Acid site regulation of the acetic acid-modified cu-MOFs: cooperative roles of defect sites and terminal hydroxyl groups in NH₃ adsorption

The release or leakage of NH₃ causes environmental pollution and poses risks to human health, underscoring the need for high-performance NH₃ adsorbents. In this study, acetic acid was used as a modulator to tailor the defect structures of Cu-MOFs, and operando infrared spectroscopy together with density functional theory (DFT) calculations was employed to systematically examine the effects of acetic acid dosage and ligand type on morphology, defect formation, thermal stability, and NH₃ adsorption performance. The structure–property relationships, cycling stability, and adsorption mechanism were clarified. Increasing the acetic acid dosage was found to promote the formation of Cu²⁺ defects, thereby enhancing NH₃ uptake, and CuBTC constructed from H₃BTC exhibited markedly higher adsorption capacity than CuBDC based on H₂BDC. Both operando IR and DFT results confirmed that the optimally modified CuBTC-Ac-64 contained abundant μ-OH groups enabling strong hydrogen-bond-mediated physisorption, while Cu²⁺ defects generated Lewis-acidic sites that strengthened chemisorption. These advantages arose from the larger surface area, smaller pore size, and high density of μ-OH and Lewis-acid sites introduced by acetic acid modification. CuBTC-Ac-64 achieved an NH₃ uptake of 16.81±0.13 mmol/g, a 16% improvement over unmodified CuBTC, and retained more than 90% of its initial capacity after five cycles. This work provides important theoretical and practical guidance for the development of efficient Cu-MOF-based NH₃ adsorbents.