Supported CuO catalysts on metal-organic framework (Cu-UiO-66) for efficient catalytic wet peroxide oxidation of 4-chlorophenol in wastewater

Yue Pan1, Songshan Jiang1[footnoteRef:1], Wei Xiong1, Derong Liu1, Min Li1, Bai He1, Xiaolei Fan2, Dong Luo3 [1: Corresponding author. E-mail: cessjiang@cqust.edu.cn(S. Jiang).]

1School of Chemistry and Chemical Engineering, Chongqing University of Science and Technology, Chongqing, 401331, P. R. China

2School of Chemical Engineering and Analytical Science, The University of Manchester, Oxford Road, Manchester, M13 9PL, United Kingdom

3College of Chemistry and Materials Science, Jinan University, Guangzhou, 510632, P. R. China

Abstract

A novel heterogeneous Fenton catalyst of Cu-UiO-66 metal-organic framework (MOF) was developed for catalytic wet peroxide oxidation (CWPO) of wastewater containing the concentrated 4-chlorophenol. Cu-UiO-66 was characterized comprehensively using XRD, N2 adsorption-desorption, SEM-EDS, TEM, H2-TPR and XPS analysis to reveal its physical and chemical properties. Specifically, XPS results indicated the interaction of Cu species with framework Zr element, and H2-TPR further proved the existence of CuO active component and thermal stability of the synthesized catalysts. The Cu-UiO-66 catalysts exhibited excellent catalytic activity in the CWPO of 4-chlorophenol wastewater (1,000 to 5,000 mg/L) with conversions of 98% achieved in 20 minutes and complete degradation in 40 minutes. Based on the detailed HPLC analysis of the reaction media, mechanistic study of the CWPO of 4-chlorophenol was performed.

Keywords: Cu-UiO-66; heterogeneous Fenton catalyst; catalytic wet peroxide oxidation (CWPO); 4-chlorophenol

1. Introduction

Phenolic wastewaters are highly toxic having various origins (such as the petrochemical, pharmaceutical and coal industries) which impose serious environmental impact and are difficult to be treated by biodegradation [1-3]. In recent years, technologies based on biodegradation [4], physisorption [5, 6] and chemical or photo-catalytic oxidation [7] were intensively studied for the treatment of phenolic wastewater. Biodegradation is suitable for treating wastewaters with the diluted pollutants of <1,000 mg/L [4, 8], while physisorption is appropriate for treating a wide range of concentrations. However, the absorbents need to be regenerated during the operation. Conversely, chemical oxidation can potentially mineralize organic pollutants completely to H2O and CO2. Among chemical oxidation methods, catalytic wet peroxide oxidation (CWPO) using hydrogen peroxide (H2O2) as the oxidant is a versatile and environmentally clean technology, being suitable for treating both diluted or concentrated phenolic pollutants [9, 10]. The exploration and development of heterogeneous Fenton catalysts is crucial to improve the CWPO process [11, 12]. Supported transition metals such as Cu [13, 14] and Fe [15, 16] are common catalysts for CWPO. However, the current catalysts are not effective to achieve the full mineralization of organics (i.e. 60–80%)[13, 16], as well as being associated with the leaching issue. Therefore, the development of effective and efficient heterogeneous Fenton catalysts for the complete mineralization of organic pollutants (before the discharge to the conventional biological wastewater treatment) still needs research effort.

Heterogeneous Fenton catalysts are commonly based on porous materials such as zeolites (e.g., ZSM-5 [17]), mesoporous MCM-41 [18, 19] and SBA-15 [20]), activated carbons [21] etc., which are used as the catalyst supports. Accordingly, the relevant properties of the porous supports are crucial to develop the appropriate supported transition metal catalysts. Recently, Metal-organic frameworks (MOFs) with the high specific surface areas and uniform pore size distributions have been extensively studied [22] for gas adsorption & separation and catalysis [23]. However, due to the generally low hydrothermal stability and poor acid resistance, the use of MOFs for developing supported Fenton catalysts for CWPO has been limited. The acidic and thermal stability of catalyst supports is crucial in developing heterogeneous Fenton catalysts because the pH value of CWPO processes is generally low (i.e. 2–4) and calcination of catalysts at temperatures >250 °C is commonly needed to activate the catalyst. Recently, many relatively stable MOFs [24] have been developed including MIL-101 [25], ZIF-8 [26], and UiO-66 [27-29]. However, some of the MOFs are not resistant to acid media, e.g., ZIF-8 decomposes readily under acidic conditions [30]. Conversely, UiO-66 MOF exhibited the relatively high thermal and chemical stability with high decomposition temperature and acid resistance (pH < 2) because the oxidation state of Zr is 4+, enabling 12 coordinations, and thus making the framework stable [27]. UiO-66 has been used to support photo catalysts for applications in the organic wastewater degradation, e.g., Ag2CO3 supported on the UiO-66 for the photo-degradation of dye (0.03 mM) [31], UiO-66/g-C3N4 hetero junction nanocatalyst used for photo catalytic degradation of methylene blue (10 mg/L) [32]. The photo-degradation is effective for treating wastewater containing diluted organic pollutants (<100 mg/L), while the mineralization of concentrated organics in wastewater (concentration > 1000 mg/L) can be further explored using CWPO. However, the CWPO of wastewater treatment using the stable MOFs based catalysts has not yet been attempted till now.

In this work, we report the development and characterization of the heterogeneous MOFs-based Fenton catalysts, i.e. Cu oxides supported on UiO-66 MOF (denoted as Cu-UiO-66) and the application of Cu-UiO-66 for the CWPO of wastewater containing concentrated 4-chlorophenol. Specifically, in comparison with phenol (which is a common model molecule for research), 4-chlorophenol is highly toxic and effluvial [33-35], being selected as the model compound for this work. Additionally, systematic kinetic and stability study of the developed catalyst and the CWPO catalysis were also carried out.

2. Experimental

2.1 Materials and Chemicals

4-chlorophenol (ClC6H4OH, analytical reagent), hydrogen peroxide (H2O2, 30 wt.% aqueous, analytical reagent) and copper nitrate trihydrate (Cu(NO3)2·3H2O, analytical reagent) were purchased from Chengdu Kelong Chemical Reagent Factory. Zircomiun tetrachloride (ZrCl4, 98%) and p-phthalic acid (HOOCC6H4COOH, PTA, 99%) were obtained from Aladdin Industrial Corporation. N,N-Dimethylformamide (C3H7NO, DMF, analytical reagent) and methanol (CH3OH, HPLC analytical reagent for UiO-66 synthesis) were purchased from Chengdu Chron Chemicals Co., Ltd. Hydrochloric acid (HCl, analytical reagent) was obtained from Chongqing Chuandong Chemical (Group) Co., Ltd. All the chemicals were used as received without any further purification. Deionized water was used in all synthesis and reaction procedure.

2.2 Synthesis of UiO-66 MOF and Cu-UiO-66 catalysts

A solvothermal method was employed to prepare the UiO-66 support. In general, ZrCl4 (1mmol) and p-phthalic acid (1mmol) were firstly added in 60 mL DMF solvent under sonication for 10 minutes to be fully dissolved. Then, HCl (1 mmol) was added in the mixture to adjust the pH value to 2. Finally, the resulting solution was transferred into a 100 mL Teflon lined autoclave and placed in an oven for the solvothermal synthesis. The synthesis temperature and time were varied in the range of 100–150 °C and 12–48h, respectively. After the synthesis, the samples were collected and washed three times with DMF and methanol and then separated by centrifugation. Finally, the synthesised UiO-66 supports were dried in a vacuum oven at 80 °C for 12 hours.

The Cu-UiO-66 catalysts were synthesized by supporting Cu active species on UiO-66 using incipient wetness impregnation. Cu(NO3)2 aqueous solutions were prepared corresponding to 1%, 3% and 5 wt.% Cu loading on UiO-66. To support the metal precursor, the support was immersed in the solution for 12 h and then dried in air at 100 °C for 12 h and calcined at various temperatures of 250, 300 and 350 °C (4 h, temperature ramp = 5°C/min). The catalysts developed were denoted as 1%Cu-UiO-66, 3%Cu-UiO-66 and 5%Cu-UiO-66, respectively.

2.3 Characterization of materials

X-ray diffraction (XRD) of the supports and catalysts were tested using a Rigaku SmartLab 9 kW XRD system with 9 kW rotating anode x-ray generator. The diffraction 2θ range was from 5° to 60°with a resolution of 0.02°.

Nitrogen adsorption-desorption measurements were conducted at −196 °C with a Quantachrome automated gas sorption analyzer (ASIQ C0500-5, Quantachrome, Boynton Beach, FL). Before the measurement, the samples were out-gassed under vacuum at 250°C for 4 h to clean the surface. The surface area (SBET) was obtained using Brunauer-Emmet-Teller (BET) method within the relative pressure (p/po) range of 0.05–0.20. The total pore volume (Vtotal) was estimated at p/p0=0.99.

The textural and morphological information of the synthesized supports and Cu-UiO-66 were characterized using a JEOLJSM-7800Fscanning electronic microscopy (SEM). The catalyst powder was coated with Au films and mounted on a carbon tape prior to the SEM analysis. The energy dispersive spectroscopy (80 mm2 X-Max N Silicon Drift Detector) for element mapping (EDS mapping) was applied to analyze the dispersion of Zr, Cu, and O in the catalysts.

The high-resolution transmission electron microscopy (TEM, FEI Tecnai G20, USA) was operated with accelerating voltage of 200 kV to observe the microstructures of the materials. Before the test, the samples (10 mg) were well dispersed in the ethanol solution by the ultrasound and then loaded on the cupper grid.

X-ray photoelectron spectroscopy (XPS) was tested on an Escalab 250Xispectrometer (Thermo Fisher Scientific) with an Al Kα (1486.6 eV) radiation source operated at 15 kV and 10 mA. The powder samples were fixed on the metal holder and tested at the specific pressure (<10−7 Pa). The binding energy (B.E.) of C1speak at 284.6 eV was taken as a reference.

Temperature-programmed hydrogen reduction experiments (H2-TPR) were carried out in a VDSORB-91i Chemisorption Analyzer with a thermal conductivity detector (TCD). The catalysts were pre-treated under a Helium atmosphere (flow rate = 30 mL/min) at 300 °C and then cool down to 50 °C. Then, a gas mixture of H2 (10 vol%)/Ar flow (flow rate = 30 mL/min) was passed over the sample, followed by a temperature ramp of 10 °C/min from 50 to 800 °C for the reduction process.

Fourier transform infrared (FT-IR) spectroscopy of the catalysts was performed on a Thermo Scientific Nicolet iS10 spectrometer with a resolution of 4 cm−1 in the range from 400 to 4000 cm−1 at room temperature. Samples (1 wt.%) were diluted with KBr to prepare pellets for the FT-IR analysis.

TG analysis of the UiO-66, Cu-UiO-66 and used catalyst were investigated in air using a thermogravimetric analyzer (TGA, HTG-4, Beijing Hengjiu) from 323 to 1073 K with a temperature rate of 10 °C·min−1.

2.4 CWPO of 4-chlorophenol wastewater over UiO-66 catalysts

Catalytic wet peroxide oxidation of 4-chlorophenolover the catalysts was operated in a batch reactor heated by a thermal bath. The model pollutant, 4-chlorophenol wastewater (500 mL, 1000 mg/L) and H2O2 (5100 mg/L) was used to evaluate the activity of the UiO-66 catalysts. The stirring rate with 200 revolutions per minute (rpm) was adjusted by the motor and the reaction conditions were at the temperature of 80 °C under atmospheric pressure. The reaction was continuously carried out for 180 minutes without stop and samples (5 mL each time) were withdrawn from the reactor every 20 minutes (for the first 2 hours and 30 minutes for the last 1 hour) and the pH values were tested immediately. The samples were diluted 10 times in a volumetric flask (50 mL) to obtain H2O2 concentration, 4-chlorophenol concentration and Cu ion concentration. H2O2 concentration was tested by iodometric titration using a 0.02 mol/L sodiumthiosulfate solution. Then, the samples were added with 0.1 g MnO2 at least 15 minutes (removing residual H2O2) before measuring the 4-chlorophenol concentration. The obtained liquid fractions were filtered prior to the high-performance liquid chromatography (HPLC) analysis. Then, the 4-chlorophenol concentration and intermediate products concentration were measured using an HPLC (SHIMADZU) equipped with an InertSustain®C18 column, a UV detector (SPD-20A, wavelength adjusted at 278 nm) and an auto-sampler (SIL-20A). The methanol and ultrapure water (v/v= 40/60) were used as the mobile phase. Cu ions leaching concentration of the final treated solution was tested by an AAS (Atomic Adsorption Spectroscopy, TAS-986F) with an external standard method. The catalytic performance was evaluated by H2O2 conversion (,%), 4-chlorophenol conversion (X4-chlorophenol, %) which were calculated by the following equations:

(1)

(2)

Where,, and (mg/L) are the concentrations of 4-chlorophenol and H2O2 at the initial time and sampling time, respectively.3. Results and discussion

To obtain the UiO-66 MOF support with good stability and crystallinity, synthesis conditions were varied by changing the synthesis temperature (i.e. at 100, 120, 140, 160 and 180 °C for 24 h) and time (i.e. at 120 °C for 12, 24, 36 and 48 h). The XRD patterns of the resulting materials are depicted in Figs. S1. It was found that the relatively high temperature of >120 °C is not beneficial to the crystallinity of UiO-66 (Fig. S1a). The XRD pattern of the samples prepared at 100 and 120 °C agrees well with the reported ones in literature [36]. By increasing the system temperature, the relative crystalline of the UiO-66 decreased, e.g. in comparison to the crystallinity of the UiO-66 prepared at 100 °C, it dropped to 25% when the synthesis temperature 140 °C was used. The resulting materials are totally amorphous when the temperature >140 °C was used. At 120 °C (Fig. S1b), the effect of the synthesis time on the crystallinity of the UiO-66 was insignificant. Accordingly, the synthesis condition at 120 °C for 24 h was chosen to prepare UiO-66 MOF support for the following study.

3.1 Characterizations of Cu-UiO-66 catalysts and CWPO of 4-chlorophenol wastewater

3.1.1 Effect of calcination temperatures

Calcination of the prepared catalyst is an important step to produce the active phase of CuO. However, the calcination at the elevated temperature may also induce the decomposition of the UiO-66 support. Accordingly, in this work, the effect of calcination temperature on the catalyst and the catalysis was investigated. The as-prepared 1%Cu-UiO-66 catalysts calcined at different temperatures were characterized by XPS, H2-TPR and XRD, as shown in Figs. 1 and 2. The XPS patterns of the Cu-UiO-66 catalysts in Fig. 1 exhibited the same Cu 2p and Zr 3d without B.E. shifting. However, the O 1s intensity at B.E. 530.5 eV increased due to the increase of the calcination temperature which could be attributed to more CuO produced at the higher temperature. This can be confirmed by the H2-TPR results in Fig. 2a in which higher CuO reduction peak intensity was observed at the calcination temperature of 300 °C. Meanwhile, XRD patterns in Fig. 2b show that the higher calcination temperature (i.e. 350 °C) jeopardised the crystallinity of the UiO-66 framework.

Fig. 1. XPS of the 1%Cu-UiO-66 catalysts calcined at different temperatures: (a) survey scans, (b) Cu 2p3/2 spectra, (c) Zr 3d5/2 spectra and (d) O 1s spectra.

Fig. 2. H2-TPR profiles (a) and XRD patterns (b) of the 1%Cu-UiO-66 catalysts calcined at different temperatures.

The effect of calcinations temperatures (250, 300 and 350°C) of the 1%Cu-UiO-66 catalysts was explored for the CWPO of 4-chlorophenol wastewater and the results were depicted in Fig. 3. As seen in Fig. 3a, 4-chlorophenol conversion reached 100% in 80 minutes, and the high calcination temperature at 350 °C was not beneficial to the CWPO process which might be due to the framework decomposition at the elevated temperature. Interestingly, the 1%Cu-UiO-66 calcined at 350 °C exhibited the highest H2O2 conversions (85% in 180 minutes in Fig. 3b) but with the relatively low 4-chlorophenol conversion indicating the low efficiency of H2O2 utilization. The relevant HPLC results of the effluent (as the function of time) together with the photos of the effluent were shown in Fig. S2 and Fig. S3, respectively, indicating the optimum calcination temperature of 300 °C.

Fig. 3. Results of the CWPO of 4-chlorophenol over the 1%Cu-UiO-66 catalysts calcined at different temperatures: (a) 4-chlorophenol conversion, (b) H2O2 conversion. Reaction conditions: mcatalysts = 0.5 g, C4-chlorophenol = 1000 mg/L, T = 80 °C, treaction= 180 min.

3.1.2 Effect of Cu loading amount

Fig. 4(a) shows the XRD patterns of Cu-UiO-66 catalysts in reference to the UiO-66 MOF support (all samples were calcined at 300 °C). One can see that, after the Cu loading (at 1%, 3% and 5%, respectively) by impregnation, the characteristic diffraction peaks of UiO-66 at 6–8° and 22° were still identifiable, suggesting that the catalyst preparation did not damage the crystallinity of the support to some extent. However, the relevant peak intensities of UiO-66 decrease by increasing the Cu loading. Comparatively, the diffraction peak at 38.7° was detected on all Cu-UiO-66 catalysts, corresponding to the CuO phase [37], and its intensity increases with an increase in the Cu.

The N2 adsorption-desorption isotherms and the relevant textual properties of the materials are shown in Fig. 4(b) and Table 1. Fig. 4(b) shows that both UiO-66 and Cu-UiO-66 catalysts produce the isotherms approximating the type I adsorption isotherm, and the Cu loading caused the decreasing of BET surface area (decreased by 2.2%, 10.7% and 13.3% after the Cu loading for 1%, 3% and 5%, respectively). Results (Fig. S4) indicated that the supported CuO particles had little influence on the pore size distribution of the UiO-66 materials.

Fig. 4. (a) XRD patterns, (b) N2 adsorption-desorption isotherms, (c) FT-IR spectra and (d) H2-TPR profiles of the as-synthesized UiO-66 support and Cu-UiO-66 catalysts.

Table 1. Textural properties of materials determined by the N2 physisorption analysis.

Catalysts

SBET [m2 g−1]

UiO-66

558

1%Cu-UiO-66

546

3%Cu-UiO-66

498

5%Cu-UiO-66

484

The FT-IR spectra of the UiO-66 MOF and Cu-UiO-66 catalysts are depicted in Fig. 4(c). All the spectra were dominated by an intense and broad absorption band centred at 3440 cm−1 due to intercrystalline water and physisorbed water condensed inside the materials. The absorption band at 1667 cm−1 is ascribed to DMF, while the intense doublet at 1589 and 1395 cm−1 is associated to the in- and out-of-phase stretching modes of the carboxylate group in the UiO-66 catalysts. The absorption bands at lower frequencies modes, e.g., 746, 726, 702, 683, 614, 556 and 475 cm−1 were due to OH and CH bending mixed with Zr–O modes. The triplet absorption bands at 725, 620 and 530 cm−1 were assigned to longitudinal and transverse modes of the Zr–O2, respectively [38]. Results indicated that the incorporate of Cu species has no effect on the framework of UiO-66.

H2-TPR analysis was performed to understand the reduction behaviours of the UiO-66 support and Cu-UiO-66 catalysts (as shown in Fig. 4(d)). The UiO-66 support showed only one reduction peak at 552 °C which could be ascribed to the reduction of Zr species in the UiO-66 framework. Accordingly, all the Cu-UiO-66 catalysts showed the similar reduction behaviour at 552 °C. Conversely, extra reduction peaks at 190 and 288 °C were measured for the Cu-UiO-66 catalysts which belong to the reduction of CuO species [37, 39] in them.

The morphology of the UiO-66 support and Cu-UiO-66 catalysts was studied by SEM (Fig. S5), showing that the crystals of all materials have the typical octahedral and intergrown morphology with well-defined faces and edges. Such morphologies are in line with the reported one in literature [40]. The aggregated crystals have the sizes of about 100–200 nm. The EDS analysis and relevant mapping images of the UiO-66 MOF and Cu-UiO-66 catalysts were shown in Fig. 5, presenting the uniform dispersion of Cu element on the UiO-66 support.

Fig. 5. EDS analysis and the relevant mapping images of the UiO-66 support and Cu-UiO-66 catalysts (calcination temperature at 300 °C).

TEM (shown in Fig. 6) analysis was used to study the microscopic feature and metal dispersion information of the as-synthesized UiO-66 support and Cu-UiO-66 catalysts. For 3%Cu-UiO-66 and 5%Cu-UiO-66, the Cu dispersion on the support is visible. The high-resolution TEM images of the catalysts show the resolved parallel interplanar fringes with the distance of d111 = 0.2 nm corresponding to the CuO phase [41] which was confirmed by the PXRD analysis as well.

Fig. 6.TEM images of the UiO-66 support (a, e, i) and 1%Cu-UiO-66 (b, f, j), 3%Cu-UiO-66 (c, g, k) and 5%Cu-UiO-66 (d, h, l), calcination temperature at 300 °C.

The surface chemical environment of the Cu-UiO-66 catalysts was studied by XPS and the results were presented in Fig. 7. The XPS survey scan of the catalysts (Fig. 7a) gives the information on the element composition and the details of Cu, Zr, and O chemical oxidation states are shown in Figs. 7b, 7c and 7d, respectively. The binding energy (B.E.) of Cu 2p3/2 and Cu 2p1/2 at 932.9 eV and 953.2 eV and two satellite peaks at 941.5 eV and 961.9 eV observed in Fig. 7b could be ascribed to the presence of Cu2+ species [39, 42]. The intensity of the XPS spectra increases with an increase in Cu content and the B.E. shifts to slightly the higher region from 932.8 eV to 933.4 eV. The shift to the higher B.E. is resulted from the losing electrons of Cu species, and hence presenting the higher oxidation states. Fig. 7c show the Zr spectra and the spectrum in the Zr 3d region can be deconvoluted into two peaks for Zr 3d5/2 and Zr 3d3/2 at around 182.9 eV and 185.2 eV, respectively [38]. The peak intensity of the Zr spectra of the catalysts is comparable but the relevant B.E. shifts to the lower region from 182.9 eV to 182.5 eV with an increase of Cu content. The B.E. shift of Zr and Cu spectra indicate the interaction between the supported Cu species and the framework Zr, i.e., Cu species lose the electrons to Zr leading to the increasing valence state of Cu species and decreasing oxidation states of Zr. XPS spectra of O 1s are shown in Fig. 7d giving the presence of framework O (with the B.E. at around 532.0 eV) and Cu–O (with the B.E. at around 530.9 eV) [14].

Fig. 7. XPS of the Cu-UiO-66 catalysts: (a) survey scans, (b) Cu 2p3/2 spectra, (c) Zr 3d5/2 spectra and (d) O 1s spectra.

Cu-UiO-66 catalysts (calcined at 300 °C) were used for the CWPO of 4-chlorophenol wastewater and the UiO-66 support was used as the control. As seen in Fig. 8a, UiO-66 exhibited the lowest activity with the 4-chlorophenol conversion of 25% after 180 min. Conversely, the Cu-UiO-66 catalysts showed the effective degradation of 4-chlorophenol with high conversions of 95% in only 20 min and full conversion in 40 min.

In CWPO, the effective decomposition of H2O2 is the crucial factor for the catalytic degradation. As shown in Fig. 8b, the H2O2 conversion on the Cu-UiO-66 catalysts increases with the increase of the supported Cu content. The 4-chlorophenol was catalytically decomposed into the low-molecular-weight acids which can be detected by HPLC (retention time around 1.5 min as shown in Fig. 9). The pH values in the solution reached 2 in 20 minutes which certificated the fast decomposition of 4-chlorophenol.

Based on the detailed HPLC analysis, the possible reaction pathway was proposed as shown in Fig. 10. Specifically, the H2O2 firstly adsorbs on the surface of Cu-UiO-66 catalysts and is rapidly decomposed into hydroxyl radicals which attack the adsorbed 4-chlorophenol producing benzoquinone and hydrochloric acid. The benzoquinone can be further attacked by hydroxyl radicals and decomposed into formic acid, acetic acid and oxalic acid. Further oxidation of these low-molecular-weight acids gives the carbon dioxide and water. The reaction pathway is similar to the reaction mechanism proposed by the literature [43, 44]. The high activity of Cu-UiO-66 catalysts is resulted from the well dispersion of CuO species on the MOF support.

The photos of the treated wastewater samples by the Cu-UiO-66 catalysts were shown in Fig. S6. Specifically, the sample by the 1%Cu-UiO-66, the gradual colour change (colourless–brown–colourless) is found, which is caused by the production of the intermediate of benzoquinone as a function of the CWPO treatment time. For 3% and 5%Cu-UiO-66 catalysts, they are very efficient, and the according colour change of the effluent samples is not visible.

The images of comparison of the fresh and used catalysts are presented in Fig. S7. Combining the post-reaction FT-IR and XRD of the catalysts (Figs. S8 and S9), the good stability of the Cu-UiO-66 catalysts is demonstrated after being used in the oxidizing and acidic system (i.e. in presence of H2O2 and pH = 2) under the hydrothermal condition (in the aqueous system at 80 °C).

Fig. 8. Results of the CWPO of 4-chlorophenol over the UiO-66 support and Cu-UiO-66 catalysts: (a) 4-chlorophenol conversion, (b) H2O2 conversion. Reaction conditions: mcatalysts = 0.5 g, C4-chlorophenol = 1000 mg/L, T = 80 °C, treaction= 180 min

Fig. 9. HPLC analysis of the treated 4-chlorophenol wastewater over the UiO-66 support and Cu-UiO-66 catalysts: peak 1, formic acid; peak 2, acetic acid; peak 3, oxalic acid; peak 4, benzoquinone and peak 5, 4-chlorophenol.

Fig. 10. Proposed reaction pathway of catalytic CWPO of 4-chlorophenol wastewater over Cu-UiO-66 catalysts.

Furthermore, 4-chlorophenol with four concentrations (1000, 2000, 3000, and 5000 mg/L) were tested for the CWPO process over the 5%Cu-UiO-66 catalyst (500 mg), and results showed that 96% 4-chlorophenol conversions were achieved in 20 min and the full degradation of 4-chlorophenol was obtained in 180 min (calculated by HPLC in Fig. S10a). As seen in Fig. S10b, the effluents become yellow brown at first 20 min with the increasing initial 4-chlorophenol concentration but gradually decolorized with the extended treatment time, as well as being inodorous. These results further proved the high activity of the synthesized Cu-UiO-66 catalysts which can be used for degradation of highly concentrated organic wastewater.

3.2 Stability of the Cu-UiO-66 catalyst

The 5%Cu-UiO-66 catalyst was used twice consecutively for the CWPO of 4-chlorophenol wastewater and the results were shown in Fig. 11. Deactivation of the catalytic system was measured due to the leaching of Cu species and the changing of UiO-66 framework as proved by the post-reaction XRD analysis (Fig. 11b). Mostly, the framework of UiO-66 was preserved after the reaction, indicating the potential of the future application of MOFs heterogeneous Fenton catalyst. The TGA results (before copper impregnation, after calcination and after the catalytic run, Fig. S11) are characterized by mass loss between 250 and 500 °C that is attributable to linker volatilization [45]. Results indicated that thermal stability of UiO-66 decreased after loading of CuO catalysts. The Cu ion leached in the solution was determined by Atomic Absorption Spectroscopy as about 15.6, 34.4 and 50.0 mg/L for the 1%Cu-UiO-66, 3%Cu-UiO-66 and 5%Cu-UiO-66 catalysts, respectively, which are lower than that reported by other works at ca. 250 mg/L [37, 39]. Although these catalysts exhibited excellent activity for the CWPO process, further research should be focused on improving the stability of Cu species supported on UiO-66.

Fig. 11. Reusability of the 5%Cu-UiO-66 catalysts: (a) 4-chlorophenol and H2O2 conversions and (b) XRD patterns of the used catalysts.

4. Conclusions

Cu-UiO-66 catalysts were developed as the heterogeneous Fenton catalyst catalytic wet peroxide oxidation of wastewater containing concentrated 4-chlorophenol (1000–5000 mg/L). Comprehensive characterization of the developed catalysts was performed using XRD, FT-IR, N2 adsorption-desorption, SEM-EDS and TEM techniques, showing that UiO-66 can promote the dispersion of Cu species without compromising its intrinsic properties to a large extent. XPS results indicated the interaction of Cu species with framework Zr element, and H2-TPR further proved the existence of CuO active component and thermal stability of the synthesized catalysts. Calcination of the as-prepared Cu-UiO-66 catalysts was necessary to produce CuO species, and 300 °C was identified as the appropriate calcination temperature to avoid the destruction of UiO-66 support framework. The Cu-UiO-66 catalysts exhibited excellent catalytic activity in the CWPO of concentrated 4-chlorophenol (1000 mg/L) in wastewater with 4-chlorophenol conversions reached 98% in 20 minutes and completely transformed in 40 minutes. The present work demonstrates that the Cu-UiO-66 is a promising MOF heterogeneous Fenton catalyst for the CWPO of high concentration organic wastewater in the environmental field. Further research should be focused on improving the stability supported Cu species to minimize the leaching, and hence the loss of activity.

Acknowledgement

The authors thank the National Natural Science Foundation of China (No. 51708075), the Natural Science Foundation of Chongqing, China (No. cstc2019jcyj-msxmX0401), Scientific & Technological Research Program of Chongqing Municipal Education Commission (Grant No. KJ1713335, KJ1713342 and KJQN201801527) and the open foundation of Guangxi Key Laboratory of Processing for Non-ferrous Metals and Featured Materials, Guangxi University (Grant No. 2019GXYSOF14) for the financial support to this work. XF thanks the partial support by The Royal Society International Exchange Award (IE161344) to enable the collaboration.

Supplementary Material

The XRD patterns of the synthesized UiO-66 support: effect of synthesis temperature and synthesis time (Fig. S1), HPLC analysis of the treated solution over the UiO-66 catalysts effect of calcination temperature (Fig. S2), pore size distribution (Fig. S4), photos of the treated solutions (Fig. S3, Fig. S6, and Fig. S10), SEM images of the UiO-66 support and Cu-UiO-66 catalysts (Fig. S5), photos of the fresh & used catalysts (Fig. S7), FT-IR of the fresh & used catalysts (Fig. S8), XRD patterns of the used Cu-UiO-66 catalysts (Fig. S9) and TGA results (Fig. S11) can be found in the Supplementary Material.

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