VOLUME 16, NUMBER 3
MAY/JUNE 2002
© Copyright 2002 American Chemical Society
Articles Hydrodesulfurization of Dibenzothiophene over Cobalt-Molybdenum Nitride Catalysts Yunqi Liu,* Chenguang Liu, and Guohe Que State Key Laboratory of Heavy Oil Processing, Department of Chemical Engineering, University of Petroleum, Dongying, Shandong 257062, China Received November 28, 2000. Revised Manuscript Received February 12, 2002
Molybdenum nitride (γ-Mo2N) and Co-promoted molybdenum nitrides (Co-Mo-N) catalysts with high surface area have been prepared from the reaction of MoO3 and Co-Mo oxides with N2-H2 and characterized by XRD and BET. The results of measurement of hydrodesulfurization(HDS) of dibenzothiophene(DBT) supported the following conclusions: (1) γ-Mo2N and Co-Mo-N have higher HDS activity and selectivity for C-S bond breakage in dibenzothiophene; (2) different pretreatment conditions affect the HDS activity and selectivity; (3) the HDS activity of the γ-Mo2N is significantly improved with the addition of a Co promoter.
1. Introduction The catalytic hydroprocessing to remove heteroatoms such as sulfur, nitrogen, and oxygen from petroleum feedstocks is a critical step in the refining processing. To meet ambitious goals for reduction in sulfur emissions, considerable research has been focused on the development of more active hydrodesulfurization (HDS) catalysts.1,2 Several transition metal nitrides and carbide catalysts have been reported to exhibit catalytic activity in a number of hydrogenation reactions, and much attention has been placed on hydroprocessing.3-5 Schlatter et al.,6 and Sajkowski and Oyama7 carried out * Corresponding author. E-mail:
[email protected]. (1) Takashi, F.; Osamu, C.; et al. Development of a High Activity HDS catalyst for Diesel Fuel: From Basic Research to Commercial Experience. Catal. Today 1998, 45, 307-312. (2) Grange, P.; Vanhaeren, X. Hydrotreating catalysts, an Old Story with new Challenges. Catal. Today 1997, 36, 375-391. (3) Oyama, S. T. Preparation and Catalytic Properties of Transition Metal Carbides and Nitrides. Catal. Today 1992, 15, 179-200. (4) Markel, E. J.; et al. Catalysis over Molybdenum Nitride and Molybdenum Carbide. J. Catal. 1990, 126, 643-657. (5) Chio, J.-G.; Brenner, J. R.; et al. Synthesis and Characterization of Molybdenum Nitride HDN Catalysts. Catal. Today 1992, 15, 201222.
the first comprehensive comparison of a series of Mo nitrides and carbides. Their results show that Mo2N and Mo2C have good activity in the hydrodenitrogenation (HDN) of quinoline. The HDS and HDN behaviors of unsupported and alumina-supported Mo carbide and nitride catalysts have been investigated and these materials have been found to have significantly higher HDS activities than conventional sulfide catalysts.8-10 However, there are still many problems that need to be solved prior to commercialization. The main two aspects need to be solved for Mo carbide and nitride catalysts (6) Schlatter, J. C.; Oyama, S. T.; et al. Preparation and Characterization of Early Transition-Metal Carbides and Nitrides. Ind. Eng. Chem. Res. 1988, 27, 1639-1648. (7) Sajkowski, D. J.; Oyama, S. T. Catalytic Hydrotreatment of Illinois No. 6 Coal-Derived Naphtha: The Removal of Individual Nitrogen and Sulfur Compound over Molybdenum Sulfide and Molybdenum Nitride. Appl. Catal. A: General 1996, 134, 339-349. (8) Lee, K. S.; Abe, H.; et al. HDN of Quinoline over High-SurfaceArea Molybdenum Nitride. J. Catal. 1993, 139, 3-8. (9) Abe, H.; Bell, A. T. Catalytic Hydrotreating of Indole, Benzothiophene and Benzofuran over Molybdenum Nitride. Catal. Lett. 1993, 18, 1-8. (10) Kim, D.-W.; Lee, D.-K.; Ihm, S.-K. CoMo Bimetallic Nitride Catalysts for Thiophene Hydrodesulfurization. Catal. Lett. 1997, 43, 91-95.
10.1021/ef000272n CCC: $22.00 © 2002 American Chemical Society Published on Web 04/03/2002
532
Energy & Fuels, Vol. 16, No. 3, 2002
Liu et al.
Table 1. Surface Areas and the Crystal Structures of CoMo Nitrides catalysts
Co/(Co + Mo) ratio (wt)
phases in CoMo oxides
phases in the products
BET surface area (m2/g)
CoMoN(1) CoMoN(2) CoMoN(3) CoMoN(4) CoMoN(5)
0 0.10 0.25 0.33 0.50
MoO3 MoO3 MoO3, CoMoO4 MoO3, CoMoO4 MoO3, CoMoO4
Mo2N Co3Mo3N, Mo2N Co3Mo3N, Mo2N Co3Mo3N, Mo2N Co3Mo3N, Mo2N
165 158 130 105 88
are as follows. (1) It is difficult to obtain in large scale the catalysts from the temperature-programmed reaction of MoO3 with NH3, which may be responsible for hydrothermal sintering and lattice fluidization caused by high H2O concentration from the fast reduction.11 (2) Much research3 shows that molybdenum nitride catalysts with high BET surface area are sensitive to H2S from the reaction of hydrosulfurization. The addition of Co can improve the HDS conversion and the specific activity of thiophene HDS is the highest at the relative Co atomic ratio of 0.5,10 but the BET surface area of catalysts from the temperature-programmed reaction of MoO3 with NH3 significantly decreases. Recently, some researchers12-16 have prepared and measured thiophene and dibenzothiophene HDS activity of a series of molybdenum nitride and bimetallic nitride catalysts. Wise et al.11 prepared an unsupported Mo2N with high BET surface area by temperature-programmed reaction of MoO3 powder with a mixture of H2 and N2. In contrast to NH3 synthesis, the use of N2 and H2 as reactants offers several advantages over the NH3 for the largescale syntheses of topotactic Mo2N. Nitrides with high surface area can be readily obtained. Virtually, 100% of the synthesis gas may be recycled after drying and the heat transfer problem associated with endothermic decomposition of NH3 in a large reactant bed may be solved. In this work, an improved method for the synthesis of high surface area CoMo bimetallic nitride by temperature-programmed reaction of CoMo oxides with a mixture of N2-H2 was introduced, and the activities and selectivity of dibenzothiophene HDS over nitride catalysts were investigated. Dibenzothiophene is a relatively simple model compound characteristic of the most refractory portion of a hydrotreater feedstock and must be removed so that new environmental regulations on sulfur emission from vehicles can be met.1,2 To investigate the HDS activity of DBT is of significance in theory and practice. 2. Experimental Section 2.1. Preparation of CoMo Nitrides. CoMo oxide precursors with various Co/(Co + Mo) ratios were prepared by (11) Wise, R. S.; Markel, E. J. Synthesis of Molybdenum Nitride with High Surface Area in Mixture of Nitrogen and Hydrogen. J. Catal. 1994, 145, 344-356. (12) Yu, C. C.; Oyama, S. T. Synthesis of New Bimetallic Transition Metal Oxynitrides V-Me-O-N (MedMo and W) by Temperatureprogrammed Reaction. J. Solid State Chem. 1995, 116, 205-207. (13) Yu, C. C.; Oyama, S. T. Synthesis and Characterization of New Bimetallic Transition Metal Oxynitrides M1-M2-O-N (M1, M2dNb, V, Mo, and W). J. Mater. Sci. 1995, 30, 4037-4042. (14) Yu, C. C.; Ramanathan, S.; Oyama, S .T. New Catalysts for Hydroprocessing: Bimetallic Oxynitrides M1-M2-O-N (M1, M2dNb, V, Cr, Mn, Co, Mo, and W) Part I. Synthesis and Charterization. J. Catal. 1998, 173, 1-9. (15) Ramanathan, S.; Yu, C. C.; Oyama, S. T. New Catalysts for Hydroprocessing: Bimetallic Oxynitrides M1-M2-O-N (M1, M2dNb, V, Cr, Mn, Co, Mo, and W) Part II. Reactivity Studies. J. Catal. 1998, 173, 10-16. (16) Zhang, Y.; Xin, Q.; et al. The Preparation and HDS Performance of Zr-Mo Bimetallic Nitrides. Chin. Sci. Bull. (Chinese) 1997, 42 (5), 488-490.
vaporizing aqueous solutions of Co(NO3)2 and (NH4)6Mo7O24‚ 4H2O (A.R) followed by calcination in air at 393 K and 773 K for 24 h and 8 h, respectively. Nitriding of CoMo oxides was performed in a quartz reactor. The temperature-programmed reaction was carried out in three stages consecutively: programmed-heating from room temperature to 573 K in 1 h, from 573 K to 673 K at rate of 0.5 K/min and from 673 to 933 K at the rate of 2.5 K/min, and then holding 4 h at 933 K. Mass flow controllers were used to measure and control the flow rate and ratio of N2 and H2. After the synthesis was finished, the furnace was opened and sample was rapidly cooled to room temperature in flowing N2. The sample was then passivated in 1% O2-Ar gases for 24 h so as to form a stable material. Alumina-supported Co-Mo oxide ((CoO-MoO3)/Al2O3) precursors with the loading of 13.8 MoO3 wt % and a Co/(Co + Mo) ratio of 0.25) were prepared by impregnation of γ-Al2O3 (BET ) 150 m2/g) with aqueous solutions of ammonium Co(NO3)2 and (NH4)6Mo7O24‚4H2O, followed by drying for 24 h at 393 K, and calcination at 773 K for 8 h in air. 2.2. Characterization of the Catalysts. The surface areas of the catalysts were measured by ASAP-2010 physisorption instrument using nitrogen as an adsorbate after the sample was evacuted at 623 K. The passivated sample was ground gently and fixed on a backless glass slice for XRD experiments using a D/MAX-IIIA powder X-ray diffractometer equipped with Cu target and Ni grating monochromatic system. The working voltage of the instrument was 40 kV and the electric current was 50 mA. 2.3. Evaluation of HDS Activities of Catalysts. The measurements of catalytic activities of catalysts for HDS of dibenzothiophene were accomplished in a trickle-bed microreactor with a central thermocouple to measure the temperature of the catalyst bed. Hydrogen flows to the reactor and was regulated by a mass flow controller. The hydrodesulfurization reactions were carried out at 593 K, 3.1 MPa, a liquid hourly space velocity (LHSV) of 12 h-1, and a ratio of hydrogen to liquid feed of 300 (v/v). The reaction feed consisting of 1% of dibenzothiophene in cyclohexane was introduced into the reactor by a pulseless pump. Before introducing the feed, the catalyst was sulfided using 1% CS2 in cyclohexane at the same conditions for 4 h. The products were collected and analyzed off-line by a Varian-3400 gas chromatograph equipped with a 0.25 mm × 30 m HP CP-1 capillary column and a flame ionization detector and the product identification was carried out by gas chromatography-mass spectrometry (GC-MS). The activities of catalysts are compared on the basis of the equal volume and presented in the formation of conversion of dibenzothiophene.
3. Results and Discussions 3.1. X-ray Diffraction Measurements and BET Results. X-ray diffraction patterns were acquired for oxidic precursors as well as nitrided catalysts using the packed powder method. The samples of nitrided catalysts were passivated prior to transferring to the X-ray diffractometer. The surface areas of the prepared CoMo nitrides and their crystal phase structures are summarized in Table 1. The XRD patterns are showed in Figure 1 and pore distribution profiles of the samples are listed in a companion paper.17 Mo2N has a very high
Hydrodesulfurization of Dibenzothiophene
Energy & Fuels, Vol. 16, No. 3, 2002 533
Figure 1. XRD profiles of Co-Mo nitrides Co/(Co + Mo) ratio (wt %), (a) 5; (b) 20; (c) 50.
surface area (165 m2/g) and the surface area of bimetallic CoMo nitride decreased slightly with the addition of Co. In contrast to the CoMo nitrides obtained from the reaction of CoMo oxides with flowing NH310 at Co atomic ratio of 0.5, the surface area of the CoMo nitrides obtained in this research at the same Co atomic ratio is as high as 88 m2/g. The characteristic XRD patterns of the CoMo bimetallic material, before and after nitriding, have been compared. Before nitriding, MoO3 and CoMoO4 appeared for various Co atomic ratio oxides. MoO3 transformed into Mo2N through the reaction with the flowing mixture gases of N2-H2. However, new XRD peaks of Co3Mo3N appeared after nitriding CoMo oxides beside the XRD pattern of Mo2N. 3.2. Dibenzothiophene HDS Activity Measurement. 3.2.1. GC-MS Analysis of Liquid Products and Reaction Network. The liquid product identification was made by GC and GC-MS analysis. Figure 2 shows a typical GC chromatogram of the product. Some products were identified by comparison of GC retention time with those of the standard samples, cyclohexane (solvent), biphenyl (BPN), and dibenzothiophene (DBT). Other products were identified by GC-MS, the data system of which searches automatically the associated database spectra of molecules and gives a best fit match. The results identified that those products are assigned as benzylcyclohexane (BCP), cyclohexylbenzene (CHB), and 1,2,3,4,-tetrahydrodibenzothiophene (4H-DBT),1,2,3,4,10,11-hexahydrodibenzothiophene (6H-DBT). On the basis of GC and GC-MS identifications, a proposed reaction network for DBT hydrodesulfurization is shown in Figure 3. This network indicates two primary reaction pathways: hydrogenation of one aromatic ring to give a mixture of 4H-DBT and 6H-DBT; hydrodesulfuriza(17) Liu, Y.; Liu, C.; Que, G. Study on the Preparation and its Hydrogenation Properties of CoMo Nitride Catalysts. Acta Petrolei Sinica (Petroleum Processing Section) (Chinese) 2000, 1, 66-70.
Figure 2. A representative gas chromatogram of liquid products for HDS of DBT. 1: hexapentane; 2: cyclohexane; 3: cyclohexylbenzene; 4: biphenyl; 5: 1,2,3,4,10,11-hexahydrodibenzothiophene; 6: 1,2,3,4-tetrahydrodibenzothiophene; 7: dibenzothiophene.
Figure 3. The reaction network for HDS of dibenzothiophene.
tion of these intermediates occurs rapidly to give cyclohexylbenzene(CHB). This combination of reactions is designated as the hydrogenation route. The second
534
Energy & Fuels, Vol. 16, No. 3, 2002
Liu et al.
Table 2. Effect of Pretreatment Conditions on the Activity and Selectivity of Mo2N Catalystsa pretreatment conditions conversion (relative value) selectivity (relative value) BPN% CHB% a
passivatednitride 100 93.3 6.7
reducednitride
sulfidednitride
88
97
67.7 32.3
63.0 37.0
BPN: biphenyl; CHB: cyclohexylbenzene.
pathway, direct hydrodesulfurization of DBT to give biphenyl (BPN), occurs by hydrogenolysis. In one experiment, a certain amount of the biphenyl was added to the reactants; however, the concentration of cyclohexylbenzene was hardly increased compared with the run with only DBT as reactant. Therefore, it is believed that the biphenyl cannot react further with hydrogen to form cyclohexylbenzene (CHB) under those conditions. 3.2.2. The Effects of Pretreatment Conditions on the Activity of Mo2N. Before the activity test, the catalysts were pretreated in situ by three different ways: (1)The nitrided catalysts were passivated in flow of 1% O2Ar; (2) The passivated nitrides were reduced by pure H2 in 673 K for 4 h; (3) The passivated nitrides were sulfided by 1% CS2 of cyclohexane in 673 K for 4 h. The three different pretreatment ways were labeled with passivated-nitride, reduced-nitride, and sulfided-nitride. The catalyst activities in different pretreatment conditions are listed Table 2. A number of studies5,8-9 have reported recently the prereduction of passivited nitride may remove the oxygen from passivation and improve the activity, and a thin layer of sulfide Mo formed on the surface under HDS conditions lowers the HDS activity. But our results show that the dibenzothiophene HDS activity of the catalysts is strongly dependent on the pretreatment conditions of the catalyst. The activity order is passivated nitride ≈ sulfided nitride > reduced nitride, which means that prereduction improved neither the activity nor the selectivity. It is very important to note that the prereduction pretreatment decreases the HDS activity for this observation and no satisfactory explanation has yet been discovered. Logan18 has recently reported a similar result for the nitride catalysts which were either reduced in H2 or sulfided in an H2S/H2. A possible reason is that by reduction metal Mo species are formed in the surface. The XRD results showed no any change in crystal phases before and after HDS reaction Figure 4. Pre-sulfided nitride has similar HDS activity but the selectivity to BPN significantly decreases, which is attributed to the formation of a nitro-sulfide layer in the surface. 3.2.3. The Relation of Surface Area and the Activity. For catalytic application it is critical for a material to have specific surface area. The nitride catalysts with different BET surface areas may be obtained from different synthesis conditions, of which high synthesis gases space velocity and low heat rate contribute to the products with highest BET surface area. Mo2N with BET surface areas of 165 m2/g and 85 m2/g, respectively, and CoMo nitrides with BET surface areas of 158 m2/g (18) Logan, J. W.; Heiser, J. L.; et al. Thiophene Hydrodesulfurization over Bimetallic and Promoted Nitride Catalysts. Catal. Lett. 1996, 56, 165-1711.
Figure 4. The XRD profile molybdenum nitride after the reaction of HDS. Table 3. Relation of Surface Area and the Activity of Nitride Catalysts catalysts Mo2N-165 Mo2N-85 CoMoNx-158 CoMoNx-43
Co/ (Co + Mo)
BET surface area (m2/g)
HDS conversion (%)
0.10 0.10
165 85 158 43
60 56 74 65
and 43 m2/g, respectively, have been prepared. The dibenzothiophene HDS activity measurements are shown in Table 3. The HDS activity is measured in the conditions below: liquid hourly space velocity (LHSV), 12 h-1, and the volume ratio of hydrogen to liquid, 300, reaction temperature, 593 K, pressure, 3.1 MPa. The results in Table 3 show that the HDS conversions of both the Mo2N and Co-Mo-N catalysts decrease with the decrease of BET surface area of the catalysts. The low HDS conversion of low BET surface area catalysts is due to the decrease of active sites number, which is similar to conventional supported catalysts. However, it is important to note that the HDS activity of the Co-Mo-N catalyst with a surface area of 43 m2/g is higher than the Mo2N catalyst with a surface area of 85 m2/g. Considering the XRD results, it is believed that the high HDS activity is due to the formation of new phase Co3Mo3N in the CoMo nitride catalysts. 3.2.4. The Relation of HDS Activity and Ratios of Co/(Co + Mo). It is well-known that Co is the promoter of Mo sulfides HDS catalysts, and the HDS activity significantly increases with the addition of Co. Therefore, it is expected that the HDS activity and stability should increase with addition of Co to the nitride catalyst. A series of Co-Mo-N catalysts with different ratios of Co/(Co + Mo) were prepared and the HDS activity for dibenzothiophene is measured in this research. The HDS activity tests are carried out at the conditions below: liquid hourly space velocity (LHSV), 12 h-1; volume ratio of hydrogen to liquid, 300 (v/v); the reaction temperature, 593 K; pressure, 3.1 MPa. The results in Table 4 show the dibenzothiophene HDS conversions over CoMo bimetallic nitride catalysts with different ratios of Co/(Co + Mo). The HDS activity increases with the increase of Co/(Co + Mo) before the ratio of 0.33. It is well-known that for the Co-Mo-S HDS catalyst, the HDS conversion has the maximum value when the ratio of Co/(Co + Mo) is 0.25-0.4. For
Hydrodesulfurization of Dibenzothiophene
Energy & Fuels, Vol. 16, No. 3, 2002 535
Table 4. HDS Conversion over Co-Mo-N Catalysts with Different Ratios of Co/(Co + Mo) catalysts
Co/ (Co + Mo)
BET surface area (m2/g)
HDS Conversion (%)
Co-Mo-N(1) Co-Mo-N(2) Co-Mo-N(3) Co-Mo-N(4)
0.10 0.25 0.33 0.50
150 130 105 88
74 77 78 70
nitride catalysts, the ratio is similar to sulfide Co-Mo catalyst. As to catalytic synergy effect in sulfide CoMo catalyst of Co promoter, there are two different proposals in recent literatures.2 One is the remote control theory, which is based on cooperation between two separate and well-identified single sulfide phase. Another is the so-called the CoMoS (CoMoS phase) model, which was not ascribed to any of the cobalt sulfide or cobalt aluminate. Topsøe et al.12 attributed it to a mixed CoMoS phase, on which cobalt is located on the edges of the MoS2 layered crystallites and presents a specific coordination. The XRD results of CoMo nitride catalysts demonstrate that the Co3Mo3N crystallite phase is presented, and therefore, it may be concluded that the catalytic synergy effect in Co-Mo nitride catalyst may be ascribed to the new phase Co3Mo3N. The HDS conversion increases with the amount of new phase of Co3Mo3N, but the rising tendency ends at the ratio of 0.33. Combining with BET data, it is suggested that the decreasing tendency is relevant to the lower BET surface area in CoMo nitrides with a higher ratio of Co/(Co + Mo). 3.2.5. The HDS Activity and Stability of Different Catalysts. Figure 5 shows a comparison of HDS activities of the passivated nitrides (Mo2N, BET ) 165 m2/g), CoMo nitride (BET ) 130 m2/g, the ratio of Co/ (Co + Mo) ) 0.25), and the Co-Mo-S/Al2O3 catalyst (MoO3 loading 13.8 wt %, the ratio of Co/(Co + Mo) ) 0.25, γ-Al2O3, BET ) 150 m2/g). The HDS activity follows the order: CoMo nitride > Mo2N > Co-Mo-S/ Al2O3. After a 48 h run, the HDS activities of three catalysts are not significantly decreased, and it implies the catalysts have reasonably good HDS stability over 50 h. Further investigation on these catalysts is in progress in our laboratory.
Figure 5. Comparison of HDS activities of Mo2N, CoMo nitride, and CoMoS/Al2O3 at 3.1 MPa and 593 K.
4. Conclusion (1) Mo nitride and CoMo bimetallic nitrides with high surface area have been prepared from the temperatureprogrammed reaction of oxide precursors with N2-H2 mixture gases in place of NH3. Compared with the temperature-programmed reaction of oxide precursors with NH3, the use of N2-H2 as reactants offers several advantages over the NH3 for the large-scale syntheses of topotactic Mo2N. Nitrides with high surface area are readily achieved. Virtually 100% of the synthesis gas may be economically recycled by drying and the heat transfer problem associated with endothermic decomposition of NH3 in a large reactant bed is solved. (2) The dibenzothiophene HDS activity over nitride catalysts was strongly dependent on the pretreatments of the catalysts. The activity order is: passivated-nitride ≈ sulfided nitride > reduced nitride, which means that pre-reduction improves neither the activity nor the selectivity. Pre-sulfiding decreased the selectivity and probably the activity of the nitride catalysts (3) The dibenzothiophene HDS activity was directly releted to the BET surface area of the nitride catalysts. The activity decreases with the decrease of BET surface area of the catalysts. Co promotes significantly the HDS activity of the nitride catalysts. EF000272N
