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Thermal Decomposition of Potential Ester Biofuels Part I: Methyl Acetate and Methyl Butanoate Jessica P. Porterfield, David H. Bross, Branko Ruscic, James H. Thorpe, Thanh Lam Nguyen, Joshua H. Baraban, John F. Stanton, John W Daily, and G. Barney Ellison J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 18 May 2017 Downloaded from http://pubs.acs.org on May 19, 2017

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The Journal of Physical Chemistry

Thermal Decomposition of Potential Ester Biofuels Part I: Methyl Acetate and Methyl Butanoate Jessica P. Porterfield,1 David H. Bross,2 Branko Ruscic,2,3 James H. Thorpe,4 Thanh Lam Nguyen,4 Joshua H. Baraban,1 John F. Stanton,4,5 John W. Daily,6 and G. Barney Ellison1 1

Dept. of Chemistry & Biochemistry, University of Colorado, Boulder, Colorado 80309. USA Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, USA 3 Computation Institute, The University of Chicago, Chicago, Illinois 60637, USA 4 Department of Chemistry, University of Texas, Austin, Texas 78712, USA 5 Department of Chemistry, University of Florida, Gainesville, Florida 32611, USA 6 Dept. of Mechanical Engineering University of Colorado, Boulder, Colorado 80309, USA 2

Submitted to J. Phys. Chem. A, March 2017 — ABSTRACT Two methyl esters have been examined as models for the pyrolysis of biofuels. Dilute samples (0.06 - 0.13%) of methyl acetate (CH3COOCH3) and methyl butanoate (CH3CH2CH2COOCH3) were entrained in (He, Ar) carrier gas and decomposed in a set of flash-pyrolysis micro-reactors. The pyrolysis products resulting from the methyl esters were detected and identified by vacuum ultraviolet photoionization mass spectrometry. Complementary product identification was provided by matrix infrared absorption spectroscopy. Pyrolysis pressures in the pulsed micro-reactor were roughly 20 Torr and residence times through the reactors were approximately 25 - 150 µs. Reactor temperatures of 300 – 1600 K were explored. Decomposition of CH3COOCH3 commences at 1000 K and the initial products are (CH2=C=O and CH3OH). As the micro-reactor is heated to 1300 K, a mixture of (CH2=C=O and CH3OH, CH3, CH2=O, H, CO, CO2) appears. The thermal cracking of CH3CH2CH2COOCH3 begins at 800 K with the formation of (CH3CH2CH=C=O, CH3OH). By 1300 K, the pyrolysis of methyl butanoate yields a complex mixture of (CH3CH2CH=C=O, CH3OH, CH3, CH2=O, CO, CO2, CH3CH=CH2, CH2CHCH2, CH2=C=CH2, HCCCH2, CH2=C=C=O, CH2=CH2, HC≡CH, CH2=C=O). Based on the results from the thermal cracking of methyl acetate and methyl butanoate, we predict several important decomposition channels for the pyrolysis of fatty acid methyl esters, R-CH2-COOCH3. The lowest energy fragmentation will be a 4-center elimination of methanol to form the ketene, RCH=C=O. At higher temperatures, concerted fragmentation to radicals will ensue to produce a mixture of species: (RCH2 + CO2 + CH3) and (RCH2 + CO + CH2=O + H). Thermal cracking of the β C-C bond of the methyl ester will generate the radicals (R and H) as well as CH2=C=O + CH2=O. The thermochemistry of methyl acetate and its fragmentation products have been obtained via the Active Thermochemical Tables (ATcT) approach, resulting in ∆fH298(CH3COOCH3) = -98.7 ± 0.2 kcal mol-1, ∆fH298(CH3CO2) = ‑45.7 ± 0.3 kcal mol-1, and ∆fH298(COOCH3) = -38.3 ± 0.4 kcal mol-1.

Introduction

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-2This paper presents a study of the flash pyrolysis of two simple methyl esters:

methyl

acetate

(CH3COOCH3)

and

methyl

butanoate

(CH3CH2CH2COOCH3). Methyl esters of fatty acids are believed to have similar combustion properties to petroleum based fuels,1,2 making them a promising class of biofuels. Such biofuels are derived from the triglyceride esters of fatty acids found in biological membranes, see Scheme 1. Transesterification of these triglycerides with methanol results in the formation of methyl esters of fatty acids, such as methyl stearate, CH3(CH2)16COOCH3. Here we focus on the pyrolysis of methyl acetate and methyl butanoate as prototypes for the behavior of these long chain fatty acid esters. An important goal of this paper is to identify all pathways for the thermal cracking of methyl acetate and methyl butanoate at pressures of roughly 20 Torr and at temperatures of 300 – 1600 K. An accurate model for the unimolecular decomposition of these simple fuels is an important first step in the development of mechanisms for larger esters.3-5 Pyrolysis of esters has been studied extensively: experiments6-8 date back to 1936. The pyrolysis of the simple ester, methyl acetate, has been studied in shock tubes,9-12 a Knudsen reactor,13 flames,14,15 jet-stirred,16 and flow reactors.17,18 There have also been a number of computational studies on the mechanisms for unimolecular decomposition of esters.19-23 In addition to pyrolysis experiments, there are many studies of the oxidation of methyl esters.5,16,18,24-32 Of all the methyl esters (RCOOCH3) the compound methyl butanoate has been of particular interest. This is because the chain length of CH3CH2CH2COOCH3 seems to represent a reasonable compromise between computational limits and


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-3carbon chain lengths characteristic of fatty acids.3,4,20,24,33-37 The pyrolysis of methyl butanoate has been studied experimentally in shock tubes10,38,39 and in rapid compression machines.40,41 In the present study, we focus on flash pyrolysis of dilute (0.06 - 0.13%) samples in the heated micro-reactor described below. If high concentrations of radicals such as CH3 or H atoms are produced, complications will ensue. Radical/methyl ester reactions will be chain reactions and it will not be easy to distinguish the primary pyrolysis products from secondary, radical/substrate products. Reactions of either H atoms or CH3 radicals with methyl esters would be expected to produce α-keto radicals such as CH2COOCH3, which will rapidly undergo β−scission in the hot micro-reactor: CH2COOCH3 → CH2=C=O + H + O=CH2. Care must be taken to avoid these radical/substrate chain reactions. That being said, we believe that there is no detectable bimolecular chemistry in these measurements owing to the dilute conditions employed in our study. Ester pyrolysis is carried out in high temperature silicon carbide (SiC) micro-reactors with short residence times. Gases exit the hot SiC micro-reactors into a vacuum of 10-6 Torr where all reactions cease. A combination of photoionization mass spectrometry and matrix isolation infrared spectroscopy is used to identify the decomposition products. These two detection methods are very powerful, and in principle enable identification of all products (stable fragments, radicals, and metastables) formed in the first 25 - 150 µs of complex fuel pyrolysis.42-48



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-4Experimental Two

flash

pyrolysis

experiments

were

performed,

employing

photoionization mass spectrometry (PIMS) and matrix isolation infrared spectroscopy (IR) as detection methods. The details of these micro-reactors can be found in previous work;49-52 however a brief description is provided here. Both the PIMS and IR experiments utilize a micro-reactor composed of a resistively heated silicon carbide (SiC) tube 1 mm in diameter and 3 cm in length. A Parker General Valve operating at 10 Hz supplies gas to the tube. Gas mixtures are composed of ≤ 0.1% fuel in helium or argon, and sufficiently dilute in order to avoid bimolecular chemistry. In the PIMS experiments, the reactor exit faces a vacuum chamber held at 1 x 10-6 Torr by a Varian VHS-6 diffusion pump (870 L s-1). A 2 mm aperture skims the molecular beam 1 mm from the reactor exit. The 9th harmonic of a Nd:YAG laser (118.2 nm or 10.487 eV) intersects the molecular beam downstream and resulting ions are accelerated into a Jordan reflectron time-offlight mass spectrometer. Mass spectra are averaged over 1000 scans, with a mass resolution ∆m/m of roughly 400. Complementary vibrational data is provided by matrix IR experiments. Gas mixtures exit the reactor into a vacuum chamber held at 10-6 Torr by an Agilent TV 81M turbo-pump. Roughly 3 cm from the reactor exit lies a cryogenic (20 K) CsI window where the gas mixture deposits, trapping the products of thermal decomposition in a matrix of frozen argon. Spectra are collected by placing the


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-5cold CsI window in the path of a Nicolet 6700 FT-IR equipped with an MCT-A detector (4000 – 600 cm-1). Spectra are averaged over 500 scans with a 0.25 cm-1 step size. Reactor temperatures are monitored by a Type C thermocouple (Omega Engineering, 1% accuracy between 270 - 2300 K) held flush against the external wall of the reactor with 0.25 mm tantalum wire. Electrodes used to resistively heat the reactor are placed 1 to 1.5 cm apart. Conventional pressure and temperature diagnostics cannot be used to measure the conditions inside the SiC tube. Pressure, temperature, and residence time approximations are further complicated due to the pulsed nature of the experiment, so proper estimations of these values are discussed below. As described in Guan et al.,50 we have carried out a series of computational fluid dynamic (CFD) simulations for steady flows inside the SiC reactor. Due to the pulsed nature of gas flow in this experiment, each pulse starts and ends with vacuum inside the reactor. The exit pressure is very low, so the gas flow accelerates along the length of the reactor and chokes (reaches the speed of sound) at the exit. This quenches the reaction chemistry and results in effective residence times in the range of 25 – 150 µs. Experimentally, the overall volumetric flow rate through the reactor was provided by monitoring the pressure loss rate within the gas mixing manifold. Through separate measurements, we also know that the General valve opens for about 2 ms with a FWHM of about 1 ms. Therefore, assuming constant flow over



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-6the 1 ms we can estimate the volumetric flow rate through the reactor. From the CFD simulations, we can estimate the reactor inlet pressure given the wall temperature and volumetric flow rate. For the PIMS and IR experiments, the inlet pressure is roughly 42 and 75 Torr, respectively. SiC is an excellent heat conductor, thus the reactor wall temperature is approximately constant between electrodes. For the PIMS experiments (helium) the peak gas temperature is close to the wall temperature. However, for the IR experiments (argon) the radially averaged gas temperature is likely to be no more than about 200 K below the wall temperature. The temperature of the flowing gas increases due to conduction until a point, then the gas velocity becomes sufficiently high and causes the gas temperature to decrease. The pressure decreases continuously with downstream distance, reaching a minimum at the exit. There is therefore a peak in the pressure-temperature product that results in a "sweet spot" within the reactor where reaction rates will be at a maximum. At the sweet spot, pressures are approximately 1/3 to 1/2 the entrance pressure. This results in pressures of about 20 Torr for both the PIMS and IR experiments.

Thermochemistry and Decomposition Pathways In order to understand the pyrolysis of methyl acetate and methyl butanoate, an accurate rendering of the thermochemistry is essential and is collected in Tables 1-3. Table 4 presents a set of relevant ionization energies. For a significant number of reactions, particularly those involved in methyl


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-7acetate pyrolysis and the related discussion, the thermochemistry has been obtained from the Active Thermochemical Tables (ATcT) approach.53-55 In contrast to conventional sequential thermochemistry (A begets B, B begets C, etc.), ATcT derives a set of internally consistent thermochemical values by constructing, statistically analyzing, adjusting, and solving a Thermochemical Network that contains the available experimental and theoretical determinations such as reaction enthalpies and free energies, bond dissociation energies, gasphase acidities, electron affinities, etc.56-59 The current ATcT results were obtained from a large Thermochemical Network (ver. 1.122e), which includes species of interest to the current study, and is an expanded progeny of versions 1.122 and 1.122b, described previously.60,61 The heat of formation of the precursor methyl acetate has been measured previously; ∆fH298(CH3COOCH3) was reported62 to be -98.4 ± 0.4 kcal mol-1. This literature value is slightly less accurate, but otherwise entirely congruent with the current ATcT value of -98.7 ± 0.2 kcal mol-1. There are no experimental values available for the heat of formation of CH3CH2CH2COOCH3, nor have we been able to incorporate this molecule (or its primary pyrolysis products) into the current

ATcT

Thermochemical

Network.55

Consequently,

a

number

of

thermochemical estimates related to the initial steps during the pyrolysis of methyl butanoate rely on published electronic structure calculations.33,63 Besides methyl acetate and methyl butanoate, we have also included the simplest methyl ester, methyl formate, in the discussion. For each reaction of relevance to the current study, Table 1 lists the 298 K enthalpy of reaction,



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-8corresponding to the current ATcT value, accompanied by an uncertainty formally corresponding to 95% confidence limits.64 Table 2 lists a summary of literature theoretical values for species not present in the ATcT Thermochemical Network. Table 3 has approximate bond energies relevant to the thermal cracking of generic alkyl methyl esters, RCOOCH3. All reaction enthalpies in Tables 1-3 are in kcal mol-1.

Results Potential Energy Surface for Methyl Acetate Decomposition To complement the experimental results, quantum chemical calculations were carried out in order to identify the most important reaction pathways for the unimolecular thermal decomposition of methyl acetate, CH3COOCH3. First, all stationary points on the PES were fully optimized using the CCSD(T) method65,66 in the frozen-core approximation, in combination with the polarized double-zeta atomic natural orbital basis set, ANO0.67-69 Harmonic vibrational analysis was then performed using the same level of theory in order to characterize all stationary points as minima, transition states or higher-order saddles and to obtain the associated zero-point vibrational energies. Last, energies of all stationary points were calculated with the larger triple-zeta ANO1 basis set,67-69 using

the

previously

calculated

(CCSD(T)/ANO0)

geometry.

This

CCSD(T)/ANO1//CCSD(T)/ANO0 approach is expected to yield an accuracy of a few kcal mol-1 for the associated energy differences and is therefore suitable to


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-9draw qualitative inferences about the process under investigation. All calculations were done with the CFOUR70 quantum chemical program. The schematic potential energy surface is displayed in Fig. 1. The computed energy differences are not listed in Table 1, but they are generally consistent within 1-3 kcal mol-1 with the current ATcT results at 0 K. Both the syn-

and

anti-conformers

of

methyl

acetate

CH3COOCH3

have

been

characterized; the anti form lies 8 kcal mol-1 (ATcT: 7.4 ± 0.4 kcal mol-1 at 0 K) higher than the syn-conformer. Under the high temperature conditions of the experiment, both conformers can undergo a rapid isomerization by CH3 rotation around the C-O axis (I1) with a low barrier of 13 kcal mol-1; see Fig. 1. The syn and anti conformers will be thermally equilibrated in the hot micro-reactor prior to pyrolytic decomposition. In addition, syn-methyl acetate can undergo a degenerate isomerization via CH3-transfer from one oxygen atom to the other (TS0) with a barrier of 54 kcal mol-1, I3. As seen in Fig. 1, pyrolysis of CH3COOCH3 can proceed through tight or loose transition states. These loose transition states – which are characterized by a barrierless reverse association step – must be treated variationally. First, we discuss the direct, tight transition state channels in Fig. 1. CH3COOCH3 can pass through four different tight TSs (TS1a to TS4) ranging in energy from 74 to 107 kcal mol-1; yielding products P1 (CH3OH + CH2CO), P2 (CH2=O + CH3CHO), P3 (CO2 + C2H6), and P4 (CO + CH3OCH3). Of these four transition states, the two four-membered ring structures (TS1a and TS2) characterized by H–migration lie significantly lower in energy than the two, three-



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- 10 membered ring saddle points accessed by CH3 migration. Consequently, the H– shift pathways are energetically preferred and their products, P1 and P2, are favored. It is notable that the decomposition of CH3COOCH3 via TS3, and consequent formation of P3 (CO2 + C2H6), is exothermic by 18 kcal mol-1 (ATcT: 16.1 ± 0.2 kcal mol-1 at 0 K). While this is the most thermodynamically favorable pathway, the large energy barrier of 97 kcal mol-1 is a severe kinetic restriction that makes it inaccessible, in line with our experimental findings discussed below. In addition to the concerted mechanism passing through TS1a, methyl acetate can undergo a stepwise decomposition via an intermediate enol (CH2=C(OH)-OCH3), leading to products P1 (CH3OH + CH2=C=O). First, CH3COOCH3 isomerizes to CH2=C(OH)-OCH3 (I2) by a 1,3 H-shift via TS1b, overcoming a barrier of 71 kcal mol-1. Next, CH2=C(OH)-OCH3 can either revert to methyl acetate or dissociate via the four-membered ring TS1c with a barrier of 42 kcal mol-1, yielding P1. Compared to the concerted decomposition of CH3COOCH3 → CH3OH + CH2=C=O, this stepwise decomposition has a slightly lower barrier (by 3 kcal mol-1), and is thus kinetically favorable. However under the high temperature conditions of pyrolysis, both mechanisms are expected to be competitive. A master-equation analysis is required to quantify the formation of P1 from both mechanisms, and such a study is underway. Next we discuss the loose (variational) transition state channels. CH3COOCH3 can break single bonds (C–C or C–O) via loose transition states to produce products such as P6 (CH3 + CH3–CO2), P7 (CH3 + CH3–O–CO), and P8 (CH3CO + CH3O) at energies of 86, 93, and 99 kcal mol-1, respectively (ATcT:


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- 11 86.4 ± 0.3, 93.6 ± 0.4, and 100.3 ± 0.2 kcal mol-1 at 0 K). Of these pathways, the formation of P7 and (especially) P6 would be dominant. However, both CH3–CO2 and CH3–O–CO have short lifetimes at high temperatures, as they face relatively low energy barriers for decomposition to P5 (CO2 + 2 CH3). The CH3-O-CO radical could also decompose to CH3 + CO + CH3O. Under the conditions of our experiments, the methoxy radical (CH3O), which has an unusually low C-H bond energy of 19.5 ± 0.1 kcal mol-1 (0 K),60 will rapidly eliminate a hydrogen atom to form formaldehyde. It should be mentioned that a rather novel concerted, threebody decomposition of CH3COOCH3 (which yields P5 directly) cannot be ruled out, since it is the most energetically feasible pathway. It is of great importance that while product P1 is formed via relatively lowlying

transition

states (TS1a,

TS1b,

and TS1c),

the channels leading

to

products P6 and P7 are entropically favored via their loose transition structures. Consequently they are likely to become competitive reaction pathways at high temperatures. The potential energy surface in Fig. 1 would suggest that the major products of the thermal dissociation of CH3COOCH3 should include CH3 radicals (and the associated products such as CO2, CO, CH2=O and H atom) as well as P1 and/or P2. However the complicated nature of these pathways necessitates the aforementioned accurate chemical kinetics (master equation) analysis to quantify the product branching ratios and thermal rate constants for these various product channels. To make sense of the observed pyrolysis products from methyl acetate and methyl butanoate, two sets of decomposition pathways have been



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- 12 developed: Schemes 2 and 3. These schemes are constructed on the basis of the thermochemistry in Table 1 and the theoretical predictions of Fig. 1. We have used the labels, P1 – P8, from Fig. 1 in Table 1. This table collects the energetics of many fragmentations of several methyl esters and includes a set of interesting oxycarbonyl radicals: formyloxy (HCO2), acetyloxy (CH3CO2), and butanoyloxy (CH3CH2CH2CO2). Oxycarbonyl radicals (RCO2) are unusual because they have “negative” bond energies: ∆rxnH298(R-CO2 → R + CO2) < 0; see (reactions H13 and P27 in Table 1). Current ATcT results provide the following O-H bond enthalpy and heat of formation of the oxyformyl radical: DH298(HCOO-H) = 112.1 ± 0.2 kcal mol-1 and ∆fH298(HCO2) = -30.5 ± 0.2 kcal mol-1(see also Table 1). For acetic acid, the relevant ATcT energies are DH298(CH3COO-H) = 109.8 ± 0.3 kcal mol-1 and ∆fH298(CH3CO2) = -45.7 ± 0.3 kcal mol-1. This thermochemistry implies a set of negative bond energies: DH298(H-CO2) = -11.5 ± 0.2 kcal mol-1 and DH298(CH3-CO2) = -13.3 ± 0.3 kcal mol-1. Other than being observed via photodetachment of the HCO2— and CH3CO2— ions, there have been no other experimental characterizations of the HCO2 or CH3CO2 radicals. No spectra exist for either of them and they have never been trapped in solution. The properties of the related CH3CH2CH2CO2 radical (relevant in pyrolysis of methyl butanoate) are likely to be similar to those of the acetyloxyl radical. Both are molecular resonances71 and are also difficult to calculate accurately by standard electronic structure methods due to strong vibronic coupling between electronic states that transform as 2A1 and 2B2 in C2v symmetry.72,73


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- 13 Another possible fragment in the pyrolysis of methyl esters is the exotic methoxycarbonyl radical, COOCH3 (or CH3O-CO). The methoxycarbonyl radical has

never

been

detected

but

it

is

likely

an

intermediate

in

the

photofragmentation74 of CH3O-CO-Cl. Photodissociation of methyl chloroformate at 193 nm yielded fragments that were photoionized by 14.8 eV synchrotron radiation: CH3OCOCl + hω193nm → fragments + hω14.8eV → (Cl+, CO+, CO2+, CH3+, CHO+, and CH2=O+). The kinematic analysis was consistent75 with the fragmentation process:

CH3OCOCl + hω193nm → Cl + [CH3OCO]* → CO, CH3O, CO2, CH3

(3)

A species related to CH3O-CO is the important hydroxycarbonyl radical, HOCO. The hydroxycarbonyl radical has been produced by reaction of fluorine76,77 or chlorine78,79 atoms with HCOOH. Alternatively, 193 nm photodissociation of CH2=CHCOOH provides80-82 a convenient source of HOCO. Recently

hydroxycarbonyl

has

been

detected

by

high

resolution

IR

spectroscopy83,84 as the adduct following reaction of OH with CO. The ground state of hydroxyl carbonyl is observed78,81,82,85,86 to be anti-HOCO X˜ 2A’ (also called trans-HOCO) and it is stable to dissociation to H and CO2 by only 1.0 ± 0.2 kcal mol-1 at 0 K (2.1 ± 0.2 kcal mol-1 at 298 K, see H12 in Table 1) although with a fairly substantial barrier that is tunneled through at low to moderate temperature.87 Table 3 shows a rough set of bond energies that will apply to all alkyl methyl esters. The last three reactions are high in energy and are unlikely to be



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- 14 in play at the temperatures of the hot SiC micro-reactor. These three reactions are the loss of H atoms α to the ester functionality (-COOCH3) or from the OCH3 group, or cleavage of the RCO-OCH3 bond.

Methyl Acetate Pyrolysis Fig. 2 shows the resulting PIMS spectra when methyl acetate (m/z 74) is diluted to 0.07% in helium and heated to 1600 K in a pulsed SiC micro-reactor. The IE(methyl acetate) is known to be 10.3 eV (see Table 4) and the only ion present at 300 K is the parent species at m/z 74. As the temperature is increased, PIMS spectra remain simple. Thermal decomposition begins around 1000 K with appearance of a peak at m/z 42. As the reactor is heated to 1300 K, an additional product at m/z 15 is observed. The only possible assignment for m/z 15 is the CH3 radical while m/z 42 could be CH2=C=O or CH2=CHCH3. A small feature at m/z 43 appears at 1000 K and becomes more intense at 1300 K; we assign this peak to the CH2COH+ cation that results from the dissociative ionization of the enol of methyl acetate. Heating methyl acetate might isomerize it to CH2=C(OH)OCH3 (reaction I2); the ionization energies of enols are typically 1 eV lower than ketones.52 The IE(CH3COOCH3) is 10.3 eV (see Table 4); consequently VUV photoionization of the enol, CH2=C(OH)OCH3, with

10.487

eV

photons

will

produce

a

chemically

activated

cation,

[CH2=C(OH)OCH3]+. The CH2=C=OH+ ion could arise from the β-scission of the [CH2=C(OH)OCH3]+ ion; [CH2=C(OH)-OCH3]+ → CH2=C=OH+ + OCH3. By 1600 K the parent methyl acetate (m/z 74) appears to be completely destroyed. Fig. 1


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- 15 does not show other possible pyrolysis products of CH3COOCH3 such as CO, CO2, CH2=O, or CH3OH because their ionization energies are too high for the 118.2 nm VUV laser (see Table 4). Infrared spectroscopy can be used to confirm the identity of the products in Fig. 2 including those with ionization potentials that are out of the range of our laser. In all matrix spectra presented, the room temperature spectrum of CH3COOCH3 is shown in black and a heated argon (1400 K) IR spectrum is shown in green as a background. Authentic samples of products deposited onto the matrix are displayed in blue when available. Strong evidence for reaction P1 is shown with the infrared spectra that result when 0.13% methyl acetate in argon is heated at 1200 K and 1500 K (Figs. 3 and 4). The presence of ketene is clearly indicated in Fig. 3 by the absorption bands ν1 (3062 cm-1), the conspicuous ν2 (2142 cm-1) and ν4 (1381 cm-1); for comparison, an authentic sample of CH2=C=O is shown in blue.88 Fig. 4 shows strong absorption bands of methanol, ν3 (2848 cm-1) and ν8 (1033 cm-1); the IR spectrum of pure CH3OH diluted in Ar is shown in blue.89 Figs. 5 and 6 demonstrate that pyrolysis of CH3COOCH3 diluted in Ar produces CH2=O, CO, and CO2 at 1200 K. Fig. 5 displays three of the observed IR absorption features of formaldehyde ν1 (2798 cm-1), ν2 (1742 cm-1), and ν3 (1499 cm-1), respectively.90 In Fig. 6, the carbonyl region of the IR spectrum is shown where the prominent C=O stretch of ketene (ν2) appears again as well as evidence for CO and CO2. Fig. 6 shows intense monomer and cluster absorption



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- 16 bands91,92 of CO at 2139 cm-1 and 2149 cm-1. Carbon dioxide is also present in Fig. 6 as ν3(CO2) at 2345 cm-1 and two cluster bands93 at 2340 cm-1 and 2339 cm-1. Scheme 2 lists a possible fragmentation pathway for CH3COOCH3 producing CH2=O and CH3CHO (reaction P2). There is no doubt that pyrolysis of methyl acetate generates formaldehyde (see Fig. 5), but the PIMS scan in Fig. 2 shows no sign of CH3CHO at m/z 44. The pyrolysis of acetaldehyde has been studied earlier and CH3CHO is stable up to 1600 K; see Fig. 3 in ref.94. The absence of CH3CHO implies that the formaldehyde observed in Fig. 5 does not result from pyrolysis of methyl acetate to CH3CHO and CH2=O. In earlier CH3COOCH3 shock tube experiments, the formation of H atoms was observed behind reflected shock waves by using atomic resonance absorption spectrometry.11 The current flash pyrolysis experiment cannot detect H atoms because the laser cannot ionize atomic hydrogen (13.6 eV, Table 4). A summary of the observed products of methyl acetate is presented here. At a pressure of roughly 20 Torr, the initial products that result at 1000 K from pyrolysis of CH3COOCH3 are ketene and methanol. By 1300 K a mixture of methyl radical, formaldehyde, H atom, carbon monoxide, and carbon dioxide appears. CH3COOCH3 (+ M) → (CH2=C=O, CH3OH) at 1000 K

(4a)

CH3COOCH3 (+ M) → (CH2=C=O, CH3OH, CH3, CH2=O, H, CO, CO2) at 1300 K (4b)


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- 17 Methyl Butanoate Pyrolysis Fig. 7 is the PIMS that results from pyrolysis of a 0.06% sample of methyl butanoate (m/z 102) in He heated to 1500 K. At 300 K the parent peak at m/z 102

is

accompanied

by

an

intense

signal

at

m/z

74.

The

IE(CH3CH2CH2COOCH3) is only 10 eV (see Table 4) and ionization by 118.2 nm (10.487 eV) photons leads to dissociative ionization. The parent cation, [methyl butanonate]+ (m/z 102), likely loses CH2CH2 via a McLafferty rearrangement95,96 to produce the [CH2C(OH)OCH3]+ cation, m/z 74. Thermal decomposition of methyl butanoate begins near 800 K when m/z 70 first appears in the PIMS spectrum. The m/z 70 feature is assigned to CH3CH2CH=C=O resulting from reaction P9. At 1300 K, products appear at m/z 42, 28 and 15. The two signals at m/z 15 and m/z 28 are certainly CH3 radical and CH2=CH2. The IE(ethylene) slightly exceeds 10.487 eV (see Table 4), consequently the m/z 28 signal in Fig. 7 must result from chemically activated ethylene, [CH2=CH2]*. The feature at m/z 42 could be either ketene, propylene, or a mixture. As we will see from the IR spectra, both CH2=C=O and CH3CH=CH2 are present. As the temperature of the micro-reactor approaches 1500 K, further decomposition ensues. Thermal cracking of CH3CH=CH2 (m/z 42) produces the allyl radical (CH2CHCH2 m/z 41), allene (CH2=C=CH2 m/z 40), and the propargyl radical (HCCCH2 m/z 39). The relevant energetics are collected in (H15 – H17, P33) of Table 1. The IR spectra (see below) confirm the presence of the allyl



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- 18 radical and allene at 1300 K. Fig. 8 is the 118.2 nm PIMS scan of the pyrolysis of an authentic sample of CH2=C=CH2. At about 1500 K the pyrolysis of allene to H atom and propargyl radical (m/z 39) is detected. Table 1 reveals that allene decomposition requires 91 kcal mol-1 (H17). Supporting evidence for these PIMS assignments was found in the infrared spectra in Figs. 9 — 15. Absorption features of methanol ν3 (2848 cm-1) and ν8 (1033 cm-1) are clearly present in Fig. 9; these are compared to an authentic sample of methanol shown in blue.89 Decomposition of methyl butanoate by alcohol elimination predicts the formation of methanol and ethylketene (CH3CH2CH=C=O). Scheme 3, P19 shows that the subsequent decomposition of CH3CH2CH=C=O can yield the following species: CH3 (m/z 15), CH2=C=C=O (m/z 54), CO, H, HC≡CH, and CH2=CHCH3. Fig. 10 displays the carbonyl stretching region that includes a strong feature at 2122 cm-1 which is assigned97 to CH3CH2CH=C=O. The broad shoulder of the ethylketene peak at 2125 cm-1 could be attributed98 to propadienone (CH2=C=C=O), although we must be cautious about a positive identification of propadienone. The IE(CH2CCO) is reported to be 9.1 eV (Table 2) so the 118.2 nm PIMS should ionize propadienone, but no feature at m/z 54 is observed in Fig. 7. Ketene is also present in Fig. 10 as revealed by the presence of ν2(CH2=C=O) at 2142 cm-1. As expected, the pyrolysis of the CH3CH2CH2COOCH3 ester generates both carbon monoxide and carbon dioxide. The intense CO2 fundamental, ν3, is present and clusters of carbon dioxide are also detected.93 Isolated carbon


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- 19 monoxide and its clusters are also present.91,92 In Fig. 10 the absorption bands for CH3CH2CH=C=O and CH2=C=C=O appear to decrease in intensity from 1100 to 1600 K while the bands for a product of their decomposition (P19) HC≡CH grow in, see Fig. 11. The acetylene Darling-Dennison doublet associated with ν3 is observed at 3289 and 3240 cm-1 and ν5 is observed99,100 at 737 cm-1. Further evidence for the formation of CH2=C=O is displayed in Fig. 12 shown by the presence of ν1 (3062 cm-1), ν2 (2142 cm-1) and ν4 (1381 cm-1) compared to an authentic sample of ketene shown in blue.88 Fig. 13 supports the appearance of CH3CH=CH2 as CH3CH2CH=C=O decomposes. The observed bands for CH3CH=CH2 are ν7, ν18, and ν19 at 1453, 998, and 908 cm-1 respectively.101 Two of the thermal products of propene, the CH2CHCH2 radical and CH2=C=CH2, were also observed in the argon matrix. Fig. 14 demonstrates the presence of allyl radical102 with absorption bands ν1 at 3112 cm-1 and ν11 at 801 cm-1. Loss of H atom from allyl radical produces allene100 (CH2=C=CH2) for which ν6 at 1957 cm-1 is displayed on the right hand side of Fig. 14. Fig. 15 presents evidence for the formation of formaldehyde and ethylene; the bands ν1(CH2=O), ν2(CH2=O) and ν7(CH2=CH2) are all present. As mentioned in the introduction, the pressure at the “sweet-spot” of the pulsed micro-reactor50 where most pyrolysis occurs is roughly 20 Torr. The products that result from pyrolysis of methyl butanoate are as follows: at 800 K the pyrolysis products are ethylketene and methanol; at 1300 K a more complex



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- 20 mixture appears. CH3CH2CH2COOCH3 (+ M) → (CH3CH2CH=C=O, CH3OH)

at 800 K (5a)

CH3CH2CH2COOCH3 (+ M) → (CH3CH2CH=C=O, CH3OH, CH3, CH2=O, CO, CO2, CH3CH=CH2, CH2CHCH2, CH2=C=CH2, HCCCH2, H, CH2=C=C=O (?), CH2=CH2, HC≡CH, CH2=C=O)

at 1300 K (5b)

Discussion A discussion of the structure of esters is essential to make sense of the thermal cracking of CH3COOCH3 and CH3CH2CH2COOCH3. To understand the molecular structures, remember that all esters have two different conformations: syn and anti. In Scheme 4, π-resonance ensures that the R-COO-CH3 backbone of methyl esters will be planar. This planarity implies that there will be two different conformations: syn and anti. The microwave spectrum103 of methyl formate has been analyzed to yield a structure for the ester and reveals that HCOOCH3 adopts a syn-conformation. The less stable anti- HCOOCH3 ester has been detected104 and the current ATcT values indicate a difference in enthalpy at 298 K of 4.9 ± 0.3 kcal mol-1 (reaction I7 in Table 1). Because the O-CH3 σ bond is polarized toward the electronegative O atom, the -CH3 group will develop a small positive character. The electrostatic attraction between the >C=O group and -CH3 can be used to rationalize the stabilization of the syn conformation. In an earlier study, a Coulombic effect such as this was found to be decisive in the conformational analysis of a set of phenyl substituted 1,3-dioxanes.105


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- 21 The structure of methyl acetate has been studied by both electron diffraction106,107 and microwave spectroscopy.108-111 The early diffraction107 and microwave109 studies conclude that the planar heavy-atom skeleton of CH3COOCH3 is in the syn conformation. The microwave spectrum of CH3COOCH3 is difficult to analyze because the ester contains two inequivalent methyl groups. Consequently there exists no precise molecular structure for methyl acetate based on rotational spectroscopy. However, there is no doubt that the syn conformer of CH3COOCH3 is lower in energy than the anti structure in Scheme 4; according to the ATcT results syn-methylacetate is more stable than anti by 7.4 ± 0.4 kcal/mol. The CH3CO–OCH3 rotational barrier104 between the syn and anti conformations of CH3COOCH3 is roughly 13 kcal mol-1 (see Fig. 1). Methyl acetate undergoes a degenerate rearrangement as it is heated. Admitting CH3C18O-16OCH3 to a Knudsen cell13 at 1400 K leads to the formation of the CH3C16O-18OCH3 (I3) isotopomer. Based on the results of the pyrolysis of acetaldehyde,94 CH3COOCH3 could also enolize to CH2=C(OH)OCH3; this is shown in Fig. 1 and in Scheme 2 (I2). According to ATcT in Table 1, the enol is higher by 26.3 ± 0.5 kcal mol-1 at 298 K. Alternatively, decomposition of the ester through a pair of four-center transition states leads to P1 (ketene, methanol) or P2 (formaldehyde, acetaldehyde). Table 1 shows the enthalpy change for these two paths to be relatively low. Both PIMS and IR spectroscopy detect formation of CH2=C=O and CH3OH in support of P1. There are no PIMS or IR signals from CH3CHO, however, even though it is known94 that an authentic sample of CH3CHO (m/z 44) persists up to 1500 K. We conclude that P1 is active by 1000



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- 22 K, but the absence of acetaldehyde in our experiments indicates that the channel leading to P2 is open under the conditions studied here. The ketene product from channel P1 is stable under all conditions of the micro-reactor. The fate of the other product, CH3OH, was not followed because the 10.487 eV laser cannot ionize methanol (see Table 4). The thermochemistry of methanol60 suggests that at the higher temperatures of the micro-reactor, CH3OH will preferentially crack apart to CH3 and OH radicals. The bottom portion of Scheme 2 shows that methyl acetate could thermally crack apart to a set of radicals. P5 is a concerted fragmentation of [CH3COOCH3]* to a pair of methyl radicals and carbon dioxide. P6 is the stepwise cleavage of the [CH3COO-CH3]* to the (CH3CO2, CH3) radical pair. The acetyloxy radical, if it exists, will rapidly produce CO2 and CH3. Alternatively, step-wise dissociation of [CH3-COOCH3]* to generate a methyl radical and the methoxycarbonyl radical offers a different pathway. Fragmentation of [COOCH3]* will produce CO and the CH3O radical, P7a. The methoxy radical will quickly decompose to H atoms and CH2=O. Alternatively, the decomposition [COO-CH3]* leads to CO2 and CH3, P7b. The experimental PIMS spectrum in Fig. 2 shows evidence of CH3 (m/z 15) but there are no signals at m/z 59 indicative of the presence of either the CH3CO2 or CH3OCO radicals. Acetoxy is an oxygen-centered radical, so the IE(CH3CO2) is expected to be too high to be to accessed by the 10.487 eV laser (ATcT: 11.14 ± 0.03 eV). However the COOCH3 radical will have a low threshold for ionization because the methoxycarbonyl radical is related to the acetyl radical,


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- 23 and IE(CH3CO) = 7.0 eV (see Table 4). The lack of any PIMS signals at m/z 59 in Fig. 2 argues that no COOCH3 or CH3CO2 radicals survive the transit through the hot micro-reactor as one would expect because of the facile pathways that exist for both of them to lose CO2. In summary, decomposition of CH3COOCH3 begins at 1000 K (eq. 4a) where it yields products P1. The 1300 K products (eq. 4b) are the radical processes shown in Scheme 2. We cannot distinguish amongst P5, P6, or P7. The observation of carbon monoxide, H atoms, and formaldehyde supports P7a with the COOCH3 radical fragmenting to either (CH3, CO2) or (CO, H, and CH2=O). Methyl butanoate decomposition pathways are constructed in a similar manner to CH3COOCH3 and are shown in Scheme 3. Isomerization of CH3CH2CH2COOCH3 to the enol is a possible initial step, I5. The ester could also fragment to P9 (CH3OH, CH3CH2CH=C=O) or to P10 (CH2O, CH3CH2CH2CHO). Fig. 7 shows no signal at m/z 72 characteristic of butyraldehyde, which is analogous to the absence of acetaldehyde from pyrolysis of methyl acetate (P2). The pyrolysis of butyraldehyde was examined112 and CH3CH2CH2CHO was stable until 1300 K when it fragmented by a variety of complex pathways. Due to the fact that there is no evidence for m/z 72, and the analogous channel P2 is inactive for methyl acetate, P10 is discounted as an active channel. The Cope reaction could lead to CH2=CH2 and CH2=C(OH)OCH3 (P18), and the resulting enol of methyl acetate could isomerize to the more stable



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- 24 CH3COOCH3. The IR spectrum in Fig. 16 is an attempt to detect P18 by observing the simultaneous appearance of methyl acetate and ethylene. As CH3CH2CH2COOCH3 is passed through a SiC micro-reactor heated to 1000 K, Fig. 16 shows the first appearance of ν7(CH2=CH2) but there are no signals from the intense carbonyl stretch of methyl acetate, ν5(CH3COOCH3). It appears that the Cope reaction, P18, is not active at 1000 K. The sources of the ethylene are likely channels P13, P14, and/or P15b. The ethyl ketene in P9 is unstable and Scheme 3 predicts the fate of CH3CH2CH=C=O. Loss of CH3 to produce the resonantly stabilized radical, CH2CH=C=O, leads to the products of P19. Propadienone, CH2=C=C=O, results from β-scission of the

CH2-CH=C=O radical. Propadienone is an unstable

molecule98 and will fragment to carbon monoxide and vinylidene: CH2=C=C=O → CH2=C: + CO. The vinylidene will then rapidly isomerize to HC≡CH on a ps time scale. Rather than loss of the methyl radical from CH3CH2CH=C=O, Scheme 3 also suggests ethyl ketene could decompose to the carbene, CH3CH2CH: and CO, P19. The singlet carbene, CH3CH2CH: X˜ 1A’, will insert on itself to yield propene, CH3CH=CH2 (m/z 42). As indicated by reaction H15 in Table 1, propene decomposes to an H atom and CH2CHCH2 radical (m/z 41). Allyl radical will lose H atoms to yield allene (reaction H16) that finally leads to the propargyl radical (reaction H17 Table 1) and H atom (Fig. 8).


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- 25 Scheme 3 shows that CH3CH2CH2COOCH3 can fragment to radicals along a variety of paths. P13 is the concerted fragmentation to the 1-propyl radical, carbon dioxide, and methyl radical. The CH3CH2CH2 radical will not be stable in the hot SiC micro-reactor and will decompose to a mixture of (CH3, CH2=CH2) and (H, CH3CH=CH2) radicals. As in the case of methyl acetate, CH3CH2CH2COOCH3

could

decompose

in

a

stepwise

manner

to

CH3CH2CH2CO2 and CH3 (Scheme 3, P14) or to the (CH3CH2CH2, COOCH3) radical pair (Scheme 3, P15), to achieve the same product distribution as the concerted fragmentation P13. Motivated by reaction P31 in Table 1, Scheme 3 also considers C-C bond breakage along the carbon chain, CH3CH2-CH2COOCH3 → CH3CH2 + CH2COOCH3. Neither of these nascent radicals will be stable in the hot SiC reactor and P16 shows the β-scission products of (CH3CH2, CH2COOCH3) as (H, CH2CH2, CH2=O, CH2=C=O). Cleavage of the CH3-CH2CH2COOCH3 bond could produce CH2=CHCOOCH3 (m/z 86) in P17 or to the mixture (H, CH2CH2, CO, CH2=O, CO2, CH3) in P20. The absence of any PIMS signals for methyl acrylate (m/z 86) argues against P17. Conclusions By means of PIMS and IR spectroscopy, we have identified the products resulting from pyrolysis of CH3COOCH3 and CH3CH2CH2COOCH3 and draw some mechanistic conclusions. Table 3 summarizes the experimental findings for all of the predicted products from Schemes 2 and 3.



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- 26 Decomposition of Methyl Acetate We summarize the case of methyl acetate first; see Table 5. The observation of CH3OH and CH2=C=O as the initial products from the thermal cracking of CH3COOCH3 at 1000 K implicates P1 in Scheme 2 as the initial decomposition pathway (eq. 4a). The PIMS and IR spectra provide no evidence for P2. The high activation barriers for P3 and P4 presented in Fig. 1 exclude these as active channels under the conditions of the micro-reactor. While the 118.2 nm laser cannot ionize CO, CO2, or CH3CH3, dimethyl ether would be detected if present (Table 4). No m/z 46 signals were observed in the PIMS. Without any unique products, we exclude channel P8 (like P2 - P4) under the conditions of the micro-reactor due to the high reaction threshold, 101 kcal mol-1 (Table 1). Heating methyl acetate to 1300 K leads to the formation of CH3, CH2=O, H, CO, and CO2 in addition to methanol and ketene, eq. (4b). These products could result from a combination of reactions P5, P6, or P7. The detection of CO and CH2=O implies the presence of the methoxycarbonyl radical, CH3O-CO, P7a. However the thermochemistry for the step-wise appearance of either the CH3CO2 or CH3O-CO radicals, reaction P6 or reaction P8 in Table 1, seemingly requires the reactor to be heated to roughly 1600 K or higher. Pyrolysis of CH3COOCH3 produces CH3 radicals at 1300 K, well below the higher temperatures implied by Table 1. We suspect that there could be a set of concerted processes in play here in which the CO2 and a pair of methyl radicals form simultaneously. The thermochemistry of the fully concerted process, P5 in


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- 27 Table 1, is only 75 kcal mol-1, and it is not unreasonable to expect this to be barrierless. This novel reaction pathway is currently being studied theoretically. Reactions P5, P6, and P7b in Scheme 2 cannot be distinguished by their products. In the parlance of organic chemists, all of the radical channels in Scheme 2, P5 – P7, differ only in the dynamics of the “bond breaking and making.” The active channels experimentally identified here, P1, P5, P6, and P7, are all compatible with the potential energy surface resulting from the calculations shown in Fig. 1. The earlier findings for methyl acetate pyrolysis are consistent with the flash pyrolysis summarized in equations (4a) and (4b). A Knudsen cell13 was used to decompose methyl acetate, labeled with

18

O as CH3C18O-16OCH3, at

temperatures up to 1400 K. Product detection by electron impact (EI) mass spectrometry

revealed

a

low

energy,

degenerate

rearrangement:

CH3C18O-16OCH3 (+ M) → CH3C16O-18OCH3 (I3 in Scheme 2). The EI-mass spectrum13 detected no CH3, CO, or CO2 and only trace amounts of CH2=C=O and CH3OH were found. Decomposition of methyl esters to ketene also agrees with much earlier, pioneering studies of ester pyrolysis. Heating CH3COOC6H5 in a sealed tube at 625 ºC produced113 an 84% yield of CH2=C=O and phenol (C6H5OH). These early pyrolysis results have been subsequently confirmed by pyrolysis-gas chromatography studies.17 There are several shock tube studies of methyl acetate decomposition. Dilute samples (0.5 %) of methyl acetate in Ar were heated9 to 1425 K – 1845 K at pressures of 1.1 atm – 5.7 atm. The only products monitored were CO2 (4.25



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- 28 µm IR emission) and CH3 (UV absorption at 216 nm). An analysis of pyrolysis in the high-pressure limit reported the only pyrolysis products to be CH3 + CO2. Other shock tube studies of 2% CH3COOCH3 in Ar have appeared10 that use a diode laser to monitor CO2 at 2.7 µm. Shock tube measurements11 of methyl acetate pyrolysis were also carried out in 0.5 atm Kr buffer gas at temperatures 1194 K – 1371 K and the decomposition was monitored by H atom detection via atomic resonance absorption spectroscopy (ARAS). The ARAS measurements revealed that decomposition to H atoms began around 1250 K. Finally the pyrolysis of 2% and 4% methyl acetate in Kr was investigated in a diaphragmless shock tube using laser schlieren densitometry.12 Experiments were performed at 63 and 122 Torr over the temperature range of 1492 K — 2266 K. The density gradient profiles indicated that the initial dissociation proceeded predominantly by the CH3COO-CH3 bond breaking, leading to two CH3 radicals and CO2 (P6). This reaction is believed to account for 83-88% of the methyl acetate loss over this temperature range. Both the pyrolysis and the oxidation of CH3COOCH3 were recently studied18 in the Princeton atmospheric flow reactor and in a low-pressure flat flame using molecular-beam mass spectrometry. The atmospheric flow reactor of (0.5 % methyl acetate in a mixture of Ar and He) was coupled to an EI mass spectrometer for product detection and the temperature was scanned from 500 K — 1150 K. The dominant pathways for decomposition of CH3COOCH3 were reported to be to (CH3 + CO2 + CH3) and (CH3OH + CH2=C=O), entirely congruent with our findings. Decomposition of CH3COOCH3 commenced at 1050


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- 29 K (Fig. 1 in ref.18) and the products (CH3CH3, CH3OH) appeared simultaneously at roughly 1100 K. Decomposition of Methyl Butanoate Flash pyrolysis of methyl butanoate in the SiC micro-reactor is more complicated than methyl acetate. Heating the reactor to 800 K induces decomposition of CH3CH2CH2COOCH3 to ethyl ketene and methanol, eq. (5a), and confirms P9 in Scheme 3. There are no m/z 72 signals characteristic of CH3CH2CH2CHO in the PIMS, so there is no support for P10 in Scheme 3. The reactions P11 and P12 are analogous to P3 and P4 for CH3COOCH3 and are expected to have prohibitively high activation barriers. At 1300 K, a complicated set of radicals and metastables appear from the pyrolysis of methyl butanoate. Eq. (5b) lists the products in addition to methanol and ethyl ketene. All of the products predicted by P13 — P15 are present. As in the case of methyl acetate, there is no easy way to distinguish P13 and P14 (similar to P5 – P7). The products characteristic of the methoxycarbonyl radical, CH3O-CO in P15, are masked by other sources of CH2=O and CO. The IR spectrum in Fig. 16 excludes in presence of P18 in Scheme 3. The detection of CH2=C=C=O and HC≡CH uniquely identifies products from the thermal cracking of ethyl ketene, P19 in Scheme 3. Ethyl ketene could also decompose to CO and the ethyl carbene, CH3CH2CH: but the resulting products (CH3CH=CH2, CH2CHCH2, CH2=C=CH2, and HCCCH2) are not unique to P19. Future studies of CH3CH2CH=C=O pyrolysis will be required to assess the relative importance of



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- 30 CH3CH2CH: and CH2=C=C=O. The detection of CH2=C=O as a 1300 K pyrolysis product of CH3CH2CH2COOCH3 confirms presence of reaction P16 in Scheme 3. There are no PIMS signals for m/z 86 of methyl acrylate (CH2=CH-COOCH3); consequently P17 is inactive. The findings in equations (5a) and (5b) can be compared to previous experimental results. The methyl butanoate ester has been decomposed by rapid compression machines.36,40 Rapid compression devices have been used to examine the intermediates formed during ignition of CH3CH2CH2COOCH3 and air mixtures. Following ignition, gas samples of the reaction products were withdrawn and analyzed by gas chromatography. Quantitative measurements of mole

fraction

time

histories

of

CH4,

CH3CH3,

CH3CH2CH3, CH2=CH2,

CH2=CHCH3, and CH3CH2CH=CH2 were obtained.40 Early shock tube studies5 of the autoignition of methyl butanoate were reported at 1 and 4 atm over the temperature range 1250-1760 K. A more extensive study38 is the shock tube measurement of the pyrolysis of CH3CH2CH2COOCH3 at 1.5 atm and temperatures of 1200 K – 1800 K. Mixtures of (0.1% – 1.0%) methyl butanoate in Ar were decomposed and concentrations of CO (4.56 µm), CO2 (2.752 µm), CH2=CH2 (10.53 µm) and CH3 radical (216 nm) were monitored simultaneously. Other shock tube experiments39 used gas chromatography for quantitative analysis of extracted gas samples. In parallel, these samples were analyzed qualitatively by FTIR. The major products of CH3CH2CH2COOCH3 pyrolysis were identified as CH4, HC≡CH, and CH2=CH2; the minor products were CH3CH3, CH2=CHCH3, CH2=CHCH=CH2, and


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- 31 CH2=CHCHO. Decomposition of Methyl Esters of Fatty Acids A set of predicted fragmentation energies for a generalized alkyl methyl ester (RCH2COOCH3) are collected in Table 3. These estimates agree with a set of recent electronic structure calculations based on the very high-level multireference

averaged-coupled

pair

functional

(CD-LMRACPF2)

method.63,73,114 The 298 K bond enthalpies reported for CH3CH2CH2COOCH3 and CH3(CH2)16COOCH3 (Figs. 5 and 9 in ref.63) match the predictions of Table 3. When CD-LMRACPF2 was applied to a set of C10 and C18 methyl esters, the bond energies for the set of five methyl esters (methyl decanoate, cis-methyl-4decenoate, trans-methyl-4-decenoate, cis-methyl-4,7-decanoate, and transmethyl-4,7-decanoate) in Fig. 11 of ref.114 were consistent with the predictions of Table 3. From the current pyrolysis experiments and the earlier referenced measurements on CH3COOCH3 and CH3CH2CH2COOCH3, we predict several important decomposition channels for the pyrolysis of a methyl ester of a generalized fatty acid, RCH2-COOCH3. The most facile fragmentation will be a 4center elimination of methanol to form a ketene. R-CH2-COOCH3 (+ M) → CH3OH + RCH=C=O

(6a)

Because ketenes are easily hydrated,115,116 some of the RCH=C=O will be converted to carboxylic acids, RCH2COOH. (Indeed a recent pyrolysis/oxidation study18 of CH3COOCH3 reported the production of CH3COOH.) Thermal



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Page 32 of 88

- 32 decomposition of the ketene will ensue to produce CH2=C=C=O, which decomposes

to

CO

and

HC≡CH.

At

higher

temperatures,

concerted

fragmentation to radicals will ensue. R-CH2-COOCH3 (+ M) → [R-CH2•••COO•••CH3] → RCH2• + CO2 + CH3•

(6b)

R-CH2-COOCH3 (+ M) → [R-CH2•••COOCH3] → RCH2• + [•CO-OCH3]

(6c)

The fragmentation75 of the syn CO-OCH3 radical leads to the production of both (CO2, CH3) and (CO, H, CH2=O). Cracking of the β C-C bond of the methyl ester will generate a mixture of radicals as well as ketene and formaldehyde. R-CH2COOCH3 (+ M) → R + [CH2COOCH3] → R• + CH2C=O + CH2=O + H• (6d) Low energy pathways to H and CH3 radicals of (6b, 6c, and 6d) will be important in the decomposition of RCH2COOCH3 esters. Reactive radicals such as H atoms or CH3 initiate abstraction reactions from the esters leading to extensive fragmentation. H-atom abstraction α to the ester group (-COOCH3) generates an α-keto radical and triggers several chain reactions: RCH2CH2COOCH3 + H• → H2 + [RCH2CH(•)-COOCH3]* → RCH2CH=C=O + CH3O• (7a)

[RCH2CH(•)-COOCH3]* → R• + CH2=CH-COOCH3 (7b)

The product radicals in (7a) and (7b) will rapidly fragment to generate a complex mixture of H atoms and alkyl radicals thereby igniting a set of chain reactions that will destroy the esters.


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- 33 The C-C bond energies of an alkyl chain (such as in methyl stearate, Scheme 1) are about 87 kcal mol-1; see P35 in Table 1. For an unsaturated fatty acid ester (for example methyl oleate) C-C bond cleavage will drop to roughly 75 kcal mol-1. If the threshold for fragmentation of methyl esters to radicals and CO2 in eq. (6) is 87 kcal mol-1 or less, then the resultant pool of (H, CH3) radicals attacks the parent ester and leads to thermal cracking processes such as eq. (7). But if the radical threshold eq. (6) is much greater than 87 kcal mol-1, ester decomposition will start by C-C cleavage somewhere along the hydrocarbon chain of the fatty acid. The resultant hydrocarbon radicals will then suffer an extensive set β-scission fragmentations leading the destruction of the ester. Acknowledgments JWD and GBE acknowledge support from the National Science Foundation (CBET-1403979). JPP is supported by a Joseph Addison Sewall Scholarship and a CU Graduate School Dissertation Completion Fellowship. The work by JHT, TLN, JHB, and JFS was supported by the U.S. Department of Energy, Office of Basic Energy Sciences under Award DE-FG02-07ER15884. The work at Argonne National Laboratory (BR and DHB) was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences, under Contract No. DE-AC02-06CH11357. The authors acknowledge helpful discussions with Nicole Labbe (Univ. of Colorado) and Ken Wiberg (Yale Univ.).



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- 34 -

Scheme 1 Methyl Esters of Fatty Acids are Biofuels


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- 35 -

Scheme 2 Pyrolysis of Methyl Acetate



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- 36 -


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- 37 -

Scheme 3 Pyrolysis of Methyl Butanoate



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- 38 -


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- 39 -

Scheme 4 Ester Structures



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- 40 -

Fig. 1 Potential energy surface for the thermal dissociation of methyl acetate constructed using CCSD(T)/ANO1 level of theory; all CCSD(T) values calculated at 298 K are accurate within ± 3 kcal mol-1. On the left and right of Figure 1 are products obtained via loose and tight transition structures, respectively. Small inset lines indicate the presence of multiple rotamers.


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- 41 -

Fig. 2 Photoionization mass spectrum 10.487 eV (118.2 nm) of 0.07% CH3COOCH3 diluted in He heated up to 1600 K in a SiC micro-reactor.



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- 42 -

Fig. 3 Matrix infrared spectrum of 0.13 % CH3COOCH3 diluted in Ar heated up to 1500 K in a SiC micro-reactor. The green trace is a background scan of Argon carrier gas heated to 1400 K. An authentic sample of ketene88 is shown in blue for reference, ν1 = 3063 cm-1 and ν4 = 1380 cm-1. The intense ν2(CH2=C=O) band at 2142 cm-1 saturated the detector and is not displayed.


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- 43 -

Fig. 4 Matrix infrared spectrum of 0.13 % CH3COOCH3 diluted in Ar heated up to 1500 K in a SiC micro-reactor. The green trace is a background scan of Ar carrier gas heated to 1400 K. An authentic sample of methanol89 is shown in blue for reference, ν3 = 2847 cm-1 and ν8 = 1034 cm-1.



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- 44 -

Fig. 5 Matrix infrared spectrum of 0.13 % CH3COOCH3 diluted in Ar heated up to 1500 K in a SiC micro-reactor. The green trace is a background scan of Argon carrier gas heated to 1400 K. An authentic sample of formaldehyde90 is shown in blue for reference; ν1 = 2798 cm-1, ν2 = 1742 cm-1, and ν3 = 1498 cm-1.


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- 45 -

Fig. 6 Matrix infrared spectrum of 0.13 % CH3COOCH3 diluted in Ar heated up to 1500 K in a SiC micro-reactor. The green trace is a background scan of Argon carrier gas heated to 1400 K. Ketene88 ν2 observed at 2142 cm-1 . The CO2 monomer band ν3 is observed at 2345 cm-1 along with multimer bands93 at 2340 cm-1. The CO monomer is observed91,92 at 2138 cm-1 and the multimer at 2148 cm-1.



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- 46 -

Fig. 7 Photoionization mass spectrum 10.487 eV (118.2 nm) of 0.06% CH3CH2CH2COOCH3 diluted in He heated up to 1600 K in a SiC micro-reactor.


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- 47 -

Fig. 8 Photoionization mass spectrum at 10.487 eV (118.2 nm) of 0.07% CH2=C=CH2 diluted in He heated up to 1600 K in a SiC micro-reactor. Decomposition of allene to H atom and HCCCH2 (m/z 39) starts about 1500 K.



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- 48 -

Fig. 9 Matrix infrared spectrum of 0.13 % CH3CH2CH2COOCH3 diluted in argon carrier gas heated up to 1600 K in a SiC micro-reactor. The green trace is a background scan of Argon carrier gas heated to 1400 K. An authentic sample of methanol is shown in blue for reference,89 see ν3 = 2847 cm-1 and ν8 = 1034 cm-1.


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- 49 -

Fig. 10 Matrix infrared spectrum of 0.13 % CH3CH2CH2COOCH3 diluted in Ar heated up to 1600 K in a SiC micro-reactor. The green trace is a background scan of Argon carrier gas heated to 1400 K. The carbonyl region for CH2=C=C=O and CH3CH2CH=C=O is severely congested. Propadienone98 is observed as a shoulder at 2125 cm-1. There are two papers reporting ν(C=O) for ethyl ketene at 2135 cm-1 (see Fig. 6 of ref.117) and at 2126 cm-1 (see Fig. 1 of ref.97). The maximum of the band system above is 2122 cm-1.



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- 50 -

Fig. 11 Matrix infrared spectrum of 0.13 % CH3CH2CH2COOCH3 diluted in Ar heated up to 1600 K in a SiC micro-reactor. The green trace is a background scan of Argon carrier gas heated to 1400 K. The acetylene ν3 doublet99 is observed at 3303 cm-1 and 3289 cm-1, as well as ν5 at 737 cm-1.


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- 51 -

Fig. 12 Matrix infrared spectrum of 0.13 % CH3CH2CH2COOCH3 diluted in Ar heated up to 1600 K in a SiC micro-reactor. The green trace is a background scan of Argon carrier gas heated to 1400 K. An authentic sample of ketene88 is shown in blue for reference, ν1 = 3063 cm-1 and ν4 = 1380 cm-1. The intense ν2(CH2=C=O) band at 2142 cm-1 saturated the detector and is not displayed.



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- 52 -

Fig. 13 Matrix infrared spectrum of 0.13 % CH3CH2CH2COOCH3 diluted in Ar heated up to 1600 K in a SiC micro-reactor. The green trace is a background scan of Argon carrier gas heated to 1400 K. Observed 101 bands are ν7 = 1453 cm-1, ν18 = 998 cm-1, and ν19 = 908 cm-1.


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- 53 -

Fig. 14 Matrix infrared spectrum of 0.13 % CH3CH2CH2COOCH3 diluted in Ar heated up to 1600 K in a SiC micro-reactor. The green trace is a background scan of Argon carrier gas heated to 1400 K. Allyl radical is indicated by observation of ν1 and ν11 at 3112 cm-1 and 801 cm-1, respectively.102 The most intense band for allene, ν6, is shown89 at 1957 cm-1.



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- 54 -

Fig. 15 Matrix infrared spectrum of 0.13 % CH3CH2CH2COOCH3 diluted in Ar heated up to 1600 K in a SiC micro-reactor. The green trace is a background scan of Argon carrier gas heated to 1400 K. An authentic sample of formaldehyde90 is shown in blue for reference; ν1 = 2798 cm-1 and ‑ν2 = 1742 cm-1. Ethylene89 ν7 is observed at 947 cm-1.


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- 55 -

Fig. 16 Matrix infrared spectrum of 0.13 % CH3CH2CH2COOCH3 diluted in Ar heated up to 1600 K in a SiC micro-reactor. The green trace is a background scan of Argon carrier gas heated to 1400 K. There appears to be no evidence of the intense carbonyl stretch (ν5 = 1761 cm-1) of methyl acetate arriving in tandem with ethylene (reaction P18 in Table 2, followed by isomerization).89



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- 56 -

Table 1: ATcT Reaction Enthalpiesa ∆rxnH298/kcal mol-1

Label

Reaction

∆rxnH298

P1

CH3COOCH3 → CH2=C=O + CH3OH

39.1 ± 0.2

P2

CH3COOCH3 → CH2=O + CH3CHO

33.0 ± 0.2

P3

CH3COOCH3 → CO2 + CH3CH3

-15.4 ± 0.2

P4

CH3COOCH3 → CO + CH3OCH3

28.4 ± 0.2

I2

CH3COOCH3 → CH2=C(OH)OCH3

26.3 ± 0.5

P6

CH3COO-CH3 → CH3CO2 + CH3

88.0 ± 0.3

P8

CH3CO-OCH3 → CH3CO + OCH3

101.5 ± 0.2

P7

CH3-COOCH3 → CH3 + COOCH3

95.4 ± 0.4

H1

CH3COOCH2-H → CH3COOCH2 + H

99.6 ± 0.4

H2

H-CH2COOCH3 → CH2COOCH3 + H

98.6 ± 0.4

P5

CH3COOCH3 → CO2 + 2 CH3

74.5 ± 0.4

H7

syn-HCOO-H → HCO2 (2A1) + H

112.1 ± 0.2

H8

syn-H-COOH → H + COOH (cis)

100.1 ± 0.2

H9

syn-H-COOH → H + COOH (trans)

97.0 ± 0.2

P22

syn-HCO-OH → HCO + OH

109.4 ± 0.1

I7

syn-HCOOCH3 → anti-HCOOCH3

4.9 ± 0.3

P23

syn-HCOOCH3 → CH2=O + CH2=O

33.8 ± 0.3

P24

syn-HCOO-CH3 → HCO2 + CH3

90.5 ± 0.3

P25

syn-HCO-OCH3 → HCO + CH3O

100.1 ± 0.3


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- 57 H10

H-COOCH3 → COOCH3 + H

99.8 ± 0.5

H11

syn-HCOOCH2-H → syn-HCOOCH2 + H

99.8 ± 0.5

I8

anti-HOCO → syn-HOCO

1.6 ± 0.2

P26

anti-HO-CO → OH + CO

26.6 ± 0.2

H12

anti-H-OCO → H + CO2

2.1 ± 0.2

H13

H-CO2 (2A1) → H + CO2

-11.5 ± 0.2

P27

CH3-CO2 (2A1) → CH3 + CO2

-13.3 ± 0.3

P28

COOCH3 → CO2 + CH3

-20.7 ± 0.4

P35

COOCH3 → CO + OCH3

17.0 ± 0.4

P29

CH3-CHO → CH3 + HCO

84.6 ± 0.1

P30

CH3-CH2COCH3 → CH3 + CH2COCH3

84.4 ± 0.4

H14

CH3O → H + CH2=O

20.9 ± 0.1

P31

CH3CO → CH3 + CO

11.0 ± 0.1

H15

CH2=CHCH3 → H + CH2CHCH2

87.6 ± 0.2

H16

CH2CHCH2 → H + CH2=C=CH2

57.3 ± 0.2

H17

CH2=C=CH2 → H + HCCCH2

90.7 ± 0.2

P32

CH2=CHCH3 → CH3 + HC=CH2

101.2 ± 0.1

P33

CH2CH2CH3 → CH3 + CH2=CH2

23.4 ± 0.2

P34

CH3CH2-CH2CH3 → CH3CH2 + CH2CH3

87.4 ± 0.1

a

Current results from version 1.122e of the Thermochemical Network.



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- 58 Table 2: Theoretical Reaction Enthalpies (∆rxnH298) in kcal mol-1 Label

Reaction

∆rxnH298a,b ∆rxnH298b,c

P9

CH3CH2CH2COOCH3 → CH3CH2CH=C=O + CH3OH

40

–

P10

CH3CH2CH2COOCH3 → CH3CH2CH2CHO + CH2=O

33

–

I5

CH3CH2CH2COOCH3 → CH3CH2CH=C(OH)OCH3

29

–

P18

CH3CH2CH2COOCH3 → CH2=CH2 + CH2=C(OH)OCH3

53

–

P14

CH3CH2CH2COO-CH3 → CH3CH2CH2CO2 + CH3

87

86

P20

CH3CH2CH2CO-OCH3 → CH3CH2CH2CO + OCH3

101

100

P15

CH3CH2CH2-COOCH3 → CH3CH2CH2 + COOCH3

93

94

H3

CH3CH2CH2COOCH2-H → H + CH3CH2CH2COOCH2

99

100

H4

CH3CH2CH(-H)COOCH3 → H + CH3CH2CHCOOCH3

94

92

H5

CH3CH(-H)CH2COOCH3 → H + CH3CHCH2COOCH3

99

100

H6

H-CH2CH2CH2COOCH3 → H + CH2CH2CH2COOCH3

101

103

P16

CH3CH2-CH2COOCH3 → CH3CH2 + CH2COOCH3

84

82

P21

CH3-CH2CH2COOCH3 → CH3 + CH2CH2COOCH3

89

89

P13

CH3CH2CH2COOCH3 → CH3CH2CH2 + CO2 + CH3

74

a

Values from reference 33 based on CBS-QB3 calculations. b Reported without uncertainties. c Values from reference 63 based on MRACPF2 calculations with a cc-pVTZ basis set.


Page 59 of 88

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Table 3: Derived Generic Alkyl Methyl Ester (RCH2COOCH3) Fragmentation Energies, ∆rxnH298/kcal mol-1 Methyl Formatea Methyl Acetatea Methyl Butanoateb Generic Esterc

Generic Fragmentation Reaction RCH2COOCH3 → RCH=C(OH)OCH3

–

26

29

28

RCH2COOCH3 → RCH2CHO + CH2=O

34

33

33

33

RCH2COOCH3 → RCH=C=O + CH3OH

–

39

40

40

RCH2CH2COOCH3 → RCH=CH2 + CH2=C(OH)OCH3

–

–

53

53

R-CH2COOCH3 → R + CH2COOCH3

–

98

84

‑‑

RCH2COO-CH3 → RCH2CO2 + CH3

91

88

87

88

RCH2-COOCH3 → RCH2 + COOCH3

–

95

93

95

RCH2COOCH3 → RCH2 + CO2 + CH3

–

75

74

75

RCH(-H)COOCH3 → RCHCOOCH3 + H

–

99

94

95

RCH2COOCH2-H → RCH2COOCH2 + H

99

100

99

100

RCH2CO-OCH3 → RCH2CO + OCH3

109

102

101

101

a

Current results from version 1.122e of the Thermochemical Network.

b

Value from reference 33 based on CBS-QB3 calculations. c Approximate Reaction Enthalpy at 298K



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Page 60 of 88

Table 4: Important Ionization Energies species

m/z 1 H

name H atom

15 CH3

methyl radical

17 OH

IE (eV)

ref 118

13.5984345

119,120

9.8406

±

0.0004

hydroxyl radical

13.01698

±

0.00025

26 HCCH

acetylene

11.400814 ±

0.000008

28 CH2=CH2

ethylene

10.51267

±

0.00006

124

28 CO

carbon monoxide

14.01362

±

0.00004

125

30 CH2=O

formaldehyde

10.8850

±

0.0002

126

31 CH3O

methoxy radical

10.716

±

0.010

127

32 CH3OH

methanol

10.846

±

0.004

128,129

39 HCCCH2

propargyl radical

8.7006

±

0.0005

130

40 CH2=C=CH2

allene

9.688

±

0.002

131

40 CH3C≡CH

propyne

10.36743

±

0.00012

132,133

41 CH2CHCH2

allyl radical

8.13145

±

0.00025

134

42 CH3CH=CH2

propene

9.7435

±

0.0005

135

42 CH2=C=O

ketene

9.6191

±

0.0004

136

43 CH3CO

acetyl

6.95

±

0.02

137

44 CH3CHO

acetaldehyde

10.2295

±

0.0007

138

46 CH3OCH3

dimethyl ether

10.025

±

0.025

54 CH2=C=C=O

propadienone

9.12

±

0.05

58 CH3CH2CH2CH3

n-butane

10.6

±

0.1

60 HCOOCH3

methyl formate

10.835

±

0.002

143

70 CH3CH2CH=C=O

ethylketene

8.80

±

0.03

144

butyraldehyde

9.73

±

0.03

145

72 CH3CH2CH2CHO 74 CH3COOCH3

methyl acetate

10.25

±

0.05

146

74 CH3CH2CH2OCH3

methyl propyl ether

86 CH2=CHCOOCH3

methyl acrylate

102

CH3CH2CH2COOCH

methyl butanoate

121 122,123

139,140 141 132,142

147

9.73 10.7

±

0.8

148

10.07

±

0.03

149

3


Page 61 of 88

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Table 5: Summary of the Products of Ester Pyrolysis: (IR) Ar Matrix Infrared Detection (PIMS) 118.2 nm (10.487 eV) Photoionization Detection (–) implies no IR or PIMS signals

Reaction

Product Detected

Channel Status

CH3COOCH3 Decompositions P1

CH3OH (IR) + CH2=C=O (PIMS, IR)

active

P2

CH2=O (IR) + CH3CHO (–)

inactive

P3

CO2 (IR) + CH3CH3 (–)

inactive

P4

CO (IR) + CH3OCH3 (—)

inactive

P5

CH3 (PIMS) + CO2 (IR)

P6

[CH3CO2] → CH3 (PIMS) + CO2 (IR)

P7a

CH3 + [COOCH3] → CH3 (PIMS) + CO (IR) + CH2=O (IR) + H (–)

P7b

CH3 + [COOCH3] → CH3 (PIMS) + CO2 (IR)

P8

[CH3CO + OCH3] → CH3 (PIMS) + CO (IR) + CH2=O (IR) + H (–)

active

CH3CH2CH2COOCH3 Decompositions


active active active inactive


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Page 62 of 88

- 62 -

P9

CH3CH2CH=C=O (IR, PIMS) + CH3OH (IR)

P10

CH3CH2CH2CHO (—) + CH2=O (IR)

inactive

P11

CO2 (IR) + CH3CH2CH2CH3 (—)

inactive

P12

CO (IR) + CH3CH2CH2OCH3 (—)

inactive

P13

CH3 (PIMS) + CO2 (IR) + [CH2CH2CH3] → CH3CHCH2 (PIMS, IR) + CH2CH2 (PIMS, IR)

active

P14

CH3 (PIMS) + CO2 (IR) + [CH2CH2CH3] → CH3CHCH2 (PIMS, IR) + CH2CH2 (PIMS, IR)

active

P15a

[CH3CH2CH2] + [COOCH3] → CO2 (IR) + CH3 (PIMS)

active

P15b

[CH3CH2CH2] + [COOCH3] → CO (IR) + CH2=O (IR) + H (—)

active

P16

[CH3CH2] + [CH2COOCH3] → CH2=CH2 (IR) + CH2=C=O (IR, PIMS) + CH2=O (IR) + H (—)

active

P17

CH3 (PIMS) + [CH2CH2COOCH3] → CH2=CH-COOCH3 (—) + H (—)

inactive

P18

CH2=CH2 (IR) + [CH2=C(OH)OCH3] → CH3COOCH3 (—)

inactive

P19

CH3 (PIMS) + H (—) + CH2=C=C=O (IR ?) → CO (IR) + HC≡CH (IR)


active

active

Page 63 of 88

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Of the 5 modes of HC≡CH, only the asymmetric CH stretch, σu ν3, and the asymmetric HCCH bend, πu ν5, are IR active. In the gas-phase ν3 HCCH is observed at 3294.9 cm-1 and 3281.9 cm-1 and is split by a Darling-Dennison resonance (DDR) of 13 cm-1. In an Ar matrix, ν3 shifts to 3302 cm-1 and 3288 cm-1; the DDR is 14 cm-1 in the cryogenic matrix. The gas phase value for ν5 is 730.3 cm-1 and shifts to 736.8 cm-1 in an Ar matrix.

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CH2=C=O, CH3OH CH3

O CH3

O

CH3

(+ M)

micro-reactor (1 mm id x 2 cm length)

CH2=O H CO CO2


Thermal Decomposition of Potential Ester Biofuels ... - ACS Publications

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