This project was broadly motivated by the need for new hardware that can process information such as images and sounds right at the point of where the information is sensed (e.g. edge computing). The project was further motivated by recent discoveries by group demonstrating that while certain organic polymer blends can be used to fabricate elements of such hardware, the need to mix ionic and electronic conducting phases imposed limits on performance, dimensional scalability and the degree of fundamental understanding of how such devices operated. As an alternative to blended polymers containing distinct ionic and electronic conducting phases, in this LDRD project we have discovered that a family of mixed valence coordination compounds called Prussian blue analogue (PBAs), with an open framework structure and ability to conduct both ionic and electronic charge, can be used for inkjet-printed flexible artificial synapses that reversibly switch conductance by more than four orders of magnitude based on electrochemically tunable oxidation state. Retention of programmed states is improved by nearly two orders of magnitude compared to the extensively studied organic polymers, thus enabling in-memory compute and avoiding energy costly off-chip access during training. We demonstrate dopamine detection using PBA synapses and biocompatibility with living neurons, evoking prospective application for brain - computer interfacing. By application of electron transfer theory to in-situ spectroscopic probing of intervalence charge transfer, we elucidate a switching mechanism whereby the degree of mixed valency between N-coordinated Ru sites controls the carrier concentration and mobility, as supported by density functional theory (DFT) .
Hydrides based on magnesium and intermetallic compounds provide a viable solution to the challenge of energy storage from renewable sources, thanks to their ability to absorb and desorb hydrogen in a reversible way with a proper tuning of pressure and temperature conditions. Therefore, they are expected to play an important role in the clean energy transition and in the deployment of hydrogen as an efficient energy vector. This review, by experts of Task 40 'Energy Storage and Conversion based on Hydrogen' of the Hydrogen Technology Collaboration Programme of the International Energy Agency, reports on the latest activities of the working group 'Magnesium- and Intermetallic alloys-based Hydrides for Energy Storage'. The following topics are covered by the review: multiscale modelling of hydrides and hydrogen sorption mechanisms; synthesis and processing techniques; catalysts for hydrogen sorption in Mg; Mg-based nanostructures and new compounds; hydrides based on intermetallic TiFe alloys, high entropy alloys, Laves phases, and Pd-containing alloys. Finally, an outlook is presented on current worldwide investments and future research directions for hydrogen-based energy storage.
An intriguing new class of two-dimensional (2D) materials based on metal-organic frameworks (MOFs) has recently been developed that displays electrical conductivity, a rarity among these nanoporous materials. The emergence of conducting MOFs raises questions about their fundamental electronic properties, but few studies exist in this regard. Here, we present an integrated theory and experimental investigation to probe the effects of metal substitution on the charge transport properties of M-HITP, where M = Ni or Pt and HITP = 2,3,6,7,10,11-hexaiminotriphenylene. The results show that the identity of the M-HITP majority charge carrier can be changed without intentional introduction of electronically active dopants. We observe that the selection of the metal ion substantially affects charge transport. Using the known structure, Ni-HITP, we synthesized a new amorphous material, a-Pt-HITP, which although amorphous is nevertheless found to be porous upon desolvation. Importantly, this new material exhibits p-type charge transport behavior, unlike Ni-HITP, which displays n-type charge transport. These results demonstrate that both p- and n-type materials can be achieved within the same MOF topology through appropriate choice of the metal ion.
Porous nanoscale carbonaceous materials are widely employed for catalysis, separations, and electrochemical devices where device performance often relies upon specific and well-defined regular feature sizes. The use of block polymers as templates has enabled affordable and scalable production of diverse porous carbons. However, popular carbon preparations use equilibrating micelles which can change dimensions in response to the processing environment. Thus, polymer methods have not yet demonstrated carbon nanomaterials with constant average template diameter and tailored wall thickness. In contrast, persistent micelle templates (PMTs) use kinetic control to preserve constant micelle template diameters, and thus PMT has enabled constant pore diameter metrics. With PMT, the wall thickness is independently adjustable via the amount of material precursor added to the micelle templates. Previous PMT demonstrations relied upon thermodynamic barriers to inhibit chain exchange while in solution, followed by rapid evaporation and cross-linking of material precursors to mitigate micelle reorganization once the solvent evaporated. It is shown here that this approach, however, fails to deliver kinetic micelle control when used with slowly cross-linking material precursors such as those for porous carbons. A new modality for kinetic control over micelle templates, glassy-PMTs, is shown using an immobilized glassy micelle core composed of polystyrene (PS). Although PS based polymers have been used to template carbon materials before, all prior reports included plasticizers that prevented kinetic micelle control. Here the key synthetic conditions for carbon materials with glassy-PMT control are enumerated, including dependencies upon polymer block selection, block molecular mass, solvent selection, and micelle processing timeline. The use of glassy-PMTs also enables the direct observation of micelle cores by TEM which are shown to be commensurate with template dimensions. Glassy-PMTs are thus robust and insensitive to material processing kinetics, broadly enabling tailored nanomaterials with diverse chemistries.
Liquid organic hydrogen carriers such as alcohols and polyols are a high-capacity means of transporting and reversibly storing hydrogen that demands effective catalysts to drive the (de)hydrogenation reactions under mild conditions. We employed a combined theory/experiment approach to develop MOF-74 catalysts for alcohol dehydrogenation and examine the performance of the open metal sites (OMS), which have properties analogous to the active sites in high-performance single-site catalysts and homogeneous catalysts. Methanol dehydrogenation was used as a model reaction system for assessing the performance of five monometallic M-MOF-74 variants (M = Co, Cu, Mg, Mn, Ni). Co-MOF-74 and Ni-MOF-74 give the highest H2 productivity. However, Ni-MOF-74 is unstable under reaction conditions and forms metallic nickel particles. To improve catalyst activity and stability, bimetallic (NixMg1-x)-MOF-74 catalysts were developed that stabilize the Ni OMS and promote the dehydrogenation reaction. An optimal composition exists at (Ni0.32Mg0.68)-MOF-74 that gives the greatest H2 productivity, up to 203 mL gcat-1 min-1 at 300 °C, and maintains 100% selectivity to CO and H2 between 225-275 °C. The optimized catalyst is also active for the dehydrogenation of other alcohols. DFT calculations reveal that synergistic interactions between the open metal site and the organic linker lead to lower reaction barriers in the MOF catalysts compared to the open metal site alone. This work expands the suite of hydrogen-related reactions catalyzed by MOF-74 which includes recent work on hydroformulation and our earlier reports of aryl-ether hydrogenolysis. Moreover, it highlights the use of bimetallic frameworks as an effective strategy for stabilizing a high density of catalytically active open metal sites. This journal is
Understanding the fundamental limits of gas deliverable capacity in porous materials is of critical importance as it informs whether technical targets (e.g., for on-board vehicular storage) are feasible. High-throughput screening studies of rigid materials, for example, have shown they are not able to achieve the original ARPA-E methane storage targets, yet an interesting question remains: what is the upper limit of deliverable capacity in flexible materials? In this work we develop a statistical adsorption model that specifically probes the limit of deliverable capacity in intrinsically flexible materials. The resulting adsorption thermodynamics indicate that a perfectly designed, intrinsically flexible nanoporous material could achieve higher methane deliverable capacity than the best benchmark systems known to date with little to no total volume change. Density functional theory and grand canonical Monte Carlo simulations identify a known metal-organic framework (MOF) that validates key features of the model. Therefore, this work (1) motivates a continued, extensive effort to rationally design a porous material analogous to the adsorption model and (2) calls for continued discovery of additional high deliverable capacity materials that remain hidden from rigid structure screening studies due to nominal non-porosity.
The thermodynamic stability and melting point of magnesium borohydride were probed under hydrogen pressures up to 1000 bar (100 MPa) and temperatures up to 400 °C. At 400 °C, Mg(BH4)2 was found to be chemically stable between 700 and 1000 bar H2, whereas under 350 bar H2 or lower pressures, the bulk material partially decomposed into MgH2 and MgB12H12. The melting point of solvent-free Mg(BH4)2 was estimated to be 367-375 °C, which was above previously reported values by 40-90 °C. Our results indicated that a high hydrogen backpressure is needed to prevent the decomposition of Mg(BH4)2 before measuring the melting point and that molten Mg(BH4)2 can exist as a stable liquid phase between 367 and 400 °C under hydrogen overpressures of 700 bar or above. The occurrence of a pure molten Mg(BH4)2 phase enabled efficient melt-infiltration of Mg(BH4)2 into the pores of porous templated carbons (CMK-3 and CMK-8) and graphene aerogels. Both transmission electron microscopy and small-angle X-ray scattering confirmed efficient incorporation of the borohydride into the carbon pores. The Mg(BH4)2@carbon samples exhibited comparable hydrogen capacities to bulk Mg(BH4)2 upon desorption up to 390 °C based on the mass of the active component; the onset of hydrogen release was reduced by 15-25 °C compared to the bulk. Importantly, melt-infiltration under hydrogen pressure was shown to be an efficient way to introduce metal borohydrides into the pores of carbon-based materials, helping to prevent particle agglomeration and formation of stable closo-polyborate byproducts.
Complex metal hydrides provide high-density hydrogen storage, which is essential for vehicular applications. However, the utility of these materials has been limited by thermodynamic and kinetic barriers present during the dehydrogenation and rehydrogenation processes as new phases form inside parent phases. Better understanding of the mixed-phase mesostructures and their interfaces may assist in improving cyclability. In this work, the evolution of the phases during hydrogenation of lithium nitride and dehydrogenation of lithium amide with lithium hydride are probed with scanning-transmission X-ray microscopy at the nitrogen K edge. With this technique, intriguing core-shell structures were observed in particles of both partially hydrogenated Li3N and partially dehydrogenated LiNH2 + 2 LiH. The potential contributions of both internal hydrogen mobility and interfacial energies on the generation of these structures are discussed.
An open question in the metal hydride community is whether there are simple, physics-based design rules that dictate the thermodynamic properties of these materials across the variety of structures and chemistry they can exhibit. While black box machine learning-based algorithms can predict these properties with some success, they do not directly provide the basis on which these predictions are made, therefore complicating the a priori design of novel materials exhibiting a desired property value. In this work we demonstrate how feature importance, as identified by a gradient boosting tree regressor, uncovers the strong dependence of the metal hydride equilibrium H2 pressure on a volume-based descriptor that can be computed from just the elemental composition of the intermetallic alloy. Elucidation of this simple structure-property relationship is valid across a range of compositions, metal substitutions, and structural classes exhibited by intermetallic hydrides. This permits rational targeting of novel intermetallics for high-pressure hydrogen storage (low-stability hydrides) by their descriptor values, and we predict a known intermetallic to form a low-stability hydride (as confirmed by density functional theory calculations) that has not yet been experimentally investigated.
HKUST-1 or Cu3BTC2 (BTC = 1,3,5-benzenetricarboxylate) is a prototypical metal-organic framework (MOF) that holds a privileged position among MOFs for device applications, as it can be deposited as thin films on various substrates and surfaces. Recently, new potential applications in electronics have emerged for this material when HKUST-1 was demonstrated to become electrically conductive upon infiltration with 7,7,8,8-tetracyanoquinodimethane (TCNQ). However, the factors that control the morphology and reactivity of the thin films are unknown. Here, we present a study of the thin-film growth process on indium tin oxide and amorphous Si prior to infiltration. From the unusual bimodal, non-log-normal distribution of crystal domain sizes, we conclude that the nucleation of new layers of Cu3BTC2 is greatly enhanced by surface defects and thus difficult to control. We then show that these films can react with methanolic TCNQ solutions to form dense films of the coordination polymer Cu(TCNQ). This chemical conversion is accompanied by dramatic changes in surface morphology, from a surface dominated by truncated octahedra to randomly oriented thin platelets. The change in morphology suggests that the chemical reaction occurs in the liquid phase and is independent of the starting surface morphology. The chemical transformation is accompanied by 10 orders of magnitude change in electrical conductivity, from <10-11 S/cm for the parent Cu3BTC2 material to 10-1 S/cm for the resulting Cu(TCNQ) film. The conversion of Cu3BTC2 films, which can be grown and patterned on a variety of (nonplanar) substrates, to Cu(TCNQ) opens the door for the facile fabrication of more complex electronic devices.
Storage of hydrogen onboard vehicles is one of the critical technologies needed to create hydrogen-fueled transportation systems that can improve energy efficiency, resiliency, and energy independence reduce oil dependency. Stakeholders in developing hydrogen infrastructure (e.g., state governments, automotive original equipment manufacturers, station providers, and industrial gas suppliers) are currently focused on high-pressure storage at 350 bar and 700 bar, in part because no viable solid-phase storage material has emerged. Early-state research to develop foundational understanding of solid-state storage materials, including novel sorbents and highdensity hydrides, is of high importance because of their unique potential to meet all DOE Fuel Cell Technologies Office targets and deliver hydrogen with lower storage pressures and higher onboard densities. However, existing materials suffer from thermodynamic and kinetic limitations that prevent their application as practical H2 storage media. Sandia's overall objectives and responsibilities within HyMARC are to: (1) provide technical leadership to the Consortium at the Director level, as well as through leadership of Task 1 (Thermodynamics), Task 3 (Gas Surface Interactions), and Task 5 (Additives); (2) provide gas sorption and other property data required to develop and validate thermodynamic models of sorbents and metal hydride storage materials, including the effects of 350 bar and 700 bar H2 delivery pressures, serving as a resource for the consortium; (3) identify the structure, composition, and reactivity of gas surface and solid-solid hydride surfaces contributing to ratelimiting desorption and uptake; (4) provide metal hydrides and Metal-Organic Framework (MOF) sorbents in a variety of formats tailored for specific consortium tasks; (5) develop sample preparation methods and experimental protocols to enable facile use of the new characterization probes employed by the Consortium; (6) apply SNL multiscale codes to discover diffusion pathways and mechanisms of storage materials; and (7) elucidate the role of additives in promoting hydrogen storage reactions.
Some of the most remarkable recent developments in metal–organic framework (MOF) performance properties can only be rationalized by the mechanical properties endowed by their hybrid inorganic–organic nanoporous structures. While these characteristics create intriguing application prospects, the same attributes also present challenges that will need to be overcome to enable the integration of MOFs with technologies where these promising traits can be exploited. In this review, emerging opportunities and challenges are identified for MOF-enabled device functionality and technological applications that arise from their fascinating mechanical properties. This is discussed not only in the context of their more well-studied gas storage and separation applications, but also for instances where MOFs serve as components of functional nanodevices. Recent advances in understanding MOF mechanical structure–property relationships due to attributes such as defects and interpenetration are highlighted, and open questions related to state-of-the-art computational approaches for quantifying their mechanical properties are critically discussed.
Because of their extraordinary surface areas and tailorable porosity, metal-organic frameworks (MOFs) have the potential to be excellent sensors of gas-phase analytes. MOFs with open metal sites are particularly attractive for detecting Lewis basic atmospheric analytes, such as water. Here, we demonstrate that thin films of the MOF HKUST-1 can be used to quantitatively determine the relative humidity (RH) of air using a colorimetric approach. HKUST-1 thin films are spin-coated onto rigid or flexible substrates and are shown to quantitatively determine the RH within the range of 0.1-5% RH by either visual observation or a straightforward optical reflectivity measurement. At high humidity (>10% RH), a polymer/MOF bilayer is used to slow the transport of H2O to the MOF film, enabling quantitative determination of RH using time as the distinguishing metric. Finally, the sensor is combined with an inexpensive light-emitting diode light source and Si photodiode detector to demonstrate a quantitative humidity detector for low humidity environments.
The efficiency of dye-sensitized solar cells (DSSCs) is strongly influenced by dye molecule orientation and interactions with the substrate. Understanding the factors controlling the surface orientation of sensitizing organic molecules will aid in the improvement of both traditional DSSCs and other devices that integrate molecular linkers at interfaces. Here, we describe a general approach to understand relative dye-substrate orientation and provide analytical expressions predicting orientation. We consider the effects of substrate, solvent, and protonation state on dye molecule orientation. In the absence of solvent, our model predicts that most carboxylic acid-functionalized molecules prefer to lie flat (parallel) on the surface, due to van der Waals interactions, as opposed to a tilted orientation with respect to the surface that is favored by covalent bonding of the carboxylic acid group to the substrate. When solvation effects are considered, however, the molecules are predicted to orient perpendicular to the surface. We extend this approach to help understand and guide the orientation of metal-organic framework (MOF) thin-film growth on various metal-oxide substrates. A two-part analytical model is developed on the basis of the results of DFT calculations and ab initio MD simulations that predicts the binding energy of a molecule by chemical and dispersion forces on rutile and anatase TiO2 surfaces, and quantifies the dye solvation energy for two solvents. The model is in good agreement with the DFT calculations and enables rapid prediction of dye molecule and MOF linker binding preference on the basis of the size of the adsorbing molecule, identity of the surface, and the solvent environment. We establish the threshold molecular size, governing dye molecule orientation, for each condition.
Ni3(2,3,6,7,10,11-hexaiminotriphenylene)2 is a π-stacked layered metal-organic framework material with extended π-conjugation that is analogous to graphene. Published experimental results indicate that the material is semiconducting, but all theoretical studies to date predict the bulk material to be metallic. Given that previous experimental work was carried out on specimens containing complex nanocrystalline microstructures and the tendency for internal interfaces to introduce transport barriers, we apply DFT to investigate the influence of internal interface defects on the electronic structure of Ni3(HITP)2. The results show that interface defects can introduce a transport barrier by breaking the π-conjugation and/or decreasing the dispersion of the electronic bands near the Fermi level. We demonstrate that the presence of defects can open a small gap, in the range of 15-200 meV, which is consistent with the experimentally inferred hopping barrier.
The first metal-organic framework exhibiting thermally activated delayed fluorescence (TADF) was developed. The zirconium-based framework (UiO-68-dpa) uses a newly designed linker composed of a terphenyl backbone, an electron-accepting carboxyl group, and an electron-donating diphenylamine and exhibits green TADF emission with a photoluminescence quantum yield of 30% and high thermal stability.
Purpose: Water vapor trapped in encapsulation materials or enclosed volumes leads to corrosion issues for critical NW components. Sandia National Laboratories has created a new diagnostic to indicate the presence of water in weapon systems. Impact: Component exposure to water now can be determined instantly, without need for costly, time-consuming analytical methods.
The atomic force microscope (AFM) offers a rich observation window on the nanoscale, yet many dynamic phenomena are too fast and too weak for direct AFM detection. Integrated cavity-optomechanics is revolutionizing micromechanical sensing; however, it has not yet impacted AFM. Here, we make a groundbreaking advance by fabricating picogram-scale probes integrated with photonic resonators to realize functional AFM detection that achieve high temporal resolution (<10 ns) and picometer vertical displacement uncertainty simultaneously. The ability to capture fast events with high precision is leveraged to measure the thermal conductivity (η), for the first time, concurrently with chemical composition at the nanoscale in photothermal induced resonance experiments. The intrinsic η of metal-organic-framework individual microcrystals, not measurable by macroscale techniques, is obtained with a small measurement uncertainty (8%). The improved sensitivity (50×) increases the measurement throughput 2500-fold and enables chemical composition measurement of molecular monolayer-thin samples. Our paradigm-shifting photonic readout for small probes breaks the common trade-off between AFM measurement precision and ability to capture transient events, thus transforming the ability to observe nanoscale dynamics in materials.
Metal organic frameworks (MOFs) are extended, nanoporous crystalline compounds consisting of metal ions interconnected by organic ligands. Their synthetic versatility suggest a disruptive class of opto - electronic materials with a high degree of electrical tunability and without the property - degrading disorder of organic conductors. In this project we determined the factors controlling charge and energy transport in MOFs and evaluated their potential for thermoelectric energy conversion. Two strategies for a chieving electronic conductivity in MOFs were explored: 1) using redox active 'guest' molecules introduced into the pores to dope the framework via charge - transfer coupling (Guest@MOF), 2) metal organic graphene analogs (MOGs) with dispersive band structur es arising from strong electronic overlap between the MOG metal ions and its coordinating linker groups. Inkjet deposition methods were developed to facilitate integration of the guest@MOF and MOG materials into practical devices.
Metal organic frameworks (MOFs) have recently attracted great attentions for the thermoelectric (TE) applications, owing to their intrinsic low thermal conductivity, but their TE efficiencies are still low due to the poor electronic transport properties. Here, various synthetic strategies have been designed to optimize the electronic properties of MOFs. Using a series of first principle calculations and band theory, we explore the effect of structural topology and redox matching between the metal and coordinated atoms on the TE transport properties. In conclusion, the presented results provide a fundamental guidance for optimizing electronic charge transport of existing MOFs, and for designing yet to be discovered conductive MOFs for thermoelectric applications.
The Sandia HyMARC team continued its development of new synthetic, modeling, and diagnostic tools that are providing new insights into all major classes of storage materials, ranging from relatively simple systems such as PdHx and MgH2, to exceptionally complex ones, such as the metal borohydrides, as well as materials thought to be very well-understood, such as Ti-doped NaAlH4. This unprecedented suite of capabilities, capable of probing all relevant length scales within storage materials, is already having a significant impact, as they are now being used by both Seedling projects and collaborators at other laboratories within HyMARC. We expect this impact to grow as new Seedling projects begin and through collaborations with other scientists outside HyMARC. In the coming year, Sandia efforts will focus on the highest impact problems, in coordination with the other HyMARC National Laboratory partners, to provide the foundational science necessary to accelerate the discovery of new hydrogen storage materials.
Metal-organic frameworks (MOFs) are highly ordered, functionally tunable supramolecular materials with the potential to improve dye-sensitized solar cells (DSSCs). Several recent reports have indicated that photocurrent can be generated in Grätzel-type DSSC devices when MOFs are used as the sensitizer. However, the specific role(s) of the incorporated MOFs and the potential influence of residual MOF precursor species on device performance are unclear. Herein, we describe the assembly and characterization of a simplified DSSC platform in which isolated MOF crystals are used as the sensitizer in a planar device architecture. We selected a pillared porphyrin framework (PPF) as the MOF sensitizer, taking particular care to avoid contamination from light-absorbing MOF precursors. Photovoltaic and electrochemical characterization under simulated 1-sun and wavelength-selective illumination revealed photocurrent generation that is clearly ascribable to the PPF MOF. Continued refinement of highly versatile MOF structure and chemistry holds promise for dramatic improvements in emerging photovoltaic technologies. (Figure Presented).
The well-known synthetic versatility of metal-organic frameworks (MOFs) is rooted in the ability to predict the metal-ion coordination geometry and the vast possibilities to use organic chemistry to modify the linker groups. However, the use of molecules occupying the pores as a component of framework design has been largely ignored. Recent reports show that the presence of these so-called guests can have dramatic effects, even when they are a seemingly innocuous species such as water or polar solvents. We term these guests non-innocent when their presence alters the MOF in such a way as to create a new material with properties different from the MOF without the guests. Advantages of using guest molecules to impart new properties to MOFs include the relative ease of introducing new functionalities, the ability to modify the material properties at will by removing the guest or inserting different ones, and avoidance of the difficulties associated with synthesizing new frameworks, which can be challenging even when the basic topology remains constant. In this article, we describe the Guest@MOF concept and provide examples illustrating its potential as a new MOF design element.
As the world transitions from fossil fuels to clean energy sources in the coming decades, many technological challenges will require chemists and material scientists to develop new materials for applications related to energy conversion, storage, and efficiency. Because of their unprecedented adaptability, metal-organic frameworks (MOFs) will factor strongly in this portfolio. By utilizing the broad synthetic toolkit provided by the fields of organic and inorganic chemistry, MOF pores can be customized to suit a particular application. Of particular importance is the ability to tune the strength of the interaction between the MOF pores and guest molecules. By cleverly controlling these MOF-guest interactions, the chemist may impart new function into the Guest@MOF materials otherwise lacking in vacant MOF. Herein, we highlight the concept of the Guest@MOF as it relates to our efforts to develop these materials for energy-related applicatons. Our work in the areas of H2 and noble gas storage, hydrogenolysis of biomass, light-harvesting, and conductive materials will be discussed. Of relevance to light-harvesting applications, we report for the first time a postsynthetic modification strategy for increasing the loading of a light-sensitive electron-donor molecule in the pores of a functionalized MIL-101 structure. Through the demonstrated versatility of these approaches, we show that, by treating guest molecules as integral design elements for new MOF constructs, MOF science can have a significant impact on the advancement of clean energy technologies.
The metal organic framework material Ni3(2,3,6,7,10,11 - hexaiminotriphenylene)2, (Ni3(HITP)2) is composed of layers of extended conjugated planes analogous to graphene. We carried out Density functional theory (DFT) calculations to model the electronic structure of bulk and monolayer Ni3(HITP)2. The layered 3D material is metallic, similar to graphene. Our calculations predict that there is appreciable band dispersion not only in-plane, but perpendicular to the stacking planes as well, suggesting that, unlike graphene, the conductivity may be nearly isotropic. In contrast, a 2D monolayer of the material exhibits a band gap, consistent with previously published results. Insight obtained from studies of the evolution of the material from semiconducting to metallic as the material is transitioned from 2D to 3D suggests the possibility of modifying the material to render it semiconducting by changing the metal center and inserting spacer moieties between the layers. Furthermore, the DFT calculations predict that the modified material will be structurally stable and exhibit a band gap.
Hydrogen diffusion impacts the performance of solid-state hydrogen storage materials and contributes to the embrittlement of structural materials under hydrogen-containing environments. In atomistic simulations, the diffusion energy barriers are usually calculated using molecular statics simulations where a nudged elastic band method is used to constrain a path connecting the two end points of an atomic jump. This approach requires prior knowledge of the "end points". For alloy and defective systems, the number of possible atomic jumps with respect to local atomic configurations is tremendous. Even when these jumps can be exhaustively studied, it is still unclear how they can be combined to give an overall diffusion behavior seen in experiments. Here we describe the use of molecular dynamics simulations to determine the overall diffusion energy barrier from the Arrhenius equation. This method does not require information about atomic jumps, and it has additional advantages, such as the ability to incorporate finite temperature effects and to determine the pre-exponential factor. As a test case for a generic method, we focus on hydrogen diffusion in bulk aluminum. We find that the challenge of this method is the statistical variation of the results. However, highly converged energy barriers can be achieved by an appropriate set of temperatures, output time intervals (for tracking hydrogen positions), and a long total simulation time. Our results help elucidate the inconsistencies of the experimental diffusion data published in the literature. The robust approach developed here may also open up future molecular dynamics simulations to rapidly study diffusion properties of complex material systems in multidimensional spaces involving composition and defects.
We demonstrate that metal-organic frameworks (MOFs) can catalyze hydrogenolysis of aryl ether bonds under mild conditions. Mg-IRMOF-74(I) and Mg-IRMOF-74(II) are stable under reducing conditions and can cleave phenyl ethers containing β-O-4, α-O-4, and 4-O-5 linkages to the corresponding hydrocarbons and phenols. Reaction occurs at 10 bar H2 and 120 °C without added base. DFT-optimized structures and charge transfer analysis suggest that the MOF orients the substrate near Mg2+ ions on the pore walls. Ti and Ni doping further increase conversions to as high as 82% with 96% selectivity for hydrogenolysis versus ring hydrogenation. Repeated cycling induces no loss of activity, making this a promising route for mild aryl-ether bond scission.
Here, thin and continuous films of porous metal-organic frameworks can now be conformally deposited on various substrates using a vapor-phase synthesis approach that departs from conventional solution-based routes.
Metal-organic frameworks (MOFs) are crystalline nanoporous materials comprised of organic electron donors linked to metal ions by strong coordination bonds. Applications such as gas storage and separations are currently receiving considerable attention, but if the unique properties of MOFs could be extended to electronics, magnetics, and photonics, the impact on material science would greatly increase. Recently, we obtained "emergent properties," such as electronic conductivity and energy transfer, by infiltrating MOF pores with "guest" molecules that interact with the framework electronic structure. In this Perspective, we define a path to emergent properties based on the Guest@MOF concept, using zinc-carboxylate and copper-paddlewheel MOFs for illustration. Energy transfer and light harvesting are discussed for zinc carboxylate frameworks infiltrated with triplet-scavenging organometallic compounds and thiophene- and fullerene-infiltrated MOF-177. In addition, we discuss the mechanism of charge transport in TCNQ-infiltrated HKUST-1, the first MOF with electrical conductivity approaching conducting organic polymers. These examples show that guest molecules in MOF pores should be considered not merely as impurities or analytes to be sensed but also as an important aspect of rational design.
Metal-Organic Frameworks (MOFs) are a rapidly expanding class of hybrid organic-inorganic materials that can be rationally designed and assembled through crystal engineering. The explosion of interest in this subclass of coordination polymers results from their outstanding properties and myriad possible applications, which include traditional uses of microporous materials, such as gas storage, separations, and catalysis, as well as new realms in biomedicine, electronic devices, and information storage. The objective of this Highlight article is to provide the reader with a sense of where the field stands after roughly fifteen years of research. Remarkable progress has been made, but the barriers to practical and commercial advances are also evident. We discuss the basic elements of MOF assembly and present a conceptual hierarchy of structural elements that assists in understanding how unique properties in these materials can be achieved. Structure-function relationships are then discussed; several are now well understood, as a result of the focused efforts of many research groups over the past decade. Prospects for the use of MOFs in membranes, catalysis, biomedicine, and as active components in electronic and photonic devices are also discussed. Finally, we identify the most pressing challenges in our view that must be addressed for these materials to realize their full potential in the marketplace. This journal is
The goal of this Exploratory Express project was to expand the understanding of the physical properties of our recently discovered class of materials consisting of metal-organic frameworks with electroactive ‘guest’ molecules that together form an electrically conducting charge-transfer complex (molecule@MOF). Thin films of Cu3(BTC)2 were grown on fused silica using solution step-by-step growth and were infiltrated with the molecule tetracyanoquinodimethane (TCNQ). The infiltrated MOF films were extensively characterized using optical microscopy, scanning electron microscopy, Raman spectroscopy, electrical conductivity, and thermoelectric properties. Thermopower measurements on TCNQ@Cu3(BTC)2 revealed a positive Seebeck coefficient of ~400 μV/k, indicating that holes are the primary carriers in this material. The high value of the Seebeck coefficient and the expected low thermal conductivity suggest that molecule@MOF materials may be attractive for thermoelectric power conversion applications requiring low cost, solution-processable, and non-toxic active materials.
Lignin is a potentially plentiful source of renewable organics, with %7E50Mtons/yr produced by the pulp/paper industry and 200-300 Mtons/yr projected production by a US biofuels industry. This industry must process approximately 1 billion tons of biomass to meet the US Renewable Fuel goals. However, there are currently no efficient processes for converting lignin to value-added chemicals and drop-in fuels. Lignin is therefore an opportunity for production of valuable renewable chemicals, but presents staggering technical and economic challenges due to the quantities of material involved and the strong chemical bonds comprising this polymer. Aggressive chemistries and high temperatures are required to degrade lignin without catalysts. Moreover, chemical non-uniformity among lignins leads to complex product mixtures that tend to repolymerize. Conventional petrochemical approaches (pyrolysis, catalytic cracking, gasification) are energy intensive (400-800 degC), require complicated separations, and remove valuable chemical functionality. Low-temperature (25-200 degC) alternatives are clearly desirable, but enzymes are thermally fragile and incompatible with liquid organic compounds, making them impractical for large-scale biorefining. Alternatively, homogeneous catalysts, such as recently developed vanadium complexes, must be separated from product mixtures, while many heterogenous catalysts involve costly noble metals. The objective of this project is to demonstrate proof of concept that an entirely new class of biomimetic, efficient, and industrially robust synthetic catalysts based on nanoporous Metal- Organic Frameworks (MOFs) can be developed. Although catalytic MOFs are known, catalysis of bond cleavage reactions needed for lignin degradation is completely unexplored. Thus, fundamental research is required that industry and most sponsoring agencies are currently unwilling to undertake. We introduce MOFs infiltrated with titanium and nickel species as catalysts for the C-O bond hydrogenolysis in model compounds, which mimic the b-O-4, a-O-4, and 4-O-5 linkages of natural lignin. The versatile IRMOF-74(n) series is proposed as a platform for creating efficient hydrogenolysis catalysts as it not only displays tunable pore sizes, but also has the required thermal and chemical stability. The catalytic C-O bond cleavage occurs at 10 bar hydrogen pressure and temperatures as low as 120 degC. The conversion efficiency of the aromatic ether substrates into the corresponding hydrocarbons and phenols varies as PhCH 2 CH 2 OPh > PhCH 2 OPh > PhOPh (Ph = phenyl), while the catalytic activity generally follows the following trend Ni@IRMOF-74>Ti@IRMOF-74>IRMOF-74. Conversions as high as 80%, coupled with good selectivity for hydrogenolysis vs. hydrogenation, highlight the potential of MOF-based catalysts for the selective cleavage of recalcitrant aryl-ether bonds found in lignin and other biopolymers. This project supports the DOE Integrated Biorefinery Program goals, the objective of which is to convert biomass to fuels and high-value chemicals, by addressing an important technology gap: the lack of low-temperature catalysts suitable for industrial lignin degradation. Biomass, which is %7E30 wt% lignin, constitutes a potentially major source of platform chemicals that could improve overall profitability and productivity of all energy-related products, thereby benefiting consumers and reducing national dependence on imported oil. Additionally, DoD has a strong interest in low-cost drop-in fuels (Navy Biofuel Initiative) and has signed a Memorandum of Understanding with DOE and USDA to develop a sustainable biofuels industry.
Electro-optical organic materials hold great promise for the development of high-efficiency devices based on exciton formation and dissociation, such as organic photovoltaics (OPV) and organic light-emitting devices (OLEDs). However, the external quantum efficiency (EQE) of both OPV and OLEDs must be improved to make these technologies economical. Efficiency rolloff in OLEDs and inability to control morphology at key OPV interfaces both reduce EQE. Only by creating materials that allow manipulation and control of the intimate assembly and communication between various nanoscale excitonic components can we hope to first understand and then engineer the system to allow these materials to reach their potential. The aims of this proposal are to: 1) develop a paradigm-changing platform for probing excitonic processes composed of Crystalline Nanoporous Frameworks (CNFs) infiltrated with secondary materials (such as a complimentary semiconductor); 2) use them to probe fundamental aspects of excitonic processes; and 3) create prototype OPVs and OLEDs using infiltrated CNF as active device components. These functional platforms will allow detailed control of key interactions at the nanoscale, overcoming the disorder and limited synthetic control inherent in conventional organic materials. CNFs are revolutionary inorganic-organic hybrid materials boasting unmatched synthetic flexibility that allow tuning of chemical, geometric, electrical, and light absorption/generation properties. For example, bandgap engineering is feasible and polyaromatic linkers provide tunable photon antennae; rigid 1-5 nm pores provide an oriented, intimate host for triplet emitters (to improve light emission in OLEDs) or secondary semiconducting polymers (creating a charge-separation interface in OPV). These atomically engineered, ordered structures will enable critical fundamental questions to be answered concerning charge transport, nanoscale interfaces, and exciton behavior that are inaccessible in disordered systems. Implementing this concept also creates entirely new dimensions for device fabrication that could both improve performance, increase durability, and reduce costs with unprecedented control of over properties. This report summarizes the key results of this project and is divided into sections based on publications that resulted from the work. We begin in Section 2 with an investigation of light harvesting and energy transfer in a MOF infiltrated with donor and acceptor molecules of the type typically used in OPV devices (thiophenes and fullerenes, respectively). The results show that MOFs can provide multiple functions: as a light harvester, as a stabilizer and organizer or the infiltrated molecules, and as a facilitator of energy transfer. Section 3 describes computational design of MOF linker groups to accomplish light harvesting in the visible and facilitate charge separation and transport. The predictions were validated by UV-visible absorption spectroscopy, demonstrating that rational design of MOFs for light-harvesting purposes is feasible. Section 4 extends the infiltration concept discussed in Section to, which we now designate as "Molecule@MOF" to create an electrically conducting framework. The tailorability and high conductivity of this material are unprecedented, meriting publication in the journal Science and spawning several Technical Advances. Section 5 discusses processes we developed for depositing MOFs as thin films on substrates, a critical enabling technology for fabricating MOF-based electronic devices. Finally, in Section 6 we summarize results showing that a MOF thin film can be used as a sensitizer in a DSSC, demonstrating that MOFs can serve as active layers in excitonic devices. Overall, this project provides several crucial proofs-of- concept that the potential of MOFs for use in optoelectronic devices that we predicted several years ago [ 3 ] can be realized in practice.
Molecular dynamics simulations were used to investigate trends in noble gas (Ar, Kr, Xe) diffusion in the metal-organic frameworks HKUST-1 and ZIF-8. Diffusion occurs primarily through inter-cage jump events, with much greater diffusion of guest atoms in HKUST-1 compared to ZIF-8 due to the larger cage and window sizes in the former. We compare diffusion coefficients calculated for both rigid and flexible frameworks. For rigid framework simulations, in which the framework atoms were held at their crystallographic or geometry optimized coordinates, sometimes dramatic differences in guest diffusion were seen depending on the initial framework structure or the choice of framework force field parameters. When framework flexibility effects were included, argon and krypton diffusion increased significantly compared to rigid-framework simulations using general force field parameters. Additionally, for argon and krypton in ZIF-8, guest diffusion increased with loading, demonstrating that guest-guest interactions between cages enhance inter-cage diffusion. No inter-cage jump events were seen for xenon atoms in ZIF-8 regardless of force field or initial structure, and the loading dependence of xenon diffusion in HKUST-1 is different for rigid and flexible frameworks. Diffusion of krypton and xenon in HKUST-1 depends on two competing effects: the steric effect that decreases diffusion as loading increases, and the “small cage effect” that increases diffusion as loading increases. Finally, a detailed analysis of the window size in ZIF-8 reveals that the window increases beyond its normal size to permit passage of a (nominally) larger krypton atom.
Despite rapid progress, solar thermochemistry remains high risk; improvements in both active materials and reactor systems are needed. This claim is supported by studies conducted both prior to and as part of this project. Materials offer a particular large opportunity space as, until recently, very little effort apart from basic thermodynamic analysis was extended towards understanding this most fundamental component of a metal oxide thermochemical cycle. Without this knowledge, system design was hampered, but more importantly, advances in these crucial materials were rare and resulted more from intuition rather than detailed insight. As a result, only two basic families of potentially viable solid materials have been widely considered, each of which has significant challenges. Recent efforts towards applying an increased level of scientific rigor to the study of thermochemical materials have provided a much needed framework and insights toward developing the next generation of highly improved thermochemically active materials. The primary goal of this project was to apply this hard-won knowledge to rapidly advance the field of thermochemistry to produce a material within 2 years that is capable of yielding CO from CO2 at a 12.5 % reactor efficiency. Three principal approaches spanning a range of risk and potential rewards were pursued: modification of known materials, structuring known materials, and identifying/developing new materials for the application. A newly developed best-of-class material produces more fuel (9x more H2, 6x more CO) under milder conditions than the previous state of the art. Analyses of thermochemical reactor and system efficiencies and economics were performed and a new hybrid concept was reported. The larger case for solar fuels was also further refined and documented.
Two of the most daunting problems facing humankind in the twenty-first century are energy security and climate change. This report summarizes work accomplished towards addressing these problems through the execution of a Grand Challenge LDRD project (FY09-11). The vision of Sunshine to Petrol is captured in one deceptively simple chemical equation: Solar Energy + xCO{sub 2} + (x+1)H{sub 2}O {yields} C{sub x}H{sub 2x+2}(liquid fuel) + (1.5x+.5)O{sub 2} Practical implementation of this equation may seem far-fetched, since it effectively describes the use of solar energy to reverse combustion. However, it is also representative of the photosynthetic processes responsible for much of life on earth and, as such, summarizes the biomass approach to fuels production. It is our contention that an alternative approach, one that is not limited by efficiency of photosynthesis and more directly leads to a liquid fuel, is desirable. The development of a process that efficiently, cost effectively, and sustainably reenergizes thermodynamically spent feedstocks to create reactive fuel intermediates would be an unparalleled achievement and is the key challenge that must be surmounted to solve the intertwined problems of accelerating energy demand and climate change. We proposed that the direct thermochemical conversion of CO{sub 2} and H{sub 2}O to CO and H{sub 2}, which are the universal building blocks for synthetic fuels, serve as the basis for this revolutionary process. To realize this concept, we addressed complex chemical, materials science, and engineering problems associated with thermochemical heat engines and the crucial metal-oxide working-materials deployed therein. By project's end, we had demonstrated solar-driven conversion of CO{sub 2} to CO, a key energetic synthetic fuel intermediate, at 1.7% efficiency.
Hydrogen is proposed as an ideal carrier for storage, transport, and conversion of energy. However, its storage is a key problem in the development of hydrogen economy. Metal hydrides hold promise in effectively storing hydrogen. For this reason, metal hydrides have been the focus of intensive research. The chemical bonds in light metal hydrides are predominantly covalent, polar covalent or ionic. These bonds are often strong, resulting in high thermodynamic stability and low equilibrium hydrogen pressures. In addition, the directionality of the covalent/ionic bonds in these systems leads to large activation barriers for atomic motion, resulting in slow hydrogen sorption kinetics and limited reversibility. One method for enhancing reaction kinetics is to reduce the size of the metal hydrides to nano scale. This method exploits the short diffusion distances and constrained environment that exist in nanoscale hydride materials. In order to reduce the particle size of metal hydrides, mechanical ball milling is widely used. However, microscopic mechanisms responsible for the changes in kinetics resulting from ball milling are still being investigated. The objective of this work is to use metal organic frameworks (MOFs) as templates for the synthesis of nano-scale NaAlH4 particles, to measure the H2 desorption kinetics and thermodynamics, and to determine quantitative differences from corresponding bulk properties. Metal-organic frameworks (MOFs) offer an attractive alternative to traditional scaffolds because their ordered crystalline lattice provides a highly controlled and understandable environment. The present work demonstrates that MOFs are stable hosts for metal hydrides and their reactive precursors and that they can be used as templates to form metal hydride nanoclusters on the scale of their pores (1-2 nm). We find that using the MOF HKUST-1 as template, NaAlH4 nanoclusters as small as 8 formula units can be synthesized inside the pores. A detailed picture of the hydrogen desorption is investigated using a simultaneous thermogravimetric modulated-beam mass spectrometry instrument. The hydrogen desorption behavior of NaAlH4 nano-clusters is found to be very different from bulk NaAlH4. The bulk NaAlH4 desorbs about 70 wt% hydrogen {approx}250 C. In contrast, confinement of NaAlH4 within the MOF pores dramatically increases the rate of H2 desorption at lower temperatures. About {approx}80% of the total H2 desorbed from MOF-confined NaAlH4 is observed between 70 to 155 C. In addition to HKUST-1, we find that other MOFs (e.g. MIL-68 and MOF-5) can be infiltrated with hydrides (LiAlH4, LiBH4) or hydride precursors (Mg(C4H9)2 and LiC2H5) without degradation. By varying pore dimensions, metal centers, and the linkers of MOFs, it will be possible to determine whether the destabilization of metal hydrides is dictated only by the size of the metal hydride clusters, their local environment in a confined space, or by catalytic effects of the framework.
A new method for spectral shape discrimination (SSD) of fast neutrons and gamma rays has been investigated. Gammas interfere with neutron detection, making efficient discrimination necessary for practical applications. Pulse shape discrimination (PSD) in liquid organic scintillators is currently the most effective means of gamma rejection. The hazardous liquids, restrictions on volume, and the need for fast timing are drawbacks to traditional PSD scintillators. In this project we investigated harvesting excited triplet states to increase scintillation yield and provide distinct spectral signatures for gammas and neutrons. Our novel approach relies on metal-organic phosphors to convert a portion of the energy normally lost to the scintillation process into useful luminescence with sub-microsecond lifetimes. The approach enables independent control over delayed luminescence wavelength, intensity, and timing for the first time. We demonstrated that organic scintillators, including plastics, nanoporous framework materials, and oil-based liquids can be engineered for both PSD and SSD.
Metal-organic frameworks (MOFs) represent a diverse and rapidly expanding class of materials comprising metal ions bridged by organic linker molecules. These robust crystalline structures have been found to exhibit exceptionally large surface areas, paving the way for diverse applications ranging from gas storage and separations to catalysis, drug delivery, and sensing. Less well understood are the intrinsic luminescence properties of MOFs, which arise from the electronic transitions within the hybrid metal-organic structure. Recently, we reported the observation of scintillation in stilbene-based MOFs, representing the discovery of the first completely new class of radiation detection materials since the advent of plastic scintillators in 1950. Photoluminescence and ion-induced luminescence spectroscopy of these materials show that both the luminescence spectrum and its timing can be varied by altering the local environment of the chromophore, establishing critical insight towards the rational design of materials for specific radiation detection applications. In this work, we describe the luminescence and scintillating properties of a series of isoreticular MOFs (IRMOFs), emphasizing the structural and electronic effects associated with systematic modification of the chromophore. Among these structures are IRMOFs based on naphthyl, biphenyl, terphenyl, and stilbene dicarboxylate linkers, for which unique structural changes and optical properties are observed. In addition to chemical changes in the structure, framework interpenetration may also be synthetically controlled, resulting in pairs of catenated and non-catenated IRMOFs based upon the same organic linker. The distinct interchromophore distances and solvate structure in these pairs lead to unique luminescence spectra that are interpreted in terms of energy transfer interactions. These spectral changes provide insight into the mechanism for radiation-induced luminescence, which for MOFs may differ significantly from the photoluminescence spectrum.
We will describe how novel nanoporous framework materials (NFM) such as metal-organic frameworks (MOFs) can be interfaced with common mechanical sensors, such as surface acoustic wave (SAW), microcantilever array, and quartz crystal microbalance (QCM) devices, and subsequently be used to provide selectivity and sensitivity to a broad range of analytes including explosives, nerve agents, and volatile organic compounds (VOCs). NFM are highly ordered, crystalline materials with considerable synthetic flexibility resulting from the presence of both organic and inorganic components within their structure. Chemical detection using micro-electro-mechanical-systems (MEMS) devices (i.e. SAWs, microcantilevers) requires the use of recognition layers to impart selectivity. Unlike traditional organic polymers, which are dense, the nanoporosity and ultrahigh surface areas of NFM allow for greater analyte uptake and enhance transport into and out of the sensing layer. This enhancement over traditional coatings leads to improved response times and greater sensitivity, while their ordered structure allows chemical tuning to impart selectivity. We describe here experiments and modeling aimed at creating NFM layers tailored to the detection of water vapor, explosives, CWMD, and volatile organic compound (VOCs), and their integration with the surfaces of MEMS devices. Molecular simulation shows that a high degree of chemical selectivity is feasible. For example, a suite of MOFs can select for strongly interacting organics (explosives, CWMD) vs. lighter volatile organics at trace concentrations. At higher gas pressures, the CWMD are deselected in favor of the volatile organics. We will also demonstrate the integration of various NFM on the surface of microcantiliver arrays, QCM crystals, and SAW devices, and describe new synthetic methods developed to improve the quality of NFM coatings. Finally, MOF-coated MEMS devices show how temperature changes can be tuned to improve response times, selectivity, and sensitivity.
Grand canonical Monte Carlo simulations were performed to investigate trends in low-pressure adsorption of a broad range of organic molecules by a set of metal-organic frameworks (MOFs). The organic analytes considered here are relevant to applications in chemical detection: small aromatics (o-, m-, and p-xylene), polycyclic aromatic hydrocarbons (naphthalene, anthracene, phenanthrene), explosives (TNT and RDX), and chemical warfare agents (GA and VM). The framework materials included several Zn-MOFs (IRMOFs 1-3, 7, 8), a Cr-MOF (CrMIL-53lp), and a Cu-MOF (HKUST-1). Many of the larger organics were significantly adsorbed by the target MOFs at low pressure, which is consistent with the exceptionally high isosteric heats of adsorption (25 kcal/mol - 60 kcal/mol) for this range of analyte. At a higher loading pressure of 101 kPa, the Zn-MOFs show a much higher volumetric uptake than either CrMIL-53-lp or HKUST-1 for all types of analyte. Within the Zn-MOF series, analyte loading is proportional to free volume, and loading decreases with increasing analyte size due to molecular packing effects. CrMIL-53lp showed the highest adsorption energy for all analytes, suggesting that this material may be suitable for low-level detection of organics.
The objective of this project is to lay the foundation for using ordered nanoporous materials known as metal-organic frameworks (MOFs) to create devices and sensors whose properties are determined by the dimensions of the MOF lattice. Our hypothesis is that because of the very short (tens of angstroms) distances between pores within the unit cell of these materials, enhanced electro-optical properties will be obtained when the nanopores are infiltrated to create nanoclusters of metals and other materials. Synthetic methods used to produce metal nanoparticles in disordered templates or in solution typically lead to a distribution of particle sizes. In addition, creation of the smallest clusters, with sizes of a few to tens of atoms, remains very challenging. Nanoporous metal-organic frameworks (MOFs) are a promising solution to these problems, since their long-range crystalline order creates completely uniform pore sizes with potential for both steric and chemical stabilization. We report results of synthetic efforts. First, we describe a systematic investigation of silver nanocluster formation within MOFs using three representative MOF templates. The as-synthesized clusters are spectroscopically consistent with dimensions {le} 1 nm, with a significant fraction existing as Ag{sub 3} clusters, as shown by electron paramagnetic resonance. Importantly, we show conclusively that very rapid TEM-induced MOF degradation leads to agglomeration and stable, easily imaged particles, explaining prior reports of particles larger than MOF pores. These results solve an important riddle concerning MOF-based templates and suggest that heterostructures composed of highly uniform arrays of nanoparticles within MOFs are feasible. Second, a preliminary study of methods to incorporate fulleride (K{sub 3}C{sub 60}) guest molecules within MOF pores that will impart electrical conductivity is described.
The consumption of petroleum by the transportation sector in the United States is roughly equivalent to petroleum imports into the country, which have totaled over 12 million barrels a day every year since 2004. This reliance on foreign oil is a strategic vulnerability for the economy and national security. Further, the effect of unmitigated CO{sub 2} releases on the global climate is a growing concern both here and abroad. Independence from problematic oil producers can be achieved to a great degree through the utilization of non-conventional hydrocarbon resources such as coal, oil-shale and tarsands. However, tapping into and converting these resources into liquid fuels exacerbates green house gas (GHG) emissions as they are carbon rich, but hydrogen deficient. Revolutionary thinking about energy and fuels must be adopted. We must recognize that hydrocarbon fuels are ideal energy carriers, but not primary energy sources. The energy stored in a chemical fuel is released for utilization by oxidation. In the case of hydrogen fuel the chemical product is water; in the case of a hydrocarbon fuel, water and carbon dioxide are produced. The hydrogen economy envisions a cycle in which H{sub 2}O is re-energized by splitting water into H{sub 2} and O{sub 2}, by electrolysis for example. We envision a hydrocarbon analogy in which both carbon dioxide and water are re-energized through the application of a persistent energy source (e.g. solar or nuclear). This is of course essentially what the process of photosynthesis accomplishes, albeit with a relatively low sunlight-to-hydrocarbon efficiency. The goal of this project then was the creation of a direct and efficient process for the solar or nuclear driven thermochemical conversion of CO{sub 2} to CO (and O{sub 2}), one of the basic building blocks of synthetic fuels. This process would potentially provide the basis for an alternate hydrocarbon economy that is carbon neutral, provides a pathway to energy independence, and is compatible with much of the existing fuel infrastructure.
Molecular simulations are used to assess the ability of metal-organic framework (MOF) materials to store and separate noble gases. Specifically, grand canonical Monte Carlo simulation techniques are used to predict noble gas adsorption isotherms at room temperature. Experimental trends of noble gas inflation curves of a Zn-based material (IRMOF-1) are matched by the simulation results. The simulations also predict that IRMOF-1 selectively adsorbs Xe atoms in Xe/Kr and Xe/Ar mixtures at total feed gas pressures of 1 bar (14.7 psia) and 10 bar (147 psia). Finally, simulations of a copper-based MOF (Cu-BTC) predict this material's ability to selectively adsorb Xe and Kr atoms when present in trace amounts in atmospheric air samples. These preliminary results suggest that Cu-BTC may be an ideal candidate for the pre-concentration of noble gases from air samples. Additional simulations and experiments are needed to determine the saturation limit of Cu-BTC for xenon, and whether any krypton atoms would remain in the Cu-BTC pores upon saturation.