J . Phys. Chem. 1989,93, 664-670
664
Appendix B (8
A(T,+’;Aw)Term Important to Eq 20.
+ + w12 Aww12 ( A W ) ~ ~ -- + 2-(0 + A ~ ) t l ) (
A(T,T’;Aw)0: {(n,- n b ) / ( n a nb))[Y2cos {Au(T’
(
(s $) -
a2
sin
a3
+(
(
Aww12
+ Aw)tl){ 2-
- -4
(AW)~W~~ 2-)
a3
+
84
sin2 a5
+x
T)
~ ~
a3
8 4
2a2
+- (Aw)2sin2+ cos + -
cos3
a3
92
02 sin 2+ sin + a2
(
(
(Aw)4
- sin2 + cos4 + - -sin4 + ) + ’/z cos {Aw(T‘+ T ) wI4 a24
8 4
wl
--82 (cos4
- 2-
(Awl4 sin4 + ++7
(-: $) +( sin
-
Aww12 ( A W ) ~ O ~ ~
-
-
&3
AwwI2
y2 sin (Aw(+’+ +) - (Aw - 8)tl){2sin2
~
a4
2%)
8 5
+ sin 2
83
+ -4
(AW)~W,~ a4
+
4 cos2
~(Aw)’
-sin2 +
cos3
2a2
+- (A.W)’ sin2 + cos + -
AW
0 2
a3
PS
sin 2+ sin
82
(
+
wI4 sin2 + cos4 a4
)
+ )/z sin {Aw(T’+
T)
Registry No. 12, 7553-56-2;CaO, 1305-78-8.
8 4
Klnetlcs of the Thermal Decomposltion of Methoxybenzene (Anisole) J. C. Mackie, K. R. Doolan, and P. F. Nelson* Department of Physical Chemistry, University of Sydney, NSW 2006, Australia (Received: October 2, 1987; In Final Form: April 21, 1988)
The thermal decomposition of anisole vapor dilute in argon has been studied in a perfectly stirred reactor over the temperature range 850-1000 K and at total pressures of (16-120) X lo-) atm. Decomposition of anisole takes place principally by the reaction C6H50CH3 C6H50+ CH,, for which the rate constant k l was found to be (2.9 A 1.0) X 10l5exp(-64.0 0.6 kcal mol-IlRT) s-l. Phenoxy radicals thus generated may decompose unimolecularly to cyclopentadienyl radicals and CO or react with methyl radicals to form cresols. Phenol is also an important secondary product. Most of the product oxygen originally contained in anisole is found in phenolic compounds rather than in carbon monoxide.
-
Introduction The alkoxy-aryl linkage plays an important role in the thermal decomposition of brown coals and other low-rank fuels. The weak alkyl-oxygen bond’ is readily ruptured by heat, leading to the formation of the phenoxy radical (C6H50). This radical is also thought to be an important intermediate in the combustion of aromatic hydrocarbons.2 In low-pressure systems phenoxy radicals have been found to undergo a unimolecular decomposition3g4at high temperatures to form carbon monoxide and the cyclopentadienyl radical. Recently *To whom correspondence should be addressed at CSIRO Division of Fossil Fuels, PO Box 136,North Ryde, NSW 2113, Australia. 0022-365418912093-0664$01.50/0
*
Lin and Lin5s6reported a shock-tube study of the decomposition kinetics of C6H50 in which C O concentration profiles were monitored by a time-resolved laser resonant absorption technique. They used anisole as a source of phenoxy radicals which were (1) McMillen, D. F.; Golden, D. M. Annu. Rev.Phys. Chem. 1982, 33, 493. ( 2 ) Venkat, C.; Brezinsky, K.; Glassman, I. Symp. (Int.) Combust., [Proc.] 1983, 19, 143. (3)Colussi, A. J.; Zabel, F.;Benson, S. W. Int. J . Chem. Kiner. 1977, 9, 161.
(4)Harrison, A. G.; Honnen, L. R.; Danben, H. J.; Lossing, F. P. J . Am. Chem. SOC.1960,82, 5593. ( 5 ) Lin, C.-Y.; Lin, M. C. I n r . J . Chem. Kinet. 1985, 117, 1025 (6) Lin, C-Y.; Lin, M. C. J . Phys. Chem. 1986, 90, 425.
0 1989 American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 2, 1989 665
Thermal Decomposition of Anisole supposed to arise via fission of the CH3-O bond: C6H@CH,
-P
C 6 H 5 0 -t CH3
(1)
The phenoxy radicals were then considered to decompose unimolecularly: C 6 H 5 0 C5H5 C O (2)
-
+
Lin and Lids6 did not measure any species other than C O in the decomposition and were unable to achieve an oxygen mass balance, particularly at the lower temperatures (- 1000 K) of their study. In earlier work, Mulcahy and Williams’ studied the reaction of methyl radicals with phenol in a perfectly stirred flow reactor. These workers showed that the combination reaction 3 C6H50
+ CH3
-
O-C6H,(OH)CH,
+ p-C6Hd(OH)CH3
(3)
between C 6 H 5 0radicals produced from phenol and methyl radicals is fast and that the 0-and p-cresols so formed dominate the reaction products under their conditions. Thus, to establish the validity of Lin and Lin’s measurement of the unimolecular rate constant for phenoxy radical decomposition, it is important to determine the extent to which reaction 3 influences the C 6 H s 0 kinetics. In addition, there has been no direct measurement of the rate constant for anisole decomposition, and the value estimated for this constant will influence, at least partially, the rate constant derived for reaction 2 by kinetic modeling. Therefore, it would seem appropriate to reexamine the thermal decomposition kinetics of anisole with particular emphasis on the distribution of oxygen in the products. Because of its ability to produce tractable amounts of products and to facilitate kinetic analysis, the perfectly stirred reactor8 has been employed in the present work.
Experimental Section Reactor. The perfectly stirred reactor was similar in design to that of Mulcahy and Williams.8 The vessel comprised a fused silica outer sphere of net volume 270 ~ m At. the ~ center of this sphere was a smaller sphere containing 28 holes of approximately 0.8-mm diameter through which reactants were radially injected into the larger sphere. Product streams were taken from three points on the circumference of the outer sphere and passed, after uniting the streams, to the trapping system. The reactor was contained in an electrically heated furnace whose temperature could be maintained a t its center to *0.2 OC at 800 OC. The temperature of the reactor was measured by a chromel-alumel thermocouple situated in a thermocouple pocket set into the reactor wall. Temperature talibrations indicated that the difference between center and wall temperatures of the reactor was less than 1 OC. The performance of the perfectly stirred reactor was tested by carrying out a series of pyrolyses of 2-chloropropane in argon between the temperatures of 710 and 870 K and a t a reactor pressure of 75 X lo-, atm and residence times of 0.14 and 0.26 s. The rate constants and Arrhenius parameters for the unimolecular elimination of HCI obtained from these pyrolyses were in excellent agreement with those of Heydtmann et aL9 Below 730 K some surface reaction was evident. The sensitivity of the reaction to surfaces is well-do~umented,~ Anisole vapor dilute in argon carrier gas was continuously passed into the reactor a t subatmospheric pressure, drawn by a mechanical vacuum pump. In addition to a liquid nitrogen trap for the vacuum pump, two traps cooled with liquid nitrogen were used to collect condensible products. Some experiments with three traps in which unreacted anisole was passed through the reactor at temperatures below the onset of decomposition demonstrated that the condensibles could be collected with an efficiency of >99% with the two-trap system. A gas buret placed before the traps was used for collection of gaseous products. (7) Mulcahy, M. F. R.; Williams, D. J. Ausr. J . Chem. 1965, 18, 20. (8) Mulcahy, M. F. R.; Williams, D. J. Ausr. J. Chem. 1961, 14, 534. (9) Heydtmann, H.; Dill, B.; Jonas, R. Inr. J. Chem. Kinet. 1975, 7, 973.
Both inlet lines and outlet lines to and from the reactor were heated by heating tape. The former were maintained at 80 OC, the latter at 120 OC. All products of significant yield had vapor pressures in excess of 0.05 atm at this temperature. The molar flow rate through the reactor was determined by a Toyo gas displacement meter placed in the exhaust line from the vacuum pump. As this meter recorded only the moles of permanent gases, a slight correction was necessary to obtain the molar flow rate and, hence, residence time in the reactor. Inlet and reactor pressures were measured with Edwards absolute pressure gauges. Two series of measurements were made: in the first total atm; in the second pressures were in the range (16-17) X atm. In the first series the total pressures were (1 10-120) X initial anisole mole fractions in argon were from 0.02 to 0.20 and in the second from 0.009 to 0.015. Two different injection systems were used. In the first, anisole vapor, initially degassed, was contained in a flask, maintained at 80 OC in a thermostat bath. The vapor pressure of anisole is about 0.07 atm at this temperature. Anisole vapor was then admitted to an argon flow via a thermostated capillary. This technique was suitable only for the first series of measurements at the lower total pressures. At higher total pressures a jet of argon impinging on the surface of anisole liquid maintained in a thermostat bath at a temperature between 25 and 40 O C was used to entrain anisole vapor, and the resultant gas mixture was injected via a heated line directly to the reactor. Residence times for the first series (Pmt= (16-17) X 10” atm) were generally about 0.14 s. For the second series ( P = (1 10-120) X atm) residence times were about 0.95 s. However, at the lower reactor pressure some runs were made at a fixed temperature with residence times varying by up to 0.33 s. Analytical Techniques. The condensibles traps were extracted with acetone and analyzed by gas chromatography (GC) with flame ionization detection on a Tenax column. Initial identification of the condensibles was made by GC/MS using capillary column separation. Retention times and fragmentation patterns were compared with authentic samples. Hexadecane was added to the solvent as an internal standard. Analyses of benzene, methylcyclopentadiene, toluene, phenol, o-cresol, p - and m-cresols, benzaldehyde, benzofuran, and unreacted anisole were made from the liquid traps. The Tenax column was unable to separate pfrom m-cresol. However, since Mulcahy and Williams7 found no evidence for significant m-cresol formation in their studies, we have assumed that the p- plus m-cresols peak is entirely due to the para isomer. Two closely separated peaks of nearly equal intensity were observed in capillary column GC and shown by mass spectrometry to have the formula C6Hg. Both exhibited the same cracking pattern. Examination of the library mass spectra and of genuine samples revealed that both methylcyclopentadiene and 1,3,5hexatriene have similar cracking patterns, presumably because they have a common (cyclic) ion. The stable trans isomer of 1,3,5-hexatriene did give a retention time similar to the second observed peak. However, methylcyclopentadiene is known to exist at room temperature as a mixture of the 1-methyl and 2-methyl isomers of nearly equal proportions.I0 Therefore, the two C6HB peaks have been assigned to the two stable isomers of methylcyclopentadiene. Analyses of methane and ethane from the gas collection system were also made on the Tenax column. CO was analyzed as CHI after methanation on a ruthenium catalyst using a Carbosphere column. Initially, attempts were made to measure the inlet anisole concentrations by GC. Samples of gas mixtures were collected in a 60-cm3 gas buret located in a side arm adjacent to the reactor inlet. It was found, however, that anisole vapor readily adsorbed onto glass surfaces and that the inlet anisole measurements using GC were systematically low. In an attempt to circumvent the adsorption problem a larger gas buret (250 cm3) was employed and extracted with solvent. Although this avoided the adsorption (10) McLean, S.; Haynes, P. Tetrahedron 1965, 21, 2313, 2343.
666 The Journal of Physical Chemistry, Vol. 93, No. 2, 1989 o'20
Mackie et al.
t
Figure 3. Yields of PhOH and C,H5(CH3)(expressed as a fraction of the initial anisole flow rate, noh) as a function of temperature. Open symbols, t,, = 0.14 s; closed symbols, t , = 0.95 s.
Temperature1 K )
Figure 1. Yields of CHI and C2H6as a function of temperature (expressed as a fraction of the initial anisole flow rate, noh). Open symbols, residence time (tra) = 0.14 s; closed symbols, t , = 0.95 s.
0
0.15
t
Lo-""oi
0.3 0% /
o-cresot/
1
O
0
i
0.2
0
I
1000
900 950 Temperature IKI
0
900 950 Temperature(K1
1000
Figure 2. Yield of CO (expressed as a fraction of the initial anisole flow rate, noAn)as a function of temperature. Open symbol, t , = 0.14 s; closed symbol, t , = 0.95 s. problem, the large volume of gas removed from the inlet system (to obtain adequate analytical sensitivity in measuring the dissolved anisole in the solution) perturbed the inlet flow significantly. Consequently, the initial anisole injected was calculated from an oxygen mass balance of all oxygenated products, both gaseous and condensible, together with unreacted anisole. Chemicals. Anisole was obtained from Aldrich and was analyzed as >99.9% pure by GC. Argon was CIG high-purity grade. Calibration standards were all >99% purity. Operating Procedure. The reactor and flow system were first evacuated then carefully purged with argon. After establishing the reactant flow to the reactor, the products stream was initially collected in a trap bypassing the two collection traps until equilibration took place. The products' flow was then routed through the collection system, and timing commenced. At the completion of a run the traps were brought up to 1 atm with dry nitrogen and extracted with solvent.
Figure 4. Yields of o- and p-cresol (expressed as a fraction of the initial anisole flow rate, noh) as a function of temperature. Open symbols, t , = 0.14 s; closed symbols, t, = 0.95 s.
.'
0
Figure 5. Yields of C6H, and PhCHO (expressed as a fraction of the initial anisole flow rate, noh) as a function of temperature. Open symbols, t,, = 0.14 s; closed symbols, t,, = 0.95 s.
Results At residence times of about 0.14 s, anisole vapor was found to commence decomposition at 860 K. At the lowest temperatures at which decomposition was detectable the principal products were found to be the cresols, phenol, methane, carbon monoxide, and ethane. Above about 930 K smaller but significant amounts of benzene, methylcyclopentadiene, benzaldehyde, toluene, benzofuran, dibenzofuran, and naphthalene were produced. Traces of ethylbenzene, ethylphenols, and xylenes were also detected. However, no cyclopentadiene or phenetole was detected. Negative tests for methanol, ketene (by FTIR),and formaldehyde (resorcinol test") were obtained. Little ethylene was detected and
even at the highest temperatures did not exceed 10%of the ethane yield. Product yield curves for the major products, scaled by the initial concentration of anisole, are given in Figures 1-5 for the two series of residence times. Scatter in the data is partly due to small variations in residence times from run to run. If it is assumed that the decomposition of anisole obeys firstorder kinetics, then at steady state if nA: and nAn respectively represent the initial molar flow rate and molar flow rate of anisole from the reactor of residence time T , the first-order rate constant is
Reinhold: New York, 1953.
The disappearance of anisole was in fact found to obey first-order
( 1 1) Walker, J. F. Formaldehyde, 2nd ed.;
The Journal of Physical Chemistry, Vol. 93, No. 2, 1989 667
Thermal Decomposition of Anisole 0
.3 0
9.6
10 0
10 I
10 8 11 2 ~ O ~ K / T
11 6
12 0
Figure 6. Arrhenius plot of the first-order decomposition rate constant of anisole. Open symbols, 1, = 0.14 s; closed symbols, t,, = 0.95 s. Points with extent of decomposition >70%, which were excluded from
the regression analysis, are indicated by crosses. kinetics up to an extent of decomposition of 70%. An Arrhenius energy plot for k is given in Figure 6. The results in Figure 6 cover an order of magnitude variation in initial anisole input and, up to 70% conversion of anisole, those from the two series of residence times collapse onto the same line. At conversions in excess of 70%, some acceleration in the decomposition rate is detectable; this begins around 980 (T = 0.14 s) and 930 K (T = 0.95 s). The Arrhenius parameters obtained from linear regression of the data of Figure 6 (omitting points at conversions in excess of 70%) are A = (2.9 f 1.0) X 1015s-l and E = 64.0 f 0.6 kcal mol-l. The activation energy associated with k is in good agreement with the enthalpy of reaction 1 (64 f 2 kcal mol-'). This together with the value of A is strong evidence that k is to be identified with reaction 1 and that other contributions to the disappearance of anisole are small. The observation of significant amounts of ethane in the products is further evidence of the occurrence of reaction 1. The value obtained for E is also close to the estimated value of Lin and L i d of 65.8 kcal mol-', but the A factor is, however, lower than their estimate by about a factor of 3. However, it should be emphasized that in the latter case their value was not directly measured but has only been derived from modeling studies. Although it seems clear that the anisole decomposes principally by reaction 1, the presence of a small proportion of benzaldehyde in the products indicates that the abstraction reaction CH3
+ C6H5OCH3
-
CbH5OCH2 + CHI
(4)
is taking place, followed by decomposition of the anisyl radical to benzaldehyde C6HsOCHz C~HSCHO +H (5)
-
as first reported by Mulcahy et al.I2 However, since no other product of the anisyl radical is apparent and the yields of benzaldehyde amount to at most a few percent, the effect of the abstraction reaction 4 on the Arrhenius parameters obtained for
reaction 1 should be negligible. Furthermore, Mulcahy et al.12 determined the rate constant for reaction 4. W e can use their value of k4 to estimate the maximum lowering of the activation energy of reaction 1 to be 0.4 kcal mol-' when benzaldehyde formation is allowed for. The unimportance of secondary reactions in anisole decomposition is presumably a consequence of the absence of weak C-H bonds and of any H-atom-elimination pathways that can proceed without rearrangement. A large proportion of the product oxygen is found in the cresols and phenol. Although in the 0.14-s residence time runs the total yield of cresols exceeds that of phenol at the lower temperatures, phenol yields overtake those of the cresols above 950 K. At a residence time of 0.95 s phenol yields exceed the combined cresols yields at all temperatures studied. In the latter series, in excess of 60% of the oxygen originally present in anisole is found in phenol and the cresols at 930 K whereas the oxygen content of the CO is only 20%. Although no loss of carbon relative to oxygen in the products was observed at the lowest temperatures studied, there was a progressive loss of carbon relative to oxygen with increasing temperature such that at 980 K (T = 0.14 s), there was a 6% loss of the carbon content of the anisole decomposed relative to oxygen. A smaller loss of hydrogen (-2% based on the hydrogen content of the decompwed anisole) occurred at this temperature. However, molecular hydrogen was not analyzed for and might comprise some of the deficit hydrogen. At the highest temperatures some tarry material was observed to deposit around the silica outlet tube passing out of the furnace. An acetone extract of this tarry matter yielded G C peaks with retention times considerably longer than the longest retention time peak (dibenzofuran) obtained from the product traps. The tarry matter was therefore assumed to be high molecular weight material rich in carbon. Formation of 0- and p-cresols also takes place via reaction 3. Up to about 60% conversion of anisole, it is found that the ortho/para ratio (assuming negligible meta isomer) is close to the statistical value of 2. At high extent of reaction the ortho/para ratio begins to decline. It would appear that the mechanism of 3 takes place, as suggested by Mulcahy and Williams,' via a cyclohexadienone intermediate and that the unpaired electron of the phenoxy radical is mainly associated with the ring: 0
0 H
CH3
To assess the consequence of significant cresols and phenol yields on the phenoxy decomposition kinetics (reaction 2), we may make the assumption, implicit in the work of Lin and Lin?" that C6H50 is the only source of CO; then at steady state where Y is the reactor volume. Also, from reaction 3, if n, is the total molar flow rate of cresols from the reactor and we assume that the cresols do not react further, then where k3 is an overall rate constant for cresol (ortho + para) formation. Since ethane must arise solely from combination of methyl radicals CH3 + CH3
(12) Mulcahy, M. F.R.; Tucker, B. G.; Williams, D. J.; Wilmshurst, J. R.Ausr. J . Chem. 1967, 20, 1155.
0
-
C2Hs
(6)
and ethane is little decomposed under the present conditions (as
668 The Journal of Physical Chemistry, Vol. 93, No. 2, 1989
Mackie et al.
evidenced by the negligible yields of ethylene), we may further write
65-
(k6 is pressure dependent,I3 and this dependence has been taken into account in the subsequent analysis.) We therefore obtain
60-
55-
. I
from which it should be possible to obtain the ratio k2/k3 from experiment. However, values of k2/k3 obtained from the first series ( T = 0.14 s) were inconsistent with those from the second (T = 0.95 s). The application of eq V requires, in addition to the single-source assumption for CO, that the cresols do not undergo subsequent reaction. Further examination shows that the latter assumption is incorrect. Runs a t two fixed temperatures (860 and 910 K) and at a fixed pressure of 17 X 10" atm showed that as the residence time was increased from 0.14 to 0.33 s the phenol yields increased and the cresol yields decreased, suggesting that, at least in part, the cresols were precursors of phenol. The elevated yields of phenol relative to cresols in the longer residence time series (although carried out at higher total pressure) were also in agreement with this postulate. It is therefore necessary to consider possible mechanisms of formation of phenol before attempting an analysis using eq V. First, however, we consider methane yields. Were the abstraction reaction 4 the only source of methane, then we would expect the yield of this product always to equal that of benzaldehyde. However, CH4 yields were always at least a factor of 3 larger than benzaldehyde yields. Mulcahy and co-workers'2 obtained the value of k4 = 10'1.7'0.3 exp(-10.5 f 0.8 kcal rnol-'/RT) cm3 mol-' s-l. They also estimated the rate constant for reaction 7 to be k7 = 10" exp(-8.0 kcal rnol-'/RT) cm3 mol-' CH3
+ C6HsOH
+
C 6 H s 0 + CH4
+ MCH
---*
MC'
+ CHI
m
-c -
1
50-
4s
(8 1
where MC' represents the radicals produced by hydrogen abstraction from 0-and pmethylcyclohexadienone, we may calculate an apparent rate constant for reaction 8. However, no systematic variation with temperature in the derived rate constants for reaction 8 was found. Furthermore, calculated rate constants under this assumption exceeded 10'' cm3 mol-' s-I, and no agreement was reached between values calculated for the two series of residence times. For a given temperature the derived rate constants for the longer residence time runs were systematically lower by a factor of between 2 and 3 compared with the series with 7 = 0.14 s. We therefore do not favor an exceptionally rapid rate for reexp(-9.2 action 8; instead we have adopted a value for k8 of kcal mol-'/RT) cm3 mol-' s-', similar to that for CH, reaction with t01uene.I~ If now we estimate the CH4 produced by abstraction reactions to be approximately that from reactions 4, 7, (13) Miller, J. A.; Kee, R. J.; Smooke, M. D.; Grcar, J. F. Paper to Western States Section, Combustion Institute, April 2-3, 1984. (14) Mulcahy, M. F. R.; Williams, D. J.; Wilmshurst, J. R. Ausr. J . Chem. 1964, 17, 1329.
-
401 96
100
I
10 4
10 8 11 2 104K/T
11 6
12 0
Figure 7. Arrhenius plot for the rate constant kp. Open symbols, t , = 0.14 s; closed symbols, tra = 0.95 s.
and 8 with the contribution from reaction 8 estimated by the rate we arrive at an "unaccounted for" yield of constant k8 = ktoluene, methane, n'cH,. This unaccounted for methane as a percentage of total methane ranges from 67% a t 860 K to 95% at 984 K ( T = 0.14 s). We have found that n',+ obeys (pseudo-) first-order formation kinetics from methyl radicals: CH3
(7)
s-*. Some methane may therefore be produced by reaction 7. With the above value of k7 this contribution to the CHI yield can be evaluated. However, the CH4 yields estimated from reactions 4 and 7 are too low to account for the observed yield of this product. Mulcahy and Williams' also recorded larger yields of CH4 in their study of methyl radical reactions with phenol than could be accounted for by reaction 7. They attributed their excess CH4 to an unusually rapid reaction between C H 3 and the methylcyclohexadienone (MCH) intermediates. If we make this assumption that the excess methane in our system arises from
CH3
c
+HX
CH4
(9)
An Arrhenius plot of k9 calculated from eq VI is given in Figure 7. Although subject to scatter, the data from both series of experiments fall on about the same line from which we obtain the approximate value of k9 = 107.5i0.5exp(-22 f 2 kcal mol-'/RT) s-1
.
Before interpreting this result, we need to consider the benzaldehyde yields. If we assume that the methyl abstraction reaction 4 and the subsequent unimolecular decomposition of the anisyl radical is the sole reaction mechanism for benzaldehyde formation, then the benzaldehyde yield should just equal that portion of the CH4 yield calculated by using the rate constant k4 of Mulcahy et However, irrespective of temperature, the experimental yields of benzaldehyde were all found to be a factor of 2.7-3.0 larger than the theoretical values (for the first series, T = 0.14 s). While an increase of k4 by this factor is a possible explanation for this systematic discrepancy, we do not favor this on two accounts. First, this increase places k4 at or slightly above the upper bound quoted by Mulcahy and co-workersl* and additionally would seem to be too fast for a CH3 abstraction reaction of this type. Second, the subsequent analysis of C6HS0(described below) would fail. Instead, we favor the alternative proposal that phenoxy radicals can abstract hydrogen from anisole (and presumably from methyl groups present on the major products 0-and p-cresols). In the case of reaction with anisole C6H5O C6HsOCH3 C6HsOH + C6HsOCH2 (IO)
+
+
this would lead to benzaldehyde formation and to part of the observed phenol yield. Thus, if ngZa represents the molar flow rate of benzaldehyde from the reactor, then ~ B =Z ~~ C ~ H S O C H ~ ~ ~ klO[C6HSOlt [ C H ~ I (VI11 under this assumption. Values of klo[C6HSO]can hence be de-
The Journal of Physical Chemistry, Vol. 93, No. 2, 1989 669
Thermal Decomposition of Anisole rived. If it is further assumed for reaction between C6H50and cresols that klo iz: k l l , we may express that portion of the phenol yield that does not arise from abstraction by nh6H50H
=
nC6H50H
- Vk10[C6H501([C6H50CH31+
2.0
[crll (VIII)
where [cr] represents the total concentration of cresols in the reactor. The calculated yields of phenol, unaccounted for by abstraction reactions, then range, as a percentage of total phenol, from 54% at 860 K to 98% at 984 K (7 iz: 0.14 s). Phenol is invariably reported to be a product of the rearrangement and thermal decomposition of many alkyl aryl ethers,”-” but its mechanism of formation has not yet been satisfactorily explained. The thermal rearrangement of, for example, benzyl phenyl ether in both liquid and gaseous states of benzylphenol has been claimed variously to be intramolecular’* or i n t e r m o l e c ~ l a r . ~Phenol ~ is generally assumed to arise from a side reaction between phenoxy radicals and a hydrogen donor, usually the solvent or the ether itself. These rearrangements are also readily by Lewis acids such as AlBr3. Vuori et a1.21thermally decomposed anisole betwen 648 and 673 K in sealed borosilicate ampules in the presence of inert gas at long residence times (0.5-6 h). They found predominantly phenol and benzaldehyde as products but observed minor amounts of 0-and pcresols together with benzene, toluene, and fused ring compounds such as naphthalene. Schlosberg et a1.22also pyrolyzed anisole at long residence times again observing phenol to be the major product. However, they did not specifically report the presence of cresols, although unidentified trace products were found. We have tested several possible steady-state relationships between the product yields to arrive at the mechanism of formation of the phenol that cannot be accounted for by abstraction. Specifically we have unsuccessfully tested a possible rapid abstraction reaction between C6H50and methylcyclohexadienone intermediate (analogous to the proposed fast reaction found not to hold for reaction 8). This was ruled out because inconsistent values of rate constants were obtained from the two sets of experiments. Another possibility is that cyclopentadienyl radicals produced by unimolecular decomposition of C6H50might be the source of hydrogen for the phenoxy radicals. This might arise via
-
C6HS0 C6H50
C5HS+ CO
(2)
+ C5H5 A C&OH + C5H4
15
-
I
--
. 2 1 0 c
I
05
00
I -
96
100
104
108 IO~KIT
112
116
Figure 8. Arrhenius plot for the rate constant = 0.14 s; closed symbols, t,, = 0.95 s.
kl3.
120
Open symbols, t,,
The only relation that was found to hold empirically was a first-order decomposition of cresol to form phenol for which nbhOH
=
k13ncr
(XI
was found to hold. The Arrhenius plot for k13 is shown in Figure 8. Least-squares exp(-35 f analysis yields the approximate value kI3 = 108.8*0.5 2 kcal mol-’/RT) s-l. Before we attempt to interpret the values of kl3 and of k9, rate constants respectively for phenol and methane production apparently not arising from bimolecular abstraction reactions, let us now return to a consideration of reaction 2, the unimolecular decomposition of phenoxy radicals. Since now it would appear that the cresols decompose in part to phenol, eq V must be modified to allow for this:
Now the origin of methylcyclopentadiene would seem to be C5H5
+ CH3
+
CSHs(CH3)
(12)
Assuming [CSH5]to be in steady state we may derive
that is, nco 3 n’phoH always. Even if other parallel routes for destruction of [C,H5] exist, nco 3 n’phoH. In fact this inequality is never obeyed experimentally. Other possible mechanisms such as a bimolecular reaction between anisole molecules and bimolecular reaction between two cresols or anisole/cresol do not obey the relevant steady-state relations, nor does a first-order formation of phenol from C6HS0. (15) Hedaya, E.; McNeil, D. J . Am. Chem. SOC.1967, 89, 4213. (16) Hart, H.; Elenterio, H. S.J . Am. Chem. SOC.1954, 76, 519. (17) Elkobaisi, F. M.; Hickinbottom, W. J. J . Chem. SOC.1959, 1873. (18) Tarbell, D. S.;Petropoulos, J. C. J. Am. Chem. SOC.1952, 74, 244. (19) Spanninger, P. A.; von Rosenberg, J. L. Chem. Commun. 1970,795. (20) Kaspi, J.; Olah, G. A. J . Org. Chem. 1978, 43, 3142. (21) Vuori, A.; Karinen, T.; Bredenberg, J. E.Finn. Chem. L e r r . 1984, 89. (22) Schlosberg, R. H.; Szajewski, P. F.; Dupre, G. D.; Danik, J. A,; Kurs, A.; Ashe, T. R.; Olmstead, W. N. Fuel 1983, 62, 690.
We now find consistent values of k 2 / k 3for both series of experiments. In Figure 9 the Arrhenius plot for the ratio k2/k3is shown. Although inevitably scattered, since individual errors in the yields propagate, the values of In (k,/k3) for both series of experiments now fall about a single line from which we derive the value of E , - E3 = 42 f 5 kcal mol-’. This value is in reasonable agreement with that obtained in Lin and L i d of 43.9 f 0.9 kcal mol-’. Again it should be pointed out that their value is not strictly for E,, but requires the assumption (reasonable for combination of two radicals) that E, i= 0. The value of A3 cannot be obtained from these studies. If, however, A, were assumed cm3 mol-* s-*, to be around 1013.0 cm3 mol-’ s-l, then A2 = also in reasonable agreement with Lin and Lin’s value6 of The postulated formation of phenol from decomposition of the cresols is qualitatively in agreement with the long residence time studiesz1fureferred to above if it is assumed that most of the cresols had decomposed into phenol. Our own studies, in which residence times were increased, support the concept of cresols as precursors for phenol. However, the Arrhenius parameters found empirically for phenol formation from cresol ( A l 3= s-l, E I 3= 35 kcal
670 The Journal of Physical Chemistry, Vol. 93, No. 2, 1989
i
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the latter by combination of methyl and cyclopentadienyl is exothermic by 70 f 2 kcal mol-' (estimated from data of ref 23). It is therefore possible that the initially formed methylcyclopentadiene might readily lose a hydrogen atom since it must first form as the 5-methyl isomer, which is known to be unstable and to undergo hydrogen migration.1° C6H7*thereby formed might be expected to rearrange to a six-membered ring and lose another hydrogen to form benzene. Benzofuran probably arises from condensation reactions of o-ethylphenol, which itself must form from o-C6H5(OH)CH2'. The latter radical would be produced as a consequence of hydrogen-abstraction reactions of o-cresol. Combination between this radical and C5H5 could also lead to dibenzofuran. In agreement with this is the experimental observation of a decrease in the ortho/para ratio of cresols at the highest temperatures and when benzofuran and dibenzofuran yields become significant. The increasing availability of H atoms at the highest temperatures studied is probably responsible for the acceleration of the rate of decomposition of anisole at high extent of conversion. H-atom reactions might also contribute to form some of the unaccounted for CH4 and phenol yields at the highest temperatures studied.
\o
-26 0-
-28.0-
-30 - 09 6
100 2
Mackie et al.
106
9loA 1 0 8K i T 112 0 116
1 2L0
Figure 9. Anhenius plot for the rate constant ratio k2/k3.Open symbols, 1, = 0.14 s; closed symbols, t,, = 0.95 s. mol-') do not suggest a homogeneous gas-phase reaction is taking place. We estimate the rate constant for the homolysis of the @-formto be 10'' exp(-80 kcal mol-'/RT) s-l. At 900 K this estimated rate constant is between 2 and 3 orders of magnitude lower than k13,suggesting the possible occurrence of a surfacecatalyzed reaction. A few runs were carried out in which 0-cresol and p-cresol were individually pyrolyzed dilute in argon. Decomposition was significant only at temperatures above 1100 K, and phenol was the only identifiable liquid-phase product of significance, although considerable amounts of high molecular weight material and some C O were produced. The Arrhenius parameters for reaction 13 seem more representative of a surface reaction than of a gas-phase decomposition. Possibly the observed high molecular weight material initially contained labile hydrogen for reduction of cresols via OH
The unaccounted for methane that was shown to obey first-order kinetics might also arise from CH3 abstraction of labile hydrogen: (9)
Indeed reactions 9 and 13 might take place sequentially on the surface. We may speculate on the mechanisms of formation of the minor products of the anisole decomposition. Benzene yields appear to parallel the yields of methylcyclopentadiene. The formation of
Conclusions Thermal decomposition of anisole between 850 and lo00 K was found to take place by unimolecular fission of the CH3-0 bond to form phenoxy radicals and methyl radicals. Unimolecular decomposition of phenoxy radicals to cyclopentadienyl radical and CO takes place and appears to be the sole source of C O observed in this temperature range. However, the majority of product oxygen is to be found in the cresols and phenol. Large yields of methane and phenol that could not be accounted for through homogeneous gas-phase abstraction reactions were observed. These yields may arise via abstraction of labile hydrogen from adsorbed high molecular weight material. Acknowledgment. We thank Dr. M. F. R. Mulcahy for his advice and encouragement and R. A. Quezada for the GC/MS analyses. Financial assistance of the Australian Research Grants Scheme, the Sir Zelman Cowen Universities Fund, and the CSIRO-University of Sydney Collaborative Fund is gratefully acknowledged. Note Added in Proof. One of the referees drew our attention to work by Stein and S ~ r y a nwhich , ~ ~ appeared after this paper was submitted. Rates and products of the thermal unimolecular decomposition of anisole and substituted anisole were determined by the very low pressure pyrolysis technique. The primary decomposition in all cases was found to be CH3-O bond homolysis, consistent with our results. Registry No. C6HS0, 2122-46-5; CH3, 2229-07-4; anisole, 100-66-3. Supplementary Material Available: Tabulation of yield data and experimental conditions (1 page). Ordering information is given on any current masthead page. (23) Harrison, A. G.;Haynes, P.; McLean, S.; Meyer, F. J . Am. Chem. SOC.1965,87, 5099. (24) Stein, S. E.; Suryan, M. M. In Int. Conf. Coal Sci., 1987 1987, 149-756.
