Generalized Reaction Mechanism for the Selective Aerobic Oxidation

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Generalized Reaction Mechanism for the Selective Aerobic Oxidation of Aryl and Alkyl Alcohols over Nitrogen Doped graphene Jeyaraj Vijaya Sundar, Manoharan Kamaraj, and Venkatesan Subramanian J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b07070 • Publication Date (Web): 10 Sep 2015 Downloaded from http://pubs.acs.org on September 21, 2015

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The Journal of Physical Chemistry

Generalized Reaction Mechanism for the Selective Aerobic Oxidation of Aryl and Alkyl Alcohols over Nitrogen Doped graphene J. Vijaya Sundar, M. Kamaraj and V. Subramanian* Chemical Laboratory, CSIR-Central Leather Research Institute, Adyar, Chennai 600 020, India

Abstract In this study, an attempt has been made to investigate the mechanistic pathway for the aerobic oxidation of alcohols over nitrogen doped graphene using density functional theory (DFT) methods employing a suitable model for graphene. The formation of activated oxygen species (AOS), upon oxidation, by dioxygen has been investigated with the aid of various possible nitrogen doped models. The detailed reaction mechanism for the oxidation of benzyl alcohol and ethanol by the three AOS obtained in the present study has been unraveled. Results indicate that the ketonic oxygen species oxidizes aromatic alcohol with minimum activation energy of about 26.5 kcal/mol. On the other hand, the activation energy for the oxidation of alkyl alcohol by AOS present at the center is the lowest which is also similar to that of ketonic oxygen species. Based on the results, a generalized reaction mechanism has been arrived for alcohol oxidation by nitrogen doped graphene. Findings reveal the valuable lead information for the optimal control over selective oxidation of alcohol by N-doped graphene based on dopant concentration and temperature Keywords: Nitrogen doped graphene, graphene models, activated oxygen species, reaction modeling and aerobic selective alcohol oxidation. AUTHOR INFORMATION Corresponding Author *Tel.: +91 44 24411630. Fax: +91 44 24911589. E-mail: [email protected], [email protected]

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1. Introduction The selective oxidation of alcohols to aldehydes and ketones is an important industrial process. Various catalysts have been used to promote the selectivity of oxidation and the yield.1-9 Utilization of a catalyst that is separable from the reaction mixture and exploitation of atmospheric oxygen as the ultimate oxidant is one of the attractive approaches. In this context, several experimental and theoretical studies have been performed.10-26 Bielawski and co-workers utilized graphene oxide as a catalyst for the aerobic oxidation of alcohols to aldehydes and ketones with >90% selectivity.27 Also, N-doped graphene has been used as a catalyst to selectively oxidize various alcohols into aldehydes and ketones in the presence of atmospheric oxygen in aqueous solutions at ambient conditions. Wang and co-workers have reported that N-doping of graphene enhances the aerobic oxidation of aromatic alcohols to aldehyde or ketone with > 99% selectivity.28 Watanabe et.al have used nitrogen doped activated carbon catalyst for the oxidation of various alcohols.29 Previous experimental and theoretical studies have provided mechanistic details and the active sites of the reaction.28, 30, 31

It is found that the presence of graphitic nitrogen in the graphene lattice is responsible for

the oxidation of alcohols. It is evident from earlier studies that this form of nitrogen is also well known in oxygen reduction reactions (ORR), which is catalyzed by N-doped graphene.32-38 Graphitic nitrogen forms three σ-bonds and one π-bond with three nearby carbon atoms. The fifth electron belongs to nitrogen p-orbital, remains as an unpaired electron and delocalized over the basal plane of graphene. Since the local domain of the graphene around the graphitic nitrogen contains an unpaired electron, it results in a net magnetic moment depending on the surface area of graphene.39-42 This magnetic moment is capable of attracting paramagnetic molecules like dioxygen closer to the surface of graphene. This proximity provides the condition for the electron transfer from the graphene to dioxygen resulting in the reduction of dioxygen and formation of activated oxygen species. This 2 ACS Paragon Plus Environment

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property of N-doped graphene has been utilized in the oxygen reduction reaction in fuel cells. Figure 1 shows the schematic representation of electron transfer from graphene mimic to dioxygen. Such reduction of dioxygen by N-doping has been studied by both computational and experimental studies even in non-graphene systems.43

N

N

O

O

O N

O

Figure 1. Schematic representation of dioxygen activation by unpaired electron on N-doped graphene with the aid of pyrene as model system. The structure and properties of oxygenated species are determined by the position of the graphitic nitrogen in the graphene lattice. There are three types of graphitic nitrogen narrated in the previous reports.3, 28, 34, 35, 44 They are shown in Figure 2. All over the text the subscript C, E and K represent the center, edge and Ketonic, respectively.



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H

H

H

H

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H

NE'

H

H H H

NC H

H H NE H

H H

H

H

H

Figure 2. Various sites for graphitic nitrogen: NE represents graphitic nitrogen at the edge bonded to three carbon atoms, NC represents graphitic nitrogen at the centre, NE’ represents graphitic nitrogen at the edge bonded to two carbon atoms and one hydrogen atom. Each of this graphitic nitrogen can activate dioxygen in different ways and it yields various activated oxygen species as given in Figure 3. These models have also been reported in previous studies.30, 37, 38, 45 O

O

O

O

(a) N

N

O O

(b) N

N

(c)

O

O

Figure 3. Schematic representation of activated oxygen species formed by (a) NE, (b) NC and (c) both NE and NC and they are represented as OE, OC and OK respectively.


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All the above mentioned activated oxygen species (AOS) can activate O–H bonds present in alcohols and convert it to aldehyde, ketone or acid through different reaction mechanisms. The overall kinetics of the oxidation reaction is the synergistic effect of oxidation by all activated oxygen species. Hence, the turn over number (TON) depends on the barrier heights of reaction and the ratios of individual AOS (i.e. OE:OC:OK ). Therefore, both the manipulation of optimal control in the reaction yield and TON require the knowledge of mechanistic details of these simultaneous parallel reactions.

The above

arguments prompted us to systematically investigate the following points: 1. To understand and quantify the activation pathway of dioxygen by each graphitic nitrogen species and the formation of AOS. 2. To explore the mechanism of oxidation of RCH2OH group to RCHO group (R=benzyl and methyl) by individual AOS using the state of the art transition state search methods.

2.

Computational Details All the geometries were optimized without any constraints using M06-2X/6-31+g** level

of theory using Gaussian 09 software package.46-52 The energies reported were Gibbs free energy as obtained by single point frequency calculations at M06-2X/6-311+g** level of theory. The minima on the potential energy surface were confirmed by the absence of imaginary frequency and the transition states were characterized by the single imaginary frequency along the reaction mode. Fukui functions53-55 were calculated using Dmol3 software package56,57 at B3LYP58,59/DND level of theory. In our earlier investigation30, we used coronene as a model to mimic graphene due to computational restrictions. However, numerous computational studies have been previously undertaken to study the properties of graphene using different model systems.60-63 For example, Sailaja et al. have studied the site selectivity of graphene nanoflakes using various model structures and reactivity descriptors.64 5 ACS Paragon Plus Environment


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In this study, various models of graphene as reported in previous studies were taken to benchmark the catalytic properties of graphene.61,63,65 The models chosen for the present study are presented in Figure 4. Both hydrogenation energies (EH) and band gap of graphene were used as the screening parameter to select suitable graphene model for further investigation.

Model-1 (M1)

Model-2 (M2)

MF=C24H12

MF=C54H18

Model-3 (M3)

Model-4 (M4)

MF=C48H18

MF=C42H18

Figure 4. Different models of graphene considered for the present investigation. MFMolecular Formula To account for the catalytic properties of graphene, the ability of lattice carbons to interconvert between sp2 and sp3 hybridization should be taken into consideration. Further, the band gap of truncated models should be comparable to that of graphene. The best way to evaluate the catalytic properties between graphene and truncated models is to compare their hydrogenation energies, which in turn compares its ability of lattice carbons to interconvert between sp2 and sp3 hybridization. In order to obtain the hydrogenation energies of graphene, 6 ACS Paragon Plus Environment

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periodic zig-zag nanoribbon (zGNR) was chosen to account for the edge effects. The zGNR structure was optimized using CPMD package66 at hybrid PBE67 level of theory using plane wave basis set with norm conserving Troullier –Martins pseudopotential68 with a cutoff of 90 Rydberg in a unit cell with dimensions of 9.8×25×12 Å3. Then zGNR was symmetrically hydrogenated at each carbon from edge to center. The position of hydrogenation and their respective hydrogenation energies (EH) in kcal/mol are shown in Figure 5.

H

H

Periodic Direction

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4 5

H

H

2 3 H

H

1

H

Position 1 2 3 4 5

EH -63.5 86.87 57.78 78.36 62.11

H

Figure 5. Schematic representation of zGNR along with position of hydrogenation and their respective hydrogenation energies (EH) in kcal/mol. The optimized geometries corresponding to zGNR and their respective hydrogenated forms are provided in supporting information (Figure S1). It is interesting to note that the hydrogenation energies at the edges are negative or exothermic whereas it is positive or endothermic at all other sites. This observation is further supported by earlier studies where the chemical reactivity of graphene is dominated at the edges.69-74 This is a very unique property of graphene when compared to other aromatic structures. A model for graphene should necessarily possess this property for studying chemical reactions. Hence for the chosen models as indicated in Figure 4, hydrogenation energies at the edges and centre were calculated at hybrid PBE67/6-31g* level of theory. The position of hydrogenation and the



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respective hydrogenation energies are depicted in Figures 6-9. The optimized geometries corresponding to these structures are given in Supporting Information (Figure S2-S5).

1

Position M1(1) M1(2) M1(3)

3

EH 28.38 52.84 57.11

2

Figure 6. Schematic representation of Model-1 (coronene) along with position of hydrogenation and their respective hydrogenation energies (EH) in kcal/mol.

3

4

1

Position M2(1) M2(2) M2(3) M2(4) M2(5) M2(6)

5

6

2

EH 37.8 5.44 68.12 70.07 78.75 43.03

Figure 7. Schematic representation of Model-2 (circumcoronene) along with position of hydrogenation and their respective hydrogenation energies (EH) in kcal/mol.


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1

Position M3(1) M3(2) M3(3) M3(4) M3(5) M3(6) M3(7) M3(8) M3(9)

4 5 3

6 7

9

2

8

EH 46.84 81.45 -31.64 82.97 50.96 62.68 47.83 104.07 22.97

Figure 8. Schematic representation of Model-3 along with position of hydrogenation and their respective hydrogenation energies (EH) in kcal/mol.

2

Position M4(1) M4(2) M4(3) M4(4) M4(5) M4(6) M4(7) M4(8)

4 8 6 7

3 5 1

EH -20.32 15.70 110.86 37.37 40.98 66.98 15.01 50.92

Figure 9. Schematic representation of Model-4 along with position of hydrogenation and their respective hydrogenation energies (EH) in kcal/mol.

The hydrogenation energies reported are not corrected for zero-point energy and entropy. Hence, they cannot be compared with the exact energies of respective compounds. It is used to compare EH values of the chosen models with that of zGNR. From Figures 6-7, it can be noted that the hydrogenation energies are all positive at the edges except for Model-3 and Model-4. However for Model-3, only one of the edge carbons shows negative hydrogenation energy and hence Model-4 appropriately reflects the properties of graphene edges. Among other models, the pattern of hydrogenation energies of Model-4 is akin to that of zGNR. A mentioned earlier, it is also necessary to compare the band gap of chosen models with that of zGNR to rationalize the selection of models. The band gap obtained for zGNR in 9 ACS Paragon Plus Environment


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this study is about 0.35 eV, whereas for Models 1-4, it is equal to 4.35, 3.07, 1.37 and 1.23 eV, respectively. Model-4 has the lowest band gap when compared to all other models. Hence, based on the hydrogenation energies and band gap, Model-4 has been chosen for further study.

3.

Results and Discussion

3.1. Dioxygen Reduction by N-doped Graphene To study the formation of activated oxygen species by N-doped graphene, Model-4 has been taken and doped with nitrogen atom at the edge and at the center as displayed in Figure 10.

` (1) NC

(2) NE

(3) NE’

Figure 10. The optimized geometries of the N-doped ribbon at (1) centre, (2) edge-tertiary and (3) edge-secondary. The site of doping is decided based on chemical intuition and also the information obtained from previous studies. Structures (1), (2) and (3) were then interacted with O2 molecule. When the dioxygen in its triplet state is interacted with the N-doped graphene with one unpaired electron, the resulting product can either be doublet or quartet state as shown in Figure 11.


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Reduction

NG( )···O2( ) NG( ) + O2( )

NG‒O2( ) doublet

NG( )···O2( )

NG( )‒O2( ) quartet

I -Anti-ferromagnetic interaction II-Ferromagnetic interaction

Figure 11. Schematic representation of the various possible interaction and bond formation between N-doped graphene (NG) and dioxygen.

The optimized geometries with dioxygen in singlet state and their respective reaction free energies (∆GR) are given in Figure 12. Bond Length (Å) C1‒O1 1.55 O1‒O2 1.29 N1‒C1 1.45

NC(O2) ∆GR =-7.98 kcal/mol

Bond Length (Å) C1‒O1 1.49 O1‒O2 1.30 N1‒C1 1.44

NE1(O2) ∆GR =-35.5 kcal/mol

Bond Length (Å) C1‒O2 1.48 C2‒O1 1.44 O2‒O1 1.45 C1‒N1 1.43 C2‒N1 1.43

NE2(O2) ∆GR =-69.62 kcal/mol 11 ACS Paragon Plus Environment


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Bond Length (Å) C1‒O1 1.34 O2‒O1 1.30 C1‒N1 1.45

NE3(O2) ∆GR =-12.89 kcal/mol

Bond Length (Å) N1–O1 3.13 N1–O1 3.00 O1–O2 1.19

NE’(O2) ∆GR =40.69 kcal/mol Figure 12. Optimized geometries of the various possible interactions between NC, NE and NE’ and dioxygen in doublet state. The relative energy differences between the quartet and doublet states are -9.31, -1.12, -56.49, -1.78 and 3.87×10-3 kcal/mol for NC(O2), NE1(O2), NE2(O2), NE3(O2) and NE’(O2). This indicates that the dioxygen reduction to its singlet state occurs preferentially in NC(O2), NE1(O2), NE2(O2) and NE3(O2) with respect to other substrates. Previous studies have shown that NE1(O2) subsequently converts into NE2(O2), which is thermodynamically more stable. Therefore for further study NC(O2), NE2(O2) and NE3(O2) were taken into account. Later, the formation of activated oxygen species (AOS) was studied using these three systems. NE2(O2) contains the most reduced form of dioxygen and hence it does not undergo any further dissociation reaction. However for NC(O2) and NE3(O2) models, the reduced form of dioxygen can further undergo dissociation to form epoxide and ketone as depicted in Figure 13.


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T1(NC/E3(O2))

O O

O O

N

N

I1(NC/E3(O2)) E(NC/E3(O2)) T2(NC/E3(O2))

O N O

PRODC/K

Figure 13. Schematic representation for the dissociation of dioxygen after bonding to the carbon adjacent to graphitic nitrogen. Figure 13 presents the possible dissociation mechanism of reduced dioxygen over Ndoped graphene. The reaction profiles corresponding to the dissociation of dioxygen in NC(O2) and NE3(O2) are presented in Figure 14.

40

Relative Free Energy (kcal/mol)

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32.3

29.5

30

NC(O2) NE3(O2)

23.7

20

14.4 9.28

10

0

2.52

5.84

0.0

-6.9 -10

E(NC)

T1(NC)

I1(NC)

T2(NC)

PRODC

E(NE3)

T1(NE3)

I1(NE3)

T2(NE3)

PRODK

Reaction Coordinate

Figure 14. Reaction profile for the dissociation of dioxygen for NC(O2) and NE3(O2) model



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In the whole text, E, T, I, P/PROD and Sub represents encounter complex, transition state, intermediate, product and substrate respectively. The optimized geometries of corresponding to the energy profile in Figure 14 are shown in Supporting Information (Figure S6). From the profile, it can be noticed that the ketone formation in NC(O2) is thermodynamically favorable but kinetically less feasible. On the other hand, the formation of ketone species in NE3(O2) is kinetically feasible at room temperature but thermodynamically less favorable by about 2.52 kcal/mol. Based on the barrier heights of these two reactions, the formation of AOS in NE3(O2) is chosen for further study for the oxidation of alcohols. The optimized structures of AOS used for the oxidation of alcohol are represented in Figure 15 along with their simplistic representation of active sites and spin densities.

O

O C

≡

C N

C

Spin Density O2 0.710 O1 0.297

SubC

N

≡ O

O

SubE



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O

≡

C C

O C

C

N

C

H


SubK

Figure 15. Optimized geometries of possible structures of AOS after reduction and dissociation of dioxygen over graphene.

The spin density over the AOS indicates that both O2 in SubC and O1 in subK have an unpaired electron and hence reactive for radical abstraction reactions. These three geometries were then interacted with aromatic and alkyl alcohols to understand the oxidative conversion of alcohols into aldehydes.

3.2. Oxidative Conversion of Benzyl Alcohol to Benzaldehyde by SubC To understand the mechanistic pathway for the aerobic oxidation of aromatic alcohols by N-doped graphene, each of the AOS was interacted with benzyl alcohol (BA). At first, the oxidation of BA by AOS present in SubC was carried out. The Fukui index for radical attack calculated for O1 and O2 are 0.029 and 0.052 respectively, indicating that O2 has high tendency for radical attack. The schematic representation of oxidative pathway of BA by AOS in SubC is given in scheme 1.



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Scheme 1. Proposed reaction pathway for the oxidation of BA by SubC. H H O

H

H

O

O

C C

H

C6H5

O

O

H O

H O C

C

N

C N

C

EC

T1C

O O O

C6 H 5

H

C

I1C

C6H5

H

H

C

H C6H5

O

C

I1C

O H

C

C N

C6H5

H

H

O

O

O

C

C

N

C N

C

T2C

H

PC

In Scheme 1, a simplistic representation for SubC is given. EC represents the interaction between SubC and BA. The optimized geometries corresponding to the encounter complex, transition states, intermediates and products provided in Supporting Information (Figure S7). The energy profile for the proposed reaction is given in Figure 16. Since O2 in SubC has tendency for radical reactions, the hydrogen atom (H1) belongs to the hydroxyl group of BA is transferred to O2 of SubC through the transition state T1C to form O1‒H1 bond. The barrier height for this hydrogen transfer reaction is 24.7 kcal/mol. In the intermediate I1C, hydrogen (H1) interacts with the oxygen of BA (O3) of SubC through hydrogen bond and it is higher in energy with reference to EC by about 18.1 kcal/mol. The next step is the transfer of hydrogen (H2) of aliphatic C‒H bond of BA to either O1 or O2 to complete oxidation. The transfer of H2 to O2 will result in water; however the transfer to O1 results in H2O2 formation.


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60

44.7 Relative Free Energy (kcal/mol)

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35.9

40

24.7 18.1

20 0

21.9 0.0

-20 -40

-50.6

-60

-67.6

-80

EC

T1C

I1C

T2C/I2C

PC/T3C

P2C

Reaction Coordinate

Figure 16. Reaction profile for the proposed oxidation reaction of BA by SubC.

The black line in Figure 16 represents the formation of H2O2 and red line depicts formation of H2O. The barrier height for H2O2 formation is 35.9 kcal/mol and the product is stable by -50.6 kcal/mol when compared to reactants. In the case of water formation, the intermediate I1C rearranges in such a way that H2 is directed towards O2. Then the migration of H2 to O2 occurs with a barrier height of 44.7 kcal/mol. The product, in the case of water formation, is thermodynamically stable by

-67.6 kcal/mol compared to reactants. However,

the higher barrier height indicates that the water formation is kinetically less feasible than H2O2 formation.

3.3. Oxidative Conversion of Benzyl Alcohol to Benzaldehyde by SubE Next, the conversion of benzyl alcohol to benzaldehyde by AOS in SubE has been studied. The AOS in SubE is of organic peroxide type, which dissociates at the O‒O bond to give two alkoxy radicals. There are two ways for the interaction of BA with AOS: (1) the phenyl ring of BA forms π‒π stacking with the graphene ribbon or (2) the phenyl ring lies



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outside the plane of graphene ribbon. The stacked structure is more stable than the other one by about 3.8 kcal/mol and hence the π‒π stacked structure was taken for further study. The possible reaction scheme is provided in Scheme 2. The optimized geometries corresponding to Scheme 2 is given in Supporting Information (Figure S8). The energy profile for the oxidation of BA by SubE corresponding to the mechanism proposed in Scheme 2 is shown in

Figure 17. In the first step, the hydrogen (H1) of hydroxyl group of BA is interacted with O2 of SubE. Then the transfer of H1 to O2 is followed by using transition state search algorithms. The barrier height for the transfer is 36.8 kcal/mol. The higher barrier height could be attributed to the high thermodynamic stability and less reactivity of AOS. In the intermediate

I1E, the unpaired electron is located over the hydroxyl oxygen (O3) of BA (spin density=0.889). For the oxidation to complete, the aliphatic hydrogen (H2) of BA should relocate to O1 of SubE. The transition state corresponding to this transfer lies at 29.9 kcal/mol above EE. The product formed is highly thermodynamically stable by -69.6 kcal/mol with respect to EE. At this point, the oxidation of benzyl alcohol to benzaldehyde is complete. However, the activated oxygen species are reduced to two hydroxyl group. In order for the active site to regenerate, the intermediate I2E should either react with another BA or undergo condensation reaction to form water. Since the hydroxyl groups of graphene have very low oxidation potential, the condensation reaction was carried out. In I2E, H2 is hydrogen bonded to O2 with a distance of 1.96 Å. The barrier height for the migration of H2 to O2 is 29.8 kcal/mol compared to I2E. During this transfer, C2‒N1 bond breaks to form an aldehyde leading to the formation of pyridinic nitrogen and thermodynamically unstable by 21.4 kcal/mol. The higher barrier height and thermodynamic instability indicate that the condensation reaction may not be probable. However, it gives one of the possibilities for the disruption to the active sites at the edges of graphene.


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Scheme 2. Proposed reaction pathway for the oxidation of BA by SubE

N

N O

N

O

O

O

O

H

H

H

O

C CH 6 5 H

O

EEC E

II1E2E

O

H

H C CH 6 5 H

O

H C CH 6 5 H

T1E

I1E

N

N

O

O H

H

H

O C

O

O

H

O

C6H5

C

H

C6H5

H

T2E

I2E

N

N

I2E O

O H

H O

O

O

H

H

C

C6H5

O

C H

H

T3E

PE


C6H5


36.8

40


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22.9

29.9

20

0

0.0

-20

-39.8

-40

-48.2

-60

-80

-69.6 EE

T1E

I1E

T2E

I2E

T3E

PE

Reaction Coordinate

Figure 17. Reaction profile for the proposed oxidation reaction of BA by SubE.

3.4. Oxidative Conversion of Benzyl Alcohol to Benzaldehyde by SubK The AOS present in SubK is of ketonic type with an unpaired electron (spin density=0.61). The Fukui index for radical attack is 0.024 for O1 in SubK indicating that O1 is more reactive and initiates the oxidation reaction. The possible mechanism for the oxidation of BA to BZ is schematically represented in Scheme 3. The optimized structures corresponding to Scheme 3 are shown in Supporting Information (Figure S9). The reaction profile for the same is given in Figure 18. Since O1 is more reactive, the hydroxyl group of benzyl alcohol is directly interacted with O1. Then the hydrogen (H1) of the hydroxyl group of BA is transferred to O1 of SubK. The barrier height for this transfer is 6.1 kcal/mol. The low barrier height is attributed to the high reactivity and spin density of O1. During the migration of H1 from BA to O1, the oxygen (O3) of hydroxyl group of BA attacks the carbon (C3) ortho to N1. In the intermediate I1K, O1 and H1 combine to form hydroxyl group whereas O3 and C3 form a bond to give adduct of BA and SubK. The intermediate is highly 20 ACS Paragon Plus Environment

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thermodynamically stable by -32.0 kcal/mol compared to encounter complex between BA and SubK. The unpaired electron is delocalized over the aromatic system. The next step is the transfer of aliphatic hydrogen (H2) of BA to O1. For this to happen, BA is slightly rotated so that H2 interacts with O1 through hydrogen bonding. The new intermediate I2K is more stable than I1K by -3.4 kcal/mol. Then both migration of H2 to O1 and breaking of C3‒O3 bond should occur for the completion of oxidation. These two events may occur either in concerted or step-wise manner. In this case, it is found that C3‒O3 bond breaks prior to the hydrogen transfer. The transition state corresponding to bond breaking is higher than EK by 6.4 kcal/mol. The intermediate I3K is thermodynamically less stable by 32.8 kcal/mol with reference to I2K due to the presence of localized spin density over O3 (spin density=0.898). After the bond breaking, the transfer of H2 to O1 takes place with a barrier height of 26.5 kcal/mol. The products formed are water and benzaldehyde and the free energy of reaction is -39.1 kcal/mol with respect to the encounter complex.



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Scheme 3. Proposed reaction pathway for the oxidation of BA by SubK C6 H5 O H

C

C 6H5 C6 H5

H H

H

O C C

O

C

C

O

C N

C

O

O

C

C

C

C

N

C

C6 H5 H

H H

O

C

H

O

O

C

C

C

C

N

T1 K

H C

H

C

O H

C

O

C

C

C

T2 K

C N

H

I3K H

C 6H 5 C

C

O

H

O

H

O

O O C C

C

C N

H

C6H5

H

H

C

H

C O

C N

C

C

H

O C

I3K

C

C6H5

H O

O C

H

C

H

T3 K

O

C

C

C

N

PK

22

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O C

H

H O

C

I 2K

C6H 5

C

O

I1K

H C

I2K

H

H

H

H

EK

C

H

C N

H

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30


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26.5

20 10 0

6.4

6.1 0.0

2.6

-10 -20

-32.0

-30

-39.1

-35.4

-40 EK

T1K

I2K

I1K

T2K

I3K

T3K

PK

Reaction Coordinate

Figure 18. Reaction profile for the proposed oxidation reaction of BA by SubK.

3.5.

Comparison between the activity of SubC, SubE, SubK The three AOS are able to convert benzyl alcohol to benzaldehyde. In all the cases,

the oxidation occurs by two one electron transfer mechanisms, wherein the electron transfer occurs along with the transfer of hydroxyl (H1) and aliphatic C‒H (H2) hydrogen. During the first hydrogen transfer, the spin density on AOS decreases and the spin density on substrate increases. The second hydrogen transfer decreases the spin density over the BA and increases the same on graphene. Also the spin density for second hydrogen transfer is always positive and high indicating that the transfer is partly radical in nature. The spin densities over H2 for

T2C, T3C, T2E and T3K are 0.13, 0.18 and 0.12 respectively. A comparative reaction profile with transition state energies are given in Figure 19.



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Figure 19. Reaction profile for the proposed oxidation reaction of BA by SubC, SubE and SubK. The profile reveals a very interesting observation that the most kinetically favorable pathway is the least thermodynamically stable. Although all the three AOS will be present in the surface of graphene, the kinetics of the oxidation is dependent on the concentration of each type of nitrogen position. When the concentration of nitrogen doping is low, the edges are occupied first than the centre.42,75 Hence reaction at the edges will be more preferred. Since the barrier height for the initial activation of BA is high for SubE, the overall oxidation process will be dominated by SubK at room temperature. At elevated temperature and at higher concentration of nitrogen doping, the role of SubC comes into play. Hence at room temperature only SubK dominates the reaction, whereas other AOS will be inhibiting the active nitrogen sites and also limiting the reaction turn over. Experimental studies have shown that the activation barrier for BA oxidation is about 13.4±0.8 kcal/mol.28 Compared to the experimental value, the present study predicts higher barrier height (26.5 kcal/mol) which


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may be due to the solvation effect that may reduce the overall barrier height by hydrogen bonding. It has also been shown in our previous studies that solvent assisted hydrogen and proton transfer can dramatically reduce the overall barrier height of the reaction.30

3.6. Oxidation of aliphatic alcohols by SubC, SubE and SubK To understand the role of aromatic ring in stabilizing the intermediates and reducing the barrier height of the oxidation reactions, aliphatic alcohols were taken for a comparative study. For this study, ethanol was considered as a model and the same mechanisms proposed in the previous sections were carried out. Comparative reaction profiles with all transition state barrier heights are shown in Figure 20.

T1E'

40


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Edge' Centre' Ketonic'

T3K'

T2E' I1E'

20

T2C'

T1C'

T2K'

I1C'

0 EE',K',C'

T1K'

PC' = H2O2 I3K'

-20 T3E'

-40

I1K'

I2K'

PK'

PC' PE'

-60

I2E'

T1E’ =37.6 T1C’ =26.5 T1K’ =5.14 T2E’ =31.0 T2C’ =26.9 T2K’ =06.9 T3E’ =-36.1 T3K’ =30.6

Reaction Coordinate Figure 20. Reaction profile for the proposed oxidation reaction of BA by SubC, SubE and SubK. The optimized geometries are similar to that of their aromatic counterpart and hence the XYZ coordinates of geometries corresponding to Figure 20 are given in Supporting Information (Figure S13-S15). From the profiles, it is clearly noticed that when compared to 25 ACS Paragon Plus Environment


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aromatic alcohols, the initial hydrogen abstraction barrier height for SubC and SubE increases, however for SubK, the same decreases. The barrier height for second hydrogen transfer increases in all the cases except SubC. This indicates that for aliphatic alcohols, the AOS at the centre might play a major role in oxidation reaction. The search for the transition state for water formation in SubC always leads to T2C’. Hence, it is assumed that the formation of water might be improbable due to the low stability of radicals formed. In this case, the AOS formed at the edges (peroxide type) are less reactive but highly thermodynamically stable among other AOS and therefore does not involve in oxidation reactions, atleast at room temperature. From this study, it is arrived that aliphatic alcohols also can undergo oxidation reactions at room temperature in which SubC plays the major role in oxidation of the substrate.

4. Conclusions In this study an attempt has been made to understand the detailed mechanisms of aerobic oxidation of both aromatic and aliphatic alcohols over N-doped graphene. For this, a model for graphene was chosen and the structure and properties of AOS were studied. Ndoped graphene can activate dioxygen to form different AOS depending on the position of Ndoping. Three models for AOS over N-doped graphene were obtained and used for the oxidation of alcohols. The following salient findings emerge from this study: 1. The AOS characterized as peroxide type (SubE) formed at the edges are highly thermodynamically stable and the energy for the conversion of oxygen from its triplet to singlet is very high. Although the formation is stable, the rate of formation is limited by spin cross over. 2. The other two species (SubK and SubC) have low energy differences between the singlet and triplet states of oxygen, leading to the faster formation of the same.


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3. All AOS except SubE exhibit high spin density, which is responsible for their high reactivity towards radical abstraction reactions. 4. All the three AOS can perform oxidation reaction, which is a two one electron redox reaction. However, the overall barrier of the reaction is lower for ketonic oxygen (SubK) for aromatic alcohols and AOS at the centre (SubC) for aliphatic alcohols. The intermediates formed are radical in nature with unpaired electron and they are highly stabilized by radical‒π interaction. 5. The turnover of the product is dependent on the concentration of each AOS. At low concentrations of nitrogen, the edges are highly populated. Since the formation of SubE is relatively slower, SubK dominates the concentration of AOS. This leads to the higher oxidation rate of aromatic alcohols. 6. However, for aliphatic alcohols the oxidation by SubK requires elevated temperature due to its high barrier height. Therefore the concentration of nitrogen doping should be increased so that the centers of graphene are largely populated for the oxidation reaction to occur. The byproduct H2O2 may either oxidize benzaldehyde which leads to over-oxidation or it is decomposed by free nitrogen sites. 7. Since formation of H2O2 is favorable only at high nitrogen doping concentration and at high temperature (high barrier height), over-oxidation is less likely in the case of aromatic alcohols. However, for aliphatic alcohols, SubC is the only form of AOS that oxidizes the alcohol, the combination of low nitrogen dopant concentration and high temperature (high barrier height for SubK) is necessary to prevent over-oxidation. This study throws light on the dependence of concentration of nitrogen doping and temperature on the rate of oxidation of alcohols and the different species involved in the



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oxidation. This work could be very useful for designing new graphene based catalysts to carry out industrially important reactions.

Acknowledgment The authors acknowledge the Multi-Scale Simulation and Modeling project – MSM (CSC0129) funded by CSIR. J. Vijaya Sundar thanks the Council of Scientific and Industrial Research (CSIR), New Delhi for financial support.

Supporting Information Available Structures of optimized geometries reported in this work with relevant geometrical parameters and spin density values. Also the XYZ coordinates for the geometries corresponding to the oxidation of alcohol is also provided. This material is available free of charge via the Internet at http://pubs.acs.org.


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Graphical Abstract


Generalized Reaction Mechanism for the Selective Aerobic Oxidation

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