An Hf(IV)-Based Metal–Organic Framework with Abundant Aromatic Rings and Uncoordinated Carboxylic Groups for Ethane/Methane Separation

The separation of ethane (C₂H₆) from methane (CH₄) is a pivotal process for increasing the calorific value of natural gas, but technically challenging due to their similar physical properties. Adsorbent-based gas separation methods utilizing metal-organic frameworks (MOFs) offer distinct advantages, yet their insufficient efficiency and stability limit their practical applications. Herein, we present a novel stable hafnium-based MOF (Hf-MOF), Hf-H₃CTTA-3COOH, constructed from a tritopic carboxylic acid ligand that incorporates high-density uncoordinated carboxylic groups attached to the central benzene ring, and further investigate its adsorption separation properties and mechanisms for C₂H₆/CH₄ mixtures. Featuring abundant aromatic rings and uncoordinated carboxyl oxygen atoms in its octahedral cages, Hf-H₃CTTA-3COOH exhibits denser and stronger C-H···π and C-H···O interactions with C₂H₆ than with CH₄. This results in an outstanding selectivity of Hf-H₃CTTA-3COOH for C₂H₆/CH₄, reaching 11.8, which is superior to many reported materials. Breakthrough experiments verified the excellent reusability and practical C₂H₆/CH₄ separation feasibility of Hf-H₃CTTA-3COOH. The mechanisms are investigated through the GCMC analysis. The results of this study underscore the promising utility of Hf-H₃CTTA-3COOH as an effective adsorbent for producing high-purity CH₄.