Leveraging Isothermal Titration Calorimetry to Obtain Thermodynamic Insights into the Binding Behavior and Formation of Metal-Organic Frameworks

Role of Metal-Organic Framework Topology on Thermodynamics of Polyoxometalate Encapsulation

Polyoxometalates (POMs) are discrete anionic clusters whose rich redox properties, strong Bro̷nsted acidity, and high availability of active sites make them potent catalysts for oxidation reactions. Metal-organic frameworks (MOFs) have emerged as tunable, porous platforms to immobilize POMs, thus increasing their solution stability and catalytic activity. While POM@MOF composite materials have been widely used for a variety of applications, little is known about the thermodynamics of the encapsulation process. Here, we utilize an up-and-coming technique in the field of heterogeneous materials, isothermal titration calorimetry (ITC), to obtain full thermodynamic profiles (ΔH, ΔS, ΔG, and Ka) of POM binding. Six different 8-connected hexanuclear Zr-MOFs were investigated to determine the impact of MOF topology (csq, scu, and the) on POM encapsulation thermodynamics.

Thermodynamic Insights into Phosphonate Binding in Metal-Azolate Frameworks

Organophosphorus chemicals, including chemical warfare agents (CWAs) and insecticides, are acutely toxic materials that warrant capture and degradation. Metal-organic frameworks (MOFs) have emerged as a class of tunable, porous, crystalline materials capable of hydrolytically cleaving, and thus detoxifying, several organophosphorus nerve agents and their simulants. One such MOF is M-MFU-4l (M = metal), a bioinspired azolate framework whose metal node is composed of a variety of divalent first-row transition metals. While Cu-MFU-4l and Zn-MFU-4l are shown to rapidly degrade CWA simulants, Ni-MFU-4l and Co-MFU-4l display drastically lower activities. The lack of reactivity was hypothesized to arise from the strong binding of the phosphate product to the node, which deactivates the catalyst by preventing turnover. No such study has provided detailed insight into this mechanism. Here, we leverage isothermal titration calorimetry (ITC) to monitor the binding of an organophosphorus compound with the M-MFU-4l series to construct a complete thermodynamic profile (Ka, ΔH, ΔS, ΔG) of this interaction. This study further establishes ITC as a viable technique to probe small differences in thermodynamics that result in stark differences in material properties, which may allow for better design of first-row transition metal MOF catalysts for organophosphorus hydrolysis.

Role of Interface of Metal-Organic Frameworks and Their Composites in Persulfate-Based Advanced Oxidation Process for Water Purification

The persulfate activation-based advanced oxidation process (PS-AOP) is an important technology in wastewater purification. Using metal-organic frameworks (MOFs) as heterogeneous catalysts in the PS-AOP showed good application potential. Considering the intrinsic advantages and disadvantages of MOF materials, combining MOFs with other functional materials has also shown excellent PS activation performance and even achieves certain functional expansion. This Review introduces the classification of MOFs and MOF-based composites and the latest progress of their application in PS-AOP systems. The relevant activation/degradation mechanisms are summarized and discussed. Moreover, the importance of catalyst-related interfacial interaction for developing and optimizing advanced oxidation systems is emphasized. Then, the interference behavior of environmental parameters on the interfacial reaction is analyzed. Specifically, the initial solution pH and coexisting inorganic anions may hinder the interfacial reaction process via the consumption of reactive oxygen species, affecting the activation/degradation process. This Review aims to explore and summarize the interfacial mechanism of MOF-based catalysts in the activation of PS. Hopefully, it will inspire researchers to develop new AOP strategies with more application prospects.

Linkage Transformations in a Three-Dimensional Covalent Organic Framework for High-Capacity Adsorption of Perfluoroalkyl Substances

Despite their many advantages, covalent organic frameworks (COFs) built from three-dimensional monomers are synthetically difficult to functionalize. Herein, we provide a new synthetic approach to the functionalization of a three-dimensional covalent organic framework (COF-300) by using a series of solid-state linkage transformations. By reducing the imine linkages of the framework to amine linkages, we produced a more hydrolytically stable material and liberated a nucleophilic amino group, poised for further functionalization. We then treated the amine-linked COF with diverse electrophiles to generate a library of functionalized materials, which we tested for their ability to adsorb perfluoroalkyl substances (PFAS) from water. The framework functionalized with dimethylammonium groups, COF-300-dimethyl, adsorbed more than 250 mg of perfluorooctanoic acid (PFOA) per 1 g of COF, which represents an approximately 14,500-fold improvement over that of COF-300 and underscores the importance of electrostatic interactions to PFAS adsorption performance. This work provides a conceptually new approach to the design and synthesis of functional three-dimensional COFs.

Special Issue "Advances in Monitoring Metabolic Activities of Microorganisms by Calorimetry"

In recent decades, the calorimetric monitoring of microbial metabolism, i [...].

Spatially confined protein assembly in hierarchical mesoporous metal-organic framework

Immobilization of biomolecules into porous materials could lead to significantly enhanced performance in terms of stability towards harsh reaction conditions and easier separation for their reuse. Metal-Organic Frameworks (MOFs), offering unique structural features, have emerged as a promising platform for immobilizing large biomolecules. Although many indirect methods have been used to investigate the immobilized biomolecules for diverse applications, understanding their spatial arrangement in the pores of MOFs is still preliminary due to the difficulties in directly monitoring their conformations. To gain insights into the spatial arrangement of biomolecules within the nanopores. We used in situ small-angle neutron scattering (SANS) to probe deuterated green fluorescent protein (d-GFP) entrapped in a mesoporous MOF. Our work revealed that GFP molecules are spatially arranged in adjacent nanosized cavities of MOF-919 to form "assembly" through adsorbate-adsorbate interactions across pore apertures. Our findings, therefore, lay a crucial foundation for the identification of proteins structural basics under confinement environment of MOFs.

On the Binding Mechanisms of Calcium Ions to Polycarboxylates: Effects of Molecular Weight, Side Chain, and Backbone Chemistry

We experimentally determined the characteristics and Langmuir parameters of the binding of calcium ions to different polycarboxylates. By using potentiometric titrations and isothermal titration calorimetry, the effects of side chain chemistry, pH value, and chain length were systematically investigated using the linear polymers poly(aspartic acid), poly(glutamic acid), and poly(acrylic acid). We demonstrate that for polymers with high polymerization degrees, the binding process is governed by higher-order effects, such as the change of apparent pK_a of carboxyl groups, and contributions arising from the whole polymer chain while the chemistry of the monomer unit constituting the polymer plays a subordinate role. In addition, primary binding sites need to be present in the polymer, thus rendering the abundance and sequential arrangement of protonated and deprotonated groups important. The detection of higher-order effects contradicts the assumptions posed by the Langmuir model of noninteracting binding sites and puts a question mark on whether ion binding to polycarboxylates can be described using solely a Langmuir binding model. No single uniform mechanism fits all investigated systems, and the whole polymer chain, including terminal groups, needs to be considered for the interpretation of binding data. Therefore, one needs to be careful when explaining ion binding to polymers solely based on studies on monomers or oligomers.

Enhancement of the Catalytic Activity of Double Metal Cyanides for the Oxidation of Styrene by the Presence of Included Alcohols

In recent years, people have focused on the development of simple and efficient heterogeneous catalysts for the styrene epoxidation reaction. In this work, a FeCo double metal cyanide (DMC) was modified with C1 to C6 linear alcohols, and the prepared materials were used to catalyze the reaction of styrene epoxidation in various solvents. It is noteworthy that the styrene conversion is mainly affected by modification with alcohols, while the selectivity in styrene oxide (SO) is obviously influenced by the solvent. FeCo DMC along with MeOH exhibits the best catalytic performance, with a conversion rate of 96% and a SO selectivity of 86%, in N,N-dimethylformamide (DMF) solvent. Various physical and chemical methods were used to analyze the structures and compositions of the materials. To clarify the mechanism of the improvement, we set up an original approach to investigate the kinetics of the adsorption process between the oxidant and the catalyst, using isothermal titration calorimetry (ITC). The obtained results illustrate that the adsorption process of the oxidant on the surface of FeCo DMC can be dramatically promoted by the presence of MeOH. Such a difference in adsorption thus explains the significant improvement of its catalytic activity by modification with MeOH. This study thus provides a new fundamental understanding of DMC catalysts for the styrene epoxidation reaction.