Ind. Eng. Chem. Res. 1989, 28, 645-654
645
KINETICS AND CATALYSIS Autocatalysis and Aryl-Alkyl Bond Cleavage in 1-Dodecylpyrene Pyrolysis P h i l l i p E. Savage,* G a r r y E. Jacobs,+ a n d Minoo J a v a n m a r d i a n Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109
1-Dodecylpyrene (DDP) pyrolysis at 375-425 "C for 10-180 min led to 1-methylpyrene, 1-undecene, 1-ethylpyrene, and n-decane as the major products at low conversions. At higher conversions, pyrene and n-dodecane were the major products. Minor products included a series of n-alkanes, a-olefins, and alkyl-substituted pyrenes. The variations of the products' molar yields with substrate conversion suggested that the reaction pathways operative a t low DDP conversions were analogous to those observed in alkylbenzene pyrolysis. However, a different pathway involving facile and apparently autocatalytic cleavage of the strong aryl-alkyl C-C bond was dominant at moderate and high DDP conversions. The kinetics of DDP disappearance were also consistent with autocatalytic decomposition. The present results suggest that thermal cleavage of aryl-alkyl C-C bonds in heavy hydrocarbon resources such as petroleum residua, asphaltenes, and coal might be more prevalent than previously thought. The majority of processes for converting and upgrading heavy hydrocarbon resources such as petroleum residua, asphaltenes, and coal, though nominally catalytic, operate at elevated temperatures where purely thermal reactions can contribute to the observed product spectra and kinetics. In fact, recent kinetics studies of direct coal liquefaction (Gollakota et al., 1985) and asphaltene hydroprocessing (Savage et al., 1988) showed that thermal reactions proceeded at rates comparable to reactions carried out in the presence of a catalyst. This affords considerable practical significance to the thermal reactions that occur. Unfortunately, the physical and chemical complexity of these heavy feedstocks frequently obscures the fundamental reaction pathways, kinetics, and mechanisms operative during the conversion and upgrading processes. Consequently, model compounds, which mimic key reactive moieties in the more complex systems, have often been used to probe and resolve their thermal reaction fundamentals. Rational selection of relevant model compounds requires knowledge of the key structural features of the complex material of interest. Fortunately, considerable effort has been expended on characterizing heavy hydrocarbon resources, and structural details such as the types and relative proportions of functional groups and chemical moieties are available. This structural scrutiny has revealed that aliphatic chains (at times, long ones) bonded to condensed aromatic rings exist in coal (Calkins, 1984; Nelson, 1987; Nelson et al., 1988) and are important moieties in heavy oils and asphaltenes (Bunger and Li, 1981; Yen et al., 1984; Speight, 1980; Suzuki et al., 1982; Savage et al., 1985). Furthermore, there is a general concensus that the number of rings in the aromatic moieties in coal, petroleum residua, and asphaltenes ranges from 1to about 6 (Speight, 1972,1986; El-Mohamed et al., 1986; Schucker and Keweshan, 1980; Speight and Pancirov, 1983;
* Corresponding author.
Present address: Fluor Daniel Co., Irvine, CA 92730.
Serio et al., 1987). Thus, it is apparent that long-chain alkyl-substituted aromatic compounds, both single-ring and condensed, are relevant model compounds. Consequently, studying the reactions of such compounds could provide considerable insight to the reactions of the analogous alkyl aromatic moieties in the more complex systems. The literature provides several accounts of the pyrolysis of long-chain (XI,,) alkylbenzenes (Billaud et al., 1988; Blouri et al., 1985; Mushrush and Hazlett, 1984; Savage and Klein, 1987a,b, 1989). These investigations revealed that the major primary products from the pyrolysis of an alkylbenzene with an aliphatic chain containing N carbon atoms were toluene plus an a-olefin with N - 1 carbon atoms and styrene plus an n-alkane with N - 2 carbon atoms. The appearance of these as the major product pairs was consistent with the relevant bond energetics. Freeradical, concerted retro-ene (Mushrush and Hazlett, 1984), and four-centered molecular schemes (Blouri et al., 1985) were all offered as possible reaction mechanisms, but Savage and Klein (1987a,b, 1989) demonstrated that the pyrolyses were entirely via free-radical chains. The pyrolysis of long-chain alkyl aromatics containing more than one aromatic ring has received considerablyless attention. The work of Billaud et al. (1988), who studied the steam cracking of tetradecylphenanthrene and octadecylnaphthalene in decane at 800 "C, and Javanmardian et al. (1988), who reported preliminary results from our present study of dodecylpyrene pyrolysis, are the only previous accounts of the pyrolysis of long-chain, polycyclic n-alkyl aromatics. Billaud et al. (1988) reported the presence of vinyl-, ethyl-, and methyl-substituted aromatics as major reaction products. They reported no kinetics and very limited reaction pathways, and interpretation of their results is complicated by the likely presence of kinetic interactions involving the n-decane solvent. Additionally, the temperature at which they accomplished the steamcracking experiments (800 "C) is significantly higher than the temperatures of interest in this study (e.g., 400 f 25
0888-58851 8912628-0645$01.50/0 0 1989 American Chemical Society
646 Ind. Eng. Chem. Res., Vol. 28, No. 6, 1989
"C). Nevertheless, Billaud et al. (1988) showed that their results were consistent with the free-radical mechanisms advanced for the pyrolysis of n-alkylbenzenes, and they concluded that the pyrolysis pathways were independent of the number of rings in the aromatic portion of the molecule. This conclusion, however, is in contrast with the results of Javanmardian et al. (1988), who noted striking differences between the pyrolysis of dodecylpyrene and dodecylbenzene. Clearly the reaction fundamentals for the pyrolysis of polycyclic alkyl aromatics, though of considerable significance, remain largely unresolved. To provide further details in this area, we report here on the pyrolysis of 1-dodecylpyrene.
Experimental Section 1-Dodecylpyrene (DDP) was pyrolyzed a t isothermal temperatures between 375 and 425 "C for batch holding times between 10 and 180 min. Materials. All chemicals were obtained commercially and used as received. DDP was obtained from Molecular Probes, and gas chromatographic analysis demonstrated that the purity exceeded 98%. 1-Butylpyrene, used in some of the background experiments described later in this section, was obtained from API Standard Reference Materials with a nominal purity of 99%. DDP pyrolyses were accomplished in batch tubingbomb microreactors fashioned from a nominal 3/8-in. Swagelok stainless steel steel port connector and two 3/8-in. stainless steel caps. Reactors were cleaned between runs by soaking them successively in acetone, 10% HN03, and acetone. Pyrolyses were performed in both new reactors and in cleaned (used) reactors to verify that the reactor cleaning procedure did not alter the results. Procedure. The tubing-bomb microreactors were typically loaded with 20-30 mg of DDP, and 10-15 mg of biphenyl, a demonstrably inert compound, was added to serve as an internal standard in the ensuing chromatographic analyses. All quantities were carefully (k0.l mg) weighed. After being purged with argon, the reactors were placed in a glovebag under an argon atmosphere for several hours to provide an inert environment for the pyrolyses. The reactors were then closed, removed from the glovebag, and immersed in a preheated, isothermal, temperaturecontrolled, fluidized sand bath. The reactors achieved the desired pyrolysis temperature after a heat-up time of about 3 min (Lawson and Klein, 1985), which is short in comparison with the typical batch holding times employed in this investigation. After the desired reaction time had elapsed, the reactors were removed from the sand bath, rapidly cooled to room temperature by immersing them in cold water, and then opened. The reaction products were extracted in toluene and analyzed chromatographically. Acetone was used as the solvent in earlier work (Javanmardian et al., 1988), but we subsequently discovered that acetone did not completely dissolve the high molecular weight, long-chain alkylpyrenes. Analytical Chemistry. The reaction products were routinely analyzed with a Hewlett-Packard Model 5890 gas chromatograph (GC) operated in the split injection mode and equipped with a flame ionization detector (FID). Helium served as the carrier gas, and the injector inlet and detector temperatures were both 275 "C. The oven temperature was programmed from 50 to 275 "C at a rate of 4 OC/min and then maintained at 275 "C until the DDP eluted from the column. Two different capillary columns were used in the quantitative analyses. A 50-m X 0.2-mm i.d. HP-5 capillary column with a 0.33-ym film thickness resolved all
of the individual reaction products but provided reliable quantitation of only the lower molecular weight (C260) compounds. The high molecular weight, pyrene-containing compounds possessed very long retention times and broad peaks, which frustrated reliable quantitative analysis. However, a 5-m X 0.53-mm i.d. HP-1 capillary column with a 2.65-pm film thickness adequately resolved the high molecular weight compounds; thus, it was used to separate and quantitate the yields of the pyrene-containing compounds. Pyrene and methylpyrene could be analyzed reliably on either column, and the molar yields calculated for these products from the two different columns typically exhibited a less than 10% difference. GC-MS analyses of selected samples were performed at the mass spectrometry lab in the Chemistry Department at the University of Michigan. Sample constituents were separated on a 30-m X 0.32-mm i.d. DB-5 capillary column with a 1-pm film thickness, and their mass spectra were obtained via electron impact ionization. The reaction products were identified by comparing their GC retention times with those of authentic standards, by coinjection, and by inspection of their GC-MS fragmentation patterns. The products' molar yields (i.e., the number of moles of product formed per mole of DDP initially loaded to the reactor) were calculated from the integrated GC peak areas and experimentally determined FID response factors (RF) via molar yield (% ) =
The subscripts i, BP, and DDP denote a reaction product, biphenyl (the internal standard), and dodecyclpyrene (the reactant), respectively. Background Experiments. We performed several background experiments to verify that our results were independent of the specific experimental procedure described above. Several of these experiments used l-butylpyrene, which displayed the same pyrolytic pathways as 1-dodecylpyrene, as the reactant because it was easier to load into the reactors and all reaction products could be analyzed on the single 50-m HP-5 capillary column. The absence of catalytic effects due to stainless steel was established by butylpyrene pyrolyses in 15-mL borosilicate glass ampules, in which the reaction products were the same as those observed from pyrolyses in the stainless steel tubing-bomb microreactors. To test for possible catalytic activity in the glass reactors, the interior surface of some ampules was neutralized with NHIOH and the interior surface of others silylated with a trimethylchlorosilane reagent (5% in toluene). Butylpyrene pyrolyses in these treated reactors, which should have possessed inert surfaces, led to the same products as did pyrolyses in untreated ampules. In a different set of experiments DDP was degassed, while in a glass ampule, and the ampule was then sealed under vacuum. Degassing did not alter the products formed during DDP pyrolysis. Finally, DDP pyrolyses with and without added biphenyl (the internal standard) also led to the same product spectra.
Results and Discussion In this section, we first present and discuss representative experimental results from DDP pyrolysis and then interpret these results in terms of the operative pyrolysis pathways, kinetics, and mechanisms. These resolved reaction fundamentals will then be compared to and contrasted with those reported in the literature for the pyrolysis of phenyldodecane, the single-ring analogue of DDP.
Ind. Eng. Chem. Res., Vol. 28, No. 6, 1989 647 SOLVENT
n
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I 5 m HP-1 column
Figure 1. Representative gas chromatograms: DDP pyrolysis a t 400 "C for 60 min.
Finally, the implications of the present results to heavy hydrocarbon processing will be explored. Temporal Variation of Product Yields from DDP Pyrolysis. Table I provides a summary of the yields of the products from DDP pyrolyses at 375,400, and 425 "C. On the basis of replicate experiments, we estimate the uncertainty in the product yields to be about &15%. Only the results from pyrolyses at 400 "C will be discussed in detail, and these results will be presented here primarily in terms of the temporal variation of the yields of the major reaction products. DDP pyrolysis at 400 "C led to 1-methylpyrene, l-undecene, 1-ethylpyrene, and n-decane as the major products at short batch holding times (e.g., 30 min). Pyrene and n-dodecane were also present, but in lower yields. A t longer holding times (e.g., 90 min), a series of n-alkanes, a-olefins, and n-alkylpyrenes were present as minor products. No pyrenylalkenes were unequivocally identified, in part because no authentic standards with which to compare GC retention times were available. GC-MS analysis revealed the presence of a minor product, the identity of which is unknown, with a molecular weight of 242. We denote this product as unknown (242) in Table I. The reaction products noted above are identified in Figure 1,which presents gas chromatograms of the products of DDP pyrolysis at 400 "C for 60 min. DDP pyrolysis also led to the formation of a dark, solid material, which we will refer to as char. This char was insoluble in acetone (Javanmardian et al., 1988), but it did dissolve in toluene. GC analysis of the toluene-soluble pyrolysis products did not reveal any new peaks that had been absent in the chromatograms of the acetone solubles; thus, the char likely contained very high molecular weight material that did not elute from the gas chromatograph. Figure 2 displays the temporal variations of the yields of the six major reaction products from DDP pyrolysis at 400 "C. The molar yields of methylpyrene and 1-undecene were nearly equal at short times, but the yield of me-
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thylpyrene increased to 8.4% at 180 min, while the yield of undecene reached a maximum value of 3.5% at 60 min and then decreased to 0.9% at 180 min. The yields of decane and ethylpyrene were 1.370 and 1.270,respectively,
648 Ind. Eng. Chem. Res., Vol. 28, No. 6, 1989 Table I. Molar Yields (%) of Products from DDP Pyrolysis holding time (min) a t 400 "C holding time (min) a t 375 "C product 30 60 90 150 180 30 45 60 90 120 180 10 hexene 0.58 0.61 0.58 hexane 0.83 1.02 2.61 heptene heptane nonene 0.29 0.31 0.38 0.60 0.66 0.41 nonane 1.67 1.55 0.59 0.96 3.94 5.49 9.42 decene 0.22 0.26 0.41 0.46 0.33 decane 0.43 0.71 1.08 1.45 1.28 2.01 2.15 3.31 3.67 5.09 1.18 undecene 0.65 1.07 1.65 1.71 2.21 2.37 3.40 3.54 3.21 2.77 0.95 2.20 undecane 0.39 0.43 0.22 0.36 1.99 2.94 6.20 dodecene 0.31 0.50 0.54 dodecane 0.33 0.73 6.08 5.37 0.66 2.01 3.18 13.4 19.8 31.6 0.42 Dvrene 0.38 0.76 8.94 7.51 0.74 2.51 4.25 20.7 29.4 43.5 methylpyrene 0.76 1.23 1.71 2.34 2.98 2.86 4.35 4.74 6.62 6.94 8.38 2.43 ethylpyrene 0.54 1.06 1.29 1.21 1.90 2.08 2.68 2.56 2.16 0.84 0.34 0.46 0.50 propylpyrene butylpyrene unknown 1.32 1.17 0.62 0.91 2.97 3.79 4.50 (242) 0.22 pentylpyrene 0.36 hexylpyrene 0.22 heptylpyrene 0.21 octylpyrene 0.33 0.88 nonylpyrene 0.12 decylpyrene 5.88 75.4 dodecylpyrene 78.4 81.5 76.2 60.7 62.6 77.8 69.1 67.5 37.1 25.2 1.60 alkanes 0.76 1.44 9.22 8.80 1.94 4.83 6.65 23.5 32.9 54.9 olefins 0.65 1.07 1.65 1.71 2.50 2.37 3.93 4.18 5.11 5.00 2.81 2.20 A
I
a t 30 min, and these remained comparable up to 60 min. After this time, the yield of decane continued to increase steadily, but the ethylpyrene yield began to decrease. Figure 2b shows that both pyrene and n-dodecane were present in low yields of
