Fengke Wang1, Chaofan Li1, Weizhen Kong1,2, Shucan Qin1,2, Qianqian Peng1,2, Lihua Zang1, Yunqian Ma This email address is being protected from spambots. You need JavaScript enabled to view it.1,2,3, Yi Nie2,3, Guihuan Yan4 

1 College of Environmental Science and Engineering, Qilu University of Technology (Shandong Academy of Science), Jinan 250353, China
2 Longzihu New Energy Laboratory, Zhengzhou Institute of Emerging Industrial Technology, Henan University, Zhengzhou 450000, China
3 Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China
4 Ecology Institute of Shandong Academy of Science (China-Japan Friendly Biotechnology Research Center, Shandong Academy of Sciences), Jinan 250000, China


Received: November 12, 2023
Revised: January 13, 2024
Accepted: January 20, 2024

 Copyright The Author(s). This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are cited.


Download Citation: ||https://doi.org/10.4209/aaqr.230264  


Cite this article:

Wang, F., Li, C., Kong, W., Qin, S., Peng, Q., Zang, L., Ma, Y., Nie, Y., Yan, G. (2024). Flexible Dual Fe-based Catalyst: MIL-53(Fe) Loaded with Deep Eutectic Solvent for Discontinuous Selective Oxidation of H2S. Aerosol Air Qual. Res. 24, 230264. https://doi.org/10.4209/aaqr.230264


HIGHLIGHTS

  • Flexible dual Fe-based DES@MOF efficiently removed H2S in discontinuous process.
  • DES could effectively improve oxidation of H2S as the guest molecular in catalyst.
  • Flexible MOF could effectively relieve passivation of catalyst as the supporting material.
  • Fe components of DES and MOF in catalysts do not play the same role.
 

ABSTRACT


Deep eutectic solvent (DES) as the guest molecule and flexible Metal-organic framework (MOF) as the supporting material can be used to provide an efficient catalyst: flexible dual Fe-based DES@MOF, which can efficiently remove H2S in discontinuous removal process (100–180°C). The materials were characterized by XRD SEM, FT-IR, BET, XPS, TG, CV and Py-FTIR. According to the result, MOF (MIL-53(Fe)) incorporating 30 wt% DES (ChCl/FeCl3) exhibited the best H2S removal performance at 180°C, with the H2S removal capacity of 2527 mg mL1 (3744 mg g1). And the H2S removal capacity reduced by 9% after two cycles of thermal regeneration. The process of desulfurization includes two stages: catalytic oxidation and adsorption, which are determined by the Fe3+ in DES and the structural characteristics, such as the aggregation of mesopores, respectively. For MIL-53(Fe), FeOOH as an intermediate and Lewis acid sites also played a catalytic role. Elemental sulfur was the predominant product with a small amount of sulfate. Both elemental sulfur and DES can induce the flexible deformation of MOF as the guest molecule to relieve the passivation of catalyst.


Keywords: Deep eutectic solvent, Metal-organic framework, Catalytic oxidation, H2S removal, Flexible deformation


1 INTRODUCTION


 Hydrogen sulfide (H2S) is a colorless, highly toxic and corrosive gas with the characteristic odor of rotten eggs (De Crisci et al., 2019). H2S can be generated from some industrial processes, such as coal pyrolysis, natural gas purification and crude oil refining. H2S in gaseous and solution form is extremely corrosive to pipelines and production facilities as well as extremely polluting to the natural environment (Garcia-Arriaga et al., 2010). The olfactory threshold for H2S in air is only 0.41 ppb. At low concentrations, it can irritate the eyes, nose and throat (Malone Rubright et al., 2017). The removal of H2S from industrial gases is therefore of paramount importance, and strict global control of H2S emission standards is in place. As a fine desulfurization method, selective catalytic oxidation is a promising technology and has also been a hot research topic because it can achieve efficient removal of H2S. The technology can be performed in continuous process (above 180°C) or discontinuous removal process (below 180°C) (Zhang et al., 2015). In H2S selective catalytic oxidation, catalysts play a crucial role. For the discontinuous removal process with low energy consumption, the main challenge is that the singlet sulfur formed is enriched in the catalyst, which accelerates the passivation of catalyst.

In the last decades, a large amount of research has been underway on Fe-based catalysts, such as iron oxide, modified iron oxides incorporate a second metal element (Cr, Mo, etc.) (Kersen and Keiski, 2009; Laperdrix et al., 2000a), and Fe-based ionic liquids (ILs) or deep eutectic solvents (DESs) (Ma et al., 2019). Bare metal oxide catalysts have poor H2S removal capacity. To improve the desulfurization performance, some porous support materials (activated carbon, clay, oxide, etc.) were modified by loading metal oxides to endow them with active phase and developed porosity (Kan et al., 2019, 2022; Terörde et al., 1993; Zhang et al., 2013). Although Fe-based catalysts have been widely used for the catalytic oxidation of H2S, the inherent problems of catalyst passivation and catalyst regeneration cannot be solved, such as corrosion and sulfation in the process of reaction, the lone pair of electrons in the structure of the elemental sulfur deposited in the catalyst has a strong coordination complex effect, which can easily lead to the poisoning of the metal catalyst, greatly reducing the catalytic efficiency; sulfur-containing compounds are easy to be oxidized due to the multivalent state of sulfur, and the catalytic conversion is subject to a number of limitations (Huang et al., 2015; Laperdrix et al., 2000a, 200b). The development of Fe-based catalysts capable of achieving efficient conversion, selectivity and stability is important and challenging.

Metal-organic frameworks (MOFs) offer a new perspective to achieve this goal as an emerging multifunctional material with unique structural properties. Fig. 1 shows the structure of MOFs. MOFs are porous materials with well-defined structures that are used in a wide range of applications such as biomedicine, catalysis, gas adsorption and separation (Aliyev et al., 2021; Altintas and Keskin, 2019; Yang et al., 2019; Zhang et al., 2021). Among them, flexible MOF materials with respiration, swelling, ligand rotation and interlayer displacement effects show great potential for gas storage, adsorption, separation and catalysis especially (Bukhtiyarova et al., 2007). In order to obtain more effective flexible MOF catalysts, many methods have been proposed to modify them, especially Fe-based MOF MIL-53(Fe), for example organic ligand-doped (Xu et al., 2023) and metal-doped (Zhu et al., 2023), two methods effectively enhance its photocatalytic activity and degradation performance on tetracycline pollutants, respectively. However, there are few studies on the removal of H2S by modified flexible Fe-based MOFs. Therefore, we explored a novel approach to modify Fe-based MOFs to remove H2S and explore its sustainable utilization in catalytic process for the first time.

Fig. 1. Schematic diagram of the MOFS structure.Fig. 1. Schematic diagram of the MOFS structure.

DESs, the third generation ILs, have the advantages of non-volatility, designability, and simple synthesis (Shi et al., 2022), and it can integrate metal salt as hydrogen bond acceptor for special application. DESs can also be loaded on the support (such as carbon materials and MOFs) to be used as solid phase catalysts or adsorbents. In recent years, the concept of "MOFs and IL composites (IL@MOFs)" has been proposed and its development has become increasingly mature, showing considerable advantages in gas adsorption and separation due to the tunable gas affinity of MOFs and ILs (Xiong et al., 2023). IL@MOFs are essentially endowed with the dual properties of IL and MOF, with great adsorption and separation performance (Gao et al., 2022). Fe-based ILs or DESs have been used for the recovery of sulfur in wet desulfurization process, while less research has been done in the dry desulfurization process. Our previous work focused on Cu-based DES supporting materials (zeolite and fumed silica) (Ma et al., 2019; Mao et al., 2020), which were found to have high desulfurization efficiency, but regeneration capabilities need to be improved.

A new catalyst was constructed for H2S selective catalytic oxidation with controllable flexible MOFs and DES composites (DES@MOFs) using MOF as the catalytic active center and DES as the enhanced reaction center, and the flexible backbone of MOFs were induced to reversible deformation to reduce sulfur deposition that should be easily regenerated for the catalyst. In this paper, the selective catalytic oxidation of H2S was completed by flexible dual Fe-based DES@MOF as the catalyst in discontinuous removal process. Specifically, a classical flexible MIL-53(Fe) MOF with ChCl/FeCl3 "type II" DES were chosen, and DES@MIL-53(Fe) were synthesized by a series of operations using solvothermal method to load MIL-53(Fe) with DES as well as the preparation of non-flexible MOF (MIL-101(Fe)) carrier material and the use of commercial Fe2O3 as a control to study its catalytic oxidation of H2S at the temperature range from 100°C to 180°C. The catalytic performance in discontinuous removal processes was discussed, which is expected to make up for the shortcomings of conventional Fe-based catalysts to improve their sustainability.

 
2 EXPERIMENTAL SECTION

 
2.1 Materials

FeCl3·6H2O, terephthalic acid (1,4-BDC), N, N-dimethylformamide (DMF), commercial Fe2O3, choline chloride (ChCl), methanol, acetone, and all of them were purchased from Shanghai Maclean Biochemical Company.

 
2.2 Preparation of the Catalysts

ChCl and FeCl3·6H2O were mixed in a molar ratio of 1:2 and stirred vigorously in an oil bath at 90°C until a viscous homogeneous brownish-red transparent liquid was formed, and the product was named as DES (ChCl/FeCl3) (Karimi et al., 2019). The flexible and rigid MOFs were prepared referring to the previous literature (Chen et al., 2022; Wang et al., 2014), which were named as MIL-53(Fe) and MIL-101(Fe), respectively. DES@MIL-53(Fe) was prepared by an impregnation method. Fe-DES was dissolved in the mixture of acetone (20 mL) and MIL-53(Fe), and then system was stirred at 57°C and 200 rpm until the organic solvent acetone was evaporated completely. The materials were further dried overnight at 105°C, and finally the catalysts were obtained with Fe-DES loadings of 10%, 30%, and 50%, named as DESx@MIL-53(Fe) (x = 10, 30 or 50). The synthesis and notes of DES@MIL-101(Fe) were similar to DES@MIL-53(Fe). The waste samples after desulfurization experiment are indicated by adding the letter "E" to the end of their names.

 
2.3 Characterization

X-ray diffraction (XRD) analysis was carried out to determine the phase and crystal structure of the materials. X-ray photoelectron spectroscopy (XPS) was performed to analyze the surface chemical composition of the material. The surface morphology of the materials was characterized using scanning electron microscopy (SEM). The specific surface area of the materials was determined by the Brunauer-Emmett-Teller (BET). Thermogravimetric analysis (TGA) was used to measure the difference in weight loss of the catalyst after adsorption of hydrogen sulfide. Fourier transform infrared spectroscopy (FTIR) was performed to determine the functional group differences between DES@MIL-53(Fe) and MIL-53(Fe). Cyclic voltammetry (CV) was used to study the electron transfer in the reactions of four catalysts with different DES loadings. The type and intensity of the acidic sites on the surface of MIL-53(Fe) and DES@MIL-53(Fe) were evaluated by Py-FTIR analysis.

 
2.4 Removal Tests

The H2S removal capacity measurements were carried using a fixed-bed column at the temperature of 100°C, 120°C, 140°C, 160°C, and 180°C. The gas stream containing H2S (2000 ppm, 200 mL min1) (30% relative humidity and total flow rate of 1200 mL min1) was passed through a glass column (0.6 cm in diameter). The removal capacity was determined by continuously passing a gas stream of air (relative humidity 30% and flow rate of 1000 mL min1) containing 0.2% H2S (2000 ppm, 200 mL min1) through a glass column of DES@MIL-53(Fe) (0.6 cm in diameter and 4.5 cm of bed height) until H2S was detected by the gas detector in the effluent gas.

The time required for a gas containing H2S to pass completely through the catalyst is called the H2S removal time and the corresponding amount of H2S adsorbed is called the removal capacity. The removal capacity of the catalyst could be calculated by the H2S removal time, the H2S inlet gas flow rate and the catalyst loading.

The H2S removal capacity of the test sample was calculated using the following equation:

 

The word C is the concentration of H2S in the air stream (%); F is the total H2S/air flow rate (1200 mL min1); T is the removal time (min); V is the actual volume of the catalyst bed in the absorber tube (mL).

The H2S removal capacity (mg-H2S g1-catalyst) is determined by the following equation:

 

where H2S removal rate, sulfur selectivity and sulfur yield are defined as follows:

 

 
3 RESULTS AND DISCUSSION


 
3.1 Characterization of the Catalysts

MIL-53(Fe) is a classical one of flexible MOFs with the topology shown in Fig. 2, which is rigid in one dimension and respiratory in the other two dimensions, while having a faceted rhombus topology. Upon adsorption of the guest, its linker-metal-linker angle is bent and diamond-shaped pores swell to cause flexible deformation.

Fig. 2. Topology diagram of MIL-53(Fe).Fig. 2. Topology diagram of MIL-53(Fe).

Fig. 3 shows the XRD plots of MIL-53(Fe), DES30@MIL-53(Fe), and DES30@MIL-53(Fe)-E. The XRD peaks of both DES30@MIL-53(Fe) and MIL-53(Fe) are clearly visible, indicating that high crystallinity and the diffraction peaks are similar to those reported previously (Dong et al., 2015), and it was suggested that the catalyst was successfully prepared and the XRD peak pattern of DES30@MIL-53(Fe) is consistent with that of MIL-53(Fe) without DES. When H2S removal experiment was completed, the XRD pattern of DES30@MIL-53(Fe)-E shows obvious characteristic sulfur peaks at 26.6° and 27.6°. In order to observe the microstructure of MIL-53(Fe) after loading DES, MIL-53(Fe) and DES@ MIL-53(Fe) were characterized by SEM. As seen in Fig. 4(a), a lamellar spindle morphology of MIL-53(Fe) is observed and its surface is smooth. Fig. 4(b) shows the SEM image of DES30@MIL-53(Fe), it is obvious that the lamellar spindle morphology of MIL-53(Fe) still maintains after loading DES, and the metallic component of DES attached to its out-surface are visible. As can be seen from Fig. S1 and Fig. S2 (Supplementary material), the distribution of Fe, O, and Cl obtained by Mapping shows that the uncompounded MIL-53(Fe) contains Fe, O, and Cl elements, and their contents are 41.9%, 41.4%, and 4.5%, respectively. The content of Fe, O, and Cl in the DES30@MIL-53(Fe) are 51.9%, 43.4%, and 16.4%, respectively. The content of the three elements in the DES30@MIL-53(Fe) was improved compared to that of the single MOF material, which is consistent with the experimental expectations.

Fig. 3. XRD plots of MIL-53(Fe), DES30@MIL-53(Fe), and DES30@MIL-53(Fe)-E.Fig. 3. XRD plots of MIL-53(Fe), DES30@MIL-53(Fe), and DES30@MIL-53(Fe)-E.

Fig. 4. SEM diagrams of (a) MIL-53(Fe) and (b) DES30@MIL-53(Fe).Fig. 4. SEM diagrams of (a) MIL-53(Fe) and (b) DES30@MIL-53(Fe).

The functional groups of the catalyst were analyzed by FT-IR, as shown in Fig. 5, and the characteristic peaks of MIL-53(Fe) are located at 1605, 1389, 748, and 555 cm1. The first two peaks are stretching vibrations and asymmetric vibrations of the carboxyl group, while the latter two peaks are vibrations of the C-H and Fe-O bonds (Ai et al., 2013; Song et al., 2014). The results are in agreement with previous reports (Zheng et al., 2018), demonstrating the successful preparation of MIL-53(Fe). For DES30@MIL-53(Fe), the amide group in the molecular structure of choline chloride undergoes C=O stretching of the amide at 1651 cm1. Consequently, it could be determined that DES is successfully loaded on the MIL-53(Fe).

Fig. 5. FT-IR plots of MIL-53(Fe) and DES30@MIL-53(Fe).Fig. 5. FT-IR plots of MIL-53(Fe) and DES30@MIL-53(Fe).

As N2 adsorption-desorption curves of DES30@MIL-53(Fe) and MIL-53(Fe) (Fig. 6(a)) showed. The N2 adsorption-desorption isotherms of DES30@MIL-53(Fe) are type IV adsorption curves with obvious hysteresis lines and mesoporous structures. The adsorption in the low-pressure region shows that the adsorbing capacity increased with the relative pressure P/P0 increasing. Below P/P0 of 0.4, it could be attributed to multilayer adsorption, while above P/P0 of 0.4, it could be concluded that the mesopores and their aggregation to improve the adsorption. Around P/P0 of 0.8, the adsorption starts to increase substantially, due to the formation of gaps between the particles. The size of the hysteresis loop shows the presence of the mesopores. The N2 adsorption-desorption isotherm of the MIL-53(Fe) clearly shows a V-shaped adsorption curve, which indicated the weak relative interaction of the catalytic material with the adsorbed gas.

Fig. 6. (a) N2 adsorption-desorption curves and (b) pore size distribution for MIL-53(Fe) and DES30@MIL-53(Fe).Fig. 6. (a) N2 adsorption-desorption curves and (b) pore size distribution for MIL-53(Fe) and DES30@MIL-53(Fe).

Fig. 6(b) shows the pore size distribution of MIL-53(Fe) and DES30@MIL-53(Fe). It can be seen that the pore size of MIL-53(Fe) is concentrated at around 1.34 nm. And the volumes of these pores are increased with the DES loading, reaching 2 nm. This phenomenon is attributed to the swelling effect of MIL-53(Fe) excited by DES, which transform the pore size of the material from microporous to mesoporous. In addition, the specific surface area of DES30@MIL-53(Fe) is 152.162 m2 g1, while that of untreated MIL-53(Fe) is 22.697 m2 g1 in Fig. 6(a). It could be concluded that the swelling effect of MIL-53(Fe) caused by DES loading increased the specific surface area of the catalyst by a factor of 7. The high surface area of DES@MIL-53(Fe) not only provides ample space for H2S adsorption and conversion but also promotes the mass transfer of H2S and S. In general, DES@MIL-53(Fe) belongs to the class of mesoporous materials, and the adsorption/desorption capacity is greatly improved to MIL-53(Fe).

Fig. 7(a) depicts the survey spectrum of DES@MIL-53(Fe), and Fig. 7(b) shows the spectra of O1s attributed to C=O (carbonyl) and C-O (hydroxyl) at 531.2 eV and 533.3 eV, respectively (Xu et al., 2008). Fig. 7(c) shows the high-resolution C1s core energy level spectrum, which convolved into three peaks at 284.8 eV, 286.5 eV, and 288.7 eV, corresponded to C=C/C-C, C-O (epoxy and hydroxyl), and O-C=O (carboxyl) in aromatic substances, respectively (Bandosz et al., 2020; Yang et al., 2020a). In Fig. 7(d), the two peaks at 711.8 eV and 725.4 eV are the characteristic peaks of Fe2p3/2 and Fe2p1/2 respectively (Masset et al., 2006). The satellite peak in 717 eV is the oxidation state of Fe3+ in DES@MIL-53(Fe) (Mills and Sullivan, 1983; Yamashita and Hayes, 2008).

Fig. 7. (a) XPS spectra of DES30@MIL-53(Fe), (b) XPS spectra of O1s, (c) C1s, and (d) Fe2p of DES30@MIL-53(Fe).Fig. 7. (a) XPS spectra of DES30@MIL-53(Fe), (b) XPS spectra of O1s, (c) C1s, and (d) Fe2p of DES30@MIL-53(Fe).

 
3.2 Evaluation of the H2S Removal Performance

The desulfurization temperature (100°C, 120°C, 140°C, 160°C, and 180°C) and the DES loading (10%, 30%, and 50%) were optimized with H2S removal efficiency and H2S removal time by H2S removal experiments. The experimental results (Fig. S3) (Supplementary material), which indicates that the prepared catalysts with different DES loadings show high H2S removal efficiency and long H2S removal time at different temperatures. For all three DES loadings, the H2S removal time at 180°C (the dew point temperature of sulfur) were longer than those at the other temperatures, with the 30% DES loading being more effective.

In the experiments, H2S removal time of the catalyst increased with the temperature increasing in discontinuous removal process. However, it has no significant relationship with DES loadings. It was found that under the adopted conditions, the H2S removal time over the catalyst follows the order of DES30@MIL-53(Fe) > DES10@MIL-53(Fe) > DES50@MIL-53(Fe). Among them, H2S removal time of DES50@MIL-53(Fe) is mainly limited by two factors: DES loading and sulfur deposition. Excessive DES loading stimulates the flexible deformation of MIL-53(Fe), some Fe atoms are exposed on the surface, and strong covalent bonds can be formed between the d orbitals of Fe atoms and the p orbitals of S atoms. The formation of inactive FeSX byproducts would significantly weaken the desulfurization performance of the catalysts, and the spatial position resistance caused by the deposition of sulfur hinders the continuation of the reaction (Chen et al., 2022).

H2S removal capacities with different units (mg L1 and mg g1) are shown in Fig. 8. At different temperatures, H2S removal capacities of the three catalysts with different DES loadings increased with temperature. DES50@MIL-53(Fe) had almost the same H2S removal capacity at 140°C and 160°C, and DES30@MIL-53(Fe) at 180°C exhibits the best H2S removal capacity, reaching 2527 mg mL1 (3744 mg g1). 

Fig. 8. H2S removal capacity (a) (mg mL–1) and (b) (mg g–1) of DES@MIL-53(Fe).Fig. 8. H2S removal capacity (a) (mg mL1) and (b) (mg g1) of DES@MIL-53(Fe).

As shown in Fig. S4 (Supplementary materials), DES@MIL-53(Fe) has the best catalytic performance in the five experimental materials selected for this study, both in terms of H2S removal efficiency, sulfur selectivity and sulfur yield. The catalytic performance of both MIL-53(Fe) and MIL-101(Fe) has been greatly improved after loading with DES. However, because of the non-flexible nature of MIL-101(Fe) and deposition of sulfur, which resulted in H2S removal efficiency of DES@MIL-101(Fe) could not reach 100% and severe passivation. As shown in Table S1 (Supplementary material), H2S removal performance of the desulfurization agents reported in the study is summarized (Bandosz, 2002; Siriwardane et al., 2017; Yang et al., 2020a; Zheng et al., 2021). Considering the cost performance, DES@MIL-53(Fe) exhibited higher H2S removal capacity and lower cost compared than both other and similar types of catalysts.

 
3.3 Analysis of the H2S Removal Performance

Fig. 9(a) depicts the measured XPS spectra of DES30@MIL-53(Fe) and DES30@MIL-53(Fe)-E, and the S2s and S2p signals can be clearly observed in DES30@MIL-53(Fe)-E. The form of sulfur species in the desulfurization process was further examined by XPS (Fig. 9(b)), and the sulfur species mainly consisted of element sulfur (S2p3/2 161.7 eV; S2p1/2 162.9 eV), and sulfates (S2p3/2 168.3 eV; S2p1/2 169.1 eV) (Feng et al., 2021; Mei et al., 2021; Yang et al., 2021; Yang et al., 2019). Fig. 9(c) shows the high resolution C1s core energy level spectrum, and there are three peaks in 284.8 eV, 286.4 eV, and 288.7 eV, corresponding to C=C/C-C, C-O (epoxy and hydroxyl), and O-C=O (carboxyl) in aromatic substances, respectively, which is consistent with the C1s core energy level spectrum of DES30@MIL-53(Fe). In Fig. 9(d), the two peaks in 711.5 eV and 725 eV are the characteristic peaks of Fe2p3/2 and Fe2p1/2, respectively. The satellite peak in 717 eV was confirmed that the oxidation state of Fe in DES30@MIL-53(Fe)-E remains Fe3+. The above results proves that the valence states of C and Fe at DES@MIL-53(Fe) do not change after H2S removal experiment. The peaks of Fe2p3/2 and Fe2p1/2 in DES@MIL-53(Fe)-E are consistent with those in DES@MIL-53(Fe). It was confirmed that DES@MIL-53(Fe)-E can be recycled due to its unchanged valence state of Fe3+.

Fig. 9. XPS spectra of (a) DES30@MIL-53(Fe) and DES30@MIL-53(Fe)-E, (b) XPS spectra of S2p, (c) C1s, and (d) Fe2p of DES30@MIL-53(Fe)-E.Fig. 9. XPS spectra of (a) DES30@MIL-53(Fe) and DES30@MIL-53(Fe)-E, (b) XPS spectra of S2p, (c) C1s, and (d) Fe2p of DES30@MIL-53(Fe)-E.

The electron transfer in the reaction of four experimental materials with different DES loadings was studied by cyclic voltammetry (CV), and the intensity of electron flow could be observed (Zhang et al., 2005). Using this method, the change in current for each experimental material at different DES loadings can be detected (Fig. 10). The CV curves (Fig. 10(a)) shows that the peak currents of DESx@MIL-53(Fe) (x = 10, 30, and 50) are higher than that of the MIL-53(Fe) at the same voltage, and the current values become larger with the DES loadings increasing from 10% to 50%. The redox potentials of catalysts were changed with different DES loadings. The oxidation and reduction peak were observed near 23 mV and –200 mV for DES30@MIL-53(Fe), respectively. The oxidation and reduction peak were observed near 8 mV and –216 mV for DES50@MIL-53(Fe), respectively. The electron transfer rate with the catalysts can be improved by loading DES, However, the peak current of the catalyst with DES loading of only 10% does not change much, leading to a unsatisfactory electron transfer rate (Fig. 10(b)).

Fig. 10. Cyclic voltammograms of (a) DES10@MIL-53(Fe), DES30@MIL-53(Fe), and DES50@MIL-53(Fe) and (b) MIL-53(Fe), DES10@MIL-53(Fe), and MIL-53(Fe).Fig. 10. Cyclic voltammograms of (a) DES10@MIL-53(Fe), DES30@MIL-53(Fe), and DES50@MIL-53(Fe) and (b) MIL-53(Fe), DES10@MIL-53(Fe), and MIL-53(Fe).

Numerous studies have shown that LAS (Lewis acid sites), which are influenced by the coordination environment of Fe3+ in MOFs, are also involved in gas adsorption and catalytic reactions (Dissegna et al., 2018). The type and intensity of acidic sites of MIL-53(Fe) have been investigated for ozone adsorption and decomposition (Yu et al., 2019). Herein, we evaluate the types and intensities of surface acidic sites of DES@MIL-53(Fe) and MIL-53(Fe) for catalytic action of H2S by Py-FTIR analysis. Fig. 11 represents the Py-FTIR spectra of MIL-53(Fe) and DES30@MIL-53(Fe) at different annealing temperatures. There are distinguishable peaks (1450, 1490, and 1610 cm1) in the curves at 30°C, 100°C, and 150°C. The two characteristic peaks at 1490 cm1 and 1610 cm1 are caused by the stretching vibration of the aromatic ring. The peak area of the characteristic absorption peak at 1450 cm1 was used to determine the acid amount of LAS, and the results are shown in Fig. 12. There is not a great difference in the number of weak and medium-strong acids of LAS before and after the DES loading on MIL-53(Fe), which indicates that DES30@MIL-53(Fe) did not expose more LAS due to the loading of DES. The removal of H2S by MIL-53(Fe) was extremely poor, which suggested that the DES in DES@MIL-53(Fe) did not enhance the adsorption capacity of the catalyst for H2S in terms of LAS, but greatly enhance the oxidizing capacity of the catalyst, which is consistent with CV. Therefore, in the H2S removal by DES@MIL-53(Fe), DES mainly plays the role of oxidation, while MIL-53(Fe) mainly acts as a support material for guest molecule DES through its flexible physical structure to enhance adsorption and reduce passivation, and plays a smaller role of catalytic oxidation with a small amount of LAS. This is the reason why a single MIL-53(Fe) catalyzed H2S oxidation is not satisfied.

Fig. 11. Py-FTIR spectra of (a) MIL-53(Fe) and (b) DES30@MIL-53(Fe) at different degassing temperatures (30°C, 100°C, and 150°C).Fig. 11. Py-FTIR spectra of (a) MIL-53(Fe) and (b) DES30@MIL-53(Fe) at different degassing temperatures (30°C, 100°C, and 150°C).

Fig. 12. Pyridine adsorption peak areas (at 1450 cm–1) normalized by catalyst weight for MIL-53(Fe) and DES30@MIL-53(Fe).Fig. 12. Pyridine adsorption peak areas (at 1450 cm1) normalized by catalyst weight for MIL-53(Fe) and DES30@MIL-53(Fe).

The reaction mechanism is investigated and shown in Fig. 13. From the above characterizations, it can be inferred that the desulfurization process consists of adsorption and catalytic oxidation, and Fe components of DES and MOF in catalysts do not play exactly the same role. Aggregation of mesopores in DES@MIL-53(Fe) improve the adsorption of H2S (Zhang et al., 2015), flexible MOFs are capable of responding to external stimuli (e.g., temperature, pressure, electrical signals, and guest molecules) changing the bond angles formed between the metal atoms and the ligands. In this process, the structure of flexible MOFs can be transited from a non-porous to a porous state and the deformation of backbone architectures such as the swelling effect, which was occurred in the catalysts of our work (Bineesh et al., 2010). DES@MIL-53(Fe) is swelled in the presence of guest molecules and temperature, in favor to the rapid release of product elemental sulfur, avoiding passivation phenomena, and the exposed Lewis acidic sites in MIL-53(Fe) also participates in the catalysis of H2S to some extent. A possible pathway is supposed that H2S adsorbs and dissociates to give HS on Fe3+ LAS in MIL-53(Fe), and then HS interacts with active oxygen species to generate sulfur (Zheng et al., 2020). Fe3+ of DES mainly plays an oxidizing role in H2S removal, producing Fe2+ and elemental sulfur (R1). Fig. S5 (Supplementary Material) shows that the CV curves of DES@MIL-53(Fe)-E are stable and there is no obvious redox peak indicating that the valence state of the product is stable and there is no iron sulfide species. This is consistent with the results of XPS (Fig. 9(b)). Reaction of Fe in MIL-53(Fe) with H2S to form Fe2S3 (R2), which is well recognized to be unstable in thermodynamic (Cao et al., 2016). Even at normal temperature, Fe2S3 could be easily decomposed into FeS2 and FeS (R3). As a result, it can be surmised that the Fe3+ of DES@MIL-53(Fe)-E also comes from FeOOH, which is generated by oxidation of FeS2 and FeS (R4). This result is similar to the findings of previous studies (Cao et al., 2016; Wang et al., 2020). FeOOH can also react with H2S to generate Fe2S3 and H2O (R5) (Wang et al., 2020), causing the reactions (R3 and R4) again. Thus, there is a chain reaction, eventually generated more elemental sulfur. moreover, the sulfates may be attributed to the oxidation of FeS2 (R6). The reactions are as follows:

 

Fig. 13. Proposed reaction mechanism of H2S over the DES@MIL-53(Fe).Fig. 13. Proposed reaction mechanism of H2S over the DES@MIL-53(Fe).

 
3.4 The Regeneration of the Catalysts

The regeneration capacity of the catalyst was evaluated from the viewpoint of economic efficiency and environmental protection. In this paper, the catalyst was regenerated using a thermal treatment method. Fig. S6 (Supplementary Material) shows the TG curves of the spent catalyst measured under N2 atmosphere. The weight loss was observed in the temperature range from 220°C to 600°C, where most of the desulfurization products were desorbed. Approximate 90 wt% of DES@MIL-53(Fe)-E did not decompose in the regeneration temperature range from 100°C to 220°C, and the loss of water inside the catalyst causes a significant weight loss of the catalyst at around 100°C. As a result, the optimal regeneration temperature of DES@MIL-53(Fe)-E can be determined to be 220°C. Thermal regeneration experiments of the catalyst were carried out in a tube furnace under N2 atmosphere (flow rate 200 mL min1) with a heating rate of 2°C min1 to 220°C and temperature maintenance for 8 hours. Fig. 14 exhibits the results of two regeneration cycles (named as C1 and C2). DES30@MIL-53(Fe) still maintains over 90% H2S removal efficiency after two cycles of thermal regeneration. It can be observed that the H2S removal capacity of different units of DES30@MIL-53(Fe) showed a significant decreasing trend with two cycles, and its H2S removal capacity decreased by 7% and 9% after two cycles of thermal regeneration, respectively.

Fig. 14. The regeneration performance of DES30@MIL-53(Fe).Fig. 14. The regeneration performance of DES30@MIL-53(Fe).

 
4 CONCLUSION


In this research, a new catalyst was synthesized to remove large amounts of H2S in discontinuous removal process with 100% removal efficiency of H2S over a long period. DES30@MIL-53(Fe) at 180°C has the best catalytic oxidation performance, reaching 2527 mg mL1 (3744 mg g1). The results indicate that the desulfurization process consists of catalytic oxidation and adsorption, which is greatly enhanced by the chemical properties of Fe3+ in DES and aggregation of mesopores, coexisting with the catalysis of Lewis acidic sites and flexible deformation of the flexible MOF. Therefore, the DES@MIL-53(Fe) effectively achieving the catalytic oxidation of H2S and avoiding passivation. In addition, no significant iron sulfide species were detected in spent catalyst, so FeOOH of MIL-53(Fe) as an intermediate may have been formed in the desulfurization process, which acts as a catalyst and leads to a chain reaction to produce more elemental sulfur. And elemental sulfur is the main desulfurization product, with a small quantity of sulfates. Further, the DES@MIL-53(Fe) can be regenerated at 220°C, without considerably diminishing on the H2S removal capacity after two regeneration cycles, which is critical for the catalyst application in industry.

 
ACKNOWLEDGEMENTS


This work is supported by Natural Science Foundation of Shandong Province (ZR2020QB199), Natural Science Foundation of Henan Province (222300420380), and the major innovation projects of science, education, industry integration of Qilu University of Technology (Shandong Academy of Science) (2022JBZ02-05), and Cultivate New Fund Qilu University of Technology (2022PX018).


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