Showing papers in "Angewandte Chemie in 2005"
TL;DR: The current knowledge in the structure and chemistry of cellulose, and in the development of innovative cellulose esters and ethers for coatings, films, membranes, building materials, drilling techniques, pharmaceuticals, and foodstuffs are assembled.
Abstract: As the most important skeletal component in plants, the polysaccharide cellulose is an almost inexhaustible polymeric raw material with fascinating structure and properties. Formed by the repeated connection of D-glucose building blocks, the highly functionalized, linear stiff-chain homopolymer is characterized by its hydrophilicity, chirality, biodegradability, broad chemical modifying capacity, and its formation of versatile semicrystalline fiber morphologies. In view of the considerable increase in interdisciplinary cellulose research and product development over the past decade worldwide, this paper assembles the current knowledge in the structure and chemistry of cellulose, and in the development of innovative cellulose esters and ethers for coatings, films, membranes, building materials, drilling techniques, pharmaceuticals, and foodstuffs. New frontiers, including environmentally friendly cellulose fiber technologies, bacterial cellulose biomaterials, and in-vitro syntheses of cellulose are highlighted together with future aims, strategies, and perspectives of cellulose research and its applications.
6,098 citations
TL;DR: The Review presents the recent developments and the use of NP catalysis in organic synthesis, for example, in hydrogenation and C--C coupling reactions, and the heterogeneous oxidation of CO on gold NPs.
Abstract: Interest in catalysis by metal nanoparticles (NPs) is increasing dramatically, as reflected by the large number of publications in the last five years. This field, "semi-heterogeneous catalysis", is at the frontier between homogeneous and heterogeneous catalysis, and progress has been made in the efficiency and selectivity of reactions and recovery and recyclability of the catalytic materials. Usually NP catalysts are prepared from a metal salt, a reducing agent, and a stabilizer and are supported on an oxide, charcoal, or a zeolite. Besides the polymers and oxides that used to be employed as standard, innovative stabilizers, media, and supports have appeared, such as dendrimers, specific ligands, ionic liquids, surfactants, membranes, carbon nanotubes, and a variety of oxides. Ligand-free procedures have provided remarkable results with extremely low metal loading. The Review presents the recent developments and the use of NP catalysis in organic synthesis, for example, in hydrogenation and C--C coupling reactions, and the heterogeneous oxidation of CO on gold NPs.
2,790 citations
TL;DR: A discussion of several strategies aimed at improving hydrogen uptake in metal-organic frameworks, including the optimization of pore size and adsorption energy by linker modification, impregnation, catenation, and the inclusion of open metal sites and lighter metals.
Abstract: Increased attention is being focused on metal-organic frameworks as candidates for hydrogen storage materials. This is a result of their many favorable attributes, such as high porosity, reproducible and facile syntheses, amenability to scale-up, and chemical modification for targeting desired properties. A discussion of several strategies aimed at improving hydrogen uptake in these materials is presented. These strategies include the optimization of pore size and adsorption energy by linker modification, impregnation, catenation, and the inclusion of open metal sites and lighter metals.
2,298 citations
TL;DR: In this Review, highlights of a number of selected syntheses are discussed, demonstrating the enormous power of these processes in the art of total synthesis and underscore their future potential in chemical synthesis.
Abstract: In studying the evolution of organic chemistry and grasping its essence, one comes quickly to the conclusion that no other type of reaction plays as large a role in shaping this domain of science than carbon-carbon bond-forming reactions. The Grignard, Diels-Alder, and Wittig reactions are but three prominent examples of such processes, and are among those which have undeniably exercised decisive roles in the last century in the emergence of chemical synthesis as we know it today. In the last quarter of the 20th century, a new family of carbon-carbon bond-forming reactions based on transition-metal catalysts evolved as powerful tools in synthesis. Among them, the palladium-catalyzed cross-coupling reactions are the most prominent. In this Review, highlights of a number of selected syntheses are discussed. The examples chosen demonstrate the enormous power of these processes in the art of total synthesis and underscore their future potential in chemical synthesis.
2,268 citations
TL;DR: In 1981, the macrocyclic methylene-bridged glycoluril hexamer (CB[6]) was dubbed "cucurbituril" by Mock and co-workers because of its resemblance to the most prominent member of the cucurbitaceae family of plants--the pumpkin.
Abstract: In 1981, the macrocyclic methylene-bridged glycoluril hexamer (CB[6]) was dubbed "cucurbituril" by Mock and co-workers because of its resemblance to the most prominent member of the cucurbitaceae family of plants--the pumpkin. In the intervening years, the fundamental binding properties of CB[6]-high affinity, highly selective, and constrictive binding interactions--have been delineated by the pioneering work of the research groups of Mock, Kim, and Buschmann, and has led to their applications in waste-water remediation, as artificial enzymes, and as molecular switches. More recently, the cucurbit[n]uril family has grown to include homologues (CB[5]-CB[10]), derivatives, congeners, and analogues whose sizes span and exceed the range available with the alpha-, beta-, and gamma-cyclodextrins. Their shapes, solubility, and chemical functionality may now be tailored by synthetic chemistry to play a central role in molecular recognition, self-assembly, and nanotechnology. This Review focuses on the synthesis, recognition properties, and applications of these unique macrocycles.
2,074 citations
TL;DR: This work reports a general approach for the fabrication of monodisperse, hydrophilic, and single-crystalline ferrite microspheres by a solvothermal reduction method, and is the first report on the synthesis of single- Crystalline magneticmicrospheres.
Abstract: It has been thought that many novel properties and potential applications would emerge from monodisperse materials with small dimensions. Therefore, the synthesis of monodisperse nanoparticles has been intensively pursued for their technological and fundamental scientific importance. The synthesis of nanostructured magnetic materials has become a particularly important area of research and is attracting a growing interest because of the potential applications such materials have in ferrofluids, advanced magnetic materials, catalysts, colored pigments, high-density magnetic recording media, and medical diagnostics. Spinel ferrites (MFe2O4; M = Fe, Mn, Zn, or Co) are among the most important magnetic materials and have been widely used in electronic devices, information storage, magnetic resonance imaging (MRI), and drug-delivery technology. 14] Magnetite (Fe3O4) has recently been considered an ideal candidate for biological applications, both as a tag for sensing and imaging, and as an activity agent for antitumor therapy. For high performance in function-specific biological applications, magnetic particles must be spherical and have smooth surfaces, narrow size distributions, large surface areas (for maximal protein or enzyme binding), high magnetic saturation (ss) to provide maximum signal, and good dispersion in liquid media. 18,19] After Sugimoto and Matijević reported the preparation of magnetite particles with a narrow size distribution in the early 1980s, monodisperse ferrite has been fabricated by various chemistry-based synthetic methods, including coprecipitation, the reverse micelle method, microwave plasma synthesis, solgel techniques, freeze drying, ultrasound irradiation, hydrothermal methods, laser pyrolysis techniques, and thermal decomposition of organometallic and coordination compounds. 9,14, 18, 20–27] However, most of these approaches were focused on the synthesis of ferrite particles limited to diameters below 30 nm. There are no reports on the synthesis of well-crystallized ferrite nanoparticles with sizes similar to protein molecules. The development of a facile and economic synthetic strategy for the synthesis of hydrophilic, biocompatible magnetite nanoparticles would benefit their technical use in biomedical fields, especially for applications in vivo. Herein we report a general approach for the fabrication of monodisperse, hydrophilic, and single-crystalline ferrite microspheres by a solvothermal reduction method. To the best of our knowledge, this is the first report on the synthesis of single-crystalline magnetic microspheres. The ferrite spheres had monodisperse diameters that were tunable in the range of 200–800 nm. This work resulted in an important method for obtaining various monodisperse, magnetic, and single-crystalline microspheres, and provided an opportunity to further apply these promising materials. Typical syntheses of Fe3O4 and ferrite microspheres were carried out in a solvothermal system by modified reduction reactions between FeCl3 and ethylene glycol. We confirmed the production of Fe3O4 by conducting controlled oxidation reactions in which aand g-Fe2O3 were produced (Supporting Information). 28–29] The crystalline structures of MFe2O4 were characterized by XRD. As shown in Figure 1, the
1,910 citations
TL;DR: The current state of knowledge, major open questions, and research perspectives on the properties and interactions of atmospheric aerosols and their effects on climate and human health are outlined.
Abstract: Aerosols are of central importance for atmospheric chemistry and physics, the biosphere, climate, and public health. The airborne solid and liquid particles in the nanometer to micrometer size range influence the energy balance of the Earth, the hydrological cycle, atmospheric circulation, and the abundance of greenhouse and reactive trace gases. Moreover, they play important roles in the reproduction of biological organisms and can cause or enhance diseases. The primary parameters that determine the environmental and health effects of aerosol particles are their concentration, size, structure, and chemical composition. These parameters, however, are spatially and temporally highly variable. The quantification and identification of biological particles and carbonaceous components of fine particulate matter in the air (organic compounds and black or elemental carbon, respectively) represent demanding analytical challenges. This Review outlines the current state of knowledge, major open questions, and research perspectives on the properties and interactions of atmospheric aerosols and their effects on climate and human health.
1,868 citations
TL;DR: In this Review, the fundamental characteristics of azide chemistry and current developments are presented and the focus will be placed on cycloadditions (Huisgen reaction), aza ylide chemistry, and the synthesis of heterocycles.
Abstract: Since the discovery of organic azides by Peter Griess more than 140 years ago, numerous syntheses of these energy-rich molecules have been developed. In more recent times in particular, completely new perspectives have been developed for their use in peptide chemistry, combinatorial chemistry, and heterocyclic synthesis. Organic azides have assumed an important position at the interface between chemistry, biology, medicine, and materials science. In this Review, the fundamental characteristics of azide chemistry and current developments are presented. The focus will be placed on cycloadditions (Huisgen reaction), aza ylide chemistry, and the synthesis of heterocycles. Further reactions such as the aza-Wittig reaction, the Sundberg rearrangement, the Staudinger ligation, the Boyer and Boyer-Aube rearrangements, the Curtius rearrangement, the Schmidt rearrangement, and the Hemetsberger rearrangement bear witness to the versatility of modern azide chemistry.
1,766 citations
TL;DR: Mesocrystals are exiting examples of nonclassical crystallization, which does not proceed through ion-by-ion attachment, but by a modular nanobuilding-block route, which makes crystallization more independent of ion products or molecular solubility, it occurs without pH or osmotic pressure changes, and opens new strategies for crystal morphogenesis.
Abstract: Controlled self-organization of nanoparticles can lead to new materials. The colloidal crystallization of non-spherical nanocrystals is a reaction channel in many crystallization reactions. With additives, self-organization can be stopped at an intermediary step-a mesocrystal-in which the primary units can still be identified. Mesocrystals were observed for various systems as kinetically metastable species or as intermediates in a crystallization reaction leading to single crystals with typical defects and inclusions. The control forces and mechanism of mesocrystal formation are largely unknown, but several mesocrystal properties are known. Mesocrystals are exiting examples of nonclassical crystallization, which does not proceed through ion-by-ion attachment, but by a modular nanobuilding-block route. This path makes crystallization more independent of ion products or molecular solubility, it occurs without pH or osmotic pressure changes, and opens new strategies for crystal morphogenesis.
1,556 citations
TL;DR: Asymmetric multicomponent reactions involve the preparation of chiral compounds by the reaction of three or more reagents added simultaneously and has some advantages over classic divergent reaction strategies, such as lower costs, time, and energy, as well as environmentally friendlier aspects.
Abstract: Asymmetric multicomponent reactions involve the preparation of chiral compounds by the reaction of three or more reagents added simultaneously. This kind of addition and reaction has some advantages over classic divergent reaction strategies, such as lower costs, time, and energy, as well as environmentally friendlier aspects. All these advantages, together with the high level of stereoselectivity attained in some of these reactions, will force chemists in industry as in academia to adopt this new strategy of synthesis, or at least to consider it as a viable option. The positive aspects as well as the drawbacks of this strategy are discussed in this Review.
1,479 citations
TL;DR: This workFloat their problematic reactions on water and to send observations of success or failure to us at onwater@scripps.edu for public dissemination with attribution.
Abstract: [*] Dr. S. Narayan, Dr. J. Muldoon, Prof. M. G. Finn, Prof. V. V. Fokin, Prof. H. C. Kolb, Prof. K. B. Sharpless Department of Chemistry and the Skaggs Institute of Chemical Biology The Scripps Research Institute 10550 North Torrey Pines Road La Jolla, CA 92037 (USA) Fax: (+ 1)619-554-6738 E-mail: sharples@scripps.edu [**] We thank Dr. Vladislav Litosh for carrying out preliminary work. Support from the National Institutes of Health, National Institute of General Medical Sciences (GM 28384), the National Science Foundation (CHE9985553), the Skaggs Institute for Chemical Biology, and the W. M. Keck Foundation is gratefully acknowledged. S.N. thanks the Skaggs Institute for a postdoctoral fellowship. We also thank Dr. Suresh Suri, Edwards Air Force Base, California, for a generous gift of quadricyclane. We urge our fellow chemists to float their problematic reactions on water and to send observations of success or failure to us at onwater@scripps.edu for public dissemination with attribution. Supporting information for this article is available on the WWW under http://www.angewandte.org or from the author. Angewandte Chemie
TL;DR: Organic porous materials—a class of advanced materials— possess enormous potential for many high-tech applications, such as bioreactors, dielectrics, sensors, microelectrophoresis, thermal insulation, and catalysts, but large porosity has rarely been reported.
Abstract: Organic porous materials—a class of advanced materials— possess enormous potential for many high-tech applications, such as bioreactors, dielectrics, sensors, microelectrophoresis, thermal insulation, and catalysts. In general, they can be prepared by phase separation, and a hard templating approach, such as those employing colloidal particles. Phase separation can be derived from organic– organic phases, while the pore structures can be formed after etching, or by dissolving one block (A) from the assembled block copolymer (A–B). However, most of the resulting porous polymer structures are disordered with wide pore size distributions because of the contraction and swelling from changes in volume, as well as the structured defects formed during template removal. Large porosities have rarely been reported. Furthermore, the resistance of the pore structure to heat and solvents is rather low because the materials are formed by weak van der Waals forces and physical twists between polymer chains, which means that the framework is not connected by covalent bonds. Recently, a procedure for cross-linking lyotropic liquid crystals (LLC) in water was introduced to prepare periodic porous organic mesostructures. Unfortunately, polymerization only occurs between nearest neighboring head groups, and the mesostructured channels are fully occupied with solution. Therefore, it is not surprising that porosity has yet to be reported. Carbon materials, including nanotubes and fullerenes, have attracted considerable attention because of their remarkable properties. The traditional carbonization process for active carbon and related materials can only generate
TL;DR: An understanding of the scope and pattern of the many posttranslational modifications in eukaryotic cells provides insight into the function and dynamics of proteome compositions.
Abstract: The diversity of distinct covalent forms of proteins (the proteome) greatly exceeds the number of proteins predicted by DNA coding capacities owing to directed posttranslational modifications. Enzymes dedicated to such protein modifications include 500 human protein kinases, 150 protein phosphatases, and 500 proteases. The major types of protein covalent modifications, such as phosphorylation, acetylation, glycosylation, methylation, and ubiquitylation, can be classified according to the type of amino acid side chain modified, the category of the modifying enzyme, and the extent of reversibility. Chemical events such as protein splicing, green fluorescent protein maturation, and proteasome autoactivations also represent posttranslational modifications. An understanding of the scope and pattern of the many posttranslational modifications in eukaryotic cells provides insight into the function and dynamics of proteome compositions.
TL;DR: It is demonstrated for the first time how high-aspectratio, self-organized, TiO2 films can be grown by tailoring the electrochemical conditions during titanium anodization by controlling the self-induced acidification of the pore bottom that is caused by the electro chemical dissolution of the metal.
Abstract: Nanotubular material surfaces produced by the electrochemical formation of self-organized porous structures on materials such as aluminum and silicon have attracted significant interest in recent years. While scientific thrust is often directed towards the elucidation of the principles of the self-organization phenomena, technological efforts target applications based on the direct use of the high surface area, for example, for sensing 6] or controlled catalysis, exploit the optical properties in photonic crystals, waveguides, or in 3D arranged Bragg-stack type of reflectors. The highly organized structures may be used indirectly as templates for the deposition of other materials such as metals, semiconductors, or polymers. Over the past few years, nanoporous TiO2 structures have also been formed by electrochemical anodization of titanium. Although several applications have been proposed, a wider impact of these structures has been hampered by the fact that the layers could only been grown to a limiting thickness of a few hundreds of nanometers. Herein we demonstrate for the first time how high-aspectratio, self-organized, TiO2 films can be grown by tailoring the electrochemical conditions during titanium anodization. Figure 1 shows scanning electron microscope (SEM) images of self-organized porous titanium oxide formed to a thickness of approximately 2.5 mm in 1m (NH4)2SO4 electrolyte containing 0.5 wt.% NH4F. From the SEM images it is evident that the self-organized regular porous structure consists of pore arrays with a uniform pore diameter of approximately 100 nm and an average spacing of 150 nm. It is also clear that pore mouths are open on the top of the layer while on the bottom of the structure the tubes are closed by presence of an about 50-nm thick barrier layer of TiO2. The key to achieve high-aspect-ratio growth is to adjust the dissolution rate of TiO2 by localized acidification at the pore bottom while a protective environment is maintained along the pore walls and at the pore mouth. In our previous work in HF and NaF solutions it was established that the thickness of the porous layer is essentially the result of an equilibrium between electrochemical formation of TiO2 at the pore bottom and the chemical dissolution of this TiO2 in an F ion containing solution (Figure 2). The solubility of TiO2 in HF, forming [TiF6] 2 , is essential for pore formation, however, it is also the reason that previous attempts to form porous layers in HF electrolytes always resulted in layer thicknesses in the range of some 100 nm. We tackled the problem by controlling the self-induced acidification of the pore bottom that is caused by the electrochemical dissolution of the metal (Figure 2 a–c). Main reason for the localized acidification is the oxidation and hydrolysis of elemental titanium [Eq. (1), in Figure 2]. The chemical dissolution rate of TiO2 is highly dependent on the pH value (see Figure 2 d). Using a numerical simulation of the relevant ion fluxes we can construct the pH profile in the pore (such as in Figure 2b), in other words, the ideal ion flux for the desired pH profile can then be determined. Furthermore we can tune the dissolution rate by the dissolution current. In other words, using a buffered neutral solution as electrolyte and adjusting the anodic current flow to an ideal value, acid can be created where it is needed, that is, at the pore bottom, while higher pH values are established at the pore mouth as a result of migration and diffusion effects of the pH buffer species (NH4F, (NH4)2SO4). Assuming equilibrium, the flux of dissolving species (leading to acidification at the pore bottom) and the flux of buffering species are equal. The calculations show that for the experimental conditions given in Figure 1 the pH value at the pore bottom is around 2 and increase to about 5 at the pore mouth, this corresponds to a drop in the local chemical etch rate of about 20 times. We used a voltage-sweep technique to achieve a steadystate current and to establish the desired pH profile. The reason to use a voltage-sweep technique rather than a Figure 1. SEM images of porous titanium oxide nanotubes. The crosssectional (a), top (b), and bottom (c) views of a 2.5-mm thick selforganized porous layer. The titanium sample was anodized up to 20 V in 1m (NH4)2SO4 + 0.5 wt. % NH4F using a potential sweep from open-circuit potential to 20 V with sweep rate 0.1 Vs . The average pore diameter is approximately 100 nm and the average pore spacing is approximately 150 nm.
TL;DR: Examples of total syntheses in which metathesis reactions of olefins, enynes, and alkynes played a crucial role and which imparted to these endeavors certain elements of novelty, elegance, and efficiency are highlighted.
Abstract: With the exception of palladium-catalyzed cross-couplings, no other group of reactions has had such a profound impact on the formation of carbon-carbon bonds and the art of total synthesis in the last quarter of a century than the metathesis reactions of olefins, enynes, and alkynes. Herein, we highlight a number of selected examples of total syntheses in which such processes played a crucial role and which imparted to these endeavors certain elements of novelty, elegance, and efficiency. Judging from their short but impressive history, the influence of these reactions in chemical synthesis is destined to increase.
TL;DR: Applications of this technique include the estimation of association constants and mapping the intermolecular interactions in multicomponent systems as well as investigating aggregation, ion pairing, encapsulation, and the size and structure of labile systems.
Abstract: Intermolecular interactions in solution play an important role in molecular recognition, which lies at the heart of supramolecular and combinatorial chemistry. Diffusion NMR spectroscopy gives information over such interactions and has become the method of choice for simultaneously measuring diffusion coefficients of multicomponent systems. The diffusion coefficient reflects the effective size and shape of a molecular species. Applications of this technique include the estimation of association constants and mapping the intermolecular interactions in multicomponent systems as well as investigating aggregation, ion pairing, encapsulation, and the size and structure of labile systems. Diffusion NMR spectroscopy can also be used to virtually separate mixtures and screen for specific ligands of different receptors, and may assist in finding lead compounds.
TL;DR: This Review classifies strategies in the asymmetric synthesis of axially chiral biaryl compounds according to their underlying concepts and critically evaluates their scope and limitations with reference to selected model reactions and applications.
Abstract: A rotationally hindered and thus stereogenic biaryl axis is the structurally and stereochemically decisive element of a steadily growing number of natural products, chiral auxiliaries, and catalysts. Thus, it is not surprising that significant advances have been made in the asymmetric synthesis of axially chiral biaryl compounds over the past decade. In addition to the classic approach (direct stereoselective aryl-aryl coupling), innovative concepts have been developed in which the asymmetric information is introduced into a preformed, but achiral-that is, symmetric or configurationally labile-biaryl compound, or in which an aryl--C single bond is stereoselectively transformed into an axis. This Review classifies these strategies according to their underlying concepts and critically evaluates their scope and limitations with reference to selected model reactions and applications. Furthermore, the preconditions required for the existence of axial chirality in biaryl compounds are discussed.
TL;DR: A versatile new strategy for producing monodisperse solid particles with sizes from 20 to 1000 mm by using a microfluidic device and shaping the droplets in a microchannel and then solidifying these drops in situ either by polymerizing a liquid monomer or by lowering the temperature of a liquid that sets thermally.
Abstract: Herein we describe a versatile new strategy for producing monodisperse solid particles with sizes from 20 to 1000 mm. The method involves the formation of monodisperse liquid droplets by using a microfluidic device and shaping the droplets in a microchannel and then solidifying these drops in situ either by polymerizing a liquid monomer or by lowering the temperature of a liquid that sets thermally. This method has the following features: 1) It produces particles with an exceptionally narrow range of sizes. 2) A new level of control over the shapes of the particles is offered. 3) The mechanism for droplet formation allows the use of a wide variety of materials including gels, metals, polymers, and polymers doped with functional additives. 4) The procedure can be scaled up to produce large numbers of particles. A number of methods exist for making inorganic and organic particles with narrow polydispersity. Inorganic colloids are typically prepared by precipitation reactions from organometallic precursors. Polymer colloids with sizes from 20 nm to approximately 1 mm are usually prepared by a variation of emulsion polymerization techniques. Larger beads are accessible through miniemulsion polymerization,
TL;DR: To overcome the problem of excessive steric hindrance, Glorius and co-workers have successfully developed ligands with “flexible steric bulk” using the conformational flexibility of cyclohexane.
Abstract: The availability of catalysts to perform specific transformations is critical for both industry and academia. Over the years, the success of homogeneous catalysis can be attributed largely to the development of a diverse range of ligand frameworks that have been used to tune the behavior of a variety of metal-containing systems. Advances in ligand design have allowed not only for improvements of known processes in terms of scope, mildness, and catalyst loadings, but also for the discovery of new selective reactions. A good illustration is given by palladium-catalyzed coupling reactions, which are applied to a wide area of endeavors ranging from synthetic organic chemistry to materials science.[1] For these catalytic processes, which represent some of the most powerful and versatile tools available for synthetic chemists, major advances have recently been reported thanks to the use of bulky, electron-rich, phosphines A and cyclic diaminocarbenes (N-heterocyclic carbenes (NHCs)) B (Figure 1).[2] These ligands stabilize the active catalytic species, and accelerate the important catalytic steps, namely oxidative addition, transmetallation, and reductive elimination. On the other hand, excessive steric hindrance can present some drawbacks for the coupling of bulky reactants.[3] To overcome this problem Glorius and co-workers have successfully developed ligands with “flexible steric bulk” using the conformational flexibility of cyclohexane.[4]
TL;DR: From the practical standpoint, SSHC exhibit very high selectivities leading to the production of sharply defined molecular products, just as do their homogeneous analogues.
Abstract: Intellectually, the advantages that flow from the availability of single-site heterogeneous catalysts (SSHC) are many. They facilitate the determination of the kinetics and mechanism of catalytic turnover-both experimentally and computationally-and make accessible the energetics of various intermediates (including short-lived transition states). These facts in turn offer a rational strategic principle for the design of new catalysts and the improvement of existing ones. It is generally possible to prepare soluble molecular fragments that circumscribe the single-site, thus enabling a direct comparison to be made, experimentally, between the catalytic performance of the same active site when functioning as a heterogeneous (continuous solid) as well as a homogeneous (dispersed molecular) catalyst. This approach also makes it possible to modify the immediate atomic environment as well as the central atomic structure of the active site. From the practical standpoint, SSHC exhibit very high selectivities leading to the production of sharply defined molecular products, just as do their homogeneous analogues. Given that mesoporous silicas with very large internal surface areas are ideal supports for SSHC, and that more than a quarter of the elements of the Periodic Table may be grafted as active sites onto such silicas, there is abundant scope for creating new catalytic opportunities.
TL;DR: It is demonstrated on the example of ammonia borane infused in the nanoporous silica that the kinetics of hydrogen release is improved while the purity of hydrogen is increased in comparison with the release from bulk ammoniaborane.
Abstract: One of the imposing barriers to realizing the promise of an energy economy based on hydrogen is onboard hydrogen storage for fuel-cell-powered vehicles. New materials that enable the release of dense, plentiful and pure hydrogen at temperatures less than 85 oC are necessary to move the world from an oil-based economy to a hydrogen economy. We report a novel approach in which we deposit a hydrogen-rich material into a nanoporous scaffold. The role of the scaffold is to impose a nano-phase structure on the hydrogen-rich material thus providing an additional handle on the kinetics and thermodynamics of hydrogen release. We demonstrate on the example of ammonia borane infused in the nanoporous silica that the kinetics of hydrogen release is improved while the purity of hydrogen is increased in comparison with the release from bulk ammonia borane. These findings suggest that hydrogen rich materials infused in nanoscaffolds offer the most promising approach to date for onboard hydrogen storage