Surface Hydrophobicity and Guest Permeability in Polydimethylsiloxane-Coated MIL-53 as Studied by Solid-State Nuclear Magnetic Resonance Spectroscopy

Experimentalcharacterization of the hydrophobic porous materialsat the atomic and molecular levels is of great significance, but exploringtheir hydrophobicity characteristics and interactions with guest moleculeswith distinct polarity is still challenging. In this work, solid-stateNMR is employed to characterize the surface hydrophobicity and explorethe guest solvent permeability in polydimethylsiloxane (PDMS)-coatedMIL-53. It was found that the PDMS-coated MIL-53 was hydrophobic towater and infiltrated to methanol, acetone, benzene, toluene, andethylbenzene solvents. In addition, two types of guest solvents (methanol,acetone, benzene, toluene, and ethylbenzene), inside the pore andoutside the pore of PDMS-coated MIL-53, were clearly identified usingtwo-dimensional H-1-H-1 homo-nuclear correlationNMR experiments. Moreover, the membrane thickness of the PDMS-coatedMIL-53 could be determined from the analysis of the H-1-H-1 spin diffusion buildup curves. Furthermore, the permeabilityof benzene, toluene, and ethylbenzene at different PDMS coating levelswas extracted from H-1 MAS NMR. The increase of the hydrophobicPDMS layer resulted in a decrease of the penetration of aromatic gueststo the internal pore of MIL-53. This work provides deep insights intothe understanding of guest solvent permeability of hydrophobic layer-coatedMOFs in the application fields of catalysis and separation.