Porphyrins as a Platform for Lithium-Selective Two-Dimensional Membranes

Two-dimensional ion-selective membranes provide an appealing avenue for extracting lithium from naturally occurring aqueous brine solutions. Most current work is focused on graphene-based membranes, where the dynamics of ion permeation is determined by noncovalent pore-ion interactions, and the main challenge is achieving precise control over the pore size. Here, we propose that porphyrins, macrocycles with atomically precise pores ∼2.6 Å in diameter, are attractive alternative building blocks for lithium-selective membranes. We use ab initio molecular dynamics with enhanced sampling to investigate the permeability of porphene, an archetypal porphyrin-based material, toward ions present in lithium brine. Our simulations show that porphene is highly permeable to lithium and impermeable to sodium and potassium, while the permeability of magnesium can be controlled by pH. These results demonstrate that membranes based on porphene, or more generally on 2D materials containing porphyrins, may be useful for sustainably extracting lithium from brine.