Polyimide/Ionic Liquids Hybrid Membranes with NH₃-Philic Channels for Ammonia-Based CO₂ Separation Processes

An efficient separation technology involving ammonia (NH₃) and carbon dioxide (CO₂) is of great importance for achieving low-carbon economy, environmental protection, and resource utilization. However, directly separating NH₃ and CO₂ for ammonia-based CO₂ capture processes is still a great challenge. Herein, we propose a new strategy for selective separation of NH₃ and CO₂ by functional hybrid membranes that integrate polyimide (PI) and ionic liquids (ILs). The incorporated protic IL [Bim][NTf₂] is confined in the interchain segment of PI, which decreases the fractional free volume and narrows the gas transport channel, benefiting the high separation selectivity of hybrid membranes. At the same time, the confined IL also provides high NH₃ affinity for transport channels, promoting NH₃ selective and fast transport owing to strong hydrogen bonding interaction between [Bim][NTf₂] and NH₃ molecules. Thus, the optimal hybrid membrane exhibits an ultrahigh NH₃/CO₂ ideal selectivity of up to 159 at 30 °C without sacrificing permeability, which is 60 times higher than that of the neat PI membrane and superior to the state-of-the art reported values. Moreover, the introduction of [Bim][NTf₂] also reduces the permeation active energy of NH₃ and reverses the hybrid membrane toward "NH₃ affinity", as understood by studying the effect of temperature. Also, NH₃ molecules are much easier to transport at high temperature, showing great application potential in direct NH₃/CO₂ separation. Overall, this work provides a promising ultraselective membrane material for ammonia-based CO₂ capture processes.