Alcohol Effect and the Related Mechanism on Fructose Dehydration

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The Alcohol Effect and the Related Mechanism on the Fructose Dehydration into 5-Hydroxymethylfurfural in the Deep Eutectic Solvent of [Emim]Cl/Alcohol Jing Zhang, Yuyan Xiao, Yaohua Zhong, Na Du, and Xirong Huang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00882 • Publication Date (Web): 13 Jun 2016 Downloaded from http://pubs.acs.org on June 20, 2016

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ACS Sustainable Chemistry & Engineering

The Alcohol Effect and the Related Mechanism on the Fructose Dehydration into 5-Hydroxymethylfurfural in the Deep Eutectic Solvent of [Emim]Cl/Alcohol Jing Zhang,†& Yuyan Xiao,†& Yaohua Zhong,‡ Na Du,† and Xirong Huang*† †

Key Lab for Colloid and Interface Chemistry of the Education Ministry of China, Shandong

University, Jinan 250100, China. ‡

State Key Laboratory of Microbial Technology of China, Shandong University, Jinan 250100,

China &

These authors contributed equally to this work and should be considered co-first authors.

*Corresponding author:

e-mail: [email protected]

ABSTRACT The ionic liquid [HNMP]Cl-catalyzed dehydration of fructose into 5-hydroxymethylfurfural (HMF) in deep eutectic solvents (formed by [Emim]Cl and different alcohols) were investigated. The experiment results indicated that the polarity of an alcohol and its stereo-structure were major factors influencing the fructose dehydration and that isopropanol was the optimum alcohol for the conversion system. Studies on the mechanism of the alcohol effect indicated that an alcohol could influence the formation of intermediates and their further transformation via the

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hydrogen bonds formed with [Emim]Cl and [HNMP]Cl. For the normal alcohols, the shorter the chain, the higher the polarity, and the stronger the ability to form the hydrogen bond; as a result, the alcohol with the shortest chain has the greatest negative effect on the fructose dehydration. For the branched chain alcohols, the existence of steric hindrance led to their weaker ability to form the hydrogen bond with the ionic liquid, so that their negative effects on the conversion were much smaller. The DES composed of equal moles of [Emim]Cl and isopropanol is the best one for the conversion of fructose into HMF catalyzed by [HNMP]Cl with an HMF yield of up to 89% after 3 h reaction at 25 °C. Keywords: fructose, 5-hydroxymethylfurfural, alcohol, hydrogen bond, steric hindrance

INTRODUCTION It is urgent to develop and utilize renewable biomass due to the speedy exhaustion of fossil resources

and

the

concomitant

deterioration

of

ecological

environment.1-6

5-Hydroxymethylfurfural (HMF) is a new important biomass-based platform chemical, from which many high value-added chemicals can be derived.7-12 Preparing HMF via hexose dehydration is the basis for the development and utilization of biomass. So much effort has been devoted to the conversion of hexose into HMF. The previous studies mostly focus on the development of new catalysts and the related catalysis mechanism, and relatively, less attention has been paid to the selection of media. Actually, a medium plays a very important role in organic reactions; it could affect the reaction rate and the equilibrium, and even the reaction mechanism.13-17


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As far as the dehydration of fructose into HMF is concerned, the most commonly used media are water, organic solvents (e.g. dimethylsulphoxide (DMSO)), or water/organic solvent mixtures (e.g. water/DMSO).6,10,18-20 Water is a green solvent, but it is not an optimal one for the preparation of HMF. In aqueous media and under high temperature condition, HMF can be easily hydrolyzed into levulinic acid and formic acid, which leads to the decrease of the HMF yield. DMSO, N, N-dimethylformamide (DMF), acetone, etc. are the commonly used organic solvents. These organic solvents are able to dissolve more sugars, and also to inhibit the side reactions via dilution of the water formed during the dehydration. In these aprotic organic solvents up to 90% HMF yield could be achieved. The major shortcomings of these media are their high volatility and their costly separation from the product. Alcohols can be regarded to some extent as a kind of green solvents.21 These cheap solvents usually have low boiling points and can be easily recycled. There have been many reports on the dehydration of sugar to prepare HMF22-28 or its derivative (its etherified product)29-36 with an alcohol as solvent or co-solvent. The results have shown that the HMF yield and selectivity varies with alcohols tested.23 However, the related mechanism has not been fully understood. Compared with molecular organic solvents, ionic liquids (ILs) could be considered as green media. ILs have good thermal stability and low vapor pressure, and their properties can be easily designed.37-41 The commonly used ILs for fructose dehydration are mainly Lewis acid ILs (especially the imidazolium salts with Cl−). These ILs not only have high sugar solubilities, but also can inhibit the polycondensation of HMF.4,42-44 Apparently, the use IL as medium is too costly, however, this cost can be compensated to some extent by its recycling.7,45,46 Protic acids



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are reported to be able to catalyze the dehydration of fructose into HMF, so the combination of Cl− (to dissolve sugars) and the protonated cation of the Brønsted acid type (to catalyze fructose dehydration)45-51 such as N-methyl-2-pyrrolidonium chloride ([HNMP]Cl) may be better in economic benefit. Unfortunately, these ILs used for fructose dehydration are generally solid at room temperature. To lower the melting points of the ILs and realize the room-temperature dehydration of fructose, the deep eutectic solvents (DESs) formed by an IL and a hydrogen-bond donor (e.g. carboxylic acids, amides, alcohols, etc.) in a certain stoichiometric ratio have been tried in recent years as the reaction media.52-55 Compared with an IL, DES is lower in melting point and cheaper in price. The use of DESs as media for the dehydration of fructose into HMF has been reported.56 As the process of fructose dehydration into HMF involves hydrogen bond, especially under low temperature condition, the alcohol used to form DES should have had impacts on the formation of HMF. This expectation has been supported by our preliminary experiments. Here, we report the alcohol effect and its related mechanism on the [HNMP]Cl-catalyzed dehydration of fructose into HMF in liquid DES systems (formed by 1-ethyl-3-methylimidazolium chloride ([Emim]Cl) and different alcohols) at room temperature.

EXPERIMENTAL SECTION Materials. [Emim]Cl and [HNMP]Cl were purchased from Shanghai Chengjie Chemicals Co. Ltd., China. Fructose, methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, 2-butanol, pyrene and acetonitrile were purchased from Sinopharm Chemical Reagent Co. Ltd., China. All reagents excluding acetonitrile (HPLC grade) were of analytical grade and used


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without further purification. Triply distilled water was used throughout the experiments. Typical procedure for fructose dehydration into HMF. All of the dehydration reactions were conducted in a 5 mL glass ware with stopper. The typical procedure for fructose dehydration into HMF was as follows. First, 5.72 mmol [Emim]Cl and 0.572 mmol [HNMP]Cl were dissolved in 10 mmol alcohol, then 0.278 mmol fructose was added and the resulting solution was stirred (600 rpm) at 25 °C. After a certain period of time, a certain amount of the sample was taken out and diluted with ultrapure water. HPLC analysis. HMF separation and detection was carried out on an HPLC apparatus equipped with a Shim-pak VP-ODS C18 column (250 × 4.6 mm), a Shimadzu LC-20AT pump, and a Shimadzu SPD-20A detector. The mobile phase was acetonitrile-water mixture (1/9 v/v). The flowing rate was 1.0 mL min-1. The detection wavelength was set at 280 nm. The actual concentration of HMF was obtained directly from the calibration curve (correlation coefficient is 0.999). All data given in the report were an average of triplicate experiments. Viscosity measurement. The viscosity of a sample was measured on a Thermo Scientific MARS III rheometer at 25 °C. Polarity measurement. The polarity of a sample was determined using pyrene (2 × 10-7 M) as fluorescent probe. An F-320 fluorescence spectrophotometer was used for fluorescence spectrum recording. The record conditions were as follow: excitation wavelength was fixed at 335 nm, excitation slit width at 10 nm, emission slit width at 2.5 nm, scanning rate at 240 nm min-1 and emission wavelength range from 355 to 450 nm. The fluorescence intensity ratio of the first and the third vibronic peaks (I1/I3) of pyrene determined the polarity of a sample.57-59 The



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larger the ratio, the stronger the polarity. The monitoring of the dehydration process. The fructose dehydration at 25 °C was monitored using 1H NMR technique on a Bruker Avance 400 MHz spectrometer. The typical procedure was as follow: 0.278 mmol fructose, 2.86 mmol [Emim]Cl and 0.572 mmol [HNMP]Cl were dissolved in 10 mmol alcohol, and the resulting mixture was transferred into a 5 mm (i.d.) NMR tube. A D2O encapsulated capillary (as external standard) was put into the tube, and then the tube was quickly inserted into the chamber of the spectrometer, followed by recording the 1H NMR spectra (rd=2 s, NS=16) of the reaction system at fixed time intervals.

RESULTS AND DISCUSSION Effects of alcohols on the fructose dehydration. The dehydration of fructose into HMF catalyzed by [HNMP]Cl in the deep eutectic solvent (DES) of [Emim]Cl/alcohol was investigated at room temperature (25 ºC). Among the alcohols tested, isopropanol is the best; after 1 h reaction, the HMF yield in isopropanol system is as high as 73.93% (Figure 1). As far as the four normal alcohols (methanol, ethanol, n-propanol and n-butanol) are concerned, the HMF yields increase with the increase of the alkyl chain length. For branched chain alcohols, e.g., isopropanol, isobutanol and 2-butanol, their DESs lead to higher HMF yields than those formed by normal alcohols. These results indicate that different alcohols have different effects on the dehydration of fructose, which probably results from the viscosity and polarity of the DES media or the spatial structure of the alcohols.


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Figure 1. Comparison of the HMF yields in different DES systems. Reaction conditions: 5.72 mmol [Emim]Cl, 0.572 mmol [HNMP]Cl and 0.278 mmol fructose were dissolved in 10 mmol alcohol, and the resulting mixture was then stirred at 25 ºC for 1 h.

Table 1. The viscosities and polarities of [Emim]Cl/alcohol DESs a Entry

Alcohols

Viscosity b / mPa s

Polarity (I1 / I3)c

HMF yield / %

1

methanol

6.27

1.860

44.67

2

ethanol

13.78

1.737

51.79

3

n-propanol

12.19

1.696

55.42

4

n-butanol

14.45

1.635

57.14

5

isopropanol

26.68

1.689

73.93

6

isobutanol

18.49

1.653

60.27

7

2-butanol

20.64

1.655

70.24

a

DESs were composed of 5.72 mmol [Emim]Cl and 10 mmol alcohol.

b

The viscosities of the DESs were measured using a rheometer.

c

The polarities of the DESs were measured using pyrene as fluorescent probe.



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The action mechanism of alcohols. Generally, a reaction could be affected by the viscosity and polarity of the medium. To clarify the action mechanism of alcohols on the dehydration of fructose and screen out the best matched alcohol, the viscosities and the polarities of the DESs formed by different alcohols were measured and listed together with the corresponding HMF yields.

Table 2. The effect of the stirring speed on the dehydration of fructose into HMF in DES HMF yield / % Stirring speed / rpm methanol

isopropanol

0

44.21

69.80

600

44.67

73.93

1000

44.95

74.01

Reaction conditions: 5.72 mmol [Emim]Cl, 0.572 mmol [HNMP]Cl and 0.278 mmol fructose were dissolved in 10 mmol alcohol, and the resulting mixture was then stirred at different speed at 25 ºC for 1 h.

The effect of viscosity. At first glance, the viscosities of DESs are seemingly correlated with HMF yields (see Table 1), but on closer examination it is found that the viscosity has little influence on the HMF yield. Table 1 shows that the isopropanol system is much more viscous than the methanol system, but the former system results in a much higher HMF yield than the later one, indicating that under present conditions, the HMF yield is not determined by the viscosity of the DES systems. For further demonstration, we studied the effect of the stirring


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speed on the yield of HMF. As shown in Table 2, the HMF yield in the methanol system is scarcely affected by the stirring speed (tested from 0 to 1000 rpm); even in isopropanol system, the stirring speed only has a limited effect on the HMF yield. It follows that under present conditions the viscosity is not the key factor influencing the dehydration of fructose. The effect of polarity. Table 1 shows a correlation between the polarity of the DES system and the HMF yield, but the relationship is more complex. For normal alcohol systems (Entries 1-4), the HMF yield increases with the decrease of the polarity; i.e., strong polarity is not conducive to the dehydration of fructose into HMF. In the case of branched chain alcohol systems (Entries 5-7), the polarities of the isobutanol and 2-butanol systems are almost the same, but the HMF yields in the two systems are quite different. These results imply that the polarity of an alcohol and its stereo-structure are major factors influencing the dehydration of fructose. This may be the reason why the isopropanol system results in highest HMF yield among the three branched alcohol systems. We speculate that in the DESs composed of alcohols with different spatial structure, the difference in the HMF yields is likely due to the involvement of an alcohol in the formation and transformation of intermediates.



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Scheme 1. Basic route for the dehydration of fructose into HMF.

Effects on intermediates. It has been shown that there are two intermediates (1 and 2, see Scheme 1) during the dehydration of fructose into HMF.56 To verify the above speculation, we monitored the characteristic proton signals of the intermediates 1 and 2 during the dehydration of fructose in different DES systems using in situ 1H NMR. As shown in Figure 2, the characteristic proton signals of HMF (labelled by ●), which appear in all tested systems (methanol, ethanol, n-propanol and 2-butanol), increase with the increase of the reaction time. In addition to the proton signals of HMF, some other proton signals (labelled by I and II) in the range of 5.0 ~ 7.0 ppm are also observed in some spectra. The intensities of these signals vary with time, showing a gradual increase or a maximum. Based on our earlier research, I and II can be separately assigned to the characteristic proton signals of the intermediates 1 and 2 formed during the fructose dehydration (see Scheme 1).56 Their time-dependent changes in intensity should be correlated with the structure and property of alcohols. Among the alcohols tested, the methanol


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containing system has the highest proton signals of the intermediates (5.35 ppm and 6.27 ppm), and moreover, the signal intensities (almost the same) increase continuously within the test time (72 min). In ethanol and n-propanol systems, however, the signal intensities of the intermediates are very low (one of the signals in n-propanol system is interfered (overlapped) by the strong signal of [HNMP]Cl at 5.75 ppm and thus difficult to identify), and moreover, they first increase and then decrease. Unlike the normal alcohols, the proton signals of the intermediates in isopropanol and 2-butanol systems are not distinct. After spectra magnification, a weak signal at 5.25 ppm (peak I) that increases first and then decreases is observed in 2-butanol system, while in isopropanol system, the proton signals of the intermediates are still invisible (HMF has been generated). The above phenomena indicate that alcohols have significant effects on the formation and transformation of the intermediates during the dehydration of fructose. For quantitive comparison of the alcohol effect on the intermediates, the HMF yields in all of the systems have been checked. It is found that the yield decreases in the following order: isopropanol > 2-butanol > n-propanol > ethanol > methanol. Based on the above results, it can be concluded that among the alcohol systems tested, the methanol system has the lowest rate of the intermediate transformation, and therefore the highest intermediate accumulation; the ethanol, n-propanol and 2-butanol systems, have a moderate rate of the intermediate transformation and therefore a moderate level of the intermediate; in isopropanol system, the intermediate transformation rate is so fast that no intermediate is accumulated.



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Figure 2. The in situ 1H NMR spectra during the fructose dehydration in different DESs. Reaction conditions: 0.278 mmol fructose, 2.86 mmol [Emim]Cl and 0.572 mmol [HNMP]Cl were added into 10 mmol alcohol.

Based on our previous research, it is suggested that the above alcohol effects result from the


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hydrogen bonds formed between the alcohol and the intermediate. As shown in Scheme 2, the alcohol (marked in red) can serve as a hydrogen bond donor to form hydrogen bonds with [HNMP]+ and/or Cl- (marked in black in Scheme 2a and 2c), leading to a negative effect on the fructose or intermediates dehydration. On the other hand, the alcohol can act as an acceptor to form hydrogen bonds with [HNMP]+ (2b) or [Emim]+ (2d), which weakens the hydrogen bond interaction between the ILs and fructose, and therefore makes the dehydration of fructose or intermediates difficult. It follows that the stronger the ability of an alcohol to form a hydrogen bond, the greater the negative effect would be.

Scheme 2. Schematic diagram showing the hydrogen bond formed between an alcohol and intermediates/ILs ([Emim]Cl and [HNMP]Cl) during the fructose dehydration.

For normal alcohols, the shorter the chain, the higher the polarity, and the stronger the ability to form a hydrogen bond; as a result, the alcohol with the shortest chain has the greatest



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negative effect on the fructose dehydration. Compared with the normal alcohols, the branched chain alcohols have a low ability to form hydrogen bonds due to their steric hindrance, therefore, their negative effect on the fructose dehydration is relatively weak. Under the same conditions, the HMF yields in branched chain alcohols are higher than that in normal alcohols.23,60 For branched chain alcohols, the isopropanol system has a better performance in fructose dehydration than the 2-butanol and isobutanol systems. The fact that different branched chain alcohols result in different negative effects could be explained as follows. Compared with 2-butanol, isopropanol has a high polarity although both have similar structures (both are secondary alcohols); similarly, isobutanol is a primary alcohol, and its steric hindrance is relatively small compared with other branched chain alcohols, so its ability to form a hydrogen bond is higher than isopropanol, which leads to a stronger negative effect on the fructose dehydration. As shown in our previous study, the sugar dissolving IL [Bmim]Cl has a promotion effect on the [HNMP][CH3SO3]-catalyzed dehydration of fructose into HMF; [HNMP][CH3SO3] mainly promotes the formation of intermediate 1, while [Bmim]Cl promotes the transformation of intermediates 1 and 2. To further clarify the promotion mechanism of [Emim]Cl in the present DES system, we studied the kinetics of the fructose dehydration process without [Emim]Cl in both methanol and isopropanol using 1H NMR. As shown in Figure 3, the heights of the proton signals of fructose (4.0 ~ 4.5 ppm) in the methanol system without [Emim]Cl change little, and the characteristic proton signals of HMF cannot be observed even the spectrum is magnified (the HPLC detection also shows that almost no HMF generated in this system within 72 min). In the


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magnified spectrum, however, the proton signals from the two intermediates can be observed. Compared with the methanol system containing [Emim]Cl (see Figure 2), the presence of [Emim]Cl makes the proton signal I (5.06 ppm) of intermediate 1 much higher than the proton signal II (6.88 ppm) of intermediate 2, indicating that without [Emim]Cl the transformation of intermediate 1 into intermediate 2 is very slow, and the intermediate 1 accumulates. In isopropanol system without [Emim]Cl, the heights of the proton signals of fructose decrease significantly and the characteristic proton signals of HMF can be clearly observed; more importantly, the proton signal I (4.83 ppm) of intermediate 1 can be seen, which was difficult to be observed in [Emim]Cl containing isopropanol system (see Figure 2). The above results indicate that in the isopropanol system without [Emim]Cl, the further transformation of intermediate 1 is very difficult. Compared with methanol, isopropanol, due to its lower hydrogen bond formation ability, is highly efficient for fructose dehydration into HMF.



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Figure 3. 1H NMR spectra during the dehydration of fructose into HMF without [Emim]Cl in both methanol and isopropanol systems. Reaction conditions: 0.278 mmol fructose, 0.572 mmol [HNMP]Cl and 10 mmol alcohol.

The effect of [Emim]Cl/isopropanol molar ratio on the fructose dehydration. As both alcohol and [Emim]Cl in DES have not only significant but also opposite effects on the [HNMP]Cl-catalyzed dehydration of fructose into HMF, it is necessary to study the effect of the [Emim]Cl/alcohol molar ratio on the dehydration of fructose into HMF.

As shown in Figure 4,

at a given time, the HMF yield increases first and then decreases with the increase of isopropanol in [Emim]Cl/isopropanol DES system, indicating that the change of the medium composition alters the reaction rate. After 3 h reaction, the HMF yields in the systems of 1 : 0.25 and 1 : 0.5 have leveled off, and those in the systems of 1 : 1, 1 : 2 and 1 : 4 have a tendency to approach an platform, but the heights of these platforms are quite different (with the increase of isopropanol they increase first and then decrease). The above results indicate that the change of the medium composition could alter both the reaction rate and the equilibrium. Among the 5 tested systems,


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the system of 1 : 1 has the fastest reaction rate and the highest HMF yield. The viscosity and/or the polarity of the DES used should be responsible for these differences (the corresponding viscosity and polarity data are listed in Table 3). With the increase of the isopropanol ratio in the DES, both the viscosity and the polarity of the DES decrease. For the DES systems of 1 : 0.25 and 1 : 0.5, both the viscosity and the polarity are very high. Based on the fact that the HMF yields in the two systems are significantly lower than that in the DES system of 1 : 1, we speculate that the viscosity may be the key factor influencing the reaction. To verify this conjecture, we studied the effect of stirring on the HMF yield. The results show that when the stirring speed increases from 0 rpm to 1000 rpm, the HMF yields from the 1 : 0.25 and the 1 : 0.5 systems increase from 35.6% and 56.0% to 54.8% and 73.3%, respectively. The above results demonstrate that at low level of isopropanol the viscosity of DES systems is the key factor affecting the yield of HMF. At high level of isopropanol, however, this is not the case. In the 1 : 1, the 1 : 2 and the 1 : 4 systems, the isopropanol content is high, the HMF yields are found to be inversely proportional to the isopropanol content. As the viscosities of these systems are relatively low, the viscosity effect could be neglected (as demonstrated earlier). It follows that at high level of isopropanol the polarity of the system as well as the concentration of [Emim]Cl (especially in the 1 : 4 system) are key factors affecting the reaction. Now it can be concluded that among the DES systems of [Emim]Cl/isopropanol, only the 1 : 1 system, whose viscosity, polarity and [Emim]Cl concentration are moderate, gives the best performance for the dehydration of fructose into HMF.



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Figure 4. The reaction kinetics of the dehydration of fructose into HMF in DESs composed of different molar ratios of [Emim]Cl to isopropanol. Reaction conditions: 5.72 mmol [Emim]Cl was first dissolved in a given amount of isopropanol, followed by adding 0.278 mmol fructose and 50 mol% [HNMP]Cl. The resulting mixture was stirred (600 rpm) at 25 °C.

Table 3. The viscosity and polarity of [Emim]Cl/isopropanol DES systema,b Molar ratio of Viscosity / mPa s

Polarity (I1 / I3)

HMF yieldc / %

1 : 0.25

293.40

1.898

62.08

1 : 0.5

143.44

1.862

77.75

1:1

73.09

1.749

88.85

1:2

18.12

1.627

71.97

1:4

1.37

1.566

52.54

[Emim]Cl/isopropanol

a

DESs were consisted of 5.72 mmol [Emim]Cl and different molar quantity of isopropanol.

b

The determination methods of viscosity and polarity were the same as Table 1.

c

The HMF yield after 3 h reaction.


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CONCLUSIONS The alcohol effect and the related mechanism on the [HNMP]Cl-catalyzed dehydration of fructose into HMF were studied in the DES system of [Emim]Cl/alcohol. The isopropanol system gives the best performance for the dehydration reaction. The polarity of an alcohol and its stereostructure are the major factors influencing the fructose dehydration. Via hydrogen bonds, an alcohol is able to affect the formation and transformation rates of the intermediates. Due to the existence of steric hindrance, the branched chain alcohols have a better performance than the normal alcohols. The DES composed of equal moles of [Emim]Cl and isopropanol is the best one for the conversion of fructose into HMF catalyzed by [HNMP]Cl at room temperature.

ACKNOWLEDGMENT We are grateful for the financial support from the Fundamental Research Funds of Shandong University (2015CJ005), the Provincial Key R & D Project of Shandong (2014BSE27153 and 2015GSF121035) and the National Natural Science Foundation of China (20973103 and 21173133).

REFERENCES (1) Mascal, M.; Nikitin, E. B. Direct, high-yield conversion of cellulose into biofuel. Angew. Chem. Int. Ed. 2008, 47, 7924. (2) Binder, J. B.; Raines, R. T. Simple chemical transformation of lignocellulosic biomass into furans for fuels and chemicals. J. Am. Chem. Soc. 2009, 131, 1979.



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For Table of Contents Use Only

The Alcohol Effect and the Related Mechanism on the Fructose Dehydration into 5-Hydroxymethylfurfural in the Deep Eutectic Solvent of [Emim]Cl/Alcohol Jing Zhang,† Yuyan Xiao,† Yaohua Zhong,‡ Na Du,† and Xirong Huang*†

Synopsis We investigated the alcohol effect and the related mechanism on the [HNMP]Cl-catalyzed dehydration of fructose into HMF in the deep eutectic solvent system of [Emim]Cl/alcohol.


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Alcohol Effect and the Related Mechanism on Fructose Dehydration

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