A Copper(II)-Paddlewheel MetalâOrganic Framework with Exceptional

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A Copper(II)-Paddlewheel Metal-Organic Framework with Exceptional Hydrolytic Stability and Selective Adsorption and Detection Ability of Aniline in Water Ya Chen, Bin Wang, Xiaoqing Wang, Lin-Hua Xie, Jinping Li, Yabo Xie, and Jian-Rong Jeff Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07920 • Publication Date (Web): 18 Jul 2017 Downloaded from http://pubs.acs.org on July 22, 2017

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ACS Applied Materials & Interfaces

A Copper(II)-Paddlewheel Metal-Organic Framework with Exceptional Hydrolytic Stability and Selective Adsorption and Detection Ability of Aniline in Water Ya Chen,† Bin Wang,† Xiaoqing Wang,†,‡ Lin-Hua Xie,†* Jinping Li,‡ Yabo Xie,† Jian-Rong Li†* †

Beijing Key Laboratory for Green Catalysis and Separation and Department of

Chemistry and Chemical Engineering, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, P. R. China. ‡

Research Institute of Special Chemicals, Taiyuan University of Technology,

Taiyuan, 030024, Shanxi, P. R. China

KEYWORDS:

copper(II) paddlewheel, octatopic ligand, hydrolytic stability, water

vapor adsorption, aniline capture and detection

Abstract: Copper(II) paddlewheel based metal-organic frameworks (CP-MOFs) represent a unique subclass of MOFs with highly predictable porous structures, facile syntheses, and functional open metal sites. However, lack of high hydrolytic stability is an obstacle for CP-MOFs in many practical applications. In this work, we report a new CP-MOF, [Cu4(tdhb)] (BUT-155), which is constructed from a judiciously designed carboxylate ligand with high coordination connectivity (octatopic), abundant hydrophobic substituents (six methyl groups), and substituent constrained geometry 1 ACS Paragon Plus Environment


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(tetrahedral

backbone),

tdhb8−

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(H8tdhb

=

3,3’,5,5’-tetra(3,5-dicarboxyphenyl)-2,2’,4,4’,6,6’-hexamethylbiphenyl)).

BUT-155

shows high porosity with a Brunauer-Emmett-Teller (BET) surface area of 2070 m2/g. Quite interestingly, this CP-MOF retains its structural integrity after being treated in water for 10 days at room temperature, or in boiling water for 24 hours. To the best of our knowledge, BUT-155 represents the first CP-MOF that is demonstrated to retain structural integrity in boiling water. The high hydrolytic stability of BUT-155 allowed us to carry out adsorption studies of water vapor and aqueous organic pollutants on it. Water vapor adsorption reveals a sigmoidal isotherm and a high uptake (46.7 wt%), which is highly reversible and regenerable. In addition, due to the availability of soft acid type open Cu(II) sites, BUT-155 shows high performance for selective adsorption of soft base type aniline over water or phenol, and a naked-eye detectable color change for the MOF sample is accompanied. The adsorption selectivity and high adsorption capacity of aniline in BUT-155 are also well-interpreted by single crystal structures of the water- and aniline-included phases of BUT-155.

Introduction

As a class of newly developed crystalline porous materials, metal−organic frameworks (MOFs), constructed from organic ligands and metal ions or metal clusters through coordination bonds, have shown great potential for gas storage, separation, heterogeneous catalysis, proton conduction, and so on.1-13 Copper paddlewheel based MOFs (CP-MOFs) represent a unique subclass of MOFs, and have 2 ACS Paragon Plus Environment

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been widely studied mainly due to the following two reasons: first, the formation of paddlewheel shaped [Cu2(−COO)4] secondary building unit (SBU) is very energetically favorable in solvothermal reactions, and the synthesis condition of CP-MOFs is mild;14-21 second, the coordinated guest molecules in Cu2 paddlewheel SBUs can be removed after activation, thus forming open metal sites (OMS) of Cu(II) ions, which are beneficial for gas storage, separation, and catalysis.22-26 For examples, Farha and co-workers discovered that HKUST-1, one of the most representative CP-MOFs that is commercially available in gram scale, exhibited a record room temperature (RT) volumetric CH4 uptake of 230 and 270 cc(STP)/cc at 35 and 65 bar, respectively.27 Yuan and co-workers presented a CP-MOF with very high C2H2 storage capability under ambient conditions. Grand canonical Monte Carlo simulation revealed that Cu(II) OMS and the suitable pore space and geometry played key roles in its remarkable C2H2 uptake.28 In addition, CP-MOFs with Cu(II) OMS can be served as highly selective Lewis acid catalysts for some important reactions, such as the isomerization of terpene derivatives29, and CO2 chemical fixing.30

Although a lot of research results have been reported, the practical application of CP-MOFs still has a long way to go and the major constraint is their poor stability, especially in aqueous environment.31 For a long time, CP-MOFs have been considered to be intrinsically unstable in water, although some CP-MOFs with coordination unsaturated Cu(II) sites blocked by thiol type ligands and some MOFs simultaneously containing paddlewheel Cu2 SBUs and SBUs of other type have been demonstrated to be hydrolytically stable.32-33 Construction of hydrolytically stable 3 ACS Paragon Plus Environment


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CP-MOFs with rich Cu(II) OMS that can be used in aqueous solutions for various applications still faces a challenge.

Many efforts have been devoted to construct hydrolytically stable MOFs, where three strategies are commonly used. First, according to the theory of soft and hard acids and bases high oxidation state metals ions (such as Fe3+, Al3+, Cr3+, and Zr4+) form stronger coordination bonds with carboxylate ligands than low-valence metal ions, and thus many highly stable MOFs with those components have been designed and synthesized, such as MIL-101(Cr) and UiO-66.34-38 The second strategy is to increase the connectivity of both organic linkers and inorganic clusters. Many MOFs with high connected building blocks are manifested to be robust even with very large pore size.39-40 The third strategy is incorporating hydrophobic groups onto linkers through direct synthesis or postsynthetic modification to enhance the hydrophobic property of MOFs, thereby protecting coordination bonds from hydrolysis to different extents.41-44

To enhance the hydrolytic stability of CP-MOFs, a straightforward idea is applying as many stability-improvement-strategies as possible simultaneously to construct them. Ligands with both high connectivity and abundant hydrophobic groups are thus attractive for the synthesis of CP-MOFs, which are expected to show high-connected frameworks, hydrophobic pore surfaces, and thus enhanced hydrolytically stability. In this work, a new octatopic carboxylic ligand with a central bi-phenyl core functionalized by six methyl groups was designed and synthesized, 4 ACS Paragon Plus Environment

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namely,

3,3’,5,5’-tetra(3,5-dicarboxyphenyl)-2,2’,4,4’,6,6’-examethyl-biphenyl

(H8tdhb) (Figure 1a). The reaction of H8tdhb and Cu(NO3)2·2.5H2O in N,N-dimethylformamide (DMF) yielded a new stable CP-MOF, [Cu4(tdhb)] (BUT-155, where BUT = Beijing University of Technology), showing an expected three-dimensional (3D) (4,8)-connected framework structure with the scu-a topology. BUT-155 exhibits high surface area, moderate pore size, as well as high stability in water at RT for a long duration. Particularly, after treatment in boiling water for 24 h, BUT-155 still retains structural integrity. Furthermore, it was demonstrated that BUT-155 is able to selectively detect and adsorb aniline over water or phenol, which is potentially useful in the removal of organic pollutions from wastewater and water quality monitoring.

Results and discussion

Synthesis and Structure

Solvothermal reaction of H8tdhb with Cu(NO3)2·2.5H2O in the presence of HBF4 as competing reagent in DMF yielded green single crystals of as-synthesized BUT-155 (BUT-155a). The TGA curve of BUT-155a reveals that it decomposes above ca. 280 °C (Figure S2, Supporting Information), similarly to some other CP-MOFs.44,45 The FT-IR spectra of free H8tdhb and BUT-155a are shown in Figure S3 of Supporting Information, from which it is shown that slight blue shifts of the characteristic bands of the carbonyl group in BUT-155a compared with that of free

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H8tdhb, illustrating that the carbonyl groups are deprotonated and coordinated with the metal ions in BUT-155a.

Single-crystal X-ray diffraction (SXRD) analysis indicates that the BUT-155a crystallizes in the tetragonal space group P4/nmm (Table S1, Supporting Information). In the asymmetric unit, there are one-eighth of the tdhb8− ligand and one Cu(II) atom which is located on a mirror plane with half occupancy. Two neighbouring Cu(II) atoms are bridged by four bi-monodentate carboxylate groups from four different but symmetrically equivalent tdhb8− ligands to form a paddlewheel [Cu2(O2C−)4] SBU. For the ligand, all the eight carboxyl groups are deprotonated and each ligand links eight different Cu2 paddlewheel SBUs. It should be noted that due to steric hindrance effect of the methyl substituents, neighbouring phenyl rings in the tdhb8− ligand are ideally perpendicular (Figure 1a). The 3D framework of BUT-155a is formed by alternate connection of the square Cu2 paddlewheel SBUs and the eight-branched tdhb8− linkers. There are two types of polyhedral nanocages in BUT-155a, which can be described as octahedrons (cage A) (Figure 1c) and cuboctahedrons (cage B) (Figure 1d), respectively. The cage A is composed of four Cu2 paddlewheel SBUs and four isophthalate moieties from two tdhb8− ligands. The centres of the ligands act as two vertices at the top and bottom of the octahedron, while the four Cu2 paddlewheel SBUs are arranged in a square geometry on the equatorial plane of the octahedron, respectively. Thus the eight faces of cage A can be seen as nearly regular triangular with edge lengths of 9.45 × 9.52 × 9.52 Å (Figure S4, Supporting Information). The inner space of cage A is mostly occupied by methyl groups on the ligands. Cage B is 6 ACS Paragon Plus Environment

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constructed by eight Cu2 SBUs and four tdhb8− ligands. The centres of the eight paddlewheel SBUs together with the centroids of the four tdhb8− ligands are considered as the twelve vertices of the cuboctahedral cage (Figure S5, Supporting Information). This cage consists of eight triangular and six square faces and can enclose a sphere of diameter 16 Å (atom-to-atom distance) inside its pore. In the structure, each cage B is surrounded by eight A cages by sharing the triangular faces and six neighbouring B cages by sharing the square faces to complete the whole framework construction (Figure 1b and Figure S6 of the Supporting Information). Thus, the framework in BUT-155a can be also regarded as the packing of cage A and cage B in the ratio of 1:1. From topology point of view, the framework of BUT-155a can be regarded as a (4,8)-connected scu-a net with the point symbol of (44·62)2(416·612) by simplifying the tdhb8− ligands and Cu2 paddlewheel SBUs as 8 and 4 connected nodes, respectively (Figure S7, Supporting Information). After removing free solvent molecules, the total solvent-accessible volume of BUT-155a is estimated to be 57.0% as determined by PLATON.46



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Figure 1. (a) Structure of the ligand H8tdhb, (b) crystal structure of BUT-155a, and (c) octahedral cage (cage A) and (d) cuboctahedral cage (cage B) in BUT-155a (color code: C, black; O, red; and Cu, blue; H atoms on ligands are omitted for clarity).

The phase purity of BUT-155a was confirmed by powder X-ray diffraction (PXRD) measurement. As shown in Figure 2a, the experimental PXRD pattern matches well with that simulated from the single-crystal data, indicating its pure phase. To evaluate its porosity, N2 adsorption experiment was performed at 77 K for the guest-free BUT-155, which was obtained after BUT-155a was guest-exchanged with methanol and subsequent evacuated under high vacuum at 100 °C. Typical type I isotherm was obtained, confirming the microporous nature of BUT-155 (Figure 2b). The BET surface area is estimated to be 2070 m2/g with a total pore volume of 0.820 cm3/g. The experimental observed pore volume is very closed to the calculated pore volume (0.821 cm3/g) from single-crystal structure of BUT-155a, indicating that the 8 ACS Paragon Plus Environment

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sample of BUT-155a is of high purity, and the framework of BUT-155a is rigid and well retain after guest removal. In addition, the CO2, CH4, and H2 adsorption isotherms of BUT-155 were also recorded. The H2 uptake of BUT-155 at 77 K and 1 bar is 233 cm3/g (Figure S8, Supporting Information). The CO2 and CH4 uptakes at 298 K and 1 bar are 62.2 and 17.0 cm3/g, and those at 273 K and 1 bar are 112.3 and 23.5 cm3/g, respectively (Figure S9 and S10, Supporting Information).

Stability Study

In order to explore the stability of BUT-155, PXRD measurements and N2 adsorption experiments at 77 K were carried out for BUT-155 samples treated under different conditions (Figure 2). A BUT-155 sample (about 15 mg) was first soaked in deionized water (about 20 mL) at RT for 24 h. PXRD pattern of the RT water treated sample shows intense peaks that well match those of pristine BUT-155, indicating that the structure of BUT-155 well retains in water. It is worthy to note that, some MOFs show kinetic stability, which transformed to other more thermodynamically stable phases eventually in extended duration.47 To see if the stability of BUT-155 in water is only kinetic, a BUT-155 sample was immersed in water at RT for 10 days. Nearly no change was observed in the PXRD pattern of the sample in comparison with that of the 24 h water treated BUT-155, suggesting a high thermodynamic stability of BUT-155 in liquid water at RT. The high thermodynamic stability of BUT-155 was further demonstrated by the PXRD of a BUT-155 sample after being treated in boiling water for 24 h, where intense diffraction peaks are still present, 9 ACS Paragon Plus Environment


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although slight shifting of the peaks towards low 2θ angles and difference of the relative intensity of the peaks were observed. Moreover, it was found that BUT-155 retains its good crystallinity after even being treated in water at RT for 13 days and boiling water for 7 days, with a longer duration (Figure S11, Supporting Information). The structural intactness of the RT and boiling water treated BUT-155 samples was also supported by their N2 adsorption isotherms recorded at 77 K, which are almost overlapped with that for the pristine BUT-155. We also checked the UV-vis spectra of the water supernatants after BUT-155 samples being treated at RT and boiling water, respectively. No detectable adsorption peak was found in the spectra (Figure S12, Supporting Information), precluding any Cu2+ leaking from BUT-155 into water during the treatments. This result also confirms the excellent water-stability of this MOF. Furthermore, we tested the stability of BUT-155 in weakly acidic and weakly basic aqueous solutions. After being treated with a HCl aqueous solution (pH = 4), or a NaOH aqueous solution (pH = 10) at RT for 24 h, PXRD patterns of the BUT-155 samples still show diffraction peaks of BUT-155, but the intensities of peaks lower compared to those of the pure water treated samples. N2 sorption isotherms for the acid and base treated samples were then measured at 77 K, which show that their N2 uptakes are lower than that of the pristine BUT-155 (540 cm3/g), being 420 and 478 cm3/g, respectively (Figure S13, Supporting Information). This indicates that BUT-155 is not so stable in acid or base; however, the structure degradation processes are slow in the weak acidic or basic condition.


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Figure 2. (a) PXRD patterns for as-synthesized BUT-155a and BUT-155 samples after being treated under different conditions and (b) N2 adsorption isotherms recorded at 77 K and SEM images (inset) for BUT-155 (1), and BUT-155 samples after being treated in water at RT for 1 (2) and 10 days (4), and boiling water for 24 h (3), respectively.

Besides PXRD and N2 adsorption measurements, scanning electron microscopy (SEM) is a complementary technique for characterizing the stability of MOFs against water.48 SEM images could provide some special information for the MOF samples, such as morphology change of MOF crystals, and formation of defects or cracks on the crystal surface. For instance, DeCoste et al. monitored the degradation process of HKUST-1 in a 90% RH atmosphere by SEM images.49 It was proposed that the 11 ACS Paragon Plus Environment


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defects (cracks and fractures) were first formed on external crystal surface of HKUST-1 before the bulk crystal structure was degraded, which could not be observed in PXRD patterns. The SEM images of BUT-155 and the water treated samples were also recorded (Figure 2b, inset). The results show that morphology of BUT-155 crystals did not change and no cracks formed on crystal surfaces after being treated with RT water or boiling water. The finding further excludes the possibility of partial degradation of the BUT-155 samples on crystal external surface.

To the best of our knowledge, BUT-155 represents the first CP-MOFs example that is demonstrated to be stable in boiling water. The high stability of BUT-155 in water should be resulted from combined effects from several aspects. First, BUT-155 is constructed from octa-carboxylate ligands and copper paddlewheel SBUs with a

scu-a topological structure. The high connectivity (eight connected) of the ligand contributes the robustness of the framework. Second, the ligand tdhb8− contains six methyl substituents in the biphenyl core. The presence of the methyl groups forces neighboring phenyl rings perpendicular to each other. Such a steric hindrance effect from ligand minimizes the distortion freedom of the framework, and thus increases the energy barrier for a reaction between water molecules and the MOF, because structural distortion is normally needed for initiating the hydrolysis reaction of a MOF.50 Third, hydrophobic and electronic effects of methyl groups in ligands should also be accountable. The methyl groups are hydrophobic, which enhance the hydrophobicity of pore surface of the framework. Moreover, as electron donors, the methyl groups increase the electron density of both the central and the terminal 12 ACS Paragon Plus Environment

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benzene rings in the ligand, leading to an increase of the electron density of the carboxylate O atoms, which may further increase the Cu−O bond strength.51 To quantitatively demonstrate such an electronic effect, DFT calculations were performed to identify the electron densities of carboxylate O atoms in H8tdhb and a counterpart without methyl substituents, by using the GAUSSIAN 03 (for details, see Section 7 of the Supporting Information). The results indeed show that with the presence of methyl substituents in ligand, carboxylate O atoms (−0.661 and −0.505) possesses slightly more negative charges than those without (−0.639 and −0.488) (Figure S14 and Table S4, Supporting Information).

Water Vapor Adsorption

Water vapor adsorption in MOFs is potentially used in many aspects, such as heat pumps, adsorption chillers, thermal batteries, and delivery of drinking water in remote areas.48, 52 Till now, a few results have been reported in the study of water adsorption of MOFs, however, most of the investigated MOFs are based on high oxidation state metals such as such as Cr3+, Fe3+, Al3+, and Zr4+.53-54 Limited works have been reported for water adsorption in CP-MOFs, because most CP-MOFs are unstable in high water vapor pressures. The high stability of BUT-155 in water encouraged us to investigate its water adsorption property. The water isotherm of BUT-155 measured at 25 ºC shows a sigmoidal profile (Figure 3a), where water uptakes gradually increase at low pressure range (P/P0 < 0.24), followed by an abrupt uptake in the pressure range from P/P0 = 0.24 to 0.30 and then a gradual increase at 13 ACS Paragon Plus Environment


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higher pressures to the maximum uptake (585 cm3/g, 46.7 wt%) at P/P0 = 0.87. The relatively low water uptakes at low pressure range (P/P0 < 0.24) indicates that the affinity between water molecules and pore surface of BUT-155 is low regardless of the presence of coordination unsaturated Cu(II) sites that should strongly interact with water molecules by coordination bonding. It may be resulted from its relatively large pore size and hydrophobicity of the organic linker. Hysteresis loop between water adsorption and desorption branches was found, which is also found in most MOFs.55

Figure 3. (a) Water vapor adsorption isotherms of BUT-155 recorded at 25 °C in five successive runs (inset shows the comparison of water uptakes of BUT-155 in the first and the fifth runs with those data of HKUST-153), (b) water molecule sorption sites in BUT-155 obtained from SXRD data of water-adsorbed phase of BUT-155. 14 ACS Paragon Plus Environment

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Generally, there are three important criteria to evaluate the water vapor adsorption performance of porous materials, namely, condensation pressure, adsorption capacity, and recyclability. Recyclability is an important criterion because it reflects the stability and practical applicability of the adsorbents. To evaluate the recyclability of BUT-155, five cycles of water adsorption-desorption isotherms were recorded (Figure 3a), where the sample was reactivated by vacuum evacuation at 25 ºC for 2 h after the previous isotherm measurement. The results show that the maximum water uptake of BUT-155 slightly drops in the first 3 cycles, from 585 to 515 cm3/g with a 12% decrease, but the uptakes well stabilized at the last 3 cycles. Compared to the reported data for some representative MOFs, both the water adsorption capacity and recyclability of BUT-155 are quite high.53 For example, the well-known HKUST-1 lost 68% maximum uptakes after 5 adsorption cycles, which should be resulted from gradual degradation of its porous structure during the water adsorption processes (Figure 3a, inset). Although BUT-155 is a CP-MOF, its water uptake retention (88%) after 5 cycles water adsorption-desorption is comparable to or even higher than those of the highly stable MOFs constructed from high-valence metal ions (Zr4+ or Al3+), such as UiO-66 (86%) and MOF-841 (94%) (Figure S15, Supporting Information). Furthermore, the water adsorption capacity (515 cm3/g) of BUT-155 is obviously higher than those of MOF-801-P (420 cm3/g), MOF-802 (130 cm3/g), UiO-66 (460 cm3/g), and CAU-10 (350 cm3/g), but slightly lower than that of MOF-841 (600 cm3/g). Clearly, the outstanding water adsorption performance of BUT-155 can be attributed to its high porosity and stability. 15 ACS Paragon Plus Environment


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To better understand the water vapor adsorption process in BUT-155, precise structure of water-included BUT-155 (water@BUT-155) was determined by using single-crystal X-ray diffraction technique. A single crystal of guest-free phase of BUT-155 was firstly obtained by heating a single crystal of methanol-exchanged BUT-155a under a heat N2 flow (100 °C) for 5 h. Structure analysis for the guest-free phase of BUT-155 reveals that after guest removal, the framework is nearly unchanged. In the final structural refinement cycles, no residual electron density peaks higher than 0.62 e Å−3 in the difference Fourier maps was observed including in the vicinity of the Cu(II) sites, suggesting that both uncoordinated and coordinated guests were completely removed. Subsequently, the BUT-155 crystals were exposed in open air (RH ≈ 60%) for one week, and a new set of diffraction data was collected for water@BUT-155. After structure determination of water@BUT-155, we found three independent residual peaks with electron density higher than 2.00 e Å−3 in the cuboctahedron cages, which are corresponding to the adsorbed water molecules. According the order of electron density, the primary adsorption sites (site I) are near the Cu(II) atoms with a distance of 2.164 Å. Water molecules (O3) were firstly adsorbed on site I due to their strong coordination interaction with the open Cu(II) metal sites. The secondary adsorption sites, site II, are located close to the site I. The distances between water molecules on sites II (O5 and O6) and those on site I (O3) are 2.823 and 2.834 Å, respectively (Figure 3b). Hydrogen bonding interactions should be involved among those water molecules. Additional adsorbed water molecules (O4) locating on the tertiary adsorption sites (site III) are hydrogen bonded 16 ACS Paragon Plus Environment

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with one type of water molecules (O6) on site II with a distance of 2.970 Å. The alternate hydrogen bonding among the two types of water molecules results in a nearly cubic arrangement of water clusters inside the cuboctahedron cage. The cubic water cluster is the smallest water cluster where each water molecule is hydrogen bonding with three neighboring water molecules, and this type of cubic water cluster is scarcely reported in literatures.56-57 It is also worthy to mention that BUT-155 even retains single crystallinity after it was treated in boiling water for 24 h, and its SXRD structure is consistent with that of the air exposed BUT-155 (Table S2, Supporting Information).

Selective Adsorption and Detection of Aniline. With the rapid development of industry and the explosive growth of the population, water pollution has been a serious threat to environment and public health. There is now a much greater demand for the detection and removal of pollutants from wastewater. Aniline and phenol are listed as two of the priority control contaminants in drinking water.58-59 Especially, even at low concentrations, aniline is highly toxic to living organisms, and is also regarded as a potential carcinogen. Thus, effective methods for the detection and removal of aniline and phenol from wastewater are in high demand. Until now, many works have shown that MOFs can be used as effective adsorbents for the removal of organic contaminants from water.60-61 However, to the best of our knowledge, relevant studies by using CP-MOFs are still scarce up to now.



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In terms of its high porosity, the availability of Cu(II) OMS on pore surface, and excellent water stability, BUT-155 was explored for the application in the detection and removal of aniline and phenol from water. Experimentally, BUT-155 samples were immersed in aqueous solution of aniline or phenol at room temperature. The adsorption capacities of BUT-155 toward aniline and phenol were firstly qualitatively evaluated by using UV−Vis spectroscopy. As shown in Figure S16 of Supporting Information, the adsorption process proceeded rapidly since the absorbance (λmax) of the supernatant solutions evidently decreased in 1 minute, and essentially kept constant after 5 minutes. Interestingly, the λmax decreasing percentages of the two supernatant solutions are different, being 42.0% and 6.9% for aniline and phenol, respectively. We further explored the adsorption isotherms of aniline and phenol in BUT-155 at RT. As shown in Figure 4a, the maximum adsorption amounts of aniline and phenol in BUT-155 are 670 and 228 mg/g at a concentration of 10 g/L, respectively. The aniline uptake of BUT-155 is high in comparison with other reported porous materials (Table S5, Supporting Information). What is more, during the aniline adsorption process, a clear color change of the MOF sample from blue to green could be observed, indicating that the coordination environment of the Cu(II) atoms in BUT-155 changed (Figure 4a, inset). In contrast, the color of BUT-155 during phenol adsorption did not change. This provides a fast and convenient method for the selective detection of aniline in water or in aqueous aniline/phenol mixture through the color change, which is detectable by naked eyes. Adsorption experiment for aniline and phenol mixture (molar ratio = 1:1) in water was also carried out for 18 ACS Paragon Plus Environment

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BUT-155. Adsorption isotherms for the two analytes are shown in Figure S18 of the Supporting Information. It was found that the aniline uptakes are all higher than those of phenol in the checked concentration range, indicating a high adsorption selectivity of BUT-155 toward aniline over phenol in their mixture. This is in accordance with their single-component adsorption result.

Figure 4. (a) Adsorption isotherms of aniline in BUT-155 (adsorption conditions: 298 K, 50 mL of solution, 15 mg of MOFs, contact time of 2 h; inset shows the colour change of BUT-155 in aniline and phenol solutions) and (b) aniline molecule sorption sites of BUT-155 confirmed from SXRD data of aniline@BUT-155.



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As the interactions between Cu(II) atoms and amine N atoms of aniline are expected to be stronger than those between Cu(II) atoms and water O atoms based on the Pearson acid base concept, the color change of BUT-155 during aniline adsorption is probably resulted from the substitution of coordinated water molecules on Cu(II) atoms by aniline molecules. To clarify this, single-crystal structure of aniline-included BUT-155 (aniline@BUT-155) was determined (Table S3, Supporting Information). Structural studies reveal that the frame-work was well retained after BUT-155 being immersed in a concentrated aniline aqueous solution, and two types of aniline molecules could be modeled inside the pore of BUT-155 (Figure 4b). As expected, one aniline molecule is adsorbed by the coordination interaction between aniline N atom and the Cu(II) atom with a Cu‒N bond of 2.232 Å. The other aniline molecules are located on the window between two neighboring large cages, with each being simultaneously hydrogen-bonded with two carboxylate O atoms (O···N distance: 2.729 Å) and the –NH2 group of a coordinated aniline molecule (N···N distance: 3.173 Å). According to the single-crystal data of aniline@BUT-155, the calculated adsorption capacity of aniline in BUT-155 is 384 mg/g, which is still less than the experimental value (670 mg/g). This could be attributed to the presence of additional aniline molecules inside the large cages in BUT-155, which are highly disordered due to the lack of strong interactions with the framework. In addition, the results of FT-IR measurements also confirm that aniline molecules were indeed adsorbed into the pores of the BUT-155 (Figure S17, Supporting Information).


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Figure 5. Aniline uptakes of BUT-155 in 5 cyclic adsorption experiments (adsorption conditions: 298 K, 50 mL solution, 15 mg MOF, and 2 h for adsorption equilibrium).

For comparison, we also explored the adsorption performance of HKUST-1 toward aniline in water, and found that it was decomposed after soaking in aniline solution for 1 hour, as confirmed by PXRD (Figure S20, Supporting Information). In contrast, the PXRD pattern of the BUT-155 after aniline adsorption experiment confirms a well-retained crystallinity (Figure S19, Supporting Information). The high performance of the BUT-155 in aniline removal can be ascribed to its large specific surface areas, suitable pore size, as well as accessible Cu(II) OMS on pore surfaces, which endows BUT-155 relatively strong coordination interactions with aniline molecules, and induced adsorption enhancement of additional guests through additional intermolecular interactions, like hydrogen-bonding interactions.

From the viewpoint of resource recycling, regeneration of the adsorbents is one of the crucial aspects. Therefore, we further explored the regeneration ability of BUT-155 at RT. As shown in Figure 5, BUT-155 almost regained its initial aniline



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adsorption capacities over five repeated cycles (after it was soaked with methanol in each retest), demonstrating its high stability and good reusability. Therefore, BUT-155 is potentially applicable in the efficient removal of aniline in waste system as a unique porous material, combining facile detection by color change, high adsorption capacity, and good reproducibility.

Conclusions

In summary, a new copper(II) paddlewheel MOF, BUT-155, showing a 3D (4,8)-connected framework structure with the scu-a topology, has been synthesized from a judiciously designed octatopic carboxylate ligand, which feature a tetrahedral backbone imposed by six methyl substituents. After guest removal, the MOF is highly porous with a BET surface area of 2070 m2/g. Notably, unlike most copper(II) paddlewheel based MOFs, BUT-155 retains its structural integrity in liquid water at room temperature or even in boiling water. Water vapor adsorption studies reveal that BUT-155 shows a sigmoidal adsorption isotherm with a maximum uptake of 46.7 wt% (585 cm3/g). After five cycles of water adsorption and desorption, the water adsorption capacity of BUT-155 stabilized at 41 wt% (515 cm3/g) with only a 12% decrease. It is also demonstrated that BUT-155 is able to selectively detect and adsorb aniline in water with a maximum uptake of 670 mg/g due to relatively strong interactions between accessible open Cu(II) sites and amine N atoms of aniline molecules, which induces a nakedeye detectable colour change of the sample. Moreover, 22 ACS Paragon Plus Environment

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the aniline adsorption performance of the MOF remains unchanged after the adsorbent being regenerated for 5 cycles. The results presented here shed some light on the design and construction of more hydrolytically stable MOFs, which are potentially useful in boarder areas.

Experimental

Materials and Instrumentation. All reagents and solvents were purchased from commercial suppliers and used without further purification. 1H-NMR spectra were collected on a Bruker Avance 400 MHz NMR spectrometer. Thermal gravimetric analyses (TGA) were performed under air condition with a heating rate of 5 °C/min using a TGA-50 (SHIMADZU) thermogravimetric analyzer. Fourier transform infrared spectroscopy (FT-IR) spectra were recorded using an F-4600 FT-IR spectrophotometer in the 4000~400 cm-1. Gas adsorption isotherms were reported by a volumetric method using a Micromeritics ASAP2020 surface area and pore analyzer. The scanning electron microscope (SEM) images were recorded with a HITACHI

SU3050.

UV−vis

spectra

were

obtained

with

a

UV-2600

spectrophotometer in the range of 250−800 nm at room temperature. PXRD patterns were recorded on a Rigaku Smartlab3 X-ray powder diffractometer equipped with a Cu sealed tube (λ = 1.54178 Å) at room temperature. Simulation of the PXRD spectra was carried out by the single-crystal data and diffraction crystal module of the Mercury program available free of charge via http://www.ccdc.cam.ac.uk/ mercury/.



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Synthesis. 3,3’,5,5’-Tetra(3,5-dicarboxyphenyl)-2,2’,4,4’,6,6’-examethyl-biphenyl (H8tdhb) was synthesized by a previous reported procedure with some modifications (Figure S1, Supporting Information).62 The detail description is provided in the Supporting Information. [Cu4(tdhb)(H2O)4]·5DMF (BUT-155a) was synthesized through a solvethermal reaction. Cu(NO3)2·2.5H2O (10 mg, 0.05 mmol), H8tdhb (5 mg, 0.005 mmol), and drops of HBF4 were ultrasonically dissolved in 3 mL DMF in a 5 mL Pyrex vial. The vial was sealed and then heated at 80 °C for 24 h in an oven. After cooling to room temperature, green crystals were collected by filtration, washed with DMF and methanol, and then dried in air (yielded 8 mg). For TGA, and FT-IR of the as-synthesized sample see Figure S2 and S3 of the Supporting Information, respectively. Elemental analyses calculated (%) for Cu2C40H54N5O15: C 49.43, H 5.60, N 7.21; found: C 49.22, H 5.88, N 6.97. HKUST-1: 1,3,5-benzenetricarboxylic acid (1.0 g) was firstly dissolved in 30 mL of 1:1 ethanol/DMF mixture solvent in a flask. In another flask, Cu(NO3)2·2.5H2O (2..00 g) was dissolved in 15 mL water. The two solutions were then mixed and stirred for 10 min, and the mixture was transferred into a Teflon-lined stainless steel autoclave and heated at 373 K for 10 h. After cooling to room temperature, green crystals were collected by filtration, washed with ethanol, and then dried in air. For the PXRD pattern of the as-synthesized sample and the N2 adsorption isotherm of its activated sample, see Figure S20 and S21 of the Supporting Information, respectively. 24 ACS Paragon Plus Environment

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Sample Activation. As-synthesized sample of BUT-155 was soaked in fresh DMF for 24 h, and the extract was discarded. Fresh methanol was subsequently added, and the samples were guest exchanged for 12 h. This procedure was repeated three times. After decanting the methanol extract, the sample was dried under high vacuum at 100 °C for 10 h, resulting in the guest-free phase of BUT-155.

Stability Test through PXRD Measurements. The BUT-155 sample, about 15 mg for each batch, was immersed in 20 mL water and aqueous solutions of HCl (pH = 4), and NaOH (pH = 10) at RT, or in boiling water for 24 hours, respectively. Another sample was immersed in water at RT for 10 days. After treatments, these samples were washed with methanol (three times) and then characterized by PXRD measurements.

N2 Adsorption for Water-treated Samples. Three batches of BUT-155 samples (∼100 mg for each) were immersed in 20 mL water at RT or boiling water for one day, or in water at RT for 10 days, respectively. After water treatment, the samples were soaked in methanol for two days (three times) and dichloromethane (two days, three times), and then degassed for 10 h at 100 °C for N2 adsorption measurement at 77 K.

X-ray Crystallographic Analysis. The diffraction data of as-synthesized phase BUT-155a, guest-free phase BUT-155, water-included phase water@BUT-155, boiling water treated BUT-155, and aniline included phase aniline@BUT-155 were collected in a Rigaku

Supernova

CCD diffractometer equipped

with a 25

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mirror-monochromatic enhanced Cu-Kα radiation (λ = 1.54184 Å). The data set was corrected by empirical absorption correction using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm. The structure was solved by direct methods and refined by full-matrix least-squares on F2 with anisotropic displacement by using the SHELXTL software package. Non-hydrogen atoms were refined with anisotropic displacement parameters during the final cycles. Hydrogen atoms of ligands were calculated in ideal positions with isotropic displacement parameters. The SQUEEZE option of PLATON was used to model the contribution of disordered guest molecules to the reflection intensities. The details of crystal data and structural refinement can be found in Supporting Information, and the provided CIF files.

Aqueous-Phase Adsorption. BUT-155 sample (15 mg) was transferred into water solution of certain analyte with a given concentration in a vial. UV−vis spectra of the solutions were recorded to characterize the adsorption performances of BUT-155 along with the soaking time at room temperature. The adsorption isotherms of aniline and phenol in single-component and mixture adsorption experiments were obtained by mixing 15 mg MOF with 50 mL solution of different concentrations at a constant temperature of 298 K with stirring for 4 h. The amounts of aniline or phenol adsorbed on BUT-155 were calculated with the following equation:

ܳ௘ =

ሺ‫ܥ‬଴ − ‫ܥ‬௘ ሻܸ ‫ܯ‬

where Qe (mg/g) is the equilibrium adsorbed amount; C0 and Ce (mg/L) are the initial and equilibrium concentrations of solution; V (L) is the volume of solution; and M (g) 26 ACS Paragon Plus Environment

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is the mass of the MOF. In order to ensure the accuracy of measurements, all the experiments were repeated at least three times, and the average values were reported. All materials were dried overnight under vacuum at 100 ºC before each repeated use.

Regeneration of Adsorbents. The BUT-155 samples used in aniline and phenol adsorption measurements were soaked in methanol (by a proportion of 100 mL methanol per 15 mg MOF) under stirring at RT for 12 h. This procedure was repeated three times by using fresh methanol. After filtration, the wet samples were dried under vacuum at 100 °C for 2 h to remove the residual solvents. The regenerated MOFs were then used again for the adsorption of aniline or phenol, up to five cycles.

ASSOCIATED CONTENT

Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI:

Detail

synthesis

method

of

H8tdhb,

more

structure

refinement,

general

characterizations, DFT calculation, additional structure figures, and aniline and phenol adsorption experiment (PDF).

Crystallographic data (CIF)



Crystallographic data (CIF) 27 ACS Paragon Plus Environment


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

*E-mail: [email protected].

Notes The authors declare no competing financial interest.

Crystallographic data of BUT-155a (CCDC 1544771), BUT-155(CCDC 1544772), water@BUT-155(CCDC 1544773), boiling water treated BUT-155(CCDC 1544774) and aniline@BUT-155 (CCDC 1544775) can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif

ACKNOWLEDGMENT We thank financial support from the Natural Science Foundation of China (21576006 and 21601008); the National Natural Science Fund for Innovative Research Groups (51621003); the China Postdoctoral Science Foundation (2015M580027); and the Importation and Development of High-Caliber Talents Project of Beijing Municipal Institutions (CIT&TCD20150309).

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A new porous copper(II)-paddlewheel based MOF has been demonstrated to be stable in boiling water, and shows great potential in water vapour adsorption, as well as the selective adsorption and detection of aniline from water.


A Copper(II)-Paddlewheel MetalâOrganic Framework with Exceptional

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A Copper(II)-Paddlewheel MetalâOrganic Framework with Exceptional