In the present review, aerogels designate dried gels with a very high relative pore volume. These are versatile materials that are synthesized in a first step by low-temperature traditional sol-gel chemistry. However, while in the final step most wet gels are often dried by evaporation to produce so-called xerogels, aerogels are dried by other techniques, essentially supercritical drying. As a result, the dry samples keep the very unusual porous texture which they had in the wet stage. In general these dry solids have very low apparent densities, large specific surface areas, and in most cases they exhibit amorphous structures when examined by X-ray diffraction (XRD) methods. In addition, they are metastable from the point of view of their thermodynamic properties. Consequently, they often undertake a structural evolution by chemical transformation, when aged in a liquid medium and/or heat treated. As aerogels combine the properties of being highly divided solids with their metastable character, they can develop very attractive physical and chemical properties not achievable by other means of low temperature soft chemical synthesis. In other words, they form a new class of solids showing sophisticated potentialities for a range of applications. These applications as well as chemical and physical aspects of these materials were regularly detailed and discussed in a series of symposia on aerogels,1-5 the last of them being held in Albuquerque in 2000.6 Reviews were also regularly published, either on both xerogels and aerogels7 or more focused on the applications of aerogels.8-13 The particularly interesting properties of aerogels arise from the extraordinary flexibility of the solgel processing, coupled with original drying techniques. The wet chemistry is not basically different for making xerogels and aerogels. As this common basis has been extensively detailed in recent books,14 it does not need to be reviewed. Compared to traditional xerogels, the originality of aerogels comes from * To whom all correspondence should be addressed. † Institut de Recherches sur la Catalyse. ‡ Laboratoire d’Application de la Chimie à l’Environnement. 4243 Chem. Rev. 2002, 102, 4243−4265

https://is.muni.cz/el/1431/podzim2006/C7780/um/Read/2711172/aerogel_Pajonk_ChRev02_4243.pdf

Chemistry of Aerogels and Their Applications

The systematic fabrication of advanced materials will require the construction of architectures over scales ranging from the molecular to the macroscopic. The basic constructional processes of biomineralization—supramolecular pre-organization, interfacial molecular recognition (templating) and cellular processing—can provide useful archetypes for molecular-scale building, or ‘molecular tectonics’, in inorganic materials chemistry.

Molecular tectonics in biomineralization and biomimetic materials chemistry

Silica aerogel; synthesis, properties and characterization

The use of nanocomposites in materials processing can lead to monophasic or multiphasic ceramics, glasses or porous materials, with tailored and improved properties. This review deals with a variety of nanocomposites such as sol–gel, intercalation, entrapment, electroceramic and structural ceramic types, which exhibit superior properties when compared to the monophasic or microcomposite alternatives. The utilization of nanocomposites in materials processing is forecasted to have a major impact in catalytic, sensor, optical, electroceramic and structural ceramic materials.

Feature article. Nanocomposites

Methane adsorption data (both experimental and simulated) under conditions of direct relevance for vehicular natural gas storage, i.e., at 500 psig and ambient temperature, has been compiled from the literature for various microporous adsorbents and discussed in this work. Characterization of microporosity has been briefly reviewed, followed with a discussion on the porous structure of natural gas adsorbents. A common trend of gravimetric methane adsorption capacity scaling with surface area among the diverse microporous adsorbents (viz., coals, carbons, zeolites, silica gel and an MCM-41 type material) is demonstrated. Further, it is substantiated and emphasized that increasing the adsorbent surface area on a volumetric basis is very important for vehicular natural gas storage where the fuel storage volume is a constraint. The effect of other adsorbent properties such as heat of adsorption and heat capacity on the natural gas storage capacity is also discussed.

Porous adsorbents for vehicular natural gas storage: a review

EXPANDABLE layer silicates such as montmorillonite can be converted to efficient heterogeneous catalysts by introducing catalytically active sites or guest species between the layers or on the external surfaces. Attempts to produce intercalated zero-valent transition-metal particles in layer silicates, by hydrogen reduction for example, have, however, failed: the layers tend to collapse1, sometimes followed by deposition of metal particles on the external surfaces2–4. Here we describe the successful intercalation of copper metal clusters of 4–5 A in montmorillonite by in situ reduction of Cu2+ ions using ethylene glycol. These metal-cluster intercalates were stable up to at least 500 °C. The clusters prop the silicate layers apart, much as metal oxides do in pillared clays5, and may thus be able to introduce unique catalytic product selectivity through a molecular sieving effect similar to that in cluster-loaded zeolites. As metal clusters of these dimensions behave very differently from the bulk metal6, intercalates of this sort may prove to be versatile catalysts.

Intercalation of copper metal clusters in montmorillonite

27Al and 29Si magic angle spinning nuclear magnetic resonance spectroscopy revealed that the maximum amount of Al that can be substituted for Si in the tobermorite structure is 15 to 20 mol%. Powder X-ray diffraction and cation exchange studies corroborate the above finding. Anomalous tobermorite structures resulted in all cases, and hydrogarnet appeared beyond the 15 to 20 mol% Al substitution limit for Si. The sorption of water molecules by synthetic [Al3++ Na+]-substituted tobermorites and their cation-exchanged forms heated at 200°C under vacuum (≥10−3 torr) was measured at 25°C in order to probe the nature of the ion-exchange cavity. Samples with ≤30 mol% of Al substitution for Si showed isotherms of Brunauer, Demming, Demming, Teller (BDDT) type I and gave better linearity with the Langmuir plot than with the BET plot. Samples with ≥35 mol% of Al substitution for Si showed BDDT type II isotherms and gave better linearity with the BET than with the Langmuir equation. The Langmuir monolayer capacity was found to depend on the Al content and on the cationic form. The Li+- and Na+-exchanged tobermorites with about 20 mol% of Al substitution for Si showed the highest Langmuir monolayer water sorption capacity. The Cs+-exchanged tobermorites showed a smaller capacity than the Li+- or Na+-exchanged samples, which can be ascribed to clogging of the ion-exchange cavity by the large Cs+ ions.

Substituted Tobermorites: 27Al and 29Si MASNMR, Cation Exchange, and Water Sorption Studies

Monolithic nanocomposite aerogels of two aluminosilicate compositions have been prepared and characterized by different techniques. The results show that high surface area and mesoporosity can be preserved in the above nanocomposite aerogels by heating at 1000 °C, unlike the single component aerogels. The presence of alumina as a second phase prevented their densification which resulted in surface areas on the order of 500–600 m2/g and mesopores of about 5–6 nm in diameter after heat treatment at 1000 °C. These novel nanocomposite aerogels are expected to find applications at high-temperatures in separations, insulation, catalysis, etc.

Nanocomposite aerogels: The SiO2–Al2O3 system

Copper metal clusters of 4–5 A have been intercalated in the interlayers of montmorillonite by in situ reduction of bulky copper acetate hydroxide species with ethylene glycol (polyol process) at 195 °C. Various amounts of CuII as copper acetate hydroxide hydrate in excess of ion-exchange capacity were introduced into the interlayers by titrating copper acetate solution with sodium hydroxide in the presence of montmorillonite. The exchange of fully developed copper acetate hydroxide hydrate interlayers in the montmorillonite gave a basal spacing of 19.6 A. The reduced samples exhibited a basal spacing of 14.4 A indicating the presence of 4.8 A copper metal clusters after subtracting the thickness of the silicate layer (9.6 A). Further evidence for formation of copper metal clusters in the interlayers was obtained from transmission electron microscopy (TEM) and UV-VIS-NIR spectroscopy. Both water sorption isotherms and nitrogen B.E.T. surface-area measurements indicated the porous nature of Cu metal intercalated phases similar to pillared clays. During the process of reduction, large amounts of copper metal particles were expelled from the interlayers which grew to ca. 0.1-0.5 µm on the external surfaces of montmorillonite.

Reduction of copper acetate hydroxide hydrate interlayers in montmorillonite by a polyol process. A new approach in the preparation of metal-supported catalysts

A new approach for the preparation of metal supported montmorillonite catalyst is reported here for the first time. The polyol process in which ethylene glycol is used as reducing agent was employed for the preparation of uniformly distributed copper metal particles on and within the montmorillonite matrix. A hydroxy copper acetate intercalated form of montmorillonite was refluxed at 195 °C with liquid ethylene glycol for 6 h. The reduction of copper acetate to copper metal was apparent from the red color of the reduced product. An X-ray powder diffraction pattern showed the deposition of copper metal on the external surface of montmorillonite and scanning electron micrographs exhibited uniformly distributed spherical copper metal particles of about 0.5 μm size. This process may also be extended for the preparation of other fine metal particles supported on montmorillonite catalysts.

P. B. Malla

Papers

Intercalation of copper metal clusters in montmorillonite

Substituted Tobermorites: 27Al and 29Si MASNMR, Cation Exchange, and Water Sorption Studies

Nanocomposite aerogels: The SiO2–Al2O3 system

Reduction of copper acetate hydroxide hydrate interlayers in montmorillonite by a polyol process. A new approach in the preparation of metal-supported catalysts

Preparation of metal supported montmorillonite catalyst: A new approach