Hydrodenitrogenation of Quinoline Catalyzed by MCM-41-Supported

Feb 27, 2007 - Jonathan L. Wagner , Emyr Jones , Asel Sartbaeva , Sean A. Davis , Laura Torrente-Murciano , Christopher J. Chuck , Valeska P. Ting...
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Energy & Fuels 2007, 21, 554-560

Hydrodenitrogenation of Quinoline Catalyzed by MCM-41-Supported Nickel Phosphides Mohong Lu,† Anjie Wang,*,†,‡ Xiang Li,†,‡ Xinping Duan,† Yang Teng,† Yao Wang,†,‡ Chunshan Song,§ and Yongkang Hu†,‡ State Key Laboratory of Fine Chemicals, Dalian UniVersity of Technology, 158 Zhongshan Road, Dalian 116012, P. R. China, Liaoning Key Laboratory of Petrochemical Technology and Equipments, Dalian UniVersity of Technology, Dalian 116012, P. R. China, and Department of Energy and Geo-EnVironmental Engineering, The Energy Institute, The PennsylVania State UniVersity, 209 Academic Projects Building, UniVersity Park, PennsylVania 16802 ReceiVed September 18, 2006. ReVised Manuscript ReceiVed January 16, 2007

A series of MCM-41-supported nickel phosphides with an initial Ni/P atomic ratio of 0.5-2 in the oxidic precursors were prepared by an in situ reduction method and characterized by X-ray diffraction (XRD), CO chemisorption, N2 adsorption, and transmission electron microscopy. Their catalytic performances were evaluated in the hydrodenitrogenation (HDN) of quinoline and compared with MCM-41-supported Ni-Mo sulfide. The supported nickel phosphides with initial Ni/P ratios of 1 or 1.25 exhibited much higher HDN activity than the supported Ni-Mo sulfide. XRD patterns of both high-performance phosphide catalysts revealed that the active phase was Ni2P. It is indicated that the HDN of quinoline on the MCM-41-supported nickel phosphides exclusively proceeds via a pathway, which involves fully saturated intermediates. The cleavage of the C-N bond in the decahydroquinoline is the rate-determining step in the HDN of quinoline on the supported nickel phosphides. In addition, the effects of H2S (CS2 as the precursor) on HDN and the performances of the prepared nickel phosphide catalysts in the simultaneous HDN of quinoline and hydrodesulfurization (HDS) of dibenzothiophene (DBT) were investigated. The presence of H2S dramatically reduced the hydrogenation of 1,2,3,4-tetrahydroquinoline to decahydroquinoline, altering unfavorably the reaction pathways involved in the HDN of quinoline. The simultaneous HDN and HDS indicated that the HDN activity of Ni-Mo sulfide was hardly affected in the presence of DBT. Whereas, the supported nickel phosphide was sensitive to the presence of DBT at low temperatures. It is favorable to perform HDN at high temperatures because the inhibiting effects of H2S and DBT on HDN were dramatically reduced at elevated temperatures.

1. Introduction The effective removal of nitrogen from middle distillates in conjunction with the deep removal of sulfur has become increasingly important.1-3 Traditionally, sulfur and nitrogen are removed simultaneously by means of the hydrodesulfurization (HDS) reaction and hydrodenitrogenation (HDN) reaction in a process called hydrotreating in a modern refinery. The catalysts for this process are mainly alumina-supported Mo or W sulfides promoted by Ni or Co. Though the supported metal sulfides exhibited substantially high activities for HDS, their HDN activities are generally quite low.1 Transition metal phosphides have recently been reported to be superior to the sulfides in both HDN and HDS.4-6 Among * Corresponding author tel.: +86-411-88993693; fax: +86-41188993693; e-mail: [email protected]. † State Key Laboratory of Fine Chemicals, Dalian University of Technology. ‡ Liaoning Key Laboratory of Petrochemical Technology and Equipments, Dalian University of Technology. § The Pennsylvania State University. (1) Kabe, T.; Ishihara, A.; Qian, W. Hydrodesulfurization and Hydrodenitrogenation: Chemistry and Engineering; Wiley-VCH: Weinheim, Germany, 1999. (2) Song, C. Catal. Today 2003, 86, 211-263. (3) Song, C.; Reddy, K. M. Appl. Catal., A 1999, 176, 1-10. (4) Oyama, S. T. J. Catal. 2003, 216, 343-352. (5) Stinner, C.; Prins, R.; Weber, Th. J. Catal. 2000, 191, 438-444. (6) Phillips, D. C.; Sawhill, S. J.; Self, R.; Bussell, M. E. J. Catal. 2002, 207, 266-273.

the phosphides investigated, nickel phosphides show excellent performance in HDN and HDS.4,7 Recently, our group found that siliceous MCM-41-supported nickel phosphides showed excellent performance in the HDS of dibenzothiophene (DBT), a refractory sulfur-containing compound in middle distillates. And the in situ reduction method not only simplifies the reduction operation but also improves the HDS activity of the prepared nickel phosphides.8 In the present study, we will investigate the performance of this series of catalysts in the HDN of quinoline. We choose quinoline as the model nitrogencontaining component because it is representative of the heterocyclic nitrogen compounds found in significant concentrations in the middle distillates. Moreover, quinoline contains both a heterocyclic ring and a benzene ring, so the reactions involved in quinoline HDN are representative of those in an industrial HDN process.9 2. Experimental 2.1. Catalyst Preparation. Siliceous MCM-41 was synthesized by using sodium silicate as the silica source and cetyltrimethylammonium bromide as the template, following the procedure in the (7) Oyama, S. T.; Wang, X.; Lee, Y.-K.; Bando, K.; Requejo, F. G. J. Catal. 2002, 210, 207-217. (8) Wang, A.; Ruan, L.; Teng, Y.; Li, X.; Lu, M.; Ren, J.; Wang, Y.; Hu, Y. J. Catal. 2005, 229, 314-321. (9) Jian, M.; Prins, R. J. Catal. 1998, 179, 18-27.

10.1021/ef060467g CCC: $37.00 © 2007 American Chemical Society Published on Web 02/27/2007

Hydrodenitrogenation of Quinoline

Energy & Fuels, Vol. 21, No. 2, 2007 555 Scheme 1. Reaction Network of Quinoline HDNa

a Q, quinoline; THQ1, 1,2,3,4-tetrahydroquinoline; OPA, ortho-propylaniline; PB, propylbenzene; THQ5, 5,6,7,8-tetrahydroquinoline; DHQ, decahydroquinoline; PCHA, 2-propylcyclohexylamine; PCHE, propylcyclohexene; PCH, propylcyclohexane.9

literature.10 The synthesized siliceous MCM-41 has a specific surface area of 938 m2 g-1, a pore volume of 1.07 cm3 g-1, and a Barrett-Joyner-Halenda average pore size of 3.8 nm. Nickel phosphides supported on MCM-41 were prepared according to a procedure adapted from the literature.11,12 In a typical preparation, 1.2 g of (NH4)2HPO4 [analytical reagent (A.R.) grade] was dissolved in 20 mL of deionized water to form a clear solution, and then 3.3 g of Ni(NO3)2‚6H2O (A.R. grade) was added in the solution to form a cloudy mixture. By adjusting the pH to 2-3 with 0.5 M HNO3 (A.R. grade), the cloudy mixture turned to a clear solution. A total of 3 g of siliceous MCM-41 was wet-impregnated with the preprepared solution for 8 h, followed by the evaporation of water and drying at 120 °C for 10 h. The obtained material was calcined in air at 570 °C for 3 h to give the oxidic catalyst precursor. The sum of the loadings of NiO and P2O5 in the oxidic precursor was 30 wt %, and the atomic ratio of Ni to P was 0.5, 1.0, 1.25, and 2.0, respectively. The MCM-41-supported nickel phosphides are denoted as Ni-P(x), where x represents the Ni/P atomic ratio in the oxidic precursors. For comparison, MCM-41-supported Ni-Mo sulfides were also investigated. Their oxidic precursors were prepared by a wet coimpregnation method. The preparation methods were described in detail elsewhere.13-15 The loading of MoO3 was 20 wt %, and the atomic ratio of Ni to Mo was 0.5. 2.2. HDN Activity Measurement. Prior to HDN reaction, either a phosphide catalyst precursor or sulfide catalyst precursor must be transformed into the active phases. For phosphide catalysts, an in situ temperature-programmed H2 reduction method was used.8 The oxidic precursor was pelleted, crushed, and sieved to 20-30 meshes. A total of 0.2 g of oxidic precursor was used for each run. The precursor was reduced in a 200 mL min-1 H2 flow by heating quickly from room temperature to 120 °C and keeping this temperature for 1 h, then to 400 °C at 5 °C min-1 and holding at 400 °C for 1 h, and finally heating at 2.5 °C min-1 to 500 °C and holding at 500 °C for 3 h. For sulfide catalysts, the precursor was sulfided at 400 °C for 3 h with a mixture of H2S in H2 (10 vol % H2S). After the precursor had been converted into nickel phosphide by reduction or metal sulfides by sulfidation, the reactor was cooled to the HDN reaction temperature. The HDN activities of the catalysts were investigated using a solution of 1 wt % quinoline (10) Wang, A.; Kabe, T. Chem. Commun. 1999, 2067-2068. (11) Wang, X.; Clark, P.; Oyama, S. T. J. Catal. 2002, 208, 321-331. (12) Stinner, C.; Tang, Z.; Haouas, M.; Weber, Th.; Prins, R. J. Catal. 2002, 208, 456-466. (13) Wang, A.; Wang, Y.; Kabe, T.; Chen, Y.; Ishihara, A.; Qian, W. J. Catal. 2001, 199, 19-29. (14) Wang, A.; Wang, Y.; Kabe, T.; Chen, Y.; Ishihara, A.; Qian, W.; Yao, P. J. Catal. 2002, 210, 319-327. (15) Wang, A.; Li, X.; Chen, Y.; Han, D.; Wang, Y.; Hu, Y.; Kabe, T. Chem. Lett. 2001, 30, 474-475.

(A.R. grade) in decalin (A.R. grade) as a model fuel. The HDN reaction conditions were as follows: temperature 300-360 °C, total pressure 5 MPa, weight hourly space velocity 24 h-1, and H2 flow rate 150 mL min-1 (at atmospheric pressure). Sampling of liquid HDN products was started 6 h after the steady reaction condition had been achieved. For each run, liquid samples were collected at an interval of 20 min and monitored by means of a gas chromatography analysis of the product composition. The compositions of both the feed and the HDN liquid products were analyzed by an Agilent-6890N gas chromatograph equipped with a flame ionization detector using a commercial HP-5 column. The HDN reaction network is more complicated than the HDS reaction network, because the heterocyclic ring has to be hydrogenated prior to the cleavage of the carbon-nitrogen bond. The reason is that the dissociation energy of double bonds in the heterocyclic rings is approximately twice that of the single bond in the hydrogenated heterocyclic rings.1 Quinoline is a typical nitrogen-containing compound in middle distillates, and its reaction network is shown in Scheme 1.9 Since there are several nitrogencontaining intermediates in the HDN of quinoline, such as 1,2,3,4tetrahydroquinoline (THQ1), 5,6,7,8-tetrahydroquinoline (THQ5), ortho-propylaniline (OPA), and decahydroquinoline (DHQ), quinoline conversion cannot be used to represent the efficiency of HDN. Therefore, HDN conversion is defined to measure the HDN efficiency: HDN conversion )

CQ0 - CQR - CNC × 100 CQ0

where CQ0 represents the quinoline concentration in the feed, CQR is the quinoline concentration in the HDN liquid product, and CNC describes the sum of the concentrations of all the nitrogencontaining intermediates in the HDN liquid product, all in micromoles per milliliter. To investigate the effect of H2S on HDN activity and the performance of the catalyst in simultaneous HDS and HDN, carbon disulfide or DBT was added in the feed. Carbon disulfide was used as a precursor of H2S,16 and the feed was a decalin solution containing 0.5 wt % carbon disulfide and 1 wt % quinoline. In the study of simultaneous HDS and HDN, the model fuel contained 0.5 wt % DBT and 0.5 wt % quinoline. 2.3. Catalyst Characterization. The nickel phosphides for characterization were prepared by reducing the precursors of nickel phosphides in a H2 flow according to the same reduction conditions as in the in situ reduction, followed by passivation with 0.5% O2 in Ar. X-ray diffraction (XRD) patterns of the supported nickel phosphides were measured to determine the crystal phase on the surface. (16) Yang, S.; Satterfield, C. N. J. Catal. 1983, 81, 168-178.

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Figure 1. XRD patterns of MCM-41-supported nickel phosphides with various initial Ni/P ratios, compared with bulk Ni2P. Table 1. Physical Characteristics of MCM-41-Supported Phosphides and Sulfide catalysts

BET surface area (m2 g-1)

CO uptake (µmol g-1)

pore volume (mL g-1)

Ni-P(0.5)/MCM-41 Ni-P(1.0)/MCM-41 Ni-P(1.25)/MCM-41 Ni-P(2.0)/MCM-41 sulfided Ni-Mo

459 564 587 642 560

14 19 21 28 36

0.469 0.552 0.523 0.610 0.431

For comparison, a bulk Ni2P was prepared according to the procedure reported in the literature.17 The XRD patterns were measured on a Rigaku D/Max 2400 diffractometer using nickelfiltered Cu KR radiation at 40 kV and 100 mA. The Brunauer-Emmett-Teller (BET) surface areas were measured on an ASAP 2400 system by nitrogen adsorption. CO chemisorption measurements were obtained on a ChemBET3000 instrument. All catalyst samples (0.10 g) were degassed in a 60 mL min-1 He flow at room temperature for 30 min and were then either reduced or sulfided prior to the chemisorption measurement. The passivated sample was reduced at 500 °C for 2 h in a 60 mL min-1 flow of H2. The sulfided sample was sulfided at 400 °C for 2 h in a 60 mL min-1 flow of 10% vol H2S/H2. All samples were flushed by He at room temperature for 2 h prior to the CO chemisorption measurement at the same temperature. Transmission electron microscope (TEM) images were obtained on a JEM-100CX II model operating at 180 kV. The sample was placed on a 200-mesh copper grid coated with carbon.

3. Results and Discussion 3.1. Catalyst Characterization. The XRD patterns of the supported nickel phosphides, together a bulky Ni2P, are shown in Figure 1. Typical diffraction peaks of Ni2P [powder diffraction file (PDF) 3-953] were observed when the atomic ratios of Ni to P were below 1.25. However, a mixture of Ni2P and Ni12P5 (PDF 22-1190) was present on the surface of MCM-41 when the Ni/P atomic ratio was 2.0. This is in good agreement with the results reported by Oyama et al.7 The low intensity and broadening of the peaks indicate that on supported catalysts Ni2P is in fine particles. The BET surface areas and the CO chemisorption capacities are present in Table 1. When the loading of NiO and P2O5 was kept constant, the BET surface areas of phosphides decreased with increasing P content, and the pore volume changed in a similar way. To produce a pure Ni2P phase, excess P is necessary because P is volatile at elevated temperatures during the reduction.7 In addition, according to Sawhill et al.,18 some P (17) Gopalakrishnan, J.; Pandey, S.; Rangan, K. K. Chem. Mater. 1997, 9, 2113-2116. (18) Sawhill, S. J.; Layman, K. A.; Van Wyk, D. R.; Engelhard, M. H.; Wang, C.; Bussell, M. E. J. Catal. 2005, 231, 300-313.

Figure 2. TEM image of Ni-P(1.25).

Figure 3. HDN conversion as a function of temperature in quinoline HDN over MCM-41-supported nickel phosphides with various initial Ni/P ratios.

remains in the form of PxOy after temperature-programmed reduction. The PxOy particles on the catalyst surface may block the access to adsorption sites, resulting in the decreases of both the BET surface area and pore volume while increasing the P content. The CO chemisorption capacities of phosphides increased with the increase of the Ni/P atomic ratio. The trend observed might be due to the increase of Ni content with the increase of the Ni/P atomic ratio, necessary to keep the sum of the loadings of NiO and P2O5 constant. And they were lower than that of sulfided Ni-Mo (0.5). A TEM image of the Ni-P(1.25)/MCM-41 catalyst is shown in Figure 2. The TEM image shows that Ni2P particles with a fairly uniform size are homogeneously dispersed on MCM-41. The particle size is about 4 nm in diameter. 3.2. Quinoline HDN. Figure 3 shows the HDN conversion in quinoline HDN catalyzed by MCM-41-supported nickel phosphides with various initial Ni/P atomic ratios as a function of the reaction temperature. It is indicated that all of the catalysts could completely denitrogenize quinoline when the reaction temperature approached 360 °C, showing substantially high HDN activities. Among them, Ni-P(1) and Ni-P(1.25) exhibited the highest HDN activities. Our previous study on the HDS of DBT showed that the crystal phase of the MCM-41-supported nickel phosphide prepared from Ni-P(1.25) by the in situ reduction method was Ni2P, and this phase exhibited the highest HDS activity.8 Oyama et al.7,19 investigated the activities of silica-supported nickel phosphides with various initial Ni/P ratios in the simultaneous HDS of DBT and HDN of quinoline at 370

Hydrodenitrogenation of Quinoline

Energy & Fuels, Vol. 21, No. 2, 2007 557

Figure 4. Liquid product composition as a function of temperature in quinoline HDN over MCM-41-supported nickel phosphides with various initial Ni/P ratios. (9) quinoline; (b) THQ1; (2) THQ5; (1) DHQ; ([) OPA; (0) PCH; (O) PB.

°C. The catalyst characterization revealed that the active phase for both HDS and HDN is Ni2P. They also found that HDS was structure-insensitive while HDN was structure-sensitive. Therefore, it can be concluded that Ni2P is responsible for the high performance in HDN and in HDS. The quinoline HDN reaction network is complicated because the heterocyclic ring has to be hydrogenated prior to the cleavage of the carbon-nitrogen bond, as shown in Scheme 1. The main products and the detectable intermediates for the HDN of quinoline on MCM-41-supported nickel phoshpides mainly include propylcyclohexane (PCH), propylcyclohexene (PCHE), propylbenzene (PB), THQ1, THQ5, OPA, and DHQ. The absence of o-propylcyclohexylamine (PCHA) suggests that PCHA might be readily converted into hydrocarbons under the reaction conditions. Figure 4 illustrates the variation of liquid product compositions in the HDN of quinoline with the reaction temperature for the MCM-41-supported nickel phoshpide catalysts. It is clear that the predominant HDN hydrocarbon product was PCH when the HDN conversion reached 100% for all the catalysts. Oyama et al.19 also reported that PCH is favored at high HDN conversion on the supported nickel phosphides. At low temperatures, at which low HDN conversions were obtained, the main intermediates are THQ1 and DHQ for NiP(0.5) and Ni-P(2.0), whereas the predominant intermediate is DHQ for Ni-P(1.0) and Ni-P(1.25). This suggests that the HDN of quinoline on the MCM-41-supported nickel phosphides exclusively takes the route through the fully saturated intermediates. (19) Oyama, S. T.; Wang, X.; Lee, Y.-K.; Chun, W.-J. J. Catal. 2004, 221, 263-273.

As shown in Scheme 1, the hydrogenation of the nitrogencontaining ring in the quinoline structure is a prerequisite step in HDN. Prior to C-N bond cleavage, quinoline has to be hydrogenated to THQ1 or DHQ. The hydrogenation of the N-containing ring is easily achieved, and a thermodynamic equilibrium between quinoline and THQ1 is readily established under typical HDN conditions.20 In order to elucidate the reaction mechanism of quinoline HDN, Jian and Prins9 investigated the HDN of DHQ on a sulfided NiMo(P)/Al2O3 catalyst. They found that quinoline, THQ1, and THQ5 were detected in the product of DHQ HDN. It is therefore suggested that there exists a thermodynamic equilibrium among quinoline, THQ1, DHQ, and THQ5. In the reaction network of quinoline HDN (Scheme 1), there are two irreversible reactions linked to THQ1 and DHQ. These two reactions are the denitrogenation steps in a real sense, in which the C-N bond is cleaved to form aromatic or aliphatic amines (OPA or PCHA). The formed OPA and PCHA are easily transformed into hydrocarbons and ammonia. Therefore, a high-performance HDN should have high activity in breaking the hydrogenated heterocyclic ring. The relative rate of each elementary reaction depends on the catalysts and reaction conditions, and the relative rates of the two hydrogenolysis reactions can influence the equilibrium state. This may be the reason why the results of HDN product distributions are quite different from each other in the literature and the distributions are affected greatly by the reaction temperature and by the presence of H2S. It is known that the reaction of OPA is greatly inhibited by quinoline and THQ1 (the precursors of OPA).20,21 Therefore, (20) Perot, G. Catal. Today 1991, 10, 447-472.

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Lu et al. Table 2. HDN Activity of Quinoline at 340 °C on MCM-41-Supported Phosphides and Sulfide

a

Figure 5. Plots of DHQ selectivity (solid symbols) and 100 - DHQ selectivity (open symbols) against reaction temperature in quinoline HDN over MCM-41-supported nickel phosphides with various initial Ni/P ratios. (9, 0) Ni/P ) 0.5; (b, O) Ni/P ) 1; (2, 4) Ni/P ) 1.25; (1, 3) Ni/P ) 2.

Figure 6. Variations of quinoline HDN conversion with temperature on MCM-41-supported Ni-P(1.25) (9) and Ni-Mo sulfide (b).

hydrodenitrogenation via the THQ1 f OPA f hydrocarbon pathway is intrinsically unfavorable due to the self-inhibiting effect. On the other hand, no self-inhibiting effect on the THQ5/ THQ1 f DHQ f PCH pathway has been observed. Ni2P is known to be active in the hydrogenation of alkenes, alkynes, dienes, and nitro compounds.22,23 It is assumed that, on Ni2P catalysts with high hydrogenation activity, THQ1 might be easily hydrogenated to yield DHQ, instead of being transformed to OPA and then denitrogenized. DHQ selectivity and the difference 100 - DHQ selectivity are plotted against the reaction temperature for each nickel phosphide catalyst in Figure 5. When comparing the curves of 100 - DHQ selectivity with HDN conversion as a function of the temperature (see Figure 3), it is interesting to note that 100 - DHQ selectivity tracks HDN conversion well for each catalyst. Since PCHA is easily transformed to hydrocarbons (PCHA was not detected in the HDN product in this study), it is reasonable to assume that the breaking of the N-containing ring in DHQ is the rate-determining step in the HDN of quinoline over the supported nickel phosphides. Figure 6 compares the Ni-P(1.25) catalyst with sulfided NiMo(0.5) in HDN activity as a function of the temperature. It is clear that Ni-P(1.25) exhibited much higher activity in the HDN of quinoline than the sulfided Ni-Mo(0.5). The HDN conversion approached 100% at 360 °C for the Ni-P(1.25), whereas (21) Ho, T. C. Catal. ReV. Sci. Eng. 1988, 30, 117-160. (22) Muetterties, E. L.; Sauer, J. C. J. Am. Chem. Soc. 1974, 96, 34103415. (23) Wang, W.; Qiao, M.; Li, H.; Deng, J. Appl. Catal., A. 1998, 166, L243-L247.

catalysts

HDN activity (nmol Q g-1 s-1)

TOFa (s-1)

Ni-P(0.5)/MCM-41 Ni-P(1.0)/MCM-41 Ni-P(1.25)/MCM-41 Ni-P(2.0)/MCM-41 sulfided Ni-Mo

345.0 540.4 546.2 431.2 230.0

0.049 0.057 0.052 0.031 0.013

Stoichiometry of CO/active site is assumed to be 1 for all samples.

only 60% was obtained for the sulfided Ni-Mo(0.5). The HDN activities of quinoline at 340 °C for both phosphides and sulfided Ni-Mo (0.5) are presented in Table 2. The turnover frequencies (TOFs) were calculated using chemisorption capacities of the catalysts as a measurement of active sites, and the stoichiometry of the CO/active site was assumed to be 1 for all catalysts. It is shown that phosphide catalysts have higher HDN activity than that of sulfided Ni-Mo (0.5). Among all phosphide catalysts, Ni-P(1.0) and Ni-P(1.25) exhibit the highest HDN activity. It also indicates that Ni-P(1.25) is twice as active in the HDN of quinoline than sulfided Ni-Mo(0.5) on a mass base, and the turnover frequency of Ni-P(1.25) is 4 times higher than that of sulfided Ni-Mo(0.5). The product compositions in the HDN of quinoline catalyzed by Ni-P(1.25) and the sulfided Ni-Mo(0.5) as a function of the reaction temperature are illustrated in Figure 7. It is shown that THQ1 and DHQ were the primary products of quinoline HDN on the sulfided Ni-Mo(0.5). This suggests that the N-containing heterocyclic ring can be easily hydrogenated on the sulfide catalyst. With the increase of reaction temperature, the concentrations of both THQ1 and DHQ decreased significantly, accompanied by an increase of hydrocarbon (PCH and PB). It is evident that the reaction rate of nitrogen removal from the hydrogenated heterocyclic ring is sensitive to the reaction temperature. In contrast to the sulfide catalyst, only DHQ is the main intermediate, and PCH predominates in the hydrocarbon products in quinoline HDN catalyzed by Ni-P(1.25). This suggests that the HDN of quinoline catalyzed by MCM-41-supported nickel phosphide exclusively proceeds via the pathway which involves the fully saturated intermediate (Q f DHQ f hydrocarbons). It implies that the supported nickel phosphide catalyst possesses higher hydrogenation activity and a higher ability of breaking C-N bonds than the supported sulfide catalyst. 3.3. Effect of Sulfur-Containing Compounds on the HDN of Quinoline. In a typical hydrotreating process, HDN and HDS take place simultaneously, and H2S is always present in the reactor as the HDS product. Therefore, the effect of H2S on HDN activity was investigated. In the present study, CS2 was used to generate H2S at the upper layer of the catalyst bed. Figure 8 shows HDN conversion as a function of the temperature on Ni-P(1.25) in the presence and absence of H2S. It is clear that the HDN conversion was dramatically decreased in the presence of H2S, indicating that H2S has an inhibiting effect on HDN catalyzed by the nickel phosphides. The product composition of quinoline HDN in the presence of H2S as a function of temperature is shown in Figure 9. It is shown that THQ1 predominated in the product at low temperatures, in distinct contrast to the case without H2S (Figure 4). Moreover, the main hydrocarbon products were PCH and PB in the presence of H2S, while the predominant hydrocarbon product was only PCH in the absence of H2S. It is therefore suggested that the presence of H2S has a negative effect on the HDN of

Hydrodenitrogenation of Quinoline

Energy & Fuels, Vol. 21, No. 2, 2007 559

Figure 7. Liquid product composition as a function of temperature in quinoline HDN on MCM-41-supported Ni-P(1.25) (a) and Ni-Mo sulfide (b). (9) quinoline; (b) THQ1; (2) THQ5; (1) DHQ; ([) OPA; (0) PCH; (O) PB.

Figure 8. HDN conversion as a function of temperature in quinoline HDN catalyzed by Ni-P(1.25) in the absence (9) and presence (b) of H2S (CS2 as the precursor).

Figure 10. Variation of HDN conversion with temperature in simultaneous HDN of quinoline and HDS of DBT over Ni-P(1.25) (9) and Ni-Mo sulfide (b).

Figure 9. Liquid product composition as a function of temperature in quinoline HDN catalyzed by Ni-P(1.25) in the presence of H2S (CS2 as the precursor). (9) quinoline; (b) THQ1; (2) THQ5; (1) DHQ; ([) OPA; (0) PCH; (O) PB.

Figure 11. Variation of DBT conversion with temperature in simultaneous HDN of quinoline and HDS of DBT over Ni-P(1.25) (9) and Ni-Mo sulfide (b).

quinoline by altering the reaction pathways. The presence of H2S reduces the pathway which involves the intermediate DHQ. The alteration of HDN pathways by H2S may be attributed to two effects. One is from the reduction of the hydrogenation of THQ1 to DHQ,16,24 the other from the promotion of the decomposition of THQ1 and OPA by H2S.20,25 Figures 10 and 11 show the HDN activity and HDS activity in simultaneous HDN of quinoline and HDS of DBT catalyzed by Ni-P(1.25) and the sulfided Ni-Mo(0.5), respectively. The (24) Egorova, M.; Prins, R. J. Catal. 2004, 221, 11-19.

sulfided Ni-Mo(0.5) catalyst exhibited comparable HDN activity to that of the Ni-P(1.25) catalyst. However, much lower HDS activity for Ni-P(1.25) was observed at low reaction temperatures. Moreover, the HDN activity on the nickel phosphide catalyst was reduced in simultaneous reactions, compared with its HDN activity without DBT. In contrast, the HDN activity of the sulfided Ni-Mo(0.5) is enhanced to some extent (Figure 6), which is assumed to be the promoting effect of H2S produced from the HDS of DBT. (25) Satterfield, C. N.; Gu¨ltekin, S. Ind. Eng. Chem. Process. Des. DeV. 1981, 20, 62-68.

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Figure 12. Liquid product composition for quinoline HDN as a function of temperature in simultaneous HDN of quinoline and HDS of DBT over Ni-P(1.25) (a) and Ni-Mo sulfide (b). (9) quinoline; (b) THQ1; (2) THQ5; (1) DHQ; ([) OPA; (0) PCH; (O) PB.

The product compositions for quinoline HDN in the presence of DBT catalyzed by Ni-P(1.25) and by the sulfided Ni-Mo(0.5) as a function of the reaction temperature are illustrated in Figure 12. It is shown that quinoline is easily hydrogenated to its tetra- or deca-hydrogenated derivatives on either Ni-P(1.25) or the sulfided Ni-Mo(0.5). In addition, the product compositions were similar, indicating that the HDN of quinoline takes place via similar pathways on both catalysts. For the Ni-P(1.25) catalyst, the hydrogenation of THQ1 to DHQ is dramatically reduced in simultaneous HDN and HDS, similar to the case of quinoline HDN in the presence of H2S (CS2 as the precursor). However, at elevated temperatures, at which DBT is almost completely converted into hydrocarbons and H2S, PCH became the predominant HDN product, indicating that the HDN of quinoline at elevated temperatures proceeds mainly via the DHQ pathway. These results suggest that only the absorbed sulfur species temporarily poison the hydrogenation catalytic sites. Therefore, it is favorable to carry out HDN and HDS at high temperatures. For example, both HDS conversion and HDN conversion approached about 100% in simultaneous HDS and HDN at 360 °C. These are in good agreement with the results reported by Oyama et al.4 For the sulfided Ni-Mo(0.5) catalyst, a HDN product distribution in simultaneous HDS and HDN is observed to be similar to that of HDN alone. These results indicate that HDN on the sulfided Ni-Mo(0.5) is not sensitive to the presence of H2S. Figure 13 shows the product selectivities of DBT HDS catalyzed by Ni-P(1.25) and by the sulfided Ni-Mo(0.5) in simultaneous HDS and HDN. It is interesting to note that their selectivities were quite close, indicating that the same pathways might be involved in HDS on the two different catalysts. 4. Conclusions The MCM-41-supported nickel phosphides prepared by an in situ reduction method from their oxidic precursors with initial Ni/P atomic ratios of 1 and 1.25 exhibited much higher HDN activity than the supported Ni-Mo sulfides. XRD patterns of the supported nickel phosphide catalysts revealed that the active phase was Ni2P. Product distribution analyses show that the HDN of quinoline on the MCM-41-supported nickel phosphides proceeds preferentially through the pathway which involves the fully saturated intermediates (decahydroquinoline). It seems that

Figure 13. HDS selectivities to CHB (b) and BP (9, 0) as a function of temperature in simultaneous HDN of quinoline and HDS of DBT over Ni-P(1.25) (open symbols) and Ni- Mo sulfide (solid symbols).

the cleavage of the C-N bond in the decahydroquinoline molecule might be the rate-determining step in the HDN of quinoline. The presence of H2S dramatically reduced the hydrogenation of 1,2,3,4-tetrahydroquinoline to decahydroquinoline, altering unfavorably the reaction pathways involved in the HDN of quinoline. In simultaneous HDN and HDS, the HDN activity of the Ni-Mo sulfide was hardly affected. On the other hand, the HDN activity of the supported nickel phosphide was dramatically reduced in the presence of DBT at low temperatures. Nevertheless, both HDN conversion and HDS conversion approached 100% in simultaneous HDN and HDS on the supported nickel phosphides at elevated temperatures, such as 360 °C. It is therefore favorable to perform HDN on the supported nickel phosphides at high temperatures. Because of the great difference in HDN activity in the presence or absence of sulfur-containing species, it is proposed that another route to achieve deep HDN is to carry out HDN on the supported nickel phosphide after the majority of sulfur in the feed has been removed. Acknowledgment. The authors acknowledge the financial support from the Natural Science Foundation of China (20003002 and 20333030), the Education Ministry of China (20030141026), and CNPC Innovation Foundation. EF060467G