Theoretical study of benzothiophene hydrodesulfurization on MoS2

Hydrotreatmentand Hydrocrackingof Oil Fractions B. Delmon,G.F.Fromentand P. Grange(Editors) 9 1999ElsevierScienceB.V.All rightsreserved.

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Theoretical Study of benzothiophene hydrodesulfurization on MoS2 S. Cristol 1, J.F. Paul 1, E. Payen 1, D. Bougeard 2 J. Hafner 3 and F. Hutschka 4 1Laboratoire de Catalyse H6t6rog6ne et Homog6ne. CNRS / U P R E S A 8010, F59655 Villeneuve d'Ascq. 2Laboratoire de Spectroscopie Infra-rouge et Raman. CNRS / UMR 8516, F-59655 Villeneuve d'Ascq. 3Institut fiir Theoretische Physik. Technische Universit~it Wien, A-1040 Wien. 4Total Raffinage Distribution, CERT, BP 27, F-76700 Harfleur.

Abstract Benzothiophene (BT) and methylbenzothiophene (MBT) desulfurization on the catalytically active MoS2 edge has been studied using density functional theory. The calculation of the stability of the (100) surface as a function of sulfur coverage indicates two potential active sites. The adsorption energies of BT, MBT and hydrogen molecules on these sites were calculated. The calculation of the binding energies of the various hydrogenated intermediates allow us to the build of energy profiles of BT and MBT desulfurization. It appears that the most endothermic step is the site regeneration (creation of the vacancy).

1. I N T R O D U C T I O N The hydrodesulfurization (HDS) is industrially performed on CoMo/A1203 or NiMo/A1203 catalysts which consist of MoS2 nanocrystallites well dispersed on an alumina support and promoted by Co or Ni atoms [1,2]. It is well admitted t h a t the active sites are located at the edges of these disulfide crystallites. Most of the theoretical work published on HDS is devoted to thiophene desulfurization. Neurock and Van Santen [3] proposed on the basis of DFT calculations on small nickel sulfide clusters that the thermodynamic limiting step is the C-S bond scission. Raybaud [4] showed from a periodic MoS2 model t h a t the most endothermic step is the creation of the vacancy on the catalyst surface.

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These results can explain the promoting effect of nickel or cobalt as discussed in ref. 4, but they do not take into account the existence of steric effects that are known to be important in deep desulfurization. Finally, DBT has a much more pronounced aromatic character than thiophene and we have to assess how this point affects the reaction path. In this work, we report density functional calculations of the adsorption of benzothiophene (BT), methylbenzothiophene (MBT) and their hydrogenated derivatives on various MoS2 surfaces. These molecules could be good models to estimate the differences between substituted and unsubstituted molecules and so between DBT and DMDBT. 2. C O M P U T A T I O N A L M E T H O D The periodic DFT calculations were performed with the Vienna Ab-initio Simulation Package (VASP) based on plane waves [5,6] which allows a good description of the MoS2 surface, by using large supercells (9.48x20x12.294 A~). As shown in a previous study [7], a model containing two layers along the z direction, three rows in the x direction and four in the y direction (Fig.l) is suitable to give a good description of the electronic and structural properties of the perfect (100) MoS2 surface. All over this work, we used a cut-off energy of Ecut = 210 eV, a Methfessel-Paxton [8] smearing with a - 0.1 eV and F point for Brillouin zone integration. The two upper rows and the adsorbed molecules were allowed to relax during the calculation while the two lower ones were kept fixed at the bulk geometry. In order to calculate reliable adsorption energies, the nonlocal functional using generalized gradient corrections (GGA) of Perdew et al [9] was applied. With these settings, the error on the adsorption energies is less t h a n 0.1 eV.

F i g u r e 1 (perfect surface) dark balls: Mo; light balls: S.

F i g u r e 2 (most stable surface)

329 3. S U L F U R C O V E R A G E AND CATALYTIC S I T E S Figure 1 is a representation of the perfect (100) surface showing alternative rows of molybdenum (metallic edge) and sulfur atoms (sulfur edge). Industrial conditions involve the presence of H2 and H2S in the gas/liquid phase, which implies t h a t the surface could be sulfur rich or sulfur deficient. The nature of the surface in the operating conditions therefore depends on the relative chemical potential of H2 and H2S [10]. The energy of the S addition or S removal on both edges can be calculated according to the reactions (1) and the relative stabilities of the various surfaces can then be deduced. Surface 1 + H2 ~-> Surface 2 + H2S

(1)

Our calculations showed, in agreement with a previous study [4] that, in the sulfiding conditions, the most stable surface is obtained by adding three sulfur atoms on the metallic edge (Fig. 2). This surface should be catalytically inactive because the Mo atoms exposed at the surface are saturated. Adsorption of molecules can only proceed on lacunary structures obtained by removing sulfur according to reaction (1). Different coordinately unsaturated sites (CUS) can be created on both edges. The first kind of potential catalytic site is obtained by removing one (site 1) or two (site 2) sulfur atom from the metallic edge of the stable surface. The second one is obtained by removing sulfur atoms 1, 2 and 3 from the sulfur edge of the stable surface (site 3). Other CUS can be created on each edge of the surface, but we only discuss in this work the more stable ones. Table 1 summarizes the creation energy of each selected site calculated according to reaction (1) the stable surface (Fig. 2) being taken as reference. Table1 Stability of different possible catalytic sites. Surface Site 1 Energy (eV) 1.3

Site 2 3.4

Site 3 2.89

4. A D S O R P T I O N O F M O L E C U L E S The adsorption of BT and MBT was studied starting from different configurations: 111 (S) adsorption is possible on each of the aforementioned sites while ~5 (thiophene) and ~16 (benzene) are only possible on site 2. Figures 3 to 6 show the adsorption geometries and the corresponding energies are reported in table 2. It appears that the methyl group does not affect the flat adsorption of MBT whereas it induces a lowering of the energy of the ~ 1 adsorption. This is due

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to the steric hindrance between the methyl group and the neighboring layer of the MoS2 surface. This steric interaction is more important on site 3 t h a n on the other ones.

Figure 6

Figure 5 BT adsorbed 1"11on site 3

MBT adsorbed 111 on site 3

Table 2 Adsorption energies of BT and MBT on different sites. Eads (eV) Site 1 Site 2 Site 3

111 (S) 111 (S) ~5 (thiophene) 116 (benzene) 111 (S)

BT 0.5 1.1 1.5 1.2 0.8

MBT 0.4 0.9 1.5 1.2 0.4

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5. E N E R G Y P R O F I L E S FOR BT A N D MBT D E S U L F U R I Z A T I O N Different mechanisms of desulfurization of BT have been proposed in the literature. Different reaction pathways are thus possible as presented on figure 7. The final product of BT hydrodesulfurization is ethylbenzene (EB). This compound can be produced either by styrene (STY) hydrogenation or by 2,3dihydrobenzothiophene (DHBT) desulfurization. Most of the authors detected DHBT during the experiments so they deduced that hydrogenation of the double bond is followed by hydrogenolysis of the C-S bonds [11,12]. On the other hand, on the basis of kinetic measurements, it has also been proposed t h a t BT desulfurization could be the result of two parallel routes, one involving DHBT as intermediate, the other one involving STY [13,14]. In these studies, it was proposed t h a t STY was not detected because its hydrogenation is too fast. This was supported by experiments showing that in the hydrodesulfurization conditions, styrene yielded 100% EB. The mechanism for MBT desulfurization is likely the same, with an overall rate constant three times less t h a n BT [2]. However there are very few literature data on MBT desulfurization. We have investigated the various reaction pathways for benzothiophene desulfurization reported on figure 7. Other intermediates are also possible but they are less stable and they will be discussed elsewhere. Assuming t h a t H atoms are coming from adsorbed H2 molecule on the surface, the hydrogenation steps are investigated in the form of successive atomic H additions. On site 2, H2 dissociates as one S-H and one Mo-H while on site 3 and site 1, only S-H are present. In all cases, H2 dissociation is exothermic (0.3 to 0.7 eV). For the clarity of figure 7, BT or the intermediates are always shown in an ~1 position, but for each step a full geometry optimization was performed. The first hydrogenation produces intermediate 1 (I1). This intermediate is stabilized by flat adsorption on site 2 whereas it is not when the hydrogenation proceed on site 3. This implies the existence of an endothermic step on site 3 t h a t does not appear on site 2. The stabilization is so important on site 2 t h a t isomerization to form orthostyrenethiolate (OSTY) is athermic. Ring opening seems therefore possible only on site 3. In this case, the last step is the C-S bond scission to produce styrene. In the other reaction path, I1 is further hydrogenated to give DHBT. Desulfurization of DHBT can then proceed through two intermediates: orthoethylbenzenethiolate (OEBT) or 2-phenylethanethiolate (PET). There is no difference on a thermodynamic point of view between these two intermediates. Both C-S bond scissions are highly exothermic as is the last one to produce ethylbenzene.

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Figure 7 Reaction path for BT desulfurization

All the results are summarized on diagram 1 and 2, which give the energy profile, including site creation from the most stable (fig.2) surface. Whatever the reaction pathway, the overall reaction is exothermic. On the molybdenum edge, creation of site 2 is assisted by BT adsorption on site 1. However, the final sulfur removal to create the CUS is more endothermic (1.61 eV) on this edge t h a n on the sulfur edge (1.33eV).

333 3.50 3.00

uxt,

x.,

u

2.50 2.00 1.50

_

1.00 ~ ~ - : N~,:site 3+S+STY 0.50 [ 0.00

DHBT ~ %

!

,

DHB'I' + H2

Stable Surface

-0.50 -1.00 site 3+S+EB -1.50 :

BT path 1 --'--BT path 2 -~-MBT1

Diagram 1 Energy profile for BT and MBT desulfurization on the sulfur edge (site 3) 3.00 BT q 1 on site 2

2.50 2.00

101e

V~BT~5

BT + H2

1.50

DHBTns~ 1.00

0.50

BT site 1

DHBT+H2

DHBT 111

~

--T

\

0.00 Stable surface -0.50 -1.00

Site I+EB

-1.50

Diagram 2 Energy profile for BT desulfurization on molybdenum edge (site2)

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F r o m t h e s e results, it can be deduced t h a t d e s u l f u r i z a t i o n of BT a n d MBT is possible on both edges of the MoS2 slab. The direct d e s u l f u r i z a t i o n to produce s t y r e n e is likely to occur on the sulfur edge while h y d r o d e s u l f u r i z a t i o n to produce e t h y l b e n z e n e t h r o u g h d i h y d r o b e n z o t h i o p h e n e could occur on both edges. On d i a g r a m 1 are r e p o r t e d the r e s u l t s o b t a i n e d with MBT, u s i n g t h e s a m e i n t e r m e d i a t e s , which also correspond to the most stable ones. On t h e m o l y b d e n u m edge, the e n e r g y profile of MBT d e s u l f u r i z a t i o n is a l m o s t t h e s a m e as BT desulfurization. A difference is found in the reaction proceed on the sulfur edge, w h e r e the a d s o r p t i o n e n e r g i e s of the various i n t e r m e d i a t e s are less i m p o r t a n t t h a n for BT ones. The lower d e s u l f u r i z a t i o n r a t e of s u b s t i t u t e d molecules s e e m s to be due to a lower a d s o r p t i o n constant. The sulfur r e m o v a l to produce the catalytic site is always the m o s t e n d o t h e r m i c step. 6. C O N C L U S I O N Different CUS on the surface of the MoS2 crystallites h a v e b e e n e v i d e n c e d on which BT a n d MBT d e s u l f u r i z a t i o n can proceed. It a p p e a r s t h a t both edges can p a r t i c i p a t e to the reaction a n d the limiting t h e r m o d y n a m i c step is the r e g e n e r a t i o n of the CUS. The e n e r g y cost of this step will d e p e n d on the c h e m i c a l p o t e n t i a l of H2 a n d H2S. Studies are in p r o g r e s s to t a k e this point into account by a c h e m i c a l p o t e n t i a l analysis. Acknowledge me nts This w o r k h a s b e e n p e r f o r m e d w i t h i n the g r o u p e m e n t de R e c h e r c h e E u r o p 6 e n " D y n a m i q u e mol6culaire q u a n t i q u e appliqu6e h la catlyse, l ' a d s o r p t i o n et h l'absorption", s u p p o r t e d by the I n s t i t u t F r a n ~ a i s du P6trole, the C e n t r e N a t i o n a l de la Recherche Scientifique, Total and TU Wien. 1. 2. 3. 4. 5. 6. 7. 8. 9.

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