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COMMUNICATIONS TO THE EDITOR (1 - f2)]vs. At will be a straight line of slope k,, the psuedo-homogeneous rate constant, when Vis the volume of the reaction solution and W is the total molar tungsten content of the macromolecular catalyst. Figure 2 20
I
-
Temp. 60’ PH 5 % W 0 3 of catalyst =18.5% 0 Run 1
Figure 1. Reactor chromatogram for dissociation of dicyclopentadiene (B) in Apiezon L (25% on Gas Chrom RZ), 10-ft x 0.25-in. column a t 209.6’: A, standard; C, cyclopentadiene; D, air; helium carrier gas, Pi/P,,t = 4; a b represents product included in reactant peak, approximated as area between bc and base line.
A Run 2 0 Run 3
0
f , seconds
,
0
500
1000
A t (seconds)
Figure 2. Epoxidation of maleic acid by hydrogen peroxide; macromolecular organometallic catalyst.
shows such plots for three different values of V / W which are linear so that IC, = 0.015 1. mo1-I sec-l. Since the epoxidation reaction does not proceed in the absence of catalyst and since the magnitude of k , in the heterogeneous reaction does not change with repeated use (15 times) of the macromolecular catalyst, the validity of the concept of macromolecular organometallic catalysis is clearly established. Additional oxidations using macromolecules containing Mol V, Os, U, and Nb have been carried out on a variety of substrates, the details of which will be reported later elsewhere.
Acknowledgment. We are indebted to the Department of Chemical Engineering for generous support of this work. (8) To whom inquiries should be addressed.
ALL AN^ DEPARTMENT OF CHEMICAL ENGINEERINGG. GRAHAM AND INSTITUTE OF FOREST PRODUCTS AMARNATH NEOGI COLLEGE O F F o m w RESOURCES OF WASHINGTON UNIVERSITY SEATTLE, WASHINGTON98105 RECEIVED MARCH15, 1969
The Gas Chromatographic Chemical Reactor. The Unimolecular Dissociation of Dicyclopentadiene
Sir: Despite the over-all potential of the gas chromatographic column as a reactor,l~zgas chromatographic
reactor studies have been limited mainly to heterogeneous reaction. Only a few homogeneous liquidphase reaction studies, of the pseudo-first-order bimolecular type with nonvolatile product, have been r e p ~ r t e d . ~Heterogeneous ,~ surface effects often cause complications.s Therefore, the validity of rate constants obtained from the gas chromatographic reactor have been largely unchecked by comparison with results from more conventional techniques. Keither a homogeneous, unimolecular reaction nor a homogeneous reaction involving volatile product has been studied by this relatively simple technique. The latter especially is troublesome to study by conventional methods, Unimolecular reactions should provide an excellent test for the method since theory predicts that they should proceed at comparable rates in the gas phase and in ideal whatever the solvent. Furthermore, the gas chromatographic method should provide a simple, rapid method for investigating solvent effects, since the chromatographic traces used for evaluating kinetic data can also give information about the solution thermodynamics.’ We chose the unimolecular dissociation of dicyclopentadiene for investigation because it has been studied in the gas phase by conventional manometric methodss and in paraffin solution by colorimetric and gas volu(1) Review included in S. H. Langer, J. E. Patton, and J. Y . Yurchak, Ind.Eng. Chem., 61, No. 4, 10 (1969). (2) D. W. Bassett and H. W. Habgood, J . Phys. Chem., 64, 769 (1960). (3) E. Gil-Av and Y . Hersberg-Minzly, Proc. Chem. SOC.,316 (1961). (4) V. G. Berezkin, V. S. Kruglikova, and N. A. Belikova, Dolcl. Akad. N a u k SSSR, 159, 182 (1964).
(5) S. H. Langer, J. Y. Yurchak, and C. M. Shaughnessy, Anal. Chem., 40, 1747 (1968). (6) A. A. Frost and R. G. Pearson, “Kinetics and Mechanism,” John Wiley & Sons, Inc., New York, N. Y., 1961, pp 128-130. (7) S. H. Langer and J . H. Purnell, J . Phys. Chem., 67, 263 (1963); 9. H. Langer, B. M. Johnson, and J. R. Conder, {bid., 72,4020 (1968). (8) J. B. Harkness, G. B. Kistiakowsky, and W. H. Mears, J . Chem. Phys., 5, 682 (1937).
Volume 79, Number 6
June 1969
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COMMUNICATIONS TO THE EDITOR where t is time and subscripts g and 1 indicate the gas and liquid phase, respectively. t, was determined from the elution time of air. t l was measured from the chromatogram. Using the known value of IC,,~ kl was calculated. The correction to obtain k~ was small (1 or 2%) since t g / t l was small and kg differs little from kl. Arrhenius plots for rate constants in the three solvents are shown in Figure 2. It is apparent that the activation energies for the reaction in all three solvents are essentially the same as that for the gas-phase reaction. The ratios of kl/k, are compared in Table I
Table I : Ratios of Liquid- and Gas-Phase Rate Constants for the Dissociation of Dicyclopent'adiene
-
kl/k*a-
Solvent T , OK
2000
2100 IOe/T (OK-')
2200
I
Figure 2. Arrhenius plot for the dissociation of dicyclopentadiene; solvent key: 0 , Apiezon L; A, silicone oil DC 550; V, polyphenyl ether (6 rings). The solid line represents the Arrhenius equation for gas-phase reaction measured by Kistiakowskg, et a1.* The broken line is for this reaction in paraffin, measured by Wasserman and Khambata.g
metric method^.^ Arrhenius plots were similar. The gas chromatographic reactor technique was applied in three solvents-silicone oil DC 550, polyphenyl ether (6 rings), hpiezon L- and at six temperatures between 180 and 230". The inert reference substance (1bromo-3-chlorobenzene) method' was used to measure first-order rate constants at different flow rates (residence times). An injection size of 0.5 pl of reactantinert substance ( 2 : 1) was used throughout. A sample reactor chromatogram is shown in Figure 1. Measured reactant peak areas needed only slight correction because marked difference in retention time between product and reactant caused only small overlap. Peak areas of reactant on the chromatogram were related to the first-order rate constants by In (reactant area/inert area) = -kt
+ constant
(1)
where 2 is the residence time in the column. A plot of the left-hand side of eq 1 vs, total residence time in the column was linear over a wide conversion range (5-90%) confirming the first-order nature of the reaction. Slopes of these plots gave the rate constant k k(t1
+
tg)
= kltl
The Journal of Phvsical Chemistry
+
JGgtg
453.0 462 9 472.8 482.8 492.9 502.8
(2)
Mean value Cnl
Silicone DC 550
1.095 0.878 0.955 0.972 0.964 1.007
Polyphenyl ether
1.242 1.030 0.974 0.965 1.074 1.185
Apiezon L
Paraffinb
0.818 0.817 0.812 0,804 0.828
0.574 0.581 0.587 0.594 0.600 0.606
...
0.979 1.078 0.8156 0.5903 zk0.025 &0.046 rt0.0035 zk0.0065
a Calculated from8 k , = 1018.00exp( -33,700/RTj(sec-l). Calculated from8 kl = 101aJo exp( -34,200/RT)(sec-I).
with corresponding values for paraffin. The gas chromatographic technique here yields values for homogeneous liquid-phase rate constants in excellent agreement with those obtained by conventional methods. The simplicity of the gas chromatographic technique does not appear to cause a loss in accuracy compared to conventional techniques. Furthermore, activity coefficient data should become a c c e ~ s i b l ein ~ ~many ~ instances. The corrected retention volumes per gram of solvent for dicyclopentadiene are: DC 550, Vglso = 66.6 ml; polyphenyl ether, V,lso = 77.9 ml; Apiezon L, V,lso = 98.8 ml; paraffin (calculateds) 51.0. Temperature variation of VR lead to the following values for the molar heat of vaporization at infinite dilution: DC 500, 9.2 kcal mol-l, polyphenyl ether, 8.9 kcal mol-l, Apiezon L, 9.2 kcal mol-'. The heat of vaporization from paraffins (not at infinite dilution) is 13.5 kcal. We hope to extend this study to a wider range of solvent types in order to investigate more fully the solution factors which cause the small variations in the rate of this unimolecular reaction. (9) B. S. Khambata and A. Wasserman, J. Chem. Soc., 375 (1939).
COMMUNICATIONS TO THE EDITOR
2097
az = 3.26 X cm3 (from Heydweiller’s5 molecular refractions) gives ep = 111 for KC1, thus predicting that KC1 should have an association constant some 8 times larger than TlCl from these effects. This larger (10) To whom inquiries regarding this work should be addressed. value arises from the much smaller value of n for KCI, despite the larger ro and smaller polarizabilities. It GRAHAM L. PRATT CHEMICAL ENGINEERING DEPARTMENT appears that the major failure of the procedure arises STANLEY H. LANGER‘~ OF WISCONSIN UNIVERSITY MADISON, WISCONSIN53706 from the choice of close-range dielectric constant, which is identified with n2 for the solid (extrapolating n to RECEIVED MARCH24, 1969 infinite wavelength). If it is assumed that the closerange dielectric constant should be the same for TlCl and KC1 then a value for this (to be compared with n2) of about 3.4, using the same polarizabilities and internuclear distances as above, would reproduce the factor of 13 difference between TlCl and KC1 while Comment on “Conductance of Thallous Chloride allowing polarizability of ions and dielectric saturation in each case. This value for the close-range dielectric in Dioxane-Water Mixtures at 25”” constant is not unreasonable. Sir: D’Aprano and Fuossl have recently determined However, the D’Aprano and Fuoss procedure would still fail to account for the observed trend in association the association constants for thallium(1) chloride in dioxane-water mixtures from conductance measureconstants for TlC1, TIBr, and T1I in water at 25”. ments at 25”. Their figure in water, 5.2 f 0.5 1. mol-’, B ~K T ~ I Using .~ data This trend is K T E L< K T ~ < is as close as can be expected to our figures2 from solufrom the sources given above we get e p values of 15.0, bility and spectrophotometric measurements, since 8.6, and 4.2, respectively, for these halides. (Our we have shown that the values obtained depend on the ep value for TIC1 is slightly different from that of mean ionic activity coefficients used and D’Aprano D’Aprano and Fuoss as we have rounded off the exand Fuoss use the Debye-Huckel limiting law. trapolated value of n to 2.1.) Again the majorcontriI n discussing the well-known finding that thallium(1) bution to these differences comes from the differences in chloride has a higher association constant than most extrapolated refractive indices (2.1, 2.2, and 2.4, reother 1: 1 salts, D’Aprano and Fuoss propose that this spectively), these increases, and increases in ro, offcan be accounted for by assuming mutual polarization setting increases in a2. But in this case, even if the of TI+ and C1- ions. They give an expression for the same value for the close-range dielectric constant is association constant assumed for each halide, the order of e p values remains the same. Since eb values in eq 1 are also in this order, the D’Aprano and Fuoss procedure predicts the trend K A = KoebeP (1) of association constants to be opposite to that observed, where K Ois an excluded volume factor, eb is a factor unless it is assumed that the close-range dielectric arising from chargecharge interactions, and ep is a constant decreases along the series. This is an unlikely factor allowing for polarization and dielectric saturaassumption, as noted below. tion effects. p is given by their model, after matheA previous’ thermodynamic analysis of association matical approximations and allowing for dielectric of TIC1 and TlBr suggests a reason for the failure found saturation, as above. This analysis shows that the reversal of stability of these salts in water, compared with the gas phase, arises from a fine balance of enthalpy and entropy terms in which, among other factors, the greater with symbols as in their paper. The factor e p (with the value 13.0 for TlCl with their choice of parameters) is claimed to account for the increased association of (1) A. D’Aprano and R. M. Fuoss, J . Amer. Chem. Soc., 91, 279 (1969). TlCl compared with that of a salt with the same size (2) J. B. Macaskill and M . H. Panckhurst, Aust. J . Chem., 22, 317 ions but whose charges remain fixed at the centers.” (1969), and earlier papers. It is shown that this factor is consistent with the differ(3) “Landolt-Bornstein Tabellen,” 6th ed, Vol. 11, Part 8, “Opences in association between TIC1 and KC1. However, tische Konstanten,” Springer-Verlag, 1962. (4) “Interatomic Distances,” Special Publications No. 11, 1958, D’Aprano and Fuoss do not consider the effect of and No. 18, 1965, The Chemical Society, London. polarization and dielectric saturation in the KC1 case. (5) A. Heydweiller, Phys. Z.,26, 526 (1925). Using their procedure we get n = 1.5 (extrapolated (6) “Stability Constants,” Special Publication No. 17, The Chemical from data listed in Landolt-Bornstein3) which, toSociety, London, 1964. a1 = 1.07 X cma, and gether with ro = 2.67 (7) M. H. Panckhurst, Aust. J . Chem., 15, 194 (1962).
Acknowledgment. The authors thank the Petroleum Research Fund, administered by the American Chemical Society, for an International Award (G. L. P.).
Volume 79, Number 6 June 1989
