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Kinetic Analyses and Inhibition by Naphthalene and H2S in Hydrodesulfurization of 4,6-Dimethyldibenzothiophene (4,6-DMDBT) over CoMo-Based Carbon Catalyst Hamdy Farag,* Kinya Sakanishi, Isao Mochida, and D. D. Whitehurst Institute of Advanced Material Study, Kyushu University, Kasuga, Fukuoka 816-0811, Japan Received June 23, 1998
Hydrodesulfurization of 4,6-dimethyldibenzothiophene (4,6-DMDBT) in decane has been investigated over a Co-Mo/C catalyst, using a magnetically stirred autoclave reactor. The hydrodesulfurization reaction was found to be pseudo-first order with respect to 4,6- DMDBT. Direct desulfurization was detected to be Arrhenius-dependent with an activation energy of 35.7 kcal/mol, while hydrogenation pathway showed a maxima at 340 °C. The activation energy of hydrogenation in the Arrhenius-dependent region was calculated to be 17.9 kcal/mol, as hydrogenation is thermodynamically limited at high temperature. The reaction of hydrodesulfurization appears to proceed via two different partially dependent routes, hydrogenation and desulfurization, over the present catalyst. Hydrogenation of 3,3′-dimethylbiphenyl at different temperatures was also measured to identify the rates of the step involved in the hydrodesulfurization network of 4,6-DMDBT. Attention was particularly paid to the effects of naphthalene and H2S on the two different reaction pathways to propose the mechanism of their inhibition. Two different catalytic active sites, one for hydrogenation and the other for both hydrogenation and direct desulfurization, are discussed to be present.
Introduction In recent years, much attention has been focused on deep desulfurization of petroleum oil. The sulfur level in diesel oil will soon be limited to less than 0.05 wt % in Japan as well as Europe. Many studies have recently appeared in the literature dealing with the kind of refractory sulfur-containing compounds in the petroleum oil. The polyaromatic sulfur-containing compounds such as dibenzothiophenes (DBT’s) are the most difficult to desulfurize in deep desulfurization of petroleum oil.1-3 Among dibenzothiophenes, 4,6-DMDBT was confirmed to have the most refractory compounds for HDS reaction. Desulfurization of such compounds has been reported by several workers.3,4 In the literature, there is a contradiction in the adsorption strength of such compounds to explain their low reactivity. Some authors5 have attributed the low reactivity of substituted DBT compounds to their weak adsorption on the catalyst surface, while others1,2 have related it to their strong adsorption on the catalytic active sites. However, it was revealed that HDS reaction of sulfur-containing compounds proceeds via two different reaction routes, a direct desulfurization route and hydrogenation route. (1) Kabe, T.; Ishihara, A.; Tajima, H. Ind. Eng. Chem. Res. 1992, 31, 1577-1580. (2) Ishihara, A.; Tajima, H.; Kabe, T. Chem. Lett. 1992, 669-670. (3) Landau, M. V.; Berger, D.; Herskowite, M. J. Catal. 1996, 158, 236-245. (4) Lamure-Meille, V.; Schulz, E.; Lemaire, M.; Vrinat, M. Appl. Catal. A: General 1995, 131, 143-157. (5) Houalla, M.; Broderick, D. H.; Sapre, A. V.; Nag, N. K.; de Beer, V. H. Z.; Gates, B. C.; Kwart, H. J. Catal. 1980, 61, 523-527.
The contribution of each route strongly depends on the location of the substituent on DBT. The desulfurization route by direct scission of the C-S bond is thoroughly inhibited by addition of methyl substituents to DBT especially on the 4- and 6-positions. Nevertheless, there is still limited information about the other factors which might affect the routes. In the present study, the HDS of 4,6-DMDBT over the laboratory-prepared sulfided Co-Mo supported on an active carbon catalyst was investigated under various experimental conditions. The principle objective is an attempt to elucidate the hydrodesulfurization mechanistic pathways of 4,6-DMDBT and the factors affect its routes. Experimental Section Materials. 3,3′-Dimethylbiphenyl, naphthalene, and tetralin were purchased from Wako Pure Chemical Industries. 4,6-DMDBT was synthesized according to the procedure in ref 6. Decane and other solvents were commercial products and used without further purification. The active carbon used as a support was obtained from Mitsubishi Chemical Co. Catalyst Preparation. Co-Mo/C catalyst was prepared by impregnation by two-step procedures in which the molybdenum phase was introduced first followed by drying in a vacuum at 120 °C overnight. The next step is the addition of the cobalt phase to this precursor, followed by the same treatment as that used for (6) Gerdil, R.; Luken, E. J. Am. Chem. Soc. 1965, 87, 213-217.
10.1021/ef980141a CCC: $18.00 © 1999 American Chemical Society Published on Web 12/22/1998
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Figure 3. Product distribution curve of HDS of 4,6-DMDBT over Co-Mo/C catalyst at 340 °C and 2.9 MPa of H2. Figure 1. Kinetics and product distribution of HDS of 4,6DMDBT over CoMo/C catalyst: 340 °C and 2.9 MPa of H2.
Figure 4. HDS reaction scheme of 4,6-DMDBT. Note, ks are expressed in 104 sec-1 g cat-1. Rate constant (reaction concerned): kD0 (desulfurization without ring hydrogenation), kD1 (desulfurization of 1 ring hydrogenated sulfur compound), kHS1 (hydrogenation of one ring of the sulfur compound), kHP1 (hydrogenation of one phenyl ring of desulfurized biphenyl), k-HS1 (dehydrogenation of one ring of the sulfur compound). Figure 2. Kinetics of pseudo-first order hydrodesulfurization of 4,6-DMDBT over Co-Mo/C catalyst: 340 °C and 2.9 MPa of H2. Note, A-X is the fraction of 4,6-DMDBT unconverted.
molybdenum. The sources of molybdenum and cobalt used in the catalyst preparation were Mo-acetylacetonate and Co-acetylacetonate. Methanol was used as the impregnating solvent. The catalyst carried cobalt and molybdenum of 2 and 10 wt %, respectively. Catalyst Sulfidation. The sulfidation of the precursor catalyst after drying in an oven at 120 °C was carried out in a flow of 5% v/v H2S in hydrogen at atmospheric pressure. The temperature was raised in 35 min from room temperature to the final temperature, 360 °C. It was kept at this temperature for 2 h, after which the sample was cooled to room temperature under continuos flow of a 5%(v/v) H2S/H2 gas mixture. Catalytic Activity. The reactor used for catalytic activity measurements was a 100 cm3 magnetically stirred autoclave. A reactant solution contained 0.1 wt % of either 4,6-DMDBT or 3,3′-dimethylbiphenyl in decane. In each run, the autoclave was charged with 10 g of the reactant solution. Freshly sulfided catalyst (0.2 g) was then added to this solution. The reactor was purged 3 times with hydrogen gas before starting the experiments. During all the experiments, the stirring speed was kept constant. The temperature was raised rapidly until the prescribed reaction temperature. At this moment, the reaction time was started. At the end of reaction, the reactor with its contents was rapidly cooled to the room temperature.
Analysis. The reaction mixtures were collected and filtered, and the catalyst was washed several times with toluene to recover any adsorbed species on the catalyst to keep the mass balance at an acceptable level. The collected samples were analyzed with the aid of adding a reference material (4,4′-diethylbiphenyl). Analysis of the samples was accomplished with a Yanaco (G3800) gas chromatograph equipped with a differential flame ionization detector and linear temperature programming. A capillary column (OV-101; 0.25 mm × 50m) was used for good separation. Chromatograms were quantified by an electronic integrator. Response factors were obtained by the aid of the reference materials. Qualitative analysis of the samples was obtained from retention time and mass spectra. Results Kinetics of HDS of 4,6-DMDBT. The products observed in this study were cis and trans-3,3′-dimethylphenylcyclohexane (cis and trans-3,3′-DMPC), 3,3′dimethylbiphenyl (3,3′-DMBP), partially hydrogenated 4,6-DMDBT (H4-4,6DMDBT), and H2S. A typical reaction course according to the reaction time is plotted in Figure 1 for the transformation of 4,6-DMDBT over CoMo/C catalyst at 340 °C. To confirm the overall order of HDS in 4,6-DMDBT, the pseudo-first-order plot of ln(a/(a - x)) vs t is illustrated in Figure 2, where a and t are the 4,6-DMDBT initial concentration and reaction time, respectively. A straight line with a 99% confidence
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Figure 5. Arrhenius plot of the direct desulfurization reaction of 4,6-DMDBT over Co-Mo/C catalyst: 2.9 MPa of H2.
Figure 7. Hydrogenation of 3,3′-dimethylbiphenyl over CoMo/C catalyst at various temperatures.
Figure 8. Effect of naphthalene concentration on the activity of HDS of 4,6-DMDBT over Co-Mo/C catalyst: 340 °C, 2.9 MPa of H2, and after a reaction time of 20 min. Figure 6. Arrhenius plot of the hydrogenation route of 4,6DMDBT over Co-Mo/C catalyst: 2.9 MPa of H2.
limit was obtained. The slope of the straight line represents the overall rate constant k. According to a previous study,7 a computer simulation plot of the product distribution vs conversion is depicted in Figure 3. The rate constants of each step in the desulfurization network are given and defined over the arrow of each reaction step in Figure 4. The calculations were accomplished based on the assumption that the main source for the production of cis and trans-3,3′DMPC compounds is H4-4,6DMDBT. Tiny percent may come from hydrogenation of 3,3′-dimethylbiphenyl, as will be discussed later. Consequently, on average the product evolved from direct desulfurization (DDS) and hydrogenation (HYD) routes as 41% and 59%, respectively, under the present conditions. (7) Farag, H.; Whitehurst, D. D.; Sakanishi, K.; Mochida, I. Symposium on catalysis in fuel processing and environmental protection. Abstracts of papers, 214th National Meeting of the American Chemical Society, Las Vegas, NV, Fall 1997; American Chemical Society: Washington, DC, 1997; pp 569-572.
Activation Energy. An Arrhenius plot (Figure 5) gave a value for the apparent activation energy of 35.7 kcal/mol for the direct desulfurization pathway. On the other hand, the rate of the hydrogenation route increased sharply up to 300 °C and very gradually up to 340 °C and then decreased with higher reaction temperature as shown in Figure 6. The apparent activation energy calculated in the temperature range of 280-340 °C was 17.9kcal/mol. Hydrogenation of 3,3′-Dimethylbiphenyl. Hydrogenation of 3,3′-dimethybiphenyl (3,3′-DMBP) was carried out over Co-Mo/C catalyst using the identical experimental conditions used in case of 4,6-DMDBT HDS reaction. The products found were exclusively cisand trans-3,3′-dimethylphenylcyclohexane. Figure 7 shows the effect of reaction temperature on the hydrogenation activity of 3,3′-DMBP at 2.9 MPa of H2. Raising the temperature increased the hydrogenation activity, indicating no restriction of equilibrium. Nevertheless, the overall conversion stayed low even at relatively high temperature, i.e., 380 °C, suggesting a minimal contribution of this route in HDS. The trans- and cis-3,3′dimethylphenylcyclohexane ratios were always found at
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Discussion
Figure 9. Naphthalene effect on the selectivities of HDS of 4,6-DMDBT over Co-Mo/C catalyst (340 °C and 2.9 MPa of H2). Selectivity was expressed as the product ratio between hydrogenation (HYD) and direct desulfurization (DDS) routes. Table 1. Naphthalene Hydrogenation in Competition with HDS of 4,6-DMDBT over CoMo/C Catalysta
a
naphthalene content in the feedstock
% naphthalene conversion (tetralin)
5 wt % 10 wt %
58.6 32.3
340 °C, 2.9 MPa of H2, and 20 min reaction time.
Table 2: Effect of H2S on HDS of 4,6-DMDBT over CoMo/ C Catalysta ratio of H2S to 4,6-DMDBT in the feedstock
% conversion
0.0 1.0
34.7 39.0
product distributionb 1 2 3 16.4 6.9
16.0 25.2
2.3 6.9
a 340 °C, 2.9 MPa of H and 20 min reaction time. b 1, 3,3′2 dimethylbiphenyl; 2, cis and trans-3,3′-dimethylphenylcyclohexane; and 3, hydrogenated 4,6-DMDBT.
a certain value, probably corresponding to the thermodynamic equilibrium between each other. Inhibition of Naphthalene and H2S on HDS. Figure 8 shows the effects of naphthalene concentration on the activity of HDS of 4,6-DMDBT over Co-Mo/C catalyst at 340 °C and 2.9 MPa of H2. The HDS activity of 4,6-DMDBT decreased proportionally with increasing naphthalene concentration. Figure 9 shows the HDS selectivity of 4,6-DMDBT vs naphthalene concentration. Increasing concentration of naphthalene in the feed increased slightly the selectivity of DDS, indicating an inhibition on both DDS and HYD routes with a slightly less effect on DDS. The major product from naphthalene in such experiments was tetralin with a negligible amount of decalin, as summarized in Table 1. It is noted that the conversion of naphthalene decreased with increasing concentration, self-inhibition being suggested. The effects of H2S on HDS of 4,6-DMDBT at 340 °C and 2.9 MPa of H2 are summarized in Table 2, where the molar ratio of H2S to 4,6-DMDBT in the feedstock was 1:1. It is interesting to note no inhibition of HDS of 4,6-DMDBT by H2S but even slight increase in the total conversion exists. The slight enhancement of total activity is due solely to the increased activity of the HYD route, which compensates for the sharp decreasing activity of DDS route.
Kinetic Analyses. The simulation of product distributions in this reaction based on the assumption of a pseudo-first-order model for each step in the network fits with the experimental one, as shown in Figure 3. Consequently, the average of the product evolved through direct desulfurization and hydrogenation routes as 41% and 59%, respectively. Such contributions of direct desulfurization are in contrast to that of dibenzothiophene (DBT) found in a previous study,8 where this rate was exclusively important. The calculated network revealed several points to be discussed. First, the HDS of 4,6-DMDBT over the present catalyst consists of two major routes: direct desulfurization and hydrogenation. Second, comparison of rate constants, kD1 . kHP1, implies that the major cis- and trans-3,3′-DMPC compounds are produced from H4-4,6-DMDBT and not through the hydrogenation of 3,3′-DMBP. This conclusion is consistent with the very slow hydrogenation of 3,3′-DMBP noticed from its separate hydrogenation reaction. In another comparison of kD1 . kHS1, kD0 indicates the much higher reactivity of H4-4,6-DMDBT to that of 4,6-DMDBT. Also, it should be noted that the hydrogenation step of 4,6-DMDBT is thermodynamically limited. The effect of temperature on the direct desulfurization of 4,6-DMDBT was found to be Arrhenius dependent in the whole temperature range, as shown in Figure 5, while the hydrogenation pathway showed a maximum at 340 °C and a higher temperature sharply reduced the rate, as shown in Figure 6. The sharp decrease of hydrogenation rate above 360 °C suggests a diminishing of surface coverage in addition to thermodynamic limitation. This suggestion has been proposed for an aromatic hydrogenation reaction over different catalysts and various conditions.9,10 Naphthalene Inhibition. A variety of polyaromatic compounds present in the diesel oil adsorb completely with sulfur species on the same catalyst surface.11-13 It was found in the present study that the increasing concentration of naphthalene in the feedstock increased the ratio of [HYD]/[DDS] slightly at a 50% conversion level, although naphthalene reduced both the rates. It should be remembered that the [HYD/DDS] ratio varies with the conversion level. In the case of HDS of polyaromatic sulfur-containing compounds, it is generally believed that the reaction takes place in two different active sites, one for the hydrogenation reaction and the other for C-S bond hydrogenolysis. Nevertheless, naphthalene addition on the feedstock inhibits both routes by a nearly equal percent. This observation combined with that in the temperature effect lead us (8) Farag, H.; Whitehurst, D. D.; Sakanishi, K.; Mochida, I. Symposium on catalysis in fuel processing and environmental protection. Abstracts of papers, 214th National Meeting of the American Chemical Society, Las Vegas, NV, Fall 1997; American Chemical Society: Washington, DC, 1997; pp 546-549. (9) van Meerten, R. Z. C.; Coenen, J. W. E. J. Catal. 1977, 46, 1324. (10) Stanislaus, A.; Copper, B. H. Catal. Rev.-Sci. Eng. 1994, 36 (1), 75-123. (11) Isoda, T.; Nagao, S.; Ma, X.; Korai, Y.; Mochida, I. Energy Fuels 1996, 10, 482-492. (12) Isoda, T.; Nagao, S.; Ma, X.; Korai, Y.; Mochida, I. Energy Fuels 1996, 10, 487-492. (13) Whitehurst, D. D.; Isoda, T.; Mochida, I. Adv. Catal. 1998, 42, 345-471.
Inhibition by Naphthalene and H2S
to support the assumption of the presence of two different catalytic active sites, one of which can be used for hydrogenation as well as direct desulfurization reactions and the other is available only for the hydrogenation reaction. This may be illustrated by considering the possible active sites of hydrogenation of naphthalene. According to the selectivity shown in Figure 9, we can conclude that naphthalene inhibits the HDS reaction via adsorption competition with the sulfur compound over both kinds of active sites with approximately the same ratio. H2S Inhibition. In the case of HDS of DBT, it is believed that hydrogen sulfide competes strongly with it for direct desulfurization active sites.13 It is interesting to note from Table 2 that there is not only no inhibition of HDS of 4,6-DMDBT by H2S, but also a slight increase in the total conversion. However, the slight enhancement of total activity is related only to the increasing activity of the hydrogenation route. As suggested before, HDS of 4,6-DMDBT performs through two different pathways, hydrogenation and direct desulfurization routes. Therefore, if adsorption is bonding through the S atom, hydrogenolysis is expected; if adsorption involves bonding through the aromatic system, hydrogenation is expected. One can notice that the direct desulfurization pathway was greatly inhibited by H2S (more than 2-fold). On the other hand, H2S promoted the activity of the hydrogenation route. The inhibition of the direct desulfurization route of 4,6-DMDBT by H2S seems to be reasonable, since there is adsorption competition between H2S and 4,6DMDBT on the direct desulfurization active sites on the catalyst surface. However, the adsorption of H2S are more preferable than 4,6-DMDBT, which might be due to a geometrical factor, i.e., 4,6-DMDBT is a large molecule that can cover an area as large as 8.0 × 12.2 Å2 (size of DBT) in a flat adsorption mode. The question still remains as to why H2S promotes the hydrogenation route of the 4,6-DMDBT HDS reaction. The fact that H2S probably adsorbs only on the direct desulfurization active sites is a good approach that can be used to demonstrate this behavior. Thus, the result suggests that 4,6-DMDBT being planar molecule is adsorbed in a plane parallel to the catalyst surface, favoring interactions of the aromatic electrons with the catalytic sites. Moreover, there might be two kind of catalytic active sites located close to each other. Due to the large size of 4,6-DMDBT and its flat adsorption on the surface, some catalytic active sites could be out of use because the shielding effect, in contrast to H2S which has comparatively a very small molecular size. In this case it is very probable that the catalytic active sites for both hydrogenation and direct desulfurization are located close to each other. Therefore, for hydrogenation, the shielding by 4,6-DMDBT when adsorbed on the catalytic active sites by its sulfur
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atom is much larger than that caused by H2S. Thus, the results demonstrate that H2S greatly inhibits the direct desulfurization route and on the other hand enhance the hydrogenation route. Furthermore, the results suggest that a majority of the two active sites might be located close to each other. Conclusions The 4,6-DMDBT HDS reaction over Co-Mo/C catalyst appears to proceed via two different pathways, hydrogenation and direct desulfurization, in agreement with the literature. The proposed theoretical model of HDS of 4,6-DMDBT was fitted with the experimental one. The hydrogenation of 3,3′-dimethylbiphenyl suggests that under the conditions employed for HDS of 4,6-DMDBT, the hydrogenated products that come from the direct desulfurization route are minimal. On the basis of our results, both routes of HDS of 4,6-DMDBT have an important part in the reaction. However, at high temperatures, it was found that the HDS reaction proceeds mainly via a direct desulfurization pathway. The effect of temperature on the direct desulfurization of 4,6-DMDBT was found to be Arrhenius-dependent with an activation energy of 35.7 kcal/mol, while in the case of the hydrogenation pathway it was Arrheniusdependent in the range of 280-340 °C with an activation energy of 32.5 kcal/mol and anti-Arrhenius at higher temperatures. The sharp increase in direct desulfurization activity at high temperatures suggests the presence of two different active sites, one can be used by both routes and the other is available only for the hydrogenation route. Thus, the rim-edge model for HDS of polyaromatic sulfur compounds over unsupported MoS2 catalyst proposed by Daage and Chianelli,14 in which hydrogenation of such compounds occurs exclusively on rim sites whereas hydrodesulfurization (C-S bond hydrogenolysis) is obtained on both the rim and edge sites, is strongly supported by this result. The presence of naphthalene in the initial feedstock was found to inhibit the HDS of 4,6-DMDBT, especially if it is introduced in high concentration. The inhibition by naphthalene was clear in both hydrogenation and direct desulfurization reactions. Furthermore, the naphthalene hydrogenation reaction was inhibited by 4,6DMDBT and by its concentration (self-inhibition). It is well-known that H2S is an inhibitor for the HDS reaction. However, based on our results, H2S has a dual character in which on one hand it inhibits the direct desulfurization reaction yet on the other hand it promotes the hydrogenation reaction, indicating that the two catalytic active sites should be close to each other. This result might be related to the geometrical factor of adsorption of 4,6-DMDBT and H2S. EF980141A (14) Daage, M.; Chianelli, R. J. Catal. 1994, 149, 414-426.
