Highly efficient bromine capture and storage using N-containing porous organic cages

Abstract

Highly volatile and toxic bromine (Br₂) molecules can be utilized safely in various chemical processes when coupled with efficient separation systems. Herein, we present two different N-containing porous organic cages (POCs), covalent cage 3-R (CC3-R) and formaldehyde tied-reduced covalent cage 3 (FT-RCC3), for vapor Br₂ capture under ambient conditions. They show outstanding sorption capacities (11.02 mmol g⁻¹ and 11.64 mmol g⁻¹, respectively) compared with previously reported adsorbents. Reversibility of the Br₂ sorption process has been elucidated experimentally and computationally by identifying bromine species adsorbed at POCs and calculating their binding energies. The strong charge-transfer interactions between adsorbed Br₂ and abundant N atomic sites of the host cages led to the dominant formation of polybromide species (Br₃⁻ and Br₅⁻). Further host–guest interaction between POCs and polybromides determined the reversibility of the Br₂ sorption process—showing partially reversible (>70% recovery) behavior for CC3-R and irreversible (<10% recovery) behavior for FT-RCC3, both of which were affected by the chemical and structural nature of different POCs. DFT calculations further indicate that the formation of carbocationic species (Br₃⁻ and Br₅⁻) and HBr is energetically favorable within the cage, which is in good agreement with the experimental results. This work demonstrates that strong host–guest interactions are essential for highly efficient Br₂ capture and storage performance.