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Unit Synthesis Approach for Creating High Dimensionally-Structured Complex Metal Oxides As Catalysts for Selective Oxidations Satoshi Ishikawa, Zhenxin Zhang, and Wataru Ueda ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02244 • Publication Date (Web): 23 Feb 2018 Downloaded from http://pubs.acs.org on February 23, 2018

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ACS Catalysis

Unit Synthesis Approach for Creating High Dimensionally-Structured Complex Metal Oxides As Catalysts for Selective Oxidations Satoshi Ishikawaa, Zhenxin Zhanga, Wataru Uedaa*

a

Department of Material and Life Chemistry, Faculty of Engineering, Kanagawa

University, 3-27, Rokkakubashi, Kanagawa-ku, Yokohama, 221-8686, Japan

ABSTRACT: Solid-state catalysts for selective oxidations have been prepared by simple traditional protocols including impregnation, precipitation, solid state reaction, etc. Although this catalyst preparation protocols are convenient to form new or improved catalysts, huge number of practical experiments have been inevitably necessary. This situation will be even more hard since solid-state catalysts become complex more and more in order to introduce multifunction. It is time that catalyst researchers should devote their efforts to establish new synthetic methodology to meet the situation and ultimately for new catalyst creation. Here, we propose a new catalyst synthesis approach, that is, unit synthesis, to produce high dimensionally structured crystalline complex metal oxide catalysts. By applying this synthesis methodology, we have been able to synthesize several new crystalline complex metal oxides active for

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catalytic selective oxidations. In this perspective, crystalline Mo3VOx oxides and crystalline porous material based on ε-Keggin polyoxometalates (POM) are introduced as successful examples of the unit synthesis. We found that the former shows extremely high catalytic activity for ethane oxidation to ethane and acrolein oxidation to acrylic acid with molecular oxygen and the latter for methacrolein oxidation to methacrylic acid.

KEYWORDS: Unit synthesis, Hydrothermal method, Transition metal, Complex metal oxides, Catalytic selective oxidation

1.

Solid-state catalysts for catalytic selective oxidation Catalytic selective oxidation of hydrocarbons is an attractive reaction since the

reaction produces valuable chemicals such as alcohols, aldehydes, acids, etc [1-3]. Recently, the selective catalytic oxidation of light alkanes has attracted much attention due to the shale gas revolution and to the concern about petroleum [4-5]. However, success in the reactions has been limited so far despite tremendous efforts [6-7]. This is due to the difficulty of the light alkane catalytic selective oxidations, which is derived from the chemical properties of light alkanes [6-9]. Generally, C-H bonds of hydrocarbons are the most effective parameter for its reactivity in catalytic oxidation. In the case of light alkanes, severe reaction condition like as high reaction temperature is usually required because of their low reactivity due to the strong C-H bond. The severe reaction condition, however, often causes undesirable side-reactions and consecutive reactions of the desired products, which makes it difficult to achieve high selectivity


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toward the desired products. These difficulties happen not only alkane oxidations but also other gas-phase selective oxidations. For tackling the difficulties, normal complex metal oxides and heteropoly acids have been frequently used as basic solid-state catalysts, since these materials have the following tunable properties to some extent; (1) high thermal stability, (2) redox property, (3) acidity and basicity, (4) structural diversity and (5) elemental diversity [2-3, 6, 10-11]. The materials have also advantages in preparation and modification. In fact, the catalysts can be simply prepared by solid state reaction, impregnation, precipitation, spray dry, etc, and be further modified by the addition of various levels of other elements in order to modify their redox and acid-base properties. While these advantages, there are also many disadvantages. For examples, tremendous research efforts based on try and error works are still unavoidable and has to be conducted under the lack of fundamental information on active site structure at nano-scale, catalytic role of constituent elements, uniformity of the catalytically active site, and so on. This situation may hinder the catalyst development for alkane selective oxidation more strongly. It appears that the following two functions have to be implemented into the solid-state oxidation catalysts; (1) special catalysis field effective for capturing and activating inert light alkanes, and (2) active site isolation effective for generating active oxygen species from molecular oxygen and for limiting the reaction between the activated light alkane and the oxygen species to a desired reaction direction. We suggest that the catalyst having the above two functions is ideal, but it is difficult to synthesize such the catalyst. In order to synthesize such the catalyst, we need to develop a material synthetic methodology that can introduce the above functions in a crystal structure in a


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designable manner. It is noted that the desired materials will be inevitably complex and be a high dimensionally structured. Once we can create the above high dimensionally structured oxides, we can expect the following additional points: Crystalline material has uniform distribution of catalytically active site at nano-scale. Crystalline structure enables the investigation of structure-activity relationship that realizes the molecular level understanding of the catalytic reaction. In turn, the understanding would allow the nano-scale design of the catalytically active site for improving the catalytic activity. Therefore, crystalline materials in which catalytically active elements are highly organized within the structure are attractive as catalysts for selective oxidations, especially for selective oxidation of light alkanes. The remaining here is how to create the materials. We need to develop synthetic methodology. Table 1 briefly lists the examples of crystalline materials showing high dimensional crystal structures. There are a lot of synthesis methods to produce the crystalline materials in organic-inorganic composites where inorganic metals are connected with organic linkers. For example, Kitagawa et al and Yaghi et al have been producing attractive crystalline organic-inorganic composite materials, so-called Metal-Organic-Frameworks (MOFs), by choosing appropriate metals and organic linkers as building units [12-14]. Mizuno et al have synthesized crystalline polyoxometalate-macrocation complexes (PMCs) by the self-assembly of the polyoxometalates and the macro cations (ex. tetrabutylammonium) [15]. Inagaki et al have reported the synthesis of Periodic-Mesoporous-Organisilicas (PMOs) by using an appropriate organic-bridged silane precursor [16-17]. These organic-inorganic composite crystalline materials are attractive since these materials have the high designability in the crystal structure and its chemical properties can be


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finely tuned by choosing appropriate organic linkers. Therefore, these materials have been widely studied for various applications including gas separator, chemical sensor, photosensitizers, drug delivery, etc [12-18]. However, utilizations of these materials as heterogeneous selective oxidation catalysts are rare since dynamic redox conditions in selective oxidations easily damage the organic linkers. Nevertheless, we can learn the usefulness of the synthetic methodology based on the structurization through an assembling of building units and linkers. There seems to be no question about that crystalline material comprised of all-inorganic metals would be better when we consider the application for catalytic oxidation. There are a few methods to produce highly organized crystalline structure using all-inorganic metals when constituent elements form a tetrahedral coordination state since sp3 geometry tends to form three dimensionally-ordered materials. Some reports in zeolite chemistry pointed out that structural units (composite building units) are formed in the hydrothermal conditions and are assembled each other to form high dimensional crystalline materials. By applying this idea, Sano et al have been developing the inter-zeolite conversion method in which locally ordered aluminosilicate units generated by the decomposition of known zeolites are assembled to form new zeolitic crystals (ex. HUS-1) [19-20]. If we consider all-inorganic solid materials for oxidation catalysts, the inorganic elements need to be redox active (transition metals). Otherwise, no electron transfer can take place between substrate and catalyst. Redox active elements usually form an octahedral coordination state, so that the catalytic materials should be comprised of octahedra to form high-dimensional crystal structure. There are a number of crystal structures, such as ReO3 structure and rutile structure, where octahedra are the


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Table 1. Materials with high dimensionally organized crystal structures

basic structural unit. The use of octahedra as building units to form highly organized three dimensionally-ordered materials seems interesting but may be a difficult issue because of less probability of high dimensional assembling of octahedra units. In fact, although there are some reports relating to the synthesis of highly organized three dimensionally-ordered octahedral-tetrahedral composite materials (e. g. transition metal phosphates), there are only a few report for the synthesis of such the materials based on only octahedra units [21-25]. Despite its difficulty, Müller et al have discovered highly organized giant polyoxometalates formed based on {M6O21}6- (M = Mo, W) pentagonal units and octahedral linkers (ex. V, Cr, Fe) [26-28]. Since these materials have the poor thermal stability, applications of these materials for catalytic selective oxidations are difficult. However, they clearly showed that the construction of highly organized crystalline structure based on the redox active elements is possible by using a large structural motif (in this case, {M6O21}6- (M = Mo, W) pentagonal units) as a structural unit.


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Organicinorganic composites

Unit

Linker

Compound

Reference

MOF

Metals

Organic parts

CPL-1, MOF-177, etc

[12-14]

PMC

Polyoxomet alate

Macrocations

TBA8[{M(OH2)2(µ3-O H)}2{Zn(OH2)}2{γ-HSi W10O36}2], etc

[15]

PMO

Silica

Organic parts

Bpy-PMO, etc

[16-17]

HUS-1, etc

[19-20]

[21-25]

Zeolite

All-Inorganic materials

Composite building units comprised of silica

Transition metal phosphate

Transition metals (Mo, V, Fe, Co, etc)

P

(Me4N)1.3(H2O)0.7[Mo4 O8(PO4)2]・2H2O, Cs3[V5O9(PO4)2]・ xH2O, etc

Giant POM

{M6O21}6pentagonal unit (M = Mo, W)

V, Cr, Fe, etc

{Mo72V30}, {Mo154}, {W72V30}, etc

[26-28]

Mo3VOx

{Mo6O21}-6 pengatonal unit

V, Mo

Orth-MoVO, Tri-MoVO

[29-30]

ε-POM materials

ε-Keggin POM

Bi, Zn, Co, etc

ε-MoVBiO, ε-MoZnO, ε-MoCoO, etc

[31]

On the basis of the above discoveries, we proposed a catalyst synthesis concept, ‘‘unit synthesis’’, for synthesizing unique all-inorganic crystalline materials using redox active metals based on group V and VI elements as constituents for selective oxidation. In this synthetic approach, structural units formed in the precursor solution are assembled in hydrothermal conditions to form high dimensionally structured complex metal oxides.

2. Unit synthesis of crystalline Mo3VOx oxides and ε-Keggin POM-based materials Here, we introduce two highly organized crystalline metal oxide materials, crystalline Mo3VOx oxides (MoVOs) and ε-Keggin POM-based materials (ε-POM materials), those of which are comprised of all-inorganic, redox-active elements [29-31].


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These materials are formed by the self-assembly of the large structural motif (MoVOs, {Mo6O21}6- pentagonal unit; ε-POM materials, ε-Keggin POM) as the building unit under hydrothermal conditions (Figure 1). In MoVOs, we focus on two crystalline materials, orthorhombic Mo3VOx oxide (Orth-MoVO) and trigonal Mo3VOx oxide (Tri-MoVO). Orth-MoVO and Tri-MoVO are formed under the hydrothermal condition using the precursor solution containing [Mo72V30O282(H2O)56-(SO4)12]36- ({Mo72V30}) which provides structural units to form these structure, such as {Mo6O21}6- pentagonal units, oligomer units (Orth-MoVO, pentamer; Tri-MoVO, trimer) and MO6 (M = Mo, V) octahedral linkers [35-36]. In fact, neither Orth-MoVO nor Tri-MoVO can be obtained when hydrothermal synthesis was carried out using the precursor solution containing no {Mo72V30}. The structural units supplied from {Mo72V30} are assembled under the hydrothermal condition to form highly organized crystalline material containing hexagonal and heptagonal channels in the crystal structure. In the channels, the heptagonal channel works as a micropore with a diameter of 0.40 nm which can adsorb small molecules such as N2, CO2, CH4 and C2H6, etc [32-34]. Orth-MoVO and Tri-MoVO are separately formed simply by altering the pH of the precursor solution. We recently reported the synthesis of crystalline Mo-V-Bi oxide (ε-MoVBiO). This material is constructed by ε-Keggin POM unit which is formed by surrounding 12 MO6 octahedra (M = Mo, V) with a central VO4 tetrahedron (chemical composition: [ε-VMo9.4V2.6O40]). The arrangement of ε-Keggin POM units and Bi linkers constructs a diamond-like framework which forms cages (0.77 nm in an internal diameter) and channels (0.34 nm in a diameter) in the crystal structure. This pore system is the same with that of FAU-type zeolites. ε-MoVBiO shows the microporosity derived from the cages and channels which can adsorb small molecules such as N2, CO2, CH4 and C2H6.


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The ε-POM materials are formed by hydrothermal synthesis or reflux of the precursor solution containing the ε-Keggin POMs which is formed by the addition of reducing agent to the mixed solution of appropriate Mo and V sources [37]. Since both the MoVOs and ε-POM materials are comprised of all-inorganic and redox-active elements, as we expected, these showed catalytic activity for several selective oxidations. From the next part, high potential of these materials as the catalysts for selective oxidations is discussed based on their highly organized crystal structure.

Figure 1. Relationship between the formation of crystalline Mo3VOx oxides or ε-Keggin based materials and the presence of {Mo72V30} or ε-Keggin POM in solution.


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{Mo6O21}6- pentagonal unit is marked with black dot circle. Pentamer unit (Orth-MoVO) and trimer unit (Tri-MoVO) are shown in red and blue circle, respectively. HAADF-STEM images of them are also shown in the bottom of each structural image. Mo, green; V, gray; Bi, purple; O, red.

3. Elemental diversity in crystalline Mo3VOx oxides and ε-Keggin POM-based material Multi-functionalization of catalysts is an attractive way to improve the catalytic activity for selective oxidation as the reaction is comprised of many elementary steps including C-H activation, adsorption of reaction intermediates, oxygen insertion, desorption of products, etc. In this view, elemental diversity of the catalyst is attractive for improving the catalytic activity in selective oxidations since various catalytic functions can be conferred. It has been well known that Mo-V-Te-Nb oxide (MoVTeNbO) is an active catalyst for the selective (amm)oxidation of propane to form acrylic acid or acrylonitrile. The basic crystal structure of MoVTeNbO is the same with that of Orth-MoVO except the presence of Te inside the hexagonal channel and the substitution of Mo with Nb in the center of the pentagonal unit. Although the basic crystal structure of Orth-MoVO and MoVTeNbO is the same, the catalytic performance between Orth-MoVO and MoVTeNbO was much different. Significant increase was observed in the acrylic acid (propane selective oxidation) or acrylonitrile (propane ammoxidation) selectivity by the introduction and the substitution of Te and Nb in the Orth-MoVO structure [38-39]. In this reaction, first, propane is activated over V site in the framework structure of Orth-MoVO and MoVTeNbO, forming propene as primary product. The formed propene is easily oxidized to COx over Orth-MoVO. On the other


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hand, the introduction of Te into the Orth-MoVO structure significantly promoted the allyl oxidation of propene which results in the enhancement of the acrylic acid or the acrylonitrile selectivity [39-41]. The substitution of Mo with Nb enhanced the desorption of the formed acrylic acid or acrylonitrile from the catalyst surface which prevents the over-oxidation of these products, resulting in the further improvement in the acrylic acid or the acrylonitrile selectivity [39, 42]. As above, high catalytic activity can be expected if various elements having the different catalytic roles are properly arranged within the crystal structure. Orth-MoVO, Tri-MoVO and ε-POM materials are very attractive since these materials are capable of substituting framework elements without altering its crystalline structure [43-53]. In addition, additional elements can be introduced into the channels and cages. These elemental diversities may allow multi-functionalization of these materials by conferring additional catalytic functions on the basis of its organized catalyst structure.

3.1. Elemental diversity of crystalline Mo3VOx oxides


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Figure 2. Elemental diversity of Orth-MoVO and Tri-MoVO.

There are three framework sites in the crystal structure of Orth-MoVO and Tri-MoVO, pentagonal units (comprised of Mo), linker position (comprised of V) and oligomer position (comprised of Mo and V) (Figure 2). It has been reported that W or Nb can partially substitute Mo in the pentagonal unit [43-44, 48]. Substitution of Mo with Nb enhances the acrylic acid or the acrylonitrile selectivity in the propane (amm)oxidation since Nb enhances the desorption of these products from the catalyst surface [39, 42]. The substitution effect of W can be observed in the selective oxidation of acrolein [43]. In this reaction, the catalytic activity of Orth-MoVO and Tri-MoVO was significantly decreased when water pressure was decreased in the reaction gas due to the slowed desorption of acrylic acid from the catalyst surface. However, the activity decrease upon the decreased water pressure was significantly improved by the partial substitution of Mo with W. The substitution of Mo with W increased the rate of acrylic acid desorption due to the modifications of the Brønsted acidity which may be the reason of the improved activity loss upon the decrease in the water pressure in the reaction gas. Additional elements can be introduced into both Orth-MoVO and Tri-MoVO. Te, Sb and Bi can be introduced into the hexagonal channel of Orth-MoVO [44-46]. The introduction of these elements significantly changed the product selectivity trend in the propane (amm)oxidation since these elements promote the allyl oxidation to form acrylic acid or acrylonitrile [39-41]. Fe or Cu can be introduced into the heptagonal channel of Tri-MoVO [48-49]. Product selectivity in allyl alcohol oxidation was clearly changed by the introduction of these elements into the heptagonal channel. Acrolein and propanal were formed as primary products with almost the same reaction


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rate over Tri-MoVO and Tri-MoVWO (no elements in the heptagonal channel). On the other hand, Tri-MoVFeO and Tri-MoVCuO containing Fe or Cu inside the heptagonal channel preferentially promoted the formation of acrolein and the formation rate of propanal was considerably slower than that of acrolein. As above, catalytic activity and product selectivity over Orth-MoVO and Tri-MoVO can be controlled by the substitution or introduction of additional elements inside the crystal structure. Such the nano-scale catalyst design would realize a high selectivity in catalytic selective oxidations in which many catalytic functions are required at the same time.

3.2. Elemental diversity of ε-Keggin POM-based material

Figure 3. Elemental diversity of ε-POM materials.

There are many structural analogues in ε-POM materials comprising of different elements, such as ε-MoVBiO, ε-MoZnO, ε-MoMnO, ε-MoFeO and ε-MoCoO, etc [50-53]. This fact clearly indicates that these materials are amenable of substituting framework elements (Figure 3). There are three framework positions in ε-POM


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materials, center position of POMs, surrounding position of POMs and linker position between two POMs. Among these positions, center position and linker position can be substituted with wide range of elements. Substitution of framework elements significantly modified the chemical properties. For example, ε-MoFeO showed an outstanding oxygen adsorption property [53]. ε-MoVBiO showed high thermal stability, so that this material could be applied for the gas-phase selective oxidations which need high reaction temperature [53]. Cations located in the cages or the channels in ε-POM materials are exchangeable with additional elements by ion exchange method and alkaline metals can be easily introduced into these sites [31]. Ion exchange of NH4+ with Na+ in ε-MoZnO significantly improved the selective molecule adsorption properties in CO2 and CH4 co-adsorption experiment [51]. The wide range of elemental diversities in ε-POM materials allows to add attractive functions. Such nano-level material designs based on the crystalline materials would realize multi-functionalization of these materials.

4. Catalytic activity of crystalline Mo3VOx oxides and ε-Keggin POM-based materials for selective oxidations MoVOs and ε-POM materials showed high catalytic activity for various selective oxidations. Herein, catalytic properties of MoVOs and ε-POM materials for selective oxidations are briefly introduced and the importance of the highly organized crystalline structure for the catalysis is discussed. Both Orth-MoVO and Tri-MoVO are active catalysts for several selective oxidations, especially for the selective oxidation of ethane and acrolein [30, 54]. MoVOs showed outstanding catalytic activity for above oxidations only when the


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ACS Catalysis

heptagonal channel was formed in the crystal structure. The relationship between the crystal structure and the catalytic activity could be deeply investigated using Orth-MoVO for the selective oxidation of ethane due to its crystalline nature (Figure 4). We found that ethane conversion takes place inside the heptagonal channel in Orth-MoVO and forms ethene [55-56]. In order to further understand the relationship between the partial crystal structure around the heptagonal channel and the ethane conversion, a series of Orth-MoVO catalysts with different heptagonal channel size was prepared by proper redox treatments since we have reported that the size of the heptagonal channel can be reversibly and continuously controlled by the change of the reduction state of the material [32-34]. Reduction state is represented as δ which means the amount of the lattice oxygen evolved from the unit cell of Orth-MoVO (Mo29V11O112-δ). Hereafter, the obtained materials are abbreviated as MoVO (δ). Microporosity was changed upon the redox treatments due to the partial structural changes around the heptagonal channel, while other catalytically affective factors (e.g. Crystal morphology, external surface area, elemental composition, basic crystal structure, etc) were almost unchanged. We found that Orth-MoVO preferentially evolved the bridging oxygen placing at the pentamer unit and facing to the heptagonal channel (α-oxygen) in the early stage of the reduction. Further reduction of Orth-MoVO after the evolution of α-oxygen caused the expansion of the {Mo6O21}6- pentagonal unit which resulted in the decrease of the heptagonal channel size. Re-oxidation restored the size of the heptagonal channel by the shrinkage of the {Mo6O21}6- pentagonal unit. However, almost no oxygen came back into the α-oxygen defect by the re-oxidation. These partial structural changes upon the redox treatments are illustrated in Figure 4 (B). Orth-MoVO without the reduction


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treatments (MoVO (0)) showed a moderate ethane conversion (ca. 13%). The moderate reduction of Orth-MoVO (MoVO (4.2)) increased the ethane conversion (ca. 35%), while further reduction (MoVO (6.8)) caused a drastic decrease in the ethane conversion (less than 1%). However, re-oxidation of MoVO (6.8) (MoVO (6.8)-AC) significantly increased the ethane conversion which was comparable with that of MoVO (4.2) and was far superior to that of MoVO (0). The decrease of the ethane conversion observed in MoVO (6.8) is derived from the decreased micropore size which prevents ethane from accessing the heptagonal channel. The interesting point is the significantly higher ethane conversion in MoVO (4.2) and MoVO (6.8)-AC than that of MoVO (0). Since the structural part commonly observed in MoVO (4.2) and MoVO (6.8)-AC and not observed in MoVO (0) is the presence/absence of the α-oxygen defect, it is reasonable to conclude that ethane is efficiently converted at the α-oxygen defect site.


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Figure 4. (A) Relationship between ethane conversion and reduction state (δ) of Orth-MoVO. δ is the amount of the lattice oxygen evolved from the unit cell of Orth-MoVO (Mo29V11O112-δ). Reduced Orth-MoVO is denoted as MoVO (δ). MoVO (δ) calcined in air atmosphere at 400 °C for 2 h is abbreviated as MoVO (δ)-AC. MoVO (0) (δ = 0) is obtained by the calcination of Orth-MoVO under air atmosphere at 400 °C for 2 h. (B) Partial structural changes around the heptagonal channel of Orth-MoVO upon the redox treatments. Dot circle in MoVO (4.2) represents the α-oxygen defect. Arrows in MoVO (6.8) and MoVO (6.8)-AC show the expansion and shrinkage of the {Mo6O21}6- pentagonal unit by the redox treatments. (C) Role of each structural parts in


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Orth-MoVO for catalytic selective oxidations.

We propose the following reaction scheme in the selective oxidation of ethane over Orth-MoVO; (1) ethane enters into the heptagonal channel, (2) molecular oxygen is activated at the α-oxygen defect site beside the heptagonal channel, (3) activated oxygen species react with ethane inside the heptagonal channel to form ethene and water as oxidation products. At this time, structural framework based on the {Mo6O21}6pentagonal unit plays a decisive role to stabilize the pentamer unit and prevents the pentamer unit from collapsing due to over-reduction or over-oxidation under the dynamic redox condition. In this manner, highly organized crystal structure of Orth-MoVO realizes an outstanding catalytic activity for the selective oxidation of ethane. ε-MoVBiO showed the catalytic activity for the selective oxidation of methacrolein (MAL). This reaction can produce methacrylic acid (MAA) which is very important intermediate for the production of value added chemicals such as methyl methacrylate. Mo-V based α-Keggin POMs catalysts are conventionally used for this reaction, but further improvements have been requested both in the catalytic activity and the structural durability [57-64]. ε-MoVBiO showed substantial catalytic activity for the selective oxidation of MAL at relatively low reaction temperature (around 280 °C) and promoted the formation of MAA with moderate selectivity. The catalytic activity observed over ε-MoVBiO was comparable with that of traditional POM-based catalysts (Table 2). In addition to the high catalytic activity, ε-MoVBiO showed excellent structural durability and no structure collapse was observed after the reaction. As described above, ε-MoVBiO has wide range of elemental diversity so that optimizations


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Table 2. Methacrolein (MAL) oxidation to methacrylic acid (MAA) over heteropoly-type Mo oxide catalysts MAAb Reaction gas formation Temperature composition. Catalyst Reference /°C MALa/O2/inert rate /µmol -1 -1 gas/H2O (mol%) gcat min 280 1.2/8.1/71.9/18.7 14.3 [53] ε-MoVBiO Cs2HPMo12O40 300 5.5/9.3/55.6/29.6 88.1 [57] Mo9V2.6W1.04Ox 300 1.5/7.3/79.2/12 4.9 [58] Co3(PMo12O40)2 320 3.3/5.0/91.7/0 2.7 [59] Cs(NH4)2PMo12O40 350 4.7/8.8/70.6/15.9 20.5 [60] H3PMo12O40 350 4.7/8.8/70.6/15.9 10.7 [60] (MoOx)0.5(PMo14O42) 350 4.7/8.8/70.6/15.9 14.7 [60] 3.3 mol% 300 3.0/6.0/76.0/15.0 48.7 [61] H4PMoVO40/SiO2 Mo12P1.5V0.6Cs0.8K0.03Cu0.5 286 5.0/9.0/61.0/25.0 14.3 [62] Sb1S0.03Rb0.2Mn0.1Bi0.05 a MAL: methacrolein. b MAA: methacrylic acid.

in the redox and acid characters can be expected. Moreover, there are more tunable factors in ε-MoVBiO which can affect the catalytic activity such as surface area and crystal morphology. As above, there are much rooms in ε-MoVBiO for improving the catalytic activity. ε-MoVBiO will be a potential candidate as the MAL oxidation catalyst.

5. Future perspective and remarks In this perspective, we introduced the unit synthesis methodology to synthesize high dimensionally structured complex metal oxide materials comprised of all-inorganic, redox-active elements based on group V and VI elements. This synthetic approach has successfully brought about two high dimensionally structured materials, MoVOs and ε-POM materials, which were found to be active for selective oxidations. It was also found that the activities were derived from their high dimensional crystalline structures. These examples clearly demonstrate the importance of the crystalline structure for selective oxidations and the usefulness of the unit synthesis concept to create active


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solid-state oxidation catalysts. After the success of the above synthesis, we continuously found many other attractive high dimensionally structured crystalline materials, one of which is (Mo, W)-(Te, Se) oxides shown in Figure 5 [65-66]. The materials are comprised of hexagonal POM units, {X(Mo, W)6O21}2- (X = Te, Se), which are stacked each other to form one-dimensional molecular wire and are further assembled into a hexagonal symmetry crystal. This one-dimensional molecular wire can work as solid-state catalyst in a separated form in water. The materials showed high catalytic activity for cellulose hydrolysis [66].

Figure 5. Scheme of unit synthesis of (Mo,W)-(Te,Se) oxide. Structural image, HAADF-STEM image and TEM image are shown. Mo or W, green; Te or Se, yellow; O, red.


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ACS Catalysis

We can expect that many more of crystalline complex metal oxide types based on group IV, V, and VI element with porous property can be synthesized by combing the structural units with appropriate inorganic linkers only since there are a lot of potential structural

units

applicable

in

the

unit

synthesis,

including

Lindqvist-type

polyoxometalate, Anderson-type polyoxometalate, Dowson-type polyoxometalate, cubane, etc. We also expect the structural diversity in the unit synthesis. For example, if the linker site of ε-POM materials could be replaced with dimer or trimer unit, we might be able to construct new materials category as highly organized crystalline microporous materials using redox-active elements as constituents. As a consequence, unit synthesis methodology has high potential to create attractive all-inorganic crystalline materials with redox-active constituents, through which revolutional oxidation catalysts based on the unique crystalline structure will be realized someday.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (W. Ueda)

Author Contributions

The manuscript was written through contributions by all authors. All authors have given approval to the final version of the manuscript.

Funding Sources


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This work was supported by JSPS KAKENHI Grant Number 15H0-2318.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

This work was supported by JSPS KAKENHI Grant Number 15H0-2318.

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