Hierarchical Nature of Nanoscale Porosity in Bone Revealed by Positron Annihilation Lifetime Spectroscopy.

Bone is a hierarchical material primarily composed of collagen, water, and mineral that is organized into discrete molecular, nano-, micro-, and macroscale structural components. In contrast to the structural knowledge of the collagen and mineral domains, the nanoscale porosity of bone is poorly understood. In this study, we introduce a well-established pore characterization technique, positron annihilation lifetime spectroscopy (PALS), to probe the nanoscale size and distribution of each component domain by analyzing pore sizes inherent to hydrated bone together with pores generated by successive removal of water and then organic matrix (including collagen and noncollagenous proteins) from samples of cortical bovine femur. Combining the PALS results with simulated pore size distribution (PSD) results from collagen molecule and microfibril structure, we identify pores with diameter of 0.6 nm that suggest porosity within the collagen molecule regardless of the presence of mineral and water. We find that water occupies three larger domain size regions with nominal mean diameters of 1.1, 1.9, and 4.0 nm-spaces that are hypothesized to associate with intercollagen molecular spaces, terminal segments (d-spacing) within collagen microfibrils, and interface spacing between collagen and mineral structure, respectively. Subsequent removal of the organic matrix determines a structural pore size of 5-6 nm for deproteinized bone-suggesting the average spacing between mineral lamella. An independent method to deduce the average mineral spacing from specific surface area (SSA) measurements of the deproteinized sample is presented and compared with the PALS results. Together, the combined PALS and SSA results set a range on the mean mineral lamella thickness of 4-8 nm.

Exceptional hydrogen storage achieved by screening nearly half a million metal-organic frameworks.

Few hydrogen adsorbents balance high usable volumetric and gravimetric capacities. Although metal-organic frameworks (MOFs) have recently demonstrated progress in closing this gap, the large number of MOFs has hindered the identification of optimal materials. Here, a systematic assessment of published databases of real and hypothetical MOFs is presented. Nearly 500,000 compounds were screened computationally, and the most promising were assessed experimentally. Three MOFs with capacities surpassing that of IRMOF-20, the record-holder for balanced hydrogen capacity, are demonstrated: SNU-70, UMCM-9, and PCN-610/NU-100. Analysis of trends reveals the existence of a volumetric ceiling at ∼40 g H2 L-1. Surpassing this ceiling is proposed as a new capacity target for hydrogen adsorbents. Counter to earlier studies of total hydrogen uptake in MOFs, usable capacities in the highest-capacity materials are negatively correlated with density and volumetric surface area. Instead, capacity is maximized by increasing gravimetric surface area and porosity. This suggests that property/performance trends for total capacities may not translate to usable capacities.

Structure activity relationships in metal-organic framework catalysts for the continuous flow synthesis of propylene carbonate from CO2 and propylene oxide.

This paper describes the systematic study of metal-organic framework (MOF) catalysts for the reaction of propylene oxide (PO) with carbon dioxide (CO2) to generate propylene carbonate (PC). These studies began with the evaluation of MIL-101(Cr) as catalyst in a flow reactor. Under the developed flow conditions, MIL-101(Cr) was found to effectively catalyze PO carbonation in the absence of a halide co-catalyst. A systematic study of catalyst performance was then undertaken as a function of MOF synthesis technique, activation conditions, metal center, and node architecture. Ultimately, these investigations led to the identification of MIL-100(Sc) as a new, active, and stable catalyst for PO carbonation.

Core-Shell Structures Arise Naturally During Ligand Exchange in Metal-Organic Frameworks.

Examination of the microstructure of metal-organic frameworks (MOFs) after postsynthetic exchange (PSE) reveals that the exchanged ligand is concentrated at the edges of the crystal and decreases in concentration with crystal depth, resulting in a core-shell arrangement. Diffusion studies of the carboxylate ligand into MOF-5 indicate that diffusion is limiting to the exchange process and may ultimately be responsible for the observed core-shell structure. Examination of PSE in UMCM-8 and single crystals of UiO-66 shows a similar trend, illustrating the applicability of PSE as a method for the creation of core-shell MOFs.

Rapid Guest Exchange and Ultra-Low Surface Tension Solvents Optimize Metal-Organic Framework Activation.

Exploratory research into the critical steps in metal-organic framework (MOF) activation involving solvent exchange and solvent evacuation are reported. It is discovered that solvent exchange kinetics are extremely fast, and minutes rather days are appropriate for solvent exchange in many MOFs. It is also demonstrated that choice of a very low surface tension solvent is critical in successfully activating challenging MOFs. MOFs that have failed to be activated previously can achieve predicted surface areas provided that lower surface tension solvents, such as n-hexane and perfluoropentane, are applied. The insights herein aid in the efficient activation of MOFs in both laboratory and industrial settings and provide best practices for avoiding structural collapse.

Purification of Chloromethane by Selective Adsorption of Dimethyl Ether on Microporous Coordination Polymers.

The use of microporous coordination polymers with coordinatively unsaturated metal centers in the removal of dimethyl ether from chloromethane via flow was explored as an alternative to current industrial methods. Three different materials examined, Co/DOBDC, MIL-100(Fe), and HKUST-1, were able to efficiently purify chloromethane through selective adsorption of dimethyl ether. The recyclability of Co/DOBDC after separation was also examined, and little loss of capacity for dimethyl ether over the course of multiple regeneration cycles was observed.

A Perylene-Based Microporous Coordination Polymer Interacts Selectively with Electron-Poor Aromatics.

The design, synthesis, and properties of the new microporous coordination polymer UMCM-310 are described. The unique electronic character of the perylene-based linker enables selective interaction with electron-poor aromatics leading to efficient separation of nitroaromatics. UMCM-310 possesses high surface area and large pore size and thus permits the separation of large organic molecules based on adsorption rather than size exclusion.

A non-regular layer arrangement of a pillared-layer coordination polymer: avoiding interpenetration via symmetry breaking at nodes.

A coordination terpolymerization strategy is introduced to alter the connectivity within layers of a pillared-layer coordination polymer. Assembling two different dicarboxylate linkers around a metal cluster in the layer suppresses interpenetration while enabling formation of a rectangular 2D grid structure.

Polymer@MOF@MOF: "grafting from" atom transfer radical polymerization for the synthesis of hybrid porous solids.

The application of a core-shell architecture allows the formation of a polymer-coated metal-organic framework (MOF) maintaining high surface area (2289-2857 m(2) g(-1)). The growth of a MOF shell from a MOF core was used to spatially localize initiators by post-synthetic modification. The confinement of initiators ensures that polymerization is restricted to the outer shell of the MOF.

The Role of Modulators in Controlling Layer Spacings in a Tritopic Linker Based Zirconium 2D Microporous Coordination Polymer.

A 2D zirconium-based microporous coordination polymer derived from the tritopic linker 1,3,5-(4-carboxylphenyl)benzene, UMCM-309a, has been synthesized. This noninterpenetrated material possesses a Zr6(µ3-O)4(µ3-OH)4(RCO2)6(OH)6(H2O)6 cluster with six hexagonal-planar-coordinated linkers. UMCM-309a is stable in an aqueous HCl solution for over 4 months. The use of different monocarboxylates as modulators leads to similar 2D structures with different layer spacings; moreover, removal of the modulator yields the parent UMCM-309a.

Porous solids arising from synergistic and competing modes of assembly: combining coordination chemistry and covalent bond formation.

Design and synthesis of porous solids employing both reversible coordination chemistry and reversible covalent bond formation is described. The combination of two different linkage modes in a single material presents a link between two distinct classes of porous materials as exemplified by metal-organic frameworks (MOFs) and covalent organic frameworks (COFs). This strategy, in addition to being a compelling material-discovery method, also offers a platform for developing a fundamental understanding of the factors influencing the competing modes of assembly. We also demonstrate that even temporary formation of reversible connections between components may be leveraged to make new phases thus offering design routes to polymorphic frameworks. Moreover, this approach has the striking potential of providing a rich landscape of structurally complex materials from commercially available or readily accessible feedstocks.

Water sensitivity in Zn4O-based MOFs is structure and history dependent.

Moisture can cause irreversible structural collapse in metal-organic frameworks (MOFs) resulting in decreased internal surface areas and pore volumes. The details of such structural collapse with regard to pore size evolution during degradation are currently unknown due to a lack of suitable in situ probes of porosity. Here we acquire MOF porosity data under dynamic conditions by incorporating a flow-through system in tandem with positronium annihilation lifetime spectroscopy (PALS). From the decrease in porosity, we have observed an induction period for water degradation of some Zn4O-based MOFs that signals much greater stability than commonly believed to be possible. The sigmoidal trend in the degradation curve of unfunctionalized MOFs caused by water vapor has been established from the temporal component of pore size evolution as characterized by in situ PALS. IRMOF-3 is found to degrade at a lower relative humidity than MOF-5, a likely consequence of the amine groups in the structure, although, in contrast to MOF-5, residual porosity remains. The presence of an induction period, which itself depends on previous water exposure of the sample (history dependence), and sigmoidal temporal behavior of the moisture-induced degradation mechanism of MOFs was also verified using powder X-ray diffraction analysis and ex situ gas adsorption measurements. Our work provides insight into porosity evolution under application-relevant conditions as well as identifying chemical and structural characteristics influencing stability.

Filling pore space in a microporous coordination polymer to improve methane storage performance.

A strategy that allows the tuning of pore size in microporous coordination polymers (MCPs) through modification of their organic linkers is presented. When large substituents are introduced onto the linker, these pendent groups partially occupy the pores, thus reducing pore size while serving as additional adsorption sites for gases. The approach takes advantage of the fact that, for methane storage materials, small pores (0.4-0.8 nm in diameter) are more desirable than large pores since small pores promote optimal volumetric capacity. This method was demonstrated with IRMOF-8, a MCP constructed from Zn4O metal clusters and 2,6-naphthalenedicarboxylate (NDC) linkers. The NDC was functionalized through the addition of substituents including tert-butylethynyl or phenylethynyl groups. High pressure methane uptake demonstrates that the IRMOF-8 derivatives have significantly better performance than the unfunctionalized material in terms of both excess volumetric uptake and deliverable capacity. Moreover, IRMOF-8 derivatives also give rise to stronger interactions with methane molecules as shown by higher heat of adsorption values.

Shear-Triggered Crystallization and Light Emission of a Thermally Stable Organic Supercooled Liquid.

Thermodynamics drive crystalline organic molecules to be crystallized at temperatures below their melting point. Even though molecules can form supercooled liquids by rapid cooling, crystalline organic materials readily undergo a phase transformation to an energetically favorable crystalline phase upon subsequent heat treatment. Opposite to this general observation, here, we report molecular design of thermally stable supercooled liquid of diketopyrrolopyrrole (DPP) derivatives and their intriguing shear-triggered crystallization with dramatic optical property changes. Molten DPP8, one of the DPP derivatives, remains as stable supercooled liquid without crystallization through subsequent thermal cycles. More interestingly, under shear conditions, this supercooled liquid DPP8 transforms to its crystal phase accompanied by a 25-fold increase in photoluminescence (PL) quantum efficiency and a color change. By systematic investigation on supercooled liquid formation of crystalline DPP derivatives and their correlation with chemical structures, we reveal that the origin of this thermally stable supercooled liquid is a subtle force balance between aromatic interactions among the core units and van der Waals interactions among the aliphatic side chains acting in opposite directions. Moreover, by applying shear force to a supercooled liquid DPP8 film at different temperatures, we demonstrated direct writing of fluorescent patterns and propagating fluorescence amplification, respectively. Shear-triggered crystallization of DPP8 is further achieved even by living cell attachment and spreading, demonstrating the high sensitivity of the shear-triggered crystallization which is about 6 orders of magnitude more sensitive than typical mechanochromism observed in organic materials.

Microporous coordination polymers as efficient sorbents for air dehumidification.

Air drying is a widespread and critical industrial process. Removal of water from air is commonly accomplished by passage through a desiccant such as alumina; modest water capacity and energy intensive regeneration are limitations of currently used sorbents. Microporous coordination polymers (MCPs) are demonstrated here to be efficient desiccants for the dehumidification of air, and a comparison of their capacity, regenerability, and efficiency with commercial activated alumina is conducted. Complete regeneration using dry air with mild heating is achieved. The attainment of high capacity for the adsorption of water coupled to facile regeneration indicates that gas dehumidification may be an important application for MCPs.

Heterogenization of homogeneous catalysts in metal-organic frameworks via cation exchange.

This paper describes the heterogenization of single-site transition-metal catalysts in metal-organic frameworks (MOFs) via cation exchange. A variety of cationic complexes of Pd, Fe, Ir, Rh, and Ru have been incorporated into ZJU-28, and the new materials have been characterized by optical microscopy, inductively coupled plasma optical emission spectroscopy, and powder X-ray diffraction. MOF-supported [Rh(dppe)(COD)]BF4 catalyzes the hydrogenation of 1-octene to n-octane. The activity of this supported catalyst compares favorably to its homogeneous counterpart, and it can be recycled at least four times. Overall, this work provides a new and general approach for supporting transition-metal catalysts in MOFs.

Interpenetration, porosity, and high-pressure gas adsorption in Zn4O(2,6-naphthalene dicarboxylate)3.

Microporous coordination polymers (MCPs) have emerged as strong contenders for adsorption-based fuel storage and delivery in large part because of their high specific surface areas. The strategy of increasing surface area by increasing organic linker length has shown only sporadic success; as demonstrated by many members of the iconic Zn4O-based IRMOF series, for example, accessible porosity is often limited by interpenetration or pore collapse upon guest removal. In this work, we focus on Zn4O(ndc)3 (IRMOF-8, ndc = 2,6-naphthalene dicarboxylate), which exhibits typical surface areas of only 1000-2000 m(2)/g even though a surface area of more than 4000 m(2)/g is expected from geometric analysis of the originally reported crystal structure. We recently showed that a high surface area could be produced with zinc and ndc by room-temperature synthesis followed by activation with flowing supercritical CO2. In this work, we investigate in detail the porosity of both the low- and high-surface-area materials. Positron annihilation lifetime spectroscopy (PALS) is used to show that the low-surface-area material suffers from near-complete interpenetration, explaining why traditional synthetic routes have failed to yield materials with the expected porosity. Furthermore, the high-pressure hydrogen and methane sorption properties of noninterpenetrated Zn4O(ndc)3 are examined, and PALS is used to show that pore filling is not operative during room-temperature CH4 sorption even at pressures approaching 100 bar. These results provide insight into how gas adsorbs in high-surface-area materials at high pressure and reinforce previous contentions that increasing surface area alone is not sufficient for the simultaneous optimization of deliverable gravimetric and volumetric gas uptake in MCPs.

Evidence of Positronium Bloch states in porous crystals of Zn4O-coordination polymers.

Positronium (Ps) is shown to exist in a delocalized state in self-assembled metalorganic crystals that have large 1.3-1.5 nm cell sizes. Belonging to a class of materials with record high accessible specific surface areas, these highly porous crystals are the first to allow direct probing with simple annihilation lifetime techniques of the transport properties of long-lived triplet Ps in what is hypothesized to be a Bloch state. Delocalized Ps has unprecedented (high) Ps mobility driven primarily by weak phonon scattering with unusual and profound consequences on how Ps probes the lattice.

Rapid and enhanced activation of microporous coordination polymers by flowing supercritical CO2.

Flowing supercritical CO(2) was used to activate a cross section of microporous coordination polymers (MCPs) directly from DMF, thus avoiding exchange with a volatile solvent. Most MCPs displayed exceptional surface areas directly after treatment although those with coordinatively unsaturated metals benefit from heating. The method presents an advance in efficiency of activation and quality of material obtained.

Non-interpenetrated IRMOF-8: synthesis, activation, and gas sorption.

The synthesis and successful activation of IRMOF-8 (Zn(4)O(ndc)(3), ndc = naphthalene-2,6-dicarboxylate) is presented. Room temperature synthesis effectively suppresses interpenetration. Although conventional activation under reduced pressure leads to structural collapse, activation by flowing supercritical CO(2) yields a guest-free material with a BET surface area of 4461 m(2) g(-1).