Iridium Clusters in KLTL Zeolite: Structure and Catalytic Selectivity for n-Hexane Aromatization

Abstract

Catalysts consisting of Ir clusters in zeolite KLTL were prepared by reduction of [Ir(NH 3) 5Cl]Cl 2 in the zeolite with H 2 at temperatures 300 or 500°C. The catalysts were tested for reactions of n-hexane and H 2 at 400, 440, and 480°C and were characterized by temperature-programmed reduction, hydrogen chemisorption, transmission electron microscopy, infrared spectroscopy of adsorbed CO, and extended X-ray absorption fine structure spectroscopy. The clusters consist of 4 to 6 Ir atoms on average and are sufficiently small to reside within the pores of the zeolite. The infrared spectra characteristic of terminal CO suggest that the support environment is slightly basic and that the Ir clusters are electron rich relative to the bulk metal. Notwithstanding the small cluster size, the support basicity, and the confining geometry of the LTL zeolite pore structure, the catalytic performance is similar to those of other Ir catalysts, with a poor selectivity for aromatization and a high selectivity for hydrogenolysis. These results are consistent with the inference that the principal requirements for selective naphtha aromatization catalysts are both a nonacidic support and a metal with a low hydrogenolysis activity, i.e., Pt.