Application of acidic deep eutectic solvents in green extraction of 5-hydroxymethylfurfural

The extraction of the 5-hydroxymethylfurfural (5-HMF), as building block for many applications, from aqueous solutions has been became an indispensable challenge. Consequently, this study investigated the extraction ability of acidic deep eutectic solvent-based aqueous two-phase system (ATPS), choline chloride as hydrogen bond acceptor (HBA) and lactic acid, oxalic acid and citric acid as hydrogen bond donor (HBD), countering polypropylene glycol 400 at T = 298.15 K. Two semi-empirical Zafarani-Moattar et al. and Merchuk equations were used to fit the measured binodal data. Further, the NRTL and UNIQUAC models were used for correlating of tie-line data. The consistency of the experimental tie-line data was determined by utilizing the Bachman-Brown and Hand correlations. Also, the performance of these ATPSs to partitioning of 5-HMF, were investigated by calculation of extraction efficiencies, EE% and partition coefficients, K. This strategy indicates DES-based ATPSs have the acceptable extraction efficiency of the 5-HMF in a single-step extraction. A greenness assessment tool, Analytical GREEnness metric (AGREE), was tested to evaluate of greenness of analytical protocol.


Scientific Reports
| (2022) 12:13113 | https://doi.org/10.1038/s41598-022-16823-x www.nature.com/scientificreports/ thermodynamics of 5-HMF extraction. However, the use of salt in the extraction process in aqueous phase can enhance the partitioning of 5-HMF into organic phase. Different salts made up of various cations and anions were investigated, such as; KCl, NaCl, K 2 SO 4 and Na 2 SO 4 , and different solvents like alcohol, ketone have been utilized on the partitioning of 5-HMF in two phase systems 21 . To prediction the LLE systems the COSMO-SAC model, and to correlation of the experimental LLE systems including 5-HMF, the NRTL and UNIQUAC models have been used 22 . Despite their attempts toward the extraction of 5-HMF, it is more urgent to find an efficient and green extraction method. Ionic liquid (IL) is a salt in the liquid state in which the ions are poorly coordinated and results in these solvents being liquid below 100 °C. Most recently, ILs based ATPSs are being developed [23][24][25] . Nevertheless, compared with organic solvents, they are more costly and difficult to synthesis, and their eco-friendliness has been challenged.
Deep eutectic solvents (DESs), as a new generation of ILs, have the shared physico-chemical properties with ILs, but their preparation is much cheaper, lower cost and simpler which formed by mixing ammonium salt as hydrogen bond acceptor (HBA), and aprotic organic compound as hydrogen bond donor (HBD) 26 . DESs were widely used in the extraction field 27,28 . The HBA (quaternary ammonium salts) shows prominent potential to the salting-out, as well as, the polarity of HBD groups influence on phase-forming ability. The main objective of this study is to find out the using DESs as extraction agent in room temperature. The term Green Analytical Chemistry (GAC) emerged in 2000 in an attempt to reduce the harmful effects of analytical practices on human beings and the environment. The principles of GAC are represented in the word SIGNIFICANCE: S-selection of a straight analytical technique, I-integration of analytical operations and processes, G-generation of minimum waste, N-never wasting energy, I-implementation of method automation, F-favoring fewer toxic reagents, I-increasing operator safety, C-carrying out in situ measurements, A-avoidance of derivatization, N-noting the use of a smaller number of samples, C-choice of multi-analytical methods, E-elimination of toxic reagents. Recently, AGREE, a novel downloadable greenness assessment software, has been proposed by Pena-Pereira and his fellows in June 2020. It relies on the twelve basics of GAC, SIGNIFICANCE, as mentioned earlier in the introduction 29,30 .
Recently, our research team has investigated the effect of DESs on thermodynamic properties of 5-HMF 31 . Continuing in this line of research, this study is focused on the effect of DESs (ChCl:lac, ChCl;ox and ChCl:cit in molar ratios 1:1) on 5-HMF extraction and experimental binodal data and tie-lines determination of (DES + PPG400 + water) at T = 298.15 K and under pressure (85 kPa). Furthermore, the NRTL and UNIQUAC, activity coefficient models, are widely applied for describing phase equilibrium data. The experimental binodals were correlated with two empirical equations involving Zafarani-Moattar and coworkers and Merchuk. The consistency of the measured LLE data were tested using the Hand and Bachman-Brown. Furthermore, the partition coefficients of 5-HMF were correlated using the Diamond-Hsu model. Then, AGREE used as a greenness assessment tool for evaluation of greenness of analytical protocol.

Experimental section
Materials. All data corresponding to the chemicals used in this study have been shown in Table 1. All materials were used without further purification. Choline chloride was purchased from Fluka company and the deionized water was utilized in preparation of solutions. 5-HMF, lactic acid, oxalic acid and citric acid were purchased from Sigma Aldrich company as well as, silver nitrate, potassium chromate, sulfuric acid and sodium hydroxide were purchased from Merck company. DES preparation. The hydrogen bond acceptor (HBA:choline chloride) is combined with hydrogen bond donor (HBD:carboxylic acids) with the same molar ratio of 1:1. The HBA and HBD were magnetically stirred and heated at 368.15 K until the mixtures became a homogeneous colorless liquid.
Liquid-liquid equilibrium measurements. For every systems, the cloud-point measurements method was applied to obtain the binodal curves 32,33 : Experimental procedure. The Carl Fischer (Metrohm 751 GPD) method was used to determinate of water content of choline chloride, carboxylic acids and obtained DESs, which it was less than 0.1% (w/w). The mixture composition was determined using an analytical balance (Shimadzu, 321-34,553, Shimadzu Co. Japan) with precision of 1 × 10 -5 g with a precision of ± 1 × 10 -4 g. To ensure the reproducibility and accuracy of the obtained data, the cloud point method for each system, were performed in three replicates. A lamp was also located near the measuring glassy cell for better visualization of the exact DESs mass fractions related to binodal curves. These binodal curves for aqueous solutions of ChCl:lac 70 wt% and PPG400 80 wt%, ChCl:ox 70 wt% and PPG400 80 wt%, ChCl:cit 60 wt% and PPG400 80 wt%, were determined. For each tie-line, a mixture point in the two-phasic region of each ternary system was gravimetrically prepared and centrifuged to phase separation and placed in water bath at 298.15 K to reach equilibrium for a minimum of 6 h. After the equilibration step, the top and bottom phases were carefully separated and weighed with a precision of ± 10 -5 g. The compositions of coexisting phases were analytically determined.
Mohr method 34 , known as argentometric method, is one of the significant methods for determination of chloride in water. Choline chloride is a quaternary ammonium salt with choline cation and chloride anion. This method determines the chloride ion concentration of a solution by titration with silver nitrate using potassium chromate as indicator.
Mohr indicator reaction is based on the following reactions: The concentration of titrant rises sharply near the equivalence point, and the solubility of Ag 2 CrO 4 is exceeded. The appearance of red precipitate marks the endpoint.
where A and B are the volumes of AgNO 3 for sample and blank, respectively. The normality of AgNO 3 is N, and V is the volume of water sample taken.
Commercial racemic, d-or l-lactic acid are usually aqueous solutions 82-95 wt%. Also, lactic acid presents dimer, trimer and oligomer states in equilibrium with monomeric lactic acid. The back-titration is an utmost important method to quantify the total acidity of lactic acid. Back-titration is applied when the reaction is too slow or no suitable sensor is available for a practical direct titration 36 . For this purpose, first, at the boiling temperature, lactic acid reacted with sodium hydroxide using phenolphthalein indicator, and then using a standard solution of sulfuric acid, the excess alkali was back titrate. The samples containing 1 gr from each phase were weighed and dissolved with 25.0 mL of deionized water, combined with 20.0 mL of 0.1 M sodium hydroxide standard solution, boiled for 10 min to hydrolyze oligomers of lactic acid completely. Eventually, back titration was applied to determine the excess alkali with the sulfuric acid standard solution of 0.05 M using phenolphthalein as a visual indicator. The total acidity was calculated as following equation 37 : where m is weight of the sample and C NaOH is the NaOH standard solution concentration.
To quantify the concentration of PPG400 in each phase, The refractometer (ATAGO DR-A 1 , Japan) with a preision of 0.0001 was used as presented by Cheluget et al. 38 . In refractive index measurements, the estimated uncertainty is 0.0002. According to this method, there is the following relation between the mass fractions of PPG400, w p , choline chloride, w c and carboxylic acids, w a and refractive index, n D : The values of constants a p , a c and a a corresponding to PPG400, choline chloride and carboxylic acids, respectively were calculated from the linear calibration plots of the refractive index of the solution and n 0 = 1.3325 is the refractive index of pure water at T = 298.15 K. The coefficient of determination values R 2 and values of these constants together are reported in Table S1. In each phase, the sum of mass fractions is equal to unity ( w w + w p + w c + w a = 1 ). To determine the concentrations of both of choline chloride and polymer, two equations with tow unknown weight fraction ( w p and w c ) were used. The uncertainty of the mass fraction of each of the ATPS components was about 0.008. Since, the Eq. (3) is only valid for dilute solutions, the samples, before refractive index measurements, were diluted in the mass fraction range (C Range(w/w)) as given in Table S1.
(3) n D = n 0 + a p w p + a c w c + a a w a  Table S2. After stirring these mixtures for 25 min and observing the phase separation, the mixtures were placed in a water bath. Then 0.002 mass fractions of 5-HMF was added to 1 mL of each phase. The samples were centrifuged at a speed of 3000 rpm for 20 min and maintained in water bath for 24 h to ensure complete separation of the phases and reach equilibrium. Finally, after separation of phases, the 5-HMF in both phases was measured by UV spectroscopy (Model: SPECORD 40-Series Analytik Jena AG-Germany) at the maximum wavelength of 286 nm.

Analytical GREEnness metric (AGREE).
In AGREE, the final score is a fraction of unity, from zero to one. The automatically resultant pictogram is divided into twelve sections with flexibility for controlling their width according to their importance. Each section has a specific color range from deep green (= 1) to deep red (= 0). The overall score appears in the middle of the circular pictogram. The AGREE too was downloaded from a specific link provided in an AGREE article on November 15, 2020 39,40 .

Results and discussion
DES-based LLE is an important separation method in research and chemical analysis. As a commercial process, it is frequently used in the chemical and mining industries and in the downstream recovery of fermentation products (antibiotics, amino acids, antioxidants) 41 .
Phase diagram. The phase diagram is a graphical representation of the physical states of a substance under different conditions and provides information about concentration of phase forming components in the top and bottom phases. Thus, the binodal curve is important for researchers to understand the liquid-liquid equilibrium. For all phase diagrams, the monophasic region is localized below the biphasic region in the binodal curve 28 . Three kinds of mono, di and tri-carboxylic acids, namely, lactic, oxalic and citric acids as the HBD, and choline chloride as the HBA, at the same mole composition ratio (1:1), were used to form of DESs. The phase diagrams of some ATPSs composed of DES + PPG400 + H 2 O were obtained by the cloud point titration method at room temperature which depicted in Fig. 1. As shown, the binodal area for citric acid is less than lactic and oxalic acid therefore, it can be seen that the phase forming ability of the DESs fall off in the order: ChCl:lac ≈ ChCl:ox ˃ ChCl:cit. In the considered ATPSs, the bottom phase corresponds to the aqueous solution enriched in DES, while the top phase is mostly composed of PPG400. There are several complicated factors spawned this difference, such as hydrophilicity factor, density and viscosity of the DESs. The hydrophilicity factor causes DES with higher carboxylic functions required more PPG400.
Determination of the tie-lines length. The tie-line length (TLL) has unit of %w/w, same as the component concentrations. The TLL, which are listed in Table S3, can be related to the equilibrium phase composition as follows 42 : where w p , w HBA and w HBD are equilibrium compositions of PPG400, choline chloride and carboxylic acids, respectively. Superscripts "bot" and "top" designate the PPG400-rich phase (top phase) and DES-rich phase (bottom phase), respectively.
Distribution coefficient (K) (sometimes referred to as partition ratios) is widely used in environmental science to relate the concentration of a chemical solute in one phase to that in a second phase between which www.nature.com/scientificreports/ equilibrium applies or is approached. The distribution coefficient, measured at equilibrium, reflect the solubility of a compound in both phases, and so is dependent on the solvent system used. Distribution coefficient and extraction efficiencies (EE%) were calculated by the following equations 43 and the obtained values were listed in Table 2 and shown in Fig. 2.
The obtained results reveal that, the 5-HMF spontaneously prefers to migrate to the PPG400-rich phase and the best partitioning with a extraction efficiency of 79% was obtained by the ChCl:lac as DES. The different partitioning behaviors of the three systems can be explained by the hydrophobicity difference between the phases. In terms of their physico-chemical properties, mono, di and tri-carboxylic acids are amphipathic substances, being composed of a hydrophobic hydrocarbon and some hydrophilic carboxylic groups. In terms of their direct effect on the phase formation ability, the more hydrophilic carboxylic groups, the more PPG400 needs and has low efficiency, therefore, the presence of three carboxylic functional groups in citric acid leads to increase hydrophilicity and decrease efficiency.
Modeling. Once the binodal curve is experimentally obtained, it is important to determine the composition of the upper and bottom phases. To this purpose, in order to avoid lengthy and minute analytical protocols, proposed methods based on least-squares regression methods by Merchuk et al. 44 (Eq. 7), Zafarani-Moattar et al. 45 (Eq. 8).
The concentrations (mass fraction) of PPG400 and DES are denoted as w p and w s , respectively, and a, b, c, α, β, γ are the fitted parameters. The adjustable parameters from fitting the binodal data are reported in Table 3. The sd reported values in Table 3 illustrated that both equations have an acceptable correlation result.
The molecular features of a binary solution are described by one parameter which developed by Guan et al., which called the effective excluded volume EEV 46 . www.nature.com/scientificreports/ The ν p−DES−w is the volume of polymer species at constant solution density in presence of DES and v * p−DES−w is effective excluded volume. These fitting parameters, obtained from the correlation of the experimental binodal data along with the corresponding standard deviations, are listed in Table 3. www.nature.com/scientificreports/ The non-random two-liquid (NRTL) 47 and universal quasi-chemical (UNIQUAC) 48 are the local composition models that used for fitting of tie-line compositions.
The values of the non-randomness parameters were obtained and their optimum value usually set in a fixed value (between 0.01 and 0.4) which in this study it was found to be 0.3. The binary interaction parameters used in the NRTL model for the ternary systems were also determined and are listed in Table 4. where G ij = exp(−α ij τ ij ) , α ij = α ji and τ ij = τ ji = 0. The UNIQUAC model, contains two different parts, including combinational and residual. This model is not fully thermodynamically consistent due to its two liquid mixture approach. In this approach the local concentration around one central molecule is assumed to be independent from the local composition around another type of molecule. These parameters always are greater than zero.  , (a, b, c), (α, β, γ), and ( v * P−DES−W , f), for {PPG400 + ChCl (HBA) + carboxylic acids (HBD) + H 2 O} systems at 298 where w1 and N represented mass fraction of PPG400 and number of binodal data, respectively.   www.nature.com/scientificreports/ The variables θ i , i , and τ ji are the volume fraction, area fraction, and interaction parameter between molecule i and j, respectively. The coordination number, Z, the number of molecules surrounding the central molecule, is set to 10. The molecular structure constants of the pure component are q and r and depend on the relative external surface areas and the molecular size (volume), respectively and reported in Table S4.
The fitting parameters and corresponding standard deviations are given in Table 4. The obtained standard deviations (sd) show that both of NRTL and UNIQUAC models can be used to reproduce the binodal data.
The interaction parameters of the NRTL and UNIQUAC models were determined by minimizing the objective function.
where ln γ here, A and B are fitting parameters. The symbol �w(PPG 400 ) is the difference of mass fraction of PPG 400 in the top and bottom phase: In Table 5, the fitting parameters with the corresponding standard deviations, sd, are given. According to the the sd values, it is concluded that correlation with this used equation is satisfactory for modelling the experimental results Journal standard requires that the first table referenced in the manuscript text should be Table 1, Consistency of tie-line data. The consistency of the experimental tie-line data was always checked by Hand 50 and Bachman-Brown 42 equations, which are expressed as: The fitting parameters and corresponding linear correlation coefficients, R 2 , are shown in Table 5, which indicates a good consistency for our LLE data. All linear correlation coefficients (R 2 ) are close to one; these values indicate of a better goodness of fit for the LLE data of the considered systems.
The AGREE tool is a simple automated and reliable tool with more detailed information about the 12 principles of GAC. The main advantage of AGREE is the clarification of strong and weak sections among the twelve principles of GAC. Besides, its overall score is reliable and informative as regards GAC bases. The AGREE overall greenness score was 0.77 with a green color which indicates the environmental sustainability of the process, (see the Supporting Information). By this technique, the weakest red sections in the AGREE pictogram were criterion 3 (off-line analytical technique).

Conclusion
Various process intensification techniques have been studied related to the production of 5-HMF, though most are limited to only lab-scale systems. Deep eutectic solvent-based aqueous two phase system was developed to extraction of 5-HMF from aqueous solution.. 5-HMF can be produced to byproducts at high temperatures, therefore, the extraction process is preferably carried out at room temperature. The ATPSs were constructed by acidic DESs (ChCl:lac, ChCl:ox and ChCl:cit) + PPG400 + water while DES-rich, polymer-poor is bottom phase and a polymer-rich, DES-poor is top phase. The experimental LLE data were satisfactorily adjusted by two semiempirical Zafarani-Moattar et al. and Merchuk equations. Also, the phase behavior of the ATPSs is also satisfactorily described by the NRTL and UNIQUAC models. According to deviations obtained from fitting tie-lines, it was found that performances of these models are good. Finally, the equation proposed by Diamond-Hsu was used for modeling the experimental data of the partition coefficients of 5-HMF and the Hand and Bachman-Brown equations were applied to verify reliability of experimental tie-line compositions. These results indicate the best partitioning with a extraction efficiency of 79% was obtained by the ChCl:lac as DES. These systems have shown the potential to become excellent integrated platforms in which the reaction and separation steps can be carried out sequentially by using their switchable behavior.