J. Phys. Chem. 1987, 91, 929-937 relevant. The preexponential factors for each radical are small which of course indicates a significant reduction in the degrees of freedom in the transmission state. In the present case this loss of freedom almost certainly arises because of a requirement that the ring ?r orbitals and the developing p orbital on the benzylic carbon align so as to allow the greatest stabilization of the product radical. This requirement will tend to "freeze" the rotation of the bond to the benzene ring. It is possible that this symmetrical conformation is disfavored in the unsymmetrical radicals from 7 and 8 and that a higher than expected activation energy might resiilt
Conclusion From CIDNP and laser flash photolysis studies, the extent of preferential type I photocleavage for unsymmetrical dibenzyl ketones has been determined. In addition, using laser flash photolysis, the absolute rates of decarbonylation of the intermediate acyl radicals have been determined. Almost certainly the main
929
factor which controls each process is the stability of the product benzylic radicals, although the results of variable-temperature studies suggest that other more subtle effects may play some role in unsymmetrical radicals.
Acknowledgment. We thank Mr. Xuegong Lei for synthesizing o,o'-dimethyldibenzyl ketone, and the NSF and AFSOR for their generous support of this research. Drs. K. U. Ingold and J. C. Scaiano of the N R C (Canada) are also thanked for discussions of related research and for agreeing to a simultaneous publication of preliminary results. Registry No. 1, 102-04-5; 2, 105457-17-8; 3, 77787-71-4; 4, 5452347-6; 5, 29903-09-1; 6, 64321-35-3; I, 23592-92-9; 8, 33500-61-7; 9, 71254-82-5; 10, 7476-1 1-1; 11, 13363-25-2; PhCHZCO', 22673-58-1; PhCDzCO', 105457-18-9; Ph"CHzCO', 105457-19-0; p MeOC6H4CH2CO', 105457-20-3; p-BrC,H,CH2CO', 105457-21-4; PhC(CH,),CO', 84175-26-8; Ph,CHCO', 105457-22-5; O MeC6H4CH2CO', 105457-23-6; p-t-BuC6H4CH2CO', 105457-24-7; PhCHCH3CO', 84175-25-7.
Evidence for Rapid Chain Growth in the Fischer-Tropsch Synthesis over Iron and Cobalt Catalysts Charles A. Mims* and L. E. McCandlish Exxon Research and Engineering Company, Corporate Research Science Laboratories, Clinton Township, Annandale, New Jersey 08801 (Received: July 7, 1986)
We have examined hydrocarbon chain growth in the Fischer-Tropsch reaction by following both the rate and position of incorporation of I3C into the hydrocarbon products after an abrupt switch from I2CO to "CO in the reactant gas. On two catalysts (cobalt supported on silica and precipitated iron combined with various promoters) we find that I3C appears after the isotope switch at the same rate in all positions of all products examined. Under the assumption that chain growth is unidirectional, this finding shows that chain growth on these catalysts is very rapid in comparison to the rate of isotope displacement in the precursors to chain growth. In addition, only a small portion of the active carbon on the surface can be in the form of hydrocarbon chains. Under these conditions no detailed information about the mechanism of chain growth is obtainable.
I. Introduction A large effort has been expended to understand the mechanism of Fischer-Tropsch synthesis.' Many ingeneous experiments have proven the plausibility of various pathways, but the difficulty in making in situ measurements of proposed intermediates has frustrated an unequivocal definition of the mechanisms. Keys to understanding the mechanism are the surface concentrations and identities of active intermediates during the reaction. We have examined the Fischer-Tropsch reacton by following both the rate and position of incorporation of I3C into the c1-c6 hydrocarbon products after an abrupt switch of I3CO for I2CO in the reactant gas. Such isotopic transients under reaction steady state provide a valuable tool for the in situ study of complex reacting systems and are a special class of transient kinetic experiments. Recently, this technique has been used by several workers to study the formation of CH, from CO-H2.2-8 The ( I ) Fpr reviews, see: (a) Anderson, R. B. The Fischer-Tropsch Synrhesis; Academic: New York, 1984. (b) Biloen, P.; Sachtler, W. M. H. Adv. Caral. 1983, 30, 165. (c) Smutek, M.; Cerny, S. Inr. Rev. Phys. Chem. 1983,3, 263. (d) Vannice, M. A. Catal. Rev.-Sci. Eng. 1976, 14, 153. (2) (a) Happel, J.; Suzuki, I.; Kokayeff, P.; Fthenakis, V. J . Catal. 1980, 65, 59. (b) Happel, J.; et al. J. Cafal. 1982, 75, 314. (c) Otarod, M.; et al. J. Catal. 1983, 84, 156. (3) Kobori, Y.; Yamasaki, H.; Naito, S.; Onishi, T.; Tamaru, K. J . Chem. SOC.,Faraday Trans. 1 1982, 78, 1473. (4) Yamasaki, H.; Kobori, Y.; Naito, S.; Onishi, T.; Tamaru, K. J. Chem. SOC.,Faraday Trans. I 1981, 77, 2913. (5) (a) Cant, N. W.; Bell, A. T. J . Caral. 1982, 73, 257. (b) Winslow, P.; Bell, A. T. J . Caral. 1984, 86, 158.
0022-3654/87/2091-0929$01.50/0
emerging picture from these studies is that a small amount of carbon on the surface of synthesis catalysts is committed to the production of hydrocarbons. Chain growth in the Fischer-Tropsch reaction has received some a t t e n t i ~ n . Biloen ~ , ~ ~ ~et aL6 placed a low bound on C-C bond formation rates on Co, Ru, and Ni by measuring the appearance rate of a new carbon isotope in C3 fragments. We have published a brief communication of a portion of the results herein which show that chain growth is very rapid on an iron catalyst.' Prior to these studies, estimates of chain growth (made from site turnover f r e q u e n c i e ~ ' ~and ~ ~ +non-steady-state '~ experim e n t ~ ' ' - ' ~were ) 2-3 orders of magnitude slower. By studying the isotopic transient in detail, including the position dependence of isotope incorporation within the molecules, we can extend our knowledge about the chain growth process in the Fischer-Tropsch reaction. We can measure or set limits on the rates of individual reaction steps and on the coverages of the catalyst surface by reactive intermediates. The experiment addresses the number and (6) Biloen, P.; Helle, J. N.; van den Berg, F. G. A,; Sachtler, W. M. J . Caral. 1983, 81, 450. (7) Mims, C. A.; McCandlish, L. E. J. Am. Chem. SOC.1985, 107,696. (8) Zhang, X.;Biloen, P. J. Caral. 1986, 98, 468. (9) Matsumato, H.; Bennett, C. 0. J . Cafal. 1978, 53, 331. (IO) Demitras, G. C.; Muertteties, E. L. J. A m . Chem. SOC.1977, 99, 2796. (1 1) Dautzenberg, F. M.; Helle, J. N.; van Santen, R. A.; Verbeek, H. J . Caral. 1977, 50, 8. (12) Feimer, J. N.; Silveston, P. L.; Hudgins, R. R. Can. J . Chem. Eng. 1984, 62, 241. (13) Peacock-Lopez, E.; Lindenberg, K. J . Phys. Chem. 1984, 88, 2270.
0 1987 American Chemical Society
Mims and McCandlish
930 The Journal of Physical Chemistry, Vol. 91, No. 4, 1987
eo
ieot
Figure 1. Compartmental model for hydrocarbon chain growth in Fischer-Tropsch synthesis. The boxes or compartments represent surface-adsorbed intermediates;C, represents the C, monomer which is the building block in chain growth, and C, ( n = 1, 2, 3, etc.) are the adsorbed
hydrocarbon chains. connectivity of pools of surface intermediates as well as the mechanism of chain growth in favorable cases. The Isotope Substitution Experiment. The application of isotopic transients at reaction steady state has been adequately discussed e l s e ~ h e r e . ~ ~For ~ ~simple ’ ~ ~ ’systems, ~ such as the hypothetical reaction given in eq 1, the application of the technique is straightforward. The rate of production of C is given by the ratio NB/TB,where NB is the number of intermediate B and Tb is the mean residence time of B on the surface. After a sudden change of reagent A to a new isotopic variant, A*, unlabeled product C can only be made by reaction of B that was on the surface at the time of the switch. Unlabeled C decays exponentially (with time constant T ~ as ) B is replaced by B*. The transient experiment thus allows one to separate the concentration . sum of the unlabeled and term, NB,and reactivity term, T ~ The labeled concentrations of each species is constrained experimentally to be constant. Thus, the reaction rate remains at steady state (barring kinetic isotope effects). In the same manner, an experiment on more complex systems will yield surface concentrations of a given element commitred to a given product. However, the translation of the measured decay times into residence times and coverages of surface intermediates is not as straightforward. For more complex systems the mechanism cannot always be uniquely determined. This is, in general, due to the limited number of points in the reaction path (typically only gas-phase species) where the transient can be observed and where an isotopic perturbation can be applied. Therefore, one must usually resort to examining models for consistency with the data. The problem of uniqueness in such experiments has been addressed and is the subject of current r e s e a r ~ h . ’ ~Nevertheless, J~ substantial detail about the mechanism can be obtained by examining the transient in as much detail as possible. For hydrocarbon growth in Fischer-Tropsch synthesis, we can learn much by monitoring the relative rates at which I3Cappears a t the various positions in the hydrocarbon chains. This can be illustrated by considering the results predicted in limiting cases of the unidirectional chain growth model shown in Figure 1. This figure shows a compartmental diagram of the simplest reaction sequence for the stepwise growth of hydrocarbon chains. The diagram represents the pathway for building linear hydrocarbon chains where each compartment represents a kinetically separate (,14) Anderson, D. In Lecture Notes in Biomathematics; Levin, S., Ed.; Springer-Verlag: West Berlin, 1983; Vol. 50. (! 5) Godfrey, K. Compartmental Models and Their Application; Academic: London, 1983. !16) Walter, E. In Lecture Notes in Biomathematics; Levin, S., Ed.; Springer-Verlag: West Berlin, 1982; Vol. 46.
intermediate on the surface. For simplicity we have included the C, and C2 intermediates in the main chain growth sequence, although the chemistry associated with the initial stages of the reaction may show distinctive behavior. In the chain growth sequence, adsorbed chains on the surface can grow by adding a carbon atom from the monomer pool, C,, or they can desorb to become hydrocarbon products. The steps are shown as irreversible in this diagram. The method of analysis remains the same if reversibility is included. A lifetime, T ~ and , an amount, N,, are associated with each reaction intermediate and can, in favorable cases,be measured by the transient experiment. Since the reaction is at steady state in these experiments, the concentrations and reactive fluxes are constant during the transient. In the model we use the nomenclature of compartmental analysis developed in ref 14. Each kinetically distinct intermediate is treated as a separate compartment containing a well-mixed pool of material. The reaction sequence is treated as a flow system. The rate of flow of material from compartment i to compartment j is proportional to A,, the “fractional transfer coefficient”, and is defined as the fraction of the total material in compartment i to flow to compartment j per unit time. Here the index 0 denotes the gas phase. The residence time of material in compartment i at steady state is given by
The rate of flow of material, Rji, from compartment i to compartment j is given by
Equations 2 and 3 establish the relationship between the residence , amounts, Ni, of the surface species. This can be times, T ~ and written
(4) where the sum is the total flux of material through compartment i. The relative fluxes through the compartments are given by reaction stoichiometry. Therefore, eq 4 states that the longer the residence time in one compartment, the more material that compartment must contain. In using this model to predict experimental results, we calculate the isotopic composition of each compartment as a function of time as the new label displaces the old from the reacting system. The isotopic composition of the gas-phase products reflects the isotopic compositipn of their respective surface precursors. The complete specification of the isotopic composition of the pool of a given surface intermediate requires the mole fractions, X(abcd ...), of each possible isotopic variant which contributes to the pool. The indices abcd, etc., represent the positional sequence of carbon isotopes along the molecule. The mass conservation equations for the various surface species depend on the assumed mechanism. It is useful to consider the outcome of the experiment in some limiting cases of the simple model in Figure 1. ( 1 ) Slow Irreversible Chain Growth. In this case the monomer changes its isotopic constitution much faster than the time required for the chain growth step ( T ,
