Salt-Promoted Glucose Aqueous Isomerization Catalyzed by

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Salt-promoted Glucose Aqueous Isomerization Catalyzed by Heterogeneous Organic Base Qiang Yang, Wu Lan, and Troy Runge ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01132 • Publication Date (Web): 03 Aug 2016 Downloaded from http://pubs.acs.org on August 6, 2016

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ACS Sustainable Chemistry & Engineering

Salt-promoted Glucose Aqueous Isomerization Catalyzed by Heterogeneous Organic Base Qiang Yang, Wu Lan, and Troy Runge* Department of Biological Systems Engineering, University of Wisconsin-Madison, 460 Henry Mall, Madison, WI 53706, United States Email: [email protected] ABSTRACT: Effective isomerization of glucose to fructose is a crucial step for 5hydroxymethylfurfural production from lignocellulose. The present study investigated polystyrene-supported organic bases with varied structural properties for aqueous isomerization of glucose. The results indicate that the heterogeneous organic bases can achieve approximately 30% fructose yields at 120-140 °C. As heterogeneous catalysts, they are generally less efficient than homogeneous organic bases and can lose catalytic activity during recycling likely from thermal-induced byproducts. However, the addition of neutral salt made the heterogeneous organic bases as efficient as homogeneous organic bases, achieving approximately 41% maximum fructose yields at 80-100 °C; and improved the reusability allowing the heterogeneous organic bases to be reused for at least four times without significant loss of catalytic activity. KEYWORDS: Isomerization, Fructose, Heterogeneous Catalyst, Organic Base, Salt-promotion

*To whom correspondence should be addressed. Tel: 608-890-3143. E-mail: [email protected]

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INTRODUCTION 5-Hydroxymethylfurfural (HMF) is considered as an important biorenewable platform chemical, which can be upgraded into liquid fuels and various chemicals such as 2,5-furandicarboxylic acid and 2,5-dimethylfuran. Glucose derived from lignocellulose is considered as a sustainable and a preferred feedstock for the HMF production.1 Mechanistically, the production of HMF from glucose involves isomerization of glucose to fructose and subsequent dehydration of fructose to HMF. Unfortunately, it has been concluded that most catalysts which can effectively convert fructose to HMF are normally ineffective for the conversion of glucose to HMF.2 Therefore, the inefficient isomerization of glucose to fructose is responsible for a low HMF yield during a onereactor process. Alternatively, the production of HMF can be achieved through two steps. Thus, an effective isomerization of glucose to fructose is a crucial step for the HMF production from glucose and lignocellulose. Additionally, fructose is also a food and beverage ingredient with higher sweetness than glucose. 3 To date, in addition to immobilized glucose isomerases, various chemical catalysts have been tested for the isomerization of glucose to fructose. Metal halides such as chromium chloride and aluminum chloride, and Brønsted bases such as sodium hydroxide and amines have been investigated as homogeneous catalysts for the isomerization of glucose.4-7 Chromium chloride and aluminum chloride can achieve 20-26% fructose yields with 48-83% selectivity at 120-140 °C.4,5 Amines such as triethylamine and tetramethylguanidine can also achieve 30-36% fructose yields with 40-73% selectivity at 60-120 °C.6,7 Although they are effective, the homogeneous isomerization catalysts have the issues of costly separation from product streams and recycling. Most investigated heterogeneous metal catalysts such as metallosilicates and metal oxides are generally not as effective as homogenous catalysts.8,9 However, Sn-beta zeolite has been shown


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to be one of the most effective heterogeneous metal catalysts, and can achieve approximately 3145% fructose yields with 51-67% selectivity at 110-140 °C.10,11 Compared to the Sn-beta zeolite, polymer-supported organic bases can be easier fabricated as heterogeneous nonmetal catalysts, and can isomerize glucose with 15-31% fructose yields with 40-73% selectivity at 60-120 °C.7 The heterogeneous isomerization catalysts are generally less efficient than the corresponding homogeneous counterparts. So, to achieve satisfactory fructose yields, the heterogeneous isomerization catalysts usually need harsh reaction conditions (longer reaction time, higher reaction temperature, and more catalyst) relative to the homogeneous isomerization catalysts. However, the harsh reaction conditions can not only consume more energy, but also result in leaching of catalyst and formation of byproducts.4-13 Therefore, it is still desirable to explore more efficient heterogeneous catalysts with better reusability for the glucose isomerization. Herein, this study explored polystyrene-supported organic bases for the glucose aqueous isomerization and investigated practical approaches to improve the isomerization performance. EPERIMENTAL SECTION Materials. Polystyrene-supported 1,5,7-triazabicyclo [4.4.0] dec-5-ene (PS-TBD, 200400 mesh, 2.6 mmol/g nitrogen, 2 % cross-linked), polystyrene-supported 1,8-diazabicyclo [5.4.0] undec-7-ene (PS-DBU, 100-200 mesh, 1.4-2.2 mmol/g nitrogen, 1 % cross-linked), polystyrene-supported

2-tert-butylimino-2-diethylamino-1,3-dimethylperhydro-1,3,2-

diazaphosphorine (PS-BEMP, 200-400 mesh, 2.0-2.5 mmol/g nitrogen, 1% cross-linked), 1,5,7triazabicyclo [4.4.0] dec-5-ene (TBD), 1,8-diazabicyclo [5.4.0] undec-7-ene (DBU), 2-tertbutylimino-2-diethylamino-1,3-dimethylperhydro-1,3,2-diazaphosphorine

(BEMP),

sodium

chloride (NaCl), potassium chloride (KCl), lithium chloride (LiCl), sodium bromide (NaBr),


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lithium bromide (LiBr), sodium iodide (NaI), potassium iodide (KI), D-fructose, D-mannose, Dglucose, and D-deuterated glucose (glucose-2-D) were bought from Sigma-Aldrich. All chemicals were used as received. Isomerization. Isomerization experiments of glucose or fructose (10-50%, wt/wt) in water were carried out in 6 mL thick-walled glass tube reactors at different temperatures (50-140 °C) for specific times (2-30 min), and then were stopped by cooling the reactors in an ice bath. Small aliquots of the filtered and diluted reaction media were taken for sugar and organic acid analysis. Characterization and Analysis Method. 1H,

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C and 2D (1H-13C) heteronuclear single-

quantum correlation (HSQC) NMR spectra were collected with a Bruker Biospin AVANCE 500 MHz NMR spectrometer. LC-MS analysis was conducted on a Shimadzu LCMS-2020 (Shimadzu, USA) system using Dual Ion Source method for ionization. Glucose, fructose and mannose were analyzed on an Agilent 1220 Infinity high performance liquid chromatography (HPLC) system equipped with an Agilent Hi-Plex H analytical column (7.7 × 300 mm), a BIORAD guard column (Cat. No. 125-0139), and a refractive index detector (RID). Yield of fructose (%) and selectivity for fructose (%) were the formed fructose as the mole percentages of the initial glucose and reacted glucose, respectively. Organic acids (acetic, formic, glycolic, and lactic acid) were measured using the Dionex ICS-3000 system (Sunnyvale, CA), equipped with a UV-vis detector and Superlcogel C-610H analytic (30 cm × 7.8 mm) and guard (5 cm × 4.6 mm) columns. RESULTS AND DISCUSSION Isomerization of glucose by heterogeneous organic base


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Due to the ability to delocalize positive charge over nitrogen atoms, amidine, guanidine and phosphazene are strong bases (Scheme S1). The 1,8-diazabicyclo [5.4.0] undec-7-ene (DUB, pKa=24.34 in CH3CN) and 1,5,7-triazabicyclo [4.4.0] dec-5-ene (TBD, pKa=25.98 in CH3CN) are bicyclic amidine and guanidine bases, respectively. Analogous to the guanidine, the 2-tertbutylimino-2-diethylamino-1,3-dimethylperhydro-1,3,2-diazaphosphorine (BEMP, pKa=27.6 in CH3CN) is a cyclic phosphazene base. Therefore, the polystyrene-supported TBD, DBU and BEMP are strong bases (Scheme S2). Their catalyzed glucose isomerization reactions were kinetically investigated with varying reaction conditions (glucose concentration, catalyst dosage, reaction time and temperature), and the results presented in Figure S1, Table 1 and Figure 1. The results show that the PS-TBD, PS-DBU and PS-BEMP can achieve approximately 30%, 30% and 32% maximum fructose yields, respectively. The rate constant (κ) at each temperature (100140 °C) was estimated from the slope of ln ([glucose concentration at time t]t/[initial glucose concentration]o) versus t plot, and the calculated rate constants were summarized in Table 1. The results indicate that the rate constant increased with increasing of the reaction temperature. And the isomerization reactions catalyzed by the PS-TBD and PS-BEMP are generally faster than that of the PS-DBU. Mannose is another isomerization product of glucose. In this study, mannose yield was typically below 2% (Table S1), and therefore was not discussed here. The basecatalyzed isomerization reaction is typically accompanied by irreversible sugar degradation, which can lead to organic acids. However, organic acids (formic, acetic, glycolic, and lactic acid) were not detectable in most cases, and thus were not reported in this study. Similar observation is also reported when triethylamine as an isomerization catalyst.18 Carbon balance based on monosaccharides (fructose, mannose and glucose) distribution was calculated and also presented in Table 1. The results show that the carbon balance decreased when increasing the reaction


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temperature likely due to the formation of degradation byproducts evidenced by the observed light yellow colors. Turnover frequency (TOF, moles of produced fructose/moles of nitrogen atom contained in catalyst/time) was also presented in Table 1. It turns out that the TOF firstly increased with increasing of the reaction temperature and then decreased due to the formation of byproducts. The isomerization performances of the commercially available polystyrene-supported organic bases in this study were similar to those of the previously reported other synthesized heterogeneous organic bases.7 Overall, in terms of fructose yield, the heterogeneous organic bases are better than some homogeneous catalysts such as chromium chloride (25.4% fructose yield) and aluminum chloride (26.3% fructose yield).4-7 Apparent activation energy (Ea) was calculated based on the kinetics data (Figure S2), and summarized in Table 1. The estimated activation energy values for the PS-TBD, PS-DBU and PS-BEMP are 33, 36 and 33 kJ.mol-1, respectively, which are lower than those of chromium chloride (58.6-64.0 kJ.mol-1), aluminum chloride (110±2 kJ.mol-1) and Sn-Beta (93±15 kJ.mol-1).4-7,10,11,14,15 For comparison, the corresponding homogeneous TBD, DBU and BEMP catalyzed isomerization reactions were also kinetically investigated and presented in Figure S3. However, the results presented in Figures S1 and S3 show that the heterogeneous organic bases are not as efficient as the corresponding homogeneous organic bases. In general, the heterogeneous organic bases require higher reaction temperatures (> 110 °C) and longer reaction time (> 20 min), and are not active below 100 °C. In comparison, the homogeneous organic bases can achieve similar isomerization performance under lower reaction temperatures and shorter time. This result is believed to be due to mass transfer limitation, steric hindrance and porous structure, with some nitrogen atoms, particularly those inside the pores, to not be available for the isomerization reaction. Therefore, the


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heterogeneous catalysts are generally less efficient than the corresponding homogeneous catalysts. Effects of catalyst dosage and glucose concentration were also investigated, and the results were presented in Figure S4. The results show that a higher glucose concentration resulted in a decreased fructose yield when the catalyst dosage is 10 mol% nitrogen relative to glucose. Increasing the catalyst dosages resulted in higher fructose yields for the PS-TBD and PS-DBU. Similar to the PS-BEMP, increasing catalyst dosage caused a higher fructose yield when the catalyst dosage is below 10 mol% nitrogen (relative to glucose). While further increasing the catalyst dosage decreased the fructose yield due to the formation of byproducts. Reusability of heterogeneous organic base The amine-catalyzed isomerization reaction of glucose will typically be accompanied with side reactions such as Maillard reaction and thermal-induced degradation of carbohydrates.6,7,18 Mechanistically, the Maillard reactions were not expected to occur in this study during the isomerization reactions, because the heterogeneous organic bases (PS-TBD, PS-DBU and PSBEMP) are tertiary amines. However, thermal-induced side reactions were believed to occur, evidenced by the observed light yellow colors when the reactions were carried out at elevated temperatures. The thermal-induced side reactions of sugars can be accelerated under basic conditions, which can produce a variety of degradation products.16,17 The formed byproducts can be soluble or insoluble in water, with the soluble byproducts typically as reaction intermediates produced under mild reaction conditions. In this study, when the isomerization reactions were carried out at a higher temperature such as 110 °C, the sugar solutions were observed to turn to light yellow, while the heterogeneous organic bases changed to yellow and then on to a dark red color. The soluble colored byproducts were identified by using a 2D (1H-13C) HSQC (heteronuclear single-quantum correlation) NMR spectrometer. As shown in Figure 1, soluble


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byproducts were clearly observed (in green color) in the isomerized glucose by PS-TBD. After the isomerization reaction, it was discovered that the colored byproducts could be selectively removed by activated carbon through adsorption.6 Similarly, during the isomerization, the soluble byproducts could be adsorbed by the heterogeneous organic bases and hinder the isomerization reactions. Heterogeneous organic bases catalyze the isomerization reactions of glucose to fructose through generated hydroxide ions from their reversible reactions with water (Scheme S3).18 Also, the organic bases are covalently attached to the polystyrene resins, and cannot be easily detached during the isomerization reactions.7 In this study, there was no TBD detected in the isomerized glucose by PS-TBD (Figure 2). However, the formed soluble byproducts can be further transformed to be insoluble under severe reaction conditions. The insoluble byproducts such as cyclic diketones can physically cover the surfaces of heterogeneous catalysts and therefore cause activity loss through limiting available nitrogen atoms for reactions.7,17 In this study, the reusability of heterogeneous organic bases was investigated by performing four consecutive isomerization cycles at 120 °C for 20 min. After each cycle, the spent PS-TBD was carefully recovered through filtration and intensively rinsed with water, and then the 10 wt% fresh glucose aqueous solution was added to start a new cycle. It was expected that the recycled heterogeneous organic bases show gradually decreased glucose conversions and fructose yields (Figure 2). In contrast, the heterogeneous organic bases after heated without sugars for 2 h at 120 °C still achieved similar isomerization performance to the fresh ones. Therefore, it was hypothesized that the thermal-induced byproducts from sugars was the reason for the activity loss.7 So, the thermalinduced byproducts should be minimized in order to maintain the catalytic activity of the heterogeneous organic bases.


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Promotion of neutral salt on isomerization performance Amines form hydrogen bonding with water that helps the compound to be water-miscible, but they also react with water. The reversible reaction (protonation) between amine and water can in situ generate hydroxide ions (Scheme S3), which are the active species for the glucose isomerization reaction.6,7,18 A proton transferred from water molecule creates an amine with a positively charged polyatomic ion (i.e. quaternary ammonium ion), which can specifically bind with anion through electrostatic attractive interaction. It was reported that salts such as sodium chloride could promote the protonation of amines in water.19,20 As a result, the specific binding can shift the acid-base reaction equilibrium between amine and water, which is favorable to generate more hydroxide ions. It was observed that addition of neutral salt slightly increased the basicity of amine aqueous solutions.

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In this study, the addition of neutral salts such as

sodium chloride also slightly increased the basicity of the heterogeneous organic base aqueous solutions (Figure S5). In addition, the heterogeneous organic base catalyst can be less swelled in an ionic environment because of the decrease in the difference of osmotic pressure between the polymeric network of the catalyst and the aqueous solution. The shrinking of the heterogeneous organic base catalyst can affect the mass transport throughout its polymeric support. However, in this study, the formed hydroxide ions in solution after the protonation of amines in the catalyst act as base catalysts for the glucose isomerization reaction. Considering that the isomerization reaction doesn’t take place on the surface or inside of the catalyst, the shrinking of the catalyst will not greatly influence the catalytic activity. Therefore, the presence of salt was expected to accelerate the heterogeneous organic base-catalyzed glucose isomerization reaction in this study. Effects of several typical neutral salts (NaCl, KCl, LiCl, NaBr, LiBr, NaI and KI) on the isomerization reactions were investigated and presented in Figure 3. Considering that the basicity


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of heterogeneous organic base aqueous solution was expected to be increased in the presence of neutral salt, the isomerization reaction was carried out under relatively mild reaction conditions (at 100 °C for 12 min). Although the neutral salts cannot convert glucose by themselves, their presence greatly promoted the heterogeneous organic bases-catalyzed isomerization reactions. Specifically, the additions of neutral salts notably increased the achievable glucose conversions and fructose yields. Remarkably, with the addition of even low amounts of salts (1 wt% based on solvent), the heterogeneous organic bases achieved approximately 41% fructose yields. The salts also increased the glucose-to-fructose selectivity (Figure S6), indicating that the thermal-induced side reactions were greatly suppressed under such mild reaction conditions. Also, there was no colored byproducts observed. In the presence of salts, increasing reaction temperature decreased the fructose yield and selectivity (Figure S7). The salts also improved the isomerization performance of spent catalysts (Figure S8). Finally, the effect of sodium chloride concentration on the isomerization reaction was also investigated and shown in Figure 3. The data indicates the optimal concentration for sodium chloride is 1 wt%. Below the 1 wt%, the catalysts cannot achieve maximum fructose yields. Further increasing the concentration of sodium chloride from 1 wt% to 20 wt% did not further promote the isomerization reaction, though the result does indicate that the heterogeneous organic bases can tolerate higher concentrations of neutral salts for this reaction. Since the salts can facilitate the glucose conversions, higher fructose yields may be achieved under more mild reaction conditions (lower reaction temperature and shorter reaction time, and less catalyst). Effects of reaction conditions on the isomerization reactions in the presence of salts were thus investigated, with the results shown in Figure 4. Supplemented with the salts, the heterogeneous organic bases were effective under lower temperatures such as 60-80 °C. In


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particular, the PS-TBD and PS-BEMP achieved approximately 30% fructose yields at 60 °C, and achieved greater than 30% fructose yields within 1 min. Because of the salt promotion, lower catalyst dosage may be used. About 30% fructose yields can be obtained when the catalyst dosage is 5 mol% nitrogen relative to glucose. Further increasing catalyst dosage from 15% to 20% decreased the achievable fructose yields and selectivity. The catalyst in the presence of salt was more tolerable to a higher glucose concentration. Specifically, the catalysts still achieved over 25% fructose yields even when the glucose concentration is up to 50%. In contrast, without the salts, the heterogeneous organic bases achieved less than 20% fructose yields (Figure S4). It can be concluded that the heterogeneous organic bases in the presence of salts can isomerize glucose as efficiently as the corresponding homogeneous organic amines (Figure S3). The heterogeneous organic bases share the same nitrogen atoms as the corresponding organic bases. To further understand the specific bindings between nitrogen atoms and anions, the isomerization reactions by the homogeneous organic bases (TBD, DBU and BEMP) in the presence of 1wt% sodium chloride were monitored by a NMR spectrometer (Figures 5 and S9S11). Upon addition of sodium chloride, the 1H NMR signals of organic bases slightly moved downfield. The observed shifts of 1H NMR signals of organic base were attributed to a slight increase of pH value in solution and an interaction between organic base and chloride ion which caused the change in pH value. However, there were no differences observed in their 13C NMR spectra, indicating that the interaction between organic base and chloride ion was weak.23,24 The comparative 1H NMR studies provided a direct evidence for the binding interaction between salt and amine. Additionally, in the presence of sodium chloride, the 1H NMR signals of sugars (glucose and fructose) also slightly shifted. The observed change in the pH value of sugar solution can result in the shifts of sugars. The hydrogen bonding between chloride ions and the


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sugar’s hydroxyl groups can also cause the shifts.23,24 To further understand the interaction between chloride ions and sugars, glucose isomerized by PS-BEMP without or with sodium chloride was also analyzed by a liquid chromatograph mass (LC-MS) spectrometer, and the results were presented in Figure 6. A negative ion peak at m/z 215 was exclusively observed in the glucose isomerized in the presence of sodium chloride, suggesting that chloride ion was combined with glucose or fructose (C6H12O6 + Cl-1) through hydrogen bonding. It was reported that chloride ion can form hydrogen bonds with glucose through the hydrogen atoms of the hydroxyl groups at C1 and C6 positions.24 So, the observed 1H NMR shifts of sugars in this study were the results of the pH change of sugar solution and interaction between chloride ions and sugars. In addition, salts may influence the degradation of sugars under basic conditions. Hu et al., concluded that the addition of sodium chloride could inhibit breaking of the carbon-carbon bond of glucose or fructose.24 In this study, sodium chloride may also inhibit thermal-induced side reactions of glucose or fructose, and therefore improve the glucose to fructose selectivity (Figures S6 and S7). The results above show that the heterogeneous organic bases can be highly efficient in the presence of salts. Therefore, to achieve a higher fructose yield, the isomerization reaction can be carried out under mild reaction conditions such as 80 °C for 5 min. It can be expected that less byproducts would be derived from the thermal degradation of sugars under mild reaction conditions. As a result, the heterogeneous organic bases were expected to have better reusability in the presence of salts under mild reaction conditions. This effects of sodium chloride on the reusability of heterogeneous organic bases was studied and presented in Figure 7. The spent catalysts were recovered through filtration and were reused after intensively rinsed with water. As expected, the heterogeneous organic bases had excellent reusability.


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Isomerization mechanism The base-catalyzed isomerization reaction of glucose proceeds through an enediol intermediate formed after deprotonation at C-2 position of acyclic glucose.6,7,14,18 Hydroxide ions generated after amine reacted with water actually act as catalysts for the amine-catalyzed glucose isomerization reactions.6,7,18 Therefore, it can be reasonably speculated that the investigated heterogeneous organic amines shared similar isomerization mechanisms to sodium hydroxide. Ring opening of glucose most likely occurs by deprotonation of O-1 and subsequent breakage of C1-O5 ether bond.18 The acyclic glucose is thermodynamically unstable and spontaneously deprotonated to an enediol intermediate.14 The deprotonation at C-2 position of acyclic glucose is known to be the rate-limiting step in the glucose isomerization reaction.6,7,14,18 Glucose deuterated at the C-2 position (glucose-2-D) was used as the reactant to investigate the deprotonation at the C-2 position. Using the glucose-2-D as the reactant, the rate constant, glucose conversion and fructose yield were found to be much lower than those for the unlabeled glucose, as shown in Figure 8. The isotopically labeled at C-2 position induced kinetic isotopic effect (KIE) is 3.27. So, it is kinetically observable that the deprotonation of the C-2 position is the rate-limiting step. Water was not only the solvent for glucose in these reactions, but was also involved in the generation of hydroxide ions as reactant, and isomerization reaction as proton donor.6,14,18,25 The water induced kinetic isotopic effect was also investigated through conducting the isomerization reaction of glucose in deuterium oxide. As a result, water (deuterium oxide in this case) was shown to not have a kinetically significant role (KIE=1) (Figure 8). The isomerized glucose-2-D was analyzed by using a 2D (1H-13C) HSQC NMR spectrometer, and the result was shown in Figure 9. The isomerized glucose-2-D displays different 1H and 13C


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signals from glucose-2-D (Figures 9 and S12). Except that the unreacted glucose-2-D remains the same (blue color), new peaks (red color) were observed, which also appear in the 1H spectrum of fructose (Figure S13). The 2D HSQC NMR spectra for the produced fructose isomerized from glucose-2-D and unlabeled glucose show no differences (Figures 1 and 9). This result indicates that the fructose isomerized from glucose-2-D does not contain a deuterium atom and therefore has to gain one proton from water. Although reacted under the same reaction conditions as the unlabeled glucose, there was no byproducts observed in the isomerized glucose-2-D. This result also verified that the isomerization of glucose-2-D was much slower due to the slow deprotonation of the C-2 position. The above NMR studies and observed kinetic isotopic effects support the hypothesis that the heterogeneous organic bases isomerized glucose to fructose without or with neutral salts through an enediol intermediate formed after deprotonation at C-2 position (Scheme 1).18,25 Given that the glucose, fructose, and enediol anions are present during the glucose isomerization, salts may interact with these anions and influence the isomerization mechanism and reaction pathways. It was reported that the carbonate ions could promote the enolisation of sugars such as talose and ribose but could not influence the enolisation of glucose and fructose.25 So, the salts in this study presumably did not change the isomerization mechanism. The isomerization reaction of glucose is known to be reversible.4-18 The equilibrium constant for the isomerization of glucose to fructose catalyzed by the PS-TBD was estimated according to the glucose and produced fructose concentrations at 30 min and was summarized in Table S2. The estimated equilibrium constant (0.32-0.57) was less than 1, clearly indicating that the glucose isomerization was reversible. To further understand the reversibility, the isomerization of fructose to glucose by the PS-TBD was also kinetically investigated, with the results are


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shown in Figure S14. Under the same reaction conditions, the PS-TBD can generally convert more glucose to fructose than fructose to glucose (Figures S1 and S14). The equilibrium constant for the isomerization of fructose to glucose catalyzed by the PS-TBD was also estimated and was summarized in Table S3. At each temperature (100-140 °C), the estimated equilibrium constant (0.09-0.16) for the reverse isomerization of fructose to glucose was also less than 1 and was much smaller than that (0.32-0.57) for the isomerization of glucose to fructose (Tables S2 and S3). These results indicated that the isomerization of glucose to fructose was more favored than the reverse isomerization of the produced fructose back to glucose when the PS-TBD used as the catalyst to isomerize glucose. CONCLUSIONS The present study demonstrated that the polystyrene-supported organic bases are effective heterogeneous catalysts for glucose aqueous isomerization. Nevertheless, they are generally less efficient than the corresponding homogeneous organic bases, and also can lose catalytic activity during recycling. However, the neutral salt-induced promotion could make the heterogeneous organic bases as efficient as the homogeneous organic bases and also stabilize the heterogeneous organic bases to allow effective catalysis after recycling. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.


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Additional schemes, figures and tables including illustrations of chemical structure and chemical reaction; estimated equilibrium conversion, rate constant, and apparent activation energy; effects of temperature, glucose concentration, catalyst dosage, sodium salts; and NMR spectra (PDF)

AUTHOR INFORMATION Corresponding Author *Phone: 608-890-3143. E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS We gratefully acknowledge the United States Department of Agriculture-National Institute of Food and Agriculture (USDA BRDI Grant number 2012-10006-19423) for their financial support. REFERENCES (1) van Putten, R-J.; van der Waal, J. C.; de Jong, E.; Rasrendra, C. B.; Heeres, H. J.; de Vries, J. G. Hydroxymethylfurfural, a versatile platform chemical made from renewable resources. Chem. Rev. 2013, 113 (3), 1499-1597. (2) Hu, L.; Zhao, G.; Hao, W.W.; Tang, X.; Sun, Y.; Lin, L.; Liu, S.J. Catalytic conversion of biomass-derived carbohydrates into fuels and chemicals via furanic aldehydes. RSC Adv. 2012, 2, 11184-11206.


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(3) Park, Y.K.; Yetley, E.A. Intakes and food sources of fructose in the United States. Am. J. Clin. Nutr. 1993, 58, 7375-7475. (4) Choudhary, V.; Mushrif, S. H.; Ho, C.; Anderko, A.; Nikolakis, V.; Marinkovic, N. S.; Frenkel, A. I.; Sandler S. I.; Vlachos, D. G. Insights into the interplay of Lewis and Brønsted acid catalysts in glucose and fructose conversion to 5-(hydroxymethyl)furfural and levulinic acid in aqueous media. J. Am. Chem. Soc. 2013, 135 (10), 3997-4006. (5) Tang, J. Q.; Guo, X. W.; Zhu, L. F.; Hu, C. W. Mechanistic study of glucose-to-fructose isomerization in water catalysed by [Al(OH)2(aq)]+. ACS Catal. 2015, 5 (9), 5097-5103. (6) Liu, C.; Carraher, J. M.; Swedberg, J. L.; Herndon, C. R.; Fleitman, C.N.; Tessonnier, J. P. Selective base-catalyzed isomerization of glucose to fructose. ACS Catal. 2014, 4 (12), 4295-4298. (7) Yang, Q.; Zhou, S. F.; Runge, T. Magnetically separable base catalysts for isomerization of glucose to frutose. J. Catal. 2015, 330 (330), 474-484. (8) Watanabe, M.; Aizawa, Y.; Lida, T.; Nishimura, R.; Inomata, H. Catalytic glucose and fructose conversions with TiO2 and ZrO2 in water at 473 K: relationship between reactivity and acid-base property determined by TPD measuremen. Appl. Catal. A Gen. 2005, 295, 150156. (9) Delidovich, I.; Palkovits, R. Catalytic activity and stability of hydrophobic Mg–Al hydrotalcites in the continuous aqueous-phase isomerization of glucose into fructose. Catal. Sci. Technol. 2014, 4, 4322-4329. (10) Bermejo-Deval, R.; Assary, R.S.; Nikolla, E.; Moliner, M.; Román-Leshkov, Y.; Hwang, S. J.; Palsdottir, A.; Silverman, D.; Lobo, R. F.; Curtiss, L. A.; Davis, M. E. Metalloenzymelike catalyzed isomerization of sugars by Lewis acid zeolites. PNAS 2012, 109 (109), 97279732. (11) Guo, Q.; Kumar, P.; Orazov, M.; Xu, D.D.; Alhassan, S.M.; Andre Mkhoyan, K.; Davis, M.E.; Tsapatsis, M. Self-pillared, single-unit-cell Sn-MFI zeolite nanosheets and their use for glucose and lactose isomerization. Angew. Chem. Int. Ed. 2015, 54, 10848-10851. (12) Osmundesn, C. M.; Holm, M. S.; Dahl, S.; Taarning, E. Tin-containing silicates: structureactivity relations. Proc. R. Soc. A 2012, 468 (2143), 2000-2016.


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(13) Lari, G.M.; Dapsens, P. Y.; Scholz, D.; Mitchell, S.; Mondelli, C.; Pérez-Ramíez, J. Deactivation mechanisms of tin-zeolites in biomass conversions. Green Chem. 2016, 18, 1249-1260. (14) Kooyman, C.; Vellenga, K.; De Wilt, H.G.J. The isomerization of d-glucose into d-fructose in aqueous alkaline solutions. Carbohydr. Res. 1977, 54, 33-44. (15) Bermejo-Deval, R.; Gounder, R.; Davis, M.E. Framework and extraframework Tin sites in zeolite beta react glucose differently. ACS Catal. 2012, 2, 2705-2713. (16) Zhang, X. C.; Tao, N. P.; Wang, X. C.; Chen, F.; Wang, M. F. The colorants, antioxidants, and toxicants from nonenzymatic browning reactions and the impacts of dietary polyphenols on their thermal formation. Food Funct. 2015, 6, 345-355. (17) Shaw, P.E.; Tatum, J.H.; Berry, R.E. Base-catalyzed fructose degradation and its relation to nonenzymic browning. J. Agr. Food Chem. 1968, 16, 979-982. (18) Carraher, J. M.; Fleitman, C. N.; Tessonnier, J. P. Kinetic and mechanistic study of glucose isomerization using homogeneous organic Brønsted base catalysts in water. ACS Catal. 2015, 5 (6), 3162-3173. (19) De Robertis, A.; De Stefano, C.; Patane, G.; Sammartano, S. Effects of salt on the protonation in aqueous solution of triethylenetetramine and tetraethylenepentamine. J. Solution Chem. 1993, 22, 927-940. (20) De Stefano, C.; Gianguzza, A.; Sammartano, S. Protonation thermodynamics of 2,2′bipyridyl in aqueous solution. Salt effects and weak complex formation. Thermochim. Acta 1993, 214, 325-338. (21) Voinescu, A.E.; Bauduin, P.; Cristina Pinna, M.; Touraud, D.; Ninham, B.W.; Kunz, W. Similarity of salt influences on the pH of buffers, polyelectrolytes, and proteins. J. Phys. Chem. B 2006, 110, 8870-8876. (22) Felippe, C.; Bellettini, I. C.; Eising, R.; Minatti, E.; Giacomelli, F. C. Supramolecular complexes formed by the association of poly(ethyleneimine) (PEI), sodium cholate (NaC) and sodidum dodecyl sulfate (SDS). J. Braz. Chem. Soc. 2011, 22, 1539-1548. (23) Jiang, Z.C.; Yi, J.; Li, J.M.; He, T.; Hu, C.W. Promoting effect of sodium chloride on the solubilization and depolymerization of cellulose from raw biomass materials in water. ChemSusChem 2015, 8, 1901-1907.


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(24) Li, J.M.; Jiang, Z.C.; Hu, L.B.; Hu, C.W. Selective conversion of cellulose in corncob residue to levulinic acid in an aluminum trichloride-sodium chloride system. ChemSusChem 2014, 7, 2482-2488. (25) De Wit, G.; Kieboom, A.P.G.; van Bekkum, H. Enolisation and isomerisation of monosaccharides in aqueous, alkaline solution. Carbohydr. Res. 1979, 74, 157-175.


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Table 1. Kinetics and Catalytic Performance for Isomerization of Glucose to Fructose by Heterogeneous Organic Bases

catalyst

PS-TBD

PS-DBU

PS-BEMP

Ea (kJ.mol-1)

temp (°C)

10-4 × κ (s-1)

YFru (%)

SFru (%)

carbon balance (%)

10-3 × TOF (molFru.mol1 -1 Nitrogen.s )

33

140 130 120 110 100

7.8 6.3 4.9 3.6 2.8

28 30 25 23 20

55 65 55 56 52

78.0 84.9 81.1 83.7 83.7

1.56 1.67 1.39 1.28 1.11

36

140 130 120 110 100

4.6 3.3 2.8 1.8 1.5

30 26 15 14 12

65 64 56 77 76

84.6 86.7 89.7 97.7 97.5

1.67 1.44 0.83 0.78 0.67

33

140 130 120 110 100

7.9 6.7 4.8 3.4 3.0

24 31 32 18 13

44 63 70 61 49

70.1 83.1 88.1 89.6 88.1

1.33 1.72 1.78 1.00 0.72

Reaction conditions: 10 wt% glucose, heterogeneous catalyst containing 10 mol% nitrogen relative to glucose, 1 mL H2O, 2-30 min, 100-140 °C. YFru: fructose yield at 30 min. SFru: fructose selectivity at 30 min. Carbon balance is defined as the ratio of moles of carbon in products (fructose and mannose) and unreacted glucose to the mole of carbon in the initial glucose. TOF: turnover frequency (moles of produced fructose/moles of nitrogen atom contained in catalyst/time).


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Figure 1. 1H-13C HSQC NMR spectrum of isomerized glucose by PS-TBD. Blue: glucose; black: mannose; red: fructose; green: byproducts. Reaction conditions: 10 wt% glucose, PS-TBD containing10 mol% nitrogen relative to glucose, 1 mL H2O, 30 min, 120 °C.


21


35

50

28

40

Glucose conversion (%)

Fructose yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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21

14

7

30

20

10

0 1

2

3

4

0 1

Run number

2

3

4

Run number

Figure 2. Reusability of the heterogeneous PS-TBD (black), PS-DBU (red) and PS-BEMP (blue) organic bases. Reaction conditions: 10 wt% glucose, the fresh (10 mol% nitrogen relative to glucose) or recycled catalyst, 1 mL H2O, 20 min, 120 °C.


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60

a

50

50

40

40

(%)

(%)

60

30

20

10

10

0

NaCl KCl

60

b

30

20

0

LiCl

NaBr LiBr

NaI

KI w/o salt

NaCl KCl

LiCl

60

c

50

50

40

40

(%)

(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


30

NaBr LiBr

a

b

NaI

KI w/o salt

c

30

20

20

10

10

0

0

0

NaCl KCl

LiCl

NaBr LiBr

NaI

KI w/o salt

3

6

9

12

15

18

21

NaCl concentration (wt%)

Figure 3. Effects of additions of neutral salts and sodium chloride concentration on glucose conversions (blue color or solid line) and fructose yields (red color or dash line) during the isomerization reactions catalyzed by the heterogeneous PS-TBD (a), PS-DBU (b), and PS-BEMP (c) organic bases. Reaction conditions: 10 wt% glucose, heterogeneous organic base containing 10 mol% nitrogen relative to glucose, 1 mL water with salt, 12 min, 100 °C.


23


75

70

a

60

45

50

b

(%)

(%)

60

30

40

15

30

0

20 40

50

60

70

80

90

100

110

0

3

6

Temperature (oC)

75

9

12

15

18

21

Reaction time (min)

70

c

d

60

60

50

45

(%)

(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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30

40

30 15

20 0 0

4

8

12

16

Catalyst dosage (mol% N)

20

10

20

30

40

50

Glucose concentration (wt%)

Figure 4. Effects of sodium chloride on glucose conversions (solid line) and fructose yields (dash line) during the isomerization reactions catalyzed by the heterogeneous PS-TBD (dot), PSDBU (square) and PS-BEMP (triangle) organic bases under different reaction conditions (a:10 wt% glucose, 10 mol% N, 12 min; b:10 wt% glucose, 10 mol% N, 90 °C; c:10 wt% glucose, 8 min, 90 °C; d:10 mol% N, 20 min, 90 °C).


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Figure 5. 1H and

13

C NMR spectra of glucose isomerized by BEMP without (red) and with

(blue) sodium chloride. Reaction conditions: 10 wt% glucose, BEMP containing 10 mol% nitrogen relative to glucose, 1 mL water without or with 1 wt% sodium chloride, 5 min, 80 °C.


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a

225

179 293 367

b

215 225

179

293

367

225 c

367

179 293

Figure 6. Negative modes of LC-Mass spectra of glucose (a), glucose isomerized by PS-BEMP with NaCl (b) and glucose isomerized by PS-BEMP without NaCl (c). Reaction conditions: 10 wt% glucose, PS-BEMP containing 10 mol% nitrogen relative to glucose, 1 mL water without or with 1wt% sodium chloride, 5 min, 80 °C.


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45

60

50

Glucose conversion (%)

36

Fructose yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


27

18

9

40

30

20

10

0

0 1

2

3

4

1

Run number

2

3

4

Run number

Figure 7. Reusability of the heterogeneous PS-TBD (red), PS-DBU (black) and PS-BEMP (blue) organic bases in the presence of sodium chloride. Reaction conditions: 10 wt% glucose, the fresh or recycled catalyst (10 mol% nitrogen relative to glucose), 1 mL water with 1 wt% sodium chloride, 5 min, 80 °C.


27


45

Glucose in D2O Glucose in H2O Glucose-2-D in H2O

36

27

(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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18

9

0 0

5

10

15

20

25

30

Reaction time (min)

Figure 8. Isomerization of glucose or glucose-2-D by PS-TBD in H2O or D2O. Glucose conversion: solid line. Fructose yield: dash line. Reaction conditions: 10 wt% glucose or glucose-2-D, PS-TBD containing 10 mol% nitrogen relative to glucose or glucose-2-D, 2-30 min, 1 mL H2O or D2O, 110 °C.


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Figure 9. 1H-13C HSQC NMR spectrum of isomerized glucose-2-D by PS-TBD. Blue: glucose2-D. Red: fructose. Reaction conditions: 10 wt% glucose-2-D, PS-TBD containing10 mol% nitrogen relative to glucose, 1 mL H2O, 30 min, 120 °C.


29


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Scheme 1. Proposed reaction mechanism of glucose isomerization catalyzed by polystyrene (PS) supported amine in the presence of neutral salt through an enediol intermediate after deprotonation at C-2 position (mannose was omitted for clarity).


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For Table of Contents Use Only

Title: Salt-promoted Glucose Aqueous Isomerization Catalyzed by Heterogeneous Organic Base

Synopsis: the neutral salt-induced promotion could make heterogeneous organic bases as efficient as homogeneous organic bases for glucose aqueous isomerization and also stabilize the heterogeneous organic bases to allow effective catalysis after recycling.


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Salt-Promoted Glucose Aqueous Isomerization Catalyzed by

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