Amino-Modified Fe-Terephthalate Metal–Organic Framework as an

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Amino-Modified Fe-Terephthalate Metal−Organic Framework as an Efficient Catalyst for the Selective Oxidation of H2S Xiao-Xiao Zheng, Li-Juan Shen, Xiao-Ping Chen, Xiao-Hai Zheng, Chak-Tong Au, and Li-Long Jiang* National Engineering Research Center of Chemical Fertilizer Catalyst, Fuzhou University, Fuzhou, Fujian 350002, P. R. China

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S Supporting Information *

ABSTRACT: Classical amino-functionalized Fe-terephthalate metal−organic framework NH2-MIL-53(Fe) and its parent framework MIL-53(Fe) were prepared via simple hydrothermal methods. The catalytic performaces of these two Fe-MOFs were explored for the selective oxidation of H2S. The physicochemical properties of the fresh and used FeMOFs catalysts were investigated by XRD, BET, SEM, FT-IR, CO2-TPD, and XPS techniques. It was found that the introduction of amino groups reduces the activation energies for H2S oxidation and endows this catalyst surface with moderate basic sites. As a result, the NH2-MIL-53(Fe) catalyst displays high H2S conversion and near 100% S selectivity in the temperature range of 130−160 °C, outperforming commercial Fe2O3 and activated carbon. Moreover, a plausible reaction route for H2S selective oxidation over NH2-MIL-53(Fe) is proposed. This work opens up the possibility of utilizing MOFs as efficient catalyst for desulfuration reactions.

1. INTRODUCTION H2S is a highly toxic pollutant generated from a diversity of chemical processes.1 With environmental laws and regulations regarding the emission of sulfur compounds becoming more stringent, advanced treatments are urgently needed to efficiently eliminate H2S.2 Currently, the multistep Claus technology is the most used for the treatment of H2Scontaining gases with elemental sulfur being recovered as an end product.3 However, because of thermodynamic limitations, there is still 3−5% of H2S that is not converted to S.4 Among the other methods for desulfurization, the selective oxidation of H2S using catalysts is attractive because the chemical processes are free from thermodynamic limitations and the capital requirement is low.5 Iron-based catalysts were widely applied to the selective oxidation of H2S.6 The disadvantages of using this class of catalysts are poor sulfur selectivity and fast deactivation of the catalysts.7 In these processes, excess oxygen is required, and poor sulfur selectivity is common. Furthermore, the Fe−S bonds that involve severe overlapping of sulfur p and iron d orbitals are strong and difficult to break,8 making the loss of active iron sites inevitable. Hence, developing efficient ironbased catalysts for selective oxidation of H2S is urgent. Metal−organic frameworks (MOFs) are ordered porous materials constructed from metal ions/clusters with multifunctional organic linkers.9 Endowed with unique physicochemical characteristics like structural diversity, versatile chemical composition, and controllable pore diameter, MOFs are widely applied to gas separation/storage, drug release, and sensing.10 In view of the presence of coordinated unsaturated metal sites © XXXX American Chemical Society

(CUSs) and catalytically active organic ligands, it is envisioned that the application of MOFs in catalysis can be greatly widened.11 It is possible to decorate MOFs with extra active sites through the functionalization of the MOF’s structure.12 For instance, Dhakshinamoorthy et al. proposed that Fe(BTC) can be used to efficiently catalyze the reaction of benzylic compounds and t-butylhydroperoxide, which is attributed to the nonframework coordination position of Fe3-μ3O clusters being used as active sites.13 Gascon et al. reported a higher catalytic activity of amino-functionalized IRMOF-3 solid base in comparison to that of MIL-53(Al).14 Furthermore, Lu et al. utilized PMOF ([Co2L0.5V4O12]·3DMF·5H2O) to achieve the efficient selective oxidation of sulfides.15 Despite many beneficial features, using functionalized MOFs as catalysts for the selective oxidation of H2S has never been reported. Herein, we demonstrate that MOFs can function as efficient catalysts for the conversion of H2S to S. MIL-53(Fe) is a classical Fe-based MOF material, which is formed by bridging FeIIIO4(OH)2 octahedra through terephthalic acid.16 This material was chosen as representative because of its high stability and ample coordinatively unsaturated centers.17 It was reported that, through the functionalization of organic ligands, extra active sites could be introduced into MIL-53(Fe).14,18 With these in mind, we prepared amino-functionalized MIL53(Fe) using a facile hydrothermal process. The catalytic performance over MIL-53(Fe) and its amino-functionalized counterpart (denoted herein as NH2-MIL-53(Fe)) was Received: May 5, 2018

A

DOI: 10.1021/acs.inorgchem.8b01232 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. (a) XRD patterns; (b) FT-IR spectra; (c) SEM images; and (d) TG profile of NH2-MIL-53(Fe) (also that of MIL-53(Fe) in the case of parts b and d). collected using a Nicolet 6700 spectrometer equipped with an HgCdTe (MCT) detector, and KBr was used as a dispersing agent. The spectra were obtained by scanning 32 times at 4.0 cm−1 resolution. Thermogravimetric analysis (TGA) was conducted over a PerkinElmer DTA7 analyzer from 50 to 650 °C with a 10 °C/min heating rate. N2 physisorption measurements of the materials were performed on a U.S. Micromeritics 3Flex analyzer. The materials were vacuum treated at 150 °C for 10 h before testing. The investigation of morphology (by SEM) and elemental composition (by EDX) was performed over an S-4800 Hitachi scanning electron microscope. Xray photoelectron spectroscopy (XPS) studies were performed over a Thermo ESCALAB 250 spectrometer with XPS binding energies corrected against the C 1s peak 284.6 eV of adventitious carbon. The distribution of sample base strength was determined by CO2temperature-programmed desorption (CO2-TPD) over a Micromeritics Autochem 2920 instrument. About 0.15 g of sample (20− 40 mesh) was pretreated at 200 °C for 1 h in a flow of pure Ar, exposed to high purity CO2 for 1 h at room temperature, and subjected to Ar purging before being heated (10 °C/min) from room temperature to 300 °C under an Ar flow for CO2 desorption recording. 2.3. Catalytic Experiments. All tests were performed at atmospheric pressure using a fixed-bed reactor with catalysts (200 mg, 20−40 mesh) securely positioned in the reactor. A gas mixture containing 0.5% H2S, 0.25% O2, and N2 as balance was introduced into the reactor at a WHSV of 3000 mL g−1 h−1, and reaction temperature was regulated at 100−190 °C. The outlet gas was monitored online using a gas chromatograph (GC9720) furnished with a thermal conductivity detector (TCD) and a flame photometric detector (FPD). The condenser located at the exit of the reactor was for capturing the S in the tail gas. H2S conversion, S selectivity, and sulfur yield were calculated according to the following formulas:

evaluated for the recovery of elemental sulfur in H2S selective oxidation. It was found that NH2-MIL-53(Fe) exhibits better performance than do commercial Fe2O3 and activated carbon. In addition, on the basis of the results of systematic studies, a plausible reaction route is presented for the H2S oxidation. As far as we know, this is the first investigation to utilize Fe-MOFs and its amino-functionalized counterpart as catalysts for H2S selective oxidation. It is anticipated that those findings would provide a new direction for the development of effective desulfurization catalysts that are based on MOFs.

2. EXPERIMENTAL METHODS 2.1. Preparation of Catalysts. All reagents were purchased from suppliers and directly used without further processing. NH2-MIL53(Fe) was synthesized as described by Han et al.19 but with some modifications. Typically, Fe(NO3)3·9H2O and H2ATA in a molar ratio of 1:1 were dispersed in DMF (20 mL). The solution was vigorously stirred at ambient temperature to reach transparency before being transferred into a Teflon autoclave (100 mL) for thermal treatment at 150 °C for 6 h. When the autoclave and its content were cooled to 30 °C, the as-generated particles were isolated by centrifugation and washed 3× with DMF and methanol. To exchange impurities inside the pores, the harvested substances were subjected to 8 h of stirring in 200 mL of methanol. Finally, the samples were collected and vacuum treated at 80 °C. The harvested powder is named herein as NH2-MIL-53(Fe). For comparison, we synthesized MIL-53(Fe) via the approach reported by Férey et al.20 Briefly, FeCl3·6H2O (15 mmol, 4.05 g) and H2BDC (7.5 mmol, 1.236 g) were dispersed in DMF (45 mL) and subjected to ultrasound agitation. The resulting transparent solution was then thermally treated (170 °C, 24 h) in a Teflon autoclave. The as-generated materials were separated by filtration and stirred in a large volume of methanol for a couple of hours to remove residual DMF. After vacuum drying at 100 °C, the MIL-53(Fe) was obtained. 2.2. Characterizations. X-ray diffraction (XRD) was analyzed on a X’Pert Powder instrument employing Co Kα radiation (λ = 1.789 Å, 40 kV, 40 mA). Fourier transformed infrared (FT-IR) spectra were B

H 2S conversion =

[H 2S]in − [H 2S]out [H 2S]in

sulfur selectivity =

[H 2S]in − [H 2S]out − [SO2 ]out [H 2S]in − [H 2S]out DOI: 10.1021/acs.inorgchem.8b01232 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. XPS spectra of NH2-MIL-53(Fe): (a) Fe 2p; (b) C 1s; (c) O 1s; and (d) N 1s.

of these two materials.18,24 The TG results (Figure 1d) indicate that MIL-53(Fe) and NH2-MIL-53(Fe) are thermally stable up to 350 and 300 °C, respectively. In addition, the characterization results presented so far demonstrate that we have successfully prepared NH2-MIL-53(Fe). With high thermal stability and ample amino groups, the NH2-MIL53(Fe) catalyst should function well for the selective oxidation of H2S. XPS characterizations were conducted to determine the chemical constitution and elemental valence of NH2-MIL53(Fe). The survey scan confirms the existence of Fe, O, C, and N (Figure S4). The Fe 2p1/2 and Fe 2p3/2 are at 724.5 and 710.9 eV binding energies (Figure 2a), respectively, showing a doublet separation of about 13.6 eV; the data indicates the presence of Fe(III).19,25 The C 1s signal of NH2-MIL-53(Fe) (Figure 2b) at 284.6, 286.1, and 288.5 eV is ascribable to phenyl, C−N, and carboxyl groups, respectively.26 The O 1s peak (Figure 2c) could be fitted to two components centered at 531.7 and 533.6 eV, which belong to Fe−O and carboxylate groups, respectively.27 The N 1s spectrum (Figure 2d) shows peaks at 399.3 and 400.7 eV, which are assignable to C−N bonds and NH2 groups, respectively.27a The XPS results further provide evidence for the successful introduction of amino groups into the MIL-53(Fe) framework. 3.2. Catalytic Performance. 3.2.1. Influence of Test Temperature. The activities of Fe-MOFs for H2S removal were evaluated at a WHSV of 3000 mL g−1 h−1 with H2S/O2/ N2 composition equal to 5/2.5/92.5. From Figure 3a, the H2S conversion of Fe2O3 is 53.1% at 100 °C, whereas those for MIL-53(Fe) and NH2-MIL-53(Fe) are much higher, ca. 73.6% and 93.7%, respectively, indicating much better performance of the latter at lower temperatures. The H2S conversion over the four catalysts increases with increasing temperature. The NH2MIL-53(Fe) catalyst outperforms MIL-53(Fe) because 100% H2S conversion can be reached at 130 °C using the former, but it is 160 °C using the latter. When the reaction temperature

sulfur yield = [H 2S conversion] × [sulfur selectivity]

3. RESULTS AND DISCUSSION 3.1. Catalyst Characterization. The physicochemical characters of Fe-MOFs were investigated by the following techniques. The crystal structure of prepared MIL-53(Fe) (Figure S1) was identical to that stated by Dong et al.21 The main diffraction peaks of synthesized NH2-MIL-53(Fe) at 2θ of 9.2, 10.4, 13.1, 18.6, 20.8, and 25.1° are similar to those of NH2-MIL-53(Fe) reported in the literature,19 suggesting that NH2-MIL-53(Fe) was successfully synthesized (Figure 1a). Nonetheless, there is a slight difference in terms of relative peak intensity as illustrated in Figure 1a with the NH2-MIL53(Fe) pattern from the literature simulated for visual comparison. The discrepancy in relative intensity could be a result of a difference in synthetic methods. Compared to MIL53(Fe), NH2-MIL-53(Fe) has three additional peaks in the FT-IR spectrum (Figure 1b). The two at 3491 and 3383 cm−1 are attributable to the stretching of νs(N−H) and νas(N−H), whereas that at 1255 cm−1 is attributable to the C−N vibration of amino groups on benzene rings.18,22 The FT-IR characterization confirms that the NH2-MIL-53(Fe) framework was indeed decorated with amino groups.23 The SEM image of NH2-MIL-53(Fe) (Figure 1c) reveals the uniform dispersion of spindlelike structures roughly 1.21 μm in length and 0.51 μm in width, whereas that of MIL-53(Fe) shown in Figure S2 displays irregular particles much bulkier in size. As one can see, the decoration of MIL-53(Fe) with NH2 results in not only significant change of morphology but also reduction of particle size, which may affect the catalytic performance. The N2 adsorption−desorption isotherms of these two Fe-MOFs are presented in Figure S3. Compared with other MOF materials, the surface areas and pore volumes of the as-prepared FeMOFs appear to be much smaller. It is considered that the measured values are not reliable because of the breathing effect C

DOI: 10.1021/acs.inorgchem.8b01232 Inorg. Chem. XXXX, XXX, XXX−XXX

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100% at 130 °C to ca. 85.9% at 190 °C. These results show that the selectivity to sulfur over the Fe-MOFs catalyst is higher than that over Fe2O3. The phenomenon is attributed to the inherent regular structure of the MOFs that enables the retention of the microenvironment of active sites, thereby effectively resisting the deep oxidation of H2S. In addition, the introduction of amino groups can further improve the performance for H2S selective oxidation, and the sulfur selectivity of NH2-MIL-53(Fe) starts to decrease from 100% at 160 °C to ca. 87.6% at 190 °C, plausibly because the introduction of amino groups can affect the acid sites and inhibit the formation of sulfur radicals.8 As mentioned, the sulfur yield over NH2-MIL-53(Fe) is close to 100% at 130 °C but starts to decrease when temperature is above 160 °C (Figure 3c). In other words, the temperature range for maximum sulfur yield is 130−160 °C. It is noted that, under the same reaction conditions, NH2-MIL53(Fe) is superior to commercial Fe2O3 and activated carbon in catalytic performance, displaying higher activities at lower temperatures. In addition, the sulfur mass balance calculated based on recovered S and the sulfur content of the used catalyst is close to the theoretical value (Figure S5). 3.2.2. Arrhenius Plots. The apparent activation energy of a catalytic reaction can be estimated from the Arrhenius plot. Generally, the smaller the gradient of the plot is, the lower the activation energy, and thus the easier the occurrence of the catalytic process is.28 During the collection of kinetic data for the Arrhenius plot, the reactions were carried out at a high WHSV where the H2S conversion was kept below 20%, thereby eliminating the effect of internal and external diffusion (Figure 4 and Table 1).The introduction of amino groups into

Figure 3. Influence of temperature on (a) H2S conversion; (b) S selectivity; and (c) sulfur yield.

was 130 °C, H2S conversion over MIL-53(Fe) is only 87%. The higher performances of NH2-MIL-53(Fe) are primarily ascribed to the amino groups that enrich the catalyst surface with basic sites, leading to an enhanced interaction with the acidic H2S molecules. According to the literature, (H2S + 1/2O2 → (1/n)Sn + H2O) is the main reaction whereas (2H2S + 3O2 → 2SO2 + 2H2O; S + O2 → SO2) are the side reactions in H2S selective oxidation.3c Increasing the temperature can accelerate the reaction rate, which would facilitate H2S deep oxidation and S oxidation, resulting in reduced sulfur selectivity. The fact that the sulfur selectivity of the three catalysts decreases with increasing temperature also confirms this view (Figure 3b). However, the difference among the three lies in the extent of decrease in terms of initial temperature and speed of the reaction. The sulfur selectivity of commercial Fe2O3 rapidly decreases from an initial value of 100% at 100 °C to 73.3% at 190 °C, while that of MIL-53(Fe) starts to decrease from

Figure 4. Arrhenius plots of H2S oxidation over the as-prepared FeMOFs. Reaction conditions: catalyst (15 mg), H2S/O2/N2 (0.5/0.25/ 99.25, wt %), and WHSV (345 000 mL g−1 h−1).

MIL-53(Fe) results in a reduction in the apparent activation energy from 17.15 to 15.66 kJ/mol. Since a lower activation energy means a higher reaction rate, NH2-MIL-53(Fe) is superior to MIL-53(Fe) as catalysts in oxidizing H2S to sulfur. Table 1. Activation Energies of Reaction over the AsPrepared Fe-MOFs

D

sample

gradient

activation energy(kJ/mol)

MIL-53(Fe) NH2-MIL-53(Fe)

−2.06 −1.88

17.15 15.66 DOI: 10.1021/acs.inorgchem.8b01232 Inorg. Chem. XXXX, XXX, XXX−XXX

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at 100%. As the number of cycles increases, H2S conversion begins to decrease. This may be due to sulfur deposition on the NH2-MIL-53(Fe) catalyst. In addition, sulfur selectivity decreases with increasing number of cycles and eventually stays at about 90% at the fifth cycle. The results suggest that the recyclability of the NH2-MIL-53(Fe) catalyst is reasonably good. In order to meet the requirements for industrial applications, we examined the structural integrity of NH2-MIL-53(Fe) using several characterization techniques. Compared to fresh NH2MIL-53(Fe), the used sample is weaker in peak intensity, and the peak at 10.2° shifts slightly to a higher angle (Figure S7). This may be due to the fact that, in the H2S oxidation reaction, H2S molecules first adsorb on Fe3+ and then react with oxygen to form S, in which the interaction between H2S and Fe3+ may slightly distort the flexible NH2-MIL-53(Fe) structure. However, the main diffraction peaks for this material remain basically unchanged after the reaction, indicating that NH2MIL-53(Fe) has good structural stability. In Figure 7a, one can see that, despite a weakening in intensity as a result of H2S interaction with the amino groups, the IR signal of NH2 can still be observed. The SEM image of the used catalyst (Figure 7b) shows that the morphology is largely the same as that of the unused catalyst. However, there is the detection of small particulates on the surface of the used catalyst due to sulfur deposition. The reaction was conducted at 190 °C, which was only slightly above the dew point (180 °C) of sulfur. It is hence possible that the sublimation of sulfur was incomplete, and there was a minute amount of sulfur left on the catalyst. According to the result of EDX analysis (Figure S8a), the amount of sulfur was 1.2 wt % of the used catalyst, and there was uniform distribution of sulfur throughout (Figure S8b). Overall, the NH2-MIL-53(Fe) catalyst displays good structural stability in the reaction. 3.3. Catalytic and Deactivation Mechanisms. In view of the amine functionality of NH2-MIL-101(Fe), Wang and Li used the material as a Brønsted base to catalyze the Knoevenagel reaction.29 Since the iron source and ligand used for the construction of NH2-MIL-53(Fe) in our study are similar to those of NH2-MIL-101(Fe), it is envisaged that the former also functions as a kind of Brønsted base to catalyze the oxidation of H2S. To prove this assumption and to gain mechanistic insight, we conducted CO2-TPD measurements over these two Fe-MOFs (Figure 8). On the basis of peak intensity and position, a comparison of the amount and strength of basic sites between the two catalysts was conducted. Despite the fact that the two are similar in the amount of weak basic sites (Tmax = 95−100 °C), the amount of moderate basic sites (Tmax = 285 °C) of NH2-MIL-53(Fe) is significantly higher. The phenomenon is caused by the existence of amino groups in NH2-MIL-53(Fe). Mechanistically, H2S mainly adsorbs on the basic sites of moderate strength before being oxidized into elemental sulfur. The abundant NH2 in the NH2-MIL-53(Fe) structure could facilitate the selective oxidation of H2S. The XPS survey spectrum of used NH2-MIL-53(Fe) confirms the presence of Fe, O, C, S, and N (Figure 9a), which matches well with the results of EDX studies. The Fe 2p spectra of fresh and used samples are similar, and both confirm the presence of Fe3+ (Figure 9b).19 According to literature reports and our experimental results, H2S oxidation over FeMOF catalysts may follow a redox mechanism that involves Fe3+.3a,7b In Figure 9c, the N 1s spectrum of used NH2-MIL-

The results of the Arrhenius plot match well with those of catalytic activities. 3.2.3. Effect of WHSV. The relationship between WHSV and catalytic activities was investigated over NH2-MIL-53(Fe) at 160 °C with WHSVs gradually increased from 3000 to 12 000 mL g−1 h−1. Each stage was kept for 60 min before WHSV was adjusted to a new level. As depicted in Figure 5,

Figure 5. Effect of WHSV on NH2-MIL-53(Fe) catalytic performance at 160 °C.

H2S conversion is 100% within a WHSV range of 3000−6000 mL g−1 h−1 but decreases with the further rise of WHSV. It is noted that the sulfur selectivity remains above 90% throughout. With the decrease of H2S conversion there is a decrease in sulfur yield. Nonetheless, the NH2-MIL-53(Fe) catalyst exhibits a certain level of catalytic activity at WHSV higher than 6000 mL g−1 h−1. 3.2.4. Stability and Recyclability of NH2-MIL-53(Fe). The stability of NH2-MIL-53(Fe) for converting H2S to S at 160 °C is shown in Figure 6. The test conditions are the same as those

Figure 6. Catalytic performance over NH2-MIL-53(Fe) versus reaction time in H2S selective oxidation at 160 °C.

in Section 3.2.1. Within the first 17 h, H2S conversion is 100% and then decreases gradually to 94% in the next 13 h. As for S selectivity, it slightly decreases at the beginning but stabilizes at ca. 90% afterwards. There is a slight decrease of sulfur yield with time, a combined result of the decrease in H2S conversion and sulfur selectivity. It should be pointed out that the deposition of sulfur on the external as well as internal surface of the catalyst could also cause a reduction of sulfur yield. The results of the stability test of NH2-MIL-53(Fe) are presented in Figure S6. During the first two cycles, H2S conversion remains E

DOI: 10.1021/acs.inorgchem.8b01232 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 7. (a) FT-IR spectra before and after reaction, and (b) SEM image of NH2-MIL-53(Fe) taken after reaction.

from 51.2 to 46.2%, indicating the loss of -NH2 sites. Furthermore, the S 2p spectrum (Figure 9d) shows peaks at 163.5, 164.7, and 168.8 eV; the first two belong to S 2p3/2 and S 2p1/2 of Sn, whereas the last peak is the signal of SO42−.6a,31 The presence of elemental sulfur and Fe2(SO4)3 species is the cause for NH2-MIL-53(Fe) catalyst deactivation. On the basis of the above discussions, we proposed a plausible mechanism for H2S selective oxidation over NH2MIL-53(Fe) (Figure 10). For this Fe-based catalyst, H2S molecules largely adsorb on the NH2 basic sites, while oxygen molecules on the surface transform to active O2− and O− species. Then surface H2S reacts with O− to form elemental S and H2O. Meanwhile, only a minute amount of sulfur continues to react with oxygen to produce SO2 and even SO42−, making sulfur selectivity always slightly below 100%. The results suggest that the formation of strong Fe−S bonds is insignificant. In other words, the involvement of a redox mechanism in which a H2S molecule first reacts with Fe3+ to

Figure 8. CO2-TPD profiles of CO2 adsorbed on the as-prepared FeMOFs catalysts.

53(Fe) also indicates the presence of C−N and NH 2 groups.7a,30 It is noted that the -NH2 relative content decreased

Figure 9. XPS spectra of NH2-MIL-53(Fe) before and after the H2S oxidation reaction: (a) survey scans; (b) Fe 2p; (c) N 1s; and (d) S 2p. F

DOI: 10.1021/acs.inorgchem.8b01232 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 10. Schematic of H2S oxidation over NH2-MIL-53(Fe).

form S2− and Fe2+, and then Fe2+ is reoxidized by oxygen species to Fe3+, is less apparent.

Development Program of China (2018YFA0209304), and the Natural Science Foundation of Fujian Province (2017J05022).

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4. CONCLUSIONS Fe-based MOF catalysts MIL-53(Fe) and NH2-MIL-53(Fe) can be easily prepared hydrothermally. The NH2 groups present in NH2-MIL-53(Fe) serve as moderate basic sites for the adsorption of H2S. As a catalyst for oxidizing H2S to S, NH2-MIL-53(Fe) far exceeds commercial Fe2O3, activated carbon, and MIL-53(Fe) in catalytic performance. The introduction of amino groups lowers the apparent activation energy for the reaction. In the temperature range 130−160 °C, H2S conversion and sulfur selectivity remain close to 100% over NH 2-MIL-53(Fe). The results not only provide mechanistic insight into the selective oxidation of H2S over Fe-based MOF catalysts but also serve as a guideline for developing stable and effective MOF-based catalysts for redox reactions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01232. XRD and SEM pictures of MIL-53(Fe); N2 physisorption studies of as-prepared Fe-MOFs; XPS survey spectrum of NH2-MIL-53(Fe); pictures of sulfur recovered from the effluent; stability test of NH2-MIL53(Fe) at 160 °C; XRD results of NH2-MIL-53(Fe) before and after H2S oxidation reaction; EDX spectrum (inset are the elemental ratio of components); and S elemental mapping of used NH2-MIL-53(Fe) (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86 591 83707796. Tel: +86 591 83731234-8201. Email: [email protected]. ORCID

Li-Long Jiang: 0000-0002-0081-0367 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (21603034), the National Key Research and G

DOI: 10.1021/acs.inorgchem.8b01232 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b01232 Inorg. Chem. XXXX, XXX, XXX−XXX