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Catalytic Synergism in Physical Mixtures Hsuan Chang and Jonathan Phillips* Department of Chemical Engineering, The Pennsylvania State University, 133 Fenske Laboratory, University Park, Pennsylvania 16802
Roland Heck Mobil Research and Development Corporation, Princeton, New Jersey 08540 Received September 21, 1995X A novel and successful approach to the design of hydrogen-shift catalysts is presented. Detailed studies of 1-butene hydroisomerization reveal that a catalytic synergism exists in physical mixtures. Mixtures of FeCe/Grafoil and Pt/Grafoil (or Pd/Grafoil) display high selectivity (.Pt) and activity levels as much as ten times larger than the sum of the separately measured activities. Thus, it is demonstrated that highly selective and active isomerization catalysts can be created by physically mixing hydrogen atom generator surfaces (e.g. Pd) with highly selective but low-activity surfaces (e.g. FeCe).
Introduction Recently a new family of very selective and active olefin double-bond shift catalysts was discovered.1 This new family of catalysts consists of one metal from the first row of the transition metals (typically iron or cobalt), one metal from the lanthanide series (typically cerium or praseodymium), and a relatively small amount of one noble metal (typically palladium). These multimetallic catalysts (e.g. Fe/Ce/Pd, 1:1:0.2) have some catalytic properties of each parent material. For example, FeCePd/Grafoil has a selectivity (excellent) similar to that of FeCe/Grafoil but high activity similar to that of Pd/Grafoil, a material with poor selectivity. The following model was proposed. First, the particle surfaces are ‘compound’. They consist primarily of ‘alloy’ transition metal-lanthanide metal zones and small zones of unalloyed noble metal. Second, each zone on the surface performs a different chemical function. Specifically, the alloy zone selectively isomerizes the 1-butene, and the noble metal zone provides hydrogen atoms to the alloy zones via ‘spillover’, thus dramatically increasing reaction rates on the alloy sites. The model is consistent with the general understanding that bond-shift isomerization of olefins occurs via hydrogen atom addition at R-carbons, creating intermediates (e.g. C4H9), followed by hydrogen atom removal at β-carbons.2-6 In this report, results supporting the earlier hypothesis are presented. It is shown that a physical mixture of the two components (e.g. FeCe/Grafoil and Pt/Grafoil) is as much as an order of magnitude more active than each component tested separately. The synergism also results in dramatically improved selectivity. This is not the first report of physical mixtures demonstrating synergistic catalytic properties, although the degree of synergism appears to be greater than any previously reported.7-10 Two models exist to explain the * Corresponding author. E-mail:
[email protected]. Fax: (814) 8657846. X Abstract published in Advance ACS Abstracts, April 15, 1996. (1) Lu, W. C.; Chang, H.; Phillips, J. J. Catal. 1994, 146, 608. (2) Macnab, J. I.; Webb, G. J. Catal. 1968, 10, 19. (3) Bond, G. C.; Wells, P. B. Adv. Catal. 1963, 15, 91. (4) Mellor, S. D.; Wells, P. B. Trans. Faraday Soc. 1969, 65, 1883. (5) Ragaina, V.; Cattania-Sabbadini, M. G. J. Catal. 1985, 93, 161. (6) Mellor, S. D.; Wells, P. B. Trans. Faraday Soc. 1969, 65, 1873. (7) Haruta, M.; Tsubota, S.; Ueda, A.; Sakurai, H. Stud. Surf. Sci. Catal. 1993, 77, 45. (8) Delmon, B. Stud. Surf. Sci. Catal. 1993, 77, 1.
S0743-7463(95)00789-X CCC: $12.00
synergism of physical mixtures: the classic ‘polyfunctional catalyst model’11 and the hydrogen spillover model.12 It is argued that the present results most strongly support the hydrogen spillover model and suggest a means to create highly selective and active catalysts from physical mixtures. Finally, it should be noted that there are many industrial applications for these catalysts. For example, selective hydrogenation (bond shift) prior to alkylation in modern gasoline refining leads to a higher octane alkylate. Increased octane is related to the fact that alkylates produced from 2-butene are more highly branched than alkylates made from 1-butene. Experimental Section Catalyst Preparation. Eight catalysts were prepared on GTA-grade Grafoil (Union Carbide), a moderately high-surfacearea (22 m2/gm), high-purity, graphitic material, via the incipient wetness technique.13-15 Four FeCePt/Grafoil multimetallic catalysts were made, each containing approximately the same amount of iron and cerium but variable amounts of platinum, as shown in Table 1. Also produced were FeCe/Grafoil, Pt/Grafoil, Fe/Grafoil, and Ce/Grafoil catalysts. Ground- and temperature-treated Grafoil16 was gradually added to a solution composed of a mixture of one or more of three aqueous solutions prepared from salts obtained from Aldrich Chemical Co. (Table 1). The average flake size after impregnation was 0.35 mm.17,18 Kinetics. The bond shift in 1-butene was studied using a differential Pyrex microreactor operated at 1 atm of pressure, as described elsewhere.19,20 Reaction gas purchased from Matheson was mixed with rotameters to yield a reaction gas with 2% butene, 20% hydrogen, and helium as balance. Analyses were conducted using a 5890 Series II Hewlett Packard gas chromatograph equipped with a TC detector and a packed column containing (9) Antonucci, P.; Truong, N. V.; Giordano, N.; Maggiore, R. J. Catal. 1982, 75, 140. (10) Flesner, R. L.; Falconer, J. L. J. Catal. 1993, 139, 421. (11) Weisz, P. B. Adv. Catal. 1962, 13, 137. (12) Conner, W. C., Jr.; Pajonk, G. M.; Teichner, S. J. Adv. Catal. 1986, 34, 1. (13) Lin, S. C.; Phillips, J. J. Appl. Phys. 1985, 58, 1943. (14) Phillips, J.; Clausen, B.; Dumesic, J. A. J. Phys. Chem. 1980, 84, 1814. (15) Bukshpan, S.; Sonnino, T.; Dash, J. G. Surf. Sci. 1975, 52, 460. (16) Gatte, R. R.; Phillips, J. J. Phys. Chem. 1987, 91, 5961. (17) Gatte, R. R.; Phillips, J. J. Catal. 1987, 104, 365. (18) Matyi, R. J.; Schwartz, L. H.; Butt, J. B. Catal. Rev. Sci. Eng. 1987, 29, 41. (19) Durr, H.; Phillips, J. J. Catal. 1990, 126, 619. (20) Silva, S. M.; Phillips, J. J. Mol. Catal. 1994, 94, 97.
© 1996 American Chemical Society
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Table 1. Catalyst Composition and Preparation catalyst
metal content
atomic ratio total metal, wt % nominal measureda,b nominal measureda
1 2 3 4 5 6 7 8
Fe/Ce/Pt Fe/Ce/Pt Fe/Ce/Pt Fe/Ce/Pt Fe/Ce Ce Fe Pt
1:1:0.20 1:1:0.10 1:1:0.07 1:1:0.04 1:1
1:0.84:0.32 1:0.91:0.18 1:1.09:0.12 1:1.07:0.06 1:0.92
5 5 5 5 5 3.6 1.4 5
5.33 4.74 4.84 4.77 4.85 3.21 1.48 4.94
a Galbraith Laboratories, Inc., Knoxville, TN. b In plotting data measured values are used in all cases. The following salts were used to prepare aqueous solutions of appropriate concentration for incipient wetness: Fe(NO3)3‚9H2O, Ce(NO3)3‚6H2O, and PtCl2‚4H2O (Aldrich Chemical Company, Inc., Milwaukee, WI).
0.19% picric acid on Carbograph ZAP 80/100 mesh (Alltech). In all studies intended for the purpose of comparing levels of activity, total conversion was kept to less than 12% in order to justify the ‘differential reactor’ approximation. Also, flow rates were always above 250 cc/min STP, allowing mass transfer limits on measured rates to be ignored.21 In order to test the impact of mixtures on catalytic activity and selectivity, physical mixtures containing different absolute amounts and different relative amounts of each ‘catalyst’ were required. To change the composition of the catalyst bed, 10 mg increments of catalyst were added through the differential reactor side arm. Next, after vigorous mixing, all the catalyst material was reduced in flowing hydrogen at 400 °C for 4 h. After cooling, the reaction gas mixture was introduced, and the system was allowed to equilibrate for 2 h before any data were taken. At least three measurements of activity/selectivity were collected under any set of conditions, although only the average values are reported. The spread in activities/selectivities was never greater than 5%. X-ray. X-ray analyses of catalysts were conducted using a Rigaku (Model Giegerflex D/Max-IIIA) diffractometer equipped with a copper target X-ray source, a curved crystal graphite monochromator, and TI drifted NaI scintillation detectors.22 Before being prepared for X-ray work, samples were reduced for 4 h in flowing H2 at 400 °C and then after cooling in He were gradually exposed for 3 h to oxygen. Specifically, oxygen exposure was conducted using a mixture of 90% He and 10% dry air. This mix was passed over the samples at a total flow rate of 10 cm3/ min for 10 h before removal from the preparation cell. Previous work indicates that for iron this prevents more than the first few layers from becoming oxidized,22 and current results support this finding.
Results Physical Mixtures. Control studies were carried out to ascertain the impact of Grafoil addition on catalytic behavior. Initially, the activity and selectivity of 18 mg of Grafoil and 2 mg of Pt/Grafoil at 373 K were measured. These activity data points (both on a total activity level basis and a per gram of Pt basis) are shown in Figure 1a, and the selectivity data are shown in Figure 1b (20 mg of Pt/Grafoil). Next, an additional 10 mg of Grafoil was added to the catalyst bed, and then the bed was shaken/mixed and re-reduced as described above. These data points are also shown in Figure 1a (30 mg of Pd/Grafoil). This procedure was repeated (10 mg of Grafoil added each time) until 90 mg of material (2 mg of Pd/Grafoil and 88 mg of Grafoil) was present in the reactor. There was a gradual decrease in the measured level of activity and very little change in the measured selectivity. A virtually identical control procedure was carried out using Pd/Grafoil (Figure 2) instead of Pt/Grafoil. The results are consistent with the expected catalytic behavior. No additional catalytic material was added, so no change in activity or selectivity was expected. The (21) Durr, H. M.S. Thesis, The Pennsylvania State University, 1990. (22) Gatte, R. R.; Phillips, J. Langmuir, 1989, 5, 758-766.
Figure 1. Synergism of platinum-containing physical mixtures. (a) Activity of the physical mixtures. (b) Selectivity of the physical mixtures measured at approximately 20% conversion in all cases. Selectivity is defined as this product ratio: 2-butenes/(2-butenes + butane). (c) Activity on a per gram mixture basis. d) Bar graph illustrating synergism. Note that the activity of 88 mg of FeCe/Grafoil is so low that it barely registers on the scale used.
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Figure 2. Synergism of palladium-containing physical mixtures. (a) Activity of the physical mixtures. (b) Selectivity of the physical mixtures measured at approximately 20% conversion in all cases. (c) Bar graph illustrating synergism. Note that the activity of 88 mg of FeCe/Grafoil is so low it does not register on the scale.
slight reduction in activity probably results from a gradual sintering of the platinum/palladium.23 Control studies on the activity of FeCe/Grafoil catalysts were also conducted. First, the activity and selectivity at 100 °C of 20 mg of the FeCe/Grafoil catalyst were measured. Next, 10 mg of FeCe/Grafoil was added to the catalyst bed, and then the bed was shaken/mixed and re-reduced. This procedure was repeated until 90 mg of material was present in the reactor. Even the maximum activity was extremely low, approximately zero on the scale used in Figure 1a (not shown). There was very little change in the measured selectivity (Figure 1b). The very (23) Wu, N. L.; Phillips, J. Surf. Sci. 1987, 184, 463.
Chang et al.
low activity is consistent with earlier reports of the activity of FeCe/Grafoil catalysts1 and possibly results from the inability of this material to dissociate hydrogen atoms. Studies of physical mixtures of Pt/Grafoil and FeCe/ Grafoil catalysts were carried out in much the same fashion as the control studies. Initially a mixture of 2 mg of Pt/ Grafoil and 18 mg of FeCe/Grafoil was studied. As shown (Figure 1a), the activity of this physical mixture was significantly higher than that of either of the control catalysts. The selectivity, however, was slightly lower than that of either control (Figure 1b). Next, an additional 10 mg of FeCe/Grafoil was added to the reactor, and the reactor was shaken/mixed and re-reduced. This resulted in increased activity and selectivity. Each additional increment led to an increase in activity and selectivity, indicating a synergism in catalytic behavior. Experiments almost identical to those described above were carried out using Pd/Grafoil (Figure 2) instead of Pt/Grafoil. The rates over Pd/Grafoil were too high to operate the reactor in a differential mode above 60 °C; thus, it was necessary to conduct experiments at lower temperatures. Qualitatively the results using Pd/Grafoil are similar to those obtained with Pt/Grafoil. Indeed, each increment of FeCe increased the total activity. The selectivity of the physical mixtures containing Pd/Grafoil also dramatically improves; however, unlike the case for the mixtures containing Pt/Grafoil (Figure 2b), the improvement occurs almost entirely upon the first addition of FeCe/Grafoil. In Figures 1d and 2c a second method of illustrating synergism for both physical mixtures is shown. The bar graphs clearly show that both physical mixtures display activity far greater than expected from the sums of the activities for the two components. Fe/Grafoil and Ce/Grafoil were physically mixed with Pt/Grafoil, and a synergism in activity was again noted; however, the selectivity in each case (300 °C), and ring opening and enlargement of some cyclic hydrocarbons.27,28 According to the polyfunctional (25) Mills, G. A.; Heinemann, H.; Milliken, T. H.; Oblad, A. G. Ind. Eng. Chem. 1953, 45, 134. (26) Evering, B. L.; D’ouville, E. L.; Lien, A. P.; Waugh, R. C. Ind. Eng. Chem. 1953, 45, 582. (27) Chow, M.; Park, S. H.; Sachtler, W. M. H. Appl. Catal. 1985, 19, 349. (28) Bai, X.; Sachtler, W. M. H. J. Catal. 1991, 129, 121.
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catalyst model a stable molecular species (intermediate) forms at one catalytic site and then diffuses via the gas phase to a second location on the surface, at which point a second catalytic process takes place, leading to the formation of the desired product. The second model is the hydrogen spillover model. Synergism is postulated to result from the ability of one catalytic site to create hydrogen atoms which spillover and activate catalytic chemistry at a second set of sites. The key differences in the two theories provide an understanding of why the spillover model is the preferred explanation for the present results. In the theory of polyfunctional catalysts it is always assumed that there is a diffusing intermediate molecular species. There is no intermediate in the spillover model. Instead, an atomic species (generally hydrogen) created at one location (generally a noble metal particle) diffuses across the surface and in some fashion ‘activates’ a catalytic process at a second location. It is also important to note the specific difference accorded the behavior of hydrogen. In the spillover model it is critical that hydrogen is dissociated and that hydrogen atoms migrate rapidly. In the classic polyfunctional catalyst model, hydrogen is deliberately left out of all the steps, as hydrogen is assumed to have no impact on the reaction rate.11 The polyfunctional model appears inadequate. It is not clear what identity to assign to the required diffusing intermediate. For example, it is not thermodynamically reasonable to postulate butane as the intermediate, as butane is overwhelmingly thermodynamically favored relative to butene over the temperature and pressure range of interest. The hydrogen spillover model is totally consistent with the reported observations. One of the catalytic materials is only present to generate ‘activator’ species, hydrogen atoms, which saturate the surface. The hydrogen atoms are not ‘used’ in the isomerization reaction and thus are not intermediates but rather ‘mobile catalysts’. As noted by many workers,2-6 hydrogen atoms are required for the metal catalyst isomerization of olefins. Indeed, deuterium exchange studies of both 1-butene isomerization and butadiene isomerization/hydrogenation29,30 show isotopic mixing fully consistent with hydrogen atom addition/ subtraction mechanisms. In particular, during 1-butene isomerization a C4H9 surface intermediate invariably forms. The extra hydrogen atom is associated with the R-carbon. Subsequently, a different hydrogen atom is removed from the β-carbon, completing the isomerization process. The principle molecule (1-butene), which only undergoes isomerization in the desired reaction, is converted at a single site (FeCe) and does not migrate at all. Moreover, the migrating activator species (hydrogen atoms) is not a stable molecular species and migrates via a surface diffusion process rather than via the gas phase. The differences in the models suggest distinct experimental outcomes. As the polyfunctional model is diffusion limited, longer distances between sites created by a larger ‘pellet’ size or the addition of pellets containing only one site type result in a reduction in observed synergism on a gram catalyst basis.28 In the hydrogen spillover model the distance between the two site types (noble metal/FeCe) is expected to have less impact. Thus, the nearly flat activity/gram measured for physical mixtures (Figure 1c) supports the hydrogen spillover model. The trends in selectivity present something of a puzzle. No existing model explains the observation that initially the selectivity of the FeCe/Grafoil + Pt/Grafoil mixture is lower than that of either individual component. The (29) Garra, S.; Ragaini, V. J. Catal. 1991, 129, 121. (30) Ragaini, V. J. Catal. 1974, 34, 1.
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Figure 5. Relative activity of palladium- and platinumcontaining mixtures. Note that the activation energy of the platinum-containing mixture is approximately 8.5 kcal/(g mol) and that of the palladium-containing mixture is approximately 4 kcal/(g mol).
difference in selectivity trends between the platinum and palladium cases are also puzzling. Although it is clear both mixtures enhance selectivity, additional study is needed for a full explanation. One additional model requires attention. It could be argued that spillover during reduction of the noble metal containing mixtures at 673 K in flowing hydrogen results in more complete reduction of the alloy relative to reduction in mixes not containing the noble metal. Activity differences simply reflect differences in the degree of reduction of the alloy. This model is considered less likely for several reasons. For example, it cannot explain differences between mixes containing platinum and palladium. As shown in Figure 5, the palladium-containing mix is far more active (e.g. more than a factor of eight at 333 K). Also, the activation energies of the platinum-containing mixture are more than twice that of the palladium-containing mixture. Moreover, the suggestion that non-noble metals are more active than noble metals for isomerization and hydrogenation runs counter to years of laboratory and industrial experience. A totally novel mechanism, or a new mechanism for hydrogen dissociation, must be postulated. In contrast, the hydrogen spillover model is consistent with differences between platinum- and palladiumcontaining mixtures. It is reasonable to postulate that the net rate reflects hydrogen atom concentration at the alloy particle surfaces. According to the spillover hypothesis this concentration should reach a steady-state value reflecting a balance between rates of hydrogen atom production at noble metal surfaces and recombination rates on all surfaces. (Note, given the high selectivities of the mixtures, the rate of hydrogen atom “use” to form butane is low.) The rate of hydrogen atom production is known to be a function of the identity of the noble metal. Indeed, in qualitative agreement with the present work, at low temperatures palladium is generally more active for processes requiring hydrogen atoms than platinum. The hydrogen spillover hypothesis is also consistent with known chemistry. First, it has been repeatedly demonstrated that hydrogen atom spillover should take place across carbon surfaces impregnated with noble metal particles.12 Second, the proposed mechanism of hydrogen atom addition/subtraction is identical to that found for all other metals. It does not require hydrogen dissociation on non-noble metals. It only requires one new postulate: The activity of some non-noble metals for isomerization/ hydrogenation is controlled by the availability of hydrogen atoms.
Catalytic Synergism in Physical Mixtures
Conclusions Physical mixtures can yield catalytic synergism in terms of both activity and selectivity. This creates a novel means to harness the phenomenon of hydrogen spillover to create significantly improved catalysts. It also suggests there is value to a shift in emphasis in catalytic research. Possibly relatively greater emphasis should be placed on understanding and controlling the concerted catalytic actions of several surfaces. For example, the present work suggests that entire families of materials presently
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considered catalytically uninteresting (e.g. FeCe) are potentially of great value when mixed (chemically or physically) with catalytically ‘complimentary’ (e.g. Pt) surfaces. Acknowledgment. The authors gratefully acknowledge the financial support for this study by the U.S. National Science Foundation under Grant CTS-9423094. LA9507895
