High-selective Oxidation of n -Propanol to Propionic Acid Catalysed by Amino-acid-based Polyoxometalates

Background: Propionic acid as a very valuable chemical is in high demand, and it is industrially produced via the oxo-synthesis of ethylene or ethyl alcohol and via the oxidation of propionaldehyde with oxygen. It is urgent to discover a new preparation method for propionic acid via a green route. Recyclable amino-acid-based organic-inorganic heteropolyoxometalates were first used to high-efficiently catalyse the selective oxidation of n -propanol to propionic acid with H 2 O 2 as an oxidant. Result: A series of amino-acid-based heteropoly catalysts using different types of amino acids and heteropoly acids were synthesized, and the experimental results showed proline-based heteropolyphosphatotungstate [ProH] 3 PW 12 O 40 exhibited excellent catalytic activity for the selective catalytic oxidation of n -propanol to propionic acid owing to its high capacity as an oxygen transfer agent and suitable acidity. Under optimized reaction conditions, the conversion of n propanol and the selectivity of propionic acid reached 88% and 75%, respectively. Over four cycles, the conversion remained at ˃80%, and the selectivity was ˃60%. [ProH] 3 PW 12 O 40 was also used to catalyse the oxidations of n -butanol, n -pentanol, n -hexanol, and benzyl alcohol. All the reactions had high conversions, with the corresponding acids being the primary oxidation product. Conclusions: Proline-based heteropolyoxometalate [ProH] 3 PW 12 O 40 has been successfully used to catalyse the selective oxidation of primary alcohols to the corresponding carboxylic acids with H 2 O 2 as the oxidant. The new developed catalytic oxidation system is mild, high-efficient, and reliable. This study provides a potential green route for the preparation propionic acid.


Introduction
Propionic acid, a very valuable chemical, is widely used as a preservative in the feed, food, and pharmaceutical industries and incorporated in the perfume, herbicide, and polymer industries [1,2] .
Propionic acid is industrially produced via the oxo-synthesis of ethylene or ethyl alcohol and via the oxidation of propionaldehyde with oxygen [3,4] . However, these oxidation reactions require the use of an oil-soluble salt or a metal complex as a catalyst under harsh reaction conditions. Therefore, the development of a mild and effective synthetic method for propionic acid is of great significance.
The oxidation of primary alcohols to the corresponding carboxylic acids is one of the most important transformations in organic chemistry [5] . Traditionally, n-propanol can be oxidised to propionic acid by using inorganic oxidants, such as chromate and potassium permanganate, which are expensive and generate a large amount of hazardous waste [6,7] . An alternative route to the oxidation of n-propanol using environment-friendly and cheap oxidants is preferable. Hydrogen peroxide (H2O2) has received considerable attention as a green oxidant over the past several decades owing to its easy availability, mild oxidation conditions, and single by-product (water) [8,9] .
Due to their high capacity as oxygen transfer agents, polyoxometalates are characterised as efficient catalysts in oxidation reactions with O2 or H2O2 [10][11][12][13] . There have been some reports on the 3 oxidation of primary alcohols using heteropolyoxometalates as catalysts [14][15][16][17] . Nonetheless, these catalysts only promote the oxidation of primary alcohols to the corresponding aldehydes.
Furthermore, most related studies have involved the oxidation of benzyl alcohol as the model substrate and benzaldehyde as the primary product [18,19] . The selective oxidation of n-propanol to the corresponding propionic acid via a green route has not been reported in the literature. In this paper, we present a highly selective oxidation of n-propanol to propionic acid with high conversion, using a recyclable organic-inorganic heteropolyoxometalate as the catalyst and H2O2 as the oxidant.
Inexpensive and readily available amino acid is selected as the cation [20][21][22] . Moreover, its weak acidity can provide a suitable catalytic environment. Amino-acid-based heteropolyoxometalates exhibit good amphiphilicity, which enhances reactivity and realises the separation and recycling of the catalyst. Among the prepared catalysts, proline-based heteropolyphosphatotungstate exhibits the best catalytic activity with good recycling efficiency.

Preparation of catalysts
The synthesis of a proline-based catalyst, [ProH]3PW12O40, was chosen as an example. A total of 0.015 mol L-proline and 10 mL of deionised water were added to a 50-mL one-neck flask. The temperature was increased to 60°C in a water bath; 0.05 mol of phosphotungstic acid was slowly dropped into 10 mL of an aqueous solution while stirring. The mixture was reacted at 60°C for 24 h. 4 After the reaction, water was removed by rotary evaporation, and the residue was further dried in a blast drying oven to obtain a white solid catalyst [ProH]3PW12O40. The synthetic method for other catalysts was similar to that of [ProH]3PW12O40.

Catalytic tests
The reaction was carried out in a 25-mL three-neck flask fitted with a reflux condenser tube.
Then, 10 mmol of n-propanol and an appropriate amount of catalyst were added to the flask. The mixture was stirred for 10 min in a 60°C water bath, and 30 mmol of 30 wt% H2O2 solution was slowly added; the reaction was continued for 6 h at 60°C. After the reaction, the catalyst was separated by centrifugation and reused after drying. The reaction solution was extracted three times with ethyl acetate, and the upper organic phase was combined for qualitative and quantitative analysis by a gas chromatograph with an FID detector. The lower water phase was titrated with 0.05 M NaOH solution for an integrated quantitative analysis of propionic acid.

Catalyst Characterization
The infrared spectra of L-proline, H3PW12O40, and [ProH]3PW12O40 are shown in Fig.1. The infrared spectrum of H3PW12O40 shows characteristic peaks at 1082 cm -1 , 988 cm -1 , 896 cm -1 , and 805 cm -1 , attributable to the stretching vibrations of P-Oa, W=Od, W-Ob-W, and W-Oc-W, respectively, characteristics of the typical Keggin structure of heteropoly acid [23] . indicating that L-proline was successfully protonated by phosphotungstic acid. The thermostability of the [ProH]3PW12O40 catalyst was studied using a thermogravimetric (TG) test. The TG curve of [ProH]3PW12O40 exhibits the stepwise decomposition of proline-based cations and heteropoly anions (Fig.3). The first decomposition peak appears above 270°C, suggesting that the catalyst has very high thermostability.

Catalytic Activity of different catalysts
Herein we synthesised a series of amino-acid-based heteropoly catalysts using different types of amino acids and heteropoly acids to identify the best selective catalysts for the oxidation of npropanol to propionic acid. The activities of these catalysts for the selective oxidation of n-propanol were fully investigated; the results are listed in Table 1. The acid strength of the catalysts was determined by n-butylamine titration [24] , and the oxidisability of the catalyst was assessed by a redox potential assay. From entries 1-3, the conversion of n-propanol and the selectivity for propionic acid increased with increasing oxidisability of thecatalyst. The catalyst with lower oxidisability primarily catalyses the H2O2 oxidation of n-propanol to propionaldehyde. The acidity of the catalyst also greatly affects the catalytic activity (Table 1), and suitable acidity is required for obtaining propionic acid. Excessive acidity of the catalyst may promote esterification to obtain propyl propionate (Entries 3-5 and 8). Among the different amino-acid-based catalysts tested 7 (entries 3, 6, and 7), the proline-based catalyst [ProH]3PW12O40 exhibited the best catalytic activity.
In summary, the catalyst with higher oxidation properties and suitable acidity is more suitable for use in the oxidation of n-propanol to propionic acid.

Optimization of Catalytic Conditions
For the selective oxidation of n-propanol catalysed by [ProH]3PW12O40, various reaction conditions were screened to obtain the optimised conditions that gave propionic acid in greater yields. 8

Fig.4 Influence of catalyst dosage on reaction (n(n-propanol) = 10 mmol, n(H2O2) = 30 mmol, T = 60°C, t = 6 h)
Increased catalyst dosage is shown to increase the conversion and selectivity of propionic acid; this effect is limited to catalyst dosages of up to 3 mol% (Fig.4). When the catalyst dosage was further increased, both the conversion and selectivity of propionic acid decreased, possibly because too much catalyst also improves the decomposition of hydrogen peroxide. Therefore, the best catalyst dosage was 3 mol%.  9 The oxidant dosage has a significant effect on the reaction. Fig.5 shows that propionic acid yield increases with increasing amount of hydrogen peroxide and reaches a maximum when the molar ratio of H2O2 to n-propanol is 3:1. Additional aqueous H2O2 dilutes the concentration of the substrate and the catalyst, resulting in low conversion and selectivity of propionic acid. .

Fig.6 Influence of temperature on reaction (n(n-propanol) = 10 mmol, n(H2O2) = 30 mmol, n(catalyst) = 0.3 mmol, t = 6 h)
The influence of temperature on the reaction is shown in Fig.6. The conversion and selectivity of propionic acid increased as the reaction temperature was increased from 40 from 60°C. The increasing trend stabilises after 60°C due to the decomposition of H2O2 at higher temperatures, resulting in low yields of propionic acid.

Scheme 1. Proposed mechanism of the catalytic oxidation of n-propanol
According to the reaction results obtained herein and those reported previously [23] , the proposed catalytic mechanism for the oxidation of After the reaction, the catalyst was recovered by centrifugation. Fig.8 shows the cycling performance of [ProH]3PW12O40 for catalysing the oxidation of n-propanol under the optimised conditions. Over the first four cycles, the conversion of n-propanol and the selectivity of propionic acid gradually declined. However, the conversion remained at ˃80%, and the selectivity was ˃60%.
After four cycles, the recovered catalyst was characterised by FT-IR (Fig.9). Compared with the fresh catalyst, the structure of the recovered catalyst was not destroyed in the first four cycles, indicating good stability. The decrease in catalytic activity may be due to a slight loss of the catalyst during separation. For the fifth cycle, an equivalent amount of the lost catalyst was added, and catalytic activity was restored (Fig.8).  All the reactions have high conversions, with the corresponding acids being the primary oxidation product. The selectivity of acids can be further improved by optimising the catalytic conditions. Therefore, [ProH]3PW12O40 as a catalyst for the selective oxidation of primary alcohols to the corresponding acids by H2O2 has good substrate adaptability. 13

Conclusions
Proline-based heteropolyoxometalate [ProH]3PW12O40 has been successfully used to catalyse the selective oxidation of n-propanol toward propionic acid with H2O2 as the oxidant. The conversion of n-propanol and the selectivity of propionic acid reached 88% and 75%, respectively.
The excellent catalytic activity of [ProH]3PW12O40 is attributed to its high capacity as an oxygen transfer agent with a suitable acidity.
This study provides a new preparation method for propionic acid via a green route, and the developed catalyst shows immense potential for the selective oxidation of other primary alcohols to the corresponding carboxylic acids with H2O2 as an oxidant.