A recent paper reports the development of a significant new method for producing biofuel from rapeseed oil-based biodiesel that resembles petroleum diesel in its physicochemical properties, enabling its use in conventional diesel engines. Unlike conventional biodiesel, this new biofuel can be used in standard diesel engines undiluted or blended in any ratio with petroleum diesel, which could improve the viability of biofuels as renewable energy resources and enable their widespread adoption.
Biodiesel is produced from rapeseed oil (or other vegetable oils) by a process called transesterification, in which the long chain fatty acids in the oil react with alcohols such as methanol to produce esters. However, the fatty acid methyl esters produced are vastly different from conventional petroleum diesel with regard to their boiling point, viscosity, and other physical and chemical properties. This makes biodiesel unsuitable for use in conventional diesel engines.
The current European Norm (EN 590) requires that the renewable fuel content of commercially sold diesel be 7% or more, and this threshold will rise to 10% in 2020. However, the high boiling points of the fatty acid methyl esters in biodiesel represent a significant barrier to its widespread adoption. Now, in a remarkable advance, a group of researchers from the universities of Kaiserslautern, Bochum, and Rostock in Germany report the development of a method to convert rapeseed oil-derived biodiesel into a biofuel with improved boiling characteristics and viscosity. In a report published in Science Advances in June 2017, the researchers describe the innovative use of a technique called isomerizing cross-metathesis to convert rapeseed oil-derived fatty acid methyl esters into the new biofuel.1
The researchers carried out one set of reactions using rapeseed oil methyl ester (RME), 1-hexene and two catalysts: an isomerizing catalyst and an olefin metathesis catalyst, and the products included a mixture of olefins, monoesters, and diesters. The isomerizing catalyst promotes the movement of double bonds along the hydrocarbon chain, while the olefin metathesis catalystex changes the alkyl residues attached to the double bonds of the 2 molecules, RME and 1-hexene.The addition of 1-hexene reduces the chain lengths of the hydrocarbon products, resulting in a more evenly rising boiling point curve that closely matched that of petroleum diesel. After the reaction, the product mixture was distilled at atmospheric pressure to determine the boiling point curve, but very narrowly failed to meet the EN 590 criteria of at least 95% recovery of product in distillate at 360°C.
In the second set of reactions, ethylene gas was used as the reactant instead of 1-hexene to reduce hydrocarbon chain length, yielding a product mixture comprised of smaller olefins, monoesters, and diesters. In this case, distillation and subsequent analysis of the distillate showed a product recovery of 95% at 360°C meeting EN 590 criteria for biodiesel fuel quality.
All reactions were carried out at temperatures between 45 and 75°C, suggesting minimal energy requirements in an industrial scale setting. Furthermore, the reaction process is environment-friendly, as it requires no additional chemical solvents. The new biofuel was successfully used to operate a 2.5 cc self-igniting model diesel engine in trials.
The technique reported in this paper is a significant step towards developing biodiesel as a viable renewable energy resource. This breakthrough technology could not only help meet targets set for the reduction of fossil fuels but help exceeds them. Biodiesel is not only sustainable but also biodegradable, clean burning, and safer compared to fossil fuels. Therefore, its adoption would go a long way towards meeting our energy needs while mitigating many of the harmful effects of fossil fuel use on the environment.
Written by Usha B. Nair, Ph.D.
- Pfister KF, Baader S, Baader M, Berndt S, Goossen LJ. Biofuel by isomerizing metathesis of rapeseed oil esters with (bio)ethylene for use in contemporary diesel engines. Sci Adv. 2017 Jun 16;3(6):e1602624. doi: 10.1126/sciadv.1602624. eCollection 2017 Jun. PubMed PMID: 28630908; PubMed Central PMCID: PMC5473673.