Propane dehydrogenation over extra-framework In(i) in chabazite zeolites

Abstract

Indium on silica, alumina and zeolite chabazite (CHA), with a range of In/Al ratios and Si/Al ratios, have been investigated to understand the effect of the support on indium speciation and its corresponding influence on propane dehydrogenation (PDH). It is found that In₂O₃ is formed on the external surface of the zeolite crystal after the addition of In(NO₃)₃ to H-CHA by incipient wetness impregnation and calcination. Upon reduction in H₂ gas (550 °C), indium displaces the proton in Brønsted acid sites (BASs), forming extra-framework In⁺ species (In-CHA). A stoichiometric ratio of 1.5 of formed H₂O to consumed H₂ during H₂ pulsed reduction experiments confirms the indium oxidation state of +1. The reduced indium is different from the indium species observed on samples of 10In/SiO₂, 10In/Al₂O₃ (i.e., 10 wt% indium) and bulk In₂O₃, in which In₂O₃ was reduced to In(0), as determined from the X-ray diffraction patterns of the product, H₂ temperature-programmed reduction (H₂-TPR) profiles, pulse reactor investigations and in situ transmission FTIR spectroscopy. The BASs in H-CHA facilitate the formation and stabilization of In⁺ cations in extra-framework positions, and prevent the deep reduction of In₂O₃ to In(0). In⁺ cations in the CHA zeolite can be oxidized with O₂ to form indium oxide species and can be reduced again with H₂ quantitatively. At comparable conversion, In-CHA shows better stability and C₃H₆ selectivity (∼85%) than In₂O₃, 10In/SiO₂ and 10In/Al₂O₃, consistent with a low C₃H₈ dehydrogenation activation energy (94.3 kJ mol⁻¹) and high C₃H₈ cracking activation energy (206 kJ mol⁻¹) in the In-CHA catalyst. A high Si/Al ratio in CHA seems beneficial for PDH by decreasing the fraction of CHA cages containing multiple In⁺ cations. Other small-pore zeolite-stabilized metal cation sites could form highly stable and selective catalysts for this and facilitate other alkane dehydrogenation reactions.