“Selectivity and reaction kinetics of methane pyrolysis to produce hydrogen in catalytically active molten salts”

Methane pyrolysis in catalytic molten salt bubble columns is an emerging method of producing low-emission hydrogen using fossil fuels. The two salt compositions MgCl2 (50):KBr (50) (EA = 254 kJ/mol) and MnCl2 (50):KBr (50) (EA = 154 kJ/mol) are promising catalysts due to their high methane conversion, each reaching about 5% conversion at 950 °C in a 12.5 cm high bubble column. While Mg-based catalysts are attractive due to their very low cost and low toxicity, their selectivity to hydrogen is lower than Mn-based catalysts. The kinetics of these two melt compositions were analysed using deuterium / hydrogen exchange experiments with methane. MnCl2 (50):KBr (50) produced the highest extent of deuterated methane’s, suggesting the transition metal cation is more effective at retaining the methane molecule on the active site during dehydrogenation, preventing reverse and side-reactions. This results in Mg-based catalysts reaching a pseudo-equilibrium much earlier than Mn-based catalysts. Both catalysts decrease selectivity to hydrogen with increasing residence time, but the Mg-based catalyst decreases selectivity to hydrogen faster as higher hydrocarbons are more likely to form due to low instances of multiple exchange occurring. It is proposed that this is the reason Mn catalysts have better selectivity to hydrogen, lower polycyclic aromatic hydrocarbon formation, less impact from pseudo-equilibrium conditions and higher extent of carbon graphitisation.