Ethylene oxide reaction over Ag catalysts in low-temperature corona discharge

Abstract

Part 1: The epoxidation of ethylene over different catalysts - namely Ag/(low-surface-area, LSA)-Al2O3, Ag/(high-surface-area, HSA)γ-Al2O3, and Au-Ag/(HSA)γ-Al2O3 - in a low-temperature corona discharge system was investigated. In a comparison among the studied catalysts, the Ag/(LSA)-Al2O3 catalyst was found to offer the highest selectivity for ethylene oxide, as well as the lowest selectivity for carbon dioxide and carbon monoxide. The selectivity for ethylene oxide increased with increasing applied voltage, while the selectivity for ethylene oxide remained unchanged when the frequency was varied in the range of 300-500 Hz. Nevertheless, the selectivity for ethylene oxide decreased with increasing frequency beyond 500 Hz. The optimum Ag loading on (LSA)-Al2O3 was found to be 12.5 wt.%, at which a maximum ethylene oxide selectivity of 12.9% was obtained at the optimum applied voltage and input frequency of 15 kV and 500 Hz, respectively. Under these optimum conditions, the power consumption was found to be 12.6x10-16 W.s/molecule of ethylene oxide produced. In addition, a low oxygen-to-ethylene molar ratio and a low feed flow rate were also experimentally found to be beneficial for the ethylene epoxidation. Part 2: The epoxidation of ethylene under a low-temperature dielectric barrier discharge (DBD) was also feasibly investigated to find the best operating conditions. It was experimentally found that the EO yield decreased with increasing O2/C2H4 feed molar ratio, feed flow rate, input frequency, and electrode gap distance, while it increased with increasing applied voltage up to 19 kV. The highest EO yield of 5.6% was obtained when an input frequency of 500 Hz and an applied voltage of 19 kV were used, with an O2/C2H4 feed molar ratio of 1:1, a feed flow rate of 50 cm3/min, and an electrode gap distance of 10 mm. Under these best conditions, the power consumption was found to be as low as 6.07x10-16 Ws/molecule of EO produced. Part 3: In this work, the epoxidation of ethylene using a low-temperature corona discharge system was investigated with various reported catalytically active catalysts: Ag/α-Al2O3, Cs-Ag/αAl2O3, Cu-Ag/αAl2O3, and Au-Ag/αAl2O3. It was experimentally found that the investigated catalysts could improve the ethylene conversion and the ethylene oxide (EO) yield and selectivity for the corona discharge system, particularly 1 wt.% Cs-12.5% Ag/αAl2O3 and 0.2% Au-12.5% Ag/αAl2O3. The power consumption per EO molecule produced in the corona discharge system, combined with the superior bimetallic catalysts, was much lower than that of the sole corona discharge system and that of the corona discharge system combined with the monometallic Ag catalyst.