Products From Pyrolysis of Gas-Phase Propionaldehyde - The Journal

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Products From Pyrolysis of Gas-Phase Propionaldehyde Brian J Warner, Emily M Wright, Hannah E Foreman, Courtney D Wellman, and Laura R. McCunn J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp5077802 • Publication Date (Web): 04 Dec 2014 Downloaded from http://pubs.acs.org on December 9, 2014

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The Journal of Physical Chemistry

Products From Pyrolysis of Gas-Phase Propionaldehyde Brian J. Warner, Emily M. Wright, Hannah E. Foreman, Courtney D. Wellman, Laura R. McCunn* Department of Chemistry, One John Marshall Drive, Huntington, WV 25755

Keywords: aldehydes, thermal decomposition, matrix-isolation FTIR, photoionization mass spectrometry

1 ACS Paragon Plus Environment


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ABSTRACT A hyperthermal nozzle was utilized to study the thermal decomposition of propionaldehyde, CH3CH2CHO, over a temperature range of 1073-1600 K. Products were identified with two detection methods: matrix-isolation Fourier transform infrared spectroscopy and photoionization mass spectrometry. Evidence was observed for four reactions during the breakdown of propionaldehyde: α-C-C bond scission yielding CH3CH2, CO, and H, an elimination reaction forming methylketene and H2, an isomerization pathway leading to propyne via the elimination of H2O, and a β-C-C bond scission channel forming methyl radical and ·CH2CHO.

The products identified during this experiment were CO, HCO, CH3CH2,

CH3CH=C=O, H2O, CH3C≡CH, CH3, H2C=C=O, CH2CH2, CH3CH=CH2, HC≡CH, CH2CCH, H2CO, C4H2, C4H4, and CH3CHO. The first eight products result from primary or bimolecular reactions involving propionaldehyde while the remaining products occur from reactions including the initial pyrolysis products.

While the pyrolysis of propionaldehyde involves

reactions similar to those observed for acetaldehyde and butyraldehyde in recent studies, there are a few unique products observed which highlight the need for further study of the pyrolysis mechanism.


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INTRODUCTION There has been renewed interest in the chemistry, particularly thermal decomposition, of small-chain aldehydes such as propionaldehyde because aldehydes occur in many atmospheric and industrial processes. Typical anthropogenic sources of aldehydes in the atmosphere include forest fires, industrial emissions, automobile exhaust, and the photo-oxidation of hydrocarbons.1 Atmospheric propionaldehyde, CH3CH2CHO, can be produced by the reactions of propene with oxygen atoms,2-3 hydroxyl radical,4 or nitrate radical.5 Propionaldehyde has also been linked to several common processes involving high temperatures, such as cooking cabbage and clams, and brewing or roasting coffee.6 It is one of many chemicals emitted by the pyrolysis of cigarette paper and other cigarette ingredients.7-8 Finally, propionaldehyde is a byproduct of both biomass processing and combustion.9-11 As biofuel use becomes more prevalent, it is important to understand the role of propionaldehyde decomposition reactions in industrial emissions and fuel contamination. There are a few published studies of thermal decomposition of propionaldehyde in the literature. Huhn et al.12 performed pyrolysis of propionaldehyde in a glass vessel and observed the following stable products at 771 K: CO, C2H6, C2H4, CH4 and C3H6.

Increasing the

temperature to 809 K leads to the additional products H2, C3H8, and C4H10. The same authors extended their experiments to determine the effect of added ethylene and propylene on the pyrolysis process.13-16 The products observed in these experiments were largely the same as those observed in their experiments on pure propionaldehyde, with C2H6 and CO as the major products. Radical intermediates, particularly ethyl, are suspected to play a key role in the mechanism of propionaldehyde decomposition. The presence of ethylene does not have any significant inhibiting effect on the rate of decomposition,16 while propylene can inhibit the rate.15



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Shock tube studies of dilute propionaldehyde in argon at 970-1350 K with reaction dwell times of ~2 ms by Lifshitz et al.17 revealed many of the same products as were observed in the glass vessel studies by Huhn’s group: CO, C2H6, C2H4, CH4, C3H6, C3H8, C4H6, C2H2, and C4H10. The decomposition process was modeled by a 52-step reaction scheme involving 22 different species.

The experimental temperature had a profound effect on the extent of

decomposition. At 975 K, over 98% of the propionaldehyde remained unreacted; at 1270 K, only ~1% remained.

The authors proposed two major dissociation channels: formation of

CH3CH2 + HCO and CH3 + CH2CHO. While identifying the thermal decomposition products of propionaldehyde has practical applications, there is also chemical significance in understanding the aldehyde dissociation mechanism. The effect of the structure or size of the alkyl chain on the reaction pathways is of particular interest. aldehydes.18-23

There are numerous published studies of the pyrolysis of small-chain

This laboratory has recently undertaken a study24 of butyraldehyde,

CH3CH2CH2CHO, by instigating gas-phase pyrolysis with a hyperthermal nozzle and identifying products

via

matrix-isolation

Fourier

transform

infrared

spectroscopy

(FTIR)

and

photoionization mass spectrometry (PIMS) in experiments similar to those presented here. Twenty different products were observed following pyrolysis, thought to originate from six initial reactions in the pyrolysis mechanism: CH3CH2CH2CHO + ∆ → CH3CH2CH2 + CO + H

(α-C-C scission)

(1)

CH3CH2CH2CHO + ∆ → CH3CH2CH=C=O + H2

(elimination)

(2)

CH3CH2CH2CHO + ∆ → CH3CH2C≡CH + H2O

(isomerization/elimination) (3)

CH3CH2CH2CHO + ∆ → CH2=CH2 + CH2=C(H)OH (H-shift/elimination)

(4)

CH3CH2CH2CHO + ∆ → CH3CH2 + CH2CHO

(5)

(β-C-C scission)


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CH3CH2CH2CHO + ∆ → CH3 + CH2CH2CHO

(γ-C-C scission).

(6)

Reactions 1-3 are analogous to the reactions observed for acetaldehyde in similar experiments conducted by Vasiliou et al.20-21, however, reactions 4-6 do not occur in acetaldehyde. Butyraldehyde’s longer alkyl chain makes these reactions possible with breakages of the extra CC bonds. Reaction 4 is thought to occur when the molecule forms a cyclic intermediate followed by an intramolecular hydrogen abstraction from the terminal carbon.23 This process is possible for butyraldehyde, but both propionaldehyde and acetaldehyde are too small for such a mechanism. The purpose of the experiments presented here was to identify the products of propionaldehyde pyrolysis by using a hyperthermal nozzle (Chen nozzle)25-27 with detection by matrix-isolation FTIR and PIMS.

The hyperthermal nozzle provided an oxygen-free

environment for thermal decomposition of gas-phase propionaldehyde and matrix-isolation FTIR or PIMS was used to detect the products. An advantage of the experiments is that they probe reactions occurring relatively early in the pyrolysis process. The typical residence time for a sample molecule in the high-temperature region of the pyrolyzer is on the order of 50-100 µs.20, 25

The expansion of pyrolysis products from the high-temperature tube into the high-vacuum

chamber rapidly cools the products and quenches further reactions. Therefore, it is possible to detect and identify radical intermediates or other reactive species that occur in the pyrolysis mechanism. The results of these experiments should reflect what happens relatively early in the pyrolysis process, hence contributing new information to what is already known from previously published pyrolysis and shock tube studies. Evaluation of the products observed from the different detection techniques should provide insight into the complete mechanism of pyrolysis and provide a basis for future studies in kinetic modeling.



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EXPERIMENTAL METHODS Thermal decomposition of propionaldehyde (purity ≥ 99 %; Sigma Aldrich) was accomplished via a hyperthermal nozzle that has been described elsewhere in the literature.25 In the matrix-isolation FTIR experiments, a matrix mixture (0.2% - 0.33%) of propionaldehyde in argon totaling approximately 700 Torr was prepared using standard manometric techniques. The mixture was expanded from a pulsed valve (General Valve Series 9) operating at 40 Hz and into the resistively heated 1.5 in. x 1 mm silicon carbide (SiC) tube. The temperature (1073-1473 K) of the SiC tube was controlled using a Series 16A temperature controller made by Love Controls. The pyrolysis products were then sprayed out of the tube and onto a cesium iodide (CsI) window mounted in a cryostat (Janis Research) with a base pressure of 1.0 x 10-6 Torr. The CsI window was cooled to 15 K by a closed-cycle helium refrigerator (Sumitomo Heavy Industries Ltd.) and regulated by a Lake Shore 331 Temperature Controller. The pyrolysis products were isolated in an Ar matrix and then cooled to 4 K prior to FTIR analysis. FTIR spectra were collected for 120 scans and a 0.5 cm−1 resolution with a Bruker Vertex 70 spectrometer that was purged with nitrogen gas. Photoionization mass spectrometry (PIMS) was also performed following the pyrolysis of propionaldehyde in separate experiments. A pyrolysis nozzle identical to the one described above was mounted on a photoionization mass spectrometer (resolution: m/∆m ~1300) that has been described previously.25 A mixture of 0.3% propionaldehyde in 2 atm of helium was expanded through the pyrolysis nozzle (1300-1600 K) into a 10−5 Torr vacuum chamber and passed through a skimmer (3 mm diameter). The resultant beam was crossed with a vacuum ultraviolet (VUV) laser operating at 118.2 nm, 30 Hz, ~0.5 µJ/pulse. The VUV light was


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generated by tripling the 3rd harmonic of an Nd:YAG in an argon cell. Products from thermal decomposition of propionaldehyde were ionized and then mass analyzed by a reflectron TOF mass spectrometer. Ions were detected by a channeltron and the signal was collected by a Tektronix digital oscilloscope.

RESULTS AND DISCUSSION Table I summarizes the products observed in the PIMS and FTIR spectra following the pyrolysis of propionaldehyde. The PIMS spectra measured after pyrolysis at 1300 K, 1400 K, and 1600 K are shown in Figure 1.

Table 1. Summary of all products observed following the pyrolysis of propionaldehyde at 12731600 K, listed in order of molecular weight. Checkmarks indicate evidence observed for each product. Question marks indicate assignments that were ambiguous or difficult to discern in the spectra. Product CH3 H2 O HCCH CH2CH2 CO HCO CH3CH2 H2CO CH2CCH CH3CCH CH3CHCH2 H2CCO CH3CHO C4H2 C4H4 CH3CHCO

PIMS

Matrix-isolation FTIR

Comments

? ?

IE prohibits PIMS detection IE prohibits PIMS detection IE prohibits PIMS detection m/z = 29 is observed, FTIR evidence is weak for both HCO and CH3CH2



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Figure 1. PIMS spectra following pyrolysis of propionaldehyde at 1300 K, 1400 K, and 1600 K obtained with a 10.5 eV photoionization laser. Pyrolysis was performed on 0.3% samples of propionaldehyde in 2 atm of He carrier gas.

The thermal decomposition of propionaldehyde led to the production of several species that were identified in the matrix-isolation FTIR spectra and the PIMS spectra. Several of these products are the direct result of four reactions below. For simplicity, these reactions are each shown as a single step, but may actually involve several steps or bimolecular reactions as discussed below. CH3CH2CHO + ∆ → CH3CH2 + HCO

(α-C-C bond scission)

(7)

CH3CH2CHO + ∆ → CH3CH=C=O + H2

(elimination)

(8)

CH3CH2CHO + ∆ → CH3C≡CH + H2O

(isomerization/elimination)

(9)

CH3CH2CHO + ∆ → CH3 + ·CH2CHO

(β-C-C bond scission) .

(10)

Subsequent thermal decomposition of the products listed above must be considered because these products may be formed with excess energy and also could spend significant time in the hot pyrolyzer tube prior to supersonic expansion. Therefore, several products not shown in the


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reactions above are attributed to subsequent reactions of the initial products of propionaldehyde pyrolysis. Furthermore, recent work21, 24 has shown that many of the reactions taking place within the pyrolyzer tube are actually bimolecular in nature. For example, the hydrogen atoms generated from decomposition of HCO in reaction 7 are quite reactive and can attack unreacted propionaldehyde molecules or other thermal decomposition products within the pyrolyzer tube. In fact, previous work on acetaldehyde21 indicates that overall reaction 8 may include bimolecular steps involving abstractions by H atoms. There is evidence that reaction 9 proceeds by formation of vinyl alcohol, the enol tautomer of the aldehyde. This is likely followed by H atom attacking the enol, followed by loss of OH, which abstracts an H from another molecule to form water. The remaining ethylene reacts with H atom, and leads to acetylene. This mechanism is difficult to test experimentally but seems to be consistent with all available data.21 The discussion that follows presents the identification of propionaldehyde pyrolysis products with hypothetical assignments of their reactions of origin, beginning with an evaluation of reactions 7-10, followed by consideration of other secondary and bimolecular reactions. A. Products of α-C-C scission The α-C-C bond scission of propionaldehyde produces the ethyl radical and HCO, which rapidly decomposes to H + CO. Figure 2 shows the appearance of carbon monoxide at 2138 cm−1.28 The CO product cannot be detected by PIMS because its ionization energy29 is 14.0 eV, above that of the photoionization laser (10.5 eV). The ethyl radical dissociates readily to H + CH2CH2,30 so observation of the radical in these experiments was not anticipated. There were no FTIR features observed at the ethyl radical’s characteristic wavenumbers31 of 2842, 2987, 3112 cm−1, however there was a hint of absorption at 540 cm−1, the most intense band associated with this radical, in spectra collected following pyrolysis at 1073 K and 1273 K (not shown). While



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the PIMS spectra do exhibit signal at m/z = 29, that may also reflect the presence of HCO, presumed to be an intermediate in reaction 7. One low-intensity band at 1862 cm−1 was observed following pyrolysis at 1073 K. This corresponds to the strongest band for HCO (literature31 values of 2483, 1863, 1087 cm−1). Experimental limitations and the presumed low concentration of the m/z = 29 species make it difficult to be certain of the presence of both HCO and ethyl, although the FTIR spectra suggest that a very small amount of each might survive the pyrolysis nozzle.

Figure 2. Matrix infrared absorption spectrum of products from the 1273 K pyrolysis of a mixture of 0.4% propionaldehyde in argon. The * symbol indicates bands that belong to unreacted propionaldehyde. The symbol indicates bands belonging to the background.

B. Products of elimination The elimination reaction of propionaldehyde produces methylketene and H2 (reaction 8). Methylketene is evidenced by bands in the FTIR spectrum at 1075 cm−1 (Figure 3), 1447 cm−1, 1471 cm−1 (Figure 4), 2128 cm−1, and 2124 cm−1 (Figure 2), which match bands observed for


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matrix-isolated methylketene in the literature.32-33 The latter two were the strongest observed for methylketene, consistent with the literature. Other frequencies from the literature were observed but are not shown here because they overlap those of other products or unreacted propionaldehyde. The methylketene cation appears at m/z = 56 in the PIMS spectra collected following pyrolysis at 1300 and 1400 K. The co-product of elimination, H2 (IE = 15.4 eV),29 cannot be observed in the mass spectra due to its ionization energy. The disappearance of methylketene in the 1600 K PIMS spectrum indicates that secondary decomposition occurs with increasing temperatures. Experimental limitations of the matrix-isolation instrument prohibited confirmation of this in the FTIR spectra. One study34 on the thermal decomposition of methylketene at 633-813 K was found in the literature, although it involved much longer timescales (>30 s) and is not directly comparable to the chemistry occurring here. Nonetheless, major products observed in that study include: CO, CO2, methane, ethylene, 2-butene, 1,2butadiene, and 2,3-pentadiene, a few of which are also observed in the spectra presented here.

* 0.04

ethylene

Absorbance

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propene

* *

0.02

methylketene

*

propene

*

*

** 1200

1150

1100

1050

1000

950

900

Wavenumber (cm-1)



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C. Products of isomerization/elimination The isomerization/elimination sequence of propionaldehyde produces propyne and water. As stated earlier, this is likely a multistep process that includes bimolecular reactions, for which there is some supporting evidence in acetaldehyde and butyraldehyde.21, 24 Current wisdom holds that the aldehyde tautomerizes to the enol, followed by reaction with an H atom. While the experiments herein cannot effectively test this hypothesized mechanism, the results will be discussed, keeping the mechanism in mind, and examined for consistency. The FTIR spectra display characteristic bands of matrix-isolated water in Figures 5 and 6. The bands at 3711 and 3776 cm−1 belong to rotational transitions associated with the ν3 band of ortho-H2O and 3757


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cm−1 belongs to para-H2O. 28, 35 Similarly, there are two rovibrational bands observed in the ν2 region, 1624 and 1608 cm−1.36 The absorbances associated with these OH stretches and bends exceed what would be expected from normal background contamination in the cryostat over the experimental deposition time. This was confirmed experimentally by passing pure argon through the hot pyrolysis nozzle and comparing the resultant intensities of the vibrational bands of water to those observed following the pyrolysis of propionaldehyde for equivalent deposition times and quantities. Therefore, at least some of the observed H2O is due to pyrolysis reactions of propionaldehyde. H2O+ does not appear in the PIMS spectra because the ionization energy29 of water (12.6 eV) is too high. In similar studies on acetaldehyde, Vasiliou and coworkers21 proposed that acetaldehyde undergoes a keto-enol tautomerism to vinyl alcohol followed by bimolecular reactions with H atoms that lead to formation of water and acetylene. If a similar mechanism governs propionaldehyde, then propen-1-ol, the enol tautomer of propionaldehyde, might also be detected in these experiments. In the FTIR spectra, there are OH stretches present at 3627 and 3640 cm−1. (Figure 6) These are just blue-shifted from the OH stretch (3619 cm−1) of vinyl alcohol.21 Similarly, there are features in the range 1710-1670 cm−1 seen in Figure 5 that are close enough to the C=C stretch of vinyl alcohol at 1662 cm−1 to suggest the presence of 1-propen-1-ol. Unfortunately, without a literature spectrum of matrix-isolated propen-1-ol, it is impossible to be certain of the presence of the enol.



0.18

* *

0.16 0.14

Absorbance

0.12 0.10

H2O 0.08

enol

0.06 0.04 0.02 0.00 1800

1750

1700

1650

1600

Wavenumber (cm-1)

Figure 5. Matrix infrared absorption spectrum of products from the 1473 K pyrolysis of a mixture

of 0.4% propionaldehyde in argon. The * symbol indicates bands that belong to unreacted propionaldehyde. The region labeled “enol” contains unassigned bands suspected to belong to propen-1-ol.

0.10

H2O

0.08

Absorbance

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0.06

0.04 H 2O

enol

H 2O

0.02

0.00 3800

3750

3700

3650

3600

Wavenumber (cm-1)

Figure 6. Matrix infrared absorption spectrum of products from the 1473 K pyrolysis of a mixture

of 0.4% propionaldehyde in argon. The features marked “enol” are unassigned but suspected to belong to propen-1-ol.


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Despite uncertainty in assigning the enol, the presence of propyne is supported by the PIMS spectra (Figure 1) at m/z = 40 and the infrared spectra37 (Figure 2). While the allene cation would overlap with propyne cation (IE = 10.36 eV for propyne and IE = 9.7 eV for allene)29 in the PIMS spectra, there were no characteristic bands of allene37 observed in the FTIR spectra, except at 1386 cm−1, which coincides with a band assigned to methylketene. The expected vibrational feature of propyne at 1446 cm−1 is obscured by ethylene absorption at 1440 cm−1 (Figure 7), but other bands were observed at 2141, 3322, and 630 cm−1 (Figures 2, 8, 9) so there is confidence in the identification of propyne. It would not have been surprising to observe both propyne and allene if the propyne had a sufficient residence time in the pyrolyzer following its formation, as the initial step of propyne pyrolysis is isomerization to allene.38-41 The thermal decomposition of propyne/allene has been extensively studied.38-45 Subsequent reactions of hot propyne may lead to its major pyrolysis products methane and acetylene, presumably via reaction with H atoms. Other possible products include ethylene, ethane, H2, diacetylene and benzene.42-43

0.08

*

0.07

* 0.06

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.05 0.04

ethylene

*

*

0.03

*

0.02

* *

*

0.01 0.00 1500

1450

1400

1350

1300

1250

1200

Wavenumber (cm-1)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 7. Matrix infrared absorption spectrum of products from the 1473 K pyrolysis of a mixture of 0.4% propionaldehyde in argon. The * symbol indicates bands that belong to unreacted propionaldehyde.

D. Products of β-C-C bond scission Reaction 10, showing the production of CH3 + CH2CHO, is considered as another possible reaction of propionaldehyde and was first suggested as part of the mechanism in a shock-tube pyrolysis study.17 This reaction may compete with the α-C-C bond scission channel (reaction 7) because the two C-C bonds of propionaldehyde have similar energies: 83.8 kcal/mol for the α-C-C bond and 83.7 kcal/mol for the β-C-C.11, 17 An analogous reaction producing ethyl and vinoxy radicals was mentioned in this laboratory’s previous study of butyraldehyde.24 There was weak evidence for the detection of vinoxy in that study, and it was suspected that both vinoxy and ethyl would decompose to products that could also be produced by other reactions in the pyrolysis mechanism, so there was some uncertainty regarding the presence of the reaction for butyraldehyde. Reaction 10 of propionaldehyde leads to the methyl radical, which is clearly evidenced by the PIMS spectra in Figure 1. A search of the FTIR spectra did not reveal any vibrational bands of CH3, but a similar situation was encountered by Vasiliou et al. in their attempts to detect methyl radicals from the pyrolysis of acetaldehyde20 and in this laboratory’s experiments on butyraldehyde.24 The co-product of reaction 10, ·CH2CHO (vinoxy radical), cannot be plainly seen at m/z = 43 in the PIMS spectra. This radical can dissociate directly to H + H2C=C=O (ketene) or isomerize to acetyl radical. These processes have nearly equal barriers of approximately 41 kcal/mol.46 The branching between the H-loss channel and the isomerization


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of vinoxy will, of course, depend on the internal energy of the radical and this branching has been examined by RRKM calculations47-48 as well as by experimentation.49-50 In the ground state, the H + ketene channel is suppressed and virtually all vinoxy isomerizes to acetyl, which ~

then proceeds to CH3 + CO.46, 50 In the Ã and B states, both reactions are observed and H + ketene are the major products.49, 51 The products CH3 and CO are clearly evidenced in the decomposition of propionaldehyde, as described above. However, CO also originates from reaction 7, and other reactions to form CH3 could certainly be conceived. Ketene (IE = 9.6 eV) may account for the signal at m/z = 42 in the PIMS spectra, but propene (IE = 9.7 eV) would overlap ketene’s signal so the FTIR spectra were examined for features of each. One of the signature bands of ketene52 is observed at 3063 cm−1 (Figure 10). The 2142 cm−1 band of ketene53 is also present, but it does overlap with bands of propyne and CO. Ultimately, it was concluded that both ketene and propene are produced during pyrolysis. The FTIR assignment and origin of propene is discussed in section F. E. Possibility of H-loss from propionaldehyde The PIMS spectra show a significant peak at m/z = 57 (C3H5O+), which suggests the loss of a hydrogen atom from the propionaldehyde molecule. This may indeed be the result of the pyrolysis mechanism, or merely dissociative ionization in the PIMS experiment. The C-H bond energy of the α-hydrogen is 90.2 kcal/mol and that of the β-hydrogen is 102.4 kcal/mol,11 and these bond fissions would lead to CH3ĊHCHO and ĊH2CH2CHO, respectively. The C-H bond energy for the aldehydic hydrogen is 89.3 kcal/mol, however, the propionyl radical co-product has a dissociation energy32 of 10 kcal/mol and likely would not survive the hot pyrolysis nozzle. Note that the C-H bond energies are significantly higher than the two C-C bond energies of ~84 kcal/mol.11 Cracking of unreacted propionaldehyde to CH3CH2CO+ during the PIMS experiment



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should be considered, as m/z = 57 is the major ion fragment observed following the multiphoton ionization of propionaldehyde at 308 nm54 and the appearance energy of C3H5O+ is 10.2 eV.55 In similar experiments on acetaldehyde, CH3CO+ was observed in the PIMS spectra following pyrolysis at 1473 K.20 On the other hand, similar dissociative ionization has not been observed in higher (C4-C8) aldehydes following photoionization at 10.5 eV.56 In this laboratory’s previous study of butyraldehyde, there was no ion fragment corresponding to H-loss in the PIMS spectra taken under similar conditions to those used here.24 To be certain of the source of C3H5O+, the FTIR spectra must be examined for evidence. Unfortunately, a literature search uncovered no FTIR spectra for the C3H5O radicals CH3ĊHCHO, CH2CH2CHO and CH3CH2ĊO for comparison. FitzPatrick57 has predicted vibrational modes for the three radical isomers using the UCCSD/aug-cc-pVDZ method. The experimental spectra were examined for unassigned bands appearing in the vicinity of FitzPatrick’s predictions, with a focus on the 1400-2000 cm-1 region, where there is the most variation among the three isomers. There were no outstanding features observed in the experimental spectra that could be plausibly assigned to any of the three radicals. While this does not definitively exclude the presence of C3H5O derived from pyrolysis reactions, dissociative ionization of unreacted propionaldehyde is the likely source of PIMS signal at m/z=57. F. Products of secondary and bimolecular reactions The products identified in this paper thus far can be attributed to primary reactions of propionaldehyde or bimolecular reactions of the pyrolysis products with either propionaldehyde or other products. However, these products may undergo further thermal decomposition with sufficient residual energy or residence time in the pyrolyzer tube. In this section, additional


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products of propionaldehyde pyrolysis are identified and possible reactions of origin are suggested for each. Ethylene is evidenced by FTIR absorption58 at 948 and 1440 cm−1 in Figures 3 and 7. It also displays PIMS signal at m/z = 28 in Figure 1; the hot, nascent ethylene (IE = 10.5 eV)59 has enough internal energy to be ionized by the 10.5 eV laser. This product appears frequently in thermal decomposition of hydrocarbons,60-61 often from several different reactions in the overall mechanism, and propionaldehyde appears to be no exception to this trend. One likely source of ethylene is the secondary reaction of ethyl radical produced in reaction 7. This could occur unimolecularly or via abstraction by a free hydrogen atom. Alternate sources of ethylene include the secondary thermal decomposition of methylketene or propyne, which are present in the mixture of pyrolysis products. Ethylene has been observed following pyrolysis of propyne at 1210 K and 1 atm in flow tube experiments.41 It was found following methylketene dissociation in a static system at 633-813 K.34 Acetylene is shown in Figures 8 and 9 at 3302 cm−1, 3288 cm−1, and 736 cm−1.20 Its ionization energy is too high (11.4 eV)29 to be detected via PIMS. There are two likely sources of acetylene in the thermal decomposition of propionaldehyde. Ethylene, a secondary product discussed above, is known to produce acetylene under pyrolysis conditions.62 Acetylene is also one of the known products of thermal decomposition of propyne.41 Lifshitz et al. included reactions forming acetylene from ethylene (via a vinyl radical intermediate) in their modeling of propionaldehyde reactions in a shock tube.17



0.020 0.018

propyne 0.016

Absorbance

0.014 0.012 0.010

acetylene

*

0.008

*

*

0.006 3400

3350

3300

3250

3200

3150

3100

Wavenumber (cm-1)


0.14

* 0.12

0.10

Absorbance

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0.08

* *

0.06

* 0.04

0.02

*

acetylene

*

900

**

*

* propyne

* 850

800

750

700

650

600

Wavenumber (cm-1)

Figure 9. Matrix infrared absorption spectrum of products from the 1273 K pyrolysis of a mixture of 0.4% propionaldehyde in argon. The * symbol indicates bands that belong to unreacted propionaldehyde.


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Propene is evidenced in the FTIR spectra at 909, 998 (Figure 3), and 3085 cm−1 (Figure 10) in the FTIR spectra. There is also PIMS signal at m/z = 42, which could be propene (IE = 9.73 eV) or ketene (IE = 9.62 eV).29 The origin of propene is not obvious from reactions 7-10, so bimolecular reaction(s) of products of propionaldehyde thermal decomposition are suspected. Huhn and coworkers observed propene following thermal decomposition of propionaldehyde at lower temperatures,12, 16 and speculated that it may come from dissociation of the n-butyl radical (formed by reaction of ethylene and ethyl radical) or the n-hexyl radical (formed by reaction of n-butyl and ethylene). C3H6 was also observed in shock-tube studies of propionaldehyde.17 It was listed as a product in the following reactions of a 52-reaction scheme: CH3 + C2H4 → C3H6 + H, i-C3H7 → C3H6 + H, and i-C3H7 + CH3 → C3H6 + CH4. 0.04

Absorbance

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propene 0.03 ketene

0.02 3100

3050

3000

2950

2900

Wavenumber (cm-1)

Figure 10. Matrix infrared absorption spectrum of products from the 1473 K pyrolysis of a mixture of 0.4% propionaldehyde in argon.

There is evidence for the production of propargyl radical in these experiments, at m/z = 39 (C3H3+) in the mass spectra in Figure 1. The characteristic vibrational frequencies of matrixisolated propargyl31 were not observed in the FTIR spectra presented here, but that may be a function of pyrolysis temperature. The feature at m/z = 39 is not evident following 1300 K pyrolysis, just barely visible at 1400 K, and appears most clearly at 1600 K. The vibrational 21 ACS Paragon Plus Environment


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spectra presented were measured following pyrolysis at a maximum temperature of 1473 K, which may have been too low in temperature to produce sufficient propargyl for detection via FTIR. Other C3H3 isomers, such as c-C3H3 and propynyl, were ruled out or had no literature spectrum for comparison. Propargyl may come from reactions of methyl radicals and ethylene or propyne,63 all products observed here, although reactions involving other species like methylene plus acetylene64-65 could also be possible. The propargyl radical is notorious as a precursor to polycyclic aromatic hydrocarbons and soot.66-67 Its appearance in this study highlights the need to consider the potential pollution contribution from pyrolysis or combustion processes that involve propionaldehyde, even when it occurs as an intermediate. The PIMS spectrum recorded following pyrolysis of propionaldehyde at 1600 K shows peaks emerging at m/z = 50 and 52, as well as 54 and 56 to a lesser extent. Pyrolysis was conducted at lower temperatures prior to the matrix-isolation FTIR analysis, and spectra for many of the C4H2 and C4H4 isomers can’t be found in the literature, so the vibrational modes cannot be used to identify the species appearing at m/z = 50 and 52. However, it should be mentioned that the FTIR spectra collected in this study did contain absorptions at 563 and 1239 cm−1, matching strong bands reported31 for cyclic-C4H4. These bands were observed following pyrolysis at 1473 K, which is consistent with the PIMS spectra containing m/z = 52 signal at a pyrolysis temperature as low as 1400 K. It seems likely that the m/z = 52 species is c-C4H4, but the presence of additional C4H4 isomers cannot be excluded. The presence of the four-carbon species (also observed following pyrolysis of butyraldehyde24) is significant as it suggests an additional pathway to bimolecular carbon-building reactions, a preliminary step to soot formation. While propargyl radical, a three-carbon species, is considered the most important precursor of soot, 66-69 there are examples in the literature of four-carbon species being formed in


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the pyrolysis of smaller molecules and participating in aromatic formation. Diacetylene (HCCCCH) is known to be formed during propyne pyrolysis, and also from reactions of acetylene.43, 70 C4H4 is a suspected precursor to benzene formation in certain flame conditions. For example, vinylacetylene can react with vinyl radical to form benzene.71 The C4H4 isomers butatriene or vinylacetylene can react with H2CCCCH to form aromatics via suspected intermediates phenylacetylene, pentalene and benzocyclobutene.72 These reactions could be significant when combustion or pyrolysis takes place at high temperatures. It is thought that reactions of even-numbered carbon species such as C4H2 and C4H4 are less likely to contribute to aromatic formation than odd-carbon species such as the propargyl radical, but the presence of C4H4 and C4H2 does offer another route to the eventual formation of polycyclic aromatic hydrocarbons and soot during the pyrolysis of propionaldehyde. The PIMS spectra contain signal at m/z = 30 following pyrolysis at temperatures below 1600 K. C2H6+ and CH2O+ were both considered as possible assignments, although the appearance of either would be somewhat surprising, as both ethane and formaldehyde have ionization energies (11.5 eV and 10.9 eV, respectively) above the photoionization laser energy in this experiment. Formaldehyde seems the more plausible of the two, as it may exit the hot nozzle with enough internal energy to make possible ionization by a 10.5 eV photon. To be thorough, the matrix-isolation FTIR spectra were evaluated for signatures of both ethane and formaldehyde.

Bands were observed at 1246 cm-1 and 2863 cm-1, matching strong bands from

the literature73-74 for formaldehyde. However, other weaker bands (1168, 1498, 2797 cm-1) were not observed and the strongest expected band, the carbonyl stretch of formaldehyde, overlaps with that of the parent propionaldehyde. The only observed vibrational band related to ethane was 1373 cm-1, which was very weak and corresponds to one of the weaker bands from literature



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spectra.73, 75 The stronger bands of ethane in the literature (2980, 2950, 2890, 1465 cm-1) were not observed. In light of all this evidence, an assignment of formaldehyde makes the most sense. The appearance of formaldehyde could be merely a contaminant in the propionaldehyde sample (although there was no evidence of this when an unheated sample of propionaldehyde was deposited in a matrix). It is more likely due to some bimolecular reaction(s) occurring in the pyrolysis nozzle. Lifshitz et al.17 suggested that H2CO could arise from reaction of HCO with propionaldehyde, or HCO + H2. They included these reactions in the 52-reaction scheme modeling of their shock tube experiments, although they did not detect formaldehyde. A similar situation was encountered in the appearance of signal at m/z = 44 in the PIMS spectra. Propane, acetaldehyde, and vinyl alcohol were all considered in the assignment. CO2 was ruled out based on its ionization energy of 13.8 eV.29 The ionization energy of propane (10.9 eV) is above the photoionization energy employed here, but that is not necessarily prohibitive, as described above in the case of formaldehyde. Acetaldehyde and vinyl alcohol would be more plausible assignments with their respective ionization energies of 10.2 eV and 9.3 eV.29, 59 To confirm an assignment of m/z = 44, the FTIR spectra were examined for distinctive features of the three products.

Propane can be ruled out, as the strongest bands associated with

matrix-isolated propane (1386, 1371, 743 cm−1)76 do not appear in any spectra collected in these experiments. Vinyl alcohol can similarly be ruled out, as no absorption was observed at 3620, 1662, 1622, or 1079 cm−1. Acetaldehyde remains, but is difficult to assign because many of the prominent bands77 overlap with those of propionaldehyde, some of which remains unreacted in the pyrolysis experiments. However, small peaks were observed in the FTIR spectra at 1720 and 508 cm−1, plus 1756 and 1112 cm−1 (Figures 4 and 5) as shoulders on larger peaks, so it seems likely that acetaldehyde is indeed present following pyrolysis.


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CONCLUSIONS The products of propionaldehyde thermal decomposition have been identified and most of them are consistent with those observed in similar experiments on acetaldehyde and butyraldehyde. While acetaldehyde decomposition can be explained by a mechanism involving three primary reactions, the longer alkyl chain structure of propionaldehyde enables more reactions and leads to a greater number of products, consistent with what has been observed in butyraldehyde. The additional C-C bonds available in these larger aldehydes make possible additional C-C bond scission reactions. Evidence for these has been observed in both propionaldehyde and butyraldehyde. A general trend that has been established for aldehyde pyrolysis is that as the pyrolysis temperature increases, the chemistry becomes more complex and alkyl-chain-building reactions emerge, leading to products such propargyl radical, C4H2, and C4H4. This implies the potential for polycyclic aromatic hydrocarbons and soot formation in processes where propionaldehyde and other aldehydes are exposed to high temperatures. Products that are unique to propionaldehyde pyrolysis include propene, acetaldehyde, and formaldehyde. The results of the experiments presented here are also quite dissimilar to those of previously published low-temperature pyrolysis and shock-tube studies of propionaldehyde. There are many new products presented here (e.g., propyne, ketene, methylketene) and many previously observed products that are absent (e.g., CH4, C3H8, C4H10). This could be due to the much shorter residence time for propionaldehyde in the pyrolysis nozzle as compared to a shock tube, or different concentrations. The increase in the variety of products underscores the need for new kinetic modeling studies of aldehyde decomposition to unravel the mechanisms at work. AUTHOR INFORMATION Corresponding Author



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*Email: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGEMENTS This work was supported by an award from Research Corporation for Science Advancement and a Faculty Start-up Award from The Camille and Henry Dreyfus Foundation. B.J.W. acknowledges a fellowship from the SURE Program funded through the West Virginia Research Challenge Fund, and administered by the West Virginia Higher Education Policy Commission, Division of Science and Research, Grant Number: HEPC.dsr.11.24; AMEND 1. The authors thank Barney Ellison, Jong Hyun Kim, and Jessie Porterfield for technical assistance in collecting the mass spectra, obtained on the Ellison Laboratory’s PIMS instrument at CU Boulder, presented in this paper.

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