Metal−Organic Frameworks Impregnated with Magnesium-Decorated Fullerenes for Methane and Hydrogen Storage

A new concept is described for methane and hydrogen storage materials involving the incorporation of magnesium-decorated fullerenes within metal−organic frameworks (MOFs). The system is modeled using a novel approach underpinned by surface potential energies developed from Lennard-Jones parameters. Impregnation of MOF pores with magnesium-decorated Mg10C60 fullerenes, denoted as Mg−C60@MOF, places exposed metal sites with high heats of gas adsorption into intimate contact with large surface area MOF structures. Perhaps surprisingly, given the void space occupied by C60, this impregnation delivers remarkable gas uptake, according to our modeling, which predicts exceptional performance for the Mg−C60@MOF family of materials. These predictions include a volumetric methane uptake of 265 v/v, the highest reported value for any material, which significantly exceeds the U.S. Department of Energy target of 180 v/v. We also predict a very high hydrogen adsorption enthalpy of 11 kJ mol−1 with relatively little decrease as a function of H2 filling. This value is close to the calculated optimum value of 15.1 kJ mol−1 and is achieved concurrently with saturation hydrogen uptake in large amounts at pressures under 10 atm

Photorearrangements of Five 1- and 2-Naphthyl Acylates in Three Unstretched and Stretched Polyethylene Films. Does Reaction Selectivity Correlate with Free Volumes Measured by Positron Annihilation Lifetime Spectroscopy?

The selectivity of photorearrangements of five 1-naphthyl and 2-naphthyl acylates has been investigated in three unstretched and stretched polyethylene films of different crystallinities and in isotropic solutions. The influence of variables such as size and position of the aryl groups of the esters, degree of crystallinity, free volume, and unstretched/stretched state of the films has been explored. The orthopositronium accessible free volume sites in the undoped unstretched and stretched polyethylenes have been measured by positron annihilation lifetime spectroscopy. These void free volumes are much smaller than the van der Waals volumes of the naphthyl molecules under investigation. The naphthyl esters inside polyethylene cavities act as templates for the formation of their photoproducts. Long alkyl chains on naphthyl myristates not only affect the shape anisotropy but also induce large van der Waals interactions with the walls of reaction cavities. Stretching enhances the templating effect and strengthens the van der Waals attractions with cavity walls in the case of naphthyl myristates, thus inducing marked increases in reaction selectivities in polyethylene films. Somewhat surprisingly, there is no correlation between void free volume of a host polyethylene film and selectivity of photoreactions of a guest naphthyl ester

Fast Synthesis of MOF-5 Microcrystals Using Sol−Gel SiO₂ Nanoparticles

The work reports on the synthesis of an archetypal metal organic framework (MOF-5) microcrystals with a narrow size distribution using SiO2 nanoparticles with tailored surface chemistry as nucleating agents. The nanoparticles boost the reaction rate by up to an order of magnitude compared to the conventional MOF-5 solvothermal synthesis and can be successfully used as nucleation seeds for the selective growth of MOF-5 on specific substrates. These results are important fundamental advances toward the controlled scale-up of MOF synthesis and directed MOF growth on suitable supports

Determination of Initial and Long-Term Microstructure Changes in Ultrahigh Molecular Weight Polyethylene Induced by Drawing Neat and Pyrenyl Modified Films

Deformation processes in gel-crystallized ultrahigh molecular weight polyethylene (UHMWPE) films with draw ratios (DR) as high as 96 have been investigated by X-ray diffraction (XRD), differential scanning calorimetry (DSC), and positron annihilation lifetime spectroscopy (PALS). In addition, low concentrations of pyrene molecules have been introduced at the time of film preparation from the gels or afterward by sorption after film preparation, and the polarization of their electronic absorption and fluorescence spectra at different draw ratios has been measured over a large temperature range extending to below the glass transition. The pyrene-doped films have been irradiated to introduce covalently attached 1-pyrenyl groups, and these films at two draw ratios have been employed to investigate over large temperature ranges (1) the steady-state fluorescence intensity and (2) the rates of diffusion of N,N-dimethylaniline (DMA). These data have been correlated with the XRD, DSC, and PALS information obtained on the unmodified films. On the basis of analyses of this body of information, a novel deformation model that explains the decreased crystallinity and increased mean free volumes in gel-crystallized UHMWPE at low draw ratios is proposed. It involves “stretch” and “flip” motions of microfibrils present in the undrawn films. The high crystallinity content and stiffer chains due to drawing UHMWPE films result in weak α- and β-relaxation processes, slower diffusion of DMA than in undrawn films, and orientation factors for doped pyrene molecules that are constant over a large temperature range. The overall picture that emerges allows several aspects of the morphology of UHMWPE, a polymer of fundamental importance in materials research, to be understood

Complete Characterization of α-Hopeite Microparticles: An Ideal Nucleation Seed for Metal Organic Frameworks

This work reports on the structural and microstructural characterization of a new class of α-hopeite microparticles, which has recently been discovered as ideal seeding agents for the formation and functionalization of metal organic framework (MOF-5) crystals. The particles have been named desert rose microparticles (DRMs), as their morphology closely resembles that of the famous gypsum and Barite mineral. The DRMs form directly inside the MOF-5 precursor solution when a block copolymer surfactant, Pluronic F-127, is added in specific amounts. The particles formation is remarkably fast, and particles are observed to form within the first minute of reaction. The DRMs formation and growth has been monitored along a 3 h synthesis, until the first nuclei of MOF-5 start to appear on their surface. Electron microscopy, energy dispersive analysis, electron diffraction, FTIR, FT-Raman, and BET give an all-around description of the chemical and morphological features that give the DRMs their remarkable MOF-seeding capacity

In Situ Crystallization of Macroporous Monoliths with Hollow NaP Zeolite Structure

Macroporous NaP zeolite monoliths (M-ZPMs) with designed shapes such as cylinder, rectangular-prism, and donut shapes were synthesized via gelcasting of the aged zeolite gel with colloidal silica as a binder and subsequent vapor-phase-transport (VPT) synthesis. X-ray diffraction (XRD), scanning electron microscopy (SEM), mercury porosimetry, and nitrogen gas adsorption were used to characterize the samples at different synthesis stages. SEM images revealed that the resulting macroporous NaP zeolite monoliths were composed of interconnected hollow particles. Mercury porosimetry showed that the macroporous NaP zeolite monoliths possessed bimodal macropore size distributions involving the textural macropores (ca. 100 μm) and skeletal macropores (ca. 3.5 μm). Furthermore, colloidal silica played a crucial role in the formation of robust macroporous NaP zeolite monoliths. Specifically, as the mass loading of colloidal silica in the zeolite-gel monoliths increases, the mechanical strength and Si/Al ratio of the resulting zeolitic monoliths increased; dispersible and individual hollow NaP zeolite particles with lower Si/Al ratios were produced in the absence of colloidal silica. The formation mechanisms of the hollow NaP zeolites in the VPT synthesis process were discussed. The incorporation of functional magnetic Fe3O4 in M-ZPMs was finally presented

Strategies toward Enhanced Low-Pressure Volumetric Hydrogen Storage in Nanoporous Cryoadsorbents

The volumetric hydrogen capacity remains one of the most challenging criteria for on-board hydrogen storage system requirements. Here a new concept for hydrogen storage of porous aromatic frameworks (PAFs) impregnated with lithium-decorated fullerenes (Li₆C₆₀) is described. The loading of Li₆C₆₀ and the effect on the adsorption of hydrogen (H₂) has been investigated by molecular simulation. It is shown that the incorporation of Li₆C₆₀ can enhance the volumetric capacity of H₂ from 12 to 44 g L^–1, a 260% increase at 10 bar and 77 K. The impregnation of Li₆C₆₀ increases the heat of adsorption and surface area at the cost of the available pore volume. However, the increase in adsorbed hydrogen outweighs any pore volume loss under optimized Li₆C₆₀ loading and operating conditions. In addition, the H₂ volumetric uptake is shown to correlate with the volumetric surface area at all pressures whereas the H₂ gravimetric uptake correlates with the heat of adsorption at low pressures, surface area at moderate pressures, and pore volume at high pressures

Finely Tuning the Free Volume Architecture in Iptycene-Containing Polyimides for Highly Selective and Fast Hydrogen Transport

Iptycene-based polyimides have attracted extensive attention recently in the membrane gas separation field due to their unique structural hierarchy and chemical characteristics that enable construction of well-defined yet tailorable free volume architecture for fast and selective molecular transport. We report here a new series of iptycene-based polyimides that are exquisitely tuned in the monomer structure to afford preferred microcavity architecture for hydrogen transport. In particular, a triptycene-containing dianhydride (TPDAn) was prepared to react with two iptycene-containing diamines (i.e., TPDAm and PPDAm) or 2,2′-bis­(3-amino-4-hydroxy­phenyl)­hexa­fluoropropane (6FAP) to produce entirely or partially iptycene-based polyimides. The incorporation of iptycene units effectively disrupted chain packing, which resulted in ultrafine microporosity in the membranes with a desired bimodal size distribution with maxima at ∼3 and ∼7 Å, respectively. Depending on the combination of diamine and dianhydride, the microporosity was feasibly tuned and optimized to meet the needs of challenging H₂ separations, especially for H₂/N₂ and H₂/CH₄ gas pairs. Particularly, a H₂ permeability of 27 barrers and H₂/N₂ and H₂/CH₄ selectivities of 142 and 300, respectively, were obtained for TPDAn-6FAP

Tailoring Physical Aging in Super Glassy Polymers with Functionalized Porous Aromatic Frameworks for CO₂ Capture

A series of chemically functionalized porous aromatic frameworks (PAFs) have been synthesized and deployed within mixed-matrix membranes for gas separation. This series of PAFs delivered for the first time simultaneous control of selective gas transport and physical aging within the membranes. New composites including native and metalated fullerenes were also prepared, and the composites exhibited exceptional increases in their porosity, which in turn resulted in ultrafast gas transport. CO₂ permeability following PAF-1-Li₆C₆₀ infusion within poly­(trimethylsilylpropyne) was as high as 50 600 Barrer, a 70% improvement. Remarkably, just 9% of this permeation rate diminished after 1 year of physical aging, compared to 74% in the native polymer. A series of characterization techniques revealed this phenomenon to be due to intercalation of polymer chains within the PAF pores, the strength of which is controlled by the levels of chemical functionalization and porosity. The membranes were exploited for gas separations, in particular the stripping of CO₂ from natural gas

Unexpectedly Strong Size-Sieving Ability in Carbonized Polybenzimidazole for Membrane H₂/CO₂ Separation

Polymers with high permeability and strong size-sieving ability are needed for H2/CO2 separation at temperatures ranging from 100 to 300 °C to enable an energy-efficient precombustion CO2 capture process. However, such polymers usually suffer from a permeability/selectivity tradeoff, that is, polymers with high permeability tend to exhibit a weak size-sieving ability and thus low selectivity. Herein, we demonstrate that carbonization of a suitable polymer precursor (i.e., polybenzimidazole or PBI) generates microcavities (leading to high H2 permeability) and ultramicroporous channels (leading to strong size-sieving ability and thus high H2/CO2 selectivity). Specifically, carbonization of PBI at 900 °C (CMS@900) doubles H2 permeability and increases H2/CO2 selectivity from 14 to 80 at 150 °C. When tested with simulated syngas-containing equimolar H2 and CO2 in the presence of water vapor for 120 h, CMS@900 exhibits stable H2 permeability of ≈36 barrer and H2/CO2 selectivity of ≈53 at 150 °C, above Robeson’s 2008 upper bound and demonstrating robustness against physical aging and CO2 plasticization