Tailoring ligand fields of metal-azolate frameworks for highly selective electroreduction of CO2 to hydrocarbons at industrial current density

Abstract

The catalytic activity of a metal complex is often contingent upon factors such as its valence state, coordination configuration of the metal ion, and coordination ability of the ligand. Hence, revealing the influence of the ligand field of the active metal center on the selectivity of the product for electrochemical CO2 reduction is crucial. Herein, three isostructural metal-azolate frameworks (MAFs) (Cu-BTP, Cu-BTTri, and Cu-BTT) with the same cyclic tetracopper(II) cluster units, namely [Cu3(BTP)2] (Cu-BTP, H3BTP = 1,3,5-tris(1H-pyrazol-4-yl)benzene), [Cu3(BTTri)2] (Cu-BTTri, H3BTTri = 1,3,5-tris(1H-1,2,3-triazol-5-yl)benzene), and [Cu3(BTT)2] (Cu-BTT, H3BTT = 1,3,5-tris(2H-tetrazol-5-yl)benzene) were synthesized using pyrazolate-, triazolate-, and tetrazolate-based ligands, respectively. The synthesized MAFs were subjected to analysis to evaluate their electrochemical capabilities for reducing CO2 under identical reaction conditions. Among them, the pyrazolate MAF, Cu-BTP, delivers a current density of 1.25 A cm2 in a flow cell device with the highest Faradiac efficiency for hydrocarbons (CH4, 60%; C2H4, 22%). Furthermore, the system shows no obvious degradation over 60 h of continuous operation. The order of selectivity of the three MAFs for hydrocarbon production is consistent with the corresponding pKa values of the azolate ligands. Theoretical calculations show that a stronger Lewis basicity of the organic ligand, resulting in a stronger ligand field strength, is conducive to strengthening the binding of metal centers with key intermediates, such as *CO and *CHO. This ultimately leads to the deep reduction of CO2 to hydrocarbons.