Reaction of nitrous oxide with methane to synthesis gas: a thermodynamic and catalytic study

Abstract

The aim of the present study is to explore the coherence of thermodynamic equilibrium predictions with the actual catalytic reaction of CH₄ with N2O, particularly at higher CH₄ conversions. For this purpose, key process variables, such as temperature (300 °C–550 °C) and a molar feed ratio (N₂O/CH₄ = 1, 3, and 5), were altered to establish the conditions for maximized H₂ yield. The experimental study was conducted over the Co-ZSM-5 catalyst in a fixed bed tubular reactor and then compared with the thermodynamic equilibrium compositions, where the equilibrium composition was calculated via total Gibbs free energy minimization method. The results suggest that molar feed ratio plays an important role in the overall reaction products distribution. Generally for N₂O conversions, and irrespective of N₂O/CH₄ feed ratio, the thermodynamic predictions coincide with experimental data obtained at approximately 475 °C–550 °C, indicating that the reactions are kinetically limited at lower range of temperatures. For example, theoretical calculations show that the H₂ yield is zero in presence of excess N₂O (N₂O/CH₄ = 5). However over a Co-ZSM-5 catalyst, and with a same molar feed ratio (N₂O/CH₄) of 5, the H₂ yield is initially 10% at 425 °C, while above 450 °C it drops to zero. Furthermore, H₂ yield steadily increases with temperature and with the level of CH₄ conversion for reactions limited by N₂O concentration in a reactant feed. The maximum attainable (from thermodynamic calculations and at a feed ratio of N₂O/CH₄ = 3) H₂ yield at 550 °C is 38%, whereas at same temperature and over Co-ZSM-5, the experimentally observed yield is about 19%. Carbon deposition on Co-ZSM-5 at lower temperatures and CH₄ conversion (less than 50%) was also observed. At higher temperatures and levels of CH₄ conversion (above 90%), the deposited carbon is suggested to react with N₂O to form CO₂