Fructose and Glucose Dissolution in Ionic Liquids: Solubility and

Solid Liquid Equilibrium of Cellobiose, Sucrose, and Maltose Monohydrate in Ionic Liquids: Experimental and Quantum Chemical Insights. Mood Mohan , Ta...
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Fructose and Glucose Dissolution in Ionic Liquids: Solubility and Thermodynamic Modeling Aristides P. Carneiro, Oscar Rodríguez,† and Eugénia A. Macedo* LSRELaboratory of Separation and Reaction Engineering, Associate Laboratory LSRE/LCM Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal ABSTRACT: Biorefining has shown the potential as an alternative source of feedstock instead of crude oil. In parallel, ionic liquids (ILs) have proved to be adequate solvents for biomass processing. Thus, phase equilibrium between carbohydrates and ionic liquids is critical for process design and optimization in biorefineries. In this work, the solubility of glucose and fructose in six ILs were measured in the temperature range 288−339 K. An isothermal method with quantification by HPLC analysis was used. Modeling of solubility was carried out using excess Gibbs energy models (NRTL and UNIQUAC). Experimental data were also compared with literature data. The thermodynamic functions of the dissolution process were calculated from the experimental solubilities.

1. INTRODUCTION Modern lifestyle and continuous population growth pushes toward an increasing consumption of fossil resources for the production of energy and materials. This causes a great impact in the planet’s management. Meanwhile, a threat begins to be seriously taken into account: a future shortage of fossil resources.1 Fortunately, concerns about the importance of renewable sources of energy and materials to face these troubles have introduced new milestones and rules for the future (United Nations, Agenda 21). One aspect of extreme relevance is the integrated use of biomass as feedstock for both materials and energy. This is the goal of the modern biorefining concept.2 Present in a great diversity of plants, wood, and residues from industries such as agriculture, biomass is indeed the major source of renewable carbon in nature, being readily available to be converted into materials and energy. Its main compounds, biopolymers such as cellulose, hemicellulose, starch, and lignin, can be independently processed to strategically produce fuels, added-value chemicals, and energy.3 The most promising route to achieve this concerted biorefining is through the platform of sugars present in biomass. Cellulose, hemicellulose, and starch can be hydrolyzed or depolymerized into their sugar monomers. Depending on the biomass nature, many different sugars can be obtained from depolymerization: glucose, fructose, xylose, galactose, sucrose, maltose, lactose, etc. Being highly functionalized chemicals, these sugars are likely to be transformed in derivatives through many reactional routes.4 Sugar derivatives, such as dicarboxylic acids, sugar alcohols, and furan compounds are high potential chemicals whether used directly as additives in food, medicines, and package materials or used as intermediate building blocks for the production of commodity chemicals and added valued products.5 Even though the biorefining concept seems to be a suitable alternative to the petrochemical industry, processing biomass is actually an extremely challenging task. After the biomass harvest, their biopolymers need to be separated. Conventional pretreatment processes6 usually involve high temperatures, large amounts of water, and the use of corrosive chemicals such as strong acids and caustic soda. The nature of these standard © 2013 American Chemical Society

processes would turn a large scale integrated biorefinery unfeasible due to very high operating costs (wastewater, energy), and the investment in processing equipment would be overly costly. Room temperature ionic liquids, ILs, are a recently explored class of materials that have been an important investigation topic in the past decade. Briefly defined as liquid salts at room temperature, their unique and unusual properties7−9 such as nonvolatility, high thermal stability, and versatility to dissolve polar, nonpolar, and polymeric chemicals have made them potential solvents to a wide range of applications, not only in reaction engineering10−12 and separation technology13−15 but also in tribology16 and electrochemistry.17 Another key feature of these novel solvents, is their tunability; that is, the ILs properties can be strategically tuned by small changes either in the cation or in the anion in order to suit a specific application. The ability of ILs as suitable solvents for biomass processing has already been demonstrated.18,19 Either acting as simple solvents to dissolve biomass biopolymers or as catalysts to yield their monomers and transform them into building blocks for biorefining, the ionic liquids nature could permit biomass treatment/conversion processes to run with a lower energetic demand, water consumption, and with a lower investment in process equipment.20,21 Nevertheless, ILs recycling has to be maximized to maintain economical feasibility of such processes, due to the actual price of the ionic liquids. The capacity of the ionic liquids to dissolve carbohydrates is primarily associated to strong hydrogen bonding between the ionic liquid’s anion and the hydrogen atoms of the hydroxyl groups.22 Playing a secondary role, the cation has also an effect in the affinity with the carbohydrates, but apart from the hydrophobicity effect and the inclusion of ether functional groups in the alkyl chains, a Received: Revised: Accepted: Published: 3424

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where ΔfusHs and Ts,fus account for the melting enthalpy and melting temperature of the pure solute, respectively. T is the equilibrium temperature, R the ideal gas constant, and γLs the activity coefficient of the solute in the liquid phase. 2.2. Activity Coefficients. To compute the activity coefficients, two local composition models were applied in this work. These models are explicit in terms of excess Gibbs free energy, gE, and the activity coefficient is calculated through fundamental thermodynamic relationships.28 In this work, we considered two well-known models: the Non-Random-TwoLiquid model, NRTL, 29 and the UNIQUAC model. 30 UNIQUAC structural parameters ri and qi for each pure compound are usually calculated using the group-contribution values of Bondi.31 However, for ILs, other methodologies are needed, and in this work, an empirical model proposed by Domanska and Mazurowska32 was used:

better elucidation of the entire role of the cation in these interactions needs more experimental and theoretical support.23 The phase equilibria knowledge of IL mixtures with carbohydrates is of extreme relevance for process design and optimization purposes. Therefore, the solubility of carbohydrates in ionic liquids is an important parameter in reaction engineering (kinetic studies and reactor sizing) as well as in separation technology where it could be useful to design and optimize liquid−liquid extractions as well as selective precipitations. Some data regarding this phase equilibria is already available in the literature.23−25 However, on one hand, these data are mostly focused on cellulose and glucose, being very scarce the data for other carbohydrates. On the other hand, the available data do not cover, for the same system, several measurements in a representative temperature range, which difficult the application of existent or the development of new thermodynamic models to describe the phase equilibrium. Providing such modeling treatment will also support the issues concerning with process design and optimization. Our previous works26,27 present solubility data for several carbohydrates and derivatives in ILs as well as modeling their phase equilibrium through thermodynamic models. In this work, in a similar perspective, the solubility of glucose and fructose in six ionic liquids were measured at several temperatures in the range from 288 to 339 K, using an isothermal approach and quantification by HPLC. Four dicyanamide ILs were tested in this work: 1ethyl-3-methylimidazolium dicyanamide, [emim][DCA], 1butyl-3-methylimidazolium dicyanamide, [bmim][DCA], Aliquat[DCA], and trihexyltetradecylphosphonium dicyanamide, [P6,6,6,14][DCA]. In addition Aliquat[NO3] and 1-ethyl3-methylimidazolium trifluoroacetate, [emim][TFA] were also used. The ionic liquids with the Aliquat cation were synthesized in this work as well as the [emim][TFA]. The stirring time to attain the equilibrium was obtained for the studied systems and solubility data were modeled with excess Gibbs energy models NRTL and UNIQUAC. Comparison with available literature data was carried out and the thermodynamic functions of dissolution were obtained from the experimental data.

qi =

2(1 − li) z−2 ri + z z

(3)

Fobj =

mod el ⎞2 N ⎛γ ∑i =p1 ⎜ s,γi exp − 1⎟ ⎝ s,i ⎠

Np

(4)

with Np representing the number of experimental points and the experimental activity coefficients, γexp s,i , calculated according to eq 1 as follows γs,exp = i

⎧ ⎛ Ts,fus ⎞ ⎪ (Δ (ΔC PsS − L) 1 fus Hs) ⎨ − − exp 1 ⎜ ⎟ ⎪ Ti ⎠ R xs,Li ⎩ RTs,fus ⎝ ⎫ ⎡⎛ ⎛ T ⎞ ⎤⎪ T ⎞ ⎢⎜1 − s,fus ⎟ + ln⎜ s,fus ⎟⎥⎬ ⎢⎣⎝ Ti ⎠ ⎝ Ti ⎠⎥⎦⎪ ⎭

(5)

After fitting the adjustable parameters, the quality of the data correlation was evaluated by means of the average absolute relative deviation, AARD: N

p xs,L,exp − xs,L,model 100 i i AARD (%) = ∑ L,exp Np i = 1 xs, i

(6)

2.4. Thermodynamic Functions of Dissolution. The approach to calculate the thermodynamic functions of dissolution proposed by Krug and co-workers32 was adopted in this work. The dissolution enthalpy is calculated through a modified Van’t Hoff equation, and it is obtained from the slope of the linear representation of ln(xs) vs (1/T − 1/Thm):

Ts,fus ⎞ (ΔfusHs) ⎛ (ΔC PsS − L) ⎜1 − ⎟− RTs,fus ⎝ T ⎠ R ⎡⎛ ⎛ Ts,fus ⎞⎤ Ts,fus ⎞ ⎢⎜1 − ⎟ + ln⎜ ⎟⎥ ⎝ T ⎠⎦ T ⎠ ⎣⎝

(2)

In the above expressions, z and li stand for the coordination number and bulk factor and assume the values of 10 and 1, respectively. M is the molar mass of the ionic liquid in g·mol−1, and ρ is the density at 298.15 K in g·cm−3. 2.3. Data Correlation. The parameters for each model were obtained by fitting them to the experimental data, through minimization of the following objective function:

2. THEORY 2.1. Solubility. The equilibrium equation that allows calculating the solubility of a solid solute in a liquid is usually derived through a thermodynamic cycle under some assumptions:28 (i) There are not solvent molecules in the solid phase. (ii) A hypothetical subcooled liquid state is assumed for fugacity of pure solute. (iii) The solute’s triple-point is not much different from the normal melting temperature, Ts,fus. (iv) Solute’s heat capacity difference between pure liquid and solid state, ΔCS−L ps , is considered temperature independent. Therefore, the expression used in this work to calculate solubilities, xLs , from thermodynamic models is ln(γs Lxs L) =

M ρ

ri = 0.029281

0 Δdiss H

(1) 3425

⎡ ⎢ ∂ ln xs = −R ⎢ ⎢∂ 1 − 1 Thm ⎣ T

(

)

⎤ ⎥ ⎥ ⎥ ⎦P

(7)

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Table 1. Sample Description Table of the Chemicals Used chemical name

source

initial purity

[emim][DCA] [bmim][DCA] [P6,6,6,14][DCA] [emim][TFA] Aliquat[DCA] Aliquat[NO3] D-(−)-fructose D-(+)-glucose

Iolitec Iolitec Iolitec synthesized synthesized synthesized Merck Merck

0.99 0.98 0.95 not measured not measured not measured 0.99 0.999

purification method

distillation extraction and distillation extraction and distillation

final purity 0.99 0.98 0.95 0.999 0.998 0.974 0.99 0.999

analysis method

Karl Fischer titration, HPLC, and 1H NMR Karl Fischer titration, Mohr method, HPLC, and 1H NMR Karl Fischer titration, Mohr method, HPLC, and 1H NMR

Figure 1. Chemical structures of the involved anions and cations.

In this approach, the harmonic average temperature, Thm, is calculated from the experimental data: Thm =

(>0.999, Labsolve) were also used in the preparation of these two ionic liquids. 1-Methylimidazole (>0.99, Merck) and ethyl trifluoroacetate (>0.999, Merck) were used for the preparation of [emim][TFA]. The ionic liquids [emim][DCA] (>0.99), [bmim][DCA] (>0.98) and [P6,6,6,14][DCA] (>0.95) were purchased from Iolitec GmbH and used as received. Silver nitrate, AgNO3 (Ph. Eur, Merck) was used for quantification of chloride anion in Aliquat[DCA] and Aliquat[NO3]. A sample description of the ionic liquids and the sugars is provided in Table 1, and the chemical structures of the ions present in the ionic liquids are presented in Figure 1. All purities are given in mass fraction. 3.2. Methods. 3.2.1. Synthesis of Ionic Liquids. Aliquat[NO3]. The preparation of this IL was carried out according to a procedure available in literature33 and applying the improvements in purification, as described in a previous work.27 About 58% in excess of NaNO3 was dissolved in deionized water. Aliquat336 at 373 K was added dropwise to the aqueous solution with 10 mL of acetone to improve phase separation. The mixture was stirred at room temperature for 6 h, and then, two liquid phases were formed. The IL-rich phase was washed with deionized water to remove sodium chloride and unreacted sodium nitrate, whereas the separation of unreacted Aliquat336 was carried out through extraction with n-heptane. Finally, the IL was dried under vacuum and moderate temperatures (343− 353 K) for 48 h to remove the volatile compounds. The water

Np N

1

∑i =p1 T

i

(8)

The intercept of the linear representation, k, represents the logarithm of the solubility at Thm, and it is used to calculate the Gibbs free energy of dissolution: 0 Δdiss G = −RThmk

(9)

Finally, the dissolution entropy is calculated from the definition of Gibbs free energy: 0 Δdiss S=

0 0 Δdiss H − Δdiss G Thm

(10)

3. MATERIALS AND METHODS 3.1. Materials. D-(−)-fructose and D-(+)-glucose were supplied by Merck with purity >0.99. Sodium dicyanamide, NaN(CN)2 (>0.97, Acros Organics), and sodium nitrate, NaNO3 (>0.99, Sigma), were used in the synthesis of Aliquat[DCA] and Aliquat[NO3], respectively. Aliquat336, supplied by Acros organics with 0.0386 of water mass fraction (measured by Karl Fischer titration) and with a molar mass of 442 g·mol−1, n-heptane (>0.99, BDH Prolabo), and acetone 3426

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Table 2. Experimental Solubility Data of Glucose in the Ionic Liquids Aliquat[NO3]

a

Aliquat[DCA]

[P6,6,6,14][DCA]

T/Kb

xs

σa × 102

T/K

xs

σ × 102

T/K

xs

σ × 102

307.75 318.19 328.05 339.75

0.0393 0.053 0.0691 0.0865 u(xs)/xsc 0.01 [emim][TFA]

0.05 0.1 0.03 0.02

299.95 307.72 317.93 328.8 339.74

0.0078 0.0114 0.01759 0.0215 0.03230 u(xs)/xs 0.009 [emim][DCA]

0.07 0.05 0.004 0.08 0.006

298.37 308.29 318.43 329.51

0.0018 0.0024 0.00365 0.0048

0.09 0.02 0.003 0.04

T/K

xs

σ × 102

T/K

xs

σ × 102

T/K

xs

σ × 102

288.47 298.29 308.25 318.05 329.07

0.261 0.292 0.314 0.3294 0.349 u(xs)/xs 0.005

0.2 0.2 0.2 0.04 0.6

288.25 298.25 308.15 318.15 328.25

0.171 0.207 0.229 0.2548 0.281 u(xs)/xs 0.003

0.1 0.6 0.3 0.06 0.2

288.15 298.35 308.05 318.15 328.45

0.1330 0.165 0.192 0.2095 0.236 u(xs)/xs 0.005

0.01 0.1 0.2 0.03 0.2

u(xs)/xs 0.03 [bmim][DCA]

Standard deviation. bUncertainty (u(T) = ± 0.01 K). cRelative uncertainty: u(xs)/xs = (1/Nm) ∑i=1Nm(u(xs)i/xsi)

content of the final IL in mass fraction was 0.0184 (measured by Karl Fischer titration) whereas the chloride anion content was 0.008 in mass fraction (measured with Mohr’s method34). The chemical structure of the cation was checked by 1H NMR in a Bruker Avance III 400 spectrometer, operating at 400 MHz.27 Aliquat[DCA]. This synthesis was performed based on a procedure available in the literature.35 Aliquat336 (200 g) was dissolved in 300 mL of dichloromethane, and 50 g of sodium dicyanamide, NaN(CN)2, was added, forming a suspension that was stirred for 48 h at room temperature. The unreacted dicyanamide salt and the sodium chloride formed in the reaction were filtered, and then, the dichloromethane was distilled in a rotary evaporator (IKA RV 10 D S40). Extractions with water and n-heptane were carried out to remove the salts dissolved in Aliquat[DCA]. Finally, the ionic liquid was dried under vacuum at 353 K to remove all the volatile compounds. Approximately 100 g of Aliquat[DCA] were obtained with 500 ppm of water (measured by Karl Fischer titration) and with chloride anion content [emim][DCA] > [bmim][DCA]. Solubility in the hydrophobic ILs is much lower than in water, following the series: Aliquat[NO3] > Aliquat[DCA] > [P6,6,6,14][DCA]. In addition to water solubility, ideal solubility of glucose and fructose are also plotted in Figures 2 and 3. This clearly shows that, in hydrophilic ionic liquids, the solutions have negative deviations (γsolute < 1) relative to an ideal solution (no interactions between carbohydrate and IL), while in hydrophobic ILs, positive deviations to the ideal solubility (γsolute > 1) were observed from the solubility data. Moreover, glucose and fructose water solubility is closer to the ideal solubility than solubilities in any IL. In accordance to this fact, the IL with higher water content, Aliquat[NO3], is the one in which solubility of the sugars are closer to ideal solubility. Glucose solubility data in [bmim][DCA] are available in literature. A comparison with the results obtained in this work is presented in Figure 4. Data from refs 38 and 42 were determined in grams per liter. Direct comparison with the data obtained in this work with those data would only be possible if the densities of the saturated solutions were known. Therefore, in this work and to enable proper conversion of these literature data to molar fractions, the density of mixtures of glucose and [bmim][DCA] were measured at five different concentrations

Figure 3. Solubility of fructose in the studied ionic liquids and in water. [emim][TFA] ∇; [emim][DCA] ×; [bmim][DCA] □; water54,55  ·  (experimental data not modeled, dash-dot line is just to guide the eye); Aliquat[NO3] ○; Aliquat[DCA] Δ and ▲ (with 1.9 wt % of water, see text for details); [P6,6,6,14][DCA] ◊; UNIQUAC ; ideal solubility ·······.

Table 4. Molar Mass of the Ionic Liquids and Water Contents before and after Solubility Measurements water/wt % after ionic liquid Aliquat[NO3] Aliquat[DCA] [emim][DCA] [bmim][DCA] [P6,6,6,14][DCA] [emim][TFA]

−1

M/g·mol 468.55 472.59 177.21 205.26 549.90 224.18

before

glucose

fructose

1.84 0.05 0.16 0.18 0.52 0.12

1.34 0.17 0.16 0.35 0.51 0.23

1.34 0.24 0.26 0.22 0.50 1.44

temperatures. Controlling this parameter is essential to ensure reliability of the experimental data. Table 4 shows the water content before and after the solubility determination for each system. Apart from the dissolution of fructose in [emim][TFA], in which water content rose considerably due to solute’s degradation (section 4.3), the variations on water content in the other systems were sufficiently small to be considered negligible for changes in ILs properties during solubility experiments. The ability of ionic liquids with dicyanamide anion to dissolve carbohydrates has been recognized,38 and it is mainly attributed to its capacity for hydrogen bonding with hydroxyl groups. However, the structure and nature of the cation can dramatically change this. As it is clear from the obtained data, hydrophobic ionic liquids are much less able to dissolve these highly polar solutes. They are barely soluble in [P6,6,6,14][DCA] (maximum solubility obtained was 0.055 for fructose) even with a higher water content than in Aliquat[DCA]. The quaternary ammonium was able to dissolve up to 0.13 (molar fraction) of fructose, showing more affinity with sugars than the phosphonium IL. The later is slightly more bulky, which partially explains the difference. In addition, the central atom could also play a role affecting the solubility: a more electronegative central atom (nitrogen) could attract more electrons from the surrounding atoms to the cation’s core, which causes the positive charge to be less effective, reducing 3429

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Figure 4. Comparison of glucose solubility in [bmim][DCA] with literature data. This work × ; ref 25 ∇; ref 42 ○; ref 38 ◊.

Figure 5. Concentration correspondence for the system glucose + [bmim][DCA].

and four temperatures, as shown in Table 5. From the knowledge of the concentrations in mole fraction, density of

Table 6. Comparison of Solubility of Glucose at 308 K with Literature Data

Table 5. Density of Mixtures of Glucose and [bmim][DCA] ρa/g·mL−1 xGlucose (mole fraction)

298 K

313 K

328 K

343 K

0.0000 0.0313 0.0937 0.1849 0.2280 0.2474

1.06059 1.06937 1.08722 1.11565 1.12902 1.13733

1.05106 1.05985 1.07772 1.10609 1.11944 1.12781

1.04167 1.05046 1.06837 1.09664 1.10987 1.11830

1.03241 1.04120 1.05912 1.08729 1.10036 1.10889

a

the solutions, ρ, and the molar mass, M, of the two components, the correspondent concentration is calculated as follows: ρ (g·mL−1) ⎛M ⎞⎛ 1 ⎞ ⎟⎜ 1 + ⎜ [bmim][DCA] − 1⎟ ⎝ Mglucose ⎠⎝ xglucose ⎠

Aliquat[DCA]

[P6,6,6,14][DCA]

0.0114 0.0321

0.0024 0.0151

al.,25 the authors do not provide information about the halide content, which could affect IL properties and increase solubility of carbohydrates (once chloride anion is likely to establish strong hydrogen bonds with the hydrogen atoms of hydroxyl groups). In the case of [P6,6,6,14][DCA], the supplier is not the same, and once again, the content of [P6,6,6,14][Cl] could differ from this work and produce the observed differences. Furthermore, as the solubilities in this IL are remarkable small, difficulties regarding experimental technique (stirring time, phase separation, sampling and analysis) could also be related to the differences observed in the comparison. The work published by Rosatella et al.25 presents the solubility of glucose at 308 K in 28 different ILs. These data are represented in Figure 6, together with the data measured in this work for glucose at the same temperature and those presented in a previous work26 for comparison and to study the effect of the different cations and anions. Some solubility data presented in Figure 6 are associated to significant water content values, which are the cases of ILs with acetate anion, some ILs with phosphonium cation, and those with the guanidinium cation, (C3O)4DMG. This causes solubility to be higher than it would be in the dried IL, and comparing ILs with very different water contents does not allow us to explain the role of different cations and anions in glucose solubility. Nevertheless, some ILs were used sufficiently dried to assume water had little effect on solubility. Some of the ILs studied in this work are among the most able to dissolve glucose: [emim][DCA] and [emim][TFA]. Connecting this capacity with the relative low viscosity for both of them, they are undoubtedly two promising ILs to be used in carbohydrate processing. Higher solubilities with low water contents are associated with hydrophilic ILs with imidazolium-based cations: [emim]+ and 1-[2-(2-methoxyethoxy)-ethyl]-3-methylimidazolium, [moeoemim]+. The solubility of glucose is slightly higher in [moeomim][DCA] than in [emim][DCA], which is in agreement with the fact that oxygen atoms in the alkyl chains of imidazolium cations give extra

Uncertainty in density, u(ρ) = ±0.00001 g·mL−1

C (g·mL−1) =

ref this work 25

(11)

Then, the correspondence between the two concentrations can be achieved through linear regression of the calculated values, as shown in Figure 5. This linear regression gave a slope of 0.9711, an intercept of 5.13 × 10−3 ,and a correlation coefficient of 0.9991, which represents a good correlation despite some dispersion observed due to density variation with temperature. With this relation, the literature data38,42 could be converted into molar fraction and therefore compared with the data measured in this work, as presented in Figure 4. Clearly, the solubility data measured in [bmim][DCA] are in good agreement with the literature. However, comparing solubilities of glucose at 308 K in Aliquat[DCA] and in [P6,6,6,14][DCA] with those measured by Rosatella et al.,25 the same agreement was not achieved (Table 6). Several reasons could be behind these differences. First, these two ILs are usually prepared from the IL with chloride anion. In this work, a purification step to remove unreacted Aliquat[Cl] from Aliquat[DCA] was carried out to decrease the chloride content to approximately 0.1 wt %. In the work of Rosatella et 3430

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Figure 6. Comparison of glucose’s solubility at 308 K with literature data.

ability to interact with carbohydrates, as was already demonstrated elsewhere.38 4.3. Solute Degradation. Some ILs are known to be acidic catalysts for the dehydration of hexoses.43,44 Whereas glucose is hardly ever dehydrated in pure IL media, fructose is easier to be converted into dehydration products (water, furfural, 5hydroxymethylfurfural, formic acid, levullinic acid, humims, etc.) without any additional catalyst.45,46 Even though this phenomenon occurs in many acidic ILs, only above a relatively high temperature (353−373 K) a significant conversion of fructose is observed. In our laboratory, during the solubility experiments, degradation products are identified by observation of yellow to brown color in the solutions, as well as by identification of 5-hydroxymethyl furfural by HPLC. When degradation has a significant impact on the conversion of fructose, the concentration of fructose begins to decrease (instead of increase) with temperature. This happened with the ionic liquid [emim][TFA], which seemed to convert significant amounts of fructose above 308 K. This effect is clearly illustrated in Figure 7. For that reason, it was not possible to measure the solubility at higher temperatures, and only three solubilities were measured for this system. Another indicator of fructose dehydration is the high water content in the solution, compared to other systems (Table 4). 4.4. Modeling. As done in our previous works,26,27 Gibbs energy models NRTL and UNIQUAC were applied to correlate the measured solubility data. As the number of solubility points is usually relatively small in SLE, the nonrandomness parameter in NRTL, α, was set to a value within the range 0.20−0.30, as recommended in the literature.47

Figure 7. Degradation of fructose in [emim][TFA].

UNIQUAC structural parameters r and q were obtained from the literature for glucose and fructose and estimated for ILs through an empirical model also available in literature.48 The binary interaction parameters for each model were then fitted to the experimental data, minimizing the differences between the activity coefficients experimentally obtained and those calculated through the model (NRTL or UNIQUAC). It should be mentioned that the water content in the ILs was not taken into account in the modeling of the solubility data. As a result, binary parameters for systems with higher water mass fractions (Aliquat[NO3] and [P6,6,6,14][DCA]; see Table 4) are 3431

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Table 7. Results for Solubility Data Correlation with NRTL and UNIQUAC NRTL

UNIQUAC

solute

IL

α

Δg12/R (K)

Δg21/R (K)

AARD (%)

Δu12/R (K)

Δu21/R (K)

AARD (%)

glucose

[emim][DCA] [bmim][DCA] [emim][TFA] Aliquat[DCA] Aliquat[NO3] [P6,6,6,14][DCA] [emim][DCA] [bmim][DCA] [emim][TFA] Aliquat[DCA] Aliquat[NO3] [P6,6,6,14][DCA]

0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.30 0.22 0.30

−828.7 −732.4 −974.8 1144.5 −118.7 976.2 −806.9 −610.7 −836.5 250.9 −250.0 1770.9

901.6 836.5 1848.9 −359.7 482.6 4122.6 1357.5 2449.5 1769.7 310.8 3162.6 −394.4

2.5 3.3 2.0 7.7 2.9 2.9 0.8 0.6 1.9 10.0 1.2 31.6

−230.5 −243.0 −82.3 165.0 −73.6 61.2 −334.3 −343.7 −363.3 95.1 −236.5 277.0

156.2 234.4 −119.7 −25.2 171.1 272.6 442.9 644.5 531.7 15.5 699.0 −32.6

2.4 3.3 2.0 6.7 2.7 2.9 0.6 1.1 1.9 8.6 1.4 38.8

fructose

rather ″apparent″ parameters that also account for the effect of water in the solubility. With the obtained parameters, the melting properties26 for glucose (ΔfusH = 32.432 kJ·mol−1 and Tfus = 423.15 K) and fructose (ΔfusH = 26.030 kJ·mol−1 and Tfus = 378.15 K) and their solid−liquid heat capacity difference (ΔCpS−L = 12049 and 9950 J·mol−1·K−1 for glucose and fructose, respectively), eq 1 was applied to compute the solubility for each system. The models accuracy to correlate the solubility data was measured by the average absolute relative deviation, AARD, as described in eq 6. Table 7 presents the results for the correlation obtained with these two models. In Table 8, the

non-randomness parameter was optimized to 0.22, obtaining a significant decrease in AARD values. Apart from this exception, α = 0.30 presented the lower values of AARD for all the other systems. The solid lines in Figures 2 and 3 represent the solubility calculated with UNIQUAC model, representing satisfactorily the experimental data. Nevertheless, deviations were too high for systems with very low solubility (Table 7). The solubility of fructose in [P6,6,6,14][DCA] has shown a high increase from 318 to 328 K. This unexpected behavior was confirmed by repeating the experiments for the same system at 328 K. Modeling the solubility in this system was found to be very sensitive to the initial guess of the parameters and also to the starting values of xsL to solve eq 1, probably due to very low solubility values. In addition, using UNIQUAC model, the experimental point at the highest temperature had to be neglected in order to obtain reasonable binary parameters and a satisfactory solubility correlation. The ILs with lower carbohydrate solubility were those which produced higher values of ARD in solubility. This is due, in part, to the larger experimental uncertainty in this range of solubility. Apart from [P6,6,6,14][DCA] and Aliquat[DCA], the correlation with NRTL and UNIQUAC was very satisfactory, with deviations (AARD) lower than 3.5% in all cases. 4.5. Thermodynamic Functions of Dissolution. The procedure based on a modified Van’t Hoff equation described by eqs 7−10, which was already applied by Panteli and Voutsas51 and more recently by our research team,26,27,52 was also applied here to obtain the thermodynamic functions of

Table 8. UNIQUAC Model Structural Parameters

a

compound

r

q

source

glucose fructose [emim][DCA] [bmim][DCA] [emim][TFA] Aliquat[DCA] Aliquat[NO3] [P6,6,6,14][DCA]

5.80 5.80 4.71 5.67 5.08 15.53 15.04 17.90

4.84 4.92 3.77 4.53 4.07 12.43 12.03 14.32

ref 56 ref 57 a a a a a a

Estimated in this work.

structural parameters for all the compounds used in this work are also presented. The values of the parameter α in NRTL were almost all set to 0.30, according to the recommendations in the literature. However, for systems with Aliquat[NO3], the

Table 9. Thermodynamic Functions of Dissolution Calculated from Experimental Data solute

IL

Thm (K)

Δ0dissH (kJ·mol−1)

Δ0dissG (kJ·mol−1)

Δ0dissS (J·mol−1)

glucose

[emim][DCA] [bmim][DCA] [emim][TFA] Aliquat[DCA] Aliquat[NO3] [P6,6,6,14][DCA] [emim][DCA] [bmim][DCA] [emim][TFA] Aliquat[DCA] Aliquat[NO3] [P6,6,6,14][DCA]

307.6 307.6 298.1 318.2 323.0 313.2 307.6 307.6 298.1 318.2 323.0 308.2

9.5 11.0 5.5 29.1 21.6 26.3 7.8 7.5 5.6 28.7 13.9 53.3

3.8 4.3 3.1 10.9 7.6 15.2 2.3 2.8 2.3 7.1 4.8 13.2

18.5 21.5 8.0 57.2 43.5 35.7 17.7 15.3 11.0 67.9 28.2 130.1

fructose

3432

dx.doi.org/10.1021/ie3024752 | Ind. Eng. Chem. Res. 2013, 52, 3424−3435

Industrial & Engineering Chemistry Research

Article

Authors also acknowledge CEMUP, University of Porto by performing the NMR analysis needed for this work.

dissolution (enthalpy, Gibbs energy, and entropy) from the experimental data. Table 9 shows these functions calculated for all the binary systems. All values presented for the thermodynamic functions are positive, which means that the dissolution of glucose and fructose in the studied ionic liquids is endothermic, nonspontaneous, and entropically favorable. The lowest values of dissolution enthalpy and Gibbs energy are associated to the systems that exhibited the highest values of solubility. These systems require less energy to break intermolecular interactions and establish new solute−solvent interactions, being able to dissolve larger amounts of solute. In addition, positive dissolution entropy reveals that the final state (solute dissolved in the ionic liquid) gives a higher degree of freedom to the molecule’s and ionic pairs’ movement than before the solute is dissolved (initial state).

■

5. CONCLUSIONS Solubility data measured in this work reveal that fructose is more soluble than glucose, which is due to the lower melting temperature and enthalpy of the former. Dicyanamide ILs used in this work exhibited a wide range of solubilities depending on the nature of the cation. The hydrophilic dicyanamide ILs were much able to dissolve the highly polar solutes. Besides, the shorter alkyl chain in [emim][DCA] improves the dissolution of the monosaccharides. Among the hydrophobic ILs, the more bulky (more hydrophobic) [P6,6,6,14][DCA] was less capable to dissolve the sugars, when compared with Aliquat[DCA]. Trifluoroacetate anion, having more free electron pairs to establish hydrogen bonds, has shown higher ability to interact with the carbohydrates than the dicyanamide anion. Dissolving fructose in [emim][TFA] caused significant degradation of the solute, observable at 308 K. A dehydration product (5hydroxymethylfurfural) was identified by HPLC. A high increase of water in the mixture was also observed, confirming this reaction. The NRTL and UNIQUAC models were successfully used to correlate the experimental data. AARDs obtained were in general