Are Amines the Holy Grail for Facilitating CO₂ Reduction?

Abstract

The selective and efficient reduction of carbon dioxide represents a key solution to producing non‐fossil‐fuel‐based feedstocks for the chemical industry, while alleviating the increasing atmospheric concentration of this greenhouse gas. A variety of catalytic methods for the CO2 reduction reaction (CO2RR) have been developed, including hydrogenations and electrochemical or photochemical reductions. For many of the most significant breakthroughs reported in the last decade, we realized that amines or closely related functional groups play a critical role for such transformations, and in several cases, are directly associated with the catalyst as a pendant group. Amines play multiple roles, such as CO2 trapping agents, proton shuttles, electron donors, or facilitators of CO2 reductions through formamide derivatives. In this Viewpoint, we compile some of these recent findings, and discuss their significance in a broader context in an attempt to provide guidelines for the design of new catalysts with enhanced activity and selectivity.

Molecular Dye-Sensitized Photocatalysis with Metal-Organic Framework and Metal Oxide Colloids for Fuel Production

“…Thus, 66-NH 2 provides an increased local CO 2 concentration available during the course of photocatalysis, thereby reducing diffusion limitations, as well as potentially activating CO 2 [59]. This is potentially shown by increased turnover frequencies (TOFs) from ZrO 2 -based, to 66-based, to 66-NH 2 -based assemblies (Figure S10).…”

Efficient Electrochemical Reduction of CO2 to Formate in Methanol Solutions by Mn Functionalized Electrodes in the Presence of Amines

“…This strongly suggests that the main source of protons in solution is MeOH rather than the ammonium moieties paired with carbamates or carbonates. 36,37 TMEDA stands as an extreme case in the series, markedly favoring H2 over formate. The conversion of this α,β-diamine into a bisammonium dication upon double carbonation is strongly disfavored, therefore its main adduct with CO2 combines a basic nitrogen moiety and an ammonium group (species 1c-TMEDA, Figure 4a).…”

“…32,33 As mentioned earlier, examples of electrochemical CO2 reduction integrated to its absorption remain scarce. [34][35][36] To our knowledge, only two studies to date reported the reduction of CO2 with a Mn-based soluble electrocatalyst, in the presence of amines (in solution or bound to the metal chelating unit). 34 Amine moieties were proposed not only to provide binding sites, hence a reservoir of CO2 under the form of carbamates, but also to stabilize and promote the formation of the hydride catalytic intermediate (HMn), thereby favouring formate production instead of CO. 36,37 These amines, which are supposed to work as proton shuttle, were either introduced in large excess with respect the catalyst 34 or upon an elaborated synthetic procedure as a side-arm of the Mn catalyst (Figure 1).…”

Outer-coordination sphere in multi-H+/multi-e–molecular electrocatalysis

“…To mimic the complex enzymatic system found in nature, simpler proton relays have been designed for CO 2 reduction. Several hydrophilic groups have been utilized to achieve this shuttling effect, including phenol (Margarit et al, 2018;Nichols et al, 2020;Sinha and Warren, 2018), triazole (Lashgari et al, 2020;Sen et al, 2019), polyethylene glycol (PEG) (Chaturvedi et al, 2021;Lashgari et al, 2020), imidazole (Sung et al, 2019), urea (Gotico et al, 2020;Haviv et al, 2018), and amine (Jakobsen et al, 2021). The relay can be as short as a single OH group in the second coordination sphere, as achieved using an asymmetric Fe-porphyrin (Amanullah et al, 2021), (B) Fe-porphyrin bearing eight pendant proton relays (FeTPPOH 8 ) (Costentin et al, 2012), (C) Co-bipyridine with a bridging pendant amine group (CoBpy-amine) (Chapovetsky et al, 2018), (D) Zn-porphyrin containing 8 triazole units each with PEG proton relays (Williams et al, 2020), (E) Feporphyrin with a single PEG proton relay (Chaturvedi et al, 2021), (F) Mn-bipyridine with imidazole in the second coordination sphere (Sung et al, 2019).…”