Methane, the main component of shale gas is considered to be an alternative feedstock for the production of liquid fuels and chemical intermediates of chemical industries. As early in 1993, activity for direct synthesis of aromatics from methane on Mo-based nanostructures supported on molecular sieves have been reported. However, the grand challenge left in the direct synthesis of aromatics is the rapid deactivation of the catalysts due to formation of coke on Mo-based catalysts at high temperatures. Up to now, almost all Mo-based nanostructures supported on molecular sieves were prepared by impregnation or high temperature synthesis of solid chemistry through introducing molybdenum species to zeolite, which inevitably leads to some Mo atoms locate on the external surface of zeolite. Likely, coke preferentially grew on surface of Mo-based nanoparticles supported on external surface of zeolite particles since these supported nanoparticles have open space for ready formation of precursor of coke, polyaromatics. Here we reported a new catalyst, Mo-based core-shell nanoparticles encapsulated in HZSM-5 (Figure 1) formed through in situ synthesis which breakdowns surface of a catalyst nanoparticle to smaller, separated surfaces via formation of nanoscale compartments around a single nanoparticle. It transfers methane to aromatics at 700 °C with a selectivity of 99% for producing aromatics including benzene, toluene and naphthalene without any decay of selectivity and activity and without formation of coke for 150 hrs. The active surface phase during catalysis is Mo2C shell formed through limited carburization of the encapsulated MoO3 nanoparticles. Both division of surface of a MoO3 NP and formation of compartments around it effectively prevent the growth of coke precursors and thus shape the reaction to aromatization, suggesting a method of developing catalysts working at high-temperature with long durability.

Direct conversion of methane to aromatics on stable Mo@HZSM-5 catalyst

2019 Spring National Meeting

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