New mechanistic insights into direct ethylene glycol synthesis from syngas over modified Rh carbonyl catalysts

Abstract

Direct ethylene glycol (EG) synthesis from syngas as one of typical atom-economic reaction is a big challenge in catalysis, attracting wide attention in scientific research. Here, the mechanistic studies on direct syngas-to-EG conversion over the rhodium carbonyl catalyst Rh4(CO)12 modified by triethylamine (Et3N), 1,3-dimethyl-2-imidalidinone (DMI), and triisopropylphosphine (PiPr3) ligands were performed using combined density functional theory (DFT) calculation and the microkinetic modeling. The results demonstrate the significant improvement of selectivity from syngas to EG on going from Rh4(CO)12 to Rh4(CO)10(Et3N)(DMI) and Rh4(CO)10(DMI)(PiPr3) systems on account of the formation of CH2OH species being more favorable than CH3O. More importantly, the effect of different ligands makes a clear change in the carbon chain growth mechanism from the CO insertion into CH2OH in the Rh4(CO)10(DMI)(PiPr3) to the CHO coupling in the Rh4(CO)10(Et3N)(DMI). According to Bader charge and canonical molecular orbital analyses, it is suggested that the facilitated CHO coupling in the Rh4(CO)10(Et3N)(DMI) is associated with stronger electrostatic attraction and π-π conjugation between two CHO intermediates, and the π and σ bonding interactions between CHO and DMI. Such a fundamental mechanistic understanding reveals the nature of tuning the catalytic performance using different ligands, and also opens a new OHCCHO-mediated pathway for direct EG synthesis from syngas over Rh carbonyl catalysts.