Publications

The 17O resonances of Zirconium-oxo clusters that can be found in porous Zr carboxylate metal-organic frameworks (MOFs) have been investigated by magic-angle spinning (MAS) NMR spectroscopy enhanced by dynamic nuclear polarization (DNP). High-resolution 17O spectra at 0.037 % natural abundance could be obtained in 48 hours, thanks to DNP enhancement of the 1H polarization by factors e(1H) = Swith/Swithout = 28, followed by 1H®17O cross-polarization, allowing a saving in experimental time by a factor of ca. 800. The distinct 17O sites from the oxo-clusters can be resolved at 18.8 T. Their assignment is supported by density functional theory (DFT) calculations of chemical shifts and quadrupolar parameters.

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The development of room temperature green syntheses of robust MOFs is of great interest to meet the demand of the sustainable chemistry and is a pre‐requisite for the incorporation of functional but fragile compounds in water stable MOFs. However, only few ambient conditions routes to produce metal(IV) based MOFs have been reported and most of them suffer from a very low yield and/or multiple steps that preclude their use for most applications. We report here a new versatile one‐step synthesis of a series of highly porous M 6 oxoclusters based MOFs (M= Zr, Hf, Ce) at room temperature, including 8 or 12‐connected micro/mesoporous solids with different functionalized organic ligands. The resulting compounds show varying degrees of defectivity, particularly for 12‐connected phases, while maintaining the chemical stability of the parent MOFs. We propose first insights for the efficient MOF preparation based on In‐situ kinetics observations. Remarkably, the synthetic versatility not only allows an efficient room temperature synthesis with a high space‐time yield, but also gives possibility to tune the particle size, which therefore paves the way for their practical use.

We analyze the application of Principal Component Analysis (PCA) for untangling the main contributions to changing diffracted intensities upon variation of site occupancy and lattice dimensions induced by external stimuli. The information content of the PCA output consists of certain functions of Bragg angles (loadings) and their evolution characteristics that depend on external variables like pressure or temperature (scores). The physical meaning of the PCA output is to date not well understood. Therefore, in this paper, the intensity contributions are first derived analytically, then compared with the PCA components for model data; finally PCA is applied for the real data on isothermal gas uptake by nanoporous framework γ –Mg(BH 4 ) 2 . We show that, in close agreement with previous analysis of modulation diffraction, the variation of intensity of Bragg lines and the displacements of their positions results in a series of PCA components. Every PCA extracted component may be a mixture of terms carrying information on the average structure, active sub-structure, and their cross-term. The rotational ambiguities, that are an inherently part of PCA extraction, are at the origin of the mixing. For the experimental case considered in the paper, the extraction of the physically meaningful loadings and scores can only be achieved with a rotational correction. Finally, practical recommendations for non-blind applications, i.e., what boundary conditions to apply for the the rotational correction, of PCA for diffraction data are given.

Crystalline materials with pore dimensions comparable to the kinetic diameters of the guest molecules are attractive for their potential use in adsorption and separation applications. The nanoporous γ-Mg(BH4)2 features one-dimensional channels matching this criterion for Kr uptake, which has been probed using synchrotron powder diffraction at various pressures and temperatures. It results in two coexisting crystalline phases with the limiting composition Mg(BH4)2·0.66Kr expecting the highest Kr content (50.7 wt % in the crystalline phase) reported for porous materials. Quasi-equilibrium isobars built from Rietveld refinements of Kr site occupancies were rationalized with a noncooperative lattice gas model, yielding the values of the thermodynamic parameters. The latter were independently confirmed from Kr fluorescence. We have also parameterized the pronounced kinetic hysteresis with a modified mean-field model adopted for the Arrhenius kinetics.

Silicoaluminophosphates (SAPOs) are a special class of zeolites that, due to their acidic and shape-selective properties, play a major role in ion exchange and separation processes and in crude oil cracking. SAPO-37 has the faujasite (FAU) topology same as zeolites X and Y, which are involved in more than 40% of the total crude oil conversion worldwide. A critical parameter that promotes detrimental structural transformations in SAPOs during real-life applications is the presence of humidity. In this study, we employ a multidisciplinary approach combining in situ synchrotron radiation powder X-ray diffraction (SR-PXRD), water adsorption, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and density functional theory (DFT) calculations to describe the mechanism and reveal the reasons why SAPO-37 collapses upon contact with humidity below 345 K. SR-PXRD revealed that the sodalite (SOD) cages (subunits of the FAU structure) have the strongest affinity to water during hydration below 345 K. Furthermore, below 345 K, the faujasite framework takes up an order of magnitude more water molecules than at temperatures above 345 K. DRIFTS confirmed the presence of Si–OH and P–OH surface structural defects that act as hydration centers, accelerating the loss of a long-range order. Finally, DFT calculations showed that the enthalpy of water adsorption in the sodalite cage and the faujasite supercage is −212 and −13 kJ/mol, respectively. The results presented in this work are highly topical for understanding the effect of water on the frameworks of the SAPO microporous catalysts family. The notorious instability of SAPO-37 is the result of the accumulative contribution of topological, physical, and chemical effects, leading to an array of rapidly evolving cascading effects. Our work shows how advancements in SR-PXRD methodology and hardware give new insight into highly dynamic features previously difficult to observe. In addition, this work introduces the conceptual insight that nonhomogeneous sorption of molecular species will induce dynamic features with dramatic consequences at both molecular and atomic levels. This is a highly impactful factor opening research paths for further work within catalysis, porous material design and chemistry, and sorption reactions and processes.

Permanent magnets are generally produced from solid metals or alloys. Less dense compositions involving lighter elements tend to demagnetize well below room temperature or under modest applied external fields. Perlepe et al. now report that chemical reduction of a low-density chromium-pyrazine network produces a magnet that remains stable above 200°C and resists demagnetization with 7500-oersted coercivity at room temperature. The straightforward synthetic route to the material shows promise for broad exploration of potential applications.

Gas adsorption by porous frameworks sometimes result in structure “breathing”, “pores opening/closing”, “negative gas adsorption”, and other fascinating phenomena which can be revealed and explained with the use of in situ diffraction methods. The time‐dependent diffraction is able to address both kinetics of the guest uptake and structural response of the host framework, since the time evolution of the crystal structure bears the information on the mechanisms and kinetic barriers of guest adsorption. Using such advanced sub‐second in situ powder X‐ray diffraction, three various intracrystalline diffusion scenarios have been evaluated from the isothermal kinetics of Ar, Kr, and Xe adsorption by nanoporous γ‑Mg(BH4)2. These scenarios are dictated by two possible simultaneous transport mechanisms: diffusion through the intra‐ (i) and interchannel apertures (ii) of γ‐Mg(BH4)2 crystal structure. The contribution of i and ii changes depending on the kinetic diameter of the noble gas molecule and temperature regime. The lowest single activation barrier for the smallest Ar suggests equal diffusion of the atoms trough both pathways. Contrary, for the medium sized Kr we resolve the contributions of two parallel transport mechanisms, which tentatively can be attributed to the smaller barrier of the migration paths via the channel like pores and the higher barrier for the diffusion via narrow aperture between these channels. Remarkably, the largest Xe atoms diffuse only along 1D channels and show the highest single activation barrier. This work demonstrates a potential of sub‐second diffraction to access site‐specific kinetics of guest uptake in multi‐adsorption site frameworks.

Isophthalic acid (IPA) has been considered to build metal-organic frameworks (MOFs), owing to its facile availability, unique connection angle-mode, and a wide range of functional groups attached. Constructing titanium-IPA frameworks that possess photoresponse properties is an alluring characteristic with respect to the challenge of synthesizing new titanium-based MOFs (Ti-MOFs). Here, we report the first Ti-IPA MOF (MIP-208) that efficiently combines the use of preformed Ti8 oxoclusters and in situ acetylation of the 5-NH2-IPA linker. The mixed solid-solution linkers strategy was successfully applied, resulting in a series of multivariate MIP-208 structures with tunable chemical environments and sizable porosity. MIP-208 shows the best result among the pure MOF catalysts for the photocatalytic methanation of carbon dioxide. To improve the photocatalytic performance, ruthenium oxide nanoparticles were photo-deposited on MIP-208, forming a highly active and selective composite catalyst, MIP-208@RuOx, which features a notable visible-light response coupled with excellent stability and recycling ability.

Mesoporous materials suffer from poor crystallinity and hydrolytic stability, lack of chemical diversity, insufficient pore accessibility, complex synthesis, and toxicity issues. Here the association of Zr-oxo clusters and isophthalate via a homometallic-multicluster-dot strategy results in a robust mesoporous metal-organic framework, denoted as MIP-206 (MIP stands for materials of the Institute of Porous Materials of Paris), that overcomes the aforementioned limitations. MIP-206, with a combination of Zr6 and Zr12 oxo-cluster inorganic building units into a single structure, exhibits meso-channels of ca. 2.6 nm and displays excellent chemical stability. Owing to the abundant variety of functionalized isophthalic acid linkers, the chemical environment of MIP-206 can be tuned without hampering pore accessibility. MIP-206 loaded with palladium nanoparticles acts as an efficient and durable catalyst for the dehydrogenation of formic acid, outperforming benchmark mesoporous materials. This paves the way toward the utilization of MIP-206 as a mesoporous platform for a wide range of potential applications.