Characteristics of Mn/H-ZSM-5 catalysts for methane dehydroaromatization

Abstract

Mn/H-ZSM-5 catalysts with various loading amounts of Mn were applied to the methane dehydroaromatization (MDA) reaction. Combined H2 TPR, NH3 TPD, and DRIFT analysis indicates that the presence of various manganese species, such as isolated Mn2+ ion, isolated Mn3+ ion, agglomerated MnO, and agglomerated Mn2O3 species, depends on the loading amount of Mn. Among these species, it is demonstrated that at Mn loading below 5 wt%, isolated Mn2+ ion and isolated Mn3+ ion species are primarily produced by a linkage of Mn2+ ion to Brønsted acid site and hydroxyl groups (exAlOH and SiOH) in zeolite, respectively. By contrast, at Mn loading above 5 wt%, agglomerated MnO and Mn2O3 species become abundant on the external surface of zeolite without anchoring to any sites, as evidenced by H2 TPR and XPS analysis. Among the manganese species, isolated Mn3+ ion species plays an essential role as a precursor of active site for the MDA reaction. On the other hand, agglomerated MnO and Mn2O3 species diminish catalytic activity by blocking the entrance of the micro channel of zeolite. Combined H2 TPR, TPO, and UV Raman analysis proves that the isolated Mn2+ ion species is produced from the reduction of isolated Mn3+ ion species and subsequently converted by CH4 into new manganese species that cannot be oxidized again. As the reaction proceeds, carbonaceous deposit species is excessively formed on the active manganese species, which results in the deactivation of the catalyst due to the blocked access of methane to active sites. In summary, the new manganese species produced from the isolated Mn3+ ion species during the initial stage of MDA reaction serves as a main active site, while the formation of excessive carbonaceous deposit species on the active manganese species is a primary reason for the deactivation of catalyst.