Metal Nitrate Catalysis for Selective Oxidation of 5-Hydroxymethylfurfural into 2,5-Diformylfuran under Oxygen Atmosphere.

Selective synthesis of various versatile compounds from biomass is of great importance to displace traditional fossil fuel resources. Here, homogeneous metal nitrate (M(NO3) x )/(2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) and M(NO3) x /TEMPO/NaNO2 catalyst systems in glacial acetic acid and acetonitrile, respectively, have been found to be highly active and practically sustainable for selective oxidation of 5-hydroxymethylfurfural (HMF) into 2,5-diformylfuran (DFF) using pure O2 or even O2 in air as the oxidant. The catalytic methods enable full HMF conversion with a nearly 100% DFF selectivity at 50 °C under atmospheric pressure using a very simple reaction setup and workup. Mechanistic aspects are discussed. The influences of reaction conditions such as different metal catalysts, catalyst loading, solvents, and reaction temperature on the promotion effect were studied. Meanwhile, the catalyst systems had also good performance for aerobic oxidation of other alcohols.

Introduction

Biomass was considered by experts as the only renewable source of energy, and it had the potential to replace petroleum in manufacturing chemicals and liquid transportation fuels.[1−5] Conversion of bio-sourced saccharides to high value-added chemicals has attracted much attention in recent years.[6−8] As an important bio-based platform chemical, 5-hydroxymethylfurfural (HMF) can be converted to various high value-added products through a variety of reactions including hydrogenation, oxidation, and hydration.[9−13] 2,5-Diformylfuran (DFF) is produced by selective oxidation of HMF[14−17] that can be used, for example, as a starting material for the synthesis of diverse poly-Schiff bases, polymer resin sealants, macrocyclic ligands, pharmaceuticals, organic conductors, and the cross-linking agent of poly(vinyl alcohol) for battery separations.[18]

Recently, chemists had paid much attention on the synthesis of DFF from HMF oxidation using molecular oxygen as the terminal oxidant catalyzed by homogeneous or heterogeneous metal catalysts, such as Ru,[19−22] Au,[23,24] Cu,[25] Mn,[26,27] Mo/V,[28] and V.[29−31] Most present protocols require the use of noble metals,[19−24] expensive ancillary ligands,[25] high oxygen pressure (up to 10 bar),[30] and/or high reaction temperatures (up to 140 °C)[26−29,31] to ensure desirable DFF selectivities at high HMF conversions. There is a need for cheaper, milder, and greener methodologies by designing inexpensive catalytic systems for selective aerobic oxidation of HMF to DFF. Accordingly, TEMPO and its analogues (N-oxyl radical catalysts) have been investigated for reaction components. However, because of the inertness of dioxygen, TEMPO alone is not able to directly catalyze the oxidation of alcohols using dioxygen as the terminal oxidant, so it is necessary to use a cocatalyst to activate dioxygen and transfer its oxidizability to the next redox cycle of TEMPO. The catalysts such as HNO3-immobilized TEMPO and TEMPO-imidazolium salts can efficiently and selectively catalyze the oxidation of alcohols to the corresponding aldehydes or ketones.[32,33] However, there are only a few reported studies on applying N-oxyl radicals to catalyze the oxidation of HMF.[34,35] Yang et al.[36] reported that benzoic acid/TEMPO obtained 86.7% conversion of HMF and 77.8% yield of DFF at 0.4 MPa O2 in acetonitrile. Although interesting achievement was obtained by this method in the field of aerobic oxidation of HMF into DFF, it required relatively high operating pressures (0.4 MPa), high catalyst loading (50 mol %), special Teflon-lined autoclave reactors, high temperatures (up to 100 °C), and long reaction time (24 h). Motivated by a desire to develop better aerobic oxidation catalysts, especially stressing the function of air oxygen in these transformations, we now describe an efficient and selective oxidation of HMF to DFF by using two novel cost-beneficial catalytic systems based on M(NO3)/TEMPO and M(NO3)/TEMPO/NaNO2 under mild, aerobic conditions (Scheme ). The oxidations were carried out in glacial acetic acid and acetonitrile solution under the same conditions.

Scheme 1

Oxidation of HMF into DFF

Results and Discussion

Our study commenced with screening the transformation of HMF to DFF in the presence of different metal nitrates using molecular oxygen as the oxidant. We initially tried this aerobic oxidation of HMF using 5 mol % Fe(NO3)3·9H2O and 5 mol % TEMPO as catalysts in glacial acetic acid as it is a good solvent for the homogeneous system of the oxidation of alcohols to aldehydes,[37,38] and this reaction could work smoothly at room temperature under atmospheric pressure of molecular oxygen affording DFF in 93% yield after 8 h (Table , entry 1). Oxygen was simply flushed into a balloon and attached to a flask containing a solution of HMF. To shorten the reaction time, the reaction temperature was raised to 50 °C (Table , entry 2). Under lower Fe(NO3)3·9H2O loading at 2 mol %, 78% yield of DFF was achieved (Table , entry 3). Only a trace amount of the product was detected in a control experiment without the Fe(NO3)3·9H2O catalyst (Table , entry 4). To examine the role of iron in the reaction, we selected two trivalent iron salts. In sharp contrast, HMF was almost not oxidized when Fe(acac)3 and FeCl3·6H2O were used instead of Fe(NO3)3·9H2O (Table , entries 5 and 6), indicating NO3– played a critical role for the aerobic oxidation of HMF. Subsequently, a series of inorganic nitrates were investigated to test whether the metal cation also had a certain effect for the reaction. We observed that all nitrates and nitric acid successfully catalyzed the conversion, and Al(NO3)3·9H2O worked only as well as Fe(NO3)3·9H2O (Table , entry 9). The surprising result may be due to the identical amount of NO3– in Fe(NO3)3·9H2O and Al(NO3)3. To test the hypothesis, we increased the amount of the catalysts Cu(NO3)2·3H2O, Zn(NO3)2·6H2O, HNO3, KNO3, and NaNO3. It was easily found in Table that conversions of the aerobic oxidation were in proportion to the amount of NO3–. The efficiency of HNO3 or other metals (Cu, Zn) containing nitrates as a viable alternative to Fe(NO3)3·9H2O was also quite surprising (Table , entries 8, 11, and 17). By employing 15 mol % NO3– in combination of TEMPO, 100% HMF conversion was obtained within 5 h with the highest DFF yield of 99%, which was complete oxidations. As far as we are aware, aluminum and zinc were applied here for the first time to mediate efficiently the TEMPO-catalyzed aerobic selective oxidation of HMF to DFF. The observation that Al(NO3)3·9H2O and Zn(NO3)3·6H2O, which contain redox-inactive Al and Zn, are competent catalysts demonstrates that a redox change at the metal is not necessary for HMF oxidation.[39] Alkali-metal nitrates such as sodium nitrate and potassium nitrate presented less catalytic activity (Table , entries 12–15). The results indicated that the metal cation also has some influence on the reaction, and the active species in the M(NO3)/TEMPO catalytic system, maybe, is not the free nitrosonium ion but the metal–TEMPO complex.[39] 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 C–H bond of HMF to the nitrogen atom of the metal–TEMPO complex.[40,41]

Table 1

Effect of Different Catalysts on HMF Conversion and DFF Yield in AcOHa

entry	catalyst, loading (mol %)	T (°C)	time (h)	conv. HMF (%)	yield DFF (%)	select.b (%)
1	Fe(NO3)3·9H2O, 5	RT	8	100	93	93
2	Fe(NO3)3·9H2O, 5	50	5	100	95	95
3	Fe(NO3)3·9H2O, 2	50	5	84	78	93
4		50	5	2	1	50
5	Fe(acac)3, 5	50	24	2	1	50
6	FeCl3·6H2O, 5	50	24	2	1	50
7	Cu(NO3)2·3H2O, 5	50	5	81	80	99
8	Cu(NO3)2·3H2O, 7.5	50	5	100	99	99
9	Al(NO3)3·9H2O, 5	50	5	95	93	98
10	Zn(NO3)2·6H2O, 5	50	5	78	72	92
11	Zn(NO3)2·6H2O, 7.5	50	5	100	97	97
12	KNO3, 10	50	5	49	57	96
13	KNO3, 15	50	5	64	61	95
14	NaNO3, 10	50	5	51	49	96
15	NaNO3, 15	50	5	64	62	97
16	HNO3, 5	50	5	72	71	99
17	HNO3, 15	50	5	100	98	98

Reaction conditions: HMF (1 mmol, 126 mg), TEMPO (0.05 mmol, 7.8 mg), 50 °C, AcOH (2 mL), and oxygen balloon; the conversion and yield were determined by HPLC. Conv. = conversion. Select. = selectivity.

S (DFF) = yield (DFF)/conversion (HMF).

Controlled experiments with or without TEMPO confirmed their necessity to obtain a desired outcome of the reaction. When Fe(NO3)3·9H2O was used without TEMPO, only little HMF was oxidized (Table , entry 1), indicating that TEMPO participated in the aerobic oxidation. Subsequently, increasing the amount of TEMPO to 2 mol % resulted in a significant increase in terms of the product yield (Table , entry 2). Five mole percent TEMPO was enough for complete transformation of HMF to DFF. Air has the obvious advantage of safety and low cost. We next examined the ability of the Fe(NO3)3·9H2O catalyst to catalyze the HMF oxidation using air instead of pure oxygen as the terminal oxidant. To our delight, DFF was still obtained in a high yield of 81% after 5 h, and the selectively of DFF was even higher (Table , entry 4). The lower DFF yield with air balloon was due to a lower concentration of oxygen. The catalytic efficiency of other stable N-oxyl radical such as N-hydroxyphthalimide (NHPI) replaced TEMPO with 5 mol % Fe(NO3)3·9H2O in AcOH under an oxygen atmosphere at 50 °C, however, exhibited a very small amount of HMF conversion (Table , entry 5). Importantly, if other common oxidants such as 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), t-BuOOH (TBHP), and H2O2 were used instead of TEMPO and oxygen, HPLC analysis of the products after 24 h of reaction revealed the further oxidation into 2,5-furandicarboxylic acid (FDCA) (14% yield at 18% HMF conversion, 45% yield at 46% HMF conversion, and 30% yield at 30% HMF conversion).

Table 2

Effect of Cocatalysts and Oxidants on HMF Conversion and DFF Yield in AcOHa

entry	cocatalyst, loading (mol %)	oxidant (mmol)	time (h)	conv. HMF (%)	yield DFF (%)	select.b (%)
1	TEMPO, 0	O2	5	17	0	0
2	TEMPO, 2	O2	5	85	69	81
3	TEMPO, 5	O2	5	100	95	95
4	TEMPO, 5	air	5	82	81	99
5	NHPI, 5	O2	24	37	3	8
6		DDQ (2)	24	18	4	22
7		TBHP (2)	24	46	1	2
8		H2O2 (2)	24	30	0	0

Reaction conditions: HMF (1 mmol, 126 mg), Fe(NO3)3·9H2O (0.05 mmol), 50 °C, AcOH (2 mL), and oxygen balloon; the conversion and yield were determined by HPLC. Conv. = conversion. Select. = selectivity.

S (DFF) = yield (DFF)/conversion (HMF).

It is known that the solvent is a key factor affecting the reaction efficiency. Thus, the aerobic oxidation of HMF to DFF was carried out in a variety of common solvents such as ethyl acetate (EtOAc) and 1,2-dichloroethane (DCE), which have been used in related alcohol oxidation methods. As shown in Table , the solvent showed a remarkable effect on DFF selectivity. Low DFF selectivity was obtained when the reaction was carried out in EtOAc and DCE (Table , entries 1 and 2). The concentration of HMF can also have some effects on the conversion of HMF during the oxidation process. When the concentration of HMF was 1 mol/L, after reaction for 5 h, the yield of product DFF was 61% (Table , entry 3); nevertheless, when HMF was set at a lower concentration of 0.5 mol/L under the same conditions, full conversion was achieved (Table , entry 4), indicating that lowering the HMF concentration, the yields of product DFF can be increased. However, a further reduction in the concentration of HMF to 0.2 mol/L resulted in a lower yield of DFF.

Table 3

Effect of the Solvent and HMF Concentration on HMF Conversion and DFF Yield with Fe(NO3)3/TEMPO Catalyst Systema

entry	solvent	solvent amount (mL)	conv. HMF (%)	yield DFF (%)	select.b (%)
1	EtOAc	2	100	57	57
2	DCE	2	100	64	64
3	AcOH	1	62	61	98
4	AcOH	2	100	95	95
5	AcOH	5	58	52	90

Reaction conditions: HMF (1 mmol, 126 mg), Fe(NO3)3·9H2O (0.05 mmol), TEMPO (0.05 mmol, 7.8 mg), 50 °C, 5 h, and oxygen balloon; the conversion and yield were determined by HPLC. Conv. = conversion. Select. = selectivity.

S (DFF) = yield (DFF)/conversion (HMF).

To make the workup procedure more simple and apply to substrates containing acid-sensitive functional groups, the catalytic system of Fe(NO3)3/TEMPO was further studied in CH3CN; however, the reaction was much slower (96% of conversion after 24 h) than that in AcOH. Thus, we started to test the effect of additives: The addition of NaNO2 helps much increase in reaction rates (Table , entry 2). The result demonstrated that NaNO2 is an activating agent for the iron–TEMPO catalytic system. The cheap and readily available inorganic halide sodium chloride was screened for such a purpose. In our hands, the addition of NaCl (5 mol %) as an additive, as reported by Ma et al.,[42] suggests that the reaction was completed within 15 h (Table , entry 3). Isobutyl nitrite (C4H9NO2) worked much slower (Table , entry 4). In addition, no reaction occurred in the absence of Fe(NO3)3·9H2O (Table , entry 5) or TEMPO under oxygen (Table 4, entry 6), suggesting that both Fe(NO3)3·9H2O and TEMPO are required for the reaction. Then, Fe(acac)3 and FeCl3 were used as catalysts in the catalytic system, and Fe(acac)3 was almost inert for the selective oxidation of HMF (Table , entry 7). As for FeCl3, much better conversion (36%) with 100% selectivity was obtained (Table , entry 8). For Cu(NO3)2·3H2O, we found that HMF conversion significantly increased from 62 to 100% with increasing catalyst loading (Table , entries 9 and 10). Further experiments with Al(NO3)3·9H2O presented a poorer catalytic activity as compared with the case of AcOH as a solvent (Table , entry 11). Increasing the amount of Zn(NO3)2·6H2O led to slight conversion of HMF (Table , entries 12 and 13). KNO3 and NaNO3 were not effective at all for the oxidation of HMF (Table 4, entries 14 and 15). These results clearly showed that employing redox-active metals such as Fe and Cu had a better effect on the reaction outcome in CH3CN. On the basis of the results of this experiment and the previous reports,[43] a plausible reaction pathway is illustrated in Scheme . In this proposed mechanism, by the oxidation action of FeIII or CuII, TEMPO is transformed into a nitrogen carbonyl cation by a single-electron oxidation, and this cation extracts a hydrogen atom from HMF to transform it to DFF. Furthermore, TEMPO is restored to its original state, whereas the FeIII or CuII ion is reduced to the FeII or CuI ion. At the same time, the NO3– ion oxidizes the FeII or CuI ion to the FeIII or CuII ion, whereas the NO3– ion is reduced to the NO2– ion. In the meantime, molecular oxygen again oxidizes the NO2– ion, and the NO3– ion is thus restored.

Table 4

Effect of Different Catalysts on HMF Conversion and DFF Yield in CH3CNa

entry	catalyst, loading (mol %)	cocatalyst	additive	time (h)	conv. HMF (%)	yield DFF (%)	select.b (%)
1	Fe(NO3)3·9H2O, 5	TEMPO		24	96	91	95
2	Fe(NO3)3·9H2O, 5	TEMPO	NaNO2	10	100	94	94
3	Fe(NO3)3·9H2O, 5	TEMPO	NaCl	15	99	98	98
4	Fe(NO3)3·9H2O, 5	TEMPO	C4H9NO2	24	58	58	100
5		TEMPO	NaNO2	24	NR
6	Fe(NO3)3·9H2O, 5		NaNO2	24	NR
7	Fe(acac)3, 5	TEMPO	NaNO2	24	2	0	0
8	FeCl3·6H2O, 5	TEMPO	NaNO2	24	36	36	100
9	Cu(NO3)2·3H2O, 5	TEMPO	NaNO2	10	62	61	98
10	Cu(NO3)2·3H2O, 7.5	TEMPO	NaNO2	10	100	99	99
11	Al(NO3)3·9H2O, 5	TEMPO	NaNO2	10	77	77	100
12	Zn(NO3)2·6H2O, 5	TEMPO	NaNO2	10	61	60	98
13	Zn(NO3)2·6H2O, 7.5	TEMPO	NaNO2	10	68	65	96
14	NaNO3, 5	TEMPO	NaNO2	10	5	1	20
15	KNO3, 5	TEMPO	NaNO2	10	2	1	50

Reaction conditions: HMF (1 mmol, 126 mg), additive (0.05 mmol), CH3CN (5 mL), 50 °C, and oxygen balloon. NR = no reaction. The conversion and yield were determined by HPLC. Conv. = conversion. Select. = selectivity.

S (DFF) = yield (DFF)/conversion (HMF).

Scheme 2

Proposed Catalytic Cycle for Aerobic Oxidation of HMF with M(NO3)x/TEMPO/NaNO2

Switching from pure oxygen to air and using the same atmospheric pressure also gave the oxidized product DFF in 88% yield after 10 h (Table , entry 1). NHPI provided a low conversion under our reaction conditions compared with the reported literature (Table , entry 2).[44] These results demonstrated that the catalytic activity of NHPI was not so effective to selectively oxidize HMF to DFF under the present reaction conditions. Both HMF conversion and the selectivity of DFF were low using H2O2 as an oxidant (Table , entry 4). Moderate HMF conversion and the DFF yield were obtained using t-BuOOH as an oxidant (Table , entry 5), which was different from the reaction in AcOH.

Table 5

Effect of Cocatalysts and Oxidants on HMF Conversion and DFF Yield in CH3CNa

entry	cocatalyst, loading (mol %)	oxidant	time (h)	conv. HMF (%)	yield DFF (%)	select.b (%)
1	TEMPO, 5	air	10	89	88	99
2	NHPI, 5	O2	24	57	54	95
3		DDQ	10	32	19	59
4		H2O2	24	11	6	55
5		TBHP	24	64	58	91

Reaction conditions: HMF (1 mmol, 126 mg); Fe(NO3)3·9H2O (0.05 mmol), NaNO2 (0.05 mmol, 3.4 mg), CH3CN (5 mL), and 50 °C; the conversion and yield were determined by HPLC. Conv. = conversion. Select. = selectivity.

S (DFF) = yield (DFF)/conversion (HMF).

To further investigate the catalytic performance of Fe(NO3)3/TEMPO/NaNO2, aerobic oxidations with different temperatures were carried out in acetonitrile. As shown in Table , it was observed that temperature played an important role in the oxidation of HMF in acetonitrile. As expected, HMF conversion and the DFF yield increased with the increase of reaction temperature from room temperature to 50 °C (Table , entries 1–4), but a rise over 50 °C resulted in a sharp decline in HMF conversion (Table , entries 5 and 6). It could be concluded that the higher temperature (above 50 °C) could result in a negative effect, and 50 °C was the preferred temperature for the Fe(NO3)3/NaNO2/TEMPO catalytic system. Importantly, the selectivity to DFF remained at a relatively constant and high level (>94%) while the reaction temperatures were varied. The decline in the yield of DFF over 50 °C may possibly be attributed to the mass transfer resistance of oxygen from oxygen gas on the liquid surface down to the liquid solution. Larger mass transfer resistance of oxygen makes the concentration of oxygen lower, and as a consequence, fewer HMF molecules will be successfully transformed into DFF.

Table 6

Effect of Reaction Temperature on HMF Conversion and DFF Yield in CH3CNa

entry	temperature (°C)	conv. HMF (%)	yield DFF (%)	select.b (%)
1	RT	16	15	94
2	30	57	55	96
3	40	72	69	95
4	50	100	94	94
5	60	54	52	96
6	70	28	27	96

Reaction conditions: HMF (1 mmol, 126 mg); Fe(NO3)3·9H2O (0.05 mmol), TEMPO (0.05 mmol, 7.8 mg), NaNO2 (0.05 mmol, 3.4 mg), CH3CN (5 mL), and oxygen balloon; the conversion and yield were determined by HPLC. Conv. = conversion. Select. = selectivity.

S (DFF) = yield (DFF)/conversion (HMF).

The solvent effect of the aerobic oxidation reaction at 50 °C was examined to select an industrial friendly solvent. The results of the oxidation of HMF in the presence of 5 mol % Fe(NO3)3·9H2O, 5 mol % TEMPO, and 5 mol % NaNO2 under the atmospheric pressure of oxygen in different solvents are summarized in Table . We were delighted to find that when using ethyl acetate as the solvent, HMF could be converted into DFF within 10 h in an almost quantitative yield of 98%. Therefore, ethyl acetate is also a desired solvent. Under the same conditions, the conversion of HMF to DFF was achieved in a high yield in 1,2-dichloroethane. However, such an environmentally unfriendly chlorinated solvent is disfavored for large-scale industrial application. The effect of the substrate concentration on the conversion of HMF and the yield/selectivity of DFF is shown in Table . As the substrate concentration increased from 0.2 mol/L to 0.5 mol/L, almost no change was observed in the DFF yield (Table , entries 5 and 6). As the concentration of HMF was further increased to 1 mol/L, the conversion of HMF decreased to 80%, which is most likely due to the evaporation of CH3CN and the mass transfer of the entire system being restricted by the increase in the substrate concentration.

Table 7

Effect of the Solvent and HMF Concentration on HMF Conversion and DFF Yield with Fe(NO3)3/NaNO2/TEMPO Catalyst System Conditionsa

entry	solvent	solvent amount (mL)	conv. HMF (%)	yield DFF (%)	select.b (%)
1			55	51	93
2	H2O	5	3	3	100
3	EtOAc	5	100	98	98
4	DCE	5	100	93	93
5	MeCN	5	100	94	94
6	MeCN	2	100	96	96
7	MeCN	1	80	77	96

Reaction conditions: HMF (1 mmol, 126 mg), Fe(NO3)3·9H2O (0.05 mmol, 20 mg), TEMPO (0.05 mmol, 7.8 mg), NaNO2 (0.05 mmol, 3.4 mg), 50 °C, 10 h, and oxygen balloon; the conversion and yield were determined by HPLC. Conv. = conversion. Select. = selectivity.

S (DFF) = yield (DFF)/conversion (HMF).

The two catalyst systems were applied to the aerobic oxidation of a variety of alcohols. As shown in Table , all benzylic alcohols were quantitatively converted into the corresponding aldehydes with >99% selectivity with both of the catalyst systems. Secondary alcohols including cyclohexanol and diphenylmethanol can be completely converted into corresponding ketones with >99% selectivity with the Fe(NO3)3/TEMPO/NaNO2 catalytic system in CH3CN, whereas the secondary aliphatic alcohol cyclohexanol was failed to afford the desired product with the Fe(NO3)3/TEMPO catalytic system in AcOH. 1,2-Diols seldom interfered with the catalytic aerobic oxidation were selectivity oxidized into ketone and aldehyde. 1,2-Diol was cleanly oxidized to α-dicarbonyl without cleavage of the 1,2-diol bond with the Fe(NO3)3/TEMPO/NaNO2 catalytic system in CH3CN. Fe(NO3)3/TEMPO was catalyzed aerobic cleavage of 1-phenyl-1,2-ethanediol into benzoic acid in the quantitative yield in AcOH. The stoichiometric oxidants that are still predominantly used for such oxidative cleavage, such as H5IO6, Pb(OAc)4, and KMnO4,[45] generate stoichiometric hazardous waste. Herein, we describe an applicable oxidative cleavage of 1,2-diols that consumes atmospheric oxygen as the oxidant, thus serving as a potentially greener alternative to the classical transformations.

Table 8

Aerobic Fe(NO3)3/TEMPO-Catalyzed Oxidation of Various Alcohols

Conditions A: substrate (1 mmol), Fe(NO3)3·9H2O (0.05 mmol), TEMPO (0.05 mmol), 50 °C, AcOH (2 mL), and oxygen balloon. Conditions B: substrate (1 mmol), Fe(NO3)3·9H2O (0.05 mmol, 20 mg), TEMPO (0.05 mmol), NaNO2 (0.05 mmol, 3.4 mg), CH3CN (5 mL), 50 °C, 10 h, and oxygen balloon.

Conversions and yields are based on gas chromatography (GC) with area normalization. NR = no reaction. Conv. = conversion.

Conclusions

We have developed two novel catalytic systems, including TEMPO and metal nitrates, for the aerobic oxidation of HMF to DFF at 50 °C in the presence of 1 atm O2. Various aliphatic and aromatic alcohols could be oxidized quantitatively to the corresponding carbonyl compounds in the catalytic systems, and pure products are generally isolable by simple extraction with an organic solvent and water. We anticipated that this protocol will be of broad interest and use to the chemical industries.

Experimental Section

Materials

All chemicals used in this study were of analytical reagent grade, commercially available, and used without further purification unless otherwise noted. Reactions were carried out in 5 or 10 mL glass flasks. HMF and balloons (500 mL) were purchased from Sigma-Aldrich and used as an air reservoir. Reactions were monitored using thin-layer chromatography on TLC silica gel 60 F254 aluminum sheets (20 × 20 cm).

Selective Aerobic Oxidation of Alcohols into Aldehydes or Ketones with the M(NO3)/TEMPO Catalytic System

For a typical procedure, a 5 mL glass flask equipped with a magnetic stirring bar was charged with HMF (126 mg, 1 mmol) and glacial acetic acid (2 mL). Fe(NO3)3·9H2O (20.2 mg, 0.05 mmol) and TEMPO (7.8 mg, 0.05 mmol) were consecutively added to the solution, and a balloon filled with oxygen (500 mL) was then attached to the flask. The reaction mixture was magnetically stirred at 50 °C, and the consumption of the starting material was monitored using TLC. After completion of the reaction, the reaction was quenched by adding EtOAc. The catalyst was removed by washing with water. Aqueous phases were combined and then extracted with EtOAc. The aqueous phase extracts were combined and dried with MgSO4. The solvent was evaporated in vacuo to yield pure products.

Selective Aerobic Oxidation of Alcohols into Aldehydes or Ketones with the M(NO3)/TEMPO/NaNO2 Catalytic System

For a typical procedure, HMF (126 mg, 1 mmol), Fe(NO3)3·9H2O (20.2 mg, 0.05 mmol), TEMPO (7.8 mg, 0.05 mmol), NaNO2 (3.4 mg, 0.05 mmol), and CH3CN (5 mL) were added to a 10 mL glass flask. The resulting mixture was stirred under the atmosphere of oxygen from a balloon at 50 °C until HMF disappeared (as monitored by TLC). Upon completion, the reaction was quenched by adding EtOAc. The catalyst was removed by washing with water. The aqueous phases were combined and extracted with EtOAc. The aqueous phase extracts were then combined and dried with MgSO4. The solvent was evaporated in vacuo to yield pure products.

Product Analysis

HMF and DFF were analyzed by HPLC 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. Other alcohols were analyzed using GC (Agilent, GC-7890A) equipped with a flame ionization detector and an Rtx-5 (30 m × 0.32 mm × 0.25 μm) capillary column and further confirmed by GC–MS. GC–MS spectra were performed using an Agilent Technologies 7890A GC system with an Agilent 5975 inert mass selective detector (EI) and an HP-5 MS column (0.25 mm × 30 m, film: 0.25 μm).