Open-framework Materials Research Articles

Skeleton Engineering of Isostructural 2D Covalent Organic Frameworks: Orthoquinone Redox-Active Sites Enhanced Energy Storage

The integration of redox-active sites into the skeleton of open-framework materials is an efficient strategy toward high-performance organic electrodes for energy storage devices. In this work, ste...

Discovering Large Isotropic Negative Thermal Expansion in Framework Compound AgB(CN)4 via the Concept of Average Atomic Volume.

Exploring isotropic negative thermal expansion (NTE) compounds is interesting, but remains challenging. Here, a new concept of "average atomic volume" is proposed to find new NTE open-framework materials. According to this guidance, two NTE compounds, AgB(CN)4 and CuB(CN)4, have been discovered, of which AgB(CN)4 exhibits a large NTE over a wide temperature range (αv = -40 × 10-6 K-1, 100-600 K). The analysis by extended X-ray absorption fine structure spectroscopy and first-principles calculation indicate that (i) the NTE driving force comes from the transverse vibrations of bridge chain atoms of C and N, corresponding to the low-frequency phonon modes; and (ii) the same transverse vibration direction of C and N atoms is a key factor for the occurrence of strong NTE in AgB(CN)4. The present concept of "average atomic volume" can be a simple parameter to explore new NTE compounds especially in those open-framework materials.

Open framework materials for energy applications

Open framework materials for energy applications

Assembly of the Thiolated [Au1 Ag22 (S-Adm)12 ]3+ Superatom Complex into a Framework Material through Direct Linkage by SbF6 - Anions.

Building framework materials with desirable properties and enhanced functionalities with nanocluster/superatom complexes as building blocks remains a challenge in the field of nanomaterials. In this study, the chiral [Au1 Ag22 (S-Adm)12 ]3+ nanocluster/superatom complex (SC, in which S-Adm=1-adamantanethiol) was employed as a building block to construct the three-dimensional (3D) superatom complex inorganic framework (SCIF) materials SCIF-1 and SCIF-2 through inorganic SbF6 - linkers. SCIF-1 is racemic due to the assembly of two SC enantiomers in a single crystal. In SCIF-2, the SC enantiomers are packed in separate crystals, thus producing larger channels and a circularly polarized luminescence (CPL) response. These two 3D SCIF materials exhibit unique sensitive photoluminescence (PL) in protic solvents. Our study provides a new pathway for creating novel open-framework materials with superatom complexes and a foundation for the further development of 3D framework materials for sensing and other applications.

Template Thermolysis to Create a Carbon Dots-Embedded Mesoporous Titanium-Oxo Sulfate Framework for Visible-Light Photocatalytic Applications.

An organo-templated titanium-oxo sulfate of the formulation (H2nep)[TiO(SO4)2] (denoted as TiOS, nep = N-ethylpiperazine) was synthesized under solvent-free conditions. The framework of TiOS is assembled from the [TiO(SO4)2]n2n- infinite chains interconnected by the H2nep cations through H-bond networks. After thermal treatment under vacuum conditions, the organic template H2nep was partially decomposed and converted into N-doped carbon dots (N-CDs), resulting in the N-CDs@TiOS composite material with retained crystallinity of the parent TiOS. The thermolysis of organic templates generates meso-cavities in the framework, rendering N-CDs@TiOS with a mesoporous structure. Photoelectrochemical and photocatalytic experiments show that the presence of N-CDs substantially improved visible-light-driven photocatalytic activity of N-CDs@TiOS compared to that of TiOS. The template thermolysis strategy gives an effective approach to construct the CDs-sensitized Ti-based mesoporous open-framework materials for visible-light photocatalytic applications.

Highly efficient capture of benzothiophene with a novel water-resistant-bimetallic Cu-ZIF-8 material

A novel water-resistant and bimetallic metal organic frameworks (MOFs), Cu-ZIF-8, was successfully synthesized by a coordination synthesis method for the first time. The structure properties of obtained MOFs can be probed and confirmed with nitrogen adsorption-desorption, X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FT-IR) and X-ray photoelectron spectroscopy (XPS), etc. In addition, the pore size can be convenient adjusted by doping amount of Zn(II) and Cu(II) that the body-centered cubic crystal lattice of the parent ZIF-8 framework is continuously maintained. The pseudo-second-order model can make a good description of the adsorption kinetics, while Langmuir model could well express the adsorption isotherms. The Cu-ZIF-8 exhibited a remarkable benzothiophene (BT) uptake capacity compared with those previously reported adsorbents in the literatures. Especially, the Cu-ZIF-8 displayed an outstanding stability in the presence of benzene, octane and water, maintaining more than 95% initial uptake capacity after recycling five times. The as-prepared Cu-ZIF-8 might be a hopeful material for BT capture toward deep desulfurization. Our findings explored a simple and powerful way to incorporate metal ions into the backbones of open framework materials without losing their properties.

(Invited) Prussian Blue Analogues Materials for Energy Storage Applications

The open framework structure of Prussian Blue analogues (PBAs) is fundamentally different from other insertion electrode materials because of its large channels and interstices1. This structure is composed of a face-centered cubic framework of transition metal cations where each cation is octahedrally coordinated to hexacyanometallate groups. Large interstitial “A Sites” within the structure can accommodate zeolitic water and hydrated alkali ions (see Figure). This results in a general chemical formula of AxPR(CN)6nH2O, where A is an alkali cation such as K+or Na+, P is a transition metal cation and R(CN)6is a hexacyanometallate anion. Both the P-site transition metal cation and the R(CN)6 3-hexacyanometallate anion can be electrochemically active in this structure. The Prussian Blue framework structure has wide channels between the A sites, allowing rapid insertion and removal of not only alkali ions but also divalent and trivalent ions from aqueous solutions2. In addition, there is little lattice strain during cycling because the A sites are larger than the hydrated ions that are inserted and removed from them. The result is an extremely stable and rapid electrode: thousands of deep discharge cycles and high capacity retention at high C-rates were demonstrated in aqueous electrolytes3,4 , 5. When operated in organic, Na-ion electrolytes they show high specific capacities, long cycle life and good rate capability6. In my talk, I will show how these exciting properties are linked to the peculiar crystal structure of PBAs and how their chemistry can be easily tuned to adapt their electrochemistry to a variety of energy storage applications. References Hurlbutt, K., Wheeler, S., Capone, I. & Pasta, M. Prussian Blue Analogs as Battery Materials. Joule 2, 1950–1960 (2018).Wang, R.Y. et al.Reversible Multivalent (Monovalent, Divalent, Trivalent) Ion Insertion in Open Framework Materials. Adv. Energy Mater.5, 1–10 (2015).Pasta, M. et al.Manganese–cobalt hexacyanoferrate cathodes for sodium-ion batteries. J. Mater. Chem. A4, 4211–4223 (2016).Pasta, M. et al.Full open-framework batteries for stationary energy storage. Nat. Commun.5, 3007 (2014).Wheeler, S., Capone, I., Day, S., Tang, C. & Pasta, M. Low-Potential Prussian Blue Analogues for Sodium-Ion Batteries: Manganese Hexacyanochromate. Chem. Mater., article ASAP (2019).Lee, H.W. et al.Manganese hexacyanomanganate open framework as a high-capacity positive electrode material for sodium-ion batteries. Nat. Commun.5, 5280 (2014). Figure 1

Pressure-induced symmetry changes in body-centred cubic zeolites.

Previous work has shown a strong correlation between zeolite framework flexibility and the nature of structural symmetry and phase transitions. However, there is little experimental data regarding this relationship, in addition to how flexibility can be connected to the synthesis of these open-framework materials. This is of interest for the synthesis of novel zeolites, which require organic additives to permutate the resulting geometry and symmetry of the framework. Here, we have used high-pressure powder X-ray diffraction to study the three zeolites: Na-X, RHO and ZK-5, which can all be prepared using 18-crown-6 ether as an organic additive. We observe significant differences in how the occluded 18-crown-6 ether influences the framework flexibility—this being dependent on the geometry of the framework. We use these differences as an indicator to define the role of 18-crown-6 ether during zeolite crystallization. Furthermore, in conjunction with previous work, we predict that pressure-induced symmetry transitions are intrinsic to body-centred cubic zeolites. The high symmetry yields fewer degrees of freedom, meaning it is energetically favourable to lower the symmetry to facilitate further compression.

Functional Electrocatalysts Derived from Prussian Blue and its Analogues for Metal‐Air Batteries: Progress and Prospects

AbstractMetal‐air batteries holding exceptionally high energy densities have been heavily explored recently amongst the promising next generation energy suppliers. However, their development for real application is restricted mainly due to the sluggish rates of the oxygen‐reduction reaction (ORR) and oxygen‐evolution reaction (OER) in the positive electrode. Thus, searching a facile way to attain highly efficient ORR and OER electrocatalysts are highly desirable. The key factors to improve the electrochemical performance of materials are to optimize their low‐dimensional feature and adjust effective compositions. Prussian blue and its analogues (PB/PBAs) with open framework have attracted growing attention as hopeful precursors and templates to diverse transition metal‐based materials and carbon hybrids for ORR and OER. Compared to other precursors, PB/PBAs are more feasible for large‐scale application due to their easy preparation, low cost, tuneable compositions and great environmental friendliness. In this review, recent progresses in the utilization of PB/PBA to rationally design and synthesize complex composites with controlled morphologies, sizes, and compositions for ORR, OER and metal‐air batteries are described in detail. Afterwards, a brief summary of the synthetic methods to complex transition metal‐based compositions with nanostructures converted from PB/PBAs are provided. Lastly, the research challenges and possible development direction of open framework materials for metal‐air batteries are outlined through analysing the merits and drawbacks of using PB/PBAs as precursors.

Fabrication of hybrids of zeolitic imidazolate frameworks supported zero‐valent copper nanoparticles as a non‐noble catalyst with high efficiency

This work integrated zero-valent copper (ZVC) onto the surface of zeolitic imidazolate framework (ZIF) to form hybrids of ZVC/ZIF-8 as a non-noble catalyst for catalytic reduction of PNP as a reliable model reaction. The ZVC/ZIF-8 exhibited superior catalytic properties and good recyclability after the fabrication. 6%ZVC/ZIF-8 hybrids complete the reduction process in 2 min with a rate constant estimated to be 2.39 min−1. The study’s findings explored a simple and powerful way to incorporate metal ions into the backbones of open framework materials without losing their properties.

Chemical control of structure and guest uptake by a conformationally mobile porous material.

Metal-organic frameworks (MOFs) are crystalline synthetic porous materials formed by binding organic linkers to metal nodes: they can be either rigid1,2 or flexible3. Zeolites and rigid MOFs have widespread applications in sorption, separation and catalysis that arise from their ability to control the arrangement and chemistry of guest molecules in their pores via the shape and functionality of their internal surface, defined by their chemistry and structure4,5. Their structures correspond to an energy landscape with a single, albeit highly functional, energy minimum. By contrast, proteins function by navigating between multiple metastable structures using bond rotations of the polypeptide6,7, where each structure lies in one of the minima of a conformational energy landscape and can be selected according to the chemistry of the molecules that interact with the protein. These structural changes are realized through the mechanisms of conformational selection (where a higher-energy minimum characteristic of the protein is stabilized by small-molecule binding) and induced fit (where a small molecule imposes a structure on the protein that is not a minimum in the absence of that molecule)8. Here we show that rotation about covalent bonds in a peptide linker can change a flexible MOF to afford nine distinct crystal structures, revealing a conformational energy landscape that is characterized by multiple structural minima. The uptake of small-molecule guests by the MOF can be chemically triggered by inducing peptide conformational change. This change transforms the material from a minimum on the landscape that is inactive for guest sorption to an active one. Chemical control of the conformation of a flexible organic linker offers a route to modifying the pore geometry and internal surface chemistry and thus the function of open-framework materials.

Tunable Thermal Expansion from Negative, Zero, to Positive in Cubic Prussian Blue Analogues of GaFe(CN)6.

The achievement of controlling thermal expansion is important for open-framework structures. The present work proposes a feasible way to adjust the coefficient of thermal expansion continuously from negative to positive via inserting guest Na+ ions or H2O molecules into a GaFe(CN)6 framework. The guest ions or molecules have an intense dampening effect on the transverse vibrations of CN atoms in the -Ga-N≡C-Fe- linkage, especially for the N atoms. This study demonstrates that electrochemical or redox intercalation of guest ions will be an effective way to tune thermal expansion in those negative thermal expansion open-framework materials induced by low-frequency phonons.

A 3D Haloplumbate Framework Constructed From Unprecedented Lindqvist‐like Highly Coordinated [Pb6Br25]13− Nanoclusters with Temperature‐Dependent Emission

Searching novel haloplumbate building units to construct three-dimensional (3D) frameworks is very important and highly desirable because such materials would possess new physical properties and potential applications. Here, by employing tetrakis(N-imidazolemethylene)methane(TIMM) as a structure-directing agent, the first 3D haloplumbate framework constructed from unprecedented Lindqvist-like highly coordinated [Pb6 Br25 ]13- nanoclusters has been successfully prepared under hydrothermal condition, where all Pb2+ centres in [Pb6 Br25 ]13- nanoclusters adopt seven-/eight-coordinated configurations. The as-obtained material is a wide-gap semiconductor (≈3.1 eV) and can be stable up to 320 °C. More importantly, this material has been demonstrated to show temperature-dependent emission. Our results could provide a new strategy to explore novel metal-halide open-framework materials.

Role of Pore Chemistry and Topology in the CO2 Capture Capabilities of MOFs: From Molecular Simulation to Machine Learning

Open framework materials (OFMs) such as metal–organic frameworks (MOFs) can provide structurally and chemically tailorable nanopores. This exceptional tunability has allowed for careful positioning of optimal adsorption sites within MOF pores to enable selective CO2 physisorption, making these materials promising for energy-efficient CO2 capture. However, given the multitude of features that can be simultaneously altered within the thousands of MOFs synthesized to date, it can be daunting to elucidate the most critical features for boosting CO2 capture capabilities. Here we use a multiscale approach—density functional theory (DFT), grand canonical Monte Carlo (GCMC), and machine learning (ML)—to investigate the role of various pore chemical and topological features in the enhancement of CO2 capture metrics of MOFs. To enable a thorough “sweep” of a target region of MOF structure-space, we used computational synthesis methods to create sets of MOFs encompassing all possible combinations of 16 topologies an...

Low-Frequency Phonon Driven Negative Thermal Expansion in Cubic GaFe(CN)6 Prussian Blue Analogues.

The understanding of the negative thermal expansion (NTE) mechanism is vital not only for the development of new NTE compounds but also for effectively controlling thermal expansion. Here, we report an interesting isotropic NTE property in cubic GaFe(CN)6 Prussian blue analogues (α l = -3.95 × 10-6 K-1, 100-475 K), which is a new example to understand the complex NTE mechanism. A combined study of synchrotron X-ray diffraction, X-ray total scattering, X-ray absorption fine structure, neutron powder diffraction, and density functional theory calculations shows that the NTE of GaFe(CN)6 originates from the low-frequency phonons (< ∼100 cm-1), which are directly related to the transverse vibrations of the atomic -Ga-N≡C-Fe- chains. Both the Ga-N and Fe-C chemical bonds are much softer to bend than to stretch. The direct evidence that transverse vibrational contribution to the NTE of GaFe(CN)6 is dominated by N, instead of C atoms, is illustrated. It is interesting to find that the polyhedra of GaFe(CN)6 are not rigid, which is a starting assumption in some models describing the NTE properties of other systems. The NTE mechanism can be vividly described by the "guitar-string" effect, which would be the common feature for the NTE property of many open-framework functional materials, such as Prussian blue analogues, oxides, cyanides, metal-organic frameworks, and zeolites.

Analysis of stacking disorder in ice I using pair distribution functions

Stacking-disordered materials display crystalline order in two dimensions but are disordered along the direction in which layered structural motifs are stacked. Countless examples of stacking disorder exist, ranging from close-packed metals, ice I and diamond to open-framework materials and small-molecule pharmaceuticals. In general, the presence of stacking disorder can have profound consequences for the physical and chemical properties of a material. Traditional analyses of powder diffraction data are often complicated by the presence of memory effects in the stacking sequences. Here it is shown that experimental pair distribution functions of stacking-disordered ice I can be used to determine local information on the fractions of cubic and hexagonal stacking. Ice is a particularly challenging material in this respect, since both the stacking disorder and the orientational disorder of the water molecules need to be described. Memory effects are found to contribute very little to the pair distribution functions, and consequently, the analysis of pair distribution functions is the method of choice for characterizing stacking-disordered samples with complicated and high-order memory effects. In the context of this work, the limitations of current structure-reconstruction approaches are also discussed.

Switching Between Giant Positive and Negative Thermal Expansions of a YFe(CN)6 -based Prussian Blue Analogue Induced by Guest Species.

The control of thermal expansion of solid compounds is intriguing but remains challenging. The effect of guests on the thermal expansion of open-framework structures was investigated. Notably, the presence of guest ions (K+ ) and molecules (H2 O) can substantially switch thermal expansion of YFe(CN)6 from negative (αv =-33.67×10-6 K-1 ) to positive (αv =+42.72×10-6 K-1 )-a range that covers the thermal expansion of most inorganic compounds. The mechanism of such substantial thermal expansion switching is revealed by joint studies with synchrotron X-ray diffraction, X-ray absorption fine structure, neutron powder diffraction, and density functional theory calculations. The presence of guest ions or molecules plays a critical damping effect on transverse vibrations, thus inhibiting negative thermal expansion. An effective method is demonstrated to control the thermal expansion in open-framework materials by adjusting the presence of guests.

Organically templated inorganic-organic hybrid metal (Zn and Cd) Sulfite-oxalates with layered and three-dimensional structures.

We report single crystal X-ray investigations of three d10 metal based oxalate-sulfite inorganic-organic hybrid open framework materials, namely [Zn2(SO3)2(C2O4)0.5(C6N2H13)]·(H2O)2, 1, [C4N2H12]0.5[Zn2(SO3)2(C2O4)0.5(H2O)2], 2 and [C4N2H12][Cd2(SO3)2(C2O4)(H2O)]·H2O, 3 which show layered and three dimensional structures with different organic templates. These materials were synthesized under hydrothermal conditions and characterized by techniques such as single crystal X-ray diffraction, dipole moment calculations, FTIR, powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The common feature for 1 and 2 is the interconnection of embedded zinc-sulfite chains via oxalate anions in the framework and in 3, cadmium-oxalate chains are interconnected via sulfite anions. The coordination geometries of metal(ii) ions vary significantly resulting in a ladder-like layered structure in the case of 1, a 3D framework in case of 2 and a layered structure with an embedded one-dimensional channel in 3. The observed structures can also be partly attributed to the influence exerted by the presence of different structure directing agents (SDA) in the lattice. The dipole moments for metal polyhedra 1, 2 and 3 were calculated from bond valence analyses and are 2.39 D for Zn(1)O4N polyhedra and 1.66 D for Zn(2)O4 polyhedra in 1, 24.90 D for Zn(1)O6 and 29.52 D for Zn(2)O4 in 2 and 5.82 D for Cd(1)O7 and 2.90 D for Cd(2)O6 in 3.

Synthesis and characterization of two layered aluminophosphates [R-C8H12N]8[H2O]2·[Al8P12O48H4] and [S-C8H12N]8[H2O]2·[Al8P12O48H4] with a mirror symmetric feature and their proton conductivity.

The template effect of chiral organic molecules to aluminophosphate open frameworks was investigated by the syntheses of two layered structures, [R-C8H12N]8[H2O]2·[Al8P12O48H4] and [S-C8H12N]8[H2O]2·[Al8P12O48H4] (denoted as AlPO-CJ72-R and -S), with a mirror symmetric feature. The two structures are obtained separately, which is the first instance in inorganic open-framework materials to our knowledge, by α-methylbenzylamine with homochirality. Single-crystal X-ray diffraction analysis reveals that the structure of either AlPO-CJ72-R or AlPO-CJ72-S crystallizes in the chiral triclinic space group P1, and the existence of AAAA-stacked Al2P3O123- layers made up of alternating AlO4 and PO3([double bond, length as m-dash]O) or PO2([double bond, length as m-dash]O)(OH) tetrahedra to form a 4 × 6 network. Their structures show self-assembled chiral H-bond chains, the chiralities of which are transferred to adjacent helical T-O (T = Al and P) chains. The frameworks are mirror symmetric to each other. The performance of AlPO-CJ72 regarding proton conductivity was tested. It shows an excellent result up to 3.01 × 10-3 S cm-1 (363 K, 98% RH), which might be attributed to the H-bond chains in the structure.

Size effects on negative thermal expansion in cubic ScF3

Scandium trifluoride (ScF3), adopting a cubic ReO3-type structure at ambient pressure, undergoes a pronounced negative thermal expansion (NTE) over a wide range of temperatures (10 K–1100 K). Here, we report the size effects on the NTE properties of ScF3. The magnitude of NTE is reduced with diminishing the crystal size. As revealed by the specific heat measurement, the low-energy phonon vibrations which account for the NTE behavior are stiffened as the crystal size decreases. With decreasing the crystal size, the peaks in high-energy X-ray pair distribution function (PDF) become broad, which cannot be illuminated by local symmetry breaking. Instead, the broadened PDF peaks are strongly indicative of enhanced atomic displacements which are suggested to be responsible for the stiffening of NTE-related lattice vibrations. The present study suggests that the NTE properties of ReO3-type and other open-framework materials can be effectively adjusted by controlling the crystal size.