Adsorption dynamics of aromatic and polar volatile organic compounds on metal-organic frameworks

The study explores the effectiveness of metal-organic frameworks (MOFs) in capturing volatile organic compounds (VOCs). Specifically, it investigates the effect of polarity and aromaticity of VOCs on their adsorption behavior on two MOFs, CuBTC and FeBTC. Cyclic adsorption breakthroughs and isotherms were completed using toluene, 2-methylpyridine, n-hexane, and 2-methyl-2-butanol as adsorbates to assess the frameworks' adsorption and regeneration capabilities. X-ray diffraction, X-ray photoelectron spectroscopy, thermogravimetry analysis, Fourier transform infrared spectroscopy and nitrogen adsorption/desorption isotherm were used to evaluate the MOFs' crystallinity, thermal stability, and surface properties. The results suggest that CuBTC demonstrates superior adsorption capabilities for all tested VOCs, due to its larger surface area and higher crystallinity. However, FeBTC has a broader pore sizes, allowing faster VOC mass transfer rates and accommodating larger molecules, albeit with slightly lower adsorption capacities. Notably, VOCs with polar and aromatic properties, such as 2-methylpyridine, exhibited higher adsorption levels due to increased π-π interactions within the frameworks. However, regeneration of 2-methylpyridine was challenging due to chemisorption, forming strong, irreversible metal–nitrogen coordination bonds. Toluene and 2-methyl-2-bustanol showed similar adsorption capacities and could be effectively regenerated, indicating physisorption mechanisms involving π–π interactions and polar interactions, respectively. N-hexane exhibited the lowest adsorption capacities, relying on weaker van der Waals forces. These results highlight the promising potential of CuBTC and FeBTC in mitigating air pollution. The research also offers valuable insights for tailoring MOFs to contaminants' molecular properties, and advances our understanding of MOF applications in air quality engineering.