Junior Professor of Inorganic Chemistry, Technical University Dortmund
I am interested in the structural chemistry as well as the functional and fundamental properties of porous metal-organic framework compounds. Metal-organic frameworks (MOFs) represent a diverse and highly tuneable family of inorganic-organic hybrid materials exhibiting large specific surface areas and huge pore volumes. The zeolite-like architectures are composed of metal ions, which are interconnected by organic linker molecules (multivalent carboxylates, imidazolates, etc.) to generate two- or three-dimensional extended networks. In view of huge potential for various applications ranging from hydrogen storage and carbon dioxide capture to catalysis and drug delivery this young family of porous materials attracts tremendous interest from academia and industry.
My research particularly focuses on the subclass of ‘soft’ frameworks (aka Soft Porous Crystals, SPCs), which represents the next generation of porous materials by combining the regularity of crystalline solids with the specific responsiveness of adaptive biomolecules (Fig. 1). SPCs undergo quite dramatic but reversible structural changes depending on external stimuli, such as guest adsorption, changes in temperature or mechanical pressure. This unique combination of properties renders SPCs particularly interesting for applications in molecular recognition, sensing or highly selective gas separations. My work concentrates on two aspects of SPCs:
- The targeted manipulation of the responsive properties of SPCs by means of synthetic chemistry. How to direct the sorption properties of SPCs (gated adsorption, breathing)? How to modulate and control intrinsic thermal responsiveness?
- The fundamental understanding of the supramolecular mechanics of these porous networks. How do SPCs react to mechanical pressure? What determines their mechanical properties? What are their limitations and advantages compared to non-responsive MOFs and other porous materials such as zeolites?
Recent Research Highlights
We showed that pillared-layered SPCs possessing functionalised 1,4-benzenedicarboxylate (fu-bdc) linkers, can be highly dynamic in response to guest adsorption (Fig. 2). The functionalised frameworks contract from a large pore (lp) form to a narrow pore (np) form upon guest removal. Importantly, the detailed structure of the np form as well as the carbon dioxide adsorption behaviour can be tuned systematically by the choice of substituent on the linker.
Mixing two differently functionalised fu-bdc linkers in one framework yields solid solutions having both linkers randomly distributed in one single phase material. Thereby linker 1 (L1) features a hydrophilic side chain, while linker 2 (L2) has a hydrophobic side chain. The parent frameworks, having only one type of linker, behave markedly different upon carbon dioxide adsorption. Both materials expand drastically (np-lp transition, ‘breathing’), but the carbon dioxide partial pressure at which this expansion happens is very different. The phase transition happens at a low pressure for the framework with the hydrophilic L1 and at high pressure for the framework with the hydrophobic L2. The carbon dioxide adsorption properties of the mixed-linker solid solutions lie in between the performance of the simple parent compounds (Fig. 3). Interestingly, we found non-linear effects, which allow tuning of the sorption properties of the solid solutions beyond the limitations of the single-linker compounds.
We synthesised a novel honeycomb-like MOF using 2,5-bis(2-methoxyethoxy)-1,4-benzenedicarboxylate (BME-bdc) and 4,4'-bipyridine as a co-linker (Fig. 4). The framework features a rigid backbone structure with one-dimensional cylindrical channels, which are populated by the flexible 2-methoxyethoxy groups of the BME-bdc linkers. These polar side chains act as molecular gates for incoming guest molecules leading to exceptionally high sorption selectivity towards carbon dioxide over nitrogen and methane. Our concept is substantiated by the syntheses of other isoreticular honeycomb-like MOFs from differently di-substituted bdc derivatives.
Massive Thermal-Expansion and Thermo-Responsive Breathing in Functionalised MOFs – Hybrid Solid-Liquid Materials
We analysed the intrinsic structural behaviour of the functionalised pillared-layered SPCs as a function of the temperature and found unprecedentedly large uniaxial positive and negative thermal expansion (Fig. 5). The magnitude of the volumetric thermal expansion is more comparable to property of liquid water rather than any crystalline solid-state material. This unique behaviour is based on the additional alkoxy side chains, which are connected to the framework skeleton but nevertheless exhibit large conformational flexibility. Thermally induced motion of these side chains induces extremely large anisotropic framework expansion and eventually triggers reversible solid-state phase transitions to drastically expanded structures.
Even though the guest-responsive nature of SPCs has been investigated extensively in recent years, knowledge on their fundamental mechanical properties is extremely rare. We analysed the mechanical properties (elastic modulus, hardness) of an entire series of pillared-layered SPCs using single-crystal nanoindentation. Very large variations in the elastic modulus and hardness triggered by exchange of the guests adsorbed in the flexible frameworks are observed. The substantial variations of the mechanical properties as a function of the guest molecules can be explained by the responsive nature of the SPCs. Crucial changes in network geometry induce a complex and dynamical mechanical behaviour.
1. S. Henke, W. Li, A.K. Cheetham, Guest-dependent mechanical anisotropy in pillared-layered soft porous crystals – a nanoindentation study, Chem. Sci. 2014, 5, 2392-2397.[DOI]
2. S. Henke, A. Schneemann, R.A. Fischer, Massive Anisotropic Thermal Expansion and Thermo-Responsive Breathing in Metal–Organic Frameworks Modulated by Linker Functionalization, Adv. Funct. Mater. 2013, 23, 5990–5996.[DOI]
3. S. Henke, A. Schneemann, A. Wütscher, R.A. Fischer, Directing the Breathing Behavior of Pillared-Layered Metal–Organic Frameworks via a Systematic Library of Functionalized Linkers Bearing Flexible Substituents, J. Am. Chem. Soc. 2012, 134, 9464–9474.[DOI]
4. S. Henke, A. Schneemann, S. Kapoor, R. Winter, R.A. Fischer, Zinc-1,4-benzenedicarboxylate-bipyridine frameworks – linker functionalization impacts network topology during solvothermal synthesis, J. Mater. Chem. 2012, 22, 909-918.[DOI]
5. S. Henke, R.A. Fischer, Gated Channels in a Honeycomb-like Zinc−Dicarboxylate−Bipyridine Framework with Flexible Alkyl Ether Side Chains, J. Am. Chem. Soc. 2011, 133, 2064–2067.[DOI]
6. S. Henke, R. Schmid, J.D. Grunwaldt, R.A. Fischer, Flexibility and Sorption Selectivity in Rigid Metal–Organic Frameworks: The Impact of Ether-Functionalised Linkers, Chem.–Eur. J. 2010, 16,14296–14306.[DOI]