Mechanistic Insights into Ruthenium-Pincer-Catalyzed Amine-Assisted Homogeneous Hydrogenation of CO₂ to Methanol

Abstract

Amine-assisted homogeneous hydrogenation of CO2 to methanol is one of the most effective approaches to integrate CO2 capture with its subsequent conversion to CH3OH. The hydrogenation typically proceeds in two steps. In the first step the amine is formylated via an in situ formed alkylammonium formate salt (with consumption of 1 equiv of H2). In the second step the generated formamide is further hydrogenated with 2 more equiv of H2 to CH3OH while regenerating the amine. In the present study, we investigated the effect of molecular structure of the ruthenium pincer catalysts and the amines that are critical for a high methanol yield. Surprisingly, despite the high reactivity of several Ru pincer complexes [RuHClPNP R (CO)] (R = Ph/i-Pr/Cy/t-Bu) for both amine formylation and formamide hydrogenation, only catalyst Ru-Macho (R = Ph) provided a high methanol yield after both steps were performed simultaneously in one pot. Among various amines, only (di/poly)­amines were effective in assisting Ru-Macho for methanol formation. A catalyst deactivation pathway was identified, involving the formation of ruthenium biscarbonyl monohydride cationic complexes [RuHPNP R (CO)2]+, whose structures were unambiguously characterized and whose reactivities were studied. These reactivities were found to be ligand-dependent, and a trend could be established. With Ru-Macho, the biscarbonyl species could be converted back to the active species through CO dissociation under the reaction conditions. The Ru-Macho biscarbonyl complex was therefore able to catalyze the hydrogenation of in situ formed formamides to methanol. Complex Ru-Macho-BH was also highly effective for this conversion and remained active even after 10 days of continuous reaction, achieving a maximum turnover number (TON) of 9900.

Hydrogenative Depolymerization of Nylons

“… 46 Similarly, Prakash and co-workers could not detect any hemiaminal intermediate during CO 2 hydrogenation via formamides. 47 In contrast, recent work from the Bernskoetter group indicates that hemiaminal dissociation is kinetically hindered and relies on proton-relay catalysis to occur readily. 48 This is in agreement with previous DFT calculations from the same group, highlighting the TOF-limiting nature of this step that is facilitated by the employed iron complex or via proton relay by a solvent/reactant molecule.…”

Homogeneous and heterogeneous catalytic reduction of amides and related compounds using molecular hydrogen

“…The latter procedure works without isolation of the oxazolidone, albeit two different steps are required to avoid catalyst deactivation in the presence of CO 2 . From 2016, the group of Prakash and Olah has been working deeply and intensively in improving the one-pot strategy 109 – 111 . As a result of their efforts, a catalyst TON up to 9900 was reached employing [Ru-MACHO-BH] and pentaethylenehexamine (PEHA) in a biphasic system suitable to be reused four times.…”

Research Progress in Conversion of CO2 to Valuable Fuels

“…For example, a number of pincer complexes can reduce CO 2 into CO, CH 4 , or other compounds [ 79 , 80 , 81 , 82 ]. The ruthenium catalysts prepared by different methods are usually used in such systems [ 83 , 84 , 85 , 86 ] in which the turnover number (TON) of CO 2 to methanol reached 9900 at an optimal condition. All of this shows the good conductivity and catalytic performance of pincer complexes, but these catalysts often rely on precious metals or supported ligands, such as bipyridines.…”