Partial oxidation of toluene to benzaldehyde and benzoic acid over model vanadia/titania catalysts: role of vanadia species

Pure and K-doped vanadia/titania prepared by different methods have been studied in order to elucidate the role of vanadia species (monomeric, polymeric, bulk) in catalytic toluene partial oxidation. The ratio of different vanadia species was controlled by treating the catalysts in diluted HNO₃, which removes bulk vanadia and polymeric vanadia species, but not the monomeric ones, as was shown by FT-Raman spectroscopy and TPR in H₂. Monolayer vanadia species (monomeric and polymeric) are responsible for the catalytic activity and selectivity to benzaldehyde and benzoic acid independently on the catalyst preparation method. Bulk V₂O₅ and TiO₂ are considerably less active. Therefore, an increase of the vanadium concentration in the samples above the monolayer coverage results in a decrease of the specific rate in toluene oxidation due to the partial blockage of active monolayer species by bulk crystalline V₂O₅. Potassium diminishes the catalyst acidity resulting in a decrease of the total rate of toluene oxidation and suppression of deactivation. Deactivation due to coking is probably related to the Brønsted acid sites associated with the bridging oxygen in the polymeric species and bulk V₂O₅. Doping by K diminishes the amount of active monolayer vanadia leading to the formation of non-active K-doped monomeric vanadia species and KVO₃.     

微反应器内甲苯气固催化氧化反应动力学

微通道反应器具有优良的传热、传质性能，能有效避免催化剂床层内热点的形成，为研究强放热反应动力学提供有利条件。开展了微反应器内的V2O5/TiO2催化剂上的甲苯气相选择氧化动力学研究，在简化反应网络的基础上建立了动力学模型，并给出动力学参数。该模型能较好地反映和预测较宽的反应条件范围内的甲苯气固相催化氧化反应转化率及产物分布，为优化操作条件提供依据。     

Vanadia/titania catalysts for gas phase partial toluene oxidation: Spectroscopic characterisation and transient kinetics study

Formation of vanadia species during the calcination of ball milled mixture of V₂O₅ with TiO₂ was studied by Raman spectroscopy in situ and at ambient conditions. It is found that calcination in air leads to fast (1–3 h) spreading of vanadia over TiO₂ followed by a slower process leading to the formation of a monolayer vanadia. The calcinated catalyst showed higher activity during toluene oxidation than the uncalcinated one, but the selectivity towards C₇-oxygenated products (benzaldehyde and benzoic acid) remains unchanged. The activity of the catalysts is ascribed to the formation of vanadia species in the monolayer. The details of the parallel–consecutive reaction scheme of toluene oxidation are presented from steady-state and transient kinetics studies. Different oxygen species seem to participate in the deep and partial oxidation of toluene. Coke formation was observed during the reaction presenting an average composition C_2nH_1.1n. The amount of coke on the catalyst was not dependent on the calcination step and the vanadium content in the catalyst. Coke formation was seen to be responsible for the deactivation of the catalyst.     

An investigation of the thermal stability and sulphur tolerance of Ag/γ-Al2O3 catalysts for the SCR of NOx with hydrocarbons and hydrogen

The sulphur tolerance and thermal stability of a 2 wt% Ag/γ-Al₂O₃ catalyst was investigated for the H₂-promoted SCR of NO_x with octane and toluene. The aged catalyst was characterised by XRD and EXAFS analysis. It was found that the effect of ageing was a function of the gas mix and temperature of ageing. At high temperatures (800 °C) the catalyst deactivated regardless of the reaction mix. EXAFS analysis showed that this was associated with the Ag particles on the surface of the catalyst becoming more ordered. At 600 and 700 °C, the deactivating effect of ageing was much less pronounced for the catalyst in the H₂-promoted octane-SCR reaction and ageing at 600 °C resulted in an enhancement in activity for the reaction in the absence of H₂. For the toluene + H₂-SCR reaction the catalyst deactivated at each ageing temperature. The effect of addition of low levels of sulphur (1 ppm SO₂) to the feed was very much dependent on the reaction temperature. There was little deactivation of the catalyst at low temperatures (≤235 °C), severe deactivation at intermediate temperatures (305 and 400 °C) and activation of the catalyst at high temperatures (>500 °C). The results can be explained by the activity of the catalyst for the oxidation of SO₂ to SO₃ and the relative stability of silver and aluminium sulphates. The catalyst could be almost fully regenerated by a combination of heating and the presence of hydrogen in the regeneration mix. The catalyst could not be regenerated in the absence of hydrogen.     

Micellar catalysis for partial oxidation of toluene to benzoic acid in supercritical CO2: effects of fluorinated surfactants

One of the key hindrances on development of solid catalysts containing cobalt species for partial oxidation of organic molecules at mild conditions in conventional liquid phase is the severe metal leaching. The leached soluble Co species with a higher degree of freedom always out-performs those of solid supported Co species in oxidation catalysis. However, the homogeneous Co species concomitantly introduces separation problems. We have recently reported for the first time, a new oxidation catalyst system for the oxidation of organic molecules in supercritical CO₂ using the principle of micellar catalysis. [CF₃(CF₂)₈COO]₂Co·xH₂O (the fluorinated anionic moiety forms aqueous reverse micelles carrying water-soluble Co²⁺ cations in scCO₂) was previously shown to be extremely active for the oxidation of toluene in the presence of sodium bromide in water–CO₂ mixture, giving 98% conversion and 99% selectivity to benzoic acid at 120 °C. In this study, we show that the effects of varying the type of surfactant counterions and the length of the surfactant chains on catalysis. It is found that the use of [CF₃(CF₂)₈COO]₂Mg·yH₂O/Co(II) acetate is as effective as the [CF₃(CF₂)₈COO]₂Co·xH₂O and the fluorinated chain length used has a subtle effect on the catalytic rate measured. It is also demonstrated that this new type of micellar catalyst in scCO₂ can be easily separated via CO₂ depressurisation and be reused without noticeable deactivation.     

Fast catalytic oxidation of phenol in supercritical water

The catalytic oxidation of phenol in water over a commercial oxidation catalyst, CARULITE 150, was investigated in a fixed bed flow reactor at 250 atm and temperatures from 380°C to 430°C. The phenol and oxygen concentrations at the reactor entrance varied between 0.070 and 1.24 mmol/l, and 9.60 and 39.6 mmol/l, respectively. The reaction conditions produced phenol conversions and selectivities to CO₂ much higher than those produced by non-catalytic oxidation. The kinetics of phenol disappearance and of CO₂ formation were both roughly first-order, and the activation energies were 31 and 47 kcal/mol, respectively. The catalyst did not undergo continuous deactivation during the catalytic oxidation, but rather maintained a high activity even after several days of continuous operation.     

Oxidation of H2S on activated carbon KAU and influence of the surface state

Properties of the oxidized activated carbon KAU treated at different temperatures in inert atmosphere were studied by means of DTA, Boehm titration, XPS and AFM methods and their catalytic activity in H₂S oxidation by air was determined. XPS analysis has shown the existence of three types of oxygen species on carbon catalysts surface. The content of oxygen containing groups determined by Boehm titration is correlated with their amount obtained by XPS. Catalytic activity of the KAU catalysts in selective oxidation of hydrogen sulfide is connected with chemisorbed charged oxygen species (O_3.1 oxygen type with BE 536.8–537.7 eV) present on the carbons surface.

Formation of dense sulfur layer (islands of sulfur) on the carbons surface and removal of active oxygen species are the reason of the catalysts deactivation in H₂S selective oxidation. The treatment of deactivated catalyst in inert atmosphere at 300 °C gives full regeneration of the catalyst activity at low temperature reaction but only its partial reducing at high reaction temperature. The last case is connected with transformation of chemisorbed charged oxygen species into CO groups.

The KAU samples treated in flow of inert gas at 900–1000 °C were very active in H₂S oxidation to elemental sulfur transforming up to 51–57 mmol H₂S/g catalyst at 180 °C with formation of 1.7–1.9 g S_x/g catalyst.     

Catalytic combustion over platinum group catalysts: fuel-lean versus fuel-rich operation

Performance data are presented for methane oxidation on alumina-supported Pd, Pt, and Rh catalysts under both fuel-rich and fuel-lean conditions. Catalyst activity was measured in a micro-scale isothermal reactor at temperatures between 300 and 800 °C. Non-isothermal (near adiabatic) temperature and reaction data were obtained in a full-length (non-differential) sub-scale reactor operating at high pressure (0.9 MPa) and constant inlet temperature, simulating actual reactor operation in catalytic combustion applications.

Under fuel-lean conditions, Pd catalyst was the most active, although deactivation occurred above 650 °C, with reactivation upon cooling. Rh catalyst also deactivated above 750 °C, but did not reactivate. Pt catalyst was active above 600 °C. Fuel-lean reaction products were CO₂ and H₂O for all three catalysts.

The same catalysts tested under fuel-rich conditions demonstrated much higher activity. In addition, a ‘lightoff’ temperature was found (between 450 and 600 °C), where a stepwise increase in reaction rate was observed. Following ‘lightoff’ partial oxidation products (CO, H₂) appeared in the mixture, and their concentration increased with increasing temperature. All three catalysts exhibited this behavior.

High-pressure (0.9 MPa) sub-scale reactor and combustor data are shown, demonstrating the benefits of fuel-rich operation over the catalyst for ultra-low emissions combustion.     

Selective oxidation of propylene to acrolein over supported V2O5/Nb2O5 catalysts: An in situ Raman, IR, TPSR and kinetic study

The vapor-phase selective oxidation of propylene (H₂CCHCH₃) to acrolein (H₂CCHCHO) was investigated over supported V₂O₅/Nb₂O₅ catalysts. The catalysts were synthesized by incipient wetness impregnation of V-isopropoxide/isopropanol solutions and calcination at 450 °C. The catalytic active vanadia component was shown by in situ Raman spectroscopy to be 100% dispersed as surface VO_x species on the Nb₂O₅ support in the sub-monolayer region (<8.4 V/nm²). Surface allyl species (H₂CCHCH_2*) were observed with in situ FT-IR to be the most abundant reaction intermediates. The acrolein formation kinetics and selectivity were strongly dependent on the surface VO_x coverage. Two surface VO_x sites were found to participate in the selective oxidation of propylene to acrolein. The reaction kinetics followed a Langmuir–Hinshelwood mechanism with first-order in propylene and half-order in O₂ partial pressures. C₃H₆-TPSR spectroscopy studies also revealed that the lattice oxygen from the catalyst was not capable of selectively oxidizing propylene to acrolein and that the presence of gas phase molecular O₂ was critical for maintaining the surface VO_x species in the fully oxidized state. The catalytic active site for this selective oxidation reaction involves the bridging VONb support bond.     

Sulphur dioxide interaction with NOx storage catalysts

The effect of SO₂ on the NO_x storage capacity and oxidation and reduction activities of a model Pt/Rh/BaO/Al₂O₃ NO_x storage catalyst was investigated. Addition of 2.5, 7.5 or 25 vol. ppm SO₂ to a synthetic lean exhaust gas caused deactivation of the NO_x storage function, the oxidation activity and the reduction activity of the catalyst. The degree of deactivation of the NO_x storage capacity was found to be proportional to the total SO₂ dose that the catalyst had been exposed to. SO₂ was found to be accumulated in the catalyst as sulphate.     

The dependence on temperature of gas-phase photocatalytic oxidation of methyl tert-butyl ether and tert-butyl alcohol

The influence of temperature on the mechanism and kinetics of gas-phase photocatalytic oxidation (PCO) and thermal oxidation (TO) of methyl tert-butyl ether (MTBE) and tert-butyl alcohol (TBA) over TiO₂ was studied in a continuous flow annular reactor. The reaction products of PCO of both substances included acetone, water, carbon dioxide and carbon monoxide. Volatile TO products of MTBE included 2-methyl-1-propene (2-MP), carbon monoxide, carbon dioxide and water; TBA decomposed to 2-MP and water. Thermal oxidation of MTBE became noticeable at 388 K, TBA started to decompose thermally at 393 K. Both PCO and TO fit well to the Langmuir–Hinshelwood (L–H) model for monomolecular reactions. The dependence of the kinetic constants on temperature was established in the form of an Arrhenius equation. The TiO₂ catalyst showed no deactivation at temperatures above 373 K but gradually lost its activity below 373 K.     

Gas-phase photo-oxidation of toluene using nanometer-size TiO2 catalysts

A modified sol–gel process was used to prepare nanostructured TiO₂ catalysts of controlled particle size (i.e. 6, 11, 16 and 20 nm). The influence of the TiO₂ particle size in the gas phase photocatalytic oxidation of toluene was investigated under both dry and humid conditions. The main products of reaction were carbon dioxide and water, although small amounts of benzaldehyde were also detected. The smaller particle size (i.e. 6 nm) lead to higher conversion and complete mineralization of toluene into CO₂ and H₂O. Both electronic and structural effects (i.e. size and ensemble effects) are responsible for the excellent performance of 6 nm TiO₂ catalyst for toluene photo-degradation. The structural differences between 6 nm TiO₂ and larger catalysts were analyzed using EPR spectroscopy.     

Transient kinetics of toluene partial oxidation over V/Ti oxide catalysts

Transient kinetics in the toluene oxidation over V/Ti oxide catalysts prepared by grafting and impregnation have been compared. V⁴⁺ cations are supposed to be the sites for the formation of electrophilic oxygen species participating in deep oxidation. Another oxygen species (probably nucleophilic) present on the oxidised catalyst surface are responsible for benzaldehyde formation. Selectivity of 80–100% can be obtained during the initial period of the reaction on the grafted catalysts in the presence of gaseous oxygen and during the interaction of toluene (without O₂ in the mixture) with partially reduced catalysts.     

Selective oxidation of ammonia over copper-silver-based catalysts

A novel catalyst based on copper-silver was developed to solve the contradiction between the high conversion temperature of Cu-based catalyst and low N₂ selectivity of Ag-based catalyst during selective oxidation of ammonium gas. The Cu-Ag-based catalyst (Cu 5 wt.%-Ag 5 wt.%/Al₂O₃) displayed a relatively low complete conversion temperature (<320 °C) with a high N₂ selectivity (>95%). Increasing loading of Cu and Ag decreases N₂ selectivity. The low N₂ selectivity of Ag-based catalyst is possibly related to the formation of Ag₂O crystals. Improvement of N₂ selectivity of Ag-based catalyst was obtained by doping Cu to decrease crystallized Ag₂O phase. The temperature programmed reaction (TPR) data show that N₂O is the main byproduct of oxidation of ammonia at temperature lower than 200 °C. Two bands of nitrate species at 1541 and 1302 cm⁻¹ were observed on Ag 10 wt.%/Al₂O₃ at the temperature higher than 250 °C, which indicates the formation of NO_x during the selective catalytic oxidation of ammonia. No nitrate species was observed on Cu 10 wt.%/Al₂O₃ and Cu 5 wt.%-Ag 5 wt.%/Al₂O₃, while only one nitrate species (1543 cm⁻¹) existed on Cu 10 wt.%-Ag 10 wt.%/Al₂O₃. We proposed that mixing Ag with Cu inhibited the formation of NO_x during the selective catalytic oxidation of ammonia over Cu-Ag/Al₂O₃.     

Studies on the deactivation of NOx storage-reduction catalysts by sulfur dioxide

The interaction of sulfur dioxide with a commercial NO_x storage-reduction catalyst (NSR) has been investigated using in situ IR and X-ray absorption spectroscopy. Two pathways of catalyst deactivation by SO₂ were identified. Under lean conditions (exposure to SO₂ and O₂) at 350 °C the storage component forms barium sulfates, which transform from surface to hardly reducible bulk sulfate species. The irreversible blocking of the Ba sites led to a decrease in NO_x storage capacity. Under fuel rich conditions (SO₂/C₃H₆) at 350–500 °C evidence for the formation of sulfides on the oxidation/reduction component (Pt) of the catalyst was found, which blocks the metal surface and thus hinders the further reduction of the sulfides.     

甲苯氧酰化反应制备苯甲醇研究进展

烯烃和芳香烃通过氧酰化反应直接制备羧酸酯具有简单、直接和高效的特点。甲苯通过氧酰化反应制备苯甲醇醋酸酯是该反应的重要应用之一,反应产物苯甲醇醋酸酯和苯甲醇醋酸酯水解生成的苯甲醇是重要的化工原料,工艺路线绿色环保。对甲苯氧酰化反应机理、催化剂组成和载体对催化剂活性的影响、工艺条件的优化以及催化剂失活及再生等方面的研究进展进行综述,现有的甲苯氧酰化催化剂活性较高并可通过再生循环使用,降低了催化剂应用成本。随着苯甲醇醋酸酯、苯甲醇以及苯甲醛等需求的增大和环保要求提高,甲苯氧酰化工艺工业应用前景广阔。     

Modified Co3O4/ZrO2 catalysts for VOC emissions abatement

The catalytic activity study of cobalt oxides dispersed on different supports evidenced first the highest performances of zirconia based catalysts in the reaction of toluene oxidation. The influence of the presence of ethylenediamine (en) during the preparation of Co/ZrO₂ and the ZrO₂ support modification by Y₂O₃ were then studied and compared with reference catalyst prepared conventionally by impregnation of ZrO₂ with an aqueous solution of Co(NO₃)₂. Addition of an aqueous solution of ethylenediamine to a cobalt nitrate solution led to a strong increase on the catalytic activity of the activated solids in the toluene deep oxidation as compared with the reference catalyst. The best catalytic results were explained in terms of cobalt oxides dispersion but also in terms of Co–support interaction. The generated cobalt species were reducible at much lower temperatures and then were more active in the toluene total oxidation. Finally an efficient catalyst for VOC oxidation was produced combining the modifications of ZrO₂ by yttrium and of the precursor.     

Effect of calcination temperature on the low-temperature oxidation of CO over CoOx/TiO2 catalysts

A 5 wt% CoO_x/TiO₂ catalyst has been used to study the effect of calcination temperature on the activity of this catalyst for CO oxidation at 100 °C under a net oxidizing condition in a continuous flow type fixed-bed reactor system, and the catalyst samples have been characterized using TPD, XPS and XRD measurements. The catalyst after calcination at 450 °C gave highest activity for this low-temperature CO oxidation, and XPS measurements yielded that a 780.2-eV Co 2p_3/2 main peak appeared with this catalyst sample and this binding energy was similar to that measured with pure Co₃O₄. After calcination at 570 °C, the catalyst, which had possessed practically no activity in the oxidation reaction, gave a Co 2p_3/2 main structure peak at 781.3 eV which was very similar to those obtained for synthesized Co_nTiO_n+2 compounds (CoTiO₃ and Co₂TiO₄), and this catalyst sample had relatively negligible CO chemisorption as observed by TPD spectra. XRD peaks indicating only the formation of Co₃O₄ particles on titania surface were developed in the catalyst samples after calcination at temperatures ≥350 °C. Based on these characterization results, five types of Co species could be modeled to exist with the catalyst calcined at different temperatures. Among these surface Co species, the Type A clean Co₃O₄ particles were predominant on a sample of the catalyst after calcination at 450 °C and highly active for CO oxidation at 100 °C, and the calcination at 570 °C gave the Type B Co₃O₄ particles with complete Co_nTiO_n+2 overlayers inactive for this oxidation reaction.     

Additional effects of cobalt precursor and zirconia support modifications for the design of efficient VOC oxidation catalysts

The influence of the ZrO₂ support modification by Y₂O₃ and the presence of ethylenediamine (“en”) during the preparation of Co/ZrO₂ were studied and compared with a reference catalyst conventionally prepared by impregnation of ZrO₂ with an aqueous solution of Co(NO₃)₂. The effect of the en/Co molar ratio (x = 1–3) was studied. Activation of cobalt species was followed by differential thermal and thermogravimetric analyses (DTA/TG) analyses and by specific surface area measurements which evidence the complete cobalt precursor decomposition at 450 °C, whatever the support composition and the en/Co molar ratio. The addition of an aqueous solution of ethylenediamine to a cobalt nitrate solution led to a strong increase in the catalytic activity of the activated solids for the toluene deep oxidation as compared to the reference catalyst. The best catalytic results were explained in terms of cobalt oxides dispersion (X-ray diffraction (XRD)) and also in terms of Co-support interaction (H₂-temperature-programmed reduction (TPR)). The generated cobalt species were reducible at much lower temperatures and were more active in the toluene total oxidation. Finally, an efficient catalyst was produced combining the modifications of the support by yttrium oxide and of the precursor (use of ethylenediamine).