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Article

Production of 5-Hydroxymethylfurfural from Glucose in Water by Using Transition Metal-Oxide Nanosheet Aggregates

by
Atsushi Takagaki
Department of Applied Chemistry, Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
Catalysts 2019, 9(10), 818; https://doi.org/10.3390/catal9100818
Submission received: 5 September 2019 / Revised: 26 September 2019 / Accepted: 27 September 2019 / Published: 28 September 2019
(This article belongs to the Section Biomass Catalysis)
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Abstract

:
Metal-oxide nanosheet aggregates were prepared by exfoliation and subsequent aggregation of layered metal oxides and used for the conversion of glucose to 5-hydroxymethylfurfural (HMF) in water. Three aggregated nanosheets, HNbWO6, HNb3O8, and HTiNbO5, yielded HMF in water at 393–413 K, whereas ion-exchange resins and H-form zeolites did not. The catalytic activity of the nanosheets decreased in the order HNbWO6 > HNb3O8 > HTiNbO5, which correlates with their acidity. The HNbWO6 nanosheets exhibited higher selectivity for HMF than niobic acid, and the selectivity was improved in the water–toluene biphasic system. The selectivity for HMF over HNbWO6 nanosheets was higher from glucose than from fructose. Kinetic analysis suggested that in addition to fructose, an intermediate species was involved in the reaction pathway of HMF production from glucose.

1. Introduction

The efficient utilization of lignocellulosic biomass is essential for building a sustainable society because biomass is the only renewable for producing liquid transportation fuels and chemicals. The catalytic transformation of carbohydrates to furfurals is an attractive process for the synthesis of biomass-derived chemicals. In particular, the conversion of glucose to 5-hydroxymethylfurfural (HMF) has attracted attention because HMF serves as a key intermediate for monomers of biopolymers. HMF has two functional groups, a hydroxyl group and an aldehyde group, which can be selectively oxidized to form 2,5-diformylfuran (DFF) and 2,5-furandicarboxylic acid (FDCA) in the presence of heterogeneous catalysts including supported Ru [1] and Au [2,3] catalysts and manganese oxide catalyst [4]. The highly selective formation of HMF from fructose was first reported by using HCl as a homogeneous acid catalyst in the water-toluene biphasic system [5]. The efficient production of HMF from glucose was demonstrated by using CrCl2 as a homogeneous Lewis acid catalyst in an ionic liquid [6]. Use of molten salts is also effective for the production of HMF [7]. The formation of fructose as an intermediate was considered as a key for achieving a high selectivity for HMF. The conversion of glucose to fructose involves aldose-ketose isomerization, which is catalyzed by a base or Lewis acid. The successive reaction of fructose to HMF is dehydration by a Brønsted acid. The use of two different heterogeneous catalysts allowed the coexistence of a base and Brønsted acid in the same pot, resulting in a high selectivity for HMF from glucose [8,9]. Heterogeneous catalysts with both Lewis acid and Brønsted acid sites also gave a high HMF selectivity [10,11,12,13,14]. The solid catalysts having Lewis acid and Brønsted acid sites were also effective for the production of furfural from xylose [15]. Most of the studies claimed that the production of HMF from glucose proceeded via fructose formation. However, Bols et al. proposed 3-deoxyglucosone as an additional intermediate [16]. Elimination of the 3-OH of glucose gives the intermediate, 3-deoxyglucosone, which can be further converted to HMF via ring closure and elimination. Density functional theory calculations indicated that 3-deoxyglucosone is more likely to be formed from glucose and their experiments using 3-deoxyglucosone and fructose as reactants showed that a higher selectivity for HMF was obtained from 3-deoxyglucosone.
Transition metal-oxide nanosheets are found to function as solid acid catalysts [17,18,19,20]. These were synthesized by exfoliation of layered transition metal oxides and subsequent aggregation of the exfoliated nanosheets. The nanosheet aggregates not only possess high surface areas but also retain the crystal structure of the parent layered oxides. The parent layered metal oxides consist of polyanion sheets of transition-metal oxides with intercalated alkaline metal cations. The metal-oxide sheets are negatively charged and regarded as macropoyanions. As the countercations, protons are located on the surface. Therefore, Brønsted acid sites, namely acidic OH groups, are dominant. These characteristics are very different from amorphous metal oxides which preferentially have Lewis acid sites formed on unsaturated metal sites. A variety of transition metal oxide nanosheets including HTiNbO5, HNb3O8, and HNbWO6 were examined as solid acids catalyzing several reactions such as esterification, hydrolysis, and Friedel–Crafts alkylation. The metal-oxide aggregated nanosheets could be simply recovered by decantation and recycled for further reaction. The activities for acid-catalyzed reactions remained unchanged after the sample was recycled for a third time [18]. These transition metal oxide nanosheets have hydroxyl groups on the surface that exhibit Brønsted acidity. The acidity can be controlled by varying the composition of metal oxides. Temperature-programmed desorption of ammonia and solid-state nuclear magnetic resonance (NMR) using trimethylphosphine oxide as a probe molecule revealed that the order of acidity is HNbWO6 > HNb3O8 > HTiNbO5, which corresponds with the catalytic activity for Friedel–Crafts alkylation. It was reported that some of these nanosheets, HTiNbO5 and HNb3O8, could dehydrate D-xylose to produce furfural in a water-toluene solvent [21]. HTiNbO5 nanosheets catalyzed the reaction to achieve a furfural yield of 55% at 92 % conversion, whereas amorphous niobium oxide (Nb2O5·nH2O) was 12%. This indicates these metal oxide nanosheet aggregates are favorable for the sugar dehydration.
This study used exfoliated transition metal oxide nanosheet aggregates for the formation of HMF from glucose in water. Three acidic nanosheet aggregates could produce HMF from glucose in water, with HNbWO6 showing the highest activity among the catalysts tested, which was not involved in the previous study on xylose and glucose dehydration [22]. Kinetic analysis of the reaction on HNbWO6 nanosheets suggested that there are two intermediates for HMF formation from glucose: fructose and, most likely, 3-deoxyglucosone.

2. Results and Discussion

Table 1 lists the surface areas and solid acid properties of the aggregated nanosheet catalysts, which have been reported previously [19]. The Brunauer-Emmett-Teller (BET) surface areas of the aggregated nanosheets were 66, 101, and 153 for HNbWO6, HNb3O8, and HTiNbO5 nanosheets, respectively, much higher than those of the parent layered oxides, ~1 m2 g-1. The acid amounts of these catalysts were in the range of 0.24-0.34 mmol g-1, which is close to that of niobic acid, 0.3 mmol g-1 [23,24]. The peak position of temperature-programmed desorption of ammonia (NH3-TPD) is indicative of the acid strength of the nanosheet catalysts. The large chemical shift observed in 31P nuclear magnetic resonance (NMR) corresponds to the strong acidity. 31P magic angle spinning (MAS) NMR measurement using trimethylphosphine oxide (TMPO) is an advanced technique to identify the type, concentration and strength of solid acids [25]. Higher chemical shifts indicate higher protonic acid strength because the 31P chemical shifts of protonated TMPO tend to move downfield. Together, these two measurements demonstrated that the strength of the acid sites in the nanosheets decreased in the order HNbWO6 > HNb3O8 > HTiNbO5. The acid amounts of aggregated nanosheets were around 0.3 mmol g-1, which is close to that of niobic acid (Nb2O5·nH2O). Sulfonic acid resins have more acid sites, 4.8 mmol g-1 for Amberlyst-15 and 0.9 mmol g-1 for Nafion NR50. H-beta zeolite has 1.0 mmol g-1 and H-ZSM5 zeolite 0.2 mmol g-1.
Table 2 shows the results of the glucose-to-HMF transformation catalyzed by aggregated nanosheets. It should be noted that three nanosheet catalysts and niobic acid yielded HMF in water, whereas the two ion-exchange resins with sulfonic groups and the two H-form zeolites with acidic hydroxyl groups did not. It is desirable to use the same amount of acid sites for the reaction in order to compare the catalytic activity. Although these ion-exchange resins and H-Beta zeolite possess higher acid amount, no formation of HMF was observed. Other study used a large amount of H-ZSM5 for the glucose transformation, but negligible activity was observed [30]. In this study, active catalysts were three nanosheets and niobic acid, which have nearly the same acid amounts. Higher acid amounts were used for other catalysts, but these were inactive.
Aqueous-phase acid-catalyzed reactions over solid acids are generally difficult because water covers Brønsted acid sites on the catalyst, resulting in no or negligible activity [26]. It is widely accepted that zeolites lose their strong acidity in water unless they improve their hydrophobicity [26]. While sulfonic acid resins were inactive for glucose transformation in water, they could dehydrate glucose in aprotic solvents to form levoglucosan [31], which is another dehydrated sugar. This indicates that the affinity of reactants against solvent is important. The metal oxide nanosheets could work as water-tolerant solid acids as well as insoluble heteropolyacids [17], which catalyze the hydrolysis of ethyl acetate [18] and sucrose in water [32,33]. The origin of Brønsted acid sites on metal oxide nanosheets resembles that of heteroplyacids. The metal oxide sheets are regarded as macropolyanions, for example [NbWO6]-, and protons are located as the countercations. Thus, the functions of acid sites on the metal oxide nanosheets would be different from those on H+-zeolites and sulfonic acid resins.
The HNbWO6 nanosheets exhibited the highest selectivity for HMF with the highest yield among the catalysts tested (entries 1, 3–9). The yield and selectivity for HMF decreased in the order HNbWO6 > HNb3O8 > HTiNbO5, which correlates with the acid strengths of these nanosheets (entries 1, 3, 4). The effect of the Brønsted acid sites of the HNbWO6 nanosheets on catalytic activity was investigated by replacing the proton with potassium in NbWO6 nanosheets, yielding KNbWO6. These KNbWO6 nanosheets were prepared by the addition of KNO3, rather than HNO3, during the aggregation of NbWO6 nanosheets. Decreases in HMF yield, HMF selectivity, and glucose conversion were observed for KNbWO6 (entry 2), suggesting that Brønsted acid sites on metal oxide nanosheets are the main active sites for HMF production. The acid types of HNbWO6 nanosheets were evaluated by using pyridine-adsorbed Fourier transform infrared (FTIR) spectroscopy (Figure S1). The peak at 1541 cm-1 attributed to Brønsted acid sites was observed, and the peak at 1456 cm-1 to Lewis acid sites was found. The ratio of Brønsted acid to Lewis acid was 3.1.
Previous studies on HMF formation from glucose in water have been summarized in Table 3. Entries 1-11 are the results using oxides without any modifications. The present study demonstrated that HNbWO6 nanosheet aggregates exhibited the highest HMF yield with the highest selectivity under the lowest reaction temperature among oxides without any modifications (entries 1,2). Recent studies showed that a hybrid TiO2 (entry 13) [34] and mesoporous metal phosphates (entries 15,16) [35,36] gave higher HMF yield as well as H3PO4-treated niobic acid (entry 14) [30]. This suggests that some modification of oxide catalysts including treatment with phosphoric acid will improve the activity.
A further investigation was carried out by adding an inhibitor to the reaction solution containing glucose and HNbWO6 nanosheets. As an inhibitor, 2,6-lutidine was used because it selectively adsorbs on the Brønsted acid sites of the catalyst [43,44]. Figure 1 shows the correlation between the HMF yield and the amount of 2,6-lutidine added. The HMF yield decreased with increasing 2,6-lutidine amount, indicating that the Brønsted acid sites of the catalyst are necessary to produce HMF. To suppress the HMF production altogether, 112 μmol of 2,6-lutidine was required, which corresponds to 2.23 mmol g cat-1. This value is much higher than the acid content of the HNbWO6 nanosheets, 0.34 mmol g-1, which is likely because of the acid-base equilibrium in water. Although 2,6-lutidine adsorbs on the Brønsted acid sites of HNbWO6 nanosheets, the interaction could be weakened in water owing to the equilibrium, necessitating the addition of a high amount of 2,6-lutidine.
Table 4 shows the effect of the reaction temperature and the solvent. Increasing the reaction temperature increased glucose conversion but decreased HMF selectivity (entries 1–4). Increasing the water content and decreasing the reaction temperature were effective in improving the HMF yield and selectivity (entry 5). Adding 1-butanol further increased the HMF yield (37%) and selectivity (52%) because the water-butanol biphasic system could extract HMF into the 1-butanol solvent, which suppresses the degradation of HMF (entry 6). Figure 2 shows the time course of the glucose-to-HMF transformation catalyzed by HNbWO6 aggregate nanosheets in water and water-1-butanol. In water, the selectivity for HMF increased with reaction time, but reached a plateau after 9 h, resulting in 44% selectivity with 23% yield after 24 h. In water-butanol, the selectivity for HMF increased continuously with increasing reaction time, reaching 52% selectivity with 71% conversion after 36 h. The addition of 1-butanol not only improved HMF selectivity but also accelerated glucose conversion.
Figure 3 compares the catalytic activities of HNbWO6 aggregated nanosheets and niobic acid, Nb2O5·nH2O. At low glucose conversions, the selectivity for HMF was low for both catalysts, suggesting that HMF was not the primary product. The selectivity for HMF over HNbWO6 aggregated nanosheets at low conversion was twice that over niobic acid. With increasing glucose conversion, the selectivity for HMF increased, eventually reaching a maximum. The maximum selectivity for HMF was 45% at 34% conversion for HNbWO6 nanosheets and 36% at 60% conversion for Nb2O5·nH2O. A further increase in glucose conversion led to a moderate decrease in selectivity for HMF, likely owing to degradation. The biphasic system was effective in improving HMF selectivity. Although the HMF selectivity at low conversion was lower than that in water, it increased monotonically with increasing glucose conversion, as mentioned above.
To investigate the reaction pathway, fructose, as well as glucose, was used as a reactant. Figure 4 shows the time course of fructose-to-HMF transformation performed by using HNbWO6 aggregated nanosheets. Only water was used as solvent in order to eliminate the effect of HMF extraction into an organic solvent and simplify the reaction network. The conversion of fructose (70% in 6 h) was faster than that of glucose. The selectivity for HMF was 27% for 6 h, almost regardless of the extent of fructose conversion. This indicates that further transformation of HMF to byproducts such as humins was very slow, and that most of the byproducts were formed directly from fructose. Also, a small amount of glucose was formed via glucose-fructose isomerization. If the condensation reactions between HMF and fructose occurred seriously, the HMF selectivity should be decreased with the increase of the reaction time. A simple simulation was carried out in which the condensation between HMF and fructose was assumed (see supplementary materials). The results indicated that a constant selectivity to HMF was obtained only at the very high conversion of fructose, which is much different from the results in the present study. Therefore, the condensation reactions between HMF and sugars were not significantly involved in this study.
Figure 5 shows the time course of the glucose-to-HMF transformation. From glucose, both HMF and fructose were obtained. With increasing reaction time, the HMF yield increased, while the fructose yield decreased, indicating that fructose was a primary product and HMF was a secondary product. Figure 6 shows the HMF selectivity against hexose conversion over HNbWO6 aggregated nanosheets. Both the yield and selectivity of HMF were higher from glucose than from fructose (Figure 4), suggesting that the reaction pathway involved not only fructose, but also another intermediate.
From these results, a reaction pathway for HMF production using HNbWO6-aggregated nanosheets is proposed (see Scheme 1). Glucose is converted into HMF via two intermediates, one of which is fructose. Also, byproducts such as humins are formed from these two intermediates and HMF. The experimental results of the time course of the product distribution shown in Figure 4 and Figure 5 were analyzed kinetically. The symbols represent the experimental results, and the curves are the calculated fits. For simplicity, a pseudo-first-order model was used. Calculations were conducted with Polymath 6.10 for solving the following differential equations.
d[G]/dt = − (k1 + k5)[G] + k-1[F] + k-5[I]
d[F]/dt = k1[G] − (k-1 + k2 + k3)[F]
d[H]/dt = k2[F] − k4[H] + k6[I]
d[B]/dt = k3[F] + k4[H] + k7[I]
d[I]/dt = k5[G] − (k-5 + k6 + k7)[I]
The constant k1 describes the isomerization of glucose (G) to fructose (F), and the constant k-1 its back reaction. The constant k2 pertains to the dehydration of fructose to HMF (H). The constants k3 and k4 describe the formation of byproducts (B) from fructose and HMF, respectively. The constant k5 describes the conversion of glucose to another intermediate (I), and the constant k-5 its back reaction. The constants k6 and k7 correspond to the transformation of the intermediate into HMF and byproducts, respectively. Good fits were obtained for all species in Figure 4 and Figure 5 using the same rate constants regardless of the reactants. Regression coefficients for fructose conversion (Figure 4) and glucose conversion (Figure 5) were 0.85 and 0.83, respectively. The rate constants of glucose conversion into two intermediates, k1 and k5, were almost the same, whereas the constants for the back reaction, k-1 and k-5, were very different. The rates of HMF formation from the two intermediates were also different.
It should be noted that the HMF formation rate from another intermediate, k6, was 10 times higher than that from fructose, k2, indicating that this hidden intermediate largely contributes to HMF formation. As mentioned above, one possible candidate for the intermediate is 3-deoxyglucosone. This pathway is attributable to the higher selectivity for HMF from glucose as compared to that from fructose. Because the rate of consumption of the intermediate was much higher than that of formation, the calculated yield of the intermediate was very low (<2%), which makes it difficult to monitor it.
An assessment was conducted by the fitting of the concentration profiles using a typical reaction pathway for HMF formation which involves glucose-to-fructose isomerization, fructose-to-HMF dehydration and degradation to byproducts from fructose and HMF but does not include a pathway via another intermediate. The rate constants used for the typical pathway, the fitted results are shown in Table S1, Figures S2 and S3. At a glance, good fits were obtained for glucose transformation to HMF. However, fairy poor fits were found for fructose transformation when the same rate constants were used. These results were very different from those using the two parallel pathways via fructose and another species as two different intermediates.
An additional experiment was conducted using 3-deoxyglucosone as a reactant. As a comparison, fructose was also used as a reactant under the same reaction condition. The results are shown in Figure 7. It was found that HMF was formed from 3-deoxyglucosone using HNbWO6 nanosheets, and its yield was much higher than that from fructose under the same reaction condition. From these results, it can be said that the primary intermediate is not fructose but another intermediate, possibly 3-deoxyglucosone. Brønsted acid sites are dominant for the metal oxide aggregated nanosheets because protons are the counterions of the negatively charged nanosheets. Here, another reaction pathway via 3-deoxyglucosone is proposed. The intermediate is obtained by dehydration of 3-OH of glucose whereas fructose is formed by isomerization. The present study showed that another route is preferable over HNbWO6-aggregated nanosheets. Because dehydration is generally catalyzed by Brønsted acid, it is considered that the water-tolerant Brønsted acid sites of the metal oxide nanosheets could eliminate 3-OH of glucose to form 3-deoxyglucosone.
Figure 8 shows the reusability of HNbWO6-aggregated nanosheets. After the reaction, the catalyst was separated by centrifugation, and washed with distilled water two times prior to further use. HMF was formed over the reused catalyst though both glucose conversion and HMF yield were gradually decreased. The decrease of the activity is likely due to the formation of humins on the catalyst which covered the active sites of the catalyst because most of unknown byproducts were attributed to insoluble humins and the color of the catalyst became dark brown after the reaction. The HNbWO6-aggregated nanosheets catalyst retained the crystal structure after the reaction (Figure S4).

3. Experimental

3.1. Chemicals

D (+)-Glucose (98%, Wako, Osaka, Japan), D(-)-fructose (99%, Wako, Osaka, Japan), 5-hydroxymethyl-2-furaldehyde (99%, Sigma-Aldrich, St. Louis, MO, USA) were used for the reactions and analysis. Metal oxides and alkaline carbonates were all purchased from Wako (Osaka, Japan). TiO2 (99%), Nb2O5 (99.9%), WO3 (99.5%), Li2CO3 (99%), K2CO3 (99.5%) were used as precursor for the layered transition metal oxides. Nitric acid and 10% tetra(n-butylammonium) hydroxide solution (Wako, Osaka, Japan) were used for proton exchange and exfoliation of the layered metal oxides, respectively. The ion-exchange resins Amberlyst-15 (Sigma-Aldrich, St. Louis, MO, USA), Nafion NR50 (Sigma-Aldrich, St. Louis, MO, USA), and niobic acid·Nb2O5·nH2O (CBMM, Araxá, Brazil) were used as comparisons for solid acids.

3.2. Catalyst Synthesis

Layered HTiNbO5, HNb3O8, and HNbWO6 were prepared by proton exchange of the precursors KTiNbO5, KNb3O8, and LiNbWO6. KTiNbO5 was obtained by calcination of a stoichiometric mixture of K2CO3, TiO2, and Nb2O5 at 1123 K for 3 h and 1373 K for 30 h with one intermediate grinding. Similarly, KNb3O8 was synthesized from K2CO3 and Nb2O5, by calcining at 1373 K for 24 h. LiNbWO6 was prepared from Li2CO3, Nb2O5, and WO3, by calcining at 1073 K for 24 h.
The proton-exchange reaction was performed by shaking 2.0 g of the alkaline form in 150 mL of 1 M nitric acid solution at room temperature for 2 weeks, exchanging the acid solution twice over that period. The product was then washed with distilled water, filtered, and dried in air at 353 K overnight.
The nanosheets constituting the layered oxides (TiNbO5–, Nb3O8–, NbWO6– nanosheets) were prepared by adding 10 wt% tetra(n-butylammonium) hydroxide (TBAOH) solution to 150 mL of distilled water containing 2.0 g of the protonated compound. The TBAOH solution was added dropwise to the suspension until the pH reached 9.5–10.0, and the resultant solution was shaken for 2 weeks. After shaking, the suspension was centrifuged, and the supernatant containing the dispersed nanosheets was collected. The exfoliated nanosheets were stable in aqueous colloidal solution because the negatively charged 2D metal oxide nanosheets are surrounded by tetrabutylammonium cations, TBA+. The addition of an aqueous HNO3 solution (0.1 M, 30 mL) to 150 mL of the nanosheet solution with vigorous stirring resulted in rapid aggregation of the nanosheets as a precipitate because of exchange of the countercations from TBA+ to H+. The aggregated nanosheet samples were then rinsed three times with 150 mL of the 0.1 M aqueous HNO3 solution to remove TBA, and then with 150 mL of distilled water to remove HNO3.

3.3. Characterization

The surface areas of the prepared catalysts were determined by N2 adsorption (BELSORP-mini II, BEL, Osaka, Japan,). Before N2 adsorption, the catalysts were pretreated at 423 K in vacuum. The acid properties of the samples were measured by ammonia temperature-programmed desorption (NH3-TPD, TPD-1-AT, BEL, Osaka, Japan). The samples were pretreated at 423 K for 1 h under He flow and then cooled to 373 K. The sample was exposed to ammonia for 20 min. After the ammonia was flushed away by He, the sample was heated to 873 K at a rate of 2 K min-1. The types of acid sites were examined by using pyridine-adsorbed Fourier transform infrared spectroscopy (FTIR, FT/IR-6600, JASCO, Tokyo, Japan). The HNbWO6 aggregated nanosheets (58 mg) were pressed into a disk (1.0 cm radius) and pretreated at 453 K in a vacuum for 1 h. Pyridine (5 Torr) was introduced into the cell at 373 K for 30 min. After the evacuation at 373 K for 30 min the spectrum was recorded.

3.4. Catalytic Tests

Quantities of 0.28 mmol (50 mg) of hexose (D-glucose or D-fructose) and 50 mg of the aggregated nanosheet catalysts were placed into 1.5–3 mL of water in a reactor vessel. The reactor vessel was heated at 393–423 K for 3 h in an oil bath under stirring. After the reaction, the reactant mixture was analyzed by high-performance liquid chromatography (HPLC; LC-2000 plus, JASCO, Tokyo, Japan) equipped with a differential refractive index detector (RI-2031 plus, JASCO, Tokyo, Japan) with an Aminex HPX-87H column (flow rate: 0.5 mL min-1, eluent: 10 mM H2SO4, Bio-Rad Laboratories, Hercules, CA, USA). For the reuse test, the catalyst was separated by centrifugation, and washed with distilled water two times prior to further use. The reaction was carried out using 50 mg of the catalyst, 50 mg of glucose in 1.5 mL of water at 413 K for 3 h.

4. Conclusions

Exfoliated nanosheets were found to be active solid acid catalysts for the formation of HMF from glucose in water. The order of HMF yield corresponded to the order of the Brønsted acid strength of the nanosheets, HNbWO6 > HNb3O8 > HTiNbO5. HNbWO6 nanosheets gave a high HMF yield with high selectivity (conversion 71%, selectivity 52%) in a biphasic system. HMF was selectively formed from glucose compared from fructose in the presence of HNbWO6. Kinetic analysis suggested that the reaction pathway of HMF production from glucose involved not only fructose but also another intermediate.

Supplementary Materials

The following are available online at https://www.mdpi.com/2073-4344/9/10/818/s1: Figure S1: FTIR spectrum of pyridine adsorbed on HNbWO6 aggregated nanosheets, Figure S2: Time course of HMF formation from glucose over HNbWO6 aggregated nanosheets, Figure S3: Time course of HMF formation from fructose over HNbWO6 aggregated nanosheets, Figure S4: XRD pattern of HNbWO6 aggregated nanosheets after the reaction, Table S1: Rate constants (units in h-1) used for a typical pathway.

Author Contributions

A.T. planned the work, obtained the funding, carried out the research and wrote the manuscript.

Funding

This work was supported by JSPS KAKENHI Grant Number JP 25709077 and 18H01785.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 09 00818 g001 550
Figure 1. Correlation between HMF yield and amount of 2,6-lutidine added. Reaction conditions: glucose (50 mg), catalyst (50 mg), water (1.5 mL), 2,6-lutidine (0-100 μmol), 413 K, 3 h.
Figure 1. Correlation between HMF yield and amount of 2,6-lutidine added. Reaction conditions: glucose (50 mg), catalyst (50 mg), water (1.5 mL), 2,6-lutidine (0-100 μmol), 413 K, 3 h.
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Figure 2. Time course of HMF formation from glucose over HNbWO6 aggregated nanosheets. Reaction conditions: (a) glucose (50 mg), catalyst (50 mg), water (3 mL), 393 K. (b) glucose (50 mg), catalyst (50 mg), water (3 mL), 1-butanol (3 mL), 393 K.
Figure 2. Time course of HMF formation from glucose over HNbWO6 aggregated nanosheets. Reaction conditions: (a) glucose (50 mg), catalyst (50 mg), water (3 mL), 393 K. (b) glucose (50 mg), catalyst (50 mg), water (3 mL), 1-butanol (3 mL), 393 K.
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Figure 3. HMF selectivity vs. glucose conversion.
Figure 3. HMF selectivity vs. glucose conversion.
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Figure 4. Time course of HMF formation from fructose over HNbWO6 aggregated nanosheets. Reaction conditions: fructose (50 mg), catalyst (50 mg), water (3 mL), 393 K. The symbols represent the experimental results, and the curves are the calculated fits.
Figure 4. Time course of HMF formation from fructose over HNbWO6 aggregated nanosheets. Reaction conditions: fructose (50 mg), catalyst (50 mg), water (3 mL), 393 K. The symbols represent the experimental results, and the curves are the calculated fits.
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Figure 5. Time course of HMF formation from glucose over HNbWO6 aggregated nanosheets. Reaction conditions: glucose (50 mg), catalyst (50 mg), water (3 mL), 393 K. The symbols represent the experimental results, and the curves are the calculated fits.
Figure 5. Time course of HMF formation from glucose over HNbWO6 aggregated nanosheets. Reaction conditions: glucose (50 mg), catalyst (50 mg), water (3 mL), 393 K. The symbols represent the experimental results, and the curves are the calculated fits.
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Figure 6. HMF selectivity vs. (a) glucose conversion and (b) fructose conversion over HNbWO6 aggregated nanosheets.
Figure 6. HMF selectivity vs. (a) glucose conversion and (b) fructose conversion over HNbWO6 aggregated nanosheets.
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Scheme 1. Proposed reaction pathway for HMF production over HNbWO6 aggregated nanosheets.
Scheme 1. Proposed reaction pathway for HMF production over HNbWO6 aggregated nanosheets.
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Figure 7. HMF formation from 3-deoxyglucosone (3-DG) and fructose over HNbWO6 aggregated nanosheets. Reaction conditions: substrate (3-deoxyglucosone or fructose, 1 mg), catalyst (2 mg), water (1 mL), 413 K.
Figure 7. HMF formation from 3-deoxyglucosone (3-DG) and fructose over HNbWO6 aggregated nanosheets. Reaction conditions: substrate (3-deoxyglucosone or fructose, 1 mg), catalyst (2 mg), water (1 mL), 413 K.
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Figure 8. Reuse of HNbWO6 aggregated nanosheets. Reaction conditions: glucose (50 mg), catalyst (50 mg), water (1.5 mL), 413 K, 3 h.
Figure 8. Reuse of HNbWO6 aggregated nanosheets. Reaction conditions: glucose (50 mg), catalyst (50 mg), water (1.5 mL), 413 K, 3 h.
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Table
Table 1. Surface areas and acid properties of aggregated nanosheet catalysts [19].
Table 1. Surface areas and acid properties of aggregated nanosheet catalysts [19].
CatalystSBET /m2 g−1Acid Amount /mmol g−1 aNH3-TPD Peak Position /K b31P MAS NMR Peal Position /ppm c
HNbWO6 nanosheets660.3456071
HNb3O8 nanosheets1010.2855070
HTiNbO5 nanosheets1530.2453563
Nb2O5·nH2O1280.30d550e65f
Amberlyst-15504.8N.A.81
Nafion NR50<0.10.9N.A.N.A.
H-Beta g4201.060078h
H-ZSM5 i3260.258086j
a From NH3-TPD. b Strong acid sites. c 31P magic-angle spinning nuclear magnetic resonance spectroscopy using trimethylphosphine oxide as a probe molecule. d Reference [26]. e Reference [18]. f Reference [27]. g SiO2/Al2O3 = 25, JRC-Z-HB25. h Reference [28] (SiO2/Al2O3 = 25, Zeolyst, CP814E). i SiO2/Al2O3 = 90, JRC-Z-5-90H. j Reference [29] (SiO2/Al2O3 = 52, Stream Chemical Inc.)
Table
Table 2. Formation of 5-hydroxymethylfurfural (HMF) from glucose in water over several solid acid catalysts a.
Table 2. Formation of 5-hydroxymethylfurfural (HMF) from glucose in water over several solid acid catalysts a.
EntryCatalystGlucose Conversion /%HMF Yield /%HMF Selectivity /%Fructose Yield /%Formic Acid Yield /%Unknown Yield /% b
1HNbWO6 nanosheets5620365525
2KNbWO6 nanosheets369257N.D.21
3HNb3O8 nanosheets4314324N.D.26
4HTiNbO5 nanosheets55112121031
5Nb2O5·nH2O6320312437
6Amberlyst-1570—007
7Nafion NR5070—007
8H-Beta cTrace0—00—
9H-ZSM5 dTrace0—00—
aReaction conditions: glucose (50 mg), catalyst (50 mg), water (1.5 mL), 413 K, 3 h. b Other water-soluble compounds showing larger signal in high-performance liquid chromatography (HPLC) than glucose, fructose, HMF and formic acid were not found, indicating that unknown products were mostly insoluble humins. c SiO2/Al2O3 = 25, JRC-Z-HB25. d SiO2/Al2O3 = 90, JRC-Z-5-90H.
Table
Table 3. Comparison of catalytic activity of solid acids for HMF production from glucose in water
Table 3. Comparison of catalytic activity of solid acids for HMF production from glucose in water
EntryCatalystHMF Yield (Selectivity) /%RS/C aTemp. /KTime /hReference
1HNbWO6 nanosheets23 (44)139324This study
2HNbWO6 nanosheets20 (36)14133This study
3HNb3O8 nanosheets14 (34)504282.5[22]
4Nb2O5·nH2O12 (12)0.13933[30]
5meso-Nb2O518 (36)14131[35]
6Nb-BEA17 (41)645324[37]
7γ-AlO(OH)17 (18)144324[38]
8SnO2/γ-Al2O312 (14)14231[39]
9SAPO-34/5A20 (N.A.)1.74633[40]
10H-ZSM-5 (Si/Al = 90)0 (0)0.14133[30]
11H-mordenite (Si/Al = 90)0 (0)0.14133[30]
12SO42-/Al2O3-SnO219 (53)23936[41]
13Hybrid-TiO245 (60)1.24037[34]
14H3PO4/Nb2O5·nH2O48 (53)0.14133[30]
15meso-NbP34 (49)14131[35]
16meso-ZrP47 (56)1.64286[36]
17MnPO418 (25)2.54331.5[42]
18Amberlyst-150 (0)0.14133[30]
19Nafion NR500 (0)0.14133[30]
a RS/C: substrate to catalyst weight ratio.
Table
Table 4. Effects of reaction temperature and solvent on HMF formation from glucose over HNbWO6 aggregated nanosheets a.
Table 4. Effects of reaction temperature and solvent on HMF formation from glucose over HNbWO6 aggregated nanosheets a.
EntrySolventTemp./ KTime /hGlucose Conversion /%HMF Yield /%HMF Selectivity /%Fructose Yield /%Formic Acid Yield /%Unknown Yield /%
1Water 1.5 mL39332711400016
2Water 1.5 mL40333714380023
3Water 1.5 mL41335620365525
4Water 1.5 mL42337024354636
5Water 3 mL393245223444531
6Water 3 mL + 1-butanol 3 mL393367137524031
aReaction conditions: glucose (50 mg), catalyst (50 mg).

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Takagaki, A. Production of 5-Hydroxymethylfurfural from Glucose in Water by Using Transition Metal-Oxide Nanosheet Aggregates. Catalysts 2019, 9, 818. https://doi.org/10.3390/catal9100818

AMA Style

Takagaki A. Production of 5-Hydroxymethylfurfural from Glucose in Water by Using Transition Metal-Oxide Nanosheet Aggregates. Catalysts. 2019; 9(10):818. https://doi.org/10.3390/catal9100818

Chicago/Turabian Style

Takagaki, Atsushi. 2019. "Production of 5-Hydroxymethylfurfural from Glucose in Water by Using Transition Metal-Oxide Nanosheet Aggregates" Catalysts 9, no. 10: 818. https://doi.org/10.3390/catal9100818

APA Style

Takagaki, A. (2019). Production of 5-Hydroxymethylfurfural from Glucose in Water by Using Transition Metal-Oxide Nanosheet Aggregates. Catalysts, 9(10), 818. https://doi.org/10.3390/catal9100818

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