Effect of Silver Cations on Propene Aromatization on H-ZSM-5 Zeolite Investigated by <sup>13</sup>C MAS NMR and FTIR Spectroscopy
Zoya N. Lashchinskaya, Anton A. Gabrienko, Igor P. Prosvirin, Alexander V. Toktarev, Alexander G. Stepanov
Abstract
Silver-modified zeolites exhibit high selectivity for alkene transformation to aromatic hydrocarbons. To inquire into the effect of silver cations in zeolite on the pathways of propene aromatization, the transformations of propene on H-ZSM-5 and Ag/H-ZSM-5 zeolites have been investigated by 13 C MAS NMR and Fourier-transform infrared spectroscopy. It is established that propene transformation on H-ZSM-5 at 296–773 K occurs on Brønsted acid sites (BAS) by the mechanism of conjunct polymerization, as evidenced by the detection of cyclopentenyl cation intermediates. On the contrary, propene forms very stable π-complexes with Ag + sites on Ag/H-ZSM-5, which prevents the alkene from converting on BAS at 296–623 K. At T ≥ 623 K, transformation proceeds with the assistance of silver cations via carbanionic allyl-like intermediate formation. Thus, the change in the reaction mechanism from conjunct polymerization on BAS to the mechanism with Ag + ion involvement provides high selectivity for aromatic hydrocarbons on Ag + /H-ZSM-5 zeolite.