Flexible Bimetallic MIL-88B MOFs for Tunable Transport in Pebax Mixed Matrix Membranes for CO ₂ /CH ₄ Separation

Dynamic control of gas transport pathways remains a key challenge in the design of high-performance mixed matrix membranes (MMMs). Here, we report a class of MMMs that leverage the dynamic framework flexibility of bimetallic MIL-88B metal–organic frameworks (MOFs) to simultaneously enhance gas sorption and diffusion. Flexible FeCo- and FeNi-MIL-88B MOFs, including their amine-functionalized analogues, were incorporated into a Pebax1657 matrix to create defect-minimized, high-loading MMMs with tunable transport properties. Unlike conventional rigid fillers, the “breathing” behavior of MIL-88B enables adsorption-induced pore expansion, creating adaptive transport pathways that preferentially facilitate CO 2 diffusion while restricting CH 4 transport. Bimetallic node engineering further modulates the local electronic environment, enhancing CO 2 affinity without excessively increasing adsorption enthalpy, thereby maintaining favorable sorption–diffusion balance. In addition, amine functionalization introduces CO 2 -philic sites that promote selective sorption and suppress competitive CH 4 permeation under mixed-gas conditions. The optimized Pebax/FeNi–NH 2 MIL-88B membrane exhibits a CO 2 permeability of 138.5 Barrer and a CO 2 /CH 4 selectivity of 16.9 under mixed-gas conditions, corresponding to a 130% increase in permeability and a 67% enhancement in selectivity relative to pristine Pebax. This work establishes a design strategy in which framework flexibility, bimetallic node chemistry, and functional group engineering act synergistically to regulate coupled sorption–diffusion transport. These findings provide insights into the role of dynamic MOF fillers in MMMs and offer a pathway toward next-generation high-performance membranes for natural gas upgrading and carbon capture.