1 Introduction

In recent years, coordination polymers (CPs) have attracted much attention due to their diverse synthetic strategies [1, 2] and potential applications like photocatalysis [3, 4], sensing [5, 6], drug delivery [7, 8], pollutants adsorption [9, 10], separation [11, 12], gas storage [13, 14], catalysis [15,16,17,18,19] and so on [20,21,22,23,24,25]. Generally, some CPs constructed from carboxylate ligands are highly stable, and exhibit structural diversities and immense applications [26] such as UiO-66 [27] and MIL-53(Cr) [28] linked by 1,4-dicarboxybenzene. As well, d-block transition metals (e.g. ZnII, CdII) usually play significant contribution in diverse structures and topologies as well as unique roles like photocatalysts [26]. From this point of view, 1,3-dibenzyl-2-imidazolidone-4,5-dicarboxylic acid (H2L) was utilized as a ligand assisted with 1,3-bis(4-pyridyl)propane (bpp), and ZnII/CdII were used as central metals to construct coordination polymers, and their photocatalytic performances were studied. Being compared to TiO2, CdS and other semiconductor photocatalysts, coordination polymers have some advantages in photocatalysis [29,30,31,32,33,34]: (i) the well-defined crystalline structures of CPs are beneficial to study the structure–property relationship of these solid photocatalysts; (ii) the modular nature of the CPs synthesis allows the rational design and fine tuning of these catalysts at the molecular level, making the electronic structure of the CPs catalysts to be easily tailored.

Wastewater containing organic compounds, generated in many industrial processes, can cause severe problems and damage to human being when they are discharged into aquatic environment [35, 36]. Organic pollutants, including dyes, are usually toxic, chemically stable and not amenable to direct biological treatment. Although dyes could be removed by photocatalysis [37,38,39,40,41,42], many photocatalysts are suffered from efficiency. Herein, four organic dyes including two cationic methylene blue (MB) & rhodamin B (RhB) and two anionic Methyl Orange (MO) & Reactive Red X-3B (X-3B), were selected as pollutant models to study photocatalytic activities of both BUC-18 and BUC-19.

2 Experimental

2.1 Materials and Instruments

All commercially available chemicals are reagent grade, and used as received without further purification. Thermogravimetric analyses (TGA) were performed from 80 to 800 °C in air stream at a heating rate of 10 °C min−1 on a DTU-3c thermal analyzer using α-Al2O3 as reference. UV–Vis diffuse reflectance spectra of solid samples were measured by Lambda 650S spectrophotometer, in which BaSO4 was used as the standard reference with 100% reflectance. The powder X-ray diffraction (PXRD) patterns were obtained on a DX-2700B X-ray diffractometer with powder diffraction procedure, using a Cu Kα radiation.

2.2 Synthesis of BUC-18

A mixture of Cd(NO3)2·4H2O (0.3 mmol, 92.54 mg), bpp (0.3 mmol, 59.4 mg) and H2L (0.3 mmol, 106.31 mg) was sealed in a 25 mL Teflon-lined stainless-steel Parr bomb containing deionized H2O (18 mL), heated at 160 °C for 72 h, and cooled down slowly to room temperature. Yellow block-like crystals of Cd(bpp)(H2O)L (BUC-18, yield 68% based on Cd(NO3)2·4H2O) were isolated and washed with deionized water and ethanol in turn. Anal. Calcd. for BUC-18, C32H32N4O6Cd: C, 56.4%; N, 8.2%; H, 4.7%. Found: C, 56.5%; N, 8.2%; H, 4.8%. FTIR (KBr) cm−1: 3674, 3409, 3085, 3064, 2943, 3028, 2875, 1959, 1708, 1612, 1496, 1405, 1345, 1306, 1232, 1210, 1168, 1122, 1069, 1046, 1030, 1015, 987, 959, 918, 877, 845, 808, 773, 748, 727, 704, 670, 642, 623, 611, 596, 566, 520, 508, 492, 474, 460, 403.

2.3 Synthesis of BUC-19

Yellow block-like crystals of Zn(bpp)L (BUC-19, yield 86% based on ZnCl2·6H2O) were synthesized following the same procedure as for BUC-18, except that Cd(NO3)2·4H2O was replaced with ZnCl2. Anal. Calcd. for BUC-19, C32H30N4O5Zn: C, 62.3%; N, 9.1%; H, 4.9%. Found: C, 62.5%; N, 9.2%; H, 5.0%. FTIR(KBr) cm−1: 3439, 3062, 2923, 1691, 1634, 1494, 1447, 1397, 1362, 1307, 1273, 1228, 1148, 1071, 1032, 989, 946, 798, 752, 702, 667, 609, 582, 518, 459.

2.4 X-ray Crystallography

X-ray single-crystal data collection for BUC-18 and BUC-19 was performed with a Bruker CCD area detector diffractometer with graphite-monochromatized MoKa radiation (λ = 0.71073 Å) using the φ − ω mode at 298(2) K. The SMART software [43] was used for data collection and the SAINT software [44] for data extraction. Empirical absorption corrections were performed with the SADABS program [45]. The structures were solved by direct methods (SHELXS-97) [46] and refined by full-matrix-least squares techniques on F2 with anisotropic thermal parameters for all of the non-hydrogen atoms (SHELEL-97) [46]. All hydrogen atoms were located by Fourier difference synthesis and geometrical analysis. These hydrogen atoms were allowed to ride on their respective parent atoms. All structural calculations were carried out using the SHELX-97 program package [46]. Crystallographic data and structural refinements for the two CPs are summarized in Table 1. Selected bond lengths and angles are listed in Table 2.

Table 1 Details of X-ray data collection and refinement for BUC-18 and BUC-19
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Table 2 Selected bond lengths (Å) and angles (o) for BUC-18 and BUC-19
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2.5 Evaluation of Photocatalytic Activity

Methylene blue (MB, 10 mg L−1), rhodamin B (RhB, 10 mg L−1), methyl orange (MO, 10 mg L−1) and reactive red X-3B (X-3B, 50 mg L−1) were selected to evaluate the photocatalytic performances of BUC-18 and BUC-19 under the UV light (500 W Hg lamp, Beijing Aulight Co., Ltd) irradiation in a photocatalytic reaction system. Fifteen microgram powder photocatalysts samples were put into 200 mL above-stated solutions containing organic dyes. Before photocatalysis reaction, the suspension was magnetically stirred in dark for 30 min to ensure the adsorption–desorption equilibrium. During the photocatalytic experiment, 3 mL aliquot was extracted at every 3 min intervals for analysis. A Laspec Alpha-1860 spectrometer was used to monitor the concentration changes determined at the maximum absorbance of 664, 554, 463 and 540 nm for MB, RhB, MO and X-3B, respectively.

3 Results and Discussion

3.1 Crystallographic Structure Analyses

3.1.1 BUC-18

The crystal structure analysis revealed that [CdL(H2O)] binuclear centrosymmetric unit is built up of two crystallographically equivalent Cd(II) atoms and two bidentate bridging carboxylate groups from two different L2−, which is further linked into 1D neutral [Cd(bpp)(H2O)L]2n chain by flexible bpp ligands, as shown in Fig. 1a, b. The Cd(II), adopting distorted octahedron geometry Fig. 1d, are coordinated by two oxygen atoms from one completely deprotonated L2− ligand, one oxygen atom from the other L2− ligand, one oxygen atom from coordinated water molecules and two nitrogen atoms from two different bpp ligands. The completely deprotonated L2− ligand exhibits cis mode to join one Cd(II) through the bidentate-chelating µ1-carboxylato-κ2O:O group (Scheme 1a) and another Cd(II) via monodentate-bridging into [Cd2L2] binuclear unit. The Cd–O distances are in the range of 2.292(2)–2.519(2) Å, and the Cd–N distances are in the range of 2.318(2)–2.347(2) Å. The Cd–O bond lengths and Cd–N bond lengths are in good agreement with previous studies [47, 48]. Hydrogen bonding interactions can be seen in binuclear centrosymmetric unit, as shown in Fig. 1c and Table 3.

Fig. 1
figure 1

a Asymmetric unit of BUC-18; b coordination polyhedron for Cd(II) atoms of BUC-18; c 1D chain of BUC-18 and d intramolecular H-bonding interactions in BUC-18

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Scheme 1
scheme 1

The coordination modes of H2L from a BUC-18 and b BUC-19

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Table 3 Hydrogen-bonding interactions in BUC-18 (Å and o)
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3.2 BUC-19

In [Zn(bpp)L] (BUC-19), the Zn(II) is tetrahedrally coordinated by two nitrogen atoms from two bpp ligands and two oxygen atoms from two L ligands, as shown in Fig. 2a, b. The deprotonated L2− ligand, adopting trans-mode, as shown in Scheme 1b, acts as bis-monodentate ligand to link two Zn(II) into an infinite zigzag [ZnL] chain along the b-axis. As shown in Fig. 3c, the adjacent [ZnL] chains are further linked by flexible bpp ligands into 2D network with undulating surface viewed from c-axis. The Zn–O distances are in the range of 1.916(4)–1.918(6) Å, and Zn–N distances are 1.993(8)–2.050(6) Å. The Zn–O bond lengths and Zn–N bond lengths are in good agreement with previous studies [47].

Fig. 2
figure 2

a Asymmetric unit of BUC-19; b coordination polyhedron for Zn(II) atom in BUC-19; c 2D network of BUC-19 viewed from c-axis (orange stick and green stick represent L2− and bpp, respectively)

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Fig. 3
figure 3

PXRD patterns of a BUC-18 and b BUC-19 and their simulated XRD patterns from the single-crystal structure

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3.3 Powder X-ray Diffraction (PXRD)

PXRD of BUC-18 and BUC-19 powder were carried out confirm their phase purities, in which the simulated PXRD patterns obtained from their corresponding single crystal structure data were utilized as references. As shown in Fig. 3, the measured PXRD patterns matched well with the simulated ones, implying the high phase purity of the both coordination polymers. The slight differences in PXRD intensities can be contributed to the preferred orientation of the crystalline powder samples [49].

3.4 FTIR

The FTIR spectrums of the coordination polymers were recorded, as depicted in Fig. 4. The band at 2923 cm−1 is ascribed to v(–CH2–) vibration, and the strong bands at 1634 and 1397 cm−1 can be assigned to the asymmetric and symmetric vibrations of carboxyl groups, respectively. The bands at 1228 and 1148 cm−1 are attributed to the v(C–N) vibrations of the phenyl rings. The weak bands at 492 and 403 cm−1 can be attributed to Zn–O vibration, while band at 518 cm−1 can be ascribed to Cd–O vibration.

Fig. 4
figure 4

FTIR of BUC-18 and BUC-19

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3.5 Thermal Properties

To investigate the thermal stability of the two CPs, thermogravimetric analyses were carried out, and the results were demonstrated in Fig. 5. It can be seen that the weight loss processes of the two CPs were very similar, although their crystal structures were different. Taking BUC-18 as an example, the TGA curve presents three stages of decomposition. The loss of water molecules was observed from 150 to 240 °C. As the temperature was increased from 240 to 490 °C, the weight loss was considered as organic ligands. The weight of the final residue was CdO, which was affirmed by a good match between the powder X-ray diffraction (PXRD) patterns of the residue and the standard patterns of CdO from the database (PDF#01-073-2245), as illustrated in Fig. 5(b).

Fig. 5
figure 5

a The TGA curves of BUC-18 and BUC-19. b PXRD patterns of the residue of the TGA of BUC-18 and the standard patterns of CdO

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3.6 Optical Energy Gap

The optical properties of BUC-18 and BUC-19 were estimated by UV–Vis diffuse reflectance spectrophotometer (UV–Vis DRS). As shown in Fig. 6a, the absorption bands were at 288 nm. The band gap (E g ) values were estimated using Kubelka–Munk function, F = (1 − R)2/2R [50], where R is the reflectance of an infinitely thick layer at a given wavelength. The F versus E plots for the two CPs are shown in Fig. 6b, where steep absorption edges are displayed and the Eg values of the sample can be assessed at 4.4 eV for both BUC-18 and BUC-19, which indicate that two CPs are potential wide gap photocatalysts [51, 52].

Fig. 6
figure 6

a UV/Vis spectra of BUC-18 and BUC-19; b Kubelka--Munk-transformed diffuse reflectance spectra of BUC-18 and BUC-19

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3.7 Photocatalytic Performance Studies

Emerging research has demonstrated coordination polymers including MOFs to be a new class of photocatalysts for potential applications in environmental field, such as organic pollutants degradation [26], Cr(VI) reduction [53] and ultra-high adsorption performance toward organic pollutants and heavy metals [9, 10]. In this study, four organic dyes (MB, RhB, MO and X-3B) were selected to evaluate the activities of the two new CPs under UV light irradiation. Control experiments were conducted to compare the removal efficiencies of organic dyes in two different systems with identical conditions except for in presence of photocatalysts in dark and absence of photocatalysts under UV irradiation, respectively. The adsorptive removals of four selected organic dyes in the presence of the two CPs in dark can be negligible, as shown in Fig. 7. After UV light irradiation for 30 min, limited dyes were decomposed in absence of any photocatalysts. BUC-19 displayed excellent photocatalytic degradation performances toward MB and X-3B, evidenced by their decomposition efficiency being 93 and 92%, respectively, as illustrated in Tables 4 and 5.

Fig. 7
figure 7

Degradation of a MB, b RhB, c MO and d X-3B solution by the CPs. Experimental conditions: photocatalyst, 15 mg; MB/RhB/MO, 10 mg L−1; X-3B, 50 mg L−1

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Table 4 Photocatalytic degradation efficiencies of the two CPs toward MB, RhB, MO and X-3B
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Table 5 Photocatalytic degradation efficiencies of MB with different catalysts
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3.8 Recyclability of BUC-19

The recyclability of BUC-19 was tested by circulating runs in the photocatalytic degradation of MB over BUC-19/UV light system. The results in Fig. 8 showed that the photocatalytic degradation performance of BUC-19 remained almost unchanged after 5 runs for MB degradation, indicating that BUC-19 was stable and could be used for the repeated of MB decomposition.

Fig. 8
figure 8

The cycling runs of the degradation of MB (10 mg L−1) over BUC-19

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To investigate the roles of the reactive species (·O2−, ·OH and h+) in the photocatalytic process, 0.2 mmol L−1 of benzoquinone (BQ), isopropyl alcohol (IPA) and ethylene diamine tetraacetic acid disodium salt (EDTA-2Na) were added into MB solution with BUC-19 as scavengers of ·O2−, ·OH and h+, respectively. As shown in Fig. 9, the photocatalytic degradation of MB was inhibited in the order of BQ > IPA > EDTA-2Na. Three reactive species were important to degrade MB molecules, while ·O2− played a dominant role for the MB degradation in the BUC-19 system.

Fig. 9
figure 9

Reactive specie trapping experiments of BUC-19

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Generally, photocatalytic mechanism of coordination polymers, including metal–organic frameworks (except MOF-5), is explained based on frontier molecular orbital theory [26]. Based on the above-stated results and analyses, a possible mechanism for the photocatalysis with BUC-19 was proposed, as illustrated in Fig. 10. Typically, the photocatalysis process involves three steps. Firstly, dye molecules are adsorbed onto the surface of BUC-19. Next, upon UV light irradiation, BUC-19 absorbs photons and excites electrons (e−) from highest occupied molecular orbital (HOMO) and leaves holes (h+) in lowest unoccupied molecular orbital (LUMO). Finally, active species are generated and decompose dyes. Briefly, the photo-induced electron in the LUMO losses and transfers to O2 generating ·O2−, which decompose dye molecules. The holes can oxide dye directly. Besides, the HOMO strongly demands electrons to return to its stable state, therefore, one electron is captured from water molecules and product ·OH active species. The ·OH radicals can further destroy organic dyes efficiently.

Fig. 10
figure 10

A simplified model of photocatalytic reaction mechanism of organic dyes in BUC-19

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4 Conclusion

In summary, the syntheses of two new coordination polymers, BUC-18 and BUC-19, have been accomplished by hydrothermal method and characterized using X-ray single crystal diffraction analysis, TGA, CNH element analysis and UV–Vis DRS. The results show that BUC-18 possesses 1D infinite chain structure, while BUC-19 displays 2D layer structures, which can be assigned to the influence of L2− ligand and flexible bpp ligands. BUC-19 exhibited excellent photocatalytic degradation performances toward organic dyes. The trapping experiences proved that ·O2−, ·OH and h+ played important roles in decomposing dyes.