Light alkanes aromatization to BTX over Zn-ZSM-5 catalysts: Enhancements in BTX selectivity by means of a second transition metal ion Original Research Article

The selectivity obtained in the aromatization of propane over Zn-ZSM-5 based catalysts (Si/Al nominal ratio=35) increased from about 75–80% (expressed on a carbon basis) to over 91% in the case of catalysts where iron (or other second transition metal) ions have been separately introduced by means of solid state ion exchange (SSIE) procedures. The observed improvements are not seen for all catalyst compositions. The ideal zinc and iron contents are 0.5 and 1.0 mass%, respectively, corresponding closely in total to the expected ion-exchange capacity of the parent ZSM-5. The introduction of iron cations is associated with the reappearance of substantial Bronsted acidity (present in the parent H-ZSM-5 but lost as a result of incorporation of Zn by SSIE as evidenced by the restoration of high temperature peak for ammonia as observed during temperature programmed desorption. Attempts are made to understand the origin of the improved BTX selectivities seen in the presence of ions of a second transition metal such as iron, chromium or manganese.