O2 Activation and Oxidative Dehydrogenation of Propane on

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

O Activation and Oxidative Dehydrogenation of Propane on Hexagonal Boron Nitride: Mechanism Revisited Hong-ping Li, Jinrui Zhang, Peiwen Wu, Suhang Xun, Wei Jiang, Ming Zhang, Wenshuai Zhu, and Hua-ming Li J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10480 • Publication Date (Web): 08 Jan 2019 Downloaded from http://pubs.acs.org on January 9, 2019

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The Journal of Physical Chemistry

O2 Activation and Oxidative Dehydrogenation of Propane on Hexagonal Boron Nitride: Mechanism Revisited

Hongping Li,a Jinrui Zhang,b Peiwen Wu,b Suhang Xun,c Wei Jiang,a Ming Zhang,a Wenshuai Zhu,*b and Huaming Li*a

a Institute b

for Energy Research, Jiangsu University, Zhenjiang 212013, P. R. China

School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang

212013, P. R. China c

School of the Environment and Safety Engineering, Jiangsu University, Zhenjiang

212013, P. R. China

*Corresponding author: Tel.:+86-511-88791800; Fax: +86-511-88791708; E-mail address: [email protected] (H. M. Li), [email protected] (W. S. Zhu)

Abstract: Since the first report of application of hexagonal boron nitride (h-BN) in oxidative dehydrogenation of propane (ODHP), numerous efforts have been devoted to the exploration of the catalytic mechanism for rational design of highly-active catalysts. The present work focuses on the mechanism of O2 activation and oxidative dehydrogenation of propane (ODHP) on h-BN by using the density functional theory (DFT). The armchair, boron atom terminated zigzag (zig-B) and nitride atom terminated zigzag (zig-N) edges were selected as the hypothetical active sites. 1

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Computational results show that the dissociative adsorption of O2 is more favorable than the molecular adsorption at all selected sites. For the mechanism of ODHP, our DFT results show that the zig-B edge is the most active sites based on kinetic and thermodynamic analysis. In addition, a possible competitive reaction model (C-C bond break vs. dehydrogenation) on the high selectivity to propene has been proposed. We propose that h-BN material with rich zig-B edges can enhance the ODHP activity based on the current results. Last, quantum analysis methods, such as electron density difference, charge distributions, and frontier orbitals have also been used to interpret the chemical nature of ODHP reaction.

1. Introduction The oxidative dehydrogenation of propane (ODHP) to propene is an important topic in catalysis and chemical industry, owing to aiming the value-added and synthetically useful chemicals and to reduce environmental pollution.1-4 The scientific challenge of ODHP process remains in the high propane conversion and high propene selectivity. Previous studies showed that the vanadium oxides and cerium oxide catalysts possess the most promising activity for ODHP.2,

4-6

However, the most

challenge for those catalysts is the low selectivity to propene, since the propane are more thermodynamically favored to be oxidized to CO or CO2. Recently, Grant et al. reported that hexagonal boron nitride (h-BN) is a charming metal-free catalyst for ODHP with a comparable catalytic performance with commercial vanadium oxide catalysts.1 2


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Understanding of the ODHP mechanism is crucial to developing more efficient catalyst and thus accelerating the process of commercial implementation. The ODHP reaction mechanism on the transitional metal oxides is generally regarded to follow the Mars–Van Krevelen (MvK) redox mechanism.7-8 Chen et al. proposed that the propane firstly weakly interacted to the M-O site, and then dehydrogenated with the help of lattice O atoms.9 For h-BN catalyst, the thermodynamic analysis of ODHP has also been explored.1,

10-11

Grant et al. also suggested that the oxygen species could

assist the dehydrogenation step.1 However, the kinetic behavior does not obey the MvK mechanism. They proposed that the active site for ODHP lies on the armchair edge of h-BN with molecular adsorption of O2 based on experimental and thermodynamic results by DFT study. In addition, they suggested that the dehydrogenation initiates by the abstraction of hydrogen atom from a secondary carbon of propane by the B-O-O-N sites, and further it breaks the O-O bond to form a B-OH species and a nitroxyl radical. However, very recently, we noted that Lu et al. argued that the B-OH groups on BN edge formed by B-O-B sites played the key role in triggering the ODHP.10-11 They also believed the dissociative adsorption of molecular oxygen is involved in the ODHP process. These two different mechanisms attract researchers’ interest. We noted that in Grant’s article, only the thermodynamic analysis on the armchair edge was calculated, whereas the activation energies and reaction pathways on armchair edge and other possible edges were not considered. For example zig-B and zig-N edges have been proposed to be the active sites in other reactions.12-15 Here, inspired by the charming catalyst and expecting a deep discussion 3



of the complex mechanism, the chemistry of armchair, zig-B and zig-N edge for ODHP was revisited and systematic studied by DFT. The outline of present work is organized as follows. First, the structures and energetics of O2 molecular and dissociative adsorption on different h-BN edges were calculated to investigate the favorable adsorption sites. Then, the activation free energies, reaction pathways and the selectivity of ODHP were discussed. In the last, the computational results were further analyzed by graphical quantum methods. Our DFT results show that the zig-B edge is more active than other two edges for ODHP. 2. Computational details Theory. The M06-2X functional has been shown to provide broad accuracy for main-group thermochemistry, kinetics, noncovalent interactions, and electronic spectroscopy, which is much better than B3LYP.16 In addition, due to the mean-field electronic structure methods like Hartree−Fock, semilocal density functional approximations do not account for long-range electron correlation (London dispersion interaction), Grimme et al. developed a dispersion correction method to enhance the computational accuracy.17 Hence, in the present work, the Minnesota hybrid meta density functional with dispersion corrected term (M06-2X+D3) was chosen as the routine method for exploring the electronic structures of h-BN and the intermediates during ODHP.17-18 Besides, a triple-zeta basis set with polarized function was selected (6-311G(d,p)) to describe the electronic wavefunction. All the coordinates were fully optimized and no imaginary frequencies were found for the reactants, intermediates, and products. The transition states (TS) were identified by confirming the orientation 4


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of imaginary vibration mode and intrinsic reaction coordinate (IRC) calculations. The temperature and pressure were set to be 763 K and 1 atm, consistent with the reaction condition. Model. The active site for ODHP on h-BN surface was proposed to be the edge sites.1, 10-11 Grant et al. suggested the active sites could be the armchair edge based on the experimental evidences of O-H vibrational frequencies, catalytic activity, and thermodynamic DFT calculations. However, Lu et al. believed that the zig-B edge is the active sites for ODHP.10-11 To model the active sites of h-BN monolayer systematically, three cluster models were selected in this work, including armchair edge, zig-B edge, and zig-N edge, respectively (Figure 1). The left of Figure 1a is the original armchair model without geometry optimization whereas the right of Figure 1a is the finally used model after geometry optimization result in a reconstruction of the edge sites. Figure 1b and Figure 1c are the zig-B and zig-N models. Other edge sites are saturated by hydrogen atoms excepting the active sites. Concepts for discussion. The transitional, rotational, vibrational, and entropy contributions for each structure at reaction condition were estimated by employing the concept of Gibbs free energy (G). Hence, for an elementary reaction, the change of free energy (∆G) and activation free energy (∆𝐺𝑎), were obtained by following formulas: ∆G = 𝐺𝑃 ― 𝐺𝑅

(1)

∆𝐺𝑎 = 𝐺𝑇𝑆 ― 𝐺𝑅

(2)

where the 𝐺𝑃 , 𝐺𝑅 are the free energy of the product and reactant, 𝐺𝑇𝑆 is the free 5



energy of transition state. In addition, the ∆Ga-rds is used to denote the activation free energy for rate determination step. The electron density difference (EDD) and charge distributions analysis were employed to understand the ODHP mechanism.19 All of the calculations were performed by Gaussian 09 program.20

3. Results and discussion 3.1 O2 activation. The O2 activation is an important topic for oxidative catalysis. In the current work, the molecular and dissociative adsorption for single O2 was firstly explored as shown in Figure 2. Figure 2a~2c illustrate the molecular adsorption (mol-ads) of O2 on armchair edge (arm-O2), zig-B edge (zig-B-O2), and zig-N edge (zig-N-O2), respectively. While Figure 2d~2f illustrate the dissociative adsorption (dis-ads) of O2 on above three active sites. The adsorption Gibbs free energies (∆G) were listed in Table 1. It can be seen that ∆G for the arm-O2 is positive (36.3 kcal/mol), whereas ∆G for zig-B-O2 and zig-N-O2 are -141.2 and -14.1 kcal/mol, respectively. Different from the conclusion proposed by Grant et al., we found that the molecular adsorption of O2 at armchair edge site will not spontaneously occur from the thermodynamic view.1 The molecular adsorption of O2 at zig-B edge site is strongly exothermic, while the molecular adsorption of O2 at zig-N edge site is a moderate reaction at such condition. For dissociative adsorption, it is found that the ∆G turns to be more negative on all edges (Table 1), indicating dissociative adsorption are more readily to occur. Especially, the dissociative adsorption at zig-B sites leads to an obvious 6


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deformation of h-BN and a B-O-B local structure is formed (Figure 2e). These findings were also supported by Lu’s experimental evidences.10-11 In addition, models which contain two O2 molecules were also studied for one reason is to study the co-adsorption effect and for the other is that Grant et al. suggested the ODHP may involve two O2 molecules (Figure 3).1 Figure 3a~3c display the molecular adsorption (mol-ads) configurations on armchair, zig-B, and zig-N edges, respectively. Figure 3d~3e are the configurations of the molecular combined dissociative adsorption (mol-dis) of O2 at zig-B and zig-N sites. The structure of the mol-dis adsorption of O2 at armchair sites was not obtained in the current model. Figure 3f~3h are the configurations of the dissociative adsorption (dis-ads) of O2 at zig-B and zig-N sites. It can be seen from Table 1 that the ∆G for the molecular adsorption at armchair sites are quite close to single O2 molecular adsorption (36.2 kcal/mol). Whereas the ∆G for the molecular adsorption at zig-B and zig-N sites doubles more or less (-284.3 and -39.7 kcal/mol). O atoms in molecular

adsorption

may

be

called

“inert

O

atoms”

since

they

are

thermodynamically not favored and with weak bonding capability. It should break the O-O bond first before the oxidative reaction. The dissociative adsorption is still favorable for the three active sites in this model. Hence, the current computational results have shown that the dissociative adsorption of O2 is more favorable than the molecular adsorption at all selected sites which are consistent with the Lu’s argument.10-11 Besides, the zig-B sites are the most active sites for O2 activation. 3.2 Mechanism of ODHP on h-BN 7



Optimized structures. In general, the dehydrogenation of propane may start at two types of H atoms (-CH3 or -CH2-). Previous works have shown that the dehydrogenation on the secondary carbon is the key step for ODHP on transition metal oxides.9, 21-22 Besides, Grant et al. also suggested the dehydrogenation start at the secondary carbon.1 Hence, in the present work, the starting point of the dehydrogenation is also considered to be the secondary carbon. It was noted that the recent experimental evidences show the nitrogen atom is inactive for the ODH reaction of alkane.11,

23-24

Hence, the related Figures and results on zig-N edge site

were placed into Supporting Information. The transition states (TS) for ODHP reaction were optimized based on the two O2 molecules adsorption structures (Figure 4) as suggested by Grant et al.1 Figure 4a~4b and Figure S1g illustrate the transition structures (TS1~TS3) of the first dehydrogenation step on armchair, zig-B, and zig-N edges in which the initial guess for the O2 species was the molecular adsorption states (mol-ads). It is found that only on armchair edge the O2 species remains the molecular adsorption states after dehydrogenation, whereas on zig-B and zig-N edge, one of the O2 molecule turns into dissociative adsorption state. The reactive C-H and O-H bond lengths of TS1 are 1.389 Å and 1.213 Å (Table 2). In TS2, the reactive C-H and O-H bond lengths are 1.166 Å and 1.478 Å. It can be known that the C-H bond length in the TS2 become shorter while the O-H bond length become longer, which is opposite to TS1. Generally speaking, the reactive bond length is related to the bond strength in the TS, the stronger the bond strength is, the longer the bond length in its TS is. This is also called “late” TS or “early” TS.25 Hence, the longer length of O-H in TS2 8


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means a more active state of B-O species as compared with TS1. Figure 4c~4d and Figure S1h illustrate the transition structures (TS4~TS6) of the first dehydrogenation step on armchair, zig-B, and zig-N edges in which the initial speculation for the O2 species was the dissociative adsorption states (dis-ads). The reactive C-H and O-H bond lengths for these TSs are similar with TS1~TS3 (Table 2). Figure 4e~4g and Figure S1i (TS7~TS9) show the transition structures of the second dehydrogenation step on armchair, zig-B, and zig-N edges, respectively. It is noted that TS7 (Figure 4e) was based on the molecular adsorption state whereas the TS7a was based on the dissociative adsorption state on armchair edge. The structures of these two TSs are quite different. Figure 5 and Figure S1a~S1f display the intermediates structures during ODHP reaction. Figure 5a~5b and Figure S1c are the intermediates for the first dehydrogenation step on armchair, zig-B, and zig-N edges. The hydrogen atom is preferred bonding to the O atom for all the situations. The CH3-CH*-CH3 (isopropyl) radical is bonded to the O atom on zig-B edge whereas on armchair edge, it is bonded to the B atom. The reason is that the O atom on armchair edge is the molecular state, preventing to form the C-O bond. Figure 5c~5d and Figure S1e are the intermediates for the second dehydrogenation step on armchair, zig-B, and zig-N edges. The CH2*-CH*-CH3 radical is bridged bonding to the O atoms on all of the situations. Whereas Figure 5e~5f and Figure S1f are the desorption structures for propene. These weak interactions can be classified into typical π-π interactions as referring to our previous works.26-28 9



Activation free energies and favorable reaction pathways. The activation free energies (∆𝐺𝑎) and favorable reaction pathways are the important topics in the present work. All of the ∆𝐺𝑎 values and reaction pathways of the three models were plotted in Figure 6 (armchair edge and zig-B edge model) and Figure S2 (zig-N edge model). The reference state for all the reaction pathways is the gas phase O2(g) + C3H8(g). In Figure 6, as mentioned above, the TS1 corresponds to the transition state of the first dehydrogenation step based on molecular adsorption state of O2. The relative free energy of TS1 is 116.1 kcal/mol, which means the ∆𝐺𝑎 is 79.9 kcal/mol for this elementary reaction. The relative free energy of the first intermediate, namely IM-arm-1 is 60.6 kcal/mol, which indicates that it is an endothermic reaction (∆G = 24.4 kcal/mol). Then the IM-arm-1 can readily dehydrogenate into IM-arm-2 (relative free energy is -58.9 kcal/mol) via TS7. Besides, there is another reaction pathway starting from the dissociative adsorption state of O2 (-41.1 kcal/mol). Then the first dehydrogenation step proceeds via TS4 (∆𝐺𝑎 = 45.7 kcal/mol). The relative free energy of TS4 is 4.6 kcal/mol, indicating this pathway is more reactive than the TS1. IM-arm-1a, an important intermediate for the formation of propyl radical, is obtained via TS1 (∆G = -23.1 kcal/mol), which is an exothermic reaction. The stabilization of the propyl radical was suggested to be the key point for the high propene selectivity and suppression of over-oxidation.1 The IM-arm-2 can be also formed by a further hydrogenation via TS7a (∆𝐺𝑎 = 49.2 kcal/mol). The IM-arm-2 (Figure 5c) is a precursor for the desorption state of propene, namely IM-arm-3 (Figure 5e). This 10


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elementary reaction goes through a two C-O bonds break, and possesses the highest activation free energy (∆Ga-rds = 53.1 kcal/mol) along the reaction pathway (via TS4 and TS7a). Then, the product can be readily desorbed from the BN surface (∆G = -13.1 kcal/mol). In summary, the results and discussions of ODHP on the armchair edge show that the dissociation adsorption state of O2 is superior to that of molecular adsorption state in terms of kinetics and thermodynamics. In Figure 6, we creatively plotted the ODHP reaction pathways on zig-B and armchair edge together on h-BN surface. On zig-B edge (blue lines), after the adsorption of O2, the starting point of ODHP is located at a relatively low energy level. The mol-dis adsorption O2 configuration can form IM-zig-B-1a through TS2 (∆ 𝐺𝑎 = 28.8 kcal/mol) while the dissociative adsorption O2 configuration can form IM-zig-B-1 through TS5 with a relatively high activation free energy (∆𝐺𝑎 = 84.5 kcal/mol). For above two elementary reactions, the isopropyl radical is consequently, rapidly combined with the B-O site, which is consistent with the radical rebound mechanism.1 Besides, the IM-zig-B-1 and IM-zig-B-1a are relative stable along the reaction pathway and more stable as compared with other two edge models. Grant et al. suggested that the prevention of the highly reactive propyl radical is the key point to understand the high propene selectivity and suppression of over-oxidation.1 Here, we suggest that the high stability of the above complexes formed by isopropyl radical may be related to the high selectivity of propene. Then these two intermediate can further dehydrogenate into IM-zig-B-2 via TS8 (∆𝐺𝑎 = 65.2 kcal/mol). IM-zig-B-2 is a precursor for the desorption state of propene, namely IM-zig-B-3 (Figure 5f), which 11



is the most stable species in the current system. Similar to IM-arm-3, this elementary reaction goes through a two C-O bond break, and possesses the highest activation free energy (∆Ga-rds = 98.2 kcal/mol). Finally, the IM-zig-B-3 can readily desorb to form propene and zig-B-2OH that contains two B-OH groups, which was ever proved to be the key intermediates in triggering the ODHP process.10-11 The activation free energies for each dehydrogenation step on different models were further listed in Table 3. Based on above results, it can be known that on armchair edge, the energy level of ODHP reaction proceeds nearly at the entrance level (0.0 kcal/mol, Figure 6). The potential energy well is relative shallow as compared with other models although the activation free energy for rate determination step is moderate. On zig-B edge, the energy level of ODHP reaction is the lowest, indicating that it holds the strongest driven force to the dehydrogenation steps although the ∆Ga-rds is the highest among all the hypothetical active sites. Hence, it would be the most favorable reaction pathway. On zig-N edge, the energy level of ODHP reaction is the moderate and the ∆Ga-rds is the lowest, showing that it is more favorable than on arm-edge. It was noted that Grant’s theoretical model was mainly based on the experimental and theoretical vibrational frequencies of -OH group.1 The infrared spectra would not be contrary to our mechanism because various kinds of OH groups may be co-existed on different edges. Lu et al. proposed a mechanism started at B-OH groups and then the active B-O-O-B sites were formed by the flow of O2 based on a B-O-B zigzag edge. Our results prove that B-O-O-B sites are indeed the active sites.10-11 In our model, the formation of B-O-B site can also be possiblely 12


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existed by re-construction of the zig-B edge (zig-B-2O and zig-B-4O). However, the O atom in B-O-B site is not active because it has no extra bonding ability. When the dehydrogenation step is started, the B-O-B species turn back to the original zig-B structure (zig-B-2O or zig-B-4O). Rate constants. In order to further confirm the current results, a simple reaction rate model based on Boltzman distribution and transition state theory (TST) under steady state approximation was established. The relative concentration of each species is obtained by Boltzmann distribution as follows: exp ( ―

𝐶𝑖 =

𝐺𝑟𝑒𝑓, 𝑖 𝑅𝑇 )

∑𝑖exp ( ―

(3)

𝐺𝑟𝑒𝑓, 𝑖 𝑅𝑇 )

Where the Ci means the relative concentration of a given species which is before the rate determination step, Gref,i means the relative free energy of each species. The relative reaction rate is obtained by Ci multiplying the Eyring equation: 𝑘𝐵𝑇

𝑟𝑖 = 𝐶𝑖

ℎ

exp (

―∆𝐺𝑎,𝑖 𝑅𝑇

(4)

)

Where the ri means the relative rate constant for each elementary reaction. kB is the Boltzmann constant, and h is the Planck constant. The calculated results were listed in Table 4. It can be seen that the predicted relative reaction rates agree well with the results from free energy profile analysis. The relative reaction rates for the dehydrogenation steps and desorption are all fastest on zig-B edge while the values on the arm edge are the smallest. The relative reaction rates on zig-N edge are moderate among three active sites. In our previous work, we proposed that the zig-N edge was the active sites for oxidative desulfurization on BN surface due to the comparable reaction heat for its reaction heat.12 This is not contrary to the current conclusion 13



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because oxidative desulfurization is an oxygen transfer reaction, which should break its B-O bond. Hence, the break of strong B-O bond makes the oxidative step difficult to occur. The current reaction is a dehydrogenation reaction. And the B-O bond is gradually and readily hydrogenated to B-OH, B-OH2, leading to a warmly break of B-O bond. The activation free energy for the formation of B···OH2 intermediate was calculated to be only 6.8 kcal/mol on zig-B edge. In summary, our calculations prove that the zig-B edge plays an important role in ODHP reaction. 3.3 Possible mechanism on high selectivity to propene The key scientific challenge for ODHP is that the prevention of the facile over-oxidation of propene product into more thermodynamically stable CO and CO2.1 Grant et al. proposed a radical rebound mechanism to interpret the selectivity. They believe that the one-dimensional nature of the h-BN edges avoids the creation of a highly reactive propyl radical, and the over-oxidation of the adsorbed species is thusly suppressed. We also believe that this is the chemical origin for the high selectivity to propene due to the high stability of isopropyl radical. Here, we try to make further discussion about the selectivity. In principle, the key point to the high selectivity to propene and low selectivity to CO and CO2 lies in the competition of C-C bond break, degree of dehydrogenation, and desorption of propene (Scheme 1).

CH3CHCH3

-H

CH2CHCH2

*

*

desorption

*

C-C bond break CH3CH* + CH3 *

*

CH3CH* + CH2* *

* 14


propene

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Scheme 1. Selectivity route for ODHP on zig-B edge. The above scheme starts with isopropyl radicals rather than propane because it is difficult to break C-C bonds directly from propane.22 Hence, the first competitive reactions are the 2nd dehydrogenation and C-C bond break. The activation free energies and free energy changes for C-C bond break at zig-B edge were calculated (Figure S3). It can be seen that the C-C bond break is not preferred from both the kinetic and the thermodynamic aspects. Hence, this chemical activity feature may be important to the selectivity. Further, the second competitive reactions are the desorption of propene and C-C bond break of propene. We propose that the C-C bond break should not be occurred because the products are both di-radicals, which will be recombined quickly in the reaction condition. As last, the desorption of propene is a spontaneous reaction (∆G = -16.9 kcal/mol), which is beneficial to the desorption. However, the high selectivity to propene and low selectivity to CO and CO2 maybe very complicated. We hope this discussion can offer some useful information to the selectivity.

3.4 Quantum analysis on the ODHP reaction Electron density difference. The electron density difference (EDD) analysis is a useful method to estimate the density change of interesting systems.29-31 Figure 7a~7c plots the EDD map of the dissociative adsorption species, namely arm-4O, zig-B-4O and zig-N-4O with the help of Multiwfn program.32 The EDD maps were obtained by differentiate the final density after adsorption and initial density when O atoms are not 15



bonding to B or N atoms. The purple areas in Figure 7 mean the increase of electron density, whereas the green areas mean the decrease of the electron density. It can be seen that the electron density of O atoms increases in arm-4O. The electron density mainly deceases between N-O bonding areas. Similarly, in Figure 7b and Figure 7c the electron densities of O atoms are both increased. All the figures have shown that there is a charge transfer from BN to O atoms during O2 activation and dehydrogenation reactions. Besides, the EDD maps of TS4, TS5, and TS6 for the first dehydrogenation step were plotted in Figure 7d~7f. EDD results have also shown a charge transfer from C-H bond to the reactive O atom. Charge distributions. In order to quantitatively obtain the information of charge transfer and interpret the activation free energies during the dehydrogenation. The natural population analysis (NPA) was implemented by natural bond orbital scheme (NBO). The NPA atomic charges in O2, C3H8, and above mentioned species were listed in Table S1. It should be noted that only the charges of related reactive atoms were listed. From Table S1, we can know the initial charges of O, H, C atom are 0.000, 0.188, and -0.375, respectively. After dissociative adsorption, the charges of reactive O atoms in three species are -0.109, -0.855, and -0.209, which shows a charge transfer from BN to the O atom, well agrees with the results discussed in EDD map. In TS4 and TS6, it is found that the negative charge of O atom increases. However, the negative charge of O atom in TS5 decreases. The electron rich state of O atom in zig-B-4O may be related to the higher activation free energy for ODHP (Table 3). 16


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Frontier orbitals. To further understand the reactivity of ODHP, the frontier orbitals of propane and the dissociative adsorption configurations mentioned above were plotted (Figure S4). As shown in Figure S4, the HOMO of propane mainly consists of C-H σ bonds, whereas the LUMO mainly consists of anti-σ bonds. For the dissociative adsorption configurations, it is found that the frontier orbitals are mainly located near to the O atoms except the HOMO of zig-B edge, which can be used to interpret the relative higher activation free energies for the dehydrogenation reactions.

4. Conclusion The mechanism of O2 activation and ODHP on three edge models of h-BN has been systematic explored by DFT. Computational results have shown that the dissociative adsorption of O2 is favorable than the molecular adsorption at all active sites. Besides, the zig-B edges are the most active sites for O2 activation due to the strong exothermic behavior. For the mechanism of ODHP, our DFT results show that the zig-B edge is the most active sites than zig-N or armchair edges based on kinetic and thermodynamic analysis. The kinetic and thermodynamic advantage of dehydrogenation reaction as compared to the C-C bond break could be the main reason for the high selectivity to propene. We propose that h-BN material with rich B zigzag edges can enhance the ODHP activity based on the current results. And we hope this new understanding will help to develop new and efficient ODHP catalysts.

17



Associated Content Supporting Information I: Other optimized structures. II: Free energy profile on zig-N model. III: Free energy profile for the selectivity. IV: Frontier orbitals of selected species. V: NPA charge distributions of important species. VI: Methods and Cartesian coordinates for the calculations. This information is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Nos. 21808092, 21722604, 2157612221606113) and National Science and Technology Program (2017YFB0306*04-1*). A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. This research work is supported by the high performance computing platform of Jiangsu University. References: 1.

Grant, J. T.; Carrero, C. A.; Goeltl, F.; Venegas, J.; Mueller, P.; Burt, S. P.; Specht, S. E.;

McDermott, W. P.; Chieregato, A.Hermans, I. Selective Oxidative Dehydrogenation of Propane to Propene Using Boron Nitride Catalysts. Science., 2016, 354, 1570-1573. 2.

Hu, P.; Lang, W. Z.; Yan, X.; Chu, L. F.Guo, Y. J. Influence of Gelation and Calcination

Temperature on the Structure-Performance of Porous VOX-SiO2 Solids in Non-Oxidative Propane Dehydrogenation. J. Catal., 2018, 358, 108-117. 3.

Atanga, M. A.; Rezaei, F.; Jawad, A.; Fitch, M.Rownaghi, A. A. Oxidative Dehydrogenation of

Propane to Propylene with Carbon Dioxide. Appl. Catal. B, 2018, 220, 429-445. 4.

You, R.; Zhang, X. Y.; Luo, L. F.; Pan, Y.; Pan, H. B.; Yang, J. Z.; Wu, L. H.; Zheng, X. S.; Jin,

Y. K.Huang, W. X. NbOx/CeO2 Rods Catalysts for Oxidative Dehydrogenation of Propane: Nb-Ceo2 18


Page 18 of 33

Page 19 of 33 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


Interaction and Reaction Mechanism. J. Catal., 2017, 348, 189-199. 5.

Fang, K.; Liu, L.; Zhang, M.; Zhao, L.; Zhou, J.; Li, W.; Mu, X.Yang, C. Synthesis of

Three-Dimensionally Ordered Macroporous Nice Catalysts for Oxidative Dehydrogenation of Propane to Propene. Catalysts, 2018, 8, 19. 6.

Xue, X. L.; Lang, W. Z.; Yan, X.Guo, Y. J. Dispersed Vanadium in Three-Dimensional Dendritic

Mesoporous Silica Nanospheres: Active and Stable Catalysts for the Oxidative Dehydrogenation of Propane in the Presence of CO2. ACS Appl. Mater. Interfaces, 2017, 9, 15408-15423. 7.

Grabowski, R. Kinetics of Oxidative Dehydrogenation of C2‐C3 Alkanes on Oxide Catalysts.

Catal. Rev. - Sci. Eng., 2006, 48, 199-268. 8.

Cavani, F.; Ballarini, N.Cericola, A. Oxidative Dehydrogenation of Ethane and Propane: How Far

from Commercial Implementation? Catal. Today., 2007, 127, 113-131. 9.

Chen, K.; Bell, A. T.Iglesia, E. Kinetics and Mechanism of Oxidative Dehydrogenation of

Propane on Vanadium, Molybdenum, and Tungsten Oxides. J. Phys. Chem. B, 2000, 104, 1292-1299. 10. Shi, L.; Wang, D.Lu, A.-H. A Viewpoint on Catalytic Origin of Boron Nitride in Oxidative Dehydrogenation of Light Alkanes. Chin. J. Catal., 2018, 39, 908-913. 11. Shi, L.; Wang, D. Q.; Song, W.; Shao, D.; Zhang, W. P.Lu, A. H. Edge-Hydroxylated Boron Nitride for Oxidative Dehydrogenation of Propane to Propylene. Chemcatchem, 2017, 9, 1720-1720. 12. Wu, P.; Yang, S.; Zhu, W.; Li, H.; Chao, Y.; Zhu, H.; Li, H.Dai, S. Tailoring N-Terminated Defective Edges of Porous Boron Nitride for Enhanced Aerobic Catalysis. Small, 2017, 13, 1701857. 13. Li, L. H.; Cervenka, J.; Watanabe, K.; Taniguchi, T.Chen, Y. Strong Oxidation Resistance of Atomically Thin Boron Nitride Nanosheets. ACS Nano, 2014, 8, 1457-1462. 14. Lee, K. H.; Shin, H. J.; Kumar, B.; Kim, H. S.; Lee, J.; Bhatia, R.; Kim, S. H.; Lee, I. Y.; Lee, H. S.Kim, G. H. Nanocrystalline ‐ Graphene ‐ Tailored Hexagonal Boron Nitride Thin Films. Angew. Chem., Int. Ed., 2015, 53, 11493-11497. 15. Dean, C. R.; Young, A. F.; Meric, I.; Lee, C.; Wang, L.; Sorgenfrei, S.; Watanabe, K.; Taniguchi, T.; Kim, P.Shepard, K. L. Boron Nitride Substrates for High-Quality Graphene Electronics. Nat. Nanotechnol., 2010, 5, 722-726. 16. Zhao, Y.Truhlar, D. G. Exploring the Limit of Accuracy of the Global Hybrid Meta Density Functional for Main-Group Thermochemistry, Kinetics, and Noncovalent Interactions. J Chem Theory Comput, 2008, 4, 1849-1868. 17. Grimme, S.; Hansen, A.; Brandenburg, J. G.Bannwarth, C. Dispersion-Corrected Mean-Field Electronic Structure Methods. Chem. Rev., 2016, 116, 5105-5154. 18. Zhao, Y.Truhlar, D. G. The M06 Suite of Density Functionals for Main Group Thermochemistry, Thermochemical Kinetics, Noncovalent Interactions, Excited States, and Transition Elements: Two New Functionals and Systematic Testing of Four M06-Class Functionals and 12 Other Functionals. Theor. Chem. Acc., 2008, 120, 215-241. 19. Reed, A. E.; Curtiss, L. A.Weinhold, F. Intermolecular Interactions from a Natural Bond Orbital, Donor-Acceptor Viewpoint. Chem. Rev., 1988, 88, 899-926. 20. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A., et al. Gaussian 09, Gaussian, Inc.: Wallingford, CT, USA, 2009. 21. Du, Y. J.; Li, Z. H.Fan, K. N. A Theoretical Investigation on the Influence of Anatase Support and Vanadia Dispersion on the Oxidative Dehydrogenation of Propane to Propene. J. Mol. Catal. A: Chem, 2013, 379, 122-138. 19



22. Alexopoulos, K.; Reyniers, M.-F.Marin, G. B. Reaction Path Analysis of Propane Selective Oxidation over V2O5 and V2O5/TiO2. J. Catal., 2012, 289, 127-139. 23. Shi, L.; Wang, Y.; Yan, B.; Song, W.; Shao, D.Lu, A. H. Progress in Selective Oxidative Dehydrogenation of Light Alkanes to Olefins Promoted by Boron Nitride Catalysts. Chem. Commun., 2018, 54, 10936-10946. 24. Grant, J. T.; McDermott, W. P.; Venegas, J. M.; Burt, S. P.; Micka, J.; Phivilay, S. P.; Carrero, C. A.Hermans, I. Boron and Boron-Containing Catalysts for the Oxidative Dehydrogenation of Propane. Chemcatchem, 2017, 9, 3623-3626. 25. Kim, S.; Evans, T. J.; Mukarakate, C.; Bu, L.; Beckham, G. T.; Nimlos, M. R.; Paton, R. S.Robichaud, D. J. Furan Production from Glycoaldehyde over HZSM-5. ACS Sustain. Chem. Eng., 2016, 4, 2615-2623. 26. Li, H. P.; Chang, Y. H.; Zhu, W. S.; Zhu, S. W.; Jiang, W.; Zhang, M.; Zhou, Y. W.; Xia, J. X.Li, H. M. The Selectivity for Sulfur Removal from Oils: An Insight from Conceptual Density Functional Theory. AIChE J., 2016, 62, 2087-2100. 27. Li, H.; Zhu, S.; Zhang, M.; Wu, P.; Pang, J.; Zhu, W.; Jiang, W.Li, H. Tuning the Chemical Hardness of Boron Nitride Nanosheets by Doping Carbon for Enhanced Adsorption Capacity. ACS Omega, 2017, 2, 5385-5394. 28. Li, H.; Wang, C.; Xun, S.; He, J.; Jiang, W.; Zhang, M.; Zhu, W.Li, H. An Accurate Empirical Method to Predict the Adsorption Strength for Π-Orbital Contained Molecules on Two Dimensional Materials. J. Mol. Graphics Modell., 2018, 82, 93-100. 29. Li, H. P.; Chang, Y. H.; Zhu, W. S.; Wang, C. W.; Wang, C.; Yin, S.; Zhang, M.Li, H. M. Theoretical Evidence of Charge Transfer Interaction between SO2 and Deep Eutectic Solvents Formed by Choline Chloride and Glycerol. Phys. Chem. Chem. Phys., 2015, 17, 28729-28742. 30. Shigemitsu, Y.; Uejima, M.; Sato, T.; Tanaka, K.Tominaga, Y. Electronic Spectra of Cycl[3.3.2]Azine and Related Compounds: Solvent Effect on Vibronic Couplings. J. Phys. Chem. A, 2012, 116, 9100-9109. 31. Zhang, W. L.; Cheng, W. D.; Zhang, H.; Geng, L.; Lin, C. S.He, Z. Z. A Strong Second-Harmonic Generation Material Cd4BiO(BO3)3 Originating from 3-Chromophore Asymmetric Structures. J. Am. Chem. Soc., 2010, 132, 1508-1509. 32. Lu, T.Chen, F. Multiwfn: A Multifunctional Wavefunction Analyzer. J. Comput. Chem., 2012, 33, 580-92.

20


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original (unoptimized)

reconstructed (optimized) (a) arm

(b) zig-B

(c) zig-N

Figure 1. Theoretical cluster models of h-BN for ODHP. (a) armchair edge model; (b) zig-B edge model; (c) zig-N edge model

21



Figure 2. Molecular and dissociative adsorption of single O2 on armchair, zig-B, and zig-N edge

models. (a) arm-O2; (b) zig-B-O2; (c) zig-N-O2; (d) arm-2O; (e)

zig-B-2O; (f) zig-N-2O.

22


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Figure 3. Molecular and dissociative adsorption of two O2 molecules on armchair, zig-B, and zig-N edge models. (a) arm-2O2; (b) zig-B-2O2; (c) zig-N-2O2; (d) zig-B-O2-2O; (e) zig-N-O2-2O; (f) arm-4O; (g) zig-B-4O; (h) zig-N-4O

23



Figure 4. Transition state structures for ODHP on armchair and zig-B models. (a) TS1; (b) TS2; (c) TS4; (d) TS5; (e) TS7; (f) TS7a; (g) TS8

24


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Figure 5. Optimized intermediates structures for ODHP on armchair and zig-B models. (a) IM-arm-1; (b) IM-zig-B-1; (c) IM-arm-2; (d) IM-zig-B-2; (e) IM-arm-3; (f) IM-zig-B-3

25



Page 26 of 33

116.1 TS1 57.7

36.2 mol-ads

0.0

60.6 IM-arm-1

TS7

4.6

0.0

Ga-rds=53.1

TS4

2O2 (g) + C3H8 (g)

-5.8 IM-arm-3

-15.0 TS7a

desorption -18.9

-41.1

C3H6 (g) + arm-2OH

dis-ads -58.9 IM-arm-2

-64.2 IM-arm-1a

arm sites TS2 mol-dis -260.9

-232.1 TS5

-208.5

-268.3

mol-ads -284.3 -293.0 dis-ads

TS8 IM-zig-B-1a

Ga-rds=98.2

-310.7

IM-zig-B-3 -314.5

-333.5 IM-zig-B-1

zig-B sites

-412.7 IM-zig-B-2

desorption -331.4 C3H6 (g) + zig-B-2OH

Figure 6. Free energy profile of ODHP reaction pathways on armchair, zig-B, and zig-N edge models at 763 K. (unit: kcal/mol)

26


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Figure 7. Electron density difference (EDD) map for dissociative adsorption of O2 and TS4~TS6 on armchair, zig-B, and zig-N models. Purple is positive, green is negative (isovalue=0.02 au for a~c, 0.01 au for d~f)

27



Page 28 of 33

Table 1. Molecular and dissociative adsorption Gibbs free energies of O2 on armchair, zig-B, and zig-N models at 763 K. (unit: kcal/mol)

O2

2 O2

adsorption type

armchair

zig-B

zig-N

mol-ads

36.3

-141.2

-14.1

dis-ads

-13.0

-253.9

-60.3

mol-ads

36.2

-284.3

-39.7

mol-dis-ads

/

-260.9

-64.3

dis-ads

-41.1

-293.0

-86.7

28


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Table 2. The C-H and O-H bond lengths for the transition states. (unit: Å) TS1

TS2

TS3

TS4

TS5

TS6

TS7

TS7a

TS8

TS9

C-H 1.389

1.166

1.291

1.338

1.165

1.301

1.250

1.581

1.421

1.364

O-H 1.213

1.478

1.217

1.187

1.481

1.216

1.323

1.049

1.244

1.155

29



Table 3. Activation free energies for dehydrogenation steps. (unit: kcal/mol) armchair

zig-B

1st dehyrogenation

45.7

84.5

2st dehyrogenation

49.2

65.2

30


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Table 4. Relative concentration and reaction rates on arm, zig-B, and zig-N edge models. Intermediates reaction dis-arm IM-arm-1a IM-arm-2 dis-zig-B IM-zig-B-1 IM-zig-B-2 dis-zig-N IM-Zig-N-1a IM-zig-N-2

1st H 2nd H desorption 1st H 2nd H desorption 1st H 2nd H desorption

relative stability (kcal/mol) -41.1 -64.2 -58.9 -293 -310.7 -412.7 -86.7 -135.6 -178.9

relative concentration a 3.61E-107 1.49E-100 4.53E-102 5.16E-35 6.06E-30 1.00E+00 4.16E-94 4.23E-80 1.07E-67

a

∆Ga (kal/mol) 45.7 49.2 53.1 84.5 65.2 98.2 19.4 25.7 43.2

relative reaction rate 4.65E-107 1.91E-101 4.43E-104 5.12E-46 2.03E-35 1.18E-15 1.83E-86 2.92E-74 7.18E-67

the relative concentration of IM-zig-B-2 was set to be 1.00E+00 as the standard because it is the most stable intermediate.

31



TOC Graphic

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Propane


O2 Activation and Oxidative Dehydrogenation of Propane on

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