Synthesis, characterization and postsynthetic modification of a novel two-dimensional Zr-based metal-organic framework

The focus in many fields of research is the generation of materials with adjustable properties for specific applications. Among porous materials, metal-organic frameworks (MOFs) have developed over the past 20 years to a material class with enormous potential. Due to the modular design of MOFs from metal cations and organic linkers, it is possible to tailor the properties of the resulting extended frameworks by selecting individual components for the needs of specific applications. Careful selection of metal ions and organic molecules with functional groups makes it possible to adjust and influence the pore system, the type of linkage and the physical and chemical properties of the framework. In the present work, the main aspect was the preparation of a novel MOF with high adjustability of its properties, based on light-induced postsynthetic reactions on the organic linker molecules. The framework is composed of Zr4+ ion-based oxo clusters and benzophenone-4,4'-dicarboxylate anions (bzpdc2–) as linker molecules. The benzophenone unit is able to react with any molecule that contains C–H bonds after excitation with photons, resulting in a covalent bond. This reaction opens the possibility of changing the properties of the framework post-synthetically. The novel Zr-bzpdc MOF is chemically and thermally stable and has a two-dimensional structure, which opens up the possibility of obtaining nanometer-thin layers through delamination methods. Further studies have shown that the adaptation of the surface chemistry by the linkage of molecules has a huge impact on the dispersibility in polar and nonpolar solvents. Furthermore, the direct polymerization of a conductive polymer, starting from the surface of the MOF, yields electrically conductive composite materials. Interestingly, in the course of this study it was found that the unmodified MOF is also electrically conductive. Furthermore, a systematic study concerned with the postsynthetic modification of Zr-bzpdc-MOF with alkanes and alcohols of different chain lengths was undertaken. This showed that small hydrophilic molecules react with all linker molecules throughout the crystal, whereas molecules with longer chain lengths and thus more hydrophobic properties only modify the surface of the MOF crystals. The so far largely unaccessed approach to employ specific photochemical modification reactions on the linker molecule the modification of MOFs has been developed here to upgrade the novel Zr-bzpdc-MOF to an unexpectedly versatile compound which is further augmented by the possibility to obtain nanosheets of this substance. Various pathways to adapt this material to specific applications have thus been opened up

Inside/Outside : Post-Synthetic Modification of the Zr-Benzophenonedicarboxylate Metal–Organic Framework

The Zr-based metal–organic framework, Zr-bzpdc-MOF, contains the photoreactive linker molecule benzophenone-4,4'-dicarboxylate (bzpdc) which imparts the possibility for photochemical post-synthetic modification. Upon irradiation with UV light, the keto group of the benzophenone moiety will react with nearly every C-H bond-containing molecule. Within this paper, we further explore the photochemical reactivity of the Zr-bzpdc-MOF, especially with regard to which restrictions govern internal versus external reactions. We show that apart from reactions with C-H bond-containing molecules, the MOF reacts also with water. By studying the reactivity versus linear alcohols we find a clear delineation in that shorter alcohol molecules (up to butanol as a borderline case) react with photoexcited keto groups throughout the whole crystals whereas longer ones react only with surface-standing keto groups. In addition, we show that with the alkanes n-butane to n-octane, the reaction is restricted to the outer surface. We hypothesize that the reactivity of the Zr-bzpdc-MOF versus different reagents depends on the accessibility of the pore system which in turn depends mainly on the size of the reagents and on their polarity. The possibility to direct the post-synthetic modification of the Zr-bzpdc-MOF (selective modification of the whole pore system versus surface modification) gives additional degrees of freedom in the design of this metal–organic framework for shaping and for applications. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA

Direct grafting-from of PEDOT from a photoreactive Zr-based MOF – a novel route to electrically conductive composite materials

The postsynthetic potential of the two-dimensional metal–organic framework Zr-bzpdc-MOF which is based on the photoreactive molecule benzophenone-4,4′-dicarboxylic acid (H2bzpdc) is used here to selectively functionalize the MOF surface. We report the direct radical-induced oxidative grafting-from polymerization of the precursor EDOT on Zr-bzpdc-MOF, leading to an electrically conductive composite material and opening the road to a variety of applications

Azobenzene Guest Molecules as Light-Switchable CO₂ Valves in an Ultrathin UiO-67 Membrane

Metal–organic frameworks (MOFs) with an exceptionally large pore volume and inner surface area are perfect materials for loading with intelligent guest molecules. First, an ultrathin 200 nm high-flux UiO-67 layer deposited on a porous α-Al₂O₃ support by solvothermal growth has been developed. This neat UiO-67 membrane is then used as a host material for light-responsive guest molecules. Azobenzene (AZB) is loaded in the pores of the UiO-67 membrane. From adsorption measurements, we determined that the pores of UiO-67 are completely filled with AZB and, thereby, steric hindrance inhibits any optical switching. After <i>in situ</i> thermally controlled desorption of AZB from the membrane, AZB can be switched and gas permeation changes are observed, yielding an uncomplicated and effective smart material with remote controllable gas permeation. The switching of AZB in solution and inside the host could be demonstrated by ultraviolet–visible spectroscopy. Tracking the completely reversible control over the permeance of CO₂ and the H₂/CO₂ separation through the AZB-loaded UiO-67 layer is possible by <i>in situ</i> irradiation and permeation. Mechanistic investigations show that a light-induced gate opening and closing takes place. A remote controllable host–guest, ultrathin smart MOF membrane is developed, characterized, and applied to switch the gas composition by external stimuli

Role of Structural Defects in the Adsorption and Separation of C3 Hydrocarbons in Zr-Fumarate-MOF (MOF-801)

status: publishe

Graphene-like metal-organic frameworks: Morphology control, optimization of thin film electrical conductivity and fast sensing applications

The metal-organic framework Cu-2,3,6,7,10,11-hexahydroxytriphenylene (Cu3hhtp2-MOF), a copper-based graphene-like framework, is one of the few MOFs featuring inherent electrical conductivity. Here, we investigate the synthesis of this material with regard to the influence of different additives. It is shown that ammonia acts as a modulator leading to platelet-like particles in a water-based synthesis system. This material is thoroughly characterized by X-ray diffraction (XRD), electron microscopy, atomic force microscopy (AFM), physisorption, thermal behaviour, and electrical conductivity. The measured conductivity value of 0.045 S cm-1 surpasses all formerly reported measurements. The obtained platelets appear especially suitable for the preparation of different devices. As an example, we prepared thin and homogenous films by spray-coating water-based dispersions of this MOF on glass and on polymer substrates. In the films, the platelets are oriented parallel to the substrate and are in intimate contact. This leads to a high electrical conductivity combined with an easily accessible pore system. The applicability of such coatings is shown in a preliminary sensing test, showing quick and strong response and fast recovery. This work shows that control of the crystal morphology combined with suitable preparation procedures can enhance the performance of MOF-based devices