Heterogeneous catalytic decomposition of hydrogen peroxide utilizing a Fe(III)-based metal–organic framework as an efficient and persistent nanozyme

Abstract

Catalases exert their crucial role in protecting living cells from damage caused by reactive oxygen species (ROS) through effectively catalyzing hydrogen peroxide decomposition to water and oxygen. Despite the fact that naturally occurring enzymes demonstrate unparalleled activity in their natural habitat, they suffer from rapid denaturation and loss of activity under harsh conditions. Thus, it is of great interest to develop an alternative material that demonstrates intrinsic enzyme-like activity mimicking the catalase behavior, but with enhanced structural stability under industrially relevant conditions. Nanomaterials with enzyme-like activity known as “Nanozymes” have been widely used in several catalytic processes. Herein, we demonstrate the superior catalytic activity of a Fe-based metal organic framework known as MIL-53(Fe) as a cheap nanozyme generated through a facile solvothermal synthesis to catalytically mediate the H₂O₂ decomposition. The data shows that the MOF demonstrated exceptional rapid catalytic activity towards H₂O₂ decomposition without applying any heat, additionally with high recyclability without significant loss in activity for an extended number of cycles.

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Introduction

Catalytic processes represent the cornerstones of many chemical transformations, ranging from those used in the industrial scale for the production of valuable feedstock and commodity products, to those occurring within living cells deriving metabolism and passage of the genetic code. The quest for an improved and optimized catalyst, designed for a specific reaction, is a multi-scale process that requires careful design and implementation. The advent of hybrid microporous solids, best represented by metal–organic frameworks (MOFs), promise unparalleled potential for their application in chemical catalysis. This is mostly due to the multitude of desirable attributes of MOFs, including their permanent porosity, which facilitates access to the active sites within the material through intriguing pore systems, associated with the underlying topology of such solids.¹ Moreover, due to their hybrid organic–inorganic composition of metal ions (or clusters), stitched through organic linkers, the chemical functionality of their molecular components can be exploited in the isolated solid. Several reports demonstrated the applicability of this design to tailor specific microporous solids for catalytic applications in the oxidation of hydrocarbons,² epoxidation of alkenes,^3,4 neutralization of nerve agents,⁵ decontamination of organic dyes,^6–8 among others.

The seminal contribution of Yan et al. of the peroxidase-like Fe₃O₄ magnetic nanoparticles⁹ stimulated the rapid growth of investigating similar constructs as Nanozymes.¹⁰ Nanozymes possess several attributes that distinguish them from natural enzymes, namely their stability against denaturation under harsh reaction conditions as well as their much lower cost of preparation, purification, and storage.¹⁰ MOFs can potentially provide a platform material capable to address the above-mentioned drawbacks of natural enzymes, yet surpass conventional nanoparticle-based catalysts due to their microporosity and hence the maximized accessibility to the true active site(s). In this study, we describe the utilization of a Fe-terephthalate MOF, known as Fe-BDC, or MIL-53(Fe), as a simple, cost-effective Nanozyme for the catalytic decomposition of hydrogen peroxide in an aqueous solution. This MOF can be readily constructed through a facile solvothermal synthesis from cheap and abundant components, where its cavities will facilitate the accessibility of target to the catalytic Fe center within the MOF structure.

Results and discussion

Fe-BDC MOF catalyst synthesis and characterization before and after H₂O₂ catalytic degradation

An iron carboxylate-based metal organic framework, named Fe-BDC MOF was successfully synthesized using a solvo-thermal method, as previously reported in the literature.¹¹ Then, several techniques were employed to characterize the MOF catalyst pre and post the catalytic degradation reaction including Fourier-transform infrared (FTIR) spectroscopy, X-ray powder diffraction (XRD), thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS) and gas sorption measurements. The FT-IR spectra (Fig. 1) reveal the characteristic peaks of the isolated pristine MOF, with C=O stretching frequency at 1660 cm⁻¹ as well as O–C–O asymmetric stretching frequency at 1505–1592 cm⁻¹.^12,13 Moreover, the FT-IR of the nanozyme after degrading the H₂O₂ indicated maintained functional groups associated with the organic backbone, albeit with evident transformation into different modes of Fe–CO₂ coordination, which can be ascribed to the formation of loosely coordinated Fe(O)_x species within the material upon oxidation with H₂O₂. Moreover, significant absorption at ∼3200 cm⁻¹ indicates the presence of –OH terminated species, most likely Fe–OH/OH₂ from the decomposed H₂O₂.¹⁴ To assess the crystallinity and phase purity of the as-synthesized Fe-BDC MOF, X-ray powder diffraction (XRD) was recorded for the MOF before adding H₂O₂, which demonstrated characteristic peaks that well matched with those reported for Fe-BDC MOF in a previous literature.¹⁵ Thus, it confirmed that the pure Fe-BDC MOF was prepared successfully under the used conditions. However, after the successive degradation of H₂O₂, it could be observed that the Fe-BDC MOF XRD pattern degraded slightly amorphized, and the peaks became broader. It seems that the degradation process impacted the crystallinity of the MOF, leading to a slight reduction in the framework crystallinity and structural alterations without the full loss of crystallinity.^16,17 As the common major diffraction peaks (Fig. 2) at 2θ = 9.24°, 13.6°, 18.3°, 19.2°, 21.3°, 25.8°, 30.6° and 44.7° were maintained after the catalytic reaction as well as being consistent with the simulated ones,^11,15,18 they indicated the survivability of Fe-BDC MOF even under a harsh degradation environment. However, the disappearance of the peaks at 2θ = 12.4°, 13.9°, 16.1°, and 21.9° and the appearance of a new strong peak at 2θ = 10.8° and other weak ones at 2θ = 13.2°, 18.4°, 19.7° and 26.7° after H₂O₂ degradation may indicate the notable breathing behavior with an adapted pore opening as a result of a possible solvent exchange or a H₂O₂ adsorption process during the catalytic process.^19–21

Fig. 1 FTIR spectra of Fe-BDC MOF prior, and post H₂O₂ degradation.

Fig. 2 X-ray diffraction patterns of the as-synthesized Fe-BDC MOF prior and post H₂O₂ degradation. Highlighting the breathing phenomena of the MOF upon degrading the H₂O₂.

This phenomenon may be attributed to the different forces imposed on the framework during the H₂O₂ degradation accompanied by the solvent/guest exchange,^17,22 resulting in Fe-BDC pore size profile swapping.^19,23 The switching of disordered guest molecules significantly affects the volume of the unit cell and leads to disordering within the structure, which results from the diverse coordination interactions with the MOF metal nodes.^24,25 Moreover, the notable shifting in some of the main diffraction peaks may be due to the dehydration of some crystalline domains during the oxidation process, which led to a minor deflection of adjacent domains and increased the number of crystallite orientations in some portion of the sample. Thermogravimetric analysis (TGA) was conducted under N₂ atmosphere to study the thermal stability of the MOF catalyst before and after the degradation (Fig. 3). The TGA pattern of Fe-BDC MOF shows that a continuous weight loss commences around 200 °C and continues up to ∼330 °C, which corresponds to the loss of coordinated water molecules. Above 330 °C, the decomposition of the BDC²⁻ is followed by conversion of the Fe species into Fe(O)_x.

Fig. 3 TGA of the as-synthesized Fe-BDC MOF prior, and post H₂O₂ degradation.

After exposure to H₂O₂, TGA indicated an additional weight loss step at 500 °C, which was not present in the pristine MOF, most likely a metal–carbonate decomposition step, which is in agreement with the XPS assignment on the partial formation of metal–carbonate species upon H₂O₂ exposure.

As shown in Fig. 4, the XPS survey spectrum of the obtained Fe-BDC MOF before and after treatment with H₂O₂ confirmed the presence of Fe, C, and O elements on the surface of the sample, albeit with the increased Fe, O/C ratio in the H₂O₂-treated sample, as shown in Table 1. For the pristine sample, the C 1s XPS spectrum demonstrated a peak at 284.88 eV assigned to C–C/C=C of the ligands, a small shoulder at 286.28 eV that could be assigned to oxygenated C–O species, and a peak at 288.74 eV characteristic of O–C=O groups of the linkers.²⁶ The O 1s spectrum can be de-convoluted into three peaks at 530.5 eV assigned to Fe–(OH) groups, 531.8 eV attributed to the C=O in carboxylate groups, and 533.3 eV attributed to the adsorbed water molecules on the surface of the material. The XPS spectrum of Fe 2p showed a peak with a binding energy of 711.68 eV assigned to Fe2p_3/2 of Fe(III) and 725.48 eV assigned to Fe2p_1/2 of Fe(III).^13,15 After adding the H₂O₂, the XPS survey spectrum as well as the EDX maps were obtained (Fig. 5) and confirmed the presence of Fe, C, and O elements on the surface of the sample.

Fig. 4 XPS detailed line scans of the as-synthesized Fe-BDC MOF (top), the MOF post degradation (bottom).

Table 1 XPS Analysis showing atomic % for the several elements in the MOF (as-synthesized) and (post degradation)

Element (atomic %)	C	O	Fe
Fe-BDC MOF (as-synthesized)	59.95	26.83	4.6
Fe-BDC MOF (post degradation)	30.24	48.45	15.67

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Fig. 5 SEM images of the as-synthesized Fe-BDC MOF (a) prior, and (b) post H₂O₂ degradation. The panel below shows the EDX elemental mapping of the as-synthesized MOF (top row) and that after catalytic reaction (below) showing the homogeneous distribution of the elements (labeled images) throughout the sample.

However, the most significant change in the MOF catalyst is in the changes in the C 1s spectrum, where the MOF exhibits strong peaks for metal-coordinated carbonate carbon (288.99 eV), presumably produced through the partial oxidation of the MOF organic backbone by the combined action of the H₂O₂ and the catalytic Fe center.¹⁴

Scanning electron microscopy (SEM) was also utilized to characterize the MOF before and after the catalytic decomposition of H₂O₂. The SEM images, Fig. 5, indicated a change in the topography of the MOF, resulting from the partial deformation of the MOF crystallite backbone after being exposed to the H₂O₂ oxidant.

Probing the catalytic degradation efficiency and recyclability of Fe-BDC MOF against H₂O₂

The addition of Fe-BDC MOF to H₂O₂ aqueous solution revealed its excellent performance as a Nanozyme for the catalytic decomposition of H₂O₂. Remarkably, the addition of 200 mg of Fe-BDC Nanozyme to 6 g of the 47 wt% H₂O₂ aqueous solution was sufficient to result in 57% weight loss of the aqueous solution in only two minutes without even applying any heat. Moreover, increasing the amount of MOF added to the hydrogen peroxide solution shortened the time required for reaching the same degradation efficiency, where adding 0.5 g of MOF was sufficient to induce a significant weight loss of the sample through the catalytic degradation of H₂O₂ accompanied by an exothermic reaction and water evaporation, altogether very rapidly within 10 seconds of mixing the catalyst with the peroxide solution (Fig. 6). The data recorded shown indicates the weight loss due to the evolution and escape of O₂ gas as a function of time after introducing the catalyst to the peroxide solution. As the experiments were done in an open vessel, unavoidable loss of moisture occurred as the reaction was highly exothermic.

Fig. 6 Degradation capabilities of different weights of Fe-BDC MOF against H₂O₂.

Interestingly, retrieving the Nanozyme through centrifugation and assessing its recyclability for 6 consecutive cycles did not show any noticeable decay in its catalytic activity, marking an industrially applicable Nanozyme towards H₂O₂ decomposition for oxygen evolution (Fig. 7). It is worth mentioning here that the weight loss calculation was very comparable to the iodometric titration for H₂O₂ determination after its catalytic degradation over Fe-BDC.

Fig. 7 Investigating the recyclability capabilities of Fe-BDC MOF (0.2 g) towards H₂O₂ degradation.

Conclusion

In conclusion, we presented the utilization of the Fe-based MOF (MIL-53(Fe)) as a nanozyme for the catalytic degradation of hydrogen peroxide. The MOF nanozyme was prepared through a simple solvothermal reaction, making it scalable for industrial applications. Furthermore, the MOF demonstrated an excellent catalytic performance against H₂O₂ in only a few seconds, with no detectable loss of activity even after six consecutive cycles. Moreover, the MOF exhibited a notable breathing behavior with an adapted pore opening as a result of a possible H₂O₂ adsorption during the catalytic process.

Experimental

Caution

H₂O₂ with relatively high concentration is highly corrosive and the reaction should be carried out in an open vessel wearing protective body equipment and face shield in a properly ventilated hood.

Materials

All reagents were used as received without further purification. Solvents and common chemicals were purchased from Sigma-Aldrich or Fisher Scientific-UK. Iron chloride (FeCl₃·6H₂O, 98%, Sigma Aldrich), 1,4 benzene dicarboxylic acid (H₂BDC, 98%, Sigma Aldrich), and N,N-dimethylformamide (DMF, 98%, Sigma Aldrich). Absolute ethanol, acetone (all from Alfa Aesar)), H₂O₂, (47%, Sigma Aldrich), and deionized water were used for washing purposes. Potassium iodide (KI, 99%, Sigma Aldrich), ammonium molybdate (99%, Sigma Aldrich), sulfuric Acid (H₂SO₄, Alfa Aesar), sodium thiosulfate (Na₂S₂O₃, 98% Sigma Aldrich), and starch indicator for H₂O₂ iodometric titration purposes were used.

Infra-red absorption spectra were recorded on a Thermo Scientific Nicolet is-10. X-ray powder diffractions were taken on a Shimadzu XD-l X-Ray powder diffractometer. SEM/EDX images were taken on a Zeiss EVO-10 microscope for the examination of the surface morphology and the elemental composition. The X-ray photoelectron spectrum (XPS) measurements were performed using a K-ALPHA XPS system (Themo Fisher Scientific, USA) with a monochromatic Al-K_α source operated at 15 keV.

Methods

Preparation of Fe-BDC

Samples were synthesized through the sequential addition of FeCl₃·6H₂O (0.27 g, 1 mmol), and 1,4-benzendicarboxylic acid (H₂BDC) (0.166 g, 1 mmol), and glacial acetic acid (1 mL) into a beaker, followed by the addition of N,N-dimethylformamide (DMF) (20 mL) under stirring until the reagents were completely dissolved. The mixture was then transferred into a scintillation vial, which was sealed and placed in isothermal oven at 100 °C for 6 h. The resulting powder was recovered by centrifugation at 6000 rpm for 30 min, and rinsed three times with DMF, acetone, ethanol, and water. Finally, the recovered crystals were placed into an oven set at 140 °C to dry overnight.

H₂O₂ degradation and recyclability investigation

In a typical degradation trial, 6 mL of 47% H₂O₂ aqueous solution containing 0.2 grams of Fe-BDC MOF powder was left at room temperature. After one second, the degradation of H₂O₂ was monitored through recording the weight loss using a digital scale. The degradation efficiency of the MOF was tested using the same volume of H₂O₂ and varying the weight of the MOF ranging from 0.01 to 0.5 grams. For recyclability, the MOF powder was isolated from the mixture by centrifugation at 6000 rpm for 30 min,washed with 50 mL DI water, and then dried at 120 °C for 2 h between each cycle. The H₂O₂ degradation and washing steps were repeated for 6 consecutive cycles.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We acknowledge the financial support from STDF (USC17-43). The authors also acknowledge Zewail City of Science and Technology for financial support (CMS-MA).

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