Oriented two-dimensional zeolitic imidazolate framework-L membranes and their gas permeation properties

A two-dimensional zeolitic imidazolate framework (ZIF), ZIF-L, with a unique anisotropic pore system makes it an excellent material for investigating ZIF membranes with crystallographically preferred orientation and their effects on gas separation properties. A b-oriented ZIF-L membrane was successfully prepared from a randomly oriented seed layer. In contrast, a c-oriented ZIF-L crystal layer was attached to a support through vacuum filtration in the presence of polyethyleneimine. After a short-time secondary growth, the oriented layer grew into a continuous membrane with a high degree of c-out-of-plane orientation. Gas separation studies demonstrated the orientation-dependent separation behavior. In single gas permeation experiments, the c-oriented ZIF-L membrane has higher ideal selectivities of 8.1 for H₂/N₂ and 24.3 for H₂/CO₂ compared to the corresponding values of 3.9 and 5.5 for the b-oriented ZIF-L membrane. The results of binary gas permeation showed decreased separation factors of the c-oriented membrane, but were still much higher than those of the b-oriented membrane. This work provides new insights into controlling ZIF membrane orientation for achieving desirable gas separation performance.