Pyrolysis of Polycyclic Perhydroarenes. 2. 1-n ... - ACS Publications

Energy & Fuels 1997, 11, 107-115

107

Pyrolysis of Polycyclic Perhydroarenes. 2. 1-n-Undecylperhydronaphthalene Tahmid I. Mizan, Phillip E. Savage,* and Brian Perry Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136 Received June 17, 1996. Revised Manuscript Received October 1, 1996X

1-n-Undecylperhydronaphthalene (UPN), a prototypical long-chain n-alkylperhydroarene, was pyrolyzed both neat and in benzene in batch microreactors at temperatures between 375 and 475 °C. The global reaction order for UPN disappearance was 1.00 ( 0.08, so UPN pyrolysis followed first-order kinetics. The Arrhenius parameters for the first-order rate constant were A (s-1) ) 1010.9 ( 2.6 and E ) 46.5 ( 8.4 kcal/mol. All of the reported uncertainties are the 95% confidence intervals. An empirical structure-reactivity correlation in the literature, which was developed for the pyrolysis of saturated cyclic compounds with short n-alkyl chains, did not accurately predict the pyrolysis kinetics of UPN and other long-chain n-alkylperhydroarenes. UPN neat pyrolysis generated numerous primary products, and the primary products with the highest initial selectivities were octahydronaphthalene plus n-undecane, methyleneperhydronaphthalene plus n-decane, and trans-decalin plus 1-undecene. These three product pairs accounted for about 40% of the primary product spectrum from UPN. The remaining 60% was apportioned in roughly equal selectivities among 18 other primary product pairs that consisted of either an n-alkane plus an alkenylperhydronaphthalene or a 1-alkene plus an n-alkylperhydronaphthalene. Secondary reactions included dehydrogenation of decalin and octahydronaphthalene, to form tetralin and eventually naphthalene, and thermal cracking of paraffins, olefins, and other primary products. This product spectrum is consistent with a free radical chain reaction mechanism for UPN neat pyrolysis.

Introduction 1-n-Undecylperhydronaphthalene (UPN) is a polycyclic hydrocarbon bearing a pendant aliphatic chain. Compounds such as UPN, which comprise a polycyclic core and a long n-alkyl chain, are representative of thermally reactive structural elements in heavy hydrocarbon resources such as coals, heavy crude oils, and petroleum residua. Because heavy hydrocarbon resources experience elevated temperatures during their processing, a knowledge of the thermal chemistry and kinetics of these materials is important.1-3 Moreover, UPN mimics the chemical structures in potential endothermic fuels for high-performance jet aircraft. Such advanced fuels will serve as an important heat sink on future aircraft and thereby be exposed to high temperatures at which pyrolytic decomposition can occur. This consideration also demonstrates the importance of experimentally investigating and establishing the thermal stability and the pyrolysis kinetics and pathways for saturated cyclic compounds such as UPN. The thermal hydrocarbon chemistry literature is very rich, and there are extensive and numerous accounts of experimental research into the reactions of model compounds that mimic most of the structural elements in coal, heavy oils, resids, and endothermic fuels.4 One notable gap in this literature, however, concerns the * Author to whom correspondence should be addressed [e-mail [email protected]; fax (313) 763-0459; telephone (313) 764-3386]. X Abstract published in Advance ACS Abstracts, November 1, 1996. (1) Miki , Y.; Tamadaya, S.; Oba, M.; Sugimoto, Y. J. Catal. 1983, 83, 371. (2) Heck, R. H.; DiGuiseppi, F. T. Energy Fuels 1994, 8, 557. (3) Szladow, A. J.; Chan, R. K.; Fouda, S.; Kelley, J. F. Energy Fuels 1989, 3, 136.

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reactions of long-chain polycyclic n-alkylnaphthenes at fuel processing temperatures of around 400 °C. Humburg and Savage5 first reported on the pyrolysis of compounds in this class. Their work provided reaction kinetics, pathways, and mechanisms for 9-n-dodecylperhydroanthracene. This second paper in the series reports results for the pyrolysis of 1-n-undecylperhydronaphthalene. Experimental Section UPN was obtained from the Thermodynamics Research Center at Texas A&M University and used as received. UPN was pyrolyzed at 375, 385, 400, 410, 425, 435, 450, and 475 °C, in batch microreactors with holding times between 10 and 180 min. These batch reactors were fashioned from nominal 1/ in. stainless steel Swagelok tube fittings (one port connector 4 and two caps). The reactor volume was approximately 0.6 mL. Previous experimental work showed that the addition of stainless steel filings had no statistically significant effect on the pyrolysis of compounds in this class.5 When conducting the neat pyrolysis experiments, we loaded approximately 10 mg of both UPN and biphenyl into each reactor, and these quantities were weighed to within (0.1 mg. Biphenyl, being thermally stable under the conditions of the experiment, served as an internal standard for determining the mass of each compound in the reactor after the pyrolysis. Benzene was used as a diluent in a set of experiments with different initial reactant concentrations. Under the conditions of these experiments, benzene can be considered to be an inert solvent.6 When conducting pyrolysis experiments in benzene, we loaded the reactors with 0.5 mL of a benzene solution with various but known amounts of UPN and biphenyl. (4) Poutsma, M. L. Energy Fuels 1990, 4, 113. (5) Humburg, R. E.; Savage, P. E. Ind. Eng. Chem. Res. 1996, 35, 2096.

© 1997 American Chemical Society

108 Energy & Fuels, Vol. 11, No. 1, 1997

Mizan et al.

In both the neat pyrolyses and the pyrolyses in benzene, the loaded and sealed reactors were placed in a preheated, isothermal, Techne SBL-2 fluidized sand bath maintained at the desired pyrolysis temperature by a Techne TC-8D temperature controller. When the desired batch holding time was reached, the reactors were removed from the sand bath, and the reaction was quenched by immersing the reactors in water at room temperature. The reactors were then opened and their contents retrieved by repeated additions of benzene. The gaseous products could not be recovered or analyzed due to the small amount of reactant used and the even smaller amount of gas produced in the experiments. The benzene-soluble reaction products were identified by using a capillary column gas chromatograph (GC) equipped with a mass spectrometer (MS). Product identification was accomplished by comparing the retention time and mass spectrum of the GC peak for a reaction product with those of authentic samples or with mass spectra in the GC/MS computer library. The reaction products were then quantified using a GC with a flame ionization detector. The GC oven temperature program used with both instruments was, for most runs, an initial temperature of 60 °C, an initial time of 5.0 min, and a ramp of 2.0 °C/min to a final temperature of 240 °C, with no final holding time. Some runs, however, were analyzed starting at an initial temperature of 40 °C, an initial time of 5.0 min, and a ramp of 1.0 °C/min to a final temperature of 240 °C. This latter set of conditions was used to provide better separation between peaks and thereby facilitate product identification. Product molar yields, calculated as the number of moles of product formed divided by the number of moles of reactant initially loaded into the reactor, were obtained from the chromatographic analysis using experimentally determined detector response factors. The response factors of compounds such as substituted decalins, which can exist as multiple isomers and hence give multiple GC peaks, were calculated using the sum of their peak areas.

Pyrolysis Kinetics The essential elements of the present kinetics study were the determination of the reaction order for UPN disappearance and the values of the rate constants and the corresponding Arrhenius parameters. The first step in this analysis is to determine the global reaction order. Reaction Order. We completed a set of experiments designed for the specific purpose of determining the global reaction order for UPN pyrolysis. These experiments provided the UPN conversion at different initial concentrations of UPN (0.0193-0.193 mol/L) but at otherwise identical reaction conditions (425 °C and 30 min). The reactant conversions (X) were between 45 and 49% for all six experiments. These conversions are nearly the same as the 41% conversion obtained for neat pyrolysis under the same reaction conditions. Pseudofirst-order rate constants (k′) were calculated from the conversions as

k′ ) -ln(1 - X)/t

(1)

and then plotted against the mean UPN concentration in the reactor on log-log coordinates, as shown in Figure 1. The slope of the line through the data on this log-log plot is equal to the reaction order minus unity.7 An unweighted linear regression showed that the slope of the best-fit line through the data in Figure 1 is -0.002 ( 0.084, so the global reaction order is 0.998 ( 0.084, where the uncertainty stated here and throughout this (6) Savage, P. E. Ind. Eng. Chem. Res. 1994, 33, 1086. (7) Savage, P. E.; Smith, M. A. Environ. Sci. Technol. 1995, 29, 216.

Figure 1. Effect of UPN concentration on the pseudo-firstorder rate constant for UPN disappearance.

Figure 2. Effect of time and temperature on UPN molar yield during neat pyrolysis. Discrete points are experimental data. The curves are calculations based on the Arrhenius equation.

paper represents the bounds of the 95% confidence interval. In the kinetics analysis that follows, we will treat UPN pyrolysis as a global first-order reaction. Rate Constants. We used data for UPN disappearance during neat pyrolysis, such as that in Figure 2, to determine the global first-order rate constant at each temperature investigated. The rate constant was determined via unweighted linear regression of the data at each temperature according to eq 2, which is the

ln(UPN molar yield) ) ln(1 - X) ) -kt

(2)

linear form of the integrated expression for a first-order reaction in an isothermal, constant-volume batch reactor. Table 1 lists the rate constants and the associated uncertainties determined at each temperature. We next used the first-order rate constants in Table 1 to estimate numerical values for the parameters a and E in the Arrhenius equation

k ) 10a exp(-E/RT)

(3)

This was accomplished by using a nonlinear leastsquares parameter estimation protocol wherein the experimental uncertainties for each rate constant were

Pyrolysis of Polycyclic Perhydroarenes. 2

Energy & Fuels, Vol. 11, No. 1, 1997 109

Table 1. Rate Constants for UPN Neat Pyrolysis temp (°C)

k (s-1)

95% CI (s-1)

375 385 400 410 425 435 450 475

2.0 × 2.7 × 10-5 3.7 × 10-5 1.0 × 10-4 1.9 × 10-4 5.3 × 10-4 5.8 × 10-4 2.4 × 10-3

0.8 × 10-5 2.3 × 10-5 2.0 × 10-5 1.3 × 10-5 0.2 × 10-4 1.1 × 10-4 0.7 × 10-4 0.5 × 10-3

10-5

used to weight the data. The results of this analysis led to Arrhenius parameters of log A (s-1) ) 10.9 ( 2.6 and E ) 46.5 ( 8.4 kcal/mol. This activation energy is nearly the same as the value of 44.7 kcal/mol recently reported for 9-dodecylperhydroanthracene,5 but it is lower than the value of 59.4 kcal/mol reported for tridecylcyclohexane.8 The covariance of the parameters a and E (Cova,E) is 3.677 kcal/mol, which becomes 22.0 kcal/mol at the 95% confidence level where the t statistic is 2.45 for the six degrees of freedom in this regression. This covariance is required to estimate the uncertainty in the rate constant at a given temperature, as described in our first paper in this series.5 The relevant equation is 2

(∆kk)

) [(ln 10)∆a]2 +

2

(∆E RT)

-

(ln 100)Cova,E (4) RT

Using the uncertainties in the parameters a and E along with the covariance of these parameters at the 95% confidence level in eq 4 allows one to estimate the uncertainty in the rate constant for UPN neat pyrolysis at any temperature within the range investigated experimentally. For example, we calculate that ∆k/k ) 0.23 at 450 °C, where ∆k is the uncertainty in the rate constant at the 95% confidence level. The nonlinear regression also led to a value of χ2ν ) 1.22. The chi-square statistic, χ2ν , is the final value of the minimized objective function (weighted sum of squares) divided by the number of degrees of freedom in the regression. The value of this statistic provides an assessment of the goodness of fit of the model. A rule of thumb in parameter estimation is that values of χ2ν = 1.0 indicate a moderately good fit of the data to a proposed model. Thus, we conclude that the Arrhenius equation provides a good fit of the experimental kinetics data for UPN pyrolysis. Figure 2 demonstrates the ability of the Arrhenius parameters above to describe experimental data for UPN disappearance during neat pyrolysis. Comparison with Kinetics of Related Compounds. The literature dealing with pyrolysis kinetics of long-chain alkylnaphthenes at fuel processing temperatures is not extensive. Data are available only for tridecylcyclohexane8,9 (TDC), dodecylperhydroanthracene,5 (DDPA), and, now, undecylperhydronaphthalene (UPN). Each of these three compounds follows firstorder disappearance kinetics, and the rate constants at 700 K are 2.1 × 10-4, 2.2 × 10-4, and 3.5 × 10-4 s-1, for UPN, TDC, and DDPA, respectively. This ranking of relative reactivities is fully consistent with that predicted by the empirical group contribution correlation (8) Savage, P. E.; Klein, M. T. Ind. Eng. Chem. Res. 1988, 27, 1348. (9) Mushrush, G. W.; Hazlett, R. N. Ind. Eng. Chem. Fundam. 1984, 23, 288.

Figure 3. Decomposition kinetics for saturated cyclic compounds at 700 K (800 °F) as a function of the characterization number of Fabuss et al.10 Data for TDC are from Savage and Klein.8 Data for DDPA are from Humburg and Savage.5

of Fabuss et al.10 These authors correlated the apparent first-order decomposition kinetics at 800 °F (700 K) for 28 different saturated cyclic compounds with a structurebased characterization number. Their working hypothesis was that the rate depended on the types of C-H bonds present in the molecule and that it was proportional to the number of each type. Thus, assigning a contribution to each type of group in the molecule and summing the contributions of the different groups led to a characterization number, n, for the molecule. For example, the characterization number for UPN is calculated as 14 for the decalin moiety, plus 40 for the CH2 groups in the alkyl chain (10 CH2 groups times 4 each), plus 2 for the terminal methyl group in the chain, minus 1 for the one C-H bond on the ring replaced by an alkyl substituent, which gives a total of n ) 55 for UPN. The authors’ structure-reactivity correlation was

k (h-1) ) 0.044 - 0.0114n + 0.008n2

(5)

and the compounds included in their experimental study had characterization numbers that ranged from 10 to 37. We note that the equation above shows 0.0114 as the coefficient for the second term, whereas the equation in Fabuss et al. shows 0.114. We believe the equation in Fabuss et al. contains a typographical error because using 0.114 in eq 5 leads to negative values for the rate constants. Figure 3 shows the correlation of eq 5 plotted along with the experimental rate constants for UPN, TDC, and DDPA at 700 K. The other discrete points in Figure 3 are experimental data reported by Fabuss et al. for n-propyl- and n-butylcyclohexane, 1-ethyldecalin, and 9-ethylperhydroanthracene. These 4 points, and 24 others, were used to develop the correlation. It is clear that the experimental kinetics for long-chain naphthenes follow the qualitative trend predicted by the Fabuss et al. correlation, but the quantitative predictions are too high by a factor of 2-3. This failure to predict the kinetics for long-chain compounds based on the kinetics of short-chain compounds emphasizes the need for experimental work such as that reported in this paper. This loss in accuracy as the correlation is extrapolated to higher characterization numbers was (10) Fabuss, B. M.; Kafesjian, R.; Smith, J. O.; Satterfield, C. N. Ind. Eng. Chem. Process Des. Dev. 1964, 3, 248.


Mizan et al.

Table 2. Molar Yields (Percent) of Major Products from UPN Neat Pyrolysis 375 °C product 1-nonene nonane 1-decene decane 1-undecene undecane naphthalene tetralin octahydronaphthalene trans-decalin methylenedecalin 1-methylnaphthalene trans-methyldecalin UPN a

400 °C

30 min

90 min

150 min

0.1 0.2 0.1 0.4 0.5 0.5

02 0.3 0.2 0.9 0.8 1.0

0.2 0.3 0.3 1.1 1.0 1.3

0.8 0.4 0.6

1.4 0.7 1.1

0.4 0.5 0.5 1.5 1.4 1.6 0.1 0.2 2.3 1.4 1.4

0.2 88

0.5 85

88

30 min

425 °C

450 °C

475 °C

120 min

180 min

15 min

49 min

120 min

180 min

30 min

90 min

180 min

20 min

40 min

60 min

0.9 2.1 0.9 5.4 1.8 5.0 0.5 2.3 5.2 7.4 3.4 0.4 3.8 21

0.8 1.1 0.7 2.2 1.5 2.2 0.8 3.5 4.0 7.1 2.4 0.7 3.3 14

0.5 1.3 0.2 2.1 0.5 1.9 3.0 5.0 2.7 8.0 1.7 1.2 3.0 3.8

0.1 0.9

0.2 0.7 0.1 1.2 0.2 0.6 4.9 4.3 1.0 4.5 0.7 1.6 1.3 8.9a

0.1 0.3 0.3 1.5

0.1 0.2 0.3 1.5

0.9 81

0.5 0.5 0.6 1.5 1.8 1.6 0.1 0.1 2.8 1.6 2.4 0.2 0.5 96

0.9 1.6 1.0 4.1 2.6 4.3 0.4 0.6 5.5 4.8 4.1

0.2 100

0.6 1.1 0.7 3.7 2.1 3.9 0.3 0.3 4.3 3.2 2.6 0.1 1.3 62

1.5 0.7 1.5 1.2 3.1 1.1

1.7 0.9 1.3

0.6 0.9 0.7 3.0 2.2 3.2 0.2 0.2 4.0 2.6 2.8

0.1 8.9 1.3 0.3 3.0 0.2 2.7 0.6 0.3a

0.2 9.2 1.5 0.4 3.3 0.2 2.9 0.6 0.1

2.3 47

1.7 5.4 4.5 5.1 2.1 29

1.2 1.0 7.7 3.2 1.2 7.0 0.7 3.0 1.9 0.4

Mean of values displayed in Figure 2.

anticipated by Fabuss et al., however, who recognized that eq 5 would become less reliable for naphthenes with long alkyl side chains. Indeed, as the length of the side chain increases, the reactivity of the compound is influenced more by the side chain than by the naphthenic group. Compounds with very long chains can thus be thought of as being more like a paraffin with a naphthenic substituent than a naphthene with an aliphatic substituent. Products and Pathways The reaction products from UPN neat pyrolysis included n-alkanes, 1-alkenes, and various decalins, tetralins, naphthalenes, and benzenes with and without pendant alkyl and alkenyl chains. Note that decalin is a shorthand name for decahydronaphthalene, which is the same compound as perhydronaphthalene. Likewise, tetralin is tetrahydronaphthalene. The hydroaromatic and aromatic products were present in measurable yields only at high temperatures and long reaction times. Table 2 provides representative results for the molar yields of selected products from UPN neat pyrolyses at temperatures of 375, 400, 425, 450, and 475 °C. Nine of the 12 products in Table 2 have been positively identified. That is, the suspected pyrolysis product was commercially available, and the GC retention time and mass spectrum of the compound matched those of the reaction product. The three products that have been identified less definitively are octahydronaphthalene, methylenedecalin, and trans-methyldecalin. These identifications are based on relative GC retention times and molecular weights (MW) as revealed upon inspection of the mass spectra. We quantified the yields of two different octahydronaphthalenes (MW ) 136) and two different methylenedecalins (MW ) 150). For both products, the results in Table 2 show the sum of the yields of the two isomers. Quantitative analysis of the larger octahydronaphthalene peak was complicated by cis-decalin eluting from the GC column at precisely the same retention time. The mass spectrum corresponding to this GC peak from pyrolyses at mild conditions (375 °C) had a large peak at a mass/charge ratio (m/z) of 136, indicating the presence of octahydronaphthalene. A much smaller peak at m/z ) 138 was also present in the mass spectrum, however, which indicates the presence of a much smaller amount of cis-decalin. At more severe reaction conditions (450 °C, 180 min) the height of the peak at m/z ) 138 exceeded that of the peak at

m/z ) 136, but both peaks were still present. This observation indicates that under these more severe reaction conditions the GC peak was due primarily to cis-decalin but also had a smaller contribution from octahydronaphthalene. Because we could not accurately determine the absolute amounts of each of these two products separately, we show the total contribution from this GC peak in the octahydronaphthalene entries in Table 2. Including the small amount of cis-decalin in the octahydronaphthalene yield will cause the octahydronaphthalene yield to be overestimated and the total decalin yield (cis- plus trans-) to be underestimated. We will return to this point later in our discussion of the product selectivities. Table 2 shows that the most abundant products at 375 °C were 1-undecene, n-undecane, trans-decalin, octahydronaphthalene, n-decane, and methylenedecalin. No other products were present in yields exceeding 1% at this temperature. The yields of these six products increased with batch holding time at 375 °C. At 400 °C, the yields of these six products continued to increase, within the uncertainty in the experimental data, and they reached values ranging from 2.5 to 4.8%. At 425 °C, the yields of all six products except trans-decalin exhibited distinct maxima between 60 and 120 min. The trans-decalin yield, in contrast, continued to increase with increasing time at 425 °C. At 450 °C, however, the trans-decalin yield reached a maximum value of 8% at 90 min. That these six products are the only ones present in yields >1% at 375 °C indicates that these are the major primary products. Moreover, that these product yields display maxima at the higher temperatures is indicative of these products reacting further as the reaction conditions become more severe. As the pyrolysis temperature increased, products in addition to the six discussed above surpassed the 1% yield level. At 400 °C and the longer times of 150 and 180 min, the yields of nonane and trans-methyldecalin exceeded 1%. The yields of both products continued to increase at 425 °C until reaching maximum values of 2.1 and 3.8%, respectively. At 425 °C, the yield of 1-decene exceeded 1% and reached a maximum of 1.4% at 60 min. The yield of tetralin likewise exceeded 1% at 425 °C, but, unlike decene, the tetralin yield increased monotonically at this temperature. The tetralin yield did reach a maximum of 5% at 450 °C and 90 min. Finally, the yields of naphthalene and 1-methylnaphthalene exceeded 1% at 450 °C. The yield of 1-methylnaphthalene was always lower than that of naphtha-



Figure 4. Delplots for products of UPN neat pyrolysis: (a) n-undecane and octahydronaphthalene; (b) 1-undecene and transdecalin; (c) n-decane and methylenedecalin; (d) n-nonane and 1-decene; (e) tetralin and naphthalene.

lene. The naphthalene yield increased monotonically to 9% at the most severe conditions investigated (475 °C, 60 min). It is noteworthy that at 475 ˚C the molar yield of naphthalene continued to rise rapidly between holding times of 10 and 60 min even though most of the UPN had been exhausted by about 20 min. This behavior indicates that naphthalene is a product of the decomposition of some other pyrolysis products. A more definitive and more quantitative definition of the different paths in the reaction network can be obtained by employing the Delplot technique,11 which involves inspection of plots of product selectivities (molar yield/UPN conversion) as a function of reactant (11) Bhore, N. A.; Klein, M. T.; Bischoff, K. B. Ind. Eng. Chem. Res. 1990, 29, 313.

conversion. Primary products, which form directly from the reactant, by definition have nonzero initial selectivities. Accordingly, products that display positive yintercepts on the Delplots are primary products. Products with y-intercepts equal to zero have zero initial selectivity, which indicates that these products appear later in the reaction network. Thus, by examining the Delplot intercepts for the different reaction products, one can discriminate between primary and nonprimary products. Figure 4 displays the Delplots for UPN pyrolysis. The UPN conversions used to make these plots were calculated from eq 2 and the rate constants in Table 1. We chose not to use the experimental conversions because of the importance of the low-conversion region in the


Delplots and the large relative uncertainty in the experimentally measured conversions when they are low. The data in Figure 4 are from experiments that span a 100 °C temperature range, and this factor contributes to the scatter in the data. Figure 4a shows that both n-undecane and octahydronaphthalene possess nonzero y-intercepts and thereby confirms that these are primary products. The selectivity to both products decreases with increasing conversion, especially at conversions >40%. This behavior is consistent with these products decomposing via secondary reactions. The octahydronaphthalene selectivity is consistently higher than that of undecane, possibly because the octahydronaphthalene yield is overestimated as discussed above. It is reasonable that octahydronaphthalene and undecane were formed in the same step since the sum of their molecular structures and chemical formulas is equal to that of the reactant, UPN. The initial selectivity to this product pair can be estimated as being roughly 15%. Figure 4b shows that both 1-undecene and transdecalin are primary products. The selectivity to 1-undecene is slightly higher than that to trans-decalin, and this difference in their selectivities might be due to the contribution from cis-decalin being excluded here. In any case, the selectivities for these two products are nearly equal at low conversions, which is the expected behavior for two products formed in the same reaction step. The initial selectivity to this product pair is about 10%. The decalin selectivity remains at about 10% until very high conversions are reached, at which point the selectivity decreases precipitously. This behavior demonstrates the thermal stability of decalin at all but the most severe conditions employed in this investigation. The selectivity to 1-undecene, on the other hand, which declines monotonically with conversion, demonstrates the reactivity of R-olefins at these temperatures. Figure 4c shows that both n-decane and methylenedecalin are primary products with an initial selectivity of about 15%. This product pair could have been formed in the same step because the sum of their molecular structures and chemical formulas is equal to that of the reactant, UPN. The behavior of the decane selectivity is similar to that of undecane in Figure 4a in that it is nearly constant until about 40% conversion, at which point the selectivity declines with conversion. The methylenedecalin selectivity data show a similar trend. The sum of the estimated initial selectivities for the six major primary products is about 40%. Therefore, the numerous minor primary products must account for the remaining 60% of the reacted UPN. If we assume that all of these minor primary products form in roughly equal amounts and further recognize that there are nine different C1-C9 alkanes and nine different C2-C10 R-olefins that can appear as minor products, then the initial selectivity to any one of these products should be about 60/18 ) 3.3%. Figure 4d is a Delplot for two of these minor products, 1-decene and n-nonane. These are minor products in that they were not present in a 1% yield at 375 °C. Figure 4d confirms that these are also primary products and that the initial selectivity is about 3%, as expected. Figure 4e shows that neither tetralin nor naphthalene is a primary product. The initial selectivities for these two products are clearly equal to zero. The selectivity

Mizan et al.

Figure 5. Reaction network for UPN neat pyrolysis.

to tetralin increases with conversion until a conversion of about 90% is reached, at which point the tetralin selectivity rapidly declines. The selectivity to naphthalene, on the other hand, increases slowly with conversion until high conversions are reached. At conversions exceeding 90%, the selectivity to naphthalene increases rapidly while the selectivity to tetralin decreases rapidly. This observation is consistent with tetralin being dehydrogenated to form naphthalene. The discussion of pyrolysis products and pathways in this section can be summarized by the reaction network in Figure 5, which shows that UPN pyrolyzes through four parallel primary paths. The path in Figure 5 with the highest total selectivity (about 60%) is the lumped set that leads to the numerous minor products. The selectivity to this product lump is high because it comprises nearly 20 different product pairs. The selectivity to any individual pair of minor products is only about 3%, however. The paths to the product pairs with the highest individual selectivities lead to octahydronaphthalene plus n-undecane (about 15%), to methylenedecalin plus decane (about 15%), and to decalin plus 1-undecene (about 10%). Dehydrogenation of the decalin and octahydronaphthalene formed in the primary reactions can lead to tetralin and naphthalene. These dehydrogenation reactions would be expected to proceed through hexahydro- and dihydronaphthalenes, but these products were not detected, perhaps because they are very reactive at the temperatures used in our experiments. Although not explicitly shown in Figure 5, all of the primary products undergo secondary decomposition reactions. This reaction network shows that the primary bond cleavage reactions occur along



the aliphatic chain and that the saturated rings in UPN are largely conserved in the primary reactions. The present results for UPN and results in the literature for tridecylcyclohexane8,9 and 9-dodecylperhydroanthracene5 allow a preliminary assessment of the effect of the number of saturated rings on the pyrolysis pathways. All three of these compounds react similarly in that the primary cleavage of C-C bonds occurs in the alkyl chain and does not involve ring-opening reactions. The reaction networks differ, however, in that cyclohexene was not a major product from tridecylcyclohexane pyrolysis, whereas the cycloalkenes octahydronaphthalene and dodecahydroanthracene were major products from UPN and dodecylperhydroanthracene neat pyrolysis, respectively. Thus, the formation of the cycloalkene and the corresponding n-alkane was a more facile reaction for these polycyclic compounds than for the alkylcyclohexane. This difference in the reaction network might be related to the n-alkyl substituent residing on a carbon atom that is also bonded to a bridgehead carbon in the polycyclic naphthenes. As will be discussed in the next section, this arrangement provides for relatively facile hydrogen abstraction and β-scission reactions that lead to the formation of the cycloalkene and the corresponding n-alkane. If this explanation is correct, then polycyclic perhydroarenes with an n-alkyl substituent further from a bridgehead carbon, as in a 2-alkyldecalin, for example, should exhibit lower selectivities for the formation of the cycloalkene and the corresponding n-alkane. Reaction Mechanism Hydrocarbon pyrolysis mechanisms usually comprise the reversible free radical reactions of homolytic dissociation, disproportionation, isomerization, β-scission, and hydrogen abstraction. The first two steps are the important initiation and termination events. The last two steps often occur together in a chain propagation sequence. That is, a radical (R•) abstracts a hydrogen atom from the reactant to form a molecule (RH) and a new radical. This reactant-derived radical then decomposes by breaking a bond β to the radical center (β-scission) to produce a molecule with a double bond involving the carbon atom that had been the radical center and to regenerate the abstracting radical. These and other elements of free radical hydrocarbon chemistry were developed years ago by Rice and Herzfeld,12 Kossiakoff and Rice,13 and Fabuss, Smith, and Satterfield,14 whose contributions provide the foundation for hydrocarbon pyrolysis mechanisms. The discussion that follows builds on this foundation. When a hydrocarbon can crack via several different free radical chain reactions that operate in parallel, as in the present case, the chains that involve cleavage of the weakest C-H and C-C bonds would be expected to lead to the most abundant pyrolysis products. In this light, we note that the relative strength of C-H bonds15 is in the order primary > secondary > tertiary. Thus, products that arise from chain reactions that include (12) Rice, F. O.; Herzfeld, K. F. J. Am. Chem. Soc. 1934, 56, 284. (13) Kossiakoff, A.; Rice, F. O. J. Am. Chem. Soc. 1943, 65, 520. (14) Fabuss, B. M.; Smith, J. O.; Satterfield, C. N. In Advances in Petroleum Chemistry and Refining; McKetta, J. J., Ed.; WileyInterscience: New York, 1964; Vol. 9, pp 156-195. (15) McMillen, D. F.; Golden, D. M. Annu. Rev. Phys. Chem. 1982, 33, 493.

Figure 6. Chain reactions that form the primary products in UPN neat pyrolysis.

abstraction of tertiary hydrogen atoms should be present in high relative yields. Most of the C-C bonds in the alkyl chain have essentially identical bond dissociation energies15 because they join two secondary carbon atoms. The only exceptions are the terminal C-C bond, which is slightly stronger because it involves a primary carbon, and the C-C bond that joins the alkyl chain to the ring, which is slightly weaker because it includes a tertiary carbon atom. Thus, products that arise from chain reactions that include β-scission of the C-C bond that joins the alkyl chain to the decalin moiety are expected to be major products. Figure 6 displays the chain reactions that most likely govern each of the four parallel primary paths shown in Figure 5. Each of the first four chains, which account for formation of the observed major primary products, includes abstraction of a tertiary hydrogen and/or β-scission of the weakest C-C bond in the alkyl chain. The first chain in Figure 6 involves hydrogen abstraction from UPN by an n-undecyl radical to produce a tertiary UPN radical and n-undecane. The tertiary


UPN radical then decomposes through β-scission to produce an octahydronaphthalene and regenerate the n-undecyl radical. The second chain likewise leads to n-undecane and an octahydronaphthalene as products. This second chain proceeds via hydrogen abstraction at the 2-position in UPN, which is a secondary carbon. Our detection of two different octahydronaphthalenes among the products of UPN pyrolysis is consistent with the operation of these two chains, but we could not assign the precise location of the double bond in either of the two different isomers. In the third chain, a decalyl radical abstracts a hydrogen atom from the β-carbon in the alkyl chain in UPN to produce a β-UPN radical and decalin. The β-UPN radical then decomposes to produce 1-undecene and regenerate the decalyl radical. The fourth chain includes hydrogen abstraction from the 1-position in UPN by an n-decyl radical to produce n-decane and a tertiary UPN radical, which then undergoes β-scission to produce methylene decalin and regenerate the n-decyl radical. The fifth and final chain reaction in Figure 6 shows a hydrocarbon radical (alkyl or alkyldecalyl), denoted by R•, abstracting one of the aliphatic hydrogens in UPN, and the thus-derived UPN radical decomposing to form a molecular product (alkene or alkenyl decalin) and the hydrocarbon radical. It is because the first four chains include the hydrogen abstraction and β-scission steps with the weakest C-H and C-C bonds that their products formed in the highest yields during UPN neat pyrolysis. The fifth chain, in contrast, involves abstraction of secondary hydrogen atoms and scission of bonds between secondary carbon atoms. Thus, the individual products from this chain should be present in lower yields. Additional discussion of the mechanism of alkylperhydroarene pyrolysis appeared in the first paper in this series.5 The initial selectivity of UPN to the cyclic olefin octahydronaphthalene being higher than that of tridecylcyclohexane (TDC) to the analogous cyclic olefin (cyclohexene) is the main difference in the product distributions for the polycyclic compound UPN and the single-ring analog, TDC. Indeed, octahydronaphthalene was one of the major primary products from UPN, whereas the initial selectivity of TDC to cyclohexene was about only half of that to the major primary products.8 In the previous section we attributed this difference to TDC having no tertiary bridgehead C-H bonds. In this section we provide a quantitative test of this hypothesis. If we assume that the amount of the cycloalkene formed is controlled by the hydrogen abstraction kinetics and that the dissociation energy of the secondary C-H bonds in cyclohexane is equal to that of the secondary C-H bonds in UPN and further recognize that there are four such C-H bonds β to the alkyl-substituted carbon in TDC but only two such secondary C-H bonds in UPN, we conclude that the single tertiary hydrogen at the bridgehead position in UPN increases the relative selectivity to the cycloalkene by roughly the same amount as six secondary hydrogen atoms. Thus, the relative reactivity ratio in this case is about 6 to 1, on a per hydrogen atom basis, which with an EvansPolanyi R of 0.5 and equal pre-exponential factors for the hydrogen abstraction steps lead to an estimated difference of about 5 kcal/mol for the dissociation energy of the tertiary C-H bond relative to the secondary C-H

Mizan et al.

bonds in UPN. Since this value is higher than the 2-3 kcal/mol difference typically ascribed to secondary and tertiary C-H bond strengths in acyclic alkanes, we performed semiempirical quantum chemistry calculations to estimate this difference in bond dissociation energies for decalin. Calculations based on MNDO16 and AM117 led to values of 5.2 and 3.3 kcal/mol, respectively, for the difference in the strengths of the tertiary and secondary C-H bonds. Our MNDO results were consistent with those of Kurtz et al.,18 who calculated heats of formation for decalin and the 9-decalyl radical. The MNDO difference of 5.2 kcal/mol is close to the estimate of 5.0 kcal/mol based on the analogy with TDC, whereas the AM1 result is lower and closer to the difference in bond strengths in alkanes. Thus, these calculations are sufficiently uncertain that they are inconclusive. It is possible that the C-H bond strengths in decalin are the same as in acyclic alkanes and that the higher yields of the cyclic olefin (octahydronaphthalene) in UPN pyrolysis are due to a higher Arrhenius pre-exponential factor for abstraction of the bridgehead hydrogen. Of course, another explanation is that the formation of octahydronaphthalenes is not governed by the kinetics of the hydrogen abstraction steps and that chain transfer steps not shown in Figure 6 influence the product distribution. Resolution of these alternate scenarios requires additional study. Summary and Conclusions This paper provides only the second report on the pyrolysis of a long-chain n-alkylperhydroarene, and the first report on the pyrolysis of a long-chain n-alkylperhydronaphthalene. 1-Undecylperhydronaphthalene (UPN) neat pyrolysis follows first-order kinetics. The Arrhenius parameters are A (s-1) ) 1010.9 ( 2.6 and E ) 46.5 ( 8.4 kcal/mol, and the uncertainties are the 95% confidence intervals. The rate constant for UPN disappearance at 700 K was lower than that predicted by the group contribution correlation of Fabuss et al.,10 which had been developed for saturated cyclic compounds with short alkyl chains. This expected result demonstrates the need for a new structure-reactivity correlation to generalize the kinetics of n-alkylperhydroarene pyrolysis. The reaction network for UPN pyrolysis includes parallel primary reactions to form undecane plus octahydronaphthalene, 1-undecene plus trans-decalin, and decane plus methylenedecalin. About 40% of the UPN reacted to form these major primary products. The remaining 60% reacted to form numerous minor primary products such as other n-alkanes, R-olefins, alkyldecalins, and alkenyldecalins. These products can be lumped together and considered to form via a fourth parallel path in the UPN reaction network. The octahydronaphthalene and decalin undergo dehydrogenation to form tetralin, and eventually naphthalene. This reaction network is consistent with a free radical reaction mechanism governed by a set of parallel chain reactions. Each chain includes a hydrogen abstraction step and a β-scission step. (16) Dewar, M. J. S.; Thiel, W. J. Am. Chem. Soc. 1977, 99, 4899. (17) Zoebisch, E. G.; Healy, E. F.; Dewar, M. J. S. J. Am. Chem. Soc. 1985, 107, 3902. (18) Kurtz, H. A.; Lloyd, R. V.; Williams, R. V. J. Org. Chem. 1987, 52, 302.


The reaction network and proposed mechanism for UPN pyrolysis are analogous with those previously advanced for a long-chain 9-n-alklyperhydroanthracene. These two polycyclic perhydroarenes, which have the alkyl substituent on a carbon atom that is also bonded to a bridgehead carbon, show elevated selectivities (relative to that from the pyrolysis of n-alkylcyclohexanes) for the primary reactions that form a cycloalkene plus an n-alkane. In all other respects the qualitative features of the reaction networks for the one-, two-, and three-ring n-alkylperhydroarenes investigated to date were identical. Thus, we conclude that the number of saturated rings in the naphthenic core has little influence on the reaction network, but the position of the substituent relative to any bridgehead carbons can affect the selectivity for one of the pyrolysis pathways.


Glossary a A E k k′ m/z MW n R t T Covi,j

log10 of the Arrhenius pre-exponential factor Arrhenius pre-exponential factor Arrhenius activation energy reaction rate constant pseudo-first-order reaction rate constant mass/charge ratio for ion in mass spectrometer molecular weight structure-based characterization number for saturated cyclic compounds gas constant time, t-statistic absolute temperature covariance of parameters i and j

Greek Letters χν2 ∆

chi-square statistic, per degree of freedom uncertainty EF960094R

Pyrolysis of Polycyclic Perhydroarenes. 2. 1-n ... - ACS Publications

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