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Development of bifunctional catalyst for the single-step synthesis of dimethyl ether

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2019
Şener, Mehmetali İlker
Depletion of fossil fuels, global warming concerns and increasing CO2 emissions necessitated the utilization of syngas in the production of alternative fuels over the last few years. Dimethyl ether (DME) is considered as promising diesel substitute or LPG additive due to its high cetane number and good burning properties. The single-step DME synthesis method, which involves consecutive methanol formation and methanol dehydration reactions, enables the production of DME directly from the syngas. Despite the advantage of breaking the thermodynamic limitation in the single-step DME synthesis, development of an bifunctional catalyst to synthesize DME from the syngas is a challenging topic. Mesoporous alumina and mesoporous carbon supports were synthesized. Cu-Zn metals were loaded for the methanol synthesis reaction whereas tungstophosphoric acid (TPA) was loaded for the methanol dehydration reaction into these supports with different amounts using impregnation and one-pot methods. Synthesized catalysts were characterized by XRD, N2 physisorption, TGA, TPR, TPD, SEM-EDX. Thermodynamic equilibrium analyses showed that the optimum molar CO/H2 ratio was 1/1 and the optimum operating temperature was 275°C under 50 bar pressure. Performance tests were carried out at these conditions in DME production system for the single-step DME synthesis from syngas. Commercial alumina and methanol synthesis catalyst mixture was the most catalytically active couple among all catalysts. TPA loaded catalysts inhibited the water-gas shift reaction and showed high CO conversion and high methanol and DME selectivities. Optimum TPA loading was determined as 10 wt. %, regardless of the loading method. The highest CO conversion was obtained as 53% with 43% DME selectivity over 10 wt. % TPA impregnated alumina catalyst mixed with commercial methanol synthesis catalyst with the molar CO/H2 ratio of 1/1 among the synthesized catalysts. The highest DME selectivity was found to be 78.5% from syngas containing 25% CO2, whereas the highest total conversion was found to be 34.5% from syngas containing 10% CO2 in the presence of commercial methanol synthesis and alumina mixture. The TPA and Cu-Zn loaded bifunctional CMK-3 catalyst showed higher DME selectivity compared to the TPA and Cu-Zn loaded bifunctional EMA catalyst.