Transient Brønsted Acid Sites in Propene Aromatization over Zn-Modified HZSM-5 Detected by Operando Dual-Beam FTIR

An operando dual-beam Fourier transform infrared spectrometer was developed to identify the transient Bronsted acid sites (BAS) in propene aromatization over acidic HZSM-5 and Zn-modified HZSM-5 catalysts under real reaction conditions. The eliminated signals include gas-phase I molecular vibrations and heat irradiation at reaction temperatures. We directly observed that the initial activation of propene over Zn2+ of Zn-modified HZSM-5 generated a substantial number of transient BAS, which serve as active sites for the subsequent aromatization reactions. Moreover, during Transient Bronsted Acid Sites the propene aromatization process, the desorption of aromatic precursors over the Zn2+ of Zn/HZSM-5 is easier than that over the H+ of HZSM-5, resulting in an enhanced aromatics productivity. A density functional theory calculation certified the priority of the metallic Zn2+ sites over BAS in the competitive activation of propene molecules. The generation of transient BAS is energetically favorable.