INFLUENCE OF THE GEOMETRIC FACTOR ON CATALYTIC PROPERTIES OF POTASSIUM GRAPHITE D E L B E R T M. O T T M E R S ’ A N D H O W A R D F. RASE Department of Chemical Engineering, The University of Texas, Austin, Tex.
All of the potassium graphites were prepared (KCs, KC24, KC36, KC48, KCso) and shown to possess high catalytic activity for double bond isomerization of 1 -pentene. A geometric factor was clearly discernible because distances between potassium layers increased in proceeding from high to low potassium concentration without significant changes in electrical properties of each layer. The layers in potassium-rich compounds were less effective because of steric hindrance, and unusual selectivity for lower olefins was demonstrated b y KCs because of this effect.
HE lamellar compounds of potassium graphite (KCs, KC24, T K C 36, KC48, and KC60) possess unusual electrical and structural characteristics which suggest interesting catalytic properties. The present study of these properties has yielded useful data on a highly active family of catalysts and further evidence of a geometric factor in catalysis. Each compound was found to be an extremely active isomerization catalyst, and this activity probably can be extrapolated to other base-catalyzed reactions such as polymerization. Previous studies on the catalytic properties of potassium graphites have been limited primarily to KCs. Because of the lamellar structure, distances between active layers are increased in proceeding from high to low potassium concentration. This change occurred without significantly altering the electrical properties of each layer, thereby offering a unique opportunity to observe possible geometric effects on activity without intervening concurrent changes in electronic properties. The layers in potassium-rich compounds were less effective because of an apparent steric hindrance. This has meaningful consequences in establishing further evidence of the role of geometry in catalysis as well as demonstrating possibilities for unusual selectivity.
Properties of Potassium Graphite
The potassium graphites are classified as lamellar compound because they retain the aromatic carbon-layer structure of graphite but contain interstitial monolayers of potassium between the carbon layers. These compounds exhibit interesting and varied structural and electrical properties (74, 28) which suggest possible unusual catalytic characteristics. Graphite crystallizes in a layer structure. The carbon atoms in each layer are lightly bound to three other atoms via bonds considerably shorter (1.415 A.) than carbon single bonds. The weak bonds between the layers result in a relatively wide separation (3.35 A.) of layers. Alternate layers are displaced relative to each other and are not superimposable by projection perpendicular to the layer planes. The hexagonal form, in which every second layer is superimposable, is by far the most predominant. Lamellar compounds of metals, metallic oxides, and metallic chlorides with graphite have been found (3, 8, 26), and all exPresent address, Esso Research and Engineering Co., Baytown, Tex. 302
l&EC FUNDAMENTALS
hibit certain common structural and electrical properties (74). X-ray diffraction studies show the reactant to be present in planar layers which are separated by a number of graphitic carbon layers-Le., with layer spacings of 3.35 A. and a hexagonal stacking of alternate layers. The electrical conductivity usually is considerably larger than the conductivity of the graphite, Lvhich is reasonable in view of the unique, electron energy-bond structure of graphite ( 2 8 ) , Tvhere one full band (valence bond) is separated by an extremely small energy gap (approximately 0.01 e.v.) from an empty band (conduction bond). Both n- and p-type lamellar compounds of graphite have been prepared. The alkali-metal graphite compounds constitute a very interesting group of “n-type” lamellar compounds. Potassium graphite (KCs) was first prepared in 1926 by Fredenhagen and Cadenbach (8). I n 1929, Fredenhagen and Suck (9) studied the formation of the alkali-metal graphites; and in 1932, Schleede and Tt’ellmann (25) investigated the structure of these compounds. More recently, Rudorff and Schulze (23), Rudorff, Schulze, and Rubisch (24), and Herold ( 7 5 ) have investigated the preparation, structure, and electrical and thermodynamic properties of the alkali-metal graphites. Much of their work and the work of others has been reviewed by Hennig (74) and Ubbelohde (28). The compounds are usually prepared by reaction of graphite with the metal as a liquid or vapor in a closed system for several hours at elevated temperatures. A two-bulb system is frequently employed for potassium graphite (8, 9, 75, 7 6 ) , the graphite being placed in one glass bulb connected to a similar bulb containing the potassium. The evacuated and sealed system is then heated to allow the potassium to vaporize and subsequently react with the graphite. The reaction is terminated by quenching the system to a low temperature, and the concentration of the resulting compound depends upon the temperatures of the trvo bulbs. Compounds KCs, KC24, KC36, K48, and KCCo have been prepared by this technique. Potassium, cesium, and rubidium graphites can be prepared by direct contact of reactants, if the alkali metal is handled and weighed accurately without oxidizing (24). Suitably degassed graphite is heated in contact with the exact amount of alkali metal. Podall, Foster, and Giraitis (22) have prepared KCs using a simple, stirred-flask technique whereby small pieces of potassium are periodically added to the well-stirred graphite a t 275’ =k 20’ C.
STAGE I
t
- A
t
STAGE 2 -/\
-
-A
----
STAGE 3
t
STAGE 4 - A
t
STAGE 5 - A
C LAYER
Podall, Foster, and Giraitis (22) reported that KCs catalyzes the polymerization of ethylene a t low pressures. Later Podall and Foster (27) stated that KCs will also catalyze both nuclear and side-chain alkylation of aromatic hydrocarbons with ethylene. Anderson (7) has patented a catalytic process for polymerizing conjugated dienes using alkali-metal graphite catalysts, but his exhibits are for KCs, KC16 (apparently a mixture of KCs and KC24 structures), and NaCs only. Each of these catalytic reactions is characteristic of strongly basic compounds. Pines (70, 78-20) has discussed the role of base catalysts in isomerization reactions, including the double-bond isomerization a t olefinic hydrocarbons containing allylic hydrogen atoms.
F: L A Y E R
Experimental Details (a)
(b)
Section o f c a r b o n skeleton i n several s t a g e s o f g r a p h i t e potassium
The experimental work performed in this study may be conveniently divided into four distinct phases : preparing and handling the compounds, x-ray analysis, measuring catalytic activity, and measuring electrical conductivity.
D i s t r i b u t i o n of r e a c t l i n t i n some C I 2 R a n d C 8 R l a m e l l a r c o m p o u n d s
Courtesy "Progress in Inorganic Chemistr/"
Figure 1.
Structure of potassium graphite compounds
Careful x-ray studies by Rudorff and Schulze (23) have characterized the structure of the first five stages of potassium graphite. Figure 1 shows the periodic sequence of carbon layers alternating with a single potassium layer and the distribution of potassium atoms in each layer for these five stages. These compounds have the ideal formulas KCs, KC24, KC36, KC48, and K C ~ Oso, th,at the potassium layer in KCs contains more potassium atoms than the potassium layers of the other compounds (Figure 1b ) . The color of the potassium graphites becomes progressively deeper as the metal content diminishes; KCs is burnt orange, KC24 is deep blue, and KC36 and higher are black. Alkalimetal graphite compounds are stable in vacuo a t ordinary temperatures, although they lose alkali metal as vapor a t higher temperatures. I n air they readily oxidize and may even ignite spontaneously (particularly the metal-rich compounds) if free access t 3 oxygen is permitted to their powders. The thermodynamics of the potassium graphites has been studied to some extent by Herold (75) and Hennig (73, 74). Numerous investigations (5-7, 76, 27) concerning the electrical properties of the lamellar graphite compounds have been reported and are summarized by Hennig (74) and Ubbelohde (28). However, the electronic structure of these compounds is still not fully elucidated. Perhaps the most plausible explanation is given by Henning (74). H e suggests that the potassium atoms are held in the graphite-type structure through ionic boniding, whereby the potassium atoms are only partially ionized. The positive ion is probably shared equally between the atoms. Hennig further asserts that the fractional ionization of potassium is approximately constant (Cn-. K + . 4K) for all stages of the compound.
D r y Box. Preparative techniques and subsequent handling procedures for these extremely pyrophoric compounds were highly dependent upon the use of a controlled-atmosphere dry box. This apparatus consisted of a helium-filled, rectangular box equipped with two glove ports and a Plexiglas front window, and a cylindrical entry-chamber capable of being evacuated so that material could be transferred without contaminating the oxygen- and moisture-free atmosphere of the dry box. Traces of oxygen in the box were eliminated by employing a helium atmosphere slightly above barometric pressure together with a "getter tray" of NaK alloy on the box floor. The box atmosphere was continuously circulated over the liquid NaK by means of a gas blower. All experimental procedures involving the use of the potassium graphites were performed so that an oxygen- and moisture-free atmosphere surrounded these compounds. This was made possible by handling them in the dry box or enclosing them in sealed containers rendered free of oxygen by evacuating or displacing with dry-box helium or degassed pentene. Preparative Techniques. The method used in this study is similar to that described by Podall, Foster, and Giraitis (22) for preparing KCs, except that stoichiometric amounts of potassium and graphite were charged to a helium-blanketed reaction flask initially rather than during the course of the reaction. The contents were then heated to the desired temperature (185' i 15' C.), and stirred until the reaction was essentially complete (usually 1 hour). Good contact between the molten potassium and the powdered graphite was assured by continual stirring with a glass-encased magnetic bar. Natural crystalline-type graphite (Dixon No. 1105) with a graphitic carbon content of 97 to 98% from Joseph Dixon Crucible Co. and Baker's analyzed purified potassium were used in these preparations. Prior to the reaction step, the graphite was sieved so that the particle sizes ranged from +90 to 145 microns and then degassed under vacuum at 400' C. for approximately 12 hours. Inside the dry box, the potassium was cut into an oxide-free lump of the appropriate size. Further details of this technique have been described by Ottmers and Rase (77). X-Ray Studies. Each potassium graphite compound prepared was verified by means of the Debye-Scherrer powder method of x-ray diffraction. In the dry box specimens of $90 to 145-micron particles were placed inside small, thinwalled capillary tubes and each end of the tube was sealed. Each specimen was exposed to Ni-filtered Cu radiation for 3 to 4 hours, using a Phillips Electronic Instruments Model 12215/0 machine. The x-ray patterns obtained were compared with those reported by Rudorff and Schulze ( 2 3 ) ; the characteristic lines of each compound agreed very well. Catalytic Activity Testing. The catalytic activity of the potassium graphite compounds in pentene isomerization was measured over the range of 75' to 235' C. using a microreactor system as described by Harrison, Hall, and Rase ( 7 7 , 72). I n this system liquid I-pentene was continuously fed by a VOL. 5
N O . 3 A U G U S T 1 9 6 6 303
PRECISION F E E D PUMP
Figure 2.
General flow arrangement for microreactor
precise metering pump to a fluidized-bed reactor immersed in a fluidized sand bath maintained a t a constant temperature. The pentene was vaporized before passing to the reactor. Reactor effluent was automatically sampled a t regular intervals and analyzed on a gas chromatograph. In this way, the isothermal isomerization of pentene could be accurately and conveniently measured a t constant values of catalyst weight and 1-pentene feed rate for the catalytic life of the compound. The apparatus consisted of a pentene reflux still to ensure an oxygen-free feed, a positive-displacement pump for metering the feed as a liquid, a microreactor immersed in a controlledtemperature sand bath, a sampling system for automatically obtaining a measured amount of reactor effluent, and a gas chromatograph for analyzing the reaction products. A general flow arrangement for the microreactor is shown in Figure 2 . The chromatograph was a standard Model 154D Vapor Fractometer manufactured by the Perkin-Elmer Corp. T h e adsorption column was a 20-foot, coiled column made of '/l-inch copper tubing and packed with 30 weight 70Of polypropylene glycol (average molecular weight approximately 500) on 60-to 80-mesh C-3 firebrick. Pure-grade (99y0 minimum) 1-pentene from Phillips Petroleum Co. was used in all tests. Each batch of pentene feed was dried over silica gel for a minimum of 12 hours and then charged to the reflux still. After the pentene had been refluxed for approximately 45 minutes, it was transferred to the feed pump using helium pressure. During the time the feed was being prepared, or hours before if it were convenient, the temperatures of the fluidizing sand bath and sampling box were adjusted to obtain the proper reaction temperature. With the feed system and reaction temperature properly regulated, the catalyst was charged to the microreactor in the dry box. The closed and charged reactor "bayonet" was removed from the dry box and quickly connected to the reaction system with pentene flowing continuously a t the proper feed rate. At this point, a n electric timer was started, so that a product sample could be taken after 5 minutes of catalyst service. The first sample in all test runs was taken after 5 minutes to allow ample time for the proper feed rate and reaction temperature to be established. All subsequent samples were taken a t regular intervals of 5, 7.5, and 15 minutes, depending upon the purpose of the test. With the automatic features of the reactor equipment, the operator could usually calculate the product compositions as the sample peaks appeared on the recorder along with the integrator counts. Electrical Conductivity. The electrical conductivity of the potassium graphites was measured as a function of temperature from 90' to 350' K. using equipment designed and constructed by Streetman ( 2 6 ) . Samples of the powdered potassium graphites were pressed into rectangular bars in 304
ILEC F U N D A M E N T A L S
vacuo and loaded into a sealed sample holder. The sample holder was then positioned in the electrical conductivity equipment, the test sample was cooled to about 90' K. by liquid nitrogen, and electrical conductivity was measured a t several successively higher temperatures a t regular intervals until 350' K. was reached. Further details are presented by Ottmers and Rase (77). Results Each of the potassium graphite compounds was found to be an extremely active isomerization catalyst, but fairly high deactivating rates were observed for the gaseous system studied. Based upon the performance of the microreactor equipment, first-order rate constants for each compound were calculated. Most reactor tests were run in duplicate, and the conversion results agreed within 3z10yo of the average. These rate constants provide a meaningful basis for comparing the various potassium graphites with one another and with a commercial silica-alumina catalyst. Catalyst Life. The catalytic behavior of these compounds over a fairly wide range of temperatures (75' to 235' C.) was explored. At low temperatures the activity dropped off sharply with time, probably because of strong physical adsorption of the reacting molecules at these low temperatures or the formation of strongly adsorbed polymers on the catalytic surface. As successively higher temperatures were employed, the deactivation rate diminished until an optimum temperature of approximately 180' C. was reached. Above this temperature the activity began to decline more rapidly again, probably because of decomposition of the potassium graphite. Figure 3 shows the catalytic activity of KCa as a function of catalyst life for three reaction temperatures. This figure displays the activity characteristics just described-for example, KCg is initially most active a t 235' C.; after 1 hour of service the conversion at 180' C . is approximately seven times that a t 235' C. Catalyst life curves of the other potassium graphites reveal similar characteristics, except that as the potassium graphite becomes less concentrated in potassium, the deactivation rate generally increases. For example, the initial activity of KCs, KCu, and KC36 a t 180' C. is approximately the same (about 68% conversion), whereas after 1 hour of service these compounds yield conversions of 35, 32, and 21%, respectively.
physical similarities between the compounds, it does not necessarily preclude the possibility of differences in internal diffusional characteristics between compounds. Detailed studies of pore-size distributions were not possible because of the inherent experimental difficulties associated with handling these compounds, but selectivity studies described below demonstrate that differences in diffusional characteristics could have played only a minor role in influencing the observed results. With this knowledge of the performance characteristics of the microreactor, a model for determining meaningful rate constants can be employed. For low conversions a t atmospheric pressure, average compositions may be used to approximate the rate constants. Thus,
70
60
Y
d
50
FI
0:
W
> 40 V
YW = t-
z W
" 0
10
20
30
40
50
60
70
CATALYST SERVICE, MINUTES
Figure 3. Catalyst life of KCg High conversion tests
The rapid decline in catalytic activity of the potassium graphites is understandable, considering their pyrophoric nature and general imtability. Although precautions were taken to prevent oxygen from entering the charged reactor and to exclude oxygen and moisture from the feedstock, the rapid decline in activity could not be prevented. These catalysts undoubtedly would be more stable in liquid phase reactions. Although rapidly declining activities always make comparisons more difficult, the differences in activity between the several compounds as rieported below were clearly discernible. Tests a t lower temperatures (125' to 180' C.) and low conversions exhibited the highest precision and, therefore, were used for the comparative study. Rate Constants. The performance of the fluidized-bed microreactor used in this study approached isothermal, plugflow operation. Based upon two generalized correlations of heat transfer in fluidized beds, the temperature rise in the reactor due to the exothermic isomerization reactions was found to be approximately 2.5' C. The gas flow in the fluidized-catalyst bed was shown to approach idealized-plug flow by injecting a sample of tracer gas into the carrier gas just upstream of the reactor. The distribution of residence time was detected by means of a microthermal conductivity cell located immediately downstream of the reactor. A highspeed recorder for maximum response was employed. Several reactor tests were conducted to verify that mass transfer of the reacting species to and from the catalyst surface was not a controlling factor in this active catalyst system. These tests showed essentially no change in conversion a t the same W/Fwhen the flow rate was doubled, whereas generalized correlations for mass transfer in fluidized beds indicate that the mass transfer coefficient should be approximately proportional to the mass f l o ~rate. Thus, mass transfer does not seem to be a controlling factor in the system studied. T o circumvent the even more difficult problem of internal diffusion, the same batch of very fine graphite particles was used for each compound. Although such a procedure will tend to minimize diffusional resistances and ensure reasonable
where
kit, klc
= rate constant for reaction of l-pen-
tene to trans- and cis-2-pentene, respectively, with dimensions of moles/mass- time = conversions of 1-pentene to cis and Axe, Axt trans, respectively, during increment A ( W/F) = change of catalyst weight to molal A(W/F) feed rate ratio across reactor ( N l ) m , (Nt),, (Nc)m= average of inlet and outlet mole fractions of 1-pentene, trans-2pentene, and cis-2-pentene, respectively P = reactor pressure (1.0 atm. in these experiments) = thermodynamic equilibrium constant Kit, K1o for reaction of 1-pentene to transand cis-2-pentene, respectively These equations are based upon a homogeneous, first-order reaction model and do not include the adsorption terms often used in equations for heterogeneous reactions. However, the rate constants determined from Equations 1 and 2 provide a good basis for comparing the catalytic activity of the potassium graphites. Since the first product sample was taken after 5 minutes of catalyst service and each potassium graphite deactivates a t slightly different rates, some correction must be made for the different states of deactivation among the various potassium graphites. In the low conversion tests, four samples of the reaction products were taken regularly a t 5-minute intervals. In order to compare each catalyst on the same basis, the mole fractions of cis- and trans-2-pentene in these samples were plotted against the catalyst service period, and the curves were extrapolated to zero time so as to obtain conversions based upon fresh catalyst. At low conversion the catalyst deactivated linearly and extrapolating was relatively simple compared to that at higher conversion such as shown in Figure 3. Rate constants calculated by means of Equations 1 and 2, using zero-time conversions, are presented in Table I. These constants based upon a unit weight of potassium graphite indicate that KCs, KC24, and KC36 have approximately the same catalytic activity, whereas KC48 and KCeo are successively less active. I t is interesting to compare these rate constants with those for a commercial silica-alumina catalyst (bottom of Table I). VOL. 5
NO. 3 A U G U S T 1 9 6 6 305
Table 1.
Rate Constants for 1 -Pentene Isomerization over Potassium Graphites Assuming First-Order Reaction
Rate Constants kit X lo2,'
a
Tpp.,
g.-moles C. p . KCn-min. 125 1.20 f 0 . 1 0 180 2.11 zt 0 . 1 2 235 2.44 f 0.35 125 1.10 i 0 . 1 0 180 2.11 zk 0.12 235 2 . 5 0 i 6.37 125 1 . 3 1 f 0.11 180 2.05 2= 0.12 235 2.58 i 0.41 KC48 180 1.67 i 0.17 KCso 125 0.69 i 0 . 1 0 180 1.45 f 0.15 235 1.77 i0.46 Silica-alumina 125 0.334 180 4.63 Based on total weight of catalyst. b Based on K weight in catalyst. Catalyst Type KCs
The silica-alumina data were obtained with the same reactor system and feedstock after heat-treating the silica-alumina a t 385' C. under vacuum for 12 hours. I t is apparent that the potassium graphites are more active catalysts at the lower temperatures, but silica-alumina is more active a t the higher temperatures. Thus, the potassium graphites could prove useful catalysts at the lower temperatures if their rapid decline in activity with service could be prevented. However, at higher temperatures, silica-alumina would be more useful because of the comparative ease of handling it as opposed to the pyrophoric potassium graphites. The olefin-isomerization mechanism for base catalysts as proposed by Pines and Schaap (79) suggests that the active constituent for the potassium graphites is the potassium ion. Therefore, rate constants based upon a unit weight of potassium are also presented in Table I ; on this basis KC36, K C o , and KC6o have about the same activity, while KC24 and KCs are successively less active. The effect of temperature upon the rate constants k i t and kl, for each potassium graphite compound can be represented well by the Arrhenius equation
where
ki, = rate constant for i reacting to form j At, = frequency factor for i reacting to f o r m j E?, = energy activation for i reacting to f o r m j R = gas law constant T = absolute temperature The slopes of log k US. 1/T curves were approximately the same for all of the potassium graphites and the activation energies, Eli and E,,, for each potassium graphite compound was determined to be 3130 and 2280 calories per gram-mole, respectively. Thus, the differences in reactivity must be associated with the frequency factor. Electrical Conductivity. The electrical conductivity, u, of pressed samples of KCs, KCU, KC36, KCm, and graphite was measured as a function of temperature from 100' to 350' K. Further details are presented by Ottmers and Rase (77). The conductivities were 10 to 100 times as large as those for pure graphite over the temperature range examined. T h e conductivities of the potassium graphites decrease with an 306
I&EC FUNDAMENTALS
kit' X
ki, X
lo2,' g.-moles g. K-min. 4.15 i 0 . 3 5 7.30 f 0.42 8.44 i 1.21 9.2 f 0.8 17.5 f 1 . 0 20.8 f 3 . 1 1 5 . 8 zk 1 . 3 24.8 f 1 . 4 31.1 i 5 . 0 24.6 zk 2 . 5 12.5 i 1 . 8 26.2 f 2 . 8 32.0 & 8 . 5
g.-moles g. KC,-min. 1.16 i 0.20 1.55 i 0.09 1.77 i 0 . 2 0 0 . 9 7 zt 0.10 1.55 i 0.09 1 . 7 8 f 0.25 1.35 f 0.20 1 . 6 3 i 0.11 2.05 i 0.38 1 . 4 4 f 0.17 0.86 f 0.10 1.28 i 0.10 1.86 f 0.47 0.260 3.25
ki,' X g.-moles g. K-min.
4.02 5.37 6.12 8.1 12.9 14.8 16.3 19.7 24.7 21.2 15.5 23.1 33.6
...
f 0.69 i 0.31 f 0.69 & 0.8 f0.8 f 2.1 2.4 f 1.3 f 4.6 f 2.5 f 1.8 f 1.8 =k 8 . 7
...
*..
increase in temperature. This behavior is not typical of semiconductors but similar to metals and is described as "degenerate semiconductivity Table I1 compares the electrical conductivities of the potassium graphites a t 70' C. The increase in conductivity of the potassium graphite minus the conductivity of graphite, (0, - u ~ )is, approximately the same for each compound when based upon a single potassium layer-Le., comparing (u, uG) (n). This fact, together with the previous work on the properties of the potassium graphites, indicates that the electrical properties of each potassium layer do not differ significantly among the various compounds.
."
-
Effectiveness of Potassium in Each Compound
The potassium graphite compounds provide an excellent opportunity for studying the geometric factor in catalysis. They are composed of the same two chemical species, potassium and carbon, and the most dramatic difference between compounds is the increased distance between potassium layers as the potassium concentration diminishes. The electrical nature of each potassium layer is not significantly different among the various compounds. As suggested by the isomerization mechanism for a base catalyst, it would be reasonable to compare the potassium graphite compounds on a unit weight of potassium. Pines and Schaap (79) propose that strongly basic compounds, such as high-surface alkali metals on activated alumina, are effective because of their ability to abstract allylic protons from the olefins involved, The same mechanism may be reasonably Table II.
Electrical Conductivities for Potassium Graphites at 70' C. Conductivities, (Ohm-Cm.)-'
Compounds KCs KC24 KC36 KCRn
Graphite
n 1 2 3 5
...
Qn
3000 1680 1330 870 150
u,, -
UGO
2850 1530 1180 720 0
- uQ)n
(u,
2850 3060 3540 3600
electrical conductivity of potassium graphite of stage n. electrical conductivity of graphite. a
u, =
...
UQ
=
applied to the potassiunil graphites. Hennig (74) has concluded from electrical measurements that a fraction of the potassium atoms in the graphite compounds is ionized and that this fraction is approximately constant a t all potassium concentrations. Some doubt concerning the latter part of this statement exists, but his first conclusion seems well founded experimentally. For the moment then, let us consider the potassium ion (K+) to be. the active constituent in the potassium graphites and assume that the fractional ionization is the same for each compound. A comparison of the rate constants, k l t and k,, (refer to Table I), should provide useful information regarding the relative catalytic activity of the K + in each compound. Since the activation energies for each compound appear to be approximately equal, the relative activity of each potassium graphite can be compared best by ratioing the rate constants a t one temperature to yield a ratio of the frequency factors ( A l t and Al,). Table I11 shows comparison of this type, whereby the frequency factor of each potassium graphite is divided by the frequency factor of KC60, the least concentrated compound studied. For convenience these ratios of frequency factors will be defined by :
where ( A l t ) , (Alc)%= frequency factor of the rate constant for 1-pentene reacting to form trans- and cis2-pentene, respectively, over the potassium graphite compound of stage n. n = 5 for KCGII
C-C-H, and H-C-H bond angles remain 109' for the single-bond atoms and in which the two double-bond carbons share edges of distorted tetrahedra such that unshared corners are occupied by hydrogen and carbon atoms with C-C-H bond angles of 120' ( 2 ) . A comparison of this length with the distances between potassium layers shown in Table I11 for the potassium graphites indicates that adsorbed pentene molecules could discourage further adsorption of 1-pentene on adjacent potassium layers in the more concentrated compounds. Figure 4 shows a possible configuration for each pentene molecule on a perfect crystal of potassium graphite. T h e position of the negative charge indicates the point a t which each molecule would be adsorbed on the catalyst. These base catalysts probably abstract the allylic proton from 1-pentene, so that the resulting carbanion is attached to a K + (on the catalyst surface) a t the third carbon. Of course, the other carbon atoms in this carbanion are continually moving and may assume any configuration consistent with the required bond angles. Thus, this adsorbed molecule would occupy a large portion of an approximately hemispherical space of radius 4 A. and would occupy each part of this space some of the time because of the continual movement of its atoms. As the reaction takes place on the catalyst surface, the double bond shifts to the second position and the negative charge moves to the end carbon. Either cis- or trans-2-pentene can be formed; but, in both instances, the carbanion must shift along the catalyst surface so that the position of the negative charge is near the K+. The unattached portions of the
The distance between potassium layers, I,, and the stage, n, are also included in Table 111. I t can be seen from these frequency ratios that the potassium atoms (or ions) in the more concentrated compounds are less effective as catalyst sites (the ratio is much less than unity) than those in less concentrated compounds. Ho\brever, the potassium ions in KC36 and KC18 are almost as effective as those in KC60. This behavior can be explained by examining the geometry of the reacting system. Steric Hindrance and Role of KC, Structure
The changes in both frequency factors and distances between potassium layers in proceeding from KCs to KCeo suggest a steric explanation for the difference in activities within the series. Each pentene molecule contains three carbon-carbon single bonds (1.54 A.), one carbon-carbon double bond (1.33 A,), and ten carbon-hydrogen bonds (1.05 A,). A maximum length of 7 A. for the molecule can be approximated, based on all possible pentene configurations in which the C-C-C,
H B. ADSORBED
TRANS-2-PENTENE
Table 111.
Relative Activity of Potassium Graphites Based on Unit Weight of Potassium Stage Ratio of Frequency Factors Compound n IC,A . G Btn B," 1 5.40 0.28 i 0.05 0.23 i 0.03 KCs 0 . 4 2 i 0.08 0.35 i0.04 (Kcla)* 1 5.40 2 8.75 KC24 0 . 6 7 i 0.12 0.56 i 0.07 3 12.10 KC36 0.95 + 0 . 1 7 0.85 i 0 . 1 2 KC48 4 15.46 0.94 i 0 . 2 2 0.92 =t0.17 KC60 5 18.80 1.00 1 .oo a Distance between potasiium layers. Obtained by using rate constant of KCa multiplied by 7.5.
H C. ADSORBED Figure 4. A. B. C.
CIS - 2-fENTENE
Pentene configurations
Adsorbed Adsorbed Adsorbed Planar
-__.
VOL. 5
1 -pentene trans-2-pentene cis-2-pentene structure
NO. 3
AUGUST 1 9 6 6
307
adsorbed 2-pentene molecules are also free to move about. They could occupy each portion of an approximately hemispherical space of radius 7 A., but the probability that the outer portions of this space would be occupied is smaller than the probable occupancy of the 1-pentene space. The minimum “radius” that the 2-pentene molecules may assume is approximately 5.5 to 6 A. The implied mobility of molecules on the catalyst surface is strongly supported by the Dutch school of DeBoer and his associates ( 4 ) ,who have urged the study of double-bond shift isomerizations of the type studied in this investigation as a means of further defining the phenomenon. With this knowledge of geometry and motion of the reacting molecules, one can readily see that reaction on K + in adjacent potassium layers is sterically hindered in the case of KCs and KC24. First, let us consider the reaction taking place on an ideal surface of KCs. KCs contains 1.5 times as many potassium atoms in each layer as the other potassium graphites. However, based on a unit weight of potassium in each layer a smaller fraction of potassium atoms are “surface” atoms in KCs than in the other compounds. If none of the “extra” potassium atoms in KCs were surface atoms, there would be 1.5 times as many “surface” potassium atoms per unit weight of potassium in the other potassium graphites as in KCs. Actually, the ratio must lie somewhere between 1 and 1.5. For this reason two values of the relative activity of KCs have been included in Table 111. The relative activities in the second (KC,,) row were obtained by multiplying the rate constant of KC8 by 1.5. Thus, if it were possible to prepare a single-stage (n = 1) potassium graphite with the same potassium layer structure as KC24, KC36, KC48, and KCoo, this compound should have a relative activity between 0.28 and 0.42 for the trans reaction and between 0.23 and 0.35 for the cis reaction. (An effort to prepare KClz produced only a “compound” whose structure was a combination of the KCs and KC24 structures.) The “corrected” relative activity for KCs is approximately one third. Figure 5 shows how the steric hindrance of adsorbed pentene molecules could explain this activity factor for KCS. Suppose one pentene molecule has already been adsorbed on the
catalyst surface and is in the process of being reacted. The probability that the 1-pentene molecule will be able to adsorb on a K + in an adjacent layer is very unlikely (the 1-pentene molecule adsorbs a t its third carbon atom). I n fact, the probability that a 1-pentene molecule will adsorb on a K + in the second layer away is also low. Therefore, the predominant type of adsorption is depicted in Figure 5. Certainly, adsorption will occur occasionally a t every second layer, but cases will also arise whereby adsorption occurs a t every fourth and fifth layer. Penetration between graphite layers is not considered possible because of the small distances (3.35 A.) * A similar argument may be developed for KC24. In this case the activity factor is approximately two thirds. Figure 6 shows the predominant type of adsorption for KC24. Figure 7 shows that pentene molecules can react a t K + in adjacent potassium layers on KC86 with only a slight chance for collision between 2-pentene molecules. Since I, is successively larger for KC48 and KCeo, no steric hindrance should occur with these catalysts. In summary, the catalytic behavior of the potassium graphites can be explained by basing the rate constants on a unit weight of potassium and by considering the geometry of the catalysts and reacting species. In each case the active constituent is considered to be the K + and the fractional ionization among the potassium graphites is assumed to be constant. If the fractional ionization changed between compounds, the change would probably be small and could be incorporated in the development presented above. General equations for the rate constants for potassium graphite compounds in pentene isomerization can now be written :
kit' = Pcnbtn(Alt)~e-E~~/RT
(6)
itl,’ = P,nbcn(Al,)~e-EIJRT
(7)
and
r
btn
=
mtdm
(8)
bcn
= mcn/mcs
(9)
-
0.75A.
REACTING MOLECULES
/
n.2
I
I
I
I
I
w I ‘ I I
I
7 I I I 04-0 I
CATALYST CROSS SECTION
.C CATALYST C R O S S SECTION
Figure 5.
308
OK
Steric hindrance of pentene reaction on
l&EC FUNDAMENTALS
W
Figure 6.
KCU
Steric hindrance of pentene reaction on
where
P,,,Pcn= steric factor for 1-pentene reacting to form transand ciss-2-pentene, respectively, on compound of stage n m,,, men = number of effective surface atoms for trans and cis reactions, respectively, per layer in compound of stage n
If a quantitative measure of the steric factors could be obtained from a probability analysis of reacting molecules adsorbing on the various surface potassium atoms, Equations 6 and 7 could be used to predict rate constants for the potassium graphite catalysts in pentene isomerization. Since little is known about the actual surface properties of these potassium graphites (cleavage planes, defects, etc.), development of some quantitative model for predicting these probabilities seems unwarranted. But the perfect crystal surfaces can be considered for qualitative purposes as representing an average picture of the surface. The experimental rate constants were used to determine the steric factors in each case, and values for the parameters to be used in Equations 6 and 7 were determined and listed in Table IV.
Table IV.
A.
Parameters for Rate Constant Equations
1-Pentene Reacting to trans-2-pentene
.AI,)^ ..,-=
k l t = Ptnbtn(Alt)je-Eit/RT
Elt = 3730 (Cal./G.-Mole) Ptn btn 0.42 0.67 1 .o 0.67 1 .o 0.95 0.94 1 .o 1 .o 1 .o
Compound
B.
(6)
1-Pentene Reacting to cis-2-Pentene kl,‘ = Pcnbcn(Alc)Se-EiclRT
(7)
.AI.)^ - _ , -=
G.-Moles
E l , = 2280 (Cal./G.-Mole) pc, bcn 0.67 0.35 1. o 0.56 1. o 0.85 1 .o 0.92 1 .oo 1 .o
Compound
Further Confirmation Ulsing Selectivity
This hypothesis based on steric hindrance and the idealized geometry of the catalyst surface was tested further by employing a mixed reactant feed of pentene and octene. T h e more reactive but larger octene should be favored on a dilute catalyst such as KC60 where steric hindrance would not be much of a problem, but the selectivity for octene would be greatly reduced on the concentrated KC8. A mixed pentene-octene (25 to 75%) feed was tested a t 180’ C . ; the ratios of octene to pentene conversions were 0.86 for KC8 and 1.82 for KC60. These results thus provide rather striking confirmation of the steric hindrance hypothesis. They also suggest that resistances to internal diffusion could have played only a minor role in these studies. As shown by Wheeler (29), “a catalyst with small pores can be expected to have a selectivity factor equal approximately to the square root of the selectivity factor observed for the same material with very large pores.” The observed differences in selectivities far exceed this criterion based on pore
I
12 IOA.
I
-I
I
I I
I I
I I I
n=3
I
I
I
I
I
II
I
p
I I I I I I 7 I I I CATALYST CROSS SECTION
Figure
KCBB
7.
e
OK
Steric hindrance of pentene reaction on
size differences and must be explained by some predominant chemical factor such as that proposed. Conclusions
The potassium graphites actively catalyze the double-bond isomerization of 1-pentene to cis- and trans-2-pentene. Several important conclusions may be drawn from the catalytic properties discovered in this research and from the known structural and electrical characteristics of potassium graphites. All the potassium graphites were extremely active isomerization catalysts. Their rate constants were four to five times those for a commercial silica-alumina catalyst at 125’ C. However, the potassium graphite constants were smaller a t 180’ C. Other base-catalyzed reactions previously reported for KCg, such as polymerization and alkylation, should be actively catalyzed by all of the potassium graphite compounds. These compounds can be prepared by a simple method which could be readily adapted to large-scale production. The potassium graphites exhibited fairly high deactivating rates in the gaseous system studied. This deactivating phenomenon was probably due largely to minute traces of oxygen in the gaseous reaction system which could be better excluded in liquid systems. T h e potassium graphites exhibited electrical conductivities 10 to 100 times those for pure graphite over the temperature range studied. Further, the conductivities of these compounds decreased as the temperature increased. Both characteristics are typical of metallic behavior, and such materials are termed “degenerate” semiconductors. A meaningful demonstration of the geometric factor in catalysis was possible with these unique compounds because their lamellar structure allowed distances between active layers to be changed without significantly altering the electrical nature of each layer. By contrast, previous studies on other series of catalysts, such as a series of different metals, have involved concurrent changes of electronic properties which made decisive experiments difficult. T h e catalytic behavior of the various potassium graphites has been explained with reference to their structural and electrical characteristics. In the hypothesis proposed, the active sites (K+) in the potassium-rich compounds are postuVOL. 5
NO. 3 A U G U S T 1 9 6 6 309
lated to be less effective because of steric hindrance of the reacting molecules. Broader use of these compounds for practical catalysis in liquid systems is indicated. Because of the steric factors, unusual and desirable selectivities could be realized with mixed reactants by the proper choice of potassium graphite. Further work is needed to establish more firmly the ionization behavior of the potassium atoms in each graphite compound. Undoubtedly, small differences in the fractional ionization between compounds may exist, but these differences could be included in the hypothesis developed. Acknowledgment
We are grateful to Hugo Steinfink for assistance with the x-ray measurements and W. H. Hartwig for use of electrical conductivity equipment. The Joseph Dixon Crucible Co. provided the graphite. D. M. Ottmers received fellowship support from the National Science Foundation and The University of Texas Graduate School. literature Cited
(1) Anderson, H., U. S. Patent 2,965,624 (December 1960). ( 2 ) ---Andrrws. D. H.. Kokes. R. J.. “Fundamental Chemistrv.” pp. 130-93, Wiley,’New Ydrk, 1962. (3) Croft, R. C., Australtan J . Chem. 9,184 (1956). (41 . . DeBoer. J. H., “Mechanism of Heterogeneous Catalysis,” p. 15, Elsevier, New York, 1960. (5) Dzurus, M. L., Hennig, G. R., J . Am. Chem. SOG.79, 1050 (1957). (6) Dzurus, M. L., Hennig, G. R., J . Chem. Phys. 27, 275 (1957). \-,
I ,
(7) Dzurus, M. L., Hennig, G. R., “Proceedings of 5th Conference on Carbon,” p. 1, Pergamon Press, New York, 1961. (8) Fredenhagen, K., Cadenbach, G., 2. Anorg. Allgem. Chem, 158, 249 (1926). ( 9 ) Fredenhagen, K., Suck, H., Z. Anorg. Chem. 178,353 (1929). (10) Haag, W. O., Pines, H., J . Am. Chem. SOG.82, 387 (1960). (11) Hall, J. W., Rase, H. F., IND.ENC. CHEM.FUNDAMENTALS 3. 158 ~~- (1964). (12) Harrison,’D. P., Hall, J. W., Rase, H. F., 2nd. Eng. C h m . 57, 18 (January 1965). (13) Hennig, G. R., “Proceedings of 2nd Conference on Carbon,” DD. 103-12. Pernamon Press. New York. 1956. (14)’ Hennig,‘G. E., Progr. Inoig. Chem. 1,‘125-65 (1959). (15) Herold, A., Bull. SOG.Chim. Frame 1955, p. 999. (16) McDonnell, F. R. M., Pink, R. C., Ubbelohde, A. R., J . Chem. SOC. 1951, p. 191. (17) Ottmers, D. M., Rase, H. F., unpublished manuscript. (18) Pines, H., Haag, W. O., J . Org. Chem. 23, 328 (1958). (19) Pines, H., Schaap, L., Advan. Catalysis 12, 117-47 (1960). (20) Pines, H., Vesely, J. A., Ipatieff, V. N., J . Am. Chem. SOC. 77. 347 11955). (21)‘Podail, H.,‘Foster, W. E., J . Org. Chem. 23,401 (1958). (22) Podall, H., Foster, W. E., Giraitis, A. P., Ibid., 23, 82 (1958). (23) Rudorff, W., Schulze, E., Z. Anorg. Allgem. Chem. 277, 156 - 7
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