Al2O3 catalysts on the existing states of Mo species and hydrodesulfurization activity

Fuel 116 (2014) 168–174

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Effect of different preparation methods of MoO3/Al2O3 catalysts on the existing states of Mo species and hydrodesulfurization activity Huifeng Li ⇑, Mingfeng Li ⇑, Yang Chu, Feng Liu, Hong Nie Research Institute of Petroleum Processing, SINOPEC, 18 Xue Yuan Road, 100083 Beijing, China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 The method of hydrothermal

treatment and XRD characterization is first presented.  The method can be used to compare the Mo dispersion of different samples.  Different preparation methods can influence the existing states of Mo species. 3  The existence of AlðOHÞ6 Mo6 O18 anions can hinder the sulfidation of Mo species.  The catalyst at the absence of AlðOHÞ6 Mo6 O3 18 anions exhibits higher HDS activity.

a r t i c l e

i n f o

Article history: Received 7 April 2013 Received in revised form 26 July 2013 Accepted 31 July 2013 Available online 14 August 2013 Keywords: Alumina Adsorption Hydrothermal Mo dispersion Hydrodesulfurization

a b s t r a c t Effect of different preparation methods of MoO3/Al2O3 catalysts on the existing states of Mo species and hydrodesulfurization activity was investigated. A series of supported Mo catalysts were prepared by equilibrium adsorption method (at 298 K or at 373 K) and incipient wetness impregnation method, with ammonium heptamolybdate as Mo precursor, c-alumina and pseudo-boehmite as initial support. The results of pre-hydrothermal treatment and XRD characterization showed that different preparation methods can greatly influence the dispersion of Mo species. The results of LRS, TPR, XPS and HRTEM characterization clearly indicated that the existence of AlðOHÞ6 Mo6 O3 18 anions can hinder the reduction or sulfidation of Mo species. The HDS tests showed that the catalyst prepared by incipient wetness impregnation method exhibits the highest HDS activity for 4,6-DMDBT, owning to the predominance of well-dispersed polymolybdate species and the absence of AlðOHÞ6 Mo6 O3 18 anions. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction The development of highly active hydrodesulfurization (HDS) catalysts has been driven by the requirement of clean fuels production for environmental protection [1–6]. So far various methods have been tried to obtain well-dispersed active phases on supports [2,4–24]. Among these, the incipient wetness impregnation method and the equilibrium adsorption method (otherwise called, the

⇑ Corresponding authors. Tel.: +86 10 82368907; fax: +86 10 62311290 (H. Li). E-mail addresses: [email protected] (H. Li), [email protected] (M. Li). 0016-2361/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2013.07.127

equilibrium deposition filtration method [21–24]) are widely used. The former involves contacting the support with a solution containing the precursor salt (e.g. ammonium heptamolybdate) in a predetermined volume of water sufficient to fill the pores. The equilibrium adsorption method consists of adsorbing Mo from an aqueous solution of ammonium heptamolybdate over an extended period of time until equilibrium is attained, followed by filtration of the leftover liquid [19]. Moreover, from the results of [21–24], compared with the conventional impregnation method, the so-called the equilibrium deposition filtration method seems more promising because the catalysts prepared by the equilibrium deposition filtration method exhibited higher HDS activity.

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In recent years, the design of surface characteristics of alumina support [5,8–14] and the introduction of chelating agents [4,15–18] exhibited more efficient role in improving HDS activity of the catalysts. In our previous studies, it was also found compared with alumina-A, alumina-B with better crystallinity and less hydroxyl groups can fully moderate the metal–support interaction, favor the formation of octahedral polymeric molybdenum species and partly inhibit inactive cobalt spinel formation, which contributes to a noticeable increase in HDS activity [12]; additionally, the well-designed NiW catalyst, which was prepared by adding citric acid in the impregnating solution and using carbon-modified alumina as support, can facilitate the formation of more well-dispersed Ni–W–S active phases and thus exhibit higher HDS activity for 4,6DMDBT [15]. From the above discussion, it clearly indicates that for one thing, the improved dispersion of MoS2 (or WS2) can play an important role in increasing HDS activity; for another, the genesis of the active phase are strongly correlated with the architecture of the oxidic precursor and metal–support interaction. Therefore, the design of new HDS catalyst demands a more complete understanding the characteristics of metal–support interface, which is the key to clarify the essential influence of various preparation methods on the existing states of Mo (or W) species. It has been shown that the alumina support is not inert and the concept of dissolution/dispersion and dissolution/precipitation has permitted to clearly define the nature of the deposited phase during the preparation of the oxidic precursor [25]. It was found that poor dispersion of the MoOx phase in the calcined samples can be brought about by the formation of bulk (NH4)3[Al(OH)6Mo6O18] during impregnation or redistribution of Mo complexes during drying [26]. And strong metal–support interaction in the deposition stage by surface dissolution followed by reaction in the liquid phase is most likely to be such an important phenomenon, not only for cationic metal precursors as previously known but also for anionic precursors such as molybdates [27,28]. Moreover, in regard to the metal–support interaction, the role of surface hydroxyl groups cannot be ignored, because it usually acts as an indicator to evaluate the dispersion of Mo (or W) species; in other words, for the given MoO3 loading, the less amount of surface hydroxyl groups left uncovered by Mo (or W) species, the better dispersion of Mo (or W) species on alumina support. In our previous study [13], the change of basic, neutral and acidic hydroxyl groups of alumina surface was examined by varying the loading of MoO3 and WO3, so as to compare the difference of the dispersion of Mo and W species on alumina surface. But unfortunately due to the limit of IR method, it failed to evaluate whether all the surface hydroxyl groups were fully used to disperse Mo (or W) species. Then the crucial problem is how to effectively determine the limited amount of uncovered alumina surface. In our preliminary experiments, it was recognized that the naked surface of c-alumina can undergo rehydration and convert into boehmite phase after severe hydrothermal treatment at 453 K for 4 h in a Teflon-lined stainless steel autoclave; and the latter can be easily

distinguished from the former by its XRD patterns. Therefore, we firstly present a method (pre-hydrothermal treatment and XRD characterization) to examine the Mo dispersion of different oxidic catalysts, and the change of the Mo dispersion upon sulfidation. It was clear to find that (1) using the method of pre-hydrothermal treatment and XRD characterization can easily determine whether alumina surface is fully covered by the Mo species; (2) after hydrothermal treatment boehmite phase appears in the sulfided sample instead of the corresponding oxidic one, and it proves the part regeneration of uncovered alumina surface upon sulfidation; (3) although the same ammonium heptamolybdate is used as Mo precursor, different preparation methods (equilibrium adsorption method at 298 K or at 373 K and incipient wetness impregnation method) of MoO3/Al2O3 catalysts can greatly change the interaction of metal–support, the existing state of Mo species and HDS activity. These findings lead to a deep insight into the metal– support interface and new design of highly active HDS catalysts. 2. Experimental 2.1. Catalyst preparation

c-Alumina (c-Al2O3) powder was prepared by calcining pseudoboehmite (PB, SINOPEC Changling Catalyst Company) at 873 K for 4 h. For equilibrium adsorption method, first, c-Al2O3 powder was added into 700 cm3 0.13 mol/L ammonium heptamolybdate solution under stirring. Second, the resulting suspension was transferred to a Teflon-lined stainless steel autoclave with stirring, sealed and kept at 298 K or at 373 K for 4 h, respectively. Third, the as-prepared products were filtered, washed with deionized water, dried at 393 K for 4 h; the dried samples were denoted as Mo-c-D and Mo-c-H373-D, respectively. Fourth, the dried samples were further calcined at 693 K for 4 h, and the calcined samples were denoted as Mo-c-C and Mo-c-H373-C correspondingly. Using the same method but with PB as the initial support, the corresponding samples (denoted as Mo-PB-D, Mo-PB-H373-D, Mo-PB-C and Mo-PB-H373-C) were prepared. For comparison, the samples (denoted as Mo/c-D and Mo/c-C) were prepared by the incipient wetness impregnation method with an aqueous solution of ammonium heptamolybdate, using c-Al2O3 powder as support, followed by drying at 393 K for 4 h and calcining at 693 K for 4 h. For the sake of clarity, the information of the samples and related preparation methods is listed in Table 1. 2.2. Characterization The metal oxide contents of the corresponding samples were determined on a ZSX100e X-ray fluorescence analyzer. X-ray diffraction (XRD) patterns of the samples were recorded on a Philips X’Pert diffractometer, using the Cu Ka radiation at 40 kV and 40 mA. The BET surface area and pore volume of the samples were

Table 1 The samples and related preparation methods. Sample

Initial support

Preparation methods

Dry

Calcination

Mo-c-D Mo-c-C Mo-c-H373-D Mo-c-H373-C Mo-PB-D Mo-PB-C Mo-PB-H373-D Mo-PB-H373-C Mo/c-D Mo/c-C

c c c c

Equilibrium adsorption at 298 K Equilibrium adsorption at 298 K Equilibrium adsorption at 373 K Equilibrium adsorption at 373 K Equilibrium adsorption at 298 K Equilibrium adsorption at 298 K Equilibrium adsorption at 373 K Equilibrium adsorption at 373 K Incipient wetness impregnation Incipient wetness impregnation

Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes

No Yes No yes No Yes No Yes No Yes

PB PB PB PB

c c

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determined by N2 physisorption using a Micromeritics ASAP 2405 N automated system. The temperature-programmed reduction (TPR) experiments of the samples were carried out on a AutoChem II 2920 apparatus in a 10% H2/Ar (by volume) flow at 50 cm3/min. The furnace was heated up to 1273 K at a rate of 10 K/min, and a cold trap was equipped to condense the water vapor. The Raman spectra were recorded on a Raman spectrometer (LabRAM HR UV-NIR, Jobin Yvon) with a microscope. The transmission electron microscopy (TEM) images of the sulfided catalysts were recorded on a Tecnai G2 F20 S-TWIN microscope. At least 20 micrographs were taken of each sample and the average slab length and the stacking number were determined by manually measuring all the slabs per sample. The XPS spectra of the sulfided catalysts were acquired on a Thermo Fischer-VG ESCALAB 250 spectrometer. Note that the above-used sulfided catalysts were freshly prepared according to the same high-pressure (4.0 MPa) sulfidation procedure as the catalytic activity evaluation in the following section, and for more characterization details see [13,16]. 2.3. Catalytic activity evaluation The HDS activity tests were carried out in a continuous flow fixed-bed micro-reactor under the conditions: 4.0 MPa, 553 K, the catalyst loading of 0.15 g (40–60 mesh), 0.45 wt.% 4,6-DMDBT and 0.45 wt.% decalin (as internal standard) in decane as the model feed, the feed flow rate of 0.20 cm3/min, the H2 flow rate of 180 cm3/min. For each run, the weighed catalyst was diluted with 1 g quartz powder (40–60 mesh), and the mixture was placed in a stainless steel reactor (Ø8 mm). The catalysts were presulfided in situ with the sulfiding feed of 5 wt.% CS2 in cyclohexane, 4.0 MPa and 633 K for 2.5 h. After steady-state conditions were reached, the liquid effluents were periodically collected and analyzed by Agilent 6890N Gas Chromatograph with a 30 m capillary column of HP-1 methyl siloxane and a flame ionization detector (FID). And the conditions of catalytic activity evaluation and the method of products analysis have been described elsewhere [16,29]. 3. Results and discussion 3.1. The MoO3 contents of the samples The metal oxide contents of the samples showed that the MoO3 contents increased in the order of Mo-c-C (11%) < Mo-PB-C (15%) = Mo-c-H373-C (15%) < Mo-PB-H373-C (16%). It clearly indicated that the reaction of Mo species with the support surface can be notably strengthened under the hydrothermal treatment at 373 K. For comparison, Mo/c-D and Mo/c-C were prepared by the incipient wetness impregnation method with the same loading of 15% MoO3 as reference catalysts. In addition, it should be noted that in our preliminary experiments the as-prepared samples always had the same loading of 11% MoO3; either the concentration of ammonium heptamolybdate solution was increased from 0.13 mol/L to 0.40 mol/L, or the treatment time was prolonged from 4 h to 6 h. Therefore, 11% MoO3 was the maximum loading, by using our equilibrium adsorption method and c-Al2O3 as support at 298 K. 3.2. Hydrothermal treatment and XRD characterization To examine the Mo dispersion of the dried and calcined samples prepared by different methods, the method of pre-hydrothermal treatment and XRD characterization were first used. The procedure was described as follows. First, the sample was hydrothermally treated at 453 K for 4 h; then the treated sample (labeled with

the suffix H453) was filtered, washed with deionized water and dried at 393 K for 4 h; finally, the dried sample was characterized by XRD. Surprisingly, for both Mo-c-D-H453 and Mo-c-C-H453, the diffraction peaks of boehmite were found in Fig. 1. It indicated that although 11% MoO3 is the maximum loading by using equilibrium adsorption method, there are still some of naked c-Al2O3 surface. On the other hand, the diffraction intensity of newly-formed boehmite is rather weaker than that of c-H453, because the dispersed Mo species on c-Al2O3 surface can remarkably impede the rehydration of c-Al2O3. In contrast, for Mo-c-H373-D, some diffraction peaks of boehmite were observed. It indicated that during equilibrium adsorption at 373 K, the rehydration of c-Al2O3 happened simultaneously. After further hydrothermal treatment at 453 K, the diffraction peaks of boehmite still exist in the XRD patterns of Mo-c-H373D-H453, but their diffraction intensity becomes weaker compared with those of Mo-c-H373-D. However, for Mo-c-H373-C, only some diffraction peaks of c-Al2O3 were observed. It showed that the newly-formed boehmite was converted into c-Al2O3 after calcination. But for Mo-c-H373-C-H453, some weak diffraction peaks of boehmite were found again. It indicated that still some naked alumina surface retained. For Mo-PB-D and Mo-PB-D-H453, the diffraction peaks of boehmite were found, but the diffraction intensity of the latter decreased notably compared with that of the former. It indicated that hydrothermal treatment can greatly strengthen the reaction of Mo species with PB surface and partly destroy the crystalline structure of PB. Interestingly, for Mo-PB-C, only some diffraction peaks of c-Al2O3 were observed, similar to Mo-c-H373-C. But compared with Mo-c-H373-C-H453, rather stronger diffraction peaks of boehmite were observed for Mo-PB-C-H453. It indicated that although Mo-PB-C and Mo-c-H373-C had the same loading of 15% MoO3, in regard to the Mo dispersion, Mo-c-H373-C was better than Mo-PB-C. Unexpectedly, no diffraction peaks of boehmite were observed in both Mo/c-D-H453 and Mo/c-C-H453. It clearly indicated that the well-dispersed Mo species on c-Al2O3 surface can effectively prevent the rehydration of c-Al2O3. And it can be concluded that c-Al2O3 surface is fully covered by 15% MoO3 loading via incipient wetness impregnation method. Finally, to compare the change of the Mo dispersion upon sulfidation, Mo/c-C was sulfided in situ according to the above-mentioned sulfiding procedure. And the sulfided sample (denoted as Mo/c-C-S) was further hydrothermally treated at 453 K for 4 h. For the treated sample (denoted as Mo/c-C-S-H453), some strong diffraction peaks of boehmite were found. It proved that upon sulfiding some bare alumina surface can be formed due to the breakage of Mo–O–Al bonds and regeneration of some of the surface alumina OH groups. And such phenomena agreed well with the results of [30], in which a new OH band was observed in the IR spectra of the sulfide catalysts. 3.3. Laser Raman spectra To determine the existing states of Mo species of the oxidic catalysts, the LRS spectra were recorded and shown in Fig. 2. According to the results of [26–28], intense peaks at 947, 899, 572, 355, and 218 cm1 indicated the existence of Anderson-type alumino heteropolymolybdate AlðOHÞ6 Mo6 O3 18 anions, and its formation was due to the extraction of Al atoms from the alumina surface by Mo species. It was clear that AlðOHÞ6 Mo6 O3 18 anions were observed in both Mo-c-H373-D and Mo-PB-D except Mo/c-D. It can be explained that first, using equilibrium adsorption at 373 K can result in part rehydration of c-Al2O3 and the formation of boehmite; second, the existence of reactive boehmite and the liquid phase environment can greatly accelerate the reaction of Mo species with

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(b)

(a)

γ-H453

Intensity

Intensity

γ-H453

Mo-γ-C-H453 Mo-γ-C

Mo-γ-H373-C-H453 Mo-γ-H373-C Mo-γ-H373-D-H453

Mo-γ-D-H453

Mo-γ-H373-D

Mo-γ-D

10

20

30

40

50

60

70

10

20

2theta (degree)

(c)

30

40

50

60

70

2theta (degree)

γ-H453

(d)

Mo-PB-C-H453

Mo-PB-C Mo-PB-D-H453

Intensity

Intensity

γ-Η453

Mo/γ-C-S-Η453 Mo/γ-C-Η453 Mo/γ-C Mo/γ-D-Η453

Mo-PB-D

10

20

30

40

50

60

Mo/γ-D 70

10

20

30

40

50

60

70

2theta (degree)

2theta (degree)

Fig. 1. XRD patterns of the samples: (a) Mo-c series, (b) Mo-c-H373 series, (c) Mo-PB series and (d) Mo/c series.

the Al atoms on the support surface and increase the formation of AlðOHÞ6 Mo6 O3 18 anions. In contrast, after calcination the characteristic peaks of AlðOHÞ6 Mo6 O3 18 anions disappeared. It indicated that on the one hand, such anions were decomposed after calcination; on the other hand, the newly formed Mo oxides were probably poorly-dispersed on alumina surface, because boehmite phase were found in XRD patterns of Mo-c-H373-C-H453 and Mo-PB-C-H453. In addition, according to the results of [31–34], the peak centered at about 320 cm1 is due to the existence of tetrahedral Mo species, which are clearly observed in the LRS spectra of Mo-PB-C and Mo-c-H373-C. Unexpectedly, for Mo/c-C, only peaks centered at about 949 and 210 cm1 were detected, which showed that the Mo species mainly existed in the form of well-dispersed polymolybdate species [31,32]. From the above discussion, it can be concluded that although Mo-c-H373-C, Mo-PB-C and Mo/c-C had the same loading of 15% MoO3, the existing states of Mo species varied remarkably from method to method.

3.4. Temperature-programmed reduction To further examine the effect of changing preparation methods on the metal–support interaction, the reducibility of Mo-c-H373-C, Mo-PB-C and Mo/c-C were compared. The TPR profiles in Fig. 3 showed that the temperatures of the reduction peaks of Mo/c-C shift to lower temperatures, compared with those of Mo-c-H373C and Mo-PB-C; moreover, there is a stronger reduction peak (around 719 K) than the corresponding peak of Mo-c-H373-C (around 735 K) and Mo-PB-C (around 753 K). It fully indicated that the polymolybdate Mo species existing on Mo/c-C are easier to be reduced. In contrast, the more tetrahedrally coordinated monomeric Mo species on Mo-PB-C resulted in its poorer reducibility in comparison with Mo-c-H373-C. In general, the reducibility decreases in the order of Mo/c-C > Mo-c-H373-C > Mo-PB-C.

3.5. N2 physisorption To investigate the effect of different preparation methods on the textural properties of the corresponding catalysts, the freshly sulfided samples (denoted as Mo-c-H373-C-S, Mo-PB-C-S and Mo/cC-S) were characterized by N2 physisorption and the results were listed in Table 2. Compared with Mo/c-C-S, Mo-c-H373-C-S had an increase of 21 m2/g in surface area and 0.03 cm3/g in pore volume. It can be probably attributed to that first, some of the Mo species reacted with newly-formed boehmite on the c-Al2O3 surface (as evidenced by XRD results in Fig. 1) and then upon calcination the newly-formed boehmite reconverted into c-Al2O3; meanwhile the Mo species interacted with it can act as structural additives, which contributes to a slight increase in both surface area and pore volume. However, Mo-PB-C-S has equivalent surface area but a decrease of 0.09 cm3/g in pore volume compared with Mo/c-C-S. Considering that c-Al2O3 is the major contributor to the textural properties of the corresponding catalysts, it could be explained that the full reaction of the Mo species with PB may severely destroy the crystalline structure of PB, and thus after calcination it was merely converted into c-Al2O3 with lower pore volume. Moreover, the pore size distribution of Mo-PB-C-S as shown in Fig. 4 considerably shifts to lower pore size compared with those of Mo/c-C-S and Mo-c-H373-C-S. It also proved the destruction of the pore structure of c-Al2O3 because of the full reaction of the Mo species with PB both in surface and bulk phase. In contrast, Mo/c-C-S and Mo-c-H373-C-S had similar pore size distribution. For Mo-c-H373-C-S, the newly-formed boehmite and its reaction with the Mo species only happed on c-Al2O3 surface, and it had a slight effect on the pore structure of c-Al2O3. 3.6. XPS To further study the sulfidation extent and the dispersion of Mo species, the freshly sulfided samples were characterized by XPS.

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H. Li et al. / Fuel 116 (2014) 168–174 Table 2 BET surface areas and pore volumes of the sulfided samples.

947

Intensity

899 Mo-γ-H373-D 572 355320218 846 562

Surface area (m2/g)

Pore volume (cm3/g)

Average pore diameter (nm)

Mo-c-H373-C-S Mo-PB-C-S Mo/c-C-S

215 195 194

0.54 0.42 0.51

10.0 8.7 10.5

320 353 210 1.6

3

Mo-γ-H373-C

Sample

Desorption Dv(log d) (cm /g)

(a)

1400

1200

1000

800

600

400

200

Raman shift (cm-1)

(b)

925

899

Mo-PB-D 355

Intensity

572

320 218

Mo-γ-H373-C-S Mo/γ-C-S Mo-PB-C-S

1.2

0.8

0.4

0.0 0

10

20

30

40

Pore Diameter (nm) 925 846

Mo-PB-C 1400

1200

Fig. 4. Pore size distribution of the sulfided samples.

320 210

1000

800

600

400

200

Raman shift (cm-1) Mo/γ-C

949

Intensity

(c)

210

940

Mo/γ-D 1400

1200

360 210 1000

800

600

400

200

-1

Raman shift (cm ) Fig. 2. LRS spectra of the samples: (a) Mo-c-H373 series, (b) Mo-PB series and (c) Mo/c series.

Intensity

719 K

Mo/γ-C

753K

Mo-PB-C 200

400

600

800

1000

3.7. HRTEM Representative TEM micrographs of the freshly sulfided samples (Mo-c-H373-C-S, Mo-PB-C-S and Mo/c-C-S) were shown in Fig. 6. The micrographs exhibit the well-known MoS2 slab-like structure. It was clear to find that the average slab length of MoS2 increase in the order of Mo-PB-C-S (2.15 nm) < Mo-c-H373C-S (2.95 nm) < Mo/c-C-S (3.18 nm). Based on the above TPR and XPS results, it can be attributed to that the weaker metal–support interaction, the easier sulfidation and further growth of MoS2. However, the average stacking number of both Mo-c-H373-C-S and Mo/c-C-S was 1.1, just slightly higher than that of Mo-PB-CS (1.0). It indicated that the loading of 15% MoO3 seemed like approaching the real threshold value of the full coverage of the alumina with the Mo species, as evidenced by the results in Fig. 1.

735K Mo-γ-H373-C

According to the deconvolution method reported by [35], the deconvolution spectra of Mo3d + S2s envelope were shown in Fig. 5. The binding energies at about 226.20 eV, 227.49 eV and 233.20 eV are assigned to S2s peak from sulfide and terminal S2 2 , S2s peak from oxysulfide and bridging S2 , and S2s peak from sul2 fate, respectively; the binding energies around 229.05 eV, 230.50 eV and 232.80 eV are ascribed to Mo3d5/2 of Mo(IV), Mo(V) and Mo(VI) species, respectively; the binding energies around 232.20 eV, 233.80 eV and 236.00 eV are ascribed to Mo3d3/2 of Mo(IV), Mo(V) and Mo(VI) species, respectively. Moreover, the analysis results obtained by the deconvolution method [35] were summarized in Table 3. From the data, it is clear that the Mo surface concentrations relative to Al decreased in the order of Mo-c-H373-C-S  Mo/c-C-S > Mo-PB-C-S, basically in agreement with the results in Fig. 1. The degree of Mo sulfidation (Mosulfide/ Mototal = Mo(IV)/Mototal) decreased in the order of Mo/c-C-S > Moc-H373-C-S > Mo-PB-C-S, consisting well with the TPR results in Fig. 3. It clearly showed that the existence of AlðOHÞ6 Mo6 O3 18 anions can impede the formation of well-dispersed MoS2 active phase.

1200

1400

3.8. Catalytic activity evaluation

Temperature (K) Fig. 3. TPR profiles of the oxidic samples.

It is generally accepted that the main HDS pathway for sterically hindered sulfur compound 4,6-DMDBT is hydrogenation

H. Li et al. / Fuel 116 (2014) 168–174

(a)

6+

Intensity (CPS)

Mo

173

4+

Mo

S2s

5+

S2s

Mo

S2s

Mo-γ-H373-C-S 240

235

230

225

220

Binding Energy (ev)

(b)

4+

Mo

6+

Intensity (CPS)

Mo

S2s

5+

Mo S2s

S2s

Mo-PB-C-S 240

235

230

225

220

Binding Energy (ev)

(c)

4+

Mo 6+

Intensity (CPS)

Mo

S2s 5+

Mo

S2s

S2s

Fig. 6. TEM image of the sulfided samples: (a) Mo-c-H373-C-S, (b) Mo-PB-C-S and (c) Mo/c-C-S.

Mo/γ-C-S 240

235

230

225

220

Binding Energy (ev) Fig. 5. XPS results: Mo3d-S2s spectra of the sulfided samples: (a) Mo-c-H373-C-S, (b) Mo-PB-C-S and (c) Mo/c-C-S.

Table 3 XPS results of the sulfided samples. Sample

Mo/Al

Mosulfide/Mototal

Mo-c-H373-C-S Mo-PB-C-S Mo/c-C-S

0.049 0.037 0.047

0.75 0.66 0.85

reaction [16,29,36,37], and the HDS of 4,6-DMDBT mainly occurs through two parallel reaction pathways [16,29,36]. In the direct desulfurization (DDS) pathway, 3,30 -dimethylbiphenyl (DM-BP) is formed. In the hydrogenation (HYD) pathway, the reactant molecule is first hydrogenated to tetrahydro-, hexahydro-, and decahydro-intermediates of DMDBT (designated TH-DMDBT, HH-DMDBT, and DHDMDBT, respectively), the C–S bonds of which are then broken to form 3,30 -dimethylcyclohexylbenzene (DM-CHB) and 3,30 -dimethylbicyclohexyl (DM-BCH) [16,29,36]. Due to the

relatively low desulfurization ability of unpromoted MoO3/Al2O3 catalysts, in our experiments the major products observed are DM-CHB, DM-BCH and especially intermediate hydrogenated sulfur containing compounds; in contrast, the yield of DM-BP is very low. So the total HDS activity was merely used to show the effect of different preparation methods on HDS performance of MoO3/Al2O3 catalysts. And the total HDS activity was calculated according to the method reported by [29]. Atotal = F0  conversion/m (mol/ kg h), where F0 is the molar flow rate of the reactant (mol/h), m is the mass of the catalyst (kg). The conversion of 4,6-DMDBT over Mo/c-C, Mo-c-H373-C and Mo-PB-C are 27.7%, 18.5% and 7.2%, respectively. Therefore, it is clear that the total HDS activity decreased in the following order: Mo/c-C (0.32 mol/kg h) > Mo-cH373-C (0.22 mol/kg h) > Mo-PB-C (0.08 mol/kg h). The reason for the lowest HDS activity of Mo-PB-C is the formation of a large amount of AlðOHÞ6 Mo6 O3 18 anions, which resulted in poor dispersion of Mo species and the lowest sulfidation degree of Mo (as evidenced by XPS results). In contrast, both Mo-c-H373-C and Mo/c-C were prepared with c-Al2O3 as initial support, but the equilibrium adsorption at 373 K caused the formation of AlðOHÞ6 Mo6 O3 18 anions on Mo-c-H373-C, which decreased the sulfidation degree of Mo and HDS activity compared with Mo/c-C. Therefore, the highest HDS activity of 4,6-DMDBT of Mo/c-C can be explained as follows. First, Mo/c-C prepared by the incipient wetness impregnation method, instead of the equilibrium

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adsorption method at 298 K or at 373 K (working in liquid phase or at higher temperature), can effectively hinder the formation of AlðOHÞ6 Mo6 O3 18 anions; second, the polymolybdate species mainly existing on the oxidic Mo/c-C were liable to convert into well-dispersed MoS2 active phase upon sulfidation. In addition, it should be noted that the average pore diameter of the sulfided catalysts decreases in the order: Mo/c-C-S (10.5 nm) > Mo-c-H373-C-S (10.0 nm) > Mo-PB-C-S (8.7 nm). Therefore, to some extent, the largest pore size of Mo/c-C may contribute to better diffusion of reactant molecules to the active sites, and thus accelerate the HDS reaction. 4. Conclusion In this study, a method of pre-hydrothermal treatment and XRD characterization for examining the change of Mo dispersion on alumina was presented. No diffraction peaks of boehmite were found in both Mo/c-D-H453 and Mo/c-C-H453, and it just indicated that the well-dispersed Mo species on c-Al2O3 surface can effectively prevent the rehydration of c-Al2O3. Boehmite phase appears in Mo/c-C-S-H453 instead of Mo/c-CH453, and it proves the part regeneration of uncovered alumina surface upon sulfidation. The reducibility of the supported Mo species, the degree of Mo sulfidation and the size of MoS2 slabs all decrease in the order of Mo/c-C > Mo-c-H373-C > Mo-PB-C. And Mo/c-C exhibits the highest HDS activity for 4,6-DMDBT because of the predominance of well-dispersed polymolybdate species and the absence of AlðOHÞ6 Mo6 O3 18 anions. Acknowledgments The authors gratefully acknowledge the funding of the State Key Project (Grant 2012CB224802 and 2012BAE05B03) and the SINOPEC project (No. 111015). References [1] Stanislaus A, Marafi A, Rana MS. Recent advances in the science and technology of ultra low sulfur diesel (ULSD) production. Catal Today 2010;153:1–68. [2] Saih Y, Nagata M, Funamoto T, Masuyama Y, Segawa K. Ultra deep hydrodesulfurization of dibenzothiophene derivatives over NiMo/TiO2–Al2O3 catalysts. Appl Catal A 2005;295:11–22. [3] Song C, Ma X. New design approaches to ultra-clean diesel fuels by deep desulfurization and deep dearomatization. Appl Catal B 2003;41:207–38. [4] Sun M, Nicosia D, Prins R. The effects of fluorine, phosphate and chelating agents on hydrotreating catalysts and catalysis. Catal Today 2003;86:173–89. [5] Breysse M, Afanasiev P, Geantet C, Vrinat M. Overview of support effects in hydrotreating catalysts. Catal Today 2003;86:5–16. [6] Topsøe H. The role of Co–Mo–S type structures in hydrotreating catalysts. Appl Catal A 2007;322:3–8. [7] Guzmán MA, Huirache-Acuña R, Loricera CV, Hernández JR, Díaz de León JN, de los Reyes JA, et al. Removal of refractory S-containing compounds from liquid fuels over P-loaded NiMoW/SBA-16 sulfide catalysts. Fuel 2013;103:321–33. [8] Carre S, Tapin B, Gnep NS, Revel R, Magnoux P. Model reactions as probe of the acid–base properties of aluminas: nature and strength of active sites. Correlation with physicochemical characterization. Appl Catal A 2010;372:26–33. [9] Trejo F, Rana MS, Ancheyta J, Rueda A. Hydrotreating catalysts on different supports and its acid–base properties. Fuel 2012;100:163–72. [10] Laurenti D, Phung-Ngoc B, Roukoss C, Devers E, Marchand K, Massin L, et al. Intrinsic potential of alumina-supported CoMo catalysts in HDS: comparison between cc, cT, and d-alumina. J Catal 2013;297:165–75. [11] Han D, Li X, Zhang L, Wang Y, Yan Z, Liu S. Hierarchically ordered meso/ macroporous c-alumina for enhanced hydrodesulfurization performance. Micropor Mesopor Mat 2012;158:1–6. [12] Li M, Li H, Jiang F, Chu Y, Nie H. Effect of surface characteristics of different alumina on metal–support interaction and hydrodesulfurization activity. Fuel 2009;88:1281–5. [13] Li H, Li M, Chu Y, Xia G, Nie H. The dispersion characteristics of molybdenum and tungsten on alumina surface. Chin J Catal 2009;30:165–70.

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