Oxidative dehydrogenation of ethane on a magnesium-vanadium aluminophosphate (MgVAPO-5) catalyst

Vanadium and/or magnesium substituted aluminophosphate with ALPO₄-5 structure have been prepared by hydrothermal synthesis. These catalysts have been tested for the oxidative dehydrogenation (ODH) of ethane. ALPO₄-5 has a low activity and low selectivity for the ODH of ethane. The presence of Mg²⁺ ions in MgAPO-5 increases the selectivity to ethene, while the presence of V⁵⁺ species in VAPO-5 increases both the activity and the selectivity for this reaction. The presence of Mg²⁺ and V⁵⁺ species in the vanadium-magnesium alumino-phosphate (MgVAPO-5) results in a more selective catalysts for the ODH of ethane. The behavior of MgVAPO-5 could be attributed to the presence of acid sites (Mg²⁺) near to the redox sites (V⁵⁺) in the molecular sieve framework.     

Similarity and differences in the oxidative dehydrogenation of C2–C4 alkanes over nano-sized VOx species using N2O and O2

The effect of O₂ and N₂O on alkane reactivity and olefin selectivity in the oxidative dehydrogenation of ethane, propane, n-butane, and iso-butane over highly dispersed VO_x species (0.79 V/nm²) supported on MCM-41 has been systematically investigated. For all the reactions studied, olefin selectivity was significantly improved upon replacing O₂ with N₂O. This is due to suppressing CO_x formation in the presence of N₂O. The most significant improving effect of N₂O was observed for iso-butane dehydrogenation: S(iso-butene) was ca. 67% at X(iso-butane) of 25%.Possible origins of the superior performance of N₂O were derived from transient experiments using ¹⁸O₂ traces. ¹⁸O¹⁶O species were detected in ¹⁸O₂ and ¹⁸O₂–C₃H₈ transient experiments indicating reversible oxygen chemisorption. In the presence of alkanes, the isotopic heteroexchange of O₂ strongly increased. Based on the distribution of labeled oxygen in CO_x and in O₂ as well as on the increased CO_x formation in sequential O₂–C₃H₈ experiments, it is suggested that non-lattice oxygen species (possibly of a bi-atomic nature) originating from O₂ are non-selective ones and responsible for CO_x formation. These species are not formed from N₂O.     

A VOx/meso-TiO2 catalyst for methanol oxidation to dimethoxymethane

Mesoporous TiO₂ was prepared by simply controlling the hydrolysis of Ti(OBu)₄ with the help of acetic acid. The mesoporous TiO₂ had a well-crystallized anatase phase and a high surface area of 290 m² g⁻¹ with a pore size of about 4 nm. The anatase phase and the mesoporous structure were maintained in the VO_x/TiO₂ catalyst with a monolayer dispersion of V₂O₅, however, the surface area decreased to 126 m² g⁻¹. The catalyst was highly active and selective for methanol oxidation, giving about 55% conversion of methanol and 85% selectivity to dimethoxymethane at 423 K.     

The retention of copper ions by AlPO4-5/VAPO-5 and their effect on reactant access

The reaction of copper salts with AlPO₄-5 or V^v-VAPO-5 under acidic (CuCl₂, pH adjusted to 2) but especially basic conditions (Cu(NH₃) ₄ ²⁺ , pH adjusted to 9) gives ion incorporations greater than expected by a simple ion exchange mechanism (both AlPO₄-5 and V^v-VAPO-5 could be expected to have no cation exchange capacity). Ion incorporation is proposed to occur initially at defect sites, and examination of the ESR spectrum of a dehydrated, evacuated CuCl₂-exchanged AlPO₄-5 shows that these defect sites give rise to a number of unique environments upon Cu^II incorporation. The CuCl₂-exchanged VAPO-5 retains a significant toluene accessibility to the V^v sites in the VAPO-5. However, the toluene accessibility in the Cu(NH₃) ₄ ²⁺ -exchanged VAPO-5 is significantly reduced and we propose this is due to a combination of the presence of crystalline CuO and structural collapse from reaction with base (NH₄OH). The ability of treatment with base (NH₄OH, pH 13) to restrict access of toluene to the V^v sites of the original VAPO-5 was verified in a separate experiment.     

The effect of potassium on the selective oxidation ofn-butane and ethane over Al2O3-supported vanadia catalysts

The catalytic properties of undoped and K-doped (K/V atomic ratio of 0.5) Al₂O₃-supported vanadia catalysts (4.5 wt% of V₂O₅) for the oxidation ofn-butane and ethane were studied. Isolated tetrahedral V⁵⁺ species are mainly observed in both undoped and K-doped samples. The incorporation of potassium decreases both the reducibility of surface vanadium species and the number of surface acid sites. Potassium-free vanadium catalysts show a high selectivity during the oxidative dehydrogenation (ODH) of ethane but a low selectivity during the ODH ofn-butane. However, the presence of potassium on the vanadium catalysts strongly influences their catalytic properties, increasing the selectivity to C₄-olefins fromn-butane and decreasing the selectivity to ethene from ethane. The role of the acid-base characteristics of catalysts on selectivity to ODH reactions is proposed.On leave from the Department of Industrial Chemistry and Materials, V. le Risorgimento 4, 40136 Bologna, Italy.     

Preparation of Ceria-Zeolite Catalysts by Different Techniques and Its Effect on Selective Catalytic Reduction of NO with NH3 at High Space Velocities

Several zeolite-based catalysts containing Ce³⁺ and/or CeO₂ were prepared by a variety of catalyst preparation techniques like ion exchange, solid-state ion exchange, impregnation and physical mixing and are characterised. Selective catalytic reduction was evaluated using simulated exhaust gas containing NO _x , NH₃, O₂ and H₂O at high space velocities (>180000 h^–1) in the temperature window 150–600 °C. The activity and selectivity in NO _x reduction was found to strongly depend on the charge compensating ions, crystallite size of the zeolite and CeO₂ content in the catalyst. CeO₂ mixed with zeolite having H⁺ or Ce³⁺ co-cations showed benificial effect and increased the NO _x conversion and selectivity. Among the different zeolite materials studied, the structure and the strength and amount of Brønsted acidity did not influence the NO _x conversion.     

The key factor determining the anodic deposition of vanadium oxides

This work demonstrates that anodic deposition of vanadium oxide (denoted as VO_x·nH₂O) can be considered as the chemical co-precipitation of V⁵⁺ and V⁴⁺ oxy-/hydroxyl species and the accumulation of V⁵⁺ species at the vicinity of electrode surface is the key factor for the successful anodic deposition of VO_x·nH₂O at a potential much more negative than the equilibrium potential of the oxygen evolution reaction (OER). The results of in situ UV-vis spectra show that the V⁴⁺/V⁵⁺ ratio near the electrode surface can be controlled by varying the applied potential, leading to different, three-dimensional (3D) nanostructures of VO_x·nH₂O. The accumulation of V⁵⁺ species due to V⁴⁺ oxidation at potentials ≥0.4 V (vs. Ag/AgCl) has been found to be very similar to the phenomenon by adding H₂O₂ in the deposition solution. The X-ray photoelectron spectroscopic (XPS) results show that all VO_x·nH₂O deposits can be considered as aggregates consisting of mixed V⁵⁺ and V⁴⁺ oxy-/hydroxyl species with the mean oxidation state significantly increasing with the applied electrode potential.     

Self Assembly and Properties of C:WO3 Nano-Platelets and C:VO2/V2O5 Triangular Capsules Produced by Laser Solution Photolysis

Laser photolysis of WCl₆ in ethanol and a specific mixture of V₂O₅ and VCl₃ in ethanol lead to carbon modified vanadium and tungsten oxides with interesting properties. The presence of graphene’s aromatic rings (from the vibrational frequency of 1,600 cm⁻¹) together with C–C bonding of carbon (from the Raman shift of 1,124 cm⁻¹) present unique optical, vibrational, electronic and structural properties of the intended tungsten trioxide and vanadium dioxide materials. The morphology of these samples shows nano-platelets in WO _x samples and, in VO _x samples, encapsulated spherical quantum dots in conjunction with fullerenes of VO _x . Conductivity studies revealed that the VO₂/V₂O₅ nanostructures are more sensitive to Cl than to the presence of ethanol, whereas the C:WO₃ nano-platelets are more sensitive to ethanol than atomic C.     

An in-situ Reduction/Oxidation XAS Study on the EL10V8 VO x /TiO2(Anatase) Powder Catalyst

The structural changes of the supported vanadium oxide in the V₂O₅/TiO₂(anatase) EUROCAT EL10V8 powder catalyst during reduction and oxidation at 420 and 490 °C were studied with in-situ X-ray absorption spectroscopy (XAS). The Vanadium K-edge XAS results are compared with pure bulk V₂O₅. For the reduction–oxidation cycle at 420 °C, similar structural changes as for bulk V₂O₅ were observed for the supported vanadium oxide: a reduction to the VO₂ structure and re-oxidation back to V₂O₅. After reduction at 490 °C however, a different structure was obtained: very regular “VO₆” octahedra with a V^2.8+ valence. This may point to a structural support effect.     

Catalyst’s Sulfur Displacement by Thiophene, as Monitored by Exchange of 35S Radiotracer

The isotopic exchange has been studied between catalyst radiosulfur and H₂S, formed in thiophene hydrodesulfurization (HDS) (named S-displace) on alumina supported molybdena, on CoMoO_x, PdMoO_x, PtMoO_x and on silica–alumina supported NiWO_x. S-displace was compared with radiosulfur exchange data between catalyst radiosulfur and gas phase H₂S (S^exc) determined previously. The extent of S^exc was higher than that of the S-displace for Mo, CoMo in and NiW, whereas the extent of S-displace from PdMoO and PtMoO was significantly higher, than that of S^exc. Thiophene HDS product distribution data are discussed in terms of increased C=C hydrogenation and C–C hydrogenolysis activity, explained by increasing H₂S production with longer circulation time of the thiophene/H₂ mixture, The C₁/C₃<1 ratios among C₄-hydrogenolysis products indicate some coke formation. The decrease of thiophene HDS activity is presumably a consequence of increasing site-blocking with the formation of more H₂S and coke with longer duration of thiophene treatment.     

Co-decorated carbon nanotube-supported Co–Mo–K sulfide catalyst for higher alcohol synthesis

Using chemical reduction-deposition method, a type of metallic cobalt-decorated multi-walled carbon nanotubes, noted as y%(mass percentage)Co/MWCNTs, was prepared. TEM, SEM and XRD measurements demonstrated that the metallic cobalt was evenly coated on the MWCNT substrate, with granule-diameter of the Co _x ⁰ -crystallites of 5–8 nm. Using the y%Co/MWCNTs as support, a type of supported Co–Mo–K sulfide catalysts, noted as x%(Co _i Mo _j K _k )/(y%Co/MWCNTs), for higher alcohol synthesis (HAS) was developed. It was experimentally shown that using the Co-modified MWCNTs in place of simple MWCNTs or activated carbon (AC) as the catalyst support led to a significant increase in activity of CO hydrogenation conversion and improvement in the selective formation of C₂₊-alcohols. Under the reaction condition of 5.0 MPa, 613 K, CO/H₂/N₂ = 45/45/10 (v/v) and GHSV = 3600 ml_STPh⁻¹ g _−cat. ⁻¹ , the observed STY of C_1–4-alcohols reached 154.1 mgh⁻¹g _−cat. ⁻¹ at 12.6% conversion of CO over the 11.6%(Co₁Mo₁K_0.6)/(6.4%Co/MWCNTs) catalyst, which was 1.76 and 2.33 times as high as that (87.7 and 66.1 mgh⁻¹g _−cat. ⁻¹ ) of the reference systems supported by simple MWCNTs and AC respectively. Ethanol became the predominant product of the CO hydrogenation, with carbon-based selectivity ratio of C_2–4-alcohols to CH₃OH reaching 3.6 in the products. It was experimentally found that using the Co-modified MWCNTs in place of simple MWCNTs or AC as the catalyst support caused little change in the apparent activation energy for the conversion of CO, but led to a slight increase in the molar percentage of catalytically active Mo-species (Mo⁴⁺) in the total Mo-amount at the surface of the functioning catalyst. Based upon the results of TPD investigation, it could be inferred that, under the reaction condition of HAS, there existed a considerably larger amount of adsorbed H-species and CO-species on the functioning catalyst, thus in favour of increasing the rate of a series of surface hydrogenation reactions in HAS.     

The effects of titania overlayers on C2H4/CO/H2 reactions over a Rh foil

The effects of submonolayer deposits of titania on the activity and selectivity of a Rh foil catalyst for C₂H₄/CO/H₂ reactions have been investigated. Reactions were carried out at 1 atm total pressure and at temperature of 488 K and 523 K. The addition of titania to the catalyst enhances the total rate of C₃-oxygenate formation. This rate enhancement is due entirely to an increase in the rate of 1-propanol formation, which reaches a maximum at a TiO _x . coverage of 0.2 ML. The rate of propanal formation, by contrast, is not enhanced. The rates of formation of methane, ethane, and C₃-hydrocarbons also exhibit rate maxima at a TiO _x . coverage of 0.2 ML. The rates of formation of C₄- and C₅-hydrocarbons, on the other hand, are suppressed by titania addition. The higher rate of 1-propanol production in the presence of titania is attributed to an interaction between Ti³⁺ ions at the edge of TiO _x . islands and the carbonyl bond of adsorbed C₃-oxygenated species. Such interactions are envisioned to facilitate hydrogenation of the carbonyl bond.     

Selective oxidation of H2S to sulfur over vanadia supported on mesoporous zirconium phosphate heterostructure

Vanadium oxide supported on mesoporous zirconium phosphate catalysts has been synthesized, characterized and tested in the selective oxidation of H₂S to sulfur. The nature of the vanadium species depends on the V-loading of catalyst. Catalysts with a V-content lower than 4wt% present both isolated vanadium species and V₂O₅ crystallites. However, V₂O₅ crystallites have been mainly observed in catalysts with higher V-content, although the presence of isolated V-species on the surface of the metal oxide support cannot be completely ruled out. The catalytic behaviour also depends on V-loading of catalysts. Thus, while the catalytic activity of catalysts can be related to the number of V-sites, the catalyst decay is clearly observed in samples with low V-loading. The characterization of catalysts after the catalytic tests indicates the presence of sulfur on the catalyst, which is favoured on catalysts with low V-loading. However, a clear transformation of V₂O₅ to V₄O₉ can be proposed according to XRD and Raman results of used catalysts with high V-loading. The importance of V⁵⁺–O–V⁴⁺ pairs in activity and selectivity is also discussed.     

Partial oxidation of methane over VOx/α-Al2O3 catalysts

The catalytic behavior of a series of VO_x/α-Al₂O₃ catalysts for the partial oxidation of methane has been evaluated. Samples with different vanadia loading were prepared from NH₄VO₃ and V(AcAc)₃. Characterization performed by TPR and oxygen uptake measurements indicates that different VO_x species are present on the samples. The catalytic patterns indicate that each V-surface species possesses different activity and selectivity. Isolated vanadates are the most active and selective towards HCHO, while V₂O₅ crystallites are detrimental to the catalytic performance.     

Electrical properties of V2O5–WO3/TiO2 EUROCAT catalysts evidence for redox process in selective catalytic reduction (SCR) deNOx reaction

Electrical conductivity measurements on EUROCAT V₂O₅–WO₃/TiO₂ catalyst and on its precursor without vanadia were performed at 300°C under pure oxygen to characterize the samples, under NO and under NH₃ to determine the mode of reactivity of these reactants and under two reaction mixtures ((i) 2000 ppm NO + 2000 ppm NH₃ without O₂, and (ii) 2000 ppm NO + 2000 ppm NH₃ + 500 ppm O₂) to put in evidence redox processes in SCR deNO_x reaction.It was first demonstrated that titania support contains certain amounts of dissolved W⁶⁺ and V⁵⁺ ions, whose dissolution in the lattice of titania creates an n-type doping effect. Electrical conductivity revealed that the so-called reference pure titania monolith was highly doped by heterovalent cations whose valency was higher than +4. Subsequent chemical analyses revealed that so-called pure titania reference catalyst was actually the WO₃/TiO₂ precursor of V₂O₅–WO₃/TiO₂ EUROCAT catalyst. It contained an average amount of 0.37 at.% W⁶⁺dissolved in titania, i.e. 1.07 × 10²⁰ W⁶⁺ cations dissolved/cm³ of titania. For the fresh catalyst, the mean amounts of W⁶⁺ and V⁵⁺ ions dissolved in titania were found to be equal to 1.07 × 10²⁰ and 4.47 × 10²⁰ cm⁻³, respectively. For the used catalyst, the mean amounts of W⁶⁺ and V⁵⁺ ions dissolved were found to be equal to 1.07 × 10²⁰ and 7.42 × 10²⁰ cm⁻³, respectively. Since fresh and used catalysts have similar compositions and similar catalytic behaviours, the only manifestation of ageing was a supplementary progressive dissolution of 2.9 × 10²⁰ additional V⁵⁺ cations in titania.After a prompt removal of oxygen, it appeared that NO alone has an electron acceptor character, linked to its possible ionosorption as NO⁻ and to the filling of anionic vacancies, mostly present on vanadia. Ammonia had a strong reducing behaviour with the formation of singly ionized vacancies. A subsequent introduction of NO indicated a donor character of this molecule, in opposition to its first adsorption. This was ascribed to its reaction with previously adsorbed ammonia strongly bound to acidic sites. Under NO + NH₃ reaction mixture in the absence of oxygen, the increase of electrical conductivity was ascribed to the formation of anionic vacancies, mainly on vanadia, created by dehydroxylation and dehydration of the surface. These anionic vacancies were initially subsequently filled by the oxygen atom of NO. N^o atoms, resulting from the dissociation of NO and from ammonia dehydrogenation, recombined into dinitrogen molecules. The reaction corresponded to

Full-size image

. In the presence of oxygen, NO did not exhibit anymore its electron acceptor character, since the filling of anionic vacancies was performed by oxygen from the gas phase. NO reacted directly with ammonia strongly bound on acidic sites. A tentative redox mechanism was proposed for both cases.     

Selective Photo-Assisted Oxidation of Methane into Formaldehyde on Mesoporous VO x /SBA-15 Catalysts

Mesoporous VO _x /SBA-15 samples have been prepared by different impregnation methods, characterized by N₂ adsorption and diffuse reflectance UV–visible spectroscopy, and their photocatalytic reactivity evaluated for the selective oxidation of methane with oxygen at 220 °C under UV irradiation. Vanadium in dehydrated VO _x /SBA-15 samples was found to be predominantly as isolated four-coordinated V⁵⁺ species, especially at V loading below 2.5 wt%. VO _x /SBA-15 catalysts prepared by impregnation with an aqueous solution of ammonium metavanadate (AMV) showed a maximum rate of formation of formaldehyde of 525 mol g^-1 h^-1 (selectivity of 93.8 mol%) at a vanadium content of 2.65 wt%. Both the rate of formation and selectivity of formaldehyde were improved when vanadium was impregnated on the SBA-15 support from a vanadyl sulfate methanolic solution (rate of formation of HCHO = 733 mol g^-1 h^-1, selectivity = 95.4 mol%). VO _x /SBA-15 catalysts were seen to be more effective for the selective photo-assisted oxidation of methane than VO _x /SiO₂ catalysts.     

Multifunctionality of Active Centers in (Amm)oxidation Catalysts: From Bi–Mo–O x to Mo–V–Nb–(Te, Sb)–O x

Catalytic centers in selective (allylic) oxidation and ammoxidation catalysts are multimetallic and multifunctional. In the historically important bismuth molybdates, used for propylene (amm)oxidation, they are composed of (Bi³⁺)(Mo⁶⁺)₂ complexes in which the Bi³⁺ site is associated with the -H abstraction and the (Mo⁶⁺)₂ site with the propylene chemisorption and O or NH insertion. An updated reaction mechanism is presented. In the Mo–V–Nb–Te–O _x systems, three crystalline phases (orthorhombic Mo_7.5V_1.5NbTeO₂₉, pseudohexagonal Mo₆Te₂VO₂₀, and monoclinic TeMo₅O₁₆) were identified, with the orthorhombic phase being the most important one for propane (amm)oxidation. Its active centers contain all necessary key catalytic elements (2V⁵⁺/Mo⁶⁺, 1V⁴⁺/Mo⁵⁺, 2Mo⁶⁺/Mo⁵⁺, 2Te⁴⁺) for this reaction wherein a V⁵⁺ surface site (V⁵⁺ = O ⁴⁺V–O) is associated with paraffin activation, a Te⁴⁺ site with -H abstraction once the olefin has formed, and a (Mo⁶⁺)₂ site with the NH insertion. Four Nb⁵⁺ centers, each surrounded by five molybdenum octahedra, stabilize and structurally isolate the catalytically active centers from each other (site isolation), thereby leading to high selectivity of the desired acrylonitrile product. A detailed reaction mechanism of propane ammoxidation to acrylonitrile is proposed. Combinatorial methodology identified the nominal composition Mo_0.6V_0.187Te_0.14Nb_0.085O _x for maximum acrylonitrile yield from propane, 61.8% (86% conversion, 72% selectivity at 420 °C). We propose that this system, composed of 60% Mo_7.5V_1.5NbTeO₂₉, 40% Mo₆Te₂VO₂₀, and trace TeMo₅O₁₆, functions with a combination of compositional pinning of the optimum orthorhombic Mo_7.5V_1.5±x Nb_1±y Te_1±z O_29± phase and symbiotic mop-up of olefin intermediates through phase cooperation. Under mild reaction conditions, a single optimum orthorhombic composition might suffice as the catalyst; under demanding conditions this symbiosis is additionally required. Improvements in catalyst performance could be attained by further optimization of the elemental distributions at the active catalytic center of Mo_7.5V_1.5NbTeO₂₉, by promoter/modifier substitutions, and incorporation of compatible cocatalytic phases (preferably epitaxially matched). High-throughput methods will greatly accelerate the rational catalyst design processes.     

Oxidative dehydrogenation of n-butane over mesoporous VO x /SBA-15 catalysts

Mesoporous VO _x /SBA-15 catalysts with different V contents were evaluated for the oxidative dehydrogenation of n-butane and characterized by N₂ adsorption, XRD, HRTEM, H₂-TPR, NH₃-TPD, XPS, and EPR techniques. Compared to conventional V₂O₅/SiO₂ catalysts, the VO _x /SBA-15 catalysts showed better performance. The good performance can be attributed to the good dispersion of V species, presence of V⁴⁺ species and the pore structure.     

Surface properties and catalytic performance of La1−xSrxCrO3 perovskite-type oxides for CO and C3H6 combustion

Catalytic CO oxidation and C₃H₆ combustion have been studied over La_1−xSr_xCrO₃ (x = 0.0–0.3) oxides prepared by solid-state reaction and characterised by X-ray diffraction (XRD), nitrogen adsorption (BET analysis) and X-ray photoelectron spectroscopy (XPS). The expected orthorhombic perovskite structure of the chromite is observed for all levels of substitution. However, surface segregation of strontium along with a chromium oxidation process, leading to formation of Cr⁶⁺-containing phases, is produced upon increasing x and shown to be detrimental to the catalytic activity. Maximum activity is achieved for the catalyst with x = 0.1 in which mixed oxide formation upon substitution of lanthanum by strontium in the chromite becomes maximised.     

An In Situ ESR Study of Pd/H-ZSM-5 Interaction with Different Adsorbents

In situ ESR at 120–473 K permits to monitor formation of transient paramagnetic ions/complexes (isolated Pd⁺ sites; Pd⁺/H₂O; Pd⁺/C₆H₆) upon interaction of isolated Pd²⁺ cations stabilized by the H-ZSM-5 matrix with different organic compounds and gas mixtures (NO, O₂, H₂O, H₂, propene, benzene). The in situ study provides insight into the elementary steps of redox processes on isolated Pd species in H-ZSM-5 zeolite under realistic conditions. Adsorbed water stabilizes the transient paramagnetic complex and decreases the rate of Pd²⁺ to Pd⁰ reduction by H₂. Strong bonding of NO _x ligands to Pd²⁺ species suppresses the reduction of Pd(II) ions. Sorption of benzene on preoxidized Pd²⁺/HZSM-5 is accompanied by an easy formation of organic cation-radicals and of a Pd⁺/benzene complex, the paramagnetic Pd⁺/benzene structure indicating a surprisingly high resistance to further reduction to Pd⁰. Illumination of the Pd/HZSM-5 by UV-visible light causes no measurable change in the redox properties of the catalyst.