STABILIZATION OF ZNO IN MESOPOROUS ZSM-5 FOR METHANE DEHYDROAROMATIZATION

摘要

The low cost of natural gas and the vast amount of accessible shale gas reserves in the US has made production of aromatics, such as benzene, via catalytic dehydroaromatization (DHA) of methane an economically attractive pathway. The overall reaction is given by 6 CH4 - 9 H2 + C6H6 and proceeds over zeolite-based catalysts at ～700-8000C with good selectivities (>60%) but low conversion. The catalysts for this reaction are bi-functional metal/zeolite systems in which the metal site activates methane, followed by oligomerization of the methyl to benzene on the Bronsted Acid Sites (BAS) of the zeolite1. The good selectivity for the desired product benzene is attributed to the zeolite pore size which is similar to the benzene diameter (～ 6A). Among the metals investigated, the most intensively studied one is Mo/HZSM-5 due to its comparatively high reactivity2. However, similar to other microporous zeolite materials, poor transport of primary reaction products results in rapid deactivation of the catalyst via coking. While this coke formation is reversible via oxidative regeneration of the catalyst, it results in an inactive oxidized form of the metal which then has to undergo a lengthy activation period to (re)form the active (oxy-carbide) phase. These issues motivate the search for more coking resistant and/or more easily regenerable catalysts for this reaction. Among those, ZnO/HZSM-5 is a promising alternative catalyst since it is active in the oxide form3 and hence does not require further activation after oxidative regeneration of a spent catalyst.