Enhancing CH₄ Capture from Coalbed Methane through Tuning van der Waals Affinity within Isoreticular Al-Based Metal–Organic Frameworks

Efficient separation of the CH4/N2 mixture is of great significance for coalbed methane purification. It is an effective strategy to separate this mixture by tuning the van der Waals interaction due to the nonpolar properties of CH4 and N2 molecules. Herein, we prepared several isoreticular Al-based metal–organic frameworks (MOFs) with different ligand sizes and polarities because of their high structural stability and low cost/toxicity feature of Al metal. Adsorption experiments indicated that the CH4 uptake, Qst of CH4, and CH4/N2 selectivity are in the order of Al-FUM-Me (27.19 cm3(STP) g–1, 24.06 kJ mol–1 and 8.6) > Al-FUM (20.44 cm3(STP) g–1, 20.60 kJ mol–1 and 5.1) > Al-BDC (15.98 cm3(STP) g–1, 18.81 kJ mol–1 and 3.4) > Al-NDC (10.86 cm3(STP) g–1, 14.89 kJ mol–1 and 3.1) > Al-BPDC (5.90 cm3(STP) g–1, 11.75 kJ mol–1 and 2.2), confirming the synergetic effects of pore sizes and pore surface polarities. Exhilaratingly, the ideal adsorbed solution theory selectivity of Al-FUM-Me is higher than those of all zeolites, carbon materials, and most water-stable MOF materials (except Al-CDC and Co3(C4O4)2(OH)2), which is comparable to MIL-160. Breakthrough results demonstrate its excellent separation performance for the CH4/N2 mixture with good regenerability. The separation mechanism of Al-FUM-Me for the CH4/N2 mixture was elucidated by theoretical calculations, showing that the stronger affinity of CH4 can be attributed to its relatively shorter interaction distance with adsorption binding sites. Therefore, this work not only offers a promising candidate for CH4/N2 separation but also provides valuable guidance for the design of high-performance adsorbents.