Kinetic Analysis of the Hydrodesulfurization of Dibenzothiophene

Energy & Fuels 2006, 20, 1815-1821

1815

Kinetic Analysis of the Hydrodesulfurization of Dibenzothiophene: Approach Solution to the Reaction Network Hamdy Farag* Department of Material Process Engineering, Graduate School of Engineering, Kyushu UniVersity, Motooka 744, Fukuoka 819-0395, Japan ReceiVed May 18, 2006. ReVised Manuscript ReceiVed July 6, 2006

The hydrodesulfurization of dibenzothiophene over CoMo/Al2O3 and MoS2 catalysts in a batch reactor at 340 °C and 3 MPa of hydrogen pressure was investigated. Overall, dibenzothiophene undergoes two parallelsequential reactions: direct desulfurization leading to biphenyl and hydrogenation leading to the partially hydrogenated dibenzothiophene. The two intermediates are solely the sources to the formation of phenylcyclohexane. Improved kinetic analysis was performed in order to develop a more accurate expression to the contribution of the two routes in the hydrodesulfurization of dibenzothiophene. The apparent rate constants for direct desulfurization, k11, and hydrogenation, k22, pathways were estimated using a developed kinetic model where each intermediate is independently treated. The rates of the intermediate components (biphenyl and the partially hydrogenated dibenzothiophene) were found to be highly dependent on the subsequent sequential reactions. With this new model, it has been possible to assess accurately the magnitude of the extreme differences in the performance of the CoMo/Al2O3 and MoS2 catalysts. However, the model would as well be readily applicable to any hydrotreating catalysts. CoMo/Al2O3 catalyst showed high selectivity to the direct desulfurization pathway while the MoS2 catalyst exhibited considerable contribution from the hydrogenation pathway. The results show that the selectivity is more accurately expressed by the individual apparent rate constants rather than by the product distribution ratio.

Introduction Hydrodesulfurization (HDS) is a catalytic process that aims to reduce the sulfur content in the petroleum feedstocks to a lower level in order to meet the set of stringent environmental legislation recently adopted in developed countries.1,2 Sulfur in the petroleum feedstocks is represented by a variety of sulfur containing species ranging mostly from relatively simple compounds such as thiophenes to the most complicated polyaromatic sulfur compounds such as dibenzothiophenes. Each individual component has its own reactivity and selectivity while being catalytically treated. Therefore, there is no consensus yet on a global mechanism to identify such reactions. However, in the hydrotreatment process, there is an agreement that as the sulfur level decreases an accumulation of most of the refractory sulfur species occurs.3-9 These are typically the dibenzothiophene and its derivatives. The low reactivity of such species reflects the existence of using a special mechanism during the reaction. Numerous studies can readily be found in

the literature that exploit dibenzothiophene as a model compound for HDS.10-16 Dibenzothiophene (DBT) HDS produces mainly biphenyl, phenylcyclohexane, and the partially hydrogenated dibenzothiophene, as has been widely reported by many studies.3,5,7,9 Therefore, consideration of a network of sequential and parallel reactions is necessary. Evaluation of the magnitude of the catalytic activity and the catalyst’s selectivity depends on how this reaction network is kinetically treated. Although it is widely accepted that this reaction is pseudo-first-order in nature, there is an uncertainty on the assessment of how much contribution results from each individual intermediate species.9,16-18 The dilemma arises due to the large number of rate constants being included in the scheme; a broad variation in the estimation could be available that may lead to erroneously ascribing the reaction. Accordingly, it is quite difficult to compare the catalytic activity results in even typical studies in the literature. Indeed, the rate of this reaction is highly influenced by other partners’ components such as H2S which might reduce the

* Corresponding author. Tel.: +81-92-802-2738. Fax: +81-92-8022792. E-mail: [email protected]. (1) Directive of the European Parliament and of the Council, 11.05.2001 241 final, Brussels COM, 2001. (2) US EPA Clean Air Act Tier 2, 1999. (3) Babich, I. V.; Moulijn, J. A. Fuel 2003, 82, 607-631. (4) Brunet, S.; Mey, D.; Perot, G.; Bouchy, C.; Diehl, F. Appl. Catal., A 2005, 278 (2), 143-172. (5) Mochida, I.; Choi, K. J. Jpn. Petrol. Inst. 2004, 47 (3), 145-163. (6) Schulz, H.; Boehringer, W.; Waller, P.; Ousmanov, F. Catal. Today 1999, 49, 87-97. (7) Song, C.; Ma, X. Int. J. Green Energy 2004, 1 (2), 167-191. (8) Vasudevan, P. T.; Fierro, J. Catal. ReV. Sci. Eng. 1996, 38, 161188. (9) Whitehurst, D. D.; Isoda, T.; Mochida, I. AdV. Catal. 1998, 42, 345471.

(10) Bataille, F.; Lemberton, J.; Michaud, P.; Perot, G.; Vrinat, M.; Lemaire, M.; Schulz, E.; Breysse, M.; Kasztelank, S. J. Catal. 2000, 191, 409-422. (11) Wang, Y.; Sun, Z.; Wang, A.; Ruan, L.; Lu, M.; Ren, J.; Li, X.; Li, C.; Hu, Y.; Yao, P. Ind. Eng. Chem. Res. 2004, 43, 2324-2329. (12) Egorova, M.; Prins, R. J. Catal. 2004, 225, 417-427. (13) Vradman, L.; Landau, M. V.; Herskowitz, M. Catal. Today 1999, 48, 41-48. (14) Steiner, P.; Blekkan, E. Fuel Process. Technol. 2002, 79, 1-12. (15) Ho, T. C. Catal. Today 2004, 98, 3-18. (16) Kabe, T.; Ishihara, A.; Zhang, Q. Appl. Catal., A 1993, 97, L1L9. (17) Houalla, M.; Nag, N. K.; Sapre, A. V.; Broderick, D. H.; Gates, B. C. AIChE J. 1978, 24, 1015-1021. (18) Kim, H.; Lee, J. J.; Moon, S. H. Appl. Catal., B 2003, 44 (4), 287299.

10.1021/ef060225g CCC: $33.50 © 2006 American Chemical Society Published on Web 08/12/2006

1816 Energy & Fuels, Vol. 20, No. 5, 2006

reactivity.19-21 Studies that treat the kinetics of such reaction with the Langmuir-Hinshelwood model often use a considerable number of constants and determine them through leastsquares analysis.22-29 Although a given mathematical expression may provide a good fit of the experimental data, this does not necessarily imply that the presumed mechanism is indeed the most feasible. Within this flexibility, it is possible to obtain a number of reliable fits of data to a mathematical expression. The number of constants included in a model, therefore, affects its feasibility. A minimum number of adjustable parameters has to be set whenever possible. In this study, we attempt to kinetically analyze the reaction network of DBT HDS so that a more subtle approach of the apparent individual rate constants could be derived. We will only consider one case where no inhibitors exist in the reaction feed. Experimental Section Two catalysts were used in this study: the commercial CoMo/ Al2O3 catalyst (Mo and Co contents are 13.7 and 3.2 wt %, respectively) and the unsupported MoS2 catalyst (synthesized by the heat treatment of ammonium thiomolybdate at 600 °C while under a flow of 60 SCCM of 10 vol/vol % H2S/H2 gas mixture). The catalyst was first subjected to sulfidation reaction through passing a continuous flow of H2S/H2 (10 vol/vol %) at atmospheric pressure and temperature of 400 °C for 3 h. Thereafter, the catalyst was flushed by argon for 20 min while maintaining the temperature at 400 °C. The catalyst was then cooled to ambient temperature and directly moved to the autoclave reactor for the HDS reaction investigation. The HDS reaction was carried out as follows. About ∼0.1 g of catalyst was loaded into a 100 mL magnetically stirrer micro-autoclave reactor. The micro-autoclave was designed so that a track follow of the rate of the reaction through sample withdrawal with time intervals could be achieved. Then, 10 g of a decane solution of 1 wt % dibenzothiophene was added. A 0.4∼0.6 g portion of copper powder was also charged into the reactor aiming to strip away the effect of H2S produced during the HDS reaction. The reactor was purged several times with H2 before adjusting the reaction pressure at 3MPa H2. The HDS reaction experiments were carried out at 340 °C and 3 MPa of H2. The qualitative and quantitative estimation of the reaction yield were accomplished with the aid of gas chromatography-flame ionization detection (GC-FID) and gas chromatography-mass spectroscopy (GC-MS), Agilent HP 6890 and HP5970, respectively, using a methylsiloxane capillary column of HP (0.32 mm × 50 m). The results were evaluated in terms of relative concentration of the component in the reaction mixture, i.e., xi (fraction of i) ) ni/n°, where ni is the number of moles for component i and n° is the initial number of moles for DBT. (19) Sie, S. T. Fuel Process. Technol. 1999, 61 (1-2), 149-171. (20) Ho, T. C. Catal. Today 2004, 98, 3-18. (21) Texier, S.; Berhault, G.; Perot, G.; Harle, V.; Diehl, F. J. Catal. 2004, 223, 404-418. (22) Vrinat, M. L. Appl. Catal. 1983, 6, 137-158. (23) Daage, M.; Chianelli, R. R. J. Catal. 1994, 194, 414-427. (24) Laredo, G. C.; Cortes, C. M. Appl. Catal., A: Gen. 2003, 252, 295304. (25) Vanrysselberghe, V.; Froment, G. F. Ind. Eng. Chem. Res. 1996, 35, 3311-3318. (26) Te, M.; Fairbridge, C.; Ring, Z. Pet. Sci. Technol. 2003, 21, 157181. (27) Vanrysselberghe, V.; Gall, R. L.; Froment, G. F. Ind. Eng. Chem. Res. 1998, 37, 1235-1242. (28) Knudsen, K. G.; Cooper, B. H.; Topsoe, H. Appl. Catal., A 1999, 189, 205-215. (29) Kagami, N.; Vogelaar, B. M.; van Langeveld, A. D.; Moulijn, J. A. Appl. Catal., A 2005, 293, 11-23.

Farag Scheme 1. Reaction Network for DBT HDS

Some Observations on the Hydrodesulfurization Reaction of Dibenzothiophene Before the details of the present manipulation to develop a proper kinetic correlation with the fewest possible adjustable parameters are discussed, it is important to look first at the following documented findings. (1) It is a common experimental finding among related studies that the HDS of DBT behaves as pseudo-first-order with respect to DBT irrespective of the catalysts involved, the experimental conditions, and either using the batch reactor and/or the fixed bed flow reactor.3,5,29-40 In most cases, the reaction rate is independent of the hydrogen concentration due to the fact that it exists in large excess. Therefore, the consumption of hydrogen in terms of the total available amount is negligible.23 Accordingly, the hydrogen rate term is supposed to be incorporated into the determined apparent rate constant of the reaction. (2) More or less, the observed HDS reaction products are mainly biphenyl, phenylcyclohexane, and the partially hydrogenated dibenzothiophenes. Biphenyl and the partially hydrogenated dibenzothiophenes are formed independently and concurrently from the parent DBT via two parallel processes. This evidence leads to the suggestion that the reaction proceeds via a network mechanism for which parallel and consecutive reactions are involved. This is illustrated in Scheme 1. This scheme can be expressed in a simpler notation, as shown in Scheme 2. (3) In the use of heterogeneous catalysis for DBT HDS, the application of the Langmuir-Hinshelwood mechanism is broadly applied.22-28 In general, this mechanism relies on the adsorption nature of the reactant and products on the catalyst active sites that make the corresponding rate equation either simple or complicated. The original simple derived form of this type of rate equation for the unimolecular reaction consists of a (30) Whitehurst, D. D.; Farag, H.; Nagamatsu, T.; Sakanishi, K.; Mochida, I. Catal. Today 1998, 45, 299-305. (31) Farag, H.; Whitehurst, D. D.; Sakanishi, K.; Mochida, I. Catal. Today 1999, 50, 49-56. (32) Farag, H.; Sakanishi, K.; Kouzu, M.; Matsumura, A.; Sugimoto, Y.; Saito, I. Prepr. Pap. Am. Chem. Soc., DiV. Fuel Chem. 2003, 48 (2), 504-505. (33) Kim, J. H.; Ma, X.; Song, C.; Lee, Y.; Oyama, S. T. Energy Fuels 2005, 19 (2) 353-364. (34) Farag, H.; Sakanishi, K.; Kouzu, M.; Matsumura, A.; Sugimoto, Y.; Saito, I. J. Mol. Catal. A 2003, 206 (1-2), 399-408. (35) Saih, Y.; Nagata, M.; Funamoto, T.; Masuyama, Y.; Segawa, K. Appl. Catal., A 2005, 295, 11-22. (36) Yoshinaka, S.; Segawa, K. Catal. Today 1998, 45 (1-4), 293298. (37) Ho, T. C.; Sobel, J. E. J. Catal. 1991, 128, 581-584. (38) Ma, X.; Sakanishi, K.; Mochida, I. Ind. Eng. Chem. Res. 1996, 35 (8), 2487-2494. (39) Yumoto, M.; Kukes, S. G.; Klein, M. T.; Gates, B. C. Catal. Lett. 1994, 26 (1-2), 1-7. (40) Zdrazil, M. Catal. Today 2003, 86, 151-171.

Kinetic Analysis of Hydrodesulfurization of DBT

Energy & Fuels, Vol. 20, No. 5, 2006 1817

Scheme 2. Reaction Network in a Symbolic Form.

nominator, assigned by the species equilibrium concentration multiplied by the intrinsic rate constant and the corresponding adsorption equilibrium constant, and a denominator, represented by the sum of unity to all the equilibrium adsorption concentrations of the species that possibly compete to occupy the active sites multiplied by the related adsorption equilibrium constant. The mathematical expression will be shown later. However, there are a number of limiting forms of this rate expression, depending on which species are strongly or weakly adsorbed. This will be reflected in the magnitudes of the various terms in the denominator relative to unity and to each other. (4) The products of the HDS reaction of DBT have almost no inhibition impacts on the rate of the reaction.19,25,26 H2S did affect the reaction but in the present study the effect was neutralized by the addition of copper powder to the reaction mixture as it has been proven to act successfully in scrubbing H2S.42,43 The kinetic treatment of the inhibition is beyond the scope of the present study. Theoretical Treatments Having taken into account these notations and on the assumption of the existence of two different kinds of catalytic active sites,5,23 the overall rate can be expressed in two parts, as described below:

k1K1CDBT RDDS ) 1 + K1CDBT + ...

(1)

k2K2CDBT 1 + K2CDBT + ...

(2)

RHYD )

where RDDS and RHYD are the rate of direct desulfurization (DDS), i.e., hydrogenolysis to produce biphenyl, and the rate of hydrogenation (HYD), i.e, production of the partially hydrogenated dibenzothiophene, respectively. CDBT is the concentration of DBT at a definite reaction time. To apply the fundamentals of this mechanism to the present case study, it is essential to eliminate the concentration term in the denominator as the only applicable solution consistent with the pseudo-firstorder observation, i.e., the magnitudes of the various terms in the denominator relative to unity should be neglected. The overall rate, RTotal, is the sum of the rates for the direct desulfurization route (RDDS) and hydrogenation route (RHYD) as shown from the suggested network mechanism of Scheme 1. Thus, eqs 1 and 2 add up to become the following:

RTotal ) RDDS + RHYD

(3)

(41) Zepeda, T. A.; Halachev, T.; Pawelec, B.; Nava, R.; Klimova, T.; Fuentes, G. A.; Fierro, J. L. G. Catal. Commun. 2006, 7, 33-41. (42) Farag, H.; Mochida, I.; Sakanishi, K. Appl. Catal., A 2000, 194195, 147-157. (43) Vogelaar, B. M.; Kagami, N.; van Langeveld, A. D.; Eijsbouts, S.; Moulijn, J. A. Prepr. Pap. Am. Chem. Soc., DiV. Fuel Chem. 2003, 48 (2), 548.

RTotal ) k1K1CDBT + k2K2CDBT

(4)

RTotal ) (k1K1 + k2K2)CDBT

(5)

RTotal ) koCDBT

(6)

where kο ) (k1K1+k2K2), k1 and k2 are the intrinsic kinetic rate constants for the direct desulfurization and hydrogenation routes, and K1 and K2 are the equilibrium adsorption constants of DBT over the catalytic active sites for direct desulfurization and hydrogenation, respectively. All these constants are lumped together as one constant, i.e., kο. As noted earlier, two intermediates are participating in the production of phenylcyclohexane. These are biphenyl and the partially hydrogenated dibenzothiophene (mostly 1,2,3,4-tetrahydrodibenzothiophene). The kinetic treatments of the rates of these intermediates with the Langmuir-Hinshelwood (LH) mechanism would lead to the same conclusion as obtained previously, in which the concentration term in the denominator of the L-H equation might be neglected in comparison with unity. The material balance equations of this model for a constant volume reactor (batch autoclave in the present study) can be listed as the following:

dCA ) -(k1K1 + k2K2)CA dt

(7)

dCB ) k1K1CA - k3K3CB dt

(8)

dCC ) k2K2CA - k4K4CC dt

(9)

dCD ) k3K3CB + k4K4CC dt

(10)

where k3, k4, K3, and K4 are the intrinsic kinetic rate constants and equilibrium adsorption constants for B and C as described in Scheme 2, respectively. CB, CC, and CD are the molar concentrations of the corresponding chemical species. These equations are linear first-order and may be solved in a variety of ways. The algebraic solutions of eqs 7-9 are the following:

CA ) CA° exp- kοt CB )

CC )

CA°k11 k33

[exp-kσt - exp-k3 t]

(12)

[exp-kοt - exp-k4 t]

(13)

3

- kο

CA°k22 k44

(11)

- kο

4

where k11, k22, k33, and k44 replaced k1K1, k2K2, k3K3, and k4K4 in eqs 7-10, respectively and CA° is the initial concentration of species A, the mother component. These three equations provide the basis that formulates the present kinetic treatment for this system. The form of eq 11 is already solvable; the forms for CB and/or CC are time-dependent variables that can be determined from the values of the apparent reaction rate constants, k11, k22, k33, and k44, the initial concentration of species A, and the overall apparent rate constant, kο. The curve fitting technique can be readily apply to obtain more precisely the overall apparent rate constant via eq 11. Once the


Farag

Figure 2. Selectivity of the products resulting from the HDS of DBT at 340 °C and 3 MPa of H2 over the CoMo/Al2O3 catalyst as a function of the reaction time. Figure 1. Pseudo-first-order plots of the HDS of DBT over the CoMo/ Al2O3 and MoS2 catalysts: 340 °C and 3 MPa of H2.

overall apparent rate constant has been determined, it can then be readily utilized in eqs 12 and 13 to fit the experimental data and achieve accurate estimate of the rate constants since we are limited to trial and error with only two constants for each intermediate. However, by dividing eqs 12 and 13 by CA° CA, one gets the following:

[ [

] ]

-k11 exp-kοt - exp-k33t CB ) 3 CA° - CA k - k exp-kοt - 1 3 ο

(14)

-k22 exp-kοt - exp-k44t CC ) 4 CA° - CA k - k exp-kοt - 1 4 ο

(15)

Equations 14 and 15 are the representatives to the selectivity consideration of the intermediates in the network reaction (selectivity in this regard is defined as the ratio of the yield of a specific product to the yield of the overall products resulting from the reaction). However, in the parallel-sequential reaction, it would be more accurate to express the selectivity as the ratio of the apparent rate constants of the direct desulfurization and hydrogenation pathways, i.e., k11/k22. Obviously, these nonlinear equations assist in establishing the selectivity in a closed, useful informative form. The optimization can be used effectively with two adjustable parameters that would be appropriate to describe the system. Results Hydrodesulfurization of Dibenzothiophene over CoMo/ Al2O3 Catalyst. Biphenyl, phenylcyclohexane, and partially hydrogenated dibenzothiophene (identification of this species with GC-MS denotes the structure to be 1,2,3,4-tetrahydrodibenzothiophene, with a double bond at the bridge between the partially hydrogenated benzene ring and the thiophene ring) were detected. Figure 1 shows the pseudo-first-order plot of the HDS of dibenzothiophene over the commercial CoMo/Al2O3 catalyst achieved at 340 °C and 3 MPa of H2. The selectivity plot is depicted in Figure 2. It could be seen that the direct desulfurization route product (biphenyl) predominates over the hydrogenation pathway product. However, the selectivity of biphenyl remained almost without change despite variation in

Figure 3. Selectivity of the products resulting from the HDS of DBT at 340 °C and 3 MPa of H2 over the MoS2 catalyst as a function of the reaction time.

the level of the overall conversion percent of dibenzothiophene. On the other hand, the selectivity of the partially hydrogenated dibenzothiophene (i.e., H4-DBT) diminishes exponentially with the reaction time. These observations are in agreement with various typical studies reported in the literature.12,23 Hydrodesulfurization of Dibenzothiophene over MoS2: The products of the HDS of DBT over the MoS2 catalyst are identical with those described for the commercial CoMo/Al2O3 catalyst, except that traces of some partially hydrogenated species and bicyclohexane were observed. Nevertheless, the 1,2,3,4-tetrahydrodibenzothiophene is still the predominant partially hydrogenated dibenzothiophene species; all these intermediates are lumped together and treated kinetically as a unit. Figure 1 shows the pseudo-first-order plot of the HDS of DBT at 340 °C and 3 MPa of H2 over the MoS2 catalyst. The selectivities of the products are depicted in Figure 3. Considerable participation from the hydrogenation route in the HDS reaction of DBT can be observed. The selectivity of a certain product, if expressed by the species product ratio, in such a consecutive-parallel reaction is a function of the reactant conversion level, Table 1. The observed yield of the partially hydrogenated dibenzothiophene is much higher over MoS2 than over the commercial CoMo/Al2O3 catalyst.



Table 1. Conversion and Selectivities for the HDS of DBT: 340 °C and 3 MPa of H2 CoMo/Al2O3

BP PC H4-DBTa a

MoS2

17.9

Conversion (%) 53.2 85.6

22.6

50.0

68.6

89.4 7.8 2.2

Selectivity (%) 89.8 89.7 7.5 10.3 0.9 0

61.5 28.3 10.2

56.0 40.0 4.0

53.9 44.3 1.7

Sum of the partially hydrogenated dibenzothiophene fractions.

Table 2. Overall and Individual Apparent Rate Constants for the HDS of DBT over the CoMo/Al2O3 and MoS2 Catalysts: 340 °C and 3 MPa of H2 rate constantsa, ×10-4‚s-1‚g cat-1 kο k11 k22 k33 k44 k11/k22 k11/k33 k22/k44 a

CoMo/Al2O3

MoS2

47.5 42.8 1.3 0.5 150.0 32.1 85.6 0.01

4.93 3.2 1.1 1.5 31.7 2.9 2.1 0.04

Figure 5. Fraction of biphenyl as a function of the reaction time for the HDS of DBT over the MoS2 catalyst.

The apparent rate constants as defined from Schemes 1 and 2.

Figure 6. Fraction of partially hydrogenated dibenzothiophene as a function of the reaction time for the HDS of DBT over the CoMo/ Al2O3 catalyst. Figure 4. Fraction of biphenyl as a function of the reaction time for the HDS of DBT over the CoMo/Al2O3 catalyst.

Nonlinear Data Fitting. We have performed least-squares data fitting, using a nonlinear curve-fitting program in Mathcad (MathSoft Inc.) on the assumption of the existence of two intermediates in the reaction network and applying eq 12 and 13 to obtain the individual apparent rate constants. The estimated results of the apparent rate constants of the direct desulfurization (k11) and the hydrogenation (k22) pathways in HDS of DBT are listed in Table 2. Calculating the apparent rate constants enables one to predict precisely the individual fractions of dibenzothiophene that react to form biphenyl and the partially hydrogenated dibenzothiophene products. The selectivity percentage of direct desulfurization and hydrogenation routes is assigned by the ratio of k11/k22. The experimental fraction of biphenyl as a function of time along with the corresponding simulated data for HDS of DBT over CoMo/Al2O3 and MoS2 catalysts are shown in Figures 4 and 5, respectively. Figures 6 and 7 present the experimental results of the partially hydrogenated dibenzothiophene and the model fitting calculations where eq 13 was applied for DBT HDS over the CoMo/Al2O3

Figure 7. Fraction of partially hydrogenated dibenzothiophene as a function of the reaction time for the HDS of DBT over the MoS2 catalyst.

and MoS2 catalysts, respectively. A close resemblance between the observation and fitting data can be observed. In addition, a plot of the observed data in accordance with eqs 14 and 15 provides a further way to account for the ratio of


Figure 8. Selectivity of biphenyl as a function of the reaction time for the HDS of DBT over the CoMo/Al2O3 catalyst.

Figure 9. Selectivity of partially hydrogenated dibenzothiophene as a function of the reaction time for the HDS of DBT over the CoMo/ Al2O3 catalyst.

k11/k22. The experimental selectivities of the intermediates of biphenyl and the partially hydrogenated dibenzothiophene as a function of the reaction time coinciding with the fitting curves from eq 14 and 15 are depicted in Figures 8-11. The observed data fit accurately with the model which was developed previously. It has the advantage that for each intermediate only two adjustable parameters, the apparent rate constants for the parallel and the consecutive reactions, are necessary in the estimation. Discussion An adequate and better understanding about the contribution percentage of the two reaction pathways in the HDS of DBT may assist in gaining an insight into the nature of the catalytic active sites. In agreement with the previously reported literature,29-40 the results in the present study showed the HDS of DBT over either CoMo/Al2O3 or MoS2 catalysts to be a firstorder reaction with respect to DBT, as shown in Figure 1. Although a number of studies have been carried out in this field, there is no consensus yet on the method for the estimation of the participation ratio for the direct desulfurization and hydro-

Farag

Figure 10. Selectivity of biphenyl as a function of the reaction time for the HDS of DBT over the MoS2 catalyst.

Figure 11. Selectivity of partially hydrogenated dibenzothiophene as a function of the reaction time for the HDS of DBT over the MoS2 catalyst.

genation routes involved in the reaction. Prins et al.,12 as well as Kim et al.,33 for instance, have determined this ratio from the total pseudo-first-order rate constant and the selectivities at low conversion levels on the assumption that the phenylcyclohexane results exclusively via the consecutive reaction of the partially hydrogenated dibenzothiophene intermediate. Other studies have reported only the yield ratio without the catalytic individual apparent rate constants.18,35,41 Equations 12 and 13 indicate that the concentration of the intermediates is a function of both the parallel and the sequential apparent rate constants. Furthermore, the determination of the k11/k22 ratio depends significantly on the sequential apparent rate constants, i.e., k33 and k44, and on the overall apparent pseudofirst-order rate constant, kο, as well. Therefore, the conclusions drawn from the relative contributions of the direct desulfurization route and the hydrogenation route may often be confusing unless the apparent values of k33 and k44 are determined. The selectivity is the result of the influence of the catalytic active sites on the reaction. It is often considered that this parallelsequential HDS reaction is the impact of two different catalytic active sites broadly based on the direct desulfurization path and


the hydrogenation pathway.3,5,9,12 Further discussion on the catalytic mechanism of such a reaction is definitely necessary in order to determine accurately the proportion of the products formed via each route. The calculation shows that the use of the initial rate points to express the reaction is not sufficient to determine the individual apparent rate constants. The reaction should be performed and pursued until a high conversion level is obtained, otherwise a misleading conclusion is likely to occur. A noteworthy observation in DBT HDS results is that while the commercial CoMo/Al2O3 catalyst furthers the reaction preferentially via the direct desulfurization route, the MoS2 catalyst shows considerable involvement of the two routes (direct desulfurization and hydrogenation) in the mechanism. It is interesting to note that the values of the apparent rate constant for the hydrogenation pathway of DBT HDS over CoMo/Al2O3 and MoS2 catalysts are almost similar, as shown in Table 2. However, the apparent rate constant for the direct desulfurization pathway was 13 times higher for CoMo/Al2O3 than for MoS2. This indicates that the extreme difference in the catalytic activity between the two catalysts is attributed to the improvement of the activity of the CoMo/Al2O3 catalyst via the direct desulfurization pathway. The qualitative similarity of the observed products over the CoMo/Al2O3 and MoS2 catalysts reveals that the HDS of DBT is ruled by identical mechanisms; although, there is a difference in the selectivities. This could imply that the presence of Co not only modifies the overall activity but also shifts the selectivity toward the direct desulfurization reaction. It could be possible that the Co promotes the reaction via enhancing the selectivity of the direct desulfurization pathway. The relation between activity and selectivity is an encouraging topic of study. A wide difference exists between the estimated values of the individual rate constants for the sequential reactions in the hydrogenation pathways for the CoMo/Al2O3 and MoS2 catalysts, i.e., k44. The value of k44 for DBT HDS over the CoMo/ Al2O3 catalyst is about 5 times higher than that obtained over the MoS2 catalyst, as shown in Table 2. This may explain why the experimental yield of the partially hydrogenated dibenzothiophene in the DBT HDS reaction over MoS2 is higher than that over the CoMo/Al2O3 catalyst. The observed experimental yield of the intermediates in the DBT HDS reaction is highly consolidated with the ratios of the individual apparent rate constants associated with their consecutive reaction pathways, i.e., k11/k33 and/or k22/k44. The higher, the ratio of k11/k33 and/or k22/ k44, the higher, the observed yield of the corresponding intermediates.


The model prescribed so far for each intermediate has mimic limitation because only two rate constants are needed for the solution since the intermediates are separately and independently treated. In addition, it is possible to express the model in another form which is a confirmation of the ability and flexibility in it’s application, as shown from the correlation of the selectivity as a function of the reaction time, eqs 14 and 15, and Figures 8-11. The selectivity is viewed to show certain physical means that would assist in predicting the progress of the reaction. In terms of the present model, if the contribution of B and/or C in production of D is minimal, then the selectivity will remain almost unchanged with any conversion level of A; a case represented exactly by the CoMo/Al2O3 catalyst, as shown in Figure 2. As the contribution percent of the intermediate increases, the magnitude of the exponential nature of the selectivity plot, as a function of the reaction time, will be more abundant and pronounced. The determination of k11 and/or k22 from the data fitting was found to be sensitive to the values adopted for k33 and/or k44 in Scheme 2. Conclusion A model kinetics expression that describes the reaction network involved in the HDS reaction of DBT was proposed. The reaction network of the HDS of DBT proceeds via two parallel-sequential reactions based on the existence of two intermediates in the scheme, namely, biphenyl and the partially hydrogenated dibenzothiophene. A phenylcyclohexane compound resulted from further sequential reactions of these intermediates. The relative contribution of the two reaction routes into the overall reaction is important when analyzing the nature of the active sites. The present study offers an example of the validity of applying such model kinetic equations to estimate the individual apparent rate constants in such a reaction network. The resulting data from two model catalysts, MoS2 and CoMo/Al2O3, were employed to investigate the applicability of the present model. A good approximation between the theoretical fitting and the observed results is evident. Interestingly, it appears that the estimations of the contribution ratio of both the direct desulfurization and hydrogenation routes, i.e., k11 and k22, were found to be very sensitive to the values of the rate constants of the sequential reactions, i.e., k33 and k44. Thus, precision on determining the individual apparent rate constants, i.e., k33 and k44, are important to permit an accurate comparison of the rates in the two parallel routes. EF060225G

Kinetic Analysis of the Hydrodesulfurization of Dibenzothiophene

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