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Preparation of hydrophobic MoS2, NiS2-MoS2 and CoS2-MoS2 for catalytic hydrodeoxygenation of lignin-derived phenols
Molecular Catalysis 477 (2019) 110537
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Preparation of hydrophobic MoS2, NiS2-MoS2 and CoS2-MoS2 for catalytic hydrodeoxygenation of lignin-derived phenols
T
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Kui Wua, Yan Liua, Weiyan Wanga,b, , Yanping Huanga, Wensong Lia, Qianqian Shia, ⁎ Yunquan Yanga,b, a b
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
A R T I C LE I N FO
A B S T R A C T
Keywords: NiS2-MoS2 CoS2-MoS2 Hydrodeoxygenation Hydrophobicity Stability
The water produced during the hydrodeoxygenation (HDO) processes has a negative effect on sulfide catalysts, which lowers the activity and stability. To solve this problem, hydrophobic MoS2, NiS2-MoS2 and CoS2-MoS2 catalysts are prepared and then applied into the HDO of 4-ethylphenol and 4-propylguaiacol. The obtained MoS2 had strong hydrophobicity and its air-water contact angle reached up to 147.5°, which might be attributed to that modification of the surface morphology due to the addition of silicomolybdic acid. During the HDO reaction on MoS2, the specific surface area played an important effect on the HDO activity. After the deposition of NiS2 or CoS2, the hydrophobicity of NiS2-MoS2 and CoS2-MoS2 reduced slightly, but the HDO activities of these two catalysts were enhanced, especially for CoS2. 99.9% Conversion was achieved with a selectivity of 99.6% ethylbenzene after the HDO of 4-ethylphenol at 225 °C for 3 h. These hydrophobic catalysts also had good activity for the HDO of 4-propylguaiacol. Moreover, because the hydrophobicity could prevent water from contacting with active sites and then protect the sulphide structure, hydrophobic sulphide catalysts exhibited high stability.
1. Introduction
based sulphides are structure-sensitive and the morphology plays an important role in the HDO activity and product distribution [19,20]. For example, the exfoliation of commercial MoS2 to few-layers or monolayer structure could enhance its HDO activity. Moreover, due to very low sulphur content in the bio-oil, Mo-based sulphide catalyst suffered from deactivation via a sulphur-oxygen exchange at the edge of MoS2 during the HDO reactions [21]. What’s worse, the presence of water enhanced this sulphur-oxygen exchange and accelerated the deactivation of sulphide catalysts [22]. In our previous investigation, we has prepared carbon-coated CoS2-MoS2 catalyst and found that the hydrophobicity enhanced the HDO activity and stability [23]. Hence, to protect sulphide structure and inhibit catalyst deactivation, the preparation of highly hydrophobic sulphide catalyst that can prevent water from the contacting with catalyst or drive away water from sulphide surface is of great importance. Until now, many methods such as chemical etching, spray coating, spin coating, templating, sol-gel process, electro-deposition and chemical vapour deposition had appeared for the preparation of hydrophobic materials [24]. MoS2 is a lubricant material, which can be used for the preparation of hydrophobic coating, but no hydrophobicity was
Many environmental pollution problems caused by the utilization of fossil fuel have impelled us to develop an environmentally friendly renewable energy. Bio-oil, derived from the pyrolysis of biomass, is an ideal candidate, but its characteristics of high oxygen content hinder its extensive utilization as a supplement or replacement for gasoline or fossil diesel [1]. HDO technology can selectively and efficiently remove the oxygen atoms from bio-oil [2], and both the deoxygenation rate and product distribution mainly depend on the catalysts [3]. Consequently, many HDO catalysts such as noble metals, borides, carbides, oxides, metals, sulphides and phosphides were developed [4–14], but the high cost, low activity or easy deactivation hindered their commercial application. Mo-based sulphide, a kind of commercial hydrodesulfurization catalyst, has also been widely applied into the HDO processes [15–17], which had even exhibited higher HDO activity than some noble metal catalysts. As reported by Likozar et al. [18], the sulphide form of NiMo/ Al2O3 presented higher activity than Pd/Al2O3 in the simultaneous direct liquefaction and HDO of lignocellulosic biomass. However, Mo-
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Corresponding authors at: School of Chemical Engineering, Xiangtan University, Xiangtan, Hunan, 411105, PR China. E-mail addresses: [email protected], [email protected] (W. Wang), [email protected] (Y. Yang).
https://doi.org/10.1016/j.mcat.2019.110537 Received 3 July 2019; Received in revised form 24 July 2019; Accepted 26 July 2019 Available online 06 August 2019 2468-8231/ © 2019 Elsevier B.V. All rights reserved.
Molecular Catalysis 477 (2019) 110537
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Fig. 1b and 2θ = 28°, 32°, 36°, 39°, 46° and 55° in Fig. 1c corresponded to NiS2 [34] and CoS2 [35], respectively, and their intensity gradually increased with the promoter content. These demonstrated a co-existence of NiS2 or CoS2 and MoS2 in NiS2-MoS2 or CoS2-MoS2 catalysts. In addition, no characteristic peak to CoS2 in Co-Mo-240-0.1 might be caused by the small CoS2 particles due to its high dispersion in CoS2MoS2 catalyst. The size and morphology of MoS2, NiS2-MoS2 and CoS2-MoS2 were examined by SEM and TEM techniques. Fig. 2 shows the SEM images of Mo-240, Ni-Mo-240-0.3 and Co-Mo-240-0.2. Mo-240 was composed of curly and interlaced nanosheets with an average wall thickness of about 10 nm, presenting its ultrathin nature, which was resulted from the oriented growth of the produced MoS2 nanoparticles. After adding NiS2 and CoS2 into MoS2, as shown in Fig. 2(b) and (c), the characteristic structure of nanosheet was not changed. Besides, some small particles were observed, which could be ascribed to NiS2 or CoS2. Fig. 3 shows the TEM and HRTEM images of Mo-240, Ni-Mo-2400.3 and Co-Mo-240-0.2. The lattice fringes had an average number of 2–6 layers with an interlayer spacing of about 0.63 nm (Fig. 3b), corresponding to the characteristic of (002) basal plane of the hexagonal MoS2 structure. When NiS2 or CoS2 was added into MoS2, their morphologies did not change obviously. Fig. 3(c) presents some cubelike particles with a size of about 100 nm, attributing to cubic NiS2 [36]. In addition, other two fringes with a interlayer spacing of about 0.26 nm and 0.25 nm, covering on the MoS2 slabs, were observed in Fig. 3(d) and Fig. 3(f), corresponding to (200) plane of the cubic NiS2 nanocrystals and (210) plane of cubic CoS2 pyramids [37,38], respectively. These demonstrated that Ni or Co did not incorporate into MoS2 slab to form Co−Mo−S analogue phases, which also gave an evidence to support the co-existence of NiS2/CoS2 and MoS2 in NiS2-MoS2 and CoS2-MoS2. The MoS2 structure in Mo-240, Ni-Mo-240-0.3 and Co-Mo-240-0.2 was further studied by Raman spectroscopy. As shown in Fig. 4, two obvious peaks appeared at 378 and 403 cm−1 in the Raman spectra of Mo-240, attributing to the in-plane E12g mode and the out-of-plane A1g mode of a hexagonal MoS2 crystal [39], respectively. The frequency (△) of E12g and A1g for Mo-240 was about 5 cm−1, indicating the formation of MoS2 nanosheets with few layers [40], which was in accordance with the HRTEM observations. In comparison with A1g peak, E12g peak presented a lower intensity, suggesting a basal−edge−rich feature and an unperfected crystal structure of MoS2 in Mo-240 [41]. After doping NiS2 or CoS2, both the frequencies of E12g and A1g peaks were almost unchanged, demonstrating that Ni-Mo-240-0.3 and Co-Mo240-0.2 had the same MoS2 structure as that of Mo-240. These suggested no incorporation of NiS2/CoS2 into MoS2 structure. The chemical compositions and element valence states in MoS2, NiS2-MoS2 and CoS2-MoS2 were analysed by XPS characterization. As shown in Fig. S1, no peak was observed in the XP spectrum of Si 2p, suggesting that no Si remained in the prepared MoS2. The other XP spectrums of Mo 3d, S 2p, Ni 2p and Co 2p are shown in Fig. 5. Three peaks were observed in the high resolution XP spectrum of Mo 3d (Fig. 5a), where the one at 226.6 eV corresponds to S 2 s of S2− in MoS2, and the other two main peaks at 229.5 eV and 232.6 eV were assigned to Mo4+ 3d5/2 and Mo4+ 3d3/2 of MoS2, respectively. Fig. 5b presents two peaks at 162.3 eV and 163.5 eV, attributing to the S 2p3/2 and S 2p1/2, respectively. Notably, the binding energy (△) of these two peaks was 1.2 eV, indicating a typical characteristic of S2 species. Fig. 5c shows two dominant peaks at 854.5 eV and 779.4 eV in Ni 2p and Co 2p spectrums, which were assigned to Ni 2p3/2 in NiS2 and Co 2p3/2 in CoS2, respectively. In comparison with the reported binding energy of Ni 2p in Ni−Mo−S phase (853.5 eV) and Co 2p3/2 in Co−Mo−S phase (778.6 eV) [42,43], it can be concluded that Ni and Co were not incorporated in MoS2 to form Ni(Co)−Mo−S phase in Ni-Mo-240-0.3 and Co-Mo-240-0.2. In addition, if Ni or Co was incorporated into the MoS2 edges, the S/Mo atomic ratio would not increase. However, the S/ Mo atomic ratios of Ni-Mo-240-0.3 and Co-Mo-240-0.2 were calculated
found for the chemically synthesized MoS2 [19,25]. Polyoxometalate, a solid acid, has been widely applied into the preparation of various heterogeneous catalysts [26,27], which could promote the dispersion of activated components and modify the surface structure. Therefore, in this study, we tried to hydrothermally synthesize MoS2 by utilizing silicomolybdic acid to substitute liquid acid and expected to obtain hydrophobic MoS2, and then used it as a substrate to further prepare NiS2MoS2 and CoS2-MoS2 via a two-step hydrothermal method. The activity and stability of these MoS2, NiS2-MoS2 and CoS2-MoS2 catalysts were tested in the HDO of 4-ethylphenol and 4-propylguaiacol as the representative compounds of lignin-derived phenols. 2. Experimental Hydrophobic sulphides were synthesized according to the similar hydrothermal method that reported before [28]. In a typical procedure, ammonium heptamolybdate (1.60 g), thiourea (2.07 g) and silicomolybdic acid (3 mmol) were dissolved in 100 mL water and then transferred to a 200 mL Teflon-lined stainless-steel autoclave and then treated at 240 °C for 12 h. Then the reactor was cooled and nickel nitrate or cobalt nitrate solution was added into it. The reactor was sealed and heated to 240 °C for 12 h again. The composition of the samples was adjusted by changing the initial Ni(Co)/Mo molar ratio. Finally, the black precipitation was collected and washed with water and absolute ethanol, and dried under vacuum at 60 °C for 5 h and stored in nitrogen atmosphere. The resultant catalysts were denoted as Ni-Mo-T-X or CoMo-T-X, where T and X represented the preparation temperature (°C) and the molar ratio of Ni(Co)/Mo in the initial solution, respectively. In addition, Mo-T catalysts were also prepared under different temperatures. These catalysts were characterized by X-ray Diffraction (XRD), Nitrogen Physisorption, Raman Spectra, Scanning Electronic Microscopy (SEM), High Resolution Transmission Electron Microscopy (HRTEM) technologies and Water Contact Angles. Their HDO activities were tested in a batch reactor using 4-ethylphenol as a representative compound of lignin-derived phenols. The detailed characterization procedure, activity tests and the definitions of conversion, selectivity and deoxygenation degree for each experiment were shown in the Supporting Information. 3. Results and discussion 3.1. Characterizations of MoS2, NiS2-MoS2 and CoS2-MoS2 The structures of MoS2, NiS2-MoS2 and CoS2-MoS2 were firstly assessed using XRD characterization. Fig. 1a presents the effect of preparation temperature on the crystallinity of MoS2. Mo-200 exhibited several broad diffraction peaks, indicating an amorphous structure of MoS2. In addition, a broad and weak diffraction peak at 2θ = 9° was observed, corresponding to the diffraction of adjacent few-layered MoS2 sheets [29]. When the preparation temperature was increased to 220 °C and 240 °C, four obvious diffraction peaks at 2θ = 14°, 33°, 36° and 59° appeared in the XRD patterns of Mo-220 and Mo-240, attributing to the reflection of (002), (100), (103) and (110) planes of hexagonal MoS2 [30,31], respectively. N2 physical adsorption results showed that Mo-200, Mo-220 and Mo-240 had specific surface area of 69 m2/g, 83 m2/g and 99 m2/g, respectively. These indicated that high preparation temperature was beneficial to increase the crystallinity and the surface area of MoS2. Co(Ni)−Mo−S phase tends to form in the one-step preparation of bi-component sulphides [32,33]. Consequently, NiSx or CoSx phases would not be observed under the low Ni(Co)/Mo molar ratio. However, in this study, MoS2 was prepared in the first step and then acted as a support to load NiS2 or CoS2 to obtain bi-component Mo-based sulphide catalysts. This preparation process indicated that Co−Mo−S analogue phases were impossible to form. The XRD results (Fig. 1) supported this conclusion. The diffraction peaks at 2θ = 27°, 31°, 35°, 38°, 45°, 53° in 2
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Fig. 1. XRD patterns of (a) MoS2, (b) NiS2-MoS2 and (c) CoS2-MoS2.
Fig. 2. SEM images of (a) Mo-240, (b) Ni-Mo-240-0.3 and (c) Co-Mo-240-0.2.
3
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Fig. 3. TEM and HRTEM images of (a, b) Mo-240, (c, d) Ni-Mo-240-0.3 and (e, f) Co-Mo-240-0.2.
be resulted from the coverage of non-hydrophobic NiS2 or CoS2 on the surface of MoS2. These suggested that this method could obtain MoS2 based catalysts with high hydrophobicity. 3.2. HDO activity of hydrophobic MoS2, NiS2-MoS2 and CoS2-MoS2 3.2.1. Effect of preparation temperature on the HDO activity of hydrophobic MoS2 Fig.7 shows the catalytic activity of hydrophobic MoS2 catalysts in the HDO of 4-ethylphenol at 300 °C. The products were ethylbenzene, ethylcyclohexane and 4-ethylcyclohexene and no oxygen-containing compound was detected, indicating the high deoxygenation activity. After reaction at 300 °C for 6 h, 4-ethylphenol conversion on Mo-200, Mo-220 and Mo-240 was 56.3%, 69.6% and 78.2%, respectively. Associated with the characterization results, the change on the conversion might be caused by the crystallinity and specific surface area. Recently, Yoosuk et al. [46] had concluded that amorphous MoS2 with more unsaturated active sites exhibited much higher HDO activity than crystalline MoS2. However, all the prepared MoS2 catalysts had low crystallinity. In addition, the catalyst, possessing larger specific surface area, will expose more active sites for the reaction. Taken these together, the surface area was a considerable factor for the HDO activity of MoS2, which was consistent with our previous study [47]. Because of the large specific surface area and high activity, Mo-240 was selected as a support to further study the promoting effect of NiS2 and CoS2 on the HDO activity of MoS2.
Fig. 4. Raman spectra of Mo-240, Ni-Mo-240-0.3 and Co-Mo-240-0.2.
to be 2.21 and 2.17, respectively, both of which were larger than that of Mo-240 (1.84). This S/Mo atomic ratio increase was caused by the introduction of extra sulphide (NiS2 or CoS2) into Mo-240, indicating the co-existence of two separate phases in NiS2-MoS2 or CoS2-MoS2 catalysts. Fig. 6 shows the hydrophobicity performance of common MoS2 (CMoS2) prepared via a normal hydrothermal method, MoS2, NiS2-MoS2 and CoS2-MoS2 catalysts. At first, these four samples were added into the mixture of dodecane and water to demonstrate the hydrophobic and hydrophilic performance. Because of the large density (4.80 g/cm3) and the low hydrophobicity, C-MoS2 powder could not disperse in dodecane and deposited in the bottom. MoS2, NiS2-MoS2 and CoS2-MoS2 in this study also had a larger density than water, but they dispersed in dodecane rather than in water, presenting their hydrophobicity. In addition, the air-water contact angle was further used to evaluate the hydrophobicity. As shown in Fig. 6, MoS2 had an air-water contact angle of 147.5°, suggesting its excellent hydrophobicity, which was attributed to the modification of surface morphology rather than chemical composition according to the XPS results [44,45]. After adding NiS2 or CoS2 into MoS2, the contact angle of NiS2-MoS2 and CoS2-MoS2 catalysts decreased slightly, being 143.9° and 140.6°, respectively, which might
3.2.2. HDO of 4-ethylphenol on MoS2: the promoter of NiS2 Table 1 shows the effect of Ni/Mo molar ratio in the NiS2-MoS2 catalysts on the activity in HDO of 4-ethylphenol. In comparison with Mo-240, after reaction at 275 °C for 6 h, the conversion and ethylcyclohexane selectivity on Ni-Mo-240-0.1 was increased by 19.5% and 22.8%, respectively, suggesting that the addition of NiS2 enhanced both the deoxygenation degree and hydrogenation−dehydration route, which was consistent with the results in the previous literatures [33,47]. Because of the co-existence of two separate phases, the HDO mechanism on NiS2-MoS2 catalysts could be well explained with the remote control model [48]. Hydrogen was firstly adsorbed on donor phase (NiS2) and then translated into spillover hydrogen, and finally 4
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Fig. 5. XP spectrums of (a) Mo 3d, (b) S 2p, (c) Ni 2p and Co 2p levels of Mo-240, Ni-Mo-240-0.3 and Co-Mo-240-0.2 catalysts.
Fig. 6. Photographs of (a) common MoS2, (b) Mo-240, (c) Ni-Mo-240-0.3 and (d) Co-Mo-240-0.2 distribution in dodecane and water and their air-water contact angles.
migrated to acceptor phase (MoS2) for the hydrotreatment reactions. Consquently, NiS2 supplied extra spillover hydrogen for benzene ring saturation and HDO reaction, which enhanced ethylcyclohexane selectivity and deoxygenation degree. Table 1 shows that the deoxygenation degree is increased in the order of Ni-Mo-240-0.1 (61.0%) < Ni-Mo-240-0.2 (70.9%) < Ni-Mo-240-0.5 (77.2%) < NiMo-240-0.3 (79.7%), indicating that NiS2-MoS2 with a Ni/Mo molar ratio of 0.3 has the highest activity. This was attributed to the synergism between NiS2 and MoS2. When NiS2 exceeded a certain value, some MoS2 active sites were covered, lowering the HDO activity. Previous investigations had also reported this similar trend [33,49].
Fig. 7. HDO of 4-ethylphenol on Mo-200, Mo-220 and Mo-240 at 300 °C for 6 h. Reaction conditions: 2.0 g evaluate, 0.03 g catalyst and 15 g dodecane.
enhanced the HDO activity but also changed the product distribution. When the reaction temperature dropped to 225 °C, 4-ethylphenol conversion on Co-Mo-240-0.05 was 24.7% after reaction for 3 h, but ethylbenzene selectivity increased to 96.3%, indicating its high direct deoxygenation activity. According to the remote control model [51], due to the change of 4-ethylphenol adsorption mode at the presence of CoS2 [50], spillover hydrogen created on CoS2 could make the direct deoxygenation going on smoothly, but it was not enough to saturate the benzene ring, leading to high ethylbenzene selectivity. With the increase of CoS2 content, the conversion increased firstly and then
3.2.3. HDO of 4-ethylphenol on MoS2: the promoter of CoS2 Table 2 shows the HDO of 4-ethylphenol on CoS2-MoS2 catalysts with different Co/Mo molar ratios. The presence of CoS2 not only 5
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detected, suggesting that the scission of Caryl-OR was prior to that of Caryl-OH, which was well consistent with previous result [52]. After adding promoters Ni and Co into hydrophobic MoS2, the HDO activity was greatly enhanced, especially for Co. When the reaction temperature dropped to 250 °C, 4-propylphenol selectivity on Co-Mo-240-0.2 was only 2.3%. These results demonstrated that the prepared hydrophobic sulphides also had good activity for the HDO of 4-propylguaiacol.
Table 1 HDO of 4-ethylphenol on NiS2-MoS2 catalysts at 275 °C for 6 h. Catalyst
Mo-240
Ni-Mo240-0.1
Ni-Mo240-0.2
Ni-Mo240-0.3
Ni-Mo240-0.5
Conversion Product distribution Ethylcyclohexane 4-Ethylcyclohexene Ethylbenzene Deoxygenation degree
44.4
63.9
73.2
81.5
79.1
8.8 3.0 88.2 36.9
31.6 0.7 67.7 61.0
46.4 0.4 53.2 70.9
49.7 0.3 50.0 79.7
48.2 0.3 51.5 77.2
3.2.5. Recycling test of hydrophobic MoS2, NiS2-MoS2 and CoS2-MoS2 Because the presence of water has a severely negative effect on the stability of MoS2 catalysts during the HDO processes [53], the hydrophobicity would be of importance. Consequently, the stability of hydrophobic MoS2, NiS2-MoS2 and CoS2-MoS2 was tested in the HDO of 4ethylphenol under different reaction conditions, as shown in Fig. 8. After 3 cycles at 300 °C, 4-ethylphenol conversion on Mo-240 only decreased by 1.7%, suggesting almost no deactivation, but ethylbenzene selectivity increased from 92.7% to 97.5%, which was attributed to the modification of MoS2 active sites during the HDO reaction [53]. Our previous study had found that the modification of the produced water on the sulphide structure during the cyclic HDO reaction caused to the increase of conversion [23]. However, Fig. 8(a) showed the conversion almost unchanged after 3 cycles. These indicated that negative effect of water was inhibited, which was attributed to that the hydrophobicity prevented water from contacting with catalyst surface. For Ni-Mo-240-0.3, as shown in Fig. 8b, the conversion changed very little but ethylbenzene selectivity increased by 18.3% after 3 cycles at 275 °C. The increase of ethylbenzene selectivity might also be resulted from the modifications of MoS2 and NiS2. NiS2, acting as the donor phase, supplied low spillover hydrogen for the HDO reaction after modification. As a result, the saturation of benzene ring was inhibited and then the direct deoxygenation was enhanced. If the promoter Ni was incorporated into MoS2 to form the so-called Ni−Mo−S phase, the ethylbenzene selectivity would be not changed so large. These also indirectly supported the co-existence of separated NiS2 and MoS2 phases in Ni-Mo-240-0.3. For Co-Mo-240-0.2, after 3 cycles at 225 °C, the conversion and ethylbenzene selectivity were still up to 99.5% and 99.7% (Fig. 8c), respectively, presenting its high stability. After HDO reaction, the water contact angles of these spent hydrophobic MoS2, NiS2-MoS2 and CoS2-MoS2 catalysts were measured again, which were 147.1°, 144.5° and 140.5°, respectively, as shown in Fig. S2, demonstrating that their hydrophobicity properties were not changed after HDO reaction at high temperature. It was generally accepted that the deactivation of sulphide catalyst during the hydrotreating reactions was mainly caused by coke deposition and sulfur-loss of active phases [21,54]. However, the HDO reaction temperature dropped to 225 °C in this study, the coke deposition could be inhibited efficiently. In addition, due to the hydrophobicity, the produced water was forced to desorb immediately and then its active structure could be protected.
Reaction conditions: 2.0 g 4-ethylphenol, 0.03 g catalyst and 15 g dodecane. Table 2 HDO of 4-ethylphenol on CoS2-MoS2 catalysts at 225 °C for 3 h. Catalyst
Mo-240
Co-Mo2400.05
Co-Mo2400.1
Co-Mo2400.2
Co-Mo2400.3
Co-Mo2400.5
Conversion Product distribution Ethylcyclohexane 4-Ethylcyclohexene Ethylbenzene Deoxygenation degree
8.5
24.7
79.0
99.9
87.5
70.2
7.9 2.7 89.4 6.5
2.8 0.9 96.3 22.2
0.3 0.3 99.6 76.6
0.3 0.1 99.6 99.9
0.4 0.4 99.2 85.9
0.4 0.5 99.1 67.2
Reaction conditions: 5.4 g 4-ethylphenol, 0.03 g catalyst and 15 g dodecane.
lowered. Co-Mo-240-0.2 presented the highest HDO activity, where the conversion and ethylbenzene selectivity reached up to 99.9% and 99.6%, respectively. The high HDO activity of Co-Mo-240-0.2 was also attributed to the synergism between CoS2 and MoS2. In comparison with the HDO of 4-ethylphenol on NiS2-MoS2, although the weight of reactant was 2.7-fold more, and the reaction temperature decreased by 50 °C, and the reaction time reduced by half, but the deoxygenation degree on CoS2-MoS2 was still higher than 20%, demonstrating the excellent HDO activity of CoS2-MoS2 catalyst. Besides, high surface area is beneficial to enhance the catalyst activity. However, Co-Mo-240-0.2 (93 m2/g) has a lower surface area than that of CoMo/Al2O3 supported catalyst (255 m2/g) [50], but showed higher HDO activity, which might be caused by the larger contact surface area between CoS2 and MoS2 phases in CoS2-MoS2 catalyst [51]. Interestingly, CoS2-MoS2 had high direct deoxygenation activity but low hydrogenation activity, leading to a low cycloalkanes selectivity in the HDO of lignin-derived phenols, even at high hydrogen pressure (4.0 MPa). This was desirable in industry and presented a great potential for application.
3.2.4. HDO of 4-propylguaiacol on MoS2, NiS2-MoS2 and CoS2-MoS2 4-Propylguaiacol, a typical monomer in lignin-biomass, was also selected to test the HDO activity of hydrophobic MoS2, NiS2-MoS2 and CoS2-MoS2. The conversion and product distribution are summarized in Table 3. After reaction at 300 °C for 6 h, 4-propylguaiacol conversion on Mo-240 reached to 99.2%, but 4-propylphenol selectivity was still high to 34.7%. During this reaction, however, 3-propylanisole was not
4. Conclusions Hydrophobic MoS2 was prepared by a hydrothermal method via introducing silicomolybdic acid and then used as support to further prepare NiS2-MoS2 and CoS2-MoS2 catalysts. The air-water contact angle of MoS2 was 147.5°, exhibiting its strong hydrophobicity. After depositing NiS2/CoS2, the hydrophobicity changed slightly, and NiS2/ CoS2 and MoS2 were co-existed in NiS2-MoS2 or CoS2-MoS2. During the HDO reactions, the activity increased with the order of MoS2 < NiS2MoS2 < CoS2-MoS2. 4-Ethylphenol conversion on CoS2-MoS2 was 99.9% with an ethylbenzene selectivity of 99.6% after reaction at 225 °C for 3 h, presenting an excellent HDO activity, which was mainly attributed to the synergism between CoS2 and MoS2 phases. Notably, the hydrophobicity forced the produced water to desorb immediately and inhibited the adsorption of water, and then the active structure was protected, presenting a high stability. After 3 cycles at 225 °C, the
Table 3 HDO of 4-propylguaiacol on MoS2, NiS2-MoS2 and CoS2-MoS2 catalysts for 6 h. Catalyst
Mo-240
Ni-Mo-240-0.3
Co-Mo-240-0.2
Temperature (°C) Conversion (%) Product distribution Propylcyclohexane (%) 4-Propylcyclohexene (%) Propylbenzene (%) 4-Propylphenol (%)
300 99.2
275 99.7
250 100
5.0 2.9 57.4 34.7
31.6 0.6 47.4 20.4
13.4 0.6 83.7 2.3
Reaction conditions: 1.84 g 4-propylguaiacol, 0.06 g catalyst and 15 g dodecane. 6
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Fig. 8. Stability test of (a) Mo-240 (Reaction conditions: 2.0 g 4-ethylphenol, 0.03 g catalyst, 15 g dodecane, at 300 °C for 6 h), (b) Ni-Mo-240-0.3 (Reaction conditions: 2.0 g 4-ethylphenol, 0.03 g catalyst and 15 g dodecane, at 275 °C for 6 h) and (c) Co-Mo-240-0.2 (Reaction conditions: 5.4 g 4-ethylphenol, 0.03 g catalyst and 15 g dodecane, at 225 °C for 3 h) in the HDO of 4-ethylphenol.
conversion and ethylbenzene selectivity in the HDO of 4-ethylphenol on CoS2-MoS2 were still remained to 99.5% and 99.7%, respectively.
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Acknowledgements This research was supported by the National Natural Science Foundation of China (21776236, 21676225), Natural Science Foundation of Hunan Province (2018JJ2384), Collaborative Innovation Centre of New Chemical Technologies for Environmental Benignity and Efficient Resource Utilization, Engineering Research Centre of Chemical Process Simulation and Optimization of Ministry of Education, and Hunan Key Laboratory of Environment-Friendly Chemical Process Integrated Technology. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.mcat.2019.110537. References [1] C. Li, X. Zhao, A. Wang, G.W. Huber, T. Zhang, Chem. Rev. 115 (2015) 11559–11624. [2] C. Liu, H. Wang, A.M. Karim, J. Sun, Y. Wang, Chem. Soc. Rev. 43 (2014) 7594–7623. [3] M. Patel, A. Kumar, Renew. Sustain. Energy Rev. 58 (2016) 1293–1307. [4] M. Saidi, F. Samimi, D. Karimipourfard, T. Nimmanwudipong, B.C. Gates, M.R. Rahimpour, Energy Environ. Sci. 7 (2014) 103–129. [5] M. Grilc, G. Veryasov, B. Likozar, A. Jesih, J. Levec, Appl. Catal. B: Environ. 163 (2015) 467–477.
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Preparation of hydrophobic MoS2, NiS2-MoS2 and CoS2-MoS2 for catalytic hydrodeoxygenation of lignin-derived phenols
T
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Kui Wua, Yan Liua, Weiyan Wanga,b, , Yanping Huanga, Wensong Lia, Qianqian Shia, ⁎ Yunquan Yanga,b, a b
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
A R T I C LE I N FO
A B S T R A C T
Keywords: NiS2-MoS2 CoS2-MoS2 Hydrodeoxygenation Hydrophobicity Stability
The water produced during the hydrodeoxygenation (HDO) processes has a negative effect on sulfide catalysts, which lowers the activity and stability. To solve this problem, hydrophobic MoS2, NiS2-MoS2 and CoS2-MoS2 catalysts are prepared and then applied into the HDO of 4-ethylphenol and 4-propylguaiacol. The obtained MoS2 had strong hydrophobicity and its air-water contact angle reached up to 147.5°, which might be attributed to that modification of the surface morphology due to the addition of silicomolybdic acid. During the HDO reaction on MoS2, the specific surface area played an important effect on the HDO activity. After the deposition of NiS2 or CoS2, the hydrophobicity of NiS2-MoS2 and CoS2-MoS2 reduced slightly, but the HDO activities of these two catalysts were enhanced, especially for CoS2. 99.9% Conversion was achieved with a selectivity of 99.6% ethylbenzene after the HDO of 4-ethylphenol at 225 °C for 3 h. These hydrophobic catalysts also had good activity for the HDO of 4-propylguaiacol. Moreover, because the hydrophobicity could prevent water from contacting with active sites and then protect the sulphide structure, hydrophobic sulphide catalysts exhibited high stability.
1. Introduction
based sulphides are structure-sensitive and the morphology plays an important role in the HDO activity and product distribution [19,20]. For example, the exfoliation of commercial MoS2 to few-layers or monolayer structure could enhance its HDO activity. Moreover, due to very low sulphur content in the bio-oil, Mo-based sulphide catalyst suffered from deactivation via a sulphur-oxygen exchange at the edge of MoS2 during the HDO reactions [21]. What’s worse, the presence of water enhanced this sulphur-oxygen exchange and accelerated the deactivation of sulphide catalysts [22]. In our previous investigation, we has prepared carbon-coated CoS2-MoS2 catalyst and found that the hydrophobicity enhanced the HDO activity and stability [23]. Hence, to protect sulphide structure and inhibit catalyst deactivation, the preparation of highly hydrophobic sulphide catalyst that can prevent water from the contacting with catalyst or drive away water from sulphide surface is of great importance. Until now, many methods such as chemical etching, spray coating, spin coating, templating, sol-gel process, electro-deposition and chemical vapour deposition had appeared for the preparation of hydrophobic materials [24]. MoS2 is a lubricant material, which can be used for the preparation of hydrophobic coating, but no hydrophobicity was
Many environmental pollution problems caused by the utilization of fossil fuel have impelled us to develop an environmentally friendly renewable energy. Bio-oil, derived from the pyrolysis of biomass, is an ideal candidate, but its characteristics of high oxygen content hinder its extensive utilization as a supplement or replacement for gasoline or fossil diesel [1]. HDO technology can selectively and efficiently remove the oxygen atoms from bio-oil [2], and both the deoxygenation rate and product distribution mainly depend on the catalysts [3]. Consequently, many HDO catalysts such as noble metals, borides, carbides, oxides, metals, sulphides and phosphides were developed [4–14], but the high cost, low activity or easy deactivation hindered their commercial application. Mo-based sulphide, a kind of commercial hydrodesulfurization catalyst, has also been widely applied into the HDO processes [15–17], which had even exhibited higher HDO activity than some noble metal catalysts. As reported by Likozar et al. [18], the sulphide form of NiMo/ Al2O3 presented higher activity than Pd/Al2O3 in the simultaneous direct liquefaction and HDO of lignocellulosic biomass. However, Mo-
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Corresponding authors at: School of Chemical Engineering, Xiangtan University, Xiangtan, Hunan, 411105, PR China. E-mail addresses: [email protected], [email protected] (W. Wang), [email protected] (Y. Yang).
https://doi.org/10.1016/j.mcat.2019.110537 Received 3 July 2019; Received in revised form 24 July 2019; Accepted 26 July 2019 Available online 06 August 2019 2468-8231/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1b and 2θ = 28°, 32°, 36°, 39°, 46° and 55° in Fig. 1c corresponded to NiS2 [34] and CoS2 [35], respectively, and their intensity gradually increased with the promoter content. These demonstrated a co-existence of NiS2 or CoS2 and MoS2 in NiS2-MoS2 or CoS2-MoS2 catalysts. In addition, no characteristic peak to CoS2 in Co-Mo-240-0.1 might be caused by the small CoS2 particles due to its high dispersion in CoS2MoS2 catalyst. The size and morphology of MoS2, NiS2-MoS2 and CoS2-MoS2 were examined by SEM and TEM techniques. Fig. 2 shows the SEM images of Mo-240, Ni-Mo-240-0.3 and Co-Mo-240-0.2. Mo-240 was composed of curly and interlaced nanosheets with an average wall thickness of about 10 nm, presenting its ultrathin nature, which was resulted from the oriented growth of the produced MoS2 nanoparticles. After adding NiS2 and CoS2 into MoS2, as shown in Fig. 2(b) and (c), the characteristic structure of nanosheet was not changed. Besides, some small particles were observed, which could be ascribed to NiS2 or CoS2. Fig. 3 shows the TEM and HRTEM images of Mo-240, Ni-Mo-2400.3 and Co-Mo-240-0.2. The lattice fringes had an average number of 2–6 layers with an interlayer spacing of about 0.63 nm (Fig. 3b), corresponding to the characteristic of (002) basal plane of the hexagonal MoS2 structure. When NiS2 or CoS2 was added into MoS2, their morphologies did not change obviously. Fig. 3(c) presents some cubelike particles with a size of about 100 nm, attributing to cubic NiS2 [36]. In addition, other two fringes with a interlayer spacing of about 0.26 nm and 0.25 nm, covering on the MoS2 slabs, were observed in Fig. 3(d) and Fig. 3(f), corresponding to (200) plane of the cubic NiS2 nanocrystals and (210) plane of cubic CoS2 pyramids [37,38], respectively. These demonstrated that Ni or Co did not incorporate into MoS2 slab to form Co−Mo−S analogue phases, which also gave an evidence to support the co-existence of NiS2/CoS2 and MoS2 in NiS2-MoS2 and CoS2-MoS2. The MoS2 structure in Mo-240, Ni-Mo-240-0.3 and Co-Mo-240-0.2 was further studied by Raman spectroscopy. As shown in Fig. 4, two obvious peaks appeared at 378 and 403 cm−1 in the Raman spectra of Mo-240, attributing to the in-plane E12g mode and the out-of-plane A1g mode of a hexagonal MoS2 crystal [39], respectively. The frequency (△) of E12g and A1g for Mo-240 was about 5 cm−1, indicating the formation of MoS2 nanosheets with few layers [40], which was in accordance with the HRTEM observations. In comparison with A1g peak, E12g peak presented a lower intensity, suggesting a basal−edge−rich feature and an unperfected crystal structure of MoS2 in Mo-240 [41]. After doping NiS2 or CoS2, both the frequencies of E12g and A1g peaks were almost unchanged, demonstrating that Ni-Mo-240-0.3 and Co-Mo240-0.2 had the same MoS2 structure as that of Mo-240. These suggested no incorporation of NiS2/CoS2 into MoS2 structure. The chemical compositions and element valence states in MoS2, NiS2-MoS2 and CoS2-MoS2 were analysed by XPS characterization. As shown in Fig. S1, no peak was observed in the XP spectrum of Si 2p, suggesting that no Si remained in the prepared MoS2. The other XP spectrums of Mo 3d, S 2p, Ni 2p and Co 2p are shown in Fig. 5. Three peaks were observed in the high resolution XP spectrum of Mo 3d (Fig. 5a), where the one at 226.6 eV corresponds to S 2 s of S2− in MoS2, and the other two main peaks at 229.5 eV and 232.6 eV were assigned to Mo4+ 3d5/2 and Mo4+ 3d3/2 of MoS2, respectively. Fig. 5b presents two peaks at 162.3 eV and 163.5 eV, attributing to the S 2p3/2 and S 2p1/2, respectively. Notably, the binding energy (△) of these two peaks was 1.2 eV, indicating a typical characteristic of S2 species. Fig. 5c shows two dominant peaks at 854.5 eV and 779.4 eV in Ni 2p and Co 2p spectrums, which were assigned to Ni 2p3/2 in NiS2 and Co 2p3/2 in CoS2, respectively. In comparison with the reported binding energy of Ni 2p in Ni−Mo−S phase (853.5 eV) and Co 2p3/2 in Co−Mo−S phase (778.6 eV) [42,43], it can be concluded that Ni and Co were not incorporated in MoS2 to form Ni(Co)−Mo−S phase in Ni-Mo-240-0.3 and Co-Mo-240-0.2. In addition, if Ni or Co was incorporated into the MoS2 edges, the S/Mo atomic ratio would not increase. However, the S/ Mo atomic ratios of Ni-Mo-240-0.3 and Co-Mo-240-0.2 were calculated
found for the chemically synthesized MoS2 [19,25]. Polyoxometalate, a solid acid, has been widely applied into the preparation of various heterogeneous catalysts [26,27], which could promote the dispersion of activated components and modify the surface structure. Therefore, in this study, we tried to hydrothermally synthesize MoS2 by utilizing silicomolybdic acid to substitute liquid acid and expected to obtain hydrophobic MoS2, and then used it as a substrate to further prepare NiS2MoS2 and CoS2-MoS2 via a two-step hydrothermal method. The activity and stability of these MoS2, NiS2-MoS2 and CoS2-MoS2 catalysts were tested in the HDO of 4-ethylphenol and 4-propylguaiacol as the representative compounds of lignin-derived phenols. 2. Experimental Hydrophobic sulphides were synthesized according to the similar hydrothermal method that reported before [28]. In a typical procedure, ammonium heptamolybdate (1.60 g), thiourea (2.07 g) and silicomolybdic acid (3 mmol) were dissolved in 100 mL water and then transferred to a 200 mL Teflon-lined stainless-steel autoclave and then treated at 240 °C for 12 h. Then the reactor was cooled and nickel nitrate or cobalt nitrate solution was added into it. The reactor was sealed and heated to 240 °C for 12 h again. The composition of the samples was adjusted by changing the initial Ni(Co)/Mo molar ratio. Finally, the black precipitation was collected and washed with water and absolute ethanol, and dried under vacuum at 60 °C for 5 h and stored in nitrogen atmosphere. The resultant catalysts were denoted as Ni-Mo-T-X or CoMo-T-X, where T and X represented the preparation temperature (°C) and the molar ratio of Ni(Co)/Mo in the initial solution, respectively. In addition, Mo-T catalysts were also prepared under different temperatures. These catalysts were characterized by X-ray Diffraction (XRD), Nitrogen Physisorption, Raman Spectra, Scanning Electronic Microscopy (SEM), High Resolution Transmission Electron Microscopy (HRTEM) technologies and Water Contact Angles. Their HDO activities were tested in a batch reactor using 4-ethylphenol as a representative compound of lignin-derived phenols. The detailed characterization procedure, activity tests and the definitions of conversion, selectivity and deoxygenation degree for each experiment were shown in the Supporting Information. 3. Results and discussion 3.1. Characterizations of MoS2, NiS2-MoS2 and CoS2-MoS2 The structures of MoS2, NiS2-MoS2 and CoS2-MoS2 were firstly assessed using XRD characterization. Fig. 1a presents the effect of preparation temperature on the crystallinity of MoS2. Mo-200 exhibited several broad diffraction peaks, indicating an amorphous structure of MoS2. In addition, a broad and weak diffraction peak at 2θ = 9° was observed, corresponding to the diffraction of adjacent few-layered MoS2 sheets [29]. When the preparation temperature was increased to 220 °C and 240 °C, four obvious diffraction peaks at 2θ = 14°, 33°, 36° and 59° appeared in the XRD patterns of Mo-220 and Mo-240, attributing to the reflection of (002), (100), (103) and (110) planes of hexagonal MoS2 [30,31], respectively. N2 physical adsorption results showed that Mo-200, Mo-220 and Mo-240 had specific surface area of 69 m2/g, 83 m2/g and 99 m2/g, respectively. These indicated that high preparation temperature was beneficial to increase the crystallinity and the surface area of MoS2. Co(Ni)−Mo−S phase tends to form in the one-step preparation of bi-component sulphides [32,33]. Consequently, NiSx or CoSx phases would not be observed under the low Ni(Co)/Mo molar ratio. However, in this study, MoS2 was prepared in the first step and then acted as a support to load NiS2 or CoS2 to obtain bi-component Mo-based sulphide catalysts. This preparation process indicated that Co−Mo−S analogue phases were impossible to form. The XRD results (Fig. 1) supported this conclusion. The diffraction peaks at 2θ = 27°, 31°, 35°, 38°, 45°, 53° in 2
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Fig. 1. XRD patterns of (a) MoS2, (b) NiS2-MoS2 and (c) CoS2-MoS2.
Fig. 2. SEM images of (a) Mo-240, (b) Ni-Mo-240-0.3 and (c) Co-Mo-240-0.2.
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Fig. 3. TEM and HRTEM images of (a, b) Mo-240, (c, d) Ni-Mo-240-0.3 and (e, f) Co-Mo-240-0.2.
be resulted from the coverage of non-hydrophobic NiS2 or CoS2 on the surface of MoS2. These suggested that this method could obtain MoS2 based catalysts with high hydrophobicity. 3.2. HDO activity of hydrophobic MoS2, NiS2-MoS2 and CoS2-MoS2 3.2.1. Effect of preparation temperature on the HDO activity of hydrophobic MoS2 Fig.7 shows the catalytic activity of hydrophobic MoS2 catalysts in the HDO of 4-ethylphenol at 300 °C. The products were ethylbenzene, ethylcyclohexane and 4-ethylcyclohexene and no oxygen-containing compound was detected, indicating the high deoxygenation activity. After reaction at 300 °C for 6 h, 4-ethylphenol conversion on Mo-200, Mo-220 and Mo-240 was 56.3%, 69.6% and 78.2%, respectively. Associated with the characterization results, the change on the conversion might be caused by the crystallinity and specific surface area. Recently, Yoosuk et al. [46] had concluded that amorphous MoS2 with more unsaturated active sites exhibited much higher HDO activity than crystalline MoS2. However, all the prepared MoS2 catalysts had low crystallinity. In addition, the catalyst, possessing larger specific surface area, will expose more active sites for the reaction. Taken these together, the surface area was a considerable factor for the HDO activity of MoS2, which was consistent with our previous study [47]. Because of the large specific surface area and high activity, Mo-240 was selected as a support to further study the promoting effect of NiS2 and CoS2 on the HDO activity of MoS2.
Fig. 4. Raman spectra of Mo-240, Ni-Mo-240-0.3 and Co-Mo-240-0.2.
to be 2.21 and 2.17, respectively, both of which were larger than that of Mo-240 (1.84). This S/Mo atomic ratio increase was caused by the introduction of extra sulphide (NiS2 or CoS2) into Mo-240, indicating the co-existence of two separate phases in NiS2-MoS2 or CoS2-MoS2 catalysts. Fig. 6 shows the hydrophobicity performance of common MoS2 (CMoS2) prepared via a normal hydrothermal method, MoS2, NiS2-MoS2 and CoS2-MoS2 catalysts. At first, these four samples were added into the mixture of dodecane and water to demonstrate the hydrophobic and hydrophilic performance. Because of the large density (4.80 g/cm3) and the low hydrophobicity, C-MoS2 powder could not disperse in dodecane and deposited in the bottom. MoS2, NiS2-MoS2 and CoS2-MoS2 in this study also had a larger density than water, but they dispersed in dodecane rather than in water, presenting their hydrophobicity. In addition, the air-water contact angle was further used to evaluate the hydrophobicity. As shown in Fig. 6, MoS2 had an air-water contact angle of 147.5°, suggesting its excellent hydrophobicity, which was attributed to the modification of surface morphology rather than chemical composition according to the XPS results [44,45]. After adding NiS2 or CoS2 into MoS2, the contact angle of NiS2-MoS2 and CoS2-MoS2 catalysts decreased slightly, being 143.9° and 140.6°, respectively, which might
3.2.2. HDO of 4-ethylphenol on MoS2: the promoter of NiS2 Table 1 shows the effect of Ni/Mo molar ratio in the NiS2-MoS2 catalysts on the activity in HDO of 4-ethylphenol. In comparison with Mo-240, after reaction at 275 °C for 6 h, the conversion and ethylcyclohexane selectivity on Ni-Mo-240-0.1 was increased by 19.5% and 22.8%, respectively, suggesting that the addition of NiS2 enhanced both the deoxygenation degree and hydrogenation−dehydration route, which was consistent with the results in the previous literatures [33,47]. Because of the co-existence of two separate phases, the HDO mechanism on NiS2-MoS2 catalysts could be well explained with the remote control model [48]. Hydrogen was firstly adsorbed on donor phase (NiS2) and then translated into spillover hydrogen, and finally 4
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Fig. 5. XP spectrums of (a) Mo 3d, (b) S 2p, (c) Ni 2p and Co 2p levels of Mo-240, Ni-Mo-240-0.3 and Co-Mo-240-0.2 catalysts.
Fig. 6. Photographs of (a) common MoS2, (b) Mo-240, (c) Ni-Mo-240-0.3 and (d) Co-Mo-240-0.2 distribution in dodecane and water and their air-water contact angles.
migrated to acceptor phase (MoS2) for the hydrotreatment reactions. Consquently, NiS2 supplied extra spillover hydrogen for benzene ring saturation and HDO reaction, which enhanced ethylcyclohexane selectivity and deoxygenation degree. Table 1 shows that the deoxygenation degree is increased in the order of Ni-Mo-240-0.1 (61.0%) < Ni-Mo-240-0.2 (70.9%) < Ni-Mo-240-0.5 (77.2%) < NiMo-240-0.3 (79.7%), indicating that NiS2-MoS2 with a Ni/Mo molar ratio of 0.3 has the highest activity. This was attributed to the synergism between NiS2 and MoS2. When NiS2 exceeded a certain value, some MoS2 active sites were covered, lowering the HDO activity. Previous investigations had also reported this similar trend [33,49].
Fig. 7. HDO of 4-ethylphenol on Mo-200, Mo-220 and Mo-240 at 300 °C for 6 h. Reaction conditions: 2.0 g evaluate, 0.03 g catalyst and 15 g dodecane.
enhanced the HDO activity but also changed the product distribution. When the reaction temperature dropped to 225 °C, 4-ethylphenol conversion on Co-Mo-240-0.05 was 24.7% after reaction for 3 h, but ethylbenzene selectivity increased to 96.3%, indicating its high direct deoxygenation activity. According to the remote control model [51], due to the change of 4-ethylphenol adsorption mode at the presence of CoS2 [50], spillover hydrogen created on CoS2 could make the direct deoxygenation going on smoothly, but it was not enough to saturate the benzene ring, leading to high ethylbenzene selectivity. With the increase of CoS2 content, the conversion increased firstly and then
3.2.3. HDO of 4-ethylphenol on MoS2: the promoter of CoS2 Table 2 shows the HDO of 4-ethylphenol on CoS2-MoS2 catalysts with different Co/Mo molar ratios. The presence of CoS2 not only 5
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detected, suggesting that the scission of Caryl-OR was prior to that of Caryl-OH, which was well consistent with previous result [52]. After adding promoters Ni and Co into hydrophobic MoS2, the HDO activity was greatly enhanced, especially for Co. When the reaction temperature dropped to 250 °C, 4-propylphenol selectivity on Co-Mo-240-0.2 was only 2.3%. These results demonstrated that the prepared hydrophobic sulphides also had good activity for the HDO of 4-propylguaiacol.
Table 1 HDO of 4-ethylphenol on NiS2-MoS2 catalysts at 275 °C for 6 h. Catalyst
Mo-240
Ni-Mo240-0.1
Ni-Mo240-0.2
Ni-Mo240-0.3
Ni-Mo240-0.5
Conversion Product distribution Ethylcyclohexane 4-Ethylcyclohexene Ethylbenzene Deoxygenation degree
44.4
63.9
73.2
81.5
79.1
8.8 3.0 88.2 36.9
31.6 0.7 67.7 61.0
46.4 0.4 53.2 70.9
49.7 0.3 50.0 79.7
48.2 0.3 51.5 77.2
3.2.5. Recycling test of hydrophobic MoS2, NiS2-MoS2 and CoS2-MoS2 Because the presence of water has a severely negative effect on the stability of MoS2 catalysts during the HDO processes [53], the hydrophobicity would be of importance. Consequently, the stability of hydrophobic MoS2, NiS2-MoS2 and CoS2-MoS2 was tested in the HDO of 4ethylphenol under different reaction conditions, as shown in Fig. 8. After 3 cycles at 300 °C, 4-ethylphenol conversion on Mo-240 only decreased by 1.7%, suggesting almost no deactivation, but ethylbenzene selectivity increased from 92.7% to 97.5%, which was attributed to the modification of MoS2 active sites during the HDO reaction [53]. Our previous study had found that the modification of the produced water on the sulphide structure during the cyclic HDO reaction caused to the increase of conversion [23]. However, Fig. 8(a) showed the conversion almost unchanged after 3 cycles. These indicated that negative effect of water was inhibited, which was attributed to that the hydrophobicity prevented water from contacting with catalyst surface. For Ni-Mo-240-0.3, as shown in Fig. 8b, the conversion changed very little but ethylbenzene selectivity increased by 18.3% after 3 cycles at 275 °C. The increase of ethylbenzene selectivity might also be resulted from the modifications of MoS2 and NiS2. NiS2, acting as the donor phase, supplied low spillover hydrogen for the HDO reaction after modification. As a result, the saturation of benzene ring was inhibited and then the direct deoxygenation was enhanced. If the promoter Ni was incorporated into MoS2 to form the so-called Ni−Mo−S phase, the ethylbenzene selectivity would be not changed so large. These also indirectly supported the co-existence of separated NiS2 and MoS2 phases in Ni-Mo-240-0.3. For Co-Mo-240-0.2, after 3 cycles at 225 °C, the conversion and ethylbenzene selectivity were still up to 99.5% and 99.7% (Fig. 8c), respectively, presenting its high stability. After HDO reaction, the water contact angles of these spent hydrophobic MoS2, NiS2-MoS2 and CoS2-MoS2 catalysts were measured again, which were 147.1°, 144.5° and 140.5°, respectively, as shown in Fig. S2, demonstrating that their hydrophobicity properties were not changed after HDO reaction at high temperature. It was generally accepted that the deactivation of sulphide catalyst during the hydrotreating reactions was mainly caused by coke deposition and sulfur-loss of active phases [21,54]. However, the HDO reaction temperature dropped to 225 °C in this study, the coke deposition could be inhibited efficiently. In addition, due to the hydrophobicity, the produced water was forced to desorb immediately and then its active structure could be protected.
Reaction conditions: 2.0 g 4-ethylphenol, 0.03 g catalyst and 15 g dodecane. Table 2 HDO of 4-ethylphenol on CoS2-MoS2 catalysts at 225 °C for 3 h. Catalyst
Mo-240
Co-Mo2400.05
Co-Mo2400.1
Co-Mo2400.2
Co-Mo2400.3
Co-Mo2400.5
Conversion Product distribution Ethylcyclohexane 4-Ethylcyclohexene Ethylbenzene Deoxygenation degree
8.5
24.7
79.0
99.9
87.5
70.2
7.9 2.7 89.4 6.5
2.8 0.9 96.3 22.2
0.3 0.3 99.6 76.6
0.3 0.1 99.6 99.9
0.4 0.4 99.2 85.9
0.4 0.5 99.1 67.2
Reaction conditions: 5.4 g 4-ethylphenol, 0.03 g catalyst and 15 g dodecane.
lowered. Co-Mo-240-0.2 presented the highest HDO activity, where the conversion and ethylbenzene selectivity reached up to 99.9% and 99.6%, respectively. The high HDO activity of Co-Mo-240-0.2 was also attributed to the synergism between CoS2 and MoS2. In comparison with the HDO of 4-ethylphenol on NiS2-MoS2, although the weight of reactant was 2.7-fold more, and the reaction temperature decreased by 50 °C, and the reaction time reduced by half, but the deoxygenation degree on CoS2-MoS2 was still higher than 20%, demonstrating the excellent HDO activity of CoS2-MoS2 catalyst. Besides, high surface area is beneficial to enhance the catalyst activity. However, Co-Mo-240-0.2 (93 m2/g) has a lower surface area than that of CoMo/Al2O3 supported catalyst (255 m2/g) [50], but showed higher HDO activity, which might be caused by the larger contact surface area between CoS2 and MoS2 phases in CoS2-MoS2 catalyst [51]. Interestingly, CoS2-MoS2 had high direct deoxygenation activity but low hydrogenation activity, leading to a low cycloalkanes selectivity in the HDO of lignin-derived phenols, even at high hydrogen pressure (4.0 MPa). This was desirable in industry and presented a great potential for application.
3.2.4. HDO of 4-propylguaiacol on MoS2, NiS2-MoS2 and CoS2-MoS2 4-Propylguaiacol, a typical monomer in lignin-biomass, was also selected to test the HDO activity of hydrophobic MoS2, NiS2-MoS2 and CoS2-MoS2. The conversion and product distribution are summarized in Table 3. After reaction at 300 °C for 6 h, 4-propylguaiacol conversion on Mo-240 reached to 99.2%, but 4-propylphenol selectivity was still high to 34.7%. During this reaction, however, 3-propylanisole was not
4. Conclusions Hydrophobic MoS2 was prepared by a hydrothermal method via introducing silicomolybdic acid and then used as support to further prepare NiS2-MoS2 and CoS2-MoS2 catalysts. The air-water contact angle of MoS2 was 147.5°, exhibiting its strong hydrophobicity. After depositing NiS2/CoS2, the hydrophobicity changed slightly, and NiS2/ CoS2 and MoS2 were co-existed in NiS2-MoS2 or CoS2-MoS2. During the HDO reactions, the activity increased with the order of MoS2 < NiS2MoS2 < CoS2-MoS2. 4-Ethylphenol conversion on CoS2-MoS2 was 99.9% with an ethylbenzene selectivity of 99.6% after reaction at 225 °C for 3 h, presenting an excellent HDO activity, which was mainly attributed to the synergism between CoS2 and MoS2 phases. Notably, the hydrophobicity forced the produced water to desorb immediately and inhibited the adsorption of water, and then the active structure was protected, presenting a high stability. After 3 cycles at 225 °C, the
Table 3 HDO of 4-propylguaiacol on MoS2, NiS2-MoS2 and CoS2-MoS2 catalysts for 6 h. Catalyst
Mo-240
Ni-Mo-240-0.3
Co-Mo-240-0.2
Temperature (°C) Conversion (%) Product distribution Propylcyclohexane (%) 4-Propylcyclohexene (%) Propylbenzene (%) 4-Propylphenol (%)
300 99.2
275 99.7
250 100
5.0 2.9 57.4 34.7
31.6 0.6 47.4 20.4
13.4 0.6 83.7 2.3
Reaction conditions: 1.84 g 4-propylguaiacol, 0.06 g catalyst and 15 g dodecane. 6
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Fig. 8. Stability test of (a) Mo-240 (Reaction conditions: 2.0 g 4-ethylphenol, 0.03 g catalyst, 15 g dodecane, at 300 °C for 6 h), (b) Ni-Mo-240-0.3 (Reaction conditions: 2.0 g 4-ethylphenol, 0.03 g catalyst and 15 g dodecane, at 275 °C for 6 h) and (c) Co-Mo-240-0.2 (Reaction conditions: 5.4 g 4-ethylphenol, 0.03 g catalyst and 15 g dodecane, at 225 °C for 3 h) in the HDO of 4-ethylphenol.
conversion and ethylbenzene selectivity in the HDO of 4-ethylphenol on CoS2-MoS2 were still remained to 99.5% and 99.7%, respectively.
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