New Bifunctional Catalysts Offers Improved Performance and Stability for Direct Synthesis of Dimethyl Ether from Syngas
|Catalytic performance on the bifunctional catalysts. CZA is unpromoted, CZA-Zr is promoted with Zr; CZA-Ga is promoted with Ga. Credit: ACS. Click to enlarge.
Researchers at the Petroleum Displacement Technology Research Center, Korea Research Institute of Chemical Technology (KRICT), have developed new bi-functional catalysts, promoted with Zr (Zirconium) or Ga (Gallium), that offer higher catalytic performance and stability for the direct synthesis of dimethyl ether (DME) from syngas. A paper on their work was published 5 January in the ACS journal Energy & Fuels.
Used as an alternative diesel fuel, dimethyl ether, which is produced by the conversion of syngas derived from coal, biomass, or natural gas, offers lower NOx emissions and near-zero smoke evolution compared to conventional petroleum diesel fuel in addition to being a non-petroleum-sourced fuel. Conventional commercial production of DME from syngas is a two step process involving first the synthesis of methanol from syngas and the subsequent dehydration of the methanol to DME.
A single-step synthesis of DME from syngas on a bifunctional catalyst is receiving attention because of the simplicity and increased cost-effectiveness of a process that uses just one fixed bed or a slurry reactor, the study authors note. The direct synthesis of DME from syngas involves three important reactions:
- Methanol formation by CO hydrogenation and/or CO2 hydrogenation,
- Dehydration of methanol to DME and H2O
- Reaction of H2O with CO to form CO2 and H2 by the water-gas shift (WGS) reaction
In the case of using a bifunctional catalyst that is usually containing simultaneously two active sites for methanol synthesis and subsequent dehydration to DME, reactions of (i) and (iii) occur simultaneously on the methanol synthesis component of Cu-ZnO-Al2O3 and reaction (ii) occurs for methanol dehydration on solid acid catalysts, such as γ-alumina, ZSM-5, and modified alumina.
—Kang et al.
The researchers modified a bifunctional catalyst to enhance DME selectivity by adding Zr or Ga promoters on a Cu-ZnO-Al2O3 (CZA) methanol synthesis catalyst, and compared their performance to an unpromoted CZA catalyst.
While the unpromoted CZA catalyst showed a low DME yield around 27% and CO conversion of 61.1%, the CZA-Zr catalyst—which showed the best catalytic activity—showed DME yield of 40.3% and CO conversion of around 72.3%.
|CO Conversion and Product Distribution on Bifunctional Catalysts
|Product distribution (mol %)
The Cu-ZnO-Al2O3/γ-Al2O3 bifunctional catalyst promoted with Zr or Ga showed a higher catalytic performance with respect to high CO conversion and DME selectivity than that of an unpromoted catalyst...The promoters, such as Zr and Ga, play an important role in augmenting the dispersion of copper species with an appropriate electronic state and acidity of γ-Al2O3, which results in showing a high formation rate for methanol and consecutive dehydration rate to DME.
—Kang et al.
Suk-Hwan Kang, Jong Wook Bae, Hyo-Sik Kim, G. Murali Dhar and Ki-Won Jun (2010) Enhanced Catalytic Performance for Dimethyl Ether Synthesis from Syngas with the Addition of Zr or Ga on a Cu-ZnO-Al2O3/γ-Al2O3 Bifunctional Catalyst. Energy Fuels, Article ASAP doi: 10.1021/ef901133z