Syntheses of Novel Lanthanide Metal–Organic Frameworks for Highly

Jilin Province Lin-Chang Environmental Technology Service Co. Ltd., Changchun 130012, China. Cryst. Growth Des. , 2017, 17 (8), pp 4189–4195. DOI: 1...
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Syntheses of Novel Lanthanide Metal−Organic Frameworks for Highly Efficient Visible-Light-Driven Dye Degradation Qiansu Xia,†,# Xiaodan Yu,†,# Hongmei Zhao,† Siping Wang,† Hong Wang,‡ Zhifen Guo,† and Hongzhu Xing*,† †

Provincial Key Laboratory of Advanced Energy Materials, College of Chemistry, Northeast Normal University, Changchun 130024, China ‡ Jilin Province Lin-Chang Environmental Technology Service Co. Ltd., Changchun 130012, China S Supporting Information *

ABSTRACT: Series of lanthanide metal−organic frameworks (Ln-MOFs) have been solvothermally synthesized using an anthracene-derived dicarboxylate ligand. The synthesized Ln-MOFs (Ce, Tb, Dy) show characteristics of broad-band visible-light absorption and efficient photoinduced charge generation. For the first time, Ln-MOFs have been employed as visible-light photocatalysts for rhodamine B (RhB) degradation in the presence of oxidant of hydrogen peroxide (H2O2). Results show that these Ln-MOFs are highly efficient for visible-light-driven RhB degradation in aqueous solution. The degradation reactions show notable reaction rate constants. It is interesting to observe that the premixing between Ln-MOFs and H2O2 is vital to improve photocatalytic performance, indicating the advantage of porous MOF catalysts. This study not only presents rare examples of visible-light-driven MOF photocatalysts constructed from lanthanide ions, but also reveals that the H2O2-involved advanced oxidation process is feasible for enhancing photocatalytic performance of aqueous dye degradation.

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INTRODUCTION Recently, the utilization of metal−organic frameworks (MOFs) as catalysts to carry out photochemical reactions has been of extensive research interest considering the energy and environment concerns.1−3 Ever since MOF-5 was first examined to behave as a photocatalyst, plenty of MOFs with light absorption properties have been designed and synthesized based on the richness of metal-containing nodes and organic bridging linkers.4,5 Studies have demonstrated that some of these lightharvesting MOFs are efficient heterogeneous photocatalysts for CO2 reduction, water splitting, pollutants degradation, etc.6,7 In particular, MOF-mediated photocatalytic dye degradation has been widely studied, in which dozens of MOFs constructed from d-block metal ions have been utilized as photocatalysts under ultraviolet and/or visible light irradiation.8,9 It is fortunate that these MOFs show high dye degradation performance as compared to that of metal oxides and sulfides.10,11 Very recently, the H2O2-involved advanced oxidation process (AOP) has been applied for MOFs-mediated dye degradation, showing enhanced photocatalytic performances compared to those using solely MOFs.3,12−17 Actually, AOPs are increasingly adopted for the destruction of organic contaminants in water into less toxic ones, due to their high efficiency and easy handling.18−20 In general, the AOP involves in situ generation of highly reactive chemical oxidants to degrade persistent organic substances into less toxic molecules, even eventually mineralizing them into innocuous CO2 and water.21,22 For H2O2-involved AOPs using MOFs as photocatalysts, it is © 2017 American Chemical Society

believed the hydroxyl free radical resulting from H2O2 serves as highly reactive species to enhance the photocatalytic performance. However, the H2O2-involved dye degradation by MOFs is primarily concentrated on MOFs constructed from d-block metal ions. Meanwhile, the reported degradation efficiencies evidenced by apparent reaction rate constants are still limited. Thus, the rational synthesis of novel MOFs photocatalysts and the study of MOFs-mediated AOP for dyes degradation are currently necessary and meaningful. Lanthanide ions with high coordination number and versatile coordination modes are widely adopted for the construction of MOFs; however, lanthanide MOFs (Ln-MOFs) have seldom been investigated for photocatalytic degradation of dyes.23−25 An emerging study has recently been reported by Wang and coworkers, in which a layered Gd-MOF exhibits photocatalytic activity under ultraviolet light, representing a rare example of Ln-MOFs photocatalysts for dye degradation.3,26 In view of the utilization of sustainable solar energy, further development of visible-light-responsive Ln-MOFs for dyes degradation is promising. However, this has never been achieved so far. We report herein the rational synthesis of a series of visible-lightresponsive Ln-MOFs (Ce, Tb, Dy) constructed from an anthracene-incorporated organic ligand, which demonstrates highly efficient visible light photocatalysts for the degradation of rhodamine B (RhB) in the presence of H2O2. The Received: April 9, 2017 Revised: June 15, 2017 Published: July 7, 2017 4189

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to a pseudo-first-order reaction when Co is small: ln(Co/Ct) = kt, where k is the apparent first-order rate constant.

degradation reactions exhibit significant degradation rate constants, superior to those shown by other MOFs photocatalysts reported so far.

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RESULTS AND DISCUSSION An orange crystal of NNU-15(Ce) at a suitable size was selected for single crystal X-ray diffraction study. Data analysis reveals it crystallizes in the triclinic space group P1̅ with a formula of [Ce(H2L)1.5(H2O)(DMF)]·DMF. The structure contains a unique Ce(III) ion in a distorted square antiprism geometry coordinated by eight oxygen atoms from six anthracene-derived organic ligands, one DMF, and one water molecule (Figure 1a). The Ce−O bond lengths are in the range

EXPERIMENTAL SECTION

Materials and Methods. All chemicals except the organic ligand were purchased from commercial suppliers in analytical grade and used without further purification. The anthracene-based organic ligand H2L was synthesized according to a reported procedure.27 The powder Xray diffraction (PXRD) experiment was carried out on Rigaku D-MAX 2550 (λ = 0.15417 nm) with 2θ ranging from 3° to 40° under ambient conditions. Thermogravimetric analysis (TGA) was measured with a PerkinElmer TG-7 analyzer at a heating rate of 10 °C min−1 from room temperature to 800 °C under air atmosphere. The Fourier transform infrared (FTIR) spectrum was recorded using KBr pellets on a Mattson Alpha-Centauri spectrometer over range of 400−4000 cm−1. The UV−vis spectrum for the solid state sample was obtained on a HITACHI U-4100 spectrophotometer. The surface photovoltage (SPV) spectrum was recorded using powder samples in a sandwich cell (ITO/sample/cathode) with the light source-monochromator lock-in detection technique ranging from 300 to 800 nm. Synthesis of NNU-15(Ce). Typically Ce(NO3)3·4H2O (4 mg, 0.0137 mmol) and H2L (5 mg, 0.0107 mmol) was first dissolved in DMF (3 mL) and H2O (0.5 mL) under stirring, and then 1 M HNO3 (0.1 mL)was added. The mixture was then transferred into a 20 mL vial and placed at 85 °C for 3 days. Orange crystals of NNU-15(Ce) were recovered by filtration and washed with DMF twice and methanol once. The yield is ca. 73.5% based on organic ligand. The bulk sample dried at ambient condition was used for further characterizations and photocatalytic studies. Synthesis of NNU-15(Tb). A mixture of Tb(NO3)·4H2O (4 mg, 0.0096 mmol) and H2L (5 mg, 0.0107 mmol) in DMF (3 mL) and H2O (0.5 mL) was added with 1 M HNO3 (0.1 mL). Then the solution was transferred to glass vial and reacted at 85 °C for 3 days. Orange crystals of NNU-15(Tb) were recovered by filtration and washed with DMF and methanol. The yield is estimated to be 55.4% based on ligand. Synthesis of NNU-15(Dy). The solvothermal reaction is very similar to that of NNU-15(Ce, Tb), except that Dy(NO3)3·4H2O (4 mg, 0.0095 mmol) was used in the reaction. After 0.1 mL of HNO3 (1 M) was added, the mixture was transferred to a glass vial and reacted at 85 °C for 3 days. The yield is about 63.8% based on the ligand. Single-Crystal X-ray Diffraction. Single crystal X-ray diffraction data for NNU-15(Ce, Tb, Dy) were collected on a Bruker Smart Apex II CCD diffractometer with graphite-monochromated using Mo−Kα radiation (λ = 0.71073 Å). Data processing was accomplished with the SAINT program. Absorption corrections were applied by using the multiscan program SADABS. The crystal structures were solved by the direct method and refined by full-matrix least-squares on F2 with anisotropic displacement using SHELXTL.27 The lanthanide ions were easily located, and then non-hydrogen atoms (N, O, and C) were placed from the Fourier-difference maps. All the hydrogen atoms were refined theoretically using the riding mode. The X-ray crystallographic coordinates of NNU-15(Ce, Tb, Dy) have been deposited at the Cambridge Crystallographic Data Centre (CCDC) under deposition numbers of CCDC 1505971−1505973. The details of the data collection and the refinement are reported in Table S1. Photocatalytic Degradation. Photocatalytic degradation of RhB by Ln-MOFs NNU-15(Ce, Tb, Dy) was investigated under visible light irradiation using a 500 W Xe arc lamp with a 420 nm cutoff filter as the light source. Typically 30 mg of photocatalyst of NNU-15 and 50 mL of RhB aqueous solution (10 ppm) and a fixed amount of H2O2 were mixed in a 250 mL quartz reactor in the dark for 2 h to achieve an adsorption−desorption equilibrium, followed by visible light irradiation. Then 1.5 mL of reaction suspension was taken out at different time intervals and treated by centrifugation. The concentration of RhB in the supernatant solution was determined at its maximum absorption wavelength centered at 554 nm using a UV−vis spectrophotometer. The photocatalytic degradation can be attributed

Figure 1. (a) Coordination environment of Ce(III) ion in NNU15(Ce); (b) the infinite chain consists of cerium ions and carboxylate groups; (c) and (d) views of the framework structure along different directions. The cerium ions are shown in polyhedral mode, and the red, purple, and black spheres represent O, N, and C atoms, respectively. All hydrogen atoms and guest molecules are omitted for clarity.

of 2.4379(2)−2.5840(2) Å, which are comparable to those of other cerium MOFs.28,29 The adjacent Ce(III) ions are bridged by carboxylate groups to build one-dimensional ceriumcarboxylate chain along the a axis (Figure 1b), in which the Ce···Ce distances are about 4.363 and 5.486 Å. These ceriumcarboxylate chains are further interconnected to each other by organic ligands to construct a three-dimensional structure (Figure 1c,d). It is notable that the organic ligand shows unexpected bending geometry, which is significantly different from the original planar configuration. NNU-15(Ce) possesses a neutral framework in which guest molecules of DMF are located in solvent accessible free volume nearby the metallic chains (Figure S1a, Supporting Information). NNU-15(Tb) with a formula of [Tb(H2L)1.5(H2O)2(DMF)]·2DMF (Figure 2) shows an isomorphic framework with a reported Eu-MOF [Eu(L)1.5(H2O)3]·(H2O)·(DMF)2.30 Each Tb(III) ion in NNU-15(Tb) is coordinated by eight oxygen atoms from five organic ligands, two water molecules, and one DMF. The Tb−O bond lengths are in the range of 2.3056(2)−2.515(2) Å, which are consistent with other reported terbium compounds.31,32 As for NNU-15(Dy) [Dy(H2L)1.5(H2O)2(DMF)]·2DMF, it shows very similar cell dimensions with Tb-MOF (Table S1) and is isostructural with NNU-15(Tb) (Figure 3). The Dy−O bond lengths in the structure are from 2.3056(12) to 2.515(2) Å, corresponding to 4190

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face distances are measured to be ca. 3.325 and 3.487 Å. In contrast, the π−π interaction between ligands in Tb- and DyMOF occurs in pairs where the face-to-face distance is ca. 3.305 and 3.338 Å, respectively (Figure S3). The bulk phase of these Ln-MOFs was examined by PXRD measurements where the experimental patterns fit well with those simulated from crystal structures (Figures S4−S6), indicating the as-prepared material is in good phase purity. Fourier transform infrared (FTIR) measurements show that they possess very similar spectra (Figure S7). The strong peaks at ca. 1576, 1509, and 1389 cm−1 are due to vibrations of carboxylate groups in ligand, and the peak at ca. 2178 cm−1 is attributed to the stretching vibration of ethynyl groups in organic ligand. The peaks at ca. 861 and 774 cm−1 can be ascribed to the out-of-plane bending vibration of the C−H bond in benzene groups of ligand. The intense peak at ca. 1663 cm−1 (vC=O) suggests the presence of DMF molecules.38 In addition, several weak peaks around 3000 cm−1 (3054, 2973, 2934 cm−1) are associated with the vibrations of C−H bonds in the DMF molecule.36 The characteristic vibration of coordinated H2O molecule is observed at ca. 3375 cm−1. The thermal weight loss of these compounds is shown in Figure S8. TGA curve of NNU-15(Ce) shows a two-step weight loss of ca. 16.3 wt % before 330 °C, which is attributed to the removal of both DMF and water molecules (calcd. 16.4 wt %) in the structure. The following weight loss of 66.7 wt % corresponds to the decomposition of organic ligand (calcd. 66.1 wt %). As for NNU-15(Tb) and NNU-15(Dy), they both exhibit multiple step weight loss (22.8 wt % for Tb-MOF, calcd. 23.0 wt %; 22.5 wt % for Dy-MOF, calcd. 22.8 wt %) before 400 °C due to the removal of DMF and water molecules. And the following sharp loss of about 60 wt % is ascribed to the decomposition of organic ligand (calcd. 59.5 wt % for both MOFs). The optical absorbance of these Ln-MOFs was first measured by UV−vis spectrometer. As shown in Figure 4a, they exhibit obvious broad-range visible light absorption due to the presence of highly conjugated anthracene-based ligand in the structures.36,37 The band gap energy of these Ln-MOFs is then calculated according to the energy dependence relation of αhν = (hν − Eg)1/2, where α and Eg are the absorption coefficient and the energy gap. The plot of (αhν)2 versus photon energy (hν) is fitted linearly (Figure S9), and the band gap energies of Ce-, Tb- and Dy-MOF are estimated to be ∼2.11, 2.20, and 2.15 eV, respectively. Then the photoinduced surface charge generation of these MOFs upon visible light illumination was investigated by SPV measurements. The SPV technique is a nondestructive approach to study heterogeneous photocatalysts by monitoring the surface voltage resulting from light-induced charge generation.39 As shown in Figure 4b, all MOFs display obvious SPV response with peaks at 468 and 480 nm in the visible-light region, indicating efficient photoinduced surface charge generation. These results suggest the synthesized LnMOFs might serve as photocatalysts to carry out visible-lightdriven photochemical reactions. A control experiment on the ligand (Figure 4b) suggests the SPV response of NNU-15(Ce, Tb, Dy) results from the visible-light-responsive organic ligand. First, the capability of NNU-15(Ce) to degrade organic pollutant of RhB in aqueous solution under visible light was investigated. Several control experiments were designed to clarify the nature of photocatalytic reaction over NNU-15(Ce) (Figure 5). The preadsorption experiment suggests that the RhB molecule could not be uploaded by Ce-MOF (Figure

Figure 2. (a) Coordination environment of the Tb(III) ion in NNU15(Tb); (b) the infinite chain consists of terbium ions and carboxylate groups; (c) and (d) views of the framework structure along different directions. The terbium ions are shown as pink polyhedra, and the red, purple, and black spheres represent O, N, and C atoms, respectively. All hydrogen atoms and guest molecules are omitted for clarity.

Figure 3. Views of framework structure of NNU-15(Dy). The dysprosium ions are shown as blue polyhedra, and the red, purple, and black spheres represent O, N, and C atoms, respectively. All hydrogen atoms and guest molecules are omitted for clarity.

the typical values in other dysprosium MOFs.33,34 The solvent accessible void spaces inside both Tb-/Dy-MOFs are occupied by DMF molecules (Figure S1). It is interesting to find that NNU-15(Ce) shows distinct structural characteristics from NNU-15(Tb, Dy), including the bending geometry of organic ligand and the different coordination modes (Figure S2) between ligand and lanthanide ion. We propose that “lanthanide contraction” may play an important role to induce the structural difference among these MOFs. In order to adapt the same framework structure, the molecular geometry of the ligand would change seriously when the radius of the metal ion is larger, as is the case in NNU15(Ce). It is notable that such a significant geometrical change of the H2L ligand has scarcely been reported.35−37 As for terbium and dysprosium ions with a quite close ion radius, they present an almost identical framework structure. Besides, the noncovalent interactions between organic ligands in these LnMOFs are different. In NNU-15(Ce), long-range π−π stacking interactions are observed owing to the aforementioned geometrical change of the ligand (Figure S3). The face-to4191

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Figure 5. (a) Photocatalytic RhB degradation using NNU-15(Ce) under different conditions, including NNU-15(Ce)/RhB/visible light (curve a), and RhB/H2O2/visible light (curve b), and NNU-15(Ce)/ H2O2/RhB/visible light (curve c); (b) time-dependent absorption for the system of NNU-15(Ce)/H2O2/RhB/visible light. For all reactions: NNU-15(Ce), 30 mg; H2O2, 0.6 M.

Figure 4. (a) Solid state UV−vis spectra of NNU-15(Ce, Tb, Dy); (b) SPV spectra of NNU-15(Ce, Tb, Dy) (solid lines) and organic ligand (dash line).

S10a), and the photobleaching experiment shows the dye of RhB is rather stable under visible light irradiation (Figure S10b). In addition, NNU-15(Ce) exhibited limited activity for RhB degradation under visible light, which suggests that the direct degradation of RhB by NNU-15(Ce) is not energetic favorable. These results promote us to investigate the H2O2assisted photocatalytic reactions. As mentioned before, it is expected that the addition of H2O2 would improve the photocatalytic performance by producing highly reactive intermediate of hydroxyl free radical.3,18,40 Before H2O2-assited photocatalytic study, the stability of NNU-15(Ce) in H2O2/H2O solution was investigated. The PXRD patterns of NNU-15(Ce) soaked in the solution at different days match well with each other (Figure S11), suggesting the MOF is stable in the solution. The photocatalytic degradation of RhB in the presence of H2O2 was performed where H2O2 was added during the preabsorption process in the dark. As shown in Figure 5, the performance of visible-light-driven RhB degradation on NNU-15(Ce) is significantly improved when H2O2 was introduced. About 99% RhB molecules in the solution (10 ppm, 50 mL) is degraded within 12 min. A control experiment on H2O2 itself shows ca. 13% RhB could be degraded during the same time. These results suggest that NNU-15(Ce) is highly efficient for RhB degradation in the presence of H2O2. Then the influence of NNU-15(Ce) and H2O2 on RhB degradation was evaluated. As shown in Figure 6a, RhB molecules in the reaction could be almost fully degraded within ca. 20 min by 20 mg of NNU-15(Ce) when H2O2 is fixed at 0.6

M. By increasing the dosage of NNU-15(Ce) to 30 mg, the photocatalytic degradation of RhB would be accomplished at ca. 10 min. It is obvious that the dosage of NNU-15(Ce) significantly affects degradation performance. Further increase of NNU-15(Ce) to 50 mg seems inoperative to improve the reaction efficiency, where it shows almost the same apparent reaction rate constant with that when 30 mg of NNU-15(Ce) was adopted. Therefore, the following photocatalytic reactions were carried out in which 30 mg of NNU-15(Ce) was added. As shown in Figure 6b, the plots of C/Co are different from each other for reactions with different H2O2 concentrations. When H2O2 in the reaction increased from 0.2 to 0.6 M, the time needed for complete RhB degradation decreased from 40 to 12 min. Meanwhile, the corresponding reaction rate constants increased from 0.0757 to 0.2397 min−1. As shown in Table 1, these rate constants are remarkable among reactions of H2O2-assisted RhB degradation by MOFs. A further increase of H2O2 leads to a decrease of reaction rate constant, suggesting the optimization of the H2O2 dosage is important to achieve better photocatalytic performance. It is interesting to find that the adsorption−desorption equilibrium between NNU-15(Ce) and H2O2 is very important to achieve a high RhB degradation rate. As shown in Figure 7a (curve c), when the adsorption−desorption equilibrium between NNU-15(Ce) and H2O2 was first achieved by premixing them in the dark for 2 h, it takes only 12 min to fully degrade RhB molecules in the reaction, whereas the 4192

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molecule cannot be loaded by NNU-15(Ce) in the solution, which has been evidenced by the aforementioned control experiment. However, the H2O2 molecule, which is much smaller than the DMF molecule inside NNU-15(Ce), can be encapsulated into NNU-15(Ce) during the adsorption− desorption process by guest exchange with DMF. Hence the improved degradation performance shown by the H2O2premixing system is reasonable because the encapsulation of H2O2 into MOFs enables close contact between H2O2 and the organic ligand, which favors rapidly production of reactive hydroxyl free radicals, thereby improving the subsequent degradation reaction. As for other Ln-MOFs of NNU-15(Tb) and NNU-15(Dy), the H2O2-assisted photocatalytic degradation of aqueous RhB was also studied (Figures 8 and S12). The reactions were performed under the same conditions as NNU-15(Ce). In the presence of H2O2, about 95% RhB in the reaction could be degraded by NNU-15(Dy) at 25 min with a reaction rate constant of 0.1020 min−1. In contrast, NNU-15(Tb) exhibits a lower photocatalytic efficiency, where about 90% RhB was degraded at 60 min. The rate constant is estimated to be ca. 0.0404 min−1. To evaluate the reusability of the synthesized Ln-MOFs catalysts, recycling experiments were carried out. As shown in Figure S19, NNU-15(Ce) shows high photocatalytic activity for aqueous RhB degradation during cycled reactions, exhibiting a minor efficiency decrease after five cycles. NNU-15(Ce) after recycling experiments was easily collected by centrifugation. The PXRD study suggests it maintains the original crystal structure. The cycling use of NNU-15(Tb) and NNU-15(Dy) has also been investigated where they maintained the catalytic activity and kept their original crystal structure after reactions (Figures S5 and S6). The stability of these Ln-MOFs during the photocatalytic reactions was also investigated by an IR study. As shown in Figures S13−S15, the Ln-MOFs after a dye degradation reaction show very similar IR spectra with those before the reaction, suggesting also the stability of these MOFs. For all Ln-MOFs, the vibrations at ca. 3375 cm−1 are attributed to H2O increases after degradation reaction. This is reasonable as the degradation reactions were carried out in aqueous solution where DMF molecules in Ln-MOFs could be exchanged by H2O molecules. Besides, the good stability of these Ln-MOFs has also been proven by fluorescence spectra

Figure 6. (a) Photocatalytic degradation of RhB with various dosages of NNU-15(Ce), including 20 mg (curve a), 30 mg (curve b), and 50 mg (curve c); (b) RhB degradation under different H2O2 additions. Curves (a) to (d) correspond to reaction with 0.2, 0.4, 0.6, and 0.8 M H2O2, respectively.

reaction needs a much longer time of 60 min to degrade ca. 80% RhB if H2O2 was added just after visible light irradiation (curve a). In contrast, the experiment shows that the preaddition of RhB is not necessary. The reaction system of premixed NNU-15(Ce)/H2O2/RhB (curve b) shows an almost equal degradation performance with the premixed NNU15(Ce)/H2O2 system (curve c). This is because the RhB

Table 1. H2O2-Assisted Photocatalytic Degradation of Aqueous RhB by MOFs under Visible Light MOFsa

Co(RhB) (mg·L−1)

CMOF (mg·L−1)

efficiency (%)

T (min)

K (min−1)

ref

MIL-53(Fe) GR-MIL-53(Fe) MHMCs-MIL-53(Fe) Co3(BPT)2(bpp) CuII(salimcy) NTU-9 Cu(ptz)(I) Cu(ptz)(II) Fe2(bhbdh) NNU-15(Ce) NNU-15(Dy) NNU-15(Tb)

10 20 10 24 12 47.9 18.7 18.7 0.2 10 10 10

400 400 400 1000 600 500 800 800 166 600 600 600

98 100 98.7 90 95 100 100 70 90 99 95 90

40 60 70 120 50 80 35 35 15 12 24 60

0.0794 0.0777 0.0513 0.0192

12 40 13 14 15 16 41 41 17 this work this work this work

0.2397 0.1020 0.0404

a

BPT = biphenyl-3,4′,5-tricarboxylic acid; bpp = 1,3-bis(4-pyridyl)propane; GR = graphene; MHMCs = MIL-53(Fe) hybrid magnetic composites; salimcy = N,N-bis-[(imidazol-4-yl)methylene]cyclohexane-1,2-diamine; ptz = 5-(3-pyridyl)tetrazole; bhbdh = bis[2-hydroxybenzaldehyde] hydrazone. 4193

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Figure 7. (a) Photocatalytic reactions at different premixing processes. (curve a) premixed NNU-15(Ce)/RhB; (curve b) premixed NNU-15(Ce)/ H2O2; (curve c) premixed NNU-15(Ce)/H2O2/RhB; (b−d) Reaction solutions at different time intervals for these premixing reactions. (b) Premixed NNU-15(Ce)/RhB; (c) premixed NNU-15(Ce)/H2O2; (d) premixed NNU-15(Ce)/H2O2/RhB.

photocatalysts by making use of their porosity to improve H2O2-assisted dye degradation.

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CONCLUSIONS In summary, a series of lanthanide MOFs have been rationally synthesized using an anthracene-derived dicarboxylate ligand. These Ln-MOFs have broad-range visible light absorption and efficient photoinduced charge generation. H2O2-assisted dye degradation experiments suggest they are highly efficient photocatalysts for RhB degradation showing notable reaction rate constants. This study not only presents rare examples of MOF photocatalysts constructed from lanthanide ions, but also reveals the H2O2-involved advanced oxidation process is indeed efficient for dye degradation. Importantly, it is demonstrated that the preloading of H2O2 into Ln-MOFs is feasible to improve photocatalytic performance by rapidly producing reactive species of the hydroxyl free radical. It is expected that the work would provide insight for visible-light-driven dye degradation using MOF photocatalysts.

Figure 8. RhB degradation by NNU-15(Tb) and NNU-15(Dy) over a H2O2/Ln-MOF/RhB system.

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(Figures S16−S18), where Ln-MOFs before and after dye degradation show almost the same spectra. A trapping experiment was performed to obtain insight into the role of active species during the reaction. The scavenger of the hydroxyl free radical tert-butyl alcohol (TBA)42 was added into the H2O2-involved photocatalytic system. As shown in Figure S20, the addition of TBA dramatically quenches the reaction, leading to very little degradation of RhB. The significant inhibitory effect indicates the hydroxyl free radical contributes to the photocatalytic process.43 Hence a plausible mechanism for H2O2-involved photocatalytic degradation of RhB by these Ln-MOFs is proposed. Following charge separation of photoexcited electron−hole pairs on the lightharvesting organic ligand, the photoinduced electron reacts with the oxidant of H2O2 producing reactive species of the hydroxyl free radical to degrade the RhB molecule. In this process, the fast reaction between photoinduced electron and oxidant of H2O2 plays an important role to enhance the photocatalytic efficiency. This speculation has been proven by our experiments where the premixing process improves the photocatalytic performance. Meanwhile, the self-decomposition of H2O2 to produce hydroxyl free radical would also contribute the high photocatalytic performance in the degradation reactions.3 This finding reveals the advantage of MOF

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.7b00504. PXRD patterns, IR spectra, TGA curves, crystal data, and structure refinement (PDF) Accession Codes

CCDC 1505971−1505973 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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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hongzhu Xing: 0000-0001-7179-0394 Author Contributions #

Q.X. and X.Y. contributed equally.

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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work is financially supported by the National Natural Science Foundation of China (21473024), Natural Science Foundation of Jilin Province (20140101228JC), and Science and Technology Research Project (JJKH20170906KJ) from Jilin Province Education Department.

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DOI: 10.1021/acs.cgd.7b00504 Cryst. Growth Des. 2017, 17, 4189−4195