We report the synthesis, characterization, and catalytic CO2 reduction activity of two LMn(CO)3Br complexes
with carbene-pyridine-carbene pincer ligands, [MnCNCMe]Br 1 and [MnCNCBn]Br (Bn = benzyl) 2.
X-ray crystallography reveals an octahedral coordination environment
with an outer sphere Br anion for 1. Catalyst 2 performs the reduction of CO2 to CO at 100 mV more positive
potential with similar current densities as 1. We hypothesize
the bulkier benzyl arms on the pincer hinder formation of a dimer.
They also alter the wingtip electronics, enabling operation at a lower
overpotential. We use normal pulse voltammetry and diffusion ordered
spectroscopy to quantify a 1e– reduction per manganese
center at the catalytic onset. We now show turnover even in the absence
of added protons..
This work describes new methods for the chemical recycling of end-use poly(ethylene terephthalate) (PET) in batch, microwave and electrochemical reactors. The reactions are based on basic hydrolysis of the ester moieties in the polymer framework and occur under mild reaction conditions with low-cost reagents. We report end-use PET depolymerization in refluxing methanol with added NaOH with 75% yield of terephthalic acid in batch after 12 h, while yields up to 65% can be observed after only 40 min under microwave irradiation at 85 °C. Using basic conditions produced in the electrochemical reduction of protic solvents, electrolytic experiments have been shown to produce 17% terephthalic acid after 1 h of electrolysis at −2.2 V vs. Ag/AgCl in 50% water/methanol mixtures with NaCl as a supporting electrolyte. The latter method avoids the use of caustic solutions containing high-concentration NaOH at the outset, thus proving the concept for a novel, environmentally benign method for the electrochemical recycling of end-use PET based on low-cost solvents (water and methanol) and reagents (NaCl and electricity).
This work studies the electrocatalytic
disproportionation of CO2 into CO and CO3
2– mediated by
a [Mn-2,6-bis(1-(alkyl)imidazol-2-ylidene)pyridine]Br pincer complex.
We identify three mechanistic scenarios involving one or two catalytically
active metal centers in the activation of carbon dioxide and use density
functional theory to map out the energy landscape for each of the
mechanistic steps. Experimentally determined second-order kinetics
in CO2 consumption, the formation of CO and CO3
2– together with an observed order in catalyst
of 0.5 determined by cyclic voltammetry using the Burés normalized
timescale method, suggest a turnover-limiting CO3
2– loss through the scission of a bimetallic species. Faradaic efficiencies
for the reduction of CO2 to CO of 86 ± 4% are observed.
Given the reaction mass balance, we estimate a Faradaic efficiency
from the conversion of CO2 to CO3
2– of 93 ± 4%. Our combined experimental−theoretical approach
suggests that two sequential CO2 insertions are followed
by a rearrangement to produce carbon monoxide and carbonate from two
molecules of CO2 at singly reduced catalyst molecules.
The disproportionation reaction reported herein combines the power
of catalytic CO2 conversion with sequestration and provides
a new chemical avenue for the conversion of carbon dioxide.
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