We have studied the catalytic activity of iron molybdate (Fe₂(MoO₄)₃) supported on ZSM-5 for the first time in non-oxidative methane dehydroaromatization (MDA). Past work has shown that adding small quantities of Fe in oxidic or metallic form as a separate phase to Mo/ZSM-5 catalysts can enhance benzene selectivity. The low Fe loading makes proper characterization of the state of Fe in the catalyst difficult, and impedes reaching an understanding of 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 potentially influence the stability and product selectivity in MDA, by employing Fe₂(MoO₄)₃/ZSM-5 as a catalyst precursor. We have compared the activity of Fe₂(MoO₄)₃/ZSM-5 catalyst with monometallic MoO₃/ZSM-5 and mixed MoO₃+ Fe₂O₃/ZSM-5 catalysts 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 under the same reaction conditions. Structural characterization indicates that Fe₂(MoO₄)₃ partially segregates to amorphous MoO_x and Fe₂O₃during thermal pretreatment in He. The MoO_xspecies migrate into the zeolite channels during pretreatment, while the Fe oxides remain on the external surface of the zeolite. We demonstrate through gas adsorption/desorption techniques and DFT calculations 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. When Fe₂(MoO₄)₃ is used as the precursor, gradual formation of an amorphous layer of MoO_x in parallel with the segregation of the Fe₂O₃ phase results in less trapping of (MoO₃)₃ clusters and consequently migration and anchoring of the MoO_x species at the Brønsted acid sites in the zeolite channels is enhanced, consequently boosting the selectivity to benzene. Fe₂(MoO₄)₃/ZSM-5 also exhibits higher stability in reaction compared to the monometallic MoO₃/ZSM-5 catalyst. Characterization of used catalysts suggests that when Fe is present in the catalyst, structured carbon nanofibers, presumably nanotubes, formed in reaction lead to a decrease in the rate of catalyst deactivation.

Sheima Jatib Khatib

Assistant Professor, Texas Tech University

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