Nano and mesoporous materials
Porous materials with an inorganic framework, such as zeolites, and characterised by good heat stability, are well adapted for industrial applications that could be as varied as catalysis, ionic exchange, or even gas separation.
These three-dimensional materials present an uninterrupted arrangement of TO4 (T = Al or Si) tetrahedrons linked by the corners, and separated by tunnels or cavities (refer to the figure below), in which alkaline-cations and water molecules are found. Zeolites are often stable at dehydration, therefore it is possible to access high-porosity structures (13.2 Å in diameter, released for clovérite).
The alkaline cations were quickly replaced by organic species (“templates” or “structuring agents”) in order to harness the size and shape of the pores. The template molecules play several roles, simultaneously. As anticipated, they exert influence on the cristallisation of zeolites and more particularly on the shape and size of the final cavities. The cyclic amines favour the formations of pores, while the linear amines more often lead to structures that contain tunnels. However, there exists no structuring zeolite-agent specificities; numerous different zeolites may be prepared from the same molecule and vice-versa. Since the zeolithic frameworks are generally anionic (defect in charge introduced when the silicon is substituted with aluminium), these molecules also act as counterions. As they are often larger in size as the alkaline cations, they are also smaller in number. This implies that the charge to compensate must be smaller. The use of large molecules therefore generally leads to phases higher in silicon concentration, thus more thermally stable. The calcination of structuring agents at temperatures of the order of 500°C, generates the porosity of these zeolites.
Fluoride anions also assume the template role, and are present in the final structure of the compound; they are located at the centre of a cube where the summits are occupied by silicon atoms.
Over time, the modifications to the synthesis atmosphere, as well as the nature of the reactives, led to the emergence of new families of composites. Of these, one of the most significant is the alumino-phosphate family, which is potentially adaptable to the same applications. This family, in which two Si atoms are substituted by an A1/P pair, presents structures that are similar to zeolites, and which can also lead to very large tunnel openings (12.7 Å for the VPI-5 alumnio-phosphate). The replacement of aluminium with cations that can easily adopt tetrahedral coordinence, led to new families such as vanado-silicates, which also offer open and stable structures.
Recently, the introduction of organic molecules into the framework permitted new types of materials to be accessed, such as the porous hybrid solids whose skeletons are constructed by the connection of inorganic entities through organic molecules. The organic molecules must be functionalised either by complexing functions (phosphonate groups (PO32-), carboxylates (CO2-), sulfonates (SO32-), etc.), or by heteroatoms in order to be able to establish a covalent or covalent-ionic bond with the metal or non-metal in the inorganic network. Two classes of materials result, issued from two different approaches. The first comes from coordination chemistry. By integrating a heteroatom into the metal coordination sphere, a discrete organometallic complex is formed, whose polymerization provides a solid with a higher dimensionality. The hybrid composites produced by this strategy are found in literature under coordination polymers. The second comes from the more traditional channel of the chemistry of solids, and consists of connecting inorganic entities with various dimensionalities to organic molecules functionalised by complexing species.
The overall dimensionality of the solid varies according to the nature of this molecule (its rigidity, the number of connecting functions and their relative position, the affinity of the heteroatom with metal, etc.), and the nature of the metal. The spacing between inorganic parts is a direct result of the size of the connecting agents. Therefore the goal of a modulation of the cavity openings in two- or three-dimensional structures is attainable.
Just as with zeolites and porous phosphates, the main significance of these porous composites is that they may be applied in gas separation, catalysis, molecular separation, etc. However, they are also significant because they exhibit remarkable potential as host structures for molecules with non-linear optical or magnetic properties.