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Metal-Organic Frameworks

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 (see figure).

 

mof-scheme_600

Based on a molecular building principle a quasi infinite number of MOFs could in principle be synthesised. Simple exchange of the inorganic and/or the organic building blocks allows for a targeted tuning of the framework properties by means of synthetic chemistry. 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.

 

Modulating Framework Flexibility in Pillared-Layered MOFs – Breathing Upon Guest Adsorption

 We showed that pillared-layered SPCs possessing functionalised 1,4-benzenedicarboxylate (fu-bdc) linkers, can be highly dynamic in response to guest adsorption (Fig. 1). 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.

 

fu-dmofs_breathing_600

Fig. 1: Library of functionalised 1,4-benzendicarboxylate (fu-bdc) linkers utilised in pillared-layered SPCs (top). Representation of the reversible large pore (lp) to narrow pore (np) phase transition as a function of guest adsorption (bottom left). Powder X-ray diffraction patterns of the different SPCs showing the structural similarity of the lp phases and the marked differences of the individual np phases (bottom right).

 

Mixed-Linker Solid Solutions – Tailoring the Sorption Properties of Soft Porous Crystals

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. 2). 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.

mtv-mofs_sorption-2_600

Fig. 2: Carbon dioxide physisorption isotherms (recorded at 195 K) of the solid solutions in comparison to the isotherms of the simple compounds having only one linker.

 

Honeycomb-Like MOFs with Gated Channels Showing Highly Selective Sorption of Carbon Dioxide

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. 3). 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.

 

gated-channels_600

Fig. 3: Structure and gas adsorption properties of the honeycomb-like MOF.

 

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. 4). 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.

 

thermo_dmof_600

Fig. 4: Lattice parameters and corresponding thermal expansion coefficients of a functionalised pillared-layered SPC as a function of temperature.

 

Huge Mechanical Anisotropy in Soft Porous Crystals

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 (Fig. 5). 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.

 

ni-dmof_600

Fig. 5: Structural response of a nitro-functionalised pillared-layered SPC as a function of adsorbed guests (DMF, DMSO, mesitylene and toluene). Load-displacement curves and the elastic modulus for different crystal orientations and different guest molecules are shown on the right.