A Porous π–π Stacking Framework with Dicopper(I) Sites and Adjacent Proton Relays for Electroreduction of CO₂ to C₂₊ Products

Abstract

Crystalline porous materials sustained by supramolecular interactions (e.g., π−π stacking interactions) are a type of molecular crystals showing considerable stability, but their applications are rarely reported due to the high difficulty of their construction. Herein, a stable π−π stacking framework formed by a trinuclear copper(I) compound [Cu 3 (HBtz) 3 (Btz)Cl 2 ] (CuBtz, HBtz = benzotriazole) with pyrazolate-bridged dicopper(I) sites is reported and employed for electrochemical CO 2 reduction, showing an impressive performance of 73.7 ± 2.8% Faradaic efficiency for C 2+ products [i.e., ethylene (44%), ethanol (21%), acetate (4.7%), and propanol (4%)] with a current density of 7.9 mA cm −2 at the potential of −1.3 V versus RHE in an H-type cell and a Faradic efficiency (61.6%) of C 2+ products with a current density of ≈1 A cm −2 and a reaction rate of 5639 μmol m −2 s −1 at the potential of −1.6 V versus RHE in a flow cell device, representing an impressive performance reported to date. In-situ infrared spectroscopy, density functional theory calculations, and control experiments revealed that the uncoordinated nitrogen atoms of benzotriazolates in the immediate vicinity can act as proton relays and cooperate with the dicopper(I) site to promote the hydrogenation process of the *CO intermediate and the C−C coupling, resulting in the highly selective electroreduction of CO 2 to C 2+ products.

Rational Design of Metal–Organic Frameworks for Electroreduction of CO2 to Hydrocarbons and Carbon Oxygenates

“…Along with efficient metal sites, the reasonable design of a chemical microenvironment around the metal sites can significantly enhance ECR performances, which may be considered to be inspired by the synergistic effect of the unique coordination geometry of metal site and its microenvironment of a biological metalloenzyme to achieve exceptional catalytic activity and selectivity. Typically, one can construct the microenvironment of an active site by introducing special functional groups around the active sites in the catalytic system. ,,− Thanks to the tailorable structures of MOFs, regulation of the chemical microenvironment around the active sites in MOFs is highly feasible, which provides an unique opportunity for the design of proton-based interactions and control of framework flexibility, and is important to tune the catalytic performances of MOFs.…”

“…(c) The corresponding Gibbs free energy barriers of the elementary steps on CuBtz during the ECR pathway. Reprinted with permission from refs and . Copyright 2022 American Chemical Society.…”

“…Actually, some properties which can improve ECR selectivity toward a single component product mentioned in the prior section, such as d-orbital upshift, electron density enhancement of the metal site, and proton source configuration, might also lead to a diminished activation energy of the rate-determining step. In the case of CuBtz, 55 the uncoordinated nitrogen atoms and N−H groups can serve as highly efficient proton relays for promoting the transfer of dissociated protons from the electrolyte to ECR intermediates (Figure 7b), thus accelerating the proton transfer process and reducing the reaction energy barrier of the key step of C−C coupling. Consequently, CuBtz exhibited a very high current density of ∼1 A cm −2 in 1 M KOH solution.…”

“…zole) into a MOF-like structure, as being characterized by powder X-ray diffraction. 55 In the well-defined porous structure of CuBtz, the dicopper(I) active sites (Figure 4b, Cu•••Cu distance = 3.52 Å) are closely adjacent to uncoordinated nitrogen atoms on the triazole ligands (Figure 7b). As has been evidenced by a substantial reduction of ECR performance through replacement of the triazole ligands with analogous indole ligands without uncoordinated nitrogen atoms and N−H groups into the trinuclear cluster, such uncoordinated nitrogen atoms and N−H groups on the triazole groups serve undoubtably as highly efficient proton relays, which can effectively reduce the Gibbs free energy barrier of the potential determining step, to facilitate the ECR of C 2+ production (FE = ∼74%) (For details about the mechanism, see the following section).…”

“…In MOFs, di- and tricopper sites (Figure b,c) usually take the form of either *CO–*CHO or *CO–*COH coupling (Figure c,d) instead of the *CO–*CO coupling since they have di- or multiple metal sites with adjacent Cu···Cu separations of ∼3.3 to ∼3.6 Å. ,, Lan’s group reported that a series of one-dimensional (1D) coordination polymers, Cu-PzX (X = H, Cl, Br, I), with dicopper sites (Figure b) can allow the C–C coupling of *CO–*COH to yield C 2 H 4 . Besides, dicopper sites in CuBtz and MAF-2E have also been reported, which led to C 2+ compounds as the main products and will be discussed in subsequent sections. We further revealed by periodic density functional theory (PDFT) calculations that a 3D MOF, [Cu 3 (μ 3 –OH)­(μ 3 -trz) 3 (OH) 2 (H 2 O) 4 ]· x H 2 O ( Cutrz , Htrz = 1 H ,1,2,4-triazole), can bind three C 1 intermediates at its tricopper active site prior the formation of *CO.…”