1 Introduction

Aliphatic carboxylic acids produced from the traditional oxidation of paraffins are valuable chemicals that are widely used in food, perfumery, herbicide, pharmaceutical, and polymer industries [1]. In accordance with environment protection laws, the current research focuses on the development of environment-friendly oxidation methods. The oxidation of primary alcohols to the corresponding carboxylic acids is one of the most important transformations in organic chemistry [2,3,4]. Although traditional inorganic oxidants, such as chromates and potassium permanganate afford high yields of acids, they are not eco-friendly [5]. Therefore, an alternative efficient oxidation method that uses clean and cheap oxidants is needed.

Hydrogen peroxide (H2O2) is a green oxidant and exhibits many unique properties [6,7,8]. However, H2O2 generally requires activation by a catalyst. Among various catalysts used in H2O2-mediated oxidation, polyoxometalates are prominent owing to their substantial capacity as oxygen-transfer agents [9,10,11]. Specifically, lacunary polyoxometalates containing an increased number of active sites are attracting increased research attention [12,13,14].

Although previous papers have reported on the oxidation of primary alcohols using H2O2 [15,16,17], the corresponding studies focused on the oxidation of aromatic primary alcohols catalyzed by polyoxometalates catalysts, such as benzyl alcohol, to aldehydes [18,19,20,21]. To the best of our knowledge, there have been few studies on the selective oxidation of aliphatic primary alcohols to the corresponding aliphatic acids using an eco-sustainable catalytic system [22,23,24]. Bi’s group reported long chain primary alcohol oxidation to acid over quaternary ammonium peroxotungstophosphate catalyst, which gave a lower yield at a higher reaction temperature [25].

This paper is the first to report on the highly selective oxidation of aliphatic primary alcohols to acids using a recyclable organic–inorganic lacunary polyoxometalate catalyst and H2O2 oxidant. The oxidation of propan-1-ol to propionic acid was used for the experiments, as propionic acid is widely used as a preservative and a reagent [26,27,28]. A green and efficient synthesis method that replaces the industrial oxo-synthesis route from ethylene or ethyl alcohol is necessary, and the proposed method addresses this need [29, 30].

2 Results and Discussion

Table 1 gives the catalytic performance of different catalysts used for the oxidation of propan-1-ol. The acidic strengths of the catalysts were measured using the potentiometric titration method [31]. In the absence of a catalyst, only trace amounts of propan-1-ol were oxidized by H2O2 (entry 1). The reaction can be obviously facilitated by phosphotungstic acid (H3PW12O40) with high acid strength (entry 2). The replacement of hydrogen ions in phosphotungstic acid by sodium ions remarkably decreased the catalyst acidity as well as the percent conversion of propan-1-ol (entry 3). This may be due to that H2O2 is generally more oxidizing under acidic conditions. In addition, the combination of hydrogen protons and alcohol hydroxyl groups under acidic conditions may increase the polarity of α-C–H bond, making α-H more susceptible to oxidation. Therefore, suitable acidity of the catalyst is crucial for the oxidation [32]. The formation of a vacancy in the saturated heteropolyanion further decreased the catalyst acidity, but with a remarkable increase in the percent conversion of propan-1-ol and the yield of propionic acid (entry 4). This observation indicates that the structure of the heteropolyanion vacancy improves the catalytic activity. Figure 1 illustrates the contrastive structures of the saturated and lacunary Keggin-type heteropolyanions. It is clear that the monolacunary anion lacks a tungsten-centered octahedral structure. This catalyst is first peroxidized at the vacancy by H2O2, and the peroxidized catalyst subsequently oxidizes propan-1-ol. The proposed catalytic mechanism is shown in Fig. 2. Therefore, the lacunary catalyst is a high-capacity oxygen-transfer agent and exhibits superior catalytic activity [33, 34].

Table 1 Catalytic performance of different catalysts
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Fig. 1
figure 1

The structures of saturated and lacunary Keggin-type heteropolyanions

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Fig. 2
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The proposed catalytic mechanism for the oxidation of propan-1-ol catalyzed by lacunary polyoxometalate

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Moreover, a catalyst with superior catalytic performance should be recyclable. The catalysts in entries 2–4 cannot be recovered owing to their water solubility. Therefore, new heterogeneous catalysts based on lacunary heteropolyanions were prepared by replacing the hydrogen ions with larger quaternary ammonium cations. These cations contain long-chain alkyl groups and exhibit good catalyst recovery (entries 5–9). However, an increase in the hydrophobicity of the catalyst decreases its activity, as the reaction occurs in an aqueous H2O2 solution. Thus, the catalyst [(CH3)3NC12H25]2Na5PW11O39 with good catalytic activity and recovery performance, due to moderate amphiphilicity, was selected for the further research in the study (entry 8).

Figure 3 illustrates the infrared spectra of [(CH3)3NC12H25]Cl, [(CH3)3NC12H25]2Na5PW11O39, and Na7PW11O39. The infrared spectrum of [(CH3)3NC12H25]Cl shows the characteristic peaks at 2916 cm−1, 2848 cm−1, and 1484 cm−1, corresponding to the stretching and bending vibrations of C–H bonds. The characteristic absorption peak at 1472 cm−1 corresponds to the stretching vibrations of the C–N bonds. In the infrared spectrum of Na7PW11O39, the P–O bond absorption peak split into two bands at 1040 cm−1 and 1080 cm−1, which indicated the presence of the lacunary heteropolyanion. This splitting occurred owing to the removal of the WO unit, which decreased the symmetry of the PO4 group. The infrared spectrum of [(CH3)3NC12H25]2Na5PW11O39 exhibited the characteristic peaks corresponding to [(CH3)3NC12H25]Cl and Na7PW11O39, indicating the successful synthesis of the target catalyst. IR spectra of all prepared lacunary polyoxometalates with different organic cations are shown in Figs. S1–5.

Fig. 3
figure 3

FT-IR spectra of the catalysts including [(CH3)3NC12H25]Cl (a), [(CH3)3NC12H25]2Na5PW11O39 (b), and Na7PW11O39 (c)

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The prepared lacunary catalysts were also characterized using 1H NMR spectroscopy and X-ray diffraction (see SI for details, Figs. S6-11). Thermogravimetric curves of these lacunary polyoxometalates demonstrated that the thermal decomposition temperature was higher than 230 °C (Figs. S12–16), indicating the high thermal stability of the catalysts.

Several factors affecting the oxidation of propan-1-ol using [(CH3)3NC12H25]2Na5PW11O39 were studied to obtain better yields of propionic acid. Figs. S17–20 illustrates the effect of the catalyst amount, oxidant dosage, reaction temperature, and reaction time on the reaction. The optimal reaction conditions determined using experimental results are as follows: the amount of catalyst is 2% of the molar amount of the substrate, H2O2:substrate molar ratio is 3:1, the reaction temperature is 60 °C, and the reaction time is 6 h. Under these conditions, the conversion of propan-1-ol and the selectivity towards propionic acid were 80% and 73%, respectively.

The kinetic study for the new catalytic system was also conducted. Because H2O2 was used in excess, the reaction was expected to exhibit pseudo first-order kinetic characteristics, wherein the reaction rate depends exclusively on the concentration of propan-1-ol. Figure 4 illustrates the variation of reaction rate constants with time at 40 °C, 50 °C, and 60 °C. According to Arrhenius equation [35], the calculated apparent activation energy (Ea) equaled 27.14 kJ/mol. The low Ea might be attributed to the lacunary and amphiphilic structure of the catalyst.

Fig. 4
figure 4

Reaction rate constants at different temperatures

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After the reaction, the catalyst was recovered using centrifugation and desiccation and was subsequently used in the next catalytic reaction without any further treatment. The catalyst [(CH3)3NC12H25]2Na5PW11O39 exhibited good recycling efficiency, as shown in Fig. 5. Under the optimized conditions, both the conversion of propan-1-ol and the selectivity towards propionic acid decreased slightly during the first four cycles. However, the conversion and selectivity remained greater than 75% and 63%, respectively. After the catalyst was reused for four times, the recovered catalyst was characterized by FT-IR spectroscopy and 31P NMR. As shown in Figs. S21 and S22, compared with the new catalyst, the recycled catalyst demonstrated an intact structure during reuse. After the reaction, the catalyst lost active oxygen and recovered its original structure. Therefore, the decrease in catalytic activity can be attributed to the small loss in the catalyst amount during the recovery process. The catalytic activity restored in the fifth cycle when an equivalent amount of the lost catalyst was supplemented by the new catalyst.

Fig. 5
figure 5

Recycling performance of the catalyst

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In addition to propan-1-ol, the oxidation of other primary alcohols, including butan-1-ol, pentan-1-ol, hexan-1-ol, and benzyl alcohol, was also studied. Table 2 gives the catalytic performance of [(CH3)3NC12H25]2Na5PW11O39 for the oxidation of these primary alcohols.

Table 2 The oxidation of different primary alcohols catalyzed by [(CH3)3NC12H25]2Na5PW11O39
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Under non-optimized conditions, these reactions exhibited ˃ 76% conversions and ˃ 69% acid selectivities. This indicated that the corresponding carboxylic acids were the primary oxidation products in all reactions, and their yields could be improved by further optimization. Furthermore, the experimental results revealed that [(CH3)3NC12H25]2Na5PW11O39 exhibits good catalytic activity not only for the selective oxidation of aliphatic primary alcohols but also for aromatic primary alcohols to the corresponding carboxylic acids using H2O2 oxidant. Therefore, [(CH3)3NC12H25]2Na5PW11O39 demonstrated good substrate adaptability for various primary alcohols, including benzyl alcohol. Further investigations in this regard are necessary.

3 Conclusions

In summary, we developed the first organic–inorganic lacunary polyoxometalate catalytic oxidation system with the H2O2 oxidant to oxidize aliphatic primary alcohols to aliphatic acids. The novel lacunary catalyst exhibited high capacity as an oxygen-transfer agent, good catalytic activity, high selectivity towards carboxylic acids, and could be easily separated for reuse. The oxidation of propan-1-ol catalyzed by [(CH3)3NC12H25]2Na5PW11O39 exhibited 80% conversion of propan-1-ol and 73% selectivity towards propionic acid. Moreover, the catalyst [(CH3)3NC12H25]2Na5PW11O39 demonstrated good substrate adaptability for various aliphatic primary alcohols and benzyl alcohol. Our method provides a highly efficient, mild, and green synthesis route for the preparation of carboxylic acids from primary alcohols.