Crown Ether Functionalized Microporous Polyimide Membranes with Hierarchical Pore Structure for Enhanced Li+/Mg2+ Separation

Abstract

Lithium, a highly sought-after element for various industries, is primarily sourced from salt-lake brines, where its extraction faces the critical challenge of separating Li+ from high concentrations of Mg2+. In this work, we present a crown ether functionalized polymer membrane with a hierarchical pore structure, designed for highly selective Li+/Mg2+ separation. By grafting 12-crown-4-ether onto a microporous polyimide, ether rings matching the size of Li+ are incorporated within the interconnected intrinsic micropores, facilitating Li+ permeation while restricting Mg2+ transport. The addition of PEG4000 during membrane formation creates unconnected macropores, reducing effective membrane thickness while preventing pore linkages that could lead to Mg2+ leakage. Consequently, the Li+ permeation rate significantly increases from 0.0095 nmol cm-1 s-1 to 0.257 nmol cm-1 s-1, while Mg2+ permeation remains low at 0.0014 nmol cm-1 s-1, achieving a remarkable Li+/Mg2+ selectivity of up to 184. This study offers a straightforward and efficient approach for developing Li+ selective membranes for efficient lithium extraction.

Keywords: Li+/Mg2+ separation, crown ether, microporous polyimide, hierarchical pore structure