Hydrogen spillover effect between Ni2P and MoS2 catalysts in hydrodesulfurization of dibenzothiophene

JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGY Volume 43, Issue 6, Jun 2015 Online English edition of the Chinese language journal Cite this article as: J Fuel Chem Technol, 2015, 43(6), 708713

RESEARCH PAPER

Hydrogen spillover effect between Ni2P and MoS2 catalysts in hydrodesulfurization of dibenzothiophene LIU Li-hua1,*, LIU Shu-qun1, YIN Hai-liang2, LIU Yun-qi3, LIU Chen-guang3 1

School of Chemistry and Materials Science, Huaibei Normal University, Huaibei 235000, China;

2

Academy of Science & Technology, China University of Petroleum, Dongying 257061, China;

3

State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), Qingdao 266555, China

Abstract: The hydrodesulfurization of dibenzothiophene over physically separated Ni2P//MoS2 catalyst beds was investigated. The results indicated that the hydrogen spillover effect appears between the Ni2P/Al2O3 and MoS2/Al2O3 catalysts in the hydrodesulfurization reaction, which can significantly enhance the concentration of active sites and the hydrodesulfurization rate over the MoS2 catalyst. The spillover factor on Ni2P//MoS2 is slightly higher than that on NiSx//MoS2, due to the higher hydrogen dissociation activity of Ni2P; as a result, Ni2P is a superior promoter to NiSx for the MoS2 catalyst. Keywords: Ni2P; MoS2; hydrodesulfurization; hydrogen spillover effect; dibenzothiophene.

Recently, more heavy oils containing refractory sulfur compounds are exploited than ever, and meanwhile, more stringent fuel specifications have been implemented to minimize air pollution and prevent the poisoning of exhaust treatment catalysts[1,2]. Therefore, it is necessary to develop better hydrotreating catalysts with superior performance and investigate the hydrodesulfurization (HDS) mechanism over metal sulfide catalyst. Many reaction models have been proposed; among them, the Co(Ni)-Mo-S model proposed by Topsøe et al[3,4] and the remote control model developed by Karroua et al[5,6] are two most recognized ones. In the Co(Ni)-Mo-S model, the effect of promoter on molybdenum sulfide catalysts has been attributed to the amount of promoter atoms that can be accommodated on the edges of MoS2 layers and also to the electronic transfer induced by the promoter atom on Mo atoms located at these sites[7]. Further insight into atomic structure of active phases has been obtained by Topsøe et al through the combination of advanced experimental and theoretical techniques; a new “brim site” model is proposed[8–11]. Due to their metallic character, the brim sites may bind the sulfur containing molecules and promote the reaction with hydrogen atoms available in neighboring SH groups. In the remote control model, the synergism is related to hydrogen spillover (Hso), where hydrogen migrates from a donor phase (such as Co9S8

and NiSx) to an acceptor phase (such as MoS2 and WS2). By using a reactor system of physically separated and layered catalyst beds, Escalona et al[12–18] provided direct evidence for the role of remote control; they demonstrated that Mn, Fe, Co, Ni, Cu and Zn sulphides could generate Hso. The investigation of synergism is also extended from hydrodesulfurization to hydrodenitrogenation reactions[19,20]. However, few studies on hydrogen spillover effect between Ni2P and MoS2 catalysts in the HDS of DBT were reported. Meanwhile, transition-metal phosphides have attracted considerable attention due to their excellent performance in hydrodesulfurization and hydrodenitrogenation[21–24]. Recently, Guan et al[25] and Liu et al[26] report a novel Ni2P-MoS2 catalyst with high activity in the HDS of DBT and 4,6-DMDBT; its catalytic performance is related to the high hydrogenation capability of nickel phosphide. Herein, the HDS of DBT over physically separated Ni2P//MoS2 catalyst beds was investigated in this work, to reveal the hydrogen spillover effect between the Ni2P and MoS2 catalysts.

1

Experimental

1.1

Catalyst preparation

WO3/SiO2 was prepared by impregnation of SiO2 in

Received: 20-Mar-2015; Revised: 04-May-2015. Foundation item: Supported by the National Natural Science Foundation of China (21206197), the Natural Science Foundation of Anhui Province (1408085QB44) the Science-Technology Foundation for Fostering Talents of Huaibei City (20140316), the Youth Foundation of Huaibei Normal University (2012xqz01), and Promotive Research Fund for Excellent Young and Middle-aged Scientists of Shandong Province (BS2013CL021). *Corresponding author: LIU Li-hua, E-mail: [email protected]. Copyright  2015, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.

LIU Li-hua et al / Journal of Fuel Chemistry and Technology, 2015, 43(6): 708713

aqueous solution of ammonium tungstate hydrate; it was dried overnight at 120°C and then calcined at 550°C for 4 h. γ-Al2O3 was used as the support of the donor and acceptor. Ni2P/Al2O3 catalyst with a Ni2P content of 4.0% was obtained through thermal decomposition of nickel hypophosphite. γ-Al2O3 support was impregnated into the Ni(H2PO2)2·6H2O solution, and then dried in a vacuum oven at 60°C for 2 h and calcined in nitrogen at 300°C for 3 h. Finally, the product was washed three times with deionized water to remove any impurities. Ammonium tetrathiomolybdate was prepared following the procedures described in the literatures[27]. The MoS2/Al2O3 catalyst containing 18.1% MoO3 was obtained by the impregnation of the support in a solution of ammonium tetrathiomolybdate and ethylenediamine. The catalyst was dried at ambient temperature for 2 h and subsequently at 60°C for 12 h and lastly calcined in a flow of nitrogen at 400°C for 3 h. The NiSx/γ-Al2O3 catalyst was prepared as reported in the previous studies[19,20]. 1.2

Catalyst characterization

Hydrogen temperature-programmed desorption (H2-TPD) was conducted with Quantachrome ChemBET 3000 instrument. Firstly, the promoter (0.1 g) was loaded in a quartz reactor and reduced in a H2 flow at 500°C for 2 h. Subsequently, it was cooled down to 100°C and purged with a He stream for 2 h to remove excessive and physically adsorbed H2. Finally, the treated catalyst sample was heated from 100 to 600°C at a rate of 10°C/min in a pure He flow. The desorbed hydrogen was detected with a thermal conduction detector. 1.3

Activity measurements

As shown in Figure 1C, the stacked bed was made as follows: the top layer consisted of 2 g Ni2P/Al2O3 (or NiSx/Al2O3) and the bottom layer was 2 g MoS2/Al2O3; they were separated by a 3 mm thick SiO2 layer to prevent the formation of a mixed phase. The remaining space in the reactor was filled with quartz sand. This “stacked bed” was denoted as Ni2P//MoS2 (or NiSx//MoS2). For comparison, another “stacked bed” shown in Figure 1D, represented as MoS2//Ni2P (or MoS2//NiSx), was located in an opposite position but with the same amounts of Ni2P/Al2O3 (or NiSx/Al2O3) and MoS2/Al2O3. The catalytic tests for HDS of DBT were carried out in a continuous high-pressure fixed-bed reactor under a hydrogen pressure of 2 MPa, 240–300°C, hydrogen/feed volumetric ratio of 600, and weight hourly space velocity (WHSV) of 3 h–1. The model oil, viz., a solution of DBT in decalin (2000 g-S/g), was fed to the reactor by a high-pressure pump.

Fig. 1 Different ways to load the catalyst in reactor : Ni2P/Al2O3 or NiSx/Al2O3; : MoS2/Al2O3;

: SiO2

The compositions of reaction products were analyzed with an Agilent 6820 gas chromatograph equipped with a HP-5 packed column (30 m × 0.32 mm × 0.5 m) and a FID detector. To evaluate the HDS activity of a catalyst, the conversion of DBT for HDS (xHDS) was calculated by following the equation: xHDS = (cS0 – cSR – cSC) / cS0 × 100% (1) where cS0 represented the DBT concentration in the feed, cSR was the DBT concentration in the product, and cSC was the total concentration of all sulfur-containing intermediates in the product. For quantitative description of the hydrogen spillover effect of the stacked bed, the spillover factor was defined as a ratio of xHDS(stacked bed) / xHDS(single bed), where xHDS(stacked bed) and xHDS(single bed) are the DBT conversion obtained in the stacked bed and the sum of DBT conversions attained on two single Ni2P/Al2O3 (or NiSx/Al2O3) and MoS2/Al2O3 beds, respectively. The first-order rate constant for the HDS of DBT (mol/(g·s)) was calculated by the equation: F 1 k= ln( ) (2) m 1–xHDS where xHDS is the total conversion of DBT, F is the molar feed rate of DBT in mol/s and m is the catalyst mass in g.

2

Results and discussion

The color change of Ni2P/Al2O3 catalyst may be used to evaluate its ability in dissociating hydrogen. It is well known that WO3 is a good indicator for the detection of H atoms[28,29]. H atoms can readily reduce WO3 to a lower valence state, which has a different color from the unreduced oxide. In this work, WO3 and a mixture of Ni2P/Al2O3 + WO3 were treated with hydrogen under the same conditions at 180°C; after the treatment, pure WO3 is still in light yellow, whereas the color of the Ni2P/Al2O3 + WO3 mixture turns to blue.

LIU Li-hua et al / Journal of Fuel Chemistry and Technology, 2015, 43(6): 708713

Fig. 2 Pictures of the WO3 particles (light yellow) (a) and the Ni2P/Al2O3 + WO3 mixture (blue) (b) after H2 treatment Table 1 HDS activity of Ni2P/Al2O3, Mo/Al2O3, Ni2P//MoS2 and MoS2//Ni2P stacked beds Catalyst bed

Fig. 3 H2-TPD profiles of the Ni2P/Al2O3 and NiSx/Al2O3 promoters

The color change of WO3 is attributed to the dissociation of hydrogen on Ni2P/Al2O3, which migrates to the yellow WO3 particles and leads to the reduction of WO3. Such a results illustrates that Ni2P/Al2O3 can dissociate molecular hydrogen into H atom. To compare the abilities in dissociating hydrogen molecule of NiSx/Al2O3 and Ni2P/Al2O3 promoters, their H2-TPD profiles are shown in Figure 3. Usually, the hydrogen species desorbed below 320°C were ascribed to H2 adsorbed on the metal sites, whereas those desorbed above 320°C were attributed to the spilt-over hydrogen species[30]. No hydrogen desorption peak is observed below 235°C, which is different from the result reported by Liu et al[31] because of the treatment conditions. Figure 3 displays that abundant spillover hydrogen species are adsorbed on the Ni2P/Al2O3 catalyst, whereas the spillover hydrogen species on the surface of the NiSx/Al2O3 catalyst is much less. NiSx has a metal center (Ni2+) with a high positive charge and a low density of states near the Fermi level, which may lead to its poor ability in dissociation of hydrogen molecules. In nickel phosphides, however, nickel has a small positive charge (Ni+0.1) and more hydrogen molecules are then adsorbed and dissociated over Ni2P[21]. All these indicate that Ni2P as a promoter is more active in dissociating hydrogen than NiSx.

Conversion xHDS/% 240°C

260°C

280°C

Ni2P/Al2O3

1.0

1.5

2.3

300°C 4.2

MoS2/Al2O3

5.6

13.1

28.9

57.9

Ni2P//MoS2

15.1

24.3

42.2

76.5

MoS2//Ni2P

6.7

14.9

30.9

62.2

NiSx/Al2O3

–

–

0.2

0.3

NiSx//MoS2

8.1

16.8

34.4

66.0

MoS2//NiSx

5.6

13.3

29.3

58.0

The HDS of DBT was used as a model reaction to investigate the hydrogen spillover effect between Ni2P/Al2O3 and MoS2/Al2O3 catalysts, as given in Table 1. The overall content of tetrahydrodibenzothiophene and hexahydrodibenzothiophene, as the sulfur-containing intermediates for DBT HDS, was less than 2%. The conversion of DBT in the single- and two-bed catalytic systems varies in a range of 1.0%–76.5%, depending on the reaction temperature. The Ni2P/Al2O3 catalyst shows a very low HDS conversion. The HDS of DBT over Ni2P catalysts is a sensitive reaction related to the temperature and catalyst loading; therefore, current observation is not contradictory to the results reported in the literatures[32,33] due to the low reaction temperature (240–300°C) and Ni2P loading (4.0%) in this work. However, the HDS conversion for the MoS2/Al2O3 catalyst is as high as 57.9% at 300°C, indicating that the monometallic MoS2/Al2O3 catalyst has a moderate activity in HDS of DBT. It can be clearly observed that the Ni2P//MoS2 stacked-bed exhibits much higher HDS activity than the sum of monometallic Ni2P/Al2O3 and MoS2/Al2O3 catalysts. Considering that the contacted activity between the Ni2P/Al2O3 and MoS2/Al2O3 beds can be excluded, these results unequivocally demonstrated that the hydrogen spillover effect for Ni2P-MoS2 works in the HDS of DBT, which can be explained by the remote control model.

LIU Li-hua et al / Journal of Fuel Chemistry and Technology, 2015, 43(6): 708713

Fig. 4 Spillover factor over the Ni2P//MoS2 and NiSx//MoS2 stacked beds Table 2 Rate constants of DBT HDS in the single- and two-bed catalytic system Catalyst

Rate constant kHDS×108 / (mol·g–1·s–1) 240°C

260°C

280°C

300°C

Ni2P

0.06

0.08

0.12

0.22

MoS2

0.30

0.73

1.78

4.50

Ni2P//MoS2

0.85

1.45

2.86

7.54

NiSx

-

-

0.01

0.02

NiSx//MoS2

0.44

0.96

2.20

5.62

Hydrogen is dissociated on the Ni2P/Al2O3 catalyst; the dissociated hydrogen species migrate to MoS2 and then remove S atoms from MoS2 phase as H2S to create more 3-fold coordinative unsaturated sites, which are considered as the active sites. As a result, the presence of Ni2P increases the amount of HDS sites and enhances the catalytic activity of MoS2 through hydrogen spillover. All these support the presence of hydrogen spillover effect between the Ni2P/Al2O3 and MoS2/Al2O3 catalysts for the HDS of DBT. On the contrary, if Ni2P/Al2O3 is situated under the MoS2/Al2O3 catalyst (Figure 1D), the HDS activity of the reversely stacked MoS2//Ni2P is close to the sum of two single beds (Table 1). In this case, almost no hydrogen spillover effect is observed, because the spillover hydrogen atoms can hardly migrate upstream from the donor (Ni2P) to the acceptor (MoS2) bed; the hydrogen spillover effect occurs only from the Ni2P/Al2O3 phase to MoS2/Al2O3. For comparison, the NiSx/Al2O3 catalyst is used as a reference donor. The existence of hydrogen spillover effect between NiSx and MoS2 is also manifested, in accordance with previous studies of Villarroel et al[15,34]. The conversion of DBT over the NiSx//MoS2 stacked bed is lower than that over Ni2P//MoS2. Figure 4 shows that for two catalytic systems, the spillover factors are around 1.13–2.29. However, the spillover factor of Ni2P//MoS2 catalyst system is always slight higher than that of NiSx//MoS2, which is closely related to their performance in dissociating hydrogen molecules; high activity

in hydrogen dissociation for the donor is favorable to the HDS of DBT. With the increase of reaction temperature, the spillover factor between Ni2P (or NiSx) and MoS2 is reduced considerably; similar trends were also reported in many literatures[14,15,17,34]. Although the donor has a stronger ability of dissociating hydrogen at higher temperature, more active hydrogen species are also desorbed from the surface of the MoS2 catalyst into the gas phase; this is probably the main reason for the lower spillover factor at higher temperature. The above results illustrate that the nature of donor and the reaction temperature are the two foremost factors influencing the hydrogen spillover effect. The spillover factor of the stacked beds is small in the present work, as the distance between Ni2P and MoS2 is as large as 3 mm. For the HDS under real conditions, the distance between the donor and the acceptor is probably less than 1 nm and the hydrogen spillover will be more efficient in a real catalyst. Guan et al[25,26] extrapolated deep hydrodesulfurization over Ni2P-MoS2/γ-Al2O3 catalyst, which shows a relatively high activity with a strong preference for the hydrogenation desulfurization pathway in comparison with the conventional NiMo and NiMoP catalysts. The effect of hydrogen spillover on the HDS activity of MoS2 catalyst was further kinetically investigated. Many researchers have observed that the HDS of DBT could be well described by pseudo-first-order reaction[35]. As presented in Table 2, the rate constants of HDS for all catalytic systems are enhanced with the increase of reaction temperature; for example, the rate constants on the MoS2 catalyst at 240 and 300°C are 0.30×10–8 and 4.50×10–8 mol/(g·s), respectively. However, the introduction of Ni2P or NiSx leads to a remarkable increase in HDS rate constant; the hydrogen spillover effect has a positive effect on the HDS of DBT. Moreover, the kHDS value of the Ni2P//MoS2 catalytic system is greater than that of the NiSx//MoS2 catalytic system, which should be attributed to their ability of dissociating hydrogen. According to the kinetic data in Table 2, it was reasonable to assume that the Ni2P-MoS2 catalyst may exhibit higher HDS activity than the conventional catalyst (NiMoS) under the same conditions. Although nickel phosphide exhibits low catalytic activity in hydrogenation and hydrogenolysis of DBT, it may play an important role in the activation of molecular hydrogen. Hydrogen spillover can increase the density of active sites and enhance the utilization of MoS2 catalyst. Therefore, nickel phosphide is a better promoter for the MoS2 catalyst in the HDS of DBT. Current study may provide a possible large-scale application of nickel phosphide composite catalysts in the HDS reaction.

3

Conclusions

LIU Li-hua et al / Journal of Fuel Chemistry and Technology, 2015, 43(6): 708713

The HDS of DBT over physically separated Ni2P//MoS2 catalyst beds was investigated. Such a simple experimental design proves the presence of hydrogen spillover effect between the Ni2P/Al2O3 and MoS2/Al2O3 catalysts in the HDS reaction, which can significantly enhance the concentration of active sites and HDS rate over the MoS2 catalyst. The nature of donor and the reaction temperature are the two foremost factors influencing the hydrogen spillover effect. The hydrogen spillover effect is more prominent at higher temperature. Compared with that on NiSx//MoS2, the spillover factor on Ni2P//MoS2 is higher, due to the higher hydrogen dissociation activity of Ni2P; as a result, Ni2P is a superior promoter to NiSx for the MoS2 catalyst. The hydrogen spillover effect can be explained by the remote control model through a migration of hydrogen spillover.

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