Precisely tuning morphology and size of ZIF-67-based nanosheets in mixed matrix membranes for advanced CO2/CH4 separation

Two-dimensional (2D) metal-organic framework (MOF) nanosheets are promising fillers for gas separation membranes, owing to their high aspect ratios and tunable pore architectures. Realizing their full application potential requires precise control over nanosheet morphology and sizes to optimize membrane performance. This work employs crystal growth kinetics modulation to tailor the morphology and size of ZIF-67-based nanosheets incorporated into mixed matrix membranes (MMMs) targeting superior CO 2 /CH 4 separation. Strategy (i): Adjusting early-stage nucleation kinetics yielded two distinct ZIF-L nanosheet types: disk-like (ZIF-L-I) and leaf-like (ZIF-L-II). ZIF-L-II, synthesized with higher ligand molar ratio, possessed larger lateral dimensions and more favorable pore characteristics, significantly enhancing gas separation. Strategy (ii): Amino-functionalized ionic liquid (IL) was introduced to further modulate the size of ZIF-L-II nanosheets. The IL's amino groups served as capping agents by coordinating preferentially with Co ions, restricting framework extension without altering crystallinity. The ILs loading amount dictated the final nanosheet size, but did not alter their crystalline phase. Critically, the IL also exhibits affinity for CO 2 , augmenting its transport. At 48 % IL addition, the CO 2 permeability of the 10 wt% IL-ZIF-L-II/PI membrane surged from 289 Barrer to 493 Barrer, without significant selectivity loss. The synergistic combination of kinetically-controlled synthesis (yielding larger and leaf-like nanosheets) and IL-mediated size regulation (providing both capping and CO 2 -philic functions) resulted in a 70 % enhancement in CO 2 permeability.