2250
WILLIAMF. TAYLOR
Catalysis in Liquid Phase Autoxidation. 11. Kinetics of the Poly (tetrafluoroethy1ene)-Catalyzed Oxidation of Tetralin
by William F. Taylor Esso Research and Engineering Company, Government Research Laboratory, Linden, New Jersey (Received September 66,1969)
A study was made of the kinetics of the poly(tetrafluoroethy1ene)-catalyzed oxidation of tetralin at 65-115". The catalyzed decomposition of tetralin hydroperoxide was also studied. The initial rate of the catalyzed oxidation of tetralin was second order in tetralin, zero order in oxygen pressure, half-order in catalyst weight and exhibited an apparent activation energy of 17.0 kcal/mol. Poly(tetrafluoroethy1ene) catalyzed the decomposition of tetralin hydroperoxide (11). This decomposition exhibited a 16.3-kcal/mol apparent activation energy and was first order in concentration and catalyst weight. The catalyzed oxidation of tetralin involves a sequence of reactions in which tetralin hydroperoxide (11) is first formed, followed by decomposition of I1 to produce equal molar quantities of the ketone (111) and the alcohol (IV); and ultimately a conversion of the alcohol (IV), first into 1,2-dihydronaphthalene(V) and then into naphthalene (VI), Independent experiments indicated that the intermediate compounds IV, V, and VI exerted no measurable influence on the catalyzed oxidation of tetralin; whereas the ketone (111) accelerated this rate. The results suggest that the principal function of the catalyst is to initiate the conventional oxidation of tetralin by generating free radicals from the hydroperoxide. The observed kinetics are discussed in terms of a proposed mechanism.
Introduction
It is well known from extensive studies that homogeneous metals can catalyze the liquid phase oxidation of a compound such as tetralin. Less extensive studies have shown that hetergeneous metal oxides are also capable of catalyzing liquid phase hydrocarbon autoxidation reactionsld6 including the oxidation of tetralin,4t6 More r e ~ e n t l y , ~itJ has been shown that polymeric surfaces also influence liquid phase autoxidation reactions. Although no detailed kinetics were reported, it was found that low-surface energy solids such as poly(tetrafluoroethy1ene) and polypropylene are surprisingly active catalysts for the oxidation of tetralin.6 I n order to further elucidate the effect of polymeric surfaces in autoxidation reactions, the kinetics of the poly(tetrafluoroethy1ene)-catalyzedoxidation of tetralin have been studied in detail. Reported in the present work are studies of both the catalyzed oxidation of tetralin in chlorobenzene over the range of 65-115" and also the catalyzed decomposition of tetralin hydroperoxide in chlorobenzene under a Nz atmosphere. Because of the importance of specific activity in studies of this nature,s-'O the influence of poly(tetrafluoroethy1ene) surface in the reaction system was studied by varying the weight of catalyst in the reactor at otherwise constant conditions. I n addition, particular emphasis was placed on following the secondary reactions which occur in the oxidation of tetralin and the decomposition of tetralin hydroperoxide as a function of reaction time, which previously has not been done. The Journal of Physical Chemistry, Vol. 74s No. 11, 1970
Experimental Section Apparatus and Procedure. A conventional liquid phase autoxidation apparatus was used. l1 The rate of oxidation was measured at atmospheric pressure. The reactor was immersed in a temperature bath controlled to 0.5". Mass transfer limitations were minimized by vibrating the reactor at a speed of 800 cpm. Experiments demonstrated that the rate of oxidation was independent of vibration speed a t this level. I n each run the tetralin, chlorobenzene, and a fresh charge of catalyst were added to the reactor and hooked into the reactionsystem; the reactants were degassed by repeated vacuum freeze-thaw cycles using liquid Nz. The reactor was then filled with O2 and immersed in the temperature-controlled bath. Aliquot samples for analy(1) J. Burger, C. Meyer, and J. C. Balaceanu, C. R. Acad. Sei.,252, 2235 (1961). (2) I. I. Ioffe, N. V. Klimova, and I. Ya. Mokrovsova, Dokl. Acad. Nauk. SSSR,169, 389 (1966). (3) N. V. Klimova and I. I. Ioffe, Kinet. Katal., 8, 565 (1967). (4) A . Mukherjee and W. F. Graydon, J . Fhya. Chem., 71, 4232 (1967) (5) P. George, Trans. Faraday Soc., 42, 210 (1946). (6) W. F. Taylor, J . Catal., 16, 20 (1970). (7) N. M. Emanuel, "Present State of the Theory of Chain Reactions in the Liquid Phase Oxidation of Hydrocarbons," Preprints, 7th World Petroleum Congress, Mexico City, 1967. (8) W. F. Taylor, J. H. Sinfelt, and D. 6 . C. Yates, J . Phya. Chem., 69, 3857 (1966). (9) W. F. Taylor and H. K. Staffin, ibid., 71, 3314 (1967). (IO) D. J. C. Yates, W. F. Taylor, and J. H. Sinfelt, J . Amer. Chem. SOC.,86, 2996 (1984). (11) J. L. Bolland, Proc. Roy. Soe., Ser. A , 186, 220 (1946). I
* c 2251
POLY (TETRAFLUOROETHYLENE)-~ATALYZEDOXIDATION OF TETRALIN sis during the run were removed via a syringe through a septum cap attached to a side arm on the reactor. Analyses were made using an F & A!t Model 500 temperature-programmed gas chromatograph with a 2f t column containing 10% QF-1 (fluoronated silicone oil) on 45/60 mesh Chromosorb W. The column was programmed from 50 to 175" a t 7.9"/min. A PerkinElmer Model 154 gas chromatograph was used for gas analyses with a 5-ft column containing 13X molecular sieve a t 35". Infrared analyses of liquid samples were obtained on a Perkin-Elmer Model 137 NaCl spectrophotometer. R/Iass spectrometric analysis was carried out in a CEC Model 21-103 mass spectrometer.12 The tetralin hydroperoxide content was determined chemically by the method of Wagner, et al.I3 Reagents. Tetralin (Matheson Coleman and Bell, practical grade) was purified by washing with concentrated sulfuric acid until the washings were colorless, followed by washing with distilled water to remove residual acidity and drying with anhydrous MgS04.4 The resulting tetralin was then repeatedly distilled in a spinning band column until a fraction was obtained which showed no impurities by glpc analyses. Chemical analyses confirmed the tetralin was free of hydroperoxide. l 3 Tetralin hydroperoxide was prepared by the method of Woodward and M e ~ r o b i a nusing ~ ~ purified tetralin and dry air. Product purity was verified by chemical titration. Glpc pure chlorobenzene (Matheson Coleman and Bell) was employed. Naphthalene was obtained from Eastman Organic Chemicals Dept., Eastman Kodak Co., Rochester, N. Y. Glpc analyses showed no detectable impurities. The 1,Zdihydronaphthalene was obtained from Aldrich Chemical Co., Milwaukee, Wis. Glpc analyses indicated it contained 5% 1,4-dihydronaphthalene. Glpc pure 1,2,3,4-tetrahydro-1-naphthol and 3,4-dihydro-l(2H)-naphthalenone were obtained from Aldrich Chemical Co. The poly(tetrafluoroethy1ene) was obtained from the E. I. du Pont de n'emours & Co. Wilmington, Del., and prepared as a 60/80 mesh powder using the method previously described.6 The catalyst had a BET surface area of 0.69 m2/g.
Results A study was first made of the effect of various kinetic parameters on the oxidation of tetralin in the presence Higher temperatures of poly(tetrafluoroethy1ene). both increased the initial oxidation rate and reduced the length of the induction period. An Arrhenius plot of the initial reaction rate, which is shown in Figure 1, yielded an apparent activation energy of 17.0 kcal/mol. A similar plot of the reciprocal of the induction period yielded a temperature dependence equivalent to 16.7 kcal/moL6 The effect of tetralin concentration, oxygen pressure, and catalyst weight in the reaction system was next studied. Results are summarized in Table I. An examination of these re-
5 x 10-2
c? 0
: w
x 10-2
Table I: Relative Rates of Tetralin Oxidation As a Function of Tetralin Concentration, Oxygen Pressure, and Catalyst Loading [TetralinI, M
Oxygen press, Torr
Catdyat loading,
3.68 3.68 3.68 3.68
660 710 760 810
0.30 0.30 0.30 0.30
1.1 1.0 1 . 0 0 (base)b 1.0
3.68 3.68 3.68
760 760 760
0.15 0.30 0.60
0.68 1.00 (base). 1.34
2.45 3.68 4.91
760 760 760
0.30 0.30 0.30
0.44 1.00 (base)d 1.86
g
Relative initial rate of oxidn, T/T#
Initial rate of oxidation in terms of moles per hour of tetralin converted at given conditions relative to initial rate at standard conditions as shown. b study conducted at 65" with 2 ml of tetralin in 2 ml of chlorobenzene. Study conducted at 90" with 2 ml of tetralin in 2 ml of chlorobenzene. d Study conducted at 90" with, respectively, 1, 2, and 4 ml of tetralin in 2 ml of chlorobenzene.
sults indicated that the initial rate of reaction is second order in tetralin concentration, zero order in oxygen pressure, and approximately half-order in catalyst (12) W. F. Taylor, J. M . Kelliher, and T. J. Wallace, Chem. Ind. (London), 651 (1968). (13) C.D.Wagner, R. H. Smith, and E. D. Peters, Anal. Chem., 19, 976 (1947). (14) A. E.Woodward and R. B. Mesrobian, J . Amer. Chem. Soc., 7 5 , 6189 (1963).
The Journal of Physical Chemistry, Vol. 74, No. 11, 1970
WILLIAMF. TAYLOR
2252
I
I
I
I
1
I
\ TIME
- MINUTES
Figure 2. Second-order rate plot for the catalyzed oxidation of tetralin in chlorobenzene at 90" and 1 atm pressure using 0.30 g of catalyst: H , 2.45 M tetralin; 0 , 3.68 M ; A, 4.91 M.
level. To confirm the second-order dependence on tetralin concentration, an integral second-order rate plot (1/C - l/Co vs. time where C is the concentration of tetralin in M a t time t and Co is the initial concentration) was prepared, which is shown in Figure 2. As can be seen, it confirms the conclusion reached from an examination of the initial rate data. A study was also made of the poly(tetrafluoroethy1ene)-catalyzed decomposition of tetralin hydroperoxide in chlorobenzene under nitrogen over the range of 90115" at atmospheric pressure. The effect of tetralin hydroperoxide concentration and catalyst weight was also studied. Results are summarized in Table 11. An examination of these results indicated that the catalyzed decomposition is a first-order process, in Table 11: Relative Rates of Tetralin Hydroperoxide Decomposition As a Function of Tetralin Hydroperoxide Concentration and Catalyst Loading" Tetralin hydroperoxide concn, wt %
Catalyst loading, g
4.37 8.30
0.30 0.30 0.30
0.58b 1.00 (base)* 1.92b
0.15 0.30 0.60
0.47" 1 . 0 0 (base). 2.21Q
15.3
8.30 8.30 8.30
Relative rate of decompn, r/TQ
105', tetralin hydroperoxide added to 4 cma a Conditions: of chlorobenzene solvent, 1 atm Nz. * Relative rates calculated from initial rate of tetralin hydroperoxide conversion in moles per hour. 0 Relative rates calculated from the initial observed first-order rate constants corrected for the thermal (noncatalytic) contribution a t the same conditions. The Journal of Physical Che?nistTy,Vol. 7& No. 11, 1970
I
I
I
1 200
100
I
300
TIME, MINUTES
Figure 3. First-order rate plot for the catalyzed decomposition of tetralin hydroperoxide in 4 cms of chlorobenzene under 1 atm Nz using 0.30 g of catalyst at 105': H , 4.37 8.30 wt %; 0 16.3 wt 70tetralin hydroperoxide. wt %;
*,
concentration, as has been reported for the uncatalyzed decomposition.l6*l8 An integral first-order rate plot is shown in Figure 3, which confirms the conclusion reached from the initial rate data. The catalyzed decomposition of tetralin hydroperoxide was also found t o be first order in catalyst weight. An Arrhenius plot of the first-order rate constants for the poly(tetrafluoroethy1ene)-catalyzed decomposition of tetralin hydroperoxide is shown in Figure 4. This plot yielded an apparent activation energy of 16.3 kcal/mol. The rate of the uncatalyzed decomposition of tetralin hydroperoxide was measured a t 115" a t the identical conditions employed in the catalyzed decomposition measurements. The uncatalyzed decomposition was slower than the catalyzed decomposition and yielded a first-order decomposition rate constant which was only 61% of the catalyzed value. Aliquot liquid and gas samples were taken during both the catalyzed oxidation of tetralin studies and the catalyzed tetralin hydroperoxide decomposition studies so as to quantitatively determine the major reaction products. Analytical techniques included quantitative chemical titration for tetralin hydroperoxide content, gas chromatographic, infrared and mass spectrometer analyses of liquid samples and gas chromatographic analyses of gas samples. Details are given in the Experimental Section. I n Figure 5 are shown the change in reactants and liquid products in(15) A. Robertson and W. A . Waters, J. Chem. Soc., 1578 (1948). (16) J. R.Thomas, J. Arne?-. Chem. Soc., 77, 246 (1955).
2253
POLY (TETRAFLUOROETHYLENE)-~ATALYZEDOXIDATION OF TETRALIN
I 2.55
5x
I
I
I
2.60
I 2.75
2.70
2.65
this point it should be noted that the oxygen uptake data obtained from volumetric changes no longer can be used as a measure of the forward progress of the reaction, i e . , tetralin conversion, as light gases are being produced which make the measured volumetric change difficult t o interpret. Thus, autoxidation studies dependent solely on oxygen uptake data for a measurement of the rate of progress of the reaction may be subject to some error if extensive secondary reactions have taken place. Similar analyses were made of the products of catalyzed decomposition of tetralin hydroperoxide. Results of the analyses of the liquid samples are shown in Figure 6. Analyses of samples of the gas phase indicated the presence of HzO,Oz,and Hg. A consideration of all the product analyses indicates that the poly(tetrafluoroethy1ene)-catalyzed oxidation of tetralin consists of a series of sequential reactions. I n analyzing these results, the light gases detected were assigned to a reaction step as dictated by the stoichiometry required by the changes in liquid constituents. As has been well established, tetralin (I) is first oxidized to tetralin hydroperoxide (11). Fol-
2.80
1000/TIDK1
Figure 4. Arrhenius plot of the first-order rate constants for the catalyzed decomposition of tetralin hydroperoxide. Conditions: 0.40 g of tetralin hydroperoxide in 4 ml of chlorobenzene under 1 atm Nz using 0.30 g of catalyst.
2.5
L
I
1
I
I
I
a -&
i
1
OOH
+
02
I
(1)
I1
lowing this, the tetralin hydroperoxide (11) decomposes to produce equal molar quantities of the ketone, 1tetralone (111), and the alcohol, 1,2,3,4-tetrahydro-1naphthol (IV). The ketone (111) is essentially stable
2
I
100
1
I $ !
200 TIME -MINUTES
1
300
0
dI1
-
.
400
Figure 5 . Reactants and liquid product changes during the catalyzed oxidation of tetralin at 115'. Other conditions: 4 ml of tetralin in 2 ml of chlorobenzene, 0.30 g of catalyst, and 1 atm pressure. V, tetralin consumed; N, oxygen consumed (by volumetric change); 0, tetralin hydroperoxide (11) content; A, ketone (111) content; ., alcohol (IV) content; 0, 1,2-dihydronaphthalene content; #, naphthalene content.
OH
0
J Y
I11
in the reaction system. However, the alcohol (IV) undergoes further reaction to the olefin 1,2-dihydronaphthalene (V). This olefin (V) then undergoes furOH
volved in the poly(tetrafluoroethy1ene)-catalyzed oxidation of tetralin at 115" as a function of reaction time. A comparison of tetralin conversion, oxygen uptake, and tetralin hydroperoxide content confirms that the initial reaction product is tetralin hydroperoxide. As the reaction progresses, however, the tetralin hydroperoxide is converted into other products. I n addition to the liquid products shown in Figure 5, HzO and HZwere also detected in samples of the gas phase. At
Iv
v
ther reaction to produce naphthalene (VI). Reactions
V
VI
The Journal of Physical Chemistry, VoL 74, No. 11, 1970
2254
WILLIAMF. TAYLOR I
1
I
1
1
1
1
L
I
30
25
1
1
I
I
I
I
4
'.,
-
I
TETRALIN HYDROPEROXIDE DECOMPOSED, g-MOLES/g SOL" X I O 5
Figure 6. Liquid products present during the catalyzed decomposition of tetralin hydroperoxide in chlorobenzene under Nz at 115': 0 , ketone (111); 0, alcohol (IV); A, 1,2-dihydronaphthalene; a, naphthalene.
1 through 4 represent the main routes for the poly(tetrafluoroethy1ene)-catalyzed oxidation of tetralin for the conditions and reaction times studied. There is a possibility that other products were made a t the end of the run a t 115" (Figure 5 ) as material balances showed a tendency to fall below their normal 100% level. A study was next made of the effect of the various products from the decomposition of tetralin hydroperoxide on the catalyzed rate of oxidation of tetralin. Included in this study was the effect of the ketone (111), the alcohol (IV), 1,2-dihydronaphthalene (V), and naphthalene (VI). I n general, these studies were carried out by sequentially injecting the compound in question into the reaction mixture after the run was in progress and observing its effect on the subsequent portion of the run. This was done in order to assure that the catalyzed oxidation was proceeding in a normal manner before the effect of the added component was assessed. Studies were made a t both 65 and 115". Results of the effect of the addition of the alcohol (IV) and the ketone (111) a t 65" are shown in Figure 7. At these conditions, data obtained from oxygen uptake and tetralin conversion are equivalent. It can be seen that the presence of the alcohol (IV) had no significant effect on the course of the reaction, whereas the addition of the ketone (111) accelerated the rate of oxidation. The results obtained a t 115" are summarized in Table 111. The presence of the alcohol (IV), dihydronaphthalene (V), and naphthalene (VI) had no significant effect on the rate of oxidation. However, the presence of the ketone (111), again increased the catalyzed rate of oxidation of tetralin.
Discussion The initial rate of oxidation of tetralin was found to be second order in hydrocarbon and zero order in oxygen pressure. Similar orders have been reported preT h e Journal of Physical Chemistry, Vol. 74,No. 11, 1970
TIME
- MINUTES
Figure 7. Effect of intermediate products on the catalyzed oxidation of tetralin at 65". Line shown for run with no product addition. a, ketone (111)addition at 90, 120, 150, and 180 min; 0, alcohol (IV)addition at 90, 120, 150, and 180 min.
Table 111: Effect of Secondary Products on the Rate of Tetralin Oxidation at 115'
Added compd
None l-Tetralone (111) 1,2,3,4-Tetrahydro-l-naphtho1 (IV) 1,2-Dihydronaphthalene (V) Naphthalene (VI)
Cumulative tetralin conversion, after addition of indicated compd relative to conversion obtained with no additiona,b
1 . 0 0 (base) 1.26 1.0 1.0 1.0
a Other conditions: 2 cma of tetralin, 2 cma of chlorobenzene, 0.30 g of catalyst, 1 atm 02 pressure. b Reaction mixture injected with 0.05 g of added compound in 0.125 cma of chlorobenzene at 25,65, and 120 min. 6 Conversions compared at 150 min.
viously for the uncatalyzed oxidation of tetralin, l7 and in the early study of George6for the heterogeneousIy catalyzed oxidation. A more recent study of the catalytic effect of various metal oxides surfaces reports a zero-order dependence on oxygen pressure and a tetralin order varying between approximately one and Poly(tetrafluoroethy1ene) was found to exhibit an apparent activation energy for the initial rate of oxidation of tetralin of 17.0 kcal/mol. Studies of the metal oxide catalyzed oxidation of tetralin showed apparent activation energies ranging from 9 to 10 kcal/ mole4 The higher apparent activation energy of poly(tetrafluoroethylene) relative to metal oxide surfaces may simply reflect a weaker bonding between the surface and the reactive species, which could occur with (17) P. George and A. Robertson, Proc. Roy. Soc., Ser. A , 185, 309 (1946).
2255
POLY (TETRAFLUOROETHYLENE)-~ATALYZEDOXIDATION OF TETRALIN such a low-surface energy solid. I n this respect, it should be noted that an interrelation between apparent activation energy and strength of the surface to reactant intermediate bond has been postulated for other catalytic reaction system^.^*^^^ A comparison with both the literature'6 and data obtained in the present work indicates that poly(tetrafluoroethylene) increases the rate of decomposition of tetralin hydroperoxide in chlorobenzene. Robertson and Waters's report a first-order decomposition of tetralin hydroperoxide in tetralin and chlorobenzene and a 24.4 kcal/mol apparent activation energy (in tetralin under Kz). ThomaslG reports a first-order decomposition of tetralin hydroperoxide and an apparent activation energy of 28.5 to 29 kcal/mol (in noctadecane and white oil under N2 in the presence of an inhibitor). It has been previously reported that poly (tetrafluoroethylene) markedly reduced the length of the induction period when oxidizing tetralin, and that an Arrhenius plot of the reciprocal of the induction period yielded a temperature dependence equivalent to 16.7 kcal/mol.6 Thus, there is close agreement between the temperature dependence for tetralin hydroperoxide decomposition (ie., 16.3 kcal/mol) and the temperature dependence of the induction period. All this suggests that a major role of the poly(tetrafluoroethylene) surface is to accelerate radical production via the decomposition of tetralin hydroperoxide. Similar observations have been made in regard to the role of homogeneous metals in the catalyzed autoxidation of alkanes and alkylbenzenes.2o Thus, during the early portion of the oxidation in the presence of an inert solvent, the catalyzed initiation step should involve adsorption on the catalyst surface, S, followed by decomposition as follows
ROOH
+ s 3ROOH A RO. + HO. I k-.
(A)
I
s Well-established propagation steps and a termination reaction between peroxy radicals should also take place. Equal molar production of the alcohol and the ketone suggest a rapid termination reaction involving alkoxy radicals. Thus
catalyzed decomposition of this hydroperoxide is also pseudo first Present results corroborate these findings. Increasing the weight of catalyst would directly increase the surface available to catalyze the decomposition of tetralin hydroperoxide. This is presumably reflected in the observed pseudo-firstorder dependence of the amount of catalyst on the catalyzed decomposition of the hydroperoxide. Studies of both the homogeneously c a t a l y ~ e d ~and ~-~ hetero~ geneously catalyzed4 oxidation of tetralin in chlorobenzene indicate the reaction is quite complex. It is generally agreed that the reaction is zero order in oxygen concentration as was found in the present study. Studies of the effect of tetralin concentration and amount of catalyst on the rate of oxidation in chlorobenzene in both the h o r n o g e n e ~ u s l y and ~ ~ ~ ~heterogeneously4 ~ catalyzed reaction indicate a complex but similar phenomena. As the catalyst amount increases at a fixed tetralin concentration, the rate increases until a critical concentration is reached above which the oxidation is inhibited and suffers a catastrophic decline. This critical catalyst concentration is a function of tetralin c ~ n c e n t r a t i o nand ~ ~ a t a given catalyst concentration the rate of oxidation will drop to zero if the tetralin concentration is decreased sufficiently (ie., a t a given catalyst concentration there is a critical-tetralin concentration and a t a given tetralin concentration there is a critical-catalyst concentration). Different kinetic schemes are postulated to account for this complex concentration-catalyst interaction on the homogeneouslyZ4and heterogeneously4 catalyzed reaction system. The heterogeneously catalyzed studies4 indicate an order in tetralin varying between one and two and an order in catalyst ranging from approximately one-half to one. The homogeneously catalyzed 0xidation2l-2~ indicates a second-order dependence of the maximum rate of oxidation on tetralin concentration, which is postulated to reflect a steady-state hydroperoxide concentratioiz For the catalyzed decomposition of hydroperoxide in the present work, application of a steady-state treatment to the fraction of the catalyst surface covered with hydroperoxide, 8, yields d8 - = k,[ROOH](l dt
- 8)
- k-,O
- k18 = 0
(G)
From which it follows
+ RH -% ROOH + R . 2RO. k6, R'=O + R-OH
R02.
(D) (E)
2R02. ks, inert products (F) The noncatalytic decomposition of tetralin hydroperoxide has been reported to be first order.l6 Ingold and coworkers have reported that the homogeneously
(18) V. Volter, J . Catal., 3, 297 (1964). (19) D. Shopov and A. Andreev, ibid., 6, 316 (1966). (20) F. R. Mayo, Accounts Chem. Res., 1, 193 (1968). (21) Y . Kamiya, S. Beaton, A. Lafortune, and K. U. Ingold, Can. J. Chem., 41, 2020 (1963). (22) Y . Kamiya, S. Beaton, A. Lafortune, and K. U. Ingold, ibid., 41, 2034 (1963). (23) Y . Kamiya and K . U. Ingold, ibid., 42, 1027 (1964). (24) Y . Kamiya and K. U. Ingold, ibid., 42, 2424 (1964).
The Journal of Physical Chemistry, Vol. 74, No. 11, 1970
2256
WILLIAMF. TAYLOR
e
K[ROOH] = 1-8
+
where K = ka/(kWa kl). The first-order dependence of the catalyzed decomposition of hydroperoxide on hydroperoxide concentration suggests that the fraction of the surface covered with hydroperoxide is low and that (ell - e) ‘v 8. An application of the conventional steady-state treatment yields the following equation for the initial oxidation rate
where w is the weight of the catalyst and kl has appropriate units. Substituting for 0 yields an equation of the form -d(02)~[RH]([ROOH]u)’iz dt
(J)
I n previous homogeneous s t ~ d i e s ~ l - 2the ~ observed second-order dependence on tetralin concentration was explained by invoking a steady-state treatment of the hydroperoxide concentration. A steady-state hydroperoxide concentration, however, was not experimentally observed for the oxidation of tetralin in chlorobenzene solvent.22 It was reported that for the homogeneously catalyzed oxidation of tetralin in chlorobenzene solvent that the measured hydroperoxide concentration was proportional to the square of the tetralin concentration at low catalyst levels.22 Substituting such a relationship into eq J would yield the following
However, in view of the obvious complexity of the catalyzed oxidation of tetralin in chlorobenzene, such a treatment must be viewed with reservation. I n this respect, Ingold and coworkers21pointed out following their derivation of a rate expression showing a secondorder hydrocarbon dependence, that similar rate forms could be obtained using different kinetic treatments. In the presence of poly(tetrafluoroethy1ene) the initial reaction products of the decomposition of tetralin hydroperoxide are the ketone (111) and the alcohol (IV). Robertson and Waters,l6 in a study of the uncatalyzed decomposition of tetralin hydroperoxide, found the ketone (111) and to a lesser extent the alcohol (IV) were major decomposition products and that water and oxygen (in chlorobenzene solvent) are also formed. This work, however, did not report the formation of
The Journal of Physical Chemistry, Vol. 74, No. 11, 1970
dihydronaphthalene and naphthalene. Thus, it would appear that the presence of the poly(tetrafluoroethy1ene) catalyst exerts a major influence on the direction of these secondary reactions. I n the study of the heterogeneously metal oxide catalyzed oxidation of tetralin no secondary products were identified other than the ketone and alcohoL4 Ketone to alcohol ratios varying from approximately 2 to 4 were reported, However, the mechanism proposed to explain these results4 does not postulate an initial equal molar production of ketone and alcohol followed by a sequential conversion of the alcohol into other products, as was found in the present study. I n this work4 no data are presented showing the reaction products as a function of the forward progress of the reaction, so it is not clear whether the varying ketone to alcohol ratio observed with metal oxides catalysts also reflects a secondary conversion of the alcohol or whether it reflects a different reaction sequence as postulated. I n the present study the ketone (111) was found to accelerate the rate of oxidation, whereas the alcohol (IV) and other products (V, VI) exerted no significant effect on the reaction. Robertson and Waters25 also observed no significant effect of alcohols on the oxidation of tetralin. However, it has been found that ketones such as cyclohexanone can accelerate the decomposition of hydroperoxide.26 Following the treatment of Emanuel,26this acceleration presumedly reflects the formation of a complex between the hydroperoxide and the ketone ROOH
+ R’=O
[XI -% free radicals (L)
Initiation involves both direct hydroperoxide decomposition (reaction A) and reaction L. The complex [XI is assumed to decompose more rapidly than the hydroperoxide itself, so that a t the latter stages of the reaction when the ketone concentration rises, reaction L would dominate the initiation process.
Acknowledgment. This work was sponsored by the Air Force Aero Propulsion Laboratory, Air Force Systems Command, United States Air Force, WrightPatterson Air Force Base, Ohio, under Contract NO. AF-33(615)-3575. Helpful discussions with H. R. Lander, Jr., J. C. Ford2 and Professor Michel Boudart are gratefully acknowledged. (25) A. Robertson and W. A. Waters, Trans. Faradau Soc., 42, 201 (1946). (26) N. M. Emanuel, E. T. Denisov, and Z. K. Maizus, “Liquid Phase Oxidation of Hydrocarbons,” Plenum Publishing Corp., New York, N. Y., 1967, p 84.
