Microreactors as tools in kinetic investigations: Ethylene oxide formation on silver catalyst

The kinetics of ethylene oxide formation from ethylene and molecular oxygen in an isothermal microreactor was studied. A set of kinetic experiments was carried out by varying the concentrations of oxygen (from 2 vol% to 30 vol%) and ethylene (from 2 vol% to 25 vol%) in the feed. The total pressure was varied between 1 and 5 bar. The reaction temperature was 250 degrees C. Pure silver microplates were used as catalysts for the experiments. The reaction products were analysed with rapid micro gas chromatography. By increasing the ethylene concentration, the chemical system was more favourable towards the partial oxidation of ethylene. The oxygen concentration had a positive effect on the conversion and selectivity towards ethylene oxide. A clearly improved catalyst stability and activity were achieved by pretreating the silver microplates during 12 h with ethylene. The reaction order was close to one with respect to oxygen, while the reaction order of ethylene was around 0.5. A new generalized kinetic model was proposed for ethylene oxide formation. The model is consistent with the experimental data, predicting correctly the observed reaction orders and selectivities. The work demonstrated that gas-phase microreactors combined to micro gas chromatography are strong tools in kinetic research. (C) 2012 Elsevier Ltd. All rights reserved.