Enhancing CH4/N2 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.