Hydrodechlorination Of CCl4 Research Articles

Influence of microwave activation on the catalytic behavior of Pd-Au/C catalysts employed in the hydrodechlorination of tetrachloromethane

This work investigates the influence of activation by microwave irradiation on the catalytic properties of Pd-Au/CSibunit catalysts in the hydrodechlorination of CCl4. Pd and Pd-Au samples were thoroughly characterized by CO chemisorption, X-ray diffraction, TPHD and STEM, and used in gas phase hydrodechlorination of tetrachloromethane. The studies showed that homogenous Pd-Au alloys are formed more efficiently if short-time microwave activation is applied instead of conventional activation by H2 reduction at 380 °C. These catalysts show higher activity, stability and higher selectivity towards the desired products (C1–C5 hydrocarbons) than the catalysts activated by conventional reduction in 10%H2/Ar flow.

Hydrodechlorination of Tetrachloromethane over Palladium Catalysts Supported on Mixed MgF₂-MgO Carriers.

Pd/MgO, Pd/MgF2 and Pd/MgO-MgF2 catalysts were investigated in the reaction of CCl4 hydrodechlorination. All the catalysts deactivated in time on stream, but the degree of deactivation varied from catalyst to catalyst. The MgF2-supported palladium with relatively large metal particles appeared the best catalyst, characterized by good activity and selectivity to C2-C5 hydrocarbons. Investigation of post-reaction catalyst samples allowed to find several details associated with the working state of hydrodechlorination catalysts. The role of support acidity was quite complex. On the one hand, a definite, although not very high Lewis acidity of MgF2 is beneficial for shaping high activity of palladium catalysts. The MgO-MgF2 support characterized by stronger Lewis acidity than MgF2 contributes to very good catalytic activity for a relatively long reaction period (~5 h) but subsequent neutralization of stronger acid centers (by coking) eliminates them from the catalyst. On the other hand, the role of acidity evolution, which takes place when basic supports (like MgO) are chlorided during HdCl reactions, is difficult to assess because different events associated with distribution of chlorided support species, leading to partial or even full blocking of the surface of palladium, which plays the role of active component in HdCl reactions.

Hydrodechlorination of CCl4 over carbon-supported palladium–gold catalysts prepared by the reverse “water-in-oil” microemulsion method

Two series of carbon-supported Pd–Au catalysts were prepared by the reverse “water-in-oil, W/O” method, characterized by various techniques and investigated in the reaction of tetrachloromethane with hydrogen at 423 K. The synthesized nanoparticles were reasonably monodispersed having an average diameter of 4–6 nm (Pd/C and Pd–Au/C) and 9 nm (Au/C). Monometallic palladium catalysts quickly deactivated during the hydrodehalogenation of CCl 4 . Palladium–gold catalysts with molar ratio Pd:Au = 90:10 and 85:15 were stable and much more active than the monometallic palladium and Au-richer Pd–Au catalysts. The selectivity toward chlorine-free hydrocarbons (especially for C 2+ hydrocarbons) was increased upon introducing small amounts of gold to palladium. Simultaneously, for the most active Pd–Au catalysts, the selectivity for undesired dimers C 2 H x Cl y , which are considered as coke precursors, was much lower than for monometallic Pd catalysts. Reasons for synergistic effects are discussed. During CCl 4 hydrodechlorination the Pd/C and Pd–Au/C catalysts were subjected to bulk carbiding.

Effect of silver on the catalytic hydrodechlorination of tetrachloromethane over silica-supported palladium-silver catalysts

AbstractTwo silica-supported palladium based catalysts characterized by overall metal loadings 10 wt.%, and atomic Pd : Ag ratios of 100 : 0 and 70 : 30, were prepared by incipient wetness impregnation, subjected to reduction in hydrogen and characterized by XRD. It was found that incipient wetness co-impregnation of silica resulted in the formation of a catalyst characterized by relatively small metal crystallites (~11 nm) and significant extent of Pd-Ag alloying. The catalytic performance was investigated in the hydrodechlorination of tetrachloromethane in the gas phase. Silver addition to palladium in the co-impregnated catalysts had very pronounced effect, vastly decreasing the overall activity (conversion) and also the selectivity towards hydrocarbons, propensity characteristic for the monometallic palladium catalysts working in CCl4 hydrodechlorination carried out in the gas phase. Instead, large amounts of C2HxCly dimeric products were formed. These dimers are considered as possible coke precursors. Post-reaction samples of both catalysts contained large amount of carbon which entered the Pd and Pd-Ag phases. This carbon can be easily removed by treatment with hydrogen at 450ºC.

Design of a Bimetallic Au/Ag System for Dechlorination of Organochlorides: Experimental and Theoretical Evidence for the Role of the Cluster Effect

The experimental study of dechlorination activity of a Au/Ag bimetallic system has shown formation of a variety of chlorinated bimetallic Au/Ag clusters with well-defined Au:Ag ratios from 1:1 to 4:1. It is the formation of the Au/Ag cluster species that mediated C–Cl bond breakage, since neither Au nor Ag species alone exhibited a comparable activity. The nature of the products and the mechanism of dechlorination were investigated by ESI-MS, GC-MS, NMR, and quantum chemical calculations at the M06/6-311G(d)&SDD level of theory. It was revealed that formation of bimetallic clusters facilitated dechlorination activity due to the thermodynamic factor: C–Cl bond breakage by metal clusters was thermodynamically favored and resulted in the formation of chlorinated bimetallic species. An appropriate Au:Ag ratio for an efficient hydrodechlorination process was determined in a joint experimental and theoretical study carried out in the present work. This mechanistic finding was followed by synthesis of molecular bimetallic clusters, which were successfully involved in the hydrodechlorination of CCl4 as a low molecular weight environment pollutant and in the dechlorination of dichlorodiphenyltrichloroethane (DDT) as an eco-toxic insecticide. High activity of the designed bimetallic system made it possible to carry out a dechlorination process under mild conditions at room temperature.

Photocatalytic Hydrodechlorination of Trace Carbon Tetrachloride (CCl4) in Aqueous Medium

The technical feasibility of the photocatalytic hydrodechlorination of CCl4 utilizing H2 produced in situ from photocatalytic water splitting was investigated using multifunctional Pd−NiO/NaTaO3:La catalysts and a O/NaTaO3:La + Pd− Au/IX (ion-exchange resin) mixture-of-catalysts approach. In the former case, the incorporation of Pd into the water-splitting photocatalyst resulted in the reduction of H2 evolution by 3 orders of magnitude. However, the multifunctional catalyst exhibited a remarkable activity for CCl4 removal. The important catalyst parameters were elucidated by the response surface methodology. The Ni and Pd loadings and the catalyst reduction temperature had significant, nonlinear effects on the catalyst activity, indicating that both NiO and Pd nanoparticles play important roles in the photocatalytic hydrodechlorination of CCl4. The strong dependence of the turnover frequency on the catalyst parameters that govern the dispersion of the catalytic phases implies that the reaction is structure-sensitive.

Supported palladium–copper catalysts: Preparation and catalytic behavior in hydrogen-related reactions

Two series of highly dispersed supported Pd–Cu catalysts were prepared using different supports: highly acidic aluminum fluoride HS-AlF3, characterized by high surface area, and active carbon, both prepared by different preparation methods (fluorolytic sol–gel method and co-impregnation). A variety of techniques: chemisorption of hydrogen, XRD, TEM, and catalytic performance in two hydrogen-related probe reactions (in n-pentane hydroisomerization and CCl4 hydrodechlorination) were employed to probe metal dispersion and the extent of interaction between two metal components. A reasonable degree of bimetal intermixing in Pd–Cu/HS-AlF3 catalysts synthesized by fluorolytic co-gelation was recognized by the catalytic behavior in bifunctional hydroisomerization of n-pentane. A significantly decreased, compared to pure palladium, hydrogenation–dehydrogenation potential of Pd–Cu alloys led to a weaker performance of bimetallic catalysts. Similarly, it was found that incipient wetness co-impregnation of active carbon resulted in the formation of catalysts characterized by good metal dispersion and significant extent of Pd–Cu alloying. Although the monometallic 2wt% Cu/carbon catalyst is nearly inactive in CCl4 hydrodechlorination, copper addition to palladium in the co-impregnated catalysts had very pronounced effect, vastly increasing the selectivity toward longer than methane hydrocarbons.

Catalytic hydrodechlorination of chlorohydrocarbons in a medium of sodium hydroxide solutions, Part 2: Transformations of hexachloroethane and other polychloroethanes

In part one of this work, it was shown that in addition to sodium formate, hexachloroethane (HCE) and perchloroethylene are the main products of the catalytic hydrodechlorination of CCl4 in a medium of sodium hydroxide solutions. These products undergo mutual transformations under the conditions of the process. Since the industrial requirements for perchloroethylene considerably exceed those for HCE, the regularities of the catalytic hydrodechlorination of HCE, pentachloroethane, 1,1,2,2-tetrachloroethane, 1,1,2-trichloroethane, and 1,2-dichloroethane are studied in part two, using 1.5 wt % Pd on sibunit as the catalyst. It is found the fraction of hydrogen substited for chlorine grows and the reactivity of the products diminishes with a reduction in the number of chlorine atoms in a polychloroethane molecule. Ethane and ethylene are the final reaction products after a rather long contact time. The kinetics of HCE catalytic hydrodechlorination is studied n the temperature range of 353–393 K and at hydrogen partial pressures of 50–810 kPa. It is shown that the only reaction product is perchloroethylene; i.e., the transformation of HCE proceeds via the elimination of two chlorine atoms, while the limiting stage of the process is product dissolution in the aqueous-alkaline reaction mass. The form of the kinetic equation is found to be w = 2.1 × 10−6 × exp[−(16200 ± 400)/RT]CHCECcat\(P_{H_2 }^{0.5} \) mol/(L s). The possibility of using the catalytic hydrodechlorination of chlorohydrocarbons in a medium of sodium hydroxide solutions for the recycling of chloroorganic wastes containing CCl4 and polychloroethanes is demonstrated.

Catalytic hydrodechlorination of chlorinated hydrocarbons in a medium of sodium hydroxide solutions. I: Conversion of carbon tetrachloride

Carbon tetrachloride, classified as an ozone-depleting substance in the Montreal Protocol, is formed as a byproduct in the production of chlorinated hydrocarbons (chloromethanes, perchloroethylene, etc.). To develop a rational method of CCl4 utilization (an alternative to burning), in this work we study the liquid-phase catalytic hydrodechlorination of CCl4 with the formation of useful products such as sodium formiate, hexachloroethane, and perchloroethylene in the presence of aqueous sodium hydroxide solutions. Experiments are performed in a 300-mL autoclave at pressures of up to 1.5 MPa within a temperature range of 80–120°C on a Pd catalyst on sibunite (5 fractions, 0.1 to 1.6 mm). The dependences of the CCl4 conversion and the selectivities with respect to the principal reaction products on the initial concentrations of CCl4 and NaOH, the temperature, the partial pressure of hydrogen, the size of catalyst particles, and the palladium content in a catalyst are studied. Using the obtained data as a basis, we propose a process flowsheet, make some recommendations on the process conditions (depending on the specified composition of products), and determine the optimum catalyst characteristics: 1.5 wt % Pd on sibunite with a grain size of 0.315–0.63 mm. The proposed process allows us not only to solve the problem of recycling CCl4-containing wastes, but also to provide the production of three marketable products.

Hydrodechlorination of tetrachloromethane on alumina- and silica-supported platinum catalysts

Two series of alumina- and silica-supported platinum catalysts, representing a wide range of metal dispersion were characterized by CO chemisorption, XRD and TEM, and investigated in the hydrodechlorination of tetrachloromethane at 343–363K. Catalytic activity of Pt/Al2O3 in CCl4 hydrodechlorination shows very strong inverse relationship with metal dispersion; highly dispersed Pt samples exhibit very low turnover frequencies. Similar, but much milder trend, was found for silica-supported Pt catalysts. This significant support effect on the catalytic behavior was attributed to an extensive surface chloriding of small Pt particles interacting with Lewis acid sites of γ-alumina. It is suggested that such interactions lead to formation of electrodeficient Pt sites, which are quickly blocked by produced chloride species. On silica, similar in kind metal-support interactions do not occur, and, in effect, deactivation of Pt/SiO2 catalysts is much less marked. Very strong dependence of catalytic activity of alumina-supported platinum on metal dispersion implies that even very small changes in Pt particle size dispersion during reaction and catalyst regeneration should not be ignored in interpreting variations in the catalytic behavior. Indeed, changes in Pt particle size caused by different regeneration procedures are found to be an important factor in shaping the ultimate hydrodechlorination behavior. Carried out in an oxidative atmosphere for an efficient removal of coke, the regeneration of a highly dispersed Pt/Al2O3 catalyst resulted in a significant growth of metal particles which, in line with the correlation between turnover frequency and metal dispersion established for this catalytic system, exhibited much better performance than that of a freshly reduced catalyst sample. In the case of highly dispersed Pt/SiO2 catalyst the regeneration also leads to decrease of metal dispersion and corresponding improvement of catalytic activity.

Highly selective hydrodechlorination of CCl4 into CHCl3 on Ag–Pd/carbon catalysts

A series of Ag–Pd/C catalysts with different Ag/Pd molar ratios were prepared by impregnation and characterized by N2 adsorption and XPS. The catalytic performance was investigated in the hydrodechlorination of carbon tetrachloride into chloroform. High selectivity toward chloroform, near 100%, was obtained over Ag–Pd/C bi-metal catalysts. A very pronounced surface enrichment with Ag in Ag–Pd/C catalysts determined by XPS may be responsible for the high selectivity of chloroform. It is suggested that hydrodechlorination of carbon tetrachloride into chloroform occurs on the surface of Ag in the Ag–Pd alloy and the role of Pd is to supply H atoms necessary.

ToF-SIMS studies of the regeneration of Pd/TiO 2 catalyst used in hydrodechlorination process

This paper is devoted to the surface studies of deactivation-regeneration process of Pd/TiO 2 catalyst used in the hydrodechlorination of CCl 4. The samples were prepared by the incipient wetness impregnation method. The time-of-flight secondary ion mass spectrometry (ToF-SIMS) was used as a main analytical tool in these investigations. The advantages of this technique result from high sensitivity of secondary ion mass spectrometers, possibility of detection of not only elemental but also molecular ions and the fact that ToF-SIMS can give information from the upper layer of the investigated material. The results of ToF-SIMS measurements of as prepared, deactivated and regenerated specimens showed that the regeneration of the deactivated sample in the O 2 atmosphere among other causes an increase in the ability of the catalyst to the formation of palladium chloride in the next working cycles. A growth in the amount of PdCl 2 was accompanied by a decrease in the quantity of surface accessible palladium atoms. It resulted in a more violent drop in the conversion of CCl 4 and faster deactivation of the regenerated catalyst. The observations described above indicate that not only carbon deposition and deterioration of the surface area of the catalyst are serious problems in the hydrodechlorination process. The bigger attention should also be focused on the prevention of the formation of palladium chloride in the next working cycles of the catalyst after its regeneration.

Hydrodechlorination of CCl4 on Pt–Au/Al2O3 Catalysts

A series of Pt/Al2O3 catalysts were prepared by the impregnation method and were characterized by TEM, XRD, H2 and CO chemisorptions, and investigated in the hydrodechlorination of tetrachloromethane. Three Pt-rich, Pt–Au/Al2O3 catalysts (Pt100, Pt95Au5 and Pt90Au10) showed a similar metal particle size (~2.5–2.7 nm), so observed changes in the catalytic behavior are ascribed to alloying effect, especially because a considerable degree of Pt–Au mixing was achieved in the bimetallic samples. It appeared that by introducing very small amount of gold (10 at.%) to platinum, the catalytic activity is increased. It is argued that the occurrence of this moderate synergistic effect is associated with a decreased tendency of surface chloriding when platinum is alloyed with gold.

Hydrodechlorination catalytic activity of gold nanoparticles supported on TiO 2 modified SBA-15 investigated by IR spectroscopy

The hydrodechlorination catalytic activity of gold nanoparticles on SBA-15 silica modified by TiO 2 promoters has been investigated. Comparing the hydrodechlorination catalytic activity platinum nanoparticles supported on TiO 2 catalyst was used in the hydrodechlorination of CCl 4 as model compound. The IR spectroscopic experimental results showed that the gold nanoparticles have higher catalytic activity, than platinum ones. The two samples were tested also in CO oxidation, in which Au/TiO 2/SBA-15 possess also somewhat higher activity than Pt/TiO 2.

Hydrodechlorination of CCl4 over Pt/γ-Al2O3 prepared from different Pt precursors

Abstract The platinum particle size on γ-Al2O3 prepared from different platinum precursors such as Pt(NH3)2(NO2)2, H2PtCl6, and K2PtCl6, and its effect on hydrodechlorination (HDC) of CCl4 with the variation of calcination temperatures was investigated. It concomitantly affects the products distribution and catalytic stability in HDC of CCl4. To verify the effects of platinum particle size with different platinum precursors on the product distribution, the catalysts have been characterized by HRTEM and CO chemisorption, FT-IR, and TP methods (TPR, TPD and TPSR). The catalysts with small platinum particles, which possess low coordination number and electron-deficient character, favor the complete dechlorination of CCl4 and produce CH4 more selectively. It could be due to the strong adsorption strength of CCl4 or the decreased activation energy of surface intermediates on small platinum particles. FT-IR studies reveal that the maximum peak position of linear-bonded CO shifted to the higher frequency with the increase of platinum particle size in all catalysts prepared from three different platinum precursors. While the selectivity to CH4 increased with the decrease in the platinum particle size, the total amount of carbonaceous species on the platinum particles was enhanced. The larger the platinum particle, the higher the selectivity to CHCl3 was obtained in all tested catalysts under the non-deactivating condition.

Effects of Tin on Product Distribution and Catalyst Stability in Hydrodechlorination of CCl4 over Pt-Sn/γ-Al2O3

The Pt−Sn/γ-Al2O3 catalyst with a moderate amount of tin showed an enhanced catalytic stability and selectivity to CHCl3 in hydrodechlorination of CCl4 compared to monometallic platinum catalyst. The tin added in consecutive order to 1.0wt%Pt/γ-Al2O3 acted as a diluting component of platinum ensemble sites and resulted in reducing the formation of C2 compounds and carbon deposition. Furthermore, tin addition modifies the electronic state of supported platinum particles by the electron transfer from tin to platinum, results in the reduction of the binding energy of intermediate species, and suppresses the complete hydrodechlorination activity of surface intermediates. Thus on Pt−Sn/γ-Al2O3 catalysts, adsorbed CCl4 is readily desorbed as CHCl3 instead of residing too long on the surface until it is completely dechlorinated to form CH4.

ToF-SIMS studies of the surface of Pd/ZrO2-TiO2 catalyst used in the hydrodechlorination process

ToF-SIMS studies of the surface of Pd/ZrO2-TiO2 catalyst used in the hydrodechlorination process Time-of-flight secondary ion mass spectrometry (ToF-SIMS) was used in order to obtain the information about the surface composition of Pd/ZrO2-TiO2 catalyst and to estimate the changes in the concentration of particular components on its surface during the hydrodechlorination of CCl4. The results demonstrated that the hydrodechlorination process led to the increase in the concentration of chlorine and the drop in the amount of surface accessible palladium, while the quantity of Pd-Cl bounds did not change considerably. It suggested that the presence of ZrO2 protected the surface of the studied catalyst against the formation of PdCl2.

Comparative IR spectroscopic study of Pt- and Pd-containing zeolites in the hydrodechlorination reaction of carbon tetrachloride

Hydrodechlorination of CCl 4 was investigated on noble metal containing zeolite catalysts by IR spectroscopy. Both the surface species and the gas phase reaction products were simultaneously analysed. Results revealed that the reaction rate and the reaction products were different on Pt- and Pd-containing samples. The enhanced formation of ethane and the suppressed generation of methane over Pd/NaY-FAU zeolite and the formation of chloroform as intermediate product over Pt/NaY-FAU zeolite were explained by the different bond strength of reaction intermediates to the metal surface. The influence of acid sites on the product distribution was traced back to the local decomposition of zeolite framework. A combination of IR spectroscopic methods including simultaneous analysis of gas phase products and surface intermediates proved to be an excellent tool for investigating the reaction mechanism.

Hydrodechlorination of dichlorodifluoromethane, carbon tetrachloride and 1,2-dichloroethane over Pt/Al2O3 catalysts

A comparative study of hydrodechlorination of dichlorodifluoromethane, carbon tetrachloride and 1,2-dichloroethane over alumina-supported Pt catalysts showed several interesting features in the catalytic behavior. Platinum proved to be very good catalyst in CCl4 hydrodechlorination (with high selectivity toward CHCl3), whereas for CCl2F2 and 1,2-dichloroethane hydrodechlorinations this metal exhibits rather low activity and, in addition, converts the reactants to less valuable products (e.g. alkanes). However, a rapid deactivation of Pt catalysts during hydrodechlorination requires their frequent regeneration. Two regeneration protocols tested in this work showed interesting, albeit somewhat different, activity evolutions. Removal of chlorine species by hydrogen at 350 °C led to a moderate increase of catalytic activity. On the other hand, a partial removal of carbonaceous residues by oxidative pretreatment brought about a considerable increase of the overall conversion level in CCl4 hydrodechlorination. Additional temperature-programmed experiments with spent and regenerated Pt/Al2O3 catalyst suggest that a partial removal of carbonaceous deposit (most probably followed by its reconstruction) leading to the formation of very active catalyst, less susceptible to poisoning by chlorine.

Effect of tungsten addition to Pd/ZrO2 system in the hydrodechlorination activity of CCl2F2

Abstract The catalytic activity of Pd on W-ZrO 2 in the hydrodechlorination of CCl 2 F 2 was studied by varying WO 3 content. The TPR results of Pd/W-ZrO 2 were related to effects caused by contact with hydrogen. XRD data of spent catalysts shows that there is no change in the structure of zirconia indicating that these catalysts are highly resistant to the corrosive reaction atmosphere. Addition of tungsten to Pd/ZrO 2 system led to the formation of CHClF 2 (HCFC-22) in the hydrodechlorination of CCl 2 F 2 . Thus, the role of WO 3 is to catalyze the partial dechlorination of CCl 2 F 2 .