Toward Combined Carbon Capture and Recycling: Addition of an Amine Alters Product Selectivity from CO to Formic Acid in Manganese Catalyzed Reduction of CO2

Abstract

Owing to the energetic cost associated with CO₂ release in carbon capture (CC), the combination of carbon capture and recycling (CCR) is an emerging area of research. In this approach, "captured CO₂," typically generated by addition of amines, serves as a substrate for subsequent reduction. Herein, we report that the reduction of CO₂ in the presence of morpholine (generating mixtures of the corresponding carbamate and carbamic acid) with a well-established Mn electrocatalyst changes the product selectivity from CO to H₂ and formate. The change in selectivity is attributed to in situ generation of the morpholinium carbamic acid, which is sufficiently acidic to protonate the reduced Mn species and generate an intermediate Mn hydride. Thermodynamic studies indicate that the hydride is not sufficiently hydritic to reduce CO₂ to formate, unless the apparent hydricity, which encompasses formate binding to the Mn, is considered. Increasing steric bulk around the Mn shuts down rapid homolytic H₂ evolution rendering the intermediate Mn hydride more stable; subsequent CO₂ insertion appears to be faster than heterolytic H₂ production. A comprehensive mechanistic scheme is proposed that illustrates how thermodynamic analysis can provide further insight. Relevant to a range of hydrogenations and reductions is the modulation of the hydricity with substrate binding that makes the reaction favorable. Significantly, this work illustrates a new role for amines in CO₂ reduction: changing the product selectivity; this is pertinent more broadly to advancing CCR.

Figure 1.

CVs of 1 mM (top orange): (bpy)Mn(CO)₃CN; (middle purple): [(bpy)Mn(CO)₃]⁻; and (bottom red): [(bpy)Mn(CO)₃(MeCN)]⁺ in 0.1 M TBAPF₆/MeCN solvent, with a scanrate of 0.1 V/s. S designates a coordinated solvent molecule. Scan directions and starting potentials are indicated by the black arrows. The electrochemical reactions that are occurring are shown; unless labeled, the reactions that occur at the same potentials correspond to the same reaction. The Mn(0) is unstable in solution and dimerizes once it is formed.

Figure 2.

CVs (100 mV/s) of the catalysts (1 mM) in the presence of CO₂ and the additives noted in the legends. For clarity, the return oxidation current is not shown. (a) Effect of increasing morpholine on the catalysis of (bpy)Mn(CO)₃CN. (b) Effect of increasing PhOH on the catalysis of (bpy)Mn(CO)₃CN in the presence and absence of morpholine. (c) Effect of increasing PhOH on the catalysis of (mesbpy)Mn(CO)₃Br in the presence and absence of morpholine.

Figure 3.

Plot of hydricity versus pK_a in MeCN. Solid lines represent boundaries for the speciation (boxed). Location of (N^N)Mn(CO)₃H are shown, with the downward arrow indicating how the hydricity is modulated by formate binding. Formate is only obtained in the gray triangle. Product distributions are shown along the x-axis as a function of pK_a. For FA, formate, and H₂, the hydricity or apparent hydricity must be sufficient enough to obtain the products.

Scheme 1.

Equilibrium between morpholine and CO₂

Scheme 2.

Effect of Acid on the Product Distribution of CO₂ Reduction

Scheme 3.

Observed Reactivity of (N^N)Mn(CO)₃(H)

Scheme 4.

Thermochemical cycle for determination of the hydricity of (N^N)Mn(CO)₃H. For simplicity, (N^N)Mn(CO)₃ is abbreviated as Mn

Scheme 5.. Thermochemical Equations Pertinent to Formate Production^a

^aFor simplicity, (N^N)Mn(CO)₃ is abbreviated as Mn. Except for eq 6, free energy values are provided for N^N = mesbpy (bpy). The hydricity of formate under the conditions presented may differ slightly, given that we are not operating at 1 atm of CO₂, and that the high concentrations of morpholine and PhOH may alter the CO₂ solubility.

Scheme 6.. Thermochemical Equations Pertinent to Direct Protonation of the Bound Formate^A

^AFor simplicity, (N^N)Mn(CO)₃ is abbreviated as Mn. Free energies are in units of kcal·mol⁻¹.

Scheme 7.. Proposed Mechanisms for the Electrocatalytic Reduction of CO₂ to Give CO (Top) or FA (Bottom)^a

^aH₂ production is indicated by gray arrows, and the stoichiometry shown is for the heterolytic pathway. Direct protonation of Mn(OCHO) is indicated by light purple arrows, and reduction followed by formate loss is indicated by dark purple arrows.