146
Energy & Fuels 1991,5, 146-155
Reactions of Polycyclic Alkylaromatics. 2. Pyrolysis of 1,3-Diarylpropanes C. Michael Smith and Phillip E. Savage* Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136 Received July 9, 1990. Revised Manuscript Received August 27, 1990
We pyrolyzed 2-(3-phenylpropyl)naphthalene (PPN) and 193-bis(l-pyrene)propane(BPP) a t temperatures between 315 and 450 "C. PPN pyrolysis followed 3/2-order kinetics with apparent Arrhenius parameters of [log A (M-'/* s-'), E* (kcal/mol)] = [13.7 i 0.1, 51.7 f 2.91. The primary pyrolysis pathway led to 2-methylnaphthalene plus styrene as one pair of major products and toluene plus 2-vinylnaphthalene as a second major product pair. Toluene and 2-methylnaphthalene were thermally stable, but both styrene and 2-vinylnaphthalene underwent rapid secondary decomposition. The selectivity of PPN to toluene plus 2-vinylnaphthalene was slightly greater than its selectivity to 2-methylnaphthalene plus styrene. This difference in selectivity was consistent with the difference in resonance stabilization energies of the benzyl and 2-methylnaphthyl radicals being about 0.8 kcal/mol. The kinetics for PPN disappearance were reliably predicted by using rate constants previously determined for 1,3-diphenylpropanepyrolysis but adjusted to reflect the relevant difference in resonance stabilization energies. BPP pyrolysis followed 3/2-order kinetics, and the apparent Arrhenius parameters were [log A (M-1/2s-'), E* (kcal/mol)] = [10.5 i 0.8, 40 i 61. The reaction network for BPP included parallel primary pathways, with one pathway being much more important. The major pathway, which was completely analogous to that for PPN, led to vinylpyrene plus methylpyrene, whereas a minor primary pathway led to the production of pyrene via hydrogenolysis of the aryl-alkyl C-C bond. Although pyrene was only a minor primary product, high yields (36%) were obtained at complete BPP conversion. These high yields were produced via secondary pathways that involved dealkylation of alkylpyrenes formed in primary reactions. Pyrolyses of BPP in the presence of 1,6-dimethylnaphthaleneY a hydrogen-transfer reporter molecule, implicated selective hydrogenolysis mechanisms as likely candidates for the aryl-alkyl bond cleavage.
Introduction a,w-Diphenylalkanes have been commonly used as chemical models of the aliphatic linkages between aromatic moieties in coal.'* The aromatic units in coal, however, can contain more than a single ring. In fact, several investigation~~-'~ suggest that these aromatic moieties can contain up to four aromatic rings. Therefore, it is clear that polycyclic a,w-diarylalkanes are relevant coal model compounds. Indeed, this fact was recognized as early as 1956 by Depp et who pyrolyzed a series of model compounds that comprised pyrene and 2,6-xylenol linked together by 1,2, and 4 carbon atom bridges. Most recent investigators, however, have used compounds containing either one, or in rare instances, two aromatic rings. Single-ring compounds appear to have been used more frequently because they are more readily available from commercial sources and they are experimentally more convenient to use than are polycyclic compounds. There are, however, assumptions implicitly made in using single-ring model compounds to represent the polycyclic aromatic moieties in coal. These are that the precise number of aromatic rings has a negligible effect on the qualitative features of the pyrolysis pathways and that quantitative estimates of the reactivity of polycyclic aromatics can be obtained from the experimentally determined reactivity of single-ringaromatics by accounting for the relevant differences in resonance stabilization energies. Several previous pyrolyses of polycyclic aromatic comp o u n d ~ ~ * 'suggest ~ - ' ~ that these assumptions are largely reasonable.
* To whom correspondence should be addressed.
Recent work with alkylpyrene pyroly~es,'~-'~ however, revealed that the two tenets above are not universally true. For instance, the pyrolysis kinetics and pathways for 1dodecylpyrene were both qualitatively and quantitatively different than the kinetics and pathways for the analogous single-ring compound, dodecylbenzene. The key difference was the formation of products exclusively through the cleavage of the strong aryl-alkyl bond. These striking (1) Sweeting, J. W.; Wilshue, J. F. K. A u t . J. Chem. 1962,15,89-105. (2) Miller, R. E.; Stein, S. E. J. Phys. Chem. 1981, 85, 580-589. (3) Vernon, L. W. Fuel 1980,59, 102-106. (4) Benjamin, B. M.; Raaen, V. F.; Maupin, P. H.; Brown, L. L.; Collins, C. J. Fuel 1978,57, 269-272. (5) Poutama, M. L.; Dyer, C. W. J. Org. Chem. 1982,47,4903-4914. (6) Gilbert, K. E.; Gajewski, J. J. J. Org. Chem. 1982,47,4899-4902.
( 7 ) Speight, J. G. Anulytical Methods for Coal and Coal Products, Vol II; Academic Press: New York, 1978; pp 75-101. (8) Whitehurst, D. D. In Organic Chemistry of Coal; American Chemical Society: Washington, DC, 1978; pp 1-35. (9) Solum, M. S.; Pugmire, R. J.; Grant, D. M. Energy Fuels 1989,3, 187-193. (10) Shinn, J. H. Fuel 1984, 63, 1186-1196. (11) Berkowitz, N. Polynuclear Aromatic Compounds; American Chemical Society: Washington, DC, 1988; pp 217-233. (12) Gerstein, B. C.; Ryan, L. M.; Murphy, P. D. Coal Structure; American Chemical Society: Washington, DC, 1981, pp 15-22. (13) Depp, E. A,; Stevens, C. M.; Neuworth, M. B. Fuel 1956, 35, 437-445. (14) Billaud, F.; Chaverot, P; Berthelin, M.; Freund, E. Ind. Eng. Chem. Res. 1988,27, 1529-1536. (15) Sato, Y. Fuel 1979,58, 318-319. (16) Korobkov, V. Y.; Aboimova, E. K.; Bykov, V. I.; Kalechitz, I. V. Fuel 1990,69,476-479. (17) Freund, H.; Matturro, M. G.; Olmstead, W. N.; Reynolds, R. P.; Upton, T. H. Prepr. Pap.-Am. Chem. Soc., Diu.Fuel Chem. 1990,35, 496-504. (18) Savage, P. E.; Jacobs, G. E.; Javanmardian, M. Ind. Eng. Chem. Res. 1989, 28, 645-654. (19) Smith, C. M.; Savage, P. E. Ind. Eng. Chem. Res, in press.
0887-062419112505-0146$02.50/0 0 1991 American Chemical Society
Reactions of Polycyclic Alkylaromatics results from 1-dodecylpyrene pyrolysis prompted us to undertake the present investigation into the pyrolysis pathways, kinetics, and mechanisms for polycyclic 1,3diarylpropanes. In this paper we report results from pyrolyses of 2-(&phenylpropyl)naphthalene (PPN) and 1,3bis( 1-pyreny1)propane (BPP).
Experimental Section The pyrolyses of BPP and PPN were conducted both neat and in a benzene diluent at temperatures between 315 and 450 "C for batch holding times between 0 and 960 min. Materials. All chemicals employed in this study were obtained from commercial sources and used as received. The model compounds PPN and BPP were obtained in high purity from API Standard Reference Materials and Molecular Probes, respectively. 1,6-Dimethylnaphthalenewas obtained with a 98% purity from Wiley Organics. The pyrolyses were accomplished in constant-volume microbatch reactors fashioned from nominal 1/4-in.316 stainless steel, Swagelok tube fittings. The reactor volume was approximately 0.6 mL. Procedure. For the PPN neat pyrolyses, the batch reactors were loaded with approximately 40 mg of a previously prepared stock solution that contained PPN and biphenyl (an internal standard). For all the pyrolyses in benzene, the batch reactors were loaded with approximately 350 mg of a previously prepared stock solution that contained the model compound, biphenyl, and benzene in known proportions. A different protocol was used for loading the reactors for the neat BPP pyrolysis experiments. For these experiments an average of 2 mg of BPP was loaded in each reactor. The reactors were then placed in a chamber that was next evacuated to about Torr, and they remained in the vacuum chamber overnight. The chamber was then brought to atmospheric pressure by using nitrogen, and the reactors were transferred to a nitrogen glovebox where they were subsequently closed. We estimatedmthe vapor pressure of BPP to be only about torr at 298 K. Therefore we do not anticipate any significant loss of BPP during the reactors' stay in the vacuum chamber. Thus, this protocol ensured the complete removal of oxygen from the system without loss of the model compound, BPP. All quantities were carefully weighed to fO.l mg with an analytical balance. After the reactors were loaded and sealed, they were next immersed in a preheated, isothermal fluidized sand bath. Upon reaching the desired holding time, the reactors were removed from the sand bath and rapidly cooled in a room temperature water bath. The reactors were then opened and their contents recovered by solvent extraction. Owing to the small amount of model compound used in these reaction studies, no attempt was made to collect or analyze the gaseous reaction products. In the experiments using a benzene diluent, the reactant concentration was calculated as the number of moles of reactant loaded into the reactor divided by the reactor volume. This treatment is reasonable because the critical temperature of a typical reactant mixture was estimatedm21to be below the reaction temperatures employed in this study. Analytical Chemistry. We used capillary column gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS) to analyze the liquid samples. Individual reaction products were identified by comparingtheir retention times with those of authentic samples and by inspection of their mass spectra. The GC-MS system used for our qualitative analyses comprised a Hewlett Packard (HP) Model 5890 Series I1 GC, a Model 5970 quadrupole mass spectrometric detector, and a computer workstation. The routine quantitative analyses employed a HP 5890 GC operated in the split injection mode and equipped with a Model 7673 automatic sampler/injector. The injection port and detector temperatures were both set at 275 "C, and helium served as the carrier gas. Sample constituentswere separated by using either ~
(SO)Macknick,A. B.; Prausnitz,J. M. Ind. Eng. Chem. Fundam. 1979,
18, 348-351.
(21) Reid, R. C.; Prausnitz, J. M.; Shemood, T. K. The Properties of Cases and Liquids; McCraw-Hill: New York, 1977.
Energy & Fuels, Vol. 5, No. 1, 1991 147 a 50 m X 0.2 mm HP-5 column or a 5 m X 0.53 mm HP-1 column and observed by a flame-ionization detector. Peak areas were determined by a HP 3392A electronic integrator. Biphenyl served as an internal standard in most of these analyses, but anthracene was used as an external standard in the BPP neat pyrolyses. Experimental response factors were determined for all commercially available compounds by analyzing standard solutions that contained the reaction products and the standard in varying, but known, amounts. Plotting the ratio of the mass of a particular compound to the mass of the standard in the solution as a function of the ratio of their integrated GC areas resulted in a straight line, which gave the response factor as the slope. The average error for these response factors was 3%. Note that results reported in a preliminary communicationz2regarding PPN pyrolysis used response factors based on a single-point calibration rather than the more accurate linear calibration used in the present work. Consequently, we have more confidence in the results reported herein. Molar yields were calculated as the number of moles of a reaction product formed divided by the number of moles of reactant initially loaded in the reactor.
Results and Discussion PPN Pyrolysis. Table I lists the molar yields ( % ) of the major reaction products from PPN pyrolysis under different reaction conditions. Toluene, styrene, 2methylnaphthalene, and 2-vinylnaphthalene were the major products from PPN pyrolysis at low conversions. At higher conversions, however, ethylbenzene and 2-ethylnaphthalene, products that were not observed initially, were present in significant yields. In addition to these major products, PPN pyrolysis also led to the production of numerous trace products. The yields of these products were always low, and they varied greatly with the reaction conditions. The molar yield of naphthalene, for example, ranged from 0.0% to 0.996, and the maximum yield was obtained from neat pyrolysis at 400 "C for 151 min. Other minor products that we have tentatively identified include isopropylnaphthalene, propylnaphthalene, bibenzyl, propenylnaphthalene, l-methyl-1,2-diphenylethane, 1,3-diphenylpropane, 1,3-diphenylbutane, stilbene, and phenanthrene. These compounds were all products from PPN neat pyrolysis a t 400 "C for 30 min. Reaction Pathways. Decisive discrimination between primary and secondary products can be accomplished by examining the variations of the reaction products' yields with conversion. The initial slopes of these yield vs conversion curves provide the initial selectivities of the reactant to each of the reaction products. Primary products (those that arise directly from the reactant) must possess nonzero initial selectivities whereas secondary products must, by definition, possess initial selectivities equal to zero. Figure 1presents the molar yield vs conversion plots for the six major products from PPN pyrolyses at 0.12 M. Figure la clearly shows that at low conversions the initial selectivities of PPN to toluene and to 2-vinylnaphthalene were essentially the same, and equal to about 0.44. These nonzero initial selectivities identify toluene and 2-vinylnaphthalene as primary products. Furthermore, the equality of their initial selectivities suggests that the two products form in the same reaction step. The data for 2-ethylnaphthalene in Figure l a clearly show an initial slope of zero, identifying this product as a secondary reaction product. Applying the foregoing logic to the data in Figure Ib leads to the conclusions that 2-methylnaphthalene and styrene are primary products formed in the same reaction step and that ethylbenzene is a secondary reaction product. (22) Javanmardian, M.; Smith, P. J.; Savage, P. E. P r e p . Pap.-Am. Chem. Soc., Diu. Fuel Chem. 1988, 33, 242-249.
Smith and Savage
148 Energy &Fuels, Vol. 5, No. 1, 1991 Table I. Molar Yields ( W )of Major Products from PPN Pyrolysis concn neat neat neat neat neat neat neat neat neat neat neat neat 0.12 M 0.12 M 0.12 M 0.12 M 0.12 M 0.12 M 0.12 M 0.12 M 0.12 M 0.12 M 0.12 M 0.12 M 0.12 M 0.12 M 0.12 M 0.12 M 0.12 M 0.12 M 0.12 M 0.12 M
0.6
temp, "C 365 365 365 365 365 365 400 400 400 400 400 400 375 375 375 375 375 375 400 400 400 400 400 400 400 400 450 450 450 450 450 450
time, min 40 40 46 76 99 158 11 17 31 47 105 151 10 30 60 95 150 240 20 30 45 60 60 90 150 153 10 15 30 45 60 90
r 1
TOL
STY
ETB
17 18 19 30 39 49 17 29 49 53 61 61 0.7 1.8 3.4 6.5 10 24 4.4 12 13 18 25 22 36 33 9.1 21 33 35 35 35
4.9 4.6 4.4 3.2 0.8 1.0 8.9 9.4 3.2 1.3 0.0 0.0 0.7 1.5 2.6 4.9 7.3 12 3.4 8.1 9.1 12 12 13 9.3 9.9 6.9 13 9.5 9.3 0.7 0.6
1.5 1.6 2.1 4.9 3.1 16 1.4 4.7 13 18 23 24 0.0 0.0 0.0 0.4 0.7 3.6 0.0 1.3 1.1 1.6 4.9 2.6 8.4 7.4 0.7 2.6 8.7 11 16 15
I I,4
2MN 14 15 17 25 30 36 16 28 37 40 42 42 0.6 1.5 2.9 5.6 9.2 23 3.6 10 11 15 21 20 32 30 7.9 19 30 31 31 31 --VI[
Toluene
2-Vinylnaphthalene
2EN 2.3 2.5 3.3 6.8 10 18 2.7 7.7 15 19 20 20 0.0 0.0 0.3 1.1 1.7 7.1 0.4 2.7 2.0 2.9 8.7 4.8 13 11 1.2 4.1 12 16 19 18
2VN 2.2 2.2 2.0 1.5 1.1 0.5 4.5 3.3 1.2 0.0 0.0 0.0 1.0 2.2 3.8 6.8 9.4 12 4.8 9.6 11 14 12 14 8.2 9.1 9.7 16 9.9 8.9 0.6 0.5
]
+
I
0.4
0.2
0.4
0.6
0.8
1 .o
PPN Conversion
o.8
2-Methylnaphthalene
\
0.6
i
Styrene
0.4
0.2
0.0 0.0
0.2
0.4
P I
I0Rapid
styrene plus 2-methylnaphthalene and secondary decomposition pathways for styrene and 2-vinylnaphthalenewith likely secondary products being ethylbenzene and 2ethylnaphthalene. Since PPN is an unsymmetrical 1,3diarylpropane, however, there are two pairs of major primary products rather than the single pair observed for diphenylpropane (Le., toluene plus styrene). Thus, the pathway in Figure 2 is completely analogous to that observed for 1,3-diphenylpropane,the single-ring analogue to PPN. Kinetics. Inspection of the PPN molar yields in Table I reveals that the amount of PPN remaining from pyrolyses at 400 "C neat and at 0.12 M were 7% and 71%, respectively, at about 30 min, and 6% and 68%, respectively, at about 45 min. These data clearly show that, for a given reaction temperature and batch holding time, PPN pyrolyses at higher concentrations led to higher reaction rates. Thus, the present data for PPN disappearance are indicative of an overall reaction order greater than 1. Further kinetics analysis using the integral method revealed that a power-law rate expression 3/2 order in PPN was consistent with the kinetics of PPN disappearance. Note that diphenylpropane pyrolysis has also been found to obey approximately 3/,-order kinetic^.^*^ Figure 3 displays the apparent 3/2-orderrate constants for PPN disappearance for pyrolyses neat and at 0.12 M. The precise value for the initial PPN concentration is unknown for the neat pyrolyses so we plotted the quantity k([PPN]o)1/2,which could be determined directly from the integral method analysis of the neat pyrolysis data, rather
' 1 0.2
+
Figure 2. PPN pyrolysis pathways.
2-Ethylnaphthalene
0.0 0.0
Rapid
+vz
PPN 53 53 46 27 19 8.0 50 23 6.6 5.9 0.0 0.0 99 98 94 84 76 42 88 71 68 58 41 48 17 23 77 47 13 6.7 1.2 0.7
0.6
0.8
1 .0
PPN Conversion
Figure 1. Variation of product molar yields with PPN conversion for pyrolyses at 0.12 M. (a, top) toluene, 2-vinylnaphthalene, and 2-ethylnaphthalene; (b, bottom) 2-methylnaphthalene, styrene, and ethylbenzene.
The analysis above leads us to propose (Figure 2) as a summary of the key reaction pathways for PPN pyrolysis. This reaction network includes primary formation of the two product pairs toluene plus 2-vinylnaphthalene and
Energy & Fuels, Vol. 5, No.1, 1991 149
Reactions of Polycyclic Alkylaromatics
-5
-
-6
-
h
Y E
d
-7-
A 0.12M (Expt)
- 0.12 M (Model)
0.0 0
50
100
150
Time (min)
Figure 4. Comparison of PPN and 1,3-diphenylpropane reactivities at 400 O C and 0.12 M.
than simply k,for these runs. Linear regression of the data in Figure 3 for the pyrolyses at 0.12 M leads to the determination of the Arrhenius parameters as [log A (M-ll2 s-l), E* (kcal/mol)] = [13.7 f 0.1, 51.7 f 2.91. Likewise, linear regression of the neat pyrolyis data results in [log (A([PPN]o)1/2)(s-l),E* (kcal/mol)] = [14.0 f 0.4,51 f 161. The uncertainties in the Arrhenius parameters represent the 95% confidence intervals. There is very good agree-
ment in the activation energies determined from the pyrolyses neat and at 0.12 M. Additionally, by assuming that the reactant was present as only a liquid phase at reaction conditions, we were able to estimatemlz1[PPNIo =3.3 M for the neat pyrolyses. Thus, the preexponential factors for the two sets of data are also in close agreement. Given the close accord between the two sets of Arrhenius parameters, but the much higher uncertainty in the data for the neat pyrolyses, we take the Arrhenius parameters determined from the pyrolyses a t 0.12 M as our most reliable representation of the apparent kinetics of P P N disappearance. Finally, to compare the kinetics of P P N with diphenylpropane, we note that the activation energy for PPN pyrolysis of 51.7 f 2.9 kcal/mol is slightly less than the value of 52.3 f 1.3 kcal/mol previously reported for diphenylpropane pyrolysis by Poutsma and Dyers but slightly higher than the value of 51.4 kcal/mol reported by Gilbert and Gajewski.e Given the uncertainties in the activation energies, however, there appears to be no statistically significant difference between the activation energies for PPN and for diphenylpropane. Therefore, to obtain a direct comparison of the reactivities of the two compounds we pyrolyzed P P N and diphenylpropane at identical reaction conditions of 400 "C and initial concentrationsof 0.12 M. Figure 4, which displays the results, shows that, for identical reaction times, the amount of PPN that remained unreacted was always less than the amount of diphenylpropane remaining. This provides clear evidence that PPN is more reactive than diphenylpropane, and this finding is consistent with the naphthyl moiety in PPN providing more resonance stabilization energy than the phenyl moieties in diphenylpropane. Mechanism. Previous studies5s6revealed that 1,3-diphenylpropane pyrolysis proceeded through a free-radical chain reaction mechanism. On the basis of these earlier works and our present results for P P N pyrolysis, we propose the reaction mechanism in Figure 5 for P P N pyrolysis. Initiation occurs through homolytic dissociation of a bond between an cy and a 0 aliphatic carbon; thus there are two initiation steps for PPN. The first leads to the
CHAIN INITIATION STEPS
CHAIN PROPAGATION STEPS
CHAIN TRANSFER STEPS
Gzr* +
PPN
_"lt,
+ 2MN
P2
CHAIN TERMINATION STEPS
Pi +
Figure 5. PPN pyrolysis mechanism.
Pj
-
PI
Smith and Savage
150 Energy & Fuels, Vol. 5, No. 1, 1991
production of benzyl and 2-ethylnaphthylradicals, whereas the second leads to the production of ethylbenzyl and 2-methylnaphthyl radcials. Propagation of the chain reaction occurs through two parallel sets of hydrogen abstraction and /3-scission steps. The first chain involves abstraction, by a benzyl radical, of a hydrogen atom a to the naphthyl substituent to produce toluene and a PPN radical. The resultant PPN radical then undergoes @-scissionto produce 2-vinylnaphthalene and regenerate the benzyl radical. This chain accounts for the formation of the major product pair toluene plus 2-vinylnaphthalene. The second chain involves abstraction of a hydrogen atom a to the phenyl substituent by a 2-methylnaphthyl radical to produce a PPN radical and 2-methylnaphthalene. The resultant PPN radical undergoes &scission to form styrene and a 2-methylnaphthyl radical. This chain accounts for the formation of the other major product pair, styrene plus 2-methylnaphthalene. The mechanism of Figure 5 also includes chain-transfer steps such as abstraction of a hydrogen atom a to the naphthyl substituent by a 2-methylnaphthylradical (rather than a benzyl radical). Steps involving abstraction of a P hydrogen atom were omitted because the resulting PPN radical does not have a kinetically significant &scission pathway available. Thus,it cannot lead to a chain reaction with an appreciable kinetic chain length. Finally, the mechanism includes all possible recombination reactions as potential chain termination steps. Reaction Modeling. The mechanism of Figure 5 can be used to derive a closed-form, analytical rate expression for PPN pyrolysis. We begin by writing the long-chain rate expression for PPN disappearance as rate = (kll + kl~)B1[PPN1+ (k21 + MP2[PPNl (1) The concentrations of the free radicals P1 and & must next be specified in terms of the reactant concentration and rate constants for the elementary steps. We accomplished this by writing the long-chain rate expressions for P1,fl2,p1,and the total radical concentration (R') and then invoking the pseudo-steady-state approximationto set each of these expressions equal to zero. Equations 2-5 show the resulting expressions. ~ 8 =, 0 = k,,~li- ( k i i + kiJPi[PPNI (2)
k,fi - (k21 + k2JP2tPPNl (3) r,, = 0 = kllB,[PPNI + k,,P,[PPNI - kPlcll (4) rR. = 0 = 2(a1 + .2)[PPNl - ~ W T ( P I cl2 +& + P 2 ) 2 rp2 = 0 =
(5) An approximation we made in writing eq 5 was that the rate constants for all termination steps in Figure 5 are equal except for the statistical factor23that renders rate constants for self-combinationterminations half those for termination reactions involving unlike species. Simultaneous solution of eq 2-5 provides explicit analytical expressions for the steady-state free-radical concentrations, which for P1 and P2 are
(23) Pryor, W. A. Free Radicals; McCraw-Hill: New York, 1966.
p2 =
--PI k12
(7)
1221
Substituting eq 6 and 7 into eq 1 gives eq 8 as the rate law for PPN disappearance.
Because our kinetics analysis showed that PPN pyrolysis was consistent with 3/2-orderkinetics, the first term in the denominator of eq 8 must be small relative to the second term. This expectation is entirely reasonable because the values of the hydrogen abstraction rate constants are about 2 orders of magnitude smaller than the values of the pscission rate constants for diphenylpropane pyr~lysis.~ Thus, the quantity in brackets, which multiplies [PPN] in the denominator of eq 8, will be of the order of and the product of this quantity and the PPN concentration will certainly be much less than unity. Therefore, we can omit the first term in the denominator and write an expression for the apparent 3/2-orderrate constant as
1+k2l
(9)
The mechanism of Figure 5 can also be used to derive an analytical expression for the ratio of the molar yields of the two different pairs of major reaction products. Equation 10 is the expression for the ratio of the yield (Y) of toluene to the yield of 2-methylnaphthalene. r a t e ~ o-~( h i + k12)Pi (10) rate2MN (k2l + k22)@2 Using eq 7 to eliminate the concentrations of p1 and p2 leads to -YTOL =-Y2MN
kll
YTOL Y2MN
1+-
kl2 k22
1+-
R21
Equation 11reveals that the ratio of the product yields is a function of two ratios of hydrogen abstraction rate constants. The ratio in the numerator, kll/kl2, gives the rate at which a benzyl radical abstracts a hydrogen a to the naphthyl group relative to the rate at which it abstracts a hydrogen a to the phenyl substituent. The ratio in the denominator, k22/k21, gives the rate at which a 2methylnaphthyl radical abstracts a hydrogen a to the phenyl substituent relative to the rate at which it abstracts hydrogen a to the naphthyl group. If we assume that the preexponential factors for all of the hydrogen abstraction rate constants are equal, then the deviation of the two ratios from unity will be due solely to the differences in the activation energies for each pair of rate constants. We can estimate these differences in activation energies for the hydrogen abstraction steps by applying the EvansPolanyi relation.24 After algebraic manipulations, we can
Energy &Fuels, Vol. 5, No.1, 1991 151
Reactions of Polycyclic Alkylaromatics A Neat A O.12M Quation 14
H
-
I
1.0 ! 350
400
450
Temperature ("C) Figure 6. Ratio of yields of toluene and 2-methylnaphthalene
from PPN pyrolysis.
write the following simple expressions for the differences in activation energies as a function of the difference in resonance stabilization energies
- E*12 = aARSE E*22 - E*z1 = -aARSE E*ll
(12) (13)
where a is a proportionality constant and ARSE is the difference in the resonance stabilization energies between 2-methylnaphthyl and benzyl radicals. Substituting eq 12 and 13 into eq 11 leads to
Equation 14 shows that the ratio of yields is a function of a single parameter, aARSE. This parameter represents the fraction of the resonance stabilization energy difference that is manifested in the transition state for the hydrogen abstraction reactions. We calculated the average value of YToL/ Y2MN for five different sets of experiments, and Figure 6 displays the results. Using eq 14 to fit the data in Figure 6 resulted in a regressed value of aARSE = 202 cal/mol. If we take a = 0.25,24*25 then we estimate ARSE as being equal to about 0.8 kcal/mol. Note that this estimate for ARSE is only approximate because of the uncertainties in the experimental values of YTOL/ Y2MN and in the precise value of a,the Evans-Polanyi factor. Nevertheless, this value of ARSE = 0.8 kcal/mol is within the broad range of values reported in the literature. ARSE values are about 0.4-0.5 kcal/mol when calculated from simple Huckel molecular orbital theory,l6Sabout 1.75 kcal/mol from self-consistent field, molecular orbital calculations,%and between 1.4 and 2.9 kcal/mol when calculated from empirical structureresonance correlation^.^'^^^ Our estimate of ARSE is in better accord with the values at the lower end of this range. In addition to theoretical and computational routes, ARSE values are also accessible from experimental kinetics data. Korobkov et al.I6 reported first-order rate constants for the homolytic dissociation of the central C-C bond in (24) Boudart, M. Kinetics of Chemical Processes; Prentice-Hall: Englewood Cliffs, NJ, 1968. (25) Malhotra, R.; McMillen, D. F. Energy Fuels 1990, 4, 184-193. (26) Unruh, J. D.; Gleicher, G. J. J . Am. Chem. SOC. 1971, 93, 2008-2014. (27) Stein, S. E.; Golden, D. M. J . Org. Chem. 1977, 42, 839-841. (28) McMillen, D. F.; Trevor, P. L.; Golden, D. M. J . Am. Chem. SOC. 1980,102,7400-7402.
a number of 1,2-diarylethanesmIf we make the reasonable assumption that the preexponential factors for all of these homolysis rate constants are essentially equal, then their differences are entirely due to differences in activation energies. These activation energy differences, in turn, are directly related to the differences in RSE. Using the rate constanta for dibenzyl and di-8-naphthylethane pyrolyses in tetralin given by Korobkov et al.le and the foregoing assumption, we calculated 0.89 kcal/mol as the difference in RSE between 2-methylnaphthyl and benzyl radicals. This experimental value is essentially the same as our rough estimate of ARSE E 0.8 kcal/mol based on PPN pyrolysis. Rate Constant Estimates. We have already demonstrated the utility of the expressions derived in the previous section, but to exploit quantitatively the expression for the intrinsic kinetics for PPN pyrolysis (eq 9) requires numerical values for the reaction rate constants for several of the elementary reaction steps in Figure 5. The following paragraphs describe our rate constant estimation procedure, which resulted in approximate, semiquantitative values. Alternate estimation procedures exist, of course, and slightly different yet equally reasonable values for the rate constants could have been determined. Clearly, more accurate estimates, if desired, could be obtained by using thermochemical kinetics procedures.m The literaturem reveals that rate constants for homolytic dissociation of C-C bonds typically have preexponential factors in the range of 1016*' s-l. Thus, we took A = 10le s-l for both of the initiation rate constants aland az.We used 68.8 kcal/mol as the activation energy for al,the initiation step that produces benzyl and 2-ethylnaphthyl radicals. This value is the bond dissociation energy (BDE) calculated by King and Stocksofor the analogous C-C bond in 1,3diphenylpropane. The rate constant a2will be larger than a1 because the corresponding C-C bond is weaker (Le., lower BDE). This difference in bond dissociation energies is directly related to the difference in resonance stabilization energies between benzyl and 2-methylnaphthyl radicals. Since the activation energy for homolytic dissociation is essentially equal to the BDE, we estimate E* for a2as 68.8 - AFtSE. Using our earier estimate of 0.8 kcal/mol for ARSE leads to an activation energy of 68.0 kcal/mol for ap. We determined values for the hydrogen abstraction rate constants by anchoring all estimates to Poutsma and Dyer's5 estimate of [log A (M-I 5-9, E* (kcal/mol)] = [8.0, 14.21 for abstraction of a secondary benzylic hydrogen by a primary benzyl radical, an example of which is the reaction we denote as h12in Figure 5. All other hydrogen abstraction rate constants were also assumed to have preexponential factors equal to 108M-' s-', thus estimating the activation energies was the only remaining task. We took E*21= 14.2 kcal/mol (Le,, the same as E*12)because the reduction in the rate for this step relative to klz due to the increased stability of the abstracting radical (Le., 2-methylnaphthyl vs benzyl) should be roughly offset by the increase in rate due to the lower C-H BDE. Finally, we estimated the activation energies for kIl and kz2 using eq 12 and 13 and our previously determined value of aARSE = 0.2 kcal/mol. This procedure led to E*ll = 14.0 kcal/mol and E*22 = 14.4 kcal/mol. We used [log A (M-I s-l), E* (kcal/mol)] = [8.5,0.0] as the Arrhenius parameters for the termination rate constant, UT. These values are at the lower end of the range (29) Benson, S. W. Thermochemical Kinetics, 2nd ed.; Wiley: New York, 1976. (30) King, H.; Stock, L. M. Fuel 1984,63,810-815.
152 Energy & Fuels, Vol. 5, No. 1, 1991
Smith and Savage
Table 11. Molar Yields (%) of Major Products from BPP Pyrolysis concn neat neat neat neat neat neat 0.005 M 0.005 M 0.005 M 0.005 M 0.005 M 0.005 M 0.005 M 0.005 M 0.005 M 0.005 M 0.005 M 0.005 M
temp, Y! 365 365 365 365 365 365 375 375 375 375 375 315 425 425 425 425 425 425
time, min 11 15 20 30 60 150 10 60 120 240 300 658 20 43 60 91 120 960
MEPY 24 39 43 45 54 29 2.2 8.8 12 26 22 59 8.6 35 40 49 52 35
of those typically reported for radical recombination reactions. The values of the estimated Arrhenius parameters described in the preceding paragraphs were then used as parameters in eq 9 to calculate the apparent 3/2-orderrate constants for PPN pyrolysis at an initial concentration of 0.12 M. The Arrhenius plot of Figure 3 displays these predictions of the reaction model along with the experimental data. Clearly, the agreement between the mechanistic pyrolysis model and the experimental data for PPN pyrolyses at 0.12 M is quite good. BPP Pyrolysis. Table 11, which provides representative results, lists the molar yields of the major reaction products from BPP pyrolyses neat a t 365 "C and in benzene a t 375 and 425 "C. These major products were methylpyrene, vinylpyrene, ethylpyrene, and pyrene. Minor products were also detected, and these included l-propylpyrene, propenylpyrene, and l-pyrenecarboxaldehyde. Additionally, the BPP pyrolyses led to the production of a benzene-insoluble material that we will refer to as char. Of the minor products, propenylpyrene was the most abundant. Its maximum yield of 4.6% was obtained from neat pyrolysis at 340 "C for 140 min. The yield of propylpyrene was too low to quantify, but the identity of this product was confirmed by GC-MS. The final minor product, 1-pyrenecarboxaldehyde,was formed only during the BPP pyrolyses in benzene. For these experiments, the reactors were not loaded in a totally inert environment (as were the BPP neat pyrolysis reactors), so the presence of minute amounts of air in the reactor is likely. The formation of l-pyrenecarboxaldehyde probably resulted from the reaction of pyrene-containing compounds with the small amount of residual oxygen remaining in the reactor after it was purged. Methylpyrene and vinylpyrene are likely participants in such a reaction because oxidation of methyl- and vinylbenzenes is known to give benzaldehyde.31 Note further that the formation of pyrene could result from the dissociation of pyrenecarboxa l d e h ~ d e .Nevertheless, ~~~~~ the highest yield of pyrenecarboxaldehyde that we ever observed was 2.3%, and the average yield was only 1.2%. No l-pyrenecarboxaldehyde was observed during any of the neat BPP pyrolysis studies reported herein. Therefore, because the pyrenecarbox(31) Stephens, H . N. J. Am. Chem. SOC.1928,50, 186-190. (32) Stein, S. E. In Chemistry of Coal Conversion, Schlosberg, R. H., Ed.; Plenum Press: New York, 1985; pp 25-26. (33) Chen, R. H.; Kafafi, S. A.; Stein, S. E. J . Am. Chem. SOC.1989, 111, 1418-1423.
VIPY
ETPY 21 35 38 39 42 19 0.0 5.1 8.7 26 21 66 3.4 21 33 46 52 36
0.0 0.0 0.0 0.0 0.0 0.0
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1.1 5.7 6.6 8.7 19 18 1.8 2.2 1.4 2.9 2.3 8.6 1.4 4.8 4.1 4.1 5.2 34
53 44 26
400
0.0 0.0 0.0
82 73 75 54 62 9.9 78 32 31 21 15 0.0
600
800
Time (min)
Figure 7. Temporal variation of product molar yields from BPP pyrolysis at 400 O C and 0.005 M.
aldehyde yields were always low and because the results from the pyrolyses in benzene were consistent with the results from the neat pyrolyses, it appears that the presence of the small amounts of pyrenecarboxaldehyde from BPP pyrolyses in benzene did not significantly influence the experimental results. Figure 7 presents the temporal variations of the molar yields of the major products from BPP pyrolyses at 400 "C and 0.005 M. Methylpyrene and vinylpyrene, which were the most abundant products at short reaction times, had yields of 3.7% and 5.9%, respectively, at 10 min. Ethylpyrene and pyrene were also present a t 10 min, but in lower yields of 2.3% and 0.8%, respectively. As the batch holding time increased, the yield of methylpyrene increased steadily until it reached a maximum value of 63% at 240 min. A t longer reaction times, the methylpyrene yield decreased. The yield of vinylpyrene exhibited a maximum value of 7.6% at 60 min, and then it decreased to 0.0% by 240 min. The yield of ethylpyrene was low initially, but it rapidly increased. At 240 min, the ethylpyrene yield reached a maximum value of 58%, and then it decreased at longer reaction times. The yield of pyrene was always less than 2% during the initial 45 min of the reaction, but this yield increased steadily thereafter and achieved an ultimate value of 36% a t 816 min. The product alignment at 816 min was methylpyrene (42%), pyrene (36%), and ethylpyrene (34%). No vinylpyrene was detected at 816 min. Figure 8 displays the results of our neat pyrolyses of BPP at 315, 340, and 365 "C in terms of the temporal variations of the molar yields of methylpyrene and ethylpyrene (Figure 8a), pyrene (Figure 8b), and BPP
Reactions of Polycyclic Alkylaromatics
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Energy & Fuels, Vol. 5, No. 1, 1991 153
I
0.8
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5
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Figure 9. Variation of product molar yields from BPP conversion for pyrolyses at 0.005 M (a, top) methylpyrene and vinylpyrene; (b, bottom) ethylpyrene and pyrene.
1.o
0.8
315°C
0.6 0.4
0.2 0.0
0
50
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Time (min)
Figure 8. Temporal variation of product molar yields from BPP neat pyrolysis: (a, top) methylpyreneand ethylpyrene;(b, middle) pyrene; (c, bottom) BPP.
(Figure 812). The yields of all three of the major reaction products increased with both time and temperature as long as the BPP conversion was less than complete. After complete conversion of BPP was achieved, however, the yields of methylpyrene and ethylpyrene reached maximum values and then decreased, whereas the yield of pyrene continued to increase. For pyrolyses at 315,340, and 365 "C,respectively, for 15 min, the product alignment was methylpyrene (yields of 2.7%,11.5%,38.5%),ethylpyrene (yields of 1.8%,9.6%,34.7%),and pyrene (yields of O.O%, 0.6%, 5.790). Pathways. The pyrolysis pathways for BPP can be deduced by using the methodology employed previously for determination of the pyrolysis pathway for PPN. Figure 9 displays the necessary data in the form of plots of the major products' molar yields as functions of the BPP conversion. All of the data in Figure 9 were obtained from the BPP pyrolyses in benzene. The initial slopes of the molar yield vs conversion curves for methylpyrene and vinylpyrene in Figure 9a both appear
to be nonzero, indicating that these are primary products. Furthermore, their yields are roughly equal at low conversions, suggesting that the two products are formed in the same reaction step. Additionally, inspection of the data at higher BPP conversions reveals that both methylpyrene and vinylpyrene exhibit maximal yields, indicative of the decomposition of these products via secondary reactions. That the maximum for vinylpyrene occurs at about 70% conversion whereas the maximum for methylpyrene occurs at about 97% conversion suggests that the decomposition of vinylpyrene is the more rapid of the two secondary steps. Figure 9b shows that the molar yield of pyrene was always less than 5% for BPP conversions less than 70%, but at higher conversions the pyrene yield increased rapidly, and it reached an ultimate value of 36%. Although the lack of precise data at very low BPP conversions (
