MICRO POROUS MATERIALS
Micro porous materials can be defined as solids, containing interconnected pores of less than 2 nm in size. Thus, they possess large surface areas, typically 300–2000 m2/g as measured by gas adsorption. Example include zeolites, AlPO4, metal organic frameworks (MOFs), clays, carbon, etc. Zeolites are the most well-known group of microporous materials. Zeolites are the aluminosilicates commonly known as “molecular sieves.” A zeolite framework is a neutral compound, comprising exclusively oxygen-sharing SiO4 tetrahedra.
Although there are natural zeolites, most of the zeolites known are synthetic. Barrer first synthesized zeolite Y in the mid-1950s. It was an attempt to imitate the conditions under which natural zeolites were supposed to have formed on the Earth. Zeolites are prepared in the laboratory by crystallization of gels containing alumina and silica in an aqueous medium at temperatures in the range of 100°C–190°C for several days or weeks. The gel can be prepared from other sources of Al, Si, and some other metals other than silica and alumina. Deville first reported the laboratory-synthesized zeolite levyne (levynite) Ca9 [Al18Si36O108], H2O in 1862. The synthetic process required heating potassium silicate and sodium aluminate in a glass ampule. Since 1950, a wide range of zeolites has been synthesized by simple isomorphous substitution of not only aluminum but also several other elements due to their excellent properties and larger pores than their counterparts. For example, the family of ZSM such as ZSM-5, ZSM-12, ZSM-22, ZSM-23, and ZSM-48 have been obtained by using various templates. A germanosilicate zeolite (ITQ-15) was first reported in the patent literature, which has a large pore volume with a channel system formed by 14 X12R pores and has been assigned as zeotype UTL.