Abstract
The activity and selectivity of Mo/ZSM-5, benchmarking catalyst for the non-oxidative dehydroaromatization of methane, strongly depend on the cluster size, spatial distribution, and chemical environment of the Mo-based active sites. This study discloses the use of an ultrasound-assisted ion-exchange (US-IE) technique as an alternative Mo/ZSM-5 synthesis procedure in order to promote metal dispersion along the zeolite framework. For this purpose, a plate transducer (91.8 kHz) is employed to transmit the ultrasonic irradiation (US) into the ion-exchange reactor. The physico-chemical properties and catalytic activity of samples prepared under the said irradiation procedure and traditional impregnation (IWI) method are critically evaluated. Characterization results suggest that US neither affects the crystalline structure nor the particle size of the parent zeolite. However, US-IE promotes molybdenum species dispersion, avoids clustering at the external fresh zeolite surface and enhances molybdate species anchoring to the zeolite framework with respect to IWI. Despite the improved metal dispersion, the catalytic activity between catalysts synthesized by US-IE and IWI is comparable. This suggests that the sole initial dispersion enhancement does not suffice to boost the catalyst productivity and further actions such ZSM-5 support and catalyst pre-conditioning are required. Nevertheless, the successful implementation of US-IE and the resulting metal dispersion enhancement pave the way toward the application of this technique to the synthesis of other dispersed catalysts and materials of interest.
Highlights
The drastic rise in shale gas production, the enormous proven reserves of natural gas, and the lack of mature sustainable alternatives to fulfil the worldwide increasing energy demand point to methane as the primary source for energy and chemicals in the near future [1,2,3]
This aluminosilicate is rapidly deactivated by coke formation in the presence of methane at high temperatures (>650 ◦C) which otherwise are required to produce aromatics in a relevant amount, since methane dehydroaromatization (MDA) is strongly limited by the thermodynamic equilibrium [15,16,17]
These metal loadings were selected according to the optimum molybdenum content reported (3–6 wt. %) to promote aromatic productivity in the MDA process on ZSM-5 supports [15,34]
Summary
The drastic rise in shale gas production, the enormous proven reserves of natural gas, and the lack of mature sustainable alternatives to fulfil the worldwide increasing energy demand point to methane as the primary source for energy and chemicals in the near future [1,2,3] The activation of such a stable molecule becomes the cornerstone of any valorization process devoted to transform methane into liquid fuels to ease its transportation into the end-users. Since the first report of this catalytic reaction in 1993 [14], many studies claimed that metal-supported ZSM-5 catalysts provide an effective shape-selective environment for methane conversion into benzene and naphthalene This aluminosilicate is rapidly deactivated by coke formation in the presence of methane at high temperatures (>650 ◦C) which otherwise are required to produce aromatics in a relevant amount, since MDA is strongly limited by the thermodynamic equilibrium [15,16,17]. The catalytic activity and selectivity toward light aromatics decrease
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