Elucidation of Retarding Effects of Sulfur and Nitrogen Compounds on

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Energy & Fuels 2003, 17, 1338-1345

Elucidation of Retarding Effects of Sulfur and Nitrogen Compounds on Aromatic Compounds Hydrogenation Atsushi Ishihara,* Jeayoung Lee, Franck Dumeignil, Masuda Takashi, Eika W. Qian, and Toshiaki Kabe Department of Chemical Engineering, Tokyo University of Agriculture & Technology, Nakacho, Koganei, Tokyo 184-8588, Japan Received December 13, 2002

We investigated the phenanthrene (PHE) hydrogenation (HYD) reaction over a Ni-Mo/Al2O3 and a Pt-Pd/Al2O3 catalyst. The catalyst deactivation in the presence of dibenzothiophene (DBT) and/or acridine (ACR) was studied. Near 300 °C, the main product of the PHE HYD reaction was 1,2,3,4,5,6,7,8-octahydrophenanthrene (1,8-OHP) in the case of the NiMo catalyst and perhydrophenanthrene (PHP) in the case of the PtPd catalyst. Further, the PHE conversion over the NiMo catalyst in the presence of DBT slightly increased with decreasing the DBT concentration (from 1 to 0 wt %). However, the reintroduction of DBT in the feed after a temporary cut did not permit recovery of the initial PHE HYD and DBT hydrodesulfurization (HDS) conversions. It was deduced that the mixed NiMoS phase deteriorated when no sulfur was present in the feed. In the case of the PtPd catalyst, the PHE conversion increased when the DBT concentration decreased. Further, the initial conversions (PHE HYD and DBT HDS) observed after a temporary DBT feed cut and a subsequent DBT reintroduction were substantially recovered. Indeed, contrary to the NiMo catalyst, the PtPd catalyst activity was hindered when sulfur was present in the system, while the absence of sulfur permitted regeneration of the active phase by reduction. In a second part, the effect of the ACR introduction was studied. The ACR introduction led to a drastic decrease in the PHE conversion over the Ni-Mo/Al2O3 catalyst while the decrease was less pronounced over the PtPd catalyst. At the same time, unlike the NiMo catalyst, the Pt-Pd/Al2O3 catalyst showed a drastic decrease in CHB yield (poisoning of the HYD active sites) leading to a drastic decrease in DBT conversion. Moreover, on both catalysts the activity was not fully recovered after the ACR feed was cut, indicating that some HYD catalytic sites were permanently poisoned.

1. Introduction The growing demand for high quality diesel fuels is accompanied with more and more severe environmental regulations imposing a progressive decrease in the pollution due to the vehicles exhaust gas emissions. Therefore, hydrotreating processes are playing a very important role in the strategy of modern refineries.1 Indeed, the new diesel fuel norms that will take place in 2004 will impose a maximal sulfur content of 0.05 wt %, a further decrease in the aromatic compounds content, with a cetane number maintained at a value of at least 40.2-5 To meet such requirements, refiners must develop new strategies to enhance the hydrogenation capabilities of the existing processes. Two approaches, a single-stage process and a two-stage process, have been proposed so far. The single-stage process, in * Corresponding author. Tel & Fax: 81-42-388-7228. E-mail: [email protected]. (1) Kabe, T.; Ishihara A.; Qian W. In Hydrodesulfurization and Hydrodenitrogenation; Kodansha Scientific, Wiley-VCH: Tokyo, New York, Berlin, 1999. (2) Van den Berg, J. P.; Lucien, J. P.; Germaine, G.; Thielemans, G. L. B. Fuel Process. Technol. 1993, 35, 119. (3) Stanislaus, A.; Cooper, B. H. Catal. Rev. Sci. Eng. 1994, 36 (1), 75. (4) Cooper, B. H.; Donnis, B. B. L. Appl. Catal. 1996, A 137, 203. (5) Cooper, B. H.; Stanislaus, A.; Hannerup, P. N. Hydrocarbon Process. 1993, June, 83.

which HDS and hydrogenation (HYD) are performed over conventional CoMo, NiMo, or NiW catalyst, permits attainment of the required aromatic saturation. However, it is costly because it needs H2 pressures substantially higher than the ones used in the actual HDS units.6 The two-stage process uses a “classical” HDS catalyst in the first reactor and a platinum-supported catalyst in the second one. This permits us to obtain a diesel stream with low aromatics content even when moderate hydrogen pressures are used.7,8 This latter process system is efficient to decrease the aromatics concentration but, on the other hand, the catalyst used in the second stage is very sensitive to sulfur poisoning.9 Therefore, considerable attention has been recently paid to develop catalysts which not only have high hydrogenation properties, but that are also sulfur-tolerant to the small amounts of sulfur inevitably present in the feed streams. Recent studies have shown that the addition of some transition metals such as Pd or Re to Pt/Al2O3 formulations can improve the sulfur-tolerance (6) Yosuda, H.; Yoshimura, Y. Catal. Lett. 1997, 46, 43. (7) Cooper, B. H.; Stanislaus, A.; Hannerup, P. N. Prepr. Pap.sAm. Chem. Soc., Div. Pet. Chem. 1992, 37 (1), 41. (8) Sogaard-Andersen, P.; Cooper, B. H.; Hannerup, P. N. AM-9250, NPRA Annual Meeting, 1992, March. (9) Barbier, J.; Lamy-Pitara, E.; Marecot, P.; Boitiaux, J. P.; Cosyns, J.; Verna, F. Adv. Catal. 1990, 37, 279.

10.1021/ef020283b CCC: $25.00 © 2003 American Chemical Society Published on Web 08/06/2003

Retarding Hydrogenation of Aromatics with S and N Compounds

of the catalysts.3,10-13 Moreover, the use of a zeolite as a support for a noble metal was also found to be a sulfurtolerant catalytic system.10,14-19 In the case of coal-derived liquids, in addition to some sulfur compounds, the feeds also contain large amounts of nitrogen compounds and aromatics, increasing the number of parameters playing a role during the global reaction. Indeed, it is well-known that the molecules treated by HDS, hydrodenitrogenation (HDN), and hydrodeoxygenation (HDO) reactions can be responsible for the hindrance of the aromatics hydrogenation reaction. Thus, it appears necessary to carry out a detailed study of the aromatics hydrogenation reaction under various conditions (presence or not of sulfur- or nitrogencontaining molecules) in order to isolate the factors influencing the reactivity. Girgis and Gates20 have reported a detailed kinetic study in a review presenting the effects of additives on aromatic hydrogenation in the presence of sulfur, oxygen, and nitrogen compounds. According to the results they presented, nitrogen compounds are the strongest inhibitors of the aromatics hydrogenation reaction. Especially, it seems that the inhibition by organic nitrogen compounds is even stronger than that induced by ammonia, although the various quantitative results presented in the different studies were not always in so good agreement. Chung and Satterfield21 have reported the effect of the ammonia presence on the hydrogenation of PHE performed concomitantly with the HDN of quinoline. They mainly focused their study on products selectivities. Vrinat et al.22 have reported a comparative inhibiting effect of polycondensed aromatics and nitrogen compounds on the alkyldibenzothiophenes HDS. They studied the inhibiting effect during the simultaneous presence of two reactants. To complete the data found in the literature, we decided to follow the hydrogenation of a model aromatic molecule (PHE) over classical hydrotreating (HDT) catalysts, i.e., Ni-Mo/Al2O3 and Pt-Pd/Al2O3 in the presence of inhibiting compounds (dibenzothiophene and acridine). In the present study, we investigated the PHE HYD reaction inhibition induced by the presence of one component in the PHE-containing feed (DBT or ACR), or even two components simultaneously present in the feed (DBT and ACR). The catalytic deactivation and the change of products selectivities over representative classical hydrotreating catalysts, i.e., Ni-Mo/Al2O3 and (10) Purnell, S. K.; Chang, J.-R.; Gates, B. C. J. Phys. Chem. 1993, 97, 4196. (11) Chiou, J.-F.; Huang, Y.-L.; Lin, T.-B,; Chang, J.-R. Ind. Eng. Chem. Res. 1995, 34, 4277. (12) Lin, T.-B.; Jan, C.-A.; Chang, J.-R. Ind. Eng. Chem. Res. 1995, 34, 4284. (13) Jan, C.-A.; Lin, T.-B.; Chang, J.-R. Ind. Eng. Chem. Res. 1996, 35, 3893. (14) Song, C.; Schmitz, A. Energy Fuels 1997, 11, 656. (15) Dossi, C.; Tsang, C. M.; Sachtler, W. M. H. Energy Fuels 1989, 3, 468. (16) Sugioka, M.; Sado, F.; Matsumoto, Y.; Maesaki, N. Catal. Today 1996, 29 (1-4), 255. (17) de Leon, S. A.; Grange, P.; Delmon, B. Catal. Lett. 1997, 47 (1), 51. (18) Yasuda, H.; Sato, T.; Yoshimura, Y. Catal. Today 1999, 50, 63. (19) Yasuda, H.; Yoshimura, Y. Catal. Lett. 1997, 46, 43. (20) Girgis, M. J.; Gates, B. C. Ind. Eng. Chem. Res. 1991, 30, 20212058. (21) Lee, C. M.; Charles N. Satterfield, C. N. Energy Fuels 1993, 7, 978-980. (22) Koltai, T.; Macaud, M.; Guevara, A.; Schulz, E.; Lemaire, M.; Bacaud, R.; Vrinat, M. Appl. Catal. A: General 2002, 231, 253-261.

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Pt-Pd/Al2O3, were followed when the relative concentration of the reactants was modified. 2. Experimental Section 2.1. Chemical Products. Decalin (solvent) and PHE (98% purity) were purchased from Kishida Chemicals. ACR (98%) and DBT (99%) were, respectively, purchased from Janssen Chemica and Acros Organics. Hydrogen (99.99%) and the used H2/H2S mixture (5.5% H2S in H2) were obtained from Tohei Chemicals. The γ-alumina used as a support was supplied from Nippon Ketjen as 1/32-in. extrudates (surface area: 256 m2/g; pore volume: 0.712 mL/g; mean pore diameter: 8.2 nm). The extrudates were crushed and screened in order to obtain grains with a size of 20 mesh-35 mesh prior to use. The Ni-Mo/ Al2O3 (MoO3 16 wt %, Ni/Mo)0.6) and the Pt-Pd/Al2O3 (2 wt % Pt, 10 wt % Pd) catalysts were prepared by the impregnation method23,24 using, respectively, (NH4)6Mo7O24‚4H2O, Ni(NO3)2‚ 6H2O, H2PtCl6, and PdCl2 as precursors. The obtained solids were then dried in an oven at 120 °C for 3 h and then calcined in air at 430 °C for 20 h. The BET surface area of the Ni-Mo/Al2O3 and the Pt-Pd/Al2O3 catalysts were, respectively, of 211 and 233 m2/g. The freshly prepared catalysts were loaded into a fixed-bed reactor. The Ni-Mo/Al2O3 catalyst was presulfided in a 50 mL/min [5.5% H2S]/H2 flow at 400 °C during 3 h under atmospheric pressure,23 while the Pt-Pd/ Al2O3 catalyst was prereduced under a pure 50 mL/min H2 flow at 400 °C for 3 h.24 The details of the preparation methods and the preactivation treatments are described in ref 23 and ref 24. 2.2. Experimental Procedure. 2.2.1. Reaction Conditions. The evaluation of the catalytic activity was carried out in a conventional fixed-bed reactor, for which details are provided elsewhere.24 Typical reaction conditions were: 0.5 g of catalyst, a reaction temperature of 280 °C for the Ni-Mo catalyst and 240 °C for the Pt-Pd catalyst, a total pressure of 50 kg/cm2, a WHSV of 28 h-1, a liquid flow rate of 16 mL/h, and a H2 flow rate of 12.5 L/h. 2.2.2. Reaction Procedure. Each experiment was carried out without interruption, the various reactant solutions being successively introduced in the reactor without any feed cut. As an example, we give the experimental procedure that was used to study the effect of the DBT concentration on the PHE hydrogenation activity: In the first part, a Decalin solution containing 1 wt % of DBT and 1 wt % of PHE was introduced into the reactor. When the conversion became stable, the reactant solution was substituted for a Decalin solution containing 1 wt % of PHE and 0.5 wt % of DBT. The catalytic activity became stable within 2 h. Similarly, the feed was then replaced by a Decalin solution containing 1 wt % PHE and 0.1 wt % DBT. The same procedure was then repeated with a Decalin solution containing 1 wt % PHE and 0 wt % of DBT. Finally, the DBT concentration was progressively re-increased, using 1 wt % PHE/DBT solutions containing, respectively, 0.1, 0.5, and 1 wt % of DBT. For each DBT concentration, the reaction products were separated by a gas-liquid separator and the liquid-phase composition was determined using a Shimadzu 17A gas chromatography apparatus equipped with a FID detector and a commercial capillary column (DB-1, 60 m × 0.25 mm). The precise identification of all the reaction products was performed by GC/MS.

3. Results and Discussion 3.1. Influence of the Presence of DBT on the PHE Hydrogenation Reaction. We presented the PHE hydrogenation reaction scheme in Figure 1.25 As (23) Kabe, T.; Qian, W.; Ishihara, A. J. Catal. 1994, 149, 171-180. (24) Kabe, T.; Qian, W.; Ogawa, S.; Ishihara, A. J. Catal. 1993, 143, 239.

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Figure 1. Hydrogenation pathways of phenanthrene.

Figure 3. PHE conversion and product yields at different temperatures in the presence of 1 wt % DBT (Pt-Pd catalyst).

shown in this figure, the main products are the 9,10dihydrophenanthrene (DHP), the primary product in which one aromatic ring has been hydrogenated, the 1,2,3,4-tetrahydrophenanthrene (THP), the 1,2,3,4,5,6,7,8octahydrophenanthrene (1,8-OHP) with two aromatic rings hydrogenated and one isomeric form, the 1,2,3,4,4a,9,10,10a-octahydrophenanthrene (1,10-OHP), and the fully hydrogenated perhydrophenanthrene (PHP). Figure 2 shows the conversion and the product yield of a PHE HYD reaction performed over the Ni-Mo/ Al2O3 catalyst in the presence of 1 wt % DBT. At low temperatures, PHE was initially hydrogenated into DHP and THP, but these products became minor products with further temperature increase. Indeed, 1,8OHP then became the main product. The PHE conversion as well as the OHPs (1,8-OHP and 1,10-OHP) yields followed a volcano curve with the highest value at 320 °C for the former (100% of conversion) and 300 °C for the latter ones. A high PHE conversion was maintained between 260 and 360 °C (>90%) but further temperature increase led to a significant conversion decrease, this latter exhibiting then a value inferior to 60% for 400 °C. As mentioned before, the OHPs yields followed a similar tendency, which suggests that at high temperatures the hydrogenation/dehydrogenation equilibrium is shifted in favor of dehydrogenation. This result is in good agreement with the work of Machida et al.26

who reported recently a similar trend for the 1-methylnaphthalene hydrogenation reaction. The DBT conversion was also calculated (results not shown). In the low-temperature region, the DBT conversion followed the same tendency as the one observed for the PHE conversion, with an increase up to 90% at 280 °C. Then, unlike PHE, the 100% conversion obtained near 320 °C was maintained up to 400 °C, supposedly because the DBT HDS reaction was not affected by equilibrium effects in this temperature region. As a remark, main products obtained during DBT desulfurization were biphenyl (BP) (82%) and cyclohexylbenzene (CHB) (18%) at 280 °C. This difference of behavior between the PHE HYD reaction and the DBT HDS reaction can be explained by the fact that as this latter reaction is mainly governed by direct hydrodesulfurization (DDS) (formation of BP as the main product), it is less sensitive to the hydrogenation-dehydrogenation equilibrium than the PHE HYD reaction. The same experiment was performed over the Pt-Pd catalyst (Figure 3) that exhibited a different behavior. First, for intermediate temperatures the main product was this time the fully hydrogenated PHP, which yield followed a volcano curve with a maxima at 320 °C (more than 80%). Moreover, the PHE conversion almost reached 100% even at low temperatures, for which the main product was DHP. This suggested that the Pt-Pd catalyst could liquefy compounds such as PHE (or other poly-aromatics) at low temperature with only a low consumption of hydrogen (the DHP formation corresponds to the hydrogenation of only one PHE aromatic ring). The Pt-Pd catalyst hydrogenation properties then increased with the temperature. Indeed, for temperatures lower than 240 °C the main product was the DHP (in which one aromatic ring is hydrogenated), for 240 °C < T < 280 °C the main product was 1,8-OHP (in which two aromatic rings are hydroge-

(25) Qian, W.; Yoda, Y.; Hirai, Y.; Ishihara, A.; Kabe, T. Appl. Catal. A 1999, 184, 81-88.

(26) Machida, M.; Takeda, Y.; Hattori, H. Sekiyu Gakkaishi 1998, 41 (4), 285-292.

Figure 2. PHE conversion and product yields at different temperatures in the presence of 1 wt % DBT (Ni-Mo catalyst).


Figure 4. Effect of DBT concentration on PHE conversion and product yield (Ni-Mo, 280 °C).

nated), for 280 °C < T 400 °C, the main product was again THP. Moreover, over 320 °C, a decrease in both the conversion and the PHP yield were observed. That reflects the fact that the PHE HYD reaction is reversible,25,27-29 implying the existence of an optimal working temperature. The DBT conversion showed a behavior similar to the one observed over the Ni-Mo catalyst. Indeed, the conversion exhibited an increase up to 100% near 320 °C and then stabilized with further increase in temperature, for which PHE conversion decreased due to thermodynamic equilibriums. As observed in the case of the NiMo catalyst, the main products observed were BP and CHB. Nevertheless, unlike the NiMo catalyst that exhibited a CHB selectivity of only 20% at 240 °C, the Pt-Pd catalyst exhibited a CHB selectivity of 76% at the same temperature, confirming that this latter exhibits strong aromatics hydrogenation properties. 3.2. Effect of the DBT Concentration on the PHE Conversion and the Product Yields. In section 3.1, we studied the influence of a relatively great quantity of DBT (1 wt %) on the PHE HYD reaction. However, petroleum- or coal-derived liquefied oils are generally prehydrotreated, which means that they might contain only a low sulfur concentration. Therefore, the HYD catalysts are likely to be exposed to low sulfur concentrations and we decided to investigate the influence of low DBT concentrations on the PHE HYD reaction. Figure 4 shows the PHE HYD reaction activity at 280 °C over the Ni-Mo catalyst exposed to various DBT concentrations. A decrease in the DBT concentration from 1 wt % to 0 wt % led to an increase in the 1,8OHP yield. PHE conversion did not change significantly because the DHP yield decreased. This means that the concentration of the hydrogenation sites (on which further DHP HYD into 1,8-OHP is possible) increased when the sulfur species concentration decreased. In a second part of the experiment, the DBT concentration was then progressively re-increased. When DBT was reintroduced in the catalytic system (first at a concentration of 0.1 wt %), the PHE conversion decreased of (27) Braekman-Danheux, C.; Fontana, A.; Laurent, P.; Lolivier, P. Fuel 1996, 75, 579. (28) Yang, S.; Stock, L. M. Energy Fuels 1996, 10, 1181. (29) Dutta, R. P.; Schobert, H. H. Catal. Today 1996, 31, 65.


Figure 5. Effect of DBT concentration on DBT conversion and CHB selectivity (Ni-Mo, 280 °C).

about 10% in parallel with a significant decrease in the 1,8-OHP yield (from ∼60% to ∼40%), while the DHP yield increased slightly. This trend was accentuated by further increase in the DBT concentration. The DBT conversion observed during this experiment is reported in Figure 5. In the first part of the experiment (decrease in the DBT concentration), the DBT conversion exhibited a slight increase. Then, in the second part of the experiment, the re-increase in DBT concentration was accompanied with a progressive decrease in DBT conversion. Indeed, the DBT conversion did not recover its initial value, with for instance an activity loss of about 10% in the case 1 wt % DBT (Figure 5). According to these results, it is likely that the surface of the sulfided Mo-based catalyst was reduced during the stage when no sulfur was present in the feed (0 wt % DBT) and/or the catalytic phase was altered, i.e., a destruction of the mixed NiMoS phase by segregation occurred. Therefore, a decrease in the number and/or reactivity of the HYD and HDS active sites was responsible for a decrease in the catalytic performances. This indicated that good aromatics hydrogenation properties together with a high HDS activity can be maintained over a classical sulfided Mo-based catalyst provided a proper sulfur concentration is present in the feed. The same experimental procedure was used for the Pt-Pd catalyst. The results are presented in Figure 6. A decrease in the DBT concentration from 1 wt % to 0.5 wt % induced only a very slight increase in the PHE conversion. Moreover, a slight decrease in the DHP yield was accompanied with an increase in the OHPs yield, indicating that the HYD properties of the catalyst were slightly improved. Then, when the DBT concentration was further decreased (0.1 wt %) or in the absence of DBT, the PHE conversion reached 100% due to enhanced hydrogenation performances. Indeed, when no DBT was introduced in the feed, PHP was the main product with a yield of more than 80%. The OHPs yields also slightly increased in the presence of 0.1 wt % DBT, but further decrease in the DBT concentration up to 0 wt % led to a decrease in these yields, supposedly due to the further hydrogenation of the OHPs into PHP. That suggests that when the sulfur concentration decreases, the number of HYD-active sites participating in the aromatic rings HYD drastically increases. In a


Figure 6. Effect of DBT concentration on PHE conversion and product yield (Pt-Pd, 240 °C).

Figure 7. Effect of DBT concentration on DBT conversion and CHB selectivity (Pt-Pd, 240 °C).

second part of the experiment, the DBT concentration was re-increased. The introduction of 0.1 wt % DBT led to only a slight decrease in the PHE conversion. Nevertheless, the PHP yield exhibited a drastic decrease, clearly indicating that a part of the catalytic sites participating into the hydrogenation were hindered. As a consequence, an increase in the DHP and the 1,8-OHP yields was observed. Further DBT concentration increase up to 0.5 wt % did not give any significant change nor in conversion neither in yield but a larger increase (1 wt %) led to significant changes in the catalytic system reactivity. Indeed, the OHPs yields significantly decreased while the DHP yield exhibited a drastic increase. Unlike the Ni-Mo/Al2O3 catalyst, the Pt-Pd/ Al2O3 catalyst PHE activity and product yield returned to their initial values after returning to the initial experimental conditions (1 wt % DBT). The DBT activity and the CHB selectivity observed during the experiments performed over the Pt-Pd/Al2O3 catalyst are reported in Figure 7. Both the DBT conversion and the CHB selectivity increased when the DBT concentration decreased. Moreover, when the DBT was reintroduced, the DBT conversion and the CHB selectivity recovered almost their initial values. This result is different from the one observed on the Mo-based system. This is likely

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Figure 8. Effect of acridine concentration on PHE conversion and product yield with 1 wt % DBT(Ni-Mo, 280 °C).

to be due to the fact that in the case of the Pt-Pd catalyst, the pretreatment was a reduction under H2 atmosphere. Therefore, when the DBT feed was cut, the experimental conditions became close to the ones used during the pretreatment (i.e., reduction). That means that during the time when the DBT feed was cut, the catalyst was under favorable experimental conditions, which permitted avoidance of the deterioration of the active phase. In brief, for both catalysts, the presence of small sulfur quantities in the system is not an obstacle for the catalytic performances of catalysts working at quite high conversion rates. Indeed, while in the case of the Ni-Mo catalyst, the presence of sulfur (up to at least 1 wt % DBT) permits preservation of the active phase structure and therefore maintenance of good catalytic performances, in the case of the Pt-Pd catalyst, small quantities of sulfur in the system (DBT concentration < 0.1 wt %) only lead to a slight modification of the product yield (lower number of hydrogenated benzenic rings), the PHE conversion remaining at about 100% for a DBT concentration inferior to 0.1 wt %. Nevertheless, in the case of the PtPd catalyst, a DBT concentration superior to 0.1 wt % altered the catalytic properties, which was not the case for the NiMo catalyst that exhibited only a slight loss in activity. Therefore, when working at high conversion rates this suggested that in order to preserve good catalytic performances over the PtPd catalyst, it is necessary to work at relatively low sulfur concentrations when compared to the ones tolerated by the NiMo catalyst. 3.3. Effect of the Acridine Concentration on the PHE Conversion and the Product Yields in the Presence of 1 wt % DBT. To investigate the effect of the addition of nitrogen-containing compounds on the activity, the PHE hydrogenation/DBT HDS was carried out in the presence of acridine. The results obtained for the Ni-Mo catalyst are shown in Figure 8. When 0.1 wt % ACR was introduced in the feed, the PHE conversion as well as the OHPs yield drastically decreased. Further increase in the ACR concentration (0.5 wt %) accentuated this phenomenon. After returning the ACR concentration to 0 wt %, the HYD activity recovered only about 70% of its initial value, indicating that the active phase was irreversibly damaged during the ACR intro-



Figure 10. Effect of acridine concentration on PHE conversion and product yield with 1 wt % DBT (Pt-Pd, 240 °C). Figure 9. Effect of acridine concentration on DBT conversion and CHB yield with 1 wt % DBT (Ni-Mo, 280 °C).

duction. In fact, two types of states can be observed during the adsorption of nitrogen compounds on catalysts: the transitory one and the permanent one.30 The activity of the transitory poisoned sites can be gradually recovered when the nitrogen source is cut, while the activity of the permanently poisoned sites cannot. That means that on the Ni-Mo catalysts, at least a part of the site poisoning is of the permanent type, indicating that the active phase is irreversibly modified when it comes into contact with nitrogen compounds. Moreover, the DBT conversion was also followed during the experiment (Figure 9). The influence of the ACR addition on the DBT conversion was different from that observed on the PHE HYD conversion. Indeed, the addition of ACR only slightly decreased the DBT conversion. Further, cutting the ACR feed allowed almost full recovery of the initial DBT conversion. The CHB yield reached a value close to 0% upon ACR introduction, indicating that the hydrogenating sites on the surface of the catalyst were completely poisoned. The CHB yield decrease was more pronounced than the DBT HDS conversion decrease. Thus, we examined the literature to explain this difference. It has been widely reported that the retarding effects due to the introduction of nitrogen compounds are different for the DBT hydrogenation reaction and the DBT hydrogenolysis reaction. For instance, Nagai et al. reported that the DBT HDS reaction is catalyzed by two types of active sites present together on the surface of sulfided Mo/Al2O3 catalysts: the first one is active for the hydrogenation reaction and very sensitive to the poisoning by nitrogen bases, while the second one is active for the desulfurization reaction (C-S bond hydrogenolysis) and less sensitive to the ACR poisoning.31-33 They also reported about the effect of the acridine introduction on the DBT HDS reaction activity over sulfided Ni-Mo/ Al2O3 and Co-Mo/Al2O3 catalysts. The presence of ACR markedly promoted the BP formation over the sulfided (30) Guenin, M.; Breysse, M.; Frety, R.; Tifouti, K.; Marecot, P.; Barbier, J. J. Catal. 1987, 105, 144-154. (31) Nagai, M. Bull. Chem. Soc. Jpn. 1991, 64 (10), 3210. (32) Nagai, M.; Sato, T.; Aiba, A. J. Catal. 1986, 97, 52. (33) Nagai, M.; Kabe, T. J. Catal. 1983, 81, 440-449.

Ni-Mo/Al2O3 catalyst, but only a slight decrease in the formed CHB quantity was observed. This indicated clearly that the hydrogenation reaction and the desulfurization reaction take place over different active sites, which is in very good agreement with our results. Indeed, the results reported in the present study show also clearly that the ACR addition selectively blocked the sites responsible for the aromatic ring hydrogenation (selective inhibition of the CHB formation). This behavior difference observed between the PHE conversion and the CHB yield after poisoning shows that transitory and permanent poisoning phenomena occurred on the same catalyst, due to the presence of two distinct types of catalytic sites. The effect of the introduction of ACR on the catalytic system performances was also studied in the case of the Pd-Pt/Al2O3 catalyst (Figure 10). The PHE conversion progressively decreased when increasing the ACR concentration. For only 0.1 wt % ACR, the PHE HYD conversion lost more than 20%. Only a very small ACR concentration (0.01 wt %) did not lead to a significant modification of the system reactivity. Indeed, when the ACR concentration was changed from 0.01 wt % to 0.05 wt %, the PHE conversion, the OHPs and the PHP yields drastically decreasedsunlike the DHP yield that increased significantly. The inverse procedure, i.e., a progressive decrease in the ACR concentration, was then performed. Then, in the second part of the experiment, we observed the effect of the progressive decrease in ACR concentration after poisoning. When the ACR concentration returned to 0 wt %, the observed PHE conversion recovered about 95% of its initial value. However, the PHP yield did not re-increase up to its initial value, unlike the DHP yield that kept the high value observed when ACR was present in the feed. During this experiment, the DBT conversion and the CHB yield were also followed (Figure 11). Unlike the observation when using the Ni-Mo/Al2O3 catalyst, the ACR addition markedly affected the reactivity of the DBT reaction over the Pd-Pt/Al2O3 catalyst. Indeed, the DBT conversion decreased drastically after the ACR addition, in parallel with the CHB yield. This can be explained by the fact that while the DBT HDS activity of the Ni-Mo/Al2O3 catalyst depends mainly on DDS (the main product is BP), the Pt-Pd/ Al2O3 catalyst DBT HDS activity is strongly dependent on the hydrogenat-


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Figure 13. Effect of DBT concentration on PHE conversion and product yield with 0.1 wt % acridine (Pt-Pd, 240 °C). Figure 11. Effect of acridine concentration on DBT conversion and CHB yield with 1 wt % DBT (Pt-Pd, 240 °C).

Figure 12. Effect of DBT concentration on PHE conversion and product yield with 0.1 wt % acridine (Ni-Mo, 280 °C).

ing properties of the catalyst, which exhibits a high CHB selectivity (main mechanism is not DDS but the hydrogenation of one aromatic ring prior to the hydrogenolysis of the C-S bond). As we pointed out before, ACR selectively poisons hydrogenation sites during the DBT HDS reaction. Therefore the HDS reaction over the Pt-Pd catalyst is greatly hindered because the activity of this catalyst is dependent on its hydrogenation performances. 3.4. Effect of the DBT Concentration on the PHE Conversion and the Product Yields in the Presence of 0.1 wt % of ACR. The effect of the DBT concentration on the PHE conversion and the product yields in the presence of 0.1 wt % ACR was investigated. The results in the case of the Ni-Mo catalyst are presented in Figure 12. First, DBT and PHE were introduced into the reactor; then, when the system became stable, ACR was introduced. We observed that the 1,8-OHP yield and the PHE conversion significantly decreased when ACR was added. Further, we decreased the DBT concentration. Both the main product (1,8OHP) yield and the PHE conversion increased. Furthermore, after an experimental step in which the DBT concentration was 0 wt % (no sulfur in the feed), the 1 wt % DBT-containing feed was introduced again in the

reactor. We observed a drastic decrease in the 1,8-OHP yield as well as in the PHE conversion. As shown in Figure 4, the conversion of PHE was slightly affected by the DBT concentration variation on the Ni-Mo/Al2O3 catalyst in the absence of acridine. The present results further showed that on the Ni-Mo catalyst, it is possible to increase activity by decreasing the sulfur concentration when ACR is present in the system. The results obtained for the Pt-Pd catalyst are reported in Figure 13. Both the 1,8-OHP and the PHP yields decreased considerably with the addition of ACR, while the DHP and the THP yields increased. Further, when the DBT concentration was decreased, only the DHP yield increased. Then, when DBT was reintroduced in the feed, only a part of the initial PHE activity was recovered, while all the product yields remained at values close to 0% except for the DHP. From these results, it was deduced that the HYD sites present on the surface of the Pt-Pd catalyst were permanently contaminated by ACR. 4. Conclusions 1. The PHE HYD reaction main products observed near 280-300 °C were the 1,8-OHP in the case of the Ni-Mo/Al2O3 catalyst and the PHP (fully hydrogenated product) in the case of the Pt-Pd/Al2O3 catalyst, reflecting the excellent hydrogenation properties of this latter. 2. The PHE hydrogenation over the Ni-Mo/Al2O3 catalyst in the presence of DBT slightly increased when the DBT concentration decreased. Furthermore, the progressive reintroduction of DBT in the feed after a reaction performed in the absence of DBT led to a significant decrease in the PHE HYD activity as well as in the DBT conversion. Indeed, both the PHE HYD reaction conversion and the DBT conversion exhibited values inferior to the initial ones when DBT was reintroduced in the feed. The PHE conversion over the Pt-Pd/Al2O3 catalyst increased when decreasing the DBT concentration from 1 wt % to 0 wt % and, unlike in the case of the Ni-Mo/Al2O3, returned to its initial value when DBT was reintroduced in the feed. The DBT conversion followed the same trend. It was deduced that in the case of the NiMo catalyst, the presence of sulfur in the feed is needed to preserve good catalytic perfor-


mances. Indeed, when the sulfur feed was cut, it is likely that the mixed NiMoS active phase was irreversibly deteriorated by segregation/reduction phenomena, leading to a decrease in the conversion. In the case of the Pt-Pd/Al2O3 catalyst, while the absence of sulfur in the system was favorable to regenerate the activity of the active phase, the presence of DBT in the feed clearly hindered the hydrogenation properties of the catalyst, leading to a decrease in the PHE conversion. 3. On the Ni-Mo/Al2O3 catalyst, a drastic decrease in PHE conversion was observed when ACR was introduced in the system, while in the case of the Pt-Pd/ Al2O3 catalyst, the decrease in PHE conversion was more moderate. However, in the same time, contrary to the NiMo catalyst, the Pt-Pd/Al2O3 catalyst exhibited a drastic decrease in DBT conversion as well as in the CHB yield. Moreover, on both catalysts, the activity was not fully recovered after ACR was removed from the feed, indicating that a part of the catalytic sites were permanently poisoned.


4. The Ni-Mo/Al2O3 catalyst exhibited an increase in activity when the sulfur concentration was decreased in the presence of a constant quantity of acridine in the feed (0.1 wt %). The same experiment performed over the Pt-Pd/Al2O3 showed that the catalyst activity increased when the DBT concentration decreased. In this case DHP was the main product, exhibiting a yield significantly higher that the one of all the other products. That suggests that the HYD properties were hindered by the introduction of acridine. Acknowledgment. This work was partly performed as “Study on the factors influencing the materials of the coal liquefaction process” received by the Japan Institute of Energy. The authors also thank the Japan Society for the Promotion of Science (JSPS) for its financial contribution. EF020283B

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