Mechanistic Aspects of Acrylic Acid Formation from CO2-Ethylene Coupling over Palladium- and Nickel-based Catalysts

The detailed mechanism of catalytic conversion of CO2 and ethylene to acrylic acid has been investigated through DFT and DLPNO-CCSD(T) calculations. Four bidentate ligands, dmpe (1,2-bis(dimethylphosphino)ethane), dcpe (1,2-bis(dicyclohexylphosphino)ethane), dtbpe (1,2-bis(di-tert-butylphosphino)ethane), and tmeda (tetramethylethylenediamine) were systematically studied to understand the different catalytic behavior of the Pd-based catalyst and the well-known Ni-based catalyst. The energy barrier of Pd-/Ni-catalyzed C-C coupling appears to increase with the larger steric hindrance of the ligand, whereas the barrier of the corresponding -H elimination shows an opposite tendency. The barrier for C-C coupling is likely higher than the associated barrier for -H elimination in both catalytic systems provided that the coordinated ligand is bulky enough. The palladium catalytic center is more effective than the nickel center for the C-C coupling and -H elimination steps in the presence of bulky ligands, whereas the nickel catalyst in turn performs better with small ligands. The palladium catalyst tends to be superior to the nickel catalyst during the following hydrogen transfer to the O atom. In general, the diamine ligand (tmeda) is less efficient than the diphosphine ligands for both systems. In the absence of the auxiliaries, a novel chelating carbene ligand was proposed to be quite efficient for Pd-catalyzed C-C coupling. Two ligands (dcpm, dtbpm) showed relatively better performance towards the palladium-catalyzed acrylic acid formation reaction among all the investigated ligands.