Article pubs.acs.org/IECR
Influence of Surfactants on the Synthesis of MoS2 Catalysts and Their Activities in the Hydrodeoxygenation of 4‑Methylphenol Weiyan Wang,†,‡ Kun Zhang,† Zhiqiang Qiao,† Lu Li,† Pengli Liu,† and Yunquan Yang*,†,‡ †
School of Chemical Engineering, Xiangtan University, Xiangtan, Hunan 411105, People’s Republic of China National & Local United Engineering Research Centre for Chemical Process Simulation and Intensification, Xiangtan University, Xiangtan 411105, People’s Republic of China
‡
ABSTRACT: MoS2 catalysts were synthesized by hydrothermal method using ammonium heptamolybdate and thiocarbamide as materials, focusing on the effects of the addition of surfactants (such as hexadecyltrimethylammonium bromide, polyvinylpyrrolidone, and sodium lauryl benzenesulfate) during the MoS2 catalyst preparation on their structure and activity in the hydrodeoxygenation (HDO) of 4-methylphenol. The addition of surfactant not only increased the surface area of MoS2, but also changed its microstructure in the stack layer. Compared with MoS prepared without any surfactant, MoSDB, Mo SCT, and MoSPV exhibited higher activity in the HDO of 4-methylphenol. According to the characterization and catalytic activity results, the structure−activity relation was revealed, and the HDO mechanism of 4-methylphenol on these catalysts was well illustrated by the Rim−Edge model. The HDO of 4-methylphenol on these MoS2 catalysts proceeded with two parallel routes (HYD and DDO), and their selectivity depended on its layer number in the stacks.
1. INTRODUCTION Continuously decreasing reserves of fossil fuels, large emissions of greenhouse gases from the utilization of fossil fuels, and rapid growth of global energy consumption have led to an increasing interest in the conversion of nonfood lignocellulosic biomass feedstocks into liquid products by fast pyrolysis technology,1−3 referred to as bio-oil. However, this bio-oil contains very important amounts of oxygenated compounds (up to 45 wt % O), which attributes to its poor heating value, high viscosity, thermal and chemical instability, as well as its tendency toward polymerization during storage and transportation.4 Aiming to improve the physical and chemical properties of bio-oil, a hydrodeoxygenation process (HDO) is required to efficiently decrease its oxygen content and meet demand as a supplement or replacement for transportation fuel. Conventional catalysts used in the HDO process are molybdenum sulfide (MoS 2 ) based catalysts originally developed for hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) reactions in petroleum refining processes. These sulfide catalysts exhibit high activity for the HDO of model compounds such as phenol and guaiacol.5−13 Presently, there are many ways to synthesize sulfide catalysts, including an in situ sulfidation method,14 a hydrothermal method,6 a thermal decomposition method,15 and a mechanical activation method.5 However, the preparation method plays an important role in the catalytic performance. For example, Smith et al.12 compared the activity of unsupported MoS2 catalysts prepared by different methods based on a rate constant from the kinetic study in the HDO of phenols, and found that exfoliated MoS2 showed the highest HDO activity. Yoosuk et al.6 reported that MoS2 catalyst prepared from ammonium tetrathiomolybdate by the hydrothermal method showed a 2.4-fold higher conversion than commercial crystalline MoS2 in the HDO of phenol, and the product distribution strongly depended on the structure of catalyst. © 2014 American Chemical Society
In addition, the catalyst with a large specific surface area and big pore diameter can enhance its catalytic activity by putting the active sites to maximum dispersion and minimizing the mass transfer influence. Alonso et al.16 have improved the synthesis method to prepare MoS2 catalysts with high surface area and found that their catalytic properties were improved in the HDS of dibenzothiophene. These indicated that increasing the surface area of MoS2 was an alternative way for improving its catalytic activity. Adding surfactant during the preparation process can adjust the morphology and structure property of materials. Afanasiev et al.17 found that the addition of surfactant to the reaction mixtures could obtain single layer and short fringes of MoS2 with a large specific surface area. Trikalitis et al.18 had prepared mesostructured molybdenum based sulfides in the presence of alkyl-pyridinium surfactant acting as a template. To our knowledge, however, there is very little literature reporting the role of the surfactant in the activity of an MoS2 catalyst in the HDO reaction. Therefore, in this study, we prepared an MoS2 catalyst by a hydrothermal method and selected hexadecyltrimethylammonium bromide (CTAB), polyvinylpyrrolidone (PVP), and sodium lauryl benzenesulfate (DBS) as representative of cationic surfactants, nonionic surfactants, and anionic surfactants, respectively, to study the effect of surfactants for the catalytic activity of MoS2 using the HDO of 4-methylphenol as a probe in a sealed autoclave.
2. EXPERIMENTAL SECTION 2.1. Preparation of Catalysts. All solvents and reagents were obtained from Sinopharm Chemical Reagent Co., Ltd. in high purity (≥99%) and used without further purification. Received: Revised: Accepted: Published: 10301
February 26, 2014 May 4, 2014 May 27, 2014 May 27, 2014 dx.doi.org/10.1021/ie500830f | Ind. Eng. Chem. Res. 2014, 53, 10301−10309
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Unsupported MoS2 catalysts were synthesized by a hydrothermal method. The catalyst synthesis was carried out in quartz reactor with a volume of 300 mL. Ammonium heptamolybdate (2.3 g) and thiourea (3.0 g) were dissolved in 250 mL ultrapure water. Different surfactants, such as hexadecyltrimethylammonium bromide (CTAB), polyvinylpyrrolidone (PVP), or sodium lauryl benzenesulfate (DBS) (0.3 g), were added into the above solution. Then, this mixed solution was added into the sealed reactor and heated to 200 °C. After 24 h, the resulting catalysts were separated and washed with absolute ethanol several times to remove the residual water and water-soluble impurities. FT-IR characterization confirmed that the surfactant was washed away completely. Finally, the resulting product was dried under vacuum at 50 °C for 8 h and stored in nitrogen environment. The prepared catalysts were denoted as MoSX, where X represented the surfactant. 2.2. Catalyst Characterization. X-ray diffraction (XRD) measurements were carried on a D/max2550 18KW Rotating anode X-ray Diffractometer with monochromatic Cu Kα radiation (λ = 1.5418 Å) 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. 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. The SEM images of the catalysts were obtained on a JEOL JSM-6360 electron microscopy. The morphology was determined by high resolution transmission electron microscopy (HRTEM) on a JEOL JEM-2100 transmission electron microscope with a lattice resolution of 0.19 nm and an accelerating voltage of 200 kV. The samples for the HRTEM study were prepared by the ultrasonic dispersing in ethanol and consequent deposition of the suspension upon a “holey” carbon film supported on a copper grid. The samples were kept under inert atmosphere until the last process. 2.3. Catalyst Activity Measurement. The HDO activity tests were carried out in a 300 mL sealed autoclave. The prepared catalyst without any further treatment (0.60 g), 4methylphenol (13.50 g), and dodecane (86.20 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 300 °C, then pressurized with hydrogen to 4.0 MPa and the stirring speed was adjusted 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, deoxygenation rate, and DDO/HYD for each experiment were calculated as follows:
deox. rate (DR, wt%) ⎛ O content final ⎞⎟ = ⎜1 − × 100% ⎝ total O content init. ⎠
where “mol resid.” is the moles of residual reactant”; “mol init.” is the moles of the initial reactant; “mol prod. (A)” is the moles of product (A); “mol. reacted react.” is the moles of reacted reactant; “total select. MCH and 4-MCH” is the total selectivity of methylcyclohexane and 4-methylcyclohexene; “select. toluene” is the selectivity of toluene; “deox. rate” is the deoxygenation rate; “ O content final” is the oxygen content in the final organic compounds; and “total O content init.” is the total oxygen content in the initial material.
3. RESULTS AND DISCUSSION 3.1. Characterization of the Unsupported Catalysts. The XRD patterns of MoS, MoSDB, MoSCT, and MoSPV are shown in Figure 1. All the prepared MoS2
Figure 1. XRD patterns of MoS, MoSDB, MoSCT, and MoSPV catalysts.
catalysts display several broad diffraction peaks at 2θ = 14°, 33°, 39°, and 59°, corresponding to the (0 0 2), (1 0 0), (1 0 3), and (1 1 0) basal planes of crystalline MoS2,19−21 respectively, which indicates a poorly crystallized MoS2 structure. The presence of the (0 0 2) peak at 2θ = 14° is representative of the multistacked slabs along the c direction, while the presence of the (1 1 0) peak at 2θ = 59° is representative of the layer of the slabs.15,19 Compared with the XRD pattern of MoS, the peak intensity at 2θ = 14° for MoSDB, MoSCT, and MoSPV decreases obviously, demonstrating an increase in the stacking slabs and a decrease in the catalyst crystallite size.19 We calculated the D (1 1 0) from the XRD data by the Scherer formula, as shown in Table 1. The D (1 1 0) gradually decreases with the order of MoSPV (3.48 nm) > MoS (3.35 nm) > MoSCT (3.23 nm) > MoSDB (2.85 Table 1. Physical Properties of MoS2 Unsupported Catalysts
⎛ mol resid. ⎞⎟ conversion (mol%) = ⎜1 − × 100% ⎝ mol init. ⎠
selectivity (A, mol%) =
HYD/DDO =
catalysts MoS Mo SDB Mo SCT Mo SPV
mol prod. (A) × 100% mol reacted react.
total select. MCH and 4‐MCH select. toluene 10302
surface area (m2/g)
pore volume (cm3/g)
pore size (nm)
D (1 1 0) (nm)
number of layers
79.8 145.3
0.38 0.79
3.0, 12.0 2.4, 10.3
3.35 2.85
4.6 3.7
157.6
1.07
2.4, 9.4
3.23
4.3
130.5
0.55
2.4, 12.2
3.48
4.9
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Figure 2. Adsorption−desorption isotherms and pore size distribution calculated by BJH method for (a) MoS, (b) MoSDB, (c) MoS CT, and (d) MoSPV catalysts.
The nitrogen adsorption−desorption isotherms and pore size distribution are shown in Figure 2. According to the IUPAC classification,26 MoS (without addition of surfactant) exhibits a type IV isotherms with a H3 hysteresis loop characteristic of mesoporous materials. The BJH pore size distribution reveals that MoS had a multimodal pore size distribution structure with a maximum around 2.4 nm and with significant contributions of larger pore diameters. When some surfactants were added, the hysteresis loops of MoSDB, MoS CT, and MoSPV show characteristics between type H1 and type H3. Compared with MoS catalyst, the pore size distribution profiles of MoSDB, MoSCT, and Mo SPV changed very little. This suggests that adding CTAB, DBS, and PVP during the preparation of MoS2 cannot modify the average pore size obviously. As shown in Figure 3, MoS prepared without any surfactant displays irregular cube particle in shape with a serious agglomeration. The particle size is hard to determine. When some surfactant was added, the agglomeration between particles was effectively inhibited due to the good dispersing effect of the surfactant, but it still exhibits that a small portion of particles get together, which was inevitable because nanoparticles tended to get together to reduce their high surface free energy. With the addition of DBS, CTAB, or DBS, the morphology of MoS2 is changed to a nanoparticle. The dominant particles sizes of MoSDB, MoSCT, or MoSPV are about 100, 70, and 120 nm, respectively. The change of particle size is in good agreement with the results of the corresponding surface area. As shown in Table 1, the surface area of MoS (79.8 m2/g) is much smaller than that of
nm), suggesting an increase in the slab layers with the addition of PVP and a reduction in the slab layers with the addition of CTAB or DBS. These results suggest that the microstructure on the stacking slab and slab layers are changed by the addition of surfactant, which will be verified with the TEM characterization. The specific surface areas and total pore volumes of the different MoS2 catalysts are summarized in Table 1. The MoS2 catalyst prepared by a one step hydrothermal method without adding any surfactant exhibits a surface of 79.8 m2/g, which is much smaller than that found in previous studies.6,22 This difference in the specific surface area mainly resulted from the precursor and synthesis conditions. As reported by Yoosuk et al.,23 with the dispersion of organic solvent, Mo sulfide, prepared from aqueous ammonium tetrathiomolybdate solution under conditions of 2.8 MPa hydrogen pressure and 350 °C, has a large surface area of 368 m2/g. The addition of the surfactant during their preparation does significantly influence the surface area values. After adding surfactant, both the surface area and pore volume are increased. The surface area of Mo SCT (157.6 m2/g) prepared by adding CTAB is almost 2.0fold larger than that of MoS (79.8 m2/g). The pore volume of MoSCT, MoSDB, and MoSPV is 1.07, 0.79, and 0.55 cm3/g, being 2.8-fold, 2.1-fold, and 1.4-fold larger than that of MoS (0.38 m2/g), respectively. These results are well in agreement with the previous studies indicating that the surfactant is beneficial to increase the surface area and pore volume of materials.24,25 Among these three surfactants, it can be see that the CTAB is helpful for the increase of the surface area and pore volume of MoS2. 10303
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Figure 3. SEM images of MoS, MoSDB, MoSCT, and MoSPV catalysts.
MoSDB (145.3 m2/g), MoSCT (157.6 m2/g), or MoSPV (130.5 m2/g). Figure 4 presents the HRTEM images of MoS, MoS DB, MoSCT, and MoSPV. It can be clearly seen in the MoS image that there are some groups of parallel dark thread-like fringes with a spacing of about 0.65 nm, characterizing the (0 0 2) basal planes of the MoS 2 structure.27,28 The average number of MoS2 stacking layers is calculated by examining more than 200 slabs on many HRTEM pictures taken from different parts of the same sample dispersed on the microscope grid and the statistical results are shown in Table 1. The HRTEM image clearly shows that the MoS has an average number of layers in the stacks of 4.6. With the addition of surfactant during the preparation process, the stack layer is changed, especially for DBS. The average stack layer is decreased from 4.6 for MoS to 3.7 for MoSDB with the addition of DBS. As shown in Table 1, the average stack layer of MoSCT and MoSPV are 4.3 and 4.9, respectively, which was well consistent with XRD results. Therefore, it can be concluded that adding surfactant during the MoS 2 preparation process also changes its microstructure in the stack layer. 3.2. Catalyst Activity Test. The catalytic performances of these different unsupported MoS2 catalysts were tested using the HDO of 4-methylphenol as a probe reaction. Sulfide catalyst, unlike the noble metal catalysts and Ni(Co) Mo(W)B amorphous catalysts with high hydrogenation activity,29−31 possesses a high deoxygenation activity but a low hydrogenation activity, making the HDO of phenols proceeded with two routes: direct deoxygenation (DDO) yielding benzene
analogue product and hydrogenation−dehydration (HYD) yielding cyclohexene and cyclohexane analogues. As shown in Figure 5(a), the product in the HDO of 4-methylphenol on MoS at 275 °C were toluene, 4-methylcyclohexene, and methylcyclohexane, and no oxygen-containing compound was detected, which indicates that MoS has a high deoxygenation activity under the studied conditions. The concentration profiles as a function of time show that 4-methylphenol concentration decreases gradually, while both the methylcyclohexane concentration and toluene concentration increases, demonstrating that there exist two parallel routes in which toluene and methylcyclohexane appeared as the end products. It has been reported previously that some cyclohexanol or cyclohexanone analogues are intermediates in small amounts, formed via hydrogenation of the corresponding aromatic ring before the CO σ-bond scission,6,23,32 but we did not detect any 4-methylcyclohexanol during the whole experiment. However, the appearance of 4-methylcyclohexene in small amounts suggested that the hydrogenation of an aromatic ring before deoxygenation has occurred. The toluene concentration is 4 times larger than the total concentration of methylcyclohexane and 3-methylcyclohexene, indicating that the DDO route is the primary route for the HDO of 4-methylphenol on MoS catalyst. When some surfactant was added during the preparation of MoS2 catalyst, its HDO activity was improved, and the product distribution in the HDO of 4-methylphenol has also changed, as shown in Figure 4b−d. After reaction at 275 for 6 h, the conversion of 4-methylphenol on MoSDB is 98.1%, which is much higher than that on MoS (70.3%). The main 10304
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Figure 4. HRTEM images of MoS, MoSDB, MoSCT, and MoSPV catalysts.
Figure 5. Changes of 4-methylphenol and products concentration versus reaction time on (a) MoS, (b) MoSDB, (c) MoSCT, and (d) MoSPV catalysts at 275 °C.
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Table 2. HDO of 4-Methylphenol on MoS, MoSDB, MoSCT, and MoSPV Catalysts at Different Temperature for 3 h MoS temperature (°C)
275
conversion (mol %) 34.6 products selectivity (mol %) 4-methylcyclohexanol 0 methylcyclohexane 17.1 3-methylcyclohexene 2.9 toluene 80.0 HYD/DDO 0.25 DR (wt %) 31.3
MoSDB
MoSCT
MoSPV
300
275
300
275
300
275
300
40.4
81.6
98.1
91.2
98.8
71.7
90.6
0 20.5 5.1 74.4 0.34 37.0
0 36.6 6.9 56.5 0.77 79.5
0 35.1 8.3 56.6 0.77 97.8
0 21.5 4.7 73.8 0.35 90.0
0 18.4 5.5 76.1 0.31 98.6
1.1 5.0 3.3 90.6 0.10 67.6
0.3 6.0 1.1 92.9 0.08 88.9
325 100 0.1 2.5 0.7 96.6 0.03 99.9
and Mo or a promoter effect of Ni. Recently, Smith et al.34 had investigated the influence of citric acid on the activity of high surface area MoP for the HDO of 4-methylphenol and reported that the highest conversion was 71% after 5 h reaction at 350 °C, 4.4 MPa of H2. We had prepared NiP catalysts from the mixed precursor of NiCl 2 and NaH 2 PO 2 by thermal decomposition method and found that 4-methylphenol conversion was only 85.0% after 10 h reaction at 350 °C, 4.0 MPa of H2.35 The previous studies had demonstrated that conversion of phenols was increased with the rising of HDO reaction temperature.35,36 However, the conversion of 4methylphenol was as high as 98.8% in this study after 3 h reaction at 300 °C, 4.0 MPa of H2. These suggested that MoS2 catalysts prepared in this study had higher HDO activity than MoS2 with other methods and phosphide catalysts. The effects of reaction temperature in the HDO of 4methylphenol on MoS, MoSDB, MoSCT, and MoSPV catalysts for 3 h are summarized in Table 2. Both the conversion and the deoxygenation rate were increased with the increasing of reaction temperature, meaning the high temperature was beneficial to deoxygenation, which is consistent with the previous studies.12,37,38 Among these four catalysts, MoSCT shows the highest HDO activity. The 4methylphenol conversion at 300 °C for 3 h reaches 98.8% with a deoxygenation rate of 98.6%. For the HYD/DDO on these catalysts, when the temperature was increased from 275 to 300 °C, this value was increased from 0.25 to 0.34 for MoS, but it decreased a little for MoSCT and MoSPV. This implies that the DDO route is enhanced as the reaction temperature increases. The reason for this change at different temperature on the HYD/DDO is not very clear, which might be related to the MoS2 morphology. Figure 6 shows the recycling tests using MoSCT catalyst under condition of 300 °C, 4.0 MPa of H2. The conversion of 4-methylphenol was reduced gradually with recycling number. Although the conversion was only decreased by 7% after five recycles, it still indicated that MoSCT catalyst was deactivated, which mainly resulted from the change of the nature of the active sites. As confirmed by Travert et al.,39 MoS2 was highly sensitive toward water and a large amount of water at the reaction temperature led to the exchange of an important fraction of edge sulfur atoms. Adding a proper amount of sulfiding agent, such H2S or CS2, could maintain the catalytic activity or decrease the deactivation rate. To illustrate the active sites of MoS2 catalyst, Rim−Edge model has been proposed and generally accepted. In this model, the MoS2 catalyst is described as stacks of several discs, the top and bottom discs are defined as rim sites, the discs between the top and bottom discs are defined as edge sites.40
products are 4-methylcyclohexane and toluene. The end product concentration decreases in the order of 4-methylcyclohexane > toluene > 4-methylcyclohexene. The corresponding HYD/DDO on this catalyst is 0.95, being much higher than that on MoS (0.25), which suggests that adding DBS during the preparation of MoS2 is favorable for the HYD route to produce much methylcyclohexane. For the HDO of 4methylphenol on MoSDB catalyst, after reaction at 275 °C for 8 h, the conversion reaches to 99.5%. Compared with the product distribution in the HDO of 4-methylphenol on MoS, the total selectivity of methylcyclohexane and methylcyclohexene on MoSCT increases from 19.1% to 31.1%, while the toluene selectivity decreases from 80.9% to 68.9%, which implies that the DDO pathway is still the dominant route, but the HYD pathway was enhanced by adding CTAB during the preparation of MoS2. For the MoSPV catalyst, oxygen-containing compound 4-methylcyclohexanol was detected with a small amount during the HDO of 4methylphenol. At the end, the selectivity of toluene is high to 87.7%, which indicates that adding PVP during the preparation of MoS2 is beneficial to enhance the DDO route. The above catalytic performance test results obviously showed that adding surfactant (DBS, CTAB, or PVP) during the preparation of MoS2 could improve its HDO activity and changed the product distribution. The previous studies had concluded that the HDO activity of Mo-based sulfide catalyst did not directly depend on the surface area but its morphology in the comparison of MoS2 and Ni/Co promoted MoS2 catalyst.6,12 However, in this study, when adding surfactant during the preparation of MoS2 catalyst, its surface area was increased greatly (see in Table 1) and the agglomeration between particles was effectively inhibited (see in Figure 3), which will expose more available active sites on the catalyst surface for the HDO action. Although the addition of surfactant also changed its microstructure in the stack layer, this change mainly influence the product distribution according to the proposed reaction mechanism on this MoS2 catalyst. Therefore, it is reasonable that the catalyst activity not only depends on its morphology, but also its surface area. Smith et al.12 reported that exfoliated MoS2 had the highest activity in the HDO of phenols, and the maximum conversion of 4-methylphenol was 75% after 7 h reaction at 350 °C, 2.8 MPa of H2. Wang et al.5,33 adopted a mechanical activation method to prepare MoWS and NiMoS. They reported that the conversion of 4-methylphenol on Mo WS and NiMoS was 50% and 77.5% after 5 h reaction at 300 °C, 3.0 MPa of H2, respectively. The HDO activity of MoS prepared by the mechanical activation method would be much lower because there is a synergistic effect between W 10306
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methylcyclohexanol HDO. Then, free hydrogen adsorbed on Mo-based sulfide catalysts quickly attacked the oxygen atom of 4-methylcyclohexanol to form the OH2+ species, and eliminated H2O to generate carbanion, and then accepted the free hydrogen to produce methylcyclohexane or generated a new double bond to form 4-methylcyclohexene that was further hydrogenated to methylcyclohexane.43 Regardless of whether the HDO of 4-methylphenol on these MoS2 catalysts proceeds with the DDO route or HYD route, the HDO activity of the catalyst can be enhanced by adding surfactant during its preparation.
4. CONCLUSIONS By adding surfactant during the preparation of MoS2 catalysts, MoS2 with different surface areas and morphologies were synthesized by a one-step hydrothermal method using ammonium heptamolybdate and thiocarbamide in water. The surface area of unsupported MoS2 was 79.8 m2/g, which was increased to 130.5, 145.3, and 157.6 m2/g by adding PVP, DBS, and CTAB, respectively. Compared with the stacking degree of MoS catalyst, the average stack layer was decreased to 3.7 for MoSDB and increased to 4.9 for MoSPV. In the HDO of 4-methylphenol, MoSDB, MoSCT, and MoSPV exhibited higher HDO activity than MoS. The high HDO catalytic activity of MoS2 catalyst depends on its large surface area, while the product distribution depended on its morphology. The morphology−activity relationship revealed that the Rim−Edge model is the right one to illustrate the active sites of these unsupported MoS2 catalysts. Two adsorption modes were simultaneously carried out on the rim site and edge site and led to two reaction routes (HYD and DDO), yielding methylcyclohexane and toluene as the final products. The hydrogenation activity and direct deoxygenation activity depended on the layer number in the stacks. Although the DDO pathway was the dominant route on these catalysts, compared with MoS catalyst prepared without any surfactant, the HYD pathway was enhanced by adding CTAB, while the DDO route was enhanced by adding PVP during the preparation of MoS2.
Figure 6. Recycling tests of MoSCT for 4-methylphenol HDO.
The activity test results confirmed that the HDO of 4methylphenol on Mo-based sulfide catalysts proceeded with two parallel routes (HYD and DDO), meaning that these catalysts copossessed two different active sites. Compared with MoS catalyst, MoSDB catalyst, presents a lower stacking degree (3.7 layers), showing higher HYD selectivity. In contrast, MoSPV catalyst, possessing a higher stacking degree (4.9 layers), showed much higher DDO selectivity than HYD selectivity. It is obvious that the HYD and DDO activity of Mo sulfide catalysts mainly depends on the layer number in the stacks. According to the hydrodesulfurization mechanism that hydrogenation reaction occurred on the rim site and the direct desulfurization occurred on the edge site of MoS2 catalyst,41 in this study, two adsorption modes are expected to occur: (1) a η1 (O) mode of adsorption precondition of the CO bond rupture or (2) a η6 flat adsorption by the aromatic ring leading to the hydrogenation of the phenyl ring. Thus, the hydrogenation activity and direct deoxygenation activity depended on the numbers of rim sites and edge sites, respectively. Therefore, the HDO mechanism of 4-methylphenol on these Mo-based unsupported sulfide catalysts are speculated and shown in Figure 7. For the DDO route, 4-methylphenol was adsorbed on the edge site located on internal layers of stacked MoS2 via the donation of lone electron pair from the oxygen and then the free hydrogen attacked on the CaromaticO bond to produce toluene.31 For HYD route, 4-methylphenol was adsorbed on the rim site located on external layers of stacked MoS2 through the π-electrons on the aromatic ring followed by hydrogenation of the aromatic ring to produce 4-methylcyclohexanol.42 The 4-methylphenol HDO was transferred into 4-
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[email protected]. Notes
The authors declare no competing financial interest.
Figure 7. HDO reaction mechanism of 4-methylphenol on MoS2 catalysts. 10307
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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (No. 21306159, 21376202), Specialized research Fund for the Doctoral Program of Higher Education (20124301120009), Natural Science Foundation of Hunan Province (13JJ4048), and Scientific Research Fund of Hunan Provincial Education Department (12C0392).
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