Energy & Fuels 1995,9, 921-927
921
Hydrotreatment of Naphtha with Molybdenum Nitride Catalysts Shuh-Jeng Liaw, Ajoy Raje, Xiang X. Bi, Peter C. Eklund, Uschi M. Graham, and Burtron H. Davis* Center for Applied Energy Research, University of Kentucky, 3572 Iron Works Pike, Lexington, Kentucky 4051 1 Received February 21, 1995@
Nanoscale, high surface area bulk molybdenum nitrides have been synthesized using two methods: a laser pyrolysis technique (LIyr) and a temperature-programmed reaction (TPR). The activities of these materials for the simultaneous hydrodenitrogenation (HDN) and hydrodesulfurization (HDS) of Illinois No. 6 naphtha were determined. Both catalysts were active for the HDN and HDS reactions and, on an unit mass basis, exhibited the same activity and selectivity.
Introduction Sulfur-containing heteroatom compounds are present in appreciable concentrations in coal-derived naphtha. A distinguishing feature of coal-derived liquids (compared to petroleum distillates) is the presence of appreciably larger quantities of nitrogen- and oxygenheteroatom compounds. Previous investigations into the hydrotreatment (removal of sulfur, nitrogen, and oxygen) of coal-derived naphtha has shown the need for catalysts with higher activity to remove the heteroatom compounds to an acceptable Transition metal nitrides have shown catalytic activity for several reactions. Anderson reported that iron nitrides have significant catalytic activity for the Fischer-Tropsch synthesis.6 Kimperamn and Temkin7 prepared MozN by reacting ammonium molybdate with ammonia, and Hillis et a1.* prepared MozN from MoOz by reduction in HZfollowed by nitriding in Nz. These materials exhibited catalytic activity for the ammonia synthesis. Unfortunately, most of the catalysts employed in the above studies had a surface area less than 1 m2/g. In order for a catalyst to be used in practical applications, a high surface area to volume ratio is desirable. Recently, Mo2N with a surface area as high as 220 m2/g has been prepared by temperature-programmed reduction of the molybdenum oxide using ammonia. This synthesis is considered to involve a topotactic r e a ~ t i o n .Since ~ then, several studies have Abstract published in Advance ACS Abstracts, August 1, 1995. (1)Frumkin, H.; Sullivan, R. F.; Strangeland, B. E. In Upgrading Coal Liquids; Sullivan, R. F., Ed., ACS Symposium Series; American Chemical Society: Washington, DC, 1981; Vol. 156, pp 75. (2)Smith, V. E.; Cha, C. Y.; Merriam, N. W.; Faky, J . ; Guffy, F. Proc. Third Annu. Oil Shale, Tar Sand Mild Gasification Contractors Reu. Meet. 1988, 166. (3) Gaeser, U. R.; Holighaus, R.; Dohms, K. D.; Langhoff, J . Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1988,33, 339. (4) Parker, R. J.; Mohammed, P.;Wilson, J . Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1988,33, 135. ( 5 ) Xu, L.; Keogh, R. A.; Huang, C.; Spicer, R. L.; Sparks, D. E.; Lambert, S.; Thomas, G. A.; Davis, B. H.Prepr. Pap.-Am. Chem. Soc., Diu. Fuel Chen. 1991,36, 1909. (6) Anderson, R. B. Catal. Rev. 1980,21, 53. 1946, (7) Kiperman, S.;Tempkin, M. Acta. Physicochim. U.R.S.S. 21, 267. (8)Hillis, M. R.; Kemball, C.; Roberts, M. W. Trans. Faraday SOC. 1988, 62, 3570. @
shown that Mo2N catalysts prepared by this method are effective and selective hydrotreatment catalysts.1°-14 Haggerty et al.I5 developed a technique to synthesize nanoscale powders using a laser pyrolysis technique. The technique involves the intersection of a tunable C02 laser with a gas stream carrying a combination of reactant gases. The rapid heating and cooling rates allowed by this technique permits the production of nanoscale powders under nonequilibrium conditions. The powder produced by this technique has been reported to have a narrower size distribution and to be less contaminated than those produced by conventional oven-based methods.15 The purpose of this work is to compare the catalytic activity of MozN prepared by a topotactic reaction with one prepared by a laser pyrolysis technique for the hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) of naphtha derived from Illinois No. 6 coal.
Catalyst Preparation Laser Pyrolysis Technique (LPT). A nanoscale molybdenum powder was generated in a laser pyrolysis system whose schematic is shown in Figure 1. The reactant gas mixture contains NH3 (liquid carbonic, anhydrous grade, 99.99%), CzH4 (Aldrich, 99.5+%), and Mo(CO)~.Gaseous NH3 is introduced a t the input to a cell that is heated at 150 "C, approximately the sublimation temperature of Mo(CO)~ (Aldrich, 99.9%)a t 1 atm, and mixes with the Mo(CO)~ vapor. CzH4 is mixed into the reactant gas stream above the Mo(COI6sublimation cell. The flow rates of the various gases are controlled by' electronic flow rate monitors. The reactant gas flows vertically out of a central stainless steel tube (or nozzle) surrounded by a second, larger diameter stainless steel tube used to establish a concentric flow of Ar gas around the nucleating nanoparticles. The (9) Volpe, L.; Boudart, M. J. Solid State Chem. 1985, 59, 332. (10) Oyama, S. T. Catal. Today 1992, 8158, 179. (11) Colling, C. W.; Thompson, L. T. J. Catal. 1994, 146, 193. (12) Schlatter, J. C.; Oyama, S. T.; Metcalfe, J. E.; Lambert, J. M. Ind. Eng. Chem. Res. 1988,27, 1648. (13)Abe, H.; Bell, A. T. Catal. Lett. 1993,18, 1. (14) Nagai, M.; Miyao, T.; Tuboi, T. Catal. Lett. 1993, 18, 9. (15) Haggerty, J . S. Laser Induced Chemical Processes, Steinfeld, J. I., Ed.; Plenum Press: New York, 1981.
0887-0624/95/2509-0921$09.00/00 1995 American Chemical Society
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922 Energy & Fuels, Vol. 9,No. 5, 1995
/
Glass ContaJner (for llquld precursors)
1
kubllmatlon Cell (for solld precursors)
NH,
Figure 1. Schematic of the COz laser pyrolysis system used for the synthesis of nanoscale particles.
particles are thereafter entrained in an inert gas and flow upward into a Pyrex trap. A continuous wave C02 laser (Laser Photonics Model 150) tuned to an absorption band (P20 line) of C2H4 was used to drive the reaction. The reaction pathway involves the thermal decompositionof Mo(CO)6 to elemental Mo and CO, the Mo then reacts with the NH3 to form the nitride, and the C2H4 acts as a passive, heat absorbing species. Thermal energy was pumped into the chemical reaction by tuning the CO2 laser to a vibrationaVrotationa1band of C2H4 (P20 line at 945 cm-l). The as-synthesized nanopowders were handled in air after an in situ T = 300 K surface passivation in a flow of 5% 0 f i e mixture. The reaction conditions used for the synthesis of molybdenum nitride include laser density of 100 w, beam width of 1mm, chamber pressure of 200 Torr, C2H4 flow rate of 70 sccm, and NH3 flow rate of 70 sccm. Details of the experimental procedure have been reported elsewhere. Temperature-ProgrammedReaction (TPR).Molybdenum nitride was also prepared by the temperatureprogrammed reduction and reaction of Moo3 (Johnson Matthey, Puratronic, 99.998%) with ammonia (Liquid Carbonic, anhydrous grade, 99.99%) at atmospheric pressure. MOO3 was placed between pads of quartz wool held in a quartz tube (3/8 in. i. d., 16 in. long) contained in a tubular furnace (Lindberg 55035); the tube was connected to the gas feed system. The temperature was raised linearly (Eurotherm 211programmable controller) at 20 "C/min to about 355 "C, programmed to about 465 "C at a rate of 0.6 "C/min, increased to about 705 "C at 1.8 "C/min, and then held at 705 "C. NH3 flowed (14 mol NHdmol MOO&) over the sample during the entire temperature range and for a period of 30 min at 705 "C. After the synthesis was completed, the sampled was cooled rapidly to room temperature in flowing ammonia. Upon reaching room temperature, the ammonia flow was stopped and the sample was allowed to contact air through the open ends of the tube for more than 18 h. (16)Bi,X. X.;Ganguly, B.; Huffman, G. P.; Huggins, F. E.; Endo, M.; Eklund, P. C. J. Muter. Res. 1993,8,1666.
Activity Test A fixed-bed reactor, operated in a concurrent downflow mode, was used for these studies. The length of the catalyst bed is approximately 7 in. A Brooks Mass flow controller, Model 5850 E, was used to deliver a constant flow of gas. A Milton Roy miniPump solvent delivery system was used for adding the naphtha at the reaction pressure. The reactor, assembled from l/4 in. 0.d. tubing (wall thickness = 0.035 in.), was placed in a 1in. x 12 in. Lindberg tube furnace held in a vertical position. A condenser, placed downstream of the reactor, allowed for collection of the reaction products as a liquid. A hand-loaded back pressure regulator, placed after the condenser and liquid product trap, was used to regulate the pressure in the reactor. The catalyst (3 g) was added to the reactor and pretreated with ammonia prior to naphtha hydrotreatment. During pretreatment, the temperature was raised to 375 "C at a rate of 1"C/min with an ammonia pressure of 1atm and then held at 375 "Cfor 4 h. After pretreatment, the gas was switched to helium (at reaction pressure) for 1h and then switched to hydrogen prior to beginning the flow of naphtha. The simultaneous heteroatom removal, HDS and HDN, was performed over the molybdenum nitride catalysts as the temperature was varied over the range of 275-400 "C while keeping the total pressure (660 psig), weight hourly space velocity (WHSV = 1 g of naphtha/h/g of catalyst), and hydrogen to naphtha g-mol ratio (2.6) constant. The density of the naphtha is 0.81; hence, a WHSV of 1corresponds to a liquid hourly space velocity of 1.23. During a period of catalyst testing, each condition was maintained for 24 h; three samples were taken during the final 6 h of each steady state period to obtain data for heterocompound conversionfor each condition. Prior to analysis, a sample was washed three times with distilled water to remove dissolved H2S and NH3. Naphtha Analyses The naphtha used as the feedstock for this study was produced during the liquefaction of a bituminous Illinois
Energy & Fuels, Vol. 9,No. 5, 1995 923
Hydrotreatment of Naphtha with M o a Catalysts
Table 2. Characterizationof Catalysts 10000 .
XRD
I catalyst
10
20
30 40 50 60 Two Theta (Degree)
70
MozN(TPR)
111
MozN(LPT)
200 220 111 200 220
80
Figure. 2. XRD data for the MozN(LPT), MozN(TPR), and MozN(standard) (ref 18). Table 1. Elemental Analysis of Illinois No. 6 Naphtha element amount 85.6 wt % C H 13.2 wt % N 1420 ppm 818 ppm S 1.24 wt % 0
crystallite size (nm) hkl size
MozN (reob
7.5 6.1 5.5 2.5 2.1 1.7
BET surface z(~oo)/ area (m2/g) Z(111) fresh spent 1.2
153
0.5
42
15.6 45
particle sizea (nm) 4.2 15
0.5
Calculated from surface area assuming a spherical particle and density of 9.5 g/cm3.l* From the powder diffraction file.'* a
Catalyst Characterization. X-ray diffraction patterns of the crystallite phase of the molybdenum nitrides produced by the two methods indicated the samples were y-MozN (Figure 2). The Scherrer equation18 was used t o calculate the crystallite dimensions. The crystallite size was taken as the average ofD(zoo),0(111),and D(zz0). The crystallite size (Table 2),determined from diffraction data, of MozN(LPT) is 2.1 nm and is approximately l/3 that of MozN(TPR) (6.4 nm). The texture, or preferred orientation, can be estimated semiquantitative by the relative intensity of appropriate peaks of the diffraction pattern. In this study, the intensity ratio 1(200)/1(111)was used to determined the
texture. For the MozN(LPT), the ratio of 1(200)/1(111) is equal to 0.5, indicating randomly distributed y-MozN crystallites of uniform dimensions.18 However, a significant degree of texturing was found for MozN(TPR1, since the 1(200)/1(111)ratio was 1.2,much larger than the expected value of 0.5. This kind of texturing, also reported by several r e s e a r ~ h e r s , ~ was J ~ probably ~~~ a consequence of the pseudomorphic nitridation of the Moo3 particles into y-MozN. The used catalysts, characterized by X-ray diffaction, indicated that the bulk structure of the two catalysts was retained and even traces of impurity phases (nitride, oxide, or sulfide) were not found. This agrees with the results reported by Markel and Van Zee.lg The BET surface areas (nitrogen adsorption) of these two catalysts (Table 2)show that MozN(TPR) has about 3 times the surface area of MozN(LPT)(153and 42 m2/ g, respectively). For the MozN(TPR)particle, the average size estimated from the BET surface area assuming a spherical particle was consistent with that calculated from the diffraction peaks; however, a significant difference was observed between these two values for the MozN(LF'T) (Table 2). This result is surprising since the MozN(LPT) sample is spherical shaped, based on XRD data, whereas there is considerable texturing of the MozN(TPR) sample, indicating deviation from a spherical particle. The Mofi(TPR) catalyst does appear to be an agglomeration of smaller particles, and the openings created during nitriding could provide easy access to the internal pores. The crystal size calculated from the BET equation is not strongly dependent upon the shape assumed for the crystal. Thus, it appears that the MozN(TPR) is composed of agglomerates of essentially single-crystal particles whereas the Mo2N(LPT) agglomerates are composed of polycrystalline spherical particles. The adsorption of isotherm hysteresis loop for MozN(TPR) (Figure 3) indicate that the pores are slit-shaped whereas the isotherms for MozN(LPT) (Figure 4) are indicative of nonporous spheres. The pore size distribution calculated from the desorption data for MozN(TPR) shows that approximately 80% of the pore sizes are in micropore range (e.g., less than 2 nm). The SEM pictures (Figure 5 ) clearly distinguish the two samples. The pictures in Figure 5A-C show increasing magnification of the elongated particles that make up the MozN(TPR)sample. The general shape of
(17)Liaw, S.-J.;Raje, A.; Chary, K. V. R.; Davis, B. H. Appl. Catal., A 1996,123,251. (18)Cullity, B. D. Elements of X-rayDiffraction; Addison-Wesley: Reading, MA, 1978.
(19)Markel, E. J.; Van Zee, J. W. J. Catal. 1990, 126, 643. (20) Choi, J. G.; Curl, R. L.; Thompson, L. T. J. Catal. 1994, 146, 218.
No. 6 coal a t the Wilsonville, AI, Advanced Integrated T w o Stage Liquefaction Plant. The sample was collected during run 261. The elemental analyses of the naphtha are shown in Table 1. Total carbon and hydrogen analyses were performed using a Leco CHN analyzer. Total nitrogen contents of the feed and product were determined using a Dohrmann DN-100 total nitrogen analyzer equipped with a chemiluminescence detector. Total sulfur content was determined using a Xertex C-300 microcoulometer. The feed and hydrotreated Illinois No. 6 naphtha samples were analyzed for individual nitrogen compounds using a thermionic specific detector (TSD) coupled to a Varian 3700 gas chromatograph fitted with a KOH treated Carbowax column (30m x 0.32 mm). The amounts of the individual sulfur compounds were obtained using a Sievers Model 350B sulfur chemiluminescence detector (SCD)coupled with a HP 5890 Series I1 gas chromatograph fitted with a SPB-1 column (30m x 0.32 mm). Identification of the nitrogen and sulfur compounds was accomplished by comparison of retention time and doping with standard compounds. Details of the naphtha analyses have been published e1~ewhere.l~
Results and Discussion
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924 Energy & Fuels, Vol. 9,No. 5, 1995 70
I
0
0.2
0.4 0.6 0.8 Relative Pressure, P/Po
1
Figure 3. Adsorption of isotherm for MozN(TPR). 80 i
I
10
0
0.2
0.4 0.6 0.8 Relative Pressure, P/Po
1
Figure 4. Adsorption of isotherm for MozN(LPT).
the 500-3000 pm particles is the same as that of the parent Moos, in agreement with the results of earlier investigators (e.g., refs 9, 19, and 20). The particle featured in Figure 5, B and C, is atypical in that it consists of a number of stacked platelets; however, the particle is atypical in showing that a large number of the particles consist of a stacking of two or more of the elongated plates. The photo in Figure 5D is a high magnification view of the titled particle, such as can be seen in Figure 5C. This plane has a (‘rough surface, as is evident by the fuzziness due to surface charging and/or changing depth. The surface is characterized by abundant cracks which, when connected, form pores throughout the solid. The large platelet particles are composed of smaller particles, both in the plane (Figure 5, D and E) and as viewed along the edge of a large particle (Figure 5F). In contrast, the Mo2N(LPT) sample consists of nearly spherical agglomerates of smaller, nearly spherical particles (Figure 5 G and HI. The SEM pictures provide data that are consistent with the XRD and surface area data. Transmission electron microscopy (TEM) pictures provide a means of identifying the ultimate particle size, and high-resolution TEM can even provide lattice imaging to provide a measure of crystal sizes. TEM data was not obtained for the sample used in this study. Efforts to relate the SEM, TEM, XRD, and surface area for other samples prepared using LPT has not resolved this question to date. Hydrodenitrogenation (HDN). Two different reactors, 114 in. 0.d. with 0.035 in. wall thickness and l12 in. 0.d. with 0.045 in. wall thickness, were used in the initial runs to identify the reactor flow regime. Mass transfer resistance was observed for the 112 in. reactor (a laminar flow); therefore, a l14 in. 0.d. with 0.035 in. wall thickness reactor was chosen for the activity test. Total Nitrogen. Each catalyst was utilized for the hydrotreatment of the Illinois No. 6 naphtha under the
same reaction conditions, and both samples exhibited significant HDN activity. Both catalysts have similar activity for the removal of total nitrogen, on a mass basis, over the temperature range 250-400 “C (Figure 6). The value of the activation energy calculated from the data in Figure 6 is 3.3 kcallmol for the Mo2N(TPR) and 4.0 kcal/mol for the MozN(LPT1. These low activation energies indicate that there may be transport limitations for HDN reactions. The surface area of Mo2N(TPR)is approximately 3.6 times larger than that of Mo2N(LPT);thus, the Mo2N(LPT)has higher specific activity of HDN, on a unit surface area basis, than that of Mo2N(TPR). Therefore, the activity, on a surface area basis, varied with particle size indicating that HDN of Illinois No. 6 naphtha depends on the particle size, as reported for the HDN of model compounds.20B21It was shown in the catalyst characterization section that Mo2N(TPR) particle contains mostly micropores; this implies that most of the surface of the Mo2N(TPR) sample is not accessed by the nitrogen compounds in the naphtha. Markel and Van Zeelg also reported that the lower surface area Mo2N catalyst exhibited a higher specific HDS activity than the higher surface area Mo2N catalyst. The activation energies are based upon rates for removal of total nitrogen. It has been demonstrated that the naphtha used in this study contains nitrogen compounds that may be lumped into a very reactive grouping and a grouping that is much more difficult to convert.22 The very reactive grouping represented approximately 50% of the total nitrogen and was converted so rapidly that conversion of this grouping was complete even at the highest flow rates. Thus, the conversion represented by the easily converted grouping will not exhibit a significant temperature dependence over the range of this study. Including this rapidly converted nitrogen, grouping will therefore yield an “apparent” activation that is significantly lower than the “true” activation energy. Thus, it appears that both the different conversion rates for the nitrogen compound classes and diffusion both play a significant role in determining the low measured activation energy or, more appropriately, the low temperature coefficient. Individual Nitrogen. Characterization of the Illinois No. 6 naphtha using the thermionic specific detector (TSD) coupled with the capillary GC shows that the naphtha contains at least 300-400 nitrogen comp o u n d ~ .Anilines ~~ comprise the major nitrogen class of the naphtha, which consists of aniline and 1- to 4-carbon substituted anilines. The next most abundant nitrogen compounds are pyridines and these are followed by the quinolines. The total nitrogen analyses provides a measure of total removal of nitrogen from the naphtha. Also, the conversion of each individual nitrogen compound, on a mass basis, X(i),was calculated using the areas from the TSD chromatogram as follows:
X(i)% = [area(i), - area[iI,Yarea(i),
x 100
(1)
where i denotes the individual compound, area denotes the GC peak area, f denotes i in the feed, and p denotes the same compound after the conversion. (21) Choi, J. G.; Brenner, J. R.; Colling, C. W.; Demczyk, G. B.; Dunning, J. L.; Thompson, L. T. Cutal. Today 1992, 15, 201. (22) Raje, A,; Liaw, S.-J.;Chary, K. V. R.; Davis, B. H. Appl. Cutul., A 1995, 123, 229.
Hydrotreatment of Naphtha with M o a Catalysts
Energy & Fuels, Vol. 9, No. 5, 1995 925
Figure 5. Scanning electron microscope pictures of the Mo2N(TPR) [A, magnification 110; B magnification 450; C, magnification 1000; D, magnification of 15 000 of particles in part C tilted to show edge and flat plane; E, magnification of 2000 of flat surface region as particles at lower left in part C; F, magnification of 15 000 of edge-on view of platelet particle] and Mo2N(LPT) [G, magnification of 6000; H, magnification of 8001.
926 Energy & Fuels, Vol. 9, No. 5, 1995
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1
-10 0.0014
i
0.002
0.0016 0.0018 l/Temp. (1IK)
Figure 6. Arrhenius plot for the HDN of nitrogen compounds in the naphtha over MozN catalysts prepared by a temperaand by a laser pyrolysis techture-programmed reaction (0) nique ( x ).
0
Figure 9. Conversion of aniline class compounds, based on catalyst mass, over MozN at 350 "C prepared by a temperatureprogrammed reaction (0)and by a laser pyrolysis technique (XI.
Q
s
Q E
0
20
10
20
0
I
0
1C-
I
Figure 7. Conversion of individual nitrogen compounds, based on mass, over MozN catalysts prepared by a temflerature-programmed reaction ( 0 )and by a laser pyrolysis technique ( x ).
-7
d
wl
,
I
02
-2
WaP
3-E-P
-10 0.0014
3,S-DIh-P
Figure 8. Conversion of pyridine class compounds, based on
0.0016 0.0018 1ITemp. (1IK)
0.002
catalyst mass, over Mo2N a t 350 "C prepared by a temperatureprogrammed reaction ( 0 )and by a laser pyrolysis technique
Figure 11. Arrhenius plot for the HDS of sulfur compounds in the naphtha over MozN catalysts prepared by a temperature programmed reaction (0) and by a laser pyrolysis technique
(XI.
(XI.
Among the three major nitrogen classes, pyridines are much easier to convert than anilines and quinolines, and this was true for both catalysts (Figure 7). For the conversion of pyridine class compounds, the activities of the two catalysts are similar (Figure 8). The data for the conversion of aniline class compounds show that for aniline and the methylanilines, the activities of the two catalysts are similar and this is consistent with the results for the pyridine class compounds. For the compounds that are harder to convert, e.g., aniline substituted with 2- to 4-carbon (Figure 9) and the quinoline class compound (Figure 101, the Mo2N(LPT) exhibits a slightly higher conversion than the Mo2N(TPR). The activity comparisons shown in Figures 7-10
are on a weight basis. If the comparison of the activity is based on a unit surface area, Mo2N(LPT) is much more active than the Mo2N(TPR). Hydrodesulfurization(HDS).Total Sulfur. Both catalysts also exhibited significant HDS reactivity, with sulfur compounds being easier to convert than the corresponding nitrogen compounds. Furthermore, the data (Figure 11) show that both catalysts exhibit a similar activity (mass basis) for the removal of total sulfur from the naphtha over the temperatures, range 250-400 "C,and therefore resemble the data for HDN. The value of the activation energy of HDS calculated from Figure 11is 7.66 kcallmol for the Mo2N(TPR) and 8.48 kcaVmol for the Mo2N(LPT). Again, since the BET
Hydrotreatment of Naphtha with M o a Catalysts I
1 9
90 0
e & * ap
Energy h Fuels, Vol.9,No. 5, 1995 927
8
70-
X 0
II
$60-
8
E:
8
6, 20-
40-
2o
lo0-
I
100,
0 Y
i
- . 0
'
100
50
150
200
Time, hr.
Figure 13. Total sulfur [ x , MozN(LR); W, Mo2N(TPR)] and nitrogen [O,MozN(LPT); +, MozN(TPR)] removal a t 350 "C and LHSV = 1with increasing time on stream t o demonstrate retention of catalytic activity.
samples have the same crystal phase, the physical structures of the two Mo2N samples differ. The Mo2N(LPT) sample consists of polycrystalline spherical particles of ca. 15 nm that agglomerated into larger particles. The MozN(TPR) consists of small (2-5 nm), largely single-crystal particles that make up large platelet particles that resemble the shape of parent Moos; these large platelets contain many cracks and voids. Each characterization technique (XRD, SEM, and surface area) produces data that is consistent with this model. Both of the catalysts exhibit catalytic activity for the HDN and HDS of a naphtha derived Illinois No. 6 naphtha. The activities, for the removal of total heteroatoms (N and S),on a mass basis, are similar when the comparison is made on a mass basis. However, when the comparison is on a unit surface area basis, the MozN(LPT) catalyst is more active than the MozN(TPR) catalyst. This implies that only about one-third of the surface of the Mo2N(TPR) sample is accessible for the HDN and HDS reactions. decline slowly with time on stream; thus, the data in the previous figures can be compared directly.
Conclusion Nanoscale catalysts have been Mo2N synthesized using two methods: a laser pyrolysis technique, and a topotactic reaction (temperature-programmed reaction of Moo3 with NH3). It was found that, while the two
Acknowledgment. This work was supported by the DOE contract DE-AC22-90PC91058 and the Commonwealth of Kentucky. The authors also thank the personnel a t the Wilsonville liquefaction facility for Providing the naphtha sample. EF9500371
