Recent Advances in Selective Propylene Oxidation over Bismuth

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Recent Advances in Selective Propylene Oxidation over Bismuth Molybdate Based Catalysts: Synthetic, Spectroscopic, and Theoretical Approaches Paul Sprenger,† Wolfgang Kleist,†,‡,§ and Jan-Dierk Grunwaldt*,†,‡ †

Institute for Chemical Technology and Polymer Chemistry, Karlsruhe Institute of Technology (KIT), 76131 Karlsruhe, Germany Institute of Catalysis Research and Technology, Karlsruhe Institute of Technology (KIT), 76344 Eggenstein-Leopoldshafen, Germany

‡

ABSTRACT: The selective oxidation of propylene to acrolein is an important reaction in the chemical industry which has been extensively studied over the last few decades. Today, spectroscopic, computational, and synthetic approaches allow a renewed view of this established and well-understood catalytic process at a fundamental level. Consequently, a revised mechanistic pathway for the selective propylene oxidation over bismuth molybdates has been suggested recently. Furthermore, studies concerning the local interaction of specific surface entities as well as concepts from semiconductor science have provided valuable information to describe the operation mode of oxidation catalysts. New synthetic methods can be used not only to tune the specific surface area and surface species of a catalyst but also to give direct access to distinct metal oxide phases or specific crystalline phases with a synergetic interplay on the nanoscale. Since complex multicomponent systems, which exhibit both higher selectivity and activity in comparison to pure bismuth molybdates, are used for industrial applications, it is important to transfer the research concepts from such model systems to those more complex systems. This also involves operando characterization techniques on multiple length scales. Recent research activities shine a renewed light on this well-studied reaction, which therefore may become one of the drivers in selective oxidation catalysis to apply and further establish new tools that have been developed in theory, modeling, synthesis, and operando spectroscopy. KEYWORDS: selective oxidation, acrolein, structure−activity relationship, operando spectroscopy, theoretical modeling

1. INTRODUCTION Selective oxidation reactions represent a very important class of transformations for the production of intermediates in chemical industry.1 Among them, the oxidation of propylene to acrolein or acrylic acid and the ammoxidation to acrylonitrile received strong attention, and these processes have been improved by a combination of catalyst preparation, formulation on a micro and macro scale, and understanding through characterization and theory, including multiscale modeling. This trend is exemplified by considering acrolein as a product, which is both an intermediate for the synthesis of acrylic acid and the starting substrate for the production of the important amino acid methionine.2 For example, Evonik Industries AG aims to produce 730000 tons of methionine per year by 2019 since it is a valuable compound for animal nutrition.3 The majority of today’s acrolein production is based on the selective oxidation of propylene over bismuth molybdate based oxidic multicomponent catalysts.4 The process is based on the work of Callahan et al.5,6 at the SOHIO Company (The Standard Oil Co. of Ohio; today, a part of BP) in the 1960s. A great deal of research on this topic was performed during the 1980s and 1990s and has been excellently reviewed in earlier works.7,8 A renewal of the research activities can be seen with each possibility to fine-tune and © XXXX American Chemical Society

understand new facets of the catalysts or new feedstocks. Recently, remarkable attention was drawn toward acrolein synthesis since the dehydration of glycerol in the presence of acidic catalysts came into focus.9,10 Although using a renewable feedstock such as glycerol seems highly attractive in terms of green chemistry,11 the classical approach via the selective oxidation of propylene still offers many facets and opportunities to improve this process, which is the basis for renewed interest.12 Since the substrate propylene is a component of natural gas and the secondmost abundant product from the petrochemical industry with an estimated demand of 89 million tons,13 both the supply and quality of the propylene feedstock are based on wellestablished infrastructures. However, obtaining a deeper understanding of the catalytic process from both the theoretical and the experimental side is important in order to improve the process on an already high level, inhibit side reactions, and reduce waste formation, pushing the selective propylene oxidation reaction toward becoming a greener process. Received: April 7, 2017 Revised: June 30, 2017 Published: July 6, 2017 5628

DOI: 10.1021/acscatal.7b01149 ACS Catal. 2017, 7, 5628−5642

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Figure 1. Three systems for the selective propylene oxidation, representing different stages of complexity and demands regarding research aspects.

Figure 2. (a) The bismuth center stabilizes the LUMO and destabilizes the HOMO of a neighboring molybdate unit. Thus, the interactions of the shown frontier orbitals on the Bi(OH)3 and MoO2(OH)2(H2O) clusters are repulsive in the singlet spin state but attractive in the triplet spin state. Reprinted with permission from ref 36. Copyright 2013 American Chemical Society. (b) Initial hydrogen abstraction happens at the bismuth-stabilized MO site (M = Mo, V), forming a symmetric π-allyl intermediate. Reprinted with permission from ref 44. Copyright 2014 American Chemical Society.

2. MECHANISM AND KINETICS OVER BISMUTH MOLYBDATE CATALYSTS: STATE OF THE ART Modern industrially relevant catalysts for the selective oxidation of propylene toward acrolein are based on bismuth molybdates.4 Depending on the Bi2O3 to MoO3 ratio, the three basic bismuth molybdate phases α-Bi2Mo3O12, β-Bi2Mo2O9, and γ-Bi2MoO6 can be obtained.16 These phases differ in both their elemental composition and crystal structure,17 resulting in different catalytic properties, as will be discussed in section 3. Interestingly, both, pure MoO318 and pure Bi2O319 show only limited activity for propylene oxidation to acrolein, unless they are mixed together. However, the mechanism for this selective oxidation over bismuth molybdates is still a matter of debate. Grasselli and co-workers proposed a mechanism for the reaction of propylene and oxygen over bismuth molybdate catalysts, which is described as follows:20−22 Early studies by Adams and Jennings23 and Sachtler24 showed that propylene is first adsorbed and an α-H abstraction of a hydrogen atom from a methyl group gives a symmetric π-allyl intermediate. Isotopic studies revealed that the initial hydrogen abstraction is the ratedetermining step.6,23 Furthermore, McCain et al.25 demonstrated the symmetric character of the π-allyl intermediate in experiments with 13C-labeled propylene. This activation occurs via a bismuth site, whereas the π-allyl radical is then coordinated to a molybdenum ion, bridged by an oxygen atom (Bi−O−Mo). Mo-bound lattice oxygen (MoO) is then inserted, introducing a C−O bond by forming a Mo-bound allyl alkoxide. A second hydrogen abstraction step followed by desorption of the product acrolein leaves the reduced molybdenum site, which is then reoxidized by bulk or gas-phase oxygen. In this four-electron

In this Perspective, we aim to point out recent developments resulting from mechanistic, synthetic, and spectroscopic studies and demonstrate that the use of new tools offers many opportunities. Starting from basic bismuth molybdates, we will highlight recent findings and move toward open questions and future research aspects regarding more complex multicomponent mixed-metal oxides. What makes a catalyst suitable for the selective oxidation of propylene? An excellent starting point for this question is given by the “seven pillars”, a set of guidelines valid to describe influencing factors for various selective oxidation reactions, as outlined by Grasselli.14,15 The principles address certain demands of the process and catalyst, regarding lattice oxygen, metal−oxygen bond strength, host structure, redox behavior, multifunctionality of active sites, site isolation, and phase cooperation.14 For propylene oxidation, most of the “seven pillars” guidelines are met for bismuth oxide based catalysts.2 In order to understand the reaction, many earlier studies focused on the mechanism, selectivity, and activity of pure bismuth molybdate phases. Today, methods such as operando spectroscopy including X-ray absorption spectroscopy (XAS) or computational approaches such as density functional theory (DFT) allow a review of these findings on a fundamental level. However, since complex multicomponent systems, which combine both higher selectivity and activity in comparison to pure bismuth molybdates, are used for industrial applications, it is important to extend the research from ideal systems toward those more complex or even hierarchically structured systems. As illustrated in Figure 1, multicomponent systems consist of multiple metal oxide phases, forming a composite of selective, unselective, promoting, and inert phases. 5629


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Figure 3. Energy diagram for the individual reaction steps of the propylene oxidation as proposed by Bell and co-workers.36,38,42 Figure based on ref 42.

reaction, Bi3+ and Mo6+ sites are both reduced to Bi2+ and Mo5+/ Mo4+, respectively.26 Since the mechanism requires partially reduced sites, their redox behavior is an important parameter for the catalytic activity.27 Several authors showed that the propylene oxidation originates from lattice oxygen and not from gas-phase oxygen in experiments with isotopically labeled oxygen, 18 O2.28−30 Thus, it is generally accepted that the selective propylene oxidation follows a Mars−van Krevelen mechanism31 with nucleophilic lattice oxygen as the oxidizing agent. This leaves a vacancy in the lattice, which is then refilled by molecular O2 through oxidation of the transition metal on one site. The oxygen anions can migrate through the lattice, and thus, acrolein can be oxidized at a different site.32 In fact, Ueda et al.33 described the kinetics of this reaction to be first order with respect to propylene and zero order with respect to molecular oxygen, as refilling of the vacant sites proceeds rapidly. In order to gain further understanding of the key factors influencing the initial rate-limiting step of hydrogen abstraction at Bi−O sites, Kou et al.34 imitated such molecular structures using homogeneous analogues, namely bismuth aryloxides. However, as Arora et al.35 pointed out during the 1990s, there had always been a disagreement as to which metal centers actually contribute to the redox cycle. Recently, Bell and coworkers proposed a new mechanism in remarkable studies utilizing advanced spectroscopic and modeling tools. By using DFT36,37 and XAS in combination with kinetic studies,38 they concluded that only Mo sites are reduced within the reaction cycle. Their studies focused on scheelite-type Bi1−x/3V1−xMoxO4 (x = 0, 1) systems, which include α-Bi2Mo3O12, and were based on previous theoretical investigations on propylene activation on various oxobismuth and oxomolybdenum structures by Goddard and co-workers.39,40 In their proposed mechanism, propylene is adsorbed and the α-H abstraction occurs at a molybdenyl oxygen, which is stabilized by the orbitals of a neighboring bismuth cation. As shown in Figure 2a, the propylene activation occurs via a spin-coupled singlet to triplet transition. Again, a symmetric π-allyl radical intermediate is formed (Figure 2b) followed by an immediate O insertion of a molybdenyl oxo group (MoO), giving an allyl alkoxide bond to Mo. A second hydrogen abstraction results in the product acrolein. After desorption of acrolein and water, two reduced Mo4+ centers are present, which can be reoxidized by molecular oxygen from the gas phase. The authors did not observe any reduction of the Bi3+ centers. Thus, the proposed mechanism includes the reduction of two Mo6+ centers to Mo4+. However, bismuth needs to be

present in order to provide a suitable electronic and structural environment for propylene oxidation. As Licht et al.41 pointed out recently, the presence of bismuth in Bi2Mo3O12 enables a certain five-coordinate Mo geometry, which is significantly more active for hydrogen abstraction. This explains why pure MoO3 is barely active for propylene oxidation. At higher temperatures (400 °C), the kinetics of the reaction seems to be, again, first order with respect to propylene and zero order with respect to gas-phase oxygen. At lower temperatures (340 °C) a zero-order dependence with respect to propylene and a positive-order dependence with respect to molecular oxygen have been postulated.38 Licht and Bell42 confirmed by DFT that the ratedetermining step is the initial hydrogen abstraction from the methyl group of propylene, preferably at bismuth perturbed MoO groups with an initial activation barrier of 162.3 kJ mol−1. Figure 3 shows the energy diagram and the reaction steps for this newly proposed mechanism. Recently, Bell and coworkers extended this concept to the ammoxidation of propylene. Again, they combined DFT42 and kinetic studies43 in order to reveal a reaction mechanism, where the initial ratedetermining steps are generally similar to the oxidation. In the presence of ammonia, the reaction turns toward the energetically favored ammoxidation, giving acrylonitrile as product. The rate is first order in propylene and zero order in oxygen and ammonia partial pressures, as long as they are present in stoichiometric ratios.

3. ORIGIN OF SELECTIVITY AND ACTIVITY FOR VARIOUS BISMUTH MOLYBDATES On comparison of the pure bismuth molybdate phases, αBi2Mo3O12, β-Bi2Mo2O9, and γ-Bi2MoO6, there is still an ongoing debate about their selectivity and activity toward the conversion of propylene to acrolein.45,46 As mentioned earlier, the three phases show different crystal structures. While γBi2MoO6 is an aurivillius-type phase, β-Bi2Mo2O9 and αBi2Mo3O12 have a (distorted) scheelite structure.17 The metastable β-Bi2Mo2O9 phase generally requires high calcination temperatures (560 °C) for its formation, which usually leads to low surface areas, whereas the α and γ phases are already accessible at lower temperatures.47 Most studies agree that βBi2Mo2O9 is the most active of the three phases,2,27 but its activity depends on the calcination temperature.48 However, there are also studies claiming that the α phase45 or the γ phase49 is more active. In general, many factors need to be taken into account. For instance, van Well et al.50 showed that the activity of γ5630


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The authors were able to formulate a reaction network on the basis of the kinetics of the byproduct formation, which is shown in Figure 4. In order to further understand the formation of

Bi2MoO6 catalysts strongly depends on the calcination conditions and is very sensitive toward a slight excess of bismuth. The differences between selectivity and activity are closely linked to the mechanism and, thus, to the nature of the active sites, which are defined by the component phases. Recently, Zhai et al.51 showed by kinetic studies that the energy barrier for an aurivillius phase, such as γ-Bi2MoO6, is 6.3 kJ mol−1 higher than for a scheelite phase, such as α-Bi2Mo3O12. This difference is attributed to the lower heat of propylene adsorption on the latter. However, other studies found that the kinetics for a reoxidation27 within the catalytic cycle and the number of oxygen vacancies32,52,53 are higher for γ-Bi2MoO6. A suitable redox behavior, which includes a quick removal and readdition of lattice oxygen, and a suitable host structure allowing a rapid transfer of electrons, vacancies, and O2− diffusion are two fundamental factors as described in the “seven pillars” by Grasselli.14 Overall, there is no simple answer to which phase is more active than the others. For comparison, other parameters such as the synthesis method, preconditioning, crystallite size, phase transitions, surface area, measurement conditions, and other parameters need to be considered. For industrial applications, the formation of byproducts on a fundamental level is a particularly important factor. Propylene and ethylene derivatives with functional aldehyde, ketone, or acid groups as well as CO, CO2, and acrylic acid occur as side products.54 The formation of CO, CO2, and acetaldehyde was discussed by Zhai et al.38 The authors showed that the products are not formed as a consequence of acrolein overoxidation but rather by an oxidation of the surface-bound allylic species through gas-phase molecular oxygen. This unselective oxidation occurs in parallel to acrolein formation by a different mechanistic pathway.29,32 Apart from molecular oxygen, lattice oxygen species can also trigger unwanted side reactions. The principle of site isolation, which was introduced by Grasselli,14,15 describes the local availability of lattice oxygen around the reactive catalyst center as the origin of unselective behavior. Hence, it is important to minimize the presence of lattice oxygen species, which are not required for propylene oxidation. For instance, γ-Bi2MoO6 shows improved lattice oxygen transport properties in comparison to the other bismuth molybdate phases but is generally less selective due to nonisolated vicinal Bi3+ sites. This results in an overly high concentration of lattice oxygen that is locally available around the active molybdenum center and, consequently, an overoxidation.15 Therefore, a partially reduced surface state is supposed to be beneficial in order to reduce the formation of byproducts.55 The oxidation of acrolein to acrylic acid was studied by Serwicka et al.56 and Tichý57 over the molybdenum-based heteropolyacid K3PMo12O40 and over MoO3−V2O5, respectively, and modeled by Wong et al.58 They concluded that the oxidation of acrolein is caused by its chemisorption to lattice oxygen sites and the formation of a surface acrylate species that reacts with a surfacebound proton. Tichý57 determined the surface protonation, the binding strength of the transition complexes, and the binding energy of lattice oxygen as important factors for the selectivity. From a mechanistic point of view, chemisorbed acrolein may undergo a C−H or C−C bond cleavage enabled by neighboring lattice oxygen sites, leading to acrylic acid or ethylene as byproducts, as shown by Bui and Bhan for a multicomponent bismuth molybdate catalyst.59 Minor byproducts, such as higher linear or cyclic hydrocarbons, may be formed by C−C bond formation of allyl alkoxide and allyl intermediates.59 Furthermore, Bui et al.54 investigated the mechanistic origins of unselective oxidation products in the presence of α-Bi2Mo3O12.

Figure 4. Proposed reaction network for the oxidation of propylene on α-Bi2Mo3O12 at 350 °C: (black arrows) reactions whose rates are independent of water pressure; (solid blue arrows) reactions promoted by water; (dashed blue arrows) reactions inhibited by water. Reprinted with permission from ref 54. Copyright 2016 American Chemical Society.

different reaction products and sequels over various phases and multicomponent systems, Schlögl60 called for an extensive formulation of such reaction networks. For pure phases of αBi2Mo3O12, β-Bi2Mo2O9, and γ-Bi2MoO6, acrolein selectivity typically decreases with increasing propylene conversion, as shown in Figure 5a. However, as mentioned above, the selectivity is governed by diverse factors. Thus, hydrothermal synthesis of bismuth molybdates at various pH values of 5−7 gave γ-Bi2MoO6 catalysts with different selectivity values at similar propylene conversions, as shown in Figure 5b and discussed in section 6.2. Here, the acrolein selectivity does not necessarily correlate with the propylene conversion. The influence of dopants and the interaction with other phases within bismuth molybdate based multicomponent catalysts adds further complexity but also presents interesting possibilities to control both selectivity and conversion, as discussed in section 4.

4. MULTICOMPONENT SYSTEMS The composition of an industrially relevant bismuth molybdate based, multicomponent catalyst is very complex, since the catalyst has to meet various criteria.62 These catalysts consist of multiple metal oxide phases, forming a composite of selective, promoting, inert, and unselective phases.63 Moro-Oka, Ueda, and Lee highlighted in their reviews64,65 that bismuth molybdates are more active for propylene oxidation when they are intermixed with other elements and phases, especially on the basis of divalent and trivalent transition-metal ions with small ion radii. A general composition of multicomponent systems can be described as Bi−Mo−MI2+−MII3+− (MIII+−X−Y−)O (e.g., with MI2+ = Co, Ni, Fe, Mg; MII3+ = Fe, Cr; MIII+ = Na, K, Cs; X = Sb, Te, W, Nb, and Y = P, B).66 Comprehensive lists of related patents for such catalysts are given in the literature.4,10 Remarkably, three-component mixtures such as Bi−Mo−MI2+ and Bi−Mo−MII3+ never exceeded the activity of pure bismuth molybdates, but four-component systems of Bi− Mo−MI2+−MII3+ did.65 A popular and well-studied example is 5631


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Figure 5. Acrolein selectivity over propylene conversion for (a) α-Bi2Mo3O12, β-Bi2Mo2O9, and γ-Bi2MoO6 prepared by flame spray pyrolysis and (b) bismuth molybdates prepared by hydrothermal synthesis at various pH values. Catalyst samples (500 mg) were tested at (a) 360 °C and (b) 320 °C in a fixed-bed reactor with 50, 80, and 120 N mL min−1 flow of 5/25/70 C3H6/O2/N2. (a) Reproduced from ref 61 with permission from The Royal Society of Chemistry. (b) Reproduced from ref 49 as licensed under the Creative Commons Attribution (CC BY 4.0).

BiMo12Co8Fe3Ox.66−69 This system consists of different phases, serving various purposes with α-Bi2Mo3O12 (selective phase) and β-CoMoO4 and FeMoO4/Fe2Mo3O12 (promoting phases).1,64 In particle models suggested by Wolfs and co-workers,70,71 the surface of an active particle seems to be rich in bismuth molybdates or molybdenum oxide,26,72 while the bulk contains cobalt- and iron-based phases, as illustrated in Figure 6. Merzlikin

et al.73 showed by low-energy ion scattering (LEIS) that the outermost surface is exclusively rich in MoOx and KOx sites, but not in BiOx, if bismuth molybdates are mixed with alkaline metals. The Fe2+/Fe3+ redox couple, present as FeMoO4/ Fe2Mo3O12, allows adsorption of molecular oxygen, reduction to nucleophilic surface oxygen, incorporation into the lattice, and transfer to the selective bismuth molybdate phases by oxygen migration through bulk diffusion and oxygen spillover at the phase interface, as illustrated in Figure 6.74 Since the oxygen sites for incorporation and reaction of lattice oxygen are isolated and the phases cooperate in a way where electrons, lattice oxygen, and anion vacancies can move between them, this interaction can be described by two of Grasselli’s “seven pillars”: namely, site isolation and phase cooperation.15 Furthermore, this phase interaction also exists for pure bismuth molybdates. This synergy effect also occurs when an α phase or β phase is mixed with the γ phase.45,75−77 Similar to multielement systems, the reaction is facilitated by an oxygen spillover from the γ phase to the α phase, which improves filling of oxygen vacancies77,78 and increases the electric conductivity.76 In a multicomponent system, β-CoMoO4 structurally stabilizes the Fe2+/Fe3+ redox couple since it is isostructural with the formed β-FeMoO4.79 The system BiMo12Co8Fe3Ox also hosts other phases such as Bi3FeMo2O12 or monometallic oxide phases. It is of crucial importance to reveal the role of these by-phases and to identify unselective phases, e.g. CoOx,63 which may be formed during the reaction. Various multielement systems can form single-phase systems instead of multiphase mixtures. Bismuth molybdates mixed with vanadium or tungsten form homogeneous phases and show the same reaction mechanism, as explained in section 3. Here,

Figure 6. Simple particle model of a Bi−Mo−Co−Fe mixed-oxide catalyst and principle of active oxygen migration through the bulk phase to the active bismuth molybdate site. Figure based on ref 64.

Table 1. Selection of Benchmarks, Illustrating the Improvement of Bismuth Molybdate Based Catalysts in Terms of Conversion and Selectivity

a

year

catalyst composition

propylene conversn/%

acrolein selectivity/%

process paramsa

ref

1960 1970 1979 1985 2006

Bi−P−Mo−O Bi−Mo−O on SiO2 Bi−Mo−Ni−Co−Fe−K−P−O Bi−Mo−Fe−Co−W−K−Si−O Bi−Mo−Fe−Co−Ni−Mn−K−P−Al−Si−Sm−O

57 65

79 84

fluidized-bed reactor, RT = 3 s, 425 °C fixed-bed reactor, RT = 1.5 s, 465 °C fixed-bed reactor, RT = 3 s, 350 °C fixed-bed reactor, RT = 2 s, 300 °C fixed-bed reactor, RT = 2.5 s, 333 °C

5 99 100 101 102

yield: 78 97 97

86 90

RT = residence time. 5632


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general description of how electronic properties are suitable to describe a catalyst is given by Haber and Witko.1,106 When a catalyst shows semiconducting properties, it is defined by the positions of its valence and conductivity bands and the electrochemical potential (Fermi level) located between these bands. This material takes electrons from an adsorbed hydrocarbon and transfers them to adsorbed oxygen. Thus, the two redox pairs should be situated above the Fermi level (hydrocarbon) and above the conductivity band (oxygen), respectively. The probability of the selective oxidation processes is a function of density of states of the valence and conductivity bands relative to the redox potential of the adsorbed species.1 Getsoian et al.44 calculated the band gap of scheelite phase bismuth molybdate based catalysts and found a linear correlation between the band gap and the activation energy for the selective oxidation of propylene. Zhai et al.51 later extended this study toward aurivillius phase systems. The observed energy barriers were generally about 6.3 kJ mol−1 higher for catalysts with the aurivillius phase than for the scheelite phase structure. Catalysts with low band gaps also showed a higher activity for propylene oxidation per active site. However, the selectivity to acrolein dramatically decreased for band gaps below 2.1 eV, but above 2.1 eV, it did never exceed 82%.51 The electrical conductivity that can easily be determined correlates to the band gap and represents to some extent oxygen and electron mobility.94 Several studies showed a correlation between the selectivity and the electrical conductivity of bismuth molybdate catalysts.76,107,108 Doping, or the introduction of defects, offers opportunities to influence the electronic properties of oxidation catalysts. Thus, Kühn et al.109,110 modified (Mo,V)5O14 by substituting oxygen with nitrogen atoms. The enhanced reducibility of the oxide−nitride model compounds correlated with their increased conductivity and selectivity toward acrolein. Recently, Le et al.94 triggered various conductivities by adding β-Bi2Mo2O9 to highly conductive supports, such as SnO2, with different methods. Again, conductivity and activity for propylene production did correlate. Nonetheless, the number of active sites, represented by the specific surface area, is still an important factor in order to achieve high catalytic activity.76 Furthermore, electrical conductivity or impedance measurements should be performed under reactionlike conditions. Eichelbaum et al.111 outlined that the bulk electronic conductivity may not be representative under reaction conditions and suggested the gas-phase-dependent surface potential barrier as a descriptor for the selective alkane oxidation. It defines the potential gradient, the so-called band bending, at the surface−bulk interface, which charge carriers need to overcome. It is caused by a surface adsorbate and takes into account the influence of the hydrocarbon molecule on the catalyst. Thus, under working conditions, the electronic structures of bulk and surface differ. Heine et al.112 showed that MoVTeNbOx forms a surface layer under alkane oxidation conditions, which is structurally and electronically different from the bulk phase. As illustrated in Figure 7, the surface barrier changes for different gas phases, as shown for ethane and nbutane, and controls the charge carrier density of the active V4+/ V5+ redox couple within the termination layer. Thus, realistic gasphase compositions need to be taken into account.

vanadate or tungstate ions replace molybdate anions in the crystal structure and the aurivillius or scheelite phases are retained.51 Bismuth molybdates mixed with large radii cations lead also to homogeneous phases with a scheelite structure. As shown by Licht et al.,41 cations such as La3+, Sb3+, and Pb2+ + H+ can replace Bi3+ and show a similar electronic perturbation of the neighboring molybdenum, facilitating the initial hydrogen abstraction within the mechanism. Another example is the scheelite-structured Bi3FeMo2O12.80 Although various combinations of mixed-metal oxides show an activity for propylene oxidation toward acrolein, bismuth molybdate based systems are the most active ones and their elemental composition has been fine-tuned over the last few decades. Table 1 demonstrates the stepwise improvement of bismuth molybdate based systems by fine-tuning its complexity (see also Figure 1). For a more detailed list of the performance of commercial acrolein catalysts, the reader is referred to a recent review by Liu et al.10 Recent approaches toward systems with different chemical composition are limited and are more of academic interest. For instance, other bulk metal oxide catalysts for the selective oxidation of propylene to acrolein are Mo−Te− O,81 U−Sb−O,82 Fe−Sb−Ti-O,83 Fe−Sb−O,84 and V2O5/ Nb2O5,85,86 while different systems include CuOx/SBA-15,87 FeOx/SBA-15,88 CuAu/SiO2,89,90 Au/MgCuCr2O491 and poly(azomethines) doped with heteropolyacids.92,93 The described multicomponent systems performed well in laboratory tests. However, a support material such as SiO2 or Al2O3 needs to be added in order to increase their mechanical strength and to shape them into catalyst bodies, which is necessary for an industrial application.2 Studies on the interaction of bismuth molybdates supported on SnO2/ZrO2/ MgO94 and SiO295,96 or MoO3 supported on Al2O3/TiO2/ SiO297 and silica-SBA-1598 indicated that these supports were not inert but also provided lattice oxygen for the selective oxidation to some extent.

5. HOW “GOOD” IS A CATALYST? IDENTIFYING APPROPRIATE PARAMETERS In order to find and rationally design catalysts which oxidize propylene in a selective and active manner, it is important to identify and understand key parameters that are most decisive in the reactive behavior.44 Jung et al.52,103 showed that the oxygen mobility within the lattice correlated with the catalytic activity for the dehydrogenation of n-butene by studying the oxygen mobility of γ-Bi2MoO6 with TPRO experiments. For scheelite phase bismuth molybdates, Ueda et al.33 showed that a correlation between the catalytic activity and the mobility of lattice oxide ions exists but cannot be used as a descriptor, since the reaction is zero order in the partial pressure of oxygen. Furthermore, the ratedetermining step, the initial hydrogen abstraction, does not involve lattice oxygen.38 In conclusion, the selectivity in this Mars−van Krevelen mechanism reaction cannot be simply associated with oxygen mobility.104 The influence of the surface acidity is barely an issue. Nonetheless, the catalyst requires acid− base properties for the C−H activation.93,105 Bing et al.77 showed for various bismuth molybdates that no changes in Lewis acid sites (cations) appeared, which could explain the synergy effect. A brief summary of further possible descriptors has been given by Getsoian et al.44 Recently, concepts from semiconductor physics have received strong attention in order to describe heterogeneous catalysts. A

6. CATALYST SYNTHESIS While the elemental composition describes a catalyst on an atomic level, the synthesis method defines material properties on larger scales.113 It affects, for instance, the catalyst’s local 5633


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Figure 8. Overview of conventional and novel preparation techniques for the synthesis of bismuth molybdates.

obtained nanorods of α-Bi2Mo3O12 via a sonochemical route with a calculated surface area of 65 m2 g−1.123 As mentioned, mixing bismuth molybdate phases with cobalt and iron molybdates can increase the surface area up to 20 m2 g−1.66,125 Though there have been different motivations behind the synthesis techniques described in the following sections, hydrothermal synthesis (see section 6.2) and flame spray pyrolysis (see section 6.3) also resulted in catalysts with high specific surface areas. The main challenge is to maintain the high surface area during activation and reaction. 6.2. Controlling the Formation of Versatile Phases. Recently, hydrothermal synthesis gained a great deal of attention as a mild and soft method to prepare nanostructured catalysts with interesting material properties at low temperatures. General reviews on hydrothermal synthesis can be found elsewhere.126,127 Beale and Sankar128 prepared bismuth molybdates by hydrothermal synthesis under halide-free conditions and at low temperatures (140 °C). α-Bi2Mo3O12 and γ-Bi2MoO6 resulted with specific surface areas of 8−10 m2 g−1. However, calcination at 560 °C was necessary to obtain β-Bi2Mo2O9 with a surface area of 3 m2 g−1. Using time-resolved energy dispersive X-ray diffraction, a three-dimensional growth of irregular α-Bi2Mo3O12 and a two-dimensional growth of platelike γ-Bi2MoO6 particles was observed. Li et al.129 showed that a low pH value and a high molybdenum concentration favor the formation of α-Bi2Mo3O12 and a high pH value while a high bismuth concentration lead to the formation of γ-Bi2MoO6. The strong dependence of the product on pH, concentration, and temperature can be explained by the stability of various polymolybdate ions under different hydrothermal conditions.130,131 For γ-Bi2MoO6, Kongmark et al.132,133 observed the formation of spherical particles and later plateletlike particles when the tetrahedral [MoO4]2− precursor was transformed to octahedral [MoO 6 ] species via a Bi2n+4MonO6(n+1) (n = 3−6) intermediate, as shown in Figure 9. Since the layered aurivillius structure of γ-Bi2MoO6 features a strong adsorption band within the visible light region, many studies on this material were related to photocatalytic applications.134−136 Schuh et al. successfully applied hydrothermally prepared MoO3137 and bismuth molybdates49,138 for the selective oxidation of propylene. MoO3 showed a rodlike structure, and bismuth molybdates, in particular α-Bi2Mo3O12 and γ-Bi2MoO6, showed a platelike morphology with surface areas of up to 32 m2 g−1. Some of these systems were highly active for propylene oxidation. Varying the synthesis parameters revealed a strong relation between the pH value and the catalytic activity, as shown in Figure 5b. However, β-Bi2Mo2O9 was only accessible via high-

Figure 7. Schematic band diagram of MoVTeNbOx in ethane (blue) and n-butane (red) feed. Definitions: ϕ = work function; eVD = diffusion energy; (Δ)χ = (difference) in electron affinity; Evac = vacuum level; EC = conduction band edge; EF = Fermi level; EV = valence band edge. Reprinted with permission from ref 112. Copyright 2013 American Chemical Society.

structure, surface, crystallite sizes, phase purity, phase interaction, and stability. The synthesis of basic bismuth molybdates and multicomponent systems is classically performed by coprecipitation,45,114−116 spray drying,47,117,118 or solid-state synthesis.119,120 For all of these synthesis methods, high calcination temperatures and harsh conditions are necessary in order to achieve pure crystalline phases. Still, multicomponent systems have a larger surface area than pure bismuth molybdates;66 the calcined systems show low surface areas of below 20 m2 g−1. These synthesis methods are well understood and are used for industrial catalysts. Using more recently established sol−gel76,121 and complexation47,122 synthesis, highly homogeneous catalysts can be obtained as phase-pure materials. However, especially for multicomponent systems, phase interaction and crystallinity of the individual phases become important. Overall, recent approaches concerning synthesis methods of bismuth molybdates and multicomponent catalysts can be assigned to one of the following challenges: (1) increase in the number of active sites and surface area, (2) control of versatile phases, (3) direct access to pure phases without further post-treatment steps, such as calcination, and (4) definition of the phase interaction on a nanoscale. Figure 8 gives an overview of the synthesis methods that are currently in focus and will be discussed in the following. 6.1. Increasing the Number of Surface Sites and Surface Area. The specific surface area of pure crystalline bismuth molybdates is rather low and often lower than 5 m2 g−1.45,46,77,118 This is a result of the high calcination temperatures required for all conventional preparation methods. Since selective propylene oxidation is a surface-related process with absorption and reoxidation, a large surface area is favored. For selective oxidations, the surface area can be correlated with the number of active sites and the catalytic activity.35 In order to obtain high surface areas, Le et al.47 combined citric acid complexation with spray drying and synthesized catalysts with surface areas of 12 and 17 m2 g−1 for α-Bi2Mo3O12 and γBi2MoO6, respectively. Nell et al.123 used mesoporous carbon templates to crystallize Bi(1−x)/3V1−xMoxO4 (x = 0, 1) with surface areas of 14−17 m2 g−1. In another study, Ghule et al.124 5634


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Figure 9. Successive transformation of tetrahedral [MoO4]2− to γ-Bi2MoO6 during hydrothermal synthesis, as identified via in situ combined highresolution powder diffraction, X-ray absorption spectroscopy, and Raman scattering and complementary ex situ experiments.132,133 Reprinted with permission from ref 132. Copyright 2012 American Chemical Society.

temperature calcination, which led to a less active catalyst. Especially with respect to multicomponent catalysts, the knowledge transfer from other hydrothermally prepared molybdates, such as FeMoO4139 and CoMoO4,140 to selective oxidation catalysis seems promising. 6.3. One-Step Synthesis of Phase-Pure Bismuth Molybdates. Coprecipitation, spray drying,50 and hydrothermal synthesis138 require high calcination temperatures (often >400 °C), which are closely linked to a loss in surface area and activity, to obtain phase-pure bismuth molybdates. Schuh et al.61 applied flame spray pyrolysis to synthesize αBi2Mo3O12, γ-Bi2MoO6, and the metastable β-Bi2Mo2O9 in a single step without further treatment or calcination. Flame spray pyrolysis is a novel method that is suitable for producing mixed metal oxide catalysts as well as supported metal catalysts. Reviews on this technique were published, for example, by Mädler and coworkers.141,142 As illustrated in Figure 10, metal precursors are dissolved in an organic solvent and, when the solution is sprayed as an aerosol through a burning methane/oxygen flame, a fine nanoparticle powder is formed at high temperatures but short residence times. The resulting bismuth molybdate catalysts had a large surface area of 18−45 m2 g−1 due to the small particle size of the nanopowder and showed high catalytic activity.61 A few selected results are given in Figure 5a. Another main advantage of this method is the well-controlled stoichiometry of the resulting catalyst. Extending this concept toward multicomponent systems seems very promising. In a similar approach, Farin et al.143 studied the suitability of solution combustion synthesis in order to directly access pure bismuth molybdates. However, complexation of the metal ions with organic ligands and a rapid heating resulted in impurities of additional phases, which made an aftertreatment step still necessary and reduced both acrolein selectivity and specific surface area. 6.4. Phase Interaction on the Nanoscale. Hydrothermal synthesis and flame spray pyrolysis represent promising techniques to obtain nanosized bismuth molybdate based catalysts. As Bell144 pointed out, the activity and selectivity of such catalyst nanoparticles strongly depend on their size, shape, and surface structure. For multiphase catalysts, not only the individual phases but also their interaction should be nanosized, though. As shown for interactions of bismuth molybdates with support material, the degree of intermixing is crucial for the catalytic performance and affects the conductivity.94 As Brazdil et al.145 postulated, minimizing the difference between the chemical

Figure 10. Principle of flame spray pyrolysis for the direct synthesis of pure bismuth molybdate phases with large surface areas, as reported by Schuh et al.61

potentials of the components in the interfacial region leads to a maximum activity. Thus, the interaction of phases brought to the nanoscale seems highly promising. Especially with respect to the activity of multicomponent catalysts, it is important to take into account structural interactions among individual nanosized domains.146 An accurate control of this phase interaction on a nano level is desired and necessary.147 Such an assembling of nanosized constituents to larger structures often takes advantage of synergy and surface effects. For instance, a structured combination of MoO3 nanobelts with Bi2Mo3O12148 and a hierarchical BiWO4 nanoflake assembly with MoO3147 led to catalysts that were highly active in propylene oxidation. The 5635


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molybdate catalyst and allows a direct observation of the active sites. To ensure the applicability of the obtained results, it is advantageous to use real and complex catalytic systems rather than model systems for in situ studies. However, the catalytic performance is not only correlated to the bulk structure. The catalytic conversion is a surface phenomenon and, consequently, the activity is related especially to the composition and structure of the outermost layers of the catalyst particle.104 Hence, techniques such as Raman and XAS as bulk techniques need to be modified to be more surface sensitive, especially in dealing with low surface areas of unsupported crystalline bismuth molybdate based catalysts.35 One option is to increase the surface to bulk ratio. New synthesis techniques may result in homogeneous catalysts with high specific surface areas, as discussed in section 6. X-ray photoelectron spectroscopy (XPS) and low-energy ion scattering (LEIS) can provide information on the surface and the outermost surface layer of a catalyst, respectively.73 Very recently, ambient-pressure XPS was achieved at 1 bar, which may pave the way for the future understanding of surface phenomena.160 Modulation−excitation spectroscopy represents another promising spectroscopic approach to study surface changes of bulk catalysts,161 which may be applied to Raman spectroscopy, 162 XRD, 163 or XAS using the QEXAFS technique.164 Engeldinger et al.79 used a Linkam reaction cell, which is similar to a fixed-bed reactor, and Zhai et al.38 and Getsoian et al.44 used a pellet reactor for their in situ studies. As the selective oxidation of propylene is performed industrially in fixed-bed reactors and shows spatial gradients of the phases, as described in section 8, optical quartz cuvette reactors,165 capillary reactors,166 or microreactors,167 which allow spatially resolved measurements along the catalyst bed, should be preferred. Figure 11 illustrates an example of spatially resolved XAS measurements in the catalytic bed of a 5% Pt−5% Rh/Al2O3 catalyst during catalytic partial oxidation of methane, performed in a capillary reactor. However, special care is required to avoid damage from laser heating during Raman studies. For such purposes, Beato et al.168 developed an in situ cell, where the probed catalyst is constantly renewed and which is like a quasi fluidized bed reactor, that may be advantageous. Furthermore, recent developments in micro- and nanospectroscopic imaging techniques offer new opportunities to study bismuth molybdates under reaction conditions, as illustrated in Figure 12. The variety of these techniques were summarized by Buurmans and Weckhuysen169 and Grunwaldt et al.170 For instance, Crozier et al.171 used environmental-TEM (ETEM) to follow the surface reduction and oxidation of ceriumbased oxide nanoparticles by detecting oxygen vacancies appearing at the surface, as shown in Figure 13. E-TEM may be useful for multicomponent bismuth molybdates in a similar manner. In contrast, synchrotron-based methods such as XRD-, XRF-, and XAS-tomography do not provide information on an atomistic level but on the submicrometer scale. Thus, depending on the resolution, changes within single catalyst particles172 or changes over catalyst bodies173 along the catalyst bed could be monitored. More recently, tomography studies have been performed under quasi in situ conditions,174 making this a very promising technique for future studies.

systems were both synthesized hydrothermally and represent promising examples with respect to multicomponent systems.

7. UNDERSTANDING THE CATALYST UNDER WORKING CONDITIONS: CHALLENGES AND OPPORTUNITIES FOR IN SITU AND OPERANDO CHARACTERIZATION AND IMAGING TECHNIQUES As pointed out, catalysts are dynamic entities rather than static materials. Therefore, in situ and operando techniques are extremely important. In particular, the application of Raman spectroscopy or synchrotron-based X-ray absorption spectroscopy (XAS) in situ or operando, i.e. under realistic gas, pressure, and temperature conditions, has been found to be very powerful to reveal the true nature of intermediate species and active sites in selective oxidation catalysis.113 Since the molybdenum coordination of the individual bismuth molybdate phases differs, XAS of the Mo K edge is suitable to distinguish phases, molybdate species, and oxidation states.149 Beale et al. used a combination of in situ energy-dispersive XRD and XAS to investigate the preparation of bismuth molybdates via spray drying150 and hydrothermal synthesis,128,151 similarly to Kongmark et al.,133 and revealed crucial reaction parameters that were responsible for the generation of undesired intermediates. Especially for hydrothermal synthesis, in situ XAS is powerful for the understanding of the growth mechanisms, as shown by Patzke and co-workers for the synthesis of, for example, MoO3152 and Bi2WO6.153 In contrast, in situ studies on bismuth molybdates during selective oxidation processes themselves are rare. For their mechanistic studies described in section 2, Zhai et al.38 treated α-Bi2Mo3O12 with pure propylene and oxygen in situ and found no reduction of Bi3+ but of Mo6+ to Mo4+, in agreement with their DFT studies.36 Getsoian et al.44 performed similar studies on Bi0.85V0.55Mo0.45O4. Raman spectroscopy is also a valuable method for such in situ studies, especially for transition-metal oxides, as reviewed by Wachs and Bañares.154 In particular, for bismuth molybdates distinct Mo−O−Mo stretching vibrations can be observed for each phase.155 Early studies confirmed the Mars−van Krevelen mechansim by detecting changes in the Mo−O−Mo stretching vibrations due to an incorporation of gas-phase 18O2 into the lattice, as described in section 2.28 As these methods give bulk information, surface-sensitive methods such as in situ DRIFTS are extremely versatile. Infrared spectroscopy could also give insight into the formation of byproducts or deposits on the surface of bismuth molybdates.156 Whereas such studies are often conducted over bimetallic mixed oxides, in situ studies on complex multicomponent systems are generally rare, as Engeldinger et al.79 pointed out. To our knowledge, only one Raman-79 and one Mössbauer-based157 study issued the phase interaction of Bi−Mo−Co−Fe−Ox systems, as described in section 4. In comparison to supported metal oxide catalysis, little is known about the atomic details of selective oxidation catalysts since in situ or operando studies on bulk catalysts have to deal with several challenges.158 Often, supported molybdenum oxides are used as model systems for in situ studies, as shown elsewhere98,158 and reviewed by Wachs and Roberts.159 However, Wachs and Bañares154 pointed out that such systems do not show the same selectivity toward acrolein as bismuth molybdates, which makes them unsuitable as model systems. Zhao et al.86 showed that a V2O5/ Nb2O5 system provides the same features as a bismuth 5636


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Figure 11. Example for spatiotemporal studies showing the evolution of reduced Pt species during the ignition of the catalytic partial oxidation of methane at the end of a catalyst bed of 5% Pt−5% Rh/Al2O3 at 330 °C. The formation of metallic platinum was measured by X-ray absorption spectroscopy with a FReLoN camera at the white line energy (Pt L3edge; 11586 eV). (A) X-ray absorption image recorded below the ignition temperature. (B−F) Images recorded as a function of time. A reddish color indicates lower absorption and thus the formation of a reduced Pt containing species. Adapted with permission from ref 175, copyright 2009, American Chemical Society, and ref 176, with permission from the Royal Society of Chemistry.

Figure 13. E-TEM images of the transformation of an identical region of a (110) surface hosting a series of (111) nanofacets into a smooth (110) surface of cerium-based oxide nanoparticles. The smooth surface can tolerate a high number of oxygen vacancies and Ce3+ ions. The catalyst was treated with 0.6 mbar of H2 and recorded in situ at (a) 270 °C, (b) 730 °C, and (c) 600 °C. Reprinted from ref 171. Copyright 2008, with permission from Elsevier.

lattice.177 Steam improves the reoxidation rates of the catalyst and increases the selectivity to acrolein by preventing the formation of strongly bonded oxygenates on the surface.178 Since the selective oxidation is highly exothermic with a reaction enthalpy of 347 kJ mol−1,179 radial and axial temperature gradients appear within the catalyst bed.180 For kinetic studies, without heat and mass transport limitation, Redlingshöfer et al.179 used a catalytic wall reactor. Studying an industrial multicomponent bismuth molybdate based catalyst, they showed that the influence of water on the reoxidation was crucial below 360 °C. Later they extended their study and formulated a kinetic model showing that catalyst reoxidation is rate determining below 360 °C while catalyst reduction by propylene is the slowest step at higher temperatures.181 The temperature of 360 °C also

8. ADDING FURTHER DIMENSIONS: FROM THE LOCAL STRUCTURE TO THE REAL REACTOR SYSTEM The industrial production of acrolein from propylene is performed in multitubular fixed-bed reactors. While the reactor is usually operated between 300 and 400 °C and at inlet pressures of 150−200 kPa, multicomponent bismuth molybdates reach a conversion of 98%. The feed stream is typically a mixture of propylene, air, and steam in a ratio of 1:8:(1−6).4 The role of steam is still a matter of debate. Surely, water vapor improves the heat transfer within the reactor. However, isotope labeling studies revealed that it actually participates in the catalytic reaction and its oxygen atoms are incorporated within the

Figure 12. Overview of suitable spatially resolved operando spectroscopic and imaging methods to study the selective oxidation of propylene. All of these techniques can provide valuable information on the reactive behavior of multicomponent systems. 5637


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Figure 14. Hot-spot formation in fixed-bed reactors occurring during the exothermic propylene oxidation. These hot spots directly influence the aging process of the bismuth molybdate based multicomponent catalyst. Figure is based on ref 190.

industrial scale. Although the formation of MoO3 enhances the selectivity, it also opens the door for a major deactivation pathway. In the presence of water vapor, also a byproduct of the reaction, volatile MoO2(OH)2 can be formed, which sublimes at higher temperatures.189 Since the catalytic bed is not isothermal, the high-temperature hot-spot zone enables the evaporation of MoO2(OH)2, which then redeposits as MoO3 in colder zones downstream from the catalyst bed. Thus, the active zone slowly depletes in MoO3,190 as illustrated in Figure 14. The reoxidation of FeMoO4 becomes irreversible, and even the formation of catalytically inactive Fe2O3 was observed.191 Since it decreases both the selectivity and the activity of the reaction, this process is called “band-aging”. Thus, the spectroscopic in situ characterization methods described in section 7 in combination with spatially resolved gas-phase reactor profiles192,193 represent promising tools for future studies in order to fully understand and correlate gas composition and bed temperature with the phase composition. Figure 15 shows an example of spatially resolved profiles of the gas composition and temperature along the catalyst bed during the oxidative dehydrogenation of ethane. Spatially resolved experiments need to be complemented by multiscale modeling studies. Touitou et al.194 used threedimensional computational fluid dynamics to correlate spatially

corresponded to the temperature where the acrolein selectivity reached its maximum. For industrial use, other reactor concepts such as fluidized beds, which are typically used for propylene ammoxidation,2 or membrane reactors182 appear attractive, although these systems are currently not in the main focus of research. Fluidized-bed reactors allow for a better temperature control during the propylene oxidation reaction, which is highly exothermic. However, using such a reactor system leads to new demands concerning the catalyst material and changes the catalyst temperature profile completely. Finally, it may cause significantly lower acrolein production rates per reactor volume and catalyst mass.183 In general, riser reactors or two-zone reactors may further improve selective oxidation reaction processes, if frequent catalyst regeneration is required. This was especially shown for the oxidation of butane to maleic anhydride.184,185 Similarly, Patience and Mills183 modeled a circulating fluidized-bed riser reactor for propylene oxidation. In their example, the byproduct formation was higher in comparison to a fixed-bed system. The oxidative dehydrogenation of butylenes over γ-Bi2MoO6 in a two-zone fluidized-bed reactor was recently reported by Rischard et al.186 This concept allows the combination of reactive and regeneration zones and improves both coke removal and catalyst reoxidation. It was shown that particle mixing within fluidized beds caused a permanent exchange of catalyst particles between both zones. Up to now, there has been no experimental proof that such reactor geometries are also beneficial for the oxidation of propylene to acrolein. Future work should be devoted to the comparison of the catalyst active states in the different operation modes. The formation of byproducts is especially critical on an industrial scale. Apart from CO and CO2, acrylic acid is one of the most common byproducts with a yield of 5−10%.4 Unfortunately, acrylic acid polymerizes easily, which makes it difficult to remove such species from the product stream. Other minor byproducts such as aldehydes and ketones can cause reactor fouling, while acetic compounds may facilitate the polymerization of acrolein and acrylic acid.54 Multicomponent bismuth molybdate based catalysts do not consist of one but many phases such as α-Bi2Mo3O12, βBi2Mo2O9, γ-Bi2MoO6, β-CoMoO4, Fe2Mo3O12, Bi3FeMo2O12, and MoO3. Under reaction conditions, such catalysts undergo dramatic changes at the phase interface, as Shashkin et al.187 showed, resulting in the evolution of some new phases while others may disappear. As an example, within the redox catalytic process, the reversible reduction of Fe2Mo3O12 gives FeMoO4 and MoO3. Within certain boundaries, MoO3 formation increased the acrolein selectivity.188 Thus, it takes time until a multicomponent system reaches its equilibrium, especially on an

Figure 15. Species and temperature profiles for the oxidative dehydrogenation of ethane measured through a MoOx/α-Al2O3 bed at 1 bar. Point α marks the maximum C2H4 concentration; point β marks the position of complete O2 conversion (GP = gas phase before and after the catalyst bed, FHS = ceramic front heat shield). Reproduced from ref 196 with permission from The Royal Society of Chemistry. 5638


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Anker Degn Jensen), the University of Zurich (Prof. Dr. Greta Patzke), and the ETH Zurich (Dr. Dorota Kozej, Prof. Dr. Markus Niederberger) for intensive discussions. Furthermore, we are grateful to Dr. Ursula Bentrup (Leibniz Institute for Catalysis, Rostock, Germany) as well as Michael Thomann, Dr. Karsten Portner, and Dr. Achim Fischer (Evonik Nutrition and Care GmbH, Hanau, Germany) for scientific input.

resolved experimental data with simulations of a packed Pt/ Al2O3 catalyst bed during CO oxidation. Dong et al.195 used a similar approach to reveal local inhomogeneities and to simulate the heat transfer along the catalyst bed for ring-type packings and highlighted the relevance of such modeling methods for catalyst beds with small reactor to particle diameter ratios, as they are commonly used for exothermic oxidation reactions.

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9. PERSPECTIVES FOR FUTURE ACTIVITIES IN SELECTIVE PROPYLENE OXIDATION Multicomponent catalysts, which are based on bismuth molybdates but strongly interact with further metal oxide phases, are needed for the selective oxidation of propylene to acrolein. Recently, the mechanistic pathway over bismuth molybdates was revised and the nature of the bismuth site was further elucidated. The selectivity and activity of the individual bismuth molybdate phases do not only depend on the composition; in addition, further material properties need to be considered. Meanwhile, principles from semiconductor physics proved themselves valuable to describe the potential of a catalyst. New synthetic methods allow control over, for instance, the specific surface area, the direct access to distinct phases, or the interaction of various phases on a nanoscale. The latter is especially relevant for multicomponent systems, which exceed the complexity of simple bismuth molybdates or MoO3 by far but are versatile to understand the catalyst during operation. Here, especially interfaces and interactions of phases and the influence of the support material are of interest. In realistic and large-scale applications, hot-spot formation leads to temperature gradients, changes in catalyst phase composition, and variations of product stream composition along the catalyst bed. Novel in situ spectroscopic and imaging techniques have been developed, offering new possibilities to understand phase interaction, deactivation processes, and conditioning, even in a spatially resolved manner. Such hierarchical characterization needs to be complemented by multiscale modeling starting from the active site in an ensemble toward the full system. This perspective shows that, in spite of the fact that selective oxidation of propylene constitutes a key process for the chemical industry, still many open questions remain. As the “European Roadmap for Catalysis”197 and an initiative of the German Catalysis Society call to strengthen the research on selective oxidation reactions,198 a joint effort from various disciplines is necessary while new tools emerge to tackle these questions.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail for J.-D.G.: [email protected]. ORCID

Wolfgang Kleist: 0000-0002-9364-9946 Jan-Dierk Grunwaldt: 0000-0003-3606-0956 Present Address

§ Laboratory of Industrial Chemistry, Ruhr-University Bochum, 44801 Bochum, Germany

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank present and former colleagues at the Karlsruhe Institute of Technology (Matthias Stehle, Dr. Thomas Sheppard, Dr. Kirsten Schuh, Prof. Dr. Olaf Deutschmann), the Technical University of Denmark (Dr. Martin Høj, Prof. Dr. 5639


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