ZSM-5

ZSM-5 (structure type MFI) is an aluminosilicate zeolite mineral belonging to the pentasil family of zeolites. Its chemical formula is Na_nAl_nSi_96-nO₁₉₂•16H₂O (0Mobil Oil Company in 1975, it is widely used in the petroleum industry as a heterogeneous catalyst for hydrocarbon isomerization reactions.

Structure

ZSM-5 is composed of several pentasil units linked together by oxygen bridges to form pentasil chains. A pentasil unit consists of eight five-membered rings. In these rings, the vertices are Al or Si and an O is assumed to be bonded between the vertices.



The pentasil chains are interconnected by oxygen bridges to form corrugated sheets with 10-ring holes. Like the pentasil units, each 10-ring hole has Al or Si as vertices with an O assumed to be bonded between each vertex. Each corrugated sheet is connected by oxygen bridges to form a structure with “straight 10-ring channels running parallel to the corrugations and sinusoidal 10-ring channels perpendicular to the sheets.” ["Zeolites and Ordered Mesoporous Materials: Progress and Prospects". (2005) Vol 157. Ed: J. Čejka, H. van Bekkum. ISBN: 0-444-52066-X] Adjacent layers of the sheets are related by an inversion point. The estimated pore size of the channel running parallel with the corrugations is 5.4 – 5.6 Å. [Modeling of Structure and Reactivity in Zeolites (1992). Ed: C.R.A. Catlow. Academic Press, Ltd.: London. ISBN: 0-12-164140-6]

ynthesis

ZSM-5 is a synthetic zeolite, closely related to ZSM-11. There are many ways to synthesize ZSM-5, a common method is as follows [Lermer, H.; Draeger, M.; Steffen, J.; Unger, K.K. "Zeolites", 1985, "5", 131-134]

:SiO₂ + NaAlO₂ + NaOH + N(CH₂CH₂CH₃)₄Br + H₂O → ZSM-5 + analcime + alpha-quartz

ZSM-5 is typically prepared at high temperature and high pressure in a Teflon coated autoclave and can be prepared using varying ratios of SiO₂ and Al containing compounds.

Uses

ZSM-5 has a high silicon to aluminum ratio. Whenever an Al³⁺ cation replaces a Si⁴⁺ cation, an additional positive charge is required the keep the material charge-neutral. With proton (H⁺) as the cation, the material becomes very acidic. Thus the acidity is proportional to the Al content. The very regular 3-D structure and the acidity of ZSM-5 can be utilized for acid-catalyzed reactions such as hydrocarbon isomerization and the alkylation of hydrocarbons. One such reaction is the isomerization of meta-xylene to

References