Emergent Frontiers in Porous Frameworks and Beyond

ConspectusPorous frameworks possess high porosity and adjustable functions. The two features conjointly create sufficient interfaces for matter exchange and energy transfer within the skeletons. For crystalline porous frameworks, including metal organic frameworks (MOFs) and covalent organic frameworks (COFs), their long-range ordered structures indeed play an important role in managing versatile physicochemical behaviors such as electron transfer or band gap engineering. It is now feasible to predict their functions based on the unveiled structures and structure-performance relationships. In contrast, porous organic frameworks (POFs) represent a member of the porous solid family with no long-range regularity. For the case of POFs, the randomly packed building units and their disordered connections hinder the electronic structural consistency throughout the entire networks. However, many investigations have demonstrated that the functions of POFs could also be designed and originated from their local motifs.In this Account, we will first provide an overview of the design and synthesis principles for porous aromatic frameworks (PAFs), which are a typical family of POFs with high porosity and exceptional stability. Specifically, the functions achieved by the specific design and synthesis of in-framework motifs will be demonstrated. This strategy is particularly intuitive to introduce desired functions to PAFs, owing to the exceptional tolerance of PAFs to harsh chemical treatments and synthetic conditions. The local structures can be either obtained by selecting suitable building units, sometimes with the aid of computational screening, or emerge as the product of coupling reactions during the synthetic process. Radical PAFs can be obtained by incorporating a persistent radical molecule as a building unit, and the rigid and porous framework may facilitate the formation of radical species by trapping spins in the organic network, which could avoid the delocalizing and recombining processes. Alternatively, radical motifs can also be formed during the formation of the framework linkages. The coupling reaction plays an important role in the construction of functional motifs like diacetylene. The highly porous, radical PAFs showed significant performance as anodes of lithium-ion batteries. To improve the charge transport within the framework, the building units and their connecting manner were cohesively considered, and the framework with a fully conjugated backbone was built up. In another case, the explicit product of the cross-coupling reaction ensured the precise assembly of two building units with electron donating and accepting abilities; therefore, the moving direction of photogenerated electrons was rationally controlled. Constructing a fully conjugated backbone or rationally designing a D-A system for charge transfer in porous frameworks introduced exciting properties for photovoltaic and photocatalysis, and their highly porous, stable frameworks improved their functional applications for perovskite solar cells and chemical productions. These investigations shed light on the designable combination of intrinsic functional motifs with highly porous organic frameworks for effective energy storage and conversion.

Pore characterization and shale facies analysis of the Ordovician-Silurian transition of northern Guizhou, South China: The controls of shale facies on pore distribution

We report the shapeable synthesis of porous silica frameworks using polyacrylamide (PAAm) gel as an organic template and hydrolyzed silicon alkoxide as a silica source. Macroscopically shaped porous frameworks—such as plates, tablets and sheets—comprised of 20- to 40-nm diameter silica particles are obtained via PAAm–silica precursor gels. The mechanical properties (i.e., hardness and Young’s modulus) of the silica frameworks depend on the packing density and are controlled by changing the silica content in PAAm gels. Shapeable synthesis of porous silica frameworks was achieved in an aqueous system using polyacrylamide (PAAm) gel as an organic template and hydrolyzed silicon alkoxide as a silica source. Macroscopically shaped frameworks comprised of 10- to 50-nm diameter silica particles are obtained via PAAm–silica precursor gels. The mechanical properties of the silica frameworks depending on the packing density are controlled by changing the silica content in PAAm gels.

The manner of bonding between constituent atoms or molecules invariably influences the properties of materials. Perhaps no material family is more emblematic of this than porous frameworks, wherein the namesake modes of connectivity give rise to discrete subclasses with unique collections of properties. However, established framework classes often display offsetting advantages and disadvantages for a given application. Thus, there exists no universally applicable material, and the discovery of alternative modes of framework connectivity is highly desirable. Here we show that chalcogen bonding, a subclass of σ-hole bonding, is a viable mode of connectivity in low-density porous frameworks. Crystallization studies with the triptycene tris(1,2,5-selenadiazole) molecular tecton reveal how chalcogen bonding can template high-energy lattice structures and how solvent conditions can be rationalized to obtain molecularly programmed porous chalcogen-bonded organic frameworks (ChOFs). These results provide the first evidence that σ-hole bonding can be used to advance the diversity of porous framework materials.

The surface structure and material composition of current collectors have significant effects on the electrochemical performances of lithium-ion batteries (LIBs). In this work, a three-dimensional (3D) porous composite framework is applied as the anode current collector in LIBs. This unique 3D skeleton is composed of conductive carbon fiber/Cu core/shell fibrous structure. With an oxidation treatment upon the copper shell, the porous framework is assembled with CuO microspheres. Using mesocarbon microbead (MCMB) graphite powders as the active material, the cell with this 3D porous composite current collector shows an improved reversible discharge-charge capacity of 415 mAh g–1 at a current rate of 0.1 C after 50 cycles, which is much higher than that of the cell with a flat Cu foil ( 127 mAh g–1 ). It is demonstrated that this unique fibrous network of the 3D current collector coupled with the morphological effects of the CuO microspheres greatly improve the cell performance in terms of electrical conductivity, reversible capacity and cycling stability.

Selective extraction of uranium from water has attracted worldwide attention because the largest source of uranium is seawater with various interferenceions (Na+ , K+ , Mg2+ , Ca2+ , etc.). However, traditional adsorbents encapsulate most of their functional sites in their dense structure, leading to problems with low selectivity and adsorption capacities. In this work, the tailor-made binding sites are first decorated into porous skeletons, and a series of molecularly imprinted porous aromatic frameworks are prepared for uranium extraction. Because the porous architecture provides numerous accessible sites, the resultant material has a fourfold increased ion capacity compared with traditional molecularly imprinted polymers and presents the highest selectivity among all reported uranium adsorbents. Moreover, the porous framework can be dispersed into commercial polymers to form composite components for the practical extraction of uranium ions from simulated seawater.

Porous framework materials for CO2 capture

Dual-active centers of porous triazine frameworks for efficient Li storage

By varying solvent systems, the solvothermal treatment of uranyl nitrate and methylenediphosphonic acid (H4PCP) afforded three new porous uranyl-organic frameworks (UOFs). All were structurally characterized by single-crystal X-ray diffraction and formulated as (Et2NH2)2[(UO2)3(PCP)2](H2O)2.5 (1), (MeNH3)(H3O)[(UO2)3(PCP)2(H2O)3] (2), and [Na(H2O)4](H3O)[(UO2)3(PCP)2(H2O)2](H2O)5 (3). These compounds crystallize with three-dimensional anionic frameworks containing U(VI) and distinct cationic species due to in situ solvent hydrolysis. The solvent systems diethylformamide (DEF), N-methyl-2-pyrrolindone (NMP), and the additive sodium vanadate (Na3VO4) significantly impact the resultant structures, affording diethyl ammonium, methyl ammonium, and sodium cations captured in channels of the anionic frameworks of 1-3. In 1, a trinuclear U3O18 unit formed by three uranyl polyhedra that share edges is connected into a three-dimensional framework. Compound 2 has a three-dimensional framework formed from a uranyl-methylenediphosphonate layer that is pillared by UO7 pentagonal bipyramids. With the inclusion of sodium cations, 3 is a porous framework containing UO7 pentagonal bipyramids within a layer, with sodium cations and UO6 square bipyramids linking the adjacent layers. Compounds 1-3 feature the uranyl/ligand ratio of 3:2, but present diverse structural building units ranging from edge-shared trinuclear to heteronuclear assemblies. The compounds have been characterized by infrared (IR), Raman, and UV-vis spectroscopies, X-ray diffraction, and thermogravimetric analysis.

Cucurbit[10]uril (Q[10] or CB[10]), with the largest rigid cavity (ca. 1.0 nm) yet characterized in the cucurbiturils family, and indeed among all artificial macrocyclic receptors to date, has been successfully exploited to construct a novel Q[10]-[Cd4Cl16]8--based pillared diamond porous supramolecular framework. Single-crystal X-ray diffraction analysis revealed that the three-dimensional open-nanotube-type porous framework is constructed from free Q[10] molecules and [Cd4Cl16]8- cluster anions through the outer surface interactions of Q[10]. Notably, the Q[10]-based porous framework host can accommodate guest dyes, such as rhodamine B (G1), pyrenemethanamine hydrochloride (G2), and bathocuproine hydrochloride (G3), to form solid materials with efficient red-green-blue (RGB) fluorescence. This work not only exemplifies a facile approach for the construction of macrocycle-based porous frameworks but also offers a simple, lower cost, yet still highly efficient means of preparing multi-emitting, including white-light-emitting, solid luminescent materials.

Polyoxometalates, a class of discrete nanoscale molecular metal-oxo clusters, have excellent catalytic and photoelectric/electrochromic properties due to their excellent electronic storage and redox capacity. They possess of plenty of potential applications as efficient catalysts in the development of clean and green fuels. However, polyoxometalates are easy to agglomerate and deactivate in the catalytic process, and possess of low specific surface area. Therefore, it is extremely urgent to introduce polyoxometalates into crystalline porous materials with well-defined structure, modifiable channels and high porosity. The introduction of transition metal into polyoxometalates can not only regulate their charge number, but also optimize their band structure. As well known, polyoxometalates, as one kind of unique metal oxide cluster, have been considered as promising secondary building units for construction of porous frameworks because of their adjustable compositions, variable topologies and oxygen-rich surface as well as their diverse physical and chemical properties. Recently, more and more attention has been paid to introducing polyoxometalates into frameworks to functionalize porous frameworks materials. So, these transition metal-centered polyoxometalates should be ideal building units for the design and synthesis of porous crystalline framework materials. These crystalline materials are expected to be used in the fields of single-molecule magnets, photocatalytic water splitting, desulfurization and so on. This review is directed to cover the main advances on the development of transition metal-centered polyoxometalate-based framework materials. In the past several decades, researchers have synthesized 1:12 Keggin type and Silverton type polyoxometalate, Anderson type polyoxometalate Waugh type polyoxometalate, Evans-Showell type polyoxometalate and heteropolyvanadate-based framework materials by hydrothermal method and conventional aqueous solution method. The water oxidation and photocatalytic degradation of organic pollutants of the framework materials were detailly studied. For example, the Keggin anion [CoW12O40]6– turned into an efficient and robust electrocatalyst upon its confinement in the well-defined void space of metal-organic framework (ZIF-8). In the electrocatalytic process, the [H6CoW12O40]@ZIF-8 composite crystalline framework materials is very stable for water oxidation that could retain its initial activity even after 1000 catalytic cycles. The catalyst has a turnover frequency (TOF) of 10.8 mol O2 (mol Co) –1s–1, one of the highest TOFs for electrocatalytic oxygen evolution at neutral pH. Further, the POM-based crystalline framework materials were used for oxidative desulfurization and the decomposition of chemical warfare agent. In these fields, the porous frameworks with catalytic active manganese(IV)-containing heteropolyvanadate [MnV13O38]7- as nodes and rare earth ions as linking units have been successfully prepared. These multifunctional polyoxometalate-based porous framework materials exhibit selective adsorption behavior and remarkable catalytic activity as heterogeneous catalysts for the oxidation of sulfides. A novel manganese(IV)-containing symmetrical heteropolyvanadate K7 [ MnV13O38] · 18H2O to K4Li2[MnV14O40] · 21H2O was also prepared, which is an deficient catalyst for the oxidation of 2-chloroethyl ethyl sulfide with tert-butyl hydroperoxide oxidizing agent. In this catalytic process, a TON of 40 is achieved in 8 h, showing potential application toward air decontamination technology. The results show that the transition metal-centered polyoxometalate-based crystalline framework materials possess of many functionalities, especially as excellent catalysts. In this paper, the research progress of crystalline framework materials constructed from transition metal-centered polyoxometalates is systematically summarized, and the challenges and opportunities in this field are also presented.

The synthesis and directed evolution of a tetranuclear copper cluster, supported by 8-mercapto-N9-propyladenine ligand, to a highly porous three-dimensional cubic framework in the solid state is reported. The structure of this porous framework was unambiguously characterized by X-ray crystallography. The framework contains about 62 % solvent-accessible void; the presence of a free exocyclic amino group in the porous framework facilitates reversible adsorption of gas and solvent molecules. Oriented growth of framework in solution was also tracked by force and scanning electron microscopy studies, leading to identification of an intriguing ripening process, over a period of 30 days, which also revealed formation of cuboidal aggregates in solution. The elemental composition of these cuboidal aggregates was ascertained by EDAX analysis.

A unique porous framework of highly ordered few-layered MoS2 was realized by using the facile solvothermal technique. The structure was composed of crystalline MoS2 in the 2H phase, with ordered, 100–150 nm wide pores and a 15 nm wall thickness. The porous framework was studied for electrochemical hydrogen evolution reaction (HER) and rechargeable Li ion batteries. The porous MoS2 showed enhanced catalytic activity for electrochemical HER, with an overpotential of −210 mV at 10 mA cm–2. In addition, in Li ion storage testing, the half-cell delivered high specific capacities: 1265 and 1256 mAh g–1 at 50 mA g–1 and 1172 and 1161 mAh g–1 at 200 mA g–1 for the first discharge and charge with Coulombic efficiencies 99.3% and 99.0%, respectively. The cyclic stability showed a reversible specific discharge capacity of 1178 mAh g–1 after 100 cycles, which is attributed to the porous MoS2 framework. An impedance study revealed an improved charge transfer process, attributed to the availability of the channels for the Li+ ion intercalation due to the porous framework of the MoS2. The prima facie observation shows that this unique morphology has significantly improved the performance of such materials without additional modifications, such as doping, indicating that such a porous framework may serve as promising bifunctional electrodes for both energy conversion and storage applications.

Ionic liquids (ILs) and porous frameworks are commonly employed in gas separation, but rarely together. To leverage their combined advantages, we designed a novel system comprising an ionic liquid inside a charged porous-polymer framework. A positively charged ionic porous aromatic framework was impregnated with an ionic liquid to confine the cations and anions inside the polymeric framework via electrostatic interactions. Molecular dynamics and free energy simulations were performed to determine the solubility, diffusivity, and hence permeability of CO2 and CH4 through the composite material. We found that the polymeric framework introduces nanoscale space in the confined IL, improving gas solubility, while the IL promotes gas transport in the porous structure. The confinement impacts CO2 and CH4 very differently regarding their solubility and diffusivity. Through tuning of the ionic-liquid loading, both high CO2 permeability and high CO2/CH4 selectivity could be achieved, overcoming the Robeson upper bound. This work highlights the advantages of confined ionic liquids in an ionic porous-polymer framework in enhancing selective gas separation.

Two new borate–sulfates form unique three-dimensional (3D) porous frameworks with embedded [SO4] units. The transition from structure I to II is driven by differences in cationic radii, leading to distinct 3D porous borate anionic frameworks.