Quantifying the n-hexane cracking activity of Fe- and Al-based acid sites in H-ZSM-5

The relative n-hexane cracking activity of Brønsted acid sites in H-[Al]-ZSM-5, H-[Fe]-ZSM-5, and H-[Al,Fe]-ZSM-5 zeolites has been investigated as a function of temperature, Cs-exchange level, and aging. Similar to H-[Al]-ZSM-5, acid sites in H-[Fe]-ZSM-5 are homogeneous. Per Brønsted acid site the reaction rate constant over H-[Fe]-ZSM-5 was found to be 15% of that of H-[Al]-ZSM-5. The acid sites in H-[Fe,Al]-ZSM-5 show inhomogeneity as expected for the coexistence of strong Al-based acid sites and weaker Fe-based acid sites. In Cs-exchanged materials, an equilibrium relationship has been found for the distribution of Cs between the two different Brønsted acid sites. Using equilibrium constants 6.6 and 75, the activity-decreasing effect of replacing protons with Cs+ ions can be described both for H-[Fe,Al]-ZSM-5 and for mildly steamed H-[Al]-ZSM-5, respectively. FTIR results also indicated preferred Cs poisoning on the more active acidic sites. The peaks representing the stronger and weaker acid sites in H-[Fe,Al]-ZSM-5 are not resolved in either FTIR or temperature-programmed ammonia desorption. The aging of H–Fe sites appears to influence that of H–Al sites in [Fe,Al]-ZSM-5.