Porous MetalâOrganic Frameworks with 5-Aminoisophthalic Acid as

Article pubs.acs.org/crystal

Porous Metal−Organic Frameworks with 5‑Aminoisophthalic Acid as Platforms for Functional Applications about High Photodegradation Efficiency of Phenol Ying Wang,† Li-Jing Zhang,† Rui Zhang,§ Yu Jin,† Yang Wang,† Yong-Heng Xing,*,† Feng-Ying Bai,*,† and Li-Xian Sun‡ †

College of Chemistry and Chemical Engineering, Liaoning Normal University, Huanghe Road 850#, Dalian 116029, P. R. China Guangxi Key Laboratory of Information Materials, Guilin University of Electronic Technology, Guilin 541004, P. R. China § Dandong Second Middle School, Rejuvenation Area Yingcai Street 2#, Dandong 118000, P. R. China ‡

S Supporting Information *

ABSTRACT: Four novel complexes, [CoL(H 2 O)] (1), [ZnL(H 2 O)] (2), [Ni(HL)2(H2O)2]·2H2O (3), and [CdL(H2O)]·H2O (4) (L = 5-aminoisophthalic acid), were successfully synthesized by the reaction of transition metal salts and 5-aminoisophthalic acid at hydrothermal conditions. The four complexes were characterized by element analysis, infrared spectra, UV−vis spectra, powder X-ray diffraction analysis, and thermogravimetric analysis. X-ray single-crystal diffraction analysis showed that the four complexes were 3D network structures and contained pores with sizes of 7.05 to 14.67 Å. In addition, we investigated the photodegradation ability of the four complexes for phenol solution under UV irradiation. The results showed that the four complexes had a different degree of degradation ability to phenol solution. At the same time, we found that the degradation process was in accordance with the pseudo-secondorder dynamics model, that is, the rate of degradation is controlled by the extra-particle process such as surface adsorption. Moreover, we further confirmed by high-performance liquid chromatography that the complexes are indeed a significant ability to degrade phenol.

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INTRODUCTION At present, the problem of environmental pollution has been given more and more attention, and water pollution is undoubtedly one of the biggest problems. Severe water pollution has aggravated the shortage of water resources faced by people’s society. Among them, the water pollution caused by the discharge of dye wastewater is a difficult problem in the process of water environment management. Data show that dye wastewater of approximately 160 million cubic meters is discharged into the environment each year in China, causing serious pollution to the water.1 Among them, because the phenolic compounds are common organic chemical raw materials, it is widely used in a variety of chemical industry. However, when the content of phenolic compounds exceeds 10 mg/L, aquatic organisms could not survive, while the amount of phenolic compounds in agricultural irrigation water is greater than 100 mg/L, it could even make crops die.2 So some means must be taken to deal with water pollution problems caused by the phenolic compounds. Currently, the traditional phenolic compound treatment methods such as physical adsorption,3 chemical oxidation,4,5 and ion exchange6,7 are often higher cost and prone to secondary pollution, though the photocatalytic degradation of semiconductor materials8−10 that deals with wastewater is also faced with a series of problems such as separation difficulty and low degradation rate. Therefore, it is imperative to seek more effective treatments to face water pollution caused by phenolic compounds. © 2017 American Chemical Society

Nowadays, the coordination polymers/metal−organic frameworks (MOFs) have been designed as efficient functional materials for the absorption and photodegradation of dyes. However, the development of the soluble and highly selective photodegradation for phenol and some other organic dyes is still a difficult problem for synthetic chemists.11−16 Due to the close correlation between MOF structures and their potential properties, the “real design” of desirable structures with expected properties becomes very important and attractive, although it is a very challenging issue. Generally speaking, most of the functional properties of MOFs are dominated by their pores. Therefore, to construct MOFs with suitable pore sizes and shapes is of great interest and importance for their environmental applications.17 For example, in 2005, Fujita et al. succeeded to synthesize porous complexes using 2,4,6-tris (4pyridyl)-1,3,5-triazine as the ligand and transition metal as a metal source.18 In 2010, Zhou et al. reported a series of transition metal complexes synthesized from polycarboxylate pyrazole ligands.19 In 2011, Rit et al. synthesized two supramolecular compounds with two bridging ring ligands and transition metals.20 In 2017, Jinhee Park et al. used metal chrome and metallic copper to synthesize two metal−organic complexes with pore structures.21 In these structures, a Received: August 27, 2017 Revised: October 6, 2017 Published: October 17, 2017 6531

DOI: 10.1021/acs.cgd.7b01190 Cryst. Growth Des. 2017, 17, 6531−6540

Crystal Growth & Design

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common characterization is that there are polycyclic and macrocyclic ligands, which are more likely to form large pore structures or porous structures. In other words, these complexes are more favorable for the degradation of pollutants in wastewater. However, similar to the structures of 1,3,5trimesic acid and 1,2,4,5-benzenetetracarboxylic acid ligands, there is little use of 5-aminoisophthalic acid to synthesize metal−organic skeleton complexes to degrade dye wastewater.22,23 However, it is ignored that these monocyclic ligands also have advantages that polycyclic ligands do not have; the stability of their structure,24 better coordination ability and rich coordination modes,25 and conjugate alkaline center.26 More importantly, such ligands can form infinite structural complexes with metal ions through covalent bonds,27,28 and hydrogen bonds are often present in the structure,29,30 which further enhances its thermal stability. Bearing the above-mentioned statement, we used 5-aminoisophthalic acid as ligand and transition metals Co(II), Zn(II), Ni(II), and Cd(II) as metal sources to synthesize four complexes [CoL(H 2 O)] (1), [ZnL(H 2 O)] (2), [Ni(HL)2(H2O)2]·2H2O (3), and [CdL(H2O)]·H2O (4) (L = 5aminoisophthalic acid; Scheme 1). The four complexes were

5.39. Found (%): C, 36.50; H, 3.08; N, 5.32. IR (KBr, cm−1): 3438, 3010, 2924, 2858, 1622, 1575, 1546, 1375, 776. [Ni(HL)2(H2O)2]·2H2O (3). NiCl2·6H2O (0.0247 g, 0.1 mmol) and L (0.0181 g, 0.1 mmol) were dissolved in a mixed solvent of ethanol (3 mL) and water (3 mL). After, the pH of the solution was adjusted to 5 with dilute nitric acid and stirred for about 3 h at room temperature to obtain a green suspension. Then the suspension was added to a 25 mL Teflon-lined stainless steel autoclave and heated at 160 °C for 3 days and cooled at room temperature for 2 days. Some green lumpy crystals suitable for X-ray diffraction analysis were obtained. Yield: 37% (based on Ni(II)). Calcd. (%) for C16H20N2O12Ni: C, 43.19; H, 4.11; N, 5.82. Found (%): C, 43.17; H, 4.07; N, 5.70. IR (KBr, cm−1): 3431, 3086, 2922, 2856, 1681, 1624, 1539, 1396, 769. [CdL(H2O)]·H2O (4). Cd(NO3)2·4H2O (0.0305 g, 0.1 mmol) and L (0.0181 g, 0.1 mmol) were dissolved in a mixed solvent of ethanol (4 mL), water (4 mL), and DMA (1 mL). After, the pH of the solution was adjusted to 5 with dilute nitric acid and stirred at room temperature for about 3 h to obtain a white suspension. Then the suspension was added to a 25 mL Teflon-lined stainless steel autoclave and heated at 160 °C for 3 days and cooled at room temperature for 2 days. Some colorless bulk crystals suitable for X-ray diffraction analysis were obtained. Yield: 45% (based on Cd(II)). Calcd. (%) for C8H9NO6Cd: C, 30.04; H, 2.87; N, 4.32. Found (%): C, 29.35; H, 2.75; N, 4.28. IR (KBr, cm−1): 3442, 3062, 2929, 2853, 1617, 1541, 1474, 1370, 780.

Scheme 1. Structure of Ligand L

RESULTS AND DISCUSSION Synthesis. Although some of the related skeleton compounds have been reported previously,31−37 their synthetic methods are different and the detail molecular packing pattern descriptions are also not specific enough; in particular, their functional properties were not investigated well. So it is necessary that we carry out a system structural and properties studies on the complexes and compare with the structural different characterization. Complexes 1−4 are synthesized by the reaction of Co(NO3)2·6H2O, ZnSO4·7H2O, NiCl2·6H2O, or Cd(NO3)2·4H2O and ligand L, respectively, under hydrothermal conditions (synthetic route shown in Scheme 2). We

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characterized by the X-ray single-crystal diffraction analysis, infrared (IR) spectra, UV−vis spectra, powder X-ray diffraction (PXRD) analysis, and thermogravimetric (TG) analysis. It is a remarkable fact that we found the complexes possess the performance of being degraded phenolic compounds under photocatalytic conditions. At the same time, we have carried on a deeper research of the photocatalytic degradation process through the establishment of the dynamic model and the highperformance liquid chromatography (HPLC) analysis method.

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Scheme 2. Synthetic Routes for Complexes 1−4

EXPERIMENTAL SECTION

The materials and details of the experiment are listed in the Supporting Information. The crystallographic data for complexes 1− 4 are given in Table S1. The selected bond distances (Å) of the complexes 1−4 are given in Table S2. Preparation of the Materials. [CoL(H2O)] (1). Co(NO3)2·6H2O (0.0294 g, 0.1 mmol) and L (0.0181 g, 0.1 mmol) were dissolved in a mixed solvent of ethanol (4 mL), water (4 mL), and DMA (1 mL). After, the pH of the solution was adjusted to 5 with dilute nitric acid and stirred for about 2 h at room temperature to obtain a pink transparent solution. The solution was then added to a vessel, heated at 100 °C for 3 days, and cooled at room temperature. Some pink lumpy crystals suitable for X-ray diffraction analysis were obtained. Yield: 52% (based on Co(II)). Calcd. (%) for C8H8NO5Co: C, 37.46; H, 3.17; N, 5.52. Found (%): C, 37.35; H, 3.11; N, 5.45. IR (KBr, cm−1): 3425, 3143, 2927, 2851, 1615, 1567, 1434, 1378, 788. [ZnL(H2O)] (2). The synthetic procedure of complex 2 is similar to that of complex 1, except that the starting material is replaced by ZnSO4·7H2O (0.0284 g, 0.1 mmol). After stirring for 2 h at room temperature, colorless transparent solution was obtained and then heated at 120 °C for 2 days to give colorless bulk crystals. Yield: 41% (based on Zn (II)). Calcd. (%) for C8H8NO5Zn: C, 36.86; H, 3.05; N,

used the same molar amounts, 0.1 mmol, of ligand and metal in the synthesis process and dilute nitric acid to adjust the solution of pH to 5. More closely, 4 mL of ethanol, 4 mL of water, and 1 mL of DMA were used in the synthesis of complexes 1, 2, and 4. However, the four complexes were different at the reaction temperature and reaction vessel. The complexes 1 and 2 were obtained in a vessel at 100 and 120 °C, respectively, and the complexes 3 and 4 were carried out in a 25 mL Teflon-lined stainless steel autoclave and heated at 160 °C. The biggest difference is that the synthesis of complex 3 does not use DMA, 6532



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but only 3 mL of ethanol and 3 mL of water. After comparing with these series of differences, we found that the synthesis of complexes is affected by many factors. Only for the four complexes we synthesized, the different proportions of mixed solvents maybe play a main role in the various influencing factors. Figure S1 shows the PXRD spectra of complexes 1−4. All peaks appearing within the measurement range correspond to the simulated peaks generated from the single-crystal diffraction data, indicating that all complexes are pure phase. IR Spectra. Figure S2 is the infrared spectra of complexes 1−4. Since the ligands used in the four complexes are the same, the infrared peak is identified only by complex 1 as an example. For complex 1, the peak appearing at 3436 cm−1 is attributed to νO−H in water. The peak at 3143 cm−1 is designated as νC−H, while the peaks of ν−C−H are at 2927 and 2851 cm−1. Peaks of 1567 and 1387 cm−1 are stretching vibration characteristic peaks of νCO. The peaks present at 1624 and 1443 cm−1 can be attributed to stretching vibration of νCC. The peak at 788 cm−1 is seen as the vibration peak of δC−H, while for complex 3 there are 1682 and 1664 cm−1 stretching vibration peaks, showing all carboxylate groups from the ligand are deprotonation. This is consistent with the structure of complex 3, namely, including an uncoordinated and deprotonated carboxyl group in ligand L. Specific references to the peaks of the remaining three complexes are listed in Table 1.

Table 2. Attribution of UV−vis Spectra (nm) of Complexes 1−4 complexes LLCT (π−π*, n−π*) LMCT MMCT (d−d*)

1

2

3

4

νO−H νC−H ν−C−H νCO νCC δC−H

3436 3143 2927, 2851 1567, 1387 1624, 1443 788

3438 3010 2924, 2858 1566, 1377 1622, 1481 776

3431 3086 2922, 2856 1682, 1664 1536, 1396 1624, 1489 769

3442 3062 2929, 2853 1548, 1377 1624, 1471 780

338 550 4T1g(F) → 4 A2g(F) 733 4T1g(F) → 4 T1g(P) 1082 3A2g(F) → 3 T2g(F) 1215 4T1g(F) → 4 T2g(F)

2

3

217, 302 333

213, 306 361 646 3A2g(F) → 3 T1g(P) 830 3A2g(F) → 3 T1g(F)

4 211, 264 318

weight loss stage occurred at 350−800 °C, and the actual weight loss was 65.47%, corresponding to the partial collapse of the ligand L skeleton. For complex 3, the first weight loss stage occurred at 30−370 °C, and the actual weight loss was 13.88% (calcd. 14.66%), corresponding to the loss of two coordination water molecules and two free water molecules. The second weight loss stage occurred at 365−800 °C, and the actual weight loss was 58.15% (calcd. 58.95%), corresponding to the collapse of the ligand L skeleton, the final remaining material for the nickel chloride. For complex 4, the first weight loss stage occurred at 30−340 °C, and the actual weight loss was 10.27% (calcd. 10.99%), corresponding to the loss of one coordination water molecule and one free water molecule. The second weight loss stage occurred at 350−800 °C, the actual weight loss was 45.74% (calcd. 44.28%), corresponding to the collapse of the ligand L skeleton, the final remaining material for the cadmium oxide. Structural Descriptions of Complexes 1−4. [CoL(H2O)] (1). X-ray single-crystal diffraction data show that complex 1 is monoclinic with a P21/c space group. The molecular structure contains one Co(II) atom, one ligand L, and one coordinated water molecule. Each of the central metal Co and the four oxygen atoms (O1, O2, O3, O4) from the ligand L, a nitrogen atom (N1) from the ligand L, and an oxygen atom (O6) from the coordination water molecule form a six-coordinated distorted octahedral configuration (Figure 1a). The linking pattern of ligand L is μ4-ηO2ηO1ηO1ηN1, where O3 and O4 take the coordination mode of bidentate chelating, and O1, O2, and N1 take the modes of monodentate coordination (Figure 1b). The bond length of the Co−O bond is in the range of 1.9892(15)−2.2619(15) Å, and the bond length of Co−N bond is 2.2233(18) Å. This is similar to the bond length range of Co−O bonds (2.098(2)−2.211(2) Å) and Co−N bonds (2.186(2) Å) reported in the literature.38 The structural unit [Co2(μ-COO)2N2O6] in complex 1 is joined into a 1D chain structure by carbon atom (Figure 1c) and further through the four oxygen atoms on the carboxyl group in the ligand L to form a 2D layered structure (Figure 1d). It is worth noting that the 2D layered structure contains two different sizes of pores: the pore sizes are 7.05 (Figure 1e) and 14.67 Å (Figure 1f), respectively, wherein the pore structure shown in Figure 1e is formed by alternately connecting two L ligands and two metals Co atoms (Figure 1f) and is formed by connecting four L ligands and four metals Co atoms by interconnections. Finally, the 2D structure is connected by N atoms on the amino group in ligand L to form a 3D network structure (Figure 1g). In addition, complex 1 contains two hydrogen bonds: O6−H6A···O3#6 (#6, −x + 1,

Table 1. Attribution of IR (cm−1) for Complexes 1−4 complex

1 215, 307

UV−vis Spectra. Figure S3 is the UV−vis spectra of complexes 1−4. The transition bands from ligand to ligand (LLCT) of the four complexes are all between 211−307 nm. For complex 1, the band present at 338 nm belongs to the transition from ligand to metal (LMCT), while the metal-tometal d−d* transition (MMCT) has a total of the three bands, at 550, 733, and 1215 nm, respectively. Likewise, the bands appearing at 333, 361, and 318 nm attributed to the ligand-tometal transition (LMCT) of the complexes 2, 3, and 4, respectively. The bands at 646, 830, and 1082 nm belong to the metal-to-metal transition (MMCT) of complex 3. However, complexes 2 and 4 have no absorption band between 400 and 1500 nm, probably because the d orbital electrons of the central metal Zn(II) and Cd(II) are fully charged, and therefore, no d− d* transitions can occur. The detailed bands for the complexes 1−4 are listed in Table 2. Thermal Analysis. Figure S4 shows the TG curves of complexes 1−4. For complex 1, the first weight loss stage occurred at 30−180 °C, and the actual weight loss was 6.04% (calcd. 7.00%), corresponding to the loss of one coordination water molecule. The second weight loss stage occurred at 180− 800 °C, and the actual weight loss was 56.95%, corresponding to the partial collapse of the ligand L skeleton. For complex 2, the first weight loss stage occurred at 30−350 °C, and the actual weight loss was 7.84% (calcd. 6.83%), corresponding to the loss of one coordination water molecule. The second 6533



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Figure 1. Complex 1: (a) molecular structure; (b) linking pattern of ligand L; (c) 1D chain structure; (d) 2D layer structure; (e) small pores; (f) big pores; (g) 3D network structure.

−y + 1, −z + 2) and O6−H6B···O4#4 (#4, x, −y + 3/2, z − 1/ 2). These two hydrogen bonds play a role in the stability of the structure. The detailed hydrogen bonds are listed in Table 3.

[ZnL(H2O)] (2). X-ray single-crystal diffraction data show that complex 2 is monoclinic with a P21/n space group. The molecular structure contains one Zn(II) atom, one ligand L, and one coordinated water molecule. Each of the central metal Zn and two oxygen atoms (O2, O4) from the ligand L, one nitrogen atom (N1) from the ligand L, and one oxygen atom (O5) from the coordination water molecule coordinated to form a four-coordinated distorted tetrahedral configuration (Figure 2a). Ligand L takes the mode of monodentate coordination, and the linking pattern is μ3-ηO1ηO1ηN1 (Figure 2b). The bond length of the Zn−O bond is in the range of 1.9587(14)−1.9779(13) Å, and the bond length of the Zn−N bond is 2.0364(16) Å. This is alike to the bond length range of Zn−O bonds (1.954(3)−1.973(3) Å) and Zn−N bonds (2.031(4) Å) reported in the literature.38 The structure of complex 2 contains two types of hydrogen bonds: (i) N−H···O, N1−H1A···O2#5 (#5, −x + 1/2, y − 1/2, −z + 1/2), N1−H1B···O4#6 (#6, −x, −y + 1, −z + 1), N1− H1B···O1#3 (#3, x − 1/2, −y + 1/2, z + 1/2); (ii) O−H···O, O5−H5A···O3#7 (#7, −x + 3/2, y − 1/2, −z + 1/2), O5− H5C···O1#8 (#8, −x + 3/2, y + 1/2, −z + 1/2). The detailed hydrogen bonds are listed in Table 3. In addition, the structural unit [ZnNO3] of complex 2 is connected into a 1D chain structure (Figure 2c) through carbon atoms in the b-axis direction and further into a 2D layered structure by carboxylate group (Figure 2d). In the 2D layered structure, a pore with a size of 8.26 Å was formed by the three ligands L and the three metals Zn atoms (Figure 2e). The 2D structure is connected into a 3D supramolecular network structure by hydrogen bonding O5−H5C···O1#8 (#8, −x + 3/2, y + 1/2, −z + 1/2) (Figure 2(f). [Ni(HL)2(H2O)2]·2H2O (3). X-ray single-crystal diffraction data show that complex 3 is monoclinic with a C2/c space group. The molecular structure contains one Ni(II) atom, two ligands L, two coordinated water molecules, and two free water molecules. Each of the central metal Ni was coordinated by the two oxygen atoms (O3#2, O3#3) (#2, −x + 1/2, y − 1/2, −z +

Table 3. Hydrogen Bond Lengths (Å) and Bond Angles (deg) of Complexes 1−4a complex 1 D−H···A

d(D−H)

O6−H6A···O3#6 O6−H6B···O4#4

0.96 0.96

d(H···A) 2.02 1.82 complex 2

d(D−H)

D−H···A N(1)−H(1A)···O(2)#5 N(1)−H(1B)···O(4)#6 N(1)−H(1B)···O(1)#3 O(5)−H(5A)···O(3)#7 O(5)−H(5C)···O(1)#8

#6

N1−H1A···O1 N1−H1B···O1#7 O1W−H1WA···O4#5

#7

N(1)−H(1A)···O(1W) N(1)−H(1B)···O(1)#8 O(1W)−H(1WB)···O(4)#6 O(1W)−H(1WC)···O(2)#9

141.3 174.7

d(D···A)

∠(DHA)

2.977(2) 2.994(2) 3.137(2) 2.641(2) 2.656(2)

153.9 130.3 125.9 168.3 159.6

d(H···A)

d(D···A)

∠(DHA)

3.049(2) 3.020(3) 2.621(3)

158.8 170.4 146.6

0.90 0.90 0.85 complex

D−H···A

∠(DHA)

2.14 2.33 2.52 1.69 1.74 3

d(H···A)

0.90 0.90 0.90 0.96 0.96 complex d(D−H)

D−H···A

d(D···A) 2.838(2) 2.775(2)

2.19 2.13 1.87 4

d(D−H)

d(H···A)

d(D···A)

∠(DHA)

0.90 0.90 0.85 0.85

2.10 2.42 2.00 2.07

2.951(4) 3.070(4) 2.781(4) 2.786(3)

156.8 129.4 151.5 141.3

Symmetry codes. Complex 1: #4, x, −y + 3/2, z − 1/2; #6, −x + 1, −y + 1, −z + 2. Complex 2: #3, x − 1/2, −y + 1/2, z + 1/2; #5, −x + 1/2, y − 1/2, −z + 1/2; #6, −x, −y + 1, −z + 1; #7, −x + 3/2, y − 1/2, −z + 1/2; #8, −x + 3/2, y + 1/2, −z + 1/2. Complex 3: #5, x, y − 1,z; #6, x, −y + 1, z + 1/2; #7, −x + 1, −y + 1, −z. Complex 4: #6, −x + 1/ 2, y + 1/2, −z + 3/2; #7, x − 1/2, −y + 3/2, z − 1/2; #8, −x,−y + 1, −z + 1; #9, −x + 1, −y + 1, −z + 1. a

Figure 2. Complex 2: (a) molecular structure; (b) linking pattern of ligand L; (c) 1D chain structure; (d) 2D layer structure; (e) pore structure; (f) 3D supramolecular network structure (#8, −x + 3/2, y + 1/2, −z + 1/2). 6534



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Figure 3. Complex 3: (a) molecular structure; (#1, −x + 1/2, −y + 1/2, −z; #2, −x + 1/2, y − 1/2, −z + 1/2; #3, x, −y + 1, z − 1/2); (b) linking pattern of ligand L; (c) 1D chain structure; (d) 2D layer structure; (e) pore structure; (f) 3D network structure.

Figure 4. Complex 4: (a) molecular structure; (#2, x + 1, y, z; #3, −x, −y, −z + 1); (b) linking pattern of ligand L; (c) 1D chain structure; (d) 2D layer structure; (e) pore structure; (f) 3D network structure.

Table 4. Structural Comparison of the Complexes 1−4

1/2; #3, x, −y + 1, z − 1/2) from the ligand L, two nitrogen atoms (N1, N1#1) (#1, −x + 1/2, −y + 1/2, −z) from the ligand L, and two oxygen atoms (O5, O5#1) (#1, −x + 1/2, −y + 1/2, −z) from the coordination water molecule to form a sixcoordinated distorted octahedral configuration (Figure 3a). Ligand L takes the coordination mode of monodentate, and the

linking pattern is μ2-ηO1ηN1 (Figure 3b). The bond length of the Ni−O bond is in the range of 2.0618(17)−2.0776(15) Å, and the bond length of the Ni−N bond is 2.1506(18) Å. This close to the bond length range of Ni−O bonds (2.030(3)− 2.156(3) Å) and Ni−N bonds (2.117(4) Å) reported in the literature.38 6535



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Figure 5. Photocatalytic degradation of phenol with different concentration: (a) 15 mg/L, (b) 25 mg/L, (c) 30 mg/L. (d) Comparison of degradation efficiency.

The structural unit [NiN2O4] of complex 3 was linked by an infinitely extending 1D chain structure of carbon atoms (Figure 3c). It is then connected into a 2D network structure by the nitrogen atom in the ligand L (Figure 3d). In the structure, it is found that eight ligands and eight metal Ni atoms successfully were connected to form a pore structure with the size of 11.82 Å (Figure 3e). Finally, the 2D network structure is further connected to a 3D network structure via nitrogen atoms (Figure 3f). In addition, complex 3 contains two types of hydrogen bonds: (i) N−H···O: N1−H1A···O1#6 (#6, −x, −y + 1, z + 1/2), N1−H1B···O1#7 (#7, −x + 1, −y + 1, −z); (ii) O− H···O: O1W−H1WA···O4#5 (#5, x, y − 1, z). Likewise, the hydrogen bonds contained in complex 3 act as the furthermore stabilizing structure. Detailed hydrogen bonds are listed in Table 3. [CdL(H2O)]·H2O (4). X-ray single-crystal diffraction data show that complex 4 is monoclinic with a P21/n space group. The molecular structure contains one Cd(II) atom, one ligand L, one coordinated water molecule, and one free water molecule. Each of the central metal Cd and the four oxygen atoms (O2, O3#2, O3#3, O4) (#2, x + 1, y, z; #3, −x, −y, −z + 1) from ligand L, one nitrogen atom (N1) from ligand L, and one oxygen atom (O5) from the coordination water molecule form a six-coordinated distorted octahedral configuration (Figure 4a). The linking pattern of the ligand L is μ4ηO2ηO1ηO1ηN1, where O3 and O4 take the coordination mode of mono/bidentate chelating, and O2 and N1 take the mode of monodentate coordination (Figure 4b). The bond length of the Cd−O is in the range of 2.230(2)−2.589(3) Å, and the bond length of Cd−N is 2.254(3) Å. This is close to the bond length range of Cd−O bonds (2.272(6)−2.508(7) Å) and Cd−N bonds (2.319(7) Å) reported in the literature.39

The structural unit [CdNO5] of complex 4 forms a 1D chain structure (Figure 4c) with the carbon atom of the ligand and is further connected into a 2D layered structure by the nitrogen atom of the ligand L (Figure 4d). Finally, the 3D network structure is formed by the carboxyl oxygen atom from the ligand (Figure 4f). In the 2D structure, three ligands and three metals are alternately connected to form 10.26 Å porous network structures (Figure 4e). In addition, complex 4 contains two types of hydrogen bonds: (i) N−H···O. N1−H1A···O1W#7 (#7, x − 1/2, −y + 3/2, z − 1/2), N1−H1B···O1#8 (#8, −x, −y + 1, −z + 1); (ii) O−H···O, O1W−H1WB···O4#6 (#6, −x + 1/ 2, y + 1/2, −z + 3/2), O1W−H1WC···O2#9 (#9, −x + 1, −y + 1, −z + 1). Detailed hydrogen bonds in complex 4 are listed in Table 3. Comparison of the Structures. The structural comparison of the complexes 1−4 is shown in Table 4. Among them, complex 2 is a four-coordinated distorted tetrahedron configuration and, by hydrogen bonds O5−H5C···O1#8 (#8, −x + 3/2, y + 1/2, −z + 1/2), formed a 3D supramolecular porous structure. While the complexes 1, 3, and 4 are sixcoordinated distorted octahedral structures, the 3D structure is finally formed by a covalent bond. Therefore, in these complexes, although there are two types of hydrogen bonds, only one played a role in the stability of the structure. In addition, it is found that coordination modes of the carboxyl oxygen atoms in the ligand L are different, resulting in a distinct packing structure difference. The bond length (M−N) formed by the central metal and N atom in the complex 4 is slightly longer than the bond length in complexes 1 and 3, and the bond lengths in 1, 3, and 4 are 2.224(2), 2.151 (2),and 2.254(3) Å, respectively; that is, the bond length order is 4 > 1 > 3. The reason for this rule is likely due to that the central 6536



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Figure 6. Photocatalytic degradation of phenol solution at a concentration of 30 mg/L: (a) 1, (b) 3, and (c) 4. (d) Comparison of degradation efficiency.

obviously different, and the degradation effect of the three concentrations was 30 mg/L > 25 mg/L > 15 mg/L. We speculate that this phenomenon may be due to the higher the concentration of phenol, the more the number of phenol molecules in the solution. So the pore structure of the complex that can enter more phenol molecules, the better the degradation performance. In order to compare with the properties of the photodegrading phenol about complexes 1−4, we referred to the experimental results of the photocatalytic degradation of complex 2 and selected the optimum phenol concentration for the degradation effect (30 mg/L) to carry out comparative experiments. Figure 6a−c shows the degradation effects of complexes 1, 3, and 4 on 30 mg/L phenol solution under the same conditions. From the figure we can clearly see that the absorbance of the solution is decreasing with the increase of time, which shows that the three complexes do have a certain degree of degradation effect. Figure 6d is a comparison of degradation efficiency of complexes 1−4 to phenol solution. We plot the line with t (min) as the abscissa and (A0 − At)/A0 as the ordinate. (A0 is the initial absorbance of the phenol solution; At is the absorbance of the phenol solution at time t; t is the time of the reaction.) The degradation effects of the four complexes were found to be significantly different. The degradation order was 2 > 4 > 3 > 1. Among them, the degradation of complexes 2 and 4 was similar, which were rapidly degraded by phenol after 80 min. For this phenomenon, we consider the following three points: the first point, since the central metals Zn and Cd lose two electrons to form divalent ions, the 3d orbit is filled with 10 electrons, and this fulfilled state makes the structure of the

metal radius is different. In other words, the larger the radius of the center metal, the longer the bond length that forms the coordination with the N atom. In the three complexes, the central metal radius is 4 > 1 > 3, so the bond length should be that 4 is the longer, and 3 is the shorter. Photocatalytic Degradation of Phenol. Phenol is a common organic chemical material, widely used in a variety of chemical industries. However, due to excessive phenol being a serious impact on the water environment, air quality, and crops, so degradation of phenol has become the top priority of current work. In this work, since the ligands used in complexes 1−4 are identical and the structure composition of the complexes are close, we only study the degradation of phenol in detail with complex 2 as an example. The research methods and experimental data on photocatalytic degradation are given in the Supporting Information. Figure 5a−c shows the degradation effects of the phenol solution at a concentration of 15, 25, and 30 mg/L using a 4-aminoantipyrine colorimetric method40 under UV irradiation. We can see from the figure, at 505 nm wavelength, with the increase of UV light irradiation time, the absorbance gradually decreases; in the other words, the amount of phenol in the solution decreases gradually with the increase of time, indicating that complex 2 does have a certain degradation effect on phenol with different concentrations. Figure 5d is a comparison of degradation efficiency of the three different concentrations of phenol solution. When adding the same amount of catalyst and the light time being less than 80 min, we found that phenol was slightly and slowly degraded in the three concentrations of phenol solution. After 80 min, the degradation of phenol was rapid, and the degradation effect of phenol solution with different initial concentrations was 6537



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Table 5. Dynamics Parameters under Different Conditions first-order dynamics equation

second-order dynamics equation

T (K)

ρ0 (mg/L)

qe exp (mg/L)

qe (mg/g)

k1 (min−1)

R2

qe (mg/g)

k2 (g/mg/min)

R2

298

15 25 30

4.331 6.173 6.618

24.011 43.965 51.892

0.003 0.035 0.051

0.9342 0.942 0.803

4.255 6.140 6.519

0.0712 0.0187 0.0096

0.9954 0.9952 0.9953

adsorption is balanced; qt (mg/g) is the adsorption capacity of complex 2 for phenol at time t; k1 (min−1) is the first-order adsorption rate constant. We used this equation to linearly fit the phenol solution of three different concentrations with ln(qe − qt) as the ordinate and t as the abscissa (Figure S6a−c). The k1 and qe are obtained by the slope and intercept of the line (Table 5). The pseudo-second-order dynamics model uses the H0-McKay equation:43 t/qt = 1/k2qe2 + t/qe; k2 (g/mg/min) is the second-order adsorption rate constant. We use this equation to linearly fit the phenol solution of three different concentrations with t/qt as the ordinate and t as the abscissa (Figure S6d−f)). The qe and k2 are obtained by the slope and intercept of the line (Table 5). From Table 5, we find that the fitting degree of the pseudosecond-order dynamics equation is relatively high for the three different concentrations of phenol solution, and the correlation coefficient is more than 0.995. This indicates that the pseudosecond-order dynamics model can correctly reflect all the processes of photocatalytic degradation of phenol and reveals that the rate of photocatalytic degradation is controlled by the extra-particle diffusion process such as surface adsorption. At the same time, when we use the pseudo-second-order dynamics model to fit the data, the theoretical adsorption capacity qe and the experimental equilibrium adsorption capacity qe,exp are closer. We also found that the value of qe increases with increasing phenol concentration. When the first-order dynamics model is used to fit the data, the theoretical adsorption capacity qe and the experimental equilibrium adsorption capacity qe,exp are very different, which further shows that the pseudo-secondorder dynamics model can more accurately describe the dynamics process of photocatalytic degradation of phenol by complex 2. We also carried out the experiments by HPLC. In the work, acetonitrile−water (50:50, V/V) was used as the mobile phase.44 After the sample was filtered through a 0.20 μm filter, the phenol, catechol, resorcinol, and hydroquinone were quantitatively analyzed by internal standard method and external standard method in which the internal standard was p-phenol. (Figure S7). The detailed experimental conditions for using an HPLC instrument are given in the Supporting Information. Then we quantitatively analyzed the liquid samples during the experiment (Figure S8). By comparison, we found that the phenol content in the solution was gradually reduced with the increase of UV light irradiation time, and the reduced phenol content was partially converted to diphenols. Stability and Reusability of Complex 2. Compared to other porous materials, MOF material has the advantage of pore regulation and can be controlled. However, at the same time, the stability is relatively poor. Therefore, the study of the stability of the complex is essential. Complex 2 selected in this work is stable at below 350 °C, and its stability in water is also good. By comparing the PXRD pattern (Figure S9), it was found that the main diffraction peak of complex 2 after adsorbing phenol was consistent with the peak value without adsorption of phenol. This proved that the skeleton structure of

complex more stable, thereby exhibiting more excellent performance in the process of photocatalytic degradation of phenol. Second point, we found that the solubility of the four complexes was obviously different. Complexes 2 and 4 are insoluble in the aqueous solution, and complex 3 is partially dissolved, whereas complex 1 is almost completely dissolved. Therefore, we deem that complexes 2 and 4 in the aqueous solution is relatively stable and that the skeleton of the structures has not been destroyed. In contrast, complex 1 is more unstable in polar solvent and easily soluble in water, maybe leading to changes in its skeleton structure, which in turn affects its photocatalytic degradation performance. Third point, the sizes of the pores formed by the different metals and ligands in complexes 1−4 are different. The sizes are 14.67, 8.26, 11.84, and 10.26 Å, respectively. Here, we believe that the smaller the size of the pore, the more able to grasp the phenol molecules; that is, the stronger the ability of phenol degradation. On the contrary, the larger the size of the pore, the more likely the phenol molecule will pass the pores, so that the degradation ability of the complex to the phenol molecule is weakened. Therefore, the degradation order was 2 > 4 > 3 > 1. Photocatalytic Degradation Mechanism. For the analysis and detection method we used in the experiment, the specific procedure was to use a buffer solution to control the pH of the phenol to 10 and to react with 4-aminoantipyrine in the presence of potassium ferricyanide to form orange-red antipyrine dyes, and the absorbance of the solution was measured at a wavelength of 505 nm. Then, the standard curve equation of phenol was obtained by the least-squares method, and the adsorption capacity (qe) of phenol was calculated by the formula 1.41 qe = (ρ0 − ρe )V /1000m

(1)

where qe (mg/g) is the adsorption capacity of the adsorbent to phenol; V (mL) is the volume of the phenol solution; ρ0 (mg/ L) is the initial concentration of phenol; ρe (mg/L) is the concentration of phenol after adsorption; and m (g) is the mass of the adsorbent. Based on the above experimental phenomena and theoretical basis, the reaction mechanism was further studied by using a dynamic model with complex 2 as an example. First of all, to a solution of different concentrations of phenol was added 0.5 mL of a buffer solution of pH = 10, 1.0 mL of potassium ferricyanide at a mass fraction of 8%, and 1.0 mL of a 4aminoantipyrine solution having a mass fraction of 2%. After mixing, the absorbance was measured at a wavelength of 505 nm (Table S3). The standard curve of the phenol is shown in Figure S5. The standard curve equation is ρ = 35.71035A − 0.03696 (ρ is the concentration of phenol, A is absorbance of a certain concentration), correlation coefficient: R2 = 0.99816. Then we use the pseudo-first-order dynamic model and the pseudo-second-order dynamic model to carry on the dynamic analysis. The pseudo-first-order dynamics model uses the Lagergren equation:42 ln(qe − qt) = ln qe − k1t; qe (mg/g) is the adsorption capacity of the complex 2 to phenol when the 6538



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Notes

complex 2 is not destroyed after adsorption of phenol, and its stability is good. In order to further verify the repeatability of complex 2, we used methanol to soak repeatedly so that the adsorbed phenol was desorbed as much as possible. The phenol solution was redegraded using complex 2 after repeated soaking under UV irradiation. After repeating experiments three times, we found that complex 2 can still be a good degradation of the phenol solution. The difference between the degradation rates of the regeneration cycle (three times) was very small. The degradation efficiency of the regeneration cycle is shown in Figure S10. Indicating that complex 2 has a good reusability.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the grants of the National Natural Science Foundation of China (Nos. 21571091, 21371086) and Guangxi Key Laboratory of Information Materials, Guilin University of Electronic Technology, P. R. China (Project No. 151002-K) for financial assistance.

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CONCLUSIONS In summary, we have successfully synthesized four complexes, [CoL(H2O)] (1), [ZnL(H2O)] (2), [Ni(HL)2(H2O)2]·2H2O (3), and [CdL(H2O)]·H2O (4), under the hydrothermal conditions by using transition metal salts and 5-aminoisophthalic acid (L). Through X-ray single-crystal diffraction analysis, we showed that the four complexes were 3D network structures. For photocatalytic degradation of phenolic properties, the four complexes have different degrees of degradation ability. At the same time, the dynamics analysis shows that the degradation process agrees with the pseudo-second-order dynamics model, and the correlation constant is as high as 0.995, which indicates that the rate of adsorption is controlled by surface adsorption. The most significant is that the adsorption capacity of complex 2 can reach 6.519 mg/g when the phenol concentration is 30 mg/L. Therefore, these complexes may have a potential effect on our degradation of phenol contaminants in wastewater.

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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.cgd.7b01190. Experimental materials, experimental details, research methods and experimental data on photocatalytic degradation, HPLC instrument, crystallographic data for complexes 1−4, absorbance values of different concentrations of phenol solution, PXRD analysis, IR spectra, UV−vis spectra, and thermal analysis of complexes 1−4, standard curve of phenol, dynamics model, HPLC of phenol and diphenol mixtures, HPLC in the course of experiment, comparison of PXRD before and after adsorption, and degradation efficiency of the regeneration cycle (PDF) Accession Codes

CCDC 1560797−1560800 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yong-Heng Xing: 0000-0002-7550-2262 6539



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