Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Porous Cubic Cesium Salts of Silicododecatungstate(molybdate)/ Borododecatungstate Blends: Synthesis and Molecular Adsorption Properties Takuya Kotabe,† Yoshiyuki Ogasawara,*,† Kosuke Suzuki,† Sayaka Uchida,‡ Noritaka Mizuno,† and Kazuya Yamaguchi*,†
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Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ‡ Department of Basic Sciences, School of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan S Supporting Information *
ABSTRACT: In this work, the synthesis and molecular adsorption properties of cubic cesium salts of Keggin-type silicododecametalates (SiM; M = W or Mo) and borododecatungstate (BW) blends were reported. Pentavalent BW, which is normally packed in a monoclinic fashion, could be packed into the desired cubic fashion by being blended with tetravalent SiW or SiMo. By simply mixing aqueous solutions containing SiM and BW in different SiM:BW molar ratios [SiM:BW = N:(100 − N), where N is the percentage of SiM in the synthetic solution and equals 100, 80, 50, 30, 20, 10, or 5] with a cesium nitrate solution, we obtained a series of cubic cesium salts (CsSiMBW-N) with POM vacancy-like pores (i.e., kinetically formed interparticle pores). Notably, the yields, compositions, and BET surface areas of the obtained CsSiMBW-N were dependent on only the N values and independent of SiW or SiMo. In CsSiMBW-N, SiM and BW were well blended at the molecular (nanometer) level. The nitrogen, water, and ethanol adsorption properties of CsSiWBW-N systematically changed according to the SiW:BW molar ratios. In the case of SiW-rich salts, nitrogen, water, and ethanol were adsorbed in the pores; the amounts of nitrogen, water, and ethanol adsorbed were nearly identical for CsSiWBW100. When the SiW:BW molar ratio was decreased, the salts selectively incorporated water and ethanol within the pores. In the case of CsSiWBW-5, barely any nitrogen was adsorbed. The plausible mechanisms for particle growth and interparticle POM vacancy-like pore formation are also discussed.
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unique properties and porosities.9−14 The class of porous materials that incorporate POMs into their framework includes POM−Cr complex ionic crystals15−17 and POM−MOFs.18−21 In these cases, organic linkers and/or substituents play an important role in the construction of unique pores in the crystal lattice. The construction of porous POM materials is also possible using appropriate inorganic counter cations, and such allinorganic POM materials are expected to be highly stable under thermal and redox conditions. All-inorganic POMs can be roughly divided into two types: (A) those utilizing ionic bonds or hydrogen bonds between clusters22−24 and (B) those linking POMs with covalent bonds.25−38 For type A POMs, several examples of aluminum cluster-paired POMs are known. Kwon et al. reported all-inorganic porous POMs composed of a cationic aluminum cluster, [Al 13O 4(OH) 24 (H 2O) 12 ] 7+
INTRODUCTION Polyoxometalates (POMs) are nanosized anionic metal− oxygen clusters formed from the condensation of early transition metal oxoacids, and they exhibit wide structural variability and unique properties.1−5 The chemical and physical properties of POMs can be optimized at the atomic level by changing their structures, constituent elements, and counter cations. Therefore, POMs have been extensively utilized in fields, such as catalysis, photochemistry, electrochemistry, analytical chemistry, magnetism, and materials science.1−5 Porous materials with molecular-sized pores or vacancies, such as zeolites and metal−organic frameworks (MOFs) (or porous coordination polymers), are useful in various applications, such as adsorption, chromatography, and catalysis.6−8 Porous POMs have also been studied with great interest because they can be utilized as building blocks with well-defined compositions, structures, sizes, and charges. The complexation of POMs with appropriate cationic complexes, or molecules, as counter cations creates solid materials with © XXXX American Chemical Society
Received: March 13, 2018
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DOI: 10.1021/acs.inorgchem.8b00657 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry ({Al13}7+), with [Al(OH)6Mo6O18]3−,22 or [V2W4O19]4−.23 Uchida et al. also reported materials composed of the {Al13}7+ cluster with Keggin-type POMs, e.g., [CoW12O40]6−.24 For type B POMs, Khan et al. reported frameworks in which [V18O42(XO4)]6− (X = V5+ or S6+) was cross-linked with [M(H2O)4]2+ (M = Co2+ or Fe2+).25 Yang et al. reported a material in which the [(PVO4)VV6VIV12O39]3− unit was directly linked through V−O−V bonds.26 Three-dimensional (3D) Keggin-type POM frameworks with K+ and Na+ were reported using [SiMo12O40]4−,27 [SiVW11O40]5−,28 or [BW12O40]5−.29 In recent years, the combinations of POMs and cations, structures, and topologies have become greatly diversified. Wang et al. reported frameworks based on [MnV13O38]7− and rare earth cations Ln (Ln = La3+ or Ce3+).30 Hill et al. reported networks containing the [Sn3(SiW9O34)2]14− unit connected by Sn−Cl−Sn bonds and ionic bonds with [Na(H2O)4]+.31 Cronin et al. reported a series of “POMzites” in which ringshaped POMs, [P8W48O184]40−, are used as building blocks and cross-linked with metal cations.32−34 Sadakane and Ueda et al. reported complex oxides constructed from ε-Keggin-type POM units with binding metal ions, such as Bi3+, Zn2+, and Mn 2+ .35−38 However, most porous all-inorganic POM materials have low skeletal stabilities. In addition, the design and synthesis of porous all-inorganic POM materials are still highly dependent on empirical methods. Among the solid POM materials, cubic A3[POM] salts composed of Keggin-type POMs ([XM12O40]n− = XM; X = P, Si, B, etc., and M = Mo, W, etc.) and monovalent cations (A = NH4+, Cs+, Ag+, etc.) are some of the most intensively studied porous inorganic POM compounds because of their high stabilities and controllable porous structures. The crystal structure of cubic A3[POM] salts can be understood using the spherical anion approximation; in the unit cell of the cubic A3[POM] structure, two POMs are packed in a body-centered cubic fashion and six monovalent cations are located between the POMs (at the center of each plane and edge of the cell), thus resulting in an A:POM ratio of 3:1 (Figure 1).39 Because
vacancies are formed because of excess negative charge compensation in the crystallite (Figure 2A).45−52 Pore formation based on mechanism A varies considerably depending on the synthesis conditions (e.g., synthesis temperature, counter cation used, and reagent concentrations).53−56 For example, the cesium salt of trivalent PW synthesized at 298 K possesses mesopores formed between randomly aggregated primary particles and that synthesized at 368 K possessed micropore forms between orderly aggregated primary particles;44 the trivalent PW ammonium salt possesses micropores between the epitaxially accumulated primary particles.44,57 When the A:POM molar ratio is greater than 3, vacancies are generated at the POM sites in the cubic A3[POM] packing. In many cases of previously reported cesium salts of tetravalent SiW synthesized at ambient temperatures (∼298 K), the vacancies would be generated at the surface of the primary particles to form interparticle pores between aggregated primary particles (we call these pores “POM vacancy-like pores”), and the salts generally exhibit large specific surface areas.45−50 Recently, we have reported a unique adsorption property of crystalline Cs3.0H0.3[SiW12O40]0.83 synthesized at a relatively high temperature (343 K) (CsSiW).51 In this CsSiW sample, micropores derived from POM vacancy are generated in the crystallite for the charge compensation while repeating dissolution and precipitation (mechanism B). Although the specific surface area of CsSiW is quite small because of its high crystallinity, such micropores derived from POM vacancy in the crystallite could incorporate relatively small polar molecules, such as water, through the ion−dipole interaction. In this study, we focused on the characteristic adsorption properties derived from POM vacancies in cubic cesium salts and aimed to develop and control their selective adsorption properties. To increase the number of POM vacancy sites, we attempted to synthesize a cubic A3[POM] cesium salt of pentavalent BW. However, the desired cubic A3[POM] salt was not formed; instead, the major product obtained was Cs5BW12O40 with a monoclinic structure.58 As such, we introduced the “molecular alloy” concept from studies of mixed POM materials59 into our synthetic strategy. By simply mixing aqueous solutions containing SiM and BW in different SiM:BW molar ratios with a cesium nitrate solution, we obtained a series of cubic cesium salts with POM vacancy-like pores (CsSiMBW-N) (Figure 2B). Notably, the yields, compositions, and BET specific surface areas of the obtained CsSiMBW-N were simply dependent on the N values and independent of SiW or SiMo. In CsSiMBW-N, SiM and BW were well blended at the molecular (nanometer) level. The nitrogen, water, and ethanol adsorption properties of CsSiWBW-N systematically changed according to the SiW:BW molar ratios. In the case of SiW-rich salts, nitrogen, water, and ethanol were adsorbed in the pores. When the SiW:BW molar ratios were decreased, the salts selectively incorporated water and ethanol within the pores. In this work, we also discuss the plausible mechanisms for particle growth and POM vacancy-like pore formation.
Figure 1. Schematic of the cubic A3[POM] structure. Light green and light purple polyhedra indicate [MO6] and [XO4] units, respectively, and purple spheres indicate monovalent cations.
the ions are densely packed, there is no intrinsic space in the crystal for the entry of exogenous molecules.40 Pore formation in A3[POM] salts has been discussed on the basis of the following mechanisms: (A) “primary particle aggregation mechanism” and (B) “dissolution−reprecipitation mechanism”. In mechanism A, trivalent POMs, e.g., PW, are typically utilized. The pores are built in the space between the nanometer-sized primary particles by controlling their aggregation states (Figure 2A).41−44 In mechanism B, tetravalent POMs, e.g., SiW, are typically utilized. POM
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EXPERIMENTAL SECTION
Materials. Cesium nitrate (CsNO3, Kanto), silicododecatungstic acid (H4SiW12O40·21H 2O, Wako), silicododecamolybdic acid (H4SiMo12O40·24H2O, Nippon Inorganic Color & Chemical Co., Ltd.), and borododecatungstic acid (H5BW12O40·29H2O, Nippon Inorganic Color & Chemical Co., Ltd.) were used as received. Water
B
DOI: 10.1021/acs.inorgchem.8b00657 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 2. Schematic of cubic A3[POM] salt pore construction. (A) Previous work. (B) This work.
Table 1. Reaction Scales, Yields, Composition Ratios, and Specific Surfaces Area of CsSiWBW-N amount of solution (mL)
elemental analysisa
amount of POM (mmol)
sample
A
B
SiW
BW
yield (mg)a,b
SiW:BW
Cs:POM
specific surface area (m2 g−1)c
CsSiWBW-100 CsSiWBW-50 CsSiWBW-30 CsSiWBW-20 CsSiWBW-10 CsSiWBW-5d
5 10 15 25 50 75 × 3
5 10 15 25 50 75 × 3
0.10 0.10 0.09 0.10 0.10 0.075 × 3
0 0.10 0.21 0.40 0.90 1.425 × 3
330 180 124 80 33 9
100:0 85:15 73:27 66:34 59:41 53:47
3.6:1 4.0:1 4.1:1 4.2:1 4.1:1 4.2:1
173 160 123 91 48 3
a For detailed results of each sample, see Table S2. bYields are normalized by amount of POM (0.10 mmol). cEstimated from a nitrogen adsorption isotherm with the BET method. dOwing to the convenience of synthetic containers, the 75 mL scale reaction was performed three times in parallel and recombined.
was purified using an Elix-UV5 or DIRECT-Q 3 UV polishing system (Millipore Corp.). Synthesis. A total of 0.1 mmol of H4SiW12O40·21H2O and H5BW12O40·29H2O was dissolved at a molar ratio of N:(100 − N) in 5 mL of water (solution A). In an additional 5 mL of water was dissolved 0.5 mmol of CsNO3 (solution B). Solutions A and B were kept at 303 K in a thermostatic chamber. Solution B was poured into solution A in a single step while it was being stirred, immediately forming a white suspension. The suspension was kept at 303 K for 3 h while being stirred. The precipitate was collected on a membrane filter (pore diameter of 0.1 μm), washed with water, and air-dried. The resulting cesium salts are denoted by CsSiWBW-N, where N represents the molar percentage of SiW12O404− in solution A. The reaction scale was adjusted to obtain a sufficient yield (Table 1). The other series of cesium salts, CsSiMoBW-N, was synthesized analogously using H4SiMo12O40·24H2O at room temperature instead of using H4SiW12O40·21H2O at 303 K. Measurements. Fourier transform infrared (FT-IR) spectra were recorded with a FT/IR-4100 (JASCO) spectrometer using KBr disks. The spectra were recorded from 250 to 4000 cm−1 with a spectral resolution of 2 cm−1. Powder X-ray diffraction (XRD) patterns were recorded with a SmartLab (Rigaku) diffractometer using Cu Kα1 radiation (1.5405 Å, 45 kV, 200 mA). The diffraction data were
collected in the 2θ range of 5−60° with steps of 0.02° at a rate of 2° min−1. Scanning electron microscopy (SEM) images were obtained with an S-4700 (Hitachi) microscope at an acceleration voltage of 10 kV and an emission current of 10 μA. SEM energy dispersive X-ray spectroscopy (SEM−EDS) mapping was performed on the same microscope with an acceleration voltage of 20 kV. Prior to SEM and SEM−EDS measurements, the compounds were sprayed on a carbon tape. Scanning transmission electron microscopy energy dispersive Xray spectroscopy (STEM−EDS) mapping was performed with a JEMARM200F Thermal FE microscope (JEOL) with an acceleration voltage of 200 kV. Prior to STEM−EDS measurements, the compounds were dispersed in ethanol and the solution was dropped on a microgrid. Elemental analyses of Si, B, Mo, and W were performed by inductively coupled plasma atomic emission spectroscopy (ICP-AES) with an ICPS-8100 spectrometer (Shimadzu). Cs analysis was performed by flame atomic absorption spectroscopy with a Z-2000 spectrometer (Hitachi). Thermogravimetric (TG) analysis was performed with a Thermo plus TG8210 system (Rigaku). The samples (∼10 mg) were heated under flowing nitrogen (150 mL min−1) from room temperature to 373 or 423 K at a rate of 5 K min−1. Thereafter, the temperature was maintained for 3 h. Nitrogen adsorption−desorption isotherms were measured with an ASAP2010 automatic adsorption apparatus (Micromeritics). Prior to the C
DOI: 10.1021/acs.inorgchem.8b00657 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 3. Relationships of the mixing ratio of tetravalent and pentavalent POMs to (A) the yields of obtained cesium salts, (B) the composition ratios of tetravalent POM to pentavalent POM in cesium salts, and (C) the composition ratios of Cs to POM in cesium salts.
Table 2. Reaction Scales, Yields, Composition Ratios, and Specific Surface Areas of CsSiMoBW-N amount of solution (mL)
amount of POM (mmol)
sample
A
B
SiMo
BW
CsSiMoBW-100 CsSiMoBW-80 CsSiMoBW-50 CsSiMoBW-20 CsSiMoBW-5
5 5 5 15 40
5 5 5 15 40
0.10 0.08 0.05 0.06 0.04
0 0.02 0.05 0.24 0.76
elemental analysisa a,b
yield (mg) 213 206 159 76 29
SiMo:BW
Cs:POM
specific surface area (m2 g−1)c
100:0 91:9 77:23 59:41 42:58
3.8:1 3.9:1 3.8:1 4.1:1 3.9:1
181 174 140 75 2
a
For detailed results of each sample, see Table S3. bYields are normalized by amount of POM (0.10 mmol). cEstimated from a nitrogen adsorption isotherm with the BET method. measurements, the samples (>100 mg) were pretreated in vacuo at 373 or 423 K for >3 h. Water and ethanol vapor adsorption− desorption isotherms were measured with a Belsorp-max (BEL Japan). Prior to the measurements, the samples were pretreated in vacuo at 423 K for >3 h. Argon adsorption−desorption isotherms were measured on the same instrument. Prior to the measurements, the samples were pretreated in vacuo at 423 K for 8 h.
Furthermore, considering that the molecular shapes and sizes of these POMs are similar and that the difference in charges can be compensated by counter cations, the formed cubic cesium salt could contain both POMs. In addition, we expected that the composition ratio of the two POMs could be controlled by the POM mixing ratio, and this could affect the adsorption properties of the resulting cubic cesium salt. Therefore, we attempted to synthesize a series of cubic cesium salts (CsSiMBW-N) using mixed solutions of tetravalent POM (SiW or SiMo) and pentavalent BW with the ratio N:(100 − N) ranging from 100:0 to 5:95. After the Cs solution was added to the mixed POM solution, cesium salts were immediately formed, thus resulting in a turbid solution. The smaller the value of N, the longer it took for the solution to become turbid. When N was large (N = 100 or 50), the solution quickly became turbid (
