Enhancing CH ₄/N ₂ Separation Performance within Aluminum-Based Metal-Organic Frameworks: Influence of the Pore Structure and Linker Polarity

Abstract

Efficient CH₄/N₂ separation is necessitated for obtaining purified methane from natural gas. Aluminum-based metal–organic frameworks (Al-MOFs) have potential for industrial separation applications due to their structure-tunable, low-cost, and scalable features. Intrigued by the impressive selectivity of a recently reported Al-MOF (Al-CDC) toward CH₄ over N₂, we herein further study the potential of Al-MOFs as adsorbent for CH₄/N₂ separation, mainly considering the effects of pore geometry and linker polarity. Utilization of two bent ligands and two linear ligands with different polarity afforded two one-dimensional square-shaped Al-MOFs i.e., CAU-10-H, MIL-160, and two rhombic-shaped counterparts i.e., Al-Fumarate (Al-Fum), MIL-53(Al)), respectively. Afterward, pure CH₄ and N₂ adsorption experiments were conducted at 273-313 K for assessing the CH₄/N₂ separation performance. The results indicated that all the Al-MOFs exhibited superior affinity toward CH₄ over N₂, and the CH₄ uptake followed the sequence of Al-Fum > CAU-10-H > MIL-53(Al) > MIL-160. Exhilaratingly, Al-Fum exhibited unprecedented CH₄/N₂ selectivity (17.2) and high CH₄ uptake at 273 K and 1.0 bar. The mechanism underlying the disparity of Al-MOFs affinity toward CH₄ was deciphered via theoretical simulation, suggesting that the synergetic effects of accessibility of strong affinity sites (μ₂-OH) on AlO₆ chains and polar pore surface induced by varying linkers highly promoted the CH₄ uptake. Furthermore, the results of cyclic adsorption-desorption experiments and binary breakthrough tests validated the feasibility of Al-Fum for practical application.

Keywords: adsorption, Al-fumarate, CH4/N2 separation, Metal-organic framework, theoretical simulation