Elucidating the role of non-covalent interactions in unexpectedly high and selective CO2 uptake and catalytic conversion of porphyrin-based ionic organic polymers

Abstract

Here, we present viologen-porphyrin based ionic covalent organic polymers (H2-ICOP and Zn-ICOP) with multiple CO₂-philic sites. The specific surface areas of H2-ICOP and Zn-ICOP were found to be 9 m² g⁻¹ and 20 m² g⁻¹, respectively. CO₂ uptake analyses reveal that H2-ICOP exhibits very high CO₂ capture uptake (62.9 mg g⁻¹), which is one of the highest values among previously reported ICOPs. The results indicate very efficient non-covalent interactions between H2-ICOP and CO₂. The possible non-covalent interactions of hydrogen (O_CO2⋯H–N), tetrel (C_CO2⋯N, C_CO2⋯Cl⁻), pnicogen (O_CO2⋯N⁺), and spodium bonds (O_CO2⋯Zn) between CO₂ and H2-ICOP and Zn-ICOP are investigated via symmetry adapted perturbation theory (SAPT) analysis and electrostatic potential maps (MEP). The strength of non-covalent interactions in H2-ICOP and Zn-ICOP is decreasing in the following order ΔE_C⋯N > ΔE_C⋯Cl⁻ > ΔE_O⋯N⁺ and ΔE_Zn⋯O > ΔE_C⋯Cl⁻ > ΔE_C⋯N > ΔE_O⋯N⁺, respectively. The major CO₂ uptake contribution comes from C_CO2⋯N tetrel bonding (−22.02 kJ mol⁻¹) interactions for H2-ICOP, whereas O_CO2⋯Zn spodium bonding (−21.065 kJ mol⁻¹) interactions for Zn-ICOP. H2-ICOP has more CO₂-philic moieties with powerful non-covalent interactions compared to Zn-ICOP, which is in good agreement with the experimental results. Furthermore, the CO₂ catalytic conversion performances of Zn-ICOP and H2-ICOP gave good yields of 83% and 54%, respectively. Surprisingly, Zn-ICOP, despite having significantly lower CO₂ uptake capacity, displayed better catalytic activity than H2-ICOP, owing to a higher number of counter anions (Cl⁻) on its surface, which shows the crucial role of the counter anion (Cl⁻) in the mechanism of this catalytic reaction.