ethanol with hydrogen sulfate ionic liquid additives and a Lewis acid ... for 5-EMF synthesis in the ethanol − 1-butyl-3-methylimidazolium hydrogen ...
Jun 22, 2018 - (1â8) Routes to 5-EMF via glucose would be preferable for 5-EMF .... 5-EMF yields, and 5-EMF yields had a maximum of 3% in standard deviation. .... One-pot, two-step process: Sn-BEA as a Lewis acid was used from 0 to 6 h, and ... (44
Jun 22, 2018 - Hydrogen sulfate ionic liquid additives with aluminum chloride catalyst in ethanol were found to promote efficient (30 min) one-pot, one-step ...
imidazolium hydrogen sulfate have been reported to afford 55 % and 54 ...... ([BMIM][HSO4]), mixtures at xIL (0: pure ionic liquid; 1: pure glucose; 2: xIL=0.4; 3:.
Jun 22, 2018 - Catalytic Conversion of Biomass-Derived Carbohydrates to Methyl Lactate by ... Abstract | Full Text HTML | PDF w/ Links | Hi-Res PDF.
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Mechanism of glucose conversion into 5-ethoxymethylfurfural in ethanol with hydrogen sulfate ionic liquid additives and a Lewis acid catalyst Haixin Guo, Alif Duereh, Yuya Hiraga, Xinhua Qi, and Richard Lee Smith Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 22 Jun 2018 Downloaded from http://pubs.acs.org on June 22, 2018
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Mechanism of glucose conversion into 5-ethoxymethylfurfural in ethanol with hydrogen sulfate ionic liquid additives and a Lewis acid catalyst Haixin Guo,*a Alif Duereh,c Yuya Hiraga,c Xinhua Qi, b and Richard Lee Smith Jr*a,c
a
Graduate School of Environmental Studies, Tohoku University, Aramaki Aza Aoba
6-6-11, Aoba, Sendai 980-8579, Japan. b
Agro-Environmental Protection Institute, Chinese Academy of Agricultural Sciences,
No. 31, Fukang Road, Nankai District, Tianjin 300191, China. c
Research Center of Supercritical Fluid Technology, Tohoku University, Aramaki Aza
Abstract Hydrogen sulfate ionic liquid additives with aluminum chloride catalyst in ethanol were found to promote efficient (30 min) one-pot, one-step transformation of glucose into 5-ethoxymethylfurfural (5-EMF) in 37 % yields. Spectroscopic measurements (FT-IR, 1H-NMR) showed that ionic liquids form multiple hydrogen bonds with glucose and promote its ring-opening through ionic liquid - AlCl3 complexes to enable formation of 5-EMF via 5-hydroxymethylfurfural (5-HMF). Reactions performed in dimethyl sulfoxide using (protic, aprotic) ionic liquid additives with and without AlCl3 catalyst showed that both the ionic liquid and AlCl3 were required for efficient transformation of glucose into 5-EMF. The proposed reaction mechanism for 5-EMF synthesis in the ethanol − 1-butyl-3-methylimidazolium hydrogen sulfate − AlCl3 reaction system consists of ring-opening of glucose to form the 1,2-enediol and dehydration to form 5-HMF that is followed by etherification to the 5-EMF product.
The
reaction system is effective for glucose transformation and has application to biomass-related compounds.
5-hydroxymethylfurfural (5-HMF) in ethanol or from fructose in ethanol using homogeneous and heterogeneous catalysts ranging from organic acids, metal chlorides, modified solid substrates and acidic ionic liquids (ILs) or with additives such as DMSO or THF as co-solvents.1-8
Routes to
5-EMF via glucose would be preferable for 5-EMF production, since glucose is the monomer of cellulose, however, a critical step for using glucose as a feedstock is its isomerization into fructose.3, 9
Catalytic systems that have been studied for glucose conversion into 5-EMF show
that low 5-EMF yields are obtained and require reaction times on the order of tens of hours.4, 10-14 The low efficiency of glucose transformation reported in many studies is probably related to the high stability of the glucopyranose ring structure of glucose.9, 14
For the case of 5-EMF, yields as
high as 46 % have been achieved at 96 ºC in 24 h reaction time with H-form zeolites and Amberlyst-15 heterogeneous catalysts in a one-pot, two-step reaction system.
4
For a one-step
reaction system, the maximum 5-EMF yield that has been obtained is 38 % at 100 ºC in 11 h reaction time.13
Chemically, the isomerization of glucose into fructose may be promoted with
either acid or base catalysts, while the steps of fructose dehydration and 5-HMF etherification with ethanol require an acid catalyst. The transformation of biomass (e.g. fructose, glucose) into 5-EMF in ethanol and dimethyl sulfoxide (DMSO) or in catalytic solvents such as ionic liquids has been previously studied.15-17 Syntheses of 5-EMF from fructose and inulin carbohydrates in ethanol with functional IL additives have been reported6,
18
that show hydrogen sulfate IL additives are effective for 3
promoting the formation of 5-EMF. Since glucose and cellulose are insoluble in many organic solvents, ionic liquids are recognized as being useful additives to improve substrate solubility and moreover, the ionic liquid additives can also act as catalysts.15-17, 19-22
For example, conversion
of glucose in DMSO and 1-hydroxyethyl-3-methylimidazolium tetrafluoroborate mixture, a 67 % yield of 5-hydroxymethylfurfural has been reported.23 Thus, the synthesis of furan derivatives from glucose can be enhanced when certain ionic liquids or additives are employed in the reaction system.24,
25
Imidazolium cation chloride-anion ionic liquids have outstanding capacity to
dissolve carbohydrates, whereas tetrafluoroborate-anion and hexafluorophosphate-anion ionic liquids are effective in reaction systems for promoting the formation of furan-related compounds.26,
combined with metal chlorides provide effective reaction systems for conversion of glucose into 5-hydroxymethylfurfural (5-HMF).28, 29 Ionic liquids having a hydrogen sulfate anion are able to promote dehydration of biomass-related compounds because of their stronger acidity compared with
other
types
of
anions.30-34
1-butyl-3-(3-sulfopropyl)-imidazolium
Two
hydrogen
bifunctional sulfate
and
group
ionic
liquids,
1-methyl-3-(3-sulfopropyl)-
imidazolium hydrogen sulfate have been reported to afford 55 % and 54 % yields of 5-EMF, respectively, from the substrate fructose.35
However, bifunctional group ionic liquids are not
widely used due to their cost and availability.
On the other hand, hydrogen sulfate ionic liquid
additives in ethanol have been found to promote conversion of carbohydrates efficiently into 5-EMF18 and they are readily available, although the yields obtained for 5-EMF are lower for carbohydrates containing glucosyl moieties. In this work, a study was made on the glucose transformation steps that occur in ethanol with 4
ionic liquid additives in the presence of a homogeneous Lewis acid catalyst (AlCl3). Factors affecting glucose transformation into 5-EMF in ethanol are identified and the dipolar aprotic solvent DMSO is used in some experiments to probe molecular interactions in the reaction system. Reaction system conditions are optimized for 5-EMF yields to allow discussion of the contribution of different factors on the reaction mechanism. The role of hydrogen sulfate ionic liquids as additives to ethanol for synthesizing 5-EMF from glucose was studied by performing comparative experiments with aprotic imidazolium-cation ionic liquid additives having chloride- or phosphateanions. Molecular interactions between functional ionic liquid additives and glucose (AlCl3) were elucidated with spectroscopic techniques (infrared and 1H-NMR) to allow a mechanism to be proposed for the transformation of carbohydrates containing glucosyl moieties into 5-EMF. 2. Experimental 2.1. Materials The solvent ethanol (99.5 %), the substrate, D-glucose (98 %), the Lewis acid homogeneous catalysts, aluminum chloride (98.0 %), iron (III) chloride hexahydrate (99.0 %) and copper (II) chloride were obtained from Wako Pure Chemical Industries Ltd (Japan).
The
5-hydroxymethylfurfural (99 %), 5-ethoxymethylfurfural (99.9 %), levulinic acid (98 %), chloroform-d (99.96 %) and tin (IV) chloride (98 %) were purchased from Sigma-Aldrich (Japan) and were used as received.
The chloride based ionic liquids 1-butyl-3-methylimidazolium
chloride ([BMIM]Cl, 95 %), 1-hexyl-3-methylimidazolium chloride ([hmim]Cl, 95 %) and -SO3H functional ionic liquid 1-butyl-3-methylimidazolium hydrogen sulfate ([BMIM][HSO4], 95 %), 1-ethyl-3-methylimidazolium hydrogen sulfate ([EMIM][HSO4], 95 %), 1-methylimidazolium hydrogen sulfate ([HMIM][HSO4], 95 %) was obtained from Sigma-Aldrich. The ionic liquid, 5
1-ethyl-3-methylimidazolium methyl phosphate ([EMIM][MP], 98 %) was supplied by Merck (Japan) and the 1-butyl-3-methylimidazolium dimethylphosphate ([BMIM][DMP], 98 %) ionic liquid was supplied by Iolitec (Japan). The bifunctional group ionic liquid 1-butyl-sulfonic acid-3-methylimidazolium chloride ([C4SO3Hmim]Cl) ionic liquid was supplied by Henan Lihua Pharmaceutical Co., Ltd (China). The dimethyl sulfoxide-d6 (99.9 %) was supplied by Kanto Chemical Co., Inc. (Tokyo) and contained 0.03 % (v/v) tetramethylsilane (TMS) as internal standard. 2.2. Conversion of glucose into 5-EMF Typical experiments were performed as follows. Glucose substrate, 0.09 g (0.5 mmol) and 0.04 g of AlCl3•6H2O catalyst were loaded into a custom-designed stainless-steel autoclave reaction vessel at room temperature. Then, the solvent (ethanol or DMSO or ethanol/DMSO or ethanol/ionic liquid or ethanol/ionic liquid/DMSO) were added into the reaction vessel. The reaction vessel was placed in an oil bath that was maintained at 130 °C and it was continuously stirred at 500 rpm with a magnetic stirrer. After a given period of time, the reaction vessel was cooled down with ice and the product solution was diluted with ultra-pure water, filtered and analyzed with HPLC. 2.3. Analyses All samples in this work were analyzed with HPLC that had a refractive index detector and an SH 1011 column. The temperature of the column oven was maintained at 60 ºC, and sulfuric acid solution (0.5 mmol/L) was used as the mobile phase (1 mL/min). The product peak was well-separated and it had a much longer retention time (e.g. 34.5 min) than the starting materials, intermediates or by-products (Fig. S1a, b). Glucose conversion (X) and fructose (Y), 5-HMF (Z) 6
and 5-EMF (Z) yields were determined as shown in Equations 1 to 4, respectively.
moles of glucose in product X = 1 − × 100 % total moles of starting glucose
(1)
Y=
moles of fructose in product × 100 % total moles of starting glucose
(2)
Z=
moles of 5-HMF in product × 100 % total moles of starting glucose
(3)
E=
moles of 5-EMF in product × 100 % total moles of starting glucose
(4)
All results were replicated for a minimum of two trials, and the reproducibility of glucose conversion, fructose yields, 5-EMF yields and 5-EMF yields had a maximum of 3 % in standard deviation. 2.4. ATR-FTIR spectroscopy Attenuated total reflectance (ATR)-FTIR was used to study interactions of glucose with the ionic liquids.
All infrared spectra of the ionic liquid-glucose mixture were measured at room
temperature (FT-IR-6300/ATR-PRO-450-S, Jasco). The ATR-FTIR were recorded at (600 to 4000) cm-1 with a resolution of 0.07 cm-1 using a 45° Ge crystal. 2.5. 1H-NMR measurements Detailed liquid-state analyses were performed with 1H nuclear magnetic resonance (1H-NMR) and a DRX-500 spectrometer (Bruker). The sample and DMSO-d6 containing 0.03 % TMS internal standard were added to a 5 mm NMR tube and then 1H-NMR spectra were taken at regular time intervals. 3. Results and discussion 3.1. Conversion of glucose into 5-HMF in DMSO 7
Results for conversion of glucose into fructose and into 5-HMF with AlCl3 catalyst and hydrogen sulfate ionic liquid additives in DMSO are shown in Figures 1a-c.
Without the
additive ionic liquid (Fig. 1a), the reaction system with AlCl3 gave a maximum 5-HMF yield of about 19 %. The maximum yield of 5-HMF increased (ca. 34 %), when the functional hydrogen sulfate ionic liquid was used as additive at the same reaction conditions (Fig. 1a). Fructose yields increased (Fig. 1b) when ionic liquid additives were used compared with experiments that used only AlCl3 (blank, Fig. 1b). To illustrate the Brønsted-acid catalytic effect of the SO3H group of additive IL for promoting 5-HMF formation from glucose substrate, results with only the ionic liquid additive and without the AlCl3 catalyst are shown in Figures 1d-f. The 5-HMF yields obtained from glucose catalyst with the [C4SO3Hmin]Cl ionic liquid (Fig. 1d) had maximum 5-HMF yields of 30 % with fructose yields being about 10 % (Fig. 1e) and results for 5-HMF were comparable with thosed obtained for the hydrogen sulfate IL-AlCl3 mixture reaction systems (Fig. 1a). The [BMIM][HSO4] ionic liquid additive containing the AlCl3 catalyst promoted glucose transformation (Figs. 1d-f, dashed line) into 5-HMF well compared with cases without the ionic liquid additive. The transformation of glucose into 5-HMF with [BMIM][HSO4] ionic liquid additive was ineffective without AlCl3 catalyst (Fig. 1d). Although [C4SO3Hmim]Cl ionic liquid additive was effective for glucose conversion (Fig. 1f), the [C4SO3Hmim]Cl ionic liquid is not widely used due to the many steps required for its synthesis. Thus, mixtures of AlCl3 with the hydrogen sulfate ILs are considered to be convenient and to have the potential to be widely used for glucose conversion. For example, acidic ionic liquids are effective for cellulose and lignocellulosic biomass conversion,21 and when they are combined with metal chlorides, they allow efficient 8
cellulose depolymerization,36 aldose isomerization37 and lignocellulosic biomass transformation into furans.38
Comparisons and details related to the reaction mechanism are discussed in the
next sections. 3.2. Ionic liquid additives with aluminum chloride catalyst Results for the transformation of glucose into 5-HMF and 5-EMF in ethanol with hydrogen sulfate ILs additives and aluminium chloride catalyst along with yields obtained for heterogeneous catalysts (Lys/PW, Glu-Fe3O4-SO3H, MIL-101-SO3H, DeAl-H-beta, H-USY (6) and Amberlyst-15) are shown in Table 1.3, 4, 11, 12, 14
Conversion of glucose into 5-HMF and 5-EMF with Lewis acid
catalyst aluminum chloride in the absence of additives (Entries 1-3, Table 1 and Fig. S2) was less efficient than when hydrogen sulfate ionic liquid additives were used (Entries 4-6, Table 1 and Fig. S2) regardless of the variation in reaction conditions. The higher efficiency of reaction systems with hydrogen sulfate ionic liquid additives (Entries 4-6, Table 1) compared with those with only ethanol (Entries 1-3, Table 1) can be attributed in some part to solvation effects of the ethanol-ionic liquid solvent mixture (basicity, dipolarity/polarizability, acidity) that allows conformational changes in glucose to occur due to preferred solvent polarity.
In experiments
performed at 25 ºC, it was found that glucose solubility increased with increasing concentration of ionic liquid additive (Section A, supporting information). Therefore, one factor in the reaction efficiencies (Entries 1-6, Table 1) can be attributed to substrate solvation and its conformation in the reaction solvent. To demonstrate that the SO3H functional group acts as a Brønsted-acid catalyst, experiments with glucose substrate were performed using aprotic ionic liquid additives (Entries 7-11, Table 1) that can dissolve glucose in substantial amounts.39 9
obtained with the hydrogen sulfate ionic liquid additives (Entries 4-6, Table 1) were higher than those obtained from the aprotic ionic liquid additives (Entries 7-11, Table 1) showing that both glucose solvation effects (conformation) and catalytic effects are important in the reaction. The 5-EMF yields were lower than 18 % for glucose substrate when other Lewis acid catalysts (e.g. FeCl3 and CuCl2) (Entries 17-18, Table 1 and Entries 1-2, Table S3).40, 41
Dutta et al. reported
conversion of lignocellulosic biomass and cellulose into 5-EMF and ethyl levulinate (9:1) with 24 % yields with Lewis acids Zr(O)Cl2/CrCl3 in one-pot [BMIM]Cl - ethanol reaction systems (Entry 19, Table 1).42
Some studies show that when functional ionic liquids are used, 5-HMF can
be efficiently prepared from biomass and that the choice of anions or cations in the ionic liquids can improve catalyst activity or selectivity under similar reaction conditions.37-40, 43 The metal chloride used in this work (AlCl3) with the acidic ionic liquids exhibited higher catalytic activity than those reported in previous studies so that enhanced conversion of glucose and glucose-containing biomass into 5-EMF is likely for the reaction system identified in this work. The mechanism of the functional ionic liquids for promoting conversion of glucose into 5-HMF and into 5-EMF is shown in Sections 3.4 and 3.5. The effect of the dipolar aprotic solvent DMSO additive on reaction efficiency was studied next. Addition of DMSO into the reaction system (Entries 12-16, Table 1) caused a dramatic increase in the required reaction time to obtain relatively low yields of 5-EMF. The DMSO solvent additive has been studied with heterogeneous catalysts (Entries 21-22, Table 1)3, 11 and these also require long reaction times.
On the other hand, high 5-EMF yields have been obtained
without addition of DMSO with heterogeneous catalysts (Entries 24-26, Table 1).4, 14
Wang et
al.5 reported that the ratio of EtOH/DMSO greatly affected 5-EMF yields from fructose substrate, 10
whereas a maximum 5-EMF yield of 64 % was achieved at an EtOH/DMSO ratio of 3/7 (v/v). However, in this work, the ratio of EtOH/DMSO did not cause large variations in the 5-EMF yields from glucose substrate (Entries 12-16, Table 1). Addition of DMSO into the reaction systems studied in this work (Entries 12-16, Table 1) results to reduction 5-EMF yields that is possibly related to a decrease in the acidity of the reaction system as shown in Section 3.3. Water is necessary for isomerization of glucose to fructose, but excess amount of water could lead to the formation of hexaaquo complex with AlCl3 that act as Brønsted acids.44
In this
solvent system (ethanol – ionic liquid), almost no change in the yield of product was observed when around 1 wt% water was added, implying a negligible role in the in situ formed hexaaquo complex during the reaction.
3.3. Effect of DMSO solvent on 5-EMF formation As shown in Sections 3.1 and 3.2, DMSO inhibited 5-EMF formation in ethanol reaction systems with ionic liquid additives. ATR-IR analyses were used to investigate the interactions of DMSO or 5-HMF with ethanol (Fig. 2). When ethanol was added to pure DMSO or pure 5-HMF, the νOH of ethanol-DMSO and ethanol-5-HMF mixtures changed to lower wavenumbers (red shift) relative to pure DMSO and to pure 5-HMF due to interactions of ethanol with DMSO and with 5-HMF (Fig. 2a). With addition of ethanol, the ∆νOH of the ethanol-DMSO mixture (Fig. 2b) exhibited a larger red shift than that of the ethanol-5-HMF mixtures (Fig. 2b), demonstrating that the hydrogen bonds between DMSO and ethanol were stronger that those of 5-HMF and ethanol. The pH of the ethanol – SO3H ionic liquid mixtures was lower (ca. 0.2) than that of ethanol- SO3H ionic liquid -DMSO mixtures (ca. 11
4.0), which means that addition of DMSO results in less acidic solutions (Section B, supporting information). It can be hypothesized that the inhibition of the formation of 5-EMF by DMSO in the reaction system is due to the preference that ethanol has for forming hydrogen bonds with DMSO rather than with 5-HMF. In other words, ethanol prefers to interact with polar aprotic solvent DMSO than with intermediate 5-HMF and this probably inhibits 5-HMF etherification. Results of 1H NMR analyses of [BMIM][HSO4] (Fig. S3) show that the chemical shift at ~9.58 ppm belongs to H9 when using CDCl3 as reference, while the invisible hyperfine structure was at ~9.58 ppm when using DMSO-d6 as reference. It is possible that the DMSO changes the solution structure of the hydrogen sulfate ionic liquid. The hydrogen bond coordination of DMSO and hydrogen sulfate SO3H group of the ILs probably causes depolarization of the H9 group of the ionic liquid, so that compared with H2, the H9 shows a larger polarization.45 Therefore, when DMSO additive is used, it hinders glucose conversion because the hydrogen sulfate anion of the functional IL interacts with DMSO which reduces the acidity of the reaction system.
There are many literature reports3,46,47 on fructose conversion using heterogeneous
catalysts demonstrating that the addition of DMSO can promote 5-EMF yield. However, the results in this study imply that for fructose substrates, DMSO mainly has the role of enhancing substrate solvation due to its high solvent polarity and when it is used with hydrogen sulfate ionic liquid additives, DMSO essentially deactivates the catalytic properties of the ionic liquid. 3.4. Interaction of ionic liquids with glucose and with AlCl3 To study the mechanism of the functional ILs for promoting transformation of glucose into 5-HMF and 5-EMF, spectroscopic measurements of hydrogen sulfate ionic liquid-glucose 12
mixtures were made with FTIR-ATR (Fig. 3 and Fig. S4) and 1H NMR (Fig. 4). IR spectral peaks of pure glucose (Fig. S4) had broad and medium bands for CH asymmetric vibrations at wavenumbers around 2920 cm-1, strong CH bending vibrations at around 1450 cm-1 and a band of CO and CC stretching at wavenumbers around 1035 cm-1, 48, 49 while [BMIM][HSO4] ionic liquid had strong ν(CH) stretching at 3107 cm-1 and SOH bending at 1160 cm-1.50 Figure 3 and Fig. S4 show how the IR spectral shifts changed with ionic liquid mole fraction. With increasing ionic liquid mole fraction, both ν(CH) stretching (Fig. 3a and Fig. S4a) and SOH bending (Fig. 3b and Fig. S4b) absorption bands exhibited a red shift for the ionic liquid. When the ionic liquid mole fraction was high (xIL > 0.7), ν(CH) stretching exhibited a red shift. The largest changes in wavenumber occurred in the mole fraction range of (0 < xIL < 0.7), for which the ν(CH) absorption band was red-shifted. When ionic liquid was added to glucose, the CH bending (Fig. 3c) and CO and νC-C stretching (Fig. 3d) of glucose showed a blue shift. In the [BMIM][HSO4]-glucose solvent system, the formation of hydrogen bonds probably weakened the vibration of CH-stretching (Fig. 3c) and CO stretching (Fig. 3d), which promoted ring-opening of glucose. The trends can be attributed to molecular interactions between glucose and [BMIM][HSO4] via hydrogen bonding between the CH group (C2 hydrogen, Fig. 3a and Fig. S4) of the ionic liquid and the carbonyl (C=O) group of glucose (Fig. 3d),51 that promotes glucose ring-opening.30, 31 The 1H-NMR spectra of pure ionic liquid, pure glucose, and [BMIM][HSO4]-glucose mixtures are shown in Figure 4a. Chemical shifts at C2 of the ionic liquid additive during the dilution process were evaluated (Fig. 4b). The CH of the ionic liquid imidazolium ring (Fig. 4b) exhibited a downfield shift typical of hydrogen bond interactions with increasing ionic liquid mole 13
fraction that is due to the influence of interaction cooperativity between functional ionic liquid and glucose.52 The 1H-NMR results for CH of the ionic liquid imidazolium ring (Fig. 4b) were in accordance with the IR results (Fig. 3). Theoretical studies reported that complex formation of ionic liquids and metal chloride catalysts promote glucose conversion.44 Interactions of AlCl3 with [BMIM][HSO4] ionic liquid were measured with ATR-FTIR (Fig. S5). Upon addition of AlCl3 to ionic liquid [BMIM][HSO4], the CH bending exhibited a blue shift (Fig. S5), which is evidence that a complex exists between the Lewis acid AlCl3 and ionic liquid.
The [BMIM][HSO4]-metal chloride forms complexes probably through hydrogen
bonding between the hydroxyl group in the ionic liquid or the chloride anions in the Lewis acid that promotes transformation of α-anomer into β-anomer group in the glucose, which would enable glucose ring-opening to form ketoses and the 1,2-enediol.44 Hydrogen sulfate ILs additives effectively promote transformation of glucose into 5-EMF and 5-HMF, and act not only as catalyst but also as co-solvent for glucose conversion (Scheme 1), which illustrates the probable conversion routes of glucose into 5-HMF and 5-EMF with AlCl3 catalyst in the ethanol – [BMIM][HSO4] mixtures reaction system.
The ν(CH) of the
imidazolium ring and the anion SO3H group forms an effective H-bond with the CH and carbonyl (CO) group of glucose to stabilize the glucose and to promote ring-opening in which the ionic liquid−AlCl3 catalyst complex promotes glucose transformation into 5-HMF and 5-EMF. 3.5. Mechanism for conversion of glucose into 5-HMF Spectroscopic results given in the previous section can be used to formulate a mechanism for 14
glucose conversion into 5-HMF in the ethanol, hydrogen sulfate IL additive and Lewis acid AlCl3 catalyst reaction system (Scheme 1). The transformation of glucose into 5-HMF is related to the following factors (step 1, Scheme 1): (i) the H atoms in -SO3H anion group and imidazolium cation (at C2) of the functional ILs act as proton donors through Brønsted-acid theory and promote glycosidic bonds in glucose rupture consistent with spectral results, (ii) interactions between functional ionic liquids with glucose via hydroxyl (OH) functional groups which probably promotes cleavage of the glycosidic bonds and facilitate 5-HMF formation,31, 51, 53 (iii) ionic liquid−AlCl3 complexes promote cleavage of glycosidic bonds. The proposed reaction pathway for 5-EMF formation from glucose substrate contains three mains steps (Scheme 1) as follows: the first step is the ring-opening reaction for glucose to form the 1,2-enediol, the second step is the dehydration reaction of the 1,2-enediol to form 5-HMF and the next step is etherification reaction of 5-HMF to 5-EMF. In the first step (Scheme 1), the ionic liquid probably promotes proton transfer and facilitates mutarotation of α-glucose by the metal chlorides AlCl3 via [AlCl3(HSO4)n]n- complexes. The ionic liquid-metal chloride complexes promote transformation of α-glucose into β-glucose through the chloride anions and the hydroxyl groups hydrogen bonds, while interactions between the hydrogen sulfate anion of the ionic liquid and the hydroxyl groups of glucose leads to ring-opening of glucose to give the 1,2-enediol form. In the second step (Scheme 1), the 1,2-enediol undergoes conversion into 5-HMF by dehydration via two possible pathways: pathway I shows glucose ring-opening and isomerization into fructose that is followed by dehydration of fructose to 5-HMF. Pathway II shows direct conversion of 1,2-enediol into 5-HMF. Subsequently, in the third step, the 5-HMF undergoes etherification with ethanol to form 5-EMF (Scheme 1). 15
4. Conclusions Hydrogen sulfate ionic liquid additives in ethanol solvent with AlCl3 catalyst provide efficient and direct transformation of glucose into 5-ethoxymethylfurfural (5-EMF) in one-step. Both hydrogen sulfate ionic liquid additive and the AlCl3 catalyst increase reaction efficiency and promote glucose conversion. hydrogen
sulfate
Of the ionic liquids studied, 1-butyl-3-methylimidazolium
([BMIM][HSO4])
provided
ethanol-[BMIM][HSO4]-AlCl3 reaction system.
the
highest
yields of
5-EMF
in
the
Hydrogen sulfate ILs additives in ethanol
solvent enhance glucose solvation and facilitate conformational changes and they form complexes with AlCl3 to promote efficient ring-opening of glucose and further, they stabilize the formed 5-HMF intermediate to allow selective etherification.
The ethanol-[BMIM][HSO4]-AlCl3
reaction system is effective for transforming glucose into 5-EMF via 5-HMF and has application to other carbohydrate systems. Acknowledgements The authors would like to acknowledge JSPS Grant-in-Aid for Scientific Research (B), contract No.25289272 (Japan) and JSPS Grant-in-Aid Scientific Research (B) No.16H04549 (Japan) for financial support of this research. The authors declare no competing financial interest. ORCID IDs Haixin Guo Alif Duereh Yuya Hiraga Xinhua Qi Richard Lee Smith Jr
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Figure and scheme captions Figure 1. Yields of 5-HMF and fructose from glucose substrate in ionic liquid reaction systems (a-c) with AlCl3 and (d-f) without AlCl3. ionic
liquid
((a)-(c))
system
are
( )
AlCl3-hydrogen sulfate
[HMIM][HSO4]-
AlCl3,
( )
[EMIM][HSO4]-AlCl3, ( ) [BMIM][HSO4]-AlCl3 and blank (no ionic liquid): (○) AlCl3.
Reaction systems without AlCl3 (d-f)) are ( ) [C4SO3Hmim]Cl, ( )
[BMIM][HSO4].
Dashed line for reference: [BMIM][HSO4]-AlCl3.
Figure 2. Plot of infrared absorption spectral shifts for (a) OH stretching (νOH) and (b) relative OH stretching (∆νOH) for ( ) ethanol−DMSO mixtures and for ( ) ethanol-5-HMF mixtures versus ethanol mole fraction at 25 °C.
Relative OH
stretching value ∆νOH is defined as ν-ν0, where ν0 is the OH stretching value of pure ethanol.
Figure 3. Wavenumber shifts in ionic liquid [BMIM][HSO4]–glucose mixtures with various mole fractions of ionic liquid. and (b) SOH bending of anion. stretching bands.
Ionic liquid: (a) CH stretching at C2 of cation
Glucose: (c) CH bending and (d) CO and CC
Experimental FTIR-ATR spectra taken at room temperature (ca.
24 ºC).
Figure 4. Plot of (a) 1H-NMR spectra of pure glucose, hydrogen sulfate ionic liquid ([BMIM][HSO4]), mixtures at xIL (0: pure ionic liquid; 1: pure glucose; 2: xIL=0.4; 3: xIL=0.5; 4: xIL=0.6; 5: xIL=0.7; 6: xIL=0.8; 7: xIL=0.9; 8: xIL=0.94; 9: xIL=0.97) and (b) chemical shift at C2 of ionic liquid with various mole fractions of ionic liquid (xIL).
Scheme 1. Proposed mechanism for 5-EMF formation from glucose catalyzed by 24
Figure 1. Yields of 5-HMF and fructose from glucose substrate in ionic liquid reaction systems (a-c) with AlCl3 and (d-f) without AlCl3.
AlCl3-hydrogen sulfate ionic liquid ((a)-(c)) system are
( ) [HMIM][HSO4]- AlCl3, ( ) [EMIM][HSO4]-AlCl3, ( ) [BMIM][HSO4]-AlCl3 and blank (no ionic liquid): ( ) AlCl3. Reaction systems without AlCl3 (d-f)) are ( ) [C4SO3Hmim]Cl, ( ) [BMIM][HSO4]. Dashed line for reference: [BMIM][HSO4]-AlCl3. Reaction conditions: 0.09 g of glucose, 0.048 g of AlCl3, 1 g of ionic liquid, 5 ml of DMSO at 130 °C.
Table 1. Conversion of glucose into 5-hydroxymethylfurfural (5-HMF) and 5-ethyoxymethylfurfural (5-EMF) in homogeneous and heterogeneous reaction systems. Entry
