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Quantum Chemical Study of CH3 + O2 Combustion Reaction System: Catalytic Effects of Additional CO2 Molecule Artem ̈ E. Masunov,*,†,‡,§,∥,⊥ Elizabeth Wait,†,‡ and Subith S. Vasu# †

NanoScienece Technology Center, ‡Department of Chemistry, §Department of Physics, University of Central Florida, 12424 Research Parkway, Suite 400, Orlando, Florida 32826, United States ∥ South Ural State University, Lenin pr. 76, Chelyabinsk 454080, Russia ⊥ National Research Nuclear University MEPhI, Kashirskoye shosse 31, Moscow 115409, Russia # Center for Advanced Turbomachinery and Energy Research (CATER), Mechanical and Aerospace Engineering, University of Central Florida, Orlando, Florida 32816, United States S Supporting Information *

ABSTRACT: The supercritical carbon dioxide diluent is used to control the temperature and to increase the efficiency in oxycombustion fossil fuel energy technology. It may affect the rates of combustion by altering mechanisms of chemical reactions, compared to the ones at low CO2 concentrations. Here, we investigate potential energy surfaces of the four elementary reactions in the CH3 + O2 reactive system in the presence of one CO2 molecule. In the case of reaction CH3 + O2 → CH2O + OH (R1 channel), van der Waals (vdW) complex formation stabilizes the transition state and reduces the activation barrier by ∼2.2 kcal/mol. Alternatively, covalently bonded CO2 may form a six-membered ring transition state and reduce the activation barrier by ∼0.6 kcal/mol. In case of reaction CH3 + O2 → CH3O + O (R2 channel), covalent participation of CO2 lowers the barrier for the rate limiting step by 3.9 kcal/mol. This is expected to accelerate the R2 process, important for the branching step of the radical chain reaction mechanism. For the reaction CH3 + O2 → CHO + H2O (R3 channel) with covalent participation of CO2, the activation barrier is lowered by 0.5 kcal/mol. The reaction CH2O + OH → CHO + H2O (R4 channel) involves hydrogen abstraction from formaldehyde by OH radical. Its barrier is reduced from 7.1 to 0.8 kcal/mol by formation of vdW complex with spectator CO2. These new findings are expected to improve the kinetic reaction mechanism describing combustion processes in supercritical CO2 medium. peroxyl radicals without changing the mechanism.12 In the third paper, we refined these reaction pathways at higher theory level.13 Here, we report the study of four other reactions, critically important in combustion:

1. INTRODUCTION Carbon capture and sequestration is a technology that can reduce the carbon dioxide (CO2) emissions produced from fossil fuel combustion.1 One of the challenges for this technology is the separation of the flue gases, which can be simplified when nitrogen is removed from the oxidant before feeding it into the combustion chamber (this process known as oxycombustion). In order to control the combustion temperature, CO2 can be used as diluent.2 The ability of CO2 to reach a supercritical state, and hence increase the efficiency of the turbine, is another advantage of oxycombustion.3,4 The effects of the large concentrations of CO2 on the combustion kinetics have not been well understood,5,6 and theoretical chemistry can greatly assist in this understanding.7−10 In the first paper of this series, we investigated the effect of CO2 on the mechanism of CO2 formation from CO and OH and found a lower activation energy pathway, involving intermediates where CO2 is covalently bonded.11 The second paper reported van der Waals complexes with CO2 that lower the activation barrier for self-reaction between two hydro© XXXX American Chemical Society

CH3 + O2 → CH 2O + OH

(R1)

CH3 + O2 → CH3O + O

(R2)

CH3 + O2 → CHO + H 2O

(R3)

CH 2O + OH → CHO + H 2O

(R4)

Reactions R1−R3 present three competitive channels for oxidation of CH3, the most stable alkyl radical appearing in many combustion processes. 14 However, their relative importance appears controversial.15−18 The channel R2 represents branching chain oxidation, important for radical Received: May 20, 2017 Revised: June 29, 2017

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DOI: 10.1021/acs.jpca.7b04897 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

Figure 1. Relative energies (kcal/mol) of the stationary points along the reaction pathways for (a) R1 and (b) R3.

reaction propagation.17 Mechanisms of R1, starting from methyl radical and oxygen diatomic in the gas phase (and continuing to H2C(O)OH and CH(OH)2), were reported by Cheung and Li.19 The reactants in R1 first form intermediate CH3OO (in03), which then rearranges by hydrogen transfer. Cheung and Li found two pathways for this transfer, illustrated in Figure 1a (with relative energies reported in the first column of Table 1). The higher energy pathway corresponds to a 1−2 hydrogen shift, which proceeds through a three-membered ring transition state (ts35) into CH2OHO intermediate (in05). Next, the same hydrogen atom is transferred from the central to the terminal oxygen atom, forming CH3OOH intermediate (in04) over another three-membered ring transition state (ts45). Alternatively, in05 may rearrange into in04 via a methylene shift over higher energy transition state ts45a, which

involves a larger rotation about the C−O bond. The lower energy pathway from in03 to in04 involves a 1−3 hydrogen shift from carbon to the terminal oxygen atoms, during which the C−O bond rotates to form the four-membered ring transition state ts34. The intermediate in04 then dissociates through a low energy transition state ts46, forming hydrogen bonded complex pc06. The low barrier, corresponding to in04, disappears after zero-point energy (ZPE) correction. The lower energy pathway for R1, along with the mechanisms for R2 and R3, was later investigated by Zhu et al.14 For R1, they reported potential surfaces for two distinct electronic states: 2A′ and 2A″. While 2A″ connects the ground state of CH3OO intermediate to the ground state of reactants CH3 + 3O2, 2A′ connects the excited state of reactants CH3 + 1 O2 (including singlet oxygen) to the excited state of CH3OO B


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contribution, we investigate the effects of carbon dioxide on the reaction R1−R4 pathways.

Table 1. Relative Energies (kcal/mol) for the Critical Points on PES for R1−R4 in the Gas Phasea Str. CH3 + O2 rc00a ts00 in03 ts34 ts35 in05 ts45 ts45a in04 ts46 pc06 ts50 HCO + H2O ts61 pc61 H2CO + OH CH3O + O ts27 in27 ts28 pc28

prior studies

DFT-D

DFT-D + ZPE

0

0 0.58 1.64 −38.13 13.46

0 −0.35 1.62 −33.79 13.69

53.44 29.08 36.9 51.04 −23.23 −23.07 −60.1

53.53 32.37 37.19 53.46 −20.06 −21.3 −58.2

44.75 −81.81

46.07 −82.64

23.72 23.71 −52.01 24.95 1.70 −29.16 −1.35 −34.69

24.57 24.72 −52.46 25.4 4.34 −24.78 −1.42 −33.18

−29.52b 18.76b 16.04d 55.1b 32.62b 40.04b 53.42b −16.85b −18.76b −55.33b −56.07d 47.4d −81.96c −82.86d

−53.5d 28.74d 5.7e −20.5e −1.0e −31.1e

ΔE

2. COMPUTATIONAL DETAILS All calculations were performed with the Gaussian 2009 suite of programs.22 Geometry optimizations and normal-mode analysis were performed using density functional theory (DFT). DFT is proven to afford superior accuracy to cost ratio among other quantum chemistry methods.23−27 For this work, we selected M11 exchange-correlation functional.28 This functional belongs to the class of meta-GGA hybrids, where larger fraction of Hartree−Fock exchange ensures the accurate description of the activated complexes, while dependence on the kinetic energy density recovers accurate description of the stable molecular configurations. Unlike older meta-GGA hybrids, M11 includes range separation so that fraction of Hartree−Fock exchange varies from 42.8% at short interelectron distances to 100% at the asymptotic limit. M11 was optimized for across-the-board superior performance (including main-group atomization energies, barrier heights, and noncovalent interaction energies). However, long-range dispersion is still missing in M11 functional.29 For this reason, we used Grimme’s three-body dispersion correction GD330 with S8 and SR6 parameters chosen to be S8 = 0.0 and SR6 = 1.619 (values, optimized for a similar M06-2X functional). The 6-311G** basis set31 was selected for compatibility with CBS-QB3 model chemistry.32 We will refer to M11/6-311G**+GD3 theory level as DFT-D in the following. The pathway search used the following protocol: 1) optimization of the product/reactant van der Waals complex; 2) relaxed potential energy scan along selected bond formation/breaking; 3) transition state (TS) optimization starting from the highest point on that scan; 4) frequency analysis for identification of that TS as first-order critical point; 5) intrinsic reaction coordinate33 (IRC) search for two (forward and backward) minimum energy pathways (MEP) starting from the transition state; 6) optimization into the local minima to continue both MEPs found in step 5, then repeat step 2 above.

−4.27 −5.07 −2.35 −1.57 −0.25 −2.85 0.04 −3.21 −2.54 −2.87 −2.13 1.33 −0.68 0.22

1.04 −3.34 −1.36 −4.28 −0.42 −2.08

a

Both literature data and DFT-D energies with and without ZPE corrections are shown (deviations from literature data are labeled as ΔE). bG2 (QCISD(T)/6-311+G(3df,2p), ZPE at MP2/6-31G(d)).19 c CC-a (CCSD(T)/cc-pVTZ, ZPE at CCSD(T)/cc-pVTZa).21 dG2M (CCSD(T)/6-311+G(3df,2p), ZPE at B3LYP/6-311G(d,p)). 14 e CCSD(T)/6-311+G(3df,2p), ZPE at CCSD/6-311++G(d,p).20

intermediate and proceeds to the products over the low energy (15 kcal/mol) transition state of 2A′ symmetry. The two alternative transition states on 2A″ surface (formed from cisand trans-conformations of in03) are more than twice higher, which contradicts experimental observations. Zhu et al. concluded that the surface crossing occurs along the reaction path from in03 to ts34. Zhu et al. also reported barrierless dissociation of in03, which describes the mechanism of R2. For R3, they reported a rather high barrier with transition state ts50, corresponding to concerted O−O bond breaking and 1−3 shift of the second hydrogen in in04, and concluded CH3 + O2 → CHO + H2O reaction channel not to be competitive with R1 and R2. Reaction R4 presents an alternative mechanism for CHO and H2O formation, which was reported by Xu et al.,20 illustrated in Figure 2. It consists of hydrogen abstraction from formaldehyde by hydroxyl starting from H-bonded complex CH2O···HO (pc06). The barrier to this abstraction (ts28 at 4.3 kcal/mol) was found to be lower than formation of adduct H2C(O)OH (ts27 at 10.1 kcal/mol). This was also confirmed in a recent study by Ali and Barker.21 One may conclude, therefore, that overall R3 reaction is more likely as a sequence of the R1 and R4 reactions, rather than a process concurrent to R2. Moreover, Zhang et al. found that the hydrogen abstraction by OH from CH2O is catalyzed by water, formic acid, and sulfuric acid. It would not be unreasonable to suggest that carbon dioxide may catalyze this final step of R3 as well. In this

3. RESULTS AND DISCUSSION 3.1. Gas Phase Reaction Mechanisms. In this work, we use the relatively new exchange-correlation functional and need to validate the DFT-D+ZPE model chemistry for the reactive systems being discussed. In Table 1, the energies of the stationary points predicted in this work at DFT-D+ZPE theory level are presented. The previously published high level theory results are also shown for comparison. The geometries of the transition states and intermediates (presented in Figures 1 and 2 and Table 1S in the Supporting Information) are in good agreement with the literature. The initial reaction step (van der Waals complex between CH3 radical and O2 triplet, and transition state for C−O bond formation) is reported here for the first time. We also found the transition state to the O−O bond breaking in in04 and van der Waals complex between CH3O radical and triplet state of the oxygen atom, not reported previously. One can see that the enthalpies at DFT+D3 level are in good agreement with G2 values. The maximal deviation (4.3 kcal/mol) is found for CH3OO intermediate and H2C(O)OH adduct. Therefore, M11/6-311G**+GD3 method is sufficiently accurate for the reactive systems considered here. C


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Figure 2. Relative energies (kcal/mol) of the stationary points along the reaction pathways for (a) R4 and (b) R2.

3.2. R1 Pathways with Spectator and Covalent CO2. Next, we include a carbon dioxide molecule into reactive system R1 in order to investigate its possible effects on reaction pathways. Two types of CO2 participation were considered: spectator (Figure 3) and covalent type (Figure 4). A spectator CO2 molecule forms no covalent bonds to the reactive system, but it may form van der Waals (vdW) complexes. In order to find those, we picked the highest point on the gas phase reaction pathway (ts34 on Figure 1a) and added a CO2 molecule to form the lowest energy adduct. It is shown as ts118 in Figure 3. IRC calculations were used to predict reactant and product intermediates (in123 and in137 respectively), and a scan along the forming and breaking bonds produced the entrance and exit transition states (ts123 and ts137). The overall pathway illustrated in Figure 3 appears to be similar to that of the gas phase (Figure 1a). It starts from the reactant vdW complex, proceeds by binding the diatomic

oxygen to methyl, and forms in123. Hydrogen is then transferred intramolecularly to the terminal oxygen on CH3OO via ts118, which contains a four-membered ring. The resulting in137 contains an OH group, which leaves by breaking the O−O bond with no barrier (ts137), after ZPE is taken into account. A hydrogen bonded complex pc137 is formed before the products finally separate. The rate limiting step for this reaction is the hydrogen transfer in ts118. The activation energy here (45.7 kcal/mol) is less than that of the same step in the absence of CO2 (47.5 kcal/mol), indicating that the formation of a vdW complex with CO2 lowers the barrier at a high pressure limit. The energy of ts118 is 8.7 kcal/ mol above the energy level of the starting products, much lower than the 13.7 kcal/mol value for the gas phase. One can see that the spectator CO2 molecule stabilizes both ts118 and (to a lesser degree) in123. However, these values above the energy level of the starting products would be important in the gas D


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Figure 3. Relative energies of reactants, transition states, intermediates, and products along the R1 reaction pathway with spectator CO2.

Figure 4. Relative energies of reactants, transition states, intermediates, and products along the R1 (black to blue), R3 (black to red), and R4 (blue to red) reaction pathways with covalent participation of CO2.

phase only for the low pressures, where some intermediates may remain hot before proceeding to the next reaction step. In this case, the energy difference between the transition state and the lowest energy of the preceding intermediate may not be as critical in determining the reaction rate since the intermediate would still be energized from the previous step. In higher supercritical pressures, one would expect sufficient collisions to stabilize the intermediates to the bottom of the energy well. This collisional stabilization of intermediates would slow down the reaction, as it is likely to be the case in the supercritical conditions considered in this work. In order to investigate the mechanism for covalent participation of carbon dioxide in R1, we hypothesized that hydrogen transfer from methyl to oxygen atom may proceed

indirectly, via attachment/detachment to CO2 molecule. This may stabilize the rate determining transition state ts34 by extending the four-membered ring to a six-membered ring, thus relieving the ring strain. This assumption proved to be correct. Such an alternative R1 pathway is illustrated in Figure 4. First, diatomic oxygen binds to methyl over a small barrier via ts123, forming intermediate CH3OO (or123). Next, oxygen atom of CO2 molecule abstracts methyl hydrogen atom from CH3OO, while its carbon atom bonds to the terminal oxygen via the sixmembered cyclic transition state ts111. The open chain intermediate or111 thus forms and immediately decomposes into CH2O and HOCO2 with no barrier. In turn, vdW complex or139 containing HOCO2 breaks apart over 21.2 kcal/mol barrier (ts139) to form the product complex (pc139). The E


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Figure 5. Relative energies of reactants, transition states, intermediates, and products along the R1 reaction pathway with covalent participation of CO2.

Figure 6. Relative energies of reactants, transition states, intermediates, and products along the R1 (black to black) and R2 (black to red) reaction pathways with covalent participation of CO2.

pathways are illustrated in Figures 5 and 6. The pathway in Figure 5 starts with the methyl binding to an oxygen atom in carbon dioxide via ts134 over a large barrier (54.4 kcal/mol). This forms intermediate CH3OCO (or133). There is then an intermolecular hydrogen transfer from methyl to O2 via ts133, resulting in intermediate of133. Thus, the formed hydroperoxide radical then rotates and bonds to carbon via ts443, which is lower in energy than of133 after ZPE correction. The open chain intermediate or142 is then decomposed into CH2O

highest barrier on this reaction pathway is associated with ts111. It is calculated to be 46.9 kcal/mol, lower than it was without CO2 but not as low as it was with the four-membered cyclic vdW complex ts118 (Figure 3). Thus, formation of the vdW complex was more effective in lowering activation energy than relieving the ring strain by using CO2 to extend the ring. We also considered alternatives to CH3−OO bond formation in the presence of CO2. Both involve methyl radical attachment to CO2 molecule. These possibilities were investigated, and the F


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Figure 7. Relative energies of reactants, transition states, intermediates, and products along the R4 reaction pathway with spectator CO2.

product complex. When this process is a part of R4 reaction, the activation barrier (ts542) is 30.2 kcal/mol, which is not competitive with the noncovalent mechanism described above. When this process is a part of R3 reaction, the activation barrier (ts111) is 46.9 kcal/mol. The barrier for direct R3 pathway without CO2 was found to be 79.9 kcal/mol (ts50). Thus, we confirm our hypothesis that CO2 catalyzes this step, as the barrier was brought down by covalent participation of CO2 in this process. Formation of vdW complex with spectator CO2 lowers the barrier for OH to abstract hydrogen from formaldehyde from 4.3 to 0.8 kcal/mol for R4 (Figures 2a and 7). However, covalent participation of CO2 in this process increases the barrier to 30.2 kcal/mol (Figure 4), making this alternative pathway unlikely. However, the activation barrier in the overall R3 process was reduced by 0.5 kcal/mol by the CO2 molecule participating covalently. 3.4. R2 Reaction Pathway with CO2 Participation. The R2 reaction pathway in the absence of CO2 environment demonstrates the highest barriers among all the processes considered here (58.3 kcal/mol). We hypothesized that intermediate or132 may dissociate, forming an oxygen atom easier than CH3OO. This hypothesis was confirmed after we located ts532 with a corresponding barrier of 42.9 kcal/mol (Figure 6). Even though the activation barrier on R2 reaction pathway with covalent CO2 participation is as high as 54.4 kcal/ mol (see ts134 on Figure 6), it is still lower than the barrier without CO2. As shown in Figure 6 in red, the alternative R2 pathway starts as a part of R1, then splits from it at or132 and proceeds by losing the terminal oxygen over ts532. Next, CO2 breaks from CH3O, resulting in product complex pc544. In comparison to the same step without CO2, in which O2 binds to methyl and then oxygen dissociates (Figure 2b), methyl binds CO2, and then this adduct binds with oxygen molecule. This results in the barrier lowering by 3.9 kcal/mol due to covalent participation of CO2, which is expected to

and HOOCO (or143). Finally, O−O bond in HOOCO breaks, and a hydrogen bonded product complex pc141 is formed. The second alternative pathway (Figure 6) proceeds to or133 similarly to the first one but involves the oxygen molecule bonding to the CH3/CO2 adduct (via ts132) and hydrogen transfer via intramolecular six-membered cyclic transition state ts135. Formed HOOCO rotates about the CO bond, moving OH away from CH2. CH2O leaves, proceeding to or143. From there, OH separates from CO2, resulting in hydrogen-bonded complex pc141. In both cases, the CH3/CO2 adduct formation is a rate limiting step with 54.4 kcal/mol energy barrier (ts134), higher compared to the gas phase R1 mechanism (47.4 kcal/mol). This makes alternative covalent pathways unlikely. 3.3. R3 and R4 Reaction Pathways with CO 2 Participation. The hydrogen abstraction from formaldehyde by OH radical (reaction R4) may be assisted by vdW complex formation with a CO2 molecule. In order to investigate this possibility, we found the most stable vdW complex between ts28 and CO2 molecule (ts128 in Figure 7). This TS corresponds to an activation barrier of 0.8 kcal/mol, significantly lower than without CO2 participation (7.1 kcal/ mol). The mechanism remains essentially the same as it was in the absence CO2, but formed vdW complexes with CO2stabilized relative energies for the stationary points. The hydrogen abstraction from formaldehyde by OH radical may also proceed via covalent intermediate or139 (CO2OH··· CH2O complex). This intermediate serves as a branching point on the R1 pathway (black to blue in Figure 4) and may be found in both R3 (black to red in Figure 4) and R4 pathways (blue to red in Figure 4), starting from CH3 + O2 + CO2 or from CH2O + OH + CO2 reactants. From or139, a terminal oxygen atom can abstract H atom from CH2O, forming intermediate CO(OH)2 (or543). One of these hydrogens is then transferred to the other OH group intramolecularly via a four-membered ring transition state (ts542), leading to the G


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The Journal of Physical Chemistry A accelerate this process, important for branching the radical chain reaction.

Subith S. Vasu: 0000-0002-4164-3163

4. CONCLUSIONS We reported quantum chemical investigation of the transition states and intermediates along the four reaction channels originating from the CH3 + O2 reactive system in the presence of a CO2 molecule. In case of reaction CH3 + O2 → CH2O + OH (R1 channel), we found that vdW complex formation stabilizes transition state (ts118 on Figure 3) and reduces the activation barrier (corresponding to the rate limiting step) by ∼2.2 kcal/mol compared to gas phase pathway. Alternatively, covalently bonded CO2 may form a six-membered ring transition state (ts111 on Figure 4) and reduce the activation barrier by ∼0.6 kcal/mol. We also found two other pathways involving methyl attachment to the CO2 molecule. Both of these alternative covalent pathways are not competitive, as they contain rate limiting step CH3/CO2 adduct formation via ts134 (Figure 5) with an activation barrier of 54.4 kcal/mol, compared to 47.4 kcal/mol in the gas phase R1 mechanism. In case of reaction CH3 + O2 → CH3O + O (R2 channel), covalent participation of CO2 (binding first to the methyl radical) lowers the barrier for the rate limiting step (corresponding to ts134 in Figure 6) by 3.9 kcal/mol, compared to the gas phase R2 pathway. This is expected to accelerate the R2 process, important for the branching step of the radical chain reaction mechanism. For the reaction CH3 + O2 → CHO + H2O (R3 channel) with covalent participation of CO2, we found the activation barrier (corresponding to ts111 on Figure 4) to be lower by 14.0 kcal/mol than the activation barrier reported for the gas phase. The reaction CH2O + OH → CHO + H2O (R4 channel) involves hydrogen abstraction from formaldehyde by OH radical. We found this barrier to be reduced drastically from 7.1 to 0.8 kcal/mol by formation of vdW complex with spectator CO2 (ts28 in Figure 2a vs ts128 in Figure 7). However, the covalent participation of CO2 increases the barrier for this same process to 30.2 kcal/mol (ts542 in Figure 4), making this pathway unlikely. Hence, all the reactions considered here are predicted to be catalyzed by carbon dioxide environment. These new findings will lead to the improvement of the kinetic reaction mechanism describing combustion processes in supercritical CO2 medium. The work on reaction rate constant calculations, incorporation of these data in AramcoMech2.0 reaction mechanism,34 and subsequent experimental verification of these improvements is presently under way.

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. The authors declare no competing financial interest.

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Notes

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ACKNOWLEDGMENTS This work was supported in part by the Department of Energy (grant number: DE-FE0025260). The authors acknowledge the National Energy Research Scientific Computing Center (NERSC) and the University of Central Florida Advanced Research Computing Center (https://arcc.ist.ucf.edu) for providing computational resources and support. A.E.M. gratefully acknowledges support from Government of the Russian Federation Act 211 (contract number: 02.A03.21.0011).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.7b04897. Cartesian coordinates and relative energies for the transition states, intermediate reactants, and product complexes (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Phone: 1-407-374-3783. E-mail: [email protected]. ORCID

Artëm E. Masunov: 0000-0003-4924-3380 Elizabeth Wait: 0000-0003-0953-2305 H


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Combustion Reaction System - ACS Publications - American

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