ZSM-5
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ZSM-5 (structure type MFI) is an aluminosilicate zeolite mineral belonging to the pentasil family of zeolites. Its chemical formula is NanAlnSi96-nO192•16H2O (0<n<27). Patented by Mobil Oil Company in 1975, it is widely used in the petroleum industry as a heterogeneous catalyst for hydrocarbon isomerization reactions.
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[edit] 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.”[1] 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.[2]
[edit] Synthesis
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[3]
- 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.
[edit] 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 para-xylene. Within the pores of the ZSM-5 zeolite, para-xylene has a much higher diffusion coefficient than meta-xylene. When the isomerization reaction is allowed to occur within the pores of ZSM-5, para-xylene is able to traverse along the pores of the zeolite, diffusing out of the catalyst very quickly. This size-selectivity allows the isomerization reaction to occur quickly in high yield.[4]
[edit] References
- ^ Zeolites and Ordered Mesoporous Materials: Progress and Prospects. (2005) Vol 157. Ed: J. Čejka, H. van Bekkum. ISBN: 0-444-52066-X
- ^ Modeling of Structure and Reactivity in Zeolites (1992). Ed: C.R.A. Catlow. Academic Press, Ltd.: London. ISBN: 0-12-164140-6
- ^ Lermer, H.; Draeger, M.; Steffen, J.; Unger, K.K. Zeolites, 1985, 5, 131-134
- ^ Dyer, Alan (1988). An Introduction to Zeolite Molecular Sieves. John Wiley & Sons. ISBN: 0-471-91981-0
