Advances in deep desulfurization

Science and Technology in Catalysis 1998 Copyright © 1999 by Kodansha Ltd.

Advances in Deep Desulfurization Henrik TOPS0E\ Kim G. K N U D S E N \ Line S. BYSKOV^ Jens K. NORSKOV^ and Bjeme S. CLAUSEN^ ^Haider Tops0e Research Laboratories, DK-2800 Lyngby, Denmark ^ Center for Atomic scale Materials Physics, Technical University of Denmark, DK-2800 Lyngby, Denmark Abstract Hydrotreating and deep hydrodesulfurization (HDS) have attracted increased attention recently due to the introduction of new legislation regarding fuel specifications. In order to meet these challenges, there is a need to modify and improve existing reactors and processes and to introduce more active and selective catalysts. The removal of the so-called sterically hindered sulfur-containing molecules, like 4,6 dimethyldibenzothiophene, is observed to be a key issue for deep HDS. Also the choice of reactor internals plays an important role for deep HDS. In order to guide rational catalyst developments, structure-activity relationships are desired and the Co-Mo-S model and the Bond Energy Model have been useful for describing many activity parameters for promoted catalysts and transition metal sulfides. It has also recently been shown that important new insight may be gained from density functional theory (DFT) calculations. The present article will focus on some of the current practical and theoretical issues. 1. INTRODUCTION AND PRACTICAL ISSUES Hydrodesulfurization (HDS) of oil fractions remains one of the most important refining processes [1]. The recent focus on HDS is related to the worldwide environmental pressures to continuously reduce the sulfur content in oil products. In this connection it could be pointed out that the EU parliament and commission in mid 1998 have decided that the maximum amount of sulfur in diesel shall be reduced significantly over the next few years (Table 1). The required reduction in sulfur will have large consequences for the refineries and significant resources will have to be devoted to Table 1 Future maximum sulfur contents in diesel decided by EU parliament and commission in 1998

Year

Maximum sulfur content wppm

2000 2005

350 50

13

14 H. Topsm et al.

Table 2 Required increases in catalyst activity (or bed temperature) to achieve different reductions in the sulfur content of the diesel products. 500 ppm is chosen as the base case and LHSV is kept constant. Product Sulfur wppm

Required catalyst activity %

Required increase in temperature °C

100 130 200 325 525

0 +7 +19 +33 +47

500 350 200 100 50

improve the processes and to introduce new and more active catalysts. The results in Table 2 illustrate that very large improvements in catalyst activities are required in order to achieve the required reductions in the sulfur content in the product. The increase in activity required to reach the year 2000 specifications is about 30% and this can be achieved by the newest generation of HDS catalysts such as TK-574. However, it is seen that about 5 times more active catalysts are required to reduce the sulfur content from 500 to 50 ppm. This applies to the situation where other process parameters are kept constant. Alternatively, the reduction in sulfur content can be achieved as illustrated in Table 2 by increasing the catalyst bed temperature. However, this will result in substantial and often unacceptable reductions in the cycle length. Removal of H2S (an inhibitor of the HDS reaction) from the recycle gas and increasing the hydrogen partial pressure are other options that may be introduced. Under all circumstances, hydrogen consumption will be increased and hydrogen availability is going to be one of the key issues that must be deah with. In order to produce products with lower and lower sulfur contents, it is clear that a good liquid flow distribution in the reactors will become of increasing importance [2,3]. For example, with just 1% bypass of the feed around a catalyst bed, it may in most cases be impossible to reach the 50 ppm level required by year 2005. Thus, besides focusing on developing more active catalysts, we have also devoted significant efforts to the development of new liquid distribution systems. Even at the quite low degrees of desulfurization employed today, it can often be advantageous to introduce new liquid distribution systems. Table 3 shows an industrial example [2,3] of the benefit gained by

Table 3 Illustration of the importance of good liquid distribution for achieving deep HDS. According to [2,3].

LHSV Ave. Bed Temp, °C Feed sulfur, wt% Product sulfur, wt% Equiv. rel. HDS activity

With chimney distribution tray

With new generation distribution tray

1 346 0.7 0.05 1

1 321 0.9 0.035 2.5

15

1 DBT 2 4-MDBT 3 4,6-DMDBT

2

3

1li lUiL 1

1 61%HDS

...JjuL^^

0 6 % HDS

* * - - - _

L^..

2

Figure 1. Sulfur compound distribution determined by GC-SCD for HGO HDS products at two conversion levels. replacing a chimney type distributor (installed in 1995) with a Topsoe Dense Pattern Flexible Distribution Tray (installed in 1996). The improvements correspond to the installment of a IV2 times more active catalyst. The operation parameters were chosen such as to double the cycle length while at the same time allowing the use of a slightly dirtier feed. For recent discussions of many of the above and other technical issues see [1-10]. In order to implement solutions for deep HDS, it is very important to take the feed complexities into account [1,8,9, 11-13]. For example, typical feedstocks contain a large variety of different sulfur compounds and these may have large variations in reactivity. Thus, the type and relative concentration of sulfur compounds in the product will depend very much on the conversion level (Fig. 1). Typical sulfur compounds with low reactivity are methyl-substituted dibenzothiophenes with the methyl groups substituted in the aromatic rings close to the sulfur. A typical example of a relevant refractory sulfur compound is 4,6 dimethyldibenzothiophene (4,6 DMDBT). The disappearance of individual sulfiir compounds has recently been studied [14] as function of 80 r

h

70 [

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• 4^MDBT D 4,6-DMBT

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200

400

600

1 800

^ 1000

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^ 1200

1400

1600

1800

Total sulfur (ppm)

Figure 2. The concentration of individual sulfur compounds versus the total concentration of sulfur in the desulfurized oil.

16 H. Tops0c et al.

Ho y

-HoS

bV

OK} Figure 3. Reaction network for HDS of dibenzothiophene. the degree of desulfurization. The results (Fig. 2) show that down to about 800 ppm sulfur in the product, only few of the 4,6 DMDBT molecules have been reacted. Thus, in order to optimize catalysts and processes for low severity operation, it is sufficient to focus on the removal of the most reactive molecules (e.g. benzothiophenes and non-sterically hindered dibenzothiophenes). In the conversion range 85-95%, it may be more appropriate to use a molecule like 4-MDBT. However, under conditions of deep HDS (more than 95% conversion), most of the reactive compounds and even molecules like 4-MDBT have been removed and it is necessary to focus on the removal of the most refractory compounds and the results show that a molecule like 4,6 DMDBT appears to be a relevant probe molecule. DBT and substituted DBT's are desulfurized by two parallel pathways (Fig. 3) [1]. One involves direct hydrogenolysis (cleavage of C-S bonds) and the other one involves prehydrogenation of an aromatic ring. For HDS of DBT the hydrogenolysis pathway is dominating while for 4,6-DMDBT prehydrogenation is the most important route. Thus, for deep HDS it may be interesting to investigate catalysts with a higher hydrogenation activity. Sulfided Ni-Mo catalysts are typically better hydrogenation catalysts than sulfided Co-Mo catalysts. In accordance with this, model compound experiments (Table 4) have shown that a Co-Mo catalyst was better than a Ni-Mo catalyst for HDS of DBT while a Ni-Mo catalyst was found to be most active for HDS of 4,6 DMDBT. Under real feed conditions, the situation is complicated by the fact that besides the HDS reactions themselves, a number of other reactions may also occur and competitive adsorption and poisoning phenomena should also be taken into account. 2. STRUCTURE-ACTIVITY CORRELATIONS AND THEORETICAL STUDIES In order to aid the development of new catalysts for HDS and deep HDS, strong efforts have been Table 4 Relative pseudo-first-order rate constants for HDS of different dibenzothiophenes over C0-M0/AI2O3 and Ni-Mo-P catalysts [11,15]. Catalyst

DBT

4-MDBT

4,6-DMDBT

Co-Mo

Too

Too

Too

Ni-Mo-P

76

101

161

17

o o (0 E Q O

Number of Co edge atoms (x1020/g catalyst)

Figure 4. Structure-activity relationship established between the HDS activity and the number of Co edge atoms in the Co-Mo-S structures (see e.g. Ref. [1]). devoted to establishing fundamental relationships between the structure of the catalyst and the reactivity towards different molecules [1]. For unpromoted Mo catalysts, the activity has been related to sites at the M0S2 edges, whereas for the Co and Ni promoted catalysts the presence of promoter edge atoms in the Co-Mo-S and Ni-Mo-S structures plays a key role. For many catalyst systems, direct relationships have been established between the catalytic activity and the number of these edge atoms (Fig. 4) and these correlations have been very useful in catalyst developments. In spite of the significant progress, many fundamental questions regarding the details of the catalytic cycle and the promoter action are still being debated [1,9,16]. It is generally believed that fully coordinated sulfided catalysts will be unable to adsorb sulfurcontaining molecules, and that sulfur vacancies must be created to provide catalytically active sites. The number of sulfur vacancies may thus be regarded as a key measure of the catalytic activity. Clearly, other features will be important but we will in the following show that many HDS activity results for both transition metal sulfides and Co-Mo-S type structures can be explained considering the variations in metal-sulfur bond strength (or the tendency to form vacancies). The Bond Energy Model (BEM) [17,18] was based on such considerations. As a starting point, one can assume that the active sites for HDS are related to sulfur vacancies, and their concentration will be given by an equilibrium-like H2(g) + S*»H2S(g) + =

(1)

Here, S* denotes the "adsorbed" S-atom on the vacancy. The number of active sites, 9*, are then given by ( e * « l ) e* = A exp (AHs/kT)

(2)

where AHs is the enthalpy for reaction (1). The reaction rate for the HDS can then be written as r = r,se*

(3)

where rris is the rate of the rate limiting step. Thus, variations in the rate from one system to the other will depend on variations in AHs although variations in r^s will clearly also be important. The relevant value of AHs relates to the vacancy formation energy at the surface of the sulfide in question. Such

18 H.Tops0e^ra/.

values are generally not available, but for analyzing the activity trends, it may be relevant to use the binding energy (or heat of formation) for the bulk sulfides [17,18]. In order to relate the binding energy to the removal of one sulfur atom (i.e. the analogous situations of Equation (1)) the binding energies must be normalized per sulfur atom. Such calculations have been carried out [17,18] using the surface area normalized activity data from the literature [19]. The results (Fig. 5a) are in agreement with Equations (2) and (3) and they confirm that the catalysts with the lowest heat of formation are the most active. Recently, Toulhoat et al. [20] found a volcano-type correlation when the activities were plotted versus the cohesive energy divided by the number of bonds in the unit cell. They used the weight average activity data of [19]. If one, however, uses the activities normalized per surface area (which may seem more reasonable), no clear correlation is observed (Fig. 5b). Thus, the Bond Energy Model which suggests that the activity increases with decreasing metal-sulfur bond strength, AHs appears to capture the activity variations best. It was also found that the BEM model could explain the origin of the large activity increase observed in promoted catalysts [17,18]. Furthermore, this model has the advantage that it is based on a simple physical picture. In the above case, the bulk parameter, AHs, is used as a measure of the metal-sulfur bond strength and it should be stressed that one cannot expect to obtain perfect correlations for surface reactions using bulk properties. Thus, it is highly desirable to obtain a more realistic and precise description of the structure and energetics of the catalyst surface. Recent DFT calculations [21-24] have provided such insight and they will be discussed below. 2.1. DFT-GGA calculations for M0S2, Co-Mo-S, Ni-Mo-S and Fe-Mo-S Before the DFT-GGA calculations on the catalyst systems were carried out [21-24], it was verified that the methods were able to give excellent structural data for such systems as Co-metal, Mo-metal and M0S2. The model, used in the calculations of the properties of M0S2 based systems, consists of M0S2 chains, two Mo atoms in cross section. These chains have the advantage that they expose both E

•OS

^Rh

a

^Re ^ p d

^

I

.0 1

0.0

.-n

1

20.0

1

40.0

60.0

80.0

AHs (kcal/mol of S-atom) ^

10.0 p .Rh •Os •^••Ru

F

b

^Re

.Pd • Pt LMH

• Ni •Co

k

• Mo

•Cr 1

1

40.0

50.0

60.0

Xg (kcal/mol "bond")

Figure 5. HDS activity correlations for transition metal sulfides. The activities (per surface area) from Ref [19] have been used, (a) activities versus AHs, the heat formation per sulfur atom; (b) Activity versus Xs, the cohesive energy normalized to the number of bonds in the unit cell. Values of Xs are taken from Ref [20]. According to [22,23].

19

the Mo terminated edge (termed "Mo-edge") and the S terminated edge (termed "S-edge"). In all the calculations, relaxations are allowed in order to find the minimum energy configuration. This is important since relaxations and surface reconstructions are observed to take place. Figure 6 illustrates this point. This figure shows the equilibrium structures after different amounts of sulftir atoms have been removed from the S-edge of M0S2. It is seen that even for the fully covered edge (Fig. 6a), the sulfiir atoms do not occupy the normal lattice sites expected from the bulk crystal structure of M0S2. Specifically, it is seen that there is a tendency for the sulfur atoms to dimerize. Raman spectroscopy investigations [25] have in fact given evidence for the existence of such disulfide species in alumina supported Mo catalysts. The DFT calculations show that also after removal of sulftir atoms, additional new "atypical" lattice sites are observed. From a catalysis point of view, this new insight has several interesting consequences. First of all, the resuhs suggest that one must reexamine the many previous models based on static vacancy structures derived from removing atoms from the bulk M0S2 structure (for a review of previous models, see e.g. [1]). Furthermore, the results indicate that quite large surface reconstructions/relaxations must take place during the catalytic cycle upon addition and removal of sulftir. This dynamical behavior is also in accordance with experimental work on M0/AI2O3 and C0-M0/AI2O3 catalysts [1] and in this connection, it is interesting that metal sulftir clusters also appear to contain such "latent vacancies" and undergo reconstructions during reactions [26]. The DFT calculations have also given important new insight into the structure of Co-Mo-S. It was found [21] that the most favored positions of the Co promoter atoms are at the S edges as illustrated in Fig. 7a. This is quite different from the structure of Co-Mo-S assumed in most recent studies (see [1] and Fig. 7b). Nevertheless, the calculated structure is in accord with EXAFS results. Besides explaining the structural features, the DFT calculations can also be used to understand the origin of promotion in Co-Mo, Ni-Mo and Fe-Mo catalysts. For this purpose, we have calculated the enthalpies (AHs) for removal of sulfiar (i.e. according to Equation (1)) from structures at the S edges, which initially have the Mo and promoter atoms, surrounded by six S atoms. The values are listed in Table 5. It is seen that it is quite difficult to remove sulfur from M0S2 and Fe-Mo-S, whereas it is favored to remove one sulftir from the Co-Mo-S and Ni-Mo-S structures. Thus, the most stable Co-Mo-S structure will under most reaction conditions have the promoter atoms surrounded by 5 sulfur atoms.

Fig. 6. Illustration of M0S2 structures. Side view seen from the S-edge side. The white circles denote S atoms, and the light grey ones are Mo atoms, (a) The dimerized S-edge. (b) One S atoms per super cell is removed from structure (a), (c) The remaining S-dimer atom is removed from structure(b). (d) One S atoms is removed from the S dimer in structure (b). According to [21].

20 H. Tops0e et al.

Top view

Side view

•D*OV

Fig. 7 Configurations of the active Co-Mo-S structure, (a) Structure derived from DFT-GGA calculations (4); (b) Model favored in previous structural characterization studies (see e.g. [1]); White circles denote S atoms; dotted circles denote S atoms in bottom layer; dashed circles denote Mo atoms; and dark grey circles denote Co atoms. Adapted from [22]. as shown in Fig. 7a. Figure 8 shows that there is a nice correlation between the degree of promotion (data from [1]) and AHs again pointing to the importance of the metal sulfur binding energy. It could be pointed out that the DFT calculations are in good general agreement with the Bond Energy Model [17,18] based on bulk properties of the sulfides [17,18].In addition, the DFT calculations have provided new insight which directly relates to the processes occurring at the relevant surfaces. The above types of calculations have not yet been done for all the situations relevant for deep HDS. However, the DFT calculations have shown that the theoretical tools have today progressed to a stage where one can address very realistic situations. Thus, theory has become an important additional tool for developing catalysts in a more rational manner. Table 5 Calculated sulfur binding energies for unpromoted and promoted sulfided Mo-structures corresponding to removal of the first S atom from the "S-edge". According to [22,24].

Structure

AHs (kJ/mole)

M0S2

1 -81 -82 -17

Co-Mo-S Ni-Mo-S Fe-Mo-S

21 1

100.0

80.0

^^ o F

J

60.0

/

40.0

•

(0

m <1

20.0

1

1

(a)

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/ // Mo

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-onn

Fe

•

•

Co

Ni

^

,(b) ^ JZ X

50.0

i f

o E o E

30.0

10.0

^

Fe

Mo

Co

Ni

Fig. 8 Effect of different transition metal atoms on the sulfur binding energy, AHs. (a) The degree of promotion for thiophene HDS (b). According to [22,24]. Acknowledgments The present work was in part financed by the Danish Research Councils through the Centre for Surface Reactivity and grant #9501775. The Centre for Atomic-scale Materials Physics is sponsored by the Danish National Research Foundation. 2.2. References [I] H. Tops0e, F.E. Masoth, and B.S. Clausen, "Hydrotreating Catalysis", Vol 11 in CatalysisScience and Technology, J.R. Anderson and M. Boudart (eds.). Springer-Verlag, 1996. [2] D.L. Yeary, J. Wrisberg and B.M. Moyse, NPRA Annual Meeting, Paper AM-97-14, 1997. [3] F.E. Bingham, M. Muller , P. Christensen and B.M. Moye, Paper presented at the European Refining Technology Conference, London, Nov. 17-19, 1997. [4] P.N. Hannerup and B.H. Cooper, Oil and Gas European Magazine (1995) 25. [5] J.W.M. Sonnemans, in "Catalyst in Petroleum Refining and Petrochemical Industries 1995",6 M. Absi-Halabi et al., (eds.), Elsevier Science B.V, 1996, p. 99. [6] T. Takatsuka, Y. Wada, H. Zuzuki, and S. Komatsu, NPRA Annual Meeting, Paper AM-91-39, 1991. [7] J.W. Gosselink, in "Transition Metal Sulphides", T. Weber et al. (eds.), Kluwer Academic Publishers, 1998,p.311. [8] I. Mochida, K. Sakanishi, X, Ma, S. Nagao, and T. Isoda, Catal. Today 29 (1996) 185. [9] D.D. Whitehurst, I. Isoda, and I. Mochida, Advances in Catalysis 42 (1998) 343. [10] M.V. Landau, Catal. Today 36 (1997) 393. [II] B.C. Gates and H. Tops0e, Polyhedron 16 (1997) 3213.

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[12] A. Amorelli, Y.D. Amos, C.P. Halsig, J.J. Kosman, R.R.J. Jonke, M. DeWind, and J. Vrieling, Hydrocarbon Process. 71 (1992) 93. [13] T. Kabe, A. Ishihara, and H. Tajima, Ind. Eng. Chem. Res. 31 (1992) 1577. [14] K.G. Knudsen, C.V. Ovesen, I.V. Nielsen, and K. Andersen, paper presented at the JECAT '97 conference, Nov. 25-28, 1997, Tsukuba, Japan [15] C.V. Ovesen, M. Brorson, I.V. Nielsen, K. Andersen, and H. Topsoe, unpublished results. [16] E. Hensen and R.A. van Stanten, CATTECH, June 1998, p. 86. [17] J.K. N0rskov, B.S. Clausen, and H. Topsoe, Catal. Lett. 13 (1992) 1. [18] H. Tops0e, B.S. Clausen, N.Y. Topsoe, J.K. N0rskov, C.V. Ovesen, and C.J.H. Jacobsen, Bull. Soc. Chim. Belg. 47 (1995)283. [19] T.A. Pecoraro and R.R. Chianelli, J. Catal. 67 (1981) 430. [20] H. Toulhoat, P. Raybaud, S. Kasztelan .G. Kresse, and J. Hafner, PREPRINTS, Div. of Petrol. Chem. ACS 42(1) (1997 114. [21] L.S. Byskov, B. Hammer, J.K. Norskov, B.S. Clausen, and H. Tops0e, Catal. Lett. 47 (1997) 177. [22] L.S. Byskov, J.K. Norskov, B.S. Clausen, and H. Topsoe, Div. of Petrol. Chem., ACS 43(1) (1998) 12. [23] L.S. Byskov, J.K. Norskov, B.S. Clausen, and H. Topsoe, in Transition Metal Sulphides, T. Weber et al. (eds)., 1998 Kluwer Academic Publishers, The Netherlands, p. 155. [24] L.S. Byskov, J.K. Norskov, B.S. Clausen, and H. Topsoe, submitted for publication. [25] J. Polz, H. Zeilinger, B. Muller, and H. Knozinger, J. Catal. 120 (1989) 22. [26] M.D. Curtis and S.H. Drucker, J. Am. Chem. Soc. 119 (1997) 1027.