Hydrothermal synthesis of carbon-coated CoS2-MoS2 catalysts with

Research Article pubs.acs.org/journal/ascecg

Hydrothermal Synthesis of Carbon-Coated CoS2−MoS2 Catalysts with Enhanced Hydrophobicity and Hydrodeoxygenation Activity Weiyan Wang,*,†,‡ Kui Wu,† Song Tan,† and Yunquan Yang*,†,‡ †

School of Chemical Engineering, Xiangtan University, Xiangtan, Hunan 411105, P. R. China National & Local United Engineering Research Centre for Chemical Process Simulation and Intensification, Xiangtan University, Xiangtan, Hunan 411105, P. R. China

‡

ABSTRACT: Carbon-coated CoS2−MoS2 catalysts were prepared by a hydrothermal method via the addition of polyvinylpyrrolidone (PVP). The effects of PVP amount on their structure properties and hydrodeoxygenation (HDO) activity were studied. The characterization results showed that carbon coated on the surface of CoS2−MoS2 and that its hydrophobicity was enhanced. During the HDO of 4-ethylphenol, these catalysts presented high direct deoxygenation activity, even at high hydrogen pressure. High reaction temperature and hydrogen pressure were beneficial to increase the conversion, but had little effect on the product distribution. After reaction at 250 °C and 4.0 MPa hydrogen pressure for 5 h, the deoxygenation degree reached 96.1% with a selectivity of 99.3% ethylbenzene. The high HDO activity of carbon-coated CoS2−MoS2 catalysts mainly resulted from its enhanced hydrophobicity. Moreover, the reusability tests also showed that these catalysts had a good stability during the HDO reaction because of the inhibition of hydrophobic coated carbon to sulfur loss. Therefore, improving the hydrophobicity of the sulfide catalyst was favored for increasing its HDO activity and inhibiting the deactivation. KEYWORDS: Co−Mo−S, Hydrothermal synthesis, Hydrophobicity, Hydrodeoxygenation, 4-Ethylphenol

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catalysts,17−19 transition metal phosphides,20,21 carbides,22−24 and oxides.25,26 Phenols, being typical and dominating oxygencontaining compounds in the bio-oil derived from lignin, are usually selected as model compounds to study the HDO activity of the prepared catalysts. Theoretically, the HDO of phenols can proceed with two main routes: hydrogenation− dehydration (HYD) and direct deoxygenation (DDO), which depends on the selected catalysts.27 Noble metals and amorphous metal borides have presented excellent hydrogenation activity and HDO activity, but consumed a lot of hydrogen because of the saturation of the benzene ring prior to the scission of the C−O bond. Moreover, it was easy for carbon deposition to form on the acid sites of the supports,28,29 which was the main deactivation reason for noble-metal-supported catalysts in the HDO reaction. In addition, the low availability

INTRODUCTION The depleting reserves of fossil fuel and some environmental pollution problems caused by its utilization have forced us to develop a new alternative energy to meet human needs and ensure sustainable development for our world.1 Lignin is a complicated aryl polymer, enriched with an aromatic structure, which is considered as a renewable resource for the production of fuels and aromatics.2 Bio-oil derived from this lignin has a lot of phenols, contributing to its higher oxygen content as compared to traditional fuels, and this leads to some disadvantages such as lower heating value, higher acidity, and mutual solubility with water. These confine its application as an alternative energy.3 One practical and effective solution is to selectively remove the oxygen atoms via hydrodeoxygenation (HDO) technology, and the deoxygenation rate depends on the selected catalysts.4 Until now, significant efforts have been concentrated on the development of promising catalysts, and there have appeared several HDO catalysts, for example, noble metal catalysts,5−9 sulfide catalysts,10−14 amorphous metal borides,15,16 metal © 2017 American Chemical Society

Received: April 10, 2017 Revised: August 20, 2017 Published: August 24, 2017 8602

DOI: 10.1021/acssuschemeng.7b01087 ACS Sustainable Chem. Eng. 2017, 5, 8602−8609

Research Article

ACS Sustainable Chemistry & Engineering

adjusted to 900 rpm. During the reaction, liquid samples were withdrawn from the reactor and analyzed by an Agilent 6890/5973N GC−MS device and 7890 gas chromatography using a flame ionization detector (FID) with a 30 m AT-5 capillary column. The experiments were repeated at least twice, and the results showed that the conversion and selectivity were within 3.0% of the average values. The conversion, selectivity, and deoxygenation degree for each experiment were calculated as follows:

and high cost of noble metals were not beneficial to the sustainability of the HDO process in industrial applications. MoS2 had also presented a high HDO activity, but its activity was closely related to the preparation method.30 When adding promoter Co into MoS2, both the HDO activity and DDO activity were markedly increased.31−33 In this case, the deoxygenation efficiency and hydrogen consumption are greatly improved, which is industrially desirable. However, oxygen is removed in the form of water, and the presence of water aggravates the deactivation during the HDO reactions.34,35 It has been reported that adding sulfiding agents could maintain its sulfide structure;36 in the meantime, the products would be contaminated. To solve this problem, we tried to prepare a hydrophobic sulfide catalyst to prevent the active site from having contact with water and expected to inhibit the deactivation, at least in part. Recently, Zhao et al.37 reported that the surface of Ni/TiO2 that was coated with a hydrophobic carbon layer could improve the stability in the present aqueous reaction system. Hence, in this study, carbon-coated CoS2− MoS2 catalysts were prepared via addition of polyvinylpyrrolidone (PVP) into its hydrothermal synthesis process. The structure properties of the resultant catalysts were determined by various characterization technologies, and their activity and reusability were tested in the HDO of 4-ethylphenol.

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conversion (mol %) ⎛ moles of residual 4‐ethylphenol ⎞ = ⎜1 − ⎟ × 100% moles of initial 4‐ethylphenol ⎠ ⎝

selectivity (A, mol %) =

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moles of product (A) × 100% moles of reacted 4‐ethylphenol

deoxygenation degree (DD, wt %) ⎛ oxygen content in final organic compounds ⎞ = ⎜1 − ⎟ × 100% total oxygen content in initial mixture ⎠ ⎝

RESULTS AND DISCUSSION Characterization of Carbon-Coated CoS2−MoS2 Catalysts. FT-IR was adopted to confirm the existence of PVP in the final catalysts. Figure 1 presents the FT-IR spectra of PVP,

EXPERIMENTAL SECTION

Preparation of Carbon-Coated CoS2−MoS2 Catalysts. Carbon-coated CoS2−MoS2 catalysts were prepared by a one-step hydrothermal method based on the previous study.33 Briefly, ammonium heptamolybdate (2.32 g), cobalt nitrate (1.91 g), PVP (0, 0.1, 0.2, 0.3, and 0.4 g), and thiourea (3.00 g) were dissolved in 400 mL of water, and hydrochloric acid was added to adjust the pH value of the mixed solution to 0.9. Then, this mixed solution was put into a sealed vessel and heated at 200 °C for 24 h. After reaction, the black precipitate was separated and washed with water and ethanol, and then dried under vacuum at 60 °C for 5 h. The resultant catalyst was denoted as Co−Mo−S−X, where X represented the weight of PVP. Catalyst Characterization. The specific surface area was measured by a Quantachrome NOVA-2100e surface area instrument by physisorption of nitrogen at −196 °C. The samples were dehydrated at 300 °C using vacuum degassing for 12 h before experiments. X-ray diffraction (XRD) measurements were carried out on a D/max2550 18KW rotating anode X-ray diffractometer with monochromatic Cu Kα radiation (λ = 1.5418 Å) at a voltage and current of 40 kV and 300 mA. The 2θ was scanned over the range 10− 90° at a rate of 10°/min. Raman spectral experiments were carried out using an inVia reflex laser micro-Raman spectroscope with a 532 nm excitation source. The laser power was kept at 0.3 mW during the experiment. The scanning electronic microscopy (SEM) images of the catalysts were obtained on a JEOL JSM-6360 electron microscope. The morphology was determined by high-resolution transmission electron microscopy (HRTEM) on a JEOL JEM-2100 transmission electron microscope with a lattice resolution of 0.19 nm and an accelerating voltage of 200 kV. The surface composition and surface electronic state were analyzed by X−ray photoelectron spectroscopy (XPS) using a Kratos Axis Ultra DLD instrument at 160 eV pass energy. Al Kα radiation was used to excite photoelectrons. The binding energy value of each element was corrected using C1s = 284.6 eV as a reference. HDO Catalytic Activity Measurement. The HDO activity tests were carried out in a 200 mL sealed autoclave, as reported in a previous study.31 The prepared catalyst (0.06 g) without any further treatment, 4-ethylphenol (10.8 g), and dodecane (25 g) were placed into the autoclave. Air in the autoclave was evacuated by pressurization−depressurization cycles with nitrogen and subsequently with hydrogen. The system was heated to the designed temperature, then pressurized with hydrogen to 4.0 MPa, and the stirring speed was

Figure 1. FT-IR spectra of PVP, Co−Mo−S−0, Co−Mo−S−0.2, and Co−Mo−S−0.4.

Co−Mo−S−0, Co−Mo−S−0.2, and Co−Mo−S−0.4. Two absorption peaks at 3443 and 1638 cm−1 corresponded to the stretching vibration and bending vibration of physically adsorbed water on the catalyst surface. For the FT-IR spectra of PVP, three peaks at 2957, 1428, and 1287 cm−1 were attributed to C−H, N−H, and C−N bonds, respectively. Co− Mo−S−0 showed no absorption peaks for PVP because it was prepared without adding PVP, and two peaks for the adsorbed water were also observed. Both Co−Mo−S−0.2 and Co−Mo− S−0.4 showed almost the same FT-IR spectra as Co−Mo−S− 0. These suggested that no PVP remained in these two catalysts. Raman spectroscopy is a powerful characterization tool for determining the structure of carbon materials. As shown in Figure 2, two broad overlapping bands at approximately 1350 and 1580 cm−1 are observed, corresponding to the characteristic D band (a defect or disorder peak) and G band (a 8603


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together. These might be caused by the coverage of the formed carbon on the surface of CoS2−MoS2, which was further confirmed by elemental mapping and surface area. As shown in the elemental mapping images of Mo, Co, S, and C for Co−Mo−S−0.4 (Figure 3), Co was uniformly dispersed in MoS2, and carbon was detected and enriched on the surface. Because of the uniform dispersion of Co sulfide in MoS2, the surface area of Co−Mo−S−0 was very low (11.1 m2/g), which was much lower than that of MoS2.41 However, the surface area decreased to 4.2 m2/g for Co−Mo−S−0.2 and then increased to 8.4 m2/g for Co−Mo−S−0.4, suggesting that adding PVP had a negative effect for the surface area of CoS2−MoS2, which would decrease the active sites for the HDO reaction. The former decrease on the surface area was caused by the pore blockage by the formed carbon while the latter increase resulted from the fact that the excessive carbon on the catalyst surface enlarged its surface area. The detailed microstructures of Co−Mo−S−0 and Co− Mo−S−0.4 were also further studied by high resolution TEM (HRTEM). As shown in Figure 4a, two groups of lattice fringe

Figure 2. Raman spectra of carbon-coated CoS2−MoS2 catalysts.

graphitic peak) of carbon materials.38 These carbons might be derived from the carbonization of PVP during the hydrothermal process.39 Notably, the peak intensity for carbon was increased with the added PVP amount, which indicated an increase of carbon content in the resultant catalysts. Usually, the ID/IG ratio represents the components of graphitic and amorphous carbon. Figure 2 shows the low ID/IG value, representing the high ratio of graphitic carbon in the catalysts,39 which would increase their hydrophobicity. In addition, two weak peaks at 373 and 403 cm−1 were observed in the Raman spectra of Co− Mo−S and Co−Mo−S−0.1, attributing to 1E2g and A1g modes of MoS2,38 respectively, but both of which were decreased and even disappeared. These weakened peaks suggested the substantial defect sites in the crystal structure of MoS2, which could act as active edge sites for reaction and enhance the catalytic activity.40 Figure 3 shows the morphologies of Co−Mo−S−0, Co− Mo−S−0.2, and Co−Mo−S−0.4 prepared by adding different

Figure 4. HRTEM images of (a) Co−Mo−S−0 and (b, c) Co−Mo− S−0.4.

with an interplanar spacing of 0.22 and 0.63 nm were observed, corresponding to the (210) plane of CoS2 and the (002) plane of MoS2,42,43 respectively. The HRTEM images of Co−Mo− S−0.4 (Figure 4b) also presented two lattice fringes corresponding to the (210) plane of CoS2 and (002) plane of MoS2. These indicated that MoS2 and CoS2 coexisted in Co−Mo−S−0 and Co−Mo−S−0.4. Moreover, Figure 4c obviously displays the coating carbon layer on the surface. In comparison with the previous literature that reported the expanded interplanar spacing for the (002) plane of MoS2,38 Co−Mo−S−0.4 did not show this enlarged interlayer distance, which might result from the low interaction between carbon and MoS2. The crystalline structure of CoS2−MoS2 catalysts prepared by adding different amounts of PVP was confirmed by XRD characterization, as displayed in Figure 5. The peak at 2θ = 14° in the XRD pattern of Co−Mo−S−0 corresponds to the (002) plane of MoS2. Compared to that of the commercial bulk MoS2,32 this (002) diffraction peak presented very weakly, and the other peaks 2θ = 33°, 39°, and 59° corresponding to the (100), (103), and (110) basal planes of MoS2 were not observed, indicating its poor crystallinity and homogeneous dispersion.44 After PVP was added during the catalyst preparation, the formed carbon was not detected in the XRD patterns, suggesting its amorphous state, which was consistent with the Raman results. The intensity of the (002) diffraction peak became weak after adding PVP, which resulted from the fact that the incorporation of carbon in the CoS2−MoS2 catalysts inhibited the continuous crystal growth of MoS2.45 On the other hand, the SEM and TEM results showed that carbon coated on the surface of CoS2−MoS2 catalysts after

Figure 3. SEM images of Co−Mo−S−0, Co−Mo−S−0.2, and Co− Mo−S−0.4; and elemental mapping images of Mo, Co, S, and C for Co−Mo−S−0.4.

amounts of PVP. The SEM image of Co−Mo−S−0 presented many curl nanosheets, which were interconnected with each other by van der Waals interaction and finally self-assembled into a disordered, crossed-linked structure. After PVP was added during the preparation of CoS2−MoS2, this nanosheet structure was not obvious, especially for the SEM image of Co− Mo−S−0.4, which presented many particles aggregated 8604


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To determine the carbon content in the final catalysts, Co− Mo−S−0, Co−Mo−S−0.2, and Co−Mo−S−0.4 were characterized by TGA from room temperature to 600 °C in air flow, and the corresponding results are shown in Figure 7a. With the increase of temperature, two weight loss stages were observed: a small weight loss below 200 °C and a large continuous weight loss in the range 250−450 °C, attributed to the vaporization of adsorbed water and the combustion of MoS2 and amorphous carbon, respectively. Assuming that the final residue was a composite of Co and Mo oxides after the combustion in the air and that the molar ratio of Co/Mo was 0.5:1, the decreased weight for Co−Mo−S−0 was only 0.5% in comparison with its weight at 200 °C, demonstrating the effectiveness of this method. According to this method, the carbon content in Co− Mo−S−0.2 and Co−Mo−S−0.4 was calculated to be 10% and 17%. Interestingly, the weight loss below 200 °C was decreased in the order of Co−Mo−S−0 (18%) > Co−Mo−S−0.2 (16%) > Co−Mo−S−0.4 (11%), indicating that the adsorption amount of water was lowered with the increase of carbon in the resultant catalyst, which might result from its enhanced hydrophobicity. For confirmation of this inference, Co−Mo− S−0, Co−Mo−S−0.2, and Co−Mo−S−0.4 were dispersed in a mixture of dodecane and water, and the corresponding image was displayed in Figure 7b. After vigorous shaking and standing for 2 min, we can find that Co−Mo−S−0 and Co−Mo−S−0.2 are dispersed in water, presenting a low lipophilicity; however, Co−Mo−S−0.4 is dispersed in both dodecane and water, and the phase interface almost disappeared, demonstrating that its hydrophobicity was enhanced compared with Co−Mo−S−0 and Co−Mo−S−0.2. These observations suggested that the hydrophobicity of the carbon-coated CoS2−MoS2 catalyst was strengthened with the increase of carbon content, which would raise the deoxygenation degree in the HDO reactions. HDO Activity of the Prepared Catalysts. HDO of 4Ethylphenol on Carbon-Coated CoS2−MoS2 Catalysts. The HDO activity of carbon-coated CoS2−MoS2 catalysts was studied by using 4-ethylphenol as a model oxygen-containing compound in the bio-oil. As shown in Table 1, after reaction at 250 °C and 4.0 MPa hydrogen pressure for 5 h, the conversion, ethylbenzene selectivity, and deoxygenation degree on Co− Mo−S−0 was 85.1%, 98.4%, and 83.2%, respectively, indicating a higher HDO activity than single MoS2,50 which was related to the promotion of CoS2. According to the remote-control model,51 CoS2 acted as a donor phase to produce spillover hydrogen, and then migrated to the MoS2 active site, which supplied more hydrogen for the HDO reaction than that on

Figure 5. XRD patterns of carbon-coated CoS2−MoS2 catalysts.

adding PVP, which also lowered the intensity of the MoS2 diffraction peak. Beyond that, peaks at 2θ = 28°, 32°, 36°, 40°, 46°, and 55° correspond to the (110), (200), (210), (211), (220), and (311) planes of CoS2,46 respectively. Dai et al.47 found no characteristic peak for Co species in the XRD pattern of Co-doped MoS2 with a Co/Mo molar ratio of 0.4 and concluded that cobalt atoms were located at Mo sites and formed a CoMoS phase. However, in this study, we observed the obvious diffraction peaks for CoS2, which demonstrated that Co in the final catalysts was in the form of a CoS2 phase rather than the so-called CoMoS phase. For further confirmation of the separated MoS2 and CoS2, fresh Co−Mo−S−0 and Co−Mo−S−0.4 were characterized by XPS. The high-resolution spectra of Mo 3d and Co 2p are shown in Figure 6. Three peaks appeared at 226.2, 229.1, and 232.5 eV in Figure 6a corresponding to the S 2s of S2−, 3d5/2 of Mo4+, and 3d3/2 of Mo4+ components of MoS2, respectively, presenting a typical MoS2 phase.48 It has been reported that the peak for Co 2p3/2 in the Co−Mo−S phase appeared at the binding energy of 778.6 eV.49 However, Figure 6b shows a strong peak at 779.4 eV, higher than the binding energy of Co 2p3/2 in the Co−Mo−S phase, which is attributed to the Co 2p3/2 of Co2+ in the CoS2 phase. These observations demonstrated that there was no Co−Mo−S phase in the prepared carbon-coated CoS2−MoS2 catalysts and that these catalysts were composed by separated MoS2 and CoS2, which is consistent with the XRD and TEM results.

Figure 6. XPS spectra of (a) Mo 3d and (b) Co 2p regions for fresh Co−Mo−S−0 and Co−Mo−S−0.4. 8605


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Figure 7. (a) TGA curves of carbon-coated CoS2−MoS2 and (b) images of Co−Mo−S−0, Co−Mo−S−0.2, and Co−Mo−S−0.4 dispersed in dodecane and water.

Table 1. HDO of 4-Ethylphenol on CoS2−MoS2 Catalysts Synthesized by Adding Different Amounts of PVP at 250 °C for 5 h catalyst conversion (mol %) Product Selectivity (mol %) ethylcyclohexane 4-ethylcyclohexene ethylbenzene DD (wt %)

Co−Mo−S−0

Co−Mo−S−0.1

Co−Mo−S−0.2

Co−Mo−S−0.3

Co−Mo−S−0.4

85.1

86.0

92.0

93.3

96.6

1.5 0.1 98.4 83.2

1.4 0.1 98.5 84.2

0.9 0 99.1 90.9

0.9 0 99.1 92.4

0.7 0 99.3 96.1

Figure 8. HDO of 4-ethylphenol on Co−Mo−S−0.4 at (a) 225 °C and (b) 250 °C.

single MoS2. As a result, the HDO activity of CoS2−MoS2 was greatly increased in comparison with that of MoS2. With the increase of PVP amount, both 4-ethylphenol conversion and deoxygenation degree were increased. For example, the conversion and deoxygenation degree on Co− Mo−S−0.4 reached 96.6% and 96.1%, 11.5% and 12.9% higher than those on Co−Mo−S−0, respectively. These suggested that the addition of PVP in the synthesis of CoS2−MoS2 was beneficial to enhance its HDO activity. In addition, ethylbenzene selectivity increased to 99.3%. In comparison with our previous study that the addition of PVP increased the DDO activity of MoS2,52 because of the presence of Co, the selectivity of DDO product ethylbenzene was very high, causing the increase to be not very obvious, but it still showed the increase of ethylbenzene selectivity with the PVP amount. Over these catalysts, all ethylbenzene selectivities were higher than 98%, suggesting that the dominant reaction route was DDO and that the promoter Co in MoS2 could enhance its DDO activity.

According to the characterization results, the high HDO activity of Co−Mo−S−0.4 was mainly attributed its enhanced hydrophobicity. The coated carbon on the catalyst inevitably covered some active sites and would then lower the HDO activity. However, the Raman spectrum results showed that Co−Mo−S−0.4 had a high ratio of graphitic carbon, resulting in an enhanced hydrophobicity and a promoted dispersion of catalyst in dodecane, as shown in Figure 7b, which inhibited water contact with sulfide species and would supply more active sites for HDO reactions. A previous investigation also reported this similar conclusion.37 Effect of Reaction Parameters on the HDO of 4Ethylphenol on Co−Mo−S−0.4. On the basis of the high HDO activity of Co−Mo−S−0.4, the effects of reaction temperature and pressure on conversion and product selectivity were studied using this catalyst, as shown in Figures 8 and 9. Figure 8a shows that the conversion increased with reaction time and that the final conversion was 54.2%. The total selectivity of ethylcyclohexane and 4-ethylcyclohexene was very 8606


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sulfide catalysts,34,35 which caused deactivation. Hydrophobicity could prevent the adsorption of water on the active sites, and then show high stability during the HDO reaction. Figure 10 shows the reusability of Co−Mo−S−0 and Co−Mo−S−0.4 in the HDO of 4-ethylphenol. The conversion on Co−Mo−S−0 was decreased with the recycling tests. 4-Ethylphenol conversion dropped to 79.2% after three recycling tests, which was 12.4% lower than that in the first test, indicating an obvious deactivation, but the product distribution changed very little. These observations suggested that the deactivation only influenced the conversion. Co−Mo−S−0.4, possessing higher hydrophobicity than Co−Mo−S−0, exhibited a better stability. The conversion was still 96.8%, which only decreased by 2.6% after three recycling tests. For an observation of the intrinsic reason for the deactivation, these two spent Co−Mo− S−0 and Co−Mo−S−0.4 were also characterized by XPS. In comparison with fresh Co−Mo−S−0 and Co−Mo−S−0.4, the ratio of S/(Co + Mo) in the surface composition of the spent Co−Mo−S−0 and Co−Mo−S−0.4 decreased by 4.1% and 0.8%, respectively, suggesting that some of the sulfur was lost from the sulfide catalyst during the HDO reaction and that the hydrophobic coated carbon could inhibit this sulfur loss, which was caused by the following reason. According to the reaction mechanism proposed by Richard et al.,31 at first, sulfur vacancies were created on the surface of carbon-coated CoS2−MoS2 for adsorbing 4-ethylphenol, which resulted in the sulfur loss after the HDO reaction. If the produced water adsorbed on these sulfur vacancies, other sulfur vacancies would be created, leading to more sulfur loss. Moreover, our previous investigation had also observed that the presence of water exacerbated this sulfur loss.35 Because of the enhanced hydrophobicity of the prepared catalysts, the produced water was forced to leave immediately, and then, the sulfur loss could be inhibited. Therefore, how to prepare sulfide catalysts with high hydrophobicity to prevent sulfur loss was of importance for increasing its HDO activity and prolonging its service life.

Figure 9. HDO of 4-ethylphenol on Co−Mo−S−0.4 at different hydrogen pressures and 250 °C for 6 h.

low during the whole reaction, indicating the low hydrogenation activity of CoS2−MoS2, which was consistent with a previous investigation.36 When the reaction temperature increased to 250 °C, as shown in Figure 8b, after reaction for 6 h, the conversion reached 99.4% with a selectivity of 98.9% ethylbenzene, which was much higher than that shown in Figure 8a. These observations indicate that high temperature was beneficial to increase the conversion and deoxygenation degree. Because the removal of oxygen is finished in the presence of hydrogen via the HDO, hydrogen pressure has a great effect on the conversion and product distribution.26 High hydrogen pressure enhances the formation of spillover hydrogen on the CoS2 active sites and supplies sufficient hydrogen for the HDO reaction, which increases the conversion. As expected, after reaction at 250 °C for 6 h, the conversion increased from 8.3% to 99.4% when the hydrogen pressure rose from 1.0 to 4.0 MPa. However, the hydrogenation product ethylcyclohexane changed little with the increase of hydrogen pressure, and ethylbenzene selectivity was higher than 99%. This showed that Co−Mo−S−0.4 had a high DDO activity even at high hydrogen pressure. In this case, the hydrogen consumption greatly decreased in comparison with the HYD route when removing the oxygen from the same amount of 4-ethylphenol, which was desirable in industry. Reusability Study of Co−Mo−S−0 and Co−Mo−S−0.4 in the HDO of 4-Ethylphenol. It has been reported that the presence of water had a negative effect on the structure of

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CONCLUSIONS Carbon-coated CoS2−MoS2 catalysts were obtained by adding PVP into its hydrothermal synthesis process. With the added amount of PVP, carbon content in the final catalyst was increased, and the nanosheet structure of CoS2−MoS2 became less sharp because of the coverage of carbon on its surface. In the HDO of 4-ethylphenol, high temperature and hydrogen pressure were beneficial to enhance the deoxygenation degree. Co−Mo−S−0.4 presented a high activity, 96.1% deoxygenation

Figure 10. Reusability study of (a) Co−Mo−S−0 and (b) Co−Mo−S−0.4 in the HDO of 4-ethylphenol. 8607


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degree with a selectivity of 99.3% ethylbenzene, demonstrating that the oxygen was removed from 4-ethylphenol via the DDO route, which greatly decreased hydrogen consumption in comparison with the HYD route. The reusability study showed that the conversion on Co−Mo−S−0.4 only decreased by 2.6% after three recycling tests. According to the characterization results, the high activity of the carbon-coated CoS2−MoS2 catalyst was mainly attributed to its enhanced hydrophobicity, which could inhibit the sulfur loss and then improve the stability. Hence, enhancing the hydrophobicity of sulfide catalysts was an important strategy to increase their activity and stability in the HDO reactions.

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AUTHOR INFORMATION

Corresponding Authors

*Phone: (+86) 731-58298581. Fax: (+86)731-58293284. Email: [email protected]. *E-mail: [email protected]. ORCID

Weiyan Wang: 0000-0003-4372-8248 Author Contributions

All authors contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (21776236, 21376202), Hunan Provincial Innovation Foundation for Postgraduate (CX2017B334), and Collaborative Innovation Center of New Chemical Technologies for Environmental Benignity and Efficient Resource Utilization.

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REFERENCES

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Hydrothermal synthesis of carbon-coated CoS2-MoS2 catalysts with

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