Zn-Promoted H‑ZSM‑5 for Endothermic Reforming of <i>n</i>‑Hexane at High Pressures

The addition of Zn to H-ZSM-5 zeolites was studied for application to endothermic reforming in hypersonic aircraft engines. Temperature-programmed-desorption (TPD)/thermogravimetric-analysis (TGA) measurements with 2-propanamine on two H­(Zn)-ZSM-5 samples showed that at low ion-exchange levels, less than 0.5 Zn/Al, each Zn cation displaces one Brønsted-acid site. Although rates for <i>n</i>-hexane conversion at 633 and 823 K and at a pressure of 137 bar decreased with the loss of Brønsted sites, Zn promotion greatly increased the production of H₂ and the formation of small aromatic molecules. FTIR of adsorbed acetonitrile-<i>d</i>₃ and calorimetric measurements of adsorbed CO at 195 K indicate that the exchanged Zn cations form Lewis-acid centers. A model in which the Zn cations, acting as Lewis-acid centers, polarize intermediates formed at Brønsted sites is presented as a way of understanding the observations