Structure and critical function of Fe and acid sites in Fe-ZSM-5 in propane oxidative dehydrogenation with N2O and N2O decomposition

Abstract

The structure and population of ion-exchanged Fe ions, Fe-oxo species, and Fe-oxo oligomers and Fe2O3 oxide particles and the concentration of acid sites in non-steamed and steamed Fe-ZSM-5 were elucidated using semi-quantitative Mössbauer and FTIR analysis supplemented by UV–Vis and H2-TPR. Differences in the concentration of the individual Fe species and acid sites were related to the activity and time-on-stream behavior of Fe-zeolites in decomposition of N2O and oxidative dehydrogenation of propane to propene with N2O. The evacuated non-steamed FeH-ZSM-5 contained high concentration of Brønsted sites and predominantly bare Fe(II) ions (72%), and less mono- and dinuclear Fe(III) (16–19%) and oligomeric Fe(III)-oxo species (9–12%). The steamed Fe(H)-ZSM-5st zeolites were greatly reorganized, resulting in a low concentration of both the Brønsted sites and bare Fe(II) ions, and the Td-coordinated Fe(III) ions in the FeAlSi extra-framework species (43–47%) and Fe-oxide-like particles (30–33%) prevailed. The redox state of Fe in both non-steamed and steamed Fe-ZSM-5 was strongly influenced by the reaction conditions of N2O decomposition and C3H8/N2O. Bare counter Fe ions balanced by AlSiSiAl sequences in 6MRs of the framework undergo reversible redox Fe(III)O−/Fe(II) cycle during N2O/evacuation treatments and represent the most active site in the decomposition of N2O to molecular components. In contrast, both mono-, dinuclear Fe(III)-oxo and Fe(III)-oxo oligomers are reduced by propane to Fe(II) at conditions of the C3H8/N2O reaction. The reduced Fe(II) species with N2O form Fe(III)O− and facilitate transfer of atomic oxygen to propane with formation of propanol dehydrated to propene. Thus, all dispersed Fe species represent highly active sites in oxidative dehydrogenation of propane to propene with N2O. The presence of high concentration of Brønsted sites and large crystal size of FeH-ZSM-5 stimulated oligomerization of formed propene decreasing its yield and increasing retention of polyolefins up to polyaromatic coke. Thus, high concentration of counter Fe(III)-oxo species easily reducible to Fe(II) is critical for high activity in N2O decomposition, whereas high yield of propene in C3H8/N2O is achieved regardless of population of the individual dispersed Fe species. The low concentration of protonic sites and small crystallites of zeolites, not supporting propene oligomerization, accumulation of polyolefins in the pores and their easy oxidation to CO/CO2, are necessary conditions for selective oxidative dehydrogenation of propane to propene over Fe-ZSM-5.