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.