J. M. MATSEN,J. W. HARDING, AND E. M. MAGEE
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Chemical Reactions in Chromatographic Columns
by John M. Matsen, John W. Harding, and Ellington M. Magee Contribution from the Central Basic Research Laboratory, Esso Research and Engineering Company, Linden, New Jersey (Received August $9, 1964)
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Chemical reactions of the sort A B C may be run by injecting pulses of A into a chromatographic column which catalyzes the reaction. Under such conditions, the products B and C will be separated from each other and the reaction should proceed to completion despite an unfavorable equilibrium constant. Experimental verification of such behavior was obtained from the dehydrogenation of cyclohexane to benzene in a column packed with a platinum-on-alumina catalyst. Under the most favorable conditions about 30% greater conversion was obtained in the chromatographic reactor than would result from allowing the same quantities of cyclohexane and carrier gas to reach a static equilibrium. An equilibrium reactor would need about four times as much diluent or carrier gas as the chromatographic one to reach the same degree of conversion.
Introduction The techniques of gas chromatography have proved to be valuable tools in the field of catalysis. The most obvious and widespread applications have been analytical. The microreactor technique developed by Kokes, et ~ 1 . is ~ 1an analytical refinement which minimizes the problems of handling minute samples and detecting low conversions. Preparative chromatography has been invaluable in the purification of reactants. Less conventional has been the application of chromatography to the measurement of adsorption isotherms and heats of adsorption for cases where adsorption-desorption rates are fast. A fourth field, in which chromatography and catalysis are much more closely intertwined, has recently been subjected to study. This is the case where a catalytic reaction occurs in a pulse of reactant traveling through a chromatographic column. Bassett and Habgood2 studied the irreversible isomerization of cyclopropane to propylene in a chromatographic column and were able to obtain rate constants for the reaction. Klinkenberg3 made a theoretical study of the reversible reaction A Q B occurring on a chromatographic column and calculated the shape and retention time of the eluted peak. Keller and Giddings4 have analyzed similar cases in which the reaction rate is slow. More interesting from an application viewpoint is the reversible reaction A -F? B C with a very low equi-
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The Journal of Physica2 C h i s t r y
librium constant. I n a chromatographic column the products B and C will be separated from each other so that they cannot re-react, and the reaction may proceed to completion despite the low equilibrium constant. This is a potentially attractive mode of operation for such equilibrium-limited reactions. The possibility has been recognized in patents by Dinwiddie5and Magee,6and Magee’ subsequently presented a mathematical analysis of such a reactor. Roginskii, et al.,s proposed chromatographic reaction independently. The same school studied the dehydrogenation of cyclohexane pulses under irreversibleg and reversiblelO conditions. A mathematical treatment was presented in ref. 10 for simple irreversible reactions. Their experimental data were analyzed according to (1) R. J. Kokes, H. Tobin, and P. H. Emmett, J . Am. Chem. Soc., 77, 5860 (1955). (2) D. W. Bassett and H. W. Habgood, J . Phys. Chem., 64, 769 (1960). (3) A. Klinkenberg, Chem. Eng. Sci., 15, 255 (1961). (4) R. A. Keller and J. C. Giddings, J . Chromatog., 3 , 205 (1960). (5) J. A. Dinwiddie, U. S. Patent 2,976,132 (1961). (6) E. M. Magee, Canadian Patent 631,882 (1961).
( 7 ) E. M. Magee, Ind. Eng. Chem. Fundamentals, 2, 32 (1963). (8) S. 2. Roginskii, M. I. Yanovskii, and G. A. Gasiev, Dokl. Alead. Nauk S S S R , 140, 1125 (1961). (9) G. A. Gasiev, 0. V. Krylov, S. 2. Roginskii, G. V. Sitmsonov, E. S. Fokina, and M. I. Yanovskii, {bid., 140,863 (1961). (10) S. 2. Roginskii, M. I. Yanovskii, and G. A. Gasiev, Kinetika i Kataliz, 3, 529 (1962).
CHEMICAL REACTIONS IN CHROMATOGRAPHIC COLUMNS
this treatment, but examination of their results indicates that the model was inapplicable to that experiment. This same treatment was extended to more complex casesll and seems far more pertinent to kinetic measurements than to chromatographic reactions. Recently,12 the mathematical model of Magee' was related qualitatively to experimental data from this laboratory, although conditions of stoichiometry and elution velocity would not allow quantitative comparison. Thus the complex theoretical problem of the chromatographic reactor has not yet been solved completely. It is, felt that chromatographic reaction can be advantageous only under a well-defined and limited set of conditions. 1. 'The equilibrium constant for the reaction must besma,ll. 2. Reaction rates should be high enough so that separation of products rather than rate of reaction limits the extent of reaction. 3. A t least, two products must be formed which are chromatographically separated in the reactor. 4. Reactants must not be separated in the reactor. For all practical purposes this limits one to a single reactant, or to two reactants where one also serves as the carrier gas.
Equilibrium Comparison Chromatographic operation of a reaction is rather complicated, and the question naturally arises as to whether this method in practice has any advantages over a conventional equilibrium reactor. The problem lies in picking a meaningful basis of comparison, since in the present case the presence of carrier gas affects conversion not only by separating the products but also by diluting the reaction mixture. Comparison was finally made on the following basis. The cliromatographic reactor was repetitively pulsed with reactant a t evenly spaced intervals. Effluent product was collected over an integral number of cycles and analyzed in order to obtain the experimental conversion. The corresponding "equilibrium" conversion was that which would result from allowing the amounts of reactant and carrier gas associated with a single pulse oycle to reach equilibrium in a static system. The ra,tio of carrier gas to cyclohexane was calculated in the present case from the equation 77+
N
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cyclohexane sample size in pl. a t 25". Total pressure was taken as 1 atm. despite the fact that it was higher than this except a t the reactor outlet. This is a rather stringent basis of comparison but it seems to be the only one justified in the present case. For given average flow rates of reactant and carrier gas this will tell whether chromatographic operation gives an advantage in conversion over conventional equilibrium operation. The problem of dilution does not exist for reactions in which the number of moles of reactants and products are equal. Any increase in conversion above equilibrium (which is here independent of dilution) would be due to chromatographic separation. One such reaction, 2HD @ H2 Dz, was examined in the present work on a reactor packed with palladium-coated asbestos. No quantitative data were obtained, but it appeared that the reaction was proceeding and that separation of the various reaction components did take place. Some earlier workers13 examined the chromatography of hydrogen-deuterium mixtures on palladium. It is interesting to note that some of their anomalous results may be explained in light of the present work by realizing that chemical reaction probably occurred on their column.
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Experimental The main reaction studied in the present work was the dehydrogenation of cyclohexane to benzene and hydrogen. CaHiz
CsHe 4- 3Hz
Fisher reagent grade cyclohexane was used without further purification. The catalyst, commercially available, consisted of 0.6% platinum on an alumina support, and the alumina served as the chromatographic adsorbent. It was ground to 40-60 mesh and activated by heating in a hydrogen atmosphere. Reactors consisted of lengths of 0.63-cm. stainless steel tubing packed with catalyst. Reactors were installed in place of the usual analytical column in an F and M Model 500 gas chromatograph, which was used without other modification. Helium was used as the carrier gas. Cyclohexane was injected through a rubber septum a t the column inlet. The thermal conductivity of the effluent gas was recorded potentiometrically. This trace will hereafter be called the product chromatogram, al-
l' .l
=:
4.43S
where N = moles of carrier gas/mole of cyclohexane, F = UJi&!r gas flow rate in cc*/min* a t 25" and 1 atm* pressure, t = time between pukes in min., and 8 =
(11) G. A. Gaziev, V. Ya. Filinovskii, and M. I. Yarovskii, Kinetika i Kataliz, 4, 688 (1963). (12) J. M. Matsen, J. W. Harding, and E. M. Magee, A.1.Ch.E. 56th Annual Meeting, Houston, Texas, Dee. 1953, Paper 19d. (13) C. 0. Thomas and H. A. Smith, J . Phys. Chem., 63,427 (1959).
Volume 69, Number B February 1966
J. M. MATSEN, J. W. HARDIN*,
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During the first few pulses of cyclohexane over a fresh catalyst, the product chromatogram changed
E. M. MAGEE
EFFECT OF PULSE SIZE ON CONVERSION
though it is not a chromatogram in the conventional sense of the word. Product samplea were trapped from the effluent stream a t -78” and were analyzed on a 2-m. Perkin-Elmer “R” column.
Results
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Figure 4. Effect of pulse size on conversion.
Figure 1. Product chromatogram for 3-pI. pulse. EFFECT OF CARRIER GAS FLOW RATE AT 210’C
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PARAMETER: AMT. CYCLOHEXANE INJECTED 20
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Figure 3. Product chromatogram for 7-fil. pulse.
The Journal of Physical Chemistry
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Figure 5. Effect, of carrier gas flow rate at 210”.
BENZENE
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40 60 CC O F CARRIER GAS
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Figure 2. Product chromatogram for 5-pl. pulse.
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considerably. This corresponded to coking of part of the catalyst surface. Any analysis of products during this period would be almost meaningless without knowledge of the coking process. After perhaps five or ten pulses, the reactor reached a steady state and there was little further change in pulse chromatograms. Material balances showed that 96 to 9801, of the reactant left the reactor once a “steady state” had been reached. In all cases only benzene and hydrogen appeared as products, and no cyclohexene or cyclohexadiene was detected. Efect of Pulse Size. Typical product chromatograms of single cyclohexane pulses are shown in Figures 1, 2, and 3. These are for a reactor 50 cm. long packed
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CHEMICAL REACTIONS IN CHROMATOGRAPHIC COLUMNS
with 8 g. of catalyst operated a t 180' with 10 cc./min. carrier gas flow. With a small sample (Figure 1) the hydrogen and benzene peaks are well separated and fairly sharp. As sample size is increased, these peaks tend to tail off toward each other (Figure 2). Such tailing is inherent in this mode of operation and is due to the fact that the products are produced in decreasing amounts as cyclohexane is consumed along the column. When large samples are introduced, a sharp peak of unreac ted cyclohexane appears between hydrogen and benzene (Figure 3). This peak increases in height very rapidly as pulse size is increased. The quantitative effect of pulse size is shown in Figure 4. At a given temperature a certain amount of cyclohexane can be almost entirely converted, but additional amounts are converted to a lesser, though essentially constant,extent. Efect of Flow Rate. Figure 5 shows the slight effect of carrier gas flow rate on conversion. This independence of conversion on residence time indicates that adsorption, reaction, and desorption were fast under the conditions studied and that the extent of reaction was equilibrium limited. Since conversion was virtually independent of residence time, the Roginskii modello (which assumes that products are separated instantaneously and that forward reaction rate limits conversion) would be clearly incorrect in the present case. The very slight effects of flow rates are explained as follows,: at high flow rates conversion begins to drop off a bit because of kinetic effects. At low flow rates slightly less carrier gas is used in eluting the products, and this means less dilution of the reacting mixture by carrier gas and hence lower conversion due to equilibrium limitations . E$et:t of Reactor Length. Examples cited thus far have been for reactors 50 cm. long packed with 8 g. of catalyst. When columns of different length were tried, It was found that the significant variable was the ratio of pulse size to column length (and hence weight of catalyst). Thus, the same conversions would result from pulsing 10 p1. of cyclohexane into a 50-cm. column and 20 p1. into a 100-cm. column. This effect is not expected on the basis of behavior in tubular flow reactors or in ordinary chromatographic columns. Its explanation awaits the solution of a realistic mathematical model of the reactor. Repetitive-Pulse Experiments. Thus far only results of single-pulse experiments have been described. When repetitive pulsing is used, the additional variable of pulse frequency enters the picture. At very low frequencies, conversions are the same as for single pulses. For a given pulse size this represents the maximiim conversion. This is wasteful of time and
carrier gas, however, and equal or greater conversions could be obtained in an equilibrium system diluted with that amount of carrier gas. Conversion drops off only slightly as frequency is increased, until pulsing is so rapid that the hydrogen peak from a fresh pulse comes soon enough to pass through part of the benzene peak from the previous pulse. When this happens the hydrogen and benzene recombine, and a new cyclohexane peak appears in the product chromatogram where the hydrogen peak had previously been. From this point on conversion decreases steadily as frequency increases. Column Treatment. In the case of ideal chromatography, adsorption isotherms are linear, and peaks are sharp and show little tendency to tail off slowly. The isotherm for benzene adsorbed on fresh alumina is very nonlinear, however, causing the benzene peak to tail off slowly. Under such conditions the reactor could be pulsed only infrequently, and conversions were no better than for a continuous reactor operating a t the same average dilution of cyclohexane by carrier gas. Data reported to this point are for such a case. Several adsorbents besides alumina were tried without success in an attempt to overcome this. Finally, an alumina was used which had been treated with a solution of 10% KOH in methanol. The treatment neutralized the very strongly acidic sites which give rise to the nonlinear isotherm. This reduced retention volume and adsorbent capacity and lessened the benzene tailing, as is shown in Figure 6. E F F E C T OF KOH TREATMENT
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Figure 6. Effect of KOH treatment on chromatogram.
The treated alumina gave slightly less reproducible results than did the untreated alumina. Small but noticeable differences between successive product chromatograms were noted, although these changes showed no apparent trend after the first few pulses on a freshly activated column. On a long-term basis, Volume 69,Number I February 1966
J. M. MATSEN, J. W. HARDING, AND E. M. MAGEE
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dependent more on time than on amount of cyclohexane injected, the treated alumina showed a greater tendency to become deactivated. The platinumon-alumina catalyst used in the remainder of the work reported here was treated in this manner. Comparison to Equilibrium. Quantitative results a t three temperatures are shown in Figures 7, 8, and 9. Equilibrium constants were obtained from the data of Rossini.l4 The "theoretical" equilibrium curve depends only on temserature and the extent of dilution of reactant by carrier gas. Additional parameters would be necessary for accurate representation of precise experimental data, and the experimental points therefore do not fall on a single curve. From Figure 9 it is seen that carrier gas flow rate has a noticeable effect when conversions are plotted in this manner. The amount of carrier gas needed to elute a peak a t a high flow rate is somewhat greater, partly because adsorption-desorption rates are finite, and partly because gas a t the column inlet is a t higher pressure so that more moles are needed for a given amount of elution. This
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CONVERSIONS A T 183°C. I I
CONVERSIONS A T 225'C.
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MOLES HELIUM PER MOL CYCLOHEXANE
Figure 7. Repetitive-pulse conversions at 183'; equilibrium constant = 4.0 X 10-6. CONVERSIONS A T 204°C.
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'
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Figure 8. Repetitive-pulse conversions at 204'; equilibrium constant = 4.2 X
The Journal of Physical C h i s t r y
1
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/
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Figure 9. Repetitive-pulse conversions at 225'; equilibrium constant = 4.8 x 10-3.
OPTIMUM REPETITIVE PULSING
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Figure 10. Optimum repetitive pulsing: temperature 225"; carrier gas flow rate 10 cc./min.; pulse size 12 pl.; time between pulses 3.5 min.; 8 g. of catalyst in 50 cm. long reactor; conversion 96.4%
effect of flow rate is not seen in single-pulse experiments because correlation in that case is not made as a function of amount of carrier gas used. Optimum Operation. The greatest improvement of experimental conversion above theoretical is about 30%. Three or four times as much carrier gas would be needed as diluent in a static system to attain the same conversion as in an optimum-pulsed case. The optimum mode of operation was to use a sample pulse as large as possible without allowing a peak of unconverted cyclohexane to appear and to pulse as frequently as possible while still keeping a peak of reconverted cyclohexane from appearing in the product chromatogram. This optimum mode of operation is shown in Figure 10.
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(14)F. D. Rossini, "Selected Values of Physical and Thermodynamic Properties of Hydrocarbons and Related Compounds," Carnegie Press, Pittsburgh, Pa., 1953.
NUCLEAR MAGNETIC RESONANCE STUDIESOF BORON TRIFLUORIDE COMPOUNDS
Discussion It has been demonstrated that reactions can be conducted in chromatographic columns so that products can be separated during the course of reaction. Higher conVersions can from this type Of Operation than in a conventional equilibrium reactor. The improvement above equilibrium is modest as yet but certainly can be improved. Ultimate Performance cannot' be estimated until a solution is obtained for a realistic mathematical model.
527
I n some cases, the fact that products are separated in the reaction step may prove io be a desirable feature quite apart from conversion considerations, since a separate separation step is unnecessary. Any factors which would tend to cause peak spreading in ordinary chromatography wfl adversely affect the conversionsin a ,,hromatographic somesuch factors are increase in packing size, increase in column diameter, and increase in carrier gas flow rate. These are potential trouble spots in scaling up a laboratory reactor to a large-scale process.
Nuclear Magnetic Resonance Studies of Boron Trifluoride Addition Compounds. 111. Rates and Mechanism for the Exchange of Boron Trifluoride
between Ethyl Ether-Boron Trifluoride and Tetrahydrofuran-Boron Trifluoride and between Ethyl Ether-Boron Trifluoride and Ethyl Sulfide-Boron Trifluoridel
by A. C. Rutenberg and A. A. Palko Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 87851 (Received August 29, 1964)
Kinetic data for the exchange of BF3 between (CzH&O*BF3and THF-BF3 and between (C2Hs)20.BF3and (C2H6)&BF3were obtained from I9F n.m.r. spectra of mixtures of (C2Hs)20,BF3,and THF or (C2H6)2S. Both pairs of addition compounds followed the rate law, R = kl[L'.BF3] [LeBFa] k2 [L'.BFs] [L] ka[L]/ [LaBFa], in which R is the rate of BF3 exchange between two BF3 addition compounds, molecule L' forming the stronger BF3addition compound. Rate constants and activation energies were calculated and compared to previously measured systems.
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Boron trifluoride forms stable 1:1 molecular addition compounds with many ethers and thioethers. In systems containing BF3 and more than one ether BF3 exchange between the different Or thioether, species is usually very rapid. The strong IgFresonance and the large chemical shifts between the different BF3 addition study Of these rapid exchanges by the n.m.r. technique. The exchanges of
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(CJl")20*BF3 with (CH3)20*BFa and CsH60CH3.BF8 were reported in earlier papers.2J It was of interest to (1) Research sponsored by the U. S. Atomic Energy Commission under contract with the Union Carbide Cow. (2) A. C. Rutenberg, A. A. Palko, and J. S. Drury, Soc., 85, 2702 (1963).
J. Am. c h m ,
(3) A. C. Rutenberg, A. A. Palko, and J. S. Drury, J . Phys. Chem., 68,976 (1964).
Volume 69, Number 2 February 1966
