Amorphous Carbon

Article pubs.acs.org/IECR

Hydrodeoxygenation of p‑Cresol on MoS2/Amorphous Carbon Composites Synthesized by a One-Step Hydrothermal Method: The Effect of Water on Their Activity and Structure Guohua Zhu,† Weiyan Wang,*,†,‡ Kui Wu,† Song Tan,† Liang Tan,† and Yunquan Yang*,†,‡ †

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

‡

S Supporting Information *

ABSTRACT: Associated with the carbonization of glucose in the hydrothermal process, MoS2/amorphous carbon composites were synthesized by a one-step hydrothermal method and characterized by various technologies. Introducing carbon into MoS2 increased its surface area and enlarged the edge-to-corner ratio of the MoS2 slab of the MoS2 phase, which contributed to a high deoxygenation degree in the hydrodeoxygenation (HDO) of p-cresol, but excessive carbon covered some MoS2 edges that acted as active sites for HDO reaction and lowered the conversion. The presence of water modified the surface structure of MoS2 and had a great effect on the HDO activity and product distribution. During the HDO reaction, sulfur−oxygen exchange at the edges of MoS2 resulted in the loss of S atoms and the intensity decrease of the (002) diffraction peak for MoS2 phase, which was accelerated by increasing the amount of added water. Carbon in MoS2 only acted as a support and had no preventing effect on water. When the added water was 0.2 g, i.e., water/p-cresol molar ratio was 0.5:1, after reaction at 275 °C for 8 h, toluene selectivity and deoxygenation degree increased to 91.3% and 94.2%, respectively.

1. INTRODUCTION

and product distribution mainly depend on the selected catalysts.4 Lignin-derived liquid fuels contain a considerable percentage of phenolic compounds, which can directly produce fuel components via a HDO treatment. Moreover, the aryl−oxygen bond was stronger than the aliphatic−oxygen bond, leading to an increase in the difficulty for oxygen removal from phenols.5 Consequently, phenols were usually selected as model compounds to study the activity of the catalysts and the HDO reaction mechanism.4 Generally, the HDO of phenols proceeds with two main routes: (1) hydrogenation−deoxygenation (HYD) via the saturation of aromatic ring followed by dehydration and (2) direct deoxygenation (DDO) via the hydrogenolysis of the aryl−oxygen bond with the aromatic

The exploitation of renewable energy to replace or supplement petroleum-based fuels has significantly increased its importance because of the depletion of nonrenewable fossil fuels and increasingly serious environmental problems.1 Biomass, a renewable carbon resource, could be converted to liquid fuels and valuable chemicals via a fast pyrolysis process. 2 Unfortunately, these liquid fuels contain many oxygenates, including phenols, furans, esters, alcohols, ketones, and aldehydes, which contribute to its high oxygen content and lower its heating value. Furthermore, these oxygenates cause some adverse properties such as high viscosity, corrosiveness, immiscibility with hydrocarbon fuels, and thermal instability.3 Hence, to obtain petroleum-like hydrocarbon fuels, an available process for the removal of oxygen atoms from the pyrolysis liquid fuels is required. Hydrodeoxygenation (HDO) is an attractive and efficient technology in whcih oxygen is removed as water in the presence of H2 and the deoxygenation degree © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

June 6, 2016 November 2, 2016 November 10, 2016 November 10, 2016 DOI: 10.1021/acs.iecr.6b02170 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research character retained.6 Significant efforts have been concentrated on the development of new catalysts with high activity for the HDO of phenols.7−13 Noble metal and metal boride catalysts had been widely applied into phenols HDO reactions,14−20 and the dominant reaction route on these catalysts was HYD because of their high hydrogenation activity. Although the HDO reaction temperature was lowered, the high cost of precious metals and high consumption of H2 for this process made it unfavorable for commercialization. Mo-based sulfide was another active catalyst for the HDO reaction where MoS2 edge acted as the primary catalytic active site; its activity was closely related to the preparation methods and supports.21−26 Alumina was usually used as a support for Mo-based catalyst because of its high surface area and thermal stability,27−29 but its weak Lewis acid sites favored the deactivation by coke formation. To lower the acidity, Nikulshin et al.30 prepared carbon-coated alumina and found its supported catalyst exhibited higher activity and lower deactivation rate than alumina supported catalyst in the HDO of guaiacol. Carbon support has a high surface area, amphoteric nature, and stable structure, which is beneficial to the active phase dispersion, and has less affinity to form coke compared to other acidic supports during the HDO reactions.31−34 Commonly, carbon-supported Mo-based sulfide catalyst was synthesized by impregnation of the molybdenum precursor followed by a sulfuration at high temperature. In such cases, the sulfuration conditions such as temperature and the activation atmosphere had a great effect on the catalyst activity.35 Moreover, the prepared catalyst had a highly ordered crystalline multilayered MoS2 structure, possessing less unsaturated S atoms at the edges, which presented HDO activity much lower than that of defective amorphous MoS2.36,37 Glucose could be converted into amorphous carbon via a hydrothermal process, which was applied into the synthesis of carbon-containing materials. For example, Chen and co-workers38 employed sodium molybdate, sulfocarbamide, and glucose as starting materials to prepare MoS2/C composites. Lin and co-workers39 obtained a highly porous MoS2−C hybrid film via an in situ hydrothermal route using glucose as carbon source. Recently, Beltramini and coworkers37 synthesized amorphous MoS2/C using an uneconomical and complex microemulsion technique and found its activity superior to that of the crystalline MoS2 catalyst obtained from a wetness impregnation method in the guaiacol HDO reaction. However, to the best of our knowledge, there is still very little literature that reports the HDO activity of MoS2/ amorphous carbon composites. In addition, during HDO reaction on Mo-based sulfides, the catalyst usually underwent a continuous deactivation caused by sulfur−oxygen exchange, and the presence of water had an inhibiting effect on the conversion of reactants and led to an additional deactivation for the catalyst. For example, Şenol et al.40 reported that the presence of water decreased the conversion of the esters and suppressed the hydrogenation and decarboxylation reactions on Ni−Mo sulfide catalyst. Travert and co-workers41 confirmed that adding water in the HDO of 2-ethyphenol on Mo/γ-Al2O3 resulted in a fast decrease in the HYD activity and additional deactivation of the catalyst because the water changed the nature of the active sites via the exchange of edge sulfur atoms. Escalona and coworkers42 concluded that the negative effect of water on conversion in the HDO of phenol over ReS2/SiO2 was attributed to the decrease in Re dispersion. However, some reactions such as hydrodenitrogenation and hydrogenation

were promoted by adding a small amount of water.43,44 Therefore, referenced to the previous investigations,38,45 we introduced glucose into the hydrothermal synthesis of MoS2 to synthesize MoS2/amorphous carbon composites and expected to enhance the activity in the HDO of p-cresol in this study. The effect of water amount on p-cresol conversion and product distribution and the change of catalyst structure after HDO reaction were also studied in detail.

2. EXPERIMENTAL SECTION All solvents and reagents were obtained from Sinopharm Chemical Reagent Co., Ltd. in high purity (≥99%) and used without further purification. MoS2/amorphous carbon composites were prepared by a one-step hydrothermal method based on the previous study.38 Ammonium heptamolybdate (1.39 g), thiourea (1.80 g), and glucose (0, 0.1, 0.2, 0.3, 0.4 g) were dissolved in 180 mL of water, and then hydrochloric acid was added into the solution to adjust the pH to 0.9. This mixture was placed in a 300 mL hydrothermal vessel, which was sealed and heated in an air environment to 200 °C for 12 h. After the reaction, the black precipitates were separated and washed with water and ethanol several times and dried under vacuum at 60 °C for 5 h. For comparison, MoS2 was also prepared by the same hydrothermal method without adding glucose under the same conditions. These as-prepared catalysts and dodecane were placed into the batch reactor and heat-treated in hydrogen at 275 °C for 5 h. The resultant catalysts were named MoS2 and MoS2-C-x, where x represents the weight of glucose. The structure properties of the prepared catalysts were characterized by X-ray diffraction (XRD), differential thermal analysis, nitrogen physisorption, Raman spectra, X-ray photoelectron spectroscopy (XPS), scanning electronic microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM) technologies. The HDO activity tests were carried out in a batch reactor using p-cresol as a model compound. The detailed procedures for characterization, water treatment, and activity tests and the definitions of conversion, selectivity, deoxygenation degree, and HYD/DDO for each experiment are provided in the Supporting Information. 3. RESULTS AND DISCUSSION 3.1. Characterizations of the Prepared MoS2/Amorphous Carbon Composites. The XRD patterns of the prepared MoS2 and MoS2/amorphous carbon composites are shown in Figure 1. The diffraction peaks at 2θ values of 14°,

Figure 1. XRD patterns of MoS2 and MoS2/amorphous carbon composites. B

DOI: 10.1021/acs.iecr.6b02170 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research 33°, 39°, and 59° were indexed to the (002), (100), (103), and (110) reflections of the MoS2 hexagonal phase, respectively; in particular, the stronger peak at 2θ = 14° indicated a stacking of the single layers and well-stacked layered structure.38,46 Besides that, not any peak was detected, suggesting that no other impurity phases were formed in this process. After the introduction of carbon, the characteristic peaks for MoS2 remained, but the diffraction peak to carbon was not observed, which was mainly a result of the amorphous structure of carbon derived from the hydrothermal carbonization of glucose.39 Compared with the XRD pattern of single MoS2, the intensity of all the diffraction peaks gradually decreased with the increase of the amount of carbon, which was caused by the inhibition of the crystal growth when incorporating carbon into MoS2.47 According to the analysis of the (002) diffraction peak, as shown in Table 1, the average crystallite size calculated by the

respectively. Compared with MoS2, the peak intensity of both E12g and A1g were decreased with carbon content, indicating the decreasing crystallinity, which suggested that the incorporation of carbon prevented the growth and promoted the distribution of MoS2 crystals in MoS2/amorphous carbon composites, in good agreement with the XRD results shown in Figure 1. In addition, two peaks located at 1342 and 1565 cm−1 were observed in the high-wavenumber region in the Raman spectra of MoS2/amorphous carbon composites with higher carbon content, attributed to the D (defect) and the G (graphite) bands,49 which confirmed the successful introduction of carbon in the composites. However, these two peaks were characteristically broad and rough, suggesting that carbon existed in an amorphous structure. Therefore, MoS2 and amorphous carbon coexist in the as-prepared composites. The electronic states of elements on the surface of MoS2 and MoS2-C-0.2 catalysts were investigated by XPS analysis, as shown in Figure 3. The survey XPS spectrum (Figure 3a) revealed that the predominant elements in the prepared sample were Mo and S. Very little oxygen was also observed, which mainly came from the adsorbed oxygen.50 Compared with MoS2, another peak appeared at about 285.0 eV, attributed to C element, which suggested that carbon was formed and doped into MoS2 after the hydrothermal carbonization of glucose. The high-resolution spectra of Mo 3d, S 2p, and C 1s are shown in panels b, c, and d of Figure 3, repsectively. After the deconvolution, three peaks located at 226.7, 229.6, and 232.8 eV in the spectrum of Mo 3d represented the S 2s of S2− and Mo4+ 3d5/2 and Mo4+ 3d3/2 components of MoS2,51,52 respectively. These indicated that Mo existed in the form of MoS2 and that there were not any Mo oxides and MoOxSy phase in the resultant catalysts.53 For the peaks of S 2p (Figure 3c), the individual doublet peaks appearing at 162.4 and 163.6 eV correspond to the S 2p3/2 and S 2p1/2, respectively, where the intensity ratio of these two peaks is about 2:1 and the separation energy is 1.2 eV, demonstrating the typical characteristics of S2 species.54 In addition, Figure 3d presents only one peak at 284.8 eV, indexing to element C, which also confirmed the successful introduction of carbon into MoS2 after the carbonization of glucose. To further confirm the content of carbon in MoS2/amorphous carbon composites, thermogravimetric analysis was carried out in air atmosphere according to the method reported in the previous investigation,55 and the corresponding results are listed in Table 1. The carbon content in MoS2-C-0.1, MoS2-C-0.2, MoS2-C-0.3, and MoS2-C-0.4 was 2.3, 3.9, 4.8, and 6.9 wt %, respectively. The increase trend was in line with the added glucose weight. The morphologies of MoS2 and MoS2/amorphous carbon composites are compared in Figure 4. MoS2 displayed a typical sheetlike morphology composed of many nanosheets stacked together in a disordered sequence. When carbon was introduced into MoS2, as shown in the SEM image of MoS2C-0.2, many nanosheets attached to each other to form a loose porous nanostructure because of the van der Waals interaction. Moreover, the thickness of sheets became thinner, which would create more edge sites and larger space for the reaction and reactant/product diffusion. However, when the adding glucose amount exceeded 0.3, more carbon produced and inserted into the space of nanosheets, the sheetlike morphology almost disappeared and the sheet interface became unobvious, e.g., the image of MoS2-C-0.4 presented that much carbon covered on the surface of MoS2. This coverage, especially at the edges of nanosheet, decreased the available active sites for reaction and

Table 1. Structure Properties of MoS2 and MoS2/ Amorphous Carbon Composites catalyst MoS2 MoS2C-0.1 MoS2C-0.2 MoS2C-0.3 MoS2C-0.4

surface area (m2/g)

C content (wt %)a

average crystallite size (nm)b

average length (nm)c

( fe/fc)wd

58.6 69.6

0 2.3

3.80 3.42

6.9 7.2

9.3 9.7

75.8

3.9

3.44

7.4

10.1

88.4

4.8

2.55

7.0

9.4

107.6

6.9

2.60

6.3

8.3

a

C content obtained from thermogravimetric analysis. bAverage crystallite size of MoS2 calculated from XRD data. cAverage length, L, of MoS2 slab analyzed from HRTEM data. dEdge-to-corner ratio of MoS2 slab calculated from HRTEM data

Scherrer equation was approximately 3.80, 3.42, 3.44, 2.55, and 2.60 nm for MoS2, MoS2-C-0.1, MoS2-C-0.2, MoS2-C-0.3, and MoS2-C-0.4, equating to about 6, 5, 5, 4, and 4 layers of MoS2, respectively. Raman spectroscopy was usually used to confirm crystallinity, defects, and structures of transition-metal dichalcogenides and carbonaceous materials. As shown in Figure 2, all MoS2/ amorphous carbon composites presented two obvious peaks at 377 and 401 cm−1, corresponding to in-layer E12g vibration mode and out-of-layer A1g vibration mode of MoS2 crystal,48

Figure 2. Raman spectra of MoS2/amorphous carbon composites. C


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Figure 3. XP spectra of MoS2 and MoS2-C-0.2 catalysts: (a) survey spectrum, (b) Mo 3d spectrum, (c) S 2p spectrum, and (d) C 1s spectrum.

lowered the catalytic activity. Brunauer−Emmett−Teller analysis results (Table 1) showed that MoS2, MoS2-C-0.1, MoS2-C-0.2, MoS2-C-0.3, and MoS2-C-0.4 had a specific surface area of 58.6, 69.6, 75.8, 88.4, and 107.6 m2/g, respectively, exhibiting an increase of surface area with carbon amount. This was attributed to the carbon from glucose carbonization, possessing a high specific surface area, acting as a carrier for MoS2. The detailed morphology on the surface and edges of MoS2 and MoS2/amorphous carbon composites was further investigated by TEM and HRTEM. As shown in Figure 5, the TEM image of MoS2-C-0.2 presented a rippled sheets morphology with many thin nanosheets. The HRTEM image of the MoS2 displayed an interlayer spacing of 0.63 nm, assigned to the (002) plane of MoS2 phase,56 which was in good agreement with that calculated from its XRD pattern. After glucose was added into the hydrothermal solution, all the HRTEM images of MoS2/amorphous carbon composites showed a typical MoS2 structure with several lamellar layers. The edge-to-corner ratio of the MoS2 slab was calculated according to the literature57,58 and is summarized in Table 1. The edge-to-corner ratio of the MoS2 slab increased from 9.3 for MoS2 to 10.1 for MoS2-C-0.2 and then decreased with the added glucose amount. Interestingly, it was measured that the interlayer distance of the (002) plane in MoS2-C-0.2 was 0.69 nm, which was in accordance with the results obtained from the XRD pattern but slightly larger than that in the MoS2 catalyst, revealing that the original spacing of the (002) crystal plane of MoS2 was expanded by introducing carbon. This MoS2/amorphous carbon composite with larger interlayer spacing and smaller crystallite size was favorable for the adsorption of reactants and then would enhance its catalytic activity. In contrast, both

MoS2-C-0.3 and MoS2-C-0.4 had an interlayer distance of the (002) plane of 0.63 nm, which is the same as that of single MoS2. This might be ascribed to the recrystallization and removal of structural defects of MoS2 because of the aggregation of carbon after carbonization at the high concentration of glucose.59 In addition, another interlayer with a spacing of 0.33 nm, characterizing the carbon,60 was also observed. Moreover, the zone attributed to carbon was increased with the added glucose amount, e.g., MoS2-C-0.4 presented that some carbon covered on the surface of MoS2, leading to the unclear or disappeared (002) plane layers, which was consistent with the results of the SEM analysis. 3.2. HDO Activities of MoS2/Amorphous Carbon Composites. As shown in Table 2, the products in the HDO of p-cresol on MoS2 and MoS2/amorphous carbon composites were methylcyclohexane, 4-methylcyclohexene, and toluene. Although toluene could be hydrogenated to methylcyclohexane, the conversion for this reaction on MoS2 was only 1.5% after reaction at 275 °C and 4.0 MPa hydrogen pressure for 8 h, which suggested that this hydrogenation reaction could be negligible. Previous study had also confirmed the low hydrogenation activity of Mo-based sulfides.61 Hence, it was reasonable to conclude that the removal of oxygen from pcresol on these Mo-based sulfide catalysts was finished via two separated routes (HYD and DDO), which is in agreement with previous investigations.23,62 Table 2 presents that the conversion on MoS2, MoS2-C-0.1, MoS2-C-0.2, MoS2-C-0.3 and MoS2-C-0.4 is 56.4%, 61.4%, 65.5%, 58.8%, and 53.5% after reaction at 275 °C for 8 h, respectively. Considering the mass of MoS2, the higher content of C in MoS2-C-0.4 would result in a reduced MoS2 and lower the conversion. However, compared with MoS2 and MoS2-C-0.1, the weight of MoS2 for MoS2-CD


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Figure 4. SEM images of MoS2 and MoS2/amorphous carbon composites.

the added glucose amount varied from 0.1 to 0.4 g. The deoxygenation degree on MoS2/amorphous carbon composites decreased in the order of MoS2-C-0.2 (62.1%) > MoS2-C-0.1 (57.9%) > MoS2-C-0.3 (55.3%) > MoS2 (52.8%) > MoS2-C-0.4 (49.8%), suggesting the highest HDO activity of MoS2-C-0.2. This could be attributed to the following reasons. Carbon acted as a carrier for MoS2 to enlarge its specific surface area, which provided more active sites on its surface for the HDO reaction. However, it also covered on the surface of MoS2 after addition of excessive glucose, as shown in the SEM image of MoS2-C0.4, leading to the decrease of the available active sites. In addition, the interlayer distance of the (002) plane in MoS2-C0.2 was the largest, which might be beneficial for adsorbing pcresol molecules for HDO reaction and then enhancing the deoxygenation degree. 3.3. Effect of Water for the HDO of p-Cresol on MoS2/ Amorphous Carbon Composites. Previous studies reported a negative effect of water on the conversion in the HDO reactions.41,64 Recently, Zhao and co-workers65 prepared a hydrophobic carbon-coated Ni/TiO2 by a hydrothermal method to prevent water contact with nickel catalytic species and improve its stability. Hence, the effects of water on the conversion and product distribution in the HDO of p-cresol were studied taking MoS2-C-0.2 as an optimized catalyst, as shown in Table 3. In the absence of water, the conversion and toluene selectivity were 65.5% and 75.6%, respectively. After water was added, the results were somewhat surprising: both the conversion and DDO product selectivity enhanced greatly.

0.2 in the reactor was decreased, but the conversion was increased. Although the mass of MoS2 in the reactor decreased with the C content, the reduction was very slight. Therefore, the mass of MoS2 in MoS2/amorphous carbon composites was not a crucial factor for the deoxygenation degree. The reaction rate constant (k) was calculated based on the equation ln(1 − x) = −k·Ccat·t, as shown in Table 2. The k for the HDO of pcresol increased to the maximum value (0.62 mL/(s·g MoS2)) on MoS2-C-0.2 and then decreased with the increase of carbon amount. Associated with the edge-to-corner ratio of the MoS2 slab (Table 1), it suggested that MoS2 possessing a higher edgeto-corner ratio exhibited a higher HDO activity. However, Mo-based sulfide was a structure-sensitive catalyst, and the adsorption methods of reactant molecules on its surface determined the reaction routes and product distribution. It had been proposed that HYD and DDO started by flat η 5 adsorption through aromatic ring and oxygen on the rim plane and the vertical η1 adsorption of phenols through oxygen on all the edge planes of MoS2,23,63 respectively. The selectivity of methylcyclohexane, 4-methylcyclohexene, and toluene was 20.5%, 6.4%, and 73.2% in the HDO of p-cresol on MoS2, respectively. According to the rim−edge reaction mechanism,63 more (002) plane layers in MoS2 structure corresponded to its higher DDO activity. The calculated layer number of MoS2 indicated that there were more exposed edge sites for direct deoxygenation, leading to the prime product of toluene. Table 2 showed that the product distribution changed little and the toluene selectivity was in the range of 71%−77% when E


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reaction route were enhanced by adding an appropriate amount of water. These findings were different from that in the literature,41 which concluded that the reduction of HDO activity resulted from a fast decrease in the HYD activity and there was almost no effect for DDO activity in the presence of water in the HDO of 2-ethylphenol on Mo/γ-Al2O3. This discrepancy might be a result of the different catalysts. During the HDO reaction, Brønsted acid sites (SH groups) on the surface of Mo-based sulfides were formed in the presence of hydrogen and at high temperature.23 For the HYD route, p-cresol was adsorbed on the active sites in a flat way first and hydrogenated to 4-methylcyclohexanol. Then, 4-methylcyclohexanol was deoxygenated via a dehydration reaction, which occurred on the acid sites. This reaction route (HYD) might lead to the destruction of the SH groups in rim sites more easily than in the DDO route. In turn, the flat adsorption of aromatic ring on the catalyst surface was inhibited and methylcyclohexane selectivity was decreased. As shown in Figure 6, because of the effect of water produced in the HDO

Figure 5. TEM and HRTEM images of MoS2 and MoS2/amorphous carbon composites.

Table 2. HDO of p-Cresol on MoS2 and MoS2/Amorphous Carbon Composites at 275 °C for 8 h catalyst conversion (mol %) k, mL/(s·g MoS2) methylcyclohexane 4-methylcyclohexene toluene D.D.

MoS2

MoS2-C0.1

56.4 0.48 product 20.5 6.4 73.2 52.8

MoS2-C0.2

61.4 0.57 selectivity 21.2 7.1 71.6 57.9

65.5 0.62 (%) 19.2 5.2 75.6 62.1

MoS2-C0.3

MoS2-C0.4

58.8 0.53

53.5 0.47

20.4 6.3 73.3 55.3

17.6 5.4 77.0 49.8

Figure 6. Change of toluene selectivity versus reaction time in the HDO of p-cresol on MoS2-C-0.2 in the presence of different weights of water.

reaction, toluene selectivity on MoS2-C-0.2 in the absent of water was increased from 68.6% to 75.6%. If some water was added in the HDO reaction at the beginning, acid sites were destroyed and the HYD route was repressed, and then the toluene selectivity was enhanced. As expected, Figure 6 presents that toluene selectivity was increased with the added weight of water, and it increased with the reaction time in each reaction. For the DDO route, p-cresol molecules were vertically adsorbed on active sites via the donation of lone electron pair from the oxygen at first.23 Satterfleld et al.43 concluded that the number or strength of acid sites increased after the adsorption of water on the catalyst surface, thereby enhancing hydrogenolysis activity. Therefore, in the presence of water, the nature of the MoS2-C-0.2 catalyst surface was modified, favoring the vertical adsorption of p-cresol, which increased the toluene selectivity. To further study whether the water had an effect on the microstructure of MoS2-C-0.2, the catalysts that were treated by 0.2 and 0.4 g water (named MoS2-C-0.2-0.2 and MoS2-C-0.20.4, respectively) and recycled after HDO reaction in the presence of 0.2 and 0.4 g water (named as MoS2-C-0.2-0.2-R and MoS2-C-0.2-0.4-R, respectively) were characterized by TEM. As shown in Figure 7, the stacking layers of MoS2 were

Table 3. Effect of Water in the HDO of p-Cresol on MoS2-C0.2 at 275 °C for 8 h water weight (g) conversion (mol %) methylcyclohexane 4-methylcyclohexene toluene D.D.

0.2

0.3

0.4

65.5 73.6 95.0 product selectivity (%) 19.2 11.1 6.9 5.2 3.9 1.8 75.6 86.0 91.3 62.1 70.8 94.2

0

0.1

94.5

70.1

5.0 1.8 93.2 93.6

6.4 2.9 90.7 66. 8

In the presence of 0.2 g of water, the conversion and toluene selectivity increased to 95.0% and 91.3%, respectively. The deoxygenation degree was 32.1% higher than that without adding water. However, when the water was further increased to 0.4 g, the conversion and deoxygenation degree declined to 70.1% and 66.8%, respectively. On the basis of these results, it could concluded that both the deoxygenation degree and DDO F


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became more obvious with the increase of water. For example, the (002)/(100) peak intensity ratio of MoS2-C-0.2 was 1.5, which reduced to 1.3, 1.0, and 0.9 after the HDO reaction in the presence of 0, 0.2, and 0.4 g of water, respectively, but the (100) peak changed very little. These indicated that the structure of the (002) plane was destroyed after the HDO reaction, and water accelerated this undesirable effect. Figure 9 presents the XP spectra of Mo 3d and S 2p levels of MoS2-C-0.2 treated with water and that of spent catalysts.

Figure 7. HRTEM images of MoS2-C-0.2, MoS2-C-0.2-0.2, MoS2-C0.2-0.4, MoS2-C-0.2-0.2-R, and MoS2-C-0.2-0.4-R.

almost unchanged after treatment with water, suggesting the increase of toluene selectivity resulted from the modification of water. However, excessive water (>0.2 g) would inhibit the adsorption of hydrogen and decrease the supply of dissociation hydrogen for HDO reaction,66 leading to the reduction of pcresol conversion. Until now, there had appeared two opposite conclusions for the effect of water on sulfide catalysts: a very limited oxidation of the molybdenum sulfide phase64 and the exchange of active edge sulfur atoms of the sulfide phase.41 The spent catalysts were also characterized by XRD, TEM, and XPS. Figure 8 shows that the characteristic peaks of MoS2 structure remained and the intensity of the (002) plane peak was higher than that of the (100) plane after treating with water, suggesting that the water had little effect on MoS2 phase. However, after HDO reaction, (002)/(100) peak intensity ratio for the spent catalysts decreased. Moreover, this decrease

Figure 9. XP spectra of (a) Mo 3d and (b) S 2p levels of MoS2-C-0.2 catalysts treated with water and spent MoS2-C-0.2 catalysts.

Figure 8. XRD patterns of (a) fresh catalysts treated with water and (b) spent catalysts in the presence of different amounts of water. G


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4. CONCLUSIONS During the synthesis of MoS2/amorphous carbon composites, with the addition of glucose, its surface area was increased and the nanosheet-like morphology of MoS2 gradually disappeared. In the HDO of p-cresol, MoS2-C-0.2 presented the highest HDO activity: the conversion and deoxygenation degree were 65.5% and 62.1%, respectively, which was mainly attributed to its large surface area and the higher edge-to-corner ratio of the MoS2 slab. However, because of the coverage of carbon on MoS2 edge active sites, the HDO activity was lowered when the added glucose weight was higher than 0.2 g. In the presence of 0.2 g of added water in the HDO of p-cresol, in comparison with adding no water, the deoxygenation degree and toluene selectivity on MoS2-C-0.2 increased by 32.1% and 15.7%, respectively, which was ascribed to the increased number or strength of acid sites on the catalyst surface under the modification of water. The characterization results of the spent catalysts indicated that the intensity of the diffraction peak to the edge of MoS2 was reduced; Mo oxide appeared and the S/Mo molar ratio was decreased in the presence of water. These provided evidence for the loss of sulfur in the edge of MoS2 structure during the HDO reaction. In addition, carbon in MoS2 only acted as a support to enhance the HDO activity but could not prevent the effect of water.

There still appeared three peaks at 226.7, 229.6, and 232.8 eV in the spectra of Mo 3d for MoS2-C-0.2-0.2 and MoS2-C-0.20.4, but these three peaks shifted to 226.3, 229.2, and 232.4 eV in the Mo 3d spectra of MoS2-C-0.2-0.2-R and MoS2-C-0.2-0.4R. Furthermore, one small peak was observed at 235.6 eV in the Mo 3d spectra of MoS2-C-0.2-0.4-R, corresponding to the Mo4+ oxide phase. Figure 9b displays two deconvolution peaks at 162.4 and 163.5 eV to S in MoS2/amorphous carbon composites treated with different water, but both decreased in the spent catalysts. S−H and one Mo−H group were formed at high temperature and in the presence of hydrogen, and sulfur−oxygen exchange occurred during the HDO reaction.23,41 After reaction, some of the S2−on the catalyst surface might be lost. Consequently, the shifts of binding energies for Mo 3d and S 2p in the spent catalysts might be attributed to the loss of S, which was further confirmed by the surface composition. According to the XPS data, the values of the S/ Mo molar ratio on the catalyst surface for MoS2-C-0.2, MoS2C-0.2-0.2, MoS2-C-0.2-0.4, MoS2-C-0.2-0.2-R, and MoS2-C-0.20.4-R were 1.62, 1.65, 1.60, 1.45, and 1.35, respectively. These indicated that the S content on the surface of MoS2/amorphous carbon composites was not affected by water and that the loss of sulfur atoms occurred during the HDO reaction. Moreover, compared with MoS2-C-0.2-0.2-R, the appearance of Mo oxide and the lower S/Mo molar ratio in MoS2-C-0.2-0.4-R suggested that the water enhanced the loss of sulfur and the introduction of carbon had no prevention effect on water. The TEM images also provided direct evidence. Travert and co-workers41 reported that the exchange of edge sulfur atoms by oxygen atoms caused the deactivation of the catalyst in the HDO reaction. As shown in Figure 7, the spent catalysts presented many more defects on the (002) plane edges of MoS2. Hence, the presence of water accelerated the loss of sulfur and increased the deoxygenation degree and DDO product selectivity. This loss could be inhibited or reduced by adding some S-containing compounds, such as H2S. The activity comparison of MoS2/amorphous carbon composites in the HDO of p-cresol in the presence of 0.2 g water is shown in Table 4. Compared with adding no water

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b02170. Detailed experimental procedures of XRD analysis, BET analysis, Raman spectra analysis, XPS analysis, SEM analysis, HRTEM analysis, activity test, and water treatment for MoS2/amorphous carbon composites; definitions of conversion, selectivity, deoxygenation degree, and HYD/DDO for each experiment (PDF)

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conversion (mol %) methylcyclohexane 4-methylcyclohexene toluene D.D.

MoS2

MoS2-C0.1

MoS2-C0.2

80.1 88.7 product selectivity 6.0 7.7 2.5 2.7 91.5 89.6 77.5 87.1

95.0 (%) 6.9 1.8 91.3 94.2

AUTHOR INFORMATION

Corresponding Authors

Table 4. HDO of p-Cresol on MoS2 and MoS2/Amorphous Carbon Composites in the Presence of 0.2 g of Water at 275 °C for 8 h catalyst

ASSOCIATED CONTENT

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

MoS2-C0.3

MoS2-C0.4

73.6

68.4

The authors declare no competing financial interest.

7.1 3.2 89.7 70.5

7.0 3.2 89.8 65.0

ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (21306159, 21376202), Scientific Research Fund of Hunan Provincial Education Department (15B234), and Collaborative Innovation Center of New Chemical Technologies for Environmental Benignity and Efficient Resource Utilization.

Notes

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(Table 2), the conversions and toluene selectivity on MoS2 increased by 23.7% and 18.3%, respectively, indicating that the water also had a promoting effect in the HDO of p-cresol on single MoS2. The deoxygenation degree on MoS2-C-0.4 was 65.0%, which was 29.2% and 15.5% lower than that on MoS2C-0.2 and MoS2, respectively. As presented in Figure 4, the SEM image of MoS2-C-0.4 indicated that much carbon covered its surface; in this case, the available active sites for HDO reaction were decreased, leading to a decrease in deoxygenation degree.

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