Density Functional Theory Analysis of Anthraquinone Derivative Hydrogenation over Palladium Catalyst

A density functional theory (DFT) analysis was conducted on the hydrogenation of 2-alkyl-anthraquinone (AQ), including 2-ethyl-9,10-anthraquinone (eAQ) and 2-ethyl-5,6,7,8-tetrahydro-9,10-anthraquinone (H₄ eAQ), to the corresponding anthrahydroquinone (AQH₂ ) over a Pd₆ H₂ cluster. Hydrogenation of H₄ eAQ is suggested to be more favorable than that of eAQ owing to a higher adsorption energy of the reactant (H₄ eAQ), lower barrier of activation energy, and smaller desorption energy of the target product (2-ethyl-5,6,7,8-tetrahydro-9,10-anthrahydroquinone, H₄ eAQH₂ ). For the most probable reaction routes, the energy barrier of the second hydrogenation step of AQ is circa 8 kcal mol^-1 higher than that of the first step. Electron transfer of these processes were systematically investigated. Facile electron transfer from Pd₆ H₂ cluster to AQ/AQH intermediate favors the hydrogenation of C=O. The electron delocalization over the boundary aromatic ring of AQ/AQH intermediate and the electron-withdrawing effect of C=O are responsible for the electron transfer. In addition, a pathway of the electron transfer is proposed for the adsorption and subsequent hydrogenation of AQ on the surface of Pd₆ H₂ cluster. The electron transfers from the abstracted H atom (reactive H) to a neighbor Pd atom (Pd_H ), and finally goes to the carbonyl group through the C₄ atom of AQ aromatic ring (C₄ ).