Green Catalytic Conversion of Some Benzylic Alcohols to Acids by NiO₂ Nanoparticles (NPNPs) in Water

Abstract

The aqueous basic systems NiSO₄·6H₂O/K₂S₂O₈ (pH = 14) and NiSO₄·6H₂O/KBrO₃ (pH = 11.5) were investigated for the catalytic conversion of benzyl alcohol and some para-substituted benzyl alcohols to their corresponding acids in 75–97% yields at room temperature. The active species was isolated and characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray powder diffraction, EDX and FT-IR techniques, and identified as comprising NiO₂ nanoparticles (NPNPs). The SEM and TEM images of the Ni peroxide samples showed a fine spherical-like aggregation of NiO₂ molecules with a nearly homogeneous partial size and confirmed the aggregation’s size to be in the range of 2–3 nm. The yields, turn over (TO) and turn over frequencies (TOF) were calculated. It was noticed that the aromatic alcohols containing para-substituted electron donation groups gave better yields than those having electron withdrawing groups. The optimum conditions for this catalytic reaction were studied using benzyl alcohol as a model. The mechanism of the catalytic conversion reaction was suggested, in which the produced NPNPs convert alcohols to acids in two steps through the formation of the corresponding aldehyde. The produced NiO because of this conversion is converted again to NPNPs by the excess of K₂S₂O₈ or KBrO₃. This catalytic cycle continues until all of the substrate is oxidized.

1. Introduction

The green protocol for the production of valuable chemicals is of great significance in the sustainable development of green economy [1]. The oxidation of benzyl alcohols to aromatic acids using green routes is very important to avoid bad effects on the environment [2]. The production of aromatic acids is important in organic synthesis, since it is used as an intermediate in the synthesis of many fine chemicals such as drugs and vitamins [3]. The use of stoichiometric amounts of inorganic transition metal salts such as copper (I) [4], copper(II) [5] permanganate [6], chromium(VI) [7] and iron(III) [8,9] is often toxic and generates a considerable amount of by-product. The use of precious metals such as Pt [10], Pd [11] and Ru [12] is industrially restricted due to their high cost and scarcity. The extremely small size of nanoparticles enlarges the surface area and allows more reactions to occur, displaying high catalytic activity [13,14,15,16,17]. Recently, benzoic acid was synthesized via the pyrolysis of xylan to toluene followed by the subsequent catalytic oxidation of toluene to benzoic acid by the bimetallic catalyst Co₂MnO₄@MCM-41 [18]. Oroujzadeh et al. [19] reported the catalytic oxidation of benzyl alcohol to benzoic acid by the [CuClL₂] (L = N-nicotinyl;N-N″-bis(tert-butyl) phosphoric triamide) complex at 78 °C in aceto-nitrile as a solvent. A mixture of benzaldehyde and benzoic acid was obtained from the catalytic oxidation of benzyl alcohol by the potassium salt of the Keggin heteropolyacids K₅PW₁₁NiO₃₉ in the presence of H₂O₂ [20]. To the best of my knowledge, few reports have been on for the applications of nickel peroxide in the catalytic oxidation of organic compounds. Goerge et al. reported a good review on the use of nickel peroxide for the oxidation of many organic compounds, including alcohols, phenols and amines [21]. The synthesis of diacetone-2-ketoL-gulonic acid, an intermediate in the synthesis of vitamin C, was reported by Weijlard via the addition of nickel salts in a solution of sodium hypochlorite [22]. Nakgawa et al. reported on the use of an aqueous solution of nickel-sulfate-treated sodium hypochlorite in an alkaline medium for the stoichiometric oxidation of primary alcohols to their corresponding acids and to their corresponding carbonyl compounds in an organic solvent [23], and also on the synthesis of 2-phenylbenzoxazoles from their corresponding Schiff bases [24,25]. As part of our ongoing interest in the catalytic oxidation of organic compounds by transition metal–oxo complexes [26], we decided to study the catalytic oxidation of a number of aromatic alcohols to their corresponding acids catalyzed by the reagent NiSO₄·6H₂O/K₂S₂O₈ or KBrO₃ in an aqueous basic medium. A number of factors were studied to optimize the reaction conditions. The active species was isolated and characterized as comprising nickel peroxide nanoparticles (NPNPs). A plausible mechanism for this catalytic reaction was suggested.

2. Results and Discussion

2.1. Synthesis of the Catalyst

The nickel peroxide nanoparticles were synthesized using the co-precipitation method by mixing two equal amounts of nickel sulfate and potassium persulfate in the presence of four equivalent parts of KOH in water according to Equation (1):

NiSO₄·6H₂O + 4KOH + K₂S₂O₈ → NiO₂ + 3K₂SO₄ + 8H₂O

(1)

K₂S₂O₈ and KBrO₃ act as co-oxidants and react with NiSO₄·6H₂O in the presence of KOH as a basic medium to generate NiO₂, for example in Equation (1). The black nickel peroxide NPNPs were filtered, washed with H₂O and dried at 60 °C for three hours.

2.2. Characterization of Nickel Peroxide Nanoparticles (NPNPs)

2.2.1. FTIR Spectrum of NiO₂ Nanoparticles (NPNPs)

The FT-IR spectrum showed several significant absorption peaks (Figure 1). The absorption bands in the range of 500–550 cm⁻¹ were assigned to the NiO stretching vibrational mode [27,28]. The weak absorption band at around 720 cm⁻¹ is characteristic of the peroxo O-O stretching vibrational mode [29]. It was noticed that a broad absorption band and a weak band at 3405 cm⁻¹ and 1630 cm⁻¹ were attributable to O–H and H-O-H bending and stretching vibrational modes, respectively [30].

These bands were due to the hydration of the FTIR sample disk when it was prepared in open air. Other bands were observed in the range of 1070–1450 cm⁻¹; these bands were assigned to the absorbed carbon dioxide by the sample disk [31].

2.2.2. EDX Analysis

The stoichiometry of the as-synthesized Ni peroxide (NiO₂) powder was examined using energy-dispersive spectroscopy (EDS), as shown in Figure 2. The atomic weight ration of nickel to oxygen (Ni/O) was found to be 1.96, which was in good agreement with the theoretically calculated value of 1.83 [32]. The results reported by Kooti et al. [28] showed that the experimental obtained values of Ni/O were 1.73 and 3.67 for NiO₂ and NiO, respectively. The present results extracted from the EDS confirmed the good stoichiometry of the prepared Ni peroxide.

2.2.3. XRD Analysis

The XRD pattern of the NiO₂ powder is shown in Figure 3. The two broad peaks around 2θ ~ 17.6° and 37.2° with the superimposed sharp peaks indicate polycrystalline ultra-nano-sized particles of NiO₂. The peaks positioned at 2θ ≈ 37.8° and 43.9 can be assigned to the (111) and (200) crystal planes, respectively, (JCPDS 47-1049) [33]. The average crystallite size of the NiO₂ powder under investigation was found to be 1.3 nm. The obtained average crystallite size suggests the synthesized NiO₂ to comprise ultra-small nanoparticles. Earlier, Seguin et al. [32] reported and confirmed the monoclinic crystal system of NiO₂. The average crystallite size (L) of NiO₂ powder can be estimated by employing the following Sherres’s equation [33,34,35]:

$$\mathrm{L}=\frac{k\lambda }{\beta \cos (\mathsf{\theta })}$$

where k = 0.95 is the wavelength of the used X-ray, θ is the full width at half maximum of the diffraction peak measured in radians, and θ is the Bragg’s angle.

2.3. SEM and TEM

The SEM images of NiO₂ samples are presented in Figure 4a,b. The SEM image in Figure 4a shows fine spherical-like aggregations of NiO₂ molecules with a nearly homogeneous particle size. The TEM image in Figure 4b confirms the aggregations’ size to be in the range of 2–3 nm. This result confirms the ultra-small size of the NiO₂ nanoparticles obtained using Sherrer’s equation. On the other hand, the electron diffraction pattern (EDP) in the inset of Figure 4b confirms the fine-sized polycrystalline phase of the NiO₂.

2.4. Catalytic Oxidation of Alcohols

We decided to investigate the catalytic activity of nickel peroxide nanoparticles (NPNPs) generated in situ from nickel salts such as NiSO₄·6H₂O, NiCl₂·6H₂O, and Ni(CH₃COO)₂·2H₂O in the presence of K₂S₂O₈ in 1.0 M aqueous KOH or KBrO₃ in 1.0 M aqueous K₂CO₃ for the catalytic conversion of benzyl alcohol and some p-substituted benzyl alcohols containing electron-withdrawing and electron-donating groups to their corresponding acids. Additionally, we also investigated the optimum conditions for conducting this catalytic reaction (Scheme 1).

Benzyl alcohol was selected as a model substrate of NiSO₄·6H₂O for the in situ generation of nickel peroxide nanoparticles (NPNPs), which were the active catalytic species in this reaction, with two co-oxidants K₂S₂O₈ in 1.0 M KOH (pH = 14) and KBrO₃ in 1.0 M K₂CO₃ (pH = 11.5) as co-oxidants.

A set of experiments were carried out using two different nickel salts instead of NiSO₄·6H₂O with two different co-oxidants additionally, and two experiments were performed at 50 °C and 80 °C. The results are presented in

Table 1. Optimization of the reaction conditions.

Entry	Co-Oxidant	Y (%)	TO	TOF (h−1)
1	K2S2O8	97	48.5	12.13
	KBrO3	75	36	9
2	K2S2O8	85	42.5	10.63
	KBrO3	60	30	7.5
3	K2S2O8	81	40.5	10.13
	KBrO3	63	31.5	7.9
4	K2S2O8	70	35	8.75
	KBrO3	54	27	6.75
5	K2S2O8	50	25	6.25
	KBrO3	50	25	6.25
6	K2S2O8	0	0	0
	KBrO3	0	0	0
7	K2S2O8	80	40	10
	KBrO3	70	35	8.75
8	K2S2O8	76	38	9.5
	KBrO3	55	27.5	6.88
9	K2S2O8	60	30	7.5
	KBrO3	40	20	5

Reaction conditions: All reactions were carried out at ambient temperature except for entries 2 and 3, which were carried out at 50 °C and 80 °C, respectively, and entries 4 and 5, where 0.2 mM of NiCl₂·H₂O or 0.2 mM of Ni(CH₃COO)₂·2H₂O was used instead of NiSO₄·6H₂O, respectively. Reaction time = 4 h; yield (Y) (%) = number of moles of produced acid × 100/number of moles of alcohol; turn over (TO) = number of moles of product/number of moles of catalyst; turn over frequency (TOF) = number of moles of product/number of moles of catalyst per hour.

.

It was found that the oxidation of benzyl alcohol (10 mM) to the corresponding benzoic acid took place smoothly in a short time (4 h) with the use of 0.2 mM of NiSO₄·6H₂O (50 fold-excess of the substrate) and 20 mM of the co-oxidants (two fold-excess of the substrate) in 100 mL of aqueous KOH (1.0 M). These conditions gave 97% and 75% benzoic acid with K₂S₂O₈ and KBrO₃ as the co-oxidants, respectively, at ambient temperature (entry 1, Table 1). The yields obtained with KBrO₃ in 1.0 M K₂CO₃ (pH = 11.5) were lower than those obtained with K₂S₂O₈ in 1.0 M KOH (pH = 14), which was probably due to K₂S₂O₈ having a stronger oxidizing power and higher aqueous solubility and stability suitable for the formation of the active species (NPNPs) than KBrO₃.

When we conducted two reactions at 50 °C and 80 °C, respectively, the yields of the benzoic acid were lower than those obtained at ambient temperature and the reaction was smelly with the formation of unidentified products. These products result from the reaction of unreacted alcohol with the acid produced when the reaction is performed at these higher temperatures, and they were difficult to isolate and identify (entries 2 and 3, Table 1).

Under the previously established reaction conditions, the oxidation of benzyl alcohol was also carried out using different nickel salts such as NiCl₂ and Ni(CH₃COO)₂·2H₂O instead of NiSO₄·6H₂O (entries 4 and 5, Table 1). It was noticed that the oxidation was quite slow and a considerably lower amount of benzoic acid was obtained. The differences in the yields obtained when the reactions were performed with the different nickel salts reflected their differences in solubility, i.e., a higher product yield was obtained with NiSO₄·6H₂O than with NiCl₂·2H₂O and Ni(CH₃COO)₂·2H₂O because NiSO₄·6H₂O is more soluble in the reaction medium than the other salts.

On the other hand, a blank experiment was conducted in the absence of NiSO₄.6H₂O, whereby the reaction did not proceed to completion and the acid was not detected (entry 6, Table 1). We separated the produced grey-green nickel oxide powder at the end of the reaction via filtration and washing with deionized water, recycled three times (entries 7, 8, and 9, Table 1) to produce nickel peroxide NPNPs, again with either excess K₂S₂O₈ in 1.0 M KOH or KBrO₃ in 1.0 M K₂CO₃, and used this for further catalytic oxidation reactions of benzyl alcohol to the corresponding acid benzoic, giving 80%, 76%, and 60% yields, respectively.

We noticed the yields obtained in the recycling of the catalyst were less than those obtained the first time, probably due to the loss of some of the catalyst during the reaction and also the loss of its active sites.

The effect of the p-derivatives on the yield of the acid was studied via the oxidation of some p-substituted electron-donating groups (p-CH₃, p-CH₃O, OH, and NH₂) and p-substituted electron-withdrawing groups (p-CHO, p-NO₂, p-CN, and p-CF₃). We found that the yields obtained with the electron-donating substituents (entries 10, 11, 12, and 13,

Table 2. The scope of the catalytic oxidation of benzyl alcohols to benzoic acids by NiSO₄·6H₂O/K₂S₂O₈.

Entry	Substrate	Product	Co-Oxidant	Y (%)	TO	TOF (h−1)
10			K2S2O8	98	49	19.25
			KBrO3	70	35.5	8.6
11			K2S2O8	97	48.5	12.13
			KBrO3	63	31.5	7.5
12			K2S2O8	95	47.5	11.88
			KBrO3	65	32.5	7.9
13			K2S2O8	95	47.5	11.88
			KBrO3	66	33	8.25
14			K2S2O8	70	35.5	8.88
			KBrO3	40	20	5
15			K2S2O8	60	30	7.5
			KBrO3	30	15	3.75
16			K2S2O8	60	30	7.5
			KBrO3	30	15	3.75
17			K2S2O8	55	27.5	6.88
			KBrO3	25	12.5	3.13
18			K2S2O8	85	42.5	10.6
			KBrO3	50	25	6.25

) were higher than those obtained with the electron-withdrawing groups (entries 14, 15, 16, and 17, Table 2). The possible reason for these observations was the fact that the electron-donating groups activate the ring, thereby enhancing the oxidation of alcohol into the corresponding acid, while the electron-withdrawing groups deactivate the phenyl ring and in this way retard the reaction. The convincing evidence was the fact that the yield obtained with the first group was higher than that obtained with the latter.

However, we noticed that increasing the reaction time more than four hours (Figure 5a) and the amount of co-oxidants (K₂S₂O₈ and KBrO₃) more than 20 mM (Figure 5b) did not improve the yield.

The reaction was self-indicating, whereby the green color of the reaction mixture changed to black upon the addition of potassium persulfate due to the formation of nickel peroxide nanoparticles (NPNPs). This color disappeared gradually with the formation of the green nickel oxide and completely disappeared when the potassium persulfate was consumed in the oxidation of the substrate.

However, this catalytic oxidation reaction is considered to be green because the reaction is selective and catalytic (alcohol is converted mainly to acid), the solvent used (water) is eco-friendly, and the oxidation is carried out at ambient temperature. However, our results are comparable with some recently reported protocols [36,37] for the catalytic oxidation of benzyl alcohols. It appears to be superior to most previously reported approaches. In our protocol, the yields of the acids were higher, and the reactions were conducted at room temperature with shorter reaction times than those reported by Han [38]. Recently, benzyl alcohol was catalytically oxidized to benzoic acid by the photocatalyst NH₂-MIL-125 (Ti) MOF in ethyacetate as a solvent and with O₂ as co-oxidant. In our recipe, we used water as a solvent, which is environmentally better than ethyl acetate, for which the reaction is longer than ours [39]. The catalyst, Ni@C/TiO₂-Z, was hydrothermally prepared and used for the oxidation of benzyl alcohol to benzoic acid at 96% in ethylacetate as a solvent for 18 h (a longer reaction time than our protocol) [40].

Finally, this protocol did not use any toxic solvent or chemical and was selective (the only product is the acid, with water as the sole by-product).

2.5. Mechanism of the Catalysis

It is very meaningful to trace the reaction process of the oxidation of alcohol to benzoic acid (BzCOOH) to understand the mechanism based on time-dependent curves (the percentages of BzOH, BzCHO, and BzCOOH with the reaction times) (Figure 6).

Firstly, benzaldehyde was produced as a mid-product with consumption of half the amount of NiO₂ (first step) (Equation (2)):

NiO₂ + PhCH₂OH →NiO + PhCHO + H₂O

(2)

Next, in the second step, the produced amount of benzaldehyde (BzCHO) decreased gradually due to its conversion to benzoic acid (BzCOOH) (Equation (3)):

NiO₂ + PhCHO → NiO + PhCOOH

(3)

In this second step, the other half of the NiO₂ was completely consumed; thus, stoichiometrically, two moles of NiO₂ reacted with one of benzyl alcohol (BzOH) to produce one mole of benzoic acid (BzCOOH) according to the following Equation (4).

2NiO₂ + PhCH₂OH → 2NiO + PhCOOH + H₂O

(4)

For this reason, the amount of the co-oxidant (K₂S₂O₈) was in two-fold excess of the benzyl alcohol (BzOH) and fifty-fold excess of the nickel salt. This means two oxidation steps occur at the same rate; therefore, we expect that the oxidation of benzyl alcohol (BzOH) to benzaldehyde (BzCHO) represents the rate-determining step [39].

However, the mechanism of this reaction occurred through the hydrogen abstraction mechanism, where benzyl alcohol (BzOH) coordinated to the nickel peroxide to form an unstable intermediate complex, which underwent intramolecular arrangement to produce the corresponding benzaldehyde (BzCHO) and nickel oxide. The produced nickel oxide reacted again with the excess of K₂S₂O₈ to produce nickel peroxide (Equation (5)):

NiO + K₂S₂O₈ + 2KOH → NiO₂+ 2K₂SO₄ +H₂O

(5)

This product similarly reacted with the produced benzaldehyde (BzCHO) to produce benzoic acid (BzCOOH). This catalytic cycle continued until the benzyl alcohol (BzOH) and benzaldehyde BzCHO) were completely consumed (Scheme 2).

3. Materials and Methods

All chemicals were purchased from Sigma-Aldrich (Berlin, Germany) (NiSO₄·6H₂O, NiCl₂·2H₂O, Ni(CH₃COO)₂·2H₂O, K₂S₂O₈, KOH, benzyl alcohol (BzOH), and p-substituted benzyl alcohols (R-BzOH; R = CH₃, OCH₃, OH, NH₂, CHO, CN, NO₂, and F₃C)). The Fourier Transmission Infrared (FTIR) spectrum of NiO₂ was recorded through Alpha-Bruker FTIR spectrophotometer (model no. 200695, Berlin, Germany). The X-ray diffraction (XRD) pattern of NiO₂ with Cu Kα radiation was obtained using PANalytical X’pert Pro X-ray diffractometer (DY 3190, Tokyo, Japan). Field Emission Scanning Electron Microscope (FESEM) Micrograph of NiO₂ was obtained by Carl Zeiss FESEM (SUPRA, 55VP, Berlin, Germany). Energy Dispersive X-ray (EDX) spectrum of NiO₂ was obtained by EDX detector of Oxford instruments (America) attached with FESEM. Transmission Electron Microscope (TEM) image of NiO₂ was taken by Jeol TEM (JEM-2100, Tokyo, Japan). ¹H NMR spectra were recorded on a Bruker DPX (600 MHz, London, UK)

3.1. Preparation of Nickel Peroxide Nanoparticles (NPNPs)

The nickel peroxide (NPNPs) were prepared using the co-precipitation method [24,25]. In a typical experiment, a solution of NiSO₄·6H₂O (13.15 g, 0.05 mol) in 75 mL of H₂O was added to the solution of K₂S₂O₈ (13.52 g, 0.05 mol) in 75 mL of H₂O with stirring. To this mixture, 150 mL of 2.0 M KOH (16.8 g, in 150 mL H₂O) was added in portions within one hour with continual stirring for a further 2 h. A black fine precipitate of nickel peroxide (NPNPs) was formed and collected by filtration, then thoroughly washed with deionized water and dried at 60 °C (yield: 90%).

3.2. General Procedure for the Oxidation of Alcohol

Alcohol (10 mM) and NiSO₄·6H₂O (0.2 mM) were added to a 250 mL flat-bottomed flask containing 100 mL of 1.0 M KOH. The reaction mixture was stirred for the 10 min, K₂S₂O₈ (20 mM) was added in three portions, then the reaction mixture was stirred for three hours. The black color of the nickel peroxide appeared then gradually disappeared, then the reaction mixture was acidified with 10 mL of 5% HCl, extracted by diethyether (3 × 10 mL) to remove the unreacted alcohol, and filtered to collect the produced acid. The IR spectrum, melting point, and ¹HNMR results were measured where appropriate and compared with authentic samples. The analysis for the produced acids is listed below.

• Benzoic acid

White solid; mp 121–122 °C (122 °C) [20]. IR: 3405–2103, 1682, 1610, 1572, 1463, 1432, 1333, 1282 cm⁻¹. ¹H NMR (400 MHz, CDCl₃): δ = 12.27 (br s, 1H, COOH), 8.43 (d, J = 7.1 Hz, 2H, Ar-H), 7.61 (t, J = 7.4 Hz, 1H, Ar-H), 7.47 (t, J = 7.8 Hz, 2H, Ar-H).

2.

4-Methylbenzoic acid

Pale yellow solid; mp 180–181.2 °C (180–182 °C) [20]. IR: 3400–2295, 1666, 1621, 1579, 1526, 1427, 1330, 1286 cm⁻¹. ¹H NMR (400 MHz, dmso-d₆): δ = 12.73 (br s, 1H, COOH), 7.67 (d, J = 8.0 Hz, 2H, Ar-H), 7.71 (d, J = 8.0 Hz, 2H, Ar-H), 2.58 (s, 3 H, CH₃).

3.

4-Methoxybenzoic acid

Pale yellow solid; mp 181.6–184 °C (182–184 °C) [20]. IR (neat): 3310–2105, 1675, 1610, 1571, 1512, 1420, 1291, 1253,1172, 1160, 1123, 1100, 1020 cm^–1. ¹H NMR (400 MHz, dmso-d₆): δ = 12.60 (br s, 1H, COOH), 7.93 (d, J = 8.8 Hz, 2H, Ar-H), 7.05 (d, J = 8.8 Hz, 2H, Ar-H), 3.82 (s, 3H, CH₃).

4.

4-Chlorobenzoic acid

White solid; mp 238.0–239.0 °C (239–240 °C) [20]. IR: 3300–2200, 1678, 1591, 1574, 1492, 1423, 1400, 1320, 1305, 1176 cm⁻¹. ¹H NMR (400 MHz, dmso-d₆): δ = 13.32 (br s, 1H, COOH), 7.78 (d, J = 8.8 Hz, 2H, Ar-H), 7.68 (d, J = 8.4 Hz, 2H, Ar-H).

5.

4-Nitrobenzoic acid

Pale yellow solid; mp 138–139 °C (139 °C) [20]. IR: 3405–2210, 1709, 1611, 1571, 1517, 1475, 1418, 1343 cm⁻¹. ¹H NMR (400 MHz, dmso-d₆): δ = 13.60 (br s, 1H, COOH), 8.43 (d, J = 8.0 Hz, 2H, Ar-H), 8.38 (d, J = 8.0 Hz, 2H, Ar-H).

6.

4-Cyanobenzoic acid

Pale yellow solid; mp 221.0–222 °C (219–221 °C) [20]. IR (neat): 3410–2300, 2221, 1675, 1620, 1560, 1439, 1325, 1276, 1173, 1122, 1126, 1030 cm⁻¹. ¹H NMR (400 MHz, dmso-d₆): δ = 13.60 (br s, 1H, COOH), 8.13 (d, J = 8.0 Hz, 2H, Ar-H), 8.03 (d, J = 8.4 Hz, 2H, Ar-H).

7.

4-Trifluoromethylbenzoic acid

White solid; mp; 220.2–222.2 °C (220–222 °C) [20]. IR (neat): 3410–2105, 1684, 1583, 1515, 1420, 1310, 1285, 1130, 1125, 1121, 1055, 1028 cm^–1. ¹H NMR (400 MHz, dmso-d₆): δ = 13.54 (br s, 1H, COOH), 8.16 (d, J = 8.0 Hz, 2H, Ar-H), 7.89 (d, J = 8.0 Hz, 2H, Ar-H).

8.

Piperonylic acid

Colorless needles; mp 224–226 °C (228–232 °C) [20]. IR (neat): 3100, 2900, 2850, 1620, 1540, 1455, 1410, 1230, 1153, 1110, 1113, 1015 cm⁻¹. ¹H NMR ¹H NMR (250 MHz, CDCl3) 7.77 (s, 1H, COOH), 7.50 (dd, 1H, J = 8 and 2 Hz, Ar), 6.22 (d, 1H, J = 2 Hz, Ar).

4. Conclusions

We introduced a straightforward and efficient catalytic method for the conversion of benzyl alcohol and some para-substituted benzyl alcohols to their corresponding carboxylic acids using NiSO₄·6H₂O (0.2 mM)/K₂S₂O₈ (20 mM) in 100 mL of 1.0 M KOH and NiSO₄·6H₂O (0.2 mM)/KBrO₃ (20 mM) in 100 mL of 1.0 m K₂CO₃. The advantages of this catalytic process are the use of non-toxic and inexpensive materials; the mild reaction conditions; the simple, safe procedure; and the short reaction times. The yields and turnover are good. This protocol can be extended for the catalytic oxidation of other organic substrates such as aromatic amines and secondary alcohols, which is currently under investigation in our laboratory.