Catalytic Methane Dehydroaromatization: Determination of the Structure of Pre-Catalysts and Effect of the Synthesis Protocol on Structure & Activity of Promoted Catalysts

Natural gas, primarily composed of methane, is an abundant resource. Unfortunately, due to its abundance and the lack of economical transport infrastructure, billions of cubic feet of natural gas are flared yearly, wasting a valuable resource and releasing CO₂ in the process^[i].

The methane dehydroaromatization (MDA) reaction provides a potential solution. MDA converts methane to aromatic compounds such as benzene in one step, thus enabling on-site processing and providing an excellent opportunity of distributed manufacturing of aromatics while minimizing transportation costs.

Mo/ZSM5 catalysts have shown promise as MDA catalysts, but further investigation is needed to understand the mechanisms of this catalyst and improve its performance. Two areas not yet fully understood are explored here: (1) the structure of the Mo oxide species in the pre-catalysts, (2) the effect of adding an iron promoter to the Mo/ZSM-5 catalysts.

While some research suggests that a (Mo₂O₅)²⁺ dimer exists^[ii], other studies show that the MoO₄ monomer is the sole MoO_x species^[iii]. Here we perform temperature-programmed oxidation (TPO) of the catalyst precursors in order to determine the stoichiometry of the MoO_x species, which will help distinguish whether monomer and dimer species are predominant in the pre-catalysts.

Additionally, the physicochemical properties of Mo-Fe/ZSM-5 catalysts are examined. Prior studies suggest that impregnation of a secondary metal can improve the performance of Mo/ZSM5 catalysts^[iv]. Here we explore various synthesis protocols to add Fe promoter to the Mo/ZSM-5 catalysts and we study the effect of the protocol on the catalyst structure and activity.

^[i] US EIA, Natural Gas Annual, 2018.

^[ii] R.W. Borry, Y.H. Kim, A. Huffsmith, J.A. Reimer, E. Iglesia, J. Phys. Chem. B 103 (1999) 5787–5796.

^[iii] J. Gao, Y. Zheng, J.-M. Jehng, Y. Tang, I.E. Wachs, S.G. Podkolzin, Science (80-.) 348 (2015), 686 LP – 690.

^[iv] A. Sridhar, M. Rahman, S.J. Khatib, ChemCatChem 10 (2018) 2571–2583.