Development of melamine-based covalent organic framework-MOF pearl-like heterostructure integrated poly(ether-block-amide) for CO2/CH4 separation

Regarding gas separation application, covalent organic frameworks (COFs) are one of the supreme ones because of their numerous merits, such as ordered pores, excellent compatibility with organic particles, and a diverse range of functionality. Unfortunately, the pore size of these nanomaterials is wider than most gas sizes, so there is an explicit limitation to using them in the gas separation field. Herein, the confined growth of metal-organic frameworks (MOFs) in a facile way in melamine-based COF pores (mCOFs) was proposed. There is some one-dimensional channel in two-dimensional COFs; therefore, there is a perfect possibility for confined growth of MOF in those channels with pearl-like structures. After forming these COF-MOF nanohybrid particles (CXMs), a polyether block amide (Pebax) membrane with excellent size sieving and high molecular transport was fabricated. The permeability and selectivity rates of neat Pebax, mCOF/Pebax, and CXM/Pebax membranes were compared systematically. It was found CXM/Pebax membrane had much better CO2/CH4 selectivity, and long-term stability was enhanced simultaneously. The CO2/CH4 selectivity reached 44.2 ± 1.3 for the optimized membrane elucidating 2.3 folds improvement compared to the neat membrane and surpassing Robeson's upper bounds. The long-term tests elucidated that the optimized membrane kept 94% of its initial CO2/CH4 selectivity, 25% higher than the unmodified one.