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Synlett 2024; 35(20): 2409-2416
DOI: 10.1055/a-2384-6371
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Special Issue to Celebrate the 75th Birthday of Prof. B. C. Ranu

NiCo2O4-Nanoparticle-Catalyzed Microwave-Assisted Dehydrogenative Direct Oxidation of Primary Alcohols to Carboxylic Acids under Oxidant-Free Conditions

Kumari Anchal
,
Ashok R. Patel
,
Subhash Banerjee  ∗

Authors are pleased to acknowledge the Department of Science and Technology, Ministry of Science and Technology, India (DST-FIST, F.No. SR/FIST/CSI-264/2015(C), Dt 26.05.2016) for providing funding to establish instrument facilities.
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Dedicated to the honorable Prof. Brindaban C. Ranu

Abstract

Here, we report the NiCo2O4-nanoparticle-catalyzed dehydrogenative direct oxidation of primary alcohols to carboxylic acid in the presence of KOH under microwave irradiation in the absence of any oxidant in good to excellent yields (75–99%) within a short reaction time (5–10 min). The polycrystalline cubic spinel phase of NiCo2O4 nanoparticles (NPs) with an average size of 25 nm were synthesized by the co-precipitation method and analyzed properly by using powder X-ray diffraction, field emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, and transmission electron microscopy measurements. The NiCo2O4 NPs were stable under the reaction conditions and reused for up to eight cycles without appreciable loss in the yield of benzoic acid. According to mechanistic insight, the KOH acts as a second oxygen source and is essential for the synthesis of carboxylic acid from alcohols. The hydrogen gas was found to be the only byproduct of this method detected by chemical reactions.


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Keywords

nano-NiCo2O4 - dehydrogenative oxidation - direct oxidation of primary alcohols - carboxylic acids - green synthesis - recyclability

The direct oxidation of primary alcohols to the corresponding carboxylic acids is one of the most important and useful reactions in functional group transformations.[1] [2] [3] Traditional methods involve the use of stoichiometric or excess amounts of reagents[4–8] or transition-metal-based homogeneous catalysts in combination with oxidants.[9–20] These methods have serious drawbacks such as the use of toxic reagents/oxidants/co-oxidants, and expensive noble-metal-based homogeneous catalysts which are expensive and are not reusable.

In this connection, acceptorless dehydrogenative oxidation of primary alcohols under oxidizing agent-free conditions has received significant attention after a report by Milstein et al. using a Ru-NNP pincer complex.[21] Later on, Sawama et al. reported a synthetic protocol using Pd/C/NaOH under reduced pressure (800 hPa).[22] Despite the progress in this era, the active metal cores of these catalysts focus on precious, expensive, and toxic noble metals, especially ruthenium, palladium, and rhodium for the direct conversion of primary alcohols into carboxylic acids.[23`] [b] [c] [d] [e] [f] [g] [h] Thus, the introduction of a robust, cost-friendly, and easily accessible heterogeneous catalyst[23i–m] for the dehydrogenative oxidation of primary alcohols to carboxylic acids is highly desirable in the context of an environmental and industrial point of view.

Recently, spinel structures (AB2O4) nanomaterials having binary and ternary mixtures of metal oxides have been established as promising redox catalysts.[24] Among others, spinel nickel cobaltite (NiCo2O4) exhibits superior redox properties and electrical conductivity compared to its individual components nickel oxide and cobalt oxide, and is extensively used as supercapacitor and charge storage materials.[25] Further, NPs exhibit unique and interesting catalytic properties due to their small sizes, larger surface-to-volume ratio, and reactive surface areas compared to bulk counterparts.

As a part of our continuous interest in nanocatalysis,[26] recently, we have demonstrated the catalytic activity of CuCo2O4 in the direct oxidative azo coupling of anilines.[27a] Further, Co3O4 NPs were employed in the synthesis of terahydrobenzo[b]pyrans and 2-aryl benzothiazoles.[27b] However, to the best of our knowledge the catalytic activities of NiCo2O4 NPs in organic transformations have not been investigated to date. Herein, we report NiCo2O4-NPs-catalyzed dehydrogenative oxidation of primary alcohols to carboxylic acids under microwave-irradiation conditions (Scheme [1]).

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Scheme 1 Previous and present methods for the dehydrogenative oxidation of primary alcohols to carboxylic acids

Initially, we prepared NiCo2O4 by co-precipitation method. Briefly, 20.0 mmol (4.7586 g) of cobalt chloride (CoCl2·6H2O) was dissolved in 500 mL double-distilled water and heated until the solution became crimson red color. Next, 10.0 mmol (1.295 g) nickel chloride (NiCl2) was added, and the color of the resulting solution turned pink. Subsequently, NaOH solution (5 M) was added drop by drop to the resulting solution with constant stirring to maintain pH ca. 10. Then, the blue precipitate was formed and the whole solution gradually turned into grey. Thereafter, it was left for 2 h with stirring. After that, the solution was made alkali-free (pH 6.0–7.0) by washing again and again with distilled water and then the precipitate was filtered and dried in a hot air oven overnight at 80 °C. Finally, the solid material was grinded and annealed at 400 °C for 3 h (Scheme [2]). The obtained black material was analyzed by different analytical techniques.

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Scheme 2 Schematic representation of NiCo2O4 preparation via co-precipitation method

In order to study the structural properties and to identify the phase of samples, powder X-ray diffraction (XRD) measurements were carried out using D8 Advance Bruker with Cu Kα source. All diffraction patterns were recorded with 2θ ranging from 10° to 70° at a low scan rate of 0.5°/min. The XRD patterns of the NiCo2O4 powder sample are shown in Figure [1a]. The diffraction peaks are observed at 2θ values of 18.77°, 31.08°, 36.86°, 38.66°, 44.63°, and 74.55°, corresponding to the diffraction plane of (111), (220), (311), (222), (400), and (533) directions, respectively. These diffraction peaks correspond to the polycrystalline cubic spinel phase of NiCo2O4 (JCPDS Card No. 20-0781).[28] The spectrum suggesting more intense and strong growth of plane in the (311) and (400) directions. This could possibly be due to the slow heating rate during the anneling process which allows any crystal to grow in a particular direction. No other phase corresponding to Ni or Co elements was found, which indicates the formation of pure NiCo2O4. However, the spectrum exhibits more intense and strong growth of planes in the (311) and (400) directions, this could be possible due to the slow heating rate which allows any crystal to grow in a particular direction. The Debye–Scherrer formula was used to determine the crystallite size D, which is expressed as D = 0.89λ/βcosθ, where λ is the wavelength, approximately 1.54 Å, β is peak width at half maxima, and θ is the Bragg diffraction angle. The XRD peak at 36.86° in Figure [1a] corresponds to an average particle size of NiCo2O4, which is approximately 28 nm. This measurement is in close agreement with those obtained through TEM analysis, which yielded a value of ca. 25 nm.

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Figure 1 The (a) powder XRD pattern, (b) FE-SEM image, (c) EDX spectra, and (d) TEM image of NiCo2O4 nanomaterial (average size ca. 25 nm).

The field emission scanning electron microscopy (FE-SEM) measurements were performed to study the morphology of the particles using a JSM-7610FPlus Schottky field emission scanning electron microscope with an energy of 5 keV. Figure [1b] shows the FE-SEM image of the NiCo2O4 sample. The formation of nano-sized nearly spherical NiCo2O4 material (size ca. 24–30 nm) is evident from the FE-SEM image. The elements present in the samples were detected by the energy-dispersive X-ray spectroscopy (EDX) measurements using 15 keV energy. The collected EDX spectrum of the NiCo2O4 samples is shown in Figure [1c]. The EDX spectra show the presence of dispersive peaks corresponding to the elements C, O, Co, Ni, and Pt (i.e., due to Pt coating during SEM measurement), and no other elements are detected within the statistical limits of counts, which represent the high purity of the sample.[29] Further, the morphology of the materials was confirmed by transmission electron microscopy (TEM) measurement. The TEM image shown in Figure [1d] demonstrates the formation of nearly spherical and uniform NPs of an average particle size of ca. 25 nm which is comparable with the particle size obtained from FE-SEM measurement (Figure [1b]).

The crystallographic structure of NiCo2O4 obtained from Vesta software is presented in Figure [2].

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Figure 2 Crsytallographic structure of NiCo2O4

Table 1 Optimization of Reaction Conditions for Dehydrogenative Oxidation of Benzyl Alcohola

Entry

Catalyst (mg)

Base (equiv)

Solvent

Temp (°C)

Time

Yield (%)b

A

B

 1

NiCo2O4 (50)

KOH (3.0)

PhMe

R.T.

12 h

–

–

 2

NiCo2O4 (50)

KOH (3.0)

PhMe

120

10 h

–

96

 3

NiCo2O4 (50)

–

PhMe

120

12 h

–

–

 4

NiCo2O4 (40)

KOH (3.0)

PhMe

120

12 h

–

93

 5

NiCo2O4 (30)

KOH (3.0)

PhMe

120

12 h

–

92

 6

NiCo2O4 (20)

KOH (3.0)

PhMe

120

12 h

–

82

 7

NiCo2O4 (30)

KOH (2.0)

PhMe

120

12 h

–

92

 8

Co3O4 (30)

KOH (2.0)

PhMe

120

12 h

10

–

 9

CuCo2O4 (30)

KOH (2.0)

PhMe

120

12 h

62

–

10

ZnCo2O4 (30)

KOH (2.0)

PhMe

120

12 h

74

–

11

NiCo2O4 (30)

KOH (2.0)

MeCN

 80

12 h

–

68

12

NiCo2O4 (30)

KOH (2.0)

DMF

120

12 h

–

82

13

NiCo2O4 (30)

KOH (2.0)

DMSO

120

12 h

–

88

14

NiCo2O4 (20)

NaOH (2.0)

PhMe

120

12 h

–

58

15

NiCo2O4 (30)

KOH (2.0)

PhMe

MW/120

 5 min

–

86

16

NiCo2O4 (30)

KOH (2.0)

PhMe

MW/120

10 min

–

99

17

NiCo2O4 (30)

KOH (2.0)

PhMe

MW/100

10 min

–

90

a Conditions: benzyl alcohol (1.0 mmol), catalyst, base, solvent (2.0 mL).

b Isolated yield after workup with 1 M HCl.

Next, the catalytic activity of well-characterized NiCo2O4 NPs was investigated in the dehydrogenative oxidation of primary alcohols. We started our investigation by taking benzyl alcohol as a model substrate and optimized the reaction conditions. The results are presented in Table [1]. At first, a mixture of benzyl alcohol (1.0 mmol), NiCo2O4 (50 mg), and KOH (3.0 equiv.) was stirred at room temperature for 12 h in 2 mL of toluene (PhMe, entry 1, Table [1]) and no reaction was observed. However, with increasing temperature the yield of the model reaction was increased gradually and an excellent yield of benzoic acid (96%) was obtained at 120 °C after acid workup (entry 2, Table [1]). The formation of benzoic acid is confirmed by melting point determination (observed 120–122 °C; reported 122 °C) and 1H NMR spectroscopic studies.

To investigate the effect of base, the reaction was performed without KOH. However, only the starting material benzyl alcohol was isolated even after at 120 °C for 12 h (entry 3, Table [1]), which confirmed the role of base is essential in this reaction. Next, we optimized the amount of catalyst and base (entries 4–7, Table [1]), and it was found that 30 mg of the catalyst (entry 5, Table [1]) and 2.0 equivalents of the KOH (entry 7, Table [1]) are sufficient for the conversion of benzyl alcohol into benzoic acid in excellent yield (entry 6, Table [1]). Next, we have compared the catalytic activity of NiCo2O4 with other cobaltite materials in the dehydrogenative oxidation of benzyl alcohol to benzoic acid. The reaction with Co3O4, CuCo2O4, and ZnCo2O4 material produced a trace amount, 62%, and 74% of benzaldehyde (entries 8–10, Table [1]). The effect solvent was screened and it was observed that toluene was the best choice of solvent over others tested in this study (entries 11–13, Table [1]). The replacement of KOH by NaOH produced a lower yield of the benzoic acid (entry 14, Table [1]). Interestingly, when the model reaction was performed under microwave-irradiation (power 100 W) rather than conventional heating, 86% benzoic acid was produced within 5 min at 120 °C (entry 15, Table [1]), and within 10 min benzyl alcohol converted quantitatively into benzoic acid in 99% isolated yield (entry 16, Table [1]). However, lowering the temperature under microwave-irradiation conditions lowered the yield (entry 17, Table [1]). Thus, microwave irradiation with a power of 80 W of a mixture of benzyl alcohol (1.0 mmol), NiCo2O4 (30 mg), KOH (2.0 equiv.), and PhMe (2 mL) and microwave irradiation at 120 °C with a power of 100 W for 10 min are established as optimized reaction conditions (entry 16, Table [1]).

Under the optimal reaction conditions, several primary alcohols were tested to explore the substrate scope and functional group tolerance. The results are presented in Scheme [3]. The benzyl alcohol derivatives having various substituents at ortho and para position in the aromatic ring such as 4-Me, 2-Me, 4-i-Pr, 4-OMe, 2-OMe, NH2, 4-Br, 4-Cl, 2,4-dichloro, 2-NH2, 4-Cl, etc. are well tolerated under these reaction conditions.

Zoom Image
Scheme 3 Scope of NiCo2O4-catalyzed chemoselective oxidation of benzyl alcohols. Reagents and conditions: benzyl alcohol (1.0 mmol), catalyst (30 mg), base (2.0), solvent (2.0 mL), MW (100 W). The yields refer to the isolated pure products obtained after workup with HCl (1 M), TOF (min–1).

Furthermore, polynuclear aromatic alcohols such as 2-naphthalenemethanol and 9-anthracenemethanol were efficiently transformed into their corresponding carboxylic acids with only a slight decrease in yield, which may be due to steric effects. Moreover, heteroaryl derivatives of benzyl alcohol like 3-pyridinemethanol (2o) and 3-thiophenemethanol (2p) were also successfully converted into their respective carboxylic acids. However, a comparatively lower yield was obtained in these conversions, which may be attributed to the electronic effect. Allylic alcohol such as cinnamyl alcohol was also oxidized to the corresponding cinnamic acid (2q). However, a comparatively lower yield was obtained. Further, aliphatic primary alcohol, e.g. n-butanol, oxidized to butanoic acid (2r).

All the reactions are fast (5–10 min) and high-yielding (70–99%). The reactions were compared under conventional heating conditions to those of microwave conditions. Under conventional heating conditions reactions also produced good to excellent yields of products at 120 °C in toluene, however, a longer time (12 h) is required to complete the reactions. The stability and reusability of nano-NiCo2O4 were investigated by choosing the oxidation of benzyl alcohol into benzoic acid as a model reaction. The results are presented in Figure [3].

Zoom Image
Figure 3 Study of reusability of the nano-NiCo2O4 using oxidation of benzyl alcohol as a model reaction

After each run, the catalyst was recovered by centrifugation, washed with methanol, dried in an oven at 80 °C for 6 h, and reused for subsequent runs. It was observed that the catalyst remained stable and active for up to eight runs. However, after the fifth cycle onwards a little loss in yield of the product was observed. This could possibly be due to the loss of catalyst during the recycling process.

To investigate the mechanistic insights, a series of experiments were performed. The yield of benzoic acid significantly decreased by lowering the amount of KOH (Scheme [4a]), indicating the crucial role of KOH to complete oxidation. The reaction using Co3O4 produced only 10% of benzaldehyde (Scheme [4b]), indicating the Ni center plays a crucial role in dehydrogenative oxidation. Further, when the oxidation of benzyl alcohol was performed in the presence of nitrobenzene in a closed vessel (Scheme [4c]), a good yield of aniline was obtained which indicates the liberation of H2 gas during the course of the reaction.[29] Again, performing the same experiment in an open vessel (Scheme [4d]), poor conversion of nitrobenzene into aniline was observed, indicating the low availability of the liberated H2 from the reaction, most probably due to H2 gas coming out from the vessel. Additionally, the liberation of H2 gas in the oxidation benzyl alcohol was assessed by performing the reaction in the presence of phenylacetylene in a closed vessel (Scheme [4e]), which converted phenylacetylene into styrene as a co-product with acid.

Zoom Image
Scheme 4 Mechanistic study for the dehydrogenative oxidation of alcohol to acid

Based on the results of control experimental and previously reported dehydrogenative oxidation of alcohols,[30] a plausible mechanism is proposed in Scheme [5]. Initially, benzyl alcohol interacts with the Ni center of NiCo2O4 resulting in O–H activation which leads to the formation of alkoxy-ligated NiCo2O4 which provides a salt of alcohol and water which further gives aldehyde and H2 gas. Next, aldehyde interacts again with the base to form the first intermediate potassium hydroxy(phenyl)methanolate that further interacts with KOH and changes into the second intermediate, Ni-coordinated phenylmethanebis(olate). Afterward, the second intermediate interacts with the base and produces potassium benzoate, KOH, and H2 gas. Finally, potassium benzoate is worked up with HCl to get the desired product benzoic acid.

Zoom Image
Scheme 5 A plausible mechanism for the NiCo2O4-catalyzed acceptorless direct oxidation of benzyl alcohol to benzoic acid

Finally, the heterogeneity of the NiCo2O4 NPs was investigated by leaching study through a hot filtration test. The NiFe2O4 NPs catalyst was separated from the reaction mixture after 2 min using ultracentrifugation under hot conditions (conversion ca. 30%). The remaining filtrate was continued to stir under the same conditions for an additional 8 min. The progress of the reaction was monitored at every 2 min. interval. However, no improvement in the yield of the product was observed after the catalyst was separated from the reaction mixture. The results presented in Figure [4] demonstrate that the NiFe2O4 NPs were stable at the reaction conditions and that there was no leaching of metal content from the NPs. On the other hand, the morphology of recycled material remains intact as observed from the FE-SEM image of recycled NiFe2O4 NPs after the 8th run (Figure [4b]).

Zoom Image
Figure 4 Results of the leaching study by hot-filtration test performed with (a) complete run (black line), (b) filtrate removed after 2 min (red line) for the dehydrogenative oxidation of benzyl alcohol to benzoic acid.

In summary, we have fabricated a polycrystalline cubic spinel phase with nearly uniform sized (ca. 25 nm) spherical NiCo2O4 NPs and demonstrated its catalytic performance in the dehydrogenative oxidation of primary alcohols into carboxylic acids under microwave irradiation.[31] The NiCo2O4 NPs efficiently furnished dehydrogenative oxidation of various benzylic, allylic, and aliphatic primary alcohols in good to excellent yields (75–99%) to the corresponding acids with good functional group tolerance. Here, reactions were carried out using reusable and inexpensive NiCo2O4 NPs under microwave-irradiation conditions, satisfying the criteria of green chemistry. By mechanistic study, it was noted that the mild base KOH used in this method plays a key role as the source of second oxygen carboxylic acid, and two moles of hydrogen gas were released in this method, proved by conversion of nitrobenzene to aniline. Most importantly, we aimed to perform dehydrogenative oxidation of primary alcohols into carboxylic acids with a nonhazardous by-product catalyzed by a cost-effective transition-metal catalyst under a green synthetic tool. Further, this is the first example of the application of NiCo2O4 nanomaterial in any organic transformations.


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Conflict of Interest

The authors declare no conflict of interest.

Supporting Information

    Supporting information for this article is available online at https://doi-org.accesdistant.sorbonne-universite.fr/10.1055/a-2384-6371.
  • Supporting Information

Corresponding Author

Subhash Banerjee
Department of Chemistry
Guru Ghasidas Vishwavidyalaya, Koni, Bilaspur, PIN 495009, Chhattisgarh
India   

Publication History

Received: 19 June 2024

Accepted after revision: 12 August 2024

Accepted Manuscript online:
12 August 2024

Article published online:
09 September 2024

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Zoom Image
Scheme 1 Previous and present methods for the dehydrogenative oxidation of primary alcohols to carboxylic acids
Zoom Image
Scheme 2 Schematic representation of NiCo2O4 preparation via co-precipitation method
Zoom Image
Figure 1 The (a) powder XRD pattern, (b) FE-SEM image, (c) EDX spectra, and (d) TEM image of NiCo2O4 nanomaterial (average size ca. 25 nm).
Zoom Image
Figure 2 Crsytallographic structure of NiCo2O4
Zoom Image
Scheme 3 Scope of NiCo2O4-catalyzed chemoselective oxidation of benzyl alcohols. Reagents and conditions: benzyl alcohol (1.0 mmol), catalyst (30 mg), base (2.0), solvent (2.0 mL), MW (100 W). The yields refer to the isolated pure products obtained after workup with HCl (1 M), TOF (min–1).
Zoom Image
Figure 3 Study of reusability of the nano-NiCo2O4 using oxidation of benzyl alcohol as a model reaction
Zoom Image
Scheme 4 Mechanistic study for the dehydrogenative oxidation of alcohol to acid
Zoom Image
Scheme 5 A plausible mechanism for the NiCo2O4-catalyzed acceptorless direct oxidation of benzyl alcohol to benzoic acid
Zoom Image
Figure 4 Results of the leaching study by hot-filtration test performed with (a) complete run (black line), (b) filtrate removed after 2 min (red line) for the dehydrogenative oxidation of benzyl alcohol to benzoic acid.