Porosity is an intersting property although it describes more or less void. Voids in porous solids can easily be occupied by guest species, creating a rich host-guest chemistry useful for the storage, transport, and separation of mixtures in the gas or liquid state. Conventional industrial adsorbents based on clays, carbons, and silicates are generally perceived as rigid brittle entities. Although an increase in porosity decreases the mechanical stability of a solid, shape-persistent silica aerogels with porosities beyond 95% can be obtained by activation procedures involving supercritical fluids. Zeolites are superior to amorphous porous solids, especially for separation applications, because of their crystalline structure and ordered porosity. However, the rather dense framework of zeolites, despite their predominantly microporous structure, hampers application in areas such as gas storage. This limitation was addressed by introducing porous coordination polymers (PCPs) and metal–organic frameworks (MOFs). The development of theat feild is nicely summarized in „The chemsitry and application of metal-organic frameworks“

In these porous crystals, metal ions or clusters are linked by multifunctional organic ligands via coordination chemistry to produce coordination networks with voids or channels. Due to abundant inorganic and organic building blocks, materials with defined porous structures are almost infinitely accessible, with functionalized surfaces and unprecedented surface areas can be established.

Many scientists in different fields are now using MOF chemistry (also termed reticular chemistry) to obtain new functionality in crystalline solids. However, reticular chemsitry provdes more possibilities than just assemblying molecules in an order solid. The selection of various network topologies allows to design features such as anisotropy and local symmetry that are otherwise unachievable in molecular solids.