Preparation of Zeolite ZSM5 and Catalysis of Xylene Isomerization

Zeolites are three-dimensional, crystalline networks of AlO₄^- and SiO₄ tetrahedra. Their crystallization is often a nucleation-controlled process from a gel and the structure that crystallizes depends on the cations present. ZSM-5, Zeolite Socony Mobil #5, is a catalyst first made by Argauer and Landolt in 1972 (U. S. Patent 3,702,886). It is a medium pore zeolite with channels defined by ten-membered rings. The synthesis involves three different solutions. The first solution is the source of alumina, sodium ions, and hydroxide ions; in the presence of excess base the alumina will form soluble Al(OH)₄^-ions. The second solution has the tetrapropylammonium cation that acts as a templating agent. The third solution is the source of silica, one of the basic building blocks for the framework structure of a zeolite. Mixing the three solutions produces supersaturated tetrapropylammonium ZSM-5, which can be heated to recrystallize and produce a solid. In subsequent steps the organic templating agent is removed and the zeolite converted to its acid form.

“A multitude of commercial processes have been developed that exploit size exclusion and selective molecular diffusivity properties based upon the nanosize pore and channel structure of zeolites… The demand for two C8 isomers, paraxylene and orthoxylene, is much greater than that for the C8 isomers metaxylene and ethylbenzene. H-ZSM5 with its 0.6 nm pore size has the ability to isomerize xylenes with little cracking of the feedstock. A second crucial property is that paraxylene has a much higher diffusivity in H-ZSM5 than do the other xylene isomers. This means that the paraxylene molecules can more easily diffuse out of the zeolite crystal, whereas the ortho and metaxylene isomers are effectively trapped within the pores.” -Vision for Nanotechnology R&D in the Next Decade,National Science and Technology Council IWGN Workshop Report, Section 9.7.4 Nanoscale Catalysts (1999).