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At the heart of the MTG process lies zeolite ZSM-5, one of the most versatile catalysts ever found. ZSM-5 is mainly used for hydrocarbon interconversion, meta-xylene to para-xylene, for example. ZSM-5 is a zeolite with a high silica to alumina ratio. The
substitution of an aluminum ion (charge 3+) for a silicon ion (charge 4+)
requires the additional presence of a proton. This additional proton gives the zeolite a high level of acidity, which causes its activity. ZSM-5 is a highly porous material and throughout its structure it has an intersecting two-dimensional pore structure. ZSM-5 has two types of pores, both formed by 10-membered oxygen rings. The first of these pores is straight and elliptical in cross section, the second pores intersect the straight pores at right angles, in a zig-zag pattern and are circular in cross section
(Fig. 2).
Fig.2 Schematic of the pore structure of ZSM-5
- This unique two-dimensional pore structure allows a molecule to move from one point in the catalyst to any where else in the particle. The large openings are the elliptical, straight pores in ZSM-5. For the MTG process it is the pores created by these 10-oxygen rings, along with the zig-zag pores intersecting them, that are essential to the formation of products that are desirable components of gasoline. An 8-oxygen ring zeolite will not produce molecules with 6 or more carbons, molecules of this size will not fit into the small pores of these zeolites. The large pores of a 12-oxygen ring zeolite produce large amounts of C-11 and C-12 compounds, which are undesirable products for gasoline.
