Rubén López-Fonseca

Enhanced activity of zeolites by chemical dealumination for chlorinated VOC abatement

Abstract The objective of this work is to evaluate the dealumination via ammonium hexafluorosilicate treatment as an effective method for enhancing the catalytic performance of H-Y zeolite in chlorinated VOC oxidative destruction. A series of Y zeolites with various Si/Al ratios was prepared from a commercial sample and then tested for the catalytic decomposition of 1,2-dichloroethane as a model reaction. In general, these modified Y zeolites exhibited a higher activity than that of the parent material, the zeolite subjected to 50% dealumination resulting the most active catalyst. This increase in activity was associated with the development of strong acidity due to dealumination. Likewise, 50% dealuminated sample showed an improved catalytic behaviour for the destruction of other typical chlorinated pollutants, namely, dichloromethane (DCM) and trichloroethylene (TCE). The ease of destruction was found to follow this trend: 1,2-dichloroethane > dichloromethane > trichloroethylene.

Gas-phase catalytic combustion of chlorinated VOC binary mixtures

Abstract The complete oxidation of 1,2-dichloroethane (DCE), dichloromethane (DCM) and trichloroethylene (TCE) as individual chlorohydrocarbons and of their binary mixtures over protonic zeolites (H-ZSM-5, H-MOR and chemically dealuminated H-Y zeolite) has been studied using a conventional fixed bed flow reactor. Its catalytic performance was associated with the presence of strong Bronsted acidity. Both H-MOR and chemically dealuminated H-Y zeolites exhibited the highest destruction activity for the abatement of single and binary mixtures. The destruction of chlorinated mixtures induced an inhibition of the reactivity of each compound leading to a significant increase in the ignition temperature, which varied form one compound to other. On the other hand, a moderate decrease in chlorinated intermediates/by-products formation was noted. Likewise, the presence of hydrogen, either as a part of the additional chlorinated molecule or as a part of water generated as a reaction product, was determined to be important for efficient combustion of the chlororganics to HCl.

State of the art in catalytic oxidation of chlorinated volatile organic compounds

Chlorine-containing organic compounds (Cl-VOC) require special attention due to their distinct toxicity, high stability and persistence in the environment. Removal of Cl-VOC by catalytic oxidation over a wide variety of catalysts has been presented in literature. This paper reviews the state of the art in this subject, including different model compounds, nature of catalysts, and oxidation activity. Catalyst selectivity (CO2 vs. CO and HCl vs. Cl2), by-products formation and the causes of deactivation are also analyzed as the most important factors in the catalyst selection for practical applications.

Role of water and other H-rich additives in the catalytic combustion of 1,2-dichloroethane and trichloroethylene.

In several practical applications gas streams containing chlorinated volatile organic compounds with variable chemical nature (namely, 1,2-dichloroethane and trichloroethylene) and a significant moisture content (15000ppm) must be addressed. In this paper the control of such emissions by catalytic oxidation over Ce/Zr mixed oxides was analysed. Results in terms of activity and selectivity were compared with those obtained when other H-rich additives (1000ppm), such as hexane or toluene, were fed. High activity was found from mixed oxides featuring a suitable combination of a large population of acid sites, easily accessible oxygen species, and hydrophobic nature attributable to cerium content. The presence of additional H-rich compounds in the feed stream (water, toluene or hexane) tended to decrease the catalytic activity due to the blockage and/or competition for actives sites. However, the increased presence of hydrogen atoms in the stream notably promoted the selectivity to hydrogen chloride instead of molecular chlorine.

Deep catalytic oxidation of chlorinated VOC mixtures from groundwater stripping emissions

The total oxidation chlorinated VOCs mixtures, with composition similar to those expected from groundwater air strippers, over Pd/Al2O3 and Pt/A2O3 catalyst at temperatures from 200 to 550°C was investigated. 1,2-Dichloroethane showed greater oxidability, followed by dichloromethane and 1,1- dichioroethylene and trichloroethylene. The addition of hydrocarbons to a chlorinated feed produced changes in the reactivity order of the chlorinated VOCs, indicating the existence of a complex reaction system due to the energetically non-uniform catalytic surface.

Catalytic behaviour of H-type zeolites in the decomposition of chlorinated VOCs

Abstract Several H-type zeolites were investigated for their activity and selectivity during TCE oxidative decomposition in dry and humid conditions. These catalysts showed high activity resulting H-MOR to be the most active. The catalytic behaviour appeared to be determined by strong Bronsted acidity. The main oxidation products were CO, CO 2 , HCl and Cl 2 . Water enhanced the catalytic activity at lower temperatures and also promoted the selectivity to deep oxidation products generation (CO 2 and HCl).

Oxidation of chlorinated alkanes over Co3O4/SBA-15 catalysts. Structural characterization and reaction mechanism

Cobalt oxide-based catalysts have been synthesised via a hard template of mesoporous silica SBA-15 in the form of massive Co3O4 nanoparticles, whose dimensions are controlled and limited by the support mesoporosity. Over this family of catalysts, both weak Bronsted and Lewis acidities have been detected, with the relative abundance a function of the cobalt content. The mesoporosity of the support leads to the growth of oxide nanoparticles, mainly in the small pores, thus improving their redox properties. Catalysts possessing cobalt oxide loadings higher than 30% present an adequate activity for the deep DCE oxidation toward carbon dioxide, hydrogen chloride and chlorine. It is believed that the reaction is markedly accelerated due to the simultaneous participation of the acid sites (where the chlorinated feed is efficiently adsorbed) and the redox sites (oxidation of the adsorbed feed with lattice oxygen anions). FTIR data on dichloroalkane oxidation evidence that lattice oxygen species are mainly involved in the Cl-VOC combustion, likely through a Mars–van Krevelen mechanism.

Effect of Pd addition on the catalytic performance of H-ZSM-5 zeolite in chlorinated VOCs combustion

The aim of this work was to evaluate the influence of the addition of palladium on the catalytic behaviour of H-ZSM-5 zeolite in the combustion of 1,2-dichloroethane (DCE) and trichloroethylene (TCE). Both catalysts showed similar activity in the oxidation of DCE, by contrast, the metal loading led to a substantial improvement in TCE combustion. Vinyl chloride was detected as an intermediate in DCE conversion, and it was appreciably suppressed when adding Pd to the zeolite. In TCE oxidation trace amounts of tetrachloethylene were identified as a by-product, Pd/H-ZSM-5 showing larger quantities of this undesired by-product. Pd/H-ZSM-5 was more selective towards CO2 formation instead of CO, which was the major carbon-containing product formed over H-ZSM-5. However, H-ZSM-5 zeolite showed a lower selectivity to Cl2 generation while the metal loaded zeolite considerably promoted the formation of this toxic by-product by the Deacon reaction.

Inverse gas chromatography as a technique for the characterization of the performance of Mn/Zr mixed oxides as combustion catalysts

Adsorption of different volatile organic compounds (trichloroethylene, TCE; 1,2-dichloroethane, DCE; n-hexane) over different manganese-zirconia mixed oxides (Mn(x)Zr(1-x)O(2)) - widely used as combustion catalysts - was studied by inverse gas chromatography. Adsorption isotherms (calculated in the Henry region), adsorption enthalpies (DeltaH(ads)), and dispersive (gamma(S)(D)) and specific (I(sp)) components of the surface energy have been determined at infinite dilution for the investigated compounds. Both the adsorption enthalpy and the specificity of the interaction of TCE and DCE over Mn(x)Zr(1-x)O(2) catalysts depend strongly on manganese content. Thus, the adsorption strength of the reactants over the active sites is closely related with both the surface acidity and the accessibility of the lattice oxygen. A great influence of the specific interaction on the catalytic pattern has been also noticed. Since I(sp) depends on the redox properties, it has been proved that the specific interaction is determined by the presence of bulk Mn(3)O(4), which hinders the mobility of the oxygen lattice, and MnO(x), with the contrary effect. Finally, the selectivity to oxidation products has been correlated with both the enthalpy of adsorption and the specific interaction parameter, decreasing the selectivity to HCl with the increase of the enthalpy of adsorption.

Catalytic Performance Of Chlorinated Ce/ZrMixed Oxides For Cl-VOC Oxidation

Contamination by chlorinated volatile organic compounds (Cl-VOCs) is currently a matter of considerable concern, since these compounds are considered as dangerous for their effect on human health and the environment. Catalytic oxidation appears as an attractive approach for the destruction of these hazardous compounds. Although the development of catalysts to produce CO2 and HCl is a topic of demanding interest, less consideration has been given to examining the catalytic stability during extended time on stream. Indeed the presence of HCl and/or Cl2 may induce alterations in the behaviour of the catalysts. The main scope of this work is to evaluate the catalytic behaviour of a series of Ce/Zr mixed oxides in the gas-phase oxidation of 1,2-dichloroethane submitted to a previous chlorination followed by calcination (550 oC for 4 hours in air) or by reduction (550 oC with 5%H2/Ar for 1 hour) and calcination. Chlorination led to remarkable changes in acidity and redox properties, which were shown to be the key catalytic properties of these catalysts for this reaction. On one hand, the chlorine reinforced the acidity of the catalytic surface. In contrast, redox properties were negatively impacted. However, these alterations were partially compensated leading to a slightly lower activity of chlorinated samples. It could therefore be concluded that Ce/Zr catalysts resulted in significant resistance to deactivation by chlorine poisoning, thereby showing a great potential for this environmental application, not only in terms of lowtemperature Cl-VOC conversion, but also of stability during extended periods of time on stream.