Research Areas

The Stock group is specialized on the synthesis and characterization of coordination polymers (CPs), particularly on metal-organic frameworks (MOFs). These types of materials are composed of inorganic nodes (also called inorganic building units (IBUs)), which are interconnected by coordinating organic ligands (linkers) to form a three-dimensional network. MOFs are obtained as single crystals or crystalline powders and exhibit well-defined pores due to their modular architecture. The latter makes them also highly designable, so that their properties can be specifically tailored for real-life applications like gas storage and separation, catalysis, sensors, heat transformation, water harvesting or medical uses.

MOFs

 

Exploratory Synthetic Investigations

The discovery of new MOFs and their synthesis optimization is the biggest challenge in the research field. Therefore, we develop and utilize high-throughput methods to enhance the speed of our screenings, while saving costs at the same time (DOI: 10.1016/j.micromeso.2009.06.007). Teflon-lined autoclaves with space for 24 or 48 miniaturized solvothermal reactions in parallel were designed to carry out experiments at temperatures of up to 200 °C. The combination with a self-developed gradient reactor also allows the investigation of multiple reaction temperatures within the same run. If microwave radiation is required as the source of heat, a microwave reactor with sealable glass vials for up to 24 reactions is available.

reactor1reactor2

An important step for the possible application of a material is its synthesis scale up. In our laboratory scale-up is carried out in larger reaction vessels, ranging from 30 mL autoclaves, to 1 and 10 L reactors, up to continuous flow systems.

synthesis methods

With these methods we mostly focus on the development of novel carboxylate and phosphonate MOFs as well as green synthesis routes. MOFs developed at the University of Kiel are denoted by the abbreviation CAU and a number X (CAU-X). Currently around 50 compounds have been developed in our laboratories. List of CAU-MOFs

 

Characterization & Tuning of Properties

To get an insight into the structure and properties of a product, sophisticated analytical techniques need to be applied. We utilize automated powder X-ray diffraction (PXRD) to quickly characterize up to 48 samples in ca. 1.5 hours. Highly crystalline phase-pure samples can be selected for follow-up long-time PXRD measurements. The resulting high-quality diffraction data in combination with routine compositional analyses allows us to perform structure determinations for microcrystalline samples, which is especially valuable when no single crystals are available.

XRD

The formation of MOFs and other crystalline samples as well as monitoring structural changes upon external stimuli or the adsorption of guest molecules can be carried out in an in-situ reactor (SynRAC, DOI: 10.1063/1.4999688), which we developed as a part of the Swedish-German Röntgen-Ångström Cluster project MATsynCELL. The SynRAC setup allows the live collection of X-ray scattering and luminescence data at a synchrotron source and can be mounted easily at any beamline with standard screw holes. The reactor is designed for homogenous and heterogeneous liquid-phase reactions and allows variable temperatures and heating rates as well as the dosing of additional solid or liquid reactants during an experiment.

synrac

A key feature of metal-organic frameworks is their porosity. To investigate surface areas, pore volumes and pore properties like hydrophilicity/hydrophobicity we utilize gas sorption measurements with different gases (e.g. N2, H2O, CO2, H2, CH4). Through computer modelling in cooperation with the Maurin group in Montpellier a deeper understanding of the adsorption processes at the molecular level is accomplished.

 

With all these tools and many more in hand, we work with collaboration partners around the world to design and characterize materials to fit certain applications like the following:

 

Water adsorption and heat transformation

Porous materials with distinct water adsorption properties can be used to create sustainable cooling applications. Recently, we developed a patented green reaction process for the large-scale synthesis of the aluminum-based MOF CAU‑10 (DOI: 10.1002/adma.201705869). Its perfectly tuned affinity and high capacity for water vapor allows the transformation of low-energy waste heat to usable cold in heat-driven chilling devices. Together with the Fraunhofer Institute Freiburg, Germany a CAU-10-coated heat exchanger was prepared and tested successfully at full scale. To study structural changes during water adsorption we also collaborate with the Lieb group from the University of Magdeburg, Germany.

water adsorption

 

Redox-active MOFs

Compounds with redox-active sites can be used as heterogenous catalysts for chemical reactions. In a MOF these sites can either be established on the IBU by choosing a redox-active metal or on the organic ligand via implementation of redox-active functional groups. Our group develops both kinds of materials – especially MOFs with cerium(IV)-containing IBUs and MOFs with ferrocene-functionalized linkers. The latter mostly feature IBUs with tri- or tetravalent metals (e.g. Al3+, Ga3+, In3+ and Sc3+) due to their exceptional stability. Below, the structures of Ce‑UiO‑66 (left, DOI: 10.1039/C5CC02606G) and Al-MIL-53-FcDC (right, DOI: 10.1039/C9DT03489G) are shown as typical examples.

 structures

Catalysis

Many organic reactions require a catalyst to be performed selectively and in sufficient yields. MOFs can act as heterogeneous catalysts themselves or adsorb and immobilize active species like noble metals into their pores. An advantage of the latter case is, that the expensive metals remain in the MOF and therefore can be recycled after the reaction. Due to the modular architecture of MOFs, active sites are well distributed all over the material. Catalytic experiments are conducted by our collaboration partners in Leuven, Belgium (De Vos group) and Oslo, Norway (Olsbye group). A recent example is the immobilization of iridium species in the functionalized pores of the zirconium MOF CAU‑27 for the borylation of arenes (DOI: 10.1002/anie.201905456).

 

Photoluminescence

The insertion of fine-tuned mixtures of luminescent rare earth metals into the pores or as an IBU of a MOF creates synergistic effects with the framework, especially the linker molecules. Emission colors can be tuned this way, making the materials interesting in applications such as solid-state lightning or sensing. For example, we developed a series of scandium-based MOFs, which cannot only emit basic green, red or blue colored light, but also white light. It is only accessible when emissions of all colors are mixed perfectly. In depth analysis of the luminescence properties are carried out in cooperation with the Terraschke group at the University of Kiel.

luminescence

 

Proton conductors

MOFs synthesized from linker molecules comprising phosphonic or sulphonic acid functionalities often exhibit acidic protons. They ease the proton conduction within a material by providing suitable pathways. Proton conducting materials are desirable in membrane development for modern fuel cells. For example, recently, we conducted a detailed study on a barium-based phosphonatosulfonate to improve the understanding of proton transport mechanisms in coordination polymers (DOI: 10.1002/cphc.202000102). The conductivity measurements are carried out by our partners in Oldenburg, Germany (Wark group) and Paderborn, Germany (Tiemann group).

 

Sensors

At the moment, we are establishing a cooperation with the Adelung group at the technical faculty of the University of Kiel, to develop MOF-based sensor materials. More information will be provided soon.