Metal-organic frameworks (MOFs) with highly ordered structures are porous materials, formed by metal centers and organic ligands. The diversity and tunability of MOFs make it possible to design desirable MOFs with high performance in separation isomers. Our group has successfully utilized morphology engineering methods to improve the separation abilities of zirconium-based MOFs as gas chromatography (GC) stationary phases. Typically, three different morphologies of nano-NU-901 from nanosheets (NS) to interpenetrated nanosheets (I-NS) and nanoparticles (NP) were first obtained. After solvent removal, the NU-901-NS, NU-901-I-NS, and NU-901-NP exhibited relatively aligned stacking, random stacking, and close packing, respectively. The different stacking modes of nano-NU-901 formed different pore environments, and the pore environments of MOFs are closely related to their separation performance. Thus, the stacked three nano-NU-901 exhibited different separation abilities, among which NU-901-NP showed the highest resolution.<br/>Actually, the separation ability of NU-901-NP was not very satisfying when comparing with other MOF-stationary phases, such as MIL-101. We supposed the reason is that the only micropores in NU-901 bring large diffusion barriers. To address this challenge, we demonstrated a one-pot solvothermal method to synthesize MOF solid solutions (MOSS) mixed micropores and mesopores by using bidentate ligands, which are half of the original tetrate ligands, as the modulators. The ratio of micropores and mesopores in MOFs could be well-tuned by modulating the ratio between the two ligands. All the MOSS showed better separation abilities than nano-NU-901. The introduction of mesopores significantly reduced the diffusion barrier and enhanced separation ability. However, too many mesopores would reduce the interaction between MOFs and analytes, resulting in lower separation performance. In this project, when the ratio between mesopores and micropores was equal to 1:1, the MOSS showed the highest separation ability. This morphology engineering method significantly improves the separation ability of MOFs and provides a new direction to design high-performance MOF-stationary phases.

Modulating separation ability of metal-organic frameworks through morphology engineering

ACS Spring Meeting 2021

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