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Aerobic oxidation of 5-(hydroxymethyl)furfural into 2,5-diformylfuran catalyzed by starch supported aluminum nitrate
Catalysis Communications 136 (2020) 105909
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Short communication
Aerobic oxidation of 5-(hydroxymethyl)furfural into 2,5-diformylfuran catalyzed by starch supported aluminum nitrate
T
⁎
Mei Honga,b, , Shuangyan Wub, Jiatong Lib, Jing Wangb, Lifen Weib, Kun Lib a b
Jiangsu Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, Nanjing Forestry University, Nanjing 210037, People's Republic of China College of Chemical Engineering, Nanjing Forestry University, Nanjing 210037, People's Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Aerobic oxidation Starch Aluminum salts 5-Hydroxymethylfurfural 2,5-Diformylfuran Catalysis
Al(NO3)3 immobilized on expanded corn starch catalyst (ECS-IL-Al(NO3)3) was prepared by silane-modified imidazolium ionic liquid reacting with expanded corn starch (ECS) and coordination of Al(NO3)3 with the imidazolium chloride grafted on the surface, as proved by FT-IR, NMR, SEM/EDS, and XPS. The catalytic activity of ECS-IL-Al(NO3)3 was evaluated in the 5-hydroxymethylfurfural (HMF) oxidation under mild conditions with 98% of 2,5-diformylfuran (DFF) production at 99% HMF conversion after 5 h at 50 °C, using molecular oxygen as the oxidant. Furthermore, this catalyst was found to exhibit excellent recyclability, making it an attractive catalytic system from economic and environmental points of view.
1. Introduction The carbohydrates of plant tissues are the major biomass feedstocks of useful and polyfunctional molecules that can replace those derived from fossil resources [1]. Acid-catalyzed dehydration of hexoses gives 5-hydroxymethylfurfural (HMF) [2]. 2,5-Diformylfuran (DFF) is one of the main furan compounds that are generated from the selective oxidation of HMF [3], which is a versatile precursor in various applications, especially for the synthesis of furanic polymers, pharmaceuticals, antifungal agents and fluorescent substances [4]. Further oxidation of DFF led to form high oxygen containing products such as 5-formyl-2furancarboxylic acid (FFCA) and furandicarboxylic acid (FDCA), as side products. Thus, the selective oxidation of HMF to DFF has been deeply studied by researchers in recent years. Classical oxidants [5,6] such as KMnO4 and NaOCl were used in several methods for oxidation HMF to DFF, but the main disadvantages are the requirement of stoichiometric quantities of toxic and corrosive oxidant and generation of hazardous waste. From the viewpoint of atom economy, cost efficiency and green and sustainable chemistry, atmospheric molecular oxygen represent the most acceptable and desirable oxidant [7]. To establish the desired reactivity with molecular oxygen as the terminal oxidant, much efforts have been directed toward the selective oxidation of HMF to DFF with transition-metal-based catalysts such as Mo [3,8–10], Cu [1,11–16], Au-Ru [17], Mn [18–21], Pt [22,23]. Our eyes have captured aluminum because the content of
aluminum is abundant in the earth's crust. However, the difficulty in separating Al3+ from the reaction mixture limits the application of homogeneous catalysts. Therefore, the supports were studied to load Al3+ ion. Natural polymers, especially polysaccharides such as starch, are attractive since they are easily available, inexpensive, nontoxic and biodegradable. Hydroxyl groups in the structure of starch are able to be modified and then immobilize metal ions. Herein, we report the straightforward synthesis of Al(NO3)3 immobilized on ionic liquid-functionalized expanded corn starch (ECS-ILAl(NO3)3). The aluminum-catalyst was demonstrated to be an efficient catalyst for the oxidation of HMF to DFF when used together with TEMPO as an electron transfer mediator under ambient pressure of dioxygen at 50 °C in glacial acetic acid. 2. Experimental 2.1. Materials and methods All chemicals were purchased from Energy Chemical Company (Shanghai, China) and used without purification. High amylose corn starch was purchased from Guomin Starch Chemistry (Shanghai, China) Co., Ltd. FT-IR spectra were recorded on a Nicolet 380 FT-IR instrument. Thermogravimetry (TG) and differential thermal analyses (DTA) were carried out with a TGA instruments thermal analyzer TG 209F1 under N2 atmosphere at a heating rate of 10 °C min−1. Scanning
⁎ Corresponding author at: Jiangsu Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, Nanjing Forestry University, Nanjing 210037, People's Republic of China. E-mail address: [email protected] (M. Hong).
https://doi.org/10.1016/j.catcom.2019.105909 Received 26 September 2019; Received in revised form 28 November 2019; Accepted 16 December 2019 Available online 17 December 2019 1566-7367/ © 2019 Elsevier B.V. All rights reserved.
Catalysis Communications 136 (2020) 105909
M. Hong, et al.
Scheme 1. Synthesis of expanded corn starch supported aluminum catalyst.
as reported for the first run. Samples were collected and analyzed via HPLC.
electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS) was used on the Quanta 200. Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/Max 2400 X-ray diffractometer using Cu Kα (λ = 0.15406 nm) radiation. X-ray photoelectron spectroscopy (XPS) spectra were recorded on a Kratos Axis Ultra DLD spectrometer, and the binding energy was calibrated by C 1 s peak. 13C CP MAS NMR spectra were recorded on Bruker DRX-600 instruments. 1H NMR and 13 C NMR spectra were recorded using Bruker DRX-600 spectrometer using CDCl3 as solvent.
2.6. Analysis and separation of the product HPLC. HMF and DFF were analyzed by HPLC (Agilent 1200) using a reversed-phase C18 column (250 × 4.6 mm) at 25 °C with a detection wavelength of 280 nm. The mobile phase was acetonitrile and 0.1 wt% acetic acid aqueous solution (65:35 v/v) at 0.5 mL/min. The HMF conversion and DFF yield were expressed as mol% in terms of the total HMF amount. The amounts of HMF and DFF in the samples were calculated by interpolation from calibration curves. Calibration curves for the observed products were constructed by injecting known concentrations of reference commercial products. NMR. After removing glacial acetic acid by a rotary evaporator, DFF was purified by flash column chromatography on silica gel, where a colorless band was collected and the solvent removed in vacuo resulting in a white crystalline compound. 1H NMR (600 MHz, CDCl3): δ/ppm 7.35 (s, 2H), 9.80 (s, 2H); 13C NMR (150 MHz, CDCl3): δ/ppm 119.35 (2C), 154.19 (2C), 179.20 (2C).
2.2. Preparation of ionic liquid [24] A mixture of 1-methylimidazole (0.82 g, 10 mmol) and (3-chloropropyl) triethoxysilane (2.41 g, 10 mmol) was stirred at 90 °C without using any solvent for 48 h under N2 atmosphere. The resulting viscous yellowish liquid was washed with diethyl ether (3 × 5 mL) to remove any unreacted material. The resulting product was vacuum dried to obtain ionic liquid (IL), 1-(triethoxysilyl)propyl-3-methylimidazolium chloride. 2.3. Synthesis of ECS-IL [25]
3. Results and discussion
The expanded corn starch (ECS) support was prepared according to our previously published method [26]. In a 50 mL flask, 2 g of the expanded corn starch were dispersed in 18 mL of dried toluene by sonicating for 1 h. After the addition of 1.0 g 1-(triethoxysilyl)propyl-3methylimidazolium chloride the mixture was stirred at reflux for 24 h under N2 atmosphere. After separation, the remaining solid was dried under vacuum and the excess of 1-(triethoxysilyl)propyl-3-methylimidazolium chloride removed by 24 h Soxhlet extraction with dichloromethane. The resulting material was denoted as ECS-IL.
3.1. Synthesis and characterization of the catalyst The strategy for preparation of silane-modified imidazolium chloride, immobilizing imidazolium-based ionic liquid on the expanded corn starch (ECS) and imidazolium successively coordinating to Al (NO3)3 is outlined in Scheme 1. N-methylimidazole reacted with 3chloropropyltriethoxysilane to produce ionic liquid (IL) 1-(triethoxysilyl)propyl-3-methylimidazolium chloride. Then, the ionic liquid was anchored on ECS by condensation reaction to obtain ECS-IL. Finally, the ECS-IL was treated with aluminum nitrate to deliver ECS-IL-Al(NO3)3 catalyst. The obtained aluminum catalyst was characterized by a number of analytic techniques, including FT-IR, 13C CP MAS NMR, XRD, TGA-DTA, SEM/EDS and XPS. The FT-IR spectra of ECS and ECS-IL-Al(NO3)3 catalyst are shown in Fig. S1. The spectrum of the ECS-IL-Al(NO3)3 catalyst exhibit one characteristic peak at the position of 1569 cm−1, which can be attributed to the C]C stretching of imidazolium ring in the ionic liquid [27–29], indicating that imidazole is present on the surface of ECS-IL-Al (NO3)3 catalyst. The signals of CeH deformation vibration and CeC, CeN stretching vibrations in imidazole ring which should be seen can't be detected due to the overlap with the peak of NO3−. The peak at 1385 cm−1 and 827 cm−1 suggests the presence of nitrate species [30,31]. The structure of the organic functional group of ECS-IL-Al(NO3)3 catalyst was also proved by solid-state 13C CP MAS NMR spectrum. As shown in Fig. S2, the 13C resonance assignment based on the chemical shifts matched well with imidazole ring [28]. The XRD patterns of ECS and ECS-IL-Al(NO3)3 catalyst were
2.4. Synthesis of ECS-IL-Al(NO3)3 1.12 g of Al(NO3)3·9H2O was dissolved in 10 mL dried alcohol followed by the addition of 1.0 g ECS-IL into the solution and kept stirring at reflux for 24 h under N2 atmosphere. After filtration, the solid product was washed completely with EtOH, dried at 70 °C under vacuum for 24 h and labeled as ECS-IL-Al(NO3)3. 2.5. General procedure for the oxidation of HMF to DFF The experiments were carried out in test tubes equipped with a balloon containing pure O2, using O2 as the oxidant. In a general procedure, HMF (125 mg, 1 mmol), ECS-IL-Al(NO3)3 (90 mg), TEMPO (8.5 mg, 0.05 mmol) and glacial acetic acid (2 mL) were added to a 10 mL test tube. The reaction mixture was stirred magnetically at 50 °C for 5 h. After completion of the reaction, the mixture was cooled down to room temperature and filtered, and the remaining solid was washed with acetonitrile and acetone to separate the catalyst. Then, the recovered catalyst was dried and used in several runs in the same manner 2
Catalysis Communications 136 (2020) 105909
M. Hong, et al.
Fig. 1. SEM image of (a) ECS, (b) ECS-IL-Al(NO3)3 and EDS mapping images of (c) Al and (d) N for ECS-IL-Al(NO3)3.
3.2. Catalytic oxidation of HMF to DFF
depicted in Fig. S3. The amorphous peaks arising from ECS between 1030° were observed. No obvious differences were seen between the ECS and the ECS-IL-Al(NO3)3 catalyst, which evidenced that the ionic liquid and Al(NO3)3 were evenly dispersed on the surface of the ECS support. The thermal stability of the support and the catalyst were investigated by TGA. As shown in Fig. S4, ECS showed a slightly weight loss below 150 °C due to the adsorbed H2O on the surface of ECS. Then the decomposition of ECS started at around 250 °C, which was possibly attributed to the gradual degradation of polymer chain. It could be found that the onset decomposing temperature of ECS-IL-Al(NO3)3 decrease dramatically as comparing with that of the parent ECS, revealing the interaction between the support and IL. To further investigate the morphological changes in the materials, SEM (Fig. 1) was performed on ECS and ECS-IL-Al(NO3)3 catalyst. The surface morphology of ECS is smooth and granular. The immobilization of the ionic liquid as well as Al(NO3)3 on ECS creates stress, which may contribute to the breakdown of ECS granules. The energy dispersive spectroscopy (EDS) mapping images clearly confirmed the presence of the uniformly dispersed Al and N element in the as-prepared ECS-IL-Al (NO3)3 catalyst as shown in Fig. 1c and d. In order to gain more insights into the surface composition of the ECS-IL-Al(NO3)3 catalyst, the as-prepared sample was investigated by the XPS analysis (Fig. S5). For the XPS of N 1 s regions, the binding energy for N 1 s located at 401.7 eV and 407.1 eV are attributed to the imidazolium nitrogen [32] and NO3− groups [33,34], respectively. The Al 2p spectrum of ECS-IL-Al(NO3)3 shows a peak at 74.6 eV, indicating the presence of aluminum.
To identify the appropriate reaction conditions for selective oxidation of HMF to DFF, initially we have carried out an optimization study. Many investigations have demonstrated that the solvent had a remarkable effect on both HMF conversion and DFF yield [4,21]. As different solvents have different properties such as the polarity, dielectric constant, steric hindrance, acid-base property, all of which show affect the chemical reactions [35]. Therefore, oxidation of HMF was carried out in various solvents with 5 mol% Al(NO3)3·9H2O as the catalyst under homogeneous conditions with molecular oxygen as the terminal oxidant. As shown in Table 1, the conversion of HMF reached 95% after a reaction for 5 h in glacial acetic acid (AcOH). Upto 100% conversion of HMF was obtained when 1,2-dichloroethane (DCE) and ethyl acetate (EtOAc) were used as the solvent. The highest DFF yield of 93% was obtained by the use of AcOH as the solvent, while that were 70% with DCE and 78% with EtOAc (Table 1, entries 1–3), respectively. Moderate HMF conversion of 77% and 67% was observed in CH3CN and γ-butyrolactone (GBL) (Table 1, entries 4 and 5), respectively. Al (NO3)3·9H2O showed no catalytic activity in the oxidation of HMF, when reactions were carried out in EtOH and DMSO (Table 1, entries 6 and 7). These results clearly indicated that the reaction solvent had a critical effect both on HMF conversion and product selectivity. The dramatic difference of conversions of HMF and DFF selectivity might be relevant to the real electrode potentials of redox pairs in these solvents [36]. The higher conversion of HMF appears to be obtained from weakly coordinating solvents (e.g. dichloroethane, ethyl acetate) toward Al(NO3)3. Better solvation of HMF in solvent (e.g. DMSO) lead to lower reactivity, and DMSO was shown to increase the LUMO energy of 3
Catalysis Communications 136 (2020) 105909
M. Hong, et al.
Table 1 Optimization study on HMF oxidation.a Entry
1 2 3 4 5 6 7 8 9 10 11 12 13 a
Catalyst
Al(NO3)3·9H2O Al(NO3)3·9H2O Al(NO3)3·9H2O Al(NO3)3·9H2O Al(NO3)3·9H2O Al(NO3)3·9H2O Al(NO3)3·9H2O AlCl3 Al2(SO4)3·12H2O ECS-IL-Al(NO3)3 ECS-IL-Al(NO3)3 ECS ECS-IL
Catalyst load
5 mol% 5 mol% 5 mol% 5 mol% 5 mol% 5 mol% 5 mol% 5 mol% 5 mol% 100 mg 90 mg 90 mg 90 mg
Solvent
AcOH DCE EtOAc CH3CN GBL EtOH DMSO AcOH AcOH AcOH AcOH AcOH AcOH
Conv. HMF (%)
95 100 100 77 67 Trace NR 8 Trace 100 99 NR NR
Yield DFF (%)
93 70 78 75 65 – – Trace – 82 98 – –
Product selectivity (%) DFF
FFCA
FDCA
98 70 78 97 97 – – – – 82 99 – –
– 17 22 – – – – – – – – – –
2 13 – 3 3 – – 7 – 18 1 – –
Reaction conditions: HMF (1 mmol), solvent (2 mL), O2 (1 atm), 50 °C, 5 h. NR = No reaction.
HMF [37]. Therefore, AcOH was recognized to be the best solvent for this aluminum catalyzed selective oxidation of HMF to DFF. To examine the role of aluminum in the reaction, two aluminum salts were subjected to the catalytic aerobic oxidation process, whereas, both AlCl3 and Al2(SO4)3·12H2O showed negligible activity (Table 1, entries 8 and 9), indicating NO3− played a critical role for the aerobic oxidation of HMF. Since free aluminum catalyst was used in the reaction the possibility of metal contamination of product was high. Then we became interested to use ECS-IL-Al(NO3)3 as heterogeneous catalyst in the reaction where aluminum is coordinated by imidazolium of ionic liquid-functionalized expanded corn starch. Employing 100 mg ECS-IL-Al(NO3)3 as catalyst in glacial acetic acid, the reaction produced 82% of DFF and 18% of 2,5-furandicarboxylic acid (FDCA) at a full conversion of HMF. Using lower amount of the catalyst, 99% HMF conversion was achieved and DFF selectivity was increased to 99%. Therefore, too much of the catalyst can cause the over-oxidation of HMF to FDCA. The blank reaction was also carried out with only the ECS and ECS-IL support as the catalyst and HPLC analysis indicated that no products were produced (Table 1, entries 12 and 13). Therefore, Al(NO3)3 was the main catalytic center in the oxidation of HMF. The active species in the Al(NO3)3/ TEMPO catalytic system, maybe, is the metal-TEMPO complex rather than the free nitrosonium ion as the oxidant [38]. The coordination of TEMPO to a Lewis acid makes TEMPO a better oxidant. We suggested that the transformation proceeds via a mechanism involving concerted proton-coupled electron transfer from the CeH bond of HMF to the nitrogen atom of the metal-TEMPO complex [39,40].
Fig. 2. Recyclability of ECS-IL-Al(NO3)3 in oxidation of HMF.
4. Conclusion The immobilized Al(NO3)3-imidazolium ionic liquid on expanded corn starch was prepared and characterized by FT-IR, NMR, XRD, TGA, SEM/EDS and XPS. ECS-IL-Al(NO3)3 acted as an efficient heterogeneous catalyst for selective aerobic oxidation of HMF into DFF under ambient pressure in acetic acid, yielding 98% DFF after 5 h. Furthermore, the catalyst could be easily recycled and reused at least 6 times without losing its activity. Another interesting aspect is the use of inexpensive, biodegradable, nontoxic, and renewable starch polymer as the support of the efficient heterogeneous catalyst which can reduce the metal contamination of products significantly.
3.3. Stability of the ECS-IL-Al(NO3)3 catalyst For practical industrial applications of heterogeneous catalysts, the life-time and the level of reusability is an important factor. Hence, the recyclability test of the starch supported catalyst was examined on the oxidation of HMF to DFF under optimized reaction conditions. After the catalytic test, the catalyst was recovered by simple filtration and extensively washed with acetonitrile and acetone. After being dried under vacuum, the catalyst was reused for the next reaction cycle and the consequences are depicted in Fig. 2. Even after 6 runs, the starch supported Al catalyst maintained a high catalytic performance without any loss. Subsequently, the FT-IR of the reused ECS-IL-Al(NO3)3 catalyst was displayed in Fig. S1d, which revealed that the typical absorption peaks of the ionic liquid and nitrate species were still present. In addition, hot filtration and ICP analysis shows no discernible amounts of Al (81 ppb), indicating the integrity of our catalytic system.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in the paper. 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). 4
Catalysis Communications 136 (2020) 105909
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Appendix A. Supplementary data [20]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.catcom.2019.105909.
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Short communication
Aerobic oxidation of 5-(hydroxymethyl)furfural into 2,5-diformylfuran catalyzed by starch supported aluminum nitrate
T
⁎
Mei Honga,b, , Shuangyan Wub, Jiatong Lib, Jing Wangb, Lifen Weib, Kun Lib a b
Jiangsu Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, Nanjing Forestry University, Nanjing 210037, People's Republic of China College of Chemical Engineering, Nanjing Forestry University, Nanjing 210037, People's Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Aerobic oxidation Starch Aluminum salts 5-Hydroxymethylfurfural 2,5-Diformylfuran Catalysis
Al(NO3)3 immobilized on expanded corn starch catalyst (ECS-IL-Al(NO3)3) was prepared by silane-modified imidazolium ionic liquid reacting with expanded corn starch (ECS) and coordination of Al(NO3)3 with the imidazolium chloride grafted on the surface, as proved by FT-IR, NMR, SEM/EDS, and XPS. The catalytic activity of ECS-IL-Al(NO3)3 was evaluated in the 5-hydroxymethylfurfural (HMF) oxidation under mild conditions with 98% of 2,5-diformylfuran (DFF) production at 99% HMF conversion after 5 h at 50 °C, using molecular oxygen as the oxidant. Furthermore, this catalyst was found to exhibit excellent recyclability, making it an attractive catalytic system from economic and environmental points of view.
1. Introduction The carbohydrates of plant tissues are the major biomass feedstocks of useful and polyfunctional molecules that can replace those derived from fossil resources [1]. Acid-catalyzed dehydration of hexoses gives 5-hydroxymethylfurfural (HMF) [2]. 2,5-Diformylfuran (DFF) is one of the main furan compounds that are generated from the selective oxidation of HMF [3], which is a versatile precursor in various applications, especially for the synthesis of furanic polymers, pharmaceuticals, antifungal agents and fluorescent substances [4]. Further oxidation of DFF led to form high oxygen containing products such as 5-formyl-2furancarboxylic acid (FFCA) and furandicarboxylic acid (FDCA), as side products. Thus, the selective oxidation of HMF to DFF has been deeply studied by researchers in recent years. Classical oxidants [5,6] such as KMnO4 and NaOCl were used in several methods for oxidation HMF to DFF, but the main disadvantages are the requirement of stoichiometric quantities of toxic and corrosive oxidant and generation of hazardous waste. From the viewpoint of atom economy, cost efficiency and green and sustainable chemistry, atmospheric molecular oxygen represent the most acceptable and desirable oxidant [7]. To establish the desired reactivity with molecular oxygen as the terminal oxidant, much efforts have been directed toward the selective oxidation of HMF to DFF with transition-metal-based catalysts such as Mo [3,8–10], Cu [1,11–16], Au-Ru [17], Mn [18–21], Pt [22,23]. Our eyes have captured aluminum because the content of
aluminum is abundant in the earth's crust. However, the difficulty in separating Al3+ from the reaction mixture limits the application of homogeneous catalysts. Therefore, the supports were studied to load Al3+ ion. Natural polymers, especially polysaccharides such as starch, are attractive since they are easily available, inexpensive, nontoxic and biodegradable. Hydroxyl groups in the structure of starch are able to be modified and then immobilize metal ions. Herein, we report the straightforward synthesis of Al(NO3)3 immobilized on ionic liquid-functionalized expanded corn starch (ECS-ILAl(NO3)3). The aluminum-catalyst was demonstrated to be an efficient catalyst for the oxidation of HMF to DFF when used together with TEMPO as an electron transfer mediator under ambient pressure of dioxygen at 50 °C in glacial acetic acid. 2. Experimental 2.1. Materials and methods All chemicals were purchased from Energy Chemical Company (Shanghai, China) and used without purification. High amylose corn starch was purchased from Guomin Starch Chemistry (Shanghai, China) Co., Ltd. FT-IR spectra were recorded on a Nicolet 380 FT-IR instrument. Thermogravimetry (TG) and differential thermal analyses (DTA) were carried out with a TGA instruments thermal analyzer TG 209F1 under N2 atmosphere at a heating rate of 10 °C min−1. Scanning
⁎ Corresponding author at: Jiangsu Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, Nanjing Forestry University, Nanjing 210037, People's Republic of China. E-mail address: [email protected] (M. Hong).
https://doi.org/10.1016/j.catcom.2019.105909 Received 26 September 2019; Received in revised form 28 November 2019; Accepted 16 December 2019 Available online 17 December 2019 1566-7367/ © 2019 Elsevier B.V. All rights reserved.
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Scheme 1. Synthesis of expanded corn starch supported aluminum catalyst.
as reported for the first run. Samples were collected and analyzed via HPLC.
electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS) was used on the Quanta 200. Powder X-ray diffraction (XRD) patterns were recorded on a Rigaku D/Max 2400 X-ray diffractometer using Cu Kα (λ = 0.15406 nm) radiation. X-ray photoelectron spectroscopy (XPS) spectra were recorded on a Kratos Axis Ultra DLD spectrometer, and the binding energy was calibrated by C 1 s peak. 13C CP MAS NMR spectra were recorded on Bruker DRX-600 instruments. 1H NMR and 13 C NMR spectra were recorded using Bruker DRX-600 spectrometer using CDCl3 as solvent.
2.6. Analysis and separation of the product HPLC. HMF and DFF were analyzed by HPLC (Agilent 1200) using a reversed-phase C18 column (250 × 4.6 mm) at 25 °C with a detection wavelength of 280 nm. The mobile phase was acetonitrile and 0.1 wt% acetic acid aqueous solution (65:35 v/v) at 0.5 mL/min. The HMF conversion and DFF yield were expressed as mol% in terms of the total HMF amount. The amounts of HMF and DFF in the samples were calculated by interpolation from calibration curves. Calibration curves for the observed products were constructed by injecting known concentrations of reference commercial products. NMR. After removing glacial acetic acid by a rotary evaporator, DFF was purified by flash column chromatography on silica gel, where a colorless band was collected and the solvent removed in vacuo resulting in a white crystalline compound. 1H NMR (600 MHz, CDCl3): δ/ppm 7.35 (s, 2H), 9.80 (s, 2H); 13C NMR (150 MHz, CDCl3): δ/ppm 119.35 (2C), 154.19 (2C), 179.20 (2C).
2.2. Preparation of ionic liquid [24] A mixture of 1-methylimidazole (0.82 g, 10 mmol) and (3-chloropropyl) triethoxysilane (2.41 g, 10 mmol) was stirred at 90 °C without using any solvent for 48 h under N2 atmosphere. The resulting viscous yellowish liquid was washed with diethyl ether (3 × 5 mL) to remove any unreacted material. The resulting product was vacuum dried to obtain ionic liquid (IL), 1-(triethoxysilyl)propyl-3-methylimidazolium chloride. 2.3. Synthesis of ECS-IL [25]
3. Results and discussion
The expanded corn starch (ECS) support was prepared according to our previously published method [26]. In a 50 mL flask, 2 g of the expanded corn starch were dispersed in 18 mL of dried toluene by sonicating for 1 h. After the addition of 1.0 g 1-(triethoxysilyl)propyl-3methylimidazolium chloride the mixture was stirred at reflux for 24 h under N2 atmosphere. After separation, the remaining solid was dried under vacuum and the excess of 1-(triethoxysilyl)propyl-3-methylimidazolium chloride removed by 24 h Soxhlet extraction with dichloromethane. The resulting material was denoted as ECS-IL.
3.1. Synthesis and characterization of the catalyst The strategy for preparation of silane-modified imidazolium chloride, immobilizing imidazolium-based ionic liquid on the expanded corn starch (ECS) and imidazolium successively coordinating to Al (NO3)3 is outlined in Scheme 1. N-methylimidazole reacted with 3chloropropyltriethoxysilane to produce ionic liquid (IL) 1-(triethoxysilyl)propyl-3-methylimidazolium chloride. Then, the ionic liquid was anchored on ECS by condensation reaction to obtain ECS-IL. Finally, the ECS-IL was treated with aluminum nitrate to deliver ECS-IL-Al(NO3)3 catalyst. The obtained aluminum catalyst was characterized by a number of analytic techniques, including FT-IR, 13C CP MAS NMR, XRD, TGA-DTA, SEM/EDS and XPS. The FT-IR spectra of ECS and ECS-IL-Al(NO3)3 catalyst are shown in Fig. S1. The spectrum of the ECS-IL-Al(NO3)3 catalyst exhibit one characteristic peak at the position of 1569 cm−1, which can be attributed to the C]C stretching of imidazolium ring in the ionic liquid [27–29], indicating that imidazole is present on the surface of ECS-IL-Al (NO3)3 catalyst. The signals of CeH deformation vibration and CeC, CeN stretching vibrations in imidazole ring which should be seen can't be detected due to the overlap with the peak of NO3−. The peak at 1385 cm−1 and 827 cm−1 suggests the presence of nitrate species [30,31]. The structure of the organic functional group of ECS-IL-Al(NO3)3 catalyst was also proved by solid-state 13C CP MAS NMR spectrum. As shown in Fig. S2, the 13C resonance assignment based on the chemical shifts matched well with imidazole ring [28]. The XRD patterns of ECS and ECS-IL-Al(NO3)3 catalyst were
2.4. Synthesis of ECS-IL-Al(NO3)3 1.12 g of Al(NO3)3·9H2O was dissolved in 10 mL dried alcohol followed by the addition of 1.0 g ECS-IL into the solution and kept stirring at reflux for 24 h under N2 atmosphere. After filtration, the solid product was washed completely with EtOH, dried at 70 °C under vacuum for 24 h and labeled as ECS-IL-Al(NO3)3. 2.5. General procedure for the oxidation of HMF to DFF The experiments were carried out in test tubes equipped with a balloon containing pure O2, using O2 as the oxidant. In a general procedure, HMF (125 mg, 1 mmol), ECS-IL-Al(NO3)3 (90 mg), TEMPO (8.5 mg, 0.05 mmol) and glacial acetic acid (2 mL) were added to a 10 mL test tube. The reaction mixture was stirred magnetically at 50 °C for 5 h. After completion of the reaction, the mixture was cooled down to room temperature and filtered, and the remaining solid was washed with acetonitrile and acetone to separate the catalyst. Then, the recovered catalyst was dried and used in several runs in the same manner 2
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Fig. 1. SEM image of (a) ECS, (b) ECS-IL-Al(NO3)3 and EDS mapping images of (c) Al and (d) N for ECS-IL-Al(NO3)3.
3.2. Catalytic oxidation of HMF to DFF
depicted in Fig. S3. The amorphous peaks arising from ECS between 1030° were observed. No obvious differences were seen between the ECS and the ECS-IL-Al(NO3)3 catalyst, which evidenced that the ionic liquid and Al(NO3)3 were evenly dispersed on the surface of the ECS support. The thermal stability of the support and the catalyst were investigated by TGA. As shown in Fig. S4, ECS showed a slightly weight loss below 150 °C due to the adsorbed H2O on the surface of ECS. Then the decomposition of ECS started at around 250 °C, which was possibly attributed to the gradual degradation of polymer chain. It could be found that the onset decomposing temperature of ECS-IL-Al(NO3)3 decrease dramatically as comparing with that of the parent ECS, revealing the interaction between the support and IL. To further investigate the morphological changes in the materials, SEM (Fig. 1) was performed on ECS and ECS-IL-Al(NO3)3 catalyst. The surface morphology of ECS is smooth and granular. The immobilization of the ionic liquid as well as Al(NO3)3 on ECS creates stress, which may contribute to the breakdown of ECS granules. The energy dispersive spectroscopy (EDS) mapping images clearly confirmed the presence of the uniformly dispersed Al and N element in the as-prepared ECS-IL-Al (NO3)3 catalyst as shown in Fig. 1c and d. In order to gain more insights into the surface composition of the ECS-IL-Al(NO3)3 catalyst, the as-prepared sample was investigated by the XPS analysis (Fig. S5). For the XPS of N 1 s regions, the binding energy for N 1 s located at 401.7 eV and 407.1 eV are attributed to the imidazolium nitrogen [32] and NO3− groups [33,34], respectively. The Al 2p spectrum of ECS-IL-Al(NO3)3 shows a peak at 74.6 eV, indicating the presence of aluminum.
To identify the appropriate reaction conditions for selective oxidation of HMF to DFF, initially we have carried out an optimization study. Many investigations have demonstrated that the solvent had a remarkable effect on both HMF conversion and DFF yield [4,21]. As different solvents have different properties such as the polarity, dielectric constant, steric hindrance, acid-base property, all of which show affect the chemical reactions [35]. Therefore, oxidation of HMF was carried out in various solvents with 5 mol% Al(NO3)3·9H2O as the catalyst under homogeneous conditions with molecular oxygen as the terminal oxidant. As shown in Table 1, the conversion of HMF reached 95% after a reaction for 5 h in glacial acetic acid (AcOH). Upto 100% conversion of HMF was obtained when 1,2-dichloroethane (DCE) and ethyl acetate (EtOAc) were used as the solvent. The highest DFF yield of 93% was obtained by the use of AcOH as the solvent, while that were 70% with DCE and 78% with EtOAc (Table 1, entries 1–3), respectively. Moderate HMF conversion of 77% and 67% was observed in CH3CN and γ-butyrolactone (GBL) (Table 1, entries 4 and 5), respectively. Al (NO3)3·9H2O showed no catalytic activity in the oxidation of HMF, when reactions were carried out in EtOH and DMSO (Table 1, entries 6 and 7). These results clearly indicated that the reaction solvent had a critical effect both on HMF conversion and product selectivity. The dramatic difference of conversions of HMF and DFF selectivity might be relevant to the real electrode potentials of redox pairs in these solvents [36]. The higher conversion of HMF appears to be obtained from weakly coordinating solvents (e.g. dichloroethane, ethyl acetate) toward Al(NO3)3. Better solvation of HMF in solvent (e.g. DMSO) lead to lower reactivity, and DMSO was shown to increase the LUMO energy of 3
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Table 1 Optimization study on HMF oxidation.a Entry
1 2 3 4 5 6 7 8 9 10 11 12 13 a
Catalyst
Al(NO3)3·9H2O Al(NO3)3·9H2O Al(NO3)3·9H2O Al(NO3)3·9H2O Al(NO3)3·9H2O Al(NO3)3·9H2O Al(NO3)3·9H2O AlCl3 Al2(SO4)3·12H2O ECS-IL-Al(NO3)3 ECS-IL-Al(NO3)3 ECS ECS-IL
Catalyst load
5 mol% 5 mol% 5 mol% 5 mol% 5 mol% 5 mol% 5 mol% 5 mol% 5 mol% 100 mg 90 mg 90 mg 90 mg
Solvent
AcOH DCE EtOAc CH3CN GBL EtOH DMSO AcOH AcOH AcOH AcOH AcOH AcOH
Conv. HMF (%)
95 100 100 77 67 Trace NR 8 Trace 100 99 NR NR
Yield DFF (%)
93 70 78 75 65 – – Trace – 82 98 – –
Product selectivity (%) DFF
FFCA
FDCA
98 70 78 97 97 – – – – 82 99 – –
– 17 22 – – – – – – – – – –
2 13 – 3 3 – – 7 – 18 1 – –
Reaction conditions: HMF (1 mmol), solvent (2 mL), O2 (1 atm), 50 °C, 5 h. NR = No reaction.
HMF [37]. Therefore, AcOH was recognized to be the best solvent for this aluminum catalyzed selective oxidation of HMF to DFF. To examine the role of aluminum in the reaction, two aluminum salts were subjected to the catalytic aerobic oxidation process, whereas, both AlCl3 and Al2(SO4)3·12H2O showed negligible activity (Table 1, entries 8 and 9), indicating NO3− played a critical role for the aerobic oxidation of HMF. Since free aluminum catalyst was used in the reaction the possibility of metal contamination of product was high. Then we became interested to use ECS-IL-Al(NO3)3 as heterogeneous catalyst in the reaction where aluminum is coordinated by imidazolium of ionic liquid-functionalized expanded corn starch. Employing 100 mg ECS-IL-Al(NO3)3 as catalyst in glacial acetic acid, the reaction produced 82% of DFF and 18% of 2,5-furandicarboxylic acid (FDCA) at a full conversion of HMF. Using lower amount of the catalyst, 99% HMF conversion was achieved and DFF selectivity was increased to 99%. Therefore, too much of the catalyst can cause the over-oxidation of HMF to FDCA. The blank reaction was also carried out with only the ECS and ECS-IL support as the catalyst and HPLC analysis indicated that no products were produced (Table 1, entries 12 and 13). Therefore, Al(NO3)3 was the main catalytic center in the oxidation of HMF. The active species in the Al(NO3)3/ TEMPO catalytic system, maybe, is the metal-TEMPO complex rather than the free nitrosonium ion as the oxidant [38]. The coordination of TEMPO to a Lewis acid makes TEMPO a better oxidant. We suggested that the transformation proceeds via a mechanism involving concerted proton-coupled electron transfer from the CeH bond of HMF to the nitrogen atom of the metal-TEMPO complex [39,40].
Fig. 2. Recyclability of ECS-IL-Al(NO3)3 in oxidation of HMF.
4. Conclusion The immobilized Al(NO3)3-imidazolium ionic liquid on expanded corn starch was prepared and characterized by FT-IR, NMR, XRD, TGA, SEM/EDS and XPS. ECS-IL-Al(NO3)3 acted as an efficient heterogeneous catalyst for selective aerobic oxidation of HMF into DFF under ambient pressure in acetic acid, yielding 98% DFF after 5 h. Furthermore, the catalyst could be easily recycled and reused at least 6 times without losing its activity. Another interesting aspect is the use of inexpensive, biodegradable, nontoxic, and renewable starch polymer as the support of the efficient heterogeneous catalyst which can reduce the metal contamination of products significantly.
3.3. Stability of the ECS-IL-Al(NO3)3 catalyst For practical industrial applications of heterogeneous catalysts, the life-time and the level of reusability is an important factor. Hence, the recyclability test of the starch supported catalyst was examined on the oxidation of HMF to DFF under optimized reaction conditions. After the catalytic test, the catalyst was recovered by simple filtration and extensively washed with acetonitrile and acetone. After being dried under vacuum, the catalyst was reused for the next reaction cycle and the consequences are depicted in Fig. 2. Even after 6 runs, the starch supported Al catalyst maintained a high catalytic performance without any loss. Subsequently, the FT-IR of the reused ECS-IL-Al(NO3)3 catalyst was displayed in Fig. S1d, which revealed that the typical absorption peaks of the ionic liquid and nitrate species were still present. In addition, hot filtration and ICP analysis shows no discernible amounts of Al (81 ppb), indicating the integrity of our catalytic system.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in the paper. 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). 4
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Appendix A. Supplementary data [20]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.catcom.2019.105909.
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