(683b) Understanding the Speciation and Propane Dehydrogenation Activity in Zn/H-ZSM-5 Catalysts | AIChE

(683b) Understanding the Speciation and Propane Dehydrogenation Activity in Zn/H-ZSM-5 Catalysts

Authors 

Yuan, Y. - Presenter, UNIVERSITY OF DELAWARE
Lobo, R., University of Delaware
The growing gap between the demand and supply of propylene and the large availability of low-price propane derived from shale gas attract increasing interest in propane dehydrogenation to propylene. Zn/H-ZSM-5 catalysts have received much attention due to their superior propane dehydrogenation and aromatization capabilities. The Zn speciation in Zn/H-ZSM-5 with varying Si/Al ratios (15 and 39) and Zn/Al ratios (0-1.7) have been thoroughly investigated. Unlike Ga/H-ZSM-5, the exchange of Zn with Brønsted acid sites (BAS) mainly occurs during the impregnation and calcination stage, and ZnO located on the external surface cannot be reduced and displace BAS at 550 °C. The relationship between BAS consumption and Zn/Al ratio was established: at Si/Al ratio of 15 with a fraction of paired Al pair sites, isolated Zn2+, [ZnOH]+, ZnOx cluster in the zeolite and crystalline ZnO species are introduced in sequence as the Zn/Al ratio rises; at Si/Al ratio of 39, the same sequence of Zn species is introduced without forming [Zn]2+. Upon H2 treatment, a hydride of Zn/H-ZSM-5 (1926 cm-1 in FTIR) was observed on Zn/H-ZSM-5 with a high Zn/Al ratio, indicating that ZnOx clusters in H-ZSM-5 dissociate H2 to form Zn hydride. The [Zn-OH]+ sites in H-ZSM-5 are more reactive than isolated Zn2+; however, the former species sublimates over time on stream of reaction, leading to the catalyst deactivation. The condensation of two adjacent [Zn-OH]+ sites to [Zn-O-Zn]2+ associated with paired framework Al atoms is stable even after high-temperature reduction treatment (750 ℃ for 60 min). We highlighted that fabrication of H-ZSM-5 with rich paired framework Al atoms is a strategy to prepare stable and reactive Zn/H-ZSM-5 catalysts for propane dehydrogenation.