Rational design of metal-ligands for the conversion of CH(4)and CO(2)to acetates: role of acids and Lewis acids

Abstract

The capture, sequestration and utilization of carbon dioxide have attracted global attention in the environmental field, science and industry. The concurrent conversion of CO(2)and CH(4)is a reasonable solution to reduce greenhouse gases, but it remains a challenging research topic. Herein, we reveal the catalytic activity of bifunctional half-sandwich complexes (metal = Ru, Rh, and Ir) in the direct conversion of methane and carbon dioxide to acetic acid (AA) using density functional theory. The use of Ru/Rh/Ir metals with ethanediphosphine (PP),N-tosylethylenediamine (NNTs), and ethylene glycol (OO) ligands shows superb performance in lowering the free energy (Delta G) barrier (Delta G(double dagger)) for the conversion. To activate CH(4)and CO(2)molecules, we used extra additives to reduce the high energy barriers required by simple metal-ligand complexes. For CH(4)activation, additives of AA and trifluoroacetic acid (TFA) were used to assist the proton abstraction in the Ru-NNTs/OO complexes, while no acid additives were used for the PP ligands. In the case of the PP ligands, the C-H activation occurs through a "concerted oxidative addition-reductive elimination" (OA-RE) process, whereas in the NNTs/OO ligands, C-H activation occurs through "cyclometallation deprotonation" (CMD) irrespective of the presence of acid additives. The reduction in Delta G(double dagger)for CH(4)activation in Ru-NNTs/OO is 5-10 kcal mol(-1)using acid additives. In contrast, in Ru-PP, this Delta G(double dagger)barrier is 20 kcal mol(-1)without additives. For CO(2)activation, AlCl3(Lewis acid) was used to insert CO(2)into the metal site of the Ru-NNTs/OO and Ru/Rh/Ir-PP complexes. The reduction in Delta G(double dagger)using AlCl(3)for CO(2)activation in Ru-NNTs center dot AA/OO center dot 2TFA and Ru/Rh/Ir-PP is 15-30 kcal mol(-1). Then, the overall activation barrier is reduced to 20-27 kcal mol(-1).