Highly efficient Fenton and enzyme-mimetic activities of NH2-MIL-88B(Fe) metal organic framework for methylene blue degradation

Here, we show that NH2-MIL-88B(Fe) can be used as a peroxidase-like catalyst for Fenton-like degradation of methylene blue (MB) in water. The iron-based NH2-MIL-88B(Fe) metal organic framework (MOF) was synthesized by a facile and rapid microwave heating method. It was characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, powder X-ray diffraction, and the Brunauer–Emmett–Teller method. The NH2-MIL-88B(Fe) MOF possesses intrinsic oxidase-like and peroxidase-like activities. The reaction parameters that affect MB degradation were investigated, including the solution pH, NH2-MIL-88B(Fe) MOF and H2O2 concentrations, and temperature. The results show that the NH2-MIL-88B(Fe) MOF exhibits a wide working pH range (pH 3.0–11.0), temperature tolerance, and good recyclability for MB removal. Under the optimal conditions, complete removal of MB was achieved within 45 min. In addition, removal of MB was above 80% after five cycles, showing the good recyclability of NH2-MIL-88B(Fe). The NH2-MIL-88B(Fe) MOF has the features of easy preparation, high efficiency, and good recyclability for MB removal in a wide pH range. Electron spin resonance and fluorescence probe results suggest the involvement of hydroxyl radicals in MB degradation. These findings provide new insight into the application of high-efficient MOF-based Fenton-like catalysts for water purification.

reaction (i.e., the water must be acidified) in the above studies are drawbacks that hinder the use of the MOFs in water purification. Most recently, MOFs have been reported to exhibit peroxidase-like catalytic activity [22][23][24][25][26] . This opens the door for development of MOF-based nanoscale platforms for sensing application in the bioanalytical field 27,28 . However, research of MOFs as enzyme mimetics in an aqueous environment is rare. The first member of enzyme-like active MOFs is PCN-222(Fe) with porphyrinic Fe(III) centers 22 . Subsequently, MIL-53(Fe), MIL-68(Fe), and MIL-100(Fe) with intrinsic peroxidase-like catalytic activity have been developed [23][24][25][26] . Application of these active MOFs as peroxidase mimetics for colorimetric biosensing has also been proposed. These studies demonstrate the potential of MOFs as mimic enzymes. However, application of MOF-based mimic enzymes is still limited. There have been no reports of using MOFs as mimic enzymes with wide pH tolerance and good recyclability for degradation of toxic dyes in water by a Fenton-like reaction.
Herein, we report removal of methylene blue (MB) by Fe-based MOFs (i.e., NH 2 -MIL-88B(Fe) MOF) to evaluate the potential of using MOFs as peroxidase mimic catalysts in the Fenton-like reaction for removal of organic dyes from contaminated water. The NH 2 -MIL-88B(Fe) MOF was synthesized by a fast and facile microwave-assisted approach 29 . Interestingly, the as-prepared NH 2 -MIL-88B(Fe) MOF shows intrinsic oxidase-like activity, peroxidase-like activity, and excellent catalytic performance for MB degradation by a Fenton-like reaction in a wide pH range. The narrow pH range remains a limitation for removal of organic pollutants from water using Fenton-based reactions 30,31 . The catalytic mechanism of NH 2 -MIL-88B(Fe) was investigated and a possible mechanism is proposed based on electron spin resonance (ESR) and fluorescence probe results for detection of •OH free radicals, which confirms the major role of •OH free radicals in degradation of MB. The adsorption isotherm for adsorption of MB on NH 2 -MIL-88B(Fe) was determined and the adsorption kinetics was investigated. The effects of NH 2 -MIL-88B(Fe) MOF and H 2 O 2 concentrations, solution pH, and reaction temperature were also investigated based on the MB and TOC removal efficiencies. Finally, the reusability of NH 2 -MIL-88B(Fe) was investigated. The results show the rapid and good recyclability of NH 2 -MIL-88B(Fe) for removal of MB from wastewater in a wide pH range by a Fenton-like reaction.

Results and Discussion
Characterization and enzyme-like activity of NH 2 -MIL-88B(Fe). NH 2 -MIL-88B(Fe) was synthesized by a microwave-assisted approach, which has attracted growing attention as a promising way to synthesize MOF materials 29,[32][33][34][35][36][37] . It has the advantages of speed, high efficiency, and high yield 29,32 . However, the microwave-assisted approach has rarely been applied for synthesis of NH 2 -MIL-88B(Fe) 29 . In this work, the NH 2 -MIL-88B(Fe) MOF was synthesized by microwave heating with a yield of ca. 30%. The crystal structure of the as-prepared material was confirmed to be NH 2 -MIL-88B(Fe) by powder X-ray diffraction (PXRD) (Fig. 1a), which is in agreement with a previous report 29 and indicates the success of NH 2 -MIL-88B (Fe) synthesis. It is noted that the PXRD pattern of NH 2 -MIL-88B(Fe) in this work is different from that in Shi's work 12 . This could be caused by the difference in the solvents used for treatment of as-prepared NH 2 -MIL-88B(Fe) 29 . Similar to previous work, the scanning electron microscopy (SEM) image shows that the synthesized NH 2 -MIL-88B(Fe) has a needle-shaped morphology 29 with needle sizes of approximately 800 nm in length and 300 nm in diameter (Fig. 1b). Successful formation of NH 2 -MIL-88B(Fe) was further confirmed by Fourier transform infrared (FTIR) spectroscopy (Fig. S1, Supporting Information). The characteristic absorption peaks associated with NH 2 -MIL-88B(Fe) are observed at around 3490 and 3370 cm −1 , corresponding to the symmetric and asymmetric stretching vibrations of the primary amine groups. The peak at around 1700 cm −1 is attributed to the carboxylic group. The FTIR spectrum is the same as those of previous studies 24,37 . The specific surface area of NH 2 -MIL-88B(Fe) MOF was calculated to be 163.9 m 2 /g. Before using NH 2 -MIL-88B(Fe) in the Fenton-like reaction, the enzyme-like activity of NH 2 -MIL-88B(Fe) was evaluated by oxidizing the typical peroxidase substrate 3,3′,5,5′-tetramethylbenzidine (TMB) in the presence and absence of H 2 O 2 . As shown in Fig. 2a, similar to other enzyme mimic reactions, the typical absorbance peak of the oxidation product of TMB is located at 652 nm 38,39 . The absorbance of the peak at 652 nm significantly increases in the presence of NH 2 -MIL-88B(Fe), confirming the peroxidase-like activity of NH 2 -MIL-88B(Fe). Role of NH 2 -MIL-88B(Fe) in Fenton-like removal of MB dye. As-synthesized NH 2 -MIL-88B(Fe) was applied as a Fenton-like catalyst for removal of MB as a model organic dye. Figure 3a shows the kinetics of MB removal in the presence and absence of H 2 O 2 and NH 2 -MIL-88B(Fe). Slight removal of MB occurs in the absence of NH 2 -MIL-88B(Fe), indicating that direct oxidation of MB by H 2 O 2 is limited. In contrast, when only the NH 2 -MIL-88B(Fe) catalyst is present in the MB solution, the removal efficiency increases with increasing reaction time. The peak removal efficiency of about 52% occurs at 50 min, which is mainly caused by adsorption of MB on NH 2 -MIL-88B(Fe). Notably, when both NH 2 -MIL-88B(Fe) and H 2 O 2 are present in the solution of MB, almost 100% removal of MB is achieved after 50 min. The above observation confirms that about half of the MB removal is attributed to NH 2 -MIL-88B(Fe) acting as a strong peroxidase mimic in aqueous media. This is probably because degradation of MB by H 2 O 2 in the presence of NH 2 -MIL-88B(Fe) mainly originates from generation of highly active •OH free radicals. The presence of •OH free radicals in the catalytic process was confirmed by ESR spectroscopy. The ESR spectrum in the presence of NH 2 -MIL-88B(Fe) shows the four-fold characteristic peak of the typical DMPO-•OH adduct with an intensity ratio of 1:2:2:1 (Fig. 3b), suggesting that NH 2 -MIL-88B(Fe) can decompose H 2 O 2 to •OH radicals. Furthermore, the intensity of the characteristic peak increases with increasing NH 2 -MIL-88B(Fe) concentration (Fig. 3b), confirming the catalytic ability of NH 2 -MIL-88B(Fe) to decompose H 2 O 2 to •OH radicals. MOFs have previously been used as photocatalysts to degrade active dyes [13][14][15][16][17] . Interestingly, as a control, when the mixture of NH 2 -MIL-88B(Fe), MB, and H 2 O 2 was placed in the dark, there was no significant change in MB removal (Fig. 3a). This suggests that NH 2 -MIL-88B(Fe) shows high catalytic activity for MB removal without the need for light irradiation, further confirming the peroxidase-like activity of   Recently, MOFs have been used for removal of hazardous organic compounds, such as benzene 36 , dyes [40][41][42][43][44][45] , nitrobenzene 46,47 , pharmaceuticals and personal care products 48,49 , phenolic compounds 50,51 , aniline 51 , herbicides 52 , and organoarsenic compounds 53 , form aqueous media for water purification. This demonstrates that MOFs are promising materials for organic pollution clean-up. In our case, a significant fraction of MB (52%) was removed by direct adsorption to NH 2 -MIL-88B(Fe). To understand the adsorption performance, the time-dependent adsorption capacity was determined to investigate the kinetics of adsorption of MB on NH 2 -MIL-88B(Fe). The kinetics of MB adsorption on NH 2 -MIL-88B(Fe) shows a rapid initial uptake stage and a subsequent stable stage (Fig. S2, Supporting Information), indicating rapid adsorption of MB on NH 2 -MIL-88B(Fe). The pseudo-first-order and pseudo-second-order equations were used to fit the adsorption kinetics data (Supporting Information). The correlation coefficient for the pseudo-second-order kinetic model is high (>0.99) (Table S1, Supporting Information). The pseudo-second-order model fits the experimental data better than the pseudo-first-order model (Table S1, Supporting Information). Similar results have been obtained for adsorption of MB on MOF-235 41 and methyl orange on Cr-BDCs, such as MIL-53 and MIL-101 42 . Interestingly, the kinetic constant for MB adsorption on NH 2 -MIL-88B(Fe) is larger than that for adsorption of MB on MOF-235 41 (Table S2, Supporting Information), confirming the fast removal rate of MB by NH 2 -MIL-88B(Fe).
The adsorption isotherm was determined after adsorption/desorption equilibrium for 1 h (Fig. S2, Supporting Information). The adsorption isotherm of MB on NH 2 -MIL-88B(Fe) is shown in Fig. S3 (Supporting Information). The Langmuir and Freundlich models were used to describe adsorption of MB on NH 2 -MIL-88B(Fe) (Supporting Information). The results show that the Langmuir model is suitable to describe adsorption of MB on NH 2 -MIL-88B(Fe) ( Table S3, Supporting Information). The maximum adsorption capacity (Q max ) of NH 2 -MIL-88B(Fe) for MB is 61.46 mg/g. This can be attributed to electrostatic interaction between the positive charge of MB and the negative charge of COO − , which is verified by the zeta potential of NH 2 -MIL-88B(Fe) (Fig. S4, Supporting Information). The surface charge of NH 2 -MIL-88B(Fe) remains negative. Thus, adsorption of positively charged MB on negatively charged NH 2 -MIL-88B(Fe) could be by electrostatic attraction. The charge-balancing anion of NH 2 -MIL-88B(Fe) 41 and π-π interaction 54 between the benzene rings of NH 2 -MIL-88B(Fe) MOF and MB could also be responsible for MB adsorption.  concentration from 0 to 0.2 g/L. This enhancement in the degradation efficiency is because of an increase in the •OH radicals (Fig. 3b). As the amount of NH 2 -MIL-88B(Fe) increases, more hydroxyl radicals are generated. However, with a further increase in the NH 2 -MIL-88B(Fe) concentration, there is only a slight change in the MB removal efficiency. It is interesting to note that TOC removal steadily increases with increasing NH 2 -MIL-88B(Fe) concentration from 0.1 to 0.5 g/L, and the maximum TOC removal of 65.1% occurs at a NH 2 -MIL-88B(Fe) concentration of 0.5 g/L. Obviously, the removal efficiency of TOC increases as the catalyst concentration increases, while that of MB gets plateau at 0.3 g/L NH 2 -MIL-88B(Fe). This is because the color removal or decolorization is ascribed to the destruction of the whole molecular or the chromophore destruction, while TOC removal is attributed to the mineralization of the organic pollutants. Hence, 100% color removal may not mean the completely mineralization of organic pollutants 55 . At a NH 2 -MIL-88B(Fe) concentration of 0.3 g/L, the color removal was as high as 97% above, while the TOC removal was only 54.6% (Fig. 4a). Thus, a further increase in catalyst dosage above 0.3 g/L caused no obvious decolorization of MB, although H 2 O 2 decomposition was accelerated as the dose of NH 2 -MIL-88B(Fe) increased and more hydroxyl radicals generated (Fig. 3b). This suggests that some organic intermediates with low molecules were generated 56  The solution pH is an important factor that can remarkably affect catalytic reactions. Thus, the effect of pH on the degradation efficiency of MB and TOC removal was investigated in the pH range 3.0-11.0 (Fig. 4c). The pH has no significant influence on degradation of MB, and the MB removal efficiency is in the range 88.1-95.5% for the studied pH range. The results for TOC removal are similar and there is a high level of mineralization of MB (47-57%) in the studied pH range. The results show that NH 2 -MIL-88B(Fe) can effectively work in a wide pH range. This is different from the conventional Fenton reaction, which usually requires acidic conditions. This wide pH range can simplify the treatment procedure for practical application and lower the treatment cost. Therefore, the pH of the MB solution was not adjusted for the subsequent experiments.
The effect of temperature on the degradation efficiency of MB and TOC removal was also investigated. The degradation efficiency of MB and TOC removal are plotted against time at different temperatures in Figs 4d and S5 (Supporting Information). The degradation efficiency of MB and TOC removal are higher at higher temperature. Only 15 min is needed for complete removal of MB when the temperature is ≥308 K (Fig. 4d). This shows that the NH 2 -MIL-88B(Fe) MOF has high temperature tolerance because it can effectively work at a high temperature (318 K), and it is superior to most natural enzymes. The TOC abatement efficiency reaches 41.44% after 30 min reaction at room temperature (Fig. S5, Supporting Information). Hence, for operational convenience and practical consideration, room temperature was chosen for the following experiments.

Possible catalytic mechanism.
To determine the catalytic mechanism of NH 2 -MIL-88B(Fe), hydroxyl radical formation was investigated using terephthalic acid (TA) as a fluorescence probe. The hydroxyl radical can readily react with TA, forming highly fluorescent 2-hydroxyterephthalic acid 57 . As shown in Fig. S6 (Supporting Information), the fluorescence intensity of TA is weak in the absence of NH 2 -MIL-88B(Fe). However, it increases with increasing amount of NH 2 -MIL-88B(Fe). This suggests that •OH is generated by H 2 O 2 decomposition. H 2 O 2 decomposition accelerates as the amount of NH 2 -MIL-88B(Fe) increases and more hydroxyl radicals are generated, which was confirmed by ESR spectroscopy (Fig. 3b). The above observations confirm that the oxidation mechanism of MB catalyzed by NH 2 -MIL-88B(Fe) can be ascribed to generation of •OH by decomposition of H 2 O 2 . p-Benzoquinone (BQ) was also used to investigate the possible active intermediate in the reaction system, because it has been reported that BQ is a good trapper of O 2 •− radicals 58 . The results indicate that MB degradation is inhibited in the presence of BQ (Fig. S7, Supporting Information), suggesting that O 2 •− radicals are formed in catalytic degradation of MB by H 2 O 2 in the presence of NH 2 -MIL-88B(Fe). To further confirm this, the effect of superoxide dismutase (SOD, the specific O 2 •− scavenger) on the removal of MB was studied. As shown in Table  S4, MB removal decreased by more than 28% in the presence of 20 U/mL SOD as the specific O 2 •− scavenger, indicating the presence of O 2 •− radical in MB removal.

Recycling of NH 2 -MIL-88B(Fe).
When a MOF is used as a heterogeneous catalyst, its stability under the reaction conditions is an important issue that needs to be considered 10 . Hence, recycling experiments were performed to evaluate the durability of NH 2 -MIL-88B(Fe). The results show that MB removal is above 80% after five cycles (Fig. 5). The loss of the catalytic activity is probably related to the nanosize and good dispersibility of NH 2 -MIL-88B(Fe) in aqueous solution, leading to its partial recovery by centrifugation. The cumulative loss of Fe ions between the cycles is probably another reason for the observed decline in MB removal, although only a small amount of Fe 3+ leaches out (<0.5%) at pH 3.0-11.0 and the concentration of dissolved iron in solution is less than 0.2 mg/L (Fig. S8, Supporting Information). In addition, after five cycles, the BET surface area of , and DMF (10 mL, 0.13 mol) were mixed by magnetic stirring for 30 min at room temperature to form a solution with a Fe 3+ /NH 2 -BDC/DMF molar ratio of 1:1:282. After the solution was degassed by shaking in an ultrasonic bath for 5 min, the resulting mixture was transferred into a 100 mL Teflon autoclave, which was sealed and placed in a microwave oven (Mars-5, CEM, maximum power 1200 W). The autoclave was heated to 150 °C in about 5 min and then maintained at this temperature for 15 min. The microwave power was set to 600 W throughout the whole synthesis process, including the heating-up stage. After the reaction, the resulting suspension was centrifuged. The obtained solid product was purified by triple treatment in DMF and ethanol at 60 °C for 1 h, and then dried under vacuum at 60 °C overnight. A reddish brown power was obtained with a yield of about 30%.
Instrumentation. The ultraviolet-visible measurements were performed with a UV-2450 Shimadzu spectrophotometer (Suzhou, China). The SEM images were recorded by a Hitachi S-4800 field emission scanning electron microscope (Hitachi, Japan) with an accelerating voltage of 10 kV. The PXRD patterns of the as-prepared products were recorded with a XD-3X-ray diffractometer (PuXi, Beijing, China) under the conditions of nickel-filtered CuKα radiation (λ = 0.15406 nm) at a current of 20 mA and a voltage of 36 kV. The scanning rate was set to 4°/min. The specific surface area was determined by the Brunauer-Emmett-Teller method using an ASAP 2020 Micromeritics instrument (Maike, USA) at 77 K. The FTIR spectra were recorded with a Nicolet 170SX spectrometer (Madison, WI, USA) in transmission mode using KBr pellets of the sample. The TOC measurements were performed with a Hach IL TOC-550 TOC analyzer (Hach, USA). A MARS-5 microwave heating apparatus was used for preparation of NH 2 -MIL-88B(Fe). The ESR spectra were recorded with an X-band Bruker ESP300 E ESR spectrometer (Bruker, Germany).  Instrument Co., Ltd., Tianjin, China) and used for MB and TOC analysis. To evaluate the stability of the as-prepared NH 2 -MIL-88B(Fe) catalyst, after the removal experiment, the resulting mixture was centrifuged and the supernatant was discarded. The isolated NH 2 -MIL-88B(Fe) catalyst was directly used for the next cycle. The concentration of MB was determined by measuring the absorbance of the solution at 662 nm. The effect of the free radical inhibitor was evaluated by adding p-benzoquinone (an O 2 •− radical quencher) 58 into the reaction solution.