While the ion is hot: Microwave irradiation of ionic liquids for natural products extraction

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  • Published: Oct 24, 2011
  • Author: Steve Down
  • Channels: Sample Preparation
thumbnail image: While the ion is hot: Microwave irradiation of ionic liquids for natural products extraction

Essential oil extraction

Essential oils extracted from plants have found widespread use in the perfumery and flavour industries and as traditional medicines, so efficient methods of extraction are indispensable. The conventional procedures like Soxhlet extraction, pressurised fluid extraction and steam distillation have all been applied with varying degrees of success but they are associated with unwanted characteristics such as high solvent volumes, high pressures, long extraction times and thermal degradation.

A new process was introduced by Chinese scientists earlier in 2011 in an attempt to circumvent some of these problems. It involved the use of ionic liquids in a pretreatment step and gave higher yields than conventional Soxhlet extraction while providing an extract that was almost the same. However, all of the problems were not solved because 8 h of ionic liquid treatment was required.

Now, the same team has improved that process to give extraction times of just 10 minutes. Shi Chen, Ling Fan and Yinghu Liu from the South China Agricultural University, Guangzhou, Ronghua Jin from the Lanzhou Institute of Chemical Physics, China, and Min An from Charles Sturt University, Wagga Wagga, Australia, combined ionic liquid treatment with microwave irradiation.

They demonstrated the new procedure with the extraction of patchouli alcohol from Pogostemon cablin, a traditional Chinese medicine. This terpenoid compound is the principal constituent and the major aroma component of patchouli oil.

Preparing patchouli alcohol with ionic liquids

Dried leaves and stems of the plant were mixed with an excess of the ionic liquid 1-n-butyl-3-methylimidazolium chloride ([C4mim][Cl]) and subjected to microwave irradiation in a domestic unit for 60 s. The temperature was held at 70°C during irradiation, which is above the melting point of the ionic liquid.

The mixture was cooled to room temperature and the whole operation was repeated for a total of 10 cycles during which the cellulose matrix was broken up and dissolved. Longer irradiation times decreased the final yield of patchouli alcohol, probably due to thermal degradation.

The alcohol was extracted from the irradiated solution with an organic solvent, diethyl ether providing the best yield of six solvents tested.

The conditions were optimised for maximum yield, which was determined by GC-FID. The size of the plant pieces, microwave power, and the ratio of plant material to ionic liquid were all varied to establish the best values.

Recovery experiments in which known amounts of patchouli alcohol were added to the plant material and extracted under the optimum conditions revealed the excellent performance of the method. The recoveries were 95.7-103.7% and the GC detection limit was 0.156 mg/L.

The extraction yield was compared with those from other methods which were all measured in the lab by the research team, rather than taken from the literature. The microwave-assisted ionic liquid procedure outperformed all methods tested, with a yield of 1.94%. For room temperature extraction, heated reflux extraction, ultrasonic extraction, Soxhlet extraction and microwave assisted extraction, the best yield was only 1.07% and the lowest was 0.58%.

The researchers attributed the good performance of their method to the ability of the ionic liquid to break down the cell walls of the plant tissue, a process that was accelerated by microwave irradiation.

The microwave-assisted ionic liquid extraction process will give researchers another option for acquiring plant components. Clearly, it can be extended to the extraction of other active constituents from medicinal plants where it should also provide enhanced recoveries in reduced amounts of time, although the conditions may need to be optimised for each component.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

The major compound responsible for the characteristic aroma of patchouli oil has been extracted in high yield from the plant by microwave-assisted ionic liquid treatment. The process should be applicable to medicinal plants in general for the extraction of active components

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