One-Pot Conversion of Corn Starch into 5-Hydroxymethylfurfural in

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One-Pot Conversion of Corn Starch into 5‑Hydroxymethylfurfural in Water-[Bmim]Cl/MIBK Biphasic Media Shrestha Roy Goswami, Agneev Mukherjee, Marie-Josée Dumont,* and Vijaya Raghavan Department of Bioresource Engineering, McGill University, 21111 Lakeshore Road, Ste-Anne de Bellevue, Québec, Canada H9X3V9 S Supporting Information *

ABSTRACT: This work reports the catalytic conversion of corn starch into 5-hydroxymethylfurfural (HMF) in the presence of AlCl3·6H2O in water-[Bmim]Cl/MIBK biphasic media. Among the varieties of corn starch investigated (regular, waxy, and native high amylose), a maximum of 64 wt % HMF was obtained from waxy corn substrates within 20 min of reaction time at 140 °C. The HMF yield was comparable to the yield obtained from fructose and glucose which were of 70 and 68 wt % respectively. Furthermore, MIBK effectively partitioned HMF from the aqueous to the organic phase, thereby preventing further HMF rehydration to levulinic acid (LA). Thus, negligible LA yields (0.1 wt %) were detected in this system. Additionally, [Bmim]Cl and AlCl3·6H2O can be recycled for several catalytic runs without compromising the HMF selectivity. In summary, the proposed process parameters are favorable to achieve high HMF yields from amylopectin-rich starches.

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INTRODUCTION With the rising awareness of finite fossil resources, the synthesis of 5-hydroxymethylfurfural (HMF) from sugar substrates has gained increasing popularity in carbohydrate chemistry since HMF is a versatile platform chemical.1 Among several polysaccharides studied, there has been limited research conducted on the microwave-assisted “one-pot” conversion of starch to HMF.2 The starch granules consist of two major polyglucans, i.e. amylose (AM) and amylopectin (AP), as shown in Scheme 1(A).3 Amylose (Mw 105−106g/mol) is comprised of long linear chains of (1 → 4)-α-D-glucose residues.4 In comparison, amylopectin (Mw 107−109 g/mol) contains much shorter chains of (1 → 4)-α-D-glucose residues heavily branched with (1 → 6)-α-D-linkages at the branch points. Based on the botanical origin of starch, the ratio of AM/ AP varies which significantly influences their functional properties. In general, regular starches contain approximately 70−80% AP and 20−30% AM; the amylose content is less than 10% for waxy starches and greater than 40% for high-amylose starches.4 Amylose exists as a major diluent to amylopectin distributed throughout the amorphous and crystalline domains of the granule along with minor components (protein, lipids, minerals).5 During acid hydrolysis at a temperature below the gelatinization temperature, the amorphous region of starch is reported to hydrolyze more rapidly than the crystalline region.6 The acidic hydrolysis of starch breaks down the glycosidic linkages to produce glucose.7,8 The monosaccharide glucose (aldose) isomerizes to fructose (ketose) which eventually dehydrates to HMF.9 It was further demonstrated that at an early stage of the catalytic depolymerization, amylopectin is preferentially hydrolyzed as amylose tends to form a resistant complex with particles of amylopectin.10 The effect is more pronounced for starches dispersed in ionic liquids containing halide ions that penetrate the amorphous portion of starch to a larger extent in preference to the crystalline zone.11 Also, under the influence of microwave heating, the kinetic energy of the polar and ionic compounds significantly increases, thereby © XXXX American Chemical Society

improving the dissolution of starches in ionic liquids. Such modifications to the physicochemical properties of starch may also influence the kinetics of glucose conversion to HMF. Among several catalysts investigated, a water compatible Lewis acid, i.e. AlCl3, proved to be a better alternative to mineral acids for driving simultaneously the “one-pot” depolymerization of starch to sugars and their subsequent dehydration to HMF.12−14 In the presence of water, metal chlorides such as AlCl3 are in equilibrium with metal hydroxide species Al(OH)x(H2O)y and HCl. It has been shown that the [Al(OH)2(aq)]+ species catalyzes the isomerization of glucose to fructose.15 The in situ formed HCl on the other hand catalyzes fructose dehydration to HMF. Although the catalyst provides high HMF selectivity, reactions in the aqueous phase can cause HMF to become highly unstable.16 In this regard, a polar aprotic solvent, i.e. dimethyl sulfoxide (DMSO), was reported to effectively suppress undesired side reactions of HMF in water.16−19 However, the low efficacy of the downstream separation of HMF from a high boiling point solvent makes the procedure difficult to commercialize. This led to the development of a system that can allow the use of an organic solvent−water biphasic media for an efficient product separation. Interesting HMF yields were reported in different biphasic media.19,20 A solid catalyst (SAPO-44) in water:methylisobutylketone (MIBK) (1:5 v/v) was reported to have attained 68% HMF yields from a starch substrate.21 The high selectivity of HMF achieved was attributed to the unique catalytic properties of SAPO such as hydrophilicity, acid amount, and strong to weak acid site ratio that allowed the “one-pot” conversion of starch to HMF without the additional requirement to produce fructose separately. In a water/ tetrahydrofuran (THF) system, AlCl3·6H2O was reported to efficiently catalyze starch and glucose dehydration to 50 and 61 Received: July 11, 2016 Revised: September 7, 2016

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DOI: 10.1021/acs.energyfuels.6b01699 Energy Fuels XXXX, XXX, XXX−XXX

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Scheme 1. (A) Amylopectin and Amylose Content of Starch Containing D-Glucose and (B) Possible Mechanism of Conversion of Starch to HMFa

a

Reprinted with permission from Roy Goswami, S.; Dumont, M.-J.; Raghavan, V. Microwave Assisted Synthesis of 5-Hydroxymethylfurfural from Starch in AlCl3·6H2O/DMSO/[BMIM]Cl System. Ind. Eng. Chem. Res. 2016, 55(16), 4473−4481. Copyright 2016 American Chemical Society.

mol % HMF yields, respectively, under microwave energy.14 Similarly, the CrCl2/HCl system provided 73 wt % HMF yields from starch in (1-octyl-3-methylimidazolium chloride) [OMIM]Cl-ethyl acetate.22 On most occasions, additional NaCl is required to improve the partitioning efficiency between the two phases.14 The extraction capacity of an organic solvent in a biphasic setup largely relies on its ability to accept protons.23 As such, the greater the H-bonding interactions between the anion of the extraction solvent with the protons in HMF, the greater the extracting efficiency of the solvent.24 Based on the literature, THF is moderately toxic and miscible in water. Furthermore, the instability of ethyl acetate at higher temperature limits its utility as a biphasic solvent.18 In this respect, water immiscible MIBK as an extracting solvent was reported to be favorable for the continuous extraction of HMF from the catalytic phase.19,25,26 However, Lewis acids in the aqueous/MIBK reaction system appeared to deliver low HMF

yields from starch.2 This may be due to the ineffective interplay between the catalyst and substrate which is often hindered, as starch is sparingly soluble in water. In this respect, an ionic liquid such as 1-butyl-3-methylimidazolium chloride ([Bmim]Cl) was reported to significantly solubilize starch providing an efficient medium for chemical reactivity.27,28 [Bmim]Cl could also act as both a proton donor (C2−H) and acceptor (Cl−) in H-bonding interactions with fructose, resulting in the improved and efficient dehydration of fructose to HMF.29 The halide ion of the ionic liquids tends to disrupt the semicrystalline structure of starch by forming H-bonds with the hydroxyl groups of polysaccharides as shown in Scheme 1(B).3 However, the interaction is insufficient to promote mutarotation (i.e., α to β conversion of glucose). Mutarotation leading to equilibrium mixes of anomers was observed to be rapid with metal halides in conjunction with ionic liquids.30 As a consequence, the synergetic effects of Lewis acid catalyst−[Bmim]Cl interactions B


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Figure 1. Effect of catalyst dosage on the conversion of waxy corn starch and glucose to HMF. Reaction conditions: starch (1 wt %, 0.6 mmol glucose equivalent), glucose (0.6 mmol), [Bmim]Cl = 2 wt %, water:MIBK = 1:5 (v/v), temperature (T) = 140 °C, total time (t) = 20 min, (a) = t = 10 min. of the microwave system cooled the reactor vessels gradually to room temperature. The product mixture containing the aqueous phase and MIBK was separated. HMF from the aqueous phase was extracted 5 times with fresh MIBK (10 mL) and pooled together for HPLC analysis. For the total reducing sugar (TRS) and glucose analysis, the aqueous product mixture was diluted with deionized water and centrifuged at 3000 rpm for 30 min to precipitate insoluble substances. The supernatants were decanted and passed through a 0.2-μm syringe filter prior to analysis. To determine the reusability of the catalytic system, MIBK was added to continuously extract HMF from the reaction media. After each reaction, MIBK was separated from the product mixture, and the aqueous phase containing the spent catalyst and ionic liquid was washed five times with 10 mL of MIBK to ensure near complete removal of HMF from the catalytic phase as determined by HPLC. Then, the starch was directly added to the catalytic phase prior to the addition of fresh MIBK. The resulting biphasic mixture was heated for 20 min at 140 °C. 2.3. Analytical Methods. 2.3.1. Determination of HMF. Quantitative analysis of HMF and LA was performed without isolation by HPLC (1260 Infinity Agilent) using a ZORBAX Eclipse Plus C18reverse phased column (100 × 4.6 mm, 3.5 μm) equipped with a UV detector (λHMF = 284 nm, λLA = 250 nm). For the detection and quantification of HMF, a 20:80 (v/v) mix of methanol and water was used as the mobile phase at a flow rate of 0.6 mL/min. The column temperature was maintained at 30 °C. The detection and quantification of LA was carried out using a mobile phase containing a mixture of acetonitrile and water (1:9 v/v, pH 2.0) at a flow rate of 0.2 mL/min and a column temperature of 60 °C. The volume of each injection was 10 μL. HMF and LA were identified in the samples by retention times determined by the use of standards. The concentrations of HMF and LA in the samples were determined from a calibration curve (available in the Supporting Information). All experiments were run in triplicates, and the results were reported as an average. The presence of HMF was confirmed by GC-MS (available in the Supporting Information). The column temperature of the GC-MS (6890N Network GC, 5973 Network MS Detector, Agilent with HP5MS column) was initially held at 40 °C for 5 min, then increased to 260 °C at a rate of 10 °C/min for 22 min, and held at 260 °C for 5 min. The pH value of the H2O−AlCl3·6H2O at 25 °C was measured on a VWR pH meter SB70P, SympHony (±0.01 pH units) calibrated with standard buffer solutions. 2.3.2. Sugar Analysis. The TRS was determined with 1% dinitrosalicylic acid (DNS) reagent according to the Miller technique.36 A 3 mL sample was added to 3 mL of the 1% DNS reagent solution and boiled for 5 min. One mL of a 40% solution of

with different carbohydrates were reported to have improved the catalytic dehydration efficiency of the substrates to HMF.18,31−35 Therefore, the primary objective of this study was to optimize microwave assisted HMF synthesis from corn starch with different amylose to amylopectin ratios. In the process, the influence of AlCl3·6H2O dosage, reaction time and temperature, water content, and the concentration of [Bmim]Cl on the conversion of corn starch in biphasic medium have been discussed. Furthermore, the recyclability of the catalytic system has been studied.

2. MATERIALS AND METHODS 2.1. Materials. Unmodified (regular) corn starch (S4126:27% amylose, 73% amylopectin), waxy corn (S9679:100% amylopectin), and native high amylose corn starch (S4180:70% amylose, 30% amylopectin) were purchased from Sigma-Aldrich (St. Louis, MO). D(+)-Glucose (99.5+%, Sigma), glucose (GO) assay kit (GAGO-20, Sigma-Aldrich), D-fructose (99%, Alfa Aesar, Haverhill MA), methylisobutylketone (MIBK) (99.5+%, Sigma-Aldrich), aluminum chloride hexahydrate (AlCl3·6H2O) (99%, Sigma-Aldrich), and 1butyl-3-methylimidazolium chloride (98+%, Aldrich) were used as purchased without further purification. 5-Hydroxymethylfurfural (99+ %, Sigma-Aldrich) and levulinic acid (LA) (98%, Sigma-Aldrich) were used for the preparation of calibration curves. Deionized water was used for the preparation of the solutions. Unless otherwise stated, waxy corn starch (S9679) was used for the experiments. 2.2. Experimental Procedure. The reactions were carried out in a microwave reactor that operated at a frequency of 2.45 GHz (MiniWAVE Digestion Module, SCP Science, Canada). In a typical experiment, a desired amount of substrate (corn starch), catalyst (AlCl3·6H2O), and an optimum volume of solvent (water-[Bmim]Cl and MIBK) were placed in 75 mL quartz reactor vessels (50 mL working volume) fitted with Teflon caps. The reason for the addition of MIBK to the aqueous phase was to allow continuous extraction of HMF from the catalytic phase to prevent its reaction with water. Prior to placing the samples into the reactor, the samples were sonicated (FS30 Ultrasonic Clear, Fisher Scientific) for 10 min to obtain a uniform suspension of the substrate in the solvent. The reactor was programmed to initiate the reaction at the desired temperature for a pre-established period of time. The sample temperature was monitored with the help of IR sensors located on the sidewalls using a single magnetron located below the floor of the treatment chamber. Once the reaction was completed, the integrated cooling unit C


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3. RESULTS AND DISCUSSION 3.1. Conversion of Starch to HMF. 3.1.1. Effect of Catalyst Dosage (AlCl3·6H2O). The effect of the amount of AlCl3·6H2O used for the conversion of waxy corn starch to HMF is presented in Figure 1. Starch dehydration to HMF is a multistep process, starting with its hydrolysis to oligomers which are further broken down to glucose. Thereafter, the glucose monomers are isomerized to fructose, which are dehydrated to HMF.9 When glucose is used as a starting substrate, there is no depolymerization step. Hence, the yields of HMF (Figure 1) were observed to be higher with glucose than with starch under identical conditions. Therefore, the hydrolysis step can be termed as rate determining in the conversion of starch to HMF over AlCl3·6H2O (see discussion in Section 3.1.2.). The hydrolysis of starch to TRS and glucose dehydration to HMF over different amounts of AlCl3·6H2O were also investigated as shown in Figure 1. It is further noticed that for glucose, HMF yield was low (60.2 wt %) at 0.05 g catalyst loading. On increasing the catalyst loading to 0.1 g and 0.15 g, HMF yields increased to 66.1 and 68 wt %, respectively. Further increase of AlCl3·6H2O to 0.3 g resulted in a decrease in HMF yield (49.7 wt %). This can be attributed to sidereactions which converted HMF to humin.13 Zhang and coworkers recently explored the possibility to reduce humin formation by 50% with maleic acid and AlCl3.38 The HMF yields from the conversion of starch showed a similar trend as glucose. At a low catalyst dosage of 0.05 g, a yield of 44.3 wt % HMF was obtained from starch. Additionally, TRS yields from starch were observed to gradually increase from 23.4 wt % to 56 wt % as the catalyst loading amount increased from 0.05 g to 0.3 g. This result is due to the higher availability of sugar monomers in the reaction mixture leading to higher HMF yields. However, HMF yields reached a maximum of 64 wt % (TRS 42 wt %) with 0.15 g of catalyst. At a catalyst loading amount greater than 0.15 g, substantial humin production reduced HMF yields. Hence, the optimal catalyst loading amount was found to be 0.15 g, which is equivalent to a loading ratio of 1.5 g catalyst/g starch. 3.1.2. Effect of Reaction Time on the Conversion of Starch. For each of the starch types, namely regular corn starch, waxy corn starch, and high amylose corn starch, yields of TRS, glucose, and HMF varied with time (Figure 2). For regular corn starch, TRS was formed within the first 2 min of reaction time, but no glucose or HMF was detectable. Under identical conditions, a similar trend was observed for both waxy and high amylose corn starch; however, for high amylose starch, TRS yields were relatively low. Warrand and Janssen (2007)

Figure 2. Time profile for the conversion of starch. Reaction conditions: starch (1 wt %, 0.6 mmol glucose equivalent), AlCl3· 6H2O = 0.15 g, [Bmim]Cl = 2 wt %, water:MIBK = 1:5 (v/v), temperature (T) = 140 °C (A) regular corn starch, (B) waxy corn starch, and (C) high amylose corn starch.

reported similar behavior for acidic hydrolysis of amylose starch.39 Hence, the observation led to the conclusion that in the first 2 min, starch was being hydrolyzed to oligosaccharides but not to glucose. Maximum glucose yields of 10 wt % (TRS 35 wt %, HMF 28 wt %), 15 wt % (TRS 42 wt %, HMF 40 wt %), and 6 wt % (TRS 18 wt %, HMF 26 wt %) were obtained within 10 min for regular, waxy, and high amylose corn starch, respectively. Hence, 10 min was considered to be the duration required to obtain a maximum amount of glucose from starch. When the reaction time proceeded to 20 min, TRS yields were observed to decrease for all three starch substrates. As more D


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Figure 3. Effect of temperature on the conversion of fructose, glucose, and waxy corn starch (WCS). Reaction conditions: substrates (1 wt %), AlCl3· 6H2O = 0.15 g, [Bmim]Cl = 2 wt %, water:MIBK = 1:5 (v/v), total time (t) = 20 min, (a) = t = 10 min.

3.1.4. Effect of Water. The influence of water content on the conversion of starch to HMF is shown in Figure 4. HMF and

sugars were accessible for conversion, the HMF yields subsequently increased. At 20 min of reaction time, HMF yields reached maximum values of 60, 64, and 51 wt % for starch from regular, waxy, and high amylose corn, respectively. HMF yield obtained from waxy corn starch was noted to be the closest to the maximum HMF yields obtained from glucose over AlCl3·6H2O at 20 min reaction time (68 wt %, Figure 1). Beyond 20 min, the TRS yields declined to a minimum, and only trace amounts of glucose were detected, indicating that starch was totally hydrolyzed to glucose which was subsequently converted to HMF. Further increase in reaction time beyond 20 min led to the slow degradation of HMF for all three varieties of corn starch. 3.1.3. Effect of Reaction Temperature on the Conversion of Starch. Experiments were carried out at different temperatures from 130 to 150 °C, for a reaction time of 20 min. The results for the conversion of fructose, glucose, and waxy corn starch are summarized in Figure 3. For temperatures ranging from 130 to 140 °C, HMF yields increased from 66 wt % to 70 wt % for fructose substrate. For glucose, 60 wt % of HMF escalated to 68 wt %. Comparatively, HMF yield was low for starch at 130 °C. An equivalent amount of waxy starch substrate under identical conditions could attain 44 wt % of HMF within 20 min at 130 °C. Longer reaction time was essential to achieve better product yields. Evidently, low HMF yields obtained could be due to the partial starch hydrolysis associated with lower temperature (TRS 18 wt % at 10 min). At 140 °C, TRS turnover of 42 wt % achieved within 10 min accelerated sugar to HMF conversion. This contributed to higher HMF yields (64 wt %) attained within 20 min reaction time. Increasing the reaction temperature to 150 °C led to a higher TRS yield (45 wt %) being achieved within 10 min of reaction time. Prolonging the reaction time to 20 min resulted in decline of HMF yield to 40 wt %. For fructose, glucose, and starch, HMF yields dropped to 59.6, 53, and 50 wt %, respectively. As observed, the accumulated humin in the aqueous phase was primarily responsible for the reduced product yields.19 Thus, it could be concluded that higher temperatures favored rapid starch hydrolysis to HMF but at the cost of a significant amount of humin formation. Hence, a reaction temperature of 140 °C was considered to be ideal for HMF synthesis from starch.

Figure 4. Effect of water on the conversion of waxy corn starch (WCS). Reaction conditions: starch (0.6 mmol), AlCl3·6H2O = 0.15 g, [Bmim]Cl = 2 wt %, MIBK = 10 mL, temperature (T) = 140 °C, time (t) = 20 min.

LA yields were observed to be negligible at 5 wt % of water content. A maximum HMF yield of 64 wt % was obtained by increasing the water content to 20 wt %. However, as the water content was increased beyond 20 wt %, the HMF yields decreased significantly. Conversely, LA yields were observed to increase while increasing the water content from 20 wt % (0.1 wt %) to 40 wt % (18.2 wt %) respectively. This behavior can be attributed to the rapid rehydration of HMF to LA. The partial hydrolysis of AlCl3·6H2O in excess of water may impart intrinsic Brønsted acidity to the catalyst, which could also catalyze the LA synthesis. To confirm this observation, a blank reaction was run with optimal amounts of water (30 wt %) and catalyst (0.15 g). The pH of the reaction mixture decreased from 2.88 to 1.70 after heating the metal chloride in water at 140 °C for 20 min. This observed drop in pH was due to a partial hydrolysis of metal chlorides which released HCl in the aqueous medium. Under identical conditions, a similar drop in pH to 1.60 was also observed at 40 wt % of water content after heating the metal chlorides in water. The partial hydrolysis of AlCl3·6H2O to HCl provided negligible LA yield until 20 wt % of water content.13 Using a solution with an equimolar E


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retained in the ionic liquid-rich aqueous phase. In the absence of MIBK, HMF yields were observed to be minimal (Table 1, entry 1). The yields improved to 62.2 wt % upon addition of MIBK (Table 1, entry 2). However, the extraction rate was only 48% (Table 1, entry 2) since HMF was distributed between the MIBK phase and the water−ionic liquid phase. A maximum yield of 64 wt % HMF and an extraction rate of 88.1% were observed in Table 1, entry 3. Therefore, the addition of more MIBK was favorable to the extraction of the product. Also, the conversion exceeded 86.5% (Table 1, entries 2, 3, and 4) as the VMIBK/VWater ratio was increased from 2 to 8. HMF yields and extraction rate remained fairly constant (Table 1, entries 3 and 4). Therefore, a ratio of VMIBK/VWater of 5 was selected as a model for studying other process parameters which may influence HMF yields. 3.1.7. Reusability of the Catalytic System. Given that AlCl3· 6H2O and [Bmim]Cl remained dissolved in the aqueous phase, it could be easily recycled for successive runs after eliminating the MIBK phase.26,42,43 As shown in Figure 6, HMF yield

concentration of HCl resulted in enhancing the LA yields to as high as 30 wt % and a minimal yield of HMF under identical conditions. Hence, 20 wt % in water content is considered to be the optimum. 3.1.5. Effect of [Bmim]Cl. [Bmim]Cl serves two purposes. First, it dissolved the starch biopolymer.28 Second, it helped to depolymerize starch to monomers.11 Thus, the ability of [Bmim]Cl to assist in the conversion of starch to HMF has been assessed as shown in Figure 5. While increasing [Bmim]Cl

Figure 5. Effect of [Bmim]Cl on the conversion of waxy corn starch (WCS). Reaction conditions: starch (0.6 mmol), AlCl3·6H2O = 0.15 g, water:MIBK = 1:5 (v/v), temperature (T) = 140 °C, time (t) = 20 min.

content from 1 wt % to 2 wt %, HMF yields improved from 60 wt % to 64 wt %. Likewise, the TRS yields within 10 min of reaction time were observed to be higher at 2 wt % of [Bmim]Cl (TRS 42 wt %) than that achieved at 1 wt % (TRS 33.7 wt %). As observed, greater [Bmim]Cl amount could influence the starch depolymerization to a greater extent, thereby improving HMF yields. However, above 2 wt % of [Bmim]Cl, HMF and TRS yields steadily dropped. The HMF and TRS yields declined to 55 wt % (TRS 35.5 wt %) and 47 wt % (TRS 30.4 wt %), respectively, with an increase in the ionic liquid content from 4 wt % to 10 wt %. The results indicated that the increasing viscosity buildup of the ionic liquid rich aqueous phase could impede efficient mixing of the watersoluble reactants and the catalyst, thus reducing HMF yields significantly.3,23,40,41 3.1.6. Effect of Biphasic Reaction Medium. The effect of MIBK/water on the conversion of waxy corn starch was determined (Table 1). In this batch system setup, HMF was extracted continuously by MIBK, while the reactants were

Figure 6. Recyclability of the catalytic system. Reaction conditions: starch (0.6 mmol), AlCl3·6H2O = 0.15 g, [Bmim]Cl = 0.2 wt %, water:MIBK = 1:5 (v/v), temperature (T) = 140 °C, time (t) = 20 min.

slightly increased in the second catalytic run. This could be due to the presence of unconverted starch and/or sugars in the catalytic phase which did not get carried over to the MIBK phase. Hence, the residual monomers dehydrated further to HMF. Thereafter, the system retained its catalytic activity for four consecutive cycles. Following the sixth cycle, the HMF yield dropped, mainly due to the accumulation of humin in the catalytic phase. However, negligible LA yields were detected under identical conditions.

Table 1. Starch Conversion to HMF in the Biphasic System of MIBK and Waterd entry

VMIBK/ VWatera

1 2 3 4

0b 2 5 8

HMFtotal yield (wt %) 10.3 62.2 64 63.4

± ± ± ±

3 2 2.5 3.8

conversion (%)

HMFOrg yield (wt %)

extraction rate (%)c

± ± ± ±

30 ± 0.8 56.4 ± 0.5 56 ± 0.2

48 ± 2.9 88.1 ± 4.8 88.32 ± 6.4

86.5 95.4 96 95.6

1.5 1.2 0.8 1.3

4. CONCLUSIONS A biphasic water-[Bmim]Cl/MIBK system with the AlCl3· 6H2O catalyst for converting corn starch to HMF with high yields has been presented. The ionic liquid [Bmim]Cl depolymerized the starch into the monomer glucose, following which the Lewis acid catalyst AlCl3·6H2O isomerized the glucose into fructose, which was readily converted to HMF. The biphasic medium used led to the extraction of the HMF from the aqueous to the organic phase, thus preventing the rehydration of HMF to LA and other byproducts. After developing the system, it was optimized in terms of catalyst loading, reaction time, reaction temperature, water content, [Bmim]Cl, and MIBK/water ratio.

a

VMIBK/VWater is the volume of MIBK to the volume of water (2 mL). Water (20 wt %), t = 60 min. cExtraction rate is the ratio of HMF yield in the MIBK phase to the total yields. dReaction conditions: waxy corn starch (0.6 mmol), AlCl3·6H2O = 0.15 g, [Bmim]Cl = 0.2 g, temperature (T) = 140 °C, time (t) = 20 min. b

F


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It was shown that an increase in the catalyst loading resulted in an increase in the HMF yield until a maximum yield of 64 wt % was obtained at a loading ratio of 1.5 g catalyst/g starch. As the catalyst loading increased further, the TRS yield obtained continued to rise, but the HMF yield decreased due to an increased humin formation. For the same reason, a similar pattern was also observed when optimizing HMF yields in terms of reaction time and temperature, with the maximum yields obtained at 20 min and 140 °C. The effect of water content was particularly significant for HMF synthesis from starch, as water is essential for hydrolyzing starch, but has a detrimental effect on HMF yields via rehydration reactions. Accordingly, it was observed that for a water content of 5 wt %, HMF yields were negligible, while an increase in water content beyond 20 wt % led to increased LA yields at the expense of HMF. A water content of 20 wt % was therefore found to be optimum. For [Bmim]Cl, the highest HMF yield was obtained at 2 wt %, since a further increase in [Bmim]Cl content led to a rise in system viscosity, impeding mixing between the reactants and catalyst. Finally, the optimum MIBK:water ratio was found to be 5:1 since this value gave the highest HMF yield along with the highest distribution ratio of HMF between the organic and aqueous phases. The reusability of the catalytic system was also tested by adding fresh starch and MIBK directly to the used catalyst from the preceding run. It was observed that the system remained reactive and selective for up to six runs, indicating a high degree of recyclability even without catalyst purification and regeneration. Hence, this system is capable of achieving high HMF yields from amylopectin-rich starches with minimal catalyst replenishment for multiple runs.

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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/acs.energyfuels.6b01699. Calibration curve of HMF (Figure S1), calibration curve of LA (Figure S2), GC-MS detection of HMF (Figure S3), TRS calibration curve (Figure S4), calibration curve of glucose (GO) assay (Figure S5) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: 1-514-398-7776. E-mail: marie-josee.dumont@mcgill. ca. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support of the Natural Science and Engineering Research Council of Canada (NSERC) is gratefully acknowledged. We are also immensely thankful to Mr. Yvan Gariépy and Dr. Darwin Lyew for their support of our work.

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REFERENCES

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One-Pot Conversion of Corn Starch into 5-Hydroxymethylfurfural in

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