Hierarchical Copper-Based Metal-Organic Frameworks for Improved Catalytic Performance

Introducing additional meso- or macroporosity into traditionally microporous metal-organic frameworks (MOFs) can greatly improve the catalytic performance of these materials, by reducing diffusional barriers during reactions. In this study, we demonstrate that HKUST-1 synthesized via supercritical CO2-assisted crystallization (referred to here as “scCO2-HKUST”) forms a hierarchically porous structure, combining intrinsic microporosity with newly formed macropores. X-ray diffraction (PXRD), scanning electron microscopy (SEM), and gas sorption analyses confirmed the preservation of the crystal structure alongside macropore formation. X-ray photoelectron spectroscopy (XPS) revealed a higher proportion of low-valent copper species (Cuo/Cu+) in scCO₂-HKUST compared to conventional HKUST-1, indicative of increased coordinatively unsaturated metal sites. In catalytic CO oxidation, scCO2-HKUST achieved higher conversion rates than normal HKUST-1 between 220 and 260 oC, with complete CO conversion at 260 °C versus 280 °C for the conventional sample. This enhanced activity is attributed to faster molecular diffusion through the macropores and greater accessibility of reactive copper sites.