Defect-Engineered UiO-66 Doped with Heterovalent Metals for Enhanced SO₂ Adsorption via Increased Active Sites

The chemical stability of UiO-66 has attracted extensive interest for gas adsorption applications. However, its intrinsic microporous structure restricts active site accessibility, thereby limiting its practical implementation. In this work, formic acid was employed as a modulator, and a postsynthetic heterovalent metal doping strategy was adopted to construct defect-engineered UiO-66 frameworks. Through systematic optimization, Mn²⁺-doped Mn-UiO-66-3 exhibited structural robustness, retaining crystallinity in aggressive acidic media (6 M HCl) and mildly basic environments (0.01 M NaOH), while maintaining thermal stability up to 400 °C. Notably, Mn-UiO-66-3 achieved a SO₂ adsorption capacity of 1.111 mmol·g^-1, representing a statistically significant 77% enhancement compared to pristine UiO-66 (0.628 mmol·g^-1). Mechanistic investigations revealed cooperative interactions between engineered defect sites and introduced Lewis acid metal centers, which synergistically increased the density of the active sites. Combined spectroscopic and kinetic analyses identified carboxylate groups (μ₃-OH), undercoordinated Zr nodes, and Mn²⁺ centers as the dominant adsorption sites, contributing to dual adsorption pathways involving physisorption (via van der Waals interactions) and chemisorption (through coordination interactions). Overall, this study validates heterovalent metal doping as a defect-engineering strategy for modulating MOF host-guest interactions, providing insights into SO₂ capture mechanisms and advancing the MOF-based adsorbent design for industrial flue gas purification.