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

Abstract

An operando dual-beam Fourier transform infrared spectrometer was developed to identify the transient Brønsted 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 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 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.

Modulating acid site spatial location of Zn/ZSM‐5 catalyst to boost the stable aromatization of hexane

“…The valence state and existing form of Zn in the Zn/ZSM‐5 and Zn/ZSM‐5‐Si catalysts were further investigated. Figure 4A shows that pure ZnO exhibits only one characteristic peak of +2 valence Zn ions at 1022.0 eV, while the Zn/ZSM‐5 and Zn/ZSM‐5‐Si catalysts display the characteristic peaks of Zn 2+ and Zn 0 at 1022.6 and 1016.9 eV 26,27 . Obviously, Zn species in the Zn/ZSM‐5 and Zn/ZSM‐5‐Si catalysts exist in a lower valence state as a whole.…”

“…For the Zn/ZSM‐5 catalyst (Figure 7A), the absorbance at 3610 cm −1 assigned to ZnOAl structure (L acid) decreases with reaction time. Meanwhile, the intensity of the peaks between 2800 and 3000 cm −1 , attributed to C‐H bond stretching vibrations, dramatically increases 26,31 . It is highly possible that the C=C bond of propene is adsorbed or activated by L acid to form an allylic carbocation.…”

Defect-driven nanostructuring of low-nuclearity Pt-Mo ensembles for continuous gas-phase formic acid dehydrogenation

“…The advantages of DB-FTIR over conventional single-beam FTIR could be explained by the fact that the former can simultaneously collect the catalyst sample and the reference spectra. As such, it could often provide more precise structural ngerprints and high-quality spectra, as have been demonstrated in the previous works [54][55][56][57] . To our delight, this superior technique was able to tackle the adsorption of formic acid on our carbon-based materials that were otherwise extremely challenging for the conventional IR.…”

1H MAS NMR In Situ Reaction Monitoring Reveals the Particular Effects of Zn2+ and ZnO Species on the Kinetics of Isobutane Transformation on Zn-Modified Zeolites

“…Zn 2+ species and BASs were further concluded to participate in the secondary reactions of the intermediate alkene transformation to aromatic hydrocarbons. 28,38,39 Recently, Liu et al 18,32 studied isobutane transformation on the fully exchanged Zn/ZSM-5 material, which contained bridged [Zn−O−Zn] 2+ species, with the BASs being lacked. Such a catalyst exhibited superior activity over H-ZSM-5 and Zn/H-ZSM-5 zeolites, with BASs being partially exchanged for Zn 2+ species.…”

“…18 They were assumed to be formed by isobutene dissociative adsorption on bridged [Zn−O−Zn] 2+ sites. Importantly, ZnO/silicalite-1 containing no BASs has been observed 32 to provide isobutene dehydrogenation and isomerization reactions. Therefore, it has been suggested that BASs of the zeolite framework are not needed for isobutane activation, and the reaction can be realized with the assistance of Zn−OH sites.…”