Article pubs.acs.org/JPCA
Pressure Dependence and Branching Ratios in the Decomposition of 1-Pentyl Radicals: Shock Tube Experiments and Master Equation Modeling Iftikhar A. Awan, Donald R. Burgess, Jr., and Jeffrey A. Manion* Chemical and Biochemical Reference Data, National Institute of Standards and Technology, Gaithersburg, Maryland, 20899-8320, United States S Supporting Information *
ABSTRACT: The decomposition and intramolecular H-transfer isomerization reactions of the 1-pentyl radical have been studied at temperatures of 880 to 1055 K and pressures of 80 to 680 kPa using the single pulse shock tube technique and additionally investigated with quantum chemical methods. The 1-pentyl radical was generated by shock heating dilute mixtures of 1-iodopentane and the stable products of its decomposition have been observed by postshock gas chromatographic analysis. Ethene and propene are the main olefin products and account for >97% of the carbon balance from 1-pentyl. Also produced are very small amounts of (E)-2pentene, (Z)-2-pentene, and 1-butene. The ethene/propene product ratio is pressure dependent and varies from about 3 to 5 over the range of temperatures and pressures studied. Formation of ethene and propene can be related to the concentrations of 1-pentyl and 2pentyl radicals in the system and the relative rates of five-center intramolecular H-transfer reactions and β C−C bond scissions. The 3-pentyl radical, formed via a four-center intramolecular H transfer, leads to 1-butene and plays only a very minor role in the system. The observed (E/Z)-2-pentenes can arise from a small amount of beta C−H bond scission in the 2-pentyl radical. The current experimental and computational results are considered in conjunction with relevant literature data from lower temperatures to develop a consistent kinetics model that reproduces the observed branching ratios and pressure effects. The present experimental results provide the first available data on the pressure dependence of the olefin product branching ratio for alkyl radical decomposition at high temperatures and require a value of ⟨ΔEdown(1000 K)⟩ = (675 ± 100) cm−1 for the average energy transferred in deactivating collisions in an argon bath gas when an exponential-down model is employed. High pressure rate expressions for the relevant H-transfer reactions and β bond scissions are derived and a Rice Ramsberger Kassel Marcus/ Master Equation (RRKM/ME) analysis has been performed and used to extrapolate the data to temperatures between 700 and 1900 K and pressures of 10 to 1 × 105 kPa.
1. INTRODUCTION An understanding of the decomposition chemistry of alkyl radicals at temperatures of 600 to 2000 K is required to correctly model combustion systems and a variety of industrial processes. Unimolecular decompositions of these species are rapid at high temperatures and involve a competition between alkene-producing β bond scission reactions and intramolecular hydrogen transfers that alter the site of the radical center. At high temperatures, the radical lifetimes are so short that bimolecular reactions cannot compete with decomposition. In such cases, the distribution of olefin products is a key quantity that impacts the subsequent chemistry. At lower temperatures, where lifetimes are longer, bimolecular oxidation processes are initiated at the radical sites in the presence of O2 and the species distribution likewise affects the course of oxidation. The potential energy surfaces for the decomposition and isomerization processes of alkyl radicals are complex and there are multiple product forming channels that may be energetically similar. An added difficulty is that the radicals are generally formed with internal energy distributions that are already above © 2012 American Chemical Society
the reaction thresholds for decomposition so that the rates are not correctly described by high pressure thermal rate constants. This in turn has consequences for simulation-based engineering models of global combustion processes such as ignition delay times or the formation of polycyclic aromatic hydrocarbons (PAH). We have recently used the shock-tube methodology to obtain direct experimental measurements of the olefin product ratios obtained in the thermal decomposition of a number of straightchain,1−3 branched,4,5 and cyclic6 alkyl radicals and have extrapolated these data with RRKM/Master Equation modeling. Other researchers have reported the results of computational studies.7−13 In addition, there are earlier reports of studies at lower temperatures,14−23 where the results have generally been inferred from models of complex systems. Received: November 30, 2011 Revised: February 13, 2012 Published: February 22, 2012 2895
dx.doi.org/10.1021/jp2115302 | J. Phys. Chem. A 2012, 116, 2895−2910
The Journal of Physical Chemistry A
Article
the weak C−I bond. Dilute concentrations of the precursor, 50−300 μL/L (ppm), are employed, together with a large excess (typically 8000 μL/L) of a radical scavenger. The very dilute conditions make bimolecular processes slow relative to unimolecular decomposition of the radical and the large excess of inhibitor removes contributions from radical chain processes. The olefin products of the unimolecular decomposition reactions are stable and unreactive on the 500 μs time scale of the reaction. In conjunction with sensitive gas chromatographic analysis with mass spectral and flame ionization detection (GC/MS/FID), we are able to quantify even minor reaction products while largely avoiding interference from secondary chemistry. The result of the above is that we are able to isolate and determine the initial fragmentation pattern of the radical with great precision. As detailed later, the olefin product concentrations have a direct correspondence with specific precursor radicals. This information can be related to relative and, ultimately, absolute rate constants with high accuracy. Apparatus and Gas Chromatographic/Mass Spectral (GC/MS) Analysis. The apparatus is described in detail in a recent publication,4 and only a brief overview is given here. The shock tube, the sample preparation system, and the product sampling system are all heated to typically 100 °C to maintain components in the gas phase. Calibrated capacitance manometers or, if the application is not critical, volumetric injection of liquids with calibrated syringes are used to establish species concentrations in the starting mixtures. Concentrations are checked by GC analyses of the unshocked mixtures. Prepared mixtures are stored in 15 L stainless steel bulbs whose interiors have been coated with a passivating silicon surface. Shock waves are produced by rupture of a cellophane diaphragm and the shock pressure profiles monitored with high frequency pressure sensors. As determined from the pressure profiles shock heating times are 500 ± 50 μs. Samples are extracted into an evacuated valve and loop sampling system immediately following each experiment by opening a port 5 cm from the end of the shock tube. Undiluted sample gas is compressed to 100 kPa and separate 1 mL volumes are injected onto two GC columns for analysis. Analyses utilize an Agilent Technologies 6890N GC equipped with twin flame ionization detectors (FIDs) and an Agilent Technologies 5973 inert mass selective detector. Mass spectral analyses are performed after separating the components on a J&W Scientific 30 m × 0.53 mm i.d. DB-1 (100% dimethypolysiloxane) fused silica column with the GC oven temperature programmed from −60 to 180 °C and the carrier gas controls set to the constant flow mode. The sample eluting from the DB-1 column is split with quantitative accuracy using a microfluidic splitter (Dean’s switch) to allow simultaneous FID and mass spectral analyses. Most components are wellseparated on the DB-1 column, but a simultaneous analysis using a Restek 30 m × 0.53 mm i.d. Rt-Alumina (aluminum oxide porous layer) capillary column allows better separation of the lighter gases. FID peak areas are determined using the Agilent Technologies ChemStation software and converted to molar quantities using responses determined from standard samples. Many components smaller than C5 are separated on both columns; in such cases, analyses typically agree within a few percent. Reaction Conditions. Compositions of the gas mixtures used in the present studies are given in Table 1. Shock temperatures are determined by a comparative rate technique, discussed in detail elsewhere,26 in which the unimolecular
In referring to intramolecular hydrogen transfer reactions, our notation for generic cases will follow that of Hardwidge et al.,24 in which they are assigned as iab processes, wherein i gives the ring size of the transition state and a and b are p or s and refer to the primary or secondary nature of the starting and ending radicals, respectively. Specific cases will be referred to as m−n processes, where the initial radical site is on the carbon in position “m” and a hydrogen is transferred from the carbon in the nth position. The previous works have shown that the rates of the intramolecular hydrogen transfer reactions can be rationalized in terms of the reaction thermochemistry and conformational energies of the transition-states. The six-center processes are found to be the most favorable. It is also found that rates of iab transfers are similar for equal values of i, a, and b, an important result that allows one to provide estimates for unstudied systems. In our models of the decomposition of C6 to C8 alkyl radicals, including 1-hexyl, 1-octyl, 4-methylpent-1-yl, and 5methylhex-1-yl radicals, we have been able to reproduce our experimentally determined product branching ratios without the need to invoke 1−2 and 1−3 H-transfer reactions. This is in accord with recent theoretical predictions, but is not fully consistent with our report from some years ago on the 1-pentyl radical. In that study, the product distribution data appeared to indicate that there was a small contribution from the 1−3 Htransfer reaction, representing about 10% of the rate of the 1−4 transfer. At the time we noted that this would imply a surprisingly low activation energy for the 1−3 process, a result not supported by theoretical studies,10 which suggest a barrier about 50 kJ mol−1 larger than for the 1−4 transfer. In the intervening years we have made a number of improvements to the analytical capabilities of our experimental apparatus, including the addition of a mass spectrometer and a move to capillary rather than packed columns in the analyses of the light gases. These improvements, together with the aforementioned inconsistencies in the various studies, have prompted us to reexamine the decomposition of 1-pentyl radical. The pressure dependence of the product branching ratio at high temperatures is investigated for the first time, and we have employed a variety of mixture compositions to check for systematic effects on the results. In addition, we have utilized computational chemistry and RRKM/Master Equation analysis to develop a model that fits the present experimental results, as well as literature data from relevant studies at lower temperatures. This model has subsequently been used to extrapolate the primary data over a wide range of pressure and temperature conditions.
2. EXPERIMENTAL METHODS25 Methodology. The technique is the same as that employed previously. Experiments are conducted in a heated single pulse shock tube. The shock tube, described more fully below, essentially functions as a pulse heater, compressing and heating the sample gas to temperatures near 950 K for about 500 μs prior to rapid cooling and quenching of the reaction. The walls of the shock tube are cold compared with reaction temperatures and reaction times are short compared with the time scale of diffusion. A consequence is that surface-induced chemistry, a major concern with static and flow systems, is unimportant and the gas phase processes can be isolated. We generate the 1-pentyl radical of interest from the thermal decomposition of 1-pentyl iodide, which undergoes fission of 2896
dx.doi.org/10.1021/jp2115302 | J. Phys. Chem. A 2012, 116, 2895−2910
The Journal of Physical Chemistry A
Article
were 99.5% for 135TMB and 99.1% for m-xylene, with the main impurities being other isomers of the respective methylbenzenes. Chemicals were degassed during preparation of the mixtures.
Table 1. Gas Mixtures Used in the Present Experiments; The Remaining Balance is Argon components in mixtures (μL/L) mixture No.
1-C5H11Ia
CCPb
13DMBc
A B C D E
290 190 200 50 50
150 100 100 100 100
10800
135TMBd
3. RESULTS 3.1. Main Products and the Reaction Mechanism. Previous work on alkyl iodides leads to the expectation that 1iodopentane will decompose via two parallel channels, molecular elimination of HI and fission of the weak C−I bond. As discussed below, the olefin product spectra from the two channels do not overlap. 1-Pentene is the alkene product of the molecular channel and this species accounts for about 25% of the reacted 1-iodopentane. On the basis of the reported rate constants,30 the subsequent decomposition of 1-pentene is unimportant at the temperatures and time scale of the present studies. The other main olefin products are ethene and propene. These species are even less susceptible to decomposition than is 1-pentene. Much smaller amounts, less than 1% on a molar basis, of (E)-2-pentene, (Z)-2-pentene, and 1-butene are also observed. Other than 1-pentene, the above olefins are attributed as direct or indirect products arising from the decomposition of the 1-pentyl radical. The high sensitivity of GC/MS analysis allows us to detect a range of other trace products. These are discussed later under secondary chemistry, but do not represent initial decomposition pathways and will be shown to have a minimal effect on the results. Product data from individual experiments are provided in Supporting Information, Table S1. The products noted above are accounted for by the mechanism shown in Scheme 1. β C−C bond scission in 1pentyl radical occurs on the time scale of a few microseconds or less under our conditions and leads to ethene and 1-propyl
8500 4000 4200 4600
a
1-C5H11I = 1-iodopentane. bCCP = chlorocyclopentane (temperature standard). c13DMB = 1,3-dimethylbenzene (m-xylene, inhibitor). d135TMB = 1,3,5-trimethylbenzene (inhibitor).
decomposition of a standard is monitored and used to deduce the reaction temperature. Thus, kstd = τ−1 ln([std]i/[std]f)], where τ is the residence time of about 500 μs and the subscripts i and f refer to the initial and final concentrations, respectively. In the present experiments we have used the standard reaction chlorocyclopentane → cyclopentene + HCl. Cyclopentene is formed solely from this reaction in our system. Our rate expression is based on our recent comparative rate shock tube studies,27,28 in which it was experimentally determined relative to several other reactions, and evaluated in conjunction with lower temperature results, a computational study, and Rice Ramsberger Kassel Marcus/Master Equation (RRKM/ME) analysis modeling. The derived high pressure limiting value, k∞(chlorocyclopentane → cyclopentene + HCl, 590−1020 K)/ s−1 = 5.62 × 1013 exp(−24514 K/T) is consistent with our rate expression for the decomposition of cyclohexene, k(cyclohexene → butadiene + ethene, 100−300 kPa)/s−1 = 1.4 × 1015 exp(−33500 K/T). Because we have examined a range of pressures in the present experiments, we have accounted for falloff effects in the rate of the standard reaction by using our previous RRKM/ME analysis27,28 to derive rate expressions specific to the conditions of each set of experiments. The pressure effects are systematic, but minor, altering the derived temperatures by less than 1 K for experiments below 1000 K, with a maximum adjustment of about 3 K at the highest temperature studied. Standard uncertainties in the relative temperatures should be about 3 K. There are no generally accepted methods for calibrating absolute temperatures in transient phenomena such as shock waves that are demonstrably more accurate and precise than the kinetic method employed here; overall, based on our critical analysis of the kinetics,27 standard uncertainties in the absolute temperatures are estimated to be about 8 K. The ideal shock equations have been used to calculate reaction pressures from the temperature and mixture composition via the ideal shock equations;29 these values are not significantly different from those derivable from the experimental pressure traces, but should provide more consistent relative values, which is important for the RRKM analysis. Temperatures in the reflected shock ranged from 880 to 1055 K with shock pressures of 80 to 680 kPa. Chemicals. 1-Iodopentane (98%, Aldrich), 1-iodohexane (99%, Aldrich), chlorocyclopentane (99%, Aldrich), and argon (Matheson, high purity grade, 99.999%) were used without further purification. The main impurities in 1-iodopentane were identified by GC/MS as 1-bromopentane (1.6%) and 1chloropentane (0.3%). The inhibitors 1,3,5-trimethylbenzene (135TMB, 99%, Aldrich) and m-xylene (99%, Aldrich) were redistilled prior to use. Purities after distillation, by GC analysis,
Scheme 1. Main Product Pathways Postulated for the Decomposition of 1-Pentyl Radicalsa
a
Paths indicated by the block arrows account for >97% of the mass flux. The starting radical is indicated by the dashed oval. The alkenes enclosed in boxes (propene and 1-butene) are marker species formed only from the indicated precursor radicals. Ethene is derived from multiple sources. Possible 1−2 and 2−3 H shift reactions are not indicated but have been considered in our RRKM/ME model and are shown to be unimportant (see text).
2897
dx.doi.org/10.1021/jp2115302 | J. Phys. Chem. A 2012, 116, 2895−2910
The Journal of Physical Chemistry A
Article
radical. The latter species rapidly decomposes to another ethene and a methyl radical. The main reaction competing with the above sequence involves a five-center 1−4 H shift that leads to the 2-pentyl radical. This species can undergo β C−C bond scission to form propene and ethyl radical, the latter of which ejects H to give ethene. Propene is formed only after isomerization and its presence confirms that isomerization and β bond scission processes in the 1-pentyl radical are competitive. Also shown in Scheme 1 is a four-center 1−3 Hshift reaction that leads to the 3-pentyl radical, which can readily eject methyl to give 1-butene. Not shown are threecenter 1−2 and 2−3 H-shift reactions that can lead to the same intermediate radicals as the 1−3 and 1−4 processes. The threecenter H-shifts were formally considered in our reaction model but, based on rate constants predicted by theory, were found to be insignificant (
