Dimethyl ether production from synthesis gas with bifunctional catalyst mixtures

Ermiş, Salih
In recent times, studies on alternative clean fuels have increased due to the depletion of crude oil reserves in the world because of increasing energy demand and the worldwide existence of severe air pollution. Dimethyl ether (DME) is, therefore, being investigated as an excellent clean fuel alternative in compression-ignition engines. DME can be produced from synthesis gas by two different methods, direct and indirect. Recently, the direct method has gained importance in the production of DME from syngas. In direct DME synthesis from the syngas, methanol synthesis and methanol dehydration occur simultaneously in a bifunctional catalyst bed in the same reactor. Moreover, the thermodynamic limitations in the methanol synthesis stage are substantially overcome, resulting in a much higher yield and, thus, a significant improvement in the process economy. Mesoporous silica aerogel support was synthesized. Silicotungstic acid (STA), tungstophosphoric acid (TPA), and both STA and alumina (Al) were loaded into this support using the impregnation method. Nitrogen physisorption technique, X-ray diffractometer, Thermogravimetric analyzer, and Diffuse reflectance infrared Fourier transform spectroscopy were used to characterize the synthesized material. According to thermodynamic analysis, operating pressure and temperature were selected 50 bar and 275 ℃, respectively. The CO/H2 molar ratio was 1/1. Activity tests were performed under these conditions in a high pressure fixed bed reactor. All synthesized catalysts were physically mixed with the commercial methanol synthesis catalyst. Sol-gel synthesis method was used to create silica aerogel support material that displayed Type IV isotherms with H3 type hysteresis loops, indicating mesoporous structure. The SA had a multipoint BET surface area of 793±14.1 m2/g, BJH desorption average pore diameter of 10.9±0.4 nm, and BJH desorption cumulative pore volume of 3.44±0.09 cm3/g. While the isotherm type remained the same, corresponding to Type IV, the metal loading into SA support changed the hysteresis loop to H1. Physically mixed commercial methanol synthesis and commercial alumina catalysts were used for repeatability tests on DME production. DME selectivity was 50.9% in the commercial catalyst mixture. It was observed that the DME selectivity increased with the increase of the amount of STA loaded on the SA support material from 10% to 35% by weight. The DRIFTS results demonstrate that increasing the amount of STA caused an increase in Brønsted acid sites, which also resulted in an increase in DME selectivity. Among the synthesized catalysts, SA-35STA yielded the highest DME selectivity of 50.6% with the CO conversion of 66.1%. The highest DME yield in all synthesized catalysts was found to be 33.5% with the SA-35STA catalyst. It was observed that SA-35STA together with MSC is a suitable catalyst candidate for direct DME synthesis.


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Citation Formats
S. Ermiş, “Dimethyl ether production from synthesis gas with bifunctional catalyst mixtures,” M.S. - Master of Science, Middle East Technical University, 2022.