Adsorption of Dibenzothiophene Derivatives over a MoS2

Energy & Fuels 2003, 17, 387-398

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Adsorption of Dibenzothiophene Derivatives over a MoS2 NanoclustersA Density Functional Theory Study of Structure-Reactivity Relations Hong Yang,* Craig Fairbridge, and Zbigniew Ring The National Centre for Upgrading Technology, Devon, AB, T9G 1A8, Canada Received August 6, 2002

Various adsorption configurations of dibenzothiophene, a series of one and two methylsubstituted dibenzothiophenes and their hydrogenated derivatives on a MoS2 nanocluster, were studied using self-consistent density functional theory with generalized-gradient approximation. The objective was to explore the relationship between the structure and catalytic hydrodesulfurization reactivity of these sulfur molecules. The calculated adsorption energies indicated that flat adsorption was more energetically favorable over perpendicular adsorption, due to the interactions of the sulfur atom, the thiophene, and aromatic rings of the sulfur molecule with the molybdenum atoms on the catalyst surface. The adsorption energy in the flat adsorption mode decreased when the aromatic ring was saturated, while the adsorption energy in the perpendicular mode increased with progressive saturation of the dibenzothiophenes. In the flat adsorption mode, dibenzothiophene, 4-methyldibenzothiophene, 2,8-, 3,7-, and 4,6-dimethyldibenzothiophenes interacted similarly with the catalyst cluster, which indicated that methyl groups on the 4- and 6-positions did not hinder the sulfur molecules from binding flat onto to the catalyst surface. However, in the perpendicular adsorption mode, it can be clearly seen from total electron density distribution of the sulfur-molecule-MoS2 cluster complex, that methyl groups in the 4and 6-positions prevented the bonding of the sulfur atom with the surface molybdenum atom. The plane of the aromatic ring system in these dibenzothiophenes was disturbed by hydrogenation of one or two aromatic rings. Ring puckering was more severe with methyl-substituted dibenzothiophenes, resulting in a reduction of steric hindrance and easier access of the sulfur atom to the catalyst surface through perpendicular binding. The atomic electron charge distribution by Mulliken population analysis, the bond lengths of the free sulfur molecule and adsorbed sulfur molecules, as well as the Mayer bond order of the S-Mo bond in perpendicular adsorption mode were also examined in this work in an attempt to understand the different hydrodesulfurization reactivities of these molecules.

Introduction MoS2-based catalysts have been widely used as hydrodesulfurization (HDS) catalysts for several decades. Commercial HDS catalysts consist of MoS2 nanoclusters well dispersed on alumina support and promoted by cobalt or nickel atoms. The activity of unpromoted catalysts is attributed to active sites located at the edges of the MoS2 nanoclusters. For promoted catalysts, Co-Mo-S and Ni-Mo-S structures are believed to be mainly responsible for the activity. More stringent environmental regulations on the sulfur level in transportation fuels have led to strong interest in exploring the relationship between the molecular structure and activity of sulfur compounds. The understanding of this relationship could guide the development of new generation HDS catalysts and help meet future fuel specifications. If sulfur content in diesel is reduced from the current level of 500 ppm to 15 ppm, alkyl-substituted dibenzothiophenes will be the major sulfur species to be removed by HDS reactions. Therefore, we focused our * Corresponding author. Fax: 1-780-987-5349. E-mail: hyang@ nrcan.gc.ca.

study on these most refractory derivatives of dibenzothiophene. Over the past decade, an extensive effort has been made to understand the HDS mechanism of dibenzothiophenes on MoS2-based catalysts.1-5 The results indicate that sulfur atoms are removed by two pathways: direct removal by hydrogenolysis and hydrogenation of one or both aromatic rings followed by C-S bond cleavage (Figure 1). It is well-known that HDS reactivity depends critically on the molecular size and structure of the sulfur molecules. The rate of dibenzothiophene (DBT) conversion is slightly greater than that of 2,8- and 3,7-dimethyldibenzothiophenes, and it is over 10 times higher than 4-methyldibenzothiophene and 4,6-dimethyldibenzothiophene.3,7,8 Alkyl (1) Girgis, M. J.; Gates, B. C. Ind. Eng. Chem. Res. 1991, 30, 20212058. (2) Topsøe, H.; Clausen B. S.; Massoth F. E. Hydrotreating Catalysis, Volume 11, Catalysis, Science and Technology; Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: New York, 1996. (3) Whitehurst, D. D.; Isoda, T.; Mochida, I. Adv. Catal. 1998, 42, 345-471. (4) Eijsbouts, S. Appl. Catal. A 1997, 158, 53-92. (5) Vasudevan, P. T.; Fierro, J. L. G. Catal. Rev. Sci. Eng. 1996, 38 (2), 161-188. (6) Houalla, M.; Nag, N. K.; Spare, A. V.; Broderick, D. H.; Gates B. C. AIChE J. 1978, 24, 1015-1021.

10.1021/ef020171k CCC: $25.00 © 2003 American Chemical Society Published on Web 02/13/2003

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Figure 1. Reaction pathways in the hydrodesulfurization of dibenzothiophenes.

substituents at the aromatic carbon adjacent to the sulfur atom are thought to sterically hinder the adsorption of these compounds on the catalyst surface. A low reaction rate in direct sulfur removal by hydrogenolysis has been attributed to the reduction in total HDS rate of 4- and 4,6-substituted benzothiophenes.3,8 For example, over a CoMo/Al2O3 catalyst, at 350 °C and 50 bar, the pseudo-first-order rate constants for the hydrogenation of DBT, 4-methyldibenzothiophene (4-MDBT), and 4,6-dimethyldibenzothiophene (4,6-DMDBT) were 15, 15, and 11 h-1, respectively. The pseudo-first-order rate constants for hydrogenolysis of DBT, 4-MDBT, and 4,6-DMDBT were, however, significantly different (126, 26, and 6 h-1).8 Therefore, it seems that substitutions at 4- and 6-positions affect only the direct sulfur removal by hydrogenolysis. In this work, using computational chemistry tools, we studied the interactions of a series of dibenzothiophene derivatives and their hydrogenated intermediates with the coordinatively unsaturated molybdenum atoms at the Mo-edge of a MoS2 nanocluster model to better understand what causes the difference in their hydrogenolysis and hydrogenation reactivity at the molecular level. Pioneered by Harris and Chianelli, theoretical studies of HDS reactions to elucidate the mechanism at the molecular level started well over a decade ago.9,10 With the advances in computer hardware and software, the costs of computational time have been falling steadily while the power and reliability of computational chemistry software have been continuously growing. This has encouraged an increasing number of researchers to use computational techniques for investigating the HDS reaction over MoS2 catalyst or Co- or Ni-promoted MoS2 catalysts. These studies can be divided into three groups: (1) nature and structure of the active sites;11-17 (7) Houalla, M.; Broderick, D. H.; Spare, A. V.; Nag, N. K.; de Beer, V. H. J.; Gates, B. C.; Kwart, H. J. Catal. 1980, 61, 523-527. (8) Gates, B. C.; Topsøe, H. Polyhedron 1997, 16 (18), 3213-3217. (9) Harris, S.; Chianelli, R. R. J. Catal. 1984, 86, 400-412. (10) Harris, S.; Chianelli, R. R. J. Catal. 1986, 98, 17-31. (11) Pis Diez, R.; Jubert, A. H. J. Mol. Catal. 1992, 73, 65-76. (12) Byskov, L. S.; Hammer B.; Nørskov J. K.; Clausen, B. S.; Topsøe, H. Catal. Lett. 1997, 17, 177-182. (13) Raybaud, P.; Hafner, J.; Kresse, G.; Toulhoat, H. Surf. Sci. 1998, 407, 237-250. (14) Byskov, L. S.; Nørskov, J. K.; Clausen, B. S.; Topsøe, H. J. Catal. 1999, 187, 109-122. (15) Raybaud, P.; Hafner, J.; Kresse, G.; Kasztelan, S.; Toulhoat, H. J. Catal. 2000, 189, 129-146. (16) Raybaud, P.; Hafner, J.; Kresse, G.; Kasztelan, S.; Toulhoat, H. J. Catal. 2000, 190, 128-143. (17) Cristol, S.; Paul, J. F.; Payen, E. J. Phys. Chem. B 2000, 104, 11220-11229.

Yang et al.

(2) sulfur binding and removal from the MoS2 clusters;12,15,16,18 and (3) adsorption of thiophenes and benzothiophenes on the active sites of the MoS2 catalyst.18-23 A few papers have also been published on hydrogen adsorption and dissociation.24 However, there are no theoretical results available in the literature on the adsorption and interaction of dibenzothiophene derivatives with MoS2 catalysts. This work presents systematic calculations, using density functional theory, of the adsorption energies of dibenzothiophene, selected methyl-substituted dibenzothiophenes, and their hydrogenated intermediates on a Mo10S18 nanocluster. We examined different adsorption configurations of these compounds on the Mo-edge of the catalyst cluster. The sulfur-molecule-Mo10S18 cluster complex was subjected to a geometry optimization to find the most stable configuration. Asdorption energy, total electron density, atomic charge distribution, Mayer bond order, and bond length were used to interpret the reactivity difference between substituted and unsubstituted molecules, and also between the parent and the hydrogenated molecules. Catalyst Model and Computational Method Molybdenum disulfide (MoS2) is a layered transition metal dichalcogenide that consists of stacks of S-Mo-S held together by van der Waals interactions. Each single-layer of S-Mo-S is composed of two hexagonal layers of sulfur atoms and an intermediate hexagonal layer of molybdenum atoms. In this study, we used a single layer Mo10S18 nanocluster with the coordinatively unsaturated sites on the Mo-edge as proposed by Ma and Schobert.18 This cluster is a simplified representation of a stoichiometric cluster Mo27S54 (Figure 2). Both clusters have the same Mo-edge planes. In their theoretical study of the HDS of thiophenic compounds over MoS2 catalyst, Ma and Schobert first designed a Mo16S32 cluster to mimic the edge structure of the Mo27S54 cluster. Then, they further reduced the Mo16S32 to Mo10S18 clusters, assuming that the influence of the atoms at positions far from the Mo-edge on its properties was negligible. They performed Mulliken orbital population analysis and calculated formal charges of the molybdenum atoms at the Mo-edges for both clusters and concluded that the Mo10S18 cluster was a reasonable approximation of the Mo27S54 cluster. Over six different binding modes of thiophenes with transition metal complexes have been reported in the literature.25 In this work, only two types of sulfurmolecule-Mo10S18 cluster coordinations were considered: flat adsorption through benzene and thiophene rings as well as the sulfur atom (η6, η5, η1S), and (18) Ma, X.; Schobert, H. H. J. Mol. Catal. A, Chem. 2000, 160, 409427. (19) Pis Diez, R.; Jubert, A. H. J. Map.ol. Catal. 1993, 83, 219235. (20) Ma, X.; Schobert, H. H. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1997, 42 (3), 657-661. (21) Ma, X.; Schobert, H. H. Prepr. Pap.sAm. Chem. Soc., Div. Pet. Chem. 1998, 43 (1), 24-27. (22) Raybaud, P.; Hafner, J.; Kresse, S.; Toulhoat, H. Phys. Rev. Lett. 1998, 80 (7), 1481-1484. (23) Cristol, S.; Paul, J. F.; Payen, E.; Bougeard, D.; Hafner, J.; Hutschka, F. Stud. Surf. Sci. Catal. 1999, 127, 327-334. (24) Li, Y. W.; Pang, X. Y.; Delmon, B. J. Mol. Catal. A., Chem. 2001, 169 (1-2), 259-268. (25) Angelici, R. J. Coord. Chem. Rev. 1990, 105, 61-67.

Adsorption of Dibenzothiophene Derivatives over MoS2

Energy & Fuels, Vol. 17, No. 2, 2003 389

Figure 2. MoS2 nanoclusters. Large light gray circles: sulfur atom; small dark gray circles: molybdenum.

perpendicular adsorption through the sulfur atom. Figure 3 shows the front view and side view of flat (3a and 3b) and perpendicular (3c and 3d) adsorption configurations. The chemical structures of unhydrogenated dibenzothiophene derivatives are shown in Figure 4. All the calculations discussed below were performed with the DMol3 simulation package (Accelrys Inc., CA). Individual sulfur molecules, Mo10S18 nanocluster, and sulfur-molecule-Mo10S18 complexes were subjected to geometry optimization before energy calculation. The surface molybdenum atoms of the Mo10S18 cluster and the adsorbed molecules were allowed to relax during the calculation, while the rest of the slab was held at the fixed bulk MoS2 geometry. The total energy of the system was determined by self-consistent density functional theory calculation, and the exchange-correlation energy was approximated by nonlocal generalized gradient corrections of PW91.26 All the electrons were included in the calculations with a double-numeric quality basis set. The real space cutoff of the atomic orbital was set at 4.0 Å, and a smearing of 0.005 Ha was used to count the orbital occupancy. The adsorption energy of a sulfur molecule on the Mo10S18 cluster was given by eq 1:

∆E (kcal/mol) ) ES-Cat - (ES + ECat)

(1)

where ES-Cat was the total energy of the sulfur-molecule(26) Perdew, J. P.; Wang, Y. Phys. Rev. 1992, B45, 13244-13249.

Figure 3. Adsorption configurations of dibenzothiophenes on the Mo-edge of a Mo10S18 nanocluster. (a) Flat adsorption: front view; (b) flat adsorption: side view; (c) perpendicular adsorption: front view; (d) perpendicular adsorption: side view. Large light gray circles: sulfur atom; small dark gray circles: molybdenum; small light circles: hydrogen; larger black circles: carbons.

Mo10S18 complex, ES is the total energy of the free sulfur molecule,and ECat is total energy of the Mo10S18 cluster. Results and Discussions Flat Adsorption of Dibenzothiophene Derivatives. The geometry-optimized sulfur molecule was laid flat on top of the Mo-edge of the optimized Mo10S18 cluster with the sulfur atom located above the middle molybdenum atom, as indicated in Figure 3, parts a and b. The initial position of the sulfur atom with respect to the molybdenum atom was similar to that in the MoS2 bulk structure. The sulfur-molecule-Mo10S18 complex was allowed to relax to obtain the minimum energy configuration. It is interesting to note that after geometry optimization, the adsorbed dibenzothiophenes are not exactly planar as in the free molecules, the sulfur atom is slightly tilted out of the plane of the aromatic rings. The adsorption energy of the sulfur molecule was calculated using eq 1. The atomic charge distribution by Mulliken population analysis, the bond length in the free sulfur molecule and in the sulfur-molecule-Mo10S18 complex, were also determined in the geometry-optimized structures. Table 1 summarizes the adsorption energies and the distances between S and Mo atoms before (in parentheses) and after adsorption, as well as atomic charges calculated for different dibenzothiophene-

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Figure 4. Dibenzothiophene and derivatives. Table 1. Adsorption Energy and Mulliken Charges for the Flat Adsorption of Dibenzothiophenes sulfur compoundsa

∆E kcal/mol

S-Mo distanceb Å

S atom c charge

Mo atom c charge

S-moleculec charge

Mo10S18 clusterc charge

DBT 4-MDBT 2,8-DMDBT 3,7-DMDBT 4,6-DMDBT

-77.4 -77.6 -82.8 -76.6 -77.1

2.593 (2.417) 2.590 (2.546) 2.587 (2.512) 2.589 (2.512) 2.593 (2.553)

0.707 (0.590) 0.709 (0.588) 0.711 (0.578) 0.707 (0.574) 0.711 (0.594)

-0.417 (-0.196) -0.424 (-0.196) -0.421 (-0.196) -0.430 (-0.196) -0.421 (-0.196)

-0.055 (-0.002) -0.067 (0.000) -0.013 (-0.004) -0.019 (-0.004) -0.082 (-0.001)

0.055 (0.001) 0.066 (0.001) 0.012 (0.001) 0.023 (0.001) 0.085 (0.001)

a DBT: dibenzothiophene; 4-MDBT: 4-methyldibenzothiophene; 2,8-DMDBT: 2,8-dimethyldibenzothiophene; 3,7-DMDBT: 3,7dimethyldibenzothiophene; 4,6-DMDBT: 4,6-dimethyldibenzothiophene. b Values in parentheses are the S-Mo distances before geometry optimization. c Values obtained for the free sulfur molecule and free Mo10S18 cluster are in parentheses.

Mo10S18 complexes. Only the charge changesson the sulfur atom of the sulfur molecule, on the middle molybdenum atom in the catalyst cluster, on the sulfurcontaining compound, and on the Mo10S18 clustersare compared in this work since these are the most meaningful electronic properties to explain the configuration change upon adsorption. The Mulliken atomic charge values obtained from the free sulfur molecules and free Mo10S18 cluster are given in parentheses. In the flat adsorption configuration, DBT and its methyl-substituted derivatives have similar adsorption energies ranging from -76.6 to -82.8 kcal/mol. The negative values indicate that the adsorption of sulfur molecules on the Mo-edge of the Mo10S18 cluster is exothermic. The equilibrium S-Mo distances are similar too, ranging from 2.587 to 2.593 Å. The results in Table 1 indicate that methyl groups do not affect the flat adsorption of dibenzothiophenes, which is in agreement with the results published by Cristol et al.23 These authors studied the adsorptions of benzothiophene and methylbenzothiophene over the Mo-edge of a MoS2 layer using periodic density functional theory calculation and obtained the same values of the adsorption energy for the two sulfur molecules in both η5(thiophene) and η6 (benzene) adsorption configurations (both flat). If hydrogenation is the primary reaction when adsorption involves binding through the aromatic rings, this result also supports the experimental results reported by Gates and Topsøe.8 These authors obtained similar hydrogenation rate constants for DBT, 4-MDBT, and 4,6-DMDBT. The strong binding in the flat adsorption mode is due to the interactions of both the ring system and sulfur atom with the Mo10S18 cluster. The Mulliken charge changes (∆MC) upon adsorption of the sulfur atom, the middle molybdenum atom as well as these of the sulfur molecule and the Mo10S18 complex

Figure 5. Mulliken charge changes upon flat adsorption for the sulfur atom in the dibenzothiophenes, the middle molybdenum atom in a Mo10S18 cluster, as well as those of the sulfur molecule and the Mo10S18 complex.

are calculated by eq 2 using the Mulliken charge values in Table 1.

∆MC ) MCF - MCAd

(2)

In this equation, MCF was the Mulliken charge before adsorption and MCAd was the Mulliken charge after adsorption. Figure 5 plots the results. The negative difference in charge indicates an increase of charge after adsorption, therefore a loss of electrons; the positive difference indicates a decrease of charge after adsorption, therefore a gain of electrons. Negative differences were observed for the sulfur atoms, and positive differences were obtained for the middle molybdenum atoms in all the cases; this indicates that there is an electron transfer from the sulfur atom to the middle molybdenum atom, to which the sulfur atom is directly bonded.



Table 2. Bond Lengths in Free Sulfur Molecules and Adsorbed Sulfur Molecule on a Mo10S18 Cluster for Flat Adsorption DBTa bonds

free

adsorbed

4-MDBTa free

adsorbed

2,8-DMDBTa free

S-CR1 S-CR2 CR1-Cβ1 CR2-Cβ2 Cβ1-Cβ2 Average bond length

1.812 1.812 1.415 1.415 1.456 1.582

1.861 1.860 1.449 1.451 1.444 1.613

1.819 1.819 1.416 1.415 1.456 1.585

Thiophene Ring 1.866 1.812 1.855 1.812 1.452 1.415 1.452 1.415 1.440 1.457 1.613 1.582

Cβ1-C1 C1-C2 C2-C3 C3-C4 C4-CR1 CR2-C6 C6-C7 C7-C8 C8-C9 C9-Cβ2 average bond length

1.406 1.395 1.407 1.397 1.394 1.394 1.397 1.407 1.395 1.406 1.400

1.422 1.383 1.436 1.463 1.451 1.450 1.459 1.434 1.388 1.417 1.430

1.405 1.395 1.404 1.403 1.399 1.394 1.397 1.407 1.396 1.407 1.401

Benzene Rings 1.422 1.406 1.381 1.400 1.435 1.412 1.472 1.397 1.461 1.394 1.452 1.394 1.460 1.397 1.435 1.412 1.385 1.400 1.420 1.406 1.432 1.402

3,7-DMDBTa

4,6-DMDBTa

adsorbed

free

adsorbed

free

adsorbed

1.857 1.857 1.450 1.449 1.440 1.611

1.815 1.815 1.415 1.415 1.455 1.583

1.860 1.856 1.449 1.450 1.443 1.612

1.819 1.813 1.415 1.414 1.456 1.583

1.864 1.863 1.451 1.451 1.442 0.807

1.419 1.390 1.444 1.459 1.449 1.451 1.463 1.446 1.385 1.421 1.43

1.405 1.395 1.412 1.402 1.393 1.393 1.402 1.412 1.395 1.405 1.401

1.421 1.384 1.441 1.464 1.451 1.447 1.459 1.438 1.391 1.414 1.431

1.405 1.395 1.404 1.403 1.399 1.400 1.402 1.407 1.395 1.407 1.402

1.420 1.382 1.435 1.470 1.460 1.457 1.469 1.438 1.382 1.422 1.434

a DBT: dibenzothiophene; 4-MDBT: 4-methyldibenzothiophene; 2,8-DMDBT: 2,8-dimethyldibenzothiophene; 3,7-DMDBT: 3,7dimethyldibenzothiophene; 4,6-DMDBT: 4,6-dimethyldibenzothiophene.

However, the gain in charge of molybdenum is more than the loss of charge from sulfur, which indicates that the π electrons of the thiophenic ring also participate in the adsorption process. For the sulfur-molecule-Mo10S18 complex system, however, a net electron transfer from the Mo10S18 cluster to the adsorbed sulfur molecule is indicated by the negative values of charge changes over the Mo10S18 cluster. This is probably due to the interactions of the Mo10S18 cluster with the benzene rings of the sulfurcontaining molecule. The other two molybdenum atoms back-donate electrons to aromatic π systems of the sulfur molecule. Charge transfer from the sulfur atom of the sulfur-containing molecule to the MoS2 cluster has been reported in several papers.18,19,22,27 In a study of molecular orbitals during thiophene hydrodesulfurization, Pis Diez and Jubert showed that a charge transfer occurred from the sulfur lone pair in the HOMO-2 orbital to the MonS2n cluster (n between 8 and 11) in both flat (η5) and perpendicular (η1S) adsorption configurations.19 When thiophene was adsorbed flat on a Mo10S18 cluster, a significant increase in the formal charge of the thiophene molecule and a decrease in the charge of the molybdenum atom were observed by Ma and Schobert.18 However, they believed that thiophene was likely to donate its π electrons to the Mo10S18 cluster. Using the ab initio Hartree-Fock method, Marshall et al. calculated the HOMO orbitals for thiophene, benzothiophene, and dibenzothiophene, and they concluded that there was a significant contribution from the sulfur atom to the benzothiophene and dibenzothiophene’s HOMO orbitals, which could result in binding of the S and metal atoms.27 Detailed analysis of the molecular orbitals is beyond the scope of this paper. For desulfurization of the thiophene ring, we would expect weakening of the SCR bonds as a result of the adsorption step. A comparison of the bond lengths in the adsorbed dibenzothiophene derivatives and those in

the free molecules in Table 2 confirms this expectation. In the thiophene ring, the double bonds CR1-Cβ1 and CR2-Cβ2 were stretched, while the single bond Cβ1-Cβ2 was shortened (see Figure 4 for numbering and symbols). In the benzene rings, the average bond distances increased, which indicate that the aromatic characters of these sulfur molecules are reduced after adsorption. These results clearly show that flat adsorption of dibenzothiophene derivatives involves interactions of benzene (η6) and thiophene (η5) rings as well as the S atom (η1S) with the catalyst cluster. They also demonstrate that both desulfurization and hydrogenation reactions could be accelerated when the sulfur molecule is adsorbed in a flat configuration. Similar results were also reported by other authors.22,25,28 Using an ab initio local density functional method, Raybaud et al. studied the different adsorption configurations of thiophene on the catalytically active MoS2 (010) surface.22 They used a Mo24S48 model of two S-Mo-S sandwiches stacked in the z direction; each slab consisted of three rows of trigonal prisms stacked in the y direction and four rows of trigonal prisms in the x direction. They found that the most favorable adsorption configuration was a flat position with the thiophene’s sulfur atom bridged between two molybdenum atoms of the top layer. The sulfur atom in this stable configuration was also tilted out of the planar aromatic ring system similar to the relaxed sulfur molecule in the sulfur-molecule-Mo10S18 complex in this study. Increased SCR bond lengths and changed geometry of the thiophene ring were observed. Angelici reviewed different coordination modes of thiophene with several organo-transition metal complexes.25 The SCR bond lengths increased for all the binding configurations of thiophene-transition metal complexes. On average, larger increases were obtained for the flat adsorption modes (η5 and η4) than for the S-binding mode (η1S). Perpendicular Adsorption of Dibenzothiophene Derivatives. The geometry-optimized sulfur molecule was placed perpendicularly to the Mo10S18 cluster, as

(27) Marshall, C. L.; Brenner, J. R.; Tilson, J. L. Prepr. Pap.sAm. Chem. Soc., Div. Pet. Chem. 1998, 43 (1), 28-31.

(28) Neurock, M.; van Santen, R, A. J. Am. Chem. Soc. 1994, 116, 4427-4439.

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Table 3. Adsorption Energy, Mayer Bond Order, and Mulliken Charge for the Perpendicular Adsorption of Dibenzothiophenes sulfur compoundsa

∆E kcal/mol

S-Mo distanceb Å

S-Moc bond order

S atom c charge

Mo atom c charge

S-moleculec charge

Mo10S18 clusterc charge

DBT 4-MDBT 2,8-DMDBT 3,7-DMDT 4,6-DMDBT

-27.9 -28.7 -28.7 -30.3 -34.1

2.422 (2.420) 2.590 (2.546) 2.456 (2.412) 2.448 (2.397) 2.548 (2.464)

0.7182 0.6856 0.7275 0.7490 0.5744

0.655 (0.590) 0.665 (0.588) 0.622 (0.578) 0.661 (0.574) 0.596 (0.594)

-0.282 (-0.196) -0.295 (-0.196) -0.281 (-0.196) -0.287 (-0.196) -0.284 (-0.196)

0.207 (-0.002) 0.296 (0.000) 0.228 (-0.004) 0.236 (-0.004) 0.375 (-0.001)

-0.206 (0.001) -0.295 (0.001) -0.226 (0.001) -0.234 (0.001) -0.372 (0.001)

a DBT: dibenzothiophene; 4-MDBT: 4-methyldibenzothiophene; 2,8-DMDBT: 2,8-dimethyldibenzothiophene; 3,7-DMDBT: 3,7dimethyldibenzothiophene; 4,6-DMDBT: 4,6-dimethyldibenzothiophene. b Values in parentheses are the S-Mo distances before geometry optimization. c Values obtained for the free sulfur molecule and free Mo10S18 cluster are in parentheses.

Figure 6. Mulliken charge changes upon perpendicular adsorption for the sulfur atom in the dibenzothiophenes, the middle molybdenum atom in a Mo10S18 cluster, as well as those of the sulfur molecules and the Mo10S18 complex.

shown in Figure 3, parts c and d. The sulfur molecule was bonded to the middle Mo atom of the cluster directly through sulfur (η1S). The sulfur-molecule-Mo10S18 complex was optimized to obtain the structure with the minimum total energy. The adsorption energies calculated using eq 1 are shown in Table 3. The Mayer bond order of the S-Mo bond, the Mulliken atomic charges of the sulfur atom in the sulfur molecules, the middle molybdenum atom in the catalyst cluster, the sulfur molecule, and the Mo10S18 cluster are also presented in Table 3. Values in parentheses are Mulliken atomic charges obtained over the free sulfur molecules and the free catalyst cluster. Similar adsorption energies ranging from -27.9 to -34.1 kcal/mol were obtained for the perpendicular adsorption of dibenzothiophenes. However, these values were considerably less than those for flat adsorption (-76.6 to 82.8 kcal/mol). The changes in Mulliken charge upon perpendicular adsorption of the sulfur atom in the dibenzothiophenes, the middle molybdenum atom in the Mo10S18 cluster, as well as those of the sulfur molecules and the Mo10S18 complex are calculated by eq 2 using the values in Table 3 (the results are plotted in Figure 6). Negative differences indicate a loss of electrons after adsorption; positive differences indicate a gain of electrons after adsorption. For each of the investigated sulfur molecules, significant electron transfer was found from the sulfur atom of all the sulfur molecules to the middle molybdenum atom of the Mo10S18 cluster, except for the 4,6-DMDBT. There were only 0.002 differences in charge between the sulfur atom of the free and adsorbed 4,6-DMDBT molecules. In this case, this shows that there is almost no electron transfer from sulfur atom to molybdenum atom. Overall,

there was net electron transfer from sulfur molecules to the Mo10S18 complex for all the dibenzothiophene derivatives in perpendicular adsorption, in contrast to the flat adsorption. In the case of the 4,6-DMDBT-Mo10S18 cluster complex, the middle molybdenum atom still shows nearly the same gain in charge, although no charge contributes directly from the sulfur atom of 4,6-DMDBT. The results suggest that thiophene and benzene rings may contribute to some extent to the perpendicular binding of 4,6DMDBT through 4,6-methyl groups. The participation of thiophene and benzene rings in the 4,6-DMDBTMo10S18 cluster complex could also explain why it has the highest adsorption energy among all the dibenzothiophenes. This result demonstrates that adsorption energy alone could not be used to explain the structurereactivity relationship. The comparison of flat adsorption and perpendicular adsorption indicates that methyl groups in the 4,6position inhibit the bonding of the sulfur atom to the catalyst surface only in perpendicular adsorption. A methyl group in the 2, 3, 7, and 8 positions has no effect on the adsorption energy, in either the flat or the perpendicular adsorption configuration. Although the charge transfer data and adsorption energy results suggest that the methyl group at the 4 position behaves similarly as those in 2, 3, 7, and 8 positions in perpendicular adsorption, the plots of volumetric total density (Figure 7) indicate a slight effect by the 4-methyl group on the formation of the S-Mo bond. For methyl groups at the 2, 3, 7, and 8 positions, there is complete overlap between electron densities from both the sulfur of the sulfur molecules and the middle molybdenum atoms of the catalyst cluster. A small gap was observed for the S-Mo bond in 4-MDBT, which indicates that the bond is weaker than that in the case of dibenzothiophene due to the presence of the 4-methyl group. A significant lack of overlap of the S-Mo electrons was observed for 4,6DMDBT in the optimized sulfur-molecule-Mo10S18 complex. Also, there was an angle of about 22.5° between the plane of 4,6-DMDBT and the plane of the Mo10S18 cluster. This kind of adsorption configuration makes the system interact easily with the Mo10S18 cluster via its 4,6 methyl groups. This is in agreement with the higher perpendicular adsorption energy of 4,6-DMDBT due to the interaction of its aromatic rings with the catalyst cluster. This result may also suggest that adsorption of 4,6-DMDBT through η1S coordination is less likely and, hence, hydrogenation followed by C-S cleavage may be the only pathway available to this molecule. To express the density overlap quantitatively, the Mayer bond order of the S-Mo bond was calculated over



Figure 7. Volumetric plots of total density distribution of different dibenzothiophenes. The symbols for different atoms are the same as in Figure 3.

the geometry-optimized structure (Table 3). The numbers increased in the order 4,6-DMDBT < 4-MDBT < DBT < 2,8-DMDBT < 3,7-DMDBT. This order is a result of the combination of electronic and steric effects. As an electron donor group, methyl substituents could enrich the electron density of the dibenzothiophenes, therefore improving the binding between their sulfur atoms and the molybdenum atoms of the catalyst cluster. The fact that the S-Mo bond order of 3,7DMDBT is larger than that of 2,8-DMDBT indicates that methyl groups near the sulfur atom probably have more of an electron enhancement effect on the S atom. The lowest bond order in the case of 4,6-DMDBT indicates the HDS rate of alkyl-substituted dibenzothiophenes is predominantly governed by steric effects. From the S-Mo bond order values, we would expect that 3,7-DMDBT and 2,8-DMDBT have higher HDS rates than DBT. Although most of the published results indicate that 3,7- and 2,8-DMDBTs have comparable or lower HDS rate than DBT,3,7,8 some authors have observed a slightly higher HDS rate for 2,8- and 3,7-DMDBTs.29,30 Kilanowski et al. carried out a study of HDS of methyl-substituted benzothiophenes and dibenzothiophenes over a Co/Mo/Al2O3 catalyst between 350 and 450 °C. The reactivity of the substituted dibenzothiophenes decreased in the order 2,8-DMDBT > DBT > 4-MDBT > 4,6-DMDBT. They proposed that the combination of the inductive and hyperconjugative effects of the methyl groups para to the two R carbons could enrich the electron density and increase the activity.30 The comparison of bond length for the free and adsorbed sulfur molecules is presented in Table 4. As expected, the bonds in the benzene rings are not affected

by the perpendicular adsorption. The SCR bonds have been lengthened, indicating their weakening. There is only a slight increase in the average bond length of the thiophene ring for the adsorbed sulfur molecules. For 4,6-DMDBT, this SCR bond length increase is more significant than for other dibenzothiophenes. To a lesser extent, this is also observed for 4-MDBT. The SCR bond (SCR1) close to the 4-methyl group increases more significantly that the other SCR bond (SCR2). We believe this is due to the presence of methyl groups at the 4 and 6 positions. However, from the much lower HDS rates of 4-methyl and 4,6-dimethyldibenothiophene reported in the literature, we conclude that this effect would improve the reactivity to a much lesser extent than it is reduced by the steric effect, which is also in concurrence with the Mayer bond order values obtained. The enhancement of the reactivity by 4 and 6 methyl groups was observed in homogeneous catalytic reaction conditions.31,32 Te et al. studied the oxidation of dibenzothiophenes catalyzed by polyoxometalate/H2O2 or by formic acid/H2O2. In the case of polyoxometalate/H2O2, they obtained the same reactivity trend as that in the heterogeneous catalytic HDS reaction. However, in the formic acid/H2O2 system, the reverse trend was obtainedsthe oxidation rate was increased in the order of DBT < 4-MDBT < 4,6-DMDBT. They indicated that alkyl groups at 4 and 6 positions imposed steric hindrance to the interaction of sulfur molecules with the active sites in the polyoxoperoxo species. The small formic acid molecule could, however, interact with sulfur molecules without any steric hindrance from alkyl groups on the 4 and 6 positions. The results clearly showed that the 4 or 4,6-substituted dibenzothiophenes have higher intrinsic reactivity than DBT.32

(29) Ma, X. L.; Sakanishi, K.; Mochida, I. Ind. Eng. Chem. Res. 1996, 35, 2487-2494. (30) Kilanowski, D. R.; Teeuwen, H.; de Beer, V. H. J.; Gates, B. C.; Schuit, G. C. A.; Kwart, H. J. Catal. 1978, 55, 129-137.

(31) Otsuki, S.; Nonaka, T.; Takashima, N.; Qian, W.; Ishihara, A.; Imai, T.; Kabe, T. Energy Fuels 2000, 14, 1232. (32) Te, M.; Fairbridge, C.; Ring, Z. Appl. Catal. A 2001, 219, 267280.

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Table 4. Bond Lengths in Free Sulfur Molecules and Adsorbed Sulfur Molecule on a Mo10S18 Cluster for Perpendicular Adsorption DBTa bonds

free

adsorbed

4-MDBTa free

adsorbed

2,8-DMDBTa free

S-CR1 S-CR2 CR1-Cβ1 CR2-Cβ2 Cβ1-Cβ2 average bond length

1.812 1.812 1.415 1.415 1.456 1.582

1.833 1.834 1.407 1.408 1.463 1.589

1.819 1.819 1.416 1.415 1.456 1.585

Thiophene Ring 1.866 1.812 1.830 1.812 1.409 1.415 1.404 1.415 1.458 1.457 1.593 1.582

Cβ1-C1 C1-C2 C2-C3 C3-C4 C4-CR1 CR2-C6 C6-C7 C7-C8 C8-C9 C9-Cβ2 average bond length

1.406 1.395 1.407 1.397 1.394 1.394 1.397 1.407 1.395 1.406 1.400

1.405 1.397 1.405 1.399 1.384 1.387 1.399 1.404 1.398 1.405 1.398

1.405 1.395 1.404 1.403 1.399 1.394 1.397 1.407 1.396 1.407 1.401

Benzene Rings 1.404 1.406 1.395 1.400 1.400 1.412 1.406 1.397 1.399 1.394 1.387 1.394 1.397 1.397 1.405 1.412 1.397 1.400 1.404 1.406 1.399 1.402

3,7-DMDBTa

4,6-DMDBTa

adsorbed

free

adsorbed

free

adsorbed

1.832 1.834 1.406 1.406 1.464 1.588

1.815 1.815 1.415 1.415 1.455 1.583

1.830 1.829 1.406 1.406 1.463 1.587

1.819 1.813 1.415 1.414 1.456 1.583

1.869 1.864 1.407 1.408 1.453 1.600

1.405 1.403 1.410 1.398 1.384 1.385 1.398 1.409 1.404 1.404 1.400

1.405 1.395 1.412 1.402 1.393 1.393 1.402 1.412 1.395 1.405 1.401

1.406 1.398 1.411 1.405 1.383 1.383 1.403 1.411 1.397 1.405 1.400

1.405 1.395 1.404 1.403 1.399 1.400 1.402 1.407 1.395 1.407 1.402

1.405 1.392 1.400 1.407 1.402 1.400 1.407 1.399 1.393 1.402 1.401

a DBT: dibenzothiophene; 4-MDBT: 4-methyldibenzothiophene; 2,8-DMDBT: 2,8-dimethyldibenzothiophene; 3,7-DMDBT: 3,7dimethyldibenzothiophene; 4,6-DMDBT: 4,6-dimethyldibenzothiophene.

Figure 8. Relaxed structures of dibenzothiophene and its hydrogenated intermediates flat-adsorbed on a Mo10S18 nanocluster. The symbols for different atoms are the same as in Figure 3.

Adsorption Energies of Dibenzothiophene and its Hydrogenated Derivatives. Dibenzothiophene, tetrahydrodibenzothiophene (4H-DBT), hexahydro-dibenzothiophene (6H-DBT), and completely hydrogenated dibenzothiophene (HY-DBT) were subjected to a structure optimization before the simulation. The flat plane of the aromatic ring system was slightly disturbed upon hydrogenation. The flat and perpendicular adsorption configurations for DBT and its hydrogenated derivatives were simulated, again using the same computation

procedure. Figure 8 shows the relaxed structures of the corresponding sulfur-molecule-Mo10S18 complexes in the flat adsorption mode, which also gives a clear picture of the dependence of the stable adsorption configuration on the molecular structure. During the simulation of the HY-DBT-Mo10S18 complex, the sulfur molecule moved away from the initial flat position on the top of the Moedge (Figure 8e). Table 5 summarizes the adsorption energies for different dibenzothiophene-Mo10S18 complexes and the Mulliken atomic charges for diben-



Table 5. Adsorption Energy and Mulliken Charges of Dibenzothiphene and Its Hydrogenated Derivatives sulfur compoundsa

∆E kcal/mol

S-Mo distance Å

DBT 4H-DBT 6H-DBT HY-DBT

-77.37 -60.4 -54.8 -40.6

2.593 (2.417) 2.570 (2.466) 2.569 (2.472) 2.531 (2.484)

DBT 4H-DBT 6H-DBT HY-DBT

-27.9 -30.6 -35.8 -40.5

2.422 (2.420) 2.403 (2.420) 2.382 (2.441) 2.368 (2.442)

S atom charge

Mo atom charge

S-molecule charge

Mo10S18 cluster charge

Flat Adsorption 0.707 (0.590) -0.417 (-0.196) 0.694 (0.552) -0.398 (-0.196) 0.522 (0.404) -0.324 (-0.196) 0.358 (0.275) -0.293 (-0.196)

-0.055 (-0.002) 0.183 (-0.003) 0.167 (-0.003) 0.365 (-0.001)

0.055 (0.001) -0.184 (0.001) -0.167 (0.001) -0.366 (0.001)

Perpendicular Adsorption 0.655 (0.590) -0.282 (-0.196) 0.640 (0.552) -0.297 (-0.196) 0.492 (0.404) -0.297 (-0.196) 0.381 (0.275) -0.295 (-0.196)

0.207 (-0.002) 0.236 (-0.003) 0.278 (-0.003) 0.336 (-0.001)

-0.206 (0.001) -0.236 (0.001) -0.277 (0.001) -0.333 (0.001)

a DBT: dibenzothiophene; 4H-DBT: tetrahydrodibenzothiophene; 6H-DBT, hexahydrodibenzothiophene; HY-DBT: completely hydrogenated dibenzothiophene.

zothiophenes and dibenzothiophene-Mo10S18 complex. The values of the Mulliken atomic charges for the free sulfur compounds and free Mo10S18 cluster are shown in parentheses. For the flat adsorption configuration, the adsorption energies decrease with the degree of hydrogenation, which is consistent with a gradual loss of interaction sites between the sulfur molecules and the Mo10S18 cluster as shown in Figure 8. The analysis of Mulliken charges of DBT and its hydrogenated derivatives demonstrates that the positive charge of the sulfur atom decreases in the order DBT (0.590) > 4H-DBT (0.552) > 6H-DBT (0.404) > HY-DBT (0.275) due to progressive hydrogenation, indicating electron density gain over the hydrogenated derivatives. This is probably because the sulfur atom regains a portion or all of its electrons previously contributed to the π system of the thiophene ring. A comparison of the Mulliken charges of the free sulfur molecules and the free Mo10S18 cluster (values in parentheses) with those of the sulfur moleculesMo10S18 complexes for DBT derivatives shows that there is electron transfer from the sulfur atom to the middle molybdenum atom. Unlike DBT, where a net electron transfer from the Mo10S18 cluster to the adsorbed sulfur molecules is observed, over the hydrogenated dibenzothiophene intermediates, net electron transfer is from the sulfur molecules to the Mo10S18 cluster. In the case of the perpendicular adsorption, the hydrogenated intermediates show higher adsorption energies than DBT. The perpendicular adsorption depends mainly on the binding strength of the S-Mo bond, which would benefit from the increase of electron density on sulfur. Few literature reports are available examining systematic comparison of the adsorption of thiophenic compounds and hydrogenated derivatives. On one hand, Cristol et al. observed a reduction in adsorption energy when benzothiophene was hydrogenated to dihydrobenzothiophene through η5 flat coordination; however, a decrease in adsorption energy from benzothiophene to dihydrobenzothiophene over perpendicular adsorption mode was also found.23 On the other hand, adsorption energies of the hydrogenated thiophenes and benzothiophenes, higher than those of the parent sulfur molecules, were reported by Ma and Schobert in the perpendicular adsorption configuration.18,20 The results obtained in our study for perpendicular adsorption confirm the findings of the latter group. In addition, experimental studies demonstrated that hydrogenated thiophenic compounds had much higher hydrogenolysis reactivity than unhydrogenated thiophenic compounds,

suggesting stronger binding between sulfur and molybdenum in the hydrogenated species.3,6,20,33 Adsorption Energies of 4,6-Dimethyldibenzothiophene and Its Hydrogenated Derivatives. 4,6Dimethyldibenzothiophene and its hydrogenated intermediates were chosen for the density functional theory calculation since they represent extreme cases of steric hindrance. The structures of tetrahydro-4,6-dimethyldibenzothiophene (4H-4,6-DMDBT), hexahydro-4,6dimethyldibenzothiophene (6H-4,6-DMDBT), and completely hydrogenated 4,6-dimethyldibenzothiophene (HY4,6-DMBDT) were first optimized as explained above. The flat plane of the aromatic ring system in 4,6dimethydibenothiophenes was disturbed by hydrogenation of the aromatic rings. Ring puckering was more severe with 4,6-DMDBT than with DBT, especially for the completely hydrogenated 4,6-dimethydibenzothiophene. The adsorption energies for flat and perpendicular configurations were calculated over the geometryoptimized sulfur molecules-Mo10S18 cluster and presented in Table 6, along with the Mulliken atomic charges. The values of Mulliken atomic charges for the free sulfur molecules and the free Mo10S18 cluster are given in the parentheses. The results are similar to those obtained in the case of dibenzothiophenes. Adsorption energy decreased when hydrogen was added gradually to 4,6-DMDBT in the flat adsorption mode. Opposite results were obtained in the perpendicular adsorption mode. Table 6 shows that the completely hydrogenated 4,6-dimethyldibenzothiohene has higher perpendicular adsorption energy (-48.4 kcal/mol) than that of 4,6-DMDBT (-34.1 kcal/ mol). Severe ring puckering was observed for fully saturated 4,6-dimethyldibenzothiophene, which could reduce or even remove the steric hindrance effect of the methyl groups and make the sulfur atom more accessible in the perpendicular configuration. This interpretation is also confirmed by Mulliken atomic charge distribution results. Electron density transfer from the sulfur atom to the middle molybdenum atom is enhanced over hydrogenated 4,6-dimethyldibenzothiophenes. The electron density transfer is much greater for the completely hydrogenated 4,6-dimethyldibenzothiophene (0.056) than for 4,6-DMDBT (0.002). Overall, there is a net electron transfer from the sulfur molecules to the Mo10S18 cluster. Our results are also consistent with the experimental results reported by Kabe et al.34 In a study of deep desulfurization of DBT, (33) Devanneaux, J.; Maurin J. J. Catal. 1981, 69, 202-205.

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Yang et al.

Table 6. Adsorption Energy, Mayer Bond Order, and Mulliken Charges of 4,6-Dimethyldibenzothiphene and Its Hydrogenated Derivatives sulfur compounds

∆E kcal/mol

S-Mo distance Å

4,6-DBT 4H-4,6-DMDBT 6H-4,6-DMDBT HY-4,6-DMDBT

-77.1 -61.1 -55.8 -51.2

2.593 (2.553) 2.603 (2.570) 2.647 (2.577) 2.554 (2.552)

4,6-DBT 4H-4,6-DMDBT 6H-4,6-DMDBT HY-4,6-DMDBT

-34.1 -36.6 -43.1 -48.4

2.548 (2.464) 2.510 (2.484) 2.451 (2.460) 2.517 (2.465)

S-Mo bond order

S atom charge

Mo atom charge

S-molecule charge

Mo10S18 cluster charge

-0.421 (-0.196) -0.315 (-0.196) -0.300 (-0.196) -0.290 (-0.196)

-0.082 (-0.001) 0.234 (0.000) 0.044 (0.0013) 0.460 (0.002)

0.085 (0.001) 0.233 (0.001) -0.043 (0.001) -0.461 (0.001)

Perpendicular Adsorption 0.5744 0.596 (0.594) -0.284 (-0.196) 0.6021 0.608 (0.568) -0.277 (-0.196) 0.7204 0.425(0.402) -0.267 (-0.196) 0.6261 0.316 (0.260) -0.271 (-0.196)

0.375 (-0.001) 0.366 (0.000) 0.385 (0.001) 0.441 (0.002)

-0.372 (0.001) -0.367 (0.001) -0.383 (0.001) -0.437 (0.001)

Flat Adsorption -0.711 (0.594) 0.614 (0.568) 0.568 (0.402) 0.327 (0.260)

4,6-DMDBT: 4,6-dimethyldibenzothiophene; 4H-4,6-DMDBT: tetrahydro-4,6-dimethyldibenzothiophene;6H-4,6-DMDBT: hexahydro4,6-dimethyldibenzothiophene; HY-4,6-DMDBT: completely hydrogenated 4,6-DMDBT.

4-MDBT, and 4,6-DMDBT over a CoMo/Al2O3 catalyst, they found that the conversion rates of partially saturated dibenzothiophenes into cyclohexylbenzenes via S-C bond cleavage were nearly the same, while the hydrogenolysis product, biphenyl from parent sulfur molecules, decreased in the order DBT > 4-MDBT > 4,6-DMDBT. They concluded that, when an aromatic ring in a dibenzothiophene was hydrogenated, the extent of steric hindrance by the methyl group was reduced and the associated differences between the HDS rates disappeared. Using Langmuir-Hinshewood rate equations, they also estimated the adsorption energies of those sulfur molecules were 12, 20, and 21 kcal/mol for DBT, 4-MDBT, and 4,6-dimethydibenzothiophene, respectively. It is worth noting that total adsorption energy was calculated without distinguishing the flat from the perpendicular adsorption energy. Nevertheless, the order of the adsorption energy values in their study agreed with the perpendicular adsorption energies obtained in this study (Table 3); however, greater differences between the two energies were observed in their study. To obtain some insight into how the electron density is distributed after adsorption, the volumetric plots of the total density of different hydrogenated intermediates adsorbed perpendicularly on the Mo10S18 cluster are considered in Figure 9. Compared with the total density plot of 4,6-DMDBT in Figure 6c, it is clear that hydrogenation improves the bonding of the sulfur atom with the middle molybdenum atom of the catalyst cluster. However, the hexahydro-4,6-dimethyldibenzothiophene has the highest degree of electron density overlap. To confirm this observation, the Mayer bond orders of the 4,6-dimethyldibenzothiophene derivatives were calculated using the geometry optimized structure, and the numbers are presented in Table 6. The results showed that the bond order increased when 4,6-DMDBT was gradually saturated, and the bond order value reached a maximum for hexahydro-4,6-dimethyldibenzothiophene, then decreased for the completely hydrogenated 4,6-dimethyldibenzothiophene. The S-Mo bond order of completely hydrogenated 4,6-dimethyldibenzothiophene was weaker than that of hexahydro-4,6dimethyldibenzothiophene, and may be used to explain why cyclohexylbenzenes and not bicyclohexanes are the most detected products. (34) Kabe, T.; Ishihara, A.; Zhang, Q. Appl. Catal. A 1993, 97, L1L9.

Figure 9. Volumetric plots of total density distribution of different hydrogenated intermediates of 4,6-dimethyldibenzothiophenes. The symbols for different atoms are the same as in Figure 3.

Figure 10 gives the final stable adsorption configurations of 4,6-DMDBT and its hydrogenated derivatives, which were placed initially in the flat adsorption mode. As in the case of dibenzothiophenes, the number of interaction points decreased gradually as 4,6-DMDBT



Figure 10. Relaxed structures of 4,6-dimethydibenzothiophene and its hydrogenated intermediates flat-adsorbed on a Mo10S18 nanocluster. The symbols for different atoms are the same as in Figure 3.

was hydrogenated. Almost identical final adsorption configurations were obtained for the completely hydrogenated 4,6-dimethyldibenzothiopene, regardless of the flat or perpendicular initial adsorption configuration, as shown in Figure 10, parts d and e. Consequently, we concluded that the perpendicular adsorption configuration is energetically most favorable for fully hydrogenated 4,6-DMDBT. Also, the most favorable adsorption configuration gradually shifts from flat to perpendicular as 4,6-DMDBT’s hydrogenation degree increases. This is supported by the adsorption energies given in Table 6. There is only 2.8 kcal/mol difference between the flat and perpendicular configurations for the fully hydrogenated 4,6-dimethyldibenzothiophene, indicating that these two adsorption configurations may be in quasiequilibrium. The results generated in this work suggest that the HDS reaction mechanism of Figure 1 is probably applicable to all simple derivatives of dibenzothiophene, with the exception of 4,6-DMDBT. For 4,6-DMDBT, a modified mechanism is probably more appropriate where the direct sulfur removal by hydrogenolysis (ks1) could be neglected by assuming that the methyl groups in 4 and 6 positions completely block the interaction between the sulfur atom and molybdenum atoms in perpendicular adsorption. Thus, 4,6-DMDBT is initially adsorbed in the flat configuration on the catalyst surface where it is hydrogenated to tetrahydro, or hexahydro, or fully saturated 4,6-dimethyldibenzothiophenes (kHYD1, kHYD2, and kHYD3). The sulfur atom could then be removed by cleavage of S-CR bonds ((ks2, ks3, and ks4). It is not clear in which adsorption configuration (perpendicular or flat) the S-CR bond cleavage of the partially hydrogenated dibenzothiophenes happens. How-

ever, for the fully saturated 4,6-DMDBT, the S-CR bond cleavage probably occurs in the perpendicular position. This proposed mechanism disagrees with the interpretations of some published experimental work.3,8,34-36 The results showed that, although at relatively much lower concentration than biphenyls from dibenzothiophene, dimethyl-substituted biphenyls were detected in the products of 4,6-dimethyldibenzothiophene. This would suggest that direct S-removal from 4,6-DMDBT is a possible pathway. However, it is also possible that dimethyl-substituted biphenyls also could be produced through dehydrogenation of corresponding cyclohexylbenzene. Since this study shows that flat adsorption is more energetically favorable over perpendicular adsorption and that similar flat adsorption energies are obtained over all the sulfur molecules studied, one could raise the question why there exists such a large difference in HDS reactivity between DBT and 4,6-DMDBT? Two possible solutions to this question are given here. First, there may be far fewer flat adsorption sites than perpendicular adsorption sites on the catalysts. According to the “Rim-Edge” model proposed by Daage and Chianelliswhere the catalyst particle was described as a stack of several MoS2 layers with rim sites consisting of the top and bottom layers, and edge sites consisting of the layers sandwiched betweensthe hydrogenation occurred exclusively on the rim sites and the hydrogenolysis was obtained on both the rim and the edges sites.37 The ratio of rim sites to the total catalytic sites (35) Vanrysselberghe, V.; Le Gall, R.; Froment, G. F. Ind. Eng. Chem. Res. 1998, 37, 1235-1242. (36) Michaud, P.; Lemberton, J. L.; Perot, G. Appl. Catal. A 1998, 169, 343-353.

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equaled 2/n (where n is the number of total layers). We could conclude that the actual catalysts have many MoS2 layers, which eventually decreased the proportion of the hydrogenation sites. Second, competitive adsorption of other fuel components, such as aromatics and nitrogen-containing compounds on the flat adsorption sites, occur during HDS reaction. It is well-known that aromatic compounds, especially multi-ring aromatics, adsorbed competitively with dibenzothiophenes on the hydrogenation sites in the flat adsorption mode. The competitive effects may become overwhelming due to the presence of aromatics that are several hundreds to several thousands times more concentrated than the sulfur-containing compounds in the real feed. All these factors may force the HDS of dibenzothiophenes to pass through mainly the perpendicular adsorption pathway, which results in a very low reactivity for 4- and 4,6substituted dibenzothiophenes due to steric hindrance. Conclusions The structure and hydrodesulfurization reactivity relationship of dibenzothiophene, a series of methylsubstituted dibenzothiophenes, and their hydrogenated derivatives were examined in this study. The optimized structures of flat and perpendicular adsorption configurations of the sulfur molecules on a Mo10S18 nanocluster were obtained by minimizing the energy of the sulfurmolecules-Mo10S18 complex. The adsorption energy was calculated using self-consistent density functional theory with generalized-gradient approximation. The results indicated that flat adsorption was energetically favorable over perpendicular adsorption, due to the multipoint interactions of the sulfur atom, thiophene, and aromatic rings with the molybdenum atoms on the catalyst surface. With the progressive hydrogenation of the sulfur molecule, the adsorption energy was gradually decreased in the flat adsorption mode, and increased in the perpendicular mode. For the completely hydrogenated 4,6-dimethydibenzothiophene molecule, the difference between the flat and perpendicular adsorption energies was only 2.8 kcal/mol. Methyl groups substituted at 4 and 6 positions did not inhibit binding of sulfur molecules on the catalyst surface in a flat adsorption configuration. However, those methyl groups prevented the bonding of the sulfur atom with the surface molybdenum atom in the perpendicular (37) Daage, M.; Chianelli, R. R. J. Catal. 1994, 149, 414-427.

Yang et al.

adsorption configuration, as deduced from the total electron distribution and the Mayer bond order of the S-Mo bond. The planar aromatic ring system in the 4,6methyl substituted dibenzothiophenes was most severely puckered by hydrogenation of one or two aromatic rings, resulting in a reduction of steric hindrance and an easier access for the sulfur atom to the catalyst surface through perpendicular binding. The atomic electron charge distributions by Mulliken population analysis were calculated for both flat and perpendicular adsorption configurations. In flat configuration, there was electron density transfer from the sulfur atom of the sulfur-containing molecule to the molybdenum atom of Mo10S18 nanocluster. For the sulfur-molecule-Mo10S18 complex, the net charge transfer was from the Mo10S18 nanocluster to the sulfur molecule, confirming the multipoint interactions of sulfur molecules with the catalyst surface, and the backdonation of electron from the Mo10S18 nanocluster. In perpendicular adsorption configuration, there was a significant electron density transfer from the sulfur atoms of all the sulfur molecules to the molybdenum atoms of the Mo10S18 clusters, except for 4,6-dimethyldibenzothiophene for which there was almost no electron density transfer due the steric hindrance effect. The elongation of SCR bonds was observed for all the absorbed sulfur molecules in both flat and perpendicular adsorption modes, indicating weakening of SCR bonds after adsorption. In the flat adsorption, the average bond distances of the benzene rings were also increased, which indicated that the aromatic characters of these sulfur molecules was reduced after flat adsorption. The results obtained from this study suggested that direct sulfur removal by hydrogenolysis is unlikely over most refractory sulfur molecules such as 4,6-dimethyldibenzothiophene. 4,6-Dimethyldibenzothiophene is first adsorbed in a flat configuration mode on the catalyst surface where it is hydrogenated to tetrahydro or hexahydro, or fully saturated 4,6-dimethyldibenzothiophenes. The sulfur atom could then be removed by cleavage of S-CR bonds of these hydrogenated intermediates. Acknowledgment. Partial funding for NCUT has been provided by the Canadian Program for Energy Research and Development, the Alberta Research Council, and The Alberta Energy Research Institute. EF020171K

Adsorption of Dibenzothiophene Derivatives over a MoS2

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