Electrocatalysis is a promising tool for utilizing carbon dioxide as a feedstock in the chemical industry. However, controlling the selectivity for different CO2 reduction products remains a major challenge. We report a series of manganese carbonyl complexes with elaborated bipyridine or phenanthroline ligands that can reduce CO2 to either formic acid, if the ligand structure contains strategically positioned tertiary amines, or CO, if the amine groups are absent in the ligand or are placed far from the metal center. The amine-modified complexes are benchmarked to be among the most active catalysts for reducing CO2 to formic acid, with a maximum turnover frequency of up to 5500 s–1 at an overpotential of 630 mV. The conversion even works at overpotentials as low as 300 mV, although through an alternative mechanism. Mechanistically, the formation of a Mn–hydride species aided by in situ protonated amine groups was determined to be a key intermediate by cyclic voltammetry, 1H NMR, DFT calculations, and infrared spectroelectrochemistry.

Carbon dioxide utilization through electrocatalysis is a promising pathway toward a more sustainable future. In this work the electrocatalytic reduction of carbon dioxide by ReI and RuII bipyridine complexes bearing pendant amines (N,N′-(([2,2′-bipyridine]-6,6′-diylbis­(2,1-phenylene))­bis­(methylene))­bis­(N-ethylethanamine) (dEAbpy)) is evaluated. In both cases, the major reduction product is carbon monoxide accompanied by some formic acid, although the yield of the latter never reaches the predominant level known from the corresponding Mn­(dEAbpy)­(CO)3Br complex. This demonstrates the profound effect of the identity of the metal center, in addition to the ligand, for the product distribution. In this work, we report the synthesis procedures and X-ray diffraction studies along with electrochemical and infrared spectroelectrochemical studies of Re­(dEAbpy)­(CO)3Cl and Ru­(dEAbpy)­(CO)2Cl2 to propose a mechanism for the CO2 reduction reaction.

Fibre-reinforced epoxy composites are well established in regard to load-bearing applications in the aerospace, automotive and wind power industries, owing to their light weight and high durability. These composites are based on thermoset resins embedding glass or carbon fibres1. In lieu of viable recycling strategies, end-of-use composite-based structures such as wind turbine blades are commonly landfilled1–4. Because of the negative environmental impact of plastic waste5,6, the need for circular economies of plastics has become more pressing7,8. However, recycling thermoset plastics is no trivial matter1–4. Here we report a transition-metal-catalysed protocol for recovery of the polymer building block bisphenol A and intact fibres from epoxy composites. A Ru-catalysed, dehydrogenation/bond, cleavage/reduction cascade disconnects the C(alkyl)–O bonds of the most common linkages of the polymer. We showcase the application of this methodology to relevant unmodified amine-cured epoxy resins as well as commercial composites, including the shell of a wind turbine blade. Our results demonstrate that chemical recycling approaches for thermoset epoxy resins and composites are achievable.

No abstract

Selective reduction of CO 2 is an efficient solution for producing nonfossil-based chemical feedstocks and simultaneously alleviating the increasing atmospheric concentration of this greenhouse gas. With this aim, molecular electrocatalysts are being extensively studied, although selectivity remains an issue. In this work, a combined experimental−computational study explores how the molecular structure of Mn-based complexes determines the dominant product in the reduction of CO 2 to HCOOH, CO, and H 2 . In contrast to previous Mn(bpy-R)(CO) 3 Br catalysts containing alkyl amines in the vicinity of the Br ligand, here, we report that bpybased macrocycles locking these amines at the side opposite to the Br ligand change the product selectivity from HCOOH to H 2 . Ab initio molecular dynamics simulations of the active species showed that free rotation of the Mn(CO) 3 moiety allows for the approach of the protonated amine to the reactive center yielding a Mn-hydride intermediate, which is the key in the formation of H 2 and HCOOH. Additional studies with DFT methods showed that the macrocyclic moiety hinders the insertion of CO 2 to the metal hydride favoring the formation of H 2 over HCOOH. Further, our results suggest that the minor CO product observed experimentally is formed when CO 2 adds to Mn on the side opposite to the amine ligand before protonation. These results show how product selectivity can be modulated by ligand design in Mn-based catalysts, providing atomistic details that can be leveraged in the development of a fully selective system.