Introducing anthracene and amino groups into Ln-OFs for the photoreduction of Cr(VI) without additional photosensitizers or cocatalysts

Abstract

The utilization of catalysts with high stability and activity for the photoreduction of Cr(VI) is desirable but has many challenges. Since metal–organic frameworks (MOFs) have unique versatilities and facile structural modulations, we fabricated a stable MOF as a highly efficient photocatalyst by incorporating chromophores into lanthanide MOFs for efficient Cr(VI) reduction. Subsequently, a novel two-dimensional (2D) layered structural Sm(III) MOF (namely LCUH-100, [Sm₄(AAPA)₆(DMA)₂(H₂O)₄][DMA]₃[H₂O]₈, AAPA²⁻ = 5-[(anthracen-9-yl-methyl)-amino]-1,3-isophthalate, DMA = N,N′-dimethylacetamide) with sql topology symbol was constructed by incorporating dinuclear clusters, {Sm₂(COO)₆} and AAPA²⁻, which demonstrated excellent chemical and thermal stability, especially retaining its complete framework in aqueous solutions at a pH range of 2–12 for 24 h. Interestingly, LCUH-100 could efficiently and rapidly photocatalyze the reduction of Cr(VI) to Cr(III) without additional photosensitizers or cocatalysts. In the photoreduction of Cr(VI), LCUH-100 showed a high rate constant (k) of 0.186 min⁻¹ and a high Cr(VI) reduction rate of 1.67 mg_Cr(VI) g⁻¹_cata min⁻¹ at pH 2. Furthermore, the photocatalyst LCUH-100 could be reused for 5 cycles without any significant loss of catalytic activity. This work provides a new strategy for the exploitation of stable and efficient catalysts for the photocatalytic reduction of Cr(VI).

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Introduction

Environmental issues are among the major issues facing the whole world today, including water contamination, which has caused wide public concern over the recent years.^1–4 Heavy metal ion pollution (i.e., Cr(VI), Hg(II), and Fe(III)) is a significant global problem.^5–9 Especially, dichromate (Cr₂O₇²⁻) ions are widely regarded as representative contaminant in water due to their high toxicity and carcinogenicity.^10–12 Therefore, the concentration of Cr(VI) in drinking water has been limited to below 0.05 mg L⁻¹ by the World Health Organization.¹³ Recently, many approaches have been exploited to efficiently adsorb and convert Cr(VI) into less toxic Cr ions.^14–16 In particular, the conversion of Cr(VI) into less toxic Cr(III) is regarded as a promising solution to alleviate the pollution problem. Since photocatalytic reduction is an eco-friendly method, solar-energy-driven Cr₂O₇²⁻ ion reduction under the action of photocatalysts into Cr(III) has been extensively investigated.^17–19 Some typical inorganic photocatalysts with multifunctional characteristics have been reported by scientific researchers, such as TiO₂, α-Fe₂O₃, In₂S₃, and ZnO.^20–24 However, there are some shortcomings, such as the electron–hole pair recombination rate, fast agglomeration speed, and low solar energy utilization, which limit the efficiency of the above catalysts for Cr(VI) removal.^25–27 Therefore, the development of efficient photocatalysts to adsorb and convert Cr(VI) into a less toxic product Cr(III) is highly desired.

In recent years, diverse MOF catalysts have been exploited for Cr(VI) photoreduction.^28–34 Compared with three-dimensional (3D) MOFs, 2D MOFs have unique properties originating from their ultrathin thickness, large surface area, excellent optical transparency, and superior electrical and thermal conductivity, which facilitate a more efficient transport of photogenerated carriers from the interior to the surface active sites. Therefore, 2D MOFs are considered as more suitable photocatalyst candidates.^35–40 However, to the best of our knowledge, only a few articles have reported successful Cr(VI) photocatalytic reduction using 2D MOFs.^41,42 Meanwhile, chemists have found that 2D MOFs can be used as semiconductor materials, whereby the highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gap of the organic ligand in MOF systems is equal to the band gap of inorganic semiconductors, so it makes these photocatalysts possess the capabilities for fine-tuning and rational design at the molecular level.^43,44 For example, as the porphyrin unit is a typical photosensitizer, Ye's group successfully modified UiO-66 by introducing a porphyrin ring into the original UiO-66, and the photoreduction activity of the modified UiO-66 was much higher than that of the original UiO-66.⁴⁵ Wu's group found that the photoreduction activity of MIL-68(In)-NH₂ was higher than the unmodified MIL-68(In), which indicated that adding amino groups in BDC²⁻ ligands could broaden the absorption edge to the visible region.⁴⁶ Therefore, an intense visible-light-absorption π-electron-rich aromatic unit and strong toxic anions adsorbent were integrated into one single MOF simultaneously to strongly improve the photoreduction capability toward Cr(VI). However, the photocatalysts obtained by the MOFs post-modification method do not have unambiguous structures, and their synthetic process is also more complicated. Based on the above considerations, we designed and obtained the 2D Sm-MOF LCUH-100 using the ligand H₂AAPA containing anthracyclines and amino groups as photosensitizers, and this LCUH-100 demonstrated excellent reactivity and good reproducibility for the reduction of Cr(VI) in water. As far as we know, this is the first example of a 2D lanthanide–organic frameworks (Ln-OFs) material among all the reported MOF-based photocatalysts for Cr(VI) photoreduction.

Herein, we rationally synthesized a stable Sm carboxylate MOF (LCUH-100) by using Sm(III) ions and the large conjugated dicarboxylate ligand H₂AAPA with amino groups (Scheme 1), which could broaden the absorption edge to the visible region. Notably, LCUH-100 demonstrated excellent chemical and thermal stability, especially retaining its complete framework in aqueous solution at pH 2–12 for 24 h. Due to the introduction of highly photosensitive anthracene units and the cochromatic amino group, LCUH-100 could achieve the complete conversion of Cr(VI) to Cr(III) within 60 min under visible-light irradiation without the need for any photosensitizer or cocatalyst. The significant photocatalytic activity of LCUH-100 could be ascribed to its rich Lewis acidic Sm(III) sites, large conjugated dicarboxylate ligands, amino groups, and good charge-separation efficiency. Meanwhile, various factors affecting the reduction of Cr(VI), including the pH value of the reaction solution, the addition of a hole scavenger, and the reusability of the photocatalyst, were also studied in detail.

Scheme 1 Schematic illustration of the synthesis processes of LCUH-100.

Results and discussion

Structure and morphology description of LCUH-100

As shown in Table S1,†LCUH-100 crystallized in a triclinic crystal system with space group P1̄, displaying a two-dimensional (2D) framework based on the AAPA²⁻ ligand and binuclear metallic [Sm₂(COO)₆] cluster. In the asymmetric unit of LCUH-100, there are four independent Sm(III) ions, six deprotonated AAPA²⁻ ligands, four coordinated H₂O molecules, and two coordinated organic solvent molecules (DMA), taking no account of the free solvent molecules. The four independent Sm(III) ions in the asymmetric unit can form two different binuclear metal clusters, namely [Sm₂(COO)₆]^a and [Sm₂(COO)₆]^b. Four oxygen atoms (O5, O6, O16, O17) of one ligand link the binuclear clusters [Sm₂(COO)₆]^a and [Sm₂(COO)₆]^b (Fig. 1a). In the binuclear cluster [Sm₂(COO)₆]^a, Sm1 is a cation octa-coordinated by eight carboxylate oxygen atoms from five different deprotonated AAPA²⁻ ligands, displaying a distorted dodecahedral geometry [Sm1–O5 = 2.32(8) Å, Sm1–O8 = 2.34(8) Å, Sm1–O4 = 2.36(8) Å, Sm1–O2 = 2.41(10) Å, Table S2†]. The Sm2 center is nine-coordinated and composed of six carboxylate oxygen atoms from four different AAPA²⁻, one oxygen atom from the coordinated DMA, and two oxygen atoms from two H₂O molecules [Sm2–O8 = 2.36(11) Å, Sm2–O3 = 2.41(8) Å, Sm2–O6 = 2.46(8) Å, Sm2–O7 = 2.46(8) Å, Table S2†]. The distance between Sm1 and Sm2 is 4.15 Å, and they are linked by two carboxylate oxygen atoms (O3 and O8) coming from two different AAPA²⁻ ligands to form a binuclear metallic cluster [Sm₂(COO)₆]^a (Fig. 1b).

Fig. 1 (a) Simplified coordination environments of LCUH-100 (green spheres represent H₂AAPA). Coordination structures of binuclear [Sm₂(COO)₆]^a (b) and binuclear [Sm₂(COO)₆]^b (c). (d) Lamellar structure of the 2D framework viewed from the c axis. (e) Reticular structure viewed from the a axis in LCUH-100. (f) Simplified topology structure of LCUH-100.

In another binuclear cluster [Sm₂(COO)₆]^b, the Sm3 center is octa-coordinated by eight oxygen atoms from five different AAPA²⁻ ligands, and displays a distorted dodecahedral geometry [Sm3–O13 = 2.35(7) Å, Sm3–O20 = 2.36(8) Å, Sm3–O18 = 2.39(7) Å, Sm3–O14² = 2.41(8) Å] (Table S2†). The Sm4 center is nine-coordinated by nine oxygen atoms and constituted by six carboxylate oxygen atoms from four different AAPA²⁻ ligands, one oxygen atom from one DMA, and two oxygen atoms from two H₂O molecules [Sm4–O26 = 2.37(9) Å, Sm4–O19 = 2.41(8) Å, Sm4–O12 = 2.42(8) Å, Sm4–O21 = 2.48(8) Å. Table S2†]. The distance between Sm3 and Sm4 is 4.12 Å, and the adjacent two Sm(III) ions are connected by two carboxylate oxygen atoms (O19 and O20) from two different AAPA²⁻ ligands (Fig. 1c). The adjacent binuclear metal clusters are linked through the deprotonated ligand AAPA²⁻ and form 2D layered reticular structures (Fig. 1e). The distance between adjacent layers is in the range of 4.97–6.96 Å, so the adjacent 2D layers can be further connected to form 3D frameworks by the π⋯π stacking and hydrogen-bonding interactions (Fig. 1d). The framework of LCUH-100 can be simplified to a 4-connected network with the sql topology of point symbol {4⁴·6²} by the topological analysis method (Fig. 1f). After removing guest molecules, the total solvent-accessible volume of LCUH-100 was assessed to be 2043.2 ų, or 25.8% of the unit volume 7914.1 ų calculated through PLATON.

The morphology and microstructure of the prepared LCUH-100 were studied by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM), as shown in Fig. 2. Given the 2D crystal structure of LCUH-100, its FESEM images showed a typical layered appearance of 30–300 nm thickness (Fig. 2a). The morphology of LCUH-100 was further observed by TEM (Fig. 2b), and the same 2D layered structure model was shown, which proved that we successfully prepared LCUH-100 with a 2D layered structure.

Fig. 2 FESEM (a) and TEM (b) images of LCUH-100.

Stability test

The powder X-ray diffraction (PXRD) patterns of LCUH-100 indicated a good match between the simulated and as-synthesized LCUH-100 in key positions, which confirmed the purity of the samples of LCUH-100. To inspect the chemical stability of LCUH-100, it was soaked in deionized water (pH 7), hydrochloric solution (pH = 2, 4, 6), and sodium hydroxide solution (pH = 8, 10, 12) at 298 K for 24 h, respectively. After the above soaking treatment, the PXRD patterns of the treated LCUH-100 matched well with that of the simulated LCUH-100 in key positions (Fig. 3a), indicating LCUH-100 had excellent chemical stability, and ensure it can be used in further applications. Furthermore, the thermostability of MOFs is a significant element for their better application in industrial production. As shown in Fig. 3b, the thermogravimetric analysis indicated that LCUH-100 had excellent thermostability and was thermally stable at 303 °C. Eight lattice water molecules were gradually lost in the range of 30–100 °C, corresponding to a 4.20% weight loss (ca. 4.15%). The further weight loss of 2.10% (ca. 2.08%) at 100–170 °C corresponded with the loss of four coordinated water molecules, and then at even higher temperature, the skeleton of LCUH-100 began to collapse after 303 °C. In addition, LCUH-100 was tested with variable temperature powders at 25–325 °C to further verify its stability, and the test results are displayed in the ESI, as shown in Fig. S2.†

Fig. 3 (a) Simulated and synthetic PXRD patterns of LCUH-100, as well as the PXRD patterns of LCUH-100 immersed in aqueous solutions of different pH values for 24 h; (b) thermogravimetric analysis (TGA) curves of LCUH-100 measured under a N₂ atmosphere from 30 °C to 750 °C at a heating rate of 10 °C min⁻¹.

Photocatalytic reduction of Cr(VI)

Photocatalytic properties. In the photocatalytic reaction, MOFs experience a process similar to that of traditional semiconductors. Note, the valence band (VB) and conduction band (CB) refer to the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in MOFs, respectively. The ultraviolet-visible-absorption band of LCUH-100 was measured by ultraviolet–visible-light diffuse reflectance spectroscopy (UV–vis DRS). As displayed in Fig. 4a, the strong visible-light absorption and broad range absorption band (300–500 nm) make LCUH-100 have potential application in visible-light photocatalysis.⁴⁷ The UV region (300–400 nm) absorption band was ascribed to the π⋯π* and n⋯π* transitions of the aromatic rings (An) and amino groups (–NH–) in the organic ligand AAPA²⁻ of LCUH-100. While the visible region (400–550 nm) absorption band could be ascribed to the charge-transfer transition from the ligand AAPA²⁻ to the {Sm₂} cluster (LMCT).⁴⁸ The band-gap energy (E_g) of LCUH-100 could be calculated based on the Kubelka–Munk equation: αhv = B(hν − E_g)^1/2.⁴⁹ The E_g of LCUH-100 was estimated to be about 2.85 eV by linear fitting the curve of αhv²versus the photon energy (hν) (Fig. 4a). Therefore, the above DRS research endorsed the utilization of LCUH-100 as a visible-light-active photocatalyst. To further investigate the band structure of LCUH-100, the semiconductor type and band potential (generally considered to be the Fermi level (E_f)) were determined by Mott–Schottky electrochemical tests. As shown in Fig. 4b, the slopes of the Mott–Schottky plots of LCUH-100 (i.e., 1000, 1300, and 1600 Hz) were all positive, indicating that LCUH-100 can be regarded as an N-type characteristic material, and the E_f value was E_f (vs. Ag/AgCl) −1.37 eV, at pH 6.8. This could be converted into reversible hydrogen electrode (RHE) potential by the following formula: E_RHE = E_Ag/AgCl + 0.198 eV + 0.0592 pH, whereby the E_f value of LCUH-100 was −0.77 eV (vs. RHE). Furthermore, the gap between E_f and VB obtained from the XPS VB spectra was 2.51 eV (vs. NHE) (Fig. 4c). The VB of LCUH-100 was determined to be 1.74 eV (vs. NHE). Combined with the band-gap values obtained above, the CB of LCUH-100 was calculated to be −1.11 eV (vs. NHE) (Table S5†), which was more negative than the Cr(VI)/Cr(III) potential (+0.51 eV, pH 6.8). Therefore, LCUH-100 is a potential catalyst for the photocatalytic reduction of Cr(VI). As shown in Fig. 4d, based on the above analysis, the energy band diagram of photocatalyst LCUH-100 was also schematically plotted.^43,50,51

Fig. 4 (a) UV–vis spectra of LCUH-100 and plot of (αhν)²versus photon energy (hν) of LCUH-100. Typical Mott–Schottky plots (b) and XPS VB spectra (c) of LCUH-100. (d) Band structure diagram of LCUH-100.

The separation efficiency of photogenerated charge carriers in LCUH-100 was then investigated by transient photocurrent measurements, electrochemical impedance spectroscopy (EIS), and steady-state photoluminescence (steady-state PL). Fig. 5a shows the apparent photocurrent response (0.5 μA cm⁻²) of the LCUH-100 electrodes over on–off cycles of intermittent visible-light irradiation. This proves that LCUH-100 was able to be stimulated to produce electron–hole pairs under visible irradiation.⁵² The semicircle in the EIS spectra was due to the contribution from the charge-transfer resistance (R_ct) and constant phase element (CPE). The R_ct should result from the resistance at the interface between the photocatalyst/electrolyte interfaces. The corresponding equivalent circuit is shown in the inset of Fig. 5b. The radius of the EIS arc is related to the charge-transfer resistance of a photocatalyst, and a smaller radius of the EIS arc corresponds to a smaller resistance. According to the EIS results, the R_ct of LCUH-100 was 2.7 kΩ, which was smaller than the R_ct of the photocatalysts reported in the literature. It is clear that LCUH-100 had less charge-transfer resistance (for example, the R_ct of H₂TCPP⊂(I⁻) Meim-UiO-66 reported by Ye's group was 5.47 kΩ), which can make the photoexciton migration more effective.^{10,15,16,23,43,45,50,51} This finding could be further confirmed by the steady-state PL spectrum (Fig. S3†). Interestingly, the PL intensity obtained over LCUH-100 was much weaker than that of H₂AAPA, thus indicating the faster electron transfer of the photogenerated electrons and holes after forming the MOF material.^45,50,51

Fig. 5 Transient photocurrent responses (a), electrochemical impedance spectroscopy plot (b) of LCUH-100, time-resolved PL spectra of LCUH-100 and LCUH-100 containing Cr₂O₇²⁻ suspensions (c), and of LCUH-100, (d) EPR analyses of LCUH-100 before and after visible-light irradiation.

Meanwhile, the time-resolved PL spectrum of LCUH-100 is shown in Fig. 5c. The average recovery lifetime (τ) of LCUH-100 was 3.176 μs. It is well known that prolongation of the exciton lifetime can provide more opportunities for photoinduced carriers to participate in surface-catalyzed reactions, thus improving the photocatalytic performance.^45,50 The electron-transfer process of the photocatalysts was further studied by electron paramagnetic resonance (EPR), which was carried out in the dark and under visible-light irradiation, respectively (Fig. 5d). After 20 min visible-light irradiation, the EPR signal at about g = 2.002 was significantly enhanced, indicating that the photogenerated electrons transferred from the excited anthracene ring to the Sm–O cluster site.^43,45 However, when the other conditions were identical, no obvious signal was observed in the dark.

Photocatalytic reduction of Cr(VI) and its influencing factors. The excellent physical properties, strong visible-light adsorption and high electron–hole separation efficiency encouraged us to assess the feasibility of LCUH-100 as a photocatalyst for Cr(VI) reduction. A negligible decrease in Cr(VI) was clearly observed in the presence of LCUH-100 in the dark conditions, which indicated that the decline in the Cr(VI) concentration was not caused by adsorption. Whereas in the presence of light and the catalyst LCUH-100, the reduction process of Cr(VI) proceeded smoothly, and a significant decline in Cr(VI) concentration was observed, indicating that the reduction of Cr(VI) was caused by LCUH-100 under visible-light irradiation. The Cr(VI) aqueous solution (50 ppm, 20 mL) could be 100% reduced within 60 min under the optimal conditions (pH 2, 3.0 mL EtOH) (Fig. 6a). As shown in Fig. 6b, the photoreduction of Cr(VI) followed a pseudo-first-order kinetics, and the reaction rate was calculated to be 0.1861 min⁻¹ under the optimal conditions.⁴⁵ Compared with other reported MOF-related photocatalysts, the reduction efficiency of LCUH-100 was at a high level for the reduction of Cr(VI) (Table 1). Moreover, the recycling tests indicated that LCUH-100 had excellent recyclable stability, as identified by the PXRD patterns, IR spectra, and XPS spectra before and after the photoreduction of Cr(VI) (Fig. 6f and S4†). However, the rate of the photocatalytic reduction reaction was strongly affected by some factors, such as the pH, hole scavenger, and electron-capture agent.⁴⁹

Fig. 6 Photocatalytic reduction of Cr(VI) to Cr(III): (a) under different conditions for Cr(VI) reduction over LCUH-100 (pH 2, 3.0 mL of ethanol) and its pseudo-first-order kinetics curves (b); (c) with different pH in the presence of 3 mL ethanol; (d) with different amounts of EtOH at pH 2; (e) visible-light-driven photocatalytic Cr(VI) reduction of LCUH-100 without (black curve) or with (red curve) the electron scavenger AgNO₃ (1 mg). (f) Cycling runs for the photocatalytic reduction of Cr(VI) over LCUH-100.

Table 1 Comparison of the photocatalytic reduction of Cr(VI) by different photocatalysts

Photocatalyst	C cata (g L^−1)	V Cr(VI) (mL); CCr(VI) (ppm)	Reduction percentage; time (min)	Reduction rate (mgCr(VI) g^−1cata min^−1)	k (min^−1)	Light source	Ref.
In2S3@MIL-53(Fe)	0.60	50; 10	97%; 20	0.81	0.143	CEL-LAX 500 (300 W)	10
Cd-MOF	1.00	30; 20	95%; 180	0.11	—	LED lamp (40 W)	25
Cu(I)-MOF	0.38	40; 10	95%; 9	2.81	—	Xenon lamp (500 W)	42
Ru-UiO-bpy	0.25	40; 50	100%; 60	3.3	0.128	PLS-SXE300 (300 W)	43
NH2-MIL-125-Ti	0.40	50; 48	91%; 60	1.82	—	Xenon lamp (500 W)	44
H2TCPP⊂(I^−)Meim-UiO-66	0.25	40; 100	100%; 30	13.3	0.154	Xenon lamp (300 W)	45
MIL-68(In)-NH2	1.00	40; 20	97%; 180	0.11	—	PLS-SXE300 (300 W)	46
Zn-MOF	1.00	40; 20	93%; 90	0.21	—	Xenon lamp (300 W)	47
Zr-MOF	0.25	40; 50	73%; 60	3.33	0.073	PLS-SXE300 (300 W)	49
LCUH-100	0.50	20; 50	100%; 60	1.67	0.186	CEL-LAX 500 (300 W)	This work

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During the redox process, the pH value of the reaction solution had a great influence on the reduction rate, which was in agreement with that of other reported MOF catalysts.^10,44 Hence, the effect of pH on the photocatalytic performance was investigated under the concentration of 50 ppm K₂Cr₂O₇ solution (20 mL) and 10 mg of the catalyst LCUH-100. The reduction ratio increased rapidly from 41% to 100% with the pH decreasing from 6.0 to 2.0 (Fig. 6c). According to the surface zeta potential versus the pH of LCUH-100, the isoelectric point was about 2.56, which indicated that the photocatalyst surface was positive at the range of pH < 2.56 (Fig. S5†). Therefore, under the condition of pH 2, the positively charged photocatalyst LCUH-100 was more likely to attract Cr₂O₇²⁻ anions and accelerate the electron migration, thus promoting the photocatalytic activity. On the contrary, the significantly less reduction at higher pH (pH > 2.56) could be attributed to the highly anionic charged surface of the photocatalyst, which repelled the Cr₂O₇²⁻ anions.

As is well known, the rate of photocatalytic reduction is greatly influenced by the presence of hole scavengers. The addition of a hole scavenger was thus used to trigger the reduction process, as it would react with the photoinduced holes. Herein, EtOH was investigated as a hole scavenger in the reaction, which could further promote the photocatalytic activities. As expected, as the amount of EtOH increased from 0 to 3 mL, the reduction rate increased rapidly from 23% to 100% (Fig. 6d). Therefore, EtOH could be beneficial to restrain the recombination of electrons and holes, which could afford sufficient electrons would be available for Cr(VI) reduction.⁴⁹

The above experiments confirmed that the introduction of the hole (h⁺)-scavenger EtOH could accelerate the reduction efficiency of Cr(VI). Herein, AgNO₃, as an electron scavenger, was introduced into the photocatalytic reduction process. As shown in Fig. 6e, the Cr(VI) reduction efficiency was obviously restrained by the solution of AgNO₃, indicating that the electron e⁻ had a dominant influence on the reduction process. The trapping of holes by EtOH may accelerate the migration of electrons e⁻ and make them fully participate in the reduction process, thereby finally improving the photocatalytic activity.¹⁰

Mechanism of photocatalytic Cr(VI) reduction

To investigate the electron-transfer process in the photocatalyst during visible-light irradiation, we then resorted to time-resolved PL spectroscopy, a robust tool to track photoexcited carriers. The τ of LCUH-100 in Fig. 5c was 3.176 μs. After the addition of Cr₂O₇²⁻, the average lifetime was reduced to 2.297 μs, indicating that the long-lived electrons generated by LCUH-100 were easily transferred to Cr₂O₇²⁻.^45,50 Meanwhile, EPR tests were performed to directly probe the photogenerated electrons in the reaction system. Here, 2,2,6,6-tetramethyl-l-piperidine-N-oxyl (TEMPO), as a typical spin label molecule, was utilized for capturing photogenerated electrons (e⁻) on the surface of LCUH-100 (Fig. 7a). The typical three signals of TEMPO with equal intensity were observed in the dark, but after visible-light irradiation for 20 min, the signal intensity dramatically declined. This could be ascribed to the TEMPO molecule being transformed into the corresponding hydroxylamine (TEMPOH) in aqueous system by trapping electrons.⁵³ The electrons e⁻ produced from the photoexcited LCUH-100 were captured by TEMPO, which led to a weakening of the signal. Similarly, it was found that there was no obvious electronic signal after 20 min of visible-light irradiation when TEMPO molecules were replaced with Cr₂O₇²⁻. The EPR data proved that the electrons e⁻ generated by photoexcited LCUH-100 were captured by Cr(VI) and then reduced to Cr(III), and then Cr₂O₇²⁻ ions were completely converted into Cr(III) ions under the irradiation of visible light, which was also proved by the XPS data of LCUH-100. As shown in Fig. 7b, the binding energies of Cr 2p_3/2 and Cr 2p_1/2 of Cr(III) corresponded to 578.0 and 587.3 eV, respectively.

Fig. 7 (a) EPR signal of TEMPO in the dark, and TEMPO and Cr₂O₇²⁻ radicals under visible light in aqueous dispersion of LCUH-100 for 20 min; (b) XPS spectra of the recovered LCUH-100 after the Cr(VI) reduction experiment.

According to the above experimental findings, a plausible mechanism is proposed for the reduction of Cr(VI) (Scheme 2). In this process, the Sm–O clusters surrounded by H₂AAPA ligands in the framework of LCUH-100 can behave as quantum dots, while the –NH– groups act as auxochromic groups, and the anthracene nucleus acts as an antennae absorbing light. Upon irradiation with visible light, the anthracene nucleus in LCUH-100 strongly absorbs visible light and efficiently transfers the photogenerated electrons to the inorganic Sm–O clusters partly via ligand-to-metal charge transfer (LMCT) (eqn (1)). The electrons on the valence band leap into the conduction band and leave behind holes in the valence band, and then the electrons on CB are accepted by Cr₂O₇²⁻ ions adsorbed on the metal sites (eqn (2)). Meanwhile, the photogenerated holes can get enough electrons from the hole scavenger EtOH adsorbed on the amine sites to oxidize EtOH into CO₂ and H₂O (eqn (3)).^43,46 Therefore, the redox process can be outlined as follows:

Sm-MOF + hν → Sm-MOF (h⁺ + e⁻) (1)

Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O (2)

EtOH + h⁺ → CO₂ + H₂O + Others. (3)

Scheme 2 Proposed mechanism for the photocatalytic reduction of aqueous Cr(VI).

Conclusions

In summary, a 2D layered Sm-MOF, LCUH-100, with a good lattice and excellent stability was successfully prepared by a solvothermal method. The visible-light-absorption edge of LCUH-100 could be extended to 550 nm by combining an anthracene group and amino group into the MOF framework, thus showing efficient Cr(VI) photocatalytic reduction performance. Remarkably, the photocatalytic reduction of Cr(VI) by LCUH-100 did not require the aid of any photosensitizer or cocatalyst under visible-light irradiation, which is quite rare among reported MOF-based photocatalysts. LCUH-100 showed a high rate constant (k) of 0.186 min⁻¹ and a high Cr(VI) photoreduction rate of 1.67 mg_Cr(VI) g⁻¹_cata min⁻¹ for the photoreduction of Cr(VI), which are higher than most reported representative MOFs. In addition, LCUH-100 demonstrated excellent photocatalytic reduction stability (5 recycling runs), making LCUH-100 an outstanding candidate for the photocatalytic reduction of Cr(VI). This work provides a new method for the exploitation of stable and efficient catalysts for the photocatalytic reduction of Cr(VI).

Experimental section

Synthesis procedure

Synthesis of {[Sm₄(AAPA)₆(DMA)₂(H₂O)₄]·DMA₃·H₂O₈}_n (LCUH-100). A mixture of SmCl₃·6H₂O (36.48 mg, 0.1 mmol), 5-[(anthracen-9-yl methyl)-amino]-isophthalic acid (H₂AAPA) (13.7 mg, 0.05 mmol), dimethylacetamide (DMA, 3 mL), and deionized water (2.0 mL) was obtained with mechanical stirring for 30 min at room temperature. The mixture was transferred into a 10 mL Teflon-lined stainless steel vessel and heated at 100 °C for 48 h, and then cooled gradually to 298 K at a rate of 0.1 °C min⁻¹. Yellow rod-shaped crystals were collected with a 70% yield based on H₂AAPA. Anal. calcd for C₁₅₈H₁₅₉N₁₁O₄₁Sm₄ (3469.22): C 54.65, N 4.44, O 18.91, H 4.58. Found: C 54.20, N 4.74, O 18.85, H 4.68. FT-IR (KBr, cm⁻¹): 3403 (m), 3018 (w), 2980 (w), 1602 (s), 1425 (s), 1012 (w), 766 (s), 742 (s), 406 (s). The Fourier transform infrared spectra (FT-IR), as shown in Fig. S1.†

Photocatalytic reduction

The Cr(VI) photocatalytic reduction was achieved in a 100 mL quartz reactor at 298 K, and the reaction process was measured by UV–vis spectroscopy. The characteristic peak of Cr₂O₇²⁻ was at 540 nm, and its absorbance gradually declined with time under visible-light irradiation. In a typical test, dichromate (Cr₂O₇²⁻) was the main form of Cr(VI) under acidic solution, and dichromate potassium (K₂Cr₂O₇) was typically chosen as a representative Cr(VI) compound to achieve the photocatalytic reaction. Here, 10 mg catalyst LCUH-100 and 20 mL K₂Cr₂O₇ aqueous solution (50 ppm) were uniformly dispersed into a 100 mL quartz reactor. The optimal pH value was 2.0 for the reaction solution, which could be adjusted with 0.2 mol L⁻¹ H₂SO₄ dilute aqueous solution. First, the mixed solution was stirred vigorously in the dark for 60 min, which ensured LCUH-100 could reach adsorption–desorption equilibrium for K₂Cr₂O₇. Second, the mixed solution was irradiated with a 300 W Xe lamp for 120 min. At regular intervals (10 min), 1.5 mL mixture was withdrawn from the 100 mL quartz reactor, and then filtered through a 0.22 μm hydrophilic membrane filter. At last, the filtered clear liquid was used for tracking tests by UV–vis spectroscopy using the previously reported diphenylcarbazide (DPC) method for the absorbance change of Cr₂O₇²⁻. The above procedure was repeated for the rest of the mixture, and finally 13 group data points (including zero point) were successfully collected in 120 min.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21401095, 21801107, 22178157), the Natural Science Foundation of Shandong Province (ZR2022MB010, ZR2019MB068, ZR2021QB123), Project of Shandong Province Higher Educational Science and Technology Program (KJ2018BZC043, J18KA113), the Liaocheng University Start-up Fund for Doctoral Scientific Research (318050104, 318052017), the Scientific Research Fund of Liaocheng University (318011913), the Youth Innovation Team of Shandong Colleges and Universities (2019KJC027, 2020KJC012), the Open Fund of Liaocheng University (2020CESNCTKL02), and the Shandong Students Innovation and Entrepreneurship Training Program (319260210).

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Footnotes

† Electronic supplementary information (ESI) available: Experimental, characterization and additional figures. CCDC 2170346. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3qi00102d

‡ These authors are contributed equally to this work.

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