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Research Article pubs.acs.org/journal/ascecg

Systematic Identification of Solvents Optimal for the Extraction of 5‑Hydroxymethylfurfural from Aqueous Reactive Solutions Lena C. Blumenthal,† Christian M. Jens,‡ Jörn Ulbrich,† Frank Schwering,§ Vanessa Langrehr,§ Thomas Turek,§ Ulrich Kunz,§ Kai Leonhard,‡ and Regina Palkovits*,† †

Institut für Technische und Makromolekulare Chemie, RWTH Aachen, Worringerweg 2, 52074 Aachen, Germany Lehrstuhl für Technische Thermodynamik, RWTH Aachen, Schinkelstraße 8, 52062 Aachen, Germany § Institut für Chemische und Elektrochemische Verfahrenstechnik, TU Clausthal, Leibnizstraße 17, 38678 Clausthal-Zellerfeld, Germany ‡

S Supporting Information *

ABSTRACT: To ensure a high efficiency of 5-hydroxymethylfurfural (HMF) synthesis, improved solvents for the extraction of HMF from a reactive aqueous solution were identified using the predictive thermodynamic model COSMO-RS. Utilizing COSMO-RS as a basis for a systematic solvent selection has the advantage of potentially saving significant time and effort by computationally screening several thousand possible solvents. Factors including temperature, concentration, and fructose addition were used for experimental validation of the predictive power of COSMO-RS. Continuous extraction experiments confirmed also kinetics and phase separation to be important for technical implementation. COSMO-RS predicted o-propylphenol and o-isopropylphenol to have partition coefficients as high as 10.02 and 9.82, which are roughly five times higher than the partition coefficient of the previously known, most effective solvent: 2-methyltetrahydrofuran (PHMF = 2). Therefore, the identification of o-propylphenol and o-isopropylphenol as improved solvents constitutes a significant efficiency improvement for the extraction, and by extension for the entire HMF synthesis. KEYWORDS: Continuous extraction, Reactive extraction, COSMO-RS, Solvent prediction, Salting-out

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INTRODUCTION In times of fossil resource depletion and climate warming, it becomes increasingly important to focus on renewable carbon sources. Within this framework, a promising platform chemical derived from cellulose is 5-hydroxymethylfurfural (HMF).1 Application examples of HMF are widespread and include the production of solvents,2 fuels,3,4 novel plastics,5 and fine chemicals.6 Starting the HMF synthesis with pure fructose as reactant yields high HMF selectivity, but it is also possible to start the synthesis directly with cellulose.7 The use of cellulose as substrate allows the combination of several reaction steps into one integrated process, omitting expensive intermediate isolation and purification steps. Such an integrated process can be further improved through a continuous removal of HMF during the synthesis. This in situ HMF removal increases the selectivity significantly, because follow up and side reactions, such as the formation of levulinic acid and humins (undefined polymer structures of sugar derivatives), are minimized (Figure 1).8 Enabling the in situ HMF removal requires suitable separation processes, e.g. distillation or extraction. Continuous © XXXX American Chemical Society

HMF separation by distillation is challenging: HMF exhibits a high boiling point and polymerizes at elevated temperatures. Thus, distillation would need to be performed at reduced pressure, leading to higher effort and cost. Therefore, several studies address the in situ HMF removal by extraction.1 Meanwhile, this possibility to increase the selectivity is wellknown, and Saha and Abu-Omar have summarized studies on HMF synthesis in biphasic reaction systems.9 For such an extraction, two solvents are necessary: One solvent that enables the reaction, the reaction solvent, and one solvent that extracts HMF, the extraction solvent. These two solvents are of paramount importance as they dictate the extraction efficiency. Thus, if the proposed integrated process is to compete with nonrenewable, established processes, highly efficient solvents are indispensable. Surprisingly, even though solvents are crucial, to the best of our knowledge, no computer-aided search for new solvents for the biphasic HMF synthesis has been performed: Thus, in this work we identify optimal solvents Received: September 9, 2015 Revised: November 10, 2015

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DOI: 10.1021/acssuschemeng.5b01036 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 1. Combination of synthesis and extraction of HMF as effective separation for further processing to valuable follow up products such as 2,5-furandicarboxylic acid (FDCA), 2,5dimethylfuran (DMF), or 2,5-bishydroxymethyltetrahydro furan (BHMTHF).

for the extraction of HMF and thereby open up an opportunity to significantly increase the efficiency of the HMF synthesis. First the reaction solvent is considered: It constitutes the medium in which HMF is formed and from which it subsequently needs to be extracted. Considering the literature, the synthesis of HMF is possible in water, ionic liquids (ILs), or polar organic solvents such as DMSO.7,10,11 Here, ILs and polar organic solvents have the disadvantage of being expensive and potentially toxic. An additional point to consider relates to the fact that, in the dehydration of sugars to HMF, 3 mol of water are released. Consequently, as water is always present, using DMSO or ILs as the reaction solvent means adding an additional species to the system, needlessly complicating the overall process. Thus, using water as reaction solvent means keeping the integrated synthesis as simple as possible. Furthermore, the intramolecular attraction between HMF and DMSO is greater than the one between HMF and water, as can be realized by considering the Henry coefficients of HMF in the = 8.82 × 10−8 bar, HHMF,COSMO‑RS two solvents (HHMF,COSMO‑RS DSMO water −5 = 1.81 × 10 bar). The stronger intramolecular attraction makes it less efficient to extract HMF from DMSO than from water. Indeed, Chheda and co-workers12 noted that the HMF partition coefficient (PHMF: a high value indicates an efficient extraction) for an extraction phase, consisting of methylisobutylketone (MIBK) and 2-butanol, decreased from 0.9 to 0.8 when the DMSO content in the reaction phase, consisting of water and DMSO, was increased from 50% to 60%. Thus, when considering all the mentioned effects for the proposed integrated process, water seems to be the most favorable proven reaction solvent. The task at hand therefore reduces to the identification of an extraction solvent for the extractive separation of HMF from an aqueous solution. The previously known solvents for this application have limited extraction efficiency (Table 1). The highest PHMF reported is 2.0 for the solvent MIBK at 120 °C.14 However, already at 180 °C, the PHMF of MIBK decreases to around 1.3,13 The second highest PHMF is reported for 1-butanol with a value of 1.7.3 With these limited extraction solvents, a complete

Table 1. Partition Coefficients of HMF (PHMF) within Biphasic Reaction Systems for Production of HMF from Fructosea Reaction solvent: Extraction solvent H2O:MIBK

H2O:1-butanol H2O:2-butanol H2O:1-hexanol H2O:2-MTHF H2O:diethylene glycole dibutyl ether H2O:toluene:2-butanol (1:1) H2O:DMSO (4:1):MIBK:2-butanol (7:3) H2O:DMSO (1:1):MIBK:2-butanol (7:3) H2O:DMSO (3:7):dichloromethane

PHMF [wt%org/wt%aq]

ref

1 2.0 1.1 1.7 1.6 0.9 not known 0.5−0.6 1.2 1.3 1.4 0.9−1.0 1.2−1.3

13 14 3 3 3 3 15 16 3 17 17 12 12

a

PHMF calculated from data reported in the literature; varying conditions, e.g. cHMF.

removal of HMF from water is only possible when using large amounts of solvent and several extraction steps. To increase the competitiveness of the HMF synthesis, it is desirable to improve the partition coefficient. This can be accomplished by two approaches: (1) Increasing the partition coefficient through addition of additives such as salts and (2) identifying completely new extraction solvents. The addition of an inorganic salt, e.g. NaCl, is widely used in extraction processes. Applying this to the separation of HMF increases the partition coefficient significantly.8,18 However, high salt concentrations lead to other difficulties and costs. The main issues are increased corrosion under reaction conditions, making more expensive equipment necessary; the energy demand for removing the salt from the wastewater; and the cost associated with salt purchase and disposal. Therefore, to avoid these additional challenges and costs, this work aims at improving the partition coefficient in a salt-free reaction system. B


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Figure 2. Flowchart of the plant for determination of partition coefficients at continuous flow conditions and of extraction kinetics.

temperature and concentration. Parameters governing technical feasibility are evaluated in a continuous extraction setup. Based on experimental data, the capability of COSMO-RS for a theoretical solvent selection is studied.

Interestingly, the high concentration of sugar can have a similar favorable impact on partition coefficients as the addition of salts. Wettstein et al.19 have given a proof of concept by publishing partition coefficients for levulinic acid in a biphasic system of water and γ-valerolactone with varying fructose concentrations. These data showed an increase of the levulinic acid partition coefficient from 1.6 to 2.9 and 3.2 when the fructose concentration was increased from 27 to 39 and 53 wt %. For the HMF synthesis, hexoses are the substrate or intermediates (when using cellulose); therefore, this effect has potential synergy with the HMF synthesis. When producing HMF from cellulose in a continuous plant, the hexose concentration needs to be at a level optimal for the extraction. This introduces just one additional parameter into a naturally complex system and will not increase the effort significantly. Until now partition coefficients of HMF under the influence of high sugar concentrations have not been reported; thus, its further investigation is worthwhile and will be done in this work. The final degree of freedom for partition coefficient improvement considered is the choice of extraction solvent. Obviously, efficient strategies to identify novel solvents with suitable partition coefficients are desirable. However, often solvents are identified using heuristic methods. Heuristic approaches are often limited, as they suggest solvents similar to those already known. Moving outside the area covered by heuristics quickly becomes prohibitively expensive, as it is heavily reliant on experimental work. An alternative to the heuristic approach is the utilization of computational methods such as COSMO-RS.20 The advantage of COSMO-RS is that it can predict the partition coefficient for potential solvents. While the predictions are not exact, COSMO-RS can screen several thousand solvents to generate a small set, which can be used as a basis for promising solvents for highly focused experiments. COSMO-RS has been used successfully in many solvent related applications.21−26 However, to the best of our knowledge, COSMO-RS has not yet been used to suggest solvents for the HMF extraction. Therefore, COSMO-RS also has to be validated for this particular application. This publication discusses the possibility to enhance HMF partitioning by additives and investigates the influence of

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MATERIALS AND METHODS

Experimental Determination of Partition Coefficients at Batch Conditions. All chemicals were obtained from commercial suppliers and used without further purification: HMF (AVA Biochem), 1-butanol, 2-butanol, MIBK, D -(−)-fructose, o-propylphenol, o-isopropylphenol (Sigma-Aldrich), 2-MTHF (ABCR). Partition coefficients for ternary and quaternary mixtures were measured in the following way: 10 mL of H2O and solvent were mixed with HMF and fructose. The concentrations of HMF and fructose reported in this work are given as weight percentages: 1 wt% solution of HMF in H2O/MIBK consists of 10 mL H2O, 10 mL MIBK (total weight of solvents 18.02 g), and 180.2 mg HMF. The mixtures were stirred at 200 rpm and 25 °C using a magnetic stirrer and a temperature controlled water bath for 1 h. Separate samples from the aqueous and organic phase were then taken and analyzed by HPLC. All experiments were conducted twice, yielding the experimental error which is presented with the data. All samples were analyzed using a Shimadzu HPLC with a Eurospher 100−5-C18 column (Knauer) and refractive index detector. Solvent: acetonitril:water (1:1), 0.3 mL min−1; 42 bar column pressure at 30 °C for 45 min. No internal standard was used. Partition Coefficient Predictions with COSMO-RS. Developed by Klamt and co-workers,20 COSMO-RS is a predictive thermodynamic model. Its predictability stems from the high degree of physics included in the model, augmented by global parameters, meaning that no system or component specific parameters are needed. Therefore, to predict the partition coefficient (or other mixture thermodynamic properties) of a solvent, only the solvent’s atomic connectivity is required. Based on the connectivity, quantum mechanics are used to obtain the solvent’s optimized geometry and charge distribution. Optimized geometries and charge distributions are available for more than 8000 molecules in the COSMO-RS database. This database was used as basis for the solvent screening. The HMF partition coefficient predictions were based on COSMORS version C30-1501 with the parametrization BP-TZVPD-FINEC30-1501. Here, the database “COSMObase-1501” includes optimized geometries for HMF, water and all potential solvents. Fructose is not present in the database and was therefore implemented using COSMOconfX13. To produce realistic fructose conformers the standard method had to be slightly modified. The standard method C


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ACS Sustainable Chemistry & Engineering generates a large number of possible conformers. For each conformer the gas phase energy is calculated. A conformer is considered relevant, if the absolute difference in gas phase energy between it and the most stable conformer is less than 6 kcal mol−1. This delivered fructose conformers which only had intramolecular hydrogen bonds. To generate conformers without intramolecular hydrogen bonds, i.e feasible in solution, the 6 kcal mol−1 were increased to 30 kcal mol−1. The final COSMO-RS fructose model is found in the Supporting Information. To enable a broad solvent screening, due to computational expense, the HMF partition coefficient of potential solvents were predicted at infinite dilution of HMF (cHMF = 0 wt% → P∞ HMF) in each binary solvent/water liquid−liquid-equilibrium (LLE). Thus, for the solvent screening the following procedure was used: First, binary LLE’s between water and potential solvents from the COSMO-RS database were predicted. If a LLE was found, the partition coefficient at infinite dilution of HMF was predicted. Furthermore, COSMO-RS was also used for calculation of partition coefficients at finite HMF concentrations. Experimental Determination of Partition Coefficients at Continuous Flow Conditions. The used HMF comes from AVA Biochem. 2-MTHF and MIBK are from Sigma-Aldrich. The multiphase continuous stirred tank reactor (CSTR, 100 cm3 excluding internals), which was used for the measurement of the partition coefficients and kinetics of extraction, can be operated at up to 10 MPa and 500 K. The stirrer consists of two spinning baskets on the stirrer shaft. All equipment is made of stainless steel; gaskets are either made of polymers (PTFE, FFKM/FFPM) or metal (gold, copper, stainless steel). Measurement and control of the whole experimental setup is managed via a National Instruments Fieldpoint programmable automation controller and a computer which are both based on the system control software package National Instruments Labview. A flowchart of the plant is shown in Figure 2. The extracting agent is dosed into the reactor by means of a Varian SD-1 high pressure piston pump (P-01). A second pump (P-02) of the same type is used for dosing the aqueous solution. The inlet of the aqueous phase is equipped with a sample loop (10 cm3). This enables a pulsed input of the aqueous solution, which is needed for kinetic experiments. The CSTR has an upper and lower outlet. The latter includes a selfdesigned flow cell with a small detector volume (0.1 cm3) for a Mettler-Toledo ReactIR 4000 FTIR-ATR spectrometer. The inspected range was between the wavenumbers 4000 and 650 cm−1 and the most useful peak of HMF was identified at 1663 cm−1. Through a calibration with solutions of different concentrations, the height of the peak (from a single point baseline at 4000 cm−1) could be related to the concentration. This setup allows the measurement of the HMF concentration in the aqueous phase in real-time. The total reactor effluent flows to the decanter (50 cm3). The position of the liquid− liquid interphase in the decanter is observed through Schott Maxos borosilicate windows. The decanter is necessary to provide the analytical system with settled phases. Samples were taken manually using valves; they were cooled immediately to avoid evaporation. The samples from both phases were analyzed in a HPLC system. Water served as eluent and transported the sample with a flow rate of 1.2 mL min−1 to a C-18 silica gel column and a refractive index detector (both from Knauer). The column is placed in an oven at 50 °C. The phase equilibrium was measured in continuous steady-state experiments in the CSTR. The residence time in the reactor was long enough to reach the liquid−liquid equilibrium. The flow rate of both phases was 4 mL min−1 and the stirrer speed was set to 1000 rpm. Multiple samples were taken during every experiment at the two sampling ports at the outlet of the decanter. They were analyzed using HPLC. Experiments were carried out for three different concentrations of HMF in the aqueous feed (1−5 wt%) and for two different temperatures (25 and 60 °C). Two organic solvents (MIBK and 2-MTHF) were used for extraction. Determination of Extraction Kinetics. Dynamic experiments were conducted in the CSTR of the continuous plant described above in order to determine the kinetics of the extraction of HMF from the

aqueous into the organic phase. The organic solvent was fed to the reactor until the hold-up reaches 50%, then the upper reactor outlet was closed and the pump was stopped. Water was dosed continuously. From the sample injection port at the reactor inlet, 10 mL of an aqueous solution of HMF were injected into the aqueous feed stream. At the reactor outlet of the aqueous phase, the concentration of HMF was measured online by FTIR. It was found that the extraction of HMF proceeds very fast. For this reason, the stirrer speed was set to the lowest value possible (about 50 min−1) and the interphase was assumed to be the same as the cross-sectional area of the reactor. Before conducting the dynamic experiments, the development of the concentration of HMF at the reactor inlet was measured after the content of the sample loop had been injected. This was necessary for the interpretation of the results because an ideal Dirac function could not be realized. The parameters for the dynamic experiments were the same as for the steady-state experiments described above. For analysis and interpretation of the results, it was assumed that the mass transfer resistance is completely located in the film at the organic phase side of the interphase i. The molar flow rate of HMF through the film is described by eq 1:

ntrans = k · A · (corg,i − corg,b) ̇

(1)

Since the partition coefficient has been determined before and the concentrations of HMF in the aqueous bulk phase caq,b and at the aqueous side of the interphase caq,i are equal, the concentration of HMF at the organic side of the interphase is given by eq 2. The partition coefficient Pc was in this case calculated based on concentrations with the unit [mol L−1].

corg,i = Pc · caq,i = Pc ·caq,b

(2)

For the organic phase, the material balance of HMF can be written as

Vorg

dcorg,b dt

= k · A · (Pc ·caq,b − corg,b)

(3)

Separation of the variables and integration leads to the following expression: ln

Pc ·caq,b Pc ·caq,b − corg,b

=

k·A ·t Vorg

(4)

Plotting of the left-hand side of eq 4 versus time gives a straight line where the slope is equal to k·A . Vorg

The concentration of HMF in the aqueous phase caq,b(t) was measured directly during the experiments. The concentration in the organic phase corg,b(t) could be calculated using a material balance for the whole reactor for the period from the beginning of the experiment until the time t:

∫0

t

c in(τ )· V̇ ·dτ =

∫0

t

caq,b(τ )· V̇ ·dτ + corg,b(t )· Vorg + caq,b(t )· Vaq

(5) From this material balance the development of the concentration of HMF in the organic phase could be determined:

corg,b(t ) =

■

1 ( Vorg

∫0

t

c in(τ )· V̇ · dτ −

− caq,b(t )· Vaq)

∫0

t

caq,b(τ )· V̇ ·dτ (6)

RESULTS AND DISCUSSION Agreement between Experimental and Calculated Partition Coefficients and Comparison to Literature Data. To validate the experimental setup as well as the calculations, comparisons with literature data were performed. The experimental data at batch conditions measured in this work (Table 2) are in good agreement with the data reported in the literature (Table 1). D


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ACS Sustainable Chemistry & Engineering Table 2. Experimentally Determined PHMF,batch and Calculated PHMF,COSMO Partition Coefficient of HMFa

Table 3. Experimentally Determined Partition Coefficient of HMF at Continuous Flow Conditions, PHMF,cont.a

cHMF

PHMF,batch

PHMF,COSMO

T

cHMF

PHMF,cont.

Solvent

[wt%]

[wt%org/wt%aq]

[wt%org/wt%aq]d

Solvent

[°C]

[wt%]

[wt%org/wt%aq]

2-MTHF

0 1 5 10 0 1 5 10 0 1 5 10 0 1 5 10

2.0396b 2.019 ± 0.226 1.849 ± 0.103 1.715 ± 0.053 1.8856b 1.879 ± 0.057 1.637 ± 0.000 1.528 ± 0.008

3.15 3.11 3.06 2.89 2.12 2.15 2.17 2.18 2.03 2.02 1.99 1.90 2.97 2.98 2.90 2.01

2-MTHF

25

0.99 2.91 4.76 0.99 2.91 4.76 1.01 3.03 5.01 1.01 3.03 5.01

1-butanol

2-butanol

MIBK

1.642 c c

1.067b 1.060 ± 0.011 0.989 ± 0.000 0.939 ± 0.004

60

MIBK

25

60

a

2.15 1.86 2.22 2.14 2.03 1.99 1.36 1.24 1.21 1.37 1.28 1.29

± ± ± ± ± ± ± ± ± ± ± ±

0.10 0.15 0.12 0.22 0.02 0.11 0.00 0.00 0.01 0.02 0.02 0.03

Error determined by experimental reproduction.

Table 4. Mass Transfer Coefficients of the Extraction of HMF at Different Temperatures

a

Error determined by experimental reproduction. bInfinite dilution; value is extrapolated linearly with HMF concentration to cHMF = 0. c One phasic system. d Error estimated from COSMO-RS: log10P ± 0.3.27

T

cHMF

k

Solvent

[°C]

[wt%]

[1 × 10−3 m s−1]

2-MTHF

25

2.92 4.78 2.92 4.78 3.02 5.00 3.02 5.00

0.0067 0.0451 0.0124 0.0928 0.1349 0.1117 0.1055 0.1206

60

Our experiments and Román-Leshkov et al.3 agree that 1butanol and 2-butanol are superior to MIBK. Additionally, Shimanouchi et al.13 report a partition coefficient similar to the one measured in this work of around 1 for MIBK. Furthermore, the ranking of the selected previously known solvents shows that 2-MTHF has the highest partition coefficient for HMF between 1.7 and 2.0 depending on the HMF concentration. Moreover, 1-butanol and 2-butanol are very similar to values between 1.5 and 1.9. For MIBK partition coefficients only around 1 could be determined. The agreement between the predicted and experimental partition coefficients is mostly within the expected COSMO-RS error of log10P ± 0.327 (Table 2). This error estimate stems from a standard deviation in the chemical potential difference between a component in the two liquid phases of 0.4 kcal mol−1.27 For 2-butanol COSMO-RS predicts the critical point of the LLE at the wrong concentration, thus predicting a phase split where none is observed experimentally. Nevertheless COSMO-RS is capable of predicting the right HMF partition coefficient trend with increasing HMF concentrations, validating a potential predictive solvent selection by COSMORS for the system in question. The two solvents 2-MTHF and MIBK have been investigated under continuous flow conditions. The data recorded at steady state listed in Table 3 show good agreement to the batch experiments. Both data sets show that the concentration of HMF in the aqueous feed stream has only little effect on the partition coefficients in the considered range. Temperature variations in the continuous setup show that also this has only very small influence on the partition coefficients. Determination of the Extraction Kinetics. The results of the dynamic experiments for the determination of mass transfer coefficients are shown in Table 4. The accuracy of the results for a concentration of 1 wt% is poor because the concentrations were apparently too small to enable exact measurement. For dynamic experiments with ca. 3 wt% HMF the accuracy is still not reliable. However, from the data at 5 wt% it can be seen

MIBK

25 60

that the mass transfer of HMF from the aqueous to the organic phase is faster for MIBK than for 2-MTHF. Fructose as Agent for Enhancing the Extraction Capacity and Inducing Phase Separation. As discussed in the Introduction a high sugar concentration can lead to increased partition coefficients. Table 5 summarizes experimentally determined partition coefficients at varying fructose concentrations. Despite the study presented by Wettstein et al.,19 the effect of high sugar concentration has not been discussed in literature before for the extraction assisted HMF synthesis. Here we present these data for the first time: With the addition of 10, 30, or 50 wt% of fructose, as a representative C6 sugar, the partition coefficient increases by up to 40% to 50% for 2-MTHF, 1-butanol and 2-butanol. Interestingly, the addition of fructose to MIBK increases PHMF only insignificantly. It becomes clear that the addition of fructose does not have the same effect on the HMF partition coefficients of all solvents, rather the effect is solvent specific. Another important effect of the fructose addition is an “anti solvent” effect. Fructose addition leads to a phase split for the solvent 2butanol where previously none was observed. Thus, fructose counteracts the co miscibility effect of HMF on the liquid liquid equilibrium (LLE) of water and 2-butanol. COSMO-RS Based Solvent Identification. The ultimate goal is certainly not only a comparison of experimental HMF partition coefficients (PHMF) and those predicted by COSMORS. Instead, it needs to be confirmed that COSMO-RS is capable of enabling a theoretical solvent screening and consequently can be used to identify superior extraction solvents. First, it has been confirmed that COSMO-RS is able E


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solvents and water were found. Here, 900 solvents were identified to have a higher HMF partition coefficient than the solvents known from the literature (Table 1). A further manual selection, with respect to availability at commercial suppliers, led to an identification of 110 possible improved solvents. These new solvents are plotted in Figure 3 and the entire solvent list can be found in the Supporting Information. A clear improvement in both HMF extraction capability and fructose retention can be observed when comparing the known solvents (triangles in Figure 3) and the novel solvents (squares in Figure 3). If the goal is to select an optimal solvent for the extraction only, then one of the red solvents should be selected as these and their subsequent neighbors are the ones with the maximal improvement in the HMF partition coefficients, while minimizing the fructose partition coefficient. Furthermore, it can be seen that short-chain-alkylated phenols have the highest HMF partition coefficients. Alkylphenols have already been reported to be promising extraction solvents for biobased products like levulinic acid and HMF.28−30 In particular o-sec-butylphenol, which is also one of the solvents identified by CORMO-RS (Figure 3, red), has been suggested for the extraction of HMF, however only in conjunction with the addition of salt. It can clearly be seen in Figure 3 that the partition coefficients of HMF and of fructose constitute a tradeoff: If the goal is to maximize HMF extraction it is inevitable to also extract fructose. Thus, two solvents were selected for experimental verification, which constitute a reasonable tradeoff between HMF extraction and fructose retention and are available at a reasonable price: o-propylphenol and o-isopropylphenol. The experiments show that the two solvents, o-propylphenol and o-isopropylphenol, have HMF partition coefficients that are significantly higher than those known in literature for salt free

Table 5. Experimentally Determined Partition Coefficient of HMF in the Presence of Fructose, PHMF,batcha cfructose

PHMF,batch

Solvent

[wt%]

[wt%org/wt%aq]

2-MTHF

0 10 30 50b 0 10 30 50b 0 10 30 50b 0 10 30 50b

1-butanol

2-butanol

MIBK

1.849 2.006 2.512 2.746 1.637 1.794 2.074 2.250 1.694 2.219 2.512 0.989 1.081 1.321 1.434

± ± ± ± ± ± ± ± c ± ± ± ± ± ± ±

0.103 0.000 0.011 0.039 0.000 0.000 0.022 0.046 0.000 0.038 0.016 0.000 0.000 0.001 0.041

a

cHMF = 5 wt%; error determined by experimental reproduction. Fructose not completely solved ⇒ saturated system with cfructose < 50 wt%. cOne phasic system. b

to describe the ternary system solvent−water-HMF (Table 2). Second, the effect of fructose addition was demonstrated experimentally: Fructose addition, as shown in Table 5, does not change the ranking of the solvents significantly, compared to the ranking without fructose. Thus, COSMO-RS can be used as a valuable tool to support the experimental investigations through a broad theoretical solvent screening. By the screening procedure described in the experimental section, about 6000 liquid−liquid equilibria between potential

Figure 3. Plotted screening results for the 110 feasible solvents. Each point represents a solvent, the triangles are the solvents used for verification of COSMO-RS, while the squares are the solvents identified through COSMO-RS to show improved extraction performance. The solvents marked red, are the solvents which embody the trade-off between HMF and fructose partition coefficient identified. o-sec-Butylphenol has earlier been reported by Pagan-Torres as extraction solvent for HMF in the presence of NaCl.28 The partition coefficients are defined so that a large partition coefficient for HMF is favorable, while the opposite is true for the partition coefficient of fructose. Thus, the optimum is toward the top left corner. F



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CONCLUSIONS In order to increase the efficiency of the HMF synthesis, we performed a systematic identification of solvents optimal for HMF extraction using the predictive thermodynamic model COSMO-RS. To the best of our knowledge, this is the first time a systematic solvent selection for the HMF extraction has been performed. Although the HMF extraction is widely investigated in literature and several solvents are already known, COSMO-RS was still able to suggest novel solvents. This highlights the potential of COSMO-RS as a solvent selection tool. The list of solvents published in this work can be used as a starting point for other solvent selection tasks, where HMF extraction is of importance. A method for the determination of partition coefficients at steady state conditions with the use of a continuously operated laboratory plant and kinetic data was developed. The two investigated solvents MIBK and 2-MTHF have different benefits: the transfer of HMF from the water phase to the organic phase is faster with MIBK, but a higher loading can be achieved with 2-MTHF. This work has also shown that large amounts of fructose can be used, not only to increase the HMF partition coefficient, but also to improve the phase settling between water and organic solvents. Several solvents such as o-propylphenol and o-isopropylphenol were identified possessing up to five times higher partitioning coefficients compared to the previously applied solvent 2-MTHF. These solvents have the potential to significantly improve the HMF synthesis.

Table 6. Experimentally Determined Partition Coefficient of HMF PHMF,batch with and without the Addition of Fructosea Solvent o-propylphenol

o-isopropylphenol

cHMF

cfructose

PHMF,batch

PHMF,COSMO‑RS

[wt%]

[wt%]

[wt%org/wt%aq]

[wt%org/wt%aq]

0 1 5 10 5 5 5 0 1 5 10 5 5 5

0 0 0 0 10 30 50b 0 0 0 0 10 30 50b

11.468

10.02 9.47

a a

10.039 12.551 8.876 11.971 10.142

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9.82 9.29

a a

14.358 11.269

a

Stable emulsion instead of clear organic phase. bFructose not completely solved ⇒ saturated system with cfructose < 50 wt%.

wt% HMF wt% HMF systems: P1o‑propylphenol = 11.468 and P1o‑isopropylphenol = 11.971 (Table 6). Here, COSMO-RS predicts infinite dilution partition ∞ coefficient to be P∞ o‑propylphenol = 10.02, and Po‑isopropylphenol = 9.82, which is within the expected error of COSMO-RS. For both solvents, o-propylphenol and o-isopropylphenol, the velocity of phase separation decreases with increasing HMF concentration. Above 5 or 10 wt% HMF, the system forms a stable emulsion, making extraction impossible. Obviously, HMF is a surface-active compound and reduces the surface tension. The addition of large amounts of fructose solves these settling problems and enables the use of o-propylphenol and o-isopropylphenol at high HMF concentrations. For o-propylphenol this happens already at 10 wt% fructose, while for oisopropylphenol it starts at 30 wt%. Moreover, for both solvents it can be seen that the HMF partition coefficient decreases when increasing the fructose concentration from 30 to 50 wt% leading to a less efficient extraction. Here probably fructose and HMF compete in the extraction and saturated systems (at cfructose = 50 wt% not all fructose was dissolved) favor fructose more than the unsaturated ones. Despite this competition, the HMF partition coefficient for o-propylphenol and o-isopropylphenol is still superior to previously known solvents. The optimum fructose concentration is dependent on the resulting overall process and its parameters, and thus would have to be determined by further investigations. Nevertheless, factors such as emulsion formation or energy efficient product isolation from the extracting solvent will also play a major role for technical feasibility. Overall, the observed partition coefficients for o-propylphenol and o-isopropylphenol appear highly promising, justifying further elaboration of a continuous HMF extraction. Additionally, constraints such as a certain fructose concentration to enable sufficient phase separation under steady state operation conditions became evident. Future studies will address the technical feasibility of reactive extraction systems for HMF synthesis with promising solvent candidates predicted by COSMO-RS. Additional factors like phase separation, kinetics and prize will be taken into account in order to obtain a more differentiated selection from the suggested solvents. Such a selection will only be possible, if the follow-up process with known requirements is specified.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b01036. Complete list of screened solvents as well as the final COSMO-RS fructose model. (ZIP)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support of the Hans Bö ckler Foundation is acknowledged. This work was performed as part of the Cluster of Excellence ”Tailor-Made Fuels from Biomass” funded by the Excellence Initiative by the German federal and state governments to promote science and research at German universities. This research was also performed as part of the DFG project KU 853/7-1. We acknowledge Jan Scheffzyck for valuable discussions.

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ABBREVIATIONS 2-MTHF 2-methyltetrahydrofuran A interphase; cross-sectional area of the reactor b bulk c concentration cont. continuous COSMO-RS conductor-like screening model for real solvents CSTR continuous stirred-tank reactor G


Research Article


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DMSO dimethylsufoxide FFKM/FFPM perfluoroelastomer FTIR(-ATR) Fourier transform infrared spectroscopy (attenuated total reflection) HMF 5-hydroxymethylfurfural i interphase IL ionic liquid IR infrared k mass transfer coefficient LLE liquid−liquid equilibrium MIBK methyl-iso-butylketone ṅ molar flow rate P partition coefficient PTFE polytetrafluorethylen t time T temperature τ time V volume V̇ volumetric flow rate wt% weight percentage

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