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Determination of aqueous phase diffusion coefficients of solutes through porous media is essential for understanding and modeling contaminant transport. Prediction of diffusion coefficients in both saturated and unsaturated zones requires knowledge of tortuosity and constrictivity factors. No methods are available for the direct measurement of these factors, which are empirical in their definition. In this paper, a new definition for the tortuosity factor is proposed, as the real to ideal interfacial area ratio. We define the tortuosity factor for saturated porous media ([τ.sub.s]) as the ratio S/[S.sub.o] (specific surface of real porous medium to that of an idealized capillary bundle). For unsaturated media, tortuosity factor ([τ.sub.a]) is defined as [a.sub.aw]/[a.sub.aw,o] (ratio of the specific air-water interfacial area of real and the corresponding idealized porous medium). This tortuosity factor is suitably measured using sorptive tracers (e.g., nitrogen adsorption method) for saturated media and interfacial tracers for unsaturated media. A model based on this new definition of tortuosity factors, termed the interfacial area ratio (IAR) model, is presented for the prediction of diffusion coefficients as a function of the degree of water saturation. Diffusion coefficients and diffusive resistances measured in a number of saturated and unsaturated granular porous media, for solutes in dilute aqueous solutions, agree well with the predictions of the IAR model. A comparison of permeability of saturated sands estimated based on [τ.sub.s] and the same based on the Kozeny-Carman equation confirm the usefulness of the [τ.sub.s] parameter as a measure of tortuosity., Introduction In the absence of significant advective flux, diffusion is an important mechanism for the transport of solutes through the saturated and vadose zones and through engineered porous materials, such [...]

Adsorption‐based cooling is an energy‐efficient renewable‐energy technology that can be driven using low‐grade industrial waste heat and/or solar heat. Here, we report the first exploration of fluorocarbon adsorption using porous covalent organic polymers (COPs) for this cooling application. High fluorocarbon R134a equilibrium capacities and unique overall linear‐shaped isotherms are revealed for the materials, namely COP‐2 and COP‐3. The key role of mesoporous defects on this unusual adsorption behavior was demonstrated by molecular simulations based on atomistic defect‐containing models built for both porous COPs. Analysis of simulated R134a adsorption isotherms for various defect‐containing atomistic models of the COPs shows a direct correlation between higher fluorocarbon adsorption capacities and increasing pore volumes induced by defects. Combined with their high porosities, excellent reversibility, fast kinetics, and large operating window, these defect‐containing porous COPs are promising for adsorption‐based cooling applications. [ABSTRACT FROM AUTHOR]

The interactions of CO2/H2 with p-tert-butylcalix[4]arene (TBC4) were studied using potential of mean force (PMF) and free energy perturbation approaches. To the best of our knowledge, the present work is one of the first to employ the constrained mean force approach to evaluate solute selectivity by the TBC4 molecule. The computed PMFs for the interaction of CO2/H2 with a single TBC4 molecule establish that the interaction of CO2 with the open end of the cage structure is attractive while the interaction with H2 is repulsive. Free energy perturbation calculations were performed for the same two guest molecules with a pair of facing TBC4 molecules used as a representative model as found in the TBC4 molecular solid. At low temperature, both CO2/H2 have favorable interactions with the TBC4 pair, with the CO2 interaction being considerably greater. These results are in agreement with recent experimental data showing considerable CO2 uptake by TBC4 at moderate pressures. [ABSTRACT FROM AUTHOR]

Magnetic core-shell microspheres were developed to extract rare earth elements (REEs) from aqueous and brine solutions with up to 99.99% removal efficiency. The shell, composed of a thermally and chemically stable functionalized metal-organic framework (MOF), is grown over a synthesized Fe3O4 magnetic core (magnetite@MOF). The composite particles can be removed from the mixture under an applied magnetic field, offering a practical, and efficient REE-removal process. [ABSTRACT FROM AUTHOR]

The oxygen electrode plays a vital role in the successful commercialization of renewable energy technologies, such as fuel cells and water electrolyzers. In this study, the Prussian blue analogue-derived nitrogen-doped nanocarbon (NC) layer-trapped, cobalt-rich, core-shell nanostructured electrocatalysts (core-shell Co@NC) are reported. The electrode exhibits an improved oxygen evolution activity and stability compared to that of the commercial noble electrodes. The core-shell Co@NC-loaded nickel foam exhibits a lower overpotential of 330 mV than that of IrO2 on nickel foam at 10 mA cm-2 and has a durability of over 400 h. The commercial Pt/C cathode-assisted, core-shell Co@NC-anode water electrolyzer delivers 10 mA cm-2 at a cell voltage of 1.59 V, which is 70 mV lower than that of the IrO2-anode water electrolyzer. Over the long-term chronopotentiometry durability testing, the IrO2-anode water electrolyzer shows a cell voltage loss of 230 mV (14%) at 95 h, but the loss of the core-shell Co@NC-anode electrolyzer is only 60 mV (4%) even after 350 h cell-operation. The findings indicate that the Prussian blue analogue is a class of inorganic nanoporous materials that can be used to derive metal-rich, core-shell electrocatalysts with enriched active centers. [ABSTRACT FROM AUTHOR]

The mixed-metal strategy, by which a MOF framework with two different metal nodes was prepared, was utilized with MOF-74 for potential adsorption chiller applications. The Ni salt precursor was partially replaced during synthesis with the relatively inexpensive Zn salt. The bimetallic MOF-74 containing only a fraction of Ni was shown to provide efficient H2O adsorption performance, while utilizing the more inexpensive Zn salt thus significantly mitigated the material costs of industrial scale synthesis/applications. [ABSTRACT FROM AUTHOR]

Metal-organic frameworks (MOFs) have recently attracted enormous interest over the past few years in energy storage and gas separation, yet there have been few reports for adsorption cooling applications. Adsorption cooling technology is an established alternative to mechanical vapor compression refrigeration systems and is an excellent alternative in industrial environments where waste heat is available. We explored the use of MOFs that have very high mass loading and relatively low heats of adsorption, with certain combinations of refrigerants to demonstrate a new type of highly efficient adsorption chiller. Computational fluid dynamics combined with a system level lumped-parameter model have been used to project size and performance for chillers with a cooling capacity ranging from a few kW to several thousand kW. These systems rely on stacked micro/mini-scale architectures to enhance heat and mass transfer. Recent computational studies of an adsorption chiller based on MOFs suggests that a thermally-driven coefficient of performance greater than one may be possible, which would represent a fundamental breakthrough in performance of adsorption chiller technology. Presented herein are computational and experimental results for hydrophyilic and fluorophilic MOFs. [ABSTRACT FROM PUBLISHER]

Magnesite (MgCO3) precipitation within the nanoconfined space of adsorbed H2O films (∼5 monolayers) was determined to have an apparent activation energy of only 36 ± 6 kJ mol−1, suggesting that Mg2+ under nanoconfinement adopts a hydration configuration that mimics that of aqueous Ca2+, at least energetically, if not also specifically in hydration structure. [ABSTRACT FROM AUTHOR]

Designing zeolites with tunable physicochemical properties can substantially impact their performance in commercial applications, such as adsorption, separations, catalysis, and drug delivery. Zeolite synthesis typically requires an organic structure-directing agent to produce crystals with specific pore topology. Attempts to remove organics from syntheses to achieve commercially viable methods of preparing zeolites often lead to the formation of impurities. Herein, we present organic-free syntheses of two polymorphs of the small-pore zeolite P (GIS), P1 and P2. Using a combination of adsorption measurements and density functional theory calculations, we show that GIS polymorphs are selective adsorbents for H2O relative to other light gases (e.g., H2, N2, CO2). Our findings refute prior theoretical studies postulating that GIS-type zeolites are excellent materials for CO2 separation/sequestration. We also show that P2 is significantly more thermally stable than P1, which broadens the operating conditions for GIS-type zeolites in commercial applications and opens new avenues for exploring their potential use in processes such as catalysis. [ABSTRACT FROM AUTHOR]

Metal–organic frameworks (MOFs) have proved to be very attractive for applications including gas storage, separation, sensing and catalysis. In particular, CO2 separation from flue gas in post-combustion processes is one of the main focuses of research among the scientific community. One of the major issues that are preventing the successful commercialization of these novel materials is their high affinity towards water that not only compromises gas sorption capacity but also the chemical stability. In this paper, we demonstrate a novel post-synthesis modification approach to modify MOFs towards increasing hydrophobic behaviour and chemical stability against moisture without compromising CO2 sorption capacity. Our approach consists of incorporating hydrophobic moieties on the external surface of the MOFs via physical adsorption. The rationale behind this concept is to increase the surface hydrophobicity in the porous materials without the need of introducing bulky functionalities inside the pore which compromises the sorption capacity toward other gases. We herein report preliminary results on routinely studied MOF materials [MIL-101(Cr) and NiDOBDC] demonstrating that the polymer-modified MOFs retain CO2 sorption capacity while reducing the water adsorption up to three times, with respect to the un-modified materials, via an equilibrium effect. Furthermore, the water stability of the polymer-functionalized MOFs is significantly higher than the water stability of the bare material. Molecular dynamic simulations demonstrated that this equilibrium effect implies a fundamental and permanent change in the water sorption capacity of MOFs. This approach can also be employed to render moisture stability and selectivity to MOFs that find applications in gas separations, catalysis and sensing where water plays a critical role in compromising MOF performance and recyclability. [ABSTRACT FROM AUTHOR]

The metal organic framework, MOF-74(Ni), was synthesized in a continuous flow microwave-assisted reactor obtaining a high space-time yield (~90 g h−1 L−1) and 96.5% conversion of reagents. Separation of the nucleation and growth steps was performed by using uniform and rapid microwave heating to induce nucleation, which allowed a substantial increase in conversion for shorter reaction times under mild pressure. High yields were achieved in minutes, as opposed to days for typical batch syntheses, with excellent control over the material's properties due to more uniform nucleation, and the separation of the nucleation and growth steps. Optimization of the microwave reactor parameters led to improvements in MOF-74(Ni) crystallinity, reagent conversion, and production rates. Differences in MOF-74(Ni) crystallinity were observed as smaller grains were formed when higher microwave zone temperatures were used. Crystallinity differences led to different final adsorption properties and surface areas. Herein we show that a continuous high space-time yield synthesis of MOF-74(Ni) allows control over nucleation using microwave heating. [ABSTRACT FROM AUTHOR]

A flexible metal–organic framework constructed from a flexible linker is shown to possess the capability of separating mixtures of polar compounds (propanol isomers) by exploiting the differences in the saturation capacities of the constituents. Transient breakthrough simulations show that these sorption-based separations are in favor of the component with higher saturation capacity. [ABSTRACT FROM AUTHOR]

Zeolite HOU-2 (LTA type) is prepared with the highest silica content (Si/Al = 2.1) reported for Na-LTA zeolites without the use of an organic structure-directing agent. The rational design of Si-rich zeolites has the potential to improve their thermal stability for applications in catalysis, gas storage, and selective separations. [ABSTRACT FROM AUTHOR]

A detailed mechanism, based on densityfunctional theory calculationsand simulation, is presented outlining the formation of the secondarybuilding unit (SBU) of MIL-101, a chromium terephthalate metal organicframework (MOF). Formation of the metal core and of the SBU is keyto MOF nucleation, the rate-limiting step in the synthesis processof many MOFs. A series of reactions that lead to the formation ofthe SBU of MIL-101 is proposed in this work. The highest barrier (∼35kcal/mol) involves the formation of a dimetal-linker intermediateand high to low spin transition as a third Cr-linker moiety joinsto form a three metal-linker group joined by a central oxygen. Theterephthalate linkers play an important, key mechanistic role withthe carboxylates first joining chromium atoms prior to the formationof bridging oxygens. Subsequent to metal core formation, stepwiselinker addition reactions generate different assembly pathways dueto structural isomers that are limited by the removal of water moleculesin the first chromium coordination shell. A simple kinetic model basedon transition state theory gave a rate of SBU formation similar toa reported rate of MOF nucleation. The least energy path was identifiedwith all linkers on the same face of the metal center added first.These first steps in developing a modeling framework for SBU formationwill hopefully lay the groundwork for future comprehensive predictivemodels of the full MOF framework structure assembly and synthesisconditions required to support the self-assembly process. [ABSTRACT FROM AUTHOR]

Crystalline metal organic framework (MOF) materials containing interconnected porosity can be chemically modified to promote stimulus-driven (light, magnetic or electric fields) structural transformations that can be used in a number of devices. Innovative research strategies are now focused on understanding the role of chemical bond manipulation to reversibly alter the free volume in such structures of critical importance for electro-catalysis, molecular electronics, energy storage technologies, sensor devices and smart membranes. In this letter, we study the mechanism for which an electrically switchable MOF composed of Cu(TCNQ) (TCNQ = 7,7,8,8-tetracyanoquinodimethane) transitions from a high-resistance state to a conducting state in a reversible fashion by an applied potential. The actual mechanism for this reversible electrical switching is still not understood even though a number of reports are available describing the application of electric-field-induced switching of Cu(TCNQ) in device fabrication. [ABSTRACT FROM AUTHOR]