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Kinetic studies for dimethyl ether and diethyl ether production

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2007
Varışlı, Dilek
Fast depletion of oil reserves necessitates the development of novel alternative motor vehicle fuels. Global warming problems also initiated new research to develop new fuels creating less CO2 emission. Nowadays, dimethyl ether (DME) and diethyl ether (DEE) are considered as important alternative clean energy sources. These valuable ethers are produced by the dehydration reaction of methanol and ethanol, respectively, in the presence of acidic catalysts. Besides DEE, ethylene which is very important in petrochemical industry, can also be produced by ethanol dehydration reaction. In the first part of this study, the catalytic activity of tungstophosphoric acid (TPA), silicotungstic acid (STA) and molybdophosphoric acid (MPA), which are well-known heteropolyacids were tested in ethanol dehydration reaction. The activities of other solid acid catalysts, such as Nafion and mesoporous aluminosilicate, were also tested in the dehydration reaction of ethanol. In the case of DME production by dehydration of methanol, activities of STA, TPA and aluminosilicate catalysts were tested. Among the heteropolyacid catalysts, STA showed the highest activity in both ethanol and methanol dehydration reactions. With an increase of temperature from 180oC to 250oC, Ethylene selectivities increased while DEE selectivities decreased. Ethylene yield values over 0.70 were obtained at 250oC. The presence of water in the feed stream caused some reduction in the activity of TPA catalyst. Very high DME yields were obtained using mesoporous aluminosilicate catalyst at about 450oC. The surface area of heteropolyacids are very low and they are soluble in polar solvents such as water and alcohols. Considering these drawbacks of heteropolyacid catalysts, novel mesoporous STA based high surface area catalysts were synthesized following a hydrothermal synthesis route. These novel catalysts were highly stable and they did not dissolve in polar solvents. The catalysts containing W/Si ratios of 0.19 (STA62(550)) and 0.34 (STA82(550)) have BJH surface area values of 481 m2/g and 210 m2/g, respectively, with pore size distributions ranging in between 2-15 nm. These catalysts were characterized by XRD, EDS, SEM, TGA, DTA, DSC, FTIR and Nitrogen Adsorption techniques and their activities were tested in ethanol dehydration reaction. Calcination temperature of the catalysts was shown to be a very important parameter for the activities of these catalysts. Considering the partial decomposition and proton lost of the catalysts over 375oC, they are calcined at 350oC and 550oC before testing them in ethanol dehydration reaction. The catalysts calcined at 350oC showed much higher activity at temperature as low as 180oC. However, the catalysts calcined at 550oC showed activity over 280oC. Ethylene yield values approaching to 0.90 were obtained at about 350oC with catalysts calcined at 350oC. DEE yield past through a maximum with an increase in temperature indicating its decomposition to Ethylene at higher temperatures. However, at lower temperatures (<300oC) Ethylene and DEE were concluded to be formed through parallel routes. Formation of some acetaldehyde at lower temperatures indicated a possible reaction path through acetaldehyde in the formation of DEE. DRIFTS results also proved the presence of ethoxy, acetate and ethyl like species in addition to adsorbed ethanol molecules on the