Metal-Free and Selective Oxidation of Furfural to Furoic Acid with an N

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Metal-free and Selective Oxidation of Furfural to Furoic Acid with an N-Heterocyclic Carbene Catalyst Navneet Kumar Gupta, Atsushi Fukuoka, and Kiyotaka Nakajima ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03681 • Publication Date (Web): 17 Jan 2018 Downloaded from http://pubs.acs.org on January 17, 2018

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ACS Sustainable Chemistry & Engineering

Metal-free and Selective Oxidation of Furfural to Furoic Acid with an N-Heterocyclic Carbene Catalyst Navneet Kumar Gupta,1,2 Atsushi Fukuoka,1 Kiyotaka Nakajima*,1,3 1

Institute for Catalysis, Hokkaido University, Kita 21 Nishi 10, Kita-ku, Sapporo, Hokkaido 001-

0021, Japan 2

Central Institute of Mining and Fuel Research (CIMFR), Digwadih Campus, PO FRI 828108,

Dhanbad, Jharkhand, India 3

Advanced Low Carbon Technology Research and Development Program (ALCA), Japan

Science and Technology (JST) Agency, 4-1-8 Honcho, Kawaguchi 332-0012, Japan

*K. Nakajima: E-mail: [email protected], Tel.: +81-11-706-9136. Fax: +81-11-7069139

ACS Paragon Plus Environment

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ABSTRACT. Aerobic oxidation of biomass-derived furfural to furoic acid was studied with an N-heterocyclic carbene as a homogeneous catalyst. Carbene species generated in situ on 1,3bis(2,4,6-trimethylphenyl)

imidazolium

chloride

with

a

strong

organic

base

(1,8-

diazabicyclo[5.4.0]undec-7-ene) was highly active and selective for the formation of furoic acid in dimethyl sulfoxide

at 40 °C. This reaction initiates the formation of a Breslow

intermediatebetween an N-heterocyclic carbene and a furfural molecule, and the subsequent activation of molecular O2. While the active carbene catalyst promoted furfural dimerization to afford furoin as a side reaction, furoin was decomposed into the Breslow intermediate and furfural through a reverse reaction, which were then converted quantitatively to furoic acid. Kinetic studies revealed that the apparent activation energy for this furfural oxidation was only 20 kJ mol-1, which is significantly lower than that with a supported Au catalyst (30.4 kJ mol-1). The N-heterocyclic carbene catalyst can oxidize various furan-based aldehydes with high selectivity; however, the electron-withdrawing group bonded to the furan ring has a negative effect on the reaction rate. Furfural can also be oxidized selectively to furoic acid, even in the presence of by-products that are formed by acid-catalyzed dehydration of xylose with Amberlyst-70. As a result, a sequential reaction system based on initial dehydration and subsequent aerobic oxidation was developed for the production of furoic acid from xylose, without the need for furfural purification, using Amberlyst-70 (a solid acid) and an Nheterocyclic carbene catalyst.

KEYWORDS. N-heterocyclic carbene, furfural, furoin, furoic acid, aerobic oxidation

INTRODUCTION


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Biomass-derived sugars such as glucose and xylose are widely accepted as abundant and renewable feedstock for sustainable chemical production.[1-3] Various synthetic strategies have been successfully proposed for the conversion of these sugars to corresponding furans, 5hydroxymethylfurfural (HMF) and furfural.[4-9] These compounds are useful as intermediates for many essential chemicals in the pharmaceutical, agrochemical and polymer industries.[4,8] For example, 2,5-furandicarboxylic acid (FDCA) was identified as a top ten value-added chemical synthesized from biomass-derived C6 sugars, and was applied to the production of polyalkylenefuranate (PAF) as a replacement for petroleum-derived polyethylene terephthalate (PET).[10,11] FDCA production is conventionally based on the aerobic oxidation of HMF, a glucose-derived furan, in the presence of supported metal catalysts.[12-14] A new synthetic route has been developed for the production of FDCA as a building block for various polymers including PAF due to its many promising applications.[15] Furoic acid can be converted to FDCA with 89% yield by the reaction of Cs2CO3 and CO2 at 200-350 °C, where furoic acid is an oxidative product of furfural, a xylose-derived furan compound.[15, 16] Based on this new reaction system, the hemicellulose moiety of lignocellulosic biomass (20-30%) is potentially available as a raw material for FDCA production. Here we have focused on the aerobic oxidation of furfural to furoic acid as an elementary reaction for overall FDCA formation from hemicellulose. Furfural oxidation is a simple oxidation reaction to convert a formyl group bonded to a furan ring to carboxylic acid (Scheme 1). Furoic acid is industrially produced from furfural with a stoichiometric amount of NaOH as a Brønsted base catalyst via the Cannizzaro reaction.[17] Various catalytic systems have been examined to improve the furoic acid yield because the theoretical yield in the Cannizzaro reaction is strictly limited up to 50%. Supported PbPt/C,[18,19] Ag2O/CuO,[20] and


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AuPd/Mg(OH)2[21] catalysts were demonstrated for the aerobic oxidation of furfural with the aid of a homogeneous base, which resulted in an almost quantitative yield toward furoic acid. Alternatively, oxidative esterification of furfural to methyl-2-furoate has been reported using supported gold catalysts with O2 as an oxidant.[22-25] Menegazzo et al. reported base-free oxidative esterification of furfural to the corresponding ester using Au/ZrO2 as an efficient catalyst in methanol with O2 (6 bar) at 120 °C.[22] On the other hand, metal-free catalytic system is also desirable to minimize contamination of an active metal species, which sometimes induces degradation of the product. Recently, metal-free and selective oxidation/oxidative esterification of aldehyde groups in aliphatic and aromatic compounds to carboxylic acid or ester has been reported using an Nheterocyclic carbene (NHC) catalyst.[26-30] In the metal-free reaction, carbon atom bridged with two quaternary-N atoms in NHCs stabilizes carbene in air by the synergy of resonance and inductive effects.[26] The nucleophilic properties of NHCs can activate various aldehyde compounds and form highly reactive Breslow intermediates.[31] These intermediates are then involved in the oxidation reaction to yield carboxylic acid with molecular O2 at relatively low temperatures.[29] This oxidation strategy is potentially profitable for the utilization of biomassderived furans because high temperature reaction of these furans is accompanied by complex side reactions that form solid by-products, the so called humin.[32] In this study, an NHC catalyst, 1,3-bis(2,4,6-trimethylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene (BTDIY), was applied to the aerobic oxidation of furfural to furoic acid in DMSO at 40 °C (Figure 1a). This NHC catalyst can be formed from 1,3-bis(2,4,6-trimethylphenyl) imidazolium chloride (BTIC; Figure 1b) in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), a strong base. Kinetic experiments were performed to determine the possible reaction pathway for furoic acid formation. The NHC


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catalyst was also examined for the oxidation of various biomass-derived aldehydes to corresponding carboxylic acids. Finally, the aerobic oxidation of furfural was combined with the solid acid-catalyzed dehydration of xylose to establish a sequential process for the formation of furoic acid from xylose in a single solution without the need for purification.

Scheme 1: Oxidation of furfural to furoic acid.

EXPERIMENTAL Catalysts and reagents 1,3-Bis(2,4,6-trimethylphenyl) imidazolium chloride (BTIC; >98%) and 1,3-bis(2,4,6trimethylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene (BTDIY; >97%) were purchased from Tokyo Chemical Industry (TCI) and used as homogeneous catalysts for furfural oxidation (Figure 1). Nb2O5 (CompanhiaBrasileira de Metalurgia e Mineração), H-BEA (SiO2/Al2O3=25, Süd Chemie Catalysts Co., Ltd.), H-ZSM-5 (SiO2/Al2O3=25, Süd-Chemie catalysts Co., Ltd.), Nafion NR-50 (Wako Pure Chemical Industries), and ion-exchange resin Amberlyst-70 (The Dow Chemical Company) were used as solid acid catalysts for xylose dehydration. 5-methyl-2furfural (99%), furfuryl alcohol (97%), 5-hydroxymethyl-2-furancarboxylic acid (HMFCA; 98%), dimethyl sulfoxide (DMSO), 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU; 97%, super dehydration grade), and xylose were obtained from Wako Pure Chemical Industries. Furfural (99%), 2-furoic acid (98%), 5-hydroxymethylfurfural (HMF; >99%), 5-methyl-2-furoic acid


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(97%), 2,5-furandicarboxylic acid (FDCA; 97%), and furoin (98%) were obtained from Sigma Aldrich. 2,5-formylfurancarboxylic acid (FFCA; 98%,) was also obtained from TCI. All reagents and solvents were used as-received without purification.

Figure 1. Molecular structure of (a) 1,3-bis(2,4,6-trimethylphenyl)-1,3-dihydro-2H-imidazol-2ylidene (BTDIY), and (b) 1,3-bis(2,4,6-trimethylphenyl) imidazolium chloride (BTIC).

Furfural oxidation Furfural oxidation was performed in a 50 mLTeflon-lined stainless-steel autoclave with a magnetic stirring bar. Typically, NHC catalyst (0.09 mmol, 30 mg), furfural (0.5 mmol, 48 mg), DBU (102mg), DMSO (2 mL), and O2 (0.1 MPa) were charged into the autoclave. The autoclave was placed in an oil bath at 313 K with a stirring rate of 600 rpm. After the reaction, the autoclave was quickly cooled in an ice bath. Unreacted furfural and all products in the reaction mixture were analyzed using high-performance liquid chromatography (HPLC; Nexera X2, Shimadzu) with refractive index and photodiode array detectors at 308 K. HPLC measurements for furfural and furoic acid were performed with an Aminex HPX-87H column (Bio-Rad Laboratories) and a 5 mM H2SO4 solution as a mobile phase. The amount of furoin, an intermediate, was analyzed with a Synergi 4µm Hydro-RP 80A column and aqueous CH3CN (50 vol%) as a mobile phase.


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For kinetic studies, the formation of furoic acid was monitored at 297-323 K to determine the reaction rates and apparent activation energy. The rates of furoic acid formation were estimated from the slopes of the linear time courses at the initial stage of the reaction (furfural conversion < 30%) where the catalyst weight was reduced from 30 mg to 5 mg in all cases.

Sequential synthesis of furoic acid from xylose in DMSO A sequential reaction for the formation of furoic acid from xylose was conducted in a 50 mL Teflon-lined stainless-steel autoclave with a magnetic stirrer. Initial xylose dehydration was performed at 413 K for 2 h after charging the autoclave with a solid acid catalyst (100 mg), xylose (0.5 mmol, 75 mg), DMSO (2 mL) and CO2 (0.5 MPa). After completion of the reaction, the autoclave was quickly cooled in an ice bath. The catalyst was separated from the reaction mixture by simple filtration. BTIC (0.09 mmol, 30 mg), DBU (102 mg) and O2 (1 MPa) were charged to the reaction solution obtained after dehydration. The mixture was heated in an oil bath at 313 K for an additional 2 h. These reaction solutions were analyzed using HPLC with an Aminex HPX-87H column (Bio-Rad Laboratories) and 5 mM H2SO4 solution as a mobile phase.

RESULTS AND DISCUSSION Catalysis with NHC catalyst for the aerobic oxidation of furfural in DMSO Figure 2 shows time courses for furfural conversion and product yields with BTIC and DBU in DMSO. More than 80% of the furfural was converted to two products within 20 min: furoic acid with 69% yield and furoin with 14% yield. An excess amount of DBU compared to the amount


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of BTIC (Catalyst/Base = 0.13) was required to complete the reaction effectively. Both furfural conversion and the yield of furoic acid then gradually increased with time and reached a maximum (>99%) at 4 h. This indicates that furfural can be selectively oxidized to furoic acid by BTIC in the presence of DBU, a strong base, and furoin can be consumed for furoic acid formation. In contrast to a combination of an NHC catalyst and a Ru-based complex effective for oxidation and oxidative esterification of various aldehydes,[33] one important feature of the reaction with BTIC and DBU is a metal-free reaction system in aldehyde oxidation. Because of low furoic acid yields in N,N’-dimethylformamide and tetrahydrofuran (Table S1), DMSO was used as a solvent in the following experiments.

Figure 2. Time courses for furfural conversion (squares), furoic acid yield (circles) and furoin yield (triangles) over BTIC at 313 K. Reaction conditions: 48 mg furfural (0.5 mmol); 2 mL DMSO; 30 mg BTIC catalyst (0.09 mmol, Substrate/Catalyst = 5.56); 102 mg DBU (0.67 mmol, Catalyst/Base = 0.13); T = 313 K; PO2 = 0.1 MPa.


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We further examined the effect of some parameters to elucidate the reaction mechanism for furfural oxidation. Table 1 summarizes furfural conversion and the yields of furoic acid and furoin at 4 h under various reaction conditions. BTIC gave >99% yield toward furoic acid with complete furfural conversion in DMSO in the presence of DBU (Entry 1). No furoic acid was formed on only DBU (Entry 2) or BTIC (Entry 3), as with the blank experiment (Entry 4), which indicates the combination of BTIC with strongly basic DBU is required to generate the NHC catalyst (BTDIY) for furfural oxidation. These results are strongly supported by the reaction of BTDIY with furfural in the absence of DBU (Entry 5). Because of the carbene species on the imidazole moiety of BTDIY, furfural was oxidized to furoic acid with high efficiency without DBU, and both furfural conversion and the furoic acid yield reached greater than 99% within 4 h. Control experiment in Figure S2 also revealed that in-situ generated carbene species from BTIC and DBU oxidizes furfural without decrease in original activity in the presence of water.

Table 1. Oxidation of furfural into furoic acid over NHCs.a Entry Catalyst Base

Reaction atmosphereb

Conversion /%

Mass balancee

Yield / % Furoic acid

Furoin

/%

1

BTIC

DBU

O2

>99

>99e

0

>99

2

BTIC

-

O2

12

0

2

90

3

-

DBU

O2

15

0

5

90

4

-

-

O2

7

0

99

>99

0

>99

6

BTIC

DBU

Arc

73

33

39

>99

7d

BTIC

DBU

Arc→ O2

>99

>99

0

>99

8

BTIC

DBU

Air

>99

>99

0

>99

a

Reaction conditions: 48 mg furfural (0.5 mmol); 2 mL DMSO; 30 mg catalyst (0.09 mmol, Substrate/Catalyst = 5.56); 102 mg DBU (0.67 mmol, Catalyst/Base = 0.13); T = 313 K; t = 4h; b PO2 = 0.1 MPa; cPAr = 3 MPa. dThe reaction in Entry 6 started again after charging O2 (0.1 MPa) for an additional 4 h. eThis yield was also obtained from 1H NMR measurement with an internal standard (see Figure S1 and Table S1 in the supporting information). eMass balance was calculated based on unreacted furfural and detectable products. Based on these results, we propose a reaction mechanism for furfural oxidation to furoic acid in the presence of BTIC and DBU with O2 as an oxidant (Figure 3). First, a carbene species can be formed on BTIC in the presence of DBU. The carbene species generated in situ then reacts with furfural to form a highly reactive Breslow intermediate (3).[34,35] Complex formation between the intermediate (3) and molecular O2 readily proceeds if O2 is present in the reaction mixture, which affords complex (4). Heat treatment enhances the reaction of activated O2 in complex (4) with an additional furfural molecule and gives two furoic acid molecules through the formation of complexes (5) and (6), and finally the active carbene species can be recovered on BTIC. The formation of these key intermediates in this cycle ((2), (3), (4), and (6)) has been identified in previous reportswith a similar oxidation reaction using nuclear magnetic resonance spectroscopy (NMR) and mass spectrometry,[36,37] so this oxygenative pathway is dominant for furfural oxidation to furoic acid with O2.


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Figure 3. Proposed reaction pathway for the conversion of furfural to furoic acid.

It is apparent that O2 can serve as an oxidant in this reaction (Table 1). However, furoic acid (33%) and furoin (39%) were obtained when the reaction was performed in an Ar atmosphere (Entry 6). The reaction in Entry 6 started again after charging O2 (0.1 MPa) into the reactor, which gave a quantitative yield of furoic acid (Entry 7). This phenomenon can be explained by the reaction mechanism shown in Figure 3. The addition of BTIC and DBU in furfuralcontaining DMSO produces complex (4) immediately in air at room temperature. The high stability of complex (4) means that it remains intact, even in an Ar atmosphere, and it can participate in furfural oxidation by heat treatment. However, the carbene catalyst regenerated after one catalytic cycle cannot promote sequential furfural oxidation, due to the absence of O2 in


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the reactor. Assuming that all BTIC (0.09 mmol) is converted into complex (4) in air and one catalytic cycle is promoted, the theoretical yield toward furoic acid in Entry 6 can be calculated as 0.18 mmol (36%), which is consistent with the experimental yield (33%). As suggested from the results of Entries 6 and 7, furfural oxidation proceeds smoothly under air at atmospheric pressure (Entry 8), which gave more than 99% selectivity toward furoic acid with complete conversion of the furfural. It should be noted that the carbene catalyst promotes dimerization of furfural to furoin (Entry 6), which is then finally converted into furoic acid (Entry 7) in parallel with the oxygenative pathway for furoic acid formation. Careful degassing treatment of all reagents in furfural oxidation further reduced furoic acid yield of 10-13% as a result of O2 removal, which strongly supports the reaction phenomenon observed in entries 6 and 7. Furoic acid formation from furoin was further studied under the same reaction conditions.

Oxidation of furoin to furoic acid Figure 4 shows time courses for furoin conversion and furoic acid yield over BTIC. Furoin oxidation proceeds quantitatively and most of the furoin can be readily converted into furoic acid at the initial stage: both furoin conversion and the furoic acid yield reached ca. 80% within 10 min. This reaction can be interpreted by the proposed mechanism in Figure 5. Based on the proposed mechanism for the dimerization of an aldehyde-containing substrate with an NHC catalyst, the dimerization of Breslow intermediate (3) with one furfural molecule results in the formation of furoin via two possible complexes, (7) and (8).[38,39] Furoin formation from Breslow intermediate (3) is a reversible reaction.[40] Carbene species formed on BTIC (2) in situ react with furoin to afford a Breslow intermediate (3) and furfural by the reverse reaction. Once Breslow


12

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intermediate (3) is formed in a reaction mixture with free furfural and molecular oxygen, they are readily involved in the oxygenated pathway by heat treatment, which yields furoic acid with high efficiency.

Figure 4. Time courses for furoin conversion (circles), furoic acid yield (squares) over BTIC at 313 K. Reaction conditions: 48 mg furoin (0.25 mmol); 2 mL DMSO; 30 mg catalyst (0.09 mmol, Substrate/Catalyst = 5.56); 102 mg DBU (0.67 mmol, Catalyst/Base = 0.13); T = 313 K; PO2 = 0.1 MPa.


13


OH

O

Fu Furoic acid OH Fu

O OH O

Fu H

H

O

O

O

Fu Furfural

O HO

Fu

R N

N R

O Fu = O

HO

Fu

R N

N R

(5) (6)

Fu

Furoic acid

(4) O2

H Base

N R

R N

N R

R N

*

O

N R

R N

Page 14 of 33

O

N R

R N

Fu

Fu OH Furoin

Fu

HO

(2)

(1)

(3)

Fu

OH

O

Fu

Fu

N R

HO

Fu

R N

N R

R N

H

O O

Fu Furfural

(8) (7)

Figure 5. Proposed reaction mechanism for furoic acid formation from furoin.

Table 2. Oxidation of furoin to furoic acid.a

Entry Catalyst Base

9

BTIC

DBU

Conversion Reaction /% atmosphereb

O2

Mass balanced

Yield / % Furoic acid

Furfural

>99

0

>99

/% 100


14

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10

-

DBU

O2

5

4

0

99

11

BTIC

-

O2

2

0

1

98

12

BTIC

DBU

Arc

88

34

52

98

a

Reaction conditions: 48 mg furoin (0.25 mmol); 2 mL DMSO; 30 mg catalyst (0.09 mmol, Substrate/Catalyst = 5.56); 102 mg DBU (0.67 mmol, Catalyst/Base = 0.13); T = 313 K; t = 4h; b PO2 = 0.1 MPa; cPAr = 3 MPa. dMass balance was calculated based on conversion and detectable products.

Furoin oxidation was also performed under various reaction conditions for 4 h (Table 2). Figure 4 shows that furoic acid can be produced from furoin quantitatively within 4 h under the reaction conditions (Entry 9). Two control experiments revealed that the carbene species formed on BTIC is catalytically active site for furoin oxidation because both BTIC and DBU alone cannot catalyze the reaction (Entries 10 and 11). Furfural formation from furoin can be directly confirmed by reaction in the absence of O2, which gave 34% and 52% yields toward furoic acid and furfural, respectively (Entry 12). In this experiment, BTIC in air was added to a DMSO solution containing furoin and DBU, and Arwas then charged into the reactor. Entry 6 of Table 1 shows that complexes 4 or 5 could be formed at room temperature immediately if free furfural is present in the reaction mixture, and furoic acid formation was initiated by heat treatment of the mixture containing the pre-formed complexes, even in Ar. The formation of furoic acid and furfural in Entry 12 suggests that the carbene species formed in the reaction mixture can convert furoin to complex (4) via a Breslow intermediate and furfural at room temperature in air, and heat treatment after charging Ar into the reactor results in the formation of furoic acid. However, this oxidation reaction stopped after one catalytic cycle due to the absence of O2. If all the BTIC participates in the formation of complex (4) from furoin before the reaction, then the theoretical


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yield for furoic acid in Entry 12 should be 0.18 mmol (36%), which is approximately comparable to the yield of furoic acid in Entry 12 (34%). Carbene species regenerated after one catalytic cycle of furoic acid formation can react further with furoin to give Breslow intermediate (3) and free furfural (Figure 5). An acidic eluent was used in the HPLC analysis, so that Breslow intermediate (3) evolved in the reaction mixture was decomposed to furfural during HPLC measurement. Therefore, the furfural yield in Entry 11 is the sum of yields toward the Breslow intermediate and free furfural.

Kinetic study for aerobic oxidation of furfural into furoic acid over NHC catalyst Furfural oxidation was performed at various temperatures with a large substrate/catalyst ratio (33.3) to estimate the rates for furoic acid formation and the apparent activation energy. Figure 6A shows time courses for furoic acid formation at different temperatures. The rates at 297, 313, and 323 K were reasonably calculated from these linear plots to be 0.0137, 0.0197 and 0.0263 mmol g-1 s-1, respectively. The apparent activation energy for this reaction can be estimated from the Arrhenius plot based on these reaction rates (Figure 6B) to be 20 kJ mol-1, which is smaller than that for a supported metal catalyst (AuPd/Mg(OH)2).[21]


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Figure 6. (A) Time courses for furfural formation during the initial stage of each reaction at 297323 K and (B) resulting Arrhenius plot. Reaction conditions: 48 mg furfural (0.5 mmol); 2 mL DMSO; 5 mg catalyst (0.015 mmol, Substrate/Catalyst = 33.3); 102 mg DBU (0.67 mmol, Catalyst/Base = 0.13); T = 297-323 K; PO2 = 0.1 MPa.

Table 3 summarizes the catalytic activities and activation energies for BTIC with those for supported metal catalysts as a reference. AuPd/Mg(OH)2[21] and PbPt/C[18]are effective catalysts for furfural oxidation with O2. Although the catalytic activity of BTIC is fairly comparable to those of the reference catalysts, the BTIC system is clearly superior in terms of activation energy and turnover frequency (TOF). Therefore, the NHC-catalyzed system reported here is confirmed as an effective strategy for the selective oxidation of furfural to furoic acid at low reaction temperatures.


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Table 3. Kinetic studies for the aerobic oxidation of furfural over BTIC and over previously reported supported Au and Pt catalysts. Entry

Catalyst

T

t

Conversion Selectivity

/K

/ min

/%

TOF

Ea

/%

/ h-1

/ kJ mol-1

13

BTIC + DBU

313

240

>99

>99

240

20

14[21]

AuPd/Mg(OH)2

303

240

87.7

>95

-

30.4

15[18]

PbPt/C

338

60

93

96

51

-

Substrate scope for aerobic oxidation with NHC catalyst The catalytic oxidation of several furan-based compounds with BTIC and DBU in the presence of O2 was investigated. Table 4 summarizes the results for oxidation of biomass-derived aldehydes and alcohol (Entries 16-19).

Table 4. Substrate scope for furan-based aldehyde oxidation with BTIC.a Entry

Substrate

Time

Targeted product

/h

Conv.

Select.

/%

/%

Mass balanceb /%

16

4

>95

96

96


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17

22

2

99

a

Reaction conditions: Aldehyde (0.5 mmol); 2 mL DMSO; 30 mg catalyst (0.09 mmol, Substrate/Catalyst = 5.56); 102 mg DBU (0.67 mmol, Catalyst/Base = 0.13); T = 313 K; PO2 = 0.1 MPa. bMass balance is calculated based on conversion and yield in this table.

When the BTIC catalyst was applied to the oxidation of 5-methylfurfural, a furfural derivative with an electron-donating group (methyl group) at the C5 position, which corresponds to carboxylic acid (5-methyl furoic acid) was obtained with high conversion (>95%) and selectivity (96%) (Entry 16). In contrast, the BTIC catalyst showed no significant activity for the oxidation of furfuryl alcohol (Entry 17). The oxidation of 5-hydroxymethylfurfural (HMF, Entry 18) and its derivative (FFCA=5-formyl-2-furancarboxylic acid (Entry 19)) were also attempted under the same reaction conditions. HMF oxidation produced 5-hydroxymethyl-2-furancarboxylic acid (HMFCA) with 70% selectivity and some by-products that could not be detected by HPLC analysis due to the lack of catalysis for alcohol oxidation. The presence of the -CH2OH group in HMF induced oxidative esterification as a side reaction to produce condensed compounds in parallel with the desired oxidation of the formyl group, and the final HMFCA selectivity was then relatively low compared to that for the oxidation of furfural (Table 1, Entry 1) and 5-


19


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methylfurfural (Table 4, Entry 16). BTIC catalyzed FFCA oxidation quantitatively (99% selectivity toward furan-dicarboxylic acid (FDCA)); however, FFCA conversion was only 29% at 7 h (Entry 19). While electron-withdrawing group bonded to furfural ring generally enhances oxidation of formyl moiety, oxidation of formyl group in FFCA proceeds more slowly than that in HMF (Entry 18). Such difference is probably due to the difference in steric hindrance of complex (5) formed from FFCA and HMF (Figure 3). Geometric limitation for complex (5) formation from FFCA may result in slow oxidation of formyl group, despite the presence of electron-withdrawing COOH moiety.

Sequential synthesis of furoic acid from xylose The strategy for a one-pot or sequential synthesis of desired compounds without the need for purification is highly relevant to minimize total energy consumption in practical applications. Based on this concept, we have conducted a sequential reaction: the dehydration of xylose to furfural with a solid acid catalyst and the subsequent oxidation of furfural to furoic acid with an NHC catalyst, without purification and isolation of the product after the first dehydration (Scheme 2).

Scheme 2. A sequential strategy for the synthesis of furoic acid from xylose.

First, various solid acid catalysts were applied for the dehydration of xylose in DMSO at 413 K under a CO2 atmosphere (Table 5) because solid acid catalysts can be easily separated from


20

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the reaction mixture by decantation or filtration, and the furfural-containing DMSO can be used for subsequent oxidation reaction with an NHC catalyst. Here Nb2O5, zeolites (H-ZSM-5 and HBEA), and sulfonated resins (Amberlyst-70 and Nafion NR-50) were used as solid acid catalysts for the dehydration of xylose.

Table 5. Xylose dehydration with various solid acid catalysts in DMSO at 413 K.a

Entry

Catalyst

Xylose conversion / %

Furfural yield / %

Othersb / %

20

Nb2O5

>99

32

68

21

H-ZSM-5

49

11

38

22

H-BEA

>99

56

44

23

Nafion NR-50

>99

53

46

24

Amberlyst-70

>99

60

39

a

Reaction conditions: 75 mg xylose (0.5 mmol); 3 mL DMSO; 100 mg catalyst; T = 413 K; PCO2= 0.5 MPa. bundetectable byproducts by HPLC. We have reported that Nb2O5 has intrinsic Brønsted and Lewis acid sites that are workable in water,[9] and Nb2O5 exhibits higher catalytic activity than homogeneous Brønsted and Lewis acid catalysts for xylose dehydration in a biphasic reaction system.[9] Despite high xylose conversion, Nb2O5 afforded only 32% selectivity toward furfural under the reaction conditions (Entry 20). Two zeolites (H-ZSM-5 with small micropores and H-BEA with medium micropores) are widely used as stable and highly active solid acids for various liquid-phase and gas-phase reactions.[41-44]


21


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The catalytic activity of H-ZSM-5 was low (Entry 21); however, H-BEA gave 56% selectivity toward furfural with >99% conversion (Entry 22). This difference can be simply explained by the microporous structure. H-ZSM-5 is an effective catalyst for the dehydration of small alcohol molecules in gas-phase reactions.[45] However, the small micropores (5.3–5.9 Å)[46] of H-ZSM-5 restrict the access of large xylose molecule (6.8 Å)[47] to catalytically active sites, which results in low catalytic activity. In contrast, H-BEA has larger micropores (5.6–7.7 Å)[48] than H-ZSM-5, and is therefore capable of readily incorporating xylose molecules into its micropores. Due to the intrinsic acidity of the SO3H group, Nafion NR-50 and Amberlyst-70 catalysts (Entries 23 and 24) also exhibited yields of 50-60% toward furfural. The activity of Nafion NR-50 with a high SO3H content (0.7 mmol g-1) was comparable to that of Amberlyst-70 with a high SO3H content (2.7 mmol g-1). This is derived from high acid strength of Nafion NR-50. Finally amberlyst-70 was determined to be an appropriate catalyst for the initial xylose dehydration because it had the highest yield among these three catalysts in terms of weight-based activity.

Figure 7. Time courses for xylose dehydration (circles) with Amberlyst-70 and sequential


22

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oxidation of furfural (squares) to furoic acid (tringles) with BTIC and DBU. Reaction conditions for xylose dehydration: 75 mg xylose (0.5 mmol); 2 mL DMSO; 100 mg catalyst; T = 413 K; PCO2 = 0.5 MPa. Aerobic oxidation of furfural: 2 mL furfural-containing DMSO, 30 mg catalyst (0.09 mmol); 102 mg DBU; T = 313 K; PO2 = 0.1 MPa.

Figure 7 shows time courses for the sequential conversion of xylose to furoic acid in DMSO through initial dehydration at 413 K and subsequent oxidation at 313 K. As shown in Entry 24 of Table 5, Amberlyst-70 gave 59% yield toward furfural with >99% conversion at 2 h. This reaction solution was then applied to the aerobic oxidation of furfural to furoic acid with BTIC and DBU after removal of the Amberlyst-70 by simple filtration, despite the presence of many by-products in the form of humin. BTIC, DBU, and O2 (0.1 MPa) were charged into the reaction vessel, and the mixture was heated at 313 K to start aerobic oxidation for the formation of furoic acid. A steep increase in the amount of furoic acid and a decrease in the amount of furfural were observed at the initial stage of the reaction, and the furoic acid yield reached a maximum (57%) after 2 h, which indicates that furfural in the reaction mixture can be quantitatively oxidized to furoic acid, even in the presence of humin. This is due to selective activation of the carbonyl group in furfural with the carbene species to give a Breslow intermediate, even in the presence of various oxygenated functional groups on humin.[49,50]

CONCLUSION Furoic acid (>99%) can be selectively synthesized from furfural by formed BTDIY in situ, an NHC catalyst, with molecular O2 as an oxidant. An oxygenative pathway that includes formation


23


Page 24 of 33

of a Breslow intermediate and subsequent activation of O2 is responsible for the efficient formation of furoic acid. Reaction of the Breslow intermediate with another furfural molecule produced furoin in the reaction mixture as a by-product. Because of a reversible reaction, the evolved furoin was decomposed into the Breslow intermediate and furfural, which were then involved in the formation of furoic acid. The NHC catalyst has advantages over some supported metal catalysts in terms of the apparent activation energy and TOF, and it facilitates the selective aerobic oxidation of various furan-based aldehydes. The direct formation of furoic acid in high yield (57%) was achieved by the combination of initial dehydration with Amberlyst-70 and subsequent aerobic oxidation with BTIC and DBU, without the need for purification. Purification of the product from the reaction mixture for the next reaction results an increase in the total energy consumption. Biomass conversion frequently accompanies the formation of many byproducts; therefore, a reduction in the number of steps shown here is an important strategy to develop sustainable chemical production from biomass resources.

ASSOCIATED CONTENT Supporting Information. The supporting information is available free of charge on ACS Publication website. Solvent screening, product characterization by 1H-NMR and moisture effect (PDF).

AUTHOR INFORMATION Corresponding Author


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*K. Nakajima: E-mail: [email protected], Tel.: +81-11-706-9136. Fax: +81-11-7069139 Notes The authors declare no competing financial interest.

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For Table of Contents Use Only

SYNOPSIS Furoic acid (>99%) can be selectively synthesized from furfural by in-situ generated NHC catalyst with molecular O2 as an oxidant.


33

Metal-Free and Selective Oxidation of Furfural to Furoic Acid with an N

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