Effect of Calcination Temperature of Unsupported NiMo Catalysts on

Mar 19, 2014 - Sustainable synthesis of ammonium nickel molybdate for hydrodesulfurization of dibenzothiophene. Huan Liu , Changlong Yin , Hongyu Zhan...
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Effect of Calcination Temperature of Unsupported NiMo Catalysts on the Hydrodesulfurization of Dibenzothiophene Huan Liu, Changlong Yin,* Bin Liu, Xuehui Li, Yanpeng Li, Yongming Chai, and Chenguang Liu* State Key Laboratory of Heavy Oil Processing, Key Laboratory of Catalysis, China National Petroleum Corporation (CNPC), China University of Petroleum, Qingdao, Shandong 266555, China ABSTRACT: The effects of NiMo catalyst calcination temperature (250, 350, 450, 550, and 650 °C) on catalytic hydrodesulfurization of dibenzothiophene (DBT) were studied. The physiochemical properties of the unsupported NiMo catalysts were characterized by various techniques, including N2 adsorption−desorption, X-ray diffraction (XRD), Fouriertransform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM). XRD and FT-IR results showed that increasing the calcination temperature brought about a phase change of the NiMo catalyst precursors from ammonium nickel molybdate to nickel molybdate. The agglomeration of precursor particles and MoS2 nanoclusters changed with various calcination temperatures as revealed by SEM and HRTEM, respectively. DBT catalytic results showed that the NiMo catalyst calcined at 350 °C exhibited higher activity than other NiMo catalysts, and the selectivity of bicyclohexane was much preferred at a reaction temperature of 320 °C over NiMo catalysts endowed with the phase of ammonium nickel molybdate. The high hydrogenation ability might be due to the efficient formation of the “Ni-Mo-S” phase and will be attractive for industrial applications.

1. INTRODUCTION During the last decades, hydrodesulfurization (HDS) has been used for the removal of sulfur atoms from diesel.1−3 With the increasing environmental concerns and stringent legislation, HDS has drawn more urgent attention to produce clean fuel, which nowadays means that the sulfur content is no more than 10 ppmw.2,4 To achieve this tough target, some attempts are undertaken, such as severe reaction conditions, increasing volume of catalyst bed, or replacement by highly active HDS catalysts, which are generally composed of MoS2 or WS2 nanoclusters promoted by sulfided Co or Ni species and deposited on high surface area supports.2,5 The applications of HDS catalysts with intrinsically high activity seem more attractive for refineries, because they do not have to rebuild the HDS plants owing to the increasing HDS activity. Therefore, many efforts have been made to develop highly active HDS catalysts. Novel preparation methods, introductions of new catalytic supports, and modifications of active phase have been testified to be efficient for the deep HDS.3,5−9 Dibenzothiophene (DBT) and its alkyl-substitutes at 4- and/ or 6-positions are of the most refractory sulfur compounds in the production of ultralow sulfur diesel (ULSD).1 It is generally accepted that two reaction routes exist for the conversion of DBT and its derivates during HDS reactions.1,10,11 One is called the direct desulfurization (DDS) route, and the other is the hydrogenation (HYD) route. These two HDS routes might happen on different catalyst sites, as was reported by Kabe et al. that there existed two adsorption modes in the HDS process:12 one is σ bonding to the catalyst surface through the sulfur atom, and the other is the π bonding by the aromatic ring. Furthermore, Egorova and Prins proposed that the DDS route predominantly takes place by σ adsorption, whereas the HYD route generally proceeds by π adsorption through the aromatic system.13 Recently, Lauritsen and co-workers confirmed the different adsorption configurations of DBT, 4© 2014 American Chemical Society

methyldibenzothiophene (4-MDBT), and 4,6-dimethyldibenzothiophene (4,6-DMDBT) on unsupported MoS2 and Copromoted MoS2 nanoclusters through scanning tunneling microscopy (STM).14,15 The better understanding of catalytic sites for the removal of sulfur from these refractory molecules might help us gain a broader and more detailed insight in the mechanism of HDS.15 Calcination is usually an essential step in the catalyst preparation, which might influence the catalyst structure and properties and the dispersion of active metals.16−19 Some transformations would occur in calcination, such as the decomposition of the precursor, the formation of oxide species, and the sintering of the precursor or the formed oxide compounds.20 Metal ions might migrate to locations that provide greater electrostatic stabilization and higher coordination to the lattice oxygen ions.20 Therefore, calcination may have distinct influence on the dispersion and distribution of the active metal compounds. In a previous paper, we reported the novel ammonium nickel molybdate (NH4 )HNi 2(OH)2(MoO4)2 as the precursor for deep HDS.7 This precursor was efficient to form the active phase and exhibited high hydrogenation ability. Besides thermogravimetry−differential scanning calorimeter−mass spectrum (TG−DSC−MS) analysis results of ammonium nickel molybdate indicated that the decomposition of the precursor and the release of small molecules (H2O and NH3) occurred over a temperature range from 350 to 500 °C.7 In this work, the effects of calcination temperature on the transformation of the precursors phases and the dispersion of active metals were investigated, and the activity toward HDS of DBT was studied. Various characterReceived: January 13, 2014 Revised: March 18, 2014 Published: March 19, 2014 2429

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ization techniques were explored to relate the physiochemical properties with catalytic performance.

2. EXPERIMENTAL SECTION 2.1. Preparation of Nickel−Molybdenum Catalyst. Nickel− molybdenum catalyst precursors were prepared by the chemical precipitation method in the following way. A mixture of ammonium heptamolybdate and nickel nitrate with the molar ratio 1:1 (Sinopharm Chemical Reagent Company, China) was dissolved in deionized water. The solution was added to a flask, then heated, and stirred at 90 °C. The addition of concentrated ammonium hydroxide (28.8% NH3) precipitated a light green solid that dissolved in excess ammonia, giving a dark green solution. After that, the solution was stirred and heated for 12 h, leading to the formation of a green precipitate. The products were isolated by vacuum filtration, washed with deionized water, and dried overnight at 100 °C and atmospheric pressure. The obtained products were then calcined at 250, 350, 450, 550, or 650 °C for 4 h with a temperature ramping rate of 3 °C/min, and these NiMo catalyst precursors were named as NiMo-250, NiMo350, NiMo-450, NiMo-550, and NiMo-650, respectively. To obtain a fixed-bed catalyst, the unsupported NiMo catalysts were tabletted without any binder and then pulverized into 20−40 mesh for catalytic reaction. 2.2. Characterization of Nickel−Molybdenum Catalysts. N2 adsorption−desorption experiments were carried out on a ChemBET 3000 (Quantachrome, USA) instrument. X-ray powder diffraction (XRD) characterization was recorded on Rigaku D/max-IIA diffractometer using a graphite-filter Cu Kα radiation. Fouriertransform infrared (FT-IR) spectra were collected on a Nexus spectrometer (Nicolet, USA) using KBr disks. Morphological analysis was collected on an FEI Quanta200 scanning electron microscope (SEM) instrument. High-resolution transmission electron microscopy (HRTEM) was carried out on a JEM 2100 microscope operated at 200 kV. 2.3. DBT Catalytic Activity of Nickel−Molybdenum Catalyst. Catalytic activity measurements of DBT desulfurization were carried out in a high-pressure fixed-bed down-flow micro reactor (10 mm i.d., 400 mm long). A 0.5 g portion of NiMo catalyst of 20−40 mesh size was diluted with quartz sand of the same mesh size to a volume of 2.5 cm3, and placed in the center of the reactor. The catalyst was sulfided in situ at 330 °C for 12 h by a liquid stream containing 3.0 wt % CS2 in petroleum ether. Then, 1 wt % DBT in petroleum ether was pumped into the reactor. The reaction was carried out at a pressure of 3.0 MPa, a H2/feed ratio of 400, a weight hourly space velocity (WHSV) of 30.0 h−1, and various reaction temperatures. The product was cooled and separated into gaseous and liquid products in a high-pressure separator. The liquid products were analyzed by a Varian 3800 gas chromatograph (flame ionization detector, a 50 m OV101 capillary column) coupled with a quadrupole mass spectrometer (Finnigan SSQ710) (GC−MS).

Figure 1. XRD patterns of nickel−molybdenum precursors calcined at various temperatures and reference patterns.

NiMo-450 precursor, but some new peaks with 2θ at 28.8, 25.3, 14.3, 32.6, 43.8, and 47.4° were detected. These peaks are ascribed to the phase of nickel molybdate (NiMoO4, JCPDS card No. 33-0948).22 The characteristic peaks of ammonium nickel molybdate could not be detected in NiMo-550 and NiMo-650 precursors, while the peaks of newly formed nickel molybdate were detected in both precursors. It could also been found that increasing the calcination temperature from 550 to 650 °C led to the higher intensity of the peaks attributing to nickel molybdate. 3.2. Textural Properties of NiMo Catalyst Precursors. The textural properties (BET surface area, pore volume, and pore size) of NiMo catalyst precursors are shown in Table 1. The NiMo-250 precursor exhibited quite low BET surface area and pore volume, which were 8.4 m2/g and 0.03 cm3/g, respectively, whereas the NiMo-450 precursor had the highest BET surface area of the series of NiMo catalyst precursors. This is in accordance with the previous work and the aforementioned XRD results. In a previous paper, we reported that the production and release of small molecules, such as NH3 and H2O, helped increase the surface area of the product.7 At the same time, the NiMo-450 precursor exhibited a quasi amorphous XRD pattern compared to other NiMo precursors,

3. RESULTS 3.1. XRD of NiMo Catalyst Precursors. The NiMo catalyst precursors calcined at various temperatures were characterized by XRD, and the results are exhibited in Figure 1. The simulated reference patterns of ammonium nickel molybdate and nickel molybdate are also shown in Figure 1 for comparison. There existed obvious phase changes of the NiMo catalyst precursor with the increasing calcination temperature from Figure 1. The phase of the NiMo-250 precursor was attributed to the ammonium nickel molybdate ((NH4)HNi2(OH)2(MoO4)2, JCPDS card No. 50-1414).7,21 The characteristic peaks of 2θ at 17.4, 23.5, 26.5, 29.6, 32.2° were detected and shown in a previous work.7 Increasing the calcination temperature to 350 °C led to the weaker intensity of ammonium nickel molybdate. The peaks attributed to ammonium nickel molybdate could still be detected in the 2430

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Table 1. BET Results of Nickel−Molybdenum Precursors Calcined at Various Temperatures NiMo precursor

BET surface area (m2/g)

pore volume (cm3/g)

pore size (nm)

NiMo-250 NiMo-350 NiMo-450 NiMo-550 NiMo-650

8.4 58.8 85.0 51.3 26.6

0.03 0.20 0.21 0.29 0.23

6.7 10.7 4.9 11.5 17.3

and the crystallizations of both ammonium nickel molybdate and nickel molybdate phases were not as high as others (as shown in Figure 1). Increasing the temperature from 250 to 450 °C led to the decomposition of ammonium nickel molybdate, which was more liable to form a larger BET surface area. Continuously increasing temperature from 450 to 650 °C led to a decrease of the BET surface area, due to the formation and crystallization of nickel molybdate. The adsorption− desorption isotherms of NiMo precursors calcined at various temperatures are presented in Figure 2. The isotherms are of type IV, which is typical for mesoporous material.

Figure 3. FT-IR spectra of the nickel−molybdenum catalyst precursors calcined at various temperatures.

characterization, which also reveals that increasing the calcination temperature from 250 to 650 °C brings about the decomposition of ammonium nickel molybdate and the formation of nickel molybdate. 3.4. SEM of NiMo Catalyst Precursors. SEM characterizations were employed to obtain detailed morphological information about the NiMo catalyst precursors calcined at various temperatures, and the SEM micrographs are presented in Figure 4. The differences in morphologies of these precursors were significant. The NiMo-250 precursor consisted of some small particles (about 50−100 nm) linked to a larger plate (around 500 nm), and NiMo-350 seemed to be formed by many smaller particles with the sizes from 20 to 40 nm. Increasing the temperature to 450 °C helped to form the agglomeration of the particles with sizes from 50 to 80 nm. As to NiMo-550 and NiMo-650 precursors, the size of particles increased to about 100 nm, and 100 to 300 nm, respectively. 3.5. HRTEM of Sulfided NiMo Catalysts. HRTEM characterizations of sulfided NiMo catalysts were carried out to gain more information about the changes of dispersion of active MoS2 nanoclusters induced by the calcination temperature. Typical HRTEM micrographs are exhibited in Figure 5. The characteristic black threadlike fringes due to stacked MoS2 with a 0.61 nm interplanar distance were observed on all sulfided NiMo catalysts.7 The NiMo-250 catalyst exhibited lowstacked MoS2 slabs, with the stacking number between 1 and 3, and the stacking number of MoS2 slabs increased to about 3−5 on the NiMo-350 catalyst. The stacked MoS2 slabs on NiMo450 and NiMo-550 catalysts seemed more agglomerated than these on NiMo-250 and NiMo-350 catalysts, with higher stacking numbers and longer slab lengths. 3.6. XRD of Sulfided NiMo Catalysts. The sulfided NiMo catalysts were characterized by XRD, and the results are exhibited in Figure 6. The characteristic XRD peaks due to MoS2 nanoclusters were detected over all the NiMo catalysts, with 2θ at 14.4, 33.0, 38.2, and 58.2°.7 The intensity of the MoS2 peaks increased with increasing calcination temperature, indicating the formation of more agglomerated MoS2 nanoparticles at higher calcination temperature.7 This is in accordance with the aforementioned HRTEM results. The diffraction peaks ascribed to sulfided nickel species were also detected over the NiMo catalysts. The XRD peaks at 21.8, 31.1,

Figure 2. N2 adsorption isotherms of nickel−molybdenum catalyst precursors calcined at various temperatures.

3.3. FT-IR of NiMo Catalyst Precursors. The FT-IR spectra of NiMo catalyst precursors calcined at various temperatures are depicted in Figure 3. The adsorption peaks from 3300 to 3050 cm−1 attributed to the ν3 N−H asymmetric stretching vibration were detected for NiMo-250 and NiMo350 catalyst precursors, and the FT-IR peak at 1410 cm−1 was due to the deformation vibration of H−N−H.21 This is in accordance with the XRD results that the phase of ammonium nickel molybdate could be detected before the calcination temperature at 450 °C. The peaks between 1000 and 700 cm−1 with different intensities of NiMo-250 and NiMo-350 precursors were assigned to the stretching vibrations of bridging oxygen in Mo−O−Mo.23 For NiMo precursors calcined at higher temperature (NiMo-550 and NiMo-650), an FT-IR peak at 933 cm−1 was attributed to ν1 deformation of the tetrahedron of the molybdate group,24 and the peak at 963 cm−1 was due to the activation of the ν1 vibration of the distorted MoO4 tetrahedron.25 The spectra of NiMo catalyst precursors were in good agreement with those reported in the literature.22,25,26 The FT-IR results are consistent with the XRD 2431

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Figure 4. SEM of nickel−molybdenum catalyst precursors calcined at various temperatures.

3.8. DBT Catalytic Results. The hydrodesulfurization reactions of DBT were carried out in a fixed-bed high-pressure reactor over the NiMo catalysts, and the influences of reaction temperature on the conversion of DBT and products distributions were investigated. The results are presented in Figure 7. Figure 7 shows that the DBT conversions at 320 °C decreased in the order: 86.5% (NiMo-350) > 78.8% (NiMo450) > 78.5% (NiMo-550) > 60.2% (NiMo-250) > 57.7% (NiMo-650). There are extensive studies on the mechanism of HDS reaction, revealing that DBT and its derivates undergo two reaction routes called DDS and HYD.10,11,13,27 The C−S bonds of DBTs are broken through hydrogenolysis or elimination (including the anti-elimination), and biphenyl (BP) is formed in the DDS route, whereas, in the HYD route, one of the aromatic rings of DBT is first hydrogenated to intermediates, such as 1,2,3,4-tetrahydrodibenzothiophene (THDBT) and hexahydrodibenzothiophene (HHDBT), and maybe the two aromatic rings are fully hydrogenated to form dodecahydrodibenzothiophene (DHDBT). Then, the sulfur is removed to form cyclohexylbenzene (CHB) and bicyclohexane (BCH).1,13 As shown in Figure 7, five products, such as BP,

37.8, 44.3, 49.8, 50.1, and 55.2° were due to the crystallized Ni3S2 phase (JCPDS card No. 76-1870).7 The peak detected at 25.7° over the NiMo-450 and NiMo-550 catalysts was due to the formation of the Ni3S4 phase (JCPDS card No. 47-1739). The phases of Ni3S2 and Ni3S4 were overlapped with 2θ at 31.1, 37.8, 49.8, and 50.1°. 3.7. Textural Properties of Sulfided NiMo Catalyst. The textural properties of the NiMo catalysts after sulfidation are exhibited in Table 2. The table shows that the BET surface area of NiMo catalysts decreases a little after sulfidation except for the NiMo-250 catalyst, while the mesoporous structure is maintained. The sulfided NiMo-450 catalyst exhibited a larger BET surface area than the other catalysts, and the BET surface area of the sulfided NiMo-250 catalyst increased to 12.5 m2/g. From the preparation section and the XRD results of NiMo catalyst precursors, it could be inferred that NiMo catalyst precursors calcined at 250 °C might be partially decomposed during sulfidation. This might release some small molecules, such as NH3 and H2O, leading to the increase of BET surface area.7 2432

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Figure 5. HRTEM photographs of sulfided NiMo catalysts.

Table 2. BET Results of Sulfided NiMo Catalysts NiMo catalyst

BET surface area (m2/g)

pore volume (cm3/g)

pore size (nm)

NiMo-250 NiMo-350 NiMo-450 NiMo-550

12.5 54.0 80.1 34.2

0.04 0.11 0.18 0.13

5.2 5.9 8.8 11.0

°C. The content of BCH was almost equal to that of BP over the NiMo-350 catalyst at 320 °C. The selectivity of BCH over NiMo catalysts at 320 °C was calculated, and the result is shown in Figure 8. From Figure 8, the selectivity of BCH was more than 20% over the NiMo-350 catalyst at a reaction temperature of 320 °C. The selectivity of BCH was almost the same for NiMo-250 and NiMo-450 catalysts, which was about 10%, and for NiMo550 and NiMo-650 catalysts, the selectivity dropped to around 4%. There existed distinct differences among the selectivity of BCH over NiMo catalysts, and the mechanism of the formation of BCH is still in debate in the literature. Some propose that the formation of BCH is due to the hydrogenation of CHB, which means that the second aromatic ring of CHB is hydrogenated to form BCH;5 others might consider that the hydrogenated intermediates, such as THDBT, HHDBT, and possibly the DHDBT, are desulfurized to form BCH.1,13 Considering the mechanism of HDS, there exists an important question about the formation of BCH, which is whether the

Figure 6. XRD patterns of sulfided NiMo catalysts.

THDBT, HHDBT, CHB, and BCH, were detected over all the NiMo catalysts, and confirmed by the combination of GC−MS. The fully hydrogenated intermediate DHDBT was not detected in our present work. The conversions of DBT increased significantly with the increasing reaction temperature, and BP and CHB were the main products over NiMo catalysts under evaluated temperatures, except on the NiMo-350 catalyst at 320 2433

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Figure 7. Products distributions of nickel−molybdenum catalysts.

hydrogenation of CHB or the desulfurization of hydrogenated intermediates more easily occurs on the catalyst surface. When we compared CHB and the two partially hydrogenated intermediates (THDBT and HHDBT) detected in our work, the molecule THDBT is nearly flat due to the bridge bond, and the double-bond conjugation with the aromatic ring and the sulfur atom makes it more aromatic than HHDBT, which is the conjugation of an aromatic ring, a cycloalkyl ring, and a sulfur atom in HHDBT. CHB is less aromatic than HHDBT because of the lack of conjugation by a sulfur atom. Therefore, the strength of π adsorption/desorption of these molecules on the catalyst surface decreases in the order: THDBT > HHDBT > CHB. As aforementioned that the HYD route proceeds π adsorption bonding to the catalyst surface through the aromatic system, the formation of BCH is more likely ascribed to the partially hydrogenated products, e.g., THDBT or HHDBT, which is then fully hydrogenated to DHDBT, and last, the sulfur atom is removed, possibly by elimination or hydrogenolysis from the cycloalkyl ring.1 On the

Figure 8. Selectivity of BCH over NiMo catalysts reacted at 320 °C.

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formation of nickel molybdate brought about the growth of particles sizes, from about 50−80 nm in NiMo-450 to 100−300 nm in the NiMo-650 catalyst precursor. HRTEM results indicated that the stacking layers and the slab length of MoS2 nanoclusters increased in the order as follows: NiMo-250 < NiMo-350 < NiMo-450 < NiMo-550. This was in accordance with the XRD results of the sulfided NiMo catalysts, because the characteristic peaks of MoS2 nanoclusters also decreased in this order. The NiMo catalyst precursors deriving from ammonium nickel molybdate through various calcination temperatures showed high hydrogenation performance toward the model compound DBT. There exist many reports covering the active sites or the catalytic active phase of MoS2-based catalysts. It is generally accepted that the active sites of MoS2-based HDS catalysts are the Mo atoms at the edges and corners of the MoS2 crystallites.13 The plain observation of the introduction of a Ni promoter into MoS2-based catalysts is that the promoter increased the HDS activity several fold than nonpromoted MoS2 catalysts.1,13 Prins and co-workers found that the promotion effect of the Ni promoter on the MoS2 catalysts was due to the faster direct desulfurization ability on both DBT and hydrogenated intermediates.10,13 Furthermore, the “NiMo-S” active phase in the Ni-MoS2 nanoclusters was detected by Lauritsen et al. with STM.14,28 In the “Ni-Mo-S” structure, the Ni promoter substituted the Mo atoms at the surface, and the full or partial substitution depended on the cluster size.14 Density functional theory (DFT) study revealed that the Ni promoter substituted Mo atoms that were trigonal-prismatically coordinated by six sulfur atoms at (1 0 0) edges of MoS2 nanoclusters, and the preferred sites for Ni were located at the metal edges surrounded by four sulfur atoms in a square-planar coordination.10,29 Therefore, the square-planar Ni atoms in NiMo catalysts are more accessible than the Mo atoms coordinated by six sulfur atoms at the surface at metal edges and might perform high hydrogenation ability, and the hydrogenation seemed to be more preferred on Ni sites.10 Catalytic results toward DBT conversion showed that the NiMo-350 catalyst exhibited higher HDS activity than other NiMo catalysts. The selectivity of BCH on the NiMo-350 catalyst was about 2-fold as high as that on NiMo-250 and NiMo-450 catalysts, and also around 5-fold as high as that on NiMo-550 and NiMo-650 catalysts. It should be emphasized that the higher selectivity of BCH was detected on NiMo catalysts calcined at temperatures below 450 °C, which contained NiMo-250, NiMo-350, and NiMo-450 catalysts. One of the similarities of these three catalysts was that the phase of ammonium nickel molybdate was detected on these NiMo catalyst precursors. This is in good accordance with our previous work that the NiMo catalyst deriving from ammonium nickel molybdate exhibited high hydrogenation ability.7 At the same time, the selectivity of BCH on NiMo-550 and NiMo-650 catalysts was rather low, indicating that the phase of their corresponding precursor, nickel molybdate, was not as efficient as ammonium nickel molybdate for the deep hydrogenation. As aforementioned, the hydrogenation sites might occur on the Ni sites of the “Ni-Mo-S” structure;10 therefore, it could be deduced that the HDS catalyst precursor endowed with the phase of ammonium nickel molybdate would be more efficient for the formation of the “Ni-Mo-S” structure than that of nickel molybdate. Since there were no other differences of preparation of this series of catalysts except the various calcination temperatures, it might be inferred that the calcination

other hand, the hydrogenation of CHB to BCH is of great difficulty in the absence of DBT, for the hydrogenation of the second aromatic ring ought to be more difficult than that of the first one.1,13 This has been confirmed by other research, that the hydrogenation of DBT to THDBT was about 3-fold faster than the hydrogenation of HHDBT to DHDBT.1 At the same time, the hydrogenation reaction of CHB to BCH can hardly happen in the presence of DBT, because that reaction is inhibited by the presence of sulfur-containing molecules. Therefore, Scheme 1 is proposed for the HDS of DBT over Scheme 1. Possible Reaction Routes in the HDS of DBT

NiMo catalysts. To distinguish the products that were detected (DBT, THDBT, HHDBT, BP, CHB, and BCH) and products that were supposed to be, but were not, observed (especially DHDBT), solid lines and dotted lines were employed to discriminate these products, respectively.

4. DISCUSSION In our previous paper, we reported that the ammonium nickel molybdate was an efficient precursor for deep hydrodesulfurization.7 This novel precursor exhibited higher hydrogenation ability toward fluid catalytic cracking (FCC) diesel than alumina-supported NiMo catalyst.7 In this paper, the effects of various calcination temperatures on the physiochemical and catalytic properties of NiMo catalysts were investigated. Increasing the calcination temperature from 250 to 650 °C caused the decomposition of ammonium nickel molybdate and the formation of nickel molybdate, which was revealed by the results of XRD and FT-IR. The decomposition of ammonium nickel molybdate released small molecules, like NH3 and H2O. The release of these small molecules helped to increase the BET surface area and pore volume, while the mesoporous structure was maintained, as shown in Figure 2. SEM micrographs showed that the agglomeration of NiMo catalyst precursors changed apparently with the various calcination temperatures. When the ammonium nickel molybdate began to decompose, the larger particles (about 500 nm) in the NiMo250 catalyst precursor changed into smaller particles with sizes in the range of 20−40 nm in the NiMo-350 precursor, which were smaller than those in other precursors. Then, the 2435

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temperature would bring significant effects on the NiMo catalyst precursor and the later HDS catalysts. The higher hydrogenation ability was promoted over NiMo catalysts calcined at temperatures below 450 °C, which may be very important in the near future. Because of the heteroatomcontaining compounds, such as DBTs and nitrogen-containing molecules, these refractory compounds could not be efficiently removed unless high hydrogenation ability was afforded.10,28,30,31 Therefore, it could be deduced that the HDS catalysts with excellent hydrogenation performance would be preferred to meet the clean diesel fuel standards. This target could be easily and technically economically achieved by adjusting the calcination temperature over NiMo catalysts.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C.Y.). *E-mail: [email protected] (C.L.). Notes

The authors declare no competing financial interest.

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REFERENCES

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5. CONCLUSION The effects of calcination temperature on the physicochemical properties and HDS activities of NiMo catalyst were investigated. The decomposition of ammonium nickel molybdate and formation of nickel molybdate were confirmed by XRD and FT-IR characterizations. SEM results revealed that increasing the calcination temperature from 250 to 350 °C led to the formation of smaller particles of ammonium nickel molybdate from about 500 to 20−40 nm, while further increasing the calcination temperature from 450 to 550 and 650 °C brought about the agglomeration of nickel molybdate with sizes to around 50−80, 100, and 100−300 nm, respectively. The stacking slab and number of MoS2 nanoclusters were detected increasing in the order: NiMo-250 < NiMo-350 < NiMo-450 < NiMo-550, as confirmed by HRTEM results. XRD results of the sulfided NiMo catalysts also indicated that the agglomeration of MoS2 nanoclusters intensified with increasing calcination temperature. DBT catalytic results showed that the NiMo-350 catalyst showed the highest DBT conversion, and the selectivity of BCH (about 20%) was about 2-fold as high as that on NiMo-250 and NiMo-450 catalysts, and 5-fold as high as that on NiMo-550 and NiMo-650 catalysts. This might be attributed to the formation of the “Ni-Mo-S” structure, which would show high hydrogenation ability. The high hydrogenation ability of the NiMo-350 catalyst will be promising in the future.

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ACKNOWLEDGMENTS

This work was financially supported by the National Key Fundamental Research Development Project of China (973 Project No. 2010CB226905), the National Natural Science Foundation of China (Grant Nos. 21106185 and 21006128), the Fundamental Research Funds for the Central Universities (Grant No. 14CX06032A), the Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20100133120007), the Shandong Provincial Natural Science Foundation of China (Grant No. ZR2011BQ002), and the Postdoctoral Science Foundation of China (Grant No. 2013M530923). Financial support from PetroChina Corporation Limited is also greatly appreciated. 2436

dx.doi.org/10.1021/ef500097u | Energy Fuels 2014, 28, 2429−2436