Study of Zn and Ga Exchange in H‑[Fe]ZSM‑5 and H‑[B]ZSM‑5 Zeolites

The catalytic properties of H-[Fe]­ZSM-5 and H-[B]­ZSM-5 were explored after addition of Zn or Ga. TPD–TGA of 2-propanamine adsorbed on Zn- and Ga-exchanged H-[Fe]­ZSM-5 showed a decrease in Brønsted-acid site densities and the formation of new dehydrogenation sites, similar to what is observed following exchange in H-[Al]­ZSM-5 and in amorphous silica–alumina. Exchanged Zn cations in [Fe]­ZSM-5 also exhibited Lewis-acid character, as demonstrated by the appearance of a υ­(CN) stretch at 2310 cm^–1 upon adsorption of CD₃CN. By contrast, the sites in H-[B]­ZSM-5 were not capable of protonating 2-propanamine, did not form dehydrogenation sites when Zn or Ga were added, and showed no evidence for sites with Lewis-acid character from the FTIR spectroscopy of CD₃CN. Neither H­(Zn)-[Fe]­ZSM-5 nor H­(Zn)-[B]­ZSM-5 catalyzed reactions of <i>n</i>-hexane at 773 K, but the TPD–TGA of adsorbed propene on H­(Zn)-[Fe]­ZSM-5 showed strong interactions between the Zn and olefins that might be responsible for the dehydrocyclization of light alkanes in Zn-exchanged, Al-containing zeolites

Endothermic Reforming of <i>n</i>‑Hexane on Metal (Pt, Ga) Containing H‑ZSM‑5 at High Pressures

The supercritical, high-pressure reaction of <i>n</i>-hexane over H-ZSM-5, with and without the addition of Pt and Ga, has been studied for application to endothermic reforming in scramjet engines. The endothermicities of the reactions were determined from the product distributions. For unpromoted H-ZSM-5, the product distribution indicated that the endothermicity is low and decreases with increasing pressure. The addition of Ga to H-ZSM-5 has a relatively small effect on <i>n</i>-hexane conversion but significantly increases the endothermicity of the reaction by increasing the selectivity to form small aromatics. The H­(Ga)-ZSM-5 catalyst showed no deactivation for at least 5 h at 633 K and 137 bar of <i>n</i>-hexane. By contrast, the addition of Pt had a minor effect on both the rate and the reaction endothermicity

Adsorption of Small Alkanes on ZSM‑5 Zeolites: Influence of Brønsted Sites

The adsorption of a series of small alkanes was studied experimentally on H-ZSM-5 zeolites using calorimetric measurements in order to determine their interactions with the Brønsted sites. Differential heats measured on four ZSM-5 samples with different Si/Al₂ ratio and with different defect concentrations were found to depend strongly on the Brønsted-site density but not on the presence of defects. The interactions for CH₄ with the Brønsted sites were minimal but the effect was significant (up to 11 ± 2 kJ/mol extra heats) for larger alkanes, such as <i>n</i>-C₆H₁₄. The affinity of the alkanes with the Brønsted sites increased with the gas-phase proton affinity of the alkanes and the calculated affinity of the alkanes for the strong acid, fluorosulfonic acid. The extra heats of adsorption in H-ZSM-5 over its siliceous counterparts can therefore be associated with the strength of hydrogen bonding between the adsorbed alkane and the Brønsted sites, which in turn increases with molecular size. Specifically, extra heats were found to vary linearly with acid affinity corrected for dispersion interactions. The comparison of the experimental and computational results, therefore, indicates that the hydrogen bonded interaction theory describes the effect of Brønsted sites for alkane adsorption on zeolites

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