English

• 

  Because of economical and pollution control aspects, up the year 2010, the international low limit of the sulfur content in fuels is to < 1.5×10^-5[1-4]. For such propos, enormous effects have been done to develop the hydrotreatment process, responsible for S and N removal. Commonly, the MoS promoted by Co or Ni active phases supported on γ-Al₂O₃ is the most active hydrotreating catalyst^{[5, 6]}. The Co-Mo based catalyst is highly selective for HDS while Ni-Mo based catalyst is known as selective for hydrodenitrogenation (HDN). So that, Ni-Mo catalysts has the drawback of consuming higher hydrogen than Co-Mo based catalysts for HDS process with identical feed^[7].

  Although the γ-Al₂O₃ support has good thermal, textural properties and high metal dispersion ability^[8], it strongly interacts with the active metal^[9]. This interaction hinder the complete metals sulfidation which decreases the required active sites^[10]. Nevertheless, this drawback motivates the researchers to suggest another supports to Co-Mo catalysts for hydrotreating^{[9, 11-14]}.

  Among many supports carbon nanotube (CNT) is an attractive alternative support. It possesses the advantages of high thermal, mechanical strength and high surface area as γ-Al₂O₃, and also other unique properties such as excellent electron transporting capability^[15]. So, it is used as catalyst support for many reactions^{[16, 17]}. It has a high surface area with controlled pore volume and pore size which reduce the carbon deposition and provide enough surface for metal dispersion^[18]. Unlike alumina, the CNT hydrophobic surface limits the metal support interaction which facilitates the metal sulfidation process. Additionally, the acid treatment of CNT creates OH^- or COO^- functional groups that enhance the Co and Mo dispersion^{[19, 20]}.

  Dong et al^[21] reported that an appreciable active MoO₂species were formation in Co-Mo/CNT catalyst and HDS activity was enhanced. Shang et al^[22] reported a considerable higher selectivity of Co-Mo/CNTs catalyst than Al₂O₃-based catalyst. This observation was attributed to the formation of MoO₂ species rather than MoO₃ formed over alumina support. Furthermore, it is also concluded that the highest catalytic performance can be achieved over the catalyst with 0.7 Co/Mo atomic ratio.

  Despite number of reports on Co-Mo/CNT for HDS were explored, there are no reports on the hydrotreating of Egyptian heavy vacuum gas oil (HVGO). Also there are rare researches in Co-Mo/CNT catalyst to feed ratio effect on the hydrotreating.

  Considering the above argument, this study investigated the Co-Mo/CNT catalytic activity toward the Egyptian HVGO hydrotreatment. In addition, the hydrotreating experimental conditions were optimized. For more convenient the catalytic performance was compared to the conventional Co-Mo/γ-Al₂O₃ catalyst.

  1   Experimental

  1.1   Feedstock

  Heavy vacuum gas oil (HVGO) feedstock in this study was purchased from Suez Oil Petroleum Company (SOPC). The main characteristics of the feedstock are indicated in Table 1.

  Table 1.  Properties of heavy vacuum gas oil feedstock

  表选项

  导出Excel

  下载全尺寸表图像

  Experiment	Method	Result
  Total sulfur content w/%	ASTM D-4294	1.66
  Aniline point /°F	ASTM D-611-82	74
  Diesel index /%	ASTM D 611	46.09
  Total aromatics w/%	-	34

  Table 1.  Properties of heavy vacuum gas oil feedstock

  1.2   Catalyst preparation

  1.3   Catalyst characterization

  1.4   Catalytic activity test

  The catalysts were presulfied prior to the activity tests (i.e. hydrotreating), in the autoclave at the following conditions: 1.5 MPa initial hydrogen pressure, 350 ℃ reaction temperature and 5 h reaction period. For this purpose, 8% (of the catalyst weight) dimethyldisulfide (DMDS) was used as sulfiding agent in light gas oil diluent.

  The sulfided catalyst is transferred out and kept in a vacuum environment to avoid contact with air till usage. The hydrotreating experiments were carried out in a static phase using batch reactor (parr.model 7575). The reactor temperature and stirring were electrically controlled, while the pressure was controlled manually. A 150 g feed and 2 g of the sulfided catalyst were loaded inside the reactor. After tightly closing the reactor, reactant and catalyst was purged in N₂ in order to check the pressure leakage. The hydrotreatment operation conditions of temperature (325-375 ℃), initial pressure (2-6 MPa), time (2-6 h) and catalyst to feed ratio of 1：75, 1：33 and 1：10 (g/g) were individually set in.

  After the required temperature, pressure and a constant catalyst/oil ratio were reached, the experiment was conducted for the desired time. Afterwards, the autoclave was maintained to cool to room temperature overnight. The Liquid products were purged out of the autoclave under sufficient N₂-pressure, and collected for analysis using the recommended standard methods (i.e. ASTM or IP methods). The catalyst was filtered and washed by solvent (naphtha). The filtered catalyst was heated in an electric oven at 110 ℃ for 2 h.

  1.2.2   Co-Mo loading

  Ammonium heptamolybdate tetrahydrate and cobalt acetate precursors were used for molybdenum and cobalt preparation respectively. The pore volume impregnation method was used to prepare the catalyst with 12% MoO₃ loading and Co/Mo atomic ratio of 0.7. The impregnation was carried out in two stages to avoid the precipitation of catalyst metal from the saturated solution. Firstly, the Mo species was loaded by dissolving the required weight of ammonium heptamolybdate tetrahydrate in an estimated doubly distilled water volume with a few H₂O₂ droplets to give a transparent solution. The solution was added to the needed CNT amount and stirred for 30 min. After impregnation, the formed moist paste was preliminary dried in an oven at 120 ℃ over night. The dried samples were calcined in a muffle furnace with a heating rate of 10 ℃/min up to 400 ℃ for 4 h. Afterwards, the Co precursor introduced by pore volume impregnation, followed by drying at 120 ℃ over night. Finally the bimetallic catalysts were calcined at 400 ℃ in a muffle furnace for 4 h.

  For alumina supported catalyst, the same procedures were used to deposit the Mo and Co over the alumina at the same drying and calcination conditions.

  1.2.1   Synthesis and purification of carbon nanotube

  Carbon nanotube were prepared by CVD method and kindly supplied by Awadallah research group (at Egyptian petroleum research institute)^[23]. The CNT were purified and functionalized with 6 mol/L HNO₃ and then stirred for 4 h. Then it was filtered and washed by doubly distilled water several times, followed by drying at 120 ℃ overnight.

  1.3.1   X-ray diffraction

  The catalysts formed phases were determined by the X-ray diffraction analysis using X’Pert PRO PANalytical apparatus. The patterns were recorded using Cu Kα radiation (λ=0.154 18 nm). The 2θ ranges from 10° to 70° at a scanning rate of 0.05 s^-1 were used to record the patterns.

  1.3.3   Raman spectroscopy

  Raman spectra of the as-grown carbon nanotube and Co-Mo/CNT samples were recorded by SENTERRA Dispersive Raman Microscope (Bruker). The apparatus was equipped with a diode Nd：YAG laser and wave length of 532 nm. The experiments were carried out at room temperature in range of 10 to 2 000 cm^-1.

  1.3.2   Transmission electron microscopy

  The CNT and Co-Mo/CNT catalysts morphology were determined using a JEOL 2010F TEM apparatus. An accelerating voltage of 200 kV was used to obtain the images. Before measurements, the samples suspensions were prepared with 5 mL ethanol. The mixture was sonicated for 20 min then a solution droplet was placed on the Cu grid. The ethanol was then slowly evaporated in open air from the Cu grid under a glass cover.

  1.3.4   Surface area measurement

  The prepared catalyst physico-chemical properties were determined by N₂ adsorption-desorption method at -196 ℃. The Autosorb 1, Quanta chrome instrument was used to measure the BET surface area. Prior to the measurement, the adsorbed gases and moisture had removed by the samples degassing at 200 ℃ for 4 h. Surface area was calculated using the adsorption data up to a relative pressure of 0.1 by BET method.

  2   Result and discussion

  2.1   X-ray diffraction

  The XRD patterns of functionalized CNT and supported catalysts after calcination are demonstrated in Figure 1. The pure functionalized CNT shows (Figure 1a) peaks at 2θ of 26.05° and 43.08° which are ascribed to the (002) graphitic basal plane and (100) plane respectively^{[22, 24, 25]}. This observation indicates the low graphitization degree of CNT used in this study. The results may generally be responsible for catalyst’s high surface area^[26].

  Figure 1.  XRD patterns of different samples

  图选项

  下载全尺寸图像

  下载幻灯片

  The XRD spectrum of the γ-Al₂O₃ (Figure 1d) shows broad diffraction peaks at 2θ=37.5°, 45.7° and 67.1° which are related to crystal phase of γ-Al₂O₃^[22].

  The XRD patterns of Co-Mo loaded CNT and S-Mo/CNT are illustrated in Figure 1b and 1c respectively. The intensities of CNT main peaks are declined by metals loading. Tan et al^[27] attributed this patterns decrease to the change in ordered structure because of metals loading.

  Figure 1b shows diffraction peaks at 2θ of 26.18° and 36.98°, which are characteristics for MoO₂ for the Mo loaded CNT catalyst^{[22, 28]}. Furthermore, very low intensity peaks for MoO₃ crystalline phase are measured. The presence of MoO₃ weak peaks may be attributed to slightly incomplete acidification of some CNT surface and hence lower functional groups^[9]. These observations reflect basically the presence of low valence Mo species on the CNT surface. This may be explained by the weak metal-support interaction due to the CNTs surface hydrophobicty^[29]. The weak metal-support interaction helps the MoO₃ to migrate and conglomerate on the CNTs surface. So, the Mo species are reduced into the lower valence state during the calcination. The same observations are also claimed previously^{[22, 25, 30]}.

  In contrary, by Mo loading on alumina (Figure 1e) diffraction lines at 2θ of 27.3°, 25.7° and 23.3° which are attributed to the (021), (040) and (110) crystallographic planes of the orthorhombic MoO₃ phase are observed. The XRD results indicate that no new compounds are formed suggesting that the reaction between MoO₃ and γ-Al₂O₃ does not occur during treatment in air at 450 ℃. The CoMoO₄ diffraction peaks at 2θ=13.2°, 19.1°, 23.3°, 25.5°, 26.5°, 32.0° and 33.7° can be found. This is explained by partial Co to Mo interaction during the drying processes.

  Also, a very tiny peak at 49°, but not peaks at 31.7° and 59.6°, attributed to CoAl₂O₄ in the XRD pattern of Mo-Co/Al catalyst, is observed. This result indicates a very weak Co-Al₂O₃ interaction which can be explained in two bases. One reason is that the almost γ-Al₂O₃ surface saturation with MoO₃. On the other hand, Mo-Co interaction may have a synergistic effect than contact the γ-Al₂O₃.

  Figure 1c shows the XRD patterns of sulfided Co-Mo/CNT. The figure demonstrates that neither Co₃O₄ nor its sulfide phases are detected. In contrary, Weak broad peaks at 14.4° and 32.71° which are attribute to (002) and (100) planes of MoS₂ are assigned. Moreover, a weak peak at 29.0° due to the low valence Co-MoS_3.13 or Co-MoS_2.17 species formation is assigned^[22]. Also, figure shows lower molybdenum oxide species peaks intensity. This may be due to the Co atoms incorporation in the MoO₃ crystals and limiting their growth. It may be concluded that the cobalt loading and the sulfiding process emphasizes the active phase's dispersion on the CNT.

  In contrary of CNT supported catalyst, the sulfided Co-MoS/γ-Al₂O₃ catalyst shows a completely different XRD spectrum than the oxidic catalyst. The XRD spectrum of sulfided catalyst (Figure 1f) shows the peaks neither related to the γ-Al₂O₃ nor Co and Mo interactions. These results may be concluded that the cobalt loading and the sulfiding process emphasizes MoO₃ dispersion in alumina.

  Furthermore, only a broad high intense peak at 26.5° attributed to the MoO₃ species is recorded. This may confirm the well dispersion of the Co species on the Mo surface layer over alumina surface. The presence of broad Mo species peak proves its small particle size. Additionally, the peaks corresponding to the Mo₃S₄ crystallite formation at 14.4° and 32.71° are not assigned. Also, the peaks for molybdenum oxide species intensity are disappeared.

  Considering the Co species, neither Co₃O₄ nor the sulfide Co species in the Co-MoS/γ-Al₂O₃ XRD spectrum are observed. These results also confirm the well dispersed Co species in MoS₂.

  2.2   Transmission electron microscopy (TEM)

  Figure 2 shows the TEM images of the pure CNT and S-CoMo/CNT catalyst. The TEM image Figure 2(a) shows the uniform diameter CNT morphological properties. It reveals that the nanotubes have 20 nm average outer diameters and about 10 nm wall thickness. Figure 2(b) illustrates the TEM images of the S-Co-Mo/CNT catalyst. The images demonstrate good metals dispersed over CNT support. This observation confirms the data obtained from XRD. Additionally, no noticeable Mo species agglomeration is observed. Furthermore, no Co metals inside the CNTs are detected. These results may be related to the CNT inner diameter and its functional groups. Comparing Figure 2(a) and 2(b), it’s obvious that the function groups on CNT act as anchoring sites that prevent the cobalt particles immigration into the tubes and the particles sintering by calcination. Also, the narrow inner CNT diameter (9.0-10.0 nm) hinders the cobalt crystallites growth inside the tube^[31]. It is known that the small cobalt crystallites are lying inside the CNT while the larger ones are growing outside^[32-34]. The EDX analysis confirms the Co and Mo loadings with good intense peaks for both.

  Figure 2.  TEM images of (a) CNT, (b) S-CoMo/CNT, and (c) EDX of CoMo/CNT

  图选项

  下载全尺寸图像

  下载幻灯片

  2.3   Textural characteristics of CoMo/CNT

  The physical adsorption-desorption isotherms of N₂ at -196 ℃ for pure CNT and CoMoS/CNT catalysts are shown in Figure 3(a). The isotherm corresponding to type IV is obtained for the pure CNT indicating the presence of mesopores. Figure 3(b) shows the pore size distribution (PSD) calculated by the BJH method. The pretreated CNT and CoMoS/CNT PSD have the average pore size of about 12 and 18 nm respectively. The specific surface area and pore volume data of CNT and CoMoS/CNT are listed in Table 2. The pure and metal loaded alumina samples show relatively higher A_BET values than the corresponding CNT samples. After metal impregnation, for all samples, a consistent decrease in the A_BET and average pore diameter are observed. The decline of the surface area and pore size may due to the blocking of some microspores and surface smoothing by the metal deposition on the support. Thus, metal loading has a great effect in decreasing textural characteristics of catalyst.

  Figure 3.  N₂ adsorption/desorption isotherm (a) and particle size distribution (b) exhibited for pure catalyst

  图选项

  下载全尺寸图像

  下载幻灯片

  Table 2.  Physical properties of supports and CoMoX/support catalysts

  表选项

  导出Excel

  下载全尺寸表图像

  Sample	BET surface area A/(m2·g-1)	Total pore volume v/(cm3·g-1)	Average pore diameter d/nm
  CNT	120.9	0.25	38.02
  CoMo/CNT	101.1	0.18	35.28
  CoMoS/CNT	73.58	0.12	32.57
  γ-Al2O3	137.2	0.19	27.7
  CoMo/ γ-Al2O3	113.3	0.16	28.8
  CoMoS/ γ-Al2O3	66.9	0.1	32.1

  Table 2.  Physical properties of supports and CoMoX/support catalysts

  2.4   Raman spectroscopy

  Figure 4 presents the Raman spectra for the supplied CNTs, CoMoO₄/CNT and sulfided catalyst. The pure CNT spectrum (Figure 4(a)) displays two main intense peaks around 1 343 and 1 575 cm^-1. The peak at 1 343 cm^-1, known as the D-band, is due to the disordered carbon^[35]. The second peak, referred as G band, is attributed to the highly graphitized CNT. It is known that the D to G band relative intensity indicates the quality and purity of the produced CNTs. The lower intensity ratio of I_D/I_G proves the higher degree of graphitization and purity of the CNTs^[36]. The obtained low I_D/I_G ratio of 0.22 form the pure CNT spectra implies the very high CNTs graphitization degree. Clearly, these results are in a good agreement with the XRD and TEM results (Figures 1 and 2).

  Figure 4.  Laser Raman spectra of (a) CNT, (b) CoMoO/CNT, (c) S-CoMo/CNT

  图选项

  下载全尺寸图像

  下载幻灯片

  The CoMoO₄/CNT Raman spectra (Figure 4(b)) show only the peaks of the pure CNT. This observation may be due to the good dispersion of loaded metal oxides over the carbon nanotubes. The CoMoS/CNT catalyst Raman spectrum is shown in Figure 4(c). The catalyst exhibits a band at 335 cm^-1 and a large sharp peak at 940 cm^-1 that can be attributed to the symmetric Mo=O stretching of tetrahedral MoO₄^2-[22]. This result proves the higher dispersion of Mo due to the low Mo-CNT interaction. Moreover, none of the known peaks for polymeric molybdates, bulk CoMoO₄ or bulk Co or Mo are recorded. The literatures state that the bands at about 320 and 920 cm^-1 are assigned for the monomeric Mo species while bands at about 220, 360 and 960 cm^-1 are ascribed to the well-dispersed octamolybdates^[37]. Additionally, the bands at ca. 820, 945 and 952 cm^-1 are attributed to the bulk CoMoO₄. While, the characteristic bands of bulk Co and Mo oxides are usually known to be at 696 and 820 cm^-1 respectively^{[22, 38]}.

  2.5   Catalytic activity

  2.5.1   Effect of reaction temperature

  Table 3 represents the effect of reaction temperature on the product quality. It is shown that yield of heavy vacuum gas oil (HVGO) is clearly affected by the reaction temperature. The liquid yields of the CoMoS/CNT catalyst decrease with the temperature due to the increasing in reaction severity. As a result, the hydrodesulfrization (HDS) activities of the CoMoS/CNT catalyst increase continuously from 61.4% to 79.26% at temperature from 325 to 375 ℃ under constant reaction conditions of 4 MPa pressure for 2 h reaction time. Also, Table 3 demonstrates the effects of temperature on aniline point and diesel index (DI). Normally, these properties give an indication on the hydrogenation activity of the CoMoS/CNT catalyst. From the table it can be seen that as the reaction temperature increasing from 325 to 375 ℃ the aromatic contents decrease from 25% to 17.6%, and the total saturates contents increase from 75% to 82.4%. This is due to the hydrogenation of aromatics which is an exothermic reaction and is favored at low reaction temperature^[39].

  Table 3.  Effect of reaction temperature on hydrotreating of HVGO by CoMoS/CNT at p=4 MPa, t=2 h and C/F=1：75

  表选项

  导出Excel

  下载全尺寸表图像

  Characteristy	Feed	325 ℃			350 ℃			375 ℃
  		CNT	alumina		CNT	alumina		CNT	alumina
  Total liquid yield w/%	-	83.66	82		78	80		54	75.66
  Total sulfur content w/%	1.66	0.64	0.82		0.38	0.76		0.34	0.61
  Sulfur reduction (HDS) /%	-	61.4	50.6		77.1	54.21		79.26	63.25
  Aniline point /°F	165.2	170.24	166.28		170.6	167		171.32	168.26
  Diesel index (DI) /%	46.09	57.37	53.64		59.65	55.1		60.56	55.57
  Total aromatics w/%	34	25	29		21.5	26.5		17.6	25
  Hydrodearomatization (HDA) w/%	-	26.47	14.7		36.76	22.05		48.23	26.47

  Table 3.  Effect of reaction temperature on hydrotreating of HVGO by CoMoS/CNT at p=4 MPa, t=2 h and C/F=1：75

  Hydrotreating results for both catalysts show that alumina based catalysts produce higher oil product but with lower valuable properties. The CoMoS/CNT catalyst with HDS of 77% produces almost the half sulfur content value than the alumina based catalyst with HDS of only 54% at 350 ℃. Also, the CoMoS/CNT reduces aromatics content by around 9% more than the alumina catalyst. More over the CNT based catalyst improves the diesel index and aniline point by 4% and 3 degrees respectively.

  From Table 3 it is clear that CoMoS/CNT catalyst has a higher catalytic activity than the alumina supported catalyst at the same reaction conditions. This behavior clearly cannot be explained on the bases of the reaction condition since both catalysts had investigated at the same conditions. Considering both catalysts texture properties and particle size (Table 2) it is clear that the alumina supported catalysts have higher A_BET than the corresponding CNT supported catalysts which may have a revers catalytic behavior. Another factor that may be considered is the type of active phase formed during the catalysts preparation method.

  This behavior may be explained by the good metal distribution over the CNT surface. On the other hand, the formation of MoO₂ species, as can be indicated from the XRD and Raman analyses, is favorable on the CNT based catalyst surface. In contrary, over the alumina supported catalyst the MoO₃ species is formed which is difficult to be sulfided and act as active phase for high activity. Furthermore, the normal Mo-Al₂O₃ interactions hinder the sulfiding process and prohibit the Co-Mo interaction and decrease the Co promotion effect. All the above mentioned reasons can support the higher catalytic activity demonstrated by the CoMoS/CNT catalyst.

  2.5.4   Effect of catalyst/oil ratio

  The amount of catalyst is also very important for hydrotreating process. Figure 5 shows the effect of weight of catalyst on the HDS of heavy gas oil.

  Figure 5.  Effect of catalyst/oil ratio of CoMoS/CNT on HVGO at 4 MPa, 350 ℃, 2 h

  图选项

  下载全尺寸图像

  下载幻灯片

  When the amount of catalyst is 2 g the HDS reaches 77.1% due to the absence of enough active sites for the reaction with the least amount of catalyst. With farther increase the weight of catalyst to 4.54 and 15 g the HDS increase to 82.7% and 94.8% respectively, due to the increased in the amount of catalyst and hence the increase in the number of acid sites available. The presence of more active sites will allow a much further hydrogen and oil to be adsorbed and hence influence the activity.

  2.5.3   Effect of time

  Table 5 represents the effect of time on the product quality of HVGO. The hydrogenation function of the catalyst increases due to the increase in the contact time between the reacting HVGO molecules and the acid hydrogenation sites of the catalyst. As a result the HDS activity of the catalyst increased from 77.1% to 83.1%, and the improvement of color, total aromatics content, aniline point and diesel index are observed. This means that improvement in the quality of heavy vacuum gas oil by increasing time from 2 to 6 h.

  Table 5.  Effect of reaction time on hydrotreating of HVGO by /CNT at p=40 MPa, t=350 ℃ and C/ F ratio=1：75

  表选项

  导出Excel

  下载全尺寸表图像

  Characteristy	Feed	2 h	4 h	6 h
  Total liquid yield w/%	150	78	75.33	69.2
  Total sulfur content w/%	1.66	0.38	0.36	0.28
  Sulfur reduction (HDS) /%	-	77.1	78.3	83.1
  Aniline point /°F	165.2	170.6	170.96	172.04
  Total aromatics w/%	34	21.5	18	16
  Hydrodearomatization (HDA) w/%	-	36.76	47.05	52.94

  Table 5.  Effect of reaction time on hydrotreating of HVGO by /CNT at p=40 MPa, t=350 ℃ and C/ F ratio=1：75

  2.5.2   Effect of hydrogen pressure

  Results of hydrogen pressure effects are shown in Table 4. As the initial hydrogen pressures increase from 2 to 6 MPa, the HDS increases from 60.24% to 79.5%, DI and aniline point increase from 56.59% to 61.18% and 169.7°F to 172.4°F respectively, while total aromatics contents decrease from 24% to 16.8 %, which can be attributed to the increase of hydrogenation activity of the catalyst with increasing total hydrogen pressure besides the increase of saturation activity and reduction of coke formation^{[39, 40]}.

  Table 4.  Effect of reaction pressure on hydrotreating of HVGO by CoMoS/CNT at t=350 ℃, t=2 h and C/F=1：75

  表选项

  导出Excel

  下载全尺寸表图像

  Characteristics	Feed	2 MPa	4 MPa	6 MPa
  Total liquid yield w/%	150	85.33	78	72
  Total sulfur content w/%	1.66	0.66	0.38	0.34
  Sulfur reduction (HDS) /%	-	60.24	77.1	79.5
  Aniline point /°F	165.2	169.7	170.6	172.4
  Total aromatics w/%	34	24	21.5	16.8
  Hydrodearomatization (HDA) w/%	-	29.41	36.76	50.58

  Table 4.  Effect of reaction pressure on hydrotreating of HVGO by CoMoS/CNT at t=350 ℃, t=2 h and C/F=1：75

  3   Conclusions

  The CoMoS/CNT hydrotreating catalyst is prepared by impregnation method. The TEM analysis shows that purified CNTs prove their morphological and structural features. Additionally, TEM indicates a good dispersion of Mo species and uniform Co particles deposition over the CNTs surface.

  The catalyst, with 12% Mo loading and 0.7 Co/Mo atomic ratios supported on CNT, exhibits an excellent catalytic performance towards hydrotreating process under the experimental conditions. Consequently, the desired promoted Co-Mo-S phase formation upon sulfidation process is confirmed. The high catalytic performance of CoMoS/CNT can be achieved at low operating condition of 350 ℃, 40 bar and catalyst/ratio of 1：10 for 2 h. A fairly high HDS of 77.1% can be achieved over a low catalyst to oil ratio of 1：75 (g/g). The CoMoS/CNT catalyst shows a higher catalytic activity than CoMoS/γ-Al₂O₃.

• 1. [1]

     TOPSФE H, CLAUSEN B S. Importance of Co-Mo-S type structures in hydrodesulfurization[J]. Catal Rev Sci Eng, 1984, 26(3/4):  395-420.

  2. [2]

     PRINS R, DE BEER V H J, SOMORJAI G A. Structure and function of the catalyst and the promoter in Co-Mo hydrodesulfurization catalysts[J]. Catal Rev Sci Eng, 1989, 31(1/2):  1-41.

  3. [3]

     MEDICI L, PRINS R. The influence of chelating ligands on the sulfidation of Ni and Mo in NiMo/SiO₂ hydrotreating catalysts[J]. J Catal, 1996, 163(1):  38-49. doi: 10.1006/jcat.1996.0303

  4. [4]

     SHIMIZU T, HIROSHIMA K, HONMA T, MOCHIZUKI T, YAMADA M. Highly active hydrotreatment catalysts prepared with chelating agents[J]. Catal today, 1998, 45(1/4):  271-276.

  5. [5]

     VAN LOOIJ F, VAN DER LAAN P, STORK W H J, DICAMILLO D J, SWAIN J. Key parameters in deep hydrodesulfurization of diesel fuel[J]. Appl Catal A: Gen, 1998, 170(1):  1-12. doi: 10.1016/S0926-860X(98)00028-3

  6. [6]

     SHIMADA H, SATO T, YOSHIMURA Y, HIRAISHI J, NISHIJIMA A. Support effect on the catalytic activity and properties of sulfided molybdenum catalysts[J]. J Catal, 1988, 110(2):  275-284. doi: 10.1016/0021-9517(88)90319-3

  7. [7]

     VISHWAKARMA S K. Sonochemical and impregnated Co-W/γ-Al₂O₃ catalysts: Performances and kinetic studies on hydrotreatment of light gas oil[D].Saskatoon University of Saskatchewan, 2007.

  8. [8]

     TOPSФE H, CLAUSEN B S. Active sites and support effects in hydrodesulfurization catalysts[J]. Appl Catal, 1986, 25(1/2):  273-293.

  9. [9]

     ESWARAMOORTHI I, SUNDARAMURTHY V, DAS N, DALAI A K, ADJAYE J. Application of multi-walled carbon nanotubes as efficient support to NiMo hydrotreating catalyst[J]. Appl Catal A: Gen, 2008, 339(2):  187-195. doi: 10.1016/j.apcata.2008.01.021

  10. [10]

      SIGURDSON S, SUNDARAMURTHY V, DALAI A K, ADJAYE J. Effect of anodic alumina pore diameter variation on template-initiated synthesis of carbon nanotube catalyst supports[J]. JMol Catal A: Chem, 2009, 306:  23-32. doi: 10.1016/j.molcata.2009.02.016

  11. [11]

      DHAR G M, SRINIVAS B N, RANA M S, KUMAR M, MAITY S K. Mixed oxide supported hydrodesulfurization catalysts-A review[J]. Catal Today, 2003, 86(1/4):  45-60.

  12. [12]

      WANG A, WANG Y, KABE T, CHEN Y, ISHIHARA A, QIAN W. Hydrodesulfurization of dibenzothiophene over siliceous MCM-41-supported catalysts: I. Sulfided Co-Mo catalysts[J]. J Catal, 2001, 199(1):  19-29. doi: 10.1006/jcat.2000.3148

  13. [13]

      MAITY S K, RANA M S, BEJ S K, ANCHEYTA-JUAREZ J, DHAR G M, RAO T S R P. Studies on physico-chemical characterization and catalysis on high surface area titania supported molybdenum hydrotreating catalysts[J]. Appl CatalA: Gen, 2001, 205(1/2):  215-225.

  14. [14]

      VRADMAN L, LANDAU M V, HERSKOWITZ M, EZERSKY V, TALIANKER M, NIKITENKO S, KOLTYPIN Y, GEDANKEN A. High loading of short WS 2 slabs inside SBA-15: Promotion with nickel and performance in hydrodesulfurization and hydrogenation[J]. J Catal, 2003, 213(2):  163-175. doi: 10.1016/S0021-9517(02)00012-X

  15. [15]

      POUR A N, RASHIDI A M, JOZANI K J, MOHAJERI A, KHORAMI P. Support effects on the chemical property and catalytic activity of Co-Mo HDS catalyst in sulfur recovery[J]. J Nat Gas Chem, 2010, 19(1):  91-95. doi: 10.1016/S1003-9953(09)60032-3

  16. [16]

      SERP P, CORRIAS M, KALCK P. Carbon nanotubes and nanofibers in catalysis[J]. Appl Catal A: Gen, 2003, 253:  337-358. doi: 10.1016/S0926-860X(03)00549-0

  17. [17]

      VAN STEEN E, PRINSLOO F F. Comparison of preparation methods for carbon nanotubes supported iron Fischer-Tropsch catalysts[J]. Catal Today, 2002, 71(3/4):  327-334.

  18. [18]

      AUER E, FREUND A, PIETSCH J, TACKE T. Carbons as supports for industrial precious metal catalysts[J]. Appl Catal A: Gen, 1998, 173(2):  259-271. doi: 10.1016/S0926-860X(98)00184-7

  19. [19]

      SHANG H Y, LIU C G, XU Y Q, ZHAO H J, SONG H H. Effect of the surface modification of multi-walled carbon nanotubes (MWCNTs) on hydrodesulfurization activity of Co-Mo/MWCNTs catalysts[J]. New Carbon Mater, 2004, 19(2):  131-136.

  20. [20]

      KYOTANI T, NAKAZAKI S, XU W-H, TOMITA A. Chemical modification of the inner walls of carbon nanotubes by HNO₃ oxidation[J]. Carbon, 2001, 39(5):  782-785. doi: 10.1016/S0008-6223(01)00013-6

  21. [21]

      DONG K, ZHANG S, WANG D, YAO X. Hydrogen bonds in imidazolium ionic liquids[J]. J Phys Chem A, 2006, 110(31):  9775-9782. doi: 10.1021/jp054054c

  22. [22]

      SHANG H, LIU C, XU Y, QIU J, WEI F. States of carbon nanotube supported Mo-based HDS catalysts[J]. Fuel Process Technol, 2007, 88(2):  117-123. doi: 10.1016/j.fuproc.2004.08.010

  23. [23]

      AWADALLAH A E, ABOUL-ENEIN A A, EL-DESOUKI D S, ABOUL-GHEIT A K. Catalytic thermal decomposition of methane to CO_x-free hydrogen and carbon nanotubes over MgO supported bimetallic group Ⅷ catalysts[J]. ApplSurfSci, 2014, 296:  100-107.

  24. [24]

      SUNDARAMURTHY V, DALAI A K, ADJAYE J. Effect of EDTA on hydrotreating activity of CoMo/γ-Al₂O₃ catalyst[J]. CatalLett, 2005, 102(3):  299-306.

  25. [25]

      HOGG J C, CHU F, UTOKAPARCH S, WOODS R, ELLIOTT W M, BUZATU L, CHERNIACK R M, ROGERS R M, SCIURBA F C, COXSON H O, PARP D. The nature of small-airway obstruction in chronic obstructive pulmonary disease[J]. N Engl J Med, 2004, 350:  2645-2653. doi: 10.1056/NEJMoa032158

  26. [26]

      SHIGAPOV A N, GRAHAM G W, MCCABE R W, PECK M P, PLUMMER H K. The preparation of high-surface-area cordierite monolith by acid treatment[J]. Appl Catal A: Gen, 1999, 182(1):  137-146. doi: 10.1016/S0926-860X(99)00003-4

  27. [27]

      TAN Z L, XIAO H N, ZHANG R D, ZHANG Z S, KALIAGUINE S. Potential to use mesoporous carbon as catalyst support for hydrodesulfurization[J]. New Carbon Mater, 2009, 24(4):  333-343. doi: 10.1016/S1872-5805(08)60056-6

  28. [28]

      ZHANG Y, ZHANG H B, LIN G D, CHEN P, YUAN Y Z, TSAI K R. Preparation, characterization and catalytic hydroformylation properties of carbon nanotubes-supported Rh-phosphine catalyst[J]. Appl Catal A: Gen, 1999, 187(2):  213-224. doi: 10.1016/S0926-860X(99)00229-X

  29. [29]

      DUJARDIN E, EBBESEN T W, HIURA H, TANIGAKI K. Capillarity and wetting of carbon nanotubes[J]. Science, 1994, 265(5180):  1850-1852. doi: 10.1126/science.265.5180.1850

  30. [30]

      DANDEKAR A, BAKER R T K, VANNICE M A. Characterization of activated carbon, graphitized carbon fibers and synthetic diamond powder using TPD and DRIFTS[J]. Carbon, 1998, 36(12):  1821-1831. doi: 10.1016/S0008-6223(98)00154-7

  31. [31]

      KARIMI A, NASERNEJAD B, RASHIDI A M. Synthesis and characterization of multiwall carbon nanotubes/alumina nanohybrid-supported cobalt catalyst in Fischer-Tropsch synthesis[J]. J Energy Chem, 2013, 22(4):  582-590. doi: 10.1016/S2095-4956(13)60076-5

  32. [32]

      TRÉPANIER M, TAVASOLI A, DALAI AK, ABATZOGLOU N. Fischer-Tropsch synthesis over carbon nanotubes supported cobalt catalysts in a fixed bed reactor: Influence of acid treatment[J]. Fuel Process Technol, 2009, 90(3):  367-374. doi: 10.1016/j.fuproc.2008.10.012

  33. [33]

      KARIMI A, NASERNEJAD B, RASHIDI A M, TAVASOLI A, POURKHALIL M. Functional group effect on carbon nanotube (CNT)-supported cobalt catalysts in Fischer-Tropsch synthesis activity, selectivity and stability[J]. Fuel, 2014, 117:  1045-1051. doi: 10.1016/j.fuel.2013.10.014

  34. [34]

      ABBASLOU R M M, TAVASSOLI A, SOLTAN J, DALAI A K. Iron catalysts supported on carbon nanotubes for Fischerâ Tropsch synthesis: Effect of catalytic site position[J]. Appl Catal A: Gen, 2009, 367(1/2):  47-52.

  35. [35]

      DRESSELHAUS M S, DRESSELHAUS G, JORIO A, SOUZA FILHO A G, SAITO R. Raman spectroscopy on isolated single wall carbon nanotubes[J]. Carbon, 2002, 40(12):  2043-2061. doi: 10.1016/S0008-6223(02)00066-0

  36. [36]

      LI Q, YAN H, ZHANG J, LIU Z. Effect of hydrocarbons precursors on the formation of carbon nanotubes in chemical vapor deposition[J]. Carbon, 2004, 42(4):  829-835. doi: 10.1016/j.carbon.2004.01.070

  37. [37]

      KOHLER S D, EKERDT J G, KIM D S, WACHS I E. Relationship between structure and point of zero surface charge for molybdenum and tungsten oxides supported on alumina[J]. Catal Lett, 1992, 16(3):  231-239. doi: 10.1007/BF00764335

  38. [38]

      JEZIOROWSKI H, KNOZINGER H, GRANGE P, GAJARDO P. Raman spectra of cobalt molybdenum oxide supported on silica[J]. J Phys Chem, 1980, 84:  1825-1829. doi: 10.1021/j100451a017

  39. [39]

      GARY J H, HANDWERK G E, KAISER M J. Petroleum refining: Technology and economics[C]. Boca Raton: CRC Press, 2007.

  40. [40]

      BARTHOLOMEW C H. Catalyst deactivation in hydrotreating of residua: A review[C]. New York: Marcel Dekker, 1994.

• 

  Figure 1  XRD patterns of different samples

  a: CNT; b: CoMoO/CNT; c: CoMoS/CNT; d: γ-Al₂O₃; e: CoMoO/γ-Al₂O₃; f: CoMoS/γ-Al₂O₃

  下载: 全尺寸图片 幻灯片
  

  Figure 2  TEM images of (a) CNT, (b) S-CoMo/CNT, and (c) EDX of CoMo/CNT

  下载: 全尺寸图片 幻灯片
  

  Figure 3  N₂ adsorption/desorption isotherm (a) and particle size distribution (b) exhibited for pure catalyst

  a: CNT; b: CoMoO/CNT; c: CoMoS/CNT

  下载: 全尺寸图片 幻灯片
  

  Figure 4  Laser Raman spectra of (a) CNT, (b) CoMoO/CNT, (c) S-CoMo/CNT

  下载: 全尺寸图片 幻灯片
  

  Figure 5  Effect of catalyst/oil ratio of CoMoS/CNT on HVGO at 4 MPa, 350 ℃, 2 h

  下载: 全尺寸图片 幻灯片

  Table 1.  Properties of heavy vacuum gas oil feedstock

  Experiment	Method	Result
  Total sulfur content w/%	ASTM D-4294	1.66
  Aniline point /°F	ASTM D-611-82	74
  Diesel index /%	ASTM D 611	46.09
  Total aromatics w/%	-	34

  下载: 导出CSV

  Table 2.  Physical properties of supports and CoMoX/support catalysts

  Sample	BET surface area A/(m2·g-1)	Total pore volume v/(cm3·g-1)	Average pore diameter d/nm
  CNT	120.9	0.25	38.02
  CoMo/CNT	101.1	0.18	35.28
  CoMoS/CNT	73.58	0.12	32.57
  γ-Al2O3	137.2	0.19	27.7
  CoMo/ γ-Al2O3	113.3	0.16	28.8
  CoMoS/ γ-Al2O3	66.9	0.1	32.1

  下载: 导出CSV

  Table 3.  Effect of reaction temperature on hydrotreating of HVGO by CoMoS/CNT at p=4 MPa, t=2 h and C/F=1：75

  Characteristy	Feed	325 ℃			350 ℃			375 ℃
  		CNT	alumina		CNT	alumina		CNT	alumina
  Total liquid yield w/%	-	83.66	82		78	80		54	75.66
  Total sulfur content w/%	1.66	0.64	0.82		0.38	0.76		0.34	0.61
  Sulfur reduction (HDS) /%	-	61.4	50.6		77.1	54.21		79.26	63.25
  Aniline point /°F	165.2	170.24	166.28		170.6	167		171.32	168.26
  Diesel index (DI) /%	46.09	57.37	53.64		59.65	55.1		60.56	55.57
  Total aromatics w/%	34	25	29		21.5	26.5		17.6	25
  Hydrodearomatization (HDA) w/%	-	26.47	14.7		36.76	22.05		48.23	26.47

  下载: 导出CSV

  Table 4.  Effect of reaction pressure on hydrotreating of HVGO by CoMoS/CNT at t=350 ℃, t=2 h and C/F=1：75

  Characteristics	Feed	2 MPa	4 MPa	6 MPa
  Total liquid yield w/%	150	85.33	78	72
  Total sulfur content w/%	1.66	0.66	0.38	0.34
  Sulfur reduction (HDS) /%	-	60.24	77.1	79.5
  Aniline point /°F	165.2	169.7	170.6	172.4
  Total aromatics w/%	34	24	21.5	16.8
  Hydrodearomatization (HDA) w/%	-	29.41	36.76	50.58

  下载: 导出CSV

  Table 5.  Effect of reaction time on hydrotreating of HVGO by /CNT at p=40 MPa, t=350 ℃ and C/ F ratio=1：75

  Characteristy	Feed	2 h	4 h	6 h
  Total liquid yield w/%	150	78	75.33	69.2
  Total sulfur content w/%	1.66	0.38	0.36	0.28
  Sulfur reduction (HDS) /%	-	77.1	78.3	83.1
  Aniline point /°F	165.2	170.6	170.96	172.04
  Total aromatics w/%	34	21.5	18	16
  Hydrodearomatization (HDA) w/%	-	36.76	47.05	52.94

  下载: 导出CSV
•