Total oxidation of chlorinated VOCs on supported oxide catalysts

Biomass-fed cogeneration units and waste incinerators have the advantages of producing efficiently heat and power and of reducing the amount of CO2 emitted per produced energy. However, they produce toxic polychlorinated VOCs (dioxins), CO and NOx. This thesis aims at developing a catalytic system for the total oxidation of chlorinated VOCs that: i) convert efficiently chlorinated VOCs below 250 °C and ii) resist to the exhaust co-pollutants (H2O, CO, NOx). Moreover, this thesis aims at having a complete understanding of the catalytic mechanism. Part I demonstrated that VOx/TiO2 based catalysts are very efficient in the total oxidation of chlorobenzene (taken as a model molecule). In particular, they proved to be highly resistant against deactivation. Moreover, Part I established that the addition of secondary phases producing VOx-WOx/TiO2 or VOx-MoOx/TiO2 induces a synergetic effect that improves the performances. Furthermore, the replacement of a classical TiO2 by a sulfated one improves the catalytic activity. In Part II, the investigation of the co-pollutants influence on the catalysts performances demonstrated their quite good resistance. Indeed, the presence of CO does not induce any deactivation of the catalysts while NOx induces a huge improvement of the catalysts ability to destroy chlorinated VOCs. This beneficial effect is explained by the in situ production of a strong oxidant (NO2) that speeds up the reoxidation of the reduced VOx sites. Nevertheless, H2O vapor can affect negatively the catalyst activity when present in a high concentration. Part III, by revisiting catalytic and characterization results exposed in Part I and II, demonstrated that the catalytic mechanism proceeds in four steps: i) adsorption of chlorinated VOCs on Brønsted sites, ii) VOx redox sites give some of their lattice oxygen atoms to oxidize the aromatic ring producing H2O and COx, iii) reoxidation of the VOx reduced sites thanks to the gas stream oxidant (O2) and iv) retrieving of the chlorine from the surface. The second and third steps compose a Mars and van Krevelen mechanism and the third one is the mechanism rate limiting step. Our work shows that the performances can be improved by tuning the redox properties of the VOx phases: i) improvement of the reducibility and ii) stabilization of the vanadium oxidation level above a limit value, estimated around 4.87.(AGRO 3)--UCL, 200

Systematic investigation of supported transition metal oxide based formulations for the catalytic oxidative elimination of (chloro)-aromatics - Part I: Identification of the optimal main active phases and supports

This paper reports, for the first time, a systematic investigation (screening) of the catalytic activity of 40 different formulations of transition metal oxides-based supported catalysts in the course of the total oxidation of benzene as a model molecule for dioxin. The catalysts consisted in 10 different transition metal oxides (CrOx, MnOx, VOx, SnOx, WOx, NbOx, TaOx, MoOx, ZrOx, and BiOx) supported on 4 different supports (2 kinds of TiO2, Al2O3 and SiO2). A theoretical coverage of 0.75 monolayer of active phase was chosen to minimize the formation of crystallites. XPS and XRD characterizations demonstrated the better spreading as monolayer of the active phases at the surface of titania than of Al2O3 and SiO2. The latter induces a poor dispersion of almost all the active phases as crystallites. The variation of spreading of the active phases on the different supports is governed by the difference in surface free energy and is fully explained by the "solid-solid wetting" concept. For almost all active phases, the conversion of benzene progressively improves when the support is changed from SiO2 to Al2O3 and, finally, titanias. The performances of the active phases exhibiting this behavior, are clearly dictated by their presence as well-dispersed monolayers at the surface of the support. The screening revealed CrOx, VOx and MnOx as the most active phases. The first two exhibit their best activity when spread as monolayer at the surface of TiO2 based supports (classical behavior). At the opposite, MnOx works best when present in the form of Mn3O4 crystallites on the SiO2 support. (c) 2006 Elsevier B.V. All rights reserved

Systematic investigation of supported transition metal oxide based formulations for the catalytic oxidative elimination of (chloro)-aromatics - Part II: Influence of the nature and addition protocol of secondary phases to VOx/TiO2

This contribution reports, for the first time, a systematic investigation of the effect of the adding secondary phases on the activity of VOx/TiO2 based catalysts in the course of the total oxidation of benzene and chlorobenzene as model molecules for dioxins. The catalysts consisted in binary formulations VOx-secondary phases (CrOx, MnOx, SnOx, WOx, NbOx, TaOx, MoOx, ZrOx or BiOx). Three synthesis pathways differing by the order of synthesis of both phases were investigated (1-2, 2-1 and 1,2 both together). Catalysts synthesized in a unique step demonstrated the best activities for the oxidation of benzene. Moreover, WO, and MOO, phases induce a synergetic effect with VOx. XPS demonstrated that this effect resides in the absence of mutual spreading between both phases preventing the VOx covering by the secondary phases and promoting the stabilization of the links VOx-TiO2. The investigation of different ratios WOx/VOx, and MoOx/VOx on the total oxidation of chlorobenzene was performed on binary formulations supported on classical TiO2 or on sulfated TiO2. For both supports and both binary formulations, the optimum ratio is equal to 1. This ratio is the highest that prevents the presence of crystallites of MoOx or WOx, which seems to be detrimental for the catalyst activity. Moreover, the resistance of the formulations in front of the chlorinated volatile organic compounds (Cl-VOCs) and the better activity of the formulations supported on sulfated TiO2 were proven. The activation effects brought by WOx, MoOx and by sulfated TiO2 are linked to the increase of the number of Bronsted acid sites as proven by FTIR with adsorbed pyridine. Moreover, the strong Lewis sites present on the sulfated TiO2 promote the dispersion of the active and secondary phases. Furthermore, these investigations pointed out the importance of the presence of Bronsted sites for the adsorption of VOCs and Cl-VOCs aromatics. (c) 2006 Elsevier B.V. All rights reserved

Catalysts for chlorinated VOCs abatement: Multiple effects of water on the activity of VOx based catalysts for the combustion of chlorobenzene

The influence of H2O on the performances of VOx/TiO2, VOx-WOx/TiO2 and VOx-MoOx/TiO2 catalysts is investigated in the combustion of chlorobenzene. H2O proves to influence in opposite manners the chlorobenzene conversion depending on its concentration and on the kind of catalysts. The overall influence of water is the sum of three effects, two negative: (1) the reduction of the vanadium phase dictated by its reducibility, itself influenced by the presence of WOx or MoOx, (2) the decrease of the number of strong Bronsted acid sites involved in the adsorption of the chlorobenzene and one positive (3) the retrieval of chlorine species from the Surface through the production of HCl. (c) 2005 Elsevier B.V. All rights reserved

Revisiting the Behaviour of Vanadia-Based Catalysts in the Abatement of (Chloro)-Aromatic Pollutants: Towards an Integrated Understanding

For several years, the authors study VO (x) /TiO2 catalytic formulations in the context of the oxidative abatement of aromatics, possibly chlorinated, from polluted air. This paper revisits their main findings and understandings. Although VO (x) /TiO2 catalysts, when upgraded with Mo or W oxides, are already used in industrial applications, it is shown that the systematic investigation (i) of the reasons why optimized synthesis and formulations are indeed efficient, (ii) of the nature of the sites responsible for the adsorption of the pollutants and their catalytic transformation, and (iii) of the corresponding mechanisms, is a valuable approach to identify guidelines and strategies for further improving the catalysts. The origin of the beneficial effects brought by WO (x) and MoO (x) to VO (x) /TiO2, and by the use of a sulphated titania support, is addressed, as well as the impact on the catalysts behaviour and performances of molecules inevitably present in a combustion exhaust gas, like CO and NO (x) co-pollutants and water. The behaviours of a chloro-aromatic molecule, like chlorobenzene, and an oxygen containing-heterocyclic molecule, like furan and dimethylfuran, are then compared and discussed in the debate of their selection as representative model compounds for the catalytic abatement of dioxins. In this context, it is shown that strategies developed to improve the performances of VO (x) /TiO2 formulations to abate Cl-aromatics, could be detrimental for cyclic molecules containing O-heteroatom, as entire dioxins. Finally, an integrated view of the catalytic process is presented by envisaging the case of a complex mixture, closer to the real application of gaseous effluent cleaning

The role of crystalline structure of molybdenum oxide catalysts onto the activity and stability in sulfoxidation of thioethers

Sulfoxidation of 2-thiomethyl-4,6-dimethyl-pyrimidine and methyl-dodecyl-sulfure was carried out in liquid phase using H2O2 in dioxane (6 or 18 wt%) and several molybdenum and mixed antimony-molybdenum oxides such as: Mo8O23, MoO3-basal, MoO3-iSO, Sb2MoO6 and a mixture of Sb2Mo10O31, and Sb4Mo10O31 (SbxMo10O31) phases as catalysts. Both catalytic performances and catalysts stability (structural modifications and leaching) depended on characteristics of the structure. The less active catalysts exhibited a higher leaching. The most stable and thus active catalysts were those for which the leaching is compensated by structural changes going in the direction of the exacerbation of their crystallinity. In the investigated series the most effective were the Mo8O23 and Sb2MoO6. (c) 2007 Elsevier B.V. All rights reserved

On the impact of the choice of model VOC in the evaluation of V-based catalysts for the total oxidation of dioxins: Furan vs. chlorobenzene

V-based catalysts, widely developed for the catalytic abatement of dioxins, are usually studied and optimized by investigating the oxidation of model chlorinated aromatic compounds (e.g. chlorobenzene). Even though the oxygenated function included in the central aromatic ring of the molecular structure of a dioxin could influence major aspects of the catalytic process, it has never been taken into account in the reported works. In this study, furan is chosen as a model for the central oxygenated ring of a polychlorinated dibenzo furan (PCDF) and its oxidation is cornpared to the case of chlorobenzene. The strategy was to check systematically if the improvements of formulations enlightened from our previous investigation on chlorobenzene also remain beneficial with furan. It turned out that the use of a sulfate containing TiO2 as support for the active VOx phase as well as the doping of the formulation with Mo or W oxides had very different impacts in the two cases. Some improvement strategies prove to be inefficient or deleterious in the case of furan. Competition tests further suggest that the adsorption behavior of dioxin could be better imitated by furan than by chlorobenzene. These observations highlight, in the case for which working with the target pollutant is difficult (as with dioxins), that the choice of the model molecule is critical. (C) 2007 Elsevier B.V. All rights reserved

Positive effect of NOx on the performances of VOx/TiO2-based catalysts in the total oxidation abatement of chlorobenzene

The influence of NO on the performances of VOx/TiO2, VOx-WOx/TiO2, and VOx-MoOx/TiO2 catalysts is investigated in the combustion of chlorobenzene. NO proves to induce an increase in the chlorobenzene conversion. However, this happens only if O-2 is present and is maximized when the catalyst contains W or Mo. The suggested mechanism for the effect is: 1) NO is oxidized to NO2 mainly on WO, and MoOx; 2) afterward, NO2 assists O-2 in the reoxidation step (as described by Mars-van Krevelen) of the VOx phase, thus speeding up the oxidation cycle, which macroscopically corresponds to the increase in chlorobenzene conversion. (c) 2005 Elsevier Inc. All rights reserved

Influence of the reduction state in the bulk and at the surface on the behavior of MoO3 catalysts in the reaction of 2-butanol (dehydration versus oxidation) in the presence of oxygen

The reaction of 2-butanol (dehydration to butenes versus total oxidation to CO2 and water) in the presence of oxygen is used as a probe of the influence of the reduction state Of MoO3-based catalysts on their behavior. Two different behaviors are revealed depending on the extent of the reduction and on the manner by which this reduction is accommodated by the catalysts. Reduction accommodated through the stabilization of bulk Mo sub-oxide phases is detrimental in terms of exchangeability of oxygen atoms. Such catalysts preferentially promote the dehydration of 2-butanol on Bronsted sites, but do not succeed to oxidize it efficiently. In other words, they do not succeed to transfer oxygen atoms into hydrocarbons through the MVK mechanism. At the opposite, a slight reduction localized at the catalyst surface through the presence of Mo (5+) species increases the exchangeability of surface lattice oxygen atoms. This makes the corresponding catalysts highly efficient in the oxidation of 2-butanol, and most generally in oxidation reaction proceeding through the MVK mechanism. (C) 2004 Elsevier B.V. All rights reserved

Oxidation of 2-thiobenzyl-4,6-dimethyl-pyrimidine with hydrogen peroxide over Mo oxides, Mo suboxides and mixed Mo-Sb oxides catalysts

The oxidation of 2-thiobeniyl-4,6-dimethyl-pyrimidine was investigated over catalysts based on MoO3, Mo suboxides and mixed Mo-Sb oxides to obtain 2-thiobenzyl-4,6-dimethyl-pyrimidine sulfoxide. The chemoselectivity was found to vary with the oxidation state of the Mo atoms, the morphology of the Mo oxide crystals and the concentration of Sb in the structure. The best chemoselectivity (about 80%) resulted from Mo8O23. The H2O2 efficiency was also found to depend on these parameters. For Mo suboxides the best efficiency was obtained on Mo8O23, which suggested that the efficient catalysts are those possessing Mo in a reduced oxidation state. The presence of Sb changed the behavior of the catalysts and the higher H2O2 efficiency resulted for the catalysts containing Mo in its highest oxidation state. (C) 2002 Elsevier Science B.V. All rights reserved