Polyethylenimines as Homogeneous and Heterogeneous Catalysts for

Research Article pubs.acs.org/journal/ascecg

Polyethylenimines as Homogeneous and Heterogeneous Catalysts for Glucose Isomerization Qiang Yang and Troy Runge* Department of Biological Systems Engineering, University of WisconsinMadison, 460 Henry Mall, Madison, Wisconsin 53706, United States S Supporting Information *

ABSTRACT: This study investigated polyethylenimines (PEIs) with varied architectures as low toxicity and efficient catalysts for aqueous isomerization of glucose to fructose. Under the investigated reaction conditions, the studied PEIs achieved 33−36% maximum fructose yields with 66−77% selectivity at 110−120 °C, with the branched PEI generally outperforming the linear and comb PEIs. Moreover, with 1 wt % sodium chloride, the PEIs achieved 35−38% maximum fructose yields with 61−72% selectivity at 110 °C. After modification through room temperature cross-linking, the branched PEI was transformed into a recyclable heterogeneous catalyst with similar isomerization performance (30% fructose yield and 78% fructose selectivity at 120 °C) to the homogeneous PEIs. Remarkably, in the presence of neutral salts, the heterogeneous PEI achieved an approximately 41% fructose yield with 72−78% selectivity at 110 °C and showed an excellent reusability. KEYWORDS: Polyethylenimine, Glucose isomerization, Homogeneous catalysis, Heterogeneous catalysis, Salt-induced effect

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INTRODUCTION Efficient and selective conversion of lignocellulosic biomass to chemicals and fuels still remains a major technical and economic challenge.1 Fructose, an isomer of glucose, can be readily dehydrated to 5-hydroxymethylfurfural (HMF), a platform chemical that can be used to produce valuable chemicals and liquid fuels,2 which has greatly increased the research interest in this reaction.3−20 The isomerization of glucose to fructose is an equilibrium-limited reaction, and therefore the maximum attainable fructose yield is governed by the thermodynamic equilibrium between glucose and fructose at the temperature.19,20 Immobilized glucose isomerases,3,4 Lewis acids such as chromium chloride and aluminum chloride,5−8 bases such as sodium hydroxide and amines,9−12 metallosilicates,13 and metal oxides14−16 have all been studied as catalysts for the aqueous isomerization of glucose to fructose. Glucose isomerases, which are biocatalysts, are generally safer and typically used in food applications, yield approximately 42− 45% fructose at 53−60 °C.17,18 Despite their advantages, the immobilized glucose isomerases have several drawbacks including high cost, longer reaction time, use of buffering solutions, and irreversible deactivation.17 On the other hand, chromium chloride can achieve 20−25% fructose yields with 48−49% selectivity at 120−140 °C.5−8 Similarly, aluminum chloride can achieve an approximately 26% fructose yield with 83% selectivity at 120 °C.8 Chromium chloride and aluminum chloride are reported to be the most effective Lewis acids for the isomerization of © 2016 American Chemical Society

glucose to fructose. However, Lewis acids are costly to separate from the product streams. Sn-beta zeolite, a heterogeneous Lewis acid catalyst, can obtain approximately 31−45% fructose yields with 51−67% selectivity at 110−140 °C.8,19,20 The traditional synthesis of Sn-beta zeolite needs a fluoride medium, multiple steps, several days, and high temperature, making it relatively complex and expensive. However, recent efforts have been made to develop lower cost, faster, and safer postsynthetic approaches.21,22 Fructose yields for reported inorganic bases such as sodium hydroxide and calcium hydroxide are typically below 10% when operated at high pH values.10 Organic Brønsted bases can achieve 10−36% fructose yields with 40−73% selectivity at 60− 120 °C.10−12 However, organic bases such as triethylamine can cause graphic disturbances in vision and systemic health effects and have strong odors, in addition to difficulty of separation.23−25 Given the significance of fructose, and the trade-offs associated with the catalysts used in their synthesis, there still is a demand for novel fructose isomerization catalysts with better yields and properties. Polyethylenimine (PEI) is a polymer with repeating iminoethylene units and can have linear, branched, comb, network, and dendrimer architectures depending upon its synthesis and modification methods. PEI has been widely Received: August 7, 2016 Revised: September 27, 2016 Published: October 16, 2016 6951

DOI: 10.1021/acssuschemeng.6b01880 ACS Sustainable Chem. Eng. 2016, 4, 6951−6961

Research Article

ACS Sustainable Chemistry & Engineering Table 1. Textural, Solubility, and Base Properties of PEIs with Different Architectures PEI

architecture

aminea

ratiob

nitrogen contentc (mmol/g)

basic sites densityd (mmol/g)

solubilitye

SABETf

B L E H

branched linear comb network

pri, sec, ter sec sec, ter sec, ter

1:2:1

18.4 23.1 15.2 8.5

7.74 3.12 3.27 0.23

soluble insoluble soluble insoluble

236.8

1:4

a

pri, primary; sec, secondary; ter, tertiary. bThe mole ratio of different amines. cDetermined through elemental analysis. dMeasured through titration at room temperature. eIn water at room temperature. fCalculated from the nitrogen adsorption isotherm according to the Brunauer−Emmett−Teller (BET) method. the reference Ag/AgCl electrode was 4 M KCl. Brønsted basic site density was measured through the titration method. Attenuated total reflectance−Fourier transform infrared (ATR-FTIR) spectra were collected on a PerkinElmer Spectrum 100 Series FT-IR spectrophotometer with a universal ATR sampling accessory. Nitrogen content was determined on a PerkinElmer 2400 Series II Elemental Analyzer 2400. Brunauer−Emmett−Teller (BET) specific surface area was determined according to the nitrogen adsorption method using an Autosorb-1 surface area analyzer (Quantachrome Instruments, Boynton Beach, Florida). 1H and 13C NMR spectra were collected with a Bruker Biospin AVANCE 500 MHz NMR spectrometer. Glucose, fructose, and mannose were analyzed on an Agilent 1220 Infinity high performance liquid chromatography system equipped with an Agilent Hi-Plex H analytical column (7.7 × 300 mm), a BIORAD guard column, and a refractive index detector. Fructose yield (%) was calculated from the formed fructose as a mole percentage of the supplied glucose. Selectivity for fructose (%) was calculated from the formed fructose as a mole percentage of the reacted glucose. Organic acids (acetic, formic, glycolic, and lactic acids) were measured using the Dionex ICS-3000 system (Sunnyvale, CA), equipped with a UV− vis detector and Supelcogel C-610H analytic (30 cm × 7.8 mm) and guard (5 cm × 4.6 mm) columns.

applied in detergents, adhesives, water treatment, cosmetics, and carbon dioxide capture.26−29 Due to its relatively low toxicity and high ionic charge density (proton sponge), PEI has become a benchmark polymer as a DNA transfection agent and has been widely investigated for drug deliveries.30−33 PEI is a weak polymeric base with pKa values of about 7.9−9.6.31−33 However, compared with other amine catalysts, PEI has some advantages, including its relatively lower toxicity, facile modification, easier separation and recycling, and lack of odor, making it attractive if it could be shown to be an effective catalyst. PEI has been widely applied as a support material for immobilization of the glucose isomerases.34−36 Because the immobilized glucose isomerases are operated in phosphate buffer, PEI does not act as a weak polymeric base to isomerize glucose. However, our preliminary study showed that the branched polyethylenimine could achieve a 21% fructose yield with a 55% fructose selectivity.12 The objectives of this study were to (1) comparatively investigate the PEIs with linear, comb, branched, and network architectures as homogeneous or heterogeneous catalysts for the isomerization of glucose to fructose in water, (2) discuss the effect of the architecture on the isomerization performance, (3) provide a simple approach to fabricate the heterogeneous base isomerization catalyst based on PEI through facile crosslinking modification, and (4) demonstrate the promotion effect of neutral salt on the isomerization reaction.

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RESULTS AND DISCUSSION Isomerization of Glucose by Homogeneous PEI. Many studies have demonstrated that the base-catalyzed isomerization of glucose to fructose proceeds through an enediol intermediate formed after deprotonation or intramolecular proton transfer at C-2 position of acyclic D-glucose.9−12,37A distinction cannot be made between the deprotonation of acyclic glucose followed by intramolecular proton transfer pathway (Scheme S1) and the proton abstraction by an external base pathway (Lobry de Bruyn−Alberda van Ekenstein mechanism).11,37 However, it appears that the intramolecular proton transfer reaction pathway is more experimentally favorable. Amines can take hydrogen ions from water and be protonated, in situ generating hydroxide ions (Scheme S2). Therefore, amines such as triethylamine can isomerize glucose in water in a similar way to sodium hydroxide.11,12 The PEIs investigated in this study were polyamines.38−42 So, it may be reasonably speculated that the PEIs could share similar isomerization mechanisms to amines such as triethylamine and sodium hydroxide.10,11,37 Branched (B), linear (L), and ethoxylated (E) PEIs are synthesized using different methods (Scheme S3) and thus differ in the architecture and amine type (Table 1).41−43 Specifically, the branched PEI synthesized by the ring opening polymerization of aziridine has a random architecture and has the primary, secondary, and tertiary amines typically in the ratio of 1:2:1. The linear PEI synthesized by the hydrolysis of poly(2-ethyl-2-oxazoline) has a linear structure and has all secondary amines with terminal primary amine groups at the polymer chain end. The ethoxylated PEI synthesized by the ethoxylation of linear PEI has a comb structure and has the secondary and tertiary amines in a ratio of 1:4. Their nitrogen

EXPERIMENTAL SECTION

Materials. Ethoxylated polyethylenimine (E, Mw = 7 × 104, 80% ethoxylated, 35−40 wt % in water), linear polyethylenimine (L, Mw = 2.5 × 104), branched polyethylenimine (B, Mw = 2.5 × 104), DLglutaraldehyde solution (Grade II, 25% in water), anhydrous ethanol, sodium chloride (NaCl), lithium chloride (LiCl), potassium chloride (KCl), sodium bromide (NaBr), lithium bromide (LiBr), sodium iodine (NaI), potassium iodide (KI), D-fructose, and D-glucose were bought from Sigma-Aldrich. All chemicals were used as received. Cross-linking of PEI. Branched PEI (4 g) was dissolved in 40 mL of anhydrous ethanol, and then glutaraldehyde (3 mL) was slowly added to the solution. The cross-linking reaction was carried out under room temperature for approximately 12 h. Then, after the crosslinking, the PEI was separated through centrifugation and intensively rinsed with ethanol. The off-pink modified PEI (H in Table 1) was then lyophilized. Isomerization of Glucose. Aqueous isomerization experiments (10−50%, wt/wt) conducted in duplicate were carried out in 6-mLthick-walled glass reactors at different temperatures (80−120 °C). The isomerization experiment was allowed to proceed for different times (1−25 min) and then was stopped by cooling the reactor in an ice bath. Small aliquots of the filtered and diluted reaction media were taken for sugars and organic acids analysis. The average values of duplicates were reported without standard errors, for they were less than 1%. Characterization and Analysis Method. Zeta potential was measured with a SZ-100 Nanoparticle Analyzer. The pH value was measured with a Thermo Scientific pH meter. The filling solution of 6952


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ACS Sustainable Chemistry & Engineering

Table 2. Kinetics, Activation Energy and Catalytic Performance for Isomerization of Glucose by Homogeneous PEIa PEI

Ea (kJ mol−1)

temp (°C)

10−4 × κ (s−1)

YFru (%)

SFru (%)

YMan (%)

carbon balance (%)

10−3 × TOF (molFru·molNitrogen−1·s−1)

B

50 ± 3.1

L

53 ± 2.0

E

48 ± 2.8

120 110 100 90 80 120 110 100 90 80 120 110 100 90 80

8.5 7.9 6.4 2.7 2.0 5.5 4.8 2.1 1.2 1.0 10.1 8.9 7.1 3.0 2.5

32 30 27 13 9 25 21 10 6 6 32 29 26 16 12

80 80 83 81 93 90 82 83 66 74 70 68 72 82 84

3.6 3.9 4.2 1.6 0.3 2.5 3.2 1.9 2.2 1.6 3.4 3.6 4.1 1.5 1.2

95.6 96.4 98.7 98.5 99.6 99.7 98.6 99.8 99.1 99.5 89.7 90.0 94.0 98.0 98.9

2.31 2.17 1.95 0.94 0.65 1.44 1.21 0.58 0.35 0.34 1.00 0.91 0.82 0.50 0.38

Reaction conditions: 10 wt % glucose, 0.05 mol % PEI relative to glucose, 1 mL H2O, 1−15 min, 80−120 °C. B, L, and E stand for branched, linear, and ethoxylated PEIs, respectively. YFru: fructose yield at 10 min. SFru: fructose selectivity at 10 min. YMan: mannose yield at 10 min. Carbon balance: 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, mol (fructose)/mol (nitrogen)/s. a

contents were determined through elemental analysis and were reported in Table 1. It turns out that the linear PEI contains more nitrogen (23.1 mmol/g) than the branched (18.4 mmol/ g) and ethoxylated (15.2 mmol/g) PEIs do. The three PEI’s isomerization performances were comparatively investigated in this study with the isomerization reactions conducted with varying glucose concentration, PEI dosage, reaction time, and temperature, and the results were presented in Figures S1−S4 and Table 2. The results showed that the glucose conversion and achievable fructose yield generally increased with increasing the PEI dosage, reaction time, and temperature and decreased with increasing the glucose concentration. The PEIs achieved the maximum fructose yields under different reaction conditions. The branched PEI was found to be generally more effective than the linear PEI, with the best performance (33% and 77% fructose yield and selectivity, respectively) observed for the branched PEI at a dosage level of 0.05 mol % relative to glucose when the reaction was carried out at 120 °C for 10 min. Comparatively, at 120 °C for 15 min, the 0.05 mol % linear PEI achieved a relatively lower maximum fructose yield (29%) but better fructose selectivity (92%). The linear PEI at 120 °C for 15 min also achieved a 33% maximum fructose yield with 66% selectivity when the dosage was increased to 0.15 mol %. However, after the modification through ethoxylation (Scheme S3), the 0.05 mol % linear PEI at 120 °C achieved a 36% maximum fructose yield with 76% selectivity. These results indicate that the isomerization performance of the PEI is dependent upon its architecture. The basicity of amine in the aqueous solution was attributed to the formed hydroxide ions after the protonation of amine. However, the basicity strength of amine depends not only upon electron releasing effect but also steric effect and hydration effect. In the tertiary amine, the hydration is the least, while the steric hindrance is the maximum. In the primary amine, the steric hindrance and inductive effect are the least, while the hydration is the maximum. Therefore, considering the combined effects of electron releasing, steric hindrance, and hydration, the secondary amine is the greatest, and the tertiary amine is almost the same as the primary amine.10−12

In this study, the PEIs are polyamines. The PEI basicity is decided not only by the amine type but also the architecture. The basicity of the amine can be predicted by the pKa value of its conjugated acid. The more basic the amine, the higher the pKa of its conjugated acid. The pKa values for the primary, secondary, and tertiary amines in the branched PEI are approximately 9.4−9.64, 6.8−8.6, and 4.4−5, respectively.38−40 So, the pKa values of the secondary amine (6.8−8.6) and tertiary amine (4.4−5) are lower than that (9.4−9.64) of the primary amine. In contrast, the linear PEI has a pKa value of about 7.9 for the secondary amine.41 These results suggest that the architecture could affect the protonation of amine and the basicity of PEI in water. It can be speculated that the branched PEI was generally more basic than the linear PEI. Brønsted basic site densities of the linear and branched PEIs were measured through the titration methods, and the results are reported in Table 1. The results verified that the branched PEI was more basic (7.74 vs 3.12 mmol/g) than the linear PEI. So, it can be concluded that the architecture greatly influences the basicity of PEI in the aqueous solution. The branched PEI was composed of a main polyethylenimine chain with multiple substituent side chains or branches. As a result, the branched PEI was amorphous and was liquid at all molecular weights. The branched architecture allows the rapid protonation of amines in the branched PEI. Conversely, the linear PEI was composed of a linear polyethylenimine chain without side chains or branches. Due to the high structural regularity, the linear PEI is crystalline (melting point: ∼ 72 °C), and thus the protonation of secondary amines was difficult at room temperature. As a result, the linear PEI was a very weak base (0.03 mmol/g). However, the linear PEI became soluble in water and more basic (3.12 mmol/g) when the temperature reached over 72 °C. Therefore, the linear PEI showed the lower glucose conversions and fructose yields when the isomerization reactions were carried out at 80−90 °C. In general, the linear PEI showed relatively poorer isomerization performance than the branched PEI did (Figures S1 and S2 and Table 2). However, the ethoxylation modification greatly disrupted the structural regularity and changed the 80% secondary amines to tertiary amines. So, the ethoxylated PEI with a comb 6953


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glucose-to-fructose selectivity with increasing reaction temperature and PEI dosage (Table 2). In addition, the byproducts could be formed through the Maillard reactions between the amines and reducing sugars with aldehyde groups.10,11,47 Mechanistically, the Maillard reaction is much faster with the primary amine than the secondary amine, while the tertiary amine was not expected to react with the reducing sugars. The occurrence of Maillard reactions was evidenced in this study by the appearance of yellow and occasionally brown color. The occurrence of Maillard reactions may explain why the linear PEI shows slightly better selectivity than the branched PEI. Similarly, it was reported that the tertiary amines such as triethylamine achieved slightly better selectivity than secondary (e.g., morpholine) and primary (e.g., ethylenediamine) amines.10 In spite of the Maillard reactions, the PEIs (0.05 mol % relative to glucose) still achieved the similar fructose yields (33−36% vs 32%) and the slightly better glucose-tofructose selectivity (66−77% vs 63%) to triethylamine (10 mol % relative to glucose).10,11 Therefore, in this study, the thermalinduced sugar degradation and condensation was likely the dominant side reactions rather than the Maillard reactions. Additionally, the ethoxylation and cross-linking modifications could greatly alleviate or even eliminate the Maillard reactions for the linear and branched PEIs, respectively. Due to their very high molecular weights relative to triethylamine, the PEIs generally needed the higher dosages (6.94 wt % relative to glucose for the branched and linear PEIs, 19.4 wt % for the ethoxylated PEI) than triethylamine (5.62 wt % relative to glucose) to achieve similar fructose yields with slightly better selectivity. The isomerization performance achieved by the PEIs in this study were also better than the reported sodium hydroxide (typically below 10% fructose yield and very poor selectivity), chromium chloride (25.4% fructose yield and 48.6% selectivity), and aluminum chloride (26.3% fructose yield and 82.7% selectivity).5,8,9 Promotion of Neutral Salt on Isomerization of Glucose. The protonation of amine in water can in situ generate hydroxide ions, which acts as a base catalyst for the isomerization reaction. However, the addition of a neutral salt can greatly influence the protonation of amine in water.48−52 Specifically, the salt can promote the protonation of amine through changing the activity coefficients of all ionic species in the solution, and the protonation constant of amine is a function of the ionic strength.51,52 Upon the addition of salt, the protonated amine can form a complex with the anion of salt through electrostatic interaction, and the stability of the complex increases with increasing the protonation. The salt ions can influence differently the water dissociation and hence shift the acid−base equilibrium and increase the pH value of the amine aqueous solution. Previous research has shown that the addition of salts such as sodium chloride and lithium chloride created conformational changes and increased the pH value of a PEI aqueous solution from 7.0 to around 9.0.51,52 The PEIs in this study were positively charged weak polyelectrolytes due to the protonation of the primary, secondary, and tertiary amines, as indicated by the observed positive zeta potentials shown in Figure S7. The positively charged PEIs can specifically bind with anions of salts through electrostatic interactions. The pH values of the PEI aqueous solutions slightly increased with the addition of sodium chloride and then plateaued after the solution reached about 1 wt % sodium chloride (Figure S8).

architecture was as soluble as the branched PEI and had a 3.27 mmol/g Brønsted basic site density. The results showed that the ethoxylated PEI was as effective as the branched PEI in the glucose isomerization (Figures S1 and S3 and Table 2). The glucose isomerization, occurring through the rearrangement of glucose, is a unimolecular reaction and therefore is a first-order reaction.5−15,37 In this study, the glucose isomerization by the PEI displayed first-order reaction characteristics, as evidenced by the observed near linearity of the glucose conversion versus reaction time within 10 min, plotted at each temperature (Figures S1−S3). The rate constant (κ) at each temperature (80−120 °C) was calculated from the slope of ln([glucose concentration at time t]t/[initial glucose concentration]o) versus t plot (Figure S5), and the calculated rate constants were summarized in Table 2. Effects of reaction temperature and PEI dosage on the rate constant were determined and shown in Figure S5. Apparent activation energy (Ea) and turnover frequency (TOF, molFru·molNitrogen−1· s−1) were calculated based on the kinetics data and were also summarized in Table 2. The TOFs increased with increasing of reaction temperature but decreased with higher PEI dosage. The conversion of glucose to fructose was governed by the thermodynamic equilibrium between both sugars at the reaction temperature. The reaction rate and thermodynamic equilibrium constant both increased when the reaction temperature increased. As a result, the TOFs increased with increasing of the reaction temperature. The TOFs decreased with the higher PEI dosage, because the higher PEI dosage resulted in a greater decomposition of fructose, and the fructose did not increase as fast as the PEI dosage. The apparent activation energies for the branched, linear, and ethoxylated PEIs catalyzed isomerization reactions were estimated to be approximately 50 ± 3.1, 53 ± 2.0, and 48 ± 2.8 kJ mol−1, respectively. The estimated apparent activation energy values for PEIs were similar to that of triethylamine (58 ± 8 kJ mol−1) and chromium chloride (58.6−64.0 kJ mol−1) and lower than that of sodium hydroxide (121 kJ mol−1), aluminum chloride (110 ± 2 kJ mol−1), or Sn-Beta (93 ± 15 kJ mol−1).5,8,9,44 Due to their very high molecular weights, the observed TOFs (0.00034−0.0023 molFru s−1 molnitrogen−1) for the PEIs in this study were noticeably lower than that (0.018 ± 0.01 molFrus−1 molnitrogen−1) of triethylamine.11 The isomerization of glucose to fructose is slightly endothermic (ΔH = 3 kJ/mol).19 Therefore, the glucose isomerization reactions catalyzed by chemical catalysts are usually carried out at high temperatures (60−140 °C),5−20 which can cause irreversible sugar degradation.5−20,37 In this study, carbon balance based on the monosaccharide (fructose, mannose, and glucose) distribution was calculated and reported in Table 2. The results showed that the carbon balance decreased when increasing the reaction temperature, suggesting nonsugar byproducts were produced during the glucose isomerization. Even without PEIs, fructose and glucose were not thermally stable (Figure S6). The base promoted irreversible sugar degradation, which led to degradation (e.g., organic acids), and aldol condensation products (Scheme S4).9−12,37,45,46 Some organic acids including the formic, acetic, glycolic, and lactic acids were not detectable in most cases and thus were not reported in this study. Similar observations were also reported for the triethylamine-catalyzed isomerization.11 Insoluble byproducts such as aromatics and polymeric melanoidins might also be formed as byproducts,45,46 which was speculated to be the cause of the observed decreased 6954


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Figure 1. Effects of neutral salts on the isomerization (blue, glucose conversion; red, fructose yield) of glucose by branched (B), linear (L), and ethoxylated (E) PEIs. Reaction conditions: 10 wt % glucose, 0.01 mol % or 0.05 mol % PEI relative to glucose, 1 mL water with 1 wt % neutral salt, 10 min for the 0.01 mol % PEI or 8 min for the 0.05 mol % PEI, 110 °C.

When the PEI dosages were 0.01 mol % relative to glucose and the reactions were carried out at 110 °C for 10 min, the 1 wt % salts enhanced the achievable fructose yields by 3−23%, 27−57%, and 5−22% for the branched, linear, and ethoxylated PEIs, respectively. Similarly, the 1 wt % salts also greatly enhanced the fructose yields for the 0.05 mol % branched (16− 26%), linear (32−38%), and ethoxylated (17−22%) PEIs, when the reactions were carried out at 110 °C for 8 min. Especially, the addition of 1 wt % sodium chloride increased the fructose yields for the branched, linear, and ethoxylated PEIs by 6−85%, 24−116%, and 6−56%, respectively, when the reactions were implemented at 80−120 °C for 10 min with the 0.05 mol % PEIs. Moreover, in the presence of sodium chloride, the maximally achievable fructose yields by the PEIs under the investigated reaction conditions were enhanced to some degree. Specifically, with the 1 wt % sodium chloride, the approximately 37%, 35%, and 38% maximum fructose yields were respectively achieved by the 0.05 mol % branched, linear, and ethoxylated PEIs. Nevertheless, under the investigated reaction conditions, the addition of salt generally decreased the fructose selectivity due to the accelerated thermal degradation of fructose (Table 3 and Figure S12). It is likely that the high fructose yields with

The salt-induced increase in the pH value (i.e., concentration of hydroxide ion) and the shift of protonation reaction equilibrium were expected to promote the glucose isomerization by the PEI. Effects of several neutral salts (1 wt %, based on water) on the isomerization reactions of glucose by the 0.01 mol % and 0.05 mol % PEIs were investigated, and the results are presented in Figures 1, S9, and S10 and Table 3. To investigate the salt effects, the isomerization reactions with salts were carried out under similar conditions to the isomerization reactions without salts. In the absence of the PEI, the neutral salts did not convert glucose, as shown in Figure S11. However, the salts greatly facilitated the glucose isomerization reactions through decreasing the apparent activation energy and increasing the rate constants, TOFs, and glucose conversions. In particular, the salts enhanced the achievable fructose yields. In the absence of salt, the higher reaction temperatures (e.g., 110−120 °C) and longer reaction times (e.g., 12−15 min) were necessary to achieve higher fructose yields (Table 2). However, in the presence of salt, the higher fructose yields could be achieved under more mild reaction conditions (lower reaction temperature, shorter reaction time, and less PEI; Table 3). 6955


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Table 3. Effects of Sodium Chloride on Kinetics, Activation Energy, and Catalytic Performance for Isomerization of Glucose by Homogeneous PEIa PEI

Ea (kJ mol−1)

temp (°C)

10−4 × κ (s−1)

YFru (%)

SFru (%)

YMan (%)

carbon balance (%)

10−3 × TOF (molFru·molNitrogen−1·s−1)

B

37

L

43

E

33

120 110 100 90 80 120 110 100 90 80 120 110 100 90 80

12.5 11.0 8.0 5.8 3.5 10.0 7.5 4.8 3.3 2.3 11.4 9.1 7.2 5.0 3.6

34 36 32 24 14 34 26 20 13 10 34 34 31 25 17

64 72 76 82 71 61 61 76 63 66 65 76 80 85 83

2.6 3.3 3.6 1.8 2.6 3.2 3.6 1.3 2.6 2.2 3.2 2.5 2.1 1.3 1.0

83.5 89.3 93.5 96.5 96.9 81.5 87.0 95.0 95.0 97.0 84.9 91.8 94.3 96.9 97.5

2.46 2.60 2.31 1.73 1.01 1.96 1.50 1.15 0.75 0.58 1.07 1.07 0.97 0.78 0.53

Reaction conditions: 10 wt % glucose, 0.05 mol % PEI relative to glucose, 1 mL H2O with 1 wt % NaCl, 1−15 min, 80−120 °C. B, L, and E stand for branched, linear, and ethoxylated PEIs, respectively. YFru: fructose yield at 10 min. SFru: fructose selectivity at 10 min. YMan: mannose yield at 10 min. Carbon balance: 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, mol (fructose)/mol (nitrogen)/s. a

Scheme 1. Illustration of the Shift of Protonation Reaction Equilibrium between PEI and Water by the Addition of Neutral Salt (M = Li+, Na+, K+; X = Cl−, Br−, I−)

peaks around δ = 3.9−4.1 ppm attributed to the formed fructose were observed in the isomerized glucose in the presence of sodium chloride (Figure S15). The comparative 1H NMR studies provided a direct evidence for the observed saltinduced facilitation on the glucose isomerization. Additionally, upon the addition of sodium chloride, the 1H NMR signals of sugars slightly moved downfield, which may suggest the interaction between the chloride ions and the hydroxyl groups of sugars (Figure S15).53,54 The weak interactions between the chloride ions and sugars might alleviate the thermal degradation of glucose and fructose, thus slowing the elimination reactions.55 Isomerization of Glucose by Heterogeneous PEI. In addition to a satisfactory isomerization performance, two desirable characteristics for a glucose isomerization catalyst include (i) easy separation from sugars and (ii) reusability with stable performance after recycling.11 In this study, after the isomerization, the separation of the PEI from the sugar medium can be achieved through three approaches. One approach was to achieve separation through a liquid chromatography. An alternate approach would be to take advantage of the size differences between sugars and PEI by using membrane filtration or dialysis for the separation. A third approach considered was to separate PEI and sugars based on the difference in their solvent miscibility. For example, water is removed through lyophilization, and then nonpolar solvents

good selectivity may be achieved under more mild reaction conditions. It appears that the salt-induced facilitation was more pronounced for the glucose isomerization by the linear PEI. The salts could presumably interact with the secondary amines and interfere with the intermolecular hydrogen bonding and structural regularity.51,52 As a result, more amines could be likely protonated after the intermolecular hydrogen bonding and structural regularity were disrupted. So, the sodium chloride increased the pH values of the linear PEI aqueous solutions (Figure S8). Consequently, the linear PEI with the salts could achieve similar fructose yields to the branched and ethoxylated PEIs without the salts. The specific bindings of anions (Cl−, Br−, and I−) of salts to the positively charged sites of the PEI chains can shift the protonation reaction equilibriums between amines and water toward the generation of hydroxide ions (Scheme 1). To further understand the observed effect of salt, the isomerization reactions by the 0.05 mol % branched PEI without or with the 1 wt % sodium chloride were monitored by the NMR spectroscopy (Figures 2 and S13−S15). The results show that the PEIs in the sugar solutions with or without sodium chloride displayed the different 1H NMR signals (Figures 2 and S15), presumably caused by the increased pH (Figure S8) and interaction between the PEI and the chloride ions.53,54 As expected, there were no differences observed in their 13C NMR spectra, indicating that the interactions between the PEI and chloride ions were weak.53,54 Stronger 1H NMR signals for the 6956


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wide variety of chemical modifications. In this study, the branched PEI was readily cross-linked with glutaraldehyde at room temperature, as illustrated in Scheme 2. After the crosslinking modification, the branched PEI became insoluble in water, with a slightly pink color. The cross-linking modification was confirmed using attenuated total reflectance-Fourier transform infrared (ATRFTIR) spectroscopy (Figure S16). In the branched PEI, the primary amine showed a pair of weak bands at about 3360 and 3430 cm−1, respectively; while the secondary amine showed a single band at 3360 cm−1, and the tertiary amine did not show any band in this region. After the cross-linking, the peak intensity of amine (N−H) stretching between 3688 and 3011 cm−1 decreased, and the peak at 3430 cm−1 disappeared. More importantly, a new peak at 1650 cm−1 attributed to the imine (CN) or enamine (CC−N) bond stretching appeared. Meanwhile, the peak intensity of alkyl (C−H) stretching between 2950 and 2850 cm−1 increased. These results prove that the branched PEI was successfully cross-linked through the imine and enamine bonds. Elemental analysis shows that the nitrogen content in the cross-linked PEI was about 8.5 mmol/g, which was lower than that (18.4 mmol/g) in the branched PEI (Table 1). The cross-linked PEI was mechanically ground to a particle size of approximately 20−40 μm. Brunauer−Emmett− Teller (BET) analysis was performed on the ground material, which determined its average surface area was 236.8 m2/g and its average pore size in diameter was about 5.95 nm (Table 1). The cross-linking modification transformed the homogeneous PEI to a heterogeneous base catalyst. This heterogeneous PEI (H in Table 1) was different from the branched PEI in terms of the basic site density, architecture, and amine type. The Brønsted basic site density of the heterogeneous PEI was about 0.23 mmol/g, which was lower than that (7.74 mmol/g) of the branched PEI. The heterogeneous PEI had a network architecture and had secondary and tertiary amines. In addition, the Maillard reactions associated with the primary and secondary amines would be greatly eliminated, because the modification nearly eliminated the primary amine and greatly reduced the secondary amine. As a heterogeneous catalyst, the isomerization reaction by the modified PEI was allowed to proceed for 25 min.12 Effects of the glucose concentration, PEI dosage, reaction time, and temperature on the isomerization reaction were reported in Table S1 and Figures S17 and S18. The results showed that the branched PEI after the crosslinking modification still achieved a 30% fructose yield with 78% selectivity when the dosage was 20 wt % relative to glucose and the reaction was conducted at 120 °C for 25 min. Nevertheless, the isomerization reaction became slower, likely due to the large steric hindrance and mass-transfer resistance caused by the high density of the cross-linked polymeric network. The modification slightly lowered (41.4 kJ mol−1 vs 50 ± 3.1 kJ mol−1) the apparent activation energy for the isomerization reaction. The lower activation energy could be

Figure 2. 1H and 13C NMR spectra of glucose solutions with (red) or without (blue) sodium chloride catalyzed by branched PEI. Glu, glucose; Fru, fructose; Man, mannose. Reaction conditions: 10 wt % glucose, 0.05 mol % PEI relative to glucose, 1 mL D2O with or without 1 wt % sodium chloride, 8 min, 110 °C.

such as dichloromethane are added to dissolve PEI from the sugars. Even with these three methods available, a more facile separation of the PEI from sugars is still desirable for commercialization. Different from small molecule amines such as triethylamine, the branched PEI in this study has highly reactive primary and secondary amines, which enables a

Scheme 2. Illustration of Cross-Linking of the Branched PEI with Glutaraldehyde

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Figure 3. Effects of neutral salts on the isomerization (blue-glucose conversion; red-fructose yield) of 10 wt % glucose catalyzed by heterogeneous (H) PEI. Reaction conditions: (a) 20 mg PEI, 25 min, 110 °C; (b) 20 mg PEI, 15 min, 60−120 °C; (c) 20 mg PEI, 2−25 min, 80 °C (dot) or 90 °C (triangle); (d) 5−20 mg PEI, 15 min, 90 °C.

Figure 4. Reusability (blue-glucose conversion; red-fructose yield) of heterogeneous (H) PEI without (a) or with (b) sodium chloride. (a) Reaction conditions: 10 wt % glucose, 20 mg the fresh or recycled PEI, 1 mL H2O, 25 min, 110 °C. (b) Reaction conditions: 10 wt % glucose, 20 mg the fresh or recycled PEI, 1 mL H2O with 1 wt % sodium chloride, 15 min, 90 °C.

which requires radical polymerization of the vinyl monomer followed by post functionalization.12 Effects of the PEI dosage, reaction time and temperature on the isomerization of glucose in the presence of 1 wt % sodium chloride were also investigated, and the results were presented in Figure 3b-d, which indicated the addition of salt enhanced the fructose yield by about 67% for the heterogeneous PEI. The results showed that a similar isomerization performance could be achieved under more mild reaction conditions (lower reaction temperature or/and shorter reaction time) in the presence of sodium chloride. The results also indicated that the addition of salt could reduce the demand for the heterogeneous PEI to achiever a satisfactory isomerization performance. Reusability of the heterogeneous PEI was preliminarily evaluated in this study by performing four consecutive isomerization cycles at 110 °C for 25 min, and the result was presented in Figure 4. After each cycle, the spent PEI was carefully recovered through filtration and intensively rinsed

attributed to the fact that the cross-linking changed the primary and secondary amines to secondary and tertiary ones, respectively.12 Increasing the glucose concentration resulted in decreases in the glucose conversions and fructose yields. However, the heterogeneous PEI could achieve better isomerization performance with increased dosage. Effects of the neutral salts on the isomerization of glucose by the heterogeneous PEI were also investigated, and the results are shown in Figure 3. Similar to the homogeneous PEIs, the salts significantly enhanced the isomerization performance of the heterogeneous PEI (Figure 3a). Remarkably, with the 1 wt % salts, the heterogeneous PEI achieved an approximately 41% fructose yield with 72−78% selectivity, which was close to the state of the art performance (a 45.3% fructose yield with 67.1% fructose selectivity) of the Sn-beta zeolite.8,19,20 However, the fabrication of the heterogeneous PEI was simpler and cheaper than that of the Sn-beta zeolite.19−22 The fabrication was also easier than that for the polystyrene resin-supported amines, 6958



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with water, and then the 10 wt % fresh glucose aqueous solution was added to start a new cycle. The recycled PEI achieved gradually decreased fructose yields (Figure 4a). Glucose and fructose are not thermally stable, and can be decomposed and condensated to a variety of small molecule, oligomeric and polymeric byproducts under basic conditions.5−20,37,45,46 It was observed that the byproducts during the triethylamine-catalyzed glucose isomerization could be selectively adsorbed by activated carbon.10 Similarly, the formed byproducts during the glucose isomerization by the heterogeneous PEI could be adsorbed or deposited onto its surface.12 However, soluble byproducts such as organic acids were not found in this study, likely due to their concentrations being below the detection limit of high-performance ion chromatography.11,12,46 Even though the spent PEI was intensively rinsed with water, insoluble byproducts such as colored aromatics, and polymeric melanoidins, likely still remained on the spent PEI surface.12,45,46 This speculated accumulation of byproducts was presumed to be responsible for the observed activity loss through impeding the protonation reactions of amines in the spent PEI.12 Further studies on the identifications of the insoluble byproducts will be needed to provide insights on the mechanism of the loss in catalytic activity and strategies on the improvement of the reusability. Interestingly, salt addition was found to be able to restore or even improve the isomerization performance of the spent PEI (Figure S19). For instance, adding 1 wt % sodium chloride helped the spent PEI after a first run to achieve a 34.7% fructose yield, which was even higher than that (a 25% fructose yield) achieved by the fresh PEI. The reusability of the heterogeneous PEI in the presence of sodium chloride was also evaluated by performing four consecutive isomerization cycles at 90 °C for 15 min. As expected, the heterogeneous PEI showed a stable performance (approximately 37% fructose yield and 88% selectivity) during four consecutive isomerization cycles (Figure 4b). The promotion of sodium chloride on the protonation of amines should be primarily responsible for the good reusability. In addition, under milder reaction conditions, less thermal-induced byproducts would presumably be generated during the isomerization reaction, which would contribute to the observed good reusability result.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b01880. Illustration of isomerization mechanism (Scheme S1); illustration of protonation reaction (Scheme S2); illustration of synthesis routes and chemical structures of PEIs (Scheme S3); illustration of possible pathways for isomerization, decomposition, and condensation reactions (Scheme S4); isomerization of glucose by heterogeneous PEI (Table S1); isomerization of glucose by homogeneous PEIs (Figures S1−S5); thermostability of glucose and fructose (Figure S6); zeta potential of PEI (Figure S7); effect of sodium chloride (Figures S8−S12); 1 H NMR spectra of isomerized glucose solutions (Figures S13−S15); ATR-FTIR spectrum of heterogeneous PEI (Figure S16); and isomerization of glucose by heterogeneous PEI (Figures S17−S19) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 608-890-3143. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the United States Department of AgricultureNational Institute of Food and Agriculture (USDA BRDI Grant number 2012-10006-19423) and the Wisconsin Energy Institute for their financial support.

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REFERENCES

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CONCLUSIONS

This study demonstrated that odor-free and low toxicity polyethylenimines (PEIs) with different architectures can be synthesized on a large scale and low cost and were efficient catalysts for aqueous glucose isomerization (33−36% fructose yields and 66−77% fructose selectivity at 110−120 °C). Furthermore, the addition of low cost salts (1 wt % based on water) was shown to greatly facilitate the isomerization reactions and increase the achievable fructose yields to 35− 38% at 110−120 °C. More importantly, after a simple roomtemperature cross-linking, the PEI was converted to a recyclable heterogeneous catalyst with similar isomerization performance to the homogeneous PEIs. Remarkably, in the presence of the 1 wt % neutral salts, the heterogeneous PEI achieved an approximately 41% fructose yield with 72−78% selectivity at 110 °C. 6959


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