Hydrotalcite-derived Co-MgAlO mixed metal oxides as efficient and stable catalyst for the solvent-free selective oxidation of cyclohexane with molecular oxygen

Molecular Catalysis 466 (2019) 130–137

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Hydrotalcite-derived Co-MgAlO mixed metal oxides as efficient and stable catalyst for the solvent-free selective oxidation of cyclohexane with molecular oxygen

T

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Peng Liua, Kuiyi Youa,b, , Renjie Denga, Zhenpan Chena, Jian Jianc, Fangfang Zhaoa, Pingle Liua,b, ⁎ Qiuhong Aia,b, He’an Luoa,b, a b c

School of Chemical Engineering, Xiangtan University, Xiangtan 411105, PR China National & Local United Engineering Research Center for Chemical Process Simulation and Intensification, Xiangtan University, Xiangtan 411105, PR China School of Chemistry and Chemical Engineering, University of Science and Technology of Hunan, Xiangtan 411201, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Cyclohexane KA-oil Hydrotalcite-derived mixed metal oxides Solvent-free Selective oxidation

Developing a highly efficient and stable catalyst for the selective aerobic oxidation of saturated hydrocarbons to functional compounds is highly desirable in chemical industry. In our present work, hydrotalcite-derived CoMgAlO mixed metal oxides as efficient and stable heterogeneous catalyst for the selective aerobic oxidation of cyclohexane to KA-oil has been developed. The results indicate that hydrotalcite-derived Co-MgAlO mixed metal oxides catalysts are active and selective; the synergistic catalysis of Co3+ and Co2+ can effectively promote the decomposition of cyclohexyl hydroperoxide (CHHP) intermediate to KA-oil. 9.1% of cyclohexane conversion and 82.0% of KA-oil selectivity over 2%Co-MgAlO catalyst with high turnover numbers (3800) was obtained at 423 K and 0.6 MPa for 2 h in this oxidation reaction. Moreover, the hydrotalcite-derived Co-MgAlO catalyst is easy to be separated and recycled, and its catalytic performance was still stable after five runs. The schematic reaction pathway for the oxidation of cyclohexane to KA-oil over Co-MgAlO mixed metal oxides catalyst was also suggested. Maybe this work employing inexpensive hydrotalcite-derived Co-MgAlO as highly efficient and environmentally friendly catalyst for the preparation of KA-oil has potential industrial application prospects.

1. Introduction Selective aerobic oxidation of cyclohexane to cyclohexanone and cyclohexanol (KA-oil) is a crucial process for chemical industry since KA-oil is an essential intermediate to product nylon-6 and nylon-6,6 [1,2]. In traditional industrial processes, the cyclohexane conversion is controlled at 3˜5% in order to obtain high selectivity of KA-oil (82˜84%), and high pressure (0.8˜1.5 MPa) and high temperature (125˜165 °C) are required [3]. Therefore, designing a highly efficient and eco-friendly catalyst for the selective aerobic oxidation of cyclohexane to KA-oil under the mild condition is highly desired. In fact, the present challenge for the aerobic oxidation of cyclohexane is to improve the conversion and selectivity to KA-oil. Designing excellent catalyst is the key to solving this problem. Thus, some research interests focus on homogeneous [4–6] and heterogeneous catalysts. Heterogeneous catalysts are more suitable to be used in industry than homogeneous catalysts because of their higher thermal stability, easy separation and recycle. Recently, many researchers have made a

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lot of efforts on heterogeneous catalysts in order to improve this oxidation process efficiency, such as transition metal oxides catalysts (MnCo mixed oxide [7], Co3O4 [8], CoFe2O4/SiO2 [9] and Ce1-xMnxO2 [10]), carbon nanotubes (CNTs) [11,12], metal organic framework (MOFS) [13,14], molecular sieves (AlPO-34 [15] and HMS [16]), hybrid nano-catalysts [17], metalloporphyrins [18,19], and oligomeric ionic liquid/heteropoly acid composite [20] etc. Hence, developing a recyclable and inexpensive catalyst with excellent catalytic performance is still highly desirable to further decrease the economic and environmental costs in the oxidation process. Hydrotalcite is a class of two-dimensional (2D) structured anionic clays that consists of positively charged layers and exchangeable interlayer anions [21–23]. The hydrotalcites-derived oxides as catalytic materials have been studied widely because of its many excellent properties, such as high thermal stability and good metal dispersion [24–31], and have been applied to various reactions, such as phenol hydrogenation [32], oxidation of mercaptans [33] and toluene, methane reforming with carbon dioxide [34–36]. In addition, Choudary

Corresponding authors at: School of Chemical Engineering, Xiangtan University, Xiangtan 411105, PR China. E-mail addresses: [email protected] (K. You), [email protected] (H. Luo).

https://doi.org/10.1016/j.mcat.2019.01.019 Received 24 September 2018; Received in revised form 24 December 2018; Accepted 16 January 2019 2468-8231/ © 2019 Elsevier B.V. All rights reserved.

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Table 1 Catalytic performance comparison of catalysts in the selective oxidation of cyclohexane with O2 a. Entry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 a b c d e

Catalyst

None MgAlO 2%Ni-MgAlO 2%Fe-MgAlO 2%Ce-MgAlO Co3O4c 0.5%Co-MgAlO 1%Co-MgAlO 2%Co-MgAlO 4%Co-MgAlO 2%Co-MCM-41 2%Co-ZSM-5 2%Co-Active carbon Co3O4-MgO-Al2O3d 2%Co-MgAlO e Nonee

TON

b

Conversion

Selectivity (%)

(%)

A

K

KA-oil

CHHP

acid

ester

(×103)

2.2 2.4 8.2 7.7 6.2 7.5 7.6 8.2 9.1 8.6 9.3 9.2 8.1 7.2 – –

18.2 17.9 37.3 36.1 30.9 48.5 47.1 48.1 53.2 50.9 42.5 43.3 42.1 49.1 – –

23.5 22.8 21.9 23.5 22.4 28.1 23.2 27.3 28.8 30.2 32.4 31.3 29.2 28.6 – –

41.7 40.7 59.2 59.6 53.3 76.6 70.3 75.3 82.0 81.1 74.9 74.6 71.3 77.7 – –

57.8 57.7 32.7 33.6 39.8 12.2 22.3 11.9 2.8 3.2 2 4.3 14.8 10.3 – –

0.5 1.6 8.1 6.8 6.9 8.5 7.4 11.6 11.3 10.2 13.8 12.6 8.4 9.2 – –

0 0 0 0 0 2.7 0 1.2 3.9 5.5 9.3 8.5 5.5 2.8 – –

– – 3.4 3.1 2.6 3.2 12.7 6.9 3.8 1.8 3.9 3.9 3.4 3.0 – –

Reaction conditions: cyclohexane 60 g, catalyst 0.05 g, temperature 150 °C, oxygen pressure 0.6 MPa, reaction time 2 h. The turnover number (TON) =moles of products (CHHP + A+K + acid + ester)/moles of cobalt in the catalysts. The amount of catalyst is 0.001 g. The physical mixture of cobalt oxide, magnesium oxide and aluminium oxide. o-dihydroxybenzene as the free-radical scavenger is added in the reaction.

2.2. Catalyst characterization

[37] has also reported the direct activation of molecular oxygen by nickel in Ni-Al hydrotalcite as demonstrated by the selective oxidation of various kinds of alcohols. Among these catalytic systems, hydrotalcites-derived metal oxides exhibit excellent catalytic performance. Herein, we report a simple and eco-friendly approach for the cyclohexane oxidation under solvent-free condition employing inexpensive hydrotalcite-derived Co-MgAlO mixed metal oxides as efficient and stable heterogeneous catalyst. The physico-chemical properties of the designed Co-MgAlO catalysts were systematically characterized by BET, H2-TPR, FT-IR, XRD, UV–vis, XPS, ICP-AES, SEM and TEM. The reaction conditions were optimized. It was found that the hydrotalcite-derived 2%Co-MgAlO catalyst showed excellent catalytic performance in the selective aerobic oxidation of cyclohexane. The obtained results from the cyclohexane oxidation will be reported in detail in this paper.

XRD data of samples were examined by using a Rigaku D/Max2550V+ diffractometer with a Cu Kα radiation. The H2-TPR of the samples is analyzed by using Quantachrome Instruments CHEMBET 3000. The FT-IR spectra of samples were recorded on a Nicolet 380 spectrometer using a potassium bromide disk in the range of 4000–400 cm−1. UV–vis spectra of samples were obtained on a UV2550 spectrometer with a scan range of 200–800 nm using BaSO4 as background standard. The chemical states of Co element were analyzed by XPS (XPS, Thermo Esclab 250Xi, a monochromatic Al Kα X-ray source) and all data were corrected with C1s peak (284.6 eV). The surface morphology of samples was investigated by a JSM-6610LV scanning electron microscopy (SEM) with the accelerating voltage is 220 kV). The transmission electron microscope (TEM) images of samples were obtained on a JEOL JEM-2100 F field emission microscope equipped with an energy dispersive X-ray spectroscopy (EDX) detector. The chemical composition of samples was determined by ICP-AES on a Agilent ICPOES730 instrument. The specific surface area, pore volume and pore diameter were characterized by using a NOVA-2200e (Quanta chrome) instruments with N2 physisorption at 77 K.

2. Experimental 2.1. Catalyst preparation The hydrotalcite-derived Co-MgAlO mixed metal oxides catalysts were prepared by the co-precipitation method reported in Ref. [36]. Typically, a certain amount of Mg(NO3)2·6H2O, Al(NO3)3·9H2O and Co (NO3)2·6H2O were dissolved into 300 ml of deionized water at 60 °C. Then the mixed alkaline solution of sodium hydroxide and sodium carbonate was added dropwise under stirring and the pH value was adjusted to ca. 9˜10. The resulting suspension was maintained at 60 °C for 24 h to get a hydrotalcite phase. Finally, the formed sediment washed several times with distilled water after filtering until getting a neutral filtrate. The obtained slurry was dried overnight at 110 °C and then calcined at 600 °C for 6 h in air atmosphere to obtain the final catalyst (defined as fresh catalyst). The hydrotalcite-derived Co-MgAlO mixed metal oxides catalysts with different cobalt content (wt%, namely 0.5%, 1%, 2% and 4%) or different metals (such as Ni, Fe and Ce) were prepared by the identical procedure. For coMParison, the cobalt-based catalysts with different supports such as MCM-41, ZSM-5 and active carbon were also prepared according to the impregnation method, respectively.

2.3. Catalytic tests The catalytic oxidation reaction of cyclohexane with molecular oxygen was carried out in a 250 mL stainless steel autoclave. Typically, 0.01 g catalyst and 60 g cyclohexane were added into a 250 mL stainless steel autoclave with Teflon lining in turn and heated to 150 °C at constant stirring of 300 rpm. Then O2 was injected into the reactor, and the oxygen pressure was maintained at 0.6 MPa. After reaction for 2 h, the reaction products were taken and dissolved in ethanol, and the catalyst was recycled by centrifugation. The products cyclohexanol (A) and cyclohexanone (K) were quantified by using GC (FID as detector and chlorobenzene as the internal standard sample). The iodometric titration is used to analyze the cyclohexyl hydroperoxide (CHHP) and the chemical titration method is used to analyze the acid and ester products.

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3. Results and discussion 3.1. Effects of various catalysts on the selective oxidation of cyclohexane with molecular oxygen Table 1 summarizes the catalytic reaction results of various catalysts on the selective aerobic oxidation of cyclohexane. Obviously, the single MgAlO exhibited little catalytic activity in this aerobic oxidation reaction (Entry 2). The conversion of cyclohexane and the selectivity to KAoil was evidently enhanced as the transition metal (Ni, Fe, Ce or Co) was introduced to MgAlO (Entry 3–6). Especially, the KA-oil selectivity was further improved over Co-modified MgAlO catalyst (Entry 7–10). In order to further screen the excellent catalysts, the effects of Comodified catalysts with different supports (such as MCM-41, ZSM-5 and active carbon) on the selective oxidation were examined. It had been seen that the hydrotalcite-derived Co-MgAlO catalyst gave the best results (Entry 10–13). Moreover, the effects of different cobalt loading (wt%, 0.5%, 1%, 2% and 4%) on the cyclohexane oxidation were also researched. The conversion was increased from 7.6% to 9.1% and selectivity of KA-oil was increased from 70.3% to 82.0%, as the Co loading was enhanced from 0.5% to 2%. The possible reason is that the active component cobalt can be well dispersed on the carrier resulting in the active sites being exposed. However, further increasing the cobalt loading to 4%, the cyclohexane conversion and selectivity of KA-oil were not enhanced (Entry 10). Maybe this is due to the active component Co with 2% loading has been able to expose completely, and further increasing the Co loading is not helpful for the improvement of its catalytic activity. In addition, as Co3O4 or physically mixed Co3O4MgO-Al2O3 (the weight percentage of Co is the same as 2% Co-MgAlO) as catalyst was added to this oxidation reaction, both the cyclohexane conversion and selectivity to KA-oil were lower than 2%Co-MgAlO (Entry 14). The results demonstrate the microstructure and composition of catalyst are related to its catalytic performance. Among these controlled catalysts, 2%Co-MgAlO exhibited better catalytic performance, and 9.1% of cyclohexane conversion with 82.0% of selectivity to KA-oil was obtained (Entry 9). However, the reaction was hardly preceded in presence of free radical scavenger (Entry 15, 16), which indicated that the oxidation of cyclohexane with molecular oxygen was a free radical reaction.

Fig. 2. H2-TPR results of MgAlO, fresh and used 2%Co-MgAlO samples.

that Co was incorporated into the layer structure of hydrotalcite. For the fresh 2%Co-MgAlO sample, the characteristic peaks of LDH structure were evidently disappeared and a periclase MgO phase was mainly observed in Fig. 1(b) and (c). The results were also consistent with the literatures characterization results [38–41]. Moreover, the XRD pattern of 2%Co-MgAlO sample is hardly changed after being repeatedly used 5 times (Fig. 1(c)). The results demonstrate that the 2%Co-MgAlO catalyst possesses excellent stability. 3.2.2. H2-TPR The H2-TPR profiles of MgAlO, fresh and used 2%Co-MgAlO samples are depicted in Fig. 2. Clearly, the H2 consumption peaks were not exhibited in the MgAlO sample. For the fresh and used 2%Co-MgAlO catalysts, we can see that there are two reduction peaks. The weak peak presented in the low-temperature (ca. 690 °C), which stands for the reduction peak of Co3+ to Co2+ and the strong peak presented in the high-temperature (ca. 999 °C), which is attributed to the reduction peak of Co2+ to Co0 [42]. Furthermore, the consumption of hydrogen for the reduction of Co2+ to Co0 is evidently greater than that of Co3+ to Co2+. The possible reason for this phenomenon is that some Co2+ species at high temperature originate from the reduction of Co3+ to Co2+ at low temperature. 3.2.3. FT-IR The FT-IR spectra of MgAlO, fresh and used 2%Co-MgAlO samples are displayed in Fig. 3. These samples exhibit the similar FT-IR spectra. The broad and intense peak at 3450 cm−1are ascribed to the stretching vibration of hydroxyl group [υ(OH)] due to surface-adsorbed water [43,44]. The low intensity peaks at 1380 and 2925 cm−1can be attributed to the trace amount of carbonate anion and CeH band, respectively [45,46]. The peak at 1630 cm−1 could be ascribed to the deformation of water molecules [∂(H2O)] [43]. The band at 660 cm−1 can be assigned to tetrahedrally coordinated Mg2+ (MgO4). It can be seen that the vibration intensity ascribed to MgO4 tetrahedron weaken while the cobalt was introduced to MgAlO, which may suggest that the

3.2. Characterization of samples 3.2.1. XRD The powder XRD patterns of 2%Co-MgAl-LDH, fresh and used 2%Co-MgAlO samples are shown in Fig. 1. The standard LDH pattern (JCPDS 89-0460, Mg0.667Al0.333(OH)2(CO3)0.167(H2O)0.5) is offered as coMParison. Clearly, the diffraction peaks of synthesized 2%Co-MgAlLDH (Fig. 1(a)) were the same as MgAl-LDH while a small amount of cobalt (ca. 2 wt%) was introduced to MgAl-LDH. This result indicated

Fig. 1. XRD patterns of (a) 2%Co-MgAl-LDH, (b) fresh 2%Co-MgAlO and (c) used 2%Co-MgAlO samples.

Fig. 3. The FT-IR spectra of MgAlO, fresh and used 2%Co-MgAlO samples. 132

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the samples is hardly changed after adding a small amount of cobalt element. Fig. 7 depicts the TEM images and EDX composition analysis for MgAlO, fresh and used 2%Co-MgAlO samples. It is observed that all three samples show conifer-like shape. Likewise, the original morphology of Co-MgAlO catalyst basically remains after five runs. This result is in agreement with the characterization of SEM. Moreover, the Mg, Al, O and Co elements in the samples are detected by EDX. EDX analysis shows the cobalt content is basically unchanged after being repeatedly used 5 times. In order to further confirm the change of cobalt content in the catalyst, the amount of cobalt in the fresh and used 2%Co-MgAlO catalysts is determinate by ICP, as shown in Table 2. The results indicate that the cobalt component is scarcely leached from the catalyst in the reaction process.

Fig. 4. The UV–vis spectra for MgAlO, fresh and used 2%Co-MgAlO samples.

3.3. Optimization of reaction conditions

Co element occupies the tetrahedral sites of the spinel oxide [47].

The reaction parameters for the selective oxidation over 2%CoMgAlO were optimized, and the obtained results were shown in shows Fig. 8. Fig. 8(A) depicted the influence of the oxygen pressure (or molar ratio) on the conversion and selectivity at 150 °C for 2 h. Clearly, the conversion of cyclohexane rapidly increased from 6.0% to 9.8% as the initial oxygen pressure increased from 0.5 to 0.6 MPa. Further elevating oxygen pressure to 1.0 MPa, the conversion increased slowly. This phenomenon indicated the reaction mechanism for cyclohexane with molecular oxygen on the catalyst surface was the Eley-Rideal mechanism. Furthermore, the selectivity to KA-oil increased first from 75.2% to 82.0% and then decreased to 74.0%. The possible reason is that the higher concentration of oxygen prefers to oxidize KA oil to acids and ester. Thus, the selectivity to KA-oil decreased. Therefore, the appropriate initial oxygen pressure is 0.6 MPa. Fig. 8(B) depicted the effects of reaction temperature on the cyclohexane conversion and KA-oil selectivity at 0.6 MPa for 2 h. Clearly, the conversion of cyclohexane rapidly increased from 1.6% to 9.8% and the selectivity of KA-oil increased first from 60.7% to 82.0%, and then decreased to 74.3% with the raised temperature from 130 °C to 170 °C. The possible reason is that Co-MgAlO catalyst is more easy to activate cyclohexane, resulting in the formation of high concentration of CHHP at high reaction temperature. The CHHP was quickly converted to KAoil under the synergistic catalysis of Co2+ and Co3+ species. However, KA-oil can be easily oxidized to acid and ester at higher temperature in this catalytic oxidation reaction [55]. Therefore, the appropriate reaction temperature was selected at 150 °C. The influence of reaction time on the catalytic reaction at 423 k for 0.6 MPa was depicted in Fig. 8(C). It is well observed that the conversion increased rapidly and the selectivity to KA-oil first increased and then decreased with the prolonged reaction time. This phenomenon may be due to the consecutive competitive reaction in this process. Firstly, the intermediate CHHP was decomposed into KA-oil, which can be further oxidized to acid and ester, thus, the selectivity to acid and ester gradually increased with the prolonged reaction time.

3.2.4. UV-vis Fig. 4 shows the UV–vis spectra of MgAlO and Co-MgAlO samples. It can be seen that the MgAlO shows no evident absorption peak in the region of 600–700 nm. For fresh 2%Co-MgAlO sample, a broad absorption band at 679 nm is observed, which is attributed to cobalt component [48,49]. Likewise, used 2%Co-MgAlO also has an absorption peak at 679 nm, indicating that the cobalt component is not leached from the catalyst. 3.2.5. XPS In order to further investigate the chemical state of Co element within the catalyst, the fresh and used 2%Co-MgAlO catalysts were characterized by XPS, as showed in Fig. 5. Obviously, both two samples contain Co3+ and Co2+ species. Two peaks at binding energy 780.8 eV and 796.4 eV are attributed to Co3+ 2p3/2 and Co3+ 2p1/2 (see the purple line in Fig. 5(a)), and two peaks at binding energy 782.9 eV and 798.4 eV are assigned to Co2+ 2p3/2 and Co2+ 2p1/2 [50,51] (see the green line in Fig. 5(a)), respectively. According to the fitted results, the content of Co3+ is about 77% and the content of Co2+ is about 23% in the 2%Co-MgAlO catalyst. Meanwhile, two peaks marked 1 and 2 are the satellite peaks of Co2+ 2p3/2 and two peaks marked 3 and 4 are the satellite peaks of Co2+ 2p1/2 [50,52]. Similarly, for the used 2%CoMgAlO sample, the same results are obtained, as shown in Fig. 5(b). It indicates that the valence state of active component cobalt in the catalyst is unchanged after being repeatedly used 5 times. The results are consistent with the above-mentioned characterization results of H2TPR. 3.2.6. SEM and TEM The SEM images of MgAlO, fresh and used 2%Co-MgAlO samples are displayed in Fig. 6. It can been seen that the three synthetic samples show the typical plate-like morphology, which are consistent with the literatures reported results [53,54]. Furthermore, the morphology of

Fig. 5. Co2p XPS spectra of (a) fresh and (b) used 2%Co-MgAlO samples. 133

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Fig. 6. SEM images of (A) MgAlO, (B) fresh and (C) used 2%Co-MgAlO samples.

Fig. 7. TEM images and EDX analyses for MgAlO, fresh and used 2%Co-MgAlO samples.

In addition, the amount of catalyst was examined in this oxidation reaction at 423 k and 0.6 MPa, as shown in Fig. 8(D). It can be observed that the cyclohexane conversion and the selectivity to KA oil seem to increase gradually with the elevated amount of catalyst from 0.01 g to 0.05 g. However, further increasing catalyst amount, the conversion and selectivity to KA oil were evidently changed. A possible reason for this phenomenon is that too much catalyst was easy to be agglomerated, thus the catalytic activity of catalyst could not be fully exhibited. According to the above single factor results, the optimal reaction conditions were that the oxygen pressure is 0.6 MPa, reaction temperature is 423 k, reaction time is 2 h and the amount of catalyst is 0.05 g.

Table 2 Textural properties and ICP of fresh and used 2%Co-MgAlO catalysts. Catalyst

Fresh 2%CoMgAlO Used 2%CoMgAlO

Elements composition (wt%)

Surface area (m2/g)

Pore volume (cm3/g)

Pore diameter (nm)

Co

Mg

Al

2.02

36.12

17.86

90.6

0.27

17.95

2.01

36.02

17.96

85.6

0.26

16.59

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Fig. 8. The optimization of reaction parameters. Reaction conditions: (A) cyclohexane 60 g, catalyst (2%Co-MgAlO) 0.05 g, reaction temperature 150 °C and reaction time 2 h. (B) cyclohexane 60 g, catalyst (2%Co-MgAlO) 0.05 g, oxygen pressure 0.6 MPa and reaction time 2 h. (C) cyclohexane 60 g, catalyst (2%CoMgAlO) 0.05 g, oxygen pressure 0.6 MPa and reaction temperature 150 °C. (D) cyclohexane 60 g, oxygen pressure 0.6 MPa, reaction temperature 150 °C and reaction time 2 h.

radical mechanism and CHHP is a crucial intermediate for the formation of KA-oil. In order to verify the present reaction whether is a freeradical reaction over Co-MgAlO catalyst, o-dihydroxybenzene as the free-radical scavenger was added in this reaction. The experimental results showed no any oxidation products were obtained under the same reaction conditions (entry 15 and 16 in Table 1), which similarly proved to be a cyclohexyl radical generated in this oxidation process. Moreover, another fact is that the CHHP exists in the initial stage of the reaction (Fig. 8(C)). It is well known that Co salt as homogenous catalyst has been widely used in the aerobic oxidation process of cyclohexane for the industrial production of KA-oil [56]. In our present work, hydrotalcite-derived 2%Co-MgAlO mixed metal oxides catalyst containing Co2+ and Co3+ also exhibits a high catalytic performance. Obviously, compared to the catalysts without Co2+ and Co3+, the amount of intermediate CHHP was much lower (entry 2–5 in Table 1), which illustrates the Co2+ and Co3+ species in the catalyst play an important role on the catalytic conversion of CHHP to KA-oil. Therefore, according to the obtained results in this work and literature reports [57–60], we suggested a possible reaction pathway for the catalytic oxidation of cyclohexane with dioxygen over hydrotalcite-derived Co-MgAlO mixed metal oxides, as shown in Fig. 10. Firstly, The CeH bond of cyclohexane was activated by the Co3+ species in the Co-MgAlO catalyst, and the formed cyclohexyl radical rapidly reacted with oxygen to form the cyclohexylperoxyl radical [61], which was further converted to CHHP (Fig. 10(1)). Then, the CHHP was decomposed to cyclohexanone or reduced to cyclohexanol under the synergistic catalysis of Co2+ and Co3+ species (Fig. 10(2)). The whole process involved the H-abstraction from cyclohexane to cyclohexyl radical under the action of catalyst, and then the formed cyclohexyl radical is easy to react with molecular oxygen to give cyclohexyloxyl radical, which further forms CHHP by abstracting the hydrogen over catalyst. Finally, Co-MgAlO catalyst was recovered and the catalytic cycle was completed.

3.4. Recycle of Co-MgAlO catalyst In order to estimate the stability of catalyst in the cyclohexane oxidation reaction, 2%Co-MgAlO was recycled for five runs. The recycling results were depicted in Fig. 9. Obviously, the conversion and the selectivity to KA oil were unchanged significantly after five runs. In addition, it had been seen that the Co content in the catalyst was not leached after five runs from the ICP-AES characterization (Table 2). These results demonstrated that 2%Co-MgAlO catalyst displayed remarkable stability in the aerobic oxidation of cyclohexane. 3.5. Possible reaction pathway of cyclohexane with molecular oxygen to KA-oil over Co-MgAlO catalyst Generally, the aerobic oxidation of cyclohexane is regarded as a free

Fig. 9. The recycling of 2%Co-MgAlO catalyst in the selective oxidation of cyclohexane with O2. Reaction conditions: cyclohexane 60 g, catalyst (2%CoMgAlO) 0.05 g, reaction temperature 150 °C, oxygen pressure 0.6 MPa and reaction time 2 h. 135

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Fig. 10. Schematic reaction pathway for selective oxidation of cyclohexane with molecular oxygen to KA-oil over Co-MgAlO mixed metal oxides catalyst.

4. Conclusions

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