Metal-organic Frameworks Crystal Structures Research Articles

Understanding the Correlation Between Structure and Entangled Photon Pair Properties with Metal–Organic Frameworks

Spontaneous parametric down conversion (SPDC) is a quantum second-order nonlinear optical process where the photons generated are frequently used in quantum information processing. Materials with large second-order nonlinearities (χ(2)) can be used as entangled photon sources with a high brightness. The source brightness scales as the square of the effective nonlinearity (deff), which is an intrinsic property of the material. Understanding material factors that can significantly alter this intrinsic property is useful in developing new materials that are SPDC efficient. In our work we focus on understanding factors affecting the entangled photon pair properties, such as the arrangements of ligands within the Zn(3-ptz)2 metal–organic framework (MOF) crystal and temperature. We find that the arrangement and alignment of the pyridine rings in the crystal structure significantly affect the deff and birefringence (Δn). Smaller pyridine ring alignments relative to the optic c-axis increase the Δn, which in turn leads to larger photon pair correlation times (τc) in coincidence measurements. Our work has significant implications in understanding the effect of ligand arrangement on deff and τc for any MOF crystal structure, providing a tool to rationalize the optimization of MOF crystals for the development of efficient nonlinear optical devices.

Water-Based Synthesis of Zr6-Based Metal–Organic Framework Nanocrystals with Sulfonate Functions: Structural Features and Application to Fructose Dehydration

A series of zirconium-based metal–organic framework (MOF) nanocrystals (95–211 nm) displaying sulfonate functions (UiO-66-SO3H) was prepared in N,N-dimethylformamide (DMF)─the conventional solvent─and water, and their physicochemical properties were thoroughly investigated. In particular, X-ray diffraction results suggest that upon replacing DMF with water, the resulting MOF crystal structure presents a highly defective structure belonging to the space group Im3̅ instead of typical Fm3̅m. The acid catalysts were applied to the fructose dehydration into 5-hydroxymethylfurfural (5-HMF). Complete conversion of fructose over UiO-66-SO3H prepared in water was reached after only 30 min at 100 °C, in line with its stronger Brønsted acidity. In comparison, its counterpart prepared in DMF showed only 30% fructose conversion. Moreover, the intrinsic catalytic effect at 80 °C was only observed with the water-based UiO-66-SO3H. Without reactivation of the catalyst, recycling tests demonstrated the preservation of its structural integrity upon nine consecutive cycles, while a gradual loss of the catalyst activity was attributed to the humin adsorption on the MOFs.

Metal–organic framework polymer composite enhancement via acyl chloride modification

AbstractIdeal mixed‐matrix composites fabricated from metal–organic frameworks (MOFs) and polymers should exhibit good compatibility between components, high adsorptivity, and fast and efficient reactivity. Often, poor compatibility between MOFs and polymers results in interfacial defects, which lead to underperforming materials. In this work the MOF UiO‐66‐NH2 was modified using various acyl chlorides to tailor MOF compatibility and dispersion within mixed‐matrix composites and electrospun fibers to enable better protective suits and filters. MOF crystal structures were unaffected and high surface areas were present after acyl chloride treatment. Most treated MOFs had increased reactivity towards soman (GD) and the simulant dimethyl p‐nitrophenylphosphate relative to the baseline MOF, and this enhanced reactivity was probably because of better wetting behavior towards the agents. Chlorine and 2‐chloroethyl ethyl sulfide uptake of modified MOFs was similar to uptake in the baseline MOF, while nitrogen dioxide and GD sorption capacity decreased for all modified MOFs due to exhaustion of amine sites and pore blockage, respectively. Overall, modification of the MOF with acyl chlorides resulted in better polymer compatibility, retained adsorption capacity for multiple chemicals and even increased reactivity towards GD. The structure–activity processing insights gained from this work will lead to the design of more effective protection equipment such as filters, clothing/suits and membranes. Published 2020. This article is a U.S. Government work and is in the public domain in the USA

Formation and growth of Pd nanoparticles in UiO-67 MOF by in situ EXAFS

We present a detailed extended X-ray absorption fine structure (EXAFS) characterization of the formation process of palladium nanoparticles (NPs) inside UiO-67 metal-organic framework (MOF). Starting from the initial UiO-67 structure with 10% of PdCl2bpydc moieties with grafted Pd ions replacing standard 4,4′-biphenyldicarboxylate linkers, we followed in situ H2-temperature programmed reduction (TPR) of the material. The cleavage of Pd–N and Pd–Cl bonds and simultaneous growth of the Pd–Pd contribution of metal NPs start at 200 °C. Below 300 °C metal NPs are small and are constrained inside the cavities of UiO-67, while at higher temperatures, the Pd–Pd coordination number (CN) increases to its bulk values, implying the aggregation of NPs into big palladium particles with probable destruction of MOF crystal structure. The formation of bigger particles is also confirmed by non-linear dependence of Debye-Waller parameter (DW) for Pd‒Pd contribution upon temperature.

Flexible SIS/HKUST-1 Mixed Matrix Composites as Protective Barriers against Chemical Warfare Agent Simulants.

We fabricated and demonstrated, for the first time, metal-organic framework (MOF), polymer mixed-matrix composites (MMCs) as effective, low burden barriers against chemical warfare agent (CWA) simulants. We incorporated the MOF HKUST-1 into elastomeric triblock copolymers of polystyrene- block-polyisoprene- block-polystyrene (SIS) for use as semipermeable barrier against the CWA simulant 2-chloroethyl ethyl sulfide (CEES). MMCs containing up to 50 wt % HKUST-1 were cast and evaluated for CEES permeation, moisture vapor transport rate (MVTR), and mechanical properties, such as elastic modulus and percent elongation. Increasing the MOF content resulted in longer protection against CEES with breakthrough times ranging from immediate breakthrough for the baseline SIS to over 4000 min for the best-performing MMC. MVTRs of high-MOF-content MMCs were approximately 5-10 times higher than either SIS or typical laboratory gloves made from nitrile and latex. The elastic moduli increased with increased MOF content corresponding to a reduction in percent elongation. The triblock copolymer also was found to protect the MOF crystal structure after exposure to CEES and liquid water, which may lead to longer usage time and shelf life. The ability to resist degradation due to moisture shows the potential utility of these composites when exposed to rain, sweat, or other moisture-rich environments. Finally, the MOF-containing composites functioned as robust colorimetric indicators of CEES exposure. Thus, these MMC materials present a potential route toward next-generation personal protective equipment with a combination of detoxification, sensing, environmental stability, and thermal/user-comfort properties not present in current materials solutions.

Metallic Metal-Organic Frameworks Predicted by the Combination of Machine Learning Methods and Ab Initio Calculations.

Emerging applications of metal-organic frameworks (MOFs) in electronic devices will benefit from the design and synthesis of intrinsically, highly electronically conductive MOFs. However, very few are known to exist. It is a challenging task to search for electronically conductive MOFs within the tens of thousands of reported MOF structures. Using a new strategy (i.e., transfer learning) of combining machine learning techniques, statistical multivoting, and ab initio calculations, we screened 2932 MOFs and identified 6 MOF crystal structures that are metallic at the level of semilocal DFT band theory: Mn2[Re6X8(CN)6]4 (X = S, Se,Te), Mn[Re3Te4(CN)3], Hg[SCN]4Co[NCS]4, and CdC4. Five of these structures have been synthesized and reported in the literature, but their electrical characterization has not been reported. Our work demonstrates the potential power of machine learning in materials science to aid in down-selecting from large numbers of potential candidates and provides the information and guidance to accelerate the discovery of novel advanced materials.

Metal–Organic Framework-Functionalized Alumina Membranes for Vacuum Membrane Distillation

Nature-mimetic hydrophobic membranes with high wetting resistance have been designed for seawater desalination via vacuum membrane distillation (VMD) in this study. This is achieved through molecular engineering of metal–organic framework (MOF)-functionalized alumina surfaces. A two-step synthetic strategy was invented to design the hydrophobic membranes: (1) to intergrow MOF crystals on the alumina tube substrate and (2) to introduce perfluoro molecules onto the MOF functionalized membrane surface. With the first step, the surface morphology, especially the hierarchical roughness, can be controlled by tuning the MOF crystal structure. After the second step, the perfluoro molecules function as an ultrathin layer of hydrophobic floss, which lowers the surface energy. Therefore, the resultant membranes do not only possess the intrinsic advantages of alumina supports such as high stability and high water permeability, but also have a hydrophobic surface formed by MOF functionalization. The membrane prepared under an optimum condition achieved a good VMD flux of 32.3 L/m2-h at 60 °C. This study may open up a totally new approach for design of next-generation high performance membrane distillation membranes for seawater desalination.

Chemistry of Metal-organic Frameworks Monitored by Advanced X-ray Diffraction and Scattering Techniques.

The research on metal-organic frameworks (MOFs) experienced rapid progress in recent years due to their structure diversity and wide range of application opportunities. Continuous progress of X-ray and neutron diffraction methods enables more and more detailed insight into MOF's structural features and significantly contributes to the understanding of their chemistry. Improved instrumentation and data processing in high-resolution X-ray diffraction methods enables the determination of new complex MOF crystal structures in powdered form. By the use of neutron diffraction techniques, a lot of knowledge about the interaction of guest molecules with crystalline framework has been gained in the past few years. Moreover, in-situ time-resolved studies by various diffraction and scattering techniques provided comprehensive information about crystallization kinetics, crystal growth mechanism and structural dynamics triggered by external physical or chemical stimuli. The review emphasizes most relevant advanced structural studies of MOFs based on powder X-ray and neutron scattering.

Deterioration of metal–organic framework crystal structure during fabrication of poly(l‐lactic acid) mixed‐matrix membranes

AbstractPoly(l‐lactic acid) (PLLA) and metal–organic framework (MOF) mixed‐matrix membranes were prepared by melt extrusion of PLLA with 5% (w/w) of either activated or water‐saturated Cu3(BTC)2 (Cu3(C9H3O6)2(H2O)3·xH2O, HKUST‐1). The morphology and the stability of injection‐molded samples were evaluated using thermogravimetric analysis, differential scanning calorimetry, gel permeation chromatography, X‐ray diffraction (XRD) and scanning electron microscopy (SEM). The presence of activated and saturated MOF crystals increased the cold crystallization onset temperature as compared to neat PLLA. This can be attributed to the MOF crystals incorporated in the PLLA matrix, which decreased the mobility of PLLA and thus impeded the crystallization process. According to the XRD results, the activated MOF crystals were successfully incorporated into the PLLA matrix without altering the crystal structure of the MOF. Moreover, the findings from permeability and tensile tests as well as SEM imaging indicated good interfacial interactions between PLLA and activated MOF. However, during melt extrusion of PLLA with saturated MOF, water molecules from the saturated MOF altered the MOF crystal structure and contributed to the degradation of the PLLA polymer by reducing its molecular weight by around 21%. © 2013 Society of Chemical Industry

Assessment of a Metal−Organic Framework Membrane for Gas Separations Using Atomically Detailed Calculations: CO2, CH4, N2, H2 Mixtures in MOF-5

Metal−organic frameworks (MOFs) have emerged as a fascinating alternative to more traditional nanoporous materials. Although hundreds of different MOF structures have been synthesized in powder form, little is currently known about the potential performance of MOFs for membrane-based separations. We have used atomistic calculations to predict the performance of a MOF membrane for separation of various gas mixtures in order to provide information for material selection in membrane design. Specifically, we investigated the performance of MOF-5 as a membrane for separation of CO2/CH4, CO2/H2, CO2/N2, CH4/H2, N2/H2, and N2/CH4 mixtures at room temperature. In every case, mixture effects play a crucial role in determining the membrane performance. Although the membrane selectivities predicted for MOF-5 are not large for the mixtures we studied, our result suggest that atomistic simulations will be a useful tool for considering the large number of MOF crystal structures that are known in order to seek membrane materials with more desirable characteristics.

Assembly of metal-organic frameworks from large organic and inorganic secondary building units: new examples and simplifying principles for complex structures.

The secondary building unit (SBU) has been identified as a useful tool in the analysis of complex metal-organic frameworks (MOFs). We illustrate its applicability to rationalizing MOF crystal structures by analysis of nine new MOFs which have been characterized by single-crystal X-ray diffraction. Tetrahedral SBUs in Zn(ADC)(2).(HTEA)(2) (MOF-31), Cd(ATC).[Cd(H(2)O)(6)](H2O)(5) (MOF-32), and Zn(2)(ATB)(H2O).(H2O)(3)(DMF)(3) (MOF-33) are linked into diamond networks, while those of Ni(2)(ATC)(H(2)O)(4).(H2O)(4) (MOF-34) have the structure of the Al network in SrAl(2). Frameworks constructed from less symmetric tetrahedral SBUs have the Ga network of CaGa(2)O(4) as illustrated by Zn(2)(ATC).(C(2)H(5)OH)(2)(H2O)(2) (MOF-35) structure. Squares and tetrahedral SBUs in Zn(2)(MTB)(H2O)(2).(DMF)(6)(H2O)(5) (MOF-36) are linked into the PtS network, which is the simplest structure type known for the assembly of these shapes. The octahedral SBUs found in Zn(2)(NDC)(3).[(HTEA)(DEF)(ClBz)](2) (MOF-37) form the most common structure for linking octahedral shapes, namely, the boron network in CaB(6). New structure types for linking triangular and trigonal prismatic SBUs are found in Zn(3)O(BTC)(2).(HTEA)(2) (MOF-38) and Zn(3)O(HBTB)(2)(H2O).(DMF)(0.5)(H2O)(3) (MOF-39). The synthesis, crystal structure, and structure analysis using the SBU approach are presented for each MOF.