(682e) Tuning External Surface of Mo/H-ZSM-5 Catalysts By Atomic Layer Deposition and Its Impact in Methane Dehydroaromatization

Methane dehydroaromatization (MDA) is a promising route of direct methane transformation to aromatics under non-oxidative conditions. For MDA process, the benchmark catalyst is Mo/H-ZSM-5 as it shows high aromatic selectivity and near equilibrium conversion (~12% yield at 700 ⁰C); however, its outstanding performance rapidly declines over time due to the formation of carbonaceous species. According to literature, Brønsted acid sites and MoO_x species on its external surface are known to cause an unselective methane conversion. Although the coke formed during MDA could be removed by combustion, Mo/H-ZSM-5 does not recover its initial catalytic activity due to zeolite dealumination. Therefore, engineering Mo/H-ZSM-5 properties to reduce undesired coke formation is needed.

In this work, to slow down the process of coke formation, we passivated the Mo/H-ZSM-5 external surface with SiO₂ and Al₂O₃ overcoats utilizing atomic layer deposition (ALD). By overcoating SiO₂ on Mo/H-ZSM-5, C₆H₆ yield was enhanced without a change in the deactivation rate constant rate (k_d). For overcoating Al₂O₃ on Mo/H-ZSM-5, C₆H₆ yield was found to be suppressed and deactivating faster than Mo/H-ZSM-5 (see attached figure). Furthermore, incorporation of SiO₂ and Al₂O₃ overcoats did not impact the prevalent reaction pathway.

Additionally, the overcoating effect was tested under isothermal oxidative regeneration. The C₆H₆ yield declined after each regeneration cycle with minimal difference between all catalyst after first cycle. For all catalysts, overcoats deposition on external surface area was verified by TEM and non-modifications of textural properties. Moreover, MoO_x redispersion due to overcoats was corroborated by UV-Vis, H₂-TPR and In situ UV-Raman which supported catalytic results. Furthermore, ²⁷Al-MAS NMR of fresh and spent catalysts were contrasted and implied that zeolite-dealumination after regeneration occurred independently of the overcoats. Overall, this targeted modification on Mo/H-ZSM-5's external surface area showed an improvement MDA’s performance which could be extrapolated to other catalytic transformations.