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Hydrothermal synthesis of carbon-coated CoS2-MoS2 catalysts with enhanced hydrophobicity and hydrodeoxygenation activity Weiyan Wang, Kui Wu, Song Tan, and Yunquan Yang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b01087 • Publication Date (Web): 24 Aug 2017 Downloaded from http://pubs.acs.org on August 26, 2017

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ACS Sustainable Chemistry & Engineering

Hydrothermal synthesis of carbon-coated CoS2-MoS2 catalysts with enhanced hydrophobicity and hydrodeoxygenation activity

Weiyan Wanga, b *, Kui Wua, Song Tana, Yunquan Yanga, b *

a

School of Chemical Engineering, Xiangtan University, Xiangtan,

Hunan, 411105, PR China b

National & Local United Engineering Research Centre for Chemical Process

Simulation and Intensification, Xiangtan University, Xiangtan 411105, P. R. China

* To whom correspondence should be addressed.

E–mail: [email protected] (W. Wang), [email protected] (Y. Yang) Tel: (+86) 731–58298581 Fax: (+86) 731–58293284

-1ACS Paragon Plus Environment


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ABSTRACT Carbon-coated CoS2-MoS2 catalysts were prepared by a hydrothermal method via adding 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 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 to 96.1% with a selectivity of 99.3% ethylbenzene. The high HDO activity of carbon-coated CoS2-MoS2 catalyst was mainly resulted from its enhanced hydrophobicity. Moreover, the reusability tests also presented that these catalyst had a good stability during the HDO reaction because of the inhibition of hydrophobic coated carbon to sulfur loss. Therefore, improving the hydrophobicity of sulfide catalyst was favored for increasing its HDO activity and inhibiting the deactivation. Keyword: Co-Mo-S; Hydrothermal synthesis; Hydrophobicity; Hydrodeoxygenation; 4-ethylphenol

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INTRODUCTION The depleting reserves of fossil fuel and some environmental pollution problems causing by its utilization had forced us to develop a new alternative energy to meet the human needs and ensured a sustainable development for our world.1 Lignin is a complicated aryl polymer, enriching with 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 then 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 of practical and effective solutions is to selectively remove the oxygen atoms via a hydrodeoxygenation (HDO) technology and the deoxygenation rate depends on the selected catalysts.4 Until now, significant efforts had been concentrated on the development of promising catalysts and there had appeared several HDO catalysts, for example, noble metal catalysts,5-9 sulfide catalysts,10-14 amorphous metal borides,15,

16

metal catalysts,17-19

transition metal phosphides,20, 21 carbides22-24 and oxides.25, 26 Phenols, being typical and dominating oxygen-containing 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: hydrogenationdehydration (HYD) and direct deoxygenation (DDO), which depends on the selected catalysts.27 Noble metals and amorphous metal borides had presented excellent hydrogenation activity and HDO activity, but which consumed much hydrogen due to



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the saturation of benzene ring prior to the scission of C−O bond. Moreover, carbon deposition was easy to form on the acid sites of the supports,28, 29 which was the main deactivated reason for noble metal supported catalysts in the HDO reaction. In addition, the low availability and high cost of noble metals were not beneficial to the sustainability of HDO process in industrial application. 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 increased markedly.31-33 In this case, the deoxygenation efficiency and hydrogen consumption are improved greatly, 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 had 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 hydrophobic sulfide catalyst to prevent the active site from the contact with water and expect to inhibit the deactivation, in part at least. Recently, Zhao et al.37 had reported that the surface of Ni/TiO2 was coated with 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 adding 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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EXPERIMENTAL SECTION Preparation of carbon-coated CoS2-MoS2 catalysts Carbon-coated CoS2-MoS2 catalysts were prepared by one–step hydrothermal method based on the previous study.33 In 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 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 the 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's 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 on a D/max2550 18KW Rotating anode X-Ray Diffractometer with monochromatic Cu Kα radiation (λ= 1.5418Å) radiation at voltage and current of 40 kV and 300 mA. The 2θ was scanned over the range of 10-90°at a rate of 10°/min. Raman spectral experiments were carried out using an inVia Reflex Laser Micro-Raman spectroscope with 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 microscopy. The morphology was determined by high resolution transmission electron microscopy



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(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 Kratos Axis Ultra DLD instrument at 160eV pass energy. Al Kα radiation was used to excited 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 previous study.31 The prepared catalyst (0.06 g) without any further treatment, 4ethylphenol (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 adjusted the stirring speed to 900 rpm. During the reaction, liquid samples were withdrawn from the reactor and analyzed by Agilent 6890/5973N GC-MS and 7890 gas chromatography using a flame ionization detector (FID) with a 30 m AT-5 capillary column. The experiments have been repeated twice at least 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: Conversion (mol %) = (1 − Selectivity (A, mol%) =

moles of residual 4―ethylphenol ) × 100% moles of initial 4―ethylphenol

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


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Deoxygenation degree (D. D. , wt%) = (1 −

oxygen content in the final organic compounds ) × 100% total oxygen content in the initial mixture

RESULTS AND DISCUSSION Characterization of carbon-coated CoS2-MoS2 catalysts 1428 1287

2957

PVP

Transmittance

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Co-Mo-S-0 Co-Mo-S-0.2 Co-Mo-S-0.4

1646 3441

4000 3500 3000 2500 2000 1500 1000

500

-1

Wavenumber/cm

Figure 1. FT–IR spectra of PVP, Co-Mo-S-0, Co-Mo-S-0.2 and Co-Mo-S-0.4 FT–IR was adopted to confirm the existence of PVP in the final catalysts. Figure 1 presents the FT–IR spectra of PVP, Co-Mo-S-0, Co-Mo-S-0.2 and Co-Mo-S-0.4. Two absorption peaks at 3443 cm-1 and 1638 cm-1 were 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 cm-1, 1428 cm-1 and 1287 cm-1 were attributed to C–H, N–H and C–N bonds, respectively. Co-Mo-S-0 showed no absorption peaks to PVP because it was prepared without adding PVP and two peaks to 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 that of Co-Mo-S-0. These suggested that no PVP remained in these two catalysts.



Co-Mo-S-0.4

Intensity (a.u)

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Co-Mo-S-0.3 Co-Mo-S-0.2 Co-Mo-S-0.1 Co-Mo-S-0 300

350

400

450

1500 2000 2500 3000 -1

Raman shift (cm )

Figure 2. Raman spectra of carbon-coated CoS2-MoS2 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 cm-1 and 1580 cm-1 are observed, corresponding to the characteristic D band (a defect/disorder peak) and G band (a graphitic peak) of carbon materials.38 These carbon might be derived from the carbonization of PVP during the hydrothermal process.39 Notably, the peak intensity to 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 the amorphous carbon. Figure 2 showed the low ID/IG value, representing the high ratio of graphite carbon in the catalysts 39, which would increase their hydrophobicity. In addition, two weak peaks at 373 cm-1 and 403 cm-1 were observed in the Raman spectra of Co-Mo-S and Co-Mo-S-0.1, attributing to E12g 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 8 ACS Paragon Plus Environment

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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 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 amount 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 disordered, crossed-linked structure. After adding PVP 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 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



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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 later increase was resulted from that the excessive carbon on the catalyst surface enlarged its surface area.

Figure 4. HRTEM images of Co-Mo-S-0 (a) and Co-Mo-S-0.4 (b, c) 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 with an inter-planar spacing of 0.22 nm 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 to (210) plane of CoS2 and (002) plane of MoS2. These indicated that there coexisted MoS2 and CoS2 in Co-Mo-S-0 and Co-Mo-S-0.4. Moreover, Figure 4c obviously displayed that coating carbon layer on the surface. In comparison with previous literature that reported the expanded inter-planar spacing for (002) plane of MoS2,38 Co-Mo-S-0.4 did not show this enlarged interlayer distance, which might be resulted from the low interaction between 10 ACS Paragon Plus Environment

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carbon and MoS2. The crystalline structure of CoS2-MoS2 catalysts prepared by adding different amounts of PVP were confirmed by XRD characterization, as displayed in Figure 5. The peak at 2θ=14° in the XRD pattern of Co-Mo-S-0 was correspond to the (002) plane of MoS2. Compared to the commercial bulk MoS2,32 this (002) diffraction peak presented very weak and the other peaks 2θ= 33°, 39° and 59° to the (100), (103), (110) basal planes of MoS2 were not observed, indicating its poor crystallinity and homogeneous dispersion.44 After adding PVP 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 (002) diffraction peak became weak after adding PVP, which was resulted from 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 adding PVP, which also lowered the intensity of MoS2 diffraction peak. Beyond that, some peak at 2θ= 28°, 32°, 36°, 40°, 46°, 55° correspond to (110), (200), (210), (211), (220) and (311) plane of CoS2,46 respectively. Dai et al.47 had found that no characteristic peak to Co species in XRD pattern of Co-doped MoS2 with a Co/Mo molar ratio of 0.4 and concluded that cobalt atoms located at Mo sites and formed CoMoS phase. However, in this study, we observed the obvious diffraction peaks to CoS2, which demonstrated that Co in the final catalysts was in the form of CoS2 phase rather than so-called CoMoS phase.



Intensity(a.u.)

Co-Mo-S-0.4 Co-Mo-S-0.3 Co-Mo-S-0.2 Co-Mo-S-0.1 Co-Mo-S-0 10

20

30

40

50

60

70

80

90

2 Theta(degree)

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

(b) Co-Mo-S-0.4

Co-Mo-S-0.4

Intensity (a.u)

(a) Intensity (a.u)

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Co-Mo-S-0

240

238

236

234

232

230

228

226

224

222

Co-Mo-S-0

792 790 788 786 784 782 780 778 776 774 772

Electron Binding Energy (eV)

Electron Binding Energy (eV)

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 To further confirm the separated MoS2 and CoS2, fresh Co-Mo-S-0 and Co-Mo-S-0.4 were characterized by XPS. The high–resolution spectrum of Mo 3d and Co 2p are shown in Figure 6. Three peaks appeared at 226.2 eV, 229.1 eV and 232.5 eV in Figure 6a corresponded to the S 2s of S2−, Mo4+ 3d5/2 and Mo4+ 3d3/2 components of MoS2, respectively, presenting a typical MoS2 phase.48 It had reported that the peak for Co 2p3/2 in 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, being higher than the binding energy of Co2p3/2 in Co-


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Mo-S phase, which attributed to the Co 2p3/2 of Co2+ in CoS2 phase. These demonstrated that there was no Co-Mo-S phase in the prepared carbon-coated CoS2-MoS2 catalysts and these catalysts were composed by separated MoS2 and CoS2, which was consistent with the XRD and TEM results. To determine the carbon content in the final catalysts, Co-Mo-S-0, Co-Mo-S-0.2 and CoMo-S-0.4 were characterized by TGA from room temperature to 600 °C in air flow and the corresponding results are shown in Figure 7(a). 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 of 250−450 °C, attributing 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 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-S0.2 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 be resulted from its enhanced hydrophobicity. To confirm 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 7 (b). After vigorous shaking and standing for 2 minutes, we can find that Co-Mo-S-0 and Co-Mo-S-0.2 dispersed in water, presenting a low lipophilicity, but



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Co-Mo-S-0.4 dispersed in both dodecane and water and the phase interface almost disappeared, demonstrating that its hydrophobicity was enhanced compared with CoMo-S-0 and Co-Mo-S-0.2. These suggested that the hydrophobicity of carbon-coated CoS2-MoS2 catalyst was strengthened with the increase of carbon content, which would rise the deoxygenation degree in the HDO reactions.

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 HDO activity of the prepared catalysts HDO of 4-ethylphenol on carbon-coated CoS2-MoS2 catalysts The HDO activity of carbon-coated CoS2-MoS2 catalysts were studies by using 4ethylphenol 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 MoS2 active site, which supplied more hydrogen for the HDO reaction than that on single MoS 2. As


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a result, the HDO activity of CoS2-MoS2 was increased greatly in comparison with MoS2. Table 1 HDO of 4-ethylphenol on CoS2-MoS2 catalysts synthesized by adding different amounts of PVP at 250 °C for 5 h Catalyst

Co-Mo-S-0

Conversion (mol %)

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

Ethylcyclohexane

1.5

1.4

0.9

0.9

0.7

4-Ethylcyclohexene

0.1

0.1

0

0

0

Ethylbenzene

98.4

98.5

99.1

99.1

99.3

D. D. (wt %)

83.2

84.2

90.9

92.4

96.1

Products selectivity (mol %)

With the increase of PVP amount, both 4-ethylphenol conversion and deoxygenation degree were increased. For example, the conversion and deoxygenation degree on CoMo-S-0.4 reached to 96.6% and 96.1%, being 11.5% and 12.9% higher than that on CoMo-S-0, respectively. These suggested that the addition of PVP in the synthesis of CoS2MoS2 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 due to the presence of Co, the selectivity of DDO product ethylbenzene was very high, leading that the increase was not very obvious, but it still presented the increase of ethylbenzene selectivity with the PVP amount. Over these catalysts, all ethylbenzene selectivity were higher than 98%, suggesting that the dominant reaction route was DDO and 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 was inevitable to cover some active sites and then would lower the HDO activity. However, the Raman spectra results showed that Co-Mo-S-0.4 had a high ratio of graphite carbon, resulting in an enhanced hydrophobicity and a promoted dispersion of catalyst in dodecane, as shown in Figure 7b, which inhibited water to contact with sulfide species and would supply more active sites for HDO reactions. Previous investigation had also reported this similar conclusion.37 Effect of reaction parameters on the HDO of 4-ethylphenol on Co-Mo-S-0.4

(a)100

(b)100 Conversion Ethylcyclohexene 4-Ethylcyclohexane Ethylbenzene

80

Conversion/Selectivity (%)

Conversion/Selectivity (%)

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60

40

20

0

80

Conversion Ethylcyclohexene 4-Ethylcyclohexane Ethylbenzene

60

40

20

0 0

1

2

3

4

5

6

7

8

0

1

2

Time (h)

3

4

5

6

7

8

Time (h)

Figure 8. HDO of 4-ethylphenol on Co-Mo-S-0.4 at (a) 225 °C and (b) 250 °C. Based on the high HDO activity of Co-Mo-S-0.4, the effects of reaction temperature and pressure on the conversion and product selectivity were studied using this catalyst, as shown in Figure 8 and 9. Figure 8a presented that the conversion increased with reaction time and the final conversion was 54.2%. The total selectivity of ethylcyclohexane and 4-ethylcyclohexene was very low during the whole reaction, indicating the low hydrogenation activity of CoS2-MoS2, which was consistent with previous investigation.36 When the reaction temperature increased to 250 °C, as shown in Figure 16 ACS Paragon Plus Environment

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8b, after reaction for 6 h, the conversion reached to 99.4% with a selectivity of 98.9% ethylbenzene, which was much higher than that in Figure 8a. These indicated that high temperature was beneficial to increase the conversion and deoxygenation degree.

Conversion/Selectivity (mol %)

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100


80 60 40 20 0 1

2

3

4

Hydrogen pressure (MPa)

Figure 9. HDO of 4-ethylphenol on Co-Mo-S-0.4 at different hydrogen pressures and 250 °C for 6 h. Because the removal of oxygen is finished in the presence of hydrogen via the HDO, the 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 raised from 1.0 MPa 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 presented that Co-MoS-0.4 had a high DDO activity even at high hydrogen pressure. In this case, the hydrogen consumption decreased greatly in comparison with the HYD route when removed the oxygen from the same amount of 4-ethylphenol, which was desirable in industry. 17 ACS Paragon Plus Environment


Reusability study of Co-Mo-S-0 and Co-Mo-S-0.4 in the HDO of 4-ethylphenol

(a)


(b) Conversion/Selectivity (mol %)

Conversion/Selectivity (mol %)

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100 80 60 40 20 0


100 80 60 40 20 0

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Recycling number

2

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Recycling number

Figure 10. Reusability study of (a) Co-Mo-S-0 and (b) Co-Mo-S-0.4 in the HDO of 4ethylphenol It had reported that the presence of water had a negative effect on the structure of sulfide catalysts,34, 35 which caused to the deactivation. The hydrophobicity could prevent the adsorption of water on the active sites, and then presented the 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 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. To reveal 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, 18 ACS Paragon Plus Environment

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suggesting that some of sulfur lost from sulfide catalyst during the HDO reaction and the hydrophobic coated carbon could inhibited 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 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, there would create other sulfur vacancies, 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. 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 unsharpness because of the coverage of carbon on its surface. In the HDO of 4-ethylphenol, high temperature and hydrogen pressure was beneficial to enhance the deoxygenation degree. Co-Mo-S0.4 presented a high activity: 96.1% deoxygenation 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



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decreased by 2.6% after three recycling tests. According to the characterization results, the high activity of 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. AUTHOR INFORMATION Corresponding Author * Tel: (+86) 731-58298581; Fax: (+86)731-58293284; E-mail: [email protected] (W. Wang), [email protected] (Y. Yang) Author Contributions #

These authors contributed equally to this work.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (No. 21776236, 21376202), Hunan Provincial Innovation Foundation for Postgraduate (CX2017B334) and Collaborative Innovation Center of New Chemical Technologies for Environmental Benignity and Efficient Resource Utilization. REFERENCES (1) Liu, Z.; Guan, D.; Wei, W.; Davis, S. J.; Ciais, P.; Bai, J.; Peng, S.; Zhang, Q.; Hubacek, K.; Marland, G.; Andres, R. J.; Crawford-Brown, D.; Lin, J.; Zhao, H.; Hong, C.; Boden, T. A.; Feng, K.; Peters, G. P.; Xi, F.; Liu, J.; Li, Y.; Zhao, Y.; Zeng, N.; He,


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Carbon-coated CoS2-MoS2 catalysts were prepared by a hydrothermal method via adding polyvinylpyrrolidone and presented high activity and stability in the HDO of 4-ethylphenol, which was mainly attributed to its enhanced hydrophobicity.

ACS Paragon Plus Environment

Hydrothermal Synthesis of Carbon-Coated CoS - American

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