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Abstract
Methane, the main component of natural gas, is an interesting source of chemicals and clean liquid fuels, and a promising alternative raw material to oil. Among the possible direct routes for methane conversion, its aromatization under non-oxidative conditions has received increasing attention, despite the low conversions obtained due to thermodynamic limitations, because of its high selectivity to benzene. Mo/H-ZSM-5, the first bifunctional zeolite-catalyst proposed for this reaction, is still considered as one of the most adequate and has been widely studied. Although the mono- or bifunctional nature of the MDA mechanism is still under debate, it is generally accepted that the Mo species activate the C-H bond in methane, producing the intermediates. These will aromatize on the Brønsted acid sites of the zeolite, whose pore dimensions will provide the shape selectivity needed for converting methane into benzene. An additional role of the zeolite’s Brønsted acid sites is to promote the dispersion of the Mo oxide precursor. Here, we show the influence of the different preparation steps—metal incorporation, calcination and activation of the Mo/ZSM-5- on the metal dispersion and, therefore, on the activity and selectivity of the final catalyst. Metal dispersion is enhanced when the samples are calcined under dynamic conditions (DC) and activated in N2, and the benefits are larger when the metal has been incorporated by solid state reaction (SSR), as observed by FESEM-BSE and H2-TPR. This leads to catalysts with higher activity, increased aromatic selectivity and improved stability towards deactivation.
Highlights
The chemical industry is rapidly evolving towards the use of more efficient and sustainable processes
Incorporation of 6 wt. % of molybdenum was confirmed by ICP for all of catalysts resulting in a Mo/Al molar ratio of 0.5, independently of the metal incorporation method or the calcination procedure
It is important to note that the PXRD patterns of the samples prepared by solid state reaction, MoZ5-SSR-SC and MoZ5-SSR-dynamic conditions (DC), present some peaks assigned to MoO3 at 12.3◦, 25.7◦, 27.4◦ and 39◦, indicating the presence of larger oxide agglomerates
Summary
The chemical industry is rapidly evolving towards the use of more efficient and sustainable processes. Two key factors in this line are the application of heterogeneous catalysts, on the one hand, and the use of alternative raw materials, on the other. One of them is the recycling and upgrading of waste materials as one of the possible routes for contributing to a circular economy by reducing the exploitation of natural resources. Some of these wastes are plastics [1,2,3] and biomass derivatives [4,5,6], which can be used for the production of chemical building blocks, such as olefins and aromatics, or fly ash that can be transformed into nanoporous materials with different applications. The interest in methane as an alternative raw material has increased substantially due to the recent discovery of large reserves of shale gas, coalbed methane and methane clathrates, resulting in a high availability and low cost of natural gas [11,12,13,14,15,16]
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