Sulfonated Metal–Organic Framework Mixed-Matrix Membrane toward Direct Lithium Extraction

Lithium supply has been limited by time-consuming and energy-intensive processing. Membranes are an attractive alternative as a low energy, timely, and continuous process to facilitate ion–ion separations. However, recent metal–organic framework (MOF) membranes are difficult to prepare and scale. Here, we realize an electrochemical LiCl/NaCl selectivity of 1.21 by use of a flexible, low-cost, and simple solution cast mixed-matrix membrane comprised of cellulose triacetate and UiO-66-SO3H MOF. Compatible chemical interactions between the MOF and polymer allowed for high loadings of up to 100% (mMOF/mpolymer) consistently, minimizing interfacial defects and aggregation. Single salt transport measurements confirmed that the selectivity of the membrane arises from high lithium diffusion (1.6 cf sodium diffusion) across the membrane overcoming high sodium solubility (1.3 cf lithium solubility). Incorporating a combination of confined sub-nanoporous (6.3 and 9.5 Å) pore windows in UiO-66-SO3H and chemically compatible high diffusivity SO3– groups achieve flexible, low-cost, and scalable membranes with desirable selectivity towards refining lithium.