Theoretical study of the post-synthetic metalation of porphyrinic metal-organic framework (pMOF) for CO2 reduction

Abstract

Exploring cutting-edge materials for the conversion of CO2 to valuable chemicals is imperative from both environmental and renewable energy perspectives. Herein, we design a Nb-doped porphyrinic metal-organic framework (Nb-pMOF) and systematically evaluate its electronic and catalytic properties as a single-atom catalyst (SAC) for CO2 conversion to formic acid using density functional theory (DFT). The computational results demonstrate successful doping and the effectiveness of Nb-pMOF as an SAC for CO2 and H2 adsorption. The preferential chemisorption and activation of CO2 on the Nb validate the Eley-Rideal mechanism for CO2 hydrogenation, which proceeds via two potential pathways: nucleophilic and electrophilic. The results show that the nucleophilic pathway takes precedence both thermodynamically and kinetically owing to its Δ G total (–0.33 eV) and RDS energy barrier (1.0 eV) relative to the electrophilic pathway, which has Δ G total of –0.13 eV and an RDS energy barrier of 1.45 eV. Whereas, given the highest energy barrier (2.71 eV) of the uncatalyzed gas-phase concerted mechanism, the lowest barriers of both the Nb-catalyzed nucleophilic and electrophilic pathways highlight the effective role of Nb-pMOF in CO2 hydrogenation. Furthermore, considering the influence of electron-donating (–NH2) and electron-withdrawing (–NO2) substituents on the RDS of the favored pathway (nucleophilic), the results unveil that the barrier significantly decreases to 0.89 eV upon –NO2 substitution. The observed decrease in the barrier is rationalized on the basis of reactivity descriptors, including the HOMO-LUMO gap, chemical potential ( μ ), chemical hardness ( η ), and electrophilicity index ( ω ). This mechanistic study provides valuable insights into fabricating effective and selective transition-metal-doped MOF-based SACs for CO2 conversion.