Study of ruthenium supported on Ta2O5–ZrO2 and Nb2O5–ZrO2 as catalysts for the partial oxidation of methane

Ru-based catalysts supported on Ta₂O₅–ZrO₂ and Nb₂O₅–ZrO₂ are studied in the partial oxidation of methane at 673–873 K. Supports with different Ta₂O₅ or Nb₂O₅ content were prepared by a sol–gel method, and RuCl₃ and RuNO(NO₃)₃ were used as precursors to prepare the catalysts (ca. 2 wt.% Ru). At 673 K high selectivity to CO₂ was found. An increase of temperature up to 773 K produced an increase in the selectivity to syngas (H₂/CO = 2.2–3.1), and this is related with the transformation of RuO₂ to metallic Ru as was determined from XRD and XPS results. At 873 K and with co-fed CO₂ an increase of the catalytic activity and CO selectivity was found. A TOF value of 5.7 s⁻¹ and H₂/CO ratio ca. 1 was achieved over Ru(Cl)/6TaZr. Catalytic results are discussed as a function of the support composition and characteristics of Ru-based phases.     

A new environmentally friendly method for the production of 2,3,5-trimethyl-p-benzoquinone

A new environmentally friendly method for the production of 2,3,5-trimethyl-1,4-benzoquinone (TMBQ, Vitamin E precursor) based on the oxidation of 2,3,6-trimethylphenol (TMP) with aqueous H₂O₂ over various Ti-containing mesoporous silicate materials is reported. Both well-organized Ti-containing mesoporous mesophase silicate (Ti-MMM), having hexagonal arrangement of uniform mesopores, and amorphous TiO₂–SiO₂ mixed oxides (aerogels and xerogels) produced TMBQ with good to high yield. All the materials studied have been proved to operate as truly heterogeneous catalysts. No titanium leaching occurred from the solid matrixes during the oxidation process. Titanium dispersion and its accessibility were found to be crucial factors determining the catalytic properties. For samples with similar titanium loading, both the catalytic activity and TMBQ yield appeared to fall in the order TiO₂–SiO₂ aerogel>Ti-MMM>TiO₂–SiO₂ xerogel and correlate with average mesopore diameter and mesopore volume. The best results (96–98% selectivity to TMBQ at 99–100% TMP conversion) were obtained with TiO₂–SiO₂ aerogels, containing 1.7–6.5 wt.% Ti.     

Oxidative carbonylation of methanol to dimethyl carbonate over CuCl/SiO2–TiO2 catalysts prepared by microwave heating: The effect of support composition

Copper(I) chloride catalysts with a loading of 20 wt%, supported on silica–titania mixed oxides with Si/Ti ratios of 1, 5, 10 and 50 were prepared by conventional and microwave heating methods and tested in the oxidative carbonylation of methanol to dimethyl carbonate (DMC). X-ray diffraction (XRD), nitrogen adsorption, X-ray photoelectron spectroscopy (XPS) and thermal gravimetric analysis (TGA) were used to examine the bulk and surface properties of the CuCl/SiO₂–TiO₂ catalysts. Quantum-chemical calculations were performed to explore the interaction of CuCl with the silica–titania support. Microwave heating showed some significant advantages over the conventional heating method, with markedly reduced preparation temperature and time, and provided improved catalytic activity in the oxidative carbonylation of methanol. The catalytic behavior of CuCl/SiO₂–TiO₂ in the test reaction studied was strongly dependent on the support composition. Incorporation of tetrahedral Ti(IV) species into the silica matrix could enhance the interaction of copper species with the oxide support. The improved catalytic performance of CuCl/SiO₂–TiO₂ in the DMC synthesis can be understood by the existence of the strong coordination interactions between the Cu⁺ centers of CuCl and the bridging oxygen atoms at the Si–O–Ti bonds in the silica–titania support.     

Chlorobenzene total oxidation over palladium supported on ZrO2, TiO2 nanostructured supports

Two series of supported Pd catalysts were synthesized on new mesoporous–macroporous supports (ZrO₂, TiO₂) labelled M (Zr and Ti). The deposition of palladium was carried out by wet impregnation on the calcined TiO₂ and ZrO₂ supports at 400 °C (Pd/Zr₄, Pd/Ti₄) and 600 °C (Pd/Zr₆, Pd/Ti₆) and followed by a calcination at 400 °C for 4 h. The pre-reduced Pd/MX catalysts were investigated for the chlorobenzene total oxidation and their catalytic properties where compared to those of a reference catalyst Pd/Ti-Ref (TiO₂ from Huntsman Tioxide recalcined at 500 °C) and of a palladium supported on the fresh mesoporous–macroporous TiO₂ (Pd/Ti). Based on the activity determined by T₅₀, the Pd/Ti and Pd/Ti₄ catalysts have been found to be more active than the reference one. Moreover activity decreased owing to the sequence: Pd/TiX  Pd/ZrX and in each series when the temperature of calcination of the support was raised. The overall results clearly showed that the activity was dependant on the nature of the support. The better activity of Pd/TiX compared to Pd/ZrX was likely due to a better reducibility of the TiO₂ support (Ti⁴⁺ into Ti³⁺) leading to an enhancement of the oxygen mobility. Production of polychlorinated benzenes PhCl_x (x = 2–6) and of Cl₂ was also observed. Nevertheless at 500 °C the selectivity in HCl was higher than 90% for the best catalysts.     

Oxidation of ethane on high specific surface SmCoO3 and PrCoO3 perovskites

An adapted sol–gel method allowed synthesizing SmCoO₃ and PrCoO₃ oxides with high specific surface (ca. 28 m² g⁻¹) and a relatively clean perovskite phase at 600 °C, a temperature much lower than the one required in ceramic methods. The perovskites were investigated as catalysts for the oxidation of ethane in the temperature range 300–400 °C. Both catalysts were very active: ethane was activated already at 300 °C, i.e., 100 °C below the temperatures previously reported for perovskites. The main product was CO₂ on both catalysts, but on PrCoO₃ oxidehydrogenation (ODH) to ethylene was observed already at 300 °C, with the low selectivity. Even so, this was quite unusual for simple perovskites, and for such a low temperature. TPR data showed that praseodymium decreases the reducibility of Co³⁺ in the perovskite, what could explain the observed ODH, and suggest it proceeds via a Mars–van Krevelen mechanism. Kinetic study showed a similar apparent activation energy for both catalysts (ca. 80 kJ/mol), but a difference in the nature of the participating oxygen species: while on PrCoO₃ both adsorbed and lattice species contribute to the reaction, on SmCoO₃ contribution of adsorbed species is practically negligible, due to its very high oxygen lability. The results show that these simple perovskites may be promising catalysts for ethane oxidation at relatively low temperatures.     

Environmentally Benign Oxidation of Alkylphenols to p-Benzoquinones: A Comparative Study of Various Ti-Containing Catalysts

Catalytic properties of Ti-containing porous solids were compared in the oxidation of 2,3,6-trimethylphenol (TMP) with H₂O₂ to produce 2,3,5-trimethyl-1,4-benzoquinone (TMBQ, vitamin E key intermediate). Mesoporous titanium–silicates with di(oligo)nuclear Ti centers, metal–organic framework MIL-125 and amorphous TiO₂ demonstrated 100 % selectivity toward TMBQ. Titanium–silicates prepared by evaporation-induced self-assembly revealed superior performance in terms of product yield and catalyst reusability.     

Selective oxidation of C3–C4 olefins over Mo-containing catalysts with tetragonal tungsten bronze structure

Mo–V–Nb–P–O-based catalysts with a tetragonal tungsten bronze-type (TTB) structure have been prepared hydrothemally from a H₃PMo₁₂O₄₀ Keggin-type heteropolyacid. These catalysts have been tested in the oxidation of C₃–C₄ olefins (propene, isobutene and 1-butene). Although the catalytic performance depends on the nature of the olefin fed the TTB-type catalysts prepared in the presence of elements of the V and VI groups such as Te, Sb and Bi have shown a high selectivity to partial oxidation products, especially that with Te. However, in the absence of these elements the TTB-catalysts present a high catalytic activity to deep oxidation. The selectivity to partial oxidation products decreases in the order: MoVNbPTe- > MoVNbPSb- > MoVNbPBi- > MoVNbP-TTB catalysts. The reaction products obtained in the oxidation of each olefin will be discussed according to their corresponding reaction mechanism and the characteristics of catalysts.     

Environmentally Benign Oxidation of Alkylphenols to p-Benzoquinones: A Comparative Study of Various Ti-Containing Catalysts

Catalytic properties of Ti-containing porous solids were compared in the oxidation of 2,3,6-trimethylphenol (TMP) with H₂O₂ to produce 2,3,5-trimethyl-1,4-benzoquinone (TMBQ, vitamin E key intermediate). Mesoporous titanium–silicates with di(oligo)nuclear Ti centers, metal–organic framework MIL-125 and amorphous TiO₂ demonstrated 100 % selectivity toward TMBQ. Titanium–silicates prepared by evaporation-induced self-assembly revealed superior performance in terms of product yield and catalyst reusability.

     

Vapor phase oxidation of dimethyl sulfide with ozone over V2O5/TiO2 catalyst

The removal of volatile and odorous emissions from pulp and paper industrial processes usually generates secondary pollution which is treated further by scrubbing, adsorption, and catalytic incineration. Studies using a flow reactor packed with 10% vanadia/titania (V₂O₅/TiO₂) catalyst showed complete conversion of dimethyl sulfide (DMS) in the presence of ozone. The molar yields of partial oxidation products were only 10–20%. Small amounts of partial oxidation products, such as and dimethyl sulfone (DMSO₂), dimethyl disulfide (DMDS), and dimethyl sulfoxide (DMSO), were also formed. The results of the oxidation of DMS using ozone only, ozone plus catalyst, and oxygen plus catalyst suggest that the combined use of O₃ with catalyst is essential for the complete destruction of DMS to CO₂ and SO₂. A Box-Behnken design was used to determine the factors that have a significant effect on the conversion and selectivity of the products. It was concluded that product selectivity is strongly influenced by temperature, gas hourly space velocity (GHSV), and ozone concentration. The catalysts were characterized using XRD, surface area measurements, and SEM techniques. Time-on-stream studies carried out in a 500 ppmv gas stream held at 150 °C for 6 h, using 2 g of the catalyst, an ozone-to-DMS molar ratio of 0.9, and a GHSV of 37,000 h⁻¹, yielded 99.9% conversion of DMS. A plausible reaction mechanism has been proposed for the oxidation of DMS based on reaction product distribution and possible intermediates formed.     

Utilization of carbon dioxide as soft oxidant for oxydehydrogenation of ethylbenzene to styrene over V2O5–CeO2/TiO2–ZrO2 catalyst

Vanadium oxide and cerium oxide doped titania–zirconia mixed oxides were explored for oxidative dehydrogenation of ethylbenzene to styrene utilizing carbon dioxide as a soft oxidant. The investigated TiO₂–ZrO₂ mixed oxide support with high specific surface area (207 m² g⁻¹) was synthesized by a coprecipitation method. Over the calcined support (550 °C), a monolayer equivalent (15 wt.%) of V₂O₅, CeO₂ or a combination of both were deposited by using wet-impregnation or co-impregnation methods to make the V₂O₅/TiO₂–ZrO₂, CeO₂/TiO₂–ZrO₂ and V₂O₅–CeO₂/TiO₂–ZrO₂ combination catalysts, respectively. These catalysts were characterized using X-ray diffraction (XRD), Raman, scanning electron microscopy (SEM), transmission electron microscopy (TEM), temperature preprogrammed reduction (TPR), CO₂ temperature preprogrammed desorption (TPD) and BET surface area methods. All characterization studies revealed that the deposited promoter oxides are in a highly dispersed form over the support, and the combined acid–base and redox properties of the catalysts play a major role in this reaction. The V₂O₅–CeO₂/TiO₂–ZrO₂ catalyst exhibited a better conversion and product selectivity than other combinations. In particular, the addition of CeO₂ to V₂O₅/TiO₂–ZrO₂ prevented catalyst deactivation and helped to maintain a high and stable catalytic activity.     

Effect of the hydrogen spillover on the selectivity of dibenzothiophene hydrodesulfurization over CoSx/γ-Al2O3, NiSx/γ-Al2O3 and MoS2/γ-Al2O3 catalysts

Dibenzothiophene (DBT) hydrodesulphurization (HDS) reaction at 3 MPa and 325–375 °C on Mo/γ-Al₂O₃ single-bed and Me/γ-Al₂O₃//SiO₂//Mo/γ-Al₂O₃ (Me = Co or Ni) double-bed catalysts were investigated. Results indicate that ratio cyclohexylbenzene (CHB)/biphenyl (BP) or selectivity is higher when using double-beds rather than a single-bed. Synergy in dibenzothiophene hydrodesulphurization on Co//Mo and Ni//Mo double-beds is also detected. Changes in selectivity and conversion are attributed to the action of spillover hydrogen (Hso) formed in the first bed that reaches the second bed.     

The interaction of cobalt species with alumina on Co/Al2O3 catalysts prepared by atomic layer deposition

Alumina supported cobalt catalysts were prepared by atomic layer deposition (ALD) of cobalt acetylacetonate precursors (Co(acac)₂ and Co(acac)₃). The main modes of interaction between the acetylacetonate precursors and the support were found to be the exchange reaction between the alumina OH-groups and the acac-ligands of the precursor and dissociative adsorption on coordinatively unsaturated Al³⁺ sites. The amount of precursor that could adsorb on the support was determined by steric hindrance. Samples were prepared using 1–5 reaction cycles, i.e. subsequent precursor addition (Co(acac)₂) and calcination, resulting in catalysts containing ca. 3–10 wt.% Co. Samples were also prepared where the last calcination step was omitted, i.e. uncalcined catalysts. Calcination at 450 °C decreased the reducibility of the Co(acac)₂/Al₂O₃ catalysts due to formation of a cobalt oxide phase strongly interacting with the support and aluminate type surface species. The reducibility increased with metal loading on both calcined and uncalcined catalysts; however the reducibility of the calcined catalysts remained lower than of the uncalcined ones. The dispersion was found to be lower on the calcined catalysts. The cobalt particle sizes on the calcined samples was ca. 8 nm and on the uncalcined 4–5 nm, for cobalt loadings of ca. 6–10 wt.%. Catalytic activity was tested by gas phase hydrogenation of toluene in temperature programmed mode (30–150 °C).     

Preferential CO oxidation in H2-rich gas mixtures over Au/doped ceria catalysts

Nanosized gold catalysts supported on doped ceria were prepared by deposition–precipitation method. A deep characterization study by HRTEM/EDS, XRD, FT-Raman, TPR and FTIR was undergone in order to investigate the effect of ceria modification by various cations (Sm³⁺, La³⁺ and Zn²⁺) on structural and redox properties of gold catalysts. Doping of ceria affected in different way catalytic activity towards purification of H₂ via preferential CO oxidation. The following activity order was observed: Au/Zn–CeO₂ > Au/Sm–CeO₂ > Au/CeO₂ > Au/La–CeO₂. The differences in CO oxidation rates were ascribed to different concentration of metallic gold particles on the surface of Au catalysts (as confirmed by the intensity of the band at 2103 cm⁻¹ in the FTIR spectra collected during CO–O₂ interaction). Gold catalysts on modified ceria showed improved tolerance towards the presence of CO₂ and H₂O in the PROX feed. The spectroscopic experiments evidence enhanced reactivity when PROX is performed in the presence of H₂O already at 90 K.     

Studies on the conditions of synthesis of picolinic acid by heterogeneous catalytic oxidation of 2-picoline

Heterogeneous oxidation of 2-picoline over binary P–Ti, Sb–Ti, P–Sb, and V–Ti oxide catalysts was studied over the temperature range of 200–300°C. The vanadium–titanium catalysts based on titanium dioxide (anatase) were found to be the most selective for picolinic acid. With binary catalysts containing 20–50% of vanadium pentoxide, the selectivity for picolinic acid was 19–22% at the 36–74% conversion of 2-picoline. A distinguishing feature of these catalysts is regular surface stacking of V₂O₅ and TiO₂ crystallites.     

Catalytic combustion of trichloroethene over Ru/Al2O3: Reaction mechanism and kinetic study

The performance of 0.5% Ru/Al₂O₃ for the deep oxidation of trichloroethene (1000–2500 ppmV, WHSV = 55 h⁻¹) in air was studied in this work. Experiments were carried out both at dry and wet (20,000 ppmV of H₂O) conditions. Catalytic performance was studied in terms of activity and selectivity for the different reaction products (CO₂, HCl, Cl₂, C₂Cl₄, CCl₄ and CHCl₃). Both the activity and the selectivity for total combustion are higher than other catalysts suggested in the literature for this process (especially Pd and Pt).The main organic by-products are CCl₄ and CHCl₃, whereas in all the other catalysts tested in the literature, tetrachloroethene is the main organic by-product. This fact suggests that the mechanism of the combustion reaction, involving a double-bond scission, is essentially specific for this catalyst.Kinetic data was fit to a pseudo-first order kinetic expression, providing fairly good fit.     

Oxidation of CH4 over Pd supported on TiO2-doped SiO2: Effect of Ti(IV) loading and influence of SO2

Titania-modified silicas with different weight% of TiO₂ were prepared by sol–gel method and used as supports for Pd (1 wt%) catalysts. The obtained materials were tested in the oxidation of methane under lean conditions in absence and in presence of SO₂. Test reactions were consecutively performed in order to evaluate the thermal stability and poisoning reversibility. Increasing amounts of TiO₂ improved the catalytic activity, with an optimum of the performance for 10 wt% TiO₂ loading. Moreover, the titania-containing catalysts exhibited a superior tolerance towards SO₂ by either adding it to the reactants or feeding it as a pure pretreatment atmosphere at 350 °C. Catalysts were characterized by XPS, XRD, FT-IR and BET measurements. According to the structural and surface analyses, the mixed oxides contained Si–O–Ti linkages which were interpreted as being responsible for the enhanced intrinsic activity of supported PdO with respect to PdO on either pure SiO₂ or pure TiO₂. Moreover, the preferential interaction of the sulfur molecule with TiO₂ and the easy SO_x desorption from high surface area silica were the determining factors for the superior SO₂ tolerance of the TiO₂-doped catalysts.     

CO hydrogenation to iso-C4 hydrocarbons over CeO2–TiO2 catalysts

CeO₂–TiO₂ (Ce:Ti = 0.25–9, molar ratio) catalysts were synthesized by a sol–gel method and the catalytic performances were evaluated in the selective synthesis of isobutene and isobutane from CO hydrogenation under the reaction conditions of 673–748 K, 1–5 MPa and 720–3000 h⁻¹. The physical properties, such as specific surface area, cumulative pore volume, average pore diameter, crystal phase and size, of the catalysts were characterized by N₂ adsorption/desorption and XRD. All the CeO₂–TiO₂ composite oxides showed higher surface areas than pure TiO₂ and CeO₂. No TiO₂ phase was detected on the samples of CeO₂–TiO₂ in which TiO₂ contents were in the range of 10–50 mol%. Crystalline Ce₂O₃ was detected in CeO₂–TiO₂ (8:2). The reaction conditions, temperature, pressure and space velocity, had obvious influences on the CO conversion and distribution of the products over CeO₂–TiO₂ (8:2) catalyst.     

Characteristics and catalytic properties of Pt–Sn/Al2O3 nanoparticles synthesized by one-step flame spray pyrolysis in the dehydrogenation of propane

The Pt–Sn/Al₂O₃ catalysts with 0.3 wt% Pt and 0.5–1.5 wt% Sn loading were prepared by one-step flame spray pyrolysis (FSP). Unlike the catalysts prepared by conventional impregnation method, the FSP-derived catalysts were composed of single-crystalline γ-alumina particles with the as-prepared primary particle size of 10–18 nm and contained only large pores. The FSP catalysts exhibited superior catalytic activity and better stability than the ones made by impregnation in the dehydrogenation of propane, while they did not alter the selectivity to propylene (in all cases, propylene selectivity ≥96%). The presence of large pores in the flame-made catalysts not only facilitated diffusion of the reactants and products but could also lessen the amount of carbon deposited during reactions. As revealed by CO chemisorption, transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS), the metal particles appeared to be partially covered by the alumina matrix (Al–O) due to the simultaneous formation of particles during FSP synthesis. Such phenomena, however, were shown to result in the formation of active Pt–Sn ensembles for propane dehydrogenation as shown by higher turnover frequencies (TOFs).     

Cu/Al2O3 catalysts for soot oxidation: Copper loading effect

Cu/Al₂O₃ catalysts with metal loading from 0.64 to 8.8 wt.% have been prepared and characterized by different techniques: N₂ adsorption at −196 °C (BET surface area), ICP (Cu loading), XRD, selective copper surface oxidation with N₂O (Cu dispersion), TPR-H₂ (redox properties), and XPS (copper surface species). The catalytic activity for soot oxidation has been tested both in air and NO_x/O₂. The activity in air depends on the amount of easily-reduced Cu(II) species, which are reduced around 275 °C under TPR-H₂ conditions. The amount of the most active Cu(II) species increases with the copper loading from Cu_1% to Cu_5% and remains almost constant for higher copper loading. In the presence of NO_x, the first step of the mechanism is NO oxidation to NO₂, and the catalytic activity for this reaction depends on the copper loading. For catalysts with copper loading between Cu_1% and Cu_5%, the catalytic activity for soot oxidation in the presence of NO_x depends on NO₂ formation. For catalysts with higher copper loading this trend is not followed because of the low reactivity of model soot at the temperature of maximum NO₂ production. Regardless the copper loading, all the catalysts improve the selectivity towards CO₂ formation as soot oxidation product both under air and NO_x/O₂.     

Selective oxidation of CO in the presence of H2, CO2 and H2O, on different zeolite-supported Pt catalysts

The selective oxidation of CO in the presence of H₂O and CO₂ has been studied on Pt supported on different zeolitic materials (MOR, ZSM-5, FAU and ETS-10) using a range of operating conditions and a variety of characterization techniques. The behavior of the Pt–ETS-10 and Pt–FAU catalysts has been investigated in more depth and the results obtained have been compared and related to the different characteristics of the supports. The best results in the presence of H₂O and CO₂ were obtained with Pt–FAU catalysts, showing stable catalytic activity and complete conversion of CO (λ = 2) at 439 K.