ORNL team discovers mechanism behind direct ethanol-to-hydrocarbon conversion; implications for energy efficiency and cost of upgrading
Researchers at Oak Ridge National Laboratory (ORNL) have discovered that the reactions underlying the transformation of ethanol into higher-grade hydrocarbons unfolds in a different manner than previously thought.
The research, supported by DOE’s BioEnergy Technologies Office (BETO), has implications for the energy efficiency and cost of catalytic upgrading technologies proposed for use in bio-refineries. Uncovering the mechanism behind the reaction helps support the potential economic viability of ORNL’s own direct biofuel-to-hydrocarbon conversion approach. An open-access paper on their findings is published in Nature Scientific Reports.
The addition of biomass derived ethanol to gasoline in the transportation sector is an important step in the utilization of renewable energy. The Energy Independence And Security Act of 2007 requires 36 billion gallons of biomass derived fuel by 2022 but ethanol demand is capped at ~14 billion gallons due to “blend-wall” at 10–15%. Although a higher concentration of ethanol containing fuel (e.g., E85) has been available for several years, such fuel can be used only in flex-fuel vehicles whose U.S. market penetration is low. This has renewed interest in the conversion of ethanol to hydrocarbon blend-stock and other industrial chemicals.
… Ethanol conversion to hydrocarbons employing zeolites as catalysts dates back to 1970s. Since then, a large number of reports have appeared in literature on ethanol conversion to hydrocarbons.[Ed note: the ORNL paper cites 35 papers.] The reaction temperature for ethanol transformation is generally >350 °C and the pressure ranges from ambient to several atmospheres. The product stream is generally high in C2 hydrocarbons (e.g., ethylene and ethane), which are not valuable for liquid fuel production or commodity chemical production (separation of pure ethylene is quite expensive from a mixed stream). The mechanism of ethanol conversion is still being debated.
… A recent review article summarizes state of the understanding of ethanol conversion mechanism over zeolites stating that ethanol first dehydrates to ethylene which then undergoes oligomerization to produce various olefins, paraffins, cyclics, and aromatics. In addition, radicals provide additional active sites for secondary reactions of ethylene. Despite these advances, the technology did not go beyond laboratory until recently due to lack of information on optimized conversion conditions, catalyst durability, and a clear understanding of mechanism to determine energy balance of ethanol conversion.—Narula et al.
As the ORNL team was developing a new type of zeolite for the conversion process, they found that the conversion technology, they found the energy-consuming intermediary dehydration step is not necessary. Instead, they found that an energy-producing “hydrocarbon pool” mechanism allows the zeolite catalysts directly to produce longer hydrocarbon chains from the original alcohols.
It challenges a long-held but incorrect assumption. It has been assumed that you must go from ethanol to ethylene, which is endothermic and requires energy. We showed this step doesn’t occur, and that the overall reaction is slightly exothermic. Our method of direct conversion of ethanol offers a pathway to produce suitable hydrocarbon blend-stock that may be blended at a refinery to yield fuels such as gasoline, diesel and jet fuel or commodity chemicals.—Chaitanya Narula, lead author
The ORNL team developed a new versatile heterobimetallic catalyst, InV-ZSM-5, that completely converts ethanol to hydrocarbons in 250–450 °C range and atmospheric pressure without added hydrogen. The InV-ZSM-5 catalyst exhibits superior performance compared to monometallic catalysts, In-ZSM-5 or V-ZSM-5 as determined by low C2 yield and high durability. All these catalysts are robust to water content in ethanol (5–95%) and volatile impurities in fermentation stream.
The researchers tracked the molecular transition in labeling experiments with deuterium to confirm the hydrocarbon pool mechanism.
The ORNL-developed catalyst and conversion process were licensed to Vertimass (earlier post), a startup company based in Irvine, CA. ORNL researchers are working with Vertimass through a separate DOE-funded project to scale the technology to the commercial level. (Earlier post.)
The study was supported by the BioEnergy Technologies Office in DOE’s Office of Energy Efficiency and Renewable Energy. Initial funding came from ORNL’s Laboratory Directed Research and Development program and DOE’s BioEnergy Science Center, which is supported by the Office of Biological and Environmental Research in DOE’s Office of Science. The project used resources at ORNL’s Center for Nanophase Materials Sciences, a DOE Office of Science User Facility. The zeolite materials were originally developed with support from DOE’s Office of Energy Efficiency and Renewable Energy.
Chaitanya K. Narula, Zhenglong Li, Erik M. Casbeer, Robert A. Geiger, Melanie Moses-Debusk, Martin Keller, Michelle V. Buchanan & Brian H. Davison (2015) “Heterobimetallic Zeolite, InV-ZSM-5, Enables Efficient Conversion of Biomass Derived Ethanol to Renewable Hydrocarbons” Scientific Reports 5, Article number: 16039 doi: 10.1038/srep16039