO2 shows better catalytic activity than the above-mentioned two catalytic systems in which two benzene rings of PCDD were saturated after the removal of chlorine atoms. Ghattas et al. studied catalytic hydrodechlorination of PCDD/Fs over Pd/[Rh(cod)Cl]2 in a silica sol–gel matrix in 1,2-C2 H4 Cl2 at 100 ◦ C, 27 atm of H2 , 24 h, and stirring speed of

Fig. 4. (a) UV/Vis spectra of 1,6-DCDD (7.90 × 10−6 M), 1-MCDD (1.53 × 10−5 M), and DD (7.33 × 10−5 M) in hexane. (b) In situ UV/Vis spectra in sc-CO2 during catalytic hydrodechlorination of 1,6-DCDD. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

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Fig. 5. Catalytic hydrodechlorination of 1,6-DCDD over Pd/m-HDPE in sc-CO2 (a) product distribution vs temperature based on GC/MS analysis. Conditions: 200 atm of CO2 , 10 atm of H2 , 30 min. (b) Product distribution vs time based on GC/MS analysis. Conditions: 200 atm of CO2 , 10 atm of H2 , 78 ◦ C.

150 rpm [7]. The final product was dodecahydro-DD. After each experiment, the catalyst was separated from products and solvents by filtration, washed and sonicated with dichloromethane, and dried for the next trial. In our case, Pd/m-HDPE was cleaned during product collection under the same experimental conditions as described in the previous paragraph and the separation was accomplished by CO2 depressurization after the collection step. No complicated recovery/cleaning procedure was required and the generation of secondary liquid waste was greatly reduced. The effect of temperature on product distribution within the range between 50 and 86 ◦ C and with a fixed 30 min reaction time is shown in Fig. 5a. At 50 ◦ C, only 18.5% of the 1,6-DCDD was converted, mostly to 1-MCDD with small amounts of DD and hexahydro-DD. On increasing the temperature from 50 ◦ C to 86 ◦ C, 1,6-DCDD decreased rapidly from 81.5 to 5%. At 64 ◦ C, a small amount (6%) of dodecahydro-DD first appeared. At 86 ◦ C, the product distribution was 5% 1,6-DCDD, 6.8% 1-MCDD, 44.3% DD, 23.6% hexahydro-DD, and 20.3% dodecahydro-DD. According to these results, a reaction temperature over 64 ◦ C is indicated to avoid lengthy reaction times for obtaining the final product, dodecahydro-DD. Fig. 5b shows the variation of product distribution with time at 78 ◦ C. 1,6-DCDD decreased rapidly with conversion over 70% in 10 min and over 90% in 40 min. 1-MCDD also increased rapidly in the beginning, reached a plateau around 10 min, and started to decrease at a rate similar to 1,6-DCDD. DD also increased rapidly in the beginning and reached a maxima point (47%) in 40 min. After that, it decreased at a slower rate than 1-MCDD. Hexahydro-DD and dodecahydro-DD increased smoothly with similar rates. Hydrodechlorination of 1,6-DCDD to DD almost complete in 80 min while subsequent hydrogenation of DD to dodecahydro-DD took a much longer time. After a two-hour reaction time, the product distribution was 0% 1,6-DD, 0% 1-DD, 11.7% DD, 48.9% hexahydro-DD, and 39.4% dodecahydro-DD. Based on the in situ UV/Vis spectra and GC/MS analysis, a proposed reaction mechanism is presented in Scheme 2. 1,6-DCDD is converted to 1-MCDD and then DD via hydrodechlorination with similar rate constants (k1 and k2 ). DD is then converted to hexahydro-DD and then to dodecahydro-DD via hydrogenation. Both rate constants (k3 and k4 ) are also similar and are lower than k1 and k2 . The higher value of k2 relative to k3 results in accumulation of DD during reactions as shown in Fig. 5b. 3.2. Catalytic hydrodechlorination of 2,8-dichlorodibenzofuran over Pd/m-HDPE in sc-CO2 The same procedure for the catalytic hydrodechlorination of 1,6DCDD in sc-CO2 over m-Pd/HDPE was applied to that for 2,8-DCDF.

Fig. 6a shows the UV/Vis spectra of the hexane trap solution at different reaction times between 200 and 320 nm. The top curve is the absorption from the 2,8-DCDF stock solution. There are two peak maxima, one at 222 and the other at 292 nm. As the reaction proceeded, both peak maxima decreased and were blue-shifted, indicating the formation of dechlorinated products, 2-MCDF and DF. A peak at 200 nm increased and then decreased until there was no absorption in the wavelength range, indicating the formation of hexahydro-DF and dodecahydro-DF. This observation is similar to the 1,6-DCDD experiment. Fig. 6b shows the product distribution vs time (min) plot. Complete conversion of 2,8-DCDF to DF through catalytic hydrodechlorination was achieved in about one-half hour and was faster than that of 1,6-DCDD to DD which required nearly 80 min. 2-MCDF increased and then decreased at a rate similar to 2,8-DCDF. DF also increased and then decreased but at a slower rate than 2-MCDF. Surprisingly, the amount of hexahydro-DF from hydrogenation of DD was quite low during the whole process. Two factors may account for this observation. One factor is that most hexahydro-DF from the hydrogenation of DF was directly converted to dodecahydro-DF on the metal surface without desorption. This phenomenon has been observed in catalytic hydrodechlorination of polychlorinated biphenyls over Pd/Al2 O3 in aqueous solution [22] and Pd/HDPE in sc-CO2 [19]. The other factor is that the rate constant of hexahydro-DF to dodecahydro-DF was much faster than that of DF to hexahydro-DF. Further investigation of this issue is necessary for it may involve other factors. Scheme 3 shows the proposed mechanism in which 2,8-DCDF is converted to DF via 2-MCDF by hydrodechlorination first with similar rate constants (k5 and k6 ). DF is then converted to dodecahydro-DF with hexahydro-DF as an intermediate by hydrogenation. The final product dodecahydro-DF reaches 100% in about an hour. 3.3. Rate constant comparison of catalytic hydrodechlorination and hydrogenation In this section the hydrogenation of DD and DF over Pd/m-HDPE in sc-CO2 was further investigated in separate experiments for rate constant information under the same experimental conditions. The experimental results show that it requires a 2 h reaction time for 100% conversion of DD to a product distribution of 0% DD, 31.8% hexahydro-DD, and 68.2% dodecahydro-DD. For DF, 100% conversion is nearly achieved in 30 min with a product distribution of 3.5% DF, 9.0% hexahydro-DF, and 87.5% dodecahydro-DF. The amount of hexahydro-DF is also low during reactions, which is similar to the hydrodechlorination of 2,8-DCDF as described in Section 3.2. The ln (A/Ao ) vs time plots for hydrodechlorination of 1,6-DCDD and 2,8-DCDF and hydrogenation reactions of DD and DF are shown in

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Scheme 2. Proposed reaction mechanism for catalytic hydrodechlorination of 1,6-DCDD over Pd/m-HDPE in sc-CO2 .

Fig. 6. Catalytic hydrodechlorination of 2,8-DCDF over Pd/m-HDPE in sc-CO2 (a) UV/Vis spectra in hexane trap solution measured at different times. (b) Product distribution vs time (min) plot based on GC/MS analysis. Conditions: 200 atm of CO2 , 10 atm of H2 , 78 ◦ C. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

Scheme 3. Proposed reaction mechanism for catalytic hydrodechlorination of 2,8-DCDF over Pd/m-HDPE in sc-CO2 .

Fig. 7 based on GC/MS analysis and obtained from separate experiments. The four curves show high R2 values (>0.97) indicating that the order of the four reactions is pseudo 1st order in the presence of excess amounts of H2 . First order kinetics in the study of catalytic hydrodechlorination of PCDD/Fs over Pd/C or Pt//C in isooctanewater is also observed [4]. The pseudo 1st order rate constants for the four reactions are 0.0293 min−1 for DD, 0.0385 min−1 for 1,

6-DCDD, 0.1016 min−1 for DF, and 0.1655 min−1 for 2,8-DCDF. The rate constants of hydrodechlorination of 1,6-DCDD and 2,8-DCDF are about 1.3 and 1.6 times faster than those of hydrogenation of DD and DF, respectively. Hydrodechlorination of 2,8-DCDF is about 4.3 times faster than that of 1,6-DCDD, while hydrogenation of DF is 3.5 times faster than that of DD. The hexagonal central structure of 1,6-DCDD gives more resonance and stability than the

Fig. 7. Pseudo 1st order rate constant comparison of hydrodechlorination of 1,6-DCDD and 2,8-DCDF and of hydrogenation of DD and DF in sc-CO2 over Pd/m-HDPE based on GC/MS analysis and obtained from separate experiments. Conditions: 200 atm of CO2 , 10 atm of H2 , 78 ◦ C.

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pentagonal central structure of 2,8-DCDF [6,12] and, additionally, chlorine atoms at the 2-position have less steric effect than those at the 1-position [5]. These two factors may account for the higher rate constants of 2,8-DCDF compared to 1,6-DCDD. 4. Conclusions This work shows that 1,6-DCDD and 2,8-DCDF can be transformed to dodecahydro-DD and dodecahydro-DF over Pd/m-HDPE in sc-CO2 via hydrodechlorination and hydrogenation under mild conditions. The dechlorinated and benzene-ring-saturated reaction products of PCDD/Fs are evidenced by in situ UV/Vis spectroscopy and GC/MS analysis. Chlorine atoms are removed and benzene rings are saturated step by step, and ring-opening or C O bond breaking is not observed. Strong sorption and interaction does not occur between Pd/m-HDPE and PCDD/Fs which may cause low mass balance and mass transfer problems. Both hydrodechlorination and hydrogenation follow pseudo 1st order kinetics. The rate constants of hydrodechlorination of 1,6-DCDD and 2,8-DCDF are higher than those of hydrogenation of DD and DF, respectively. 2,8DCDF has a much higher rate constant than 1,6-DCDD due to the position of the chlorine atoms and the symmetry of the chemical structure. Complete hydrodechlorination of 2,8-DCDF to DF occurs in 30 min, while it takes 80 min for complete conversion of 1,6DCDD to DD at 78 ◦ C. The monolithic physical form of Pd/m-HDPE makes it easier to handle than conventional catalysts with powdered forms. The presented work has the potential to serve as a green and efficient remediation method for destruction of PCDD/Fs and can be directly coupled with supercritical fluid extraction for treating PCDD/F-contaminated matrices, such as device, soil, or water. Acknowledgment The work was supported by the National Science Council, Taiwan, ROC (NSC 99-2116-M259-005). References [1] R. Weber, Relevance of PCDD/PCDF formation for the evaluation of POPs destruction technologies – review on current status and assessment gaps, Chemosphere 67 (2007) S109–S117. [2] P.S. Kulkarni, J.G. Crespo, C.A.M. Afonso, Dioxins sources and current remediation technologies—a review, Environ. Int. 34 (2008) 139–153.

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