The defining and unique characteristic of these materials is that they have well defined pores and cavities of molecular dimensions. Due to the porosity and cavities of these materials, they have a large internal surface where molecules may enter and subsequently become attached or undergo a chemical transformation. It is possible to fine tune both the framework topology, i.e. the size and shapes of the pores and cavities, as well as the density and nature of the active sites. This makes these materials extraordinarily flexible in applications within catalysis, gas adsorption, and separation.
Zeolites are porous oxide minerals. Some are found in nature, but many more may be synthesized in the laboratory. The zeolites are aluminosilicates, consisting of frameworks built up from corner sharing SiO4 and AlO4 tetrahedra. Each substitution of trivalent Al for tetravalent Si leads to the introduction of a charge deficiency that may be compensated for in a manner leading to the formation of strongly acidic Brønsted sites.
The above leads to the use of zeolites as catalysts in acid catalyzed reactions. For example, zeolite Y is employed in catalytic cracking, a refinery process to manufacture gasoline from heavy oil fraction. One can estimate that one third of all gasoline molecules have seen the inside structure of a zeolite. It is customary to distinguish between small, medium, and large pore zeolites, depending on the number of T-atoms defining the circumference of the pores, i.e. 8-ring, 10-ring, or 12-ring materials.
The term zeotype is used to describe topologically similar materials comprising other T-atoms than Si and Al. To date, the International Zeolite Association has assigned their well known three letter codes to nearly 200 unique framework types.
Four different zeolite topologies. Subtle differences in pore architecture may lead to substantial differences in properties in applications such as catalytic reactions and separation processes. Note how the benzene molecule fit differently into the various pores.
Metal organic frameworks
Metal organic frameworks (MOFs) are hybrid materials, built up from an organic part, the linker, and an inorganic part, the cornerstone. This increased complexity leads to much more structural variation than is conceivable for the zeolites described above, and the skilled synthetic chemist will have many more possibilities to design new and unique materials.
The MOFs are a relatively new group of materials. Many different MOFs have been synthesized from a broad range of linkers and metals with different coordination geometries, resulting in a large number of structures. In 2008, the material called UiO-66 was synthesized in the Catalysis group at the Department of Chemistry, University of Oslo. This is a metal organic framework build up from terephthalic acid and a zirconium-containing cornerstone. UiO-66 and various modified version of this material have attracted great interest. This is in part due to their exceptional thermal and chemical stability compared to most other MOFs. These properties, in combination with a high surface area, make the material an excellent candidate for applications in the fields of adsorption and catalysis.
Professor Unni Olsbye of the Catalysis group contemplates possible applications of the UiO-66 metal organic framework material. This material was invented by her colleague, Professor Karl Petter Lillerud (Foto: Håvard Simonsen, Forskningsrådet).