Dimethyl ether from synthesis gas over bifunctional hybrid catalyst mixtures

Bayat, Ayşegül
Due to increasing prices of crude oil based transportation fuels and ascending rate of global warming caused by high emission levels of conventional fuels with excessive use, alternative fuels have been considered as alternates. Dimethyl ether (DME) has received growing attention as an alternative clean fuel with low NOx formation and particulate emission, smokeless combustion and high cetane number. DME is mainly synthesized by two methods. In the first method, synthesis gas which is a mixture of carbon monoxide and hydrogen is converted to methanol over a copper-based catalyst; then, methanol dehydration takes place in the presence of a solid acid catalyst, resulting in the production of DME. The second method is direct synthesis of DME from synthesis gas, which is a single step process that requires catalysts having two active sites for methanol synthesis and methanol dehydration. In this work, direct synthesis of DME from synthesis gas was investigated using different catalyst mixtures containing methanol synthesis and dehydration components. Bifunctional catalyst mixtures containing copper-based methanol synthesis catalysts and commercial γ-Al2O3 methanol dehydration catalyst were tested in a high pressure, fixed bed flow reactor system. The reaction conditions were selected as 50 bar, temperature range of 200-300oC with a feed gas composition of H2/CO=50/50 based on volume ratio. The highest CO conversion was achieved as 15.6% at 300oC using alumina promoted catalyst mixture. Bifunctional catalyst mixtures containing alumina and zirconia promoted catalysts yielded the best DME selectivities of 68.8% and 66.4%, respectively, at 200oC. Zirconia promoted catalyst (CZZr) was found to be the most stable copper-based catalyst. Catalyst mixture containing zirconia promoted catalyst calcined at higher temperature showed high initial DME selectivity of 73.9% at 225oC which decreased sharply at higher temperatures. In the second part of the work, mesoporous alumina (MA) was successfully synthesized with surface area of 322 m2/g. Commercial methanol synthesis catalyst (MSC) and CZZr were separately mixed with MA at the same reaction conditions. The activity test of MSC + MA showed that both CO conversion and DME selectivity increased with temperature and reached 36.6% and 55.8%, respectively, with a DME yield of 20.4% at 300oC. To increase the acidic strength, Silicotungstic acid (STA) was impregnated on MA (STA@MA). The amount of STA@MA was also increased. The highest CO conversion, DME selectivity and DME yield were achieved as 49.4%, 60.8% and 30.1%, respectively, at 300oC by using MSC + STA@MA. MSC + STA@MA was tested using different volumetric flow rate ratios of H2, CO and CO2. The overall DME selectivity obtained with 10% of CO2 containing feed gas was 90% at 275oC and decreased to 82.2% and 78.2% with increasing CO2 to 25% and 40%, respectively. The feed gas mixture containing 10% of CO2 could be considered as the best feed gas composition. Experimental CO conversion and CO and CO2 compositions were compared with the predicted equilibrium values. None of the experimental results exceeded their corresponding equilibrium CO conversion, CO and CO2 compositions. Results proved that, synergetic effect of occurrence of methanol synthesis and dehydration reactions simultaneously in the same system considerably enhanced DME yield.


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Citation Formats
A. Bayat, “Dimethyl ether from synthesis gas over bifunctional hybrid catalyst mixtures,” M.S. - Master of Science, Middle East Technical University, 2013.