Host–guest molecular interaction promoted urea electrosynthesis over a precisely designed conductive metal–organic framework

Abstract

The highly selective electrocatalytic activation of N₂ and CO₂ to synthesize value-added urea via a C–N coupling reaction is an extremely challenging reaction that is largely inhibited by the poor chemisorption and coupling abilities of the reactant molecules. Herein, the novel conductive MOF Co–PMDA–2-mbIM (PMDA = pyromellitic dianhydride; 2-mbIM = 2-methyl benzimidazole) is designed, attaining a record-high urea yield rate of 14.47 mmol h⁻¹ g⁻¹ with a FE of 48.97% at −0.5 V vs. RHE. The host–guest interactions involved not only generate desirable local electrophilic and nucleophilic regions but they also allow evolution from high-spin state Co³⁺ (HS: t⁴_2ge²_g) to intermediate-spin state Co⁴⁺ (IS: t⁴_2ge¹_g) in CoO₆ octahedrons. Thus, N₂ and CO₂ can be adsorbed in a targeted fashion and activated to produce the desired *NN* and *CO intermediates. Subsequently, the low e_g orbital occupied Co⁴⁺ (t⁴_2ge¹_g) easily accepts electrons from σ orbital of *NN* and effectively triggers the C–N coupling reaction to emerge *NCON* urea precursor.