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The role of catalysts in the decomposition of phenoxy compounds in coal: A density functional theory study
Applied Surface Science 428 (2018) 541–548
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The role of catalysts in the decomposition of phenoxy compounds in coal: A density functional theory study Jiang-Tao Liu, Ming-Fei Wang, Zhi-Hua Gao, Zhi-Jun Zuo ∗ , Wei Huang ∗ Key Laboratory of Coal Science and Technology of Ministry of Education and Shanxi Province, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China
a r t i c l e
i n f o
Article history: Received 26 May 2017 Received in revised form 15 September 2017 Accepted 20 September 2017 Available online 21 September 2017 Keywords: DFT Phenoxy compound Anisole Catalytic pyrolysis
a b s t r a c t The pyrolysis mechanisms of anisole (C6 H5 OCH3 ), as a coal-based model compound, on CaO, ZnO and ␥Al2 O3 catalysts were studied using density functional theory (DFT). In contrast to the products of pyrolysis (C6 H6 , H2 and CO), the products of catalytic pyrolysis on CaO, ZnO, and ␥-Al2 O3 are C6 H5 OH and C2 H4 ; CO, C5 H6 and C2 H4 ; and C6 H5 OH and C2 H4 , respectively. Our results indicate that CaO, ZnO and ␥-Al2 O3 catalysts not only decrease the energy barrier of C6 H5 OCH3 decomposition but also alter the pyrolysis process and the products. It is also found that the pyrolysis gas H2 alters the products on CaO (C6 H5 OH and CH4 ), but it does not affect the products on ZnO and ␥-Al2 O3 . In sum, these catalysts are beneficial for phenoxy compound decomposition. © 2017 Published by Elsevier B.V.
1. Introduction Coal pyrolysis is considered one of the most important technologies to make the use of coal clean, efficient and comprehensive [1]. Coal pyrolysis is also an initial step in the coal conversion processes of liquefaction and gasification, which strongly influences the complete combustion process, the char reactivity and the product distributions [2]. It has been found that catalysts in coal pyrolysis not only increase the total conversion and product yield [3] but also make the pyrolysis conditions milder. To obtain high-quality oil, gas and chemicals, a large number of catalysts have been used [3–13], including ZnO [13,14], CaO [5], Al2 O3 [6], MgO [9], etc. [15–17]. The organic components of coal mainly include polycyclic aromatic hydrocarbons, heterocyclic compounds and aromatic hydrocarbons [18], and the aromatic rings are normally substituted by groups such as alkyl side chains, phenolic hydroxyl and ether groups [19]. Because the structure of coal is complex and variable, the pyrolysis mechanisms are difficult to describe clearly [20]. To understand the role and improve the efficiency of the catalysts for coal pyrolysis, many theoretical studies have been carried out using model compounds, such as anisole, phenyl ethyl ether,
benzoic acid, phenol, etc. [18,20–24], which has been proven to be an effective method to understand the reaction process for coal pyrolysis over the past few decades [25]. However, these studies only focus on the reaction mechanism of the model compounds and do not consider the role of catalysts. Recently, C6 H5 COOH decomposition, as a model compound, on ZnO, ␥-Al2 O3 , CaO, and MgO surfaces was studied. It was found that ZnO, ␥-Al2 O3 , CaO, and MgO catalysts not only change the energy barrier but also alter the reaction pathways. However, they do not influence the products of C6 H5 COOH decomposition. Additionally, ZnO, MgO, and CaO are beneficial to C6 H5 COOH decomposition, but ␥-Al2 O3 is disadvantageous to C6 H5 COOH decomposition [26]. In other words, it is necessary to consider the role of catalysts in coal pyrolysis. To better understand the role of catalysts for phenoxy compounds, which are common groups in coal [18,22], the mechanisms of C6 H5 OCH3 pyrolysis on CaO, ZnO and ␥-Al2 O3 surfaces were investigated in this paper using density functional theory (DFT), in which C6 H5 OCH3 is used as a model compound, as it is the most simple phenoxy compound. The difference between the pyrolysis mechanism and the catalytic pyrolysis mechanism is compared. Finally, the influence of the pyrolysis gas H2 for C6 H5 OCH3 decomposition on CaO, ZnO and ␥-Al2 O3 surfaces is also considered. 2. Computational methods and models
∗ Corresponding authors. E-mail addresses: [email protected] (Z.-J. Zuo), [email protected] (W. Huang). http://dx.doi.org/10.1016/j.apsusc.2017.09.170 0169-4332/© 2017 Published by Elsevier B.V.
All calculations were performed using DFT methods, as implemented in the Dmol3 package developed by Accelrys, Inc. [27,28],
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¯ Fig. 1. Top views of the (a) CaO (100), (b) ZnO 1010 and (c) ␥-Al2 O3 (110) surfaces.
which has already been extensively used to study the adsorption behaviours and reaction mechanisms on different catalyst surfaces [29–34]. All-electron relativistic DFT was used for core electrons by employing the generalized gradient approximation and Perdew and Wang function (PW91) [35]. Double-numerical basis sets with polarization functions (DNP) were used to describe the valence orbitals of the atoms [36]. The transition states (TS) were located by the complete linear/quadratic synchronous transit method [37]. A Fermi smearing of 0.005 hartree was used. During all calculations, the self-consistent Kohn−Sham equation was applied to optimize all structures geometrically under spin-unrestricted condition [38,39]. According to our previous results, we used p(3 × 4) CaO (100) with a four-layer slab, p(3 × 3) ZnO (100) with a six-layer slab and p(1 × 2) ␥-Al2 O3 (110) with a four-layer slab [26], as these surfaces are the main surfaces for CaO, ZnO and non-spinel ␥-Al2 O3 [40–43]. During the calculation, the bottom two layers were fixed, and other layers and adsorbates were allowed to relax. A vacuum region thickness of 15 Å was chosen to eliminate interaction effects of neighbouring slabs, and 2 × 2 × 1 k-points were used. The top views of the selected three surfaces are shown in Fig. 1. 3. Results and discussion 3.1. Pyrolysis reaction C6 H5 OCH3 pyrolysis has three possible pathways: C6 H5 OCH3 bond scission (C6 H5 OCH3 → C6 H5 + OCH3 ), C6 H5 O CH3 bond scission (C6 H5 OCH3 → C6 H5 O + CH3 ) and C H bond of the methyl group scission (C6 H5 OCH3 → C6 H5 OCH2 + H). According to the three pathways, C6 H5 O and CH3 are preferentially produced via C6 H5 OCH3 → C6 H5 O + CH3 (E = 3.02 eV, Ea = 3.47 eV), rather than C6 H5 OCH3 → C6 H5 + OCH3 (E = 4.63 eV, Ea = 4.97 eV) and C6 H5 OCH3 → C6 H5 OCH2 + H (E = 4.46 eV, Ea = 5.39 eV). Then, CO and C5 H5 are formed from C6 H5 O → C5 H5 + CO (E = 1.67 eV, Ea = 1.76 eV), because the energy barrier of CO and C5 H5 formation is clearly lower than that of C6 H5 and O formation from C6 H5 O → C6 H5 + O (E = 5.87 eV). In further reactions with C5 H5 , the reaction energy of C5 H5 + CH3 → C5 H5 CH3 (E = −3.28 eV, Ea = 0.49 eV) is obviously lower than the reaction energy of C5 H5 → C5 H4 + H (E = 5.09 eV) or CH3 dehydrogenation (CH3 → CH2 + H, E = 5.07 eV), indicating that C5 H5 CH3 formation is possible. Finally, C6 H6 and H2 are formed from C5 H5 CH3 → C6 H6 + H2 (E = −0.20 eV, Ea = 5.04 eV), rather than C5 H5 CH3 →C5 H5 CH2 + H (E = 4.27 eV, Ea = 5.56 eV). The potential energy diagrams with the TS are shown in Fig. 2, and the corresponding IS and FS of each elementary reaction are shown in Fig. S1 of the Supporting Information. As shown in Fig. 2, the C6 H5 OCH3 pyrolysis reaction pathway is C6 H5 OCH3 → C6 H5 O + CH3 → C5 H5 + CO + CH3 → C5 H5 CH3
+ CO→ C6 H6 + H2 +CO, which C5 H5 CH3 → C6 H6 +H2 has the highest energy barrier (Ea = 5.04 eV). Previous experimental studies have also indicated that the pyrolysis of C6 H5 OCH3 begins with the removal of methyl [44–49], followed by ejection of a CO to form C5 H5 . Then, the majority of benzene is formed by a ring expansion reaction of (C5 H5 CH3 ). Finally, C6 H6 is formed. Our DFT results are in accordance with the previous experimental results [49]. Combing with our previous studies [26], the intramolecular hydrogen migration reaction and radical reaction are the first reaction path for C6 H5 COOH and C6 H5 OCH3 pyrolysis, which the corresponding energy barriers are 2.72 and 3.47 eV, respectively. The result shows that the initial decomposition temperature of C6 H5 COOH is lower than that of C6 H5 OCH3 . Previous experiment study shows that C6 H5 COOH converts to C6 H6 at 623 K using C6 H5 COOH as a model compound [50]. Using C6 H5 OCH3 as a model compound, the decomposition temperature is around 1000 K [18,47]. Therefore, our calculation results are in accordance with the experiment results [18,47,50]. 3.2. Catalytic pyrolysis reaction
¯ The reaction intermediates on the CaO (100), ZnO 1010 and ␥-Al2 O3 (110) surfaces are shown in Fig. 3, and their corresponding adsorption configurations and adsorption energies are listed in Table 1. In addition, the adsorption energy and geometrical parameters of some other species are shown in Table S1 of the Supporting Information. 3.2.1. Catalytic pyrolysis reaction on a CaO (100) surface C6 H5 OCH3 prefers to adsorb on the Ca top site of the CaO (100) surface, and the adsorption energy of C6 H5 OCH3 is −0.90 eV. Similar to the pyrolysis reaction of C6 H5 OCH3 , The energy barrier of *C6 H5 OCH3 + * → *C6 H5 O + *CH3 (E = −0.13 eV, Ea = 2.81 eV) is lower than that of *C6 H5 OCH3 + * → *C6 H5 OCH2 + *H (E = 4.27 eV, Ea = 4.82 eV) or *C6 H5 OCH3 + * → *C6 H5 + *OCH3 (E = 0.27 eV, Ea = 3.84 eV), indicating that *C6 H5 O and *CH3 formation are possible. C6 H5 O and CH3 prefer to adsorb on the Ca top and O top site, and the corresponding adsorption energies are −1.65 and −0.58 eV, respectively. In further reactions of *CH3 , the energy barrier of *CH3 + *H → *CH4 + * (E = −4.89 eV, Ea = 0.42 eV) is smaller than that of *CH3 + * → *CH2 + *H (E = 0.97 eV, Ea = 1.53 eV) or *CH3 + *CH3 → *C2 H6 + * (E = −6.18 eV, Ea = 1.60 eV). The result shows that *CH4 formation is possible. Although the energy barriers of *CH3 + * → *CH2 + *H and *CH3 + *CH3 → *C2 H6 + * are similar, the *CH3 coverage is low due to the high energy barrier of *C6 H5 OCH3 + * → *C6 H5 O + *CH3 . Therefore, *CH2 and *H formation is possible. Then, *C2 H4 formation is likely, because the energy barrier of *CH2 + *CH2 → *C2 H4 + * (E = −2.97 eV, Ea = 2.25 eV) is lower than that of *CH2 + * → *CH + *H (E = 2.03 eV, Ea = 2.82 eV).
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Fig. 2. Potential energy diagrams and corresponding TS of the C6 H5 OCH3 pyrolysis reaction.
¯ Fig. 3. C6 H5 OCH3 catalytic pyrolysis reaction intermediates on the CaO (100), ZnO 1010 and ␥-Al2 O3 (110) surfaces.
CH2 adsorbs on the Ca bridge and O top sites, and the adsorption energy is −2.80 eV. After optimization, CH4 and C2 H4 are far away the surface, and the adsorption energies are −0.24 and −0.51 eV, respectively.
In further reactions of *C6 H5 O, the energy barrier of *C6 H5 O + *H → *C6 H5 OH + * (E = 1.00 eV, Ea = 1.02 eV) is smaller than that of *C6 H5 O + * → *C6 H5 + *O (E = 2.35 eV, Ea = 5.29 eV) or *C6 H5 O + * → *C5 H5 + *CO (E = 1.78 eV, Ea = 3.67 eV), indicating
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Table 1 Adsorption energies and geometrical parameters for relevant species on the surfaces. catalysts
species
site
Eads (eV)
bond length d (Å)
CaO (100)
C6 H5 OCH3 C6 H5 O CH3 H CH2 C2 H4 CH4 CO C6 H5 OH
Ca top Ca top O top O top Ca bridge and O top O top O top Ca top Ca top
−0.90 −1.65 −0.58 −0.73 −2.80 −0.51 −0.24 −0.43(−0.55) [52] −0.82
dCa O :2.758 dCa O :2.276 dO C :1.431 dO H :1.028 dCa C :2.601,dO DO H :2.516 dO H :2.379 dCa C :2.714 dCa C :2.539
¯ ZnO 1010
C6 H5 OCH3 C6 H5 O CH3 C6 H5 OCH2 H CH2 C2 H4 CO C5 H5 C5 H6
Zn top Zn top O top Zn bridge O top Zn top and O top Zn top and O top Zn top Zn top Zn top
−0.75 −1.41 −2.25 −1.05 −3.23 −3.56 −0.62 −0.63(−0.68)[53] −1.31 −0.75
dZn O :2.436 dZn O :1.997 dO C :1.449 dZn O :2.156,dZn C :2.043 dO H :0.975 dZn C : 2.063,dO C :1.490 dZn C : 2.005,dO C :1.501 dZn C :2.137 dZn C :2.286 dZn C : 2.278
␥-Al2 O3 (110)
C6 H5 OCH3 C6 H5 O CH3 C6 H5 OCH2
Al3 Al3 Al3 Al3 Al3 and O4 O4 Al3 and O4 Al3 Al3 Al3
−1.71 −2.65 −1.53 −1.73 −1.71 −2.59 −3.22 −1.07 −0.96 −1.98
dAl3 O :1.951 dAl3 O :1.797 d Al3 C :1.980 dAl3 O :.2.057 dAl3 O :2.039,dO4 dO4 H :1.063 dAl3 C :1.938,dO4 dAl3 C :2.445 dAl3 C :2.166 dAl3 O :1.978
H CH2 C2 H4 CO C6 H5 OH
that *C6 H5 OH formation is likely. H prefers to adsorb on the O site with an adsorption energy of −0.73 eV. Fig. 4 shows the potential energy diagrams and TS diagrams for C6 H5 OCH3 decomposition on the CaO (100) surface, and Fig. S2 of the Supporting Information shows the corresponding IS and FS of each elementary reaction. In summary, the catalytic pyrolysis reaction pathway of C6 H5 OCH3 decomposition on the CaO (100) surface is C6 H5 OCH3 (g) → *C6 H5 OCH3 + * → *C6 H5 O + *CH3 → *C6 H5 O + *CH2 + *H → *C6 H5 OH + *CH2 → *C6 H5 OH + 0.5 *C2 H4 → C6 H5 OH(g) + 0.5C2 H4 (g). Our results for the formation of C6 H5 OH are in accordance with the previous experimental result in which aromatic ether carbons were the precursor structures of phenols during pyrolysis [51]. Meanwhile, the catalytic pyrolysis reaction pathway of C6 H5 OCH3 under the pyrolysis gas H2 on the CaO (100) surface is C6 H5 OCH3 (g) + H2 (g) → *C6 H5 OCH3 + *2H + * → *C6 H5 O + *CH3 + *2H → *C6 H5 O + *CH4 → *C6 H5 OH + *CH4 → C6 H5 OH(g) + CH4 (g). The rate-determining step of the two pathways is *C6 H5 OCH3 + * → *C6 H5 O + *CH3 , which has an energy barrier of 2.81 eV. Therefore, the pyrolysis gas H2 alters the reaction pathway and the products of the catalytic pyrolysis of C6 H5 OCH3 on CaO surfaces.
¯ 3.2.2. Catalytic pyrolysis reaction on a ZnO 1010 surface ¯ On the ZnO 1010 surface, C6 H5 OCH3 prefers to adsorb on the Zn top site with an adsorption energy of −0.75 eV. The energy barrier of *C6 H5 OCH3 + * → *C6 H5 OCH2 + *H (E = 0.14 eV, Ea = 1.99 eV) is lower than that of *C6 H5 OCH3 + * → *C6 H5 + *OCH3 (E = 0.06 eV, Ea = 3.24 eV) or *C6 H5 OCH3 + * → *C6 H5 O + *CH3 (E = −0.41 eV, Ea = 2.41 eV), indicating that *C6 H5 OCH2 and *H formation is possible. Comparing with pyrolysis reaction, the reaction pathways are different (C6 H5 OCH3 → C6 H5 O + CH3 vs. *C6 H5 OCH3 + * → *C6 H5 OCH2 + *H), showing that the ZnO catalyst changes the reaction pathway. C6 H5 OCH2 prefers to adsorb on the
C :1.500
C :1.392 C :1.497
Zn bridge site with an adsorption energy of −1.05 eV, and H prefers to adsorb on the O site with an adsorption energy of −3.23 eV. Then, the energy barrier of *C6 H5 OCH2 + * → *C6 H5 O + *CH2 (E = 0.16 eV, Ea = 1.91 eV) is lower than that of *C6 H5 OCH2 + * → *C6 H5 + *OCH2 (E = −2.15 eV, Ea = 2.37 eV) or *C6 H5 OCH2 + * → *C6 H5 OCH + *H (E = −1.62 eV, Ea = 3.43 eV), and thus *C6 H5 O and *CH2 are formed from further reactions of *C6 H5 OCH2 . The adsorption energies of C6 H5 O and CH2 are −1.41 and −3.56 eV, respectively. In further reactions of *CH2 , there are three possible reaction pathways: *CH2 + * → *CH + *H, *CH2 + *H → *CH3 + * and *CH2 + *CH2 → *C2 H4 + *. The results show that *C2 H4 formation is possible (E = −1.44 eV, Ea = 1.95 eV), because the energy barrier of *C2 H4 formation is smaller than that of *CH (E = 2.44 eV, Ea = 4.06 eV) or *CH3 (E = −0.81 eV, Ea = 3.26 eV) formation. The adsorption energy of C2 H4 is −0.62 eV. In further reactions of *C6 H5 O, there are also three possible reaction pathways: *C6 H5 O + * → *C6 H5 + *O, *C6 H5 O + * → *C5 H5 + *CO and *C6 H5 O + *H → *C6 H5 OH + *. *C5 H5 and *CO (E = 0.75 eV, Ea = 0.79 eV) can be more easily formed than *C6 H5 and *O (E = 1.66 eV, Ea = 2.64 eV) or *C6 H5 OH formation (E = 0.61 eV, Ea = 1.21 eV). Finally, a high energy barrier is obtained for *C5 H5 dehydrogenation (E = −0.29 eV, Ea = 2.88 eV), while the energy barrier and reaction energy of *C5 H6 formation from *C5 H5 hydrogenation are 0.88 and 0.28 eV. Therefore, *C5 H6 formation seems likely. The adsorption energies of C5 H5 , CO and C5 H6 are −1.31, −0.63 and −0.75 eV, respectively, and all of them prefer to adsorb on the Zn top site. The for the catalytic pyrolysis of C6 H5 OCH3 on pathway ¯ ZnO 1010 is C6 H5 OCH3 (g) → *C6 H5 OCH3 + * → *C6 H5 OCH2 + *H → *C6 H5 O + *CH2 + *H → *C6 H5 O + 0.5 *C2 H4 + *H →*CO + *C5 H5 + 0.5 *C2 H4 + *H → *CO + *C5 H6 + 0.5 *C2 H4 → CO(g) + *C5 H6 + 0.5C2 H4 (g). The potential energy diagrams, TS, IS and FS diagrams for *C6 H5 OCH3 decomposition are shown in Fig. 5and Fig. S3. *C6 H5 OCH3 + * → *C6 H5 OCH2 + *H has the highest reaction
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Fig. 4. Potential energy diagrams and corresponding TS for C6 H5 OCH3 decomposition on the CaO (100) surface.
¯ Fig. 5. Potential energy diagrams and corresponding TS for C6 H5 OCH3 decomposition on the ZnO 1010 surface.
barrier (Ea = 1.99 eV). As shown in Fig. 5, the pyrolysis gas H2 will not change the reaction pathway of C6 H5 OCH3 decomposition on ¯ surfaces. ZnO 1010
3.2.3. Catalytic pyrolysis reaction on a -Al2 O3 (110) surface The adsorption energy of C6 H5 OCH3 on the ␥Al2 O3 (110) surface is −1.71 eV. The energy barriers of *C6 H5 OCH3 + * → *C6 H5 O + *CH3 (E = −0.47 eV, Ea = 2.11 eV) and *C6 H5 OCH3 + * → *C6 H5 OCH2 + *H (E = 0.41 eV, Ea = 2.04 eV) are similar and are lower than that of *C6 H5 OCH3 + * → *C6 H5 + *OCH3 (E = 0.01 eV, Ea = 2.96 eV). The results indicate that *C6 H5 O and *CH3 or *C6 H5 OCH2 and *H formation are possible. The adsorption energies of C6 H5 O, CH3 , C6 H5 OCH2 and H are −2.65, −1.53,
−1.73 and −2.59 eV, respectively. In addition, C6 H5 OCH2 has two different adsorption configurations. In further reactions of *CH3 , the energy barrier is 1.48 eV for *CH3 migration from the O4 site to the Al3 site. Then, the energy barrier of *CH3 + * → *CH2 + *H (E = 1.05 eV, Ea = 1.15 eV) is smaller than that of *CH3 + *H → *CH4 + * (E = 0.61 eV, Ea = 1.93 eV) or *CH3 + *CH3 → *C2 H6 + * (E = −0.23 eV, Ea = 4.00 eV). The results show that *CH2 forms more easily than *CH4 or *C2 H6 . Finally, *C2 H4 is formed, because the energy barrier of *CH2 + *CH2 → *C2 H4 + * (E = −3.35 eV, Ea = 2.97 eV) is smaller than that of *CH2 + * → *CH + *H (E = 1.24 eV, Ea = 3.48 eV). In further reactions of *C6 H5 O, the energy barrier of *C6 H5 O + *H → *C6 H5 OH + * (E = 1.22 eV, Ea = 1.44 eV) is smaller than that of *C6 H5 O + * → *C6 H5 + *O (E = 2.33 eV, Ea = 2.39 eV)
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Fig. 6. Potential energy diagrams and corresponding TS for C6 H5 OCH3 decomposition on the ␥-Al2 O3 (110) surface.
¯ Fig. 7. The PDOS of one CaO, ZnO and AlO1.5 unit on the first slab of the CaO (100), ZnO 1010 and ␥-Al2 O3 (110) surfaces and DOS of C6 H5 OCH3.
or *C6 H5 O + * → *C5 H5 + *CO (E = 2.70 eV, Ea = 3.76 eV), indicating that *C6 H5 OH formation is likely. The adsorption energies of C2 H4 , CH2 and C6 H5 OH are −1.07, −3.22 and −1.98 eV, respectively. When C6 H5 OCH2 adsorbs on Al3 sites, the reaction energies and energy barriers of *C6 H5 OCH2 + * → *C6 H5 O + *CH2 , *C6 H5 OCH2 + * → *C6 H5 OCH + *H and *C6 H5 OCH2 + * → *C6 H5 + *OCH2 are −0.16 and 2.59 eV, 1.68 and 6.60 eV, −0.76 and 2.30 eV, respectively. However, the corresponding energy barriers and reaction energies of C6 H5 OCH2 adsorption on Al3 and O4 sites are only −0.18 and 0.47 eV, 1.66 and 4.36 eV, −0.78 and 1.55 eV, respectively. Therefore, *C6 H5 OCH2 first needs to migrate from the Al3 sites to the O4 sites, which has an energy barrier 1.72 eV. Subsequently, *C6 H5 O and *CH2 are formed. According to the above results, it can be obtained that there are two catalytic pyrolysis pathways for *C6 H5 COCH3 on ␥-Al2 O3 (110) surfaces: one is C6 H5 OCH3 (g) → *C6 H5 OCH3 + * → *C6 H5 O + *CH3 → *C6 H5 O + *CH2 + *H → *C6 H5 O
+ 0.5 *C2 H4 + *H → *C6 H5 OH + 0.5 *C2 H4 → C6 H5 OH(g) + 0.5C2 H4 (g), while the other is C6 H5 OCH3 (g) → *C6 H5 OCH3 + * → *C6 H5 OCH2 + *H → *C6 H5 O + *CH2 + *H → *C6 H5 O + 0.5 *C2 H4 + *H → *C6 H5 OH + 0.5 *C2 H4 → C6 H5 OH(g) + 0.5C2 H4 (g). The potential energy diagrams and TS diagrams are shown in Fig. 6, and the corresponding IS and FS of each elementary reaction are shown in Fig. S4 of the Supporting Information. The last result of C6 H5 OH is in accordance with the previous experimental result [51]. As shown in Fig. 6, it can be seen that *CH2 → 0.5 *C2 H4 is the rate-determining step (Ea =2.97 eV). Similar to C6 H5 OCH3 decom¯ position on the ZnO 1010 surface, the pyrolysis gas H2 will not change the reaction pathway on the ␥-Al2 O3 (110) surface. In general, the energy barriers of C6 H5 COOH on the highest ¯ CaO (100), ZnO 1010 and ␥-Al2 O3 (110) (2.46, 1.58 and 2.66 eV) are lower than that of C6 H5 OCH3 decomposition (2.81, 1.99 and of C6 H5 COOH 2.97 eV), indicating that the reaction temperature ¯ and ␥-Al2 O3 (110) catalytic pyrolysis on the CaO (100), ZnO 1010
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is lower than that of C6 H5 OCH3 . Comparing with C6 H5 COOH and C6 H5 OCH3 pyrolysis,the highest energy barriers of C6 H5 COOH on ¯ the CaO (100), ZnO 1010 and ␥-Al2 O3 (110) decrease by 0.26, 1.14 and 0.06 eV, and the highest energy barriers of C6 H5 OCH3 on ¯ CaO (100), ZnO 1010 and ␥-Al2 O3 (110) decrease by 2.23, 3.05 and 2.07 eV. The results show that the catalytic effect of C6 H5 OCH3 decomposition is higher than that of C6 H5 COOH decomposition using CaO, ZnO and ␥-Al2 O3 as catalysts. 3.3. Electronic properties In order to explain the results of the section 3.1 and 3.2, the electronic structures are studied. The band gaps of the CaO, ZnO and ␥-Al2 O3 bulk are 3.94, 2.72 and 5.17 eV using DFT + U, which are smaller than that of experiment results (6.25, 3.37 and 8.70 eV) [54–56], but are similar with other calculation results (3.63, 3.10 and 4.90 eV) [43,57,58]. The projected density of states (PDOS) of ZnO one CaO, and AlO1.5 unit on the first slab of the CaO (100), ¯ ZnO 1010 and ␥-Al2 O3 (110) surfaces and DOS of C6 H5 OCH3 are shown in Fig. 7. As shown in Fig. 7a, the band gaps are in the order ZnO (2.38 eV) < ␥-Al2 O3 (110) (3.14 eV) < CaO (100) (3.28 eV). Comparing with the band gap of bulk, the band gaps of the CaO (100), ¯ and ␥-Al2 O3 (110) surfaces decrease due to the surface ZnO 1010 relaxation [58]. It is well known that the band gap, intensity of (P)DOS and hybridization between catalyst and reactant effect the activity of the catalysts. In general, the catalyst with a small band gap is more reactive than that with a high value, and it with a high intensity (hybridization) of valence band is more reactive than that with a low intensity (hybridization) [29,59]. The PDOS intensities of one CaO, ZnO and AlO1.5 unit on the first slab on the CaO (100), ¯ ZnO 1010 and ␥-Al2 O3 (110) surfaces in the valence band are the order ␥-Al in 2 O3 (110) (59.98 eV) < CaO(100) (107.95 eV) < ZnO ¯ ¯ 1010 (155.72 eV). Therefore, the ZnO 1010 surface shows the lowest energy barrier for C6 H5 OCH3 decomposition. The band gap of the CaO(100) is slightly larger than that of the ␥-Al2 O3 (110), but the intensity of the ␥-Al2 O3 (110) is smaller than that of the CaO(100). Therefore, it could not determine the activity of the ␥Al2 O3 (110) and CaO(100) surfaces by the electronic structures, which must consider the hybridization with the reactant. 4. Conclusion In this paper, we studied the catalytic pyrolysis of phenoxy compounds in coal using DFT, in which C6 H5 OCH3 was selected as a ¯ coal-based model compound and CaO (100), ZnO 1010 and ␥Al2 O3 (110) surfaces were selected as catalysts. The results show that the products of pyrolysis are C6 H6 , H2 and CO, and the highest energy consumption step is C5 H5 CH3 → C6 H6 + H2 (Ea = 5.04 eV). The results show that the products of catalytic pyrolysis on the CaO (100) surface are C6 H5 OH and C2 H4 , and the highest energy consumption step is *C6 H5 OCH3 + * → *C6 H5 O + *CH3 ¯ (Ea = 2.81 eV). On the ZnO 1010 surface, the products are CO, C5 H6 and C2 H4 , and the highest energy consumption step is *C6 H5 OCH3 + * → *C6 H5 OCH2 + *H (Ea = 1.99 eV). On ␥-Al2 O3 (110), the products are C6 H5 OH and C2 H4 , and the highest energy consumption step is *CH2 → 0.5 *C2 H4 (Ea = 2.97 eV). According to the highest energy consumption step of pyrolysis and catalytic pyrol¯ ysis on CaO (100), ZnO 1010 and ␥-Al2 O3 (110) surfaces for C6 H5 OCH3 decomposition, the highest energy consumption step shows the order of Ea (ZnO) < Ea (CaO) < Ea (␥-Al2 O3 ) < Ea (no catalyst). These results show that these catalysts not only decrease the energy barrier of C6 H5 OCH3 decomposition but also alter the reaction products of pyrolysis. In addition, it is found that the products on the CaO (100) surface under the pyrolysis gas H2 are C6 H5 OH and CH4 , but the pyrolysis gas H2 does not affect the products on the
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¯ ZnO 1010 and ␥-Al2 O3 (110) surfaces. We should consider the limitations of using anisole (C6 H5 OCH3 ) as the coal-based model compound in the experiments. The decomposition mechanisms of phenoxy compounds on a CaO catalyst may be somewhat different from the catalytic product of coal. H easily produced during the catalytic pyrolysis of coal, which becomes the H source for the decomposition of phenoxy compounds to finally obtain C6 H5 OH, CH4 and C2 H4 . However, no H is produced using C6 H5 OCH3 as the coal-based model compound, and the products are C6 H5 OH and C2 H4 . Acknowledgments The authors gratefully acknowledge by the National Natural Science Foundation of China (21776197), Shanxi Province Science Foundation for Youths (201701D211003), the Program for the Outstanding Innovative Teams of Higher Learning Institutions of Shanxi and the key project of Basic Industrial Research of Shanxi (201603D121014). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2017.09. 170. References [1] X.J. Kong, Y.H. Bai, L.J. Yan, F. Li, Catalytic upgrading of coal gaseous tar over Y-type zeolites, Fuel 180 (2016) 205–210. [2] P.R. Solomon, T.H. Fletcher, R.J. Pugmire, Progress in coal pyrolysis, Fuel 72 (1993) 587–597. [3] F. Pinto, I. Gulyurtlu, L.S. Lobo, I. Cabrita, The effect of catalysts blending on coal hydropyrolysis, Fuel 78 (1999) 761–768. [4] X.W. Zou, J.Z. Yao, X.M. Yang, L. Song, W.G. Lin, Catalytic effects of metal chlorides on the pyrolysis of lignite, Energ. Fuel. 21 (2007) 619–624. [5] T.Y. Zhu, S.Y. Zhang, J.J. Huang, Y. Wang, Effect of calcium oxide on pyrolysis of coal in a fluidized bed, Fuel Process. Technol. 64 (2000) 271–284. [6] Q.R. Liu, H.Q. Hu, Q. Zhou, S.W. Zhu, G.H. Chen, Effect of inorganic matter on reactivity and kinetics of coal pyrolysis, Fuel 83 (2004) 713–718. [7] B. Dwijen, N. Hiroshi, Y. Tadashi, Hydropyrolysis of Alberta coal and petroleum residue using calcium oxide catalyst and toluene additive, Catal. Today 45 (1998) 385–391. [8] M. Seitz, W. Heschel, T. Nägler, S. Nowak, J. Zimmermann, T. Stam-Creutz, W. Frank, J. Appelt, S. Bieling, B. Meyer, Influence of catalysts on the pyrolysis of lignites, Fuel 134 (2014) 669–676. [9] E. Pütün, Catalytic pyrolysis of biomass: Effects of pyrolysis temperature, sweeping gas flow rate and MgO catalyst, Energy 35 (7) (2010) 2761–2766. [10] W.J. Su, M.X. Fang, J.M. Cen, C. Li, Z.Y. Luo, K.F. Cen, Influence of metal additives on pyrolysis behavior of bituminous coal by TG-FTIR analysis, in: H. Qi, B. Zhao (Eds.), Cleaner Combustion and Sustainable World, Springer, Berlin, Germany, 2013, pp. pp. 149–159. [11] Y. Fu, Y.H. Guo, K.X. Zhang, Effect of three different catalysts (KCl CaO, and Fe2 O3 ) on the reactivity and mechanism of low-rank coal pyrolysis, Energ. Fuel. 30 (2016) 2428–2433. [12] X.Z. Gong, Z.C. Guo, Z. Wang, Reactivity of pulverized coals during combustion catalyzed by CeO2 and Fe2 O3 , Combust. Flame 157 (2010) 351–356. [13] R. Kandiyoti, J. Lazaridis, B. Dyrvold, C.R. Weerasinghe, Pyrolysis of a ZnCI2 -impregnated coal in an inert atmosphere, Fuel 63 (1984) 1583–1587. [14] R. Jolly, H. Charcosset, J. Boudou, J.M. Guet, Catalytic effect of ZnCl2 during coal pyrolysis, Fuel Process. Technol. 20 (1988) 51–60. [15] E. Abbasi-Atibeh, A. Yozgatligil, A study on the effects of catalysts on pyrolysis and combustion characteristics of Turkish lignite in oxy-fuel conditions, Fuel 115 (2014) 841–849. [16] X.H. Cheng, X.M. He, C. Chen, S. Yi, Influence of Fe2 O3 /CaO catalysts on the pyrolysis products of low-rank coal, Energy Technol. 3 (2015) 1068–1071. [17] J.L. Yu, F.J. Tian, M.C. Chow, C.Z. Li, Effect of iron on the gasification of Victorian brown coal with steam enhancement of hydrogen production, Fuel 85 (2006) 127–133. [18] G. Li, L. Li, L. Shi, L.J. Jin, Z.C. Tang, H.J. Fan, Experimental and theoretical study on the pyrolysis mechanism of three coal-based model compounds, Energ. Fuel. 28 (2014) 980–986. [19] M.L. Poutsma, Free-radical thermolysis and hydrogenolysis of model hydrocarbons relevant to processing of coal, Energ. Fuel. 4 (1990) 113–131. [20] L.H. Kong, G. Li, L.J. Jin, H.Q. Hu, Pyrolysis behaviors of two coal-related model compounds on a fixed-bed reactor, Fuel Process. Technol. 129 (2015) 113–119.
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The role of catalysts in the decomposition of phenoxy compounds in coal: A density functional theory study Jiang-Tao Liu, Ming-Fei Wang, Zhi-Hua Gao, Zhi-Jun Zuo ∗ , Wei Huang ∗ Key Laboratory of Coal Science and Technology of Ministry of Education and Shanxi Province, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China
a r t i c l e
i n f o
Article history: Received 26 May 2017 Received in revised form 15 September 2017 Accepted 20 September 2017 Available online 21 September 2017 Keywords: DFT Phenoxy compound Anisole Catalytic pyrolysis
a b s t r a c t The pyrolysis mechanisms of anisole (C6 H5 OCH3 ), as a coal-based model compound, on CaO, ZnO and ␥Al2 O3 catalysts were studied using density functional theory (DFT). In contrast to the products of pyrolysis (C6 H6 , H2 and CO), the products of catalytic pyrolysis on CaO, ZnO, and ␥-Al2 O3 are C6 H5 OH and C2 H4 ; CO, C5 H6 and C2 H4 ; and C6 H5 OH and C2 H4 , respectively. Our results indicate that CaO, ZnO and ␥-Al2 O3 catalysts not only decrease the energy barrier of C6 H5 OCH3 decomposition but also alter the pyrolysis process and the products. It is also found that the pyrolysis gas H2 alters the products on CaO (C6 H5 OH and CH4 ), but it does not affect the products on ZnO and ␥-Al2 O3 . In sum, these catalysts are beneficial for phenoxy compound decomposition. © 2017 Published by Elsevier B.V.
1. Introduction Coal pyrolysis is considered one of the most important technologies to make the use of coal clean, efficient and comprehensive [1]. Coal pyrolysis is also an initial step in the coal conversion processes of liquefaction and gasification, which strongly influences the complete combustion process, the char reactivity and the product distributions [2]. It has been found that catalysts in coal pyrolysis not only increase the total conversion and product yield [3] but also make the pyrolysis conditions milder. To obtain high-quality oil, gas and chemicals, a large number of catalysts have been used [3–13], including ZnO [13,14], CaO [5], Al2 O3 [6], MgO [9], etc. [15–17]. The organic components of coal mainly include polycyclic aromatic hydrocarbons, heterocyclic compounds and aromatic hydrocarbons [18], and the aromatic rings are normally substituted by groups such as alkyl side chains, phenolic hydroxyl and ether groups [19]. Because the structure of coal is complex and variable, the pyrolysis mechanisms are difficult to describe clearly [20]. To understand the role and improve the efficiency of the catalysts for coal pyrolysis, many theoretical studies have been carried out using model compounds, such as anisole, phenyl ethyl ether,
benzoic acid, phenol, etc. [18,20–24], which has been proven to be an effective method to understand the reaction process for coal pyrolysis over the past few decades [25]. However, these studies only focus on the reaction mechanism of the model compounds and do not consider the role of catalysts. Recently, C6 H5 COOH decomposition, as a model compound, on ZnO, ␥-Al2 O3 , CaO, and MgO surfaces was studied. It was found that ZnO, ␥-Al2 O3 , CaO, and MgO catalysts not only change the energy barrier but also alter the reaction pathways. However, they do not influence the products of C6 H5 COOH decomposition. Additionally, ZnO, MgO, and CaO are beneficial to C6 H5 COOH decomposition, but ␥-Al2 O3 is disadvantageous to C6 H5 COOH decomposition [26]. In other words, it is necessary to consider the role of catalysts in coal pyrolysis. To better understand the role of catalysts for phenoxy compounds, which are common groups in coal [18,22], the mechanisms of C6 H5 OCH3 pyrolysis on CaO, ZnO and ␥-Al2 O3 surfaces were investigated in this paper using density functional theory (DFT), in which C6 H5 OCH3 is used as a model compound, as it is the most simple phenoxy compound. The difference between the pyrolysis mechanism and the catalytic pyrolysis mechanism is compared. Finally, the influence of the pyrolysis gas H2 for C6 H5 OCH3 decomposition on CaO, ZnO and ␥-Al2 O3 surfaces is also considered. 2. Computational methods and models
∗ Corresponding authors. E-mail addresses: [email protected] (Z.-J. Zuo), [email protected] (W. Huang). http://dx.doi.org/10.1016/j.apsusc.2017.09.170 0169-4332/© 2017 Published by Elsevier B.V.
All calculations were performed using DFT methods, as implemented in the Dmol3 package developed by Accelrys, Inc. [27,28],
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¯ Fig. 1. Top views of the (a) CaO (100), (b) ZnO 1010 and (c) ␥-Al2 O3 (110) surfaces.
which has already been extensively used to study the adsorption behaviours and reaction mechanisms on different catalyst surfaces [29–34]. All-electron relativistic DFT was used for core electrons by employing the generalized gradient approximation and Perdew and Wang function (PW91) [35]. Double-numerical basis sets with polarization functions (DNP) were used to describe the valence orbitals of the atoms [36]. The transition states (TS) were located by the complete linear/quadratic synchronous transit method [37]. A Fermi smearing of 0.005 hartree was used. During all calculations, the self-consistent Kohn−Sham equation was applied to optimize all structures geometrically under spin-unrestricted condition [38,39]. According to our previous results, we used p(3 × 4) CaO (100) with a four-layer slab, p(3 × 3) ZnO (100) with a six-layer slab and p(1 × 2) ␥-Al2 O3 (110) with a four-layer slab [26], as these surfaces are the main surfaces for CaO, ZnO and non-spinel ␥-Al2 O3 [40–43]. During the calculation, the bottom two layers were fixed, and other layers and adsorbates were allowed to relax. A vacuum region thickness of 15 Å was chosen to eliminate interaction effects of neighbouring slabs, and 2 × 2 × 1 k-points were used. The top views of the selected three surfaces are shown in Fig. 1. 3. Results and discussion 3.1. Pyrolysis reaction C6 H5 OCH3 pyrolysis has three possible pathways: C6 H5 OCH3 bond scission (C6 H5 OCH3 → C6 H5 + OCH3 ), C6 H5 O CH3 bond scission (C6 H5 OCH3 → C6 H5 O + CH3 ) and C H bond of the methyl group scission (C6 H5 OCH3 → C6 H5 OCH2 + H). According to the three pathways, C6 H5 O and CH3 are preferentially produced via C6 H5 OCH3 → C6 H5 O + CH3 (E = 3.02 eV, Ea = 3.47 eV), rather than C6 H5 OCH3 → C6 H5 + OCH3 (E = 4.63 eV, Ea = 4.97 eV) and C6 H5 OCH3 → C6 H5 OCH2 + H (E = 4.46 eV, Ea = 5.39 eV). Then, CO and C5 H5 are formed from C6 H5 O → C5 H5 + CO (E = 1.67 eV, Ea = 1.76 eV), because the energy barrier of CO and C5 H5 formation is clearly lower than that of C6 H5 and O formation from C6 H5 O → C6 H5 + O (E = 5.87 eV). In further reactions with C5 H5 , the reaction energy of C5 H5 + CH3 → C5 H5 CH3 (E = −3.28 eV, Ea = 0.49 eV) is obviously lower than the reaction energy of C5 H5 → C5 H4 + H (E = 5.09 eV) or CH3 dehydrogenation (CH3 → CH2 + H, E = 5.07 eV), indicating that C5 H5 CH3 formation is possible. Finally, C6 H6 and H2 are formed from C5 H5 CH3 → C6 H6 + H2 (E = −0.20 eV, Ea = 5.04 eV), rather than C5 H5 CH3 →C5 H5 CH2 + H (E = 4.27 eV, Ea = 5.56 eV). The potential energy diagrams with the TS are shown in Fig. 2, and the corresponding IS and FS of each elementary reaction are shown in Fig. S1 of the Supporting Information. As shown in Fig. 2, the C6 H5 OCH3 pyrolysis reaction pathway is C6 H5 OCH3 → C6 H5 O + CH3 → C5 H5 + CO + CH3 → C5 H5 CH3
+ CO→ C6 H6 + H2 +CO, which C5 H5 CH3 → C6 H6 +H2 has the highest energy barrier (Ea = 5.04 eV). Previous experimental studies have also indicated that the pyrolysis of C6 H5 OCH3 begins with the removal of methyl [44–49], followed by ejection of a CO to form C5 H5 . Then, the majority of benzene is formed by a ring expansion reaction of (C5 H5 CH3 ). Finally, C6 H6 is formed. Our DFT results are in accordance with the previous experimental results [49]. Combing with our previous studies [26], the intramolecular hydrogen migration reaction and radical reaction are the first reaction path for C6 H5 COOH and C6 H5 OCH3 pyrolysis, which the corresponding energy barriers are 2.72 and 3.47 eV, respectively. The result shows that the initial decomposition temperature of C6 H5 COOH is lower than that of C6 H5 OCH3 . Previous experiment study shows that C6 H5 COOH converts to C6 H6 at 623 K using C6 H5 COOH as a model compound [50]. Using C6 H5 OCH3 as a model compound, the decomposition temperature is around 1000 K [18,47]. Therefore, our calculation results are in accordance with the experiment results [18,47,50]. 3.2. Catalytic pyrolysis reaction
¯ The reaction intermediates on the CaO (100), ZnO 1010 and ␥-Al2 O3 (110) surfaces are shown in Fig. 3, and their corresponding adsorption configurations and adsorption energies are listed in Table 1. In addition, the adsorption energy and geometrical parameters of some other species are shown in Table S1 of the Supporting Information. 3.2.1. Catalytic pyrolysis reaction on a CaO (100) surface C6 H5 OCH3 prefers to adsorb on the Ca top site of the CaO (100) surface, and the adsorption energy of C6 H5 OCH3 is −0.90 eV. Similar to the pyrolysis reaction of C6 H5 OCH3 , The energy barrier of *C6 H5 OCH3 + * → *C6 H5 O + *CH3 (E = −0.13 eV, Ea = 2.81 eV) is lower than that of *C6 H5 OCH3 + * → *C6 H5 OCH2 + *H (E = 4.27 eV, Ea = 4.82 eV) or *C6 H5 OCH3 + * → *C6 H5 + *OCH3 (E = 0.27 eV, Ea = 3.84 eV), indicating that *C6 H5 O and *CH3 formation are possible. C6 H5 O and CH3 prefer to adsorb on the Ca top and O top site, and the corresponding adsorption energies are −1.65 and −0.58 eV, respectively. In further reactions of *CH3 , the energy barrier of *CH3 + *H → *CH4 + * (E = −4.89 eV, Ea = 0.42 eV) is smaller than that of *CH3 + * → *CH2 + *H (E = 0.97 eV, Ea = 1.53 eV) or *CH3 + *CH3 → *C2 H6 + * (E = −6.18 eV, Ea = 1.60 eV). The result shows that *CH4 formation is possible. Although the energy barriers of *CH3 + * → *CH2 + *H and *CH3 + *CH3 → *C2 H6 + * are similar, the *CH3 coverage is low due to the high energy barrier of *C6 H5 OCH3 + * → *C6 H5 O + *CH3 . Therefore, *CH2 and *H formation is possible. Then, *C2 H4 formation is likely, because the energy barrier of *CH2 + *CH2 → *C2 H4 + * (E = −2.97 eV, Ea = 2.25 eV) is lower than that of *CH2 + * → *CH + *H (E = 2.03 eV, Ea = 2.82 eV).
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Fig. 2. Potential energy diagrams and corresponding TS of the C6 H5 OCH3 pyrolysis reaction.
¯ Fig. 3. C6 H5 OCH3 catalytic pyrolysis reaction intermediates on the CaO (100), ZnO 1010 and ␥-Al2 O3 (110) surfaces.
CH2 adsorbs on the Ca bridge and O top sites, and the adsorption energy is −2.80 eV. After optimization, CH4 and C2 H4 are far away the surface, and the adsorption energies are −0.24 and −0.51 eV, respectively.
In further reactions of *C6 H5 O, the energy barrier of *C6 H5 O + *H → *C6 H5 OH + * (E = 1.00 eV, Ea = 1.02 eV) is smaller than that of *C6 H5 O + * → *C6 H5 + *O (E = 2.35 eV, Ea = 5.29 eV) or *C6 H5 O + * → *C5 H5 + *CO (E = 1.78 eV, Ea = 3.67 eV), indicating
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Table 1 Adsorption energies and geometrical parameters for relevant species on the surfaces. catalysts
species
site
Eads (eV)
bond length d (Å)
CaO (100)
C6 H5 OCH3 C6 H5 O CH3 H CH2 C2 H4 CH4 CO C6 H5 OH
Ca top Ca top O top O top Ca bridge and O top O top O top Ca top Ca top
−0.90 −1.65 −0.58 −0.73 −2.80 −0.51 −0.24 −0.43(−0.55) [52] −0.82
dCa O :2.758 dCa O :2.276 dO C :1.431 dO H :1.028 dCa C :2.601,dO DO H :2.516 dO H :2.379 dCa C :2.714 dCa C :2.539
¯ ZnO 1010
C6 H5 OCH3 C6 H5 O CH3 C6 H5 OCH2 H CH2 C2 H4 CO C5 H5 C5 H6
Zn top Zn top O top Zn bridge O top Zn top and O top Zn top and O top Zn top Zn top Zn top
−0.75 −1.41 −2.25 −1.05 −3.23 −3.56 −0.62 −0.63(−0.68)[53] −1.31 −0.75
dZn O :2.436 dZn O :1.997 dO C :1.449 dZn O :2.156,dZn C :2.043 dO H :0.975 dZn C : 2.063,dO C :1.490 dZn C : 2.005,dO C :1.501 dZn C :2.137 dZn C :2.286 dZn C : 2.278
␥-Al2 O3 (110)
C6 H5 OCH3 C6 H5 O CH3 C6 H5 OCH2
Al3 Al3 Al3 Al3 Al3 and O4 O4 Al3 and O4 Al3 Al3 Al3
−1.71 −2.65 −1.53 −1.73 −1.71 −2.59 −3.22 −1.07 −0.96 −1.98
dAl3 O :1.951 dAl3 O :1.797 d Al3 C :1.980 dAl3 O :.2.057 dAl3 O :2.039,dO4 dO4 H :1.063 dAl3 C :1.938,dO4 dAl3 C :2.445 dAl3 C :2.166 dAl3 O :1.978
H CH2 C2 H4 CO C6 H5 OH
that *C6 H5 OH formation is likely. H prefers to adsorb on the O site with an adsorption energy of −0.73 eV. Fig. 4 shows the potential energy diagrams and TS diagrams for C6 H5 OCH3 decomposition on the CaO (100) surface, and Fig. S2 of the Supporting Information shows the corresponding IS and FS of each elementary reaction. In summary, the catalytic pyrolysis reaction pathway of C6 H5 OCH3 decomposition on the CaO (100) surface is C6 H5 OCH3 (g) → *C6 H5 OCH3 + * → *C6 H5 O + *CH3 → *C6 H5 O + *CH2 + *H → *C6 H5 OH + *CH2 → *C6 H5 OH + 0.5 *C2 H4 → C6 H5 OH(g) + 0.5C2 H4 (g). Our results for the formation of C6 H5 OH are in accordance with the previous experimental result in which aromatic ether carbons were the precursor structures of phenols during pyrolysis [51]. Meanwhile, the catalytic pyrolysis reaction pathway of C6 H5 OCH3 under the pyrolysis gas H2 on the CaO (100) surface is C6 H5 OCH3 (g) + H2 (g) → *C6 H5 OCH3 + *2H + * → *C6 H5 O + *CH3 + *2H → *C6 H5 O + *CH4 → *C6 H5 OH + *CH4 → C6 H5 OH(g) + CH4 (g). The rate-determining step of the two pathways is *C6 H5 OCH3 + * → *C6 H5 O + *CH3 , which has an energy barrier of 2.81 eV. Therefore, the pyrolysis gas H2 alters the reaction pathway and the products of the catalytic pyrolysis of C6 H5 OCH3 on CaO surfaces.
¯ 3.2.2. Catalytic pyrolysis reaction on a ZnO 1010 surface ¯ On the ZnO 1010 surface, C6 H5 OCH3 prefers to adsorb on the Zn top site with an adsorption energy of −0.75 eV. The energy barrier of *C6 H5 OCH3 + * → *C6 H5 OCH2 + *H (E = 0.14 eV, Ea = 1.99 eV) is lower than that of *C6 H5 OCH3 + * → *C6 H5 + *OCH3 (E = 0.06 eV, Ea = 3.24 eV) or *C6 H5 OCH3 + * → *C6 H5 O + *CH3 (E = −0.41 eV, Ea = 2.41 eV), indicating that *C6 H5 OCH2 and *H formation is possible. Comparing with pyrolysis reaction, the reaction pathways are different (C6 H5 OCH3 → C6 H5 O + CH3 vs. *C6 H5 OCH3 + * → *C6 H5 OCH2 + *H), showing that the ZnO catalyst changes the reaction pathway. C6 H5 OCH2 prefers to adsorb on the
C :1.500
C :1.392 C :1.497
Zn bridge site with an adsorption energy of −1.05 eV, and H prefers to adsorb on the O site with an adsorption energy of −3.23 eV. Then, the energy barrier of *C6 H5 OCH2 + * → *C6 H5 O + *CH2 (E = 0.16 eV, Ea = 1.91 eV) is lower than that of *C6 H5 OCH2 + * → *C6 H5 + *OCH2 (E = −2.15 eV, Ea = 2.37 eV) or *C6 H5 OCH2 + * → *C6 H5 OCH + *H (E = −1.62 eV, Ea = 3.43 eV), and thus *C6 H5 O and *CH2 are formed from further reactions of *C6 H5 OCH2 . The adsorption energies of C6 H5 O and CH2 are −1.41 and −3.56 eV, respectively. In further reactions of *CH2 , there are three possible reaction pathways: *CH2 + * → *CH + *H, *CH2 + *H → *CH3 + * and *CH2 + *CH2 → *C2 H4 + *. The results show that *C2 H4 formation is possible (E = −1.44 eV, Ea = 1.95 eV), because the energy barrier of *C2 H4 formation is smaller than that of *CH (E = 2.44 eV, Ea = 4.06 eV) or *CH3 (E = −0.81 eV, Ea = 3.26 eV) formation. The adsorption energy of C2 H4 is −0.62 eV. In further reactions of *C6 H5 O, there are also three possible reaction pathways: *C6 H5 O + * → *C6 H5 + *O, *C6 H5 O + * → *C5 H5 + *CO and *C6 H5 O + *H → *C6 H5 OH + *. *C5 H5 and *CO (E = 0.75 eV, Ea = 0.79 eV) can be more easily formed than *C6 H5 and *O (E = 1.66 eV, Ea = 2.64 eV) or *C6 H5 OH formation (E = 0.61 eV, Ea = 1.21 eV). Finally, a high energy barrier is obtained for *C5 H5 dehydrogenation (E = −0.29 eV, Ea = 2.88 eV), while the energy barrier and reaction energy of *C5 H6 formation from *C5 H5 hydrogenation are 0.88 and 0.28 eV. Therefore, *C5 H6 formation seems likely. The adsorption energies of C5 H5 , CO and C5 H6 are −1.31, −0.63 and −0.75 eV, respectively, and all of them prefer to adsorb on the Zn top site. The for the catalytic pyrolysis of C6 H5 OCH3 on pathway ¯ ZnO 1010 is C6 H5 OCH3 (g) → *C6 H5 OCH3 + * → *C6 H5 OCH2 + *H → *C6 H5 O + *CH2 + *H → *C6 H5 O + 0.5 *C2 H4 + *H →*CO + *C5 H5 + 0.5 *C2 H4 + *H → *CO + *C5 H6 + 0.5 *C2 H4 → CO(g) + *C5 H6 + 0.5C2 H4 (g). The potential energy diagrams, TS, IS and FS diagrams for *C6 H5 OCH3 decomposition are shown in Fig. 5and Fig. S3. *C6 H5 OCH3 + * → *C6 H5 OCH2 + *H has the highest reaction
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Fig. 4. Potential energy diagrams and corresponding TS for C6 H5 OCH3 decomposition on the CaO (100) surface.
¯ Fig. 5. Potential energy diagrams and corresponding TS for C6 H5 OCH3 decomposition on the ZnO 1010 surface.
barrier (Ea = 1.99 eV). As shown in Fig. 5, the pyrolysis gas H2 will not change the reaction pathway of C6 H5 OCH3 decomposition on ¯ surfaces. ZnO 1010
3.2.3. Catalytic pyrolysis reaction on a -Al2 O3 (110) surface The adsorption energy of C6 H5 OCH3 on the ␥Al2 O3 (110) surface is −1.71 eV. The energy barriers of *C6 H5 OCH3 + * → *C6 H5 O + *CH3 (E = −0.47 eV, Ea = 2.11 eV) and *C6 H5 OCH3 + * → *C6 H5 OCH2 + *H (E = 0.41 eV, Ea = 2.04 eV) are similar and are lower than that of *C6 H5 OCH3 + * → *C6 H5 + *OCH3 (E = 0.01 eV, Ea = 2.96 eV). The results indicate that *C6 H5 O and *CH3 or *C6 H5 OCH2 and *H formation are possible. The adsorption energies of C6 H5 O, CH3 , C6 H5 OCH2 and H are −2.65, −1.53,
−1.73 and −2.59 eV, respectively. In addition, C6 H5 OCH2 has two different adsorption configurations. In further reactions of *CH3 , the energy barrier is 1.48 eV for *CH3 migration from the O4 site to the Al3 site. Then, the energy barrier of *CH3 + * → *CH2 + *H (E = 1.05 eV, Ea = 1.15 eV) is smaller than that of *CH3 + *H → *CH4 + * (E = 0.61 eV, Ea = 1.93 eV) or *CH3 + *CH3 → *C2 H6 + * (E = −0.23 eV, Ea = 4.00 eV). The results show that *CH2 forms more easily than *CH4 or *C2 H6 . Finally, *C2 H4 is formed, because the energy barrier of *CH2 + *CH2 → *C2 H4 + * (E = −3.35 eV, Ea = 2.97 eV) is smaller than that of *CH2 + * → *CH + *H (E = 1.24 eV, Ea = 3.48 eV). In further reactions of *C6 H5 O, the energy barrier of *C6 H5 O + *H → *C6 H5 OH + * (E = 1.22 eV, Ea = 1.44 eV) is smaller than that of *C6 H5 O + * → *C6 H5 + *O (E = 2.33 eV, Ea = 2.39 eV)
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Fig. 6. Potential energy diagrams and corresponding TS for C6 H5 OCH3 decomposition on the ␥-Al2 O3 (110) surface.
¯ Fig. 7. The PDOS of one CaO, ZnO and AlO1.5 unit on the first slab of the CaO (100), ZnO 1010 and ␥-Al2 O3 (110) surfaces and DOS of C6 H5 OCH3.
or *C6 H5 O + * → *C5 H5 + *CO (E = 2.70 eV, Ea = 3.76 eV), indicating that *C6 H5 OH formation is likely. The adsorption energies of C2 H4 , CH2 and C6 H5 OH are −1.07, −3.22 and −1.98 eV, respectively. When C6 H5 OCH2 adsorbs on Al3 sites, the reaction energies and energy barriers of *C6 H5 OCH2 + * → *C6 H5 O + *CH2 , *C6 H5 OCH2 + * → *C6 H5 OCH + *H and *C6 H5 OCH2 + * → *C6 H5 + *OCH2 are −0.16 and 2.59 eV, 1.68 and 6.60 eV, −0.76 and 2.30 eV, respectively. However, the corresponding energy barriers and reaction energies of C6 H5 OCH2 adsorption on Al3 and O4 sites are only −0.18 and 0.47 eV, 1.66 and 4.36 eV, −0.78 and 1.55 eV, respectively. Therefore, *C6 H5 OCH2 first needs to migrate from the Al3 sites to the O4 sites, which has an energy barrier 1.72 eV. Subsequently, *C6 H5 O and *CH2 are formed. According to the above results, it can be obtained that there are two catalytic pyrolysis pathways for *C6 H5 COCH3 on ␥-Al2 O3 (110) surfaces: one is C6 H5 OCH3 (g) → *C6 H5 OCH3 + * → *C6 H5 O + *CH3 → *C6 H5 O + *CH2 + *H → *C6 H5 O
+ 0.5 *C2 H4 + *H → *C6 H5 OH + 0.5 *C2 H4 → C6 H5 OH(g) + 0.5C2 H4 (g), while the other is C6 H5 OCH3 (g) → *C6 H5 OCH3 + * → *C6 H5 OCH2 + *H → *C6 H5 O + *CH2 + *H → *C6 H5 O + 0.5 *C2 H4 + *H → *C6 H5 OH + 0.5 *C2 H4 → C6 H5 OH(g) + 0.5C2 H4 (g). The potential energy diagrams and TS diagrams are shown in Fig. 6, and the corresponding IS and FS of each elementary reaction are shown in Fig. S4 of the Supporting Information. The last result of C6 H5 OH is in accordance with the previous experimental result [51]. As shown in Fig. 6, it can be seen that *CH2 → 0.5 *C2 H4 is the rate-determining step (Ea =2.97 eV). Similar to C6 H5 OCH3 decom¯ position on the ZnO 1010 surface, the pyrolysis gas H2 will not change the reaction pathway on the ␥-Al2 O3 (110) surface. In general, the energy barriers of C6 H5 COOH on the highest ¯ CaO (100), ZnO 1010 and ␥-Al2 O3 (110) (2.46, 1.58 and 2.66 eV) are lower than that of C6 H5 OCH3 decomposition (2.81, 1.99 and of C6 H5 COOH 2.97 eV), indicating that the reaction temperature ¯ and ␥-Al2 O3 (110) catalytic pyrolysis on the CaO (100), ZnO 1010
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is lower than that of C6 H5 OCH3 . Comparing with C6 H5 COOH and C6 H5 OCH3 pyrolysis,the highest energy barriers of C6 H5 COOH on ¯ the CaO (100), ZnO 1010 and ␥-Al2 O3 (110) decrease by 0.26, 1.14 and 0.06 eV, and the highest energy barriers of C6 H5 OCH3 on ¯ CaO (100), ZnO 1010 and ␥-Al2 O3 (110) decrease by 2.23, 3.05 and 2.07 eV. The results show that the catalytic effect of C6 H5 OCH3 decomposition is higher than that of C6 H5 COOH decomposition using CaO, ZnO and ␥-Al2 O3 as catalysts. 3.3. Electronic properties In order to explain the results of the section 3.1 and 3.2, the electronic structures are studied. The band gaps of the CaO, ZnO and ␥-Al2 O3 bulk are 3.94, 2.72 and 5.17 eV using DFT + U, which are smaller than that of experiment results (6.25, 3.37 and 8.70 eV) [54–56], but are similar with other calculation results (3.63, 3.10 and 4.90 eV) [43,57,58]. The projected density of states (PDOS) of ZnO one CaO, and AlO1.5 unit on the first slab of the CaO (100), ¯ ZnO 1010 and ␥-Al2 O3 (110) surfaces and DOS of C6 H5 OCH3 are shown in Fig. 7. As shown in Fig. 7a, the band gaps are in the order ZnO (2.38 eV) < ␥-Al2 O3 (110) (3.14 eV) < CaO (100) (3.28 eV). Comparing with the band gap of bulk, the band gaps of the CaO (100), ¯ and ␥-Al2 O3 (110) surfaces decrease due to the surface ZnO 1010 relaxation [58]. It is well known that the band gap, intensity of (P)DOS and hybridization between catalyst and reactant effect the activity of the catalysts. In general, the catalyst with a small band gap is more reactive than that with a high value, and it with a high intensity (hybridization) of valence band is more reactive than that with a low intensity (hybridization) [29,59]. The PDOS intensities of one CaO, ZnO and AlO1.5 unit on the first slab on the CaO (100), ¯ ZnO 1010 and ␥-Al2 O3 (110) surfaces in the valence band are the order ␥-Al in 2 O3 (110) (59.98 eV) < CaO(100) (107.95 eV) < ZnO ¯ ¯ 1010 (155.72 eV). Therefore, the ZnO 1010 surface shows the lowest energy barrier for C6 H5 OCH3 decomposition. The band gap of the CaO(100) is slightly larger than that of the ␥-Al2 O3 (110), but the intensity of the ␥-Al2 O3 (110) is smaller than that of the CaO(100). Therefore, it could not determine the activity of the ␥Al2 O3 (110) and CaO(100) surfaces by the electronic structures, which must consider the hybridization with the reactant. 4. Conclusion In this paper, we studied the catalytic pyrolysis of phenoxy compounds in coal using DFT, in which C6 H5 OCH3 was selected as a ¯ coal-based model compound and CaO (100), ZnO 1010 and ␥Al2 O3 (110) surfaces were selected as catalysts. The results show that the products of pyrolysis are C6 H6 , H2 and CO, and the highest energy consumption step is C5 H5 CH3 → C6 H6 + H2 (Ea = 5.04 eV). The results show that the products of catalytic pyrolysis on the CaO (100) surface are C6 H5 OH and C2 H4 , and the highest energy consumption step is *C6 H5 OCH3 + * → *C6 H5 O + *CH3 ¯ (Ea = 2.81 eV). On the ZnO 1010 surface, the products are CO, C5 H6 and C2 H4 , and the highest energy consumption step is *C6 H5 OCH3 + * → *C6 H5 OCH2 + *H (Ea = 1.99 eV). On ␥-Al2 O3 (110), the products are C6 H5 OH and C2 H4 , and the highest energy consumption step is *CH2 → 0.5 *C2 H4 (Ea = 2.97 eV). According to the highest energy consumption step of pyrolysis and catalytic pyrol¯ ysis on CaO (100), ZnO 1010 and ␥-Al2 O3 (110) surfaces for C6 H5 OCH3 decomposition, the highest energy consumption step shows the order of Ea (ZnO) < Ea (CaO) < Ea (␥-Al2 O3 ) < Ea (no catalyst). These results show that these catalysts not only decrease the energy barrier of C6 H5 OCH3 decomposition but also alter the reaction products of pyrolysis. In addition, it is found that the products on the CaO (100) surface under the pyrolysis gas H2 are C6 H5 OH and CH4 , but the pyrolysis gas H2 does not affect the products on the
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¯ ZnO 1010 and ␥-Al2 O3 (110) surfaces. We should consider the limitations of using anisole (C6 H5 OCH3 ) as the coal-based model compound in the experiments. The decomposition mechanisms of phenoxy compounds on a CaO catalyst may be somewhat different from the catalytic product of coal. H easily produced during the catalytic pyrolysis of coal, which becomes the H source for the decomposition of phenoxy compounds to finally obtain C6 H5 OH, CH4 and C2 H4 . However, no H is produced using C6 H5 OCH3 as the coal-based model compound, and the products are C6 H5 OH and C2 H4 . Acknowledgments The authors gratefully acknowledge by the National Natural Science Foundation of China (21776197), Shanxi Province Science Foundation for Youths (201701D211003), the Program for the Outstanding Innovative Teams of Higher Learning Institutions of Shanxi and the key project of Basic Industrial Research of Shanxi (201603D121014). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2017.09. 170. References [1] X.J. Kong, Y.H. Bai, L.J. Yan, F. Li, Catalytic upgrading of coal gaseous tar over Y-type zeolites, Fuel 180 (2016) 205–210. [2] P.R. Solomon, T.H. Fletcher, R.J. Pugmire, Progress in coal pyrolysis, Fuel 72 (1993) 587–597. [3] F. Pinto, I. Gulyurtlu, L.S. Lobo, I. Cabrita, The effect of catalysts blending on coal hydropyrolysis, Fuel 78 (1999) 761–768. [4] X.W. Zou, J.Z. Yao, X.M. Yang, L. Song, W.G. Lin, Catalytic effects of metal chlorides on the pyrolysis of lignite, Energ. Fuel. 21 (2007) 619–624. [5] T.Y. Zhu, S.Y. Zhang, J.J. Huang, Y. Wang, Effect of calcium oxide on pyrolysis of coal in a fluidized bed, Fuel Process. Technol. 64 (2000) 271–284. [6] Q.R. Liu, H.Q. Hu, Q. Zhou, S.W. Zhu, G.H. Chen, Effect of inorganic matter on reactivity and kinetics of coal pyrolysis, Fuel 83 (2004) 713–718. [7] B. Dwijen, N. Hiroshi, Y. Tadashi, Hydropyrolysis of Alberta coal and petroleum residue using calcium oxide catalyst and toluene additive, Catal. Today 45 (1998) 385–391. [8] M. Seitz, W. Heschel, T. Nägler, S. Nowak, J. Zimmermann, T. Stam-Creutz, W. Frank, J. Appelt, S. Bieling, B. Meyer, Influence of catalysts on the pyrolysis of lignites, Fuel 134 (2014) 669–676. [9] E. Pütün, Catalytic pyrolysis of biomass: Effects of pyrolysis temperature, sweeping gas flow rate and MgO catalyst, Energy 35 (7) (2010) 2761–2766. [10] W.J. Su, M.X. Fang, J.M. Cen, C. Li, Z.Y. Luo, K.F. Cen, Influence of metal additives on pyrolysis behavior of bituminous coal by TG-FTIR analysis, in: H. Qi, B. Zhao (Eds.), Cleaner Combustion and Sustainable World, Springer, Berlin, Germany, 2013, pp. pp. 149–159. [11] Y. Fu, Y.H. Guo, K.X. Zhang, Effect of three different catalysts (KCl CaO, and Fe2 O3 ) on the reactivity and mechanism of low-rank coal pyrolysis, Energ. Fuel. 30 (2016) 2428–2433. [12] X.Z. Gong, Z.C. Guo, Z. Wang, Reactivity of pulverized coals during combustion catalyzed by CeO2 and Fe2 O3 , Combust. Flame 157 (2010) 351–356. [13] R. Kandiyoti, J. Lazaridis, B. Dyrvold, C.R. Weerasinghe, Pyrolysis of a ZnCI2 -impregnated coal in an inert atmosphere, Fuel 63 (1984) 1583–1587. [14] R. Jolly, H. Charcosset, J. Boudou, J.M. Guet, Catalytic effect of ZnCl2 during coal pyrolysis, Fuel Process. Technol. 20 (1988) 51–60. [15] E. Abbasi-Atibeh, A. Yozgatligil, A study on the effects of catalysts on pyrolysis and combustion characteristics of Turkish lignite in oxy-fuel conditions, Fuel 115 (2014) 841–849. [16] X.H. Cheng, X.M. He, C. Chen, S. Yi, Influence of Fe2 O3 /CaO catalysts on the pyrolysis products of low-rank coal, Energy Technol. 3 (2015) 1068–1071. [17] J.L. Yu, F.J. Tian, M.C. Chow, C.Z. Li, Effect of iron on the gasification of Victorian brown coal with steam enhancement of hydrogen production, Fuel 85 (2006) 127–133. [18] G. Li, L. Li, L. Shi, L.J. Jin, Z.C. Tang, H.J. Fan, Experimental and theoretical study on the pyrolysis mechanism of three coal-based model compounds, Energ. Fuel. 28 (2014) 980–986. [19] M.L. Poutsma, Free-radical thermolysis and hydrogenolysis of model hydrocarbons relevant to processing of coal, Energ. Fuel. 4 (1990) 113–131. [20] L.H. Kong, G. Li, L.J. Jin, H.Q. Hu, Pyrolysis behaviors of two coal-related model compounds on a fixed-bed reactor, Fuel Process. Technol. 129 (2015) 113–119.
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