MgO Nanosheet Assemblies Supported CoMo Catalyst with High

Article pubs.acs.org/IECR

MgO Nanosheet Assemblies Supported CoMo Catalyst with High Activity in Hydrodesulfurization of Dibenzothiophene Lei Zhang,†,‡ Wenqian Fu,†,‡ Mei Xiang,† Wenchang Wang,‡ Mingyang He,‡ and Tiandi Tang*,†,‡ †

College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou, Zhejiang 325027, P. R. China Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, College of Chemistry and Chemical Engineering, Changzhou University, Changzhou, Jiangsu 213164, P. R. China

‡

Downloaded by UNIV OF CAMBRIDGE on August 31, 2015 | http://pubs.acs.org Publication Date (Web): May 18, 2015 | doi: 10.1021/acs.iecr.5b00452

S Supporting Information *

ABSTRACT: A magnesium oxide nanosheet assembly (NS-MgO) with high surface area (255 m2/g) and nanopore volume (0.30 cm3/g) was prepared by a pressure-assisted carbonation method at large scale. After loading of cobalt and molybdenum (CoMo) species and followed by sulfidation, the NS-MgO supported CoMo catalyst (Co-MoS2/NS-MgO) exhibits high activity (94.2%) and good nitrogen tolerance in the hydrodesulfurization (HDS) of dibenzothiophene (DBT), compared with a conventional γ-alumina-supported CoMo catalyst (64.1%). These results are attributed to the difference in the basicity of the NSMgO and γ-Al2O3 supports. The large amount of the middle strong basic sites on NS-MgO can avoid the polymerization of Mo species and form small Mo oxide clusters. After sulfidation, the small MoS2 clusters with shorter lengths and less stacking formed on the NS-MgO contribute to an increase in the sites available for Co promotion, resulting in the Co-MoS2/NS-MgO catalyst with high HDS activity. catalysts.26,29 In this paper, we develop a facile method to prepare MgO nanosheet assemblies (NS-MgO) with high surface area at a large scale. After introduction of CoMo species followed by sulfidation, the obtained catalyst (Co-MoS2/NSMgO) shows high HDS activity and good nitrogen tolerance, compared with conventional CoMo/γ-Al2O3 catalyst in the HDS of the dibenzothiophene. This feature is due to the fact that the Mo species are highly dispersed on the basic NS-MgO surface and easily formed small MoS2 clusters, which can contribute to an increase in the sites available for Co promotion, resulting in the Co-MoS2/NS-MgO catalyst with high HDS activity.

1. INTRODUCTION Development of highly active hydrodesulfurization (HDS) catalyst is of great importance for producing ultraclean fuel.1−3 Conventional metal sulfide catalysts (CoMo and NiMo) supported on alumina (γ-Al2O3) are widely used in oil refining.4−8 However, the catalysts should be operated at high temperature and pressure to produce clean fuel because of their low activity, which greatly increases fuel cost. Much effort has been attempted all the time to improve the catalytic performance of metal sulfide catalysts. For instance, adding phosphorus,9 fluorine,10 boron,5 and acidic zeolites11 into γAl2O3 is designed to modify the acidity of catalysts or increase the metal dispersion, but the deep hydrodesulfurization over this kind catalyst is still a challenge. On the other hand, use of binary supports such as TiO2−Al2O3,12 SiO2−Al2O3,13 and ZrO2−Al2O314 is to weaken the interaction between the metal and supports to form more active phases but resulted in the undesirable agglomeration of metals. With this aspect, developing new carrier for preparation of high active metal sulfide catalyst is very necessary. For example, ordered mesoporous molecular sieves (MCM-41 and SBA-15) with high surface area were used to increase the metal sulfide dispersion, leading to high HDS activity.15−19 However, the relatively low hydrothermal stability of this type of material severely limits their industrial applications.20 Recently, metal sulfides (CoMoS2 and NiMoS2) catalysts supported on mesoporous zeolites show unprecedented hydrodesulfurization activities,21 while the relatively strong acidity of the catalysts could induce the coke formation.22,23 It has been reported that the magnesium oxide (MgO) with basicity benefits the dispersion of the acidic MoO3 species on its surface, and the sulfided catalyst shows intrinsic high HDS activity.24−28 However, because of its low surface area, it is still a challenge for MgO as a support to prepare highly active HDS © 2015 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Synthesis of MgO Nanosheet Assemblies. The main experimental apparatus with a volume of 5.0 L and inside diameter of 160 mm for synthesizing MgO nanosheet assemblies is shown in Figure 1. In a typical run, 100 g of magnesium hydroxide (purity 98%) and 3.5 L of water were added into the reactor. Next, the carbon dioxide (CO2) (purity 99.9%) was supplied into the reactor and pressurized to 2.0 MPa and kept at this pressure during the carbonation process. After stirring at ambient temperature for 1 h, the obtained clear aqueous solution containing magnesium bicarbonate is pumped and then quickly heated to 100 °C by the heating reactor. Then the magnesium carbonate salt precursor was obtained by filtration and dried at 60 °C overnight. Finally, the MgO nanosheet assemblies, denoted as NS-MgO, are prepared by the Received: Revised: Accepted: Published: 5580

February 2, 2015 May 5, 2015 May 6, 2015 May 6, 2015 DOI: 10.1021/acs.iecr.5b00452 Ind. Eng. Chem. Res. 2015, 54, 5580−5588

Article

Industrial & Engineering Chemistry Research

sample was heated from room temperature to 1000 °C with a heating rate of 15 °C/min. Transmission electron microscopy (TEM) images of the sulfided catalysts were obtained on a JEM-2100F microscope with a limited line resolution capacity of 1.4 Å, operating at 200 kV. Before the measurement, the samples were dispersed ultrasonically in an ethanol solution and dropped onto a Cu grid coated with carbon membrane. X-ray photoelectron spectroscopic (XPS) experiments were performed for sulfided catalysts, using an ESCALAB MK II system. The sulfided catalysts were transferred into a bottle filled with absolute cyclohexane under nitrogen stream. The cyclohexane was removed, and the residual catalyst was quickly moved to a sample holder before the sample loading to the XPS chamber. For analysis, the obtained Mo 3d and Co 2p spectra were decomposed by using “XPS processing” software and applying a Shirley background subtraction and Gaussian−Lorentzian decomposition parameters with 30/70 Gaussian/Lorentzian. The rules applied during the decomposition for the Mo 3d spectra were as follows. (1) Binding energy between the doublets: ΔE(3d3/2−3d5/2) ≈ 3.2 eV. (2) Relative area of doublet: A(3d5/2)/A(3d3/2) = 1.5. The decomposition for the Co 2p spectra was performed according to the method reported in literature.30 2.4. Activity Tests. The HDS tests were carried out in a high-pressure laboratory-scale setup equipped with a stainless steel fixed-bed reactor. Prior to the HDS test, the catalysts (40− 60 mesh) were presulfided in a gas mixture of H2−H2S (10 vol % H2S) from room temperature to 400 °C with a heating rate of 2 °C/min and held for 3 h. The sulfided catalyst (0.2 g) was then diluted with SiC (40−60 mesh, 1.5 g) and loaded into the fixed-bed reactor. The HDS experiments are performed at a temperature of 260 °C, a pressure of 5.0 MPa, a weight hourly space velocity (WHSV) of 21.0 h−1, a H2 flow of 110 mL/min, and a liquid feed of 0.6 wt % dibenzothiophene (DBT) in decalin. After the reaction was stabilized for 10 h, the products were collected every 60 min and analyzed using an Agilent 7890A GC instrument equipped with an FID detector. The selectivity of the products was calculated as follows:


Figure 1. Schematic diagram for synthesizing NS-MgO.

calcination of the magnesium carbonate salt precursor at 450 °C for 5 h in air. 2.2. Catalyst Preparation. The CoMo catalysts were prepared by incipient wetness impregnation method using an ammonia solution containing the required amount of ammonium heptamolybdate ((NH4)6Mo7O24·4H2O), cobalt nitrate (Co(NO3)2·6H2O), and ethylenediaminetetraacetic acid (EDTA). The molar ratio of Co/Mo/EDTA was 1:2:1, and the desired Mo loading was 10.7 wt %. The pH value of the impregnation solution was about 10.0. After impregnation, the sample was dried under ambient temperature for 12 h in air and then further dried at 100 °C for 12 h. The catalysts with NSMgO and γ-Al2O3 supports were designated as CoMo/NSMgO and CoMo/γ-Al2O3, respectively. In addition, the NSMgO and γ-Al2O3 supported Co (3.3 wt %), and Mo (10.7 wt %) catalysts were also prepared in the same way. 2.3. Characterization. An X-ray diffraction (XRD) pattern was obtained with a RIGAKU Ultimal V diffractometer using Cu Kα radiation. Thermogravimetric (TG) analyses of the dried NS-MgO sample were carried out with increasing rate of 10 °C/min under flowing air (100 mL/min) on a SDT2960 instrument. Nitrogen sorption isotherm was obtained using a Micromeritics ASAP 2020M apparatus at the temperature of liquid nitrogen (−196 °C). Before the measurements, all the samples were degassed for 10 h at 350 °C. Specific surface area was calculated from the adsorption data, using the Brunauer− Emmett−Teller (BET) equation. The mesopore size distribution was calculated using the Barrett−Joyner−Halenda (BJH) model by the adsorption branch. Scanning electron microscopy (SEM) experiments were performed on a NanoSEM200 apparatus. The basicity of the supports was measured using temperature-programmed desorption of CO 2 (CO 2 -TPD) on a Micromeritics ASAP2920 instrument. 300 mg of the sample was heated to 450 °C and maintained for 2 h in the flowing helium. After the sample was cooled to 40 °C, the CO2 (99.999%) was passed over the sample for 30 min. And then after a purging of physically adsorbed CO2 by flowing helium at 40 °C for 2 h, the sample was heated from 40 to 500 °C at a rate of 10 °C/min. The CO2 consumption was also obtained by calibrating the CO2 with the TCD detector. UV−vis diffuse reflectance (UV−vis) spectra were obtained on a PerkinElmer Lambda 25 spectrometer with an integration sphere. Temperature-programmed reduction (TPR) of the catalysts was carried out on a Micromeritics ASAP 2920 instrument using a H2−Ar mixed gas (10 vol % H2). The

DDS selectivity =

[CHB] [CHB] + [BP]

HYD selectivity =

[CHB] [CHB] + [BP]

The CHB is the cyclohexylbenzene from the HDS of DBT by the hydrogenation (HYD) pathway, and the BP is the biphenyl from the HDS of DBT by the direct desulfurization (DDS) pathway. The promotion effect of Co species is also evaluated under the same feedstock by the comparison of rate constant kHDS. Assuming a pseudo-first-order reaction for the HDS of DBT, the rate constant can be expressed as follows:

kHDS =

F ⎛⎜ 1 ⎞⎟ ln m ⎝1 − x ⎠

where x is the total conversion of DBT, F is the molar feed rate of DBT in mol s−1, m is the catalyst mass in g, and kHDS is the rate constant of HDS in mol g−1 s−1.31 Another liquid feed containing 0.6 wt % DBT and 300 ppm of nitrogen as indole was used to examine the hydrodenitrogenation (HDN) effect on HDS activity over the catalyst. 5581

DOI: 10.1021/acs.iecr.5b00452 Ind. Eng. Chem. Res. 2015, 54, 5580−5588

Article



3. RESULTS AND DISCUSSIONS 3.1. Characterization. Figure 2 shows the TG curve of the dried magnesium carbonate salt. Clearly, there is no weight loss

0.95, which is typically assigned to the presence of mesoporous structure (Figure 4). Correspondingly, the pore size diameter

Figure 4. Nitrogen isotherms and pore size distributions of the (a) NS-MgO and (b) γ-Al2O3 supports. Isotherm of γ-Al2O3 has been offset by 100 cm3/g along the vertical axis for clarity. The pore size distribution of γ-Al2O3 has been offset by 0.8 cm3/g along the vertical axis for clarity.

Figure 2. TG curve of the precursor of NS-MgO sample.

after the temperature reached about 450 °C, and the calculated weight loss is 64.2%. Thus, the NS-MgO support was prepared by calcination at 450 °C. The XRD patterns of the dried magnesium carbonate salt precursor and the NS-MgO sample are shown in Figure 3. The

for the NS-MgO and γ-Al2O3 samples is centered at 3.1 and 10.4 nm (insert, Figure 4). After loading of metals and sulfidation, the pore size diameter of the Co-MoS2/NS-MgO catalyst is increased to 6.1 nm (insert, Figure 5). The textual

Figure 3. XRD patterns of the prepared (a) NS-MgO and (b) precursor samples (★, MgO, JCPDS 45-0946; ■, MgCO3·3H2O, JCPDS 20-0669).

Figure 5. Nitrogen isotherms and pore size distributions of the (a) Co-MoS2/NS-MgO and (b) Co-MoS2/γ-Al2O3 catalysts. Isotherm of Co-MoS2/γ-Al2O3 has been offset by 130 cm3/g along the vertical axis for clarity. The pore size distribution of Co-MoS2/γ-Al2O3 has been offset by 0.7 cm3/g along the vertical axis for clarity.

typical peaks at 13.66°, 23.06°, 35.76°, 47.14° for the magnesium carbonate salt precursor are associated with MgCO3·3H2O (JCPDS, 20-0669), and the typical diffraction peaks at 36.70°, 42.9°, 62.30°, 74.60°, and 78.61° for the NSMgO are corresponding to MgO (JCPDS, 45-0946). The formation of NS-MgO may be formulated as

parameters of the supports and catalysts are presented in Table 1. The specific surface areas of the two catalysts are almost unchanged and consistent with the supports. The mesopore volume of the Co-MoS2/NS-MgO was increased from 0.30 to 0.36 cm3/g, while the mesopore volume of the Co-MoS2/γAl2O3 is decreased from 0.50 to 0.32 cm3/g (Table 1). The SEM images reveal that the NS-MgO has a flower-like structure with sheet thickness of about 20 nm (Figure 6). In this manner, the nanoporous channels are formed in the assemblies, which is very helpful for the mass transfer of the bulky molecules. Figure 7 shows the CO2-TPD curves of the NS-MgO and γ-Al2O3

Δ

MgCO3·3H 2O → MgO + H 2O + CO2

The ideal weight loss calculated by above formula is 71.0%, higher than that of weight loss (64.2%) from TG analysis. The difference in weight loss may be due to the loss of some crystal water for the precursor during the dry process at 60 °C. The nitrogen sorption isotherms of the NS-MgO and γAl2O3 exhibit a hysteresis loop at a relative pressure of 0.45− 5582


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Industrial & Engineering Chemistry Research Table 1. Textural Parameters of the Support and Catalyst Samples


a

sample

SBET a (m2/g)

Vmeso b (cm3/g)

D c (nm)

NS-MgO γ-Al2O3 Co-MoS2/NS-MgO Co-MoS2/γ-Al2O3

255 205 222 215

0.30 0.5 0.36 0.32

5.5 9.5 7.0 5.0

BET surface area. bMesoporous volume. cAverage pore diameter.

samples, and the curve of the blank desorption experiment in He flow for the γ-Al2O3 sample was shown in Figure S1. The profile with lower temperature peak at 80 °C for the NS-MgO and γ-Al2O3 can be attributed to the desorbed CO2 in interaction with the weak basic sites corresponding to hydroxyl groups.32 The profile with relatively higher temperature peak at 140 °C for the NS-MgO should be associated with the desorbed CO2 in the interaction with middle basic sites (oxygen in the Mg2+ and O2− pairs).32 Obviously, the desorbed quantity of CO2 for NS-MgO is much higher than that of γAl2O3, indicating the basic sites on NS-MgO are larger than that on γ-Al2O3. The total CO2 consumption over the NS-MgO and γ-Al2O3 is 0.88 and 0.38 mmol/g, respectively. In addition, there is a large amount of middle basic sites on NS-MgO (0.7 mmol/g), while the middle basic sites are absent on the γAl2O3. Figure 8 presents the UV−vis spectra of the dried CoMo/ NS-MgO and CoMo/γ-Al2O3 catalysts. The absorption band in the range of 200−320 nm for CoMo/NS-MgO and CoMo/γAl2O3 could be associated with the O2− → Mo6+ charge transfer transitions of Mo species in the tetrahedral and octahedral coordination.33,34 Apparently, tetrahedral and octahedral coordination Mo species are both present on the NS-MgO and γ-Al2O3 supported catalysts. The absorption edge energy (Eg) can be obtained from UV−vis data through the linear correlation hv − (F(R)hv)2, which could be increased with the decrease of the average size of molybdenum oxide cluster.35 In our case, the Eg value for CoMo/NS-MgO (3.7 eV) is higher than that of CoMo/γ-Al2O3 (3.1 eV), indicating smaller Mo oxide clusters are better dispersed on the CoMo/NS-MgO than that on CoMo/γ-Al2O3 catalyst. To investigate the difference in the reduction behavior of Mo species on the NS-MgO and γ-Al2O3 supports, the TPR experiments were carried out and the obtained results are shown in Figure 9. Generally, the profile with low-temperature

Figure 7. CO2-TPD curves of (a) NS-MgO and (b) γ-Al2O3 samples.

Figure 8. UV−vis diffuse reflectance spectra of the dried (a) CoMo/ NS-MgO and (b) CoMo/γ-Al2O3 samples.

peak can be attributed to the first step reduction of Mo species (Mo6+ → Mo4+) in weak interaction with the support and the profile with high-temperature peak could be associated with the deep reduction of all Mo species (Mo4+ → Mo0).36,37 In our case, the profile with peak at low temperature of 350 °C could be assigned to the reduction step of octahedral Mo6+ to Mo4+ species on the NS-MgO. Correspondingly, this reduction step over CoMo/γ-Al2O3 catalyst occurred in relatively high

Figure 6. SEM images of (a) the as-synthesized NS-MgO and (b) the magnified SEM image of the selected area in (a). 5583


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Figure 9. H2-TPR curves of (a) CoMo/NS-MgO and (b) CoMo/γAl2O3 samples.

temperature range with peaks at 410 and 470 °C. These results indicate that the interaction of octahedral Mo6+ species with NS-MgO is weaker than that with γ-Al2O3. Considering the catalyst sulfidation was conducted at 400 °C, the hydrogen consumption was calculated from the peak area by controlling the reduction temperature at 400 °C and holding for 60 min. The obtained results show that the hydrogen consumption over CoMo/NS-MgO (0.32 mmol/g) is higher than over CoMo/γAl2O3 (0.23 mmol/g). A comparison of the TPR results of the CoMo/NS-MgO and CoMo/γ-Al2O3 catalysts shows that Mo species are more easily reduced on the NS-MgO. In addition, the TPR experiments of the NS-MgO and γ-Al2O3 supported Co or Mo catalysts were also measured (the details were discussed in the Supporting Information). To investigate the state of the metals in the sulfided catalysts (Co-MoS2/NS-MgO and Co-MoS2/γ-Al2O3), the XPS experiments were performed and the spectra are shown in Figure 10. The decomposition of Mo 3d spectra of the Co-MoS2/NSMgO and Co-MoS2/γ-Al2O3 catalysts is shown in Figure 10a, and the Mo species can exist as disulfide MoS2, MoOx, and MoOxSy phase with binding energy at 228.7 ± 0.1 eV (Mo4+ 3d5/2), 232.6 ± 0.2 eV (Mo6+ 3d5/2), and 230.2 ± 0.2 eV (Mo5+ 3d5/2),38−40 respectively. Additionally, the S 2s peak with the binding energy at 226.1 ± 0.1 eV was also observed on both catalysts. And the related parameters are listed in Table 2. The atomic percentage of Mo4+ in the form of MoS2 is comparable on both Co-MoS2/NS-MgO (76.4%) and Co-MoS2/γ-Al2O3 (73.5%) catalysts. The Co 2p spectra were also decomposed according to the reported literature,30,41 as shown in Figure 10b. For the sulfided CoMo catalysts, Co species could be present as CoMoS, Co9S8, and Co(II), which corresponds to the binding energy at 778.6, 781.6, and 778.1 eV, respectively.30,41 The relative quantities of Co species (CoMoS, Co9S8, and Co(II) oxide) on the Co-MoS2/NSMgO and Co-MoS2/γ-Al2O3 catalysts are shown in Table 3. Apparently, a higher proportion of the CoMoS phase was obtained on the Co-MoS2/NS-MgO (57.8%) than on the CoMoS2/γ-Al2O3 (33.3%) catalysts. These results suggest that the proportion of Co−Mo interaction is much higher on NS-MgO than on γ-Al2O3, which could lead to the formation of more CoMoS active phases. The representative TEM images of the Co-MoS2/NS-MgO and Co-MoS2/γ-Al2O3 catalysts are shown in Figure 11. It is clear that the less-stacked MoS2 crystallites with shorter lengths

Figure 10. XPS decomposition of (a) Mo 3d and (b) Co 2p spectra of Co-MoS2/NS-MgO and Co-MoS2/γ-Al2O3 catalysts.

are highly dispersed on the surface of NS-MgO, but the multistacked MoS2 particles (2−3 layers) with relatively longer lengths are located on the surface of γ-Al2O3. A qualitative comparison was carried out by calculation of lengths and stacking degree distributions of MoS2 active phase according to the method reported in the literature,42,43 and the results are shown in Table 4. The lengths of the MoS2 slabs on the CoMoS2/NS-MgO catalyst are mainly 2−4 nm, smaller than that of MoS2 slabs on the Co-MoS2/γ-Al2O3 catalyst (2−6 nm). The stackings of the MoS2 slabs on the Co-MoS2/NS-MgO catalyst are mainly 1−2 layers, which is lower than that of MoS2 slabs on the Co-MoS2/γ-Al2O3 catalyst (2−3 layers). As a result, the MoS2 active phase on the Co-MoS2/NS-MgO catalyst has a better dispersion than that on the Co-MoS2/γAl2O3. 3.2. HDS Activity. Figure 12 shows the catalytic data in the HDS of DBT over Co-MoS2/NS-MgO and Co-MoS2/γ-Al2O3 catalysts. Table 5 gives the product selectivity in the HDS of the DBT at the same conversion level over the two catalysts. The DBT conversion over Co-MoS2/NS-MgO (94.2%) is much higher than that over Co-MoS2/γ-Al2O3 (63.9%), and the selectivity of direct desulfurization product (biphenyl, BP) over Co-MoS2/NS-MgO (94.2%) is higher than that over CoMoS2/γ-Al2O3 (88.3%, Table 5). These results indicate that the Co-MoS2/NS-MgO catalyst exhibits very high HDS activity and prefers direct desulfurization in the HDS of the DBT compared to Co-MoS2/γ-Al2O3 catalyst. The higher HDS 5584


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Industrial & Engineering Chemistry Research Table 2. XPS Parameters of the Different Contributions Mo 3d5/2 Obtained for Sulfide Catalysts Mo6+

a

Mo5+

catalyst

BE (eV)

% atom

fwhm

Co-MoS2/NS-MgO Co-MoS2/γ-Al2O3

232.5 232.8

15.8 13.8

1.7 1.5

a

Mo4+ a

BE (eV)

% atom

fwhm

BE (eV)

% atom

fwhma

230.3 230.1

7.9 12.7

1.7 1.5

228.6 228.7

76.4 73.5

1.6 1.0

Full width at half-maximum.

Table 3. XPS Parameters of the Different Contributions Co 2p3/2 Obtained for Sulfide Catalysts CoMoS


a

Co9S8

catalyst

BE (eV)

% atom

fwhm

Co-MoS2/NS-MgO Co-MoS2/γ-Al2O3

778.6 778.5

57.8 33.3

3.5 1.3

a

Co(II) a

BE (eV)

% atom

fwhm

BE (eV)

% atom

fwhma

781.6 781.6

37.7 58.5

3.0 1.0

778.1 778.1

4.4 8.2

3.9 2.4

Full width at half-maximum.

Figure 11. TEM micrographs of (a) Co-MoS2/NS-MgO and (b) Co-MoS2/γ-Al2O3 catalysts.

Table 4. Slab Lengths and Stack Layer Number Distribution of the MoS2 Clusters over the Co-MoS2/NS-MgO and CoMoS2/γ-Al2O3 Catalysts frequency (%) slab lengths (nm)

MgO Nanosheet Assemblies Supported CoMo Catalyst with High

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