(289c) Propane Dehydrogenation over Metal-Exchanged Small-Pore Zeolites

Small-pore zeolites ion-exchanged with metal cations such as In-CHA have been recognized as selective catalyst for non-oxidative alkane dehydrogenation, the impact of the zeolite structure on the indium speciation, however, has yet been investigated. In this work, indium on silica, alumina and zeolite CHA have been used to investigate the effect of support on the indium speciation and corresponding influence on propane dehydrogenation performance. On In-CHA, In₂O₃ initially located on the external surface of the H-CHA zeolite was found to displace Brønsted acid sites (BAS) in the presence of H₂ at high temperature. The formation of In⁺ in CHA was evidenced by the stoichiometric ratio (1.5) of formed H₂O to consumed H₂ during H₂ reduction pulses. The reduced indium species is different depending on the support—In/SiO₂, In/Al₂O₃ and In₂O₃—in which a portion of the In₂O₃ was reduced to form In(0) species (as determined by X-ray diffraction patterns and H₂ temperature-programmed reduction profiles, pulse reactor measurements and in-situ transmission FTIR spectroscopy). It is clear that the presence of BAS prevents the deep reduction of In₂O₃ to In(0) in CHA. We will show that the In⁺ in CHA can be oxidized to form indium oxide in the micropores, and reduced again to form stable In⁺, showing promising redox properties. At comparable conversion, In-CHA shows better stability and C₃H₆ selectivity (~85%) than In₂O₃, In/SiO₂ and In/Al₂O₃. The ratio of C₃H₈ dehydrogenation rate to C₃H₈ cracking rate is the highest on the In-CHA, a result consistent with the low C₃H₈ dehydrogenation activation energy and high C₃H₈ cracking activation energy. These results all point to the conclusion that the dehydrogenation properties of In⁺ species in CHA are superior than In(0) in other supports. This investigation highlights that small-pore-zeolite-stabilized metal cations could find practical application as a highly stable and selective catalyst on alkane dehydrogenation reactions.