Researchers from the universities of Kaiserslautern, Bochum, and Rostock have developed a method for producing a petroleum diesel-like fuel from conventional biodiesel (rapeseed oil methyl ester, RME).
Catalyzed by a Pd/Ru system, RME undergoes isomerizing metathesis in a stream of ethylene gas, leading to a defined olefin, monoester, and diester blend. This innovative refining concept requires negligible energy input (60 °C) and no solvents and does not produce waste. The new biofuel fulfills the current EU and US requirements, and can be used undiluted in modern diesel engines or mixed in any ratio with petroleum diesel. The researchers present their work in an open access paper in Science Advances.
The main obstacle to increasing the biodiesel content in motor fuels arises from its unfavorable boiling behavior. The standard EN 590 for commercial diesel fuel suitable for powering standard diesel engines calls for a smoothly rising boiling point curve within strict limits that ensure optimal fuel ignition and combustion. Petrodiesel, which fulfills these requirements, consists of a mixture of linear and branched hydrocarbons with various chain lengths. In contrast, a typical biodiesel based on rapeseed oil methyl ester (RME) mainly contains linear molecules with 19 carbon atoms, that is, 65% methyl oleate [18:1], 22% methyl linoleate [18:2], 8% methyl linolenate [18:3], 1% methyl stearate [18:0], and 4% methyl palmitate [16:0]. As a result, its boiling range starts at too high a temperature and is disadvantageously narrow (330° to 400°C), clearly outside the EN 590 specifications. This adversely affects its ignition behavior and precludes late-stage injections as are required by modern diesel engines with particulate filters. In pure form, RME and related biodiesels can, thus, only be used in dedicated engines especially engineered to cope with these challenging physical and chemical properties.
The only known strategy to convert vegetable oils into biofuel suitable for use in standard diesel engines is their conversion into mixtures of saturated hydrocarbons by energy-intensive hydroprocessing. Products obtained by conventional cross-metathesis of RME do not meet the specifications of EN 590. An alternative, low-temperature refining concept that allows converting vegetable oils into biodegradable product blends with petrodiesel-like boiling ranges would be of tremendous interest. This key challenge is addressed by the isomerizing metatheses process disclosed herein, which enables generating an EN 590–compatible fuel from fatty acid esters and ethylene, which is abundantly available from bioethanol or shale gas.—Pfister et al.
Isomerizing metatheses is a known chemical reaction in which, an isomerization catalyst constantly moves double bonds up and down along alkyl chains within a molecule, while an olefin metathesis catalyst continually shuffles the alkyl residues attached to the double bonds between two molecules.
This iterative, cooperative action of two orthogonal catalysts converts single olefins into olefin blends with carbon-chain lengths evenly distributed around the chain length of the starting material.Although as a technology isomerizing olefin metathesis has made a number of recent advances, it has never been envisaged as a tool for diesel refining due to a number of challenges, the German team noted. In earlier work, the team had been unable to obtain full conversion or homogeneous product distributions even in the presence of solvent.
The latest process uses a ternary catalyst system that is able to catalyze isomerization, ethenolysis, and long-chain olefin cross-metathesis, respectively. To demonstrate the new biofuel’s suitability as motor fuel, the team used it in a 2.5 cc model diesel engine.
|Isomerizing ethenolysis of RME. Conditions: 400 mmol of RME, ethylene stream, Ru-11, Ru-CAAC, IC-1, neat, 60°C, 16 hours. Pfister et al. Click to enlarge.|
The particular advantage of this new technique is that the researchers are able to precisely adjust the chemical properties of the mixture. The process is environmentally friendly: it neither requires solvents, nor produces waste.
… isomerizing olefin metathesis with (bio)ethylene allows converting RME into olefin mixtures that match the boiling behavior of diesel fuel as specified in EN 590. This technology may turn out to be a decisive breakthrough toward increasing the content of renewables in diesel fuel. The activity, compatibility, and stability of the catalysts are sufficient to demonstrate the viability of this low-temperature refining concept but do not fulfill the industrial manufacturing standards. Many steps are still required to establish an industrial production of an EN 590–compatible biofuel. At the end of this development, we envision a fully continuous flow process in which an RME/ethylene feed passes through alternating sections of immobilized isomerization and metathesis catalysts dimensioned to provide the ideal ratio of turnover frequencies. The limiting challenge at this stage is the discovery of long-lived, economically viable, heterogeneous or heterogenizable catalysts.—Pfister et al.
The research was carried out within the collaborative research centre “3MET” (SFB/TRR 88 “Cooperative Effects in Homo and Heterometallic Complexes”) at the University of Kaiserslautern and the cluster of excellence Resolv (Ruhr Explores Solvation) at Ruhr-Universität Bochum. It was also supported by the German Federal Environmental Foundation (DBU) and the Carl Zeiss Foundation.
Kai F. Pfister, Sabrina Baader, Mathias Baader, Silvia Berndt, Lukas J. Gooßen (2017) “Biofuel by isomerizing metathesis of rapeseed oil esters with (bio)ethylene for use in contemporary diesel engines” Science Advances Vol. 3, no. 6, e1602624 doi: 10.1126/sciadv.1602624