(337ar) Gas-Phase Surface Modification to Control Catalyst Structure and Yields in Methane Dehydroaromatization

Methane dehydroaromatization (MDA) is a promising route of direct methane transformation to aromatics and hydrogen 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 on stream due to the formation of carbonaceous species. According to literature, Brønsted acid sites and MoO_x species on the 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 the inevitable zeolite dealumination. Therefore, it is required to engineer Mo/H-ZSM-5 catalyst properties to reduce undesired coke formation under MDA’s operating conditions.

In this work, in order to slow down the process of coke formation on Mo/H-ZSM-5, we passivated the external surfaces with SiO₂ and Al₂O₃ overcoats utilizing atomic layer deposition (ALD). By partially overcoating SiO₂ on Mo/H-ZSM-5, the yield of C₆H₆ was enhanced without being accompanied by a change in the deactivation rate constant rate as function of contact time (k_d). For overcoating Al₂O₃ on Mo/H-ZSM-5, the yield of C₆H₆ was found to be suppressed, deactivating faster than the bare Mo/H-ZSM-5 (see attached figure). Under a one-pass evaluation, incorporation of SiO₂ and Al₂O₃ overcoats did not impact the prevalent reaction pathway as suggested by benzene’s carbon-based selectivity as a function of methane conversion.

Additionally, the overcoating effect was tested under isothermal oxidative regeneration. After each regeneration cycle, the C₆H₆ yield declined after each regeneration with minimal difference between all catalyst after first regeneration cycle. For each overcoated Mo/H-ZSM-5, a deposition on the external surface area was verified by TEM imagines and non-modifications of textural properties based on N₂-adsortion. Moreover, MoO_x redispersion due to overcoats was corroborated by UV-Vis, H₂-TPR and In situ UV-Raman which support catalytic results observed. Furthermore, overcoated and bare Mo/H-ZSM-5’s ²⁷Al-MAS NMR of fresh and spent catalysts were contrasted and implied that zeolite-dealumination after regeneration occurred independently of the external surface modification. Overall, this targeted modification on external surface area on Mo/H-ZSM-5 showed an improvement in MDA’s performance which could be extrapolated to other catalytic transformations.