Theoretical Study on Catalytic Pyrolysis of Benzoic Acid as a Coal-Based Model Compound

Abstract

Benzoic acid (C 6 H 5 COOH) is selected as a coal-based model compound, and its catalytic pyrolysis mechanisms on ZnO, γ-Al 2 O 3 , CaO, and MgO catalysts are studied using density functional theory (DFT) compared to the non-catalytic pyrolysis mechanism. DFT calculation shows that the pyrolysis process of C 6 H 5 COOH in the gas phase occurs via the direct decarboxylation pathway (C 6 H 5 COOH → C 6 H 6 + CO 2 ) or the stepwise decarboxylation pathway (C 6 H 5 COOH → C 6 H 6 COO → C 6 H 6 + CO 2 ). For C 6 H 5 COOH catalytic pyrolysis on the ZnO (101̅ 0) surface, the preferred reaction pathway is C 6 H 5 COOH → C 6 H 5 COO + H → C 6 H 6 + CO 2 , whereas the preferred reaction pathway on γ-Al 2 O 3 (110), CaO (100), and MgO (100) surfaces is C 6 H 5 COOH → C 6 H 5 COO + H → C 6 H 5 + CO 2 + H → C 6 H 6 + CO 2 , indicating that the presence of catalysts changed the pyrolysis mechanism of C 6 H 5 COOH. In addition, dissociative adsorption of C 6 H 5 COOH is observed on these surfaces. It is found that ZnO (101̅ 0), MgO (100), and CaO (100) are beneficial to C 6 H 5 COOH decomposition, but γ-Al 2 O 3 (110) is disadvantageous to the C 6 H 5 COOH decomposition. At the same reaction temperature, the rate constants show the order: k(ZnO) > k(MgO) > k(CaO) > k(no catalyst) > k(γ-Al 2 O 3 ).

Theoretical Study on Influence of Cobalt Oxides Valence State Change for C6H5COOH Pyrolysis

“…The energy barriers of pathways 1 and 2 are similar, which are far lower than that of pathway 3. Therefore, pathways 1 and 2 for C6H5COOH pyrolysis are possible, which is similar with our previous study [20]. The potential energy diagrams of C6H5COOH pyrolysis and the corresponding initial states (IS), transition states (TS) and final states (FS) are shown in Figure 2.…”

“…The potential energy diagrams and corresponding IS, TS and FS geometrical structures of C6H5COOH catalytic pyrolysis on Co3O4(110)-B surface are shown in Figure 4. It is demonstrated that the energetically preferred pathway of C6H5COOH catalytic pyrolysis on Co3O4(110)-B is C6H5COOH(g) →*C6H5COO + *H →*C6H5 + *CO2 +*H→*CO2+*C6H6 →CO2(g) + C6H6(g), which is similar with C6H5COOH pyrolysis on MgO, CaO and γ-Al2O3 surfaces [20]. The highest energy barrier (2.28 eV) on Co3O4(110)-B surface is smaller than that of C6H5COOH pyrolysis without a catalyst (2.78 and 2.89 eV).…”

“…The corresponding adsorption energies and geometrical parameters are shown in Table 1. The O-H bond scission of *C6H5COOH after optimization indicates that C6H5COOH is dissociative adsorption on Co3O4(110)-B, which is the same as HCOOH adsorption on ZnO surface [28,29], and C6H5COOH adsorption on ZnO, MgO, CaO and γ-Al2O3 surfaces [20]. *C6H5COO energy of -1.91 eV.…”

“…However, those theoretical studies focus on the coal pyrolysis [18,19], and the role of catalysts are not considered. Recently, the role of some catalysts (ZnO, γ-Al 2 O 3 , CaO, and MgO) for coal catalytic pyrolysis is studied by using DFT, which C 6 H 5 COOH, C 6 H 5 CHO and C 6 H 5 OCH 3 are selected as the model compounds [20][21][22]. It is found that the catalysts alter the energy barrier and reaction pathway.…”

Quantitative study of the pyrolysis of levoglucosan to generate small molecular gases

“…According to the current prevailing opinion, 9,24,39 the decarboxylation reaction during pyrolysis produces CO 2 . Wang et al 40 studied the decarboxylation mechanism of benzoic acid and proposed three possible pathways for decarboxylation: direct decarboxylation, stepwise decarboxylation, and a stepwise radical process. According to the results of Wang's calculations, the energy barrier of the direct decarboxylation (246 kJ mol À1 ) and stepwise decarboxylation (245 kJ mol À1 ) were much lower than that of the stepwise radical process (470 kJ mol À1 ).…”