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 In(2)O(3) is formed on the external surface of the zeolite crystal after the addition of In(NO(3))(3) to H-CHA by incipient wetness impregnation and calcination. Upon reduction in H(2) 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 H(2)O to consumed H(2) during H(2) pulsed reduction experiments confirms the indium oxidation state of +1. The reduced indium is different from the indium species observed on samples of 10In/SiO(2), 10In/Al(2)O(3) (i.e., 10 wt% indium) and bulk In(2)O(3), in which In(2)O(3) was reduced to In(0), as determined from the X-ray diffraction patterns of the product, H(2) temperature-programmed reduction (H(2)-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 In(2)O(3) to In(0). In(+) cations in the CHA zeolite can be oxidized with O(2) to form indium oxide species and can be reduced again with H(2) quantitatively. At comparable conversion, In-CHA shows better stability and C(3)H(6) selectivity (∼85%) than In(2)O(3), 10In/SiO(2) and 10In/Al(2)O(3), consistent with a low C(3)H(8) dehydrogenation activation energy (94.3 kJ mol(-1)) and high C(3)H(8) 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.