Kinetics of Guest-Induced Structural Transitions in Metal-Organic-Framework MIL-53(Al)-NH2 Probed by High-Pressure Nuclear Magnetic Resonance

A nuclear magnetic resonance (NMR) study of a pore opening in amino-functionalized metal-organic framework (MOF) MIL-53(Al) in response to methane pressure variation is presented. Variations of both NMR signal intensities and transversal relaxation rates for methane are found to reveal hysteretic structural transitions in the MOF material, which are smeared out over broad pressure ranges. Experiments with pressure reversals upon an incomplete adsorption/desorption gave deeper insight into the microscopic transition mechanisms. These experiments have unequivocally proven that the non-stepwise pore opening/closing transitions observed in the experiments are governed by a distribution of the opening/closing pressures over different MOF crystallites, for example, due to a distribution of the crystal sizes or shapes. The slow kinetics of the structural transitions measured in the hysteresis regime revealed a complex free energy landscape for the phase transition process.

Automated summary

A nuclear magnetic resonance (NMR) study shows that a pore opening occurs in amino-functionalized metal-organic framework (MOF) MIL-53(Al) in response to changes in methane pressure. Variations in NMR signal intensities and relaxation rates reveal hysteretic structural transitions in the MOF material, which are spread out over a broad pressure range. Additional experiments with pressure reversals provide insight into the microscopic transition mechanisms and confirm that the non-stepwise pore opening/closing transitions observed in the experiments are influenced by a distribution of opening/closing pressures across different MOF crystallites, possibly due to variations in crystal sizes or shapes. The slow kinetics of the structural transitions measured in the hysteresis regime highlight the complexity of the phase transition process.