A metal–organic framework modulated to site-isolate Cl˙ pendants via radical inter-conversion for degrading hard-to-ionize aqueous organic wastes

Abstract

Compared with conventional ˙OH, Cl˙ is longer-lived, more selective to destabilizing refractory electron (e⁻)-donating aqueous aromatics via radicalization, and renewable via e⁻ transfer from aromatics to enable Cl˙ ↔ Cl⁻ inter-conversion. To demonstrate the merits of Cl˙, a Cl pendant (Cl_SUP)-functionalized Zr-based metal–organic framework (UiO-66-Cl) was synthesized/modulated to impart mesoporosity for facilitating the diffusion of bulky aromatics into the porous architecture. UiO-66-Cl could site-isolate Cl⁻ anions (Cl⁻_SUP) near Lewis acidic Zr⁴⁺ cations (LA) and Brønsted acidic –OH (BA), on which ˙OH was produced via homolytic H₂O₂ dissection, desorbed, and bound to Cl⁻_SUP to yield Cl˙_SUPvia exothermic radical inter-conversion of ˙OH → Cl˙_SUP (referred to as the overall ˙OH → Cl˙_SUP route). UiO-66-Cl provided greater LA/BA strengths than UiO-66 un-functionalized with Cl_SUP/Cl⁻_SUP, thus requiring a lower energy for ˙OH desorption, which was identified as the rate-determining step of homolytic H₂O₂ dissection on UiO-66 or the overall ˙OH → Cl˙_SUP route on UiO-66-Cl. Consequently, Cl˙_SUP productivity on UiO-66-Cl was higher than ˙OH productivity on UiO-66 (activity↑). Moreover, UiO-66-Cl exploited Cl˙_SUP as the major decomposer of e⁻-donating aromatics (selectivity↑) via the e⁻ transfer pathway (recyclability↑), as proved by DFT calculations, EPR spectroscopy, and filtration/scavenging/isotope control runs. Furthermore, UiO-66-Cl was more resistant to structural deformation upon exposure to extreme reaction environments than UiO-66 (stability↑), as verified by DFT calculations/XRD analysis. Hence, UiO-66-Cl (Cl˙_SUP) outperformed UiO-66 (˙OH), SO₄²⁻-functionalized iron oxide (SO₄˙⁻_SUP), or NO₃⁻-modified Mn oxide (NO₃˙_SUP) in degrading e⁻-donating, ionization-resistant aqueous aromatics (phenol, aniline, acetaminophen, sulfanilamide, and sulfamethoxazole) in terms of activity, selectivity, stability, and/or reusability.