Adsorption of Mn2+, Co2+, and Ni2+ in MOF-808 and pyrazole-modified MOF-808: Selectivity trends assessed with microcalorimetry and spectroscopic analysis

Abstract

Zirconium(IV)-based metal-organic framework (MOF) MOF-808 and pyrazole-functionalized MOF-808 were investigated for the selective adsorption of Mn²⁺, Co²⁺, and Ni²⁺, the key components of lithium-ion battery cathodes. Batch adsorption experiments were conducted to assess selectivity for each metal, while flow microcalorimetry was employed to assess thermodynamic signatures of adsorption. To understand molecular interactions between the MOFs and adsorbing metal ions we utilized attenuated total reflectance Fourier transform infrared (ATR-FTIR) and Ni K-edge extended X-ray absorption fine structure (EXAFS) spectroscopic analyses. For MOF-808, the adsorption followed Mn²⁺ > Co²⁺ ≈ Ni²⁺trend, following the changes in hydration energy of the examined ions. Pyrazole functionalization altered this trend by increasing the affinity of MOF-808-pyrazole for Ni only, while resulting in similar affinity for the other two ions (Ni²⁺ > Mn²⁺ > Co²⁺). Microcalorimetry revealed that Mn²⁺ adsorption on both MOFs was exothermic, whereas Co²⁺ and Ni²⁺ adsorption was endothermic. Importantly, ATR-FTIR spectra showed peak growth and vibrational band shifts assigned to direct interactions of Co²⁺ and Ni²⁺ with pyrazole groups, indicating that Ni²⁺ coordinates to N in pyrazole. Additionally, the shell-by-shell fitting of EXAFS data also indicated that some Ni²⁺ adsorption took place at the Zr oxo-cluster sites. These results demonstrate that metal adsorption by MOF-808 and MOF-808-pyrazole is metal-specific, governed by both hydration energy and the electrostatic interactions described by the Irving-Williams series. Importantly, pyrazole functionalization enables selective Ni²⁺ capture, offering new insights into ligand design within MOFs for target element separation from mixed aqueous systems.