X'OTES
2070
Vol. 66 CATALYTIC ISORIERIZATION OF
2-PENTENE BYJ. C. ROHRER AND J. H. SINFELT Esso Research and Eneineerino Co., Linden, New Jersey Received April 11, 196%
This note presents kinetic data on the skeletal isomerization of 2-pentene over a chlorided alumina catalyst. Included are data on the effect of hydrogen, which are of interest in connection with the observed complex effect of hydrogen on the isomerization of saturated hydrocarbons over a chlorinecontaining platinum on alumina catalyst.lS2 Considerable evidence indicates that the latter reaction involves a step in which olefinic intermediates undergo skeletal rearrangement on acidic alumina ~ites.3~4It was suggested, however, that the effect of hydrogen pressure was not associated with this step.lrz The results of the present study serve to check this point. Experimental Procedure.-The 2-pentene was contacted with the catalyst in the presence of hydrogen or nitrogen, using a flow reactor technique described previously.6 A catalyst charge of 4.0 g. was used throughout. The reaction products were 0 0.4 0.8 1.2 1.6 2 . 0 (x10-31 analyzed by a chromatographic procedure which also has been described previously.a RA~NO, Materials.-The 2-pentene used in this study wm a mixture of 85% cis-2-pentene and 15% trans-2-pentene, and w t t ~ Figure 1. obtained from Matheson Coleman and Bell. A chromatographic analysis showed no other components to be present in detectable amounts (about 0.05%). The alumina catalyst used in this work contained 1.2% chlorine. The catalyst was calcined in air for 4 hr. a t 593", and had a surface area TABLE I of 210 m.a/g. X-Ray diffraction measurements of the alumina prior to calcination showed it to be p-alumina triCALCULATED ASSOCIATION CONSTANTS IX MOLEFRACTION hydrate, a form of alumina which has been described preUNITSAND DERIVED PARAMETERS viously.' T = 350°
T = 385"
T
436'
K I (in mole fraction units)
553 f 20 4 6 0 f 15 3 1 5 i 1 2
- AE1
(kcal./mole) for Z =
1'
4 612
6 21
6 17
5 585 ( 6 562
5 93 5 69
5 86 5 62
Ti2 (in mole fraction
units
215 f 25 169 f 20 117 i 12 6.15
6 18
6 16
581 1 56 5 5 2
5 82 5 53
5 7; 5 45
4 -A&
for Z
(kcal./mole) =
0. The difference makes clear the necessity of a careful analysis of data in order to obtain values of the association constants precise enough for comparison with theory. The calculated values of AE1 and AEz within the estimated error are constant at all three temperatures and for the three values of 2. It is indicated, therefore, that eq. 1 and 2, with co istant values of AEl and AE2, and for any reasonable choice of 2, can be utilized to predict the temperature coefficients of K1 and K 2 which are correct within the experimental precision of the measurements.
Results When 2-pentene is passed over the promoted alumina catalyst, both double bond migration and skeletal isomerization are observed, the former yielding 1-pentene and the latter a mixture of methylbutenes. Interconversion between cis- and trans-Zpentene also is observed. Some hydrogenation to pentanes and cracking to C1-C4 hydrocarbons also are observed, particularly a t the higher temperature (471"). Typical product distribution data are shown in Table I. Interconversion between cis- and trans-Zpentene and the formation of 1-pentene occur more readily than skeletal isomerization to the methylbutenes. Thus, a t the lowest reactant flow rate used ( F / W = 0.18 gram mole 2-pentene charged per hour per gram catalyst), the distribution of cis- and trans-2pentene and 1-pentene is roughly in accord with equilibrium data,8 whereas the ratio of total methyl(1) J. H. Sinfelt and J. C. Rohrer, J . Phya. Chem., 66, 978 (1961). (2) J. C. Rohrer, H. Hurwitz, and J. H. Sinfelt, ibid., 66, 1458
(1961). (3) F. G. Ciapetta, Ind. Eng. Chem., 46, 162 (1953). (4) G. A. Mills, H. Heinemann, T. H. Milliken, and A. C . Oblad, ibzd., 46, 134 (1953). (5) J. H. Sinfelt, H. Hurwitz, and J. C. Rohrer, J . Phys. Chem., 64, 892 (1960). (6) J. C. Robrer and J. H. Sinfelt, ibid., 66, 950 (1962). (7) H. S. Stumpf, A. 8. Russell, J. W. Newsome, and C. M. Tucker. I d . Eng. Chem., 42, 1398 (1950).
2071
NOTES
Oct., 1962
TABLE I Temp.,, "C.
FIW"
TYPICAL PRODUCT DISTRIBUTION DATAFOR 2-PEXTENE 372 372 372 47 1 0.79 0.18 0.82 0.80
47 1 0.18
47 1 0.78
5.3 0.53
5.3 0.53
21.3 0.53
Pressure, atm. 21.3 5.3 5.3 0.53 0.53 0.53 8-Pentene Productt distribution, mole % ' CI-C: 7.4 8.7 7.2 I-Pentene 38.5 44.6 32.2 tran8s-2-Pentene 48.9 21.3 56.3 cis-2-Pentene 0.3 0.9 3-Methyl-1-butene 1.3 5.8 1.2 2-Methyl-1-butene 2-Methyl-2-butene 3.7 15.2 3.1 0.2 Isopentane n-Pentane Expressed as mole % of Gram moles 2-pentene charged per hr./g. catalyst. not determined.
Hz
Temp., "C. Pressure, atm.
T.4BLE 11 RATEDATAFOR SKELETAL ISOMERIZATION OB' PENTEK EKE 372 372 372 372 47 1
Hz
@
5.3 I .3 2-Pontene 0.13 0.13 Ijate, ra 0.026 0.020 Gram moles 2-pentcne converted pcr hr, per g.
5.3 0.53 0,042 catalyst to
butenes to the sum of cis- and trans-2-pentene and 1-pentcne is far from thc value a t equilibrium, which averages about 3.5 a t the temperatures used in this study.8 IIowevcr, the distribution of the various methylbut cries formed iii the reaction corresponds roughly with that! at equilibrium, m-hirh is further evidenw 1hat double bond migration occurs more readily than skeletal isomerization. The ratio of isopentane to n-pentane in the products a t 47 lo is nbout 3 or 4 to 1. Increasing hydrogen pressure a t a given value of F/W increases the extent of hydrogenation to form both n-pentane and isopentane. Rates of skelet a1 isomeriaaiion of 2-pentenc are shown in 'I'able I1 as a function of temperature and partial prossures of hydrogen and 2-pentene. The reaction rates were determined from data where the levels of conversion to methylbutenes were low (2.6 to 17.4%) and hence represent initial rates. The reaction rates were evaluated using the relation
F r=-Ax W where F represents the feed rate of 2-pentene in g. moles/hr., W is the weight of catalyst in grams, and Ax is the fraction of the 2-pentene converted to methylbutenes. The rates shown in Table I1 actually represent rates of conversion of a mixture of cis and trans-2-pentenes and 1-pentene to a mixture of methylbutenes. The 2-pentene used as a reactant was a mixture of cis and trans isomers, and in addition, extensivc cis-trans interconversion and double bond migration take place before the extent of skeletal isomerization becomes appreciable, (8) "Selected Values of Physical and Thermodynamic Properties of Hydrocarbons a n d Related Compounds," API Research Project 44, Carnogie Press, Inc., New York. N. Y.,19%.
IN
21.3 1.3 0.13 0.53 0.035 0.10 methylbutcnes.
0.9 2.6 . .c 6.1 8.5 8.0 27.3 34.5 39.1 19.1 39.7 33.6 1 .o 1.9 0.8 5.0 11.6 4.3 11.4 25.3 8.8 0.8 4.6 2.6 0.2 1.5 0.7 2-pentene converted to CJ-C~. CI-CI
PREREXCE OF Hz 47 1
47 1
5.3 0.13 0.080
5.3 0.53 0.15
471 21.3 0.53 0.13
The data on the effect of tcmperatuye on the rate of skeletal isomerization indicate an apparent activation energy of about 13 kcal./mole. Regarding pressure effects, the rate is roughly 0.5 order with respect to 2-penteiie partial pressure and nearly independcn t of hydi ogeii partial pressure. Since the experiments in the presence of hydrogen showed little effect of hydrogen prcssure, it was decided to determine the effert of eliminating hydrogen completely. In some side experiments on the same alumina, after it had been somewhat deactivated due to prolonged exposure to 2-pentene, it was found that the skeletal isomerization of 2pcntene proceeded satisfactorily when nitrogen was substituted for hydrogen. The rate in the presence of nitrogen relative to that in the presence of hydrogen ( r n / r h ) is Temp., "C. rn/rha
* He or
416 1.1
47 1 0.85
Ne pressure = 5.3 atm., 2-pentene pressure = 0.53 atm.
Thus, the effect of hydrogen on the rate of skeletal isomerization generally is not very great. In previous studieson the isomerization of methyleyclopentanel and %heptane2 over a platinumalumina catalyst containing chlorine, it was found that the presence of hydrogen was necessary for isomerization and that at moderate hydrogen pressures (below 6 atm. a t 471') the rate of isomerization increased markedly with increasing hydrogen pressure. It was suggested that this effect of hydrogen was not associated with the isomerization of olefin intermediates over acidic sites on the alumina, but rather with the reaction on the platinum sites, the role of hydrogen being to keep the platinum free of surface residues. The observation of the present study that the presence of hydrogen is not necessary
NOTES
2072
for isomerization of olefins over an acidic alumina at similar conditions is in agreement with this conclusion.
THE POLAROGRAPHIC DIFFUSIQK COEFFICIENT BY
Vol. 66
which is the experimental verification of the Onsager reciprocal relatioq2 it has become increasingly clear that eq. 1 is inadequate even if the mobility is supposed to be a function of composition. The more general equations which are now being used to replace eq. 1 may be written in the following form3
RICHARD J. BEARX4X
Department of Chemzstr y, Unzcerszty of Kansas, Lawrence, K a n s a s
rap =
Recezzed 4Iaich 9, 1963
It is well known that the diffusion coefficient is one of the important parameters of the limjting current equations (e.g., the llkovic equation) of polarography. Although numerous measurements have been carried out to evaluate polarographic diffusion coefficieats, little effort has been expended thus far to compare these with diffusion coefficients determined by other standard techniques. Such comparisons would be valuable in providing further information conccruing electrode processes and in checking the fundamental equations of both polarography and diffusion. Professor Adams of this department has undertaken an experimental program of tracer diffusion measurements with these objectives. In this paper, we proiride a theoretical framework for the interpretation of his data. By now, it is commoiily realized that in a given system there exist several diffusion coefficients of interest, namely all of the mutual and tracer coefficients. However, the simple derivations of the limiting current equations to be found in standard textbooks1 do not concern themselves with the distinctions among them. Clearly, for meani ngful comparisons be tween the polarographic diffusion coefficients and the other coefficients, these distinctions must be made. With the purpose of determining the relationship of the polarographic diffusion coefficient to the others, we study in this note the derivation of Fick’s law as it applies to common polarography experiments. We find that the polarographic diffusion coefficient is closely related to the tracer diffusion coefficient of the electroactive ion. Strictly speaking. the two coefficients are not equal. Severtheless, it may be found in practice that they are identical within the limits of validity of the limiting current equations themselves. The Fundamental Phenomenological Equations. -Fick’s Law for the diffusion of an ion CY along the line of the x direction is often regarded as a consequence of the simple equation1 u, =
ua(-apu + i,P+Y)
--
(1)
dx where u, is the velocity of the ion, U , is its mobility, dp,/dx is the gradient of its chemical potential, xu is its (signed) charge number, dpldx is the gradient of electric potential, and F is the faraday. For a number of reasons, the most compelling of F
dx
(1) G. W. C. Milner, “The Principles and Bpplioations of Polarography ahd Other Electroanalytical Processes,” Green and Co., London, 1957.
(2)
baa
-ivhere c p is the concentration of p and r a p is the concentration dependent friction coefficient between species CY and p. The summation is over all x species in the system. Equation 2 reduces to eq. 1 when all interactions are negligible except those between the ion a and the solvent. The Diffusion Coefficients.--We consider a system with only one electroactive species, which we conventionally assume to be a positive ion A. Equation 2 for species A contains dp,’dx which generally is not accessible by direct measurement. In order to discuss only measurable quantities, it is convenient to eliminate the electric potential from the equation. We may do this by considering any negative ion R in the system and iatroducing simple positive integers V R and v-4 with the property that v g / v ~ = - X A / X R . Multiplying eq. 2 for A by V A ,eq. 2 for R by U R and adding the two equations gives dz 7I
7I
V = VA
4
UR
(3)
To derive eq. 3, we have written the chemical potential as the sum of a standard potential term and an activity term pa =
+
pao(T,P) RT In casa
(4)
with R the gas constant, T the absolute temperature, and f a the single ion activity coefficient. In eq. 3. we have introduced the measurable mean ionic activity coefficientf A R Y = fAyAf R ~ ~ . The deril-ation of the limiting current equations requires eq. 3 only in a small region of the diffusion layer near the electrode surface.j Because of various surface effects, it is unlikely that eq. 3 or its precursor, eq. 2, applies directly at the surface. Severtheless, in phenomenological discussions like this one it is ordinarily assumed that the equations do apply for all macroscopic distances from the surface, and that surface effects play a significant role only insofar as they help to determine bound(2) P. J. Dunlop and L. J. Gosting, J . Phys. Chem., 63, 86 (lB59). (3) (a) R. W. Laity, i b z d , 63, 80 (1959); (b) E. Helfand, f. Chem. Phys., 33, 319 (1960); (0) 8. Ljunggren, Trans. Roy. I n s t . Technol. Stockholm, Slceden, Nr. 172 (1961); (d) It. J. Bcarman and J. G. Kiikwood, J . Chem. Phys., 28, 136 (1958). (4) E. A. Guggenheim, “Thermodynamics,” 31d Ed,, Nul th-llolland Publishing Co., Amsterdam, 1957. (5) Reference 1, p. 32.
