Nitrogen-Decorated, Ordered Mesoporous Carbon Spheres as High

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Nitrogen-Decorated, Ordered Mesoporous Carbon Spheres as HighEfficient Catalysts for Selective Capture and Oxidation of H2S Xun Kan, Xiaoping Chen, Wei Chen, Jinxing Mi, Jia-Yin Zhang, Fujian Liu, Anmin Zheng, Kuan Huang, Lijuan Shen, Chaktong Au, and Lilong Jiang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05852 • Publication Date (Web): 26 Feb 2019 Downloaded from http://pubs.acs.org on March 3, 2019

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ACS Sustainable Chemistry & Engineering

Nitrogen-Decorated, Ordered Mesoporous Carbon Spheres as High-Efficient Catalysts for Selective Capture and Oxidation of H2S Xun Kan, †Xiaoping Chen, † Wei Chen, § Jinxing Mi, † Jia-Yin Zhang, ‡ Fujian Liu*,† Anmin Zheng, § Kuan Huang, ‡ Lijuan Shen, † Chaktong Au, † and Lilong Jiang*† †

National Engineering Research Center of Chemical Fertilizer Catalyst (NERC-CFC), School of

Chemical Engineering, Fuzhou University, 523, Gongye Road, Fuzhou, 350002, P. R. China. E-mail: [email protected]; [email protected] ‡

Key Laboratory of Poyang Lake Environment and Resource Utilization of Ministry of

Education, School of Resources Environmental and Chemical Engineering, 999, Xuefu Road, Nanchang, Jiangxi, 330031, China. §

National Center for Magnetic Resonance in Wuhan, State Key Laboratory of Magnetic

Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, West 30, Xiaohongshan, Wuhan, Hubei, 430071, China. KEYWORDS Nitrogen-doped carbons; micropore and ordered mesopores; selective removal of H2S; green and sustainable chemistry; self-assembly.

ABSTRACT 1 Environment ACS Paragon Plus

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Hydrogen sulfide (H2S) is highly toxic and corrosive, and its selective removal from fuel and flue gases is of significance. It is a challenge to develop a dual-role material for efficient H2S capture as well as selective H2S oxidation. In the present work, we designed a class of nitrogendecorated ordered mesoporous carbon spheres (denoted herein as N-OMCS-T, where T stands for carbonization temperature) for such an end. The N-OMCS-T with ordered mesopores are large in BET surface area (1201~1500 m2/g), and enriched with nitrogen sites of pyridinic and pyrrolic nature, and exhibit strong interaction with H2S as verified in DFT calculations. With large BET surface areas, ordered mesopores and abundant nitrogen sites, the N-OMCS-T materials show extraordinary efficacy for the capture of H2S: H2S capacities are up to 13.4 mmol/g (0 °C, 1 bar), and the Ideal Adsorption Solution Theory selectivity in cases of H2S/CO2, H2S/CH4 and H2S/N2 is 2.1~8.5, 11.0~25.5, and 30.4~81.7, respectively, much higher than those of porous materials such as activated carbon, OMCS, Zeolite A, UiO-66 and SBA-15. Moreover, N-OMCS-T could act as efficient and long-lived metal-free catalysts for selective oxidation of the captured H2S to sulfur under mild conditions.

INTRODUCTION

Hydrocarbon streams, such as those originated from industrial processes of natural gas, syngas, and biogas are widely used as feedstock for energy generation and chemicals production.1-3 Hydrogen sulfide (H2S), which is highly toxic and corrosive, is commonly present in these resources as a result of coal and biomass gasification as well as crude oil hydrodesulfurization in high-temperature and low-oxygen situations.4-6 H2S existence in fuel and flue gases is unacceptable because it is not only harmful to health, but also destructive to facilities and metal catalysts.7,8 So far, the selective removal of H2S is required for safe

2 Environment ACS Paragon Plus

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transportation and vital fuel-source utilization.9,10 For H2S removal, the traditional Claus process has been widely applied, but it is uneconomical.11,12 Because of the limit of thermodynamics, there is still 3−5% of H2S left in the tail gas, which is far behind the target of H2S elimination.13,14 Alternative methods such as cryogenic separation, membrane separation, adsorption and selective oxidation show improved performance for H2S removal in comparison with the Claus process.15-17 Furthermore, aqueous solutions of tertiary amines are widely used for selective removal of H2S from fuel gases.18 Nonetheless, the volatile and corrosive nature, and the degradation of amine absorbents strongly constrains their applications.19,20 In recent years, ionic liquids (ILs) were found to be extremely active for the selective capture of H2S under ambient conditions.21 However, ILs is costly to make and/or buy, and its regeneration complicate.22 Unlike liquid absorbents, solid adsorbents show negligible volatility, and are less corrosive and easy to regenerate.22,23 Various kinds of solid materials such as metal oxides, metal organic frameworks (MOFs), zeolites, carbons, and composites that are based on these materials have been used to capture H2S from fuel gases.7,24-26 A solid adsorbent should be abundant in nanopores. It is desirable that the nanopores are tunable during adsorbent fabrication for enhanced accessibility of adsorption sites. The porous materials should be richly decorated with basic species for the uptake of H2S.27 In addition to H2S adsorption capacity, the removal of adsorbed H2S is of significance in terms of adsorbent reusability.28 In this aspect, the selective oxidation of H2S has high potential because the process is not only green and low cost, but also free from thermodynamic limitation.29,30 Nonetheless, there are few reports on structural engineering of porous materials that are endowed with dual functionality for both selective capture and oxidation of H2S.



For H2S oxidation, much attention has been paid to the development of metal-free catalysts such as nitrogen-doped porous carbons, which show superior activities in selective oxidation of H2S.23,30,31 In comparison with the metal-based catalysts, the metal-free ones are superior in sulfur tolerance,32,33 and are more controllable in the generation of basic sites.26 Moreover, porous carbons are large in BET surface area, high in thermal stability and versatile in functionality.22,23 In spite of the promising results of carbon materials, none of them are satisfactory in selective capture and oxidation of H2S. In this paper, we report a class of nitrogendecorated ordered mesoporous carbon spheres (herein denoted as N-OMCS-T where “T” stands for carbonization temperature), which are synthesized through steps of self-assembly of block copolymer template with m-aminophenol, followed by hydrothermal curing at high temperature (180 °C) in the presence of hexamethylenetetramine, and activation with KOH at 700−900 °C. The synthesized N-OMCS-T are large in BET surface area (1201~1500 m2/g), abundant in nanopores, and are uniquely endowed with pyridinic, pyrrolic and graphitic nitrogen. According to the outcomes of DFT calculation, the interaction of H2S with pyridinic and pyrrolic nitrogen sites are strong. Besides acting as active sites for H2S adsorption, the basic surface nitrogen sites also participate in the dissociation of H2S to HS- and H+, thus initializing the selective oxidation of H2S to elemental sulfur. As a result, the N-OMCS-T synthesized in this work show excellent performance for selective capture of H2S [13.4 mmol/g at 0 °C, 1.0 bar, and IAST (Ideal Adsorption Solution Theory) selectivity up to 8.5~81.7 in cases of H2S/CO2, H2S/CH4, and H2S/N2], and superior activity for selective oxidation of H2S under rather mild conditions (near 100% conversion of H2S with close to 100% selectivity to elemental sulfur at 160 °C). The H2Sremoval performance of N-OMCS-T is much better than those of metal-free materials reported in the literature.23,24


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EXPERIMENTAL DETAILS Chemicals and reagents All the chemicals were analytical grade and used directly without further purification. Triblock copolymer poly (ethylene oxide)-b-poly (propylene oxide)-b-poly (ethylene oxide) of Pluronic

F127

(PEO106-PPO70-PEO106,

Mw=12600),

resorcinol,

m-aminophenol

and

hexamethylenetetramine were purchased from Sigma-Aldrich Co., Ltd. Other chemicals and materials were obtained from Macklin Chemical Reagents Co., Ltd. Catalysts preparation The nitrogen-decorated mesoporous carbon spheres were prepared via a self-assembly process of block copolymer template (F127) with m-aminophenol in the presence of hexamethylenetetramine (HMTA), through steps of hydrothermal curing at 180 °C, followed by carbonization at a designated temperature in the 700–900 °C range, and activation with KOH at 800 °C. Typically, 0.654 g of m-aminophenol and 0.42 g of HMTA were dissolved in 75 mL of distilled water under vigorous stirring. Then, an aqueous template solution containing 0.47 g of triblock copolymer (F127) and 5 mL of distilled water was introduced. The resultant precursor solution was heated to 50 °C and stirred for 16 h. The resulted brown precipitate was collected and transferred into an autoclave and hydrothermally treated at 95 °C for 12 h. The as-generated pristine ordered mesoporous polymer spheres (OMPS) was collected through filtration, washed with abundant water, and dried at 60 °C for 12 h under vacuum. The N-OMCS-T was synthesized through carbonization of OMPS at a selected temperature followed by controlled activation with KOH. As a typical run for the synthesis of N-OMCS-700, the OMPS was heated to 250 °C (1 °C/min) and subjected to oxidation at this temperature for 2 h under a flow of air. As shown in Scheme S1, the resulted substance was heated in flowing N2 from room temperature 5 Environment ACS Paragon Plus


to 400 °C within 30 min, and then rapidly to 700 °C and held at 700 °C for 20 min. Finally, the carbonized material was activated with KOH according to the following procedures: 1.0 g of carbonized material was mixed with 1.0 g of KOH, and the mixture was thoroughly ground within 10 min. The obtained powder was heated to 800 °C (1 °C/min) and subjected to activation at 800 °C for 2 h in flowing nitrogen (Scheme S2). For purification, the as-synthesized material was stirred in 50 mL of aqueous HCl (0.5 M) solution for the removal of residual inorganic salts, and then rinsed with plenty of deionized water to reach neutrality. The material denoted as NOMCS-700 was harvested after the washed material was dried at 80 °C under vacuum. Likewise, N-OMCS-800 and N-OMCS-900 were synthesized at a carbonization temperature of 800 °C and 900 °C, respectively (Scheme S1). For comparison purposes, ordered mesoporous carbon with no nitrogen decoration (denoted herein as OMCS) was produced following the procedure for the generation of N-OMCS-700, where resorcinol was employed as the precursor instead of maminophenol. Characterizations X-ray diffraction (XRD) measurements were performed on an X'Pert3 Powder diffractometer using Cu Kα radiation (40 mA, 45 kV). Scanning electron microscopic (SEM) analysis was performed over S-4800 Hitachi at an acceleration voltage of 5 kV. Transmission electron microscope (TEM) images were collected on a Zeiss Libra200 TEM at an acceleration voltage of 200 kV. Specific surface areas and pore volumes were determined from adsorptiondesorption isotherms of nitrogen at −196 °C using a Micromeritics ASAP 2020M system, with the sample degassed under vacuum (1 ×10−5 Pa) at 180 °C for 6 h prior to measurement. X-ray photoelectron spectroscopic (XPS) analysis was performed on a Thermo Fisher Scientific EscaLab 250Xi instrument. Elemental analysis (EA) was carried out on a Vario EL III elemental


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analyzer. Laser Raman spectra were recorded on a Renishaw InVia Reflex spectrometer at wavelength of 532 nm (500 mW). Thermogravimetric analysis (TG) was recorded under an air flow (30 mL/min) with a Setsys Evolution analyzer using 5 mg of sample at a heating rate of 10 °C/min. Computational Details Density functional theory (DFT) with wB97XD hybrid exchange-correlation functional was employed to perform theoretical calculations using def2-SVP basis set.32,33 The related information could be found in “Supporting Information”. Selective capture and oxidation of H2S The H2S adsorption isotherms of various samples were conducted on a set of self-made equipment, which included an adsorption tank and a buffer tank from Feiyu Petroleum Technology Development Co. LTD (Nantong, China). Before test, the sample was placed into a vacuum drying chamber at 150 °C for 12 h to remove adsorbed species such as water. Typically, 100 mg of the treated sample (precisely weighed) was enclosed and sealed into the adsorption tank; the adsorption tank and buffer tank were then transferred into a water bath of constant temperature. The changes of pressure in the buffer tank can reflect even a slight change of system temperature. To confirm the tightness of the system and calculate the free volume of adsorption tank, the buffer tank was filled with helium (He) to a pressure of 1.5 bar, while the adsorption tank was outgassed. Then, the adsorption tank was opened to the buffer tank until the He pressure reached equilibrium. Afterwards, the He in the adsorption tank and buffer tank was evacuated, and the buffer tank was filled with about 2.5 bar of H2S. Finally, through adjusting



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the pressure between buffer tank and adsorption tank, 0.1 bar of H2S was injected into the adsorption tank every 2 h, and the data of balance pressure were recorded. The capacities of various samples for H2S adsorption were calculated according to the following equation:

n (Pg )=g ( P1 ,T )V1   g ( P1' , T )V1   g ( Pg , T )(V2  w /  y )

Equation 1

Where T is the adsorption temperature, V1 , V2 , and w /  y represent the volume of buffer tank, adsorption tank and adsorbent, respectively; g ( P1 ,T ) is the initial H2S density in the buffer tank ' ' at T,  g ( P1 , T ) is the H2S density in the buffer tank at T and P1 ,  g ( Pg , T ) is the H2S density in

the buffer tank at T and Pg . The schematic of experimental set-up is shown in the Figure S1. The cycling of H2S adsorption: After equilibrium adsorption, the captured H2S could be removed by treating the sample at 150 °C for 6 h under vacuum condition. The regenerated sample was used directly for the next run. Interestingly, there was no significant decrease of H2S adsorption capacities across the 5 cycles of adsorption. Selective catalytic oxidation of H2S over the N-OMCS-T samples was performed in a continuous flow fixed-bed reactor at atmospheric pressure. First 0.1 g of catalyst (100−120 mesh) was placed in the central section of the reactor. A mixture gas containing 5000 ppm of H2S, 2500 ppm of O2 and balance gas (N2) was introduced into the reactor at a total flow rate of 30 mL/min (WHSV = 12 000 mL g-1 h−1) for reaction in the temperature range of 100−200 °C. After reaction, the effluent stream was analyzed by a gas chromatograph (GC9790Ⅱ) equipped with a FPD and TCD. A condenser was located at the bottom of the reactor to trap sulfur in the


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effluent stream. Instantaneous fractional conversion of H2S, sulfur selectivity, and sulfur yield are defined as follows:

H2S conversion =

H2S selectivity =

(H2S)in  (H2S)out (H2S)in (H2S)in  (H2S)out  (SO2)out (H2S)in  (H2S)out

Sulfur yield = (H2S conversion)  (H2S selectivity)

Equation 2

Equation 3

Equation 4

RESULTS AND DISCUSSION Scheme 1 illustrates the synthetic process of N-OMCS-T. In the first stage, m-aminophenol, hexamethylenetetramine and triblock copolymer template (F127) were well mixed in an aqueous solution. The self-assembly and cross-linking processes occurred during heat treatment. Due to strong hydrogen bonding between the polymer precursors and triblock copolymer template, there was the formation of nitrogen-decorated polymer of spherical morphology. To obtain N-OMCST with large BET surface areas, a series of treatment including pre-oxidization, rapid carbonization, and activation with KOH were carried out. Then the synthesized N-OMCS-T were systematically characterized and used for the selective capture and oxidation of H2S.



Scheme 1 Synthetic processes of N-OMCS-T. Figure 1 shows the XRD patterns and Raman spectra of N-OMCS-T. Notably, N-OMCS700, N-OMCS-800, and N-OMCS-900 show two broad peaks at around 2θ = 24° and 43.5° associated with (002) and (100) reflection, respectively, which indicates amorphous carbon network and abundant structural defects.34 The amorphous carbon network was attributed to the relatively low carbonization temperatures; while the KOH activation process could enhance microporosity of the samples (Figure S2). The peak at around 2θ = 43.5° becomes more intense with increasing carbonization temperature from 700 to 900 °C. The Raman spectra of N-OMCST show two peaks centered at around 1325 and 1400 cm-1 which are, respectively, associated with D- and G-mode. The D-band is attributed to sp3 carbon with amorphous characteristics, while the G-band to sp2 carbon in a graphitic 2D hexagonal lattice.35,36 It was found that the ID/IG ratios of N-OMCS-T are around 0.95–1.03, indicating the presence of amorphous network and abundant structural defects, in good agreement with the XRD results.


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Figure 1 (A) XRD patterns and (B) Raman spectra of N-OMCS-T samples. Figure 2 shows the N2 adsorption-desorption isotherms and the pore size distribution of the N-OMCS-T samples. Notably, they show typical-IV isotherms with a steep increase at relative pressure of 0 < P/P0 N-OMCS-900. It is understandable because H2S adsorption ability is strongly dependent on the number of available nitrogen sites. A large BET surface area and abundance in exposed pyridinic and pyrrolic nitrogen sites would result in enrichment of surface H2S molecules, and the pyridinic, pyrrolic and graphitic nitrogen sites are responsible for selective H2S oxidation.22,31,49 Furthermore, the abundance of structural defects could result in expansion of microporosity, promoting the mass transport, anti-carbon deposition and accessibility of nitrogen sites in NOMCS-T, which are favorable factors for improving the performance of H2S capture and oxidation. It is noteworthy that the N-OMCS-T catalysts are superior to the widely reported metal-oxide catalysts such as Al2O3, CeO2, and MgAlxV1-x, especially in terms of lowtemperature performance (Table S4).7,14,50 Figure 9 (B) shows the selectivity to sulfur during H2S oxidation over the AC and NOMCS-T samples at different reaction temperatures. Notably, despite low in H2S conversion, AC shows selectivity to sulfur close to 100% in the whole temperature range (100–200 °C). For the N-OMCS-T catalysts, nearly 100% selectivity to sulfur was found below 120 °C, which


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decreases to 88% with temperature increasing to 200 °C. The phenomenon is attributed to deep oxidation of H2S (H2S + 3/2O2 = SO2 + H2O) and sulfur (S + O2 = SO2),14 noting that SO2 is an intermediate in H2SO4 production. The variation of H2S conversion and selectivity to sulfur show a similar trend of N-OMCS-900 < N-OMCS-800 < N-OMCS-700, suggesting that too high a carbonization temperature would result in poor catalytic performance. The relationship between sulfur yield and reaction temperature is shown in Figure 9C. Over the N-OMCS-T catalysts, the variation trend of sulfur yield is similar to that of H2S conversion, because the selectivity to sulfur is close to 100%. The sulfur yield over N-OMCS-T and AC increases with increasing reaction temperature. As the most superior, N-OMCS-700 exhibits a maximum sulfur yield of 96% at 140 °C, which is much higher than those of reported carbonaceous catalysts. Moreover, the weight of sulfur recovered from the effluent and catalyst (Figure S11) is close to the sulfur yield, indicating good mass balance. The Arrhenius plots of H2S oxidation over the N-OMCS-T catalysts are displayed in Figure S12. The experiments were performed at the conditions of low temperature (100–180 °C) and high WHSV (48 000 mL·g−1·h−1) to eliminate internal and external diffusion limitation. The obtained apparent activation energies of the N-OMCS-T catalysts are summarized in Figure S8, and the values are close to those reported by Steijns et al.51 The values of activation energy follow an order of NOMCS-700 (26.52 kJ/mol) < N-OMCS-800 (34.09 kJ/mol) < N-OMCS-900 (38.88 kJ/mol), and all of them are lower than that of AC (44.56 kJ/mol). In terms with the understanding that lower activation energy means higher reaction rate, the trend of activation energy is in good agreement with that of catalytic activity. The good linear plots (0.97 < R2 < 0.99) of N-OMCS-T suggests that diffusional effect is insignificant across the catalysts.



In addition to catalytic performance, the durability of a catalyst is also an important factor for practical application. As shown in Figure 9D, the N-OMCS-700 catalyst exhibits stable performance in a span of 16 h, constantly showing H2S conversion of ca. 99%. Further prolonging of reaction time results in gradual decline of activity, and H2S conversion at 22 h is 93.2%. Unlike the N-OMCS-700 catalyst, N-OMCS-800, N-OMCS-900 and AC show early decline of H2S conversion at around 14, 13 and 10 h, respectively. Nevertheless, all the NOMCS-T catalysts show much higher H2S conversion and better durability than AC. The gradual decrease of H2S conversion over the N-OMCS-T catalysts should be attributed to the deposition of elemental sulfur and accumulation of SO42- species on the external and internal surfaces of the catalysts. This is in consistent with the XPS result that indicates the presence of a minute amount of sulfate on the surface of used catalyst (Figure S13).

CONCLUSIONS In summary, a class of nitrogen-decorated ordered mesoporous carbon spheres (N-OMCST) was prepared from controlled carbonization and activation of nitrogen-doped ordered mesoporous polymer spheres. The materials could be rationally designed and fabricated through self-assembly of block copolymer templates with selected nitrogen-containing monomers through hydrothermal curing at high temperature (~180 °C). The N-OMCS-T adsorbents have large BET surface areas, hierarchical and well-defined nanopores together with controllable amount of nitrogen sites. As a result, the N-OMCS-T adsorbents show superior performance for selective capture of H2S, and the capacity for H2S adsorption is up to 13.4 mmol/g (0 °C, 1 bar) with IAST selectivity in cases of H2S/CO2, H2S/CH4, H2S/N2 as high as 8.5, 25.5 and 81.7, respectively, which are much better than those of H2S adsorbents reported in the literature.


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Moreover, the captured H2S could be selective converted to elemental sulfur under mild oxidation conditions. This work develops a new kind of dual functional porous materials with extremely high adsorption capacity for selective removal of toxic H2S, which may find applications in a wide variety of industrial processes.

ASSOCIATED CONTENT Supporting Information. Details of Density functional theory (DFT) calculation, temperature programs for the synthesis of samples, TG curves, XPS spectra, adsorption isotherms of H2S/CH4, H2S/N2 and H2S/CO2 over N-OMCS-T, and picture of sulfur recovered from the effluent. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by the National Science Fund for Distinguished Young Scholars of China (21825801) and Natural Science Foundation of China (21573150, 21203122). REFERENCES



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Nitrogen-decorated, ordered mesoporous carbon spheres were used as efficient and reusable materials for selective removal of H2S from industrial gases.


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Nitrogen-Decorated, Ordered Mesoporous Carbon Spheres as High

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