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 (101̅0) 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 (101̅0), 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 ).