Article Cite This: J. Am. Chem. Soc. 2018, 140, 993−1003
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A New Class of Metal-Cyclam-Based Zirconium Metal−Organic Frameworks for CO2 Adsorption and Chemical Fixation Jie Zhu,† Pavel M. Usov,† Wenqian Xu,‡ Paula J. Celis-Salazar,† Shaoyang Lin,† Matthew C. Kessinger,† Carlos Landaverde-Alvarado,§,# Meng Cai,† Ann M. May,† Carla Slebodnick,† Dunru Zhu,∥ Sanjaya D. Senanayake,⊥ and Amanda J. Morris*,†,# †
Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, United States X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, United States § Department of Chemical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, United States ∥ College of Chemical Engineering, State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University, Nanjing 210009, China ⊥ Department of Chemistry, Brookhaven National Laboratory, Upton, New York 11973-5000, United States # Macromolecules Innovation Institute, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, United States ‡
S Supporting Information *
ABSTRACT: Metal−organic frameworks (MOFs) have shown great promise in catalysis, mainly due to their high content of active centers, large internal surface areas, tunable pore size, and versatile chemical functionalities. However, it is a challenge to rationally design and construct MOFs that can serve as highly stable and reusable heterogeneous catalysts. Here two new robust 3D porous metal-cyclam-based zirconium MOFs, denoted VPI-100 (Cu) and VPI-100 (Ni), have been prepared by a modulated synthetic strategy. The frameworks are assembled by eight-connected Zr6 clusters and metallocyclams as organic linkers. Importantly, the cyclam core has accessible axial coordination sites for guest interactions and maintains the electronic properties exhibited by the parent cyclam ring. The VPI-100 MOFs exhibit excellent chemical stability in various organic and aqueous solvents over a wide pH range and show high CO2 uptake capacity (up to ∼9.83 wt% adsorption at 273 K under 1 atm). Moreover, VPI-100 MOFs demonstrate some of the highest reported catalytic activity values (turnover frequency and conversion efficiency) among Zrbased MOFs for the chemical fixation of CO2 with epoxides, including sterically hindered epoxides. The MOFs, which bear dual catalytic sites (Zr and Cu/Ni), enable chemistry not possible with the cyclam ligand under the same conditions and can be used as recoverable stable heterogeneous catalysts without losing performance.
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INTRODUCTION
One of the highly desired types of MOF functionalities is organic linkers that can serve as secondary coordination sites for additional metal ions that do not directly construct the framework. Such metalloligands can support coordinatively unsaturated metal centers and endue the resultant MOF with a unique set of reactivities difficult to attain otherwise. Cyclam (1,4,8,11-tetraazacyclotetradecane) is a 14-membered tetraamine macrocyclic ligand with rich coordination chemistry.17 This molecule features four secondary amine groups, which are capable of chelating various transition metal cations in a tetradentate fashion (cis or trans). 18 The metal axial coordination sites can then bind different molecules and facilitate chemical or electrochemical transformations.19 For example, [Ni(cyclam)]2+ has been shown to be a low-cost,
Metal−organic frameworks (MOFs), also referred to as multidimensional porous coordination polymers, are constructed from metal ions/clusters and organic linkers1,2 and have wide-ranging applications, including gas storage and separation,3−5 sensing,6,7 drug delivery,8 and catalysis.9−12 The extraordinary scope of available organic and inorganic components enables fine-tuning of MOF structures and chemical properties, demonstrating the beauty of rational framework design via crystal engineering.13 The porous, highly ordered nature of these structures provides a unique platform to extend their applications as multifunctional hybrid materials.14,15 Generally, there are two strategies for introducing the functional moieties into MOFs−design of an appropriate ligand or post-synthetic modification of existing frameworks.16 © 2017 American Chemical Society
Received: October 5, 2017 Published: December 22, 2017 993
DOI: 10.1021/jacs.7b10643 J. Am. Chem. Soc. 2018, 140, 993−1003
Article
Journal of the American Chemical Society
Caution! The perchlorate salts described are potentially explosive and should be handled with care and prepared in small quantities. Synthesis of 6,13-Dicarboxy-1,4,8,11-tetraazacyclotetradecane)copper(II) Perchlorate [CuL(ClO4)2] (1). The Cu-based cyclam ligand [CuL(ClO4)2] (1) was synthesized according to a literature procedure.45 In a typical synthesis, the tetracarboxylate precursor [Cu(tetacH4)](ClO4)2 (630 mg, 0.99 mmol) was added to 10 mL of 0.1 M HClO4 solution and heated at reflux for 1 h. After the reaction mixture cooled down to room temperature, 3 mL of 60% HClO4 was added and the solution refrigerated overnight. The product was filtered off as a red crystalline solid (0.38 g, 46% yield). The phase purity of 1 was verified by powder X-ray diffraction (PXRD). Synthesis of 6,13-Dicarboxy-1,4,8,11-tetraazacyclotetradecane (L) (2). The free-base ligand 2 was prepared by the demetalation of 1. The copper was removed by refluxing [CuL(ClO4)2] (0.32 g, 0.58 mmol) in 20 mL of 37% HCl for 1 h. The resultant white solid was filtered and washed with ethanol. After drying under vacuum, a white powder (0.16 g, 95% yield) was obtained. ESI-MS: [M − H]+ m/z = 289.19; 1H NMR, 13C NMR, and HSQC spectra are shown in Figures S1−S3. Synthesis of 6,13-Dicarboxy-1,4,8,11-tetraazacyclotetradecane)nickel(II) Perchlorate [NiL(ClO4)2]·2H2O (3). The Ni-based cyclam ligand [NiL(ClO4)2]·2H2O (3) was synthesized under solvothermal conditions. In a 20 mL vial, NiCl2·6H2O (70 mg, 0.295 mmol) and L (70 mg, 0.242 mmol) were dissolved in 5 mL of H2O with addition of 0.02 g of NaOH (0.5 mmol). The vial was heated in a 100 °C oven for 6 h. After the solution cooled to room temperature, 2 mL of 60% HClO4 was added and the solution refrigerated overnight. The product was filtered off as an orange crystalline solid (38 mg, 26.9% yield). The phase purity of 3 was verified by PXRD. Synthesis of VPI-100 (Cu). In an 8 mL vial, ZrCl4 (14 mg, 0.06 mmol) was dissolved in 4 mL of dry dimethylformamide (DMF) with 0.44 mL (200 equiv.) of formic acid. The vial was heated in an 80 °C oven for 1 h. After the solution cooled to room temperature, [CuL(ClO4)2] (33 mg, 0.06 mmol) was added and the solution sonicated for 10 min. The mixture was then heated in a 120 °C oven for 24 h. After cooling to room temperature, the resultant solid was isolated by centrifugation and washed with fresh DMF (3 × 1 mL) and acetone (3 × 1 mL). The powder was dried under vacuum at 60 °C, and a violet crystalline powder was obtained (32 mg, 86% yield). The phase purity of VPI-100 (Cu) was verified by PXRD. Synthesis of VPI-100 (Ni). In an 8 mL vial, ZrCl4 (14 mg, 0.06 mmol) was dissolved in 2 mL of dry DMF with 0.44 mL (200 equiv.) of formic acid. In a separate vial, NiCl2·6H2O (14 mg, 0.06 mmol) and free-base ligand 2 (17.3 mg, 0.06 mmol) were added to 2 mL of dry DMF and sonicated for 10 min. Both vials were then heated in an 80 °C oven for 1 h. The solutions were combined into one vial and heated in a 120 °C oven for 24 h. After cooling to room temperature, the resultant solid was isolated by centrifugation and washed with fresh DMF (3 × 1 mL) and acetone (3 × 1 mL). The product was dried under vacuum at 60 °C. Light pink crystalline powders were obtained (22 mg, 60% yield). The phase purity of VPI-100 (Ni) was confirmed by PXRD. Powder X-ray Diffraction (PXRD) Analysis. Structure Determination and Refinement. Both VPI-100 (Cu) and VPI-100 (Ni) were obtained as microcrystalline powders. Their structures were determined by means of the PXRD technique using synchrotron radiation at Beamline 17-BM at Advanced Photon Source (APS) at Argonne National Laboratory (ANL). The X-ray wavelength was 0.45336 Å. Sample powder was packed in a 1.1 mm diameter Kapton capillary. Two-dimensional (2D) diffraction data were collected with a PerkinElmer flat panel amorphous Si area detector in the transmission geometry. Reduction of the 2D images to a 1D XRD pattern of intensity versus 2θ was carried out through the GSAS-II program.46 XRD indexing and Rietveld analysis were performed with TOPAS version 5. Structure Confirmation and Phase Purity. The phase purity of the synthesized MOF samples was analyzed on a Rigaku Miniflex diffractometer with Cu Kα radiation (λ = 1.5418 Å) over a 2θ range of 3−50° in continuous scanning mode (1.0°/min) and a resolution of
highly selective CO2 electroreduction catalyst.20−22 [Co(cyclam)]3+, on the other hand, can act as a photocatalyst for CO2 reduction.23,24 Both complexes are promising candidates for CO 2 utilization applications. 25 trans-[Cr(cyclam)(ONO)2]+ is a known precursor complex for light-induced NO release, making it useful in many therapeutic areas.26,27 Motivated by the favorable properties of cyclam derivatives and their metal complexes, past researchers have explored incorporation of these moieties into MOF structures.28−30 Cyclam-containing MOFs have the potential to combine the unique reactivity described above with the high porosity and stability of MOFs in heterogeneous systems. In order to tap the great potential held by cyclam-MOFs, two synthetic challenges must first be addressed. (1) MOF structures with open axial coordination sites must be developed. Thus far, in reported cyclam-based MOFs, the core metal participated in the structural backbone of the framework, consequently precluding guest binding and subsequent metal-mediated reactivity.31−34 (2) The synthetic modification of the parent cyclam molecule to incorporate MOF binding groups (carboxylic acids) must not electronically deactivate the compound. A common strategy is to bind the carboxylic groups to the coordinating amines. Such modification can reduce activity by altering the electron density on the central metal. Indeed, such a strategy has been used to incorporate a related macrocycle, cyclen, into MOFs.35,36 In this work, we address the challenges by first chemically modifying the cyclam periphery at the 6 and 13 positions. Such modification allows the cyclam to serve as a linear linker while minimizing the electronic effects on the metal− tetraamine core. Second, the carboxylic acid substituents utilized are minimally flexible, which restricts the ability for the groups to interact with the metallic core of the same cyclam, as is commonly seen in reported structures.31−34 Inspired by the outstanding chemical and thermal stability of Zr(IV)-based MOFs,37−41 we use Zr(IV) in the construction of the metal nodes for the cyclam-based MOFs. By combining the versatility of the metallocyclam motif with the durability of Zr-carboxylate structures, we report two new porous metal-cyclam-based zirconium MOFs, [Zr 6 (μ 3 OH)8(OH)8(M-L)4], where M = Cu(II) or Ni(II), L = 6,13dicarboxy-1,4,8,11-tetraazacyclotetradecane), denoted as VPI100 (Cu) and VPI-100 (Ni) (VPI = Virginia Polytechnic Institute), respectively. To the best of our knowledge, this is the first report of a metal-cyclam-based Zr-MOF, which exhibits extraordinary stability and promising catalytic activity. Unlike previously reported frameworks29,30,42,43 featuring similar tetraamine linkers, the metallocyclam has its axial positions occupied by labile chloride ligands. The chloride ligands are readily displaced by the desired guest molecules without compromising the structural integrity of the material. This study undertakes a model approach for designing atomically precise Zr-based MOFs with non-aromatic macrocyclic ligands, and opens a new window for the further development of metal−metallocyclam frameworks and their unique functionalities. This framework family could potentially feature a wide range of analogues, similar to the extensively studied metal− metalloporphyrin frameworks.44
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EXPERIMENTAL SECTION
Materials and Methods. All chemicals were purchased from commercial sources and used without further purification unless otherwise mentioned. 994
DOI: 10.1021/jacs.7b10643 J. Am. Chem. Soc. 2018, 140, 993−1003
Article
Journal of the American Chemical Society 0.05°. The powder samples were loaded onto a zero-background Si (510) disk. Single-Crystal X-ray Diffraction. Single-crystal X-ray data of [CuL(ClO4)2] (1) and [CuLCl2] (4) were collected on a Rigaku Oxford Diffraction Gemini diffractometer equipped with a Mo Kα Xray source (λ = 0.71073 Å) and a low-temperature device operating at 100 K. X-ray data collection of [NiL(ClO4)2]·H2O (3) and [NiLCl2]· 2DMF (5) was performed on an Rigaku Oxford Diffraction Nova diffractometer operating with a Cu Kα radiation source (λ = 1.5418 Å). Collection temperature was 250 K for 3 and 100 K for 5. The data collection routine, unit cell refinement, and data processing were carried out with the program CrysAlisPro.47 The structures were solved using SHELXT-201448 and refined using SHELXL-201449 via Olex2.50 Molecular graphics generation was carried out using Olex2.50 The final refinement model for all four structures involved anisotropic displacement parameters for non-hydrogen atoms. A riding model was used for all hydrogens not involved in hydrogen bonding. The H-atom positions on the carboxylic acids were located from the residual electron density map and refined independently. The isotropic displacement parameters of the H-atom were constrained to 1.5Ueq of the attached O-atom. CCDC 1572991, 1572992, 1572993, 1572994, 1576953, and 1576954 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. Structure [CuL(ClO4)2] (1). The sample crystallized as a pink plate (0.195 × 0.320 × 0.360 mm3). The Laue symmetry was consistent with the triclinic space groups P1 and P1̅. The centrosymmetric space group P1̅ was chosen. Structure [NiL(ClO4)2]·H2O (3). The sample crystallized as a yellow plate (0.039 × 0.100 × 0.214 mm3). The Laue symmetry and systematic absences were consistent with the monoclinic space group P21/c. A 2-position disordered model was used for the perchlorate with relative occupancies that refined to 0.585(12) and 0.415(12). Structure [CuLCl2] (4). The sample crystallized as a purple plate (0.061 × 0.104 × 0.201 mm3). The Laue symmetry and systematic absences were consistent with the monoclinic space group P21/n. Structure [NiLCl2]·2DMF (5). The sample crystallized as a colorless needle (0.16 × 0.11 × 0.05 mm3). The Laue symmetry was consistent with the triclinic space groups P1 and P1̅. The centrosymmetric space group P1̅ was chosen. The final refinement model involved anisotropic displacement parameters for non-hydrogen atoms. Attenuated Total Reflectance Fourier-Transform Infrared Spectroscopy (ATR-FTIR). ATR-FTIR spectra were obtained on a Varian 670 FT-IR spectrometer equipped with a diamond Specac Golden Gate attachment. All spectra were an average of 96 scans for powdered samples and were recorded from 4000 to 500 cm−1 with 4 cm−1 resolution. A background spectrum collected on air was subtracted from the sample spectra. The spectra were not corrected for the depth of wavelength penetration. Thermogravimetric Analysis (TGA). A TA Instruments Q500 was used to analyze thermal stability. Powder samples (∼5 mg) were placed on an aluminum pan and heated at a rate of 10 °C/min under N2 atmosphere over the temperature range of 25−600 °C. Sample Activation and N2/CO2 Adsorption Measurement. Before a gas sorption experiment, as-synthesized VPI-100 (Cu)/VPI100 (Ni) (∼50 mg) samples were washed with DMF three times and acetone three times, followed by soaking in acetone for 3 days to allow solvent exchange. During the solvent exchange process, the acetone was decanted and replaced with fresh solvent three times every 24 h. After that, the MOF powders were isolated by centrifugation. The resulting exchanged frameworks were activated under vacuum for 12 h and then degassed under vacuum for 12 h at 100 °C prior to gas adsorption/desorption measurements. X-ray Photoelectron Spectroscopy (XPS). The XPS spectra were collected on a PHI 5000 Vera Probe III spectrometer using an aluminum anode X-ray source (photon energy of 1486.6 eV). Survey and elemental spectra were collected using a 100 μm beam size and 25 W, 15 kV source. The data were collected in the range of 1100−0 eV with a pass energy of 225 eV at a step size of 0.8 eV/step. Elemental spectra were collected at the following energy ranges using a pass
energy of 69 eV and step size of 0.125 eV/step: N 1s (390−410 eV), Ni 2p3/2 (845−865 eV), Cu 2p3/2 (925−940 eV), and Zr 3d (174−194 eV). Each element was scanned at 15 sweeps, except for the Cu, Ni, and N regions, which were scanned for 450 sweeps each. The pressure inside the sample chamber was kept below 1 × 10−7 Torr for all the measurements. Scanning Electron Microscopy (SEM). A LEO (Zeiss) 1550 field-emission scanning electron microscope, equipped with an in-lens detector, operating at 5.0 kV was used to obtain high-resolution images of the MOF powders. Stability Tests. For a solvent stability study, small amounts of freshly synthesized VPI-100 (Cu) and VPI-100 (Ni) (8−12 mg) were added into different vials containing 1 mL of solvent. The MOF powders were soaked for 24 h, after which they were isolated by centrifugation, washed with fresh solvent three times, and dried under vacuum at room temperature. The resultant samples were analyzed by PXRD and compared to the pristine MOFs. Thermal stability of VPI-100 (Cu) and VPI-100 (Ni) was assessed by variable-temperature PXRD experiments. The measurements were performed using a Bruker AXS D8 Advance powder diffractometer at 40 kV, 40 mA for Cu Kα (λ = 1.5406 Å), with a scan speed of 0.2 s/ step and a step size of 0.02°. Inductively Coupled Plasma Mass Spectrometry (ICP-MS). The MOF samples were digested in 70% nitric acid and heated at 90 °C for 1 h. After filtration through a syringe filter (
