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

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

Solvent exchange is a crucial step in ensuring the complete activation of metal-organic frameworks (MOFs); however, the conditions for solvent exchange vary among MOFs, even within the isostructural variants. This study examines the factors contributing to solvent exchange by investigating the isostructural M2(dobdc) (M═Mg, Co, Zn) series. Common solvents N,N-dimethylformamide (DMF), ethanol (EtOH), and methanol (MeOH) are employed to assess the solvent exchange at coordinatively unsaturated sites (CUS) of M2(dobdc). By monitoring both solvents released from the MOF during solvent exchange and the coordination environment of metals within the MOF, a picture is constructed of exchange rates during early stages of solvent exchange as well as expulsion of the last traces of bound solvents. This differentiation is achieved by a combination of bulk monitoring of solvent phase composition and microscopic application of Raman spectroscopy on the single-crystal level. The kinetics of solvent replacement is revealed to have a substantial contribution from cooperativity; this phenomenon is observed in both the forward and reverse directions. Thermogravimetric analysis coupled with IR spectroscopy and density functional theory (DFT) calculations are employed to elucidate the relationship between solvent exchange rates and solvent binding energy. The solvent exchange rates are determined by the kinetic barriers of solvent exchange that do not follow the order of the solvent binding affinity. This work contributes to understanding the solvent exchange of MOFs by examining the interplay among the binding strength, exchange kinetics, and cooperativity. It further provides valuable insights for scrutinizing MOF activation protocols.

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

Postsynthetic exchange (PSE) is a key technique for integrating sensitive linkers into metal-organic frameworks (MOFs). Despite its importance, investigations into linker distributions have primarily focused on micrometer-sized crystals due to the analytical limitations, leaving nanoparticles less explored, although they are commonly synthesized and used in applications. In particular, the emergence of core-shell nanostructures via PSE has shown potential for applications in CO2 adsorption and selective catalysis. This study addresses this gap by investigating the formation of core-shell structures on nanoparticles under diffusion-controlled PSE conditions. By analyzing volume-to-surface ratios and conducting time-dependent experiments, we confirmed that these conditions facilitate the development of core-shell architectures. We also developed a straightforward method to calculate the minimum incorporation depth using basic parameters such as particle size and the total amount of incorporated linker. The accuracy of our approach was validated against data obtained from transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy. These findings enhance the understanding of PSE in MOF nanoparticles and open up promising avenues for developing advanced MOF core-shell structures for various applications.

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

Currently, metal-organic frameworks (MOFs) have emerged as viable candidates for enduring electrode materials in nonenzyme glucose sensing. However, given the inherent water susceptibility of MOFs and their complete self-reconstruction during the process of electrochemical oxygen evolution in alkaline conditions, we are motivated to explore the truth of MOFs catalyzing glucose oxidation. In this work, we fabricated a two-dimensional cobalt-based zeolitic imidazolate framework (ZIF-L) as the electrode material for catalyzing glucose oxidation in alkaline conditions. Our explorations revealed that while the initial glucose catalytic response varied among ZIF-L samples with differing thicknesses, the ultimate steady-state catalytic performance remained largely consistent. This phenomenon arose from the transformation of ZIF-L with distinct thicknesses into CoOOH with uniform morphological and structural characteristics during the glucose catalysis process. And in situ Raman spectroscopy elucidated the sustained equilibrium within the glucose catalytic system, wherein the dynamic interconversion between CoOOH and Co(OH)2 governs the overall process. This study contributes to an enhanced understanding of the glucose catalytic mechanism aspects of nonenzymatic glucose sensor electrode materials, offering insights that serve as inspiration for the development of advanced glucose electrode materials.