The selective liquid phase oxidation of cyclic hydrocarbons is one of the most widely used industrial processes producing oxygenates ^[1]. Because of the importance of cyclohexanol (A) and cyclohexanone (K) in the production of adipic acid and caprolactam, compounds that in turn are used in the manufacture of nylon-6 and nylon-6,6 polymers, the selective oxidation of cyclohexane has become a very important reaction ^{[2, 3]}. In general, there is the trade-off between selectivity and high conversion in hydrocarbon liquid phase oxidation reactions. Thus, conditions generating low conversions (less than 5%) are generally preferable as a means of preventing over-oxidation to form undesired by-products ^{[4, 5, 6]}. The greatest challenge associated with conventional industrial processes for cyclohexane oxidation is therefore the inherent difficulty in controlling the selectivity for the target products while obtaining a high conversion ^[7].

To date, a large number of catalysts have been reported for the oxidation of cyclohexane, such as metalloporphyrins ^{[8, 9]}, metal oxides [10−12], molecular sieves [13−16], carbon nanotubes ^{[17, 18]}, metal-organic frameworks materials ^{[19, 20]}, and small organic molecules ^[21]. Recently, polyoxometalate-based organic-inorganic hybrids demonstrated superior catalytic activity, producing a 29.4% yield of KA oil using H₂O₂ as the oxidant with acetonitrile as the solvent ^[22]. This same work also investigated changing the structure of the ionic liquid (IL) cation and obtained homogeneous catalysts having the structure [MimC₄]₅PMoV₂ IL-cation. Using this catalyst, a 28.5% yield of KA oil was obtained from the cyclohexane oxidation. However, a homogeneous catalyst such as this cannot be separated from the reaction mixture and recycled. Therefore, significant research work has been devoted to the development of heterogeneous catalysts for the selective oxidation of cyclohexane as substitutes for conventional homogeneous catalysts.

Iglesia et al. ^[23] reported that Mn species in MnAPO materials can serve as active sites during cyclohexane oxidation when using O₂ as the oxidant. Xu et al. ^[24] also studied the catalytic activity of MnAPO materials with regard to cyclohexane oxidation, and reported 3.7% conversion of cyclohexane and 64.5% selectivity for KA oil. However, the complicated process required to synthesize these catalysts and the uneconomical raw materials limit the potential industrial applications of these materials. Various hetero-mixed oxide catalysts have also been studied for the oxidation of cyclohexane, and WO₃/V₂O₅ has exhibited high catalytic activity with H₂O₂ as the oxidant ^[11]. Unfortunately, this approach suffers from the toxicity of the vanadium-based catalyst, and so effective, environmentally friendly, inexpensive and safe catalysts for the oxidation of cyclohexane are still required.

The catalytic oxidation of cyclohexane with oxygen as the oxidant has also been widely studied because of the inherently environmentally friendly nature of this process, although the resulting cyclohexane conversion is generally lower compared with that obtained with H₂O₂ or tert-butyl hydroperoxide (TBHP). Yang et al. ^[18] proposed that cyclohexane oxidation proceeds through a radical mechanism with O₂ as the oxidant. Hence, in some cases, the use of a radical initiator such as H₂O₂ or TBHP in the reaction mixture decreases the induction period and increases the conversion of cyclohexane. Chen et al. ^[25] also added TBHP to a cyclohexane reaction mixture as a free radical initiator to increase both the reaction rate and the conversion of cyclohexane. Recently, hollow sulfated V-doped TS-1 photocatalysts were evaluated for the selective photooxidation of cyclohexane with molecular oxygen and exhibited high efficiency (TOF = 2.37 h^-1, based on the molar amount of V sites) and chemoselectivity (cyclohexanone/cyclohexanol molar ratio = 3.46) ^[26].

Compared with molecular sieves and metal-organic frameworks materials, transition metal oxides are much easier to prepare and to apply in industrial processes. In the present work, we selected a transition metal oxide (manganese oxide) as the catalyst for the solvent-free oxidation of cyclohexane with molecular oxygen because this oxide is inexpensive, readily available, and environmentally friendly. This study investigated the effect of the calcination temperature on the physicochemical properties of manganese oxide catalysts prepared by a precipitation method. Based on the relationship between the calcination temperature and the resulting physicochemical properties and catalytic performance of the manganese oxide catalysts, we attempted to synthesize a highly effective and stable oxide catalyst for the solvent-free oxidation of cyclohexane with molecular oxygen.

Manganese oxide catalysts were prepared by a precipitation method using an aqueous Mn(NO₃)₂ solution as the source of Mn. In a typical procedure, a quantity of a 50 wt% Mn(NO₃)₂ solution was added to a 250-mL three-neck flask followed by the dropwise addition of a aqueous NaOH solution (1 mol/L) at 60 °C with stirring until the mixture reached a pH of 8. The solution was subsequently held at 60 °C for 4 h with stirring during which time its pH was maintained at 8 by adding the NaOH solution as required. The solid product was collected by filtration, washed several times with de-ionized water, dried at 120 °C overnight, and finally calcined in air at either 350, 400, 450, or 500 °C for 4 h. The corresponding samples are designated as MnO_x-350, MnO_x-400, MnO_x-450, and MnO_x-500, respectively.

X-ray diffraction (XRD) patterns were recorded on a Bruker AXS D8 Focus X-ray diffractometer with Cu K_α radiation (λ = 0.15406 nm) at a scanning rate of 6°/min. N₂ adsorption-desorption isotherms were acquired on an ASAP 2010 analyzer (Micromeritics) at −196 °C. Prior to these measurements, all samples were degassed at 300 °C for 6 h under vacuum to remove moisture and any volatile impurities. The Brunauer-Emmett-Teller (BET) method was used to calculate the specific surface areas of samples. X-ray photoelectron spectroscopy (XPS) was performed using a Thermo ESCALAB 250 spectrometer with a monochromatized Al K_a X-ray source (1486.6 eV), employing the C 1s (binding energy 284.6 eV) peak of adventitious carbon as a reference. Thermogravimetry-differential thermal analysis (TG-DTA) was carried out using a PerkinElmer Pyris Diamond instrument together with a WCT-2 thermal analyzer at a rate of 10 °C/min in air.

H₂ temperature-programmed reduction (H₂-TPR) profiles were collected with a PX200 apparatus (Tianjin Pengxiang Technology Co., Ltd., China) equipped with a thermal conductivity detector (TCD). The samples were heated from room temperature to 800 °C at a rate of 10 °C/min under 5 vol% H₂ in N₂ (40 mL/min). The hydrogen consumption during this process was measured quantitatively by the TCD. O₂ temperature-programmed desorption (O₂-TPD) was performed using a conventional flow system equipped with a quadrupole mass spectrometer (MS, INFICON IPC400). In these trials, a 200-mg sample was pretreated in Ar at its original calcination temperature for 1 h. After cooling to 50 °C, the sample was again pretreated in 3 vol% O₂ in He (30 mL/min) for 1 h, after which the system was flushed with Ar for 30 min to sweep residual oxygen from the sample surface. Finally, the sample was heated from 50 to 800 °C at a heating rate of 10 °C/min, and the gas released was analyzed online using the MS.

The catalytic performance of the catalysts during cyclohexane oxidation was assessed in a 50-mL Teflon-lined stainless steel autoclave while stirring. Molecular oxygen was used as the oxidant without any solvent. In a typical reaction, 8 mL cyclohexane and 20 mg catalyst were introduced into the reactor. Subsequently, O₂ was introduced until the desired initial pressure was obtained at room temperature, after which the reactor was heated to 140 °C with stirring and these conditions were maintained for 4 h. The post-reaction mixture was analyzed using a PerkinElmer Clarus 500gas chromatograph equipped with a PE-2 capillary column (25 m × 0.32 mm × 1.0 μm) and a flame ionization detector (FID). Methylbenzene was used as the internal standard. The main by-products of the reaction were found to be hexanedioic acid, hexanoic acid, dicyclohexyl adipate, and cyclohexyl caproate (Scheme 1). The concentration of acids in the reaction solution was determined by titration with 0.1 mol/L NaOH.

Scheme 1. Oxidation of cyclohexane.

Recycling trials were carried out using solely the MnO_x-400 catalyst. Following completion of each reaction trial, the catalyst was separated by filtration from the reaction solution, washed with ethanol three times in a glass beaker with stirring, dried at 100 °C for 6 h, and calcined in air at 400 °C for 2 h, thus producing the regenerated catalyst. The catalytic performance of the regenerated catalyst together with a supplement of fresh catalyst was then examined using the same reaction procedures applied with the fresh catalyst.

The powder XRD patterns of MnO_x samples calcined at different temperatures are shown in Fig. 1. The diffraction peaks of the MnO_x-350 sample are consistent with those of hausmannite Mn₃O₄ (JCPDS 24-0734) and Mn₅O₈ (JCPDS 39-1218). The Mn₃O₄ can be considered to have a spinel structure composed of Mn²⁺(Mn³⁺)₂O₄, in which Mn²⁺ and Mn³⁺ occupy the tetrahedral and octahedral sites of the structure, respectively ^[27]. During the calcination process, the octahedral Mn³⁺ ions in Mn₃O₄ are oxidized to Mn⁴⁺ preferentially over Mn²⁺, leading to the formation of (Mn²⁺)₂(Mn⁴⁺)₃O₈ ^[28]. In the case of the MnO_x-400 and MnO_x-450 samples, the intensities of the Mn₃O₄ and Mn₅O₈ diffraction peaks are markedly strengthened. When the calcination temperature is further increased to 500 °C, Mn₂O₃ peaks appear in the pattern of the MnO_x-500 sample, and this evidently becomes the main MnO_x phase.

Fig. 1. XRD patterns of MnOx-350 (1), MnOx-400 (2), MnOx-450 (3), and MnOx-500 (4) samples.

The N₂ adsorption-desorption isotherms and the pore size distributions of samples are shown in Fig. 2. All isotherms exhibit typical V-type shapes as defined by IUPAC. The BET surface areas, average pore diameters, and pore volumes of samples are listed in Table 1. The BET surface areas of the MnO_x samples are seen to have decreased with increases in the calcination temperature, such that the BET surface areas of the MnO_x-350 and MnO_x-500 were 31.4 and 20.5 m²/g, respectively. The pores in these samples are secondary particle- piled pores composedof MnO_x, and both the pore volume and pore diameter show only a very slight variation with increasing calcination temperature.

Fig. 2. N2 adsorption-desorption isotherms and pore size distributions (inset) of MnOx-350 (1), MnOx-400 (2), and MnOx-500 (3) samples.

Table 1 Textural properties of MnOx-350, MnOx-400, and MnOx-500 samples. Sample Surface area (m2/g) Pore volume (cm3/g) Aaverage pore diameter (nm) MnOx-350 31.4 0.13 16.3 MnOx-400 24.0 0.13 16.0 MnOx-500 20.5 0.12 17.0

Table 1 Textural properties of MnOx-350, MnOx-400, and MnOx-500 samples.

The XPS Mn 2p and O 1s spectra of the MnO_x-400 and MnO_x-500 samples are shown in Fig. 3. The Mn and O atom surface concentrations are summarized in Table 2. It can be seen that the surface concentrations of Mn and O on the MnO_x-400 sample are close to those on the MnO_x-500.

Fig. 3. XPS Mn 2p (a) and O 1s (b) spectra of MnOx-400 (1) and MnOx-500 (2) samples.

The Mn 2p peaks can be deconvoluted to three peaks, and the peaks at binding energy (BE) values of 640.8 and 641.9 eV are assigned to Mn³⁺ and Mn⁴⁺, respectively, while the peak at 643.6 eV is a satellite peak of manganese ^{[29, 30]}. The Mn⁴⁺/(Mn⁴⁺+Mn³⁺) atomic ratios were calculated from these XPS spectra using XPSPEAK 4.1 and are presented in Table 2. It is evident that the Mn⁴⁺/(Mn⁴⁺+Mn³⁺) ratio of the MnO_x-400 sample is higher than that of the MnO_x-500, as result of the appearance of a Mn₂O₃ phase in the MnO_x-500 (Fig. 1). This phase is responsible for the greater oxidation ability of the MnO_x-400 sample compared with the MnO_x-500.

Table 2 Surface atom concentrations on MnOx-400 and MnOx-500 samples as determined by XPS. Sample Surface atom concentration (%) Mn4+/ (Mn4++Mn3+) Oads/ (Oads+Olat) O Mn Mn/O MnOx-400 49.9 30.6 0.61 0.25 0.30 MnOx-500 50.9 31.2 0.61 0.22 0.27

Table 2 Surface atom concentrations on MnOx-400 and MnOx-500 samples as determined by XPS.

The XPS O 1s spectra of the MnO_x-400 and MnO_x-500 samples are also provided in Fig. 3, and here two peaks can be distinguished at 530 and 531 eV. The peak at 530 eV is assigned to lattice oxygen (O^2-, O_lat), while the peak at 531 eV is attributed to surface oxygen (O_ads). These species include surface- adsorbed oxygen and weakly bonded oxygen, such as O₂²⁻ or O⁻, belonging to defect-oxide or hydroxyl-like surface groups ^{[31, 32]}. The O_ads/(O_ads+O_lat) ratios of both MnO_x samples were calculated and are given in Table 2. It is interesting to observe that the O_ads/(O_ads+O_lat) ratio for the MnO_x-400 sample is slightly higher than that for the MnO_x-500 sample, indicating that the MnO_x-400 has more surface oxygen. This suggests greater oxidation ability for this material compared with the MnO_x-500, which in turn favors the selective oxidation of cyclohexane when using O₂ as the oxidant.

The H₂-TPR profiles of the MnO_x-400 and MnO_x-500 samples are shown in Fig. 4. There are two reduction peaks in the MnO_x-400 sample, at 315 and 450 °C, attributed to the reductions Mn₅O₈ → Mn₃O₄ and Mn₃O₄ → MnO, respectively ^[33]. With regard to the MnO_x-500 sample, four reduction peaks are seen at 161, 348, 376, and 500 °C. The small reduction peak at 161 °C can be attributed to the reduction of Mn(III) in tetrahedral sites ^{[34, 35]}, while the two overlapped peaks at 348 and 376 °C are assigned to Mn₅O₈ → Mn₃O₄ and Mn₂O₃ → Mn₃O₄ respectively, and the peak at 500 °C is assigned to Mn₃O₄ → MnO ^{[28, 36, 37]}. Compared with the MnO_x-400 sample, the main reduction peaks of the MnO_x-500 sample are obviously shifted to higher temperatures, showing the lower reducibility of the MnO_x-500 sample.

Fig. 4. H2-TPR profiles of MnOx-400 (1) and MnOx-500 (2) samples.

Figure 5 presents the O₂-TPD profiles of the MnO_x-400 and MnO_x-500 samples. The MnO_x-400 sample exhibits three desorption peaks at 487, 613, and 772 °C. The peaks at 487 and 613 °C are attributed to the desorption of adsorbed and lattice oxygen from Mn₅O₈ to Mn₂O₃, while the peak at 772 °C is assigned to the desorption of lattice oxygen from Mn₂O₃ to Mn₃O₄ ^[35]. The MnO_x-500 sample only generates a weak peak at 540 °C and a strong peak at 844 °C, assigned to the desorptions of adsorbed and lattice oxygen from Mn₂O₃ to Mn₃O₄ ^{[35, 38]}. It can be seen that the MnO_x-400 sample has a greater capacity for O₂ adsorption compared with the MnO_x-500, which could account for the higher reactivity of the MnO_x-400 sample for the selective oxidation of cyclohexane by O₂.

Fig. 5. O2-TPD profiles of MnOx-400 (1) and MnOx-500 (2) samples.

Figure 6 shows the TG-DTA curves obtained from the MnO_x-400 sample before and after the selective oxidation of cyclohexane. Following the reaction, the MnO_x-400 sample was collected by filtration, washed several times with deionized water, and dried overnight at 120 °C. The resulting sample was assessed by TG-DTA analysis with the results presented in Fig. 6. It can be seen that the mass of the unreacted MnO_x-400 sample exhibits a very slow decrease on heating due to the loss of adsorbed water, and does not generate an obvious exothermic peak in its DTA curve. This result indicates that the MnO_x-400 sample is stable when heated to 800 °C. After being used in the reaction, the MnO_x-400 sample undergoes a mass loss of 83% as the temperature is raised to 400 °C, accompanied by a strong exothermic peak at approximately 400 °C, which is due to the combustion of carbon deposited on the MnO_x-400 sample during the reaction ^{[39, 40]}.

Fig. 6. TG-DTA curves of the MnOx-400 sample before (1) and after (2) the reaction.

Table 3 summarizes the catalytic performance of the MnO_x catalysts for the selective oxidation of cyclohexane with O₂ as the oxidant. These results show that a 1.2% conversion of cyclohexane with 92% total selectivity for KA oil was achieved in the blank test. Using MnO_x as the catalyst, the conversion of cyclohexane was greatly increased while the selectivity for KA markedly decreased. The calcination temperature evidently had a significant effect on the catalytic activity of the MnO_x. As the calcination temperature applied to the MnO_x catalysts was increased, the conversion of cyclohexane first increased and then decreased, although the total selectivity for KA oil exhibited a continual gradual increase. As shown in Table 3, when employing the MnO_x-400 catalyst, an 8.0% conversion of cyclohexane was achieved with 62% total selectivity for KA oil, such that the yield of KA oil was 5.0%. When using the MnO_x catalyst calcined at 500 °C, the conversion of cyclohexane decreased to 6.1%, but the selectivity for KA oil increased to 75%. Compared with the MnO_x-400 catalyst, the MnO_x-500 catalyst exhibited a lower surface area and lower concentrations of surface Mn⁴⁺ and surface oxygen (O_ads), which account for its reduced ability for O₂ adsorption and activation and the lower conversion of cyclohexane that it provides. In contrast, the MnO_x-500 catalyst’s lower ability for O₂ adsorption and activation also limited the over-oxidation of cyclohexanol and cyclohexanone, leading to the increase in its selectivity for KA oil. The reported performance of other catalysts intended for the selective oxidation of cyclohexane is also listed in Table 3. When using O₂ as the oxidant, the MnO_x-400 catalyst showed higher activity for the selective oxidation of cyclohexane than has been reported for Co-TUD-1, Au/MgO, and Mn₃O₄ in the literature ^{[41, 42, 43]}. However, the yield of KA oil over the MnO_x-400 catalyst was much lower than that obtained from a Mn-TUD-1 catalyst when employing TBHP as the oxidant ^[44].

Table 3 Catalytic performance of the MnOx catalysts for the selective oxidation of cyclohexane with O2. Catalyst Time (h) Atmosphere (MPa) Conversion (%) Selectivity (%) Yield of KA oil (%) Cyclohexanol (A) Cyclohexanone (K) KA oil a Acids b Blank test 5 O2/0.5 1.2 43 49 92 4 1.1 MnOx-400 4 N2/0.5 — — — — — — MnOx-350 4 O2/0.5 7.5 30 29 59 8 4.4 MnOx-400 4 O2/0.5 8.0 32 30 62 6 5.0 MnOx-450 4 O2/0.5 6.6 34 33 67 7 4.4 MnOx-500 4 O2/0.5 6.1 38 37 75 8 4.6 Co-TUD-1 c [41] 24 8% O2/N2/1.5 3.8 56 31 87 — 3.3 Au/MgO d [42] 17 O2/0.3 5.0 50 29 79 19 4.0 Mn3O4 e [43] 6 O2/1.0 5.2 36 27 63 35 3.3 Mn-TUD-1 f [44] 24 TBHP 13.5 24 60 84 — 11.3 Reaction conditions: cyclohexane 4 ml, catalyst 20 mg, 140 °C. a Cyclohexanol (A) and cyclohexanone (K). b Adipic acid, glutaric acid, and succinic acid. c Reaction conditions: cyclohexane 175 mmol, cyclohexyl hydroperoxide (radical initiator) 0.05 mmol, active metal species 0.1 mmol, 120 °C. d Reaction conditions: cyclohexane 10 mL, tert-butylhydroperoxy (radical initiator) 0.05 mmol, catalyst amount 6 mg, 140 °C. e Reaction conditions: cyclohexane10 g, catalyst amount 0.1 g, 140 °C. f Reaction conditions: cyclohexane 9.8 g, tert-butylhydroperoxy 4.4 g, Mn catalyst 0.1 mmol, 70 °C.

Table 3 Catalytic performance of the MnOx catalysts for the selective oxidation of cyclohexane with O2.

The effects of the reaction temperature, reaction time, and the initial oxygen pressure on the catalytic activity of the MnO_x-400 catalyst for the cyclohexane oxidation were evaluated. As shown in Fig. 7(a), with increases in the reaction temperature from 120 to 160 °C, the conversion of cyclohexane gradually increased, while the selectivity for cyclohexanol and cyclohexanone markedly decreased at first, then plateaued. This occurs because the higher reaction temperature enhances the activation of cyclohexane but also promotes the over-oxidation of cyclohexanol and cyclohexanone to esters, which can be detected by GC-MS. Considering both the conversion of cyclohexane and the selectivity for KA oil, it can be seen that the most appropriate reaction temperature is 140 °C.

Fig. 7. Effect of the reaction temperature (a), reaction time (b), and initial oxygen pressure (c) on the catalytic activity of MnOx-400 for cyclohexane oxidation. Reaction conditions: cyclohexane 4 mL, catalyst 20 mg; (a) O2 0.5 MPa, 4 h; (b) O2 0.5 MPa, 140 °C; (c) 140 °C, 4 h.

The effect of the reaction time on the activity of the MnO_x-400 catalyst for cyclohexane oxidation is summarized in Fig. 7(b). Increasing the reaction time evidently results in a slight enhancement of the cyclohexane conversion, while the selectivity for cyclohexanol and cyclohexanone decreases gradually because of the over-oxidation of cyclohexanol and cyclohexanone ^{[45, 46]}. The generation of acidic by-products changed only minimally with variations in the reaction time, although esters resulting from over-oxidation of cyclohexanol and cyclohexanone were detected by GC-MS. From these data, the optimal reaction time appears to be 4 h.

Figure 7(c) shows the effect of the initial O₂ pressure on the cyclohexane oxidation over the MnO_x-400 catalyst. The results demonstrate that raising the initial oxygen pressure can enhance both the conversion of cyclohexane and the selectivity for acids, and thus reduce the selectivity for cyclohexanol and cyclohexanone. This phenomenon may be caused by the over-oxidation of cyclohexanol and cyclohexanone under high pressure O₂. Taking into account both the conversion of cyclohexane and the selectivity for KA oil, the optimal initial O₂ pressure is evidently 0.5 MPa.

After the reaction finished, the catalyst was separated by filtration from the reaction solution, washed with ethanol three times, dried at 100 °C for 6 h, then calcined in air at 400 °C for 2 h. The catalytic activity of this regenerated MnO_x-400 catalyst was subsequently assessed, and the results obtained from 10 such recycling trials are shown in Fig. 8. It can be seen that the conversion of cyclohexane and the selectivity for KA oil are barely changed by the repeated usage. These results indicate that the MnO_x-400 catalyst exhibits good stability for the selective oxidation of cyclohexane by O₂ in a solvent-free system.

Fig. 8. Effect of recycling on the catalytic activity of MnOx-400 for cyclohexane oxidation.

The effect of calcination temperature on the physicochemical and catalytic properties of MnO_x catalysts prepared by the precipitation method was investigated. These MnO_x catalysts exhibited high catalytic performance during the selective oxidation of cyclohexane with O₂ in a solvent-free system. The calcination temperature markedly affected the catalytic activity, such that the catalyst calcined at 400 °C exhibited the highest activity, giving an 8.0% cyclohexane conversion and a 5.0% yield of cyclohexanol and cyclohexanone at 0.5 MPa initial O₂ pressure and 140 °C over 4 h. After 10 reaction cycles, the catalytic activity of the MnO_x-400 catalyst was unchanged, demonstrating that this material is highly stable during the selective oxidation of cyclohexane by O₂ in a solvent-free system. Compared with the MnO_x-500 catalyst, the MnO_x-400 catalyst, consisting of both Mn₃O₄ and Mn₅O₈, possessed a higher surface area and elevated concentrations of surface adsorbed oxygen and surface Mn⁴⁺ ions. These species can promote oxygen mobility, O₂ adsorption, and the reducibility of the catalyst, leading to the observed superior catalytic activity of the MnO_x-400 catalyst for the selective oxidation of cyclohexane in a solvent-free system.