“Hard” Emulsion-Induced Interface Super-Assembly: A General Strategy for Two-Dimensional Hierarchically Porous Metal–Organic Framework Nanoarchitectures

Abstract

Two-dimensional (2D) hierarchically porous metal-organic framework (MOF) nanoarchitectures with tailorable meso-/macropores hold great promise for enhancing mass transfer kinetics, augmenting accessible active sites, and thereby boosting performance in heterogeneous catalysis. However, achieving the general synthesis of 2D free-standing MOF nanosheets with controllable hierarchical porosity and thickness remains a challenging task. Herein, we present an ingenious "hard" emulsion-induced interface super-assembly strategy for preparing 2D hierarchically porous UiO-66-NH₂ nanosheets with highly accessible pore channels, tunable meso-/macropore sizes, and adjustable thicknesses. The methodology relies on transforming the geometric shape of oil droplet templates within appropriate oil-in-water emulsions from conventional zero-dimensional (0D) "soft" liquid spheres to 2D "hard" solid sheets below the oil's melting/freezing point. Subsequent surfactant exchange on the surface of 2D "hard" emulsions facilitates the heterogeneous nucleation and interfacial super-assembly of <i>in situ</i> formed mesostructured MOF nanocomposites, serving as structural units, in a loosely packed manner to produce 2D MOF nanosheets with multimodal micro/meso-/macroporous systems. Importantly, this strategy can be extended to prepare other 2D hierarchically porous MOF nanosheets by altering metal-oxo clusters and organic ligands. Benefiting from fast mass transfer and highly accessible Lewis acidic sites, the resultant 2D hierarchically porous UiO-66-NH₂ nanosheets deliver a fabulous catalytic yield of approximately 96% on the CO₂ cycloaddition of glycidyl-2-methylphenyl ether, far exceeding the yield of approximately 29% achieved using conventional UiO-66-NH₂ microporous crystals. This "hard" emulsion-induced interface super-assembly strategy paves a new path toward the rational construction of elaborate 2D nanoarchitecture of hierarchical MOFs with tailored physicochemical properties for diverse potential applications.