Effect of Linker Substituent Nature on Performance of Active Sites in UiO-66: Combined FT-IR and DFT Study

The nature of organic linker substituents plays an important role in gas sorption and separation as well as in catalytic applications of metal-organic frameworks. Zirconium-based UiO-66 is one of the most tunable members of this class of materials. However, the prediction of its properties is still not a fully solved problem. Here, the infrared spectroscopic measurements using highly sensitive CO probe molecules, combined with DFT calculations, are used in order to characterize the performance of different acidic sites caused by the presence of different organic linker substituents. The proposed model allowed differentiation between various active sites over the UiO-66 and clarification of their behavior. The experimental IR bands related to CO adsorption can be unambiguously assigned to one type of site or another. The previously undescribed highly red-shifted band is attributed to CO adsorbed on coordinatively unsaturated zirconium sites through an O atom. The results confirm the lower and higher Lewis's acidity of coordinatively unsaturated Zr sites on linker defects in the UiO-66 structure when electron-withdrawing and electron-donating groups are, respectively, included in a terephthalate moiety, whilst the Brønsted acidity of zirconium oxo-cluster remains almost unchanged.

Latest insights on separation and storage of carbon compounds in buildings towards sustainable environment: Recent innovations, challenges, future perspectives and application of machine learning

Throughout the past few decades, scientific agencies have paid a lot of attention to environmental issues such as acid rain, water poisoning, and global warming. In order to solve these environmental problems, metal-organic frameworks (MOFs), which are made up of metal ions and/or clusters attached to organic ligands, have shown some promise. With a focus on the usage of MOFs, this paper examines the most recent developments, difficulties, and potential future directions in the separation and storage of carbon compounds in buildings for a sustainable environment. The importance of using MOFs in decarbonizing water systems and lowering environmental concerns in buildings is highlighted in the research. It addresses the most recent developments in the use of MOFs for renewable energy, such as the elimination of dangerous gases like CO2 and CH4 from water systems. The article also looks at how MOFs might be used to decarbonize water systems in structures, with a focus on how carbon-containing compounds are stored chemically and physically using artificial neural network models. MOFs are a potential solution for renewable energy and environmental remediation in buildings because they have special physical and chemical characteristics like adjustable pores, high porosity, and tiny pore size. The report offers insights into existing treatments and invites academics to investigate MOFs' potential for resolving environmental problems in order to create a sustainable environment in buildings.

Metal-organic frameworks as promising solid-phase sorbents for the isolation of third-generation synthetic cannabinoids in biological samples

In this work, metal-organic frameworks (MOFs) were used for the first time as solid-phase extraction (SPE) sorbents for the isolation of synthetic cannabinoids (SCs) from oral fluids and subsequently quantified by LC-fluorescence detection (FLD). In this context, different MOF families were synthesized and tested under SPE mode. UiO-66 was the family selected, being the amino functionalized (NH₂-UiO-66) the best candidate in terms of extraction performance. After the method optimization, several analytical parameters of interest were obtained, reaching limits of detection (LODs) as low as 0.6-0.8 μg L^-1 and precision values (expressed as RSD) lower than 10.6%. The developed method was successfully applied to the determination of 8 SCs in different oral fluids at three spiked levels with recoveries between 67 and 114%. This method claims to be a real alternative for screening purposes, being a cost-effective procedure due to the price of the sorbent (<0.5 €/cartridge) and its recyclability (up to 12 uses), among others good features.

Active site identification and CO oxidation in UiO-66-XX thin films

Metal-organic frameworks (MOFs) offer an intrinsically porous and chemically tunable platform for gas adsorption, separation, and catalysis. We investigate thin film derivatives of the well-studied Zr-O based MOF powders to understand their adsorption properties and reactivity with their adaption to thin films, involving diverse functionality with the incorporation of different linker groups and the inclusion of embedded metal nanoparticles: UiO-66, UiO-66-NH₂, and Pt@UiO-66-NH₂. Using transflectance IR spectroscopy, we determine the active sites in each film upon consideration of the acid-base properties of the adsorption sites and guest species, and perform metal-based catalysis with CO oxidation of a Pt@UiO-66-NH₂film. Our study shows how surface science characterization techniques can be used to characterize the reactivity and the chemical and electronic structure of MOFs.

Nanoconfinement and mass transport in metal-organic frameworks

The ubiquity of metal-organic frameworks in recent scientific literature underscores their highly versatile nature. MOFs have been developed for use in a wide array of applications, including: sensors, catalysis, separations, drug delivery, and electrochemical processes. Often overlooked in the discussion of MOF-based materials is the mass transport of guest molecules within the pores and channels. Given the wide distribution of pore sizes, linker functionalization, and crystal sizes, molecular diffusion within MOFs can be highly dependent on the MOF-guest system. In this review, we discuss the major factors that govern the mass transport of molecules through MOFs at both the intracrystalline and intercrystalline scale; provide an overview of the experimental and computational methods used to measure guest diffusivity within MOFs; and highlight the relevance of mass transfer in the applications of MOFs in electrochemical systems, separations, and heterogeneous catalysis.

Tuning the Catalytic Properties of UiO-66 Metal-Organic Frameworks: From Lewis to Defect-Induced Brønsted Acidity

The Lewis/Brønsted acidity and catalytic properties of UiO-66-type metal-organic frameworks are studied in the context of tunable acid catalysts based on the presence of linker defects that create coordinatively unsaturated Zr⁴⁺ centers. Fourier transform infrared spectroscopy of adsorbed CO and direct pH measurements are employed to characterize hydrated and dehydrated UiO-66 containing different number of Zr⁴⁺ sites associated with defects. These sites can strongly polarize coordinated water molecules, which induces Brønsted acidity in the hydrated material. Upon dehydration of the solid, the coordinated water molecules are removed, and the underlying coordinatively unsaturated Zr⁴⁺ cations become exposed and available as Lewis acid sites. Herein we show, for various acid-catalyzed reactions, how it is possible to shift from a Brønsted acid to a Lewis acid catalyst by simply controlling the hydration degree of the solid. This control adds a new dimension to the design and engineering of MOFs for catalytic applications.

Metal-Organic Framework- and Polyoxometalate-Based Sorbents for the Uptake and Destruction of Chemical Warfare Agents

The threat of chemical warfare agents (CWAs), assured by their ease of synthesis and effectiveness as a terrorizing weapon, will persist long after the once-tremendous stockpiles in the U.S. and elsewhere are finally destroyed. As such, soldier and civilian protection, battlefield decontamination, and environmental remediation from CWAs remain top national security priorities. New chemical approaches for the fast and complete destruction of CWAs have been an active field of research for many decades, and new technologies have generated immense interest. In particular, our research team and others have shown metal-organic frameworks (MOFs) and polyoxometalates (POMs) to be active for sequestering CWAs and even catalyzing the rapid hydrolysis of agents. In this Forum Article, we highlight recent advancements made in the understanding and evaluation of POMs and Zr-based MOFs as CWA decontamination materials. Specifically, our aim is to bridge the gap between controlled, solution-phase laboratory studies and real-world or battlefield-like conditions by examining agent-material interactions at the gas-solid interface utilizing a multimodal experimental and computational approach. Herein, we report our progress in addressing the following research goals: (1) elucidating molecular-level mechanisms of the adsorption, diffusion, and reaction of CWA and CWA simulants within a series of Zr-based MOFs, such as UiO-66, MOF-808, and NU-1000, and POMs, including Cs₈Nb₆O₁₉ and (Et₂NH₂)₈[(α-PW₁₁O₃₉Zr(μ-OH)(H₂O))₂]·7H₂O, (2) probing the effects that common ambient gases, such as CO₂, SO₂, and NO₂, have on the efficacy of the MOF and POM materials for CWA destruction, and (3) using CWA simulant results to develop hypotheses for live agent chemistry. Key hypotheses are then tested with targeted live agent studies. Overall, our collaborative effort has provided insight into the fundamental aspects of agent-material interactions and revealed strategies for new catalyst development.

On the importance of combining bulk- and surface-active sites to maximize the catalytic activity of metal–organic frameworks for the oxidative dehydrogenation of alcohols using alkyl hydroperoxides as hydride acceptors

The combination of spectroscopically characterized bulk- and surface-active zirconium sites makes Zr-MOF crystals efficient catalysts for alcohol dehydrogenation under mild conditions.