Metal-Phenolic network and metal-organic framework composite membrane for lithium ion extraction

To tackle the increased demand of lithium production, the separation of monovalent and divalent cations is crucial for lithium recovery from brine. Recently, metal-organic framework (MOF) has shown great potential in such application due to its tuneable pore size and pore channel chemistry in the sub-atomic scale. Commercialization of ion-selective MOF-based membrane requires the fabrication of MOF thin film on a polymeric support that enables scale-up procedure. Herein, we specifically design a facile method of fabricating zeolitic imidazolate framework (ZIF-8) on a flexible polymeric membrane, using versatile tannic acid and iron complexes (TA-FeIII) as an intermediate layer to promote heterogenous MOF nucleation and growth. The TA-FeIII/ZIF-8 film demonstrated ion selectivity ratio of K+/ Mg2+ (4.49), followed by Na+/ Mg2+ (4.0) and Li+/ Mg2+ (3.87); while the selectivity ratio of Ca2+/ Mg2+ is 1.1. Further investigation suggests that partial dehydration-hydration process plays a role for ion transport mechanism across the TA-FeIII/ZIF-8. However, the separation between monovalent ions remains a difficult challenge due to the trade-off between low dehydration energy of K+ and fast ion transport of dehydrated Li+. This study provides an insight in utilizing the versatile TA-FeIII coating for scale-up growth of MOF thin film for molecular separations.