1146
I n d . Eng. Chem. Res. 1989, 28, 1146-1152
Allara, D. L.; Shaw, R. A Compilation of Kinetic Parameters for the Thermal Degradation of n-Alkane Molecules. J . Phys. Chem. Ref. Data 1980, 9(3), 523-559. Crabtree, R. H.; Brown, S. H. US Patent 4,725,342, 1988. Davis, W. The Gas-Phase Photochemical Decomposition of the Simple Aliphatic Ketones. Chem. Rev. 1947, 40, 201-250. Echols, L. S.; Pease, R. N. Kinetics of the Decomposition of n-Butane. I. Normal Decompositon. J . Am. Chem. SOC.1939, 61, 208-212. Hansen, D. A.; Lee, E. K. C. Radiative and Nonradiative Transitions in the First Excited Singlet State of Symmetrical Methyl-Substituted Acetones. J . Chem. SOC.Phys. 1975, 62(1), 183-189. Hunjan, M. S.Photolytic Formation of Free Radicals and Their Effect on Hydrocarbon Chemistry in a Concentrated Solar Environment. M.S. Thesis, University of Hawaii at Manoa, 1987. IMSL, Inc. 2500 Park West Tower One, 2500 City West Blvd., Houston, TX, 77042-3020, 1987. Mok, W.; Hunjan, M.; Antal, M. J.; Evans, R. J.; Milne, T. A. Direct Observation by Molecular Beam Mass Spectrometry of the Photolytic Enhancement of Hydrocarbon Reactions by Concentrated Sunlight. Energy 1987, 12(3-4), 283-290. Noyes, W.; Leighton, P. A. The Photochemistry of Gases; Reinhold: New York, 1941.
Noyes, W. A.; Porter, G . B.; Jolley, J. E. The Primary Photochemical Process in Simple Ketones. Chem. Reu. 1956, 56, 49-94. Pitts, J. M. Relations Between Molecular Structure and Photodecomposition Modes. J. Chem. Ed. 1957, 34(3), 112-121. Sagert, N. H.; Laidler, K. J. Kinetics and Mechanisms of the Pyrolysis of n-Butane. Can. J. Chem. 1963, 41, 838-857. Smith, J. 0.; Taylor, H. S. The Reactions of Methyl Radicals with Deuterium, Ethane, Neopentane, Butane and Isobutane. J. Chem. Phys. 1939, 7 , 390-396. Smith, J. 0.;Taylor, H. S. The Reactions of Methyl Radicals with Benzene, Toluene, Diphenyl Methane and Propylene. J . Chem. P h y ~ 1940, . 8, 543-546. Steacie, E. W. R. Atomic and Free Radical Reactions; Reinhold: New York, 1946. Taylor, H. S.; Jungers, J. C. The Polymerization of Ethylene and Acetylene Photosensitized by Acetone. Trans Faraday Soc., 1937, 33, 1353-1360. Trotman-Dickenson, A. F.; Steacie, E. W. R. The Reactions of Methyl Radicals. 111. The Abstraction of Hydrogen Atoms from Olefins. J . Chem. Phys. 1951, 19, 169-171.
Received for review October 21, 1988 Accepted May 3, 1989
Solvent and Catalytic Metal Effects on the Decomposition of Cumene Hydroperoxide Galen J. Suppes and Mark A. McHugh* Department of Chemical Engineering, T h e Johns Hopkins University, Baltimore, Maryland 21218
Kinetic information is presented on the decomposition of cumene hydroperoxide at 110 " C and 60-380 bar in supercritical krypton, xenon, carbon dioxide, propane, and chlorodifluoromethane and a t ambient pressure in liquid octane, l-octene, l-hexanol, and cyclohexanol. The supercritical reaction studies are performed with a high-pressure reactor constructed of 316 stainless steel (316SS), gold-plated 316SS, and Teflon-coated 31621s. The magnitude of the reaction rate constant is a strong function of the metals present during the reaction and of the ability of the solvent to hydrogen bond or complex with the activated complex of cumene hydroperoxide, and it is a weak function of hydrostatic pressure and solution viscosity over the pressure ranges investigated. Gold, 316SS, and aluminum facilitate the formation of geminate radicals, while gold also affects the selectivity of the reaction. Transition-state theory is used to interpret the reaction data. The focus of the present study is to investigate the effect of solvent properties, pressure, and metals on the reaction mechanisms for the unimolecular, free-radical decomposition of cumene hydroperoxide. By concentrating on an elementary decomposition reaction in an array of supercritical and liquid solvents, certain conclusions can be made about reaction engineering with supercritical solvents. We chose to study the decomposition of cumene hydroperoxide because of its half-life characteristics at temperatures in the vicinity of 100 "C (Casemier et al., 1973; Kharasch et al., 1951), its solubility in various solvents, its direct application as an initiator in the partial oxidation of cumene (Occhiogrosso and McHugh, 1987; Occhiogrosso, 1987),and the amount of available literature information to which our study could be compared (Casemier et al., 1973; Kharasch et al., 1951). The present study adds to the current body of information on reactions in supercritical fluid media (Kramer and Leder, 1975; Alexander and Paulaitis, 1984; Subramaniam and McHugh, 1986; Kim and Johnston, 1987; Johnston and Haynes, 1987; Occhiogrosso and McHugh, 1987; Tiltscher and Hofmann, 1987; Dooley and Knopf, 1987; Townsend et al., 1988).
* Author to whom correspondence should be addressed. 0888-5885/89/2628-l146$01.50/0
Scheme I. Reaction Mechanism for the Homolytic Scission of Cumene Hydroperoxideo homolytic scission ROOH
solvent
[RO"OH]
solvent cage effects ROOH + [RO"OH]
-
RO'
+ 'OH
6-scission or hydrogen abstraction
RO' RO'
-+
R1=O
+ RBH
+
+ R2'
ROH
+ RB'
Supercritical solvents as reaction media have certain advantages not normally associated with liquid solvents. In a supercritical solvent, it is possible to devise novel product separation schemes and, in certain cases, to reduce catalyst deactivation (Tiltscher and Hofmann, 1987; McHugh and Krukonis, 1986). Certain physicochemical properties of supercritical solvents allow easy manipulation of the reaction environment and possibly in situ catalyst regeneration. Scheme I shows that the homolytic decomposition of cumene hydroperoxide is influenced by the reaction solvent and the metals present in the reactor. The cumyloxy 0 1989 American Chemical Society
Ind. Eng. Chem. Res., Vol. 28, No. 8, 1989 1147 Table I. Physical Properties at Reaction Conditions for the Five Supercritical and Four Liquid Solvents Used as Reaction Media for the Decomposition of Cumene Hydroperoxidea solvent Tc,bOC Pc,bbar Tr pr p : lo3 mol/cm3 7, lo4 g/(cm.s) w,d D krypton -63.8 54.7 1.83 3.8-6.3 7.3-11.5 5.5-9.1' 0.0 .. xenon 16.6 58.6 1.32 2.6-5.9 7.5-13.2 6.2-1 1.3e 0.0
co2
CFM propane octane 1-octene 1-hexanol cyclohexanol
31.1 96.1 96.7 295.7 293.5 336.9 351.9
73.9 49.0 42.2 24.5 25.9 40.0 37.0
1.25 1.04 1.04 0.7 0.7 0.6 0.6
2.3-5.1 3.5 2.4-4.7 0.04 0.04 0.03 0.03
6.8-16.6 11.9d 8.6-9.8 5.6 5.8
4.0-7.9' 9.40 6.3-7.7e 24.4' 22.9' 68.8*
0.0 1.4 0.0 0.0 0.3 1.7 1.8
OThe properties of krypton, xenon, chlorodifluoromethane (CFM), and propane are determined at 110 OC and the pressures listed in the table. The properties of octane, 1-octene, 1-hexanol,and cyclohexanol are determined at 100 "C and 1 bar. bReid et al., 1977. cVargaftik, 1975. McClellan, 1963. e Bird et al., 1960.
radical produced from the decomposition either undergoes (?-scission to form acetophenone or undergoes hydrogen abstraction to form cumyl alcohol. A modest increase in the reaction rate is expected in polar reaction solvents since the activated complex of cumene hydroperoxide is slightly more polar than the parent compound (Reichardt, 1979). Reaction rates should also increase if the viscosity of the solvent decreases since the geminate radicals can more easily diffuse from their solvent cage (Neuman and Behar, 1971). Furthermore, the decomposition rate can be influenced by certain metal catalysts (Bulatov et al., 1985; Casemier et al., 1973; van Ham et al., 1971). In this study, the reactor was constructed of 316 stainless steel (316SS), gold-plated 316SS, and Teflon-coated 316SS. The effect of these metals and aluminum on the rates and selectivities will be discussed. The kinetics of the decomposition of cumene hydroperoxide is studied in supercritical krypton, xenon, carbon dioxide, propane, and chlorodifluoromethane and in liquid octane, 1-octene, 1-hexanol, and cyclohexanol. Table I lists the critical properties, dipole moments, densities, and viscosities of the solvents at reaction temperatures and pressures. The reaction conditions with the supercritical fluids are 110 "C, pressures from 60 to 350 bar, and concentrations of cumene hydroperoxide near 1.0 mol %. Also, in certain cases, approximately 1.3 mol % styrene or cumene is added as a scavenger to minimize backbiting of the parent cumene hydroperoxide (Swain et al., 1950). The liquid-phase reactions are performed a t 100 "C without any scavenger. Except for propane and COS, solvent densities are quite similar for both the supercritical and the liquid solvents. Although the range of solvent viscosities was from 0.05 to 0.70 cP, the range as a function of pressure for a given supercritical solvent varied by less than a factor of 2. The actual pressure range in each case is set by phase-equilibrium considerations-that is, the minimum pressure investigated was high enough to ensure that a single phase was present during a run.
Experimental Section The experimental apparatus and procedures used for the high-pressure reactions are very similar to those reported by Occhiogrosso and McHugh (1987) and, therefore, are only briefly described. The volume of the high-pressure reactor (5.1-cm 0.d. X 1.9-cm i.d.) shown in Figure 1 can be varied between approximately 5 and 40 cm3 by displacing an internal piston which communicates with a pressure generator filled with hydraulic fluid. The system pressure, measured with a bourdon-tube heise gauge, can be maintained constant within f1.4 bar. A Pyrex window, secured at one end of the reactor with a threaded end cap, allows for visual verification that only one phase is present during the course of the reaction. The reaction mixture is well mixed with a glass-coated stirring bar driven by a
PRESSURE GAUGE
5
r-----1
2
I
L
I I
I I
n / Y OVEN
d)Y
I I
I
rs1
OPTICS
READOUT
IJ
-
u
Figure 1. Schematic diagram of the experimental apparatus used in this study to obtain kinetic information.
magnet located below the reactor. A relatively constant area for catalysis is provided by the internal surface of the reactor which was either 316 stainless steel (316SS) or gold-plated 316SS. The ratio of volume to surface area is 0.48 cm for these reactions. To obtain information on the homogeneous reaction a t high pressures, the reactor was coated with a 0.002-cm-thick film of a high-temperature fluoropolymer (ISPA Company, Teflon S958-203). Since it was desirable to work with a large stock solution and since the rate of decomposition is minimal at room temperature, a premixed solution is quickly transferred from a large, variable-volume cell at room temperature to the heated reactor housed in a forced-convection oven. Within 5 min, the reaction mixture reaches the desired operating temperature, maintained constant to within f0.1 "C as measured with a three-wire platinum RTD. Single-phase samples are obtained by displacing 0.5 cm3 of the reaction mixture into a sample loop connected to the reactor. The system pressure is maintained constant by displacing the piston to compensate for the material removed from the cell. The sample is slowly vented into chilled acetonitrile to trap the relatively nonvolatile reactants and products while allowing the volatile solvent to escape. The total time for sampling is about 3 min. High-performance liquid chromatography is used to analyze the sample. An internal standard of approximately 0.05 mol % propiophenone is added to the premixed feed solution to improve the accuracy of the analysis. This small amount of propiophenone, which is chemically similar to acetophenone, one of the products of the decomposition reaction, is not expected to affect the rate of reaction.
1148 Ind. Eng. Chem. Res., Vol. 28, No. 8, 1989 x 0
A
CUMENE HYDROPEROXIDE CUMYL ALCOHOL ACETOPHENONE
0 O .';f 8
x
0
0
02
04
06
08
10
12
14
Time ( m i n Figure 2. Example of the fit of the Himmelblau-Jones-Bischoff method (Froment and Bischoff, 1979) to the decomposition data obtained in krypton at 110 "C and 345 bar in the gold-plated reactor. In this case, the profile for phenol is left off the graph since only a very small amount is produced. The reaction profile is defined as the number of moles of i at any time divided by the initial number of moles.
Although several of the liquid-phase reactions were run in the variable-volume reactor, most of these reactions were run in Pyrex test tubes. For these liquid-phase reactions, catalyst was supplied in the form of 316SS balls, gold powder, and aluminum foil. Interpretation of Conversion-Time Data. Kharasch and co-workers (1951) have shown that the decomposition of cumene hydroperoxide (CHP) follows a first-order mechanism In (C/Co) = k,t
(1)
where Co is the concentration at the start of the reaction. It should be noted that the volume change on reaction is extremely small for the high-pressure reactions in our study. The Himmelblau-Jones-Bischoff (HJB) method (Froment and Bischoff, 1979) is used to analyze the data to obtain first-order rate constants and selectivity information since this method utilizes all of the available data. Rate expressions for CHP, phenol, cumyl alcohol (CUAL), and acetophenone (ACP) are derived using SI,which is the selectivity to phenol, Sz, which is the selectivity to CUAL, and SB,which is the selectivity to ACP (Suppes, 1988): CHP CHP CHP
S A
phenol
(24
cumyl alcohol
(2b)
acetophenone
(2c)
SZk,
S3k1
Figure 2 shows the good fit of the rate data with the HJB method for this system. The reaction profile for phenol is not shown in this figure since its concentration is extremely low. The rate constants reported here are averaged values of at least three data points with an estimated error of *lo%. They are reproducible to within f15%.
Results Table I1 shows the observed rate constants and the calculated catalytic rate constants for cumene hydroperoxide (CHP) decomposition in supercritical krypton, xenon, carbon dioxide, propane, and chlorodifluoromethane (CFM) at 110 "C and 345 bar in the presence of a scavenger. The rate constants obtained in the Teflon-coated reactor are assumed to represent homogeneous-phase rate constants. The catalytic rate constants are obtained by
Table 11. Decomposition Rate Constants for Cumene Hydroperoxide Obtained in Krypton, Xenon, COz, Propane, and Chlorodifluoromethane at 110 "C and 345 bar with Approximately 1.3 mol % Scavenger Added to the Reaction Mixture 104k,(apparent), 104k,(catalytic), solvent reactor min-I cm min-' krypton gold-plated 25.9 3.2 316SS 7.6 35.1 Teflon-coated 19.3 xenon gold-plated 12.4 2.3 14.0 3.1 316SS Teflon-coated 7.6 C02 gold-plated 8.2 2.8 316SS 12.2 4.8 Teflon-coated 2.3 propane gold-plated 19.7 316SS Teflon-coated CFM gold-plated 65.7 316SS Teflon-coated Table 111. Decomposition Rate Constants for Cumene Hydroperoxide Obtained in COz, Chlorodifluoromethane, and Propane a t 110 OC and 345 b a r without Scavenger Added to the Reaction Mixture 104kl(apparent), 104kl(catalytic), solvent reactor min-l cm min-' C02 gold-plated 19.2 6.8 316SS 11.1 2.9 Teflon-coated 5.0 CFM gold-plated 316SS 24.3 9.7 Teflon-coated 4.1 propane gold-plated 3165% Teflon-coated 1.8
subtracting the apparent rate constant in the Teflon-coated reactor from the apparent rate constant in the metal reactor and multiplying the resultant number by the reactor volume-to-surface ratio. A comparison of the rate constants listed in Table I1 shows that both 316SS and gold are catalytic, with 316SS being just slightly more catalytic than gold. The rate constants obtained in the Teflon-coated reactor show that the decomposition rate is highest in krypton, then xenon, then COB. Interestingly, at 345 bar, the viscosity is lowest in COzand highest in xenon. Evidently, viscosity has only a minor influence on the rate of decomposition at these conditions. Notice, also, that the rate constant in propane in the presence of 3165% is more than 3 times smaller than that in CFM, even though both of these solvents are virtually at the same reduced pressure and temperature. In fact, the density and viscosity of CFM are both slightly higher than those of propane, suggesting that other physical properties of these solvents, such as polarity, have a greater effect on the rate constant. Table I11 lists the observed rate constants and the calculated catalytic rate constants for CHP decomposition in supercritical COB,CFM, and propane at 110 "C and 345 bar when no scavenger is used. For COz,the observed rate constant is slightly larger in gold than in 316SS, a trend opposite to that found when scavenger is used with krypton, xenon, and COB. Although, as expected, the rate constant obtained with the Teflon-coated cell in COZ without scavenger is slightly more than double that when the scavenger is added to the mixture (see Table 11) (Swain et al., 1950), good first-order fits of the rate data are obtained with or without scavenger. Therefore, it is assumed that very little induced decomposition occurs.
Ind. Eng. Chem. Res., Vol. 28, No. 8, 1989 1149 Table IV. Decomposition Rate Constants for Cumene Hydroperoxide Obtained in Liquid Solvents at 100 "C without Scavenger Added to the Mixture l0'k1(apparent), 1O4kl(catalytic), solvent metal present min-' cm mi& 18.0 8.5 octane gold-plated reactor 0.2 0.6 316SS 1.1 2.4 aluminum 0.3 Pyrex reactor 153 1-octene gold powder 10.2 316SS 3.6 Pyrex reactor 2.2 1-hexanol gold powder 314 61 177 316SS 48.2 Pyrex reactor 5214 cyclohexanol gold powder 337 758 gold-plated reactor 316SS 180 431 a1umin um 102 260 55.5 Pyrex reactor 52.4 Pyrex + glass beads
Table I11 also shows that similar rate constants are obtained with the Teflon-coated cell using COz and CFM even though each of these solvents are at very different reduced conditions. However, both of these solvents are polar; CFM has a dipole moment and carbon dioxide has a quadrapole moment. In addition, the rate constants in CFM are more than twice those in propane, even though these two solvents are at virtually identical reduced conditions. The similarity in rate constants for the polar supercritical fluids may be a manifestation of polar interactions which stabilize the CHP-activated complex more than the slightly less polar parent compound (Reichardt, 1979; Troe, 1986; Johnston and Haynes, 1987; Niki et al., 1969; Hendry and Russell, 1964; Swain et al., 1950; Spirin, 1969). Table IV compares the observed and the calculated catalytic rate constants for CHP decomposition in liquid octane, 1-octene, 1-hexanol, and cyclohexanol at atmospheric pressure. With cyclohexanol, the observed rate constants are nearly equal in the Pyrex reactor with or without glass beads. Therefore, the rate constants obtained in the Pyrex vessel are assumed to represent the homogeneous-phase rate constants. The rate constant reported for 1-octene is in good agreement with the temperature-corrected rate value reported by Kharasch et al. (1951). The observed homolytic rate constant obtained in liquid octane at 100 "C in the Pyrex reactor can be compared to that obtained in supercritical propane in the Teflon-coated reactor at 110 "C if the rate constant in octane is double from 0.3 X to 0.6 X min-' (Stannett and Mesrobian, 1950). The rate constant at 110 "C in propane is 3 times greater than that in octane. This increase in rate constant is directly proportional to the reduced viscosity of propane relative to liquid octane (see Table I). However, the temperature-adjusted rate constant for 1-octene in the Pyrex reactor is greater than that of propane and is similar to those obtained in the Teflon-coated reactor with CFM and COz even though these supercritical solvents are less viscous than liquid octene. The larger rate constant in octene is probably a consequence of the specific interactions between the cumyloxy radical (ROO) and the .rr electrons of the double bond in octene (Reichardt, 1979; Niki et al., 1969; Hendry and Russell, 1964). The large values of &(apparent) for hexanol and cyclohexanol obtained in the Pyrex vessel can be explained by assuming
10 HOMOGENEOUS
0.8-
-Q)
-
b ..
0.6c
0
t
04-
0
E
o
, 0
,
2
,
,
, 4
,
, 6
,
S
,
,
IO
,
,
12
,
,
14
,
Time ( m i n ) Figure 3. Effect of different metals on the decompositonof cumene hydroperoxide in liquid cyclohexanol at 100 "C. Only the cumene hydroperoxide reaction profile is shown in this figure. The reaction profile is defined as the number of moles of cumene hydroperoxide at any time divided by the initial number of moles.
Table V. Selectivities to Cumyl Alcohol and Acetophenone Relative to Cumene Hydroperoxide as a Function of Liquid Solvent and Reactor Surface selectivity ACp, solvent metalpresent CUAL ACP CUAL
