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.     

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.     

Selective oxidation of ethane: Developing an orthorhombic phase in Mo–V–X (X = Nb, Sb, Te) mixed oxides

Mo–V–X (X = Nb, Sb and/or Te) mixed oxides have been prepared by hydrothermal synthesis and heat-treated in N₂ at 450 °C or 600 °C for 2 h. The calcination temperature and the presence or absence of Nb determines the nature of crystalline phases in the catalyst. Nb-containing catalysts heat-treated at 450 °C are mostly amorphous solids, while Nb-free catalysts heat-treated at 450 °C and samples treated at 600 °C clearly contain crystalline phases. TPR-H₂ experiments show higher H₂-consumption on catalysts with amorphous phases. Catalytic results in the oxidative dehydrogenation of ethane indicate that the selective production of the olefin is strongly related to the development of the orthorhombic Te₂M₂₀O₅₇ or (SbO)₂M₂₀O₅₆ (M = Mo, V, Nb) phase (the so-called M1 phase), which is mainly formed at 600 °C. This active and selective crystalline phase is characterized to show moderate reducibility and active centers enough for the selective oxidative activation of ethane with the minimum quantity possible of active centers for ethylene activation. In this sense, the best yield to ethylene has been achieved on a Mo–V–Te–Nb mixed oxide.     

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.     

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.     

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.     

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.     

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.     

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.     

High-throughput synthesis and screening of V–Al–Nb and Cr–Al–Nb oxide libraries for ethane oxidative dehydrogenation to ethylene

High-throughput synthesis and screening of mixed metal oxide libraries for ethane oxidative dehydrogenation to ethylene have been developed. A 144-member catalyst library was prepared on a 3 in. quartz wafer. An apparatus for screening catalytic activity and selectivity of a 144-member catalyst library consists of a reaction chamber, where each member can be heated individually by a CO₂ laser and reactant gases can be delivered locally to each member. The reaction products, ethylene and CO₂, are detected by photothermal deflection spectroscopy and by mass spectrometry. A 144-member catalyst library can be screened in slightly more than 2 h. V–Al–Nb oxide and Cr–Al–Nb oxide libraries are illustrated as examples. V–Al–Nb oxide catalysts are high temperature catalysts and Nb did not affect the catalytic activity of the V–Al oxides in contrast to the effect of Nb found in Mo–V–Nb oxides. However, for the Cr–Al–Nb oxide library, the most active catalyst contains about 4% Nb. These results suggest that a fine composition mapping is necessary for discovery of new heterogeneous catalysts in those ternary systems.     

Influence of the V + Mo/Al ratio on vanadium and molybdenum speciation and catalytic properties of V–Mo–ZSM-5 prepared by solid-state reaction

V–Mo–ZSM-5 catalysts with various composition prepared by solid-state ion exchange were investigated with respect to their physico-chemical characteristics using chemical analysis, XRD, BET, DRIFT, UV–vis, ²⁷Al MAS-NMR spectroscopy, H₂ TPR and TPD of NH₃. It was found that all the preparations leads to either metal ions sitting at the bridging oxygen of Si–OH–Al or anchored at Si–OH groups or deposited as oxide. These different solids were tested in the selective catalytic reduction of NO_x by ammonia. The main result is that upon addition of small amount of Mo to V–ZSM-5, catalytic performances were enhanced.     

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.     

Effect of Potassium Doping on the Catalytic Behavior of Mo–V–Sb Mixed Oxide Catalysts in the Oxidation of Propane to Acrylic Acid

Small addition of potassium to a Mo–V–Sb mixed oxide catalyst (previously prepared by hydrothermal synthesis) strongly modifies its catalytic behavior. Thus, while acetic acid is mainly observed in the K-free catalysts, acrylic acid is selectively obtained in K-doped catalysts. In this case, a selectivity to acrylic acid of about 30% is achieved at propane conversions of 30%. This catalytic behavior is apparently not due to modification of the crystalline phases in the K-doped catalysts but to the elimination of the acid sites of the undoped Mo–V–Sb mixed oxide catalyst.     

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.     

Oxidation of benzyl alcohol using supported gold–palladium nanoparticles

A key discovery in the last two decades has been the realisation that gold, when prepared as supported nanoparticles, is exceptionally effective as an oxidation catalyst, particularly for the oxidation of alcohols. The catalytic efficacy is enhanced further by the alloying of gold with palladium. In this paper we study the effect of the method preparation of gold–palladium alloy nanoparticles supported on titania and investigate the activity of the materials for the selective oxidation of benzyl alcohol. We contrast impregnation and deposition–precipitation methods and demonstrate that the most active catalysts are prepared using the deposition–precipitation method.     

Selective oxidation and ammoxidation of propane on a Mo–V–Te–Nb–O mixed metal oxide catalyst: a comparative study

A comparative study on the selective oxidation and the ammoxidation of propane on a Mo–V–Te–Nb–O mixed oxide catalyst is presented. The catalyst has been prepared hydrothermally at 175 °C and heat-treated in N₂ at 600 °C for 2 h. Catalyst characterization results suggest the presence mainly of the orthorhombic Te₂M₂₀O₅₇ (M = Mo, V and Nb) bronze in samples before and after use in oxidation and ammoxidation, although some little modifications have been observed after its use in ammoxidation reaction. Propane has been selectively oxidized to acrylic acid (AA) in the 340–380 °C temperature range while the ammoxidation of propane to acrylonitrile (ACN) has been carried out in the 360–420 °C temperature interval. The steam/propane and the ammonia/propane molar ratios have an important influence on the activity and the selectivity to acrylic acid and acrylonitrile, respectively. The reaction network in both oxidation and ammoxidation reactions as well as the nature of active and selective sites is also discussed. The catalytic results presented here show that the formation of both ACN and AA goes through the intermediate formation of propene.     

K–V–Ca catalysts supported on porous alumina ceramic substrate for soot combustion: Preparation and characterization

The KCl, KNO₃, CaCl₂, Ca(NO₃)₂, V–Ca and K–V–Ca catalysts supported on alumina ceramic substrates have been prepared. X-ray diffraction and thermogravimetry/differential scanning calorimetry were used to characterize the catalysts, and their catalytic activities were evaluated by a soot oxidation reaction using the temperature-programmed reaction system. The catalytic activity of KNO₃ is higher than KCl, and the catalytic activity of Ca(NO₃)₂ is as much as that of CaCl₂. The catalyst containing a higher KNO₃ content exhibits CO₂ adsorption, whereas higher CaCl₂ and Ca(NO₃)₂ content can restrain the adsorption of CO₂. The K–V–Ca catalyst with a molar ratio of 6:1:1 had the lowest soot onset combustion temperature. The melting and oxidation–reduction of KNO₃, oxygen content of catalyst surface, and formation of some eutectic phase may be the key factors in improving catalytic activity of catalysts.     

Synthesis of Higher Alcohols from Syngas over Ultrafine Mo—Co—K Catalysts

Ultrafine Mo–Co–K catalysts were prepared and tested for higher alcohol synthesis. The catalysts exhibited high catalytic activity. The effect of the mole ratio of cobalt and molybdenum in the catalysts upon the catalytic performance of higher alcohol synthesis was investigated. Among the ultrafine Mo–Co–K catalysts, the best one corresponded to the Co/Mo mole ratio of 1:7. The XPS spectra revealed that molybdenum was present in two species: Mo⁶⁺ and Mo⁴⁺ on the surface of reduced catalysts, and the Mo⁴⁺ species content depended strongly on the Co/Mo mole ratio. The selectivity towards higher alcohols was found to be related to the Mo⁴⁺ species content. A linear relation between the selectivity and Mo⁴⁺ species content led to the conclusion that the Mo⁴⁺ species was the main active species for higher alcohol synthesis over the ultrafine Mo–Co–K catalysts.     

The selective oxidative activation of light alkanes. From supported vanadia to multicomponent bulk V-containing catalysts

V-containing materials have been proposed as active and selective catalysts in the selective oxidative transformation of C₂–C₄ alkanes to the corresponding olefins, oxygenates and nitriles. Although vanadium ions are generally assumed as the active sites in the oxidative activation of short chain alkanes, the coordination, the oxidation state and the environment of V atoms strongly influence both the catalytic activity and the nature of the reaction products. A comparative study on the catalytic behaviour of V-containing catalysts, i.e. metal vanadates, supported vanadium oxides, V-containing microporous/mesoporous materials and multicomponent bulk Mo–V–Te(Sb)–Nb mixed metal oxides, on the alkane oxidation is presented. The influence of the structure (including the V-coordination and V-environment) and the physico-chemical properties (especially redox and acid–base characteristics) of the catalysts on their behaviour in the selective oxidation of paraffins and alkenes is discussed.     

K-doped Mo–V–Sb–O crystalline catalysts for propane selective oxidation to acrylic acid

Mo–V–Sb–O complex metal oxide catalysts were synthesized hydrothermally and potassium was doped to the catalysts either during the hydrothermal synthesis or by ion-exchange method. The obtained catalysts were characterized by ammonia TPD and tested for propane selective oxidation to acrylic acid (AA) in order to investigate relationship between surface acid property and catalytic properties. The K-doped catalysts showed higher selectivity to AA than the un-doped catalyst but poorer in the propane conversion. It was observed that potassium clearly decreased the ammonia adsorption capacity on the surface without affecting bulk structural phases and furthermore a linear relationship was obtained between the reaction rate of propane and the ammonia adsorption capacity. The results strongly suggest that surface protonic acid sites involve in the oxidative activation step of propane which is regarded as the rate-determining step. On the basis of a demonstrative test using the K-doping by ion-exchange and the grinding procedure, it is concluded that the selectivity increase by the K-doping was due to the less influence of potassium on the rate of propene selective oxidation. We discuss a multi-functional character derived from high dimensional structures of the catalysts and mechanism of the selective oxidation of propane.