Ulrich Stoeck

A Mesoporous Metal–Organic Framework

A new class of porous materials namely metal–organic frameworks (MOFs) has set records in recent years regarding specific surface area and pore volume. Nevertheless, the search for compounds with very large pores and higher specific surface area remains a key challenge in the rapidly expanding field of MOFs, especially for applications in catalysis, drug delivery, and high-pressure gas storage. Compounds containing small windows or pores that are inaccessible for anchoring molecular catalysts, impregnation with catalyst precursors, or larger drug molecules pose limitations for MOFs in fine chemical transformation, nanoparticle formation, or drug delivery. For energy-storage applications at 200 bar, larger pores (2-3 nm) are essential to achieve a shift of the excess adsorption maximum towards higher pressure. Despite a somewhat reduced heat of adsorption, in practice such large pore MOFs outperform small pore MOFs as a result of the higher pore volume. A common concept to enhance the pore size in MOFs uses a linear extension of the linker in a given network topology. In such MOFs the pore diameter achievable is limited by interpenetration. A prominent example is the IRMOF series (isoreticular MOFs). Other examples of increasing pore size through linker extensions are [Cu3(btc)2] [11] (btc = benzene-1,3,5-tricarboxylate; tbo-topology), PCN-6 ([Cu3(tatb)2], tatb = 4,4’,4’’-s-triazine-2,4,6-triyltribenzoate; tbo), or MOF-14 ([Cu3(btb)2], btb = benzene1,3,5-tribenzoate; pto), built from paddle wheel clusters and tritopic linkers. However, in PCN-6 and MOF-14, which have the btb linker (a longer version of the btc linker), the porosity is reduced because of the presence of two interwoven 3D nets in the structure. The non-interpenetrated analogue of PCN-6 (PCN-6’) is obtained using a templating strategy, while a non-interpenetrated analogue of MOF-14 is unknown. Herein, we report an approach that avoids interpenetration by using a secondary linker to stabilize a highly open framework structure by crosslinking an extended Pt3O4topology. The resulting new mesoporous MOF material, DUT-6 (DUT= Dresden University of Technology), shows no interpenetration and has an extremely high gas adsorption capacity for n-butane, hydrogen, and methane. Single crystals of [Zn4O(2,6-ndc)(btb)4/3(def)16(H2O)9/2] (DUT-6; def = N,N-diethylformamide, 2,6-ndc = 2,6-naphthalenedicarboxylate) suitable for X-ray diffraction analysis were obtained from the reaction of H3(btb), H2(2,6-ndc), and zinc nitrate in a ratio of 3:2:14. The compound crystallizes in the cubic space group Pm3̄n. Dodecahedral mesoporous cages 2.5–3 nm in diameter are formed by twelve Zn4O 6+

A pressure-amplifying framework material with negative gas adsorption transitions

Adsorption-based phenomena are important in gas separations, such as the treatment of greenhouse-gas and toxic-gas pollutants, and in water-adsorption-based heat pumps for solar cooling systems. The ability to tune the pore size, shape and functionality of crystalline porous coordination polymers—or metal–organic frameworks (MOFs)—has made them attractive materials for such adsorption-based applications. The flexibility and guest-molecule-dependent response of MOFs give rise to unexpected and often desirable adsorption phenomena. Common to all isothermal gas adsorption phenomena, however, is increased gas uptake with increased pressure. Here we report adsorption transitions in the isotherms of a MOF (DUT-49) that exhibits a negative gas adsorption; that is, spontaneous desorption of gas (methane and n-butane) occurs during pressure increase in a defined temperature and pressure range. A combination of in situ powder X-ray diffraction, gas adsorption experiments and simulations shows that this adsorption behaviour is controlled by a sudden hysteretic structural deformation and pore contraction of the MOF, which releases guest molecules. These findings may enable technologies using frameworks capable of negative gas adsorption for pressure amplification in micro- and macroscopic system engineering. Negative gas adsorption extends the series of counterintuitive phenomena such as negative thermal expansion and negative refractive indices and may be interpreted as an adsorptive analogue of force-amplifying negative compressibility transitions proposed for metamaterials.

Dye Encapsulation Inside a New Mesoporous Metal–Organic Framework for Multifunctional Solvatochromic‐Response Function

Metal–organic frameworks (MOFs), hybrid materials built up from metal clusters and organic linkers, have shown a huge potential for a wide range of applications. In recent years, MOFs have set new records in terms of specific surface areas and pore volumes and therefore are highly suitable as storage materials for small and large molecules. The development of new materials is crucial for the improvement of storage devices, but MOFs are also ideal candidates for functionalization. One functionalization strategy is the integration of complex organic linker molecules containing secondary functional groups. However, this approach is synthetically demanding and not general, because functional donor atoms may affect the linker connectivity resulting in unexpected network topologies. A second powerful strategy is postsynthetic modification of the framework. In this case, the range of functions is restricted due to the limited stability of MOFs against aggressive chemicals. A modular and more versatile approach may be the encapsulation of functional guest molecules into the MOF material. However, for these systems leaching is critical. A crucial requirement in all cases is also the accessibility of MOF functionalities for guest molecules. In this context, a large pore size is highly beneficial. However, the development of such mesoporous frameworks is challenging, because expanded frameworks are often more fragile leading to a collapse of the framework during removal of included guests molecules. A very effective concept to achieve robust frameworks lies in the creation of hierarchical pore structures by combination of two different linker molecules. The most prominent examples of such copolymerization approach are UMCM-1/ 2/ 3, DUT-6 and DUT-23 or MOF210. In the case of DUT-23, auxiliary linker was used successfully to avoid interpenetration and to enhance the robustness, which resulted in highly porous structures. To date, this concept was restricted only to the combination of triand ditopic linkers. On the other hand, DUT-10(M) (M= Zn, Cu, Co) compounds based on the tetratopic N,N,N’,N’benzidinetetrabenzoate (benztb) ligand and the paddlewheel secondary building unit (SBU) undergo structural change upon solvent removal. By enhancing the connectivity of the framework by using a six-connecting [Zn4O] 6+

Modular construction of a porous organometallic network based on rhodium olefin complexation.

We describe the rational design and synthesis of the first member of a new class of microporous materials. It is built from rhodium and a polyolefinic ligand featuring a rigid tetraphenylsilane backbone via metal olefin complexation, creating a truly organometallic network. The resulting framework, denoted as DUT-37 (Dresden University of Technology no. 37) exhibits considerable porosity and unprecedented stability under ambient conditions. Furthermore, it is catalytically active in transfer hydrogenation.

Synthesis of the homochiral metal–organic framework DUT-129 based on a chiral dicarboxylate linker with 6 stereocenters

A dicarboxylate linker with an exceptionally high density of stereogenic centers for the development of new chiral metal–organic frameworks (MOFs) was synthesized. A bicyclic functionality forms the backbone of the linker, creating a linear structural analogue of terephthalic acid. The linker includes two side chains bearing secondary amine functionalities, which could induce catalytic activity of the resulting MOF structures for nucleophilic organocatalysis. The new chiral linker was used to synthesize a zinc-based MOF structure, named DUT-129. Single crystal X-ray analysis identified the porous framework with a sodalite (sod) topology. The bulky side groups of the chiral linker limit the pore accessibility of DUT-129, which was proven by liquid phase adsorption experiments. The calculated pore limiting diameter of 5.2 A indicates pore accessibility exclusive for small molecules, such as linear and branched alkanes. Therefore, the new MOF DUT-129 is expected to show high size selectivity in adsorption and separation applications in combination with high enantiomeric discrimination.

A Stimuli‐Responsive Zirconium Metal–Organic Framework Based on Supermolecular Design

A flexible, yet very stable metal-organic framework (DUT-98, Zr6 O4 (OH)4 (CPCDC)4 (H2 O)4 , CPCDC=9-(4-carboxyphenyl)-9H-carbazole-3,6-dicarboxylate) was synthesized using a rational supermolecular building block approach based on molecular modelling of metal-organic chains and subsequent virtual interlinking into a 3D MOF. Structural characterization via synchrotron single-crystal X-ray diffraction (SCXRD) revealed the one-dimensional pore architecture of DUT-98, envisioned in silico. After supercritical solvent extraction, distinctive responses towards various gases stimulated reversible structural transformations, as detected using coupled synchrotron diffraction and physisorption techniques. DUT-98 shows a surprisingly low water uptake but a high selectivity for pore opening towards specific gases and vapors (N2 , CO2 , n-butane, alcohols) at characteristic pressure resulting in multiple steps in the adsorption isotherm and hysteretic behavior upon desorption.

Surface and Electrochemical Studies on Silicon Diphosphide as Easy-to-Handle Anode Material for Lithium-Based Batteries-the Phosphorus Path.

The electrochemical characteristics of silicon diphosphide (SiP2) as a new anode material for future lithium-ion batteries (LIBs) are evaluated. The high theoretical capacity of about 3900 mA h g-1 (fully lithiated state: Li15Si4 + Li3P) renders silicon diphosphide as a highly promising candidate to replace graphite (372 mA h g-1) as the standard anode to significantly increase the specific energy density of LIBs. The proposed mechanism of SiP2 is divided into a conversion reaction of phosphorus species, followed by an alloying reaction forming lithium silicide phases. In this study, we focus on the conversion mechanism during cycling and report on the phase transitions of SiP2 during lithiation and delithiation. By using ex situ analysis techniques such as X-ray powder diffraction, formed reaction products are identified. Magic angle spinning nuclear magnetic resonance spectroscopy is applied for the characterization of long-range ordered compounds, whereas X-ray photoelectron spectroscopy gives information of the surface-layer species at the interface of active material and electrolyte. Our SiP2 anode material shows a high initial capacity of about 2700 mA h g-1, whereas a fast capacity fading during the first few cycles occurs which is not necessarily expected. On the basis of our results, we conclude that besides other degradation effects, such as electrolyte decomposition and electrical contact loss, the rapid capacity fading originates from the formation of a low ion-conductive layer of LiP. This insulating layer hinders lithium-ion diffusion during lithiation and thereby mainly contributes to fast capacity fading.