Oxygen oxidation of ethylbenzene over manganese porphyrin is promoted by the axial nitrogen coordination in powdered chitosan

Abstract

To understand the role of a cysteinate axial ligand in cytochrome P450 enzyme activity and to explore the actions of the axial ligand in a biomimetic catalyst, tetrakis(4-carboxyphenyl)porphyrin manganese chloride (Mn TCPP) was acylated and ligated onto powdered chitosan (pd-CTS). The grafted material was used as a catalyst for the oxidation of ethylbenzene with molecular oxygen. Common spectroscopic techniques, in particular X-ray photoelectron spectroscopy, were used to characterize the grafted material. We found that the coordination of chitosan to manganese porphyrin changed the electron cloud densities of the manganese ion in the catalytic center. This promoted cleavage of the Mn–Cl bond in the Mn TCPP/pd-CTS, it tripled the catalytic activity of manganese tetrakis(4-carboxyphenyl)porphyrin and the catalytic efficiency of the metalloporphyrin. The catalyst decreased the environmental impact of the oxidation of ethylbenzene to acetophenone and phenethyl alcohol.

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1. Introduction

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.¹ There is a great deal of research that has used the heterogeneous metalloporphyrins^2–9 and the homogeneous metalloporphyrins^10–19 to mimic the modulation function of the cysteinate axial ligand in the cytochrome P450 enzyme.²⁰ These studies are highly informative, especially because it is known from previous research^19,20 that the different axial ligands of metalloporphyrins (PM^III [L]) can influence the coordination of an oxidant. The axial ligand effect is that the more strongly an axial ligand binds to metalloporphyrin, the more stable the PM^III (L) will be and the more reactive the (P⁺˙)M^IVO (L) will be. This action should have a crucial effect on the catalytic activity of the center metal ion in metalloporphyrins. That is to say, when the metalloporphyrins are coordinated because they are immobilized on the support, the electron cloud densities of the center metal ions and the elements around the metal ions should be affected.²¹ Our goal is to decrease the environmental impact of the process rather than to mimic the catalysis of cytochrome P450 enzymes. A far as possible the simple catalyst preparation and mild oxidation conditions are in line with the desired biomimetic catalysis system proposed by Lyons,²² which requires only the catalyst, reaction substrate and oxygen, but nothing else. At present, many new techniques for the catalytic oxidation of ethylbenzene are available that offer high yields of the related main products.^23–33 But they employ solvents and nonoxygen (O₂) oxidizing agents, and a complex catalyst preparation process, which result in the discharge of a large amount of waste liquid.

Based on the interesting findings mentioned previously, and the need to reduce the environmental impact of the production of acetophenone and phenethyl alcohol, we used powdered chitosan (pd-CTS) as a support, to immobilize tetrakis(4-carboxyphenyl)porphyrin manganese(III) chloride (Mn TCPP), forming the Mn TCPP/pd-CTS catalyst. This was used to catalyze oxygen oxidation of ethylbenzene under catalytic reaction conditions that produce less pollution, and it revealed an important role in tuning catalytic reactivity of metalloporphyrins by the axial ligand for oxidation of hydrocarbon.

2. Experimental

2.1. Chemicals

All reagents and solvents were of analytical grade and were obtained commercially. Tetrakis(4-carboxyphenyl)porphyrin (TCPP), manganese (Mn) TCPP was synthesized as previously described in a similar method.³⁴ Powdered chitosan (MW ∼ 7.7 × 10⁴ Da, degree of deacetylation was 90.3%, 4000 mesh) was purchased from JINKE Biochemistry Co Ltd (Zhejiang China). Ethylbenzene was analyzed by gas chromatography before use to ensure it was free from oxidation products.

2.2. Synthesis of powdered chitosan-grafted metal tetrakis(4-carboxyphenyl)porphyrin

1.0 mL SOCl₂ was added to a stirred solution of 250 mL dry CH₂Cl₂ containing 0.0407 g Mn TCPP Cl, under protection of nitrogen gas. The resulting mixture was refluxed for 3 h. After evaporation, 20 mL dry N,N-dimethylformamide (DMF), 2 mL dry Et₃N, and 20.0 g powdered chitosan (pd-CTS) were added and refluxed for 3 h.³⁵ The grafted solid catalyst was filtered and washed thoroughly with CH₃OH and CH₂Cl₂, and the filter cake was further extracted with CH₂Cl₂ in a Soxhlet apparatus until no Mn TCPP Cl could be detected in the CH₂Cl₂, as measured by UV-Vis spectrophotometry. Drying the cake in vacuum yielded 18.04 g of a deep green solid. The filtrate was used to analyze the amount of Mn TCPP Cl in the solid material. The amount of grafted manganese porphyrin in the solid material was determined using atomic emission spectroscopy with inductively coupled plasma (ICP-AES Spectroflame model FVMØ3). The sample was digested using a traditional acid method (HNO₃ and HCl), diluted adequately, and analyzed for manganese. The amount of grafted Mn TCPP Cl per gram of the solid material was 2.02 mg, a value consistent with that determined using UV-Vis spectrophotometry.³⁶

2.3. Characterization of the Mn TCPP/pd-CTS materials

Ultraviolet-visible (UV-Vis) spectra of the Mn TCPP/pd-CTS material in DMF suspension and the ungrafted catalyst in DMF solution were collected on a Perkin-Elmer L-17 spectrometer. Fourier transform infrared (FT-IR) spectra of the Mn TCPP/pd-CTS material was obtained on a Perkin-Elmer model 783 IR spectrophotometer. X-ray photoelectron spectroscopy (XPS) measurements of the Mn TCPP/pd-CTS material were performed with an X-ray photoelectron spectrometer (XPS) (Kratos Ultra Axis DLD), equipped with an Al K_α radiation source, at 150 W with a pass energy of 40 eV. The particle size of the Mn TCPP/pd-CTS material was measured with a Tecnai G2F20 S-TWIN transmission electron microscope (TEM), with a 100 kV accelerating voltage. Thermal gravimetric study of a 65 mg sample was performed with a Netzsch Sta 409 Pc thermogravimetric analyzer in air over the temperature range 0–1000 °C, at a heating rate of 10 °C min⁻¹.

2.4. Ethylbenzene oxidation over the grafted material

The catalytic reaction was performed with the Mn TCPP/pd-CTS material. A KCF-10 250 mL autoclave reactor equipped with a magnetic stirrer and a frozen water re-condenser 10 °C was used for the oxidation.³⁷ Typically, 200 mL of ethylbenzene and 0.5 g of the grafted catalyst (containing 1.14 × 10⁻⁶ mol Mn TCPP) were placed in the reactor, charged with nitrogen at a desired pressure, and heated to a desired temperature. Afterwards, the reactor was charged with a desired pressure of air, and the pressure of air was maintained by continuously feeding the gas. A rotameter was used to measure the oxygen flow and a CYS-1 digital oxygen detector was employed to determine the oxygen concentration of the tail gas. Oxidation products taken at regular intervals were identified by gas chromatography-mass spectrometry and were quantified with an internal standard (p-dichlorobenzene) method with a Shimadzu GC-16A chromatograph, equipped with a 30 m × 0.32 mm × 0.5 μm FFAP capillary column and a flame ionization detector.³⁷ The oxidation was terminated at 4 h for further reuse of the grafted catalyst. It was obtained by simple filtration from the oxidation mixture, followed by washing with ethanol and air-drying, for extracting any oxidation product that might have remained in the grafted material. This was then used in the next ethylbenzene oxidations.

3. Results and discussion

3.1. Appearance and UV-Vis spectra of the grafted material

To investigate the promotion of the coordinating nitrogen atom in the powdered chitosan to the catalysis of Mn TCPP for oxidation of ethylbenzene, the metal tetrakis(4-carboxyphenyl)porphyrin was grafted onto the 4000-mesh chitosan by acylation. Manganese TCPP grafted onto chitosan, which is gray, produces a deep green solid (Fig. 1). The TEM micrograph of the Mn TCPP/pd-CTS is also shown in Fig. 1. It shows that Mn TCPP/pd-CTS consists of the rounded microparticles, which will display the duel catalytic performance of the grafted Mn TCPP. The presence of Mn TCPP on the pd-CTS can also be established by UV-Vis spectroscopy. Fig. 2 shows the UV-Vis spectra of the metal tetrakis(4-carboxyphenyl)porphyrin and the pd-CTS-grafted Mn TCPP. The UV-Vis spectral data of Mn TCPP agree well with the reference values,³⁸ After the Mn TCPP was grafted to pd-CTS, however, its Soret band was present 2 nm from the original position. The Soret band was blue-shifted slightly, which relates to the changes of the Mn TCPP plane due to the coordination of the amino group in pd-CTS.^39,40 This is probably because the carboxyl groups of the Mn TCPP had acylated the amino groups of pd-CTS, and conversely, the other amino groups had coordinated with the manganese ions of the Mn TCPP. These reactions promoted tight binding between Mn TCPP and pd-CTS, resulting in the change in the peak position of the Soret band observed in its UV-Vis spectra. The combination can be further seen in the FT-IR spectra and XPS data.

Fig. 1 Image of chitosan (left), and an image (middle) and transmission electron micrograph (right) of Mn TCPP/pd-CTS.

Fig. 2 UV-Vis spectra at room temperature: DMF solution of Mn TCPP (ε = 6.43 × 10⁴ L mol⁻¹ cm⁻¹) and DMF suspension of Mn TCPP/pd-CTS (ε = 6.47 × 10⁴ L mol⁻¹ cm⁻¹).

3.2. FT-IR spectra and thermal properties of Mn TCPP/pd-CTS

The IR spectrum of the pd-CTS (Fig. 3) shows a strong absorption band around 3298 cm⁻¹ and around 1596 cm⁻¹ for the corresponding amino groups. The absorption band for its hydroxyl groups is at a higher value (>3298 cm⁻¹). Bands at about 1650 cm⁻¹ are for its carbonyl groups; bands at 2922 cm⁻¹ and 1382 cm⁻¹ are for its methyl groups; and bands at 1081 cm⁻¹ are for its glycosidic linkage (–C–O–C–) of the polymer, CTS.^41,42 The IR spectrum of the Mn TCPP (Fig. 3) shows a middle absorption band around 3419 cm⁻¹, at 1698 cm⁻¹ and at 1264 cm⁻¹ for the stretching vibration of O–H groups, C=O groups and C–O groups of the carboxyl groups. The bands at 1606 cm⁻¹ and 1536 cm⁻¹ are for the stretching vibration of C=C (C–C) bonds (benzene rings). The band at 1384 cm⁻¹ is for the stretching vibration of C=N bonds.⁴³ The bands at 1011 cm⁻¹ and 801 cm⁻¹ are respectively assigned to the rocking and bending vibrations of C–H bonds for the pyrrole and benzene (two-substituted benzene) rings.⁴⁴ When Mn TCPP was grafted on pd-CTS to form Mn TCPP/pd-CTS, the broad bands at 3298 cm⁻¹ and the shoulder band at 1596 cm⁻¹ observed for pd-CTS almost disappeared, and the intensity of absorption band at 3436 cm⁻¹ and 1634 cm⁻¹ increased for the Mn TCPP/pd-CTS.⁴⁵ These changes occurred because the partial amino groups in pd-CTS were acylated by the benzoyl chloride converted from the Mn TCPP. The bands at 3436 cm⁻¹ and at 1634 cm⁻¹ for the Mn TCPP/pd-CTS should be attributed to the stretching vibration of O–H groups and the vibration of carbonyl groups, respectively.⁴⁵ Further, we used Mn TCPP to graft the powdered chitosan in an acid–base reaction to form Mn TCPP^#/pd-CTS and confirm that the manganese porphyrin was acylated to pd-CTS. The IR-spectra are compared with those of Mn TCPP/pd-CTS in Fig. 3. It is apparent that the band at 1558 cm⁻¹ was a stretching vibration of COO⁻ groups in Mn TCPP^#/pd-CTS (Por-COO^{− +}NH₃-CTS). However, no band was detected for the Mn TCPP/pd-CTS at 1558 cm⁻¹. In fact, stretching vibrations of the carbonyl groups for the solid secondary amide were present at 1680–1630 cm⁻¹, corresponding to 1667–1634 cm⁻¹ in Mn TCPP^#/pd-CTS and 1634 cm⁻¹ in Mn TCPP/pd-CTS. These results suggest that pd-CTS was acylated by Mn TCPP. Next, we analyzed the IR-spectra for the Mn TCPP and powdered chitosan (Mn TCPP + pd-CTS), mixture. The broad band at 3567–3429 cm⁻¹ was attributed to stretching vibrations of the O–H groups in the Mn TCPP carboxyl groups, which did not react with the pd-CTS –NH₂ groups. Conversely, the band at 1593 cm⁻¹ in the (Mn TCPP + pd-CTS) mixture indicates that the amino groups did not react with the hydroxyl groups. In addition, there was a new band at 461 cm⁻¹ for the Mn TCPP/pd-CTS, which is likely to be the coordination of N atoms in the partial amino groups of pd-CTS to the manganese ions of Mn TCPP.

Fig. 3 Fourier-transform infrared spectra of Mn TCPP/pd-CTS, CTS, and Mn TCPP, with an effective frequency range of 4000 to 400 cm⁻¹ and a TG curve of Mn TCPP/pd-CTS.

Fig. 3 shows the heat resistance data for the grafted material. It can resist temperatures of at least 250 °C only losing 0.3 mg% of moisture contained in the material. The weight reduction is likely the result of water loss from pd-CTS,⁴⁶ because Mn TCPP exhibited no decomposition at 300 °C for 4 h.⁴² In addition, based on the loss of Fe (TPFPP) observed as an endothermal effect centered at 370 °C,⁴⁷ indicating that Mn TCPP/pd-CTS is stable under our oxidation reaction conditions.

3.3. XPS analysis for Mn TCPP/pd-CTS

The results of XPS and main spectral bands based on their binding energy (BE) of the key elements for pd-CTS, Mn TCPP, and Mn TCPP/pd-CTS, and electron binding energies are shown in Fig. 4 and Table 1, respectively.

Fig. 4 X-ray photoelectron spectroscopy, and main spectral bands based on binding energy (BE) of the key elements for pd-CTS, Mn TCPP, and Mn TCPP/pd-CTS.

Table 1 Main spectral band assignments based on their binding energy (BE) for the key elements of Mn TCPP and Mn TCPP/pd-CTS

XPS spectra	Existential form of the key elements	Binding energy/eV
		Mn TCPP/pd-CTS	Mn TCPP	pd-CTS
Mn 2p	Mn–N	641.78	642.38
		642.33	643.38
N 1s	N=C (N–C=)	401.13	399.18
	N–Mn	401.68	400.28
	O=C–NH	399.53	—	401.23
	H2N–C	399.33	—	399.28
C 1s	O–C–O, O=C	286.63	289.28	287.83
	O–C,C–O–C, N–C, N=C	284.88	288.88	286.33
	C=C, C–C	284.73	284.83	284.88
O 1s	H–O–C, O=C	532.58	533.08	532.63
Cl 2p	Cl–Mn	197.99	199.78

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The wide scan XPS spectra of pd-CTS, Mn TCPP, and Mn TCPP/pd-CTS show the presence of oxygen, nitrogen, and carbon on the three material surfaces, and manganese and chlorine on the last two material surfaces. These elements have corresponding binding energies at about 532.58–532.63 eV (O 1s), 399.18–401.68 eV (N 1s), 532.58–533.08 eV (C 1s),^9,48–50 641.78–643.38 eV (Mn 2p),⁵¹ and 197.99–199.78 eV (Cl 2p), as shown in Table 1, and their attributions of the binding energies agree with previous reports.^9,48–51 This indicates that Mn TCPP had been immobilized on the pd-CTS.

When Mn TCPP was grafted to pd-CTS, we noted two interesting and important outcomes. First, the binding energies of the N (1s) in the H₂N–C unit and in the N–Mn unit increased from 399.28 and 401.23 eV to 399.33 and 399.53 eV; 399.18 and 400.28 eV to 401.13 and 401.68 eV, respectively. These results suggest that there is increasing coordination of the nitrogen atom in those units to the manganese ion in Mn TCPP, and that the amino groups of chitosan were acylated by the acyl groups of Mn TCPP. This would make (pd-CTS–H₂N)/TCPP Mn^III more stable, and increase the strength of the bond between Mn TCPP and chitosan. Second, the binding energies of the Mn (2p) in the Mn–N unit and the Cl (2p) in the Cl–Mn unit decreased from 642.38 and 643.38 eV to 641.78 and 642.33 eV, and 199.78 eV to 197.99 eV, respectively. This means that, at increased temperature, there is a strong potential trend of a homolytic cleavage of the Cl–Mn bond in the Mn TCPP Cl, because both Cl and Mn possessed higher electron cloud densities after Mn TCPP was grafted. Therefore, there is a repulsive Coulomb force between elements of like charge, between the Mn ion and the chloride ion.⁵²

Scheme 1 shows the changes in electron cloud density of Mn ion in the Mn TCPP after it was grafted to pd-CTS. When the amino groups of pd-CTS were not ligated to Mn TCPP, the binding energy of the N (1s) in the H₂N–C unit was 399.28 eV. It increased to 399.33 eV after ligation. Obviously, the electron cloud density of the N atom in the H₂N–C unit was lower than ever, the nitrogen atom was in a state of lower electron cloud density (noted as “l.e.”). Considering the equilibrium within the bonding electron cloud of H₂N–Mn, the Mn ion in Mn TCPP/pd-CTS should possess a higher electron cloud density (noted as “h.e.”), which is compared to the N–Mn (l.e.) [Mn TCPP], as shown in part 1 of Scheme 1. Because of the coordination of the amino groups to Mn TCPP, the binding energy of the N (1s) in the N–Mn unit of Mn TCPP was changed from 399.28 and 401.23 eV to 401.68 eV. For the same reason (from 400.28 eV to 401.68 eV) for the bond of N–Mn in Mn TCPP, Mn ion in the Mn TCPP/pd-CTS also possessed higher electron cloud density (in part 2 of Scheme 1). Secondly, the binding energy of the Cl (2p) in the Cl–Mn unit of Mn TCPP decreased from 199.78 eV to 197.99 eV. Similarly, considering the equilibrium of the Cl–Mn bond of Mn TCPP, the Mn ion in Mn TCPP/pd-CTS possessed a lower electron cloud density (in part 3 of Scheme 1). To sum up, the Mn element possessed higher electron cloud density after Mn TCPP was immobilized (in part 4 of Scheme 1), in addition, the Cl ion in Mn TCPP/pd-CTS possessed a higher electron cloud density (in part 3 of Scheme 1). Therefore, there is a strong potential trend of a homolytic cleavage of the Cl–Mn bond in Mn TCPP/pd-CTS, causing free radical pyrolysis.⁵² Next, after the homolytic cleavage of the Cl–Mn bond occurs, the Mn ion in the Mn TCPP/pd-CTS should be more active than that in Mn TCPP, because it will possess a lower electron cloud density than the latter, so it could more easily attract and activate molecular oxygen.

Scheme 1 Electron cloud density state of the Mn ions in Mn TCPP and Mn TCPP/pd-CTS.

3.4. Catalytic performance of Mn TCPP/pd-CTS for ethylbenzene oxidation with O₂

Ethylbenzene oxidation was used to investigate the catalytic performance of the manganese porphyrin and how it was promoted by nitrogen coordination in powdered chitosan. The effects of temperature, air pressure, and amount of catalyst on the oxidation of ethylbenzene catalyzed by Mn TCPP/pd-CTS was studied (Fig. 5). The results revealed that the optimal catalytic reaction conditions were at 155 °C and 0.8 MPa with 1.0 mg Mn TCPP. Under these reaction conditions, oxidation of ethylbenzene over the catalyst gives about 21% conversion of ethylbenzene, 18% molar yields of acetophenone and phenethyl alcohol, and a catalyst turnover number of 2.47 × 10⁵. Obviously, the control experiment (known as autooxidation, chitosan and free base porphyrin) was compared to the grafted catalyst, showing relatively low catalytic activity for ethylbenzene oxidation. The ethylbenzene conversion was about 4.1%, and the yield of acetophenone and phenethyl alcohol was about 3.5% (Table 2). When Mn TCPP was used as a catalyst for ethylbenzene oxidation under the same reaction conditions, the ethylbenzene conversion was about 11%, and the yield of acetophenone and phenethyl alcohol was about 10%, and the catalyst turnover number was about 1.32 × 10⁵ (Table 2). This means that, although the Mn TCPP is responsible for catalyzing the oxidation, the catalytic power of the Mn TCPP was almost doubled by grafting it onto the pd-CTS. The twofold increase of the catalytic power occurs not only at the 2.5 h reaction time, but also throughout the whole oxidation process. Fig. 6 shows that the ratios of ethylbenzene conversion to reaction time and yields to reaction time for the Mn TCPP/pd-CTS are about twice those for Mn TCPP.

Fig. 5 Changes in conversion rate and yields (ketone + alcohol) with reaction temperature, pressure, and amount of catalyst for the first run of ethylbenzene air oxidation over the grafted catalyst. Reaction conditions: 200 mL ethylbenzene, 0.040 m³ h⁻¹ airflow, and 2.5 h reaction time.

Table 2 Comparison of the catalytic performance of the two catalysts reused under the optimal reaction conditions^a

Catalysts	Recycling times	Conversion mol%	TON/×10^5	Yield/mol%	Selectivity/%
					-on	-ol	-al	-ac	-es
a TON = catalyst turnover numbers; -on: acetophenone; -ol: 1-phenylethanol; -al: benzaldehyde; -ac: benzoic acid; -es: 1-phenylethyl benzoate.
Mn TCPP/pd-CTS	1	20.74	2.47	17.74	53.91	31.63	4.63	4.55	5.28
	2	13.90	1.88	11.43	58.17	24.03	5.78	5.31	6.71
	3	12.64	1.67	10.32	51.95	29.72	6.95	5.82	5.56
	Average	15.76	2.01	13.16	54.68	28.46	5.79	5.23	5.85
Mn TCPP	1	11.08	1.32	9.50	51.32	34.45	7.08	4.12	3.03
Mn TCPP + pd-CTS	1	17.20	2.27	13.56	43.12	35.73	8.80	8.86	3.48
CTS or no catalyst		4.13		3.50	52.83	31.90	10.29	1.37	3.61

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Fig. 6 Changes in conversion rate and yields (ketone + alcohol) with reaction time for the first run of ethylbenzene air oxidation over the two catalysts. Reaction conditions: 200 mL ethylbenzene, 1.0 mg of Mn TCPP, 0.040 m³ h⁻¹ airflow.

The increase probably results from two processes. First, there is the stronger trend of the homolytic cleavage of the Cl–Mn bond for the Mn TCPP/pd-CTS (or pd-CTS–H₂N) than for Mn TCPP. When the grafted catalyst is heated, it would be changed from (pd-CTS–H₂N)/TCPP Mn^IIICl to (pd-CTS–H₂N)/TCPP Mn^II. The latter would quickly bind to O₂ forming (pd-CTS–H₂N)/TCPP Mn^IIIO₂˙, which further changed into an important intermediate, (pd-CTS–H₂N)/TCPP Mn^IV = O˙+, referred to in previous reports.^53,54 The intermediate is responsible for the oxidation of ethylbenzene into acetophenone and phenethyl alcohol. We suggest that it is the first reason that the grafted catalyst possesses higher catalytic activity than Mn TCPP. Second, the more stable (pd-CTS–H₂N)/TCPP Mn^III in the (pd-CTS–H₂N)/TCPP Mn^III Cl and the (pd-CTS–H₂N)/TCPP Mn^III O₂˙ can create nitrogen-ligated activated species, can promote homolytic cleavage of the Cl–Mn bond and combine with O₂, and then can quickly cleave oxygen molecules into the very active oxygen atom and hydroxylate ethylbenzene into phenethyl alcohol to perfectly fulfill its catalysis mission.

Mn TCPP was immobilized on pd-CTS by acylation and coordination between chitosan and manganese porphyrin. It was firmly attached to the support and did not leach from the grafted material in the catalytic oxidation process. Although, the surface of pd-CTS did not withstand high temperatures for such long periods, and a small area gradually carbonized, which results in the some reduction of catalytic performance, the amino groups of pd-CTS play a crucial role in promoting the catalytic activity of Mn TCPP. This coordination means that only 1 mg Mn TCPP is needed to triple the catalytic efficiency of the corresponding unsupported metalloporphyrin. Therefore, on average, each catalytic oxidation in the three runs gives 13.2% molar yields of acetophenone and phenethyl alcohol. The yields are more than 9.50% of those obtained from the oxidation over the Mn TCPP catalyst, and is close to 13.56% of that obtained from the oxidation over the mixture material (Mn TCPP + pd-CTS), which could be not reused for oxidation (Table 2).

The only requirement of a catalyst for modern green industrial catalysis is practicability. Although various chemical means of ethylbenzene oxidation to the corresponding main products in high yields can be offered, some techniques produce unexpected pollution, such as solvent (acetonitrile-like) emissions, reductant emissions from the oxidant (TBHP); and/or complex catalyst preparation processes. As shown in Table 3, the production technology results in high yields of the acetophenone, phenethyl alcohol, and phenylaldehyde. However, reducing pollution emissions should be considered, a priority, so that public health needs can be met now and in the future.

Table 3 Comparison of reference catalysts and our catalyst system for ethylbenzene oxidation

Entry	Catalytic system	Reaction condition	Yields (one + ol + al)^a/mol%
a One = acetophenone; ol = phenethyl alcohol; al = phenylaldehyde.
1	Li et.al.^23	{[Cd-(DMF)2MnIII(DMF)2TPyP](PW12O40)} 2DMF·5H2O catalyst, TBHP, H2O, 80 °C, 12 h	92.7 (one)
2	Murugesan et.al.^24	Mn–Ti-SBA-15(50) catalyst, TBHP, acetonitrile, 80 °C, 2 h	92.0 (one + ol)
3	Xu et.al.^25	Pr–Co–SiO2 catalyst, O2, solvent-free, 120 °C, 7 h	66.1 (one + ol)
4	Li et.al.^26	MMO-0.5/A catalyst, TBHP, solvent-free, 120 °C, 12 h	64.3 (one + al)
5	Ghiaci et.al.^27	Si/Al–pr–NH-et-N-methyl-2-pyridylketone–Mn solvent-free, 80 °C, 24 h	64.1 (one + al)
6	Liu et.al.^28	Co–N–C-500/SiO2 catalyst, O2, solvent-free, 120 °C, 5 h	30.4
7	Peng et.al.^29	Fe@CNTs-5 catalyst, O2, CH3CN, 155 °C, 3 h	28.8
8	Liu et.al.^30	Co–N–C/MO catalyst, O2, solvent-free, 120 °C, 5 h	24.8 (one)
9	Liu et.al.^31	CoTPP–(CoOx/CeO2)@SiO2 catalyst, O2, solvent-free, 120 °C, 5 h	22.0 (one)
10	Guo et.al.^32	MnCP@SiO2 catalyst, O2, solvent-free, 100 °C, 10 h	15.5 (one + ol)
11	Liu et.al.^33	CS-MnCP catalyst, O2, solvent-free, 100 °C, 10 h	12.6 (one)
12	This work	Mn TCPP/pd-CTS catalyst, O2, solvent-free, 155 °C, 2.5 h	17.7 (one + ol)

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4. Conclusion

Powdered chitosan with a high degree of deacetylation is a convenient carrier, providing sufficient amino groups for the acylation and ligation of Mn TCPP and highly dispersing the metalloporphyrins on itself. The simple immobilization results in formation of the more efficient (pd-CTS–H₂N)/TCPP Mn, which more easily cleaves the Mn–Cl bond, binding and activating O₂ and further hydroxylating ethylbenzene. The catalytic activity and the catalytic efficiency of Mn TCPP was greatly increased by pd-CTS primarily because the ligation of the nitrogen atom in the amino groups of pd-CTS to Mn TCPP changed the electron cloud densities of the Mn ion and the elements around it. This technique gives a less wasteful production method, including simple grafting of metalloporphyrins and oxidation of ethylbenzene that emits lower levels of environmental pollutants.

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 (201510593308).

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