Synthesis of Crystalline Orthorhombic Mo-V-Cu Oxide for Selective

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Synthesis of Crystalline Orthorhombic Mo-VCu Oxide for Selective Oxidation of Acrolein Satoshi Ishikawa, Yudai Yamada, Chuntian Qiu, Yoshito Kawahara, Norihito Hiyoshi, Akihiro Yoshida, and Wataru Ueda Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b04962 • Publication Date (Web): 30 Jan 2019 Downloaded from http://pubs.acs.org on February 2, 2019

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Chemistry of Materials

Synthesis of Crystalline Orthorhombic Mo-V-Cu Oxide for Selective Oxidation of Acrolein Satoshi Ishikawa a, Yudai Yamada a, Chuntian Qiu ,b,†, Yoshito Kawahara a, Norihito Hiyoshi c, Akihiro Yoshida a,††, Wataru Ueda a* a

Department of Material and Life Chemistry, Faculty of Engineering, Kanagawa University, 3-27-1, Rokkakubashi, Kanagawa-ku, Yokohama, 221-8686, Japan b

Catalysis Research Center, Hokkaido University, N-21, W-10, Sapporo 001-0021, Japan

c

National Institute of Advanced Industrial Science and Technology (AIST), 4-2-1 Nigatake, Miyagino, Sendai 9838551, Japan †

SZU-NUS Collaborative Center and International Collaborative Laboratory of 2D Materials for Optoelectronic Science & Technology of Ministry of Education, College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China ††

Current address: Institute of Regional Innovation, Hirosaki University, 2-1-3 Matsubara, Aomori, 030-0813, Japan

ABSTRACT: Crystalline orthorhombic Mo-V-Cu oxide (MoVCuO) was successfully obtained by hydrothermal synthesis using methylammonium cation as a structure-directing agent, and the role of Cu for selective oxidation of acrolein (ACR) to acrylic acid (AA) was evaluated using crystalline Mo3VOx (MoVO) as a comparison. Cu was located at the corner of a heptagonal channel micropore in the MoVO structure with a square planner configuration. The introduction of Cu substantially improved AA selectivity up to 98% from 93% obtained over the MoVO catalyst. The introduction of Cu would moderate activation of molecular oxygen at the heptagonal channel site and prevent side reactions such as oxidative C-C scission to form COx and acetic acid, resulting in enhancement of AA selectivity.

■ INTRODUCTION Acrylic acid (AA) is an important intermediate for various valuable products including esters, superabsorber polymers, detergents, and water treatment agents1-6. Currently, AA is mainly produced via a two-step oxidation process from propylene. The first step is selective oxidation of propylene to form acrolein (ACR) over Mo-Bibased mixed metal oxides, and the following step is oxidation of ACR over Mo-V-based mixed metal oxides to form AA1-11. Both of the oxidation steps have been well developed, and AA now obtained from propylene is over 90% yield. However, the catalysis in the above reactions has remained poorly understood due to the complexity of the mixed metal oxides. This situation is largely derived from nonuniformity of the crystal structure since the mixed metal oxide catalysts are normally amorphous or polycrystalline in nature. The lack of fundamental information about catalytically active sites makes rational catalyst design for further catalyst development difficult, and trial and error works are still required to make progress in the development of catalysts. The situation will become more complex even if some progresses is made by the above methodology due to the increased complexity. Mo-Vbased mixed metal oxides, which are industrially used for selective oxidation of ACR, is faced with this serious issue. These catalysts are normally produced by the addition of

various amounts of elements as promoters, including W, Cu and Sb, under the condition of a lack of fundamental information about their catalytic functions1-4, 12-13. Future catalyst development should be conducted on the basis of fundamental information about the catalytic functions of constituent elements. We believe that this concept will finally result in the production of evolutional oxidation catalysts containing a catalytically active site in a controlled manner. We recently reported that crystalline orthorhombic Mo3VOx (MoVO) is an extremely active catalyst for selective oxidation of ACR to AA13-16. MoVO is comprised of a network arrangement based on a pentagonal {Mo6O21}6unit and {MO6} octahedra (M = Mo, V), forming a framework structure (Figure 1). The void space of the framework structure is filled with a pentamer unit constituted by five {MO6} octahedral, which results in the formation of hexagonal and heptagonal channels in the crystal structure. Among these structural sites, the local crystal structure around the heptagonal channel was found to work as a catalysis field for ACR oxidation. Since the catalysis field for ACR oxidation is now clear, the catalytic roles of promoters for ACR oxidation can be evaluated if promoters can be introduced into the MoVO structure without altering its crystal structure. However, synthesis of MoVO containing promoters periodically in the crystal

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hydrothermal reaction was started at 175 °C for 48 h under static conditions in an electric oven. The formed gray solid was washed with distilled water and dried in air at 80 °C overnight. The obtained material was abbreviated as MoVO-fresh. Purification with oxalic acid was then conducted for MoVO-fresh in order to remove an impurity derived from amorphous-type materials. To 25 mL of an aqueous solution (0.4 mol L-1, 60 °C) of oxalic acid (Wako), 1 g of MoVO-fresh was added and the solution was stirred for 30 min, followed by washing with more than 500 mL of distilled water during filtration. The sample after the purification is abbreviated as MoVO-oxa. Air calcination of MoVO-oxa was carried out under static air at 400 °C for 2 h prior to the catalytic reaction. The sample after the air calcination is abbreviated as MoVO-AC. Synthesis of Mo-V-Cu oxide catalyst. For the synthesis of orthorhombic Mo-V-Cu oxide (MoVCuO) with the same crystal structure as that of MoVO, methylammonium heptamolybdate (MAHM, (CH3NH3)6Mo7O24) was used as a Mo source instead of (NH4)6Mo7O24・4H2O (AHM). MAHM was prepared by our previously reported method17. First, 21.594 g of MoO3 (0.150 mol, Kanto) was dissolved in 33.2 mL of 40% methylamine solution (methylamine: 0.300 mol, Wako). After being completely dissolved, the solution was evaporated under vacuum (P/P0 = 0.03) at 75 °C and then a solid powder was obtained. The powder was dried in air at 80 °C overnight. Figure 1. Structural model of MoVO. Mo, green; V, gray; O, red. structure has been hardly achieved so far, and the catalytic roles of promoters have remained poorly understood. We successfully synthesized a crystalline orthorhombic Mo-V-Cu mixed metal oxide (MoVCuO) having the same crystal structure as that of MoVO by a hydrothermal method using an appropriate structure-directing agent, and the catalytic role of Cu, which is widely used as a promoter for ACR oxidation, could be clarified. We believe the findings obtained here will pave a way for the rational design of the Mo-V based mixed metal oxide based on the information about the catalytically active site. ■ EXPERIMENTAL SECTION Synthesis of Mo3VOx catalyst. Crystalline orthorhombic Mo3VOx (MoVO) was synthesized by a hydrothermal method. First, 8.828 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 3.288 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. The mixed solution (pH 3.2) was introduced into an autoclave with a 300-mL Teflon inner vessel and a thin Teflon sheet of 4000 cm2 occupying about half of the Teflon inner vessel space. N2 was then fed into the prepared solution in the tube in order to remove residual oxygen for obtaining well-crystallized material. Then the

First, 1.783 g of MAHM (Mo: 10 mmol) was dissolved in 30 mL of distilled water. Separately, an aqueous solution of VOSO4 was prepared by dissolving 0.658 g of hydrated VOSO4 (V: 2.5 mmol) in 10 mL of distilled water. The two solutions were mixed at ambient temperature and stirred for 10 min before the addition of 0.156 g of CuSO4·5H2O (Cu: 0.63 mmol, Wako). The pH value of the solution was not changed by the addition of CuSO4·5H2O and was 3.2. This mixed solution was introduced into an autoclave with a 50-mL Teflon inner vessel and a thin Teflon sheet of 800 cm2 occupying about half of the Teflon inner vessel space. After being introduced, N2 was fed into the solution in order to remove residual oxygen. Then the hydrothermal reaction was carried out at 175 °C for 20 h with rotation at 1 rpm min-1. Rotation is necessary to obtain a highly pure material. The formed solid was separated by filtration, washed with distilled water, and dried in air at 80 °C overnight. The obtained material was abbreviated as MoVCuO-fresh. In order to remove an amorphous type of impurity concomitantly formed with MoVCuO-fresh, purification with oxalic acid was performed. To 25 mL of an aqueous solution (0.4 mol L-1, 60 °C) of oxalic acid (Wako), 1 g of MoVCuO-fresh was added and the solution was stirred for 30 min, followed by washing with more than 500 mL of distilled water during filtration. The sample after the purification is abbreviated as MoVCuO-oxa. Air calcination of MoVCuO-oxa was then conducted under static air at 400 °C for 2 h. The sample after the air calcination is abbreviated as MoVCuO-AC. If necessary, purification using 1.2 M HCl solution was conducted for MoVCuO-AC in order to remove impurities related to Cu species. To 20 mL of 1.2 M HCl solution, 1 g of MoVCuO-


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AC was dispersed and the solution was stirred for 30 min at room temperature, followed by washing with more than 1000 mL of distilled water during the filtration. The obtained material was dried in air at 80 °C overnight and then calcination was conducted under static air at 250 °C for 2 h. The thus-obtained material is abbreviated as MoVCuO-AC-HCl. When no Cu source was used in the synthesis, the obtained material was denoted as MoVOMA-fresh. After oxalic acid treatment, the resulting material is abbreviated as MoVO-MA-oxa. The sample obtained after air calcination of MoVO-MA-oxa at 400 °C for 2 h is abbreviated as MoVO-MA-AC. For comparison, Mo-V-Cu oxide was synthesized using (NH4)6Mo7O24·4H2O (AHM) as a Mo precursor. First, 1.766 g of AHM (Mo: 10 mmol) was dissolved in 30 mL of distilled water. This solution was mixed with an aqueous solution prepared by dissolving 0.658 g of hydrated VOSO4 (V: 2.5 mmol) in 10 mL of distilled water. The two solutions were mixed at ambient temperature and stirred for 10 min before the addition of 0.156 g of CuSO4·5H2O (Cu: 0.63 mmol). The pH value at that time was 3.1. Then N2 bubbling was carried out for 10 min, following hydrothermal reaction at 175 °C for 48 h with rotation at 1 rpm min-1. The formed material was separated by filtration, washed with distilled water, and dried in air at 80 °C overnight. The obtained material was abbreviated as MoVCuO-A-fresh. Characterization of materials. The obtained materials were characterized by the following techniques. Powder XRD patterns were recorded with a diffractometer (RINT Ultima+, Rigaku) using Cu-K radiation (tube voltage: 40 kV, tube current: 40 mA). For Rietveld refinement, samples were ground with Si standard for 10 min with the help of 5 mL of acetone in order to correct the peak position and to exclude an orientation effect. Diffractions were recorded in the range of 4°~80° with 1° min1 scan speed. SEM-EDX analysis was carried out with an electron microscope (SU8010, Hitachi) equipped with an EDX detector (EMAX Evolution X-MAX, Horiba). Elemental compositions in the bulk were measured by ICPAES (ICPE-9820, Shimadzu). XPS (JPC-9010MC, JEOL) with non-monochromatic Mg-K radiation was used for measuring binding energy values of Mo and V. Spectra were calibrated by using Au (4f7/2, 84.0 eV) coated using an Au coater (SC-701 MkII, Sanyu Electron) before XPS measurement. N2 adsorption isotherms at liq. N2 temperature were obtained using an auto-adsorption system (BELSORP MAX, Nippon BELL). Prior to the N2 adsorption, the samples were pre-treated under a vacuumed condition at 300 °C for 2 h. The external surface area and micropore volume were determined by the s-plot method. CO2 and CH4 adsorption at room temperature was also conducted using the same instrument. The micropore volumes were determined by the DA method. FTIR analysis was carried out using a spectrometer (FT/IR4700, JASCO). IR spectra were obtained by integration of 64 scans with a resolution of 4 cm-1. Temperatureprogrammed desorption (TPD) was carried out by an auto-chemisorption system (BEL JAPAN). The sample (ca.

50 mg) was sandwiched by two layers of quartz wool. The temperature was increased from 40 °C to 600 °C with 50 mL min-1 of He or 45/5 mL min-1 of He/O2 mixed gas flow. Temperature-programmed desorption (TPD) of ammonia, NH3-TPD, was used to measure oxide surface acidity. The experiment was carried out using the same apparatus. The sample (ca. 50 mg) was sandwiched by two layers of quartz wool and pre-heated under He (50 mL min-1) at 400 °C for 10 min. Then 50 mL min-1 of 12.5% NH3/He was flowed at 200 °C for 30 min, followed by flushing with He for 30 min at the same temperature. The desorption profile from 200 °C to 600 °C was recorded with a mass spectrometer under He flow (50 mL min-1). Aberrationcorrected STEM images were obtained using ARM-200F (JEOL Ltd, Japan) equipped with a cold field emission gun at an acceleration voltage of 200 kV. The convergence semi-angle of the probe was 24 mrad. The collection semi-angle for high-angle annular dark-field (HAADF) imaging was adjusted in the range of 110-170 mrad. The powder diffraction program of Materials Studio 7.1 (Accelrys Software Inc.) was used for XRD simulation experiments. XRD simulation was carried out by changing the location site of Cu in the MoVO structure obtained by single crystal analysis18. Geometry optimization calculation was carried out using the DMol3 program of Materials Studio 7.119. Perdew-Burke-Ernzerhof (PBE) generalized gradient functional and DND basis set were employed. The Rietveld program of Materials Studio 7.1 was used for Rietveld refinement. XRD patterns after correcting the peak position with Si were subjected to the refinement. The occupancy of metals in the framework (comprised of Mo and V) and temperature factors of all atoms were fixed without further refinement from the structural model of MoVO obtained by single crystal analysis18. All metal positions were refined. The positions of oxygen atoms were arranged to be a proper metaloxygen length. The pattern parameters were refined for obtaining the lowest Rwp value. Atom positions are shown in Table S1, and Rietveld analysis parameters are shown in Table S2. Selective oxidation of acrolein. Selective oxidation of acrolein (ACR) was carried out by using a conventional vertical flow system with a Pyrex tubular reactor at an ambient pressure. Catalysts were ground with an agate mortar for 5 min, followed by calcination under static air at 400 °C or at 250 °C (in the case of MoVCuO-AC-HCl) for 2 h. Generally, 0.15 g of the catalyst was diluted with 2.0 g of SiC and put into the tubular reactor. The reactor was heated at a rate of 10 °C min-1 to 250 °C under N2 flow (20 mL min-1). The temperature was measured with a thermocouple inserted in the middle of the catalyst zone. When the temperature reached 250 °C, a reactant gas with the composition of ACR/O2/(N2+He)/H2O = 0.8/4.0/31.8/12.0 mL min-1 was flowed. ACR was supplied by N2 bubbling to liquid ACR solution at 0 °C. Water was supplied by helium bubbling to hot water at 80 °C. After the reaction at 250 °C, the temperature was decreased to 240, 230, 220 and 210 °C with analysis of the reactant and product gases at each reaction temperature. Reactants



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and products were analyzed with three on-line gas chromatographs (Molecular Sieve 13X for O2, N2, and CO with a TCD detector, Gaskuropack54 for CO2 and H2O with a TCD detector, and Porapak Q for ACR, acetic acid and AA with an FID detector). Blank runs showed that no reaction took place without catalysts under the experimental conditions used in this study. Carbon balance was always ca. 98~100%. ■ RESULTS AND DISCUSSION Synthesis and characterization of MoVCuO. First, we tried to prepare a crystalline orthorhombic Mo-V-CuO mixed metal oxide having the same structural framework as that of crystalline orthorhombic Mo3VOx (MoVO). For this purpose, the counter cation involved in the crystal formation of MoVO was altered since the role of a counter cation is sometimes crucial for formation of the MoVO structure20-21. Figure 2 (A) shows XRD patterns of MoVO-fresh and MoVCuO-fresh prepared by using (NH4)6Mo7O24 ・ 4H2O (AHM) and (CH3CH3)6Mo7O24 (MAHM) as the Mo precursors, respectively. MoVO-fresh showed XRD peaks at 2 = 6.7°, 7.9°, 9.0° and 22.2° attributable to the diffraction of (020), (120), (210) and (001) planes of the MoVO structure, respectively, with a small XRD halo below 2 = 10° derived from amorphous materials. No such diffraction peaks were observed when a Cu source was added in this synthesis and an unidentified crystal structure was formed instead of the MoVO structure. When MAHM was used as the Mo precursor (MoVO-MA-fresh), MoVO-MA-fresh showed almost the same XRD pattern as that of MoVO-fresh, indicating formation of the MoVO structure as we reported previously17. However, the crystallinity was obviously poorer than that of MoVO-A-fresh as can be seen in the weak XRD diffraction peaks. Very interestingly, crystalline XRD peaks all attributable to the MoVO structure were observed when a Cu source was added in this synthesis, and the crystallinity derived from the MoVO structure was clearly better than that of MoVO-MA-fresh. The addition of a Cu source was found to be crucial for formation of the Orth-MoVO structure. MoVO and MoVCuO after purifications were used for further investigations. MoVO-fresh was purified with oxalic acid in order to remove amorphous impurities concomitantly formed with the MoVO structure (MoVO-oxa). MoVCuO-fresh was firstly purified with oxalic acid (MoVCuO-oxa). After the air calcination of MoVCuO-oxa (MoVCuO-AC), the material was further purified with 1.2 M HCl in order to remove a small fraction of impurities related to the CuO moiety formed during the calcination of MoVCuO-oxa. The resulting material was abbreviated as MoVCuO-AC-HCl. Firstly, elemental compositions of bulk of MoVCuO-oxa and MoVCuO-AC-HCl were measured by ICP (Figure S1). V/Mo ratios were 0.32 ~ 0.36 in all cases and were slightly lower than that of MoVO-oxa (V/Mo = 0.38). The bulk Cu/(Mo+V) ratio of MoVCuOoxa continuously increased with increase in the preparative Cu/(Mo+V) ratio. In the case of MoVCuO-AC-HCl, the bulk Cu/(Mo+V) ratio increased with increase in the preparative Cu/(Mo+V) ratio up to ca. 3%. However, the

Figure 2. (A) XRD patterns of as-synthesized materials: (a) MoVO-fresh, (b) MoVCuO-A-fresh, (c) MoVO-MA-fresh, and (d) MoVCuO-fresh. (B) XRD patterns of purified materials before and after calcination: (a) MoVO-oxa, (b) MoVCuO-oxa, (c) MoVO-AC, (d) MoVCuO-AC, and (e) MoVCuO-AC-HCl.

Figure 3. N2 adsorption isotherm (A), CO2 adsorption isotherm (B) and CH4 adsorption isotherm (C) over MoVOAC, MoVCuO-AC and MoVCuO-AC-HCl. MoVO-AC, circle; MoVCuO-AC, square; MoVCuO-AC-HCl, triangle. bulk Cu/(Mo+V) ratio became constant despite further increase in the preparative Cu/(Mo+V) ratio. XPS spectra of MoVO-AC, MoVCuO-AC and MoVCuO-AC-HCl are shown in Figure S2. MoVCuO-AC showed an XPS signal derived from Cu2+ as can be judged by the XPS peak position (934.5 eV) and the satellite peak around 943 eV. On the other hand, the XPS signal derived from Cu was significantly decreased by HCl treatment (Table 1, Figure S2), indicating that the Cu2+ species over MoVCuO-AC was removed by the HCl treatment. Accordingly, it can be considered that the linear increase in the bulk Cu/(Mo+V) ratio with increase in the preparative Cu/(Mo+V) ratio is derived from not only the introduction of Cu inside the


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The maximum amount of Cu capable of being introduced into the MoVO structure is thought to be Cu/(Mo+V) = ca. 3%. This value might be decided by the crystal structure of MoVCuO. XPS spectra of Mo and V in MoVCuO-AC(HCl) were almost the same with those of MoVO-AC, indicating that the oxidation state of Mo and V is comparable. Charge balance in the solid might be compensated by lattice oxygen as described latter. The V/Mo ratios near the catalyst surfaces measured by XPS were 0.13 ~ 0.18 and they were similar in all samples. A lower V/Mo ratio on the surface than that in the bulk was also reported by several authors21-23. In the case of MoVO, the XRD halo observed in MoVO-fresh had completely disappeared after purification with oxalic acid, and a highly crystalline XRD pattern attributed to the MoVO structure was observed. The XRD pattern remained unchanged after air calcination and no XRD peaks related to impurities were observed. In the case of MoVCuO, MoVCuO-oxa showed diffraction peaks all attributable to the MoVO structure and the pattern remained unchanged after air calcination or HCl treatment. However, some differences were observed between MoVO and MoVCuO in XRD peak intensities and peak positions. Table 1 shows the lattice parameters of MoVO-oxa, MoVO-MA-oxa, MoVCuO-oxa and MoVCuO-AC-HCl before and after air calcination. The lattice parameters of MoVO-oxa were a = 2.105 nm, b = 2.647 nm and c = 0.3996 nm. These parameters were decreased to the a-b axes and increased to the c axis by air calcination, and the lattice parameters of MoVO-AC were a = 2.100 nm, b = 2.644 nm and c = 0.4014 nm. In the case of MoVO-MAoxa, the lattice parameter to the a axis was slightly larger than that of MoVO-oxa, possibly due to the location of different counter cations in the crystal structure (MoVOoxa, NH4+; MoVO-MA-oxa, CH3NH3+), and the lattice parameters were a = 2.115 nm, b = 2.648 nm and c = 0.3997 nm. The lattice parameters were decreased to the a-b axes and increased to the c axis by air calcination as were those of MoVO-oxa and became a = 2.104 nm, b = 2.648 nm and c = 0.4008 nm. In the case of MoVCuO-oxa, the lattice

parameters were a = 2.118 nm, b = 2.650 nm and c = 0.4023 nm. Despite the similarity of lattice parameters to those of MoVO-MA-oxa in the a-b axes, the lattice parameter to the c axis was clearly larger than those of MoVO-oxa and MoVO-MA-oxa. The parameter to the c axis was almost unchanged even after air calcination, while the lattice parameters to the a-b axes were decreased as were those of MoVO-oxa and MoVO-MA-oxa (lattice parameters of MoVCuO-AC: a = 2.109 nm, b = 2.646 nm and c = 0.4025 nm). HCl treatment caused almost no changes in the lattice parameters (lattice parameters of MoVCuO-AC-HCl: a = 2.107 nm, b = 2.647 nm and c = 0.4020 nm). The introduction of Cu was found to influence the lattice parameters of MoVO particularly that in the c direction, implying that Cu is located at a structural site where the lattice parameter in the c direction is influenced, such as the intervening spaces between the a-b planes. In addition to the shift in XRD peaks, XRD peak intensities were changed by the introduction of Cu due to local structural changes as will be discussed in the following section. N2 adsorption at liquid N2 temperature was carried out. Figure 3 (A) shows the N2 adsorption isotherms of MoVOAC, MoVCuO-AC and MoVCuO-AC-HCl. MoVO-AC showed clear N2 adsorption from a relative pressure (P/P0) lower than 1.0×10-5, indicating the presence of micropores in the crystal structure. The external surface area and the micropore volume estimated by an s-plot were 6.1 m2 g-1 and 12.2×10-3 cm3 g-1, respectively. We previously reported that the empty heptagonal channel in MoVO can work as a micropore with an average diameter of 0.4 nm to adsorb small molecules, and the observed micropore adsorption should thus be derived from the empty heptagonal channel in the MoVO structure 16, 18, 24-25. The theoretical micropore volume of the empty heptagonal channel was calculated to be 23.4×10-3 cm3 g-1 based on crystallographic data of the MoVO structure. The micropore volume measured by N2 adsorption was clearly smaller than the theoretical value. It has been reported that N2 adsorption at liquid N2 temperature gives an unrealistically small pore volume due to the diffusional limi-

Table 1. Elemental compositions and lattice parameters of the catalysts Lattice parametersb /nm

Elemental composition (Mo/V/Cu) Sample Bulk (ICP)

MoVO-oxa

1/0.38/-

MoVO-MA-oxa

1/0.24/-

MoVCuO-oxa

1/0.36/0.07

MoVCuO-AC-HCl

1/0.32/0.04

a Elemental b

Surface (XPS)a 1/0.13/-

a

b

c

2.105(4) (2.099(6))

2.646(8) (2.644(4))

0.3996(2) (0.4014(1))

2.114(6) (2.104(2))

2.648(4) (2.648(3))

0.3996(9) (0.4008(4))

(1/0.16/0.11)

2.117(7) (2.109(1))

2.650(4) (2.646(3))

0.4023(1) (0.4025(3))

1/0.15/0.07

2.106(7)

2.646(8)

0.4019(5)

(1/0.13/-) 1/0.18/1/0.14/0.13

composition after the air calcination are shown in parenthesis.

Determined by Rietveld refinement. Lattice parameters after the air calcination are shown in parenthesis.



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Table 2. External surface areas and micropore volumes of catalysts Micropore volume /10-3 cm3 g-1 a

Sample

2

-1

External surface area /m g

VN2a MoVO-AC

6.1

MoVCuO-AC

5.0

MoVCuO-AC-HCl

4.1

12.2

VCO2b

VCH4b

25.8

26.7

0.8

23.4

3.1

6.6

25.5

6.4

Measured by N2 adsorption at liquid N2 temperature and calculated by an s plot. VN2 represents the micropore volume measured by N2 adsorption. a

b Micropore

volume measured by CO2 and CH4 adsorption and calculated by the DA method.

tations of N2 into the micropores when the pore mouth diameter is small (less than 0.60 nm)26. When Cu was introduced, micropore adsorption was significantly decreased. The external surface area and micropore volume were 5.0 m2 g-1 and 0.8×10-3 cm3 g-1, respectively, in MoVCuO-AC and 4.1 m2 g-1 and 6.6×10-3 cm3 g-1, respectively, in MoVCuO-AC-HCl. The observed decrease in micropore volume implies occupation of the heptagonal channel either by Cu or organic molecules that possibly remained even after air calcination. However, the latter possibility can be easily excluded. Figure S3 (A) shows IR spectra of MoVCuO-oxa and MoVCuO-AC. The spectra below 1000 cm-1 derived from metal-oxygen bonds were almost unchanged by air calcination. The IR bands derived from the counter cation are expanded in Figure S3 (B). MoVCuO-oxa showed characteristic IR bands of CH3NH3+ at 1254 cm-1 assigned to the rocking of CH3NH3+, at 1433 cm-1 assigned to the symmetric deformation of CH3, at 1462 and 1473 cm-1 assigned to the asymmetric deformation of CH3, and at 1499 cm-1 assigned to the deformation vibration of NH3, indicating the location of CH3NH3+ in MoVCuO-oxa27-28. Since we reported that CH3NH3+ can be located at the heptagonal channel in the MoVO structure, CH3NH3+ is thought to be located inside the heptagonal channel of MoVCuO-oxa17. Almost no bands related to CH3NH3+ were observed in MoVCuOAC, implying that CH3NH3+ inside the crystal structure was desorbed by the air calcination. Figure S4 (A) and (B) show TPD profiles of MoVCuO-oxa in the presence of oxygen (10% O2/He) and of MoVCuO-AC in the absence of oxygen (only He), respectively. Mass numbers (m/z) of 18, 30, 31 and 44 are ascribed to H2O, NO, CH3NH2 and CO2, respectively. Desorption of H2O, NO and CO2 was observed in MoVCuO-oxa, though almost no desorption of CH3NH2 was observed. This fact indicates that CH3NH3+ inside the crystal structure of MoVCuO-oxa was oxidized with lattice oxygen under the measurement condition and transformed into NO and CO2. The desorption of the above chemicals was completed up to 400 °C. In fact, MoVCuO-AC showed almost no desorption derived from the above chemicals except desorption of physisorbed water (completed up to 200 °C). These results

strongly suggest that MoVCuO-AC-(HCl) contained almost no organic compounds inside the crystal structure. Accordingly, the location of Cu inside the MoVO structure was found to decrease the micropore volume, indicating that Cu should be located at the heptagonal channel micropores. The increase in micropore volume by HCl treatment might be derived from the removal of Cu species located on the mouth of the heptagonal channel. In order to further investigate the microporosity changes by the introduction of Cu, adsorption experiments were carried out for MoVO-AC, MoVCuO-AC and MoVCuO-AC-HCl using CO2 (0.33 nm) and CH4 (0.38 nm) as adsorbates18, 29. These molecules are known to have activated diffusion, and theoretical micropore volume is known to be obtained when CO2 adsorption and CH4 adsorption are conducted. Figure 3 (B) and (C) show adsorption isotherms over MoVO-AC, MoVCuO-AC and MoVCuO-AC-HCl using CO2 and CH4, respectively, as adsorbates. In the case of CO2 adsorption, all of the samples showed comparable adsorption, and the micropore volumes estimated by the DA method were 25.8×10-3 cm3 g-1, 23.4×10-3 cm3 g-1 and 25.5×10-3 cm3 g-1 in MoVO-AC, MoVCuO-AC and MoVCuO-AC-HCl, respectively. These values were comparable with that of the theoretical micropore volume (23.4×10-3 cm3 g-1). In the case of CH4 adsorption, MoVO-AC showed clear CH4 adsorption in a manner similar to that of CO2 adsorption and the micropore volume estimated by the DA method was 26.7×10 3 cm3 g-1. On the other hand, MoVCuO-AC-(HCl) showed clearly less CH4 adsorption than that of MoVO-AC. The estimated micropore volumes were 3.1×10-3 cm3 g-1 and 6.4×10-3 cm3 g-1 in MoVCuO-AC and MoVCuO-AC-HCl, respectively. According to these facts, Cu is thought to be located at the heptagonal channel site where CH4 adsorption is disturbed but CO2 adsorption is not influenced. Most probably, Cu is located at the heptagonal channel site apart from its center. HAADF-STEM analysis was further carried out for MoVCuO-AC in order to obtain additional information about the location site of Cu. HAADF-STEM is a useful method for identifying positions of heavy metal elements because the intensity of the white spot is roughly propor


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Figure 4. (A) HAADF-STEM image of MoVCuO-AC measured from the (001) direction. (B) Enlarged HAADF-STEM image of the red enclosure of (A).

Figure 5. Simulation XRD patterns of MoVCuO calculated after setting Cu at positions 1 ~ 7. Mo, light blue; V, gray; O, red; yellow, Cu. tional to the square of the atomic number (Z), providing an enhanced Z-contrast image30-31. Figure 4 shows an HAADF-STEM image of MoVCuO-AC. The HAADFSTEM image of MoVCuO-AC was almost identical to that of MoVO as reported previously, and almost no elements in both the hexagonal and heptagonal channels were observed, although the location of Cu inside the heptagonal channel was suggested by adsorption experiments30-31. The obtained results are understandable if Cu is assumed to be located at the corner of the heptagonal channel since observation of Cu is expected to be difficult due to the overlapping with contrast of lattice oxygen, even if the oxygen contrast is weak. This assumption does not conflict with the adsorption results. Based on the results of XRD, gas adsorptions and HAADF-STEM, we can speculate that Cu is placed at the corner of the heptagonal channel between the a-b planes. Therefore, Cu is allowed to be located at seven sites as shown in Figure 5. An XRD pattern was then simulated using the structural model of MoVO obtained by single crystal analysis. In this experiment, Cu was set at each of the location sites as shown in Figure 5 and the XRD pattern was simulated for comparison with the experimentally obtained XRD pattern. The occupancy of Cu was set to be 32% according to the results of ICP. The simulation showed that XRD patterns were significantly influenced by the location of Cu. When Cu was located at positions 4~6, an XRD peak attributable to (110) that was not intense in the experimentally obtained XRD pattern ap-

peared. The location of Cu at positons 2~3 gave comparable XRD peak intensity ratios between (020) and (120) (Int(020)/Int(120)), though the ratio was almost half in the observed XRD pattern. The peak intensity ratio between (210) and (120) (Int(210)/Int(120)) was less than 1 when Cu was located at position 7, while the ratio was clearly larger than 1 in the observed XRD pattern. The observed XRD pattern matched well only when Cu was located at position 1. In the cases that Cu was located at the hexagonal channel, the simulated XRD patterns were also different with experimentally obtained XRD pattern in terms of the XRD peak intensity ratios in the similar way as described above (Figure S5). Accordingly, it can be speculated that Cu is located at the intervening spaces between the bridging oxygen in the pentamer unit facing the heptagonal channel. Geometry optimization calculation also supported this assumption. After the geometry optimization, Cu was located at position 1 (Figure S6). The Cu-O distances were 1.932 Å~ 2.083 Å and O-Cu-O angles were 83.7° ~ 89.3°, respectively, implying that Cu has a distorted square planar geometry32-33. In order to confirm the crystal structure of MoVCuO, Rietveld analysis was further carried out for MoVCuOAC-HCl. In this experiment, Cu was simply introduced into position 1 with occupancy of 32% (based on ICP results) and no changes were made for the elemental composition of the MoVO framework since the V/Mo ratios were similar for MoVO and MoVCuO-AC-HCl. Figure 6 shows the results of Rietveld refinement. The experimentally obtained pattern (red) and the simulation pattern calculated from the crystal structure (blue) matched well, and the Rwp value, which represents the difference between these patterns, was 8.1%. The reasonable fitting results strongly support the validity of the proposed crystal structure. Through the above characterizations, we here conclude that Cu can be introduced into the MoVO structure without altering its framework structure. Cu was located under the bridging oxygen in the pentamer unit facing the heptagonal channel with a distorted square planar geometry. Charge balance in the solid might be compensated by lattice oxygen. We reported that the bridging oxygen in the pentamer unit facing the heptagonal channel ( oxygen. Axial oxygen of Cu in Figure 6 (A)) can be easily removed without altering MoVO structure. We speculate that the occupancy of  oxygen is not 1 in MoVO-AC even after the air calcination. In fact, oxygen is hardly return back to the  oxygen defect site by the air calcination18. Occupancy of  oxygen in MoVCuO-AC-(HCl) might be larger than that of MoVO-AC which may result in the charge compensation of the solid. In the next section, the catalytic performance of MoV(Cu)O-AC-(HCl) for selective oxidation of acrolein will be discussed. Selective oxidation of acrolein over MoV(Cu)O. We have reported that the local catalyst structure around the heptagonal channel of MoVO is responsible for the catalysis for selective oxidation of acrolein (ACR) to form



Figure 6. (A) Structural model of MoVCuO-AC-HCl obtained after Rietvelt refinement. Mo, light blue; V, gray; O, red; mixture of Mo and V, light green; Cu, dark red. The crystal structure in the c direction and the local structure around Cu are shown on the right. (B) Difference (black), calculated (blue line), and observed (red) patterns from Rietveld refinement obtained by using XRD data of MoVCuO-AC-HCl. XRD peaks of 27.8°∼28.8°, 46.8°∼47.7°, 55.7°∼56.4°, 68.9°∼69.3°, and 76°∼76.7° were attributed to powdered Si added in order to remove orientation effects and were excluded in Rietveld refinement. acrylic acid (AA) 14-16. In this work, the catalytic role of Cu was evaluated using MoVCuO-AC-(HCl) containing Cu inside the MoVO structure in order to evaluate the catalytic role of Cu, which is mainly used as a prominent promoter for this reaction. Selective oxidation of ACR was carried out over MoVOAC, MoVCuO-AC and MoVCuO-AC-HCl. The XRD patterns of the above catalysts remained unchanged after the catalytic reaction, indicating high thermal stability during the reaction (Figure S7). The characteristic changes in XRD peak intensities caused by the introduction of Cu were still observed after the catalytic reaction. Therefore, the location site of Cu was considered to be not significantly changed regardless of the reaction, while an increase of the Cu amount was observed in the catalyst surface after the reaction (Table 3) possibly due to the formation of Cu metal or migration of bulk Cu to catalyst surface as Cu0 or Cu1+ as can be judged by XPS signal posi

Page 8 of 13

Figure 7. (A) ACR conversion change as a function of reaction temperature over MoVO-AC (circle), MoVCuO-AC (square) and MoVCuO-AC-HCl (triangle). Closed symbols, ACR conversion; open symbols, oxygen conversion. (B) AA selectivity change as a function of reaction temperature over MoVO-AC (circle), MoVCuO-AC (square) and MoVCuO-AC-HCl (triangle). (C) Changes in acetic acid (closed circle), CO (closed triangle) and CO2 (open triangle) selectivities as a function of reaction temperature. MoVO-AC, black; MoVCuO-AC, red; MoVCuO-AC-HCl, blue. (D) AA selectivity change as a function of ACR conversion over MoVO-AC (circle) and MoVCuO-AC-HCl (triangle). tion (932.6 eV) (Figure S8). External surface areas of the above catalysts were changed only slightly after the reaction as shown in Table 3. In contrast to the external surface areas, the micropore volumes were considerably decreased after ACR oxidation in terms of N2 adsorption. The decrease in micropore volumes might be due to the reduction of the MoVO structure after the catalytic reaction since the heptagonal channel size is known to be decreased by the reduction18. In fact, Mo and V were both reduced after the reaction as can be seen in the UV absorption around 550 ~ 600 nm derived from the charge transfer of V4+-O-Mo6+, V5+-O-Mo5+ and Mo5+-O-Mo6+ and that around 720 ~ 780 nm derived from d-d transition of V4+ and Mo5+ (Figure S9)34-35. ACR conversion and product selectivity changes as a function of reaction temperature are shown in Figure 7. MoVO-AC showed catalytic activity for ACR conversion and almost full ACR conversion was achieved at 250 °C. The main reaction product was AA and the selectivity toward AA was ca. 92%~93% for all of the reaction temperatures. Acetic acid, CO and CO2 were observed as byproducts. MoVCuO-AC showed lower ACR conversion than that observed over MoVO-AC. The observed decrease in ACR conversion would be derived from the


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Table 3. Physicochemical properties of MoV(Cu)O-AC-(HCl) after ACR oxidation External surface areaa /m2 g-1

Sample

a

Micropore volumea /10-3 cm3 g-1

NH3 adsorption amountb /mmol g-1

Elemental composition of the surfacec (Mo/V/Cu)

MoVO-AC

6.4

0.3

671

1/0.14/-

MoVCuO-AC

7.2

0.4

830

1/0.12/0.29

MoVCuO-AC-HCl

5.4

0.1

762

1/0.13/0.15

Measured by N2 adsorption at liquid N2 temperature and calculated by an s plot.

b Estimated

by NH3 adsorption TPD.

c Determined

by XPS.

Figure 8. Oxygen activation over MoVO (A) and MoVCuO (B). Molecular oxygen is activated over the  oxygen defect site. The degree of activation of molecular oxygen might be moderated by the location of Cu under the  oxygen defect site. Mo, light blue; V, gray; O, red; dark red, Cu. Activated oxygen is shown in blue over MoVO and in green over MoVCuO. blocking of Cu in the heptagonal channel where ACR oxidation occurs. In fact, the ACR conversion was substantially improved by HCl treatment possibly due to the removal of surface Cu impurities, although the ACR conversion was still lower than that observed over MoVO-AC. Although the ACR conversion was decreased, MoVCuOAC and MoVCuO-AC-HCl showed higher AA selectivity than that of MoVO-AC, and AA selectivity of 97% ~ 98% was achieved at all of the reaction temperatures examined. Figure 7 (D) shows AA selectivity as a function of ACR conversion over MoVO-AC and MoVCuO-AC-HCl at 250 °C. ACR conversion was controlled by changing the catalyst amount. In both the catalysts, AA selectivity was almost constant upon the change of ACR conversion, and AA selectivity observed over MoVCuO-HCl (ca. 97%~98%) was clearly higher than that over MoVO-AC (ca. 92%~93%). The AA selectivity over MoVCuO-AC-HCl extrapolated to 0% ACR conversion was also clearly better than that of MoVO-AC, indicating that direct transformation of ACR toward COx or acetic acid is prevented by the introduction of Cu into the MoVO structure. Similar result was reported by Kuznetsova et al who reported that an addition of Cu to V2O5 ・9MoO3/SiO2 catalyst improved the AA selectivity in the ACR oxidation36. As the reason for the improvement of AA selectivity, redox state

change is not likely since XPS spectra of Mo and V were comparable for all of the catalysts after the catalytic reaction (Figure S8). Change of Cu state either by the formation of Cu metal or migration of bulk Cu to near catalyst surface during the reaction were also not the reason because AA selectivities were almost the same for MoVCuO-AC and MoVCuO-AC-HCl despite very different surface amounts of Cu as measured by XPS (Table 3 and Figure S8). Acid property would also not be the reason for the improved AA selectivity since the acid amounts and acid strengths over the studied catalysts estimated by NH3 adsorption TPD were comparable for all of the catalysts (Table 3 and Figure S10). We have proposed that the oxidation catalysis over MoVO takes place at  oxygen site16, 18, 37. Since Cu is located under the  oxygen, Cu is expected to influence the activity of the active oxygen species formed at the  oxygen defect site (Figure 8). We now consider that the location of Cu under the  oxygen defect site makes the oxygen activation moderate and prevents side reactions such as C-C scission to form COx and acetic acid, resulting in improvement of AA selectivity38-40. In fact, Andrushkevih et al reported that binding energy of oxygen to Mo-V based mixed metal oxide estimated by an adsorption heat decreased by the introduction of Cu2-3. However, an activation energy with respect to ACR conversion estimated by Arrhenius plot was not significantly changed between MoVO-AC (152.5 kJ mmol-1 g-1) and MoVO-AC (157.7 kJ mmol-1 g-1) (Figure S11). It has been suggested that the rate limiting step of this reaction over Mo-V based mixed metal oxide is the desorption of AA from the catalyst, so that almost no change might be observed in the apparent activation energy2. Further details about the catalytic role of Cu for ACR oxidation are currently under investigation in our laboratory. ■ CONCLUSIONS In the present work, we successfully synthesized crystalline orthorhombic Mo-V-Cu oxide (MoVCuO) by hydrothermal synthesis using methylammonium cation as a structure-directing agent. Structural analyses revealed that Cu was located under the bridging oxygen in the pentamer unit facing the heptagonal channel with a distorted square planar geometry. Selective oxidation of



acrolein (ACR) was carried out over MoVCuO and crystalline orthorhombic Mo3VOx (MoVO) having the same structural framework as that of MoVCuO in order to reveal the catalytic role of Cu at the molecular level. The introduction of Cu decreased the ACR conversion possibly due to blockage of the heptagonal channel where the catalytic reaction takes place. On the other hand, the selectivity toward acrylic acid (AA) was substantially increased by the introduction of Cu (AA selectivity: MoVO, 92%~93%; MoVCuO, 97%~98) and the AA selectivity was not dependent on ACR conversion. We here propose that Cu located at the heptagonal channel site makes the activation of molecular oxygen moderate, preventing undesirable side reactions such as C-C scission to form COx or acetic acid. The experimental results obtained in this work contribute to an understanding at the molecular level of the catalytic role of Cu in Mo-V-based mixed metal oxide that is industrially used for selective oxidation of ACR. We believe that we can approach the ideal catalyst for ACR oxidation by arranging various promoter elements in appropriate structural sites in a controlled manner. The work presented here substantially progresses the catalyst synthesis methodology of an ideal ACR oxidation catalyst and opens up the way for rational catalyst design. ASSOCIATED CONTENT Supporting Information. ICP data for MoVCuO-oxa and MoVCuO-AC-HCl; XPS spectra of MoVO-AC, MoVCuO-AC and MoVCuO-ACHCl; IR spectra of MoVCuO-oxa and MoVCuO-AC; TPD spectra of MoVCuO-oxa and MoVCuO-AC; XRD simulation of MoVCuO-AC-HCl; Structural model of MoVCuO obtained by geometry optimization; XRD patterns of MoVO-AC, MoVCuO-AC and MoVCuO-AC-HCl after ACR oxidation; UV spectra of MoVO-AC, MoVCuO-AC and MoVCuO-AC-HCl after ACR oxidation; XPS spectra of MoVO-AC, MoVCuO-AC and MoVCuO-AC-HCl after ACR oxidation; results of ammonia TPD of MoVO-AC, MoVCuO-AC and MoVCuO-AC-HCl after ACR oxidation. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (S. Ishikawa) 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 This work was supported by JSPS KAKENHI Grant Number 18K14058. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by Technical Division of Institute for Catalysis, Hokkaido University. REFERENCES

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Synthesis of Crystalline Orthorhombic Mo-V-Cu Oxide for Selective

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