Heteroepitaxially Grown Zeolitic Imidazolate Framework Membranes with Unprecedented Propylene/Propane Separation Performances

Propylene/propane
separation is one of the most challenging separations, currently achieved
by
energy-intensive cryogenic distillation. Despite the great potential
for energy-efficient membrane-based separations, no commercial membranes
are currently available due to the limitations of current polymeric
materials. Zeolitic imidazolate framework, ZIF-8, with the effective
aperture size of ∼4.0 Å, has been shown to be very promising
for propylene/propane separation. Despite the extensive research on
ZIF-8 membranes, only a few reported ZIF-8 membranes have displayed
good propylene/propane separation performances presumably due to the
challenges of controlling the microstructures of polycrystalline membranes.
Here we report the first well-intergrown membranes of ZIF-67 (Co-substituted
ZIF-8) by heteroepitaxially growing ZIF-67 on ZIF-8 seed layers. The
ZIF-67 membranes exhibited impressively high propylene/propane separation
capabilities. Furthermore, when a tertiary growth of ZIF-8 layers
was applied to heteroepitaxially grown ZIF-67 membranes, the membranes
exhibited unprecedentedly high propylene/propane separation factors
of ∼200 possibly due to enhanced grain boundary structure.