Beyond vacancy defects: creation of coordinatively modulated defective sites in HKUST-1 for unprecedented enhancement of peroxymonosulfate activation

Abstract

By pioneering coordinative modulation in defective metal–organic frameworks (MOFs), a transformative leap in catalytic efficiency was achieved in this work, surpassing conventional vacancy-based designs. Coordinatively modulated (CM) Cu–NH₂ sites are created in Cu₃(BTC)₂ (HKUST-1, H₃BTC is 1,3,5-benzene tricarboxylic acid) via functionalization of missing carboxylate defective sites. For this, a secondary carboxylate linker is used so that one carboxylate is replaced with –NH₂ groups. Structural characterization studies reveal that amine groups are directly oriented toward inorganic nodes and coordinated to Cu-sites in the framework to generate CM-Cu–NH₂ sites. Tritopic BTC³⁻ linkers inside HKUST-1 were partially replaced with secondary linkers with the same molecular geometry, 5-amino isophthalic acid (5-AIPA), and isophthalic acid (IPA). Coordinatively modulated frameworks with different ratios of CM-Cu–NH₂ sites were applied for activation of peroxymonosulfate (PMS) as a proof-of-concept reaction with a complex mechanism based on Lewis acidity and redox activity. The HN-50 framework (Cu_2.167^IICu_0.833^I(BTC)_1.123(5-AIPA)_0.472), with the highest ratio of CM-Cu–NH₂ and defective sites, achieves >99% pollutant degradation in 5 minutes via peroxymonosulfate activation—a 65-fold kinetic enhancement over HKUST-1, driven by enhanced Cu electrophilicity and electron transfer, as revealed by experimental and DFT studies. Experimental studies, as evidenced by theoretical simulations, reveal that two structural changes are observed in the presence of amines; (i) charge redistribution on CM-Cu–NH₂ sites through increases in the density of positive charge on Cu atoms, and (ii) improved electron transfer kinetics through electronic delocalization facilitated by the coordination environment of the CM-Cu–NH₂ sites. These two structural factors are dominant for the highly efficient PMS activation by improving MOF-PMS affinity and MOF-to-PMS electron transfer. The strategy of this work reveals how defect engineering in HKUST-1 can tune the electronic structure of Cu active sites and promote Cu(II)/Cu(I) redox cycling for PMS activation, redefining MOF catalyst design.