Energy Fuels 2010, 24, 789–795 Published on Web 12/03/2009
: DOI:10.1021/ef901084f
Hydrodesulfurization (HDS) and Hydrodenitrogenation (HDN) Performance of an Ex Situ Presulfided MoNiP/Al2O3 Catalyst: Model Compounds Study and Pilot Test for Fluidized Catalytic Cracking (FCC) Diesel Oil Yanpeng Li,*,†,‡ Dapeng Liu,‡ and Chenguang Liu*,†,‡ †
State Key Laboratory of Heavy Oil Processing and ‡Key Laboratory of Catalysis, China National Petroleum Corporation (CNPC), China University of Petroleum, Qingdao 266555, Shandong, China Received September 24, 2009. Revised Manuscript Received November 12, 2009
An ex situ presulfided MoNiP/γ-Al2O3 catalyst was prepared, and its hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) performances were evaluated with dibenzothiophene and quinoline as the model compound, respectively. Results indicated that the ex situ presulfided catalyst presented a similar sulfur removal rate as the in situ presulfided catalyst but its hydrogenation activity and HDN efficiency were relatively lower. However, its hydrogenation activity increased with longer time on stream (TOS). In a pilot test with fluidized catalytic cracking (FCC) diesel oil as feed, close HDS and HDN efficiencies were acquired for both presulfided catalysts. Furthermore, along with the running period, the ex situ presulfided catalyst exhibited incremental HDN efficiency as well as hydrogenation activity. An active-phase study was performed by means of Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy (XPS). Results showed that the ex situ presulfided catalyst presented relatively weaker MoS2 diffraction peaks and a lower stacking number and length of MoS2 slabs than those of the in situ presulfided catalyst with short TOS but those could be significantly enhanced with long TOS. XPS data also indicated that the ex situ presulfided catalyst only became partially sulfurized during the ex situ presulfiding process but could be further sulfided with long TOS. The conclusion was made that it was the distinction lying in the MoS2 active phase that caused the ex situ presulfided catalyst to have a lower hydrogenation activity with short TOS, and the distinction would be far more diminished with long TOS or in a real diesel oil system.
less pollution. Up to now, several commercial ex situ presulfiding processes have been developed, such as SULFICAT,2 SUPER PLUS,3 and SURECAT4 processes from the Eurecat company in France and the actiCAT5 process from the CRI company in the U.S.A. However, few work have been performed about the detailed study of the ex situ presulfided NiMo hydrotreating catalyst, except for several early reports by Dufresne et al.6 and Labruyere et al.7 Researchers have nearly no idea of the behavior of the ex situ presulfided catalyst in model compounds system and, still further, the practical application in fluidized catalytic cracking (FCC) diesel oil hydrotreatment, which is what is focused on in this paper. In this paper, an ex situ presulfided MoNiP/Al2O3 catalyst was prepared and its HDS and HDN activities were evaluated with dibenzothiophene (DBT) and quinoline (Q) as the model compound, respectively. A pilot test of the scaled-up prepared ex situ presulfided catalyst with feed of FCC diesel oil had also been carried out. The in situ presulfided MoNiP/Al2O3 catalyst was used as the reference within both the model compound study and the pilot test. The different hydrotreating performances for the two kinds of presulfided catalyst were presented,
1. Introduction The new environmental regulations in most of the developed countries impose the reduction of sulfur compounds (SOx) and nitrogen compounds (NOx) produced from fuel combustion. This calls for the production of cleaner fuels containing low contents of S and N and thereby an improvement of the efficiency of the hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) of the petroleum feedstock. For the commonly used NiMo/Al2O3 hydrotreating catalyst, the Ni and Mo active components present best hydrogenation activity only in their sulfide form.1 Therefore, the presulfiding process, which transforms the catalyst from the oxide form to the sulfide form is of great importance. The conventional in situ presulfiding process has several drawbacks, such as difficulties in operation, toxicity of sulfiding reagents, hazards to operators, risk for temperature runaway of the catalyst bed, etc. Attention has been paid to the ex situ presulfiding process recently for its simplicity, high efficiency, safe handling, and *To whom correspondence should be addressed. Telephone: þ86-53286984723 (Y.L.); þ86-532-86981716 (C.L.). Fax: þ86-532-86981711 (Y.L.; C.L.). E-mail:
[email protected] (Y.L.); cgliu@hdpu. edu.cn (C.L.). (1) Bataille, F.; Lemberton, J. L.; Michaud, P.; Perot, G.; Varinat, M.; Lemaire, M.; Schulz, E.; Breysse, M.; Kasztelan, S. J. Catal. 2000, 191 (2), 409–422. (2) Berrebi, G. U.S. Patent 4,530,917, 1985. (3) Berrebi, G; Le Gall, B. U.S. Patent 5,139,983, 1992. (4) Roumieu, R; Boitiaux, J P. U.S. Patent 5,153,163, 1992. r 2009 American Chemical Society
(5) Blashka, S.; Bond, G.; Ward, D. Oil Gas J. 1998, 96 (1), 36–40. (6) Dufresne, P.; Brahma, N.; Labruyere, F.; Lacroix, M.; Breysse, M. Catal. Today 1996, 29 (1-4), 251–254. (7) Labruyere, F.; Dufresne, P.; Lacroix, M.; Breysse, M. Catal. Today 1998, 43 (1-2), 111–116.
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Energy Fuels 2010, 24, 789–795
: DOI:10.1021/ef901084f
Li et al.
Figure 1. Flowchart of the ex situ presulfiding process.
and several analytical methods were adopted for the fundamental research about the MoS2 active phase to interpret the difference.
2.3. Catalyst Characterizations. The pore structures of the calcined oxide-form catalyst, used EX and IN catalysts, were determined by N2 adsorption-desorption isotherms at 77 K with a Micromeritics Tristar 3000 equipment. Powder X-ray diffraction (XRD) patterns were recorded by a PANalytical X’Pert MPD Pro diffractometer using Cu KR radiation at 40 kV and 40 mA. High-resolution transmission electron microscopy (HRTEM) employed a JEM-2100UHR with a magnification of 300 000 to measure the micromorphology. After each catalyst sample was ground with pestle and mortar, it was dispersed in ethanol and deposited on a carbon-coated copper grid. The specimen was dried in air prior to analysis. At least 10 representative micrographs were taken for each catalyst to obtain the statistical data of the distribution of MoS2 slab length and the stacking number. Typically, the length and stacking number of at least 200 slabs were measured for each catalyst. X-ray photoelectron spectroscopy (XPS) were collected by a Kratos AXIS HSI 165 with a monochromatized Al Ka X-ray source (1486.71 eV photons), and the narrow scan of Mo 3d spectra acquired was deconvoluted by the vendor-supplied XPSpeak software (version 4.1). The bulk sulfur content of the fresh and used catalyst was determined with an Elementar VARIO EL III CHNS/O elemental analyzer. On the basis of the data of sulfur content, the ratio of detected bulk sulfur content to the catalyst stoichiometric sulfur content (this value was calculated on the basis of the presupposition that the ultimate sulfide form of Mo and Ni were MoS2 and NiS separately) was defined as the bulk sulfidation degree.
2. Experimental Section 2.1. Preparation of the ex Situ Presulfided Catalyst. The ex situ presulfiding process adopted in this paper was shown in Figure 1, which had been described elsewhere.8,9 The oxide-form MoNiP/Al2O3 catalyst was prepared with the incipient wetness impregnation method using calcinated γ-Al2O3 as support. Molybdenum trioxide and basic nickel carbonate powders were dissolved in phosphoric acid of a given concentration to obtain the mother solution for impregnation. The thus-obtained catalyst was dried at 393 K and calcined at 773 K for 4 h in the air to obtain the oxide-form catalyst, which was the precursor of both ex situ and in situ presulfiding processes. The metallic active components in the oxide-form catalyst precursor were 22 wt % MoO3 and 6 wt % NiO. The ex situ presulfided catalyst was denoted as the EX catalyst, and the traditional in situ presulfided catalyst was denoted as IN catalyst hereafter in this paper. 2.2. Performance Evaluation of Catalysts. 2.2.1. Model Compound Study. Model compound studies were carried out on a self-assembled high-pressure fixed-bed reactor, and the dosage of the catalyst in each run was fixed to 10 mL. The EX catalyst should first be activated in situ under a H2 atmosphere (2 MPa) at 573K for two hours and then shifted to feed. A 2 wt % DBT/ toluene solution and a 2 wt % Q/0.5 wt % CS2/toluene solution were used as model compounds, respectively. The evaluation conditions were as follows: reaction temperature, 573 K; H2 pressure, 2.0 MPa; H2/oil (v/v), 300; space velocity, 2.0 h-1. Samples were collected at different time on stream (TOS) and then analyzed with a Varian 3800 gas chromatography-flame ionization detector (GC-FID) and a VG7070E GC-mass spectrometry (MS) instrument to identify each product. The IN catalyst was primarily presulfided with a liquid stream containing 3 wt % CS2 in cyclohexane at 573 K for 6 h at a H2 pressure of 2.0 MPa and space velocity of 2.0 h-1, and then the feed was shifted to DBT or Q solution. 2.2.2. Pilot Test for FCC Diesel Oil. For the pilot study for FCC diesel oil, process conditions were as follows: H2 pressure, 6.0 MPa; H2/oil (v/v), 500; reaction temperature, 623 K; space velocity, 1.5 h-1, and the dosage of catalyst was 100 mL in each run. Density data of feed and hydrogenated oils were measured by a densimeter based on Chinese National Standard GB1884-92. Sulfur and nitrogen content in oil samples was detected with a Jena MultiEA3100 instrument. The cetane numbers of feed and hydrogenated oils were calculated on the basis of Chinese National Standard GB/T11139-89.
3. Results and Discussion 3.1. HDS of DBT. DBT is a widely selected model compound for HDS reaction.10,11 Figure 2 shows the HDS results, including the sulfur removal rate and yield of biphenyl (BP) and cyclohexylbenzene (CHB) over the two kinds of presulfided catalysts. Samples were taken with the TOS of 10 and 72 h, respectively. From Figure 2, it can be seen that the EX catalyst has similar desulfurization activity of DBT to the IN catalyst but the HDS reaction product distribution is different. HDS reaction network study of DBT11 suggests that the desulfurization of DBT undertakes two pathways, i.e., the hydrogenation (HYD) pathway and direct desulfurization (DDS) or hydrogenolysis pathway. The amount of BP and CHB can roughly represent the relative reaction level through each pathway. The more CHB means the higher hydrogenation activity of the catalyst, and the more BP means the higher hydrogenolysis activity of the catalyst.
(8) Dufresne, P.; Valences, B.; Le Gall, B.; Berrebi, G. U.S. Patent 5,397,756, 1995. (9) Dufresne, P.; Brahma, N.; Murff, S. R. U.S. Patent 5,985,787, 1994.
(10) Egorova, M.; Prins, R. J. Catal. 2004, 224 (2), 278–287. (11) Nagai, M.; Fukiage, T.; Kurata, S. Catal. Today 2005, 106 (1-4), 201–205.
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: DOI:10.1021/ef901084f
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Thus, from Figure 2, it can be implied that the EX catalyst favors the DDS pathway more to attain desulfurization and shows lower hydrogenation activity, as indicated by its lower CHB yield but higher BP yield. This distinction between the two catalysts is more notable with short TOS (10 h). In addition, the lower hydrogenation reactivity of the EX catalyst can be further verified by the HDN reaction of the Q with the same TOS of 10 h as section 3.2. However, with regard to the EX catalyst with longer TOS (72 h), the yield of CHB is increased dramatically (from 7.9 to 24.9 wt %) and the sulfur removal rate is also enhanced (from 95.1 to 98.6 wt %). Because the higher yield of CHB corresponds to the higher hydrogenation activity, the EX catalyst thus must become further developed in its HDS active phase during the reaction. As the comparison, the IN catalyst keeps high HDS efficiency and high hydrogenation activity all along during the reaction with only a slight increase. 3.2. HDN of Q. Q has many advantages over other nitrogen-containing hydrocarbons as the model reactant in the HDN reaction.12,13 All reactions that take place in an industrial HDN process also undergo within the reaction network of Q. The HDN results over each catalyst were collected in Table 1. From the data in Table 1, it was found that the EX catalyst showed obviously a lower HDN ratio than the IN catalyst under given reaction conditions, while the HDN products of Q over both the IN and EX catalysts were similarly complicated. Jian and Prins14 proposed the HDN reaction network of Q over the conventional NiMo/Al2O3 catalyst, shown in Figure 3, which was also consistent with our results fairly well. From Table 1, it can be found that there are almost the same three types of reaction products over both kinds of presulfided catalysts: the hydrogenation derivatives of Q (THQ5, THQ1, and DHQ), amines resulting from the C-N bond cleavage (OPA and PCHA), and denitrogenation products (PB, PCHE, and PCH). It is suggested that the HDN reaction of Q over the NiMo/Al2O3 catalyst undergoes two pathways: the hydrogenolysis pathway (pathway I, THQ1 f OPA f PB) and the hydrogenation pathway (pathway II, DHQ f PCHA f PCHE or PCH).8 It has been demonstrated that PB could hardly be further hydro-
genated to PCH and/or PCHE in the presence of Q-type compounds (such as Q, THQ5, THQ1, and DHQ).14-16 Therefore, the value of PCH/PB can roughly represent the relative contribution of each reaction pathway. The higher the value, the more contribution of pathway II to the final denitrogenation of Q. From the value of PCH/PB in Table 1, it is indicated that, in the HDN reaction of Q, pathway II is much more preferable than pathway I, because the value of PCH/PB over each catalyst is much larger than 1. Therefore, it can be deduced that the HDN of Q over both EX and IN catalysts proceeds mainly via the fully saturated intermediates DHQ and PCHA, i.e., pathway II. With short TOS (10 h here), the EX catalyst has a relatively lower PCH/PB value compared to the IN catalyst, which suggests that the former has a relatively weaker denitrogenation ability through full hydrogenation and, therefore, a lower hydrogenation activity than that of the latter. This result was consistent with our early research work17 and former results in section 3.1. 3.3. Pilot Test for FCC Diesel Oil Hydrotreatment. In sections 3.1 and 3.2, the different behaviors of the two kinds of presulfided catalysts in the model compound study have been illustrated. However, things may be different in the real feed system, such as FCC diesel oil. Because the composition of diesel oil is considerably complicated, there possibly exist some unpredictabilities. Thus, a pilot test with FCC diesel oil as feed oil carried out over the EX and IN catalysts was also chosen as the comparison. Related results are shown in Table 2. As a whole, the EX catalyst has a similar hydrotreating performance to the IN catalyst, seeing the data of d420, aniline point, cetane number, and HDS and HDN efficiencies in Table 2.This is remarkably different from the results in the aforementioned model compound study in sections 3.1 and 3.2 (with short TOS of 10 h). With regard to the detail, hydrogenated oil over the EX catalyst has the same aniline point and even a little better desulfurization rate than the IN catalyst. However, it still shows a fairly weaker hydrogenation activity, which is reflected from the higher d420 and lower denitrogenation efficiency. Another noteworthy result is that the difference in the HDN performance between the EX and IN catalysts decreases clearly compared to that shown in the model compound study. Considering the reasonable errors in sampling and analyzing, there is nearly no difference in the hydrotreating activity between the EX and IN catalysts. Therefore, it should say that the EX catalyst has been successfully applied in the real diesel oil system. With very close HDN and hydrogenation reactivity but a slightly higher HDS efficiency, the EX catalyst can be qualified for its potential application in the future refining industry. Additionally, it is interesting to notice that the hydrotreating performance of the EX catalyst can be gradually improved with longer TOS, as indicated in Figure 4. Although the EX catalyst shows weaker HDN activity at the beginning, its HDN activity increases with the reaction time prolonging and reaches a comparable level to the IN catalyst after 5 days. This may be interpreted as the development of the hydrogenation active phase during a long reaction time. Here, the various sulfur compounds in diesel oil probably act
(12) Satterfield, C. N.; Gultekin, S. Ind. Eng. Chem. Proc. Des. Dev. 1981, 20 (1), 62–68. (13) Eijsbouts, S.; van Gestel, J. N. M.; van Veen, J. A. R.; de Beer, V. H. J.; Prins, R. J. Catal. 1991, 131 (2), 412–432. (14) Jian, M.; Prins, R. J. Catal. 1998, 179 (1), 18–27.
(15) Jian, M.; Kapteijn, F.; Prins, R. J. Catal. 1997, 168 (2), 491–500. (16) Cocchetto, J. F.; Satterfield, C. N. Ind. Eng. Chem. Proc. Des. Dev. 1981, 20 (1), 49–53. (17) Li, Y. P.; Liu, X.; Liu, D. P.; Liu, C. G. Prepr. Pap.-Am. Chem. Soc., Div. Pet. Chem. 2005, 50 (3), 321–323.
Figure 2. HDS results of EX and IN catalysts with different TOS.
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: DOI:10.1021/ef901084f
Li et al.
Table 1. HDN Results of Q over Different Catalystsa HDN productsb (wt %) catalysts
PCH
PCHE
PB
PCHA
DHQ
THQ5
OPA
Q
THQ1
conversion of Q (wt %)
HDN efficiency (wt %)
PCH/PB
EX IN
4.87 7.53
2.77 5.35
0.19 0.27
0.37 0.52
9.21 10.77
4.13 7.79
10.78 16.73
0.88 2.60
66.80 48.44
99.12 97.40
7.83 13.15
24.4 26.6
a
Samples were taken with a TOS of 10 h. b Abbreviation of each product was shown in Figure 3.
Figure 3. Reaction network of HDN of Q.8 Q, quinoline; THQ5, 5,6,7,8-tetrahydroquinoline; DHQ, decahydroquinoline; THQ1, 1,2,3,4tetrahydroquinoline; PCHA, 2-propylcyclohexylamine; OPA, ortho-propylaniline; PCHE, propylcyclohexene; PCH, propylcyclohexane; PB, propylbenzene. Table 2. Characters of Hydrotreated Oil with Different Presulfided Processesa hydrotreated oil characters
feed oil
d420 (g cm-3) aniline point (°C) cetane number sulfur content (μg g-1) percentage of desulfurization (wt %) nitrogen content (μg g-1) percentage of denitrogenation (wt %)
0.912
