Operando dual beam FTIR spectroscopy unravels the promotional effect of Zn on HZSM-5 in iso-butane aromatization

Abstract

Although zinc-promoted HZSM-5 catalysts have been widely used in the aromatization of short-chain hydrocarbons, the precise role of Zn in catalysis is still unclear due to the complex reaction mechanism and the lack of in situ or operando techniques. A home-built dual beam Fourier transform infrared (DB-FTIR) spectrometer, designed to eliminate the interference of gas-phase molecular vibration and heat irradiation through collecting reference and sample spectra simultaneously, can present an unimpeded view of the dynamic surface processes involved in gas/solid heterogeneous catalysis under real reaction conditions. With the aid of the DB-FTIR spectrometer, we directly observed the reaction process of iso-butane aromatization over HZSM-5 and Zn/HZSM-5. Unlike conventional studies, the aromatization was firstly performed under sub-atmospheric pressures using a home-built fixed-bed reactor. Comprehensive characterization results demonstrate that, besides their good dehydrogenation abilities, Zn species undergo strong interactions with olefin intermediates, even at sub-atmospheric pressures, but weaker interaction (or adsorption) with final aromatic products. Density functional theory studies certify that strong interactions could inhibit the desorption of olefin intermediates, which is beneficial for secondary chain-growth reactions relating to aromatics.