Hydrotreatment Activities of Supported Molybdenum Nitrides and

668

Energy & Fuels 1997, 11, 668-675

Hydrotreatment Activities of Supported Molybdenum Nitrides and Carbides G. M. Dolce, P. E. Savage, and L. T. Thompson* Department of Chemical Engineering, The University of Michigan, Ann Arbor, Michigan 48109-2136 Received June 4, 1996X

The growing need for alternative sources of transportation fuels encourages the development of new hydrotreatment catalysts. These catalysts must be active and more hydrogen efficient than the current commercial hydrotreatment catalysts. Molybdenum nitrides and carbides are attractive candidate materials possessing properties that are comparable or superior to those of commercial sulfide catalysts. This research investigated the catalytic properties of γ-Al2O3supported molybdenum nitrides and carbides. These catalysts were synthesized via temperatureprogrammed reaction of supported molybdenum oxides with ammonia or methane/hydrogen mixtures. Phase constituents and compositions were determined by X-ray diffraction, elemental analysis, and neutron activation analysis. Oxygen chemisorption was used to probe the surface properties of the catalysts. Specific activities of the molybdenum nitrides and carbides were competitive with those of a commercial sulfide catalyst for hydrodenitrogenation (HDN), hydrodesulfurization (HDS), and hydrodeoxygenation (HDO). For HDN and HDS, the catalytic activity on a molybdenum basis was a strong inverse function of the molybdenum loading. Product distributions for the HDN, HDO, and HDS of a variety of heteroatom compounds indicated that several of the nitrides and carbides were more hydrogen efficient than the sulfide catalyst.

Introduction As light crude oil supplies diminish, there is increasing interest in the conversion of coal-derived liquids and heavy oils into transportation fuels. Current commercial hydrotreatment catalysts are typically optimized for the removal of sulfur from light crudes.1 Catalysts based on Ni-Mo are generally preferred for hydrodenitrogenation (HDN), while Co-Mo catalysts are preferred for hydrodesulfurization (HDS) and hydrodeoxygenation (HDO).1-6 Both types of catalysts possess high hydrogenation activities and are typically not as efficient at removing sulfur, oxygen, and nitrogen from heavy liquids. Therefore, new hydrotreatment catalysts are being developed that can efficiently remove unwanted heteroatoms while consuming a minimum amount of hydrogen. These catalysts might also be effective for deep HDS. Molybdenum nitride and carbide catalysts have been reported to be comparable or superior to sulfided NiMo/Al2O3 and Co-Mo/Al2O3 catalysts for the HDN, HDS, and HDO of model compounds.7-15 Recently we * Telephone: (313) 936-2015. FAX: (313) 763-0459. E-mail: [email protected]. X Abstract published in Advance ACS Abstracts, March 15, 1997. (1) Ho, T. C. Catal. Rev.sSci. Eng. 1988, 30, 117-160. (2) Satterfield, C. N. Heterogeneous Catalysis in Industrial Practice; McGraw-Hill, New York, 1991; p 376. (3) Chianelli, R. R. Catal. Rev.sSci. Eng. 1984, 26, 361-393. (4) Katzer, J. R.; Sivasubramanian, R. Catal. Rev.sSci. Eng. 1979, 20, 155-208. (5) Shabtai, J.; Guohe, Q.; Balusami, K.; Nag, N. K.; Massoth, F. E. J. Catal. 1988, 113, 206-219. (6) Shabtai, J.; Nag, N. K.; Massoth, F. E. J. Catal. 1987, 104, 413423. (7) Schlatter, J. C.; Oyama, S. T.; Metcalfe, J. E.; Lambert, J. M. Ind. Eng. Chem. Res. 1988, 27, 1648-1653. (8) Ramanathan, S.; Teixeira da Silva, V. L. S.; Oyama, S. T. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1994, 618. (9) Abe, H.; Bell, A. T. Catal. Lett. 1993, 18, 1-8.

S0887-0624(96)00083-7 CCC: $14.00

reported that Al2O3-supported molybdenum nitrides were more active than sulfide-based catalysts for pyridine HDN and that their activities were a strong inverse function of the loading.15 Our current research focuses on evaluating the activities of supported molybdenum nitrides and carbides for the hydrotreatment of heavier heteroatom compounds including methylcarbazole, dibenzothiophene, and dibenzofuran. These model compounds are more representative of industrial feeds than pyridine. In this paper, we describe the activities of a series of γ-Al2O3-supported molybdenum nitrides and carbides for the hydrotreatment of two- and three-ring heteroatomic compounds. These catalysts were prepared by the temperature-programmed reaction (TPR) of supported molybdenum oxides.15-18 We systematically varied the heating rates, a parameter that has been reported to affect the composition and structure of the final product as well as the solid-state reaction intermediates.18-20 For example, the surface areas of unsupported molybdenum nitrides can be regulated by con(10) Nagai, M.; Miyao, T.; Omi, S. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1994, 577. (11) Nagai, M.; Miyao, T. Catal. Lett. 1992, 15, 105-109. (12) Lee, K. S.; Abe, H.; Reimer, J. A.; Bell, A. T. J. Catal. 1993, 139, 34-40. (13) Markel, E. J.; Van Zee, J. W. J. Catal. 1990, 126, 643-657. (14) Choi, J. G.; Brenner, J. R.; Colling, C. W.; Demczyk, B. G.; Dunning, J. L.; Thompson, L. T. Catal. Today 1992, 15, 201-222. (15) Colling, C. W.; Thompson, L. T. J. Catal. 1994, 146, 193-203. (16) Ranhotra, G. S.; Haddix, G. W.; Bell, A. T.; Reimer, J. A. J. Catal. 1987, 108, 24-39. (17) Volpe, L.; Boudart, M. J. Solid State Chem. 1985, 59, 332347. (18) Volpe, L.; Boudart, M. J. Solid State Chem. 1985, 59, 348356. (19) Choi, J. G.; Curl, R. L.; Thompson, L. T. J. Catal. 1994, 146, 218-227. (20) Jaggers, C. H.; Michaels, J. N.; Stacy, A. M. Chem. Mater. 1990, 2, 150-157.

© 1997 American Chemical Society

Molybdenum Nitrides and Carbides

trolling the heating rate.19 The supported molybdenum nitride and carbide catalysts were structurally characterized via X-ray diffraction, and their compositions were analyzed using CHN elemental analysis and neutron activation analysis. The sorptive properties were studied using oxygen chemisorption. The catalysts were evaluated for quinoline HDN, benzothiophene HDS, and benzofuran HDO. A subset of these catalysts was also tested for 9-methylcarbazole HDN, dibenzothiophene HDS, and dibenzofuran HDO. The heavier compounds more closely represent the character of compounds in coal-derived liquids and heavy crudes. Experimental Section Catalyst Synthesis. A series of γ-Al2O3-supported catalysts was prepared by the nitridation or carburization of supported molybdates. The molybdates were prepared using an incipient wetness technique.15 An aqueous solution of ammonium heptamolybdate, (NH4)6Mo7O24‚4H2O (Mallinckrodt), was added dropwise to γ-Al2O3 (Alfa-Aesar, activated acidic, 96% metals basis, 60 mesh, 190 m2/g) which had been previously calcined at 773 K for 5 h in dry air. The solution was stirred into the support until the point of incipient wetness was reached, at which time the mixture was calcined again for 5 h at 773 K. The resulting molybdenum loading was determined on the basis of the initial concentration of the ammonium heptamolybdate solution. We attempted to prepare materials with loadings of 4, 8, and 14 wt % molybdenum. Only one impregnation was needed for the 4 and 8 wt % molybdenum materials; however, because of the solubility limits of ammonium heptamolybdate in water, the 14 wt % molybdenum samples required two impregnations. The materials were calcined following each impregnation. It has been reported that multiple impregnations do not affect the character of the resulting molybdate.21 The supported nitrides were prepared by reacting the molybdates with ammonia in a temperature-programmed fashion. The nitridation protocol was similar to that previously used by our group and others.15-17,20 Approximately 2.5 g of the supported molybdate were placed in a quartz reaction tube. The material was supported on a plug of glass wool which was held up by a fritted disk. The reactor was then placed in a furnace. The molybdate was exposed to ammonia (Scott, 99.998%) flowing at 150 cm3/min, then heated from room temperature to 623 K in 0.5 h, from 623 to 723 K at 40 or 100 K/h, and finally from 723 to 973 K in 1.25 h. The temperature was held at 973 K for 1 h, and then the nitrided material was cooled quickly by removing the quartz tube from the furnace. The sample remained in flowing ammonia for an additional 1 h. After being cooled, the material was passivated for 2 h in a flowing stream of 0.98% O2/He (Scott) at 20 cm3/min. Passivation was performed in an attempt to prevent bulk oxidation of the catalyst. The supported Mo carbides were synthesized via TPR of the supported molybdates with a mixture of 49.0% CH4 in H2 (Scott). The experimental protocol was based on several other established procedures.16,22-24 The supported molybdate was placed in the quartz reaction tube and exposed to the methane mixture flowing at 150 cm3/min. The material was heated from room temperature to 823 K in 0.5 h and then to 1093 K at 60 or 120 K/h where it was held for 1 h. The carburized material was reduced in flowing hydrogen (Cryogenic, 99.99%) (120 cm3/min) at 1093 K for 1 h and then cooled to room (21) Deo, G.; Wachs, I. E. J. Phys. Chem. 1991, 95, 5889-5895. (22) Lee, J. S.; Yeom, M. H.; Park, K. Y.; Nam, I. S.; Chung, J. S.; Kim, Y. G.; Moon, S. H. J. Catal. 1991, 128, 126-136. (23) Lee, J. S.; Oyama S. T.; Boudart, M. J. Catal. 1987, 106, 125133. (24) Choi, J. G.; Brenner, J. R.; Thompson, L. T. J. Catal. 1995, 154, 33-40.

Energy & Fuels, Vol. 11, No. 3, 1997 669 Table 1. Synthesis Parameters and Catalyst Composition catalyst code

Mo loading (wt % Mo)

β (K/h)

X/Moa

O2 uptake (µmol of O2/gcat.)

MN04+ MN04MN08+ MN08MN14+ MN14-

3.8 ( 0.1 3.6 ( 0.1 6.5 ( 0.1 6.5 ( 0.2 11.2 ( 0.3 10.9 ( 0.3

100 40 100 40 100 40

1.01 1.02 0.75 1.26 0.79 0.96

4.66 5.54 8.37 13.9 33.2 26.9

MC04MC04+ MC08MC08+ MC14MC14+

3.0 ( 0.1 2.9 ( 0.1 4.8 ( 0.1 4.9 ( 0.1 8.2 ( 0.1 8.6 ( 0.1

60 120 60 120 60 120

Co-Mo/Al2O3

9

a

29.1 22.9 26.0 24.4 14.3 13.5

7.94 8.50 1.71 2.46 14.0 14.1 93.9

For nitrides, X ) N. For carbides, X ) C.

temperature in hydrogen over 1 h. The catalyst was purged with helium (Cryogenic 99.995%) for 10 min to remove any remaining methane or hydrogen and was finally passivated for 2 h in a flowing stream of 0.98% O2/He at 20 cm3/min. The heating rates used during TPR were varied to determine their effects on the structural and catalytic properties of the materials. The heating rates and final temperatures were chosen on the basis of our previous work with molybdenum nitrides and carbides.15,24 These parameters were initially chosen in an attempt to vary the selectivity of intermediate solid-state reactions while forming the final product. The intermediate reactions affect the properties of the final product.19 The molybdenum loading was also varied so that its effects on the catalytic properties could be evaluated. The synthesis parameters for the catalysts are shown in Table 1. The catalysts were coded as follows: XXyy where XX represents the catalyst (e.g., MN ) molybdenum nitride), and yy indicates the intended molybdenum loading. This is followed by a + or - mark which indicates the level of the first heating rate (β). Catalyst Characterization. The passivated materials, produced as described above, were then characterized by several complementary techniques. X-ray diffraction (XRD) was used to determine the phase constituents of the materials. Diffraction patterns were collected using a Rigaku DMAX-B diffractometer and Cu KR radiation (λ ) 1.542 Å). Data acquisition, peak identification, and plotting were performed using a Gateway 2000 486/33C computer, which was interfaced to the diffractometer. Neutron activation analysis (NAA) was used to measure the aluminum and molybdenum in the materials. The materials were bombarded with neutrons, and the resulting radiation was measured as the radioisotopes decayed. Aluminum produced an isotope with radiation at about 1779 keV, and molybdenum produced an isotope with radiation at 140.5 keV. We used NIST-SRM-1633b as a standard for aluminum and NIST-330 for molybdenum. The amounts of carbon, hydrogen, and nitrogen in the asprepared catalysts were determined using a Perkin-Elmer 2400 CHN elemental analyzer equipped with a thermal conductivity detector. Approximately 0.5 mg of catalyst was loaded into a tin capsule and placed in the analyzer. The catalyst then underwent combustion at 1198 K, and the gaseous products were reduced at 913 K. The amounts of CO2, H2O, and N2 were measured and recorded. In addition, control runs of carburized and nitrided alumina were performed. Oxygen chemisorption was measured using a Quantasorb Sorption Analyzer (Model QS-17). Approximately 100 mg of material was placed in a quartz U-tube and reduced in hydrogen at 20 cm3/min for 3 h. The nitrides were reduced at 673 K,7,14 and the carbides were reduced at 753 K.24 Following reduction, the catalyst was purged with helium at the reduc-

670 Energy & Fuels, Vol. 11, No. 3, 1997

Dolce et al.

Figure 1. X-ray diffraction patterns for (a) MoOx/Al2O3, (b) MN14-, and (c) MC14-. tion temperature. The material was then cooled to room temperature in helium flowing at 20 cm3/min and finally placed in a dry ice/acetone bath at 195 K. Pulses of a gas mixture containing 10.1% O2 in helium (AGA) were injected into a flowing stream of helium and passed over the catalyst. The amount of oxygen that did not adsorb on the catalyst was measured by a thermal conductivity detector (TCD) and recorded. When several consecutive injections produced a relatively constant TCD signal, the catalyst was assumed to be saturated. Oxygen uptake was then calculated on the basis of the TCD signal and the number of injections. Catalytic Activity. Reaction rate measurements were made using a batch microreactor system. This type of system has been employed by several other groups in recent studies.25-29 The catalytic reactions were performed in 2 mL stainless steel reactors. The reactor was made of Swagelok components including two caps (3/8 in.) and a port connector (3/8 in.). The reactor was connected to a Whitey Union Bonnet needle valve and a Swagelok quick connect via a 4 in. length of 1/4 in. tubing. The reactor was loaded with approximately 10 mg of catalyst and 100 mg of reactant. The reactants were quinoline (Aldrich, 98%), benzothiophene (Aldrich, 99%), benzofuran (Aldrich, 99.5%), 9-methylcarbazole (Aldrich, 99%), dibenzothiophene (Aldrich, 99%), and/or dibenzofuran (TCI, 98%). Two glass beads (d ) 4 mm) were added to the reactor to enhance mixing. The reactor was then sealed and pressurized with hydrogen to 1000 psig at room temperature. The reactor portion was immersed in a heated fluidized sand bath so that only the valve remained above the surface of the bath. The bath temperature was maintained at 663 K for all reactions except benzothiophene HDS, which was run at 593 K. These temperatures were chosen so as to achieve no more than 10% conversion, based on heteroatom removal. The reactor was agitated manually every 10 min. The reactions were run for 1-4 h, after which the reactor was removed from the bath and immediately cooled to room temperature. The pressure in the reactor was measured, then the gases were vented. The liquid products were extracted with hexadecane (Aldrich, 99%), and the reactor was cleaned with acetone and baked overnight for subsequent use. Liquid product identification was accomplished using a Hewlett-Packard 5890 gas chromatograph (GC) equipped with (25) Hajdu, P. E.; Tierney, J. W.; Wender, I. Energy Fuels 1996, 10, 493-503. (26) Benito, A. M.; Martı´nez, M. T.; Ferna´ndez, I.; Miranda, J. L. Energy Fuels 1996, 10, 401-408. (27) Beasley, T. M.; Curtis, C. W. Energy Fuels 1996, 10, 209-215. (28) Walter, T. D.; Klein, M. T. Energy Fuels 1995, 9, 1058-1061. (29) Kim, H.; Curtis, C. W. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1990, 35, 1064.

a Hewlett-Packard 5970 mass spectrometer (MS). All subsequent quantitative measurements were carried out on a separate HP 5890 GC equipped with a flame ionization detector (FID). Both systems were equipped with HP-1 crosslinked methyl silicone gum columns (12 m × 0.2 mm × 0.33 µm film thickness). The molybdenum nitrides and carbides were compared to a sulfided commercial Co-Mo/Al2O3 catalyst (Crosfield 477). The commercial catalyst was sulfided in a mixture of 2% hydrogen sulfide in hydrogen at 673 K for 4 h. It was then cooled to room temperature in the hydrogen sulfide mixture and purged with helium. The commercial catalyst was ground to the same mesh size (60 mesh) as the nitrides and carbides prior to use.

Results and Discussion Catalyst Characterization. No peaks other than those associated with the support were observed in diffraction patterns for the low- and medium-loaded materials, presumably because the oxide, nitride, and carbide domains were too highly dispersed. The alumina support produced broad peaks that may have masked peaks that were due to other crystalline species in the 4 and 8 wt % loaded materials. Clear evidence of crystalline phases was, however, observed for the high-loaded materials. Figure 1 shows the diffraction patterns for several of these materials. Diffraction patterns for the molybdate precursor (top curve) indicated the presence of MoO3. The absence of oxide peaks in the nitride (center curve) and carbide (bottom curve) patterns suggested that the molybdate was completely converted during nitridation and carburization. The phases that are expected for the conditions used to nitride and carburize the molybdates are γ-Mo2N and β-Mo2C, respectively (see Figure 1). There was no evidence of either of these phases in the supported materials, even at the highest loadings, suggesting that the nitride and carbide domains were very small and lacked long-range order. Table 1 shows the compositions of the nitrides and carbides as determined by NAA and CHN analysis. The molybdenum loadings were lower than those expected on the basis of the precursor solution concentrations. We believe that some of the Mo was removed during calcination in the form of volatile molybdates. Additionally, differences may be partially due to experi-


Energy & Fuels, Vol. 11, No. 3, 1997 671 Table 2. Quinoline Hydrodenitrogenation Activity

nmol/gcat./s

activitya µmol/mol of Mo/s

mmol/mol of O2/s

3.8 3.6 6.5 6.5 11.2 10.9

124 84 121 100 110 140

310 223 177 148 94 123

26.6 15.2 14.5 7.2 3.3 5.2


3.0 2.9 4.8 4.9 8.2 8.6

135 141 49 58 107 104

435 464 97 113 126 116

17.0 16.6 28.7 23.6 7.6 7.4

Co-Mo/Al2O3

9

1260

1300

Mo loading (wt %)

MN04+ MN04MN08+ MN08MN14+ MN14-

catalyst

Figure 2. Effect of Mo loading on oxygen uptake for nitrides and carbides.

mental inaccuracy in determining the point of incipient wetness during impregnation. The measured loadings, however, were consistent with the experimental design. That is, materials with low, medium, and high loadings were produced. The nitrided materials contained significant amounts of nitrogen, indicating that nitrides were formed even for the low loaded catalysts. The N/Mo ratios were somewhat higher than the values expected for stoichiometric Mo2N; however, these ratios are consistent with those reported by Colling and Thompson15 for “as-prepared” nitrides. There was also evidence of some nitrogen on the support. A nitrided alumina contained approximately 0.4 wt % nitrogen, while the lowest loaded nitrides contained about 0.6 wt % nitrogen. These results suggest that some ammonia was adsorbed on the alumina support. The presence of small amounts of hydrogen in the nitrided materials supports this claim. The C/Mo ratios for the carbides were less informative. As shown in Table 1, the C/Mo ratios are considerably higher than the expected stoichiometric ratio. This excess carbon was likely graphitic. Results for related unsupported Mo carbides indicated that hydrogen post treatment is an effective method for removing excess carbon from the surface of carbides. Unsupported carbides prepared under similar conditions had an average C/Mo ratio of 0.77, which is close to that expected for Mo2C. A carburized alumina specimen contained approximately 0.5 wt % carbon, while the supported carbides contained approximately 10-15 wt % carbon. Oxygen chemisorption was used to quantify the dispersions of the nitride and carbide catalysts. For unsupported molybdenum nitrides and carbides, the oxygen uptake scaled linearly with the surface area.14,24 The average O/Mo stoichiometries were 0.21 ( 0.05 for the unsupported nitrides and 0.13 ( 0.03 for the unsupported carbides. Oxygen uptakes for the supported nitrides and carbides are given in Table 1. Oxygen does not adsorb onto γ-Al2O3 under the chemisorption conditions employed.15 For the nitrides, the amount of adsorbed oxygen increased as the molybdenum loading increased. The carbides, however, exhibited no distinct trend. The oxygen uptake was lowest for the medium-loaded carbides. We suspect that excess carbon may have blocked sites and may be responsible for the large variations in oxygen uptake by the carbides. A plot of the O/Mo ratio versus loading characterizes changes in the dispersion and particle morphology (Figure 2). Repeated oxygen chemisorption experi-

10

Activities measured at 663 K, ≈2000 psi H2, and e10% HDN conversion. a

Table 3. Benzothiophene Hydrodesulfurization Activity activitya µmol/mol of Mo/s

Mo loading (wt %)

nmol/gcat./s

MN04+ MN04MN08+ MN08MN14+ MN14-

3.8 3.6 6.5 6.5 11.2 10.9

1180 1060 1670 1620 2080 2080

2970 2820 2450 2390 1790 1830

0.25 0.19 0.20 0.12 0.06 0.08


3.0 2.9 4.8 4.9 8.2 8.6

570 1010 429 681 892 759

1830 3320 850 1340 1050 844

0.07 0.12 0.25 0.28 0.06 0.05

Co-Mo/Al2O3

9

10130

10420

0.11

catalyst

mol/mol of O2/s

Activities measured at 593 K, ≈2000 psi H2, and e10% HDS conversion. a

ments resulted in a standard deviation of 10% as shown by the error bars in Figure 2. Using the O/Mo ratios for unsupported materials as our basis, we estimated that the dispersions for the supported nitrides ranged from 10% to 30% while those for the carbides ranged up to 40%. For the nitrides, the O/Mo ratio increased slightly as the loading increased. There are at least two explanations for this observation. The nitride particle size may have increased with decreasing loading. The somewhat low O/Mo ratios for the low-loaded nitrides could also be a consequence of difficulties in nitriding these materials. High-loaded materials are typically easier to completely nitride than low-loaded materials.15 The presence of oxygen in the molybdenum nitride lattice is known to suppress the oxygen uptake.15 Because there was no evidence of large particles in the low-loaded materials, we have concluded that the nitride domains, particularly for the low-loaded materials, were only partially nitrided. The O/Mo ratios for the carbides again showed no discernible trend, although the data seemed to suggest a lower dispersion at higher loadings, perhaps due to the presence of larger carbide particles. Oxygen chemisorption is also useful for measuring the dispersion of supported and unsupported molybdenum sulfides.30,31 The sulfide catalyst adsorbed significantly (30) Concha, B. E.; Bartholomew, C. H. J. Catal. 1983, 79, 327333.


Dolce et al.

Table 4. Benzofuran Hydrodeoxygenation Activity

nmol/gcat./s

activitya µmol/mol of Mo/s

mmol/mol of O2/s

3.8 3.6 6.5 6.5 11.2 10.9

68 80 160 206 398 260

170 214 234 304 342 229

14.6 14.4 19.1 14.8 12.0 9.7


3.0 2.9 4.8 4.9 8.2 8.6

91 152 49 44 349 507

293 498 97 87 409 564

11.5 17.9 28.7 17.9 24.9 36.0

Co-Mo/Al2O3

9

5130

5280

Mo loading (wt %)

MN04+ MN04MN08+ MN08MN14+ MN14-

catalyst

50

Activities measured at 663 K, ≈2000 psi H2, and e10% HDO conversion. a

more oxygen than the nitrides and carbides. On the basis of O/Mo ratios reported in the literature for molybdenum sulfide catalysts,30-32 it is evident that the promoted sulfide used in our study was more highly dispersed than the nitrides and carbides. Catalytic Activity. Tables 2-4 display the activities of the supported molybdenum nitride and carbide catalysts for quinoline HDN, benzothiophene HDS, and benzofuran HDO. We report three measures of activity, and these activities were reproducible to within approximately 10%. The first activity is based on the catalyst mass, the second is based on the moles of molybdenum in the catalyst, and the third is based on the amount of oxygen that chemisorbed on the catalyst. The first activity measure is probably the most relevant for commercial applications because a high activity per unit mass implies low catalyst requirement and a low reactor volume. Indeed commercial catalysts are optimized to have a high activity per unit mass. The catalysts we synthesized, on the other hand, are not optimized for commercial use. Consequently, the final activity measure is probably the most relevant to use to compare the different catalysts in this study since it is a site-based measure of the performance. We verified that the measured activities were not influenced by mass transfer using the Weisz-Prater and Mears criteria. The Weisz-Prater criterion33 is used to determine whether a system is pore-diffusion limited. This criterion states that if

CWP )

-rAFR2 ,1 DeCAs

then the reaction is not pore-diffusion limited (-rA is the measured reaction rate, F is the catalyst density, R is the particle radius, De is the effective diffusivity, and CAs is the surface concentration of reactant). We estimated CWP values on the order of 10-1, which indicates that pore diffusion did not limit the measured reaction rates. We used the Mears criterion33 to assess the potential influence of external mass transfer effects. (31) Muralidhar, G.; Concha, B. E.; Bartholomew, G. L.; Bartholomew, C. H. J. Catal. 1984, 89, 274-284. (32) Brenner, J. R. Ph.D. Dissertation, The University of Michigan, Ann Arbor, MI, 1994; p 21. (33) Fogler, H. S. Elements of Chemical Reaction Engineering, 2nd ed.; Prentice Hall: Englewood Cliffs, NJ, 1992; pp 625-628.

Figure 3. Effect of Mo loading on quinoline HDN activity (663 K, ≈2000 psi of H2).

This criterion states that when

CM )

(-rA)FRn < 0.15 kcCA

mass transfer from the bulk fluid to the catalyst surface does not limit the reaction rate (n is the reaction order, assumed to be 1, kc is the mass transfer coefficient, and CA is the reactant concentration in the bulk fluid). Even when using a minimum value of kc (Sherwood number ) 2), we estimated CM to range from 0.01 to 0.1, which indicates that mass transfer from the bulk fluid to the catalyst surface did not control the reaction rates. For quinoline HDN, the nitrides and carbides exhibited similar activities on a molybdenum basis (Table 2). The sulfided Co-Mo/Al2O3 catalyst was the most active catalyst for HDN on both a catalyst mass and a molybdenum basis. Variations in the heating rates used during catalyst synthesis led to as much as a 40% difference in the molybdenum-based activities for both the nitrides and carbides. The molybdenum loading had a much more significant effect on their activities. For both the nitrides and carbides, the HDN activities on a molybdenum basis generally decreased as the loading was increased (Figure 3). This trend does not appear to be a consequence of dispersion effects since changes in the dispersion were significantly smaller than changes in the activities. Our observations are consistent with those reported by Colling and Thompson15 for pyridine HDN over γ-Al2O3-supported molybdenum nitrides. On the basis of results from thermal desorption spectroscopy and transmission electron microscopy, they concluded that the most active sites for pyridine HDN were at the interface between the nitride domains and the support. As the loading increases, the relative number of Mo atoms at the perimeter of the particles decreases. Assuming that the active sites are concentrated at the particle perimeter, the activity on a molybdenum basis should decrease. It is also possible that, as the loading was varied, the quality of the sites present on the supported nitrides and carbides changed. While we have tentatively concluded that the most active sites resided at the particle perimeters, more detailed characterization of the supported nitrides and carbides using techniques including transmission electron microscopy and infrared spectroscopy is required. Activities based on the chemisorptive uptake allow a more direct comparison of the intrinsic properties of the nitride, carbide, and sulfide catalysts. On an oxygen


Energy & Fuels, Vol. 11, No. 3, 1997 673 Table 5. 9-Methylcarbazole Hydrodenitrogenation Activity activitya catalyst code

nmol/gcat./s

µmol/mol of Mo/s

mmol/mol of O2/s

MC04+ MN04+ Co-Mo/Al2O3

51 12 640

170 30 680

6.1 2.5 7.0

a Activities measured at 663 K, ≈2000 psi H , and e10% HDN 2 conversion.

Table 6. Dibenzothiophene Hydrodesulfurization Activity activitya

Figure 4. Product distributions for quinoline HDN (663 K, ≈2000 psi of H2).

catalyst code

nmol/gcat./s

µmol/mol of Mo/s

mmol/mol of O2/s


1450 1840 4420

4780 4650 4710

171 388 49

a Activities measured at 663 K, ≈2000 psi H , and e10% HDS 2 conversion.

Table 7. Dibenzofuran Hydrodeoxygenation Activity activitya catalyst code

nmol/gcat./s

µmol/mol of Mo/s

mmol/mol of O2/s


230 46 3710

758 116 3950

27 10 41

a Activities measured at 663 K, ≈2000 psi H , and e10% HDO 2 conversion.

Figure 5. Product distributions for benzofuran HDO (663 K, ≈2000 psi of H2).

chemisorption basis, activities of the nitrides and carbides were similar or superior to those of the sulfide catalyst. Considering that the commercial sulfide catalyst has been optimized for hydrotreatment via the addition of promoters, these observations are encouraging. Unpromoted molybdenum sulfide catalysts can be more than one order of magnitude less active than their promoted counterparts.34 Products from quinoline HDN included substituted benzenes, cyclohexenes, and cyclohexanes (Figure 4). Overall, approximately 65 mol % of the product was 1,2,3,4-tetrahydroquinoline and approximately 20% was substituted anilines. These products have been observed in previous studies of quinoline HDN over molybdenum nitrides and carbides.8,12 Inspecting the product distributions for each catalyst provided information about the amount of H2 required to remove the heteroatoms. The lowest hydrogen consumption (highest hydrogen efficiency) would correspond to the exclusive formation of aromatic products. The formation of partially and fully hydrogenated products comes at the expense of higher hydrogen consumption. Figure 4 shows that the nitrides formed significant amounts of benzenes and cyclohexenes and smaller quantities of fully hydrogenated cyclohexanes. The carbides formed more cyclohexanes than the nitrides, but their overall HDN product distributions exhibited significantly less hydrogenation than those of the sulfide catalyst. Thus, the nitrides and carbides were more hydrogen efficient (34) Topsøe, H.; Clausen, B. S.; Massoth, F. E. Hydrotreating Catalysis: Science and Technology; Springer: New York, 1996; pp 162-165.

than the sulfide catalyst. That is, the sulfide catalyst consumed more hydrogen than the nitrides or carbides in achieving the same HDN conversion. Hydrogen consumption is a large contributor to the overall cost of hydrotreatment, so a higher efficiency such as that accomplished by the nitrides and carbides is desired. The differences in the product distributions apparent in Figure 4 suggested differences in reaction pathways. Reaction pathways for the nitrides and carbides appeared to be more selective toward C-N hydrogenolysis than those of the sulfide catalyst. Table 3 shows the results for benzothiophene HDS. The nitrides and carbides were less active than the sulfide catalyst on a molybdenum basis. The activities on an oxygen uptake basis, however, were similar for the nitrides, carbides, and sulfided Co-Mo/Al2O3 catalyst. The synthesis conditions affected the overall activities of the catalysts much less than the loading. The HDS activity on a molybdenum basis was a strong function of the loading with the low-loaded nitrides possessing the highest activities on a molybdenum basis. The similarity between the HDN and HDS activity trends suggested that these reactions occurred on the same types of sites for the nitrides and carbides. The product distributions for benzothiophene HDS were similar for all of the catalysts. The primary nonHDS product was 2,3-dihydrobenzothiophene, which accounted for 70-80 mol % of the product spectrum. The primary HDS product for all of the catalysts was ethyl benzene, as was also observed by Abe and Bell9 for benzothiophene HDS over γ-Mo2N. Hydrogen efficiencies for the nitride, carbide, and sulfide catalysts were essentially the same. Small amounts of cyclohexanes were produced, and only trace amounts of cyclohexenes were observed.


Dolce et al.

Figure 6. Product distributions for 9-methylcarbazole HDN (663 K, ≈2000 psi of H2).

Table 4 shows the activities for benzofuran HDO. The carbides and nitrides exhibited similar activities, but they were less active than the sulfided Co-Mo/Al2O3 catalyst on all three activity bases. The synthesis parameters led to differences of up to 50% in the molybdenum-based HDO activities of the nitrides and carbides. Over the nitrides, the activity per gram of catalyst increased slightly with the loading. However, the loading did not affect the HDO activity to the same extent that it affected the HDN and HDS activities, suggesting that HDO took place at sites different from those at which HDN and HDS occurred. Substituted phenols were the most common products and accounted for 80-90 mol % of the product spectrum. The HDO product distributions indicated that the nitrides and carbides were as hydrogen efficient as the Co-Mo/Al2O3 catalyst (Figure 5). The nitrides formed mostly benzenes and less than 10 mol % cyclohexanes. The sulfide catalyst, however, formed approximately 20% cyclohexanes. Product distributions for the carbides were also similar to those of the sulfide catalyst. Two of the 12 catalysts (MN04+ and MC04+) were consistently more active on a Mo basis than the other nitrides and carbides and were therefore chosen to be tested for 9-methylcarbazole HDN, dibenzothiophene HDS, and dibenzofuran HDO. These molecules are among the most difficult to hydrotreat.35 Results of the reaction studies are shown in Tables 5-7. These initial experiments demonstrate that the nitrides and carbides are quite active for the hydrotreatment of multiple-ring heteroatom compounds. In some cases, the nitrides and carbides were as active as the sulfided Co-Mo/Al2O3 catalyst. On an oxygen uptake basis, the HDN and HDO activities of the nitrides and carbides were comparable to those of the Co-Mo/Al2O3 catalyst, and the HDS activities of the nitrides and carbides were higher. In general, the results suggest that with further development the molybdenum nitride and carbide catalysts could be competitive with or superior to currently available hydrotreatment catalysts. Figure 6 shows the product distributions for methylcarbazole HDN. The nitrides and carbides were competitive with the sulfided Co-Mo/Al2O3 catalyst in terms (35) Girgis, M. J.; Gates, B. C. Ind. Eng. Chem. Res. 1991, 30, 20212058.

of hydrogen efficiency. All of the catalysts formed significant amounts of bicyclic compounds, indicating a low selectivity toward C-C bond cleavage. Figure 7 displays the product distributions for dibenzothiophene HDS. The nitride and carbide, which were the more active catalysts, also appeared to be the most hydrogen efficient. Almost all of the HDS products were bicyclic and aromatic compounds over these catalysts. The sulfide catalyst exhibited higher selectivities toward hydrogenation and C-C hydrogenolysis than the nitride and carbide, indicating that the nitride and carbide were more hydrogen efficient. Figure 8 shows the dibenzofuran HDO product distributions. For this reaction, all of the catalysts had a high C-C hydrogenolysis activity, but the sulfided Co-Mo/Al2O3 catalyst also exhibited high hydrogenation activities. The nitride and carbide formed smaller amounts of cyclohexanes and more aromatics and cyclohexenes, indicating a greater hydrogen efficiency. Summary and Conclusions A series of γ-Al2O3-supported Mo nitrides and carbides was synthesized via the temperature-programmed nitridation or carburization of supported molybdates. The molybdenum loadings ranged from 3 to 11 wt %. Various heating rates were used during the temperature-programmed reactions. Even at the highest loading, the domains were too highly dispersed to be detected by XRD. Elemental analysis indicated that large amounts of free carbon were deposited on the carbides. We believe that carbon deposition was facilitated by the support. Oxygen chemisorptive uptake provided an indication of the dispersion of the molybdenum nitride or carbide domains.14,24 The oxygen uptake for the nitrides increased as the molybdenum loading increased, but the uptake for the carbides showed no distinct trend. The dispersion appeared to vary on the carbides, presumably because of the high levels of free carbon on the surface. The uptake for the nitrides indicated that the dispersion was fairly constant. We have tentatively concluded that the increase in O/Mo ratios was a consequence of particles in the low-loaded materials not being fully nitrided.


Energy & Fuels, Vol. 11, No. 3, 1997 675

Figure 7. Product distributions for dibenzothiophene HDS (663 K, ≈2000 psi of H2).

Figure 8. Product distributions for dibenzofuran HDO (663 K, ≈2000 psi of H2).

The materials were tested for quinoline HDN, benzothiophene HDS, and benzofuran HDO. Their activities for heteroatom removal were compared to a sulfided commercial Co-Mo/Al2O3 catalyst. In some cases, the nitride and carbide catalysts were superior to the sulfide catalyst. Product distributions showed that, for HDN, the nitrides and carbides were more hydrogen efficient than the sulfide catalyst. This was particularly true for the nitrides. Product distributions for HDS were similar for all materials, including the sulfide catalyst. For both HDN and HDS, the activity on a molybdenum basis was an inverse function of the molybdenum loading. This result suggested that HDN and HDS occurred at the same sites on the nitrides and carbides. For the nitrides, we believe that HDN and HDS occurred at the perimeter of the nitride particles, as reported by Colling and Thompson.15 It appeared that HDO was not as site specific over the nitrides and carbides.

Two catalysts were tested for the hydrotreatment of 9-methylcarbazole, dibenzothiophene, and dibenzofuran. The selected nitride and carbide catalysts had specific activities for HDS that were superior to the sulfide catalyst. In addition, the nitrides and carbides appeared to be at least as hydrogen efficient as the sulfide catalyst for these reactions. Together the results suggest that with optimization the nitrides and carbides could be superior hydrotreatment catalysts with very high activities and hydrogen efficiencies. Acknowledgment. We are grateful for the financial support of the Department of Energy University Coal Research Program (DE-PS22-92PC92520) administered through the Pittsburgh Energy Technology Center (PETC). EF960083Y

Hydrotreatment Activities of Supported Molybdenum Nitrides and

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