Template-Free Synthesis of Hierarchical Porous Metal–Organic Frameworks

Abstract

A template-free synthesis of a hierarchical microporous-mesoporous metal-organic framework (MOF) of zinc(II) 2,5-dihydroxy-1,4-benzenedicarboxylate (Zn-MOF-74) is reported. The surface morphology and porosity of the bimodal materials can be modified by etching the pore walls with various synthesis solvents for different reaction times. This template-free strategy enables the preparation of stable frameworks with mesopores exceeding 15 nm, which was previously unattained in the synthesis of MOFs by the ligand-extension method.

Template synthesis of hierarchical porous metal–organic frameworks with tunable porosity

“…All of the Cu-BTC_An samples exhibit lower Brunauer-EmmettTeller (BET) surface areas (S BET ) than C-Cu-BTC, which can be attributed to the increased fraction of meso-and macropores decreasing S BET . Similar conclusions were drawn from the works of Qiu et al, 20 Yue et al, 22 and Huang et al 14 However, all Cu-BTC_An products have much higher total pore volumes (V t ) and mesopore volumes (V meso ) than C-Cu-BTC, conrming the generation of mesopores in the Cu-BTC_An samples. In addition, as the amount of template is increased from 0.45 to 4.5 mmol during synthesis, V t of the MOF products is increased from 0.66 (for Cu-BTC_A1) to 0.81 cm 3 g À1 (for Cu-BTC_A3), and V meso is also increased from 0.19 to 0.31 cm 3 g À1 .…”

“…To solve the diffusion and accessibility problems that guest species face in conventional MOFs with micropores, hierarchical porous MOF materials (HP-MOFs; MOFs with micro-and mesopores, or MOFs with micro-, meso-, and macropores) with tunable porosity, such as Cu-BTC, 20 ZIF-8, 21 and Zn-MOF-74, 22 have been successfully synthesized since 2008. The extended pore structures (mesopores or macropores, or both mesopores and macropores) in HP-MOF materials can improve the diffusion rate and facilitate mass transfer of reactants or products.…”

Highly mesoporous metal–organic framework assembled in a switchable solvent

“…The larger mesopores allow for improved molecular accessibility, whereas smaller micropores provide high surface areas. Some methods have been developed for the formation of hierarchically meso- and microporous MOFs, including surfactant–template method6789, ligand extension method1011121314, microwave-assisted method15, solvent evaporation diffusion method1617, ionic liquid/supercritical CO 2 emulsion route18 and so on. However, most of the meso-MOFs adopt smaller mesopores (<5 nm) and the preparation of large-pore meso-MOFs is sporadic at best, which usually needs the supramolecular or cooperative template strategy69.…”

Elaboration of metal organic framework hybrid materials with hierarchical porosity by electrochemical deposition–dissolution

“…As conventional MOFs have sole micropores and large crystal sizes, many molecules with relatively large dimensions compared to the pore size of MOFs can experience severe diffusion limitations when penetrating the framework. In order to improve their performance, various synthetic strategies have been employed to create MOFs with pores of an increased size such as mesopores (2-50 nm) and macropores (>50 nm), [20][21][22][23][24] thus allowing shorter diffusion path lengths when they are used as thin films, nanoaggregates or nanoparticles. 20,[25][26][27][28] It also has been pointed out recently that structuring of MOFs in a hierarchical order opens up new opportunities to improve the material performance via the design of their physical form rather than altering the chemical components.…”