Ind. Eng. Chem. Res. 1999, 38, 4525-4530
4525
Isoprene/Methyl Acrylate Diels-Alder Reaction in Supercritical Carbon Dioxide Bin Lin and Aydin Akgerman* Chemical Engineering Department, Texas A&M University, College Station, Texas 77843-3122
The Diels-Alder reaction between isoprene and methyl acrylate was carried out in supercritical carbon dioxide in the temperature range 110-140 °C and the pressure range 95.2-176.9 atm in a 300 cm3 autoclave. The high-pressure phase behavior of the reaction mixture in the vicinity of its critical region was determined in a mixed vessel with a sight window to ensure that all the experiments were performed in the supercritical single-phase region. Kinetic data were obtained at different temperatures, pressures, and reaction times. It was observed that in the vicinity of the critical point the reaction rate constant decreases with increasing pressure. It was also determined that the reaction selectivity does not change with operating conditions. Transition-state theory was used to explain the effect of pressure on reaction rate and product selectivity. Additional experiments were conducted at constant temperature but different phase behaviors (two-phase region, liquid phase, supercritical phase) by adjusting the initial composition and pressure. It was shown that the highest reaction rate is in the supercritical region. Introduction The utilization of supercritical fluids (SCFs), specifically supercritical carbon dioxide (scCO2), as the reaction medium for organic chemical synthesis reactions has been the subject of many recent studies.1-3 scCO2 has many advantages to make it a favorable reaction medium. It is inert to most reactions, nontoxic, cheap, readily available, and environmentally acceptable. Moreover, scCO2 is nonflammable, so its use does not introduce a safety hazard during operation. In addition, its relatively mild critical properties (Tc ) 31 °C, Pc ) 72.9 atm) are particularly attractive for supercritical reaction applications. Research indicated that, in the supercritical region, the pressure could affect not only the reaction rate4,5 but also the reaction selectivity.6-8 The Diels-Alder reaction, which is a very important organic synthesis reaction, is the most powerful method for the construction of six-membered ring systems. Many patents cite the use of Diels-Alder chemistry in the synthesis of complex compounds including insecticides, fragrances, plasticizers, and dyes.9 The DielsAlder reaction has the same rate and activation energy when carried out in the gas phase or in inert organic solvents, implying the same mechanism. It is a bimolecular second-order reaction and has no significant side reactions. Therefore, it is an excellent reaction to study the rate and selectivity changes due to the partial molar volume effect when the reaction is carried out in a supercritical fluid.10 In the Diels-Alder reaction between isoprene and methyl acrylate, the product methyl 4-methyl-3-cyclohexene-1-carboxylate (para-substituted product) is preferred over the methyl 3-methyl-3-cyclohexene-1-carboxylate (meta-substituted product) under atmospheric conditions. However, Ikushima et al.11,12 reported a large effect of varying reaction conditions on the DielsAlder reaction regioselectivity in the scCO2 reaction * To whom correspondence should be addressed. Phone: (409) 845-3375. Fax: (409) 845-6446. E-mail: a-akgerman@ tamu.edu.
medium. They observed dramatic changes in the isomer distribution when the reaction was carried out near the critical pressure of carbon dioxide. Ikushima et al.12 explained their regioselective results in terms of steric effects. They argued that the clustering of solvent molecules around the activated complex disfavors the more stable para isomer, resulting in higher selectivity to the meta addition. Recently, Renslo et al.13 studied the regiochemical course of several Diels-Alder reactions in scCO2, and their findings dispute the results reported by Ikushima et al. Renslo et al. claimed that the erroneous results of Ikushima came from the combination of unknown phase behavior and partial sampling of the reaction mixture. The conflicting results on the same reaction by two different groups intrigued us to take a systematic investigation of the pressure effect on the reaction rate and selectivity of the DielsAlder reaction between isoprene and methyl acrylate. Experimental Section A major concern in using a supercritical reaction medium is to make sure that the reaction is carried out in the single-phase supercritical region. Therefore, we first studied the reaction in a reactor with a sight window to observe the phase behavior and determined the conditions under which there is a single phase throughout the course of the reaction (as reactants are converted to products). Furthermore, we made sure that this single phase is in the supercritical region. To observe the phase behavior of the reaction mixture (the reactants, carbon dioxide, and the products) under the reaction conditions, a small reactor cell was fabricated. The cell serves as a high-pressure vessel that has windows made of unoriented sapphire (Insaco, 1 in. diameter by 0.25 in. thickness). The windows allowed the reactor interior to be monitored visually, and enabled us to confirm the reaction mixture was in the single-phase region. The volume of the cell was about 8 mL. The cell was placed on a small stir plate, and agitation was achieved by a magnetic stir bar. The cell was heated with a heating tape. This reactor was used
10.1021/ie990120j CCC: $18.00 © 1999 American Chemical Society Published on Web 09/25/1999
4526
Ind. Eng. Chem. Res., Vol. 38, No. 12, 1999
Figure 1. Experimental assembly for the supercritical Diels-Alder reaction.
only for phase observation, and no kinetic determinations were made. A schematic diagram of the experimental setup used to study the reaction is shown in Figure 1. The main part of the system consisted of a 300 cm3 vessel with a magnetic drive stirrer (Autoclave Engineers). The pressure was measured with a Heise 12401 gauge. The temperature was monitored with a thermocouple (Omega 115KC). The kinetic studies were performed in this batch reactor system. The reactor was cleaned thoroughly before the start of each experiment. Carbon dioxide was used to flush the whole system of air. The reactant mixture, isoprene and methyl acrylate, 0.1761.174 and 0.088-0.587 mol, respectively, was then injected into the vessel via a syringe, and the reactor was sealed. The reactor was then pressurized by introducing CO2 and heated to the desired temperature (110-140 °C). When the temperature was about 20 °C less than the desired temperature, additional liquid CO2 was pumped through an ice bath to the reactor to the desired pressure by an LDC Analytical mini pump. The final pressure adjustment was made when the desired temperature was reached. The procedure of getting the reactor system to the desired conditions took about 1 h. The reactor temperature is lower than the reaction temperature during this period, and the reaction is very slow; therefore, we consider that the reaction starts when the reactor is brought to the reaction conditions and the stirrer is switched on. The stirrer speed was 1250 rpm for each run. Samples were taken by a doublevalve sampling system. Opening and then closing the valve V4 trapped the SCF samples in the tubing line
between V4 and V5. The pressure was then released slowly through V5, and the effluent was bubbled through toluene in an ice bath to trap the products and unreacted reactants. The sample line was then washed with toluene to collect any precipitated product and by lowpressure carbon dioxide to dry the loop for the next sample. Reactants were analyzed by SRI GC with a FID detector, and products were analyzed by GC/MS. Results and Discussions Although the critical point and the phase behavior of pure carbon dioxide are well-known and there are abundant data on the phase behavior of supercritical mixtures, the phase behavior of the reaction mixtures in the Diels-Alder reaction is not well characterized in most studies reported in the literature. Even a small amount of reactants can greatly change the phase behavior and the critical loci of the reaction mixture. A group contribution method14 was used to estimate the critical properties of the reaction mixtures. Our objective in using group contribution to predict the critical loci was to obtain a rough estimate and then to run the experiments at the vicinity of these conditions in the reactor with the sight window to experimentally determine the critical loci. In determining the critical loci, the cell was flashed with carbon dioxide, and we added the desired amount of reactants into the cell at a temperature much lower than the estimated critical temperature of the mixture; then we pressurized the cell with carbon dioxide. We first observed the two-phase region, followed by the single liquid region, as the
Ind. Eng. Chem. Res., Vol. 38, No. 12, 1999 4527 Table 1. Critical Properties of Reaction Mixtures C1
C2
Cco2
mol/L
x1
mol/L
x2
mol/L
x3
vessel pressure (atm)
Pc (atm) (calcd)
Tc (°C) (calcd)
Tc (°C) (exptl)
0.624 0.624 0.624 0.624
0.054 0.096 0.138 0.152
0.312 0.312 0.312 0.312
0.026 0.048 0.069 0.076
10.839 5.524 3.585 3.156
0.92 0.855 0.793 0.772
107.5 106.8 91.2 92.5
85.0 83.9 81.0 80.0
65.8 87.1 107.4 114.1
51 81 110 140
amount of carbon dioxide (hence the pressure) was increased. Once the single liquid phase is formed, an increase in temperature with small increments enables determination of the critical temperature within a few degrees. Alternatively, at a lower pressure, the twophase mixture would be heated until the critical temperature is exceeded and then the pressure adjusted. At high concentrations of the reactants, it may not be possible to obtain a supercritical phase at the temperatures of interest (T < 150 °C), and increases in pressure would yield a single liquid phase. Table 1 summarizes the critical temperatures at different concentration levels. Both simulated and observed values are presented in the table. The vessel pressure was always above the critical pressure, and our technique does not give the critical pressure accurately. In addition, we were not interested in the exact critical properties, but in properties within a few degrees (or atmospheres) so that we make sure that we operate in the supercritical region. It should be noted that the pressure in the constant-volume reactor is adjusted by the amount of carbon dioxide and higher pressures indicate more carbon dioxide in the reactor and hence lower mole fractions of the reactants and a lower critical temperature. The first six columns in Table 1 give the concentrations (C1, isoprene; C2, methyl acrylate; Cco2, carbon dioxide) in moles per liter and as mole fractions (x1, x2, and x3 denote mole fractions of isoprene, methyl acrylate, and carbon dioxide, respectively). As can be seen, although the concentrations (mol/L) of the reactants are the same under all four experimental conditions, as the vessel pressure increases the concentration of carbon dioxide increases and the mole fractions of the reactants decrease. These changes in mole fractions affect the critical temperatures as noted in the last three columns; both simulated and experimental critical properties are given. Our phase determinations in the reactor with a sight window indicated that the experimental results reported by Ikushima et al.11,12 were in the two-phase region, and the comments by Renslo et al.13 concerning Ikushima et al.’s data were correct. Renslo et al. ran their experiments in the single-phase region by carefully observing the phase behavior through a window when they were running the experiments, and we verified that their data were taken in the single-phase region. However, our observations indicate that, at their reactant concentration, 1.168 mol/L isoprene and 0.584 mol/L methyl acrylate, respectively, the critical temperature of the reaction mixture is much higher than 50 °C, their reaction temperature. At 50 °C and at sufficiently high pressures a single liquid phase forms. Therefore, under their reaction conditions, their system was in the single liquid phase region and not in the supercritical region. To demonstrate the unique effect of pressure on reaction rate and selectivity, the reaction should be observed in the near critical region where the compressibility is significantly high. The pressure dependency of the rate constant is greatest in that region.
Figure 2. Pressure dependency of conversion at 140 °C.
We anticipate that the pressure dependency of the rate constant would be higher than that in the liquid region. Our first set of data were taken at the fixed temperature of 140 °C, at pressures of 95.2-176.9 atm, and at fixed initial reactant concentrations of 0.624 mol/L for isoprene and 0.312 mol/L for methyl acrylate. At these initial concentrations, the expected critical temperature in the reactor is 51-140 °C depending on the initial composition in the reactor. Using the reactor with a sight window, we made sure that all experiments were in the supercritical region. Figure 2 shows the conversion profiles at 140 °C at four different pressures with a repeat run at the lowest pressure. Since the temperature and initial concentrations were constant in all experiments, the change in the rate is due to the pressure effect. This can be explained in terms of the transition-state theory.15,16 A bimolecular reaction is represented by
A + B / M* f products
(1)
where M* is the transition state or the activated complex. The transition state is in equilibrium with the reactants, and the rate of the reaction is the rate of product formation from the transition state, given by
r)
k BT dCA k BT dCP ))κ CM* ) κ K C C (2) dt dt h h C A B
where KC is the concentration-based equilibrium constant, and is given by
CM* 1 Ka ) KC ) CACB F Kγ
(3)
where Ka and Kγ are the equilibrium constants in terms of activity and the activity coefficient, respectively, and F is the molar density of the reaction mixture. Hence, the concentration-based rate constant kC is defined as
1 kB T K a F h Kγ
kC ) κ
(4)
Since Ci ) xiF, where xi is the mole fraction of species i,
4528
Ind. Eng. Chem. Res., Vol. 38, No. 12, 1999
The pressure dependency of the rate constant kx is given by
(
)
∂ ln kx ∂P
Figure 3. Second-order fits for the rate constant kx at 140 °C.
Figure 4. Pressure dependency of the rate constant kx at 140 °C.
expressing the reaction rate in terms of mole fractions of the species rather than the concentration yields
r ) kCCACB ) k B T Ka 1 kB T K a CACB ) κF x x (5) F h Kγ h Kγ A B
κ
Expressing the rate in terms of the mole fractions
rx )
dxA kBT Ka r ))κ x x ) kxxAxB F dt h Kγ A B
(6)
where rx and kx are the mole fraction-based rate and rate constant, respectively, which have units of (time)-1. Expressing mole fractions in terms of conversion x and integration of eq 6 yields
(
)
1 - (xAo/xBo)x 1 ln k xt ) o 1-x xB - xAo
(7)
where xAo and xBo are the initial mole fractions of the limiting reactant and the second reactant, respectively. This equation was fit to the data given in Figure 2, and the fits are shown in Figure 3. Both data sets for the repeat run at 95.2 atm were fit together. Figure 4 shows how the rate constant kx decreases with pressure. The highest reaction rate constant is observed at 95.2 atm when the reaction temperature is in the vicinity of the critical temperature of the mixture in the reactor. The rate constant is reduced by a factor of 3 as the pressure is increased from 95.2 to 176.9 atm. The decrease in the rate constant is rapid until about 120 atm of pressure, and then the rate constant levels off. There is not much difference in the rate constants determined at 136.1 and 176.9 atm.
T
)-
V h M* - V hA - V hB ∆V* )RT RT
(8)
where ∆V* is the activation volume, or the difference between the partial molar volume of the activated complex M* and the partial molar volumes of the reactants A and B. The partial molar volumes of the components become very large and negative in the vicinity of the critical point. However, their difference, the activation volume, can have a positive or negative value depending on the relative magnitude of the partial molar volumes of the species involved. On the basis of the transition-state theory, if the activation volume is positive, then the reaction will be hindered by pressure, whereas if the activation volume is negative, then the reaction will be enhanced by pressure. In the literature there are examples of both increasing3,9,17-19 and decreasing20-23 rate constants with pressure. In any case, the reaction rate could be varied significantly by pressure, when it is run under conditions in the vicinity of the critical point if ∆V* is a large positive or negative number. Our results indicate that the isoprene/methyl acrylate Diels-Alder reaction apparently has a positive ∆V* since the reaction rate constant increases with decreasing pressure. However, it should also be noted that the pressure increase is achieved by adding more carbon dioxide in the constant-volume reactor. This results in (1) an increase of the density of the mixture in the reactor and (2) lowering of the critical temperature of the reaction mixture, although the concentrations of the reactants were maintained constant. So the reaction temperature of 140 °C becomes further removed from the critical temperature as the pressure increases by addition of carbon dioxide. Hence, that may also be affecting the reaction rate. Our data indicate that the rate constant is affected by slight variations in pressure in the supercritical region close to the critical point. Only when the reactions are run in the vicinity of the critical point (95.2136.1 atm at 140 °C), the rate constant rapidly changes with pressure from 0.95 to 0.31 L/(mol h). At higher pressures the rate constant levels off. We have shown that it is very important to determine the phase behavior of the reaction mixture correctly to observe this effect. Unfortunately, many literature data were obtained without carefully determining the phase behavior of the reaction mixture. Paulaitis and Alexander9 studied Diels-Alder reaction in scCO2, but they did not specify the concentration of the reactants. Therefore, no conclusion can be made on the phase behaviors of their reactions. On the other hand, many studies were carried out in the nonsupercritical region. Table 2 summarizes some of the data from the literature on Diels-Alder reactions in scCO2 where sufficient information is given concerning the reactant concentrations and experimental conditions, enabling calculation of the critical properties. The reported reaction conditions and experimental observations are given together with the critical properties calculated by the group contribution method.14 In Table 2, again the concentrations are given both in moles per liter and as mole fraction. We have already mentioned above the data of Ikushima et al.11 and Renslo et al.13 Renslo et al. (1997) ran their experiments in the single liquid phase region. Their results did not indicate a rate or selectivity change with pressure varied
Ind. Eng. Chem. Res., Vol. 38, No. 12, 1999 4529 Table 2. Literature Data, Experimental Conditions, and Simulated Critical Properties ref
C1 ((mol/L)/x1)
11 1.1/0.177 13 1.17/0.131 1.17/0.081 6 1.51/0.110 1.51/0.073 1.510.067 24 0.444/0.0751 0.444/0.0216 25 0.101/0.0148 0.101/0.0062 0.101/0.0054 0.101/0.0051
C2 ((mol/L)/x2)
C3 ((mol/L)/x3)
0.56/0.09 0.585/0.065 0.585/0.041 0.83/0.061 0.83/0.040 0.83/0.037 0.2427/0.0410 0.2427/0.0118 0.0923/0.0135 0.0923/0.0057 0.0923/0.0050 0.0923/0.0047
4.57/0.733 7.18/0.804 12.61/0.878 11.36/0.829 18.48/0.887 20.25/0.896 5.23/0.8839 19.87/0.9666 6.62/0.9716 16.04/0.9881 18.38/0.9896 19.48/0.9902
reaction reaction Pc (atm) Tc (°C) pressure (bar) temp (°C) (calcd) (calcd) 74.5 95.2 117 100 200 300 70 210 80 110 162 210
in a narrow range (95.2-117 bar). Ikushima et al.11 reported a dramatic change in reaction selectivity, while our calculation shows that the reaction was run in the two-phase region. Hence, observations indicate that selectivity change was an experimental artifact due to sampling. Kim and Johnston6 studied the Diels-Alder reaction between methyl acrylate and cyclopentadiene. They reported that the reaction selectivity changes slightly with pressure. When pressure increases from 100 to 300 bar, the selectivity increases from 2.78 to 2.87. Our group contribution calculations show that, under their reaction conditions, the critical temperature of the reaction mixture is much higher than their reaction temperature of 45 °C. We believe that at their experimental conditions they most probably had a liquid phase. Since their data were taken in the liquid phase, it is not surprising that the pressure effect on the selectivity is very small. Clifford et al.24 also studied the Diels-Alder reaction between cyclopentadiene and methyl acrylate. They reported that the ratios of endo- to exoproduct changed from 2.83 to 2.94 at 35 °C as the pressure changed from 70 to 230 bar. The group contribution calculations of their phase behavior are also shown in Table 2, which indicates that the critical temperature of the reaction mixture is much higher than their reaction temperature of 35 °C, and their experiments were probably in the single liquid phase region as well. In addition, the change in selectivity in both studies is too small and certainly within experimental error in standard kinetic measurements. Weinstein et al.25 studied the Diels-Alder reaction between cyclopentadiene and ethyl acrylate at 38 °C constant temperature and in the pressure range 80-210 bar. The reactants’ concentrations were so low that the reaction mixtures’ critical properties and density were assumed to be equivalent to that of pure CO2. They explained the change in the rate constant in terms of density dependence. The group contribution simulations indicate that their experiments were run in the supercritical region, but the first data set may have been in the liquid phase. They did not report any change in selectivity. We have also studied the reaction at 110 °C but under different phase conditions. At this temperature, by adding different amounts of reactants and carbon dioxide, it is possible to obtain mixtures that exhibit different phase behaviors. Table 3 summarizes the second-order rate constants determined under different conditions such as the two-phase region, the liquid phase and the supercritical phase. First, a supercritical phase mixture was formed at initial concentrations of Cisoprene ) 0.624 mol/L and Cma ) 0.312 mol/L at Tc ) 110 °C and Pc ) 95.2 atm, which is just above the mixture critical point of Tc ) 110 and Pc ) 91.2 atm.
50 50 50 45 45 45 35 35 38 38 38 38
77.4 80.5 83.6 83.3 85.2 85.4 86.2 81.4 83.6 77.9 76.0 75.9
123.2 105.2 80.5 104.6 83.3 79.6 86.7 47.2 49.1 39.1 38.1 37.7
reaction rate and selectivity selectivity does change dramatically no change in reaction rate and selectivity selectivity changes slightly with pressure selectivity changes slightly with pressure rate constant increases with pressure
Table 3. Second-Order Rate Constants at T ) 110 °C and Different Phase Behaviors pressure (atm) 95.2 95.2 176.8
initial concn initial concn of isoprene of methyl (mol/L) acrylate (mol/L) 0.624 1.232 4.078
0.312 0.616 2.039
phase behavior
rate constant kx (h-1)
supercritical gas-liquid liquid
0.2222 0.1471 0.1219
The second reaction was carried in the two-phase region. When the initial concentrations were increased to Cisoprene ) 1.25 mol/L and Cma ) 0625 mol/L, the mixture critical properties were changed and both critical temperature and critical pressure became higher than the previous conditions. Therefore, at T ) 110 °C and P ) 95.2 atm, the reaction mixture exhibits a gas-liquid two-phase behavior. The experimental procedure under this condition was modified. To eliminate sampling errors, the experiments were run for a certain period of time and samples were collected after the reactor was cooled for analysis. Finally, when the initial concentrations were increased to a high level, Cisoprene ) 4.16 mol/L and Cma ) 2.08 mol/L, the critical temperature was much higher than 110 °C; therefore, a single liquid phase mixture was formed when pressure was sufficiently high. As can be observed from Table 3, different phase behaviors resulted in different reaction rate constants. The highest rate constant was in the supercritical region followed by the two-phase mixture and the liquid phase. This result supports the hypothesis that phase behavior plays a vital role in rate. At all experimental conditions we have also determined the product selectivity and observed that the selectivity remains the same under all conditions. This observation is consistent with Renslo et al.’s13 observation. In accordance with the transition-state theory, when two parallel reactions take place (such as isoprene/ methyl acrylate reaction leading to para- and metasubstituted isomers), the selectivity is given by8,23
-RT
(
)
∂ ln(kpara/kmeta) ) ∂P T ∂ ln S -RT ∂P
( )
T
) V*para,M - V*meta,M (9)
where V* values are the partial molar volumes for the activated complex for the reaction to each isomer. These partial molar volumes are closely related to the partial molar volumes of the isomers themselves. Hence, there will be a change in selectivity with pressure if and only if the difference between the partial molar volumes is significant.8 In this reaction it appears that this is not the case, and hence although the rate changes with
4530
Ind. Eng. Chem. Res., Vol. 38, No. 12, 1999
pressure, the selectivity does not. In fact we know of only two studies where a significant change in selectivity was observed. Guo and Akgerman3,8 studied propylene hydroformylation in scCO2 and showed that the selectivity of the linear to branch isomer ratio changes from 1.6 to 2.6 at constant temperature as the pressure increases from 90.6 to 194.1 bar. Similarly Fu¨rstner et al.7 reported on the ring closure metathesis and showed that for a specific diene the large ring was produced in excellent yields at high densities; mainly oligomers were obtained at low densities. Guo and Akgerman8 measured the partial molar volumes of the product isomers and explained their selectivity variation in terms of partial molar volume variation. Fu¨rstner et al.7 attributed the density effect to the compressibility of the supercritical phase. Increasing the density at constant volume leads to an increasing number of inert solvent molecules and mimics the dilution effect in conventional solvents. Acknowledgment This project has been funded by Grants 027TAM0640 and 028TAM2640 in part with Federal Funds as part of the program of the Gulf Coast Hazardous Substance Research Center which is supported under cooperative agreement R815197 with the United States Environmental Protection Agency and in part with funds from the State of Texas as part of the program of the Texas Hazardous Waste Research Center. Literature Cited (1) Subramaniam, B.; McHugh, M. A. Reactions in Supercritical FluidssA Review. Ind. Eng. Chem. Process Des. Dev. 1986, 24, 1. (2) Savage, P. E.; Gopalan, S.; Mizan, T. I.; Martino, C. J.; Brock, E. E. Reactions at Supercritical Conditions: Applications and Fundamentals. AIChE J. 1995, 41, 1723. (3) Guo, Y.; Akgerman, A. Hydroformylation of Propylene in Supercritical Carbon Dioxide. Ind. Eng. Chem. Res. 1997, 36, 4581. (4) Kim, S.; Johnston, K. P. Molecular Interactions in Dilute Supercritical Fluid Solutions. Ind. Eng. Chem. Res. 1987, 26, 1206. (5) Johnston, K. P.; Haynes, C. Extreme Solvent Effects on Reaction Rate Constants at Supercritical Fluid Conditions. AIChE J. 1987, 33, 2017. (6) Kim, S.; Johnston, K. P. Adjustment of the Selectivity of A Diels-Alder Reaction Network Using Supercritical Fluids. Chem. Eng. Commun. 1988, 63, 49. (7) Fu¨rstner, A.; Koch, D.; Langemann, K.; Leitner, W.; Six, C. Olefin Metathesis in Compressed Carbon Dioxide. Angew. Chem., Int. Ed. Engl. 1997, 36, 2466. (8) Guo, Y.; Akgerman, A. Determination of Selectivity for Parallel Reactions in Supercritical Fluids. J. Supercrit. Fluids 1999, 15, 63. (9) Paulaitis, M. E.; Alexander, G. C. Reactions in Supercritical Fluids. A Case Study of the Thermodynamic Solvent Effects on a
Diels-Alder Reaction in Supercritical Carbon Dioxide. Pure Appl. Chem. 1987, 59, 61. (10) Poling, B. E.; Eckert, C. A. A Study of Homogeneous Catalysis by High-Pressure Kinetics. The Mechanism of Catalysis of Diels-Alder Reaction. Ind. Eng. Chem. Fundam. 1972, 11, 451. (11) Ikushima, Y.; Ito, S. A Diels-Alder Reaction in Supercritical Carbon Dioxide Medium. J. Chem. Eng. Jpn. 1990, 23, 96. (12) Ikushima, Y.; Saito, N. Supercritical Carbon Dioxide as Reaction Medium: Examination of Its Solvent Effects in the nearCritical Region. J. Phys. Chem. 1992, 96, 2293. (13) Renslo, A.; Weinstein, R. D.; Tester, J. W.; Danheiser, R. L. Concerning the Regiochemical Course of the Diels-Alder Reaction in Supercritical Carbon Dioxide. J. Org. Chem. 1997, 62, 4530. (14) Li, L.; Kiran, E. Estimation of Critical Properties of Binary Mixtures Using Group Contribution Methods. Chem. Eng. Commun. 1990, 94, 131. (15) Evans, M. G.; Polanyi, M. Some Applications of the Transition State Method to the Calculation of Reaction Velocities, Especially in Solution. Trans. Faraday Soc. 1935, 31, 875. (16) Glasstone, S.; Laidler, K. J.; Eyring, H. The Theory of Rate Process, 1st ed.; McGraw-Hill: New York, 1941. (17) Brennecke, J. F.; Tomasko, D. L.; Eckert, C. A. Naphthalene/ Triethylamine Exciplex and Pyrine Excimer Formation in Supercritical Fluid Solutions. J. Phys. Chem. 1990, 94, 7692. (18) Johnston, K. P.; Haynes, C. Extreme Solvent Effects on reaction rate Constants at Supercritical Fluid Conditions. AIChE J. 1987, 33, 2017. (19) Peck, D. G.; Mehta, A. J.; Johnston, K. P. Pressure Tuning of Chemical Reaction Equilibria in Supercritical Fluids. J. Phys. Chem. 1989, 93, 4297. (20) Randolph, T. W.; Carlier, C. Free Radical reactions in Supercritical Ethane: A Probe of Supercritical Fluid Structure. J. Phys. Chem. 1992, 96, 5146. (21) Chateauneuf, J. E.; Roberts, C. B.; Brennecke, J. F. Laser Flash Photolysis Studies of Benzophenone in Supercritical Carbon Dioxide. ACS Symp. Ser. 1992, 488, 106. (22) Penninger, J. M. L.; Kolmschate, J. M. M. Chemistry of Methoxynaphthalene in Supercritical Water. ACS Symp. Ser. 1989, 406, 242. (23) Yoshimura, Y.; Kimura, Y. Solvent Density Dependence of the Unimolecular Reaction Rate: Dissociation Reaction of 2-Methyl-2-Nitrosopropane Dimer in Carbon Dioxide from Gas to Liquid States. Chem. Phys. Lett. 1991, 181, 517. (24) Clifford, A. A.; Pople, K.; Gaskill, W. J.; Bartle, K. D.; Rayner, C. M. Potential Tuning and Reaction Control in the DielsAlder Reaction Between Cyclopentadiene and Methyl Acrylate in Supercritical Carbon Dioxide. J. Chem. Soc., Faraday Trans. 1998, 94, 1451. (25) Weinstein, R. D.; Renslo, A. R.; Danheiser, R. L.; Harris, J. G.; Tester, J. W. Kinetic Correlation of Diels-Alder Reactions in Supercritical Carbon Dioxide. J. Phys. Chem. 1996, 100, 12337.
Received for review February 18, 1999 Revised manuscript received August 2, 1999 Accepted August 8, 1999 IE990120J
