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Comparative investigation of homogeneous and heterogeneous Brønsted base catalysts for the isomerization of glucose to fructose in aqueous media
Applied Catalysis B: Environmental 261 (2020) 118126
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Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb
Comparative investigation of homogeneous and heterogeneous Brønsted base catalysts for the isomerization of glucose to fructose in aqueous media ⁎⁎
Season S. Chena,b,c, Daniel C.W. Tsangc, , Jean-Philippe Tessonniera,b,
T
⁎
a
Department of Chemical and Biological Engineering, Iowa State University, Ames, IA, 50011, United States NSF Engineering Research Center for Biorenewable Chemicals (CBiRC), Ames, IA, 50011, United States c Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Biomass conversion Glucose isomerization Brønsted base catalysis Molecular structure Principal component analysis
Glucose isomerization to fructose via Brønsted base catalysis is sparking increasing interest due to the importance of fructose as a feedstock molecule for the production of renewable chemicals. The work reported here stems from an interest in understanding the key parameters governing glucose isomerization in addition to solution basicity, and in identifying catalyst features that are key to fructose selectivity. A wide range of homogeneous and heterogeneous Brønsted bases were investigated under identical reaction conditions to enable their cross comparison. The results showed that glucose conversion was positively correlated with the basicity of the solution, although exceptional cases were observed owing to the specific structure of that catalyst. Interactions between the cations associated with metal hydroxide catalysts and carbohydrates affected the isomerization of glucose. Likewise, cations formed through protonation of organic catalysts in water (e.g. amines) were shown to alter the glucose isomerization process and impact both conversion and selectivity under iso-pH conditions. Principal component analysis revealed that the catalytic patterns were dependent on the catalyst nature and structure. Overall, meglumine showed a superior catalytic performance compared to other homogeneous bases with a yield of 35% fructose and approximately 80% selectivity. These results obtained under identical experimental conditions will help to identify promising catalyst structures for future heterogeneous catalyst design.
1. Introduction The production of renewable chemicals from biomass has gained increasing attention in recent decades to alleviate our dependence on fossil resources, transition to a sustainable chemical industry, and revitalize this industry by supplying new molecules that are not easily accessible from petroleum [1–4]. The most common strategy for producing biobased chemicals consists in deconstructing biomass to separate its carbohydrate fraction (starch, cellulose, hemicellulose) from lignin, followed by chemical or biochemical hydrolysis of the polysaccharides to produce sugars, notably glucose [5–7]. Glucose can be subsequently fermented using metabolically-engineered microorganisms to produce biologically-accessible chemicals (e.g., lactic acid, succinic acid, muconic acid, etc.). However, its thermocatalytic conversion necessitates an additional isomerization step to fructose, which is critical for the production of biobased platform chemicals, such as 5hydroxymethylfurfural (HMF), levulinic acid (LA), and their derivatives
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such as 2,5-furandicarboxylic acid (FDCA) and 5-ethoxymethylfurfural (EMF) [8–10]. Industrial-scale glucose-to-fructose conversion currently relies on an enzymatic process developed for the production of high fructose corn syrup (HFCS). This process operates near thermodynamic equilibrium and yields a sugar stream containing ∼42% fructose, which can be subsequently separated from glucose using simulated moving bed chromatography [11]. While well-established and robust, the HFCS process generates food-grade fructose, making it challenging to manufacture biobased platform chemicals at a low cost [12–14]. Scalable chemo-catalytic conversions are therefore critically needed for the industrial production of technical fructose for use as a feedstock in the emerging biobased chemical industry. The isomerization of glucose to fructose proceeds in the presence of Lewis acids and Brønsted bases via two distinct mechanisms that involve, respectively, an intramolecular hydride shift and an intramolecular proton transfer (Lobry de Bruyn-Alberda van Ekenstein
Corresponding author at: Department of Chemical and Biological Engineering, Iowa State University, Ames, IA, 50011, United States. Corresponding author. E-mail addresses: [email protected] (D.C.W. Tsang), [email protected] (J.-P. Tessonnier).
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https://doi.org/10.1016/j.apcatb.2019.118126 Received 11 January 2019; Received in revised form 25 July 2019; Accepted 23 August 2019 Available online 20 September 2019 0926-3373/ © 2019 Elsevier B.V. All rights reserved.
Applied Catalysis B: Environmental 261 (2020) 118126
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resins containing primary, secondary, tertiary amines or quaternary ammonium. Aqueous alkaline solutions were prepared using sodium hydroxide (> 99.8%), calcium hydroxide (> 98%), and barium hydroxide (> 98.1%) purchased from Fisher Scientific, and magnesium hydroxide (95%) obtained from Sigma Aldrich. Desired amounts of hydroxides were dissolved in an aqueous solution of 10 wt.% glucose to obtain a target concentration. The divalent hydroxides mixtures were filtered using a 0.22-μm syringe nylon filter (Celltreat®) to ensure an homogeneous reaction without undissolved residues. Homogeneous aminecontaining compounds were all purchased from Sigma Aldrich. For example, compounds containing primary amines were L-arginine (≥98%, Sigma Aldrich), branched polyethylenimine (PEI) (Mw ∼ 25,000), ethoxylated PEI (80% ethoxylated, 35∼40 wt% in H2O), guanidine carbonate salt (99%), tris(2-aminoethyl)amine (96%), and N,N-dimethylethylenediamine (95%). Compounds containing secondary amines were morpholine (≥99%), meglumine (99%), piperazine (99%), 1-methylpiperazine (99%), and piperidine (≥99.5%). Triethylamine ((≥99%) and 1,4-dimethylpiperazine (98%) were investigated as homogeneous tertiary amines. Various styrene-divinylbenzene-based commercial anion exchange resins were used as heterogeneous catalysts such as AmberlystTM A21 and AmberliteTM IRA96 with tertiary amines, as well as AmberlystTM A26 OH and AmberliteTM IRA900RF Cl with quaternary ammoniums (DOW Chemical). The anion exchange resins were first activated in 1 M NaOH for 1 h at a 1:3 resin-to-solution volume ratio and rinsed with copious amounts of deionized water. (3-aminoproply)triethoxysilane (APTES, 99%, Sigma Aldrich), (N,N-diethyl-3-aminopropyl)trimethoxysilane (> 95%, Gelest), and 2Chloro-N,N-dimethylethylamine hydrochloride (99%, Sigma Aldrich) were used to synthesize microcrystalline cellulose-based (microcrystalline cellulose powder, Sigma Aldrich) solid catalysts and aminemodified carbon nitride with primary amines and tertiary amines, respectively. A detailed synthesis procedure for these heterogeneous catalysts is provided in the Supporting Information. Polyethylenimine on silica gel (20–60 mesh, Sigma Aldrich) was tested for comparing immobilized PEI and homogeneous PEI.
mechanism) [15,16]. Lewis acids such as metal chlorides and tin-beta zeolite have been studied in great details, both experimentally and theoretically. They produce fructose in relatively high yields but recurring issues related to their long-term stability in aqueous media and recycling have hampered the translation of this technology for the industrial production of technical fructose [17]. In contrast, the Brønstedbase-catalyzed route received significantly less attention as early studies reported poor selectivities to fructose and yields below 10% [18–20]. It is only recently that glucose was selectively isomerized to fructose with yields comparable to Lewis acids using organic aminebased homogeneous and heterogeneous catalysts [16,21–25,50]. It has been demonstrated that homogeneous amines can catalyze glucose isomerization, with triethylamine affording the highest fructose yield (32%) among the tested amines because of the suppression of the Maillard reactions that accompanies the reactions with primary or secondary amines [21,26]. Yang et al. also reported an effective fructose production with a maximum of 31% yield and 76% selectivity under optimized conditions using a basic amino acid (L-arginine) [23]. However, the corresponding heterogeneous catalysts, e.g. polyethylenimine, exhibited variable reactivity depending on the structure of the polymer (linear, branched, and ethoxylated), indicating that the chemical environment of the amino groups may alter the performance of these materials [24]. These results are intriguing as a detailed kinetic and mechanistic study revealed that the hydroxide ions (OH−) generated in situ through interaction between the amine and water, rather than the amine itself, were the active species involved in the basecatalyzed isomerization of glucose to fructose [16]. Similar inconsistencies were noted when comparing the catalytic activity of free amines and the corresponding groups tethered onto polystyrene beads [22,27] and mesoporous silica [25,50]. Although the large and uniform pores of the mesoporous silica support facilitate mass transfer, the reaction rates were found to be much lower for the heterogeneous catalysts than for the corresponding homogeneous organic bases. This result was likely associated with the immobilization of the amine on the silica support, where surface silanols may either affect the reaction directly or indirectly through interaction with the grafted amino groups. Experiments performed with various inorganic bases such as metal oxides (e.g., MgO, CaO, TiO2, and ZrO2), magnesium-modified zeolites, and naturally occurring minerals (e.g., attapulgite and imogolite) did not yield any clear structure-activity relationships either [28–31]. Rational design with lessons learned from previous studies is critical to devise effective heterogeneous base catalysts. For the base-catalyzed isomerization of glucose to fructose, it requires the assessment of multiple variables including catalyst basicity, molecular structure, recyclability, product selectivity, etc. to evaluate the catalyst’s suitability for this process [32,33]. However, cross comparison of previous studies and attempts to relate the catalyst structure to glucose isomerization activity have, so far, failed owing to important differences in experimental protocols. Therefore, this study aims to (i) evaluate the catalytic performance of various groups of catalysts including homogeneous inorganic bases, organic amines, and heterogeneous anion exchange resins under identical reaction conditions; (ii) reveal the critical parameters for the basecatalyzed glucose isomerization via correlation analysis; (iii) characterize the catalyst behaviors in relation to catalyst nature and structure by principal component analysis (PCA); and (iv) identify potentially promising homogeneous bases for future design of heterogeneous catalysts.
2.2. Catalytic tests A 10 wt% glucose solution (≥99.5%, Sigma Aldrich) was prepared in deuterium oxide (D2O, 99.9 atom% D, Sigma Aldrich) using a 50 mL 3-neck round-bottom flask equipped with an olive shape stir bar, a bubbler, and a septum with a needle. The solution was purged with Ar gas under vigorous stirring for at least 30 min to remove any dissolved CO2 that may titrate some of the basic sites of the homogeneous/heterogeneous catalysts. The system was air-tight but allowed the release of excess Ar through the needle. Liquid catalysts were transferred to the glucose solution using a syringe to keep the solution free of air and CO2, and the obtained mixture was further purged with Ar for 10 min. The amount of organic catalyst was set at 12 mol% of nitrogen relative to glucose, which was based on our previous studies [16,21]. The prepared mixture was then transferred to Ar-purged 5-mL thick-walled glass reactors (Chemglass Life Sciences) each containing a V-shaped stir bar. Solid catalysts were added in powder form directly to Ar-purged reactors. The amount of inorganic catalyst was set to achieve a pDo of 10 to facilitate the comparison with the organic bases. The reactors were subsequently sealed and placed in a preheated oil bath (digital IKA RCT stirring hot plate equipped with PT100 thermocouple) at 100 °C (time 0). Preliminary tests showed that the reaction mixture reached the target temperature within 2 min and that inserting reactors at room temperature into the oil bath did not change the temperature of the oil bath by more than 2 °C. The reactors were removed from the oil bath after 30 min and immediately transferred to an ice bath to quench the reaction. The pH values before and after reaction were measured using a pH meter (Metter Toledo) at room temperature calibrated using
2. Materials and methods 2.1. Chemicals The tested catalysts can be divided into the following categories: aqueous alkaline solutions, homogeneous compounds containing primary, secondary, or tertiary amines, and heterogeneous anion exchange 2
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buffer solutions at pH 4, 7, and 10 before use. The pH probe was fitted with a septum to prevent the introduction of atmospheric CO2 into the solutions during measurement. The pD value in D2O was determined by conversion of pH*, the direct reading in a D2O solution with the H2Ocalibrated pH-meter, using the relationship proposed by Krȩżel and Bal (i.e., pD = 0.929pH*+ 0.42) [34].
(Table 1, Y = 31% at 12 N mol%) or the number of basic groups (Table S2, Y = 29% at ca. 6 N mol%) in the solution was fixed. More importantly, for polymeric compounds, fixing the amount of catalyst on a per mole basis may lead to potentially obscure conclusions due to variations in the average molecular weight and the average number of basic groups (concentration) in the tested polymers. For example, branched PEI and ethoxylated PEI showed substantial differences in catalytic performance at 12 mol.% N relative to glucose (Table 1), in good agreement with the differences in turnover frequencies reported in a previous study [24]. However, fixing the amount of PEI (0.05 mol% relative to glucose) led to very similar fructose yields (27% for branched PEI and 26% for ethoxylated PEI at 100 °C and 10 min) [24]. Hence, these results clearly demonstrate the need for standard testing conditions and benchmarks to facilitate the comparison of catalytic results across research teams. These results also reveal the need for a broad study exploring the performance of a range of homogeneous and heterogeneous catalysts under identical reaction conditions. It can be seen from the correlation matrix (Fig. 1) that compiles the reaction data from Table 1 that fructose yield is strongly and positively correlated with glucose conversion (coefficient of correlation r = 0.778) and initial pD of the solution (pDo) (r = 0.784). These results are consistent with our previous kinetic study, which demonstrated that hydroxide anions generated through interaction of the organic base and water are the main drive for the triethylamine-catalyzed isomerization of glucose to fructose [16]. Fig. 1 also reveals that fructose selectivity is not significantly correlated with most variables (r < 0.49), except with the pD of the reacted solution (pDf) (r = 0.603), although it is generally true that the selectivity decreases with increasing glucose conversion. Notably, the selectivity is widely distributed at each of the two sides of the linear fitting for glucose conversions below 20%. This result implies that some catalysts may excel the others at low conversion, especially homogeneous catalysts that generate a pDo close to 10 (vide infra). Heterogeneous catalysts will be discussed in details in the next section. Since the pKa value of fructose is similar to that of glucose (∼12), pDf can also serve as an indication for the formation of acidic byproducts and provide insights into factors responsible for low fructose yields. pDf was found to be poorly correlated with most variables (r < 0.58), except with the change of pD over the course of the catalytic test (ΔpD) (r = 0.765) and selectivity (r = 0.603), which is fairly sensible because a high pDf suggests a small change in basicity (pD) during the reaction, hence reduced Maillard and alkaline degradation reactions, indicating also higher selectivity. Interestingly, while a correlation between pDf and pDo does exist (r = 0.585), the pDo-ΔpD matrix revealed that the pDf value can drop by 0.5–3 units regardless of the corresponding pDo value. As a matter of fact, glucose conversion showed a higher correlation with ΔpD (r = 0.614) than with pDo. Therefore, it can be hypothesized that high basicity is not the only driving force for sugar degradation and that some catalysts may promote or inhibit undesired side reactions regardless of the initial pH (pDo) and the extent of glucose conversion (as shown in the matrix of conversion versus pDf). In other words, this analysis suggests that some catalyst structures may outperform the others for the isomerization of glucose because of suppressed side reactions, which advocates for further investigations on homogeneous Brønsted base catalysts and how their molecular structure may contribute to the reaction.
2.3. Chemical and statistical analysis Glucose conversion, fructose yield, and fructose selectivity were calculated from 1H NMR spectra (AVIII600, Bruker 600 MHz NMR) using N,N-dimethylformamide (≥99.8%, Fisher Scientific) or dimethyl sulfone (98%, Sigma Aldrich) as an internal standard where appropriate. To illustrate the differences in catalyst performance (i.e., conversion and selectivity) and their relationships with catalyst features (i.e., basicity, nature of the N-contain group(s), molecular structure of the organic catalyst), exploratory data analysis and unsupervised pattern recognition were employed in this study [32]. The specific signals obtained from 1H NMR spectra (i.e., conversion, yield, and selectivity derived from normalized peak intensities) were used in the data processing (Fig. S1 and Table S1) [35]. Relationships between variables were initially investigated using a correlation matrix. Principal component analysis (PCA) was then performed in OriginPro 2017 to reduce variables to a smaller number of components and to understand how catalytic patterns (i.e., conversion, selectivity, yield, and change in pH) vary among different groups of catalysts. 3. Results and discussion 3.1. Relationships between variables for the base-catalyzed isomerization of glucose to fructose Glucose conversion, fructose selectivity, fructose yield, initial pD before reaction, and final pD determined after 30 min at 100 °C were collected for both homogeneous and heterogeneous catalysts (Table 1), and compiled in a correlation matrix (Fig. 1). Each scatter plot shows the correlation between the variables listed on the corresponding X- and Y-axes. The red line and red circle in each scatter plot represent the linear fit and 95% confidence ellipse, respectively. The histograms in diagonal cells display the frequency of occurrence for the variables listed on the X-axes. Among all the experiments, the glucose conversion ranged from 0 to 70% and the highest fructose yield was recorded at 35% with 40% glucose conversion (Table 1). The fructose selectivity exhibited a bell-shaped distribution with the highest frequency between 50 and 70%. pD of the initial solutions ranged from 8.0 to 11.0, and approximately 70% of the catalysts tested resulted in a solution pD higher than 9. The pD of the solutions dropped over the course of the catalytic tests due to side reactions such as Maillard reactions, which are known to consume primary and secondary amines through coupling of the amines with reducible sugars, and alkaline degradation reactions, which produce acidic byproducts [26,36]. The pD dropped by up to 3 units, but was found to decrease by only 1 or 2 units for most tests. The catalytic results obtained for triethylamine were consistent with those previously reported by Carraher et al., hereby validating our reactor design and experimental conditions [16]. Under the same reaction temperature and reaction time, degassing the solution prior to the catalytic tests performed with, for instance, morpholine, piperazine and piperidine, demonstrated beneficial effects on fructose selectivity as compared to the catalytic results reported by Liu et al. [21]. Differences with previous works were also observed for L-arginine. Yang et al. reported that L-arginine exhibits a higher fructose yield than triethylamine under identical reaction conditions when the catalyst concentration is set on a per mole basis [23]. However, each L-arginine molecule contains two basic sites, specifically the guanidine and primary amine groups. As shown in our tests, L-arginine generated a lower fructose yield (19%) than triethylamine when the nitrogen concentration
3.2. Glucose reaction sensitivity to catalyst structures As informed by Fig. 1, glucose conversion demonstrated a positive correlation with pDo. However, a closer look at glucose conversion as a function of pDo (Fig. 2) revealed large differences in conversion over a small pH range and, simultaneously, little change in conversion over a large pH range. Two areas of interest, which each present a diverse set of homogeneous and heterogeneous catalysts with similar pDo (Zone 1) or conversion (Zone 2), are indicated in Fig. 2 to guide the eye and 3
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Table 1 Results obtained for the Brønsted base-catalyzed isomerization of glucose to fructose using a broad range of homogeneous and heterogeneous catalysts. Catalyst
Type
a
−
X (%)
b
S (%)
c
Sodium hydroxide Calcium hydroxide Barium hydroxide Magnesium hydroxide (3-aminoproply)triethoxysilane Guanidine carbonate salt L-arginine Tris(2-aminoethyl)amine N,N-dimethylethylenediamine Meglumine Morpholine 1-methylpiperazine Piperazine Piperidine Triethylamine 1,4-dimethylpiperazine Branched PEI e Ethoxylated PEI e
OH OH− OH− OH− Pri pri, sec pri, sec pri, ter pri, ter Sec Sec Sec Sec Sec Ter Ter pri, sec, ter pri, sec, ter
39 14 27 13 28 49 29 35 49 40 28 41 46 59 59 29 20 9
63 79 60 96 68 60 65 63 47 87 86 63 57 54 52 94 94 > 100
Silica based-PEI Cellulose-APTES Microcrystalline cellulose-APTES C3N4-Et2N Cellulose-Et2N IRA-96 f IRA-96 g IRA-96 h Amberlyst-21 f Amberlyst-21 g Amberlyst-21 h Amberlyst-26 f Amberlyst-26 g Amberlyst-26 h IRA-900 f IRA-900 h Mg(OH)2 (powder) Mg(OH)2 (powder, recycled once)
pri, sec, ter Pri Pri Ter Ter Ter Ter Ter Ter Ter Ter Quat Quat Quat Quat Quat OH− OH−
8 10 7 12 8 8 3 11 18 21 8 50 69 41 30 43 28 15
1 82 > 100 38 36 89 > 100 15 25 34 41 39 23 48 48 39 62 99
Y (%)
#
#
#
d
pDo
pDf
24 11 17 17 19 30 19 22 23 35 24 26 26 32 31 27 19 13
9.80 10.02 10.25 9.91 10.25 9.74 9.76 10.62 10.44 10.37 10.04 10.16 10.28 11.02 10.90 9.77 9.72 9.54
7.06 9.03 9.53 9.23 7.86 7.04 8.60 8.00 8.49 9.14 9.21 8.99 8.90 7.43 8.19 8.73 8.35 8.42
<1 8 7 5 3 7 3 2 4 7 3 20 16 20 14 17 17 15
7.36 9.00 9.02 8.85 8.13 8.64 8.85 8.51 8.61 9.43 8.65 8.67 9.52 8.71 8.97 8.43 9.91 9.28
6.58 8.02 8.03 5.68 7.92 7.77 7.81 6.71 6.13 6.68 6.45 5.36 5.70 5.48 5.70 5.59 9.23 9.16
Reaction conditions: 10 (g/g)% glucose/D2O solution, catalyst corresponding to 12 mol% N or other basic sites relative to glucose, 100 °C, 30 min. a basic groups: OH− = hydroxide, pri = primary amine, sec = secondary amine, ter = tertiary amine; quat = quaternary ammonium; b glucose conversion; c fructose selectivity; d fructose yield; e viscous under ambient conditions and soluble in water, therefore act as homogeneous catalysts under reaction conditions; f activated using 1 M NaOH; g used as received; h reused once and regenerated using 1 M NaOH before a second run. # glucose conversion was too low to obtain a reliable selectivity.
isomerization and degradation processes when triethylamine is used as a catalyst. Specifically, compounds that generate the same pDo should lead to the same conversion and selectivity [16]. Yet, the present results reveal clear deviations in catalytic performance for homogeneous bases that generate similar pDo. Comparing 1,4-dimethylpiperazine with piperazine and L-arginine, it is reasonable to attribute these deviations to (i) differences in the structure of the N-containing sites and (ii) the presence of other functional groups in the vicinity of the N-containing group(s). It should also be noted that the organic bases are present in solution in their protonated form due to their interaction with water and the formation of a BH+ OH− acid-base pair. The associated BH+ cation is not directly involved in the reaction [16]. However, it may stabilize/destabilize the negatively charged enediol intermediate through electrostatic interactions. This interpretation is actually consistent with the detailed kinetic analysis we performed previously for triethylamine [16]. We demonstrated that the same enediol intermediate leads to either fructose through a unimolecular process or to decomposition products through a bimolecular process that involves the hydroxide ions. Electrostatic interactions between the highly reactive enediol anion and the organic cation (protonated organic base) may stabilize this intermediate and hamper the undesired bimolecular processes that lead to sugar degradation and selectivity losses. This hypothesis is supported by the large negative values of ΔS‡ (ca. −140 J/molK) for the unimolecular isomerization of glucose to fructose that were attributed to the rearrangement of solvent molecules to accommodate the intermediate [16]. Specifically, the solvation of the
illustrate this observation. The results in Fig. 2 contrast with our previous kinetic study, which established OH− as the active species involved in both the isomerization and degradation reactions; hence glucose conversion should increase monotonically with pH [16]. The performance of the catalysts identified in Zone I of Fig. 2 is reported in Table 2. Under similar pDo environment, the glucose conversion in the presence of L-arginine was 20% lower than for guanidine carbonate. It was also reflected from the pDf that more acidic byproducts were generated in the presence of guanidine carbonate salt, which resulted in a slightly lower fructose selectivity than for L-arginine. Both guanidine carbonate salt and L-arginine possess a guanidino group and generate the same pDo in solution. Yet, their catalytic performances are drastically different, which further suggests that the molecular structure of the organic base and the presence of additional functionalities (e.g., hydroxyls, carboxylic acids) in the vicinity of the basic N-containing groups do play a role in the reaction. Similarly, substantial differences in glucose conversion and fructose selectivity were observed for piperazine and 1,4-dimethylpiperazine (Table 2) despite their similar molecular structures and basicity (based on initial pDo). The most striking difference in catalytic performance was revealed when comparing 1,4-dimethylpiperazine and L-arginine: the two compounds present the same pDo (9.73–9.74), similar pDf (8.57–8.70), same conversion (29%) but critically different selectivity (65–94%). This observation seems at first inconsistent with our previous work that identified OH− as the sole active species involved in both the 4
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Fig. 1. Correlation matrix compiling the reaction data presented in Table 1. The scatter plots show the correlations between selected variables. The red line and red circle in each scatter plot represent the linear fit and 95% confidence ellipse, respectively. The histograms in diagonal cells show frequency distributions for each reactivity parameter (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).
Finally, bulky organic cations may also shield the enediol intermediate from OH− through steric hindrance and, hereby, hamper the bimolecular pathways that lead to sugar degradation. Either of these effects or a combination of them may be responsible for the variations in selectivity observed for similar pDo. The catalysts marked in Zone II of Fig. 2 and the corresponding data in Table 3 further illustrate how the molecular structure impacts the catalytic performance. Specifically, Zone II shows that nearly identical
enediol intermediate and interactions with other species in solution (e.g., the protonated organic bases) play a significant role in the isomerization process. Obviously, the electrostatic interactions between the negatively charged enediol and positively charged organic base are expected to be sensitive to the molecular structure of the base, i.e. the nature and accessibility of NR3H+ (protonated nitrogen-containing group) and the presence of vicinal functional groups that alter the solvation of the organic base through polar and H-bonding interactions.
Fig. 2. Glucose conversion as a function of the initial pH of the aqueous reaction mixture. Two areas of interest, Zones I and II, are indicated to guide the reader’s eye. Zone I presents catalysts giving similar basicity in solution but resulting in different conversions; Zone II presents catalysts with varying basicity but resulting in similar glucose conversions.
5
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that Ca(OH)2 favors retro-aldolization by-products via cation-ketose complexation [19,43]. Recent quantum mechanical studies showed that divalent ions bind more strongly and preferentially to α-glucose than monovalent ions [44]. The glucose anomeric distributions were identical for the alkaline solutions tested in this study (Fig. S1), corroborating that the alkaline environment played a more dominant role on anomeric distribution than cations [45]. Nevertheless, varying binding strength between alkali/alkaline earth metals with sugars in various conformations probably determines the stability of sugars [46], which may in turn influence the ring-opening and ring-closing rate [45]. The above factors could account for the observed glucose reactivity that was not completely dependent on the pH in alkaline solutions.
Table 2 Catalysts marked in Zone I of Fig. 2 and their catalytic performance. Catalyst
Basic Groups
Guanidine carbonate salt
1° & 2° amines OH− 2° amines 3° amines 1° amines 1°, 2° & 3° amines
NaOH Piperazine 1,4-dimethylpiperazine L-arginine Branched PEI
a b c
pKa
pDo
pDf
X (%)
S (%)
a
9.74
7.04
49
60
13.46 9.73b 8.38b 12.48a 9.4-9.64c
9.80 9.82 9.77 9.76 9.72
7.06 8.69 8.73 8.60 8.35
39 35 29 29 20
63 86 94 65 94
13.6
Yang et al., 2016. Khalili et al., 2009. Yang and Runge, 2016.
3.3. Identification of key structural features through principal component analysis
Table 3 Catalysts marked in Zone II of Fig. 2 and their catalytic performance. Catalyst
Basic Groups
pKa
pDo
pDf
X (%)
S (%)
IRA900RF
Quaternary Ammonium 1° amines 3° amines 2° amine 1° amine
9.25
8.97
5.70
30
48
12.48 8.38 8.4a 9.8b
9.76 9.77 10.04 10.25
8.60 8.73 9.21 7.86
29 29 28 28
65 94 86 69
L-arginine 1,4-dimethylpiperazine Morpholine APTES a b
The catalytic results presented in Table 1 were also analyzed by principal component analysis (PCA) to identify trends and correlations for the broad set of homogeneous and heterogeneous catalysts tested in this work. The dataset used for PCA also included additional tests performed with various catalyst concentrations (2–75 mol.% N relative to glucose) (Table S2). Fig. 3 revealed that the performances for homogeneous and heterogeneous catalysts were clearly different from each other, strongly suggesting that the polymeric framework of the resins and PEI influence the catalytic performance. The deviations displayed by heterogeneous catalysts may involve effects due to: (1) increased local pH in the vicinity of clustered nitrogen functional groups compared to bulk solutions; (2) hindered accessibility to sites in the polymeric structure; and (3) repeated sugar adsorption resulting in secondary reactions and low selectivity. Meanwhile, the solid bases with quaternary ammoniums (anion exchange resins) clustered more in the PCA profile than the other heterogeneous catalysts (Fig. 3). This was possibly because the resins with quaternary ammoniums carry hydroxide counterions that may behave as strong bases. Hence, they exhibited higher conversions than resins with tertiary amines. Homogeneous catalysts with primary, secondary, and tertiary amines also clustered, implying that the catalytic activity was highly related to the type of amines (Fig. 3). One of the homogeneous tertiary amines (no. 4 in Fig. 3) fell into the same region as homogeneous secondary amines. This outlier represented 1,4-dimethylpiperazine, which was a heterocyclic amine. Interestingly, the other samples in this region of the PCA plot were also heterocyclic amines (no. 1 to 5 in Fig. 3). Acyclic amines contain nitrogen atoms with a pyramidal configuration that allows free inversion. While still possible, this inversion is markedly slower in heterocyclic amines. The clustering of the organic amines, independently of their basicity, inferred that the stereochemistry of amines may govern the glucose isomerization reaction in some way. In contrast, the data points corresponding to metal hydroxides were dispersed, which was consistent with our previous analysis about the role of cations and their interactions with glucose and its products (section 3.2). Therefore, different cations contributed to the overall reactivity and resulted in higher deviations in the PCA analysis. As these results indicate how the catalytic results are affected by the nature and structure of the base catalysts, they can also shed light on promising catalyst candidates and guide the rational design of nextgeneration heterogeneous catalysts. Ideally, the desirable catalysts should attain satisfactory fructose yield without compromising on selectivity in order to facilitate the downstream separation process. Fig. 4 illustrates that meglumine stood out from other catalysts with an approximate fructose yield of 35% and selectivity of 80%. Meglumine is an amino sugar, which can be derived from glucose or sorbitol. Considerable differences in glucose conversion and fructose selectivity were observed for meglumine and triethylamine under similar pDo condition (Table S2). Yet the apparent activation energy for the meglumine-catalyzed glucose isomerization was 74 kJ mol−1 (Fig. S2), which was close to that of triethylamine (61 kJ mol−1) [16], suggesting that while
Liu et al., 2014. Rosenholm and Linden, 2008.
glucose conversions of ∼30% were obtained for various homogeneous and heterogeneous catalysts and pDo ranging from 8.94 to 10.22. The anion exchange resin (IRA900RF) showed the lowest pDo value (8.94) in the initial glucose solution, yet the glucose conversion (30%) was as high as for the other catalysts. Glucose adsorption onto the macroporous resin may contribute to the high conversion [22]. Meanwhile, the corresponding fructose selectivity was noticeably low (S < 50%), which could also be related to its porous structure. Fructose generated from glucose isomerization may further react within the pores, where more N-groups may be located, and the localized basicity could result in secondary reactions that consume the fructose [37,38]. Amino-functionalized silane (APTES) showed the highest pDo value (10.22) but conversion remained steady at 28%. APTES can undergo hydrolysis and self-condensation under aqueous conditions (> 40 °C) to form a polymeric structure with active sites that are less accessible [39,40]. A recent study also demonstrated that amine-silanol or silanolglucose interactions reduced the catalytic activity for glucose isomerization [25]. However, theoretical calculations may be needed in the future to acquire a detailed understanding of these contributions. The reactions with divalent hydroxides were maintained in homogeneous conditions by syringe filtration in order to rule out the contribution of multiple homogenous and heterogeneous sites. It was interesting that in spite of the higher basicity of the Mg(OH)2, Ca(OH)2, and Ba(OH)2 solutions compared to NaOH, glucose conversion was not necessarily higher (Table 4). This indicated the impact of divalent metal ions on the base-catalyzed glucose isomerization, in addition to the critical role of hydroxide ions (Brønsted base). The effects of divalent metal ions could be attributed to their complexation with sugars or interference with sugar degradation [41,42]. It has been demonstrated Table 4 Glucose isomerization by metal hydroxides under homogeneous conditions. Catalyst
pDo
pDf
X (%)
S (%)
NaOH Mg(OH)2 Ca(OH)2 Ba(OH)2
9.80 9.91 10.02 10.25
7.06 9.23 8.65 7.40
39 13 14 27
63 96 79 60
6
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Fig. 3. Principal component (PC) biplot for the isomerization of glucose to fructose using Brønsted base catalysts (PC 1 accounts for 41.56% of the total variance and PC 2 accounts for 33.36% of the total variance. The top and right axes stand for vector loadings on PC 1 and PC 2, respectively; the smaller the angle between two vectors, the more positively correlated they are (< 90°); a near 90° angle means they are not likely to be correlated; the larger the angle between two vectors, the more negatively correlated they are (90° - 180°). The bottom and left axes stand for PC 1 score and PC 2 score, respectively. ΔpD is pD drop after reaction (pDo – pDf); S (%) = fructose selectivity; Y (%) = fructose yield; X (%) = glucose conversion; type = catalyst type).
heterogeneous Brønsted base catalysts for the isomerization of glucose to fructose and evaluated them under identical reaction conditions. The overall trends between glucose conversion, fructose selectivity, and basicity were consistent with those reported earlier by our group and others. However, significant differences with previous studies were occasionally observed (e.g., PEI), which calls for the use of a standardized catalyst testing method and/or the systematic use of benchmark catalysts (e.g., NaOH, TEA). Our results revealed that some catalysts outperformed the others by displacement of the equilibrium between glucose isomerization and its side reactions (i.e., Maillard reactions, alkaline degradation through β-elimination and retro-aldol reactions), which may be structure-sensitive. Apart from corroborating that the hydroxide ions were the active species involved in the glucose conversion, the PCA results underlined that the performance of organic Brønsted bases is directly related to their nature and molecular structures, for instance, availability of basic sites, type of amine (i.e., primary, secondary, tertiary amine and quaternary ammonium), the stereochemistry of the amines, and the co-existence of other functionalities in the vicinity of the N-group (e.g., hydroxyls). Among the catalysts we tested, meglumine outperformed the other homogeneous catalysts in view of its high fructose yield and selectivity, which may be related to its abundant hydroxyl groups that induced intermolecular hydrogen
the meglumine structure enhances the selectivity of the reaction, the overall reaction mechanisms for meglumine and triethylamine are likely similar. These results further support that the molecular structure of the base plays a role in the isomerization reaction, in addition to the basicity-dependence of the reaction system as articulated in our previous study [16]. Therefore, the observed differences may be due to synergistic effects between the basic sites and meglumine’s hydroxyl groups, similarly to the effects reported for Sn and neighboring hydroxyl groups in zeolite structures [47,48]. As solvent effects and in particular the solvation of glucose were shown to play an important role in the Lewis acid-catalyzed isomerization of glucose, we cannot exclude similar effects for the Brønsted base-catalyzed route as well [49]. Meglumine may alter the first solvation shell of glucose and/or of the enediol intermediate, thus favoring the selective isomerization over the unselective alkaline degradation pathways. The mechanistic details should be further confirmed by computational modeling in future studies and the developed understanding should guide the future design of basic heterogeneous catalysts. 4. Conclusions This study investigated a wide range of homogeneous and
Fig. 4. Fructose yield and selectivity as a function of pD in glucose isomerization (12 N mol% homogeneous catalysts relative to glucose at 100 °C in 30 min). 7
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bonding with glucose or water molecules. The standardized testing conditions and statistical data analysis used in this work enable the current dataset to be a good starting point for identifying structureactivity relationships. Theoretical calculations are now needed to fundamentally understand the interactions between the various species in solution, in particular the enediol-countercation-water interactions, and identify additional descriptors that will ultimately facilitate the design of catalysts with tailored functional properties.
[20]
[21]
[22] [23]
Declaration of Competing Interest
[24]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[25]
[26] [27]
Acknowledgments
[28]
This work was supported in part by Iowa State University, the National Science Foundation Grant Number EEC-0813570, and the Hong Kong Research Grants Council (PolyU 15217818). The first author (Season S. Chen) would like to thank Fulbright-RGC Hong Kong Research Scholar Award Program.
[29]
[30] [31]
Appendix A. Supplementary data [32]
Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcatb.2019.118126.
[33] [34]
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Contents lists available at ScienceDirect
Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb
Comparative investigation of homogeneous and heterogeneous Brønsted base catalysts for the isomerization of glucose to fructose in aqueous media ⁎⁎
Season S. Chena,b,c, Daniel C.W. Tsangc, , Jean-Philippe Tessonniera,b,
T
⁎
a
Department of Chemical and Biological Engineering, Iowa State University, Ames, IA, 50011, United States NSF Engineering Research Center for Biorenewable Chemicals (CBiRC), Ames, IA, 50011, United States c Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Biomass conversion Glucose isomerization Brønsted base catalysis Molecular structure Principal component analysis
Glucose isomerization to fructose via Brønsted base catalysis is sparking increasing interest due to the importance of fructose as a feedstock molecule for the production of renewable chemicals. The work reported here stems from an interest in understanding the key parameters governing glucose isomerization in addition to solution basicity, and in identifying catalyst features that are key to fructose selectivity. A wide range of homogeneous and heterogeneous Brønsted bases were investigated under identical reaction conditions to enable their cross comparison. The results showed that glucose conversion was positively correlated with the basicity of the solution, although exceptional cases were observed owing to the specific structure of that catalyst. Interactions between the cations associated with metal hydroxide catalysts and carbohydrates affected the isomerization of glucose. Likewise, cations formed through protonation of organic catalysts in water (e.g. amines) were shown to alter the glucose isomerization process and impact both conversion and selectivity under iso-pH conditions. Principal component analysis revealed that the catalytic patterns were dependent on the catalyst nature and structure. Overall, meglumine showed a superior catalytic performance compared to other homogeneous bases with a yield of 35% fructose and approximately 80% selectivity. These results obtained under identical experimental conditions will help to identify promising catalyst structures for future heterogeneous catalyst design.
1. Introduction The production of renewable chemicals from biomass has gained increasing attention in recent decades to alleviate our dependence on fossil resources, transition to a sustainable chemical industry, and revitalize this industry by supplying new molecules that are not easily accessible from petroleum [1–4]. The most common strategy for producing biobased chemicals consists in deconstructing biomass to separate its carbohydrate fraction (starch, cellulose, hemicellulose) from lignin, followed by chemical or biochemical hydrolysis of the polysaccharides to produce sugars, notably glucose [5–7]. Glucose can be subsequently fermented using metabolically-engineered microorganisms to produce biologically-accessible chemicals (e.g., lactic acid, succinic acid, muconic acid, etc.). However, its thermocatalytic conversion necessitates an additional isomerization step to fructose, which is critical for the production of biobased platform chemicals, such as 5hydroxymethylfurfural (HMF), levulinic acid (LA), and their derivatives
⁎
such as 2,5-furandicarboxylic acid (FDCA) and 5-ethoxymethylfurfural (EMF) [8–10]. Industrial-scale glucose-to-fructose conversion currently relies on an enzymatic process developed for the production of high fructose corn syrup (HFCS). This process operates near thermodynamic equilibrium and yields a sugar stream containing ∼42% fructose, which can be subsequently separated from glucose using simulated moving bed chromatography [11]. While well-established and robust, the HFCS process generates food-grade fructose, making it challenging to manufacture biobased platform chemicals at a low cost [12–14]. Scalable chemo-catalytic conversions are therefore critically needed for the industrial production of technical fructose for use as a feedstock in the emerging biobased chemical industry. The isomerization of glucose to fructose proceeds in the presence of Lewis acids and Brønsted bases via two distinct mechanisms that involve, respectively, an intramolecular hydride shift and an intramolecular proton transfer (Lobry de Bruyn-Alberda van Ekenstein
Corresponding author at: Department of Chemical and Biological Engineering, Iowa State University, Ames, IA, 50011, United States. Corresponding author. E-mail addresses: [email protected] (D.C.W. Tsang), [email protected] (J.-P. Tessonnier).
⁎⁎
https://doi.org/10.1016/j.apcatb.2019.118126 Received 11 January 2019; Received in revised form 25 July 2019; Accepted 23 August 2019 Available online 20 September 2019 0926-3373/ © 2019 Elsevier B.V. All rights reserved.
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resins containing primary, secondary, tertiary amines or quaternary ammonium. Aqueous alkaline solutions were prepared using sodium hydroxide (> 99.8%), calcium hydroxide (> 98%), and barium hydroxide (> 98.1%) purchased from Fisher Scientific, and magnesium hydroxide (95%) obtained from Sigma Aldrich. Desired amounts of hydroxides were dissolved in an aqueous solution of 10 wt.% glucose to obtain a target concentration. The divalent hydroxides mixtures were filtered using a 0.22-μm syringe nylon filter (Celltreat®) to ensure an homogeneous reaction without undissolved residues. Homogeneous aminecontaining compounds were all purchased from Sigma Aldrich. For example, compounds containing primary amines were L-arginine (≥98%, Sigma Aldrich), branched polyethylenimine (PEI) (Mw ∼ 25,000), ethoxylated PEI (80% ethoxylated, 35∼40 wt% in H2O), guanidine carbonate salt (99%), tris(2-aminoethyl)amine (96%), and N,N-dimethylethylenediamine (95%). Compounds containing secondary amines were morpholine (≥99%), meglumine (99%), piperazine (99%), 1-methylpiperazine (99%), and piperidine (≥99.5%). Triethylamine ((≥99%) and 1,4-dimethylpiperazine (98%) were investigated as homogeneous tertiary amines. Various styrene-divinylbenzene-based commercial anion exchange resins were used as heterogeneous catalysts such as AmberlystTM A21 and AmberliteTM IRA96 with tertiary amines, as well as AmberlystTM A26 OH and AmberliteTM IRA900RF Cl with quaternary ammoniums (DOW Chemical). The anion exchange resins were first activated in 1 M NaOH for 1 h at a 1:3 resin-to-solution volume ratio and rinsed with copious amounts of deionized water. (3-aminoproply)triethoxysilane (APTES, 99%, Sigma Aldrich), (N,N-diethyl-3-aminopropyl)trimethoxysilane (> 95%, Gelest), and 2Chloro-N,N-dimethylethylamine hydrochloride (99%, Sigma Aldrich) were used to synthesize microcrystalline cellulose-based (microcrystalline cellulose powder, Sigma Aldrich) solid catalysts and aminemodified carbon nitride with primary amines and tertiary amines, respectively. A detailed synthesis procedure for these heterogeneous catalysts is provided in the Supporting Information. Polyethylenimine on silica gel (20–60 mesh, Sigma Aldrich) was tested for comparing immobilized PEI and homogeneous PEI.
mechanism) [15,16]. Lewis acids such as metal chlorides and tin-beta zeolite have been studied in great details, both experimentally and theoretically. They produce fructose in relatively high yields but recurring issues related to their long-term stability in aqueous media and recycling have hampered the translation of this technology for the industrial production of technical fructose [17]. In contrast, the Brønstedbase-catalyzed route received significantly less attention as early studies reported poor selectivities to fructose and yields below 10% [18–20]. It is only recently that glucose was selectively isomerized to fructose with yields comparable to Lewis acids using organic aminebased homogeneous and heterogeneous catalysts [16,21–25,50]. It has been demonstrated that homogeneous amines can catalyze glucose isomerization, with triethylamine affording the highest fructose yield (32%) among the tested amines because of the suppression of the Maillard reactions that accompanies the reactions with primary or secondary amines [21,26]. Yang et al. also reported an effective fructose production with a maximum of 31% yield and 76% selectivity under optimized conditions using a basic amino acid (L-arginine) [23]. However, the corresponding heterogeneous catalysts, e.g. polyethylenimine, exhibited variable reactivity depending on the structure of the polymer (linear, branched, and ethoxylated), indicating that the chemical environment of the amino groups may alter the performance of these materials [24]. These results are intriguing as a detailed kinetic and mechanistic study revealed that the hydroxide ions (OH−) generated in situ through interaction between the amine and water, rather than the amine itself, were the active species involved in the basecatalyzed isomerization of glucose to fructose [16]. Similar inconsistencies were noted when comparing the catalytic activity of free amines and the corresponding groups tethered onto polystyrene beads [22,27] and mesoporous silica [25,50]. Although the large and uniform pores of the mesoporous silica support facilitate mass transfer, the reaction rates were found to be much lower for the heterogeneous catalysts than for the corresponding homogeneous organic bases. This result was likely associated with the immobilization of the amine on the silica support, where surface silanols may either affect the reaction directly or indirectly through interaction with the grafted amino groups. Experiments performed with various inorganic bases such as metal oxides (e.g., MgO, CaO, TiO2, and ZrO2), magnesium-modified zeolites, and naturally occurring minerals (e.g., attapulgite and imogolite) did not yield any clear structure-activity relationships either [28–31]. Rational design with lessons learned from previous studies is critical to devise effective heterogeneous base catalysts. For the base-catalyzed isomerization of glucose to fructose, it requires the assessment of multiple variables including catalyst basicity, molecular structure, recyclability, product selectivity, etc. to evaluate the catalyst’s suitability for this process [32,33]. However, cross comparison of previous studies and attempts to relate the catalyst structure to glucose isomerization activity have, so far, failed owing to important differences in experimental protocols. Therefore, this study aims to (i) evaluate the catalytic performance of various groups of catalysts including homogeneous inorganic bases, organic amines, and heterogeneous anion exchange resins under identical reaction conditions; (ii) reveal the critical parameters for the basecatalyzed glucose isomerization via correlation analysis; (iii) characterize the catalyst behaviors in relation to catalyst nature and structure by principal component analysis (PCA); and (iv) identify potentially promising homogeneous bases for future design of heterogeneous catalysts.
2.2. Catalytic tests A 10 wt% glucose solution (≥99.5%, Sigma Aldrich) was prepared in deuterium oxide (D2O, 99.9 atom% D, Sigma Aldrich) using a 50 mL 3-neck round-bottom flask equipped with an olive shape stir bar, a bubbler, and a septum with a needle. The solution was purged with Ar gas under vigorous stirring for at least 30 min to remove any dissolved CO2 that may titrate some of the basic sites of the homogeneous/heterogeneous catalysts. The system was air-tight but allowed the release of excess Ar through the needle. Liquid catalysts were transferred to the glucose solution using a syringe to keep the solution free of air and CO2, and the obtained mixture was further purged with Ar for 10 min. The amount of organic catalyst was set at 12 mol% of nitrogen relative to glucose, which was based on our previous studies [16,21]. The prepared mixture was then transferred to Ar-purged 5-mL thick-walled glass reactors (Chemglass Life Sciences) each containing a V-shaped stir bar. Solid catalysts were added in powder form directly to Ar-purged reactors. The amount of inorganic catalyst was set to achieve a pDo of 10 to facilitate the comparison with the organic bases. The reactors were subsequently sealed and placed in a preheated oil bath (digital IKA RCT stirring hot plate equipped with PT100 thermocouple) at 100 °C (time 0). Preliminary tests showed that the reaction mixture reached the target temperature within 2 min and that inserting reactors at room temperature into the oil bath did not change the temperature of the oil bath by more than 2 °C. The reactors were removed from the oil bath after 30 min and immediately transferred to an ice bath to quench the reaction. The pH values before and after reaction were measured using a pH meter (Metter Toledo) at room temperature calibrated using
2. Materials and methods 2.1. Chemicals The tested catalysts can be divided into the following categories: aqueous alkaline solutions, homogeneous compounds containing primary, secondary, or tertiary amines, and heterogeneous anion exchange 2
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buffer solutions at pH 4, 7, and 10 before use. The pH probe was fitted with a septum to prevent the introduction of atmospheric CO2 into the solutions during measurement. The pD value in D2O was determined by conversion of pH*, the direct reading in a D2O solution with the H2Ocalibrated pH-meter, using the relationship proposed by Krȩżel and Bal (i.e., pD = 0.929pH*+ 0.42) [34].
(Table 1, Y = 31% at 12 N mol%) or the number of basic groups (Table S2, Y = 29% at ca. 6 N mol%) in the solution was fixed. More importantly, for polymeric compounds, fixing the amount of catalyst on a per mole basis may lead to potentially obscure conclusions due to variations in the average molecular weight and the average number of basic groups (concentration) in the tested polymers. For example, branched PEI and ethoxylated PEI showed substantial differences in catalytic performance at 12 mol.% N relative to glucose (Table 1), in good agreement with the differences in turnover frequencies reported in a previous study [24]. However, fixing the amount of PEI (0.05 mol% relative to glucose) led to very similar fructose yields (27% for branched PEI and 26% for ethoxylated PEI at 100 °C and 10 min) [24]. Hence, these results clearly demonstrate the need for standard testing conditions and benchmarks to facilitate the comparison of catalytic results across research teams. These results also reveal the need for a broad study exploring the performance of a range of homogeneous and heterogeneous catalysts under identical reaction conditions. It can be seen from the correlation matrix (Fig. 1) that compiles the reaction data from Table 1 that fructose yield is strongly and positively correlated with glucose conversion (coefficient of correlation r = 0.778) and initial pD of the solution (pDo) (r = 0.784). These results are consistent with our previous kinetic study, which demonstrated that hydroxide anions generated through interaction of the organic base and water are the main drive for the triethylamine-catalyzed isomerization of glucose to fructose [16]. Fig. 1 also reveals that fructose selectivity is not significantly correlated with most variables (r < 0.49), except with the pD of the reacted solution (pDf) (r = 0.603), although it is generally true that the selectivity decreases with increasing glucose conversion. Notably, the selectivity is widely distributed at each of the two sides of the linear fitting for glucose conversions below 20%. This result implies that some catalysts may excel the others at low conversion, especially homogeneous catalysts that generate a pDo close to 10 (vide infra). Heterogeneous catalysts will be discussed in details in the next section. Since the pKa value of fructose is similar to that of glucose (∼12), pDf can also serve as an indication for the formation of acidic byproducts and provide insights into factors responsible for low fructose yields. pDf was found to be poorly correlated with most variables (r < 0.58), except with the change of pD over the course of the catalytic test (ΔpD) (r = 0.765) and selectivity (r = 0.603), which is fairly sensible because a high pDf suggests a small change in basicity (pD) during the reaction, hence reduced Maillard and alkaline degradation reactions, indicating also higher selectivity. Interestingly, while a correlation between pDf and pDo does exist (r = 0.585), the pDo-ΔpD matrix revealed that the pDf value can drop by 0.5–3 units regardless of the corresponding pDo value. As a matter of fact, glucose conversion showed a higher correlation with ΔpD (r = 0.614) than with pDo. Therefore, it can be hypothesized that high basicity is not the only driving force for sugar degradation and that some catalysts may promote or inhibit undesired side reactions regardless of the initial pH (pDo) and the extent of glucose conversion (as shown in the matrix of conversion versus pDf). In other words, this analysis suggests that some catalyst structures may outperform the others for the isomerization of glucose because of suppressed side reactions, which advocates for further investigations on homogeneous Brønsted base catalysts and how their molecular structure may contribute to the reaction.
2.3. Chemical and statistical analysis Glucose conversion, fructose yield, and fructose selectivity were calculated from 1H NMR spectra (AVIII600, Bruker 600 MHz NMR) using N,N-dimethylformamide (≥99.8%, Fisher Scientific) or dimethyl sulfone (98%, Sigma Aldrich) as an internal standard where appropriate. To illustrate the differences in catalyst performance (i.e., conversion and selectivity) and their relationships with catalyst features (i.e., basicity, nature of the N-contain group(s), molecular structure of the organic catalyst), exploratory data analysis and unsupervised pattern recognition were employed in this study [32]. The specific signals obtained from 1H NMR spectra (i.e., conversion, yield, and selectivity derived from normalized peak intensities) were used in the data processing (Fig. S1 and Table S1) [35]. Relationships between variables were initially investigated using a correlation matrix. Principal component analysis (PCA) was then performed in OriginPro 2017 to reduce variables to a smaller number of components and to understand how catalytic patterns (i.e., conversion, selectivity, yield, and change in pH) vary among different groups of catalysts. 3. Results and discussion 3.1. Relationships between variables for the base-catalyzed isomerization of glucose to fructose Glucose conversion, fructose selectivity, fructose yield, initial pD before reaction, and final pD determined after 30 min at 100 °C were collected for both homogeneous and heterogeneous catalysts (Table 1), and compiled in a correlation matrix (Fig. 1). Each scatter plot shows the correlation between the variables listed on the corresponding X- and Y-axes. The red line and red circle in each scatter plot represent the linear fit and 95% confidence ellipse, respectively. The histograms in diagonal cells display the frequency of occurrence for the variables listed on the X-axes. Among all the experiments, the glucose conversion ranged from 0 to 70% and the highest fructose yield was recorded at 35% with 40% glucose conversion (Table 1). The fructose selectivity exhibited a bell-shaped distribution with the highest frequency between 50 and 70%. pD of the initial solutions ranged from 8.0 to 11.0, and approximately 70% of the catalysts tested resulted in a solution pD higher than 9. The pD of the solutions dropped over the course of the catalytic tests due to side reactions such as Maillard reactions, which are known to consume primary and secondary amines through coupling of the amines with reducible sugars, and alkaline degradation reactions, which produce acidic byproducts [26,36]. The pD dropped by up to 3 units, but was found to decrease by only 1 or 2 units for most tests. The catalytic results obtained for triethylamine were consistent with those previously reported by Carraher et al., hereby validating our reactor design and experimental conditions [16]. Under the same reaction temperature and reaction time, degassing the solution prior to the catalytic tests performed with, for instance, morpholine, piperazine and piperidine, demonstrated beneficial effects on fructose selectivity as compared to the catalytic results reported by Liu et al. [21]. Differences with previous works were also observed for L-arginine. Yang et al. reported that L-arginine exhibits a higher fructose yield than triethylamine under identical reaction conditions when the catalyst concentration is set on a per mole basis [23]. However, each L-arginine molecule contains two basic sites, specifically the guanidine and primary amine groups. As shown in our tests, L-arginine generated a lower fructose yield (19%) than triethylamine when the nitrogen concentration
3.2. Glucose reaction sensitivity to catalyst structures As informed by Fig. 1, glucose conversion demonstrated a positive correlation with pDo. However, a closer look at glucose conversion as a function of pDo (Fig. 2) revealed large differences in conversion over a small pH range and, simultaneously, little change in conversion over a large pH range. Two areas of interest, which each present a diverse set of homogeneous and heterogeneous catalysts with similar pDo (Zone 1) or conversion (Zone 2), are indicated in Fig. 2 to guide the eye and 3
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Table 1 Results obtained for the Brønsted base-catalyzed isomerization of glucose to fructose using a broad range of homogeneous and heterogeneous catalysts. Catalyst
Type
a
−
X (%)
b
S (%)
c
Sodium hydroxide Calcium hydroxide Barium hydroxide Magnesium hydroxide (3-aminoproply)triethoxysilane Guanidine carbonate salt L-arginine Tris(2-aminoethyl)amine N,N-dimethylethylenediamine Meglumine Morpholine 1-methylpiperazine Piperazine Piperidine Triethylamine 1,4-dimethylpiperazine Branched PEI e Ethoxylated PEI e
OH OH− OH− OH− Pri pri, sec pri, sec pri, ter pri, ter Sec Sec Sec Sec Sec Ter Ter pri, sec, ter pri, sec, ter
39 14 27 13 28 49 29 35 49 40 28 41 46 59 59 29 20 9
63 79 60 96 68 60 65 63 47 87 86 63 57 54 52 94 94 > 100
Silica based-PEI Cellulose-APTES Microcrystalline cellulose-APTES C3N4-Et2N Cellulose-Et2N IRA-96 f IRA-96 g IRA-96 h Amberlyst-21 f Amberlyst-21 g Amberlyst-21 h Amberlyst-26 f Amberlyst-26 g Amberlyst-26 h IRA-900 f IRA-900 h Mg(OH)2 (powder) Mg(OH)2 (powder, recycled once)
pri, sec, ter Pri Pri Ter Ter Ter Ter Ter Ter Ter Ter Quat Quat Quat Quat Quat OH− OH−
8 10 7 12 8 8 3 11 18 21 8 50 69 41 30 43 28 15
1 82 > 100 38 36 89 > 100 15 25 34 41 39 23 48 48 39 62 99
Y (%)
#
#
#
d
pDo
24 11 17 17 19 30 19 22 23 35 24 26 26 32 31 27 19 13
9.80 10.02 10.25 9.91 10.25 9.74 9.76 10.62 10.44 10.37 10.04 10.16 10.28 11.02 10.90 9.77 9.72 9.54
7.06 9.03 9.53 9.23 7.86 7.04 8.60 8.00 8.49 9.14 9.21 8.99 8.90 7.43 8.19 8.73 8.35 8.42
<1 8 7 5 3 7 3 2 4 7 3 20 16 20 14 17 17 15
7.36 9.00 9.02 8.85 8.13 8.64 8.85 8.51 8.61 9.43 8.65 8.67 9.52 8.71 8.97 8.43 9.91 9.28
6.58 8.02 8.03 5.68 7.92 7.77 7.81 6.71 6.13 6.68 6.45 5.36 5.70 5.48 5.70 5.59 9.23 9.16
Reaction conditions: 10 (g/g)% glucose/D2O solution, catalyst corresponding to 12 mol% N or other basic sites relative to glucose, 100 °C, 30 min. a basic groups: OH− = hydroxide, pri = primary amine, sec = secondary amine, ter = tertiary amine; quat = quaternary ammonium; b glucose conversion; c fructose selectivity; d fructose yield; e viscous under ambient conditions and soluble in water, therefore act as homogeneous catalysts under reaction conditions; f activated using 1 M NaOH; g used as received; h reused once and regenerated using 1 M NaOH before a second run. # glucose conversion was too low to obtain a reliable selectivity.
isomerization and degradation processes when triethylamine is used as a catalyst. Specifically, compounds that generate the same pDo should lead to the same conversion and selectivity [16]. Yet, the present results reveal clear deviations in catalytic performance for homogeneous bases that generate similar pDo. Comparing 1,4-dimethylpiperazine with piperazine and L-arginine, it is reasonable to attribute these deviations to (i) differences in the structure of the N-containing sites and (ii) the presence of other functional groups in the vicinity of the N-containing group(s). It should also be noted that the organic bases are present in solution in their protonated form due to their interaction with water and the formation of a BH+ OH− acid-base pair. The associated BH+ cation is not directly involved in the reaction [16]. However, it may stabilize/destabilize the negatively charged enediol intermediate through electrostatic interactions. This interpretation is actually consistent with the detailed kinetic analysis we performed previously for triethylamine [16]. We demonstrated that the same enediol intermediate leads to either fructose through a unimolecular process or to decomposition products through a bimolecular process that involves the hydroxide ions. Electrostatic interactions between the highly reactive enediol anion and the organic cation (protonated organic base) may stabilize this intermediate and hamper the undesired bimolecular processes that lead to sugar degradation and selectivity losses. This hypothesis is supported by the large negative values of ΔS‡ (ca. −140 J/molK) for the unimolecular isomerization of glucose to fructose that were attributed to the rearrangement of solvent molecules to accommodate the intermediate [16]. Specifically, the solvation of the
illustrate this observation. The results in Fig. 2 contrast with our previous kinetic study, which established OH− as the active species involved in both the isomerization and degradation reactions; hence glucose conversion should increase monotonically with pH [16]. The performance of the catalysts identified in Zone I of Fig. 2 is reported in Table 2. Under similar pDo environment, the glucose conversion in the presence of L-arginine was 20% lower than for guanidine carbonate. It was also reflected from the pDf that more acidic byproducts were generated in the presence of guanidine carbonate salt, which resulted in a slightly lower fructose selectivity than for L-arginine. Both guanidine carbonate salt and L-arginine possess a guanidino group and generate the same pDo in solution. Yet, their catalytic performances are drastically different, which further suggests that the molecular structure of the organic base and the presence of additional functionalities (e.g., hydroxyls, carboxylic acids) in the vicinity of the basic N-containing groups do play a role in the reaction. Similarly, substantial differences in glucose conversion and fructose selectivity were observed for piperazine and 1,4-dimethylpiperazine (Table 2) despite their similar molecular structures and basicity (based on initial pDo). The most striking difference in catalytic performance was revealed when comparing 1,4-dimethylpiperazine and L-arginine: the two compounds present the same pDo (9.73–9.74), similar pDf (8.57–8.70), same conversion (29%) but critically different selectivity (65–94%). This observation seems at first inconsistent with our previous work that identified OH− as the sole active species involved in both the 4
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Fig. 1. Correlation matrix compiling the reaction data presented in Table 1. The scatter plots show the correlations between selected variables. The red line and red circle in each scatter plot represent the linear fit and 95% confidence ellipse, respectively. The histograms in diagonal cells show frequency distributions for each reactivity parameter (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).
Finally, bulky organic cations may also shield the enediol intermediate from OH− through steric hindrance and, hereby, hamper the bimolecular pathways that lead to sugar degradation. Either of these effects or a combination of them may be responsible for the variations in selectivity observed for similar pDo. The catalysts marked in Zone II of Fig. 2 and the corresponding data in Table 3 further illustrate how the molecular structure impacts the catalytic performance. Specifically, Zone II shows that nearly identical
enediol intermediate and interactions with other species in solution (e.g., the protonated organic bases) play a significant role in the isomerization process. Obviously, the electrostatic interactions between the negatively charged enediol and positively charged organic base are expected to be sensitive to the molecular structure of the base, i.e. the nature and accessibility of NR3H+ (protonated nitrogen-containing group) and the presence of vicinal functional groups that alter the solvation of the organic base through polar and H-bonding interactions.
Fig. 2. Glucose conversion as a function of the initial pH of the aqueous reaction mixture. Two areas of interest, Zones I and II, are indicated to guide the reader’s eye. Zone I presents catalysts giving similar basicity in solution but resulting in different conversions; Zone II presents catalysts with varying basicity but resulting in similar glucose conversions.
5
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that Ca(OH)2 favors retro-aldolization by-products via cation-ketose complexation [19,43]. Recent quantum mechanical studies showed that divalent ions bind more strongly and preferentially to α-glucose than monovalent ions [44]. The glucose anomeric distributions were identical for the alkaline solutions tested in this study (Fig. S1), corroborating that the alkaline environment played a more dominant role on anomeric distribution than cations [45]. Nevertheless, varying binding strength between alkali/alkaline earth metals with sugars in various conformations probably determines the stability of sugars [46], which may in turn influence the ring-opening and ring-closing rate [45]. The above factors could account for the observed glucose reactivity that was not completely dependent on the pH in alkaline solutions.
Table 2 Catalysts marked in Zone I of Fig. 2 and their catalytic performance. Catalyst
Basic Groups
Guanidine carbonate salt
1° & 2° amines OH− 2° amines 3° amines 1° amines 1°, 2° & 3° amines
NaOH Piperazine 1,4-dimethylpiperazine L-arginine Branched PEI
a b c
pKa
pDo
X (%)
S (%)
a
9.74
7.04
49
60
13.46 9.73b 8.38b 12.48a 9.4-9.64c
9.80 9.82 9.77 9.76 9.72
7.06 8.69 8.73 8.60 8.35
39 35 29 29 20
63 86 94 65 94
13.6
Yang et al., 2016. Khalili et al., 2009. Yang and Runge, 2016.
3.3. Identification of key structural features through principal component analysis
Table 3 Catalysts marked in Zone II of Fig. 2 and their catalytic performance. Catalyst
Basic Groups
pKa
pDo
X (%)
S (%)
IRA900RF
Quaternary Ammonium 1° amines 3° amines 2° amine 1° amine
9.25
8.97
5.70
30
48
12.48 8.38 8.4a 9.8b
9.76 9.77 10.04 10.25
8.60 8.73 9.21 7.86
29 29 28 28
65 94 86 69
L-arginine 1,4-dimethylpiperazine Morpholine APTES a b
The catalytic results presented in Table 1 were also analyzed by principal component analysis (PCA) to identify trends and correlations for the broad set of homogeneous and heterogeneous catalysts tested in this work. The dataset used for PCA also included additional tests performed with various catalyst concentrations (2–75 mol.% N relative to glucose) (Table S2). Fig. 3 revealed that the performances for homogeneous and heterogeneous catalysts were clearly different from each other, strongly suggesting that the polymeric framework of the resins and PEI influence the catalytic performance. The deviations displayed by heterogeneous catalysts may involve effects due to: (1) increased local pH in the vicinity of clustered nitrogen functional groups compared to bulk solutions; (2) hindered accessibility to sites in the polymeric structure; and (3) repeated sugar adsorption resulting in secondary reactions and low selectivity. Meanwhile, the solid bases with quaternary ammoniums (anion exchange resins) clustered more in the PCA profile than the other heterogeneous catalysts (Fig. 3). This was possibly because the resins with quaternary ammoniums carry hydroxide counterions that may behave as strong bases. Hence, they exhibited higher conversions than resins with tertiary amines. Homogeneous catalysts with primary, secondary, and tertiary amines also clustered, implying that the catalytic activity was highly related to the type of amines (Fig. 3). One of the homogeneous tertiary amines (no. 4 in Fig. 3) fell into the same region as homogeneous secondary amines. This outlier represented 1,4-dimethylpiperazine, which was a heterocyclic amine. Interestingly, the other samples in this region of the PCA plot were also heterocyclic amines (no. 1 to 5 in Fig. 3). Acyclic amines contain nitrogen atoms with a pyramidal configuration that allows free inversion. While still possible, this inversion is markedly slower in heterocyclic amines. The clustering of the organic amines, independently of their basicity, inferred that the stereochemistry of amines may govern the glucose isomerization reaction in some way. In contrast, the data points corresponding to metal hydroxides were dispersed, which was consistent with our previous analysis about the role of cations and their interactions with glucose and its products (section 3.2). Therefore, different cations contributed to the overall reactivity and resulted in higher deviations in the PCA analysis. As these results indicate how the catalytic results are affected by the nature and structure of the base catalysts, they can also shed light on promising catalyst candidates and guide the rational design of nextgeneration heterogeneous catalysts. Ideally, the desirable catalysts should attain satisfactory fructose yield without compromising on selectivity in order to facilitate the downstream separation process. Fig. 4 illustrates that meglumine stood out from other catalysts with an approximate fructose yield of 35% and selectivity of 80%. Meglumine is an amino sugar, which can be derived from glucose or sorbitol. Considerable differences in glucose conversion and fructose selectivity were observed for meglumine and triethylamine under similar pDo condition (Table S2). Yet the apparent activation energy for the meglumine-catalyzed glucose isomerization was 74 kJ mol−1 (Fig. S2), which was close to that of triethylamine (61 kJ mol−1) [16], suggesting that while
Liu et al., 2014. Rosenholm and Linden, 2008.
glucose conversions of ∼30% were obtained for various homogeneous and heterogeneous catalysts and pDo ranging from 8.94 to 10.22. The anion exchange resin (IRA900RF) showed the lowest pDo value (8.94) in the initial glucose solution, yet the glucose conversion (30%) was as high as for the other catalysts. Glucose adsorption onto the macroporous resin may contribute to the high conversion [22]. Meanwhile, the corresponding fructose selectivity was noticeably low (S < 50%), which could also be related to its porous structure. Fructose generated from glucose isomerization may further react within the pores, where more N-groups may be located, and the localized basicity could result in secondary reactions that consume the fructose [37,38]. Amino-functionalized silane (APTES) showed the highest pDo value (10.22) but conversion remained steady at 28%. APTES can undergo hydrolysis and self-condensation under aqueous conditions (> 40 °C) to form a polymeric structure with active sites that are less accessible [39,40]. A recent study also demonstrated that amine-silanol or silanolglucose interactions reduced the catalytic activity for glucose isomerization [25]. However, theoretical calculations may be needed in the future to acquire a detailed understanding of these contributions. The reactions with divalent hydroxides were maintained in homogeneous conditions by syringe filtration in order to rule out the contribution of multiple homogenous and heterogeneous sites. It was interesting that in spite of the higher basicity of the Mg(OH)2, Ca(OH)2, and Ba(OH)2 solutions compared to NaOH, glucose conversion was not necessarily higher (Table 4). This indicated the impact of divalent metal ions on the base-catalyzed glucose isomerization, in addition to the critical role of hydroxide ions (Brønsted base). The effects of divalent metal ions could be attributed to their complexation with sugars or interference with sugar degradation [41,42]. It has been demonstrated Table 4 Glucose isomerization by metal hydroxides under homogeneous conditions. Catalyst
pDo
X (%)
S (%)
NaOH Mg(OH)2 Ca(OH)2 Ba(OH)2
9.80 9.91 10.02 10.25
7.06 9.23 8.65 7.40
39 13 14 27
63 96 79 60
6
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Fig. 3. Principal component (PC) biplot for the isomerization of glucose to fructose using Brønsted base catalysts (PC 1 accounts for 41.56% of the total variance and PC 2 accounts for 33.36% of the total variance. The top and right axes stand for vector loadings on PC 1 and PC 2, respectively; the smaller the angle between two vectors, the more positively correlated they are (< 90°); a near 90° angle means they are not likely to be correlated; the larger the angle between two vectors, the more negatively correlated they are (90° - 180°). The bottom and left axes stand for PC 1 score and PC 2 score, respectively. ΔpD is pD drop after reaction (pDo – pDf); S (%) = fructose selectivity; Y (%) = fructose yield; X (%) = glucose conversion; type = catalyst type).
heterogeneous Brønsted base catalysts for the isomerization of glucose to fructose and evaluated them under identical reaction conditions. The overall trends between glucose conversion, fructose selectivity, and basicity were consistent with those reported earlier by our group and others. However, significant differences with previous studies were occasionally observed (e.g., PEI), which calls for the use of a standardized catalyst testing method and/or the systematic use of benchmark catalysts (e.g., NaOH, TEA). Our results revealed that some catalysts outperformed the others by displacement of the equilibrium between glucose isomerization and its side reactions (i.e., Maillard reactions, alkaline degradation through β-elimination and retro-aldol reactions), which may be structure-sensitive. Apart from corroborating that the hydroxide ions were the active species involved in the glucose conversion, the PCA results underlined that the performance of organic Brønsted bases is directly related to their nature and molecular structures, for instance, availability of basic sites, type of amine (i.e., primary, secondary, tertiary amine and quaternary ammonium), the stereochemistry of the amines, and the co-existence of other functionalities in the vicinity of the N-group (e.g., hydroxyls). Among the catalysts we tested, meglumine outperformed the other homogeneous catalysts in view of its high fructose yield and selectivity, which may be related to its abundant hydroxyl groups that induced intermolecular hydrogen
the meglumine structure enhances the selectivity of the reaction, the overall reaction mechanisms for meglumine and triethylamine are likely similar. These results further support that the molecular structure of the base plays a role in the isomerization reaction, in addition to the basicity-dependence of the reaction system as articulated in our previous study [16]. Therefore, the observed differences may be due to synergistic effects between the basic sites and meglumine’s hydroxyl groups, similarly to the effects reported for Sn and neighboring hydroxyl groups in zeolite structures [47,48]. As solvent effects and in particular the solvation of glucose were shown to play an important role in the Lewis acid-catalyzed isomerization of glucose, we cannot exclude similar effects for the Brønsted base-catalyzed route as well [49]. Meglumine may alter the first solvation shell of glucose and/or of the enediol intermediate, thus favoring the selective isomerization over the unselective alkaline degradation pathways. The mechanistic details should be further confirmed by computational modeling in future studies and the developed understanding should guide the future design of basic heterogeneous catalysts. 4. Conclusions This study investigated a wide range of homogeneous and
Fig. 4. Fructose yield and selectivity as a function of pD in glucose isomerization (12 N mol% homogeneous catalysts relative to glucose at 100 °C in 30 min). 7
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bonding with glucose or water molecules. The standardized testing conditions and statistical data analysis used in this work enable the current dataset to be a good starting point for identifying structureactivity relationships. Theoretical calculations are now needed to fundamentally understand the interactions between the various species in solution, in particular the enediol-countercation-water interactions, and identify additional descriptors that will ultimately facilitate the design of catalysts with tailored functional properties.
[20]
[21]
[22] [23]
Declaration of Competing Interest
[24]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[25]
[26] [27]
Acknowledgments
[28]
This work was supported in part by Iowa State University, the National Science Foundation Grant Number EEC-0813570, and the Hong Kong Research Grants Council (PolyU 15217818). The first author (Season S. Chen) would like to thank Fulbright-RGC Hong Kong Research Scholar Award Program.
[29]
[30] [31]
Appendix A. Supplementary data [32]
Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcatb.2019.118126.
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