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Extraction of adipic, levulinic and succinic acids from water using TOPO-based deep eutectic solvents
Journal Pre-proofs Extraction of adipic, levulinic and succinic acids from water using TOPObased Deep Eutectic Solvents Elisa Riveiro, Begoña González, Ángeles Domínguez PII: DOI: Reference:
S1383-5866(19)34847-6 https://doi.org/10.1016/j.seppur.2020.116692 SEPPUR 116692
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
22 October 2019 29 January 2020 8 February 2020
Please cite this article as: E. Riveiro, B. González, A. Domínguez, Extraction of adipic, levulinic and succinic acids from water using TOPO-based Deep Eutectic Solvents, Separation and Purification Technology (2020), doi: https://doi.org/10.1016/j.seppur.2020.116692
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Extraction of adipic, levulinic and succinic acids from water using TOPO-based Deep Eutectic Solvents Elisa Riveiroa,b, Begoña Gonzáleza, Ángeles Domíngueza,* a
Laboratory of Advanced Separation Processes, Department of Chemical Engineering, University of Vigo, Campus Lagoas-Marcosende, 36310 Vigo, Spain.
bEscuela
Internacional de Doctorado UNED, Universidad Nacional a Distancia, 28040, Madrid, Spain *Corresponding author. E-mail address: [email protected]
Abstract Organic acids are extremely useful as starting materials for chemical industry. Adipic, levulinic and succinic acids, are becoming in such interesting alternatives to petrochemical feedstock in the production of a wide range of compounds in many different industries such as polymer, pharmaceutical or food manufacture. The extraction process of these acids often use organic solvents which are environmental hazardous. In this paper, two hydrophobic trioctylphosphine oxide (TOPO)-based deep eutectic solvents (DESs) were studied as alternative solvents for the organic acid extraction. They were prepared and analysed using differential scanning calorimetry (DSC) and nuclear magnetic resonance (NMR) to confirm the DESs formation and to check their stability in water. Density, speed of sound, refractive index and viscosity of the prepared TOPO-based DESs were determined from 293.15 to 343.15 K. Even tough DES used to extract adipic, levulinic or succinic acid demonstrate high capacity to remove the above mentioned acids from aqueous solutions, TOPO is still exhibiting the best extraction capacity.
1
Keywords. Liquid-liquid extraction, deep eutectic solvents, organic acids, TOPO. 1. Introduction Organic acids are a key group of compounds as they have a big potential as precursors in the manufacture of a wide range of materials in the chemical industry, as well as pharmaceutical, food or polymer industry [1,2]. Among the organic acids, adipic, levulinic and succinic acids are used to produce a wide range of polymers and biopolymers. Some of the applications of adipic acid are in the production of Nylon 6,6, a polymer with very high demand due to the textile and automotive industries. It also can be used in the production of polyester resin increasing flex resistance as well as intermediate in the production of different compounds such as cyclopentanone, adiponitrile or 1,6-hexanediol and in food industry as a flavouring agent [3,4]. Succinic acid is an important feedstock of many industrially chemicals including adipic acid, 1,4butanediol, aliphatic esters and biodegradable plastics [5,6]. On the other hand, levulinic acid, mainly derived from lignocellulosic materials, is used to produce a number of bio-chemicals, resins, herbicides, pharmaceuticals or flavouring agents [7,8]. Nowadays, organic acids are usually obtained from fossil feedstock, even though the growing commercial interest has stimulated their bio-production to offer a solution to their high demand in the industry [9,10]. The development of efficient strategies to obtain organic acids is the main goal in order to achieve sustainable and competitive processes [11], taking into account that within the
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production process, one of the most challenging steps is the efficient separation of the acid from fermentation broths. Liquid-liquid extraction of carboxylic acids has been performed for many decades using different solvents such as ethyl acetate or methyl ter-butyl ether [12]. These typical organic solvents are generally toxic, flammable and volatile, causing harmful environment effects. Therefore, it is necessary to find new solvents capable of replacing traditional organic solvents in order to reduce the environmental impact, as well as the energy consumption in the extraction process. The demand for environmentally benign, less toxic, biodegradable, natural and low-cost solvents has encouraged the development of novel alternative solvents such as ionic liquids (ILs) and more recently deep eutectic solvents (DESs) [13– 19]. DES consists in a mixture of two or more components leading to the formation a eutectic mixture with a melting point much lower than the starting materials. [20–22] They are often simple to prepare by mixing hydrogen bond acceptors (HBA) such as quaternary ammonium salts or phosphonium salts and hydrogen bond donors (HBD) such as sugars, alcohols, carboxylic acids or amino acids. The number or HBA in combination with the HBD leads to the possibility to prepare a wide range of DESs with the appropriate characteristics depending on their application. Due to fact that the trioctylphosphine oxide (TOPO) as HBA has been proved to be successful to prepare hydrophobic DESs [23–25], in the present study two TOPO-based/long chain organic acid (decanoic or dodecanoic acids) were 3
prepared for the extraction of adipic, levulinic and succinic acids. The behaviour of these DESs in water and their physical properties were studied to analyse their suitability as extraction agents in aqueous solution and to characterise them, respectively. Once their stability in water was verified, they were applied as extraction agents to remove adipic, levulinic and succinic acids from water at 298.15 and 313.15 K. The performance of the DESs was compared to TOPO as reference solvent in the extraction of organic acids. 2. Materials and methods 2.1. Chemicals All chemicals were used as received from the supplier without further purification. The purity and supplier for each chemical are listed in Table 1. 2.2. Apparatus and procedure 2.2.1. DESs preparation DESs were prepared, following the literature [24], in molar ratio (1:1) by mixing the HBD (decanoic acid or dodecanoic acid) and the HBA (TOPO) in an Erlenmeyer flask with mechanical stirrer at 750 rpm and heated up to 343.15 K in a silicon bath. The temperature was controlled with an IKA ETS-D5 temperature controller with an uncertainty of ± 0.1 K. Once a homogeneous liquid appeared, and no solid is observed, the prepared DES, TOPO-DecAc and TOPO-DodecAc were kept for 24 hours at room temperature to test their stability. All the mixtures were prepared by weighing known masses of each component using a Mettler AX-205 Delta Range Balance with an uncertainty of ± 3.10⁻4 g. 2.2.2. Melting point analysis 4
The analysis of the melting point of the DES gives information about the temperature at which the solvents remain liquid. So that, a differential scanning calorimetry (DSC) analysis of both TOPO-DecAc and TOPO-DodecAc was carried out using a DSC Q 1000 TA Instruments. Samples of both DES, as well as the pure components (2-4 mg) were heated from 298.15 K to 343.15 K and then they were cooled to 193.15 K with a rate of 10 K. min⁻1 under inert atmosphere (50 mL.min-1 of N2). 2.2.3. Stability of solvents in water In order to establish the suitability of TOPO-DecAc, TOPO-DodecAc and TOPO as extraction agents in aqueous solution, the behaviour of these solvents in water was studied. According to previous studies conducted in our laboratory,[26,27] the stability of water-DESs mixtures was studied at 298.15 K and 313.15 K; while the stability of water-TOPO mixtures was studied at 313.15 K because of TOPO is solid at room temperature. So that, 50% (v/v) watersolvent mixtures were introduced in glass cells with silicon covers and they were stirred for 4 hours in a thermostatic circulating bath PoliScience Digital Temperature Controller coupled to a Phoenix Instrument RSM-0310K magnetic stirrer. The temperature was controlled with a digital thermometer ASL model F200 with an uncertainty of ± 0.01 K. To achieve the phase separation, the mixtures were centrifuged at 7000 rpm for 30 minutes in a Hettich Universal 320 centrifuge. Samples of both aqueous and solvent phases were analysed by 1H NMR spectroscopy using a Bruker DPX 400 at 400 MHz. Water-rich phases were prepared by dissolving an aliquot in deuterated water (D2O), while solventrich phases were dissolved in deuterated chloroform (CDCl3).
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2.2.4. Physical properties The knowledge of the physical properties of the solvents is very important in order to design an extraction process. Thus, density and viscosity influence the mass transport phenomena, there by affecting the suitability of the solvents for particular applications. On the other hand, speed of sound can be used to calculate volumetric derived properties that give us information about volume variation with pressure and all of them can be used to check the purity of these solvents. Density (ρ), speed of sound (u), refractive index (nD) and viscosity (η) of two prepared DESs were determined as a function of temperature from 293.15 K to 343.15 K at atmospheric pressure (0.1 MPa). Densities and speeds of sound were measured using an Anton Paar DSA5000M digital vibrating-tube densimeter. The uncertainties in the measurements of the density and the speed of sound were estimated to be ± 3.10⁻4 g.cm⁻³ and ± 0.3 m.s⁻1 respectively. Refractive index measurement was performed with an automatic digital refractometer ABBEMAT-WR Dr.Kernchen that has an uncertainty of ± 4.10-5. Dynamic viscosity was determined by using a microviscosimeter Lovis 2000/ME connected to the Anton Paar DSA-5000M densimeter, based on the Hoeppler´s falling ball principle. The measurement uncertainty is ± 0.03 mPa·s. 2.2.5. Liquid –liquid extraction
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In order to determine the extraction capability of the solvents (TOPO-DecAc, TOPO-DodecAc and TOPO to extract adipic, succinic and levulinic acids, liquidliquid extraction was carried out at 298.15 K and 313.15 K. Optimisation of the stirring time Prior to the determination of the extraction capability of the solvents used in this work, an optimisation of the stirring time study was carried out. An aqueous solution of adipic acid with a concentration of 3 g.L-1 was prepared. Mixtures of 50 % v/v of aqueous solution and solvent were mixed and stirred in an interval of time between 1 and 4 hours, in order to stablish the proper stirring time. Extraction process Once the stirring time was stablished, liquid-liquid extraction process was carried out. Aqueous solutions of adipic, levulinic and succinic acids were prepared with a concentration between 0.5 g.L-1 and 15 g.L-1. For each solution, 50 % v/v of aqueous solution and solvent were mixed and stirred for 1 hour in a PoliScience Digital thermostatic bath coupled to a Phoenix Instrument RSM0310K magnetic stirrer. The temperature was controlled with a temperature controller ASL model F200 with an uncertainty of ± 0.01 K. In order to assure the correct separation of the phases, the mixtures were centrifuged for 30 minutes at 7000 rpm using a Hettich Universal 320 centrifuge. The quantification of the organic acids in the aqueous phases was carried out by UV-Vis spectrometry using a Jasco V-750 UV/Vis spectrophotometer with an accuracy of ± 0.0015 Abs (from 0 to 0.5 Abs) and ± 0.0025 (from 0.5 to 1 Abs) and the working wavelength for adipic and succinic acids was 210 nm and 266 nm for levulinic acid. 7
To determine the concentration of the absorbing species, a calibration curve was determined through the Lambert-Beer law: 𝐼
𝐴 = 𝑙𝑜𝑔𝐼0 = 𝜀.𝑙.𝐶
(1)
where A is the absorbance, I0 is the intensity of the incident light at a given wavelength, I is the transmitted intensity, l is the path length through the sample, ε is the molar absorptivity for each species and wavelength and C is the concentration of the absorbing species. Subsequently used to calculate the concentration of each acid in the aqueous phase. The fitting parameters for all the acids studied in this work are presented in Table SI 1 of the supplementary material. The concentration of the acid was quantified by UV-Vis spectroscopy in the initial aqueous sample and in the aqueous phase once the equilibrium was reached. Knowing the volume of each sample, the difference between the initial amount of acid in the aqueous phase before and after the equilibrium, the concentration of the acid in the organic phase can be calculated. To know how the acid is distribute between both aqueous and organic phases, the distribution coefficient (K) was calculated according to the following expression: 𝐶𝑂𝐹
𝐾 = 𝐶𝐴𝐹
(2)
where COF and CAF are the concentration of the acid in the organic phase and in the aqueous phase, respectively.
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The extraction efficiency for all the studied organic acids was calculated as follows: 𝐸(%) =
(𝐶0 ― 𝐶𝐴𝐹) 𝐶0
𝑥100
(3)
where C0 refers to the initial concentration of the organic acid in the aqueous phase. 3. Results and discussion 3.1. Melting point analysis Two DESs, TOPO-DecAc and TOPO-DodecAc were prepared under the abovedescribed experimental conditions. Both solvents presented a homogeneous aspect; TOPO-DecAc was light yellow coloured, whereas DES TOPO-DodecAc was colourless. In order to determine their melting point, a DSC analysis was conducted for both DES and starting components and the thermograms are shown in Figure SI 2. The melting temperature of TOPO reported from the DSC analysis showed a melting peak temperature at 328.15 K, however the melting process begins before this value, allowing to get experimental results at 313.15 K (Figure SI 2 A), whereas the melting temperature for decanoic and dodecanoic acids were 300.15 K ((Figure SI 2 B), and 310.15 K (Figure SI 2 C), respectively. On the other hand, TOPO-DecAc presented a melting point at 255.15 K (Figure SI 2 D), whereas TOPO-DodecAc (Figure SI 2 E) showed a melting point at 270.15 K. In both cases, the melting temperature of the DES was lower than the melting temperature of the pure components. Regarding the results obtained for the melting temperature, to our knowledge there is no literature referring the melting point of TOPO-DecAc (1:1), whereas in the case of TOPO-Dodec (1:1) 9
the results are in good agreement with the results reported by Van den Bruinhorst et al. [24]. 3.2. Stability of the solvents in water The behaviour of the solvents in water is an important factor to be taken into account, since the viability of the extraction will depend on their stability. To this end, water/solvent mixtures were prepared as it was described in section 2.2.3. Both solvent and water phases were analysed by 1H NMR. The spectra showed in Figure SI 3 confirm the stability of both DESs at 298.15 K and 313.15 K since there is no signal of DES or starting DES components in the water phase, neither there is water in the DES phase, consequently TOPO-DecAc and TOPO-DodecAc are stable in water at studied temperatures. On the other hand, the stability of TOPO in water was only conducted at 313.15 K (Figure SI 1) because this compound is solid at room temperature. Therefore, all the studied solvents are suitable for the extraction of compounds present in aqueous media. 3.3. Physical properties of DESs Density (ρ), viscosity (η), refractive index (nD) and speed of sound (u) of the solvents were measured in a temperature range between 293.15 K and 343.15 K at 0.1MPa of pressure. The experimental physical properties are reported in Table SI 2. Density, speed of sound and refractive index were fitted to a linear expression as a function of the temperature using the following equation: 𝑍 = 𝑎 + 𝑏·𝑇
(4)
10
where Z is density (ρ), speed of sound (u) or refractive index (nD), a and b are the fitting parameters and T is the temperature. The fitting parameters a and b together with the correlation coefficient squared R2 and the standard relative deviations (σ) (equation 5) are given in Table SI 3.
{
𝑛
(
(𝑧 ― 𝑧𝑐𝑎𝑙)
)
𝑧𝑐𝑎𝑙
σ = ∑𝑖 𝑑𝑎𝑡
1 2 2
𝑛𝑑𝑎𝑡
}
(5)
where z and zcal are both the experimental and calculated values, respectively and ndat is the total experimental data. As it can be observed from Table S3 the linear equation properly fits these experimental data. Dynamic viscosities were fitted using Vogel-Fulcher-Tamman (VFT) equation [28–30] 𝜂 = 𝐴exp
(
𝐵
)
𝑇 ― 𝑇0
(6)
where η is the dynamic viscosity, A (mPa.s), B (K) and T0 (K) are the fitting parameters. The values of these parameters together with the relative standard deviations (σ) calculated with equation 5 are also presented in Table SI3. Density and viscosity are key properties of solvents since they influence dissolution, reaction and separation processes, determining their viability. Besides that, refractive index and speed of sound can provide more information about DES and their purities and they are useful in terms of design and optimization of industrial processes. In Figure 1, the experimental values of density, refractive index, speed of sound and viscosity for the studied DESs as
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a function of the temperature are plotted, together with the corresponding fitting curve. As it can be observed in this figure, the density (A), the refractive index (B) and the speed of sound (C) decrease linearly when the temperature increases, while an increase in the temperature leads to an exponential decrease in the dynamic viscosity values (D). These DES exhibit lower densities than water (0.9978 g/cm3 at 298.15K), varying between 0.8813 g/cm3 and 0.8795 g/cm3 at 298.15 K for TOPO-DecAc and TOPO-DodecAc, respectively. The highest density was observed for TOPO-DecAc (0.8512 g/cm3 at 343.15 K). Experimental viscosity data also showed lower values in comparison with viscosity values of water. So that, DES formed of TOPO-DecAc had a viscosity of 39.03 mPa.s at 298.15 K and the viscosity of TOPO-DodecAc was 46.51 mPa.s at 298.15 K, which means both DES exhibit lower values than water (100 mPa.s at 298.15 K). The same HBA (TOPO) forms both DES, but they differ in the HBD (decanoic or dodecanoic acids). Regarding the change of the HBD, the DES involving dodecanoic acid as HBD shows higher values of refractive index, speed of sound and viscosity in the whole range of the studied temperatures, and whereas DES formed of decanoic acid shows higher values of density. In this case, we can observe that an increase in the alkyl chain of the HBD leads to an increase in the viscosity values and a decrease in the density values. 3.4. Liquid-liquid extraction With the aim of designing safe and environmentally benign separation processes, to the development of a new generation of solvents is essential. In 12
this way, the utility of TOPO-DecAc and TOPO-DodecAc for liquid-liquid separation of organic acids (adipic, levulinic and succinic acids) in aqueous solution was tested and the performance of the DESs was compared with TOPO. An optimisation of the stirring time was firstly carried out with the aim of testing the best performance time of the above mentioned solvents. In Figure SI 1 are shown the extraction efficiencies at different stirring times using an aqueous solution of adipic acid with a concentration of 3 g.L-1. From this graph, it is demonstrated that 1 hour of stirring time is sufficient to perform the LLE. Additionally, there are not major differences between the extraction efficiencies of the solvents at different times in an interval within 4 hours of stirring, for instance it was decided to perform all the extraction processes at a stirring time of 1 hour. The experimental data for the liquid-liquid extraction of adipic, levulinic and succinic acids at 298.15 K and 313.15 K are presented in Tables SI 4, SI 5 and SI 6 respectively. In these tables, the initial concentration of the acid (C0), the concentration of the acid in both the organic phase (COF) and the aqueous phase (CAF) together with the distribution coefficient (K) and the extraction efficiency (E (%)) of the solvents are reported. As it is shown in Table SI 4, distribution coefficients for adipic acid were, in general, higher than one, especially when TOPO-Dodec was used as extraction solvent (4.96 at an initial concentration of 10 g/L at 298.15 K). From low to high concentration of the acid, the distribution coefficients increase when acid concentration increases. Regarding the extraction of levulinic acid (Table S5) the distribution coefficients at 298.15 K were higher when the solvent was TOPO-DecAc,(4.67 at an initial concentration of 10 g/L) whereas this distribution coefficients were higher in the extraction of succinic acid when the 13
solvent was TOPO-DodecAc (Table SI 6) (1.64 at an initial concentration of 10g/L). The distribution coefficients sharply increase when the extraction solvent is TOPO regardless the extracted acid. Thus, in the extraction of adipic acid it is possible to reach a distribution coefficient of 41.89 at an initial concentration of 15 g/L Which means that the higher the concentration the higher the distribution coefficient. In the case of the distribution coefficients obtained for the extraction of levulinic and succinic acids using TOPO, those coefficients were not as high as in the case of the extraction of adipic acid, leading to values from 4.09 for levulinic acid and 6.65 for succinic acid. In the literature, similar extraction results were reported for solvent systems containing TOPO. [31,32] It can be concluded that the extraction capability of the solvents increases when the concentration of the acid increases, regardless of the acid to be extracted. 3.4.1. Influence of temperature in the extraction capability Liquid-liquid extraction was carried out with DESs at 298.15 K and 313.15 K in order to study the effect of the temperature in the extraction process. The experimental values for the extraction efficiency (E (%)) of DES at 298.15 K and 313.15 K are presented in Tables SI 4, SI 5 and SI 6 for adipic, levulinic and succinic acids, respectively. Figure 2 shows the influence of temperature on the extraction yield, E (%) and it was divided in 6 graphs, where A and B corresponds with the extraction of adipic acid, C and D corresponds with levulinic acid and E and F corresponds with succinic acid. It is possible to observe in this figure that the extraction yield is higher at a temperature of 298.15K for both adipic, levulinic and succinic acids (A,C and E, 14
respectively), rather than at a temperature of 313.15 K (B ,D and F, respectively). That is an increase in temperature does not improve the extraction capacity of DES to extract adipic acid (A,B) or levulinic acid (C,D). In the case of the extraction of succinic acid (E,F), an increase in the temperature promotes a better extraction capacity of succinic acid at very low concentrations when the solvent used is TOPO-DecAc. Whereas when the solvent is TOPODodecAc the increase in the temperature is almost negligible. The choice of temperature is an important factor to be taken into account, in the design a liquid-liquid extraction process, especially in terms of energy saving. Thus, according to the results obtained from the study of the effect of the temperature in the extraction capacity of these DESs, the best working temperature in the extraction process of adipic, levulinic and succinic acids using both TOPO-DecAc and TOPO-DodecAc would be 298.15 K. 3.4.2. Influence of the extracted acid structure Based on the results obtained before, the best extraction was achieved using TOPO-DecAc and TOPO-DodecAc at 298.15 K, therefore the study of the influence of the acid structure was carried out at 298.15 K. The organic acids consider in this study have different structures. Adipic and succinic acids are both dicarboxylic acids with five and four carbon atoms, respectively. Levulinic acid is formed of five atoms of carbon with only one acidic group and a ketone group at the fourth carbon of the chain. So that, the study of the influence of the structure of the acid is evaluated in order to determine the best extraction performance of DESs.
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In Figure 3, the extraction efficiencies (E (%)) of DESs and TOPO according to the acid to be extracted are presented. Figure 3 A shows the extraction efficiencies of TOPO-DecAc to extract adipic, levulinic and succinic acids. Figure 3 B shows the extraction efficiencies of TOPO-DodecAc to extract adipic, levulinic and succinic acids and Figure 3 C shows the extraction efficiencies of TOPO to extract adipic, levulinic and succinic acids. The results show better extraction efficiencies in the extraction process of adipic, acid, getting a 47.47 % of extraction efficiency using TOPO-Dec and 52.93 % using TOPO-Dodec at low concentrations. The extraction efficiencies showed and increasing trend when the concentration of adipic acid increases. In Table SI 3 is possible to observe values of 84.58 % of extraction efficiency using TOPO-DecAc and 83.14 % using TOPO-Dodec an initial concentration of 10 g/L. The extraction efficiencies of levulinic acid (Table SI 4) were found to be around 14.05 % at 0.5 g/L of initial concentration using TOPO-DecAc and 61.16 % using TOPO-DodecAc. When the initial concentration of the acid is increased, the extraction efficiencies increase as well. So that, at an initial concentration of 10 g/L is possible to get 82.32 % of extraction efficiency when using TOPO-DecAc. and 65.87 % when the solvent was TOPO-DodecAc. Finally, in the case of succinic acid (Table SI 5), similarly to levulinic acid, the extraction efficiency was also very low at low concentrations (6.64 % using TOPO-DecAc and 30.41 % at an initial concentration of 0.5 g/L using TOPODodecAc). For what the following sequence of extraction with DESs can be stablished: Adipic acid>levulinic acid>succinic acid
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This trend can be explained because of the solvation of these acids in water is favoured by the presence of shorted chains and carboxylic groups that can form hydrogen bonds with water molecules. When the adipic acid and succinic acid are compared, it is possible to observe that the increase in the acidic alkyl chain favours the extraction process, possibly due to their higher solubility in the organic phase. Whereas in the comparison between succinic acid and levulinic acid, the extraction process is disadvantaged by the presence of two carboxylic acid groups in front of a carboxylic acid group and a ketone group, probably due to the formation of a larger number of hydrogen bonds. In this figure (Figure 3 C) is shown that the use TOPO as extraction solvent gives a maximum extraction efficiency of 97.67 % for the extraction of adipic acid, 88.07% for succinic acid and around 80.37 % for levulinic. In the case of the extraction with TOPO the trend is: Adipic acid>succinic acid> levulinic acid The best extraction efficiencies obtained with TOPO can be due to the hydrogen bond formation between the oxygen of the P=O group and the OH group of the extracted acid, while in the case of DESs the P=O group is involved in the formation of the DES and it is not available to form hydrogen bonds with the extracted acid.
3.4.3. Influence of the DES structure The two DESs formed under specified conditions, are made of an HBA (TOPO) and an HBD (decanoic or dodecanoic acid). Therefore, the structural difference
17
between them is the length of the alkyl chain of the HBD. The study of the influence of the structure of the DES can be observed in Figure 3 A and B. TOPO-DodecAc (Figure 3 B) exhibits the best extraction capability to extract the studied acids especially for low initial concentrations. It is possible to conclude that an increase in the HBD chain length leads to an increase in the extraction efficiency. 4. Conclusions In this work two hydrophobic TOPO-based DESs TOPO-DecAc and TOPODodecAc were studied as separation agents to extract adipic, levulinic and succinic acids from water solutions. The study of the behavior of DESs in water showed that both TOPO-DecAc and TOPO-DodecAc are stable in water confirming their potential as extracting agents of components present in aqueous solutions. Similarly, the behavior of TOPO in water was studied and its suitability as extraction solvent was confirmed. The increase of the temperature in the extraction process did not show a benefit in the extraction efficiencies for the three studied acids. Therefore, it can be concluded that the best working temperature to use these solvents as extracting agents for adipic, levulinic and succinic acids is 298.15 K. The extraction capability of DESs regarding the structure of the extracted acid is: adipic acid>levulinic acid>succinic acid. The extraction capabilities of TOPO-DecAc and TOPO-DodecAc
were
compared with the extraction capability of TOPO. The extraction capacity of TOPO showed a very high efficiency for both the three acids.
18
The following sequences of extraction capability can be established: TOPO>TOPO-DodecAc>TOPO-DecAc. After having determined the economical and simple procedure for separating these organic acids, the next stage would be to find an effective way to recover the extracted acids and to reuse the solvents.
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Acknowledgements This work was supported by Comisión Interministerial de Ciencia y Tecnología (Spain) (Project CTQ2016-77422-C2-1-R). Appendix A. Supporting Information Fitting parameters for adipic, levulinic and succinic acids calibration curves are shown in Table SI 1.
Experimental densities (ρ), viscosities (η), refractive
indices (nD) and speeds of sound (u) of TOPO-DecAc (1:1) and TOPO-Dodec (1:1) as well as their fitting parameters are presented in Table SI 2 and Table SI 3, respectively. Experimental data of initial concentration (C0), acid concentration in the organic phase (COF), acid concentration in the aqueous phase (CAF), distribution coefficients (K) of the acid and extraction efficiencies (E (%)) of the solvents at 298.15 K and 313.15 K are shown in Table SI 4, Table SI 5 and Table SI 6 respectively for adipic, levulinic and succinic acids. In Figure SI 1 the 1H NMR of organic and aqueous phases of the study of DESs and TOPO stability are presented. References [1]
M. Djas, M. Henczka, Reactive extraction of carboxylic acids using organic solvents and supercritical fluids: A review, Sep. Purif. Technol. 201 (2018) 106–119. doi:10.1016/j.seppur.2018.02.010.
[2]
D. Painer, S. Lux, A. Grafschafter, A. Toth, M. Siebenhofer, Isolation of Carboxylic Acids from Biobased Feedstock, Chemie-Ingenieur-Technik. 89 (2017) 161–171. doi:10.1002/cite.201600090.
[3]
K. Raj, S. Partow, K. Correia, A.N. Khusnutdinova, A.F. Yakunin, R. Mahadevan, Biocatalytic production of adipic acid from glucose using engineered Saccharomyces cerevisiae, Metab. Eng. Commun. 6 (2018) 28–32. doi:10.1016/j.meteno.2018.02.001. 20
[4]
A. Corona, M.J. Biddy, D.R. Vardon, M. Birkved, M.Z. Hauschild, G.T. Beckham, Life cycle assessment of adipic acid production from lignin, Green Chem. 20 (2018) 3857–3866. doi:10.1039/c8gc00868j.
[5]
E.N. Dragoi, S. Curteanu, D. Cascaval, A.I. Galaction, Separation of succinic acid from fermentation broths. Modelling and optimization, Environ.
Eng.
Manag.
J.
14
(2015)
533–539.
doi:10.30638/eemj.2015.057. [6]
H. Song, S.Y. Lee, Production of succinic acid by bacterial fermentation, Enzyme
Microb.
Technol.
39
(2006)
352–361.
doi:10.1016/j.enzmictec.2005.11.043. [7]
T. Brouwer, M. Blahusiak, K. Babic, B. Schuur, Reactive extraction and recovery of levulinic acid, formic acid and furfural from aqueous solutions containing sulphuric acid, Sep. Purif. Technol. 185 (2017) 186–195. doi:10.1016/j.seppur.2017.05.036.
[8]
D.W. Rackemann, W.O. Doherty, The conversion of lignocellulosics to levulinic acid, Biofuels, Bioprod. Biorefining. 5 (2011) 198–214. doi:10.1002/bbb.267.
[9]
Q.Z. Li, X.L. Jiang, X.J. Feng, J.M. Wang, C. Sun, H.B. Zhang, M. Xian, H.Z. Liu, Recovery processes of organic acids from fermentation broths in the biomass-based industry, J. Microbiol. Biotechnol. 26 (2016) 1–8. doi:10.4014/jmb.1505.05049.
[10] A.J.J. Straathof, S. Panke, A. Schmid, The production of fine chemicals by biotransformations., Curr. Opin. Biotechnol. 13 (2002) 548–56. http://www.ncbi.nlm.nih.gov/pubmed/12482513. [11] C.S. López-Garzón, A.J.J. Straathof, Recovery of carboxylic acids produced by fermentation, Biotechnol. Adv. 32 (2014) 873–904. doi:10.1016/j.biotechadv.2014.04.002. [12] L.M.J. Sprakel, B. Schuur, Solvent developments for liquid-liquid extraction of carboxylic acids in perspective, Sep. Purif. Technol. 211 (2019) 935–957. doi:10.1016/j.seppur.2018.10.023.
21
[13] K.N. Marsh, J.A. Boxall, R. Lichtenthaler, Room temperature ionic liquids and their mixtures - A review, Fluid Phase Equilib. 219 (2004) 93–98. doi:10.1016/j.fluid.2004.02.003. [14] J. Marták, S. Schlosser, Phosphonium ionic liquids as new, reactive extractants
of
lactic
acid,
Chem.
Pap.
60
(2006)
395–398.
doi:10.2478/s11696-006-0072-2. [15] F.S. Oliveira, J.M.M. Araújo, R. Ferreira, L.P.N. Rebelo, I.M. Marrucho, Extraction of l-lactic, l-malic, and succinic acids using phosphoniumbased
ionic
liquids,
Sep.
Purif.
Technol.
85
(2012)
137–146.
doi:10.1016/j.seppur.2011.10.002. [16] C. Florindo, L.C. Branco, I.M. Marrucho, Development of hydrophobic deep eutectic solvents for extraction of pesticides from aqueous environments,
Fluid
Phase
Equilib.
448
(2017)
135–142.
doi:10.1016/j.fluid.2017.04.002. [17] C. Florindo, L. Romero, I. Rintoul, L.C. Branco, I.M. Marrucho, From Phase Change Materials to Green Solvents: Hydrophobic Low Viscous Fatty Acid-Based Deep Eutectic Solvents, ACS Sustain. Chem. Eng. 6 (2018) 3888–3895. doi:10.1021/acssuschemeng.7b04235. [18] Y.R. Lee, K.H. Row, Comparison of ionic liquids and deep eutectic solvents as additives for the ultrasonic extraction of astaxanthin from marine
plants,
J.
Ind.
Eng.
Chem.
39
(2016)
87–92.
doi:10.1016/j.jiec.2016.05.014. [19] M. Rogošić, K.Z. Kučan, Deep eutectic solvents based on choline chloride and
ethylene
glycol
as
media
for
extractive
denitrification/desulfurization/dearomatization of motor fuels, J. Ind. Eng. Chem. 72 (2019) 87–99. doi:10.1016/j.jiec.2018.12.006. [20] A.P. Abbott, G. Capper, D.L. Davies, R.K. Rasheed, V. Tambyrajah, Novel Solvent Properties of Choline Chloride /Urea Mixtures, Chem. Commun. 0 (2003) 70–71. [21] A.P. Abbott, D. Boothby, G. Capper, D.L. Davies, R.K. Rasheed, Deep 22
Eutectic Solvents formed between choline chloride and carboxylic acids: Versatile alternatives to ionic liquids, J. Am. Chem. Soc. 126 (2004) 9142–9147. doi:10.1021/ja048266j. [22] A. Paiva, R. Craveiro, I. Aroso, M. Martins, R.L. Reis, A.R.C. Duarte, Natural deep eutectic solvents - Solvents for the 21st century, ACS Sustain. Chem. Eng. 2 (2014) 1063–1071. doi:10.1021/sc500096j. [23] M. Gilmore, É.N. McCourt, F. Connolly, P. Nockemann, M. SwadźbaKwaśny, J.D. Holbrey, Hydrophobic Deep Eutectic Solvents Incorporating Trioctylphosphine Oxide: Advanced Liquid Extractants, ACS Sustain. Chem.
Eng.
6
(2018)
17323–17332.
doi:10.1021/acssuschemeng.8b04843. [24] A. van den Bruinhorst, S. Raes, S.A. Maesara, M.C. Kroon, A.C.C. Esteves, J. Meuldijk, Hydrophobic eutectic mixtures as volatile fatty acid extractants,
Sep.
Purif.
Technol.
216
(2019)
147–157.
doi:10.1016/j.seppur.2018.12.087. [25] D.J.G.P. Van Osch, C.H.J.T. Dietz, J. Van Spronsen, M.C. Kroon, F. Gallucci, M. Van Sint Annaland, R. Tuinier, A Search for Natural Hydrophobic Deep Eutectic Solvents Based on Natural Components, ACS
Sustain.
Chem.
Eng.
7
(2019)
2933–2942.
doi:10.1021/acssuschemeng.8b03520. [26] O.G. Sas, R. Fidalgo, I. Domínguez, E.A. Macedo, B. González, Physical properties of the pure deep eutectic solvent, [ChCl]:[Lev] (1:2) DES, and its binary mixtures with alcohols, J. Chem. Eng. Data. 61 (2016) 4191– 4202. doi:10.1021/acs.jced.6b00563. [27] O.G. Sas, M. Castro, Á. Domínguez, B. González, Removing phenolic pollutants using Deep Eutectic Solvents, Sep. Purif. Technol. 227 (2019) 115703. doi:10.1016/J.SEPPUR.2019.115703. [28] H. Vogel, The law of the relation between the viscosity of liquids and the temperature, Phys. Z. 22 (1921) 645–645. [29] G.S. Fulcher, Analysis of recent measurements of the viscosity, J. Am. Ceram. Soc. 8 (1925) 339–355. 23
[30] G. Tammann, W. Hesse, The dependence of viscosity upon the temperature of super cooled liquids, ZAAC. 156 (1926) 245–257. [31] E. Alkaya, S. Kaptan, L. Ozkan, S. Uludag-Demirer, G.N. Demirer, Recovery of acids from anaerobic acidification broth by liquid-liquid extraction,
Chemosphere.
77
(2009)
1137–1142.
doi:10.1016/j.chemosphere.2009.08.027. [32] B.H. Um, B. Friedman, G.P. Van Walsum, Conditioning hardwood-derived pre-pulping extracts for use in fermentation through removal and recovery of acetic acid using trioctylphosphine oxide (TOPO), Holzforschung. 65 (2011) 51–58. doi:10.1515/HF.2010.115. Table 1. List of chemicals used: CAS numbers, purity and supplier. Compound name
CAS
Purity wt %
Supplier
Adipic acid
124-04-9
99.5
Scharlau
Succinic acid
110-15-6
99
Sigma-Aldrich
Levulinic acid
123-76-2
99
Sigma-Aldrich
Decanoic acid
334-48-5
≥98
Sigma
Dodecanoic acid
143-04-7
99
Scharlau
TOPO
78-50-2
99
Acros Organics
24
Figure. 1. Experimental physical properties as function of temperature and atmospheric pressure: (A) density (ρ), (B) refractive index (nD), (C) speed of sound (u) and (D) dynamic viscosity (η), and fitting from the equation 5 and fitting with VFT equation 6 of the DESs TOPO-DecAc (1:1) (●) and TOPO-DodecAc (1:1) (●).
25
A
B
C
D
E
F
Figure. 2. Extraction efficiency of DESs (E (%)) vs initial concentration ((C0 (g/L)) of the acid at different working temperatures. Adipic acid: (A) 298.15 K, (B) 313.15 K, levulinic acid: (C) 298.15 K, (D) 313.15 K and succinic acid: (E) 298.15 K, (F) 313.15 K. The colour code of the bars refers to the solvent used: (▬▬) TOPO-DecAc (1:1) at 298.15 K , (▬▬) TOPO-DodecAc (1:1) at 298.15 K (▬▬) TOPO-DecAc (1:1) at 313 K.15 and (▬▬) TOPO-DodecAc (1:1) at 313.15 K.
26
B
A
C
Figure. 3. Extraction efficiency (E (%)) of the solvents at 298.15 K vs organic acid. (A) TOPO-DecAc, (B) TOPO-Dodec and (C) TOPO; vs (▬▬) adipic acid (▬▬) levulinic acid and (▬▬) succinic.
27
CRediT author statement
Elisa Riveiro: Investigation, Writing - Original Draft
Angeles Dominguez: Conceptualization, Validation, Writing - Review & Editing
Begoña Gonzalez: Conceptualization, Writing - Review & Editing, Supervision, administration
Project
28
Declaration of interests
X The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
29
Highlights
Organic acids are extracted using trioctylphosphine oxide- based eutectic solvents.
The values of the studied physical properties (viscosity and density) of the solvents decrease when the temperature increases.
Deep
eutectic
solvents
show
extraction
efficiencies:
adipic>levulinic>succinic acids.
30
GA
31
S1383-5866(19)34847-6 https://doi.org/10.1016/j.seppur.2020.116692 SEPPUR 116692
To appear in:
Separation and Purification Technology
Received Date: Revised Date: Accepted Date:
22 October 2019 29 January 2020 8 February 2020
Please cite this article as: E. Riveiro, B. González, A. Domínguez, Extraction of adipic, levulinic and succinic acids from water using TOPO-based Deep Eutectic Solvents, Separation and Purification Technology (2020), doi: https://doi.org/10.1016/j.seppur.2020.116692
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© 2020 Published by Elsevier B.V.
Extraction of adipic, levulinic and succinic acids from water using TOPO-based Deep Eutectic Solvents Elisa Riveiroa,b, Begoña Gonzáleza, Ángeles Domíngueza,* a
Laboratory of Advanced Separation Processes, Department of Chemical Engineering, University of Vigo, Campus Lagoas-Marcosende, 36310 Vigo, Spain.
bEscuela
Internacional de Doctorado UNED, Universidad Nacional a Distancia, 28040, Madrid, Spain *Corresponding author. E-mail address: [email protected]
Abstract Organic acids are extremely useful as starting materials for chemical industry. Adipic, levulinic and succinic acids, are becoming in such interesting alternatives to petrochemical feedstock in the production of a wide range of compounds in many different industries such as polymer, pharmaceutical or food manufacture. The extraction process of these acids often use organic solvents which are environmental hazardous. In this paper, two hydrophobic trioctylphosphine oxide (TOPO)-based deep eutectic solvents (DESs) were studied as alternative solvents for the organic acid extraction. They were prepared and analysed using differential scanning calorimetry (DSC) and nuclear magnetic resonance (NMR) to confirm the DESs formation and to check their stability in water. Density, speed of sound, refractive index and viscosity of the prepared TOPO-based DESs were determined from 293.15 to 343.15 K. Even tough DES used to extract adipic, levulinic or succinic acid demonstrate high capacity to remove the above mentioned acids from aqueous solutions, TOPO is still exhibiting the best extraction capacity.
1
Keywords. Liquid-liquid extraction, deep eutectic solvents, organic acids, TOPO. 1. Introduction Organic acids are a key group of compounds as they have a big potential as precursors in the manufacture of a wide range of materials in the chemical industry, as well as pharmaceutical, food or polymer industry [1,2]. Among the organic acids, adipic, levulinic and succinic acids are used to produce a wide range of polymers and biopolymers. Some of the applications of adipic acid are in the production of Nylon 6,6, a polymer with very high demand due to the textile and automotive industries. It also can be used in the production of polyester resin increasing flex resistance as well as intermediate in the production of different compounds such as cyclopentanone, adiponitrile or 1,6-hexanediol and in food industry as a flavouring agent [3,4]. Succinic acid is an important feedstock of many industrially chemicals including adipic acid, 1,4butanediol, aliphatic esters and biodegradable plastics [5,6]. On the other hand, levulinic acid, mainly derived from lignocellulosic materials, is used to produce a number of bio-chemicals, resins, herbicides, pharmaceuticals or flavouring agents [7,8]. Nowadays, organic acids are usually obtained from fossil feedstock, even though the growing commercial interest has stimulated their bio-production to offer a solution to their high demand in the industry [9,10]. The development of efficient strategies to obtain organic acids is the main goal in order to achieve sustainable and competitive processes [11], taking into account that within the
2
production process, one of the most challenging steps is the efficient separation of the acid from fermentation broths. Liquid-liquid extraction of carboxylic acids has been performed for many decades using different solvents such as ethyl acetate or methyl ter-butyl ether [12]. These typical organic solvents are generally toxic, flammable and volatile, causing harmful environment effects. Therefore, it is necessary to find new solvents capable of replacing traditional organic solvents in order to reduce the environmental impact, as well as the energy consumption in the extraction process. The demand for environmentally benign, less toxic, biodegradable, natural and low-cost solvents has encouraged the development of novel alternative solvents such as ionic liquids (ILs) and more recently deep eutectic solvents (DESs) [13– 19]. DES consists in a mixture of two or more components leading to the formation a eutectic mixture with a melting point much lower than the starting materials. [20–22] They are often simple to prepare by mixing hydrogen bond acceptors (HBA) such as quaternary ammonium salts or phosphonium salts and hydrogen bond donors (HBD) such as sugars, alcohols, carboxylic acids or amino acids. The number or HBA in combination with the HBD leads to the possibility to prepare a wide range of DESs with the appropriate characteristics depending on their application. Due to fact that the trioctylphosphine oxide (TOPO) as HBA has been proved to be successful to prepare hydrophobic DESs [23–25], in the present study two TOPO-based/long chain organic acid (decanoic or dodecanoic acids) were 3
prepared for the extraction of adipic, levulinic and succinic acids. The behaviour of these DESs in water and their physical properties were studied to analyse their suitability as extraction agents in aqueous solution and to characterise them, respectively. Once their stability in water was verified, they were applied as extraction agents to remove adipic, levulinic and succinic acids from water at 298.15 and 313.15 K. The performance of the DESs was compared to TOPO as reference solvent in the extraction of organic acids. 2. Materials and methods 2.1. Chemicals All chemicals were used as received from the supplier without further purification. The purity and supplier for each chemical are listed in Table 1. 2.2. Apparatus and procedure 2.2.1. DESs preparation DESs were prepared, following the literature [24], in molar ratio (1:1) by mixing the HBD (decanoic acid or dodecanoic acid) and the HBA (TOPO) in an Erlenmeyer flask with mechanical stirrer at 750 rpm and heated up to 343.15 K in a silicon bath. The temperature was controlled with an IKA ETS-D5 temperature controller with an uncertainty of ± 0.1 K. Once a homogeneous liquid appeared, and no solid is observed, the prepared DES, TOPO-DecAc and TOPO-DodecAc were kept for 24 hours at room temperature to test their stability. All the mixtures were prepared by weighing known masses of each component using a Mettler AX-205 Delta Range Balance with an uncertainty of ± 3.10⁻4 g. 2.2.2. Melting point analysis 4
The analysis of the melting point of the DES gives information about the temperature at which the solvents remain liquid. So that, a differential scanning calorimetry (DSC) analysis of both TOPO-DecAc and TOPO-DodecAc was carried out using a DSC Q 1000 TA Instruments. Samples of both DES, as well as the pure components (2-4 mg) were heated from 298.15 K to 343.15 K and then they were cooled to 193.15 K with a rate of 10 K. min⁻1 under inert atmosphere (50 mL.min-1 of N2). 2.2.3. Stability of solvents in water In order to establish the suitability of TOPO-DecAc, TOPO-DodecAc and TOPO as extraction agents in aqueous solution, the behaviour of these solvents in water was studied. According to previous studies conducted in our laboratory,[26,27] the stability of water-DESs mixtures was studied at 298.15 K and 313.15 K; while the stability of water-TOPO mixtures was studied at 313.15 K because of TOPO is solid at room temperature. So that, 50% (v/v) watersolvent mixtures were introduced in glass cells with silicon covers and they were stirred for 4 hours in a thermostatic circulating bath PoliScience Digital Temperature Controller coupled to a Phoenix Instrument RSM-0310K magnetic stirrer. The temperature was controlled with a digital thermometer ASL model F200 with an uncertainty of ± 0.01 K. To achieve the phase separation, the mixtures were centrifuged at 7000 rpm for 30 minutes in a Hettich Universal 320 centrifuge. Samples of both aqueous and solvent phases were analysed by 1H NMR spectroscopy using a Bruker DPX 400 at 400 MHz. Water-rich phases were prepared by dissolving an aliquot in deuterated water (D2O), while solventrich phases were dissolved in deuterated chloroform (CDCl3).
5
2.2.4. Physical properties The knowledge of the physical properties of the solvents is very important in order to design an extraction process. Thus, density and viscosity influence the mass transport phenomena, there by affecting the suitability of the solvents for particular applications. On the other hand, speed of sound can be used to calculate volumetric derived properties that give us information about volume variation with pressure and all of them can be used to check the purity of these solvents. Density (ρ), speed of sound (u), refractive index (nD) and viscosity (η) of two prepared DESs were determined as a function of temperature from 293.15 K to 343.15 K at atmospheric pressure (0.1 MPa). Densities and speeds of sound were measured using an Anton Paar DSA5000M digital vibrating-tube densimeter. The uncertainties in the measurements of the density and the speed of sound were estimated to be ± 3.10⁻4 g.cm⁻³ and ± 0.3 m.s⁻1 respectively. Refractive index measurement was performed with an automatic digital refractometer ABBEMAT-WR Dr.Kernchen that has an uncertainty of ± 4.10-5. Dynamic viscosity was determined by using a microviscosimeter Lovis 2000/ME connected to the Anton Paar DSA-5000M densimeter, based on the Hoeppler´s falling ball principle. The measurement uncertainty is ± 0.03 mPa·s. 2.2.5. Liquid –liquid extraction
6
In order to determine the extraction capability of the solvents (TOPO-DecAc, TOPO-DodecAc and TOPO to extract adipic, succinic and levulinic acids, liquidliquid extraction was carried out at 298.15 K and 313.15 K. Optimisation of the stirring time Prior to the determination of the extraction capability of the solvents used in this work, an optimisation of the stirring time study was carried out. An aqueous solution of adipic acid with a concentration of 3 g.L-1 was prepared. Mixtures of 50 % v/v of aqueous solution and solvent were mixed and stirred in an interval of time between 1 and 4 hours, in order to stablish the proper stirring time. Extraction process Once the stirring time was stablished, liquid-liquid extraction process was carried out. Aqueous solutions of adipic, levulinic and succinic acids were prepared with a concentration between 0.5 g.L-1 and 15 g.L-1. For each solution, 50 % v/v of aqueous solution and solvent were mixed and stirred for 1 hour in a PoliScience Digital thermostatic bath coupled to a Phoenix Instrument RSM0310K magnetic stirrer. The temperature was controlled with a temperature controller ASL model F200 with an uncertainty of ± 0.01 K. In order to assure the correct separation of the phases, the mixtures were centrifuged for 30 minutes at 7000 rpm using a Hettich Universal 320 centrifuge. The quantification of the organic acids in the aqueous phases was carried out by UV-Vis spectrometry using a Jasco V-750 UV/Vis spectrophotometer with an accuracy of ± 0.0015 Abs (from 0 to 0.5 Abs) and ± 0.0025 (from 0.5 to 1 Abs) and the working wavelength for adipic and succinic acids was 210 nm and 266 nm for levulinic acid. 7
To determine the concentration of the absorbing species, a calibration curve was determined through the Lambert-Beer law: 𝐼
𝐴 = 𝑙𝑜𝑔𝐼0 = 𝜀.𝑙.𝐶
(1)
where A is the absorbance, I0 is the intensity of the incident light at a given wavelength, I is the transmitted intensity, l is the path length through the sample, ε is the molar absorptivity for each species and wavelength and C is the concentration of the absorbing species. Subsequently used to calculate the concentration of each acid in the aqueous phase. The fitting parameters for all the acids studied in this work are presented in Table SI 1 of the supplementary material. The concentration of the acid was quantified by UV-Vis spectroscopy in the initial aqueous sample and in the aqueous phase once the equilibrium was reached. Knowing the volume of each sample, the difference between the initial amount of acid in the aqueous phase before and after the equilibrium, the concentration of the acid in the organic phase can be calculated. To know how the acid is distribute between both aqueous and organic phases, the distribution coefficient (K) was calculated according to the following expression: 𝐶𝑂𝐹
𝐾 = 𝐶𝐴𝐹
(2)
where COF and CAF are the concentration of the acid in the organic phase and in the aqueous phase, respectively.
8
The extraction efficiency for all the studied organic acids was calculated as follows: 𝐸(%) =
(𝐶0 ― 𝐶𝐴𝐹) 𝐶0
𝑥100
(3)
where C0 refers to the initial concentration of the organic acid in the aqueous phase. 3. Results and discussion 3.1. Melting point analysis Two DESs, TOPO-DecAc and TOPO-DodecAc were prepared under the abovedescribed experimental conditions. Both solvents presented a homogeneous aspect; TOPO-DecAc was light yellow coloured, whereas DES TOPO-DodecAc was colourless. In order to determine their melting point, a DSC analysis was conducted for both DES and starting components and the thermograms are shown in Figure SI 2. The melting temperature of TOPO reported from the DSC analysis showed a melting peak temperature at 328.15 K, however the melting process begins before this value, allowing to get experimental results at 313.15 K (Figure SI 2 A), whereas the melting temperature for decanoic and dodecanoic acids were 300.15 K ((Figure SI 2 B), and 310.15 K (Figure SI 2 C), respectively. On the other hand, TOPO-DecAc presented a melting point at 255.15 K (Figure SI 2 D), whereas TOPO-DodecAc (Figure SI 2 E) showed a melting point at 270.15 K. In both cases, the melting temperature of the DES was lower than the melting temperature of the pure components. Regarding the results obtained for the melting temperature, to our knowledge there is no literature referring the melting point of TOPO-DecAc (1:1), whereas in the case of TOPO-Dodec (1:1) 9
the results are in good agreement with the results reported by Van den Bruinhorst et al. [24]. 3.2. Stability of the solvents in water The behaviour of the solvents in water is an important factor to be taken into account, since the viability of the extraction will depend on their stability. To this end, water/solvent mixtures were prepared as it was described in section 2.2.3. Both solvent and water phases were analysed by 1H NMR. The spectra showed in Figure SI 3 confirm the stability of both DESs at 298.15 K and 313.15 K since there is no signal of DES or starting DES components in the water phase, neither there is water in the DES phase, consequently TOPO-DecAc and TOPO-DodecAc are stable in water at studied temperatures. On the other hand, the stability of TOPO in water was only conducted at 313.15 K (Figure SI 1) because this compound is solid at room temperature. Therefore, all the studied solvents are suitable for the extraction of compounds present in aqueous media. 3.3. Physical properties of DESs Density (ρ), viscosity (η), refractive index (nD) and speed of sound (u) of the solvents were measured in a temperature range between 293.15 K and 343.15 K at 0.1MPa of pressure. The experimental physical properties are reported in Table SI 2. Density, speed of sound and refractive index were fitted to a linear expression as a function of the temperature using the following equation: 𝑍 = 𝑎 + 𝑏·𝑇
(4)
10
where Z is density (ρ), speed of sound (u) or refractive index (nD), a and b are the fitting parameters and T is the temperature. The fitting parameters a and b together with the correlation coefficient squared R2 and the standard relative deviations (σ) (equation 5) are given in Table SI 3.
{
𝑛
(
(𝑧 ― 𝑧𝑐𝑎𝑙)
)
𝑧𝑐𝑎𝑙
σ = ∑𝑖 𝑑𝑎𝑡
1 2 2
𝑛𝑑𝑎𝑡
}
(5)
where z and zcal are both the experimental and calculated values, respectively and ndat is the total experimental data. As it can be observed from Table S3 the linear equation properly fits these experimental data. Dynamic viscosities were fitted using Vogel-Fulcher-Tamman (VFT) equation [28–30] 𝜂 = 𝐴exp
(
𝐵
)
𝑇 ― 𝑇0
(6)
where η is the dynamic viscosity, A (mPa.s), B (K) and T0 (K) are the fitting parameters. The values of these parameters together with the relative standard deviations (σ) calculated with equation 5 are also presented in Table SI3. Density and viscosity are key properties of solvents since they influence dissolution, reaction and separation processes, determining their viability. Besides that, refractive index and speed of sound can provide more information about DES and their purities and they are useful in terms of design and optimization of industrial processes. In Figure 1, the experimental values of density, refractive index, speed of sound and viscosity for the studied DESs as
11
a function of the temperature are plotted, together with the corresponding fitting curve. As it can be observed in this figure, the density (A), the refractive index (B) and the speed of sound (C) decrease linearly when the temperature increases, while an increase in the temperature leads to an exponential decrease in the dynamic viscosity values (D). These DES exhibit lower densities than water (0.9978 g/cm3 at 298.15K), varying between 0.8813 g/cm3 and 0.8795 g/cm3 at 298.15 K for TOPO-DecAc and TOPO-DodecAc, respectively. The highest density was observed for TOPO-DecAc (0.8512 g/cm3 at 343.15 K). Experimental viscosity data also showed lower values in comparison with viscosity values of water. So that, DES formed of TOPO-DecAc had a viscosity of 39.03 mPa.s at 298.15 K and the viscosity of TOPO-DodecAc was 46.51 mPa.s at 298.15 K, which means both DES exhibit lower values than water (100 mPa.s at 298.15 K). The same HBA (TOPO) forms both DES, but they differ in the HBD (decanoic or dodecanoic acids). Regarding the change of the HBD, the DES involving dodecanoic acid as HBD shows higher values of refractive index, speed of sound and viscosity in the whole range of the studied temperatures, and whereas DES formed of decanoic acid shows higher values of density. In this case, we can observe that an increase in the alkyl chain of the HBD leads to an increase in the viscosity values and a decrease in the density values. 3.4. Liquid-liquid extraction With the aim of designing safe and environmentally benign separation processes, to the development of a new generation of solvents is essential. In 12
this way, the utility of TOPO-DecAc and TOPO-DodecAc for liquid-liquid separation of organic acids (adipic, levulinic and succinic acids) in aqueous solution was tested and the performance of the DESs was compared with TOPO. An optimisation of the stirring time was firstly carried out with the aim of testing the best performance time of the above mentioned solvents. In Figure SI 1 are shown the extraction efficiencies at different stirring times using an aqueous solution of adipic acid with a concentration of 3 g.L-1. From this graph, it is demonstrated that 1 hour of stirring time is sufficient to perform the LLE. Additionally, there are not major differences between the extraction efficiencies of the solvents at different times in an interval within 4 hours of stirring, for instance it was decided to perform all the extraction processes at a stirring time of 1 hour. The experimental data for the liquid-liquid extraction of adipic, levulinic and succinic acids at 298.15 K and 313.15 K are presented in Tables SI 4, SI 5 and SI 6 respectively. In these tables, the initial concentration of the acid (C0), the concentration of the acid in both the organic phase (COF) and the aqueous phase (CAF) together with the distribution coefficient (K) and the extraction efficiency (E (%)) of the solvents are reported. As it is shown in Table SI 4, distribution coefficients for adipic acid were, in general, higher than one, especially when TOPO-Dodec was used as extraction solvent (4.96 at an initial concentration of 10 g/L at 298.15 K). From low to high concentration of the acid, the distribution coefficients increase when acid concentration increases. Regarding the extraction of levulinic acid (Table S5) the distribution coefficients at 298.15 K were higher when the solvent was TOPO-DecAc,(4.67 at an initial concentration of 10 g/L) whereas this distribution coefficients were higher in the extraction of succinic acid when the 13
solvent was TOPO-DodecAc (Table SI 6) (1.64 at an initial concentration of 10g/L). The distribution coefficients sharply increase when the extraction solvent is TOPO regardless the extracted acid. Thus, in the extraction of adipic acid it is possible to reach a distribution coefficient of 41.89 at an initial concentration of 15 g/L Which means that the higher the concentration the higher the distribution coefficient. In the case of the distribution coefficients obtained for the extraction of levulinic and succinic acids using TOPO, those coefficients were not as high as in the case of the extraction of adipic acid, leading to values from 4.09 for levulinic acid and 6.65 for succinic acid. In the literature, similar extraction results were reported for solvent systems containing TOPO. [31,32] It can be concluded that the extraction capability of the solvents increases when the concentration of the acid increases, regardless of the acid to be extracted. 3.4.1. Influence of temperature in the extraction capability Liquid-liquid extraction was carried out with DESs at 298.15 K and 313.15 K in order to study the effect of the temperature in the extraction process. The experimental values for the extraction efficiency (E (%)) of DES at 298.15 K and 313.15 K are presented in Tables SI 4, SI 5 and SI 6 for adipic, levulinic and succinic acids, respectively. Figure 2 shows the influence of temperature on the extraction yield, E (%) and it was divided in 6 graphs, where A and B corresponds with the extraction of adipic acid, C and D corresponds with levulinic acid and E and F corresponds with succinic acid. It is possible to observe in this figure that the extraction yield is higher at a temperature of 298.15K for both adipic, levulinic and succinic acids (A,C and E, 14
respectively), rather than at a temperature of 313.15 K (B ,D and F, respectively). That is an increase in temperature does not improve the extraction capacity of DES to extract adipic acid (A,B) or levulinic acid (C,D). In the case of the extraction of succinic acid (E,F), an increase in the temperature promotes a better extraction capacity of succinic acid at very low concentrations when the solvent used is TOPO-DecAc. Whereas when the solvent is TOPODodecAc the increase in the temperature is almost negligible. The choice of temperature is an important factor to be taken into account, in the design a liquid-liquid extraction process, especially in terms of energy saving. Thus, according to the results obtained from the study of the effect of the temperature in the extraction capacity of these DESs, the best working temperature in the extraction process of adipic, levulinic and succinic acids using both TOPO-DecAc and TOPO-DodecAc would be 298.15 K. 3.4.2. Influence of the extracted acid structure Based on the results obtained before, the best extraction was achieved using TOPO-DecAc and TOPO-DodecAc at 298.15 K, therefore the study of the influence of the acid structure was carried out at 298.15 K. The organic acids consider in this study have different structures. Adipic and succinic acids are both dicarboxylic acids with five and four carbon atoms, respectively. Levulinic acid is formed of five atoms of carbon with only one acidic group and a ketone group at the fourth carbon of the chain. So that, the study of the influence of the structure of the acid is evaluated in order to determine the best extraction performance of DESs.
15
In Figure 3, the extraction efficiencies (E (%)) of DESs and TOPO according to the acid to be extracted are presented. Figure 3 A shows the extraction efficiencies of TOPO-DecAc to extract adipic, levulinic and succinic acids. Figure 3 B shows the extraction efficiencies of TOPO-DodecAc to extract adipic, levulinic and succinic acids and Figure 3 C shows the extraction efficiencies of TOPO to extract adipic, levulinic and succinic acids. The results show better extraction efficiencies in the extraction process of adipic, acid, getting a 47.47 % of extraction efficiency using TOPO-Dec and 52.93 % using TOPO-Dodec at low concentrations. The extraction efficiencies showed and increasing trend when the concentration of adipic acid increases. In Table SI 3 is possible to observe values of 84.58 % of extraction efficiency using TOPO-DecAc and 83.14 % using TOPO-Dodec an initial concentration of 10 g/L. The extraction efficiencies of levulinic acid (Table SI 4) were found to be around 14.05 % at 0.5 g/L of initial concentration using TOPO-DecAc and 61.16 % using TOPO-DodecAc. When the initial concentration of the acid is increased, the extraction efficiencies increase as well. So that, at an initial concentration of 10 g/L is possible to get 82.32 % of extraction efficiency when using TOPO-DecAc. and 65.87 % when the solvent was TOPO-DodecAc. Finally, in the case of succinic acid (Table SI 5), similarly to levulinic acid, the extraction efficiency was also very low at low concentrations (6.64 % using TOPO-DecAc and 30.41 % at an initial concentration of 0.5 g/L using TOPODodecAc). For what the following sequence of extraction with DESs can be stablished: Adipic acid>levulinic acid>succinic acid
16
This trend can be explained because of the solvation of these acids in water is favoured by the presence of shorted chains and carboxylic groups that can form hydrogen bonds with water molecules. When the adipic acid and succinic acid are compared, it is possible to observe that the increase in the acidic alkyl chain favours the extraction process, possibly due to their higher solubility in the organic phase. Whereas in the comparison between succinic acid and levulinic acid, the extraction process is disadvantaged by the presence of two carboxylic acid groups in front of a carboxylic acid group and a ketone group, probably due to the formation of a larger number of hydrogen bonds. In this figure (Figure 3 C) is shown that the use TOPO as extraction solvent gives a maximum extraction efficiency of 97.67 % for the extraction of adipic acid, 88.07% for succinic acid and around 80.37 % for levulinic. In the case of the extraction with TOPO the trend is: Adipic acid>succinic acid> levulinic acid The best extraction efficiencies obtained with TOPO can be due to the hydrogen bond formation between the oxygen of the P=O group and the OH group of the extracted acid, while in the case of DESs the P=O group is involved in the formation of the DES and it is not available to form hydrogen bonds with the extracted acid.
3.4.3. Influence of the DES structure The two DESs formed under specified conditions, are made of an HBA (TOPO) and an HBD (decanoic or dodecanoic acid). Therefore, the structural difference
17
between them is the length of the alkyl chain of the HBD. The study of the influence of the structure of the DES can be observed in Figure 3 A and B. TOPO-DodecAc (Figure 3 B) exhibits the best extraction capability to extract the studied acids especially for low initial concentrations. It is possible to conclude that an increase in the HBD chain length leads to an increase in the extraction efficiency. 4. Conclusions In this work two hydrophobic TOPO-based DESs TOPO-DecAc and TOPODodecAc were studied as separation agents to extract adipic, levulinic and succinic acids from water solutions. The study of the behavior of DESs in water showed that both TOPO-DecAc and TOPO-DodecAc are stable in water confirming their potential as extracting agents of components present in aqueous solutions. Similarly, the behavior of TOPO in water was studied and its suitability as extraction solvent was confirmed. The increase of the temperature in the extraction process did not show a benefit in the extraction efficiencies for the three studied acids. Therefore, it can be concluded that the best working temperature to use these solvents as extracting agents for adipic, levulinic and succinic acids is 298.15 K. The extraction capability of DESs regarding the structure of the extracted acid is: adipic acid>levulinic acid>succinic acid. The extraction capabilities of TOPO-DecAc and TOPO-DodecAc
were
compared with the extraction capability of TOPO. The extraction capacity of TOPO showed a very high efficiency for both the three acids.
18
The following sequences of extraction capability can be established: TOPO>TOPO-DodecAc>TOPO-DecAc. After having determined the economical and simple procedure for separating these organic acids, the next stage would be to find an effective way to recover the extracted acids and to reuse the solvents.
19
Acknowledgements This work was supported by Comisión Interministerial de Ciencia y Tecnología (Spain) (Project CTQ2016-77422-C2-1-R). Appendix A. Supporting Information Fitting parameters for adipic, levulinic and succinic acids calibration curves are shown in Table SI 1.
Experimental densities (ρ), viscosities (η), refractive
indices (nD) and speeds of sound (u) of TOPO-DecAc (1:1) and TOPO-Dodec (1:1) as well as their fitting parameters are presented in Table SI 2 and Table SI 3, respectively. Experimental data of initial concentration (C0), acid concentration in the organic phase (COF), acid concentration in the aqueous phase (CAF), distribution coefficients (K) of the acid and extraction efficiencies (E (%)) of the solvents at 298.15 K and 313.15 K are shown in Table SI 4, Table SI 5 and Table SI 6 respectively for adipic, levulinic and succinic acids. In Figure SI 1 the 1H NMR of organic and aqueous phases of the study of DESs and TOPO stability are presented. References [1]
M. Djas, M. Henczka, Reactive extraction of carboxylic acids using organic solvents and supercritical fluids: A review, Sep. Purif. Technol. 201 (2018) 106–119. doi:10.1016/j.seppur.2018.02.010.
[2]
D. Painer, S. Lux, A. Grafschafter, A. Toth, M. Siebenhofer, Isolation of Carboxylic Acids from Biobased Feedstock, Chemie-Ingenieur-Technik. 89 (2017) 161–171. doi:10.1002/cite.201600090.
[3]
K. Raj, S. Partow, K. Correia, A.N. Khusnutdinova, A.F. Yakunin, R. Mahadevan, Biocatalytic production of adipic acid from glucose using engineered Saccharomyces cerevisiae, Metab. Eng. Commun. 6 (2018) 28–32. doi:10.1016/j.meteno.2018.02.001. 20
[4]
A. Corona, M.J. Biddy, D.R. Vardon, M. Birkved, M.Z. Hauschild, G.T. Beckham, Life cycle assessment of adipic acid production from lignin, Green Chem. 20 (2018) 3857–3866. doi:10.1039/c8gc00868j.
[5]
E.N. Dragoi, S. Curteanu, D. Cascaval, A.I. Galaction, Separation of succinic acid from fermentation broths. Modelling and optimization, Environ.
Eng.
Manag.
J.
14
(2015)
533–539.
doi:10.30638/eemj.2015.057. [6]
H. Song, S.Y. Lee, Production of succinic acid by bacterial fermentation, Enzyme
Microb.
Technol.
39
(2006)
352–361.
doi:10.1016/j.enzmictec.2005.11.043. [7]
T. Brouwer, M. Blahusiak, K. Babic, B. Schuur, Reactive extraction and recovery of levulinic acid, formic acid and furfural from aqueous solutions containing sulphuric acid, Sep. Purif. Technol. 185 (2017) 186–195. doi:10.1016/j.seppur.2017.05.036.
[8]
D.W. Rackemann, W.O. Doherty, The conversion of lignocellulosics to levulinic acid, Biofuels, Bioprod. Biorefining. 5 (2011) 198–214. doi:10.1002/bbb.267.
[9]
Q.Z. Li, X.L. Jiang, X.J. Feng, J.M. Wang, C. Sun, H.B. Zhang, M. Xian, H.Z. Liu, Recovery processes of organic acids from fermentation broths in the biomass-based industry, J. Microbiol. Biotechnol. 26 (2016) 1–8. doi:10.4014/jmb.1505.05049.
[10] A.J.J. Straathof, S. Panke, A. Schmid, The production of fine chemicals by biotransformations., Curr. Opin. Biotechnol. 13 (2002) 548–56. http://www.ncbi.nlm.nih.gov/pubmed/12482513. [11] C.S. López-Garzón, A.J.J. Straathof, Recovery of carboxylic acids produced by fermentation, Biotechnol. Adv. 32 (2014) 873–904. doi:10.1016/j.biotechadv.2014.04.002. [12] L.M.J. Sprakel, B. Schuur, Solvent developments for liquid-liquid extraction of carboxylic acids in perspective, Sep. Purif. Technol. 211 (2019) 935–957. doi:10.1016/j.seppur.2018.10.023.
21
[13] K.N. Marsh, J.A. Boxall, R. Lichtenthaler, Room temperature ionic liquids and their mixtures - A review, Fluid Phase Equilib. 219 (2004) 93–98. doi:10.1016/j.fluid.2004.02.003. [14] J. Marták, S. Schlosser, Phosphonium ionic liquids as new, reactive extractants
of
lactic
acid,
Chem.
Pap.
60
(2006)
395–398.
doi:10.2478/s11696-006-0072-2. [15] F.S. Oliveira, J.M.M. Araújo, R. Ferreira, L.P.N. Rebelo, I.M. Marrucho, Extraction of l-lactic, l-malic, and succinic acids using phosphoniumbased
ionic
liquids,
Sep.
Purif.
Technol.
85
(2012)
137–146.
doi:10.1016/j.seppur.2011.10.002. [16] C. Florindo, L.C. Branco, I.M. Marrucho, Development of hydrophobic deep eutectic solvents for extraction of pesticides from aqueous environments,
Fluid
Phase
Equilib.
448
(2017)
135–142.
doi:10.1016/j.fluid.2017.04.002. [17] C. Florindo, L. Romero, I. Rintoul, L.C. Branco, I.M. Marrucho, From Phase Change Materials to Green Solvents: Hydrophobic Low Viscous Fatty Acid-Based Deep Eutectic Solvents, ACS Sustain. Chem. Eng. 6 (2018) 3888–3895. doi:10.1021/acssuschemeng.7b04235. [18] Y.R. Lee, K.H. Row, Comparison of ionic liquids and deep eutectic solvents as additives for the ultrasonic extraction of astaxanthin from marine
plants,
J.
Ind.
Eng.
Chem.
39
(2016)
87–92.
doi:10.1016/j.jiec.2016.05.014. [19] M. Rogošić, K.Z. Kučan, Deep eutectic solvents based on choline chloride and
ethylene
glycol
as
media
for
extractive
denitrification/desulfurization/dearomatization of motor fuels, J. Ind. Eng. Chem. 72 (2019) 87–99. doi:10.1016/j.jiec.2018.12.006. [20] A.P. Abbott, G. Capper, D.L. Davies, R.K. Rasheed, V. Tambyrajah, Novel Solvent Properties of Choline Chloride /Urea Mixtures, Chem. Commun. 0 (2003) 70–71. [21] A.P. Abbott, D. Boothby, G. Capper, D.L. Davies, R.K. Rasheed, Deep 22
Eutectic Solvents formed between choline chloride and carboxylic acids: Versatile alternatives to ionic liquids, J. Am. Chem. Soc. 126 (2004) 9142–9147. doi:10.1021/ja048266j. [22] A. Paiva, R. Craveiro, I. Aroso, M. Martins, R.L. Reis, A.R.C. Duarte, Natural deep eutectic solvents - Solvents for the 21st century, ACS Sustain. Chem. Eng. 2 (2014) 1063–1071. doi:10.1021/sc500096j. [23] M. Gilmore, É.N. McCourt, F. Connolly, P. Nockemann, M. SwadźbaKwaśny, J.D. Holbrey, Hydrophobic Deep Eutectic Solvents Incorporating Trioctylphosphine Oxide: Advanced Liquid Extractants, ACS Sustain. Chem.
Eng.
6
(2018)
17323–17332.
doi:10.1021/acssuschemeng.8b04843. [24] A. van den Bruinhorst, S. Raes, S.A. Maesara, M.C. Kroon, A.C.C. Esteves, J. Meuldijk, Hydrophobic eutectic mixtures as volatile fatty acid extractants,
Sep.
Purif.
Technol.
216
(2019)
147–157.
doi:10.1016/j.seppur.2018.12.087. [25] D.J.G.P. Van Osch, C.H.J.T. Dietz, J. Van Spronsen, M.C. Kroon, F. Gallucci, M. Van Sint Annaland, R. Tuinier, A Search for Natural Hydrophobic Deep Eutectic Solvents Based on Natural Components, ACS
Sustain.
Chem.
Eng.
7
(2019)
2933–2942.
doi:10.1021/acssuschemeng.8b03520. [26] O.G. Sas, R. Fidalgo, I. Domínguez, E.A. Macedo, B. González, Physical properties of the pure deep eutectic solvent, [ChCl]:[Lev] (1:2) DES, and its binary mixtures with alcohols, J. Chem. Eng. Data. 61 (2016) 4191– 4202. doi:10.1021/acs.jced.6b00563. [27] O.G. Sas, M. Castro, Á. Domínguez, B. González, Removing phenolic pollutants using Deep Eutectic Solvents, Sep. Purif. Technol. 227 (2019) 115703. doi:10.1016/J.SEPPUR.2019.115703. [28] H. Vogel, The law of the relation between the viscosity of liquids and the temperature, Phys. Z. 22 (1921) 645–645. [29] G.S. Fulcher, Analysis of recent measurements of the viscosity, J. Am. Ceram. Soc. 8 (1925) 339–355. 23
[30] G. Tammann, W. Hesse, The dependence of viscosity upon the temperature of super cooled liquids, ZAAC. 156 (1926) 245–257. [31] E. Alkaya, S. Kaptan, L. Ozkan, S. Uludag-Demirer, G.N. Demirer, Recovery of acids from anaerobic acidification broth by liquid-liquid extraction,
Chemosphere.
77
(2009)
1137–1142.
doi:10.1016/j.chemosphere.2009.08.027. [32] B.H. Um, B. Friedman, G.P. Van Walsum, Conditioning hardwood-derived pre-pulping extracts for use in fermentation through removal and recovery of acetic acid using trioctylphosphine oxide (TOPO), Holzforschung. 65 (2011) 51–58. doi:10.1515/HF.2010.115. Table 1. List of chemicals used: CAS numbers, purity and supplier. Compound name
CAS
Purity wt %
Supplier
Adipic acid
124-04-9
99.5
Scharlau
Succinic acid
110-15-6
99
Sigma-Aldrich
Levulinic acid
123-76-2
99
Sigma-Aldrich
Decanoic acid
334-48-5
≥98
Sigma
Dodecanoic acid
143-04-7
99
Scharlau
TOPO
78-50-2
99
Acros Organics
24
Figure. 1. Experimental physical properties as function of temperature and atmospheric pressure: (A) density (ρ), (B) refractive index (nD), (C) speed of sound (u) and (D) dynamic viscosity (η), and fitting from the equation 5 and fitting with VFT equation 6 of the DESs TOPO-DecAc (1:1) (●) and TOPO-DodecAc (1:1) (●).
25
A
B
C
D
E
F
Figure. 2. Extraction efficiency of DESs (E (%)) vs initial concentration ((C0 (g/L)) of the acid at different working temperatures. Adipic acid: (A) 298.15 K, (B) 313.15 K, levulinic acid: (C) 298.15 K, (D) 313.15 K and succinic acid: (E) 298.15 K, (F) 313.15 K. The colour code of the bars refers to the solvent used: (▬▬) TOPO-DecAc (1:1) at 298.15 K , (▬▬) TOPO-DodecAc (1:1) at 298.15 K (▬▬) TOPO-DecAc (1:1) at 313 K.15 and (▬▬) TOPO-DodecAc (1:1) at 313.15 K.
26
B
A
C
Figure. 3. Extraction efficiency (E (%)) of the solvents at 298.15 K vs organic acid. (A) TOPO-DecAc, (B) TOPO-Dodec and (C) TOPO; vs (▬▬) adipic acid (▬▬) levulinic acid and (▬▬) succinic.
27
CRediT author statement
Elisa Riveiro: Investigation, Writing - Original Draft
Angeles Dominguez: Conceptualization, Validation, Writing - Review & Editing
Begoña Gonzalez: Conceptualization, Writing - Review & Editing, Supervision, administration
Project
28
Declaration of interests
X The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
29
Highlights
Organic acids are extracted using trioctylphosphine oxide- based eutectic solvents.
The values of the studied physical properties (viscosity and density) of the solvents decrease when the temperature increases.
Deep
eutectic
solvents
show
extraction
efficiencies:
adipic>levulinic>succinic acids.
30
GA
31