Doping of MoVNbTeO (M1) and MoVTeO (M2) Phases for Selective Oxidation of Propane and Propylene to Acrylic Acid

The effectiveness of MoV(Nb,Ta)(Te,Sb)O catalysts for the selective oxidation of propane to acrylic acid (AA) and ammoxidation to acrylonitrile (AN) is well known and recorded in the literature. One of the best known catalyst systems is comprised of two phases: M1 (orthorhombic) Mo_7.8V_1.2NbTe_0.94O_28.9 and M2 (pseudo-hexagonal) Mo_4.67V_1.33Te_1.82O_19.82, usually in a 60/40 ratio. The M1 structure performs all of the catalytic functions needed for converting propane to acrylic acid or acrylonitrile, since all key metals having the desired catalytic functions are located at the active center of the catalyst and within bonding distance of each other to perform this complex task. The M2 phase is a co-catalyst in symbiosis with M1, performing a mop-up operation converting free intermediately formed propylene to the respective desired end products (AA or AN). Various attempts have been reported in the literature, with varying degrees of success, to substitute select elements of the two respective structures to enhance the yields of the desired end products. A yield improvement in either phase should theoretically lead to an enhancement of the overall desired yield and of M1/M2 optimal mixtures. Our current study concentrates on the selective doping of the M1, as well as, M2 structure in the selective oxidation of propane and propylene, respectively, to acrylic acid; it centers at doping these structures with low levels of elements having acidic (P, B, W) or redox (Cu) properties. Higher acrylic acid yields were obtained with the doped M1 (2–5%) and M2 (up to 15%) systems. Directed, high throughput methodology was employed as the experimental technique. The study of low doping levels is being extended to include a broader base of elements, as well as, M1/M2 mixtures (optimum compositions) aimed at achieving still higher useful product yields (AA or AN).     

M1 to M2 Phase Transformation and Phase Cooperation in Bulk Mixed Metal Mo–V–M–O (M=Te, Nb) Catalysts for Selective Ammoxidation of Propane

In the present study we systematically explored hydrothermal synthesis of bulk mixed metal Mo–V–(Te–Nb)–O catalysts and investigated the bulk characteristics of the resulting M1 and M2 phases as well as their roles in the selective ammoxidation of propane. It was found that unlike Mo–V–Te–Nb–O M1 phases, the Mo–V–Te–O M1 phases may be quantitatively transformed into M2 phases of the same chemical composition indicating that Nb stabilizes the M1 structure. The stabilizing role of Nb and Te was further observed in high resolution TEM studies of Mo–V–(Te–Nb)–O catalysts which indicated that structural order and the M1 phase domain size progressively decreased in this order: Mo–V–Te–Nb–O > Mo–V–Te–O > Mo–V–O. The cooperation between the M1 and M2 phases in propane ammoxidation to acrylonitrile was observed only at low propane conversions suggesting that the M1 phase is the only crystalline phase required for the activity and selectivity of the Mo–V–Te–Nb–O catalysts in propane ammoxidation to acrylonitrile at practical propane conversions.     

Activation of propane and butanes over niobium- and tantalum-based 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).     

Selectivity issues in (amm)oxidation catalysis

Selectivity is currently taking center stage in heterogeneous oxidation catalysis as the cost of feed materials escalates. Particularly important and imperative for commercial processes is selectivity at acceptably high conversions. Dealing with this demanding quest we proposed, some 40 years ago, the concept of site isolation, defining one of the key requirements needed to achieve selectivity in oxidation catalysis. This principle continues to be useful in the conceptual design of new selective oxidation catalysts and has successfully described the selectivity behavior of many commercial (amm)oxidation catalysts, including now the MoVNbTeO system for propane ammoxidation to acrylonitrile (or oxidation to acrylic acid). In its catalytically optimum form, this system is comprised of at least two crystalline phases, orthorhombic Mo_7.8V_1.2NbTe_0.94O_28.9 (M1) and pseudo-hexagonal Mo_4.67V_1.33Te_1.82O_19.82 (M2). The M1 phase is the key paraffin activating and ammoxidation catalyst, its active centers containing all of the key elements V⁵⁺, Te⁴⁺, Mo⁶⁺, properly arranged to catalytically transform propane to acrylonitrile, and four Nb⁵⁺ centers, each surrounded by five molybdenum-oxygen octahedra, isolating the active centers from each other, thereby preventing overoxidation and leading to the observed high selectivity of the desired acrylonitrile product. Symbiosis between the M1 and M2 phases occurs when the two phases are synthesized concurrently in one vessel; or between physical mixtures of the two separately prepared phases provided they are finally divided (≤5 μm), thoroughly mixed and in micro-/nano-scale contact with each other. This phenomenon is particularly pronounced at high propane conversion when the M2 phase begins to serve as a co-catalyst to the M1 paraffin activating phase, converting extraneous, desorbed propylene intermediate, emanating from the M1 phase, effectively to acrylonitrile in a phase cooperation mode. The M2 phase is incapable of propane activation, lacking V⁵⁺ sites, but is a better propylene to acrylonitrile catalyst than the M1 phase since it possesses a higher concentration of Te⁴⁺ sites (i.e., propylene activating sites). Reaction networks for propane (amm)oxidation are proposed for these catalysts.     

Influence of gel composition in the synthesis of MoVTeNb catalysts over their catalytic performance in partial propane and propylene oxidation

MoVTeNb mixed oxides catalysts have been prepared by a slurry method with different molar compositions (Mo/Te ratio from 2 to 6 and Nb/(V + Nb) ratio from 0 to 0.7) in the synthesis gel leading to different crystalline phases distribution and catalytic behaviour in the partial oxidation of both propane and propylene to acrylic acid. Chemical analysis indicates that the composition of samples before and after the heat-treatment changes, especially the Te-content, since a significant amount of Te is lost during the heat-treatment step when the amount of oxalate (from niobium oxalate) increases in the synthesis gel. Thus, the nature of the crystalline phases and the catalytic performance of heat-treated materials will be related to the final chemical composition. On the other hand, only the catalysts presenting Te₂M₂₀O₅₇ (M = Mo, V, Nb) crystalline structure, the so-called M1 phase, were active and selective in the partial oxidation of propane to acrylic acid. Moreover, all catalysts were active and relatively selective to the formation of O-containing products, i.e. acrolein and/or acrylic acid, during the partial propylene oxidation although the more active ones were those presenting M1 phase.     

Effect of preparation conditions on the phase composition of the MoVTe(Nb) oxide catalyst for the oxidative conversions of propane

The replacement of expensive propylene by propane, which requires the development of catalysts for the direct oxidation of propane into acrylonitrile, is an important and insufficiently studied problem. Multicomponent Mo _m V _n Te _k Nb _x oxide systems are promising in one-stage ammoxidation of propane to acrylonitrile. Despite considerable attention of various authors to the preparation methods for these catalysts, the reproducibility of their physicochemical and catalytic properties is low. To optimize the technology of catalyst synthesis, we studied the effect of drying method (evaporation or spray drying) for the aqueous suspension of the initial compounds on the formation of the Mo1V_0.3Te_0.23(Nb_0.12) oxide catalyst. It is shown that the method of drying determines the chemical and phase composition of solid catalyst precursors and the phase composition of the final catalyst in high-temperature treatment. The use of spray drying provides the required physicochemical characteristics of the catalyst (the specific surface area and the phase composition) that determine the high activity and selectivity in the selective conversion of propane. These catalysts contain two crystalline phases: orthorhombic M1 and hexagonal M2 in an optimal ratio of 3: 1.     

Alumina supported Mo–V–Te–O catalysts for the ammoxidation of propane to acrylonitrile

The effect of Te addition over Mo–V–O catalysts supported on alumina is discussed for the ammoxidation of propane to acrylonitrile. Catalyst composition and atmosphere of activation are evaluated. Catalysts are characterized before and after catalytic reaction by XPS, XRD and in situ Raman spectroscopies. The absence of Te in catalysts formulation and the presence of a high amount of vanadium induce the presence of V⁵⁺ species and the formation of V₂O₅ oxide; associated with a decrease in acrylonitrile selectivity. The presence of Mo-based polyacids structures decreases the selectivity to acrylonitrile. V⁵⁺ sites are responsible of propane activation and of the subsequent -H abstraction to form the intermediate propylene. Then, a Mo–V rutile-like structure in which vanadium species are reduced as V⁴⁺, is responsible for nitrogen insertion and acrylonitrile formation. The formation of such structure is favoured when Te is added to catalysts and is promoted during propane ammoxidation.     

Substituted Mo–V(Ti)–Te(Ce)-oxide M2 Catalysts for Propene Ammoxidation

One of the most effective propane to acrylonitrile ammoxidation catalyst is comprised of the two phases M1 (orthorhombic) Mo_7.5V_1.5NbTeO₂₉ and M2 (pseudo-hexagonal) Mo₄V₂Te₂O₂₀. Under reaction conditions, the two phases work in symbiosis with each other where M1 is the paraffin activating component and M2 is the olefin activating component. Since the catalytic improvement of either phase should result in an enhancement of the overall acrylonitrile yield, controlled substitution of certain elements in either or both phases might result in the desired improvement. Our current study concentrates on the partial substitutions of V with Ti and Te with Ce in the M2 phase. Ti substitution results in a considerable propene activity improvement, whereas the selectivity to acrylonitrile is unaffected. Substitution with Ce, on the contrary, substantially improves the selectivity to acrylonitrile. Also, a minor improvement of the activity is notable. The acrylonitrile selectivity improvement is a result of better NH₃ utilization and comes at the expense of reduced acrolein make. XRD reveals that all of the substituted compositions retain the M2 structure and essentially are monophasic. XANES recordings show for the bulk that the Mo is 6+, the V is 4+, or 4+ and 5+ when Ce is present, the Ti is 4+, the Ce is 3+, and the Te 4+ with some 6+ also present. According to the ESR data, in the M2 with Ce (7Te/3Ce) only 21% of the V is 4+, the remainder being 5+, which tentatively can be explained by the existence of some cation vacancies in the hexagonal channels. HRTEM imaging reveals little if any differences between the materials, all have the typical pseudo-hexagonal habit of the M2 phase and expose a 1–2 nm thick surface layer without any apparent long-range ordering. XPS data show that all catalysts, including the base, are highly enriched at the surface with Te at the expense of other metals. The 7Te/3Ce composition exhibits also substantial Ce surface enrichment. Moreover, the valences of the cations at the surface differ from the bulk in that for all fresh catalysts V is 5+ and Te is 6+ on the surface. Characterization by XPS of catalysts used in propene ammoxidation, reveals reduction of Te and, except when Ce is present, also Mo. Therefore, it might be inferred that the surfaces of the catalysts studied here are comprised essentially of one or a few monolayers of TeMoO or TeCeMoO on an interacting M2 crystalline base.     

Preparation, Characterisation and Catalytic Behaviour of a New TeVMoO Crystalline Phase

TeM_xMo_1.7O 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.     

Towards an Understanding of the Reaction Pathways in Propane Ammoxidation Based on the Distribution of Elements at the Active Centers of the M1 Phase of the MoV(Nb,Ta)TeO System

The M1 phase of the MoV(Nb,Ta)TeO system is one of the most effective catalysts for the ammoxidation and selective oxidation of propane to acrylonitrile (AN) and acrylic acid, respectively. The active centers of the M1 phase reside on the ab planes of this crystalline material (i.e., the (001) lattice face). Early on we proposed that the thus located active centers contain all key catalytic elements strategically placed for the conversion of propane to AN. These seven element comprising active centers contain: five metal oxide octahedra (2 V _0.32 ⁵⁺ /Mo _0.68 ⁶⁺ , 1 V _0.62 ⁴⁺ /Mo _0.38 ⁶⁺ , 2Mo _0.5 ⁶⁺ /Mo _0.5 ⁵⁺ ) and two Te⁴⁺??oxygen sites. In this contribution we analyze the various compositional probabilities of the seven element active centers and their additional eight element surroundings and conclude that there are 32 possible compositional arrangements of this 15 element assembly. From the diverse structural arrangements, diverse catalytic properties can be assigned to the individual sites, leading to diverse propane reaction pathways. We conclude that there are 22% AN forming, 22% propylene, 10% waste and 46% inert sites. After normalization these sites are deemed to lead to the following product yields: 41% AN, 41% propylene and 18% waste. The highest experimentally attained AN yield from propane is 42%, employing M1 phase only, which coincides with the predicted value of a concerted mechanism. Higher AN yields are, however, anticipated, up to a lofty upper limit of 82%, by allowing also for a consecutive mechanism (C₃° ?? C ₃ ⁼ ??AN). This possibility can be rationalized on the basis of the existence of vicinal C₃° ?? C ₃ ⁼ /C ₃ ⁼ ??AN sites whose presence is plentiful on the catalytically important ab planes of M1. The placement and efficiency of these sites is, however, not perfect; therefore the upper AN yield limit is not realized in practice. Our analysis of the elemental distribution at the active centers and their immediate surroundings provides us with new insights into the relationship between structure and catalytic reaction mechanisms of the M1 phase and might serve as a guide towards a redesign of the M1 composition, so as to attain higher AN yields from propane. It provides a challenging task for the synthetic chemist.     

Catalytic Consequences of a Revised Distribution of Key Elements at the Active Centers of the M1 Phase of the MoVNbTeOx System

In an earlier publication (Grasselli et al., Top Catal 54:595, 2011) [1] we analyzed the distribution of key catalytic elements at the active site of the M1 phase of the MoVNbTeO_x catalyst system based on the definition of this center put forth in our original work (Grasselli et al., Top Catal 23:5, 2003) [2]. From that analysis we derived the probabilities of the metal element distributions at the active center and its immediate surroundings, and based on those results, proposed a model for propane ammoxidation on M1. Recently, the occupancies and concomitant charges of the various elements of the M1 phase have been revised (Li et al., Top Catal 54:614, 2011) [3]. Based on this revised structural model we have now recalculated the elemental probabilities at the active center and its immediate surroundings, and describe here the catalytic consequences in the ammoxidation mechanism of propane that these changes portend. Our revised model (REV) predicts more closely the actual experimental results of propane ammoxidation over MoVNbTeO_x than does our first model (ORIG). The results obtained are: ORIG Model: 41 % AN (acrylonitrile) concerted, 82 % total possible AN; REV Model: 43 % AN concerted, 59 % total possible AN; experimental: 42 % AN concerted, 62 % total possible AN using both the M1 + M2 phases. Comparing the original (Grasselli et al., Top Catal 23:5, 2003) [2] and current (Li et al., Top Catal 54:614, 2011) [3] elemental distributions at the active centers of M1 and the ORIG and REV ammoxidation reaction pathways (from the derived models), it is readily apparent that higher concentrations of V⁵⁺ at the active centers lead to undesirable overoxidation of propane and thus lower AN selectivity. Therefore, decreasing the surface concentration of vanadium in M1 (to favor more site isolated V⁵⁺ sites) should be beneficial and lead to better AN selectivities and yields. Additionally, selective doping or selective isomorphous substitution of M1 (and/or M2) should also be useful approaches towards improved AN yields.     

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

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

Selective oxidation of light alkanes over hydrothermally synthesized Mo-V-M-O (M=Al, Ga, Bi, Sb, and Te) oxide catalysts

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.     

Highly Efficient Fe-silicalite Zeolite in Direct Propane Ammoxidation with N2O and O2

A novel process for the direct ammoxidation of propane over steam-activated Fe-silicalite at 723–823 K is reported. Yields of acrylonitrile (ACN) and acetonitrile (AcCN) below 5% were obtained using N₂O or O₂ as the oxidant. Co-feeding N₂O and O₂ boosts the performance of Fe-silicalite compared to the individual oxidants, leading to AcCN yields of 14% and ACN yields of 11% (propane conversions of 40% and products selectivity of 25–30%). The beneficial effect of O₂ on the propane ammoxidation with N₂O contrasts with other N₂O-mediated selective oxidations over iron-containing zeolites (e.g. hydroxylation of benzene and oxidative dehydrogenation of propane), where a small amount of O₂ in the feed dramatically reduces the selectivity to the desired product. It is shown that the productivity of ACN and especially AcCN, expressed as mol product h⁻¹ kg_cat⁻¹, is significantly higher over Fe-silicalite than over active propane ammoxidation catalysts reported in the literature. Our results open new perspectives to improve the performance of alkane ammoxidation catalysts.     

Catalytic behaviour of M1, M2, and M1/M2 physical mixtures of the Mo–V–Nb–Te–oxide system in propane and propene ammoxidation

Essentially pure orthorhombic M1 and pseudo-hexagonal M2 phases were prepared using the precursor method. Consistent with literature the M1 phase was shown to be effective for propane ammoxidation to acrylonitrile while the M2 phase was essentially inert for propane activation. Both phases convert propene efficiently to acrylonitrile. Both phases show a significant selectivity dependence on the ammonia and oxygen concentrations in the feed, revealing thereby additional insights into the reaction mechanism.

Physical mixtures of the two separately prepared phases exhibited symbiosis in the ammoxidation of propane when finally divided (5 μm), thoroughly mixed and brought into intimate contact with each other. Acrylonitrile yields significantly higher than those obtained with the M1 phase alone were demonstrated with a 50 wt.% M1/50 wt.% M2 physical mixture having a corresponding surface area ratio of about 4:1. The phase cooperation effect is particularly large at high propane conversions and non-existent when the particle size of the phases is too large (e.g. >250 μm) and the inter-particle contact is poor.     

Active centers, catalytic behavior, symbiosis and redox properties of MoV(Nb,Ta)TeO ammoxidation catalysts

Selective as well as waste forming active centers were defined for MoVNbTeO and MoVTaTeO catalysts in the ammoxidation of propane to acrylonitrile and all catalytic functionalities were assigned to specific elements at the respective active centers. Symbiosis between M1 and M2 phases of these catalysts was observed, with phase cooperation being more extensive in the Nb than Ta containing compositions. The difference in catalytic effectiveness arises most likely because contact and surface area exposure of the two respective, cooperating phase pairs are not equal. The M1 phase of the catalysts is reducible by propane and ammonia in the absence of dioxygen and is regenerable to its original, fully oxidized state by dioxygen (air). No structural collapse is observed even after 120 C₃H₈ + NH₃ reduction pulses. The so induced reduction of the catalyst extends up to 70 layers deep. The product distribution over the first few pulses is very similar to that under catalytic conditions, supporting the concept that lattice oxygen is involved in the catalytic ammoxidation process. Therefore, the ammoxidation of paraffins is a redox process, as is of course the well-known olefin ammoxidation process.     

New Class of Catalysts for the Propane Ammoxidation Process Based on Vanadium Aluminum Oxynitrides

Amorphous high-surface-area vanadium aluminum oxynitrides (VAlON) prepared by nitridation of an amorphous oxide precursor exhibit a high acrylonitrile yield in propane ammoxidation at very low contact time, indicating the participation of ammonia dehydrogenated species in the ammoxidation of alkanes. The productivity of the VAlON catalysts (l of ACN/kg catalyst/h) was markedly improved as compared with known oxide systems.     

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

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

Propane Oxidation on MoVTeNbO Mixed Oxide Catalysts: Study of the Phase Composition of Active and Selective Catalysts

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.     

Model bulk Mo–V–Te–O catalysts for selective oxidation of propane to acrylic acid

The crystal structures and chemical compositions of the M1 and M2 phases proposed as active and selective in propane oxidation to acrylic acid over the bulk mixed Mo–V–Te–O catalysts were investigated by the transmission electron microscopy, coupled with energy dispersive analysis of X-rays. The results revealed that the crystal structure of the M1 phase is orthorhombic with space group Pbca, lattice parameters a=21.25 Å, b=27.14 Å, c=4.03 Å and composition Mo_0.64V_0.32Te_0.1O_3.05 (Mo/V ∼2), whereas M2 is hexagonal with space group P6mm, lattice parameters a=7.10 Å, c=4.05 Å and composition Mo_1.79V_1.85Te_0.1O_11.36 (Mo/V ∼1). The M1 phase was dominant in the Mo–V–Te–O catalyst. The results obtained indicated that the bulk Mo–V–Te oxides represent a well-defined model catalytic system for the studies of the surface molecular structure-activity/selectivity relationships in propane oxidation to acrylic acid.