English

• Phenolic substances (phenol, 4-chlorophenol, 4-nitrophenol, etc) are widely used in food, medicine and industry, but many of these phenolic substances are discharged into the water as effluent, which has resulted in serious water pollution around the world [1–3]. We need an eco-friendly settlement that can detect and removal phenolic substances in wastewater [4–6]. Phenol is the most common pollutant of phenolic substances exist in wastewater and the World Health Organization (WHO) listed it as priority control pollutant in water due to its carcinogenic nature. As for the U.S. Environmental Protection Agency (EPA), the maximum permitted limit of phenol in drinking water is 1 µg/L [7–9]. To detect phenol in aqueous medium, some methods have been reported including chromatographic, spectrophotometric and electrochemical. Nevertheless, the colorimetric analysis is an effective method for detecting phenol owing to its high degree of precision, selectivity and sensitivity [10,11]. Besides that, the removal of phenol is extremely important, consequently, scientists are devoted to develop a new promising method with prominent efficiency. Many methods have been obtained for removing phenolic substances in wastewater such as biological treatment, adsorption extraction, chemical precipitation and membrane separation [12–14]. Compared with above methods, the photodegradation of phenolic substances is one of the promising method due to its low treatment cost and no secondary pollution [15,16]. Therefore, design and synthesis new catalysts for both trace detection and photodegradation of phenol is a challenging and meaningful work.

  Polyoxometallates (POMs) with oxygen-rich surfaces and highly negative charges, have attracted more and more attention due to their interesting structural diversity and potential applications [17–26]. POM-based metal-organic complexes (POMOCs) as a branch of POMs, are often used as a catalytic material widely used in photo/electro-catalysis, electrocatalysis, adsorption, magnetism and other fields due to their excellent stability [27–32]. Researchers have reported several POMOCs to detect or removal phenolic substances. Ma et al. has obtained a new POMOC [Co₂(btap)₄(H₂O)₄][SiW₁₂O₄₀] (btap = 3,5-bis(triazol-1-yl)pyridine) with two fold interpenetrated and displayed excellent colorimetric detection performance to phenol with LOD of 1.32 µmol/L [33]. Su et al. has synthesized a new POMOC hybridy {((Cu(bipy))₂(µ-PhPO₃)₂Cu(bipy))₂H(PCuW₁₁O₃₉)·3H₂O}_n (bipy = 2,2′-bipyridine) used as biocatalyst for degradation of phenol, 4-chlorophenol (4-CP) and 2,4-dichlorophenol (2,4-DCP) [34]. To our knowledge, the colorimetric analysis and photodegradation of phenol using a single material has not been well-explored. Hence, this study is highly significant and useful for industrial application.

  In this work, we obtained a new POMOC by using Keggin-type [SiW₁₂O₄₀]^4– (SiW₁₂) and 3-HPA ligand in presence of dap under solvothermal reaction, namely {[Cu(dap)(3-PA)]₄(SiW₁₂O₄₀)(H₂O)₂}·2H₂O (dap = 1,2-diaminopropane, 3-HPA = 3-pyridineacrylic acid) (1), and characterized it by elemental analysis, IR spectra, PXRD and UV–vis diffuse-reflectance. 1 as a peroxidase, evaluated through H₂O₂, and the oxidative coupling reaction of phenols and 4-aminoantipyrine (4-AAP) proves the good catalytic activity of 1, a rapid colorimetric method for the determination of phenols was established. The LOD is only 0.36 µmol/L using 1 as a peroxidase mimic. Furthermore, 1 exhibits excellent photocatalytic degradation performance toward phenol with the removal efficiency of 96% for 100 min, and has satisfactory selectivity, stability and reliability.

  Synthesis of {[Cu(dap)(3-PA)]₄(SiW₁₂O₄₀)(H₂O)₂}·2H₂O (1): A mixture of Cu(NO₃)₂·3H₂O (0.50 g, 2.07 mmol), H₄[SiW₁₂O₄₀]·xH₂O (0.50 g, 0.17 mmol), 3-HPA (0.40 g, 2.68 mmol), 1,4-naphthalenedicarboxylic acid (1,4-H₂NDC) (0.20 g, 0.92 mmol), dap (0.2 mL), H₂O (4 mL) and acetonitrile (2 mL) was stirred for 2 h (pH_start = 3.77). The resulting mixture was added to a 25 mL Teflon-lined stainless steel reactor, then kept at 120 ℃ for 4 days and cooled to room temperature (pH_end = 3.71). Blue block crystals were filtered from solution, washed by ultrapure water and acetonitrile with a yield of 54% based on H₄[SiW₁₂O₄₀]·xH₂O. Anal. calcd. for C₄₄H₇₀Cu₄N₁₂O₅₂SiW₁₂: C, 12.92; H, 1.73; N, 4.12. Found: C, 12.98; H, 1.81; N, 4.10. IR (KBr, cm^–1): 3558 (w), 3438 (w), 3324 (w), 3245 (w), 2954 (w), 2869 (w), 1641 (m), 1555 (s), 1464 (w), 1441 (s), 1362 (m), 1287 (w), 1259 (m), 1185 (w), 1071 (m), 1009 (m), 999 (m), 969 (m), 912 (m), 873 (m), 775 (s), 696 (w), 633 (w), 537 (m). The crystallographic data structure refinement information and selected bond distances (Å) of 1 are summarized in Tables S1 and S2 (Supporting information), respectively.

  Complex 1 used a mixture of Cu(NO₃)₂·3H₂O, H₄(SiW₁₂O₄₀)·xH₂O, 3-HPA, 1,4-H₂NDC, and H₂O at 120 ℃ for 4 days under solvothermal conditions. Although 1,4-H₂NDC was used as a starting material for the synthesis of 1, 1,4-H₂NDC was not present in 1. Exploratory experiments showed that 1 is not obtained under the same conditions after removal of 1,4-H₂NDC. The addition of 1,4-H₂NDC may provide a suitable solution environment for high-quality crystalline materials and play a role in mineralization [35]. During the synthesis of complex 1, we carried out the parallel experiments under the same conditions. When replaced Cu(NO₃)₂·3H₂O by Cu(OAc)₂·H₂O or CuCl₂·2H₂O, no blue crystal produced; when we added 0.1, 0.15, 0.2, 0.25 or 0.3 mL of dap respectively, the results showed that the same product was obtained in the range of 0.15–0.25; and we tried 100 ℃, 120 ℃, 140 ℃, and only obtained crystals at 120 ℃.

  The results of Bond-valence-sum (BVS) calculations show that the valence of Cu, Si and W atoms in 1 are +2, +4 and +6 (Table S3 in Supporting information), verifying the charge balance of 1 [36]. Single crystal X-ray diffraction indicated that complex 1 crystallizes in triclinic space group P-1. The structure unit of 1 is consists of one [SiW₁₂O₄₀]⁴⁻ anion, two {[Cu(dap)(3-PA)]₂(H₂O)}²⁺ metal-organic units, and two lattice water molecules (Fig. 1a). [SiW₁₂O₄₀]^4– anion presents a classic Keggin-type structure, and eight oxygen atoms surround the central atom Si in a semi-occupied manner [Si–O: 1.57(3)–1.71(3) Å, W–O: 1.66(3)–2.50(3) Å]. The 3-PA ligands exhibit cis and trans coordination modes: 3-PA-a and 3-PA-b. Two crystallographically independent Cu²⁺ ions (Cu1, Cu2) in 1, which adopt pentacoordinated environment in a distorted square-pyramid geometry. Cu1 is coordinated with two N atoms from one dap ligand, one N atom from 3-PA ligand and one O atom from {SiW₁₂} anion, while Cu2 bonds to two N atoms from one dap ligand, one N atom from 3-PA ligand and one O atom from coordinated water molecule (Figs. S1a and b in Supporting information). Thus, the crystallographically independent Cu1 and Cu2 ions are linked by 3-PA-a and 3-PA-b in an alternate manner to form a wave-like 1D metal-organic chain along the b-axis (Fig. 1b, Figs. S1c and d in Supporting information). One dap and a coordinated water molecule decorate on both sides of the chain by coordinating with each Cu ion. Two mutually parallel {[Cu(dap)(3-PA)]₂(H₂O)}_n²ⁿ⁺ chains are bridged by SiW₁₂ anions and coordinated with Cu1 ions to generate a inorganic-organic nano-ladder (Fig. 1c and Fig. S1e in Supporting information). The Cu–O distances are in the range of 1.92(2)–2.15(2) Å and the Cu–N distances are from 1.97(2) to 2.06(2) Å] (Table S2). The adjacent chains were connected through H-bonds between carboxyllate oxygen and coordination water to form 2D supermolecular layer (Table S4 and Fig. S2 in Supporting information).

  Figure 1

  

  Figure 1.  (a) The structure unit of complex 1. (b) Wave-like {[Cu(dap)(3-PA)]₂(H₂O)}_n²ⁿ⁺ chain. (c) Nano-ladder (dap and coordinated water molecules are omitted for clarity).

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  Up to now, only a few POMOCs constructed with 3-HPA have been reported, such as (3-H₂PA)[(3-HPA)₂Ag][AgAlMo₆H₆O₂₄]·3H₂O (2), [(3-HPA)₂Ag][(H₂O)₂Ag]₂[AlMo₆H₆O₂₄]·2H₂O (3) and HNa₂[(3-PA)(3-HPA)Ag]₂[AlMo₆H₆O₂₄]·8H₂O (4) [37]. As we know, 3-HPA have shown free ligand (in 2), monodentate ligand (in 3) and cis-bidentate ligand (in 4), respectively. It is interesting that 3-PA exhibits cis and trans bidentate coordination modes simultaneously in complex 1.

  Firstly, powder X-ray diffraction (PXRD) and IR spectroscopy were used to characterize the complex 1. The detail information can be found in the supporting information. The simulated and experimental diffraction peaks are matches well, indicating that the phase purity of 1 is well (Fig. S3 in Supporting information). The FT-IR spectrum shows the bands at 999 cm^–1, 969 cm^–1, 948 cm^–1 and 873 cm^–1 are attributed to v(W=O_d), v(Si–O_a), v(W–O_b–W), and v(W–O_c–W), respectively. The absorption bands at 2869–2954 cm^–1 and 1441–1555 cm^–1 are corresponded to –NH₂ and –CH₂ group (Fig. S4 in Supporting information) [38]. Above results showed that the complex 1 were synthesized from the corresponding starting materials.

  As we know that excessive amounts of phenolic compounds are harmful to biological tissue. Therefore, the detection of phenolic contaminants is very important [39,40]. Colorimetry can use color change to qualitatively or quantitatively detect substances. The determination process is simple and rapid, which attracts people's attention [41,42]. In order to explore the peroxidase-like characteristics of 1 (Fig. 2a), H₂O₂ was used as oxidant for the oxidative coupling reaction between phenol and 4-AAP. In the control experiment, only phenol and 4-AAP were added. In this system, when H₂O₂ and 1 are added, the solution changes from colorless to obvious pink by naked eyes, and the UV–vis peak at 503 nm is enhanced, which indicates that oxidative coupling reaction occurs. When only H₂O₂, 1, SiW₁₂, 3-HPA, or Cu²⁺ were added into the system, no obvious pink color was observed by naked eyes, and the UV–vis peak at 503 nm did not increase obviously, indicating that oxidative coupling reaction did not occur. The above phenomena indicated that 1 as a peroxidase mimic, could catalyze the H₂O₂ reaction of phenol and 4-AAP to form quinoneimine. In addition, the same molar mass of 1, SiW₁₂, 3-HPA and Cu²⁺ were used in the system of phenol, 4-AAP and H₂O₂, the UV absorption peak was measured at 503 nm. The absorbance follows the order of 1 > Cu²⁺ > 3-HPA > SiW₁₂ (Fig. 2b). The experimental results show that the activity of 1 as peroxidase mainly comes from the coordination of Cu²⁺ and 3-HPA, and the increase of absorbance may be due to the synergistic effect of metal and 3-HPA.

  Figure 2

  

  Figure 2.  (a, b) UV–vis absorbance spectra of diverse systems and various compounds (1, SiW₁₂, 3-HPA, Cu²⁺ at the same µmol). (c) ·OH-trapping photoluminescence spectra (0.5 mmol/L of TA, 0.5 mg/mL of 1, 5 mmol/L of H₂O₂). (d) Proposed mechanism of phenol catalyzed by 1.

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  In previous reports, hydroxyl radicals (·OH) were usually involved in the reaction catalyzed by peroxidase mimetic. Therefore, photoluminescence experiments were used to explore the mechanism of 1 as a peroxidase mimetic [33]. Since peroxidase generate ·OH when carry out a catalytic reaction. Terephthalic acid (TA) was selected as a fluorescence probe to identify hydroxyl group, and carried out as follows: (I) TA; (II) TA and 1; (III) TA and H₂O₂; (IV) TA, H₂O₂ and 1 in a 5 mL centrifuge tube and set the total volume to 3 mL, react in the dark for 12 h and centrifuge. Then, the fluorescence spectrum of the mixed solution was detected at an excitation wavelength of 326 nm. As shown in Fig. 2c, the fluorescence test results show that III and IV both have an absorption peak at 420 nm, and IV is higher than III solution. The results indicated that the peroxidase-like activity of 1 was mainly derived from ·OH generated by the decomposition of H₂O₂. ·OH can take electrons from the hydroxyl of phenols to produce quinone free radicals due to the outstanding oxidation capacity. Subsequently, the pink quinoneimine can be formed by the oxidative coupling of 4-AAP and quinone radicals (Fig. 2d) [43,44].

  To make better use of the peroxidase activity of 1, a colorimetric sensor for the quantitative analysis of phenolic pollutants (phenol/4-chlorophenol/o-cresol/4-nitrophenol/phloroglucinol) was designed. In order to make the best use of the phenol sensor system, the experimental conditions were optimized in a certain range. Similar to other reported enzyme mimics, the catalytic efficiency of 1 also depends on the amount of 4-AAP, pH, catalyst and H₂O₂. The best experimental conditions were explored by using phenol as a model. When the amount of 4-AAP is in the range of 0.5–2 mg/mL, the absorbance is the strongest at 1.5 mg/mL (Fig. 3a). Complex 1 exhibited peroxidase-like activity at acidic pH conditions and maximum activity was achieved at pH 3 (Fig. 3b). Meanwhile, the absorbance increases with the amount of catalyst 1 in the range of 0.1–1.0 mg/mL (Fig. 3c). Under the selected assay conditions, the absorbance is the strongest when the amount of H₂O₂ is 4 mmol/L (Fig. 3d). Therefore, the optimum conditions for determination are as follows: the concentration of 4-AAP is 1.5 mg/mL, the pH value is 3, the concentration of catalyst is 0.1 mg/mL, and the concentration of H₂O₂ is 4 mmol/L.

  Figure 3

  

  Figure 3.  Effect of (a) concentration of 4-AAP, (b) pH, (c) concentration of catalyst, (d) concentration of H₂O₂ on the colorimetric quantification of phenol.

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  Under the optimal conditions, the absorbance was tested every 5 min, and the absorbance had no obvious change after 30 min (Fig. 4a). Then, phenol of different concentrations was added to 3 mL of solution containing 1 and 4-AAP, H₂O₂ (Fig. 4b). With the increase of phenol concentration, the color of the solution gradually changed from colorless to a distinct pink (Fig. 4c). The absorbance and the phenol concentration show a linear relationship over the range of 0.001–0.1 mmol/L (Fig. 4d). The limit of detection (LOD) for phenol (S/N = 3) was 0.36 µmol/L. As shown in Table 1, the LOD of composite materials are in the range of 0.86–3.33 µmol/L [9,15,16,45]. The corresponding value of POM-based complex is 1.32 µmol/L [33]. The LOD of 1 is apparently lower than most of the enzyme mimetics that have been reported as colorimetric detectors for phenol. The detection limits for 4-chlorophenol, o-cresol, 4-nitrophenol and phloroglucinol were also satisfying (1.89, 2.12, 1.67 and 5.02 µmol/L) (Figs. S5a–h in Supporting information).

  Figure 4

  

  Figure 4.  (a) The absorbance (503 nm) changes of the phenol and 4-AAP system vs. time. (b) UV–vis absorbance spectra of the colorimetric detection of phenol in the 1 + H₂O₂+4-AAP system. (c) Photographs of different concentration of phenol added to the 1 + H₂O₂+4-AAP system. (d) The linear calibration curve for detection at varied phenol concentrations.

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  Table 1

  

  Table 1.  Comparative table for the detection of phenol using different enzyme mimics.

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  In order to verify the selectivity of phenol, the possible interference of coexisting species such as ethanol, acetone, NaCl and 2-ethylimidazole on phenolics was studied (Fig. S6a in Supporting information). Add above substances to the 1 + H₂O₂ + 4-AAP system, the color of the solution still colorless (Fig. S6b in Supporting information). The influence of the above reactions on phenol is almost negligible, which shows that they have good anti-interference performance in phenol analysis.

  The method has been applied to the analysis of tap water (Jinzhou), industrial discharge water (a petrochemical plant) and lake water (Jinzhou, Bohai University). As shown in Table 2, the range of the recoveries toward above three samples is 95%–105%, and the corresponding RSDs were from 1.00% to 5.87% (n = 3). The concentration of phenol is proportional to the absorption intensity with good linear relationship (Fig. S7 in Supporting information). The above results indicated that the method was simple, reliable and sensitive for the visual detection of phenol derivatives in water environment.

  Table 2

  

  Table 2.  Results for the determination of the phenol in three water samples.

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  In order to understand the photocatalytic performances of complex 1 in detail, firstly, the photocatalytic activities of 1 were evaluated by measurement of UV–vis absorption spectrum (Fig. S8a in Supporting information), the band gap energy (E_g) of 1 is 3.21 eV, which displays that 1 can be considered as a semiconductor material (Fig. S8b in Supporting information) [46,47]. As is well known, the Mott-Schottky (MS) electrochemical test can be used to obtain the Fermi level (E_f). As shown in Fig. 5a, the Fermi level E_f is –0.63 eV vs. Ag/AgCl (–0.43 eV vs. NHE) and the MS curve features positive slope. The above results show that 1 is a typical n-type semiconductor. The gap between the E_f and VB potentials of 1 was measured by the VB-XPS method, which is 2.41 eV (2.17 eV vs. NHE) (Fig. 5b). Therefore, the VB of 1 is 1.74 eV (vs. NHE) and the corresponding CB is –1.47 eV (vs. NHE) combining the band gap value (3.21 eV).

  Figure 5

  

  Figure 5.  (a) Mott-Schottky plot of 1. (b) The VB-XPS of 1.

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  Complex 1 was used as photocatalyst to degradate phenol under visible light, and the removal efficiency is affected by various reaction conditions, including the pH, molar ratio of H₂O₂/phenol, amount of catalyst and concentration of phenol. Firstly, the pH range of 1–14 was selected due to the wide pH range in wastewater. As shown in Fig. 6a, with the increase of pH, the removal efficiency of phenol by complex 1 reaches the maximum (96%) at pH 7.5 in 100 min, and then it begins to decrease as pH continues to increase. The detected pH range (1–14) is apparently wider than most reported materials (Table S5 in Supporting information) [9,15,34]. The removal rate of phenol in 30 min is 65.8%, which is apparently higher than that of reported POM-based complex (Table S6 in Supporting information) [34]. As shown in Fig. 6b, the most appropriate molar ratio of H₂O₂/phenol is 46:1 at pH 7.5. The amount of catalyst is one of the key factors affecting the phenol removal efficiency, as shown in Fig. 6c, with the amount of 1 increase from 2.5 mg to 20 mg, the phenol removal efficiency reached the maximum at 5 mg. In addition, 1 exhibits outstanding removal ability for phenol solutions at the concentration of 200 mg/L and 400 mg/L. When the concentration is higher than 400 mg/L, the removal ability for phenol presents a reduction trend (Fig. 6d). Based on these results, we established the optimal photocatalytic degradation conditions; phenol concentration (400 mg/L), pH (7.5), reaction time (100 min), H₂O₂/phenol molar ratio (46:1) and catalyst 1 amount (5 mg) are used for photocatalytic degradation of phenol with the removal efficiency at 96%.

  Figure 6

  

  Figure 6.  The effects of (a) different pH, (b) molar ratio of H₂O₂/phenol, (c) amount of catalyst and (d) concentration of phenol on the phenol removal efficiency. Reaction conditions for (a): Reaction time (100 min), amount of 1 (5.0 mg), molar ratio of H₂O₂/phenol (46:1) and phenol concentration (400 mg/L); the conditions for (b-d) are the same as (a) except that the pH is 7.5.

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  Fig. 7a shows the catalytic activity of complex 1 for phenol degradation under the optimal catalytic condition. The reaction kinetic of photocatalytic process is matched with the pseudo-first-order mode corresponding to Langmuir-Hinshelwood model [48,49], The color of the solution changed from pink to light yellow as the photocatalytic process proceeded (Fig. S9 in Supporting information). The ln(c₀/c) and reaction times of phenol shows an excellent linear relationship with R² of 0.999 [50,51]. The appearance rate constant and corresponding half-life of 1 for phenol is 0.0255 min^–1 and 27.18 min (Fig. 7b), respectively. And we also study the catalytic activity of other starting material for phenol degradation under the optimal catalytic condition (Fig. S10 in Supporting information). We found that the phenol removal efficiency of 3-HPA, SiW₁₂ and Cu²⁺ are only 38%, 43%, 46%. The above results prove that 1 is an efficient catalyst for the removal of phenol. Furthermore, the removal efficiency of phenol for complex 1 indicated no apparent changes after four recycling experiments (Fig. 8a). The XPS (Fig. 8b), PXRD (Fig. S11 in Supporting information) and IR (Fig. S12 in Supporting information) before and after detection and four times photocatalytic degradation reactions were not changed obviously, indicating that 1 has good structural stability.

  Figure 7

  

  Figure 7.  (a) The photocatalytic degradation of phenol in presence of 1 under different irradiation times. (b) The linear relationship between ln(c₀/c) and reaction time (t) of phenol.

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  Figure 8

  

  Figure 8.  (a) The removal efficiency of phenol after four cycles. (b) The XPS spectra patterns of 1 before and after detection/photoreduction of phenol.

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  In order to study the mechanism of photocatalytic degradation process, the reactive species trapping experiments were carried out [52,53]. SN (silver nitrate, 0.01 mol/L), AO (ammonium oxalate, 0.01 mol/L), TBA (tert‑butanol, 0.01 mol/L), and BQ (benzoquinone, 0.01 mol/L) were introduce to scavenge e^–, h⁺, ·OH, and ·O₂^–, the removal efficiency of phenol decrease to 43%, 38%, 51% and 35%, respectively (Fig. 9). The results show that the photocatalytic degradation process of phenol is affected by e^–, h⁺, ·OH, ·O₂^– together, while ·O₂^– plays a major role.

  Figure 9

  

  Figure 9.  The phenol removal efficiency in the presence of different sacrificial agents under visible light irradiation.

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  As shown in Fig. 10, the mechanism of phenol photocatalytic degradation is explicated. The photogenerated electrons and holes on the surface of 1 are separated under light irradiation. The {[Cu(dap)(3-PA)]₂(H₂O)}²⁺ metal-organic units connecting [SiW₁₂O₄₀]^4–, thus facilitate the electrons transfer between [SiW₁₂O₄₀]^4– fragments through {[Cu(dap)(3-PA)]₂(H₂O)}²⁺ units. This impetus can prevent from recombining electron-hole. Subsequently, the interplay between photogenerated electrons and oxygen molecules could to generate superoxide radicals, which can oxidize phenols to produce water and carbon dioxide. Meanwhile, the photogenerated holes and phenols directly response to the form products. The reduced [SiW₁₂O₄₀]^4– can be rapidly re-oxygenated in the presence of O₂ to enter the next cycle [23,54,55].

  Figure 10

  

  Figure 10.  Catalytic mechanism of 1 in the phenol degradation process.

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  In this paper, a new keggin-type POMOC was successfully synthesized and acted as a bifunctional catalyst to detect and degrade phenol. The results display that complex 1 is an excellent colorimetric detector with the detection limit (LOD) of 0.36, 1.89, 2.12, 1.67 and 5.02 µmol/L (S/N = 3) toward phenol, 4-chlorophenol, o-cresol, 4-nitrophenol and phloroglucinol, possessing satisfactory anti-interference. Moreover, complex 1 exhibits outstanding photocatalytic degradation activity toward phenol in the wide pH range with the highest removal efficiency at 96% for 100 min at pH 7.5, and the photocatalytic degradation process obeyed the pseudo-first-order reaction kinetic with the appearance rate constant and corresponding half-life of 0.0255 min^–1 and 27.18 min, respectively. This work provides much stimulation to develop new POMOCs as bifunctional catalysts to detect and degrade phenolic substances in wastewater.

  Declaration of competing interest

  The authors declare no conflict of interest.

  Acknowledgments

  This work was supported by the National Natural Science Foundation of China (NSFC, Nos. 21901018, 21971024, 22271021), the Natural Science Foundation and Education Department of Liaoning province (Nos. 2022-MS-373, 2021-MS-312, LJ2020008).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2023.109148.

  
  
• 1. [1]

     D.W. Li, Y. Cheng, H. Zuo, et al., J. Colloid. Interface. Sci. 603 (2021) 771–782. doi: 10.1016/j.jcis.2021.06.155

  2. [2]

     S. Ray, S. Panjikar, R. Anand, ACS Sens. 2 (2017) 411–418. doi: 10.1021/acssensors.6b00843

  3. [3]

     Y. Gong, X. Zhao, H. Zhang, et al., Appl. Catal. B 233 (2018) 35–45. doi: 10.1016/j.apcatb.2018.03.077

  4. [4]

     X.H. Zhong, Y. Lu, F. Luo, et al., Chem. Eur. J. 24 (2018) 3045–3051. doi: 10.1002/chem.201705677

  5. [5]

     W. Xu, Y.F. Zhang, X. Zhang, X.X. Xu, Q. Wang, J. Haz. Mat. 457 (2023) 131776. doi: 10.1016/j.jhazmat.2023.131776

  6. [6]

     O.G. Sas, M. Castro, Á. Domínguez, B. González, Sep. Purif. Technol. 227 (2019) 115703. doi: 10.1016/j.seppur.2019.115703

  7. [7]

     T. Dogan, E. Bayram, L. Uzun, S. Senel, A. Denizli, Inc. J. Appl. Polym. Sci. 132 (2015) 41981–41987. doi: 10.1002/app.41981

  8. [8]

     Q. Wang, X.Y. Wang, H. Wei, Anal. Chem. 94 (2022) 10198–10205. doi: 10.1021/acs.analchem.2c01695

  9. [9]

     Y. Wang, M.Z. Zhao, C. Hou, et al., J. Taiwan. Inst. Chem. E 102 (2019) 312–320. doi: 10.1016/j.jtice.2019.06.019

  10. [10]

      N.N. Xing, Y.S. Lyu, J. Yang, et al., Environ. Sci. Nano 9 (2022) 2815–2826. doi: 10.1039/D2EN00322H

  11. [11]

      S.Z. Huang, Q. Tang, K.N. Wei, et al., Anal. Chim. Acta 1233 (2022) 340504. doi: 10.1016/j.aca.2022.340504

  12. [12]

      T. Song, C. Xie, Q. Che, P. Yang, J. Ind. Eng. Chem. 122 (2023) 415–425. doi: 10.1016/j.jiec.2023.02.041

  13. [13]

      Y. Yuan, W.G. Pan, R.T. Guo, et al., Sep. Purif. Technol. 297 (2022) 121538. doi: 10.1016/j.seppur.2022.121538

  14. [14]

      X. Chen, R.T. Guo, W.G. Pan, et al., Appl. Catal. B: Environ. 318 (2022) 121839. doi: 10.1016/j.apcatb.2022.121839

  15. [15]

      C. Hou, L.H. Fu, Y. Wang, et al., Carbohyd. Polym. 273 (2021) 118548. doi: 10.1016/j.carbpol.2021.118548

  16. [16]

      S.W. Wu, D.Z. Guo, X.C. Xua, J.M. Pan, X.H. Niu, Sensor. Actuat. B: Chem. 303 (2020) 127225. doi: 10.1016/j.snb.2019.127225

  17. [17]

      D. Zhang, W.Q. Zhang, Z.G. Lin, et al., Inorg. Chem. 59 (2020) 9756–9764. doi: 10.1021/acs.inorgchem.0c00976

  18. [18]

      Z. Zhang, Y.L. Wang, Y. Liu, S.L. Huang, G.Y. Yang, Nanoscale 12 (2020) 18333–18341. doi: 10.1039/D0NR02945A

  19. [19]

      L. Yang, Z. Zhang, C.N. Zhang, et al., Inorg. Chem. Front. 9 (2022) 4824–4833. doi: 10.1039/D2QI00838F

  20. [20]

      X.T. Wang, W.J. Mao, D.S. Wang, et al., Talanta 257 (2023) 124270. doi: 10.1016/j.talanta.2023.124270

  21. [21]

      B.X. Zeng, Y.Z. Li, G.P. Liu, et al., Inorg. Chem. 62 (2023) 10351–10358. doi: 10.1021/acs.inorgchem.3c01218

  22. [22]

      R.D. Lai, Z.K. Zhu, Y.L. Wu, et al., Inorg. Chem. 61 (2022) 21047–21054. doi: 10.1021/acs.inorgchem.2c03576

  23. [23]

      Y. Guan, H.P. Xiao, X.X. Li, S.T. Zheng, Polyoxometalates 2 (2023) 9140023. doi: 10.26599/POM.2023.9140023

  24. [24]

      X.X. Li, C.H. Li, M.J. Hou, et al., Nat. Commun. 14 (2023) 5025. doi: 10.1038/s41467-023-40685-0

  25. [25]

      S.R. Li, W.D. Liu, L.S. Long, L.S. Zheng, X.J. Kong, Polyoxometalates 2 (2023) 9140022. doi: 10.26599/POM.2023.9140022

  26. [26]

      X.X. Li, T. Ji, J.Y. Gao, et al., Chem. Sci. 13 (2022) 4573–4580. doi: 10.1039/D1SC06779F

  27. [27]

      J.C. Liu, J.F. Wang, Q. Han, et al., Angew. Chem. Int. Ed. 60 (2021) 11153–11157. doi: 10.1002/anie.202017318

  28. [28]

      Q.Z. Wang, B.J. Xu, Y.Y. Wang, et al., Inorg. Chem. 60 (2021) 7753–7761. doi: 10.1021/acs.inorgchem.1c00135

  29. [29]

      X.L. Wang, J.Y. Zhang, Z.H. Chang, et al., Inorg. Chem. 60 (2021) 3331–3337. doi: 10.1021/acs.inorgchem.0c03738

  30. [30]

      D. Liu, B.K. Chen, J. Li, et al., Inorg. Chem. 60 (2021) 3909–3916. doi: 10.1021/acs.inorgchem.0c03698

  31. [31]

      Y.N. Liu, J. Wang, L.N. Li, et al., Chem. Mater. 35 (2023) 3941–3950. doi: 10.1021/acs.chemmater.3c00120

  32. [32]

      S.Z. Chang, H.Y. An, Y.H. Chen, et al., ACS. Sustain. Chem. Eng. 10 (2022) 4728–4740. doi: 10.1021/acssuschemeng.2c00351

  33. [33]

      J.J. Xin, H.J. Pang, Z.X. Jin, et al., Inorg. Chem. 61 (2022) 16055–16063. doi: 10.1021/acs.inorgchem.2c02454

  34. [34]

      Y. Lu, T. Zhang, Y.X. Zhang, et al., Dalton. Trans. 50 (2021) 15198–15209. doi: 10.1039/D1DT02644E

  35. [35]

      Z.Y. Du, Z. Chen, R.K. Kang, et al., Inorg. Chem. 59 (2020) 12876–12883. doi: 10.1021/acs.inorgchem.0c01941

  36. [36]

      I.D. Brown, D. Altermatt, Acta. Crystallogr. B 41 (1985) 244–247.

  37. [37]

      Y. Hu, H.Y. An, X. Liu, et al., Dalton. Trans. 43 (2014) 2488–2498. doi: 10.1039/C3DT52987H

  38. [38]

      R.J. Liu, X.K. Shang, C.X. Li, et al., Int. J. Hydrogen. Energ. 38 (2013) 9954–9960. doi: 10.1016/j.ijhydene.2013.06.009

  39. [39]

      Y.J. Jang, V.K.H. Bui, P.T. Nguyen, Y.C. Lee, M.I. Kim, Chemosensors 9 (2021) 219–228. doi: 10.3390/chemosensors9080219

  40. [40]

      T.S. Cheng, M.Z.M. Nasir, A. Appl. Mater. Today 9 (2017) 212–219. doi: 10.1016/j.apmt.2017.07.005

  41. [41]

      A. Saeed, M. Umer, N. Yamasaki, et al., ChemElectroChem 7 (2020) 3943–3950. doi: 10.1002/celc.202000544

  42. [42]

      S.R. Jia, X.K. Zhang, F. Yuan, T. Xia, ChemistrySelect 7 (2022) e202202984. doi: 10.1002/slct.202202984

  43. [43]

      D.P. Li, Y.L. Tong, J. Huang, et al., J. Mol. Catal. A: Chem. 345 (2011) 108–116. doi: 10.1016/j.molcata.2011.06.002

  44. [44]

      Y.H. Xiong, S.H. Chen, F.G. Ye, et al., Anal. Methods 7 (2015) 1300–1306. doi: 10.1039/C4AY02687J

  45. [45]

      H.J. Liu, Y.J. Du, J. Gao, et al., Ind. Eng. Chem. Res. 59 (2020) 42–51. doi: 10.1021/acs.iecr.9b04391

  46. [46]

      Z.W. Guo, L.H. Lin, J.P. Ye, et al., Angew. Chem. Int. Ed. 62 (2023) e202305260. doi: 10.1002/anie.202305260

  47. [47]

      R.G. Yang, Y.M. Fu, X. Meng, et al., Inorg. Chem. Front. 10 (2023) 3699. doi: 10.1039/D3QI00217A

  48. [48]

      F.L. Wang, Y.P. Feng, P. Chen, et al., Appl. Catal. B 227 (2018) 114–122. doi: 10.1016/j.apcatb.2018.01.024

  49. [49]

      M.F. Atitar, A. Bouziani, R. Dillert, M. Azzouzib, D.W. Bahnemann, Catal. Sci. Technol. 8 (2018) 985–995. doi: 10.1039/C7CY01903C

  50. [50]

      R. Farouq, J. Chin. Chem. Soc. 65 (2018) 1333–1339. doi: 10.1002/jccs.201800029

  51. [51]

      Z.H. Wang, C.Z. Wang, C.W. Zhang, W.J. Li, Innov. Food. Sci. Emerg. 44 (2017) 224–234. doi: 10.1016/j.ifset.2017.02.013

  52. [52]

      T.T. Cao, T.F. Xu, F.X. Deng, W.W. Qiao, C.W. Cui, J. Photoch. Photobio. A 407 (2021) 113025. doi: 10.1016/j.jphotochem.2020.113025

  53. [53]

      E.F. Grosu, C. Gabriela, R. Froidevaux, Res. Chem. Intermediat. 44 (2018) 7731–7752. doi: 10.1007/s11164-018-3583-x

  54. [54]

      L.H. Yuan, J. Chai, S.W. Wang, et al., Environ. Technol. Innov. 30 (2023) 103085. doi: 10.1016/j.eti.2023.103085

  55. [55]

      K.F. Zhang, Y.X. Liu, J.G. Deng, et al., Appl. Catal. B: Environ. 224 (2018) 350–359. doi: 10.1016/j.apcatb.2017.10.044

• 

  Figure 1  (a) The structure unit of complex 1. (b) Wave-like {[Cu(dap)(3-PA)]₂(H₂O)}_n²ⁿ⁺ chain. (c) Nano-ladder (dap and coordinated water molecules are omitted for clarity).

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  Figure 2  (a, b) UV–vis absorbance spectra of diverse systems and various compounds (1, SiW₁₂, 3-HPA, Cu²⁺ at the same µmol). (c) ·OH-trapping photoluminescence spectra (0.5 mmol/L of TA, 0.5 mg/mL of 1, 5 mmol/L of H₂O₂). (d) Proposed mechanism of phenol catalyzed by 1.

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  Figure 3  Effect of (a) concentration of 4-AAP, (b) pH, (c) concentration of catalyst, (d) concentration of H₂O₂ on the colorimetric quantification of phenol.

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  Figure 4  (a) The absorbance (503 nm) changes of the phenol and 4-AAP system vs. time. (b) UV–vis absorbance spectra of the colorimetric detection of phenol in the 1 + H₂O₂+4-AAP system. (c) Photographs of different concentration of phenol added to the 1 + H₂O₂+4-AAP system. (d) The linear calibration curve for detection at varied phenol concentrations.

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  Figure 5  (a) Mott-Schottky plot of 1. (b) The VB-XPS of 1.

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  Figure 6  The effects of (a) different pH, (b) molar ratio of H₂O₂/phenol, (c) amount of catalyst and (d) concentration of phenol on the phenol removal efficiency. Reaction conditions for (a): Reaction time (100 min), amount of 1 (5.0 mg), molar ratio of H₂O₂/phenol (46:1) and phenol concentration (400 mg/L); the conditions for (b-d) are the same as (a) except that the pH is 7.5.

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  Figure 7  (a) The photocatalytic degradation of phenol in presence of 1 under different irradiation times. (b) The linear relationship between ln(c₀/c) and reaction time (t) of phenol.

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  Figure 8  (a) The removal efficiency of phenol after four cycles. (b) The XPS spectra patterns of 1 before and after detection/photoreduction of phenol.

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  Figure 9  The phenol removal efficiency in the presence of different sacrificial agents under visible light irradiation.

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  Figure 10  Catalytic mechanism of 1 in the phenol degradation process.

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  Table 1.  Comparative table for the detection of phenol using different enzyme mimics.

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  Table 2.  Results for the determination of the phenol in three water samples.

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•