Transition Metal Mediated C-O Bond Cleavage: From CO2 Activation to Lignin Degradation
Abstract: CO2 activation and conversion mediated by transition metal(TM) catalysts were investigated. Homogeneous catalysis of the reverse water gas shift reaction CO2+H2→H2O+CO was studied as a means to reduce CO2. β-diketiminatometal models L’MI ( L’ =C3N2H5-; M = first-row TMs) were considered as potential catalysts. The thermodynamics of prototypical reaction pathways were simulated using B3LYP/aug-cc-pVTZ. Results show that middle series metal complexes result in more thermodynamically favorable properties; therefore, more detailed thermodynamic and kinetic studies were carried out for Mn, Fe, and Co complexes.
On the other hand, heterogeneous catalysis of the reduction of CO2 to CO were carried out on Fe, Co, Ni, and Cu surfaces, using the PBE functional. Reaction barriers were calculated using the climbing image nudged elastic band method. Lignin degradation is one of the most significant challenges to achieve the full potential of lignocellulosic bio-fuels. To aid in addressing this challenge, the C-O bond activation of the β-O-4 linkage of lignin was investigated. Late 3d and 4d transition metal ion (Fe, Co, Ni, Cu, Ru, Rh, Pd, and Ag) mediated activation of dimethyl ether was studied to investigate the intrinsic catalytic properties of metals on C-O bond cleavage.
A set of density functional theory (DFT) methods (BLYP, B3LYP, M06, M06-L, B97-1, B97-D, TPSS, and PBE) with aug-cc-pVTZ basis sets were utilized in calibration with CCSD(T)/CBS calculations on reaction energies and barriers. Based on the thermodynamic favorability of transition metals, group VIII metals (Fe, Ru, and Os) with “pincer” type ligands were chosen for the catalysis of the C-O bond activation of β-O-4 linkage model compound (Ph-CH(OH)-CH2-O-Ph). B3LYP/CEP-31-G(d) was used for geometry optimizations and energy calculations.