Natural gas is the cleanest of fossil energy resources in terms of CO2 emissions per unit energy generated. Non-oxidative methane dehydroaromatization (MDA) over zeolite-supported Mo-carbides continues to be one of the most promising options for directly converting natural gas into aromatic hydrocarbons and hydrogen [1,2]. The major hurdle in realizing a technological process is rapid coking deactivation of the zeolite catalyst. Here, we present two novel ways to mitigate formation of coke in Mo/HZSM-5 at typical reaction temperatures. First, pulsing small amounts of oxygen in the feed can selectively combust the coke. Periodic pulsing of oxygen into the methane feed results in substantially higher cumulative product yield with synthesis gas with a H2/CO ratio close to 2 being the main side-product of coke combustion [3]. Second, we show that lowering the Mo content of Mo-ZSM-5 results in improved hydrothermal stability to the extent that the zeolite catalyst can be regenerated at reaction temperature in (artificial) air.  

Supplying small pulses of oxygen to a continuous methane feed over a fixed catalyst bed containing Mo/HZSM-5 greatly stabilizes the catalytic performance in methane dehydroaromatization at 700°C (Fig. 1, left). By using 13C isotope labelling of methane, we show that oxygen predominantly reacts with the Mo-carbides, the formed Mo-oxides catalyze coke oxidation. We also explored the high temperature (550-700°C) air stability of Mo/ZSM-5. At loadings of 1-2 wt% Mo, Mo is predominantly dispersed in the zeolite micropores as cationic Mo-oxo complexes. At higher loading, aggregated Mo-oxide at the external surface become mobile above 600°C and react with framework Al, irreversibly damaging the zeolite framework. High oxidative stability of Mo/HZSM-5 with low Mo loading (< 2 wt%) allows operating the catalyst in a stable manner using an isothermal (700 °C) reaction – air regeneration protocol. Whereas 5 wt% Mo/HZSM-5 rapidly lost its activity, an optimized 2 wt% Mo/HZSM-5 catalyst retained more than 60% of its initial activity after 100 reaction-regeneration cycles.