Nanoporous Materials: From Catalysis And Hydrogen Storage To Wastewater Treatment
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.5
The second largest known group of microporous materials is the aluminophosphate family, members of which were first synthesised in 1978 using amines as templates a.k.a. structure-directing agents. The aluminophosphate AlPO4 frameworks are formed from vertex-sharing AlO4 and PO4 tetrahedra. The Al/P ratio is usually one, making the framework electrostatically neutral with no active sites present. The isomorphous substitution of framework aluminium or phosphorus atoms with divalent transition metals, such as Mn, Co, Fe, etc. generates negative framework sites and makes these microporous materials very successful selective catalysts. An example of industrially very successful aluminophosphate catalyst is SAPO-34. Research has now progressed to form new microporous metallo-phosphates from many elements other than aluminium, such as gallophosphates and transition-metal phosphates, with differing chemistry and coordination geometry that have lead to new structures and new possible applications.6
The careful selection of organic ligands as building blocks in the formation of novel metal-organic (MOF) and inorganic-organic (hybrid) microporous frameworks has recently open a promising route to a variety of new advanced materials exhibiting the flexibility and functionality of organic and hydrothermal stability of inorganic components.7 The robustness of these networks depends on the strength and dimensionality of the bonds, i.e. the metal-ligand interactions. The research has focused on the metal phosphates carboxylates8 and metal carboxylates9, where carboxylate groups, like oxalates and higher dicarboxylates, have acted as a linker between inorganic moieties, usually PO4 or MeO4 and MeO6 polyhedra (Me=Zn, Fe, Co, Cd, Mn, etc.).
Figure 1: Structure and possible cation sites (SI, SII, etc.) in zeolite faujasite. Due to the presence of aluminium, zeolites exhibit a negatively charged framework, which is counter-balanced by positive cations resulting in a strong electrostatic field on the internal surface. These cations can be exchanged to fine-tune the pore size or the adsorption characteristics [reproduced with permission from ref. 106].
Figure 2: Mesoporous silicates were reported by Mobil Oil scientists in 1992; (a) MCM-41, which stands for Mobil Composition of Matter No. 41, shows a highly ordered hexagonal array of one-dimensional pores with a very narrow pore size distribution. The walls, however, very much resemble amorphous silica. (b) MCM-48 has a cubic mesostructure, i.e. arrangement of the pores. 120 Acta Chim. Slov. 2006, 53, 117–135 Zabukovec Logar and Kaučič Nanoporous Materials: From Catalysis ….
The advantage of the shape confinements inside the pores of microporous materials can turn into severe limitations when large, bulky reactants have to be processed. These limitations seemed to be overcome by the discovery of MS41 family of silicate amorphous mesoporous materials with large internal surface areas and narrow pore size distributions by Mobil Oil scientists in 1992.10
Their most known and studied material is MCM-41 mesoporous silicate with one-dimensional hexagonal arrangement of the pores. The cubic MCM-48 material exhibits three-dimensional pore system that is more resistant to pore blocking and allow faster diffusion of reactants than a one-dimensional array of pores (Figure 2). The long-range ordering of the pores and the potential for isomorphous substitution with transition metals, enabling formation of catalytically-active centres, have incited applications in areas such as adsorption, separation and catalysis, especially in processes where bulkier molecules are used.11 A variety of mesoporous silicates, aluminosilicates, aluminophosphates, and other transition metal oxides and phosphates have been developed and synthesised using several supermolecular assembly pathways.12
Most of the silica- and phosphate-based mesoporous materials have relatively lower catalytic activity and hydrothermal stability than their microporous analogues. This is attributed to the low acidity or low oxidation ability of catalytically active species that are strongly related to the amorphous nature of the pore walls and which severely hinder their practical applications.13 Additionally, molecular size exclusion or shape-selective processing is not as readily obtained on solids with amorphous pore walls because they usually have a broader distribution of pore sizes compared to microporous crystalline materials. Various strategies have been employed to overcome these problems. One of them is to combine major advantages of crystalline microporous and mesoporous material and prepare so-called microporous/mesoporous composites.14 Different methodologies have been described for their preparation.15 The two-step preparation of microporous/mesoporous solid, which encompasses the synthesis of nanosized microporous crystallites that are later organised in the mesoporous structure by using large surfactant molecules, has recently drawn greatest attention.
1.2. Designing nanoporous materials for clean technology, green chemistry, and sustainable development
There is an increasing need for cleaner fuels and industrial processes that would minimise the consumption of energy, production of waste, or the use of corrosive, explosive, volatile, and nonbiodegradable materials and a range of nanoporous materials already meet the majority of these demands.16
Microporous materials are mostly used as heterogeneous acid and redox catalysts in petroleum industry and in the production of chemicals for various types of shape-selective conversion and separation reactions.17 They form the basis of new environment-friendly technologies, involving cheaper, more efficient and more environment-friendly ways for carrying out chemical reactions. Transition metal-modified microporous molecular sieves with aluminosilicate and aluminophosphate frameworks catalyse a wide variety of synthetically-useful oxidative transformations with clean oxidants such as hydrogen peroxide or oxygen under relatively mild conditions with the advantage of facile recovering and recycling, if compared to homogeneous liquid phase catalyst, like sulphuric acid.18 A number of applications in waste treatment processes, including removal of heavy metals and radioactive species, as well as ammonia, different phosphates and toxic gasses from water, soil and air are due to the unique structural and surface physico-chemical properties of microporous materials, such as excellent absorption and ion-exchange capacities. Among very recent applications of microporous materials are also optical-electronic devices, biological materials and implants, sensors, and membranes for gas separation.19 Gas sorption studies on new inorganic-organic (hybrid) and metal-organic (MOF) framework structures have shown their great potential in the field of solid state hydrogen storage technologies.20
Somewhat less developed, but still highly important are applications of mesoporous solids. Expansion of the functionality and improved control over the hydrothermal and chemical stability of ordered mesoporous solids by modified and optimised synthetic or post-synthetic routes in recent years have been crucial for their increasing application in the fields of catalysis and also optics and semiconductor industry.21 Intensive research efforts have also been driven by the emerging applications such as biosensors, drug delivery, gas separation, energy storage and fuel cell technologies.22
The main challenges in research of nanoporous materials include the understanding of structure-property relations and tailor-design of nanostructures for specific applications. A substantial progress has been made in the control of the pore shape and sizes and active sites’ activity and distribution (Figure 3). Tailoring of acid catalysis and other processes over zeolites and zeolite-like materials is probably as close as one can get. For mesoporous solids the main goal, the control of homogeneous surface with single active sites, i.e. the regular distribution of active sites that would lead to an optimised catalytic performance, has still not been achieved.121 Acta Chim. Slov. 2006, 53, 117–135 Zabukovec Logar and Kaučič Nanoporous Materials: From Catalysis ….
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