Enhancing CH4 separation from surplus N2 in Al-based metal organic frameworks through liner ligand regulation

Recovery of CH4 from unconventional natural gas with surplus N2 is of great significance for improving energy utilization efficiency and mitigating global warming. In this study, Al-CDC, Al-Fum, and Al-BDC compounds with similar linear ligands were synthesized to investigate the effects of ligand modulation on the separation of CH4 through experimental characterization and quantitative calculations. The results show that at 298 K and 100 kPa, Al-CDC exhibits the highest CH4 adsorption capacity (1.43 mmol/g), which is 1.27 and 1.99 times higher than that of Al-Fum and Al-BDC, respectively. Furthermore, Al-CDC demonstrates exceptional CH4/N2 selectivity with an adsorbent selectivity parameter of 68.57, surpassing most reported adsorbents. Characterization studies reveal that Al-CDC has an extremely narrow pore channel of 0.54 nm, abundant saturated C-H groups, oxygen-containing groups, and moderate polar ligands. Therefore, the selective adsorption force on CH4 is increased through strong London forces owing to the primary CH4 adsorption sites in Al-CDC are AlO6 clusters and MIDLINE HORIZONTAL ELLIPSIS OOC groups. Additionally, the high dispersion of Al-CDC crystal structure is critical in increasing the dynamic desorption rate of CH4. In contrast, the Henry's law constant of Al-Fum for N2 is 1.45 times higher than that of Al-CDC and that of Al-BDC. This enhancement is attributed to the fact that the adsorption sites for N2 in Al-Fum are mainly located near-C=C-, leading to increased London and Debye forces and improved selective adsorption of N2 through the synergistic effect of strong ligand polarity and microporous structure. This study establishes Al-CDC as a suitable adsorbent for CH4 separation and gives useful suggestions for optimizing adsorbents for CH4 separation from unconventional natural gas.