Oxidative chlorination of methane over Fe2O3/CeO2 catalyst

Abstract

Methane, which accounts for higher than 80% of natural gas components, is an abundant alternative feedstock to petroleum. However, efficient utilization of methane especially for chemical production has been challenging task in catalysis community due to the highly stable C–H bonds of methane. Although there are commercialized methane activation processes such as steam–reforming, partial oxidation and auto–thermal reforming, they are disadvantageous in the terms of capital and energy.

Methane activation through oxychlorination is in the spotlight due to the relatively mild reaction conditions at atmospheric pressure and in the temperature range of 450–550 ℃. CeO2 is known to exhibit good activity for oxychlorination which is arising from its redox properties (Ce4+ ↔ Ce3+), however, significant amounts of by–products such as CO2, CO and carbon deposits are produced during the reaction over CeO2. At 510 ℃, where the yield of chlorinated products is maximized, the selectivity of 56.8% for chloromethane (CM) and 17.2% for dichloromethane (DCM) are obtained, while by–products (CO2, CO and carbon deposits) are produced with about 26% selectivity.

In this study, various metals were introduced into CeO2 to promote the activity of CeO2 for methane oxychlorination. Metal/CeO2 catalysts were prepared by applying wet impregnation method, and all catalysts were calcined at 600 ℃ for 6 hours. From the result of activity test, iron showed the best promoting effect on CeO2 for methane oxychlorination, and thus, the effect of iron in Fe2O3/CeO2 catalysts on methane oxychlorination were extensively studied. The activity test result of Fe2O3/CeO2 with different iron loading shows that although there are no advantages in the perspective of methane conversion, the production of by–products is suppressed by adding iron irrespective of iron loading. Fe2O3/CeO2 with 3 wt% iron shows the maximum yield at 510 ℃ with 23% conversion of methane and 65% selectivity of chloromethane. To clarify the cause of the promotion by addition of iron, the characterizations of catalysts, e.g. XPS, BET, XRD, TEM and H2 TPR, were conducted. XRD and H2 TPR results indicate that iron–cerium solid solution is formed, resulting in the production of more easily reduced cerium oxide. In addition, the suppression of catalyst sintering during the reaction is confirmed by XRD and TEM. Consequently, the selectivity of by–products decreases more significantly over Fe2O3/CeO2 than over cerium oxide which can be attributed to the facilitation of HCl oxidation arising from the enhanced reducibility of the former sample.