Ind. Eng.
Chem. Prod. Res. Dev. 1983, 22, 519-526
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Benzene Hydrogenation over Iron. 1. Specific Activities and Kinetic Behavior over Unsupported Iron and Iron Dispersed on Si02, A1203, Carbon, and Doped Carbon KI J. Yoon,+ Phllllp L. Walker, Jr.,* Laxman N. Mulay,f and M. Albert Vannlce'+ Department of Chemical Engineering and Department of Materials Science and Engineering, The Pennsylvania State Universlty, University Park, Pennsylvania 16802
Specific activities for benzene hydrogenation have been measured for supported iron catalysts over a range of welldefined reaction conditions in a differential reactor. Iron surface areas based on CO chemisorption at 195 K were used to estimate particle sizes and to calculate turnover frequencies on supported iron for the first time. The turnover frequency on iron is the lowest of the group 8 metals wRh final values centered near 3.5 X lo3 s-I at 448 K under 90 kPa H, and 7 kPa CeH,. All iron catalysts showed an activity maximum near 473 K with pressure dependencies near 3 for hydrogen and between 0 and -1 for benzene. No significant effect of crystallite size on initial TOF values was observed over a range of 2-6700 nm; however, deactivation occurred extremely rapidly on the smaller iron particles. After substantial time in the reactor, the active iron catalysts had very similar turnover frequencies at 448 K, indicating little effect of the support on specific activity; however, the highest achievable actiiies occurring at the activity maximum were obtained with the borondoped carbon catalysts, which were 2-5 times more active than the other iron catalysts.
Introduction Benzene hydrogenation is frequently used as a model reaction for hydrogenation of aromatic hydrocarbons and it has been extensively studied over many of the group 8 metals. It is of current interest because it is the simplest reaction which might be used to study the kinetic behavior of metal catalysts for the upgrading of coal liquids by hydrogenation. Inexpensive catalysts, such as Fe, would be highly desirable; however, few studies have dealt with iron (Anderson and Kemball, 1957; Badilla-Ohlbaum et al., 1977; Beeck and Ritchie, 1950; Deberbentsev et al., 1972; Emmett and Skau, 1943; James and Moyes, 1978; Long et al., 1934; Phillips and Emmett, 1961; Schuit and van Reijen, 1958) and no kinetic studies have been reported for iron dispersed on a support. This study was undertaken to thoroughly investigate the kinetic behavior of iron in benzene hydrogenation over a wide range of conditions in a differential reactor. Iron surface areas were measured not only so that specific activities could be determined, but also to see if very small iron crystallites possessed different catalytic properties. Finally, an emphasis was placed on the use of various carbon supports, because graphitic carbon is an electron conductor and the electronic properties of both graphitic and amorphous carbons can be altered by doping with donor or acceptor elements, such as potassium or boron, respectively. However, typical oxide supports were also utilized to disperse the iron, and unsupported iron was also studied. This paper describes and compares the catalytic properties of these iron catalysts in benzene hydrogenation. Experimental Section A. Catalyst Preparaton. The catalysts employed in this investigation are listed in Table I. All supported iron catalysts were prepared by an incipient wetness technique which employed only a quantity of an aqueous solution of Fe(N03)3.9H20(Reagent grade, Fisher Scientific) sufficient
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to fill the pore volume of the support. The details of this approach are described elsewhere (Palmer and Vannice, 1980). After impregnation, the samples were oven-dried in air for 16 h at 393 K, bottled, and then stored in a desiccator. The two unsupported iron samples, ultrapure Fez03 powder and ultrapure Fe powder ( 90 30 0 5 50 5 0 40 15
molecule s-l Fe;’ 5.7 7.8 15 6.8 26 8.9 26 -8 16 5.2
a The initial activity was estimated by extrapolation to zero time on-stream; reaction conditions: T = 448 K ; P H7 ~ 680 torr; PB = 50 torr. The initial specific activity at T,, = 493 K for reduced Fe2O3,T,, = 473 K for the others; reaction conditions: PH2= 680 torr; PB = 50 torr.
to describe the data very well and also to predict the rate maximum (Yoon, 1982; Yoon and Vannice, 1983). After completion of the kinetic studies, activity maintenance was studied during continuous on-stream operation under a standard set of reaction conditions ( T = 448 K; PH, = 680 torr; PB = 50 torr). The behavior typical of the catalysts exhibiting no, little, or moderate activity loss is shown in Figure 7, and the degree of activity loss after 5 h on-stream for all samples studied is reported in Table V. Also in Table V are listed the maximum TOF values observed for each catalyst. Discussion The hydrogenation of benzene has been studied on iron films (Anderson and Kemball, 1957; Beeck and Ritchie, 1950; James and Moyes, 1978),iron powder (Deberbentsev et al., 1972; Emmett and Skau, 1943; Long et al., 1934), bulk promoted iron (Badilla-Ohlbaum et al., 1977; Phillips and Emmett, 1961),and silica-supported iron (Schuit and Reijen, 1958). However, a thorough kinetic analysis was reported only in the study of Badilla-Ohlbaum et al. (1977), although James and Moyes (1978) reported partial pressure dependencies at 273 K. No kinetic studies have been conducted on supported iron or on unsupported, nonpromoted bulk iron. Even with inexpensive metals such as iron, dispersing the metal on a support can be beneficial because higher metal surface areas per gram of catalyst
can be achieved along with greater stabilization toward sintering under reaction conditions. In addition, the type of support can affect catalytic activity in certain reactions. For example, Sagert and Pouteau (1972,1973) have found that the specific activity of Graphon-supported platinum in the D2-H20 exchange reaction increased with the degree of graphitization of the carbon, and Aika et al. (1972) have reported that carbon-supported Ru and Fe catalysts showed enhanced activity for ammonia synthesis when potassium was deposited on the carbon surface. In the benzene hydrogenation reaction, Taylor and Staffin (1967) reported that carbon-supported cobalt had a similar specific activity but a lower activation energy compared to silica- and alumina-supported cobalt. Finally, highly dispersed iron can be prepared on certain carbon supports, and the catalytic behavior of very small iron crystallites for CO hydrogenation is significantly different from that of large iron crystallites (Vannice et al., 1981; Jung et al., 1982a,b. As a consequence of these studies it was of considerable interest not only to determine the specific activities and kinetic behavior of iron dispersed on various supports including boron- and potassium-doped carbons, but also to learn whether the support or iron crystallite size has any significant effect on the catalytic properties of iron for benzene hydrogenation. A number of studies have indicated that CO chemisorption at 195 K is an accurate method of measuring reduced iron surface area (Emmett and Brunauer, 1937; Boudart et al., 1975; Jung et al., 1982a) and the results of this investigaton support this conclusion. With the unsupported iron, quite satisfactory agreement w8s obtained between surface areas calculated by BET measurements and those calculated from CO chemisorption. As indicated in Table 11, only the 5.0% Fe/C-1 catalyst was well-dispersed although the 10% Fe/A120, did appear to have iron crystallites well below 10 nm. The chemisorption measurement on the K-doped sample (3.4% Fe/K-GC) was complicated by the presence of the potassium, and the larger particle size indicated by XRD may be a more accurate indication because of the high-temperature treatment utilized during preparation (Moreno-Castilla et al., 1980). Agreement between particle sizes obtained from chemisorption and XRD measurements was only moderate, although values typically agreed within a factor of 2, particularly for the C-supported catalysts. The higher CO uptakes on the used 10% Fe/A1203samples are probably a consequence of more complete reduction because of the repeated regeneration steps a t 673 K. The average iron crystallite size determined by CO chemisorption is almost certainly more accurate than that from XRD because of the insensitivity of the latter to crystallites below 4 nm. Regardless, the assumption of a CO,d,/Fe, ratio of one-half provides reasonable agreement and allows the calculation of turnover frequencies. These turnover frequencies (TOF) indicate consistent behavior among these iron catalysts even with the complication of rapid deactivaton in some systems, as mentioned previously. With the exception of 10% Fe/A1203, s-l TOF values at 393 K were between 0.2 and 0.9 x while a somewhat wider range of initial activities occurred at 448 K; however, for most supported catalysts a TOF s-l existed. After long periods on-stream, the near 8 x final activities were similar, with a value of (3.5 f 2) X s-l at 448 K describing a l l catalysts. There are few previous values to which these turnover frequencies can be compared. Estimating a value for Fe, from the BET measurement on the promoted catalyst studied by BadillaOhlbaum et al. (1977) and extrapolating their rate equation
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 4, 1983
to the pressures in Table IV provides the values listed. The agreement is not unreasonable considering the approximations. Using the apparent activation energies and the TOF values at 393 K and PH of 680 torr, specific activities at 273 K can be estimated. These values of 9.8 x s-l for 5.3% Fe/BC-11 (sample 2), 7.8 X lo4 s-l for 10% Fe/A1203(sample l),and 8.4 X s-l for iron powder are all lower than the value of 2.2 X s-l reported by James and Moyes (1978) on iron T i s . This last value was obtained by dividing the reported rate by 6 as it represented the rate in terms of H atoms reacted. The hydrogen partial pressure (20 torr) used therein was much lower, and, in addition, the Fe surface area was measured by D2 chemisorption. When compared to other group 8 metals at identical conditions, iron is clearly the least active and has a TOF that is from one to three orders of magnitude lower than those on other metals (Yoon, 1982; Yoon and Vannice, 1983). The cause for the rapid deactivaton of three catalysts-5.0% Fe/C-1, 10% Fe/A1203, and iron powder-is not precisely known, but this behavior was quite reproducible because at least two different samples of each catalyst gave essentially identical results. However, significant differences existed among the three catalysts as the 10% Fe/A1203catalyst could be completely regenerated by the standard pretreatment whereas the other two could not. This result plus the absence of deactivation with some catalysts and the similar deactivation pattern with two different benzene purities, as shown in Figure 2, strongly argue against the possibility of poisoning by trace amounts of sulfur (or other impurities). Also, sintering cannot adequately describe loss of activity because iron surface areas of these catalysts did not change significantly as shown in Tables I1 and 111. The pretreatment (and regeneration) procedure used in this study is known to reduce the iron (Jung et al., 1982a); therefore, deactivation by oxygen entering via an air leak or in the benzene is not a satisfactory explanation because complete regeneration should have been achieved after rereduction of the iron. In addition, tests were run to check for air leaks and the benzene had been degassed and kept under flowing helium. These considerations lead us to conclude at this time that deposition of carbonaceous material on the iron surface and/or the formation of iron carbide phases represent the most likely explanation of the deactivation process. The small iron crystallites in the 5.0% Fe/C-1 catalyst are known to have suppressed H2adsorption, lower CO hydrogenation activity, and to readily form iron carbides (Jung et al., 1982a) and this could account for their rapid activity loss. Such explanations have been proposed for the deactivation observed during benzene hydrogenation over Ni and Pt catalysts (Huang and Richardson, 1978; Marangozis et al., 1979; Akyurtlu and Stewart, 1978). Graphitic carbons can be obtained by passing benzene and hydrogen over iron at higher temperatures of 613-1423 K (Kawasumi et al., 1981), and three different forms of carbon have been identified by Bonze1 and Krebs (1980) on an Fe (110) surface during CO hydrogenation. The graphitic form of carbon was very unreactive toward hydrogenation. Therefore, the possibility exists that residual benzene or carbonaceous material on the iron in some cases might even be converted into unreactive graphitic carbon during the regeneration step, thereby nullifying its effectiveness. To determine if the very slow activity decline observed for 5.3% Fe/BC-11 might be reversed by an activation in air to remove carbon species, sample 2 of this catalyst was heated in situ in flowing air at 673 K for 1 h and then given the standard pretreatment. The specific
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activity of this sample (sample 3) was not regenerated and was somewhat lower than that for sample 2, while CO chemisorption also decreased (Table 111). This result is consistent, however, with the supposition of a very stable unreactive graphitic carbon species. The activity maximum that was always observed as the temperature increased, as shown in Figures 3 and 6, and the partial pressure dependencies represented in Figures 4 and 5 are consistent with a reaction model that assumes that the rate-determining step is the addition of the last H atom to an adsorbed C6Hll species. Such a model predicts the rate equation given previously. This has been discussed in detail in another paper (Yoon and Vannice, 1983). However, the appearance in this study of an activity maximum which was clearly reversible has been observed over other metals (van Meerten and Coenen, 1975,1977; Kubicka, 1968) and is not a consequence of thermodynamic limitations on conversion as equilibrium conversions of benzene under the conditions employed here are 97.5% or higher. No significant trend in TOF values was observed among the various catalysts; however, the B-doped carbons did provide the highest values and the K-containing catalyst had no detectable activity. This behavior is more apparent when the maximum activities obtained with each catalyst are compared, and Table V shows that the two carbon supports doped with 1% boron produced catalysts which were 2-5 times more active than the other catalysts. The iron dispersed on these two B-doped carbons also showed very good activity maintenance;however, Table V indicates that this may well be a feature of the graphitic state of the carbon because the two graphitized carbon supports (GMC and V3G) also produced catalysts with essentially constant activity during 7 h on-stream as shown in Figure 7. The difference in behavior for the Si02-supportediron is clearly evident. Although the 4.8% Fe/BC-1 and 4.8% Fe/MC catalysts exhibited noticeable activity loss during the first hour or two on-stream, their activity was constant for the remainder of the 6-h run while stable activities on-stream were never achieved for the 10% Fe/A1203and reduced Fe203powder catalysts (Yoon, 1982). The 5.0% Fe/C-1 which had the smallest crystallites deactivated very rapidly and unsupported iron also showed continuous large activity declines. Because of the influence of deactivation and the use of different supports, 2 to 3-fold variations in final activity at 448 K were not considered significant. However, similar variations in activity loss for a given catalyst were assumed significant as all final activities were internally consistent because of the comparison to initial activity for that particular catalyst.
Summary This study has measured specific activities for benzene hydrogenation over a variety of supported iron catalysts through a range of well-defined reaction conditions by use of a differential reactor. Iron surface areas based on CO chemisorption at 195 K were to estimate particle sizes and to calculate turnover frequencies on supported iron for the first time. In addition, any influence of crystallite size or of the support could be clarified. The turnover frequency on iron is the lowest of the group 8 metals; however, the activity per gram of iron can be greatly increased by dispersion on a support material. All iron catalysts showed an activity maximum near 473 K, a pressure dependence on H2 near 3, and on benzene between 0 and -1. After substantial time in the reactor, the active iron catalysts had very similar turnover frequencies at 448 K, indicating little effect of the support on specific activity; however, the highest achievable activities occurring at the
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activity maximum were obtained with the boron-doped carbon catalysts, which were 2-5 times more active than the other iron catalysts. Carbon-supported iron possessed superior activity maintenance, with two graphitized carbon supports (GMC and V3G) showing stable activity and the two 1% B-doped graphitized carbons showing almost no decline in activity. In contrast, large continuous decreases occurred for unsupported iron and iron supported on silica and alumina. Evidence at this time favors formation of surface carbon and/or iron carbide as the explanation for this deactivation process and the possibility exists that carbon supports may retard or eliminate this problem. No significant effect of crystallite size on TOF values was observed over a range of 2-6700 nm; however, deactivation occurred extremely rapidly on the smaller iron particles. This is attributed to the decreased Hzadsorption and reduced hydrogenation activity observed over small iron crystallites and the strong possibility of rapid iron carbide formation (Jung et al., 1982a). Addition of potassium to an Fe/C system had only negative effects. However, of the iron catalysts studied, the carbon-supported systems, especially B-doped carbon, appear to be superior catalytic systems. Acknowledgment We are grateful to the National Science Foundation for support of this work through Grant CPE-7915761. This grant was co-sponsored by the Army Research Office. Registry No. Fe, 7439-89-6; A1,0,, 1344-28-1; Cab-0-Sil, 7631-86-9;benzene, 71-43-2; boron, 7440-42-8;carbon, 7440-44-0. Literature Cited
Badilla-Ohlbaum, R.; Neuburg, H. J.; Graydon, W. F.; Phillips, M. J. J . Catal. 1977, 47,273. Beeck. 0.; Ritchie. A. W. Discuss. Faraoky SOC.1950, 8 , 159. Benson, J. E.; Boudart, M. J . Catal. 1965, 4 , 704. Bonzel, H. P.; Krebs, H. J. Surf. Sci. 1980, 91, 499. Boudart, M.; Deibouilie, A.; Dumesic, J. A.; Khammouma, S.;Topscle. H. J . Catal. 1975, 37,486. Debertbentsev, Y. I.; Paal, Z.; TBtBnyi, P. Z . Phys. Chem. (frankfurt am Main) 1972 80, 51. Emmett, P. H.; Brunauer, S.J . Am. Chem. SOC. 1937, 59, 310. Emmett, P. H.; Brunauer, S. J . Am. Chem. Soc. 1937, 59, 1553. Emmett, P. H.; Skau, N. J . Am. Chem. SOC.1943, 65,1029. Huang, C. P.; Richardson, J. T. J . Catal. 1978, 52, 332. faraday Trans. 1 1978, 74, James, R. G.;Moyes, R. B. J . Chem. SOC., 1666. Jung, HJ.; Vannice, M. A.; Mulay, L. N.; Stanfield, R. M.; Delgass, W. N. J . Catal. 1982a, 76,208. Jung, HJ.; Walker, P. L., Jr.; Vannice, M. A. J . Catal. l982b, 75,416. Kawasumi, S.;Egashira, M.; Katsuki, H. J . Catal. 1981, 68, 237. Kubicka. H. J . Catal. 1988, 12, 223. Long, J. H.; Frazer. J. C. W.; Ott, E. J . Am. Chem. SOC. 1934, 5 6 , 1101. Marangozis, J. K.; Mantzouranis. B. G.; Sophos, A. N. Ind. Eng. Chem. Proo'. Res. Dev. 1979, 78, 61. Moreno-Castilla, C.; Mahajan. 0. P.; Walker, P. L., Jr.; Jung. H-J.; Vannice, M. A. Carbon 1980, 18, 271. Mulay, L. N.; Rao, A. V. Prasad; Yoon, K. J.; Vannice, M. A. Paper to be published. Palmer, M. B.; Vannice, M. A. J . Chem. Techno/. Biotech. 1980, 30,205. Phillips, M. J.; Emmett, P. H. Int. Congr. Pure Appl. Chem. 18th 1961, Paper 81-28, Sagert. N. H.; Pouteau, R. M. L. Can. J . Chem. 1972, 5 0 , 3686. Sagert, N. H.; Pouteau. R. M. L. Can. J . Chem. 1973, 5 1 . 4031. Schuit, G . C. A.; van Reijen, L. L. Adv. Catal. 1958, IO, 243. Seideil. A.; Linke, W. F. "Solubility of Inorganic and Metai-Organic Compounds", 4th ed., American Chemical Society. Washington, DC, 1958; p 1234. Taylor, W. F.; Staffin, H. K. J . Phys. Chem. 1987, 71,3314. van Meerten, R. 2 . C.; Coenen, J. W. E. J . Catal. 1975, 37,37. van Meerten, R. Z. C.; Coenen, J. W. E. J . Catal. 1977, 46, 13. Vannice. M. A.; Walker, P. L.. Jr.; Jung, HJ.; Moreno-Castilla, C.; Mahajan, 0. P. R o c . Int. Congr. Catal. 7th 1981, 460. Yoon, K. J. Ph.D. Thesis, The Pennsylvania State University, 1982. Yoon. K. J.; Vannice, M. A. J . Catal. 1983, 82,457.
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Received for review February 18, 1983 Accepted June 9, 1983
Hydroisomeriration and Hydrocracking. 4. Product Distribution from n-Octane and 2,2,4-Trimethylpentane Herman Vanslna, Mlguel A. Baltanas, and Gilbert F. Froment' Laboratorium voor Petrochemlsche Technlek, Rijksunlversiteit Gent, Krijgslaan, 28 7, 59000 Gent, Belgium
n -Octane and isooctane hydroisomerizationand hydrocracking on an ultrastable Y zeolite containing 0.5 wt % Pt were investigatedin a Berty-type reactor over a wide range of temperatures (80-240 "C)and pressures (5-100 bar). The primary products of the reaction of n-octane are the monobranched isomers. Multibranched feed isomers and cracked products are formed in consecutive reactions. For isooctane (2,2,44rimethylpentane),the primary products are multibranched isomers and isobutane. The formation of dibranched, monobranched, and linear octanes proceeds over a consecutive mechanism. The product distribution from hydroisomerization and hydrocracking is a unique function of the total conversion, regardless of the degree of branching of the hydrocarbon fed. Carbenium ion rearrangementsvia protonated cyclopropane rings and cracking by 0scission are the only observed catalytic reactions besides the usual proton and methyl shifts.
Introduction Hydrocracking is a highly versatile petroleum refining process currently used for converting various petroleum fractions, from light naphthas to deasphalted vacuum residua, t~ more valuable quality products (Billon et al., 1975; Bolton, 1976; Kelley et al., 1979). As early as 1964 bifunctional hydrocracking catalysts based on zeolites and noble metals were shown to have a much better resistance to nitrogen and sulfur compounds, greater activity, and high selectivity for producing a maximum gasoline (Ward,
1975). The use of ultrastable Y zeolites has also led to remarkable resistances to deactivation and improved thermal and hydrothermal stabilities (McDaniel and Maher, 1967; Kerr, 1969; Vaughan, 1979). A considerable effort has been invested recently in the understanding of the reaction networks that lead to the formation of isomers, as well as cracked products, from normal and branched paraffins (Steijns et al., 1981; Weitkamp et al., 1978). With certain types of catalysts a t least, normal paraffins have been shown to isomerize to
0196-4321/83/1222-0526$01.50/00 1983 American Chemical Society
