English

• 1. INTRODUCTION

  Commercial dyes with strong and stable color structures are serious pollutants of water^[1]. Dyes could have a great influence on the growth of animals and plants in water and on human health due to their harmful substances, toxic heavy metals, and carcinogenic and mutagenic substances^[2]. Therefore, numerous methods, such as flocculation^[3], photocatalysis^[4], adsorption^[5], and membrane separation^[6], have been applied to purify dye wastewater. Among these methods, photocatalysis offers energy savings and high efficiency, and it avoids secondary pollution^[7]. There have been enormous efforts to utilize metal oxides and metal sulfides as photocatalysts to decompose dyes into enviromentally benign species^[8].

  Recently, increasing interest has been focused on exploring the metal-organic frameworks (MOFs) materials as a new type of photocatalysts due to their ligand-metal charge transfer^[9]. Compared with traditional inorganic photocatalysts, there exhibit tremendous catalytically active sites throughout the MOFs materials and they can be reached via their open channels. Furthermore, many physical properties such as light-harvesting, porosity, adsorption and transportation of guest molecules, which are very important for photocatalysts, can be tailored in the MOFs materials^{[10, 11]}.

  Several factors can highly influence the selfassembly progress of MOFs, for example, ligand/metal nature, temperature, solvent, reaction time, pH value and reactant stoichiometry^[12-15]. In recent years, more and more organic diverse carboxylate ligands are chosen in synthesizing MOFs^[16-18] because carboxylates can display diverse bridging modes. Among them, flexible organic carboxylate ligands as good candidates for the construction of metal-organic frameworks are relatively rare and challenging^[19-21]. Therefore, we reported a metal-organic framework (complex 1) based on flexible multidentate ligand H₃tci^[22-25].

  Compared to rigid tripodal spacers, like 1,3,5-benzenetricarboxylic acid and 1,3,5-tris(4-carboxyphenyl)benzene, the flexible H₃tci ligand is more sensitive to the reaction condition. As H₃tci ligand features highly flexible arms and three rotation-free carboxyl groups, it is likely to be advantageous to form high-dimensional metal-organic frameworks^[26-31]. Previously, Kitagawa group used this ligand to generate several coordination polymers^[29].

  In this paper, the synthesis and structure analysis of complex 1 have been studied. We chose the flexible H₃tci as a main ligand to construct a novel MOF and selected the rigid 4,4΄-bipyridine (bpy) ligand as an auxiliary ligand to increase the stability of skeleton. Complex 1 shows a two-fold interpenetrating 3D network and its topology, thermal stability and fluorescent and CO₂ adsorption properties were analyzed in detail. Significantly, the photocatalytic degradation performance of 1 has been intensively investigated.

  2. EXPERIMENTAL

  2.1 General procedure

  All chemicals were of reagent grade and used as commercially obtained without further purification. Elemental analyses (C, H or N) were carried out on a Perkin-Elmer 240 elemental analyzer. Powder X-ray diffraction measurements were performed with a Bruker AXS D8 Advance instrument. The FT-IR spectra were recorded in the range of 4000~400 cm^-1 on a Nicolet 330 FTIR Spectrometer using the KBr pellet method. Thermogravimetric analysis (TGA) experiments were performed using a Perkin-Elmer TGA 7 instrument (heating rate of 10 ℃·min^-1, nitrogen stream). Solid-state photoluminescence spectra were performed with a Hitachi F-7000 Fluorescence Spectrophotometer. The gas adsorption isotherm was performed on the surface area analyzer ASAP-2020. Ultraviolet-visible (UV-vis) data were collected from a UV-vis spectrophotometer (UH4150, Hitachi). The photocatalytic activities were carried out in a photochemical reactor (XPA, Nanjing Xujiang).

  2.2 Synthesis of complex 1

  Complex 1 was synthesized by solvothermal reaction of Zn(NO₃)₂·6H₂O (14.87 mg, 0.05 mmol), H₃tci (10.35 mg, 0.03 mmol) and bpy (3.124 mg, 0.02 mmol) in the mixed solvents of 2 mL N, N΄-dimethylformamide (DMF) and 2 mL acetonitrile (CH₃CN). The reaction system was sealed in a 25 mL Teflon-lined stainless-steel autoclave and heated at 120 ℃ for 3000 min, and then slowly cooled to 30 ℃ at a rate of 0.1 ℃·min^-1. Colorless block crystals of complex 1 were obtained in 71% yield. Elemental analysis (%): calcd. for C₂₆H_28.5N_7.5O₁₃Zn₂ (798.29): C, 39.79; H, 3.66; N, 13.39. Found (%): C, 39.65; H, 3.68; N, 13.42.

  2.3 X-ray structural determination

  Single-crystal structure analysis of complex 1 was performed at 293(2) K on an Agilent Super-Nova diffractometer equipped with a copper microfocus X-ray sources (λ = 1.54178 Å). The data were collected with an ω-scan mode in an arbitrary φ-angle. Data reduction was performed with the CrysAlisPro package, and an analytical absorption correction was performed. The structure was solved by direct methods and refined by full-matrix least-squares on F² with anisotropic displacement using the SHELXL software package^[32]. The non-H atoms were treated anisotropically, whereas the aromatic and hydroxyl-hydrogen atoms were placed in calculated, ideal positions and refined as riding on their respective carbon or oxygen atoms. The structure was examined using the Addsym subroutine of PLATON to assure that no additional symmetry could be applied to the models^[33]. A total of 12592 reflections were collected in the range of 4≤θ≤70.42° (–25≤h≤25, –11≤k≤15, –31≤l≤29), of which 6122 were independent (R_int = 0.0185). The final refinement gave R = 0.0621, wR = 0.1773 (w = 1/[σ²(F_o²) + (0.1059P)² + 41.7503P], where P = (F_o² + 2F_c²)/3), S = 1.041, (Δ/σ)_max = 0.000, (Δρ)_max = 2.664 and (Δρ)_min = –1.464 e/ų. Selected bond lengths and bond angles for 1 are collected in Table 1.

  Table 1

  

  Table 1.  Selected Bond Lengths (Ǻ) and Bond Angles (°) for Complex 1

  DownLoad: CSV

  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Zn(1)–O(1)	2.020(3)		Zn(1)–O(2)	2.001(3)		Zn(1)–O(3)	2.071(3)
  Zn(1)–O(4)	2.193(3)		Zn(1)–O(5)	2.294(3)		Zn(1)–N(4)	2.147(3)
  Zn(2)–O(7)	1.921(3)		Zn(2)–O(8)	1.931(3)		Zn(2)–O(9)	1.911(3)
  Zn(2)–N(5)	2.030(3)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(1)–Zn(1)–O(3)	95.40(14)		O(1)–Zn(1)–O(4)	150.67(12)		O(1)–Zn(1)–O(5)	93.74(12)
  O(1)–Zn(1)–N(4)	87.18(12)		O(2)–Zn(1)–O(1)	116.38(13)		O(2)–Zn(1)–O(3)	99.16(13)
  O(2)–Zn(1)–O(4)	92.39(12)		O(2)–Zn(1)–O(5)	149.44(12)		O(2)–Zn(1)–N(4)	90.64(12)
  O(3)–Zn(1)–N(4)	84.86(12)		O(3)–Zn(1)–O(5)	82.01(12)		O(3)–Zn(1)–N(4)	167.43(12)
  O(4)–Zn(1)–O(5)	57.16(11)		N(4)–Zn(1)–O(4)	86.89(12)		N(4)–Zn(1)–O(5)	85.55(11)
  O(7)–Zn(2)–O(8)	110.28(15)		O(7)–Zn(2)–N(5)	105.61(13)		O(8)–Zn(2)–N(5)	108.80(13)
  O(9)–Zn(2)–O(7)	121.08(18)		O(9)–Zn(2)–O(8)	114.09(19)		O(9)–Zn(2)–N(5)	94.82(14)

  2.4 Photocatalytic degradation activity

  The photocatalytic activities of complex 1 were evaluated by monitoring the degradation of rhodamine B(RhB), methylene blue (MB) and methyl orange (MO) in the liquid phase under UV light irradiation at room temperature. Typically, 30 mg of complex 1 was dispersed in a 50 mL solution of RhB (5×10^–5 mol·L^–1) and the mixture was put in a photochemical reactor. The UV light source used in the experiment was a 300 W Hg lamp. The suspensions were magnetically stirred in the dark for 30 min to ensure adsorption-desorption equilibrium before the light irradiation. During the tracking reaction, 3 mL liquid was taken out at 10 min intervals and centrifuged to remove the catalyst. The remaining dye concentration was determined with a UV-Vis spectrophotometer by detecting the maximum absorption wavelength for RhB at 554 nm, for MB at 663 nm, and for MO at 464 nm, respectively.

  3. RESULTS AND DISCUSSION

  3.1 Crystal structure of complex 1

  Crystal structural analysis reveals that complex 1 is a three-dimensional neutral porous framework and crystallizes in monoclinic space group C2/c. The asymmetric unit contains two Zn(Ⅱ) ions, one tci^3- anion ligand, one coordinated NO₃^- ion, one coordinated 4,4΄-bipy, one uncoordinated DMF molecule and one uncoordinated CH₃CN molecule. As can be seen from Fig. 1, the neighboring Zn(Ⅱ) ions have different coordination number and modes. The Zn(1) is six-connected by two O atoms from two different tci^3- ligands, two O atoms from one NO₃^- ion and one N atom from coordinated 4,4΄-bipy molecule to form a distorted trigonal prism coordination geometry. The Zn(1)–O bond distances range from 2.001 to 2.294 Å and O–Zn(1)–O bond angles from 57.34 to 167.26 Å. The Zn(2) is four -coordinated by three O atoms from three different tci^3- ligands and one N atom from coordinated 4,4΄-bipy molecule to form a tetrahedral coordination geometry. The Zn(2)–O bond distances range from 1.912 to 1.929 Å and O–Zn(2)–O bond angles from 110.17 to 121.5 Å. All the bond lengths and bond angles are consistent with the previously reported results^{[34, 35]}. The Zn(1) and Zn(2) ions are linked together by three carboxylic O and two pyridine N atoms to form a binuclear unit.

  Figure 1

  

  Figure 1.  (a) Coordination environment of Zn(Ⅱ) ions in complex 1. (b) Projection view of 3D open framework along the b axis. (c) Schematic representation of a simplified 3D network for complex 1 with hms topology (Symmetry codes: a (x, 1 + y, z), b (x, 2 – y, 0.5 + z), c (0.5 + x, 0.5 + y, z), d (0.5 + x, 1.5 – y, 0.5 + z))

  DownLoad: Full-Size Img PowerPoint

  The tci^3- ligand exhibits the same coordination mode as μ₂-η₁: η₁ and each carboxyl group bridges two Zn(Ⅱ) ions to form a binuclear Zn unit with the Zn–Zn distances of 3.464 Å. The binuclear units as SBUs are united together by three tci^3- ligands and two 4,4΄-bipy ligands to construct a 3D structure and the large window within this single framework is filled via mutual interpenetration of the other identical framework, generating a 2-fold interpenetrating 3D architecture. Viewed along the b axis, there is a narrow 1D rhombic channel with a side dimension of 10.5Å × 15.3Å, and the calculated free volume in the framework is 10.6% by PLATON. A better method to understand this intricate structure can be achieved by the application of topological analysis, which reduces the complicated structures to simple nodes and connection nets. Therefore, if the binuclear Zn unit can be regarded as 5-connected node and tci^3- ligand as 3-connected node, its topology can be described as a rare doubly-interpenetrating 3,5-connected net of hms with point symbol of {6^3}{6^9.8}.

  3.2 IR spectrum

  FT-IR spectrum of bulk complex 1 was also investigated. As shown in Fig. 2, the sharp bands at about 1752 and 1686 cm^-1 are attributed to asymmetric stretching at 1467 and 1411 cm^-1 for symmetric stretching vibrations of carboxylic group. The separations (Δ) between ν_asym (CO₂) and ν_sym (CO₂) indicate the presence of chelating (176 cm^-1) and monoatomic (234 cm^-1) coordination modes, which is in compliance with the crystal data of 1.

  Figure 2

  

  Figure 2.  IR spectrum of complex 1

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  3.3 Powder X-ray diffraction and thermogravimetric analyses

  Phase purity of bulk complex 1 is sustained by the powder X-ray diffraction pattern. The most peak positions of simulated and experimental patterns are in good agreement with each other. The differences in intensity may be due to the preferred orientation of the powder samples (Fig. S1).

  To investigate the thermal stability of complex 1, thermogravimetric analysis (TGA) of 1 was performed under a N₂ atmosphere from room temperature to 800 ℃ at a heating rate of 10 ℃·min^-1. Complex 1 has two identifiable weight loss steps: the first one is consistent with the removal of one uncoordinated DMF molecule and half uncoordinated acetonitrile molecule (obsd. 11.87%, calcd. 11.93%), which appears below 164 ℃. The second one is attributed to the collapse of the framework above 260 ℃, and the remaining residue corresponds to the formation of ZnO (Fig. 3).

  Figure 3

  

  Figure 3.  TG curve of complex 1

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  3.4 Photoluminescence property

  Due to the coordination polymers with d₁₀ metal centers and organic ligands with π→n or π﹡→π electronic transitions usually exhibiting excellent photoluminescent properties, the solid-state luminescent emission spectra of free ligands and complex 1 were studied at room temperature. As depicted in Fig. 4, the emission spectra of the free H₃tci show a main peak at 437 nm upon excitation at 330 nm, while 1 exhibits a strong fluorescent emission peak at 464 nm upon excitation at 300 nm. Obviously, it can be observed that the emission spectra for 1 exhibit red-shifts compared to the free H₃tci ligand, which may be ascribed to the deprotonation of the H₃tci ligand and the coordination effects of the tci ligand to the Zn(Ⅱ) ions. This kind of strong luminescence material can be used as a sensor, such as metal ions, NACs and so on.

  Figure 4

  

  Figure 4.  Solid state emission spectra for free ligand H₃tci and complex 1 at room temperature

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  3.5 Gas absorption property

  First of all, the N₂ uptake of complex 1 was measured at 77 K on a Micromeritics ASAP 2020. But 1 takes almost no nitrogen, which may be contributed to its small pores. Furthermore, in order to certify the porous structure of 1, we investigated its CO₂ adsorption properties at 195, 273 and 295 K. Before this measurement, the freshly prepared crystals of 1 were guest-exchanged with acetone for 36 hours, then activated at 60 ℃ under high vacuum to get the solvent-free sample. Complex 1 can adsorb 46.2 cm³·g⁻¹ CO₂ gas at 195 K and 1 bar, and the BET surface area, calculated from the CO₂ adsorption profile, was found to be approximately 145 m²·g⁻¹. Although 1 has limited solvent accessible volume (10.6% of the cell volume), it also has an adsorption value of 17.7 and 2.5 cm³·g⁻¹ at 273 and 298 K and 1 bar. The enthalpy of adsorption of CO₂ was obtained by measuring two isotherms at T = 273 and 298 K, followed by a fit of the data to virial equation. At zero loading, ΔH_ads = 37.5 kJ·mol⁻¹, which clearly indicates moderate interaction between 1 and CO₂. Clearly, the CO₂ adsorption isotherm for 1 displays a small hysteresis, which possibly because the MOF takes enough CO₂ to open up the pores due to the flexible nature of MOF. However, at 298 K, since there is not much CO₂ uptake, the pores are in a close form and most of the uptake is from the surface of the crystals (Fig. 5).

  Figure 5

  

  Figure 5.  CO₂ adsorption capacities for complex 1 at 195 K (Left), 273 K and 298 K (Right)

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  3.6 Photocatalytic degradation activity

  Rhodamine B was chosen as a representative dye to evaluate the capability of complex 1 for organic pollution degradation. Based on the tentative experiment, the color of RhB was gradually faded when adding the catalyst under UV light irradiation. As shown in Fig. 6a, after irradiation, the maximum absorbance of RhB gradually decreased with time, which illustrated complex 1 possessed remarkable photocatalytic degradation activities of RhB. In the absence of a catalyst, the degradation of RhB under UV irradiation was slow. Only 29% of RhB was decomposed after 150 min, while approximately 96% RhB was degraded in the same reaction time (Fig. 6d). As shown by the linear plot of ln(C₀/C_t) vs reaction time, the degradation of RhB followed pseudo-first-order kinetics(Fig. 6f). The pseudo-first-order reaction equation can be written as:

  $ \ln \left(\mathrm{C}_0 / \mathrm{C}_{\mathrm{t}}\right)=K_{a p p} \mathrm{t} $

  Figure 6

  

  Figure 6.  Photocatalytic properties of complex 1. Time-dependent absorption spectra of RhB (a), MO (b) and MB (c) solution in the presence 1. Degradation rate of RhB (d), MO and MB (e) over 1. Kinetic data of photodegradation of RhB in the presence of 1 (f)

  DownLoad: Full-Size Img PowerPoint

  where C_t = concentration of dye at time and C₀ = concentration of dye at t = 0, K_app = apparent rate constant. From the plot of ln(C₀/C_t) versus time, the rate constant was calculated as 1.80 × 10^-2 min^-1, which was comparable with other MOFs reported^[36-38].

  To further investigate the photocatalytic activity of complex 1 on organic dyes, the photocatalytic degradation of MO and MB was explored under consistent conditions with RhB. As shown in Fig. 6b and 6c, the intensity of their peaks gradually decreased with time until they completely disappeared. The photocatalytic efficiency of MO was found to be 98% after 60 min and the photocatalytic efficiency of MB was found to be 91% after 90 minutes (Fig. 6e). Complex 1 exhibits a higher photocatalytic activity over MO among the three dyes. The considerable photocatalytic activity of 1 indicates the advantage of porous MOF catalysts. As 1 shows considerable photodegradation of organic dyes under UV irradiation, it is expected to be semiconducting in nature.

  4. CONCLUSION

  In conclusion, a novel doubly-interpenetrating Zn-MOF (complex 1) with tripodal flexible ligand has been successfully obtained by solvothermal method. 1 features a 3,5-connected 3D hms network with the point symbol of {6^3}{6^9.8}. Importantly, complex 1 can adsorb 46.2 cm³·g⁻¹ CO₂ gas at 195 K and 1 bar. What's more, 1 shows highly encouraging photocatalytic degradation of toxic dye molecules with a potential application in wastewater purification.

  
  
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  Figure 1  (a) Coordination environment of Zn(Ⅱ) ions in complex 1. (b) Projection view of 3D open framework along the b axis. (c) Schematic representation of a simplified 3D network for complex 1 with hms topology (Symmetry codes: a (x, 1 + y, z), b (x, 2 – y, 0.5 + z), c (0.5 + x, 0.5 + y, z), d (0.5 + x, 1.5 – y, 0.5 + z))

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  Figure 2  IR spectrum of complex 1

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  Figure 3  TG curve of complex 1

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  Figure 4  Solid state emission spectra for free ligand H₃tci and complex 1 at room temperature

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  Figure 5  CO₂ adsorption capacities for complex 1 at 195 K (Left), 273 K and 298 K (Right)

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  Figure 6  Photocatalytic properties of complex 1. Time-dependent absorption spectra of RhB (a), MO (b) and MB (c) solution in the presence 1. Degradation rate of RhB (d), MO and MB (e) over 1. Kinetic data of photodegradation of RhB in the presence of 1 (f)

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  Table 1.  Selected Bond Lengths (Ǻ) and Bond Angles (°) for Complex 1

  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Zn(1)–O(1)	2.020(3)		Zn(1)–O(2)	2.001(3)		Zn(1)–O(3)	2.071(3)
  Zn(1)–O(4)	2.193(3)		Zn(1)–O(5)	2.294(3)		Zn(1)–N(4)	2.147(3)
  Zn(2)–O(7)	1.921(3)		Zn(2)–O(8)	1.931(3)		Zn(2)–O(9)	1.911(3)
  Zn(2)–N(5)	2.030(3)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(1)–Zn(1)–O(3)	95.40(14)		O(1)–Zn(1)–O(4)	150.67(12)		O(1)–Zn(1)–O(5)	93.74(12)
  O(1)–Zn(1)–N(4)	87.18(12)		O(2)–Zn(1)–O(1)	116.38(13)		O(2)–Zn(1)–O(3)	99.16(13)
  O(2)–Zn(1)–O(4)	92.39(12)		O(2)–Zn(1)–O(5)	149.44(12)		O(2)–Zn(1)–N(4)	90.64(12)
  O(3)–Zn(1)–N(4)	84.86(12)		O(3)–Zn(1)–O(5)	82.01(12)		O(3)–Zn(1)–N(4)	167.43(12)
  O(4)–Zn(1)–O(5)	57.16(11)		N(4)–Zn(1)–O(4)	86.89(12)		N(4)–Zn(1)–O(5)	85.55(11)
  O(7)–Zn(2)–O(8)	110.28(15)		O(7)–Zn(2)–N(5)	105.61(13)		O(8)–Zn(2)–N(5)	108.80(13)
  O(9)–Zn(2)–O(7)	121.08(18)		O(9)–Zn(2)–O(8)	114.09(19)		O(9)–Zn(2)–N(5)	94.82(14)

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