Metal-Organic Frameworks in Germany: from Synthesis to Function

Metal-organic frameworks (MOFs) are constructed from a combination of inorganic and organic units to produce materials which display high porosity, among other unique and exciting properties. MOFs have shown promise in many wide-ranging applications, such as catalysis and gas separations. In this review, we highlight MOF research conducted by Germany-based research groups. Specifically, we feature approaches for the synthesis of new MOFs, high-throughput MOF production, advanced characterization methods and examples of advanced functions and properties

Magnetic coupling of divalent metal centers in postsynthetic metal exchanged bimetallic DUT-49 MOFs by EPR spectroscopy

EPR measurements at X- (9.5 GHz), Q- (34 GHz) and W-band (94 GHz) on paddlewheel (PW) type post-synthetic metal exchanged DUT-49(M,M): M- Zn, Mn, Cu MOFs are here reported (DUT–Dresden University of Technology). Temperature-dependent X-band measurements are recorded from T = 7 K to T = 170 K on monometallic DUT-49(Cu), DUT-49(Mn), and bimetallic DUT-49(Cu0.7Zn0.3), DUT-49(Cu0.5Mn0.5) MOFs. In the case of the CuII - CuII dimers in DUT-49(Cu), an isotropic exchange coupling of the metal ions (2J = −240(11) cm−1) determined from the EPR intensity of the S = 1 spin state of the CuII–CuII dimers using the Bleaney Blowers equation. The sign of the found isotropic exchange coupling constant confirms an antiferromagnetic coupling between the cupric ions. Also, the MnII ions in the paddle wheels of DUT-49(Mn) are antiferromagnetically coupled. However, at low temperatures, EPR measurements reveal the presence of CuII and MnII monomers in DUT-49(Cu) and DUT-49(Mn), respectively, either associated with extra framework sites or defective paddle wheels. Otherwise, EPR signals observed for bimetallic DUT-49(Cu0.7Zn0.3) and DUT-49(Cu0.5Mn0.5) MOFs reveal the formation of mixed ion CuII–ZnII and CuII–MnII paddle wheels with SCuZn =1/2 and SCuMn = 2 spin states, respectively

Carbon dioxide capture by metal organic frameworks

1223-1230The design and synthesis of functionalized metal organic framework  materials (MOFs) for reversible physisorption of CO2 is discussed. This strategy of CO2 adsorption in MOFs requires less energy for regeneration than materials relying on chemisorption. As a result the MOFs have received considerable attention as sorbent materials for strategic gases such as CO2 and H2. <span style="font-size:9.0pt;mso-bidi-font-size:10.0pt; mso-ansi-language:EN-US" lang="EN-US">In this review, we have discussed different MOFs and hybrid materials containing MOFs which can adsorb CO2 at room temperature.<span style="font-size:9.0pt;mso-bidi-font-size: 10.0pt" lang="EN-GB"> In order to achieve high adsorption capacity, fast CO2 adsorption-desorption and low energy requirement for regeneration are necessary. Several avenues for increasing the CO2 adsorption capacity of such materials, for instance, introduction of open metal sites and the use of ligand molecules with specific functionalities (like -OH or -NH2) have been described. <span style="font-size: 9.0pt;mso-bidi-font-size:10.0pt;mso-ansi-language:EN-US" lang="EN-US">It has been observed that CO2 loading capacity of MOFs increases with functionalization. Herein, we have discussed how N-containing and fluorinated MOFs are designed to achieve higher CO2 loading than their non-functionalized counterparts. Nanocarbons (e.g. carbon nanotubes, carbon nanofibres, etc.) are porous materials and a blend of these porous materials with porous MOFs or porous carbon derived from MOFs may act as a better adsorbate than even the pure materials. Enhancement of CO2 loading by nanocarbon-MOF hybrid material is also discussed. </span

Functionalization and Isoreticulation in a Series of Metal–Organic Frameworks Derived from Pyridinecarboxylates

The partially fluorinated metal–organic frameworks (F-MOFs) have been constructed from 3-fluoro-4-pyridinecarboxylic acid and <i>trans</i>-3-fluoro-4-pyridineacrylic acid linkers using Mn²⁺, Co²⁺, and Cd²⁺ metals via the solvothermal method, which show isostructural isomerism with their nonfluorinated counterparts synthesized using 4-pyridinecarboxylic acid and <i>trans</i>-4-pyridineacrylic acid, respectively. The simultaneous effect of partial fluorination and isoreticulation on structure and H₂ adsorption has been studied systematically in isostructural nonfluorinated and partially fluorinated MOFs, which shows that the increment in the hydrogen uptake properties in F-MOFs is not a universal phenomenon but is rather system-specific and changes from one system to another

Elucidating the Structural Evolution of a Highy Porous Responsive Metal-Organic Framework (DUT-49(M)) upon Guests Desorption by Time-Resolved In-Situ Powder X-Ray Diffraction

Variation in the metal centres of M-M paddle-wheel SBU results in the formation of isostructural DUT-49(M) frameworks. However, the porosity of the framework was found to be different for each of the structures. While a high and moderate porosity was obtained for DUT-49(Cu) and DUT-49(Ni), respectively, other members of the series [DUT-49(M); M= Mn, Fe, Co, Zn, Cd] show very low porosity and shapes of the adsorption isotherms which is not expected for op phases of these MOFs. Investigation on those MOFs revealed that those frameworks undergo structural collapse during the solvent removal at the activation step. Thus, herein, we aimed to study the detailed structural transformations that are possibly occurring during the removal of the subcritical fluid from the framework

Elucidating the Structural Evolution of a Highly Porous Responsive Metal–Organic Framework (DUT-49(M)) upon Guest Desorption by Time-Resolved in Situ Powder X-ray Diffraction

Removal of the guest molecules from the pores of metal–organic frameworks (MOFs) is one of the critical steps in particular for highly porous frameworks associated with high internal stress. In the case of isostructural mesoporous DUT-49(M) (M = Cu, Ni, Mn, Fe, Co, Zn, Cd) frameworks, only DUT-49(Cu) and DUT-49(Ni) could be successfully desolvated so far and only by using supercritical activation. To get a deeper insight into the processes occurring upon the desorption of the solvent from the pores of DUT-49(M), the desolvation was monitored in situ by synchrotron powder X-ray diffraction (PXRD). Analysis of the time-resolved PXRD data shows the full structural transformation pathway of the solid, which involves continuous and discontinuous phase transitions from the open pore (op) to the intermediate pore (ip) phase and from the ip to the contracted pore (cp) phase for DUT-49(Cu) and DUT-49(Ni). For DUT-49(Zn), the op to ip transition is directly followed by amorphization of the framework. All other frameworks show direct amorphization of the op phase

Reversible Switching Between Positive and Negative Thermal Expansion in a Metal-Organic Framework DUT-49

Three-dimensional architectures constructed via coordination of metal ions to organic linkers (broadly termed as metal-organic frameworks, MOFs), are highly interesting for many demanding applications such as gas adsorption, molecular separation, heterogeneous catalysis, molecular sensing etc. Being constructed from heterogeneous components, such framework solids show characteristic features from both of the individual components as well as framework-specific features. One such interesting physicochemical property is thermal expansion, which arises from thermal vibration from the organic linker and metal ions. Herein, we show a very unique example of thermal responsiveness for DUT-49 framework, a MOF well-known for its distinctive negative gas adsorption (NGA) property. In the guest-free form, the framework shows another counter-intuitive phenomenon of negative thermal expansion (NTE), i.e. lattice size increase with decrease of temperature. However, in the solvated state, it shows both NTE and positive thermal expansion (i.e. lattice size decreases with lowering of temperature, PTE) based on a specific temperature range. When the solvent exists in liquid form inside the MOF pore, it retains the pristine NTE nature of the bare framework. But freezing of the solvent inside the pores induces a strain, which causes a structural transformation through in-plane bending of the linker and this squeezes the framework by ~10 % of the unit cell volume. This effect has been verified using 3 different solvents where the structural contraction occurs immediately at the freezing point of individual solvent. Furthermore, studies on a series of DUT-49(M) frameworks with varying metal confirm the general applicability of this mechanism.<br /

Tunable Flexibility and Porosity of the Metal-Organic Framework DUT-49 Through Post-Synthetic Metal Exchange

As a prominent and representative example of flexible metal-organic frameworks (MOFs), DUT-49(Cu) has gained attention due to the unique phenomenon of Negative Gas Adsorption (NGA); originating from an unprecedented struc-tural contraction during the gas adsorption. Herein, post-synthetic metal exchange is demonstrated to afford DUT-49 frameworks with a wide variety of metal cations, e.g. Mn2+, Fe2+, Ni2+, Zn2+, Cu2+ and Cd2+. The single-crystal-to-single-crystal conversion allowed to characterize the new MOFs by single crystal X-ray diffraction, indicating identical struc-ture and topology, as that of previously explored DUT-49(Cu) framework. This approach is proven successful in achieving Mn-Mn and Cd-Cd dimers, which are rare examples of M-M paddle-wheel SBUs. The relative stability and flexibility of the resulted frameworks are observed to be highly sensitive to the metal ion of the framework, following the trends predicted by Irving-Williams series. DUT-49(Ni) was recognized as a second material from DUT-49 series showing adsorption-induced transitions. A sequential increase in framework flexibility from rigid to flexible and from flexible to NGA has been achieved through selective incorporation of metal centers into the structure. Finally, hetero-metallic structures are formed by selective and controlled exchange of metal ions to finely tune the flexibility and NGA phenomenon of the framework.<br /

A Distinctive PdCl₂‑Mediated Transformation of Fe-Based Metallogels into Metal–Organic Frameworks

Simple, efficient conversion of viable Fe³⁺-based metallogels into Fe-metal–organic frameworks (MOFs) has been achieved by PdCl₂-mediated gel degradation. The metallogels and the resulting MOFs have been characterized, and a probable mechanism for the event has been elucidated