H/D Exchange of Propane on Indium-Modified Zeolite BEA Investigated by 1H MAS NMR In Situ

The pathways of propane molecule activation on indium-modified zeolite have been studied by 1H MAS NMR spectroscopy in situ at temperatures of 453-573 K. The kinetics of H/D hydrogen exchange between propane -d8 and Bri nsted acid sites (BAS) of the H-BEA zeolite, modified with indium species in the form of either InO+ or In+ cations (InO+/H-BEA and In+/H-BEA samples), were monitored. It is shown that the presence of either InO+ or In+ species in zeolite pores provides regioselectivity of the H/D exchange in methyl groups of the propane molecule. For the InO+/H-BEA zeolite, the process is characterized by greater rates and a lower activation energy compared to those for the unmodified H-form zeolite. The two-exponential kinetics of the exchange for InO+/H-BEA is rationalized in terms of two mechanisms: a direct mechanism assisted by the InO+ species and an indirect mechanism initiated by a propene intermediate. Both mechanisms provide the regioselectivity of the exchange and low activation energy. In+/H-BEA zeolite demonstrates less activity with regard to the rate of the exchange, but it shows a lower activation energy compared to H-BEA zeolite. It is inferred, for In+/H-BEA, that the mechanism of the direct regioselective exchange competes for the same BAS with the mechanism of unselective H/D exchange. The latter results in a specific growth of protium concentration in the methyl and methylene groups of propane -d8 upon the increase of the reaction temperature.