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Ind. Eng. Chem. Res. 1996, 35, 1517-1523
1517
Pathways, Kinetics, and Mechanisms for 2-Dodecyl-9,10-dihydrophenanthrene Pyrolysis Phillip E. Savage* and Kelly L. Baxter Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136
We pyrolyzed 2-dodecyl-9,10-dihydrophenanthrene (DDPh) in batch microreactors. The reaction conditions included neat pyrolyses between 375-450 °C for times of 15-240 min and also pyrolyses in benzene at 400 °C and 90 min but at different initial DDPh concentrations ranging from 0.0095 to 0.238 mol/L. The disappearance of DDPh followed first-order kinetics, and the global first-order rate constant had Arrhenius parameters of (log10 A (s-1) ) 13.6 ( 2.8 and E (kcal/mol) ) 54.5 ( 9.1, where the uncertainties are the 95% confidence intervals. The decomposition of DDPh can be described by a reaction network that possesses four parallel primary pathways. The major primary path, which involves dehydrogenation, leads to 2-dodecylphenanthrene. The other three primary paths involve C-C bond cleavage, and they lead to 2-methyl-9,10-dihydrophenanthrene plus undecene, to 2-vinyl-9,10-dihydrophenanthrene plus decane, and to numerous minor products. Important secondary and tertiary reactions include the rapid reduction of 2-vinyl-9,10-dihydrophenanthrene to 2-ethyl-9,10-dihydrophenanthrene and the facile dehydrogenation of 2-methyl- and 2-ethyl-9,10-dihydrophenanthrene to form 2-methyl- and 2-ethylphenanthrene, respectively. The identities and relative abundances of the major products are consistent with a free-radical chain reaction mechanism for DDPh pyrolysis. Introduction Advances in analytical chemistry and computer technology have provided the opportunity to take an increasingly molecular perspective on the processing of hydrocarbon resources such as coals and heavy crude oils. Analytical techniques can now provide detailed structural information about these materials. Integrating this knowledge of the structure with an understanding of the reaction fundamentals for the different structural elements has led to the development of molecule-based reaction models (Gray, 1990; Neurock et al., 1990; Quann and Jaffe, 1992; Shinn, 1992; Savage and Klein, 1989; Freund, 1992). The emergence of these molecule-based models has also been catalyzed by environmental forces. Government regulations are focusing more on molecules (e.g., specifying maximum aromatic content in gasoline) so refiners must be able to “engineer” the molecular content of their products. This need to control the molecular composition of products shows the utility and necessity of moleculebased process models. Central to the successful implementation of a moleculebased modeling approach is a knowledge of the reaction pathways and kinetics for the different structural elements in heavy crudes and coal and in their processderived products. This knowledge can, in principle, be obtained from studies of the reactions of model compounds that mimic the key structural features of the complex material. It is generally agreed that clusters of condensed rings bearing alkyl chains are important structural features of coals (Solum et al., 1989; Shinn, 1984; Nelson, 1987) and heavy oils and residua (Ali et al., 1990; Speight, 1989; Rovere et al., 1989; Waller et al., 1989). Moreover, Wiehe (1994) showed that a “pendant-core” structural model for petroleum residua that considers the molecules to be combinations of a core (comprising rings) decorated by pendants (including alkyl chains) could * E-mail:
[email protected]. Fax: (313) 763-0459.
explain several important trends in resid conversion. The polycyclic moieties in heavy hydrocarbons can be fully aromatic (arenes), partially hydrogenated (hydroarenes), and fully hydrogenated (naphthenes or perhydroarenes). This structural picture suggests that polycyclic cores bearing long n-alkyl chains would be good model compounds. Having identified a class of model compounds to investigate, we next consider the types of reactions to investigate. Many of the processes for upgrading heavy hydrocarbon resources use catalysts, but they also employ elevated temperatures so thermal reactions play a very important role. In fact, experimental studies repeatedly show that thermal reactions can control the product yields and the reaction rates even in nominally catalytic processes (Heck and DiGuiseppi, 1994; Sanford, 1994; Savage et al., 1988; Miki et al., 1983; Khorasheh et al., 1989). This dual recognition of the significance of thermal hydrocarbon chemistry and the utility of long-chain n-alkyl-substituted model compounds has motivated numerous pyrolysis studies with such compounds. Poutsma’s review (1990) of the thermal chemistry of model compounds and a search of the more recent literature showed that most of these studies were limited to n-alkylarenes and n-alkylnaphthenes. We found no previous reports of pyrolyses of any long-chain n-alkylhydroarenes at the temperatures of interest (around 400 °C) in this study. The published accounts describe only the pyrolysis of unsubstituted hydroarenes (see Poutsma, 1990 and references therein) and the pyrolysis of n-alkyltetralins either with short chains (Miller et al., 1993; Savage and Klein, 1988, Trahanovsky and Swensen, 1981) or at temperatures around 700 °C (Murata et al., 1995). The relevance of the reactions of long-chain n-alkylsubstituted hydroaromatics to the processing of heavy hydrocarbon resources and the very limited amount of information available on the reaction fundamentals for these compounds motivated the research reported here. This paper discusses the thermal reaction pathways,
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1518 Ind. Eng. Chem. Res., Vol. 35, No. 5, 1996 Table 1. Molar Yields (%) of Major Products from DDPh Neat Pyrolysis yields at various times 375 °C product undecene 2-methyl-9,10-dihydrophenanthrene 2-methylphenanthrene decane 2-ethyl-9,10-dihydrophenanthrene 2-ethylphenanthrene 2-dodecylphenanthrene 2-dodecyl-9,10-dihydrophenanthrene phenanthrene sum
400 °C
450 °C
60 (min)
120 (min)
240 (min)
30 (min)
90 (min)
180 (min)
15 (min)
45 (min)
90 (min)
0.13 0.39
0.20 0.53 0.05 0.11 0.03
0.20 0.47 0.09 0.13 0.09
0.83 1.08 0.31 0.48 0.39 0.09 15.1 78.3 95.9
2.07 2.48 1.01 1.26 0.99 0.37 24.5 58.3 89.0
2.63 2.85 1.81 1.54 1.28 0.76 37.8 58.4 105
4.81 4.24 6.22 3.75 2.78 4.31 34.9 35.0 93.4
4.42 2.08 22.8 5.61 1.39 19.9 21.2 7.22 85.0
1.10 0.20 39.7 5.63
14.0 113 127
12.5 90.4 103
kinetics, and mechanisms for 2-dodecyl-9,10-dihydrophenanthrene. Experimental Section 2-Dodecyl-9,10-dihydrophenanthrene (DDPh) was obtained from the Thermodynamics Research Center at Texas A&M University and used as received. Gas chromatographic analysis of the reactant revealed that it was 98% pure. The only impurity was 2-dodecylphenanthrene. DDPh was pyrolyzed at temperatures between 375 and 450 °C in stainless steel, tubing bomb batch microreactors fashioned from nominal 1/4 in. Swagelok tube fittings (one port connector and two caps). To conduct the neat pyrolysis experiments, we loaded the reactors with approximately 10 mg of the model compound and 5 mg of biphenyl, which served as an internal standard for subsequent analyses, each weighed to within (0.1 mg. To conduct experiments at different initial reactant concentrations, we used benzene as the diluent. Benzene can be taken to be an inert solvent under the conditions of these experiments (Savage, 1994). The reactors were loaded with 0.5 mL of a benzene solution containing the reactant and biphenyl in known amounts. In both cases, after the reactors were loaded and sealed, they were placed in a preheated, isothermal, fluidized sand bath set at the desired pyrolysis temperature. Upon reaching the desired holding time, the reactors were removed from the sand bath, and the reaction was quenched by immersing the reactors in room-temperature water. The reactors were then opened and their contents recovered by repeated extraction with benzene. Gaseous products could not be analyzed because of the small amount of reactant used in the experiments. The reaction products were analyzed by capillary column gas chromatography using both flame ionization and mass spectrometric detectors. Product identification was accomplished by comparing retention times with those of authentic standards and by inspecting mass spectra. Product molar yields, calculated as the number of moles of product formed divided by the number of moles of the reactant initially loaded into the reactor, were obtained from the chromatographic analysis using experimentally determined detector response factors. When reaction products were not available commercially, we estimated their response factors as being equal to that of a chemically similar compound. Results Table 1, which provides representative experimental data, lists the molar yields of the reactant and the major products from neat pyrolysis experiments at 375, 400,
12.3 81.8 94.8
32.4 0.86 0.06 94.9
and 450 °C. Data from experiments at 425 °C appear later in Figure 1. We also report the sum of the molar yields of all phenanthrene- and dihydrophenanthrenecontaining compounds that we identified and quantified. We refer to this quantity as the phenanthrene sum. It is a measure of our ability to recover and quantify the yields of these products. The values of the phenanthrene sum in Table 1 are most often between 90 and 110%. The phenanthrene sum and the molar yield of the reactant exceeding 100% in a few instances are a manifestation of the uncertainty in the experimental data. We discarded all data obtained from experiments with a phenanthrene sum outside the range of 100% ( 35%. In addition to the major products listed in Table 1, 2-dodecyl-9,10-dihydrophenanthrene pyrolysis also led to numerous minor products. These included other n-alkanes, 1-alkenes, 9,10-dihydrophenanthrenes bearing n-alkyl and 1-alkenyl substituents, phenanthrenes bearing n-alkyl and 1-alkenyl substituents, 9,10-dihydrophenanthrene, and phenanthrene. These products were typically present in low yields. Indeed, only at conversions greater than 80% were any of these other alkanes or alkenes produced in yields exceeding 1%, and only at 450 °C did the yields of any of the other substituted phenanthrenes and dihydrophenanthrenes exceed 1%. The product with the highest molar yield under nearly all conditions investigated was 2-dodecylphenanthrene. For example, at 375 °C, the 2-dodecylphenanthrene yield always exceeded 10% whereas the yield of any other product was always less than 1%. The highest 2-dodecylphenanthrene yield of 47% occurred at a 2-dodecyl-9,10-dihydrophenanthrene conversion of 85%. At higher conversions, the 2-dodecylphenanthrene yield decreased, and at the most severe conditions investigated (450 °C, 90 min, >99% conversion) the yields of other products (2-methyl- and 2-ethylphenanthrene) finally surpassed the yield of 2-dodecylphenanthrene. 2-Methyl-9,10-dihydrophenanthrene was the product present in the second highest yield at conversions of less than 50%. The yield of this product increased to a maximum value of nearly 5% before it began to decrease at higher conversions. The yield of 1-undecene was typically close to the yield of 2-methyl-9,10-dihydrophenanthrene. A final pair of major products observed in the lowconversion (
