Figure 1. Experimental results of DOMtM. (a) Cu-MOR catalysts prepared by ion exchange, (b) Cu-MOR catalysts prepared by wetness impregnation, (c) Comparison of “only Cu-MOR IE-3 catalyst”, “only plasma”, “plasma + MOR”, and “plasma + Cu-MOR IE-3” for CH3OH selectivity and CH4 conversion, and (d) for energy consumption of CH3OH production, (e) Comparison of this work with literature results for CH3OH yield (or productivity) by stoichiometric chemical looping using different catalysts, (f) Comparison of this work with literature results of CH3OH selectivity and CH4 conversion by CH4/O2 plasma using different catalysts. Reaction conditions: 160 ml/min CH4, 40 ml/min O2, 20 °C circulating water, 1.25 g catalyst, 25 W input power and 1.178 s residence time.
Figure 1c presents a comparative analysis of the DOMtM reaction outcomes for different modes. In the case of using only a catalyst (Cu-MOR IE-3), the CH4 conversion is zero, indicating that CH4 cannot be converted at 20 °C and atmospheric pressure without plasma assistance. For “plasma only” conditions, a CH4 conversion of 3.6% is achieved with 32% CH3OH selectivity. Introducing MOR as the packing material for plasma results in a CH4 conversion and CH3OH selectivity of 4.0% and 34.5%, respectively, hence similar to the plasma-only results. This suggests that pure MOR zeolite lacks active sites for DOMtM. However, substituting MOR with the Cu-MOR IE-3 catalyst significantly enhances the reaction performance, indicating a synergistic effect between plasma and the copper active sites on Cu-MOR IE-3 catalyst for DOMtM. This enhancement leads to improved CH4 conversion (7.9%) and CH3OH selectivity (51%).
Figure 1d depicts the comparison of these modes in terms of energy consumption, which also serves as a crucial indicator for plasma-catalytic DOMtM. The energy consumption for CH3OH synthesis in the ”plasma + Cu-MOR IE-3” system is 20.6 kJ/mmol, which is much lower than that of ”plasma only” (143.9 kJ/mmol) and ”plasma + MOR” (78.7 kJ/mmol).
Comparison with literature results is presented in Figure 1e and 1f. Figure 1e reveals that the CH3OH productivity (13877 μmol·gcat-1·h-1) surpasses the best outcomes achieved through stoichiometric chemical looping by an order of magnitude.15 Additionally, as depicted in Figure 1f, the CH3OH selectivity exceeds literature results from plasma catalysis using various catalysts, albeit at a lower CH4 conversion. Notably, a high CH4conversion and high CH3OH selectivity is challenging to achieve simultaneously, as observed by many researchers.16 Consequently, the CH3OH yield will always be limited in the DOMtM process.
In summary of the above experimental findings, the catalytic performance of Cu-MOR catalysts prepared via ion exchange generally surpasses that of the catalysts prepared through wetness impregnation. Notably, the Cu-MOR IE-3 catalyst, synthesized via ion exchange, exhibits the highest catalytic performance in plasma-assisted DOMtM. Furthermore, the Cu-MOR catalyst prepared through wetness impregnation, particularly with low loading (2 wt.%), also demonstrates reasonable catalytic performance, although slightly lower than that of Cu-MOR IE-3. It is noteworthy that altering the preparation method (ion exchange vs wetness impregnation), increasing the number of ion exchange cycles, and enhancing the loading of wetness impregnation can lead to the formation of different metal species on zeolites. Consequently, the diverse catalytic performance of the catalysts is closely linked to the variation in copper species present on the Cu-MOR catalysts.