Zinc Speciation and Propane Dehydrogenation in Zn/H-ZSM-5 Catalysts

Abstract

Zn/H-ZSM-5 catalysts have been frequently investigated for propane dehydrogenation (PDH); however, the active site remains unresolved due to the complexity of the system. We employed in situ FTIR spectroscopy and a kinetics method to correlate the Zn speciation and PDH activity in Zn/H-ZSM-5 with two Si/Al ratios (15 and 39) and a range of Zn/Al ratios (0–1.7). Incremental additions of zinc show that Zn2+ sites are preferentially formed on H-ZSM-5 over a fraction of paired Al sites followed by [Zn-O-Zn]2+ and [ZnOH]+ sites and then ZnO x clusters. The [Zn-OH]+ and [Zn-O-Zn]2+ sites in H-ZSM-5 are more active and selective than isolated Zn2+ for PDH. [Zn-OH]+ species sublimate over time on stream, leading to catalyst deactivation, while [Zn-O-Zn]2+ species are stable even after high-temperature reduction (750 °C for 60 min). Three distinct Zn sites ([Zn-O-Zn]2+, Zn2+, and [ZnOH]+) show a similar propane reaction order (close to 1) and H2 reaction order (close to 0). Combined with the lack of Zn hydride when propane flows over the catalyst at 550 °C, it is concluded that propane adsorption and dissociation is a rate-determining step and H2 desorption is fast. This work indicates that the preparation of H-ZSM-5 with abundant Al pairs may be a strategy to form stable and selective Zn/H-ZSM-5 catalysts for propane dehydrogenation. It is also highlighted that examining the effect of both metal/Al ratios and Al distribution of the zeolite is crucial in identifying the metal cations in metal–zeolite systems.

Regioselective Distribution of Zinc Hydroxyl within Straight Channels in MFI Zeolite Nanosheets for Propane Dehydrogenation

“…The propylene selectivity remained above 95% throughout the test. Compared to the state-of-the-art Zn-based PDH systems, ,− ,,− ZnO@MFI-P exhibits a higher propylene formation rate and relatively lower deactivation rate, as summarized in Table S1.…”

“…The higher amounts of hydroxyl nests contribute to Zn oxo formation and achieve exceptional PDH performance. Based on the above studies, it can be found that various synthetic methods lead to the formation of multiple Zn species including isolated Zn 2+ , [ZnOH] + , Zn oxo , ZnO x clusters, and bulk ZnO aggregates . Moreover, several studies have observed the in situ Zn site evolution during the PDH reaction or H 2 pretreatment. , The multiple Zn species and dynamic evolution make it challenging to identify the structure of the real active Zn sites under working conditions, which restrains further catalyst optimization.…”

Recent Advances in Metal−Zeolite Catalysts for Direct Propane Dehydrogenation

“…ZnO-supported zeolites are potential high-temperature dehydrogenation catalysts. ,− Zhao et al utilized the physical mixing method of ZnO and silicalite-1 zeolite to synthesize supported ZnO x species by the reduction treatment above 550 °C (Figure m) Figure n).…”

Regulation of Active ZnO Species on Si‐based Supports with Diverse Textural Properties for Efficient Propane Dehydrogenation

“…[8] Recently, Yuan et al proposed insights on the formation of ZnO species (single Zn 2 + site, [ZnÀ OH] + site, [ZnÀ OÀ Zn] 2 + site, and ZnO x clusters) on H-ZSM-5 catalysts with different Si/Al and Zn/Al ratios. [9] The results showed that the [ZnÀ OH] + and [ZnÀ OÀ Zn] 2 + sites were more superior than the single Zn 2 + site in PDH reaction, especially for the stability of ZnÀ OÀ Zn. In addition, amorphous SiO 2 supported with Zn single sites (tetrahedrally coordinated single Zn (II) sites) were also developed as efficient PDH catalyst, in which the heterolytic cleavage mechanism was favorable for increasing the selectivity for alkane dehydrogenation.…”