Ionic liquids for the GC of biodiesel blends

Biodiesel was the first advanced biofuel to make it to market, where it is generally sold as blends with petroleum distillates. It has already become a significant alternative fuel due to three main factors: its sources, lack of toxic content, and combustion characteristics.

The fuel is produced from domestic renewable resources such as vegetable oils, recycled cooking oils, animal fats and, lately, algae. Contrary to one of the most common criticisms, biodiesel production does not contribute to land clearage for new crops because it is produced from co-products of existing crops. So, rather than contributing to greenhouse gas emissions, it reduces lifecycle carbon emissions by 60-80%. This makes it "the best carbon reduction tool of any liquid fuel commercially available" according to the US National Biodiesel Board.

Apart from assuring its biodegradability, the biological sources ensure that there are no toxic sulphur or nitrogen compounds present. In diesel engines, a blend of 20% biodiesel in petroleum (B20) showed similar fuel consumption, horsepower, torque and haulage rates as conventional diesel fuel over 50 million miles of tests. Biodiesel also has superior lubricity as well as the highest BTU content of any alternative fuel.

Biodiesel consists of mixtures of fatty acid methyl esters (FAMEs) and their quantities in blends is gauged by one of two official methods. ASTM D7371 uses mid-IR spectroscopy to assess FAME levels of 1-20% whereas the European UNI EN 14331 procedure measures the FAME profile using HPLC fractionation followed by GC. The latter procedure has been used for middle distillates containing up to 5% FAMEs but it is relatively complex and time consuming.

Italian scientists have developed an alternative method for measuring FAMEs in biodiesel blends that is quicker and simpler and can be applied to blends containing up to 40% esters. Luigi Mondello, Carla Ragonese, Peter Quinto Tranchida and Danilo Sciarrone from the University of Messina and University Campus Biomedico of Rome used GC-FID in a procedure that required no pre-fractionation of the fuel blend.

Their method relies on an ionic liquid as stationary phase. This is not a novel concept for GC, with reports of ionic liquids being used for the separation of PAHs, essential oils and chlorinated pesticides. Their low volatility, good wetting properties, high thermal stability and selectivity towards particular classes enhance their suitability.

Mondello and colleagues used a commercial column containing 1,9-di(3-vinylimidazolium)nonane bis(trifluoromethyl)sulphonylimidate (SLB-IL100). In initial experiments using a 30 m column, it performed better on a mixture of C4-C24 FAMEs than a conventional 100% polyethylene glycol (PEG) stationary phase under the same conditions.

Resolution was good for both columns but the ionic liquid column accomplished the separation in a quicker time, the C24 methyl ester eluting after 48 minutes, compared with 65 minutes on the PEG column. In addition, plots of the elution temperatures versus the carbon number revealed that the ionic liquid column should elute the C32 methyl ester at its highest operating temperature (230°C) compared with the C28 ester for the PEG column at its maximum temperature of 280°C.

The polarity of the IL100 column was calculated to be 4437, almost double that of the PEG column and even greater than that of the highly polar tris(2-cyanoethoxy)propane stationary phase.

For the analysis of B20 blends, each FAME eluted separately on a 30 m IL100 column without interference from the other more polar diesel components such as PAHs. In contrast, the PEG column brought about coelution of some components.

The same good separation was achieved with a 12 m IL100 column, but in a fraction of the time, taking just 2.5 minutes to elute the FAMEs.

The FAME contents in a soybean B20 blend were measured on the 12 and 30 m columns from calibration curves, based on the peak areas of each component corrected for detector response. The calibrations curves were linear for both columns up to 40% (by volume) of FAMES.

Both columns gave low coefficients of variation for retention time and concentration and very similar contents for each of 6 FAMEs. The methyl esters of linoleic, oleic, palmitic, linolenic, stearic and eicosanoic acid were present at 55.4-56.1, 21.5-21.8, 12.7-11.8, 6.3-6.6, 3.3-3.0 and 0.7%, respectively.

The ionic liquid GC stationary phase gave excellent performance in short and long GC columns, its high polarity ensuring complete separation of the target FAMEs from each other and the diesel hydrocarbons in a faster time than conventional GC columns. Combined with the absence of a pre-fractionation step, the method should be useful for the routine quality control of biodiesel blends.

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Journal of Chromatography A 2009, 1216, 8992-8997: "Conventional and fast gas chromatography analysis of biodiesel blends using an ionic liquid stationary phase"