Aerobic Oxidations of Light Alkanes over Solid Metal Oxide Catalysts

Review pubs.acs.org/CR

Aerobic Oxidations of Light Alkanes over Solid Metal Oxide Catalysts Joseph T. Grant,†,§ Juan M. Venegas,‡,§ William P. McDermott,† and Ive Hermans*,†,‡ †

Department of Chemistry, University of WisconsinMadison, 1101 University Avenue, Madison, Wisconsin 53706, United States Department of Chemical and Biological Engineering, University of WisconsinMadison, 1415 Engineering Dr., Madison, Wisconsin 53706, United States

‡

ABSTRACT: Heterogeneous metal oxide catalysts are widely studied for the aerobic oxidations of C1−C4 alkanes to form olefins and oxygenates. In this review, we outline the properties of supported metal oxides, mixed-metal oxides, and zeolites and detail their most common applications as catalysts for partial oxidations of light alkanes. By doing this we establish similarities between different classes of metal oxides and identify common themes in reaction mechanisms and research strategies for catalyst improvement. For example, almost all partial alkane oxidations, regardless of the metal oxide, follow Mars−van Krevelen reaction kinetics, which utilize lattice oxygen atoms to reoxidize the reduced metal centers while the gaseous O2 reactant replenishes these lattice oxygen vacancies. Many of the most-promising metal oxide catalysts include V5+ surface species as a necessary constituent to convert the alkane. Transformations involving sequential oxidation steps (i.e., propane to acrylic acid) require specific reaction sites for each oxidation step and benefit from site isolation provided by spectator species. These themes, and others, are discussed in the text.

CONTENTS 1. Introduction 1.1. Rules for Inclusion 2. Description of Metal Oxides 2.1. Supported Metal Oxides 2.1.1. Synthesis Methods 2.1.2. Supported Metal Oxide Surface Structures in Oxidative Environments 2.1.3. Influence of Metal Oxide Dispersion on Catalytic Activity 2.1.4. Role of the Oxide Support Structure on Catalytic Activity 2.2. Mixed Metal Oxides 2.3. Zeolites 2.3.1. Shape Selectivity and Confinement Effects 2.3.2. Zeolite Synthesis 3. Common Themes in Reaction Kinetics Using Metal Oxide Catalysts 4. Common Characterization Techniques for Metal Oxide Catalysts 4.1. Infrared Spectroscopy 4.2. Raman Spectroscopy 4.3. Diffuse Reflectance Ultraviolet−Visible (DRUV−vis) Spectroscopy 4.4. X-ray Photoelectron Spectroscopy (XPS) 4.5. X-ray Absorption Spectroscopy (XAS) 4.6. X-ray Diffraction (XRD) 5. Applications of Metal Oxide Catalysts for Aerobic Oxidation of Alkanes 5.1. Methane 5.1.1. Activation of Methane © 2017 American Chemical Society

5.1.2. Oxidative Coupling of Methane 5.1.3. Aerobic Oxidation of Methane to Methanol and Formaldehyde 5.2. Ethane 5.2.1. Oxidative Dehydrogenation of Ethane 5.2.2. Aerobic Oxidation of Ethane to Acetaldehyde and Acetic Acid 5.3. Propane 5.3.1. Oxidative Dehydrogenation of Propane 5.3.2. Aerobic Oxidation of Propane to Acrolein 5.3.3. Aerobic Ammoxidation of Propane to Acrylonitrile 5.3.4. Aerobic Oxidation of Propane to Acrylic Acid 5.4. Butane 5.4.1. Oxidative Dehydrogenation of n-Butane 5.4.2. Oxidative Dehydrogenation of Isobutane 5.4.3. Aerobic Oxidation of n-Butane to Maleic Anhydride 6. Conclusions Author Information Corresponding Author ORCID Author Contributions Notes

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Special Issue: Oxygen Reduction and Activation in Catalysis

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Received: May 1, 2017 Published: November 7, 2017 2769

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Biographies Acknowledgments References

O2 for selective oxidations where it is not currently employed presents a “holy grail” for researchers due to the significant cost savings that may be achieved. Many oxidation processes in the chemical industry employ heterogeneous catalysts. A heterogeneous catalytic system describes processes in which the reactants are in a separate phase from the catalyst. In most cases, the reactants are gases and/or liquids that flow over a solid catalyst material. This phase difference makes them highly suitable for industrial implementation as processes can conveniently operate in continuous flow while eliminating the need to separate the product from the catalyst. Many of the considerations that are made when studying and developing heterogeneous catalysts are similar to those made with homogeneous catalysts. Just as the choice of catalyst precursor, ligand set, and solvent are critical to the performance of a homogeneous catalyst, the catalytic response of a heterogeneous catalyst is sensitive to the metal precursor employed in its synthesis, the local chemical environment of the active metal, and the composition of the reactant feed. The catalysts of choice for a wide variety of aerobic oxidations are metal oxides, which possess many attractive qualities. Metal oxide catalysts are stable enough under oxidative reaction conditions to maintain their bulk structure while exhibiting a reactive surface with unique electronic properties. These surface characteristics stem from the metal coordination environment, its redox properties, and its acidity or basicity.6 Combined, these characteristics lead to the activation of O2 to form surface species capable of performing alkane activation. Many metal oxides also contain functionalities that can perform O- and N-insertion reactions beyond the initial C−H abstraction that lead to the formation of oxygenates and nitriles, respectively. A catalyst system that exemplifies the versatility of metal oxide catalysts is the M1/M2 catalyst which is capable of alkane oxidations to create olefins or carboxylic acids and even alkane ammoxidation to form nitriles.7,8 Furthermore, the catalyst carries out all of these transformations selectively, lending itself as a robust platform for potential industrial implementation. Despite the desirable qualities of metal oxides, few metal oxidecatalyzed aerobic oxidations have been industrially implemented to date; most notably the oxidative transformation of n-butane to maleic anhydride using vanadium pyrophosphate (VPO) catalysts.9 Not only must a metal oxide catalyst be able to activate C−H bonds, but it must do so in a manner that selectively produces the desired product and halts the oxidation before the complete combustion of the alkane to carbon oxides (COx). Poor yield is the main challenge facing potential industrial implementation of many aerobic alkane oxidations,13 as will be highlighted in section 5 of this review. This selectivity issue is tied directly to the types of oxygen species present on the metal oxide surface, which can be nucleophilic, generally associated with selective reaction pathways, or electrophilic, associated with complete combustion of the alkane to COx.14 Therefore, in formulating new catalyst compositions, researchers look to improve selectivity of catalytic active sites while eliminating the sites that are active for alkane combustion or product overoxidation. Despite the relatively high selectivity to COx observed with use of metal oxides, research continues within this field as oxidative processes have considerable cost-savings over nonoxidative processes. For example, the implementation of the oxidative dehydrogenation of ethane to form ethylene could lead to a

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1. INTRODUCTION The production of olefins, oxygenates, and nitriles are key pillars of the chemical industry. Olefins (e.g., ethylene, propylene, and butenes), oxygenates (e.g., methanol, acetaldehyde, acetic acid, acrolein, acrylic acid, and maleic anhydride), and nitriles (e.g., acrylonitrile) are some of the most important feedstocks in the production of polymers and specialty chemicals. To satisfy global demand, the chemical industry reports massive nameplate capacities for these compounds, highlighted in Table 1. Typically, olefins are produced through the steam Table 1. Global Production Capacities of Selected Olefins and Oxygenates chemical 10

methanol formaldehyde11 ethylene12 acetaldehyde11 acetic acid11 propylene12 acrolein11 acrylic acid11 acrylonitrile11b linear butenes isobutene 1,3-butadiene12 maleic anhydride11

global production capacity (metric tons × 106) 100 70 160 2 20 110 5a 6 7 20 22 15 3

a

This includes the acrolein produced but not isolated for the acrylic acid synthesis. Only small amounts of acrolein are actually isolated, for instance, for methionine, glutaric aldehyde, and 1,3-propanediol. b Produced through an ammoxidation.

cracking and fluid catalytic cracking (FCC) of oil fractions, and these olefins are used as feedstocks for oxyfunctionalization processes to produce oxygenates.1 In recent years, the use of natural gas, rather than petroleum, as a source for feedstock chemicals has become advantageous; the proven reserves of natural gas have doubled over the past decade within the United States, thanks in part to the increases in extraction from unconventional sources such as those found in shale deposits.2 Processes exist that convert natural gas to olefins, such as Fischer−Tropsch to olefins (FTO) and methanol to olefins (MTO).3 However, these first require the conversion of light alkanes to syngas (CO and H2) through steam reforming before ultimate transformation to olefins. Therefore, the direct synthesis of olefins and oxygenates through the selective catalytic oxidation of C1−C4 alkanes is an attractive field for industrial and academic researchers alike. When choosing an oxidant for a specific process, cost-effectiveness is the major factor. Safety hazards, poor selectivity, and the need for separations increase both the capital and operation costs of an industrial process. Although oxidants such as H2O2 and N2O may lend themselves to facile and selective oxidations,4,5 they can be expensive relative to the limited value increase of the products. Nonetheless, these costly oxidants are sometimes utilized in industrial applications when inexpensive O2 cannot selectively form a desired product. Thus, the use of 2770

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• Rule (3) excludes metal-nanoparticle catalysts which are deposited atop an oxide surface (e.g., Pd-nanoparticles for the oxidative dehydrogenation of ethane and Ag-nanoparticles for ethylene oxidation to ethylene oxide).18,19 In these examples the catalysis occurs predominantly on the surface of the metal-nanoparticle without engaging an oxide layer at all. These guidelines essentially focus our attention to catalysts commonly described in the literature as “supported metal oxides”, “mixed metal oxides”, and “zeolites”.

potential savings of approximately 35% on energy consumption when compared to conventional ethane steam cracking.15 The objective of this review is to examine the common themes found within the literature regarding the aerobic (amm)oxidation of C1−C4 alkanes catalyzed by metal oxides. This includes information regarding (i) descriptions of the types of metal oxide catalysts, specifically supported metal oxides, mixed metal oxides, and zeolites; (ii) common themes in reaction kinetics when employing metal oxide catalysts; and (iii) spectroscopic techniques used to characterize metal oxides. These items will then be used in the description of some of the most effective and promising catalytic systems investigated for various light alkane transformations to olefins, oxygenates, and nitriles. We note that this review is not meant to be an exhaustive list of all catalysts and reactions found within this literature; rather, we wish to summarize and compare the main challenges for specific applications of aerobic alkane (amm)oxidations and outline the strategies researchers use to overcome these challenges.

2. DESCRIPTION OF METAL OXIDES 2.1. Supported Metal Oxides

Supported metal oxide catalysts are unique from other metal oxide catalysts in that the active metal oxide is completely contained on the surface of an inert (or relatively inert) oxide support structure. These catalysts see advantages with their relatively simple synthesis procedures, reactive metal economy, and high thermal stability. One important disadvantage is that it can be very difficult to understand the composition of the catalyst active site. Multiple types of surface metal oxide species may exist simultaneously on the support with varying degrees of coordination. A common example of a supported metal oxide catalyst, which appears throughout C1−C4 alkane partial oxidation literature, is vanadium oxide supported on oxides such as alumina (VOx/Al2O3) or silica (VOx/SiO2). 2.1.1. Synthesis Methods. Supported vanadium oxide catalysts are prepared by deposition of a metal oxide complex onto an oxide support structure, usually by incipient wetness impregnation (IWI)20−23 but also by chemical grafting and chemical vapor deposition (CVD).24,25 Use of IWI methods requires the precursor to exist in the liquid phase as a solute with this solution dripped onto an oxide support material. The liquid volume added should be equal to the pore volume of the oxide support. The wet powder may then be dried and calcined at elevated temperatures to properly anchor the supported metal oxide to the surface by forming support−oxygen−metal groups (S-O-M).24 Grafting and chemical vapor deposition require the metal precursor to be in gaseous form, flowing the metal precursor over the solid oxide support material at elevated temperatures to form the desired metal oxide surface sites.24,25 These grafting synthetic methods are commonly described as “precise” techniques, considering that the types of metal oxide anchoring sites (e.g., Si−OH) and quantities of these sites can be controlled in the absence of solvent. Additionally, these anchor sites, and metal oxide surface species itself, can be monitored during synthesis using proper surface characterization methods (e.g., infrared spectroscopy). 2.1.2. Supported Metal Oxide Surface Structures in Oxidative Environments. When describing surface structures of supported metal oxides (such as supported vanadia), emphasis is traditionally placed on the existence of three-dimensional V2O5 nanoparticles or the degree of two-dimensional (VOx) dispersion made up of site-isolated monomeric species or polymeric-like metal oxide species. Conceptual drawings of these different surface structures are included in Figure 1.21 2D metal oxide surface species must have at least one bridging oxygen atom (S-O-M) between the support oxide structure and every individual metal atom. Metal oxide surface species that satisfy this requirement and include only the S-O-M sites or terminal oxo groups are identified as “monomeric” or “isolated” metal oxide species. 2D species that additionally include one or

1.1. Rules for Inclusion

Reflecting the importance of small building-block molecules, there exists a great variety of heterogeneous catalysts studied in the literature which oxidize C1−C4 alkanes. Similarly, for many of these small molecules, there are multiple reaction strategies to synthesize each important downstream chemical. For example, acrylic acid can be synthesized by oxidation of acrolein, but it can also be synthesized in one step by the oxidation of propylene or even propane. We aim to focus this review by only including extensive discussion of catalysts and processes that abide by the following rules: The catalyst must: 1. partially (amm)oxidize a C1−C4 alkane into an important building block molecule, i.e., olefin, oxygenate (alcohol, aldehyde, and carboxylic acid), or nitrile. 2. utilize molecular oxygen as the oxidant to drive the desired chemical transformation. 3. include one or more oxygen atoms as part of the catalyst’s active site. 4. be part of a different phase than the reactant/product stream (heterogeneous catalyst). These criteria reflect our understanding of true aerobic oxidations of C1−C4 alkanes using heterogeneous metal oxide catalysts. By complying with these rules, we can more easily examine important themes in metal oxide catalysis by excluding discussion of catalysts and chemical processes which detract from our message. Some notable exclusions are given below: • Rule (1) focuses attention on partial oxidations of only alkanes as a starting material. Many researchers alternatively offer applications of olefin oxidation to oxygenates, e.g., propylene oxidation to acrolein or acrylic acid. • Rule (1) also limits discussion to partial oxidation reactions and omits any examination of complete oxidation applications. Total oxidation catalysts are important for environmental chemistry applications in minimizing atmospheric pollutants.16,17 • Rule (2) excludes any catalysts that require common oxidants which are not O2 (i.e., H2O2, CO2, NOx, etc.) or which require an oxidant to “regenerate” the catalyst when the desired chemical transformation has terminated (i.e., reactor downtime). 2771

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clusters of 3D V2O5 nanoparticles on a surface can lower reaction rates when normalized by metal content, due to bulk metal oxide sites within a nanoparticle never being exposed to a reacting molecule. Moreover, supported 3D V2O5 nanoparticles are sometimes shown to accelerate rates of undesired reactions, lowering selectivity to the desired products.20,21 In this regard, it is often desired to synthesize solely 2D supported VOx surface structures and to avoid the formation of 3D metal oxide nanoparticles. 2.1.4. Role of the Oxide Support Structure on Catalytic Activity. The identity of the oxide support structure chosen to immobilize a reactive metal oxide active site has a great influence on the observed catalytic activity. For example, VOx/SiO2 and VOx/Al2O3 are much more selective to propylene than VOx/TiO2 for the oxidative dehydrogenation of propane reaction, yet VOx/TiO2 is much more reactive than VOx/SiO2 and VOx/Al2O3. Though these support effects are consistently made and great emphasis is placed on this research topic, debate remains regarding the source of this oxide support influence. Some believe the aforementioned effect indicates that the oxygen atoms bridging the metal and the support (S-O-M) plays the most substantial role in the catalytic reaction. Others believe the main role of the support oxide is to provide an electron source/sink for the redox chemistry taking place at the supported metal oxide sites atop the surface. Specific supported metal oxide catalysts and their applications will be used to illustrate these points in sections 5.2.1, 5.3.1, 5.4.1, and 5.4.2.

Figure 1. Two- (monomeric and oligomeric/polymeric) and threedimensional group V supported metal oxide structures. S = support atom (Si, Al, Ti, etc.), and M = supported metal (V, Nb, and Ta). Reprinted with permission from ref 21. Copyright 2015 American Chemical Society.

more bridging oxygen atoms between metal centers (M-O-M) typically occur when metal oxide species are forced into close proximity and are identified as “polymeric” or “oligomeric” metal oxide species. Typically only early transition metal oxides (i.e., VOx, TiOx, NbOx, etc.) can be effectively dispersed as 2D species. Meanwhile, 3D metal oxide nanoparticles contain at least some metal atoms that are not bound to bridging oxygen atoms to the support structure. The maximum allowable 2D metal oxide loading is frequently referred to as the “monolayer coverage”. At monolayer coverage, all anchoring sites to the support oxide are occupied, and adding any additional metal oxide will form 3D nanoparticles. When exposed to oxidative atmospheres, the metal atoms of supported metal oxide active sites typically exist in their most oxidized state. For example, vanadium atoms of supported VOx surface structures remain V5+ when exposed to oxidative environments. Because of this, monomeric 2D supported metal oxide surface structures are heavily dependent on the mostoxidized state of the metal. For example, the monomeric species of supported Group V metal oxides (V-, Nb-, and Ta-oxide) arrange as tetrahedral tripodal mon-oxo sites (i.e., OV(−O−S)3, S = support atom).26 Supported Group VI metal oxides, meanwhile, arrange primarily as dipodal dioxo sites (i.e., (O)2Cr(−O−S)2.27 Three-dimensional supported metal oxides, however, take on the structure of their bulk oxide (i.e., 3D vanadium oxide supported on SiO2 has the distorted trigonal bipyramidal geometry of V2O5). STM images of a VOx/CeO2(111) surface with various V-loadings is included in Figure 2, displaying isolated tetrahedral VOx at moderate V-loadings and the formation of V2O5 with increased loadings. 2.1.3. Influence of Metal Oxide Dispersion on Catalytic Activity. A common topic of research in supported vanadium oxide catalysis is to identify structure−reactivity relationships between metal oxide surface structures and the response to the catalytic activity of these sites. Indeed, the presence of large

2.2. Mixed Metal Oxides

Mixed metal oxide (MMO) catalysts contain metal oxide clusters scattered throughout the bulk oxide material resulting from coprecipitation synthesis methods, giving MMOs well-ordered crystalline phases. Though it is true that the inclusion of metal oxide clusters within the bulk (where they will never directly interact with an alkane) diminishes reactive atom efficiencies of MMOs, the reactive metals in MMOs are almost always abundant early transition metals and do not warrant much concern for metal conservation. Some of the most industrially attractive and well-studied metal oxide catalysts for C1−C4 alkane oxidations are MMOs, including the vanadium pyrophosphate (VPO) catalyst and the M1/M2 phase catalyst, which are highlighted in this section. Since its discovery as an active oxidation catalyst in 1966,28 vanadium pyrophosphate (VPO) has been industrially implemented for light alkane oxidation using O2. VPO is the main catalyst for the production of maleic anhydride via oxidation of n-butane. Further details on this reaction are discussed in section 5.4.3. Despite its economic success and decades of scientific research, there is still debate on the nature of the catalytic active site and overall catalyst structure.29−31 One point where the scientific community has come to an agreement is the in situ restructuring of the catalyst during reaction.32−34 This restructuring leads to complications in catalyst characterization, as synthesis technique and reaction conditions may alter the catalyst surface. A current point of debate is the role of various crystalline phases on the oxidation process. The observation of the pyrophosphate phase, (VO)2P2O7, as the main constituent of the bulk catalyst has led to the suggestion that it is this phase which leads to selective activation of n-butane.30,35 On the other hand, surface-sensitive characterization via HRTEM and in situ XPS suggest that an amorphous layer on the catalyst

Figure 2. STM images of VOx/CeO2(111) for various V-loadings (indicated within the figure). Increased V-loading leads isolated VOx sites (a and b) to agglomerate to form V2O5 nanoparticles (c). Adapted with permission from ref 26. Copyright 2009 John Wiley and Sons. 2772

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catalyst made up of only M2 phase shows no direct conversion of alkane, yet high conversion of olefin to form carboxylic acids or nitriles.8,45 It is believed that the activation of alkane in the M1 phase is due to both its inclusion of V5+ and a heptagonal ring structure (orthorhombic M1 is a mixed phase of pentagonal, hexagonal and heptagonal ring structures).46 Further discussion over this phase cooperation is included in specific applications located in sections 5.3.3 and 5.3.4. Some advancements to the M1/M2 catalyst have come in the last two decades. One to note, especially, is the method of synthesis. The initial M1/M2 catalyst was prepared with a slurry method, where stoichiometric amounts of Mo, V, Nb, and Te salts were mixed and evaporated before finally treating at high temperatures (∼600 °C). While this is a simple synthesis method and large quantities could be produced, reproducibility was poor48 and inspired the development of morereproducible synthesis procedures. Such an advance came in the form of hydrothermal synthesis,49,50 which is now a common synthesis method for MMO catalysts. The hydrothermal synthesis method involves the mixing of aqueous salt solutions of Mo, V, etc. inside an autoclave and heated to temperatures of ∼175 °C for ∼48 h before rinsing the formed solid product with water and drying in air. Much research also has been devoted to the discovery of the role of each of the elements in the M1/M2 catalyst and the optimization of their included ratios. With careful experimentation and reaction modeling, it is suggested that the V5+ site is the alkane adsorption site and responsible for alkane conversion to the olefin, the Te4+ site abstracts an H atom of the olefin, and the Mo6+ site might be responsible for N or O insertion.51 The Nb atoms are suggested to isolate Mo atoms from one another by providing spatial separation.52 There are also some indications that Nb atoms may occasionally arrange in the structural position normally occupied by V, resulting in increased selectivity to carboxylic acids due to suppression of overoxidation.46 Applications of the M1/M2 catalyst are discussed in sections 5.2.1, 5.3.3, and 5.3.4.

surface is responsible for catalytic activity via formation of isolated VOx sites segregated from the bulk VPO material.31,34,36,37 The VPO catalyst is synthesized via reaction of V2O5 and H3PO4 in either aqueous or organic media. This reaction leads to the formation of VOHPO4·0.5H2O, which is then heated to eliminate water, either in situ in the butane oxidation reactor or in a dedicated synthesis reactor.38 The now-dehydrated (VO)2P2O7 can be activated in an air/n-butane mix at reaction temperatures. This activation process may take up to 1000 h on-stream to generate an “equilibrated” catalyst for industrial operation. Addition of metal dopants during synthesis leads to surface modifications that alter VPO’s reactivity. Co, Fe, Ce, Ga, and Nb have all been reported to alter the reactivity of VPO.9 These promoters may act as defect sites that enhance the in situ formation of the VPO active site. Under working conditions, the roles of V, P, and O on catalyst activity and selectivity are still under investigation. As mentioned previously, there is evidence to suggest that, on the surface of the working catalyst, isolated VOx species are responsible for catalytic activity. The role of phosphate is primarily that of an “insulator” to structurally isolate active sites and prevent the formation of larger VOx motifs.34,39 Increasing conversions, with ensuing formation of water, lead to hydrolysis of surface VOPO4 species to form H3PO4 and VOx.39 The mobility of phosphate species under reaction conditions has been supported by in situ XPS investigations showing significant changes in surface P concentrations.31,40 Other studies found that dopants such as Nb led to weaker binding of maleic anhydride on the surface, decreasing the rate of overoxidation.9,37 More detailed discussion on the VPO catalyst will be presented in section 5.4.3 of this review. The Mo−V−Te−Nb-O catalyst, also referred commonly as the “M1” or “M1/M2” catalyst, was discovered in 1994 by Mitsubishi Chemicals who filed patents claiming the material’s ability for alkane ammoxidations to nitriles41 and alkane oxidations to carboxylic acids.42 Since then it has been commercialized by PTT Asahi Chemical Company Limited for direct aerobic ammoxidation of propane to produce acrylonitrile.43 Both phases of the catalyst, that is, the orthorhombic M1 phase (displayed in Figure 3) and the pseudohexagonal M2 phase, contribute to different roles of the overall observed catalysis. The M1 phase is responsible for alkane conversion to the olefin, while the M2 phase is responsible for subsequent olefin (amm)oxidation to the final product. Indeed, a catalyst containing only the M1 phase shows quite high reactivity of alkane conversion to the olefin without much formation of aldehyde, carboxylic acid, or nitrile.44 As well, a Mo−V−Te−Nb-O

2.3. Zeolites

Zeolites, also known as “molecular sieves”, are crystalline microporous materials primarily comprised of tetrahedrally connected SiO4− and AlO4− structural units. These materials form intricate structures of channels and cages with a wide range of geometrical dimensions. Though known to occur in nature, the importance of zeolites in chemical applications became evident with the development of synthetic routes for their controlled production. In the realm of catalysis, the industrial application of zeolites for fluid catalytic cracking of heavy petroleum distillates fundamentally transformed the petrochemical industry. The unique chemical reactivity of zeolites originates in their tunable structures. By 2017, the International Zeolite Association has recognized 232 unique zeolite structures.53 Figure 4 shows structures of a narrow selection of zeolites and their most relevant pore dimensions. Unlike other types of metal oxides, the dimensions shown in Figure 4 are similar to the kinetic diameters of organic molecules (Table 2). The consequences of this feature on catalysis will be discussed in subsequent sections. In addition to geometrical considerations, zeolites can also be designed to contain metal cations within their cage structures. The difference in charge on Si and Al within the framework leads to a net negative charge that must be compensated.

Figure 3. Active center if the M1 phase (Mo7.5V1.5NbTeO29) in the [001] projection (left). ChemDraw depiction of this site (right). Adapted with permission from ref 47. Copyright 2003 Springer. 2773

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2. Reaction product shape selectivity: Upon formation of reaction products within the zeolite structure, formed species with dimensions larger than those of channels and pores leading out of the zeolite framework will diffuse at slower rates than smaller reaction products. 3. Transition state shape selectivity: Upon reactant adsorption, reaction pathways that show transition states incompatible with framework geometry will be hindered or even fully eliminated. As further details on confinement effects and shape selectivity lie outside the scope of this review, we refer the reader to refs 55 and 59−61 for additional details. 2.3.2. Zeolite Synthesis. Though found in nature, zeolites are typically produced synthetically in order to accurately control their structure and properties. Every zeolite framework is synthesized with different reactants and conditions; however, most synthetic approaches share some common features. The traditional hydrothermal synthesis of zeolites typically involves the formation of a gel containing Si and Al species, templating agents, and mineralizers. Hydrothermal treatment of the gel under moderate heat leads to nucleation of zeolite crystals. A key feature is the use of structure-directing agents that act as templates around which the zeolite crystal grows. By modifying the geometry, rigidity, and hydrophobicity of the template, different zeolite frameworks can be obtained.62 For a more detailed overview of structure-directing agents, we direct the reader to refs 58 and 59. As discussed above, ion exchange can be used to substitute charge-balancing cations within the zeolite framework with metal cations that possess desired catalytic activities. Due to the negative charge of the trivalent Al tetrahedra in the framework, the ion exchange capacity of the zeolite increases at low Si/Al ratios.57 Thus, a balance must exist between the desired degree of ion exchange and the desired Brønsted acidity, which is lost at low Si/Al ratios. Zeolite synthesis is undoubtedly a broad topic beyond the scope of this review, thus we refer the reader to refs 58, 62, and 63 for further details and to ref 57 for details on metal cation exchange.

Figure 4. Selection of zeolite framework structures and relevant pore and cage dimensions. Arrows indicate pore geometries and dimensions formed by each framework. Adapted with permission from ref 54. Copyright 2000, Elsevier.

Table 2. Kinetic Diameter of Common Species in Alkane Oxidationsa molecule

kinetic diameter [Å]b

O2 H2O CH4 C3H8 n-C4H10 i-C4H10

3.46 2.65 3.8 4.3 4.3 5

b Approximated by Lennard-Jones potential collision diameters, σ, in Φ(r) = 4ε[(σ/r)6 − (σ/r)12]. See ref 55. aAdapted from ref 55.

Protons, ammonium ions, and metal cations can all balance the framework charge. This interaction allows various ions to be exchanged into the framework cages and channels in order to tailor a zeolite material for a desired chemistry. An alternative way to modify zeolites with metal cations is the substitution of framework Al with other metals.54,56,57 An important advantage of framework substitution is a decrease in metal leaching rates when compared to simple ion-exchanged cations.58 Furthermore, the catalytic properties of the framework can be modified by varying the Si/Al ratio. As discussed previously, the charge difference between the tetravalent Si and the trivalent Al lead to a net negative charge on the zeolite framework. The compensating charge, in many cases a proton, will interact with different strengths based on the Si/Al ratio, leading to varying Brønsted acid strength and density.58 Thus, the combination of acidity, metal content, and framework structure allow for flexible catalyst design. 2.3.1. Shape Selectivity and Confinement Effects. The zeolite framework channels and pores are critical to catalytic activity. The microporous structure of zeolites can lead to transport limitations of reactant molecules within pores and channels, as well as the control over entire reaction pathways. The consequence of these factors to chemical reactivity is termed “shape selectivity”. There are three main types of shape selectivity:54 1. Reactant shape selectivity: Given a particular zeolite framework and multiple reactant species, those reactants that have kinetic diameters close (or smaller) to the dimensions found in the zeolite pores and channels will diffuse and react preferentially within the framework.

3. COMMON THEMES IN REACTION KINETICS USING METAL OXIDE CATALYSTS Aerobic oxidation of an alkane requires the cofeed of alkane and molecular oxygen to produce (ideally) only the desired oxidized product (olefin, alcohol, aldehyde, and carboxylic acid) and water as a necessary byproduct (Scheme 1). Aerobic Scheme 1. Representative Aerobic Oxidation of Alkane Reaction, in This Case Forming the Desired Olefin and Water as a Necessary Byproducta

a

In all examples of partial oxidations, the desired reaction pathway competes with undesired overoxidation and alkane combustion to COx.

ammoxidation of an alkane to produce a nitrile can also occur by cofeeding alkane, oxygen, and ammonia while still producing water as a byproduct. The ability of the oxide catalysts to activate dioxygen, as well as the high thermal stability under oxidative conditions, makes metal oxides effective catalysts for 2774

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cleavage to form an olefin or an oxygen transfer to form an oxygenate. Alternatively the radical itself may react with gaseous O2 which largely contributes to the formation of undesired COx. It is common to find that the kinetics of aerobic oxidations of alkanes are described by the Mars−van Krevelen (MvK) mechanism when using metal oxide catalysts.70−72 It is important to note that interpretations of this mechanism have evolved since its conception as an approach to explain the kinetics of SO2 oxidations. In its current interpretation, the mechanism assumes that the reduced metal centers formed after alkane conversion are reoxidized by oxygen atoms supplied by the bulk oxide itself (i.e., “lattice” oxygen atoms). It is therefore the role of the gaseous molecular oxygen to reoxidize the support oxide by replenishing the used lattice oxygen atoms of the bulk material at the oxygen vacancies.70,72 The MvK mechanism features a first-order dependence in alkane concentration and zero-order dependence with respect to oxygen concentration. As will be discussed in the ensuing paragraph, the low reaction order in oxygen does not discount its significance; indeed, the electronic properties of the surface oxygen species are critical to the reactivity of the metal oxide. Despite its broad use to describe alkane oxidation, the MvK mechanism omits the contribution of other types of oxygen species on catalytic activity. In this way, the MvK serves as a working hypothesis for the study of partial oxidations of alkanes. A depiction of the MvK mechanism is shown in Figure 5.73

these transformations. Still, common challenges endure that overlap all types of metal oxide catalysts and light alkane aerobic oxidations. The common challenge between all applications of aerobic oxidations of light alkanes is to halt the oxidation at a desired product rather than oxidize into COx. This is especially challenging considering that the desired partially oxidized product is often more-reactive than the parent alkane substrate due to the reactive functional groups contained in these olefin, alcohol, aldehyde, and carboxylic acid products.64 An ideal catalyst, then, selectively targets cleavage of the alkane’s C−H bonds while ignoring the functional groups of the partial oxidation products. We must note that the kinetic triangle displayed in Scheme 1 greatly simplifies the actual complex reaction network, yet it is included here (and in many previous works) to represent the overarching dehydrogenation and overoxidation reaction pathways in these partial oxidations. Many aerobic oxidations of light alkanes follow the same initial mechanistic steps along their route to the desired product. Each of the light alkanes in this review (methane, ethane, propane, n-butane, and isobutane) must be initially activated by the cleavage of a C−H bond. This is not an easy endeavor: the C−H bond strength varies between 400 and 440 kJ/mol, with the following order of increasing strength: tertiary, secondary, primary, and methyl carbon atoms.65 It is intuitive to assume that the weakest C−H bond of the alkane will react before the others,66 and it is frequently observed that this initial C−H cleavage is the rate-determining step for the overall alkane transformation.13,67 With metal oxide catalysts, the alkane is seldom proposed to coordinate directly to an exposed metal atom itself but rather at oxygen atoms bonded to the reactive metal. The initial C−H cleavage of the alkane is almost always considered to proceed via H atom transfer, with an electron from a terminal MO site donating into a metal’s d orbital and the H atom from the alkane accepted by an oxygen atom bound to the metal.68 A visual representation of this H atom transfer is included in Scheme 2. Thus, after an initial C−H cleavage of the alkane, it Scheme 2. Hydrogen Atom Transfer from an Alkane to the Active Site of a Metal Oxide Catalysta

The H-atom transfer forms a hydroxyl site and alkyl radical while an electron adds to a vacant d orbital to create a reduced metal center.

Figure 5. (Top left to top right) 1-Butene oxidized to butadiene using a reducible metal oxide of face-centered cubic arrangement of oxygen atoms (open circles); (top right to bottom) filling of the oxygen vacancy (open square) by lattice oxygen migration; (bottom to top left) gas phase oxygen filling the lattice oxygen vacancy. Note that metal centers are not shown. Reprinted with permission from ref 73. Copyright 2002 Springer.

is believed that the catalyst’s active site is a reduced metal atom with a bound hydroxyl species. In many aerobic oxidation applications, the reactive metal atom is fully oxidized, and thus the energy required to donate an electron into the metal d orbital is identified as the ligand-to-metal-charge-transfer (LMCT) band.69 The mechanisms of further alkane transformation after this initial C−H cleavage are less uniform and vary depending on the type of partial alkane oxidation and the type of metal oxide catalyst employed. It is true that the homolytic C−H cleavage also leaves behind a highly reactive carbon-centered radical. Great speculation can be made about the fate of this radical: obviously some fraction of the radical species continues to react as desired to form product, through either a second C−H

Considering that the catalysis occurs at oxygen atoms bound to the metal center, it is essential to examine the nature of these oxygen species and attempt to correlate reactivity to the properties of the bound oxygen atoms. Surface oxygen atoms of metal oxide catalysts are commonly described as being either “electrophilic” or “nucleophilic” oxygen species, differentiated by their electron affinity and consequently their M−O bond strength. Electrophilic oxygen species are described as adsorbed peroxo groups and can bond to a lone metal atom as a peroxo (M>O2), a superoxo (M-O2−), or as a peroxo bridging two metal centers (M−O−O−M). Alternatively, nucleophilic oxygen species can bond to a lone metal atom as an oxo group (MO) or a bridging oxygen atom between metal centers (M−O−M).74,75 Representative drawings of these oxygen species are shown

a

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Table 3 presents a brief overview of the techniques and typical applications for the study of heterogeneous catalysts. As an exhaustive discussion of analytical techniques is beyond the scope of this review, we will also refer the reader to appropriate references.

in Figure 6. The preferred oxygen species varies by the application. For example, the more reactive electrophilic oxygen

4.1. Infrared Spectroscopy

Functional groups in organic molecules, such as C−H, CO, and R−OH, readily undergo a change in dipole moment during bond vibration. This feature makes IR spectroscopy a valuable tool in probing interactions between organic functionalities and the surface of the metal oxide catalyst. A classical example of this interaction is the use of pyridine as a probe molecule to investigate the acid properties of a metal oxide surface. This basic molecule forms hydrogen bonds with weak Brønsted acid sites, forms a coordination complex with Lewis acid sites, and is protonated into a pyridinium ion by strong Brønsted acid sites.80 These acid−base interactions lead to changes in the vibrational modes of pyridine that are used to identify surface acidity in a metal oxide material. Figure 7 shows the shift of vibrational mode frequency in liquid phase pyridine upon interaction with acid functionalities.80 Beyond determining the presence of acid sites on a metal oxide surface, controlled chemisorption of pyridine onto the catalyst surface is useful for quantification of acid sites. This quantification follows the Beer−Lambert law (eq 1):81

Figure 6. Representative drawings of electrophilic and nucleophilic oxygen species of metal oxide catalysts.

species are necessary to activate the strong C−H bonds of CH4 (see section 5.1), while they predominantly lead to total oxidation of other alkanes.76 Selective dehydrogenations and oxygen insertions of C2−C4 alkanes (sections 5.2−5.4) require surface nucleophilic oxygen species to prevent the overoxidation of desired species. Additionally, more than one of these oxygen species may be present on the catalyst surface during catalysis. This is due to the required reoxidation of lattice oxygen species from gas phase O2. Attention is sometimes given to the acid/base characteristics of a catalyst surface rather than focusing solely on its redox properties. The common definitions of Lewis acids and bases, that is, chemical species that accept or donate electron pairs, respectively, applies to the surfaces of metal oxides as well. The metal cation center is treated as a Lewis acid site while lattice oxygen atoms are considered to be Lewis bases. More acidic support oxides see increases to the metal−oxygen bond strength, which can have profound impacts on the observed reactivity of the metal oxide catalyst due to the lability of oxygen atoms.64 Additionally, in some cases the surface Lewis acid or base sites are believed to aid or hinder the desorption of reaction products. In the oxidative dehydrogenation (ODH) of ethane, for example, it is suggested that close proximity between redox sites and acidic M+ sites improves selectivity to the olefin due to fast desorption of the olefin from the nearby M+ site.77 Interestingly this effect might be specific to the length of the alkane, with acidic sites favoring ODH of short alkanes (i.e., ethane) and basic sites favoring ODH of longer alkanes (i.e., n-butane).78

A=ε

n s

(1)

where A is the integrated absorbance, ε is the molar extinction coefficient, n is the moles of probe molecule dosed, and S is the surface area of the catalyst pellet sample. By plotting absorbance as a function of the amount of dosed probe molecule, the extinction coefficient can be determined for a particular catalyst system. Upon saturation of the surface acid sites, the absorbance will become constant. The moles of probe molecule dosed at saturation denote the acid site concentration on the surface of the catalyst. Similar studies can be carried out for other probe molecules such as CO2 (to probe basicity), CO (to probe acidity and metal sites), and methanol (to probe basicity, redox properties, and the presence of oxide defects).80,82 As mentioned previously, the surface of metal oxides may terminate with M−O−M, M−OH, and MO species. Understanding the distribution of such species on a catalyst may provide important information regarding catalytic active sites. Unfortunately, IR spectroscopy is typically not used to directly probe the structures of metal oxides on the catalyst surface. The strong absorbance of typical metal oxides within the 100−1100 cm−1 region makes it difficult to study MO and M−O−M vibrations, which occur in this range.83 The one exception to this disadvantage of IR spectroscopy is in the study of M−OH species. M−OH stretching vibrations occur between 3500 and 3800 cm−1, which is a region well suited for IR study. M−OH species on metal oxide surfaces are particularly important in the study of zeolite catalysis. As was discussed in section 2.3, The Brønsted acidity of these materials arises from the Si−OH···Al species formed to balance the net negative charge on the zeolite framework or isolated Si−OH groups arising from defects in the zeolite framework. In acidcatalyzed reactions, the zeolite Si−OH IR band at 3748 cm−1 or the Si−OH···Al band at 3600 cm−1 can be monitored to quantify the number of acid sites being used under reaction conditions.84

4. COMMON CHARACTERIZATION TECHNIQUES FOR METAL OXIDE CATALYSTS Understanding the surface structures involved in catalysis is of paramount importance for development of novel catalytic systems. Surface characterization via spectroscopy is one of the most valuable approaches available to probe catalysts, especially under reaction conditions. The spectroscopic characterization of metal oxide catalysts is particularly challenging due to the wide array of structures on their surface (e.g., variation of metal oxidation states, multiple crystalline phases, and varying local coordinations).79 Spectroscopic techniques (e.g., infrared, Raman, and UV−vis) are critical tools to evaluate both the chemical properties of catalytic active sites as well the surface motifs that give rise to said chemical properties. Complementing vibrational spectroscopy, electronic spectroscopy, and X-ray based spectroscopic techniques can provide a wealth of information on the local coordination of metal cations, relative concentrations at the surface (and in the bulk), and oxidation state. In the following sections, we present an overview of the most widely used characterization techniques for studying metal oxide catalysts, as these tools will be referred to throughout our discussion of light alkane transformations. 2776

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Table 3. Overview of Material Characterization Techniques and Examples of Their Application to Heterogeneous Catalysis characterization technique infrared spectroscopy (section 4.1)

probed property Change in bond dipole moment during vibrational excitation using infrared radiation.

sensitivity

common application to heterogeneous catalysis

Sensitive to organic adsorbates and surface structures such as hydroxyls, carbonates, and nitrates. Metal oxides show significant absorption of incident beam at low wavenumbers (30%) or selectivity (>80%). Figure 18 shows the most promising catalyst systems with reported C2 yields above 25%.124 In particular, three main types of catalysts families were evidenced: (i) alkaline earth metal oxides with alkali metal dopants, (ii) lanthanide metal oxides with alkali or alkaline earth metal dopants, and (iii) Mn-based oxides with alkali and transition metal dopants. From these catalyst families, it is apparent that basicity is an important property of OCM catalysts, which may be a key factor in activating methane. To illustrate more detailed mechanistic considerations related to OCM, we will present an overview of some of the most studied catalysts compositions to date. One of the “simplest” systems studied for OCM is magnesium oxide (MgO) doped with alkali and alkaline earth metals. The OCM reaction appears to be sensitive to catalyst preparation technique as well as reaction conditions, making direct comparisons between catalysts reported in the literature

Scheme 3. Oxidative Methane Coupling to form Ethane

Scheme 4. Oxidative Methane Couple to form Ethylene

This reaction is exothermic and is run at a wide range of temperatures (500−1000 °C)119 and pressures (up to 10 bar).120 The difficulties involved in direct transformation of methane (e.g., C−H bond strength and high symmetry) require harsh reaction conditions where control of undesired reactions is the main challenge. The formed ethane or ethylene products react 2782

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80%. XPS analysis of spent Ba/MgO samples showed the presence of peroxide (O22−) species that were not present in the unpromoted MgO. These peroxide species were attributed to BaO2, which was proposed as the active site. In contrast to the work by Kwapien et al., Rosynek et al. propose a homolytic C−H activation pathway to form methyl radicals: [Ba 2 +O2 2 −] + CH4 → Ba 2 + + H − O2 2 − + •CH3

To discern between the two pathways (i.e., homolytic versus heterolytic C−H abstraction), Rosynek et al. compared the performance of Ba/MgO catalysts for OCM and CH4/CD4 exchange reactions with a CO2 cofeed. The idea behind this study was the fact that CH4/CD4 exchange proceeds via heterolytic C−H cleavage, which is catalyzed by basic surface sites. These types of sites would be poisoned by a CO2 cofeed, evidencing their presence on the Ba/MgO surface. Similarly, if the OCM reaction proceeded primarily via heterolytic C−H bond cleavage, CO2 cofeed would lower reaction rates by the poisoning of surface basic sites. In contrast, if OCM proceeds via homolytic C−H cleavage, the poisoning of basic sites would not affect the reaction rate because basic sites do not catalyze this type of reaction. The study showed a strong inhibition of the rate of CH4/CD4 exchange while presenting no change on the rates of methane conversion during OCM. Thus, the authors conclude that OCM in supported MgO systems likely proceeds via homolytic C−H activation. This step has been proposed to be the rate-determining step of OCM via kinetic isotope effect experiments.131 Beyond the role of lattice oxygen species present on the catalyst, weakly adsorbed surface oxygen species may play a significant role in the OCM reaction as well. Mallens et al. used temporal analysis of products (TAP) studies to elucidate the role of both lattice and weakly adsorbed oxygen on OCM activity with Li/MgO.132 The authors found that at 700 °C two oxygen species are present on the surface, one bound more weakly and present in lower quantities. The weakly adsorbed species desorbed from the Li/MgO catalyst within 4 s, while the strongly bound species desorbed in a 3 min time frame. To probe the reactivity of the weakly adsorbed oxygen, sequential pulses of oxygen and methane were introduced into the TAP reactor. The authors noted a methane conversion of 66% when the O2 and CH4 pulses were dosed simultaneously, and this conversion decreased to 44% when the pulses were 10 s apart. This decrease in activity was rationalized by the disappearance of weakly adsorbed, and reactive, oxygen surface species. During these pulses, the formation of ethane and CO remained constant, while the formation of CO2 decreased. This observation suggests that weakly adsorbed oxygen primarily leads to conversion of methane to CO2. As mentioned previously, the primary role of the catalyst during OCM is in the activation of methane to form methyl radicals. As such, the study of the gas phase reactions occurring during OCM is of great importance for reaction engineering. Chen et al. developed a reaction network of gas phase species under a wide range of OCM conditions (1−10 bar, 630− 960 °C).133 This study determined that the two most important radicals in the system were methyl and hydrogen peroxy (HO2•), with the latter being formed by oxygen activation. Building upon this gas phase reaction network, Couwenberg et al. sought to study the role of Sn/Li/MgO on the generation of radicals as an additional factor in the OCM reaction network.134 By combining both gas phase and surface reaction

Figure 18. OCM catalysts with combined ethane and ethylene yields greater than 25%. Reproduced with permission from ref 124. Copyright 2011 John Wiley and Sons.

difficult. Early reports on activity of 5% Li/MgO show methane conversions of 9% with C2 selectivities up to 80%.125 Other authors show that 4% Li/MgO catalysts prepared via different synthesis techniques achieve methane conversions between 0 and 3% after noticeable deactivation.126 Elemental analysis of the used Li/MgO catalysts after different times on stream showed a significant decrease in total Li content, suggesting volatilization of Li as a factor in catalyst deactivation. After 40 h on stream, catalysts with initial Li loadings between 0.5 and 8 wt % showed only 0.01−0.03 wt % Li remaining. Interestingly, the C2 selectivity at all times was constant and suggests that MgO species rather than Li species are responsible for catalytic activity. Kwapien et al. explored this possibility via a combined theoretical and experimental approach.127 The authors propose Li as a dopant that aids in the formation of active MgO sites, in particular, steps and corners on the MgO lattice. TEM images of spent Li/MgO catalysts show significant disturbance of the MgO structure. Theoretical calculations point to a mechanism for methyl radical formation that is nearly thermoneutral (ΔH = 37 kJ/mol), involving heterolytic C−H abstraction on a [Mg2+−O2−] pair (reaction 4) and subsequent adsorption of O2 as a superoxo species (reaction 5): [Mg 2 +O2 −]MgO + H − CH3 → [HO−(Mg − CH3)+ ]MgO → [HO−Mg •+]MgO + •CH3 [HO−Mg •+]MgO + O2 → (O2•−)[HO−Mg +]MgO

(6)

(4) (5)

This superoxo species is more reactive than gas phase O2 and could possibly contribute to additional surface reactions. Studies by Schwach et al. on MgO model catalysts reported the presence of surface superoxo species via EPR spectroscopy during dosing of methane and oxygen.128,129 While this proposed mechanism on MgO may help explain important reaction pathways on MgO, there is evidence to suggest the direct role of the dopant element on the OCM reaction. Rosynek et al. studied the role of Ba when supported on various basic oxides during OCM.130 In their experimental conditions, the authors found a nearly 3-fold increase in methane conversion (from 5% to 15% conversion) when MgO was impregnated with Ba, while improving C2 selectivity up to 2783

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alternative systems is the on-stream instability caused by Li loss in the reactor as well as the limited yield improvements possible. In recent years, significant research efforts have begun focusing on Mn−Na2WO4/SiO2 catalysts.132,137,138 While significantly more complex due to the presence of three metallic elements as well as multiple metal oxide phases, this catalytic system is highly selective and stable under OCM conditions. At atmospheric pressure and 800 °C, methane conversions between 20 and 40% and C2 selectivities between 66 and 80% are reported.138,139 Difficulties in improving this catalyst further stem from the complex structure of the catalyst, which is modified under reaction conditions. To develop a reaction mechanism for this system, research efforts focus on identifying the role of each individual component (i.e., role of each element and/or crystalline phase) under reaction conditions. One of the critical features of the Mn−Na2WO4/SiO2 catalyst is the crystalline phase of the SiO2 support. During calcination of the catalyst, amorphous SiO2 transforms to crystalline α-cristobalite.140 Palermo et al. studied this transformation and noted several factors that make the presence of α-cristobalite critical to OCM activity.140 Amorphous SiO2 was found to be very active in complete oxidation of methane to COx. In contrast, α-cristobalite was inert under OCM reaction conditions, suggesting that the phase transformation is critical to prevent unselective methane combustion. Furthermore, the authors determined that Na played a role in catalyzing the phase transformation of amorphous SiO2 to α-cristobalite at lower temperatures (phase transition at 750 °C with Na promotion versus 1600 °C with no Na promotion). The authors attempted to support the Na2WO4 phases on preformed α-cristobalite, but found that methane conversion was nearly 6 times lower. Thus, the transformation of amorphous SiO2 to α-cristobalite in the presence of Na2WO4 is critical for high catalytic activity. The authors hypothesize that Na acts as a chemical promoter that enhances the dispersion of the tetrahedrally coordinated WO4 active phase. Follow up studies found that Na was not unique in its ability to generate an active and selective OCM catalyst.141 Bimetallic K2WO4/SiO2 or Rb2WO4/SiO2 catalysts showed enhanced selectivity at comparable levels of conversion to the Na2WO4/SiO2 reference, showing 10−20% higher C2 selectivities. Addition of Mn led to an increase in selectivity for the K-promoted system but decreased selectivity of the Rbpromoted sample. These experiments suggest that multiple alkali metals can be used for OCM catalysts, but the synergy of the multiple elements on the catalyst must be further resolved. Beyond the role of the supported elements on dispersion, the structure of the SiO2 support plays a role in the optimal transformation to the α-cristobalite phase. Yildiz et al. reported that the use of SBA-15, an ordered mesoporous SiO2, as the support precursor led to markedly improved catalytic performance.142 The SBA-15-derived catalyst showed nearly a 2-fold improvement in methane conversion while enhancing C2 selectivity when compared to a catalyst prepared with amorphous SiO2. Overall, this catalyst achieved a stable 14% conversion with C2 selectivity above 60%. The authors analyzed the elemental distribution on the catalyst surface of the tested catalysts and found a homogeneous distribution of Mn and W on the surface of the SBA-15 derived catalyst after its transformation to α-cristobalite, compared to large clusters of each element on the amorphous SiO2-derived sample. Further tuning of SBA-15 pore diameter and volume may lead to future improvements of this approach to improving catalytic performance.

networks, the authors found strong radical concentration gradients within the catalyst pore volume. In particular, methyl radicals were highly concentrated within the catalyst pores while the hydrogen peroxy radical was present in lower concentrations. This latter observation was due to significant radical quenching reactions on the catalyst surface. The authors also showed that models that did not account for the radical concentration gradient between the catalyst pellet and the gas phase failed to predict experimental selectivities to ethane and ethylene. This work exemplifies the inherently coupled surface and gas phase reactions during OCM. As presented in Figure 18, MgO-based catalysts have achieved C2 yields between 25 and 30%. The challenging catalyst design required to maximize active sites that activate methane must be balanced by the propensity of the surface to catalyze unselective pathways, such as methoxy formation. To rationalize the design of future MgO catalysts, Thybaut et al. developed a microkinetic model combining the reaction networks on the Li/MgO surface and the gas phase with catalyst properties.135,136 Sensitivity analysis of the 53 elementary steps studied found that the reaction enthalpy for methane C−H abstraction and the heat of oxygen adsorption on Li/MgO are the two most critical properties of the catalyst surface during OCM. To determine possible optimizations of the MgO system, the authors varied the microkinetic model parameters at reaction conditions reporting high C2 yields with Li/MgO of 27%. Figure 19

Figure 19. Yields of C2 products versus oxygen enthalpy of adsorption and C−H abstraction enthalpy on Li/MgO catalysts. Results based on optimization of microkinetic model as presented on ref 136. Reproduced with permission from ref 136. Copyright 2011, Elsevier.

shows the resulting relation between C2 yield, the oxygen adsorption enthalpy, and the C−H abstraction reaction enthalpy. At the tested conditions, the experimental yield of 27% is already essentially the optimum for the modeled Li/MgO system. A wider optimization varied the reaction conditions in addition to the catalyst properties and found a maximum C2 yield of 35%. Thus, it appears that future research on the Li/MgO system may be limited on the overall yields possible; however, the rich literature developed over the last decades on Li/MgO may inform new catalyst development. After being studied extensively until the early 2000s, research on Li/MgO has stalled due to shortcomings in its performance as an OCM catalyst. The key factor that motivates research on 2784

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monitored the reactor effluent to determine the temperatures where methane conversion occurred. Additionally, purging cycles between oxygen and methane feeds led the authors to assume that no weakly adsorbed oxygen species were present on the surface; thus, the TPSR experiments ascribed all catalytic activity to strongly bound lattice oxygen species. The authors determined that there are two lattice oxygen sites present in the catalyst surface, one selective and one unselective toward methane coupling toward ethane. These oxygen sites are proposed to be electrophilic (selective for methane activation) and nucleophilic (selective for dehydrogenation and deep oxidation). TPSR experiments with ethane or ethylene feeds were carried out, which highlighted significant oxidative dehydrogenation, nonoxidative dehydrogenation, and cracking reactions. In particular, ethane can be converted to ethylene via surface oxidative or nonoxidative dehydrogenation as well as gas phase nonoxidative dehydrogenation. CO2 is only observed during ethane or ethylene feeding, indicating it is not a primary product during methane activation. A more detailed OCM reaction network summarizes their findings in Figure 21.

Elkins and Hagelin-Weaver characterized Mn−Na2WO4 systems supported on SiO2 and MgO.98 X-ray diffraction studies showed that the cristobalite-supported system formed Mn2O3 while the MgO support hindered formation of this Mn3+ phase. The role of Mn2O3 is hypothesized to involve oxygen activation on the catalyst surface. This hypothesis is supported by the reactivity results of the cristobalite- and MgO- supported samples, where the latter showed 6 times less methane conversion. Li found the presence of Mn2O3 critical for O2 activation via temperature-programmed desorption experiments.138 Elkins and Hagelin-Weaver hypothesize that the particular Mn3+ oxidation state in Mn2O3 is compatible with the electronic and structural properties of the W active center. The MgO-supported catalyst showed the presence of redox active MnO2 species which were able to activate oxygen, but the selectivity of this catalyst favored formation of COx products. This observation suggests that the Mn4+ center in MnO2 may be too reactive compared to Mn3+, leading to a loss of selectivity. Furthermore, Malekzadeh et al. studied the electrical conductivity of Metal-Na2WO4/SiO2 and found a correlation between C2 yield and electrical conductivity.143 In their studies, Mn showed the highest electrical conductivity and C2 yield, suggesting that electron transfers involving the Mn center to the WO4 site may be critical for catalyst selectivity. Such an interaction may feature oxygen spillover from the Mn2O3 active sites and onto WO4 centers, where Li has proposed that methane activation occurs (see Figure 20).138

Figure 21. OCM reaction network involving only lattice oxygen species. Adapted with permission from ref 76. Copyright 2016 Elsevier.

Figure 20. Two-site mechanism for OCM. Methane activation occurs on W with oxygen spillover from Mn. Adapted with permission from ref 138. Copyright 2001 John Wiley and Sons.

In this mechanism, lattice oxygen is proposed to activate methane, leading to the formation of methyl radicals on W. Oxygen activation occurs on neighboring Mn sites, which regenerate lattice oxygen species. The generated methyl radicals combine in the gas phase to form ethane, which can be further dehydrogenated to form ethylene. This mechanism has been debated, primarily due to the limited spectroscopic data available.137 Wang et al. propose Mn and Na as the critical components for an active catalyst, and they show a linear correlation between specific activity and Mn content of the catalyst.139 This correlation is questionable, however, as their study showed significant diffusion of Mn into the bulk oxide when supported on MgO, making a significant portion of the Mn unavailable for catalytic reactions. Future studies of the structure−activity relationships of Mn−Na2WO4/ SiO2 catalysts require implementation of in situ surface studies to refine current mechanistic hypotheses. The difficulties of such studies lie in the complex network of gas phase and surface reactions involved under steady state conditions. To separate the contributions of gas phase oxygen species and surface oxygen to OCM activity, Fleischer et al. studied Mn−Na2WO4/SiO2 via temperature-programmed surface reactions (TPSR) and transient feed experiments.76 The TPSR experiments involved a temperature ramp under methane, ethane, or ethylene flow over a preoxidized catalyst sample. The authors

Furthermore, the authors carried out transient OCM reactions under a pure methane flow to determine the amount of available lattice oxygen for methane activation. A preoxidized catalyst was maintained at 750 °C under inert gas, which was succeeded by a step change to pure methane flow. The catalyst reactivity was monitored via mass spectrometry to follow product formation over time. Once the catalyst was mostly deactivated, the total oxygen amount used to produce all products was established. The available oxygen for reaction was determined to be 20 oxygen atoms/nm2. This oxygen surface density is too large based on the catalyst composition, suggesting that significant oxygen migration from sublayers to the surface is involved. This finding supports a reaction mechanism similar to the Mars−van Krevelen mechanism commonly modeled in oxidative dehydrogenation reactions. Recent research on the Mn−Na2WO4/SiO2 system proposes hydroxyl radicals, HO•, as critical contributors to methane activation. Takanabe and Iglesia carried out a thorough kinetic study of OCM using Mn−Na2WO4/SiO2 which noted a positive effect of water toward methane conversion and C2 selectivities.144 The authors determined that, under conditions of quasi-equilibrated oxygen adsorption on the surface, reactive surface oxygen can activate water to form both surface OH and gas phase HO•. These gas phase radicals readily activate methane via reaction 7 and were determined to be the primary 2785

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review of the fundamental understanding of MgO-based catalysts for OCM. Reference 137 presents a review of the field’s understanding of the Mn−Na2WO4/SiO2 system, as well as specific gaps that need to be further studied for a clearer picture on the mechanism of OCM using this catalyst system. 5.1.3. Aerobic Oxidation of Methane to Methanol and Formaldehyde. Methanol is one of the most important chemical feedstocks in use today, primarily in the manufacture of formaldehyde and dimethyl ether and recently as a feedstock for olefin and gasoline production.149,150 With a worldwide demand of approximately 96 million tons per year, any prospective efficiency gains in its manufacture would have significant economic impacts.151 Currently, methanol is produced from syngas obtained from methane reforming, operating at temperatures of 210−270 °C and 50−100 atm.136 These relatively mild conditions have led to process improvements focused primarily on the efficiency of the syngas production process, which is estimated to account for 60% of the capital costs of gas-to-liquid industrial processes.152 It is not surprising then, that research efforts focus on developing one of the “holy grails” of industrial chemistry: direct oxidation of methane to methanol (Scheme 5).

route for C−H abstraction (with surface oxygen species accounting for a minor degree of methane activation). CH4 + HO• → CH3•+H 2O

(7)

This radical-mediated methane activation still requires the presence of oxygen on the reactor feed for water activation, and its predominance is diminished at high conversions. This change is due to oxygen depletion and the concomitant reduction in surface oxygen species available for radical formation. Follow up studies on the role of HO• during OCM have focused on determining the catalyst features that lead to their formation. Liang et al. determined that Mn−Na2WO4/SiO2 is not unique in its kinetic behavior under OCM reaction conditions.145 The authors found that Mn was not necessary to replicate the radical-mediated C−H activation. The authors suggest that Mn enhances the surface-mediated C−H activation pathway of the catalyst, which may have led other authors to deem it critical for reactivity under anhydrous conditions. Furthermore, substituting W with Mo and even SiO2 with Al2O3 led to catalysts with similar radical-mediated kinetic behavior in the presence of steam. The authors concluded that the alkali metal is the critical species on the catalyst for the formation of gas phase HO•. Takanabe et al. recently carried out in situ characterization of these materials under OCM conditions to gain spectroscopic evidence of radical formation.146 Laser-induced fluorescence evidenced the formation of gas phase HO• radicals when a Na2WO4/SiO2 was exposed to an O2/H2O mixture at 800 °C. Furthermore, the authors hypothesize that Na plays an active role in oxygen activation and OH generation via reactions 8−11: 2Na 2O + O2 → 2Na 2O2

(8)

2Na 2O2 + 2H 2O → 2Na 2O + 2H 2O2

(9)

2H 2O2 → 4HO•

(10)

O2 + 2H 2O → 4HO•

(11)

Scheme 5. Aerobic Oxidation of Methane to Methanol

To date, however, modest methane conversions (less than 1%) are achieved under conditions that lead to favorable methanol selectivities using oxygen as an oxidant.139 The high temperatures required to activate methane lead to fast decomposition of methanol intermediates to formaldehyde and COx products. Indeed, Otsuka et al. showed complete oxidation of a methanol feed under reaction conditions required for methane activation.154 Thus, compared to methanol production, partial oxidation of methane to formaldehyde shows improved product yields (3−5%).155−158 To date, these modest yields, however, have not been improved upon. The evident difficulty in selectively converting methane to useful oxygenated products has led most of the research in this field to focus on mechanistic studies, in hope of aiding future catalyst research. Promising developments in the area of methane oxidation to methanol take inspiration on enzymatic processes. Methane monooxygenase enzymes are able to activate methane and selectively convert it to methanol at room temperature.159,160 The active sites in these enzymes consist of Fe2O2 or Cu2O2 species, which are stabilized within the larger enzyme structure. Mimicking such an environment in heterogeneous catalysts is suited to the confined pore and cage environments of zeolites. Successful production of methanol from methane at room temperature was achieved when using N2O or H2O2 as an oxidant with Fe-exchanged ZSM-5.161−163 While detailed description of the use of N2O and H2O2 as oxidants is outside the scope of this review, these studies highlight the validity of the analogous reactivity of Fe sites in enzymes to those in zeolites. Unfortunately, there are no studies showing successful use of oxygen as an oxidant with the Fe-ZSM-5 system. Cu-exchanged zeolites attract more attention in systems using O2 as an oxidant, with Groothaert et al. first reporting its successful implementation in 2005.164 The authors reported a stepwise reaction sequence where Cu-ZSM-5 or Cu-MOR were preoxidized with oxygen at 450 °C, followed by reaction with methane at 175 °C. The authors did not observe any methanol

The presence of Na2O2 under reaction conditions was studied with in situ XRD to determine any change in the Na2WO4 crystal structure. Indeed, at 800 °C the characteristic reflections of Na2WO4 disappear while the support material remains unchanged. Cooldown of the sample under air led to the recovery of the original crystal structure. Ambient-pressure XPS analysis complemented the XRD study by probing the surface electronic environment of the sample under O2/CH4/H2O feeds. The sample shows a 1 eV shift of its Na 1s binding energy upon heating from 20 to 800 °C (1072 to 1073 eV). This Na 1s signal was assigned to Na2O2 from previous literature studies. Furthermore, the surface was enriched with Na at the expense of W, which the authors propose to be due to the presence of a molten alkali peroxide salt film on the catalyst surface. The complex interactions of surface structures with adsorbed reactants, as well as gas phase reactions, make complete understanding of the OCM reaction mechanism difficult. As shown in this section, there is evidence for various reaction mechanisms using the same catalyst systems. Beyond reaction mechanism studies, future research needs to focus on reaction engineering to minimize unselective gas phase reactions, as well as heat transfer as a possible way to overcome the unavoidable interactions of radicals and desired reaction products.142,147,148 For complementary reviews of OCM research, we refer the reader to refs 125 and 137. The former presents a comprehensive 2786

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in the reactor outlet during methane flow and instead extracted the product using a water−acetonitrile mixture. Analysis of the extracting solution indicated the presence of methanol. The authors proposed the active site shown in Figure 22a based on

the zeolite surface. The role of the zeolite structure on catalytic activity has been computationally studied by Göltl et al., who found that confinement effects by zeolite frameworks significantly stabilize reaction intermediates and products compared to cluster models.61 Regardless of which methanol formation path occurs, it appears that water is necessary to complete a catalytic turnover. To understand the reaction mechanism occurring in the Cu active site, significant research has been devoted to the synthesis and characterization of well-defined Cu-exchanged zeolites. The early work by Groothaert et al. estimated that only 4.6% of Cu atoms were involved in methane oxidation.164 This incomplete participation of Cu atoms implies a heterogeneity in the types of Cu sites exchanged into the zeolite structure, complicating any unequivocal characterization of the active structure. To address this shortcoming in the literature, Grundner et al. carried out studies on the preparation of Cu-MOR zeolites with well-defined structures.167,168 The authors chose mordenite as the zeolite framework due to its preferential ion exchange in constrained 8-member side pockets. This focused the study to a single zeolite framework location. The synthesis protocol highlighted strict control of the exchange solution pH as well as the elimination of alkali cations from the parent catalyst. Catalytic testing for methane conversion was carried out in sequential oxidation-liquid extraction fashion, which showed selective formation of methanol. The authors found a linear relationship between methanol yield and Cu concentration in the zeolite, with a calculated stoichiometry of three Cu centers involved in the conversion of one methane molecule (Figure 23).

Figure 22. Possible active site structure on Cu-exchanged zeolites used in oxidation of methane to methanol. (A) Dioxo Cu2O2 proposed in ref 164 and (B) revised Cu2O presented in ref 165.

UV−vis analysis of the oxidized zeolite, which showed a Obridge → Cu charge transfer band at 22 700 cm−1 previously assigned to the Cu2O2 structure. In situ UV−vis analysis showed the disappearance of this band after just 3 min of methane flow. Lowering the reaction temperature to 125 °C led to the disappearance of the band after 25 min. Follow-up studies explored the structure and C−H activation mechanism on the Cu active site.165 Resonance Raman enhancement of the charge transfer band observed at 22 700 cm−1 provided further spectroscopic evidence to refine the structural assignment of the active site. The authors established that the active site was in fact not Cu2O2 species but instead consisted of a single Cu−O−Cu bridge (Figure 22b). DFT calculations of this site during methane activation showed the formation of a [Cu− OH−Cu]2+ intermediate after C−H atom abstraction. The O−H bond strength (376 kJ/mol) in this intermediate was proposed to be an important driving force in the reaction pathway, allowing methane to be activated at moderate temperatures. As is expected for methane activation, initial hydrogen atom abstraction was proposed as kinetically relevant, since the authors observed a kinetic isotope effect when utilizing CD4 as the substrate. Furthermore, an activation energy of 66 kJ/mol was calculated based on both the rate of product formation and the rate of UV−vis band disappearance upon reaction with methane. Unfortunately, this study was also carried out in a stepwise fashion where methanol was quantified via solvent extraction, so insight on the reaction mechanism during a complete catalytic cycle (i.e., methanol desorption and site generation processes) could not be obtained. The difficulty in detecting methanol in the product effluent rather than after solvent extraction may lie in two possibilities:166 (i) the intermediate adsorbed on the active site is not fully formed methanol, but instead is present as a methoxy species and (ii) the produced methanol is confined to the active site environment due to strong hydrogen bond interactions within the zeolite cage. In the first scenario, methanol production requires a subsequent methoxy hydrolysis step and solvent extraction with water may provide the necessary proton for this transformation. Starokon et al. found that product extraction by dry solvents from Fe-ZSM-5 after methane exposure showed only trace amounts of methanol.162 In contrast, addition of water to the extraction solvent leads to nearly complete methanol quantification. The similar reactivities of Fe-ZSM-5 and Cu-ZSM-5 (although the Fe catalyst uses N2O as oxidant) make it reasonable to hypothesize a similar mechanism in Cu-based catalysts. In the second scenario (i.e., strongly bound methanol to zeolite via hydrogen bonds), water extraction may weaken the hydrogen bonding forces enough to allow displacement of methanol from

Figure 23. Total methane converted as a function of Cu2+ exchanged onto mordenite zeolites with different alkali cations present. The solid line represents the reactivity of the sample reported to only contain Cu3O3 clusters as active sites. Reproduced with permission from ref 167. Copyright 2016 Royal Society of Chemistry.

Notably, the authors point to the linearity of the relationship to suggest that all Cu sites on the most active zeolites were involved in methane oxidation. These results, coupled to FTIR and XAS characterization led the authors to conclude that the active site contained a [Cu3(μ-O)3]2+ structure instead of previously reported two metal centers.168 These studies emphasized the role of synthetic parameters on the successful preparation of such an active site. The pH chosen for synthesis (pH 5.7), was selected to maximize the presence of Cu(OH)+ while preventing the hydrolysis of Cu(OH)2 and the deprotonation of framework Si−OH.167 2787

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step of the transformation via kinetic isotope effect experiments with CD4. Furthermore, the authors determined that the reaction showed reaction orders of 1 for CH4, 0.5 for H2O, and 0 for O2. While this contribution presents a proof-of-concept for continuous conversion of methane to methanol, the overall methane conversion of the system amounted to approximately 0.002%. Even by considering alternative oxidants such as H2O2163 and N2O,153 methane conversion is not reported to reach 1% while maintaining substantial methanol selectivities. These modest results exacerbate the tremendous process improvements still needed to make implementation of direct aerobic conversion of methane to methanol a reality. Since methanol is used to a large extent in the production of formaldehyde, direct aerobic oxidation of methane to formaldehyde (Scheme 6) has been studied for several decades in

Deviating from this pH caused either an incomplete Cu exchange, or the formation of Cu(OH)2, which upon calcination forms inactive Cu sites. The presence of inactive sites was evidenced by the deviation from linearity observed in Figure 23 in some samples. Inactive Cu species were also observed when Na-MOR, Mg-MOR, and Ca-MOR were used as parent zeolites. High concentrations of alkali cations led to a decrease in methanol yield, which was rationalized by competition between Cu and alkali ions for the most stable framework exchange sites. Despite thorough catalyst characterization and compelling evidence for the reactivity of Cu3O3 as an active site for methane oxidation, the authors point out that both Cu2O and Cu3O3 sites are selective for the production of methanol. Valuable insight on this system may be gained by future studies on the relative reactivity of these sites and any differences in reaction pathways during methane oxidation. These types of studies, however, would greatly benefit from a continuous methane to methanol system rather than the cumbersome and time-consuming oxidation treatment followed by liquid extraction described so far. Alayon et al. proposed a modified reactor operation sequence where the Cu-ZSM-5 catalyst is preoxidized, then contacted with a methane feed at 200 °C, and finally steam at 200 °C is used to desorb the methanol product.166 This process could be cycled multiple times with comparable activity, providing a semicontinuous approach to methanol production from methane. Unfortunately, the authors did not provide conversion and selectivity values in their experiments to determine the potential for future process optimization. In an improvement to the sequential methanol production reactor, Narsimhan et al. recently reported the successful production of methanol in continuous flow mode.169 The authors used a feed mixture of 98.1 kPa CH4, 3.2 kPa H2O, and 0.0025 kPa O2 at 210 °C and saw steady formation of methanol in the reactor effluent (see Figure 24). The rate of methanol production amounted to

Scheme 6. Aerobic Oxidation of Methane to Formaldehyde

hope of improving process efficiencies. The two most studied catalyst systems for aerobic oxidation of methane to formaldehyde are supported MoOx/SiO2 and VOx/SiO2. MoOx catalysts show higher selectivities toward formaldehyde, but they are also less reactive at comparable reaction conditions.170 Faraldos et al. studied these two systems by synthesizing catalysts with the same active metal loading and exposing the catalysts to the same reaction feed and temperature. At a methane conversion of 1%, MoOx/SiO2 had a formaldehyde selectivity of ∼80% while VOx/SiO2 had 60% formaldehyde selectivity (see Figure 25). To achieve this conversion, however,

Figure 25. Selectivity to formaldehyde (circles), CO (triangles), and CO2 (black diamonds) versus methane conversion. (A) 0.8MoOx/ SiO2 and (B) 0.8VOx/SiO2. Reproduced with permission from ref 170. Copyright 1996 Elsevier.

VOx/SiO2 required a temperature of 530 °C while MoOx/SiO2 required 590 °C. Higher conversions led to a precipitous drop in formaldehyde selectivity down to ∼10% at 5.5% methane conversion with VOx/SiO2. While the authors did not test MoOx/SiO2 at conversions higher than 3%, other studies reported approximately 30% formaldehyde selectivity at 5−7% conversion, albeit with different MoOx loadings.155,171 In all studies, the loss of formaldehyde selectivity was accompanied by a significant increase of COx selectivity. As observed in other alkane activation reactions described in this review, the formation of COx products was attributed to overoxidation of the more reactive methanol and formaldehyde products.156,170 Faraldos et al. attribute the higher selectivity of MoOx catalysts to their decreased oxygen activation capability when

Figure 24. Methanol partial pressure as a function of time during steady state operation using Cu-ZSM-5 catalyst and a reaction feed of 98.1 kPa CH4, 3.2 kPa H2O, and 0.0025 kPa O2 at 210 °C. Reproduced with permission from ref 169. Copyright 2016 American Chemical Society.

1.81 μmol h−1 gcat−1 over the course of 288 h. The total amount of methanol produced was 1.4 times the total Cu content of the catalyst, evidencing the catalytic origin of the detected methanol. The authors confirmed C−H abstraction as the rate-determining 2788

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Regardless of where this reduced Mo4+ site originates from, the peroxo species appears to be critical for any type of methane activation. From their calculated reaction mechanism (Figure 27),

compared to VOx, as determined by oxygen adsorption experiments. This result led the authors to hypothesize that the VOx/ SiO2 surface has higher concentrations of adsorbed reactive oxygen species, making the catalyst more reactive but less selective toward desired oxygenates.170 Ohler and Bell carried out further investigations on the formation of formaldehyde over MoOx/SiO2 catalysts.172 The authors synthesized fully dispersed MoOx species and characterized the Mo oxidation state via XAS, O2 titration, and Raman spectroscopy. The authors determined that in the fully oxidized state the Mo is in a Mo6+ state. Under H2 treatment, these MoOx species are reduced to Mo4+. In contrast, reduction with CH4 showed no significant change in the Mo K-edge, which suggests that CH4 does not reduce MoOx species. This hypothesis was further verified by O2 titration of samples treated with either H2 or methane. H2-treated samples showed O2 consumption amounting to ∼1 O atom per Mo on the catalyst, whereas methane-treated samples consumed about 0.5 O atom per Mo atom. This result suggested the deposition of coke on the catalyst surface during treatment with methane, which was verified by Raman spectroscopy. Under a flow of methane and oxygen, the authors determined that approximately 50−500 ppm of total MoOx species existed in a reduced state, which led them to hypothesize that the oxygen involved in methane activation is not originating from lattice oxygen. Instead, their proposed reaction mechanism involved the formation of peroxide species on a Mo4+ species (see Figure 26) which are responsible for methane

Figure 27. Proposed formaldehyde formation mechanism via peroxide species on isolated dioxo MoOx/SiO2 catalysts. Structures within rectangles are transition states. Reproduced with permission from ref 173. Copyright 2007 Elsevier.

the most likely structure for surface MoOx species was a dioxo with two terminal O bonds, in agreement with their experimental reaction kinetic models.157,172 This type of site agrees with Raman studies by Lee and Wachs.89,174 Future studies of the MoOx/SiO2 catalytic system should focus on the determination of peroxo species, which would validate the proposed mechanism described by Bell and co-workers. The use of high surface area materials such as mesoporous silicas could present a catalyst surface with higher quantity of isolated Mo sites per mass of catalyst, making peroxide detection more likely than in conventional amorphous silica supports. While much progress has occurred in the mechanistic study of active catalysts for aerobic oxidation of methane to methanol and formaldehyde, little progress has been achieved in terms of methanol yields under continuous production. This is the result of the higher reactivity of reaction products when compared to methane. An interesting alternative to direct production of methanol shown to be feasible in homogeneous catalysis is the formation of “protected” methanol derivatives such as methyl esters and methyl formates. In a recent review, Ravi et al. highlight the significantly higher methane conversions and selectivities possible during production of such species.175 Design of heterogeneous catalysts for such a transformation with subsequent “deprotection” steps may prove a more feasible route to production of methanol and formaldehyde from methane.

Figure 26. Proposed C−H activation mechanism via peroxide species on isolated MoOx/SiO2 catalysts. Reproduced with permission from ref 172. Copyright 2006 American Chemical Society.

5.2. Ethane

5.2.1. Oxidative Dehydrogenation of Ethane. Ethylene is the most-produced organic chemical worldwide and is primarily used to create low- and high-density polyethylene plastics, as well for its further oxidation to valuable oxygenates. Traditionally, ethylene is produced in the steam cracking of naphtha fractions of crude oil. With the recent surge in shale gas resources, chemical manufacturers see economic benefit in substituting naphtha with ethane-rich shale gas as a way to produce low-cost ethylene. The high temperatures required for this transformation (∼850 °C) present a process inefficiency that can be improved by employing alternative processes which require lower operating temperatures. The aerobic oxidation of ethane to form ethylene (Scheme 7), more commonly identified in literature as the “oxidative

activation. This Mo4+ species is formed by reduction from H2 produced during formaldehyde decomposition. While the mechanism is plausible, it is unclear how any significant H2 concentration would be available for Mo6+ reduction, given the aerobic reaction conditions. Follow-up DFT studies by Chempath and Bell studied the surface structures involved in methane conversion to formaldehyde assuming a monoxo or dioxo MoOx active site.173 This study also indicated the formation of peroxo species on the Mo active sites but postulated that the reduced Mo4+ sites would be formed during reduction by methane. This assumption appears to contradict their previous study which experimentally determined that Mo6+ was not significantly reduced by methane. 2789

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The substitution of Sb still allows for the formation of an orthorhombic M1 phase, which is necessary to convert the alkane, but yields with Te were still found to be superior. Other recent work by Nguyen et al.183 investigates the inclusion of silica to the preparation slurry during M1/M2 synthesis, as well as employing a postsynthetic heat treatment of the M1/M2 catalyst to 600 °C under N2 flow. The authors found that the addition of silica increases conversion without influencing ethylene selectivity, while the heat treatment negatively impacts ethane conversion. These observations are explained by considering the morphology of the active species, with the addition of silica providing antiagglomeration qualities, and high-temperature treatment serving to sinter the M1 phase to decrease accessibility of an active site. Although much work has been done to understand the M1/M2 catalyst’s reaction pathways and phase cooperation properties under propane (amm)oxidation to acrylic acid/acrylonitrile (see sections 5.3.3 and 5.3.4), this type of analysis is largely absent from ODHE literature up to now. Another popular MMO to use for ODHE is nickel oxide (NiO).184 Heracleous and Lemonidou reported that MMOs containing Nb as dopants to the NiO catalyst can show ethylene yields up to ∼50% at a temperature of 400 °C.185,186 Without the presence of Nb, the NiO catalyst successfully converted ethane but almost entirely to CO2.185 These authors attribute the enhanced selectivity upon the incorporation of Nb to the elimination of the electrophilic oxygen sites of the Ni−Nb−O surface, which are thought to be abundant on NiO and contribute to ethylene oxidation to COx. They further suggest that the Ni−Nb−O catalyst proceeds via the typical Mars− van Krevelen reaction mechanism with participation of lattice oxygen atoms of the MMO framework.186 More recent work with these Ni−Nb−O MMOs vary synthetic procedures and suggests that the synthetic strategy greatly affects catalyst performance.184 For example, NiO alone can offer up to 24% ethylene yield if using a sol−gel synthetic method187 or as low as ∼4% if using a microemulsion method.188 Savova et al. note that the inclusion of Nb atoms as a dopant significantly improves the ethane conversion of their microemulsion-synthesized NiO catalyst, resulting in an improved ethylene yield of 26% (up from ∼4%).188 Zhu et al. show that their NiO catalyst synthesized via sol−gel techniques reveal an increase in ethylene selectivity upon inclusion of Nb dopants, but it is accompanied by a decrease in the conversion of ethane to ultimately give the same yield as was achieved without Nb dopants.187 It is not clear why these sol−gel and microemulsion synthetic techniques result in such large discrepancies in catalytic performance. This example serves as an illustration of the difficulties in the inclusion of dopants to achieve reproducible catalytic performance. Supported VOx catalysts are historically some of the moststudied catalysts for ODHE, despite their inferior performance to the M1/M2 catalyst. Some of the best-performing supported VOx catalysts report ethylene yields of ∼25%, which use a K+-doped VOx/Al2O3 catalyst.78 It is well established that ODHE follows Mars−van Krevelen kinetics using supported VOx catalysts, showing first-order and zero-order dependencies in ethane and oxygen, respectively.189 Kinetic isotopic tracer experiments reveal that the rate of ethylene formation is significantly faster using ethane than with deuterated ethane (Figure 29, rC2H6/rC2D6 > 2), suggesting that the initial C−H cleavage of ethane is the kinetically relevant step to form ethylene.189 There is debate concerning the nature of the active site that

Scheme 7. Oxidative Dehydrogenation of Ethane to form Ethylene

dehydrogenation of ethane” (ODHE), improves upon the process inefficiencies of currently utilized steam cracking methods. The exothermicity of the ODHE reaction drives down the required reaction temperature to ∼500 °C and catalysts are quite stable in these oxidative conditions. Despite the gained process efficiency with use of lower reaction temperature, which is taken into account in recent techno-economic analyses for this process,176,177 inexpensive ethylene produced via steam cracking of shale gas is still projected to remain cost-competitive against ODHE technologies. The abundance of catalyst literature reports in this topic may likely be due to the achievable high ethylene selectivity, relative to the olefin selectivities offered by ODH of other light alkanes. Indeed, high selectivity to the desired olefin product simplifies potential reaction networks and kinetic interpretations. Mixed metal oxide catalysts (MMO), particularly the M1/M2 catalyst comprised of Mo−V−Nb−Te−O atoms, are some of the best-performing catalysts for ODHE. After similar Mo−V−O MMOs initially showed promising ethylene selectivity decades ago,178 the M1/M2 catalyst now reports >80% ethylene selectivity at >80% ethane conversion, resulting in ethylene yields of ∼75%.179−181 Aside from the noted high yields, M1/M2 catalysts additionally benefit from activating ethane at lower temperatures than other ODHE catalysts, requiring temperatures of ∼340−400 °C. One of the main reasons for the high selectivity offered by M1/M2 is that it does not catalyze sequential oxidation of ethylene. This is in strong contrast to use of C3−C4 alkane feedstocks, which show high yields not to olefins but rather their respective sequential oxidation products, such as acrylic acid or maleic anhydride (Figure 28).180 Thus, activation of the C−H bonds of an allylic carbon atom is required for further oxidation of an olefin using the M1/M2 catalyst. Most recent work with the M1/M2 catalyst seeks to further optimize ethylene yields, perhaps by substituting Te for Sb.182

Figure 28. Variation of the selectivity to the main reaction products achieved during the oxidation of C2−C4 alkanes over M1/M2 catalysts at 400 °C. Reprinted with permission from ref 180. Copyright 2010 Elsevier. 2790

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ethylene decreases upon K+ incorporation, while selectivities to propylene and 1-/2-butene increase upon K+ incorporation (Figure 30).191 Therefore, the presence of acid sites improves

Figure 29. Ethylene formation rate as a function of contact time during ODHE using C2H6 and C2D6 as alkane feeds. Reprinted with permission from ref 189. Copyright 2002 American Chemical Society.

Figure 30. Selectivity to corresponding olefins during the ODH of ethane, propane, and n-butane using undoped VOx/Al2O3 and K+-doped VOx/Al2O3. Reprinted with permission from ref 191. Copyright 1997 Elsevier.

abstracts the first H atom. There are several possibilities, including terminal VO, bridging V−O−V, or V−O−S. Regardless, it is accepted that neighboring O−H groups combine to form and desorb H2O while the reduced V3+ center is reoxidized by lattice oxygen.13,64 Reactivity of supported VOx catalysts is greatly influenced by the choice of the support oxide. Gao et al. show that the TOF of supported VOx catalysts increases in the order of VOx/SiO2 < VOx/Al2O3 < VOx/ZrO2.190 In addition, they use temperatureprogrammed reduction (TPR) to show that the reducibility of supported V5+ structures increases in the same order, therefore establishing a correlation between the reactivity of the supported VOx catalyst and the reducibility of V sites. An alternative supported metal oxide which also catalyzes ODHE is supported MoOx. Overall, supported MoOx catalysts are less-active than supported VOx catalysts and have not been studied as extensively.13 This same drop in reactivity was established when using supported MoOx for ODH of propane as well (see section 5.3.1). The active surface structure of supported MoOx is believed to be a Mo6+ dioxo, dipodal site (O)2Mo6+(−O-S)2.64 H atom abstractions proceed at MoO sites and ultimately lead to formation of two hydroxyl species at the Mo-site creating a reduced Mo4+ center.178 Like supported VOx, these hydroxyl species can reorganize to form and desorb H2O before reoxidizing MoOx with lattice oxygen. Also similar to supported VOx catalysts is the observance of structure− reactivity relationships in supported MoOx catalysts: higher selectivity to ethylene is realized with two-dimensionally dispersed MoOx surface structures. Regardless of the catalyst type for ODHE, whether it be supported VOx or the M1/M2 catalyst, there is agreement among most in this field that surface acidity/basicity plays a role in the observed reactivity and selectivity, yet there is debate about how the acidity contributes. A nice example of the effects of surface acidity is given by Galli et al., who test supported VOx/Al2O3 catalysts and VOx/Al2O3 doped with K+ (VOx/K/ Al2O3) catalysts for their reactivity for ODH of ethane, propane and n-butane.78 The incorporation of K+ decreases the acidic character of the surface by blocking some of the acidic Al−OH sites available on VOx/Al2O3, as well as increasing the reducibility of the V5+, evidenced by TPR. The authors notice interesting trends in the olefin selectivities using VOx/Al2O3 or VOx/K/Al2O3 for ODH of these light alkanes: selectivity to

selectivity to ethylene, while surface basicity favors selectivity to butenes. This interesting observation is rationalized by the authors in terms of the desorption of the olefins, suggesting that the basicity of olefins increases with increasing chain length, and thus the most-acidic olefin (ethylene) should desorb from a surface with more-acidic character.192 Another consideration to include is the strength of the metal− oxygen bond and the impact this might have on the selectivity to ethylene. It is intuitive to think that if a metal−oxygen bond is weak, the catalyst could be very active but not selective, as sequential oxidation steps are facile. Conversely, strong metal− oxygen bonds make the catalyst surface less reactive but would potentially result in lower selectivity to COx. Such an analysis helps to justify why three-dimensional V2O5 (V5+ coordinated to five O atoms) shows lower selectivity to ethylene and higher selectivity to COx than two-dimensional supported VOx (V5+ containing terminal VO).193 For additional reading on ODHE, we refer the reader to several influential reviews of this topic.13,64,184,191,194 Each of these references help to put the aforementioned catalysts in perspective toward their industrial relevance, especially refs 13, 191, and 194. Reference 64 gives further details on mechanistic aspects and includes concepts of advanced reactor designs. Reference 184 gives further details on the NiO MMOs and dopants used for these catalysts. 5.2.2. Aerobic Oxidation of Ethane to Acetaldehyde and Acetic Acid. Aside from ODHE to create ethylene, other applications using metal oxides for aerobic oxidations of ethane are rare. It is much preferred to use ethylene as a feedstock for further oxidations, but these transformations are outside the scope of this review. We detail here a few examples of aerobic oxidations of ethane to form acetaldehyde and acetic acid (HOAc). Acetaldehyde is an important chemical precursor to form pyridine and pyridine derivatives using the Chichibabin synthesis, also forming acrolein as an intermediate. Acetaldehyde is primarily produced today by oxidizing ethylene using the Wacker process, using a homogeneous Pd/Cu catalyst. The literature available for use of metal oxides for direct oxidation of ethane to form acetaldehyde (Scheme 8) is not as impressive as the other applications outlined in this review. Zho et al. report 2791

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the Introduction) creates a growing gap between propylene demand and propylene supplied by traditional steam cracking methods.15,203 This gap between propylene demand and supply can no longer be ignored; propylene is the second-highest produced organic chemical worldwide, behind only ethylene. Most propylene is polymerized into polypropylene, but large fractions of propylene are used to produce other useful chemical building blocks, including acrolein, acrylic acid, and propylene oxide. New methods, termed “on-purpose” propylene processes, are emerging to address this growing gap. Although nonoxidative dehydrogenation of propane is the technique of choice for on-purpose propylene production,1 the aerobic oxidation of propane to propylene, more commonly discussed in literature as the “oxidative dehydrogenation of propane” (ODHP) offers improvements to the process inefficiencies of its nonoxidative counterpart (Scheme 10). These improvements include

Scheme 8. Aerobic Oxidation of Ethane to form Acetaldehyde

that use of a Cs-doped supported VOx catalyst results in selectivity to acetaldehyde as high as 30% but at very low ethane conversion of only ∼3%.195 Interestingly these authors also see relatively high selectivity to acrolein (15%) and low selectivity to ethylene (∼7%), with the remaining products mostly COx. Ensuing work by Zhang and Kobayashi revealed that a SiO2 catalyst doped with Cs and Bi at the surface could achieve similar selectivity to acetaldehyde (27%) at the same ethane conversion as noted before (∼3%), yet the selectivity to acrolein was almost cut in half (∼8%).196 The fact that the vanadium-free catalyst shows similar reactivity to a catalyst with its inclusion might suggest that a redox V center is surprisingly not required in this particular catalytic system. The very low selectivities to acetaldehyde and the sparse literature on this topic make it clear that this transformation currently attracts little interest to pursue as an application. Most HOAc is produced via the carbonylation of methanol, but small fractions are also produced by the oxidation of acetaldehyde or ethylene. HOAc is considered an important precursor to form the vinyl acetate monomer, as well as acetic anhydride and esters (ethyl acetate, isobutyl acetate, propyl acetate, etc.). Literature related to the direct aerobic oxidation of ethane to form HOAc (Scheme 9) using metal oxide

Scheme 10. Oxidative Dehydrogenation of Propane to form Propylene

favorable thermodynamics which lowers reaction temperatures and eliminates coke production on the catalyst surface. The inclusion of oxygen makes these catalysts highly stable without the need for catalyst regeneration. However, despite the potential process efficiencies gained when switching to ODHP, low selectivity to propylene compared to levels achieved with nonoxidative dehydrogenation currently limits the industrial application of ODHP.13 Supported metal oxide catalysts, especially those employing supported VOx as the reactive metal oxide, are the most selective and most studied metal oxide catalysts in the literature for the ODHP reaction.22 Some of the best-performing supported VOx catalysts utilize mesoporous silica material as the support, VOx/MCM-48, and result in propylene yields of ∼17%.204 Though supported VOx materials have been studied for decades, substantial improvements to the achievable propylene selectivity have not been observed. Researchers attempt to improve selectivity and reactivity by strategically focusing attention on a few areas: (1) surface dispersion of more selective VOx sites, (2) altering the identity of the oxide support, in some cases using a high surface area support, and (3) introducing an additional surface metal oxide species to have a combinatorial effect with surface VOx. The work of Chen et al. helped to establish supported VOx as the most-preferred supported metal oxide catalyst for ODHP by supporting VOx, MoOx, and WOx on ZrO2 and comparing the reactivity of each material.205 They observe that activation energy increases in the order of VOx/ZrO2 < MoOx/ZrO2, WOx/ZrO2, while the reaction rate of propane dehydrogenation decreases in the same sequence and is a reflection of the reducibility of the metal oxide species. The authors suggest that Lewis acidity of the metal cation (increasing in the order of V5+ < Mo6+ < W6+) plays a role in the adsorption enthalpies of propylene and propane, which would also benefit the desorption of propylene from the VOx/ZrO2 catalyst, in particular, as propylene should interact more strongly with metal centers of increasing Lewis acidity.206 Some reports in the early 2000s also investigated supported MoOx catalysts for ODHP.207−210

Scheme 9. Aerobic Oxidation of Ethane to form Acetic Acid

catalysts is somewhat more common than what is available for acetaldehyde synthesis, but studies for this transformation still remain rare. MMO catalysts containing typical Mo−V−Nb atoms are the most common in this topic.197−202 Of these, Mo−V−Nb catalysts doped with Pd are the best-performing, showing yields to HOAc as high as ∼3−5% in the academic literature.197,201,202 These catalysts propose that two sites are necessary for HOAc formation: ethane is proposed to react with lattice oxygen of the MMO at reactive V5+ sites to form ethylene, whereas ethylene finds dispersed Pd2+ atoms to undergo a Wacker-like reaction to form HOAc. Linke et al. make an interesting observation that the presence of water vapor plays a crucial role in the increased conversion of ethylene and heightened selectivity to HOAc (at the expense of COx).201 They show that when feeding ethylene and oxygen over the Pd−Mo−V−Nb-O catalyst, they observe ∼20% conversion of ethylene with 73% and 24% selectivity to HOAc and COx, respectively. By adding steam to the feed stream, they see that ethylene conversion increases to 99.4%, with selectivity to HOAc increasing to 93% and COx selectivity lowering to 7%. They assume that the role of water assists the Wacker-like process at the Pd2+ reactive centers by helping to facilitate HOAc desorption from the surface. 5.3. Propane

5.3.1. Oxidative Dehydrogenation of Propane. The recent substitution of shale gas to steam crackers (detailed in 2792

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an incoming V-alkoxide to act as an anchoring site during synthesis. Raman spectroscopy is one of the most employed characterization methods used to differentiate between 2D VOx or 3D V2O5.20,21,211,212 Particular focus is placed on the VO stretching vibration, which appears in the Raman spectrum at 995 or ∼1035 cm−1 depending on the existence of 3D V2O5 or 2D VOx, respectively. Therefore, any observance of a Raman feature at 995 cm−1 indicates the formation of undesired 3D V2O5. Alternative methods to Raman can also reveal the presence of 3D V2O5, including diffuse reflectance UV−vis (DRUV−vis) spectroscopy,20,21,94,212 51V-MAS NMR,20,21,214,215 and XRD.212 Reactivity of supported VOx catalysts is frequently reported as propylene selectivity plotted as a function of propane conversion (Figure 31). The decreasing propylene selectivity with increasing propane conversion is representative of the overoxidation of propylene to COx. Researchers typically work 80% and providing ∼7 billion kg of ACN per year.223 Although the SOHIO process shows suitable yields and is commercially implemented, favorable economics of using propane rather than propylene motivates research to use propane as a feedstock instead (Scheme 12). Indeed, the cost of propylene in the

Figure 34. Ammoxidation of propane using physical mixtures of orthorhombic M1 and pseudohexagonal M2 phases. Open shapes are pure M1 phase, filled squares (a) are physical mixtures of separate M1 and M2 catalyst particles within a catalyst bed. The other filled shapes (b−d) are M1/M2 catalysts in very close contact, pressed together as powders into catalyst particles. Reprinted with permission from ref 8. Copyright 2004 Elsevier.

Scheme 12. Aerobic Ammoxidation of Propane to form Acrylonitrile

showing selectivity to ACN with only the M1 phase and physical mixtures of M1 and M2.8,47 Pure M1 and M2 phases can be synthesized using specific ratios of chemical precursor and calcination cycles, with an Nb/ Te ratio below 0.2 giving the M2 phase and a ratio of Nb/Te above 1 giving the M1 phase.8,47 Additionally, the M1 phase can be isolated by treating the synthesized M1/M2 catalyst with dilute H2O2 to dissolve away the M2 phase.45 Interestingly, similar phase cooperation is proposed for the bimolybdate catalyst used for the SOHIO process mentioned at the start of this section, helping to reinforce the fundamental concepts of selective oxidation catalysis proposed by Grasselli.73,225,227 The common heterogeneous catalyst theme of site isolation also plays a role in the success of the M1/M2 catalyst. The site isolation hypothesis states that when using metal oxides for partial oxidations the reactive lattice oxygen atoms of a catalyst surface must be structurally isolated from one another to achieve high selectivity.228 Groupings of reactive oxygen atoms in too close of proximity lead to the production of COx waste. In the M1 phase, active centers are separated from one another by four Nb bipyramids.225 These active centers appear on the “ab” plane or the (001) basal plane.47 This spatial separation of active centers was captured with STEM imaging and is displayed in Figure 35.229,230 Considering this, Nb atoms play an important “spectator” role in the ammoxidation reaction, meaning they do not directly convert propane into ACN but rather structurally separate the reactive centers. It should therefore be possible to substitute Nb with another atom to act as an isolating-agent. Indeed, when substituting Nb with Ta, the orthorhombic M1 phase is preserved, and similar yields to ACN are observed. This effect is displayed here in Figure 36, while again showing the importance of phase cooperation between

SOHIO process is substantial: propylene feedstock alone accounts for upward of ∼70% of the production cost of ACN.224 The most-successful and well-studied catalyst for the one-step ammoxidation of propane to ACN process is the M1/M2 catalyst. The difficult transformation of propane to ACN requires sequential oxidations and thus requires multiple types of reaction sites, which the M1/M2 catalyst provides. As already detailed in section 2.2, the M1/M2 catalyst is a mixed metal oxide (MMO) catalyst comprising Mo, V, Te, Nb, and O atoms. Following its discovery by Mitsubishi Chemicals in 1994 for alkane (amm)oxidation into nitriles41 or carboxylic acids,42 two decades of research has served to improve the performance of the M1/M2 catalyst and investigate its functional properties. This technology is in the beginning stages of commercialization by PTT Asahi Chemical Company Limited for the production of ACN via the aerobic ammoxidation of propane.43 Operating conditions using M1/M2 vary among studies of propane ammoxidation to form ACN, but it is typical to use temperatures of ∼400 °C with high concentrations of oxygen and ammonia and only moderate concentrations of propane. Some of the best reported ACN yields using M1/M2 are ∼65%, showing high conversion of propane (∼90%) and reasonable selectivity to ACN (∼70%).47,225 Perhaps the most intriguing aspect of the M1/M2 catalyst is the observed cooperation between the separate M1 and M2 phases. The M1 phase is orthorhombic, containing all key catalytic elements to form ACN (V5+, Mo6+, and Te4+), while the hexagonal M2 phase contains these same elements but notably V4+ rather than the fully oxidized V atom. A structural model of the catalytic active center of the M1 phase is shown in Figure 3 (section 2.2).47 The isolated M1 or M2 phases do not provide 2795

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process, with reaction intermediates still coordinated to the Mo site until ACN is formed and desorbs. A visual representation of the proposed mechanism described above is provided in Figure 37.

Figure 35. Enlargement of HAADF-STEM image of the M1 phase, highlighting spatial separation. Reprinted with permission from ref 229. Copyright 2014 Springer.

Figure 37. Proposed propane ammoxidation mechanism using the M1/M2 catalyst. Only the M1 phase can activate propane, while both phases can contribute to propylene ammoxidation. Reprinted with permission from ref 223. Copyright 2011 Springer.

Some of the complexities of the proposed catalytic mechanism might be simplified by considering only the M1 phase and the statistical probabilities of the elemental structures of the active centers. By comparing a statistical analysis of elemental distributions of the active centers to the experimentally determined product yields, assumptions can be made about the identities of structural arrangements which lead to production of ACN, propylene, and COx, as well as those that remain inert.223 The four most prominent elemental arrangements of the M1 catalyst are displayed in Figure 38.223 It is suggested

Figure 36. Yields to acrylonitrile (ACN) using Mo−V−Nb−Te−O and Mo−V−Ta−Te−O catalysts. Reprinted with permission from ref 51. Copyright 2006 Springer.

the M1 and M2 phases as described above.51 Although the pure M1 Nb- or Ta-substituted catalysts show almost identical ACN yields (100% orthorhombic), the catalyst containing Nb shows much higher yield to ACN than the Ta-containing catalyst at an optimal M1 content of 60%. However, the authors do not attribute this to differences in specific properties between Ta or Nb but rather postulate that it could be due to differences between surface areas and contact behavior of each catalyst.51 Like many other alkane transformations elaborated in this review, the alkane is proposed to activate via the V5+O site, where an H atom abstraction takes place. Some authors note special reactivity of this VO site due to the resonance radical structure that can be drawn, V4+-O•.47 The remainder of this mechanism is highly speculative and has hardly changed since its conception in 2003.47 A second H atom abstraction takes place at an O atom of an adjacent Te4+, forming propylene. The Π-electrons of propylene then coordinate with an adjacent Mo6+ center while a neighboring O atom from a Te4+ site abstracts another H atom, forming an allyl radical intermediate. The NH3 + O2 flow causes the creation of a stable (O)(HN)Mo6+(−O−) site, which must be the same Mo6+ center coordinated to the allyl radical in order to provide NH insertion to the allyl radical. This insertion is said to be aided by the spectator oxo group (MoO).231 The remaining mechanistic steps to form ACN (H atom shifts, water desorption, and surface oxidation) are proposed to follow typical olefin ammoxidation routes similar to bimolybdates in the SOHIO

Figure 38. Four most prominent elemental arrangements of the M1 phase catalyst. Reprinted with permission from ref 223. Copyright 2011 Springer.

that at least one V5+ site must exist within the inner core of the active center in order to activate propane. Having only Mo6+ atoms within this core makes the site inactive, while having two V5+ sites leads to production of COx. Considering the statistical probabilities of elemental arrangement, 22% form ACN, 2796

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Researchers at Mitsubishi filed patents for use of M1/M2 for propane to AA in 1995.42 Although yields to AA have not improved significantly since the conception of M1/M2, significant research on this topic has helped to determine a reaction network and to define the reactive constituents of the catalyst surface.237−242 Many of the themes laid out in section 5.3.3 when discussing the M1/M2 catalyst for propane to ACN also apply to its use for propane to AA. The phase cooperation between the M1 and M2 phases is still evident: only the orthorhombic M1 phase can activate propane since it contains the necessary V5+ site, while the pseudohexagonal M2 phase cannot react with propane but instead efficiently oxidizes propylene into AA.238 Though catalysts containing only the pure M1 phase do observe appreciable yields to AA of ∼40%,239,240 maximum AA yields can only be achieved when both the M1 and M2 phases are contained together in a catalyst bed.45,243 Although the development of hydrothermal synthesis practices for the M1/M2 catalyst helped to ensure reproducible reactivity data between research groups, catalysts may still have varying surface terminations, various crystalline and amorphous phases which may contribute to side reactions.242 Recently Schlögl, Trunschke, and co-workers sought to address these potential discrepancies between research groups by simplifying the catalytic material and only studying well-defined, phasepure M1 catalyst.237,239,240 Use of the phase-pure M1 catalyst allowed for insights into the role of each elemental constituent of M1 and the development of mechanistic hypotheses for the conversion of propane to AA. Schlögl and co-workers envision that vanadium centers are essentially responsible for all catalysis occurring in the propane to AA transformation. Two H atom abstractions occur from propane to form propylene using two V5+O sites forming V4+−OH groups, which can combine to form H2O and are reoxidized by lattice oxygen. Propylene is activated by V clusters containing at least one V5+ center, creating a hydroxyl group and a propoxyl intermediate (OV−O− VO + C3H6 → HO−V−O−V−OC3H5). A series of ensuing O transfers and additions while the C3 substrate remains adsorbed to the V cluster eventually forms AA. This mechanistic interpretation conflicts with the mechanism proposed by Grasselli and co-workers for propane ammoxidation using M1/M2, which suggests that the propylene formed on V5+O sites is coordinated by nearby Mo sites which are ultimately responsible for N insertion (or O insertion to produce AA). Schlögl and co-workers do not dispute the importance of Mo atoms in the M1 catalyst, but they rather describe that Mo provides nothing more than structural stability of the M1 phase (Mo being the most abundant cation) and to also provide spatial separation of V-based active sites. As evidence they point to the fact that AA yield is inversely proportional to the surface concentration of Mo atoms (Figure 40b),240 as well that there are no observed changes to the Mo electronic states (determined by XPS) during in situ operation, in contrast to surface V species. Though the reaction pathways for further propylene (amm)oxidation are interpreted differently by Grasselli and Schlögl, they share agreement in other mechanistic aspects. Both agree that these reactions proceed through a concerted mechanism that requires propylene to remain adsorbed to a specific reaction site in order to further react. Schlögl notes that his interpretation of the overall catalysis occurring on M1 shares commonalities with traditional selective oxidation principles put forward by Grasselli and others, most notably the theme of site

22% propylene, 10% COx, and 46% remain inert. With these arrangements, the anticipated yields should be 41% ACN, 41% propylene, and 18% COx, which compares well with an experimentally determined ACN yield of 42% using only the M1 phase. This same work suggests that the sites inactive for propane ammoxidation should still be active for propylene ammoxidation, effectively giving an expected ACN yield of 82% if generated propylene were to further react to ACN. Since ACN yields >42% are not observed with M1, this implies that a concerted reaction pathway occurs with M1 in which any generated propylene must remain adsorbed to the active site in order to further react to form ACN. This rationale is explained further in Figure 39.223

Figure 39. Distribution of active centers in the M1 phase, the expected product distribution, and maximum expected ACN product yield. Reprinted with permission from ref 223. Copyright 2011 Springer.

For further reading on the aerobic ammoxidation of propane to ACN using metal oxide catalysts, we refer the reader to several nice review articles and book chapters.43,232−235 References 232 and 235 give historical perspectives of the M1/M2 catalyst for ACN production, while ref 234 particularly details its phase cooperation properties. References 43 and 233 explain the industrial potential of the M1/M2 catalyst for the production of ACN from propane. 5.3.4. Aerobic Oxidation of Propane to Acrylic Acid. Manufacturing of acrylic acid (AA) is currently a two-step process, first aerobically oxidizing propylene to acrolein and then further aerobically oxidizing acrolein into AA in a second step.236 Catalysts for each of these steps consist of mixed metal oxides (MMOs) and offer yields of ∼90% and ∼98% to acrolein and AA, respectively (overall AA yield ∼88%).237 Most AA is polymerized into poly(acrylic acid) used as super absorbents. A highly desired alternative is to produce AA in one step directly from propane (Scheme 13). This is a difficult reaction Scheme 13. Aerobic Oxidation of Propane to form Acrylic Acid

that involves three subsequent oxidations (propylene and acrolein as intermediates) and ultimately requires the transfer of eight electrons, abstraction of four H atoms, and addition of two O atoms to the hydrocarbon without initiating C−C cleavage. The most successful catalyst for this difficult transformation is the M1/M2 catalyst, achieving AA yields up to ∼50%. 2797

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Nb is serving to isolate V sites from one another, helping to limit the amount of overoxidation of AA and other byproducts to COx. Schlögl and co-workers expand on this site isolation principle to further suggest that three distinct reaction sites should be responsible for different reaction pathways (though details of these sites are not discussed): one site responsible for dehydrogenation of propane to form propylene, a second site responsible for propylene oxidation to AA, and a third site responsible for steam reforming producing CO.239 It is observed that the inclusion of steam in the reactor feed can improve both the reactivity of the M1 phase and its selectivity to AA by decreasing selectivity to COx products (Figure 42).7,237,240,245 d’Alnoncourt et al. used in situ XPS

isolation. The observation that AA yield increases as a function of Te surface concentrations (Figure 40a)240 might suggest that

Figure 42. Selectivity to CO vs propane conversion measured at temperatures 623−663 K using gas mixtures 3% propane, 4.5−12% O2, and 0−40% steam. The inclusion of steam in the reactor feed significantly decreases COx production, increases selectivity to AA, and does not influence selectivity to propylene (not shown). Adapted with permission from ref 237. Copyright 2014 Elsevier.

Figure 40. Yield of acrylic acid normalized to the specific surface area of the catalysts in the oxidation of propane at 673 K over phase-pure M1 as a function of (a) the Te percentage and (b) the Mo percentage. Surface atoms are represented as filled shapes, and framework (bulk) atoms are represented as open shapes. Numbers above each shape correspond to the catalyst ID within the referenced work. Reprinted with permission from ref 240. Copyright 2010 American Chemical Society.

techniques to monitor an M1 phase catalyst surface in the presence and the exclusion of steam in the reactant feed.237 When steam is present, these authors show that the catalyst surface is altered, with an increase in the atomic concentrations of Te and V and a decrease in Mo concentration (Figure 43). It is

Te effectively assists in containing and isolating V-based active sites. A similar V-site isolation theory is made by Vitry et al. with their observation that Nb exclusion does not appear to change the M1 structure, but inclusion of Nb increases selectivity to AA by 15−20% (Figure 41).244 It could be that

Figure 43. (a) Normalized elemental composition with respect to Mo (red), Te (green), and V (blue) near the catalyst surface. (b) V5+ (blue) and V4+ (black) abundance at the surface. Reprinted with permission from ref 237. Copyright 2014 Elsevier.

postulated that the structural changes that occur upon the introduction of steam contribute to the required V-site isolation240 and that additionally steam may facilitate the desorption of formed AA from the surface, improving selectivity.237 Prior to the discovery of the relatively successful M1/M2 catalyst, the VPO catalyst was frequently studied as well for the one-step aerobic oxidation of propane to AA. This is the same VPO catalyst which is an industrially employed catalyst for n-butane oxidation into maleic anhydride (see section 5.4.3).246 However, yields of AA from propane using VPO are significantly lower than for n-butane to maleic anhydride, even though maleic anhydride synthesis requires more oxidations

Figure 41. Propane conversion (square) and acrylic acid selectivity (circle) over Mo−V−Te−O (open shape) and Mo−V−Te−Nb−O (closed symbol) catalysts as a function of reaction temperature. Reprinted with permission from ref 244. Copyright 2003 Elsevier. 2798

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focus within the n-butane oxidation community is probing the factors that lead to the observed product distribution. Vanadium-based metal oxide catalysts are found to be active for ODH of n-butane. Chaar et al. reported that VMgO MMO reached up to 60% selectivity of combined C4 olefins and dienes at 28% n-butane conversion.250 The pure V2O5 and MgO reference compounds showed low conversions and selectivities toward C4 olefins, suggesting that the chemical interaction of V and Mg is critical for catalytic activity. Catalysts with 20−60 wt % V2O5, which showed the highest selectivities to dehydrogenation products, revealed the presence of Mg3(VO4)2 as the only V-containing phase on the surface. The tetrahedral coordination of vanadium, leading to the presence of VO terminal atoms was speculated to be the active site. As discussed in section 5.3.1, tetrahedrally coordinated 2D VOx species are the most selective V structures for ODH of propane, and this result agrees well with the structure of Mg3(VO4)2 found by Chaar et al. Interestingly, Chaar et al. saw no formation of maleic anhydride, which was attributed to the basic character of the VMgO MMO, as acidic catalysts such as pure V2O5, VOx/Al2O3, and VOx/TiO2 readily catalyze the formation of maleic anhydride. The basicity of VMgO possibly leads to weak adsorption of butenes and butadiene, preventing further oxidation steps to maleic anhydride. To support this hypothesis, the authors noted that the 1-butene/cis-2-butene/trans-2-butene ratio was always 3:1:1, corresponding to statistical control of the second hydrogen abstraction step (Figure 44). Stronger adsorption of

and electron transfers of the alkane. One reason for this could be that maleic anhydride is a fairly stable oxidation product, forming a 5-membered ring, while acrylic acid is not as stable and thus more prone to overoxidation to COx. The VPO catalyst was first reported to catalyze propane oxidation to AA in 1986 by Ai et al.236 This work, as well as subsequent studies, varied concentrations of vanadium, phosphorus, and tellurium to find an optimal atomic ratio. Ultimately, Ai reports that a Te/P/V ratio of 0.15:1.15:1 gives the best acrylic acid yield of 10.5%, with the particular improvement in selectivity owing to the inclusion of Te. More recent work conflicts with this report and instead suggests that the inclusion of Te may not be necessary, for VPO catalysts containing no Te show acrylic acid yields slightly higher than reported with its inclusion (11−15%).247 For additional reading on aerobic oxidations of propane to AA using metal oxide catalysts, we refer the reader to some influential reviews on this topic.234,246 Reference 246 gives a wide overview of AA production, both via propylene oxidation and propane oxidation, and highlights the M1/M2 and VPO catalysts in particular for aerobic oxidation of propane to AA. Reference 234 highlights the phase cooperation properties of the M1/M2 catalyst in particular. 5.4. Butane

5.4.1. Oxidative Dehydrogenation of n-Butane. While n-butane and isobutane are only minor components of natural gas (up to 2 mol %),248 their efficient utilization has garnered significant research interest. The ODH of n-butane reaction was often studied in parallel to the ODH of ethane and propane due to similarities between these light alkanes, providing insights on the reaction mechanisms involved.191,249 Furthermore, selective catalyst systems for ODH of ethane and propane are often the starting points for development of ODH of n-butane catalysts. Unlike ethane and propane, which produce only one olefin, n-butane oxidation can lead to multiple valuable ODH products: 1-butene, cis-/trans-2-butene (Scheme 14), and 1,3-butadiene (Scheme 15). Thus, an important research

Figure 44. Selectivity-determining step during n-butane ODH after initial secondary hydrogen atom abstraction. HA denotes a hydrogen atom located in the neighboring methyl group. HB denotes a hydrogen located in neighboring secondary carbon.

Scheme 14. Oxidative Dehydrogenation of n-Butane to form 1-/2-Butene

butenes on the surface is expected to lead to isomerization reactions, resulting in an equilibrium 1-butene/cis-2-butene/ trans-2-butene ratio of 1:1:1. To investigate the proposed role of acid−base characteristics on ODH of n-butane, Blasco et al. synthesized supported VOx catalysts using support oxides with varying degrees of acidity and basicity.251 The authors reported the highest selectivities toward 1-butene on catalyst supported on basic MgO and hydrotalcite. The acidic Al2O3 and sepiolite supports showed higher selectivity toward cis-/trans-2-butene. Additionally, butadiene selectivity increased with conversion on the basic supports, while the opposite trend was observed with the acidic supports. From the catalytic tests, the authors verify the observations found by Chaar et al. with VMgO MMO. To test whether acid catalysts show stronger adsorption of reaction products, a separate study carried out in situ IR spectroscopy of these catalysts during 1-butene adsorption and subsequent temperature-programmed desorption.252 When 1-butene is adsorbed on VOx/Al2O3 and heated, IR features related to alkoxides and carbonyls are present throughout the temperature range investigated. These features change with increasing temperature, leading to 1-butene decomposition and eventual formation of coke on the catalyst surface. In contrast, when

Scheme 15. Oxidative Dehydrogenation of n-Butane to form Butadiene

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lysts have been the focus of intense research efforts due to their higher selectivities when compared to alumina-supported catalysts. Similar to the early report by Chaar et al.,250 studies by Lopez Nieto et al. showed that the most selective ODH catalysts contain low V loadings (≤25 wt %).257−259 XPS analysis of VOx/MgO samples prepared via coprecipitation or impregnation indicated that, at low V loadings, catalysts containing a Mg3(VO4)2 phase revealed the highest reactivity and selectivity. This phase contains vanadium in tetrahedral coordination. Thus, even if the VOx species are not present as 2D isolated species at and below monolayer coverage, the tetrahedral motif appears to lead to the higher selectivity toward ODH products during ODH of n-butane. While the mentioned studies establish tetrahedrally coordinated VOx species as the most selective active sites for ODH of n-butane, a study involving well-defined V catalysts with monomeric, oligomeric, monolayer, and nanoparticle structures is still needed to fully elucidate the effect of V speciation on TOF and selectivity. Indeed, most reports present reaction rates on a per mass basis rather than per site. Furthermore, studies on mixed phases such as Mg3(VO4)2 require the accurate determination of active sites (beyond calculating activity based on V content) to compare the catalytic activity of mixed phases to that of supported systems. The VOx-containing catalyst studies described thus far for ODH of n-butane show few examples of ODH product selectivities above 65%. Though this may be a function of operation of ODH reactors at high conversions, there seems to be a limit to the selectivity of VOx-based catalysts under reaction conditions required for n-butane activation for the production of butenes. Alternative catalyst materials may be of interest to improve ODH product selectivity further. Mixed metal oxide systems have been studied for ODH of n-butane in part due to the success of these types of systems in catalyzing ODH of ethane (see section 5.2.1) and the formation of maleic anhydride from n-butane (see section 5.4.3). Nickel molybdates, NiMoO4, in particular are active catalysts for n-butane ODH to butenes and 1,3-butadiene. While Ozkan and Schrader reported selective oxidation of 1-butene and 1,3-butadiene using nonstoichiometric NiMoO4,260,261 direct oxidation of n-butane was not reported. Madeira et al. extended this initial study of NiMoO4 to n-butane feeds and found that the α- and β- phases of NiMoO4 were active ODH catalysts.262 In particular, it was found that α-NiMoO4 converted more of the alkane feed than β-NiMoO4 at comparable contact times. Furthermore, the β-NiMoO4 catalyst showed higher selectivity toward C4 ODH products at comparable n-butane conversions (∼45% C4 selectivity for β phase versus ∼30% for α phase). The Mo coordination in these two phases appears to be responsible for the differences in reactivity. α-NiMoO4 shows a distorted octahedral coordination of Mo5+, while β-NiMoO4 has Mo5+ tetrahedrally coordinated.263 The nature of this active site, however, has not been fully understood. Mazzocchia et al. proposed that lower selectivity toward ODH products is caused by the presence of MO terminations on the surface.264 This was rationalized by correlating IR signals related to MoO with catalytic activity, where the presence of such MO signals correlated with low ODH selectivity. The β-NiMoO4 catalyst shows a shift of this vibration from 980 to 930 cm−1. This shift in band position was attributed to an increased lability of the MoO, thus reducing their activity. This hypothesis, however, requires further investigation via surface-sensitive techniques to determine the role of MO species on ODH activity. The tetrahedral coordination of Mo, however, agrees with

adsorbed on VOx/MgO, 1-butene reacts rapidly and desorbs, leading to a relatively clean catalyst surface until 300 °C. Above this temperature, characteristic vibrations of 1,3-butadiene appear, suggesting readsorption at high temperatures. Thus, acidic catalysts do appear to bind adsorbates more strongly than catalysts with basic support oxides. In conjunction with the 1-butene adsorption experiments, catalytic tests using 1-butene and 2-butene as reactants during ODH showed that the more acidic catalysts led to significant isomerization reactions (i.e., 1-butene ↔ 2-butenes).252 The VOx/MgO catalyst showed only minor isomerization products and primarily led to 1,3-butadiene, an ODH product. Thus, the combined studies described in refs 250−252 postulate the reaction network shown in Figure 45. For basic catalysts, ODH of

Figure 45. Reaction network for ODH of n-butane over basic or acidic oxide catalysts proposed by Blasco et al.253

n-butane proceeds via formation of mono-olefinic compounds, which may readsorb for further dehydrogenation to 1,3-butadiene. In acidic catalysts, both mono- and di- olefins are formed on the surface without significant desorption and readsorption steps. The acid−base character of the tested catalysts also had an effect on catalyst reactivity. At comparable contact times, the more acidic supports led to higher conversions, likely due to higher rates of formation of deep oxidation products. Similar to studies using VOx catalysts for ODH of propane, the effect of VOx surface structures on ODH of n-butane was studied. As shown with ODH of propane (see section 5.3.1), the presence of V2O5 nanoparticles has a negative effect on catalyst selectivity. While it may be reasonable to expect similar structure sensitivity during ODH of n-butane, the exact effect of nanoparticles on catalytic activity requires careful catalyst preparation and proper characterization techniques. Owens and Kung explored the effect of V loading on VOx/SiO2 catalyst for ODH of n-butane254 and indeed showed by Raman spectroscopy that at V loadings of 0.5−0.6 wt % only tetrahedral VOx species were present. When their catalytic activity was compared to samples with 3D V2O5 particles, the total selectivity toward ODH products was significantly higher in the fully dispersed samples (64% selectivity versus 40%). Unfortunately, the authors reported coking in the highly loaded samples, suggesting complete consumption of oxygen in the reactor. This made direct comparison of catalytic activity qualitative at best. Other authors studied VOx supported on mesoporous silicas by varying V loading during synthesis.255,256 While the same trends observed by Owens and Kung are reported, all samples contained 3D V2O5 crystallites (detected by XRD or Raman). Nevertheless, authors noticed a difference in n-butane activity and selectivity, with low-loaded samples showing improved selectivity toward ODH products. Supported VOx/MgO cata2800

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and (2) higher selectivity due to lower rates of consecutive reactions. These results are in agreement with the results from Chaar et al.250 and Blasco et al.251 for vanadium-based materials. Reaction kinetic studies have sought to determine the rates of the steps involved in the overall n-butane ODH network shown in Figure 47. On basic VMgO MMO materials,

conclusions drawn for vanadium-based catalysts, where VOx tetrahedra are the most selective active species. Madeira et al. also studied the effect of alkali and alkaline earth dopants on the activity of NiMoO4.263,265,266 While the doped catalysts all lost reactivity, many of the samples showed significantly enhanced selectivity toward ODH products. Cs-doped materials showed the highest selectivities toward ODH products, reaching nearly 100% selectivity.265 This however was coupled with the most dramatic decrease in conversion. Comparison of the undoped NiMoO4 and Cs-doped catalysts at isoconversion indicates that alkali doping does lead to a change in the inherent selectivity of the catalyst. Ba doping did not show as improved total selectivity as Cs, but it did lead to a significant change in product distribution. At comparable levels of conversion, the Ba-doped samples showed higher selectivities toward 1,3-butadiene compared to Cs-doped catalysts. To rationalize this finding, the authors carried out CO2 temperature-programmed desorption (TPD) experiments.266 In a typical experiment a fully oxidized sample was pretreated under flowing CO2 at 30 °C, allowing the basic sites on the surface (typically O species) to saturate with the slightly acidic CO2.80 After a purging period to remove any physisorbed CO2 from the surface, the sample was heated at a constant rate. The effluents from the TPD cell were monitored by a thermal conductivity detector (TCD) to determine the quantity of CO2 desorbing from the surface. The temperature at which the desorption occurs can be linked to the strength of the basic sites on the catalyst sample. Furthermore, the quantity of CO2 detected at a particular desorption temperature allowed for quantification of site density. Figure 46 shows the CO2 TPD

Figure 47. Reaction network studied via cofeed experiments by Lemonidou. Reproduced with permission from ref 267. Copyright 2001, Elsevier.

Lemonidou investigated the effect of addition of reaction products to the reactor feed.267 By analyzing the effect that low concentrations of reaction products had on the rate of n-butane conversion and product selectivity, the most dominant reaction kinetic pathways may be studied. The authors determined that direct oxidation of n-butane to COx products is significant compared to the formation of primary dehydrogenation products (butene and 1,3-butadiene). While this result contrasts with ODH of propane literature (which proposes propene oxidation as the main pathway to COx), Lemonidou’s findings may be influenced by the presence of active Mg species on the catalyst surface, a possibility that was studied by Dejoz et al.249 By studying VOx/hydrotalcite catalysts, the researchers proposed Mg2+ or Al3+ sites as significant contributors to deep oxidation reactions via direct attack on electron-rich C−C bonds. In terms of the mechanism for the selective oxidative dehydrogenation reaction, there appears to be agreement within the community on the role of a redox cycle on V or Mo metal centers. In a Mars−Van Krevelen catalytic cycle, these metal centers are rapidly reoxidized by lattice oxygen (see section 3). In a study of the contribution of lattice oxygen to dehydrogenation activity, Lopez Nieto et al. carried out pulse experiments where n-butane pulses were dosed onto either an oxidized or prereduced VOx/MgO catalyst.259 While the preoxidized catalyst showed conversion and selectivity similar to that of steadystate experiments, the prereduced catalyst showed markedly lower conversions. Only at 823 K did the prereduced catalyst show catalytic activity with high selectivity, suggesting the involvement of nonoxidative dehydrogenation pathways. The activity of the preoxidized catalyst indicates that the oxygen species involved in the ODH mechanism are not originating from adsorbed oxygen species and instead involve lattice oxygen. Lemonidou studied the loss of ODH activity of VMgO MMO materials under anaerobic conditions and found near total loss of activity after 20 min on stream.267 During this period, however, there was still a significant presence of COx species in the reactor effluent, indicating the involvement of lattice oxygen species on unselective reactions. Many researchers have come to a consensus regarding the importance of tetrahedrally coordinated metal centers. VOx and MoOx species in tetrahedral coordination have been found to be the most selective in ODH of n-butane. Chaar et al.250 and

Figure 46. CO2 temperature-programmed desorption profiles on Cs- and Ba- doped NiMoO4 catalysts. Adapted with permission from ref 266. Copyright 1997, Elsevier.

profiles of the various doped samples. Cs doping showed a significant increase in the amount of basic sites on the catalysts (i.e., a larger area under the TPD desorption peak) while simultaneously shifting the peak maximum to lower temperatures. This shift indicated that the majority of basic sites are weakly basic, as the stronger sites (peak maximum of 600 °C) remained nearly constant between the undoped and doped materials. The Ba doping shows only a modest increase in the amount of basic sites on the catalyst, and the peak maximum only slightly shifts. Comparison between Cs and Ba TPD experiments suggests that alkali ions lead to a significantly more basic catalyst surface. Combined with reactivity data which shows lowered reactivity with Cs-doped catalysts, there is indication that basicity may lead to weaker adsorption of reactants and/or intermediates. These weaker interactions would lead to (1) lower reactivity 2801

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Mazzochia et al.264 attribute this selectivity to the low concentration of MO bonds observed in IR experiments. While their investigations relate the MO vibration to sites that lead to oxygen functionalization of the alkane, there was no clear evidence of this relationship. Wachs et al. reported in situ Raman studies of supported VOx during n-butane oxidation which found no perturbation of the VO bond during production of the oxygenated maleic anhydride, indicating that the VO bond is not involved in oxygen insertion onto the alkane.269 The reported studies then suggest that the species involved in ODH of n-butane may be the V−O−S oxygen. Wachs et al. observed that altering the identity of the support metal oxide resulted in order of magnitude differences in n-butane TOF, in agreement with what is observed during ODH of propane. Similar insight has been gained with NiMoO4 systems, where the substitution of Ni with other metal cations such as Co and Mg lead to changes in n-butane consumption rates.270 Further reading on the reaction kinetics of VOx catalysts for the ODH of n-butane may be found in refs 267 and 268. A broader review of catalytic oxidative dehydrogenation of n-butane may be found in ref 270. 5.4.2. Oxidative Dehydrogenation of Isobutane. Isobutene is used in the manufacture of polyisobutene (butyl rubber), as well as in the production of gasoline additives such as ethyl-tert-butyl ether.271 Alkylation of isobutene with isobutane and n-butane leads to branched compounds used in the production of high-octane gasoline. Deeper oxidation of isobutene is used to manufacture methacrolein and methacrylic acid, both precursors for resins and epoxides.271 Similarly to linear n-butane, aerobic oxidation of isobutane to isobutene (Scheme 16) has garnered research interest over the last decades.

The VPO catalyst allows a three-carbon chain to interact with both V centers simultaneously leading to cracking via lattice oxygen. Additionally, VPO may lead to oxygen insertion into the cracked products due to the more nucleophilic oxygen species found near the V4+ centers of VPO. The Mg2VO7 catalyst similarly binds two carbon atoms, but its higher V5+ oxidation state leads to more electrophilic oxygen species that are more prone to complete oxidation. The authors, however, do not show direct evidence of these interactions. Other supported vanadium catalysts show rather poor selectivities, with SiO2249,273,274 and TiO2275 supports showing selectivities below 40% at comparable conversions to those reported by Michalakos et al. using VMgO catalysts. While the references show varying feed conditions and temperatures, there appears to be a significant difference in the performance of supported vanadium systems for ODH of isobutane when compared to ODH of propane249 which may be attributed to the higher reactivity of isobutyl intermediates formed under reaction conditions. Ovsitser et al. showed that the reactivity of alkanes on V/SiO2 catalysts increases as C2H6 < C3H8 < n-C4H10 < isoC4H10.249 This is related to the bond dissociation energy of the weakest C−H bond in the molecule, which is lowest in isobutane. Improved catalyst systems may require active sites that are able to activate isobutane at lower temperatures than those required with vanadium (e.g., 475−550 °C) to maintain selectivity. An alternative metal explored in ODH of isobutane research is chromium. Grabowski et al. studied the ODH of isobutane on CrOx supported on SiO2, Al2O3, TiO2, ZrO2, and MgO.276 Of the tested materials, Al2O3 and TiO2 supported CrOx species showed the highest selectivity toward isobutene (70% at 5% conversion and 60% at 10% conversion). An important factor in the higher selectivity of these materials may be the moderate temperatures required for isobutane activation (220− 260 °C) compared to other support materials that required higher temperatures, up to 420 °C when using CrOx/MgO, to activate isobutane. Furthermore, the authors found that K+ promotion of the TiO2 and Al2O3 systems enhanced selectivity at the expense of activity. In a follow-up contribution, Sloczynski et al. supported CrOx on Al2O3 and TiO2 at approximately monolayer coverage.277 Monolayer coverage minimized the contributions of the support to catalyst reactivity. The authors concluded that the Al2O3-supported system was more reactive and selective than the TiO2 system. Oxygen chemisorption experiments on the supported catalysts and Cr2O3 reference compounds indicated a dissociative adsorption mechanism. Kinetic analysis of the oxygen chemisorption experiments showed a much higher activation barrier for chemisorption in the Al2O3-supported system than in TiO2 (213 vs 44 kJ/mol). Both of the supported systems showed slower rates of oxygen chemisorption than the bulk Cr2O3 samples. Further reaction kinetic analysis at low conversions showed that the ODH of isobutane reaction had a first order rate dependence with respect to isobutane concentration and 1/2 order dependence on oxygen concentration. While this relationship held true at all studied levels of conversion for the bulk Cr2O3, the supported samples deviated significantly from this rate dependence at high conversions. Based on this deviation and the rate of oxygen chemisorption on all samples, the authors concluded that the supported CrOx species reoxidized slowly when compared to the bulk Cr2O3. This slow reoxidation step could be a factor in preventing overoxidation of the isobutene product, leading to the observed

Scheme 16. Oxidative Dehydrogenation of Isobutane to form Isobutene

As may be expected from their isomeric nature, n-butane and isobutane share similar reactivity under ODH conditions. This has resulted in development of V-based catalyst materials that are reactive in both ODH of n-butane and isobutane. Michalakos et al. studied VMgO MMO materials for ODH of isobutane after determining these were selective toward butene formation using n-butane as feedstock.272 A Mg3(VO4)2 catalyst with tetrahedral V coordination presented isobutene selectivity of 70% at 4% conversion. This isobutene selectivity was nearly identical to the combined C4 olefin (i.e., 1-butene, cis-/trans-2butene, and 1,3-butadiene) selectivity during ODH of n-butane at the same level of conversion. Thus, the authors suggested that butane isomers show analogous reaction mechanisms. To explore the role of active site structure, the authors compared the V coordination in Mg3(VO4)2 with the dimeric Mg2V2O7 (corner-sharing V tetrahedrons) and dimeric VPO (V in octahedral coordination, sharing two oxygen atoms). During ODH of isobutane using the VPO catalyst, no isobutene was produced, and instead acetic acid was the main oxidation product. In the Mg2V2O7 catalyst, isobutane ODH resulted primarily in COx compounds. Such differences in product distribution were attributed to the distance between V atoms. 2802

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in the +6 oxidation state, and Raman characterization suggests the presence of 2D polymeric species. These Cr6+ species require higher temperatures to undergo reduction in hydrogen TPR experiments than unsupported Cr2O3, once again suggesting that the less labile oxygen species on the supported catalysts may be a factor in determining selectivity. The surprising difference in reactivity of ethane, propane, n-butane, and isobutane with CrOx catalysts highlights the subtle, yet important differences between substrates during their oxidation. Chain length, geometry, and electronic properties play critical roles that must be accounted for during catalyst design. Furthermore, the formation of different oxidation products (e.g., structural isomers, oxygenates) may occur differently for a given catalyst. A comprehensive review exploring commonalities and differences in kinetic models of ODH of light alkanes is presented in ref 281. 5.4.3. Aerobic Oxidation of n-Butane to Maleic Anhydride. Maleic anhydride (MA) is a versatile chemical intermediate in the chemical industry. It is used in the production of phthalic resins, coatings and lubricants. Its annual production is approaching 3 million tons per year with continued yearly growth.11 In the first half of the 20th century, its main production route was via the oxidation of benzene using supported VOx and MoOx. While a selective process, benzene oxidation requires the elimination of two carbon atoms per produced MA molecule, thus making this process somewhat atom inefficient. In the mid 1960s, a patent by Bergman and Frisch28 disclosed the selective aerobic oxidation of n-butane to MA with a vanadium pyrophosphate (VPO) catalyst. This process has proven more economical than the benzene oxidation route and is estimated to account for 80% of installed capacity today.38 Beyond its industrial importance, the aerobic oxidation of n-butane to MA is one of the rare examples of selective light alkane aerobic oxidations commercialized today.9 At a chemical level, this oxidation reaction is complex. In a single catalytic turnover, 14 electrons must be transferred to the catalyst surface from the substrate and three oxygen atoms must be inserted into the final molecule (see Scheme 17).109,282

high selectivity of the supported CrOx catalysts. The higher activation energy for oxygen chemisorption on Al2O3-supported catalysts may explain the improved reactivity of this system compared to the TiO2 support. When comparing the performance of the CrOx/Al2O3 system for ODH of different alkanes, Grzybowska et al. found a stark contrast in its selectivity toward isobutane when compared with ethane, propane, and n-butane (Figure 48).278 At all tested

Scheme 17. Aerobic Oxidation of n-Butane to form Maleic Anhydride Figure 48. Alkane conversion versus selectivity to olefin (top), CO2 (middle), and CO (bottom) with CrOx/Al2O3. Symbols represent different alkane feeds: ethane (□), propane (○), n-butane (▽), and isobutane (Δ). Reproduced with permission from ref 278. Copyright 2001, Elsevier.

To perform such a reaction selectively, the catalytic active site requires a balance of reactivity, being initially active enough to activate n-butane, but not reactive enough to facilitate the overoxidation of the reaction intermediates to COx. This requirement is similar to the role Mo, V, Te, and Nb play in the M1/M2 MMO where the reactive V5+ activates the alkane and the less reactive Mo and Te complete the transformation. While the industrial process has been implemented for several decades, there is still debate as to the structure of the active site, the reaction mechanism, and strategies to further improve this catalyst system. It has long been observed that the active phase of the material is formed in situ under n-butane/air mixture at reaction temperatures of 390−430 °C.38 The catalyst precursor, VOHPO4·0.5H2O, restructures during the induction period, forming the (VO)2P2O7 phase that is the bulk of the active catalyst (see Figure 49).33 X-ray diffraction, TEM, and in situ

levels of conversion, selectivity toward the desired olefinic product was less than 20% except between ODH of isobutane. This is different from what was observed by Michalakos et al. with VMgO MMO catalysts, which showed nearly identical performance of their catalysts during ODH of n-butane and isobutane.272 Similarly to Michalakos et al., the authors provide a geometric argument to explain the reactivity difference of CrOx catalysts during ODH of isobutane. However, no detailed structural characterization was done to determine what type of CrOx surface structures were present on the catalyst to clarify how substrate dimensions affect reactivity. To further understand the unique reactivity of CrOx catalysts for ODH of isobutane, follow-up contributions have studied the role of surface acid/base properties and Cr oxidation state on reactivity.279,280 Catalyst activity increased with higher Cr loadings up to monolayer coverage. Surface Cr species are primarily 2803

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Figure 49. Structure of the V4+ dimeric structure of vanadium pyrophosphate. Adapted with permission from ref 282. Copyright 2004, Elsevier.

Figure 50. (VO)2P2O7 surface termination with phosphate groups exposed and a proposed furan intermediate bonded to a vanadium site. This configuration leads to vanadium being present in cavities or clefts within the (VO)2P2O7 structure. In color version: P is orange, V is light gray, O is red, and C is dark gray. Adapted with permission from ref 284. Copyright 1993, Elsevier.

Raman spectroscopy reveal that VOPO4 phases with a V5+ oxidation state are formed as well but are primarily confined to the surface of the catalyst.33,37 During this transformation, the conversion of n-butane and selectivity to MA increase. The “steady state” activity of the catalyst under industrial operation results in 53−65% molar yields of MA, with an overall n-butane conversion up to 85%.9 Reaction side products include COx, acrylic acid, and acetic acids. Catalyst stability is limited, with a significant loss of phosphorus from the surface as well as significant hotspot formation. Operators cofeed phosphorus compounds to compensate for any losses. Lesser et al. studied this process and noted that addition of phosphorus and/or steam to the reactor feed led to a decrease in activity, avoiding hotspot generation.9 Thus, catalyst activity and durability can be modulated to ensure longterm MA yields. Furthermore, the effect that feed conditions have on catalyst stability and activity indicate a dynamic material. These studies of complex gas−surface interactions aim to improve catalytic activity as well as, in a broader context, to gain fundamental understanding into the factors that affect selective oxidation activity of light alkanes. The combined catalyst restructuring, relative material instability, and the relation between feed conditions and changing surface sites has hindered the development of structure− activity relationships for the VPO catalyst. There are still conflicting views on the role of the most prevalent (VO)2P2O7 phase of the equilibrated catalyst. Much like the M1/M2 catalyst phases have been shown to be critical for propane ammoxidation to acrylonitrile, it is possible that (VO)2P2O7 crystallites are responsible for catalytic activity during n-butane oxidation. Guliants et al. studied the equilibration period of VPO catalysts for 23 days on-stream to relate structural transformation to catalytic activity.283 The authors found that, early in the equilibration period, a noticeable amorphous layer was visible via HRTEM. This layer decreased in size with equilibration time and eventually disappeared within 23 days. In parallel, Raman spectroscopy showed the formation of (VO)2P2O7 crystalline structures on the surface of the catalyst. The appearance of (VO)2P2O7 features correlated well with MA selectivity. Thus, the authors concluded that (VO)2P2O7 crystals were responsible for catalytic activity. This conclusion was supported by Ebner and Thompson, who studied the structure of industrial VPO catalysts after 200−1000 h of equilibration.284 The lack of VOPO4 in the equilibrated catalysts led the authors to propose different terminations of (VO)2P2O7 as responsible for MA production. In particular, the slight P excess detected (P/V = 1.05) was modeled by surface termination by phosphate tetrahedra (Figure 50). In turn, this termination would lead to surface clefts or cavities where the vanadium

centers are located. The authors argue, in accordance with Grasselli, that such isolated sites stabilize reactive intermediates while simultaneously providing access to multiple types of O species (VO, V−O−V, V−O−P, and PO) for different steps of the transformation of n-butane to MA. While (VO)2P2O7 formation during equilibration is undisputed, analysis of this VPO phase as the active site presents important shortcomings. The presence of water, MA, oxygen, and hydrocarbon products in the gas phase present a fundamentally different environment to the surface than what is observed ex situ. Hutchings et al. studied the equilibration period of VPO using in situ Raman spectroscopy and detected the noticeable presence of VOPO4 phases on the surface.285 The authors did not relate MA activity on this surface structure, but its presence and its relationship to (VO)2P2O7 sites such as the ones proposed by Ebner and Thompson must be determined to develop a complete structure−activity relationship for MA production. In response to this research question, Wachs et al. developed supported VOx catalysts on various metal oxide supports to determine their n-butane oxidation activity.30 The goal of this study was to eliminate the overpowering signals of VPO phases to identify possible active surface species. When below monolayer coverage (see section 2.1), the VOx species were in a tetrahedral V5+ state that was only partially reduced upon exposure to the n-butane/air mixture. Notably, all tested supports showed MA formation, even fully monomeric VOx/SiO2, suggesting that the (VO)2P2O7 is not the sole prerequisite for catalytic activity. Further, impregnation of the support with both phosphate and vanadium led to a significant increase in MA selectivity (from 23% to 56%) while Raman spectroscopy did not detect the presence of (VO)2P2O7 phases after catalyst synthesis. This study, coupled with evidence that completely amorphous VPO catalysts286 can also catalyze MA formation put into question the validity of reaction mechanisms based solely on the (VO)2P2O7 phase as the active site. Instead, studying the surface dynamics in situ and the role of each structural component has quickly become the key research focus with VPO systems. To gain insight into the factors that lead to selective MA production from n-butane over VPO, researchers investigated the individual roles of V, P, and O atoms. Schlögl et al. used in situ near ambient pressure XPS studies to show that within 1 nm from the catalyst surface the vanadium oxidation state was somewhat higher than 4, with an average value of 4.3.31,282 The bulk oxidation state did show the expected V4+ state in the 2804

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VPO catalyst. This discrepancy between surface and bulk oxidation states suggests the presence of small quantities of V5+ species primarily at the catalyst surface. In an attempt to confirm the surface oxidation state with a complementary technique, Coulston et al. investigated active VPO catalysts via time-resolved XAS.287 The study involved principal component analysis of consecutive XAS spectra while simultaneously monitoring reactor effluents during a step change from an O2/He feed to n-butane. Figure 51 shows that the rate of formation of

Figure 52. Proposed transition state during C−H activation by PO. Some V−O bonds omitted for clarity. Adapted with permission from ref 29. Copyright 2013, American Chemical Society.

VOPO4 or VOx species are not involved in the n-butane oxidation pathway. Wachs et al. determined via in situ Raman spectroscopy that the V−O−P or V−O−support bonds in supported vanadium phosphate systems were involved in the ratedetermining step of the n-butane oxidation reaction.30 Order of magnitude changes of catalyst TOF were observed when the support metal oxide or phosphate concentration was varied. While Cheng and Goddard’s contribution is a new perspective on this catalyst system, further experimental evidence is needed to establish the role of PO bonds in catalytic activity. The role of oxygen on the catalyst surface is equally complex. Analogous to many of the processes discussed in this review, the Mars−van Krevelen mechanism is expected to lead to formation of MA. A question arises, however, due to the large number of electron and oxygen transfers required for formation of MA. If the reoxidation of the catalyst is faster than the reduction cycle occurring during hydrogen atom abstractions, the constant high reactivity of the surface would lead to overoxidation of the alkane reactants.109 Thus, during MA formation, it may be valid to hypothesize that surface reoxidation by lattice oxygen must be relatively slow to prevent complete overoxidation of n-butane to COx. Recent work by Eichelbaum et al. provides compelling evidence to support the slow reoxidation hypothesis.40 The authors developed an ambient pressure XAS reactor, allowing study of the VPO catalyst under reaction conditions at 10, 100, and 1000 mbar. At these various pressure conditions, different gas flows were used: n-butane/O2 mix, O2 only, and n-butane only. Analysis of NEXAFS spectra (particularly, the V L3 edge) of the catalyst under O2 and reaction mixtures at 10 and 100 mbar showed no significant variation in V oxidation state on the surface. At 1000 mbar total pressure, the presence of n-butane noticeably shifted the V L3 edge signal toward a more reduced state, and simultaneously the yield of MA increased significantly. The authors propose that at 1000 mbar (i.e., close to normal reactor conditions) the rate of surface reoxidation is rate limiting due to high conversions that deplete the active site oxygen species. In contrast, at lower pressures the rate of alkane activation is rate limiting due to the relatively low number of adsorbed alkane reactants. Thus, the VPO catalyst active site, when fully oxidized, is reactive enough to activate the n-butane substrate. As the reaction progresses, however, the active site is not reoxidized fast enough to lead to overoxidation. The segregation of the active surface species by phosphate species may limit the available lattice oxygen to only the surface region of the catalyst. This surface insulation is evidenced by temperature-programmed oxidation and reduction studies, which show that only 0.35% of lattice oxygen was available for exchange in an activated catalyst.36

Figure 51. Rate of change of V5+ concentration compared to maleic anhydride production at 320 °C. Adapted with permission from ref 288. Copyright 1997, American Association for the Advancement of Science.

MA follows with the rate of V5+ disappearance, suggesting that the presence of V5+ species is indeed needed to form MA. The role of phosphate is not without controversy. While the observation by Lesser et al. of phosphorus loss suggests that this element may be an active species in the reaction, other authors speculate the role of the phosphate as primarily an “insulator.”9,34,109 In the insulating role, phosphate prevents the formation of long-range order the vanadium surface structures, which is in agreement with the site-isolation concept proposed by Grasseli. Ballarini et al. showed that catalysts with phosphorus-rich stoichiometries had higher selectivity than those with phosphorus-lean compositions. This observation was rationalized by the easier hydrolysis of VOPO4 surface species into a mixed VOx/H3PO4 phase. Bluhm et al. noted that, under reaction conditions, the surface of the working catalyst was significantly enriched with oxygen.34 The stoichiometry of both (VO)2P2O7 and VOPO4 did not agree with this enrichment and suggests the formation of H3PO4 species. Thus, there is significant experimental evidence for phosphate segregation, isolating vanadium active centers in the surface and simultaneously isolating the surface structures from the bulk VPO structure.109 A compelling, and somewhat opposing, view is proposed by Cheng and Goddard.29 Using quantum chemical calculations, the authors propose PO bonds as the active sites for hydrocarbon activation (see Figure 52). In their studies, only hydrogen atom abstraction by the PO bond to form P−O−H species led to an activation barrier in line with the experimental value of 13−26 kcal/mol. Furthermore, upon H-abstraction, it was noticed that the electron gained from the bound H atoms was not localized on the P but instead was delocalized to the neighboring V atoms. Thus, the H atom abstraction by PO still led to reduction of vanadium centers, as established in the literature. While this proposed reaction mechanism contrasts with the primarily insulating effect of phosphate species discussed above, some experimental evidence suggests that the VO bonds in 2805

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it stepwise at various temperatures in either 1-propanol or 1-butanol.289 Once the stepwise heat treatment was completed, the precipitates were comprised of a mix of VOPO4·0.5H2O intercalated with alcohol molecules. The spacing of this intercalation was a function of alcohol chain length. Thus, the authors propose that the heat treatment led to exfoliation of VOPO4·0.5H2O. Treatment under typical MA reaction conditions led to the formation of the working VPO catalyst. These VPO samples showed thin, fractured crystallites. VPO samples without the exfoliated treatment lacked the flaky morphology and instead showed large platelet morphology. The exfoliated catalysts showed surface areas of 40 m2/g and higher activity (per mass of catalyst) than conventionally prepared VPO catalysts. The authors rationalized these results by two factors: (1) The exfoliated crystallites were about half the thickness of a nonexfoliated sample, which leads to greater exposure of basal planes of the VPO surface, and (2) the basal planes of VPO materials are believed to be more selective toward MA production. Thus, the exfoliated catalysts were more active due to the higher surface area while remaining selective toward MA due to enhanced exposure of the VPO basal planes. Wang et al. were able to enhance the surface area of VPO materials by introducing polyethylene glycol (PEG) in the precursor synthesis process.290 The PEG was proposed to prevent the agglomeration of precursor particles during activation, leading to high surface area materials (40−60 m2/g) with small crystallite dimensions. These catalysts showed higher activity and selectivity than conventional VPO materials, highlighting the potential gains of achieving surface area increases in VPO materials. As previously discussed, the working catalyst appears to be formed by isolated VOx species formed from hydrolysis of surface VOPO4. This requires a significant induction period to allow the VPO precursor to restructure under reaction conditions, but elimination of the ordered precursor phase may lead to higher concentrations of active surface species. Hutchings et al. sought to exploit this possibility by synthesizing amorphous VPO materials that may readily rearrange to active surface sites.286 The authors used supercritical CO2 and isopropanol to form amorphous VPO spheroids which remained amorphous for 100 h on-stream. These catalysts were active for n-butane oxidation and showed higher intrinsic activity than conventionally prepared catalysts. The low surface areas of the catalysts, however, limited the benefits gained from their amorphous nature. Notably, these catalysts required no induction period to reach their steady state reactivity, which suggests that their amorphous nature reflects the active species formed during the induction period of conventional VPO materials. In addition to changes to the morphology of the VPO macrostructure itself, studies show the benefit of supporting VPO on other materials. Ledoux et al. developed a synthesis procedure to support VPO on β-SiC, a conductive ceramic material.291 This catalyst showed significant improvements over the bulk VPO catalyst, doubling the rate of n-butane conversion while retaining MA selectivity comparable to the bulk system. However, the authors noticed complete consumption of oxygen, which limited their overall reactivity. Preliminary testing with higher oxygen contents in the feed allowed the supported VPO system to obtain a total MA yield of 54%. The authors attribute the enhanced activity of the supported system to a higher surface area of the material but, more importantly, to the role of the conductive SiC as a heat sink to remove the heat generated during oxidation. This possibly plays a significant role in

While the role of lattice oxygen in slow reoxidation of the active species in VPO agrees with the established redox mechanisms proposed for other types of alkane activations discussed in this review, chemisorbed oxygen may also play a direct role during n-butane oxidation. Wang and Barteau studied the active oxygen species in VPO catalysts using in situ gravimetric analysis.289 A reactor located within a tapered element oscillating microbalance allowed quantification of mass changes of the catalyst sample during different gas treatments. At steady state conversion, the authors determined that 68 μg/s/gcat of O2 were consumed. They then proceeded to quantify mass changes during oxidation and reduction treatments with O2 and n-butane, respectively. For these treatments, the changes in mass of the catalyst represented a rate of oxidation/reduction of 0.34−3.7 μg/s/gcat. This represents only 5% of the total oxygen consumed during n-butane oxidation, which suggests that the majority of catalytic activity during reaction is due to surfacebound oxygen species. Wang and Barteau further determined, based on the overall n-butane conversion, that these adsorbed oxygen species were approximately 62% selective during reaction. While this study presents an important aspect to consider during all alkane oxidations (i.e., the role of nonlattice oxygen), it may be difficult to directly compare a catalyst’s activity from separate reduction and oxidation cycles to performance observed under reaction conditions. From the discussion of the role of the elemental components of VPO in the active catalyst, it is evident that the reactive atmosphere in contact with the catalyst surface affects the type of surface species present on the catalyst. This is exemplified in transient reaction studies carried out by Ballarini et al.,39 where VPO catalysts with different P/V ratios were exposed to comparable reaction conditions. The reactivity of each catalyst was monitored with time to determine when a new “steady state” was reached. As an example, the authors reported that a catalyst with a P/V ratio equal to 1.03 showed its optimal selectivity above 350 °C. In contrast, a catalyst with a P/V ratio equal to 0.99 did not develop a comparable selectivity until a temperature above 380 °C was reached. The authors postulate that excess P on the surface leads to the hydrolysis of VOPO4 at lower temperatures, forming the segregated, and selective, VOx species. Lower P content leads to prolonged establishment of VOPO4 phases, which require higher temperatures to generate enough surface moisture for significant hydrolysis of VOPO4. Academic researchers make great efforts in developing a reaction mechanism for MA production with VPO catalysts. These studies will ideally aid in development of new catalyst synthesis techniques that may improve MA yields in commercial implementation. As mentioned throughout the discussion of VPO catalysts, typical MA yields of 65% are obtained during the commercial process, achieved at 80% n-butane conversion.9,35 Trifirò and Grasselli compare these MA yields to other commercial oxidation processes (e.g., ammoxidation and acrylic acid synthesis from propylene) where yields are above 80%.35 While this comparison omits the inherent difficulty in alkane activation compared to alkene activation, it highlights the fact that the MA process can still be improved, likely with new catalyst synthesis techniques. Industrially utilized VPO catalysts prepared using alcohol and organic media typically have surface areas between 20 and 30 m2/g. Avenues for improvement of these catalysts may involve enhancing their surface area and/or modifying their morphology to improve activity per unit mass of catalyst. Kamiya et al. prepared a VOPO4·0.5H2O precursor and heated 2806

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oxide catalyst to effectively perform each of these transformations does not exist. This reality speaks to the complex nature of partial alkane oxidations and the need to consider the effect a particular reaction environment (i.e., alkane identity, reaction temperatures, feed composition, surface oxygen speciation, etc.) has on the structural and electronic properties of a catalyst. Ignoring these dynamics will hinder efforts to optimize a catalytic system. As with all catalytic processes, there still exists room to improve our fundamental understanding of the structure of the active catalyst and the mechanism by which substrates are transformed into products. Although many metal oxide materials have been explored for decades for their ability to catalyze aerobic oxidations and some have even found industrial application, there is still not a complete knowledge of the state of the working catalyst in some cases. In attempts to characterize both the bulk and surface of these materials, researchers employ both ex and in situ spectroscopic techniques. In this way, catalyst structure−activity relationships are developed that inspire the strategic design of new materials that increase the selectivity to olefins and oxygenates while decreasing the selectivity to undesirable COx. The addition of dopants that alter either the electronic or acid−base characteristics of the catalytically active site is a common theme within the area of metal oxide-catalyzed transformations. Moreover, further exploration into ternary metal oxide systems may lead to materials with vastly improved reactivity. Ultimately, an emphasis on precision materials synthesis with thorough spectroscopic interrogation will lead to the creation of new catalyst systems that will allow for the harnessing of O2 as a benign oxidant for more efficient, atom economical, and greener processes.

maintaining MA selectivity at high conversions. Thus, optimization of catalyst composition in addition to improved heat transfer solutions may present new routes to improve industrial MA production.

6. CONCLUSIONS Solid metal oxides, namely supported metal oxides, mixed metal oxides, and zeolites, are frequently studied as catalysts for aerobic partial (amm)oxidations of C1−C4 alkanes to form olefins, alcohols, aldehydes, carboxylic acids, and nitriles. The use of alkanes as reactants presents economic incentive due to the lower cost of alkanes compared to more oxidized compounds (i.e., olefins or oxygenates), much in part due to their current availability from natural gas. Indeed, petrochemical processes which currently use olefins or oxygenates as reactants show substantial production costs, even in some cases a majority, attributed to the feedstock. The main challenge for all partial alkane oxidations remains achieving high selectivity to the desired product without overoxidizing to undesired COx. Despite structural and electronic differences between the metal oxide catalysts used for these transformations, some mechanistic conclusions are uniform across these applications. It is generally accepted that partial alkane (amm)oxidations using metal oxide catalysts proceed via the Mars−van Krevelen mechanism, with lattice oxygen atoms being responsible for reoxidation of reduced metal centers formed after alkane conversion. Gas-phase O2 then serves to replenish oxygen vacancies of the bulk metal oxide. In many of the best-performing metal oxide catalysts, a V5+ surface species is a necessary constituent to activate the alkane. The alkane transformation initiates with a C−H bond cleavage, most commonly via H atom abstraction forming an alkyl radical, a reduced metal site, and a surface −OH group. Transformations that entail sequential oxidations (i.e., alkane → olefin → carboxylic acid) require separate reaction sites specific for each step and benefit from isolation of these sites provided by spectator atoms which do not directly participate in the reaction. The use of some metal oxide catalysts, in particular, result in product yields attractive for industrial implementation of these partial alkane oxidation processes. For example, the VPO mixed metal oxide catalyst remains the industrial catalyst for the production of maleic anhydride from n-butane since the 1960s. The ammoxidation of propane to acrylonitrile using the M1/ M2 catalyst, another mixed metal oxide, is in the beginning stages of commercialization. The M1/M2 catalyst appears to be a particularly robust metal oxide catalyst for aerobic oxidations, showing high yields for several different alkane transformations, including propane to acrylonitrile (∼65%), propane to acrylic acid (∼50%), and ethane to ethylene (∼75%). The other applications included in this review (propane to propylene, isobutane to isobutene, etc.) do not show the same encouraging product yields with use of metal oxide catalysts. For example, the state of the art supported VOx catalysts for the oxidative dehydrogenation of propane (ODHP) result in yields of only ∼17%. It is worth noting that variations of this supported VOx catalyst remain heavily investigated as a catalyst for ODHP despite these achievable propylene yields remaining low for decades. Considering this, alternative materials to these metal oxides and/or novel process operations should be thoroughly explored to substantially increase product yields. It is interesting to note that oxidations of methane, ethane, propane, and butanes are proposed to initiate through a common H atom abstraction of the alkane, yet a uniform metal

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Juan M. Venegas: 0000-0002-3603-4312 Ive Hermans: 0000-0001-6228-9928 Author Contributions §

J.T.G. and J.M.V. contributed equally to this work.

Notes

The authors declare no competing financial interest. Biographies Joseph T. Grant earned his Bachelor of Science degree in chemical engineering from the University of MichiganAnn Arbor in 2012. At Michigan he worked in the laboratory of Prof. Vincent Pecoraro to synthesize Gd-based metallacrown complexes and characterize their binding properties using electrochemistry. He began his Ph.D. studies at the University of WisconsinMadison in the Fall of 2013. There he is advised by Prof. Ive Hermans, where he studies reaction kinetics and surface properties of heterogeneous catalysts for applications in the oxidative dehydrogenation of light alkanes. Juan M. Venegas was born in Bogotá, Colombia in 1991. He obtained his bachelor’s degree in chemical engineering at Worcester Polytechnic Institute in 2014. During his time at Worcester, he studied the effect of thermal and mechanical treatments on biochar via Raman spectroscopy. He started his Ph.D. under the supervision of Prof. Ive Hermans at the University of WisconsinMadison. Currently, he studies the 2807

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(11) Teles, J. H.; Hermans, I.; Franz, G.; Sheldon, R. A. Oxidation. In Ullmann’s Encyclopedia of Industrial Chemistry; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2015; pp 1−103. (12) LyondellBasell; Data Book; https://www.lyondellbasell.com/ globalassets/investors/company-reports/2015/2015-august-investorsdata-book.pdf (accessed Apr 26, 2017). (13) Cavani, F.; Ballarini, N.; Cericola, a. Oxidative Dehydrogenation of Ethane and Propane: How far from Commercial Implementation? Catal. Today 2007, 127, 113−131. (14) Albonetti, S.; Cavani, F.; Trifirò, F. Key Aspects of Catalyst Design for the Selective Oxidation of Paraffins. Catal. Rev.: Sci. Eng. 1996, 38, 413−438. (15) Ren, T.; Patel, M.; Blok, K. Olefins from Conventional and Heavy Feedstocks: Energy Use in Steam Cracking and Alternative Processes. Energy 2006, 31, 425−451. (16) Kim, S. C.; Shim, W. G. Catalytic Combustion of VOCs over a Series of Manganese Oxide Catalysts. Appl. Catal., B 2010, 98, 180− 185. (17) Larsson, P. O.; Berggren, H.; Andersson, A.; Augustsson, O. Supported Metal Oxides for Catalytic Combustion of CO and VOCs Emissions: Preparation of Titania Overlayers on a Macroporous Support. Catal. Today 1997, 35, 137−144. (18) Lu, J.; Fu, B.; Kung, M.; Xiao, G.; Elam, J.; Kung, H.; Stair, P. Coking- and Sintering-Resistant Palladium Catalysts Achieved Through Atomic Layer Deposition. Science 2012, 335, 1205−1208. (19) Carucci, J. R. H.; Halonen, V.; Eränen, K.; Wärnå, J.; Ojala, S.; Huuhtanen, M.; Keiski, R.; Salmi, T. Ethylene Oxide Formation in a Microreactor: From Qualitative Kinetics to Detailed Modeling. Ind. Eng. Chem. Res. 2010, 49, 10897−10907. (20) Grant, J. T.; Love, A. M.; Carrero, C. A.; Huang, F.; Panger, J.; Verel, R.; Hermans, I. Improved Supported Metal Oxides for the Oxidative Dehydrogenation of Propane. Top. Catal. 2016, 59, 1545− 1553. (21) Grant, J. T.; Carrero, C. A.; Love, A. M.; Verel, R.; Hermans, I. Enhanced Two-Dimensional Dispersion of Group V Metal Oxides on Silica. ACS Catal. 2015, 5, 5787−5793. (22) Carrero, C. A.; Schloegl, R.; Wachs, I. E.; Schomaecker, R. Critical Literature Review of the Kinetics for the Oxidative Dehydrogenation of Propane over Well-Defined Supported Vanadium Oxide Catalysts. ACS Catal. 2014, 4, 3357−3380. (23) Carrero, C.; Kauer, M.; Dinse, A.; Wolfram, T.; Hamilton, N.; Trunschke, A.; Schlögl, R.; Schomäcker, R. High Performance (VOx)n−(TiOx)m/SBA-15 Catalysts for the Oxidative Dehydrogenation of Propane. Catal. Sci. Technol. 2014, 4, 786−794. (24) Love, A. M.; Carrero, C. A.; Chieregato, A.; Grant, J. T.; Conrad, S.; Verel, R.; Hermans, I. Elucidation of Anchoring and Restructuring Steps during Synthesis of Silica-Supported Vanadium Oxide Catalysts. Chem. Mater. 2016, 28, 5495−5504. (25) Zhu, H.; Ould-Chikh, S.; Dong, H.; Llorens, I.; Saih, Y.; Anjum, D. H.; Hazemann, J. L.; Basset, J. M. VOx/SiO2 Catalyst Prepared by Grafting VOCl3 on Silica for Oxidative Dehydrogenation of Propane. ChemCatChem 2015, 7, 3332−3339. (26) Baron, M.; Abbott, H.; Bondarchuk, O.; Stacchiola, D.; Uhl, A.; Shaikhutdinov, S.; Freund, H. J.; Popa, C.; Ganduglia-Pirovano, M. V.; Sauer, J. Resolving the Atomic Structure of Vanadia Monolayer Catalysts: Monomers, Trimers, and Oligomers on Ceria. Angew. Chem., Int. Ed. 2009, 48, 8006−8009. (27) Lee, E. L.; Wachs, I. E. In Situ Spectroscopic Investigation of the Molecular and Electronic Structures of SiO 2 Supported Surface Metal Oxides. J. Phys. Chem. C 2007, 111, 14410−14425. (28) Bergman, R. I.; Frisch, N. W. Production of Maleic Anhydride by Oxidation of N-Butane. U.S. Patent 3,293,268, 1966. (29) Cheng, M.-J.; Goddard, W. A. The Critical Role of Phosphate in Vanadium Phosphate Oxide for the Catalytic Activation and Functionalization of N -Butane to Maleic Anhydride. J. Am. Chem. Soc. 2013, 135, 4600−4603. (30) Wachs, I. E.; Jehng, J.-M.; Deo, G.; Weckhuysen, B. M.; Guliants, V. V.; Benziger, J. B. In Situ Raman Spectroscopy Studies of

oxidative dehydrogenation of light alkanes with metal oxide and boron nitride catalysts. William P. McDermott was born in New Jersey, U.S.A., in 1993. He received his B.Sc. in Chemistry at The College of New Jersey in 2015. There he studied the synthesis of novel chalcogenide materials for development of new thermoelectric materials with unique crystal structures and magnetic properties. He began working on his Ph.D. in chemistry under the supervision of Prof. Ive Hermans at the University of WisconsinMadison. He currently studies the selective partial oxidations of alkanes using metal-free catalysts. Ive Hermans obtained his M.Sc. in chemistry from K.U. Leuven University in Belgium in 2002. There, at K.U. Leuven, under the supervision of Profs. Pierre Jacobs and Jozef Peeters, he studied the mechanism of catalytic autoxidation processes and obtained his Ph.D. alongside a Masters in Business Administration in 2006. He then pursued postdoctoral work under Prof. Alfons Baiker and continued pursuing work in catalytic autoxidation. In 2008, he became Assistant Professor at ETH Zurich, and in 2013, he accepted a Professorship at UWMadison with appointments in both the Department of Chemistry and the Department of Chemical and Biological Engineering. He has received several awards for his work within the area of catalysis, such as the 2009 ExxonMobil Chemical European Science & Engineering Award (2009) and the 2014 Emerging Research Award by the American Chemical Society Division of Energy and Fuels.

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