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Ind. Eng. Chem. Res. 2009, 48, 2826–2835
Hydrodechlorination of Light Organochlorinated Compounds and Their Mixtures over Pd/TiO2-Washcoated Minimonoliths Carlos A. Gonza´lez,† Michael Bartoszek,‡ Andreas Martin,‡ 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, and Leibniz-Institut fu¨r Katalyse e.V. an der UniVersita¨t Rostock (Aussenstelle Berlin), Richard-Willsta¨tter-Strasse 12, D-12489 Berlin, Germany
The gas-phase catalytic hydrodechlorination (CHD) of dichloromethane (DCM), tetrachloroethylene (TTCE), and chloroform (CF) and their mixtures was studied over Pd/TiO2-washcoated cordierite minimonoliths. Experiments were carried out in a flow reactor at 120-300 °C, 1 bar, and 0.45 g min/mL. Catalytic runs with the pure compounds at 200 °C led to 60-100% conversion following the sequence CF > TTCE > DCM. Catalyst deactivation and regeneration were also examined. Lower conversions (between 30% and 95%) and catalyst deactivation were observed when mixtures of organochlorinated compounds where fed as reactants. DCM was the most affected when binary and ternary mixtures were used. Catalyst samples were characterized before and after reaction by various temperature-programmed studies (H2 TPR, He TPD, NH3 TPD, and TPO), H2 chemisorption, and XPS measurements. Most characterization studies were carried out using an online coupled mass spectrometer, thus allowing the parallel detection of different fragments and species corresponding to several hydrocarbons and HCl. In some experiments, a fast GC-ToF-MS system was used. The catalytic activity of spent samples was partially recovered by heating them in flowing air and then in 5% H2/N2. However, the initial reaction rate decreased by 62% over samples used in three consecutive runs when ternary mixtures were fed. Carbonaceous deposits of different nature and changes in the oxidation state of Pd (Pd0 to Pd2+ and Pd4+) appear to play key roles in catalyst deactivation, whereas acidity was found to remain almost the same for fresh and used catalyst samples. Carbonaceous deposits were removed by heating at temperatures lower than 400 °C in flowing air. 1. Introduction The release of halogenated compounds into the environment has been associated with stratospheric ozone depletion, smog formation, global warming, and a range of human health effects.1-3 Although such releases have been curtailed through stringent environmental regulations, chlorinated compounds still find widespread use in a wide variety of industries, such as electronics, dry cleaning, and adhesives.1,3,4 Dichloromethane (DCM), chloroform (CF), and tetrachloroethylene (TTCE) are among the light chlorinated hydrocarbons mainly found in Colombian industrial emissions.5 These compounds are included in the list of the 17 highly dangerous chemicals targeted in the emissions reduction effort (33/50 Program) of the U.S. Environmental Protection Agency.6 Since the early 1990s, there has been an increasing need to develop technologies to convert harmful compounds into environmentally benign products.7-10 Catalytic hydrodechlorination (CHD) represents an interesting alternative because it is suitable for the treatment of both concentrated and dilute chlorinated streams.11 CHD works under mild conditions, does not lead to the formation of dioxins, and avoids unwanted CO2 releases into the environment.12 It is wellknown that noble metals, especially Pd, are active catalysts for carbon-chlorine bond hydrogenolysis.13-16 However, these catalysts suffer from relatively fast deactivation, which discourages their use in commercial processes.1,17-19 Moreover, research on catalyst deactivation has received little attention in comparison to the discovery of new catalysts.20 In gas-phase operation, deactivation of noble metal catalysts has been linked * To whom correspondence should be addressed. Tel.: +5742196605. Fax: +5742196609. E-mail:
[email protected]. † Universidad de Antioquia. ‡ Leibniz-Institut fu¨r Katalyse e.V. an der Universita¨t Rostock (Aussenstelle Berlin).
to HCl/Cl poisoning, carbon deposition, loss of metal through the formation of volatile chlorides, and metal sintering.2,10,19,21 Additionally, changes in the properties of the supports used in CHD reactions can affect the active phase and consequently influence the catalytic activity. Silica and alumina can be attacked by hydrogen chloride formed during reaction, leading to a decrease in surface area and an increase in surface acidity (because of the high Lewis acidity of aluminum and silicon halides). This would intensify the trend toward catalyst fouling by carbonaceous deposits.22,23 Other alternative supports, such as AlF3, MgO, ZrO2, and modified zeolites, are also unstable under the aforementioned aggressive conditions or exhibit decreased performance.18,24,25 However, deactivation and regeneration of these catalysts during CHD are still under research.17 In this context, the development of poison-resistant catalysts has gained significant importance in recent years.21 There is evidence that the support can inhibit HCl poisoning, and in any case, the support must be resistant to the corrosive effects of high HCl concentrations generated at elevated temperatures.16,26-28 Titania appears to provide better resistance to coke and HCl. Moreover, titania exhibits a strong metalsupport interaction (SMSI) with group VIII metals under high reduction temperatures (usually above 300 °C), resulting in an improved catalytic performance. After reduction, it has been observed that the TiOx species formed (Ti2+, Ti3+) could have an important effect in different reactions. Panpranot et al.29 reported that the presence of Ti3+ in contact with Pd can probably diminish the adsorption strength of ethylene, resulting in an ethylene gain, whereas Li et al.30 explained that metal particles can be blocked by the mobile TiOx phase, thus affecting hydrogen chemisorption.26,31-33 In general, different applications of titania-based materials have been considered as highly interdisciplinary, embracing inorganic, organic, organometallic,
10.1021/ie8013742 CCC: $40.75 2009 American Chemical Society Published on Web 02/05/2009
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and polymeric research areas. In particular, those materials containing cyclopentadienyl ligands have been of interest in organic chemistry as reducing agents or as catalysts in hydrodehalogenation reactions. Hara et al.35 obtained effective dechlorinations of various chloroanisoles, chloronapthalenes, and chlorophenols by means of Grignard reagents in the presence of catalytic amounts of dicyclopentadienyl titanium dichloride. Schwartz et al.36 also reported dehalogenation of a broad variety of aryl halide environmental pollutants such as 4-halobiphenyls, octachlorodibenzo-p-dioxin, and tetrachloronapthalene. On the other hand, Burris et al.37 obtained comparable results during the reduction of TTCE and trichloroethylene by vitamin B12 using titanium(III) citrate as the bulk reactant and observed no loss in activity with repeated use of the heterogeneous catalysts. In addition to high activity, selectivity, and resistance to deactivation, it is necessary to develop an appropriate catalyst regeneration method7,33 considering that a drawback of catalytic processes is the high cost of catalysts.20,38 Catalyst regeneration has been carried out using different solvents and thermal procedures.10,17,33,39-43 Light carbon deposits can be removed with solvents, and thermal treatments can eliminate catalyst poisons depending on the treatment time and the nature of the catalysts. However, in most studies, catalysts have been submitted to one regeneration operation only, without any mention of the effects of additional cycles of treatment on the performance of reused catalysts.10,21,44 Reports dealing with CHD of mixtures of organochlorinated compounds such as DCM, TTCE, chlorobenzene, and trichloroethylene45,46 or chlorinated olefins47 have shown important effects regarding the number of chlorine atoms and the presence of double bonds.44,46,48,49 In general, CHD rates increase with the number of chlorine atoms for aliphatic chlorinated compounds. However, in the case of olefinic compounds, chlorine atoms affect the double-bond density, and CHD is more difficult. Additionally, appreciable differences in the reactivity of chlorofluorocarbon compounds have also been observed depending on the number of chlorine atoms in the molecule.48 All of these works confirm the existence of structural sensitivity in chlorinated compounds for CHD. In a previous work from our group,31 the effects of different binders on the preparation of Pd/TiO2-washcoated minimonolith samples, resistance tests, and characterization studies were reported. The most promising catalysts, in terms of activity and resistance, were selected for a study of the effects of using mixtures of organochlorinated compounds on deactivation and regeneration treatments. In this paper, the hydrodechlorination of binary and ternary mixtures of DCM, CF, and TTCE was studied over Pd/TiO2-washcoated cordierite minimonolith samples considering that the simultaneous hydrodechlorination of mixtures of chloro-organic compounds has been poorly studied up to now. Fresh, used, and regenerated catalyst samples were characterized by various temperature-programmed studies [H2 temperature-programmed reduction (TPR), He temperatureprogrammed desorption (TPD), NH3 TPD, temperature-programmed oxidation (TPO)], H2 chemisorption, and X-ray photoelectron spectroscopy (XPS). Two mass spectrometers [quadruple and a fast gas chromatograph-time-of-flight-mass spectrometer (GC-ToF-MS) system] were used to analyze and confirm the nature of the desorbed species during temperatureprogrammed studies. The use of mass spectrometric detection methods provides detailed information about catalyst changes due to pretreatment, CHD reaction, and regeneration. Therefore, the main goals of this work were to examine the effect of the chloro-organic compound structure on the CHD reaction, to
assess the behavior and possible deactivation causes of Pd/TiO2 minimonoliths, and to determine the efficiency of the proposed regeneration treatment. 2. Experimental Section 2.1. Catalytic Activity Tests. Pd/TiO2-washcoated minimonolith samples were prepared as previously reported.31 The CHD reaction was carried out in a Pyrex glass tubular reactor heated with an electrical oven equipped with a temperature controller. 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). It consisted of 5500 ppmv H2; 5500 ppmv toluene; and 550 ppmv of either dichloromethane, chloroform, tetrachloroethylene or their binary or ternary mixtures at 1:1 and 1:1:1 molar ratios, respectively, with a balance of N2. The Pd loading (0.8 wt %) of Pd/TiO2 was always the same as determined by elemental analysis. Therefore, the lowest Pd-to-Cl ratio (1:9) was obtained with the ternary mixtures. All experiments were carried out by allowing the organochlorinated mixture in toluene to flow through Pd/TiO2-washcoated minimonolith samples. Toluene was chosen as the solvent because it allows the simulation of a real chlorinated waste and reacts to a very low extent (conversion below 0.05%).47-50 All feed components were supplied from available compressed gas cylinders before reaching the catalyst. The total gas flow rate was maintained at 145 mL/ min. The temperature range was 120-300 °C. K-type thermocouples were used for temperature measurements. All gas lines were heated at 120 °C to prevent water condensation during regeneration runs. Prior to reactions, catalyst samples were calcined in air at 400 °C for 2 h (2 °C/min) and reduced in flowing 5% H2/N2 (75 cm3/min), with the temperature ramped from 20 to 300 °C (at 2 °C/min) and held at 300 °C for 1 h. Several runs were repeated under the same initial reaction conditions over fresh, used (spent catalysts after 12 h at 300 °C on stream), reused (used in further runs without any pretreatment), and regenerated catalysts. Before the CHD reaction, all catalyst samples (fresh, used, reused, and regenerated) were loaded into a Pyrex glass tube, and the reaction mixture was allowed to flow at 120 °C until steady-state conditions were reached. The regeneration treatments were done under the same pretreatment conditions as used for fresh catalysts, but the time of pretreatment ranged between 2 and 6 h. 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. The catalytic reactor and operating conditions to ensure negligible heat-/mass-transport limitations on minimonolith catalysts were tested by means of the Madon-Boudart test and the Thiele module (considering the washcoat thickness). Reaction rates over catalyst samples with different metal loadings but similar dispersions were determined.31,51-55 Catalytic runs were repeated up to three times, and the results were within (5%. 2.2. Catalyst Characterization. Samples of fresh, used, and regenerated Pd/TiO2-washcoated minimonolith samples were obtained with a diamond wire saw. Parallel and perpendicular cuts to the minimonolith axis were performed. Catalyst samples were all calcined in air at 400 °C prior to characterization studies. The samples were characterized by temperatureprogrammed techniques, thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS) in order to investigate possible causes of catalyst deactivation. Ammonia temperatureprogrammed desorption (NH3 TPD), temperature-programmed
2828 Ind. Eng. Chem. Res., Vol. 48, No. 6, 2009
Figure 1. Reaction rates [mmol/(gcat min)] of DCM, TTCE, and CF on 65 mg of both Pd/TiO2 powder (p) and Pd/TiO2-washcoated minimonolith (m) at 200 °C. Chloro-organic compound concentration ) 550 ppm, flow rate ) 145 mL/min. Time on stream ) 12 h/run.
oxidation (TPO), and temperature-programmed desorption in flowing He (He TPD) were carried out in a Micromeritics AutoChem II 2910 instrument connected to a quadrupole mass spectrometer (Pfeiffer Vacuum Omnistar). Additional He TPD experiments were carried out by coupling the microreactor outlet of the TPD instrument to a 250-µL injection loop of the fast GC-ToF-MS system (Pegasus II, LECO Instruments) equipped with a DB5-MS column (10 m, i.d. ) 0.1 mm, d ) 1.2 µm, isothermal run at 60 °C). Repetitive loop injections were carried out at 40-s intervals. The MS acquisition rate was 10 full spectra per second. Pretreatment conditions before the application of each technique were similar to those used before reaction. Temperature-programmed reduction (H2 TPR) measurements and H2 chemisorption studies were carried out using an AMI-1 TPD apparatus (Altamira Instruments Inc.). H2 TPR, TPO, He TPD, and NH3 TPD experiments were conducted from room temperature to 900 °C at 10 °C/min using 5% H2/Ar, 20% O2/He, and helium, respectively, at 400 °C. For NH3 TPD experiments, calcined and reduced catalyst samples were kept in a flow of NH3/He for 2 h at 100 °C. The reactor was then purged in flowing He for an additional hour to remove weakly adsorbed NH3. A linear temperature program was used to follow NH3 desorption, with evolved gas detected by a quadrupole mass spectrometer. Palladium dispersion by hydrogen chemisorption was determined at 100 °C. 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. The XPS measurements were carried out using a VG ESCALAB 220iXL instrument, with Al KR radiation. 3. Results and Discussion 3.1. Catalytic Hydrodechlorination over Pd/TiO2 Powder and Washcoated Minimonolith Samples. The CHD reaction rates of each chloro-organic compound over Pd/TiO2-washcoated minimonolith samples are illustrated in Figure 1. CF was more reactive than TTCE and DCM. For comparison purposes, the reaction rate of CF hydrodeclorination is also presented in Figure 1. Washcoated minimonoliths were more active than powder samples, most likely because the size of Pd particles in powder materials was much smaller than that in the minimonoliths.31 It has been observed that small metal particles can be more quickly saturated with HCl formed during reaction, which, in turn, can poison the catalyst surface, leading to low activity.56,57 As displayed in Figure 1, the CF reaction rate at 200 °C over Pd/TiO2-washcoated minimonolith was more than
twice as high as that on powder Pd/TiO2. Under reaction conditions, toluene underwent a very low conversion (
