Communication Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Ionothermal Synthesis of Open-Framework Metal Phosphates Using a Multifunctional Ionic Liquid Kangcai Wang,†,§ Ting Li,†,§ Hongmei Zeng,† Guohong Zou,† Qinghua Zhang,*,‡ and Zhien Lin*,† †
College of Chemistry, Sichuan University, Chengdu 610064, China Institute of Chemical Materials, China Academy of Engineering Physics, Mianyang 621900, China
‡
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S Supporting Information *
[H2PO4] (Hmim = N-methylimidazolium).9 Besides being a solvent and a structure-directing agent, [Hmim][H2PO4] has a functionial inorganic anion that can act as a framework building unit for metal phosphates. Two new metal phosphates (metal = Be and Al) were prepared in the presence of [Hmim][H2PO4] without the addition of phosphoric acid.10 The beryllium phosphate (denoted SCU-1) has intersecting extra-large 24 MR channels running along the [−111], [12−1], and [211] directions. Such a pore system is unprecedented in open-framework metal phosphate systems. The aluminum phosphate (denoted SCU-2) has a two-dimensional structure with 8 MR windows. It possesses a robust framework that can remain stable under hydrothermal conditions. Notably, this compound shows a high proton conductivity of 1.92 × 10−3 S cm−1 at 25 °C under 98% relative humidity (RH). In a typical ionothermal synthesis, a mixture of [Hmim][H2PO4] (0.724 g) and BeO (0.050 g) was sealed in a 25 mL Teflon-lined bomb at 180 °C for 10 days and then cooled to room temperature. Colorless crystals of SCU-1 were obtained in ∼64.4% yield based on beryllium. Caution! Berylliumcontaining compounds are toxic. Appropriate precautions should be taken when handling these phases. SCU-2 was prepared under similar ionothermal conditions. The phase purities of SCU-1 and SCU-2 were confirmed by powder X-ray diffraction (XRD) (Figures S1 and S2). SCU-1 crystallizes in the trigonal space group R-3 (No. 148). The asymmetric unit contains three unique beryllium atoms and four unique phosphorus atoms, all of which are tetrahedrally coordinated by oxygen atoms. The linkages between BeO4 tetrahedra and HPO4 tetrahedra give rise to a three-dimensional open-framework structure. An instructive view of the framework structure involves cyclic and chainlike building blocks (Figure 1). The cyclic building block has a 12ring aperture made up of six BeO4 tetrahedra and six HPO4 tetrahedra. The chainlike building block is helical and runs along the [001] direction. The right- and left-handed helices coexist in the structure with a ratio of 50:50. These helices are assembled by cyclic 12-ring building blocks through Be−O−P linkages to create the open-framework structure. H2P(4)O2 tetrahedra attach to the wall as pendants to reduce its free void. The framework density (FD) of SCU-1 measured by the number of tetrahedra per 1000 Å3 is 10.8, which is comparable to the valuse of some open-framework metal phosphates such as ND-1 (12.1) and NTHU-1 (10.9).11
ABSTRACT: Two crystalline metal phosphates (metal = Be and Al) were prepared under ionothermal conditions using a multifunctional ionic liquid as a solvent, a structure-directing agent, and a phosphorus source. The beryllium phosphate has a three-dimensional structure with intersecting 24-membered ring (24 MR) channels. The aluminum phosphate has a two-dimensional structure containing 8 MR windows. It displays exceptional hydrothermal stability and shows a high proton conductivity on the order of 10−3 S cm−1 at 25 °C under high humidity conditions.
O
pen-framework metal phosphates are a well-known class of zeolite-like materials that are potentially used in catalysis, ion-exchange, separation, and fuel cells.1 These crystalline materials are usually synthesized in the presence of different structure-directing agents under hydrothermal or solvothermal conditions.2 The replacement of water or organic solvents by ionic liquids in a process called ionothermal synthesis has resulted in the formation of a number of new open-framework materials.3 A notable example is the microporous aluminophosphate DNL-1 with extra-large 20-membered ring (20 MR) channels.3c Compared with molecular solvents, ionic liquids (e. g., 1-ethyl-3-methyl imidazolium bromide, [EMIm][Br]) possess several unique features such as negligible vapor pressure and exceptional structure-directing ability.4 This makes it possible to prepare open-framework metal phosphates in ambient pressure without the addition of structure-directing agents. During the past years, a great deal of attention has been paid to the structure-directing roles of the organic cations of ionic liquids because the anions of ionic liquids are usually not occluded into the framework structures.5,6 It has been demonstrated that the anions of ionic liquids may have an induction effect on the formation of different metal−organic frameworks.7 For example, Morris and co-workers reported the ionothermal synthesis of three different cobalt-based coordination polymers from the ionic liquids EMIm-X (X = Br and Tf2N, where Tf2N is triflimide).8 These ionic liquids have the same cation but with different anions. By tuning the anions of the ionic liquids, different framework structures may be prepared. Inspired by all-in-one ionic liquids, we report here the ionothermal synthesis of open-framework metal phosphates using a multifunctional ionic liquid formulated as [Hmim]© XXXX American Chemical Society
Received: May 31, 2018
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DOI: 10.1021/acs.inorgchem.8b01509 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
Hmim cations, and water molecules, are accommodated within the interlayer region of SCU-2 (Figure 3a). Two types of Him
Figure 1. View of the three-dimensional structure of SCU-1 assembled from helical chains and cyclic 12 MR building blocks. Color code: BeO4, green; PO4, pink. Figure 3. (a) View of the two-dimensional structure of SCU-2 intercalated with Him cations, Hmim cations, and water molecules. (b) A supramolecular chain consists of Him cations, water molecules, and framework oxygen atoms. (c) A view of the π···π interactions between Him and Hmim cations. Color code: AlO4, green; PO4, pink; O, red; N, blue; C, gray.
A striking structural feature of SCU-1 is the existence of a pore system with 24 × 24 × 24 MRs. Prior to this work, the largest channels in beryllium phosphate systems are delimited by 16 tetrahedra.12 The extra-large 24 MR channels in SCU-1 run along the [−111], [12−1], and [211] directions, respectively (Figure 2a). They have similar pore apertures
cations are observed in the structure. Type I Him cations and the water molecules interact with the host framework through hydrogen bonds to form supramolecular chains running along the [010] direction (Figure 3b). The closest N···O and O···O distances vary from 2.624(7) to 2.816(7) Å. Type II Him cations interact with Hmim cations through π···π interactions (Figure 3c). The centroid-to-centroid distance between two adjacent Him and Hmim cations is ∼3.83 Å, and the average distance between the least-squares planes of two adjacent Him and Hmim cations is ∼3.55 Å. Thermogravimetric analyses were carried out under nitrogen gas flow with a heating rate of 10 °C min−1 (Figures S3 and S4). For SCU-1, the initial weight loss of 0.5% below 50 °C is attributed to the departure of water molecules (calcd, 0.4%). The second weight loss of 31.1% between 150 and 500 °C is assigned to the decomposition of mim molecules (calcd, 32.4%). The last weight loss of 3.6% between 530 and 680 °C corresponds to the dehydration of HPO42− and H2PO4− anions. SCU-2 remains stable up to 130 °C. The weight loss of 31.7% between 130 and 720 °C is caused by the departure of water molecules and the decomposition of organic molecules (calcd, 32.9%). The crystals of SCU-1 and SCU-2 remained almost unchanged when they were soaked in water at ambient temperature for 3 days. By immersing their polycrystalline samples in boiling water for 8 h, the structure of SCU-1 collapsed, whereas SCU-2 could sustain its layered structure. Figure 4a shows powder XRD patterns of pristine SCU-2 and its polycrystalline samples after hydrothermal treatment. The diffraction peaks on the two patterns correspond well in position and intensity, indicating the integrity of the structure after hydrothermal treatment. It has been demonstrated that the incorporation of imidazole molecules or other proton carriers into porous
Figure 2. (a) View of the extra-large pore structure of SCU-1 along the [−111] direction showing that H2PO4 groups attach to the walls as pendants. (b) Polyhedral representation of the extra-large pore delimited by 24 TO4 (T = Be and P) tetrahedra.
with a size of 4.5 × 16.4 Å2 (Figure 2b). Hmim cations and water molecules locate within the extra-large channels. These extraframework species occupy 55.6% of the crystal volume. Although a number of metal phosphates and phosphites with 24 MRs, such as ND-1, NTHU-1, VSB-1, VSB-5, ZnHPO-CJn (n = 1−4), Cr-NKU-24, SCU-24, and 24-NTHU-13, have been reported, the extra-large channels in these compounds are unidirectional.11,13 Therefore, SCU-1 represents the first threedimensional metal phosphate containing multidirectional 24 MR channels. SCU-2 crystallizes in the orthorhombic space group Pnma (No. 62). It has a two-dimensional structure that is similar to the aluminum phosphate AlPO-CJ12.14 The inorganic layers have 8 MR windows constructed from corner-sharing AlO4 tetrahedra and PO4 tetrahedra. They are stacked in an AB sequence along the [001] direction. In comparison, the inorganic layers in AlPO-CJ12 are stacked in an AA sequence. Extraframework species, including imidazolium (Him) cations, B
DOI: 10.1021/acs.inorgchem.8b01509 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
Figure 4. (a) Powder XRD patterns of pristine SCU-2 and its powder sample soaked in water at 100 °C for 8 h. (b) Nyquist plots of SCU-2 between 25 and 85 °C under 98% RH. (c) Arrhenius plot of SCU-2 between 25 and 85 °C under 98% RH.
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metal−organic frameworks could greatly enhance their proton conduction.15 The presence of imidazole and its derivative within the robust structure of SCU-2 prompts us to study its proton conduction behaviors. The temperature-dependent proton conductivities of SCU-2 were measured by alternating current impedance spectroscopy using a compacted pellet of the polycrystalline sample. As determined from the Nyquist plot, the conductivity of SCU-2 at 25 °C under 98% RH is estimated to be 1.92 × 10−3 S cm−1 (Figure 4b). Such a high value is comparable to those of SCU-12 (1.0 × 10−3 S cm−1) and Na6[(AlPO4)8(OH)6]·8H2O (3.59 × 10−3 S cm−1) under similar measurement conditions.16 When the measurement temperature increases to 85 °C, the conductivity of SCU-2 reaches a value of 5.94 × 10−3 S cm−1. The activation energy (Ea) for the proton transfer is estimated to be 0.20 eV according to the Arrhenius equation σT = σ0exp(−Ea/kBT) (Figure 4c). This result indicates that the proton conduction behavior of SCU-2 follows the Grotthuss mechanism (Ea = 0.1−0.4 eV).15−17 The hydrogen-bonded networks and the π···π packing structure of imidazole and its derivative within the interlayer region of SCU-2 may provide a fast and efficient pathway for proton transfer. In conclusion, we have demonstrated the ionothermal synthesis of two new metal phosphates with three-dimensional and layered structures. The utilization of a multifunctional ionic liquid as a solvent, a structure-directing agent, and a phosphorus source creates exciting possibilities in the preparation of crystalline metal phosphates with interesting structures and appealing properties. In particular, this synthetic approach is well suited for the creation of proton-conducting materials through the incorporation of imidazole and its derivative into their structures. Given the large variety of multifunctional ionic liquids, we believe that more openframework inorganic materials will be prepared in the near future.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Guohong Zou: 0000-0003-4527-0058 Qinghua Zhang: 0000-0002-4162-7155 Zhien Lin: 0000-0002-5897-9114 Author Contributions §
K.W. and T.L. contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the Development Foundation of CAEP (No. 2015B0302056). REFERENCES
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01509. X-ray crystallographic data in CIF format, experimental details, powder XRD patterns, TGA curves, IR spectra, and additional figures (PDF) Accession Codes
CCDC 1846602−1846603 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by C
DOI: 10.1021/acs.inorgchem.8b01509 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
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DOI: 10.1021/acs.inorgchem.8b01509 Inorg. Chem. XXXX, XXX, XXX−XXX
