Carboxylic acid ligand engineering and lipophilic modification for enhanced methane storage in metal–organic frameworks

Methane is a potent greenhouse gas. The efficient capture of methane is regarded as a key strategy for mitigating its environmental impacts. Metal-organic frameworks (MOFs) are recognized as promising candidates for methane capture. However, further structural optimization is required to maximize their adsorption capacity. In this work, the structure and methane adsorption capacity of HKUST-1 were systematically tuned through lipophilic surface modification and carboxylic acid ligand engineering. The effects of varying molar ratios of formic, propionic, ortho-phthalic, and isophthalic acids as ligands on methane uptake were investigated. Results indicated that methane uptake in carboxylic acid-modified HKUST-1 varied by changes in surface area and pore size. Owing to optimal pore size tuning and preserved framework stability, the PA-modified sample at a 1:1 M ratio achieved the highest methane uptake (184.29 cm 3 /g at 35 bar), surpassing HKUST-1 by 37.94 cm 3 /g. Although isophthalic acid-modified IPA-HKUST-1 exhibited the highest BET surface area (1288.68 m 2 /g), representing a 46 % increase over HKUST-1, the excessive formation of ultra-micropores hindered methane diffusion, leading to reduced uptake. Lipophilic and formic acid modifications markedly reduced both surface area and micropore fraction. Furthermore, changes in surface polarity of lipophilic-modified SA-HKUST-1 failed to generate effective adsorption sites, while grafted ester functional groups obstructed accessible pore channels, resulting in a pronounced reduction in methane uptake (90.57 cm 3 /g). These findings provide valuable insights into enhancing methane capture performance in MOFs through targeted modifications.