(149b) CO2 As a Carbon Source in Direct Synthesis of Dimethyl Ether from Syngas over Silicotungstic Acid Incorporated Bifunctional Catalysts

Dimethyl ether (DME) is one of the most promising environmentally friendly diesel fuel alternates with a high cetane number and clean-burning properties. Its synthesis directly from syngas has significant thermodynamic advantages over the conventional two-step process involving methanol synthesis and dehydration reactors [1, 2]. The use of CO₂ as the carbon source in the synthesis of DME has added advantages of converting one of the most abundant greenhouse gases to a valuable chemical. Direct synthesis of DME from CO₂ requires the development of novel bifunctional catalysts containing both methanol synthesis and dehydration sites within the same proximity. In the present study, core-shell type bi-functional catalysts and an STA incorporated Cu-Zn-Al type methanol synthesis catalyst (25STA@CZA:6/3/1) were synthesized and tested in direct synthesis of DME from syngas containing different CO₂/CO mole fractions, ranging between 1/0 and 0/1. In the case of core-shell type catalytic materials (25STA@CZA-EMA), γ-Al₂O₃ was deposited over the methanol synthesis catalyst, and then STA was impregnated into this layer. Activity test results proved that the overall conversion of CO+CO₂ and DME yield values advance with an increase in CO₂/CO ratio until a feed composition of H₂/CO₂/CO = 50/40/10. The highest overall conversion of CO₂+CO and the corresponding DME yield values were obtained as 73.4% and 51%, respectively, over the 25STA@CZA catalyst, at 275^oC and 50 bar. In the case of using a feed mixture containing only CO₂ and H₂, lower overall conversion and DME selectivity values were obtained (20% and 55%, respectively). In this case, conversion of CO₂ to CO and H₂O through reverse-water-gas-shift reaction gains importance, and the increase of H₂O in the product stream negatively affects the in-situ dehydration reaction of produced methanol. Results proved that the new STA incorporated methanol synthesis catalysts were highly promising for the conversion of CO₂- containing syngas to DME, and the optimum CO₂/CO ratio for the highest DME yield was 4/1.

[1] Karaman, BP, Oktar, N., Dogu, G, Dogu, T (2020). Catal. Letters, 150: 2744-2761.

[2] Bayat, A, Dogu T. (2016). Ind. Eng. Chem. Res., 55:11431-39.