Energy & Fuels 1995,9, 590-598
690
Hydrogen-TransferMechanisms in l-Dodecylpyrene Pyrolysis Phillip E.Savage? Chemical Engineering Department, University of Michigan, Ann Arbor, Michigan 481 09-2136 Received January 10, 1995@
The pyrolysis of l-dodecylpyrene (DDP), which can serve as a chemical model of the alkylaromatic structural elements in heavy hydrocarbon resources, is modeled using a detailed free-radical reaction mechanism and estimated rate constants for the elementary steps. This mechanism differs from previous work in that it includes hydrogen-transfer reactions from 4,5dihydropyrene and 1-(l-dodecenyl)pyrene,which are products of DDP pyrolysis, to DDP. The model is validated by comparing its predictions with experimental results for the effects of time, temperature, and initial concentration on the kinetics of DDP disappearance and product formation. Removing the radical hydrogen-transfer (RHT)steps from the validated model revealed that the more conventional hydrogen-transfer steps of reverse radical disproportionation and H atom addition are insufficient to describe all of the experimental observations. Moreover, varying the resonance stabilization energy, the C-H bond dissociation energies for hydropyrenyl radicals, and the additional stabilization energy attributed to the double bond in l-(l-dodeceny1)pyrene cannot bring the results of the model without RHT into agreement with experimental observations. Including radical hydrogen transfer or phenomenologically similar steps appears to be necessary t o model DDP pyrolysis.
Introduction Polycyclic alkylaromatic compounds are good chemical models of the alkylaromatic moieties in heavy oils, asphaltenes, and coal. Consequently, understanding the reaction kinetics, pathways, and mechanisms of these compounds is central to understanding the reactions of these structural elements during heavy hydrocarbon processing. This resolution of the reaction fundamentals supports the current industrial trend1-3 toward developing models that take a decidedly molecular perspective. Polycyclic aromatics are also important because of their role in hydrogen-transfer reactions in heavy hydrocarbon pro~essing.~ Previous work in our laboratory showed that the very strong aryl-alkyl C-C bonds in l-dodecylpyrene (DDP) and other polycyclic alkylaromatics are susceptible to cleavage by thermal hydrogen~lysis.~,~ The reaction proceeds by the addition of a hydrogen atom to the substituted aromatic carbon atom, followed by displacement of the aliphatic substituent and formation of the unsubstituted arene. Mechanisms possibly responsible for the hydrogen-transfer reaction include the reverse of radical disproportionation (RRD), radical hydrogen transfer (RHT), and the addition of a free H atom. A schematic depiction of each of these three mechanisms, using DDP as an example, appears as Figure 1. Note that this dealkylation pathway for polycyclic compounds operates in parallel with the conventional pathway for E-mail:
[email protected]. Abstract published in Advance ACS Abstracts, May 15, 1995. (1)Quann, R. J.; Jaffe, S. B.Ind. Eng. Chem. Res. 1992,31,2483. (2) Shinn, J. H. Prepr. Pup.-Am. Chem. SOC.,Diu. Fuel Chem. 1992, 37,50. (3) Freund, H. Energy Fuels 1992,6,318. (4) Stein, S. E. Acc. Chem. Res. 1991,24, 350. (5) Savage, P. E.; Jacobs, G. E.; Javanmardian, M. Ind. Eng. Chem. Res. 1989,28, 645. (6) Smith, C. M.; Savage, P. E. AIChE J. 1991,37,1613.
alkylbenzenes, which leads to the methyl- and vinylarene as major products. Smith and Savage' developed a free-radical reaction mechanism for DDP pyrolysis, and it included each of the hydrogen-transfer steps noted above. A quantitative kinetics model based on their reaction mechanism showed that RHT from non-ipso l-hydropyrenyl radicals and from u-DDP radicals were the hydrogen-transfer steps with the fastest reaction rates for neat DDP pyrolysis a t 400 "C. RRD was nearly an order of magnitude slower, and H atom addition was even slower. This paper extends the original mechanism of Smith and Savage' and investigates these hydrogentransfer steps in more detail. We focus attention on the necessity of including hydrogen-transfer steps other than RRD and H atom addition to model DDP pyrolysis.
The RHT Controversy Focusing attention on hydrogen-transfer reactions and mechanisms in systems with polycyclic aromatic compounds is timely given the current controversy on the significance of radical hydrogen-transfer steps during coal conversion and heavy oil processing.8-11 Much of this controversy stems from a recent ab initio quantum chemistry calculation of the intrinsic barrier height for hydrogen transfer from an ethyl radical to ethylene, and subsequent estimates of the barrier height
+
@
0887-0624/95/2509-0590$09.00/0
(7) Smith, C. M.; Savage, P. E. Chem. Eng. Sci. 1994,49,259.
(8) Franz, J. A.; Ferris, K. F.; Camaioni, D. M.; and Autrey, S. T. Energy Fuels 1994,8, 1016. (9) Autrey, T.; Alborn-Cleveland, E.; Camaioni, D. M.; Franz, J . A. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1994,39,627. (10)McMillen, D. F.; Malhotra, R. Prepr. Pup.-Am. Chem. SOC.,Diu. Fuel Chem. 1994,39,632. (11)Camaioni, D . M.; Autrey, S. T.; Franz, J. A. Prepr.Pup.-Am. Chem. SOC.,Diu. Fuel Chem. 1994,39,638.
0 1995 American Chemical Society
H-Transfer Mechanisms in 1-Dodecylpyrene Pyrolysis
Energy & Fuels, Vol. 9, No. 4, 1995 591
r-@-@-
REVERSE OF RADICAL DISPROPORTIONATION (RRD)
H
2
00
-
DDP (A)
a-DDPradical (pg)
RADICAL HYDROGEN TRANSFER (RHT)
ipso 1-alkylhydropyrenyl radical (p4) H
Internal Olefin (19
non-ipso 1-hydropyrenyl radical (15)
H ATOM ADDITION H
+
w Figure 1. Hydrogen-transfer mechanisms for DDP pyrolysis.
for RHT in aromatic systems based on semiempirical methods.8 These calculations suggest that the activation energy for RHT may be higher than was previously thought. The need to invoke RHT steps in aromatic systems has also come into question through a reexamination of mechanistic kinetics models reported in the literature. Billmers et a1.,12for example, studied hydrogen-transfer reactions between dihydroanthracene and ethylanthracene, and they explained the observed kinetics by including an RHT step in their mechanism. Camaioni et al.,I3 however, showed that the data of Billmers et al. could be described equally well using a mechanism that relied solely on reverse radical disproportionation steps for hydrogen transfer and excluded this RHT step. They concluded that there was no need to invoke the RHT step and that greater consideration should be given to conventional pathways (e.g., RRD) when modeling thermal hydrogen transfer. As another example, we note that Smith and Savage14 used a model that included RHT steps to describe their experimental data for the pyrolysis of methyl- and ethylpyrene. One of the authors’ goals was to assess the relative significance of rival hydrogen-transfer mechanisms in this system so their model included hydrogen transfer by RRD, RHT, and free H atoms. They found that RRD steps were the most important hydrogenolysis steps for methylpyrene and that RRD and RHT had comparable rates for ethylpyrene hydrogenolysis. The addition of free H atoms was less important. Franz et recently reported that the experimental data for methyl- and ethylpyrene pyrolysis could also be represented by a model that excluded RHT (12) Billmers, R.; Grifith, L. L.; Stein, S. E. J. Phys. Chem. 1986,
90, 517. (13)Camaioni, D. M.; Autrey, S. T.; Franz, J. A. J. Phys. Chem. 1993,97,5791. (14)Smith, C. M.; Savage, P. E. Energy Fuels 1992, 6 , 195.
but had faster RRD steps (by a factor of 2) than did the model developed by Smith and Savage. So again, two different models were largely consistent with the same set of experimental observations. The ability of the two different models (one with and one without RHT) to describe the same data simply confirms that RHT steps are not the dominant hydrogenolysis steps during methyl- and ethylpyrene pyrolysis. Thus, methyl- and ethylpyrene pyrolyses are not good systems to study if one is interested in determining whether RHT steps are necessary in detailed chemical kinetics models. A more convincing demonstration that solely RRD steps and the addition of free H atoms can generally model hydrogen transfer in polycyclic aromatic systems would be to model a system where RHT has been reported to be the dominant hydrogenolysis mechanism. The pyrolysis of DDP provides just such a system since the rate of hydrogenolytic dealkylation of DDP by RHT steps was calculated t o be about an order of magnitude faster than hydrogenolysis by RRD.’ Thus, if the mechanism for DDP pyrolysis can be modified so that RHT steps are excluded and the resulting kinetics model is still faithful to the experimental results, then the case for avoiding “unconventional” hydrogen-transfer steps in mechanistic models will have been strengthened. Conversely, if the omission of RHT steps leads only to poorer descriptions of the experimental data, then the case for including RHT steps in mechanistic models will be strengthened. This paper extends the DDP pyrolysis mechanism of Smith and Savage’ (hereafter referred to as the “original” model) and then explores the ability of this extended model to describe experimental data when the RHT steps are removed.
The Extended DDP Pyrolysis Mechanism and Model This section is apportioned into two subsections. The first subsection describes the refinements and additions
592 Energy & Fuels, Vol. 9, No. 4, 1995
Savage
Table 1. Thermochemical Data Used in Extended DDP Pvrolvsis Model parameter
value (kcallmol)
ARSE (1-position in pyrene) BDE of primary benzylic C-C bond BDE of C-H bond in 1-hydropyrenyl radical BDE of C-H bond in 4-hydropyrenyl radical BDE of primary benzylic C-H bond BDE of secondary benzylic C-H bond BDE of primary aliphatic C-H bond BDE of secondary aliphatic C-H bond
5.1 72.5 37.0 33.0 89.5 87.0 100.5 98.0
made t o the original mechanism for DDP pyrolysis t o develop an improved and more complete mechanism. We then use the mechanism as the basis for a quantitative kinetics model and compare model predictions with experimental data to validate the model. Mechanism and Model Development. The original DDP pyrolysis mechanism7included hydrogenolysis by RHT, RRD, and H atoms (see Figure 11, as well as the conventional free-radical reaction steps of C-C bond homolysis, hydrogen abstraction, p-scission, isomerization, disproportionation, and recombination. The thermochemical data and rate constant estimation protocol were adapted from previous work done by groups at the National Institute of Standards and Technology, the Stanford Research Institute, and the Oak Ridge National Laboratory. Essentially the same thermochemical data and rate constant estimation protocol were used in the extended model presented herein. The only change is that the extended model used an EvansPolanyi relation for radical disproportionation (RD) kinetics whereas the original model took all RD steps to proceed with equal rates. The numerical values used for the thermochemical data and the Evans-Polanyi parameters appear in Tables 1 and 2. The Acuchem software package15 was used for the numerical calculations. The present mechanism differs from the original mechanism of Smith and Savage in two ways. First it considers only disproportionation as a termination route for 1-hydropyrenyl radicals @4 and p5 in Figure 1). All other radicals can terminate by recombination, and in some cases by both recombination and disproportionation. Second, the present model includes two sets of new elementary steps. These new steps deal with two new potential hydrogen donors, 4,Ei-dihydropyreneand a DDP molecule with a double bond between the a and p carbons in the aliphatic chain (internal olefin, I2 in Figure 1). The first set of new steps involves hydrogen transfer to the 4-, 5-, 9-, and 10-positions in pyrene, and the formation of the hydrogen donor, 4,5-dihydropyrene. The original DDP pyrolysis model7considered hydrogen transfer only to the 1-,3-, 6-, and 8-positions in pyrene nuclei. Although the 4-, 5-, 9-, and 10-positions are less reactive for hydrogen addition than are the 1-,3-, 6-, and 8-positions,hydrogen transfer to these positions can lead t o the formation of an additional hydrogen donor molecule, 4,5-dihydropyrene. The original model did not include the possibility of forming 4,5-dihydropyrene. Only traces of 4,5-dihydropyrene and its substituted analogs were observed in our experiments. The set of elementary steps added appears in Table 3. New species appearing in this extended model (15) Braun, W.; Herron, J. T.; Kahanen, D. K. Int. J. Chem. Kinet. 1988,20, 51.
include a nonipso alkylhydropyrenyl radical b 6 ) formed by H addition to the 4-, 5-, 9-, or 10-position, and 4 5 dihydropyrene k6H). S is a pseudospecies that denotes the concentration of 4-, 5-, 9-, and 10-positionsin pyrene nuclei in the reaction mixture. All other species appeared in the original model,7and the same notation is used here. The first reaction in Table 3 is an RRD step whereby an a hydrogen in DDP is transferred to the 4-, 5 - , 9-, or 10-position in a pyrene moiety. After the 4-hydropyrenyl radical b6) is formed, 4,5-dihydropyrene can be produced by disproportionation of two 4-hydropyrenyl radicals or by hydrogen abstraction from DDP by a 4-hydropyrenyl radical. 4,5-Dihydropyrene can then participate in RRD steps and serve as a hydrogen donor, thereby generating ipso- $4) and nonipso (4)l-hydropyrenyl radicals. Additionally, we included all possible radical recombination reactions involvingp6 (not shown in Table 3) as potential termination steps. We used the rate constant estimation regimen and the thermochemical data in Tables 1and 2 to estimate the rate constants for these new steps. We took the bond dissociation energy (BDE)of the C-H bond formed by adding H t o the 4-position in pyrene to be 33 kcall moll6 for our base case model. The uncertainty in this value is probably about f2 kcallmol. We estimated the C-H bond dissociation energy in 4,Ei-dihydropyrene to be 81.2 kcallmol. This value was determined from the set of reactions listed in Table 4. The summation of these four reactions leads to no net reaction, so the sum of the individual heats of reaction must be zero, Thus, the desired BDE is readily calculated as 81.2 kcallmol. A second set of new steps incorporated in the extended mechanism accounts for reactions involving the “internal olefin”. These steps appear in Table 5 along with their associated rate constants at 400 “C. Freund et a1.18 observed that the formation of an internal olefin (a substituted pyrene with a double bond between the a and ,i3 carbons) was coupled to the formation of pyrene during the neat pyrolysis of 1,20-di(1-pyreny1)eicosane. This observation implicated the a-alkylpyrene radical as an important hydrogen source, because removing a hydrogen atom from the p carbon in this radical produces the internal olefin. Freund et a1.18postulated an RHT step t o account for this hydrogen transfer, because other paths (H atoms, multistep paths) seemed unlikely. The original mechanistic model7for DDP also included this RHT step from the a-DDP radical (~42)to form the internal olefin, as shown in Figure 1. Moreover, the model results showed that this RHT to the ipso position in DDP was the second most rapid hydrogentransfer step leading to hydrogenolysis. If RHT were excluded from the original DDP mechanism, however, no path remained t o form the internal olefin. Since the internal olefin does form and since we are interested in determining whether a mechanism without RHT steps can describe DDP pyrolysis, we now discuss other paths for internal olefin formation. One might consider p-scission of a C-H bond in the a-alkylpyrene radical, for example. Aliphatic C-H bonds are roughly 12 kcdmol stronger than C-C bonds, (16)Stein, S. E.; Brown, R. L. J. Am. Chem. SOC.1991,113, 787. (17)Johnston, K. P. Fuel 1984, 63, 463. (18)Freund, H.; Matturro, M. G.; Olmstead, W. N.; Reynolds, R. P.; Upton, T. H. Energy Fuels 1991,5 , 840.
H-Transfer Mechanisms in 1-Dodecylpyrene Pyrolysis
Energy & Fuels, Vol. 9, No. 4, 1995 593
Table 2. Arrhenius Preexponential Factors and Evans-Polanyi ParametersaUsed in Extended DDP Pyrolysis Model reaction family
log Ab
homolytic dissociation reverse radical disproportionation (RRD) H atom elimination H atom addition radical hydrogen transfer (RHT)
16 8.6 13.6 10.3 7.8
Eo 0 9 9 6.5 16.5
a 1.0 0.82 0.78 0.11 0.35 exothermic 0.65 endothermic
H abstraction between 2 benzylic centers
8.0
16
between aliphatic and benzylic centers
8.0
13.5
between 2 aliphatic centers
8.0
12.6
by H atoms radical recombination radical disproportionation (RD)
10.4 10.5 8.8
13.3 0 9
0.35 exothermic 0.65 endothermic 0.35 exothermic 0.65 endothermic 0.35 exothermic 0.65 endothermic 0.25 exothermic 0 0.18
+
a The Evans-Polanyi relation has the form E , = Eo a AHH,,where E, is the Arrhenius activation energy and AHr is the heat of reaction. Units are kcal, moles, liters, seconds. log A given here does not include reaction path degeneracy.
Table 3. Reactions in Extended DDP Pyrolysis Mechanism to Account for H Addition to 4-Positions in Pyrene with Kinetics Used in Base Case Model reaction step
type
AHm (kcallmol)
A+S-p6+p2 p6H+S-p6+p6 p6H+A+pd+p6 p6H + R-p5 + p 6 p6 f A p6H p1 p6 + A p6H pz p6 + A ,&H p3 H S-pg p6'H+S p6+pz-A+S ps+/~-p~gH+A p~g+,&-p~gH+R p6+p6-p6H+S
RRD RRD RRD RRD H-abstr H-abstr H-abstr H addition H elimination RD RD RD RD
48.9 48.2 44.2 44.2 16.8 0.7 16.8 -33 33 -48.9 -44.2 -44.2 -48.2
--
+
+ + +
+
K(4OO"C) (L.mo1.s) 9.0 x 2.8 x 3.2 x 3.2 x 2.3 x 9.1 x 2.1 x 2.3 4.2 x 1.1 2.9 x 5.8 x 4.9 x
10-8 10-7
loo
lo2 lo1 109 lo2 109 108 lo8 lo8
Table 4. Thermochemical Cycle for Estimating the C-H BDE in 4,5-Dihydropyrene
AH=,, (kcaVmo1)
reaction step
-
+
-10a BDE 336 -104.2b
pyrene Hz p6H p6H-p6 4-H p6 pyrene H H+H-H2
-
a
+
Reference 17. Reference 16.
Table 5. Reactions in Extended DDP Pyrolysis Mechanism To Account for H Transfer from the Internal Olefin with Kinetics Used in Base Case Model " K (400 "C) reaction step
type
(kcal/mol)
12+A-p7+p4 IzfR-p7+p~ Iz A-pz + p2 IZ+ P6H ~2 + p6 12 + s -p7 + p6 Iz Iz -p7 p2 p7 A I2 p1 p7 A 12 p2 p7 A 12 + p3 p7 olefin /33 pz +pz Iz + A
RRD RRD RRD RRD RRD RRD H-abstr H-abstr H-abstr p-scission RD RD RD RD RD RD
41.9 41.9 31.8 31.1 45.9 28.8 19.1 3.0 19.1 8.1 -31.8 -28.8 -45.9 -31.1 -41.9 -41.9
+
-
+
+ + + +
--- ++ - -+ + - + PI + - 12 + s + -p6H + Iz p7
pup
12
12
p6
p6
pz
p7+p4-Iz+A p7 +p5-12 +R
(L.mo1.s) 6.6 x 6.6 x 3.2 9.9 5.7 x 4.1 x 7.7 x 3.0 x 6.9 x 1.5 x
10-3 10-3 10-7
10-1 lo2
loo
loo
2.2 x 108
7.3 107 7.3 x 108 9.9 x 10' 2.1 x 108 4.2 x lo8
however, so p-scission of the C-C bond to form vinylpyrene would be favored over /3-scission of the C-H bond t o form the internal olefin. Indeed if we take just half of this 12 kcaYmo1 difference in bond energies to
be felt a t the transition state and assume equal preexponential factors we find that the relative rate of C-C to C-H bond scission is 9O:l at 400 "C. Another potential route to the internal olefin is radical disproportionation. The a-alkylpyrene radical could donate hydrogen to another radical t o form the internal olefin and a second stable molecule. The a-alkylpyrene radical is the radical present in the highest concentration during DDP neat pyrolysis7 so disproportionation between two a-alkylpyrene radicals is a possibility. Of course, radical disproportionation must compete with radical recombination. The ratio of the disproportionation to recombination rate constants for the structurally similar a-ethylbenzene radicals is 0.0233.19 Moreover, this rate constant ratio for resonance stabilized radicals tends to decrease as the disproportionation exothermicity decreases.lg Using the data in Table 1 and in Freund et al.ls we estimated that the disproportionation exothermicity is about 7 kcaYmol lower for DDP than it is for ethylbenzene. The correlation of Manka and Steinlg then leads to the ratio of the disproportionation t o recombination rate constants being about 0.005. (Note: the rate constant estimation regimen of Table 2 used in the extended model leads to a ratio of 0.007, in good agreement.) We note, however, that the recombination reaction is reversible and that the rate constant for the reverse (homolyticdissociation) is very fast. Thus, an equilibrium might be established rapidly between the a-DDP radicals and their recombination product. The presence of this equilibrium would give a-DDP radicals an opportunity t o disproportionate even though the rate constant is lower than that for recombination. Thus, disproportionation might be able t o compete with recombination. The extended DDP mechanism includes this and other related disproportionation steps. In addition to adding a new route to form the internal olefin, the extended model also considers reactions wherein the internal olefin is a reactant. Indeed, the molar yield calculated for the internal olefin by the original model was nearly as high as the molar yields of pyrene and dodecane from DDP neat pyrolysis. These high yields were calculated because the original model treated this molecule as a stable product. We detected only trace amounts of this product in our experiments (19) Manka, M. J.; Stein, S. E.J. Phys. Chem. 1984, 88, 5914.
594 Energy & Fuels, Vol. 9,No. 4, 1995
Savage
RRD
L
Internal Olefin (12)
Figure 2. Hydrogen transfer from the internal olefin.
DDP pyrolysis. These comparisons will consider the (in a closed system), however. Since the internal olefin effects of time, temperature, and initial concentration remained in the reactor afier its formation during o u r on DDP disappearance and the molar yields of the major experiments, it was free t o participate in secondary products methylpyrene and either pyrene or dodecane. reactions. Our detecting only traces of this product attests to its reactivity under the conditions used in our Figure 3 shows the predictions of the extended model experiments. Developing a more complete model of along with experimental data for DDP pyrolysis at 400 DDP pyrolysis requires that we account for the second"C and at initial DDP concentrations of 0.005,0.02, and ary reactions of this internal olefin. 0.03 M. These figures display the temporal variations of DDP, methylpyrene, and pyrene. Overall, the model Secondary reactions that have been postulated for does a good job of predicting DDP disappearance and vinylarenes include polymerization t o form "char", methylpyrene formation at all three concentrations. The reduction to form ethylarenes, and cracking to form model and experimental results for pyrene are also in methylarenes and a r e n e ~ , ~For , ~ Jthe ~ present case of close accord at 0.005 and 0.02 M, but the model failed the internal olefin formed from DDP, similar reaction to predict the high pyrene yield observed at 1050 min paths should be available, and these likely contribute at 0.03 M. to the disappearance of this compound in our experiFigure 4 shows the model predictions and experimenments. Unfortunately, these reaction paths do not tal data for neat DDP pyrolysis at 375, 400, and 425 appear to have been modeled quantitatively using a set "C. Again, one observes good agreement for DDP of elementary reaction steps. disappearance and for methylpyrene formation at all A n additional reaction path that is available for the three temperatures. In fact, this extended model does internal olefin is hydrogen transfer. The internal olefin of predicting these yields than did the a better job can participate in RRD reactions whereby a hydrogen original model. The extended model also does a good atom on the y-carbon in the aliphatic chain is transjob of predicting dodecane yields a t high conversions ferred to a peripheral aromatic carbon in a pyrene ( >60%), but it consistently overpredicts the dodecane moiety in another molecule, as depicted in Figure 2. To yields a t low conversions. The original model showed explore the effect of RRD from the internal olefin on the the same deficiency. kinetics and product selectivities predicted for DDP, the Figure 5 shows the predictions of the extended model extended DDP mechanism includes these RRD steps along with experimental data for DDP pyrolysis at 400 and other related ones. Table 5 lists these new steps. "C for 300 min and a t different initial DDP concentraThe internal olefin can transfer hydrogen t o either the tions. Once more, it is clear that the model captured ipso position or t o nonipso 1-and 4-positions in DDP. the key trends in the experimental data. These trends The radical $7) formed by hydrogen transfer from the are a precipitous decrease in the DDP yield and an internal olefin can then abstract hydrogen from the a, accompanying increase in the dodecane yield at high y , or other aliphatic carbons in DDP to regenerate the DDP concentrations. internal olefin and produce p1, pz, or p3 radicals, To summarize, the improvements made to the original The radical p7 can also decompose via respectively. mechanism and model have led to a better quantitative P-scission. description of DDP pyrolysis kinetics. The extended The rate constant for hydrogen transfer by RRD from model can accurately predict the effects of batch holding the internal olefin to 1-positionsin pyrene nuclei should time, temperature, and initial concentration of the be at least as large as the rate constant for the RRD kinetics of DDP disappearance and on product yields. steps that transfer an a hydrogen in DDP to these positions. This is because the y C-H bond in the DDP Pyrolysis without RHT internal olefin is weaker than the a C-H bond in DDP. The radical formed by removing an allylic hydrogen in The extended model developed and validated in the the internal olefin enjoys the resonance stabilization previous section includes RHT steps. To determine the energy associated with the aromatic system plus the role that RHT plays in this extended model we removed additional stabilization associated with the double bond. these steps from the mechanism and show the model As a first attempt at estimating rate constants for predictions in Figure 6. It is clear that the removal of reactions involving the y-position in the internal olefin the RHT steps has little impact on the model results at or the p7 radical, we took the y C-H bond in the internal low DDP concentrations, where the hydrogenolysis olefin to be 3 kcal/mol weaker than the a C-H bond in reaction and dealkylation are unimportant. At higher DDP. We explore the sensitivity of the model results concentrations, however, the model without RHT steps to this value in the next section. fails to predict the precipitous decrease in the DDP yield Model Validation. Having extended the original and the formation of high yields of n-dodecane. The DDP mechanism to include additional hydrogen donors same behavior was observed with the original DDP (4,Ei-dihydropyrene and the internal olefin), we now validate the model by demonstrating its ability t o (20) Smith, C. M. Ph.D. Dissertation, University of Michigan, 1992. reproduce trends observed e ~ p e r i m e n t a l l y for ~ ~ ~ , ~ ~(21) ~ ~Smith, ~ C. M.; Savage, P. E. Ind. Eng. Chem. Res. 1991,30,331.
H-Transfer Mechanisms in 1-Dodecylpyrene Pyrolysis
Energy & Fuels, Vol. 9, No. 4, 1995 595
i n
0.2-
A 0
Pyrene
0.0
0
400
600 TIME (minutes)
800
30
60
90 120 TIME (minutes)
150
180
1000
-
0.8
s
#
\
+
. 0.4
0.2
0 Methylpyrene
30
60
90 120 TIME (minutes)
Dodecane
150
Pyrene
A 0.0
0
300 TIME (minutes)
200
100
400
500
Dodecane 0.2
0.0 0 0.2-
0
30
60
90 120 TIME (minutes)
150
Figure 4. Temporal variations of DDP, methylpyrene, and dodecane yields from DDP neat pyrolysis: (a, top) 375 "C; (b, middle) 400 "C; (c, bottom) 425 "C.
zoo
400
600 800 TIME (minutes)
1000
1200
Figure 3. Temporal variations of DDP (squares), methylpyrene (triangles), and pyrene (circles) yields from DDP pyrolysis a t 400 "C at different initial concentrations. In this and all following figures, discrete points are experimental data and curves are calculations based on t h e extended model. (a, top) 0.005 M; (b, middle) 0.02 M; (c, bottom) 0.03 M.
mechanism and model when RHT steps were removed. Thus, one can conclude that the presence of 4,5dihydropyrene and the inclusion of hydrogen transfer by RRD from the internal olefin in the extended model cannot compensate for the absence of RHT steps in the mechanism. That is, the inability of the original model without RHT t o describe DDP pyrolysis was not due to the omission of the RRD and RD steps included in this extended mechanism.
The balance of this section focuses on determining whether the model without RHT can be brought into consonance with the experimental results by adjusting the numerical values of some of the rate constants. This exercise was undertaken because of recent reports8J3 that slight changes in a few rate constants in other mechanistic kinetics models obviated the need for RHT steps. Rather than varying individual rate constants independently, however, we will systematically vary the numerical values of important thermochemical data used to calculate many of the rate constants. To determine which thermochemical data would be important, we took advantage of a sensitivity analyses conducted by Smith and Savage' for the original DDP pyrolysis mechanism. The object of their analysis was to identify the elementary reactions that most strongly influenced the calculated species concentrations. Their
Savage
596 Energy & Fuels, Vol. 9,.No. 4, 1995 1.0
,
I
0.91
0.9{
0.7; 0
.
0.6:
a 0.3:
Dodecane*
0.2
0.2,
,
0.0 . 0.0001
0.0 0.0001
0.01
0.001
0.1
1
DDP Initial Conc. (mollliter)
initial concentrations. I a
0.7 0.6 0.5
a 0.4
0,31
Dodecane
0.2
0.0001
0.001
. ,=,,
:
0,001
0.01
0.1
1
10
Figure 7. Molar yields of DDP and dodecane from DDP pyrolysis at 400 "C for 300 min at different DDP initial concentrations. Model results are from the extended mecha-
nism, but with the RHT steps removed. The ARSE values used are indicated on the curves.
1.0
3
,
DDP Initial Conc. (molfliter)
Figure 5. Molar yields of DDP, dodecane, and methylpyrene from DDP pyrolysis at 400 "C for 300 min at different DDP
0.94 0.8
...'
Dodecane
0.1
0.01
0.1
1
10
DDP Initial Conc. (mollliter)
Figure 6. Molar yields of DDP, dodecane, and methylpyrene from DDP pyrolysis at 400 "C for 300 min at different DDP initial concentrations. Model results are from the extended mechanism, but with the RHT steps removed.
analysis showed that the DDP conversion and the pyrene yield were most strongly influenced by the RRD initiation steps, RHT from an a-DDP radical, and the recombination of two a-DDP radicals and the subsequent dissociation of the product so formed. Figure 1 depicts the RRD and RHT steps. The heats of reaction and hence activation energies for these steps will be influenced by the strengths of the C-H and C-C bonds involving the a carbon in the alkyl chain and the strength of the C-H bond formed by hydrogen addition to the 1-position in pyrene. Consequently, the resonance stabilization energy (RSE) associated with the a-carbon residing at the 1-position in pyrene and the dissociation energy of the C-H bond formed by adding H to the 1-position in pyrene are key thermochemical data. Additionally, the new steps in the present extended mechanism involve hydrogen transfer to the 4-position in pyrene nuclei and transfer of an allylic hydrogen in the internal olefin to form a p7 radical. So additional thermochemical data that are possibly important include the dissociation energy of the C-H bond formed by adding H to the 4-position in pyrene and the additional stabilization energy enjoyed by the p7 radical relative to the pz radical. Effect of ARSE. As Table 1shows, a value of ARSE = 5.1 kcaVmol was used in the extended model for DDP
pyrolysis. This value was taken from Poutsma's review article,22and it is based on a correlation of experimental kinetics data.23 ARSE is the additional resonance stabilization energy associated with the a-carbon residing at the 1-position in pyrene relative to the same substituent in benzene. The literature provides other estimates for this quantity. Stein and Goldenz4published correlations based on structure-resonance theory that lead to ARSE = 6.1 and ARSE = 8.2. They also reported the results of SCF-MO calculations that gave ARSE = 8.1 kcaVmol. Herndon'sz5structure-resonance theory correlation gave ARSE = 6.3 kcaVmo1. Both Satoz6and Stein and Brow@ provide data from Huckel molecular orbital theory calculations that lead to 4.1 and 6.0 kcaVmol, respectively, for ARSE for the 9-position in anthracene. The ARSE for the 1-position in pyrene will be less than that for the 9-position in anthracene, so one can view the preceding values as upper bounds. The foregoing discussion shows that, although the precise value of ARSE is uncertain, the range of reasonable values is probably between 4.0 and 8.0 kcallmol. To determine whether this uncertainty in ARSE improves the predictions of the mechanistic model without RHT steps for DDP pyrolysis, we performed a set of calculations with different values of ARSE. Specifically, we used ARSE = 5.1 (base case), 6.6, and 8.1 kcaVmo1, recalculated numerical values for all rate constants influenced by ARSE, and then performed the model calculations using Acuchem. We only considered values of ARSE higher than that used in the base case because increasing the ARSE will increase the DDP conversion and dodecane yield predicted by the model. The base case model without RHT severely underpredicts the conversion and dodecane yield at high DDP concentrations. Figure 7 displays the results of this examination of the effect of ARSE. Inspection of Figure 7 shows that changing ARSE did not greatly alter the shapes of the curves predicted by the model. Rather it largely shifted the curve for DDP to lower yields (higher conversions). (22) Poutsma, M. L. Energy Fuels 1990,4, 113. (23) McMillen, D. F.; Trevor, P. L.; Golden, D. M. J . Am. Chem. SOC. 1980,102, 7400. (24) Stein, S. E.; Golden, D. M. J . Org. Chem. 1977,42, 839. ( 2 5 ) Herndon, W. C. J . Org. Chem. 1981,46, 2119. (26) Sato, Y. Fuel 1979,58,318.
H-Transfer Mechanisms in 1-Dodecylpyrene Pyrolysis
Energy & Fuels, Vol. 9, No. 4, 1995 597
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The final piece of thermochemical data, and one to which we expect the model results to be sensitive, is the dissociation energy of the C-H bond formed by adding H t o the 1-position in pyrene. This quantity is important because it influences the rate of hydrogen addition to the 1-position. This hydrogen addition is a prerequisite for cleavage of the aryl-alkyl C-C bond. Table 1shows that we used a value of 37 kcal/mol for this C-H bond dissociation energy (BDE). Stein and Brown16 calculated this value using Huckel molecular orbital theory. The uncertainty in their estimate is about &2 kcal/mol. Freund et al.l* estimated BDE = 35.9 kcal/mol based on a linear free energy relationship between the enthalpy of H atom addition and the kinetics of methyl radical addition t o a series of polycyclic aromatics. These two independent estimates are in good agreement. This examination of the effect of changes in this BDE on the model predictions will consider only values of BDE higher than that used in the base case. Increasing the BDE will increase the RRD rates and hence increase the DDP conversion and dodecane yield predicted by the model at high DDP concentrations. Moreover, we will consider BDE values up to 41 kcdmol, even though this takes us beyond the range of "reasonable" values. Note that this highest value for the BDE leads to rate constants for the RRD steps that are 11 times higher than the RRD rate constants in the base case model. A related but more preliminary studyzs of this system considered RRD rate constants up to 1000 times higher than a base case model. Figure 10 shows the results of the Acuchem simulations of the DDP pyrolysis model with different values for the BDE of the C-H bond in a 1-hydropyrenyl radical. The value of this BDE has very little effect on the model results at low concentrations. This result occurs because the bimolecular RRD hydrogen-transfer reactions to the 1-positions in pyrene moieties are unimportant at low concentrations. Instead, unimolecular initiation (e.g., homolytic dissociation) dominates. As the DDP concentration increases, however, the model results for the different BDE values begin to ~
(27) Lide, D. R., Ed. CRC Handbook ofchemistry and Physics, 75th ed.; CRC Press: Boca Raton, FL, 1994; p 9-64.
(28)Savage, P. E.; Kaza, S. Prepr.-Am. Chem. Soc., Diu. Pet. Chem. 1994, 39, 313.
Savage
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CONVERSION Figure 11. Reaction rate for DDP neat pyrolysis a t 400 "C a s a function of DDP conversion. The empirical model is from ref 21, the base case is the extended model, which includes RHT, and t h e third curve is from the extended mechanism, but with the RHT steps removed and the 1-hydropyrenyl radical C-H BDE = 41 kcallmol.
separate. It is clear that the higher values for the BDE give a better representation of the experimental results than do the lower values. Indeed, at the highest value of BDE = 41 kcaYmol the model results appear to be nearly as good as those from the extended model with RHT (see Figure 5). Keep in mind, however, that 41 kcaYmo1 is probably outside the range of reasonable values for this bond strength. Moreover, the model without RHT would need to be validated by comparing its predictions with additional experimental observations. One observation that this model is incapable of describing is the autocatalytic kinetics determined for neat DDP pyrolysis. Smith and Savage2I report an empirical autocatalytic rate law for DDP with parameter values determined by fitting experimental data. Figure 11shows the variation of the reaction rate with conversion for neat DDP pyrolysis at 400 "C. The three curves correspond t o the empirical rate law, the model without RHT and with a 41 kcaYmol BDE, and finally, the base case extended model, which includes RHT. It is evident that the base case reflects the experimental trend whereas the model without RHT does not. When RHT steps are removed, the model predicts a monotoni-
cally decreasing rate with conversion, which is not consistent with experimental results. To summarize, the results presented herein show that a mechanistic kinetics model for DDP pyrolysis that includes RHT (or phenomenologically similar) steps does a better job of representing the available experimental data than does a model that relies only on RRD and H atom addition reactions. An example of a reaction that is phenomenologically similar to RHT is a three-step addition, intramolecular hydrogen transfer, and elimination The absence of the RHT steps cannot be compensated for by increasing the ARSE, by increasing the C-H BDE for hydropyrenyl radicals, by including hydrogen transfer t o the 4, 5 , 9, and 10positions, or by including hydrogen transfer from the internal olefin. Summary and Conclusions
This work resulted in a more complete reaction mechanism and quantitative kinetics model for l-dodecylpyrene pyrolysis. The mechanism, which includes RHT steps, also included hydrogen transfer to 4-, 5-, 9-, and 10-positions in pyrene and hydrogen transfer from the internal olefin. The quantitative kinetics model based on this mechanism accurately predicted the effects of time, temperature, and initial concentration on the pyrolysis kinetics and on the product yields. The results of this mechanistic modeling study show that removing RHT steps from the extended DDP pyrolysis mechanism leads to a much poorer description of experimental results a t high initial concentrations. The absence of RHT could not be offset by adjusting (within reasonable bounds) the resonance stabilization energy, the C-H bond dissociation energies for hydropyrenyl radicals, or the additional stabilization associated with the double bond in the internal olefin. Adjusting the BDE for the C-H bond in 1-hydropyrenyl radicals came the closest to giving quantitative results that rivaled those from the model with RHT steps. This adjustment failed to predict the maximum rate observed experimentally a t a nonzero conversion, however. Of course, the ability of a mechanism-based model to describe experimental results is not proof of the validity of the mechanism. Nevertheless, the results of this study suggest that alkylpyrene pyrolysis might be a system in which RHT (or phenomenologically similar) steps are operative and kinetically significant. A more detailed and complete mechanism for alkylpyrene pyrolysis at high conversions, more refined estimates of the kinetics and thermochemical data, and comparisons with additional experimental data are required to test this hypothesis more fully.
Acknowledgment. Sarita Kaza, John Kolakowski, and John Santini assisted with this project. This work was supported in part by the Exxon Education Foundation. EF950005V (29) Franz, J. A.; Camaioni, D. M.; Alnajjar, M. S.; Autrey, T., Linehan, J. C. Prepr. Pap-Am. Chem. SOC.,Diu.Fuel Chem. 1995,
40, 203.
