Computational Kinetic Study for the Unimolecular Decomposition

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Computational Kinetic Study for the Unimolecular Decomposition Pathways of Cyclohexanone Aristotelis M. Zaras, Philippe Dagaut, and Zeynep Serinyel J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp506227w • Publication Date (Web): 29 Oct 2014 Downloaded from http://pubs.acs.org on November 6, 2014

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The Journal of Physical Chemistry

Computational Kinetic Study for the Unimolecular Decomposition Pathways of Cyclohexanone Aristotelis M. Zaras a*, Philippe Dagaut a, Zeynep Serinyel a,b a

CNRS-INSIS, Institut de Combustion, Aérothermique, Réactivité et Environnement (ICARE),

1C, Avenue de la recherche scientifique, 45071 Orléans cedex 2, France b

Université d’Orléans, 6 Avenue du Parc Floral, 45100 Orléans, France

*

Corresponding author: Aristotelis M. Zaras

E-mail address: [email protected] Tel: +33238257613 Abstract There has been evidence lately that several endophytic fungi can convert lignocellulosic biomass into ketones amongst other oxygenated compounds. Such compounds could prove useful biofuels for internal combustion engines. Therefore their combustion properties are of high interest. Cyclohexanone was identified as an interesting second generation biofuel1, 2. However until recently3, no previous studies on the kinetics of oxidation of that fuel could be found in the literature. In this work we present the first theoretical kinetic study of the unimolecular decomposition pathways of cyclohexanone, a cyclic ketone that could demonstrate important fuel potential. Using the quantum composite G3B3 method we identified five different decomposition pathways for cyclohexanone and computed the corresponding rate constants. The rate constants were calculated using the G3B3 method coupled with Rice-Ramsperger-Kassel-Marcus theory in the temperature range of 800 to 2000 K. Our calculations show that the kinetically more favorable channel for thermal decomposition is pathway 2 that produces 1,3-butadien-2-ol, which in turn can isomerize easily to methyl vinyl ketone through a small barrier. The results presented here can be used in a future kinetic combustion mechanism.

Keywords Biofuels, cyclohexanone, ab initio method, unimolecular decomposition, rate constants

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1. Introduction Petroleum based fuels are most frequently used worldwide in various sectors such as transportation and industry. Nevertheless, due to their significant impact on the environment, such as enhancement of the greenhouse effect attributed to increased carbon dioxide emissions and concerns about energy security, the development and evaluation of alternative fuels has become crucial. In this context, biofuels appear as promising renewable alternative fuels. They can be produced from biomass and their use can result in an important reduction of greenhouse gas emissions. Specifically second generation biofuels, fuels that arise from lignocellulosic biomass, are expected4 to be more efficient in reducing greenhouse gas emissions compared to first generation biofuels. Recently it has been shown5-7 that several endophytic fungi can convert that kind of biomass into a plethora of volatile organic compounds (VOCs) including saturated as well as unsaturated hydrocarbons, esters, acids, alcohols and ketones. Such compounds can present considerable biological and fuel potential, which is interesting nowadays due to the decrease of fossil fuel supplies in conjunction with their negative environmental impact. Ketones are a category of potential biofuels generated by lignocellulosic biomass conversion from fungi, as well as from other methods8 and their combustion is of great interest9,

10

.

Cyclohexanone is a representative molecule for cyclic ketones and its combustion is not well known. In the very past, a few experimental studies regarding cyclohexanone photochemical and thermal decomposition have been reported in the literature11-14 but these are more than 40 years old. Norrish et al11, 12 first studied the photochemical decomposition of cyclic ketones, including cyclohexanone. In their first paper11, as an attempt to throw some fresh light on the mechanism of photolysis of cyclic ketones, they proposed that cyclic ketones primarily decompose through α fission reactions usually referred to as Norrish type I reactions. Particularly, a unimolecular elimination of carbon monoxide occurs with simultaneous production of mainly cyclic saturated hydrocarbons, a change in which free radicals are initially produced. Especially for cyclohexanone, they proposed a main decomposition pathway involving production of cyclopentane and carbon monoxide and a very small amount of cyclopentane’s olefinic breakdown products, i.e. ethylene and propylene along with carbon monoxide. Subsequently12 they revised their mechanism by proposing that the production of 1-pentene and carbon 2 ACS Paragon Plus Environment

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monoxide occurs via CO elimination and formation of a five carbon straight-chain diradical. Eventually, this diradical isomerizes mainly to 1-pentene and to a smaller but notably larger extent, compared to their first report, to ethylene and propylene. Benson and Kistiakowsky13 suggested that the hydrocarbon identified as C5H10 in Norrish’s experiments was actually a mixture of cylopentane and 1-pentene, with this mixture being the major product of the photochemical decomposition along with carbon monoxide. Furthermore, they supported the polymethylene-acyl diradical formation mechanism, which in their hypothesis, initially breaks into carbon monoxide and polymethylene diradicals. Then, the diradicals either close the ring yielding unsaturated cyclic hydrocarbons, or rearrange into olefins with a necessary hydrogen atom shift. On the other hand, De Mayo and Verdun

14

experimentally observed ethylene and

methyl vinly ketone as major products of cyclohexanone thermal decomposition in a flow system at 1050°C andsuggested that the major pathway for thermolysis involves β cleavage producing methyl vinyl ketone. In that case, initial homolysis of the β carbon-carbon bond occurred followed by hydrogen atom migration and finally collapse of the formed diradical into methyl vinyl ketone and ethylene. That was the only reported work in the literature, at least to our knowledge so far, that involves thermal fragmentation of cyclohexanone. Very recently3, the oxidation of cyclohexanone was studied in a jet-stirred reactor at high pressure and a kinetic modelling was performed. In the present paper we attempt to investigate the unimolecular thermal decomposition of cyclohexanone, using ab initio methods and master-equation calculations. This study intends to (i) provide theoretical insights regarding several possible decomposition pathways of cyclohexanone and (ii) propose rate constants for these reactions over a wide temperature range.

2. Computational Methodology All electronic structure calculations were carried out using tsian 09 program suite15. The quantum composite G3B3 method16 which is a variation of G3 theory17 was chosen to perform all corresponding calculations. This compound method consists of geometry optimization and vibrational frequency calculations at the B3LYP18/6-31G(d)19 level of theory. Thereafter, a series of single-point energy calculations at four different higher levels of theory QCISD(T)/6-31G(d),



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MP4/6-31+G(d), MP4/6-31G(2df,p) and MP2/G3Large are being performed upon the obtained optimized structure. For all computed moieties rigid rotor-harmonic oscillator approximations were used. Regarding decomposition pathways, potential energy surfaces connecting reactants with products were computed by performing intrinsic reaction coordinate (IRC) calculations at the same level of theory. These calculations were performed in order to verify that the transition state is connected to the assumed reactants and products and therefore the decomposition reaction pathways are properly configured. Each IRC calculation involved at least 70 steps at each reaction path direction (reverse and forward) coupled with small step intervals in the range between 0.03 and 0.05 a.u.. Since the transition states were verified, RRKM/Master Equations were applied by using the MESMER20 code, in order to calculate high pressure limit rate constants in a wide temperature range for all pathways involved in this study. A singleexponential down model21,

22

was used to represent the collisional energy transfer probability,

whereas down= 200 (T/300)0.85 cm−1 was the relationship applied in the Master Equation calculations. The Lennard-Jones parameters for cyclohexanone, σ = 6.21 Å and ε / kB = 700 K, used in the present study were based on the Tee-Gotoh-Steward correlation model23 based on thermophysical data taken from the literature24. The He bath gas Lennard-Jones parameters, σ = 2.55 Å and ε / kB = 10.2 K were taken from the literature25.

3. Results and discussion

At first it should be noted that our computed structure is in good agreement with the known experimental structure26 as shown in Figure 1, even though its determination involved several assumptions. Moreover our values of computed entropy 333.2 J mol-1K-1 and constant pressure heat capacity 111.7 J mol-1K-1 are in rather good agreement with literature27, 335.5 J mol-1K-1 and 121.1 J mol-1K-1 respectively, given the already large discrepancies in reported literature values.


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Figure 1. Calculated structure of cyclohexanone and comparison with experiment26. Figure 2 reports the six different unimolecular thermal decomposition pathways for cyclohexanone that were identified in this study. Both pathways 2 and 3 require the initial ketoenol isomerization step, namely pathway 1, where hydroxy-cyclohexene is produced in order to occur. This isomerization step demonstrates a barrier height of 66.1 kcal mol-1 as calculated at the G3B3 level of theory. This value is in very good agreement with the previously reported values of Simmie et al28 67.0 at CBS-QB3 and 66.1 kcal mol-1 at CBS-APNO for propanal↔propenol and butanal↔butenol isomerization, as well as with the values of Silva et al29 65.6 kcal mol-1 at CBS-APNO and Yang et al30 67.3 kcal mol-1 at QCISD(T)/6-311++G(d,p)//B3LYP/631++G(d,p), for ethenal↔ethenol and butanone↔butenol respectively. Consequently, hydroxy-cyclohexene can either give a retro-Diels Alder reaction yielding ethylene and 1,3-butadien-2-ol as described in Pathway 2, or an internal hydrogen abstraction reaction from hydroxyl that finally produces 1,2-cyclohexadiene (Pathway 3a) or cyclohexyne (Pathway 3b) and water as described in Pathway 3.



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Figure 2. Unimolecular decomposition pathways of cyclohexanone that were identified in this study. A detailed reaction scheme for Pathways 2&3 is shown in Figure 3. This distinction of pathways has been necessary since different structures of transition states have been identified and our IRC calculations result in different structures of reactants and products as well. Pathway 2a proceeds through a transition state that lies 68.8 kcal mol-1 above hydroxy-cyclohexene, while the transition state for pathway 2b lies 62.9 kcal mol-1 above its parent molecule. Additionally, Pathway 3a occurs via a transition state which lies 87.5 kcal mol-1 higher than hydroxycyclohexene, while the corresponding transition state for Pathway 3b lies 82.5 kcal mol-1 above its parent molecule. It should be noted that structure H2 can be produced by H1 through a small rotational barrier of 3.5 kcal mol-1, as calculated at the G3B3 level of theory In addition, the enolization enthalpies for these two isomers at 0 K are 9.7 and 11.0 kcal mol-1 at G3B3 for H1 and H2 respectively. The comparison with the reported value of 11.1 kcal mol-1 at CBS-4 of Zhang et al31 reveals the very good agreement regarding structure H2.


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Figure 3. Detailed reaction scheme for pathways 2 & 3. Pathways 4 to 6 demonstrate significantly higher reaction barrier heights than pathways 2 and 3 and therefore are expected to exhibit notably slower rate constants. Between these three different channels of decomposition, pathway 4 is the reaction with the lowest barrier height calculated as 99.3 kcal mol-1 while pathways 5 and 6 proceed through reaction barriers of 102.4 and 122.0 kcal mol-1 respectively. Moreover, to account for the entropy contribution to the rate constant, the standard entropy of reactant, transition state and products were computed within the G3B3 calculation, for each pathway identified in this study. In addition, the entropy of activation which is the entropic difference between each corresponding transition state and the reactant was determined. Table 1 reports the reaction barrier height ∆‡E at 0 K, the entropy of activation ∆‡S298 as well as the reaction enthalpy ∆Hr at 0 K for each pathway, along with Figure 4 which illustrates the potential energy surfaces.



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Table 1. Barrier height (kcal mol-1), entropy of activation (cal mol-1 K-1) and reaction enthalpy (kcal mol-1) for reaction pathways 1-6 at the G3B3 level of theory.

Pathway

∆‡E

∆‡S298

∆Hr(0 K)

1

66.1

-1.3

9.7

2a

68.8

4.5

42.2

2b

62.9

3.0

39.8

3a

87.5

0.7

53.7

3b

82.5

-0.3

60.5

4

99.3

7.4

21.0

5

102.4

0.8

23.2

6

122.0

4.4

24.9

Figure 4. Potential energy surfaces of reaction pathways 1-6 with main organic products denoted.


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Between all the investigated pathways, pathway 2b has the lowest barrier height and therefore is expected to demonstrate the highest value of rate constant while pathway 6 exhibits the highest reaction barrier height and is expected to have the lowest value of rate constant. Interestingly, pathways 2, 4, and 6 exhibit higher values of activation entropy than pathways 1, 3 and 5. This can be attributed to the fact that in pathways 1, 3 & 5 the ring structure is maintained after the hydrogen abstraction, while in the cases of pathways 2, 4, and 6 there is a ring opening which is obviously entropically more favorable than the preservation of the ring structure. Once the transition states were identified, a vibrational analysis was performed so as to ensure that each structure demonstrates a unique imaginary frequency which corresponds to the proper mode of vibration for each pathway. The vibrational analysis was performed in terms of visualization by using the Molden32 software. With optimized geometries as well as vibrational frequencies of reactant, corresponding transition states and products, as computed with the G3B3 method, rate constants of pathways 16 were calculated using RRKM/ME. Figure 5 reports the pressure dependent rate constants of pathways 1-4 at 2000K since values of rate constants for pathways 5 and 6 are negligible compared with the others. Figure 6 shows the calculated rate constants of pathways 1-6 as a function of temperature, in the temperature range between 800 K and 2000 K.



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Figure 5. Pressure dependent rate constants of pathways 1-4 at 2000K.

Figure 6. Calculated rate constants of pathways 1-6 at the high pressure limit. 10 ACS Paragon Plus Environment

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Subsequently the rate constants are fitted to a modified Arrhenius equation of the form k = B Tn exp(-Ea/RT) and the parameters for the six different pathways obtained from the fit are reported in Table 2.

Table 2. Arrhenius parameters (units: s, K, kcal, mol) for all the decomposition pathways of cyclohexanone at high pressure limit. Pathway

B

n

Ea

1

1.00×1013

-0.130

64.6

2a

8.53×1014

-0.180

68.6

2b

4.98×1014

-0.179

64.0

3a

6.32×1013

-0.173

86.8

3b

4.10×1013

-0.211

82.2

4

8.57×1014

-0.150

95.2

5

2.90×1013

-0.170

98.4

6

2.08×1014

-0.183

117.4

As expected, pathway 2b demonstrates the highest value of rate constant among all channels that were identified in this study. This can be attributed to the fact that it exhibits the lowest activation barrier and a large value of entropy of activation. This combination makes this pathway kinetically more favorable in both energetic and entropic terms. Moreover, the trend of the barrier determines the trend of the rate constant for all channels investigated. Specifically the larger the reaction barrier is, the lower the rate constant of the pathway as can be seen in Figure 6. Interestingly, pathways 3 and 4 seem to compete in high temperature regimes although pathway 3 demonstrates a significantly lower activation barrier. This fact can be attributed to the larger value of activation entropy of pathway 4, value which is the greatest among all pathways involved. Pathway 2 which is kinetically more favorable than every other pathway, could be the precursor of methyl vinyl ketone which was the major product reported in the thermal study of De Mayo et al. although they proposed a different mechanism for its production. According to our calculations 1,3-butadien-2-ol, one of the two products of pathway 2, can isomerize easily to methyl vinyl ketone through a 1,3 shift reaction with a reaction barrier which was calculated 52.3 11 ACS Paragon Plus Environment


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kcal mol-1 as shown in Figure 7. This isomerization can only come from structure B2, as our IRC calculations revealed; yet again this structure can be produced by B1 through a small rotational barrier which was calculated to be 2.6 kcal mol-1. B2 conformation (OH towards the double bond) is clearly more stable (Supporting Information section) than B1 and this finding is consistent with the previous study of Ribeiro-Claro33. On the other hand, pathway 3 which yields 1,2 cyclohexadiene or cyclohexyne and water as products is the second energetically more favorable one. However, especially cyclohexyne a six-carbon cycloalkyne, is expected to be highly reactive and therefore unstable due to increased ring strain. Two pathways have been identified regarding the fate of cyclohexyne and are also reported in Figure 7. The first is rearrangement to carbene and the second is cleavage to butatriene and ethylene in a retro-Diels-Alder reaction. The calculated barrier heights for the two reactions are 24.0 and 42.7 kcal mol-1 which are slightly different from the MP2/6-31G* values of Shevlin et al34 but the difference between the two remains and therefore the rearrangement to carbene is still more favourable. Finally, no pathway regarding the fate of 1,2 cyclohexadiene could be identified, at least at this point.


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Figure 7. Fates of 1,3 butadienol and cyclohexyne.

Moreover, pathway 3 is competing with pathway 4 in higher temperatures as denoted earlier. The latter leads to the formation of 1-pentene and carbon monoxide, products which are also consistent with the work of both Norrish et al.12 and Benson et. al.13. Hence, this pathway could play an important role in higher temperatures. Regarding pathways 5 and 6, they present progressively higher activation barriers, thus this is the reason that they demonstrate the slowest rate constants. Nevertheless, analogous organic products have been identified in previous experimental work35 regarding the thermal decomposition of cyclopentanone. The corresponding products were 2-cyclopentenone and 4-pentenal respectively to 2-cyclohexenone and 5-hexenal that were identified in this work and therefore these two pathways should not be disregarded. Finally, as denoted in the Introduction section, this work involves the study of several unimolecular thermal decomposition pathways of cyclohexanone. Bond scission reactions can occur as well but they should involve higher reaction barriers than the fastest pathways identified 13 ACS Paragon Plus Environment


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in this study. This hypothesis arises from the values of bond dissociation energies (BDEs) computed with the G3B3 method, as can be seen in Table 3. Bond fissions are in general reactions without an intrinsic barrier and should be treated with a completely different approach and methodology than the one used in this study. In such cases, transition states are located at lower bond lengths than bond lengths of full dissociation. This results in barrier heights somewhat lower than the values of BDEs. Nevertheless it is our intention to study these bond scission reactions separately, in a future work. Table 3. Bond dissociation energies of cyclohexanone at 298.15K (kcal mol-1).

Bond

BDE

C1C2

81.4

C2C3

84.8

C4C5

90.5

C2 H

91.5

C3 H

98.3

C4 H

99.1

Conclusion

This work reports the first theoretical kinetic investigation of the unimolecular decomposition pathways of cyclohexanone by using DFT/ ab initio methods. Cyclohexanone is a cyclic ketone with almost unknown combustion properties that could prove to be an important biofuel. Previously

reported

experimental

work

concerning

the

decomposition

pathways

of

cyclohexanone were taken into account in this study. The structures and properties of all moieties involved in the unimolecular decomposition pathways were calculated by means of the quantum composite G3B3 method. The rate constants were calculated using the G3B3 method coupled with RRKM theory as implemented in Mesmer software in the temperature range of 800 to 2000 K. The computed rate constants reveal that the major pathway of decomposition of cyclohexanone is the one that produces 1,3-butadien-2-ol, which can convert easily to methyl 14 ACS Paragon Plus Environment

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vinyl ketone through a small barrier. This finding is consistent with previous experimental pyrolysis studies. The rest of the pathways investigated in this study lead to products that are in agreement with those identified in previous experimental studies. Contribution of these product channels is of greater importance at high temperatures.

Acknowledgements

The authors would like to thank Dr. Marianna Dakanali, Professor Barney Ellison and Jessie Porterfield for useful comments and discussions. In addition they would like to acknowledge helpful feedback from Dr. Robin Shannon concerning MESMER. The research leading to these results has received funding from the European Research Council under the European Community’s Seventh Framework Programme (FP7/2007–2013)/ERC grant agreement No.291049 – 2G-CSafe.

Supporting Information Available: Optimized geometries and vibrational frequencies of reactants, transition states and products as calculated with the G3B3 compound method. Tables for pressure dependent (0.01, 0.1 and 1 atm) rate constants in the temperature range 800-2000K. This material is available free of charge via the Internet at http://pubs.acs.org



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