Deterpenation of Citrus Essential Oils Using Glycerol-Based Deep

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Deterpenation of Citrus Essential Oils Using Glycerol-Based Deep Eutectic Solvents Baranse Ozturk,† Jesus Esteban,‡ and Maria Gonzalez-Miquel*,† †

School of Chemical Engineering and Analytical Sciences, Faculty of Science and Engineering, The University of Manchester, Manchester M13 9PL, United Kingdom ‡ Chair of Technical Chemistry, Department of Biochemical and Chemical Engineering, Technical University of Dortmund. Emil-Figge Straße 66, 44227 Dortmund, Germany S Supporting Information *

ABSTRACT: Citrus essential oils are complex hydrocarbon mixtures mainly composed of terpenes and terpenoids and are widely used as raw materials in food, pharmaceutical, and fine chemical industries. However, essential oil deterpenation (i.e., separation of terpenes and terpenoids) is required to preserve the quality of the final product for practical applications. Currently, there is a need to find efficient and environmentally friendly solvents to replace the harmful volatile organic compounds that are conventionally used as extraction solvents. Therefore, alternative solvents with more benign and environmentally friendly characteristics are crucial to develop sustainable citrus essential oil deterpenation processes. In this work, biorenewable deep eutectic solvents (DES) composed of glycerol (Gly) and choline chloride (ChCl) are evaluated as sustainable solvents for citrus essential oil deterpenation, using model mixtures and real citrus crude orange essential oils (COEO). The liquid−liquid extraction process for essential oil deterpenation using DES was performed at 298.15 K and 101.3 kPa, and the solvent performance was evaluated in terms of the experimental solute distribution coefficients and selectivity values, which were compared against those predicted using the conductor-like screening model for real solvents (COSMO-RS). The effect of solvent composition (i.e., hydrogen bond acceptor/donor ratio) and the addition of water (pure DES vs diluted DES) were also explored. Overall results indicate the feasibility of using DES as extraction solvents for citrus essential oil deterpenation, with pure ChCl:Gly 1:2 providing the highest extraction yield, while the addition of water decreased the distribution coefficient but increased the selectivity of the process.

1. INTRODUCTION Essential oils (EO) are complex mixtures of aromatic hydrocarbons obtained as byproducts of fruit and plant processing.1 They present inherent antioxidant, antibacterial, insecticidal, and organoleptic properties, which are broadly exploited in the pharmaceutical, cosmetic, food, and flavor industries.2 The global EO market exceeded USD 6 billion in 2015 and is expected to reach USD 14 billion by 2024, driven by increasing consumer’s preference for natural products rather than synthetic ones. Citrus EO are the most commonly used EO worldwide, with orange EO emerging as the major product (nearly 30% of total market share in 2015).3 Orange EO are formed of around 95 wt % terpenes (mainly limonene), 5 wt % terpenoids (i.e., linalool), and less than 1 wt % of nonvolatile compounds. Despite being the major compound, terpenes are prone to oxidation and contribute to off-flavors hindering the commercial applications of the essential oil.4 Therefore, deterpenation of EO (i.e., separation of terpenes from terpenoids) is crucial to improve the stability, solubility, and storage requirements of the final product for industrial value.5 Deterpenation of citrus EO has been previously performed via vacuum distillation,6 and emerging extraction methods such as supercritical fluids,5 microwave assisted extraction,7 membrane separation,8 subcritical water,9 and pervaporation;10 © XXXX American Chemical Society

however, these methods have disadvantages such as low yields, undesirable byproducts, and high heat application. Liquid− liquid extraction is preferred over these methods to maintain the organoleptic and bioactive properties of the oil as it is performed at mild conditions, presenting several advantages including lower water and energy requirements and lower operating cost.11 Since this extraction method is based on the different solubilities of the terpenes and oxygenated compounds in a given solvent,12 the choice of the latter is critical to ensure the operating performance, economic viability, and environmental sustainability of the process.13 Conventional liquid−liquid extraction processes for essential oil deterpenation rely on petroleum-based volatile organic compounds (VOCs), which are widely recognized hazardous air pollutants that cause harmful effects in human health and environment (EPA, 2016).14 In fact, such solvents are currently being regulated by different international and European Directives including REACH (Registration, Evaluation, AuthorSpecial Issue: Emerging Investigators Received: October 31, 2017 Accepted: February 16, 2018

A

DOI: 10.1021/acs.jced.7b00944 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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isation and Restriction of Chemicals).15 Moreover, recovery of these solvents after extraction increases the energy consumption and required investment.16 Consequently, replacement solvents are being proposed for developing more sustainable deterpenation processes, including glycols,17 short chain alcohols,4 and ionic liquids.18 Recently, deep eutectic solvents (DES) have emerged as a new generation of fluids with promising applications in separation processes due to their advantageous properties such as negligible vapor pressure, broad liquid range, tuneability, high thermal stability, solvation ability, and nonflammability, among others.19 DES are commonly composed of quaternary ammonium salts and hydrogen-bond donors as complexing agents, with the eutectic mixture presenting a lower freezing point than the individual components due to the formation of strong hydrogen-bonding interaction.20,21 These solvents can be synthesized from readily available nontoxic biorenewable materials by simple preparation methods such as heating, mechanical grinding, or stirring.21 Moreover, most DES do not react with water22 and can be applied economically to large-scale processes.23 Many studies report DES as nontoxic, biodegradable components,24,25 with a recent study by Hayyam et al.26 suggesting that the toxicity of DES is dependent on the structure of the individual components; therefore, it is important to consider this when choosing a suitable hydrogen-bond acceptor and donor to have negligible toxicity of the resulting solvent. To this date, DES have proved promising in several liquid−liquid extraction processes;27−29 however, deterpenation of citrus EO using DES has not been reported yet. Thus, the goal of this paper is to assess the feasibility of biobased DES in EO deterpenation as a means to develop more sustainable separation processes for isolation of high-value added compounds from natural sources. Choline chloride (ChCl), a biodegradable and biocompatible compound with very low toxicity and mass-produced as food additive,30 is chosen as hydrogen-bond acceptor (HBA). Glycerol (Gly), a benchmark building block in the global biobased industry, which is produced in large surplus as a major byproduct of the biodiesel manufacturing process31 is chosen as hydrogen-bond donor (HBD). First, validation of the experimental methodology was conducted using hydroethanolic solutions (aq. ethanol 70 wt %) as extraction solvent for deterpenation of EO model mixtures composed of limonene (terpene) and linalool (terpenoid), and compared against available literature data. Afterward, the liquid−liquid extraction of EO model mixtures and real crude orange essential oil (COEO) was performed using DES at different compositions under mild operating conditions (T = 298.15 K and P = 101.3 kPa) and evaluated in terms of experimental solute distribution coefficients and selectivity values. In particular, DES composed of ChCl:Gly at 1:2 and 1:3 molar ratios were used to evaluate the effect of the hydrogen-bond concentration on the separation process. Moreover, since previous studies have suggested the impact of water addition on conventional deterpenation processes,32,33 this was further evaluated here by using diluted DES (mixtures containing 70 wt % DES plus 30 wt % water) as extraction solvents. Lastly, the quantum chemical approach COSMO-RS (conductor-like screening model for real solvents)34 was evaluated as a tool to predict key thermodynamic parameters of the separation process.

2. EXPERIMENTAL PROCEDURE 2.1. Materials. The following materials were used for DES preparation: Glycerol (>98%, CAS no. 56-81-5) was purchased from WVR Chemicals UK and choline chloride (99%, CAS no. 67-48-1) was purchased from Sigma-Aldrich UK. Glycerol and choline chloride were vacuum-dried (Vacutherm, ThermoScientific) for 24 h at 323.15 K and 150 mbar to eliminate any remaining water. The most representative components of orange citrus EO include limonene as main terpene and linalool as main terpenoid; therefore, such components were chosen as a model mixture to mimic the composition of citrus EO. Limonene (>98%, CAS no. 5989-27-5) was purchased from Fischer Scientific and linalool (>97%, CAS no. 78-70-6) was purchased from Acros Organic. Natural cold compressed crude orange essential oil (origin California, USA) was purchased from Sigma-Aldrich UK (CAS no. 8008-57-9). The components used for the experimentation (glycerol, choline chloride, limonene, linalool, and COEO) were used without further purification. The chemical structures of the components used in this work can be seen in Figure 1. In addition, ethanol (>98%,

Figure 1. Chemical structures of the chemicals used in this work.

CAS no. 64-17-5) for experimental methodology validation was purchased from Sigma-Aldrich UK, and Milli-Q water was used for the preparation of aqueous solvents. 2.2. Preparation of DES. The preparation of DES was carried out by mixing choline chloride and glycerol at ratios of 1:2 and 1:3 following a previous method.35 The required amount of the individual components according to their molar ratio and molecular weight (Table 1) was measured using an analytical balance (New Classic MS, Mettler Toledo) with an accuracy of ±0.0001 g. The two components were heated on an IKA hot plate with stirrer (±0.05 K) at 323.15 K until a homogeneous transparent liquid was formed. Composition, abbreviations, and molecular weights of synthesized DES, calculated using an equation provided by Ghaedi et al.36 can also be found in Table 1. 2.3. Experimental Determination of Solute Distribution Coefficients and Selectivities. EO model systems were prepared by mixing known quantities of limonene (terpene) and linalool (terpenoid), and afterward extracted using DES 1 {ChCl:Gly 1:2}, DES 2 {ChCl:Gly 1:3}, DES 3 {ChCl:Gly 1:2 70 wt % + water 30 wt %}, and DES 4 {ChCl:Gly 1:3 70 wt % + water 30 wt %}, respectively. Both the extraction solvents and the model systems were placed in 15 mL polypropylene centrifuge tubes and weighed with the analytical balance. The extraction time was 60 min with 2800 rpm speed at 298.15 K using a shaking incubator (Labnet VorTemp 1550) to ensure satisfactory contact between both phases. To promote phase separation after the extraction, the tubes were centrifuged at the same temperature for 20 min (Labnet Spectrafuge 6C Compact B



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Table 1. Abbreviation, Composition, and Molecular Weights of Deep Eutectic Solvents (DES) at T = 298.2 K and P = 101.3 kPa abbreviations DES DES DES DES

1 2 3 4

hydrogen bond acceptor (HBA)

hydrogen bond donor (HBD)

molar ratio HBA/HBD

mass fraction of HBA/ HBD (%)

mass fraction of water (%)

ChCl ChCl ChCl ChCl

Gly Gly Gly Gly

1:2 1:3 1:2 1:3

100 100 70 70

0 0 30 30

molar mass (M) (g·mol−1) 107.94 103.98

Table 2. Chemical Names, CAS Numbers, Suppliers, Mass Fraction Purities and Experimental Density (ρ) and Viscosity (η) Values at T = 298.2 K and P = 101.3 kPaa material (chemical name)

CASRN

supplier

purity

molar mass (g mol −1)

ρ (g cm−3)

limonene (1-methyl-4-prop-1-en-2-ylcyclohexene) linalool ((±)-3,7-dimethyl-1,6-octadien-3-ol) crude orange essential oil (COEO) ethanol glycerol

5989-27-5

Fischer

0.98

136.23

0.84073 ± 0.00005

78-70-6 8008-57-9 64-17-5 56-81-5

0. 97

154.25

0.98 0.98

46.07 92.09

0.85625 0.83960 0.78610 1.25770

choline chloride

67-48-1

Acros Organics Sigma-Aldrich UK Sigma-Aldrich UK WVR Chemicals UK Sigma-Aldrich UK

0.99

139.62

a

Standard uncertainties u are u(T) = 0.05 K, u(P) = 0.5 kPa, u(ρ) = 0.003 g cm

−3

± ± ± ±

0.00002 0.00002 0.00002 0.00009

η (mPa·s) 0.928 ± 0.005 4.469 ± 0.001 0.980 ± 0.002 1.040 ± 0.003 873.600 ± 0.005

and ur(η) = 0.06.

of HBA/HBD has been fixed for each DES used as the extraction solvent; however, HBA/HBD molar ratio of DES may not remain constant in each phase after the extraction. To validate the experimental results, the deviation was calculated following the equation proposed by Marcilla et al.:38

Research Centrifuge) with 5000 rpm speed. Then the samples were left standing overnight in a Labnet AccuBlock Digital Dry Bath under the same conditions to allow complete thermodynamic equilibrium between the phases. These operating conditions were selected to provide sufficient contact time between the two phases and complete separation after the extraction.2,32 Afterward, deterpenation of real COEO was performed with all solvents (DES 1−4) at oil to solvent ratios of 1:2, 1:1, and 2:1, 3:1, 4:1, 5:1 following a similar operating procedure. The samples were withdrawn from the top and bottom layers after extraction using a syringe to quantify limonene (terpene) content and linalool (terpenoid) content in both terpene-rich and solvent-rich phase for further analysis via gas chromatography with mass spectrometry (GC−MS). GC−MS analysis was conducted using Agilent Technologies 5973 equipped with ZB-1 (30m x 0.32 mm x 1 μm) (Restek Chromatography, UK) using a method proposed by Tang et al.37 with slight modifications. Briefly, pure hydrogen (99.99%) was used as a carrier gas at a flow rate of 1 mL min−1, setting the temperature program at 313.15 K (holding for 3 min) to 393.15 K at a rate of 278.15 K min−1, then increasing the temperature to 493.15 K (holding for 5 min) at a rate of 288.15 K min−1. Detection temperature was kept at 573.15 K, and the injection temperature was 533.15 K with a split ratio of 50:1. The determination of the mass fractions (w) of limonene and linalool (for model mixture) or terpenes and terpenoids (for real COEO) in both phases after the extraction was performed by GC-MS, constructing external calibration method using tetradecane samples diluted in ethanol. The water content in the phases, when extraction was completed with diluted DES, was determined by Karl Fischer titration method (Metrohm 899 Coulometer). As shown in Table 1, all DES were assumed to be single solvent systems for quantification purposes; after limonene/terpene and linalool/terpenoid mass fractions were determined from GC−MS analysis, the mass fraction of the extraction solvent present in each phase was calculated through the mass balance. Therefore, individual quantification of choline chloride and glycerol after the extraction has not been performed in this work; this means that the initial molar ratio

δ=

|(M TP + MSP) − MOC| 100 MOC

(1)

where MTP and MSP represent the mass of the components in the terpene phase and the solvent phase, respectively, and MOC represents the mass fraction of the overall phase. Afterward, distribution coefficient (β) and selectivity values (S) are calculated using eqs 2 and 3, respectively, to evaluate the solvent performance in the extraction process as suggested in previous works.2,4,17,18 βlin =

S=

DES phase wlin terp phase wlin

βlim =

DES phase DES phase wlin wlim terp phase wlin phase wDES lin

terp phase wlim

DES phase wlim terp phase wlim

=

(2)

βlin βlim

(3)

phase wterp lin

where and denote the mass fraction of linalool in DES phase and terpene phase, respectively, while phase phase wDES and wterp refer to the mass fraction of limonene in lim lim the DES phase and terpene phase, correspondingly. For the physical characterization of the oil, an Anton Paar DMA-4500 oscillating U-tube densitometer with an accuracy of ±0.00001 g cm−3 and an Anton Paar automated microviscometer (AMVn) with the falling ball procedure with repeatability lower than 0.5% were used to measure the density and viscosity of limonene, linalool, crude orange essential oil, ethanol, and glycerol, as reported in Table 2.

3. COSMO-RS METHODOLOGY The conductor-like screening model for real solvents (COSMO-RS) is a unimolecular quantum chemical-based method developed by Klamt and co-workers which predicts thermodynamic properties and phase equilibrium of pure fluids and its mixtures.39,40 COSMO-RS has been previously used to C



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predict the thermodynamic behavior of systems containing neoteric solvents, including ionic liquids41−43 and mixtures composed of DES and hydrocarbons.21,44 In this work, the performance of COSMO-RS as a tool to predict the solvent performance in terms of solute distribution coefficient and selectivity values for deterpenation of citrus EO using DES as extraction solvents has been evaluated. COSMO-RS calculations were performed using COSMOtherm software, version C30, release 1601 at the BVP86/ TZVP/DGA1 quantum chemical level and the corresponding parametrization (BP_TZVP_C30_1601) following the standard procedure described in previous works.45 For all the calculations, DES were treated as a mixture of HBA and HBD of corresponding stoichiometric ratios following the electroneutral approach suggested in previous refs.44 This means that the molar ratio of HBA/HBD in DES has been set as a constant throughout the solvent extraction calculations in COSMO-RS, which may not be necessarily the case for the real systems.

Figure 3. Comparison of the selectivity to linalool of aq.ethanol (70 wt %) between this work (solid blue squares) and the literature data reported by Gonçalves et al.4 (open squares) in the ternary system {limonene + linalool+ aq. ethanol (70 wt %)} at T = 298.15 K and P = 101.3 kPa.

4. RESULTS AND DISCUSSION 4.1. Validation of Experimental Methodology. To validate the experimental methodology and analytical procedure described in section 2.3, the experimental solute distribution coefficient and selectivity values for the benchmark system composed of model citrus EO and aq. ethanol (70 wt %) at 298.15 K and 101.3 kPa were determined, and the obtained results were evaluated against the study reported by Gonçalves et al.4 Known quantities of limonene and linalool were used to construct the model mixture and extracted with the mentioned hydroalcoholic solution using a solvent-to-feed mass ratio of 1:1. The experimental mass fractions of limonene and linalool (w) as well as the resulting solute distribution coefficients (β) and selectivity values (S) can be found in Table S1. In addition, distribution coefficients of limonene and linalool and selectivity values are plotted in Figures 2 and 3 with available literature data reported by Gonçalves et al.4 It can be seen that generally β and S are in good agreement with the reference literature values and that they follow the same trend, by which both decrease as the linalool mass fraction increases in the solventrich phase. Small differences may arise from using different analytical methods (i.e., GC−MS was used in this work,

whereas GC with flame ionization detector was used by Gonçalves et al.4). The small mass balance deviations (≤0.14%) support the consistency of the experimental values. 4.2. Deterpenation of Essential Oil Model Mixtures Using DES. 4.2.1. Liquid−Liquid Extraction of Linalool from Essential Oil Model Mixtures Using DES. The liquid−liquid extraction of linalool from the EO model mixtures using DES as extraction solvents was performed at 298.15 K and 101.3 kPa. The experimental mass fractions of limonene and linalool in each phase after the extraction are shown in Tables 3−6, and the consistency of the results is supported by the small mass balance deviation (≤0.1%). Table 3. Experimental Solute Distribution Coefficient (β) and Selectivity (S) for the System {Limonene(1) + Linalool(2) + DES 1(3)} at T = 298.15 K and P = 101.3 kPaa terpene-rich phase

DES rich phase

selection parameter

w1

w2

w1

w2

δ (%)

β1

β2

S

0.907 0.873 0.808 0.718 0.679 0.581 0.557 0.507

0.050 0.117 0.173 0.258 0.295 0.413 0.437 0.486

0.007 0.005 0.005 0.004 0.004 0.004 0.004 0.003

0.007 0.007 0.008 0.008 0.008 0.010 0.011 0.011

0.020 0.040 0.050 0.050 0.030 0.030 0.050 0.050

0.008 0.006 0.006 0.006 0.006 0.006 0.006 0.007

0.147 0.062 0.046 0.031 0.028 0.024 0.024 0.022

18.993 10.964 7.530 5.373 4.834 3.983 3.892 3.445

a

Standard uncertainties u are u(T) = 0.05 K, u(P) = 0.5 kPa, and u(w) = 0.005.

Overall, all DES appear suitable solvents to separate linalool from limonene by liquid extraction, promoting higher affinity toward the target polar solute which can be ascribed to preferential interactions through hydrogen bonding. Additionally, results have shown that the increase of linalool in the initial composition resulted in lower migration from the terpene phase to solvent phase due to the increase in phase solubility as observed in previous studies.11,46−48 The high viscosity of glycerol (1410 mPa·s at 293.15 K)49 could be a drawback for glycerol-based DES since it may imply

Figure 2. Comparison of the distribution coefficient of linalool between the aq. ethanol (70 wt %)-rich and the limonene-rich phases obtained in this work (solid blue squares) and the literature data reported by Gonçalves et al.4 (open squares) for the system {limonene + linalool+ aq. ethanol (70 wt %)} at T = 298.15 K and P = 101.3 kPa. D



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terpenic hydrocarbons (limonene) and oxygenated terpenoids (linalool) as the mutual solubility decreases among the components in the systems. Accordingly, to evaluate the effect of water on the deterpenation performance when using DES as extractants, the solvents were mixed with water to obtain diluted DES with a composition of 30 wt % in water. The experimental mass fractions of limonene and linalool in each phase after deterpenation when using the diluted DES (DES 3 and DES 4) are collected in Tables 5 and 6, with results showing that the addition of water limits the extraction of the target compound due to the reduced mutual solubility in agreement with previous studies.51 Moreover, the addition of water may also influence the interaction between DES and linalool as choline chloride (HBA) and glycerol (HBD) are able to interact through hydrogen bonding with water due to the high polarity of this molecule. Furthermore, the addition of water breaks apart DES;29,52 therefore, individual components glycerol, choline chloride, and water dissociate and try to interact with linalool forming complex interactions and weakening the clusters forming strong hydrogen-bond networks in DES. Glycerol (HBD) and water are likely to have strong hydrogen-bonding interactions among themselves that is hard to overcome which could hinder interactions between diluted DES and linalool, reducing the miscibility region. 4.2.2. Calculation of Distribution Coefficients and Selectivities. The distribution coefficients and selectivity values of the solutes when using DES 1−4 as deterpenation solvents are presented in Tables 3−6 and Figures 4 and 5. When the

Table 4. Experimental Solute Distribution Coefficient (β) and Selectivity (S) for the System {Limonene(1) + Linalool(2) + DES 2(3)} at T = 298.15 K and P = 101.3 kPaa terpene-rich phase

DES-rich phase

selection parameter

w1

w2

w1

w2

δ (%)

β1

β2

S

0.920 0.875 0.791 0.756 0.635 0.632 0.593 0.529

0.075 0.108 0.184 0.212 0.304 0.337 0.380 0.417

0.008 0.008 0.006 0.006 0.006 0.005 0.005 0.005

0.011 0.011 0.014 0.014 0.015 0.015 0.015 0.016

0.030 0.020 0.010 0.040 0.070 0.020 0.060 0.040

0.009 0.009 0.008 0.008 0.009 0.009 0.008 0.009

0.145 0.102 0.074 0.066 0.048 0.044 0.041 0.038

16.219 11.194 9.348 8.140 5.580 5.176 4.902 4.297

a Standard uncertainties u are u(T) = 0.05 K, u(P) = 0.5 kPa, and u(w) = 0.005.

Table 5. Experimental Solute Distribution Coefficient (Β) And Selectivity (S) For the System {Limonene(1) + Linalool(2) + DES 3(3)} at T = 298.15 K and P = 101.3 kPaa terpene-rich phase

DES-rich phase

selection parameter

w1

w2

w1

w2

δ (%)

β1

β2

S

0.937 0.851 0.792 0.735 0.661 0.627 0.576

0.023 0.092 0.144 0.219 0.271 0.321 0.410

0.004 0.005 0.005 0.004 0.003 0.003 0.003

0.002 0.004 0.005 0.005 0.006 0.006 0.007

0.090 0.060 0.102 0.086 0.087 0.081 0.079

0.005 0.006 0.006 0.005 0.005 0.005 0.005

0.095 0.043 0.033 0.023 0.021 0.019 0.016

20.753 7.097 5.495 4.425 4.564 4.171 3.488


Table 6. Experimental, Solute Distribution Coefficient (β) and Selectivity (S) for the System {Limonene(1) + Linalool(2) + DES 4(3)} at T = 298.15 K and P = 101.3 kPaa terpene-rich phase

DES-rich phase

selection parameter

w1

w2

w1

w2

δ (%)

β1

β2

S

0.924 0.865 0.800 0.741 0.720 0.689

0.037 0.096 0.157 0.191 0.200 0.245

0.004 0.007 0.011 0.016 0.016 0.017

0.003 0.009 0.011 0.017 0.017 0.019

0.005 0.008 0.014 0.021 0.022 0.025

0.005 0.008 0.014 0.021 0.022 0.025

0.092 0.089 0.072 0.087 0.085 0.078

19.297 10.941 5.140 4.062 3.793 3.125

Figure 4. Experimental (solid shapes) and prediction by COSMO-RS (open shapes) of the solute distribution coefficient values for the systems {limonene + linalool+ DES} using DES 1−4 at T = 298.15 K and P = 101.3 kPa.

extraction performance of the solvents in thermodynamic aspects is compared, high β and S values are targeted. The β values indicate the migration of solute from the terpene-rich phase to the DES-rich phase, giving an idea of the amount of solvent required to carry out the extraction process (i.e., the lower the solute distribution the higher the amount of solvent required). The S values indicate the affinity of the solvent to the solute; hence, higher selectivity implies an enhanced capability of the solvent to separate target compounds. As seen in Tables 3−6, linalool β values are always higher than those of limonene denoting the preferential migration of linalool from the terpene phase to the solvent phase, which is promoted by the hydrogen bonding interactions between the hydroxyl groups of such solute and the glycerol-based extraction solvent. Furthermore,

a


higher mixing costs and increasing mass transfer limitations in the system, affecting the migration of terpenoids from the terpene phase to the solvent phase. The addition of ChCl to glycerol at 1:2 ratio decreases the viscosity to around 473 mPa·s at 293.15 K50 minimizing the issues occurring due to high viscosity. While the addition of water can decrease the viscosity of the extraction solvents further, as the viscosity of diluted DES 3 is around 14 mPa·s,50 previous studies4,47 have reported that the presence of water reduces the solvent’s capability to extract E



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their capability to separate linalool from citrus EO. The trend of selectivity values is as follows: DES 3 (S = 20.80) > DES 4 (S = 19.30) > DES 1 (S = 18.99) > DES 2 (S = 16.22). These selectivity values are higher than those obtained with conventional solvents such as aq. ethanol (S = 13)46 and comparable to those obtained with ionic liquids such as 1-ethyl3-methylimidazolium ethylsulfate (S = 24.80).55 Higher selectivity values obtained by DES 3 and 4 are due to the increased polarity of the solvents with the addition of water, which increases solvent affinity toward polar components. The selectivity values decrease with increasing concentration of linalool in the terpene phase with all four solvents, in agreement with results obtained by previous researchers using a variety of solvents for deterpenation of EO.18,55 The distribution coefficients and selectivity values computed with COSMO-RS can be found in Tables S2 to S5 in the Supporting Information, and are shown in Figures 4 and 5 along with the values obtained from experimental data. Overall, COSMO-RS predictions slightly underestimate β and S values for all solvents proposed but reproduce the experimental trend supporting the conclusion that higher β but lower S values were obtained with pure DES in comparison to aq. DES solutions, therefore allowing a prediction of the effect of addition of water in the extraction process. Also, note that the mole ratios of HBA to HBD in all DES have been set to constant for the purpose of calculating the solute distribution coefficients and selectivities through the COSMO-RS approach; however, this may not be necessary in the case for real systems, for which the composition ratio of individual DES components (i.e., ChCl and Gly) may not remain constant in each phase after the extraction. The extraction efficiency parameters (β and S) proved that DES are promising solvents to fractionate citrus EO for practical applications. The selected HBD and HBA could be used in cosmetics and food formulations for which solvent recovery is not required, which is the major benefit of the chosen DES. DES 1 seems more suitable for deterpenation in order to avoid further solvent cost (more material required for DES 2 than DES 1 in relationship with the amount of linalool to be extracted) and fluctuations arising by the viscosity of DES with increasing glycerol concentration. The addition of water reduces the distribution coefficient of linalool but increases the solute selectivity values; therefore, although more solvent could be required when performing the separation with aqueous mixtures, DES 3 and 4 could be more selective toward fractionating terpenoids with similar structures. Therefore, a comprehensive consideration for suitable solvent selection will

Figure 5. Experimental (solid shapes) and prediction by COSMO-RS (open shapes) of the selectivity of DES 1−4 to linalool in the systems {limonene + linalool+ DES} at T = 298.15 K and P = 101.3 kPa.

the higher the difference between the distribution coefficient of limonene (βlim) and linalool (βlin,), the more linalool is migrating from the terpene phase to the solvent phase. The results show that higher differences observed for DES 1 and 2 in comparison to DES 3 and 4. In particular, the distribution coefficients of linalool (βlin), obtained when using pure solvents for deterpenation (DES 1 and DES 2) are in the range of 0.02−0.15, whereas the addition of water (DES 3 and DES 4) reduces this range to 0.01−0.10 as the migration of linalool from the terpene-rich phase to DES rich-phase decreases with the decreased miscibility region. This could be explained by the chemical affinity between water and linalool, which decreases the extraction capacity of the solvent due to lower solubility as previously reported by Chiyoda et al.47 In comparison to other novel solvents proposed for deterpenation of citrus EO, the distribution coefficients of linalool obtained with DES are in the same order of magnitude than those reported for polyols such as ethylene glycol (βlin < 0.2)11 and higher than those reported for ionic liquids such as 1-ethyl-3-methylimidazolium methanesulfonate (βlin = 0.03).53 Moreover, it is worth noting that glycerol is an approved solvent by the U.S. Food and Drug Administration (FDA) as “generally recognized as safe” (GRAS)54 while choline chloride is a nontoxic salt widely used as feed additive;30 therefore, the linalool extracts obtained using DES as deterpenation solvents could be applied in food and fine chemical formulations without the need to perform additional steps for solvent recovery, which can be a significant advantage over other extraction solvents. The selectivity values obtained when using the proposed DES as deterpenation solvents are higher than 1, confirming

Table 7. Chemical Composition Characterization of Crude Orange Essential Oil (COEO) component group terpenes α pinene β phellandrene myrcene limonene terpenoids linalool R-citronellal decanal dodecanal

retention time (min)

identification

molar mass (g mol−1)

4.338 5.059 5.334 6.378

NIST-MS NIST-MS NIST-MS NIST-MS

136.24 136.24 136.24 136.23

7.913 8.859 10.717 15.745

NIST-MS NIST-MS NIST-MS NIST-MS

154.24 156.27 156.2 184.32 F

composition in mass fraction (Wc)

composition in molar fraction (Xd)

0.930 0.028 0.083 0.005 0.814 0.070 0.026 0.009 0.027 0.008

0.939 0.029 0.084 0.005 0.821 0.061 0.024 0.008 0.023 0.006 DOI: 10.1021/acs.jced.7b00944 J. Chem. Eng. Data XXXX, XXX, XXX−XXX


Article

be required according to the final target application of the purified EO and related techno-economic implications. 4.3. Deterpenation of Crude Orange Essential Oils (COEO) Using DES. 4.3.1. Characterization of COEO. Characterization of real crude orange essential oil (COEO) was performed by the identification and quantification of the chemical compositions using a GC−MS analytical method following the procedure described in section 2.3. The retention times of the main compounds were compared with the retention times of the standards injected under the same conditions. The initial composition can be found in Table 7, where eight compounds were identified with limonene being the major component in terpenes and linalool the major component in terpenoids. COEO density and viscosity values can be found in Table 2 alongside with density and viscosity values of limonene and linalool. The density and viscosity of COEO are very close to those of limonene, as 81% of the total mass fraction of COEO is represented by such a compound. As seen by the chemical compositions, orange EO can be divided into two fractions: terpene fraction with limonene as the main component and oxygenated terpenoid fraction with linalool as the major component. Therefore, in this work, COEO was treated as containing two pseudocomponents for the purpose of studying the phase equilibrium after extraction, as suggested in previous reports by Gonçalves et al.4 and Koshima et al.48 4.3.2. Liquid−Liquid Extraction of Linalool from COEO Using DES. The experimental mass fractions of terpenes (1) and terpenoids (2) after COEO deterpenation using DES 1−4 (3) at 298.15 K and atmospheric pressure are shown in Tables 8−11. The small mass balance deviations (δ) (≤0.06%)

Table 9. Experimental Solute Distribution Coefficient (β) and Selectivity (S) for the System {Terpenes(1) + Terpenoids(2) + DES 2(3)} at T = 298.15 K and P = 101.3 kPaa terpene-rich phase

DES-rich phase

w2

w1

w2

δ (%)

β1

β2

S

0.707 0.712 0.673 0.669 0.621 0.629 0.608 0.601 0.601 0.592 0.593 0.592

0.013 0.013 0.014 0.013 0.014 0.015 0.015 0.016 0.017 0.017 0.017 0.016

0.179 0.180 0.199 0.200 0.202 0.202 0.214 0.222 0.215 0.214 0.216 0.221

0.062 0.061 0.063 0.063 0.064 0.065 0.065 0.064 0.065 0.066 0.066 0.066

0.020 0.031 0.028 0.014 0.031 0.025 0.037 0.020 0.039 0.033 0.039 0.016

0.253 0.253 0.295 0.299 0.325 0.321 0.352 0.369 0.358 0.361 0.365 0.373

4.639 4.712 4.696 4.760 4.458 4.330 4.285 3.915 3.868 3.830 3.994 4.068

18.351 18.661 15.900 15.901 13.704 13.493 12.183 10.616 10.792 10.607 10.952 10.904

w1

w2

δ (%)

β1

β2

S

0.764 0.779 0.702 0.696 0.682 0.697 0.669 0.671 0.669 0.661 0.651 0.648

0.018 0.017 0.019 0.019 0.019 0.019 0.019 0.019 0.020 0.019 0.020 0.019

0.173 0.184 0.196 0.198 0.194 0.239 0.235 0.257 0.255 0.256 0.296 0.300

0.061 0.060 0.064 0.063 0.064 0.063 0.065 0.064 0.066 0.064 0.067 0.067

0.021 0.020 0.025 0.028 0.029 0.023 0.044 0.025 0.035 0.020 0.041 0.029

0.227 0.237 0.279 0.284 0.284 0.343 0.351 0.383 0.382 0.386 0.455 0.463

3.463 3.494 3.435 3.324 3.335 3.380 3.391 3.432 3.354 3.428 3.347 3.448

15.279 14.752 12.305 11.709 11.724 9.863 9.673 8.963 8.785 8.873 7.350 7.451


selection parameter

w1

selection parameter

w2



DES-rich phase

w1

DES rich phase

selection parameter

w1

w2

w1

w2

δ (%)

β1

β2

S

0.870 0.852 0.846 0.832 0.791 0.788 0.790 0.781 0.780 0.776 0.757 0.762

0.031 0.030 0.033 0.032 0.035 0.035 0.037 0.037 0.039 0.037 0.042 0.039

0.071 0.069 0.076 0.076 0.081 0.083 0.102 0.111 0.134 0.135 0.140 0.135

0.048 0.049 0.042 0.043 0.041 0.041 0.040 0.042 0.039 0.042 0.036 0.040

0.045 0.026 0.047 0.022 0.040 0.037 0.038 0.023 0.037 0.020 0.032 0.046

0.082 0.081 0.089 0.091 0.103 0.105 0.129 0.142 0.171 0.174 0.185 0.177

1.545 1.628 1.291 1.352 1.178 1.171 1.078 1.145 0.994 1.112 0.839 1.020

18.898 20.043 14.448 14.791 11.483 11.114 8.325 8.046 5.804 6.380 4.547 5.756

a


of DES 1 and DES 2 suggests that the former promotes higher terpenoid content in the DES-rich phase at similar compositions. Moreover, the addition of water and diluting DES (DES 3 and DES 4) alters the solvent polarity decreasing the solubility of the EO, reducing the terpenoid concentration in the solvent phase. 4.3.3. Calculation of Distribution Coefficients and Selectivities. The performance of DES solvents for deterpenation of real COEO is further evaluated by the calculation of distribution coefficients (β) and selectivity (S) values of the terpenoid solutes. As previously mentioned, β values refer to migration of the extracted solutes from the oil phase to the solvent phase while S values indicate affinity of the solvent toward the solute. Feasible deterpenation processes can be further determined by differences in the β values for terpenes and terpenoids. The β and S values in mass fraction obtained for COEO deterpenation using DES 1−4 as extraction solvents


confirm the consistency of the measured data considering COEO initial composition as well as terpene and terpenoid mass fractions after extraction. Experimental mass fraction results show that increasing oil content in the initial composition increases the terpenoid concentration in the DES-rich phase after extraction, indicating migration of terpenoids from the terpene to the DES-rich phase. Additionally, comparison of deterpenation performance G



Article

Distribution coefficient (β) values for terpenoids were higher than those for terpenes with all the solvents, indicating a preferential migration of oxygenated terpenoids toward the DES phase. In addition, higher differences between βterpenoids and βterpenes, are found for DES 1 and 2 in comparison to DES 3 and 4, indicating the higher solvent capacity of pure DES. In particular, for DES 1 and 2, the range for βterpenoid values was 3.33−4.76, whereas this reduces to 1.02−1.64 with DES 3 and DES 4. The addition of water as mentioned previously reduces the solubility of the solutes in the solvents and decreases the miscibility region, resulting in lower migration of the components. Furthermore, the addition of water may favor hydrogen-bonding interactions between DES and water while limiting those with oxygenated terpenoids, hence reducing the extraction ability of the solvent. Increasing oil to solvent ratio slightly decreases the distribution coefficient and significantly reduces the selectivity values. Overall, the experimental values obtained in this work for distribution coefficients of terpenoids are significantly higher than those reported in the literature for benchmark ethanolic solutions,4 which were in the range of 0.5−0.7; this supports the feasibility of DES to be used as sustainable extraction solvents for effective purification of real EO. Selectivity (S) of terpenoids is an important parameter to consider for real EO deterpenation, as variations in the compositions of COEO with respect to the model mixture can alter the extraction ability of the solvents. Overall, the selectivity values of terpenoids yielded by the proposed DES were over 1, which indicates the capability of the solvents to extract target solutes. The obtained selectivity values using DES 1 and 2 were in the range of 7.35−18.66, whereas those provided by DES 3 and DES 4 were ranging from 4.55 to 27.85. Therefore, according to the pattern observed for the case of EO model mixtures, the addition of water increases the selectivity of DES toward oxygenated compounds for the deterpenation of real COEO, as increasing polarity of solvent can promote affinity toward polar terpenoids. The experimental selectivity values obtained in this work for terpenoids after COEO deterpenation with DES are comparable or even higher than those obtained in the literature for ethanolic solutions4 which were in the range of 15−21, thus confirming the suitability of using DES as extraction solvents for separating target terpenoid solutes from real COEO.


DES rich phase

selection parameter

w1

w2

w1

w2

δ (%)

β1

β2

S

0.852 0.848 0.816 0.809 0.799 0.790 0.773 0.773 0.747 0.732 0.732 0.716

0.033 0.032 0.036 0.036 0.038 0.038 0.042 0.044 0.045 0.050 0.048 0.052

0.051 0.050 0.053 0.053 0.060 0.063 0.063 0.064 0.068 0.070 0.069 0.071

0.052 0.052 0.055 0.055 0.056 0.057 0.059 0.060 0.060 0.062 0.060 0.061

0.062 0.042 0.060 0.026 0.048 0.032 0.041 0.021 0.042 0.042 0.036 0.037

0.060 0.059 0.065 0.065 0.075 0.080 0.082 0.083 0.091 0.096 0.095 0.100

1.593 1.636 1.538 1.522 1.483 1.497 1.397 1.369 1.348 1.239 1.266 1.173

26.556 27.848 23.845 23.293 19.789 18.653 17.116 16.435 14.863 12.935 13.377 11.757


are shown in Tables 8−11 and represented graphically in Figures 6 and 7.

Figure 6. Experimental distribution coefficients of terpenoids between the terpene-rich and DES-rich phases with changing oil to solvent mass ratios using DES 1−4 at T = 298.15 K and P = 101.3 KPa.

5. CONCLUSIONS The feasibility of essential oils (EO) deterpenation via liquid− liquid extraction using renewable glycerol-based deep eutectic solvents (DES) has been proven in this work. The solute distribution coefficients and selectivity values of model mixtures and real crude orange citrus essential oils (COEO) were experimentally determined using glycerol-based DES, namely, DES 1 {ChCl:Gly 1:2}, DES 2 {ChCl:Gly 1:3}, DES 3 {ChCl:Gly 1:2 70 wt % + water 30 wt %}, and DES 4 {ChCl:Gly 1:3 70 wt % + water 30 wt %} at 298.15 K and atmospheric pressure. Experimental results indicate that pure ChCl:Gly 1:2 provides the highest extraction capacity; whereas the dilution of DES results in lower distribution coefficients but higher selectivity values due to the increased polarity of the solvent, promoting the affinity toward the target linalool solute. Additionally, thermodynamic parameters estimated using COSMO-RS were in good agreement with the experimental trends in terms of experimental distribution coefficient and

Figure 7. Experimental selectivities of DES 1−4 with respect to terpenoids as a function of the oil to solvent mass ratios at T = 298.15 K and P = 101.3 KPa.

H



Article

(11) Sevgili, L. M.; Şahin, S.; Kirbaşlar, Ş. I. Liquid-liquid equilibria of (limonene + linalool + ethylene glycol or diethylene glycol or triethylene glycol or 1,2-propylene glycol) ternary systems. J. Chem. Eng. Data 2008, 53, 737−741. (12) American Institute of Chemical Engineers (AIChE) Design Principles for Liquid-Liquid Extraction, 2015. https://www.aiche.org. uk/resources/publications/cep/2015/november/design-principlesliquid-liquid-extraction (accessed Aug 8, 2017). (13) Soh, L.; Eckelman, M. J. Green Solvents in Biomass Processing. ACS Sustainable Chem. Eng. 2016, 4, 5821−5837. (14) United States Environmental Protection Agency (EPA). Health Effects Notebook for Hazardous Air Pollutants, 2017. https://www. epa.gov/haps/health-effects-notebook-hazardous-air-pollutants.(accessed Oct 19,2017). (15) Yara-Varon, E.; Balcells, M.; Canela-Garayoa, R.; Bily, A.; Chemat, F.; Fabiano-Tixier, A. S. Is it possible to substitute hexane with green solvents for extraction of carotenoids? RSC Adv. 2016, 6, 27750−27759. (16) Meindersma, G. W.; Podt, A. J. G.; de Haan, A. B. Selection of ionic liquids for the extraction of aromatic hydrocarbons from aromatic/aliphatic mixtures. Fuel Process. Technol. 2005, 87, 59−70. (17) Romero, M. D.; Díez, E.; Díaz, I. Feasibility of 1,3-butanediol as solvent for limonene and linalool separation. Chem. Eng. Process. 2010, 49, 1183−1187. (18) Matsuda, H.; Norizuki, Y.; Kawai, M.; Kurihara, K.; Tochigi, K.; Ochi, K. Liquid−Liquid Equilibria for Extraction of Citrus Essential Oil Using Ionic Liquids. J. Solution Chem. 2014, 43, 1561−1573. (19) Zhang, Q.; De Oliveira Vigier, K.; Royer, S.; Jérôme, F. Deep eutectic solvents: syntheses, properties and applications. Chem. Soc. Rev. 2012, 41, 7108−7146. (20) López-Porfiri, P.; Brennecke, J. F.; Gonzalez-Miquel, M. Excess Molar Enthalpies of Deep Eutectic Solvents (DESs) Composed of Quaternary Ammonium Salts and Glycerol or Ethylene Glycol. J. Chem. Eng. Data 2016, 61, 4245−4251. (21) Gouveia, A. S. L.; Oliveira, F. S.; Kurnia, K. A.; Marrucho, I. M. Deep Eutectic Solvents as Azeotrope Breakers: Liquid-Liquid Extraction and COSMO-RS Prediction. ACS Sustainable Chem. Eng. 2016, 4, 5640−5650. (22) Chen, Z.; Zhu, W.; Zheng, Z.; Zou, X. One-pot extraction combined with metal-free photochemical aerobic oxidative desulfurization in deep eutectic solvent. J. Fluorine Chem. 2010, 131, 340−344. (23) Abbott, A. P.; Capper, G.; McKenzie, K. J.; Ryder, K. S. Electrodeposition of zinc-tin alloys from deep eutectic solvents based on choline chloride. J. Electroanal. Chem. 2007, 599, 288−294. (24) Hayyan, A.; Mjalli, F. S.; Alnashef, I. M.; Al-Wahaibi, T.; AlWahaibi, Y. M.; Hashim, M. A. Fruit sugar-based deep eutectic solvents and their physical properties. Thermochim. Acta 2012, 541, 70−75. (25) Abbott, A. P.; Boothby, D.; Capper, G.; Davies, D. L.; Rasheed, R. Deep Eutectic Solvents Formed Between Choline Chloride and Carboxylic Acids. J. Am. Chem. Soc. 2004, 126, 9142. (26) Hayyan, M.; Hashim, M. A.; Hayyan, A.; Al-saadi, M. A.; Alnashef, I. M.; Mirghani, M. E. S.; Saheed, O. K. Are deep eutectic solvents benign or toxic? Chemosphere 2013, 90, 2193−2195. (27) Gonzalez, A. S. B.; Francisco, M.; Jimeno, G.; De Dios, S. L. G.; Kroon, M. C. Liquid-liquid equilibrium data for the systems {LTTM +benzene+hexane} and {LTTM+ethyl acetate+hexane} at different temperatures and atmospheric pressure. Fluid Phase Equilib. 2013, 360, 54−62. (28) Naik, P. K.; Dehury, P.; Paul, S.; Banerjee, T. Evaluation of Deep Eutectic Solvent for the selective extraction of toluene and quinoline at T = 308.15 K and p = 1 bar. Fluid Phase Equilib. 2016, 423, 146−155. (29) Florindo, C.; Branco, L. C.; Marrucho, I. M. Development of hydrophobic deep eutectic solvents for extraction of pesticides from aqueous environments. Fluid Phase Equilib. 2017, 448, 135−142. (30) European Food Safety Authority (EFSA). Scientific Opinion on safety and efficacy of choline chloride as a feed additive for all animal

selectivity values related to the deterpenation process using DES.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.7b00944. Experimental solute distribution coefficients and selectivity values for the system {limonene + linalool + aq. ethanol (70 wt %)} at T = 298.15 K and P = 101.3 kPa; COSMO-RS predicted solute distribution coefficients and selectivity values for the systems {limonene + linalool + DES} at T = 298.15 K; GC−MS spectrum for crude orange essential oil (COEO) characterization (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jesus Esteban: 0000-0003-3729-5378 Maria Gonzalez-Miquel: 0000-0003-3978-8299 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS Baranse Ozturk would like to thank EPSRC (Engineering and Physical Sciences Research Council) for the Ph.D. scholarship provided. In addition, authors would like to thank the company Treatt (UK) for valuable discussions and industrial input.

■

REFERENCES

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Deterpenation of Citrus Essential Oils Using Glycerol-Based Deep

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