Elucidating the role of Fe-Mo interactions in the metal oxide precursors for Fe promoted Mo/ZSM-5 catalysts in non-oxidative methane dehydroaromatization

Abstract

Literature shows that adding Fe as a separate phase to MoO3/ZSM-5 catalysts can improve benzene selectivity in methane dehydroaromatization (MDA), but only when added in small quantities, making it difficult to characterize the state of Fe in the catalyst and understand the role of Fe-Mo interactions on the catalytic properties. Here, we explore how the nature of the Mo-Fe interactions in the catalyst precursor can influence the stability and product selectivity in MDA, by employing for the first time Fe2(MoO4)3/ZSM-5 as a catalyst precursor in MDA. We have compared the activity of Fe2(MoO4)3/ZSM-5 with monometallic MoO3/ZSM-5 and mixed MoO3 + Fe2O3/ZSM-5 containing equivalent Mo and Fe loadings and found that Fe2(MoO4)3/ZSM-5 shows higher benzene selectivity than the mixed MoO3 + Fe2O3/ZSM-5 catalyst and exhibits higher stability in reaction compared to the monometallic MoO3/ZSM-5 catalyst. Structural characterization suggests that Fe2(MoO4)3 partially segregates to Fe2O3 and amorphous MoOx during thermal pretreatment. The MoOx species migrate into the zeolite channels during pretreatment, while Fe oxides remain on the external surface of the zeolite. Gas adsorption/desorption techniques and density functional theory calculations demonstrate that the preexisting Fe2O3 phases on the external surface of the zeolite in the mixed MoO3 + Fe2O3/ZSM-5 precursor trap (MoO3)3 clusters preventing them from migrating into the zeolite channels during pretreatment, whereas gradual formation of amorphous MoOx together with the segregation of the Fe2O3 phase when using the Fe2(MoO4)3 precursor diminishes trapping of (MoO3)3 and consequently enhances migration and anchoring of the MoOx species in the zeolite channels, boosting selectivity to benzene. Characterization of used catalysts suggests that the presence of Fe promotes formation of structured carbon nanofibers which reduce the rate of catalyst deactivation.