Enhanced CO2/N2 Separation of Mixed-Matrix Membranes Containing Hierarchical UiO-66 Synthesized from Hard Templates

The metal–organic framework (MOF) has attracted widespread attention due to its high porosity, adjustable framework, and tailorable functionality and is widely used as a filler of mixed-matrix membranes (MMMs). However, a high diffusion barrier hinders the fast permeation of guest molecules in the framework. The introduction of a hierarchy in the MOF represents an effective protocol for reducing guest diffusion resistance. In this work, a polystyrene sphere was utilized as a hard template to prepare hierarchical UiO-66 (H-UiO-66). Compared with bulk UiO-66, H-UiO-66 contained a hierarchical pore structure (e.g., 80–250-nm-sized macropores and 0.57-nm-sized micropores), which established fast diffusion pathways for guest molecules; moreover, a hard template facilitated more sufficient exposure of coordinatively unsaturated Zr6-oxo sites in the framework, which was conducive to preferential CO2 adsorption. Prepared MMMs with a loading of 10 wt % H-UiO-66 exhibited an ideal CO2/N2 selectivity of 43.48, with CO2 permeability reaching 809.26 barrer, which well exceeded the 2008 Robenson upper bound and the majority of MOF-based MMMs, showing promising prospects for efficient CO2 capture from flue gas.