In Situ Observation of Solvent Exchange Kinetics in a MOF with Coordinatively Unsaturated Sites

Solventexchange of synthesis solvent within metal-organicframeworks (MOFs) is an essential process for the activation of coordinativelyunsaturated sites (CUS) to achieve an optimal surface area; activationof the CUS is required to exploit the versatile applications of MOFs.However, it is challenging to replace CUS-bound synthesis solventprior to MOF activation, which can lead to a structural collapse andreduced surface area post-evacuation. Herein, we quantify the exchangebehavior of a copper paddlewheel-based CUS-MOF (HKUST-1) in the presenceof three different solvents: ethanol (EtOH), dichloromethane (DCM),and N,N-dimethylformamide (DMF).The DMF release profiles are monitored via in situ observation ofthe exchange solvent composition via H-1 NMR and Raman spectroscopyat the macroscopic scale. Furthermore, the change in solvent withina single crystal is measured to directly elucidate the exchange behavior.We demonstrate the DMF release profile from HKUST-1 exhibits differentrate laws depending on whether the solvent exchange occurs at theCUS or is purely diffusive through the pores. This contribution representsthe first characterization of release from a CUS-MOF as a functionexchange solvent and reveals that solvent exchange in a CUS-MOF isnot diffusion-limited, but rather is limited by the solvent exchangekinetics at the metal center. Insights from this study can be generalizedto the variety of copper-paddlewheel-based MOFs, informing best practicesfor solvent exchange.

Automated summary

Solvent exchange within metal-organic frameworks (MOFs) is crucial for activating coordinatively unsaturated sites (CUS) and achieving optimal surface area, enabling versatile applications of MOFs. This study quantifies the exchange behavior of a copper paddlewheel-based CUS-MOF (HKUST-1) in the presence of different solvents and reveals that solvent exchange in CUS-MOFs is not diffusion-limited, but rather determined by the solvent exchange kinetics at the metal center. The insights from this study can inform best practices for solvent exchange in various copper-paddlewheel-based MOFs.