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BY 4.0 license Open Access Published by De Gruyter Open Access August 21, 2019

Ionic liquids modified cobalt/ZSM-5 as a highly efficient catalyst for enhancing the selectivity towards KA oil in the aerobic oxidation of cyclohexane

  • Yun Hong , Yanxiong Fang , Dalei Sun EMAIL logo and Xiantai Zhou EMAIL logo
From the journal Open Chemistry

Abstract

The industrial oxidation of cyclohexane is currently performed with very low conversion level, i.e. 4-6% conversion and poor selectivity for cyclohexanone and cyclohexanol (K-A oil), i.e.70-85%, at above 150oC reaction temperature and above 10atm reaction pressure using molecular oxygen oxidant and homogeneous catalyst. Several disadvantages are, however, associated with the process, such as, complex catalyst-product separation, high power input, and low safe operation. Therefore, the oxidation of cyclohexane using heterogeneous catalyst oxygen oxidant from air at mild conditions has received particular attention. Aerobic oxidation of cyclohexane over ionic liquids modified cobalt/ZSM-5 (IL-Co/ZSM-5) in absence of solvents was developed in this article. The prepared catalysts were characterized by XRD, FT-IR, N2 adsorption-desorption, SEM, TEM and XPS analyses. The influence of reaction parameters on the oxidation of cyclohexane was researched, such as the various catalysts, reaction temperature, reaction time, and the reaction pressure, on the process. Highly selective synthesis of KA oil was performed by aerobic oxidation of cyclohexane using ionic liquids modified cobalt/ZSM-5 (IL-Co/ZSM-5) as the catalyst in absence of solvents for the first time. A selectivity of up to 93.6% of KA oil with 9.2% conversion of cyclohexane was produced at 150℃ and 1.5 MPa after 3 h, with about 0.1 mol cyclohexane, C7mimHSO4-Co/ZSM-5 catalyst equal to 6.0 wt%, respectively. The induction period of oxidation was greatly shortened when the ionic liquid was supported on ZSM-5. The catalyst was easy to centrifuge and was reused after five cycles. It was found that both the characterization and performance of the catalysts revealed that both the presence of oxygen vacancies with incorporation of Co ions into the framework of ZSM-5 and the introduction of C7mimHSO4 into the ZSM-5 leads to the both satisfactory selectivity and robust stability of the C7mimHSO4-Co/ZSM-5 heterogeneous catalyst.

1 Introduction

In the modern chemical industry, selective oxidation of cyclohexane is an important chemical process. Partial oxidation of cyclohexane to cyclohexanol and cyclohexanone (KA oil), which are intermediates in Nylon-6 and Nylon-6-6 manufacturing, has attracted commercial interest [1, 2, 3]. The selectivity towards KA oil is significant in the commercial process under mild conditions [4, 5, 6]. Therefore, many catalytic systems, such as cobalt salt [7], metal complex [8], biomimetic [9, 10], transition metal oxide [11, 12, 13] and molecular sieve [14] combined with molecular oxygen were developed to improve the selectivity and efficiency.

In the chemical industry, aluminosilicates zeolite ZSM-5 has been used as one of the important heterogeneous catalysts [15, 16]. Hydrogen peroxide is often used as a green oxidant in the oxidation of cyclohexane catalyzed by metal-supported ZSM-5 catalysts. Copper-doped ZSM-5 presented excellent catalytic performance and selectivity for KA oil in the presence of H2O2 as oxidant [17].

Aerobic oxidation of hydrocarbons has a low cost and is an environmentally friendly nature of oxidant, which has generated interest[18]. Due to the high energy of C-H bond, the aerobic oxidation of cyclohexane could be conducted under more benign conditions by using NHPI (N-hydroxyphthalimide) or TBHP (tert-butylhydroperoxide) as free radical initiators [19, 20]. Gold containing ZSM-5 can achieve the direct aerobic oxidation of cyclohexane without any additives [21]. But the extensive application of this catalytic process is seriously limited due to the expensive cost of catalyst. Therefore, the development of non-noble metal supported ZSM-5 catalytic system is particularly important. However, the selectivity of KA oil is generally not high in the system of direct catalytic cyclohexane oxidation of metal/ZSM-5. It has been reported that the stability of molecular sieves in ionic liquids could be improved [22]. The anionic or cationic groups of ionic liquids may interact with the surface of the molecular sieve, which will be favorable for improving the selectivity of products.

Even though this is promising research is related to industrialization, there is little reported, especially for the aerobic oxidation of cyclohexane in absence of solvents, are relatively scanty. In this work, ionic liquids were used to modify the metal supported on ZSM-5 to prepare one kind of stable catalyst, which was applied in the aerobic oxidation of cyclohexane under solvent-free conditions. The effects of different reaction conditions on cyclohexane oxidation were studied, such as the various catalysts, reaction temperature, reaction time, and the reaction pressure, on the process.

2 Experimental

2.1 General

The reagents involved are as follows: ZSM-5, Co(NO3)2×4H2O, Cyclohexane (A.R., Sinopharm Chemical Reagent Co. Ltd.); C7mimHSO4, C7mimH2PO4, C7mimNO3 and C7mimTsO (A.R., Lanzhou Institute of Chemical Physics, 1-heptylimidazole, abbreviated as C7mim); Before use, ZSM-5 was first milled and screened by a 20-mesh sieve.

2.2 Preparation of Co/ZSM-5

Co/ZSM-5 was obtained as followed: 0.5g of the ZSM-5 calcined at 550oC was immersed in 1mol Co(NO3)2×4H2O . The mixture was stirred at 333K for 10 h, filtered and washed with ethanol, separated by centrifugation. At 120oC, the product was dried in an air oven overnight. Co/ ZSM-5 was obtained after the calcination at 550oC for 10h.

2.3 Preparation of ionic liquids modified Co/ ZSM-5

IL-Co/ZSM-5 was obtained as followed: In a three-neck glass flask, 0.5g Co/ZSM-5 and 0.75g ionic liquid were added to 30 mL of ethanol. The mixture was stirred at 60oC for 4 h. The product was obtained after filtration, washing with ethanol, separation by centrifugation and drying, and is denoted as IL-Co/ZSM-5.

2.4 Characterization of the catalysts

Microphotography (SEM) images were generated on a Hitachi S3400N instrument equipped.

Transmission electron microscope (TEM) images were carried out using a JEM-2010HR transmission electron microscopy.

On a Phi Quantum 2000 Scanning ESCA Microprobe, X-ray photoelectron spectroscopy (XPS) was performed with Al Ka radiation. A C1s binding energy of 284.6 eV was used as the reference.

X-ray diffraction (XRD) spectroscopy was performed on a Rigaku-Ultima III with Cu with Cu Ka1 radiation. Its Scanning radiation were a range of 2θ = 6-80º and a rate of 1º/min.

FT-IR spectra was recorded by Bruker-TENSOR 27. The precise Co loading was measured by ICP-OES on a PerkinElmer-Optima 8300. Thermogravimetric analysis was conducted by a STA 449F3 Jupiter. The BET specific surface areas were measured by N2 adsorption in an ASAP 2020C.

2.5 Catalytic activity studies

The cyclohexane was oxidized in a 100 mL stainless steel autoclave equipped with an automatic temperature controller and a magnetic stirrer. The cyclohexane reacted with IL-Co/ZSM-5 catalyst in the autoclave. The autoclave was rinsed with air three times, then it was pressurized to the desired pressure and heated to the desired temperature with stirring. After the reaction was over, the autoclave was cooled with ice to room temperature and slowly depressurized. The products were analyzed by GC-MS and quantified by GC (Agilent-7890A, capillary column: HP-5, 30 m × 0.25 mm×0.25 μm).

Ethical approval: The conducted research is not related to either human or animal use.

3 Results and discussion

3.1 Characterization results

Figure 1 shows the powder XRD patterns of ZSM-5, Co/ ZSM-5 and C7mimHSO4-Co/ZSM-5. The parent ZSM-5 exhibited five XRD diffraction peaks assigned to reflections at (111), (220), (311), (511) and (440), which are characteristic of the lattice structure in hexagonal, mesoporous molecular sieves. Compared to Co/ZSM-5, the XRD diffraction peaks of C7mimHSO4-Co/ZSM-5 at 2q =31.3o, 37.0o, 59.5o and 65.4o were decreased. It indicates that it may be the modification of the ZSM-5 with the ionic liquid and the immobilization of cobalt.

Figure 1 XRD patterns of samples (a) ZSM-5, (b) Co/ZSM-5 and (c) C7mimHSO4-Co/ZSM-5.
Figure 1

XRD patterns of samples (a) ZSM-5, (b) Co/ZSM-5 and (c) C7mimHSO4-Co/ZSM-5.

Figure 2 shows the FT-IR spectra of Co/ZSM-5, C7mimHSO4 ionic liquid and C7mimHSO4-Co/ZSM-5. The strong broad absorption peak at 1080 cm-1 corresponds to the asymmetric and symmetric stretching vibration of Si-O-Si. The peak at 666 cm-1 can be due to Si-O bending vibrations. The peaks at 2853 cm-1 and 2933 cm-1 can be associated to the vibration of C-H bond in 1-heptyl group. The peak 1634 cm-1 can be due to the stretching vibration of C-N, and both the peaks at 1218 cm-1 and 1460 cm-1 can be associated to the vibration of the imidazole ring. Therefore, the results confirmed the existence of both imidazole ring and 1-heptyl group.

Figure 2 FT-IR spectra of (a) C7mimHSO4 ionic liquid, (b) Co/ZSM-5 and (c) C7mimHSO4-Co/ZSM-5.
Figure 2

FT-IR spectra of (a) C7mimHSO4 ionic liquid, (b) Co/ZSM-5 and (c) C7mimHSO4-Co/ZSM-5.

Figure 3 shows isotherms and pore size distribution of ZSM-5 by nitrogen adsorption-desorption, Co/ZSM-5 and C7mimHSO4-Co/ZSM-5. It can be observed that ZSM-5 and Co/ZSM-5 exhibited a type IV isotherm, which was characteristic of highly ordered mesoporous materials. No obvious change in average pore size was observed when ZSM-5 was loaded with cobalt ion, and the surface area and pore volume decreased slightly with the loading of cobalt ion.

Figure 3 N2 sorption isotherms of (a) ZSM-5, (b) Co/ZSM-5 and (c) C7mimHSO4-Co/ZSM-5.
Figure 3

N2 sorption isotherms of (a) ZSM-5, (b) Co/ZSM-5 and (c) C7mimHSO4-Co/ZSM-5.

However, when the sample Co/ZSM-5 was modified by the ionic liquid, the isotherm had a hysteresis effect under high relative pressure, which was characteristic of macroporous materials. Meanwhile, a sharp decrease in surface area and pore volume were observed. These changes could be attributed to the blockage of micropores and the filling of aggregated particles (Table 1).

Table 1

BET surface area and pore characters of catalysts.

CatalystBET surface area (m2g-1)Pore volume (cmg-1)Average pore diameter (nm)
ZSM-5249.480.162.51
Co/ZSM-5196.320.132.59
C7mimHSO4-

Co/ZSM-5
25.480.058.22

The characteristics of the surfaces of the particles were studied with TEM and field emission scanning electron microscopy (SEM) images of ZSM-5, Co/ZSM-5 and C7mimHSO4-Co/ZSM-5. Figure 4(a)and Figure(d)are the SEM and TEM images of ZSM-5, respectively. It can be seen from the images that the surface of the ZSM-5 molecular sieve particle is smooth and the particle size is about 1.0 to 2.5 microns. After the cobalt ion was loaded, the surface of the particles was coarse, and the cobalt ions are mainly supported on the surface of the ZSM-5. No significant changes of ZSM-5 pore structure was observed with the introduction of cobalt ion (Figure 4(b)and Figure

Figure 4 SEM and TEM images of (a, d) ZSM-5, (b, e) Co/ZSM-5 and (c, f) C7mimHSO4-Co/ZSM-5.
Figure 4

SEM and TEM images of (a, d) ZSM-5, (b, e) Co/ZSM-5 and (c, f) C7mimHSO4-Co/ZSM-5.

4(e)). The cobalt nanoparticles were still clearly observed when Co/ZSM-5 was modified by the ionic liquid. It can be noticed that the pores and surfaces of ZSM-5 were almost covered with ionic liquid (Figure 4(c)and Figure 4(f)).

In order to obtain the valence states and binding energy of cobalt atoms and oxygen atoms, Co/ZSM-5 and C7mimHSO4-Co/ZSM-5 were characterized by XPS spectroscopy. The binding energy values in eV corresponding to the individual peaks are presented in Table 2. The binding energy of the Co2p3/2 and O1s peaks (Table 2) for Co/ZSM catalyst is respectively found to be 780.49 eV and 532.19 eV, higher than that for Co2O3 (780.0eV for the Co2p3/2 and 530.0 eV for O1s [23]). This indicates that Co (III) ions, replacing the Al(III) and/ or Si(IV), are incorporated into the ZSM-5 framework, resulting in oxygen vacancies from Co (III) doping, and a stronger ability of attracting electrons compared with Co2O3. This result is also consistent with that of both the spectra of FT-IR and XRD for Co/ZSM-5. When Co/ZSM-5 was modified by the ionic liquid, the binding energy for Co2p3/2 was 0.57 eV higher than that of Co/ZSM-5. While, the binding energy for O1s decreased by 0.90 eV relative to that of Co/ZSM-5. The results suggested that, on the one hand, the lattice oxygen existed on the surface of the C7mimHSO4-Co/ZSM-5 catalyst, and on the other hand, the introduction of C7mimHSO4 results in a strong electron

Table 2

XPS analysis of catalysts.

CatalystCo2p3/2(eV)O1s(eV)Atomic ratio
C/SiCo/SiO/SiN/Si
Co/ZSM-5780.49532.198.320.0681.80-
C7mimHSO4-Co/ZSM-5781.06531.297.600.0743.731.24

modification. Thus, the catalytic activity of cobaltous oxide in the oxidation reactions is improved.

3.2 Catalytic oxidation of cyclohexane

3.2.1 Effect of catalyst on the aerobic oxidation of cyclohexane

Table 3 shows the results of cyclohexane aerobic oxidation over various catalysts at 150oC and 1.5 MPa for 3h. As shown in Table 3, the main products in the oxidation of cyclohexane were cyclohexanol and cyclohexanone (KA oil). In the control experiment, almost no obvious conversion was observed (entry 1). In the presence of molecular oxygen, the pure ZSM-5 can promote the oxidation of cyclohexane, which may be due to its large surface area and small amount of acid sites (entry 2). When cobalt was supported on ZSM-5, the activity of the catalyst was increased, in which the selectivity of KA was 72.9% (entry 3). When this type supported cobalt catalyst was modified by the ionic liquid, the activities of catalyst was enhanced, accompanied with the obvious improved selectivity towards KA (entries 4~7). It seems that the activity is related with the anionic group of the ionic liquid with same cation group (1-heptylimidazole). In contrast to the relative catalytic activity of the anionic group of the ionic liquid, the following decreasing order was observed: HSO4>NO3>TsO>H2PO4. The same decreasing order about the selectivity towards KA was observed. Hydrogen bonds could be generated from an interaction between the anionic group and silicon hydroxyl of ZSM-5. The differences in catalytic activity and selectivity towards KA oil maybe related to the intensity of the hydrogen bonds. The mechanistic studies on the influence of the ionic liquids are in progress.

Table 3

Effect of catalyst on the aerobic oxidation of cyclohexanea.

EntryCatalystConv./%Select./%
alcoholketoneKA
1None1.212.324.036.3
2ZSM-53.132.128.060.1
3Co/ZSM-56.438.634.372.9
4C7mimHSO4-Co/ZSM-59.248.445.293.6
5C7mimNO3-Co/ZSM-58.347.841.989.7
6C7mimTsO-Co/ZSM-57.943.843.787.5
7C7mimH2PO4-Co/ZSM-57.545.240.285.4
  1. a Cyclohexane (0.1 mol), catalyst (6.0 wt%), O2 (1.5MPa), 150oC, 3h.

3.2.2 Effect of temperature and pressure on the aerobic oxidation of cyclohexane

The effects of reaction temperature and pressure were researched, as seen in Table 4. The conversion of cyclohexane increased from 2.5% to 9.2% (entries 1~3) when the reaction temperature was increased from 130oC to 150oC. No significant increase was obtained with the continual increase of reaction temperature (entry 4). With increasing temperature, selectivity towards KA oil slightly decreased. However, when the temperature was over 160oC, the reaction rates of side reactions were also obviously accelerated (entry 4). The conversion of cyclohexane increased with increasing molecular oxygen pressure, whereas the selectivity for KA slightly declined with increasing pressure to 2.0 MPa.

Table 4

Effect of temperature and pressure on the oxidation of cyclohexanea.

EntryT/oCP/MPaConv./%Select./%
alcoholketoneKA
11301.52.553.442.896.2
21401.56.650.543.994.4
31501.59.248.445.293.6
41601.59.745.642.788.3
51501.08.548.445.994.3
61502.09.345.145.090.1
  1. a Cyclohexane (0.1 mol), C7mimHSO4-Co/ZSM-5 catalyst (6.0 wt%), 3h

3.2.3 Effect of time on the aerobic oxidation of cyclohexane

Figure 5 shows the conversion of cyclohexane and selectivity for KA oil at different reaction times. Within the first 1 h, the conversion of cyclohexane slowly increased as the reaction rate rapidly increased. The aerobic oxidation of cyclohexane has an obvious induction period just like any other free radical. For verifying the free radical mechanism, 2,6-di-tert-butylphenol (1 mmol), serving as the free radical inhibitor was used. After the addition of this inhibitor, the oxidation was subsequently quenched.

Figure5 Profile of the aerobic oxidation of cyclohexane catalyzed by C7mimHSO4-Co/ZSM-5 catalyst. Cyclohexane (0.1 mol), catalyst (6.0 wt%), O2 (1.5MPa), 150oC.
Figure5

Profile of the aerobic oxidation of cyclohexane catalyzed by C7mimHSO4-Co/ZSM-5 catalyst. Cyclohexane (0.1 mol), catalyst (6.0 wt%), O2 (1.5MPa), 150oC.

3.3 Catalyst reuse and stability

The stability of C7mimHSO4-Co/ZSM-5 catalyst was monitored using multiple sequential aerobic oxidations of cyclohexane. After the catalyst was recovered by centrifugation, filtration, washed with cyclohexane and dried, it was used in the subsequent run. The results are shown in Figure6.

Figure 6 Stability and reusability of C7mimHSO4-Co/ZSM-5 catalyst in the aerobic oxidation of cyclohexane. Cyclohexane (0.1 mol), catalyst (6.0 wt%), O2 (1.5MPa), 150oC, 3h.
Figure 6

Stability and reusability of C7mimHSO4-Co/ZSM-5 catalyst in the aerobic oxidation of cyclohexane. Cyclohexane (0.1 mol), catalyst (6.0 wt%), O2 (1.5MPa), 150oC, 3h.

From Figure 6, the catalyst was without a significant loss of the activity and selectivity for KA oil after it was consecutively reused five times. The results showed that the supported catalyst has good activity and excellent stability for the oxidation of cyclohexane. This can be attributed to the channel range of the ZSM-5 molecular sieve that results in a difficult loss of the C7mimHSO4 ionic liquid, and on the other hand, a micro-reactor with high concentration of C7mimHSO4 ionic liquid was constructed with the C7mimHSO4 ionic liquids were introduced into the pores of ZSM-5 molecular sieve. This further led to good both cyclic stability and reactive activity of the C7mimHSO4-Co/ZSM-5 heterogeneous catalyst.

4 Conclusions

C7mimHSO4-Co/ZSM-5 was identified for the the first time as an effective and robust heterogeneous catalyst for the aerobic oxidation of cyclohexane. A selectivity of KA oil for the aerobic oxidation of cyclohexane was obtained as high as 93.6% with 9.2% conversion of cyclohexane using C7mimHSO4-Co/ZSM-5 as catalyst under relatively mild reaction conditions (150℃,3 h, and 1.5 MPa). The activity of C7mimHSO4-Co/ZSM-5 heterogeneous catalyst remains almost unchanged after five consecutive cycles. Further studies for both the characterization and performance of the C7mimHSO4-Co/ZSM-5 catalyst revealed that both the presence of oxygen vacancies with incorporation of Co ions into the framework of ZSM-5 and the introduction of C7mimHSO4 into the ZSM-5 led to the higher catalytic activity of C7mimHSO4-Co/ZSM-5. In-depth studies for the mechanism of the oxidation of cyclohexane with molecular oxygen over the cobalt/ZSM-5 modified by the ionic liquid catalysts are under way.

AcknowledgmentsŁ

This study was supported by the National Natural Science Foundation of China (21206019, 21276052, 21776049, 21878055), and the Science and Technology Program of Guangzhou, China (201607010166).

  1. Conflict of interest: Authors declare no conflict of interest.

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Received: 2018-08-19
Accepted: 2019-02-12
Published Online: 2019-08-21

© 2019 Yun Hong et al., published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 Public License.

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