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Interesting green catalysis of cyclohexane oxidation over metal tetrakis(4-carboxyphenyl)porphyrins promoted by zinc sulfide Guan Huang, Wei Lai Wang, Xing-xing Ning, Yao Liu, Shu Kai Zhao , Yong An Guo, Su Juan Wei, and Hong Zhou Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b00061 • Publication Date (Web): 07 Mar 2016 Downloaded from http://pubs.acs.org on March 7, 2016

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Industrial & Engineering Chemistry Research

Interesting green catalysis of cyclohexane oxidation over metal tetrakis(4-carboxyphenyl)porphyrins promoted by zinc sulfide

Guan Huang*, Wei Lai Wang, Xing Xing Ning, Yao Liu, Shu Kai Zhao, Yong-An Guo, Su Juan Wei, and Hong Zhou College of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, China Abstract To mimic the activity regulated by a cysteinate axial ligand in cytochrome P450 enzymes, metal tetrakis(4-carboxyphenyl)porphyrins (M TCPP) were immobilized on ZnS. They were characterized by a variety of spectroscopic techniques and were used as a catalyst for cyclohexane oxidation. The small amount of metal (Co, Fe, or Mn) TCPP (1.1 µmol) in the immobilized catalyst could be reused three times for oxidation, providing more than 6.5 ×105 turnover number and 20% molar yield of cyclohexanone and cyclohexanol (KA oil) on average. The coordination of ZnS played a key role. These catalysts were applied to green catalytic oxidation of cyclohexane to produce KA oil products. Key words: zinc sulfide, promotion, metal porphyrin, oxidation, dioxygen, cyclohexane. 1. Introduction Cytochrome P450 is one of the thiolate-ligated enzymes, which are unique among heme proteins in that they catalyze insertion of an oxygen atom into a variety of organic ______________ *Corresponding author. Tel.: +86 771 3237868; fax: +86 771 2851043. E-mail address: [email protected] (G. Huang) 1

ACS Paragon Plus Environment


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substrates.1 The cysteinate proximal axial ligand plays a crucial role in cytochrome P450 enzymes, chloroperoxidase, and nitric oxide synthase. The deprotonated cysteine is thought to exert a "push" effect on the heme iron and is very important in maintaining the monooxygenase activity in cytochrome P450 enzymes.2 However, the inactive forms of cytochrome P450 and chloroperoxidase do not retain their thiolate ligation in the ferrous state.3 Many scientists wanted to determine how the thiolate ligation affected heme catalytic activity4,5 and how to mimic its function using metalloporphyrins as a model. The goal was to understand the purpose of donating thiolate in cytochrome P450 function in vivo,6–13 so that thiolate ligation could be incorporated into the catalytic activity of metalloporphyrins in homogeneous catalytic systems. Currently, stronger thiolate coordination to the metalloporphyrins in catalytic systems has been proposed only for theoretical consideration.14 However, few studies have focused on additional exploration to increase catalytic activity of metalloporphyrins via the thiolate ligand for aerobic oxidation of hydrocarbons in heterogeneous systems, or even in homogeneous systems, in practice. 14,15

To prevent the undesired reaction in which the thiolate ligand is oxidized to disulfide by

molecular oxygen, bulky protecting groups were added around the thiolate axial ligand.16 First, based on the previously discussed guidelines and studies where an oxygen or a nitrogen atom with a lone electron pair was included in the supporting medium, such as Al2O3,17 AlOOH,18,19 MCM-41,20,21 ZnO,22–26 silica modified with imidazole groups (CPTMS)27 and the porous porphyrinic framework materials28 for axial ligation to the metalloporphyrins, based on enhanced catalytic performance of the Fe- and Mn-porphyrins in which the imidazole and pyridine groups served as the axial ligands.29–32 Second, many new techniques for catalytic cyclohexane oxidation are available that offer high yields of the 2


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corresponding main products,33–39 but applications of solvents and non-oxygen(O2) oxidizing agents,33–39 photochemical techniques,34,35 and the relatively long oxidation time have been disappointing,33–38 which will eventually increase energy consumption and pollution. The biomimetic catalysis system proposed by Lyons is environmentally friendly because it is a green catalysis system that only requires the catalyst, reaction substrate, and oxygen but does not require co-reductants, stoichiometric oxidants or photochemical techniques, even any solvents.22 Based on these advantages of the Lyons catalytic system, we used ZnS as the support for metalloporphyrins. Sulfur was chosen because it is one of chalcogenides and is less electronegative than oxygen or nitrogen. Furthermore, the supported metalloporphyrins are desired to use as better catalysts in a green catalytic oxidation system, the Lyons catalysis system22 in practical chemical industry. Although we have reported that iron tetrakis(4-carboxyphenyl)porphyrin can be supported on zinc sulfide, Fe TCPP/ZnS catalyzed the aerobic oxidation of cyclohexane to KA oil with moderate yields, the evidence for the promotion of the thiolate ligand(S2-) on the catalysis of the metalloporphyrin for the oxidation of cyclohexane has not been disclosed.40 In this work, ZnS

was

used

as

a

thiolate

ligand

resource,

on

which

metal

tetrakis(4-carboxyphenyl)porphyrins were immobilized and then used to catalyze oxidation of cyclohexane with O2 under the conditions of green industrial catalysis. We chose these materials because one of our other goals was to explore how the type of thiolate ligand affected the catalytic power of the metalloporphyrins. 2. Experimental 2.1. Chemicals All reagents and solvents were of analytical grade and were obtained commercially. 3



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Tetrakis(4-carboxyphenyl)porphyrin (TCPP), cobalt (Co) TCPP, iron (Fe) TCPP, and manganese (Mn) TCPP were synthesized as previously described.41–43 Cyclohexane was analyzed by gas chromatography before use to ensure that no oxidation products were present. 2.2. Equipment A PerkinElmer L-17 spectrometer was used to record the UV-Vis spectra of the immobilized catalyst in an ethanol or dimethylformamide (DMF) suspension, and the unsupported catalyst in either of the same solvents. A PerkinElmer ( model 783) IR spectrophotometer (Waltham, MA, USA) was used to record Fourier transform infrared (FTIR) spectra of the M TCPP/ZnS catalysts in the range of 4000 to 400 cm−1 at a resolution of 4 cm−1, using the potassium bromide pellet method. X-ray photoelectron spectroscopy (XPS) measurements were performed with an X-ray photoelectron spectrometer (XPS) (AXIS Ultra DLD; Kratos, Manchester, UK), equipped with an Al Kα radiation source, at 150 W with a pass energy of 40 eV. A Tecnai G2 F20 S–TWIN transmission electron microscope (Hillsboro, OR, USA) with a 100 kV accelerating voltage was employed to measure the particle sizes of the M TCPP/ZnS catalysts and ZnS support. A NETZSCH STA 409 PC thermal analyzer (Burlington, MA, USA) was used to record thermoanalytical curves (thermogravimetric [TG]) for the ZnS and M TCPP/ZnS samples. The measurements were carried out using 65-mg samples in air over a temperature range of 0 to 1,000 ºC, with a heating rate of 10ºC/min. 2.3. Preparation of metal tetrakis(4-carboxyphenyl)porphyrins immobilized on ZnS 0.26 mol zinc sulfate (ZnSO4 • 7H2O) was dissolved in 0.20 L distilled water, and then 4


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0.20 L of 1.30 M sodium sulfide (Na2S • 9H2O) was slowly added as the precipitating agent. The white ZnS precipitate that formed was filtered and washed with distilled water until no SO4–2 ion could be detected. Then the precipitate was added to 150 mL ethanol in a three-necked flask with stirring at high speed for 0.5 h. Subsequently, 25 mg metal (Co, Fe or Mn) tetrakis(4-carboxyphenyl)porphyrins( M TCPP ) dissolved in 20 mL absolute ethanol was slowly added to the suspension. The mixture was heated to 70°C with rapid stirring for 5 h. The porphyreus (Co), laurel green (Fe) or ink green (Mn) suspension was filtered and washed with bulk distilled water, and the cake was dried at 0.08 MPa and 160°C for 5 h to give 25 g of the supported catalyst, M TCPP/ZnS. The amount of immobilized metalloporphyrin in the M TCPP/ZnS catalyst was determined by atomic emission spectroscopy with inductive coupled plasma (ICP-AES, Spectroflame – FVMØ3). The samples were digested by a traditional acid method (HF, HClO4, HNO3, and HCl), diluted adequately, and analyzed for Co, Fe, and Mn.44 The amounts of Co TCPP, Fe TCPP, and Mn TCPP immobilized per gram of the supported catalyst were 1.00 mg, 1.01 mg, and 1.19 mg (1.13 µmol, 1.15 µmol, and 1.35 µmol, respectively). These values agreed with those determined by ultraviolet–visible (UV-Vis) spectrophotometry.45 2.4. Oxidation of cyclohexane over the M TCPP/ZnS catalyst Cyclohexane oxidation was performed over the catalyst obtained by immobilization of the M TCPP on ZnS nanoparticles. The oxidation was carried out in a KCF-10 250-mL autoclave reactor equipped with a magnetic stirrer and a frozen ethanol re-condenser at –20ºC.24 Cyclohexane (200 mL) and catalyst (containing 1.15 × 10–6 mol M TCPP) were placed in the autoclave reactor, charged with nitrogen at a desired pressure and heated to a desired temperature. Afterward, oxygen was continuously pumped into the reaction 5



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system, and the desired pressure was maintained. The oxygen flow was measured with a rotameter, and the oxygen concentration of the tail gas was determined with a CYS-1 digital oxygen detector. The reaction mixture samples taken at regular intervals were identified by gas chromatography–mass spectrometry (GC-MS) and were quantified by a Shimadzu GC-16A chromatograph equipped with a 30 m × 0.32 mm × 0.5 µm FFAP capillary column and a flame ionization detector. Chlorobenzene was used as the internal standard.19 Generally, the oxidation stage lasted 4 h, and it was terminated so that the support catalyst could be reused. It was recovered by simple filtration from the reaction mixture, followed by washing with ethanol and air-drying to extract any reaction product that might have been retained in the catalyst. This was then used in subsequent cyclohexane oxidation reactions. 3. Results and discussion 3.1. Supported catalyst characterization 3.1.1. UV-Vis spectra study of M TCPP/ZnS The changes of UV-Vis spectroscopy from M TCPP to M TCPP/ZnS demonstrate the presence of immobilized M TCPP in the M TCPP/ZnS. The UV-Vis spectra data of M TCPP and M TCPP/ZnS are presented in Table 1, which show the characteristic Soret peak of immobilized M TCPP. When the M TCPP samples were immobilized on the ZnS support, the Soret peaks of Co TCPP and Fe TCPP were blue-shifted about 13 nm and 3 nm, respectively. The blue-shift phenomenon is opposite to the red-shifts reported elsewhere. 46–50

These mean that the original non-planar M (Co and Fe) TCPP molecules are in a

co-planar configuration on the surface of the ZnS crystals. We suggest that, when the original non-planar M (Co and Fe) TCPP are immobilized on the surface of the ZnS crystal, the interaction between the polar carboxyl groups on the phenyl rings of the metal porphyrin 6


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and the polar ZnS on the surface of the ZnS crystal cause it to become more planar. The immobilization process on ZnS sterically pushes the stationary metal porphyrin, so that the porphyrin a2u orbital (the highest occupied molecular orbital) is farther away than the porphyrin eg orbital (the lowest unoccupied molecular orbital). This causes a blue-shift in the Soret band.22 The immobilization situations are quite different, because the blue-shifted values are not the same (13 nm and 3 nm). In addition, when Mn TCPP was Table 1 UV-Vis data of tetrakis(4-carboxyphenyl)porphyrins, metalloporphyrins, and the corresponding supporting catalysts Samples

Solvent

ε

Soret Band

Q Band

/×105 (Lmol-1cm-1)

/nm

/nm

H2 TCPP

Ethanol

1.65

416

512, 547,590, 646

Co TCPP

Ethanol

0.34

430

541

Co TCPP/ZnS

Ethanol

5.23

417

594

Fe TCPP

DMF

0.28

418

503, 562, 683

Fe TCPP/ZnS

DMF

1.09

415

507, 568, 681

Mn TCPP

DMF

0.35

467

566, 598

Mn TCPP/ZnS

DMF

1.49

473

614, 654

immobilized on the ZnS, the Soret peak was red-shifted about 6 nm. It is possible that porphyrin ring distortion is different for each M TCPP, which results in different energy gaps between the a2u orbital and the eg orbital.50 3.1.2. FTIR characterization of M TCPP/ZnS FTIR spectra of ZnS, M TCPP, and M TCPP/ZnS are shown in Fig. 1. The wide band at 7



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3449 cm−1 and the sharp bands at 1640 cm−1 are assigned to the stretching and bending vibrations of O–H groups from the water contained within the ZnS support, as suggested by previous studies.51–53 The band at 1402 cm−1 may be assigned to Zn–OHCO2, and the two small bands at 1117 cm−1 and 1001 cm−1 are assigned to the stretching vibration of SO4−2.51 The band at 655 cm−1 is assigned to the Zn–S vibrations.52 For the spectra of M TCPP, the broad bands at 3420 to 3445 cm−1 and the sharp bands at 1686 to 1698 cm−1 are assigned to the vibrations of O–H and C=O groups, respectively, from the carboxyl

Fig. 1 Fourier-transform infrared spectra of M TCPP/ZnS (for Co, Fe, and Mn) and ZnS, respectively, with an effective frequency range of 4000 to 400 cm-1 groups of M TCPP. The sharp bands at 1507 to 1638 cm−1 are assigned to the vibrations of C=C in the porphyrin ring. The sharp bands at 1386 to 1401 cm−1 are assigned to the vibrations of C–N bands, and the sharp bands at 1263 to 1273 cm−1 are assigned to the vibrations of C–OH bands54. The bands at 1000 to 1011 cm−1 and 690 to 801 cm−1 are respectively assigned to the rocking and bending vibrations of C–H bands for the pyrrole 8


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and benzene rings.55,56 After each M TCPP is immobilized on the ZnS, the original peaks for the ZnS and the M (Co, Fe, and Mn) TCPP appear at 655 cm−1, 612 cm−1, 619 cm−1 and 623 cm−1, respectively. There are characteristic peaks at 441 cm−1, 472 cm−1, and 468 cm−1 for the M (Co, Fe, and Mn) TCPP/ZnS, which may be the vibrations of M (Co, Fe, and Mn)–S, respectively. 3.1.3. X-ray photoelectron spectroscopy analysis for M TCPP/ZnS The electron binding energies and results of X-ray photoelectron spectroscopy (XPS) of M (Co, Fe, and Mn) TCPP, M (Co, Fe, and Mn) TCPP/ZnS, and ZnS are shown in Table 2 and Fig. 2, respectively. In Table 2, the peak values at 284.72 to 284.83 eV and 288.48 to 289.18 eV can be attributed to the C (1s) in the C=C unit and the C–H unit, respectively, for both the M (Co, Fe, and Mn) TCPP and M (Co, Fe, and Mn) TCPP/ZnS. The peak values at 531.94 to 531.56 eV and at 531.44 to 531.42 eV can be attributed to the O (1s) in the HO–C unit and the O=C unit, respectively, for M (Co, Fe, and Mn) TCPP/ZnS. The peak values at 532.48 to 532.30 eV and at 532.11 to 532.17 eV can be attributed to the O (1s) in the HO–C unit and the O=C unit, respectively, for the M (Co, Fe, and Mn) TCPP. The peak values at 398.68 to 400.08 eV and 398.78 to 401.13 eV can be attributed to the N (1s) in the N–C= unit and the N–M unit, respectively, for the M (Co, Fe, and Mn) TCPP and M (Co, Fe, and Mn) TCPP/ZnS. Of these, the peak values of Fe TCPP are quite close to those previously reported.57 The two N 1s peaks present at lower and at higher binding energies can be attributed to the N–C= unit and the N–M unit, respectively, for both the M (Co, Fe, and Mn) TCPP/ZnS and M (Co, Fe, and Mn) TCPP. Interestingly, the oxidation state of the M (Co, Fe, and Mn) ion in the M (Co, Fe, and

Mn) TCPP/ZnS and M (Co, Fe, and

Table 2 Binding energy of core electrons for the key elements of ZnS, 9



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M TCPP/ZnS (for Co, Fe, and Mn) and M TCPP (for Co, Fe, and Mn) XPS spectra

Existential form of

the

Binding energy/eV

key ZnS

elements S 2p

M 2p

N 1s

C 1s

Cl 2p

Fe,

Mn)

TCPP/ZnS

(Co,

Fe,

Mn)

–

–

TCPP

S–Zn

163.27

162.38 162.38

S–Zn

161.87

161.73

161.53 161.48

–

–

–

162.48

–

–

–

–

S–M

–

162.58

162.48 162.68

M–N

–

781.03

702.83

641.37 781.53

711.53 642.82

–

780.48

702.23

639.62 780.73

711.18 642.12

N–C=(N=C)

–

398.68

400.08

399.92 398.93

398.73 399.02

N–M

–

400.13 401.13

C=C

–

C–H O 1s

(Co,

–

400.77

399.48 398.78 399.87

284.83

284.78 284.72

288.48

288.73

288.62 289.18

288.98 288.67

531.56 532.48

532.39 532.30

O–C

–

531.94

531.75

O=C

–

531.44

531.43 531.42

Cl–M

–

198.99

199.05

284.78 284.78 284.77

532.11

199.10 200.00

532.14 532.17 201.04

201.11

Mn) TCPP is +3. The binding energies of the oxidation state for the M (Co, Fe, and Mn) TCPP decreased when each M (Co, Fe, and Mn) TCPP was immobilized on the ZnS. This suggests that these shifts to lower binding energies were related to an electronic interaction of the bound metal ion with the sulfur anion in the support ZnS. The coordination of the sulfur anion to the M (Co, Fe, Mn) ion increased the electron density around the M (Co, Fe, Mn) ion, and then the corresponding electron binding energy was reduced, for example, from 781.53 eV to 781.03 eV, and so on. Similar results were previously reported.58–60 More 10


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importantly, for the ZnS support, there are two S 2p peaks at 161.87 eV and 163.27 eV (Fig. 2), respectively.61,62 However, when each M (Co, Fe, Mn) TCPP was immobilized on the ZnS, the binding energies of the two S 2p peaks decreased for all metals (Table 2). This suggests that the coordination of the sulfur anion to the zinc ion decreased the electron cloud density around the sulfur anion in ZnS, and the sulfur element has a higher electron binding energy. Whereas during immobilization, the electron cloud around the sulfur anion in ZnS was shared with the M (Co, Fe, Mn) ion in the metalloporphyrin; so a part of the electron cloud ligated to the M ions, resulting in a lower electron binding energy, for example, from 161.87 eV to 161.73 eV and so on. This explanation agrees with the decrease in the corresponding electron binding energy around the M (Co, Fe, Mn) ion in the supported materials, M (Co, Fe, Mn) TCPP/ZnS. All results above just discussed indicate that the metalloporphyrins were immobilized on ZnS mainly in coordination and next in electrostatic retention.39 However, all electron cloud densities changed for the M (Co, Fe, Mn) ions in the supported material at this tiime. Scheme 1 gives a state of electron cloud density for the metal ion in M (Co, Fe, Mn) TCPP and M (Co, Fe, Mn) TCPP/ZnS. First, in both M (Co, Fe, Mn) TCPP and M (Co, Fe, Mn) TCPP/ZnS (part 1 in Scheme 1), the sizes of the electron cloud density increased in the same order in both series. The cobalt ion had the lowest electron cloud density (noted as ‘l.e.’) in both series and the manganese ion possessed the highest electron cloud density (noted as ‘h.e.’) in both series. These results are based on the binding energy data of the electrons in the 2p orbital of the metal ions shown in Table 2. Because the cobalt ion had the lowest density, it would be the most

11



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Fig. 2 X-ray photoelectron spectroscopy spectra of N 1s, M (Co, Fe, Mn) 2p, and sulfur 2p for M (Co, Fe, Mn) TCPP, M (Co, Fe, Mn) TCPP/ZnS, and ZnS.

12


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Electron cloud density changed to be lower __

N Co*(l.e.)

N__Fe*(m.e.)

N__Mn*(h.e.)

M TCPP

1. Electron cloud density changed to be lower

N__Fe*(m.e.)

S

__

Zn __

__

N*(m.e.) Co(m.e.)

N*(h.e.) Fe(l.e.) N*(l.e.)__Fe(h.e.)

N*(h.e.)__Co(l.e.)

S

__

Zn

3.

N Co(m.e.)

S*(m.e.) Zn

Zn __

N Fe(l.e.)

M TCPP M TCPP/ZnS

S Zn

N__Mn(h.e.)

S*(h.e.) Zn

S*(l.e.) Zn

__

__

N*(m.e.)__Mn(m.e.)

__

S

N*(l.e.)__Mn(h.e.)

__

__

Zn

Zn

M TCPP/ZnS

S

__

S

2.

N__Mn*(h.e.)

M TCPP/ZnS

__

N__Co*(l.e.)

__

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Scheme 1 State of electron cloud density for the M ions in M TCPP and M TCPP/ZnS (for Co, Fe, and Mn) active, followed in sequence by the iron ion and then the manganese ion. At first glance, it seems that the S anion in ZnS did not act on the M (Co, Fe, Mn) ion. In fact, after M (Co, Fe, Mn) TCPP was bound to ZnS, the electron cloud densities around the nitrogen atom and the sulfur atom adjacent to the M (Co, Fe, Mn) ion were changed, as shown in parts 2 and 3 of Scheme 1. These conclusions are based on the binding energy data of electrons in the 1s orbital of nitrogen and the 2p orbital of sulfur from Table 2. XPS data only give the electron binding energies rather than a quantum chemical analysis of the electron cloud densities. Considering the equilibrium within the bonding electron cloud of the bond, the nitrogen atom in the N–Co unit of Co TCPP possessed the middle binding energy values (399.48 eV and 398.93 eV), compared with those in the N–Fe unit of Fe TCPP and in the N–Mn unit of Mn TCPP. Therefore, it would be a nitrogen atom with the middle electron cloud density (m.e.), and then the corresponding cobalt ion should have the same m.e. Then the iron ion would have the lowest electron cloud density (l.e.), and the manganese 13



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ion would have the highest electron cloud density (h.e.), as shown for the M TCPPs in part 2 of Scheme 1. In the same way, we assessed the size of the electron cloud densities around the M (Co, Fe, Mn) ion in M (Co, Fe, Mn) TCPP/ZnS, which were affected by the changes in electron cloud density from the nitrogen and sulfur atoms in the supported materials. We found that the effects of the nitrogen atoms on the M (Co, Fe and Mn) ion resulted in the lowest electron cloud density (l.e.) for the cobalt ion [denoted as (l.e.)N], the highest electron cloud density (h.e.) for the iron ion [denoted as (h.e.)N], and the middle electron cloud density (m.e.) for the manganese ion [denoted as (m.e.)N]. The effects of the sulfur atoms on the M (Co, Fe and Mn) ions resulted in the middle electron cloud density (m.e.) [denoted as (m.e.)S], the lowest electron cloud density (l.e.) [denoted as (l.e.)S], and the highest electron cloud density(h.e.) [denoted as (h.e.)S], respectively. Therefore, on average, Co TCPP/ZnS would be the most active catalyst material, because it possesses the lowest electron cloud density in the cobalt ion, [(l.e.)N+(m.e.)S]/2, the next would be Fe TCPP/ZnS, because it possesses the middle electron cloud density in the iron ion, [(h.e.)N+(l.e.)S]/2}, and the least active would be Mn TCPP/ZnS, because it possesses the highest electron cloud density in the manganese ion, [(m.e.)N+(h.e.)S]/2. 3.1.4. Micro and macro characteristics of M TCPP/ZnS For understanding the characteristics of the catalyst material particles, transmission electron microscopy (TEM) images of ZnS and M (Co, Fe, Mn) TCPP/ZnS were studied. In Fig. 3, 10-nm and 30- to 50-nm particles of ZnS and M (Co, Fe, Mn) TCPP/ZnS, respectively, are shown. The particle sizes of M (Co, Fe, Mn) TCPP/ZnS are about four times larger than ZnS. This may be due to the polarity of the four carboxyl groups in the

14


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Fig. 3 TEM images (Left) and appearance characteristics (Right) of ZnS and M (Co, Fe, Mn) TCPP/ZnS immobilized M (Co, Fe, Mn) TCPP, which would attract the polar ZnS. Inversely, the support would promote an increasing electropositivity in the M (Co, Fe, Mn) ions, which would further enhance the coordination of the sulfur anion to the metalloporphyrins and regulate their catalytic activity. However, at a macro level, only the larger size of M (Co, Fe, Mn) TCPP/ZnS can be seen (Fig. 3). These interesting structural characteristics of the immobilized catalysts will promote the oxidation of cyclohexane. 3.1.5. Thermogravimetric analyses of M TCPP/ZnS The thermal stability of M (Co, Fe, Mn) TCPP/ZnS and ZnS is very important when they are used as catalysts for oxidation. The results of an investigation of their thermostability are shown in Fig. 4. The weight loss for the solid catalyst and support was smaller than 5% at temperatures below 200 ºC, and this is attributed to the loss of the surface moisture contained in the ZnS.63 Our data indicate that the M (Co, Fe, Mn) TCPP/ZnS frameworks are very stable. 15



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Fig. 4 Thermogravimetric and differential thermogravimetry analyses of ZnS and M (Co, Fe, Mn) TCPP/ZnS 3.2. Oxidation of cyclohexane over M TCPP/ZnS The oxidation of cyclohexane over M (Co, Fe, Mn) TCPP or ZnS immobilized M (Co, Fe, Mn) TCPP produced mainly cyclohexanol and cyclohexanone. The oxidation of cyclohexane is described in Scheme 2. The support compound, ZnS, cannot by itself catalyze the oxidation. M TCPP/ZnS or M TCPP

OH

O

+

+

by-products

O2 , T. P.

Scheme 2 The reaction products were identified by GC-MS, and were quantified by GC using internal standard methods and chemical analysis. The analyses show that the by-products were cyclohexyl hydroperoxide, hexanedioic acid, and dicyclohexyl adipate. However, the 16


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amount of cyclohexyl hydroperoxide was very small. In spite of the differences in catalytic activity among metalloporphyrins and their immobilized materials, the catalysis mechanisms are quite similar, except the cobalt porphyrins.24,64–66 So, we suggest that (ZnS/TCPP M[Ⅳ]=O)

＋

is the important intermediate

for the catalysts made by immobilizing M(Co, Fe, Mn) TCPP with ZnS to oxidize cyclohexane under our experimental reaction conditions. We also suggest that M[Ⅲ] TCPP/ZnS and M[Ⅲ] TCPP/ZnS are responsible for the decomposition of cyclohexyl hydroperoxide into the main products. Based on the our best understanding of hydrocarbon oxidation with O2,67–69 the very small amount of cyclohexyl hydroperoxide detected by chemical analysis, and especially the comparison of the binding energies of metal core electrons for M (Co, Fe, Mn) TCPP/ZnS with those of M (Co, Fe, Mn) TCPP, a plausible mechanism for the catalytic oxidation of cyclohexane is shown in Scheme 3.

Scheme 3 A plausible mechanism for cyclohexane oxidation with the ZnS-immobilized metalloporphyrins The differences in catalytic performance of the two types of materials, M (Co, Fe, Mn) TCPP/ZnS and M (Co, Fe, Mn) TCPP, may originate in two aspects. First, the ZnS-immobilized metalloporphyrins are well protected by the coordination between the 17



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metalloporphyrins and the ZnS, because their M (Co, Fe, Mn) ions have lower binding energy than those in the unsupported compounds (Table 2) (i.e., those that have the higher electron cloud densities). Second, the coordination of the sulfur atom of ZnS to M (Co, Fe, Mn) TCPP promotes the catalysis of the metalloporphyrins for the hydrocarbon oxidation, probably because, after immobilization, the binding energies of Cl 2p in the ZnS-immobilized metalloporphyrins are decreased the most (Table 2), i.e. around which are the higher electron cloud density, while the corresponding M (Co, Fe, Mn) ion have also higher electron cloud densities. Therefore, there is a repulsive Coulomb force between elements of like charge between the M (Co, Fe, Mn) ion and the chloride ion. This repulsion would more easily promote a homolytic cleavage of a M (Co, Fe, Mn)–Cl bond under certain thermal reaction conditions,18,70,71 changing M (Co, Fe, Mn)[Ⅲ] TCPP/ZnS to M (Co, Fe, Mn)[Ⅱ] TCPP/ZnS (Scheme 3). 3.2.1. Effect of reaction temperature on M TCPP/ZnS–catalyzed cyclohexane oxidation The most important factor for the oxidation is temperature. There are no free oxygen atoms below 227 ºC in the absence of catalysts, because a Kp for the dissociation of molecular oxygen is 2.4 × 10-46 at 227 ºC.72 Tolman and co-workers reported that the auto-oxidation of cyclohexane yielded a small amount of cyclohexanol and cyclohexanone 69

. Their similar results (not more than 4% yields of KA oil) were obtained from oxidation

over ZnS with no catalyst. So, the M (Co, Fe, Mn) TCPP should be a catalyst for the oxidation of cyclohexane. The changes in yields (ketone + alcohol) and catalyst turnover number (TON) with reaction temperature for the oxidation over M (Co, Fe, Mn) TCPP/ZnS with a 4 h reaction time are shown in Fig. 5A. Results suggest that a reaction temperature that is either too low 18


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or too high would decrease the catalytic efficiency of the ZnS-immobilized catalysts. Generally, too high a temperature results in over oxidation of the main products into by-products and damage to the core catalyst, consequently resulting in a decrease in yields.22,73 In addition, Fe TCPP/ZnS seems to be the best catalyst, because it produces the highest catalyst TON and the highest yields of KA oil under the same reaction conditions of 0.8 MPa and 1.1 × 10-6 mol M TCPP. As far as the effect of temperature on oxidation over the M (Co, Fe, Mn) TCPP/ZnS, the optimal oxidation temperatures for Fe (or Mn) TCPP/ZnS and Co TCPP/ZnS are 160 °C and 165 °C, respectively. Increasing reaction temperature benefits the oxidation by cleaving the M–Cl bonds in M (Co, Fe, Mn) TCPP/ZnS and the O–O bonds in the peroxo complex O2M(Co, Fe, Mn)[Ⅲ] TCPP/ZnS (Scheme 3). However, a reaction temperature that is too high (e.g., higher than 170 °C) does not increase catalyst TON. 3.2.2. Effect of reaction pressures and the amount of M TCPP on M TCPP/ZnS–catalyzed cyclohexane oxidation O2 pressure is the second most important factor for the oxidation. Figure 5B gives the changes in yield (KA oil) and catalyst TONs with O2 pressure with a 4 h reaction time and under their optimal reaction temperatures for cyclohexane oxidation over M (Co, Fe, Mn) TCPP/ZnS. The same amount of metalloporphyrin was used in each test. The oxygen concentration in the autoclave was the second main factor, affecting the cyclohexane oxidation at a given optimal reaction temperature and the same amount of metalloporphyrin. Similar to the effect of reaction temperature on the oxidation, a proper O2 pressure is needed for obtaining high catalyst TON and yield of KA oil from the oxidation. An oxygen concentration that was too high destroyed a small fraction of the metalloporphyrins on the 19



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ZnS surface and resulted in decreased yields.73–75 Compared with the catalytic efficiency of the two other immobilized catalysts, Co TCPP/ZnS was the most active catalyst, because it gave the highest catalyst TON and the greatest yields of KA oil at the optimal reaction conditions. The third most active was the Mn TCPP/ZnS catalyst. Even using different amounts of M (Co, Fe, Mn) TCPP in the M (Co, Fe, Mn) TCPP/ZnS as the catalyst for the oxidation, the order of catalytic activity is the same as that just described.

Fig. 5 Changes in yields (KA oil) [black] and catalyst turnover numbers [blue] with reaction temperatures (A), reaction pressures (B), and catalyst amount (C) for cyclohexane oxidation over M (Co, Fe, Mn) TCPP/ZnS. Changes in yields (KA oil) [black] and catalyst turnover numbers[blue] with reaction time(D) for cyclohexane oxidation over Co TCPP/ZnS and Co TCPP (1.1 × 10-6 mol Co TCPP) at optimum reaction conditions. Reaction conditions: 200 mL cyclohexane, 0.040 20


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m3/h oxygen flow, and reaction time of 4.0 h. Figure 5C provides the changes in yields (KA oil) and catalyst TONs with different amounts of metalloporphyrin, used in a 4 h reaction time and under the optimal reaction temperature and pressure for oxidation over M (Co, Fe, Mn) TCPP/ZnS. The natural law relating their activity changes to the use of different amounts of catalyst is similar to previous studies, i.e. so-called “catalyst inhibitor conversion”.18,74–76 Table 3 lists the data for the first use of six catalysts for cyclohexane oxidation at their Table 3 Comparison of the catalytic

Catalysts

performance

of the six catalysts at first use

Optimal reaction

Conversion Selectivity TON

conditions

Yield

/%

/%

/(×105)

/%

Co TCPP/ZnS

165 °C, 0.9 MPa

72.9

37.1

9.4

27.0

Fe TCPP/ZnS

160 °C, 0.8 MPa

64.9

37.6

8.6

24.4

Mn TCPP/ZnS

160 °C, 0.8 MPa

33.4

63.9

4.9

21.3

Co TCPP

165 °C, 0.9 MPa

38.9

51.7

5.5

20.1

Fe TCPP

160 °C, 0.8 MPa

35.6

47.9

5.1

17.1

Mn TCPP

160 °C, 0.8 MPa

21.9

62.1

3.3

13.6

optimal reaction conditions, further proving our conclusions about the order of activity. When we investigated the reuse of the immobilized catalysts, the data indicate that their average catalytic power also follows the same order of activity (Table 4). This means that the order of the catalytic activity of the three immobilized catalysts is highly consistent with the order of changes in the binding energies for the metal ion in the catalytic active center of immobilized metalloporphyrins. 21



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3.2.3. Recycling and practicability of the M TCPP/ZnS catalyst A comparison of the catalytic power of each M (Co, Fe, Mn) TCPP/ZnS for the oxidation reaction with that of the corresponding unsupported metalloporphyrins (Tables 3 and 4) reveals that all ZnS-immobilized M (Co, Fe, Mn) TCPP have better catalytic efficiency at the time of reuse. First, this would be due to the stability of M (Co, Fe, Mn) TCPP/ZnS frameworks, which enhances reusability of the M (Co, Fe, Mn) TCPP for the oxidation reaction. The ZnS support also increases the catalyst TON and the yields of KA oil; based on the former, the catalytic activity of M TCPP increase about 48% to 71%. Secondly, the catalytic results should be due to the coordination of ZnS to the metalloporphyrins, changing the electron cloud density around the center ion of the M (Co, Fe, Mn) TCPP. This is true not only at the end of the 4 h reaction time but also during the entire course of catalysis. As shown in Fig. 5D, not only the rake ratio (3.2861) of yields (KA oil), but also the rake ratio (1.2481) of catalyst TON by oxidation over the Co TCPP/ZnS are Table 4 Comparison of the catalytic performance of the three kinds of supported catalysts reused for the same number of cycles Catalysts

Optimal reaction Conversion Selectivity TON

Yield*

Recycling

conditions

*/%

*/%

*/(×105)

/%

times

Co TCPP/ZnS

165 °C, 0.9 MPa

67.8

34.2

8.9

23.3

3

Fe TCPP/ZnS

160 °C, 0.8 MPa

51.8

42.3

7.1

21.6

3

Mn TCPP/ZnS

160 °C, 0.8 MPa

46.1

46.1

6.5

20.0

3

*Average values

higher than those (2.1693 and 0.6954 respectively) of the Co TCPP. Catalysts such as, Fe TCPP/ZnS and Mn TCPP/ZnS, would show similar trends.22 These results indicate that 22


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ZnS promotes oxidation with M (Co, Fe, Mn) TCPP. M (Co, Fe, Mn) TCPP/ZnS and M (Co, Fe, Mn) TCPP were at their highest activation levels at different reaction conditions (Table 3 or 4), which is probably due to the different bond strength of

O O

M( ) in the intermediate

in Scheme 3. This finding is a very interesting and important issue for future work. Table 5 Comparison of reference catalysts and our catalyst systemfor cyclohexane oxidation Entry Catalytic

Reaction

Yields(one+ol)

system

condition

/%

1

Bhaumik and co-workers33

V–Y catalyst, TBHP, solvent–free, 30 ◦C, 72 h.

2

Liu and co-workers34

Au/CQDs photocatalyst, H2O2, 30 ◦C, 48 h.

3

Huang and co-workers35

4 Balakumar and co-workers36

Chowdhury and co-workers38

This work

37.5

VPO-TUD catalyst, H2O2, acetonitrile, 60 ◦C, 24 h.

7

44.0

MOR-D/Fe catalyst, H2O2, acetonitrile, 30 ◦C, 10 h.

6

57.0

Cu(II)/Co(II)/Ni(II)]/SiO2 catalyst, H2O2, acetonitrile, 70 ◦C, 12 h,

Martins and co-workers37

63.7(one)

Ag/CQDs photo-catalyst, TBHP, solvent–free,60 ◦C, 48 h.

5

97.8(one)

31.2

Co TCPP/ZnS catalyst, O2, solvent–free, 165 ◦C, 4 h.

23.3

Practicability is not the only requirement of a catalyst for modern green industrial 23



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catalysis. The pursuit of high yields in modern chemical industry forces scientists to use chemical means to obtain benefits, but this produces unexpected pollution, such as solvent (acetonitrile-like) emissions, and reductant emissions from the oxidant(TBHP). As shown in Table 5, KA oil production technology results in high yields; however, reducing pollution emissions should be considered, as public health is most important. Therefore, Lyon's green catalytic industry concept is worthy of praise. 4. Conclusion The immobilization of M TCPP on ZnS increases its catalytic power for cyclohexane oxidation. On average, the oxidation reaction produced more than a 20% molar yield of KA oil, the catalyst turnover numbers are more than 6.5 ×105 and used 1.1 × 10-6 mol (about 1 mg) M TCPP for three catalytic reaction periods. Based on the catalyst turnover numbers, the catalytic activity of M TCPP increased about 50%. The M TCPP/ZnS complex mimics the mechanism underlying the catalytic activity of cytochrome P450 enzymes by the coordination of a cysteinate proximal axial ligand. After each M (Co, Fe, Mn) TCPP was immobilized on ZnS, we first observed that the coordination of the sulfur anion changes the binding energies of the sulfur atom in ZnS and the nitrogen atoms in the metalloporphyrins, which ultimately changes the binding energies of the catalytic active centers of the metal ion. Second, the binding energies of the catalytic active centers, the M (Co, Fe, Mn) ions, were decreased to their lowest levels. So the immobilized catalysts all have better catalytic performance. Third, the coordination of the sulfur anion also more easily causes a homolytic cleavaging of the M (Co, Fe, Mn)–Cl bond by increasing the electron cloud density around the metal atoms and the chlorine. Fourth, the highest catalytic activity of the immobilized catalysts displayed at different optimal reaction conditions is related to the 24


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different bond strength of

O O

O

M( ) in the O M( ) TCPP/ZnS, which will be the focus of

interesting future investigations. Fifth, the catalysts should be suitable for green production and the health of human beings. Acknowledgements This research was supported by the National Natural Science Foundation of China (No: 51363001), the Guangxi Natural Science Foundation (2014GXNSFDA118009), the Guangxi scientific and technological project (12118008-12-3) and the Experimental Innovation Project Foundation of Guangxi University, PR China(20120322).

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Graphical abstract

We have developed a green technology using a catalytic system that mimics cytochrome P450. The system uses metal tetrakis(4-carboxyphenyl)porphyrins bound to ZnS to oxidize cyclohexane with O2. The technology advocates Lyon's green catalytic industry concept and provides high KA oil yieldls.

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Interesting Green Catalysis of Cyclohexane ... - ACS Publications

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