Au/Ag/Cu-Mixed Catalysts for the Eco-Friendly Oxidation of 5-Hydroxymethylfurfural and Related Compounds to Carboxylic Acids under Atmospheric Oxygen in Water

Abstract

A green method for the oxidation of alcohols to carboxylic acids was developed using a novel co-catalytic system based on gold, silver, and copper catalysts. This reaction system was conducted under atmospheric oxygen in water and mild conditions to selectively oxidize 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid, as a building block for polyethylene furanoate, which is a 100% bio-based, future alternative to the petroleum-based polyethylene terephthalate. Furthermore, various primary alcohols were conveniently oxidized to their corresponding carboxylic acids in up to quantitative yields.

Introduction

Aromatic dicarboxylic acids are important substrates in industry and are widely utilized as basic sources for plastics, pharmaceutical products, and pesticides. For example, terephthalic acid is used in large quantities for the production of polyethylene terephthalate (PET) as a representative polyester.¹ In the production of PET, ethylene glycol can be supplied from natural resources, whereas terephthalic acid is obtained from oil resources. Recently, marine pollution by plastic waste has led to serious environmental issues, and therefore, the production of polyester from biomass resources is of particular interest.² As an aromatic dicarboxylic acid derived from biomass, 2,5-furandicarboxylic acid (FDCA) has attracted growing attention. FDCA is expected to be produced by the oxidation of 5-hydroxymethylfurfural (HMF), which can be derived from a saccharide or fructose, by dehydration (Scheme 1).³

Scheme 1. FDCA Synthesis from Fructose.

Therefore, a polyester derived from FDCA and ethylene glycol, namely, polyethylene furanoate (PEF), can be considered a 100% biomass resin, and this resin is expected to be biodegradable. Furthermore, PEF has a higher gas barrier property; for example, its barrier properties toward O₂ and water vapor are 10 and 2 times higher than those of PET, respectively.⁴ In the context of green chemistry, the production of FDCA should ideally be eco-friendly. Thus, for the conversion of HMF to FDCA, the development of a catalytic oxidation process using molecular oxygen as a co-oxidant⁵ in water as the solvent⁶ would be ideal owing to the safety, abundance, and renewable nature of these resources.

Recently, we successfully developed a series of transition-metal-catalyzed or organocatalytic systems for the oxidation of alcohols⁷ and amines⁸ using molecular oxygen or aqueous hydrogen peroxide, wherein oxidovanadium complexes⁷ and copper sulfate,^8b8c were found to act as an oxidation catalyst even in water. In addition, we developed a Pt-C/AgO/CuO catalytic system, which catalyzes the oxidation of HMF to FDCA using O₂ in water.⁹

In addition to a series of Co/Mn/Br-mixed systems, which are widely used in the autoxidation of hydrocarbons to carboxylic acids, several metal catalyst systems, such as Pd-, Pt-, and Au-based systems, are known to catalyze the oxidation of HMF to FDCA.¹⁰ Considering the standard oxidation–reduction potentials (i.e., Cu, E⁺ = 0.340 V (Cu²⁺ + 2e^– → Cu); Ag, E⁺ = 0.7991 V (Ag⁺ + e^– → Ag); Pd, E⁺ = 0.915 V (Pd²⁺ + 2e^– → Pd); Pt, E⁺ = 1.188 V (Pt²⁺ + 2e^– → Pt); and Au, E⁺ = 1.52 V (Au³⁺ + 3e^– → Au)),¹¹ gold is a promising candidate catalyst for the green oxidation process. Thus, we herein report a novel combination of gold, silver, and copper oxides for the efficient and eco-friendly catalysis of the oxidation of HMF to FDCA in water under an oxygen atmosphere (eq 1).

1

The application of this method to the oxidation of benzylic alcohol derivatives to carboxylic acids is also described.

Results and Discussion

Initially, the oxidation of HMF in water under an O₂ atmosphere was examined using only the Ag₂O/CuO-mixed catalyst (Table 1, entry 1). In this reaction, the desired FDCA 2a was obtained in 10% yield, and the major product was 5-(hydroxymethyl)furan-2-carboxylic acid 3a (56%), which is likely formed by selective oxidation of the HMF formyl group to a carboxylic group. As the oxidation abilities of Ag₂O and CuO are not particularly high, the combination of Ag₂O/CuO and metal oxides exhibiting more powerful oxidation potentials is desirable to facilitate the eco-friendly oxidation reaction under mild conditions. Therefore, we examined the oxidation of HMF through the combination of several metal oxides with Ag₂O/CuO. However, metal oxides such as Co₃O₄, CeO₂, IrO₂, PdO, and ZnO were ineffective (entries 2–6, respectively). Interestingly, in the case of Au₂O₃, the desired FDCA 2a was obtained as the major product in 38% yield, along with 3a (entry 7). When AuCl₃ (0.05 mmol) was used instead of Au₂O₃, the yield of 2a was improved to 51% (entry 8). However, the use of a cationic gold complex, PPh₃AuNTf₂ (0.05 mmol), gave no improvement in the yield of 2a (entry 9).

Table 1. Optimization of the Combination of Metal Oxides^a.

		yield (%)
entry	catalyst	2a	3a
1	–b	10	56
2	Co3O4	5	57
3	CeO2	7	61
4	IrO2	9	46
5	PdO	10	30
6	ZnO	17	39
7	Au2O3	38	20
8	AuCl3 (0.05 mmol)	51	20
9	PPh3AuNTf2 (0.05 mmol)	13	7

^aYields were determined by ¹H NMR spectroscopy.

^bThe oxidation of 1a was conducted using a combination of only Ag₂O and CuO.

As AuCl₃ is hygroscopic and is difficult to handle, we subsequently attempted the optimization of reaction conditions using Au₂O₃, Ag₂O, and Cu catalysts under an O₂ atmosphere (Table 2). Doubling the amount of Ag₂O and CuO in comparison to the conditions of entry 1 effectively afforded 2a in 93% yield along with a trace amount of 3a (entry 2). Decreasing the gold catalyst loading to 0.013 mmol resulted in a lower yield of 2a likely owing to retarded conversion from 3a to 2a (entry 3). Using the conditions of entry 2, optimization of the copper catalysts was also investigated. More specifically, Cu(acac)₂, CuBr₂, and CuCl₂ were ineffective for the oxidation of 1a to 2a, and 3a was obtained as the major product (entries 4–6). In contrast, CuSO₄, Cu(NO₃)₂·3H₂O, and Cu(OAc)² were effective for the oxidation reaction (entries 7–10), and 2a was obtained in excellent yields (entries 8–10), wherein the use of Cu(OAc)₂ as the co-catalyst allowed the loading of Ag₂O to decrease to 0.11 mmol (entry 10). Although Cu(NO₃)₂·3H₂O and Cu(OAc)₂ exhibited excellent oxidation abilities in the mixed catalyst system, we selected CuO as the optimized copper catalyst from the viewpoints of safety and ease of handling.

Table 2. Optimization of the Reaction Conditions^a,13.

				yield (%)
entry	Au2O3 (mmol)	Ag2O (mmol)	Cu catalyst (mmol)	2a	3a
1	0.026	0.11	CuO (0.22)	38	20
2	0.026	0.22	CuO (0.44)	93	trace
3	0.013	0.22	CuO (0.44)	42	57
4	0.026	0.22	Cu(acac)2 (0.44)	12	48
5	0.026	0.22	CuBr2 (0.44)	12	55
6	0.026	0.22	CuCl2 (0.44)	45	54
7	0.026	0.22	CuSO4 (0.44)	73	12
8	0.026	0.22	Cu(NO3)2·3H2O (0.44)	93	trace
9	0.026	0.22	Cu(OAc)2 (0.44)	88	trace
10	0.026	0.11	Cu(OAc)2 (0.44)	95	trace
11	0.395			53	22
12		0.395		2	54
13			CuO (0.79)	6	45
14	0.026	0.395		5	85
15	0.026	0.79		40	53
16	0.026		CuO (0.44)	82	trace
17	0.013		CuO (0.44)	82	trace
18b	0.013		CuO (0.22)	91	trace

^aYields were determined by ¹H NMR spectroscopy.

^bReaction time: 32 h.

It is important to know the catalytic abilities of the individual metals consisting of the Au₂O₃/Ag₂O/CuO catalyst system, and therefore, several control experiments were conducted. When Au₂O₃ (0.395 mmol) was employed in the absence of both Ag₂O and CuO, 1a was mainly converted into 2a along with 3a in a moderate yield (entry 11). When Ag₂O (0.395 mmol) or CuO (0.79 mmol) was used independently, 3a was mainly obtained with trace amounts of 2a (entries 12 and 13). These results suggest that the gold catalyst may be involved in the oxidation of the hydroxymethyl group of 1a, whereas Ag₂O and CuO may contribute to oxidation of the formyl group of 1a. We also investigated how the combination of metal catalysts varied the product selectivity of the oxidation reaction. The use of Au₂O₃ (0.026 mmol) with Ag₂O (0.395 mmol) gave 3a and 2a in 85 and 5% yields, respectively, and a Ag₂O loading of 0.79 mmol improved the yield of 2a (entries 14 and 15). Based on these results and the standard oxidation–reduction potentials of these metals, the gold reactive species may be regenerated via oxidation by Ag₂O. In this process, the generated Ag⁰ may be stable under O₂, undergoing only minor oxidation to Ag⁺ by molecular oxygen.

Therefore, excess amounts of Ag₂O are necessary in the oxidation of 1a and 3a to 2a. Notably, the combination of Au₂O₃ and CuO efficiently catalyzed the oxidation of 1a and 3a to form 2a in 82% yield with an excellent selectivity (entry 16). This Au₂O₃/CuO-mixed catalytic system remained active even upon reducing the amount of Au₂O₃ to 0.013 mmol (entry 17). Interestingly, prolonging the reaction time to 32 h was also effective, and FDCA 2a was obtained from HMF in 91% yield (entry 18). These results suggest that the Au⁺ species generated in situ via the oxidation of 1a with Au₂O₃ could be reoxidized by CuO. The generated Cu⁺ species are then easily oxidized under O₂ to regenerate the Cu²⁺ species.¹² Compared with the results of entries 2 and 16 in Table 2, it appears that Ag₂O promotes the catalytic cycle, whereby the Cu⁺ species undergoes oxidation by O₂, while Ag₂O assists in the regeneration of the Au³⁺ species through the oxidation cycle (Scheme 2). We note here that many previously reported methods require high pressures of O₂ and high reaction temperatures, and the purification of FDCA 2a was difficult owing to the contamination with byproducts. In contrast, our method could be conducted under mild conditions in atmospheric oxygen, and the excellent product selectivity of FDCA 2a could lead to its practical application in the synthesis of PEF.

Scheme 2. Proposed Pathways for the Catalytic Oxidation of Alcohols.

With the optimized reaction conditions in hand (Table 2, entry 2), the scope of the Au/Ag/Cu-catalyzed oxidation reaction was further investigated using various primary alcohols (Table 3).¹⁴ As presented in Table 3, various heterocyclic alcohols (1a–1e) were converted to their corresponding carboxylic acids in almost quantitative yields (2a–2e, respectively). Furthermore, the Au/Ag/Cu-mixed catalyst system was applied in the oxidation of benzyl alcohols. More specifically, o-methyl, m-methyl, p-methyl, p-methoxy, p-nitro, and p-chlorobenzyl alcohols were successfully converted to their corresponding carboxylic acids (2f–2l) in excellent yields without the formation of aldehydes. The use of NaOH (2.5 mmol, 3.2 equiv) could successfully be reduced to 2.0 and 1.0 equiv for the oxidation of HMF 1a and benzyl alcohol 1f, respectively (see footnotes b and c in Table 3). However, the oxidation of p-t-butylbenzyl alcohol 1m gave 2m in only 30% yield owing to its low solubility in water. When 2-naphthalenemethanol 1n and aliphatic alcohols such as cyclohexanemethanol 1o, which are less soluble in water, were used as substrates, poor yields of 2n and 2o were obtained, respectively.

Table 3. Scope of Au/Ag/Cu-Catalyzed Oxidation of Alcohols^a.

^aYields were determined by ¹H NMR spectroscopy (isolated yields).

^bNaOH (1.58 mmol, 2.0 equiv), 60 °C, along with 22% yield of 3a.

^cNaOH (0.79 mmol, 1.0 equiv), 60 °C.

To examine the durability of the Au/Ag/Cu-mixed catalyst, recycling experiments were conducted for the HMF oxidation reaction (eq 2). In the first step, 1a was oxidized to 2a in good yield (93%). Subsequently, 1a (0.79 mmol) and base were added, and the second reaction was conducted under O₂ to give 2a in 93% yield. In the third step, 2a was formed in 73% yield along with 15% yield of 3a.

2

The catalysts could also be recycled by washing the reaction residue with H₂O. After the reaction (first cycle) was completed, H₂O was added to the reaction residue, and the centrifugation was carried out. The precipitate was collected and used as the catalyst for the second cycle. Therefore, the recovered catalyst showed a good catalytic ability toward oxidation of 1a to afford 2a in 96% yield along with 3a in 4% yield. Unfortunately, this catalyst recycling system was difficult to apply in the third cycle, and 3a was obtained as the main product (Table 4).

Table 4. Recycling Experiments of the Au/Ag/Cu-Mixed Catalyst for Oxidation of HMF^a.

	yield (%)
cycle	2a	3a
first	94	6
second	96	4
third	23	59

^aYields were determined by ¹H NMR spectroscopy.

To gain insight into the reaction pathway, additional experiments were conducted (Table 5). Aldehydes can be transformed to geminal diols in the presence of a base and H₂O.¹⁵ Thus, we considered that alcohols are first oxidized to aldehydes and then to carboxylic acids via the formation of geminal diol intermediates in the present oxidation reaction. However, it is difficult to isolate the geminal diols themselves. Therefore, we selected 2-(hydroxymethyl)benzaldehyde 1p as the substrate for the control experiment (Table 5). The aldehyde 1p can be synthesized and isolated as an equilibrium mixture of the free aldehyde 1p-I and the ring-closed hemiacetal 1p-II (1p-I/1p-II = 1:2).¹⁶ When an equilibrium mixture of 1p-I and 1p-II was oxidized in the presence of Au/Ag/Cu-mixed catalyst under atmospheric oxygen, 2p was successfully obtained in 70% yield (entry 1). The sole use of Au₂O₃ or Ag₂O^17,18 also afforded 2p in moderate yield (entries 2 and 3, respectively). In addition, when CuO was used as the catalyst, the oxidation of 1p did not proceed smoothly to produce 2p in only 4% yield.

Table 5. Oxidation of 2-(Hydroxymethyl)benzaldehyde to Phthalide^a,b.

entry	catalyst	yield 2p (%)
1	Au2O3 (0.026 mmol), Ag2O (0.22 mmol), CuO (0.44 mmol)	70
2	Au2O3 (0.026 mmol)	58
3	Ag2O (0.22 mmol)	31
4	CuO (0.44 mmol)	4
5c	Au2O3 (0.026 mmol), Ag2O (0.22 mmol), CuO (0.44 mmol)	19d

^a1p was an equilibrium mixture of the free aldehyde 1p-I and the ring-closed hemiacetal 1p-II (1p-I/1p-II = 1:2).

^bYields were determined by ¹H NMR spectroscopy.

^cBenzaldehyde was used instead of 1p.

^dThe yield of benzoic acid (2f).

In contrast, the oxidation of benzaldehyde, conducted under the same conditions as in entry 1, barely proceeded at all (entry 5). In the case of 1p, intramolecular cyclization of 1p-I proceeds smoothly to generate hemiacetal 1p-II, and the oxidation of 1p-II to phthalide 2p might be accelerated by the rapid isomerization of 1p-I to 1p-II. In contrast, the addition of H₂O to benzaldehyde was much slow in the absence of a base, which might result to the inefficient oxidation of benzaldehyde to benzoic acid. These results strongly suggest that the oxidation of aldehyde to carboxylic acid might proceed via the formation of hemiacetal 1p-II or geminal diols (generated from aldehydes and H₂O in the presence of the base) as key intermediates.

Based on the results of entries 11–18 in Tables 2 and 5, possible reaction pathways were constructed (Scheme 2). Initially, alcohol A is oxidized by Au³⁺ (Au₂O₃) to generate aldehyde B, which reacts with OH^– in the aqueous NaOH solution to form geminal diol C. gem-Diol C is oxidized by Au³⁺, Ag⁺, and/or Cu²⁺ to produce the corresponding carboxylic acid D.¹⁹ The Au⁺ formed in situ is also oxidized to regenerate Au³⁺ in the presence of Ag/Cu catalysts under molecular oxygen. In this case, the Au³⁺ species can be regenerated by the Cu catalyst with O₂.

It is noteworthy that the selective oxidation of HMF to FDCA could be attained using Au/C (0.89 wt % on activated carbon), instead of Au₂O₃ when using the Ag/Cu co-catalytic system (Table 6). Using 25 mg of Au/C with Ag₂O (0.11 mmol) and CuO (0.22 mmol) resulted in the oxidation of 1a to form 2a and 3a in 87 and 6% yields, respectively (entry 3). Furthermore, the oxidation of HMF using Au/C proceeded even in the absence of Ag₂O and afforded 2a in an excellent yield with a good FDCA selectivity (entry 4). Surprisingly, the amount of the gold catalyst could be decreased to 0.001 mmol in this reaction. For further decreasing the loading of the metal catalysts, the effects of the reaction temperature and time on the yields and the product selectivity were also investigated in detail. When the reaction was conducted using Au/C (25 mg) and CuO (0.10 mmol) at 60 °C for 24 h, the yield of 2a was 40% yield along with 3a in 7% yield (entry 5). The reaction at 80 °C did not improve the conversion of 1a to 2a and 3a (entry 6). Interestingly, when the reaction was conducted in the presence of Au/C (25 mg) and CuO (0.10 mmol) at 60 °C for 48 h, 2a was obtained in 59% yield with 87% selectivity (entry 7). Moreover, increasing the amount of CuO to 0.15 mmol improved the yield of 2a, and 2a was obtained in 71% yield with 97% selectivity (entry 8).

Table 6. Catalytic HMF Oxidation by Combination of Au/C and Ag/Cu^a.

						yield (%)
entry	Au/C (mg)b	Ag2O (mmol)	CuO (mmol)	temp. (°C)	time (h)	2a	3a
1	50	0.22	0.44	42	16	19	55
2	25	0.22	0.44	42	16	53	44
3	25	0.11	0.22	42	16	87	6
4	25		0.22	42	16	80	trace
5c	25		0.10	60	24	40	7
6c	25		0.10	80	24	36	13
7c	25		0.10	60	48	59	9
8c	25		0.15	60	48	71	2

^aYields were determined by ¹H NMR spectroscopy.

^b0.89 wt % Au on activated carbon.

^c1a (1.0 mmol), H₂O (1.0 mL), and NaOH (3.1 mmol) were used.

As for the oxidation of HMF, the total amount of the metal catalysts was successfully decreased (Table 6, entry 8). Therefore, we next examined the oxidation of some benzylic alcohols using the same catalyst loading as in Table 6, entry 8. As a result, alcohols (1f and 1j) could be transformed to the corresponding carboxylic acids in quantitative yields, respectively (eqs 3 and 4).

3

Conclusions

A green process for the oxidation of alcohols to carboxylic acids was developed using a novel co-catalytic system based on Au₂O₃, Ag₂O, and CuO in water in the presence of molecular oxygen under mild conditions. Herein, 5-hydroxymethylfurfural was selectively oxidized to 2,5-furandicarboxylic acid, as a building block of PEF, in an excellent yield. Although many previously reported methods require high pressures of O₂, high reaction temperatures, and/or tedious isolation operations due to less product selectivities, the present method could be conducted under mild conditions in atmospheric oxygen, with excellent product selectivity of FDCA. Furthermore, various primary alcohols were conveniently oxidized to their corresponding carboxylic acids in up to quantitative yields. Moreover, Au/C instead of Au₂O₃ was effective in this Au/Ag/Cu catalytic oxidation system.

We expect this novel catalytic system to aid in the development of new eco-friendly synthetic processes.

Experimental Section

General Remarks

Unless otherwise stated, all starting materials and catalysts were purchased from commercial sources and used without further purification. 2-(Hydroxymethyl)benzaldehyde 1p was prepared according to the previously reported procedure.¹⁶ All solvents were distilled and degassed with nitrogen before use. ¹H NMR spectra were recorded on a JEOL JNM-ECS400 (400 MHz) FT NMR system or a JEOL JNM-ECX400 (400 MHz) FT NMR system in CDCl₃ with Me₄Si as an internal standard. ¹³C{¹H} NMR spectra were recorded on a JEOL JNM-ECX400 (100 MHz) FT NMR or JEOL JNM-ECS400 (100 MHz) FT NMR system in CDCl₃.

General Procedure for the Au/Ag/Cu-Catalyzed Oxidation of Alcohols to Carboxylic Acids (Table 3)

Alcohol 1 (0.79 mmol), Au₂O₃ (11.5 mg, 0.026 mmol), Ag₂O (51.0 mg, 0.22 mmol), CuO (35.0 mg, 0.44 mmol), NaOH (100 mg, 2.5 mmol), and H₂O (0.75 mL) were added to a 10 mL two-neck flask equipped with an O₂ balloon at 25 °C and stirred at 42 °C under an O₂ atmosphere for 16 h. After this time, the resulting mixture was filtered and washed with H₂O (10 mL) and 1 M NaOH aq (1.0 mL). Then, 3 M HCl aq (4 mL) was added to the filtrate to precipitate the product. After concentrating the solvent under reduced pressure, the residue was dissolved in CH₃CN (10 mL, 2b–2k) or acetone (10 mL, 2h) and filtered over cotton. Finally, the filtrate was concentrated under reduced pressure to obtain the corresponding carboxylic acid 2.

2-Furancarboxylic Acid (2b)²⁰

White solid, 81.3 mg, 86%; ¹H NMR (400 MHz, CDCl₃): δ 11.68 (s, 1H), 7.66 (t, J = 0.9 Hz, 1H), 7.35–7.34 (m, 1H), 6.57 (q, J = 1.7 Hz, 1H); ¹³C{¹H} NMR (100 MHz, CDCl₃): δ 163.8, 147.6, 143.9, 120.3, 112.4.

2-Thiophenecarboxylic Acid (2c)²¹

White solid, 95.1 mg, 96%; ¹H NMR (400 MHz, CDCl₃): δ 11.97 (s, 1H), 7.91 (s, 1H), 7.65 (d, J = 4.6 Hz, 1H), 7.14 (s, 1H); ¹³C{¹H} NMR (100 MHz, CDCl₃): δ 168.1, 135.2, 134.2, 133.0, 128.2.

4-Bromothiophene-2-carboxylic Acid (2d)²²

Brown solid, 102.9 mg, 64%; ¹H NMR (400 MHz, CDCl₃): δ 7.79 (s, 1H), 7.55 (s, 1H); ¹³C{¹H} NMR (100 MHz, CDCl₃): δ 166.1, 137.1, 133.6, 131.3, 111.1.

Benzoic Acid (2f)²⁰

White solid, 91.9 mg, 92%; ¹H NMR (400 MHz, CDCl₃): δ 11.98 (s, 1H), 8.13 (d, J = 7.3 Hz, 2H), 7.61 (t, J = 7.3 Hz, 1H), 7.48 (t, J = 7.8 Hz, 2H); ¹³C{¹H} NMR (100 MHz, CDCl₃): δ 172.7, 133.9, 130.3, 129.4, 128.6.

2-Methylbenzoic Acid (2g)²³

White solid, 107.7 mg, 100%; ¹H NMR (400 MHz, CDCl₃): δ 12.13 (s, 1H), 8.08 (d, J = 7.3 Hz, 1H), 7.47–7.43 (m, 1H), 7.28 (t, J = 7.3 Hz, 2H), 2.67 (s, 3H); ¹³C{¹H} NMR (100 MHz, CDCl₃): δ 173.7, 141.5, 133.1, 132.1, 131.7, 128.5, 126.0, 22.3.

3-Methylbenzoic Acid (2h)²³

White solid, 109.6 mg, 100%; ¹H NMR (400 MHz, CDCl₃): δ 12.34 (s, 1H), 7.93 (d, J = 7.8 Hz, 2H), 7.42 (d, J = 7.8 Hz, 1H), 7.36 (t, J = 7.6 Hz, 1H), 2.42 (s, 3H); ¹³C{¹H} NMR (100 MHz, CDCl₃): δ 172.9, 138.4, 134.7, 130.8, 129.4, 128.5, 127.5, 21.3.

4-Methylbenzoic Acid (2i)²⁰

White solid, 106.8 mg, 99%; ¹H NMR (400 MHz, CDCl₃): δ 12.27 (s, 1H), 8.01 (d, J = 8.2 Hz, 2H), 7.27 (d, J = 8.2 Hz, 2H), 2.42 (s, 3H); ¹³C{¹H} NMR (100 MHz, CDCl₃): δ 172.6, 144.7, 130.4, 129.3, 126.7, 21.9.

4-Methoxybenzoic Acid (2j)²⁰

White solid, 128.2 mg, 100%; ¹H NMR (400 MHz, DMSO-d₆): δ 7.91 (d, J = 9.1 Hz, 2H), 7.03 (d, J = 9.1 Hz, 2H), 3.83 (s, 3H); ¹³C{¹H} NMR (100 MHz, DMSO-d₆): δ 167.5, 163.4, 131.9, 123.5, 114.3, 55.9.

4-Nitrobenzoic Acid (2k)²⁰

Yellow solid, 73.4 mg, 56%; ¹H NMR (400 MHz, DMSO-d₆): δ 8.35 (d, J = 8.2 Hz, 2H), 8.20 (d, J = 8.6 Hz, 2H); ¹³C{¹H} NMR (100 MHz, DMSO-d₆): δ 166.3, 150.5, 137.0, 131.2, 124.2.

4-Chlorobenzoic Acid (2l)²⁴

White solid, 114.9 mg, 94%; ¹H NMR (400 MHz, DMSO-d₆): δ 7.90 (d, J = 8.7 Hz, 2H), 7.51 (d, J = 8.2 Hz, 2H); ¹³C{¹H} NMR (100 MHz, DMSO-d₆): δ 167.0, 138.3, 131.7, 130.2, 129.2.

The production of 2a(10a) and 2e(25) was also determined and characterized from the reported ¹H and ¹³C{¹H} NMR data.

Recycle Utilization of Au/Ag/Cu-Mixed Catalyst for Oxidation of HMF to FDCA (First Cycle, Table 4)

5-Hydroxymethylfurfural 1a (0.79 mmol), Au₂O₃ (11.5 mg, 0.026 mmol), Ag₂O (51.0 mg, 0.22 mmol), CuO (35.0 mg, 0.44 mmol), NaOH (100 mg, 2.5 mmol), and H₂O (0.75 mL, the concentration of 1a: 132.8 mg/mL) were added to a 10 mL two-neck flask equipped with an O₂ balloon at room temperature and stirred at 42 °C under an O₂ atmosphere for 16 h. After this time, the resulting mixture was filtered and washed with H₂O (10 mL) and 1 M NaOH aq (1.0 mL). Then, 3 M HCl aq (4 mL) was added to the filtrate to precipitate the product. The yields of 2a and 3a were determined by ¹H NMR spectroscopy (DMSO-d₆; internal standard, 1,3,5-trioxane). After the reaction (first cycle) was completed, H₂O (5 mL) was added to the filtered residue, and the centrifugation was carried out. This procedure repeated twice, and the residue insoluble to H₂O was collected. This recovered catalyst was directly used for the second cycle without any further purification.

Au/Ag/Cu-Catalyzed Oxidation of 2-(Hydroxymethyl)benzaldehyde to Phthalide (Table 5, Entry 1)

2-(Hydroxymethyl)benzaldehyde 1p (0.79 mmol), Au₂O₃ (11.5 mg, 0.026 mmol), Ag₂O (51.0 mg, 0.22 mmol), CuO (35.0 mg, 0.44 mmol), and H₂O (0.75 mL, the concentration of 1p: 143.4 mg/mL) were added to a 10 mL two-neck flask equipped with an O₂ balloon at room temperature and stirred at 42 °C under an O₂ atmosphere for 16 h. After this time, the resulting mixture was filtered and washed with H₂O (10 mL). The filtrate was concentrated under reduced pressure. The yield of phthalide 2p was determined by ¹H NMR spectroscopy using 1,3,5-trioxane as the internal standard.²⁶

General Procedure for the Au/C and CuO-Catalyzed Oxidation of Alcohols to Carboxylic Acids (Eq 2)

Alcohol 1 (1.0 mmol), Au/C (25 mg, 0.89 wt % Au on activated carbon), CuO (11.9 mg, 0.15 mmol), NaOH (3.1 mmol), and H₂O (1.0 mL) were added to a 10 mL two-neck flask equipped with an O₂ balloon at room temperature and stirred at 60 °C under an O₂ atmosphere for 48 h. After this time, the resulting mixture was filtered and washed with H₂O (10 mL) and 1 M NaOH aq (1.0 mL). HCl aq (3 M, 4 mL) was then added to the filtrate to precipitate the product. After concentrating the solvent under reduced pressure, the residue was dissolved in CH₃CN (10 mL) and filtered over cotton. Finally, the filtrate was concentrated under reduced pressure to obtain the corresponding carboxylic acid 2.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c05526.

• Reaction optimization and copies of ¹H NMR and ¹³C{¹H} NMR spectra (PDF)

The authors declare no competing financial interest.