Hydrodechlorination Of Carbon Tetrachloride Research Articles

Constructing Ni4/Fe@Fe3O4-g-C3N4 nanocomposites for highly efficient degradation of carbon tetrachloride from aqueous solution

Catalytic hydrodechlorination is one of the most potential remediation methods for chlorinated organic pollutants. In this study, Ni4/Fe@Fe3O4-g-C3N4 (NFFOCN) nanocomposites were synthesized for carbon tetrachloride (CT) removal and characterized by SEM, XPS and FTIR. The characterization results demonstrated that the special functional groups of g-C3N4, especially NH groups, effectively alleviated the aggregation of nanoparticles. In addition, the C and N groups of g-C3N4 enhanced the catalytic dechlorination of CT by providing binding sites. The experimental results showed that NFFOCN could effectively remove CT over a wide initial pH range of 3–9, and the CT removal efficiency reached 94.7% after 35 min with only 0.15 g/L of NFFOCN at pH 5.5. The Cl−, SO42−, and HCO3− promoted the removal of CT, while HA and NO3− had the opposite effect. Furthermore, good sequential CT removal by NFFOCN nanocomposites was observed, and the CT removal efficiency reached 77.3% after four cycles. Based on the identification of products, a possible degradation pathway of CT was proposed. Moreover, the main mechanisms regarding CT removal included the direct reduction of nZVI (about 40.51%), adsorption (around 34.79%), and hydrodechlorination of CT by Ni0 using H2 (about 19.40%).

Hydrodechlorination of carbon tetrachloride with nanoscale nickeled zero-valent iron @ reduced graphene oxide: kinetics, pathway, and mechanisms.

In recent years, carbon tetrachloride (CT) has been frequently detected in surface water and groundwater around the world; it is necessary to find an effective way to treat wastewater contaminated with it. In this study, Ni/Fe bimetallic nanoparticles were immobilized on reduced graphene oxide (NF@rGO), and used to dechlorinate CT in aqueous solution. Scanning electron microscopy (SEM) demonstrated that the two-dimensional structure of rGO could disperse nanoparticles commendably. The results of batch experiments showed that the 4N4F@rGO (Fe/GO = 4 wt./wt., and Ni/Fe = 4 wt.%) could reach a higher reduction capacity (143.2 mgCT/gcatalyst) compared with Ni/Fe bimetallic nanoparticles (91.7 mgCT/gcatalyst) and Fe0 nanoparticles (49.8 mgCT/gcatalyst) respectively. That benefited from the nickel metal as a co-catalyst, which could reduce the reaction activation energy of 6.59 kJ/mol, and rGO as an electrical conductivity supporting material could further reduce the reaction activation energy of 4.73 kJ/mol as presented in the conceptual model. More complete dechlorination products were generated with the use of 4N4F@rGO. Based on the above results, the reductive pathway of CT and the catalytic reaction mechanism have been discussed.

Deactivation and regeneration of Pt/Al2O3 catalysts during the hydrodechlorination of carbon tetrachloride

Deactivation and regeneration of Pt/Al2O3 catalysts during the hydrodechlorination of carbon tetrachloride were studied. The effect of reactant partial pressures and temperature on the catalyst deactivation was investigated. A deactivation model with residual activity was developed to quantify the kinetic deactivation parameters. The effect of the regeneration atmosphere was also investigated. Regeneration under air allowed for the full recovery of the catalytic performance of fresh catalysts while treatments under flowing hydrogen resulted in a superior catalytic performance, increasing both the initial and residual activities. This was ascribed to a combined effect, redispersion of the metallic phase and formation of surface defects.

Multi-Wall Carbon Nanotubes as a Support for Platinum Catalysts for the Hydrodechlorination of Carbon Tetrachloride and Dichlorodifluoromethane

Multi-wall carbon nanotubes (MWCNTs) were used as a support for the deposition of highly dispersed platinum. After characterization by several physical techniques, the catalyst was studied in reactions for: hydrodechlorination of carbon tetrachloride and the hydrodechlorination of dichlorodifluoromethane. For the first reaction Pt/MWCNTs were very effective catalysts in terms of both the overall activity and the selectivity to CHCl3; both quantities appeared high and stable. For CCl2F2 hydrodechlorination the catalyst was rather moderate, although very stable, activity and product selectivities were established at a constant level in a relatively short time-on-stream. The MWCNTs-supported Pt particles do not undergo great changes during the reactions, i.e. neither substantial metal sintering occurred nor extensive surface carbonization/chloriding took place.

Hydrodechlorination of chloroalkanes on supported platinum catalysts

A comparative study of hydrodechlorination of carbon tetrachloride and 1,2-dichloroethane over supported Pt catalysts showed a large effect of support (C, Al 2 O 3 , SiO 2 ) on the catalytic behavior. High selectivity towards CHCl 3 (from CCl 4 ), -80%, was found for Al 2 O 3 - and SiO 2 -supported Pt. In 1,2-dichloroethane reaction, Pt/Al 2 O 3 appeared more active than the other catalysts but the least selective towards C 2 H 5 Cl, giving mainly ethane. Al 2 O 3 - and C-supported catalysts contained more chlorine and carbon after hydrodechlorination.

Hydrodechlorination over Zeolite Supported Catalysts—Clarification of Reaction Mechanism—

Chlorinated compounds emitted into the atmosphere are responsible for the current thinning and shrinking of the ozone layer in the stratosphere. Two main potential methods have been suggested for their transformation: oxidative destruction to form environmentally safe products or hydrodechlorination to form potentially valuable chemical compounds. In this review, performances of halogenated compounds over the zeolites and the noble metal-containing zeolite catalysts are characterized by different spectroscopic methods (IR, NMR, etc.) to clarify the reaction mechanism of hydrodechlorination over the catalysts. NMR studies provided evidence that chlorofluorocarbon reacts more aggressively with zeolites at elevated temperatures. The extent and nature of reaction depended on the zeolite type and the exchanged cation. Infrared spectra of the gas phase showed that HCl, CO2 and COCl2 were detected as the reaction products in the hydrodechlorination reaction of CCl2F2 over HY. The phosgene intermediate is believed to be the key to the structural damage of the zeolite. Pt-containing zeolite catalyst was more active in the hydrodechlorination of carbon tetrachloride than the Pd-containing catalyst, and beside the main products (methane and HCl) chloroform as intermediate product was formed. On Pd-containing zeolite no chloroform, but high amounts of ethane as final products together with methane were formed. The activity and selectivity of the hydrodechlorination reaction of various chlorine-containing compounds (C1, C2, with and without fluorine content) on different metal-containing zeolites of various types is summarized.

Hydrodechlorination of carbon tetrachloride over PtNaX zeolite: Deactivation studies

Platinum-containing NaX zeolite showed high activity and high methane selectivity in the hydrodechlorination of carbon tetrachloride. The catalyst showed fast deactivation. To identify the possible causes of deactivation, the fresh and used catalysts were studied by thermogravimetric analysis, X-ray diffraction, thermoprogrammed oxidation, infrared spectroscopy and diffuse reflectance spectroscopy. The results indicate that the deactivation is probably caused by the attack of the HCl produced during the reaction to the zeolite structure forming an amorphous aluminosilicate. The HCl adsorption can also be pointed as a cause for deactivation. It is worthy to mention that no coke formation (oligomers) was observed, which is not consistent to some other observations made for other platinum-containing catalysts.

Interaction of –CClx (x=1–3) with Ru2 and RuSn dimers: a density functional study

Abstract Density functional calculation were performed on the Ru2 and RuSn metal dimers and the species formed from their interaction with –CClx (x=1–3) fragments. The importance of these fragments in the hydrodechlorination of carbon tetrachloride has been motivated this study aiming to contribute to understand the effect of the tin in the performance of the noble metals based catalysts. We have observed that the carbon tetrachloride does not form a precursor with the metal dimers. The CCl4 readily dissociates forming adsorbed chlorine and –CCl3 complexes. The chlorine atoms prefer to adsorb on the bridge sites and the RuSn–Cl binding energy is about 5 kcal mol−1 larger than the Ru2–Cl binding energy. The Ru2–CClx (x=1–3) binding energies are larger than the respective RuSn–CClx (x=1–3) binding energies. However, the reaction energy of the dechlorination of CCl4 leading to adsorbed –CClx (x=1–3) and chlorine is thermodynamically more favorable for RuSn than the Ru2. The differences between Ru2 and RuSn systems have been discussed based on the different interaction mechanism due to the presence of the tin center and its affinity for the chlorine atoms.

Hydrodechlorination of carbon tetrachloride in the liquid phase on a Pd/carbon catalyst: kinetic and mechanistic studies

Activity and selectivity of a 1wt.% Pd/carbon catalyst in the hydrodechlorination of carbon tetrachloride into chloroform was studied in the liquid phase as a function of temperature (353–413K) and reaction time. The catalyst was prepared by incipient wetness impregnation and reduced at 523K, and the reaction performed in a three-phase bubble column reactor at 3MPa. Reaction is highly selective towards chloroform, with dichloromethane, monochloromethane, hexachloroethane, tetrachloroethylene and methane as by-products. Increasing the temperature mainly favors the production of hexachloroethane and tetrachloroethylene, though the chloroform selectivity remains always above 90% even at conversion levels even higher than 90%. The reaction fits a zero order related to carbon tetrachloride up to 60% conversion at 413K and even to higher conversions at lower temperatures. From the analysis of both product distribution versus time and reactivity of the reaction products, a reaction pathway is proposed. The results indicate that chloroform, hexachloroethane and methane are primary products, and that the carbon tetrachloride transformation proceeds via three parallel and independent paths. Chloroform, dichloro- and monochloromethanes are formed from tetrachloromethane by successive hydrodechlorination reactions (Path I), while hexachloroethane and tetrachloroethylene follow other parallel consecutive path (Path II). Methane is directly produced from tetrachloromethane (Path III). Paths I and II are conventional consecutive reactions in which the intermediates are desorbed to the gas/liquid phase as stable molecules and further readsorbed in order to give the following chemical species. In contrast, for methane the overall consecutive process occurs in the adsorbed phase, on the same active site, without the appearance of any intermediate molecular species.

Modification of the supported palladium catalysts surface during hydrodechlorination of carbon tetrachloride

Hydrodechlorination (HDC) of carbon tetrachloride (CCl 4) was studied in the presence of 0.5–5% palladium catalysts supported on carbon material Sibunit and TiO 2. Experiments were performed in a flow-type reactor in hydrogen at 150–220 °C. Non-chlorinated C 1–C 5 hydrocarbons were formed in the presence of all catalysts. Stable work (more than 4 h) was observed in the most cases. Conversion of carbon tetrachloride was 100%. It was assumed, that Cl, which was formed as intermediate during dissociative adsorption of CCl 4, oxidizes palladium on the catalytic surface generating active sites of oligomerization. The surface of the catalyst is covered by carbonaceous deposits during the HDC, resulting in deactivation of the catalyst. These deposits are probably a little different in nature for Pd/Sibunit and Pd/TiO 2 catalysts. For Pd/Sibunit carbonaceous deposits are more hydrocarbonaceous in nature. These carbonaceous deposits can be removed by heating in argon-flow to regenerate the catalyst.

Hydrodechlorination of CCl4 over group VI transition metal carbides

Hydrodechlorination of carbon tetrachloride (CCl4) was investigated on group VI metal carbides (WC, W2C, Mo2C) prepared by temperature programmed carburization of WO3 and MoO3 in 10–20% CH4/H2 mixtures. Their catalytic performances have been compared with those of 1wt.% Pt/γ-Al2O3 catalyst. Experiments were carried out at atmospheric pressure in a fixed bed flow reactor at 120 and 250°C with a molar ratio H2/CCl4=12. WC shows the best selectivity towards chloroform (CHCl3), whatever the temperature and particularly at 120°C (selectivity above 99%) while W2C and Mo2C produce tetrachloroethylene in substantial amount. Mo2C and W2C deactivate rapidly with time irrespective of the temperature, whereas WC exhibits a good stability at 120°C. Surface characterization indicates that the deactivation on M2C (M=Mo, W) could be correlated to the formation of a chlorocarbonaceous polymer.

Carbon tetrachloride hydrodechlorination with organometallics-based platinum and palladium catalysts on MgO

Hydrodechlorination of carbon tetrachloride was performed at atmospheric pressure over Pd and Pt catalysts supported on various inorganic oxides (Al 2O 3, MgO, CeO 2) and obtained from different organometallic precursors (Pd(acac) 2, [Pd(C 3H 5)Cl] 2, Pt(acac) 2 and Pt(CH 3CN) 2Cl 2. Generally, Pd-based catalysts show low conversion levels and high selectivity to C 1–C 7+ hydrocarbons with a distribution that fits a typical Schulz–Flory mechanism for alkyl polymerization. On the other hand, Pt catalysts show high and stable activity, the major products are CHCl 3 and in lesser extent methane. Catalytic performances depend on the stability, under reaction conditions, of the metal phase, that could be related to the nature of chlorine species pre-adsorbed on catalysts. Owning to their superior catalytic performances, the structural modifications of Pt catalysts under reaction conditions were characterized by EXAFS spectroscopy.

Effects of Pt Precursors on Hydrodechlorination of Carbon Tetrachloride over Pt/Al2O3

Hydrodechlorination of carbon tetrachloride (CCl4) has been studied over 1% Pt/Al2O3prepared from various Pt precursors. The Pt/Al2O3catalyst prepared from Pt(II) precursors showed stable, high conversion of CCl4above 99% and selectivity of CHCl3above 78% at the reaction temperature of 393 K, H2/CCl4mole ratio of 9 and weight hourly space velocity of 9000 liters/kg/h. Relative to fresh catalyst, amounts of carbon and chlorine increased in catalysts which had been used for the reaction and, in particular, when the catalysts showed deactivation. Used catalyst showed a decrease in the amount of H2chemisorption and the decrease was greater for catalysts prepared from Pt(IV) precursors than those from Pt(II) precursors. The catalyst prepared by Pt(NH3)2(NO2)2indicated a retarded coke formation compared to that from H2PtCl6. From temperature-programmed reduction, temperature-programmed oxidation, and X-ray absorption fine structure measurements, it was concluded that the difference between the catalysts prepared from Pt(II) and Pt(IV) precursors was due to the different oxidation states of Pt.

Hydrodechlorination of Carbon Tetrachloride over Pt/MgO

Hydrodechlorination of carbon tetrachloride (CCl4) has been studied at various reaction temperatures, mole ratios (H2/CCl4), and WHSVs over 1% Pt/MgO. The main factors determining conversion of CCl4and deactivation of the catalyst were reaction temperature and H2/CCl4mole ratio. The optimum reaction conditions for obtaining stable and high conversions of CCl4above 90% were the reaction temperature of 413 K, H2/CCl4mole ratio of 9, and WHSV of 9000 liters/kg/h. The surface area of catalyst decreased due to phase change of MgO to MgCl2·xH2O during reaction. Relative to fresh catalyst, the amounts of carbon and chlorine increased and that of oxygen decreased in used catalyst, especially in deactivated catalyst. The catalyst used for the steady-state reaction did not chemisorb H2at room temperature, but chemisorbed nearly the same amounts of H2as those observed for the fresh catalyst at 373 K. From the XPS and XAFS measurements the active phase of Pt during hydrodechlorination appeared to be a surface Pt(II) species with Cl ligands, while the bulk remained as Pt metal.