Zeolite induced chemical selectivity during toluene alkylation

The role of the acid base properties of a catalyst to direct the surface chemistry and hence the selectivity in catalyzed reactions, i.e., the toluene, methylation with methanol, was investigated. It is shown that the selectivity depends critically upon the nature of the surface complexes and the concentration of the two reactants in the zeolite, pores. In the presence of Brönsted acid sites, only xylenes are found as primary products. Methanol is preferentially adsorbed over toluene, the rate of methylation being directly proportional to the surface concentration of chemisorbed methanol. The aromatic ring of toluene is only weakly adsorbed and interacts with the sorbed methanol species. On strong basic zeolites (e.g., Rb-X), toluene is preferentially adsorbed over methanol. Toluene is strongly bound via the aromatic ring to the Rb cation and the methyl group interacts with the lattice oxygens of the zeolite. At reaction temperatures, methanol forms a formaldehyde like species at the surface. The positively charged carbon-atom of chemisorbed formaldehyde reacts with the carbon atom of the side chain of sorbed toluene to yield styrene as the main product.