Impact of the preparation method and iron impurities in Fe-ZSM-5 zeolites for propylene production via oxidative dehydrogenation of propane with N2O

The performance of differently prepared Fe-ZSM-5 zeolites in the N2O-mediated propane oxidative dehydrogenation to propylene has been investigated at 723 K using a tapered element oscillating microbalance (TEOM) coupled to on-line GC analysis. Catalysts were prepared by hydrothermal synthesis followed by calcination and steam treatment, liquid-ion exchange, and chemical vapor deposition, and contain molar Fe/Al ratios in the range of 0.26-1. Propylene yields in the range of 22-25% were attained over steam-activated Fe-ZSM-5, much higher over the iron zeolites prepared by post-synthesis methods (9-16%), with propylene selectivities around 40%. The deactivation behaviour due to coking differs among the catalysts investigated. Calcined H-ZSM-5 with impurities of iron (Fe/Al = 0.007) leads to an initial yield of propylene of 8%, and is increased to 16% upon steam treatment, obtaining a selectivity to propylene of 90%. The turnover frequency for propylene production is up to three orders of magnitude higher in steamed H-ZSM-5 than in the synthesized iron zeolites. Our results suggest that traces of well-isolated (mononuclear) Fe species released from framework positions by steaming are extremely active in the oxidative dehydrogenation of propane to propylene, while large iron clusters enhance deep oxidation of important reaction intermediates to COx.