Application of hard and soft acid base theory to uncover the destructiveness of Lewis bases to UiO-66 type metal organic frameworks in aqueous solutions

Abstract

Metal organic frameworks (MOFs), emerging adsorbents and catalysts in the wastewater treatment field, are subject to destruction by Lewis bases in wastewater, such as F⁻ and PO₄³⁻. To promote the application of MOFs in wastewater treatment, it is imperative to clarify the mechanism of the destructiveness of L-bases to MOFs. Accordingly, in this study, we performed a series of deliberately designed experiments and theoretical analyses. The ligand release of MOFs in solutions of various L-bases indicated that the order of destructiveness of L-bases to UiO-66-type MOFs was OH⁻ > F⁻ > PO₄³⁻ > AsO₄³⁻ > SO₄²⁻ > CO₃²⁻ = Ac⁻ = Cl⁻ = Br⁻ = NO₃⁻ = 0. To elaborate the intrinsic nature of the destructiveness of L-bases to MOFs based on hard and soft acid base theory, density functional theory was used to calculate the various physicochemical parameters of L-bases related to their strength, including chemical potential (μ), Mulliken electronegativity (χ = −μ), chemical hardness (η), local softness for electrophilic attack (s⁻) and charges of coordinating atoms (CCA). Among these parameters, μ (−χ) and CCA can reflect the destructiveness of L-bases to MOFs to a large extent, with higher μ (−χ) and CCA indicating higher destructiveness. To more accurately judge the destructiveness of L-bases to MOFs, integrated use of both μ (−χ) and CCA is suggested. Additionally, the stability of some UiO-66-type MOFs cannot be well judged by μ (−χ) and CCA due to the deformation of their ligand structure caused by the steric hindrance of the functional groups of the ligand.