MOF Nanosheet-Based Mixed Matrix Membranes with Metal–Organic Coordination Interfacial Interaction for Gas Separation

In the mixed matrix membrane (MMM), the interface between the filler and the polymer matrix will directly affect the gas separation performance of the membranes. Reasonable interfacial design in MMMs is thus important and necessary. In this work, metal-organic coordination interaction is used to construct the interface in metal-organic framework (MOF) nanosheet-based polyimide MMMs where ultrathin Co-benzenedicarboxylate MOF nanosheets (CBMNs) with a thickness less than 5 nm and a lateral size more than 5 μm are synthesized as fillers and a carboxyl-functionalized polyimide (6FDA-durene-DABA) is used as a polymer matrix. Because of the high aspect ratio (>1000) of CBMNs, abundant metal-organic coordination bonds are formed between Co²⁺ in CBMNs and the -COOH group in 6FDA-durene-DABA. As a result, the 6FDA-durene-DABA/CBMN MMMs exhibit improved separation performance for the CO₂/CH₄ and H₂/CH₄ gas pairs with H₂/CH₄ and CO₂/CH₄ selectivities up to 42.0 ± 4.0 and 33.6 ± 3.0, respectively. The enhanced interfacial interaction leads to the comprehensive separation performance of CO₂/CH₄ and H₂/CH₄ gas pairs approaching or surpassing the 2008 Robeson upper bound. In addition, the CO₂ plasticization pressure of the MMMs is significantly enhanced up to ∼20 bar, which is 2 times that of the pure 6FDA-durene-DABA membrane. When separating a mixed gas of CO₂/CH₄, the selectivity of CO₂/CH₄ remains stable at around 23 and the CO₂ permeability keeps around 400 barrer during the long-term test.