Selective Adsorption Mechanism of Borderline Acid Metals by Hard Base-Functionalized UiO-66: A Combined Experimental and DFT Study.

Abstract

The principle "hard acids prefer hard bases" from the Hard and Soft Acid-Base (HSAB) theory, has gained extensive validation. However, the interaction mechanisms between borderline acid metals and hard base groups remain unclear, limiting the rational design of highly selective adsorption. This study systematically investigates the synthesis of hard base-functionalized UiO-66 materials (UiO-66-X, X = -NH₂, -OH, and -COOH) and their efficacy in adsorbing borderline acid metal (Cu(II), Co(II), Ni(II), Pb(II)). Comprehensive characterization confirms the preservation of the parent framework and the successful introduction of functional groups. Batch adsorption experiments reveal that the solution pH and the pairing between functional groups and metal ions are critical factors affecting adsorption capacity and selectivity. The -OH group exhibits strong affinity for Cu(II), Co(II), and Pb(II), -COOH for Pb(II) and Cu(II), and -NH₂ is exceptionally selective for Ni(II). The mechanism, elucidated through XPS and DFT calculations (ESP, DOS, and adsorption energy), verifies coordination between metal ions and the N/O atoms of hard base groups. Additionally, the introduction of -COOH groups through para-functionalization improves the adsorption rate and selectivity per functional group by modulating the electronic structure. This study provides fundamental insights into the structure-activity relationship, aiding in the design of highly selective MOF-based adsorbents.