(712e) Manganese As a Redox Reactant for Sustainable Thermochemical Ammonia Synthesis

Fixed nitrogen, as ammonia (NH₃), is the main ingredient of most fertilizers and its availability is the most common yield-limiting factor in industrial agricultures. Responsible for up to 50% of crop yields, ammonia fertilizers provide the nitrogen necessary to sustain over half of the global population. Annual global ammonia production, currently greater than 140 million metric tons, will need to expand along with the expected increase in demand for fertilizer for food production. Catalytic ammonia synthesis via the Haber-Bosch process, operates at some of the most intense process conditions for chemical production, with temperatures as high as 500°C and pressures reaching 300 atmospheres. The Haber-Bosch process consumes 0.6 tons of natural gas and emits 2.3 tons of carbon dioxide per ton of ammonia produced.

A chemical loop is experimentally demonstrated here using manganese as a cycled redox reactant for ammonia production from nitrogen, water, and methane at atmospheric pressure. Manganese is used to reduce the nitrogen which is then reacted with steam to form ammonia in the presence of a sodium promotor. The corresponding manganese oxide is reduced by a dilute methane stream also producing syngas. The role of sodium hydroxide in the promotion of ammonia liberation is investigated. The thermodynamic predictions of the system are discussed, and the reaction kinetics are evaluated.

The thermochemical cycle appears a promising method to sustainably produce ammonia at near-ambient pressures. Separating the ammonia synthesis into nitrogen reduction and subsequent protonation steps introduces an additional degree of freedom for optimizing the redox reactant and process conditions. Reduced carbon dioxide emissions, and valuable syngas coproduct are advantageous for this novel approach going forward.