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

The separation of ethane (CH) from methane (CH) 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-HCTTA-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 CH/CH mixtures. Featuring abundant aromatic rings and uncoordinated carboxyl oxygen atoms in its octahedral cages, Hf-HCTTA-3COOH exhibits denser and stronger C-H···π and C-H···O interactions with CH than with CH. This results in an outstanding selectivity of Hf-HCTTA-3COOH for CH/CH, reaching 11.8, which is superior to many reported materials. Breakthrough experiments verified the excellent reusability and practical CH/CH separation feasibility of Hf-HCTTA-3COOH. The mechanisms are investigated through the GCMC analysis. The results of this study underscore the promising utility of Hf-HCTTA-3COOH as an effective adsorbent for producing high-purity CH.