Porous inorganic solids have found great utility as catalysts and sorption media because of their large internal surface area, i.e. the presence of voids of controllable dimensions at the atomic, molecular, and nanometer scales. With increasing environmental concerns worldwide, nanoporous materials have become more important and useful for the separation of polluting species and the recovery of useful ones. Their prospective applications include the use as templates for the production of electrically conducting nanowires and also for highly selective biosensors and biomembrane materials. Inorganic-organic or hybrid nanoporous crystalline materials have recently attracted much attention and increasing interest due to their promising use in gas processing and hydrogen storage. This review covers our recent developments in the synthesis, characterisation and property evaluation of new nanoporous inorganic and some hybrid solids with the emphasis on the silica and phosphate-based frameworks by using hydrothermal and microvawe procedures with X-ray diffraction, spectroscopic (XAS, NMR) and electron microscopy characterization techniques. The functionalization of nanoporous materials by physical and/or chemical treatments, studies of their fundamental properties, such as catalytic effects or adsorption and their applications, emphasising (1) catalysis, (2) hydrogen and energy storage, and (3) environmental pollution control are also reviewed.
Keywords: nanoporous materials, microporous materials, mesoporous materials, zeolites, inorganic-organic hybrids, catalysts, hydrogen storage, wastewater treatment
1.1. Nanoporous materials on micro- and meso-scale.
International Union of Pure and Applied Chemistry (IUPAC) classifies porous materials into three categories1 – microporous with pores of less than 2 nm in diameter, mesoporous having pores between 2 and 50 nm, and macroporous with pores greater than 50 nm. The term nanoporous materials has been used for those porous materials with pore diameters of less than 100 nm. Many kinds of crystalline and amorphous nanoporous materials such as framework silicates and metal oxides, pillared clays, nanoporous silicon, carbon nanotubes and related porous carbons have been described lately in the literature.2 This review will focus on the microporous and mesoporous silica- and phosphate-based materials with ordered pore structures.
Microporous materials are exemplified by crystalline framework solids such as zeolites, whose crystal structure defines channels and cages, i.e. micropores, of strictly regular dimensions (Figure 1). They can impart shape selectivity for both the reactants and products when involved in the chemical reactions and processes. The large internal surface area and void volumes with extremely narrow pore size distribution as well as functional centres homogeneously dispersed over the surface make microporous solids highly active materials. Over the last decade, there has been a dramatic increase in synthesis, characterization and application of novel microporous materials.3 The 119 Acta Chim. Slov. 2006, 53, 117–135 Zabukovec Logar and Kaučič Nanoporous Materials: From Catalysis composition of crystalline microporous materials ranges from aluminosilicates to aluminophosphates and gallophosphates or recently discovered inorganic-organic hybrids.4 Zeolites, which represent the largest group of microporous materials, are crystalline inorganic polymers based on a three-dimensional arrangement of SiO4 and AlO4 tetrahedra connected through their oxygen atoms to form large negatively-charged lattices with Brønsted and Lewis acid sites. These negative charges are balanced by extra-framework alkali and/or alkali earth cations. The most known zeolites are silicalite-1, ZSM-5, zeolite Beta and zeolites X, Y, and A. The incorporation of small amounts of transition metals into zeolitic frameworks influences their properties and generates their redox activity. Zeolites with their well–organised and regular system of pores and cavities also represent almost ideal matrices for hosting nanosized particles e.g. transition metal oxides that can also be involved in catalytic applications.