Effective Gas Separation Performance Enhancement Obtained by Constructing Polymorphous Core-Shell Metal-Organic Frameworks

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.

Interfacial Engineering of Heterogeneous Reactions for MOF-on-MOF Heterostructures

Metal-organic frameworks (MOFs), as a subclass of porous crystalline materials with unique structures and multifunctional properties, play a pivotal role in various research domains. In recent years, significant attention has been directed toward composite materials based on MOFs, particularly MOF-on-MOF heterostructures. Compared to individual MOF materials, MOF-on-MOF structures harness the distinctive attributes of two or more different MOFs, enabling synergistic effects and allowing for the tailored design of diverse multilayered architectures to expand their application scope. However, the rational design and facile synthesis of MOF-on-MOF composite materials are in principle challenging due to the structural diversity and the intricate interfaces. Hence, this review primarily focuses on elucidating the factors that influence their interfacial growth, with a specific emphasis on the interfacial engineering of heterogeneous reactions, in which MOF-on-MOF hybrids can be conveniently obtained by using pre-fabricated MOF precursors. These factors are categorized as internal and external elements, encompassing inorganic metals, organic ligands, lattice matching, nucleation kinetics, thermodynamics, etc. Meanwhile, these intriguing MOF-on-MOF materials offer a wide range of advantages in various application fields, such as adsorption, separation, catalysis, and energy-related applications. Finally, this review highlights current complexities and challenges while providing a forward-looking perspective on future research directions.

Effect of Spatial Heterogeneity on the Unusual Uptake Behavior of Multivariate-Metal-Organic Frameworks

The uniqueness of multivariate metal-organic frameworks (MTV-MOFs) has been widely explored to discover their unknown opportunities. While mesoscopic apportionments have been studied, macroscopic heterogeneity and its spatial effects remain unexplored in MTV-MOFs. In this study, we investigated the effect of macroscopic heterogeneity on MTV-MOFs on their uptake behaviors by comparing three types of MTV-MOFs having the functional groups in inner, outer, or entire parts of crystals. Their adsorption behavior for carbon dioxide (CO₂) and water (H₂O) brought out that functional groups located in the outer part of the crystals dominantly influence the sorption behavior of MTV-MOFs. These results are also visualized by observing iodine adsorption in the three types of MTV-MOFs using scanning transmission electron microscopy-electron energy loss spectroscopy. We believe that this finding provides new ways to decipher and design MTV-MOFs for their unusual properties.

Designing optimal core-shell MOFs for direct air capture

Metal-organic frameworks (MOFs), along with other novel adsorbents, are frequently proposed as candidate materials to selectively adsorb CO₂ for carbon capture processes. However, adsorbents designed to strongly bind CO₂ nearly always bind H₂O strongly (sometimes even more so). Given that water is present in significant quantities in the inlet streams of most carbon capture processes, a method that avoids H₂O competition for the CO₂ binding sites would be technologically valuable. In this paper, we consider a novel core-shell MOF design strategy, where a high-CO₂-capacity MOF "core" is protected from competitive H₂O-binding via a MOF "shell" that has very slow water diffusion. We consider a high-frequency adsorption/desorption cycle that regenerates the adsorbents before water can pass through the shell and enter the core. To identify optimal core-shell MOF pairs, we use a combination of experimental measurements, computational modeling, and multiphysics modeling. Our library of MOFs is created from two starting MOFs-UiO-66 and UiO-67-augmented with 30 possible functional group variations, yielding 1740 possible core-shell MOF pairs. After defining a performance score to rank these pairs, we identified 10 core-shell MOF candidates that significantly outperform any of the MOFs functioning alone.

An X-ray Photoelectron Spectroscopy Study of Postsynthetic Exchange in UiO-66

Postsynthetic exchange (PSE) is a method that is widely used to change the composition of metal-organic frameworks (MOFs) by replacing connecting linkers or metal nodes after the framework has been synthesized. However, few techniques can probe the nature and distribution of exchanged species following PSE. Herein, we show that X-ray photoelectron spectroscopy can be used to compare the relative concentrations of exchanged ligands at the surface and interior regions of MOF particles. Specifically, PSE of iodobenzene dicarboxylate ligands results in a gradient distribution from surface to bulk in UiO-66 nanoparticles that depends on PSE time. X-ray photoelectron spectroscopy also reveals differences between the surface chemistry of the PSE product and that of the direct synthesis product.

Synthesis of MOF-on-MOF architectures in the context of interfacial lattice matching

This highlight summarises the previously reported MOF-on-MOF systems, with a focus on the presented crystallographic information and classification of the systems according to lattice parameter matching.

Solvent Choice in Metal-Organic Framework Linker Exchange Permits Microstructural Control

Linker exchange is a widely applied, robust technique for elaboration of metal-organic frameworks (MOFs) post-synthesis. The observation of core-shell microstructures under certain conditions was hypothesized to arise from diffusion rates into the MOF that are slower than linker exchange. Here the relative contributions of these processes are manipulated through solvent choice in order to modulate shell thickness and exchange extent. The findings allow tailoring MOF microstructure to application.