TiO

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Ind. Eng. Chem. Res. 2010, 49, 490–497

Catalytic Hydrodechlorination of Tetrachloroethylene over Pd/TiO2 Minimonoliths Carlos A. Gonza´lez and Consuelo Montes de Correa* EnVironmental Catalysis Research Group, Sede InVestigacio´n UniVersitaria, UniVersidad de Antioquia, Calle 62 52-59 Torre 2-332/333 Medellı´n, Colombia

The gas phase catalytic hydrodechlorination (CHD) of tetrachloroethylene (TTCE) over 0.8% Pd/TiO2 (Hombikat uv-100) washcoated cordierite minimonoliths has been studied in the temperature range 120-180 °C. Experiments were carried out operating under differential regimes at a gas hourly space velocity (GHSV) of 0.45 (g min)/mL and using different concentrations of TTCE (300-1000 ppmv), hydrogen (0-10000 ppmv), ethane (0-850 ppmv), and hydrogen chloride (0-550 ppmv). The turnover frequency, specific rate constant, reaction order, and activation energy were determined. The pseudo-first-order Langmuir-Hinshelwood models adequately represent experimental results. The best adjustment corresponds to models featuring TTCE adsorption (associative or dissociative) as the limiting step. The CHD reaction is favored as TTCE and hydrogen concentrations are increased, while HCl negatively affects kinetic parameters. In order to study the causes of deactivation, fresh and used catalyst samples were characterized by nitrogen adsorption (BET), H2 chemisorption, thermogravimetric analysis (TGA), Fourier transform infrared (FTIR), and temperatureprogrammed oxidation coupled to mass spectrometry. Characterization results indicate that carbonaceous deposits were insignificant under reaction conditions, while the presence of HCl was the main cause of catalyst deactivation. 1. Introduction Chlorinated hydrocarbons are widely used not only as solvents, dry cleaners or degreasing media, but also as chemical intermediates in the production of plastics, synthetic resins, and pharmaceuticals. On the other hand, there are great concerns because of their high toxicity and carcinogenicity.1-3 Chloroemissions into the environment are now stringently regulated due to the associated adverse health effects and ecological damage.4 Specifically, tetrachloroethylene (TTCE) is the most commonly used organochlorinated compound so it is released into the environment in large amounts. For this reason, TTCE is usually chosen as a model compound for studying available technologies for the treatment of emissions of these compounds.5,6 Catalytic hydrodechlorination (CHD) represents an alternative and innovative approach whereby the hazardous substance is transformed into recyclable products in a closed system with limited toxic emissions under mild conditions.7 Heterogeneous CHD has been reported in liquid8,9 and gas phase reactions, involving aliphatic, olefinic, and aromatic reactants.10-13 Pd, Pt, Rh, and Ni on several supports are suitable catalysts for this reaction. Notwithstanding, Pd appears to be the most active metal at lower operating temperatures.14,15 In most works, 0.5 wt % Pd over Al2O3, SiO2, carbon, and TiO2 supports has been used.5,6,9,14 More recently 0.8 wt % Pd/TiO2 was found to be more active.12,13 There have been several attempts at kinetic modeling4,10,14 and pseudo-first-order kinetics for the CHD has been mainly postulated.18,16,17 In the development of hydrodechlorination models reported so far, the authors have worked with catalysts having high affinity to hydrogen and organochlorinated compounds. The mechanisms have been described by Langmuir-Hinshelwood,10,11,13,14 where two possibilities are considered: chemisorption of hydrogen and organochlorinated compounds on the same active sites or on different active sites. Dissociative or molecular adsorption depends on the type of * To whom correspondence should be addressed. Tel.: +5742106605. Fax: +5742106609. E-mail: [email protected].

organochlorinated compound. In the case of olefinic compounds as TTCE, the rate limiting step has been mainly linked with C-Cl scission.16 In general, kinetic studies have been performed with different catalysts, organochlorinated compounds, and reaction conditions. Keane et al.10 reported the CHD of chlorobenzene over Ni/SiO2 catalysts. The best model represented the noncompetitive dissociative adsorption of chlorobenzene and hydrogen. Similar kinetic expressions have been reported for different organochlorinated compounds.11,14,17 Ordo´n˜ez et al. evaluated the CHD of organochlorinated olefins such as TTCE14,18 on 0.5% Pd/γ-Al2O3. The activation energies and specific reaction rates were between 33-71 kJ/mol and 24-221 mmol/(min g cat MPa), respectively. In another study, Ordo´n˜ez et al.,19 used “red mud” for the CHD of TTCE obtaining specific reaction rates between 0.124 and 0.198 mmol/ (g cat min). Kim et al.20 achieved specific reaction rate constants of 69 mL/(g cat min) for the CHD of TTCE over NiMo/Al2O3 catalysts. Coq et al.21 found activation energies larger than 84 kJ/mol for the CHD of chlorobenzene over Pd catalysts. Reaction orders different than one were estimated by Go´mez et al.,9 who obtained a zero reaction order during the CHD of carbon tetrachloride (CCl4) over Pd/C at 140 °C, suggesting a very strong adsorption of CCl4 on the catalyst. The specific reaction rates were in the range of 0.43-2.60 mmol/(g cat min). Prakash et al.,22 during the CHD of 4-chlorophenol over Ru-Pd/ TiO2, obtained reaction orders from 0.22 to 0.40 with respect to 4-chlorophenol and from 0.81 to 0.95 for hydrogen. Hashimoto et al.23 found reaction rates between 1.08 × 10-5 and 9.50 × 10-4 mol/(h g) for the CHD of monochlorobenzene over Pd and Pt catalysts supported on alumina, titania, silica, and silica-alumina at room temperature. Ribeiro et al.24 obtained activation energies between 92 and 109 kJ/mol and turnover frequencies (TOFs) of 0.3 1/s for the CHD of CF3-CFCl2 (CFC) over Pd alloys at temperatures around 80-200 °C and 770 torr. Heinrichs et al.,17 working with Ag-Pd/SiO2 on the CHD of 1,2-dichloroethane, found that HCl is an inhibitor of the reaction.

10.1021/ie901027y  2010 American Chemical Society Published on Web 11/23/2009

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The goal of the present work is to obtain a kinetic expression for the CHD of TTCE over Pd/TiO2 washcoated minimonolith samples. This catalyst has been previously tested on CHD of organochlorinated mixtures,12-15,25-30 and its activity demonstrated. To the best of our knowledge there are few studies of CHD focused on the influence of structured materials and their interactions with noble metals, especially under operating conditions of interest for industrial application. An important aspect to be considered in the application of an industrial CHD process is catalyst inhibition, especially since hydrodechlorination takes place in a reductive environment (risk of coke formation) and hydrogen chloride is produced (risk of catalyst poisoning).25,26 Ordo´n˜ez et al.6 carried out catalyst deactivation studies during the gas-phase CHD of TTCE over Pd/Al2O3 catalyst. The formation of carbonaceous deposits was the main cause of deactivation. In the same way, other reports indicate that these phenomena are important for several noble metals (i.e., Pd) suggesting that sintering of metal particles and coke deposition are the main causes of the observed deactivation.27-29 2. Experimental Section 2.1. Catalyst Activity Tests. Titania Hombikat uv-100 (previously calcined at 400 °C) was wet impregnated with palladium(II) acetylacetonate diluted in acetone. Pd/TiO2 samples were mixed in a ratio of 1/2.3 with water. Meanwhile, a 5 wt % alumina solution was added as a binder maintaining a pH of 4.12 The slurries were milled at 90 rpm for 36 h in a planetary ball-mill at room temperature (24 cycles of 60 min each and 30 min cooling between each cycle). Then, the slurry was stirred at 800 rpm using a mixer (Ultraturrax T 25). The slurry contained 25 wt % solids. Cordierite minimonoliths 10 × 10 × 12 mm, 36 channels, previously treated with HNO3 and calcined at 600 °C for 2 h were washcoated with the milled slurry as previously reported.12 Washcoated minimonoliths were calcined in air at 2 °C/min from room temperature to 400 °C for 2 h, which is crucial for ensuring good adherence of catalytically active components to the monolith. Prior to catalytic reactions, the last operation was repeated and catalyst samples were reduced at 300 °C in flowing 5% H2/N2 for 2 h. The CHD reaction was carried out at atmospheric pressure in a Pyrex glass tubular reactor heated with an electrical oven equipped with a temperature controller and a K-type thermocouple. Minimonolith samples were wrapped and fixed to the reactor with fiberglass layers. The reaction mixture was obtained by blending the gas flows of the desired compounds using electronic mass flow controllers (Brooks 5850 TR series), i.e. TTCE (300-1000 ppmv), hydrogen (0-10000 ppmv), hydrogen chloride (0-550 ppmv), ethane (0-850 ppmv), and N2 balance. The concentration of generated HCl was also monitored by titrimetric analysis of a NaOH trap solution. The total concentration of chlorides in solution was quantified by titration with AgNO3 using K2CrO4 as the indicator. The Pd loading of Pd/ TiO2 was 0.8 wt % as determined by absorption spectroscopy. The total gas flow rate was maintained at 145 mL/min. The temperature range was 120-80 °C. Reaction times (time on stream) between 12-20 h were employed. An initial operational period of 4 h was allowed for the system to reach steady state before taking representative samples. A Fourier transform infrared (FTIR) gas analyzer (Temet) equipped with a 2-L cell and a 240-cm optical step and operated at 120 °C was used to monitor reactants and products. All gas lines were heated at 120 °C to prevent water condensation during catalytic runs. The catalytic reactor and operating conditions to obtain negligible

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Figure 1. Effect of Pd loading on CHD of TTCE, C2Cl4 550 ppmv, H2 5500 ppmv; GHSV 0.45 (g min)/mL.

mass-transport limitations on minimonolith samples were ensured by means of the Madon-Boudart test.31 Catalytic runs were repeated up to three times with different samples from the same batch, and the results were within 5%; the values quoted in this paper are mean values. Several rate expressions10,11,14,20,21,24 previously proposed for catalytic hydrodechlorination were tested as a first approach to evaluate kinetic parameters. 2.2. Catalyst Characterization. Fresh and used catalyst samples (powder or washcoated minimonoliths) were characterized by several techniques. N2 adsorption-desorption at 77 K was performed in a Micromeritics ASAP 2010. Thermogravimetric studies were carried out in a 2950 TGA HR V6.1° thermobalance in an oxidant atmosphere using synthetic air to evaluate the presence of carbonaceous deposits on used catalyst samples. Temperature-programmed oxidation (TPO) in flowing 5% O2/Ar was carried out in a Micromeritics AutoChem II 2920 instrument equipped with a thermal conductivity detector (TCD) coupled to a quadrupole mass spectrometer (Pfeiffer Vacuum Omnistar) following the evolution of HCl, CO2, O2, and CO in the outlet gas with increasing temperature. The temperature and detector signals were continuously recorded while heating at 2 °C/min from 25 to 900 °C. Palladium dispersion was determined at 100 °C by hydrogen chemisorption in the same apparatus. Catalyst samples were initially reduced in flowing 5% H2/Ar heating from room temperature to 300 at 2 °C/min and kept at this temperature for 1 h. Afterward, samples were cooled down at 100 °C in flowing Ar, and finally, hydrogen pulses were introduced until saturation. FTIR measurements were obtained in transmission mode using pressed disks (∼20 mg) in a Cary/ 5E Varian apparatus. Before analyses, samples were heated in flowing dry 4% O2/Ar at 200 °C for 1 h to remove undesired impurities from air. 3. Results and Discussion 3.1. EvaluationofDiffusionalLimitations.TheKoros-Nowak method modified by Madon-Boudart is a useful criterion to measure diffusional limitations in a catalytic heterogeneous system.11,31 Two different minimonolith samples with Pd loadings of 0.8 and 1.6 wt % were tested for the CHD of TTCE using the same spatial velocity. As can be observed in Figure 1, the TOF values are similar at the temperatures tested, which indicate that until 180 °C, the reactor operated in the kinetic control regime. Experiments carried out under the same reaction conditions over powder Pd/TiO2 samples with different particle diameters (0.180-0.090 µm) and over Pd/TiO2 washcoated minimonoliths showed similar reaction rates at temperatures between 120 and 180 °C (Figure 2). Results from Madon-Boudart and particle size tests suggest the absence of internal mass transfer limitations in powder and monolith catalyst samples.14 The results also

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Figure 2. CHD of TTCE: intraparticle limitations on powder (particle diameter 0.180-0.090 µm) 0.8% Pd/TiO2 Hombikat and minimonolith samples; GHSV 0.45 (g min)/mL.

Figure 3. Effect of C2Cl4 during CHD over 0.8% Pd/TiO2 minimonolith samples. H2 ) 5500 ppmv, C6H5CH3 ) 5500 ppmv; GHSV 0.45 (g min)/ mL.

Figure 4. Effect of H2 (0-10000 ppmv) during the CHD of TTCE (550 ppmv) over 0.8% Pd/TiO2 minimonolith samples. H2 ) 5500 ppmv, C6H5CH3 ) 5500 ppmv; GHSV 0.45 (g min)/mL.

show that the CHD of TTCE is proportional to the reaction rate for all particle sizes at different temperatures used. In a series of blank tests, the reaction mixture flowing through the empty reactor (only with fiberglass) did not result in any detectable conversion.12 Toluene used as solvent did not show appreciable conversion (less than 5%). In general, Pd catalysts are active for TTCE total hydrodechlorination and almost inactive for the hydrogenation of toluene.21,18,32 3.2. Effect of Tetrachloroethylene Concentration. TTCE concentrations varied between 300 and 950 ppmv keeping either concentration of H2 and C6H5CH3 at 5500 ppmv. As can be observed in Figure 3, the CHD rate increased as the TTCE concentration increased at temperatures between 135-180 °C. 3.3. Effect of Hydrogen. Figure 4 shows the effect of the H2 on the catalytic HDC of TTCE. In the absence of hydrogen, the reaction rate is too low even at the highest tested temperature (180 °C). Figure 4 also shows that the reaction rate of TTCE hydrodechlorination increases as hydrogen concentration increases. Other authors have found that the saturation is attained in the presence of large amounts of hydrogen and reaction rates

Figure 5. Effect of HCl during the CHD of TTCE (550 ppmv) over 0.8% Pd/TiO2 minimonolith samples in the presence and absence of HCl. H2 ) 5500 ppmv, C6H5CH3 ) 5500 ppmv; time on stream 44 h at 180 °C; GHSV 0.45 (g min)/mL.

remain constant.19,33,34 The increase of TTCE hydrodechlorination with H2 could be linked to mechanisms involving double bond hydrogenation steps, followed by hydrogen chloride release and regeneration (depending on the catalyst and reaction conditions) of the double bond.28,35,36 Chen et al.11 discussed that the last steps occur quickly on Pd/C catalysts. On the other hand, the deposition of carbon and chlorine, which is due to a homolytic cleavage of the C-Cl bond has been confirmed using a linear free energy method on the catalyst surface. In hydrogendeficient environment, CHD reactions might allow a higher chlorinedepositionandproduceimportantdeactivationeffects.11,24,37 Schneider et al.38 have reported that at the highest Cl:H ratio significantly larger amounts of byproduct were obtained in the CHD of dichloromethane.33,38 Therefore, in several kinetic expressions, H2 appears to favor the CHD of TTCE. It is important recalling that at a constant hydrogen concentration, the activity of Pd/TiO2 catalyst samples did not vary after 20-40 h reaction at the maximum temperature (180 °C). 3.4. Hydrogen Chloride Effects. From the experiments carried out to evaluate the role of HCl (with all the other operating parameters remaining constant), it was found that the rate of TTCE hydrodechlorination decreased with increasing concentrations of HCl in the feed (Figure 5). The effects of adding HCl allow us to evaluate not only deactivation but also the simulation of recycling unreacted H2 to the reactor.29 It is important considering that there is no agreement in the literature on the effect of hydrogen chloride on the activity, selectivity, and stability of CHD catalysts. Some authors indicate that hydrogen chloride is a reversible inhibitor, whereas other authors suggest an irreversible poisoning effect.29 Besides, HCl can react with inorganic supports, causing an increase of surface acidity leading to the formation of carbonaceous deposits on the catalyst surface.1,13,39,40 We did not observe significant acidity changes over Pd/TiO2 catalysts by NH3 temperature-programmed desorption (TPD).30 In this work, CHD inhibition was only observed when HCl(g) was fed to the reaction mixture (Figure 5). The effect of feeding HCl was more pronounced with the time on stream. However, when no HCl(g) was introduced to the feed stream, the catalyst recovered its activity exhibiting long time durability (more than 60 h). In other reports,29,35 it has been observed that HCl deposited over the catalyst can be released at high reaction temperatures, and this effect is more appreciable at temperatures around 200-300 °C. Nonetheless, in other reports, the presence of HCl produced new active sites and/or modified the acidity of the catalyst, leading to coke formation over the catalytic surface.41 Ordo´n˜ez et al.29 found carbon deposits due to HCl formed over Pd/Al2O3 during the CHD of TTCE at 225 °C and without HCl in the feed. Therefore, the effect of HCl depends on the type of catalyst, considering that some metals (active phaseorsupport)canstronglylinkchloridespeciesproduced.28,41-44

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Figure 6. Effect of C2H6 (250-850 ppmv) on the CHD of TTCE (550 ppmv) over 0.8% Pd/TiO2 minimonolith samples. H2 ) 5500 ppmv, C6H5CH3 ) 5500 ppmv; GHSV 0.45 (g min)/mL.

So, it is probably that the presence of HCl in the feed only blocked active sites of Pd/TiO2 and/or increased the Cl:H ratio leading to a less efficient hydrodechlorination of TTCE. It is also important to remark that the presence of HCl can generate an agglomeration of the active phases (attributed to the mobility of low active metal-chloride phases produced), which can be regenerated by catalyst treatment.29,30,45-47 The changes in the properties of our catalyst samples are analyzed in the characterization section, vide infra. 3.5. Effect of Ethane. Preliminary experiments showed that ethane was the main hydrocarbon produced over Pd/TiO2 minimonolith catalyst samples under the reaction conditions used in this study. As can be observed in Figure 6, changes in the concentration of this hydrocarbon has little effect on initial HDC reaction rate of TTCE in the temperature range 135-180 °C. Consequently, in agreement with previous reports,11,48,49 where hydrocarbons are not included in the kinetic expressions, it appears that ethane is weakly adsorbed.28 Additionally, the C-H bonds are very stable, and consequently, the organochlorinated conversion to hydrocarbons is irreversible.11 3.6. Kinetic Results. The experimental data of the present work was evaluated by least-squares regression using the software Statgraphics. Most mechanisms considered for this type of reactions are related with Langmuir-Hinshelwood models, where the adsorption (associative or dissociative) of the organochlorinated compound is considered as a limiting step. Table 1 lists rate expressions that led to better experimental data fit of the CHD of TTCE at 180 °C. Models A, B, C, and M11,16,17,50,52 have been applied to mechanisms where dissociative adsorption of the organochlorinated compound and/or hydrogen were assumed. In models D, G, and H-L,19,21,49 molecular adsorption of the organochlorinated compound and dissociative adsorption of hydrogen are considered, except for model G, where molecular adsorption of both the organochlorinated compound and hydrogen was assumed. Model I is explained by means of reaction between hydrogen and the organochlorinated compound as limiting step.51 It can be observed that the effect of hydrocarbons and HCl is not included in several models. When fitting experimental data, it was assumed that the equilibrium adsorption constants were much lower than one (taking into account that dilute concentrations of TTCE were used). Therefore, the reaction rate of TTCE appears to be directly proportional to the changes in concentration of this compound as shown in Figure 5 when HCl was not added to the feed. This suggests that HCl formed during reaction did not affect the CHD of TTCE. Under the reaction conditions used in this work, most models tested fit our data quite well confirming a pseudo-first-order reaction with respect to TTCE. Only model E applied to

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chlorofluorocarbon compounds did not provide good results. On the other hand, the negative effect of HCl added to the feed suggests its inclusion in the kinetic expressions. Model C, obtained during the study of 1,2-dichloroethylene hydrodechlorination by Heinrichs et al.17 over Pd-Ag/Al2O3 catalysts and using different reaction conditions (300-375 °C, powder materials, GHSV, etc.) appears to predict the observed effects in Figures 3-6. It is important to point out that models D, M, and H showed similar behavior when the effect of HCl was considered, but they were applied for the CHD of aromatic compounds or catalytic oxidation. Anyway, model D cannot be discarded since it could probably explain molecular adsorption of TTCE with dissociative adsorption of H2.35,53 The mechanism reported by Heinrichs17 (model C) for hydrodechlorination of olefinic chlorinated compounds over Pd catalysts was selected in this work. Model C assumes a Langmuir-Hinshelwood mechanism with C-Cl scission as a limiting step. To evaluate and compare the reaction constants of model C, model D was also used in order to have more insights regarding reaction mechanisms. We considered only one active site and constant hydrocarbon concentration (ethane). Experimental and predicted values for model C are shown in Figure 7. There is good agreement between experimental and predicted values for this model. In general, the experimental results show a fairly good fit with the proposed model (deviation lower than 10%). In order to evaluate the kinetic parameters, the reaction rates were estimated at different temperatures (135-180 °C) and TTCE concentrations (300-1000 ppm), holding constant the hydrogen to toluene ratio (H2/toluene ) 10). Reaction rate constants were calculated for models C and D. The Arrhenius equation (eq 1) describing the relationship between reaction rate constant and temperature was applied to evaluate the activation energy for the hydrodechlorination of TTCE. Table 2 lists the estimated k values. The standard deviation shows models C and D fit data quite well. kobs ) Ae-Ea/RT

(1)

The reaction rate constants obtained in this work are within the range reported for most organochlorinated compounds, between 0.044 and 3339 mL/(g cat min). Activation energies (Ea) for TTCE hydrodechlorination larger than 67 kJ/mol suggest that the reactor operated in the kinetic control regime. The activation energy was similar to that reported in other works for TTCE hydrodechlorination (without toluene) on powder catalysts.13 The reaction constants from Table 3 were estimated by the Van’t Hoff method54 using the reaction rate constant obtained at 180 °C. By means of the relationship between each constant and the activation energy, we obtained similar order of magnitude values. 3.7. Catalyst Characterization. 3.7.1. Nitrogen Adsorption. Textural properties of the TiO2 Hombikat support and fresh and used Pd/TiO2 catalyst samples are listed in Table 4. The specific surface area of the support decreased after Pd impregnation. Romero et al.55 have attributed this behavior to the incorporation of active phases, which alters the pore size. The pore size distribution for tested catalyst samples is around 10 nm, indicating that these materials are mesoporous.56 The surface area of fresh and spent catalysts was almost the same. 3.7.2. Hydrogen Chemisorption. Table 5 shows that Pd dispersion decreased, i.e. particle size increased on used Pd/ TiO2 minimonolith samples compared to fresh ones. This phenomenon has widely been ascribed to reaction of palladium

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Table 1. Fitting Rate Expressions Used for Kinetic Modeling of Organochlorinated Compoundsa goodness of fit, r2

kinetic equation r ) k(Ca)(CH2)1/2

r)

1+

r)

CH21/2

r)

0.965

Model B; Immaraporn et al.16

0.964 st ) 1

Model C; Chen; Heinrichs et al.11,17

+

CH21/2

)

0.965

Model D; Coq et al.21

st

K1Ca + (K2CH2)

0.5

+ K3CHCl

kKCa(CH2)0.5

nd

Model E; Ribeiro et al.24

CHCl

kKaKH2CaCH2

0.965

Model G; Ordo´n˜ez et al.19

0.980

Model H; Park et al.49

0.973

Model I; Borgna et al.51

0.975

Model J; Ordo´n˜ez et al.18

0.972

Model K; Park et al.49

(1 + KaCa + KH2CH2)

2

kKaKH2CaCH2 (1 + KaCa + KHClCHCl) + (1 + (KH2CH2))2

r)

kKaKH2CaCH2 (1 + KaCa)(1 + KH21/2CH21/2)2

r)

kKaCa(KH2CH2)1/2 (1 + (KH2CH2)1/2 + KaCa)2

r)

r)

K2CHClCHC

k(K1Ca)(K2CH2)0.5

r)

Model A; López et al.14

(1 + K1Ca)

K1CHCl

r)

0.965

0.8

kCa

(

r)

k(K1Ca)0.8

models and refs

kKaKH2CaCH2 (1 + KaCa + (KH2CH2))3

kKaKH2CaCH2

nd

Model L; Park et al.49

(1 + KaCa + KHClCHCl)(1 + (KH2CH2) )

1/2 2

r)

kCa (1 + KaCa + KHClCHCl)

0.962

Model M, Corella et al.52

a Ca: concentration of organochlorinated compound. k: rate constant. K: adsorption constant. st: number of active sites. m and n: reaction orders. nd: not determined.

with chlorine atoms during the CHD reaction, producing PdCl2, which has some mobility over supports leading to agglomeration of the active phase. On the other hand, considering that in these experiments the catalytic surface must be clean, coke residues can also have an additional influence on dispersion since the presence of carbon can block metallic active sites. The effect of deposits over used catalysts has been analyzed by TGA and TPO.28,57

Although an unambiguous link between catalyst structure and CHD activity has yet to emerge, several studies have demonstrated a dependence of CHD activity and stability on metal dispersion.1,4,13,58 Some authors have suggested a higher initial specific CHD activity for “larger” supported Pd particles. This observation is in line with the work of Juszczyk et al.,59 who reported higher turnover frequencies of both CF3CFCl2 and CCl2F2 for larger Pd particles supported on Al2O3, an effect

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Figure 7. Experimental and predicted values from model C for the CHD of TTCE over 0.8% Pd/TiO2 minimonolith samples. C2Cl4 ) 550 ppmv, H2/C2Cl4 ) 10, C6H5CH3/C2Cl4 ) 10. Table 2. Specific Rate Constants, k (mL/(g cat min)) for the Hydrodechlorination of TTCE temperature (°C)

C2Cl4a σc ) 7.75

C2Cl4b σ ) 6.69

125 135 150 165 180

62.43 127.18 265.00 423.68

67.58 136.12 257.00 399.6

a

Figure 8. TGA analyses of 0.8% Pd/TiO2 minimonolith samples. C2Cl4 ) 550 ppm, H2/C2Cl4 ) 10, C6H5CH3/C2Cl4 ) 10. Catalyst used at 180 °C.

Model C. b Model D. c σ ) standard deviation.

Table 3. Experimental Reaction Rate Constants Obtained from Model C and Calculated by the Van’t Hoff Method (Reference Temperature 180 °C) C2Cl4 temperature (°C)

experimental

Van’t Hoff method

135 150 165

62.43 127.18 265.00

59.8 120.3 231.55

Table 4. Textural Characteristics of Powder TiO2 Hombikat Support and Pd/TiO2 Samples

catalyst

BET, specific surface area (m2/g)

TiO2 Hombikat 0.8% Pd/TiO2, fresh 0.8% Pd/TiO2, used

195.00 126.00 124.00

pore volume (cm3/g)

pore equivalent diameter (nm)

0.53 0.37

10.74 10.00

Table 5. H2-Chemisorption on Fresh and Used Pd/TiO2 Minimonolith Catalyst Samples catalyst

dispersion (%)

average particle size (nm)

fresh used C2Cl4

19.40 7.85

5.81 14.56

attributed to an ensemble effect. In our study, although the Pd particle size increased during long time runs (Table 5), no changes in activity were detected. Besides, produced coke did not affect the CHD reaction. The most pronounced effects were only observed when HCl was added to the feed (Figure 5). When no HCl was fed, a slight activity increase was observed at first, but it remained stable afterward. 3.7.3. TGA. Figure 8 shows that used Pd/TiO2 catalyst samples loss more weight (2.5 wt %) between 200 and 800 °C than fresh ones (1 wt %), suggesting formation of carbon deposits over used Pd/TiO2 samples. Therefore, although the presence of these deposits did not affect catalyst activity during the CHD reaction (without HCl in the feed), they could have some influence on the signals observed on used samples analyzed by chemisorption (Table 5) and FTIR (Figure 11). The increase of the TGA signal on used samples between 200 and 400 °C (attributed to carbon deposits over the active phase) was 0.5 wt %, while between 400 and 800 °C (linked to coke accumulation on supports),60 it was 1.03 wt %, suggesting that coke deposits are insignificant. Therefore, larger metal particle size observed by chemisorption are ascribed to agglomeration,

Figure 9. TPO profiles of 0.8% Pd/TiO2 minimonolith samples. Fragments of CO2 (E44) and chlorine intermediates (E50) 7 times the signal of CO2. C2Cl4 ) 550 ppm, H2/C2Cl4 ) 10, C6H5CH3/C2Cl4 ) 10. Catalyst used at 180 °C.

in agreement with other works.57 Further, BET results (Table 4) showed small area decrease of used catalyst samples, suggesting that after the CHD reaction of TTCE catalyst pores might not be blocked. This means that the low amount of coke observed, (much lower than 2 wt %) was not an important cause of deactivation during TTCE hydrodechlorination under the reaction conditions of the present work. 3.7.4. TPO. During the CHD of TTCE, carbon deposits and possible chlorine intermediates were formed at temperatures below 400 °C (Figure 9). The high intensity of the signal assigned to CO2 released, compared to the signal of the chlorinated compound confirms peak assignments obtained by TGA suggesting that carbon deposits are mainly formed by TTCE adsorption. Coke deposits have mainly been attributed to adsorption of organochlorinated compounds by means of its chlorine atoms.14 In previous works, the existence of carbonaceous deposits around 6 wt % were demonstrated to be among the main causes of deactivation but at higher temperatures and in the presence of mixtures of organochlorinated compounds.30 In that case, the beneficial effect of the regeneration procedure for removing coke deposits was demonstrated. On the other hand, HCl release was not observed by TPO. Therefore, HCl appears to be released as it is produced.11,36 However, a mild interaction of HCl or Cl with active sites promoting some agglomeration is not discarded.

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Figure 10. TPO profiles of used 0.8% Pd/TiO2 minimonolith samples. C2Cl4 ) 550 ppm, H2/C2Cl4 ) 10, C6H5CH3/C2Cl4 ) 10, CO2 fragments. (a) Reaction at 400 °C. (b) Reaction at 180 °C after adding HCl (550 ppm) in the feed. (c) Reaction at 180 °C.

Increasing the concentrations of TTCE (300-1000 ppm) and hydrogen (0-10000 ppm) in the feed favored TTCE CHD, whereas HCl added to the feed partially inhibited the reaction. However, the activity was recovered when HCl was removed. On the other hand, ethane does not affect the hydrodechlorination reaction of the organochlorinated compound. Except for those reactions where HCl was added to the feed, Pd/TiO2 minimonolith samples showed to be stable during time on stream. Therefore, recycling of unreacted hydrogen requires HCl removal. Carbon deposits over tested catalysts were detected by TGA and TPO. However, they did not appear to deactivate minimonolith catalyst samples. The presence of surface carbon and rather negligible amounts of chlorine intermediates were detected by TPO. Mild acidity changes and agglomeration of the active phases after the CHD reaction were confirmed by TGA, BET surface area, TPO, FTIR, and chemisorption analyses of used catalyst samples. However, they do not significantly influence catalyst activity for tetrachloroethylene hydrodechlorination. The reaction mechanism appears to be in agreement with the scission of C-Cl bonds on TTCE previously adsorbed as the limiting step. Then, a quick release of HCl and hydrogenation of the double bond occur. Acknowledgment

Figure 11. FTIR spectra of 0.8% Pd/TiO2 fresh and used minimonolith samples. CHD of C2Cl4 at 180 °C, W/F ) 4.45 × 104 (g min)/mL.

TPO profiles of CO2 desorption over three different catalyst samples used for CHD of TTCE are illustrated in Figure 10. The most intense signal observed is for the sample used at 400 °C (Figure 10a), followed by the used sample at 180 °C in the presence of HCl in the fed (Figure 10b). A small signal is observed for the used sample at 180 °C without adding HCl to the feed (Figure 10c). These results confirm that HCl does promote carbonaceous deposits on Pd/TiO2 minimonolith catalyst samples. 3.7.5. FTIR Analysis. The comparison between FTIR spectra of fresh and used catalyst samples (Figure 11) shows that the absorbance signal in the OH region slightly decreased and shifted to the right, suggesting acidity changes during the CHD. This might contribute to the agglomeration of the active phase as already explained. Under more severe conditions, it was found that modifications of the support structure do not appreciable affect the acidity of titania.30 However, we can not completely discard possible effects ascribed to HCl29 at a reaction temperature similar to that used in the present work, where HCl can have a better contact with the active phase. 4. Conclusions The results presented in this paper demonstrate that structured catalytic reactors are effective for kinetic investigations of catalytic hydrodechlorination. Several heterogeneous models for the CHD of TTCE over 0.8% Pd/TiO2 minimonoliths were compared. Reaction kinetics based on Langmuir-Hinshelwood mechanisms are in good agreement with the experimental data. The reaction was first order with respect to TTCE. The calculated activation energy was 67 kJ/mol, and the reaction rate constants were between 62 and 424 mL/(g cat min) in the temperature range of 135-180 °C. The reaction rate constants were comparable with those observed in powder catalysts.

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ReceiVed for reView June 24, 2009 ReVised manuscript receiVed October 16, 2009 Accepted November 2, 2009 IE901027Y