The Oxidative Dehydrogenation Of Propane With Nitrous Oxide Catalyzed By Fe-ZSM-5: Reaction And Deactivation Mechanism

The oxidative dehydrogenation of propane with greenhouse gas N₂O as oxidant is very important process for the production of propylene. Fe-ZSM-5 catalysts exhibit unique catalytic activity in the oxidative dehydrogenation of propane with N₂O. The major drawbacks of Fe-ZSM-5 catalysts are their insufficient activity and quick deactivation by coke. Therefore, it is great challenge to prepare highly active Fe-ZSM-5 catalysts for the oxidative dehydrogenation of propane with N₂O via suitable method.In the present study, Fe-ZSM-5 samples were prepared by conventional solid-state ion exchange method and surface organometallic chemistry method. Characterization results indicate the existence of a large number of isolated Fe3+ and a bit oligonucler Fe3+ clusters in Fe-ZSM-5 prepared by surface organometallic chemistry method, which are further confirmed to be extra-framework Fe-O-Al species. The quantitative analysis of active Fe species is achieved by N₂O pulse titration and ca. 60 % of total iron species are active iron sites in Fe-ZSM-5 prepared by surface organometallic chemistry method. O— radicals can be formed upon the activation of N₂O on extra-framework Fe-O-Al species.The reaction pathway and mechanism of propane oxidative dehydrogenation with N₂O catalyzed by Fe-ZSM-5 were investigated by means of temperature-programmed experiments and in situ FTIR spectroscopy. The results confirm the involvement of O—radicals in the oxidative dehydrogenation process and the detailed pathway is described as following: N₂O + * â†' N2 + *-O—; C3H8 + *-O— â†' *-C3H6 + H2O; *-C3H6 â†' * + C3H6; *-C3H6 â†'（allyl alcohol,acrolein, bicarbonate） â†' COx + *We further investigated the effects of reaction parameters on the catalytic performances of Fe-ZSM-5 in the propane oxidative dehydrogenation with N₂O. The results suggest that 748 K is the optimistic reaction temperature and reaction temperature does not show great impacts on the catalyst deactivation. With decreasing N₂O/C3H8 ratio, the coke content formed in unit interval decreases while the catalyst lifetime increases. With of OPDH N₂O/C3H8 ratio of 1/2 and temperature of 748 K, the lifetime of Fe-ZSM-5 can reach 48 h and the activity can be fully recovered after regeneration. Thermogravimetric analysis and GC-MS results aromatics from aromatization are main reason for the formation of coke and catalytic deactivation. With the increase of N₂O/C3H8 ratio, the formation rate of coke also increases.In summary, Fe-ZSM-5 prepared by surface organometallic chemistry method exhibited excellent activity in propane oxidative dehydrogenation with N₂O and extra-framework Fe-O-Al species are proposed to be the active Fe sites. The reaction parameters show great impacts both on the catalytic performances and on the deactivation behavior of Fe-ZSM-5.