Abstract:
The objective of this study is to examine the catalytic performance of bi-functional catalyst systems in direct synthesis of dimethyl ether (DME). Direct synthesis method involves two consecutive steps: methanol synthesis followed by methanol dehydration. Hence, a commercial methanol synthesis catalyst (Cu-Zn based HiFuel-R120) was coupled with different methanol dehydration catalysts in a dual-bed micro-reactor. Methanol dehydration catalysts were prepared by incipient-to-wetness impregnation by varying CeO2 loading on δ-Al2O3. Syngas-to-DME performance of the bi-functional catalyst system was studied in an Autoclave Engineers' BTRS-Jr-PC high-pressure, high-temperature reaction test system with a down-flow fixed-bed reactor designed to operate up to 600oC and 100 atm. Temperature, pressure, feed composition and CeO2 loading were tested for their effects on catalyst performance expressed in terms of CO conversion; DME, methane, carbon dioxide and methanol yields, and DME selectivity. Temperatures of 250, 275 and 300oC and pressures of 25 and 34 bar were tested with CeO2 loadings of 5%, 10% and 20% CeO2 on δ-Al2O3 for methanol dehydration. Results on 5% and 10% CeO2/δ-Al2O3 catalysts indicate that increasing the temperature enhances CO conversion and increases both DME selectivity and DME yield, while CO conversion on 20% CeO2/δ-Al2O3 is not altered, which may be due to metal sintering. Increasing the pressure leads to higher catalytic activity on 5% and 10% CeO2/δ-Al2O3, due to Le Chatelier’s principle operative in methanol synthesis. Effect of H2/CO molar feed ratio on CO conversion and DME selectivity is studied at ratios of 1 and 2, and results show that a H2-rich medium increases DME selectivity. Effect of decreasing CeO2 loading is to enhance CO conversion and DME selectivity. Highest CO conversion (36.6%) and DME selectivity (74.4%) are obtained when HiFUEL R120 is coupled with 5% CeO2/δ-Al2O3 at 300 °C and 34 bar using a H2/CO molar feed ratio of 2.
