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Methane dehydroaromatization (MDA) is a promising approach for direct methane transformation to aromatics and hydrogen. The benchmark catalyst Mo/H-ZSM-5 struggles to find commercial adoption because of thermodynamically-limited yields and rapid coking on Brønsted acid and molybdenum carbide species, especially on zeolite external surfaces. Here, gas-phase atomic layer deposition (ALD) overcoats H-ZSM-5 external surfaces with SiO 2 or Al 2 O 3. NH 3 -TPD, HRTEM, and textural properties show that these overcoats exclusively passivate zeolite external surfaces. Under MDA conditions, SiO 2 gives softer coke and increases cumulative benzene yields by 25 %, while Al 2 O 3 strongly decreases yields. H 2 -TPR and UV–visible and Raman spectroscopy show how the overcoats redisperse the MoO x precatalysts, especially over multiple deactivation and isothermal oxidative regeneration cycles. Combined with 27Al-MAS NMR, MoO x redistribution and dealumination are seen as the causes of long-term deactivation over multiple regeneration cycles, and this process continues to occur regardless of the overcoat. Overall, the deposition of a small amount of silica on the outer surface of Mo/H-ZSM-5 reduces the formation of hard coke, which could be regenerated by milder methods such as hydrogen treatment. [Display omitted] • Atomic layer deposition (ALD) of Al 2 O 3 and SiO 2 on zeolite external surfaces. • Their impact on MDA cumulative aromatic formation increases by 25 % for SiO 2 and decreases by 50 % for Al 2 O 3. • Oxide overcoats consume external acid sites and redistribute MoO x domains was key for improved C 6 H 6 yields. • A small amount of silica on the outer surface of Mo/H-ZSM-5 reduces the formation of hard coke. • Permanent deactivation under oxidative-regeneration driven by dealumination and further MoO x redistribution. [ABSTRACT FROM AUTHOR]

Oxidative coupling of methane (OCM) presents a direct route to upgrade methane to higher value hydrocarbons in the presence of an oxidant. There have been extensive efforts dedicated to studying the initial coupling reactions to form ethylene, but there has been much less emphasis on further molecular weight growth to C 3 + species since it was first noted in the 1990′s. Here, catalysts were first screened for OCM activity and production of C 3 + during operation at high methane conversion, especially within the more-recently developed family of A 2 WO 4 -MnO x /SiO 2 (A=none, Li, Na, K, Rb, Cs) catalysts. Within these, K and Rb presented a similar OCM performance to that of the more popular Na-system. Ag- and La- co-doping were also assessed to follow up on recent reports of high performance, but the latter had minimal impact on C 2 + formation under these conditions. Ethylene and propylene concentrations rose in proportion, independent of catalyst composition, suggesting that C 3 + formation was a gas-phase, radical process, not occurring directly on the catalyst surface. As such, ethylene-methane co-feeds were investigated over a range of reactor conditions for Na 2 WO 4 -MnO x /SiO 2 , which was relatively stable over multiple days at high conversion and the most active under conventional OCM conditions. Ethylene co-feeds increased the C 3 + selectivity over a range of reactor conditions, but it also promoted CO x formation. Nonetheless, this work shows that further growth of C 3 + species under OCM conditions can be achieved, which may be desirable under certain scenarios. [Display omitted] • Family of (alkali) 2 WO 4 -MnO X /SiO 2 screened for OCM, including La and Ag dopants. • At high conversion, C 3 H 6 yields scale with C 2 H 4 production, regardless of catalyst. • With ethylene co-feed, Na 2 WO 4 -MnO X /SiO 2 improves C 3 H 6 production. • Results motivate two-stage OCM or OCM after cracking using modern catalysts. [ABSTRACT FROM AUTHOR]