CuO and Ag2O/CuO Catalyzed Oxidation of Aldehydes to the Corresponding Carboxylic Acids by Molecular Oxygen

Furfural was oxidized to furoic acid by molecular oxygen under catalysis by 150 nm-sized Ag2O/CuO (92%) or simply CuO (86.6%). When 30 nm-size catalyst was used, the main product was a furfural Diels-Alder adduct. Detailed reaction conditions and regeneration of catalysts were investigated. Under optimal conditions, a series of aromatic and aliphatic aldehydes were oxidized to the corresponding acids in good yields.


Introduction
The transformation of aldehydes to carboxylic acids is an important reaction in organic synthesis. The commonly used oxidants for this transformation are NaClO 2 [1][2][3][4][5][6][7], K 2 Cr 2 O 7 [8][9], pyridinium chlorochromate [10], quinolinium dichromate [11][12][13], KMnO 4 [14][15][16], H 2 O 2 [17][18] and so on. Organic solvents such as CH 3 CN, CH 2 Cl 2 , cyclohexane, formic acid and benzene are usually needed. Although some of them have been widely used, they still have several disadvantages: low selectivity, environmental contamination and so on. Catalytic oxidation by molecular oxygen is an eco-friendly method. However, most reported catalysts such as Ru/CeO 2 [19] and Ag 2 O/Cu 2 O [20] are too expensive or difficult to prepare. Comparatively, Ag 2 O/Cu 2 O is an economical catalyst. Even so, it still needs to be regenerated, and there are no reports on its use for the above purpose, because it is almost impossible to keep Cu 2 O unchanged at high temperature.
Furoic acid, an important intermediate, is mainly obtained by oxidation of furfural. A series of reagents have been developed for this reaction, such as chromium species [9,11,13,21,22] and chlorite [2,3,23]. For the reasons mentioned above, we first investigated the oxidation of furfural to furoic acid by oxygen catalyzed by Ag 2 O/CuO. Detailed reaction conditions and catalyst regeneration were studied. In order to demonstrate the applicability of this method, some other aromatic aldehydes were oxidized to the corresponding aromatic carboxylic acids under optimized conditions in good yields. We also found that 150 nm-size CuO could catalyze the oxidation in a comparative yield to that obtained with an Ag 2 O/CuO system. This method has been used in industrial production of furoic acid.

Reaction conditions: Reaction temperature
Sodium hydroxide is needed to transform the formed acid to its salt, which is soluble in water. Otherwise the catalyst will be encapsulated by the formed acid. Aldehydes can undergo Cannizzaro reaction under basic conditions. The temperature has different effects on the kinetics of the desired oxidation and the undesired Cannizzaro reaction. Thus, it is crucial to find a suitable temperature for the reaction. As shown in Figure 1, the best temperature for both CuO and Ag 2 O/CuO catalyzed oxidation of furfural to furoic acid was 70˚C, while higher temperatures would decrease the adsorption of oxygen and give faster side reactions.

pH value
Acid can react with the catalyst to form corresponding silver and copper carboxylates which can inhibit the catalytic reaction. Therefore, it is essential to neutralize the formed acid. Meanwhile, as mentioned above, alkali will promote the undesired Cannizzaro reaction of aldehydes. The effects of

Amount of catalyst
More catalyst can shorten the reaction time and raise the yield. However, the reaction rate is also controlled by other conditions, such as the distribution of oxygen. Figure 3 shows that when the catalyst was increased to 30% wt of the mixture, the yield of furoic acid reached its maximum.

Size of catalyst
In general, a smaller-sized catalyst will have a larger specific surface area and a better catalytic activity [24]. However, this was not the case in the Ag 2 O/CuO and CuO-catalyzed oxidation of furfural to furoic acid. When small nano-scale CuO and Ag 2 O/CuO (30 nm, Figure 4) powder were used, the main product showed a molecular weight of 300 by ESI-MS ( Figure 5). Its structure is shown in Figure 6. This result suggests that small nano-scale CuO and Ag 2 O/CuO catalyze the Diels-Alder reaction more efficiently than the oxidation of aldehydes.

Cation poisoning
Catalysts are always sensitive to certain cations. When deionized water was replaced with running water, no furoic acid could be obtained. Detailed experiments showed that Ca 2+ inhibited the reaction and Mg 2+ just made the product darker while Na + , K + , Cu 2+ and Fe 3+ did little harm.

Catalyst regeneration and aging
Catalyst regeneration is essential for industrial production. The recycled catalyst could not be used directly because some organic matter was adsorbed on the catalyst and part of the CuO had been reduced to Cu 2 O and Cu (Figures 7 and 8). The recycled catalyst was first washed with hot deionized water to get rid of most organic adsorbates. Calcination was then performed to oxidize the reduced Cu 2 O and Cu to CuO.  Experiments showed that the calcination temperature greatly affected the recovery of catalyst activity and the optimal temperature was around 250 °C (Figure 9).
With recycling, the catalyst aged gradually. The product acid could erode the catalyst and cause remodification of the catalyst surface. Calcination also caused a granular aggregate of CuO powder ( Figure 10). These factors caused the specific surface area of CuO powder to decrease, so less oxygen could be adsorbed. For the Ag 2 O/CuO catalyst, friction also caused Ag 2 O to desquamate from the CuO ( Figure 10). Therefore, the catalysts gradually became aged, as shown in Figure 11.  Recycle Times many special advantages, such as low cost, easy preparation and regeneration. The results are shown in Table 1.

Conclusions
Under optimized conditions, 150 nm-size Ag 2 O/CuO and CuO can catalyze the oxidation of aromatic aldehydes to their corresponding carboxylic acids by molecular oxygen in good yields. Obviously, CuO is a better choice for the industrial production in that it can be easily collected and regenerated. Small nano-scale (30 nm) CuO and Ag 2 O/CuO catalysts favor Diels-Alder reactions instead of the oxidation of furfural.

General
All chemical reagents were of analytical grade. XRD experiments were carried out on a D8 Advance Bruker X-ray Diffractometer with monochromatized CuKα (λ=1.5418 Ǻ) radiation. The morphology and particle size were measured using a Hitachi H-800 transmission electron microscope (TEM). The accelerating voltage of the electron beam was 200 kV. 1 H-and 13 C NMR spectra were recorded on a JEOL JNM ECA-300 (300 MHz) instrument. Mass spectra were obtained on a Bruker ESQUIRE-LC.

Preparation of Ag 2 O/CuO
CuO (150 nm) powder (23.7 g) was added to a solution of AgNO 3 (1.83 g) in deionized water (20 mL). After stirring well for 10 minutes, the mixture was heated to vaporize the water. The precursor was then calcined at 400 ˚C for 3 hours to promote the decomposition of AgNO 3 . The heating rate was maintained at 5 ˚C/min.

Oxidation of aldehydes
To a 500 mL five-necked round bottom flask were added catalyst (5 g) and deionized water (150 mL). Solutions of NaOH (6 g) in deionized water (30 mL) and aldehyde (0.15 mol) were placed separately in two constant pressure dropping funnels. Then oxygen was blown into the flask while ts contents were heated and vigorously stirred. When the temperature reached 70 ˚C, the NaOH aq and the aldehyde were dropped into the flask in proportional amounts. It took about 20 min to add all the reagents while the temperature was kept at 70 ˚C. After that, the mixture was filtered to recycle the catalyst. 30% (wt) H 2 SO 4 (25 mL) was added to the filtrate while stirring. The product precipitated and was filtered, washed with water and dried. The used catalyst was first washed thoroughly with hot water to remove adsorbates. Then the catalyst was calcined for regeneration.