Anion pillar inserted MOF-74-Cu with customized pore environment for efficient purification of natural gas

Abstract

Natural gas (NG), a kind of effective and clean low-carbon fossil energy, contains small amounts of non-CH₄ impurities (such as CO₂, C₂H₆ and C₃H₈) during extraction, which must be separated to improve the quality of NG and safeguard the security of transportation. However, the adsorptive separation technology based on traditional adsorbent always suffers from a trade-off effect, which is the balance of adsorption capacity and separation selectivity. Owing to the metal–organic framework-74 (MOF-74) with abundant open metal sites (OMSs), electronegative anion pillar inserted MOF-74-Cu composites with customized pore size/shape and chemistry were firstly prepared by grafting a series of functionalized anion pillars, including −F and −O. Therein, the sample M/TIFSIX_1/8 obtained from hexafluorotitanate (TIFSIX) anion exhibited optimal pore structure, showing excellent adsorption ability for CO₂, C₂H₆ and C₃H₈ due to the generated host–guest interactions of F···C = O and F···H-C, respectively. And the uptake for C₃H₈ (1.77 mmol·g⁻¹, 5 kPa) is comparable to that of C₂H₆ (1.81 mmol·g⁻¹, 10 kPa), much higher than that of CH₄. Meanwhile, the ideal adsorption solution theory selectivity (S_IAST) of C₂H₆/CH₄ (10: 90, v: v) and C₃H₈/CH₄ (5: 95, v: v) for M/TIFSIX_1/8 are 51 and 1969, surpassing that of pristine MOF-74-Cu and reported representative C₂H₆-/C₃H₈- selective MOFs. In addition, the adsorption breakthrough experiments for C₂H₆/CH₄, C₃H₈/CH₄ and C₃H₈/C₂H₆/CH₄ mixtures further confirm the excellent separation ability and recyclability, demonstrating its great potential for the engineering application.