Amorphous Nb2O5 as a Selective and Reusable Catalyst for Furfural

Research Article pubs.acs.org/acscatalysis

Amorphous Nb2O5 as a Selective and Reusable Catalyst for Furfural Production from Xylose in Biphasic Water and Toluene Navneet Kumar Gupta,† Atsushi Fukuoka,† and Kiyotaka Nakajima*,†,‡ †

Institute for Catalysis, Hokkaido University, Kita 21 Nishi 10, Kita-ku, Sapporo, Hokkaido 001-0021, Japan Advanced Low Carbon Technology Research and Development Program (ALCA), Japan Science and Technology (JST) Agency, 4-1-8 Honcho, Kawaguchi 332-0012, Japan

‡

S Supporting Information *

ABSTRACT: Aqueous-phase dehydration of xylose into furfural was studied in the presence of amorphous Nb2O5 with water-compatible Brønsted and Lewis acid sites. Nb2O5 was determined as a more active and selective catalyst for xylose dehydration than typical homogeneous Brønsted and Lewis acid catalysts including HCl and Sc(OTf)3, and Nb2O5 converted 93% of xylose with 48% selectivity toward furfural in water at 393 K. No significant loss of the original catalytic activity was observed after Na+-exchange treatment, which indicates that the reaction proceeded only on Lewis acid sites. Isotope-labeling experiments using D2O and xylose suggested that furfural is formed through stepwise dehydration via a highly reactive dicarbonyl intermediate on Nb2O5, whereas typical Lewis acids such as CrCl3 and Sc(OTf)3 convert xylose to furfural in water through hydride transfer and subsequent dehydration via xylulose as a ketose-type intermediate. The difference in the reaction mechanism accounts for the lower activation energy (83 kJ mol−1) with Nb2O5 than those with Sc(OTf)3 and HCl (107−131 kJ mol−1). Continuous extraction of evolved furfural with toluene enabled a large increase in the selectivity toward furfural from 48% to 72% and prevented deactivation of the Lewis acid sites by covering with heavy byproducts, represented by humin. KEYWORDS: xylose, furfural, water-tolerant Lewis acid, biomass, dicarbonyl intermediate

1. INTRODUCTION Green synthesis of value-added chemicals from biomass has received widespread attention in recent years due to the availability of wood or agricultural residues as renewable feedstocks and an increase in CO2 emissions associated with the consumption of fossil fuels.1−5 Lignocellulose, the main component of abundant biomass resources, consists mainly of cellulose (35−50%), hemicellulose (20−35%), and lignin (10− 25%).6 A variety of hydrothermal, chemocatalytic, and biological techniques have been proposed for the depolymerization of cellulose and hemicellulose into constituents such as glucose, xylose, and arabinose.7−12 These sugars are readily available as raw materials and can be transformed by chemocatalytic routes into useful compounds such as 5hydroxymethylfurfural (HMF) and furfural with an aim toward the production of industrial chemicals. Furfural can be synthesized from xylose and arabinose by acid-catalyzed dehydration.13,14 Furfural has a wide variety of potential applications as a versatile intermediate for the production of furan derivatives,15,16 fuel additives,17,18 diols and dicarboxylic acids for polyester synthesis,19−21 and hydrocarbons,22 which is why furfural is industrially produced using H2SO4 as a Brønsted acid catalyst.23 However, one serious drawback of the commercialized production process is the use of liquid acid, which is inevitably accompanied by energy-inefficient separa© 2017 American Chemical Society

tion of the catalyst from the reaction mixture or the removal of inorganic salt byproducts such as gypsum by neutralization treatment.24 Although heterogeneous catalysts such as zeolites,25,26 Amberlyst-15,26 and Nafion27 have been examined as alternative catalysts for furfural production at 413−473 K, such a high-temperature reaction in water generally results in the formation of large amounts of byproducts,28 which limits the furfural yield to 40%. A combination of Lewis and Brønsted acid catalysts were recently reported to convert xylose into furfural in water at low temperatures.29,30 The combined system is distinct from a Brønsted acid system in terms of its reaction mechanism. Lewis acids such as Sn-beta29 or CrCl330 enable the formation of xylulose as a possible intermediate through a Meerwein−Ponndorf−Verley (MPV)-type hydride transfer, where xylulose is readily dehydrated to furfural by HCl. Continuous extraction of furfural with an organic solvent from an aqueous reaction mixture containing CrCl3 and HCl was found to effectively prevent complex byproduct formation and improve furfural yields that exceeded 70%.30 However, despite the high catalytic performance of the combined system, which uses toxic and hazardous homogeneous acids, the Received: December 26, 2016 Revised: February 17, 2017 Published: February 23, 2017 2430

DOI: 10.1021/acscatal.6b03682 ACS Catal. 2017, 7, 2430−2436

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times with water and then analyzed using high-performance liquid chromatography (HPLC; Nexera X2, Shimadzu) with refractive index and photodiode array detectors. The HPLC apparatus was equipped with an Aminex HPX-87H column (Bio-Rad Laboratories) and used at 308 K with 5 mM H2SO4 as an eluent at a flow rate of 0.5 mL min−1. For kinetic studies, the amounts of furfural formed were monitored during the initial stage of the reactions at 383−413 K to determine the reaction rates and activation energies. The rates for furfural formation were estimated from the slopes of the linear time course lines. To obtain a linear correlation between the amount of furfural formed and the reaction time, the weight of the Nb2O5 catalyst was reduced from 100 mg to 20 mg in all cases. Toluene, p-xylene, and THF were used as solvents to extract furfural produced in the reaction mixture and to suppress the formation of byproducts. Typically, 3 mL of organic solvent was added to a mixture containing catalyst (100 mg), xylose (0.5 mmol), and water (2 mL), and stirred at 393 K for 3 h. The resulting water and organic phases were analyzed separately using HPLC and gas chromatography (GC; GC-2025, Shimadzu) with a capillary column (DB-FFAP, GL Sciences) and chlorobenzene as an external standard. 2.3. Isotope-Labeling Experiment Using Xylose and D2O. In a typical experiment, 0.5 mmol of xylose was dissolved in 5 mL of D2O (99.9%, Cambridge Isotope Laboratories). After addition of 100 mg of catalyst, the reaction solution was heated at 393 K for 3 h and then analyzed using HPLC to estimate xylose conversion and selectivity toward furfural. The incorporation of D atoms in furfural was evaluated using 1H nuclear magnetic resonance (NMR) spectroscopy (Jeol, ECX400, 400 MHz) with dimethyl sulfone as an internal standard. The D content at each carbon in the resultant furfural was calculated from the theoretical and measured band areas of furfural on the basis of furfural yield to the normalized band area of the internal standard.

chemical industry requires an easily separable and highly active solid acid catalyst for the environmentally benign and largescale production of furfural from xylose. We have investigated the sustainable conversion of sugar compounds over a stable and reusable solid acid catalyst. Heterogeneous Lewis acids can selectively catalyze several sugar transformations. Sn-containing zeolite promotes the hydride transfer of glucose to form fructose in water, which is one of the important elementary steps in HMF production from cellulosic biomass. Among the insoluble metal oxides with water-tolerant Lewis acid sites,31−33 anatase TiO234−36 and amorphous Nb2O537 can successfully convert glucose to HMF in water or a water/tetrahydrofuran (THF) mixture in the presence of phosphate groups on their surfaces. Here, we have focused on acid catalysis with amorphous Nb2O5 because the combination of intrinsic Brønsted and Lewis acidity on the surface of Nb2O5 is potentially effective for furfural production (Scheme 1). Scheme 1. Conversion of Xylose to Furfural in Water over Nb2O5

Amorphous Nb2O5, known as niobic acid, is a stable and active solid acid catalyst that is workable in water. Such unique catalysis makes Nb2O5 applicable for various water-involving reactions.38−40 NbO4 tetrahedra as Lewis acid sites on Nb2O5 alone catalyze both the hydride transfer of sugar-derivatives41 and the dehydration of sugars;37 therefore, the role of Brønsted and Lewis acid sites for the transformation of xylose to furfural is clarified in this work.

2. EXPERIMENTAL SECTION 2.1. Catalysts and Reagents. Amorphous Nb2O5, denoted simply as Nb2O5, was received from CBMM (Companhia Brasileira de Metallurgia e Mineraçaõ , BET surface area = 132 m2 g−1) and used without pretreatment. Scandium trifluoromethanesulfonate (Sc(OTf)3) was purchased from Tokyo Chemical Industry. Furfural was purchased from Sigma-Aldrich. D-Xylose was obtained from Wako Pure Chemical Industries. Concentrated HCl solution (35−36 wt %) was obtained from Kanto Chemical. Phosphoric acid immobilization and sodium-exchange treatment of Nb 2 O 5 was performed using the following procedures.37 One gram of Nb2O5 was continuously stirred in 100 mL of 1 M H3PO4 solution for 48 h. The sample was thoroughly washed with distilled water until no phosphate ions were detected in the filtrate, and then dried at 353 K overnight. The resulting solid is denoted as phosphate/Nb2O5. Na+exchange treatment of Nb2O5 was conducted with 0.2 M NaCl solution. One gram of Nb2O5 was stirred in 150 mL of NaCl solution while maintaining the pH in the range of 5.5−5.8 by the addition of 0.05 M NaOH. After continuous stirring of the mixture for 24 h, the sample was washed thoroughly with water until Cl− was no longer detected, and then dried at 353 K for 12 h. The Na+-exchanged sample is denoted as Na+/Nb2O5. 2.2. Furfural Formation from Xylose with Acid Catalysts. Catalytic reactions were typically performed in a pressure-resistant quartz reactor using 100 mg of solid catalyst, 0.5−1 mmol of xylose, and 2−5 mL of water at 383−413 K. After the reactions, the resultant solutions were diluted 10−30

3. RESULTS AND DISCUSSION 3.1. Acid Catalysis of Nb2O5 for Furfural Production from Xylose in Water. Nb2O5 and some heterogeneous catalysts, including basic and amphoteric oxides, were first examined for xylose conversion to furfural in water at 393 K (Table S1 and Figure S1). Screening of the catalysts clearly indicated that acidic Nb2O5 had higher xylose conversion and selectivity toward furfural than the other tested catalysts. The catalytic activity of Nb2O5 was further compared in detail with that of HCl under the same reaction conditions. Figure 1 shows time courses for xylose conversion and furfural yield over Nb2O5 and HCl. While both of these catalysts converted xylose into furfural, Nb2O5 was clearly superior in both xylose conversion and furfural yield. This suggests that the combination of Brønsted and Lewis acid on Nb2O5 is apparently effective for the conversion of xylose to furfural. Table 1 summarizes the acid site densities and catalytic activities of Nb2O5 and homogeneous catalysts at 393 K. HCl and Sc(OTf)3 were adopted as Brønsted and Lewis acids for comparison. The densities of water-tolerant Lewis and Brønsted acid sites on Nb2O5 were 0.03 and 0.14 mmol g−1, respectively, as previously estimated from FTIR measurements of a pyridineadsorbed hydrated sample.37 Nb2O5 gave a xylose conversion of 93% and 48% selectivity toward furfural over 3 h (entry 1). Despite a high density of Brønsted acid sites, xylose conversion over HCl was lower than 40% (entry 2), which revealed that 2431


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catalysts were further examined under the same reaction conditions. Figure S2 shows time courses for furfural formation during the initial stage of the reaction, and Table 2 summarizes the Table 2. Kinetic Parameters of Nb2O5 and Reference Catalysts for the Conversion of Xylose to Furfurala

Figure 1. Time courses for xylose conversion (solid lines) and furfural yield (dashed lines) over Nb2O5 (diamonds) and HCl (circles) at 393 K. Reaction conditions: 75 mg of xylose (0.5 mmol); 5 mL of water; 100 mg of catalyst; T = 393 K.

solvent

LASb

BASc

conversion (%)

selectivity (%)

1 2 3 4 5

Nb2O5d HCl Na+/Nb2O5 Sc(OTf)3 Sc(OTf)3

0.03 − 0.03 2.00 2.00

0.14 9.90 − − −

93 31 89 95 79

48 39 45 18 38

6

Nb2O5

H2O H2O H2O H2O HCl aq.e HCl aq.e

0.03

0.14

92

46

rate for furfural formationa /mmol g-cat−1 h−1

TOFd (h−1)

Eae /kJ mol−1

7 8 9

Nb2O5 Sc(OTf)3 HCl

2.10b 0.54c 0.18c

70 0.27 0.018

83 107 131

Reaction conditions: 75 mg of xylose (0.5 mmol); 5 mL of water; T = 393 K. bCatalyst weight: 20 mg. cCatalyst weight: 100 mg. dTurnover frequency (TOF) values were calculated simply by dividing the rate for furfural formation by acid site density shown in Table 1. eAll reactions were performed at 383−413 K.

initial rates for furfural formation at 393 K and the apparent activation energies at 383−413 K. Despite the low selectivity toward furfural (Table 1, entries 2 and 4), the reaction rate over Sc(OTf)3 was more than twice as large as that over HCl (Table 2, entries 8 and 9). It should be noted that Nb2O5 has the smallest activation energy (83 kJ mol−1) and the largest reaction rate (2.1 mmol g−1 h−1) among the tested catalysts (Table 2, entry 7). The small activation energy for xylose conversion to furfural over Nb2O5 clearly indicates that the NbO4 tetrahedra in Nb2O5 promote furfural formation more preferably than these homogeneous Brønsted and Lewis acid catalysts at 383−413 K, whereas Nb2O5 has only a small amount of Lewis acid sites (0.03 mmol g−1). This tendency was strongly supported by TOF values. 3.2. Reaction Mechanism for the Conversion of Xylose to Furfural over Nb2O5 and Sc(OTf)3. Lewis acid catalysis over Sc(OTf)3 and Nb2O5 was examined with an isotope-labeling experiment. The reaction mechanism for glucose dehydration over Lewis acid catalysts was previously studied for reactions of D-containing glucose in water or bare glucose in D2O.36 The distribution of D atoms in the resultant HMF is an indicator for elucidation of the reaction mechanism. Here, a mixture of bare xylose and catalyst in D2O was heated at 393 K for 3 h, and the resulting solution was analyzed using 1 H NMR spectroscopy to elucidate the incorporation of D atoms by H−D exchange reaction, and HPLC was used to calculate the xylose conversion and furfural yield (Table 3). H−D exchange reactions of xylose and furfural in D2O were examined first as control experiments. Dimethyl sulfone was used as an internal standard to calculate the amount of D atoms bonded to xylose and furfural (Table S2). When xylose was heated in D2O at 393 K for 3 h in the absence of acid catalysts, no D atoms were incorporated at any carbons of the xylose molecule (entry 10). In addition, heat treatment of furfural in D2O at 393 K in the presence of Sc(OTf)3 or Nb2O5 resulted in no H−D exchange reaction between furfural and D2O (Figure S3 and Table 3, entries 11 and 12). These results suggest that D atoms in the furfural product are clearly derived from a H−D exchange reaction that proceeds during furfural formation from xylose. There was no significant difference in xylose conversion and furfural selectivity for Sc(OTf)3 and Nb2O5 in pure water (Table 1, entries 1 and 4) or in D2O (Table 3, entries 13 and 14). The selectivity of Sc(OTf)3 toward furfural was 17% with a xylose conversion of 95% (Table 3, entry 13). No incorporation

acid site density /mmol g−1 catalyst

catalyst

a

Table 1. One-Pot Synthesis of Furfural from Xylose Using Lewis and Brønsted Acid Catalystsa

entry

entry

a

Reaction conditions: 75 mg of xylose (0.5 mmol); 5 mL of water; 100 mg of catalyst; T = 393 K; t = 3 h. bLewis acid sites. cBrønsted acid sites. dAmounts of Lewis acid and Brønsted acid sites on hydrated Nb2O5 were estimated by pyridine-adsorption experiments with Fourier transform infrared spectroscopy (FTIR), as described in ref 37. e0.1 M HCl solution was used as the solvent instead of water.

typical Brønsted acids are less active for this reaction at 393 K. There was no decrease in the original conversion and selectivity over Nb2O5, even after deactivation of the Brønsted acid sites by ion-exchange treatment with sodium cations (entry 3), which indicates that the Lewis acid sites of Nb2O5 are responsible for the formation of furfural. Sc(OTf)3 with a high density of Lewis acid also exhibited high xylose conversion (95%), although the selectivity toward furfural was low (18%) (entry 4). Lewis acid catalysis over Nb2O5 is different from that with a typical Lewis acid such as Sc(OTf)3, which is also suggested from entries 3, 5, and 6 of Table 1. When HCl was added to the Sc(OTf)3 system, the selectivity toward furfural was significantly improved from 18% to 38% (entry 5). This improvement was also reported in a CrCl3 system, where the addition of HCl to a mixture of CrCl3, xylose, and H2O resulted in an increase of the furfural selectivity (30−40%),30 whereas CrCl3 alone produced furfural with moderate selectivity (10−15%). In contrast, no effect was observed by deactivation of the Brønsted acid sites on Nb2O5 (entry 3) and the addition of HCl to the Nb2O5 system (entry 6). These results indicate that the unique catalysis over Nb2O5 can be explained by neither the cooperative action of Brønsted and Lewis acid sites nor simple catalysis over a water-tolerant Lewis acid. To reveal these differences, the reaction kinetics of Nb2O5 and the reference 2432


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Table 3. Catalytic Activities of Nb2O5, Sc(OTf)3, and TiO2 for the Formation of Furfural from Xylose in D2Oa

D content (%)

a

entry

substrate

catalyst

conversion (%)

selectivity (%)

C1

C3

10 11 12 13 14 15

xylose furfural furfural xylose xylose xylose

− Nb2O5 Sc(OTf)3 Sc(OTf)3 Nb2O5 TiO2

− − − 95 92 59

− − − 17 45 27

NDb NDb NDb NDb 28 21

NDb NDb NDb NDb 53 57

Reaction conditions: 75 mg of xylose (0.5 mmol); 5 mL of D2O; 100 mg of catalyst; T = 393 K; t = 3 h. bND: not detected.

Figure 2. Proposed reaction pathways for the formation of furfural from xylose through (A) isomerization and dehydration, and (B) stepwise dehydration.

molecules, as shown in Figure 2A. These consecutive reactions include no H−D exchange reaction as an elementary step (Figure 2); therefore, D-containing furfural cannot be formed on Sc(OTf)3. This result is consistent with that for the dehydration of glucose; pure HMF was formed through the hydride transfer of glucose and subsequent dehydration of fructose in D2O over Sc(OTf)3.36 The selectivity of Nb2O5 toward furfural was 45%, and the xylose conversion was 92% (Table 3, entry 14). In contrast with

of D atoms into furfural produced over Sc(OTf)3 was observed from 1H NMR measurements (Figure S4). It is apparent that the formation of furfural from xylose in water by Lewis acid catalysis over Sc(OTf)3 is analogous to that over CrCl3. Heterogeneous and homogeneous Lewis acids have been reported to convert xylose to xylulose in water through the MPV-type hydride transfer.29,30,42 Sc(OTf)3 in the absence of a Brønsted acid is considered to promote the hydride transfer of xylose to xylulose, followed by dehydration of three water 2433


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degradation and polymerization of the products.44−46 This method has received much attention, especially for selective HMF formation from glucose in water using a Sn-containing zeolite and homogeneous Brønsted acid.44 THF, toluene, and xylene were employed as organic solvents with this approach, and Table 4 shows the catalytic activities of Nb2O5 in these solvent mixtures.

Sc(OTf)3, furfural formed over Nb2O5 includes D atoms at the C1 and C3 positions with contents of 28% and 53%, respectively (Figure S4). A similar phenomenon was reported for the conversion of glucose to HMF in D2O with Lewis acidic TiO2, and HMF was produced by Lewis acid catalysis in the presence of TiO4 tetrahedra through a stepwise dehydration mechanism,36 where the product was a mixture of pure HMF and HMF with D at the C1 and C3 positions.36 TiO2 was also used as an additional reference catalyst to confirm H−D exchanged reaction during xylose dehydration in D2O. TiO2 gave a xylose conversion of 59% and 27% selectivity toward furfural, and also introduced D atom at C1 and C3 positions in the resulting furfural (Table 3, entry 15). The difference in catalytic performance between Nb2O5 and TiO2 was separately discussed in the Supporting Information (Table S1 and Figure S1). These results strongly suggest that the formation of furfural over Nb2O5 and TiO2 also proceeds through a stepwise dehydration mechanism in the same manner as shown in Figure 2B. In this mechanism, activation of the OH group at the C3 position is initiated with NbO4 tetrahedra to form an αdicarbonyl compound after the elimination of one water molecule. Due to a variety of resonance structures in cyclic and linear form, a H−D exchange reaction occurs during keto−enol tautomerization (Figure S5) with the simultaneous formation of a five-membered cyclic intermediate by intramolecular acetalization. Furfural is evolved as a final product after the additional dehydration of two water molecules. Thus, the large difference in both the activation energy and catalytic performance over Nb2O5 and Sc(OTf)3 is reasonably explained by the different reaction mechanisms. 3.3. An Efficient System for Furfural Production from Xylose over Nb2O5. Two effective strategies have been established to improve the catalytic activity of some heterogeneous catalysts for glucose conversion to furan derivatives. One is immobilization of phosphate groups on the surface by postsynthetic treatment.37,43 According to the reported procedure,37 Nb2O5 (1 g) was treated with phosphoric acid solution (1 M, 100 mL) at room temperature for 2 days. The solid recovered by vacuum filtration was washed with excess water to remove acidic residues. It was confirmed that there is no decrease in the original density of Lewis acid sites after the immobilization of phosphate groups.37 Although the selectivity toward furfural increased from 48% to 67% after phosphoric acid treatment, xylose conversion was approximately half (48%) that with the original Nb2O5. This modification is therefore not very effective for furfural production with respect to the reaction rate. The role of phosphate group is probably ascribed to steric hindrance. The density of phosphate group on Nb2O5 (ca. 1 mmol g−1) is much larger than that of Lewis acid site (0.03 mmol g−1), suggesting that Lewis acid site on Nb2O5 is surrounded by a lot of phosphate groups.37 The high density of phosphate group prevents access of reactant molecule to Lewis acid site, which decreases the reaction rate. In addition, intermolecular reaction for humin formation can be suppressed by this steric hindrance more effectively than intramolecular reaction for xylose to furfural transformation due to the decrease in accessibility of substrate and intermediate molecules. As a result, furfural selectivity was increased with the decrease in xylose conversion after introduction of phosphate group on Nb2O5 surface. Another strategy is the in situ extraction of furfural from aqueous solution into an organic phase to avoid catalytic

Table 4. Dehydration of Xylose to Furfural in Biphasic Systems Using Nb2O5a selectivity (%) entry

solvent

conversion (%)

organic phase

water phase

1 16 17 18b 19c 20

water water + toluene water + xylene water + toluene water + toluene water + THF

93 >99 97 89 75 >99

− 63 56 65 55

48 9 11 9 8 44

total 48 72 65 74 63 44

a

Reaction conditions: 75 mg of xylose (0.5 mmol); 2 mL of water and 3 mL of organic solvent; 100 mg of catalyst; T = 393 K; t = 3 h. b1 mmol and. c2 mmol of xylose correspond to 7.5 and 15 wt % in the aqueous phase, respectively.

Xylose conversion over Nb2O5 in water was 93% with 48% selectivity toward furfural (entry 1). The addition of a waterimmiscible solvent such as toluene and xylene to aqueous xylose solution forms a biphasic system where hydrophilic Nb2O5 is only suspended in the water phase during the reaction. These systems had 65−72% selectivity toward furfural with xylose conversion of more than 95% (entries 16 and 17). High furfural selectivity exceeding 70% with complete conversion of xylose (entry 16) clearly indicates that toluene is effective as an extraction solvent for the one-pot production of furfural. The effect of the xylose concentration on the selectivity toward furfural in the toluene-water system was also examined, whereby similar yields and selectivities were obtained, even with 1.0 mmol of xylose (7.5 wt % solution) at 393 K for 3 h (entry 18). Further increase in the xylose content up to 2.0 mmol (15 wt % solution) led to the slight decrease in the furfural selectivity from 74% to 63% (entry 19). Separation of furfural from the reaction mixture is highly beneficial for efficient furfural production from concentrated xylose solution. No significant improvement in separation was obtained in the case of the THF−water system (entry 20), which is due to the miscibility of THF with water to form a single-phase solution, and Nb2O5 particles were highly dispersed in the water−THF mixture, which induces the formation of byproducts among the target product and substrate. Finally, the reusability of Nb2O5 in pure water and a biphasic solution was tested (Figure 3). After each reaction, the catalyst was washed thoroughly with water and toluene to remove organic contaminants from the surface and then reused for subsequent reactions. Although a slight decrease in xylose conversion and furfural selectivity occurred in the aqueousphase system (Figure 3A), no significant loss of activity was observed in the biphasic reaction system (Figure 3B). The deposition of polymerized species, represented by humin, generally deactivates catalytically active sites on the Nb2O5 catalyst (Figure S6), but suppression of byproduct formation in 2434


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Figure 3. Catalytic activity of fresh and reused Nb2O5 for furfural production from xylose at 393 K in (A) water and (B) biphasic media. Reaction conditions: 75 mg of xylose (0.5 mmol); 5 mL of water for monophasic reaction, and 2 mL of water and 3 mL of toluene for biphasic reaction; 100 mg Nb2O5; T = 393 K; t = 3 h. (4) Kumar, A.; Kumar, N.; Baredar, P.; Shukla, A. Renewable Sustainable Energy Rev. 2015, 45, 530−539. (5) Liu, B.; Zhang, Z. ACS Catal. 2016, 6, 326−338. (6) Saha, B. C. In Handbook of Industrial Biocatalysis; Hou, C. T., Ed.; CRC Press: Boca Raton, FL, 2005; pp 24-1−24-12. (7) Suganuma, S.; Nakajima, K.; Kitano, M.; Yamaguchi, D.; Kato, H.; Hayashi, S.; Hara, M. J. Am. Chem. Soc. 2008, 130, 12787−12793. (8) Rinaldi, R.; Palkovits, R.; Schüth, F. Angew. Chem., Int. Ed. 2008, 47, 8047−8050. (9) Kobayashi, H.; Yabushita, M.; Komanoya, T.; Hara, K.; Fujita, I.; Fukuoka, A. ACS Catal. 2013, 3, 581−587. (10) Fan, J.; De bruyn, M.; Budarin, V. L.; Gronnow, M. J.; Shuttleworth, P. S.; Breeden, S.; Macquarrie, D. J.; Clark, J. H. J. Am. Chem. Soc. 2013, 135, 11728−11731. (11) Kayser, H.; Rodriguez-Ropero, F.; Leitner, W.; Fioroni, M.; Maria, P. D. RSC Adv. 2013, 3, 9273−9278. (12) Zhou, L.; Yang, X.; Xu, J.; Shi, M.; Wang, F.; Chen, C. Green Chem. 2015, 17, 1519−1524. (13) Karinen, R.; Vilonen, K.; Niemela, M. ChemSusChem 2011, 4, 1002−1016. (14) Lange, J. P.; van der Heide, E.; van Buijtenen, J.; Price, R. ChemSusChem 2012, 5, 150−166. (15) Manzoli, M.; Menegazzo, F.; Signoretto, M.; Cruciani, G.; Pinna, F. J. Catal. 2015, 330, 465−473. (16) Jiménez-Gómez, C. P.; Cecilia, J. A.; Durán-Martín, D.; Moreno-Tost, R.; Santamaría-González, J.; Mérida-Robles, J.; Mariscal, R.; Maireles-Torres, P. J. Catal. 2016, 336, 107−115. (17) Mascal, M.; Nikitin, E. B. ChemSusChem 2009, 2, 423−426. (18) Lew, C. M.; Rajabbeigi, N.; Tsapatsis, M. Ind. Eng. Chem. Res. 2012, 51, 5364−5366. (19) Liu, S.; Amada, Y.; Tamura, M.; Nakagawa, Y.; Tomishige, K. Green Chem. 2014, 16, 617−626. (20) Banerjee, A.; Dick, G. R.; Yoshino, T.; Kanan, M. W. Nature 2016, 531, 215−219. (21) Jenness, G. R.; Wan, W.; Chen, J. G.; Vlachos, D. G. ACS Catal. 2016, 6, 7002−7009. (22) Faba, L.; Diaz, E.; Ordonez, S. ChemSusChem 2014, 7, 2816− 2820. (23) Cai, C. M.; Zhang, T.; Kumar, R.; Wyman, C. E. J. Chem. Technol. Biotechnol. 2014, 89, 2−10. (24) Adams, J. F.; Papangelakis, V. G. Hydrometallurgy 2007, 89, 269−278. (25) Gürbüz, E. I.; Gallo, J. M. R.; Alonso, D. M.; Wettstein, S. G.; Lim, W. Y.; Dumesic, J. A. Angew. Chem., Int. Ed. 2013, 52, 1270− 1274. (26) Bruce, S. M.; Zong, Z.; Chatzidimitriou, A.; Avci, L. E.; Bond, J. Q.; Carreon, M. A.; Wettstein, S. G. J. Mol. Catal. A: Chem. 2016, 422, 18−22. (27) Lam, E.; Majid, E.; Leung, A. C. W.; Chong, J. H.; Mahmoud, K. A.; Luong, J. H. T. ChemSusChem 2011, 4, 535−541.

this biphasic system enables the high activity and reusability of Nb2O5 to be maintained.

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CONCLUSIONS Efficient one-pot synthesis of furfural, one of the most important raw materials derived from biomass, can be realized using amorphous Nb2O5 as a water-tolerant Lewis acid catalyst at 393 K. Lewis acid sites on Nb2O5 are crucial for the selective conversion of xylose to furfural through a stepwise dehydration mechanism. The combination of aqueous-phase dehydration of xylose over Nb2O5 and the continuous extraction of furfural with an immiscible organic solvent resulted in a high selectivity toward furfural of approximately 72%, even from a highly concentrated xylose solution of more than 7 wt %. Facile recovery and high reusability of Nb2O5 in the biphasic system is essential for the development of an environmentally benign and efficient process for the production of furfural.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b03682. Additional results of catalytic activities, characterization of Lewis acid strength for Nb2O5 and TiO2, 1H NMR spectra of furfural evolved in D2O, and proposed mechanism for the formation of furfural containing D atom (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +81-11-706-9136. Fax: +81-11-706-9139. ORCID

Navneet Kumar Gupta: 0000-0002-4204-4680 Atsushi Fukuoka: 0000-0002-8468-7721 Kiyotaka Nakajima: 0000-0002-3774-3209 Notes

The authors declare no competing financial interest.

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Amorphous Nb2O5 as a Selective and Reusable Catalyst for Furfural

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