Production of Adipic Acid from Mixtures of Cyclohexanol-Cyclohexanone using Polyoxometalate Catalysts

Adipic acid production through catalytic conversion of cyclohexanol-cyclohexanone using polyoxometalat e H5[α-BW12O40] and H4[α-SiW12O40] as catalysts was carried out systematically. Poly oxometalates H5[α-BW12O40] and H4[α-SiW12O40] were synthesized using an inorganic synthesis metho d and were characterized using Fourier transform in frared spectroscopy (FTIR). Adipic acid was formed from conversion of c yclohexanol-cyclohexanone and was characterized by using melting point measurement, identification of functional gro up using FTIR spectrophotometer, analysis of gas ch romatography-mass spectrometry (GC-MS), and H and C NMR (nuclear magnetic resonance) spectrophotometer. Th is research investigated the influence of reaction time and temperature on c onversion. The results showed that adipic acid was formed successfully with a yield of 68% by using H 5[α-BW12O40] as catalyst at the melting point of 150-152 °C after optimization. In contrast, using H4[α-SiW12O40] as catalyst, formation of adipic acid was only 3. 7% Investigation of time and temperature showed 9 h as the optimum reaction time and 90 °C as the optimum temperature for conversion of up t o 68% adipic acid. Identification using FTIR, H, and C NMR showed that the adipic acid from conversion o f cyclohexanol-cyclohexanone was in agreement with the standard adipic acid data in the literatures. GC-MS analysis indicated that several by-products were formed in conversion of cyclohexanol-cyclohexanone using H5[α-BW12O40] and H4[α-SiW12O40] as catalysts.


Introduction
Green synthesis of adipic acid is extremely urgent for environmental reasons. Adipic acid is a precursor for nylon 6,6 and polyurethane plastic synthesis. The main method of synthesizing adipic acid through catalytic methods using nitric acid releases toxic nitrous oxide into the environment [1]. Thus, a catalytic conversion that forms adipic acid using an eco-friendly catalyst is needed. An approach to synthesizing adipic acid by oxidation of cyclohexanone, cyclohexanol, cyclohexane, or mixtures of these has been developed, using H 2 O 2 as a green oxidant and appropriate catalyst, as shown in Figure 1.
Various catalysts have been tested for adipic acid synthesis, including ionic liquid by two catalytic functions [2], hybrid porous tin (IV) phosphonate [3], manganese catalyst [4], and tungstate-based catalyst [5]. Recently, development of tungstate-based catalyst for adipic acid synthesis has been explored deeply using H 2 WO 4 [6], silica functionalized ammonium tungstate [7], sodium tungstate with stearyl dimethyl benzyl ammonium chloride as surfactant [8], and polyoxometalate compounds. Polyoxometalates are oxygen-metal clusters with various structures and oxidation states and can be used as potential catalyst due to very high acidity and non-toxic solid materials. Polyoxometalates are used intensively as catalyst for adipic acid synthesis. Keggin-type polyoxometalate H 3+x PMo 12-x V x O 40 (x=1 and 2) has catalyzed conversion of cyclohexanone to adipic acid [9]. Effect of addenda and heteroatoms in polyoxometalate has been studied by Zhu et al. using peroxotungstate and peroxomolobdates as catalyst [10]. Common Keggin-type H 3 PW 12 O 40 and H 3 PMo 12 O 40 with different addenda atoms have been converted into glycine phosphotungstate, and glicine phosphomolybdate acted as catalyst for conversion of cyclohexanone to adipic acid with high yield and selectivity [11]. In addition, structure effect has been investigated in the polyoxometalate, which was Anderson type [(C 18 H 37 ) 2 N(CH 3 ) 2 ] 6 Mo 7 O 24 , as catalyst for the conversion of cyclohexane to adipic acid [12].
Herein, we demonstrate the basic structure with different heteroatoms of polyoxometalate H 5 [α-BW 12 O 40 ] and H 4 [α-SiW 12 O 40 ] and with similar addenda atoms (tungsten) but different heteroatoms (boron and silica) used as catalyst for converting a 1:1 mixture of cyclohexanol-cyclohexanone to adipic acid. Based on our knowledge, this is the first report to investigate these polyoxometalate catalysts in conversion of cyclohexanol-cyclohexanone. In addition, several factors affecting formation of adipic acid, such as time and temperature reactions, were investigated in cyclohexanol-cyclohexanone conversion.   [14]. Adipic acid was characterized using an FTIR spectrophotometer and 1 H and 13 C NMR analysis.

Results and Discussion
Polyoxometalates H 5 [α-BW 12  The data in Table 2 show that H 5 [α-BW 12 O 40 ] is more effective for conversion than H 4 [α-SiW 12 O 40 ]. Probably, a Keggin-type heteroatom is key to conversion of cyclohexanol-cyclohexanone into adipic acid. Boron is more acidic than silicon, according to the Lewis acid concept; therefore, activation of H 2 O 2 needs a more acidic heteroatom [16]. Adipic acid was isolated and characterized using FTIR spectroscopy and then compared with the adipic acid standard, as shown in Figure 3.  Figure 3 shows that isolated adipic acid (B) is similar to standard adipic acid (A). Vibrations of isolated adipic acid appear at wave number of 2954 cm -1 for O-H, 2877 cm -1 for aliphatic methylene (CH 2 ), 1697 cm -1 for C = O, 1280 cm -1 for C-O, and 1195 cm -1 for aliphatic C-C. All these vibrations are in agreement with those of standard adipic acid reported in the literature [17]. Characterization of isolated white crystals of adipic acid was conducted using 1 H, 13 C NMR, and mass spectroscopy. Figure 4 shows the 1 H NMR spectrum.

Figure 3. FTIR Spectra of (A) Standard Adipic Acid and (B) Isolated Adipic Acid
The structure of adipic acid having a symmetric molecule can be identified using 1 H NMR and various deuterated solvents. A peak of DMSO-D6 used as solvent appears at 2.50 ppm. Other peaks related to adipic acid appear at 1.49 ppm (CH 2 ), δ 2.20 ppm (CH 2 ), and δ 12.02 ppm (COOH). The isolated adipic acid from this research has high purity compound indicated by no other peaks besides deuterated DMSO and adipic acid peaks were observed. The 1 H NMR result is supported by the 13 C NMR result, as shown in Figure 5.
Similar to the 1 H NMR spectrum, the 13 C NMR spectrum for adipic acid detects only three peaks. Acetone D6 was used to measure 13 C NMR of isolated adipic acid. As mentioned earlier, adipic acid can be measured using various solvents, and acetone appeared at 206.4 ppm. Peaks of adipic acid can be observed at 25.2 ppm, 33.9 ppm, and 17 4.7 ppm. Further characterization was conducted using a mass spectrometer ( Figure 6).
Adipic acid has a molecular weight of 146 g/mol and a molecular peak of m/z 146. The molecular peak at m/z 146 is cannot be observed clearly in Figure 6. However, other peaks related to molecular fragments at m/z 128, 112, and 100 are obvious, with the highest intensity appearing at m/z 100 (100%) [18]. Figure 7 shows the fragmentation pattern of adipic acid.
Although all characterizations of isolated adipic acid show high purity of the compound, moderately isolated adipic acid obtained from cyclohexanol-cyclohexanone using H 5 [α-BW 12 O 40 ] indicated formation of several byproducts and remaining cyclohexanol-cyclohexanone. Figure 8 shows three by-products identified using GC-MS.
On the basis of these results and from comparison with the literature [3,[19][20], we propose a plausible mechanism for converting cyclohexanol-cyclohexanone into adipic acid, as shown in Figure 9. Oxygen from peroxide was activated with boric tungsten from polyoxometalate to attack cyclo species from cyclohexanol-cyclohexanone. Intermediate derivative lactone was formed, followed by re-arrangement of lactone to adipic acid and oxygen as a green by-product. This study also investigated reaction time and temperature of conversion. Data are presented in Figures 10 and 11. According to Alcañiz-Monge et al., conversion of cyclohexene to adipic acid was achieved at 3 h [21]. In this study, the reaction time needed to convert the mixture of cyclohexanone-cyclohexanol was 7-10 h. As shown in Figure 10, at 9 h, up to 61% of cyclohexanolcyclohexanone can be converted to adipic acid. The yield of adipic acid obtained at 7 h is not significantly different from that obtained at 8 h, 59.5%. By increasing Temperature is an important factor in the catalytic conversion of cyclohexanol, cyclohexanone, and cyclohexanol-cyclohexanone mixtures into adipic acid [22]. In this study, temperature was adjusted to 100 °C due to stability of hydrogen peroxide. Figure 11 shows that a temperature of 90 °C yields isolated adipic acid of up to 68%. At lower temperatures, adipic acid formation is about 50-60%. At 100 °C, formation of adipic acid falls to 49%. All results indicated that the 1:1 mixture of cyclohexanol-cyclohexanone could form adipic acid with moderate isolated yield.

Conclusions
After optimization of the cyclohexanol-cyclohexanone mixture, adipic acid was synthesized successfully at the yield of 68% at a reaction time of 9 h and a temperature of 90 °C, using H 5 [α-BW 12 O 40 ] as the catalyst and with the melting point of 150-152 °C. Characterization using FTIR spectrophotometer, 1 H, and 13 C NMR showed that highly pure white crystalline adipic acid was obtained from synthesis.
Sriwijaya, for the use of laboratory facilities for this research.