施偉東 任 傲 劉春波 白紅葉 車廣波 孫赫一 叢 堯

(江蘇大學化學化工學院，鎮(zhèn)江 212013)

Much attention has been paid to metal-organic frameworks (MOFs) as promising candidates for gas absorption, molecular recognition, heterogeneous catalysis, optics, and so on[1-7]. Recently, MOFs as photocatalytic materials have attracted a lot of interests because they provide a green method to solve the pollution problems caused by organic dyes[8-12].Garcia et al. firstly reported the photocatalytic behaviors of MOF-5 as a microporous semiconductor for degrading phenol in aqueous solutions upon light excitation. Compared to traditional photocatalysts TiO2and ZnO, MOF-5 can absorb UV radiations of wavelengths longer than 350 nm to realize photodegradation under visible light[13]. Yuan et al.employed a novel series of photocatalysts based on MIL-53(M) (M=Fe, Cr, Al) MOFs for methylene blue(MB) decolorization under UV-Vis light and visible light irradiation. 99%, 24% and 47% MB degradation were achieved at 20 min in the presence of MIL-53(Fe) and three electron acceptors (H2O2, KBrO3and(NH4)2S2O8), respectively. Their results demonstrate that the electron acceptors had beneficial effect on improving the rate for MB decolorization and the metal centers of MIL-53 show nil effect on the photocatalytic activity for MB photodegradation[14]. In addition, Wen et al. reported X3B (an anionic organic dye) was degraded about 60% and 80% by using new Cd (Ⅱ)and Co (Ⅱ) compounds based on aromatic polycarboxylate acids and a rigid imidazole-based bidentate ligand as photocatalysts after 9 hours visible irradiation. They concluded that the excellent photocatalytic capability of Co(Ⅱ)compound was originated from its small band gap[15]. All the above achievements indicated that MOFs as new photocatalytic materials could be successfully applied in degradation of organic pollutants. However, comparing with the traditional photocatalysts, less researches were done regarding the photocatalysis of MOFs.

To attain better photocatalytic activity, herein, Co is selected as central atom due to the single electron in its molecular orbit and HDPPZC (dipyrido[3,2-a∶2′,3′ -c]-phenazine-2-carboxylic acid) is used as a organic ligand thanks to its excellent coordinating ability and large conjugated system which can easily form π-π interaction. The synthesis and characterization of lead complex based on HDPPZC were previously reported by our group[13]. In addition, we employed phosphotungstic acid as the second ligand for its oxidation-reduction property and high photocatalytic activity.

Under hydrothermal conditions,a novel compound,namely [CoCl0.5(H2O)0.5(HDPPZC)2](PW12O40)0.5·3.5H2O(1), has been successfully prepared with an unusual 2D supramolecular framework via hydrogen bonds and π-π stacking interactions. Compound 1 exhibits excellent photocatalytic performance in degradation of Rhodamine B.

1 Experimental

1.1 Materials

The HDPPZC ligand was prepared according to the literature method[16]and all other reagents were purchased from commercial sources without further purification.

1.2 Synthesis of [CoCl0.5(H2O)0.5(HDPPZC)2](PW12O40)0.5·3.5H2O (1)

The starting materials HDPPZC (0.05 mmol, 16.3 mg), α-H3[PW12O40] (0.05 mmol, 1 444 mg) and CoCl2·6H2O (0.2 mmol, 47.6 mg) were dissolved in distilled water (12 mL) The resulting suspension was stirred for 10 min and sealed in the 25 mL Telfon-lined reactor.After heating to 423 K for 3 d and then cooling to room temperature, orange block crystals of 1 were obtained. The crystals were filtered, washed with distilled water and dried at ambient temperature.Yield： 59% based on CoCl2·6H2O. Calcd. for C76H40O56N16PW12Cl2Co2(%)： C, 20.43; H, 0.91; N, 5.02.Found(%)： C, 20.48; H, 0.90; N, 5.09.

1.3 General characterization and physical measurements

Thermal analysis (TGA) was carried out under air condition on a NETZSCH STA 449C analyzer in flowing N2with a heating rate of 5 ℃·min-1. X-ray diffraction (XRD) technique was used to characterize the crystal structure. In this work, XRD patterns were obtained with a D/max-RA X-ray diffractometer(Rigaku, Japan) equipped with Ni-filtrated Cu Kα radiation (40 kV, 200 mA). The 2θ scanning angle range was 5°～50° with a step of 0.02° per 0.2 s. UVVis diffuse reflectance spectra (UV-Vis DRS) of photocatalysts powder was obtained for the drypressed disk samples using Specord 2450 spectrometer(Shimazu, Japan) equipped with the integrated sphere accessory for diffuse reflectance spectra, using BaSO4as the reflectance sample. Elemental analysis (C, H,and N) were performed on a Perkin-Elmer 2400 CHSN elemental analyzer. A crystal with dimensions of 0.20 mm×0.20 mm×0.20 mm was put on a Bruker SMART APEX CCD area-detector diffractometer, and the data were collected by using a graphitemonochromatized Mo Kα radiation (λ=0.071 073 nm)at 292(2) K in the range of 3.06°≤θ≤25.03°. A total of 18 835 reflections were collected and 9 028 were independent (Rint=0.049 3), of which 6 829 were observed (I＞2σ(I)). The structure of the compound was solved with the direct method of SHELXS-97[17]and refined with full-matrix least-squares techniques using the SHELXL-97[18]program within WINGX[19]program.Nonhydrogen atoms were refined with anisotropic temperature parameters.

Table 1 Crystal data and structure refinements for compound 1

The detailed crystallographic data and structure refinement parameters for compounds 1 are summarized in Table 1. Selected bond distances and angles for compounds 1 are given in Table S1. The structures were analyzed by using Diamond program.

CCDC： 968312.

2 Results and discussion

2.1 Description of crystal structure

Fig.1 ORTEP drawing of compound 1

Single crystal X-ray diffraction analysis reveals that compound 1 crystallizes in the triclinic space group, and its molecular weight is 4 463.72. As shown in Fig.1,in the[CoCl0.5(H2O)0.5(HDPPZC)2]1.5+ion,each Co atom adopts a slightly distorted octahedron structure defined by four N atoms of two HDPPZC ligands, one water molecule and half Cl atom (Fig.1).Bond distances of Co-N which conform to the bond distances of literature referred are between 0.210 6(10)and 0.214 3(11) nm. The N(1), N(2), N(5) and Cl(11)atoms comprise the equatorial plane and the axial position is occupied by N(6) and O(W1) atom. In the[PW12O40]3-atom, P atom is surrounded by eight O atom to form a cube and each O atom exists in the form of half possession which usually appears in the α-Keggin-type polyoxometalate. The P-O bond lengths range from 0.145 3(17) to 0.155 3(13) nm. The W atom has a distorted octahedral geometry defined by six O atoms forming a WO6unit and three WO6units constitute a W3O13trinuclear metal cluster. Each Keggin structure has four W3O13. The bond distances of W-O can be divided into three groups, W-Ot(tip oxygen) 0.166 8(9)～0.170 0(9) nm, W-Ob (bridge oxygen) 0.184 9(9)～0.194 3(9) nm and W-Oc(center oxygen) 0.2436(17)～0.2528(15) nm. Notably, the HDPPZC ligands are arranged in a parallel fashion,leading to a structure suitable to form weak π-π stacking interactions. And there are two types of the weak π-π stacking interactions in the structure as follows： (i) hexatomic ring A (C37i, C32i, C33i, C34i,C36i, C35i) and hexatomic ring B (N7ii, C31ii, C30ii,N8ii, C33ii, C32ii), the distance of Cg of A (P1iii(0.093 877, 0.164 733, 0.103 358)) and Cg of B (P2iv(0.115 679, 0.146 648, 0.094 427)) is 0.364 54(12)nm, see the lines marked in black in Fig.2(a). The dihedral angle of A and B is 2.074 (355)° ; (ii)hexatomic ring C (C15v, C14v, C13v, C18v, C17v, C16v)and hexatomic ring D (N4vi, C14vi, C13vi, N3vi, C12vi,C11vi), the distance of Cg of C (P3iii(0.198 957,0.208 909, 0.057 293)) and Cg of D (P4vii(0.184 832,0.189 072, 0.053 613)) is 0.368 92(9) nm, see the lines marked in blue in Fig.2(a). The dihedral angle of A and B is 3.032(409)°. They link the discrete structures to form one-dimensional wave chains.Meanwhile, as shown in Fig.2(a), the adjacent chains are connected together by hydrogen bonding (dashed line) formed by uncoordinated carboxyl group (O(48)-H(48)…O(46) with the distance of 0.253 4(2) nm between H(48) and O(46)). Because of the polyoxanions, a large amount of disorder water molecules exist in the structure. As displayed in Fig.2 (b), the[PW12O40]3-groups which fill in the wavelike pore along a axis are only used as structure-directing agent rather than coordinate with Co2+. Therefore, a twodimensional supramolecular framework is formed via hydrogen bonding and π-π stacking interactions.

Fig.2 (a) 2D supermolecular structure via hydrogen bonding and π-π interactions of compound 1;(b) 2D packing structure of compound 1 along the a axis

2.2 Thermogravimetric analyses

The phase purity of the compound 1 is confirmed by XRD as shown in Fig.S1. Thermal stability was investigated by TGA under the atmosphere of N2in temperature ranging from room temperature to 1 000℃with a heating rate of 10 ℃·min-1(Fig.S2). In the TGA curve, the first 5.35% weight loss (Calcd.2.82%) from room temperature to 120 ℃is ascribed to the loss of the solvent and free water molecules.Then the 2.98% (Calcd. 2.40%) weight loss was attributed to the Cl ion and combined water molecules from 400 to 420 ℃. The range of decomposition of the four HDPPZC ligands occurs at 580～830 ℃with the loss of 26.67% (Calcd. 29.12%).

2.3 Photocatalytic property

The UV-Vis spectrum of compound 1 was recorded between 200 and 800 nm in the background of BaSO4(Fig.3a). The UV-Vis spectrum of compound 1 consists of absorption components in the ultraviolet and visible regions. The absorption bands of ligand are at 259, 357 nm, while the main UV absorption bands are at 255, 354 nm for 1, which can be assigned to ligand-to-metal charge transfer (LMCT)[20]. Considering that RhB is commonly used as a representative of widespread organic dyes that is very difficult to decompose in waste streams under ultraviolet or visible light irradiation[21], we selected RhB as a model pollutant in aqueous media in GHX-2 photocatalytic reactor under tungsten lamp at room temperature to evaluate the photocatalytic effectiveness of compound 1 herein.

As shown in Fig.3b, the changes in the degradation of RhB were observed in the following reaction conditions： (1) in the dark; (2) without catalysts; (3) with catalysts under light irradiation. The degradation rates of RhB in the dark and without catalyst are about 10 percent after 120 min, whereas the degradation rate with catalysts under light irradiation increases to 90% after merely 80 min,which is comparable with those of the reported semiconductors TiO2, ZnO, SrTiO3, et al.[22]. Moreover,the absorption peaks of RhB on photocatalytic degradation under the third condition decreased obviously as the reaction time prolongs in Fig.3c. The effect of catalyst dosage on the photodegration of RhB was also investigated under the same conditions. The absorbency was determined at 554 nm, which is the maximal absorption wavelength of RhB (Fig.3d). It can be clearly seen that the photocatalytic activity increases with the addition of catalyst dosage at first due to the increase of hydroxyl radical. However,when the catalyst dosage is more than 15 mg, the photodegradation ability begins to decrease owing to the catalyst particles being sheltered each other. Our results show that compound 1 may be a good candidate for the photocatalytic degradation of RhB.

Fig.3 (a) UV-Vis spectrum of compound 1; (b) Degradation rate of RhB under different conditions： (1) in the dark;(2) without catalysts; (3) with catalysts under light irradiation; (c) UV-Vis spectra of RhB on photocatalylic degradation under the third condition; (d) Effect of catalyst dosage on the photodegration of RhB

3 Conclusions

The photocatalytic study on compound 1 indicates that it is a rare example of MOFs that exhibit high photocatalytic activity for RhB degradation under ultraviolet and visible light. The degradations of other dyes are in progress.

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