Effect of imidazolium ionic liquids as microwave absorption media for the intensification of microwave-assisted extraction of Citrus sinensis peel essential oils

Highlights

• Ionic liquid-based microwave-assisted extraction (MAE-IL) was studied.

• MAE-IL was evaluated for the extraction of essential oil (EO) from orange peel.

• Two imidazolium ionic liquids (ILs) were tested at two different concentrations.

• ILs influenced EO extraction, thermal behavior, and chemical composition.

• [C₂mim]OAc was more effective, increasing both extraction velocity and efficiency

Abstract

Ionic liquid-based microwave-assisted extraction (MAE-IL) has been applied for the separation of essential oils (EOs) from Citrus sinensis var. Valencia peels. Two imidazolium ionic liquids, 1-ethyl-3-methyl imidazolium acetate ([C₂mim]OAc) and 1-butyl-3-methyl imidazolium chloride ([C₄mim]Cl), were investigated for extraction efficiency, thermal behavior of the process, and chemical composition of the EOs obtained. During extractions, two different concentrations of ionic liquids (ILs) were studied (5% or 10 %), and the proposed MAE-IL was evaluated in comparison with usual microwave-assisted extraction (MAE) using water as solvent. Results indicated that both concentrations of ILs influenced EO extraction, thermal behavior, and chemical composition. However, [C₂mim]OAc was more efficient, increasing extraction velocity by 10 times, enhancing extraction efficiency of EOs (39 %), and increasing the presence of oxyterpene compounds in the EOs. However, the great ability of ILs to interact with microwaves had an impact on the thermal history of the sample, reaching almost 110 °C, promoting the presence of compounds related to thermal effect.

Introduction

Room-temperature ionic liquids (ILs) are a group of organic salts that are composed of organic cations and organic or inorganic anions with melting points that range from −81 °C to 125 °C [1]. These compounds are considered green solvents due to their unique properties, namely, chemical and thermal stability, nonflammability, practically negligible volatility at the usual process conditions, high conductivity, wide liquid range, great ability to solvate a wide variety of compounds, and negligible vapor pressure [2,3]. Moreover, ILs have recently been proved to dissolve with high efficiency various types of biomacromolecules such as cellulose, chitin, starch, and proteins [4,5]. The matrix structure composed of cellulose may block the release of volatile compounds in plant cells, so the use of ILs to treat plant matrices is a promising approach to overcome cell wall limitations for isolation of intracellular active constituents [6]. Different ILs have been successfully used as solvents in the extraction of various useful substances from plant samples, such as phenols, alkaloids, glycoside, lignans, organic acids, and essential oils (EOs), among others [7]. Additionally, it has been reported that ILs can act as surfactants for the extraction of hydrophobic EOs by forming emulsions in water. Therefore, application of IL treatment should not only give a better and higher yield access to the valuable intracellular components via dissolution of plant cell walls but also facilitate the direct distillation and separation of EOs, taking advantage of their negligible volatility and their surfactant effect [4].

Nowadays, traditional hydro-distillation and/or steam-distillation processes are employed to obtain most essential oils from plants, which requires long processing times and high-energy consumption. Furthermore, in order to increase the EOs stability and quality for pharmaceutical, cosmetic or food applications, such natural extracts need to be treated by deterpenation processes, which also increases their production costs. Deterpenation processes for citrus essential oils by means of liquid-liquid extraction, vacuum distillation, and extractive distillation aided by ionic liquids have been recently explored, noticing that the differences between the gas liquid partition coefficients for terpenes and low volatile terpenoids allow to contemplate ILs for appropriate purification of EOs [8,9]. It is clear that there is a great area of opportunity for process intensification during EOs extraction, looking to reduce the extraction times, improve the obtained yields and selectivity, reduce energy requirements, and minimize the environmental impact. In this field, microwave-assisted extraction (MAE), has been gaining application for extraction of EOs since this technique has proved to present many advantages such as convenience, less time consuming, and high efficiency. For MAE extraction, water has been widely utilized as solvent due its high microwave absorbing properties that allow efficient desorption and solubilization [10] and because it can be easily separated from the EO after the process, due to its immiscibility. Furthermore, since most EOs are destined to pharmaceutical, cosmetic, and food applications, solvents such as water and ethanol are preferred over other solvents.

It has been reported that ILs are very suitable for MAE because they can rapidly and effectively absorb microwave energy [7]. Hence, it is noteworthy that application of ILs as solvents, co-solvents, and additive media under microwave irradiation has attracted the attention of the scientific community since addition of an ionic liquid into the extraction system can enhance process efficiency [4,5,11,12]. An advantage of using ILs in water solutions, is that EO is recovered in the vapor phase with water, but free of ionic liquid. Furthermore, ILs can enhance the extraction not only due to their dielectric properties [13,14], but also due to their solvation properties and surfactant activity, allowing a better interaction between water and EO [15,16].

In recent years, several authors have reported the use of ILs from imidazolium family for EO extraction from different plants via MAE. Particularly, the alkyl chain imidazolium salts are the most often used ILs as solvents for a wide range of inorganic and organic materials [17,18]. In most reported cases, higher yields and shorter extraction times than those obtained using conventional solvents were reported, making the use of ILs, potential, green, and highly efficient options for the extraction of volatile fractions from aromatic plant material via MAE. To date, there are reports of successful extraction for leaves [15,19,20], barks [21], fruits [11], and spices [22] using imidazolium IL family as solvents for MAE. Imidazolium based ILs behave as amphiphilic compounds, displaying an interface phenomenon that depends on the chain length [23]. Additionally, the character of the anion affects the heating behavior of IL during microwave irradiation and the ability to accept hydrogen bonding [24,3].

The effect of ILs on the extraction of EOs from citrus species has been reported, and their ability to enhance the sensory properties of their EOs has been demonstrated [3]. [25] reported the use of three imidazolium-based ILs, 1-butyl-3-methylimidazolium chloride ([C₄mim]Cl), 1-allyl-3-methylimidazolium chloride ([Amim]Cl), and 1-ethyl-3-methylimidazolium acetate ([C2mim]OAc) on the extraction of EO from orange peels by means of a pretreatment in IL solution followed by vacuum distillation (60–65 °C/12–15 mbar). These authors found that, in general, the best results were obtained using [C₂mim]OAc as solvent, with the highest yield of 5%. However, the impact of these green solvents on the extraction of EOs from orange peels by MAE and information on their composition is scarce.

The objective of the present work was to determine the effect of two imidazolium ILs with different chain length and anion nature, [C₂mim]OAc and [C₄mim]Cl, as microwave absorption media for the extraction of EOs from Citrus sinensis peels by MAE. The impact of the type and concentration of both ILs on yield, thermal behavior of the mixture treated, and chemical composition of the obtained EOs was also addressed.