highly efficient cobalt doped carbon nitride polymers for solvent free selective oxidation of cyclohexane

Lu, Highly Efficient Cobalt-doped Carbon Nitride Polymers for Solvent-Free Selective Oxidation of Cyclohexane, Green Energy & Environment 2017, doi: 10.1016/j.gee.2017.01.006.. M ANUS C

Highly Efficient Cobalt-doped Carbon Nitride Polymers for Solvent-Free Selective

Oxidation of Cyclohexane

Yu Fu, Wangcheng Zhan, Yanglong Guo, Yun Guo, Yunsong Wang, Guanzhong Lu

DOI: 10.1016/j.gee.2017.01.006

Reference: GEE 51

To appear in: Green Energy and Environment

Received Date: 9 January 2017

Revised Date: 23 January 2017

Accepted Date: 26 January 2017

Please cite this article as: Y Fu, W Zhan, Y Guo, Y Guo, Y Wang, G Lu, Highly Efficient

Cobalt-doped Carbon Nitride Polymers for Solvent-Free Selective Oxidation of Cyclohexane, Green Energy & Environment (2017), doi: 10.1016/j.gee.2017.01.006.

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Highly Efficient Cobalt-doped Carbon Nitride Polymers for Solvent-Free Selective Oxidation of Cyclohexane

Key Laboratory for Advanced Materials and Research Institute of Industrial Catalysis, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237, P R China

*Corresponding author, Email: zhanwc@ecust.edu.cn; gzhlu@ecust.edu.cn

Abstract: Selective oxidation of saturated hydrocarbons with molecular oxygen has

been of great interest in catalysis, and the development of highly efficient catalysts for this process is a crucial challenge A new kind of heterogeneous catalyst,

oxidation of cyclohexane X-ray diffraction, Fourier transform infrared spectra and high resolution transmission electron microscope revealed that Co species were highly dispersed in g-C3N4 matrix and the characteristic structure of polymeric g-C3N4 can

be retained after Co-doping, although Co-doping caused the incomplete polymerization to some extent Ultraviolet–visible, Raman and X-ray photoelectron

Co(II)–N bonds For the selective oxidation of cyclohexane, Co-doping can markedly

content exhibited the highest yield (9.0%) of cyclohexanone and cyclohexanol, as

catalysts was elaborated

Keywords: Selective oxidation of cyclohexane; Oxygen oxidant; Carbon nitride;

Co-doping

1 Introduction

C–H activation is always a spotlight for the development of chemical industry owing to the ubiquity of C–H bonds in organic molecules [1-3] However, saturated hydrocarbons consist of only strong and localized single bonds, C–C and C–H bonds,

so that they generally have no empty orbitals of low energy or filled orbitals of high

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energy that can be readily stimulated to react with other molecules [4-6] What is worse is that the reaction products are generally far more reactive than saturated hydrocarbons and thus are prone to further transform to by-products Among various selective oxidation reactions of hydrocarbons, selective oxidation of cyclohexane is a very attractive reaction because its oxidation products, cyclohexanone (K) and cyclohexanol (A), are the key intermediates for producing nylon-6 and nylon-66 [7]

To date, a large number of heterogeneous catalysts have been reported for the selective oxidation of cyclohexane, including transition metal oxides [8], carbon nanotubes [9,10], molecular sieves [11-13], etc Even though significant progress has been achieved, it is still a significant challenge to control the selectivity for the target products while obtaining a high conversion of cyclohexane because of the trade-off between selectivity and high conversion in selective oxidation reactions On the other hand, in order to reconcile the demand of economy and environment, there is an unremitting drive to pursue a solvent free system using molecular oxygen as the oxidant to exploit KA oil [14, 15]

excellent physicochemical stability, as well as unique electronic structure with a large band gap of 2.7 eV So far, most of the research efforts have been focused on its

metal-free heterogeneous catalysts [22-24] Wang et al reported boron and fluorine co-doped mesoporous carbon nitride as a metal-free catalyst for selective oxidation of

remarkably high selectivity to cyclohexanone [25] In addition, Yang and co-workers

found that it is highly effective for the selective oxidation of cyclohexene to

effort to reveal the catalytic activity of metal-doped carbon nitride polymers for solvent-free selective oxidation of cyclohexane Interestingly, metalloporphyrins possessing the similar structure with metal-doped carbon nitride have been found to

be effective for the selective oxidation of cyclohexane with air as homogeneous catalysts [27] Since homogeneous catalysts definitely suffer from the problems of separation and recycling, metal-doped carbon nitride polymers provide us the

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opportunity to develop scalable and effective heterogeneous catalysts for selective

polymers with different Co content were synthesized and their catalytic performances for selective oxidation of cyclohexane in a solvent free system were investigated

oxidation of cyclohexane with excellent conversion of cyclohexane and selectivity to

KA oil simultaneously Meanwhile, controlled experiments were made to elucidate the mechanism of this catalytic system

2 Experimental section

2.1 Synthesis of the catalysts

continuous stirring at 80 °C until the water was evaporated Consequently, the

2 °C/min The sample obtained was ground into powder and washed thoroughly with hot water to remove the residual Co precursor on the surface Finally, the solid was dried at 120 °C for 12 h, and the obtained samples with different Co contents were

dicyandiamide in the synthesis mixture The actual Co content in samples was detected by energy disperse spectroscopy and shown in Table 1

2.2 Characterization of catalysts

X-ray diffraction (XRD) data were collected on a Bruker D8 Focus diffractometer using Cu Kα radiation (40 kV, 40 mA) at room temperature The composition of

electron microscope equipped with TEAMEDS energy disperse spectroscopy (EDS) The Fourier transform infrared (FT-IR) spectra of samples were carried out on a Nicolet Nexus 670 FT-IR spectrometer, and the samples were ground with anhydrous KBr and pressed into thin wafers The Ultraviolet–visible (UV-Vis) spectra of

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samples were performed on a Varian Cary 500 spectrometer by using the diffuse

spectrometer Thermal gravimetric (TG) analysis was performed at a heating rate of

TGA thermal analyzer The structure information and elemental mapping images of the samples were measured on a high resolution transmission electron microscope (HR-TEM) (JEM-2100, JEOL) at an accelerating voltage of 200 kV X-ray photoelectron spectroscopy (XPS) was analyzed on a Kratos Axis Ultra-DLD system with Al Kα radiation and the XPS results were calibrated using the C 1s line at 284.8

eV

2.3 Testing of catalytic activity

The selective oxidation of cyclohexane was carried out in a 50 mL autoclave lined with polytetrafluoroethylene (PTFE) and equipped with an explosion-proof pressure sensor 4 g of cyclohexane, 20 mg of catalyst and 10 µL TBHP as initiator were added

to 1.0 MPa, and then the reactor was heated to a certain temperature under stirring at

300 rpm After the reaction, the reactor was quenched with ice water to avoid the losses due to evaporation of volatile organic compounds The reaction mixture was diluted with ethanol to thoroughly dissolve the side products After the catalyst was

added to completely convert the intermediate cyclohexylhydroperoxide (CHHP) to cyclohexanol The reaction products were analyzed by Agilent gas chromatograph 7890B equipped with an HP-5 capillary column and a flame ionization detector Methylbenzene was used as an internal standard substance In addition, the side-products were identified with Agilent 7890A-5975C gas chromatograph-mass spectrometry (GC-MS)

The recycling experiments of the catalyst were carried out under the same conditions mentioned above and the catalyst was repeatedly optimized for five times

in the selective oxidation reaction After each run, the catalyst was recovered from the reaction solution by centrifugation, washed with ethyl alcohol for three times, and then dried at 100 °C to constant weight in the air

3 Results and discussion

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3.1 XRD

at 2θ = 27.4°, which is associated with the typical interlayered stacking of the aromatic systems Another diffraction peak can also be discerned at 2θ = 13.1°, assigned to the in-plane structural packing motif of tri-s-triazine units Compared with

sharply decreases with the increase in the content of cobalt, in agreement with other

polymeric condensation of dicyandiamide during the synthesis process of the catalysts Meanwhile, no diffraction peaks of any crystalline phase of Co species appear in the

Co − g-C3N4(20)

Co − g-C

3 N

4 (15)

Co − g-C3N4(10)

Co − g-C3N4(5)

2Theta (Degree)

g-C3N4

3.2 TEM

a sheet-like structure (Fig 2a) and no agglomerated Co species can be observed Furthermore, elemental mapping results confirm that Co and N elements are

XRD results

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3.3 FT-IR

the acceleration of deamination by cobalt salts during the self-polymerization process

of dicyandiamide precursors On the contrary, the intensity of absorption peak at

Co-doping, which was in accord with XRD results [29, 30] In addition, a group of

characteristic stretching modes of aromatic CN heterocycles in the polymeric melon network and the triazine units, respectively [31, 32] On the whole, there is no

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4000 3500 3000 2500 2000 1500 1000 500

Co − g-C3N4(20)

Co − g-C3N4(15)

Co − g-C3N4(10)

g-C

3 N 4

Co − g-C3N4(5)

Wavenumber (cm -1)

3.4 Thermal analysis

TG curves of all the catalysts are present in Fig 4 All the catalysts exhibit a

catalysts shifts significantly to the lower temperature compared with that for pure

matrix due to the incomplete polymerization of the matrix as shown in XRD and FT-IR results

g-C3N4 Co-g-C3N4(5) Co-g-C3N4(10) Co-g-C3N4(15) Co-g-C3N4(20)

0 20 40 60 80 100

Temperature ( o C)

3.5 UV-vis and Raman

The UV-vis and Raman spectra were carried out to confirm the state of Co and

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energy transition of tetrahedral Co(II) species, is existed in UV-vis spectra of all

Raman patterns of the catalysts are presented in Fig 5b Neither peaks due to cobalt

(a)

Wavelength (nm)

g-C3N4

Co − g-C3N4(5)

Co − g-C3N4(10)

Co − g-C3N4(15)

Co − g-C3N4(20)

(b)

Raman Shift (cm -1)

Co−g-C3N4(20) Co−g-C3N4(15) Co−g-C3N4(10)

g-C3N4 Co−g-C3N4(5)

with different Co contents

3.6 XPS

catalysts There are three distinguishable peaks of N1s located at 398.8, 400.1 and 401.2 eV (Fig 6b), assigned to pyridinic N, pyrrolic N and graphitic N respectively, indicating the typical tri-s-triazine (melem) repeating building blocks of graphitic

deconvoluted into three peaks at 781.7, 785.2 and 788.0 eV The main peak at 781.7

matrix in the form of Co–N bonds [43]

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1200 1000 800 600 400 200 0

(a)

Co−g-C3N4(20) Co−g-C3N4(15) Co−g-C3N4(10) Co−g-C3N4(5) g-C3N4 C1s N1s O1s Co2p

Binding Energy (eV)

404 402 400 398 396 394

400.1

398.8

Co−g-C3N4(20) Co−g-C3N4(15) Co−g-C3N4(10) Co−g-C3N4(5) g-C3N4

(b)

Binding Energy (eV)

788.0 781.7 Co 2p

785.2

(c)

Binding Energy (eV)

Co − g-C3N4(20)

Co − g-C3N4(15)

Co − g-C3N4(10)

Co − g-C3N4(5)

catalysts with different Co contents

3.7 Catalytic performance of the catalysts for selective oxidation of cyclohexane 3.7.1 Effect of Co content

cyclohexane were evaluated and the results are shown in Table 1 In the blank testing, 3.5% conversion of cyclohexane and 89.1% selectivity to KA oil are

hybrids Furthermore, the conversion of cyclohexane monotonically increases with

conversion of cyclohexane gradually increases from 9.3% to 11.2%, when the content of Co increases from 3.2 wt% to 11.9 wt% On contrary, the selectivity to

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to the deep oxidation of products KA oil Furthermore, the ratio of cyclohexanol to cyclohexanone exhibits the similar trend as the selectivity to KA oil Comparing

of cyclohexanol to cyclohexanone and the lowest ratio is obtained over

conversion of cyclohexane and 84.6% selectivity to KA oil On the other hand,

selective oxidation of cyclohexane under the same reaction conditions for comparison As shown in Table 1, transition metal (Mn and Cu) or rare earth (Ce)

Catalyst Co content

(wt%)

Conversion (%)

Selectivity (%)

K/A

A K Others Blank test - 3.5 39.6 49.5 10.9 1.28 g-C 3 N 4 - 7.2 39.7 50.1 10.2 1.29

Co–g-C 3 N 4 (5) 3.2 9.3 33.5 57.4 9.1 1.75

Co–g-C 3 N 4 (10) 6.3 9.8 31.4 57.6 11.0 1.87

Co–g-C 3 N 4 (15) 9.0 10.6 29.4 55.2 15.4 1.92

Co–g-C 3 N 4 (20) 11.9 11.2 27.2 51.1 13.2 1.92

CoCl 2 +g-C 3 N 4

b

- 7.8 44.9 45.1 10.0 1.03

Co–g-C 3 N 4 (15)c 9.0 trace - - - -

Mn–g-C 3 N 4 8.8 8.0 36.1 37.5 26.4 1.06

Cu–g-C 3 N 4 9.2 8.8 36.6 49.7 13.7 1.39

Ce–g-C 3 N 4 8.7 9.1 32.8 56.3 10.9 1.75

a

Reaction conditions: 4 g cyclohexane, 20 mg catalyst, 10 µL TBHP as initiator, initial p(O2 ) = 1.0 MPa, at 140 °C for 4 h By-products mainly include diacids (succinic acid and adipic acid) and esters (dicyclohexyl adipate and hexanolactone)

b

Physical mixture of 4 mg CoCl 2 and 16 mg g-C 3 N 4 as catalyst

c

120 mg of hydroquinone was added.

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3.7.2 Effect of reaction temperature

Fig 7a shows the effect of reaction temperature on the selective oxidation of

cyclohexane sharply increases from 3.0% to 10.6% when the reaction temperature increases from 120 to 140°C However, continually increasing the reaction temperature to 160 °C, the conversion of cyclohexane remains steady On the contrary, the selectivity to KA oil gradually decreases from 93.4% to 78.8% with the increase

in the reaction temperature In summary, the reaction temperature of 140 °C is optimal, with an attractive conversion of cyclohexane and a moderate selectivity to

KA oil

0

5

10

15

20

25

30

Temperature (o C)

(a)

0 20 40 60 80 100

Conversion of cyclohexane Selectivity to Cyclohexanol

Selectivity to Cyclohexanone

0 5

10 20 40 60 80 100

Amount of Catalyst (mg)

(b)

Fig 7 The effect of reaction temperature (a) and amount of catalyst (b) on cyclohexane

oxidation over the Co–g-C 3 N 4 (15) catalyst.

3.7.3 Effect of catalyst amount

Fig 7b shows the effect of catalyst amount on the selective oxidation of

from 5 mg to 25 mg, the conversion of cyclohexane increases firstly and gradually comes to a standstill, while the selectivity to KA oil decreases Meanwhile, the highest ratio of cyclohexanone to cyclohexanol can be obtained when 20 mg of

is an optimal amount of catalyst under applied reaction conditions

3.7.4 The stability of the Co–g-C 3 N 4 (15) catalyst

oxidation of cyclohexane in solvent-free reaction system, its reusability was examined