Understanding the Effects of Binders in Gas Sorption and Acidity of Aluminium Fumarate Extrudates.

Understanding the impact of shaping processes on solid adsorbents is critical for the implementation of MOFs in industrial separation processes or as catalytic materials. Production of MOF-containing shaped particles is typically associated with loss of porosity and modification of acid sites, two phenomena that affect their performance. Herein, we report a detailed study on how extrusion affects the crystallinity, porosity, and acidity of the aluminium fumarate MOF with clays or SiO2 gel binders. Thorough characterization showed that the clay binders confer the extrudates a good mechanical robustness at the expense of porosity, while silica gel shows an opposite trend. The CO2 selectivity towards CH4 , of interest for natural gas separation processes, is maintained upon the extrusion process. Moreover, probe FTIR spectroscopy revealed no major changes in the types of acid sites. This study highlights that these abundant and inexpensive clay materials may be used for scaling MOFs as active adsorbents.

Visualizing defects and pore connectivity within metal-organic frameworks by X-ray transmission tomography.

Metal-Organic Frameworks (MOFs) have the potential to change the landscape of molecular separations in chemical processes owing to their ability of selectively binding molecules. Their molecular sorting properties generally rely on the micro- and meso-pore structure, as well as on the presence of coordinatively unsaturated sites that interact with the different chemical species present in the feed. In this work, we show a first-of-its-kind tomographic imaging of the crystal morphology of a metal-organic framework by means of transmission X-ray microscopy (TXM), including a detailed data reconstruction and processing approach. Corroboration with Focused Ion Beam-Scanning Electron Microscopy (FIB-SEM) images shows the potential of this strategy for further (non-destructively) assessing the inner architecture of MOF crystals. By doing this, we have unraveled the presence of large voids in the internal structure of a MIL-47(V) crystal, which are typically thought of as rather homogeneous lattices. This challenges the established opinion that hydrothermal syntheses yield relatively defect-free material and sheds further light on the internal morphology of crystals.

Influence of Pore Structure and Metal-Node Geometry on the Polymerization of Ethylene over Cr-Based Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) have received increasing interest as solid single-site catalysts, owing to their tunable pore architecture and metal node geometry. The ability to exploit these modulators makes them prominent candidates for producing polyethylene (PE) materials with narrow dispersity index (Ð) values. Here a study is presented in which the ethylene polymerization properties, with Et2 AlCl as activator, of three renowned Cr-based MOFs, MIL-101(Cr)-NDC (NDC=2,6-dicarboxynapthalene), MIL-53(Cr) and HKUST-1(Cr), are systematically investigated. Ethylene polymerization reactions revealed varying catalytic activities, with MIL-101(Cr)-NDC and MIL-53(Cr) being significantly more active than HKUST-1(Cr). Analysis of the PE products revealed large Ð values, demonstrating that polymerization occurs over a multitude of active Cr centers rather than a singular type of Cr site. Spectroscopic experiments, in the form of powder X-ray diffraction (pXRD), UV/Vis-NIR diffuse reflectance spectroscopy (DRS) and CO probe molecule Fourier transform infrared (FTIR) spectroscopy corroborated these findings, indicating that indeed for each MOF unique active sites are generated, however without alteration of the original oxidation state. Furthermore, the pXRD experiments indicated that one major prerequisite for catalytic activity was the degree of MOF activation by the Et2 AlCl co-catalyst, with the more active materials portraying a larger degree of activation.

Spectroscopy, microscopy, diffraction and scattering of archetypal MOFs: formation, metal sites in catalysis and thin films.

Metal-organic frameworks (MOFs) are a class of porous crystalline materials showing great potential for applications such as catalysis, gas storage, molecular separations, energy storage and drug delivery. The properties that render them interesting stem from their structure (e.g. morphology, porosity or metal coordination and geometry). Thus, gaining a deeper understanding strongly relies on the availability and adequate use of advanced characterization tools, which can interrogate MOFs under realistic synthesis as well as catalysis (or sorption) conditions. Herein, we present an overview of the various characterization techniques specifically suitable for the study on the underlying chemistry of the formation mechanisms and adsorption properties of three archetypal MOFs, namely MIL-100, ZIF-8 and HKUST-1. A section on using MOFs as supports for metal atoms or complexes that can be used for catalysis on the robust Zr6 nodes of UiO-66 or NU-1000, and the characterization techniques used thereof, is presented as well. In addition, we discuss recent developments on the application of nano-spectroscopic characterization for MOF thin-films and explore the potential of MOFs as model systems in catalysis. The conclusions and outlook provide future research possibilities in the field of MOF characterization.

In Situ Spectroscopy of Calcium Fluoride Anchored Metal-Organic Framework Thin Films during Gas Sorption.

Surface-mounted metal-organic frameworks (SURMOFs) show promising behavior for a manifold of applications. As MOF thin films are often unsuitable for conventional characterization techniques, understanding their advantageous properties over their bulk counterparts presents a great analytical challenge. In this work, we demonstrate that MOFs can be grown on calcium fluoride (CaF2 ) windows after proper functionalization. As CaF2 is optically (in the IR and UV/Vis range of the spectrum) transparent, this makes it possible to study SURMOFs using conventional spectroscopic tools typically used during catalysis or gas sorption. Hence, we have measured HKUST-1 during the adsorption of CO and NO. We show that no copper oxide impurities are observed and also confirm that SURMOFs grown by a layer-by-layer (LbL) approach possess Cu+ species in paddlewheel confirmation, but 1.9 times less than in bulk HKUST-1. The developed methodology paves the way for studying the interaction of any adsorbed gases with thin films, not limited to MOFs, low temperatures, or these specific probe molecules, pushing the boundaries of our current understanding of functional porous materials.

Correction: Micro-spectroscopy of HKUST-1 metal-organic framework crystals loaded with tetracyanoquinodimethane: effects of water on host-guest chemistry and electrical conductivity.

Correction for 'Micro-spectroscopy of HKUST-1 metal-organic framework crystals loaded with tetracyanoquinodimethane: effects of water on host-guest chemistry and electrical conductivity' by Miguel Rivera-Torrente et al., Phys. Chem. Chem. Phys., 2019, 21, 25678-25689.

Multi-Spectroscopic Interrogation of the Spatial Linker Distribution in Defect-Engineered Metal-Organic Framework Crystals: The [Cu3 (btc)2-x (cydc)x ] Showcase.

In the past few years, defect-engineered metal-organic frameworks (DEMOFs) have been studied due to the plethora of textural, catalytic, or magnetic properties that can be enhanced by carefully introducing defect sites into the crystal lattices of MOFs. In this work, the spatial distribution of two different non-defective and defective linkers, namely 1,3,5-benzenetricarboxylate (BTC) and 5-cyano-1,3-benzenedicarboxylate (CYDC), respectively, has been studied in different DEMOF crystals of the HKUST-1 topology. Raman micro-spectroscopy revealed a nonhomogeneous distribution of defect sites within the [Cu3 (btc)2-x (cydc)x ] crystals, with the CYDC linker incorporated into defect-rich or defect-free areas of selected crystals. Additionally, advanced bulk techniques have shed light on the nature of the copper species, which is highly dynamic and directly affects the reactivity of the copper sites, as shown by probe molecule FTIR spectroscopy. Furthermore, electron microscopy revealed the effect of co-crystallizing CYDC and BTC on the crystal size and the formation of mesopores, further corroborated by X-ray scattering analysis. In this way we have demonstrated the necessity of utilizing micro-spectroscopy along with a whole array of bulk spectroscopic techniques to fully describe multicomponent metal-organic frameworks.

Nanoweb Surface-Mounted Metal-Organic Framework Films with Tunable Amounts of Acid Sites as Tailored Catalysts.

Metal-organic frameworks (MOFs) are a promising class of materials for many applications, due to their high chemical tunability and superb porosity. By growing MOFs as (thin-)films, additional properties and potential applications become available. Here, copper (II) 1,3,5-benzenetricarboxylate (Cu-BTC) metal-organic framework (MOF) thin-films are reported, which were synthesized by spin-coating, resulting in "nanowebs", that is, fiber-like structures. These surface-mounted MOFs (SURMOFs) were studied by using photoinduced force microscopy (PiFM) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The optimal concentration of precursors (10 mm) was determined that resulted in chemically homogeneous, pure nanowebs. Furthermore, the morphology and (un)coordinated Cu sites in the web were tuned by varying the rotation speed of the spin-coating process. X-ray diffraction (XRD) analysis showed that rotation speeds ≥2000 rpm (with precursors in a water/ethanol solution) generate the catena-triaqua-µ-(1,3,5-benzenetricarboxylate)-copper(II), or Cu(BTC)(H2 O)3 coordination polymer. X-ray photoelectron spectroscopy (XPS) highlighted the strong decrease in number of (defective) Cu+ sites, as the nanowebs mainly consist of coordinated Cu2+ Lewis acid sites (LAS) and organic linker-linker, for example, hydrogen-bonding, interactions. Finally, the Lewis-acidic character of the Cu sites is illustrated by testing the films as catalysts in the isomerization of α-pinene oxide. The higher number of LAS (≥3000 rpm), result in higher campholenic aldehyde selectivity reaching up to 87.7 %. Furthermore, the strength of a combined micro- and spectroscopic approach in understanding the nature of MOF thin-films in a spatially resolved manner is highlighted.

Micro-spectroscopy of HKUST-1 metal-organic framework crystals loaded with tetracyanoquinodimethane: effects of water on host-guest chemistry and electrical conductivity.

Composite materials, consisting of a metal-organic framework (MOF) and a guest molecule, further denoted as guest@MOF composites, have gained strong interest due to the guest-induced tunability of the host properties, for example in sensing or electroconductivity applications. However, decoding the complexity of these guest@MOF composites and extracting structure-performance relationships are far from trivial and require the use of a gamut of characterization tools. In this work, we use various micro-spectroscopic tools both under static (ex situ) and dynamic (in situ) conditions to map the properties and diffusion behavior of TCNQ (7,7,8,8-tetracyanoquinodimethane) as a guest molecule in single HKUST-1 crystals as the host. Raman micro-spectroscopy allowed us to map the spatial distribution of TCNQ within HKUST-1 single crystals, thereby revealing a heterogeneous distribution of TCNQ after initial TCNQ-infiltration, concentrated at the crystal edges, and a homogeneous redistribution upon water vapor treatment. These insights are correlated to I-V scans at different temperatures and to electrochemical impedance spectroscopy (EIS), which allowed us to verify the different contributions to conductivity. These data showed changes in electrical conductivity after exposing the sample to moisture and air. In situ FT-IR micro-spectroscopy during treatment with moisturized nitrogen gas suggests lower transient diffusion rates for water inside TCNQ@HKUST-1 relative to pristine HKUST-1, likely due to steric hindrance of the pore-filling TCNQ molecules in the HKUST-1 framework. The application of micro-spectroscopic techniques is crucial to uncovering MOF intracrystal heterogeneities and yielding rationally-derived instructions for the improved design of guest@MOF systems.

Metal-Organic Frameworks as Catalyst Supports: Influence of Lattice Disorder on Metal Nanoparticle Formation.

Because of their high tunability and surface area, metal-organic frameworks (MOFs) show great promise as supports for metal nanoparticles. Depending on the synthesis route, MOFs may contain defects. Here, we show that highly crystalline MIL-100(Fe) and disordered Basolite® F300, with identical iron 1,3,5-benzenetricarboxylate composition, exhibit very divergent properties when used as a support for Pd nanoparticle deposition. While MIL-100(Fe) shows a regular MTN-zeotype crystal structure with two types of cages, Basolite® F300 lacks long-range order beyond 8 Šand has a single-pore system. The medium-range configurational linker-node disorder in Basolite® F300 results in a reduced number of Lewis acid sites, yielding more hydrophobic surface properties compared to hydrophilic MIL-100(Fe). The hydrophilic/hydrophobic nature of MIL-100(Fe) and Basolite® F300 impacts the amount of Pd and particle size distribution of Pd nanoparticles deposited during colloidal synthesis and dry impregnation methods, respectively. It is suggested that polar (apolar) solvents/precursors attractively interact with hydrophilic (hydrophobic) MOF surfaces, allowing tools at hand to increase the level of control over, for example, the nanoparticle size distribution.