A Robust Approach to Inorganic Aerogels: The Use of Epoxides in Sol–Gel Synthesis

Abstract

Over the last decade, the diversity of metal oxide materials prepared using sol-gel techniques has increased significantly. This transformation can be attributed, in part, to the development of the technique known as epoxide-initiated gelation. The process utilizes organic epoxides as initiators for the sol-gel polymerization of simple inorganic metal salts in aqueous or alcoholic media. In this approach, the epoxide acts as an acid scavenger in the sol-gel reaction, driving the hydrolysis and condensation of hydrated metal species. This process is general and applicable to the synthesis of a wide range of metal oxide aerogels, xerogels, and nanocomposites. In addition, modification of synthetic parameters allows for control over the structure and properties of the sol-gel product. This method is particularly amenable to the synthesis of multi-component or composite sol-gel systems with intimately mixed nanostructures. This chapter describes both the reaction mechanisms associated with epoxide-initiated gelation as well as the variety of materials that have been prepared using this technique. IntroductionAerogels [1, 2] are a special class of open-cell foams that exhibit many interesting properties, such as low mass densities, continuous porosities, and high surface areas. These unique properties are derived from the aerogel microstructure, which typically consists of three-dimensional networks of interconnected nanometer-sized primary particles. Because of their unusual chemical and textural properties, aerogels have been investigated for a wide variety of applications, including catalysis, sorption, insulation, energy storage, and even for cosmic dust collection (Parts VIII-XV). To further expand the utility of these materials, recent efforts have focussed the development of new synthetic processes that can be used to tailor both the composition and structure of aerogels for different applications. Aerogels are typically prepared using sol-gel chemistry, a process that involves the transformation of molecular precursors into highly cross-linked inorganic or organic gels that can then be dried using special techniques to preserve the tenuous solid network. For organic and carbon

Porous Materials for Applications in Energy and Environment

“…Organic epoxides allow the formation of the three-dimensional network of the gel, because the protonation of the oxygen of the epoxide occurs by an acid, followed by the opening of the ring by nucleophilic attack by the conjugate base as shown in Fig. 6 [46].…”

“…Different organic epoxides have been used in the synthesis of metal oxide aerogels, the difference between each of them is the time it takes to reach of gel point, although it also depends on the nature of metal precursors and/or metal salts being used. Table 2 shows some organic epoxides and the gelling time they take to form a gel of iron oxide (III) [46].…”

Hierarchically porous monoliths based on low-valence transition metal (Cu, Co, Mn) oxides: gelation and phase separation

“…The second category of precursor is ionic salts of metallic elements. Most metal oxide-based monolithic gels and HP monoliths so far reported were prepared via an epoxide-mediated method, as developed by Gash's group [ 27 ]. Compared with most metal alkoxides, the inorganic metal salts are less reactive toward hydrolysis and can generally be purchased at lower cost.…”

Anomalous Anisotropic Nanoparticle Aggregation in Cu2(OH)3Br Gels

“…The sol–gel method is a versatile approach for the synthesis of monolithic nanoporous materials such as aerogels. − In this method, an unstable, typically polydisperse, colloid spontaneously aggregates to form a 3D percolating solid network . In particular, Cu II aerogels are useful as precursors for nanoporous Cu metal monoliths .…”