Metallated porphyrinic metal−organic frameworks for CO2 conversion to HCOOH: A computational screening and mechanistic study

Catalytic CO2 conversion is a promising and increasingly needed strategy to reduce carbon footprint, stockpile renewable energy in the form of fuels and high value-added chemicals. In recent years, porphyrinic metal-organic frameworks (PMOFs) have emerged as potential catalysts due to their isolated active sites, tunable structures and functionalities. Herein, we report a computational exploration for CO2 conversion into formic acid (HCOOH) on 9 metallated porphyrinic MOF-525(M) (M = CrIII, MnIII, FeIII, CoIII, NiII, CuII, ZnII, RhIII and IrIII) via density functional theory (DFT) calculations. The confinement effect of MOF-525(M) framework on CO2 conversion is found to be negligible. A reaction mechanism involving H2 dissociation and subsequent CO2 hydrogenation is proposed and investigated in detail, and the former is revealed to be rate-determining. From the analysis of natural bonding orbital (NBO) charges, charge transfer among different atoms is observed during H2 dissociation and the magnitude of charge is governed by the internal charge redistribution and the spin-state of metal. Moreover, d–σ* back-donation is unraveled to be significant in H2 dissociation on MOF-525(Rh) and MOF-525(Ir). Good correlations are established between the energy barriers of H2 dissociation and certain geometric/thermodynamic properties. During CO2 hydrogenation, there is significant non-covalent interaction between CO2 and dissociated H atom, and a strong attractive interaction is revealed to be favorable for hydrogenation. All the 9 MOF-525(M) are predicted to be catalytic active over the gas-phase reaction, particularly MOF-525(Rh) and MOF-525(Ir) are screened out as the best catalysts for CO2 conversion to HCOOH. This study demonstrates the capability of metallated porphyrinic MOF-525 to catalyze CO2 conversion and would assist in the future rational design of more efficient MOFs for CO2 conversion.