RELATIONSHIP BETWEEN CATALYST STRUCTURE AND HDS REACTION MECHANISM

Narinobu Kagami1, Bas M. Vogelaar, A. Dick van Langeveld2,

and Jacob A. Moulijn*

Reactor & Catalysis Engineering Group, Faculty of Applied Sciences, Delft University of Technology, Julianalaan 136, 2628 BL Delft,

The Netherlands 1Manufacturing Deparment, Idemitsu Kosan Co., Ltd.

26,Anesakikaigan Ichihara-city, Chiba, 299-0107, Japan 2 Particle Optics Group, Faculty of Applied Sciences, Delft

University of Technology, Lorentzweg 1, 2628CJ Delft, The Netherlands

Introduction

The hydrodesulfurization (HDS) process is receiving more and more attention, because the legislative requirements on transportation fuels are becoming more severe. Therefore, it is very important to elucidate the effect on the relationship between catalyst structure and reaction mechanism of the sulfur compounds.

CoMo/Al2O3 and NiMo/Al2O3 catalysts are the work-horses in HDS processes. The so-called type II CoMoS phase has a higher activity for thiophene HDS than the so-called type I. For catalyst design such a conclusion is of crucial importance. However, this has not been unequivocally proven to hold for dibenzothiophene (DBT) HDS1). It should be noted that DBT is a better model compound than thiophene to represent diesel fuel liquid phase HDS.

It is widely accepted that DBT desulfurizes via both direct desulfurization (DDS) and hydrogenation followed by desulfurization (HG-HDS) of benzene ring. This work focuses on the structure type I and type II catalyst structure and the possible relationship with the different routes of DBT HDS. Experimental Catalysts

NiO-MoO3/Al2O3 catalysts (type I and type II) were prepared via liquid phase pore volume impregnation. A high purity γ-Al2O3 in the form of 1.5 mm cylindrical extrudates was impregnated using aqueous solutions containing the required amounts of Ni and Mo, according to the literature2).

Activity measurement

The activity for liquid-phase DBT HDS was determined in a batch reactor. Details on the experimental procedure can be found in the literature1). Before the reaction catalysts, they were presulfided at 370 °C. Subsequently, the activity tests were carried out at 350 °C and 5 MPa hydrogen pressure. The feed consisted of 0.2wt% DBT in hexadecane (n-C16).

The inhibiting effect of H2S is quite large on DBT HDS3). In order to mimic the realistic conditions of the commercial reactor bottom, dimethyldisulfide (DMDS) was added to adjust the H2S partial pressure to 1.4bar.

Results and Discussion

In Fig. 1 a simplified reaction scheme for DBT HDS is shown. Under the applied reaction conditions the main products are cyclohexylbenzene (CHB) and biphenyl (BP). From the yields of

these two products between the direct HDS (DDS) and the hydrogenation route can be discriminated.

In all cases, the time dependency of the DBT concentration followed first order kinetics.

k(HG)

Dibenzothiophene

k(DDS)

k(HG)

Dibenzothiophene (DBT)S

Cyclohexylbenzene (CHB)

Biphenyl (BP)

k(HDS)

STetrahydroDBT (THDBT)

k(HG)

Dibenzothiophene

k(DDS)

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Dibenzothiophene (DBT)SS

Cyclohexylbenzene (CHB)

Biphenyl (BP)

k(HDS)

SSTetrahydroDBT (THDBT)

Figure 1. Simplified reaction scheme for DBT HDS In Fig. 2, the effect of active metals loading on the DBT HDS

activity for both NiMo type I and type II catalysts is shown. At increasing loading, type I catalyst did not show higher activity, probably due to formation of MoO3 crystals, in agreement with literature. Diaz and Bussell reported that at a metal loading above 4.2 atoms Mo per nm2, three-dimensional MoO3 particles are formed4). In contrast, type II catalyst showed higher activity at increasing loading.

* Corresponding author, email: J.A.Moulijn@tnw.tudelft.nl

Figure 2. The effect of metal loading for DBT HDS

Figure 3. The effect of metal loading for hydrogenation selectivity

TypeII

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Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem. 2003, 48(2), 601

In Fig.3, the selectivity of cyclohexylbenzene, representative for the hydrogenated route is shown. Note that type II catalysts have a higher selectivity for cyclohexylbenzene. Therefore, it can be concluded that the reaction rate of hydrogenated route on type II catalyst is higher than on type I. Moreover, the higher loading catalysts show higher selectivity for cyclohexylbenzene.

Table 1. The Rate Constants of Each Reaction Step

Figure 4. The yield of products HDS DBT on type II NiMo catalysts

In Fig.4, the yields of the products of high and low loading type II catalysts are shown. Time yields of cyclohexylbenzene and biphenyl became higher at increasing loading, but the former increased most. Therefore, the higher HDS activity of type II high loading catalysts is mostly dependent on an acceleration of the hydrogenation route.

Figure 5. The yield of products HDS DBT over type I NiMo catalysts

In Fig.5, the yields of the products of high and low loading type

I catalysts are shown. Time yields of cyclohexylbenzene and biphenyl decrease with increasing loading, but the former becomes much lower. Therefore, it is concluded that due to the formation of MoO3 crystals, the direct desulfurization reaction rate decreases while hydrogenation route does not change significantly.

To unravel the reaction scheme described in Fig.1, the rate constant of each step was determined numerically by fitting the data to the kinetic model. The inverse reaction of THDBT to DBT, and the hydrogenation reaction of BP to CHB could be neglected because the rate constants were very low.

kDBT k(DDS) k(HG) k(HDS) k(HG)/k(DDS)

k(HDS)/k(DDS)

TypeI Low loading 3.6 2.2 1.4 14.0 0.65 6.5

TypeI High loading 2.5 1.3 1.2 10.9 0.91 8.4

TypeII Low loading 2.9 1.7 1.2 15.9 0.69 9.4

TypeII High loading 5.9 3.0 2.9 27.5 0.99 9.3

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DBT(low loading)BP yield(low loading)CHB yield(low loading)DBT(high loading)BP yield(high loading)CHB yield(high loading)

In Table 1, the rate constants of each reaction step are given. All

data show higher k (HDS) than k (DDS). Therefore, THDBT is supposed to desulfurize easier than DBT. Moreover, type II high loading had a higher k (HG)/ k (DDS) than type II low loading. Therefore, the higher HDS activity of type II high loading catalysts mainly originates from the acceleration of hydrogenation route activity.

These results can contribute to a more rational catalyst design in

hydrodesulfurization. Work in that direction is in progress. Conclusions

The various DBT HDS routes significantly depend on the catalyst structure (type I, type II). The higher HDS activity of the high loading type II catalysts mainly originates from the increased hydrogenation activity.

Acknowledgement. We thank Dr. S. Eijsbouts (Akzo-Nobel,P.O.Box 37650,1030 BE Amsterdam, The Netherlands) for the stimulating discussions. References

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(1) Reinhoudt, H.R.; Boons, C.H.; Van Langeveld, A.D.; Van Veen, J.A.R.; Sie, S.T.; and Moulijn, J.A., Appl. Catal. A. 2001, 207, 25.

(2) Van Veen, J.A.R.; Gerkema, E.; Van der Kraan, A.M.; and Knoester, A. J. Chem. Soc., Chem. Commun. 1987, 1684.

(3) Kabe, T.; Aoyama,Y.; Wang, D.; Ishihara, A.; Qian, W.; Hosoya, M.; and Zhang, Q., Appl. Catal. A. 2001, 209, 237.

(4) Diaz, A.L.; and Bussell, M.E.; J. Phys. Chem. 1993,97, 470.

Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem. 2003, 48(2), 602