Bi-functional nanostructured novel catalysts for dimethyl ether synthesis

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2012
Çelik, Gökhan
Excessive use of fossil fuels shall result in the significant energy problems in the coming century and causes global warming by CO2 emission. Use of petroleum in transportation constitutes the dominant part of total petroleum use. Researches on non-petroleum based, environmentally friendly alternative fuels have been ascended in last decades. Among the alternative fuels, DME has been considered as an attractive fuel alternate due to high cetane number, low PM (particulate matter) and low NOx emission. Synthesis of DME is possible with gasification of biowastes or coal and steam reforming of natural gas. DME is produced in two different methods. In the first method, methanol is formed from the synthesis gas, followed by methanol dehydration to DME. In the second method, called as direct synthesis of DME from synthesis gas, methanol formation and dehydration occurs simultaneously at the same location within the reactor. For the direct synthesis of DME, bi-functional catalysts must be used; one site is responsible for methanol synthesis and other site is responsible for methanol dehydration. Throughout this thesis work, several catalysts were prepared to be used as methanol synthesis component or methanol dehydration component of bi-functional direct DME synthesis catalyst and bi-functional catalysts were also prepared for the direct synthesis of DME from synthesis gas. Materials were characterized by XRD, EDS, SEM, N2 physisorption, and DRIFTS characterization techniques. Activity tests were conducted in a high pressure, fixed bed flow reactor at 50 bar and for the feed gas compositions of H2:CO=50:50 and H2:CO: CO2=50:40:10. Addition of zirconia and alumina promoters, long aging time, calcination temperature of 550 °C and reduction at 250 °C were found to be beneficial in methanol synthesis from the equimolar composition of CO and H2. Precipitated catalysts were usually active and selective to methanol. However, bi-functional co-precipitated catalyst was not successful in situ conversion of methanol into dimethyl ether. Furthermore, tungstosilisic acid impregnated SBA-15 was physically mixed with commercial methanol reforming catalyst and activity results revealed that high DME yield and selectivity were obtained. By physically mixing commercial methanol synthesis and reforming catalysts with γ-Al2O3 and TRC-75(L) in appropriate proportions or by preparing the reactor bed in a sequential arrangement, very high DME yields were obtained and superiority of direct synthesis to conventional two step synthesis was proven. Presence of CO2 in the feed stream not only enhanced the catalytic activity but also utilization of the most important greenhouse gas was accomplished. It was seen that synthesized catalysts are very promising in the direct synthesis of dimethyl ether from synthesis gas.

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Citation Formats
G. Çelik, “Bi-functional nanostructured novel catalysts for dimethyl ether synthesis,” M.S. - Master of Science, Middle East Technical University, 2012.