Modulating the cavity micro-environments of Fe-MOFs for high-performance gas chromatographic separations

Precise modulation of cavity micro-environment in metal-organic frameworks (MOFs) is important for achieving significant separation performance. Herein, the Fe-MOF (MIL-142-BTB-BDC) with different tridentate and bidentate ligands to form cavities, was chosen as the platform to precisely modulate the cavity micro-environment and investigate the influence of cavity windows and cavity walls on gas chromatographic separations. Changing tridentate ligands on the cavity walls led to MIL-142-TATB-BDC while changing bidentate ligands on the cavity windows produced MIL-142-BTB-BDC-NH₂. Mechanism investigation revealed that for MIL-142-BTB-BDC and MIL-142-TATB-BDC, altering the ligands of cavity walls had little influence on the thermodynamic interactions between MOFs and analytes while slightly reducing the kinetic diffusion rate of analytes. On the contrary, introducing amino groups on cavity windows in MIL-142-BTB-BDC-NH₂ not only increased the thermodynamic interactions with analytes but also slowed down the kinetic diffusion of analytes, which resulted in poor separation performance of the MIL-142-BTB-BDC-NH₂ coated column. This work provides a guide to precisely modulating the cavity micro-environment and analyzing the relationship between the cavity micro-environment and application properties of MOFs.