Redox Treatment of Orthorhombic Mo29V11O112 and Relationships

Article pubs.acs.org/JPCC

Redox Treatment of Orthorhombic Mo29V11O112 and Relationships between Crystal Structure, Microporosity and Catalytic Performance for Selective Oxidation of Ethane Satoshi Ishikawa,*,†,‡ Daichi Kobayashi,† Takeshi Konya,† Shunpei Ohmura,† Toru Murayama,† Nobuhiro Yasuda,§ Masahiro Sadakane,∥ and Wataru Ueda*,† †

Catalysis Research Center, Hokkaido University, N-21, W-10, Sapporo 001-0021, Japan Japan Society for the Promotion of Science, Kojimachi Business Center Building, 5-3-1 Kojimachi, Chiyoda-ku, Tokyo 102-0083, Japan § JASRI/SPring-8, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan ∥ Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527, Japan ‡

S Supporting Information *

ABSTRACT: Redox treatments of an orthorhombic Mo29V11O112 catalyst (MoVO) were conducted and its crystal structure, microporosity, and catalytic activity were investigated. TPR and TG revealed that MoVO evolved two kinds of lattice oxygen (α-oxygen and β-oxygen) from the structure by reduction treatment. In the early stage of reduction, αoxygen was evolved from the structure, causing expansion of the micropore channel. With further reduction, the atoms in the pentagonal [Mo6O21]6− unit moved toward the micropore channel, resulting in a decrease in micropore size. Expansion of the micropore drastically increased catalytic activity for selective oxidation of ethane, but the activity was decreased by a reduction in the micropore channel size. Strong relationships were found between crystal structure, microporosity, and catalytic activity for selective oxidation of ethane.

1. INTRODUCTION

of this material since it is impossible for other crystalline porous materials such as zeolites and MOFs, because zeolites are comprised of elements that are redox-inactive, and the organic moiety of MOFs is unstable under redox conditions. Apart from the sorption behavior, we have reported that the heptagonal channel converts ethane to ethene in the channel without diffusion effects, resulting in outstanding catalytic performance.2,3 This fact suggests that the catalytic performance for selective oxidation of ethane should depend on the properties of the heptagonal channel. More specifically, the microporosity derived from the heptagonal channel may be a key factor for the catalysis of ethane. It is important to reveal the relationship of microporosity and catalytic activity for selective oxidation of ethane in order to understand the catalysis of this reaction at the molecular level. We also believe that such findings will lead to a deep understanding of general selective oxidations.

Orthorhombic Mo3VOx oxide (MoVO) has attracted much attention because of its outstanding catalytic performance for selective oxidation of ethane1−8 and acrolein.3,9−12 A structural model of MoVO constructed on the basis of HAADF-STEM images is shown in Figure 1.13,14 The a−b plane is composed of pentagonal [Mo6O21]6− units (shown by purple) and {MO6} (M = Mo, V) octahedra which stacks on each other to form layered material of MoVO. In the a−b plane, the network arrangement of the pentagonal [Mo6O21]6− units and Mo/V octahedra forms the framework unit (shown by the blue enclosure). Voids in the framework are filled with pentamer units comprised of five octahedra (shown by a red circle), resulting in the formation of hexagonal and heptagonal channels. In these channels, the heptagonal channel provides molecular sieving properties on the basis of the channel size (0.40 nm) and can adsorb small molecules such as N2 or light alkanes.15−18 We have reported that the sorption behavior of MoVO derived from the heptagonal channel is continuously and reversibly tunable by redox treatment.17 This is a unique feature © 2015 American Chemical Society

Received: December 24, 2014 Revised: March 17, 2015 Published: March 17, 2015 7195

DOI: 10.1021/jp512848w J. Phys. Chem. C 2015, 119, 7195−7206

Article

The Journal of Physical Chemistry C

water, and dried at 80 °C overnight. Since the ontained solid contained amorphous types of materials as an impurity phase, so the dried sample was treated with oxalic acid for purification. To 25 mL of an aqueous solution (0.4 mol l−1, 60 °C) of oxalic acid (Wako), 1 g of the dried material was added, and the mixture was stirred for 30 min and then washed with 500 mL of distilled water. The obtained solid was dried at 80 °C overnight. MoVO was ground for 5 min with an agate mortar before heat treatment under air as an oxidative gas at 400 °C for 2 h with a 10 °C min−1 ramp in a muffle furnace. The oxidized MoVO is abbreviated as MoVO(0). The formula from elemental analysis was Mo28.8V11.2O112(N2)2(O2)0.5(H2O)4.5 (Anal. Calcd: Mo, 52.7; V, 11.0; O, 35.8; N, 1.0; H, 0.2. Found: Mo, 52.3; V, 10.8; O, 35.6; N, 1.1; H, 0.2; total, 100.7). 2.2. Redox Treatment. Redox treatment for MoVO was carried out by heat treatment under an oxidative or reductive gas atmosphere. As a reduction treatment, the oxidized sample (MoVO(0)) was heat-treated under 70 mL min−1 of 5% H2/Ar flow with a fixed bed Pyrex tubular furnace equipped with a TPR apparatus. The temperature was increased from room temperature to 400 °C with a 10 °C min−1 ramp rate and was kept for x h (0, 0.2, 0.5, 1.0, 1.5, and 2.0), 0 h meaning that the temperature was decreased soon after the temperature had reached 400 °C. The obtained samples are abbreviated as MoVO(δ) (δ = 0.8, 2.9, 4.2, 5.4, 6.1, 6.8), where δ represents the amount of lattice oxygen evolved from the unit cell of MoVO (Mo29V11O112‑δ) and was estimated by a TCD equipped with a TPR apparatus. As an oxidative treatment, MoVO(2.9) and MoVO(6.8) were heat-treated under air atmosphere at 400 °C for 2 h with a 10 °C min−1 ramp in a muffle furnace, and the obtained samples are abbreviated as MoVO(2.9)-AC and MoVO(6.8)-AC, respectively. 2.2. Characterization of Synthesized Materials. The obtained catalysts were characterized by the following techniques. TPR was conducted by using CHEMBET 3000 (Quantachrome). First, 1.7 g of MoVO(0) was put into a Utype quartz tube and was set in a furnace equipped with a TPR apparatus. The temperature was increased to 400 °C with a 10 °C min−1 ramp and kept for x h (0, 0.2, 0.5, 1.0, 1.5, 2.0) under 70 mL min−1 of 5% H2/Ar flow. The amount of lattice oxygen evolved was estimated from the signal area of TPR from 200 °C to the signal end. Prior to the calculation, the TPR signal area was calibrated with CuO (99.9%, Wako). The amount of lattice oxygen evolved was calculated by assuming that consumed H2 is used to reduce MoVO and consumed H2 was removed as H2O, which is confirmed by TG-DTA measurements. TG analysis was conducted with a TG/DTA 7200 (SEIKO Instrument). After placing 0.01 g of Al2O3 as a reference and 0.01 g of MoVO(0) in a furnace equipped with a TG, the temperature was increased to 400 °C with a 10 °C min−1 ramp and kept for 2 h under 70 mL min−1 of 5% H2/Ar flow or 70 mL min−1 Ar flow. After holding for 2 h at 400 °C, the gas was switched to air and kept for another 2 h at the same temperature. The amount of lattice oxygen evolved was estimated from the weight loss from 200 °C. Powder XRD patterns were recorded with a diffractometer (RINT Ultima+, Rigaku) using Cu−Kα radiation (tube voltage, 40 kV; tube current, 40 mA). For XRD measurements, 0.16 g of the catalyst was mixed with 0.04 g of Si, as an external standard to correct the peak positions, and was set in a rotation attachment (Rigaku). Diffractions were recorded in the range of 4−80° with 1° min−1 at a rotation rate of 10 rpm. Unit cell parameters were refined by the Rietveld program with Materials Studio 6.1

Figure 1. Structural model of MoVO. The blue enclosure represents a framework structure comprised of pentagonal [Mo6O21]6− units. The red circle represents pentamer units embedded in the framework structure. Key: purple, pentagonal unit (Mo); gray, linker of two pentagonal units (mixture of Mo and V); green, pentamer unit (mixture of Mo and V).

In this study, we carried out redox treatments of MoVO and we evaluated the relationships between crystal structure, microporosity, and catalytic activity for selective oxidation of ethane.

2. EXPERIMENTS 2.1. Catalyst Preparation. The orthorhombic Mo29V11O112 oxide (MoVO) was synthesized by the hydrothermal method. First, 8.83 g of (NH4)6Mo7O24·4H2O (Mo: 50 mmol, Wako) was dissolved in 120 mL of distilled water. Separately, an aqueous solution of VOSO4 was prepared by dissolving 3.29 g of hydrated VOSO4 (V: 12.5 mmol, Mitsuwa Chemicals) in 120 mL of distilled water. The two solutions were mixed at ambient temperature and stirred for 10 min. At this stage, the pH value of the solution was 3.2. Then the obtained mixed solution was introduced into an autoclave with a 300 mL-Teflon inner vessel and 4000 cm2 of a Teflon thin sheet to occupy about half of the Teflon inner vessel space. This sheet is indispensable for obtaining well-crystallized samples. After the introduction, N2 was fed into the solution in the vessel to remove residual oxygen. The hydrothermal reaction was started at 175 °C for 48 h under static conditions in an electric oven. The gray solid formed on the Teflon sheet was separated by filtration, washed with 1000 mL of distilled 7196


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The Journal of Physical Chemistry C (Accelrys) using the XRD patterns after the peak positions had been corrected. In situ XRD experiments were carried out to evaluate the structural behavior upon redox treatment. After 0.20 g of MoVO(0) had been set in a furnace attachment, the temperature was increased from room temperature to 400 °C with a 10 °C min−1 ramp under 50 mL min−1 air flow (oxidative gas). When the temperature had reached 400 °C, the gas was switched to 100 mL min−1 of 5% H2/N2 mixture (reductive gas) and kept for 1 h. Then the gas was switched to 50 mL min−1 of air (oxidative gas) and kept for 0.5 h, and the gas was again switched to 100 mL min−1 of 5% H2/N2 mixture (reductive gas) and kept for 1 h. This cycle was repeated 4 times. In-situ XRD measurement by using 100 mL min−1 of 10% C2H6/N2 mixture as a reductive gas instead of 100 mL min−1 of 5% H2/N2 mixture was also performed at 300 °C. In addition, in situ XRD measurement at 300 °C using C2H6/O2/ N2 = 10/10/80 mL min−1 mixture was conducted for 12 h to evaluate the structural behavior of MoVO during the reaction. Molecular adsorption (N2, CO2, CH4, C2H6, C3H8) experiments were performed to estimate the external surface area and the micropore volume. When CO2, CH4, C2H6, and C3H8 were used, isotherms were obtained at 25 °C using an autoadsorption system (BELSORP MAX, Nippon BELL). N2 adsorption isotherms at liquid N2 temperature were obtained by using the same apparatus. Prior to the adsorption, the catalysts were evacuated under vacuum at 300 °C for 2 h. FTIR spectra were obtained using a spectrometer (Paragon 1000, PerkinElmer) at room temperature in the range of 500−2000 cm−1. Raman spectra (inVia Reflex Raman spectrometer, RENISHAW) were obtained in air on a static sample with an Ar laser (532 nm). Elemental compositions of the bulk were determined by ICP-AES (ICPE-9000, Shimadzu). Complete elemental analysis for MoVO(0) was carried out by Mikroanalytisches Labor Pascher (Remagen, Germany). Single crystal analysis was carried out for MoVO(0). Crystals of MoVO(0) are too small (cross-sectional size, 2.00σ(I)) and wR = [Σw(Fo2 − Fc2)2/ Σw(Fo2)2]1/2, respectively, were used. Nitrogen atoms of a nitrogen molecule, oxygen atoms of an oxygen molecule, and oxygen atoms of water were modeled as oxygen atoms because nitrogen atoms could not be distinguished from oxygen atoms. All oxygen atoms were refined isotropically, and Mo and V atoms were refined anisotropically. Obtained crystallographic data of MoVO(0) are shown in Table 1. The Mo/V atom ratio (The numbers of Mo and V atoms were 7.2 and 2.8, respectively.) estimated by complete elemental analysis was different from that obtained by single crystal structure analysis (the numbers of Mo and V atoms were 7.6 and 2.4,

Table 1. Crystallographic Data for Orthorhombic MoVO(0) MoVO(0) formula formula weight crystal system space group a (Å) b (Å) c (Å) volume (Å3) temperature (K) Z ρcalcd (g cm−3) F000 λ (Å) absorption coefficient (mm−1) measured reflections unique reflections R1 (I > 2σ(I)) wR2 (all data) goodness of fit

Mo7.62V2.38O29.23 1320.25 orthorhombic Pba2 20.9864(5) 26.4023(6) 3.98755(9) 2209.46(9) 100(2) 4 3.969 2435 0.83077 7.986 38157 4016 0.0351 0.0832 1.049

respectively), due to the difference in the crystal sample and bulk sample. The total number of oxygen atoms of water, oxygen atoms, and nitrogen atoms was smaller than the total number of oxygen atoms of water, oxygen atoms and nitrogen atoms estimated by elemental analysis. This is also due to the difference in the crystal sample and bulk sample. The sample for elemental analysis may contain surface waters. CIF files are available in Supporting Information. CSD-428978 contains crystallographic data for MoVO(0) (data available from ref 50). In order to obtain structural models of the catalysts with various reduction states, Rietveld refinement was carried out by using the structural model of MoVO(0) obtained by single crystal analysis as a base structure. The Rietveld program with Materials Studio 6.1 (Accelrys) was used for refinement. The XRD patterns after peak position corrections were subjected to the refinement. The occupancy of metals in the framework and temperature factors of all atoms were fixed without further refinement from those of MoVO(0). As shown the Results and Discussion, there are two kinds of lattice oxygen in the structure (α-oxygen and β-oxygen). α-oxygen was assumed to be O29 (Figure 8, Tables S1−S10) and β-oxygen was assumed to be axial oxygen (Tables S1−S10: O1−O3, O15, and O17− O23). Occupancies of these oxygen atoms were calculated based on the results of TPR, assuming that the evolution oxygens from MoVO(6.8)-AC was all attributed all to β-oxygen. All metal atom positions were refined. After the refinement of metal positions, oxygen atom positions were refined to set a proper metal−oxygen length. The pattern parameters were refined for obtaining the lowest Rwp value. Atom positions are shown in Tables S3−S10, and Rietveld analysis parameters are shown in Tables S11−S19. 2.3. Catalytic Test. Selective oxidation of ethane in gas phase was carried out at atmospheric pressure in a conventional vertical flow system with a fixed bed Pyrex tubular reactor. After 0.50 g of the catalyst had been diluted with 2.30 g of SiO2, the mixture was put into the tubular reactor for ethane oxidation. The reactor was heated gradually from room temperature at a rate of 10 °C min−1 to 300 °C under a nitrogen flow (40 mL min−1 from the top of the reactor). The temperature was measured with a thermocouple inserted in the middle of the 7197


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The Journal of Physical Chemistry C catalyst zone. When the temperature had reached 300 °C, a reactant gas with the composition of C2H6/O2/N2 = 10/10/80 (mol %) was fed in at a total flow rate of 50 mL min−1 and the catalytic test was started at that temperature. Reactants and products were analyzed with three online gas chromatographs (molecular sieve 13X for O2, N2 and CO with a TCD detector, Gaskuropack for CO2, C2H4 and C2H6 with a TCD detector, and Porapak Q for acetic acid with an FID detector). Blank runs showed that homogeneous gas-phase reactions were negligible under the experimental conditions used in this study. Carbon balance was always ca. 98−100%. After catalytic oxidation, the catalysts were cooled down to room temperature under an N2 flow and were subjected to various characterizations.

molybdenum oxides.25,30−34 Neither Mo atoms nor V atoms were found in the hexagonal and heptagonal channels, being consistent with HAADF-STEM results for orthorhombic Mo29V11O112.13 3.2. Physicochemical Properties of MoVO after Redox Treatment. Table 2 shows the physicochemical properties of MoVO catalysts after redox treatment. The redox treatment caused no change in the V/Mo ratio, which was in the range of 0.37−0.39. TPR spectra and TG spectra of MoVO(0) are shown in Figures S2 and S3, respectively, and the estimated amounts of lattice oxygen evolved (δ) from the unit cell (shown as Mo29V11O112‑δ) are shown in Table 2. For TG, weight loss was measured under 70 mL min−1 of Ar or 70 mL min−1 of 5% H2/Ar. Under Ar flow, weight loss was only observed below 150 °C and the weight loss was due to desorption of N2, O2, and H2O trapped in the heptagonal channel.17 On the other hand, under 5% H2/Ar flow, as well as the weight loss at 150 °C, weight loss was observed from 250 °C and proceeded when the temperature was held at 400 °C. This weight loss corresponds to the evolution of lattice oxygen from the structure since weight increase was observed when 5% H2/Ar was switched to air at the same temperature because of the incorporation of gaseous O2 into the structure. For TPR, the peak around 150 °C was derived from the desorption of N2, O2, and H2O from the heptagonal channel as was also observed by TG. With increase in the temperature, the TPR signal rapidly increased and the signal rapidly decreased and became flat when the temperature was held at 400 °C. As indicated earlier, this TPR signal corresponds to the evolution of lattice oxygen by the reduction.17 The values of δ estimated from TPR and TG were almost the same. Therefore, for further experiments, δ estimated from TPR was used. Figure 3 shows TPR spectra of MoVO(0), MoVO(2.9)-AC, and MoVO(6.8)AC. The temperature was increased to 400 °C with a 10 °C min−1 ramp and kept at 400 °C for 2 h under 70 mL min−1 of 5% H2/Ar flow. As already shown in Figure S3, the TPR signal of MoVO(0) rose rapidly with increase in temperature and was maximum when the temperature reached 400 °C. The signal decreased and became flat when the temperature was held at 400 °C. The TPR spectrum of MoVO(2.9)-AC was similar to that of MoVO(0), but a small decrease in the initial rise at 400 °C was observed. MoVO(6.8)-AC showed no initial rise in the TPR signal. These experimental results suggest that there are two kinds of lattice oxygen in the structure. One is evolved in the early stage of the reduction and hardly returns to the structure even with reoxidation. The other is evolved continuously by the reduction and reversibly comes back to the structure with reoxidation. We abbreviated the former oxygen as α-oxygen and the latter oxygen as β-oxygen. We calculated the total amount of α-oxygen to be 2.7 on the basis of the TPR signal area by assuming that the TPR signal area of MoVO(6.8)-AC is all attributed to the evolution of β-oxygen. Based on this assumption, it was implied that the evolution of α-oxygen was almost completed by the reduction at MoVO(4.2). Figure 4 shows XRD patterns, lattice parameters, and peak intensity ratios of the catalysts. All of the catalysts showed peaks at 6.6°, 7.9°, 9.0°, and 22.1°, corresponding to diffractions of (020), (120), (210), and (001) in MoVO, respectively.1 No XRD peaks related to impurities appeared during the reduction. With increase in δ, XRD peaks attributed to (020), (120), and (210) were shifted to lower degrees, but the XRD peak attributed to (001) was shifted to a higher degree.

3. RESULTS AND DISCUSSION 3.1. Single Crystal Analysis of MoVO. First, we introduce the detailed crystal structure of MoVO(0) as a base structure for redox treatments. Single crystal structural analysis revealed the detailed structure of MoVO(0), and metal coordinates, occupancies, and metal−oxygen bond lengths are summarized in Tables S1 and S2. It was shown that the pentagonal units consist exclusively of Mo atoms and that V prefers linking polyhedra connecting the pentagonal units. Both Mo and V atoms occupy nonpentagonal unit metal sites (M7, M8, M9, M10, and M11) with Mo occupancy ranging from 0.11 to 0.75, which is consistent with HAADF-STEM results for orthorhombic Mo29V11O112 and single crystal structure analysis results for orthorhombic Mo−V−Sb oxide.13,23 Mo and V in octahedral sites have out-of-center distortions with significant elongation of one apical M−O bond and with subsequent shortening of the other apical M−O bond. These findings are similar to the results of a previous study obtained by using combined Rietveld analysis of synchrotron X-ray and neutron powder data for orthorhombic Mo−V−Nb−Te oxide and results of single crystal structure analysis of orthorhombic Mo− V−Sb oxide.23,24 The apical short distance ranged from 1.63 to 1.73 Å, the long distance ranged from 2.28 to 2.36 Å, and equatorial M-O bonds ranged from 1.76 to 2.08 Å; these distances are reasonable for molybdenum oxides containing pentagonal units.24−29 Disordering of some metal sites (M7, M8, and M9) along the z direction was detected (black balls and gray balls in Figure 2). Such disordering is often observed in single-crystal structural analyses of polyoxomolybdates and

Figure 2. Site map of an asymmetric unit in MoVO(0) obtained from single crystal structural analysis: metals with high occupancy (black ball), low occupancy (gray ball), and oxygen (small white ball). 7198


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The Journal of Physical Chemistry C Table 2. Physicochemical Properties of MoVO δb/−

micropore volume/10−3 cm3 g−1

lattice parametere /nm

catalyst

bulk V/Moa

TPRc

TGd

a

b

C

external surface areaf/m2 g−1

VN2f

VCO2g

VCH4g

V C2 H 6 g

VC3H8g

MoVO(0) MoVO(0.8) MoVO(2.9) MoVO(4.2) MoVO(5.4) MoVO(6.1) MoVO(6.8) MoVO(2.9)-AC MoVO(6.8)-AC

0.38 0.39 0.37 0.37 0.38 0.37 0.38 0.39 0.37

− 0.8 2.9 4.2 5.4 6.1 6.8 0.5 2.7

− 1.0 1.8 2.8 5.1 6.6 7.3 − −

2.100 2.109 2.112 2.114 2.118 2.121 2.122 2.099 2.100

2.645 2.646 2.653 2.662 2.668 2.671 2.674 2.645 2.647

0.4015 0.4010 0.4004 0.3998 0.3988 0.3979 0.3977 0.4013 0.4010

5.8 5.8 5.7 6.5 5.8 6.2 6.3 5.7 5.8

13.0 15.4 13.3 17.6 4.4 1.3 1.2 11.2 10.7

27.7 26.1 26.2 25.5 15.1 2.6 0.0 24.3 23.9

27.8 25.0 23.6 23.0 1.8 0.3 0.0 25.4 23.0

15.8 20.5 22.1 22.0 1.5 0.7 0.0 16.3 20.8

3.7 2.8 6.2 13.8 1.6 0.1 0.0 5.5 10.0

Determined by ICP. bNumber of lattice oxygen atoms evolved from the unit cell. cEstimated from the TPR signal area from 200 °C. dEstimated from the weight loss based on the weight loss at 200 °C. eMeasured by XRD and calculated by Rietveld refinement. fMeasured by N2 adsorption at liquid N2 temperature and calculated by a t-plot. VN2 represents the micropore volume measured by using N2. gMicropore volume measured by CO2, CH4, C2H6, and C3H8 adsorption and estimated by the DA method. a

axis continuously decreased. The lattice parameter changes are due to cation size changes of Mo and V, which were suggested by UV analysis to be changes in the oxidation states of Mo and V (Figure S4).17,35−40 A drastic increase in the lattice parameter to a axis between MoVO(0) to MoVO(0.8) was observed. The reduction of MoVO(0) may cause desorption of oxygen incorporated in the heptagonal channel (see Figure S10). This drastic change in the lattice parameter to the a axis between MoVO(0) to MoVO(0.8) is thought to be caused by the desorption of oxygen in the heptagonal channel as well as cation size changes. It is notable that the lattice parameters of MoVO(2.9)-AC and MoVO(6.8)-AC were almost the same as that of MoVO(0) (Table 2). The difference between MoVO(2.9)-AC and MoVO(6.8)-AC is only the amount of α-oxygen in the structure. This experimental fact clearly indicates that the evolution of α-oxygen does not cause any change in lattice parameters. Therefore, the change in lattice parameters with reduction would be caused solely by the evolution of β-oxygen. As shown in Figure 1, MoVO is composed of the framework and pentamer units. It has been reported that the pentamer unit is structurally less stable than the framework unit.41 Therefore, we propose that evolution of

Figure 3. TPR spectra of MoVO(0) (black solid line), MoVO(2.9)AC (brown solid line), and MoVO(6.8)-AC (blue solid line). Temperature (green dotted line) was increased at a 10 °C min−1 ramp rate and was held at 400 °C for 2 h.

Lattice parameters of the catalysts were calculated by Rietveld refinement and are shown in Figure 4 (B) and Table 2. With increase in δ, the lattice parameters in the a−b plane continuously increased and the lattice parameter in the c

Figure 4. (A) XRD patterns of MoVO: (a) MoVO(0), (b) MoVO(0.8), (c) MoVO(2.9), (d) MoVO(4.2), (e) MoVO(5.4), (f) MoVO(6.1), (g) MoVO(6.8), (h) MoVO(2.9)-AC, and (i) MoVO(6.8)-AC. Lattice parameters (B) and peak intensity ratios based on (120) (C) of MoVO as a function of δ. 7199


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Figure 5. IR (A) and Raman (B) spectra of MoVO: (a) MoVO(0), (b) MoVO(0.8), (c) MoVO(2.9), (d) MoVO(4.2), (e) MoVO(5.4), (f) MoVO(6.1), (g) MoVO(6.8), (h) MoVO(2.9)-AC, and (i) MoVO(6.8)-AC. (C) IR (circle) and Raman (triangle) bands attributed to the Mo−O− Mo bond in the pentagonal [Mo6O21]6− unit as a function of δ.

In this section, we showed that (1) MoVO has two kinds of lattice oxygen (denoted as α-oxygen and β-oxygen) and (2) the evolution of lattice oxygen with reduction treatment causes a partial structural change in the a−b plane that is involved in the decrease in strength of the Mo−O−Mo bond in the pentagonal [Mo6O21]6− unit. 3.3. Microporosity Changes Caused by Redox Treatment. It has been reported that the heptagonal channel of MoVO works as a micropore with a size of 0.40 nm.1,15−17 The change in microporosity of MoVO caused by redox treatment was evaluated by means of molecular adsorption using N2, CO2, CH4, C2H6, and C3H8. The kinetic diameters of N2, CO2, CH4, C2H6, and C3H8 have been reported to be 0.36, 0.33, 0.38, 0.40, and 0.43 nm, respectively.15,47 N2 adsorption was performed at liquid N2 temperature (−196 °C), and CO2, CH4, C2H6, and C3H8 adsorptions were performed at room temperature (25 °C). Micropore volumes were estimated by the DA method for CO2, CH4, C2H6, and C3H8 adsorptions (VCO2, VCH4, VC2H6, VC3H8) and by the t-plot method for N2 adsorption (VN2). The adsorption isotherms and t-plots are shown in Figures S5−S9. Micropore volumes estimated from molecular adsorptions are shown in Table 2 and Figure 6.

oxygen from the framework strongly affects the lattice parameters rather than the pentamer units. Based on this idea, β-oxygen should be located in the framework unit and αoxygen should be located in the pentamer unit. The peak intensity ratios of (020) to (120), Int(020)/Int(120), and (210) to (120), Int(210)/Int(120), are shown in Figure 4 (C). Small differences in Int(020)/Int(120) were observed in the range of MoVO(0)−MoVO(4.2) and the ratio was continuously decreased by reduction above MoVO(5.4). The change in Int(020)/Int(120) implies a continuous structural change in the a− b plane above MoVO(5.4). In the case of Int(210)/Int(120), the ratio was almost the same in the range of MoVO(0)− MoVO(2.9). The ratio was decreased and became constant by reduction above MoVO(4.2). In the TPR part, it was implied that the evolution of α-oxygen was almost completed by reduction up to MoVO(4.2). The change in Int(210)/Int(120) may be related to the structural change caused by evolution of α-oxygen from the structure. Figure 5 shows IR and Raman spectra of the catalysts. Characteristic IR absorption at 915 cm−1 attributed to VO, 874, 800, 720, and 652 cm−1 to Mo−O−Mo, 604 cm−1 to V− O−Mo, and 458 cm−1 to Mo−O were observed. All of those were characteristic absorption of MoVO.1,42−44 For the band at 874 cm−1, the wavenumber rapidly decreased in the range of MoVO(0) to MoVO(0.8), possibly due to the desorption of oxygen incorporated in the heptagonal channel. In the range of MoVO(0.8) to MoVO(4.2), there was almost no peak shift, but the continuous downward shift of the wavenumber was observed with reduction above MoVO(5.4) (Figure 5C). This wavenumber was reversibly shifted from 862 to 874 cm−1 by reoxidation. For the Raman spectrum, the Raman band at 873 cm−1 was shifted in a lower direction by reduction above MoVO(5.4) and was restored by reoxidation in the same manner as IR. The band around 873−874 cm−1 found by IR and Raman is attributed to the Mo−O−Mo bond in the pentagonal [Mo6O21]6− unit.44−46 This band shift indicates that the strength of the Mo−O−Mo bond in the pentagonal [Mo6O21]6− unit was weakened when MoVO was reduced above MoVO(5.4). The continuous structural change by reduction above MoVO(5.4), suggested by XRD, is thought to be involved in the decrease in strength of the Mo−O−Mo bond in the pentagonal [Mo6O21]6− unit.

Figure 6. Micropore volumes of MoVO estimated by the Dubinin− Astakhov (DA) method using CO2 (circle), CH4 (triangle), C2H6 (lozenge), and C3H8 (square) plotted against δ. 7200


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Figure 7. (A) Structural model of MoVO(Mo, light green; V, gray; O, red). Fractional coordinates of (B) Mo1, (C) Mo2, (D) Mo3, (E) Mo4, (F) Mo5, and (G) Mo6 as a function of δ. Atomic numbers are represented in the structural model. Circle, x (a axis); triangle, y (b axis).

completed at MoVO(4.2). In conjunction with TPR and C3H8 adsorption, the evolution of α-oxygen is thought to be related to the enlargement of the heptagonal channel. As mentioned in the XRD part, we proposed that α-oxygen is located in the pentamer unit of the structure. Since the evolution of α-oxygen affected the microporosity, we speculated that α-oxygen is the bridging oxygen between the metals in the pentamer unit that faced the heptagonal channel. For reoxidized catalysts, the micropore volumes of MoVO(2.9)-AC estimated by various molecules were almost the same as those of MoVO(0). For MoVO(6.8)-AC, the micropore volumes were drastically increased from MoVO(6.8) and were almost the same as those of MoVO(4.2). This astonishing result indicates that the decreased heptagonal channel size by reduction is restored by the reoxidation most probably due to the reversible structural change.17 VC3H8 of MoVO(6.8)-AC was close to that of MoVO(4.2) since no α-oxygen came back with reoxidation of MoVO(6.8). These adsorption results together with IR and Raman band shifts and XRD profile changes support the suggestion that evolution of α-oxygen is completed at MoVO(4.2). In this section, we showed that the partial structural change of the a−b plane strongly affects microporosity. Results of detailed structural analysis are presented in the next section. 3.4. Structural Refinement. As demonstrated above, continuous structural change in the a−b plane suggested by XRD may be strongly related to the microporosity change. In order to understand the relationship between crystal structure and microporosity, Rietveld refinement was carried out. At this time, the crystal structures of the catalysts after redox treatments were refined on the basis of the crystal structure of MoVO(0) obtained by single crystal analysis. Actually, the simulated XRD pattern fitted well with the obtained XRD pattern in MoVO(0) (Rwp value =7.8%), indicating that the crystal structure obtained by single crystal analysis is applicable for Rietveld refinement. An occupancy of oxygen in the unit cell was set on the basis of δ estimated from TPR. At that time, αoxygen was assumed to be located in the pentamer unit that

External surface areas obtained by N2 adsorption are also shown in Table 2. External surface areas of MoVO were not changed by redox treatment and were in the range of 5.7−6.5 m2 g−1. As observed in our previous study, MoVO(0) showed micropore adsorption.16 Reduction treatment of MoVO(0) caused small changes in measured micropore volumes, possibly due to the desorption of oxygen incorporated in the heptagonal channel. VCO2, VCH4, VC2H6, and VN2 were almost unchanged in the range of MoVO(0.8)−MoVO(4.2) and were 20−25 × 10−3 cm3 g−1. VN2 was slightly lower than those volumes and was 13.3−17.6 cm3 g−1. It is well-known that N2 adsorption at liquid N2 temperature gives unrealistically small pore volumes due to the diffusional limitations of N2 into the pore if the pore mouth diameter is small (less than 0.60 nm).48,49 Actually, the micropore volume expected from the crystallographic data (23.4 × 10−3 cm3 g−1) matches well with VCO2, VCH4, and VC2H6 of MoVO(0.8)−MoVO(4.2).2 When the catalyst was reduced up to MoVO(5.4), VCO2 became about half of those of MoVO(0.8)−MoVO(4.2) and VCH4, VC2H6, and VN2 became almost 0. With further reduction above MoVO(6.1), all of the estimated micropore volumes became almost 0. This is due to the decrease in size of the heptagonal channel by the reduction.15,17 As discussed in XRD, IR, and Raman analyses, reduction above MoVO(5.4) causes continuous structural change in the a−b plane, which is involved in the decrease in strength of the Mo−O−Mo bond in the pentagonal [Mo6O21]6− unit. This continuous structural change may be related to the decrease in heptagonal channel size. In the case of propane adsorption, VC3H8 increased up to MoVO(4.2) and then decreased with further reduction (Figure 6). The kinetics diameter of C3H8 is 0.43 nm and is slightly larger than that of the size of the heptagonal channel (0.40 nm). Since the moderately reduced catalyst is able to adsorb C3H8, the size of which is larger than the size of the heptagonal channel, the size of the heptagonal channel was found to be increased by reduction. With further reduction, the heptagonal channel size decreased. As implied in TPR, the evolution of α-oxygen was 7201


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Figure 8. (A) Structural model of MoVO(Mo, light green; V, gray; O, red). (B) Diameters of the heptagonal channel. Diameters were determined on the basis of atomic positions of oxygen. Red bar, D1 (O24−O26, long axis); blue bar, D2 (O6−O27, short axis).

faced the heptagonal channel, and β-oxygen was assumed to be located in the framework unit in the XRD part. Since the evolution of β-oxygen is involved in the lattice parameter change not only for the a−b plane but also for the c-axis, βoxygen was assumed to be the axial oxygen bridging to the c direction. An occupancy of the α-oxygen and β-oxygen was determined on the basis of the TPR signal area. In this time, evolved oxygen of MoVO(6.8)-AC was assumed to be all attributed to the β-oxygen. The results of Rietveld refinements for the catalysts are shown in Figures S10−S18. The weighted agreement factors that represent a difference between the observed XRD pattern and simulated pattern obtained from the structural model were in the range of 7.1−8.2%, indicating that the crystal structure was adequately refined for all of the catalysts. We evaluated the detailed structure at each reduction state of MoVO. Figure 7 shows the structural model (A) and structural coordinates in x and y of the pentagonal [Mo6O21]6− unit as a function of δ ((B−G)). The atomic number of oxygen is marked in the structural model. Mo1−Mo5 are edge-shared octahedra and Mo6 is the center of the [MoO7] pentagonal bipyramidal in the pentagonal [Mo6O21]6− unit. The fractional coordinates of Mo1, Mo5, and Mo6 were greatly changed between MoVO(0) to MoVO(0.8), suggesting that the desorption of oxygen in the heptagonal channel cause changes in the metal positions. The fractional coordinates of Mo2, Mo3, Mo5, and Mo6 were substantially increased in the x and y directions (a and b axes, respectively) by reduction above MoVO(5.4). These movements indicate that these metals are close to the heptagonal channel (Figure 7, parts C, D, F, G). On the other hand, the fractional coordinates of Mo1 and Mo4 slightly decreased with increase in δ (Figure 7, parts B, E). The different movements of Mo1 and Mo4 from those of Mo2, Mo3, Mo5, and Mo6 expand the pentagonal [Mo6O21]6− unit due to the increase in bond length. This is in good agreement with results of IR and Raman analyses indicating that the strength of the Mo−O−Mo bond in the pentagonal [Mo6O21]6− unit is weakened by reduction above MoVO(5.4). The fractional coordinates of these atoms, of the reoxidized catalysts, were almost the same as those of MoVO(0) (Tables S1, S9, and S10). The expanded pentagonal [Mo6O21]6− unit was found to shrink with reoxidation. Figure 8 shows the structural model (A) and the diameters of the heptagonal channel in the short axis (D1) and long axis (D2) (B) obtained by Rietveld refinement. The diameters of the heptagonal channels were determined on the basis of the atomic positions of oxygen. The atomic number of oxygen is marked in the structural model.

The size of the heptagonal channel was changed by the movement of metals. The diameter in the long axis (D1) increased with increasing δ due to the lattice expansion (from 0.652 nm for MoVO(0) to 0.663 nm for MoVO(6.8)) and decreased with reoxidation (Figure 8B, red bar). As well as D1, the diameter in the short axis (D2) increased with increasing δ in the range of MoVO(0) (0.605 nm)−MoVO(4.2) (0.612 nm), but it drastically decreased with further reduction and D2 of MoVO(6.8) was 0.582 nm (Figure 8 (B), blue bar). It was found that the movements of Mo2, Mo3, Mo5, and Mo6 toward the heptagonal channel decreased D2. The decrease of D2 should be related to the loss of microporosity of the catalysts above MoVO(5.4). Actually, D2 was increased by reoxidation, which explains the recovery of the microporosity as observed in the adsorption analyses. For enlargement of the heptagonal channel by reduction up to MoVO(4.2), MoVO was found to release α-oxygen, which is assumed to be located at O29, in the early stage of the reduction. The evolution of O29 should increase the heptagonal channel size. Therefore, C3H8, the size of which is larger than that of the heptagonal channel, may be able to enter the heptagonal channel due to the generation of a void space by evolution of α-oxygen (O29). Based on the above results, we propose that the structural change involving the evolution of α-oxygen and the expansion of the pentagonal [Mo6O21]6− unit is strongly related to the microporosity. From the next section, we discuss the relationship between microporosity and catalytic activity for selective oxidation of ethane. 3.5. Selective Oxidation of Ethane. Based on the Rietveld refinement, we proposed that the microporosity of MoVO is strongly related to the structure around the heptagonal channel. We have reported that ethane is converted not on the crystal surface but in the heptagonal channel.2,3 This situation suggests that the microporosity change caused by the partial structural change around the heptagonal channel affects the catalytic activity for selective oxidation of ethane. We tested the catalysts for selective oxidation of ethane in order to understand the relevance of crystal structure, microporosity, and catalytic activity for selective oxidation of ethane. Ethane conversion of the catalysts at 300 °C over the course of the reaction time is shown in Figure 9 (A). Ethane conversion and product selectivity at 10 min and at 69 h from the start of the reaction are shown in Table S20. The products detected were ethene, acetic acid, and COx. The initial ethane conversion of MoVO(0) was 13.7% and was slightly increased with increase in reaction time, showing 17.9% ethane conversion at 69 h from the start of the reaction. The product 7202


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conversion rates reached 34.5% and 24.6% at 69 h, respectively. The selectivities to ethene were 92.4% and 93.3% at the initial stage and 89.8% and 89.7% at 69 h. For the reoxidized catalysts, the initial conversion of MoVO(2.9)-AC was 10.0%. The conversion was slightly increased with increase in reaction time and was 12.7% at 69 h. The selectivities to ethene were 89.4% at the initial stage and 86.5% at 69 h. For MoVO(6.8)-AC, the initial ethane conversion was 31.2% and that at 69 h was 34.6%, and the selectivities to ethene were 84.4% at the initial stage and 87.1% at 69 h. Surprisingly, the conversion of MoVO(6.8)AC was about 3-times higher than that of MoVO(2.9)-AC, although both of them were calcined in air under the same conditions after the reduction. As discussed above, the difference between MoVO(2.9)-AC and MoVO(6.8)-AC was only the presence of α-oxygen in the structure. Therefore, the evolution of α-oxygen is one of the main reasons for the increase in catalytic activity for selective oxidation of ethane. Ethane conversion at 10 min from the start of the reaction as a function of δ is shown in Figure 9B. The initial ethane conversion was increased with increasing δ in the range of MoVO(0)−MoVO(4.2), but the conversion drastically decreased with further reduction. As discussed for the catalytic activity difference between MoVO(2.9)-AC and MoVO(6.8)AC, the evolution of α-oxygen should strongly contribute to the increase in catalytic activity. Therefore, the increase in catalytic activity in the range of MoVO(0)−MoVO(4.2) should be due to the evolution of α-oxygen. In the case of MoVO(5.4)− MoVO(6.8), structural analysis showed that Mo2, Mo3, Mo5, and Mo6 in the pentagonal [Mo6O21]6− unit moved toward the heptagonal channel, resulting in the loss of microporosity. Taking into account the fact that ethane is converted in the heptagonal channel, the drop in the initial ethane conversion with reduction above MoVO(5.4) would be due to the inaccessibility of ethane into the heptagonal channel. We then investigated the reasons why the ethane conversion of MoVO(5.4), MoVO(6.1), and MoVO(6.8) increased drastically with increase in reaction time despite the fact that only small increases in catalytic activity were found for the other catalysts. Figure S19 and Figure S20 show the results of in situ XRD under 100 mL min−1 of 5% H2/N2 and 50 mL min−1 of air at 400 °C or under 100 mL min−1 of 10% C2H6/ N2 and 50 mL min−1 of air at 300 °C. Measurements were carried out on the (210) plane because this plane was especially active for redox treatment. The gas atmosphere was switched for each, 1 h under the reductive condition and 0.5 h in the

Figure 9. (A) Ethane conversion as a function of reaction time: closed circle, MoVO(0); closed triangle, MoVO(0.8); closed square, MoVO(2.9); closed lozenge, MoVO(4.2); open circle, MoVO(5.4); open triangle, MoVO(6.1); open square, MoVO(6.8); open circle in closed square, MoVO(2.9)-AC; cross mark in open square, MoVO(6.8)-AC. (B) Ethane conversion at 10 min from the start of the reaction as a function of δ. Reaction conditions: catalyst weight, 0.5 g; reaction temperature, 300 °C; reaction gas feed, C2H6/O2/N2 = 5/ 5/40 mL min−1.

selectivity to ethene was almost unchanged during the reaction and was 88.5% at the start of the reaction and 87.1% at 69 h. When the catalyst was slightly reduced, MoVO(0.8) showed 15.4% initial ethane conversion with 87.1% ethene selectivity. The conversion was slightly increased with increase in reaction time and reached 17.9% at 69 h. The selectivity to ehene was almost unchanged during the reaction and was 87.3% at 69 h. With further reduction, ethane conversion was substantially increased. The initial conversion rates of MoVO(2.9) and MoVO(4.2) were 24.1% and 33.0% and the conversion rates at 69 h were 23.6% and 35.7%, respectively. The selectivities to ehene were decreased by reduction and were 79.6% and 76.8% at the initial stage and 82.9% and 81.4% at 69 h for MoVO(2.9) and MoVO(4.2), respectively. These decreases in ethene selectivity were due to the increase in COx selectivity. For more reduced catalysts, the initial conversion of MoVO(5.4) was 8.8%, which was far less than that of MoVO(4.2), but the conversion was increased drastically with increase in reaction time and 41.6% conversion was achieved at 69 h. The selectivity to ethene of MoVO(5.4) was higher than that of MoVO(4.2) for all reaction times and was 88.3% at the initial stage and 88.5% at 69 h, due to the decrease in COx selectivity. In the case of MoVO(6.1) and MoVO(6.8), the initial ethane conversion rates were both negligible, but their conversion rates were increased with increase in reaction time and the

Table 3. Physicochemical Properties of MoVO after Selective Oxidation of Ethane micropore volume/10−3 cm3 g−1

lattice parameterb/nm catalyst

bulk V/Moa

a

b

c

external surface areac/m2 g−1

VN2c

VCO2d

VCH4d

VC2H6d

V C3 H 8 d

MoVO(0) MoVO(0.8) MoVO(2.9) MoVO(4.2) MoVO(5.4) MoVO(6.1) MoVO(6.8) MoVO(2.9)-AC MoVO(6.8)-AC

0.39 0.39 0.39 0.39 0.39 0.39 0.39 0.38 0.39

2.102 2.106 2.104 2.108 2.108 2.109 2.109 2.103 2.107

2.647 2.647 2.647 2.649 2.654 2.654 2.655 2.647 2.651

0.4008 0.4005 0.4008 0.4005 0.4002 0.4003 0.4001 0.4007 0.4002

3.6 3.5 3.5 4.8 4.6 5.4 4.8 4.9 4.8

9.0 10.7 9.5 11.9 13.8 12.2 9.2 13.9 12.0

23.5 25.1 21.9 22.6 22.1 17.9 16.1 23.2 21.3

20.8 22.9 18.1 23.8 17.9 15.8 13.1 21.0 23.3

19.3 19.9 17.1 22.5 20.4 17.0 12.3 20.2 20.5

5.7 5.3 6.2 16.1 21.2 16.9 13.9 5.4 18.9

a

Determined by ICP. bMeasured by XRD and calculated by Rietveld refinement. cMeasured by N2 adsorption at liquid N2 temperature and calculated by a t-plot. VN2 represents the micropore volume measured by using N2. dMicropore volume measured by CO2, CH4, C2H6, and C3H8 adsorption and estimated by the DA method. 7203


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MoVO(6.8) in the early stage of analysis, suggesting partial structural change in the a−b plane as indicated by ex-situ XRD analyses. It is notable that the partial structural change was almost completed at about 360 min after the start of the reaction, although the catalytic activity at 360 min was still very low. Detailed investigation of this difference is now in progress. The experimental results further confirmed that the increase in catalytic activity of the catalysts above MoVO(5.4) during the reaction is surely derived from the structural change in the a−b plane. Hence, we investigated the active structure for selective oxidation of ethane. As discussed above, the evolution of αoxygen increases VC3H8. Therefore, if the evolution of α-oxygen contributes to the increase in catalytic activity, a proportional relationship should be found between catalytic activity and VC3H8. Figure S25 shows ethane conversion at 69 h as a function of VC2H6 or VC3H8 obtained after the catalytic test. Although no relationship between ethane conversion and VC2H6 was found, there was a positive correlation between ethane conversion and VC3H8. This implies that the void space produced by the evolution of α-oxygen contributes to the catalysis for selective oxidation of ethane. Figure S26 shows the Arrhenius plot of MoVO(2.9)-AC and MoVO(6.8)-AC. The difference between these two catalysts is only the number of α-oxygen atoms in the structure. The slopes of the lines were almost the same, indicating that the activation energy of ethane was almost the same and was ca. 82 kJ mol−1, being in agreement with our previous report.1 However, the intercepts were very different. Calculated frequency factors of MoVO(2.9)-AC and MoVO(6.8)-AC were 0.86 × 109 mmol s−1 gcat−1 and 2.20 × 109 mmol s−1 gcat−1, respectively. The frequency factor of MoVO(6.8)-AC was three times higher than that of MoVO(2.9)-AC. These results indicate that the catalytically active site for ethane activation is the same regardless of the presence of α-oxygen; however, the number of active sites is increased by the evolution of α-oxygen. These results support our proposal that the space produced by the evolution of αoxygen contributes to the catalysis. Based on these results, the void space generated by evolving α-oxygen should be the active site for the reaction. The space produced by the evolution of αoxygen may moderately activate molecular oxygen, leading to high catalytic activity for selective oxidation of ethane. Investigation of the active oxygen species is now in progress.

oxidative condition, in order to elucidate the partial structural change in the a−b plane. The d value and the peak intensity of (210) increased under 5% H2/N2 flow and decreased under air flow. This d value change and intensity change were reversible for the redox treatment, being in agreement with the results of ex situ XRD analysis (Figure 4). The same trend was observed when 10% C2H6/N2 was used as a reductive gas instead of 5% H2/N2. The experimental results clearly show that ethane can cause partial structural change in the a−b plane. Therefore, partial structural change of the catalysts may occur during the selective oxidation of ethane. Structural analysis of the catalysts after the reaction was therefore carried out. Figure S21 shows the XRD, IR, and Raman results, and Table 3 shows the physicochemical properties of the catalysts after selective oxidation of ethane. No elemental composition changes were observed. The external surface areas of the catalysts used were slightly smaller than those before the reaction and were in the range of 3.5−5.4 m2 g−1. After the reaction, no peaks related to impurities were observed in XRD, indicating high stability of the catalyst structures under the catalytic test. However, slight changes in peak positions and in peak intensity ratios were observed, particularly in the low angle region (2θ = 4−10°). This finding indicates that the structure, especially that in the a−b plane, is changed during selective oxidation of ethane. At the same time, IR or Raman bands attributed to the Mo−O−Mo bond in the pentagonal [Mo6O21]6− unit were shifted and the band positions of the catalysts became almost the same, suggesting that bond lengths in the pentagonal [Mo6O21]6− units were almost the same in all of the catalysts. Rietveld refinement performed on the catalysts after the catalytic test revealed that the lattice parameters of the catalysts were almost the same regardless of the initial reduction states (Table 3). This implies that the amount of β-oxygen was almost the same for all of the catalysts used, because of the evolution of β-oxygen (for MoVO(0), MoVO(0.8), MoVO(2.9), MoVO(2.9)-AC, and MoVO(6.8)-AC) or incorporation of gaseous oxygen into the framework structure as βoxygen (for MoVO(4.2), MoVO(5.4), MoVO(6.1), and MoVO(6.8)). This can be speculated from the lattice parameter change of the catalysts before the reaction. The structural models obtained by Rietveld refinement indicated that the diameter of the heptagonal channel in the short axis (D2 in Figure 8A) was increased in MoVO(5.4), MoVO(6.1), and MoVO(6.8) after the reaction. D2 values of the MoVO(5.4), MoVO(6.1), and MoVO(6.8) catalysts were 0.608, 0.608, and 0.606 nm, respectively (data not shown). By the increase in D2, the micropore volumes estimated from adsorption of various molecules of MoVO(5.4), MoVO(6.1), and MoVO(6.8) were significantly increased (Table 3). We concluded that the partial structural change around the heptagonal channel during the catalysis restored the microporosity of MoVO(5.4), MoVO(6.1), and MoVO(6.8), contributing to the increase in catalytic activity. For further confirmation of the partial structural change during the reaction, in situ XRD analysis was carried out. Figures S22, S23, and S24 show the results of in situ XRD analysis for MoVO(0), MoVO(4.2), and MoVO(6.8) at 300 °C under C2H6/O2/N2 = 10/10/80 mL min−1 gas flow. Peak intensities of (020), (120), and (210) are plotted as a function of holding time at 300 °C. Figures S22 and S23 show that the peak intensities of MoVO(0) and MoVO(4.2) were almost unchanged, indicating almost no structural change in the a−b plane during the in situ XRD. However, clear change in peak intensity was observed for

4. CONCLUSION Redox treatments of orthorhombic Mo29V11O112‑δ catalyst (MoVO) were carried out in order to obtain catalysts with different reduction states. Single crystal analysis combined with Rietveld refinement provided a detailed structural model of MoVO for each reduction state that revealed partial structural change in the a−b plane caused by the redox treatment. The microporosity of MoVO was strongly dependent on the crystal structure in the a−b plane. The change in microporosity significantly affected the catalytic activity for selective oxidation of ethane. Strong dependence was found between the crystal structure, microporosity, and catalytic activity for the selective oxidation of ethane. This work has provided a comprehensive understanding for the catalysis of ethane involving the micropore as the catalysis field and has enabled us to express the catalytic reaction at the molecular level. 7204


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ASSOCIATED CONTENT

are grateful to Dr. Greg Shaw at Cardiff Catalysis Institute, Cardiff University for careful reading of the manuscript.

S Supporting Information *

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Figures showing the crystal image that was used for single crystal analysis, weight losses of MoVO(0), TPR spectra of MoVO(0), UV spectra of MoVO, adsorption isotherms and tplots of MoVO, Rietveld refinement of MoVO, d value change and peak intensity change of the (210) plane of MoVO(0) measured by in situ XRD, XRD patterns, IR spectra, and Raman spectra of MoVO after ethane oxidation, change in peak intensities of MoVO(0), MoVO(4.2), and MoVO(6.8) as a function of time under C2H6/O2/N2 = 10/10/80 mL min−1 at 300 °C measured by in situ XRD, ethane conversion at 69 h as a function of micropore volumes measured by C3H8 or C2H6 for the catalysts after the reaction, Arrhenius plot for ethane conversion over MoVO(2.9)-AC and MoVO(6.8)-AC and tables of atomic coordinates and occupancy of the framework of MoVO(0), bond distances of MoVO(0), atomic coordinates and occupancy of the framework of MoVO(0.8), atomic coordinates and occupancy of the framework of MoVO(2.9), atomic coordinates and occupancy of the framework of MoVO(4.2), atomic coordinates and occupancy of the framework of MoVO(5.4), atomic coordinates and occupancy of the framework of MoVO(6.1), atomic coordinates and occupancy of the framework of MoVO(6.8), atomic coordinates and occupancy of the framework of MoVO(2.9)AC, atomic coordinates and occupancy of the framework of MoVO(6.8)-AC, refined parameters and agreement factors of Rietveld refinement for MoVO(0), refined parameters and agreement factors of Rietveld refinement for MoVO(0.8), refined parameters and agreement factors of rietveld refinement for MoVO(2.9), refined parameters and agreement factors of Rietveld refinement for MoVO(4.2), refined parameters and agreement factors of Rietveld refinement for MoVO(5.4), refined parameters and agreement factors of Rietveld refinement for MoVO(6.1), refined parameters and agreement factors of Rietveld refinement for MoVO(6.8), refined parameters and agreement factors of Rietveld refinement for MoVO(2.9)-AC, refined parameters and agreement factors of Rietveld refinement for MoVO(6.8)-AC, and ethane conversion and product selectivity at the initial stage and at 69 h after the start of the reaction, and cif files for the crystal structures. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (W.U.). *E-mail: s.ishikawa(@)cat.hokudai.ac.jp (S.I.). Author Contributions

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI Grant Number 2324-6135. The synchrotron radiation experiments were performed at the BL40XU of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal Nos. 2011B1181, 2012A1161, and 2014A1316). We 7205


Article

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Redox Treatment of Orthorhombic Mo29V11O112 and Relationships

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