Total Oxidation Of Chlorobenzene Research Articles

Elaboration and characterization of sulfated and unsulfated V2O5/TiO2 nanotubes catalysts for chlorobenzene total oxidation

This paper examines the use of TiO2 nanotubes (HNTs) as supports for V2O5 based catalysts in the total oxidation of chlorobenzene. The effect of the addition of SO42− onto the support is also discussed. Vanadium was introduced either by direct incorporation of V during the elaboration of the nanotubes (in situ elaboration), or by the impregnation of V on the surface of the supports (ex situ elaboration). The obtained catalysts have been characterized by means of ICP-AES, N2 adsorption–desorption at 77K, XRD, DRIFTS, XPS, H2-TPR and NH3-TPD. We demonstrated that sulfating step highly improves the catalytic performances of V-HNTs catalysts. This is due to an increased global acidity and a higher reactivity of redox sites thanks to the electronic interaction between sulfated titania and VOx species. Moreover, it seems that the ‘in situ’ or ‘ex situ’ elaboration route of sulfated V-catalysts influences the environment of vanadium species. In particular, the ‘in situ’ route leads to a more efficient catalyst.

Effect of support on V2O5 catalytic activity in chlorobenzene oxidation

The present paper investigates the influence of the direct incorporation of ceria in TiO2 aerogels support (sulfated and unsulfated) prepared by a sol–gel route on the catalytic properties of vanadia based materials in the total oxidation of chlorobenzene. ICP-AES, N2 physisorption, XRD, DRIFTS, Raman spectroscopy, XPS, H2-TPR and NH3-TPD were employed for catalyst characterization. This study demonstrated that cerium oxide is present as CeO2 form at the catalyst surface, which can improve the catalyst redox properties and so enhance the catalytic activity at high temperature (400°C). Besides, sulfate containing vanadia–titania samples (doped or not with ceria) had their catalytic performances considerably improved (due to sulfates) at lower temperatures (range 200–300°C). This is due to an increased global acidity and higher reactivity of redox sites thanks to the superficial interaction between (i) vanadia and sulfate, and (ii) when ceria is present, between ceria and sulfate leading to higher efficiency of catalysts in the chlorobenzene oxidation.

Influence of the impregnation order on the synergy between Ag and V2O5/TiO2 catalysts in the total oxidation of Cl-aromatic VOC

Silver is added to V2O5/TiO2 catalysts by a wet impregnation method and the influence of the impregnation order is studied. Silver and vanadia were introduced by co-impregnation or by impregnation in 2 steps (V first or Ag first). Vanadia loading always corresponds to 2.49wt% while 9 different loadings of silver were explored (0.02–12.5wt%). Whatever the way of synthesis, the addition of silver to V2O5/TiO2 formulation induces: (i) a decrease of the specific surface; (ii) a higher vanadium oxidation state and (iii) a decrease of the Brønsted acidity. A synergy can be evidenced for the co-impregnated Ag–V2O5/TiO2 catalysts in the total oxidation of chlorobenzene. This synergy is observed for several loadings of Ag with a maximum at a loading of 0.05wt%. This synergy can be related to the higher oxidation state of vanadium induced by silver, to the minimum decrease of the specific surface and to the absence of AgCl at the surface of the catalysts. This last aspect is confirmed via the investigation of the catalytic performances in the total oxidation of benzene. In this system, a synergy is observed for the co-impregnated catalysts and when the vanadium is impregnated first. On the contrary, no synergy was observed when silver is impregnated first. This result suggests that the synergy can be obtained only if the vanadium keeps a direct interaction with the support and if a good “degree of accessibility” of the gaseous phase to the silver species is maintained.

Necessary conditions for a synergy between Ag and V 2O 5 in the total oxidation of chlorobenzene

V 2O 5/TiO 2 catalysts are widely developed for the total oxidation of dioxins. However, this study presents an innovative way to improve the performances of V 2O 5/TiO 2 catalysts by the addition of silver. Three kinds of catalysts were synthesized: Ag/TiO 2, V 2O 5/TiO 2 and Ag–V 2O 5/TiO 2. The vanadia loading always corresponds to a 0.75 theoretical monolayer while 9 different loadings of silver were explored (0.02–12.5%). This new kind of catalysts is tested in the total oxidation of chlorobenzene which is a classical model molecule for dioxins. The samples were characterized by BET, XPS, XRD, ICP-AES and TEM before and after catalytic tests. The specific surface decreases with the increase of the silver loading for Ag/TiO 2 and Ag–V 2O 5/TiO 2 catalysts. XPS results show that the addition of silver to a V 2O 5/TiO 2 formulation induces a higher oxidation state of the vanadium which should be beneficial on the performances. A synergy is indeed observed for several loadings of silver with a maximum at a loading of 0.05 wt.%. This synergistic effect between silver and vanadia can only be achieved in the total oxidation of chlorobenzene by choosing appropriate operating conditions, namely conditions avoiding AgCl formation. This synergy is directly linked to an optimal silver loading which offers the best compromise between a high oxidation state of vanadia without a significant decrease of the specific surface of the catalyst.

Influence of the meso-macroporous ZrO 2–TiO 2 calcination temperature on the pre-reduced Pd/ZrO 2–TiO 2 (1/1) performances in chlorobenzene total oxidation

The effect of two calcining temperatures (400 °C, 600 °C) on the texture, structure and thermal stability of a meso-macroporous ZrO 2–TiO 2 of equal molar amount (ZrTi x ; x = 4, 6) synthesized from hydrothermal conditions using a surfactant and metal alkoxide precursors was studied. Pd (0.5 wt.%) was dispersed on these innovative supports (Pd/ZrTi x ; x = 4, 6) by a classic wet impregnation and calcined at 400 °C. The resulting catalysts were characterized by Elemental analysis (EA), X-Ray Diffraction (XRD), N 2 adsorption–desorption, H 2-Temperature Programmed Reduction (H 2-TPR), Pd dispersion and tested as pre-reduced in the total oxidation of chlorobenzene. It was found that the calcination temperature did not provoke a significant effect on the physicochemical properties of the resulting binary oxide except the acidity which significantly decreased. The support was stable against phase transformation upon calcination as no crystalline phase could be detected indicating a homogeneous mixing of the Zr and Ti components. The Specific Surface Areas (SSAs) were superior to 400 m 2/g even after calcination at 600 °C. The Pd dispersion was around 40% and the palladium was totally reduced before catalytic testing. Lowering the temperature of calcination of the support enhanced the performances of the catalyst. Indeed PhCl conversion increased as the calcination temperature decreased and correlates with the Pd dispersion and SSA of the catalyst. A significant amount of polychlorinated benzenes PhCl x ( x = 2–6) was detected in the course of the reaction which practically disappeared at 100% PhCl conversion. This amount was reduced by a factor 2 as the temperature of calcination decreased. Compared to the performances of Pd/Ti x and Pd/Zr x for the same reaction, all things being kept equal, calcined ZrO 2–TiO 2 supports show higher surface areas and higher thermal stability than their ZrO 2 and TiO 2 counterparts. In terms of activity for PhCl total oxidation based on T 50 values the ZrO 2–TiO 2 supports showed an intermediate behaviour while a beneficial effect was observed regarding the by-products production which is the lowest herein for the Pd/TiZr 4 catalyst.

Flame-made vs. wet-impregnated vanadia/titania in the total oxidation of chlorobenzene: Possible role of VO x species

Vanadia/titania particles with a specific surface area (SSA) around 50 m 2 g −1 and a V 2O 5 content up to 30 wt.% (corresponding to a V surface density up to 33 V nm −2) were prepared by flame spray pyrolysis as well as by classic wet-impregnation. The catalysts were characterized by nitrogen adsorption, X-ray diffraction, temperature programmed reduction, Raman spectroscopy, X-ray photoelectron spectroscopy and tested in the total oxidation of chlorobenzene. Depending on vanadia content, monomeric, polymeric and crystalline vanadia species were formed. The dispersion of the VO x species was in general higher for flame-made catalysts. While the classic wet-impregnated catalysts already showed crystalline V 2O 5 when the V surface density reaches 8 V nm −2, the flame-made ones exhibited only amorphous VO x species up to 16 V nm −2. The activity of flame-made and wet-impregnated catalysts increased with increasing V 2O 5 loading and therefore depended on the VO x species structure: catalysts exceeding a V surface density of 8 V nm −2 containing high amounts of amorphous polymeric and/or crystalline VO x species showed significantly higher activity than catalysts with lower V surface density. Wet-impregnated catalysts with numerous V–O–V bonds as involved in polymeric and crystalline VO x species showed superior activity than FSP-made ones of similar composition. This contribution proposes a discussion aiming at understanding the role played by the different types of VO x species in the total oxidation of volatile organic compounds on the example of chlorobenzene.

Elucidation of deactivation or resistance mechanisms of CrOx, VOx and MnOx supported phases in the total oxidation of chlorobenzene via ToF‐SIMS and XPS analyses

AbstractVOx, CrOx and MnOx supported on TiO2 are all efficient catalysts in the total oxidation of benzene. However, in the oxidation of chlorobenzene, they exhibit different behaviors in terms of their resistance to deactivation by the chlorinated reactant and/or products of the reaction (Cl2, HCl). Precisely: VOx catalysts present a very good resistance; conversely, MnOx catalysts present a huge deactivation and CrOx catalysts exhibit an intermediate behavior. This contribution leads to a better understanding of the mechanisms and origins of the respective deactivation (or resistance to deactivation) of the catalysts via a postmortem characterization by ‘X‐ray photoelectron spectroscopy’ (XPS) and ‘time of flight—secondary ion mass spectroscopy’ (ToF‐SIMS). (i) The different behaviors are correlated to the atomic ratio of chlorine/transition metal at the surface of the used catalysts and (ii) the nature of the chlorinated species responsible for the deactivation is elucidated: namely, CrOx catalysts deactivate because of a firm adsorption at the surface of the chlorinated volatile organic compounds (VOC) or of chlorinated intermediates of reaction, while MnOx catalysts deactivate because of the formation of (oxy)chlorides at their surface. Copyright © 2008 John Wiley & Sons, Ltd.

Effect of CeO2 Redox Behavior on the Catalytic Activity of a VOx/CeO2 Catalyst for Chlorobenzene Oxidation

Abstract Three CeO 2 samples were prepared by precipitation with urea, ammonia, and sodium carbonate. Vanadium oxide was supported on these CeO 2 materials, and their catalytic activity in the total oxidation of chlorobenzene was investigated. Their turnover frequencies for chlorobenzene oxidation are 4.9, 1.6, and 0.8 h −1 at 300°C, respectively, and depend on the redox properties of the CeO 2 supports. These results demonstrate that the oxygen of the V–O–Ce bond plays an important role in determining the catalytic activity of the VO x /CeO 2 catalyst.

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, WOx and MoOx 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 Brønsted 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 Brønsted sites for the adsorption of VOCs and Cl-VOCs aromatics.

Catalytic properties of nanocomposites based on cobalt ferrites dispersed in sol–gel silica

In this work, composites based on cobalt ferrite (CoFe2O4) dispersed in a porous silica matrix have been prepared via sol–gel method using tetraethylorthosilicate (TEOS) as a silica precursor, and iron and cobalt nitrates as precursors. In this process, Co and Fe ions dispersed in a sol–gel matrix reacted upon thermal treatment to form Co–Fe crystallites with nanometric dimensions. The structural, morphological, and textural changes of the composites treated at various temperatures were studied by X-ray diffraction, electronic microprobe, and gas adsorption. The treated composites had surface areas in the range 263–359m2g−1 with mesopore contribution. These materials were active as catalysts in the total oxidation of chlorobenzene in air. The most active catalyst was the composite treated at 500°C, which was stable at a temperature as high as 600°C for 72h.

Oxidative Destruction of Chlorobenzene and o-Dichlorobenzene on a Highly Active Catalyst: MnOx/TiO2–Al2O3

A highly active catalyst, MnOx/TiO2–Al2O3, was prepared by impregnating MnOx species on TiO2-modified Al2O3. The TiO2 species in TiO2–Al2O3 support is in a monolayer dispersion, and the MnOx species is again highly dispersed on TiO2–Al2O3 support. The total oxidation of chlorobenzene and o-dichlorobenzene on MnOx/TiO2–Al2O3 catalyst can be achieved at 300°C and 250°C respectively, at the space velocity of 8000 h−1. The activity of MnOx/TiO2–Al2O3 catalyst (Mn loading 11.2 wt%) is gradually increased in the first 10–20 h and then keeps stable at least for the measured 52 h at 16,000 h−1. Furthermore, no chlorinated organic byproducts are detected in the effluent during the oxidative destruction of chlorobenzene and o-dichlorobenzene. It is proposed that the partially chlorinated and highly dispersed manganese oxide on a monolayer TiO2-modified Al2O3 is responsible for the high and stable activity for the total oxidation of chlorinated aromatics.