Abstract
The effect of temperature and hydrogen addition on undesired carbonaceous deposit formation during methane coupling was studied in DBD-plasma catalytic-wall reactors with Pd/Al2O3, using electrical power to drive the reaction. Experiments with thin catalyst layers allowed comparison of the performance of empty reactors and catalytic wall reactors without significantly influencing the plasma properties. The product distribution varies strongly in the temperature window between 25 and 200 °C. Minimal formation of deposits is found at an optimal temperature around 75 °C in the catalytic-wall reactors. The selectivity to deposits was c.a. 10% with only 9 mg of catalyst loading instead of 45% in the blank reactor, while decreasing methane conversion only mildly. Co-feeding H2 to an empty reactor causes a similar decrease in selectivity to deposits, but in this case methane conversion also decreased significantly. Suppression of deposits formation in the catalytic-wall reactor at 75 °C is due to catalytic hydrogenation of mainly acetylene to ethylene. In the empty reactor, H2 co-feed decreases conversion but does not change the product distribution. The catalytic-wall reactors can be regenerated with H2-plasma at room temperature, which produces more added-value hydrocarbons.
Highlights
Availability of large natural gas reserves rich in CH4 and, more recently, the development of shale gas in USA, has revived interest in developing direct routes for methane conversion to higher value chemicals [1,2]
Methane coupling in a dielectric barrier discharge (DBD) reactor is possible at mild temperatures close to ambient [8,9] without the need of a catalyst
The goal of this work is to study the effect of temperature and H2 co-feeding in DBD plasma-catalysis reaction when Pd/Al2O3 catalyst is used inside the plasma zone for non-oxidative coupling of methane, mainly aiming at decreasing the formation of carbonaceous deposits and improving the stability of the catalyst
Summary
Availability of large natural gas reserves rich in CH4 and, more recently, the development of shale gas in USA, has revived interest in developing direct routes for methane conversion to higher value chemicals [1,2]. Oxidative and non-oxidative coupling of methane has been extensively studied [3,4]. The former struggles with extensive deep oxidation to CO and CO2, while the later suffers from carbon formation, low methane conversion and high operation temperatures (>600 °C). Non-oxidative coupling of methane via non-equilibrium plasma is attracting attention in the last decades [5,6,7,8], offering an avenue to electrify the chemical industry by using electrical power to drive an endothermic and endogenic reaction. Methane coupling in a dielectric barrier discharge (DBD) reactor is possible at mild temperatures close to ambient [8,9] without the need of a catalyst.
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