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Crystalline Mo-V-O Based Complex Oxides as Selective Catalyst of Propane
Abstract
Three single crystalline MoVO based oxides, MoVO, MoVTeO and MoVTeNbO, all of which have the same orthorhombic layer-type structure with the particular arrangement of MO6 (M = Mo, V, Nb) octahedra forming slabs with pentagonal, hexagonal, and heptagonal rings in (1 0 0) plane, were synthesized by hydrothermal method and their catalytic performance in the selective oxidation of propane to acrylic acid were compared in order to elucidate the roles of constituent elements and crystal structure in the course of the propane oxidation. It was observed that the rate of propane oxidation was almost the same over all three catalysts, revealing that Mo and V, which were indispensable elements for the structure formation, were responsible for the catalytic activity for propane oxidation. The Te-containing catalysts showed much higher selectivity to acrylic acid than the MoVO catalyst. Since propene was formed as a main product at low conversion levels over every catalyst, it can be concluded that Te located in the central position of the hexagonal ring promoted the conversion of intermediate propene effectively to acrylic acid. The catalyst with Nb occupying the same structural position of V clearly showed the improved selectively to acrylic acid particularly at high conversion region, because the further oxidation of acrylic acid to COx was greatly suppressed. These conclusions were further supported by the additional studies of the determination of activation energy and catalytic oxidations of intermediate products of the propane oxidation.
... Metallic oxides are the most widely studied and effective catalysts. Mo-V based catalysts for the two-step oxidation reactions have been an active topic of research in recent years [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. Mo and/or V is usually considered to be the basic redox element [4,5]. ...
... Some transition metals such as Fe can decrease the loss of lone-pair elements and stable the catalysts [16]. The presence of Nb improves the selective oxidation reactions which is the result of moderate acid sites and better stability of acrylic acid [17,18]. A better investigation of the element composition, pore structure and chemical properties of the catalyst is quite necessary for improvement of an efficient catalyst for the oxidation reactions. ...
... Orthorhombic M1 phase and hexagonal M2 phase are not observed, probably because the two phases have been destroyed after calcination in air. This is consistent with the current reports [17,40,41]. ...
Novel Mo-V-PMMA and Mo-V-PS catalysts are prepared by addition of hard polymethyl methacrylate (PMMA) and polystyrene (PS) nanospheres into Mo/V compounds in the preparation process, respectively. The catalytic tests in selective oxidation of acrolein reveal that Mo-V-PMMA catalyst shows very high acrolein conversion (99.1%) and the yield of acrylic acid (90.7%). The BET, DLS, SAXS, XRD, XPS, H2-TPR and NH3-TPD measurements reveal that the addition of PMMA and PS nanospheres causes the obvious changes of porous structure, crystal phases composition and chemical properties of catalysts. These differences between Mo-V-PMMA and Mo-V-PS catalysts are attributed to the totally different “real” nano–environment during heat treatment in the high–concentration component mixture. PS nanospheres are in a state of adhesion or agglomeration or not uniformly distributed in the active component solution, while PMMA nanospheres with much better hydrophilicity and monodispersed state promote Mo and V ions more easily and uniformly dispersed in the mixture.
Graphic abstract
Novel Mo-V catalysts are prepared by addition of hard polymethyl methacrylate (PMMA) and polystyrene (PS) nanospheres into Mo/V mixture. Obvious changes of porous structure, crystal phases and chemical properties of catalysts are caused by the nanospheres introduction, showing very high acrolein conversion (99.1%) and the yield of acrylic acid (90.7%) in selective oxidation of acrolein.
... 1,2 To date, MoVTeNbO x -mixed oxides have been prepared by different synthesis routes, including slurry, co-precipitation, hydrothermal, dry-up and solid state reactions. [3][4][5][6][7] Among them, slurry and hydrothermal methods are the most common options; however, both of the processes need long reaction times that can take dozens of hours. [8][9][10] Furthermore, in order to improve the crystallinity and stability of products, a necessary post-heat treatment at 550-600 C under a given atmosphere, lasting for several hours, is also involved in current routes. ...
... [8][9][10] Furthermore, in order to improve the crystallinity and stability of products, a necessary post-heat treatment at 550-600 C under a given atmosphere, lasting for several hours, is also involved in current routes. For instance, Ueda et al. 5 synthesized three Mo-V-O based oxides with single crystals by using a hydrothermal method at 175 C for 48 h followed by calcination under nitrogen atmosphere for 2 h at 600 C. Mazloom et al. prepared MoVTeNbO x using a slurry method, in which continuous evaporation at 60 C was needed, rstly to remove the water in the precursor solution and a subsequent calcination at 600 C for 2 h under nitrogen atmosphere was then carried out. 6 For the mixed oxides, most notably, these aforementioned complicated synthesis processes make complete reproduction difficult for other researchers. ...
... In addition, Nb was also considered to improve the selectivity to acrylic acid by reducing the overoxidation rate of acrylic acid. 5,35 Based on this, it was speculated that the more supercial niobium ions with low valence states of S250 (Fig. 4e) had an adverse impact on its selectivity. ...
A fast and simple sub-/supercritical water synthesis method is presented in this work in which MoVTeNbOx-mixed metal oxides with various phase compositions and morphologies could be synthesized without post-heat treatment. It was demonstrated that the system temperature for synthesis had a significant influence on the physico-chemical properties of MoVTeNbOx. Higher temperatures were beneficial for the formation of a mixed crystalline phase containing TeVO4, Te3Mo2V2O17, Mo4O11 and TeO2, which are very different from the crystalline phases of conventional Mo–V–Te–Nb-mixed metal oxides. While at lower temperatures, Mo4O11 was replaced by Te. At high temperature, the as-prepared samples presented distinct nanoflake morphologies with an average size of 10–60 nm in width and exhibited excellent catalytic performances in the selective oxidation of propylene to acrylic acid. It is illustrated that the large specific surface area, presence of Mo4O11 and superficial Mo⁶⁺ and Te⁴⁺ ions are responsible for the high propylene conversion, while suitable acidic sites and superficial Nb⁵⁺ ions improved the selectivity to acrylic acid.
... The first work on oxidative dehydrogenation of ethane (ODE) over mixed V-Mo-X oxide systems (X = Ti, Cr, Mn, Fe, Co, Ni, Nb, Ta, or Ce) appeared back in 1978 [1]. This publication began an intensive study of the processes of low-temperature oxidation of light alkanes [2][3][4][5][6][7][8][9][10][11][12][13][14]. One of the most promising catalytic systems that has shown the best results in ODE is the mixed oxide catalyst Mo-V-Te-Nb-O [2,3]. ...
... One of the most promising catalytic systems that has shown the best results in ODE is the mixed oxide catalyst Mo-V-Te-Nb-O [2,3]. Mo-V-Te-Nb oxide catalysts obtained by hydrothermal synthesis are widely studied in the ODE process using O 2 as an oxidizing agent at atmospheric pressure [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]. So, in [6], using these catalytic systems, it was possible to achieve high selectivity for ethylene of 80-97% and ethane conversion of 7-80% in the temperature range of 340-400 °C. ...
... However, the selectivity of the latter was better. It turned out that M1 phase active in the ODE process [8][9][10][11][12][13] is formed only at 600 °C, while at 450 °C the phases of the catalyst component are predominantly amorphous. In [9], the effect of the gas used in the process of preparation the catalyst at the calcination stage was considered. ...
One of the most active and high-selective catalytic systems for oxidative dehydrogenation of ethane is the MoVNbTeOx catalyst. However, this catalytic system has the low thermal stability and is prone to deactivation at temperatures above 350 °C. At the first time it was proposed to use oxygen ((Tcr = –119 °C, Pcr = 48 atm) and ethane (Tcr = 32 °C, Pcr = 51 atm) in supercritical state in the oxidative dehydrogenation process. Carrying out the process at supercritical conditions (T = 240–300 °C, P = 60–100 atm) leads to high selectivity up to 90% for ethylene under growth conversion of ethane and oxygen. At the same time, it manages to keep M1 phase crystallinity unchanged throughout the experiment (more than 40 h).
Graphic Abstract
Dependences of ethylene productivity on pressure (Reaction conditions: T = 280 °C, C2H6:O2:N2 = 31:23:46, 2 g of catalyst, the density of the reagent mixture varies from 0.65 (at 1 atm ) to 63.15 kg/m3 (at 100 atm )
... The ternary MoV M1 oxide can activate propane but leads to nonselective oxidation to carbon oxides. 12,13 Highly performing and well-defined bulk catalysts composed of the M1 structure 14−16 provide a suitable opportunity to address structure−function relationships in complex oxidation reaction. Based on the site isolation concept and considering classic organic chemistry as well as the crystal structure of M1, 17 Grasselli and co-workers proposed that propane can be converted to acrylic acid over specific sites existing in the basal (001) plane of the M1 structure without desorption of intermediates. ...
... 23 Irrespective of the specific active site model, it is generally assumed that V is the key element for the propane activation, while the presence of Nb and particularly Te seem to be related to the selectivity to acrylic acid. 7,12,13,21,23 The reaction network and underlying catalytic functions of MoVTeNb M1 oxide have been addressed by kinetic investigations, as well as reactivity studies of potential intermediates. 7,11,12,24−28 Propene is a major intermediate, which undergoes further oxygenation to acrylic acid. ...
... The M1 phase efficiently transforms potential intermediates in the acrylic acid formation (i.e., propene, allyl alcohol, and acrolein) at reaction temperature lower than that of propane. 7,11,12,28 The ternary MoV M1 oxide serves as excellent catalyst for acrolein oxidation to acrylic acid at temperature as low as 463 K. 29 Interestingly, the M1 phase shows versatile performance in selective oxidation of reactants beyond alkanes. Besides abovementioned propene, allyl alcohol, and acrolein, oxidation of alcohols at low temperature has been explored. ...
Propane oxidation at 653-673 K and benzyl alcohol oxidation at 393 K over phase-pure MOV(TeNb) M1 oxide catalysts were studied to gain insight into the multiple catalytic functions of the surface of the M1 structure. Electron microscopy and X-ray diffraction confirmed the phase purity of the M1 catalysts. Propane oxidation yields acrylic acid via propene as intermediate, while benzyl alcohol oxidation gives benzaldehyde, benzoic acid, benzyl benzoate, and toluene. The consumption rates of benzyl alcohol and propane level in the same range despite huge difference in reaction temperature, suggesting high activity of M1 for alcohol oxidation. Metal-oxygen sites on the M1 surface are responsible for the conversion of the two reactants. However, different types of active sites and reaction mechanisms may be involved. Omitting Te and Nb from the M1 framework eliminates acrylic acid selectivity in propane oxidation, while the product distribution in benzyl alcohol oxidation remains unchanged. The results suggest that the surface of M1 possesses several types of active sites that likely perform a complex interplay under the harsh propane oxidation condition. Possible reaction pathways and mechanisms are discussed.
... Complex oxides, defined here as oxides with more than one cation in their formula unit, find application in a wide range of established and emerging technologies, from nuclear waste forms [1] to energy materials such as batteries [2][3][4][5] and solid oxide fuel cells [6,7] to catalysts [8,9] and thermal barrier coatings [10,11]. Many complex oxides that are of technological interest for energy applications exhibit high levels of ionic transport. ...
... The migration entropy is more challenging. In the harmonic approximation, it can be calculated via Vineyard's equation: (9) where ν min are the normal frequencies, found by diagonalizing the Hessian, at the relevant minimum energy configuration and ν sad are the same for the saddle point for the path. Oftentimes, however, the challenge is in determining the pathway in the first place. ...
Mass transport is one of the most fundamental properties of a material and dictates its potential use for a range of applications. In simple materials, mass transport is determined by migration of point defects along simple and well defined pathways. When considering complex oxides, however, new considerations arise that complicate both our understanding and our ability to describe mass transport. Chemical disorder, the mixing of cations across different sublattices, is one of these considerations. This disorder leads to a complex potential energy landscape that impacts both the thermodynamics and kinetics of the defects responsible for mass transport through the material. We review the recent literature to highlight new insights that have been revealed by both experiment and computational modeling about the coupling between disorder and transport. We highlight points of agreement and disagreement. Our survey raises open scientific questions about our understanding of this coupling which, if answered, provide new possibilities for tailoring functionality in these materials.
... To what the catalyst concerns, researchers' efforts have been focused on designing a suitable formulation, with the capacity to activate propane and, at the same time, favoring the route of acrylic acid production over the undesired deep oxidation reactions that, in turn, are very exothermic and yields worthless by-products [2]. V 2 O 5 -P 2 O 5 (VPOs) and MoVTe(Nb)(Sb)O are the catalytic systems reporting the most promising performance, the later exhibiting a notably higher yield to acrylic acid [6,[11][12][13][14][15]. A judicious selection of the reaction conditions, namely, temperature, pressure, feed composition, space-time, etc., is of a paramount importance to identify the real potential of a catalytic system [2]. ...
... Propane conversion augments as a result of increasing temperature. Qualitatively, it is clear that temperature, pressure and space-time affect both propane conversion and products distribution, whereas water content in the feed basically impacts on products distribution, results that agree with information reported by others [12][13][14][15]20]. ...
An attractive route for producing acrylic acid is the direct partial oxidation of propane for which, MoVTeNb-based mixed oxides have exhibited promising results. In this work, formal design of experiments (DOE) is applied to investigate the simultaneous impact of four key independent reaction variables (factors): temperature (613–693 K), pressure (4–55 kPa–g), space–time (55–167 gcath/mole) and water content in the feed (0–20 mole%) on the performance of a MoVTeNb-based catalyst to produce acrylic acid from propane. Catalytic experiments are performed in a lab-scale fixed-bed reactor feeding a gaseous mixture containing propane, oxygen, nitrogen and water. DOEs results are analyzed by combining graphical (main effects and interaction plots) and statistical tools (ANOVA, regression analysis as well as surface responses). Most of the factors accounted for in the DOEs have a linear effect on propane conversion and a quadratic effect on acrylic acid selectivity; binary interaction between factors is common for acrylic acid selectivity. Water addition to the reaction mixture is essential to improve acrylic acid selectivity. Regression models predict that adding 18 mole% of water to the feed maximizes acrylic acid selectivity when operating at moderate levels of temperature (648 K) and pressure (10 kPa-g). Also, operation at high severity in pressure, medium severity in space–time and low-medium severity in temperature increases further acrylic acid selectivity.
... 18-21 Chemical composition, drying and the conditions of heat treatment also have a strong inuence on the phase composition of the samples. M1 oxide phase is prepared either by hydrothermal synthesis [22][23][24][25] or by the "slurry" method, which implies mixing of aqueous solutions of initial reagents followed by solvent removal and thermal treatment in combined mode. 26,27 The essential synthetic steps and required conditions are mostly known from empirical ndings, whereas the structural transformations that occur at each step are still ill-understood. ...
... 13 Mo 5 O 14 -type of the structure is usually formed in the VMoNb mixed oxides and has a similar structural arrangement to M1. 33 The only difference between Mo 5 O 14 and M1 in the arrangement of the polygonal grids is the absence of big heptagonal channels in the Mo 5 O 14 structure. 22 The goal of the present work is to study the development of the local structure of the M1 phase in the VMoNbTe oxide samples with cationic composition V 0.3 Mo 1 Nb 0.12 Te 0.23 , typical for highly active catalysts. 34 We started with the V 0.3 Mo 1 Te 0.23 solution and analyzed the local structure changes during all the synthetic steps using EXAFS and pair distribution function (PDF) methods. ...
The so-called M1 phase (the common formula (TeO)x(Mo, V, Nb)5O14) is a very promising catalyst for ethane oxidative dehydrogenation (ODE). It shows 90% selectivity to ethylene at 78% ethane conversion (400 °C, contact time-5.5 s). The active crystal structure is formed under certain synthetic conditions in VMoNbTe mixed oxides. This paper is devoted to the analysis of how the local and average structure of the M1 phase is developed during the synthesis and what happens at particular synthetic steps. The analysis of the local structure was performed using the EXAFS and pair distribution function (PDF) methods. The EXAFS analysis of the initial VMoTe water solution and VMoNbTe slurry showed that Anderson-type heteropoly anions are formed in the solution and are preserved after fast spray-drying of the slurry. Nb cations do not enter the structure of the polyanions, but form an extended hydrated oxide matrix, where distorted NbO6 and NbO7 polyhedrons are connected to each other. The hydrated oxide matrix with captured polyanions provides the compositional homogeneity of the precursor. The distances in the second coordination shell are redistributed after thermal treatment at 310 °C. After being heated at T > 350°, the local structure of the M1 phase is organized and pentagonal domains are formed. These domains consist of a NbO7 pentagonal bipyramid and five MeO6 adjacent octahedra (Me = Mo, V). In the first stages, the building blocks are stacked along the [001] direction. The crystallization process results in the connection of the pentagonal domains to the extended polygonal grid. The formation of the regular grid with TeOx containing channels is accompanied by the increase in ethane conversion and ethylene selectivity of the catalysts.
... The role of Nb was studied by Concepcion et al. [14] and Zhang et al. [78]. Although the MoVTeO sample already has the M1 phase, the presence of Nb +5 ions in the MoVTeNbO samples decreases the number of acid sites, stabilizes the crystalline structures, and reduces further degradation of acrylic acid to carbon oxides (CO, CO2, etc.), eventually resulting in much-improved acrylic acid yields [13,53,79]. Although the XRD patterns show the same M1 phase for MoVTeO and MoVTeNbO catalysts, Raman spectroscopy was able to detect the presence of NbOx species whose presence decrease the strength of acidity as Elemental metal surface composition as determined by operando XPS with the simultaneously-recorded acrylic acid abundance. ...
... The role of Nb was studied by Concepcion et al. [14] and Zhang et al. [78]. Although the MoVTeO sample already has the M1 phase, the presence of Nb +5 ions in the MoVTeNbO samples decreases the number of acid sites, stabilizes the crystalline structures, and reduces further degradation of acrylic acid to carbon oxides (CO, CO 2 , etc.), eventually resulting in much-improved acrylic acid yields [13,53,79]. Although the XRD patterns show the same M1 phase for MoVTeO and MoVTeNbO catalysts, Raman spectroscopy was able to detect the presence of NbO x species whose presence decrease the strength of acidity as detected by the NH 3 -TPD. ...
Light alkanes are abundant in shale gas resources. The bulk mixed metal oxide MoVTe(Sb)NbOx catalysts play a very important role in dehydrogenation and selective oxidation reactions of these short hydrocarbons to produce high-value chemicals. This catalyst system mainly consists of M1 and less-active M2 crystalline phases. Due to their ability to directly monitor the catalysts under the relevant industrial conditions, in situ/operando techniques can provide information about the nature of active sites, surface intermediates, and kinetics/mechanisms, and may help with the synthesis of new and better catalysts. Sophisticated catalyst design and understanding is necessary to achieve the desired performance (activity, selectivity, lifetime, etc.) at reasonable reaction conditions (temperature, pressure, etc.). This article critically reviews the progress made in research of these MoVTe(Sb)NbOx catalysts in oxidation reactions mainly through in situ/operando techniques and suggests the future direction needed to realize the industrialization of these catalysts.
... In contrast, the combination of these elements, preferably with more complex structural features, e.g. MoVOx with M1-structure type [60][61][62] , have a much higher capability to selectively oxidize the reactant. By introducing Te and Nb into the M1 structure, MoVTeNbOx was obtained as an excellent catalyst for the oxidative dehydrogenation of ethane (ODHE) and the direct oxidation of propane to AA 15,[63][64][65] . ...
Based on the concept that most reaction steps proceed only at the surface layer of a bulk catalyst, the catalytic impact of the surface-modification with POx, BOx, and MnOx in the selective oxidation of ethane, propane, and n-butane is systematically studied. Three different promoter elements are deposited as a sub-monolayer on the surface of oxidation catalysts with high dispersion by sequential and self-limiting reactions at the solid-gas interface, using atomic layer deposition. Oxygenate and olefin selectivities are tuned by the surface deposition of POx and BOx, leading to improved product yields. The mixed metal oxide MoVTeNbOx is used as a case study to demonstrate the effect of the modification in different reactions with yield improvements of up to 24% in the propane oxidation towards acrylic acid. It is shown that the beneficial performance is related to a change in surface composition, a modification in the electronic properties of the redox active element vanadium, and a decrease in acidity. A comparative study considering several bulk catalysts and deposited elements revealed further promoting effects for different oxidation catalysts. In particular, the deposition of POx on V-containing oxides suppresses COx formation. Precisely adjusted surface modifications leading to enhanced product yields demonstrate the potential of atomic layer deposition as a powerful tool for tuning catalytic properties of bulk catalysts.
... 51 In the field of selective oxidation, the most prominent Nb-containing example is the mixed metal oxide MoVTeNbO x that still provides the maximum yields for the partial oxidation of propane to acrylic acid and the oxidative dehydrogenation of ethane. 52,53 The insertion of Nb into the M1 crystal structure is described as a director for a variable V occupancy enabling increased product yields. 54 A promoting effect of Nb in VPP catalysts for the selective oxidation of n-butane is also known for decades. ...
... Therefore, it is reasonable to deduce that the catalytic performance of the catalyst can be enhanced by increasing the specific surface area of the catalyst and especially the exposure of (0 0 1) plane [11,31]. However, the current studies on the role of the (0 0 1) plane in M1 MoVNbTeO x are mainly focused on the selective oxidation reaction of propane and the conclusions are conflicting with each other [22,28,[32][33][34][35][36][37][38][39]. Celayasanfiz et al. [21] investigated the activity of (0 0 1) plane by selectively exposing the basal surface of M1 phase by SiO 2 coating and crushing method. ...
The original phase-pure M1 MoVNbTeOx catalyst was mechanically treated with stirring water for several days under ambient conditions to obtain particles with different sizes. The catalytic performance was significantly improved and stabilized after about 7 days of treatment. Among all the catalysts, the sample treated in water for 7 days can obtain 57.6% ethane conversion and 0.79 kgC2H4/kgcat/h ethylene productivity while the original phase-pure M1 catalyst was only 19.2% and 0.27 kgC2H4/kgcat/h at reaction temperature of 400 °C and contact time of 18.5 gcat·h/molC2H6. Meanwhile, the catalyst was systematically characterized and the results showed that there was essentially no difference in microstructure between the original and treated catalysts. TEM and SEM morphological characterizations presented that the particle size and aspect ratio of the catalysts changed significantly, in which the particle size of the catalyst become smaller and the proportion of basal surface was enhanced. The activity and selectivity of the catalysts exhibited a correlation with the amount and proportion of basal surface. It can be deducted that the basal surface is the most active and selective surface during oxidative dehydrogenation of ethane (ODHE) process, and the intrinsic activity based on the basal (001) plane of the catalyst remains stable after the mechanical treatment. Samples treated in water for 7 days showed excellent long-term stability in a 200 hours stability test at reaction temperature of 440 °C and contact time of 9.3 gcat·h/molC2H6. The present work demonstrated that the differences in the catalytic properties of different M1 samples depend mainly on the amount and proportion of exposed basal surface, and the possibility of enhancing the activity of orthorhombic M1 phase by fragmentation method.
... Note that the structure of the catalytically active surface of the M1-based MoVNbTe oxide system is still under discussion [9,49,[51][52][53]. Some researchers suggest that catalytically active structures form immediately under the reaction conditions, and this leads to compositions that are different from the pristine M1 surface [9,49]. ...
MoVNbTe mixed oxide, which is used as a catalyst for the oxidative dehydrogenation of ethane, was investigated by the XPS technique during prolonged treatment with X-rays. This treatment caused a modification of the MoVNbTe oxide surface related to the reduction of V, Mo, and Te species. This behavior is supposed to be related to the interaction of oxide with secondary electrons emitted during X-ray absorption. Firstly, the reduction of Te⁴⁺ into Te⁰ species, accompanied by the removal of no more than ∼5% of overall oxygen from the surface/subsurface region, was found during ∼8.5 h under X-rays. After ∼28 hours of exposure, the significant surface restructuring resulted in a change in the relative amounts of V and Te. This was accompanied by a high degree of surface reduction and the appearance of V³⁺ species. Under X-rays, an abnormal broadening of Mo 3d spectral lines was observed in contrast to other photoelectron regions. The possible reasons for this phenomenon were discussed. The limit for the amount of lattice oxygen, which might be extracted with the preservation of the pristine surface structure of MoVNbTe oxide, was in the range from 5 to 7%.
... This discovery is consistent with experimental results, showing that the rate of propane consumption is correlated with the surface concentration of vanadium in the M1 phase [36]. Experiments by Ueda and co-workers showed that the rate of propane oxidation catalyzed by Mo-V-O is almost the same as that catalyzed by Mo-V-Nb-Te-O, suggesting that initial C-H cleavage may not involve Te=O, which seems to conflict with our DFT results [52]. However, it is likely that in the Mo-V-O system, propane C-H activation is not through the ROA pathway. ...
In the oxidative dehydrogenation (ODH) of alkanes, some important advances of the last decade have made it possible to accelerate the development and industrial insertion of the M1 based catalytic systems. These catalysts may be fine-tuned to account for the inevitable variability of different chemicals and impurities in the feedstocks. The latter may include, among others, different blends of shale gases, with different ratios of the C1-C3 alkanes and impurities such as sulfur, phosphorus, etc. In this article, we review the recent progress achieved in our understanding of the crystal structures and the oxidative dehydrogenation (ODH) reaction mechanisms of the multi-metal oxide (MMO) M1 catalyst. Firstly, the complex crystal structure of the M1 phases has been examined using quantum mechanics (QM), reactive force field (ReaxFF), and machine learning (ML) approaches. Secondly, we discussed the ODH mechanism on the M1 phase based on the QM simulations including the finite cluster model and the periodic slab model. Finally, we proposed a catalyst design approach to improve the selectivity of the M1 phase based upon the ODH reaction mechanism. We also briefly discuss the concept of the CE (“Concurrent Engineering”, introduced by the European Space Agency). The development of the CE concepts may be applied to the M1 catalytic systems in the future allowing businesses to be agile and react fast to the changing production conditions, thereby making them uniquely competitive in the ODH of alkanes and other areas.
... This discovery is consistent with experimental results, showing that the rate of propane consumption is correlated with the surface concentration of vanadium in the M1 phase[60]. Experiments by Ueda and co-workers showed that the rate of propane oxidation catalyzed by Mo-V-O is almost the same as that catalyzed by Mo-V-Nb-Te-O, suggesting that initial C-H cleavage may not involve Te=O, which seems to conflict with our DFT results[65].However, it is likely that in the Mo-V-O system, propane C-H activation is not through the ROA pathway. This would lead to a higher activation barrier. ...
Accepted for publication in Topics in Catalysis on July 07, 2020
... Recent synthesis efforts have succeeded in preparing the orthorhombic M1 phase mixed metal oxides consisting of Mo and V without Te and Nb, which exhibited similar C 2 H 4 selectivity as those with the additives [12]. The structure of these oxide is well-studied and consists of layers of linked octahedral units of MO 6 (M = Mo, V) that form five, six, and seven membered rings in the (001) crystal planes that form one-dimensional pores [12][13][14][15][16][17][18]. ...
Mo and V containing oxides are among the most important oxidative dehydrogenation catalysts. The effects of differences in structure and compostion among SiO2 supported VOx, unsupported V2O5 and MoO3 and M1 phase MoV mixed oxide catalysts on catalytic proerties are probed using their reactivity and dehydrogenation selectivity in oxidative conversion of ethane (C2H6) and cyclohexane (C6H12). The C2H6 and C6H12 activation rates are nearly insensitive to VOx loading on SiO2 at low loadings that predominantly form monovanadate species, but decrease at high loadings due to the formation of V2O5 nanoparticles with low V dispersion. The C-H activation enthalpies are lower at high loadings and in unsupported V2O5, suggesting that intrinsic reactivity of V2O5 nanoparticles is higher than monovanadates. The C2H6/C6H12 rate ratios are below 0.01 on all VOx/SiO2 catalysts, consistent with weaker C-H bonds in C6H12, but are higher on V2O5 nanoparticles than on low loading VOx/SiO2 samples. MoO3 samples exhibit lower rates and higher activation energies than VOx/SiO2 and V2O5 samples, and similar C2H6/C6H12 rate ratios as V2O5. M1 phase MoVTeNb and MoV mixed oxides contain one-dimensional micropores of size similar to C2H6 but much smaller than C6H12; preparation methods significantly affect their elemental composition, accessible micropore volumes and surface areas. Post-synthesis treatment of MoVTeNbO with H2O2 improves M1 phase purity, and increases in C2H6 and C6H12 activation rates are consistent with increase in their intrapore and external surface areas. The C2H6 and C6H12 activation rates in MoVO without Te and Nb are higher than values predicted from MoVTeNbO and their surface micropores and external surface areas, because higher V content in MoVO increases their reactivity by slightly decreasing activation energies. The C2H6/C6H12 rate ratios in these samples are much higher than VOx/SiO2, V2O5, and MoO3 and roughly correlate with internal/external surface ratios, which is consistent with C2H6 and C6H12 activation occurring inside and outside the pores, respectively. The M1 phase samples exhibit much higher selectivity than VOx/SiO2, V2O5, and MoO3, but among the M1 phase samples the selectivity is slightly lower in MoVO than in MoVTeNbO. Local structure and composition affect reactivity in M1 phase oxides and oxides without heptagonal micropores, but C2H6/C6H12 rate ratios and C2H4 selectivities are much higher in the M1 phase, which confirms for a broad range of oxides previously proposed roles of micropores in activating C2H6 selectively.
... Orthorhombic Mo−V oxide is one of the most active solidstate catalysts for selective oxidation of ethane to ethylene, 1−3 propane to acrylic acid 3,4 or acrylonitrile, 5 acrolein to acrylic acid, 1 and alcohols to carboxyl compounds. 6 Orthorhombic Mo−V oxide features slabs comprising 6-and 7-membered rings of corner-sharing MO 6 octahedra (M = Mo or V) and pentagonal Mo 6 O 21 units with a MoO 7 pentagonal bipyramidal unit and five edge-sharing MoO 6 octahedra (Figure 1a,b). ...
Orthorhombic Mo–V oxide is one of the most active solid-state catalysts for selective oxidation of alkane, and revealing its detailed structure is important for understanding reaction mechanisms and for the design of better catalysts. We report the single-crystal X-ray structure analysis of orthorhombic Mo–V oxide heated under a N2 flow; V is present in 6-membered rings with partial occupancy, similar to the structure reported by Trunschke’s group for orthorhombic Mo–V oxide heated under an Ar flow (Trunschke, ACS Catal. 2017, 7, 3061). Our previous paper (Ishikawa, J. Phys. Chem. C, 2015, 119, 7195) reported that V is not present in the 6-membered rings when orthorhombic Mo–V oxide is calcined in the presence of oxygen. Furthermore, Trunschke’s paper reported that V in the 6-membered rings moves to the surface of the crystals under oxidation reaction conditions in the presence of H2O. Our present results provide additional evidence for V migration in the 6-membered rings during heat treatment. We also report the differences in the thermal behaviors, ultraviolet–visible absorptions, N2 isotherms, and elemental analysis results of Mo–V oxide heated in air and under a N2 flow. Furthermore, we report the solid-state transformation of orthorhombic Mo–V oxide to tetragonal Mo–V oxide by controlled heat treatment.
... Approximately 75% of applications in the chemical industry use mixed oxide catalysts [22], which have been studied for many gas phase reactions including ethane oxidation [23,24], ethane oxidative dehydrogenation [25], selective oxidation of light alkanes [26], propane oxidation [27,28], glycerol dehydration to acrolein [29], and more recently the conversion of glycerol to acrylic acid [10,20,30]. Some works have also reported the transformation of glycerol to acrolein by a liquid phase reaction, giving a great catalytic activity [31,32]. ...
... Commercially, MA is produced by esterification of acrylic acid (AA) and methanol, prior to the synthesis of AA by selectivity oxidation of propene with air. Although the multi-component metal oxides were found to be catalytically active for selectivity oxidation, they were rather sensitive to preparation parameters [4][5][6][7][8][9] and not efficient enough for practical application. Furthermore, propene, which is the essential component feedstock for AA production, is mostly based on petroleum, so the price of it is greatly influenced by the crude oil price. ...
Vapor phase aldol condensation of methyl acetate with formaldehyde was first studied over Al2O3 with different calcination temperatures and Al2O3-supported barium with different amounts of barium catalysts. The catalysts were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, N2 adsorption–desorption, the pyridine absorption performed via Fourier transform infrared spectroscope, NH3 and CO2 temperature-programmed desorption. The results indicated that the calcination temperature affected the strength and the number of surface acid and base sites over the alumina catalysts. In addition, the moderate Lewis acid and weak base sites were critical to promote this aldol condensation reaction. Adding barium species, which could effectively modified the acid–base properties of Al2O3, clearly improved the catalytic activity and selectivity. The stability and regeneration of the optimum catalyst were also investigated and did not exhibit an obvious decrease in efficiency.
Graphical Abstract
Al2O3-supported barium catalyst was found to be an effective catalyst for vapor phase aldol condensation of methyl acetate with formaldehyde as a result of the appropriate intensity of acid and base.
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... This discovery is consistent with experimental results, showing that the rate of propane consumption is correlated with the surface concentration of vanadium in the M1 phase [30]. that the rate of propane oxidation catalyzed by Mo-V-O is almost the same as that catalyzed by Mo-V-Nb-Te-O, suggesting that initial C-H cleavage may not involve Te=O, which seems to conflict with our DFT results [31]. However, it is possible that in the Mo-V-O system, propane C-H activation is not through the ROA pathway, or it is through the ROA mechanism but involves different motifs (such as V=O and Mo, or Mo=O and V pairs). ...
We report here first principles predictions (density functional theory with periodic boundary conditions) of the structures, mechanisms, and activation barriers for the catalytic activation and functionalization of propane by the M1 phase of the Mitsubishi-BP America generation of Mo–V–Nb–Te–O mixed metal oxide (MMO) catalysts. Our calculations show that the reduction-coupled oxo activation (ROA) principle, which we reported at Irsee VI to play the critical role for the selective oxidation of n-butane to maleic anhydride by vanadium phosphorous oxide, also plays the critical role for the MMO activation of propane, as speculated during Irsee VI. However for MMO, this ROA principle involves Te=O and V rather than P=O and V. The ability of the Te=O bond to activate the propane CH bond depends sensitively upon the number of V atoms that are coupled through a bridging O to the Te=O center. Based on this ROA mechanism, we suggest synthetic procedures aimed at developing a single phase MMO catalyst with dramatically improved selectivity for ammoxidation. We also suggest a modified single phase composition suitable for simultaneous oxidative dehydrogenation of ethane and propane to ethene and propene, respectively, which is becoming more important with the increase in petroleum fracking. Moreover, we also suggest some organometallic molecules that activate alkane CH bonds through the ROA principle.
... 21 In view of their unique structures, more interest has been paid to the catalytic applications of Mo-V-O materials in the selective oxidation of hydrocarbons. For example, the ammoxidation of propane to acrylonitrile and propane oxidation to acrylic acid, [1][2][3]11,13,[21][22][23][24][25][26][27][28][29][30][31][32] selective oxidation of isobutane 14,19,[33][34][35][36][37] 53 reported epoxidation/alcoholysis and epoxidation/hydrolysis of glucal and galactal derivatives by Mo catalyst. These studies expand the application of Mo-V-O-based materials into broad catalytic processes. ...
Crystalline Mo-V-O oxides have been used as a catalyst for the hydrolysis and alcoholysis of propylene oxide to diols and ethers, respectively. Relationships between the active crystal facet, the acidity of Mo-V-O catalysts and the activity have been established. Our results indicate that the a-b plane is the active facet for the hydrolysis reaction.
... Ni, Mo, V) at temperature lower than 500 @BULLET C. Among all the catalytic systems, a multicomponent catalyst based on MoVNbTe mixed oxides is one of the most outstanding catalysts for ODHE [21][22][23][24][25][26][27][28][29][30][31][32][33][34], which has great potential to achieve the economic feasibility of the process at 60–80% ethane conversion, more than 90% ethylene selectivity and 1.00 kg C2H4 /kg cat /h catalyst productivity below 500 @BULLET C [35][36][37][38][39]. Unfortunately, the existing research in the litera-ture has not reached the goal so far for lacking the effective way to promote the catalyst performance. The MoVNbTeO x catalyst usually consists of M1 and M2 crystalline phases as well as minor amounts of other phases such as Mo 5 O 14 -type structures, MoV and MoTe oxides [21][22][23][24]. ...
This work presented a comprehensive study on structure sensitivity of phase-pure M1 catalyst in oxidative dehydrogenation of ethane (ODHE). We proposed an effective method to modulate the morphology of phase-pure M1 catalysts by oxalic acid treatment. The proposed approach significantly promoted the catalyst surface area by restraining the anisotropic growth of the basal (001) plane of the M1-phase in calcination and decreased the diameter of M1 needles. Experimental results showed that phase-pure M1 catalyst treated with 1.0 mmol/L oxalic acid solution exhibited remarkably promoted catalyst performance of 73% ethane conversion, 85% ethylene selectivity and 0.77 k C2H4/kg cat/h catalyst productivity at 400 °C. The serial catalyst tests also demonstrated the structure sensitivity of M1 phase in the ODHE reaction. Active sites were located on the lateral faces as well as the basal plane of the M1 needles, but different effects on the catalyst selectivity were observed. The phase-pure M1 catalyst with a lower aspect ratio was suggested to provide better ethylene selectivity in ODHE process.
... These two active sites form a complex network of oxidation. For example, ternary MoV M 1 oxide catalyst selectively oxidizes acrolein to acrylic acid at moderate temperature (190°C) when the Mo-O-V sites are working, but it unselectively oxidizes propane at temperature required for the activation of propane ([320°C) when both bridging and terminal oxygen sites are involved [10,27]. Hence, the E a at 475-525°C is generated by the synergism of the two sites. ...
A series of bulk vanadium mixed oxides (Ce, Al, Fe or Cr) were successfully prepared. Methanol and propane were proposed as probe molecules to investigate the redox properties of this series of bulk vanadium mixed oxides and a qualitative picture of the redox properties was obtained. Specifically, we tested steady-state methanol oxidation at range 250–300 °C and oxidative dehydrogenation (ODH) of propane to propene at range 350–550 °C. We also explored the turn over frequency of redox products (TOFredox) and found that the TOFredox values in methanol oxidation and ODH of propane changed in the order as CeVO4 > AlVO4 > V2O5 > FeVO4 > CrVO4.
Graphical Abstract
... The very short residence times at high temperatures and the fast cooling rates, due to quenching in the flame, [80] do not always allow enough time for complete crystallization, as, for example, observed for BiVO 4 . [167] This leads to a high fraction of amorphous materials without any defined structure (texture), which might be both an advantage, as well as a disadvantage, depending on the material composition and need for well-defined crystal structures as for example in the case of the V-Mo-M-O (M = Te, Sb) system [168][169][170][171] or even more complex systems like polyoxometalates [172] in Keggin-type [173] structures. For such materials, new approaches in the FSP set-up are necessary to achieve the desired crystal phase. ...
... The second major objective of this study was to examine the location, concentration and catalytic role of Nb (and Ta) in propane ammoxidation to acrylonitrile. Although the presence of Nb is known to significantly improve the activity and selectivity of the M1 phase in propane (amm)oxidation [10], its location in the M1 phase could not be directly established by X-ray and neutron diffraction methods due to similar scattering properties of Nb and Mo centers. Instead, Pyrz et al. [11] investigated the Ta-substituted M1 phase by the HAADF-STEM and determined that Ta was located in so-called pentagonal bipyramidal site 9. ...
The MoVTeTaO M1 phases were prepared by conventional hydrothermal (HT) and
microwave-assisted HT synthesis methods (MW) employing two different Ta
precursors, Ta ethoxide and a custom-made Ta oxalate complex. The profile
intensity analysis of the HAADF-STEM image of M1 phases oriented along [hk0]
directions from the surface to bulk region of HAADF-STEM images indicated that
the chemical composition of surface ab planes is very similar to their
composition in the bulk. The HAADF-STEM image analysis showed that synthesis
methods have a significant impact on the Mo/V distribution in the MoVTeTaO M1
phases and their reactivity in propane ammoxidation. Enhanced acrylonitrile
yield and 1st order irreversible reaction rate constants for propane
consumption, normalized to the estimated surface ab plane areas, correlated
with increased V content in the proposed catalytic center (S2-S4-S4-S7-S7).
These observations lend further support to the idea that multiple VOx sites
present in the surface ab planes may be responsible for the activity and
selectivity of the M1 phase in propane ammoxidation.
... Extensive studies have concluded that the initial methylene CAH bond breaking is the rate-determining step in the propane oxidation reaction, and the surface V 5+ @O site is generally considered to be the active center [51][52][53][54][55]. In the case of 3%VTe z O x -MS catalysts, vanadium species existed predominantly as tetrahedral V 5+ sites with a terminal V@O bond. ...
A mesoporous silica with incorporated vanadium (V) and tellurium (Te) (3%VTezOx-MS; z = 0-0.33), synthesized by an evaporation-induced self-assembly route, was used as a model catalyst to investigate the synergetic effect of VOx and TeOx species on propane oxidation to acrolein. It was found that V in the catalyst was predominantly present as a highly dispersed VO4 species, while Te existed as a highly dispersed TeOx species when the Te/V atomic ratio was below 0.2. Acrolein formation rate was positively correlated with TeOx content when both VOx and TeOx in the-catalysts-were in a highly dispersed state. Study of the propane oxidation pathways further indicated that highly dispersed VO4 in close contact with highly dispersed TeOx constituted the active sites for direct oxidation of propane to acrolein. These binary sites satisfied all the required functions for the reactions, including consecutive sequence of activation of propane and propylene intermediate followed by inserting oxygen into the as-formed allylic species.
Nanostructured solids with controlled porosities and morphologies are important for catalysis, separation, optics, electronics, energy storage, drug delivery, and so on. In order to design nanostructured solids, templated synthesis, using various molecules, molecular assemblies, and porous materials as templates, has been developed. In this chapter, preparative methods for nanostructured solids by templating are reviewed. In typical cases, metal cations are used for the structural control of metal oxides with various frameworks. Organic molecules are used as templates of microporous materials such as zeolites. Assemblies of surfactants or block copolymers are used as templates of mesoporous materials. Colloidal crystals consisting of latex particles or colloidal silica are used as templates of three‐dimensionally ordered macroporous materials. Emulsions, solvent droplets, ice, and biominerals are also used as templates of macroporous materials. These processes are classified according to templates, framework components, solidification methods, and removal methods of templates. Morphologies are also controlled as particles, rods or nanowires, and films by the deposition of materials in shaped nanospace or by sol–gel methods. These methods are combined for the control of hierarchical structures containing pores with different scales and/or controlled morphologies. Nanostructured materials are applied as catalysts, catalyst supports, photocatalysts, separation media, optical materials, secondary batteries, fuel cells, supercapacitors, and so on. Templated synthesis is quite useful to control nanostructures, morphologies, and functions of solid‐state materials.
Undoped and Te-doped MoV-Oxide (M1 phase) catalysts have been prepared hydrothermally (Te/Mo ratio in the synthesis gel from 0 to 0.17; and heat-treated at 400 or 600ºC in N2 atmosphere), characterized by several physicochemical techniques and tested in the oxidative dehydrogenation (ODH) of ethane. The morphology and microporosity of the catalysts, the nature of V-species on the catalyst surface and the catalytic performance strongly depend on the composition and the heat-treatment temperature. When calcined at 400ºC, the selectivity to ethylene decreases when the amount of tellurium increases, whereas when heat treated at 600ºC, the selectivity to ethylene increases when the Te-loading increases. These trends have been explained on the basis of the good correlation between selectivity to ethylene and the concentration of V⁴⁺ species on the surface of catalysts, in which the most selective catalyst is that prepared with a Te/Mo ratio of 0.17 and heat-treated at 600ºC.
Ternary Mo–V oxide nanocrystals (Nano-MoVO) were hydrothermally synthesized in the confined space of a mesoporous carbon template and tested in the oxidative dehydrogenation (ODH) of ethane and propane. The synthesized nanocrystals are approximately 60 nm in length, 20 nm in diameter on average, and possess a structure resembling orthorhombic MoVO (Orth-MoVO) as indicated by spectroscopic and microscopy characterization. Yet, the Nano-MoVO catalyst has a 5-fold higher mesopore volume and a 4-fold larger external surface area than an Orth-MoVO synthesized by a conventional method (Orth-MoVO) as characterized through N2 adsorption analysis. Nano-MoVO shows a similar activation energy in the ODH of ethane compared with other conventional MoVO catalysts while exhibiting significantly higher propane/ethane activation ratios and higher propene selectivity even in the absence of elements such as Te and Nb to suppress overoxidation of propane-derived species to COx. The results suggest the benefits of the nanocrystalline morphology to limit overoxidation.
Adding a small quantity of K or Bi to a MoVTeNbO
x
via impregnation with inorganic solutions modifies its surface acid and redox properties and its catalytic performance in propa(e)ne partial oxidation to acrylic acid (AA) without detriment to its pristine crystalline structure. Bi-doping encourages propane oxydehydrogenation to propene, thus enlarging the net production rate of AA up to 35% more. The easier propane activation/higher AA production over the Bi-doped catalyst is ascribed to its higher content of surface V leading to a larger amount of total V5+ species, the isolation site effect of NbO
x
species on V, and its higher Lewis acidity. K-doping does not affect propane oxydehydrogenation to propene but mainly acts over propene once formed, also increasing AA to a similar extent as Bi-doping. Although K-doping lowers propene conversion, it is converted more selectively to acrylic acid owing to its reduced Brønsted acidity and the presence of more Mo6+ species, thereby favoring propene transformation via the π-allylic species route producing acrylic acid over that forming acetic acid and CO
x
via acetone oxidation and that yielding directly CO
x
.
The scientific exploration of solid materials represents one of the most important, fascinating and rewarding areas of scientific endeavour in the present day, not only from the viewpoint of advancing fundamental understanding but also from the industrial perspective, given the immense diversity of applications of solid materials across the full range of commercial sectors. Turning Points in Solid-State, Materials and Surface Science provides a state-of-the-art survey of some of the most important recent developments across the spectrum of solid-state, materials and surface sciences, while at the same time reflecting on key turning points in the evolution of this scientific discipline and projecting into the directions for future research progress. The book serves as a timely tribute to the life and work of Professor Sir John Meurig Thomas FRS, who has made monumental contributions to this field of science throughout his distinguished 50-year career in research, during which he has initiated, developed and exploited many important branches of this field. Indeed, the depth and breadth of his contributions towards the evolution and advancement of this scientific discipline, and his critical role in elevating this field to the important position that it now occupies within modern science, are demonstrated recurrently throughout the chapters of this book. Individual chapters are contributed by internationally leading experts in their respective fields, and the topics covered include solid-state chemistry of inorganic and organic materials, heterogeneous catalysis, surface science and materials science, with one section of the book focusing on modern developments in electron microscopy and its contributions to chemistry and materials science. The book serves as a modern and up-to-date monograph in these fields, and provides a valuable resource to researchers in academia and industry who require a comprehensive source of information on this important and rapidly developing subject.
A series of nanosized-bulk-supported Mo-V-Nb-Te catalysts has been prepared. With a new synthesis approach; nanocristalline aggregates of active phase are deposited on a support; this results in active and selective catalysts for propane oxidation into acrylic acid reaction, that require a low amount of active phase, and that exhibit better mechanical properties and are more economic than their bulk counterparts. The presence of nanocrystalline domains of the Mo-V-Te-Nb-O active phases (M1, M2 and rutile) for the supported-bulk catalyst is confirmed by HRTEM, Raman and SAED. These nanoscaled materials present activity performances during the oxidation of propane into acrylic acid similar to those delivered by conventional bulk phase based catalysts. These values are much higher for the supported nanoscaled active phase when normalized per gram of MoVNbTe oxide. When the active phase is stabilized in the nanoscale, it maximizes the exposed fraction of active sites. In addition to its economic advantage, stabilization and higher exposure of active phase; alumina support endows better mechanical resistance and an easier control to conform of catalyst pellets. In terms of acrylic acid produced per amount of active phase required to produce the catalysts, the catalytic results indicate that best catalytic performances are obtained with the supported-samples, indicating that this is a very promising route/concept for the preparation of catalytic materials.
Interest in the transformation of light alkanes to valuable oxygenated compounds and olefins has been growing again in recent years due to the possibility of developing new processes of lower environmental impact and of lower cost, and to the large availability of hydrocarbons from shale‐gas shells. Here we summarize the most recent findings and perspectives of developments for the industrial gas‐phase oxidation and oxidative dehydrogenation of C2‐C4 alkanes to oxygenated compounds and to light olefins.
M1 phase MoVTeNb mixed oxides exhibit unique catalytic properties that lead to high C 2 H 4 yields in oxidative conversion of C 2 H 6 at moderate temperatures. The role of the heptagonal channel micropores of the M1 phase in regulating reactivity and selectivity is assessed here using reactant size-dependent kinetic probes and density functional theory (DFT) treatments for C 2 H 6 and cyclohexane (C 6 H 12) activations inside and outside the micropores. The sizes of C 2 H 6 and the micropores suggest a tight guest−host fit, but C 6 H 12 cannot access intrapore sites. Measured C 2 H 6 to C 6 H 12 activation rate ratios on MoVTeNbO are much higher than those measured on nonmicroporous vanadium oxides (VO x / SiO 2) and estimated by DFT on external surfaces, suggesting that most C 2 H 6 activations on MoVTeNbO occur inside the micropores under typical conditions. C 2 H 6 exhibits higher activation energy than C 6 H 12 on VO x /SiO 2 , consistent with the corresponding C−H bond strengths; the activation energy difference between C 2 H 6 and C 6 H 12 is lower on MoVTeNbO because micropores stabilize C−H activation transition states through van der Waals interactions. Product selectivities for C 2 H 6 and C 6 H 12 suggest that the ability of VO x /SiO 2 to activate C−H bonds and resist O-insertion in products is similar to the external surfaces of MoVTeNbO, but the micropores in the latter oxides are more selective for C−H activation. DFT calculations show that the tight confinement in micropores hinders the C−O contact necessary for O-insertion. These insights provide guidance for utilizing shapes and sizes of confining voids to mitigate selectivity limitations dictated by thermodynamics of sequential oxidation reactions and electronic properties of redox catalysts.
Synthesized via the slurry method and activated at high temperature (873 K), MoVTeNb multimetallic mixed oxides are applied to catalyze the oxidative dehydrogenation of ethane to ethylene (ODHE). Mixed oxides typically contain M1 and M2 crystalline phases, the relative contribution of these phases and the respective catalytic behavior being notably influenced by the preparation conditions of the metallic aqueous solution precursor, given the complexity of the chemical interactions of metal species in solution. Thus, detailed in situ UV-Vis and Raman studies of the chemical species formed in solution during each step of the synthetic procedure are presented herein. The main role of vanadium is to form decavanadate ions, which interact with Mo species to generate an Anderson-type structure. When niobium oxalate solution is added into the MoVTe solution, a yellow-colored gel is immediately formed due to a common ion effect. When liquid and gel phases are separated, the M1 crystalline phase is produced solely from the gel phase. Attention is also devoted to the influence and role of each metal cation (Mo, V, Te and Nb) on the formation of the active M1 crystalline phase and the catalytic behavior in the ODHE. The catalyst constituted mostly of M1 crystalline phase is capable to convert 45 % of the fed ethane, with a selectivity to ethylene of around 90 %.
Heterogeneous metal oxide catalysts are widely studied for the aerobic oxidations of C1-C4 alkanes to form olefins and oxygenates. In this review, we outline the properties of supported metal oxides, mixed-metal oxides, and zeolites and detail their most common applications as catalysts for partial oxidations of light alkanes. By doing this we establish similarities between different classes of metal oxides and identify common themes in reaction mechanisms and research strategies for catalyst improvement. For example, almost all partial alkane oxidations, regardless of the metal oxide, follow Mars-van Krevelen reaction kinetics, which utilize lattice oxygen atoms to reoxidize the reduced metal centers while the gaseous O2 reactant replenishes these lattice oxygen vacancies. Many of the most-promising metal oxide catalysts include V(5+) surface species as a necessary constituent to convert the alkane. Transformations involving sequential oxidation steps (i.e., propane to acrylic acid) require specific reaction sites for each oxidation step and benefit from site isolation provided by spectator species. These themes, and others, are discussed in the text.
Acrylic acid is an important industrial chemical, and efficient catalysts for its direct preparation by propane oxidation are highly desirable. For this purpose, neutral silica networks were introduced on the surface of MoVTeNb mixed oxide catalysts by controlled silylation using a methyl silicate oligomer (MS-51). The modified catalysts gave ∼56.5% yield of acrylic acid with a selectivity of 77.1% in the oxidation of propane at 380 °C. The catalysts were characterized by X-ray fluorescence, Fourier-transform infrared spectroscopy, Brunauer–Emmett–Teller specific surface area, X-ray diffraction (Rietveld analysis), pyridine desorption, and scanning electron microscopy. MoVTeNb mixed oxide was found to be composed of 90.9% M1 phase and 2.3% M2 phase, and upon silylation, the surface was uniformly covered by a thin SiO2 layer with 0.14 molar ratio with respect to Mo and an estimated thickness of 2.4 nm. The amount of acid sites decreased after the first three silylation cycles, but was not affected by repeated cycles. The results of the kinetic study based on the comparison of the simulated contribution of each side reaction were consistent with those of the model reactions using acrylic acid and other reactants: the controlled silylation effectively suppressed acrylic acid oxidation, especially after repeated silylation cycles, which is responsible for the superior performance of the silylated catalysts. Considering the relatively large size of acrylic acid compared to propane and the efficient propane activation by silica-covered catalysts, the controlled silylation was proposed to have two roles, by which further consecutive oxidation is prevented effectively to exhibit excellent performance in oxidation of propane: i) to block the unfavorable acidic sides, ii) to generate a silica layer with pore mouth openings on the surface of MoVTeNb mixed oxide, which allow the entrance of propane but inhibit re-entrance of the produced acrylic acid.
Bismuth (Bi) was successfully introduced into the crystalline orthorhombic Mo3VOx (MoVO) structure for the first time by using ethylammonium cation (EtNH3+) as a structure-directing agent in hydrothermal synthesis, and the catalytic ac-tivities of MoVO containing Bi (MoVBiO) for selective oxidation of ethane and ammoxidation of propane were compared with those of ternary MoVO. Bi and EtNH3+ were located in hexagonal and heptagonal channels in the MoVO structure, respectively. EtNH3+ could be removed without collapse of the crystal structure by appropriate heat treatment, leaving the heptagonal channels empty. The introduction of Bi had only a little effect on the catalytic activity for selective oxida-tion of ethane. On the other hand, the conversion of propane was significantly enhanced in propane ammoxidation. Acry-lonitrile selectivity was also enhanced by the introduction of Bi, especially at high temperatures (> 440 °C).
The catalytic performance of (i) crystalline MoVTeNb oxide that exhibits the electronic properties of a n-type semiconductor, (ii) sub-monolayer vanadium oxide supported on meso-structured silica (SBA-15) as an insulating support, and (iii) surface-functionalized carbon nanotubes that contain neither a redox active metal nor bulk oxygen, but only surface oxygen species have been compared in the oxidative dehydrogenation of ethane and propane under equal reaction conditions. The catalytic results indicate similarities in the reaction network over all three catalysts within the range of the studied reaction conditions implying that differences in selectivity are a consequence of differences in the rate constants. Higher activity and selectivity to acrylic acid over MoVTeNb oxide as compared to the other two catalysts are attributed to the higher density of potential alkane adsorption sites on M1 and the specific electronic structure of the semiconducting bulk catalyst. Microcalorimetry has been used to determine and quantify different adsorption sites revealing a low Vsurface/C3H8ads ratio of 4 on M1 and a much higher ratio of 150 on silica-supported vanadium oxide. On the latter catalyst less than one per cent of surface vanadium atoms adsorb propane. Barriers of propane activation increase in the order P/oCNT (139 kJ mol-1) ≤ M1 (143 kJ mol-1) < 6V/SBA-15 (162 kJ mol-1), which is in agreement with trends predicted by theory.
Applications of renewable biomass provide facile routes to alleviate the shortage of fossil fuels as well as to reduce the emission of CO2. Glycerol, which is currently produced as a waste in the biodiesel production, is one of the most attractive biomass resources. In the past decade, the conversion of glycerol into useful chemicals has attracted much attention, and glycerol is mainly converted by steam reforming, hydrogenolysis, oxidation, dehydration, esterification, carboxylation, acetalization, and chlorination. In this review, we focused on the catalytic hydrogenolysis of glycerol into C3 chemicals, which contain many industrially important products such as 1,2-propanediol, 1,3-propanediol, allyl alcohol, 1-propanol and propylene. In the hydrogenolysis of glycerol into propanediols, advantages and disadvantages of liquid- and vapor-phase reactions are compared. In addition, recent studies on catalysts, reaction conditions, and proposed pathways are primarily summarized and discussed. Furthermore, new research trends are introduced in connection with the hydrogenolysis of glycerol into allyl alcohol, propanols and propylene.
The direct oxidation of propane to acrylic acid has become a hot spot in catalytic oxidation of light alkanes in recent years. The MoVTe (Sb) NbO mixed metal oxide catalysts are the most important catalysts for this reaction. This article intends to review recent progress in preparation methodologies and in understanding vital crystalline structures which are closely related to catalytic performance. The latest knowledge on the functions of the constituent elements in the catalysts is summarized in terms of the reaction pathways of propane oxidation. It is recognized that modification of particle size and morphology, phase composition and structure, surface acidity and redox property of catalyst are critical to achieve superb catalyst performance and stability.
Recent developments of crystalline Mo3VOx catalysts (MoVO), a new type of oxidation catalysts for selective oxidations of ethane to ethene and of acrolein to acrylic acid, are reviewed. MoVO are formed by the building unit assembly of polyoxomolybdates under hydrothermal conditions. These catalysts are composed of a network arrangement based on a {Mo6O21}6− pentagonal unit and a {MO6} (M = Mo, V) octahedral unit to form a hexagonal channel and a heptagonal channel. Between these channels, the heptagonal channel acts as a micropore of 0.40 nm in diameter which can adsorb small molecules such as CO2, N2, methane, ethane, etc. The size of the heptagonal channel micropore is reversibly and continuously tunable by redox treatment. Interestingly, the heptagonal channel activates ethane inside and acrolein on the channel located over the external surface. Tuning of the heptagonal channel size significantly modifies the catalytic performance for the selective oxidation of ethane. Strong relationships among crystal structure, microporosity, and catalytic performance were observed here.
Abstract The M1 phase of MoVNbTeOx mixed metal oxide is one of the most attractive catalysts for the oxidative dehydrogenation of ethane (ODHE). This work presents the performance of pure-phase M1/CeO2 nanocomposite catalysts in the ODHE process. Nanosized CeO2 is successfully added to pure-phase M1 catalysts by physical mixing and by a sol-gel method, so that M1/CeO2 nanocomposites with different CeO2 particle sizes are formed. The experimental results show that the introduction of CeO2 can increase the abundance of V5+ on the catalyst surface by a self-redox solid-state reaction during activation at 400 °C in air. Improvement of catalyst performance can be observed with the decrease of CeO2 dimensions. The nanocomposite catalyst consisting of M1 particles and 4.4 nm CeO2 exhibits the best catalyst productivity of 0.66 kgC2H4/kgcat h with 20% lower cost than for pure-phase M1 catalysts at 400 °C. It is anticipated that a high-throughput and low-cost ODHE process could be realized on M1/CeO2 nanocomposites.
This review deals with an analysis of the main structural and reactivity properties of mixed oxide systems containing vanadium oxide as the key component in catalysts for gas-phase reactions, mainly oxidations. Particular focus is placed on catalysts showing intrinsic bi-functional properties, where the combination of both acidic/basic properties and redox V-sites is a requisite for achieving optimal catalytic performance. For the selected catalytic systems, structure–reactivity correlations that have been proposed in many decades of literature were analysed, with the aim of shedding light on the molecular-level aspects of current processes and facilitating a more rational design of future catalysts.
Methanol is oxidised to formaldehyde by the Formox process, in which molybdenum oxides, usually doped with iron, are the catalyst. The active phase of the catalysts and the reasons for the selectivity observed are still unknown. We present a density functional theory based study that indicates the unique character of Mo(VI) Mo(IV) pairs as the most active and selective sites and indicates the active sites on the surface, the controlling factors of selectivity, and the role of the dopant. Iron reduces the energy requirements of the redox Mo(VI) Mo(IV) pair by acting as an electron reservoir that sets in if required. Our present study paves the way towards a better understanding of the process.
© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
There is a growing interest in alcohol oxidation electrochemistry due to its role in renewable energy technologies. The goal of this work was to develop active non- precious metal electrocatalysts based on the Mo-V-(M)-O (M is Nb, Te) lattice. Selective gaseous alkane oxidation had been previously observed on these catalysts at elevated temperatures above 300 °C. In this study, the activity of the catalysts at lower temperatures, 25-60 °C, was investigated. Hydrothermal conditions were used to synthesize the Mo-V-(M)-O mixed oxides. Physical characterization of the catalysts were obtained by Powder X-ray diffraction (XRD), Scanning electron micrography (SEM) equipped with energy dispersive x-ray (EDX), Transmission electron micrography (TEM), and X-ray photoelectron spectroscopy (XPS). The catalytic activity for the oxidation of cyclohexanol was studied electrochemically. Chronoamperometric studies were used to evaluate the long term performance of the catalysts. The onset of alcohol oxidative current was observed between 0.2 V to 0.6 V vs Ag/AgCl. Gas chromatography-mass spectrometry analysis was used to determine the nature of the oxidative products. The mild oxidation products, cyclohexanone and cyclohexene, were observed after oxidation at 60 °C. The catalytic activity increased in the order Mo-V-O < Mo-V-Te-O < Mo-V-Te-Nb-O. Mo-V-(Te,Nb)-O based electrocatalysts efficiently catalyzed the oxidation of alcohols at low temperatures.
The sections in this article areIntroductionOxidation of Propylene to Acrolein and Acrylic AcidGeneralActive Phases and Reaction MechanismOxidation of Propane to Acrylic AcidIntroductionThe MoVNbTeO Catalyst SystemReaction Mechanism over MoVNbTeO CatalystsFuture ApproachesEpoxidation of Propene to Propene OxideOxidation of Isobutene to Methacrolein and Methacrylic AcidOxidation of Isobutane to Methacrylic AcidAcknowledgmentsKeywords:active phases;reaction mechanism;MoVNbTeO catalysts;epoxidation
Phase-pure M1 catalysts with different post-treatments have been prepared from the same precursor slurry by hydrothermal synthesis and used to study the key factors influencing their performance in the oxidative dehydrogenation of ethane (ODHE) process. Different purification processes (i.e. steam treatment and hydrogen peroxide treatment) result in different tellurium (Te) contents and V5+ concentration in the catalysts. Catalytic tests reveal that there is a direct correlation between the amount of V5+ present in the catalysts and the catalytic activity. A hydrogen peroxide treatment increases the V5+concentration and decreases the Te content which can improve the catalytic activity and stability in comparison with the steam treatment. A post-treatment with oxalic acid improves the catalyst surface area (54 m2/g) but causes some vanadium leaching. The phase-pure M1 catalyst calcined at 650 ̊C and purified by H2O2 shows the best catalyst productivity of at 73% ethane conversion and 85% ethylene selectivity in ODHE process. The formation of reduced Te(0) aggregates blocking the active sites is identified as a main reason for catalyst deactivation. A low Te content favors a stable catalyst with less risk of Te aggregation. However, at harsh operating conditions (i.e. high oxygen concentration and high reactor temperature) the performance of phase-pure M1 catalysts with a low Te content can also reduce obviously due to the formation of a new (V, Nb)-substituted θ-Mo5O14 and MoO2 phases.
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 α-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–Ox
systems, three crystalline phases (orthorhombic Mo7.5V1.5NbTeO29, 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 α-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 Mo0.6V0.187Te0.14Nb0.085Ox
for maximum acrylonitrile yield from propane, 61.8% (86% conversion, 72% selectivity at 420 °C). We propose that this system, composed of 60% Mo7.5V1.5NbTeO29, 40% Mo6Te2VO20, and trace TeMo5O16, functions with a combination of compositional pinning of the optimum orthorhombic Mo7.5V1.5±x
Nb1±y
Te1±z
O29±δ
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 Mo7.5V1.5NbTeO29, by promoter/modifier substitutions, and incorporation of compatible cocatalytic phases (preferably epitaxially matched). High-throughput methods will greatly accelerate the rational catalyst design processes.
Partial oxidation of acrolein is a commercially important reaction, its product—acrylic acid—being widely used industrially for producing resins, dyes, glues, nonwoven fabrics, etc.Partial oxidation of acrolein is also a convenient model reaction because: (1) the number of reaction products is moderate (CO, CO2, acrylic acid) and (2) their difference in acid-base properties from the starting material makes it possible to select desirable catalysts by applying directly and efficiently Boreskov's concept of intermediate chemical interaction of a catalyst with reaction mixture components. According to this concept [1], the transformation of surface intermediates (SI) formed in the interaction of reactants with a catalyst's surface is determined by the structure and bond energy of these SI.The study of the reaction mechanism includes determination of structures and energy characteristics of the surface intermediates and the elucidation of their connection with catalyst chemical composition and reaction routes to particular products. This reliable information helps us to understand the nature of catalyst action and to elaborate the theory of catalyst selection. We have used this method to approach the problem of the systematic selection of catalysts for the oxidation of acrolein to acrylic acid. The review summarizes the research done in the lnstitute of Catalysis of the Siberian Branch of the Russian Academy of Sciences during recent years.
Unit cell parameters and space group symmetry have been
determined for the two main phases of the MoVTeNbO catalysts; models of
their structures are proposed based on electron micrographes, EDX data and
comparison with other crystal structures.
The structure of the orthorhombic phase in the MoVNbTeO propane ammoxidation catalyst system has been characterized and refined using a combination of TEM, synchrotron X-ray powder diffraction (S-XPD), and neutron powder diffraction (NPD). This phase, designated as M1 by Ushikubo et al. [1], crystallizes in the orthorhombic space group Pba2 (No. 32) with a = 21.134(2) , b = 26.658(2) , and c = 4.0146(3) . The formula unit is Mo7.5V1.5NbTeO29. Bond valence sum calculations indicate the presence of d
1 metal sites neighbored by d
0 metal sites. The d
1 sites are occupied by a distribution of Mo5+ and V4+, whereas the d
0 sites are occupied by a distribution of Mo6+ and V5+. Out-of-center distortions in d
0 octahedra are consistent with the second-order Jahn–Teller effect and lattice effects. We argue that the V5+–O–V4+/Mo5+ moieties adjacent to Te4+ and Mo6+ sites in the [001] terminal plane provide a spatially isolated active site at which the selective ammoxidation of propane occurs.
Several phases reported as minor or major phases in the active MoVTeNbO catalysts have been prepared and investigated for the oxidation of propane into acrylic acid. Activity and selectivity of pure phases and mixtures of phases obtained either directly from synthesis or by co-grinding have been compared. The results obtained confirmed that the orthorhombic M1 phase is the most active and selective phase and is responsible for the major part of the efficiency of the best catalysts. However, they also clearly demonstrated that a synergism due to a cooperation between phases occurs, similar to that previously proposed between the M1 [(Te2O)M20O56] and M2 [(TeO)M3O9] phases for the ammoxidation of propane. The origin of this phase cooperation is discussed.
This review covers the recent developments and the present state of selective oxidation of propane to acrylic acid with molecular oxygen. The current commercial manufacturing process of acrylic acid, as well as the possible oxidation pathways of propane are included as background information. Special attention is given to three classes of leading catalysts: vanadium pyrophosphate, heteropoly acids and salts and mixed metal oxides. Topics covered include the development and the effectiveness of the catalyst systems, the oxidation pathways, and some structural aspects.
The high temperature oxidation behavior in air atmosphere and air-mist cooling property of 1 ton ingot are investigated using plain carbon steel (P.C. steel), chromium bearing steel (Cr steel) and chromium and nickel bearing steel (Cr-Ni steel) in order to clarify the effect of surface oxidation on cooling property. The results obtained are summarized as follows.
At 1573K, Cr enriched outer scale layer and internal oxide consist of Fe2SiO4 eutectic oxide are formed on Cr steel, whereas characteristic subscale of Ni bearing steel is formed on Cr-Ni bearing steel. These characteristics disappear at lower oxidation temperature.
At mist cooling, cooling rate, and hence heat transfer coefficient of Cr steel is significantly reduced compared with P.C. steel and Cr-Ni steel. Characteristics of scale morphology and cooling property due to chromium bearing disappear by Ni addition. Comparing with other cooling experiments, specimen dimension has significant effects on cooling properties.
As air-mist cooling intensity is mild in this work, insulation effect of surface scale or subscale layer is insufficient to explain the cooling property. It is considered that the morphology change of cast surface is caused by the formation of chromium oxide layer at the inside of outer scale and hence prevention of FeO or Fe2SiO4 formation which clamp scale layer.
Publisher Summary This chapter focuses on the multicomponent bismuth molybdate catalyst which is a highly functionalized catalyst system for the selective oxidation of olefin. Some progress is made in explaining the splendid catalytic performance of multicomponent bismuth molybdates that are used widely for the industrial oxidations and ammoxidations of lower olefin. The catalytic activity and selectivity are enhanced by the multifunctionalization of the catalyst systems. Many functions newly introduced are deeply associated with lattice vacancies formed by the introductions of third and fourth elements. The design of the excellent oxidation catalyst depends seriously on the method of selecting these additives by considering their valencies, electronegativities, and ionic radii. The rapid migration of oxide ion and electron transfer are also important in enhancing the catalyst stability. Thus, the appropriate introduction of additional elements into the catalyst system makes it more flexible and durable under the working conditions.
Preparation of a new orthorhombic Mo-V-O catalyst was succeeded by hydrothermal synthesis for the first time. This orthorhombic structure showed high activity for propane oxidation. Selectivity to acrylic acid dramatically increased by the introduction of Te into the orthorhombic Mo-V-O catalyst.
It is shown that the major effect of water in the oxidation of propane to acrylic and acetic acids on Mo1V0.3Sb0.25Nb0.08On
catalysts is to stabilise the active sites and increase the rates of formation of both acids. The usual effect of favoring desorption of the products is considered to be secondary.
Reaction pathways for propane oxidation over the Mo-V-Te-Nb-O catalyst are suggested by Ai[6] for a Te-modified vanadium pyrophosphate (PVO) catalyst. Propane is first oxidized to propylene, which is further oxidized to acrylic acid (AA) through acrolein intermediate. Acetone is then oxidized to acetic acid and COx.The pathway involves the formation and reaction of isopropanol intermediate from propane or propylene to acetone. If an equilibrium exists between propylene and isopropanol, it might be able to also D2O or H2O18 as vapor feed for propylene oxidation since a hydration step is proposed by Ai.
This work aims at demonstrating a relationship between the propane oxidation reaction mechanism and the chemical/physical properties of the Mo–V–Sb–Nb mixed oxide catalyst, calcined (500/600°C) and activated (500°C) under oxidative and inert conditions. Calcination and activation under inert conditions yields a catalyst with complex mixed oxides phases (Sb4Mo10O31, (My5+Moz5+Mo6+1−y−z)5O14, etc.) and a partial reduced total oxidation state, which favours the formation of propene as the sole primary product and acrylic acid as a major secondary product. In contrast, catalysts calcined and/or activated in an oxidative atmosphere give more oxidised phases (MoO3 as a major phase), which results in the preferable formation of acetic acid via a route which bypasses propene as an intermediate.
Kinetic and mechanistic investigations were made of propane oxidation to acrylic acid over Mo1V0.3Sb0.25Nb0.08On catalyst, calcined at 600°C in air, and activated at 500°C in O2/He. The kinetic study allowed determination of the orders of propane disappearance and major products formation, and also emphasised the crucial effect of water on the activation energies of propane and oxygen. Propane and independently propene oxidation at various contact times enabled us to differentiate primary from secondary products and to propose a reaction scheme with three pathways: two major pathways leading to acrylic and acetic acids and a minor pathway leading to propionic acid. It was shown that acetic acid is formed via a route bypassing propene as an intermediate.
MoVTeNbO catalysts with several Mo/Te/V/Nb (1/0.15–0.7/0–0.5/0–0.9) contents have been prepared by hydrothermal synthesis and tested in the selective oxidation of propane and propene to acrylic acid. Characterization results (XRD, FTIR, SEM–EDX, and XPS) and catalytic tests show important differences, depending on the composition of the catalysts. In this way, several crystalline phases have been observed in the active and selective catalysts, i.e., TeMo5O16, (Mo0.93V0.07)5O14, 3MoO2Nb2O5, and/or Nb0.09Mo0.91O2.80, and a new TeVMo oxide crystalline phase. Vanadium is the key element in the activation of propane and the selective achievement of acrylic acid while V- and/or Nb-doped MoTe-containing crystalline phases are related to the selective transformation of propene to acrolein/acrylic acid. However, the role of Nb ions is still unclear. Nb-containing MoVTe catalysts present both high activity and high selectivity to acrylic acid. Space time yields of acrylic acid closer than 70 and 600 gAA Kgcat−1 h−1 can be obtained at 380°C during the oxidation of propane (keeping selectivities of about 55%) and propene (keeping selectivities of about 80%), respectively.
The reaction pathways for the oxidation of propane over VO-H-beta and Mo1V0.3Te0.23Nb0.12Ox are investigated. Two methods are used in this study: (i) overall product selectivities are recorded as a function of conversion, and (ii) those species observed or speculated to exist are reacted individually over the catalysts. With VO-H-beta, propene is the primary product of propane oxidation and acetic acid is a sequential oxidation product of the propene, possibly forming through an acetone intermediate. Mo1V0.3Te0.23Nb0.12Ox also gives propene as the primary product of propane oxidation, and the propene thus formed oxidizes further to acrylic acid and acetone. Reactions of individual oxygenated compounds, e.g., propanal, acrolein, etc., confirm the superior oxidation features of the mixed metal oxide catalyst relative to the zeolite-based catalyst.
The partial oxidation of propane and propene is investigated over a multi-component oxidic catalyst. Kinetic measurements were carried out in an integrally operated fixed-bed reactor with distributed local sampling. The selectivities to acrylic acid are above 60%. In the case of propane oxidation the highest yield obtained so far compares favorably with the values given in the open literature. In both cases the results can be described quantitatively by a network of parallel and consecutive first-order reactions.
This work presents a simple synthesis route for MoVNbTe(Sb)Ox with the metastable structure that is catalytically active for the selective oxidation of light alkanes. The addition of an oxoacid to the partially reduced raw material mixture assisted in the formation of the active structure around 600 °C in an O2-excluded atmosphere. This simple synthesis using a reductant and oxoacid is remarkable since it allows a combinatorial approach to diversify Mo- and V-based mixed oxides such as catalytic materials.
The catalytic effects of molybdenum oxides and vanadium oxides in the conversion of propane are described. The temperature dependence is discussed.
Vapor-phase oxidations of ethane, propane, and acrolein were carried out over crystalline (orthorhombic) and amorphous Mo-V-O catalysts. The Mo-V-O crystalline catalyst was prepared by hydrothermal synthesis for the first time in a new orthorhombic structure. The orthorhombic Mo-V-O catalyst showed extremely high activity for the reactions compared to the amorphous one. Te in the orthorhombic Mo-V-O catalyst increased the selectivity to acrylic acid from propane dramatically.
The bulk mixed Mo-V-Sb-Nb oxides, which is a candidate catalytic system for the selective oxidation of propane to acrylic acid, were prepared using an automated synthesis workstation. Two catalyst compositions, Mo0.3V0.3Sb0.125Nb0.125 and Mo1.0V0.3Sb0.125Nb0.125, were characterized by XRD and Raman spectroscopy and shown to contain Mo6V9O40, Mo0.61–0.77V0.31–0.19Nb0.08–0.04Ox (a niobium-stabilized defect phase of a vanadium-rich molybdate), Mo3Nb2O11, and MoO3 as the major phases. The results of the structural characterization and kinetic screening demonstrated good reproducibility of the phase compositions and catalytic properties of the model Mo-V-Sb-Nb-O system and provided new insights into the transformation processes that occur in these mixed oxides with time on stream. These results suggest the promise of emerging high-throughput in situ characterization techniques for establishing fundamental structure–activity/selectivity relationships and developing novel multicomponent mixed metal oxides for selective oxidation of lower alkanes.
The bulk mixed Mo–V–Sb–Nb–O catalysts, which are a candidate catalytic system for the selective oxidation of propane to acrylic acid, were investigated to elucidate the bulk structure and catalytic behavior of these complex materials. These mixed oxides were prepared via a redox reaction between V5+ and Sb3+ in the presence of Mo6+ and Nb5+ and characterized by potentiometric titrations, XRD, Raman spectroscopy, electron microscopy (TEM), and bulk elemental analysis. A potentiometric titration method was used to determine concentrations of metal cations in various oxidation states. XRD and Raman spectroscopy identified Mo6V9O40, MoO3, SbVO4, and a Nb-stabilized defect phase of a V-rich molybdate as the major phases present. Electron microscopy illustrated the heterogeneity of the bulk oxide phases present in the model Mo–V–Sb–Nb–O system on the submicron scale. MoO3 comprised the bulk of this mixed metal oxide system, while the surface region of these model catalysts contained mixed Mo–V–Sb–Nb oxides. The rutile SbVO4 phase was inefficient in propane oxidation to acrylic acid, while mixed Mo–V–Nb oxides were capable of producing acrylic acid at ∼20 mol% yield.
A new MoVTeO mixed oxide has been obtained during the calcination of the corresponding catalyst precursor mixture at 600 °C in N2. The crystal structure has been studied by means of X-ray diffraction, electron diffraction, high-resolution electron microscopy, and diffuse reflectance spectroscopy. Its XRD pattern, similar to those observed in hexagonal tungsten bronze (HTB) K0.13-0.33WO3, was indexed on the basis of a hexagonal cell of parameters a = 0.727 nm and c = 0.4012 nm. However, the electron microscopy study reveals the crystals as being formed by structural domains of an orthorhombically distorted unit cell derived from the HTB structure. Microstructural details are discussed in terms of tellurium location inside the hexagonal tunnels of the structure. The results obtained constitute the first step for elucidating the role of this new orthorhombic phase in the effectiveness of Mo−V−Te−Nb mixed oxide catalysts.
Niobium and tantalum are important elements for the activation of alkanes in the viewpoints of acidic property and the formation of unique mixed metal oxides. And the difference of the ability of alkane activation between niobium- and tantalum-based oxide catalysts is studied. Although hydrated niobium and tantalum oxides show strong acid property, only hydrated tantalum oxide is activated to a solid superacid by the treatment with sulfuric acid, and isomerizes n-butane to isobutane at room temperature. The sulfuric acid treated tantalum oxide activates P–Mo–V heteropolyacid compounds for the selective oxidation of isobutane to methacrolein (MAL) and methacrylic acid (MAA). The difference of ability of alkanes activation between niobium and tantalum is studied by using surface science technique. Mo–V–Nb–Te mixed metal oxide catalysts are active for the ammoxidation of propane to acrylonitrile (AN). However, Mo–V–Ta–Te mixed metal oxide is less active. The effect of catalyst preparation condition is studied. Mo–V–Nb–Te mixed metal oxide catalysts are also active for the oxidation of propane to acrylic acid (AA).
Several single phasic MoVO-based mixed oxides, all of which have a layer structure in the direction of c-axis and a high dimensional arrangement of metal octahedra in a–b plane, were synthesized by hydrothermal method and their catalytic performance in the selective oxidation of propane to acrylic acid were compared in order to elucidate structure effects on catalytic property and roles of constituent elements. It was clearly demonstrated that the catalyst with the particular arrangement of MO6 (M = Mo, V) octahedra forming slabs with pentagonal, hexagonal and heptagonal rings in (0 0 1) plane of orthorhombic structure was exclusively superior both in the propane oxidation activity and in the selectivity to acrylic acid to the other related Mo- and V-based layer oxide catalysts consisting of either pentagonal or hexagonal ring unit. The role of constituent elements was clarified by the comparison of catalytic performance of MoVO, MoVTeO and MoVTeNbO, all of which have the same orthorhombic structure. Mo and V, which were indispensable elements for the structure formation, were found to be responsible for the catalytic activity for propane oxidation. Te located in the central position of the hexagonal ring promoted the conversion of intermediate propene effectively, resulting in a high selectivity to acrylic acid. The introduced Nb occupied the same structural position of V and the resulting catalyst clearly showed the improved selectively to acrylic acid particularly at high conversion region, because the further oxidation of acrylic acid to COx was suppressed.
Several methods for preparing Mo–V–Nb–Te mixed oxides were examined. Hydrothermal treatment gives a precursor of ammoxidation catalyst which shows twice as high activity after calcination as the catalyst prepared by the known dry-up method. Mixed oxides prepared by solid state reaction give rather poor activity. It has been suggested that the higher activity of hydrothermal treated catalyst is related primarily to its higher surface area.
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.
Mo-V-M(=Al, Ga, Bi, Sb and Te)–O mixed oxide catalysts were synthesized hydrothermally for the first time, characterized structurally, and tested for ethane and propane oxidation after activation by various ways. These catalysts were black solids of rod-shaped (fiber like) crystals, which had a layer structure in the direction of fiber axis and a high dimensional arrangement of metal octahedra in the cross-section plane. These fresh crystalline materials became active for catalytic oxidation of alkanes after heat-treatment at 600 C and subsequent grinding in order to increase exposed plane of the cross-section. The resulting catalysts were very active for an oxidative dehydrogenation of ethane with 80% of the ethylene selectivity in the reaction temperature range of 300 to 400 C and also showed about 50% selectivity to acrylic acid in the propane oxidation. Multi-functional character which derived from the high dimensional structure of the catalysts and mechanism of the selective alkane oxidation were discussed.
TeMxMo1.7O mixed oxides (M = V and/or Nb; x = 0-1.7) have been prepared by calcination of the corresponding salts at 600 C in an atmosphere of N2. A new crystalline phase, with a Te/V/Mo atomic ratio of 1/0.2-1.5/1.7, has been isolated and characterised by XRD and IR spectroscopy. This phase is observed in the TeVMo or TeVNbMo mixed oxide but not in the TeNbMo mixed oxide. The new crystalline phase shows an XRD pattern similar to Sb4Mo10O31 and probably corresponds to the M1 phase recently proposed by Aouine et al. (Chem. Commun. 1180, 2001) to be present in the active and selective MoVTeNbO catalysts. Although these catalysts present a very low activity in the propane oxidation, they are active and selective in the oxidation of propene to acrolein and/or acrylic acid. However, the product distribution depends on the catalyst composition. Acrolein or acrylic acid can be selectively obtained from propene on Nb-free or Nb-containing TeVMo catalysts, respectively. The presence of both V and Nb, in addition to Mo and Te, appears to be important in the formation of acrylic acid from propene.
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.
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, Mo1V1Sb10Ox exhibited the highest selectivity to methacrolein at 440C. The structure analyses by XRD, laser Raman spectroscopy and XPS showed the coexistence of highly dispersed molybdenum suboxide, VSbO4 and -Sb2O4 phases in the Mo1V1Sb10Ox. The high catalytic activity of Mo1V1Sb10Ox 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.
MoVNbTe mixed oxides have been prepared by both hydrothermal synthesis and slurry methods and have been tested in the selective oxidation of propane to acrylic acid. For comparative purpose, ternary metal oxides have also been prepared and tested. Characterisation results (X-ray diffraction and EPR) show important differences between the catalysts prepared hydrothermally and one prepared by a slurry method. The catalysts prepared hydrothermally show a higher activity and selectivity to acrylic acid than those prepared by slurry method. A reaction network for the partial oxidation reaction is tentatively proposed from the catalytic results obtained during the oxidation of propane and propylene on these catalysts.
An arrangement of catalytically active elements of Mo, V, and Te in an oxide solid with a single crystallographic phase was successfully done by the hydrothermal synthetic method. A black solid powder with a rod-shape (by SEM) was obtained. This catalyst material was first air-treated at 280C for 2 h, by which Te was stabilized in the structure. The air-treated sample was then heat-treated at 600C in a nitrogen stream. It was revealed by XRD analysis that this treatment made the solid in a well-crystallized state. Finally, in order to break the rods into fine powders, the well-crystallized rod-shaped material was ground, by which a face of the cross-section of the rods seems to be preferentially appeared. Thus obtained catalyst, Mo6V3Te1O
x
, showed a high activity for the selective oxidation of propane to acrylic acid at 360C. Since the grinding was found to be the most effectual determinant in the propane conversion and the acrylic acid formation, the surface on the cross-section part of the rod-shaped crystals is active for the selective oxidation. It was assumed that all the elements of Mo, V, and Te arrange in this surface and effectively promote the consecutive oxidation from propane to acrylic acid via propene and acrolein.
Syntheses of Mo–V–Sb–Nb–O bulk materials, which are candidate catalyst systems for the selective oxidation of propane to acrolein and acrylic acid, were made using soluble precursor materials. The products were characterized by X-ray powder diffraction and Raman spectroscopic studies. The objectives of this work were to explore the utility of liquid phase automated synthesis for the preparation of bulk mixed metal oxides, and the identification of the oxide phases present in the system. This is the first published study of the phase composition for these materials. After calcination of these bulk oxides under flowing nitrogen at 600°C, and using stoichiometric ratios of Mo–V–Sb–Nb (1:1:0.4:0.4) and Mo–V–Sb–Nb (3.3:1:0.4:0.4) it was demonstrated that a mixture of phases were obtained for the syntheses. X-ray powder diffraction studies distinguished SbVO4, Mo6V9O40, MoO3, and a niobium-stabilized defect phase of a vanadium-rich molybdate, Mo0.61–0.77V0.31–0.19Nb0.08–0.04Ox, as the major phases present. Complementary data were provided by the Raman spectroscopic studies, which illustrated the heterogeneity of the phases present in the mixture. Raman also indicated bands attributable to the presence of phases containing terminal MO bonds as well as M–O–M polycrystalline phases. Previous studies on this system have identified SbVO4 and niobium-stabilized vanadium molybdate species as the active phases necessary for the selective oxidation of alkanes.
Catalytic oxidation and ammoxidation of propane to acrolein and acrylonitrile, respectively, were carried out over Mo–V–Sb mixed oxide catalysts. V–Sb mixed oxide showed the activity for the oxidative dehydrogenation of propane to propene, and the selectivity to propene remarkably increased with increasing the concentration of cation vacancy in VSbO4 phase. It is likely that the oxidative dehydrogenation of propane on the VSbO4 phase is initiated via H-abstraction by acid–base concerted mechanism. The selectivity to acrolein and acrylonitrile increased by the addition of molybdenum species to V–Sb mixed oxide catalyst. Among a series of Mo–V–Sb oxide catalysts, Mo1V1Sb10Ox exhibited the highest selectivity to acrolein and acrylonitrile at 430 and 480°C, respectively. The highly dispersed molybdenum suboxide was formed together with the both phases of VSbO4 and α-Sb2O4 in the Mo1V1Sb10Ox. The high catalytic activity of Mo1V1Sb10Ox can be explained by the bifunctional mechanism of the highly dispersed molybdenum suboxide and the VSbO4 phases as follows: the oxidative dehydrogenation of propane proceeds on the VSbO4 phase followed by the oxidation of propene into acrolein or the ammoxidation into acrylonitrile on the molybdenum suboxide phase. When large size of MoO3 crystallites were formed, cracking reaction, i.e., C–C bond cleavage, occurred leading to non-selective total oxidation, resulting in decreasing the selectivities to acrolein and acrylonitrile.
Selective oxidations of propane and propene to acrylic acid were investigated over hydrothermally prepared Mo-V-Te-O and Mo-V-Te-Nb-O mixed metal oxide catalysts. Mono-phasic crystalline materials were obtained for both catalysts with the same orthorhombic layered structure characterized by sharp peaks at 6.6, 7.9, 9.0, and 22.2° in XRD (Cu Kα) patterns, so that niobium was not necessary for creating the structure. However, the addition of niobium resulted in a clear change in catalyst morphology. Long rod-shaped crystals (30–50 μm) in the case of the Mo-V-Te-O system were observed by SEM, whereas aggregates of small cylinder-shaped crystals (300–500 nm length) in the case of the Mo-V-Te-Nb-O system. Both catalysts were highly active for the oxidations of propane and propene and both showed almost the same catalytic performance in terms of substrate conversions. The results indicate that niobium was not influential in the oxidative activation of propane and propene. On the other hand, the selectivity to acrylic acid in both reactions increased by about 15–20% in the niobium-containing catalyst. This effect was much clearer at higher conversion. We discuss the formation of active orthorhombic structure during the hydrothermal synthesis and the role of niobium on the structure formation and on the catalytic performance of Mo-V-Te-O.
Selective oxidations of ethane to ethene and acetic acid and of propane to acrylic acid were carried out over hydrothermally synthesized Mo-V-M-O (M=Al, Ga, Bi, Sb, and Te) complex metal oxide catalysts. All the synthesized solids were rod-shaped crystallites and gave a common XRD peak corresponding to 4.0 Å d-spacing. From the different XRD patterns at low angle region below 10° and from the different shape of the cross-section of the rod crystal obtained by SEM, the solids were classified into two groups: Mo-V-M-O (M=Al, possibly Ga and Bi) and Mo-V-M-O (M=Sb, and Te). The former catalyst was moderately active for the ethane oxidation to ethene and to acetic acid. On the other hand the latter was found to be extremely active for the oxidative dehydrogenation. The Mo-V-M-O (M=Sb, and Te) catalysts were also active for the propane oxidation to acrylic acid. It was found that the grinding of the catalysts after heat-treatment at 600°C in N2 increased the conversions of propane and enhanced the selectivity to acrylic acid. Structural arrangement of the catalytic functional components on the surface of the cross-section of the rod-shaped catalysts seems to be important for the oxidation activity and selectivity.
The oxidation of propane to acrylic and acetic acids has been studied using a Mo1Nb0.08Sb0.25V0.3 mixed oxide catalyst, calcined and activated before reaction under different conditions (T=500 or 600 °C in atmospheres of N2, static air, and flowing air (dry or with water or ammonia addition)). Catalytic testing was performed at 400 °C in a plug flow microreactor and the characterisation of the catalysts was carried out by XRD, XPS, BET, FT-IR and ammonia TPD. The best conditions for acrylic acid (AA) formation are calcination at 500 °C under N2, followed by activation at 500 °C prior to reaction under helium. The selectivity to acetic acid was found to be rather high, which was assigned to the presence of Sb instead of Te, which is used most often. The presence of all four elements, Mo, Nb, Sb and V, was found necessary to achieve high selectivities in acrylic and acetic acids. The presence of several crystalline phases, such as Sb4M10O31/Sb2M10O31 with M: Mo (major), Nb and/or V (minor), SbVO4, sub-oxides of MoO3, such as Mo8O23 and (MxMyMo1−x−y)5O14 with x and y referring to low amounts of V and Nb, and some amorphous phases, such as Sb2O4, was shown to be important in order to orientate the reaction toward the production of acrylic and/or acetic acids. The presence of MoO3 in all air-calcined samples was found to be detrimental to the selective oxidation of propane.This work exemplifies further the general concept of multicomponent oxide catalysis for heterogeneous selective oxidation, which involves low acidity, a synergistic effect between several solid phases and the role of shear/defective and even XRD amorphous, structures facilitating the redox mechanism of the reaction.
Mo-V-Te-Nb-O catalysts have been prepared by hydrothermal synthesis and have been tested in the selective oxidation of propane to acrylic acid. The catalysts have been prepared by using different Te- and Mo-starting compounds. Both the catalyst characterization results (XRD, SEM–EDX, FTIR and XPS) and the catalytic tests show important differences depending on the Te- and Mo-starting compounds used in the hydrothermal synthesis. The most active and selective catalysts for the oxidation of propane to acrylic acid were those prepared from Te(VI)-compounds, i.e. telluric acid or an Anderson-type telluromolybdate. However, catalysts prepared from Te(IV)-compounds, i.e. TeO2 or (NH4)4TeMo6O22·2H2O, presented both low activity and low selectivity to acrylic acid. In the best catalysts, several crystalline phases were observed: Mo5TeO16, (Mo0.93V0.07)5O14, and/or Nb0.09Mo0.91O2.80 and new Te-V-Nb-Mo-oxide crystalline phases. The presence of small crystals of MoO3 in active and selective catalysts can also be proposed. The different catalytic performance of these catalysts could be related to the different incorporation of Te, V and Nb ions, which depends strongly on the Te-compound introduced in the synthesis gel.
The oxidation state and local geometry of tellurium in MoVTeNbO catalysts used in the ammoxidation or oxidation of propane were characterized by X-ray absorption, Mössbauer, and X-ray photoelectron (XPS) spectroscopies. The results obtained by Mössbauer, and XPS spectroscopies showed that the catalysts contained Te(IV) in the bulk and mainly Te(VI) at the surface. X-ray absorption fine structure (EXAFS) measurements allowed to determine that the tellurite entities corresponded to TeO4E trigonal bipyramid in the hexagonal phase and to TeO3E somewhat distorted trigonal pyramid in the orthorombic one. The completely determined environment appeared to correspond in both phases to crystallographic sites in hexagonal channels. These results allowed to determine the stoichiometries of the two phases which are TeM3O10 for the hexagonal phase and Te2M20O57 for the orthorombic phase (M=Mo, V, Nb). 125Te Mössbauer isomer shift and pre-peak surface of the X-ray absorption spectra of the Te LIII-edge have been correlated and contributions of the Te(5s) and Te(2p3/2) to the structure have been analyzed.
Current topics of catalysts containing niobium and tantalum, especially in the field of solid acid catalysis and selective oxidation of hydrocarbons are reviewed. Hydrated niobium oxide and hydrated tantalum oxide are highly acidic. Hydrated niobium oxide is active for the hydration of ethene to ethanol, and Nb–W mixed metal oxide is more active for the reaction. Acid properties of tantalum oxide are changed by being supported on SiO2. Ta oxide/SiO2, prepared by the chemical reaction between tantalum alkoxide and surface hydroxyl groups of SiO2, is active and selective for the gas phase Beckmann rearrangement of cyclohexanoneoxime to caprolactam. Niobium oxide and tantalum oxide easily react with many other oxides to form mixed metal oxide phases with complex structure. Mixed metal oxide catalysts, containing molybdenum, vanadium, certain elements together with niobium are active for the selective oxidation of hydrocarbons. Especially, the selective oxidation of propane by such mixed metal oxide catalysts has been paid attention. Additionally, recent progress of environmental catalysts, promoted by niobium and tantalum compounds, namely catalysts for the pollution abatement is reviewed.
The synthesis of MoVNbTe(Sb)O(x)() composite oxide catalysts based on the self-organization of polyoxometalates (POMs) was investigated. The catalysts which were synthesized via reduction of POMs by using reducing agents under mild conditions and/followed by calcination in an O(2)-excluded atmosphere which superior performance for propane (amm)oxidation. It was suggested that the metastable phase formed at an elevated temperature with a specific oxidation state corresponds to the catalytic activity.
Mo-V-Te-Nb metal oxide catalysts prepared by hydrothermal synthesis and heat-treated in N2 at high temperatures (600-700 degrees C) show high activity and selectivity for the oxidative dehydrogenation of ethane to ethene. Yields of ethene of 75% have been obtained at 400 degrees C on the best catalysts.
- Moro-oka