Article

Multifunctionality of Crystalline MoV(TeNb) M1 Oxide Catalysts in Selective Oxidation of Propane and Benzyl Alcohol

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Abstract

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.

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... 4 Multi-metallic oxide networks are active components in catalysis for selective oxidation of alkanes as they provide the best compromise between selectivity and site isolation. [5][6][7][8] Hence, complex oxides have been synthesized to establish site isolation on a structural basis. In particular molybdenum-vanadium based multimetal oxide catalysts are considered among the most versatile catalysts in the selective oxidation of ethane 9 , propane 10 and propene. ...
... In fact, the multifunctionality and the site selectivity of this class of catalysts has been explored both experimentally and theoretically in various oxidation reactions. 5,[12][13][14][15][16][17][18][19][20][21][22][23] It has been shown however, that under reaction conditions the catalyst surface is structurally different from the bulk and highly dynamic with respect to changes in the composition of the surrounding gas phase. 5 Therefore, establishing key structure-reactivity relationships is a highly desirable task to rationally design catalysts for selective oxidation. ...
... 5,[12][13][14][15][16][17][18][19][20][21][22][23] It has been shown however, that under reaction conditions the catalyst surface is structurally different from the bulk and highly dynamic with respect to changes in the composition of the surrounding gas phase. 5 Therefore, establishing key structure-reactivity relationships is a highly desirable task to rationally design catalysts for selective oxidation. This requires the execution of systematic spectroscopic investigations into structural, electronic and surface properties of the catalysts. ...
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Absorption and multiwavelength resonance Raman spectroscopy are widely used to investigate the electronic structure of transition metal centers in coordination compounds and extended solid systems. In combination with computational methodologies that have predictive accuracy, they define powerful protocols to study the spectroscopic response of catalytic materials. In this work, we study the absorption and resonance Raman spectra of the M1 MoVO x catalyst. The spectra were calculated by time-dependent density functional theory (TD-DFT) in conjunction with the independent mode displaced harmonic oscillator model (IMDHO), which allows for detailed bandshape predictions. For this purpose cluster models with up to 9 Mo and V metallic centers are considered to represent the bulk structure of MoVO x . Capping hydrogens were used to achieve valence saturation at the edges of the cluster models. The construction of model structures was based on a thorough bonding analysis which involved conventional DFT and local coupled cluster (DLPNO-CCSD(T)) methods. Furthermore the relationship of cluster topology to the computed spectral features is discussed in detail. It is shown that due to the local nature of the involved electronic transitions, band assignment protocols developed for molecular systems can be applied to describe the calculated spectral features of the cluster models as well. The present study serves as a reference for future applications of combined experimental and computational protocols in the field of solid-state heterogeneous catalysis.
... Catalytically, the M1 phase alone was found to be active and selective in the propane (amm)oxidation reactions, while the pure M2 phase was not able to activate propane. 10,11,25,26 Nevertheless, the best reported catalysts still contain significant amount of the M2 phase (about 40 wt % in the MoÀVÀTeÀNb oxide system). 27 The reason behind this is unclear. ...
... The identity of the propane activation sites in the M1 phase catalyst were believed by some to be located on the (001) basal plane of the crystal structure, 4,27,28,39 while others suggested that the side planes can also be active. 10,25 This catalyst (A) provided a rare example where the side planes of the catalyst particle largely appear to be "M1-like", while the basal planes remain to be the M2 phase. Lack of catalytic activity strongly supports the hypothesis that the basal plane of the M1 phase is crucial. ...
Article
In recent decades, catalysis research has transformed from the predominantly empirical field to one where it is possible to control the catalytic properties via characterization and modification of the atomic-scale active centers. Many phenomena in catalysis, such as synergistic effect, however, transcend the atomic scale and also require the knowledge and control of the mesoscale structure of the specimen to harness. In this paper, we use our discovery of atomic-scale epitaxial interfaces in Molybdenum-Vanadium based complex oxide catalysts systems (i.e. Mo-V-M-O, M = Ta, Te, Sb, Nb and etc.) to achieve control of the mesoscale structure of this complex mixture of very different active phases. We can now achieve true epitaxial intergrowth between the catalytically critical M1 and M2 phases in the system that are hypothesized to have synergistic interactions, and demonstrate that the resulting catalyst has improved selectivity in the initial studies. Finally, we highlight the crucial role atomic scale characterization and mesoscale structure control play in uncovering the complex underpinnings of the synergistic effect in catalysis.
... Moreover, the promoted charge separation and transfer will accelerate the activation of oxygen by transferring more photoelectrons to the absorbed oxygen [12,21]. In addition, the interaction between the reactant, the product, and semiconductors also influence the reaction efficiency and products distribution [20][21][22][23][24]. Previous works have demonstrated that the surface properties of photocatalysts greatly affect the absorption of benzyl alcohol (BA) and desorption of benzaldehyde (BZH), and thus tuning the conversion (con.) and selectivity (sel.) of the photosynthetic reactions [20][21][22][23][24]. Based on these understandings, the synergistic modulation of the structure and surface properties of photocatalysts may be an effective approach to advance photocatalytic organic transformations, such as the conversion of benzyl alcohols to benzaldehydes. ...
... Moreover, the promoted charge separation and transfer will accelerate the activation of oxygen by transferring more photoelectrons to the absorbed oxygen [12,21]. In addition, the interaction between the reactant, the product, and semiconductors also influence the reaction efficiency and products distribution [20][21][22][23][24]. Previous works have demonstrated that the surface properties of photocatalysts greatly affect the absorption of benzyl alcohol (BA) and desorption of benzaldehyde (BZH), and thus tuning the conversion (con.) and selectivity (sel.) of the photosynthetic reactions [20][21][22][23][24]. Based on these understandings, the synergistic modulation of the structure and surface properties of photocatalysts may be an effective approach to advance photocatalytic organic transformations, such as the conversion of benzyl alcohols to benzaldehydes. ...
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Light-driven selective oxidation of aromatic alcohols allows a sustainable and eco-friendly manner to convert solar energy into highly valuable aromatic aldehydes. However, the low separation and transfer efficiency of photogenerated charge carriers and the sluggish reaction kinetics seriously restrict the efficiency of the organic photosynthesis of desired compounds. Herein, a facile strategy is adopted to regulate the boron carbon nitrides (BCN) semiconductors by hydrogen reduction to precisely tune the structural and surface properties. The reduced BCN materials can effectively enhance charge separation and migration as well as promote O2 activation and mass transfers. As a result, this BCN catalyst therefore displays a remarkable enhancement in photosynthesis of aromatic aldehydes from the alcohols with high conversion and selectivity compared to pristine BCN.
... Especially,t he crystal structure may favor desorption of benzyl aldehydes, thus increasing the selectivity and yield toward benzyla ldehydes. [16] Considering all these beneficialp roperties, it is expected that the photoconversion of benzyl alcohols to benzyla ldehydes can be achieved efficiently by manipulating the crystallinityd egree of g-CN. To the best of our knowledge, the effect of crystallinity of g-CN on photocatalytic alcohol oxidation hasnot been studied thus far. ...
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Exploiting efficient photocatalysts with strengthened structure properties for solar-driven alcohol oxidation is of great significance. Herein, we found that the photocatalytic performance of graphitic carbon nitrides can be sharply promoted by modulating the crystallinity. Results confirmed that the high crystallinity accelerates the separation and transfer of photogenerated charge carriers, thus providing more free-charges for photoredox reactions. More importantly, the crystallinity facilitated the adsorption of benzyl alcohol and desorption of benzyl aldehyde, and simultaneously lowered the energy barrier for O2 activation. As a result, the crystalline carbon nitride exhibited ~ 12-fold promotion with respect to the normal carbon nitride. The remarkable enhancement of activity can be attributed to the synergistic effects of the increased electron/hole separation and boosted surface reaction kinetic. These findings will open up new opportunities in modulating the structure of the polymers for a wide variety of organic transformations.
... Oxygen activation seems to be fast over M1 under the applied reaction conditions as reflected in the low reaction order with respect to O 2 ,b ut more demanding over the vanadia monolayer catalyst and, in particular,o ver P/oCNT ( Table 2). Fast oxygen activation over M1 is in agreement with the experimentalo bservations that molecular O 2 can be activated at low reactiont emperature for oxidationr eactions in the liquid phase, [24] and with the absence of electrophilic oxygen species as intermediates of oxygen reduction under conditions of propane oxidation. [5b, 25] In summary,t he main differences observed in oxidative dehydrogenation over the three different types of catalysts are (1) an order of magnitude higheri ntrinsic activity of M1 compared to 6V/SBA-15 andP /oCNT, (2) acids are formed over M1, but occur only in traces over 6V/SBA-15 and P/oCNT, (3) facilitated oxygen activation over M1 compared to 6V/ SBA-15 and P/oCNT. ...
Article
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.
... Figure 4 shows the layout of the reaction network following from the activation of propane over a bulk catalyst such as M1 or VPP in the presence of oxygen and water. Experiments from our group [96,99,100] and a discussion of literature results supporting that network are collected in a book [101]. ...
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The design of heterogeneous selective oxidation catalysts based upon complex metal oxides is governed at present by a set of empirical rules known as “pillars of oxidation catalysis”. They serve as practical guidelines for catalyst development and guide the reasoning about the catalyst role in the process. These rules are, however, not based upon atomistic concepts and thus preclude their immediate application in for example computer-aided search strategies. The present work extends the ideas of the pillar rules and develops the concept of considering a selective oxidation catalyst as enabler for the execution of a reaction network. The enabling function is controlled by mutual interactions between catalyst and reactants. The electronic structure of the catalyst is defined as a bulk semiconductor with a surface state arising form a terminating over layer being different from the structure of the bulk. These components that can be identified by in situ analytical methods form a chemical system with feedback loops, which is responsible for generating selectivity during execution of the reaction network. This concept is based upon physical observables and could allow for a design strategy based upon a kinetic description that combines the processes between reactants with the processes between catalyst and reactants. Such kinetics is not available at present. Few of the constants required are known but many of them are accessible to experimental determination with in situ techniques.
... These elements were chosen to cover a range of acidities, where potassium represents a typical alkali group metal of nominal acidity and cesium represents a much stronger acid. Tellurium was chosen as the third acid/base component due to its overwhelming precedence in catalyzing EPO to promote greater catalytic activity and the more selective formation of acetic acid [14,23,26,37,44]. It is important to note that, while tellurium can contribute catalytic activity via its redox cycling between the +4 and +6 state, its contribution to the overall reducibility of the catalyst is significantly lower than the chosen redox elements for this study; warranting categorization as an acid/base element in this study and not a redox element. ...
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The catalytic performance of Mo8V2Nb1-based mixed-oxide catalysts for ethane partial oxidation is highly sensitive to the doping of elements with redox and acid functionality. Specifically, control over product distributions to ethylene and acetic acid can be afforded via the specific pairing of redox elements (Pd, Ni, Ti) and acid elements (K, Cs, Te) and the levels at which these elements are doped. The redox element, acid element, redox/acid ratio, and dopant/host ratio were investigated using a three-level, four-factor factorial screening design to establish relationships between catalyst composition, structure, and product distribution for ethane partial oxidation. Results show that the balance between redox and acid functionality and overall dopant level is important for maximizing the formation of each product while maintaining the structural integrity of the host metal oxide. Overall, ethylene yield was maximized for a Mo8V2Nb1Ni0.0025Te0.5 composition, while acetic acid yield was maximized for a Mo8V2Nb1Ti0.005Te1 catalyst.
... 5 In complex reactions, such as selective oxidation of hydrocarbons, multi-functionality is necessarily required and can be achieved either chemically or by nano-structuring. 6,7 Solvothermal techniques have been efficiently used in the preparation of metastable phases or oxides with particular morphological properties. [8][9][10][11][12][13] However, the underlying preparation strategies are oen based on experience and parameter variation. ...
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The mechanism of C-H activation in selective oxidation reactions of short-chain alkane molecules over transition metal oxides is critically affected by the balance of acid-base and redox sites at the surface of the catalyst. Using the example of manganese tungstate we discuss how the relative abundance of these sites can be controlled via synthetic techniques. Phase-pure catalysts composed of the thermodynamic stable monoclinic MnWO4 phase have been prepared by hydrothermal synthesis. Variation of the initial pH value resulted in rod-shaped nano-crystalline MnWO4 catalysts composed of particles with varying aspect ratio. The synthesis products have been analysed by transmission elecron microscopy, X-ray diffraction, infrared, and photoelectron spectroscopy. In-situ Raman spectroscopy was used to investigate the dissolution-re-crystallization processes occuring under hydrothermal conditions. Ethanol oxidation was applied to probe the surface functionalities in terms of acid-base and redox properties. Changes in the aspect ratio of the primary catalyst particles are reflected in the product distribution induced by altering the fraction of acid-base and redox sites exposed at the surface of the catalysts in agreement with the proposed mechanism of particle growth by re-crystallization during ageing under hydrothermal conditions.
... 2,4 One prominent example of a highly active and selective catalyst in ethane oxidative dehydrogenation to ethylene, propane selective oxidation to acrylic acid, and ammoxidation to acrylonitril is the Mo−V− Te−Nb oxide composed of the so-called M1 structure. 5,6 The surface of crystalline mixed Mo−V−(Te−Nb) oxide catalysts under reaction conditions of propane oxidation to acrylic acid is enriched in V 5+ , whereas molybdenum is present in the oxidation state 6+. 7−9 The presence of surface oxide species other than vanadia may, however, affect the reaction pathways of alkanes and reaction intermediates. ...
Article
Full-text available
The cooperation of metal oxide sub-units in complex mixed metal oxide catalysts for selective oxidation of alkanes still needs deeper understanding to allow for a rational tuning of catalyst performance. Herein we analyze the interaction between vanadium and molybdenum oxide species in a monolayer supported on mesoporous silica SBA-15. Catalysts with variable Mo/V ratio between 10 and 1 were studied in the oxidation of propane and characterized by FTIR, Raman, and EPR spectroscopies, temperature programmed reduction, UV/Vis spectroscopy in combination with TD-DFT calculations and time-resolved experiments to analyze the redox properties of the catalysts. Molybdenum oxide (sub-)mono-layers on silica contain mainly di-oxo (Si-O-)2Mo(=O)2 species. Dilution of silica-supported vanadium oxide species by (Si-O-)2Mo(=O)2 prevents the formation of V-O-V bonds, which are abundant in the pure vanadium oxide catalyst that predominantly contains two-dimensional vanadium oxide oligomers. Existing single vanadyl (Si-O-)3V(=O) sites and neighboring (Si-O-)2Mo(=O)2 sites do not strongly interact. The rates of reduction in propane and of oxidation in oxygen are lower for single metal oxide sites compared to those for oligomers. The rate of propane oxidation correlates with the overall redox rates of the catalysts, but not linearly with the chemical composition. Retarded redox behavior facilitates selectivity towards acrolein on single-site catalysts. The abundance of M-O-M bonds is more important in terms of activity and selectivity compared to the nature of the central atom (molybdenum versus vanadium).
... For example, ZnO-ZrO 2 solid solution for the CO 2 hydrogenation to methanol reaction [13], Mn-Ni-Ti-O x solid solution for the selective catalytic reduction of NO x by NH 3 [14] and Mn x Co 3-x O 4 solid solution for the low-temperature oxidation of formaldehyde [15]. Furthermore, a classic successful case is the Mo-V-Nb-Te-O solid solution mixed metal oxide (single phase, so-called M1 phase) for the conversion of propane to acrylonitrile or acrylic acid [16][17][18][19][20]. ...
... The problem that is still faced in the utilization of this catalyst is the control of the complexity of the reaction network consisting of consecutive reactions and parallel reactions [3,[8][9][10]. The phase-in MoVTeNb which is responsible for the production of acrylic acid from direct oxidation of propane is referred to as "M1" phase [2,4,[11][12][13]. Mo and V are very suitable for the oxidation of alkanes via alkenes to carboxylic acids [14,15]. ...
Article
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The effect of the water stream to the propane oxidation on diluted MoVTeNb catalyst has been investigated. The present work has elucidated that careful operation of high throughput instrumentation can be used in various beneficial ways to speed up the discovery process of improved catalysts in other forms than enabling efficient trial-and-error testing of compositional variations of a given catalyst system. The result shows that the addition of massive amounts of water to the feed should have a negative influence on the kinetics, as water will compete with all other polar molecules in the system for adsorption sites. This work also investigated the effect of catalyst leach process toward propane oxidation. From the result, it can be described that catalyst leach process tends to reduce the phase of the catalyst that responds to the total oxidation of propane. This work also proposed the reaction network and gave the comparison between the propane oxidation reaction kinetic using leached and un-leached catalyst. The result showed that the activation energy of the acrylic acid formation on the leached catalyst was slightly higher than that of on un-leached catalyst. On the other hand, the activation energy of the carbon dioxide formation on the leached catalyst was much higher than that of on un-leached catalyst. It can be described that the leaching process to the catalyst can reduce the phase of the catalyst responsible for the total oxidation of propane.
... Certainly, these reactions are important in synthetic organic chemistry and the products found applications in various fields such as pharmaceuticals and fine chemicals (Jude & Paul, 2010;Xiao, Xu, Ma, & Fang, 2012). Benzaldehyde is an important intermediate for the perfumery, pharmaceutical, dyestuff and agrochemicals (Amakawa et al., 2013). Similarly, the piperazine motifs often play a key role in the preparation of biologically active molecules and drugs (Bartoli et al., 2005). ...
... The role of reducible Te-O units adjacent to the active sites of M1 has been subject of vivid debates for these materials. It has been proposed that Te-O increases activity by enhancing alkane adsorption [17], aiding reoxidation of V 4+ spent species [18], or participating in the abstraction of first H from the alkane [19]. In situ studies combined with theory have pointed to electronic effects due to the reduction of Te sites as origin of additional catalytic activity [20]. ...
Article
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The pathways of ethane oxidative dehydrogenation and total combustion have been elucidated for M1 phase type Mo–V oxide catalysts with different metal composition. The ethane oxidation mechanism is not affected by the presence of Te or Nb. Conversely, the selectivity is strongly affected by stoichiometry of M1 catalysts. This is attributed to the facile oxidation of ethene to COx upon formation of unselective VOx species in the absence of Te and Nb.
... of tellurium, a key constituent of the obtained nanocatalysts which translates into enhanced catalytic activity (Amakawa et al. 2013;Csepei and Muhler 2011). Moreover, the presence of small particles with rod-like morphology (see HR-TEM results) that corresponds to increased specific surface area with more active sites might be responsible for rapid (in 10 s) degradation of dye with S1 as compared to S2 and S3 (Singh et al. 2018). ...
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... [2,4,5] The multiple catalytic functions of the M1 phase capable of activating various functional groups (e.g., alkanes, alkenes, alcohols, aldehydes) are considered a key aspect that enables the direct formation of acrylic acid with high selectivity. [6,7] Structural assignment of the catalytic functions is not straightforward, as the catalytic functions of M1 are closely related to the dynamics of the M1 surface changes in response to the reaction atmosphere (e.g., gas composition, temperature). [8][9][10] It is likely that the gradient of the gas composition within the catalyst bed influences the catalytic properties of M1. ...
Article
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... Multicomponents MoVTeNbOx catalysts have been recognized as the most promising catalyst in giving high activity and selectivity in propane partial oxidation to acrylic acid [1]. Typically, MoVTeNbOx catalysts present as main crystalline phases of orthorhombic M1 phase Te 2 M 20 O 57 (M = Mo, V or Nb) [2][3][4][5][6][7][8][9][10][11] and hexagonal M2 phase ...
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A sustainable catalyst for the selective oxidation of benzyl alcohol (BnOH) to benzaldehyde (BzH) was developed by mineralizing Ti(SO4)2 on graphene oxide foam (GOF) surface. The Ti(SO4)2/GOF was characterized by scanning electron microscopy (SEM), energy disperse spectroscopy (EDS), X-ray diffraction (XRD), solid-state NMR (SSNMR), thermal gravimetric analysis (TGA), infrared (IR) and BET analysis. Recycling experiments proved that Ti(SO4)2/GOF possessed excellent reusability and durability. The industrial perspective of Ti(SO4)2/GOF was demonstrated by the preparation of BzH in a large-scale and solvent-free oxidation process.
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The oxidation of propane has been studied over silica-supported vanadium oxide and polycrystalline, bulk MoVTeNb oxide with M1 structure. Temperature-programmed reaction experiments were performed, and the reactivity of propane molecules labeled with deuterium and 13C, respectively, was analyzed under steady-state conditions. The measurement of kinetic isotope effects reveals fundamental differences in the activation of propane over the two catalysts. The reaction network of consecutive and parallel reactions of the formed propylene is comparable. However, oxygen insertion into the CHO group of acrolein under formation of acrylic acid is faster over M1 than oxidation at the CH2 group and decarbonylation to acetaldehyde. In contrast, the latter process is preferred over silica-supported vanadium oxide resulting in lower selectivity to unsaturated oxygenates.
Article
An efficient oxidation catalyst has been synthesized by anchoring oxo-vanadium (IV) onto the amino-functional microporous organic nanotube frameworks (NH2-MONFs). The amino groups are used to immobilize oxo-vanadium (IV) into the porous organic polymer supporter. The resulting oxo-vanadium (IV) complexes have a high surface area, large pore volume, hierarchically porous structure and robust organic network, which have great potential application in heterogeneous catalysis. The introduction of mesoporous tubular channels in the network units improves mass transfer in the catalytic system and the well distribution of catalytic sites in the channels facilitates the accessibility of active sites, which will be beneficial for good catalytic performance in heterogeneous catalysis. The prepared oxo-vanadium heterogeneous catalyst shows high catalytic activity and excellent stability in the selective oxidation reactions of thiols to disulfides.
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Here, we have described the synthesis, characterization and catalytic activity of a dioxo-molybdenum(VI) complex supported on functionalized Merrifield resin (MR-SB-Mo). The functionalization of Merrifield resin (MR) was achieved in two-steps viz. carbonylation (MR-C) and Schiff base formation (MR-SB). The compounds, MR-C, MR-SB and MR-SB-Mo, were characterized at each step of the synthesis by elemental, SEM, EDX, thermal, BET and different spectroscopic analysis. The catalyst, MR-SB-Mo, efficiently and selectively oxidized a wide variety of alcohols to aldehydes or ketones using 30% H2O2 as an oxidant with reasonably good TOF (660 h⁻¹ in case of benzyl alcohol). The catalyst acted heterogeneously under solventless reaction conditions and did not lead to over oxidized products under optimized conditions. The catalyst afforded regeneration and can be reused for at least five reaction cycles without loss of efficiency and product selectivity. A reaction mechanism for the catalytic activity of MR-SB-Mo was proposed and a probable reactive intermediate species isolated.
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Herein we report the first example of the catalytic aerobic partial oxidation of allyl ether to its acrylate ester derivative. Many partial oxidations often need an expensive oxidant such as peroxides or other species to drive such reactions. In addition, selective generation of esters using porous catalysts has been elusive. This reaction is catalyzed by a Li ion promoted mesoporous manganese oxide (meso-Mn2O3) under mild conditions with no precious metals, a reusable heterogeneous catalyst, and easy isolation. This process is very attractive for the oxidation of allyl ethers. We report on the catalytic activity, selectivity, and scope of the reaction. In the best cases presented, almost complete conversion of allyl ether with near complete chemo-selectivity towards acrylate ester derivatives is observed. Based on results from controlled experiments, we propose a possible reaction mechanism for the case in which N-hydroxyphthalimide (NHPI) is used in combination with trichloroacetonitrile (CCl3CN).
Article
In this work, a simple and efficient strategy for fabrication of a novel encapsulated MnO2 nanoparticles inside the spherical mesoporous silica hollow-nanoparticles was described. It was synthesized by consecutively anchoring the MnO2 nanoparticles on the poly(styrene-co-methacrylic acid) particles, coating by mesoporous silica shell, and subsequently removing the polymeric core by resolving in acetone. The catalytic activity of the nanoparticles was examined in the aerobic oxidation of various primary and secondary alcohols that were shown the good activity and selectivity for transformation of primary alcohols to corresponding esters through the oxidative esterification process and secondary alcohols to ketones in short reaction times under mild reaction conditions. In addition, the catalyst system was utilized for oxidation of primary alcohols to aldehydes using tert-butyl hydroperoxide (TBHP) as oxidant under mild conditions and produced the excellent product yield.
Article
The conversion of solar energy to chemical energy has received significant attention in recent years, achieved by photocatalysis comprising homogeneous transition-metalbased systems, organic dyes, or semiconductors. Among those photocatalysts, boron carbon nitride (BCN) materials, as an emerging class of metal-free heterogeneous semiconductors, have extended the field of photocatalysts due to good performance and earth-abundant constitute. The combination of boron (B), carbon (C), and nitrogen (N) constitutes a ternary system with the large extent, large surface area, and abudant activity sites, which together contributed to good performance for reduction reactions, oxidation reactions and orchestrated both reduction and oxidation reactions. This Minireview concludes the methods to the synthesis of nanoscale hexagonal boron carbonitride (h-BCN) and describes the latest advances in applying h-BCN as semiconductor photocatalysts for sustainable photosynthesis, such as water splitting, reduction of CO2, acceptorless dehydrogenation, oxidation of sp3 C-H bonds, and sp2 C-H functionalization. The h-BCN materials would have a potential application in other organic transformations and industrial manufacture in the future.
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A new dioxo‐molybdenum (VI) complex supported on functionalized Merrifield resin (MR‐Mo ) has been synthesized and characterized by elemental, scanning electron mcroscopy, energy‐dispersive X‐ray analysis, TGA, Brunauer–Emmett–Teller method, powder‐X‐ray diffraction, Fourier transform infrared, X‐ray photoelectron spectroscopy and DRS–UV–vis analysis. The virgin Merrifield resin (MR ) was functionalized by carbonylation followed by Schiff base formation with ethanolamine (MR‐SB ). Experimental data showed that the Schiff base coordinated with the MoO2²⁺ moiety via O‐ and N‐atoms. The catalytic activity of MR‐Mo was explored under solventless conditions toward the oxidation of organic sulfides and alcohols using 30% aqueous H2O2 as oxidant. The oxidation reactions were conducted under microwave and conventional methods. The microwave‐assisted oxidation reactions were found to be many times faster than the conventional methods. The oxidation reactions were selective and formed sulfoxides or aldehydes as the sole product with superior TOF values among the molybdenum (VI)‐based complexes. Besides these, the MR‐Mo was purely heterogeneous in nature and can be recycled for at least five reaction cycles without the loss of catalytic efficiency and product selectivity.
Article
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.
Article
Development of efficient catalyst for conversion of benzyl alcohol to benzaldehyde using molecular oxygen is of great importance in virtue of the economic and environmentally friendly superiority. In this study, a cobalt-based zeolitic imidazolate framework, ZIF-67, has been used as precursor to synthesize different catalysts for study in benzyl alcohol oxidation by air. The catalysts were characterized by XRD, SEM, ICP-MS, N2 adsorption-desorption and XPS. The cobalt oxide catalyst, which was obtained via calcination of ZIF-67 in air, showed low activity and selectivity toward benzaldehyde. While the [email protected], prepared from pyrolysis in argon, exhibited good activity and recyclability under mild conditions. The results suggested that the cobalt species, either metallic cobalt or Co²⁺, were the main active sites. In addition, the nitrogen species also played a role for the efficient conversion of benzyl alcohol.
Article
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Selective oxidation of higher alcohols using heterogeneous catalysts is an important reaction in the synthesis of fine chemicals with added value. Though the process for primary alcohol oxidation is industrially established, there is still a lack of fundamental understanding considering the complexity of the catalysts and their dynamics under reaction conditions, especially when higher alcohols and liquid-phase reaction media are involved. Additionally, new materials should be developed offering higher activity, selectivity, and stability. This can be achieved by unraveling the structure‒performance correlations of these catalysts under reaction conditions. In this regard, researchers are encouraged to develop more advanced characterization techniques to address the complex interplay between the solid surface, the dissolved reactants, and the solvent. In this mini-review, we report some of the most important approaches taken in the field and give a perspective on how to tackle the complex challenges for different approaches in alcohol oxidation while providing insight into the remaining challenges.
Article
A series of MoVTeNbOx-mixed metal oxides were synthesized by high-pressure hydrothermal method. The effects of temperature and time during the preparation process on the structure and catalytic performance of MoVTeNbOx were investigated. It was demonstrated that high content of M1 phase (94.5%) MoVTeNbOx with typical needle-shape morphology and high surface abundance of V⁵⁺ (68.9%) could be fabricated by high-pressure hydrothermal method in a short time (30 min to 60 min) without hydrogen peroxide post-treatment. The catalytic results of propane selective oxidation showed that the MoVTeNbOx sample synthesized at 200 °C for 30 min possessed the highest surface V⁵⁺ abundance and largest proportion of terminating (001) plane, displaying the highest selectivity (74.1%) for acrylic acid. Moreover, the prepared MoVTeNbOx showed excellent stability in high temperature continuous reaction.
Article
MoVTeNb oxide catalysts exclusively composed of the M1 phase (ICSD No. 55097) have been studied in the direct oxidation of propane to acrylic acid applying a broad range of reaction conditions with respect to temperature (623–633–643–653–663 K), O2 concentration in the feed (4.5–6.0–9.0–12.0%), steam concentration in the feed (0–10–20–40%), and contact time (0.06–0.12–0.18–0.24–0.36–0.48–0.72–1.44 s gcat Nml−1). The molar fraction of propane was kept at 3.0%. Model experiments were performed to study the reactivity of possible intermediates propene, acrolein, and CO. The impact of auxiliary steam on the chemical nature of the catalyst surface was analyzed by in situ photoelectron spectroscopy, while in situ X-ray diffraction has been carried out to explore the structural stability of the M1 phase under stoichiometric, oxidizing, and reducing reaction conditions. Phase purity apparently accomplishes absolute stability in terms of the crystalline bulk structure and the catalytic performance over month even under extreme reaction conditions. In contrast, the catalyst surface changes dynamically and reversibly when the feed composition is varied, but only in the outermost surface layer in a depth of around one nanometer. The addition of steam causes enrichment in V and Te on the surface at the expense of Mo. Surface vanadium becomes more oxidized in presence of steam. These changes correlate with the abundance of acrylic acid detected in the in situ photoelectron spectroscopy experiment. Analysis of the three-dimensional experimental parameter field measured in fixed bed reactors revealed that the complexity of the reaction network in propane oxidation over MoVTeNb oxide is reduced compared to less-defined catalysts due to phase purity and homogeneity. The oxidative dehydrogenation of propane to propene followed by allylic oxidation of propene comprises the main route to acrylic acid. The oxygen partial pressure was identified as an important process parameter that controls the activity in propane oxidation over phase-pure M1 without negative implications on the selectivity. High O2 concentration in the feed keeps the catalyst in a high oxidation state, which provides an increased number of active sites for propane activation. Auxiliary steam increases activity and selectivity of M1 by changing the chemical nature of the active sites and by facilitating acrylic acid desorption.
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The phase evolution of MoVTeNb mixed oxides derived from a novel acrylamide-gelation approach combined with subsequent washing by hydrogen peroxide and/or annealing in argon was investigated by using powder X-ray diffraction and scanning electron microscopy in conjunction with energy-dispersive X-ray spectroscopy. The calcination of acrylamide-gelation derived carbon-based xerogels in air at 410 °C afforded at unexpected low temperature the formation of a phase mixture containing a significant amount of small MoVTeNb oxide crystallites consisting of the M1 phase (ICSD no. 55097). Hydrogen peroxide treatment drastically changed the phase evolution upon annealing in argon at 600 °C. Whereas further enrichment of the M1 phase took place for the untreated precursor, annealing of the washed material resulted in complete collapse of the M1 phase, yielding a ternary MoVNb mixed oxide with tetragonal Mo5O14-type structure (ICSD no. 27202). The reducing effect of organic residues on phase evolution during calcination of the carbon-inorganic xerogel in air is discussed. The presence of side phases and the deficiency in tellurium upon annealing have a critical impact on the phase composition of the final product. The catalysts exhibit significant activity in partial oxidation of propane to acrylic acid. The analysis of the product distribution suggested that the activation of both propane and propylene intermediates proceed exclusively over the M1 phase, while side phases do not play a significant role in the reaction.
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The structures of M1 and M2 in MoVNbTeO propane ammoxidation catalysts have been solved using a combination of TEM, neutron powder diffraction, and synchrotron X-ray powder diffraction. The unit cell of M1 is Pba2 (No. 32) with a = 21.134(2) Angstrom, b = 26.658(2) Angstrom, c = 4.0146(3) Angstrom and Z = 4. The formula unit is Mo7.8V1.2NbTe0.937O28.9. The unit cell of M2 is Pmm2 (No. 25) with a = 12.6294(6) Angstrom, b = 7.29156(30) Angstrom, c = 4.02010(7) Angstrom and Z = 4. The formula Unit is Mo4.31V1.36Te1.81Nb0.33O19.81. Tellurium sites in hexagonal channels of both phases are displaced toward vanadium-occupied framework sites, whereas Te in the heptagonal channel of M1 is near the channel center. The chemical topology resulting from oxidation states and Madelung site potentials presents active moieties for the ammoxidation of propane in M1 and propene in M2. EPR confirmed the presence of V4+ and possibly Mo5+ in M1 and V4+ in M2.
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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.
Article
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An improved structural model for the M1 phase in the Mo–V–Nb–Te–O propane (amm)oxidation catalyst has been refined after accounting for a molybdenum-substituted-V2O5 impurity and by making adjustments based on aberration-corrected imaging results. The newly refined unit cell has Pba2 symmetry with a=21.134(1) Å, b=26.647(1) Å, c=4.0140(2) Å, and Z=4, in good agreement with our earlier findings (DeSanto et al. Top Catal 23:23 [20], DeSanto et al. Z Kristallogr 219:152 [22]). From the newly refined occupancies, the formula unit is {TeO}0.86(1)·Mo7.48(6)V1.52(6)NbO28. As in the earlier models, V is concentrated in sites that link the pentagonal rings of M1. Careful analysis of bond valences, in combination with the electroneutrality constraint, suggest that the linking sites S3, S4, and S7 all have mixed Mo/V occupancies and valences (d 1/d 0). Furthermore, these sites may contain a mix of Mo5+ and V5+, which is consistent with the proposed catalytic mechanism in which V5+ plays an important role in propane activation. Keywords M1 catalyst–Propane (amm)oxidation–Rietveld refinement–Bond valence sum–Hydrogen abstraction
Article
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In situ X-ray absorption spectroscopy (XAS) and in situ X-ray photoelectron spectroscopy (XPS) have been applied to study the active surface of vanadium phosphorus oxide (VPO) catalysts in the course of the oxidation of n-butane to maleic anhydride (MA). The V L3 near edge X-ray absorption fine structure (NEXAFS) of VPO is related to the details of the bonding between the central vanadium atom and the surrounding oxygen atoms. Reversible changes of the NEXAFS were observed when going from room temperature to the reaction conditions. These changes are interpreted as dynamic rearrangements of the VPO surface, and the structural rearrangements are related to the catalytic activity of the material that was verified by proton transfer reaction mass spectrometry (PTR-MS). The physical origin of the variation of the NEXAFS is discussed and a tentative assignment to specific V-O bonds in the VPO structure is given. In situ XPS investigations were used to elucidate the surface electronic conductivity and to probe the ground state of the NEXAFS spectra.
Book
The first comprehensive survey of the principles and applications of heterogeneous catalysis! With contributions from more than two hundred leading scientists, this book is indispensable for every scientist concerned with heterogeneous catalysis.
Chapter
Reactions Involving Carbon MonoxideReactions with the Participation of HydrocarbonsTransformations of Aldehydes and KetonesTransformations of AlcoholsTransformations of Nitrogen-containing Compounds
Article
Reliable procedures for the controlled synthesis of phase-pure MoVTeNb mixed oxides with M1 structure (ICSD 55097) and tunable crystal dimensions were developed to study the structure sensitivity of the selective oxidation of propane to acrylic acid. A series of powdered M1 catalysts was successfully prepared on a gram scale by using a hydrothermal-based route, purification of biphasic M1-M2 (M2 phase – ICSD 55098) oxide systems, and an innovative approach utilizing a superheated water vapor treatment of calcined precursors. The influence of the preparation technique on the particle morphology and the size is discussed. Detailed experimental studies highlight that the as-derived catalytic materials were indeed phase-pure and compositionally uniform MoVTeNb oxide M1 powders, composed of single-crystalline and structural defect-free crystals grown along the c axis. The morphologically different catalysts were studied in the selective oxidation of propane to acrylic acid, revealing that active sites appear on the entire M1 surface and illustrating the high sensitivity of catalyst performance on the catalyst synthesis method.
Article
In the mix with metals: Multi-component MoVNbTe mixed oxides, in contrast to three-component MoVNb and MoVTe oxides show exclusive efficiency in selective oxidation of ethanol by molecular oxygen to obtain acetic acid.
Article
Bulk mixed oxide catalysts are widely used for various applications (selective oxidation catalysts, electrocatalysts for solid oxide fuel cells, and solid oxide electrolyzers for the production of hydrogen), but fundamental understanding of their structure–performance relationships have lagged in the literature. The absence of suitable surface composition and surface structural characterization techniques and methods to determine the number of catalytic active sites, with the latter needed for determination of specific reaction rates (e.g., turnover frequency (1/s)), have hampered the development of sound fundamental concepts in this area of heterogeneous catalysis. This Perspective reviews the traditional concepts that have been employed to explain catalysis by bulk mixed oxides (molybdates, vanadates, spinels, perovskites, and several other specific mixed oxide systems) and introduces a modern perspective to the fundamental surface structure–activity/selectivity relationships for bulk mixed oxide catalysts. The new insights have recently been made available by advances in surface characterization techniques (low-energy ion scattering, energy-resolved XPS, and CH3OH-IR) that allow for direct analysis of the outermost surface layer of bulk mixed metal oxide catalysts. The new findings sound a note of caution for the accepted hypotheses and concepts, and new catalysis models need to be developed that are based on the actual surface features of bulk mixed oxide catalysts.
Article
Novel crystalline MoVO oxide was employed as the catalyst in the aerobic oxidation of alcohols to the corresponding carbonyl compounds. Reactions were mainly conducted at 353 K in pure oxygen or air (1 atm). The selectivities for benzaldehydes were more than 95% in all cases. The conversions of benzyl alcohols varied from 10% to 99% depending on the substituent. A Hammett plot gave a moderate ρ-value of −0.249 (r2 = 0.98), suggesting that the reaction processes may involve hydride abstraction. The oxidation of primary alkanols afforded aldehydes, and secondary alcohols were mainly dehydrated to olefins. It was found that the conversion of linear alkanols decreased with the length of alkanols. Kinetic analysis showed that catalytic reaction rate was first-order dependent on the concentrations of substrate and of catalyst. The apparent activation energy was estimated to be 45.7 kJ mol−1. Catalytic reactions took place on the 6- or 7-member rings on the a–b basal plane, where highly dense unsaturated metal cation centers and oxygen anion might serve as catalytic active sites.
Article
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.
Article
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 0.325+/Mo 0.686+, 1 V 0.624+/Mo 0.386+, 2Mo 0.56+/Mo 0.55+) 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.
Article
The selective oxidation of propane to acrylic acid is studied over a series of nearly pure M1-phase MoVTeNbOx catalysts. Quantitative analysis of the reaction network shows that the ratio of the rate constants for propane oxidative dehydrogenation to propene and for the further oxidation of propene is constant. The rates towards acrolein and acetone, however, vary subtly with the concentration of vanadium and the location of its substitution. The reaction of acrolein to acetic acid and carbon oxides, associated with accessible metal cations, contributes two-thirds towards the non-selective pathway. The other third is associated with acetone formation. Vanadium is first substituted selectively at sites that are inactive for propane activation. Depending on the selectivity of this substitution two groups of materials have been identified, which show a distinctly different dependence on the concentration of vanadium. Statistic distribution of vanadium in the M1 phase appears to be the most promising strategy to improve the performance of MoVTeNbOx catalysts for a given vanadium concentration.
Article
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.
Article
A series of MoOâ/SiOâ samples prepared by several research groups, employing different silicas and preparation methods, have been studied by in situ Raman spectroscopy, X-ray photo-electron spectroscopy, and methanol oxidation as a probe reaction. The in situ Raman spectroscopy studies show that under dehydrated conditions the molecular structure of the silica-supported molybdenum oxide surface species is independent of the preparation methods used in the present study. The surface molybdenum oxide species is assigned to an isolated, highly distorted octahedral mono-oxo Mo structure. The appearance of new structures in some samples is due to the presence of calcium impurities in the silica supports which result in the formation of calcium molybdate. Neither the preparation method nor the specific silica used affected the methanol oxidation activity of the MoOâ/SiOâ catalysts. The surface molybdenum oxide coverage on silica is the only relevant factor that determines the catalytic properties during methanol oxidation. In situ Raman spectroscopy during methanol oxidation shows aggregation of surface molybdenum oxide species to crystalline beta-MoOâ. The extent of aggregation increases with the surface molybdenum coverage, even at very low coverages, and accounts for the decrease in the methanol oxidation catalytic activity with increasing surface molybdenum coverage. 56 refs., 12 figs., 4 tabs.
Article
The catalytic properties of MoVTeNbO catalysts during the selective oxidation of short chain alkanes and olefins (C2–C4) have been comparatively studied. The main reaction products have been: ethylene from ethane, acrylic acid from propane, maleic anhydride from n-butane and methacrolein from isobutane. FTIR studies of the adsorption of the main reaction products, i.e. olefins and aldehydes, over MoVTeNbO catalyst has been carried out. Accordingly, the reaction pathway is explained on the basis of the characteristics of the alkane fed, the stability and reactivity of both the intermediates and the reaction products, and the nature of the catalytic sites involved in each reaction.
Article
The catalytically active centers of the MoVNbTeOx system for propane ammoxidation to acrylonitrile have been identified. The catalytic system is comprised of three crystalline phases: orthorhombic Mo7.8V1.2NbTe0.94O28.9 (M1) (Pba2: a=21.1337 Å; b=26.6440 Å; c=4.01415 Å; z=4), pseudo-hexagonal Mo4.67V1.33Te1.82O19.82 (M2) (Pmm2: a=12.6294 Å; b=7.29156 Å; c=4.02010 Å; z=4) and a trace of monoclinic TeMo5O16 (P21/C: a=10.0349 Å; b=14.430 Å; c=8.1599 Å; β=90.781°; z=1). The catalytically active and selective centers reside on the surface of the basal plane of the M1 phase and are comprised of an assembly of five metal oxide octahedra (2V0.325+/Mo0.686+, 1V0.624+/Mo0.385+, 2Mo0.56+/Mo0.55+) and two tellurium–oxygen sites (2Te0.944+), which are stabilized and structurally isolated from each other (site isolation) by four Nb5+ centers, each surrounded by five molybdenum–oxygen octahedra. The V5+ surface sites, distinguished through their ( ) resonance structure, are the paraffin activating sites capable of methylene-H abstraction; the Te4+ sites (lone pair of electrons) for the α-H abstraction of the chemisorbed propylene molecule, once formed; and the adjacent Mo6+ sites for the NH insertion into the π-allylic surface intermediate. Herewith, all key catalytic elements needed to transform propane directly to acrylonitrile are contained, strategically arranged and within bonding distance of each other, at the active center of the M1 phase.
Article
Vanadium oxide catalysts supported on activated carbon (V/AC) with V loadings ranging from 1 to 20wt.% were prepared by a wet-impregnation method. Various physicochemical characterization techniques, including nitrogen physisorption, X-ray diffraction (XRD), Raman spectroscopy, X-ray absorption (XANES and EXAFS), X-ray photoelectron spectroscopy (XPS), and electron spin resonance (ESR), were employed to understand the nature of vanadium species on activated carbon. The results revealed that vanadium oxide mainly existed in a highly dispersed state for 10wt.% or less vanadium loadings; a large amount of vanadium resulted in aggregated microcrystalline phase. Vanadium species on activated carbon surface showed a similar local coordination structure to that of NH4VO3 with a distorted tetrahedral symmetry at low vanadium loadings, whereas octahedral coordination was dominant at high vanadium loadings (>10wt.%). All V/AC samples showed V5+ as the major oxidation state. Nevertheless, V4+ centered in a distorted tetrahedral symmetry could be detected at a vanadium loading greater than 4wt.%. The catalytic activity for the benzyl alcohol oxidation largely depended on the dispersion, oxidation state, and local coordination of vanadium oxides on activated carbon. Highly dispersed vanadium (5+) species with a distorted tetrahedral coordination were postulated to account for the excellent catalytic performances of V/AC catalysts (TOF=39.1h−1).
Article
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 (∼5μm), 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 50wt.% M1/50wt.% 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. >250μm) and the inter-particle contact is poor.
Article
The surface of a highly crystalline MoVTeNb oxide catalyst for selective oxidation of propane to acrylic acid composed of the M1 phase has been studied by infrared spectroscopy, microcalorimetry, and in situ photoelectron spectroscopy. The acid–base properties of the catalyst have been probed by NH3 adsorption showing mainly Brønsted acidity that is weak with respect to concentration and strength of sites. Adsorption of propane on the activated catalyst reveals the presence of a high number of energetically homogeneous propane adsorption sites, which is evidenced by constant differential heat of propane adsorption qdiff,initial=57kJmol−1 until the monolayer coverage is reached that corresponds to a surface density of approximately 3 propane molecules per nm2 at 313K. The decrease of the heat to qdiff,initial=40kJmol−1 after catalysis implies that the surface is restructured under reaction conditions. The changes have been analyzed with high-pressure in situ XPS while the catalyst was working applying reaction temperatures between 323 and 693K, different feed compositions containing 0mol.% and 40mol.% steam and prolonged reaction times. The catalytic performance during the XPS experiments measured by mass spectrometry is in good agreement with studies in fixed-bed reactors at atmospheric pressure demonstrating that the XPS results taken under operation show the relevant active surface state. The experiments confirm that the surface composition of the M1 phase differs significantly from the bulk implying that the catalytically active sites are no part of the M1 crystal structure and occur on all terminating planes. Acrylic acid formation correlates with surface depletion in Mo6+ and enrichment in V5+ sites. In the presence of steam in the feed, the active ensemble for acrylic acid formation appears to consist of V5+ oxo-species in close vicinity to Te4+ sites in a Te/V ratio of 1.4. The active sites are formed under propane oxidation conditions and are embedded in a thin layer enriched in V, Te, and Nb on the surface of the structural stable self-supporting M1 phase.
Article
Catalytic selective oxidation of benzyl alcohol with molecular oxygen under mild conditions was carried out over novel crystalline Mo-V-O oxide. The present research is focused on investigation of recycling, reusability and stability of the crystalline oxide in the liquid-phase reaction. The Mo-V-O oxide catalyst was used at least four times with comparable activities to that of fresh catalyst. The separation of the catalyst from reaction medium can stop the conversion of benzyl alcohol, and the addition of the catalyst to the reaction medium can trigger the reaction immediately. The catalytic oxidation of 2,3,6-trimethylphenol as a reference reaction suggested that there were no leached active species in the reaction mixture. The results of the ICP–MS analysis, XRD, and SEM characterization confirmed that the structure and composition of the catalyst were stable. Besides, the Mo-V-O oxide can catalyze the oxidation of a series of alcohols with high selectivities for corresponding carbonyl compounds.
Article
The partial oxidation of propane has been investigated at a multicomponent oxidic catalyst in the temperature range 360
Article
A full frozen phonon multislice simulation of high angle annular dark field scanning transmission electron microscopy (HAADF STEM) images from the M1 phase of the Mo-V-Nb-Te-O propane oxidation catalyst has been performed by using the latest structural model obtained using the Rietveld method. Simulated contrast results are compared with experimental HAADF images. Good agreement is observed at ring sites, however significant thickness dependence is noticed at the linking sites. The remaining discrepancies between the model based on Rietveld refinement and image simulations indicate that the sampling of a small volume element in HAADF STEM and averaging elemental contributions of a disordered site in a crystal slab by using the virtual crystal approximation might be problematic, especially if there is preferential Mo/V ordering near the (001) surface.
Article
Bulk metal oxide catalysts, especially bulk mixed-metal molybdates such as Fe2(MoO4)3, often exhibit high methanol oxidation activity and selectivity. However, the difficulties involved in determining active surface site densities on these catalysts have, in the past, generally prevented side-by-side comparisons of their intrinsic activities, or turn-over frequencies (TOFs). In the present study, high temperature (110 °C) methanol chemisorption and in-situ infrared spectroscopy have been employed to directly and quantitatively determine the number of active metal oxide surface sites available for methanol oxidation. The IR spectra indicate that methanol chemisorption on these catalysts produces both associatively adsorbed, intact Lewis-bound surface methanol species (CH3OHads, species I) on acidic sites, as well as dissociatively adsorbed surface methoxy species (−OCH3, species II) on less acidic or basic sites. In fact, the Lewis acidity of bulk mixed-metal molybdates relative to the methanol probe molecule was found to decrease as follows:  Fe2(MoO4)3, NiMoO4 (species I predominates) > MnMoO4, CoMoO4, ZnMoO4, Al2(MoO4)3 > Ce(MoO4)2 > Bi2Mo3O12 > Zr(MoO4)2 (species II predominates). It also appears that Mo cations are the primary methanol chemisorption sites in many of the bulk mixed-metal molybdates, including commercially important Fe2(MoO4)3. By quantifying the surface concentrations of the adsorbed methoxylated reaction intermediates from the IR spectra, it was then possible to normalize the catalytic methanol oxidation activities for the calculation of TOFs. The methanol oxidation TOFs of bulk molybdates were shown to be relatively similar to those of model supported catalysts with the same co-cation (e.g., MoO3/NiO vs NiMoO4)possibly due to the formation of a “monolayer” of surface molybdenum oxide species on the surfaces of the bulk metal molybdates. In addition, the bulk mixed-metal molybdates were found to exhibit the same ligand effect as that discovered previously in supported metal oxide catalysts, in which the TOF generally decreases with increasing ligand cation electronegativity due to electronic variations in localized M−O−Ligand bonds.
Article
The surface chemistry of anatase-supported vanadium oxide catalysts whose loading corresponds to that needed to complete the geometric monolayer has been studied by FT-IR spectroscopy of adsorbed probe molecules. The suppression of the linear chemisorption of CO indicates that the support surface is no longer exposed on the supported catalysts. Adsorption of ammonia, pyridine, and acetonitrile indicates that very strong Lewis acid sites and medium-strong Brønsted acid sites are present on the vanadium oxide monolayer, identified as coordinatively unsaturated VO2+ ions and VOH groups, respectively. Acidic OH groups are also active in the adsorption of propylene to produce isopropoxy groups and of alcohols to produce alkoxy groups that are further easily oxidized to the corresponding carbonylic compounds. The stability of adsorbed benzaldehyde and the weak adsorption of CO2 seem to indicate that basic and nucleophilic surface anions are very weak.
Article
Effects of framework and near surface composition of quinternary, phase-pure M1 MoVTeNb oxide catalysts on their catalytic performance in selective oxidation of propane to acrylic acid have been studied. The catalysts were prepared by hydrothermal synthesis, spray-drying, and superheated water vapor treatment. Electron microscopy, chemical analysis, nitrogen physisorption, and in situ photoelectron spectroscopy have been used to characterize the materials. The yield of acrylic acid normalized to the specific surface area of the catalyst increases with decreasing percentage of Mo and increasing molar ratio of Te/V at the surface. The metal stoichiometry at the surface differs from the stoichiometry in the crystalline bulk and changes in response to the composition of the gas phase. In situ valence band spectroscopy at 623 K in the presence of all reactants revealed a substantial covalent character of the metal−oxygen bonds in M1. The surface restructuring under formation of V- and Te-containing clusters anchored on crystalline, semiconducting M1 is, therefore, considered to establish structurally and electronically isolated active sites. The mobility of Te especially in the presence of water vapor may contribute to the development of site isolation under reaction conditions and to the enhanced selectivity to acrylic acid in the presence of steam in the feed.
Article
Isolated molybdenum centers bearing either one (oxomolybdenum system) or two (dioxomolybdenum system) terminal oxo ligands are considered, which are modeled by appropriate mononuclear oxomolybdenum methoxides and oxomolybdasilsesquioxanes. Although the oxidation process in both systems is characterized by the same fundamental steps, that is, dissociative addition of methanol followed by rate-determining hydrogen abstraction from the methoxy group, the mechanism of the oxidation reaction differs in each case. In the oxomolybdenum system, the first step leads to cleavage of a bond in a Mo−O−Si sequence and the formation of a surface molybdenum methoxide species. Hydrogen is then abstracted from the methoxide ligand by a terminal oxo ligand in a process entailing a closed-shell transition structure. In contrast, the preferred mechanism in the dioxomolybdenum system involves a hydroxomolybdenum methoxide intermediate formed without cleavage of a bond in a Mo−O−Si sequence. Furthermore, the hydrogen abstraction in the second step is effected by the hydroxide ligand formed in the first step and proceeds via an open-shell singlet transition structure.
Article
We report results of aberration-corrected STEM imaging of the orthorhombic M1 phase in the MoVNbTeO selective oxidation catalyst prepared using two different solution techniques. Atomic coordinates and cation site occupancies for both sample preparations are compared with a previously reported Rietveld-based model for this structure. The high angle annular dark-field (HAADF) images from the two preparations varied significantly only in the occupancy of the heptagonal channels. The M1 sample prepared under ambient conditions exhibited partial occupancy, whereas the hydrothermally prepared sample had heptagonal channels that were primarily vacant. Compared to the Rietveld model, both the atomic positions and the occupancies were found to be consistent, with the exception of the Mo4 and the two Te sites. The Mo4 site shows a lower HAADF intensity than expected, suggesting that appreciable V content may be present, whereas both Te sites had low Te occupancy as compared with the refined model, likely due to e-beam sublimation. Z-contrast imaging may become a valuable tool for rapidly obtaining initial model parameters for the Rietveld refinement of complex structures.
Article
We investigate possible mechanisms of oxidative dehydrogenation of propane using density functional theory. Monomeric vanadium oxide species supported on silica are modeled by vanadyl-substituted silsesquioxane. Similarly to other catalysts with transition metal oxo bonds, the initial C−H bond activation step is hydrogen abstraction by the vanadyl (OVV) group yielding a diradical intermediate in which a propyl radical is bound to a HO−VIV site. This is followed by a propyl rebound mechanism yielding alkoxide or alcohol attached to a VIII(OSi)3 surface site from which propene can be formed. Propene is also directly obtained by a second hydrogen abstraction from the diradical intermediate. Desorption of propyl radicals leads to a stationary concentration of propyl in the gas phase and leaves reduced HO−VIV sites on the surface. Due to fast reoxidation their concentration is much smaller than the concentration of OVV sites. Therefore the rate of propene formation after readsorption on OVV sites is much larger than the rate of isopropyl alcohol (or propene) formation after readsorption on HO−VIV sites. Generation of surface propyl radicals by the first hydrogen abstraction becomes rate limiting. We predict that at 750 K the apparent activation energy is 123 ± 5 kJ/mol and the rate constant is about 0.26 s-1, in close agreement with experiments. The first hydrogen abstraction occurs exclusively on OVV sites, while the second hydrogen abstraction can also occur on V−O−Si bridging oxygen sites.
Article
Five single-phase Mo−V−O-based mixed metal oxides, Mo−V−O, Mo−V−Te−O, Mo−V−Sb−O, Mo−V−Te−Nb−O, and Mo−V−Sb−Nb−O, all of which assume the same orthorhombic structure, were prepared by hydrothermal method, and propane ammoxidation to acrylonitrile (AN) using these mixed oxides as catalysts was performed in order to clarify roles of the constituent elements. All of the catalysts were found to be active for propane ammoxidation. The AN selectivity was increased by approximately a factor of 2 by the introduction of Te or Sb to the Mo−V−O oxide catalyst and further increased by the introduction of Nb to the Mo−V−Te−O and Mo−V−Sb−O catalyst. At the same time, the oxidative decomposition of ammonia to nitrogen was retarded by the introduction of Te, Sb, and Nb. Reaction network analyses for each catalyst revealed that Mo and V in a framework structure are responsible for oxidative activation of propane to propene, which is the rate-determining step, and that Te or Sb clearly promotes the conversion of the formed propene to AN, whereas catalysts without Te or Sb clearly promote the destructive conversion of propene to COx. The data also revealed that Nb suppresses the further reaction of AN to undesired products. Active sites for the selective ammoxidation of propane to AN are discussed on the basis of the catalyst crystal structure.
Chapter
The sections in this article are
Article
Theoretical and experimental work is described which leads to a novel hypothesis for explaining the selectivity in heterogeneous vapor phase hydrocarbon oxidation catalysis. Two essential postulates of the hypothesis are: The oxygen atoms must be distributed on the surface of a selective oxidation catalyst in an arrangement which provides for limitation of the number of active oxygen atoms in various isolated groups. The metal-oxygen bond energy of the active oxygen atoms, at the conditions of reaction, must be in a range where rapid removal (hydrocarbon oxidation) and addition (regeneration by oxygen) is assured. Monte Carlo methods are used to illustrate the distribution of isolated sites on catalytic surfaces under the dynamic conditions of hydrocarbon oxidation reactions. Differences in catalyst and process requirements between oxidation processes operating in an overall oxidizing or reducing atmosphere are discussed.
Article
Catalytic pathways are described for reactions of ethanol to acetaldehyde by oxidative dehydrogenation and of ethanol to diethyl ether by condensation over VOx-Al2O3, MoOx-Al2O3, and WOx-Al2O3. Isotopic labeling shows that acetaldehyde formation occurs via rate-determining C-H bond cleavage of the CH2 group in an adsorbed alkoxide followed by removal of surface oxygen in a Mars and van Krevelen redox mechanism (as confirmed by in situ X-ray absorption, diffuse reflectance infra-red Fourier transform spectroscopy and UV-visible spectroscopy); diethyl ether formation occurs in parallel via coupling and condensation of two adjacent ethoxy species. Using a combination of in situ spectroscopic and kinetic analysis, catalyst properties influencing the formation of acetaldehyde and ether from the common adsorbed ethoxy intermediate are elucidated. X-ray absorption analysis during anaerobic ethanol titration is used to preclude the involvement of terminal M=O bonds during the reaction. A study of the activity of catalysts with the same MoOx domain size on Al2O3, TiO2, and CeO2 supports and binary oxides of MoOx and WOx on Al2O3 are used to prove that the active redox oxygen for acetaldehyde formation is the oxygen atom linking the active metal oxide domain to the support oxide. Ether formation ability of the metal oxide is related to the electronegativity of the active metal atom. (C) 2011 Published by Elsevier Inc.
Article
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.
Article
The role of vanadyl and peroxovanadate oxygen species in the oxidative dehydrogenation of propane (ODP) was analyzed by density functional theory (DFT). The dehydrogenation of C3H8 to C3H6 and the oxidation of C3H6 occurred over vanadyl oxygen of VOx species, yielding reduced VOx species. The vanadyl oxygen species were restored via reoxidation of reduced VOx species with O2 and N2O. The ODP reaction with N2O occurred via a VV(d0)/VIII(d2) redox couple, whereas both VV(d0)/VIV(d1) and VV(d0)/VIII(d2) redox cycles were active with O2. O2 was a more active oxidizing agent than N2O. Peroxovanadates as precursors of vanadyl species were formed on reoxidation of reduced vanadium oxide species with O2, but not with N2O. Peroxovanadates were highly reactive for propene oxidation. The absence of peroxovanadates may explain the superior performance of N2O compared with O2 in the selective ODP reaction over highly dispersed VOx species.
Article
The Mo–V–O crystalline oxide with novel pore structure and consisting uniform six- and seven-member rings on the a–b plane is investigated for the selective oxidation of alcohols of different steric hindrance in liquid phase. The research target is to correlate catalytic activity with the pore structure of the crystal. Specially, substituted pyridines are employed as probe molecules to study poison effect which is closely related to the steric hindrance. As a result, the oxidation of benzyl alcohol, 1-hexanol and cyclohexanol produces aldehydes or ketones as main products. The oxidation of substrates with methyl groups on the carbon next to alcohol group mainly affords dehydrated products as olefins. The catalytic results with adding substituted pyridines in the oxidation of benzyl alcohol, 1-hexanol and 2-hexanol suggest that the active sites are located around the pore area, and are reachable by pyridine, not by substituted pyridines, such as 2-methylpyridine, 2-ethylpyridine and 2, 6-dimethylpyridine. Competitive adsorption on active sites between pyridine and benzyl alcohol remarkably decreases catalytic activity, which 2, 6-dimethylpyridine affects slightly. We have discussed that the adsorption-activation model of substrate is greatly dependent on its steric hindrance.
Article
Based on experimental observations about structure and dynamics of the reactive surface of the M1 phase some considerations are made about the nature and size of active sites. In analyzing the stoichiometry of the reactions following activation of propane it occurs that for dehydrogenation small sites are desirable being just sufficient to re-activate oxygen without kinetic hindrance. For deeper oxidation to acrylic acid (AA) the sites should be larger to accommodate all redox equivalents and oxygen species required for one transformation. A qualitative model for size and composition of the active site is made. It is likely that the active site consists on a VxOy species of strictly 2-D nature. This follows the structural suggestion for the active site on the a–b-plane of the M1 structure. The role of Te in moderating the active site is discussed. The suggestions are discussed in comparing requirements for oxidative dehydrogenation of propane (ODP) with those for AA synthesis. KeywordsSelective oxidation–Propane oxidation–Dehydrogenation of propane–AA synthesis
Article
This paper is a survey of main facts and concepts, presented in the last thirty years, in the field of selective oxidation of hydrocarbons on mixed oxide catalysts. The accent is put on the phenomena, which may occur in the catalysts for different modes of arrangement of the oxide components, and on their bearing on the structure and catalytic performance. The role of different oxygen species and of acid–base properties in determining the activity and selectivity is also described. The dynamics of the catalytic surface under conditions of the oxidation reactions is briefly discussed. Attention is paid to quantum chemical calculations as a tool to determine the reaction mechanism on a molecular sacle.
Article
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.
Article
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.
Article
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 Mo7.8V1.2NbTe0.94O28.9 (M1) and pseudo-hexagonal Mo4.67V1.33Te1.82O19.82 (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.
Article
The role of the (001) crystallographic plane of the M1 phase of MoVTeNb mixed-oxide catalysts in selective oxidation of propane to acrylic acid was addressed by investigating a phase-pure M1 material preferentially exposing this surface. A model catalyst was prepared by complete silylation of M1, followed by breakage of the SiO2-covered needles. Using this approach, the reactivity of the M1 (001) surface was investigated by combining a microreactor study of propane oxidation with high-sensitivity low-energy ion scattering (HS-LEIS). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to study the shape and microstructure of the model system and verify the surface exposure of the model catalyst. The specific rate of formation of acrylic acid on the model catalyst was found to be similar to that on the phase-pure M1 reference material, indicating that the (001) plane of the M1 crystal structure did not have better catalytic properties compared with the lateral surface of M1 needles in propane oxidation.
Article
Technically relevant partial oxidation reactions represent complex reaction networks. Establishing a kinetic model for a system of multiple consecutive and parallel reaction steps is a challenging goal. The synthesis of acrylic acid by oxidation of propane using MoVTeNb mixed oxide as catalyst is such a reaction network. In an on-going study, a 10- fold parallel reactor set-up is used to vary systematically reaction conditions in a broad range over a single, well-defined MoVTeNb oxide. Selectivity and product yield in a multidimensional parameter space can give insight into the reaction network. Apparent activation energies and reaction orders of propane are derived for several conditions. Optimum reaction conditions within the investigated parameter space are specified. The results presented within this contribution contain about 200 data points measured in steady states each corresponding to reaction conditions that differ in temperature, contact time, and propane feed concentration. The fact that this data was collected in less than two months shows clearly the advantage of parallel screening of reaction conditions for mechanistic studies.
Article
CO oxidation, although seemingly a simple chemical reaction, provides us with a panacea that reveals the richness and beauty of heterogeneous catalysis. The Fritz Haber Institute is a place where a multidisciplinary approach to study the course of such a heterogeneous reaction can be generated in house. Research at the institute is primarily curiosity driven, which is reflected in the five sections comprising this Review. We use an approach based on microscopic concepts to study the interaction of simple molecules with well-defined materials, such as clusters in the gas phase or solid surfaces. This approach often asks for the development of new methods, tools, and materials to prove them, and it is exactly this aspect, both, with respect to experiment and theory, that is a trade mark of our institute.
Article
In situ X-ray absorption spectroscopy (XAS) and in situ X-ray photoelectron spectroscopy (XPS) have been applied to study the active surface of vanadium phosphorus oxide (VPO) catalysts in the course of the oxidation of n-butane to maleic anhydride (MA). The V L-3 near edge X-ray absorption fine structure (NEXAFS) of VPO is related to the details of the bonding between the central vanadium atom and the surrounding oxygen atoms. Reversible changes of the NEXAFS were observed when going from room temperature to the reaction conditions. These changes are interpreted as dynamic rearrangements of the VPO surface, and the structural rearrangements are related to the catalytic activity of the material that was verified by proton-transfer reaction mass spectrometry (PTR-MS). The physical origin of the variation of the NEXAFS is discussed and a tentative assignment to specific V-O bonds in the VPO structure is given. In situ XPS investigations were used to elucidate the surface electronic conductivity and to probe the ground state of the NEXAFS spectra.
Article
The outermost surfaces and subsurface layers of the orthorhombic (M1) Mo-V-O catalysts promoted with Te, Nb, and Sb oxide species at submonolayer surface coverage were examined by low-energy ion scattering (LEIS). This study indicated that the Nb oxide species was preferentially located at the topmost surface, while the subsurface Te and Sb concentrations declined gradually into the bulk. Although the original Mo-V-O catalyst was essentially unselective in propane oxidation to acrylic acid, significant improvement in the selectivity to acrylic acid was observed when Te, Nb, and Sb oxides were present as the surface species at submonolayer coverage. These findings further suggested that the formation of the surface V-O-M bonds (M = Nb, Te, or Sb) was highly beneficial for both the activity and selectivity of the orthorhombic Mo-V-O catalysts in propane oxidation to acrylic acid. The highest selectivity was observed when both Nb and Te (or Sb) oxide species were present at the surface. The selectivity trends established for the surface-promoted Mo-V-O catalyst parallel those found previously for the corresponding bulk Mo-V-M-O catalysts. These results further indicated that the introduction of surface metal oxide species is a highly promising method to prepare well-defined model catalysts for studies of the structure-activity/selectivity relationships as well as optimize the catalytic performance of the bulk mixed Mo-V-M-O catalysts for selective (amm)oxidation of propane.
Article
Methanol and allyl alcohol chemisorption and surface reaction in combination with low energy ion scattering (LEIS) were employed to determine the outermost surface compositions and chemical nature of active surface sites present on the orthorhombic (M1) Mo-V-O and Mo-V-Te-Nb-O phases. These orthorhombic phases exhibited vastly different behavior in propane (amm)oxidation reactions and, therefore, represented highly promising model systems for the study of the surface active sites. The LEIS data for the Mo-V-Te-Nb-O catalyst indicated surface depletion for V (-23%) and Mo (-27%), and enrichments for Nb (+55%) and Te (+165%) with respect to its bulk composition. Only minor changes in the topmost surface composition were observed for this catalyst under the conditions of the LEIS experiments at 400 degrees C, which is a typical temperature employed in these propane transformation reactions. These findings strongly suggested that the bulk orthorhombic Mo-V-Te-Nb-O structure may function as a support for the unique active and selective surface monolayer in propane (amm)oxidation. Moreover, direct evidence was obtained that the topmost surface VO(x) sites in the orthorhombic Mo-V-Te-Nb-O catalyst were preferentially covered by chemisorbed allyloxy species, whereas methanol was a significantly less discriminating probe molecule. The surface TeO(x) and NbO(x) sites on the Mo-V-Te-Nb-O catalyst were unable to chemisorb these probe molecules to the same extent as the VO(x) and MoO(x) sites. Our findings suggested that different surface locations for V(5+) ions in the orthorhombic Mo-V-O and Mo-V-Te-Nb-O catalysts may be primarily responsible for vastly different catalytic behavior exhibited by the Mo-V-O and Mo-V-Te-Nb-O phases. Although the proposed isolated V(5+) pentagonal bipyramidal sites in the orthorhombic Mo-V-O phase may be capable of converting propane to propylene with modest selectivity, the selective 8-electron transformation of propane to acrylic acid and acrylonitrile may require the presence of several surface VO(x) redox sites lining the entrances to the hexagonal and heptagonal channels of the orthorhombic Mo-V-Te-Nb-O phase. The study of allyl alcohol oxidation over the Mo-V-O and Mo-V-Te-Nb-O catalysts further suggested that water plays a critical role during the oxidation of acrolein intermediate to acrylic acid over the orthorhombic (M1 phase) Mo-V-Te-Nb-O catalysts. Finally, the present study strongly indicated that chemical probe chemisorption combined with low energy ion scattering (LEIS) is a novel and highly promising surface characterization technique for the investigation of the active surface sites present in the bulk mixed metal oxides.
Article
The CH 3OH temperature programmed surface reaction (TPSR) spectroscopy was employed to determine the chemical nature of active surface sites for bulk mixed metal oxide catalysts. The CH 3OH TPSR experiments provides fundamental surface information about the nature of the active surface sites present on the outermost surface layer of bulk Mo-V-Te-Nb-O mixed metal oxides. The surface of the bulk Mo-V-Te-Nb-O mixed metal oxides revealed that the active surface sites cations may primarily be as promoter ligands. The proposed metal oxides is universal in nature and is applied to other bulk mixed metal oxide systems to determine the chemical nature of the active surface sites.