Julia Hitzbleck

Heterogeneous Catalytic Oxidation by MFU‐1: A Cobalt(II)‐Containing Metal–Organic Framework

Porous metal–organic frameworks (MOFs) are a rapidly emerging class of multifunctional hybrid materials that might be useful for diverse technical applications, such as gas or liquid adsorption and separation, molecular recognition, or catalysis. Combining polycarboxylate ligands and (transition) metal ions, moderately robust MOFs can be prepared; 1,4-benzenedicarboxylate (bdc, terephthalic acid) and 4,4’biphenyldicarboxylate (bpdc) are often used as linkers. Highly porous non-interpenetrated frameworks, such as the well-known MOF-5 ([Zn4O(bdc)3]) [2] or IRMOF-9 ([Zn4O(bpdc)3]) [3] then form. These microporous MOFs generally show good thermal stabilities (decomposition occurs at T> 350 8C). A fundamental disadvantage, however, is their low hydrolytic stability: Decomposition of the framework occurs rapidly when the gas or liquid phase contains a small amount of water, which imposes severe limitations on their usage in catalytic oxygenation reactions, in which water is a major reaction product. Preliminary attempts to use MOF-5 as a photocatalyst have been reported recently; however, the fact that these frameworks contain Lewis acidic zinc(II) ions only imposes severe limitations on their use in redox catalytic applications in general. Conceptually different approaches have been reported to circumvent the intrinsic disadvantages of MOF-5-type frameworks. Fischer et al. reported the gas-phase deposition of volatile organometallic complexes in the open cavities of MOF-5. Subsequent photolytic or reductive cleavage of the precursors led to catalytically active metal clusters (Cu, Pd, Au) that are finely dispersed in the MOF-5 framework. Nguyen, Hupp et al. were among the first to present oxidations using a MOF catalyst. They used an enantiomerically pure manganese complex of a modified salen ligand as a building block to construct a three-dimensional porous framework. A distinct approach towards heterogeneous asymmetric catalysis based on a homochiral metal–organic framework was recently proposed by Lin et al. However, industrial oxidation or oxygenation reactions typically require very high turn-over numbers (TONs) and frequencies (TOFs), which have not been achieved to date by current MOF catalysts. To produce thermally and hydrolytically stable redoxactive MOFs, our initial efforts focused on the isostructural replacement of a single zinc ion by an open-shell transition metal ion M within the tetranuclear {Zn4O} coordination unit of MOF-5. However, all attempts in this direction led to heteronuclear MOFs containing trinuclear coordination units (for example, [MZn2(bpdc)3(dmf)2], M = Co , Ni, Cd), which are structurally different from MOF-5. A search of the CSD database, however, led to the tetranuclear complex [Co4O(3,5-dmpz)6] (3,5-dmpz = 3,5-dimethylpyrazolate), [10]

Electroactive Polymers: Developments of and Perspectives for Dielectric Elastomers

We present the development and applications of dielectric elastomers. For the last 10 years the significance of this class of polymers has risen as more applications seem possible and first products have been commercialized.

Dielectric elastomer–based energy harvesting: Material, generator design, and optimization:

Electroactive polymers are soft capacitors made of thin elastic and electrically insulating films coated with compliant electrodes offering a large amount of deformation. They can either be used as actuators by applying an electric charge or used as energy converters based on the electrostatic principle. These unique properties enable the industrial development of highly efficient and environmentally sustainable energy converters, which opens up the possibility to further exploit large renewable and inexhaustible energy sources like wind and water that are widely unused otherwise. Compared to other electroactive polymer materials, polyurethanes have certain advantages over silicones and acrylates. Due to the inherently higher permittivity as well as the higher dielectric breakdown strength, the overall specific energy, a measure for the energy gain, is better by at least factor of 10, that is, more than 10 times the energy can be gained out of the same amount of material. In order to reduce conduction losses on the electrode during charging and discharging, a highly conductive bidirectional stretchable electrode has been developed. Other important material parameters like stiffness and bulk resistivity have been optimized to fit the requirements. We also report on different measures to evaluate and improve electroactive polymer materials for energy harvesting by, for example, reducing the defect occurrence and improving the electrode behavior.

New DEA materials by organic modification of silicone and polyurethane networks

Dielectric elastomer actuators (DEAs) can be optimized by modifying the dielectric or mechanical properties of the electroactive polymer. In this work both properties were improved simultaneously by a simple process, the one-step film formation for polyurethane and silicone films. The silicone network contains polydimethylsiloxane (PDMS) chains, as well as cross-linker and grafted molecular dipoles in varying amounts. The process leads to films, which are homogenous down to the molecular level and show higher permittivities as well as reduced stiffnesses. The dipole modification of a new silicone leads to 40 times higher sensitivities, compared to the unmodified films. This new material reaches the sensitivity of the widely used acrylate elatomer VHB4905. A similar silicone modification was obtained by using smart fillers consisting of organic dipoles and additional groups realizing a high compatibility to the silicon network. Polyurethanes are alternative elastomers for DEAs which are compared with the silicones in important properties. Polyurethanes have an intrinsically high dielectric constant (above 7), which is based on the polar nature of the polyurethane fragments. Polyurethanes can be made in roll-to-roll process giving constant mechanical and electrical properties on a high level.

Copper(II) Nanoballs as monomers for polyurethane coatings: synthesis, urethane derivatization and kinetic stability

The self-assembly of copper(II) ions and 5-(2-hydroxyethoxy)benzene-1,3-dicarboxylate (1) leads to Nanoballs in which twelve dinuclear copper(II) paddle-wheel units are interconnected via 24 ligands. The structure of the spherical coordination compound decorated with 24 hydroxy groups has been determined by single crystal X-ray structure analysis. As a model for the integration of Nanoballs into bulk polyurethane polymers and coatings, its reaction with phenylisocyanate is investigated. The stability of Nanoballs against hydrolytic decomposition is studied under acidic conditions and compared to simple copper(II) complexes. Release of copper(II) ions from Nanoballs is much slower than from discrete copper(II) paddle-wheel complexes, suggesting the use of Nanoballs as monomers for polyurethane-based antifouling coatings.

Optimized energy harvesting materials and generator design

Electroactive polymers are soft capacitors made of thin elastic and electrically insulating films coated with compliant electrodes offering a large amount of deformation. They can either be used as actuators by applying an electric charge or they can be used as energy converters based on the electrostatic principle. These unique properties enable the industrial development of highly efficient and environmentally sustainable energy converters, which opens up the possibility to further exploit large renewable and inexhaustible energy sources like wind and water that are widely unused otherwise. Compared to other electroactive polymer materials, polyurethanes, whose formulations have been systematically modified and optimized for energy harvesting applications, have certain advantages over silicones and acrylates. The inherently higher dipole content results in a significantly increased permittivity and the dielectric breakdown strength is higher, too, whereby the overall specific energy, a measure for the energy gain, is better by at least factor ten, i.e. more than ten times the energy can be gained out of the same amount of material. In order to reduce conduction losses on the electrode during charging and discharging, a highly conductive bidirectional stretchable electrode has been developed. Other important material parameters like stiffness and bulk resistivity have been optimized to fit the requirements. To realize high power energy harvesting systems, substantial amounts of electroactive polymer material are necessary as well as a smart mechanical and electrical design of the generator. In here we report on different measures to evaluate and improve electroactive polymer materials for energy harvesting by e.g. reducing the defect occurrence and improving the electrode behavior.