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

Abstract

Solvent exchange of synthesis solvent within metal− organic frameworks (MOFs) is an essential process for the activation of coordinatively unsaturated sites (CUS) to achieve an optimal surface area; activation of the CUS is required to exploit the versatile applications of MOFs. However, it is challenging to replace CUS-bound synthesis solvent prior to MOF activation, which can lead to a structural collapse and reduced surface area post-evacuation. Herein, we quantify the exchange behavior of a copper paddlewheel-based CUS-MOF (HKUST-1) in the presence of three different solvents: ethanol (EtOH), dichloromethane (DCM), and N,N-dimethylformamide (DMF). The DMF release profiles are monitored via in situ observation of the exchange solvent composition via 1 H NMR and Raman spectroscopy at the macroscopic scale. Furthermore, the change in solvent within a single crystal is measured to directly elucidate the exchange behavior. We demonstrate the DMF release profile from HKUST-1 exhibits different rate laws depending on whether the solvent exchange occurs at the CUS or is purely diffusive through the pores. This contribution represents the first characterization of release from a CUS-MOF as a function exchange solvent and reveals that solvent exchange in a CUS-MOF is not diffusion-limited, but rather is limited by the solvent exchange kinetics at the metal center. Insights from this study can be generalized to the variety of copper-paddlewheel-based MOFs, informing best practices for solvent exchange.

Solvent Exchange Dynamics in M2(dobdc): An Interplay among Binding Strength, Exchange Kinetics, and Cooperativity

“…When the coordinated ligand in an octahedral complex is replaced, a dissociative mechanism is generally operative; , this mechanism involves a reduced coordination number of the intermediate during the ligand replacement, making the exchange rate independent of the nature of the entering ligand. In such cases, the substitution of CUS-bound solvents proceeds at similar rates regardless of the exchange solvent, as observed in the case of copper paddlewheel MOFs . However, if the solvent exchange follows an associative mechanism, then, the solvent exchange rate is sensitive to the nature of the entering ligand.…”

“…Unlike the copper paddlewheel MOFs , that exhibit metal–metal stretching frequencies, M 2 (dobdc) does not display such vibrations. Instead, the stretching frequency of phenolate CO, which coordinates directly to the metal, is measured to indirectly monitor the change in the metal–solvent interaction.…”

“…In certain MOFs, coordinatively unsaturated sites (CUS) are obtained via the liberation of solvent molecules bound to the metal clusters while preserving the MOF structure; CUS-MOFs find extensive applications in gas capture/separation, − catalysis, , and sensing. , Exchange of the coordinated solvents is challenging because CUS-bound solvents are more strongly bound than pore-filled solvents, which are physically held in the pore environment. Solvent exchange kinetics depend on whether the solvent is CUS-bound or only pore-filled . In the solvent exchange of metal complexes, exchange rates of the coordinated solvents are known to depend on the nature of metal ions and solvent molecules. , When octahedral metal complexes are formed with divalent first-row transition metal cations, the stability of complexes generally increases across the periodic table because the ionic radius of the metal gradually decreases across the period. , While such effects are expected to manifest in MOFs, the fact that these are extended coordination materials makes extrapolation uncertain, and there is a dearth of experimental data.…”

Fast and In-Depth Reconstruction of Two-Dimension Cobalt-Based Zeolitic Imidazolate Framework in Glucose Oxidation Processes

“…Practically, MOFs are reported to release metal ions and ligands in water media, − concurrently experiencing structural reconstruction stimulated by electrochemical environment. − Furthermore, it has been observed that MOFs undergo surface reconstruction to generate oxyhydroxides during glucose electrocatalytic oxidation. − These phenomena raise two critical inquiries: first, the veritable source of catalytic activity governing glucose oxidation and, second, the extent to which this reconstruction transpires, whether it remains confined to the surface of MOFs or penetrates deeper as the reaction proceeds. Nevertheless, these aspects have not been explored in depth within the current literature.…”

A Method for Determining Incorporation Depth in Core–Shell UiO-66 Nanoparticles Synthesized Via Postsynthetic Exchange

“…The mechanisms of the PSE process are complex and not fully understood, though recent research has begun to elucidate some aspects. Factors influencing PSE include a choice of solvent, − reaction time, ,− temperature, ,,, and the functional groups ,, on the linker. The particle size of the original MOF used in the PSE process is also crucial and typically remains unchanged after PSE. − ,,,, Nair and co-workers demonstrated that, under identical reaction conditions, the incorporation rate for nanoparticles significantly increases compared to the micrometer-sized crystals .…”