Ind. Eng. Chem. Res. 1992,31,1445-1449 Chen, J. P.; Yang, R. T. Mechanism of Poisoning of the Vz05/Ti02 Catalyst for the Reduction of NO by NHB. J. Catal. 1990,125, 411. C l d e l d , A. Recent Advances in Pillared Clays and Group IV Metal Phosphates. NATO ASZ Ser., Ser. C 1988,231,271. Czarnecki, L.J.;Anthony, R. G. Selective Catalytic Reduction of NO over Vanadia on Pillared Titanium Phosphate. AZChE J. 1990, 36,794. Drezdon, M. A. Synthesis of Isopolymetalate-Pillared Hydrotalcite via Organic-Anion-Pillared Precursors. Znorg. Chem. 1988,27, 4628. Figueras, F. Pillared Clay as Catalysts. Catal. Rev.-Sci. Eng. 1988, 30,457. He, M. Y.; Liu, 2.; Min, E. Acidic and Hydrocarbon Catalytic Properties of Pillared Clay. In Pillared Clays; Burch, R., Ed. Catal. Today 1988,2,321. Jones, W. The Structure and Properties of Pillared Clays. In Pillared Clays; Burch, R., Ed. Catal. Today 1988,2,357. Kung, M. C.; Kung, H. H. IR Studies of NH3, Pyridine, CO, and NO Adsorbed on Transition Metal Oxides. Catal. Rev.-Sei. Ena. 1985,27,425. Lahav, N.; Shani, N.; Shabtai, J. Cross-Linked Smectite. 1. Synthesis and Properties of Hydroxy-Aluminum-Montmorillonite. Clays Clay Miner. 1978,26;107.Loeppert, R. H.; Mortland, M. M.; Pinnavaia, T. J. Synthesis and Properties of Heat-Stable Expanded Smectite and Vermiculite. Clays Clay Miner. 1979,27,201. Occelli, M. L. Catalytic Cracking with an Interlayered Clay. A Two-Dimensional Molecular Sieve. Znd. Eng. Chem. Prod. Res. Dev. 1983,22,553. Occelli, M. L. Surface Properties and Cracking Activity of Delaminated Clay Catalysts. In Pillared Clays; Burch, R., Ed. Catal. Today 1988,2,339. Occelli, M. L.; Tindwa, R. M. Physical Chemical Properties of Montmorillonite Interlayered with Cationic Oxyaluminum Pillars. Clays Clay Miner. 1983,31,22. Occelli, M. L.; Pinnavaia, T. J.; Landau, S. D. The Cracking Activity of A Delaminated Clay Catalyst. J. Catal. 1984,90,256. Occelli, M. L.; Hsu, J. T.; Galya, L. G. Propylene Oligomerization with Pillared Clays. J. Mol. Catal. 1985a,33,371. Occelli, M. L.; Inns, R. A.; Hwu, F. S. S.; Hightower, J. W.; Sorption and Catalysis on Sodium-Montmorillonite Interlayered with Aluminum Oxide Clusters. Appl. Catal. 1985b,14,69. Pinnavaia, T. J. Intercalated Clay Catalysts. Science 1983,220,365. Pinnavaia, T. J.; Tzou, M. S. Pillared and Delaminated Clays Containing Iron. US. Patent 4,665,044,May 1987.
1445
Pinnavaia, T. J.; TZOU, M. S.; Landau, S. D.; Raythatha, R. H. On the Pillaring and Delamination of Smectite Clay catalysts by Polyoxo Cations of Aluminum. J . Mol. Catal. 1984.27, 196. Pinnavaia, T. J.; Tzou, M. S.; Landau, S. D. New Chromia Pillared Clay Catalysts. J. Am. Chem. SOC.1985,107,4783. Rajadhyasksha, R. A.; KnBzinger, H. Ammonia Adsorption on Vanadia Supported on Titania-Silica Catalyst. An Infrared Spectroscopic Investigation. Appl. Catal. 1989,51,81. Rajadhyasksha, R. A.; Hausinger, G.; Zeilinger, H.; Ramstetter, A.; Schmelz, H.; Knazinger, H. Vanadia Supported on Titania-Silica. Physical Characterization of the Activity for SCR of NO. Appl. Catal. 1989,51,67. Rightor, E.G.;Tzou, M. S.; Pinnavaia, T. J. Iron Oxide Pillared Clay with Large Gallery Height: Synthesis and Properties as a Fischer-Tropsch Catalyst. J. Catal. 1991,130,29. Robie, C. P.; Ireland, P. A.; Cichanowicz, J. E. Technical Feasibility and Economics of SCR NO, Control in Utility Applications. 1989 Joint Symposium on Stationary Combustion NO, Control, San Francisco, 1989. Sprung, R.; Davies, M. E.; Kauffman, J. S.; Dybowski, C. Pillaring of Magadiite with Silicate Species. Znd. Eng. Chem. Res. 1990, 29,213. Sterte, J. Synthesis and Properties of Titanium Oxide Croea-Linked Montmorillonite. Clays Clay Miner. 1986,34,658. Sterte, J. Hydrothermal Treatment of Hydroxycation Precursor Solutions. In Pillared Clays; Burch, R., Ed. Catal. Today 1988, 2,219. Tennakoon, D. T.; Jones, W.; Thomas, J. M. Structural Aspects of Metal-Oxide-Pillared Sheet Silicates. J. Chem. SOC.,Faraday Trans. I 1986,82,3081. Topsoe, N. Y. Characterization of the Nature of Surface Sites on Vanadia-Titania Catalyst by FTIR. J. Catal. 1991,128,499. Tuenter, G.; Leeuwen, W.F. V.; Snepvangers, L. J. M. Kinetics and Mechanism of NO, Reduction with NH3 on V2O5-WO3-TiO2 Catalyst. Znd. Eng. Chem. Prod. Res. Dev. 1986,25,633. Vaughan, D. E. W.; Lussier, R. J.; Magee, J. S. Pillared Interlayered Clay Materials Useful as Catalyst and Sorbents. U.S. Patent 4,176,090,1979. Wong, W. C.; Nobe, K. Reduction of NO with NH3 on A1203and Ti0,-Supported Metal Oxide Catalysts. Znd. Eng. Chem. Prod. Res. Dev. 1986,25,179. Yang, R. T.; Baksh, M. S. A. Pillared Clays as a New Class of Sorbents for Gas Separation. AZChE J. 1991,37,679. Received for review December 26, 1991 Accepted March 19, 1992
Nitrogen Bases Resistant to Hydrodenitrogenation: Evidence against Using Quinoline as a Model Compound Paula L.J o k u t y a n d M u r r a y R.Gray* Department of Chemical Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6
A retention chromatographic method was used to concentrate basic nitrogen compounds from a commercial synthetic crude oil derived from Athabasca bitumen. The most abundant ring systems were alkyl-substituted 5,6,7,8-tetrahydroquinolines and octahydrobenzoquinolines. The largest gas chromatographic peaks due to individual components were from alkylpyridines and alkyltetrahydroquinolines. Higher ring systems were also found. 13C and 'H NMR spectroscopy indicated a high degree of substitution but with protons frequently found in the position meta to the nitrogen. The distribution and degree of substitution of product ring types were consistent with a multistep mechanism for hydrodenitrogenation (HDN), including hydrogenation of carbon rings followed by cracking of the hydrogenated rings to give alkylpyridines. This mechanism of HDN is significantly different from that reported for quinoline. Introduction As world reserves of conventional crude oil dwindle, processing of heavier feeds grows in importance. These heavier feeds contain more aromatic nitrogen compounds than their lighter counterparts. This development of heavy and synthetic crude oils has, therefore, generated a great
deal of interest in the chemistry and kinetics of hydrodenitrogenation. Nitrogen bases are ubiquitous in distillates derived from petroleum residues, heavy oils,and coal liquids. Removal of these substituted pyridines, quinolines, and higher benzologues is required before the distillates can be
0888-5885/92/2631-1445$03.00/00 1992 American Chemical Society
1446 Ind. Eng. Chem. Res., Vol. 31, No. 6, 1992
blended into refinery feedstocks. Catalytic hydrodenitrogenation of quinoline has been intensively studied as a model for removal of nitrogen from hydrocarbon mixtures, and this literature has been summarized in recent reviews (Ho, 1988; Girgis and Gates, 1991). Bitumen extracted from the Athabasca tar sands is used to produce a synthetic crude oil which constitutes about 15% of total Canadian oil production. Two main types of aromatic nitrogen compounds have been found in Athabasca bitumen: neutral pyrrole benzologues and basic pyridine benzologues (Mojelsky et al., 1986; Frakman et al., 1987). Alkyl-substituted carbazoles are the major type of neutral nitrogen compounds in synthetic crude oil derived from Athabasca bitumen (Jokuty and Gray, 1991). The present paper focuses on the basic nitrogen compounds found in the same synthetic crude oil product. The objectives of the current work were to recover gram quantities of nitrogenous material from a hydrotreated coker gas oil and to characterize the basic nitrogen components in the recovered materials. Experimental Apparatus and Procedure The oil was a commercially produced hydrotreated coker gas oil, derived from Athabasca bitumen. The material had been hydrotreated in a commercial trickle-bed reactor over a Ni/Mo on y-alumina catalyst, at a temperature of 360-400 OC and a pressure of 9.5-11 MPa. A sulfur concentration of 4% in the coker gas oil ensured that the catalyst was completely sulfided. Nitrogen compounds were retained by silica gel in a g h 100 cm X 3 cm 0.d. column. The details of the frontal retention technique have been described elsewhere (Jokuty and Gray, 1991). The mass ratio of sample to sorbent was 9:l. One liter of oil was used. All solvents were of HPLC grade. A methylene chloride extract and a methanol extract were produced. Analytical Methods For both extracts, C, H, N, and 0 were measured by the Microanalytical Lab, Chemistry Department, University of Alberta, using a Carlo Erba EA 1108 Elemental Analyzer. Sulfur was determined in a Leco S-132 sulfur determinator. For the oil, C, H, and S were determined as for the extracts. N was determined with an Antek pyroreactor and digital nitrogen analyzer. Infrared spectra were recorded on a Nicolet 730 FTIR spectrometer by coadding 32 scans at a resolution of 4 cm-'. The samples were dissolved in methylene chloride and run in 0.05-cm NaCl cells. GC-FTIR was performed on a Hewlett-Packard HP5765 IRD using a Supelco Ultra 2 column. The temperature program was 150 OC for 2 min, then heating at 3 OC/min to a final temperature of 290 "C. A GC-mass spectrum of the methanol extract was obtained using a DB-1 column (0.25 mm i.d. X 30 m) coupled to a Varian VG 7070E mass spectrometer. 13Cand 'H NMR spectra were obtained with a 300 MHz Bruker WH-300 spectrometer. Potentiometric titration of basic nitrogen was carried out using a procedure similar to that of Buell (1977). A 90-100-mg sample was dissolved in 10 mL of benzene and 20 mL of either acetonitrile or acetic anhydride to determine strong bases and very weak bases, respectively. The solution was potentimetrically titrated with approximately 0.1 N perchloric acid in dioxane. Results The elemental analyses of the gas oil and extract fractions are given in Table I. Other characteristics of the extract fractions are given in Table 11. The high sample
Table I. Elemental Analysis of Hydrotreated Gas Oil and Extract Fractions gas oil CHzClzextract methanol extract wt % mo1/100a w t % mo1/100a w t % mo1/100~ c 86.75 7.22 87.41 7.28 82.30 6.85 H 11.96 11.84 8.39 8.31 9.87 9.77 3.76 0.27 N 0.09 0.01 3.76 0.27 0 0.01' 0.00 2.87 0.18 0.10 0.01 s 1.20 0.04 0.34 0.01 1.21 0.04 "Calculated from extract data. Table 11. Characteristics of Extract Fractions
yield (9) yield (wt % of gas oil) nitrogen (wt % ) pyrrolic N (mo1/100 g) basic N (mo1/100 g) 'H NMR aromatic H (mo1/100 g) aliphatic H (mo1/100 g) 13C NMR aromatic C (mo1/100 g) aliphatic C (mo1/100 g)
CHzClzextract 12.075 1.33 3.76 0.14 0.046
methanol extract 1.406 0.15 3.76
0.00 0.21 0.61 9.16 2.61 4.24
to sorbent ratio of 9:l gave only 61% recovery of total nitrogen, but the two solvent extracts were highly enriched in specific nitrogen types. Both extracts contained 3.8 w t % N. In the methylene chloride extract, 50% of the nitrogen was pyrrolic (determined by IR) and 16% was basic (determined by potentiometric titration). The infrared spectrum of the methanol extract showed no signal at 3500 cm-'; therefore, the pyrrolic nitrogen content was negligible. Potentiometric titration of this fraction gave a single inflection point with a half-neutralization potential of 152 mV. This potential was typical of substituted pyridines and quinolines (Buell, 1967),so that this class accounted for 74% of the nitrogen in the sample. There was no evidence from the titration for stronger bases in the methanol extract. Because methanol would desorb even very polar compounds from the silica gel (e.g., Choi and Gray, 1991), these data indicated that primary, secondary, and tertiary amines were absent from the synthetic crude oil. The methanol extract was highly enriched in nitrogenous bases; therefore, subsequent analysis concentrated on this fraction. Nine types of ring systems containing a pyridine ring were identified by GC-MS analysis of the methanol extract (Figure 1). The largest peaks due to individual components were from C9-pyridine and C2- and C,-tetrahydroquinoline. The relative abundance5 of the different ring systems were estimated by summing base peak intensities as voltages (see Table 111). In all cases the base peak was considered to represent the molecular ion (M) if odd or M - 1if even. Only peaks with an intensity of at least 10% of the maximum were included. Using this method, the most abundant compounds were tetrahydroquinolines (THQ) and octahydrobenzoquinolines (OHBQ). Because the mixture did not contain any strong bases (by potentiometric titration), the THQ and OHBQ were not hydrogenated in the nitrogen-containingring. Furthermore, the mass spectra showed no evidence for elimination of HCN (mass = 27), which would be expected if cyclic amines were present. The isomers of THQ, therefore, were substituted 5,6,7,8THQ, with an intact pyridinic ring. The MS fragmentation patterns indicated that multiple methyl substitutions were dominant; i.e., the base peaks had masses 14 units or more above the mass of the unsubstituted ring structure.
Ind. Eng.Chem. Res., Vol. 31, No. 6,1992 1447 I
I
I
I
I
I
I
I
I
I
Table 111. Relative Abundance of Ring Systems in Methanol Extract mol 90 of identified rina svstem structure comDds'
I
V
280
0
~
V
266
m
AV
252
88 8
b 224
0
I
e,
f
0.e
mm A
210
0
3
Z
12
THQ
34
quinoline
13
OHBQ
29
THBQ
4
BQ
1
OHDBQ
4
THDBQ
2
DBQ
2
0
A 4
238
pyridine
cmm
196 fJY
0
=
182
aB* 168
0 Dibenzoquinoline
**
V Tetrahydrodibenzoquinoline 8 Octohydrodibenzoquinoline Senzoquinoline 0 Tetrahydrobenzoquinoline 0 Octahydrobentoquinollne A Quinoline Tetrohydroquinoline 0 Pyridine
154
0
0 140 126
0 0
5
IO 15 20 25 30 35 40 45 50 55 60
Retention Time, min Figure 1. Ring systems identified by GC-MS analysis of the methanol extract.
'Based on mass spectrometry by addition of base peak intensities assuming the same response factor for all nitrogen species.
0.U
8.5
8,O
7.5
7.0
6.5
6.0
ppm
Figure 2. Aromatic portion of the 'H NMR spectrum of the methanol extract.
'H and 13CNMR spectroscopy of the methanol extract showed that the aromatic rings were highly substituted. The molar ratio of aromatic hydrogen to aromatic carbon was 0.23. A sharp signal at 6.8 ppm in the lH NMR spectrum (Figure 2) indicated that about 11% of the aromatic hydrogens were meta to the ring nitrogen (Chamberlain, 1974). This observation implied that the meta position was usually protonated rather than alkylated. The abundance of these meta protons also indicated that the pyridinic ring tended to be a t the periphery of fused ring systems. This result was in agreement with the conclusion of Mojelsky et al. (1986) that the benzo[h]quinoline nucleus was probably the dominant structural
4000
3500
3000
2500
2000
Frequency, cm
1500
1000
500
-1
Figure 3. IR spectrum of the methanol extract at a GC retention time of 9.1 min.
unit for triaromatic ring systems in the basic nitrogen component of Athabasca maltene. The absence of a strong signal 150-160 ppm downfield of TMS in the 13C NMR spectrum of the methanol extract indicated that the carbons ortho to the nitrogen were almost always substituted. IR spectra derived from peaks appearing at early retention times (6-15 min) in the GC-FTIR analysis of the methanol extract had a small peak at approximately 850 cm-' (Figure
1448 Ind. Eng. Chem. Res., Vol. 31, No. 6,1992 DIBENZOQUINOLINE
THDBQ
OHDBQ
t
t
QUINOLINE
P (Cx) = most abundant type in product *
Mojelsky et a\, 1986
Figure 4. Proposed reaction network for the pyridinic ring systems found in hydrotreated gas oil.
3) as expected from isolated hydrogen atoms in a polymethylpyridine (Katritzky and Ambler, 1963). The absence of any overtone bands in the 2000-1650-cm-' region was also consistent with polyalkyl substitution of the pyridinic ring.
Discussion The basic nitrogenous components of Athabasca bitumen consist mostly of quinolines, benzoquinolines, and tetrahydrodibenzoquinolines(THDBQ) (Mojelsky et al., 1986). Pyridines and THQ, if present, are insignificant in comparison to these other types of compounds. The oil used in this work was the product of coking followed by hydrotreating. Coking of the Athabasca bitumen may have caused some dehydrogenation of the nitrogen ring systems as well as some shortening of alkyl chains on the rings. Subsequent hydrotreating would definitely have resulted in hydrogenation of these ring systems. Therefore, THQ, OHBQ, and octahydrodibenzoquinoline (OHDBQ) resulted from catalytic hydrogenation. Since pyridines were absent from the feed, in order for them to appear in abundance in the product they must have been formed by hydrogenation of higher benzologues to THQ, followed by cracking. Figure 4 shows the probable sequence of reactions for conversion of benzo- and dibenzoquinolines in the feed to THQ and alkylpyridines in the products. The most abundant number of carbons in substituents, in the bitumen, and in the product, is shown for each ring system. The hydrogenated ring systems had a low degree of substitution, Le., THQ with C2substitution and OHBQ with methyl substitution. This observation would seem to indicate that, on the whole, these ring systems tended to form from hydrogenation (possibly accompanied by some dealkylation) of ring systems present in the feed. On the other hand, completely aromatic ring systems (benzoquinolines, quinolines, and pyridines) showed a much higher degree of substitution, suggesting formation primarily from the cracking of hydrogenated rings formed during hydrotreating. The pyridines would form from ring opening of THQ and OHBQ and would be more extensively substituted. In summary, the distribution and degree of substitution of product ring types were consistent with a multistep mechanism for HDN, including hydro-
genation of carbon rings followed by cracking of the hydrogenated rings to give alkylpyridines. In contrast to a combination of hydrogenation and cracking reactions as in Figure 4, studies on HDN of quinoline as a model compound show the following dominant pathway (Girgis and Gates, 1991): quinoline
- -+2Hz
1,2,3,4-THQ
decahydroquinoline
+2H2
+3H2
propylcyclohexane
+ NH3
Shabtai et al. (1989) found a similar pathway for HDN of 5,6-benzoquinoline. Both 1,2,3,4THQ and decahydroquinoline are secondary amines and are much more basic than quinoline. The present study found no evidence for such intermediates; instead, substituted 5,6,7,8-THQ was a significant component. This observation is consistent with the work of Claret and Osborne (1982), who found that alkyl substituents on the nitrogen-containingring shifted the selectivity of the hydrogenation from 1,2,3,4-THQto 5,6,7,8-THQ. For example, while catalytic reduction of unsubstituted quinoline yielded 100% 1,2,3,4-THQ, 2,4-dimethylquinoline yielded 80% 5,6,7,8-THQ. The present study shows that the implications of substitution of the nitrogen ring go beyond the question of which ring is hydrogenated first. As illustrated in Figure 4, the accumulation of alkylpyridines suggests that the substituted nitrogen ring is resistant to conversion, and that cracking of saturated carbon rings is a significant pathway. These results may be relevant to any feed containing alkylated benzoquinolines, such as bitumens and petroleum residues, but the exact effects of alkyl substitution require further study. The substituted pyridine benzologues in synthetic crude oil appear to denitrogenate via a very different pathway from model studies to quinoline. If alkyl substitution strongly affects reaction pathways, as suggeated by this study, then effective models for HDN should be based on the more resistant alkylquinolines and alkylbenzoquinolines. Furthermore, reaction networks should be studied a t temperatures representative of commercial reactors. This study also indicates that rate-limiting steps identified in model compound
Ind. Eng. Chem. Res. 1992,31, 1449-1457 studies may not be relevant to the HDN of actual mixtures. Conclusions 1. Alkyl-substituted tetrahydroquinolines and octahydrobenzoquinolines with an intact pyridinic ring are the most abundant basic nitrogen compounds found in synthetic crude oil. 2. These compounds were formed by catalytic hydrogenation of higher benzologues followed by cracking of the saturated hydrocarbon rings. 3. The substituted nitrogen ring is resistant to conversion, and HDN of these pyridine benzologues appears to proceed via a different pathway from quinoline. 4. To effectively model HDN of complex feeds, the more resistant alkylquinolines and alkylbenzoquinolines should be used. Reaction temperatures should be representative of commercial reactors. Acknowledgment We are grateful to the Alberta Oil Sands Technology and Research Authority for their generous financial support. Literature Cited Buell, B. E. Nonaqueous, Differential Titration Applied to a Classification of Basic and Very Weak Basic Nitrogen Compounds in Petroleum. Anal. Chem. 1967,39,756-761.
1449
Chamberlain, N. F. The Practice of NMR Spectroscopy; Plenum: New York, 1974, Choi, J. H. K.; Gray, M. R. Identification of Nitrogen Compounds and Amides from Spent Hydroprocessing Catalyst. Fuel Process. Technol. 1991,28,77-93. Claret, P. A.; Osborne, A. G. Alkylquinolines and Arylquinolines. In Quinolines; Jones, G., Ed.; Wiley: New York, 1982; Part 11. Frakman, Z.; Ignasiak, T. M.; Montgomery, D. S.; Strausz, 0. P. Nitrogen Compounds in Athabasca Asphaltene: The Carbazoles. AOSTRA J. Res. 1987,3, 131-138. Girgis, M. J.; Gates, B. C. Reactivities, Reaction Networks, and Kinetics in High Pressure Catalytic Hydroprocessing. Znd. Eng. Chem. Res. 1991,30, 2021-2058. Ho, T. C. Hydrodenitrogenation Catalysis. Catal. Rev.-Sci. Eng. 1988,30, 117-160. Jokuty, P. L.; Gray, M. R. Resistant Nitrogen Compounds In Hydrotreated Gas Oil from Athabasca Bitumen. Energy Fuels 1991, 5, 791-795. Katritzky, A. R.: Ambler, A. P. Infrared Spectra. In Physical Methods in Heterocyclic Chemistry; Katritzy, A. R., Ed.; Academic: New York, 1963; Vol. 11. Mojelsky, T. W.; Montgomery, D. S.; Strausz, 0. P. The Basic Nitrogen Compounde in Athabasca Maltene. AOSTRA J. Res. 1986, 3, 25-33. Shabtai, J.; Yeh, G. J. C.; Russel, C.; Oblad, A. G. Fundamental Hydrodenitrogenation Studies of Polycyclic N-Containing Compounds Found in Heavy Oils. 5,6-Benzoquinoline, Znd. Eng. Chem. Res. 1989,28,139-146. Received for review November 7, 1991 Accepted March 9, 1992
Characterization Studies of Plasma-Sprayed Cobalt and Iron Catalysts Ajay K. Dalai? Narendra N.Bakhshi,* and M. Nabil Esmail Catalysis and Chemical Reaction Engineering Laboratory, Department of Chemical Engineering, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N OW0
Tube-wall reactor surfaces were prepared by plasma-spraying five catalysts, namely, Fe, 75Fe/25Co, 50Fe/50Co, 25Fe/75Co, and Co (weight percent basis), and the catalyst surface was characterized for the first time by using BET surface area measurements, chemisorption measurements, X-ray diffraction (XRD), scanning electron microscopy (SEM),and electron probe microanalysis (EPMA). It was interesting to observe that despite the low BET surface areas for "plasma-sprayed" catalysts, the hydrogen and carbon monoxide uptakes were found to be quite high. XRD studies showed that the various catalyst phases present on the surface were stable up to 350 OC and also that the surface consists of three types of particles, namely, Fe and Co oxides and cemented particles of CoO-Fe20B. The SEM studies confirmed that these particles were uniformly distributed throughout the catalyst layer. The crystallite sizes determined from hydrogen chemisorption measurements are fairly close to those obtained from X-ray line-broadening experiments. EPMA measurements show that, usually, the plasma-sprayed-catalyst surface possessed a higher concentration of iron particles than cobalt particles, suggesting that iron tends to migrate to the surface a t the expense of cobalt. Introduction A number of Fischer-Tropsch (FT) synthesis studies have been carried out in tube-wall reactors (TWR) to analyze their performance in terms of conversion, selectivity, and catalyst life (Haynes et al., 1972;Smith and Carberry, 1975;Senken et al., 1976;Schehl et al., 1977; Pennline et d,1979;Goyal et al., 1983;Kapoor et al., 1986, Dalai and Bakhshi, 1991,1992;Dalai et al., 1992). Also, Goyal (1984)and Dalai (1989)have provided a detailed
* To whom correspondence should be addressed. Present address: Department of Chemical and Petroleum Engineering,University of Calgary, Calgary, Alberta,Canada T2N 1N4. f
review of the work in this area. In the case of TWRs, most of the work has been carried out using an iron catalyst (Haynes et al., 1978;Baird et al., 1980;Zarochak et al., 1982;Kapoor et al., 1986). In recent literature, carbon monoxide hydrogenation has been carried out using bimetallic catalysts ( N a m a k u r a et al., 1980;Arai et al., 1984;Butt et al., 1984,Ishihara et al., 1987;Dalai and Bakhshi, 1991). It has been reported by Burch (1982)that alloying of the metal components greatly enhances the activity and selectivity of metal catalysts due to electronic interactions between metal species. Such an interaction should make it possible to design catalysts to improve product selectivity. For example, carbon monoxide is adsorbed by metals in linear and bridged forms and the proportion of each varies with alloy composition 1992 American Chemical Society
