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Theoretical study on catalytic pyrolysis of benzoic acid as coal based model compound Ming-Fei Wang, Zhi-Jun Zuo, Ruipeng Ren, Zhi-Hua Gao, and Wei Huang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00132 • Publication Date (Web): 17 Mar 2016 Downloaded from http://pubs.acs.org on March 22, 2016

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Theoretical study on catalytic pyrolysis of benzoic acid as coal based model compound Ming-Fei Wang, Zhi-Jun Zuo*, Rui-Peng Ren, Zhi-Hua Gao, Wei Huang* Key Laboratory of Coal Science and Technology of Ministry of Education and Shanxi Province, Taiyuan University of Technology, Taiyuan 030024, Shanxi China; * Corresponding author. Fax/Tel.: +086 3516018073, E-mail address: [email protected] (Z. J. Zuo) and [email protected] (W. Huang). Abstract: The benzoic acid (C6H5COOH) is selected as a coal based model compound, and its catalytic pyrolysis mechanisms on ZnO, γ-Al2O3, CaO and MgO catalysts are studied using density functional theory (DFT) compared with the non-catalytic pyrolysis mechanism. DFT calculation shows that the pyrolysis process of C6H5COOH in gas phase occurs via the direct decarboxylation pathway (C6H5COOH → C6H6 + CO2) or the stepwise decarboxylation pathway _

(C6H5COOH → C6H6COO → C6H6 + CO2). For C6H5COOH catalytic pyrolysis on ZnO (1010) surface, the preferred reaction pathway is C6H5COOH → C6H5COO + H → C6H6 + CO2, whereas the preferred reaction pathway on γ-Al2O3 (110), CaO (100) and MgO (100) surface is C6H5COOH → C6H5COO + H → C6H5 + CO2 + H → C6H6 + CO2, indicating the presence of catalysts changed the pyrolysis mechanism of C6H5COOH. In addition, dissociative adsorption of _

C6H5COO is observed on these surfaces. It is found that ZnO (1010), MgO (100), and CaO (100) are beneficial to C6H5COOH decomposition, but γ-Al2O3 (110) is disadvantageous to the C6H5COOH decomposition. At the same reaction temperature, the rate constants show the order: k(ZnO) >k(MgO) > k(CaO) > k(no catalyst)

> k(γ-Al2O3 ).

Keywords: DFT, coal based model, benzoic acid, catalytic pyrolysis

1


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1. Introduction Coal is a solid heterogeneous and structurally complex fossil fuel in which the major components are organic materials and the minor components are inorganic materials

1, 2

. The

rapidly increasing demand for natural gas and crude oil has stimulated intensive research for coal conversion due to scarcity of natural gas and oil reserves in China3-5. The main method of the coal conversion processes is thermal conversion

1, 6

, such as combustion, gasification, pyrolysis and

liquefaction7. Pyrolysis has been considered to be an effective method for the clean conversion of coal has been applied to produce coke, gas, oil and high value chemicals

9, 10

8, 9

, it

. Catalysts used in

pyrolysis could activate the molecules, and startup the reaction at moderate conditions. Therefore, more and more researchers focus on the influence of catalysts for the pyrolysis11-19. For example, the formation of ZnO from

ZnCl2 transition detected by X-ray Powder Diffraction affects the

pyrolysis product distribution12,17. Liu et al. study the influence of CaO and Al2O3 for coal pyrolysis. It is found that inherent mineral in coal had no evident effect on the reactivity of coal pyrolysis. CaO and Al2O3 show a catalytic activity, which is closely related to temperature region and coal types15. MgO can promote biomass pyrolysis, with which almost all of the long chain alkanes and alkenes can be converted to short chains. In our recent study21, we also found different product distributions when different catalysts were added. In order to investigate the catalytic pyrolysis mechanism and give guidance for designing catalysts, plenty of theoretic works have been done to study the reaction pathway7, 22-27. Due to the complexity, variability, and heterogeneity of coal, the coal based model compounds have been widely used to study the pyrolysis behaviors of coal. C6H5COOH is a common model compound 2


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in the study of coal pyrolysis using density functional theory (DFT)

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22, 23, 27

. It is found that the

direct decarboxylation mechanism and the stepwise decarboxylation mechanism are the main pathways of C6H5COOH pyrolysis

22, 23

. According to the experiment results, the catalysts will

affect the product distribution 28-33. However, to our knowledge, no paper has studied the catalytic pyrolysis mechanism of coal based model compound C6H5COOH on different catalysts. What are the differences between catalytic pyrolysis mechanism and non-catalytic pyrolysis mechanism? What are the differences of catalytic pyrolysis mechanism on different catalysts? To answer these questions, the possible reaction pathways of C6H5COOH with and without ZnO, γ-Al2O3, CaO and MgO catalysts are studied at the molecular level. These candidate catalysts are chosen because of their wide application in the catalytic pyrolysis 12, 14, 15, 18. 2. Computational methods and models 2.1. Computational methods DFT plane-wave calculations with periodic boundaries were carried out using Dmol3 program in Materials Studio software package34, 35. All structures were geometrically optimized by solving the

Kohn-ham

equation

self-consistently

under

spin-unrestricted

conditions36,37.

The

exchange-correlation energy was calculated within the PW91 generalized gradient approximation (GGA)38. A double-numeric quality basis set with polarization functions was chosen39. The complete linear/quadratic synchronous transit method was used to locate the transition states (TS) of reactions40. A Fermi smearing of 0.005 hartree was used to improve the computational performance. In this study, the adsorption energy, Eads, is defined as follows: Eads = E (adsorbate/slab) -E (slab) -E (adsorbate) 3


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where E (adsorbate/slab), E (adsorbate) and E (slab) are the total energies of the slab with adsorbate, the free adsorbate in the gas phase and the clean slab. A negative Eads signifies exothermic adsorption. The reaction barrier, Ea, is defined as follows: Ea = Etot (TS/slab) - Etot (IS/slab) Where Etot (TS/slab) and Etot (IS/slab) are the total energies of the slab with transition state (TS) and the slab with initial state (IS). 2.2. Models The energies of all free molecules and radicals were calculated in a 15×15×15 Å cube. The optimized bulk ZnO lattice parameters were a=3.249 Å, c=5.205 Å, which were in good agreement with the experimental values (a=3.250 Å, c=5.207 Å)41. The optimized parameters of bulk γ-Al2O3 using the nonspinel model42-44 were a=5.595 Å, b=8.425 Å, c=8.072 Å, which were consistent with the experimental values (a=5.587 Å, b=8.413 Å, c=8.068 Å)44. Cubic crystal structure was selected for CaO and MgO, with optimized parameters of 4.811 Å and 4.211 Å respectively, which were also in accordance with the previous results45, 46. _

Previous studies showed that ZnO (1010), γ-Al2O3 (110), CaO (100) and MgO (100) surfaces were the main surfaces of ZnO, γ-Al2O3, CaO and MgO 41, 44-46. Therefore, these four surfaces were used in the paper, and the top views of the selected four surfaces were shown in Fig. 1. ZnO _

(1010), γ-Al2O3 (110) , CaO (100) and MgO (100) surfaces were modeled using a p (3 × 3) super cell with six-layer, p (1 × 2) super cell with four-layer, p (3 × 4) super cell with four-layer and p (3 × 4) super cell with four-layer. Meshes of 2 × 2 × 1 k-points were used for these surfaces (see Table. S1). A vacuum thickness of 15 Å in the direction perpendicular to the surface was 4


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employed to avoid any significant interaction between repeated slabs. In all calculations, the _

bottom two layers of the ZnO (1010), γ-Al2O3 (110), CaO (100) and MgO (100) surfaces were fixed, whereas the other layers and adsorbate were allowed to relax. 3. Results and discussions 3.1. Without catalyst There are three possible mechanisms for C6H5COOH decomposition22. The corresponding energy diagrams with the TS, IS and final state (FS) are shown in Fig. 2 and Fig. S1. The first possible reaction pathway is direct decarboxylation to form C6H6 and CO2, where H of COOH directly migrates to ipso-C of benzene ring. The reaction barrier and reaction energy are 2.88 and -0.14 eV. The second possible reaction pathway is the stepwise decarboxylation mechanism. First, H of COOH transfers to the ortho-C of benzene ring. A reaction barrier of 2.72 eV is observed in this step with reaction energy of 2.63 eV. Then, H migrates to the ipso-C which forms CO2 and C6H6. This process needs to overcome a barrier of 1.15 eV with reaction energy of -2.77 eV. The third possible reaction pathway is the stepwise radical process. The first step is the scission of the O-H bond, which forms C6H5COO and H. This step needs to overcome a barrier of 4.61 eV， with an endothermicity of 4.59 eV. The second step is the formation of C6H5 and CO2 from C6H5COO. The barrier of this step is 0.69 eV, with an endothermicity of 0.41 eV. At last, C6H6 is formed via C6H5 hydrogenation, which the reaction barrier and reaction energy are 1.23 and -5.06 eV. Comparing with the reaction barriers of the three possible reaction pathways as shown in Fig. 2, it is found that the first two pathways need to overcome similar barriers (2.88 vs. 2.72 eV), 5


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whereas the third pathway has a higher barrier (4.61 eV). Therefore, the pyrolysis process of C6H5COOH occurs via the direct decarboxylation pathway or the stepwise decarboxylation pathway. Liu et al. also studied C6H5COOH decomposition22. It was found that the stepwise radical pathway had a higher reaction barrier (4.87 eV), and the preferred reaction pathways were direct decarboxylation pathway and the stepwise decarboxylation pathway which the reaction barriers were 2.55 and 2.54 eV respectively. Our results are in accordance with the previous results. _

3.2 On ZnO (1010) surface The optimized adsorption configurations of all possible intermediates involved in the _

C6H5COOH pyrolysis on ZnO (1010) at their favorable sites are shown in Fig. 3, and the _

corresponding adsorption energies are listed in Table 1. For C6H5COOH adsorption on ZnO (1010) surface, after optimization, the O-H bond breaks, indicating that C6H5COOH is dissociatively _

_

adsorbed on ZnO (1010) surface. The result is similar with HCOOH adsorption on the ZnO (1010) 47, 48

. For C6H5COO, the preferred adsorption site is bridge site of Zn via two O atoms. The

distance between Zn and O is 1.997 Å, and the adsorption energy is -2.72 eV. H tends to adsorb on the O top site, and the adsorption energy is -3.22 eV. C6H5 prefers to adsorb on the O top site via the ipso-C atom with an adsorption energy of -3.03 eV. In the case of CO2, it prefers to adsorb on a Zn-O-Zn site, where two O atoms and C atom bind to two Zn atoms and O atom, yielding a carbonate species. The corresponding adsorption energy is _

-1.11 eV. The adsorption configuration and adsorption energy of CO2 on ZnO (1010) surface are in agreement with previous theoretical49 and experimental results50. C6H6 adsorbs flat on the surface. The bond length of C-Zn is 2.684 Å, and the adsorption energy is -0.74 eV. The adsorption 6


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configurations of C6H6 adsorption on different metal surface is similar with Yildirim et al.’s results51. _

Due to the dissociative adsorption of C6H5COOH on ZnO (10 1 0) surface, the direct decarboxylation and stepwise decarboxylation mechanism are impossible. Therefore, we only consider the stepwise radical process. For C6H5COO further decomposition, there are only two possible pathways: one is C6H6 and CO2 formation with H assistance which from the dissociative adsorption of C6H5COOH, the other is C6H5 and CO2 formation without H assistance. _

The potential energy diagrams and TS diagrams for C6H5COOH decomposition on ZnO (1010) surface are shown in Fig. 4, and the corresponding IS and FS are shown in Fig. S2. As shown in Fig. 4, with H assistance, the reaction barrier and reaction energy of C6H6 and CO2 formation are 1.58 eV and 0.15 eV. Without H assistance, the preferred reaction pathway has an adsorption configuration on Zn top site via one O atom (Fig. S2) with reaction barrier and reaction energy of 1.07 and 1.05 eV. Then, the reaction barrier and reaction energy of C6H5 and CO2 formation are 2.16 eV and -0.47 eV. The reaction barrier of C6H5 and CO2 formation from C6H5COO adsorbed on Zn bridge site is 3.05 eV. Finally, the reaction barrier of C6H6 formation from C6H5 hydrogenation is 0.47 eV. Comparing with the reaction barriers with and without H assistance, the reaction barrier of C6H6 and CO2 formation with H assistance is obviously smaller than that of C6H5 and CO2 formation (1.58 vs. 2.16 eV) without H assistance, indicating that H is beneficial for C6H5COO decomposition. Therefore, the preferred reaction pathway of C6H5COOH catalytic pyrolysis on _

ZnO (1010) surface is C6H5COOH → C6H5COO + H → C6H6 + CO2. 3.3 On γ-Al2O3 (110) 7


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The optimized adsorption configurations of all possible intermediates involved in C6H5COOH pyrolysis on γ-Al2O3 (110) at their favorable sites are listed in Fig. 5, and the corresponding adsorption energies are shown in Table 1. A dissociative adsorption model of C6H5COOH on γ-Al2O3 (110) is obtained after optimization, where H and C6H5COO adsorb on O4 site, Al2 and Al3 sites via two O atoms. The result is in agreement with experimental result from fourier transform infrared spectroscopy52. For the adsorption of C6H5COO, it prefers to adsorb on Al2 and Al3 sites via two O atoms, and the corresponding adsorption energy is -4.02 eV. H binds to O4 site with an adsorption energy of -2.60 eV. There are two possible adsorption sites for C6H5 adsorption. One is O4 site, where the adsorption energy is -2.01 eV. The other is Al3 site, where the adsorption energy is -2.05 eV. In the case of CO2, there are also two possible adsorption sites. The first adsorption configuration is that two O atoms bind to Al3 and Al1 sites, and C atom binds to O4 atom with an adsorption energy of -0.73 eV. The other adsorption configuration is that only one O atom adsorbs on Al3 atom with an adsorption energy of -0.74 eV. The adsorption configuration of C6H6 on γ-Al2O3 (110) surface is _

similar with that on ZnO (1010) surface, where the C6H6 molecule plane is parallel to the surface, which has an adsorption energy of -0.85 eV. _

Similar with C6H5COOH on ZnO (1010) surface, we only consider the stepwise radical process due to dissociative adsorption of C6H5COOH on γ-Al2O3 (110). There are also two possible reaction pathways for C6H5COO further decomposition with or without H assistance. Fig. 6 shows the potential energy diagrams and TS for C6H5COOH decomposition on γ-Al2O3 (110) surface, and the corresponding IS and FS are shown in Fig. S3. As shown in Fig. 6, with H assistance, the reaction barrier and reaction energy of C6H6 and CO2 formation are 3.25 and 2.42 8


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eV. Without H assistance, the reaction barrier and reaction energy of C6H5 and CO2 formation are 2.66 and 2.03 eV. For C6H6 formation from C6H5 hydrogenation, C6H5 adsorption on O4 site shifts to Al3 site and the reaction barrier is 2.04 eV. Then, the reaction barrier and reaction energy of C6H6 formation are 1.97 and 0.43 eV. According to the above results, it is found that H is not a promotion for C6H5COO decomposition, not only due to the high reaction barrier of 0.59 eV but also associated with a low stability by 0.39 eV. Therefore, the preferred reaction pathway of C6H5COOH decomposition on the γ-Al2O3(110) is C6H5COOH → C6H5COO+ H → C6H5 + CO2 + H → C6H6 + CO2, which C6H5COOH is dissociative adsorption. 3.4. On CaO (100) The most stable adsorption configuration of intermediates involved in C6H5COOH pyrolysis on CaO(100) surface are listed in Fig. 7, and the corresponding adsorption energies are shown in Table 1. _

Similar with C6H5COOH adsorption on ZnO (1010) and γ-Al2O3 (110) surface, C6H5COOH is also dissociatively adsorbed on CaO (100) surface. For C6H5COO, it prefers to adsorb on Ca bridge site through two O atoms, and the adsorption energy is -2.78 eV. H tends to adsorb on the O top site with an adsorption energy of -0.72 eV. In the case of C6H5, it tends to adsorb on O top site through ipso-C, and the adsorption energy is -1.29 eV. CO2 preferably adsorbs onto a Ca-O-Ca site, where two O atoms and C atom adsorb on two Ca atoms and O atom. The adsorption configuration is similar with previous experimental and theoretical results

54-56

. The adsorption

energy of CO2 is -1.59 eV, and the binding strength is larger than that of previous theoretical value (-1.24 eV)53. The adsorption configuration of C6H6 on CaO (100) surface is similar with that on 9


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_

ZnO (1010) and γ-Al2O3 (110) surfaces, that the C6H6 molecule plane is parallel to the surface, and the adsorption energy is -0.63 eV. Similarly, the stepwise radical process is considered due to the dissociative adsorption of C6H5COOH on CaO (100). The potential energy diagrams and TS of C6H5COOH decomposition on CaO (100) surface are shown in Fig. 8, and the corresponding IS and FS are shown in Fig. S4. With H assistance, the reaction barrier of the process (C6H5COO+H→C6H6+CO2) is 6.35 eV with an exothermicity of 2.86 eV. Without H assistance (Fig. S4), C6H5COO adsorbed on bridge site migrates to Ca-O-Ca site, with reaction barrier and reaction energy of 0.44 and 0.41 eV. Then, C6H5 and CO2 are formed, with reaction barrier and reaction energy of 2.46 and 0.57 eV. Finally, C6H6 is formed from C6H5 hydrogenation, with reaction barrier and reaction energy of 1.55 and -3.84 eV. The above results show the reaction pathway of C6H5COOH catalytic pyrolysis on CaO (100) surface is C6H5COOH → C6H5COO + H → C6H5 + CO2 + H → C6H6 + CO2. 3.5. On MgO (100) The most stable adsorption configurations of all possible intermediates of C6H5COOH decomposition on MgO (100) surface are shown in Fig. 9, and the adsorption energies are listed in Table 1. C6H5COOH is also dissociatively adsorbed on MgO (100) surface. For C6H5COO, it binds to Mg bridge site through two O atoms, the adsorption energy is -2.27eV. H tends to bind O top site and the adsorption energy is -1.00 eV. The binding strength of H on MgO(100) surface is larger than that of H on MgO(001) (-0.56 eV)56. In the case of C6H5, it prefers to adsorb on O top site through ipso-C, and the adsorption energy is -1.18 eV. CO2 occupies Mg-O-Mg site with two O atoms binding to two Mg atoms and C binding to O atom. The adsorption configuration is in 10


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agreement with previous works 53-55, 57. The adsorption energy is -0.54 eV. The binding strength is larger than that of Allenet et al.’s result (-0.20 eV)53, but it is smaller than that of Kim et al.’s result (-0.71 eV)57. The adsorbed C6H6 is parallel to MgO (100) surface, and the adsorption energy is -0.61 eV. Similarly, only the stepwise radical process is considered due to the dissociative adsorption of C6H5COOH on MgO (100) surface. The potential energy diagrams and TS for C6H5COOH decomposition on MgO (100) surface are shown in Fig. 10, and the corresponding IS and FS are shown in Fig. S5. With H assistance, the reaction barrier of the process (C6H5COO + H → C6H6 + CO2) is 3.59 eV with an exothermicity of 3.03 eV. Without H assistance, C6H5COO adsorbed on Mg bridge site migrates to Mg-O-Mg site (Fig. S5), with reaction barrier and reaction energy of 0.60 and 0.56 eV. Then, C6H5 and CO2 are formed, with reaction barrier and reaction energy of 1.85 and 1.74 eV. Finally, C6H6 is formed from C6H5 hydrogenation, with reaction barrier and reaction energy of 0.79 and -5.33 eV. The above results show the preferred reaction pathway of C6H5COOH catalytic pyrolysis on MgO (100) surface is C6H5COOH → C6H5COO + H → C6H5 + CO2 + H → C6H6 + CO2, where C6H5COOH is dissociatively adsorbed. In order to probe into the effect of the reaction temperature on C6H5COOH decomposition without and with catalysts, the highest energy consumption pathways are studied using Eyring’s transition state theory (TST)58-60. According to experiments, the reaction temperature of coal pyrolysis ranges from 400 to 700 oC11,

14, 15, 21

, so the rate constants of the highest energy _

consumption pathways with and without ZnO (1010), γ-Al2O3 (110), CaO (100) and MgO (100) surfaces are studied at 500, 600 and 700 oC, and the rate constants are shown in Table 2. 11


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As shown in Table 2, the rate constant increases with temperature increasing. At a certain _

reaction temperature, the rate constants rank as follows: k(ZnO (1010)) > k(MgO (100)) > k(CaO (100))

> k(no catalyst)

>

k(γ-Al2O3 (110)). Comparing with C6H5COOH _

decomposition without catalyst, the rate constants of C6H5COOH decomposition on ZnO (1010), MgO (100) and CaO (100) surface at 700 oC are faster than that without catalyst by about 5.52 × 103, 4.17× 102 and 4.63× 101 times, respectively. However, the reaction rate of C6H5COOH decomposition on γ-Al2O3 (110) surface at 700 oC is slower than that without catalyst with a factor _

of 2.32× 10-2. The result shows that ZnO (1010), MgO (100), CaO (100) are beneficial for C6H5COOH decomposition, but γ-Al2O3 (110) is disadvantageous for C6H5COOH decomposition. 4. Conclusions In this work, the periodic DFT calculation has been carried out to investigate the catalytic pyrolysis mechanism of a coal based model compound, C6H5COOH, with and without the ZnO _

(1010), γ-Al2O3 (110), CaO (100) and MgO (100) surfaces. It is found that the pyrolysis processes of C6H5COOH have two pathways: direct decarboxylation pathway and the stepwise decarboxylation. The corresponding highest energy consumption steps are H migration from COOH to ipso-C and ortho-C of benzene ring (2.88 vs. _

2.72 eV). However, on ZnO (1010), γ-Al2O3 (110), CaO (100) and MgO (100) surfaces, the direct decarboxylation pathway or the stepwise decarboxylation pathway do not occur due to the _

dissociative adsorption of C6H5COOH on these surfaces. On ZnO (1010) surface, the preferred pathway is C6H5COOH → C6H5COO + H → C6H6 + CO2, where the highest energy consumption step is C6H6 and CO2 formation (1.58 eV). On γ-Al2O3 (110), CaO (100) and MgO (100) surfaces, the preferred pathway is C6H5COOH → C6H5COO + H → C6H5 + CO2 + H → C6H6 + CO2. The 12


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highest energy consumption steps on γ-Al2O3 (110), CaO (100) and MgO (100) surfaces are same, which is C6H5 and CO2 formation (2.66, 2.46 and 1.85 eV). The result shows that catalyst can change the reaction pathway for pyrolysis processes of C6H5COOH. Based on the TST result, it is found that, at the same reaction temperature, the rate constants are _

in such an order: k(ZnO (1010))

> k(MgO (100))

> k(CaO (100))

> k(no catalyst)

>

k(γ-Al2O3 (110)). Therefore, we think that the acid material (γ-Al2O3) is not favorable for C6H5COOH decomposition, but the base (MgO and CaO) and neutral (ZnO) materials are beneficial for C6H5COOH decomposition.

Supporting Information _

IS and FS for C6H5COOH decomposition with and without ZnO (1010), γ-Al2O3(110), CaO (100) and MgO (100) surfaces, the surface energies using differ layer and k-points.

Acknowledgements The authors gratefully acknowledge the financial support of this study by the International Cooperation Program between China and Japan (2013DFG 60060), the National Natural Science Foundation of China (21306125), and Program for the Outstanding Innovative Teams of Higher Learning Institutions of Shanxi.

13


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Energy & Fuels

Figure caption: _

Fig. 1 The top views of ZnO (1010)(a), γ-Al2O3 (110)(b), CaO (100)(c) and MgO (100)(d) surfaces. Fig. 2 The potential energy diagrams and corresponding TS of C6H5COOH pyrolysis without catalyst. Fig. 3 The most stable adsorption configurations of all possible intermediates involved in the _

decomposition of C6H5COOH on ZnO (1010) surface. Fig. 4 The potential energy diagrams and the corresponding TS for C6H5COOH decomposition on _

ZnO (1010) surface. Fig. 5 The most stable adsorption configurations of all possible intermediates involved in the decomposition of C6H5COOH on γ-Al2O3 (110) surface. Fig. 6 The potential energy diagrams and the corresponding TS for C6H5COOH decomposition on γ-Al2O3 (110) surface. Fig. 7 The most stable adsorption configurations of all possible intermediates involved in C6H5COOH decomposition on CaO (100) surface. Fig. 8 The potential energy diagrams and the corresponding TS for C6H5COOH decomposition on CaO (100) surface. Fig. 9 The most stable adsorption configurations of the reactants and all possible intermediates involved in C6H5COOH decomposition on the MgO (100) surface. Fig. 10 The potential energy diagrams and the corresponding TS for C6H5COOH decomposition on the MgO (100) surface.

18


Energy & Fuels

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

(a)

(b)

(c)

(d) Fig. 1

19


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TS1

TS2

TS5

TS3

TS6 Fig. 2

20


TS4

Energy & Fuels

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

C6H5COO

H

C6H5

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CO2 Fig. 3

21


C6H6

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TS7

TS8

TS9 Fig. 4

22


TS10

Energy & Fuels

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

C6H5COO

CO2

H

Page 24 of 31

C6H5

CO2

C6H6 Fig. 5

23


C6H5

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TS11

TS12

TS13 Fig. 6

24


TS14

Energy & Fuels

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

C6H5COO

C6H5

H

CO2 Fig. 7

25


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C6H6

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TS15

TS16

TS17 Fig. 8

26


TS18

Energy & Fuels

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

C6H5COO

H

C6H5

CO2 Fig. 9

27


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C6H6

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TS19

TS20

TS21 Fig. 10

28


TS22

Energy & Fuels

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_

Table 1. Adsorption energies and geometrical parameters for relevant species on ZnO (1010), γ-Al2O3 (110), CaO (100) and MgO (100) surfaces Catalysts

_

ZnO (1010)

Species C6H5COO H C6H5 CO2

Site Zn bridge O top O top Zn-O-Zn parallel to the surface

Eads(eV) -2.72 -3.22 -3.03 -1.11

Bond length d (Å) dZn-O:1.997 dO-H:0.973 dO-C:1.412 dO-C:1.374,dZn-O:2.090

-0.74

dZn-C:2.684

Al2 and Al3 O4 Al3 O4 Al3

-4.02 -2.60 -2.05 -2.01 -0.74

Al3-O4-Al1

-0.73

C6H6

parallel to the surface

-0.85

dAl2-O:1.868, dAl3-O:1.829 dO4-H:1.061 dAl3-C:1.994 dO4-C:1.400 dAl3-O:2.047 dO4-C:1.359,dAl1-O:1.968, dAl3-O:1.993 dAl3-C:2.472

C6H5COO H C6H5 CO2

Ca bridge O top O top Ca-O-Ca parallel to the surface

-2.78 -0.72 -1.29 -1.59

dCa-O:2.376 dO-H:1.026 dO-C:1.404 dO-C:1.418, dCa-O:2.409

-0.63

dCa-C:3.570

Mg bridge O top O top Mg-O-Mg parallel to the surface

-2.27 -1.00 -1.18 -0.54

dMg-O:2.084 dO-H:1.088 dO-C:1.425 dO-C:1.456, dMg-O:2.200

-0.61

dMg-C:3.549

C6H6 C6H5COO H C6H5 γ-Al2O3 (110)

CaO (100)

CO2

C6H6

MgO (100)

C6H5COO H C6H5 CO2 C6H6

29


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Energy & Fuels

Table 2 The rate constant k(s-1) of the highest energy consumption pathways for C6H5COOH _

decomposition with and without ZnO (1010), γ-Al2O3 (110), CaO (100) and MgO (100) Catalysts

no catalyst

_

ZnO (1010)

(eV)

500 oC

600 oC

700 oC

C6H5COOH→C6H6+CO2

2.88

1.37E-03

2.15E-01

1.23E+01

C6H5COOH→C6H6COO

2.72

4.77E-03

5.66E-01

2.59E+01

C6H5COO+H→C6H6+CO2

1.58

7.29E+02

1.46E+04

1.43E+05

2.66

7.70E-05

6.27E-03

6.00E-01

2.46

4.36E-01

1.68E+01

1.20E+03

1.85

2.85E+01

5.33E+02

1.08E+04

γ-Al2O3 (110) CaO (100)

Rate constant k (s-1)

Ea

Reaction

C6H5COO+H → C6H5 + CO2 + H

MgO (100)

30


Theoretical Study on Catalytic Pyrolysis of Benzoic ... - ACS Publications

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