Remarkably High Li+ Adsorptive Separation Polyamide Membrane by Improving the Crown Ether Concentration and Electron Density

Lithium has become an important element in the last two decades owing to its critical applications in many fields, including rechargeable batteries, adhesives and ceramic drugs. In addition to being the lightest alkali metal, it exhibits other attractive properties like high specific heat capacity, high electrochemical activity and redox properties. In fact, lithium has been regarded as one of the most important elements for driving the creation of a sustainable economy, especially the transition from gasoline-driven vehicles to electric-driven ones. To this end, the demand for lithium has increased dramatically, necessitating the need to develop effective and green lithium extraction methods.

Currently, over 60% of lithium exists in brine and seawater. While there are several methods for extracting lithium from aqueous solutions, they suffer from several drawbacks that limit their applications. Selective adsorption separation, for example, is limited by the high loss rate, low adsorption capacity and low service life. Recently, crown ethers have been used to overcome these challenges, leading to the construction of a membrane-based solid-liquid adsorptive separation method by immobilizing crown ethers on carriers to capture lithium ions (Li⁺). Despite their remarkable performance, the uneven crown ether distribution and influence of crown ether site-induced mass transfer resistance are the main barriers to realizing efficient solid-liquid adsorptive lithium separation using crown ether-supported materials.

Previous results have revealed that incorporating crown ether into the polymer main chain could improve not only the even distribution of the load but also the crown ether density. This offers numerous advantages in the main chain of polyimide (PI) polymers. Inspired by these findings, researchers at Tiangong University: Dr. Liuyong Mao, Dr. Rui Chen, Dr. Jintao He, Professor Hongchang Pei, Professor Benqiao He, Professor Xiaohua Ma and Professor Jianxin Li synthesized a novel DAB14C4-based polyamide (14C4 PA) via polycondensation of DAB14C4 with 2,2-bis-(4-carboxyphenyl)-hexafluoropropane. Consequently, non-solvent-induced phase separation method was adopted to prepare polymer membranes and films. The 14C4 PA structure, the lithium adsorption performance and the adsorption kinetics and isotherms were studied. Their work is currently published in the peer-reviewed journal, ACS Sustainable Chemistry and Engineering.

The authors showed that the resulting polyamide membrane exhibited high porosity (72.7%), high mechanical strength (6.5 MPa) and significantly higher maximum Li⁺ adsorption capacity (40.1 mg g^‑1). Compared with 14C4 PI membrane, 14C4 PA membrane achieved a 25% increase in the amount of Li⁺ adsorbed. This was partially attributed to the higher concentration (7.3% higher) of the crown ether. Consequently, the adsorption energy of Li⁺ on 14C4 PA was significantly larger than that on 14C4 PI. Furthermore, the adsorption of Li⁺ on the 14C4 PA polymer membrane was by chemical monolayer adsorption as validated by the adsorption kinetics and isotherms. The adsorptive selectivity for Li⁺ of the 14C4 PA membrane was remarkably higher, especially in the presence of Na⁺, K⁺ and Mg²⁺ interfering ions. The selective separation factors of Li⁺ to Mg²⁺, Na²⁺ and K²⁺ were 8.47, 14.0 and 25.7, respectively.

In summary, the study reported the synthesis of a novel 14C4 PA with high crown ether charge density and loading, good repeatability, high glass transition temperature, high molecular weight, remarkable thermal stability and excellent adsorption/desorption stability. The experimental results were consistent with the simulation results, suggesting the potential application of 14C4 PA in membrane-based Li⁺ adsorption separation. In a statement to Advances in Engineering, the corresponding author Professor Jianxin Li explained that their study provided a feasible approach for designing highly efficient Li⁺ adsorption membranes for selective separation and green extraction of Li⁺.

NOTE: The technology is now protected by intellectual property.

PATENT: Jianxin Li, Xiaohua Ma, Quanji Zhu, Hong Wang, Dibenzo crown ether polyimide polymer and the preparation method and application thereof, China Patent No. ZL202010073422.7 (issued date: January 03, 2023; Application date: January 22, 2020)