Selective oxidation of propane and ethane on diluted Mo–V–Nb–Te mixed-oxide catalysts

Te-free and Te-containing Mo–V–Nb mixed oxide catalysts were diluted with several metal oxides (SiO₂, γ-Al₂O₃, α-Al₂O₃, Nb₂O₅, or ZrO₂), characterized, and tested in the oxidation of ethane and propane. Bulk and diluted Mo–V–Nb–Te catalysts exhibited high selectivity to ethylene (up to 96%) at ethane conversions <10%, whereas the corresponding Te-free catalysts exhibited lower selectivity to ethylene. The selectivity to ethylene decreased with the ethane conversion, with this effect depending strongly on the diluter and the catalyst composition. For propane oxidation, the presence of diluter exerted a negative effect on catalytic performance (decreasing the formation of acrylic acid), and α-Al₂O₃ can be considered only a relatively efficient diluter. The higher or lower interaction between diluter and active-phase precursors, promoting or hindering an unfavorable formation of the active and selective crystalline phase [i.e., Te₂M₂₀O₅₇ (M = Mo, V, and Nb)], determines the catalytic performance of these materials.     

Effect of preparation and reaction condition on the catalytic performance of Mo–V–Te–Nb catalysts for selective oxidation of propane to acrylic acid by high-throughput methodology

Combinatorial screening technique has been applied to investigate the catalytic activity and selectivity of quaternary Mo–V–Te–Nb mixed oxide catalysts treated with various chemicals during preparation for selective oxidation of propane to acrylic acid. The catalyst libraries were prepared by the slurry method and catalytic activities were examined in 32-channel high-throughput screening reactor system coupled with a mass spectrometer and/or gas chromatograph.The obtained results provided substantial evidence that the sample preparation condition would have strong effect on the catalytic performance for propane selective oxidation. Among screened samples, Mo–V–Te–Nb treated with HIO₃ solution presented a better performance. The reaction results of promising catalysts selected from the libraries were applied to further scale-up of the system and confirmed catalytic performance. Quantification of the result of Mo–V–Te–Nb treated with HIO₃ solution was realized by combination of GC and MS and relationship between the MS data and the GC results can be established.     

Oxidation of isobutane over Mo–V–Sb mixed oxide catalyst

The catalytic performances of Mo–V–Sb mixed oxide catalysts have been studied in the selective oxidation of isobutane into methacrolein. V–Sb mixed oxide showed the activity for oxidative dehydrogenation of isobutane to isobutene. The selectivity to methacrolein increased by the addition of molybdenum species to the V–Sb mixed oxide catalyst. In a series of Mo–V–Sb oxide catalysts, Mo₁V₁Sb₁₀O_x exhibited the highest selectivity to methacrolein at 440°C. The structure analyses by XRD, laser Raman spectroscopy and XPS showed the coexistence of highly dispersed molybdenum suboxide, VSbO₄ and -Sb₂O₄ phases in the Mo₁V₁Sb₁₀O_x. The high catalytic activity of Mo₁V₁Sb₁₀O_x can be explained by the bifunctional mechanism of highly dispersed molybdenum suboxide and VSbO₄ phases. It is likely that the oxidative dehydrogenation of isobutane proceeds on the VSbO₄ phase followed by the oxidation of isobutene into methacrolein on the molybdenum suboxide phase.     

Propane Selective Oxidation Over Monophasic Mo–V–Te–O Catalysts Prepared by Hydrothermal Synthesis

Two distinct phases, orthorhombic and hexagonal, of Mo–V–Te–O mixed oxide catalysts were prepared separately by the hydrothermal synthetic method and solid-state reaction, and these catalysts were tested for propane selective oxidation to acrylic acid. The hydrothermally synthesized orthorhombic phase of the Mo–V–Te–O catalyst showed high activity and selectivity for the oxidation of propane into acrylic acid. This catalyst also showed extremely high catalytic performance in the propene oxidation, producing acrylic acid in a high yield. The hexagonal Mo–V–Te–O catalyst was formed via the solid-state reaction between the orthorhombic Mo–V–Te–O and -TeVO₄. This phase showed poor activity to both propane and propene oxidations, although the hexagonal phase was constructed with the octahedra of Mo and V similar to the orthorhombic phase. Reaction kinetics study over the catalyst with orthorhombic structure revealed that propane oxidation was of first order with respect to propane and nearly zero order with respect to oxygen, suggesting that the rate-determining step of the reaction is C–H bond breaking of propane to form propene. Structural effects on the catalytic oxidation performance were discussed.     

W-promoted Co–Mo–K/γ–Al2O3 catalysts for water–gas shift reaction

The effects of incorporating tungsten into the traditional Co–Mo–K/γ–Al₂O₃ catalysts on the catalytic performances for water–gas shift reaction were investigated. Activity tests showed that W-promoted Co–Mo–K/γ–Al₂O₃ catalysts exhibited higher activity than W-free Co–Mo–K/γ–Al₂O₃ catalyst. Raman and H₂-TPR studies indicated that part of the octahedrally coordinated Mo–O species on Co–Mo–K catalysts transformed into tetrahedrally coordinated Mo–O species in the presence of W promoter.     

V–Mg–O catalysts for oxidative dehydrogenation of alkylpyridines and alkylthiophenes

Vanadium appears to be the element that is most frequent (along with molybdenum) used in the catalyst formulations for oxidative dehydrogenation (ODH) of hydrocarbons and alcohols. In the present work the employment of ODH reaction in the presence of air has been extended for the preparation of vinyl substituted pyridines and thiophenes using vanadium (and for comparison molybdenum) oxide catalysts.The efficiency of vanadium–magnesium oxide catalysts in the production of vinylpyridines and vinylthiophenes has been evaluated. A strong dependence of the yield and selectivity of the latter upon the vanadium (molybdenum) oxide loading and the conditions of heat treatment were observed. In optimized reaction conditions V–Mg–O catalysts at the temperature approximate 450 °C ensured the formation of vinylpyridines and vinylthiophenes with the yield of 40–60% at the selectivity of 90%. In prolonged runs no visible changes in the performance of the catalyst were observed. DTA–DTG, XRD, IR ESR, NMR methods have been used detecting the formation of species of V–Mg–O catalysts that appear to be responsible for the catalyst efficiency in the reactions under consideration.     

A Study of the Functionalities of the Phases in Mo–V–Nb–Te Oxides for Propane Ammoxidation

Catalysts belonging to the Mo–V–Nb–Te–O system have been prepared with both a slurry method and hydrothermal synthesis and were tested for propane and propylene ammoxidation to acrylonitrile. All samples were characterized with BET, XRD, ICP and XPS. The catalysts were found to consist of three phases, to which activity and selectivity correlations were made. The results indicate that both an orthorhombic phase and a hexagonal phase are needed to have an active and selective catalyst. The orthorhombic phase is the most active for propane conversion although less selective than the hexagonal phase for the conversion of formed propylene to acrylonitrile.     

On the effect of La–Cr–O– phase composition on diesel soot catalytic combustion

A series of samples of La–Cr–O– perovskites were designed as catalysts for diesel soot combustion. They were prepared by using co-precipitation method, at ambient temperature using ammonia followed by a hydrothermal treatment (T = 200 °C, P = 20 atm, t = 24 h). All the chromium-containing precursors were then calcined at high temperature to develop the oxide catalyst. Two phase composition 86%LaCrO₃–(14%) La₂CrO₆ or 94%LaCrO₃–6%La₂O₃ were formed depending on the atmosphere of calcination (oxygen or hydrogen respectively) used. Their respective BET surface areas were 1.1 and 6.5 m² g⁻¹. Highly crystalline, pure phase of LaCrO₃ and La₂CrO₆ powders were also prepared, with BET area of 4 and 3 m² g⁻¹, respectively. The crystalline structure and properties of all samples were characterised by X-ray diffraction (XRD), using Rietveld refinement, and temperature-programmed reduction (TPR) techniques. O₂-TPD measurements performed on all samples showed the presence of suprafacial, weakly chemisorbed oxygen only for LaCrO₃, which contributes actively to soot combustion. TPR study performed on all catalysts showed that while pure LaCrO₃ and La₂O₃ samples did not reduce, the biphasic catalysts showed the presence of oxygen depletion peaks characteristic of lattice oxygen mobility in the samples at ca. 665 °C. Catalytic combustion of diesel soot was studied over all catalysts. The results showed that pure LaCrO₃ exhibited significant catalytic activity which was sensitively affected by the modifier La₂CrO₆ or La₂O₃.     

Selective oxidation of propane over alkali-doped Mo–V–Sb–O catalysts

Alkali metal-doped MoVSbO catalysts have been prepared by impregnation of a MoVSbO-mixed oxide (prepared previously by a hydrothermal synthesis) and finally activated at 500 or 600 °C in N₂. The catalysts have been characterized and tested for the selective oxidation of propane and propylene. Alkali-doped catalysts improved in general the catalytic performance of MoVSbO, resulting more selective to acrylic acid and less selective to acetic acid than the corresponding alkali-free MoVSbO catalysts. However, the specific behaviour strongly depends on both the alkali metal added and/or the final activation temperature. At isoconversion conditions, catalysts activated at 600 °C present selectivity to acrylic acid higher than that achieved on those activated at 500 °C, both K-doped catalysts presenting the highest yield to acrylic acid. The changes in the number of acid sites as well as the nature of crystalline phases can explain the catalytic behaviour of alkali-doped MoVSbO catalysts.     

Thermal behaviour of Cu–Mg–Mn and Ni–Mg–Mn layered double hydroxides and characterization of formed oxides

Thermal behaviour of synthetic Cu–Mg–Mn and Ni–Mg–Mn layered double hydroxides (LDHs) with M^II/Mg/Mn molar ratio of 1:1:1 was studied in the temperature range 200–1100 °C by thermal analysis (TG/DTA/EGA), powder X-ray diffraction (XRD), Raman spectroscopy, and voltammetry of microparticles. Powder XRD patterns of prepared LDHs showed characteristic hydrotalcite-like phases, but further phases were indirectly found as admixtures. The Cu–Mg–Mn precipitate was decomposed at temperatures up to ca. 200 °C to form an XRD-amorphous mixture of oxides. The crystallization of CuO (tenorite) and a spinel type mixed oxide of varying composition Cu_xMg_yMn_zO₄ with Mn⁴⁺ was detected at 300–500 °C. At high temperatures (900–1000 °C), tenorite disappeared and a consecutive crystallization of 2CuO·MgO (gueggonite) was observed. The high-temperature transformation of oxide phases led to a formation of Cu^I oxides accompanied by oxygen evolution. The DTA curve of Ni–Mg–Mn sample exhibited two endothermic effects characteristic for hydrotalcite-like compounds. The first one with minimum at 190 °C can be ascribed to a loss of interlayer water, the second one with minimum at 305 °C to the sample decomposition. Heating of the Ni–Mg–Mn sample at 300 °C led to the onset of crystallization of oxide phases identified as Ni_xMg_yMn_zO₄ spinel, (Ni,Mg)O oxide containing Mn⁴⁺ cations, and easily reducible XRD-amorphous species, probably free Mn^III,IV oxides. At 600 °C (Raman spectroscopy) and 700 °C (XRD), the (Ni,Mg)₆MnO₈ oxide with murdochite structure together with spinel phase were detected. Only spinel and (Ni,Mg)O were found after heating at 900 °C and higher temperatures. Temperature-programmed reduction (TPR) profiles of calcined Cu–Mg–Mn samples exhibited a single reduction peak with maximum around 250 °C. The highest H₂ consumption was observed for the sample calcined at 800 °C. The reduction of Ni–Mg–Mn samples proceeded by a more complex way and the TPR profiles reflected the phase composition changing depending on the calcination temperature.     

Hydrothermal synthesis (200 °C) of Co–kaolinite and Al–Co–serpentine

In the systems CoO–Al₂O₃–SiO₂–H₂O and CoO–Al₂O₃–SiO₂–HCl–H₂O, at initial pH between 5.5 and 8.1 and temperature of 200 °C, kaolinite is unstable and the following phases form through a dissolution-precipitation process: a) kaolinite and Co-bearing kaolinite; b) Al–Co–serpentine; and c) poorly crystalline phases. Identification of the several phases was carried out from a combination of X-ray diffraction and transmission/analytical electron microscopy.Co–kaolinite shows variable morphologies: a) Platy lath-shaped particles with very low Co content; b) Spherical particles, with relatively constant Co contents (in the order of 0.10 apfu); c) Kaolinite stacks with very variable Co contents (up to 0.25 apfu). Analytical data indicate that the presence of Co(OH)₂ in the system favors the dissolution process as well as serpentine formation but it leads to the parallel formation of abundant poorly crystalline phases. The Co-content in kaolinite increased as a function of the Co(OH)₂/CoCl₂ ratio in the initial systems, and it is reflected by a parallel increase of the b-cell parameter of kaolinite. The average composition of the coexisting Al–Co–serpentine is: (Al_1.20Fe_0.11Co_1.27)(Si_1.64Al_0.36)O₅(OH,Cl)₂, with Cl contents in the order of 0.14 apfu.The assemblage Co–kaolinite + Al–Co–serpentine, which appears to be stable at 200 °C, has not been described in natural environments, probably because it requires unusual Al- and Co-rich chemical systems.     

Mo–V–Nb mixed oxides as catalysts in the selective oxidation of ethane

Mo–V–Nb–O mixed metal oxides, obtained by heat-treatment in N₂ at 425 °C, have been studied as catalysts in the oxidative dehydrogenation of ethane. They present higher catalytic activity, while maintaining the same selectivity to ethylene, than the corresponding metal oxides calcined under air. Both amorphous and crystalline phases are present on active and selective catalysts. The implications of the presence of these phases as well as their physicochemical characteristics on the nature of active and selective sites are discussed.     

Direct synthesis of mesoporous silicalite-1 supported TiO2–ZrO2 for the dehydrogenation of EB to styrene with CO2

Direct synthesis route was developed to support TiO₂–ZrO₂ binary metal oxide onto the carbon templated mesoporous silicalite-1 (CS-1). Metal hydroxide modified carbon particles could play a role as hard template and simultaneously support metal components on the mesopores during the crystallization of zeolites. Such supported TiO₂–ZrO₂ binary metal oxides (TZ/CS-1) showed better resistance to deactivation in the oxidative dehydrogenation of ethylbenzene (ODHEB) in the presence of CO₂. These catalysts were found to be active, selective and catalytically stable (10 h of time-on-stream) at 600 °C for the dehydrogenation of ethylbenzene (EB) to styrene (Sty).     

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.     

Surface active sites present in the orthorhombic M1 phases: low energy ion scattering study of methanol and allyl alcohol chemisorption over Mo–V–Te–Nb–O and Mo–V–O catalysts

The outermost surface compositions and chemical nature of active surface sites present on the orthorhombic (M1) Mo–V–O and Mo–V–Te–Nb–O phases were determined employing methanol and allyl alcohol chemisorption and surface reaction in combination with low energy ion scattering (LEIS). These orthorhombic phases exhibited vastly different behavior in propane (amm)oxidation reactions and, therefore, represented highly promising model systems for the study of the surface active sites. The LEIS data for the Mo–V–Te–Nb–O catalyst indicated surface depletion for V (−23%) and Mo (−27%), and enrichments for Nb (+55%) and Te (+165%) with respect to its bulk composition. Only minor changes in the topmost surface composition were observed for this catalyst under the conditions of the LEIS experiments at 400 °C, which is a typical temperature employed in these propane transformation reactions. These findings strongly suggested that the bulk orthorhombic Mo–V–Te–Nb–O structure is terminated by a unique active and selective surface layer in propane (amm)oxidation. Moreover, direct evidence was obtained that the topmost surface VO _x sites in the orthorhombic Mo–V–Te–Nb–O catalyst were preferentially covered by chemisorbed allyloxy species, whereas methanol was a significantly less discriminating probe molecule. The surface TeO _x and NbO _x sites on the Mo–V–Te–Nb–O catalyst were unable to chemisorb these probe molecules to the same extent as the VO _x and MoO _x sites. These findings suggested that vastly different catalytic behavior exhibited by the Mo–V–O and Mo–V–Te–Nb–O phases is related to different surface locations of V⁵⁺ ions in the orthorhombic Mo–V–O and Mo–V–Te–Nb–O catalysts. Although the proposed isolated V⁵⁺ pentagonal bipyramidal sites in the orthorhombic Mo–V–O phase may be capable of converting propane to propylene with modest selectivity, the selective 8-electron transformation of propane to acrylic acid and acrylonitrile may require the presence of several surface VO _x redox sites lining the entrances to the hexagonal and heptagonal channels of the orthorhombic Mo–V–Te–Nb–O phase. Finally, the present study strongly indicated that chemical probe chemisorption combined with low energy ion scattering (LEIS) is a novel and highly promising surface characterization technique for the investigation of the active surface sites present in the bulk mixed metal oxides.     

Selective hydrogenation of maleic anhydride to γ-butyrolactone and tetrahydrofuran by Cu–Zn–Zr catalyst in the presence of ethanol

A series of Cu–Zn–Zr catalysts were prepared by a coprecipitation method and characterized by X-ray diffraction, X-ray photoelectron spectroscopy, temperature programmed reduction, and N₂ adsorption. The catalytic activity of the Cu–Zn–Zr catalyst in the hydrogenation of maleic anhydride using ethanol as a solvent was studied at 220–280 °C and 1 MPa. Maleic anhydride was mainly hydrogenated to γ-butyrolactone and tetrahydrofuran while ethanol dehydrogenated to ethyl acetate. After reduction, CuO species present in the calcined Cu–Zn–Zr catalysts were converted to metallic copper (Cu⁰). The presence of ZrO₂ favored the deep hydrogenation of γ-butyrolactone to tetrahydrofuran while the presence of ZnO was beneficial to the formation of the intermediate product γ-butyrolactone. The molar ratios of the hydrogen produced in ethanol dehydrogenation to the hydrogen consumed in maleic anhydride hydrogenation increased with the increase of the reaction temperature.     

Surface texturing of Mo–V–Te–Nb–O x selective oxidation catalysts

The paper concentrates on the study of Mo–V–Te–Nb oxide mixtures by electron microscopy combined with catalytic investigation of these materials in the partial oxidation of propane. Surface texturing of catalyst particles composed of two phases referred to in the literature as M1 and M2 is revealed by high-resolution transmission electron microscopy of high performing catalysts. The chemical composition of the catalyst surface is modified by treatment in water to obtain a significant increment in yield of acrylic acid. A chemical realization of the site isolation concept recurring on a supramolecular arrangement of catalyst and reactant rather than on atomic site isolation is suggested. A complex Mo–V–Te–Nb–O _x precursor phase carries nanoparticles made from a network of oxoclusters active as catalyst for the conversion of propane to acrylic acid. The designed synthesis of the multi-element oxide bulk and of the surface structure with a different composition than the precursor phase improved the performance by a factor of 4.     

Catalytic combustion of chlorobenzene over MnOx–CeO2 mixed oxide catalysts

A series of MnOx–CeO₂ mixed oxide catalysts with different compositions prepared by sol–gel method were tested for the catalytic combustion of chlorobenzene (CB), as a model of volatile organic compounds of chlorinated aromatics. MnOx–CeO₂ catalysts with different ratios of Mn/Ce + Mn were found to possess high catalytic activity in the catalytic combustion of CB, and MnOx(0.86)–CeO₂ was identified as the most active catalyst, on which the temperature of complete combustion of CB was 254 °C. Effects of systematic variation of reaction conditions, including space velocity and inlet CB concentration on the catalytic combustion of CB were investigated. Additionally, the stability and deactivation of MnOx–CeO₂ catalysts were studied by various characterization methods and other assistant experiments. MnOx–CeO₂ catalysts with high Mn/Ce + Mn ratios present a stable high activity, which is related to their high ability to remove the adsorbed Cl species and a large amount of active surface oxygen.     

Development of cobalt–copper nanoparticles as catalysts for higher alcohol synthesis from syngas

The synthesis of higher alcohols from syngas has been studied over different types of Cu-based catalysts. In order to provide control over the catalyst composition at the scale of a few nanometers, we have synthesized two sets of Co–Cu nanoparticles with novel structures by wet chemical methods, namely, (a) cobalt core–copper shell (Co@Cu) and (b) cobalt–copper mixed (synthesized by simultaneous reduction of metal precursors) nanoparticles. These catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and temperature programmed reduction (TPR). The catalysts were tested for CO hydrogenation at temperatures ranging from 230 °C to 300 °C, 20 bar and 18,000 scc/(hr.gcat). It was observed that the Co–Cu mixed nanoparticles with higher Cu concentration exhibit a greater selectivity towards ethanol and C₂₊ oxygenates. The highest ethanol selectivity achieved was 11.4% with corresponding methane selectivity of 17.2% at 270 °C and 20 bar.     

Mo–V–M–O (M = Te, W, Fe, Ga) catalysts for selective ammoxidation of propane

We report the incorporation of Ga, Fe, and W, well-known activity and selectivity promoters in lower alkane activation, into Te-free Mo–V–O M1 phase in order to improve its stability under propane ammoxidation conditions. The Mo–V–M–O (M = W, Fe, Ga) M1 phases displayed improved stability as compared to the parent Mo–V–Te–O M1 phase due to higher Tammann temperatures of M oxides and activity for direct conversion of propane to propene and acrylonitrile.