Two-dimensional sulfonate-functionalized covalent organic frameworks for large-capacity adsorption of lithium ions

Abstract

Adsorption using porous materials is an effective and promising approach to the separation of lithium ions (Li⁺) from aqueous solutions, but the current adsorbent materials generally suffer from the problems of low lithium adsorption capacity and poor stability. The covalent organic frameworks (COFs) with tunable pore size, modifiable surface and chemically stable structure show great potential to overcome the above issues. Herein, we reported the design and solvothermal batch synthesis of sulfonated (-SO₃H) decorated COFs: TpPa-SO₃H and TpBd-SO₃H for effective separation of Li⁺. It was found that the negatively charged frameworks demonstrated excellent adsorption ability for Li⁺, especially TpPa-SO₃H with a maximum practical lithium adsorption capacity of 145 mg/g, far higher than those reported state-of-the-art adsorbents. Meanwhile, TpPa-SO₃H exhibits superior affinity for Li⁺ in the presence of Li⁺, Na⁺, K⁺ and Mg²⁺. The spectroscopic characterization and density functional theory revealed that the extremely high adsorption capacity mainly arised from the densely and uniformly distributed -SO₃H groups on the surface of TpPa-SO₃H, unlike porous materials with post-grafted functional groups, the adsorption of metal ions for which was limited due to the low grafting rate of functional groups. Meanwhile, the unique π-π conjugated skeleton structure of COF materials and the electronegative groups as CO also provide sufficient binding sites for the adsorption of Li⁺. This work fully demonstrates the great potential of COF as a designable porous material for adsorption and separation of strategic metal lithium ions.