Porous Inorganic Solids Research Articles

Catalysis in Confined Spaces of Metal Organic Frameworks

AbstractCompared to homogeneous catalysis, heterogeneous catalysis can achieve product selectivity due to spatial constraints and differences in molecular dimensions of substrates and products by performing the reaction in a confined space. Shape selectivity for substrates, products or transition states is a well‐established phenomenon observed in porous inorganic solids like zeolites with considerable impact in petrochemistry. More recently, metal organic frameworks (MOFs) have emerged as heterogeneous solid catalysts for a broad range of organic reactions due to their unique properties of large surface area, tuneable pore volume and high density of sites. The present mini‐review aims to describe the current evidences showing MOFs as shape‐selective catalysts. After a brief introduction on the concept, types and examples of shape‐selectivity, the available data on MOFs as shape‐selective catalysts have been organized according the nature of active sites promoting the reaction, starting from metal nodes as Lewis acid sites and commenting examples of encapsulated metal nanoparticles (NPs) and molecular guests as centers in various reactions such as addition to carbonyl compounds, hydrogenations, oxidations and oxidative dehydrogenations. The final section provides our view on the current achievements and prospective for future developments in the field.

A dynamic and multi-responsive porous flexible metal\u2013organic material

Stimuli responsive materials (SRMs) respond to environmental changes through chemical and/or structural transformations that can be triggered by interactions at solid-gas or solid-liquid interfaces, light, pressure or temperature. SRMs span compositions as diverse as organic polymers and porous inorganic solids such as zeolites. Metal–organic materials (MOMs), sustained by metal nodes and organic linker ligands are of special interest as SRMs. SR-MOMs have thus far tended to exhibit only one type of transformation, e.g. breathing, in response to one stimulus, e.g. pressure change. We report [Zn2(4,4′-biphenyldicarboxylate)2(4,4′-bis(4-pyridyl)biphenyl)]n, an SR-MOM, which exhibits six distinct phases and four types of structural transformation in response to various stimuli. The observed structural transformations, breathing, structural isomerism, shape memory effect, and change in the level of interpenetration, are previously known individually but have not yet been reported to exist collectively in the same compound. The multi-dynamic nature of this SR-MOM is mainly characterised by using in-situ techniques.

Mechanical characterization of highly porous inorganic solids materials by instrumented micro-indentation

This study focuses on the mechanical characterization of highly porous inorganic solids through spherical micro-indentation. The mechanical properties of a porous plaster, used here as a model material, at two different porosity ratios (30 and 60vol.%) were determined using analytical methods and numerical simulation. The Young’s modulus, hardness, contact stress–strain curves and parameters associated with the Drücker–Prager yield criterion (cohesion, friction and dilatancy) were determined through an inverse method. Porous inorganic solids show a specific mechanical behaviour, with densification of the indented area promoted by the amount of porosity, without initiation of macroscopic brittle cracks.

Transposition of chirality from diphosphonate metal–organic framework precursors onto porous lanthanide pyrophosphates

Chiral porous inorganic materials, lanthanide pyrophosphates, were prepared from chiral porous metal-organic framework precursors, which upon thermal decomposition transpose their chirality and porosity onto the inorganic framework. It is argued that this synthesis concept may be extended to other chiral porous inorganic solids.

ChemInform Abstract: Emerging Concepts in Solid‐State Hydrogen Storage: the Role of Nanomaterials Design

AbstractReview: 264 refs.

Mesoporous Metal Oxides by Vapor Infiltration and Atomic Layer Deposition on Ordered Surfactant Polymer Films

Catalysis, chemical separations, and energy conversion devices often depend on well-defined mesoporous materials as supports or active component elements. Herein, we show that ordered assembled organic surfactant films can directly template porous inorganic solids with surface area exceeding 1000 m(2)/g by infusing the polymers with reactive inorganic vapors, followed by anneal. The specific surface area, pore size, chemical composition, and overall shape of the product material are tuned by choice of the polymer and precursor materials as well as the influsion and postinfusion treatment conditions. X-ray diffraction, infrared spectroscopy, and electron microscopy show that vapor infusion changes both the physical and chemical structure of the starting ordered polymer films, consistent with quantified trends in specific surface area and pore size distribution measured by nitrogen adsorption after film annealing. This method yields porous TiO(2) films, for example, that function as an anode layer in a dye-sensitized solar cell.

Bulk hydrolysis and solid–liquid sorption of heavy metals in multi-component aqueous suspensions containing porous inorganic solids: Are these mechanisms competitive or cooperative?

Fundamental aspects of the removal of heavy metals from aqueous streams under conditions of competition among the various species have been studied between pH 3 and 9 on Spherosil XO75LS, ordered mesoporous MCM-41 and MCF silicas, as well as a MCF sample grafted with (3-aminopropyl)methoxydimethylsilane (AMPS-MCF). Cd(II), Co(II), Pb(II), or Sr(II) nitrate solutions were used to determine the percentage of metal uptake by each solid at 298K as a function of the pH of the equilibrium solution, at an initial metal concentration of 0.0001molL−1 and the ionic strength being fixed with 0.01molL−1 NaNO3. Almost complete retention of the heavy metals on the four solid samples was observed, with the process beginning at pH values smaller than those marking the onset of “bulk” precipitation of a given metal in “free” solution. The heavy metal-uptake mechanism was regarded as hydrolysis-like phenomenon in metal-containing solid suspensions. Weak adsorption of metal species from slightly acidic and neutral solutions was a kind of nucleation step. Adding cadmium to an equimolar solution containing cobalt, lead, or strontium showed no significant effect on the retention of the main metal component. This indicated the great independence of the retention mechanisms.

Emerging concepts in solid-state hydrogen storage: the role of nanomaterials design

This perspective highlights the state-of-the-art solid-state hydrogen storage and describes newly emerging routes towards meeting the practical demands required of a solid-state storage system. The article focuses both on the physical and chemical aspects of hydrogen storage. Common to both classes of storage material is the concept of nanostructure design to tailor kinetics and thermodynamics; whether this be control of functionalised porosity or crystalline growth on the nanoscale. In the area of chemical storage, different processing and nanostructuring techniques that have been employed to overcome the barriers of slow kinetics will be discussed in addition to new chemical systems that have emerged. The prospects of porous inorganic solids, coordination polymers (metal organic frameworks; MOFs) and other polymeric matrices for physical storage of hydrogen will be highlighted. Additionally the role of inorganic nanostructures as evolving materials “intermediate” between physical and chemical storage systems will be discussed and their place within the fine thermodynamic balance for optimum hydrogen uptake and release considered.

Free-standing mesoporous silica films with tunable chiral nematic structures

Chirality at the molecular level is found in diverse biological structures, such as polysaccharides, proteins and DNA, and is responsible for many of their unique properties. Introducing chirality into porous inorganic solids may produce new types of materials that could be useful for chiral separation, stereospecific catalysis, chiral recognition (sensing) and photonic materials. Template synthesis of inorganic solids using the self-assembly of lyotropic liquid crystals offers access to materials with well-defined porous structures, but only recently has chirality been introduced into hexagonal mesostructures through the use of a chiral surfactant. Efforts to impart chirality at a larger length scale using self-assembly are almost unknown. Here we describe the development of a photonic mesoporous inorganic solid that is a cast of a chiral nematic liquid crystal formed from nanocrystalline cellulose. These materials may be obtained as free-standing films with high surface area. The peak reflected wavelength of the films can be varied across the entire visible spectrum and into the near-infrared through simple changes in the synthetic conditions. To the best of our knowledge these are the first materials to combine mesoporosity with long-range chiral ordering that produces photonic properties. Our findings could lead to the development of new materials for applications in, for example, tuneable reflective filters and sensors. In addition, this type of material could be used as a hard template to generate other new materials with chiral nematic structures.

Materials chemistry: macroporous crystalline vanadium oxide foam.

Porous inorganic solids can be synthesized in complex forms, and here we describe a simple method to create an ultralight, macroporous, crystalline vanadium oxide foam by bubbling oxygen gas produced in situ through a viscous vanadium oxide gel. This foaming process could be extended to other metal oxides.

Nanocrystalline metal oxides as unique chemical reagents/sorbents.

A new family of porous inorganic solids based on nanocrystalline metal oxides is discussed. These materials, made up of 4-7 nm MgO, CaO, Al2O3, ZnO, and others, exhibit unparalleled destructive adsorption properties for acid gases, polar organics, and even chemical/biological warfare agents. These unique sorption properties are due to nanocrystal shape, polar surfaces, and high surface areas. Free-flowing powders or consolidated pellets are effective, and pore structure can be controlled by consolidation pressures. Chemical properties can be adjusted by choice of metal oxide as well as by incorporating other oxides as monolayer films.

Fullerenes in Sol-Gel Materials

The family of fullerene molecules is composed of a large variety of compounds that have been synthesized following the discovery of C60 in 1985. The chemistry of fullerenes, developed in these last years, has allowed designing the properties of this family of molecules for specific applications in materials science. One of the main tasks to build up solid state devices based on fullerenes is the synthesis of materials doped with a highly dispersed and homogeneous distribution of fullerenes. Many of the peculiar photophysical properties, such as the reverse saturable absorption used to obtain a solid state optical limiter, are in fact lost in the aggregates of fullerenes. Sol-gel processing allows preparing inorganic oxides and hybrid organic-inorganic materials at low temperatures and presents an interesting alternative to organic polymers to entrap molecules of the fullerene family in a solid matrix. Porous inorganic solids and aerogels are also important classes of materials that can be synthesized via sol-gel and can act as hosts of fullerenes. In the present article we have reviewed the main achievements of sol-gel processing of fullerene based nanocomposite materials.

Synthesis of oriented films of mesoporous silica on mica

POROUS inorganic solids find wide application in, for example, catalysis and separation technologies. One of the most promising routes to such materials exploits cooperative interactions between a supramolecular organic template and inorganic precursor molecules to create an ordered porous solid1–7. This general approach offers considerable flexibility, and has led to materials with a wide range of bulk morphologies and controlled pore diameters spanning several orders of magnitude8–10. From a technological point of view, the preparation of such materials in the form of mesoporous thin films is desirable, but attempts to achieve this aim have been largely unsuccessful, resulting in disordered materials11. Here we report the synthesis of thin (∼0.2–1.0μm) ordered films of mesoporous silica on freshly cleaved mica substrates. The films are stable on removal from the substrate. We propose a model to explain the formation of these films, in which the surface structure and reactivity of the mica substrate controls the orientation of the micellar precursor species, which in turn imposes order on the resulting solid film.