Bio-based green solvent for metal-free aerobic oxidation of 5-hydroxymethylfurfural to 2,5-diformylfural over nitric acid-modified starch

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Contents lists available at ScienceDirect

Catalysis Communications

journal homepage: www.elsevier.com/locate/catcom

Short communication

Bio-based green solvent for metal-free aerobic oxidation of 5-

hydroxymethylfurfural to 2,5-diformylfural over nitric acid-modified starch

Mei Hong

^a,b,^⁎

, Shuangyan Wu

^b

, Himanshu Sekhar Jena

^c

, Jiatong Li

^b

, Linfei Ding

^b

, Jing Wang

^b

, Lifen Wei

^b

, Zhi Ling

^b

, Kun Li

^b

, Shifa Wang

^b

a Jiangsu Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, Nanjing Forestry University, Nanjing 210037, People’s Republic of China

b College of Chemical Engineering, Nanjing Forestry University, Nanjing 210037, People’s Republic of China

c COMOC, Center for Ordered Materials, Organometallics and Catalysis, Department of Chemistry, Ghent University, 9000 Ghent, Belgium

A R T I C L E I N F O Keywords:

Aerobic oxidation Starch Nitric acid

5-Hydroxymethylfurfural 2,5-Diformylfuran

A B S T R A C T

The non-toxic biologic material of expanded corn starch (ECS)-supported nitric acid (ECS-HNO3 and ECS-NH2- HNO3) has been prepared via simple procedures. Both the materials are used as heterogeneous catalysts for the selective aerobic oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-diformylfuran (DFF) with an up to 98%

yield in γ-butyrolactone (GBL, a bio-based green solvent) containing as little as 10% (v/v) acetic acid (AcOH) after less than 5 h under ambient pressure of dioxygen at 50 ^oC. The ECS-NH2-HNO3 proved to be more stable than ECS-HNO3 and was repeatedly used five times with almost no change in its catalytic performance.

1. Introduction

Biomass has been regarded as an attractive candidate to fossil-based energy sources, fuels, and chemicals [1]. 5-Hydroxymethylfurfural (HMF) can be obtained from cellulose, which is the most abundant biopolymer comprising 35-45% of renewable lignocellulosic biomass [2].

Oxidation of HMF to 2,5-diformylfuran (DFF) is an important reaction, and the latter is a versatile compound for the synthesis of pharmaceu- ticals, furan-containing polymers, poly-Schiff bases, antifungal agents, organic conductors, and cross-linking agents of poly(vinyl alcohol) for battery separations [3]. However, oxidative transformation of HMF to DFF always produces a series of furan derivatives including 5-hydroxymethyl-2-furancarboxylic acid (HMFCA), 5-formyl-2-furancarboxylic acid (FFCA), and 2,5-furandicarboxylic (FDCA) as byproducts [4].

Therefore, efficient and selective oxidation of HMF into DFF is a great challenge. In this regard, the development of a novel catalytic system is needed.

In recent years, molecular oxygen has been widely utilized as the terminal oxidant with a variety of homogeneous and heterogeneous catalysts containing Ru [5,6], Cu [7], Cu/V [8,9], Mn/Cu [10], Mn/Co [11], Fe [12,13], Fe/Co [14], and V [15]. Regarding the potential catalytic material, a metal-free catalyst is a better choice than the metal-based catalyst which is corrosive/toxic in nature, expensive, and introduces metal contamination in the product mixture.

Hou et al. reported the graphene oxide (GO)/TEMPO as efficient

catalysts for selective oxidation of HMF into DFF (100% conversion and 100% selectivity) at 100 ^oC for 18 h [16]. Yang et al. developed a benzoic acid/TEMPO system with 86.7% conversion of HMF and nearly 77.8% yield of DFF at 100 ^oC under 4 bars of O2 in 24 h over 0.5 eqv.

benzoic acid and TEMPO [17]. Estrine et al. reported that DFF was obtained in 85% yield by catalytic conversion of HMF over NaBr in DMSO at 150 ^oC after 18 h [2]. The reported metal-free catalysts for the oxidation of HMF to DFF mostly suffer from several disadvantages such as use of toxic organic solvents, no catalyst recyclability, low yields, long duration, notable waste generation, and harsh reaction conditions;

besides, the preparation of the heterogeneous catalysts are relatively complicated. Therefore, there is a requirement to develop more efficient and environmentally friendly protocols with recyclable catalysts, green solvents, mild conditions, and with O2 as the terminal oxidant for solving the problems.

Recently, HNO3 was used as a cocatalyst in HNO3-imobilized TEMPO catalytic system for alcohol oxidation under 5-bar oxygen pressure in 1,2-dichloroethane [18]. Nitric acid is produced in- dustrially. However, This method required high operating pressures (up to 5 bars) and an environmentally undesirable chlorinated solvent (1,2- dichloroethane) using homogeneous catalyst of nitric acid. Thus, from the environmental and economic perspectives, it is still highly desirable to establish milder, cheaper, cleaner, and recyclable HNO3 catalytic systems for the selective aerobic oxidation of HMF into DFF.

Starch is a widely studied natural biopolymer due to its beneficial

https://doi.org/10.1016/j.catcom.2020.106196

Received 27 July 2020; Received in revised form 9 October 2020; Accepted 11 October 2020

⁎Corresponding author.

E-mail address: [email protected] (M. Hong).

Available online 16 October 2020

T

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properties such as biocompatibility, biodegradability, non-toxicity, abundance, and ready availability from a variety of waste biomasses.

Furthermore, the reactive free-OH groups on the starch can easily be modified [19]. The expanded starch (ECS) as the support material has high surface, high pore volume, low density, and chemical stability [20].

A metal-free aerobic HMF oxidation protocol (Scheme 1) is reported in this paper to efficiently produce DFF in AcOH-GBL (1:9, v/v) solvent using expanded corn starch-supported nitric acid (ECS-HNO3 and ECS- NH2-HNO3) as eco-friendly, biodegradable, and recyclable catalysts under mild reaction conditions.

2. Experimental Section 2.1. Materials and reagents

All chemicals were purchased from commercial suppliers (Sigma- Aldrich and Energy Chemical Company) and used without further purification. High-amylose corn starch was purchased from Guomin Starch Chemistry (Shanghai, China) Co., Ltd.

2.2. Anchoring of nitric acid onto expanded corn starch (ECS-HNO3 and ECS-NH2-HNO3)

The expanded corn starch (ECS) support and APTES-functionalized expanded corn starch (ECS-NH2) were synthesized using our previously published procedure [21]. ECS-HNO3 and ECS-NH2-HNO3 catalysts were prepared by the slow addition of concentrated HNO3 (0.19 g, 2 mmol) to a stirred suspension of ECS and ECS-NH2 (1.0 g), respectively, in n-hexane (5 mL) at 0 ^oC for 2 h. The mixture was further stirred for another 2 h, filtered and washed with acetone, and dried at room temperature to afford ECS-HNO3 or ECS-NH2-HNO3 as white powder.

2.3. Back titration analysis of ECS-HNO3 and ECS-NH2-HNO3

To an Erlenmeyer flask containing ECS-HNO3 or ECS-NH2-HNO3

(100 mg), NaOH solution (20 mL, 0.1 N) was added, and the entire mixture was allowed to stir for an additional 10 min. The excess amount of base was neutralized by adding HCl solution (1 N). NaOH solution (20 mL, 0.1 N) was added to ECS or ECS-NH2 (100 mg) without HNO3

and subjected to back titration under similar conditions, and no change was observed in the strength of NaOH. The possible reaction of NaOH with hydroxyl groups in the structure of starch can be ruled out.

2.4. Catalyst characterization

Fourier Transform Infrared Spectroscopic (FTIR) spectra were collected on a Nicolet 380 FTIR infrared spectrometer, and the samples were ground with KBr and pressed into a thin wafer. The specific surface area (SBET) values of the samples were measured on a Autosorb-iQ instrument through N2 adsorption-desorption isotherms at 77 K using the Brunauer-Emmett-Teller (BET) method. Thermogravimetry (TG) and differential thermal analyses (DTA) were conducted using a TGA instruments thermal analyzer TG 209F1 under N2 atmosphere at a heating rate of 10 ^oC min^-1. The X-ray photoelectron spectrometry (XPS) spectra of the catalysts were performed on a Kratos Axis Ultra DLD spectrometer. A Quanta 200 field emission scanning electron mi- croscope (SEM) was used to determine the morphology of the catalysts.

2.5. General procedure of the oxidation of HMF

Typically, HMF (126 mg, 1 mmol) and AcOH-GBL (1:9, v/v, 2 mL) were added to a 5 mL test tube equipped with a magnetic stirring bar.

TEMPO (7.8 mg, 0.05 mmol) and ECS-HNO3 or ECS-NH2-HNO3 (40 mg) were consecutively added to the solution, and then a balloon filled with oxygen (500 mL) was attached to the test tube. The reaction mixture was magnetically stirred at 50 ^oC. Samples were taken at appropriate intervals through a silicon septum using a hypodermic needle.

2.6. Product analysis

The furans were analyzed using an UltiMate 3000 HPLC system equipped with a UV detector. Before analysis, all samples were filtered through a syringe filter (VWR, 0.22 μm PTFE). The furan products were isolated by a reversed-phase C18 column (250 ×4.6 mm), and the UV detector wavelength was 280 nm. Acetonitrile and 0.1 wt% acetic acid aqueous solution in a volume ratio of 65:35 was used as the mobile phase at 30 ^oC at a flow rate of 0.5 mL/min. The calibration of HPLC was controlled for drift by applying a standard solution of reference compounds of known concentration prior to every run.

HMF conversion and DFF yield are defined as the following equa- tions:

HMF conversion = (1-moles of HMF/moles of starting HMF)×100%

DFF yield = moles of DFF/moles of starting HMF ×100%

3. Results and discussion 3.1. Catalyst Characterization

The catalysts were prepared by adding nitric acid to a stirred suspension of expanded corn starch or modified expanded corn starch in n- hexane. The number of H⁺sites on ECS-HNO3 and ECS-NH2-HNO3 was measured by back titrations and found to be 1.1 mmol g^-1and 1.3 mmol g^-1, respectively.

The surface areas of the samples were assessed using N2 adsorption- desorption analysis. The surface area of ECS calculated using the BET method was 135 m²g^-1. The surface area of ECS-NH₂was found to be 52 m²g^-1. In the case of the ECS-HNO3 and ECS-NH2-HNO3 catalyst, the surface areas were determined to be 24 m²g^-1and 16 m²g^-1, respectively. The decrease in the surface area of the ECS-HNO₃and ECS-NH₂- HNO3 catalysts compared to the ECS and ECS-NH2 material was due to the presence of HNO3 anchored on the surface of the ECS material.

All ECS-anchored catalytic systems, ECS-HNO3 and ECS-NH2-HNO3, were characterized by FT-IR spectroscopy as can be seen in Fig. S1.

Several absorbance bands resulting from starch in the region of 1000- 1200 cm^-1, corresponding to C-O, C-C, and C-O-H bond stretching and CO-H bending, are evident together with a broad band centered at 3410 cm^-1related with O-H stretching of hydroxyl groups in expanded corn starch (Fig. S1a) [22]. The successful binding of APTES onto ECS seems to be confirmed by the appearance of the new weak intensity bands near 1563 and 665 cm^-1of the ECS-NH2 spectrum produced by H–N–H and N–H bending vibration of the introduced amino group (Fig. S1b) [23,24]. The values 1390 and 836 cm^-1can probably be assigned to the adsorption of nitric acid (Fig. S1c and S1d) [4,25].

To evaluate the thermal stability of ECS-HNO3, ECS-NH2-HNO3, and their precursors, the thermogravimetric analyses were carried out, and the TG curves are illustrated in Fig. S2. The thermogram for ECS-NH2

shows two regions of weight loss between 25 and 800 ôC. The first weight loss observed at 50-150 ôC was allocated to the adsorbed water and organic solvents (ca. 5%), and the second one to the combustion of the starch polysaccharide structure and aminopropyl fragment, as also reported in the literature [21]. As expected, the curves for the ECS- HNO₃and ECS-NH₂-HNO₃samples show a pronounced weight loss, centered at about 300 ôC (ca. 76% of total weight loss) and assigned to Scheme 1. Oxidation of HMF to DFF using ECS-HNO3 or ECS-NH2-HNO3.

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the complete decomposition of the loaded HNO3 along with the grafted organic functionality and starch.

Furthermore, XPS was used to identify the surface species of ECS- HNO3, ECS-NH2-HNO3 catalyst, and ECS. It is observed in the XPS spectrum in Fig. 1 that ECS, ECS-HNO3, and ECS-NH2-HNO3 samples have an intense N 1s peak located at around 400 eV. This N1s peak is attributed to the nitrogen atoms present in the amide bonds in both the polypeptide chain and amino acids [26] as corn starch contains minimal amounts of protein. Both ECS-HNO3 and ECS-NH2-HNO3 surfaces exhibit a peak in their N1s binding energy region at about 407 eV, which can be attributed to the nitrate ion (NO3-) with an oxidation state of +5. The N1s peak of ECS-NH2-HNO3 catalyst shows another peak at about 401.5 eV, which can originate from ammonium-like nitrogen (−NH3+). [27–30].

To study the surface morphology of ECS, ECS-NH2, ECS-HNO3, and ECS-NH₂-HNO₃catalyst, we used SEM imaging, as shown in Fig. S3. It was observed that the expanded corn starch granules present smooth regular spherical shapes. The grafting of nitric acid (ECS-HNO3 and ECS-NH2-HNO3) had no significant effect on the surface morphology, indicating that the structure of expanded corn starch granules had not been destroyed during the chemical modification.

3.2. The evaluation of catalytic performance for the oxidation of HMF to DFF

Our previous studies [31] showed that the oxidation of HMF to DFF exhibited good reactivity with 15 mol% HNO3 as catalyst in AcOH. In the present work, we continue to search for an economical, greener, and more efficient solvent which can act as an activator for supported nitric- acid-catalyzed aerobic oxidations (Table 1). Generally, the solvent played a principal role in determining the yield and selectivity of DFF [32]. Preliminary homogeneous experiments were devoted to the transformation with HNO3 as catalyst at 50 ^oC. The aerobic oxidation of HMF to DFF was carried out in a variety of common solvents (Table 1, entries 2-5) such as ethyl acetate (EtOAc) and 1,2-dichloroethane (DCE), which have been used in related alcohol oxidation methods.

When aerobic oxidation of HMF occurred in AcOH, EtOAc, and DCE, a complete conversion of HMF was obtained; however, significant dif- ferences in the selectivity to DFF was observed among the solvents used. A large quantity of DFF was further oxidized to form FDCA as a byproduct in EtOAc and DCE. AcOH, GBL, and CH3CN showed a selectivity to DFF higher than 97%, which can be attributed to the solvent properties, including viscosity, polarity, and dissolving capacity.

When 100 mg of ECS-HNO3 was used as the catalyst in comparison with HNO3 as the catalyst in different solvents, a high DFF selectivity was still achieved in AcOH and GBL (Table 1, entries 6 and 9). The result indicated that AcOH apparently plays an important role in ob- taining a good catalytic activity and selectivity for conversion of HMF to DFF. With a prolonged reaction time up to 9 h, the yield of DFF increased to 95% in GBL. Considering the environmental issues, a green solvent with low toxicity, chemical and thermal stability, non-corrosion under the operation conditions, and biodegradability are preferred [33]. GBL can be produced from lignocellulose and is marked as a natural, nontoxic dietary supplement. More importantly, GBL has ex- tremely similar physicochemical properties to γ-valerolactone (GVL) and is a very cheap commodity. To avoid sacrificing the reaction effi- ciency, AcOH/GBL (1:1, v/v) mixture was investigated. To our delight, 98% DFF yield was still achieved after 5 h (Table 1, entry 11). An amount of 40 mg catalyst of ECS-HNO3 is sufficient to obtain the maximum yield of DFF when the GBL to AcOH ratio increased up to 9:1 (Table 1, entry 14).

When the catalyst was changed to ECS-NH2-HNO3, a full conversion of HMF with 93% DFF selectivity was obtained after 3 h in AcOH.

However, ECS-NH2-HNO3 catalyst showed much lower catalytic effi- ciency when the reaction medium switched to DCE and GBL. With the aim of improving the selectivity to DFF, AcOH-GBL mixture medium Fig. 1. N1s XPS for (a) ECS, (b) ECS-HNO3, and (c) ECS-NH2-HNO3.

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was used. The catalyst loading and the volume ratio for AcOH-GBL mixture was also studied. Indeed, as indicated in entries 20-27 of Table 1, almost the same results as ECS-HNO3 were observed, but the reaction time was shortened to 3 h. The results obtained so far indicate that the solvent effects in heterogeneous catalytic oxidation could be attributed to the interactions between substrate and solvent and/or those between support and solvent.

The effect of the reaction temperature on the aerobic oxidation of HMF into DFF was studied over ECS-HNO3 or ECS-NH2-HNO3 (Table 1, entries 16-19 and 28-30). The oxidation of HMF hardly occurred at room temperature and 40 ^oC. In addition, as the reaction comprised the oxidation of HMF into DFF and the subsequent oxidation of DFF into FFCA, the selectivity of DFF depended on the reaction temperature.

With the same HMF conversion of 100%, the highest selectivity of DFF (98%) was obtained at 50 ^oC.

To provide more insights into the oxidation of HMF into DFF, the time course of the HMF conversion and DFF yield was recorded during the process of the oxidation of HMF over ECS-HNO3 or ECS-NH2-HNO3. As shown in Fig. S4a and S4b, the conversion of HMF and the yield of DFF gradually increased. The reaction was complete after 4.5 h and 2.5 h over ECS-HNO3 and ECS-NH2-HNO3, respectively, with a high yield and conversion (98% yield, 100% conversion); thereafter, the yield of DFF remained stable. Furthermore, FFCA was determined as the byproduct of the reaction in low yields.

3.3. Recycling of the catalysts

Recyclability of heterogeneous catalysts is an important factor from the economical and sustainable chemistry standpoints. In this regard, we studied recyclability of the ECS-HNO3 and ECS-NH2-HNO3 catalysts for the oxidation of HMF with molecular oxygen under the optimized reaction conditions, and the results are shown in Fig. 2. After reaction, the ECS-HNO3 and ECS-NH2-HNO3 catalysts were collected by cen- trifugation and washed with water. Subsequently, the spent catalysts were dried under reduced pressure and used for the next cycle. In a series of 5 consecutive runs, ECS-NH2-HNO3 exhibited basically stable catalytic activity and selectivity (i.e. conversion: 100-92%, DFF selectivity: 94-98%). In contrast, the catalyst activity of ECS-HNO3

sharply decreased after the fourth addition of HMF. The results indicated that the ECS-NH₂-HNO₃catalyst was more stable than the ECS- HNO3 catalyst.

Similar patterns of FT-IR (Fig. S1e and f) and SEM analyses (Fig. S3e and f) were observed for the recovered catalysts with some low intensity, which may be due to the catalyst deactivation or some changes in the morphology of the catalyst after a few runs. This fact was also checked by measuring the concentration of the residual H⁺on the recovered ECS-HNO3 and ECS-NH2-HNO3 catalysts through the titration method. The used ECS-HNO3 (0.7 mmol g^-1) and ECS-NH2-HNO3 catalysts (1.0 mmol g^-1) showed some loss of H⁺sites compared to the fresh catalysts.

Table 1

Effect of the reaction solvent on the aerobic oxidation of HMF into DFF.a

Entry Catalyst Catalyst load Solvent Time (h) Conv.

HMF (%) Yield DFF (%) Select. (%)^b

1^c HNO3 15mol% AcOH 5 100 98 98

2 HNO3 15mol% EtOAc 5 100 78 78

3 HNO3 15mol% DCE 5 100 75 75

4 HNO3 15mol% GBL 5 66 65 99

5 HNO3 15mol% CH3CN 5 85 82 97

6 ECS-HNO3 100 mg AcOH 4 100 99 99

7 ECS-HNO3 100 mg EtOAc 5 100 56 56

8 ECS-HNO3 100 mg DCE 5 100 54 54

9 ECS-HNO3 100 mg GBL 5 85 84 99

10 ECS-HNO3 100 mg GBL 9 97 95 98

11 ECS-HNO3 100 mg AcOH-GBL (1:1, v:v) 5 100 98 98

15 ECS-HNO3 20 mg AcOH-GBL (1:9, v:v) 5 Trace - -

16^d ECS-HNO3 40 mg AcOH-GBL (1:9, v:v) 5 Trace - -

17^e ECS-HNO3 40 mg AcOH-GBL

(1:9, v:v) 5 Trace - -

18^f ECS-HNO3 40 mg AcOH-GBL (1:9, v:v) 5 100 96 96

19^g ECS-HNO3 40 mg AcOH-GBL (1:9, v:v) 5 100 90 90

20 ECS-NH2-HNO3 100 mg AcOH 3 100 93 93

21 ECS-NH2-HNO3 100 mg DCE 9 100 68 68

22 ECS-NH2-HNO3 100 mg GBL 9 94 92 98

23 ECS-NH2-HNO3 100 mg AcOH-GBL (1:1, v:v) 3 100 98 98

28^e ECS-NH2-HNO3 40 mg AcOH-GBL (1:9, v:v) 3 7 7 100

29^f ECS-NH2-HNO3 40 mg AcOH-GBL (1:9, v:v) 3 100 97 97

30^g ECS-NH2-HNO3 40 mg AcOH-GBL (1:9, v:v) 3 100 92 92

a Reaction conditions: HMF (1 mmol, 126 mg), TEMPO (0.05 mmol, 7.8 mg), solvent (2 mL), 50 ^oC, oxygen balloon; the conversion and yield were determined by HPLC.

b FDCA and FFCA were the by-products. ^cRef. 33.

dRoom temperature.

e 40 ^oC.

f 60 ^oC.

g 70 ^oC.

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3.4. Possible reaction mechanism

The mechanistic insights [34,35] are summarized in Scheme S1. The acid catalyzed the decomposition of HNO3 to NO2. It is well known that two TEMPO molecules can disproportionate, acid-catalyzed, into one TEMPOH and one oxoammonium ion. The oxoammonium cation is supposed to oxidize HMF and is thereby converted to TEMPOH.

TEMPOH can be re-oxidized to the oxoammonium ion, thus closing a catalytic cycle.

4. Conclusions

In this study, nitric acid was successfully immobilized on expanded corn starch. The heterogeneous ECS-HNO₃and ECS-NH₂-HNO₃catalysts showed high catalyst activity and selectivity in the oxidation of HMF into DFF with molecular oxygen. Under optimal conditions, up to 98% DFF yield at full HMF conversion was obtained after 3 h with oxygen at 50 ^oC in AcOH-GBL (1:9, v/v). More importantly, the catalysts were stable during the reaction and could be reused several times without loss of activity. It is shown that under reaction conditions, the ECS-NH2-HNO3 catalyst is more stable than the ECS-HNO3 catalyst.

These methods represent the development of a promising metal-free catalyst for the oxidation of hydroxyl groups into aldehyde groups with oxygen.

CRediT authorship contribution statement

Mei Hong: Conceptualization, Methodology, Supervision.

Shuangyan Wu: Formal analysis, Writing - original draft. Himanshu Sekhar Jena: Writing - review & editing. Jiatong Li: Visualization, Investigation. Linfei Ding: Investigation. Jing Wang: Investigation.

Lifen Wei: Validation. Zhi Ling: Validation. Kun Li: Writing - review &

editing. Shifa Wang: Writing - review & editing.

Declaration of Competing Interest There are no conflicts to declare.

Acknowledgements

This work was financially supported by the Open Foundation of Jiangsu Key Laboratory of Biomass Energy and Materials (JSBEM201915). This work was also financially supported by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). H.S.J. acknowledges FWO [PEGASUS]2 Marie Sklodowska-Curie Grant Agreement No. 665501 for an incoming post- doctoral fellowship to Ghent University, Belgium.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://

doi.org/10.1016/j.catcom.2020.106196.

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