Efficient Oxidative Transformation of Furfural into Succinic Acid over

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Efficient oxidative transformation of furfural into succinic acid over acidic metal-free graphene oxide Wanzhen Zhu, Furong Tao, shiwei Chen, Ming Li, Yongxing Yang, and Guangqiang Lv ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03373 • Publication Date (Web): 22 Nov 2018 Downloaded from http://pubs.acs.org on November 25, 2018

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ACS Sustainable Chemistry & Engineering

Efficient oxidative transformation of furfural into succinic acid over acidic metal-free graphene oxide Wanzhen Zhu,[a] Furong Tao,[a] Shiwei Chen,[b] Ming Li,[c] Yongxing Yang,*[d] Guangqiang Lv,*[a] a Shandong

Provincial Key Laboratory of Molecular Engineering, School of Chemistry and

Pharmaceutical Engineering, Qilu University of Technology (Shandong Academy of Sciences), Jinan, 250353, People’s Republic of China b

School of Materials Science and Engineering, Qilu University of Technology (Shandong

Academy of Sciences), Jinan, 250353, People’s Republic of China c College

of Chemical Engineering, Qingdao University of Science & Technology, Qingdao,

266042, People’s Republic of China d School

of Chemistry and Chemical Engineering, Shanxi University, Taiyuan, 030006, People’s

Republic of China *Corresponding author: E-mail: [email protected]; [email protected]

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Abstract Graphene oxide (GO), prepared by modified Hummers method, was applied as highly active and selective solid acid catalyst for oxidative transformation of furfural (FAL) into succinic acid (SA) with H2O2 as the oxidant. Over GO catalyst, 88.2 % yield of SA from FAL was achieved under relatively mild reaction conditions. By characterization of the prepared GO and various control experiments, the higher efficiency of GO than conventional catalysts was attributed to its unique atomic layered structure and suitable acidity. Based on the results in this work and previous reports, a reasonable reaction pathway was proposed. Different from other acid catalysts, it was suggested that the aromatic rings in GO and the edges decorated with SO3H groups worked synergistically in FAL oxidation.

The GO catalyst was found reusable

and only a slight decrease in the catalytic activity observed after 6 cycles. Keywords: graphene oxide; furfural oxidation; succinic acid; metal-free catalysis; synergistic effect Introduction Recent years, biomass refinery are growing into an important supplementary process alternative to fossil resources1. Succinic acid (SA) was identified as one of the top 12 value-added chemicals produced from biomass2. SA is widely used as a C4 building block for fuel additives, solvents, food/cosmetic/pharmaceuticals, biopolymers/polyesters, polyurethane, plasticizers and other fine chemicals3. Bio-base SA production methods mainly involve bacterial fermentation4 and catalyzed chemical synthesis5. Bacterial fermentation processes are applicable for various kinds of sugar feedstocks and supply a large amount of SA productivity. With glucose as the substrate6, SA was produced via the metabolism of corynebacterium glutamicum strain. Under oxygen deprivation conditions, SA was produced up to 1.24 M (146 g L-1) in concentration within 46 h by


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intermittent addition of sodium bicarbonate and glucose. However, the tedious recovery or purification process of SA from the fermentation broth, and the high cost of bioreactor and strict control of reaction conditions (eg. pH and temperature) both limit the large-scale SA production. For a wider utilization of SA, development of catalyzed chemical synthesis method with high efficiency is an important issue. Maleic anhydride (or maleic acid) was the oxidation product from n-butene or butadiene7 over metal-cathodes in electronic reactions 8 or precious metal-catalysts under various conditions 9-13. Traditionally, SA was produced by hydration and/or hydrogenation of fossil-derived maleic anhydride or maleic acid (MA). In 1960s, in the homogeneous catalytic system, reduction of MA by chromous sulfate in water gave the SA in 86% yield.14 Hydrogenation of MA to SA over Ru/Al2O3 gave a 45% MA conversion.12 Besides, galvanostatic electrolysis for SA synthesis from MA using stainless steel, Cu or Pd cathodes in an ion conducting polymer electrolyte flow cell

obtained 95 % yield of SA with a coulombic

efficiency of 80–90 % .15 Recently, researchers developed various catalytic system and obtained considerably high bio-based MA selectivity from 5-hydroxymethylfrufural (HMF) 16-17, furfural

(FAL)18-19 and levulinic acid (LA).20 However, tedious procedure and high cost would

be concerned if MA hydrogenation method is employed in SA production. From the perspective of green chemistry, direct synthesis of SA from economic biomass platform chemicals with high efficiency, low cost, and simple procedure under mild conditions is mostly needed. In this filed, Prof. Kohki Ebitani groups 3, 21 found that SA could be produced easily from FAL and HMF over Amberlyst-15 catalyst in H2O2-containing conditions under relatively mild conditions (80 oC, 24 h). In FAL oxidation, the authors compared the catalytic activity of various common acidic catalysts, such as Amberlyst-15 (SA yield 74 mol.%),

Nafion NR50 (41 mol.%), Nafion

SAC13 (28 mol.%), γ-Al2O3 (26 mol.%), Nb2O5 (24 mol.%), ZSM-5 (17 mol.%), ZrO2 (17



mol.%), SO4/ZrO2 (10 mol.%), p-TsOH (72 mol.%) and also homogeneous acids, such as HCl (49 mol.%), H2SO4 (45 mol.%). The best SA yield of 74 mol.% was achieved. By comparasion of the structures of

Amberlyst-15 and p-TsOH with those of other catalysts, the authors

claimed that the π-π electronic interaction between catalyst surface and furan ring, combined with acid groups, favored the intermediate in a suitable conformation and thus enhanced the SA selectivety in FAL oxidation. Inspired by this work and in combination with our experience in the application of graphene-based catalyst in bio-refinery, graphene oxide (GO) prepared by modified Hummers method and exfoliated by ultrasonic was employed in the highly selective oxidation of FAL into SA. With time go on and improvement in GO production technologies in lab or industry in recent years, the studies on GO material are becoming more prevalent22-23and has been developed into an inexpensive, environmental friendly, metal-free carbocatalyst24-29 in various chemical synthesis reactions23, 25-26, 30-34 . In our lab, we are interested in excavating the inherent catalytic ability of GO to facilitate useful synthetic transformation, relying on its unique structural, electronic and chemical properties, as well as high surface area.35-36 In this work, the prepared GO material contains 1-10 layered nanosheets and abundant –SO3H groups at the edge of the nanosheets. The intrinsic 2D structure of aromatic domain on GO basal plane and plenty of –SO3H at GO sheets edges provide the possibility for efficient SA production from FAL. Direct synthesis of SA from FAL, HMF and LA with a green and facile method under relatively mild conditions was investigated. High SA yield from FAL was obtained with GO as the catalyst and H2O2 as the oxidant. Catalytic reaction and analysis General procedure for oxidation


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In a typical run, the procedure for FAL oxidation was as follows: furfural (5 mmol, 480.4 mg), GO (50 mg), H2O (15 mL) and 30 wt.% H2O2 (H2O2, 30 mmol, determined by iodometric titration) were added into a 50 mL three-necked round bottom flask. The reactor was equipped with a total reflux condenser to avoid solvent evaporation. A thermostatic oil bath was used as the heating source. The reaction was performed under vigorous stirring (600 rpm) and maintained at the reaction temperature for a specific time. Samples were taken from the reaction mixture at specified time for products analysis. Analytical procedures The reaction mixture was diluted with deionized water and filtered with a 0.45 µm syringe filter prior to analysis. The yields of SA, MA, FuA and other organic acid by-products, as well as the conversion of FAL, HMF were determined by using a Shimadzu high-pressure liquid chromatograph (LC-10AT) equipped with a UV-VIS detector (SPD-10A). A 4.6 mm id×250 mm Shimadzu Ultra Aqueous C18 column was used. The mobile phase was acetonitrile and 0.3 wt. % phosphoric acid aqueous solution (20:80 v/v) at 0.5 mL min-1. The UV-VIS detector was set at the wavenumber of 215 nm for organic acid analysis, and 280 nm for FAL, HMF analysis. The substrates conversion and the yields of the products were calculated on the basis of external standard curves constructed with authentic standards. Results and Discussion Characterization The specific surface area (SBET) of prepared GO was measured by N2 adsorption/desorption at liquid nitrogen temperature using a Micromeritics ASAP2020HD88 apparatus after outgassing under vacuum at 150 oC for 3 h, the area was calculated by the Brunauer-Emmett-Teller (BET) model (Figure 1). Ultrasonic treatment is an effective method in graphene oxide exfoliation.35, 37



Typically, 1-10 layered, mesopores graphene nanosheets with relative high surface area can be obtained by ultrasonic exfoliation. In this work, the achieved GO has a BET surface area of 379.0 m2/g with an average pore diameter of 14.4 nm. For comparison, the specific surface areas, average pore volumes and average pore diameters of as-prepared GO-HT, -SO3H/GO-HT and COOH/GO-HT were also measured and provided in Table S1 (supporting information ). BET analysis data shows that high temperature treatment under reduction atmosphere followed by sulfonation or oxidation of GO-HT, which derived –SO3H/GO-HT and COOH/GO-HT, did not induce obvious micro structure changes within the prepared graphene based materials. 1000 0.10

400 200 0 0.0

0.08

-1 3

600

3

-1

Pore Volume (cm g )

800 V ads (cm g )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.06 0.04 0.02 0.00

0.2

0.4

0.6

0.8

1.0

0

3

Relative Pressure (P/P0)

6

9

50

Pore diameter (nm)

Figure 1. N2 sorption and pore distribution of GO material


100 150 200 250

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b

a

200 nm

1 μm c

200 nm

d

d= 0.34 nm

o

28.8

o

11.6

Intensity (a.u.)

42.6

o

GO-HT o

26.8

GO Graphite oxide 15

10 nm

10 nm

30

45

60

75

90

o

2( )

Figure 2. SEM (a), TEM (b), HRTEM (c) images and XRD (d) pattern of prepared GO materials b

a S 2p3/2

Average ID/IG

O 1s

C 1s

D

G

1.23

COOH/GO-HT

1.21

GO-HT

Intensity (a.u.)

COOH/GO-HT

intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


GO-HT

0.91

GO

GO 0

200

400

600

800

600

900

1200

1500

1800

2100

-1

Raman Shift (cm )

Binding Energy (eV)

Figure 3. XPS (a) and Visible-Raman (b) spectra of GO, GO-HT and COOH/GO-HT samples



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Table 1. Elemental analysis and Boehm titration results of prepared materials Samples GO GO-HT COOH/GO-HT SO3H/GO-HT SO3H-AC

C 49.6 95.6 70.8 75.4 89.5

Element content (wt.%) H O S 2.4 46.8 1.2 1.1 2.4 0.2 1.6 26.2 0.1 1.5 22.1 1.0 1.5 7.2 1.8

n (RCOOH) (mmol/g) 0.74 0.04 0.62 -----

The morphology and structure of the prepared materials were studied by SEM, TEM and HRTEM. As shown in Figure 2 a, with GO as example, the sample exhibits the typical layered nanosheets structure of graphene. The morphology of GO showed randomly aggregated, crumpled, fluffy nanosheets under SEM and TEM observations (a-b). HRTEM characterization further indicates that these nanosheets consist of 1-10 layers of graphenes (c). The interlayer distance is about 0.34 nm, as indicated in figure 2 c. To get the main elemental composition, EDX mapping was recorded to indicate elements distribution.

As shown in Fig S1 (supporting

information), C, O and S were detected on a random selected GO surface area and distributed uniformly. It was reported that GO layers would be refabricated when temperature38 and might be aggregated under

annealed at a high

hydrothermal conditions. In our work, however,

the as-prepared GO-HT at 1000 oC and sulfonated GO-HT (-SO3H/GO-HT) or oxidized GO-HT by HNO3 vapor (COOH/GO-HT) all show a layered morphology, similar to that of GO (Supporting information Fig. S2). This phenomenon indicates the as-prepared GO materials by ultrasonic are relatively stable and not influenced obviously by the following treatments. XRD patterns of the prepared GO, GO-HT, and graphite oxide were recorded and provided in Figure 2 d. Graphite oxide represents one main reflection peak centered at 11.6 o and one minor peak centered at 42.6 o, corresponding to the c-axis interlayer spacing of 0.81 nm and 0.21 nm, respectively; the results indicate that the graphite is oxidized nearly completely under


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conditions selected in this work. By comparison, the exfoliated GO shows one main broad peak centered at 26.8o (d = 0.34 nm), consistent with HRTEM analysis (c), and one very weak peak centered at 42.6o (d = 0.21 nm). The XRD pattern of GO suggested that graphite oxide was not exfoliated completely uniformly under these conditions. After thermal reduction under relatively high temperature, GO-HT shows a similar XRD pattern to that of GO, and one main peak centered at 28.8o (d = 0.32 nm) was observed. The peak shift towards high angle from 26.8o to 28.8o implies the removal of functional groups in GO basal plane under thermal conditions. A rough elemental scan by XPS in Figure 3 a shows that GO possesses the highest oxygen signal intensity, which decreases sharply in GO-HT, obtained by thermal treatment of GO at 1000 oC under reduction atmosphere. Oxidation of carbon material with vapor-phase HNO3 is an effective method in the introduction of various oxygen groups, such as hydroxyl, epoxides, carbonyl, carboxyl, lactones et al. into carbon materials surface. Compared with GO-HT, the oxygen signal intensity in oxidized GO-HT (COOH/GO-HT) shows an obvious increase. It should be noted that weak signals attributed to S element could be distinguished in all XPS spectra. As reported in previous literatures, S mainly exhibites as -SO3H group covalently bonded to GO edges 31 and can function as efficient acid catalyst in biomass transformation39. Figure 3 b compares the Raman spectra of GO, GO-HT and COOH/GO-HT samples. All samples display two intense bands. The D band (~1350 cm-1), corresponding to a breathing mode of k-point phonons in A1g symmetry, is attributed to local defects and disorders, particularly the defects located at the edges of graphene. The G band (~1580 cm-1) is generally assigned to the E2g phonon of the sp2 bond of carbon atoms.36 The larger ID/IG peak intensity ratio of the Raman spectrum indicates higher defects and disorders of the graphitized structures in GO. In Figure 3 b, the D and G bands are displayed at about 1345 cm-1 and 1592 cm−1. The ID/IG ratio increased



from 0.91 for the GO to 1.21 for the GO-HT. This increase was due to the high reduction of GO, removal of oxygen functionalities and accompanying formation of the pores (as defects and disorders) in the graphene sheets at high temperature. Oxidation of GO-HT with vapor-phase HNO3 introduced more oxygen groups into COOH/GO-HT and the ID/IG ratio in COOH/GO-HT further increased to 1.23 (although not obvious), indicating more defects were caused in COOH/GO-HT by oxidation. To get an accurate element distribution in graphene-based materials, the element composition of GO, GO-HT and COOH/GO-HT was analyzed and provided in Table 1. Consistent with the XPS analysis results, GO had the highest oxygen content, reaching to 46.8 wt.%.

High temperature reduction made the oxygen content in GO-

HT reduce to 2.4 wt.%. Oxidation of GO-HT with vapor-phase HNO3 made the oxygen content increase again to ca. 26.2 wt.%. Notably, the S contents were detected as 1.2 wt.% in GO and 0.2 wt.% , 0.1 wt.% in GO-HT, COOH/GO-HT respectively, indicating the –SO3H groups can be removed efficiently under thermal condition. To understand the contents of specific oxygen groups, especially the carboxyl groups in GO materials, the de-convoluted C 1s XPS spectra of GO, GO-HT and COOH/GO-HT were depicted in Figure S3 (Supporting information). The O contents assigned to C-O/C-O-C, C=O and HO-C=O from Fig. S3 were calculated and tabulated in Table S2 (supporting information). The de-convoluted peak located at the binding energy of ca. 285.0 eV was attributed to the C-C, C=C and C-H bonds. The peaks centered at the binding energies of ca. 286.0, 287.5, and 289.5 eV were assigned to the C-OH, C=O and O=C-OH oxygen containing canbonaceous bonds, respectively.36 The spectra in Figure S3 and Table S2 indicated that the oxygen-containing functional groups in GO can be largely reduced, especially the hydroxyl groups. After oxidation by gas phase HNO3, oxygen-containing functional groups can be recovered to some extent, but without introduction of sulfur-containing functional groups.


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This feature facilitates the subsequent investigation of the effect of –SO3H and -COOH (or COH, -C=O etc.) on their catalytic activity in FAL oxidation. It is always not much reliable for quantification of elements in different valence states by XPS. We did not give the de-convoluted S 2p XPS spectra in this work due to its relatively low content. For different oxygen-containing groups, Boehm titration was employed as an auxiliary method to analyze the contents of them in above three samples. The data shown in table S3 indicated that 0.74 mmol/g –COOH could be detected in GO, and the content decreased to 0.04 mmol/g in GO-HT, nearly removed completely. The oxidation made the –COOH content in COOH/GO-HT recover to 0.62 mmol/g (Summarized in Table 1). Other oxygen-containing functional groups also show similar trends during reduction and oxidation. Oxidation of FAL over various catalysts with H2O2

O

O

GO, 30 wt. % H2O2 o

o

H2O, 60 C- 80 C Furfural (FAL)

O HO

O O OH

HO

O

O

OH

O H

O Succinic acid (SA)

Maleic acid (MA)

Furoic acid (FuA)

Scheme 1. Detected products in FAL oxidation reaction In a preliminary experiment, 5 mmol FAL dissolved in 15 mL water was heated to 70 oC in the presence of 30 wt. % H2O2 and GO for 24 h. Subsequent filtration afforded a mixture of SA and trace amount of MA and FuA, as detected and determined by HPLC (LC-MS) method. The FAL oxidation and observed products distribution were given in Scheme 1. Under optimized conditions, up to 88.2 % SA yield was obtained with nearly full FAL conversion, as depicted in Table 2, Entry 1.



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Table 2. Catalytic oxidation of FAL into SA over different catalyst [a] Entry Catal.

CFAL[b] SSA[c] YSA

Product yield YMA YFuA

Carbon balance

1

GO

99.8

88.4

88.2

1.1

0.8

72.2

2

GO (no H2O2)

5.5

0

0

0

0

94.5

3

blank (with H2O2)

32

0

0

1.6

8.1

77.4

4

GO-HT

8.8

13.6

1.2

0.8

0.5

93.7

5

SO3H/GO-HT

79.8

82.2

65.6

1.2

0.7

87.7

6

AC

25.2

1.6

0.4

1.7

4.2

82.7

7

SO3H/AC

99.7

31.3

31.2

1.8

1.1

27.8

8

COOH/GO-HT

25.2

60.7

15.3

0.5

0.6

91.2

9

benzoic acid [d]

38.6

3.6

1.4

2.2

4.2

68.5

10

acetic acid [d]

47.3

2.3

1.1

13.7

3.1

67.6

11

HCl [d]

99.6

46.4

46.2

2.3

0.9

40.1

HCl[Ref]

>99

49

49

11

0

Ref. 8; 25

H2SO4[d]

99.9

41.7

41.6

1.2

0.6

35.0

H2SO4[Ref]

>99

45

45

6

0

Ref. 8; 25

13

H3PW12O40[d]

99.2

48.6

48.2

0.4

0

39.7

14

γ-Al2O3[e]

99.6

22.7

22.8

1.1

0.7

24.6

γ-Al2O3[Ref]

>99

26

26

4

99

17

17

2

0

Ref. 8; 25

p-TSA

99.8

70.2

70.1

6.7

1.4

63.0

p-TSA[Ref]

>99

72

72

11

1

Ref. 8; 25

Amberlyst-15

99.8

68.4

68.3

5.3

1.5

60.1

Amberlyst-15[Ref]

>99

74

74

11

2

Ref. 8; 25

18[f]

GO

10.1f

0

0

0.2

---

---

19[g]

GO

1.3g

0

0

0

---

---

20[h]

GO

11.4

0

0

2.5

0

---

12

15 16 17


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[a] Reaction conditions: Furfural, 5 mmol, 480.4 mg; catalyst, 50 mg; H2O, 15 mL; H2O2, 30 mmol, determined by iodometric titration; reaction temperature 70 oC; reaction time, 24 h; stirring speed, 600 rpm. [b] FAL conversion. [c] SA selectivity. [d] 5 mmol acid (H+) was added. [e] Acid density in γ-Al2O3 and ZSM-5 was determined by NH3-TPD method. Equimolar acid amount of acid is added. [f] With HMF as substrate. [g] With LA as the substrate. [h] With 2(5H)-furanone as the substrate. [Ref] The experimental data were cited from the references and compared with our results in table 2. To explain the high efficiency of this catalytic system in FAL oxidation, catalytic oxidation of FAL over various catalysts was investigated and compared, as listed in Table 2. Control experiment without H2O2 addition (Table 2, Entry 2) shows that FAL is relatively stable under current conditions, and no oxidation product was detected, indicating the importance of H2O2 in FAL oxidation. In the absence of GO (Entry 3), analysis results show that H2O2 alone gives a moderate FAL conversion and small amount of FuA and trance amount of MA. Oxidation of FAL into MA is a free radical reaction19, the lower MA yield in this situation can be attributed to the relatively mild reaction conditions and negative solvent effect (H2O). In the absence of any catalyst, aldehyde group was oxidized into carboxyl group to give 8.1% yield of FuA, without any SA observed. These results imply that GO is crucial for a high SA selectivity in H2O2containing oxidation conditions. To determine the vital groups in GO in FAL oxidation, GO material was thermally reduced at 1000 oC in Ar/H2 flow for 5 h. -SO3H groups endow GO the with acidic properties.39 Thermal treatment makes GO highly reduced and the -SO3H groups, as well as most of other oxygen groups were removed36, 40. Elemental analysis indicates the S content in GO-HT decreased to 0.2 wt.% from ca. 1.2 wt.% in GO



(Table 1). Experiment with GO-HT as catalyst in entry 4 (Table 2) shows that the catalytic activity decreased obviously. After re-grafting -SO3H onto GO-HT by sulfonation in concentrated H2SO4, 65.6% SA yield was recovered, which could be attributed to the partial recurrence of acidity in SO3H/GO-HT (Table1, entry 5). Infrared (IR) spectroscopy proved the presence of –SO3H groups in SO3H/GO-HT (Figure S4). In addition, very close SA selectivity but a lower FAL conversion over SO3H/GO-HT, compared with GO, may presumably be attributed to the more hydrophilic property of GO imparted by various functional groups in GO. To confirm the effect of -SO3H in FAL oxidation, active carbon (AC) was applied as the catalyst and found that AC is nearly inert in FAL oxidation (entry 6). After sulfonation in H2SO4, SO3H/AC shows an obvious increase in catalytic ability (entry 7). -SO3H is a typical acid group in GO, and the above results reveal that –SO3H in GO is important for FAL oxidation. Carboxyl can also be an effective acid group in GO; to check its catalytic ability in FAL oxidation, functionalization of GO-HT with –COOH by vapor-phase HNO3 was conducted. Various oxygen groups like carboxyl, carbonyl, hydroxyl, epoxides, lactones et. al can be introduced into GO-HT nanosheets by this method41. However, COOH/GO-HT shows no obvious increase in catalytic ability in FAL oxidation (Table 2, entry 8). This result implied that –COOH group (or –OH, -C-O-C-,-C=O et.al) in GO is not the crucial catalytic site in FAL oxidation. Control experiments with pure benzoic acid or acetic acid (entry 9-10) also demonstrate that weak acidic carboxyl groups cannot effectively catalyze FAL oxidation. It is worth noting that ca. 13.7% MA yield was achieved from FAL oxidation in the presence of H2O2 and acetic acid (entry 10). It can be explained that the oxidation of FAL into MA is a radical route; H2O2 could function as a free radical


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initiator and acetic acid is a free radical acceptor that facilitated the FAL oxidation into MA.19 It had been demonstrated that manganese in the active site (Mn3O4) owns the catalytic ability in aerobic oxidation of HMF.42 Besides, Mn impurities in GO were also demonstrated to be a key factor that had crucial effect on oxygen reduction reaction.43 Based on ICP analysis, we noticed that the GO materials contained an impurity of manganese. As shown in Table S4, GO and GO-HT contained Mn impurities at concentrations of ca. 237 ppm and 272 ppm, respectively. To check whether the residual Mn component in GO material contribute to the observed catalytic activity in FAL oxidation, the Mn contents in GO and GO-HT were adjusted (Detailed procedure was provided in Supporting information Table S4).44 The purification process makes the Mn content in GO decrease to 61 ppm after thorough washing with HCl aqueous solution. In following catalytic tests, the reaction time was set at 6 hours. Compared with GO, the significant decrease of Mn impurity in washed GO did not induce a significant change on its catalytic activity (Supporting information Table S4, Entry 1-2). GO-HT owns a Mn content up to 272 ppm (Table S4) and a low S content (0.2 wt.%) and O content (2.4 wt.%). Catalytic activity tests showed that GO-HT gave 6.8% FAL conversion and trace amount of SA. GO-HT supported Mn catalytic materials with Mn content up to 2700, 5400 and 8100 ppm, respectively, were prepared by impregnation method; catalytic tests showed that no obvious increase in catalytic ability could be observed with the Mn content increasing in GO-HT (supporting information, Table S4, entry 4-6). These results imply that the catalytic activity of GO is not originated from the residual Mn component.



After confirmation of the importance of -SO3H group in GO in FAL oxidation, the effectiveness of GO was compared with conventional Brønsted acid and Lewis acid catalysts, as listed in Table 2, entry 11-17. As stated in previous reports3, 21, acid type and strength affected the FAL oxidation significantly. Brønsted acid gave better SA yield and selectivity than Lewis acid in FAL oxidation (entry 11-15, based on equimolar amount of acid). HCl (entry 11), H2SO4 (entry 12) and HPW (entry 13) with high acid strength gave moderate SA yield (ca. 40-50%), pTSA and Amberlyst-15 with moderate acid strength gave relatively high SA yield (Entry 16-17), while acetic acid and benzoic acid with weak acid strength gave negligible SA yield (Entry 910). In terms of C balance, as shown in Table 2, SO3H/AC, Brønsted acid or Lewis acid (Entry 11-15) gave a relatively low carbon balance, indicating much undesired side reactions occurred. No identified product except for SA, MA, FuA and formic acid was detected by LC-MS, the low carbon balance may be attributed to the formation of humins over acid catalysts. However, moderate carbon balances over p-TSA, Amberlyst-15 and GO (Entry 1, 16, 17) ranging from 60.0% to 72.2% were obtained, implying the importance of acid strength in FAL oxidation. For comparison, the experiment results over common acid catalysts (homogeneous or heterogeneous) were compared with previous reports, as cited in entry 11-17, Table 2. At the same concentration of FAL dissolved in H2O and a relatively lower temperature (80 oC used in literature), a slightly lower FAL conversion and SA yield were achieved in our work (Entry 11-17, Table 2). In SA synthesis,3 the best result of 74 % SA yield was obtained over Amberlyst-15. In this work, at 70 oC

and over GO, the higher SA yield (88.2 % in entry 1) demonstrated the effectiveness of GO as

a highly active catalyst in FAL oxidation. The superior activity of GO to Amberlyst-15 can presumably be attributed to the suitable acid strength in GO and the higher surface area of GO than that of Amberlyst-15.


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Inspired by the high efficiency of GO/H2O2 in FAL oxidation, experiments with HMF and LA as substrates were conducted under optimized conditions. However, only small amount of HMF was consumed in the presence of H2O2 and negligible SA or MA

was

observed (entry 18). In case of LA, nearly no reaction occured under current reaciton conditons. Reaction pathway in FAL oxidation Solid acids containing -SO3H functional group in Table 2 show the best catalytic activity in FAL oxidation. Previous studies indicated the π-π interaction between a tolyl ring in catalyst and the furan ring in FAL could favor a suitable conformation via fivemembered ring intermediate involving the sulfur atom of -SO3H group to form 2(3H)furanone type intermediate and thus enhanced the SA selectively in FAL oxidation 3. By comparing the catalytic activity of SO3H/AC, p-TSA and GO (entry 1, 7, 16), It can be found that the catalytic ability increased with the increasing number of the aromatic rings in catalysts. It was also confirmed that the sulfonic acid group in the catalysts had pronounced effect on FAL oxidation. It is speculated that the super catalytic activity of GO was derived from its unique structure and suitable acidity. We suggested that the hydrophilic property of GO and its unique 2D atomic-layered structure with adjacently decorated –SO3H faciliated the oxidation and afforded an enhanced SA selectivity in FAL oxidation. According to previous reports 3, 14, 21 in FAL oxidation over Amberlyst-15 and p-TSA and experiment results in this work, a plausible reaction pathway was depicted in Scheme 2, following the proposal by Kul’nevich et al. in 197021. The reaction was proposed to undergo Baeyer-Villiger oxidation to form SA via furan-2(3H)-one. The 2D basal plane



structure in GO facilitated a favorable spatial conformation between GO and FAL molecular, in which a suitable conformation via five-membered ring intermediate involving the sulfur atom of -SO3H group from GO was formed (A). With the assistance of -SO3H group, intermediate A was oxidized into 2-formyloxyfuran (B) by H2O2.

Scheme 2. A plausible reaction pathway for FAL oxidation into SA Under acidic conditions, B can be transformed into 2-hydroxyfuran (C), and then C has isomers as 2(3H)-furanone and 2(5H)-furanone, the former provides SA whereas the latter serves MA14. We suggest that 2(3H)-furanone was more favored to form than 2(5H)furanone over GO and thus enhanced the SA selectivity during reaction. Control


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experiment with 2(5H)-furanone as substrate gave only trance amount of MA and no SA product (Table 2, entry 20). Although reactions from intermediates such as 2(3H)-furanone were hardly investigated owing to their undersupplies and difficult syntheses, the reaction with 2(5H)-furanone as substrate failed to give SA, indicating that the oxidation of FAL into SA was more likely to undergo with 2(3H)-furanone as the intermediate. In previous report,19 FAL could be oxidized into maleic anhydride even without any catalyst in acetic acid. H2O2 is often used as a free radical initiator and acetic acid was considered to be beneficial for FAL oxidation because acetic acid is a free radical acceptor and could be oxidized to acetic acid peroxide. In this work, it is speculated that the high efficiency of the reaction was derived from the strong oxidizing property of free radical (HO•) from H2O2 during FAL oxidation. H+/H2O

O

OH

keto-enol tautomerisation

OH

O

O

O

H 2O 2

O

OH

HO

O

O

O HCOOH

FAL

(2Z,4Z)-2,5-dihydroxypenta-2,4-dienal

a

2-oxopentanedial

SA

b

Scheme 3. Brønsted acid catalyzed ring opening-up mechanism in FAL oxidation In 1949, Bunton had proposed an furfural ring opening mechanism over Brønsted acid, as shown in Scheme 3.45 Different from Kul’nevich’s proposal, C-O bond cleavage in furfural ring catalyzed by Brønsted acid in the presence of H2O initiated the reaction. In the reaction mechanism, the hydration product (a) could be converted into 2-oxopentanedial (b).

After

that, the C-C cleavage in (b) in the presence of H2O2 gave SA and side-product formic acid. Considering the existence of the –SO3H groups on GO edge and its typical Brønsted acid property, here, the route of FAL oxidation via Scheme 3 cannot be excluded completely in this work.



Effect of reaction conditions on FAL oxidation Catalytic oxidation of FAL over GO in the presence of H2O2 was studied in detail.

a

100

FAL converison/SA yield (mol. %)

FAL conversion/SA yield (mol.%)

Effect of usage of H2O2 and GO on FAL oxidation were illustrated in Figure 4 a and b.

90 80

CFAL YSA

70 60 50

0

10

20

30

40

100

b

90

CFAL

80

YSA

70 60 50 40 30

10

50

20

30

c FAL conversion/SA yield (mol. %)

100 90 80

CFAL

70

YSA

60 50 40 30 20 0

3

6

9

12 15

18

Reaction time (h)

40

50

60

70

GO loading (mg)

Usage of H2O 2 (mmol)

FAL conversion/SA yield (mol. %)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 28

21

24

27

100

d

80 60 40 20 0

1

2

3 4 Recycle number

5

6

Figure 4. (a) Effect of usage of H2O2 on

FAL oxidation. Reaction conditions: furfural, 5 mmol; GO, 50 mg; H2O 15 mL; reaction temperature, 70 oC; stirring speed, 600 rpm; reaction time, 24 h. (b) Effect of GO loading on FAL oxidation. reaction conditions: furfural, 5 mmol; H2O, 15 mL; H2O2, 30 mmol; reaction temperature, 70 oC; stirring speed, 600 rpm; reaction time, 24 h. (c) Reaction kinetics of FAL over GO. Reaction conditions: furfural, 5 mmol; GO, 50 mg; H2O 15 mL; H2O2, 30 mmol; reaction temperature, 70 oC; stirring speed, 600 rpm; (d) GO reusability test. Reaction conditions: furfural,


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5 mmol; GO, 50 mg; H2O 15 mL; H2O2, 30 mmol; reaction temperature, 70 oC; stirring speed, 600 rpm; reaction time, 6 h. H2O2 and GO were demonstrated to be significant in FAL oxidation, as discussed above and shown in Figure 4 a, b. It was found that FAL conversion and SA yield increased monotonously with the loadings of H2O2 and GO increasing. When H2O2 loading increased to 25-30 mmol (5~6 times

FAL mol content) and GO loading

increased to ca. 50 mg in reaction, the increase of FAL conversion and SA yield started to level out, and further addition of H2O2 or GO led to only a minor increase in FAL conversion and SA yield under selected reaction time. It is well known that H2O2 can be easily decomposed in the presence of catalyst and high temperatures; the amount of H2O2 in the reaction mixture and its efficiency after 24 h were determined. As shown in Figure S5 (supporting information), more residual H2O2 can be detected after reaction with the increasing of H2O2 in reaction. The H2O2 efficiency21 of 83.1% can be achieved when 10 mmol H2O2 is added; the H2O2 efficiency dropped to 67.2% when 40 mmol H2O2 was added. It should be noted that the H2O2 efficiency does not represent its complete utilization in the FAL oxidation process. It is speculated the more H2O2 was added, the more H2O2 was directly decomposed into H2O and O2 without participation into FAL oxidation under selected reaction conditions. The effect of reaction temperature on FAL oxidation was also investigated. As shown in Figure S6, FAL conversion increased with the temperature increasing and full FAL conversion could be achieved at 70 oC. As for products, the yields of MA and FuA kept relatively low, and the highest MA yield of 6.7 % was observed at 100 oC. The highest SA yield of 88.2% was achieved at 70 oC, and further increasing of the reaction



temperature induced an obvious decrease in SA yield. Under current reaction conditions (80-100 oC), few unidentified weak peaks at a retention time of 40-45 min in HPLC appeared. Besides, higher temperature may also favor the cyclization of 1,4-dicarbonyl compounds (SA and MA) to furan derivatives, as well as humins. These undesired sidereactions at high temperature may induce the low carbon balance during reaction. To explore the kinetic behavior of reaction, the FAL conversion and SA yield as a function of reaction time are displayed in Figure 4 c. The FAL converted quickly and the conversion above 90 % could be achieved in ca. 9 h. The SA yield also increased to ca. 80% in 9 h. Full FAL conversion with ca. 88.2% SA yield was obtained with reaction time prolonged to 24 h. In GO reusability test, the reaction time was set at 6 h to avoid the impact of excess reaction time or GO loading. The GO catalyst was recycled by simple filtration, washed by water to remove adsorbed reaction mixture and dried at 80 oC. GO material shows a good reusability in FAL oxidation (Figure 4 d). There is little difference in both FAL conversion and SA yield up to the fourth cycle test. Only slight decrease in FAL conversion and SA yield were observed in 5-6 cycles. SEM, TEM and HRTEM indicate that the recycled GO retained its porous, layered nanosheet structure (Supporting information, Figure S7). The morphology of GO was not affected obviously after 6 cycles. To better understand the changing of oxygen functional groups before and after reactions, the C 1s peaks in high-resolution XPS spectra of recycled GO were provided in Figure S8. The different oxygen group distributions in recycled GO were analyzed by deconvoluted C 1s XPS and compared with those in fresh GO (supporting information Figure S8). Oxygen contents in different groups all show slight decrease after 6 cycles (Table S5). S content in recycled GO shows a slight decrease, which can be attributed to


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the long thermal treatment under above reaction conditions and also regarded as the reason for the slight loss of the catalytic reactivity after 5-6 catalytic cycles. Conclusion In summary, with modified Hummers method, acidic GO with ca. 1.2 wt.% S content was prepared. By comparison with various catalysts, GO has been shown to be a highly active, selective and reusable catalyst in furfural oxidation. Control experiments and characterization revealed that GO materials containing aromatic ring and –SO3H acidic functional group show remarkable catalytic activity and product selectivity in furfural oxidation than other conventional acid catalysts, such as HCl, H2SO4, HPW, γ-Al2O3 and ZSM-5. Super high succinic acid selectivity over GO was attributed to the unique 2D basal plane structure with edge decorated with –SO3H group, which facilitated a favorable spatial conformation between GO and FAL molecular. Hydrophilic property of GO derived from various oxygen-containing groups leads to its good dispersion in reaction medium. H2O2 was also demonstrated to be crucial in the reaction. With the progress of the production of graphene oxide and its reduction in cost, it can be reasonably envisaged that such an environmental friendly and cheap carbocatalyst will hold great potential in a wide range of acid-catalyzed reactions. Appendix- Supplementary data Supplementary data to this article was provided. Detailed procedure in materials preparation and complementary characterization of GO, GO-HT and COOH/GO-HT were provided in supporting information.

Complementary characterization s of SO3H/GO-HT, recycled GO and

complementary experiments were also provided in supporting information. Acknowledgements



The authors acknowledge the financial support from Science and Technology Plan Project of Shandong Provincial University (J12LA02), Program for Scientific Research Innovation Team in Colleges and Universities of Shandong Province, and Shanxi Provincial Natural Science Foundation of China (201601D011024). References 1.

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For Table of Contents Use Only hydrophilic GO favored intermediate towards product SO3H

O O O

O S

acidity SO3H

O

favored O O

OH

HO

GO, H2O, H2O2, 70 oC

O O

unfavored

HO

OH

O OH

or

O O

Over graphene oxide, the biomass-derived, renewable furfural was oxidized into succinic acid efficiently (SA, 88.4 % selectivity) under relatively mild reaction conditions.


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Efficient Oxidative Transformation of Furfural into Succinic Acid over

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