3D hollow CoNi-LDH nanocages based MMMs with low resistance and CO2-philic transport channel to boost CO2 capture

Layered double hydroxide (LDH) bearing numerous hydroxyl groups shows high affinity with CO2 and is promising in mixed matrix membranes (MMMs) based CO2 capture. However, its non-porous structure limits the further improvement of CO2 permeability. In this work, 3D hollow CoNi-LDH nanocages were developed to construct low-resistance and CO2-philic transport channel in MMM by in-situ etching of ZIF-67 template. The resultant material exhibits hollow structure with cavity size of about 500 nm, mesopores and micropores from LDH stacking and ZIF-67 template. The outer layered CoNi-LDH nanosheets can attract more CO2 molecules by their abundant hydroxyl groups, resulting in an improvement in dissolution selectivity of CO2 over N2. Meanwhile, the tortuous gas diffusion paths originated from 2D nanosheet stacking can further enhance the CO2 selectivity over N2. Moreover, the hollow nanocages provide CO2 molecules with fast transport path, resulting in high CO2 permeability. As a result, compared with pure Pebax membrane and 2D LDH/Pebax MMM, all the 3D hollow CoNi-LDH/Pebax MMMs present enhanced CO2 permeability. The 3D hollow CoNi-LDH-2h/Pebax MMM performs the highest CO2 permeability of 135.49 Barrer, which are increased by 141.95% and 71.50% than those of the above two membranes, respectively. The selectivity of 3D hollow CoNi-LDH-2h/Pebax MMM is about 26.30% higher than that of the pure Pebax membrane. The present membrane presents comparable CO2 capture capacity in contrast with most reported Pebax based MMMs and surpasses the Robeson upper bond. Together with the good long-term stability, the 3D hollow CoNi-LDH nanocages are believed to be a promising candidate for MMMs to improve CO2 capture.