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 ZnOx 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.