Tailoring carboxyl-rich micro-environment of metal-organic framework via mixed linkers strategy for enhanced adsorption of lithium ion

Abstract

Lithium ion (Li⁺) uptake from salt lakes is highly significant while still faces challenges. In this work, a new highly stable metal–organic framework (MOF) with mixed linkers was rationally designed for Li⁺ capture. To construct carboxyl-rich active traps, an additional functional linker of benzenehexacarboxylic acid (named H₄ for which contains four free carboxyl groups after coordination) was in-situ introduced into MIL-121-H₂ framework, which owns inherent benzenetetracarboxylic acid (H₂) linker. Dense carboxyl groups in the local region surround to form a distinct 4-oxygen adsorption trap, endowing the prepared MIL-121-H₂/H₄ with stronger affinity toward Li⁺ ion than the inherent 3-oxygen trap of MIL-121-H₂. On this basis, MIL-121-H₂/H₄ exhibits short adsorption equilibrium time of 45/ 60 min (C₀, 100/1000 mg L^-1) and high capacity of 35.6 mg g⁻¹. Neutral pH was found to facilitate the adsorption and selective adsorption especially for K⁺ and Ca²⁺ ions was observed. Besides, even after four adsorption–desorption cycles, MIL-121-H₂/H₄ still remained 93.5 % of the adsorption amount of the original sample. Thus, our work proposes a new and feasible strategy for the design of highly effective MOF-based adsorbents.