Fe3O4@C Core–Shell Carbon Hybrid Materials as Magnetically

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Article Cite This: ACS Omega 2019, 4, 1652−1661

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Fe3O4@C Core−Shell Carbon Hybrid Materials as Magnetically Separable Adsorbents for the Removal of Dibenzothiophene in Fuels Chunxia Wang,*,† Huangliang Zhong,† Wenjie Wu, Caiwen Pan, Xiaoran Wei, Guanglin Zhou, and Fan Yang*

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State Key Laboratory of Heavy Oil Processing, Institute College of New Energy and Materials, China University of Petroleum (Beijing), Beijing 102249, China S Supporting Information *

ABSTRACT: Herein, we demonstrate a new class of core− shell magnetic carbon hybrid materials (Fe3O4@C) for remarkable adsorptive desulfurization of dibenzothiophene (DBT), which have been successfully prepared through hydrocarbonization of glucose on the surface of Fe3O4 and the subsequent pyrolyzation process. The as-obtained Fe3O4@C retains amorphous nature of carbon shells with a large surface area and displays an increase of iron atoms as active sites under elevated pyrolyzation temperature which is favorable in the adsorption of sulfur-containing species through physical and chemical adsorption, respectively. We investigate the adsorption capacity and efficiency of Fe3O4@C as a magnetically adsorbent for the removal of DBT in model oils under various experimental conditions including the adsorbent obtained at different temperatures, the amount of adsorbents, the DBT initial concentration, the regeneration approach, as well as the interference species. Our results demonstrated that the as-obtained Fe3O4@C at 650 °C (Fe3O4@C-650) displays a remarkable estimated adsorption performance (57.5 mg DBT/g for 200 ppmw), extraordinary high desulfurization efficiency (99% for 200 ppmw), and a high selectivity for DBT compared with its derivatives. Moreover, Fe3O4@C can be recovered in a quite easy, economical, and ecofriendly manner by an external magnet after five cycles without significant weight loss, which significantly simplifies the operation procedure and favors the recycle of Fe3O4@C. Combined with the economic and eco-friendly merits, Fe3O4@C offers a new avenue to employ the magnetic carbon materials for industrial applications and provides a promising substitute for adsorptive desulfurization in view of academic, industrial, and environmental aspects.

1. INTRODUCTION The sulfur-containing compounds that are generally existed in liquid fuels are released to air in the form of harmful sulfur oxides, leading to serious environmental pollution and human health issues.1,2 To eliminate the sulfur-containing substance levels in fuels, various desulfurization approaches have been developed. So far, the most widely used industrial technology to lower the sulfur levels in fuels is hydrodesulfurization in which the sulfur contaminant is converted into hydrogen disulfide and corresponding hydrocarbon.3,4 Despite its good performance in the removal of thiol compounds, the inefficient adsorption of thiophene sulfides, the high energy and hydrogen consumption, and harsh experimental conditions accompanied with the loss of octane number have hampered its large-scale applications and limited its performances.5−8 Therefore, tremendous efforts have been made to search for alternative technology toward desulfurization.9,10 Among them, adsorptive desulfurization (ADS) approach, because of its prominent adsorptive performance toward organosulfur compounds, is © 2019 American Chemical Society

considered as a prospective approach for desulfurization because of the ambient experimental conditions, effective removal of thiophene derivatives, excellent recycling efficiency, as well as environment friendliness.11,12 Until now, a large quantity of adsorbents has been chemically synthesized and/or explored in the ADS process including metal−organic frameworks (MOFs),13,14 molecular imprinted polymers (MIPs),15,16 carbon-based materials,17−21 mixed metal oxides,22 and mesoporous materials.23−25 Among these adsorbents, carbon materials especially graphene, MOFbased carbons, and mesoporous carbons have gained significant attention because of their controllable structure and chemical modifications.17,26−28 In most cases, the porous carbon materials endow themselves with a large surface area, appropriate pore size, and plenty of active sites deposited on Received: November 13, 2018 Accepted: December 26, 2018 Published: January 18, 2019 1652

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composed of physical adsorption and chemical adsorption. It should be noted that Fe3O4@C displays a quite simple separation procedure and high recovery efficiency via a magnet, which greatly reduces its economic cost and simplifies the operation in an eco-friendly manner. This study demonstrated here paves a new way to apply the magnetically adhered carbon materials to industrial applications and offers a promising substitute for adsorptive desulfurization from the viewpoints of academic, industrial, and environmental aspects.

their surface and thus could realize highly efficient desulfurization through chemical and/or physical adsorption process.29−35 More interestingly, recent attempts demonstrated that tailoring the surface of the carbon materials with oxygenrich functionalities and depositing metals such as iron and nickel onto activated carbon have been proved to realize effective removal of sulfides in fuels.36,37 Unfortunately, most of these carbon-based materials usually suffer from the complex separation procedure that possibly hinders the recovery of the materials and causes pollution to the fuel which thereby perturbs the regeneration. In this regard, the preparation of new ADS adsorbents with effective separation unit could meet the requirements and greatly simplify the separation procedure. Fe3O4, one of the important magnetic materials, is used as an ideal material in biological magnetic applications, water treatment, and biomedicines because of its biocompatibility, superparamagnetic property, nontoxicity, and high chemical stability.38−42 Moreover, Fe3O4 can be encapsulated by carbon materials to give a core−shell structure that is beneficial for protecting Fe3O4 from agglomeration and avoiding instability as well as maintaining the magnetic fields with biocompatibility in many organic and inorganic media.43 Recently, further advancement has been achieved to capture aromatic sulfur and nitrogen compounds by controlling the thickness of the carbon shell-wrapped magnetic Fe3O4.44 However, the introduction of metal atoms as active sites combined with a porous carbon material-encapsulated Fe3O4 as a highly active desulfurization carbon hybrid material has not been explored. In this paper, core−shell magnetic carbon hybrid materials (Fe3O4@C) were synthesized by a facile approach through hydrocarbonization of glucose and the subsequent pyrolysis process, which were employed for the dibenzothiophene (DBT) adsorption in model oils under mild conditions (Figure 1). The as-obtained Fe3O4@C features with a porous carbon

2. RESULTS AND DISCUSSION 2.1. Structure Characterization of Fe3O4@C. The highly active ADS adsorbent was synthesized through hydrothermal carbonation of glucose at 180 °C, followed by pyrolysis at 650 °C in a tube furnace to form Fe3O4@C-650. The typical scanning electron microscopy (SEM) image of Fe3O4@C-650 is shown in Figure 2A, showing a microsphere wrapped with wrinkle carbon coating, which coincides with the morphology of the carbohydrate-derived structure by a hydrothermal approach. In the center of this image, some undesired fragments are formed, which may be due to the incomplete hydrocarbonation process. We note that Fe3O4@C-650 displays a rough surface that possibly arises from the formation of carbon pores which is produced from the gasification of carbon under high temperature.45−47 In addition, the iron atoms intercalated into the carbon lattices leading to the distortions of the carbon microstructure cannot be seen in the SEM image,45 whereas the existence of iron atoms can be detected in the following X-ray diffraction (XRD) analysis. From the typical transmission electron microscopy (TEM) image of Fe3O4@C-650, a core−shell structure constructed by Fe3O4 nanoparticles and carbon shells can be seen clearly, the Fe3O4 nanoparticles remain intact with an average size of 150− 200 nm (Figure 2B, red arrow), and the carbon shells are composed of carbon layers (Figure 2B, yellow arrow) and graphite layers in the space between carbon shells with a relatively low graphitization degree. From the XRD patterns of Fe3O4@C-650 in Figure 2C, we find out several kinds of characteristic peaks existed in the samples. The peak at 26.4° (Figure 2C, green star) is assigned to the (002) diffraction of graphite sheets with a slight increase compared with that of graphite (26.1°),48 whereas the peaks at 44.6°, 65.0°, and 82.3° are interpreted as the (110), (200), and (211) diffraction peaks of iron atoms, respectively (Figure 2C, JCPDS no. 06-0696, red triangle). We found out that the peaks appeared in the region of 30.1°, 35.4°, 56.9°, 62.5°, and 73.9° are indexed to (220), (311), (511), (440), and (533) of Fe3O4 species (Figure 2C, JCPDS no. 85-1436, blue dots). As for the formation of iron atoms, we deduce that under the pyrolysis process, Fe3O4 could be reduced to iron atoms by carbon oxide that are produced by the hydrothermal method of glucose with a large amount of hydroxyl and carbonyl functional groups under mild or high temperature.49,50 Whereas under 450 and 550 °C, as indicated in Figures S1 and S2, we note that a negligible signal of iron atoms in Fe3O4@C-450 and Fe3O4@ C-550 can be detected in the XRD pattern, but the diffraction peaks of Fe3O4 can be clearly seen. This is possibly due to the incomplete carbonization at low temperature. These characterizations reveal that coating Fe3O4 with carbon shells at elevated pyrolyzation temperature will not only increase the content of iron atoms as active sites of Fe3O4@C but also improve the graphitization degree to a certain extent, leading to multiple functions in this hybrid material, thus the performance of

Figure 1. Schematic illustration of the synthesis and adsorption performance of Fe3O4@C toward DBT.

surface as an adsorptive cavity and provides a location for the deposition of metal active sites, which is favorable for DBT adsorption, whereas magnetic Fe3O4 acts as a separation unit that could guarantee the separation efficiency. Moreover, the adsorption desulfurization results demonstrate that Fe3O4@C shows a remarkable calculated adsorption capacity (57.5 mg DBT/g for 200 ppmw), an extraordinary adsorption efficiency (99% for 200 ppmw), as well as a relatively high selectivity for DBT compared with its derivatives. Experimental results and theoretical simulation indicate that the adsorption process are 1653

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Figure 2. (A) SEM images, (B) TEM images, (C) XRD, and (D) N2 sorption isotherm of Fe3O4@C-650.

adsorption will be greatly enhanced. On the other hand, Fe3O4@C obtained at higher temperature with a high graphitization degree could create much more pores on the surface of Fe3O4@C and thereby contribute to the adsorption capacity and favor the diffusion of adsorbents.52 Furthermore, Brunauer−Emmett−Teller (BET) analysis of Fe3O4@C-650 in Figure 2D displays a specific surface area of 314 m2 g−1, comparable to some carbon-based materials,30 which is higher than that of Fe3O4@C-450 (245 m2 g−1) and Fe3O4@C-550 (250 m2 g−1) as shown in Figures S3 and S4, implying that the carbon hybrids of Fe3O4@C-650 possess a larger surface area. While the corresponding total pore volume reaches 0.18 cm3 g−1, which is slightly different from Fe3O4@ C-450 (0.29 cm3 g−1) and Fe3O4@C-550 (0.21 cm3 g−1) (Table S1). Moreover, the N 2 adsorption−desorption isotherms of Fe3O4@C-650 follow a combination of type I and IV isotherms, suggesting that the coexistence of micropores and mesopores and this kind of pores would possibly be beneficial for the adsorption of small molecules (i.e., DBT) in fuels.51,52 Taking into account all the above characterization, we expect that a core−shell structure of Fe3O4@C-650 containing iron atoms as active sites, carbon hybrids with a high surface area as the adsorbent, and magnetic Fe3O4 as the separation unit could exhibit high adsorption activity and easy separation capability for DBT as will be demonstrated below. 2.2. Desulfurization Performance of Fe3O4@C. Initially, the adsorption capacity of the Fe3O4@C-650 adsorbent toward DBT was evaluated by the RPP-2000S instrument in model oils (200 ppmw DBT in dodecane) and compared with that of Fe3O4@C-450 and Fe3O4@C-550 (Figure 3). The adsorption capacity (q) was defined as mg DBT per gram adsorbents. Figure 3 shows that the adsorption capacities for all the adsorbents increase rapidly in the initial 5 min and almost reach equilibrium in the first 20 min, suggesting that the adsorption process occurs quickly. Moreover, we found out that both the adsorbents display a moderate to excellent adsorption capability toward DBT, while the adsorption performance depends on the morphology and component of adsorbents. Among the three adsorbents, Fe3O4@C-650 shows

Figure 3. Adsorption capacity of Fe3O4@C-650 (red line), Fe3O4@C550 (black line), and Fe3O4@C-450 (blue line) toward DBT vs time.

a higher equilibrium adsorption capacity for DBT, about 30.7 mg of DBT per gram adsorbent can be removed from the model oils, exhibiting an increase of 4 times in comparison with that of Fe3O4@C-550 (black curve, 6.76 mg of DBT) and 5 times (blue curve, 4.98 mg of DBT) higher than that of Fe3O4@C-450. The adsorption capacity over Fe3O4@C-650 in our study is comparable with MOFs,53 ion-exchange zeolite, graphene,54 and various carbon-based materials.22,55 This result suggests that Fe3O4@C-650 demonstrates excellent performance with respect to Fe3O4@C-450 and Fe3O4@C-550 adsorbents obtained at lower pyrolysis temperature. In another aspect, the desulfurization efficiency which was defined as E = 1 − C/C0 was employed to assess the capability of the adsorbent as shown in Figure 4. C0 and C are the sulfur content in the absence and presence of adsorbents, respectively. For all of the adsorbents, we found that the different desulfurization efficiency was observed depending on the time, and the desulfurization efficiency increases as the time increases in the whole time range and almost reached equilibrium in the first 5 min (94% for Fe3O4@C-650, 14% for Fe3O4@C-550, and 12% for Fe3O4@C-450), indicating a fast adsorption rate. Because in the mass-producing system, a long contact time to reach equilibrium is commonly avoided from a practical viewpoint, the fast equilibrium in this system favors low operation cost. Moreover, Fe3O4@C-650 exhibits the most 1654

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contact time increases, negligible further adsorption efficiency enhancement can be observed for 30 mg of the adsorbent. This is because the adsorption sites overlapped with each other, which decreases the access of DBT to the adsorption sites of the adsorbent and causes difficulty in the diffusion process. These observations are consistent with those reported previously for the absorption of aromatic compounds by different materials.54,56,57 In the following experiment, 30 mg will be selected as the optimal conditions for the ADS. Moreover, as shown in in the inset of Figure 5, after the adsorption reaction, the highly dispersed mixture of absorbents and model oils can be separated and easily recovered by a magnet, which makes the separation process an easy and economic. All the above strongly demonstrates that Fe3O4@C650 can be potentially used as a magnetically separable adsorbent. 2.3. Investigation of Adsorption Mechanism. Moreover, further discussions were performed to acquire a deep understanding of the ADS mechanism because various factors influence the adsorption process. The most important factors that possibly influence the adsorption capacity are the abovementioned high surface area and pore volume of the adsorbent. Considering all of the adsorbents, the presence of high surface area in the adsorbents contributes to the adsorption of DBT, leading to a larger sulfur capacity (Table S1). In addition, as demonstrated in Figure S5, micropores of around 4 nm and mesopores ranging from 10 to 40 nm were observed in all of the adsorbents, constructing a micro- and mesoporous structure, favoring the adsorption and diffusion of DBT, thereby giving rise to high desulfurization efficiency. However, we found out that the adsorption capacity for DBT increased sharply as the calcination temperature increases. First, the different adsorption capacity between Fe3O4@C-650, Fe3O4@C-550 and Fe3O4@C-450 is partly ascribed to the different surface area-induced physical absorption. Second, the chemical adsorption might contribute to the adsorption performance because the aromatic ring tends to interact with the benzene rings of DBT to form conjugated complexation.58 In addition, the highly active adsorption sites of iron atoms that existed in the corresponding Fe3O4@C-650 also determine the different adsorption among the three adsorbents. It is revealed that the Fe-containing adsorbent is liable to interact with the sulfur atom (σ-atom) of DBT molecules accompanied with the appearance of the stretching vibration band of Fe−S (1065 cm−1) appearing after the adsorption.59,60 In this case, we speculate that the high adsorption capacity of Fe3O4@C-650 may partly arise from the similar reason. To clarify this hypothesis of adsorption mechanism, we investigated the surface state of Fe3O4@C650 before and after the adsorption of DBT by Fourier transform infrared (FT-IR) analysis. As shown in Figure 6, Fe3O4@C-650 shows the typical bands at 3383, 2903, 1597, and 1377 cm−1 (Figure 6, black line), which are ascribed to carbon shells produced from the carbon precursor under calcination. Because of the coating of carbon shells, two peaks at 1597 and 1377 cm−1 are assigned to carboxylate groups, whereas the band at around 580 cm−1 is associated with the Fe−O vibration band.61 After the adsorption of DBT, another two bands located at 1701 and 966 cm−1 emerge, which can be deemed to be the weak interaction between DBT and Fe3O4@ C-650 and the vibration of Fe−S bonds,59 respectively. This means that the adsorption process also arises from chemical adsorption. However, we note that the band at 1701 cm−1

Figure 4. Adsorption efficiency of Fe3O4@C-650 (red line), Fe3O4@ C-550 (black line), and Fe3O4@C-450 (blue line) toward DBT.

effective desulfurization efficiency at equilibrium toward DBT (red curve, 100%), which is much higher than that of Fe3O4@ C-550 (black curve, 20%) and Fe3O4@C-450 (blue curve, 17%). Similar to the variation trend of adsorption capacity, the order of the desulfurization efficiency (E) follows the same trend as observed for the sulfur capacity. In combination with the excellent sulfur adsorption capacity and high adsorption efficiency of the absorbent as demonstrated above, all these results suggest that Fe3O4@C-650 demonstrates excellent performance with respect to the other adsorbents. Therefore, Fe3O4@C-650 was employed as the optimal adsorbents in the following experiments. Since enough amount of the adsorbent could ensure sufficient adsorption of sulfur-containing compounds. Then, different amounts of the adsorbents were employed to investigate the ADS efficiency of Fe3O4@C-650 toward DBT versus time in model oils (200 ppmw DBT in dodecane), as plotted in Figure 5. The desulfurization efficiency which was

Figure 5. Adsorption efficiency of different amounts of Fe3O4@C-650 [10 mg (red line), 20 mg (black line), and 30 mg (blue line)] toward DBT. Inset: the separation process with a foreign magnet.

defined as E = 1 − C/C0 was employed to evaluate the capability of the adsorbent. Under the constant DBT concentration, with a further increase in the amount of the absorbent, the desulfurization efficiency increases and reaches an equilibrium value after the amount of the adsorbent is raised up to 30 mg, indicating that the sufficient adsorption of DBT occurs. This phenomenon could be easily understood that the enhancement of the adsorbent surface area brings about the increase of available adsorption sites. In addition, a fast equilibrium time can be observed for 30 mg of the absorbent compared to the other two lower amounts of the absorbent, which implies a higher amount of the absorbent, enabling a sufficient and fast adsorption. We note that even when the 1655

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concentration, as illustrated in Figure S7. This phenomenon can be easily understood because of low initial concentration that enables effective availability of adsorption sites and the DBT can be readily adsorbed, whereas at higher original concentration, the overall adsorption sites are confined, thereby leading to insufficient adsorption and the decrease of desulfurization efficiency. 2.5. Regeneration Performance. Among various methods employed in adsorptive desulfurization, solvent extraction and thermal treatment have been thought to be the commonly utilized regeneration method.36,64,65 In this work, solvent extraction and thermal treatment were both employed to investigate the regeneration performance of Fe3O4@C-650. As for the solvent exaction method, after the completion of the first adsorption cycle, Fe3O4@C-650 was collected by an external magnet and washed by adding methanol as the extraction solvent. Afterward, Fe3O4@C-650 was dried for the next run. As illustrated in Figure 8, it is found that the Fe3O4@

Figure 6. FT-IR spectra of Fe3O4@C-650, Fe3O4@C-650 with DBT, and Fe3O4@C-650 and DBT washed with methanol.

disappears, whereas the peak located at 966 cm−1 still remains after Fe3O4@C-650 and DBT were washed with methanol, indicating that the physical adsorption indeed occurs. Moreover, the chemical component of Fe3O4@C-650 was also investigated by the thermogravimetric (TG) measurement to certify the above results. As demonstrated in Figure S6, the TG analysis reveals that Fe3O4@C-650 shows a significant weight loss of 80 wt % when the temperature rises from 300 to 600 °C and leaves 20 wt % of the weight residue, which is assigned to the iron component, implying that the chemical component of Fe3O4@C-650 was composed of 80% carbon content, which endows Fe3O4@C-650 with abundant surface area and pore volume (Table S1), thereby enabling the sufficient physical adsorption. On the basis of the above investigation, we propose that the adsorption process is possibly mainly dominated by physical adsorption and partly ascribed to chemical adsorption. 2.4. Effect of DBT Concentration on Desulfurization Performance. In view of the facts that high concentrations provide more contact sites and generally result in high sulfur capacities, therefore a high original concentration is used in order to obtain a high adsorption capacity in experimental aspects. To address this issue, we then investigated the ADS performance of Fe3O4@C-650 when the original concentration ranged from 200 to 1000 ppmw DBT (Figure 7) and found

Figure 8. Regeneration capability of Fe3O4@C-650 with solvent extraction (light gray) and thermal treatment (dark gray). (DBT: 200 ppmw, absorbent, 30 mg, solvent: methanol). Therefore, in this work, the further thermal treatment of the regenerated adsorbent is highly desirable for the sake of retaining the adsorption capacity in an expected range.

C-650 absorbent (light gray column) can be easily regenerated with solvent washing for the first two runs. This is probably because physical adsorption dominates the adsorption process and most of the deactivated adsorbents caused by contamination could be regenerated by the solvent extraction method for the initial cycles. However, after five successive regeneration operations, the adsorbent keeps 67% of the initial desulfurization efficiency (corresponding to 20.22 mg DBT/g) with the fresh adsorbent. With an increase in the number of cycles, the much more active sites interact with DBT and fewer active sites were left for the latter run, thereby resulting in the decreased desulfurization efficiency in the subsequent regeneration runs. To improve the regenerative effect of the Fe3O4@C-650 adsorbent, the thermal treatment approach was introduced. After regenerated by solvent extraction, the obtained adsorbent was activated at 350 °C for 1 h under nitrogen atmosphere. As illustrated in Figure 8, Fe3O4@C-650 regenerated by thermal treatment (Figure 8, dark gray column) afforded 98 and 96% of the primary adsorption efficiency for the second and third runs, which are 29.53 and 28.96 mg DBT/g, respectively (Figure S8, dark gray column), while the corresponding values are 92, 91, and 87% for the successive recycle. The regeneration efficiencies herein are comparable to those of reported carbon adsorbents in the previous literature,64−67

Figure 7. Adsorption capacity of Fe3O4@C-650 toward DBT under different sulfur initial concentrations.

out that the maximum adsorption capacity of DBT on Fe3O4@ C-650 increased from 32.6 to 56.0 mg with the increasing DBT concentration from 200 to 1000 ppmw at the equilibrium state; this is because the high concentration at the original stage usually prompts the mass transfer between the oil and solid phases.62,63 However, it is found that the adsorption efficiency decreased with the increasing original DBT 1656

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whereas the expected regeneration efficiency cannot be realized by the solvent extraction approach only. 2.6. Interference Experiments. Generally, aromatic molecules and sulfur-containing compounds in the fuel hinder the selective adsorption of adsorbent toward the target molecule. Therefore, the influence of these compounds should be investigated in detail to attain deep desulfurization. Because aromatic molecules are naturally existed in fuels, effective removal of DBT with the coexistence of aromatic molecules is an urgent issue.68,69 In this work, the adsorptive removal of DBT in the presence of tetralin was investigated by Fe3O4@C650. Figure 9 demonstrates that the adsorption capacity Figure 10. Adsorption capacity of Fe3O4@C-650 toward BT, DBT, 4MDBT, and 4,6-DMDBT.

other thiophenic compounds. This trend is consistent with many previous reports in which the reason was attributed to the differences in adsorption energy and interaction strength. In summary, the different adsorption performance of Fe3O4@ C-650 toward various sulfur compounds, indicating that the adsorption of sulfur compounds on the carbon surface, is possibly dominated by many aspects including π- interactions, strength of interaction that depends on the number of π electrons or the size of the aromatic ring systems, and the number of electron-donating groups.72 2.7. Theoretical Simulation. Langmuir and Freundlich isotherms are considered as the typical standard to describe the adsorption behavior between the adsorbents and the target molecules.73 In the present study, both the models were used to investigate the adsorption capacity toward DBT. The experimental data of Fe3O4@C-650 toward DBT adsorption were analyzed by using Langmuir (eq 1) and Freundlich (eq 2) isotherm models.56,74,75

Figure 9. Adsorption capacity of Fe3O4@C-650 toward DBT in the presence of tetralin with different mass fractions (0, 10, and 20 wt %).

decreased by increasing the proportion of tetralin probably because DBT and tetralin compete each other in the adsorption process. When the percentage of tetralin reached up to 10%, a larger drop of DBT adsorption capacity from 30.42 to 19.94 mg DBT/g was observed, and when the fraction of tetralin increased from 10 to 20%, a relatively slower drop of DBT adsorption amount from 19.94 to 13.25 mg DBT/g was observed. This anti-interference phenomenon is superior to the reported results that boron nitride was utilized as the adsorbent, in which the DBT adsorption performance was reduced by 23% in the presence of an interference substance.54 The decreased adsorption capacity in the presence of tetralin may be attributed to tetralin is prone to form π−π interaction with graphitic planes in the carbons.70 The adsorption difference between tetralin and DBT lies in oxygen-containing functionalities acting as acid centers on the carbon surface,68,71 favoring DBT adsorption, whereas such interaction seems too weak to be observed or absent between tetralin and carbon. In addition, further experiments have been performed by using benzothiophene (BT), 4-methylDBT (4-MDBT), and 4,6-dimethylDBT (4,6-DMDBT) as interferences to investigate the selectivity of the adsorbent. The adsorption capacities for BT, 4-MDBT, and 4,6-DMDBT are summarized in Figure 10. The adsorption capacity of Fe3O4@C-650 toward BT is calculated to be 24.7 (mg/g) of BT, which is slightly lower than that of 27.3 (mg/g) of 4,6-DMDBT. In the case of 4-MDBT, the adsorption capacity reaches up to 30.4 (mg/g) of 4-MDBT. Nevertheless, the adsorption capacity on DBT increases to 33.9 (mg/g) of DBT under the same conditions, although DBT lacks a methyl group compared to 4-MDBT, indicating that the molecular size of sulfur compounds did not play a leading role in the adsorption performance. Compared to other thiophenic derivatives, DBT and 4-MDBT display higher adsorption capacity, indicating that this kind of adsorbent favors the DBT and 4-MDBT removal than the

qe =

qmKLCc (1 + KLCe)

ln qe = ln K f +

(1)

1 ln Ce n

(2) −1

where Ce and qe refer to the concentration (mg·L ) and the amount adsorbed (mg S·g−1) at equilibrium, respectively. KL and qm are the Langmuir-type constants and adsorption capacity, respectively. Kf and n denote the Freundlich constants and adsorption intensity, respectively. As displayed in Figure 11 and Table 1, we can observe that the adsorption of Fe3O4@C-650 toward DBT can be described simultaneously by the Langmuir and Freundlich models.

Figure 11. Langmuir and Freundlich isotherms of Fe3O4@C-650 toward DBT. 1657

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the economic and environmental merits of the prepared process, the advantages of easy preparation procedure and environmental compatibility endow them as a promising candidate for practical applications.

Table 1. Adsorption Parameters Based on Langmuir and Freundlich Isotherm Models Langmuir

Freundlich

qm

KL

R2

Kf

1/n

R2

9.99

1.16

0.9877

6.23

0.06

0.9715

4. EXPERIMENTAL SECTION 4.1. Materials and Chemicals. Fe3O4 used in this study (Figure 1) was prepared in accordance with the reported iterature.80 All chemicals and reagents were of analytical grade and used without further treatment. FeCl3·6H2O, urea, glucose, and ethylene glycol were purchased from Fuchen Chemical Reagents Co. Ltd. BT, DBT, 4-MDBT, and 4,6DMDBT were obtained from Aladdin. Model oils were prepared by mixing different concentrations of DBT into dodecane to give a final sulfur concentration of 200, 400, 600, 800, and 1000 ppmw and then were used for adsorptive desulfurization analysis. 4.2. Preparation of Fe3O4@C. Fe3O4 was prepared by the solvothermal reaction according to the reported method with slight modification.80 Typically, FeCl3·6H2O (0.5 g) and urea (1.0 g) were dissolved in ethylene glycol (35 ml) with stirring for 30 min at room temperature and were then transferred into a 50 mL stainless-steel autoclave, and the obtained mixture was heated at 200 °C with 3 °C/min for 12 h. After the reaction, the solid product was gathered by an external magnet, washed sequentially with distilled water and ethanol, and then dried under vacuum to give the final Fe3O4 product. The synthesis of Fe3O4@C comprised the following procedures: (i) hydrothermal carbonation of glucose at 180 °C and (ii) activation with pyrolysis at high temperature in a tube furnace. In a typical synthesis, 30 mL of glucose (0.6 mol/ L) and 120 mg of Fe3O4 were added into a 50 mL capacity stainless-steel autoclave with the temperature held at 180 °C for 6 h. After the reaction, the solid product was separated, washed with distilled water and ethanol, and then dried in an oven at 120 °C overnight. Subsequently, the obtained solid was heated at 650 °C for 6 h in a furnace tube under nitrogen atmosphere to give Fe3O4@C-650. For comparison, Fe3O4@ C-450 and Fe3O4@C-550 were obtained under the same procedure, while the pyrolysis temperature was held at 450 and 550 °C, respectively. 4.3. Apparatus and Characterization. Desulfurization measurements were conducted by a UV fluorescence sulfur analyzer (RPP-2000S) (Taizhou Zhonghuan Analysis Instrument Co., Ltd., China). The FT-IR spectra were recorded on a TENSOR-27 spectrophotometer (Bruker, USA). The morphologies of the adsorbent were characterized by using SEM (S-4800, Hitachi, Japan) and TEM (TEM-2100F, JEOL, Japan). XRD was conducted on an X-ray diffractometer (Rigaku D/max 2500) with Cu Kα radiation in the range from 5° to 90°. X-ray photoelectron spectroscopy was taken on an X-ray source (ESCALab220I-XL) with aluminum Kα line at 1486.6 eV. The thermal decomposing curve of the adsorbent was recorded by the TG analysis (TGA, Netzsch/STA449F3) system. 4.4. Adsorptive Desulfurization Measurements. Model oils were prepared by dissolving DBT in dodecane with sulfur contents of 200, 400, 600, 800, and 1000 ppmw. Adsorptive desulfurization experiments were conducted by adding 30 mg of adsorbents (i.e., Fe3O4@C-650) to 1 mL of the model oil in the microreactor at room temperature and then monitored by the RPP-2000S instrument. The adsorptive

Nevertheless, the value of correlation coefficient R2 values indicates that the Langmuir isotherm model fits well to the experimental data than that of the Freundlich isotherm, suggesting that the Langmuir model was favorable for describing the ADS of DBT. On the basis of the Langmuir model, the maximum adsorption capacity of Fe3O4@C-650 toward DBT was calculated to be 57.5 mg DBT/g. As shown in Table 2, the above-mentioned adsorption capacity of Table 2. Various Adsorbents for Desulfurization adsorbent Fe3O4@C-650 Cu(II)M-3 C−Fe AgNO3/Fe3O4@mSiO2 carbon aerogel (4 nm) activated alumina MOF-5 GO-P3 CMK-3 activated carbon porous glass beads activated carbon porous glass beads

DBT capacity (mg DBT/g)

sulfur capacity (mg S/g)

57.5

9.99

25.1 64.4 13.8 52.3 37.3 62.6 58.6 59.2 58.6 59.2

4.60 4.40 4.70 11.2 2.40 9.10 6.50 10.9 10.2 10.3 10.2 10.3

refs this work 14 22 24 29 37 53 54 64 76 77 78 79

Fe3O4@C-650 was superior or comparable than most of the reported MIPs and activated carbon adsorbents.22,74,76,77 We note that the carbon aerogel demonstrates a higher adsorption capacity than our synthesized adsorbents and those kinds of adsorbents.29 However, in view of the environment and economical aspects, high adsorption capacity and magnetically separable adsorbents will be favorable for the desulfurization process in practical industrial applications.

3. CONCLUSIONS In summary, a novel class of core−shell magnetic carbon hybrid materials (Fe3O4@C) derived from the pyrolyzation of glucose and Fe3O4 have been successfully prepared and utilized for the removal of DBT under mild conditions. The adsorbents possess a magnetic core, a porous carbon shell, and metal ions as active sites. The porous carbon shell makes a great contribution to the adsorptive desorption of DBT through physical adsorption, whereas the metal ions act as the active sites, which are responsible for the chemical adsorption. By integrating the combination of the two adsorption process, an enhanced adsorption performance (57.5 mg DBT/g for 200 ppmw), a high desulfurization rate (99% for 200 ppmw), and a relatively high selectivity for DBT were obtained, which is higher than most of the existed adsorbents. In addition, the adsorbent can be recovered in a quite easy, economical, and eco-friendly manner by a magnet for five successive runs without appreciable loss in the adsorptive efficiency, which significantly simplifies the operation procedure. By integrating 1658

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(2) Zhu, W.; Dai, B.; Wu, P.; Chao, Y.; Xiong, J.; Xun, S.; Li, H.; Li, H. Graphene-Analogue Hexagonal BN Supported with Tungstenbased Ionic Liquid for Oxidative Desulfurization of Fuels. ACS Sustainable Chem. Eng. 2014, 3, 186−194. (3) Yu, C.; Fan, X.; Yu, L.; Bandosz, T. J.; Zhao, Z.; Qiu, J. Adsorptive Removal of Thiophenic Compounds from Oils by Activated Carbon Modified with Concentrated Nitric Acid. Energy Fuels 2013, 27, 1499−1505. (4) Sattler, A.; Parkin, G. Carbon-Sulfur Bond Cleavage and Hydrodesulfurization of Thiophenes by Tungsten. J. Am. Chem. Soc. 2011, 133, 3748−3751. (5) Aslam, S.; Subhan, F.; Yan, Z.; Liu, Z.; Ullah, R.; Etim, U. J.; Xing, W. Unusual Nickel Dispersion in Confined Spaces of Mesoporous Silica by One-Pot Strategy for Deep Desulfurization of Sulfur Compounds and FCC Gasoline. Chem. Eng. J. 2017, 321, 48− 57. (6) Li, Y.-X.; Jiang, W.-J.; Tan, P.; Liu, X.-Q.; Zhang, D.-Y.; Sun, L.B. What Matters to the Ddsorptive Desulfurization Performance of Metal-Organic Frameworks. J. Phys. Chem. C 2015, 119, 21969− 21977. (7) Peralta, D.; Chaplais, G.; Simon-Masseron, A.; Barthelet, K.; Pirngruber, G. D. Metal-Organic Framework Materials for Desulfurization by Adsorption. Energy Fuels 2012, 26, 4953−4960. (8) Kou, J.; Lu, C.; Sun, W.; Zhang, L.; Xu, Z. Facile Fabrication of Cuprous Oxide-Based Adsorbents for Deep Desulfurization. ACS Sustainable Chem. Eng. 2015, 3, 3053−3061. (9) Fox, B. R.; Brinich, B. L.; Male, J. L.; Hubbard, R. L.; Siddiqui, M. N.; Saleh, T. A.; Tyler, D. R. Enhanced Oxidative Desulfurization in a Film-Shear Reactor. Fuel 2015, 156, 142−147. (10) Danmaliki, G. I.; Saleh, T. A. Influence of Conversion Parameters of Waste tires to Activated Carbon on Adsorption of Dibenzothiophene from Model Fuels. J. Cleaner Prod. 2016, 117, 50− 55. (11) Yang, R. T.; Hernández-Maldonado, A. J.; Yang, F. H. Desulfurization of Transportation Fuels with Zeolites Under Ambient Conditions. Science 2003, 301, 79−81. (12) Ahmed, I.; Jhung, S. H. Composites of Metal-Organic Frameworks: Preparation and Application in Adsorption. Mater. Today 2014, 17, 136−146. (13) Tang, W.; Gu, J.; Huang, H.; Liu, D.; Zhong, C. Metal-Organic Frameworks for Highly Efficient Adsorption of Dibenzothiophene from Liquid Fuels. AIChE J. 2016, 62, 4491−4496. (14) Qin, J.-X.; Tan, P.; Jiang, Y.; Liu, X.-Q.; He, Q.-X.; Sun, L.-B. Functionalization of Metal−organic Frameworks with Cuprous Sites using Vapor-Induced Selective Reduction: Efficient Adsorbents for Deep Desulfurization. Green Chem. 2016, 18, 3210−3215. (15) Qin, L.; Liu, W.; Yang, Y.; Liu, X. Functional Monomer Screening and Preparation of Dibenzothiophene-Imprinted Polymers on the Surface of Carbon Microsphere. Monatsh. Chem. Chem. Mon. 2014, 146, 449−458. (16) Hager, R. A.; Heba, H. E.-M.; Fouad, Z.; Yasser Mohamed, M. A Novel Surface Imprinted Polymer/Magnetic Hydroxyapatite Nanocomposite for Selective Dibenzothiophene Scavenging. Appl. Surf. Sci. 2017, 426, 56−66. (17) Kumar, D. R.; Srivastava, V. C. Studies on Adsorptive Desulfurization by Activated Carbon. Acta Hydrochim. Hydrobiol. 2012, 40, 545−550. (18) Peng, T.; Ding-Ming, X.; Jing, Z. Hierarchical N-Doped Carbons from Designed N Rich Polymer Adsorbents with a Record High Capacity for Desulfurization. AIChE J 2018, 64, 3786−3793. (19) Saleh, T. A. Simultaneous Adsorptive Desulfurization of Diesel Fuel over Bimetallic Nanoparticles Loaded on Activated Carbon. J. Cleaner Prod. 2018, 172, 2123−2132. (20) Saleh, T. A.; AL-Hammadi, S. A.; Abdullahi, I. M.; Mustaqeem, M. Synthesis of Molybdenum Cobalt Nanocatalysts Supported on Carbon for Hydrodesulfurization of Liquid Fuels. J. Mol. Liq. 2018, 272, 715−721. (21) Saleh, T. A.; AL-Hammadi, S. A.; Al-Amer, A. M. Effect of Boron on the Efficiency of MoCo Catalysts Supported on Alumina for

desulfurization capacity (eq 3) and desulfurization efficiency (eq 4) were determined by the following equations. q=

M m × (C0 − C) × DBT 10−3 M MS

E=1−

C × 100% C0

(3)

(4)

where q represents the adsorption capacity (mg/g) and m and M refer to the mass of model oil (g) and adsorbent (g), respectively. MDBT and MS are the molecular weight of DBT and S (g/mol), respectively. C 0 and C denote the concentration of S before and after the addition of adsorbent, respectively. E is the desulfurization efficiency. Adsorption of thiophenic compounds was studied by adding the same molar concentration of BT, DBT, 4-MDBT, and 4,6DMDBT separately into dodecane. The interference experiment of aromatic compounds (i.e., tetralin) was conducted by adding different amounts of tetralin into dodecane. The liquid layer was analyzed to estimate the selectivity of thiophenic compounds and interferences from arenes. Regeneration experiments: the solvent extract approach was conducted by washing the adsorbent with methanol three times and then dried in an oven for the next run. As for the thermal treatment method, after being washed with methanol by solvent extraction, the resulting adsorbent was heated to 350 °C for 1 h under nitrogen atmosphere and then used directly for the next adsorption cycles.

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b03157.

■

Adsorbent characterization including XRD and BET analysis, adsorption efficiency, and regeneration performance (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +86-10-89739062 (C.W.). *E-mail: [email protected] (F.Y.). ORCID

Chunxia Wang: 0000-0002-8214-6905 Author Contributions †

C.W. and H.Z. contributed equally.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work is supported by the Natural Science Foundation of China (nos. 21804139 and 21776302) and the Science Foundation of China University of Petroleum (Beijing) (nos. 2462016YJRC027, 2462017BJB04, and C201604).

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