Ind. E n g . Chem. Res. 1988, 27, 1639-1648 treating Catalysts Studied by X-ray Photoelectron and Ultraviolet-Visible Spectroscopies”. J. Chem. Soc., Faraday Trans. 1, 1985,81,1047. Morales, A.; Garcia, J. J.; Prada, R. “Mechanism and Kinetic Model for Hydrodemetallation Reactions”. In Proceedings of the 8th International Congress on Catalysis; Verlag Chemie: Weinheim, 1984;Vol. 11, p 341. Mosby, J. F.; Hoekstra, G. B.; Kleinherz, T. A.; Sroka, J. M. ‘Pilot Plant Proves Resid Process”. Hydrocarbon Process 1973,52(5), 93. Oleck, S. M.; Sherry, H. S. “Fresh Water Manganese Nodules as a Catalyst for Demetalizing and Desulfurizing Petroleum Residua”. Ind. Eng. Chem. Process Des. Dev. 1977,16,525. Pazos, J. M.; Aquino, L.; Pachano, J. “Upgrading of High Metals Venezuelan Residua”. Prepr.-Am. Chem. SOC.,Diu. Pet. Chem. 1981,26,456. Rankel, L.A. “Reactions of Metalloporphyrins and Petroporphyrins with H,S and H,”. PreDr.-Am. Chem. SOC..Diu. Pet. Chem. 1981,26,689. Riley, K. L. “The Effect of Catalyst Properties on Heavy Feed Hvdromocessing”. PreDr.-Am. Chem. SOC.,Diu. Pet. Chem. 1978,23,1104. Shah. Y. T.: Paraskos. J. A.: “IntraDarticle Diffusion Effects in Residue Hydrodesulfurization”. Ind. Eng. Chem. Process Des. Dev. 1975,14, 368. Silbernagel, G. B.; Riley, K. L. “Heavy Feed Hydroprocessing Deactivation: The Chemistry and Impact of Vanadium Deposits”. In Catalyst Deactivation; Delmon, B., Foremont, G. G., Eds.;
1639
Elsevier: Amsterdam, 1980;p 313. Speight, J. G. The Desulfurization of Heavy Oils and Residua; Marcel Dekker: New York, 1981. Sugihara, J. M.; Branthaver, J. F.; Wu, G. Y.; Weatherbee, C. “Research on Metal Compounds in Petroleum-Present and Figure”. Prepr.-Am. Chem. SOC.,Diu. Pet. Chem. 1970,15,C5. Tamm, P. W.; Harnsberger, H. F.; Bridge, A. G. “Effects of Feed Metals on Catalyst Aging in Hydroprocessing Residuum”. Ind. Eng. Chem. Process Des. Deu. 1981,20,262. Van Ginneken, A. J. J.; van Kessel, M. M.; Ponk, K. M. A,; Renstorm, G. “Shell Process Desulfurizes Resid”. Oil Gas J. 1975, 17(3),59. Ware, R. A.; Wei, J. ‘‘Catalytic Hydrodemetallation of Nickel Porphyrins. I. Porphyrin Structure and Reactivity”. J.Catal. 1985a, 93, 100. Ware, R. A.; Wei, J. “Catalytic Hydrodemetallation of Nickel Porphyrins. 11. Effects of Pyridine and of Sulfiding”. J. Catal. 1985b,93, 122. Weitcamp, J.; Gerhardt, W.; Scholl, D. “Hydrodemetalation of Nickel Porphyrins over Sulfided and Reduced CoO-Mo03/yA1203”. In Proceedings of the 8th International Congress on Catalysis; Verlag Chemie: Weinheim, 1984;Vol. 11, p 269. Yen, T. F. “Chemical Aspects of Metals in Native Petroleum”. In The Role of Trace Metals in Petroleum; Yen, T. F., Ed.; Ann Arbor Science: Ann Arbor, MI, 1975;p 1. Received for review August 3, 1987 Accepted May 27, 1988
Preparation and Characterization of Early Transition-Metal Carbides and Nitrides S. Ted Oyama**+and James C. Schlatter Catalytica, 430 Ferguson Drive, Building 3, Mountain View, California 94043
Joseph E. Metcalfe, 111, and Joseph M. Lambert, Jr.l Standard Oil Company of Ohio, 9101 East Pleasant Valley Road, Independence, Ohio 44131
Temperature-programmed synthesis in a flowing reactive gas stream is used to prepare transition-metal carbides and nitrides from groups IVB-VIB in high surface area form. The materials are characterized by X-ray diffraction, elemental analysis, thermogravimetric analysis, temperature-programmed reduction, selective chemisorption of carbon monoxide, and surface area measurements. It is deduced that the space velocity of the synthesis gas is an important factor in obtaining high surface area. Activation energies for the synthesis, calculated from the reactiontemperature profiles, yield values close to those for the diffusion of oxygen in the oxides. The synthesis temperature is found to correlate with the melting point of the parent oxides. These results indicate that a critical step in the synthesis involves disruption of metal-oxygen bonds. Transition-metal carbides and nitrides combine extreme hardness with high thermal and mechanical stability and thus find applications as cutting tools, wear-resistant parts, and high-temperature structural components. Their use in catalysis and the ceramics industry has been limited because the traditional preparation methods yield materials of low surface areas (typically less than a few squared meters/gram). Processing in ceramics requires small and uniform particle sizes for the crucial sintering step. Usage in catalysis requires exposure of a substantial fraction of the material to the surrounding reactive medium. A substantial leap in the preparation of high surface area refractory materials was the development of a temperature-programmed method of synthesis (Oyama, 1981). Present address: Department of Chemical Engineering, Clarkson University, Potsdam, NY 13676. Present address: Chemical Data Systems, 7000 Limestone Road, Oxford, PA 19263.
*
0888-5885/88/2627-1639$01.50/0
This method has been applied in the preparation of Mo2N (Volpe et al., 1983; Boudart et al., 1985), Mo2C (Boudart et al., 1985; Lee et al., 1987),MoC1, (Volpe and Boudart, 1985b), W2N (Volpe and Boudart, 1985a), and WC1, (Volpe and Boudart, 1985b)with specific surface areas of about 100-200 m2 g-l. This investigation extends the temperature-programmed method to the synthesis of transition-metal carbides and nitrides spanning groups IVB-VIB.
Experimental Section The gases employed in this study consisted of H2 (Linde High Purity Grade, 99.995%), 50% H2/Ar (Linde Custom Grade, 99.995%), CH, (Linde Research Grade, 99.99%), NH, (Linde UHP Grade, 99.995%), O2 (Linde Specialty Grade, 99.99%), and CO (Linde Research Grade, 99.97%). The H2 and 50% H2/Ar mixture were purified by passage through a Matheson oxygen-removing purifier (Model OR-10-HP) and through a trap containing Drierite (Ald0 1988 American Chemical Society
1640 Ind. Eng. Chem. Res., Vol. 27, No. 9, 1988 PRESSURE GAUGE
CHqlH2 REACTOR
CONDUCTIVITY 1 - 1
I
I
VENT
GAS CHROMATOGRAPH
TEMPERATURE PROGRAMMER'
2-PEN RECORDER L
rich) and 5A molecular sieves. The CH4was purified from any sulfur impurities by passage through a bed of Ni/Si02 catalyst previously reduced at 673 K. The NH3 was dried by passage through a bed of BaO (Fisher Technical Anhydrous Grade) in porous lump form. The CO was purified by passage through a bed of activated carbon (VWR Y-12). The chemicals used in this study consisted of T i 0 2 (Baker Analyzed Grade, 99.99%), HfOz (Aldrich Gold Label, 99.99%), V205 (Aldrich Gold Label, 99.999%), Nbz05(Aldrich Gold Label, 99.99%), Moos (Aldrich Gold Label, 99.999% ), W03 (Aldrich Gold Label, 99.999% ), and RezO, (Alfa Products, 99.99%). The chemicals were employed without further purification. However, because the chemicals were powders unsuitable for packing in a bed, they were formed into particles before use. This was done by wetting them with distilled water to form a thick paste, drying them in crystallizing dishes, and breaking the resulting cakes into 1-mm chunks. Calculations show that there are no diffusion limitations to reaction with this size of particles. Figure 1shows the catalyst preparation unit. Gases from a delivery manifold (omitted in the figure) pass through the reference side of a thermal conductivity detector (TCD) and through a reactor holding the raw material. A gauge measures the pressure above the packed bed. The effluent gases pass through a dessicant trap containing Drierite (or KOH for drying NH3), enter the sample side of the TCD, and go out to vent. Flow controllers at the inlets to the TCD maintain both flows at 20 Fmols-l, while small bore tubing connected in parallel (dashed lines in Figure 1)allows the remainder of the flows to bypass the TCD. (Flow rates reported in units of micromole/second may be changed to units of cubic centimeters (NTP)/ minute by multiplication by 1.5.) This allows the use of flow rates as high as 3000 kmol s-l in the synthesis apparatus, while permitting the TCD to operate in its most sensitive flow regime. With these flow rates, quantities in excess of 100 g can be processed in a single batch. The output of the TCD is analyzed with the bridge circuit of a gas chromatograph (Wilkens Aerograph Ago), and the signal is recorded on one channel of a two-pen chart recorder (Linear Model 202). The reactor is a quartz tube provided with a quartz frit for holding the solid starting materials. It is constructed with a thermocouple well located close to the center of the bed position. Depending
on the size of the synthesis batch, different reactor diameters were utilized. These ranged from 12 to 32 mm. The reactor was placed inside a furnace (Thermcraft Model 114). The temperature of the reactor was monitored by using a chromel-alumel thermocouple, and the output was recorded on the second channel of the two-pen recorder. The temperature of the furnace was set by a controller (Research Inc. Model 61011) and a programmer (Research Inc. Model 61010). The preparation procedure consisted of loading the metal oxide precursor in the reactor tube, establishing the reactant gas flows, and starting the temperature program. Oftentimes the temperature was initially raised rapidly to a level just below that at which the main solid-state transformation began to occur. After the synthesis was over, the clam-shell furnace was opened and the reactor tube was swung away from the furnace to be rapidly cooled in the flowing reactant mixture. The gas stream was changed to a helium purge stream, and to this was added a small amount of O2 The O2concentration was increased from 0.01% to 20% over the course of many hours. After this passivation procedure, the product batch was taken out and characterized. The bulk crystallographic phase was determined by powder X-ray diffraction (XRD) by using a Philips Norelco 42275/0 diffractometer. The bulk composition was determined by elemental analysis using atomic absorption (Schwarzkopf Microanalytical Laboratory, Woodside, NY). Characterization of the surface was carried out by the irreversible chemisorption of CO at room temperature and the physisorption of N2 at liquid nitrogen temperature. The measurements were carried out in a volumetric adsorption apparatus (Hanson, 1975) equipped with a Texas Instruments differential pressure gauge. Samples of about 1 g were employed in the measurements. Pretreatment generally consisted of reduction of the samples in H2 at a set temperature for 3.5 h. In certain cases the samples were reduced in a temperature-programmed manner in a 50% H2/Ar mixture at a heating rate of 0.017 K s-l by using the same apparatus and procedure as employed in the synthesis of the materials. These two types of pretreatment are designated as IS0 and TPR, respectively. Some of the TPR traces are shown in Figures 7-9. The irreversible CO uptake is obtained by taking the difference between two CO adsorption isotherms, the first measured on the fresh catalyst and the second measured on the same sample after evacuating the sample cell at room temperature to Pa. The evacuation process removes weakly adsorbed CO, leaving behind the chemisorbed CO. The N2physisorption isotherms were used to calculate the BET specific surface area (Brunauer et al., 1938) of the samples. In addition some of the materials were studied by thermogravimetric analysis (TGA) using a Du Pont 1090 TGA apparatus. Samples of about 50 mg, flowing H2 gas, and heating rates of 0.17 K s-l were used in the TGA experiments.
Results Material Preparation. Figures 2-4 depict the time evolution of the synthesis of the various refractory materials. Each figure shows the temperature program and the recorded thermal conductivity response. In most cases the temperature is initially raised quickly to an appropriate level and then is slowly ramped linearly over the region of the transformation. Table I reports the exact preparation conditions. The final heating rate ranged from 0.0011 to 0.065 K s-l. The transformation itself is reflected in the TCD signal. A return of the signal to base line indicates no difference
Ind. Eng. Chem. Res., Vol. 27, No. 9, 1988 1641 Table I. Summary of P r e m r a t i o n Conditions ~
precursor amt, g 23 80 48 12 20 150 14 82 23 1
material prepared
Tic
vc
NbC NbN MozC-1 Mo&-2 MozN-1 MozN-2
wc Re
gas composition, % Ar HZ 95 96 96
CHI 5 4 4
P, kPa 160 170
NH3
200
100 10 20
45
180 240 160 130 170 170 120
45 80 100 100
14 17
43
43 83
MOLYBDENUM CARBIDE
a
TITANIUM CARBIDE
ramp, K s-l 0.0011 0.0016 0.0023 0.065 0.017 0.010 0.025 0.017 0.042 0.017
flow rate, pmol s-l 300 2700 2700 1300 1100 2500 1300 1500 1100 200
1500
TIK
Ti203 -TIC
600 300 0
1
I b
1500
1
I
I
I
I
VANADIUM CARBIDE
1 1
L
I
1200 900 TIK
600 300
CJ
YP3-
woz - wc
300 0
NIOBIUM CARBIDE
C
1500
-
2
1
3
v
I 5
L
6
Ti m e l H ours
Figure 3. Temperature-programmed synthesis of refractory compounds: (a) molybdenum carbide, (b) tungsten carbide. TIK
600
-
300
- 2
0
I
I
0
L
I 8
I
I
I
I
12
16
20
2L
la
MOLYBDENUM NITRIDE
I
Ib
REDUCTION OF Re207
I
Ti me IH ours
Figure 2. Temperature-programmed synthesis of refractory compounds: (a) titanium carbide, (b) vanadium carbide, (c) niobium carbide.
in composition between the gases going in and out of the reactor and, thus, the end of the reaction. (In some cases instrumental drift causes the signal to cross the base line at the end of the reaction.) In carburizations with CH4/Hz mixtures, the TCD signal obtained is complex. A positive signal results from a decrease in the Hzconcentration in the gas stream (decrease in thermal conductivity) and is indicative of the reduction of the oxide. A negative signal is due to a net increase in the H2 concentration in the stream and is indicative of carburization of the material. At times both reduction and carburization processes occur simultaneously so that there is partial cancellation in the TCD signal; however, in general, a single process dominates, and the magnitude of the TCD signal represents the rate of the process. The synthesis of titanium carbide (Figure 2a) proceeds in two stages. The first stage involves the reduction of TiOz to Tiz03at a temperature of 1270 K. The Ti203was identified by XRD in a separate experiment. The second stage involves the reduction/carburization of Tiz03to T i c in a slow process occurring at around 1410 K. The synthesis of vanadium carbide (Figure 2b) also proceeds in two stages. First V205 is reduced to Vz03at 760 K, and then Vz03is transformed to VC at around 1350
01
1
I
0
2
I
I
L
6
8
10
12
TI me IH ours
Figure 4. Temperature-programmed synthesis of refractory compounds: (a) molybdenum nitride, (b) rhenium metal.
K. The V203was isolated in a separate experiment and identified by XRD. The initial reduction of Vz05to VzO3 is strongly exothermic, giving rise to a substantial temperature increase in the reactor bed. The synthesis of niobium carbide (Figure 2c) is very similar to that of vanadium carbide. Again the TCD signal shows two features, the first at about 1060 K and the second centered at 1330 K. In analogy to the vanadium system, an intermediate reduction of Nb205to Nbz03 is likely.
1642 Ind. Eng. Chem. Res., Vol. 27, No. 9, 1988 Table 11. Elemental Analysis compd Tic
metal 79.95 80.92 88.55 86.90 94.11 93.20 93.20 93.87
vc
NbC NbN Mo&-2 Mo2N-1 Mo~N-2
wc
theoretical wt % N C 20.05 19.08 11.45 13.10 5.89 6.80 6.80 6.13
total 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00
metal 87.31 77.17 89.25 84.74 93.30 84.66 91.82 91.09
actual w t % C N 11.94 23.15 11.88
total 99.25 100.32 101.13 95.86 99.09 93.04 99.83 99.23
11.12
5.79 8.38 8.01 8.14
TiO,
t
I
I
b
1
I
1
wc
I
tb
1
20
LO
Mo2C
!I 1 ,
60
I
e
I
80
20
LO
NbC
60
80
Degrees 2 8
Figure 6. X-ray diffraction patterns of refractory compounds: (a) molybdenum trioxide, (b) molybdefium carbide, (c) molybdenum nitride, (d) niobium oxide, (e) niobium carbide, (f) niobium nitride + oxide. 20
LO
60
80
20
LO
60
80
Degrees 28
Figure 5. X-ray diffraction patterns of refractory compounds: (a) titanium dioxide, (b) titanium carbide, (c) vanadium oxide, (d) vanadium carbide, (e) tungsten trioxide, (f) tungsten carbide, (9) rhenium oxide, (h) rhenium metal.
The preparations of molybdenum carbide and tungsten carbide (Figures 3a and 3b) proceed through, first, the formation of an intermediate metal dioxide and, second, the carburization of the dioxide. The Moo3 to Mooz transformation occurs at around 810 K, whereas the W03 to W02 transformation occurs at around 930 K. A temperature increase is observed in the former, indicating that the reduction is exothermic. Both of the intermediate oxides were isolated in pure form as shown by XRD in separate experiments. The Mooz to MozC and WOz to WC conversions occur at around 1090 and 1140 K, respectively. Two different samples of molybdenum carbide, designated MozC-1and Mo2C-2,were prepared, differing primarily in the scale of their preparation. The synthesis of molybdenum nitride (Figure 4a) was difficult to follow with the TCD because in the latter part of the reaction, at temperatures above -700 K, all of the reactant NH3 decomposed in passing through the bed, and the resulting TCD signal was flat. When the scale of the synthesis was decreased from 20-150 g to about 1 g, the end point of the synthesis could be reached before total
decomposition of the NH, occurred. The end point for the large-scale syntheses was obtained by trial and error. An intermediate in the nitridation of Moo3 was a molybdenum oxynitride. Two different samples of molybdenum nitride, designated Mo2N-1and MozN-2,were prepared. The synthesis of niobium nitride was similar to that of molybdenum nitride, with full decomposition of NH3 occurring before the end point of the reaction. The synthesis of rhenium carbide from rhenium oxide by the temperature-programmed method did not occur. Instead, a reduction of Rez07 to Reozat around 470 K and a further reduction of Reoz to Re at around 1000 K took place (Figure 4b). The processes were accompanied by volatilization of rhenium metal. Preparation of hafnium carbide from its oxide was also attempted with the temperature-programmed method. The maximum temperature employed approached 1500 K. The TCD signal obtained showed no deviation from base line, and XRD analysis showed no change in the parent oxide starting material. Characterization. Figures 5 and 6 display the X-ray diffraction patterns of the various synthesized materials and of their respective starting material oxides. The patterns were identified by comparison with standard spectra from a powder diffraction file (McClune, 1979). Table I1 reports elemental analysis of the synthesis products.
Ind. Eng. Chem. Res., Vol. 27, No. 9, 1988 1643 102.4
a
TITANIUM CARBIDE TGA
b
TITANIUM CARBIDE TPR
NIOBIUM CARBIDE TGA
z
-._ u)
3 C
200
LOO
600
800
1000
1200
1400
Temperaturel K
z @-100.2 m C 0
/
V Jz
2
100.1
D
$ 100.0
~
-
VANADIUM CARBIDE TPR
!A
I
I
1
I
600
800
1000
1200
1400
3 C
t.
-e
Figure 9. Thermal analysis of niobium carbide: (a) TGA, (b) TPR. Table 111. Surface Properties of Materials surface area, m2 CO uptake, g-' pmol g-1 material treatment: K Tic ISO-673 4.7 0.08 0.5 Tic TPR-1253 4.7 4.0 TPR-1253 VC 5.3 0 10 NbC ISO-773 11 55 TPR-1313 NbC 1.2 3.6 NbN ISO-718 36 38 Mo2C-1 ISO-673 25 29 ISO-673 MozC-2 28 Mo~C-2 TPR-843 33 280 88 Mo2N-1 ISO-673 22 51 MozN-2 ISO-673 4.5 wc 6.7 ISO-673
VANADIUM CARBIDE TGA
LI
1
LOO
Temperaturel K
Figure 7. Thermal analysis of titanium carbide: (a) TGA, (b) TPR. 100.3
200
/
site density, 10-14 cm-2 0.010 0.064 0.45 0 3.0 0.20 0.64 0.52 0.71 1.9 1.4 0.40
a IS0 refers to isothermal reduction in flowing hydrogen at the indicated temperature for 3.5 h. TPR refers to temperature-programmed reduction in flowing 50% H2/Ar to the indicated temperature at a heating rate of 0.017 K s-l.
9
4
0
+ 200
400
600
800
1000
1200
1400
Temperaturel K
Figure 8. Thermal analysis of vanadium carbide: (a) TGA, (b) TPR.
Parallel TGA and TPR measurements were carried out to determine the optimal temperature for reduction of some of the materials prior to chemisorption measurements. Figures 7-9 present TGA and TPR data from several of the compounds. The TGA results show that T i c , VC, and NbC all increase in weight with increasing temperature. If the increase in weight is due to formation of a bulk hydride, then the calculated compositions at the end of each measurement are H,,TiC, HOsVC, and H1.lNbC. The corresponding TPR traces for VC and NbC (Figures 8b and 9b) show a peak in the same temperature region where the TGA shows a weight increase. The peak is caused by a decrease in the thermal conductivity of the effluent gas as a result of hydrogen consumption. In contrast to VC and NbC, the TPR trace for T i c (Figure 7b) shows no detectable signal even though the TGA indicates substantial hydrogen uptake. The TPR trace of
MozC (not shown) showed a hydrogen consumption feature at 820 K. Table 111reports the specific surface area, irreversible CO uptake, and number density of sites of the various materials. The number density of sites is obtained by dividing the CO uptake value by the specific surface area. The materials are pretreated in isothermal (ISO) or temperature-programmed (TPR) manners, as indicated. The Mo2C-1and the WC samples have in addition been subjected to a mild treatment in a 1% 02/He stream at 673 K prior to reduction.
Discussion Material Preparation and Properties. Many methods have been employed in the past for preparing metal carbides and nitrides, and these are compiled by Toth (1971). Several of the methods involve contacting a metal oxide with solid carbon to achieve reduction of the oxide. Other methods comprise carburization or nitridation of the reduced metals. Table IV lists typical specific surface areas obtained by these methods. Clearly these techniques do not result in the production of high surface area materials. In the case of the reaction between a metal oxide and solid carbon, the carburization must be carried out at high
1644
Ind. Eng. Chem. Res., Vol. 27, No. 9, 1988
Table IV. Literature Reports of Carbide a n d Nitride Catalysts surface area, m2 catalvst use E-l ref Tic 2.7 dehydrogenation Samsonov et al., 1969 0.6 hydrogen oxidation Il'chenko, 1977 Tic Tic 0.8 Kojima et al., 1979 ethylene hydrogenation Tic Fischer-Tropsch 0.5 Kojima et al., 1980 synthesis 0.9 Samsonov et al., vc dehydrogenation 1969 vc hydrogen oxidation 0.3 Il'chenko, 1977 NbC 0.9 Samsonov et al., dehydrogenation 1969 Il'chenko, 1977 NbC 1.5 hydrogen oxidation 7.5 Sinfelt and Yates, M o ~ C ethane hydrogenolysis 1971 1.0 Rycheck and Mo& olefin hydrogenation Pennella, 1978 Boudart et al., 1981 M o ~ C ammonia synthesis 11.0 M o ~ C Fischer-Tropsch 6.8 Saito and Anderson, 1980 synthesis M o ~ N ethylene 5.6 Shigehara, 1977 hydrogenation 12.0 M o ~ N ammonia synthesis Boudart et al., 1981 M o ~ N Fischer-Tropsch 7.3 Saito and Anderson, synthesis 1980 0.4 wc olefin hydrogenation Rycheck and Pennella, 1978 wc neopentane 5.0 Levy and Boudart, isomerization 1972 wc cyclohexane Lemaitre et al., 1986 30.0 dehydrogenation Vidick et al., 1986
temperatures over a long period of time to allow the carbon to diffuse into the oxide matrix and replace the lattice oxygen. In the case of the reaction of a reduced metal with a carburizing or nitriding gas, the metal sinters before reaction. In the method first employed by Oyama (1981) and developed extensively here, the primary aim is the achievement of high specific surface area. This is done by passing a reactive gas over a precursor oxide to produce the carbide or nitride. The use of a gas rather than solid carbon avoids the problem of poor contact between two solids. The use of an oxide allows the bypassing of the metallic state, the state most prone to sintering. Furthermore, the transformation is carried out in a temperature-programmed manner with constant monitoring of the extent of reaction. The gradual rise in temperature allows a balance between synthesis and sintering processes which minimizes the loss of surface area. The measurement of the extent of reaction allows the synthesis to be quenched a t the precise end point of the transformation. The flow rates of the gases employed ranged up to 3000 pmol to yield space velocities based on bed volume of up to about 15000 h-l. From the general shape of the temperature-programmed reaction traces (Figures 2-41, the maximum rate of transformation of the solid by the reaction is estimated to be about twice the average rate of reaction over the synthesis period. Therefore, it can be calculated that the space velocities correspond to a throughput of reactive gas 100-400 times greater than the maximum rate of reaction. Thus, the gas composition is relatively uniform down the length of the bed. The only exceptions are for the nitrides, NbN and Mo2N, and for Mo2C-2. For the nitrides, because of the relatively large batch size of 10-80 g, the reactant NH3 reaches full decomposition before the end of the reaction, causing a severe
Table V. Effect of Space Velocity on MozC a n d MozN Surface Areas space velocity, h-' volumetricd product molare surface area, m2 g-' MoZC-2' 2 600 9 29 Mo~C-1" 8 600 28 36 Mo2Cb 31 000 100 100 Mo~N-2" 2 900 10 22 Mo2N-ln 14 000 48 88 Mo~N' 55 000 180 190 "This study. *Boudart et al., 1985. CVolpeand Boudart, 1985b. *Ratio of volumetric flow rate of gas to volume of precursor. e Ratio of molar flow rate of gas to moles of precursor.
concentration gradient through the bed. For this reason, higher temperatures were required for the nitridations than for the smaller batches of 1 g reported by Volpe and Boudart (1985a) for Mo2N. The effect of flow rate is tabulated for various molybdenum carbide and nitride samples in Table V. The table lists two space velocities, one based on volumetric flow rate and bed volume and the other based on molar flow rate and moles of precursor oxide. There is clearly a correlation between surface area and reactive gas flow rate. For MozC-2 and Mo2C-1,the throughputs of reactive gas are 30 and 100 times, respectively, the maximum rates of reaction, whereas it is 1500 for the MozC sample of Lee et al. (1987). The most likely reason for the positive effect of flow rate on product surface area is the rapid removal of water vapor from the vicinity of the reacting solid. Water vapor can cause hydrothermal sintering (Horlock et al., 1963) even in small concentrations as found here. From this discussion, the higher surface area of Mo,N-1 over Mo2N-2and Mo2C-1over MozC-2can be understood. Similarly, it can be suggested that higher surface areas for Tic, VC, and NbC might be obtained by increasing the space velocity of the synthesis. In past reports, the importance of space velocity in the synthesis was not explicitly stated. The X-ray diffraction data of Figures 5 and 6 show that the product materials are composed of only the one reported phase. The only exception is the pattern for NbN which shows the presence of substantial amounts of a lower niobium oxide. This is expected because of the observation during the synthesis that all of the reactant NH3 decomposed at the higher temperatures. This means that the lower portion of the packed bed was not exposed to NH, but to a mixture of Nz and H2, which is known to be much less reactive toward nitride formation. The elemental analyses reported in Table I1 confirm that the products obtained have elemental compositions close to those expected. Among the carbides, only Tic has a substantially lower carbon content than the theoretical. This is consistent with the observation that T i c required the highest synthesis temperature and was thus the most difficult material to prepare. Among the nitrides, NbN and MozN-1 show significant discrepancy in their total composition. As discussed above, for NbN this is due to incomplete reduction of the oxide. For Mo2N-1,which has the largest surface area of all the samples, this is probably due to the presence of substantial amounts of surface oxygen deposited during passivation. After synthesis the materials are passivated to allow them to be exposed to the ambient atmosphere. Passivation involves the treatment of the samples in an 02/He mixture to lay down a layer of chemisorbed oxygen on the surface. This layer protects the solid from bulk oxidation. In order to deposit only one layer of oxygen, very low oxygen concentrations are employed. This limits the local
Ind. Eng. Chem. Res., Vol. 27, No. 9, 1988 1645 Table VI. Crvstallite and Particle Size of the Materials compd DXRD, nm DBET,nm 63 260 TIC vc 100 200 NbC 69 73 NbN 62 200 Mo~C-2 16 24 Mo~N-2 15 29 wc 22 57
heat evolved in the chemisorptionprocess. It is well-known that when the produced heat is not controlled, high temperatures promote the oxidation of many subsurface layers. The materials synthesized in this investigation have a high affinity for oxygen, and therefore, precautions were taken to use extremely low oxygen concentrations, of the order of 0.013'%, and long passivation times. Nevertheless, it is likely that several layers were affected by the passivation. Prior to CO adsorption measurements or to catalytic testing, the passivated materials must be activated in order to remove their protective coating of oxygen. Activation at an appropriate temperature is critical to remove this oxygen. The effect of temperature on the catalytic activity is also substantial. This was demonstrated for WC, Tic, and TaC in the hydrogenation of ethylene (Miyazaki and Fuse, 1972; Kojima et al., 1979). It was found that evacuation of the materials above 873,1273, and 1373 K, respectively, tremendously increased their catalytic activity. The surface areas of the materials were not affected by the activation. However, X-ray photoelectron spectroscopy (XPS) revealed that oxygen, which had been present originally, was completely removed from the surface at precisely the optimum temperature of activation. On the other hand, carbon, in the form of carbide as seen by its XPS signature, was present at all times. In this study, activation of the passivated samples is carried out by TPR. The amount of strongly bound CO is then used as a measure of the surface sites. These sites are the most likely to be involved in catalytic reactions. In addition to TPR, another thermal method, TGA, was employed to characterize some of the samples. The VC and WC samples had some excess carbon, but it is unlikely that the carbon contributed to the measured surface area because activation in hydrogen at high temperature is expected to remove amorphous carbon. Comparisons of the TPR and TGA traces for Tic, VC, and NbC are shown in Figures 7-9. The TGA traces show an increase in weight for these carbides, indicating the probable formation of bulk hydride compounds. At the maximum temperatures reached in the measuremenb, the compositionsattained were Ho.,TiC, H,,.2VC, and H1.lNbC. The TPR traces for VC and NbC show peaks in the precise temperature regions where the TGA traces show weight increases. These peaks are caused by hydrogen consumption due to both formation of the hydrides and to reduction of surface oxide or carbide. In the case of Tic, the TPR trace shows no hydrogen consumption, whereas the TGA trace shows substantial hydrogen uptake. We cannot explain this discrepancy. Evidence that the TPR peaks observed correspond, at least in part, to cleaning of the surface is given by the adsorption data summarized in Table 111. The data show that samples that have been activated by temperature programming have higher CO uptakes than those that have been activated isothermally at lower temperatures. The specific surface area, on the other hand, does not vary appreciably with either treatment. Table VI lists crystallite sizes and particle sizes for several of the materials synthesized in this study. The
crystallite size is obtained from X-ray diffraction linebroadening analysis using the Scherrer equation (Scherrer, 1918), DXRD= KX/(@cos e), where K is a constant, here taken to be 1.2, X is the wavelength of the X-ray radiation, /3 is the peak width, and 6 is the angle of the diffraction line. The peak width is taken to be the full-width at half-maximumof the most intense line in each of the XRD spectra. The particle size is obtained from the BET specific surface area, s,,by use of the equation, D B m = 6 / S g p , where p is the density of the material. Table VI shows that for most of the materials the crystallite size is smaller than the particle size, indicating that the particles are polycrystalline agglomerates. Theory of the Synthesis Method. The traces of TCD signal versus temperature recorded for each synthesis indicate the temperatures at which transformations occur. The transformations include not only the production of the final carbide or nitride but also the formation of intermediate phases. The transformation temperature, taken to be at the peak of the TCD signal, corresponds to the maximum rate of reaction. Temperature-programmed reaction theory predicts that the transformation temperature, T,, is related to the heating rate, @,by the following equation (Hurst et al., 1982): 2 In Tt - In /3 = E/RT, constant
+
The constant is related to the characteristics of the solid, the reactive gas composition, and the kinetics of the transformation and contains the units that make the lefbhand side of the equation dimensionless. The equation predicts that the lower /3 is, the lower the transformation temperature will be. Throughout the experimentation, the values of heating rate have ranged from 0.0011 to 0.065 K s-l. These are very low values and are not expected to shift Tt appreciably. The constant in the above equation is difficult to evaluate, and to determine the activation energy of the synthesis process, several experiments at different heating rates must be carried out. Usually a span of at least an order of magnitude in heating rate must be employed, including values at high rates. This is frequently a problem because at high heating rates very high temperatures are necessary to complete the synthesis. In order to circumvent this problem, a simplified analysis is carried out. The synthesis process is modeled as a first-order process: df/dt = k ( l - f)'.' In the above equation, f is the extent of reaction and k is the synthesis constant. Substituting the following expressions k = A exp(-E/RT) @ = dT/dt and integrating, we obtained the following expression:
Assuming that the transformation temperature (Tt) corresponds to an extent of reaction of f = 0.9 and the preexponential factor A = 1013s-l, the equation is solved iteratively for the activation energy, E. The results for the various refractory compounds are shown in column 2 of Table VII. It was found that the calculations were not sensitive to the assumed value off. For a value off = 0.5, the activation energies increased by about 2 kJ mol-'. Changing the assumed value of the preexponential to A = 10l2s-l had a greater effect on the activation energies, decreasing them by about 30 k J mol-'.
1646 Ind. Eng. Chem. Res., Vol. 27, No. 9, 1988 Table VII. Activation Energies for Synthesis and Diffusion in the Metal Compounds activation energies, kJ mol-' 0 cN -. compd synth. diffusionbb diffusionbb diffusionbb 220," 250b TIC 280 400,e 4101 300'
vc NbC
270 260
MozC-2
200
MoZN-2
200
wc
200
270,' 280d 250j 180," 200"' 210" 200," 200" 25OW 200," 200" 25OW 260YcC
440,g 46oh 280,g 360k 310: 32W 370,q 400' 4308 350," 3808
240' 470t
350," 3808
loo 0
370,' 420M
"Elyutin et al., 1971. *Haul and Duembgen, 1965. "undy et al., 1973. dDerry e t al., 1971. eSarian, 1968. fKohlsted et al., 1970. BAndrievskii e t al.! 1967. "Eremeev and Panov, 1967. 'Levinskii e t al., 1968. 'Pavlov e t al., 1972. kSarian, 1972. 'Adelsberg and Cardoff, 1968. "Massiani et al., 1967. "Chen and Jackson, 1967. "Vilk et al., 1967. PMeshcheryakov and Zagryazkin, 1971. qKokon e t al., 1973. 'Andrievskii e t al., 1971. 'Andrievskii e t al., 1969. LLevinskiie t al., 1973. UMeshcheryakov e t al., 1970. "Pavlov e t al., 1975a. wPavlov et al., 1975b. Eremeev and Panov, 1968. YFrantsevich e t al., 1963. Buhsmer, 1968. aaFries e t al., 1967. bbSelf-diffusionof the element in the respective oxide, carbide, or nitride. cc Oxidation of the metal.
. I
. x
i
c
Synthesis
A
Oxides
C C a r b i d e s and Nitrides
1
I
I
I
I
I
, /
0
L
1500 2 2 8 1000 .I )
0
The use of the first-order description of the temperature-programmed reaction process is a significant oversimplification of physical reality. The simplification only provides a tractable starting point for analyzing a very complex process. It is interesting to note (Table VI1 and Figure 10) that the calculated activation energies of synthesis are of similar order of magnitude as the activation energies for diffusion of oxygen in the metal oxides and for oxidation of the metals. Presumably the latter two quantities are related, as oftentimes the mechanism of oxidation involves diffusion of oxygen through an oxide layer. It is also noteworthy that the activation energies for diffusion of carbon or nitrogen in the respective metal carbides or nitrides are considerably different than those of the synthesis. There is an apparent correlation between the transformation temperature and two-thirds of the melting point of the parent oxides, as shown in Figure 11. No firm rationale can be given for the value of the slope of twothirds. In the past, other fractions of the bulk melting point have been suggested to correspond to the onset of various physical transformations. Thus, one-half of the melting point, which has become known as the Tammann temperature, has been associated with the temperature at which sintering rates become appreciable. Burton and Cabrera (1949) have suggested that one-half of the melting point also marks the point where surface atoms are able to jump onto the top of a crystal plane to produce molecular roughness. The original references (Tammann and Mansuri, 1923; Tammann, 1926) actually provide more detail. For the relation T, = CUT,,where T , is the temperature at which sintering commences and T , is the bulk Table VIII. Predicted Synthesis Temperature (K) of Carbides" scZ03 Tic VC Cr3C2
Ln C
+e
500
/ /
0 0
I
I
I
I
I
500
1000
1500
2000
2500
P a r e n t Oxide M e l t i n g P a i n t / K
Figure 11. Reduction, carburization, and nitridation of oxides.
melting point, different values of a are given for different classes of compounds. Thus, a = 0.33 for metals, a = 0.52 for metal oxides, a = 0.57 for salts, and a = 0.90 for carbon. For the temperature for surface diffusion on metal oxides, T,', Huttig and collaborators (1935, 1939, 1943) propose T i = 0.59Tz or T,' = 0.31Tm. It is perhaps not surprising that a relationship exists between the synthesis temperature and the oxide melting point. The processes of reduction and carburization or nitridation involve the replacement of oxygen atoms by carbon atoms or nitrogen atoms and require atomic mobility. A t the melting point of the oxide, oxygen-metal bonds are ruptured and movement of atoms commences. Thus, the two processes have in common the disruption of metal-oxygen bonds in the initial oxide material. This disruption also occurs in the diffusion of oxygen in the oxides and for this reason there is a similarity in the activation energies for oxygen diffusion and fo-r synthesis (Table VII). The increase in surface area with space velocity was earlier suggested to be caused by sintering promoted by water vapor produced from the reduction of the oxide. This would occur sequentially, after the metal-oxygen bond breaking steps which would remain the kinetically
Mn,C
Fe&
C03C
1780
1600
1490
1690
1220
1220
1380
1500
yZc3
ZrC
NbC
MoZC
TcC
Ru
Rh
Pd
osc
Ir
Pt
1790
1980
1370
710
1780
Lac,
HfC
TaC
WC
ReC
1720
2040
1430
1430
850
OMelting points obtained from Lundy et al. (1973).
3000
610
Ni3C
Ind. Eng. Chem. Res., Vol. 27, No. 9, 1988 1647 important steps of the synthesis. Further experiments with water vapor added to the reactants are desirable to clarify the sintering mechanism. The correlation shown in Figure 11 implies that the transformation temperatures required for synthesizing of transition-metal carbides can be predicted on the basis of the melting point of the corresponding oxide. Some such predictions are shown in Table VIII. It should be noted that the greatest confidence is for the predictions for carbides of Groups IVB-VIIB,the groups from which data were obtained for the correlation. The temperatures predicted for the first row group VI11 metal carbides appear too high. Fe, Co, and Ni are known to form carbides at temperatures of about 650 K in the course of reactions such as the Fischer-Tropsch synthesis. Nevertheless, the correlation helps explain why the synthesis of hafnium carbide failed in this study: the temperature used in the preparation, 1500 K, was significantly lower than the predicted temperature of 2040 K. The results of this paper indicate that the large-scale production of high surface area materials is possible with attention to several critical parameters of the synthesis: a uniform temperature rise, high throughput of gases, and minimization of water vapor concentration. The latter two requirements can be met by recirculation of reactants with intermediate removal of the product water.
Conclusions (1)The temperature-programmed method of synthesis of refractory carbides and nitrides is superior to traditional isothermal methods in producing materials of high surface area. (2) In addition to parameters such as gas composition and programming rate, space velocity is an important synthesis parameter. Molar gas flow rates in excess of a factor of 1500 over the maximum molar rate of substrate transformation ensure the achievement of high specific surface area values. From this standpoint, the syntheses reported in this study have not been optimized. Water vapor is implicated as the chief cause of nonoptimal results using the temperature-programmed synthesis method. (3) Carbides and nitrides may be passivated in a dilute 02/He stream and handled in the atmosphere. However, they must then be activated at high temperature in H2 prior to chemisorption and catalysis applications. (4)The activation energy for the synthesis of the refractory compounds is close to the activation energy for oxygen diffusion in the metals. An important step in both processes is the disruption of metal-oxygen bonds. (5) Synthesis temperatures for group IV-VI carbides and nitrides were approximately two-thirds the melting point of the parent oxide. Acknowledgment The authors thank the Standard Oil Company of Ohio and Catalytica for permission to publish this work. Registry No. TiOz, 13463-67-7; HfOz, 12055-23-1; VzO5, 1314-62-1; NbzO5, 1313-96-8; MoOB, 1313-27-5; WOB, 1314-35-8; Re2O7, 1314-68-7; T i c , 12070-08-5; VC, 12070-10-9; NbC, 12069-94-2; NbN, 24621-21-4; M o ~ C12069-89-5; , M o ~ N 12033, 31-7; WC, 12070-12-1; CHI, 74-82-8; Hz, 1333-740; NH,, 7664-41-7.
Literature Cited Adelsberg, L. M.; Cardoff, L. H. J. Am. Ceram. SOC.1968,51,213. Andrievskii, R. A.; Eremeev, V. S.'; Zagryazkin, V. N.; Panov, A. S. Izu. Akad. Nauk. S S S R , Neorgan. Mater. 1967,3,2158. Andrievskii, R. A.; Khromov, Yu. F.; Alekseeva, I. S. Fiz. Met. Metalloued. 1971,32,664.
Andrievskii, R. A.; Klimenko, V. V.; Khormov, Yu. F. Fiz. Met. Metalloued. 1969,28,298. Boudart, M.; Oyama, S. T.; Volpe, L. U S . Patent 4515763, 1985 (Assigned to Stanford University). Boudart, M.; Oyama, S. T.; Leclercq, L. Proceedings, 7th International Congress on Catalysis, Tokyo, 1980; Seiyama, T.,Tanabe, K., Eds.; Elsevier: Amsterdam, 1981; p 578. Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. SOC.1938,60, 309. Buhsmer, Ch. P. Diss. State Uniu. N.Y. 1968, 167, (Avail. Univ. Microfilms, Ann Arbor Mich. Order no 68-16, 616). Burton, W. K.; Cabrera, N. Discuss. Faraday SOC.1949,5,33. Chen, W. K.; Jackson, R. A. J. Chem. Phys. 1967,47,1144. Derry, D. J.; Lees, D. G.; Calvert, J. M. Proc. Br. Ceram. SOC.1971, 19,77. Elyutin, V. P.; Lenskaya, T. G.; Pavlov, Yu. A.; Polyakov, V. P. Dokl. Akad. Nauk. SSSR. 1971,199,62. Eremeev, V. S.; Panov, A. S. Sou. Powder Met. Metal Ceram. 1967, 4,306. Eremeev, V. S.;Panov, A. S. Izu. Akad. Nauk. S S S R , Neorgan. Mater. 1968,4, 1507. Frantsevich, I. N., et al. High Temperature Oxidation ofMetals and Alloys; Gostekhizdat Ukr. SSR Leningrad, 1963. Fries, R. J.; Cumming, J. E.; Hoffman, C. G.; Daily, S. A. US Atomic Energy Comm. Report LA-3795, 1967, p 32. Hanson, F. V. Ph.D. Dissertation, Stanford University, Stanford, CA, 1975. Haul, R.; Duembgen, G. J. Phys. Chem. Sol. 1965,26,1. Horlock, R. F.; Morgan, P. L.; Anderson, P. J. Trans. Faraday SOC. 1963,59,721. Hurst, N.W.; Gentry, S. J.; Jones, A.; McNicol, B. D. Catal. Rev.-Sci. Eng. 1982,24,233. Huttig, G. F. In Handbuch der Katalyse; Schwab, G. M., Ed.; Springer-Verlag: Wien, 1943; Vol. 6, p 318. Huttig, G. F.; Marcus, G. Kolloid Z. 1939,88, 274. Huttig, G. F.; Meyer, T.; Kittel, H.; Cassirer, S. Z. Anorg. Allg. Chem. 1935,224,225. Il'chenko, N. I. Kinet. Katal. 1977,18, 153. Kohlsted, D. L.; Williams, W. S.; Woodhouse, J. B. J.Appl. Phys. 1970,41,4476. Kojima, I.; Miyazaki, E.; Inoue, Y.; Yasumori, I. J . Catal. 1979,59, 472. Kojima, I.; Miyazaki, E.; Yasumori, I. J. Chem. SOC.,Chem. Commun. 1980,573. Kokon, H.; Son, P.; Miyake, M.; Sano, R. J. J p n . Inst. Met. 1973, 37, 1065. Lee, J. S.;Oyama, S. T.; Boudart, M. J. Catal. 1987,106,125. Lemakre, J.; Vidick, B.; Delmon, B. J . Catal. 1986,99, 415. Levinskii, Yu. V., et al. Izu. Akad. Nauk. S S S R , Neorgan. Mater. 1968,4,2068. Levinskii, Yu. V.; Kiparisov, S. S.; Strogonov, Yu. D. Izv. Akad. Nauk. S S S R , Metal 1973,1,70. Levy, R. B.; Boudart, M. Science (Washington,D.C.) 1973,181,547. Lundy, T. S.; Padgett, R. A.; Banus, M. D. Met. Trans. 1973,4,1179. Massiani, Y.; Crousier, J. P.; Streiff, R. C. R. Seanses Acad. Sci., Ser. C 1967,282,567. McClune, W. F., Ed. Powder Diffraction File; Alphabetical Index Inorganic Materials, International Center for Diffraction Data: Washington, DC, 1979. Meshcheryakov, G. Ya; Zagryazkin, V. N. Fiz. Met. Metalloued. 1971,32,883. Meshcheryakov, G. Ya.; Pavlov, Yu. A.; Polyakov, V. P.; Sidorov, V. P.; Shelboldaev, S. B. Fiz. Khim. Obrab. Mater. 1970,6,50. Miyazaki, E.; Fuse, K., Nippon Kagaku Kaishi 1972,4,815. Oyama, S.T. Ph.D. Dissertation, Stanford University, Stanford, CA, 1981. Pavlov, Yu. A.; Skrobut, Yu. S.; Polyakov, V. P.; Meshcheryakov, G. Ya.; Zamalin, E. Yu. Izu. Vyssh. Ucheb. Zaved., Chern. Met. 1972, 7,8. Pavlov, Yu. A.; Kryukov, S. N.; Sheboldaev, S. B.; Meshcheryakov, G. Ya. Izu. Akad. Nauk. S S S R , Neorg. Mater. 19758, 11, 661. Pavlov, Yu. A.; Polyakov, V. P.; Skrobut, Yu. S.; Meshcheryakov, G. Ya.; Zamalin, E. Yu. Izu. Vyssh. Uchebn. Zaued., Chern. Metall. 1975b,5,26. Rycheck, M. R.; Pennella, F. U. S. Patent 4 101592, 1978 (Assigned to Philips Petroleum). Saito, M.; Anderson, R. B. J. Catal. 1980,63,438. Samsonov, G. V.; Bulankova, T. G.; Khodak, P. A.; Prshedromirskaya, E. M.; Sinel'nikova, V. S.; Sleptsov, V. M. Kinet. Katal. 1969,10, 1057.
1648
I n d . E n g . Chem. Res. 1988,27, 1648-1653
Sarian, S. J. Appl. Phys. 1968, 39, 3305. Sarian, S. J . Phys. Chem. Sol. 1972, 33, 1637. Scherrer, P. Gott. Nachr. 1918, 2, 98. Shigehara, Y. Nippon Kagaku Kaishi 1977,4, 474. Sinfelt, J. H.; Yates, D. J. C. Nature (London) Phys. Sci. 1971,229, 27. Tammann, G. Z. Anorg. Allg. Chem. 1926, 39, 869. Tammann, G.; Mansuri, Q. A. 2.Anorg. Allg. Chem. 1923,126,119. Toth, L. E. Transition Metal Carbides and Nitrides; Academic: New York, 1971.
Vidick, B.; Lemakre, J.; Leclercq, L. J . Catal. 1986, 99, 439. Vilk, Yu. N.; Nikolskii, S. S.; Avarbe, R. G. Teplofiz. Vys. Temp. 1967, 5, 607. Volpe, L.; Oyama, S. T.; Boudart, M. Prep. Catal. 3, Int. Symp., 3rd 1983, 147. Volpe, L.; Boudart, M. J. Solid State Chem. 1985a, 59, 332. Volpe, L.; Boudart, M. J. Solid State Chem. 198513, 59, 348. Received f o r review March 30, 1987 Accepted March 22, 1988
Catalytic Behavior of Selected Transition-Metal Carbides, Nitrides, and Borides in the Hydrodenitrogenation of Quinoline James C. Schlatter and S. Ted Oyama*,+ Catalytica, 430 Ferguson Drive, Building 3, Mountain View, California 94043
Joseph E. Metcalfe, 111, and Joseph M. Lambert, Jr.$ Standard Oil Company of Ohio,9101 East Pleasant Valley Road, Independence, Ohio 44131 High surface area carbides and nitrides have been synthesized and tested for hydrodenitrogenation activity. Some of the novel materials, particularly MozC and MozN, provided hydrodenitrogenation activity of the same magnitude as commercial sulfided Ni-Mo/Alz03. In the absence of sulfur in the feed, the refractory catalysts could achieve denitrogenation with less hydrogen consumption than the commercial sample. Sulfur addition was detrimental to the selectivity, but not the activity, of MozC and MozN. Heteroatom removal is an important processing step in upgrading hydrocarbon feedstocks to commercially useful products. Organic nitrogen and sulfur are most commonly removed via reaction with hydrogen at 300-400 "C and 50-150 atm of pressure. Under these severe conditions, hydrogen is consumed not only in breaking carbon-nitrogen and carbon-sulfur bonds but also in saturating aromatic components in the feed. In fact, the latter reactions generally consume at least as much hydrogen as the desired denitrogenation and desulfurization reactions (Katzer and Sivasubramanian, 1979). There are economic incentives to decrease hydrogen consumption in hydrotreating operations-not only is hydrogen an expensive raw material in many situations, but also the required capital investment is directly related to the amount of hydrogen consumed (Kirk-Othmer Encyclopedia of Chemical Technology, 1980). Sulfur can be extracted from aromatic organosulfur compounds using only enough hydrogen to react with the sulfur atom and to heal the broken carbon-sulfur bonds (Katzer and Sivasubramanian, 1979; Rollmann, 1977). On the other hand, removing a nitrogen heteroatom using existing catalysts requires complete saturation of the associated aromatic rings in the molecule (Katzer and Sivasubramanian, 1979; Rollmann, 1977; Cocchetto and Satterfield, 1981). Thus, development of a catalyst for denitrogenation without saturation of the surrounding aromatic rings could represent a significant advance in treating high-nitrogen feedstocks. Processing of shale-derived materials in particular would benefit from such a catalyst, since nitrogen levels of about 2 wt % are typical of shale oil. Hydrogen consumptions in excess of 1500 scf/bbl (standard cubic feet per barrel) are common in hydrotreating shale oil, while the amount theoretically t Present address: Department of Chemical Engineering, Clarkson University, Potsdam, NY 13676. f Present address: Chemical Data Systems, 7000 Limestone Road, Oxford, PA 19363.
0888-5885/88/2627-1648$01.50/0
required for selective heteroatom removal is only about 600 scf/bbl (Robinson, 1978). The reaction pathways accessible using sulfided Co-Mo or Ni-Mo hydrotreating catalysts (standard in industrial practice) favor ring saturation prior to denitrogenation. Because transition-metal sulfides are able to catalyze sulfur removal with high hydrogen selectivity, we reasoned that nitrides might behave analogously for selective nitrogen removal. Also, certain carbides have shown interesting catalytic properties for hydrogenation reactions (Levy, 1977). For these reasons, we investigated several novel catalytic materials, specifically transition-metal carbides, nitrides, and borides which might allow direct hydrogenation and removal of the nitrogen atom without the need for prior ring saturation. The primary point of interest is the selectivity for denitrogenation in preference to ring saturation, since such selectivity will decrease the amount of hydrogen required for heteroatom removal. Thus, for our model substrate, quinoline, the desired product is n-propylbenzene rather than the saturated analogue, npropylcyclohexane (see Figure 1). The theoretical hydrogen consumption for the selective pathway is 43% lower than for the pathway involving ring saturation (4 mol versus 7 mol of hydrogen).
Experimental Section The gases employed in this study consisted of H2 (Linde High Purity Grade, 99.995%),CH4 (Linde Research Grade, 99.99%), NH3 (Linde UHP Grade, 99.995%), HzS (Matheson Certified Purity Grade, 99.5%), Nz (Linde High Purity Grade, 99%), Oz (Linde Specialty Grade 99.99%), and CO (Linde Research Grade, 99.97%). The H2, H2S, N2, and CO were used without further purification. The CH, was purified from sulfur impurities by passage through a bed of Ni/Si02 catalyst previously reduced at 673 K. The NH3 was dried by passage through a bed of BaO (Fischer Technical Anhydrous Grade) in the form of porous lumps. The CO was purified by passage through 0 1988 American Chemical Society
