A Robust, Water-stable, and Multifunctional Praseodymium-organic Framework Showing Permanent Porosity, CO2 Adsorption Properties, and Selective Sensing of Fe3+ Ion
Qing-Xia YAO , Miao-Miao TIAN , Yao WANG , Yu-Jie MENG , Jun WANG , Qing-Yuan YAO , Xin ZHOU , Hua YANG , Huai-Wei WANG , Yun-Wu LI , Jie ZHANG
Considerable efforts have been devoted to the design and synthesis of metal-organic frameworks (MOFs or so-called porous coordination polymers) because of their potential applications in gas storage, separation, catalysis, and sensing[1-6]. However, due to the lability of metal-ligand bonds, many reported MOFs are sensitive to water/moisture, leading to structural collapse and nonporosity of the materials[7]. For instances, the well-known MOF-5 could gradually decompose upon exposure to moisture[8]. The water-exposed HKUST-1 loses approximately 50% of the original surface area[9]. The instability of MOF materials has severely limited their real-world applications, since water or moisture is present in most industrial processes like the preparation, storage, transportation and application of the material[10]. Hence, water-stable MOFs are in high demand from an application perspective. Recent studies reveal that several factors are considered as the critical contributors towards the MOF stability in water content. These factors include metals with high oxidation state, ligands with high pKa value, and the shielding of coordination sites by hydrophobic organic groups[10-12]. In this regard, lanthanide ions have attracted special attention, not only because of their high oxidation states and higher coordination number which would bring forth robust frameworks but also because of their unique optical and magnetic properties. However, lanthanide based metal-organic frameworks (Ln-MOFs) with permanent porosity are less developed[13].
On the other hand, the desired structures and functions of MOFs can be modified by judicious selection of functional organic ligands, such as salens, NHCs and viologens[14-16]. Among these, triphenylamine ligands are unique for their potential optoelectronic applications, such as light-emitters, photo conductors, and hole-transporters[17]. 4, 4΄, 4΄΄-Nitrilotribe-nzoic acid (H3NTB) is a typical triphenylamine ligand and attracts ongoing efforts to construct multifunctional MOF materials[18, 19]. In recent years, the assemblies of H3NTB with lanthanide ions have afforded several Ln-NTB MOF structures. For example, Duan et al. reported a Tb-NTB structure with its catalytic properties and a different Eu-NTB structure with fluorescent sensing property respectively[20, 21]. Some of us reported a unique La-NTB structure with 1D nanochannels[22]. As the structure and properties of such lanthanide based MOFs are strongly dependent on the nature of the rare-earth, it is necessary to extend to other lanthanide cations. Herein, in continuing our previous work, we report the synthesis, structure, and characterization of a new member of lanthanide organic frameworks based on H3NTB, Pr-NTB (1), which demonstrates remarkable thermal stability and tolerance towards water with pH values ranging from 3 to 11. Moreover, Pr-NTB exhibits permanent porosity with the BET surface area of 156.2 m2·g-1 based on Ar adsorption analysis and can adsorb suitable CO2 (1.14 mmol·g-1 at 273 K/1 bar) with the binding energy of 28.5 kJ·mol-1. These combined features promise the porous material 1 as a good adsorbent candidate for CO2 capture in practical application. Furthermore, the integration of fluorescent property, porosity and water stability in 1 allows itself to be used for selective sensing of Fe3+ cation in water.
All chemicals except the ligand were commercially available and used without further purification. The ligand, 4, 4΄, 4΄΄-nitrilotribenzoic acid (H3NTB), was synthesized according to the literature[23]. IR spectra were recorded on a Nicolet-iS50 FT-IR spectrophotometer with KBr pellets in the region of 4000~400 cm-1. The powder X-ray diffraction (PXRD) data were collected on a Bruker D8 Advance diffractometer with CuKα radiation (λ = 1.5418 Ǻ) and a graphite-monochromater at 298 K. Thermogravimetric analysis (TGA) and mass spectrum were performed under nitrogen atmosphere on a Netzsch STA 449F5-QMS403C simultaneous TG/DSC-QMS analyzer at a heating rate of 20 ℃/min. The photoluminescence spectra were measured on a Hitachi F-7000 spectrofluorometer. Ar sorption isotherms were measured at 87 K on a Micromeritics ASAP 2460 system. The samples were degassed at 200 ℃ for 12 h prior to the measurements.
A mixture of Pr(NO3)3·6H2O (43.1 mg), H3NTB (43.8 mg), DMF (4.5 mL) and HCl (0.05 M, 3.0 mL) was sealed in a 23.0 mL Teflon-lined stainless-steel container, which was heated at 100 ℃ for 21 hours to afford block crystals. The collected crystals were dried in air at room temperature. It is worthy to note that the purity of the ligand permits good reproducibility of the compounds.
{[Pr3(NTB)3(H2O)3]·(DMF)3(H2O)4}n (1). Yield > 30%. IR/cm-1 (KBr): 3392 (br), 2928 (w), 1674 (m), 1590 (s), 1523 (m), 1506 (w), 1366 (s), 1314 (s), 1273 (m), 1175 (m), 1142 (w), 1102 (w), 1014 (w), 845 (w), 783 (m), 701 (w), 673 (m), 649 (w), 540 (w), 442 (w).
A suitable crystal with dimensions of 0.20mm × 0.15mm × 0.10mm was selected for diffraction analysis. Single-crystal X-ray diffraction data were recorded at room temperature on a Bruker SMART-1000 CCD diffractometer with graphitemonochromated with MoKα radiation (λ = 0.71073 Å) in the ω scan mode. Data reduction was performed using the SAINT program[24]. The structure was solved by direct methods and refined by full-matrix least-squares methods with SHELX program[25]. All non-hydrogen atoms of the framework were refined anisotropically; other non-hydrogen atoms of the free DMF and water molecules were refined isotropically. The hydrogen atoms of the ligands and free DMF molecules were placed in idealized positions using a riding model and refined isotropically. The hydrogen atoms of the solvent water molecules can not be allocated from the difference Fourier maps. The PLATON program[26] was used for void analysis. Crystal data for 1: C63H36N3O21Pr3, Mr = 1878.98, monoclinic space group P21/c, a = 14.5746(3), b = 23.3584(4), c = 24.9936(5) Å, β = 102.564(2)°, V = 2056.3(3) Å3, Z = 4, ρcalcd = 1.503 g·cm-3, μ = 1.811 mm-1, 36305 reflections measured, 18830 independent reflections, R (wR) = 0.0663 (0.1374) for 12343 reflections (I > 2σ(I)) and 914 parameters, GOF = 1.026.
Ar adsorption-desorption isotherm was recorded at 87 K on a Micromeritics ASAP2460 analyzer. The sample was activated under vacuum by heating at 200 ℃ overnight. The specific surface area was calculated from the data in the adsorption branch at p/p0 = 0.05~0.30. The total pore volume was calculated from the uptake at p/p0 of 0.950. The pore size distribution was calculated by the Horvath-Kawazoe (HK) method using the slit model. CO2 adsorption isotherm was performed at 196 and 273 K, respectively.
Analytes were prepared as follows: 1.8 mg powdered 1 was suspended in 10.0 mL water and ultrasonicated for 45 min to give suspension. To 0.5 mL such suspension was added a series of aqueous solution (0.01 M, 5 μL), including fresh CrCl3, Cr(NO3)3, MnCl2, FeCl3, Fe(NO3)3, CoCl2, NiCl2, CuCl2, Zn(NO3)2, AgNO3, Al(NO3)3, HCl and NaNO3. The emission spectra of each analyte were recorded at 340 nm on a Hitachi F-7000 spectrofluorometer. In addition, in order to examine the fluorescence quenching effect by Fe3+, more diluted FeCl3 (0.001 M) with different amounts was added into the above suspension of 1 (0.5 mL).
Solvothermal reaction of Pr(NO3)3·6H2O with H3NTB in DMF-HCl mixture affords crystalline Pr-NTB (1). Singlecrystal X-ray diffraction (SCXRD) shows that 1 crystallizes in the monoclinic space group P21/c and has the composition of {[Pr3(NTB)3(H2O)3]·(DMF)3(H2O)4}n which is in good accordance with TG analysis. The structure of 1 is intrinsically isostructural to the Eu-NTB[21]. The asymmetric unit of 1 contains three crystallographically independent Pr3+ cations, three NTB3- ligands, and three coordinated water molecules. As seen in Fig. 1a, the coordination geometry of each Pr cation is complex. Pr1 is coordinated by nine oxygen atoms from two H2O molecules and five carboxylate groups that belong to five different ligands. Pr2 is also coordinated by nine oxygen atoms that pertain to six different ligands. However, Pr3 is eight-coordinated by oxygen atoms from one H2O molecule and six carboxylate groups that are from six different ligands. Interestingly, the polyhedron of each Pr cation is fused via edge-sharing or corner-sharing carboxylate oxygen atoms to form 1D Pr-carboxylate inorganic chains running along [001] (Fig. 1b). Each ligand connects to five or six different Pr3+ cations through three coordination modes: chelating, bridging (μ2-η1: η1), and chelating-bridging (μ2-η2: η1) (Fig. 1c). These Pr-carboxylate inorganic chains, as rod-shaped secondary building units (SBUs), are bridged by NTB3- ligands to construct the 3D porous framework (Fig. 2). Different from our previous La-NTB framework with 1D straight channels[22], Pr-NTB has nanosized cages throughout the network, which can accommodate a sphere with the diameter up to ~8.1 Å (Figs. 2 and 3). Furthermore, these cages are accessible through the rhombus windows/channels of 3.3 × 3.8 Å2 (measured between the opposite atoms of H(62)···H(63) and H(45)···H(46), and considering the van der Waals radii) along the [001] direction. There exist guest DMF and disordered water molecules in the cages. Free void analyzed by PLATON is up to 37.2% of the crystal volume. Therefore, 1 is a highly microporous framework with restricted yet accessible windows, which would be suitable for gas adsorption. Moreover, the structural stability can also be expected because of the high coordination numbers of Pr3+, the inorganic metal-carboxylate chain, and the rigid ligand.
A thermogravimetric analysis (TGA) experiment under N2 atmosphere was carried out to determine the thermal stability of 1. The TGA curve (Fig. 4) demonstrates a weight loss of 19.0%, which corresponds to the release of all guest and coordinated solvent molecules (calcd. 18.9% weight loss based on structural composition of {[Pr3(NTB)3(H2O)3]· (DMF)3(H2O)4}n). There is no mass loss between 251.6 and 500 ℃. Upon heating to 500 ℃, a dramatic weight loss indicates the decomposition of the framework. As shown in Fig. 5, the experimental PXRD patterns of the as-synthesized and desolvated 1 are almost the same as the simulated one from the single-crystal structure data, indicating the phase purity of the crystalline powders and structural stability. It is worthy to note that 1 ranks among MOFs with the highest thermal stabilities[5, 22, 27-30].
Water stability is another important physical property for MOF adsorbent. As mentioned earlier, the synthesis of water stable MOFs is a key challenge in the MOF field. Hence, the as-synthesized samples of 1 were soaked in water with different pH values (pH = 3 and 11) to examine its water stability. The resulting PXRD patterns demonstrate the crystalline nature of the samples, indicating the high water stability (Fig. 5). In addition, it is also necessary to explore the stability of the evacuated framework under moist environment. When exposed to air at room temperature for one day, the resulting PXRD pattern is nearly identical with that of the original phase, suggesting the moisture stability of the evacuated framework. This water stability of 1 is comparable to those water-stable MOFs[5, 22, 27]. Taking the crystal structure into consideration, the high water stability as well as the thermal stability could be attributed to the combination of high coordination numbers of Pr3+, rod-shaped chains, and the rigidity of the ligand.
To demonstrate the permanent porosity of 1, an Ar adsorption-desorption experiment was performed at 87 K. Prior to gas sorption measurement, the sample was vacuum activated at 473 K for 12 hours. The adsorption isotherm (Fig. 6) exhibits the typical type-I adsorption at low pressure that is characteristic of micopores in 1. Accordingly, the BET surface area calculated from the Ar adsorption isotherm is 156.2 m2·g-1 and the pore volume is 0.064 cm3·g-1. These values are lower than that of our La-NTB[22], but comparable to that of many porous lanthanide MOFs[5, 27, 28]. Furthermore, the distribution of pore width simulated by the Horvath-Kawazoe (HK) method is about 3.5 Å, which is in good agreement with that derived from single crystal structural analysis (window size of 3.3 × 3.8 Å2).
In recent years, MOFs have been extensively investigated as new alternatives for CO2 capture, which is central to the reduction of greenhouse gas emission[31]. Encouraged by the permanent porosity and structural stability of 1, we investigated its CO2 adsorption behaviors at 196 and 273 K (Fig. 7). It is clearly demonstrated that the porous framework of 1 is readily accessible to CO2. At 196 K/1 bar, 1 shows a maximum CO2 uptake of 4.08 mmol·g-1, corresponding to about 2.09 molecules per NTB ligand. In addition, the volumetric gas storage capacity, as one of the important parameters of adsorbents in feasible applications, should be considered[32]. When considering the high density (1.236 g·cm-3) of the desolvated 1, its volumetric CO2 storage capacity can reach 221.9 g·L-1 at 196 K/1 bar, higher than that of La-BTN[27] (167.1 g·L-1 at 196 K/1 bar) which is constructed by the larger ligand H3BTN. Importantly, even at 273 K/1 bar, the CO2 uptake of 1 is 62.0 g·L-1, which is higher than that of some important lanthanide based MOFs (MIL-103[33]: 54.8 g·L-1; La-BTN[27]: 56.5 g·L-1 under the same conditions of 273 K/1 bar), but lower than that of the extremely porous Yb-BTB[30]. Among Ln-MOFs, the high CO2 uptake of 1 may be attributed to the small cages in the framework, which strengthens the electrostatic interactions between the highly-dense porous surface and CO2 molecules. To better understand the framework-CO2 interactions, isosteric heat of adsorption (Qst) was calculated by using the viral equation based on the isotherms at 196 and 273 K. As seen in Fig. 7b, the Qst value for 1 is 28.5 kJ·mol-1, which is comparable to some benchmark Ln-MOFs like La-BTN[27] (26.5 kJ·mol-1), Yb-BTB[30] (34.8 kJ·mol-1), Y-ftw-MOF-2[34] (27.0 kJ·mol-1), and TbL[35] (28.23 kJ·mol-1), reflecting the moderate interactions between the framework and guest CO2 molecules.
Due to the integration of fluorescent triphenylamine ligands, the porosity and water stability in the framework, the fluorescence and sensing properties of 1 were investigated. As expected, the suspension of 1 shows a broad emission centered at 450 nm which can be ascribed to the π*-π transition of the NTB3- ligand. The porosity and water stability of 1 allow its fluorescence property to be used for sensing metal cations in water. To the prepared suspension of 1 (0.5 mL) was added a series of metal salt aqueous solution (0.01 M, 5.0 μL), including fresh CrCl3, MnCl2, FeCl3, Fe(NO3)3, CoCl2, NiCl2, CuCl2, Zn(NO3)2, AgNO3, Al(NO3)3, HCl, and NaNO3. The emission spectra of each analyte were recorded (Fig. 8). We find that only Fe3+ ion causes a notable fluorescence quenching effect, while other metal ions show negligible effect on the emission of the suspension of 1. These results indicate that 1 may be a good fluorescent probe for selective sensing of Fe3+ ion. Furthermore, fluorescence quenching titration of suspension of 1 with various concentrations of Fe3+ aqueous solution ranging from 10 to 120 μM was also performed. The observed fluorescence intensity of 1 was inversely proportional to the concentration of Fe3+ ion. Notably, the fluorescence intensity of 1 is decreased by 25% at the quite low Fe3+ concentration of 50 μM. These results suggest 1 is quite sensitive in sensing Fe3+ ion in water.
In summary, we have successfully synthesized a new multifunctional lanthanide organic framework, Pr-NTB (1), based on 4, 4΄, 4΄΄-nitrilotribenzoic acid (H3NTB). 1 has a robust and water-stable framework exhibiting permanent porosity. Moreover, 1 can adsorb suitable CO2 (62.0 g·L-1 at 273 K/1 bar) with the binding energy of 28.5 kJ·mol-1. These combined features make 1 a good adsorbent candidate for CO2 capture in practical application. Furthermore, the integration of fluorescent property, porosity and water stability in 1 promises it as a selective fluorescent sensor for Fe3+ cation in water.
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Figure 1 (a) Coordination geometry of three crystallographically independent Pr3+ cations; (b) 1D Pr-carboxylate inorganic chain running along the [001] direction; (c) Coordination modes of three crystallographically independent NTB3-ligands
Figure 2 3D porous framework containing nanosized cages which are represented by yellow balls with the diameter of 8.1 Å
Figure 3 A cage with accessible rhombus window 3.3 × 3.8 Å2 (measured between the opposite atoms of H62···H63 and H45···H46 with considering the van der Waals radii)
Figure 5 PXRD patterns (from bottom to top) of the simulated, as-synthesized, the desolvated after vacuum treatment at 250 ℃ for 12 hours, the exposed in air for 1 day, and the one soaked in pH = 3 and 11, respectively
Figure 6 Ar sorption isotherm at 87 K. Inside: pore width distribution calculated by the Horvath-Kawazoe (HK) method using the slit model
Figure 7 (a) CO2 adsorption isotherms at 196 and 273 K; (b) Isosteric heat of adsorption (Qst) calculated by the viral method
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