Article pubs.acs.org/JPCA
Pyrolysis of Cyclopentadienone: Mechanistic Insights from a Direct Measurement of Product Branching Ratios Thomas K. Ormond,†,‡ Adam M. Scheer,§ Mark R. Nimlos,† David J. Robichaud,† Tyler P. Troy,∥ Musahid Ahmed,∥ John W. Daily,⊥ Thanh Lam Nguyen,# John F. Stanton,# and G. Barney Ellison*,‡ †
National Renewable Energy Laboratory, 15013 Denver West Parkway, Golden, Colorado 80401, United States Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0215, United States § Combustion Research Facility, Sandia National Laboratory, PO Box 969, Livermore, California 94551-0969, United States ∥ Chemical Sciences Division, Lawrence Berkeley National Laboratories, Berkeley, California 94720, United States ⊥ Center for Combustion and Environmental Research, Department of Mechanical Engineering, University of Colorado, Boulder, Colorado 80309-0427, United States # Institute for Theoretical Chemistry, Department of Chemistry, University of Texas, Austin, Texas 78712, United States ‡
ABSTRACT: The thermal decomposition of cyclopentadienone (C5H4O) has been studied in a flash pyrolysis continuous flow microreactor. Passing dilute samples of ophenylene sulfite (C6H4O2SO) in He through the microreactor at elevated temperatures yields a relatively clean source of C5H4O. The pyrolysis of C5H4O was investigated over the temperature range 1000−2000 K. Below 1600 K, we have identified two decomposition channels: (1) C5H4O (+ M) → CO + HCCCHCH2 and (2) C5H4O (+ M) → CO + HCCH + HCCH. There is no evidence of radical or H atom chain reactions. To establish the thermochemistry for the pyrolysis of cyclopentadienone, ab initio electronic structure calculations (AE-CCSD(T)/aug-ccpCVQZ//AE-CCSD(T)/cc-pVQZ and anharmonic FC-CCSD(T)/ANO1 ZPEs) were used to find ΔfH0(C5H4O) to be 16 ± 1 kcal mol−1 and ΔfH0(CH2CHCCH) to be 71 ± 1 kcal mol−1. The calculations predict the reaction enthalpies ΔrxnH0(1) to be 28 ± 1 kcal mol−1 (ΔrxnH298(1) is 30 ± 1 kcal mol−1) and ΔrxnH0(2) to be 66 ± 1 kcal mol−1 (ΔrxnH298(2) is 69 ± 1 kcal mol−1). Following pyrolysis of C5H4O, photoionization mass spectrometry was used to measure the relative concentrations of HCCCHCH2 and HCCH. Reaction 1 dominates at low pyrolysis temperatures (1000−1400 K). At temperatures above 1400 K, reaction 2 becomes the dominant channel. We have used the product branching ratios over the temperature range 1000−1600 K to extract the ratios of unimolecular rate coefficients for reactions 1 and 2. If Arrhenius expressions are used, the difference of activation energies for reactions 1 and 2, E2 − E1, is found to be 16 ± 1 kcal mol−1 and the ratio of the pre-exponential factors, A2/A1, is 7.0 ± 0.3.
I. INTRODUCTION The cyclopentadienone molecule, C5H4O, is found to be a central intermediate in the thermal decomposition of various lignin monomers: methoxyphenol,1 dimethoxybenzene,2 dihydroxybenzene,3 vanillin,3,4 and syringol.3 It has also been well established that C5H4O is an intermediate in the combustion of aromatic hydrocarbons.5−12 The role of C5H4O in both biomass pyrolysis and hydrocarbon combustion is summarized in Scheme 1. Due to its importance in these processes, kinetic models have been developed for the thermal decomposition of C5H4O in both combustion and pyrolysis environments.5,7,9,13−15 It has been suggested that cyclobutadiene (C4H4) is a primary decomposition intermediate,5 and that CO + 2 HCCH are the primary fragmentation products with the lowest activation energies.5,7,16 However, several other potential decomposition mechanisms exist that include carbene and diradical intermediates.5 Scheme 2 shows some of these intermediates and their final products. © XXXX American Chemical Society
The objective of this paper is to isolate the nascent steps in the pyrolysis of cyclopentadienone. The goal is to identify the initial, unimolecular pathways for the thermal fragmentation of C 5H4O. As Pilling has succinctly stated,17 chemical mechanisms must have a quantitative foundation. Mechanisms consist of explicit, coupled chemical reactions, together with rate coefficients and product yields. In order to model the pyrolysis of C5H4O, the initial fragmentation products must be identified and quantified. Over the past decade, we have used a set of microreactors to examine the pyrolysis of complex organic molecules. These Special Issue: 100 Years of Combustion Kinetics at Argonne: A Festschrift for Lawrence B. Harding, Joe V. Michael, and Albert F. Wagner Received: November 13, 2014 Revised: January 15, 2015
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DOI: 10.1021/jp511390f J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A
microwave spectroscopy is used to identify all the decomposition products. This package of spectroscopies is very powerful and enables identification of all products (atoms, radicals, metastables) that are formed in the first 100 μs pyrolysis of these prototypes for complex fuels.18−23 For the particular case of C5H4O, it has been found1 that the products of C5H4O pyrolysis include CO, HCCH, and HCCCHCH2. The ratio of these products varies with the temperature of the microreactor. The target C5H4O is a metastable species and is not easily interrogated. This reactive ketone dimerizes at temperatures as low as −80 °C, so it is not possible to work with a gas mixture in a sample reservoir. However, using a suitable precursor, the rotational23 and vibrational21 spectra of C5H4O have been measured and detailed assignments have been reported. These assignments were guided by CCSD(T) ab initio electronic structure calculations. From an analysis of the microwave spectrum, the re structure of C5H4O was found with all bond lengths established to an uncertainty of ±0.001 Å. The C5H4O molecule is a rigid, planar C2v species with an electric dipole moment24 of 3.132 ± 0.007 D. In this paper, we use tunable synchrotron VUV PIMS to measure the temperature and pressure-dependent branching ratios for the thermal fragmentation of C5H4O. The initial decomposition pathways of C5H4O are (1), (2), and, at the highest temperatures achievable in the microreactor, probably (3).
Scheme 1
C5H4O ( +M) → CO + HCCCHCH 2
(1)
C5H4O ( +M) → CO + HCCH + HCCH
(2)
C5H4O ( +M) → CO + HCCCCH + H 2
(3)
As mentioned earlier, C5H4O is a reactive molecule. Measurement of the initial C5H4O pyrolysis products and the branching ratio of reactions 1−3 is therefore a difficult task. More traditional approaches, such as pyrolysis and product collection followed by gas chromatographic/mass spectrometric detection (GC/MS), are difficult to interpret due to the fact that C5H4O reacts with HCCH or HCCCHCH2 and dimerizes25,26 at an encounter-controlled rate. Coupling the microreactor with tunable VUV PIMS is one way to isolate and quantify the product distributions resulting from the thermal cracking of C5H4O.
23,25,26
resistively heated silicon carbide (SiC) reactors are typically 0.5−1 mm inner diameter (i.d.) and 2−3 cm long. Target molecules are delivered to the reactor as dilute mixtures (typically 0.1% or less) in He, Ar, or Ne buffer gas at pressures of roughly 400 Torr. The microreactors can be either pulsed or continuous flow and are heated to temperatures up to 2000 K. Residence times are in the range 25−150 μs. Gases exit into a pressure of 10−4 Torr, and all chemistry is quenched. A combination of photoionization mass spectrometry (PIMS), matrix isolation infrared spectroscopy (IR), resonanceenhanced multiphoton ionization spectroscopy (REMPI), and Scheme 2
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DOI: 10.1021/jp511390f J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A Recent computational fluid dynamics (CFD) simulations27 have revealed that these reactors are complex, nonlinear devices. Chemical reactions in the microreactors vary exponentially with the gas temperature (which is rising) and quadratically with the pressure (which is falling). Consequently, there is a small region (“sweet-spot”) wherein most of the chemical reaction occurs. The size and location of the sweetspot can vary dramatically with changes in mass flow-rate, reactor dimensions (diameter, length), material (SiC, quartz, or Al2O3), the nature of the buffer gas (He, Ne, Ar), or other factors that we do not yet appreciate. Consequently, all of the measurements in this paper use the same SiC microreactor with a constant and regulated continuous flow of He buffer gas.
The characteristics of continuous and pulsed reactors are quite different. In steady-state laminar flow, the time-dependent terms in the Navier−Stokes equation drop out. In our situation, the geometry is fixed and the mass flow rate is controlled, thus making CFD numerical solutions tractable.27 The simulations produce detailed flow field information. For example, centerline pressure and temperature distributions for helium and argon as carrier gas are shown in Figure 1 for a volumetric flow rate of
II. EXPERIMENTAL SECTION A. Sample Preparation. Continuous molecular beams containing C5H4O and C5D4O were generated from the flash pyrolysis of o-phenylene sulfite. Both C6H4O2SO and C6D4O2SO (see Scheme 3) were subjected to pyrolysis in a Scheme 3
Figure 1. Calculated27 centerline pressure and temperature distributions with Twall = 1500 K and flow rate fixed at 280 sccm.
SiC microreactor, which has been described.28−33 The synthesis of C6H4O2SO has been reported previously.34 The pyrolysis of C6H4O2SO proceeds via the pathway shown in Scheme 3, with o-benzoquinone (o-OC6H4O) as an intermediate. C6H4O2SO diluted in He (approximately 0.05%) was passed through a microtubular (2 mm o.d. × 0.66 mm i.d. × 2.5 cm long), resistively heated (up to 2000 K with 10 A current) SiC continuous flow reactor. A mass flow controller (MKS P4B) regulated the flow of He (280 sccm) over a sample heater that contained the weakly volatile liquid C6H4O2SO. Experimental sample temperatures were 50−65 °C. The concentration of C6H4O2SO in He was estimated by taking the ratio of the vapor pressure at the sample temperature to the backing pressure of He passed over the sample. The gas exiting the SiC reactor was expanded into a vacuum (10−6 Torr), skimmed through a 2 mm aperture, and ionized by synchrotron radiation in a pulsed extraction field, described in the next subsection. Recent computational fluid dynamics (CFD) simulations27 have dramatically clarified the nature of the pyrolysis in the microreactors. We operate these devices in a steady-state, or continuous flow, mode when using synchrotron photons (LBNL Advanced Light Source and Paul Scherrer Institute Swiss Light Source) or in a pulsed mode in Colorado, as befits the nature of the associated radiation sources. For the continuous flow cases, we have run with a carrier gas volumetric flow rate of 280 sccm. At 280 sccm (standard cm3 min−1), the flow in the reactor is mostly in the continuum domain and a well-developed supersonic expansion forms downstream. During an experiment, the volumetric flow rate is controlled using a mass flow controller, and the pressure, just upstream of the reactor and in the downstream chambers, is measured. The upstream pressure typically ranges from 100 to 300 Torr depending on the heating temperature and flow rate.
280 sccm and a wall temperature of 1500 K. Given a reaction mechanism and kinetic parameters, one can include chemistry in the simulations, predicting, for example, the overall conversion percentage of the reactant. Thus, simulations can be used to test reaction mechanisms. The wall temperature (300−2000 K) of the SiC reactor was monitored with a type C thermocouple mounted to the outer wall of the SiC tube that was shielded with an alumina housing to reduce radiative losses from the active thermocouple junction.35 The accuracy of the thermocouple is reported by Omega Engineering as 1.0% from 0 to 2320 °C. An additional small amount of error (
