Methane dehydroaromatization under nonoxidative conditions over Mo/HZSM-5 catalysts: Identification and preparation of the Mo active species

Complete reduction of Mo species interacting strongly with a HZSM-5 zeolite support in methane is difficult, and the reduction product, MoCxOy, is a key active species in methane dehydroaromatization (MDA) under nonoxidative conditions. Pretreatment in H2 at 623 K could lead to topotactic transformation of the Mo species from hexagonally close packing (hcp) to face-centered cubic (fcc) structures; the fcc MoCxOy species are more active and more stable than the hcp MoCxOy in MDA. After initial H2 pretreatment at 623 K, followed by a CH4-treatment, the Mo species on a MoO3/HZSM-5 catalyst prepared by mechanical mixing was reduced and carburized to form fcc α-MoC1−x species on the extra surface of the zeolite. Methane molecules activated by the α-MoC1−x were converted mainly into carbon deposits instead of benzene. Both the most active Mo species and the neighboring Brönsted acid sites are essential in obtaining a perfect catalyst in the reaction of MDA.