Fabrication of a Robust Lanthanide MetalâOrganic Framework as a

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Fabrication of a robust lanthanide metal-organic framework as multifunctional material for Fe(III) detection, CO capture and utilization 2

Tan Jing, Lian Chen, Feilong Jiang, Yan Yang, Kang Zhou, Muxin Yu, Zhen Cao, Shengchang Li, and Maochun Hong Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00068 • Publication Date (Web): 20 Apr 2018 Downloaded from http://pubs.acs.org on April 20, 2018

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Crystal Growth & Design

Fabrication of a robust lanthanide metal-organic framework as multifunctional material for Fe(III) detection, CO2 capture and utilization Tan Jing,a,b Lian Chen,*a Feilong Jiang,a Yan Yang,c Kang Zhou,a Muxin Yu,a,b Zhen Cao,a,b Shengchang Li,ad and Maochun Hong*a a

State Key Laboratory of Structure Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China. b University of the Chinese Academy of Sciences, Beijing, 10049, China c Liaocheng Univ, Coll Chem & Chem Engn, Liaocheng 252059, Peoples R China. d ShanghaiTech Univ, Sch Phys Sci & Technol, Shanghai 201210, Peoples R China.

Abstract By employing a tricarboxylate ligand 1-(4-carboxybenzyl)-1H-pyrazole-3,5-dicarboxylic acid (H3L), four lanthanide metal-organic frameworks (Ln-MOFs) formulated as {[LnL(H2O)2]∙ H2O}n (Ln = Eu, 1; Gd, 2; Tb, 3; Dy, 4) have been prepared under hydrothermal conditions. Single-crystal X-ray analyses reveal that compounds 1-4 are isomorphous and exhibit a three-dimensional network structure featuring a one-dimensional rectangular channel with the size of ca. 7.8 Å×12.1 Å along b axis. The framework shows strong stabilities towards high temperature, humid air, water as well as acid/base environments. Efficient ligand-sensitized characteristic luminescence can be observed in the visible region for Eu- and Tb- based compounds. Detailed property investigation shows that TbL is a promising multifunctional material, which can quantitatively detect Fe(III) ions in the solution mixtures of Fe(Ⅱ)/Fe(Ⅲ), selectively adsorb CO2 over CH4 and be applied as catalysts in cyclization reaction with epoxides and CO2. Keyword: metal-organic framework, Fe(Ⅲ) sensor, gas separation, CO2 conversion

Introduction Metal–organic frameworks (MOFs), a flourishing subclass of porous crystalline materials, have been developed quickly ascribed to their prosperous applications in gas

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Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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storage/separation,1-5 catalysis,6-11 drug delivery,12-16 and sensing17-29 in the past two decades. Among all these applications, MOFs incorporating functional sites and proper channels for host-guest recognition have attracted extensive attention, especially in sensing cations, anions, small organic molecules and biomarkers as well as pH and temperature. As we know, Fe(Ⅲ) ion is one of the indispensable elements for human body and other biological systems.30 Both shortage and surplus of Fe(Ⅲ) ions may result in somatic function disorders, such as iron deficiency anemia (IDA), skin diseases, agrypnia and decreased immunity.31-33 Therefore, selective and quantitative detection of Fe(Ⅲ) ions is of great importance. Compared with other detection methods, fluorescence-based detection has proved to be a preeminent avenue for rapid detection of Fe(Ⅲ) ions attributed to its excellent sensitivity, unique selectivity, convenient operation, low cost and visual detection.19 As a subgroup of MOFs, Ln-MOFs are considered as the fascinating and promising sensing materials which combine the unique spectroscopic features of lanthanide ions and porous advantage of MOFs.34 In terms of fluorescence sensing, Eu-MOFs and Tb-MOFs, are drawing increasing attention as effective materials for fluorescent probes due to their excellent luminescence properties, such as extremely sharp pure red or green emission, large Stokes shifts, high quantum yields.35-37 On the other hand, in recent years, CO2 has caught people’s attention not only because it is the predominant greenhouse gas causing global warming but also because it can be widely used in organic syntheses as the nontoxic, inexpensive and renewable C1 resource. Numerous efforts have been devoted to CO2 storage, separation, fixation and utilization.38-41 One of the most efficient ways is the additive reactions between epoxides and CO2 to form cyclic carbonates, critical reagents widely used in fine chemical industrial and pharmaceutical syntheses. While stable Ln-MOFs are considered as the promising materials in CO2 storage, separation and catalysis due to their regular channels and Lewis acid/basic sites. Based

on

the

above

discussion,

1-(4-carboxybenzyl)-1H-pyrazole-3,5-dicarboxylic

acid

we (H3L)

employed ligand

to

the construct

Ln-MOFs, because this tricarboxylate linker with versatile coordination modes and Lewis base sites could promote the formation of a high-dimensional and multifunctional


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framework material. Finally, we have successfully obtained four novel 3D Ln-MOFs {[LnL(H2O)2]∙H2O}n (Ln =Eu, 1, Gd, 2, Tb, 3, Dy, 4), with high air, water, pH and thermal stabilities which can be used as the Fe(III) sensor, CO2 separation material and the catalyst in the reaction of CO2 cyclization with epoxides. Materials and methods All reagents and solvents were of AR grade commercially available and used as received without further purification. The German Elementary Vario EL III instrument was used for Elemental analyses. Thermogravimetric analyses (TGA) were measured using the Netzsch Model STA 449C instrument from ambient temperature to 900 °C at a heating rate of 10 °C/min under flowing nitrogen atmosphere. The IR spectra in the 400-4000 cm-1 region were collected with KBr slices on a PerkinElmer Spectrum One FT-IR spectrometer. The temperature-dependent X-ray powder diffraction patterns were recorded on the Ultima-IV X-ray diffractometer. Powder X-ray diffraction (PXRD) data were collected on the Rigaku MiniFlex 600 diffractometer. Gas adsorption measurements were carried out on ASAP 2020 M gas adsorption analyser. UV-vis study was recorded on a Lambda 365. Photoluminescence spectra of solid state samples were collected on a Horiba Jobin-Yvon Fluorolog-3 spectrofluorometer with a xenon arc lamp as the light source. Luminescence lifetimes were obtained on an Edinburgh Analytical Instruments FLS920 and the overall fluorescence quantum yields were measured by a calibrated integrating sphere at room temperature on Edinburgh Instruments FLS920 spectrofluorometer. Nuclear magnetic resonance (NMR) spectroscopy was recorded on the Burker AVANCE 400 (400 MHz). Synthesis of [LnL(H2O)2]•H2O (Ln = Eu, Gd, Tb and Dy) (Compounds EuL, GdL, TbL and DyL) A mixture of H3L (29mg, 0.10 mmol) and Ln(NO3)3·6H2O (45mg, 0.10 mmol) was dissolved in the 10 mL binary solution (CH3CN : H2O = 1: 1), which was sealed in a 25 mL Teflon-lined stainless steel autoclave. Then they were kept in a 130 °C oven for 72 h. After slowly cooling to room temperature, the colorless crystals were collected by filtration, washed with CH3CN and ethanol, and dried in air. The total yields of EuL, GdL, TbL and DyL are ca. 60%, 65%, 63% and 61%, respectively, based on the metal ions. The elemental analyses are as follows.



[EuL(H2O)2]•H2O (EuL) Elemental analysis (%): Calcd for EuC13H13N2O9, C, 29.68/29.49; H, 3.15/3.17; N, 5.27 /5.22 [GdL(H2O)2]•H2O (GdL) Elemental analysis (%): Calcd for GdC13H13N2O9, C, 29.61/29.21; H, 3.16/3.21; N, 5.26 /5.24. [TbL(H2O)2]•H2O (TbL) Elemental analysis (%): Calcd for TbC13H13N2O9, C, 29.63/29.12; H, 3.13 /3.20; N, 5.30/5.22. [DyL(H2O)2]•H2O (DyL) Elemental analysis (%): Calcd for DyC13H13N2O9, C, 29.08/28.93; H, 3.15 /3.17; N, 5.20 /5.19. X-ray Crystallographic Crystallographic data of compounds 1-4 were collected on the SuperNova diffractometer equipped with a multilayers mirror Cu Kα radiation (λ = 1.5418Å). All structures were solved by direct methods and refined by full-matrix least-squares techniques on F2 with SHELX-97.42, 43 All the non-hydrogen atoms were refined anisotropically on F2 using the full-matrix least-squares technique using the SHELXL-97 program except some disordered water molecules. All the hydrogen atoms were placed geometrically and refined using a riding model. The disordered solvents were removed by the SQUEEZE process.44, 45 The final chemical formulas of the compounds 1-4 were calculated from solved results combined with thermogravimetric and elemental analyses. The CCDC numbers for 1-4 are 1814661-1814664. Detailed crystallographic data are attached in Table S1.

Results and discussion Structural description The collected PXRD patterns (Fig. S2) are almost the same as the corresponding simulated ones, indicating phase purities of the compounds. Single-crystal X-ray diffraction analyses (SCXRD) confirm that these four compounds are heterogeneous isomorphous and possess similar three-dimensional network. Therefore, we choose TbL as a model to illustrate the structure in detail. TbL crystallizes in P-1 space group, whose unit cell contains one Tb(III) ion, one fully deprotonated [L]3- ligand and two coordinated water molecules (Fig 1a). The Tb(III) center is nine-coordinated and its coordination geometry is completed by five µ2-bridging and two monodentate carboxylate oxygen atoms from four [L]3- ligands as well


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as two oxygen atoms from two coordinated water molecules. A pair of µ2-bridging carboxylic groups connect two symmetry-related lanthanide ions and produce a dual-core structure, which is further linked with each other to form an infinite [Tb2(µ2-COO)2]n chain (Fig. 1b). Each [L]3- ligand connects five Tb(III) atoms through a µ 5-bridging coordination mode as Fig. 1c shown. Every dual-core cluster is bridged with other eleven dual-core clusters by eight L3- (Fig. S3), finally generating a neutral 3D framework. A 1D rectangular channel with the size of ca. 7.8 Å×12.1 Å can be seen from the direction of b axis (Fig.

1d). Fig. 1 (a) Coordination environment of the Tb(III) ion in TbL. (Symmetry codes: A, 1-x, -y, -z; B, x, y-1, z-1; C, -x, -y-1, -z; D, x, y-1, z. Tb, green; N, blue; O, red; C, gray). (b) 1D [Tb2(µ2-COO)2]n chain in TbL. (c) The coordination mode of ligand. (d) The 3D network of TbL viewed along the b axis. Framework Stability In spite of high performance in many fields, a crucial weakness which hinders the applications of MOFs is their low stability. Thus, a comprehensive stability investigation of the material was then carried out. The TGA data (Fig. S4) demonstrates that the decomposition temperature of the framework is high up to 440 oC. The first weight loss appears in the range of ca. 30–100 oC, which corresponds to the removal of the water molecules in the pores. And the weight loss in the range of ca. 100–240 oC corresponds to the removal of the coordinated water. The X-ray thermodiffractogram of TbL was



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investigated to further explore the thermal stability of the framework (Fig. 2a). The result shows no obvious phase transition occurs when we heat up to 180

o

C, but some

characteristic peaks become unconspicuous when we raise temperature to 200

o

C,

suggesting that this framework could keep stability until heating up to 180 oC. For air stability test, 40mg TbL was exposed to the open air for 7 d, 15 d, and 30 d with an average humidity of ca. 55%. The PXRD patterns were then recorded to test its air stability. The result shown in Fig. 2b suggests that TbL can keep its framework after exposed to the open air for more than 30 days. To test its water stability, 40mg TbL was soaked in 25mL water for 3d, 7d, 15d and 30d. The corresponding PXRD results suggest that TbL possesses excellent stability to water (Fig. 2c). Chemical stability test was assessed by soaking the 40mg TbL in the aqueous solutions of pH = 1–14 for 48 hours. Then, all the samples were filtered and washed 3 times and dried in air before PXRD experiments for pH stability tests. The result reveals that the framework can keep stable in a broad pH range (pH = 2–12) (Fig. 2d). Overall, this framework shows strong stabilities towards high temperature, humid air, water as well as acid/basic environments.

Fig. 2 X-ray thermodiffractogram (a), air stability (b), water stability (c) and pH stability (d) analyses of TbL.


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Luminescent Properties The luminescence emission intensity of LnMOFs depends on the efficiency of energy transfer from the organic ligand to lanthanide ions to a certain extent. When excited, the emission spectra of ligand, GdL, EuL, TbL and DyL were measured and recorded in Fig. S5, Fig. S6, Fig. 3a, Fig. 3b and Fig. S7, respectively. The luminesence spectra of EuL, TbL, and DyL all show characteristic emission lines of the corresponding lanthanide ions: at 580, 592, 617, 653 and 699 nm for EuL, at 491, 543, 586, 653 and 696 nm for TbL, at 480, 573, 660 and 750 nm for DyL. The decay lifetimes and luminescent quantum yields of EuL, TbL, DyL are shown in the Table. S2. The larger decay lifetimes and luminescent quantum yields of EuL and TbL indicate that the Eu(III) and Tb(III) can be well sensitized by the ligand compared with Dy(III) ion.

Fig. 3 The excitation and emission spectra of EuL (a) and TbL (b) at room temperature. Sensing for Fe(Ⅲ Ⅲ) cations In spired by the excellent luminescent properties and water stability of TbL, the application of TbL as luminescent probes for sensing metal ions was explored. 5 mg of TbL was dispersed in 4 mL aqueous solution, then the suspension was ultrasonicated for 30 minutes to form well-distributed MOF suspension whose luminescence intensity was recorded as the blank. Then, 1mL different metal salts, Sr(NO3)2, KNO3, Mg(NO3)2, CsNO3, Fe(NO3)2, NaNO3, Mn(NO3)2, Ni(NO3)2, Co(NO3)2, Ba(NO3)2, Cu(NO3)2, Fe(NO3)3, with the molar concentration of 5 х 10-3 mol/L were added respectively to form well-distributed metal ion incorporated MOFs suspensions for luminescence experiments. As shown in Fig. 4a, there is no significant change in fluorescence intensity for most metal ions suspensions, but pronounced quenching effect of Fe(Ⅲ) ions can be observed,



indicating that TbL can selectively detect Fe(Ⅲ) ions through luminescence quenching effect. The extent of quenching effect can be evaluated by quenching effect coefficient (Ksv), which is calculated according to the Stern–Volmer equation46: I0/I = 1 + Ksv[M], where I0 and I are the luminescence intensity of TbL before and after adding metal ions, and [M] is the molar concentration of metal ions. It shows the highest Ksv value of 1.9ⅹ 103 L/mol for Fe(Ⅲ) ions. It is interesting to note that, in spite of prominent quenching effect to Fe(Ⅲ) ions, the emission of TbL remains almost unchanged to Fe (II) ions. Thus, TbL may be act as the luminescent probe for Fe(Ⅲ) ions in the Fe(Ⅲ)/Fe(II) solution mixtures and could monitor oxidation extent of Fe in complicated systems, which plays significant roles not only in biology systems but also in industry. Here, we utilize TbL to measure the ratio of Fe(Ⅲ) ions as a prototype system to demonstrates the potential of TbL as a new tool for detecting the extent of hydrogen peroxide oxidization reaction of Fe(Ⅱ) to Fe(Ⅲ) ions in industry by luminescence measurements. The photoluminescent properties of TbL in the mixtures of Fe(Ⅱ) and Fe(Ⅲ) ions with different molar ratios were investigated, in which the total concentration of Fe(Ⅱ) and Fe(Ⅲ) ions is 1 mmol/L. As figure 4b shown, when the concentration of Fe(Ⅲ) ions increased, the luminescence emission of the suspension quenched gradually. It shows obvious linear relation between fluorescence intensity and the concentration of Fe(Ⅲ) ions in the range of 0-0.8 mmol in the Fe(Ⅲ)/Fe(II) mixed solution (Fig. 4c), indicating the concentration of Fe(Ⅲ) ions in this range can be well estimated. In order to understand the mechanism of luminescence quenching of Fe( Ⅲ ) ions suspension, the PXRD of the sample after fluorescence testing was measured (Fig. S10). The unchanged pattern suggests that the quenching of photoluminescence has no relation with the crystallographic transformation, indicating that Fe(Ⅲ) ions would not induce framework collapse. According to UV-vis absorption spectra(Fig. S11), the strong adsorption of the aqueous solution of Fe(Ⅲ) ions is in the range of 250-450 nm, while others do not overlap in this range, especially near the 283nm. Therefore, there exists obvious competition of the absorption in excitation wavelength (283 nm) between TbL


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and the Fe(Ⅲ) ions water solution. Such competitive adsorption will significantly decrease the transfer efficiency of excitation energy.

Fig. 4. (a) Luminescence intensity of the 5D4 → 7F5 transitions (543 nm) of TbL in 5 × 10-4 M different metal cations. (b) The luminescence image of corresponding Fe(Ⅲ) ions and Fe(II) ions samples under the irradiation of UV light. (c) The fluorescence intensity of Fe(Ⅲ) ions and Fe(II) ions aqueous solution with 1 mmol/L total concentration. Gas sorption properties In order to explore the porosity of 1-4, we choose the TbL as an example to investigate the porosity of the framework through the gas sorption experiments. The fresh TbL firstly was guest-exchanged with dry CH2Cl2 for three times in three days, then activated at 150oC under 10 µmHg for 10 hours. The sorption isotherms for N2, CH4 and CO2 are expressed in Fig. 5a. The sorption isotherms of CO2 and CH4 show the typical Type-I, indicating the retention of the micro-porous structures after the removal of guests from the crystalline samples. At 77 K, nearly no N2 absorption was observed, while CH4 and CO2 show obvious absorption at 273 K and 295 K. The max uptakes of CO2 are of 38.21 cm3 g-1 at 273 K and 33.10 cm3∙g-1 at 295 K and 1.0 bar, which are much higher compared with CH4 (10.13 cm3∙g-1 at 273 K and 7.61 cm3∙g-1 at 295 K and 1.0 bar). For TbL, it exhibits obvious selective adsorption of CO2 over CH4, although the uptake values are not high compared with some reported results. The adsorption performance



could be ascribed to the host-guest interactions and microporous structure. TbL contains open imidazolyl active sites and unsaturated metal sites in the framework, which may lead to an electric field to induce a dipole in CO2.47, 48 The lowest isosteric heat (Qst) of CO2 has been estimated more than 30 kJ mol-1 (Fig. 5b), which reflects a strong adsorbent−adsorbate interaction. Ideal Adsorbed Solution Theory was utilized to study the feasibility of TbL to selectively separate CO2 from binary CO2/CH4 mixtures. As shown in Fig. 5c, the selectivity exhibits a rapid decline trend in the low pressure. The values of selectivity are all above 80, which indicates the good selectivity compared with some reported MOFs.49, 50 Although the adsorption capacity of TbL for CO2 is not remarkable, it shows particular capturing ability of CO2 over some gases with high selectivity, indicating that TbL may be a potential candidate in selective gas separation.

Fig. 5 (a) CO2 and CH4 uptake capacity for TbL at 295 K, and N2 uptake capacity at 77 K. (b) The Qst of TbL for CO2. (c) The CO2/CH4 selectivity of TbL for the 15/85 CO2/CH4 mixture at 1 bar and 273 K. Catalytic properties Considering that the TbL has coordinative unsaturated metal sites after activation 3 hours in 150°C, it may possess catalytic performance.51, 52 We chose the styrene oxide as a typical reactant to find the optimal condition in the reaction of CO2 cycloaddition with epoxides, and the experimental results are shown in Table 1. Firstly, the influence of TbL towards this reaction was investigated under the solvent-free conditions with 0.1 MPa CO2 at 70°C. When we added TbL catalyst and TBBA (tetrabutylammonium bromide) cocatalyst simultaneously (entry 3), it has a notably higher yield (89.7%) compared with adding only one of them (entry 2,

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