TCE Conversion Research Articles

Regulation of the liquid–solid interface of Cs catalysts for the synthesis of 1,1-Dichloroethylene from 1,1,2-Trichloroethane

There remain considerable challenges for Cs-based catalysts in gas-phase dehydrochlorination of 1,1,2-trichloroethane (TCE) due to their poor catalytic performance and harsh reaction temperature requirements. In this work, we report a novel supported ionic liquid (IL) phase Cs-based system and evaluate its catalytic performance and mechanism, combining the kinetic and DFT calculations. The optimal Cs-IL/SiO2 catalyst achieved the TCE conversion of 60% and the selectivity to 1,1-Dichloroethylene (VDC) of 86% under the mild temperature of 200 °C. It is demonstrated that IL additives can significantly improve the dispersion of Cs species during the catalyst preparation and evaluation process. Moreover, IL promotes the activation of TCE and enhances the desorption efficiency of HCl, which is the main reason for its high activity. The selectivity is improved by changing the adsorption behaviors of the products, which ensures the preferential formation of the main product VDC. The enhanced activity and selectivity of Cs-IL/SiO2 suggest a promising candidate for dehydrochlorination reaction.

Tuning the band-gap and enhancing the trichloroethylene photocatalytic degradation activities of flower-like Ni-doped SnS2/SnO2 heterostructures by partial oxidation

A hydrothermal procedure was used to synthesis flower-like 0–0.5 wt% Ni-doped SnS2, oxidation extent of which was tuned in air at 350 °C for various times. Photocatalysts were dip-coated on coiled wire-meshes including a light source for oxidation of 500 ppm trichloroethylene in air. FE-SEM revealed that the SnS2 flower-like structure integrity was preserved after partial oxidation, while surface areas were dramatically enhanced. XRD, HR-TEM, Raman, XPS and PL showed formation of fine crystalline nanoparticles and defects by oxidation and resulted in enhanced photocatalytic activity. Band gaps and defects recombination rate of heterostructures were tuned by partial oxidation and Ni doping. 43% oxidation of 0.34 wt% Ni-doped SnS2 results in increased surface area by 7.7 times to 100 m2/g, quenched electrons and holes recombination, increased the band gap, and showed the highest TCE conversion of 88%, while the photocatalyst with no oxidation showed no activity. Further addition of Ni declined the surface area and photocatalytic conversion.

Ce-modified zeolite BEA catalysts for the trichloroethylene oxidation. The role of the different and necessary active sites

This paper reports the activity of different Ce-BEA zeolites for the catalytic oxidation of trichloroethylene and it is focused on determining the nature of the catalyst active sites. The study was made by using a microporous zeolite BEA, two types of desilicated BEA zeolites and mildly steamed desilicated BEA zeolites. The catalysts were prepared by introducing Ce to the zeolites with incipient wetness impregnation and their structural, textural, and acidic properties were established. The evolution of TCE conversion was correlated with the physicochemical properties of the zeolites. It is shown that highly developed mesopore surface area, well-dispersed cerium species and a high number of Brønsted sites results in the highest activity. The activity and selectivity of the Ce-loaded zeolites were found to be dependent on the number of high strength Brønsted acid centres. The hierarchical materials with a higher density of hydroxyls showed higher yields to HCl while the formation of chlorine was prevented.

A Novel Synthetic Route to Prepare High Surface Area Mayenite Catalyst for TCE Oxidation

Mayenite (Ca12Al14O33) was synthesized by a novel route based on the use of polymethyl methacrylate (PMMA) as a soft templating agent. The material was tested for the total oxidation of trichloroethylene in the gas phase and the catalytic performance was analysed when using different initial amounts of PMMA in the catalyst synthesis. The results were compared with those obtained with a mayenite synthetized by a classical hydrothermal method. The highest activity in terms of TCE conversion was achieved in the presence of mayenite prepared using 10% w/w of PMMA; its activity was also higher than that of the hydrothermal mayenite. The surface area and the number of superoxide anions (O2−) seem to be the main properties determining the catalytic activity of the material.

Highly efficient Mg(OH)Cl/SiO2 catalysts for selective dehydrochlorination of 1,1,2-trichloroethane

A series of Mg catalysts supported on SiO2 were prepared by an incipient wetness impregnation method and tested for gas phase dehydrochlorination of 1,1,2-trichloroethane. It was found that these catalysts were very active and selective for the reaction. The catalytic performance depended on the Mg loading rather than the Mg precursors as the catalysts using Mg(NO3)2·6H2O and MgCl2·6H2O as the precursors showed the similar performance. A catalyst containing 10wt.% of Mg showed the best performance with a steady state TCE conversion of 92% and cis-dichloroethene selectivity of 91%. Moreover, characterizations of the catalysts revealed the formation of Cl-containing Mg species on the surface during the reaction. The analyses of the compositions of the stable catalysts under working conditions indicated a Cl/Mg ratio of 1, suggesting that Mg(OH)Cl could be the active sites for the reaction.

Photochemical degradation of trichloroethylene by sulfite-mediated UV irradiation

This study investigates the degradation of trichloroethylene using advanced reduction processes (ARP), which was developed by combining activation methods and reducing agents to produce reactive reducing radicals that can destroy persistent oxidized contaminants. Screening experiments were performed using a combination of three reducing reagents (sulfite, sulfide, and dithionite) and three UV light sources (monochromatic light at 254nm and two UV lamps with primary energy peaks at 365 and 312nm, respectively). The screening test results showed that the highest TCE removal efficiency was obtained by the combination of sulfite with low-pressure UV light, monochromatic at 254nm. The effectiveness of sulfite/UV-L ARP for dechlorinating TCE with regard to the initial TCE concentration, sulfite dose, solution pH, and light intensity was evaluated. Additionally, the reaction mechanisms and kinetics of TCE degradation were proposed. The photochemical degradation of TCE in ARP is attributed to sulfite radicals and hydrated electrons and generally followed first-order decay kinetics. At high pH and high sulfite dose, TCE was completely degraded, with chloride ions as the major reaction product. The TCE dechlorination rate constant increased as the sulfite dose, pH, and light intensity increased. This study provides the effectiveness of sulfite/UV-L ARP for TCE dechlorination and suggests optimum conditions that result in complete TCE conversion to a nonhazardous byproduct, chloride ions, and a rapid TCE dechlorination rate.

Plasma-catalysis of low TCE concentration in air using LaMnO3+δ as catalyst

The oxidative decomposition of trichloroethylene (TCE; 500–600ppmv) was investigated in synthetic (dry air) and humid air (RH=18%) with NTP (non thermal plasma) at atmospheric pressure, both in the absence and presence of lanthanum manganite catalyst at 150°C. In the absence of catalyst, TCE removal is enhanced with humidity (RH=18%) while O3 production is decreased. However, whatever the carrier gas the carbon mass balances are poor amounting to 25–30% due to the presence of polychlorinated by-products such as phosgene, dichloroacetyl chloride (DCAC) and trichloroacetaldehyde (TCAD). In the presence of catalyst, the TCE conversion significantly increased as compared to that of the plasma alone system. The perovskite catalyst can dissociate the in plasma produced ozone to oxygen radicals that decompose TCE. In dry air, the carbon mass balance keep rather poor (35% – 460J/L) due to the structural transformation of the catalyst with chlorine. In dry air, an optimum temperature of the catalytic reactor is found to be 100°C resulting from a compromise between catalyst deactivation and enhancement of the catalytic reactions. However, water remarkably promotes the carbon mass balance (75% – 460J/L) due to the beneficial role of water which acts as a chlorine scavenger at the surface of the catalyst, therefore retarding the perovskite degradation.

Behaviour of TiO2–SiMgOx hybrid composites on the solar photocatalytic degradation of polluted air

The photocatalytic performance of TiO2–SiMgOx ceramic plates for trichloroethylene abatement in gas phase has been evaluated under sun irradiance conditions. A continuous flow Pyrex glass reactor fixed on the focus of a compound parabolic collector has been used. The performance of the hybrid photocatalyst has been evaluated as the variation of TCE conversion and reaction products formation with the solar irradiance at different total gas flow, TCE concentration, and water vapour content. SiMgOx not only provides adsorbent properties to the photocatalyst, but it also allows the effective use of the material during low solar irradiance conditions. The adsorption–desorption phenomena play a pivotal role in the behaviour of the system. Thus, TCE conversion curves present two different branches when the sun irradiance increases (sunrise) or decreases (sunset). CO2, COCl2 and DCAC were the most relevant products detected. Meanwhile CO2 concentration was insensitive to the branch analysed, COCl2 or DCAC were not indicating the ability of these compounds to be adsorbed on the composite. An increase of the UV irradiation at total TCE conversion promotes the CO2 selectivity. The excess of energy arriving to the reactor favours the direct reaction pathway to produce CO2. The photonic efficiency, calculated as a function of the rate of CO2 formation, decreases linearly with the solar irradiance up to around 2mWcm−2, where it becomes constant. For decontamination systems high TCE conversion is pursuit and then high solar irradiance values are required, in spite of lower photonic efficiency values. The present photocatalyst configuration, with only 17% of the reactor volume filled with the photoactive material, allows total TCE conversion for 150ppm and 1Lmin−1 in a wide sun irradiance window from 2 to 4mWcm−2. The incorporation of water vapour leads to an increase of the CO2 selectivity keeping the TCE conversion around 90%, although significant amounts of COCl2 were observed.

Reactivity and antimicrobial properties of nanostructured titanium dioxide

The photo-induced reactivity of three nanostructured titanium dioxides including TiO 2 nanoparticles (i.e., nano-TiO 2), titania–silica aerogel and nanotextured TiO 2 film, were investigated for gas phase photocatalytic oxidation of trichloroethylene and inactivation of Bacillus subtilis and Escherichia coli microbial cells. A correlation was observed between photoreactivity for TCE conversion and bactericidal activity within TiO 2 of similar nanostructures and composition (i.e., elemental and phase). This indicates that the photogenerated radical species from UV-irradiated TiO 2 responsible for TCE photo-oxidation also play a major role in the inactivation of microbial cells. The superb bactericidal activities of titania–silica aerogels (i.e., up to 6log reductions in viable B. subtilis) and nanotextured TiO 2 film (3.4log reduction) compared to commercial Degussa P25 TiO 2 (0.64log reduction) and optimum nano-TiO 2 (1.3log reduction) far exceeds the observed activity enhancement for TCE photo-oxidation. This implies that nanoscale chemical and structural environment also contribute towards the bactericidal activities.

Improved performance of non-thermal plasma reactor during decomposition of trichloroethylene: Optimization of the reactor geometry and introduction of catalytic electrode

The decomposition of trichloroethylene (TCE) by non-thermal plasma was investigated in a dielectric barrier discharge (DBD) reactor with a copper rod inner electrode and compared with a plasma-catalytic reactor. The particularity of the plasma-catalytic reactor is the inner electrode made of sintered metal fibers (SMF) coated by transition metal oxides. In order to optimize the geometry of the plasma reactor, the efficiency of TCE removal was compared for different discharge gap lengths in the range of 1–5mm. Shorter gap lengths (1–3mm) appear to be more advantageous with respect to TCE conversion. In this case TCE conversion varies between 67% and 100% for input energy densities in the range of 80–480J/l, while for the 5mm discharge gap the conversion was lower (53–97%) for similar values of the input energy. As a result of TCE oxidation carbon monoxide and carbon dioxide were detected in the effluent gas. Their selectivity was rather low, in the range 14–24% for CO2 and 11–23% for CO, and was not influenced by the gap length. Several other chlorinated organic compounds were detected as reaction products.When using MnOx/SMF catalysts as the inner electrode of the DBD reactor, the TCE conversion was significantly enhanced, reaching ∼95% at 150J/l input energy. The selectivity to CO2 showed a major increase as compared to the case without catalysts, reaching 58% for input energies above 550J/l.

The influence of surface properties on the photocatalytic activity of nanostructured TiO 2

Nanostructured TiO 2 with 11- (P11) and 5-nm (P5) crystal sizes were prepared by a modified sol–gel method. Controlled crystallization and a pretreatment process were employed to obtain TiO 2 with different surface roughness and degree of hydroxylation, while maintaining an identical crystal (i.e., 11 or 5 nm) and aggregate (i.e., 100 nm) sizes, phase structure (i.e., anatase), and crystallinity (i.e., X-ray diffraction peak intensity). Using the photooxidation of airborne trichloroethylene as a probe reaction, we were able to identify that the hydroxyl groups on low-coordinated titanium atoms are responsible for the generation of dichloroethylene and dichloroacetaldehyde by-products. Their presence usually means lower TCE conversion and in some cases leads to catalyst deactivation.

Pd-catalyzed TCE dechlorination in water: effect of [H2](aq) and H2-utilizing competitive solutes on the TCE dechlorination rate and product distribution.

The aqueous-phase H2 concentration ([H2](aq)) and the presence of H2-utilizing competitive solutes affect TCE dechlorination efficiency in Pd-based in-well treatment reactors. The effect of [H2](aq) and H2-utilizing competing solutes (cis-DCE, trans-DCE, 1,1-DCE, dissolved oxygen (DO), nitrite, nitrate) on the TCE transformation rate and product distribution were evaluated using 100 mg/L of a powdered Pd-on-Al2O3 catalysts in batch reactors or 1.0 g of a 1.6-mm Pd-on-gamma-Al2O3 catalyst in column reactors. The TCE dechlorination rate constant decreased by 55% from 0.034 +/- 0.006 to 0.015 +/- 0.001 min-1 when the [H2](aq) decreased from 1000 to 100 microM and decreased sharply to 0.0007 +/- 0.0003 min-1 when the [H2](aq) decreased from 100 to 10 microM. Production of reactive chlorinated intermediates and C4-C6 radical coupling products increased with decreasing [H2](aq). At an [H2](aq) of 10 microM (P/Po = 0.01), DCE isomers and vinyl chloride accounted for as much as 9.8% of the TCE transformed at their maximum but disappeared thereafter, and C4-C6 radical coupling products accounted for as much as 18% of TCE transformed. The TCE transformation rate was unaffected by the presence of cis-DCE (202 microM), trans-DCE (89 microM), and 1,1-DCE (91 microM), indicating that these compounds do not compete with TCE for catalyst active sites. DO is twice as reactive as TCE but had no effect on TCE conversion in the column below a concentration of 370 microM (11.8 mg/L), indicating that DO and TCE will not compete for active catalyst sites at typical groundwater DO concentrations. TCE conversion in the column was reduced by as much as a factor of 10 at influent DO levels greater than 450 mM (14.3 mg/L) because the [H2](aq) fell below 100 microM due to H2 utilized in DO conversion. Nitrite reacts 2-5 times slower than TCE and reduced TCE conversion by less than 4% at a concentration of 6630 microM (305 mg/L). Nitrate was not reactive and did not effect TCE conversion at a concentration of 1290 microM (80 mg/L).

Trichloroethene degradation in a two-step system by methylosinus trichosporium OB3b. Optimization of system performance: use of formate and methane.

The breakdown of dissolved TCE in a two-step bioremediation system is described. In the first reactor, the organism Methylosinus trichosporium OB3b is grown; in the second reactor, consisting of three 17-L column reactors in series, the cells degrade TCE. A special design allowed both for the addition of air (uG,s = 0.01-0. 04 mm s-1) in the conversion reactor to prevent oxygen limitation while minimizing stripping of TCE, and for the use of methane as exogenous electron donor. In two-step systems presented thus far, only formate was used (excess, 20 mM). We found formate additions could be reduced by 75% (15 degrees C), whereas small amounts of methane (0.02-0.04 mol CH4/g cells) could replace formate and led to equally optimal results. Example calculations show that up to 90% reduction in operating cost of chemicals can be obtained by using methane instead of formate. A model was developed to describe each of the conditions studied: excess formate and optimal methane addition, suboptimal formate addition and suboptimal methane addition. Using parameters obtained from independent batch experiments, the model gives a very good description of the overall TCE conversion in the two-step system. The system presented is flexible (oxygen/methane addition) and can easily be scaled up for field application. The model provides a tool for the design of an effective and low-cost treatment system based on methane addition in the conversion reactor.

Effects of catalyst composition on dual site zeolite catalysts used in chlorinated hydrocarbon oxidation

Cobalt exchanged and chromia impregnated Y zeolite catalysts (Co-Y/CA ) have been found to show good activity and selectivity for low-molecular-weight chlorohydrocarbon (CH 2Cl 2, C 2HCl 3, CCl 4, etc.) oxidation in the range of 250–350°C. For such dual site catalysts, the effects of cobalt and chromia composition on the catalytic properties were investigated. Higher cobalt exchange increased the acidity and the oxygen adsorption capacity of the Co-Y zeolite without affecting the surface area, whereas increasing Cr 2O 3 impregnation caused increased dealumination and consequent loss of acidity. Higher loading of Cr 2O 3 also reduced catalyst surface area by combination of structural loss and pore/channel blockage. Increasing cobalt exchange produced improved conversion of methylene chloride (CH 2Cl 2) while higher loading of impregnated Cr 2O 3 was detrimental for methylene chloride conversion due to the associated loss of surface area and acidity. Methylene chloride conversion appeared to be primarily controlled by carbonium ion formation at the acidic sites followed by oxidation with the oxygen adsorbed at the cationic sites. Conversely, trichloroethylene (C 2HCl 3) oxidation was primarily determined by the impregnated Cr 2O 3 sites; low or no Cr 2O 3 produced poor conversion while increasing Cr 2O 3 loading increased the conversion. However, an optimum impregnation level (⋍5%) seemed to be necessary to avoid poor activity either due to lack of sites or due to structural damage by overloading. Additionally, increasing cobalt exchange level in the Co-Y/CA catalysts also showed improved TCE conversion probably due to the increasing acidity. Therefore, TCE conversion seemed to be controlled by the initial adsorption at the Cr 2O 3 sites with subsequent reactions which may involve the acidic and/or cationic sites.