A shock tube laser schlieren study of cyclopentane ring opening

Transcription

1 8 th U. S. National Combustion Meeting Organized by the Western States Section of the Combustion Institute and hosted by the University of Utah May 19-22, 2013 A shock tube laser schlieren study of cyclopentane ring opening Christopher J. Annesley 1, Patrick T. Lynch 1, David C. Garon 2, Robert S. Tranter 1 1 Chemical Science and Engineering Division Argonne National Laboratory, 9700 South Cass Ave., Argonne, IL Department of Mechanical and Industrial Engineering, University of Illinois at Chicago, 842 W. Taylor St, Chicago, IL The pyrolysis of cyclopentane, 0.5% dilute in krypton, was investigated in a diaphragmless shock tube (DFST) using laser schlieren densitometry (LS). Experiments were performed at 148±5 Torr over the temperature range of K. Fitting to density gradient profiles obtained from the LS experiments provide rates for the initial step for cyclopentane to ring open to 1-pentene. 1-Pentene then quickly dissociates to allyl and ethyl radicals. The ethyl product undergoes a fast and well understood dissociation to yield a hydrogen atom. The ring opening step is compared to other cycloalkanes. Further, in these LS experiments dissociation of the allyl radical is noncompetitive with H abstraction, so the studies can yield information regarding the reaction of allyl radicals and H-atoms. Rate coefficients from RRKM modeling for ring opening were found to give a rate of k 150Torr = (3.614± 0.72) T exp(-60300/t) s Introduction Saturated cyclic molecules are prevalent in combustion, primarily as constituents of fuels, but also as intermediates formed during combustion. Cyclic species, including saturated molecules, form a larger fraction in non-traditionally sourced fuels, and thus are of increasing importance. At high temperatures, dissociation of these species may be many times faster than reaction of the molecule with oxygen (Kiefer et al. 2009), therefore the pyrolytic reactions contribute to establishing the initial radical pool. Thus kinetic and mechanistic data for the pyrolysis of cyclic molecules at combustion conditions is required for the development of accurate models. Recently, the pyrolysis of cyclohexane and its sole dissociation product, 1-hexene, has been studied (Kiefer et al. 2009; Peukert et al. 2011; Tsang 1978) and the dissociation of five and six membered molecules with oxygen hetero atoms have also received theoretical and experimental study (Yang et al. 2011; Simmie and Metcalfe 2011; Lifshitz et al. 1998). The dissociation of these cyclic alkanes follows a general scheme: ring opening followed by 1

2 dissociation (see Fig. 1). However, the different cyclic alkanes ring open differently. As shown in Fig. 1a, cyclohexane primarily ring opens to 1-hexene, followed by dissociation to allyl and propyl radicals. 1,4-Dioxane, (Fig. 1b) on the other hand, ring opens to two different linear alkenes (ethylene-glycol-vinyl-ether and 2-ethoxyacetaldehyde), each having their own dissociation rates and branching ratios (Yang et al. 2011). To complement these recent works, we have studied dissociation of cyclopentane in a diaphragmless shock tube by laser schlieren densitometry (LS). There is some disagreement in the literature about the rate of this dissociation that arises partly because of difficulties in extrapolation to high temperature, as well as the likely effects of high-temperature falloff. Previous studies have found rates that when extrapolated to 1500 K vary by a factor of 20 (Sirjean et al. 2006; Tsang 1978; Brown and King 1989; Kalra et al. 1979). Sensitive, pressure resolved, high temperature studies of this rate are necessary. Figure 1: Decomposition mechanisms of various cycloalkanes. While cyclohexane is largely single channel (Kiefer et al. 2009), 1,4 dioxane branches between radicals and stable molecules. Previous studies on cyclopentane (Kalra et al. 1979, Feinstein, and Lewis 1979; Brown, King, and Nguyen 1986; Tsang 1978; Sirjean et al. 2006) have given various different branching ratios for both molecular and radical channels. Furthermore the literature diverges on the product channels of cyclopentane. Like 1,4- dioxane, there are two potential ring opening pathways, full ring opening and rearrangement to 1-penene (ΔH r,298k = 23.5 kcal/mol), or ring contraction to cyclopropane and ethylene (ΔH r,298k = 43.7 kcal/mol). While thermochemistry favors the rearrangement to 1-pentene, the cyclopropane and ethylene channel may contribute due to the ring opening diradical intermediate. Tsang (1978) has this as about a 20% pathway, but it should likely exhibit greater falloff and is not expected to contribute in high temperature, low pressure experiments as conducted in this present work. Laser schlieren densitometry was able to discern mechanistic information between these two pathways because the secondary chemistry of each pathway was sufficiently well 2

3 understood. This short paper focuses on the dissociation mechanism and kinetics of cyclopentane at elevated temperatures. 2. Methods The LS experiments were performed in a diaphragmless shock tube, DFST, which has been fully described elsewhere (Tranter and Giri 2008). The driver section of the DFST contains a fast acting, bellows driven valve which replaces the more traditional diaphragm. When the valve is closed the driver and driven sections can be filled to the desired loading pressures. The DFST is fired by rapidly opening the valve. By varying both the driver section pressure, P 4, and the driven section pressure, P 1, the pressure behind the incident shock wave, P 2, can be constrained to narrow ranges over a wide range of temperatures (Tranter and Giri, 2008). The laser schlieren technique utilizes deflection of a narrow beam from a He/Ne laser to measure density gradients behind the incident shock wave. The technique has been fully described previously (Kiefer 1981; Kiefer et al. 1981). The driven section of the shock tube has an internal diameter of 6.35 cm and the quartz windows, through which the laser beam passes, are located sufficiently far downstream to allow the shock wave to fully develop after firing the DFST. A set of five piezo-electric time-of-arrival transducers with 120 mm spacing are centered around the LS windows, and incident shock wave velocities were obtained by interpolation of time intervals taken for the shock wave to pass between successive transducers. From these velocities and the loading conditions, the temperature and pressure behind the incident shock wave are calculated assuming frozen conditions. The uncertainty in velocity is estimated as 0.2%, corresponding to a temperature error of less than 0.5%, here amounting to the order of K. The molar refractivity of Kr = 6.367,(Gardiner et al. 1981) while that of cyclopentane, 22.89, was calculated from its refractive index and molar density. The normal assumption was made that the mixture molar refractivity does not vary with extent of reaction, which is a good assumption for low concentration species dilute in krypton. Mixtures containing 0.5% cyclopentane dilute in krypton were prepared manometrically in a 50 L glass vessel that had been evacuated to <10-3 Torr. Krypton (AirGas %), was used as supplied. Cyclopentane (Aldrich Chemical Co., 98%) was degassed by repeated freeze pump thaw cycles with liquid nitrogen. Reagent mixtures were stirred for 1 hour using a Teflon-coated magnetic stirrer before use. 3. Results and Discussion 11 experiments with cyclopentane mixtures (0.5% in Kr) were conducted with incident shock pressures of 148±5 Torr over the temperature range 1620 < T 2 < 2055 K. Example raw LS signals are shown in Fig. 1, and the corresponding semi-log density gradient plots are shown in Fig. 2. Density gradients are proportional to the product of enthalpy of reaction and reaction rates. Negative minima, like those in Fig. 2b, indicate net exothermic reactions. The first few points in each plot of Fig. 2 are due to the shock front/laser beam interaction (Kiefer et al. 1981) which is also responsible for the large peak and preceding valley seen in both panels of Fig. 1. The small valley evident near 1.3 s in Fig. 2a is a residual of diffraction from this interaction. 3

4 Figure 2: Example raw laser schlieren profiles, showing dissociation of shock heated cyclopentane/kr mixtures. Figure 3: Density gradients corresponding to Fig 2. Shown in the open circles are the density gradients generated by pyrolysis of cylcopentane. The solid line is the best fit line for these experiments. The dashed line shows the effect of raising k R1 by 30%, while the dot-dash line is reduced by this same amount. They are seen in all but the lowest pressure LS experiments and are always ignored. The shock front / laser interaction masks the location of t 0, the time origin at the onset of reaction. Consequently, t 0 is determined by a well-established method (Kiefer 1981, Kiefer et al. 1981) and is typically located to an accuracy of s. For cyclopentane pyrolysis a 32 reaction mechanism was developed assuming initial dissociation of cyclopentane entirely to 1-pentene and thus 1-pentene dissociation was also studied by DFST/LS although it is not discussed here. A small channel to cyclopropane + 4

5 ethylene (and subsequent secondary chemistry) was tested; however the best simulations were generated without this channel. The most important reactions are summarized in Table 1. Dissociation of cyclopentane is only mildly endothermic and thus generates only a weak initial density gradient. The rapid increase in the simulated density gradients at short time, Fig. 3, is mainly due to dissociation of the product 1-pentene which at the conditions studied is comparable to that of precursor cyclopentene. Thus it is not possible to separate these decompositions as was possible in previous work with cyclohexane (Kiefer et al. 2009). The relatively facile dissociation of 1-pentene generates prompt H atoms (from the tertiary dissociation of ethyl radical) which significantly complicate the overall chemistry of the system. Early in reaction these hydrogen atoms will abstract hydrogen atoms from cyclopentane, forming cyclopentyl. Decomposition of cyclopentyl radicals was studied before (Awan et al. 2011), and at these temperatures, dissociation to ethylene and allyl is very rapid. This overall reaction of H abstraction from cyclopentane was summed into Reaction 7. For every cyclopentane molecule that dissociates an allyl radical is generated. Recent work from this laboratory (Lynch et al. 2013) has studied recombination of allyl radical at temperatures up to 1700 K, and these are incorporated in the model. However, at the higher temperatures of the current study dissociation of allyl to allene + H is significant and the results of Fernandes et al. (2005) were adopted for this reaction. Large numbers of H-atoms and allyl radicals are present simultaneously in cyclopentane dissociation and the simulations are sensitive to the rate and reaction path of H + allyl. New calculations (A. W. Jasper personal communication) have shown that the abstraction rate to allene is far faster than the addition rate to propene at the low pressures of the current experiments, although there is significant uncertainty in not only the pressure dependent branching fraction, but the overall rate. A branching ratio almost exclusively to abstraction (Reaction 6) is needed to fit this low concentration data, however we are still relatively insensitive to the actual rate. With higher concentration data, it may be possible to fit these rates, but currently uncertainties in the other H reactions preclude this. Table 1: Core reactions for the dissociation of cyclopentane. Rates are given as modified Arrhenius rates. k=at n exp-(e a /RT). Units; mol, cm, s, kcal. Reaction log A N Ea H rxn (298 Source K) 1 Cyclopentane = 1-pentene ( This work torr) 2 1-pentene = allyl + ethyl This work 3 ethene + H = ethyl (150 torr) Yang X. et al C 3 H 5 + C 3 H 5 = C 6 H Lynch et al. 5 C 3 H 5 + C 3 H 5 = C 3 H 4 + C 3 H Lynch et al. 6 C 3 H 5 + H = C 3 H 4 + H A. Jasper private communication 7 H + cc5h10 = C 2 H 4 + C 3 H 5 + H Awan et al

6 Minimizing the initial composition present in the experiment, by going from our standard conditions 2% compositions down to 0.5%, the signal generated from the direct decomposition can be maximized without as much influence from other competing processes at the expense of total signal. We fit the decomposition rate for many individual experiments and the resultant rates at time zero is shown as an Arrhenius plot in FIG. 4. Fig. 2 shows the sensitivity to reaction (1). Figure 4 Arrhenius plot. Shown here are the fit rates for the different cyclopentane ring opening (R 1 ) experiments in the squares. Shown are also the rates from the various other previous studies with the broken lines. The long dashes are the rate proposed by (Sirjean et al. 2006). The dotted line is the high pressure limit (HPL) from the calculations of (Brown, King, and Nguyen 1986). The small dashed line is the extrapolation of the work of (Tsang 1978), while that of Kalra et al. (1979) is hidden behind the solid black line of this calculated HPL. The range of the data for Kalra et al. (1979) is denoted by open triangles. The HPL of this work follows that of (Kalra, Feinstein, and Lewis 1979) well, and the 150 torr calculation of this RRKM calculation falls off to our data. Deviations are discussed in the text. An RRKM model for this dissociation was created in order to extrapolate to other pressures. The model is constructed with vibrational frequencies from Sirjean et al. (2006) and varying the ΔE <down> and critical energy to fit the data. This resulted in a critical energy of 78.5 kcal/mol and ΔE <down> = 250 cm -1. The critical energy is lower than the calculated barrier of 82 6

7 kcal/mol. However, a similar apparently lower barrier occurs in cyclohexane dissociation (Kiefer et al. 2009) and this will need to be investigated further. At low temperatures our experimental points intersect the calculations of Sirjean et al. (2006), however this masks significant disagreement since we observe high temperature falloff, and the characteristic decreased apparent activation energy. The resulting rate for 150 Torr is k 150Torr = (3.61± 0.72) T exp(-60300/t) s -1 and our extrapolated high pressure limit rate is k = (1.12±0.22) T exp(-41695/t) s -1. In all our high pressure limit is about a factor of five lower than Sirjean et al. s (2006). Additionally we find that high pressure limit of Brown et al. (1986) is about a factor of 2-3 larger than our calculated high pressure limit. The model fits well our experimental data with small deviations at high and low temperature likely due to low signal with this low concentration, and we have good agreement between our high pressure limit and the previous work of Kalra et al. (1979), which was itself a stated improvement on the high pressure limit obtained by Tsang (1978). 4. Conclusions The rate of pyrolysis of cyclopentane in krypton has been measured at a pressure of 150 torr and the temperature range of K. The initial dissociation is two steps in which the cyclopentane ring opens to 1-pentene, which, in turn, decomposes into ethyl and allyl. The ethyl product undergoes a fast and well understood dissociation to yield a hydrogen atom. The secondary reactions are reduced by the use of a highly dilute mixture for this technique of 0.5%. This will allow a thorough fitting of rates for secondary reactions at higher concentration, as well as the refinement of initial rates to understand the system as a whole. Acknowledgements This work was performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences, U.S. Department of Energy, under contract number DE-AC02-06CH Special thanks go to Professor John Kiefer for helpful discussions. References Awan, I.A., Burgess Jr. D.R., Tsang, W., Manion, J.A., Shock tube study of the decomposition of cyclopentyl radicals Proceedings of the Combustion Institute, (1), Brown, Trevor C., Keith D. King, and Tam T. Nguyen. Kinetics of primary processes in the pyrolysis of cyclopentanes and cyclohexanes. Journal of Physical Chemistry (3), Gardiner Jr., W.C, Hidaka, Y Tanzawa, T. Refractivity of combustion gases : Kalra, B.L., Feinstein, S.A. Lewis, D.K. Pyrolysis of cyclopentane behind reflected shock waves. Canadian Journal of Chemistry (11): Kiefer, J. H., M. Z. Al-Alami, and J-C. Hajduk. Physical optics of the laser-schlieren shock tube technique. Applied Optics., ,(2):

8 Kiefer, J. H., K. S. Gupte, L. B. Harding, and S. J. Klippenstein Shock Tube and Theory Investigation of Cyclohexane and 1-Hexene Decomposition. The Journal of Physical Chemistry A 113 (48): Kiefer, J.H The Laser Schlieren Technique in Shock Tube Kinetics. Edited by M. Dekke, Shock Waves in Chemistry New York. Lifshitz, A., Tamburu, C., and Shashua, R Thermal Decomposition of 2,5-Dimethylfuran. Experimental Results and Computer Modeling. The Journal of Physical Chemistry A 102 (52): Lynch, P.T., Annesley, C.J., Aul, C.J., Yang, X. and Tranter, R.S. Recombination of allyl radicals in the high temperature fall-off regime. Submitted. Peukert, S., Naumann, C., Braun-Unkhoff, M. and Riedel, U. Formation of H-atoms in the pyrolysis of cyclohexane and 1-hexene: A shock tube and modeling study. International Journal of Chemical Kinetics (3): Simmie, J. M., and Metcalfe, W.K Ab Initio Study of the Decomposition of 2,5- Dimethylfuran. The Journal of Physical Chemistry A 115 (32): Sirjean, B., P. A. Glaude, M. F. Ruiz-Lopez, and R. Fournet Detailed Kinetic Study of the Ring Opening of Cycloalkanes by CBS-QB3 Calculations. The Journal of Physical Chemistry A 110 (46): Tranter, R.S., and Giri B.R., A diaphragmless shock tube for high temperature kinetic studies. Review of Scientific Instruments 79 (9). Tsang, W. Thermal decomposition of cyclopentane and related compounds. International Journal of Chemical Kinetics (6): Tsang, W., Thermal stability of cyclohexane and 1-hexene. International Journal of Chemical Kinetics (11): Yang, X.; Goldsmith, C.F.; Tranter, R.S., Decomposition and Vibrational Relaxation in CH3I and Self-Reaction of CH3 Radicals. Journal of Physical Chemistry A 2009, 113, Yang, X., Jasper, A. W., Giri, B. R., Kiefer, J. H., and Tranter, R. S., A shock tube and theoretical study on thepyrolysis of 1,4-dioxane. Physical Chemistry Chemical Physics (9):