Uncommon Pyrazoyl-Carboxyl Bifunctional Ligand-Based

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Uncommon Pyrazoyl-Carboxyl Bifunctional Ligand-Based Microporous Lanthanide Systems: Sorption and Luminescent Sensing Properties Gao-Peng Li,† Ge Liu,† Yong-Zhi Li,† Lei Hou,*,† Yao-Yu Wang,† and Zhonghua Zhu‡ †

Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of the Ministry of Education, Shaanxi Key Laboratory of Physico-Inorganic Chemistry, College of Chemistry and Materials Science, Northwest University, Xi’an, Shaanxi 710127, P. R. China ‡ School of Chemical Engineering, The University of Queensland, Brisbane, Queensland 4072, Australia S Supporting Information *

ABSTRACT: Seven new isostructural lanthanide metal−organic frameworks (Ln-MOFs), [Ln(Hpzbc)2(NO3)]·H2O (1-Ln, Ln = Nd3+, Sm3+, Eu3+, Gd3+, Tb3+, Er3+, and Yb3+ ions, H2pzbc = 3-(1H-pyrazol-3-yl) benzoic acid), with one-dimensional (1D) channels decorated by nitrate anions and pyrazoyl groups have been constructed. 1-Ln, as revealed by structural analysis, represent uncommon microporous 3D Ln-pyrazoyl-carboxyl systems using pyrazoyl-carboxyl bifunctional ligands as bridges. The luminescent investigations show that 1-Eu is an excellent MOF-based fluorescent probe, with high sensitivity, selectivity, and simple regeneration, for environmentally relevant Fe3+ and Cr2O72− ions. 1-Eu also presents highly selective capture for CO2 over N2 and CH4 due to the multiple binding sites for CO2 molecules, which were supported by Grand Canonical Monte Carlo (GCMC) simulations.

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INTRODUCTION Fe and Cr2O72− ions are two well-known important ions, in which Fe3+ is an indispensable biological element and is also widely used in industry production, while Cr2O72− is an important oxidant in industry.1 Accordingly, the massive utilizations of these two ions have brought severe environmental pollutants because Cr2O72− is very carcinogenic and Fe3+ causes health problems.2 Therefore, a material with selectivity and sensitivity for probing Fe3+ and Cr2O72− ions is urgently required. In the known detection methods, the fluorometric determination has been intensively explored owing to high sensitivity, simplicity, short response time, and so on. In this regard, fluorescent metal−organic framework (MOF) as a new type of sensor has gained ever-increasing attention of chemists due to not only regular and tunable structures but also intense and visible luminescence to naked eyes.3 These MOFs displayed nice luminescent sensing for metal ions, such as K+, Mg2+, Co2+, Cu2+, Al3+, and organic molecules,3c−g however, fewer examples were engaged in probes for Fe3+ and Cr2O72− ions.3i−l Compared to transition metal-based MOFs, lanthanide MOFs (Ln-MOFs), for example, Eu- and Tb-MOFs, due to their unique optical advantages, such as large Stokes shift, visible and very bright luminescent colors, high color purity, relatively long decay lifetimes, and undisturbed emissive energy, have been regarded as very promising luminescent sensing materials.4 For this goal, a variety of strategies, such as generation of exposed Ln3+ sites and immobilization of open

Lewis basic sites and carboxylic groups in MOFs, have been adopted,3l,4 although Ln-MOFs are presently not as well developed as their competitors. In particular, very sporadic LnMOFs were observed to show luminescent sensing for Fe3+ or Cr2O72− ions (Table S1),3l,5 and meanwhile, only one example reveals sensing for these two ions.5 On the other hand, the rising content of CO2 in the atmosphere has induced the most serious environmental issue as the result of rapidly increasing consumption of fossil fuels.6 In the context of clean energy, CH4, a primary component of natural gas and biogas, is a very ideal candidate to mitigate this problem for its lower sulfur and nitrogen content.7 Therefore, developing a suitable material for CH4 and CO2 separation at room temperature is vital from economic and environmental considerations. Due to the high valence and flexible coordination number of Ln3+ ions, great efforts have been made by chemists on Ln-MOFs for CO2 capture and separation.8 Notably, for either luminescent sensing or CO2 capture in Ln-MOFs, the majority of Ln-MOFs were prepared by pure carboxylate or pyridyl-carboxylate ligands.3a,9 It is known that pyrazole not only forms strong coordination with various metal ions but also can combine mixed carboxylate ligands to form stable MOFs.10 The latest CCDC search (version 5.36) indicates that, although a large number of Ln-pyrazoyl

3+

© XXXX American Chemical Society

Received: January 27, 2016

A

DOI: 10.1021/acs.inorgchem.6b00217 Inorg. Chem. XXXX, XXX, XXX−XXX

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Synthesis of [Er(Hpzbc)2(NO3)]·H2O (1-Er). Yield: 46%. Anal. Calcd for C20H16ErN5O8: C, 38.65; H, 2.58; N, 11.27. Found: C, 38.59; H, 5.50; N, 11.34%. IR (KBr, cm−1): 3282(s), 1547(s), 1432(s), 1305(m), 1053(w), 762(m), 715(m), 580(w). Synthesis of [Yb(Hpzbc)2(NO3)]·H2O (1-Yb). Yield: 41%. Anal. Calcd for C20H16YbN5O8: C, 38.28; H, 2.55; N, 11.16. Found: C, 38.38; H, 2.41; N, 11.23%. IR (KBr, cm−1): 3379(s), 3153(s), 1532(s), 1403(s), 1278(m), 1095(m), 941(w), 762(m), 705(m), 462(w). X-ray Crystallographic Measurements. A Bruker Smart CCD area-detector was utilized to get the crystal data of complexes 1-Eu, 1Tb, and 1-Er at 296(2) K using ω rotation scans with widths of 0.3° and Mo Kα radiation (λ = 0.71073 Å). The structures were solved by the direct methods and refined by full-matrix least-squares refinements based on F2 with the SHELXTL program.12 All non-hydrogen atoms were refined anisotropically. The hydrogen atoms were added to their geometrically ideal positions. Relevant crystallographic data were given in Table 1, and the selected bond lengths and angles were listed in Table S2.

complexes have been documented, only rare Ln-pyrazoylcarboxyl coordination frameworks were recorded.11 Moreover, the overwhelming majority in those systems was based on pyrazole-3,5-dicarboxylate ligand chelating with Ln3+ centers.11b−f Thus, the fabrication of Ln-MOFs through a pyrazoyl and carboxyl separated bifunctional ligand is an unprecedented and challenging project. Meanwhile, the incorporation of pyrazole with relatively high N contents in Ln-MOFs would strengthen the affinity of the framework toward CO2. We are interested in an unexplored ligand, 3-(1H-pyrazol-3yl) benzoic acid (H2pzbc), which contains one pyrazoyl and one carboxyl unit spaced by one phenyl ring and combines versatile coordination modes of carboxylic acid and pyrazole. H ere i n , se v e n i s o s t r u c t u r a l 3 D Ln - M O F s , [L n(Hpzbc)2(NO3)]·H2O (1-Ln, Ln = Nd3+, Sm3+, Eu3+, Gd3+, Tb3+, Er3+, and Yb3+), have been constructed by H2pzbc, which represent the novel pyrazoyl-carboxyl ligand-incorporated 3D microporous Ln-MOF systems. Strikingly, 1-Eu displays excellent selective and sensitive fluorescent probes for Fe3+ and Cr2O72− ions and highly selective capture for CO2 over N2 and CH4 as well.

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Table 1. Crystal Data and Structure Refinement for 1-Eu, 1Tb, and 1-Er complexes formula formula weight crystal system a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dcalc (g cm−3) F(000) Rint GOF on F2 R1a [I > 2σ(I)] wR2b (all data)

EXPERIMENTAL SECTION

Materials and General Methods. All chemicals are commercially available and were used without further purification. An infrared (IR) spectrum was obtained through an EQUINOX-55 FT-IR spectrometer together with a KBr pellet from 4000 to 400 cm−1. Elemental analyses for C, H, and N were recorded on a PerkinElmer 2400C Elemental Analyzer. Thermogravimetric analyses (TGA) were carried out in a N2 stream using a Netzsch TG209F3 instrument at a heating rate of 10 °C min−1. Powder X-ray diffraction (PXRD) data were collected on a Bruker D8 ADVANCE with Cu Kα radiation (λ = 1.5418 Å). A luminescent spectrum was measured on an Edinburgh FLS55 luminescence spectrometer. UV−vis spectroscopic studies were measured on a Hitachi U-3310 spectrometer. An Axis ultra spectrometer was used to measure X-ray photoelectron spectroscopy (XPS). All gas sorption isotherms were measured by ASAP 2020 M adsorption equipment. Grand Canonical Monte Carlo (GCMC) simulations were performed by the Sorption module of Material Studio (Supporting Information). Synthesis of [Ln(Hpzbc)2(NO3)]·H2O (1-Ln). A mixture of Ln(NO3)3·6H2O (0.1 mmol), H2pzbc (0.018 g, 0.1 mmol), and CH3CN (9 mL) was sealed in a 25 mL Teflon-lined stainless steel container. The container was heated at 110 °C for 72 h and then cooled to room temperature at a rate of 5 °C h−1 to afford bulk crystals of 1-Ln. Synthesis of [Nd(Hpzbc)2(NO3)]·H2O (1-Nd). Yield: 39%. Anal. Calcd for C20H16NdN5O8: C, 40.13; H, 2.69; N, 11.70. Found: C, 40.03; H, 2.82; N, 11.61%. IR (KBr, cm−1): 3344(s), 1544(s), 1430(s), 1297(m), 1096(m), 940(w), 770(m), 716(m), 576(w). Synthesis of [Sm(Hpzbc)2(NO3)]·H2O (1-Sm). Yield: 43%. Anal. Calcd for C20H16SmN5O8: C, 39.74; H, 2.68; N, 11.69. Found: C, 40.01; H, 2.72; N, 11.76%. IR (KBr, cm−1): 3422(s), 3147(s), 1527(s), 1402(s), 1279(m), 1097(m), 941(w), 762(m), 703(m), 522(w). Synthesis of [Eu(Hpzbc)2(NO3)]·H2O (1-Eu). Yield: 42%. Anal. Calcd for C20H16EuN5O8: C, 39.60; H, 2.64; N, 11.55. Found: C, 39.73; H, 2.59; N, 11.49%. IR (KBr, cm−1): 3263(s), 1550(s), 1432(s), 1305(m), 1097(m), 941(w), 771(m), 715(m), 578 (w). Synthesis of [Gd(Hpzbc)2(NO3)]·H2O (1-Gd). Yield: 42%. Anal. Calcd for C20H16GdN5O8: C, 39.28; H, 2.62; N, 11.46. Found: C, 39.35; H, 2.72; N, 11.53%. IR (KBr, cm−1): 3372(s), 1493(s), 1280(s), 1063(m), 932(w), 761(m), 702(m), 572(w). Synthesis of [Tb(Hpzbc)2(NO3)]·H2O (1-Tb). Yield: 37%. Anal. Calcd for C20H16TbN5O8: C, 39.15; H, 2.61; N, 11.42. Found: C, 39.22; H, 2.48; N, 11.51%. IR (KBr, cm−1): 3291(s), 1553(s), 1401(s), 1307(m), 1098(m), 942(w), 771(m), 715(m), 580(w).

a

1-Eu

1-Tb

1-Er

C20H16EuN5O8 604.34 monoclinic 19.974(14) 13.135(10) 9.901(7) 90 113.308(11) 90 2386(3) 4 1.683 1192 0.0523 1.014 0.0350 0.0846

C20H16N5O8Tb 613.30 monoclinic 20.010(4) 13.193(3) 9.8950(19) 90 113.142 90 2402.1(8) 4 1.696 1200 0.0508 1.024 0.0327 0.0751

C20H16ErN5O8 621.64 monoclinic 19.810(10) 13.153(7) 9.806(5) 90 113.196(8) 90 2349(2) 4 1.785 1212 0.0406 1.036 0.0290 0.0627

R1 = Σ∥F0| − |Fc∥/Σ|F0|. bwR2 = [Σw(F02 − Fc2)2/Σw(F02)2]1/2.

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RESULTS AND DISCUSSION Crystal Structure. Single-crystal X-ray diffraction analysis reveals that complexes 1-Eu, 1-Tb, and 1-Er show the isotypic structures with monoclinic C2/c space group (Figure S1). It failed to determine the structures of 1-Nd, 1-Yb, 1-Gd, and 1Sm by X-ray single crystal diffraction due to very small sizes of crystals. However, PXRD confirmed that they are isostructural with 1-Eu (Figure S2). The structure of 1-Eu is taken as an example. In 1-Eu, the asymmetry unit consists of half a Eu3+ ion, one monodeprotonated Hpzbc, and half a coordinated NO3− anion (Figure 1a). Eu3+ ion with a distorted bicapped trigonal prism is eight-coordinated by six O atoms, from four carboxylate O atoms of four Hpzbc and two O atoms of one NO3−, and two pyrazole N atoms of two Hpzbc. The carboxylate group of Hpzbc with a syn-syn μ2-fashion bridges Eu3+ centers to form an infinite chain-like secondary building unit (SBU) running along the c axis (Figure 1b), which is characteristic of double helixes with opposite chirality but the same axis. The neighboring chains are interlinked by the coordination of pyrazole N of B

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Figure 1. (a) Coordination environment of Eu3+ ion in 1-Eu (symmetry codes: #1 = 0.5 − x, 0.5 − y, 1 − z; #2 = −0.5 + x, 0.5 − y, −0.5 + z; #3 = −x, y, 0.5 − z; #4 = x, −y, 0.5 + z; #5 = −x, −y, −z); (b) 1D helical chain; (c) 3D framework; (d) 1D channel viewed along the c axis (green: inner surface of pores; yellow: outer surface of pores).

Figure 2. Luminescent intensity at 614 nm of 1-Eu treated with 1.0 × 10−3 M various cations (a) and anions (b) for 6 h and the luminescent spectra of 1-Eu in the presence of Fe3+ (c) and Cr2O72− (d) ions with different concentrations (0−10−3 M). Insets: the linear correlation for the plot of (I0 − I)/I0 vs concentration of Fe3+ and Cr2O72− ions, respectively, in low concentration range.

Hpzbc to afford a 3D framework (Figure 1c), which contains one-dimensional (1D) channels with the window sizes of ca. 4.5 × 3.5 Å2 (excluding van der Waals radii of the atoms) along the c axis (Figure 1d). The uncoordinated O atoms of NO3−

stand in the porous surface, which could behave as potential active sites for sensing and adsorption of guests. The −NH group of pyrazolyl in Hpzbc is nondeprotonated and forms a N−H···O hydrogen bond with one coordinated O atom of C

DOI: 10.1021/acs.inorgchem.6b00217 Inorg. Chem. XXXX, XXX, XXX−XXX

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the detection limit of 2.6 × 10−5 M is obtained by the ratio of 3δ/slope, in which δ is the standard deviation of luminescent intensity of blank solution for ten times.14 Compared with the reported MOFs enumerated in Table S3, this detection limit is significantly low, implying that 1-Eu is very promising in the sensitive and selective detection for Fe3+ ion. Notably, although recyclability is one of the important indices of sensors, the related investigation on recyclable capacity of MOF sensors was scarcely explored. 1-Eu was soaked in an ethanol solution of 1 × 10−3 M Fe3+ ion for minutes to form Fe3+@1-Eu, which was washed several times to yield the recycled 1-Eu, in which no Fe3+ ion was remaining, as verified by XPS (Figure S9a). Importantly, for three recycles, the luminescent intensity of each recycle is almost unchanged compared to that of 1-Eu (Figure 3a). Meanwhile, the PXRD

NO3−. Topologically, by regarding Eu3+ center and Hpzbc as 6and 3-connected nodes, respectively, the extended framework of 1-Eu can be simplified as a binodal (3,6)-connected ant net with the point symbol of (426)2(44628810) (Figure S3). Notably, although some Ln-pyrazolyl-carboxyl coordination polymers were observed in the past,11 to the best of our knowledge, the corresponding porous framework was reported only in a La-MOF.11f However, differing from 1-Eu, in that MOF, the coordination of La3+ ions and pyrazole-3,5dicarboxylates forms 2D layers, which have to be connected by CO32− to produce a 3D framework. Thereby, 1-Ln represents the unprecedented microporous 3D Ln-pyrazoylcarboxyl systems. PXRD and TGA. The experimental powder X-ray diffraction patterns of 1-Ln agreed well with those simulated from the respective crystal structures, demonstrating phase purity of 1Ln (Figure S2a). 1-Ln showed the similar weight loss processes under the N2 environment, in accordance with their similar structures (Figure S4). TGA of 1-Eu is representatively discussed. The first weight loss of 3.5% in 1-Eu below 145 °C corresponds to the release of all water molecules (calcd: 3.0%). The main framework is thermally stable up to 305 °C and then decomposes at a higher temperature. Luminescent Properties. The solid-state luminescent properties of 1-Eu and free H2pzbc ligand were studied at room temperature (Figure S5). H2pzbc shows the strongest emission at 350 nm at an excitation of 322 nm. 1-Eu has a maximum of excitation at 394 nm, and under this excitation, 1Eu displays the typical luminescence of Eu3+ ion, wherein the four characteristic emission peaks at 586, 593, 614, and 698 nm, originate from 5D0−7F0, 5D0−7F1, 5D0−7F2, and 5D0−7F3 f−f transitions of Eu3+ ion, respectively. The strongest 5D0−7F2 transition at 614 nm resulted from the magnetic-dipole induced transitions leads to the strong red luminescence of 1-Eu. 1-Eu displays the double-exponential decays with the lifetimes of 4.96 and 612.81 μs obtained by the decay lifetime curve (Figure S6). In light of the nitrate O atom active sites exposed in pores and the bright red luminescence of 1-Eu, the potential luminescent detection for cations and anions was further evaluated. In this experiment, 5 mg of 1-Eu was dispersed in an 1 × 10−3 M ethanol solution containing M(NO3)x (M = Cu2+, Zn2+, Na+, K+, Hg2+, Mn2+, Pb2+, Co2+, Cd2+, Ni2+, Mg2+, Ca2+, Al3+, and Fe3+). The emission spectra were shown in Figures 2a and S7a. Interestingly, it was found that Cu2+, Zn2+, Na+, K+, and Hg2+ ions slightly enhanced luminescent intensity of 1-Eu, while other metal ions (Pb2+, Co2+, Cd2+, Ni2+, Mg2+, Ca2+, and Al3+) decreased luminescence to a different extent. The most striking phenomenon is that Fe3+ ion causes a very significant quenching effect on luminescence of 1-Eu. The obvious change of luminescent intensities affected by Fe3+ relative to other metal ions implies the potential of 1-Eu for recognizing and sensing Fe3+ ion. The plot of I0/I vs concentration of Fe3+ ion does not match with the Stern−Volmer equation, indicating the coexistence of the dynamic and static quenching processes,13 which can be well fitted by I0/I = 1.029 × exp(c/339.445) − 0.266 (I0 and I are the luminescent intensity of 1-Eu in the absence and presence of Fe3+, respectively, and c is the molar concentration of Fe3+) (Figure S8a). Thereby, the quenching process can be quantitatively controlled by the concentration of Fe3+ ion. In addition, a good linear correlation is observed for the plot of (I0 − I)/I0 vs concentration of Fe3+ ion in the range of 0−220 × 10−6 M (Figure 2c, inset). By the calculated slope,

Figure 3. Luminescent intensity at 614 nm of 1-Eu after three recycles (c1, c2, c3) in Fe3+ (a) and Cr2O72− (b) solutions (10−3 M).

pattern of the recycled 1-Eu shows structural integrity (Figure S10). The study of the quenching effect with different immersion times illustrates that the emission of 1-Eu is almost totally quenched after 120 s of the Fe3+ ion (1 × 10−3 M) addition (Figures 4 and S11a), which is greatly shorter than

Figure 4. Luminescent intensity of 1-Eu at 614 nm at different reaction times in Fe3+ and Cr2O72− solutions. Inset: color changes of 1Eu induced by the addition of Fe3+ and Cr2O72− ions.

those in [H2NMe2][Eu(C33H24O12)(H2O)]4h and [Tb(Hbtca)(H2O)2] (Table S3).4e The results indicate that 1-Eu could be used for the fast and recycle fluorescent probe for Fe3+ ion. In previous studies, the reasons for luminescent quenching caused by Fe3+ ion were basically attributed to collapse of the framework, cationic exchange, competition absorption between Fe3+ ion and Ln-MOFs, and strong framework-Fe3+ interactions.3l,4h−j As reflected by PXRD, the framework of 1-Eu treated in metal ion solutions remains intact (Figure S12). It is also very difficult for the neutral 1-Eu to capture Fe3+ by the D

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Figure 5. Gas sorption isotherms of 1-Eu for (a) N2 at 77 K, CO2 and CH4 at 195 K; (b) CH4 at 298 K, CO2 at 273.15, 285, and 298 K. Filled and open symbols represent adsorption and desorption curves, respectively.

Figure 6. IAST adsorption selectivities and isotherms of 1-Eu for CO2 over CH4 at different compositions.

(Figure 3b) and the quenching effect with different immersion times in Cr2O72− solution (1 × 10−3 M) indicates 1-Eu is very smart for Cr2O72− ion probe (Figures 4 and S11b). Sorption Properties. H2O solvent molecules in 1-Eu can be completely removed by heating sample at 160 °C for 3 h under vacuum, which was confirmed by TGA (Figure S16). Due to the existence of active sites in 1-Eu, gas sorption capacities were assessed on the desolvated sample by gas adsorptions of N2 at 77 K and of CO2 and CH4 at 195 K, respectively (Figure 5a). At 1 atm, 1-Eu reveals a minimum N2 loading of 28.1 cm3 (STP) g−1 but more CO2 and CH4 uptakes of 56.1 and 36.5 cm3 (STP) g−1, respectively. This sorption isotherm of CO2 displays typical type-I microporous adsorption character, and the BET surface area of 158.2 m2 g−1 (Langmuir surface area is 188.1 m2 g−1) and a mean pore width of 3.9 Å based on Horvath−Kawazoe mode (Figure S17) are obtained, respectively. The adsorption capacities of 1-Eu for CO2, CH4, and N2 were also conducted at 298 K (Figure 5b). It is found that 1-Eu at 1 atm is nonadsorptive for N2 (the adsorption amount is too low to be detected by our instrument) and very low CH4 uptake (6.4 cm3 (STP) g−1), but a remarkable CO2 loading of 31.2 cm3 (STP) g−1, indicating the significant gas adsorption selectivities for CO2 over N2 and CH4. To predict CO2/CH4 selectivity in 1-Eu for a CO2/CH4 binary mixture, the ideal adsorbed solution theory (IAST)16 was employed on the basis of the adsorption curves of CO2 and CH4 at 298 K (Figure S18). For CO2/CH4 mixtures with general feed compositions of landfill gas (CO2/CH4 = 50:50) and natural gas (CO2/CH4 = 10:90 and 5:95), the CO2/CH4 selectivities calculated at 1 atm were 12.8, 10.3, and 10.4, respectively (Figure 6). Compared to most of the known MOFs which possessed good CO2/CH4 selectivity at similar conditions (Table 2), the values of 1-Eu are even higher. The remarkable

cationic exchange. Meanwhile, the UV−vis absorption spectrum of Fe3+ ion solution shows little overlap with the excitation spectrum of 1-Eu, so there is no clear evidence for competitive adsorption between Fe3+ ion and 1-Eu (Figure S13). We inferred that Fe3+ ion diffused into the channels of 1Eu and formed contacts with uncoordinated O atoms of NO3−, leading to the luminescent quenching of 1-Eu.4i,15 Simultaneously, ethanol (aq. 90%) solutions containing various anions (F−, Cl−, I−, ClO4−, BrO3−, IO4−, PO43−, H2PO4−, CO32−, SO42−, and Cr2O72−) at the same concentration (1.0 × 10−3 M) were selected to evaluate their effect on the luminescent intensity of 1-Eu. As shown in Figures 2b and S7b, the luminescent intensities of the different suspensions are closely related to the types of anions. Uniquely, Cr2O72− completely quenches the luminescence of 1-Eu, implying the great potential of 1-Eu for Cr2O72− ion sensing. Upon increasing the concentration of Cr2O72−, the luminescence of 1-Eu was gradually quenched, and the luminescent intensity obeys equation I0/I = 1.463 × exp(c/345.886) − 0.545 (Figure S8b), similar to the situation for Fe3+ ion. The luminescent quenching of 1-Eu induced by Cr2O72− ion can, on one hand, be attributed to the interactions between Cr2O72− ion and framework and, on the other hand, result from the competitive adsorption of excitation wavelength energy between 1-Eu and Cr2O72− ion because the UV−vis adsorption spectra of K2Cr2O7 in ethanol shows the moderate overlap on the excitation spectra of 1-Eu (Figure S14).3j,4h The detection limit (3δ/slope) for Cr2O72− reaches as low as 2.2 × 10−5 M (Figure 2d, inset). Notably, there are rare Ln-MOFs that displayed luminescent quenching for Cr2O72− ion compared to other metal ions (Table S1).5 PXRD patterns confirmed the samples of 1-Eu soaked in different anionic salts hold the structural integrity (Figure S15). In addition, the study of recyclability E

DOI: 10.1021/acs.inorgchem.6b00217 Inorg. Chem. XXXX, XXX, XXX−XXX

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273.15, 285, and 298 K (Figure S19). The initial Qst is 39.2 kJ mol−1, which is relatively high and compares with the MOFs containing open metal sites and other activity sites (Table S4).8b,34 Although Qst displays a gradual decrease with the increasing of CO2 coverage, Qst still reaches to 26.8 kJ mol−1 at the maximum loading of 31.2 cm3 (STP) g−1, reflecting the strong framework−CO2 interactions which lead to the significant selectivity for CO2. GCMC Simulation. For a better understanding of the interaction details of 1-Eu with CO2, GCMC simulation has been employed at 298 K and at different pressures (0.1 and 100 kPa, Figures S20 and 7a). The obtained density contours revealed that, at both low and high pressures, the mostly populated sites are located in the vicinity of uncoordinated O atoms of NO3− ions, pyrazoyl, and phenyl rings of Hpzbc linkers in channels. As revealed by the preferential locations derived from simulation (Figure 7b), one uncoordinated electronegative O atom of NO3− attracts two electropositive C atoms of two CO2 molecules, in which the O···C distances of 3.346 and 3.589 Å approximate with the sum of van der Waals radii of carbon (1.70 Å) and oxygen (1.52 Å) atoms, indicating moderate contacts. Two CO2 molecules have similar environments, and also form intermolecular interactions as one O atom of one CO2 interacts with the C atom of the other CO2 (O···C = 3.589 Å) by a T-shaped fashion. The O atoms of each CO2 also form O···H (2.547−2.769 Å) hydrogen bonds with the −CH groups of phenyl and pyrazoly rings of Hpzbc. Meanwhile, the C atom of each CO2 is also involved in C···π interactions with the pyrazoly rings (C···πcentroid = 3.775 and 4.137 Å and 3.864 and 4.088 Å, respectively).10g,35 However, no C···π interactions (C···πcentroid = 4.698−5.289 Å) between CO2 and phenyl rings in 1-Eu were observed, which is possibly due to less electronic density in phenyl relative to pyrazyl rings. These multipoint framework-CO2 contacts and CO2−CO2 interactions are responsible for relatively high sorption heat and selectivity for CO2.

Table 2. Comparison of CO2/CH4 Selectivity Calculated by IAST Method for the Equimolar Mixture at 1 atm and 298 K of 1-Eu with the Selected MOFs

a

MOFs

selectivity

ref

UTSA-49 Cu-TDPDA MAF-X7 1-Eu [Cu(bpy)2(SiF6)] [Mn2(Hcbptz)2(Cl)(H2O)]Cl ZIF-97 ZIF-93 UiO-66-AD4 SNU-151′ UiO-66 Zr-UiO-67AcOH Co9−INA [Zr6O4(OH)4(FDCA)6] [CH3NH3][In3(L1)2(H2O)2.5] [Cu(INIA)] dia-7i-1-Co DMOF UiO-67 ZIF-25 MOF-205 UMCM-1

33.7 13.8 12.6, 12.0a 12.8, 10.3a, 10.4b 10.5 10.3, 8.8a 9.14 8.19 8.04 7.20 6.87 6.8a 6.2 5.1b 4.6, 4.3a 4.3 4.1, 4.0a 3.2 2.7b 2.53 2.2 1.82

17 18 19 this work 20 21 22 22 23 24 23 25 26 27 28 29 30 31 23 22 32 33

CO2/CH4 = 5:95. bCO2/CH4 = 10:90.

selectivities for CO2 over CH4 and N2 render 1-Eu to be a promising material in postcombustion CO2 capture, natural gas upgrading, and landfill gas purification. The significant sorption selectivity of 1-Eu for CO2 is closely related to the existence of NO3− groups and rich-N pyrazole rings, which makes the framework very polar, as a result, to form specific affinity for CO2, which has a larger quadrupole moment and a higher polarizability value (CO2, 29.1× 10−25 cm−3; CH4, 25.9 × 10−25 cm−3; N2, 17.4 × 10−25 cm−3) compared to CH4 and N2. In particular, the uncoordinated O atoms in NO3− can directly draw CO2 by dipole−quadrupole interactions. The adsorption affinity of 1-Eu for CO2 can be evaluated by the isosteric heat (Qst) of adsorption calculated by the virial equation from the adsorption isotherms of CO2 at

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CONCLUSIONS In conclusion, a series of uncommon microporous Ln-pyrazoylcarboxyl systems have been constructed by employing a pyrazoyl-carboxyl bifunctional ligand. The obtained frameworks feature 1D channels decorated by O atoms and pyrazoyl groups. As a result, 1-Eu reveals excellent luminescent sensing

Figure 7. (a) Density contours of CO2 adsorption in pores of 1-Eu obtained from GCMC simulation at 298 K under pressure 100 kPa and (b) view of CO2 molecules in pores of 1-Eu. F

DOI: 10.1021/acs.inorgchem.6b00217 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry for Fe3+ and Cr2O72− ions with high sensitivity, selectivity, and simple and quick regeneration, as well as remarkably selective capture for CO2 over N2 and CH4 at ambient temperature. GCMC simulations confirmed the multiple CO2-philic sites in 1-Eu. These facts indicate that 1-Eu can potentially be applied not only as an efficient luminescent sensor for Fe3+ and Cr2O72− detection but also as a promising material for CO2 capture and separation in some industry processes. This contribution also corroborates a less-investigated but feasible strategy by employing pyrazoyl-carboxyl bifunctional ligands to broaden functional Ln-MOFs.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00217. Additional structural figures, TGA, PXRD, excitation and emission spectra, the detailed calculations on sorption and bond length/angle, and GCMC simulation methodology (PDF) X-ray crystallographic data (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (L.H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by NSFC (21471124, 21531007, and 21371142), NSF of Shannxi province (2013KJXX-26, 2014JQ2049, and 15JS113), the Australian Research Council Future Fellowship FT12010072, Open Foundation of Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education (338080060), and NFFTBS (J1210057).

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DOI: 10.1021/acs.inorgchem.6b00217 Inorg. Chem. XXXX, XXX, XXX−XXX