Hydrodesulfurization of 4,6-Dimethyldibenzothiophene over CoMo

Article pubs.acs.org/IECR

Hydrodesulfurization of 4,6-Dimethyldibenzothiophene over CoMo Catalysts Supported on γ‑Alumina with Different Morphology Xuehui Li, Yongming Chai, Bin Liu, Huan Liu, Jingfeng Li, Ruiyu Zhao, 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, PR China S Supporting Information *

ABSTRACT: Nanostructured γ-alumina with two different morphologies (rod-like and cube-like) was used as support for CoMo hydrodesulfurization catalyst. Both γ-aluminas were prepared by thermal decomposition of ammonium aluminum carbonate hydroxide precursor, which was synthesized by a convenient hydrothermal method at two pH values. Fourier transform infrared spectroscopy of prydine adsorption, thermogravimetric analysis, and 27Al magic angle spinning (MAS) NMR showed that the rod-like γ-alumina exhibited a lower acidity than the cube-like γ-alumina. The result of X-ray diffraction and temperature-programmed reduction indicated that CoMo oxidic catalysts supported on the rod-like γ-alumina presented higher reducibility compared to those of cube-like γ-alumina, because more β-CoMoO4 was formed on the surface of the rod-like γ-alumina than that of the cube-like γ-alumina. After sulfidation, a large stack with slightly longer MoS2 slabs was formed on the rod-like γ-alumina supports, thereby creating a catalyst with higher hydrodesulfurization activity and hydrogenation selectivity. The morphology of γ-alumina has an influence on the activity and selectivity of the as-synthesized CoMo catalyst. and template.5,11 Inevitably, impurity elements are easily introduced into the materials during the preparation of boehmite and, therefore, affect the surface properties of the alumina support. Elemental chlorine can enhance the surface acidity of the alumina support;12 however, sodium can moderate its surface acidity.13 Both elements will hamper the explicit investigations of the morphological effects on the catalytical performance of the as-synthesized catalysts. Therefore, it is important to employ a simple method to synthesis γ-alumina with high purity and homogeneous morphology. A novel alumina precursor, nanocrystals of ammonium aluminum carbonate hydroxide (AACH, NH4Al(OH)2CO3), with many morphologies, has attracted much attention during recent years.14,15 The preparation of AACH is generally carried out by precipitation from aqueous solutions of aluminum salts or Al(OH)3 suspensions with aqueous solutions of NH4HCO3 or (NH4)2CO3.16−19 Through moderate thermal treatment, the nanostructure of AACH undergoes an isomorphous transformation to nanocrystalline γ-alumina. The product retains the morphology of the parent AACH nanostructure, and the byproducts of thethermal decomposition are only NH3, CO2, and H2O.20,21 Consequently, AACH is a promising precursor for the preparation of γ-alumina with high purity and homogeneous morphology. The main objective of the present paper is to study the relationship between the morphology of γ-alumina and the catalytic performance of the as-synthesized catalysts. To exclude the influence of impurities and maintain the homogeneous morphology, one kind of γ-alumina was synthesized but with

1. INTRODUCTION As environmental regulations become more stringent,1 the sulfur content of diesel and gasoline must be reduced to ultralow levels to decrease harmful exhaust emissions and to improve air quality. Hence, it is urgent to enhance the catalytic properties of current hydrodesulfurization (HDS) catalysts. Industrial HDS catalysts utilize Mo(W)-based sulfides as active components and Co(Ni) as promoters supported on alumina,2 and the strategies to improve catalyst performance are mainly focused on the preparation method, the genesis of active phase, and the support properties. It was found that the dispersion and reducibility of supported CoMo active phases were significantly affected by the type and structure of the support.3,4 Therefore, selecting an appropriate material as support is a vital step. γ-Alumina is one of the most widely used supports in HDS, due to its thermal, mechanical, and chemical stability.5,6 The morphology of γ-alumina is a key factor in affecting its physiochemical properties, which can influence the catalytic activities of the catalysts.7,8 For instance, the nanorod γ-alumina9 can provide a bimodal mesoporous structure to improve the activity of CoMo-based HDS catalysts. The lathlike mesostructured γ-alumina supported catalysts10 maintains the good Mo/Co dispersion under reaction conditions, which is benefical to improve the conversion of dibenzothiophene (DBT). However, most researchers have just focused on the textural structures of γ-alumina with different morphologies, and the surface properties induced by the morphological effect of γ-alumina supports were not mentioned. The surface property of γ-alumina, especially the surface acidity2,4 is very important to affect the dispersion of the active phases and adjust the metal−support interaction. Hence, the morphological effect of γ-alumina support on the catalytic performance of the HDS catalyst needs a further investigation. γ-Alumina with different morphologies are usually produced by thermal dehydration of boehmite in the persence of surfactant © 2014 American Chemical Society

Received: Revised: Accepted: Published: 9665

February 22, 2014 May 14, 2014 May 16, 2014 May 16, 2014 dx.doi.org/10.1021/ie5007504 | Ind. Eng. Chem. Res. 2014, 53, 9665−9673

Industrial & Engineering Chemistry Research

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different morphologies by thermal decomposition of AACH precursors, which were prepared in a single step, low temperature hydrothermal technique without adding any template. The textural, structural, and surface properties of the corresponding γ-aluminas were characterized with X-ray powder diffraction, scanning electron microscopy, N2-sorption, Py-FTIR, and 27Al magic angle spinning (MAS) NMR. CoMo catalysts were loaded on these carriers. The dispersion of the active components influenced by the γ-alumina morphology was characterized by temperature-programmed reduction tests on calcined oxide CoMo/γ-Al2O3 samples. Finally, the HDS of 4,6-dimethyldibenzothiophene (4,6-DMDBT) was chosen to evaluate the activity of the catalysts.

derived from the desorption branch of the isotherms utilizing the Barrett−Joyner−Halenda (BJH) method. The types of surface acid sites on the catalysts were determined through FT-IR pyridine adsorption measurement, performed on a Newus Fourier transform infrared spectrometer (Nicolet, USA). Before analysis, the samples were degassed in He at 350 °C for 3 h; then, they were cooled and exposed to a saturated pyridine atmosphere at room temperature for 2 h. After adsorption, the infrared spectrum was recorded at a resolution of 4 cm−1 at 100 °C while outgassing. Thermogravimetric (TG) analysis was employed to record the weight loss of pyridine adsorbed on the surface of the aluminas, so that the acidity of calcined samples could be quantitatively determined. Experiments were carried out on a NETZSCH STA 449C analyzer (Germany) using a heating rate of 10 °C min−1 and a nitrogen flow of 20 cm3 min−1 from room temperature to 500 °C. 27 Al MAS NMR spectra was acquired on a Bruker Avance 300 spectrometer (Switzerland) at a frequency of 104.3 MHz using a 5 mm MAS probe with a spinning rate of 10 kHz. Short single pulses (π/18) were used at a reprocessing time of 1.0 s. The 27Al chemical shifts were referenced to an Al(NO3)3 aqueous solution of 0.1 mol L−1. The population of (AlOx) species was estimated by fitting Lorentzian lines to the peaks, calculating the areas of the peaks, and dividing them by the total area. Temperature-programmed reduction (TPR) measurements were conducted with a Quantachrome CHEMBET-3000 instrument. The catalyst sample (about 100 mg) was treated in 10 vol % H2/Ar with a flow rate of 90 mL min−1 and was heated at a rate of 10 °C min−1 to 1000 °C. During this process, a thermal conductivity detector was used to measure the hydrogen consumption. The morphology of the precursors and calcined samples was obtained with a Hitachi S-4800 field emission scanning electron microscope (FE-SEM, Japan) with an accelerating voltage of 1.5 kV. Transmission electron microscopy (TEM) was carried out on a JEM-2100UHR electron microscope (JEOL, Japan) at an accelerating voltage of 200 kV. The distribution of the MoS2 slab size was manually measured by taking about 300−400 particles from at least 15 representative micrographs. The average stacking number N̅ and particle length L̅ were calculated according to the equation:

2. EXPERIMENTAL SECTION 2.1. Supports and Catalyst Preparation. All chemicals were analytical-grade reagents and were used without further purification. Aluminum nitrate (Al(NO3)3·9H2O, 7.5 g) was dissolved in 100 mL of deionized water to form a transparent solution. Then, 100 mL of ammonium bicarbonate solution (NH4HCO3, 15.8 g) was added under vigorous stirring. The pH of the solution was adjusted to 6.0 or 10.0 by dropwise adding ammonia solution (25 wt %), and the mixture was kept at 80 °C for 24 h. Afterward, the product was collected and washed several times with deionized water. The resulting solid (AACH) was dried under vacuum at 60 °C for 12 h and will be called preACH-1 (pH = 6.0) and pre-ACH-2 (pH = 10.0). Finally, the product was calcined at 550 °C for 4 h with a ramp rate of 2 °C min−1. On the basis of the pH value of the final solution, the samples were named ACH-1 and ACH-2. For comparison, alumina (CA) was made from a commercial boehmite (pre-CA), from Condea Chemical Co. (Germany). The γ-alumina support was extruded by a twin-screw extrude rate of 300 r/min, and before extrusion, 1.0 g of nitric acid, 70 g of deionized water, and 1.0 g of lubricant (sesbania powder) were added to 100 g of precursor and stirred well. Afterward, the extrudates were dried at 120 °C for 12 h and calcined at 550 °C for 4 h with a ramp rate of 2 °C min−1. CoMo/γ-Al2O3 catalysts with 15.5 wt % MoO3 and 4 wt % CoO were prepared by impregnating the γ-alumina extrudates with a mixed solution of ammonium heptamolybdate ((NH4)6Mo7O24·4H2O) and cobalt nitrate (Co(NO3)2·6H2O). The impregnated materials were dried overnight in air at 80 °C and calcined at 500 °C for 3 h with a 2 °C min−1 heating ramp. The corresponding CoMo catalysts were labeled CoMo/ACH-1, CoMo/ACH-2, and CoMo/CA. 2.2. Characterization. X-ray powder diffraction (XRD) patterns of the samples were collectedon a Panalytical X’Pert Pro MPD diffractometer (Netherlands) using Cu Kα radiation at a 2θ scan rate of 0.05° s−1. The accelerating voltage and applied current were 40 kV and 30 mA. The crystallite size of the samples was calculated according to the Scherrer formula D = (0.9λ)/ (βcos θ), where D is the crystallite size, λ is the wavelength (1.54060 Å), β is the full width at half-maximum intensity (fwhm) of the diffraction peaks in radians, and θ is Bragg’s diffraction angle. Information about pore size distributions and specific surface areas was obtained from nitrogen adsorption−desorption isotherms, which were measured on a Micromeritics TRISTAR 3020 adsorption analyzer (USA) at −196 °C. Before the adsorption measurements, all samples were degassed at 140 °C in vacuum for 6 h. The surface areas were calculated by the Brunauer− Emmett−Teller method, and the pore size distributions were

n

L̅ =

∑i = 1 niLi n

∑i = 1 ni

n

N̅ =

∑i = 1 niNi n

∑i = 1 ni

where Li is the length of particle i, ni is the number of particles with a Li length or Ni layer, and Ni is the number of layers in the particle i. 2.3. Catalytic Activity Assessment. Catalytic activity tests for the HDS of 4,6-DMDBT were carried out in a high-pressure continuous-flow microreactor with 10 mL catalysts. Prior to the reaction, the oxide catalysts were sulfided with a 3 wt % CS2 in cyclohexane solution at 400 °C for 8 h under a hydrogen pressure of 2.0 MPa. Then, 4,6-DMDBT at a concentration of 0.5 wt % in toluene was introduced at an H2/oil ratio of 300/1 (v/v) and a liquid hour space velocity (LHSV) of 2 h−1. The tests were stabilized at 300 °C for 5 h before the liquid products were collected and analyzed with a Varian 3800 gas chromatograph equipped with CP-5 capillary column and flame ionization detector. 9666

dx.doi.org/10.1021/ie5007504 | Ind. Eng. Chem. Res. 2014, 53, 9665−9673


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Figure 1. XRD patterns of the as-synthesized samples (a) and the calcined samples (b).

Figure 2. SEM micrographs of the AACH precursors pre-ACH-1 (a) and pre-ACH-2 (b) and of their calcined γ-Al2O3 products ACH-1 (d) and ACH-2 (e) and of the commercial boehmite precursor pre-CA (c) and γ-Al2O3 sample CA (f).

3. RESULTS AND DISCUSSION

The XRD patterns of the pre-ACH-1 and pre-ACH-2 samples (Figure 1a) can be indexed to crystalline ammonium aluminum carbonate hydroxide (AACH) with a composition of NH4Al(OH)2HCO3 (JCPDS 01-076-1923). The high intensity of the

3.1. Support Characterization. 3.1.1. Textural Properties of AACH and γ-Alumina Synthesized at Different pH Values. 9667



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aggregation ratio of these products, which is calculated according to the equation:23,24

XRD peaks of the as-synthesized samples indicates that the AACH phase synthesized in this work is highly crystalline at pH 6.0 and 10.0. Figure 1b presents the XRD patterns of the samples calcined at 550 °C for 4 h. All diffraction peaks can be indexed to γ-Al2O3 (JCPDS Card 10-425). No peaks from other phases were observed, indicating a high purity of the products. Furthermore, in Figure 1b, the peak heights of the γ-Al2O3 samples are larger than those of the commercial alumina (CA), indicating that the ACH-1 and ACH-2 have a better crystallinity than that of the CA. Figure 2 shows the SEM images of the AACH and γ-Al2O3 samples prepared at different pH values. Figure 2a,b is the SEM images of the AACH. They show a regular morphology and uniform size. Different pH values lead to different shapes of the AACH crystallites. At pH 6.0, the pre-ACH-1 mainly consists of rod-like nanoparticles with a length of about 400−500 nm and a diameter of about 100−130 nm (Figure 2a). When the pH value was tuned to 10.0, the morphology of the AACH changed. The pre-ACH-2 is composed of cube-like nanoplatelets (Figure 2b), and the size of the AACH nanocrystals was smaller with a dimension of about 100 nm. The increase in pH value of solution finally leads to the increase in nucleation rate. Due to the high nucleation rate, a large number of small nanocubes are formed that cannot grow to long nanorods. The γ-Al2O3 calcined from pre-ACH-1 and pre-ACH-2 (Figure 2d,e) kept the rod-like and cube-like structure, but their size decreased due to the phase transformation. These results coincide with those of Zhu et al.,22 in that the transformation of AACH to γ-Al2O3 is pseudomorphic, which preserves the morphology and textural structure of AACH. The commercial alumina (CA) has an nonuniform morphology and very small size. The results of the porous structure and pore size distribution of the as-obtained calcined products are shown in Figure S1, Supporting Information. According to the IUPAC classification, all samples exhibit a type IV isotherm, revealing the existence of abundant mesoporous structure. The hysteresis loops are of type H3, which is often associated with slit-like pores as a result of the aggregation of the wire-like particles. Together with the SEM results, we conclude that the mesopores arise from the space between the particles. The textural properties of the calcined samples are listed in Table 1. Compared with the commercial

ψ = SA theor /SABET

where SA is surface area (m2 g−1) and SAtheor = 6000/(ργ‑Al2O3 × D) (ργ‑Al2O3 = 3.65 g cm−3, the theoretical density of the γ-Al2O3 phase, and D is the average particle size (nm) derived from the X-ray diffraction data). From the obtained aggregation ratio ψ, we estimate that the nanocrystals are more loosely packed in the rod-like alumina particles of ACH-1. Vidruk et al.23 proposed that the surface acidity and catalytic activity of γ-alumina can be controlled by adjusting the nanocrystalline contact interface. The tetrahedral-coordinated aluminum atoms (AlO4) responsible for the Lewis acidity of γ-alumina can be created and stabilized in grain boundary (GB) areas proportional to the nanocrystal interface. In our case, the contact interface in ACH-1 is smaller than in ACH-2 and CA. The data suggest that the nanostructured γ-alumina with different morphology may possess different acidity. 3.1.2. Relationship between Morphology and Surface Acidity of γ-Alumina Samples. The chemical properties of γ-alumina are usually affected by the surface composition, particularly, the type, quantity, and acidity of the surface catalytic sites.25 Figure 3 shows the FT-IR spectra of pyridine adsorbed on

Figure 3. DRIFT spectra of pyridine adsorption on the alumina samples.

Table 1. Characteristic Properties of Alumina and CoMoBased Catalysts sample ACH-1 ACH-2 CA CoMo/ ACH-1 CoMo/ ACH-2 CoMo/ CA

average pore size (nm)

pore volume (cm3g−1)

BET surface area (m2g−1)

13.8 13.3 10.8 10.3

0.88 0.83 0.71 0.50

249 256 265 191

8.9

0.42

196

8.3

0.43

207

crystal size (nm)

crystal aggregation ratio ψ

4.2 3.8 2.8

1.5 1.8 2.3

the different γ-alumina morphologies. There are five spectral bands in the 1400−1700 cm−1 region. The band at 1613 cm−1 is attributed to pyridine coordinated to the Lewis acid site of moderate strength, and the band at 1593 cm−1 is attributed to the weak interaction between pyridine and aprotic hydroxyl groups.26,27 The band at 1444 cm−1 is assigned to pyridine species bonded to Lewis acid sites.28 The band at 1575 cm−1 is due to the physisorption of pyridine. The band at 1487 cm−1 is also attributed to pyridine coordinated to Lewis acid sites. Besides, Brønsted acid sites were not detected in all samples. Deduced from the area of the curves, the total acidity is higher in the sample ACH-2 than in the sample ACH-1, and sample AC has the highest total acidity. To quantitatively calculate the total acidity of γ-alumina samples, thermogravimetric analysis of pyridine (TG-Py) adsorbed on γ-alumina was employed. The results are shown in Table 2. Considering the physisorption of water and pyridine, the total acidities of the prepared samples are calculated on the basis of the weight loss from 100 to 500 °C. According to the results in ref 21, the amount of weak acidity is defined as the amount of pyridine desorbed in the temperature range of 100−200 °C, and the

alumina CA, the alumina synthesized in the present work possess a higher pore volume, but their surface areas are almost the same (240−270 m2 g−1). We know that the specific surface area can vary widely depending on the crystallite size, shape, and porosity. The SEM results show that the particle shape and size of all products are different (ACH-1 > ACH-2 > CA). This means that the aggregation ratio of the products is different. Table 1 lists the 9668



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surface area, pore volume, and average pore diameter are mainly caused by the loading of the Mo and Co species on the supports. The XRD patterns of the prepared calcined catalysts are shown in Figure 5. Besides the XRD peaks of γ-alumina, several peaks are

Table 2. Amount of Lewis Acid Sites and Relative Population of Aluminum Species on Alumina Samples relative population (%)

acid site density (μmol Py m−2) samples

weak

moderate

strong

total

AlO4

AlO6

ACH-1 ACH-2 CA

0.57 0.97 1.24

0.76 1.47 1.32

0.12 0.15 0.22

1.45 2.59 2.78

18 20 21

82 80 79

moderate acidity of the samples is related to the range of 200− 400 °C, while the strong acidity is defined as the desorption of pyridine above 400 °C. Compared to the ACH-2 and CA, the total Lewis acidity of ACH-1 is the lowest, which is in accordance to the results in Figure 3. The acidity of γ-alumina is essentially originated from the coordination of Al atoms and the chemical nature of their neighbors. Consequently, creating 4-fold (AlO4) and 5-fold (AlO5) coordinated aluminum species are the keys to control its acidity. Figure 4 shows the coordination environment of the Figure 5. XRD patterns of as-synthesized calcined catalysts.

observed in the range of 20°< 2θ < 35° in all samples. These peaks are identified as CoMoO4 crystallites. The peaks at 23.5°, 26.5°, 27.3°, and 33.0° are attributed to β-CoMoO4 phase, which is characterized by Co in octahedral coordination and Mo in tetrahedral coordination29.30 The crystallinity of β-CoMoO4 in catalyst CoMo/ACH-1 is better than that in catalyst CoMo/ ACH-2 and CoMo/CA, evidenced by the increased intensity in the diffraction peaks at 26.5° and 27.3°. This indicates that the Co and Mo species interact more easily on ACH-1 than on ACH-2 and CA, due to less Lewis acid sites at the surface of ACH-1 as evidenced by the FTIR spectra (Figure 3). On one hand, Co(NO3)2 could react with (NH4)6Mo7O24 to form β-CoMoO4 during the catalyst perparation. On the other hand, γ-alumina not only scattered Co(NO3)2 and (NH4)6Mo7O24 in its porous structure but also adsorbed these active components on its surface hydroxyl groups and acid sites to form the Anderson type-heteropolymodybdate [Al(OH)6Mo6O18] and CoAl2O4, respectively. Therefore, it appears that the morphology difference of γ-alumina leads to a change in the interaction between the support and the oxide species. The TPR results on the calcined oxidic CoMo/γ-Al2O3 samples are shown in Figure 6. All samples exhibited three main peaks within the range of 400−1000 °C. The low-temperature

Figure 4. 27Al-mass NMR spectra of the γ-Al2O3.

γ-alumina samples that was confirmed by 27Al MAS solid-state NMR measurements. The signal at 4.7 ppm corresponds to the octahedral-coordinated Al atoms in γ-Al2O323 or in amorphous polymeric aluminum oxide phases. The signal around 63 ppm is attributed to the tetrahedral-coordinated Al atoms, also typically found in alumina.8 Integration of the peaks yielded 18% AlO4 for ACH-1 and 20% AlO4 for ACH-2 (Table 2). As the pH value increased from 6.0 to 10.0, the morphology of the synthesized γ-alumina changed form rod-like to cube-like, and the relative number of tetrahedral sites (AlO4) is slightly larger in the sample ACH-2 than that in the sample ACH-1. 5-fold coordinated aluminum species were not found in all samples. These changes may be associated with the crystallographic structure of the γ-alumina samples. Compared with γ-alumina nanocubes and CA, the bigger γ-alumina nanorods lead to a lower crystal aggregation ratio. According to Vidruk et al.,23 the lower aggregation ratio is evidence of the more loosely packing of the nanoparticles and AlO4 can be created in the grain bound areas proportional to the nanocrystal interface. Hence, less AlO4 was created in ACH-1. This suggests that the surface acidity of nanostructured γ-alumina can be controlled by adjusting the morphology of the nanocrystals. 3.2. Catalyst Characterization. Nitrogen adsorption− desorption isotherms of the catalysts are shown in Figure S2a,b, Supporting Information, and the texture parameters calculated by the BJH method are presented in Table 1. The reduction in

Figure 6. Temperature-programmed reduction profiles of the CoMo catalysts. 9669



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Figure 7. HRTEM micrographs of MoS2 crystallites observed with spent CoMo/ACH-1 (a); CoMo/ACH-2 (b); CoMo/CA (c) catalysts.

Figure 8. Distribution of MoS2 slab lengths (a) and stacking degree of MoS2 slabs (b).

ACH-2 and CA γ-alumina. Furthermore, the peak area represents the hydrogen consumption. The relative areas under the reduction peak of the β-CoMoO4 over the CoMo/ACH-1 is higher than those of CoMo/ACH-2 and CoMo/CA, indicating that more β-CoMoO4 could be reduced at the surface of ACH-1. Some studies demonstrated that this intermediate oxide is a good precursor for sulfide HDS catalysts, because the weak interaction between β-CoMoO4 and support could lead to the formation of the active CoMoS Type II phases. Moreover, the formation of β-CoMoO4 prevents the formation of less active CoAl2O4 and Co3O4 species.34−36 The changes of all the TPR patterns confirm the conclusion that the differences in the morphology of γ-alumina may lead to a change in the degree of interaction between the carrier and the supported oxide species. The typical TEM micrographs of the sulfided CoMo catalysts are presented in Figure 7. The views exhibited the well-known MoS2 slab-like structure, which were homogeneously dispersed on the support. The stacking degree and slab length distributions of the MoS2 slabs are shown in Figure 8. About 28% of the MoS2

peak at 520 °C can be assigned to the first reduction step of Mo6+ to Mo4+ in amorphous multilayered Mo oxides or dispersed polymeric Mo structures (octahedral Mo species),31 and the second overlapped peak at 570 °C is generally associated with the partial reduction of cobalt molybdate (β-CoMoO4) to Co2Mo3O8.32 The highest-temperature peak at ∼890 °C can be attributed to the deep reduction of all Mo species, including the further reduction of partially reduced MoOx species obtained at lower temperatures and the partial reduction of tetrahedral coordinated Mo6+ species which strongly interact with the alumina supports.31,33 The first and third reduction peak of CoMo/CA shifts to higher temperature, compared with the other catalysts. It means that the interaction between MoOx species and CA is stronger than that of ACH-1 and ACH-2. However, the second reduction peak shifts to higher temperature in the order of CoMo/ACH-1 < CoMo/ACH-2 < CoMo/CA, this indicates that the interaction of the β-CoMoO4 and the ACH-1 is weaker than that with the 9670



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Table 3. Conversion and Products Distribution for the HDS of 4,6-DMDBT over Different Catalysts products (%) catalyst

average length (nm)

stacking layer number

TH

CoMo/ACH-1 CoMo/ACH-2 CoMo/CA

4.16 4.01 3.71

2.19 1.73 1.69

2.2 0.8 0.6

DMBP MCHT 8.8 15.2 18.8

37.3 23.0 39.9

DMBCH

HYD/DDS ratioa

conversionb (%)

desulfurization ratioc (%)

28.2 20.9 5.7

7.4 2.9 2.4

75.6 59.9 65.0

73.4 59.1 64.4

a c

HYD/DDS ratio: (MCHT + DMBCH)/DMBP. bConv. (%) = 4,6-DMDBT conversion (%) = TH + HH + MCHT + DMBCH + DMBP. Desulfurization ratio (%) = MCHT + DMBCH + DMBP = conversion − HH.

slabs in catalyst CoMo/ACH-1 is present as single layers, 45% as double layers, and 27% as more than three layers. For the sample CoMo/ACH-2, more than 50% of the slabs is present as single layers, 26% as double layers, 17% as three layers, and 10% as crystallites with between 4 and 6 layers. For the CA support catalyst, the stacking degree of the MoS2 slabs is almost the same as for ACH-2, except for the higher single layer distribution. The average slab length and stacking number are presented in Table 3. The interplanar distance of the MoS2 crystallites in CoMo/ACH-1 is 0.618 nm, and the average length is 4.16 nm. On the contrary, MoS2 looks more dispersed in CoMo/ACH-2 and is mainly composed of small and poorly stacked MoS2 particles with an average length of 4.01 nm and stacking number of 1.73. The MoS2 slabs in sample CoMo/CA have nearly the same stacking (1.69 slabs) as CoMo/ACH-2; its average length is smaller than those of CoMo/ACH-1 and CoMo/ACH-2. It could be seen that the ACH-1 supported catalyst has a higher stacking number than that of the ACH-2 and CA catalysts. This confirms the weaker interaction between the ACH-1 support and the CoMo species. The support can determine the stabilization and dispersion of the CoMoS active phase for the HDS catalyst reaching the best performance.37 In our work, the morphologies of the γ-alumina particles are related to the acidic nature, which can influence the dispersion of the CoMo oxide species by influencing the interaction between CoMo oxide species and the supports. The dispersion of CoMo oxide species is closely related to the dispersion of MoS2 slabs; hence, the acidity could influence the dispersion of MoS2 slabs. The decreased surface acidity of γ-alumina may lead to the increased particle size of MoS2. The morphology of γ-alumina particles are also related to the dispersion of MoS2 slabs by affecting the acidity. A large stack with slightly longer MoS2 slabs was formed on the rod-like γ-alumina supports which possessed the lowest acidity among the three supports. 3.3. Catalytic Activity Assessment. The results of the HDS of 4,6-DMDBT over the supported catalysts are shown in Table 3. The conversion of 4,6-DMDBT is the highest over catalyst CoMo/ACH-1 compared to catalyst CoMo/ACH-2 and CoMo/CA. However, the production distributions change significantly with the support morphology. The amount of 3,3′dimethylcyclohexylbenzene (MCHT) + 3,3′-dimethylbicyclohexyl (DMDCH) produced over these catalysts follows the order CoMo/ACH-1 > CoMo/ACH-2 ≈ CoMo/CA, while the amount of 3,3-dimethylbiphenyl (DMBP) for the catalysts is in the order CoMo/CA > CoMo/ACH-2 > CoMo/ACH-1. Previous studies show that the transformation of 4,6-DMDBT takes place by two reaction routes (Figure 9): direct desulfurization (DDS) and hydrogenation (HYD).38−40 The DDS route yields DMBP, and the HYD route involves mainly 4,6-dimethyltetrahydrodibenzothiophene (TH-DMDBT) and 4,6-dimethylhexahydrodibenzothiophene (HH-DMDBT); both intermediates can be further desulfurized to MCHT and DMBCH.

Figure 9. Reaction network for 4,6-DMDBT hydrodesulfurizaiton.

The DMBCH selectivity over catalyst CoMo/ACH-1 is higher than those over catalyst CoMo/ACH-2 and CoMo/CA, indicating the superior HDS activity by the HYD route. The activity and selectivity of the HDS catalysts are strongly affected by the morphology of the sulfide catalytic phase. In order to understand the difference in catalysis performance, it is necessary to correlate the activities of the two catalysts with their structures. Known form the aforementioned results, the morphologies of the different γ-alumina particles were related to the acidity, which could influence the dispersion of the CoMo oxide species by influencing the interaction between CoMo oxide species and the supports, as proved by XRD and TPR. The results of the XRD has shown that catalyst CoMo/ACH-1 has bigger β-CoMoO4 crystals than catalyst CoMo/ACH-2, indicating the weaker interaction between the supported oxide species and the rod-like γ-alumina (ACH-1). The TPR results have also proved this conclusion and have further revealed that more β-CoMoO4 could be reduced on the surface of the rod-like γ-alumina. The acidity of γ-alumina can influence the dispersion of MoS2 slabs, because the loosely bound species are highly reducible and easily sulfided.41 The distribution frequencies of the MoS2 slabs stacked in the range of 2−4 layers are larger in catalyst CoMo/ACH-1 (60%) than that in the catalyst CoMo/ ACH-2 (40%). Hensen et al.42 proposed that the HYD rates increased with an increasing stacking degree of MoS2 due to the less hampered planar adsorption geometry model of reactants; hence, the amount of MCHT + MDCH decreased as the acidity of the alumina support increased. Consequencely, the morphology of γ-alumina could influence the activity and selecivity of as-synthesized CoMo/γ-Al2O3 catalyst.

4. CONCLUSION Nanosized rod-like and cube-like γ-alumina powders were obtained by thermal decomposition of AACH precursors, which were successfully synthesized with a simple hydrothermal route by regulating the pH of the reaction solution. Compared 9671



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with the cube-like γ-alumina and commercial γ-alumina, the rodlike γ-alumina possesses a lower amount of acidity, because of the higher crystallinity and lower aggregation ratio, which could represent the loose packing of the randomly stacked particles. The acidity of the supports has an effect on the dispersion of the CoMo oxide active phase, because more β-CoMoO4 is formed on the rod-like γ-alumina than on the cube-like and commercial γ-alumina. After sulfidation, the stacking number of the MoS2 slabs supported on γ-alumina decreased in the order CoMo/ ACH-1 > CoMo/ACH-2 > CoMo/CA. This is in accordance with the order of HYD selectivity toward 4,6-DMDBT. The catalyst CoMo/ACH-1 exhibited the highest HDS activity due to higher hydrogenation activity. Hence, the morphology of γ-alumina could influence the activity and selecivity of assynthesized CoMo/γ-Al2O3 catalysts.

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ASSOCIATED CONTENT

S Supporting Information *

The nitrogen adsorption−desorption isotherms and pore size distributions of the alumina supports and the as-synthesized CoMo oxide catalysts. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 532 8698 1716. Fax: +86 532 8698 1787. E-mail: cgliu. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by grants from the Major State Basic Research Development Program of China (973 Program, 2010CB226905), National Natural Science Foundation of China (Grant Nos. 21006128 and 21106185), Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20100133120007), Shandong Provincial Natural Science Foundation of China (ZR2011BQ002), and Fundamental Research Funds for the Central Universities.

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Hydrodesulfurization of 4,6-Dimethyldibenzothiophene over CoMo

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