Catalytic properties of N-hydroxyphthalimide immobilized on a novel porous organic polymer in the oxidation of toluene by molecular oxygen

Abstract

By molecular design, using a micro-sized macroporous silica mold, N-hydroxyphthalimide (NHPI) was synthesized and immobilized on a novel porous organic polymer (POP) polystyrene (pSt). The POP pSt was prepared using surface-initiated graft polymerization and the silica matrix was etched away. Subsequently, with 1,4-dichloromethoxybutane used as a chloromethylation reagent, a chloromethylated POP was obtained through a Friedel–Crafts alkylation reaction. Next, phthalic anhydride was bonded onto the POP through an esterification reaction, which was carried out between the chloromethyl groups on the POP and the carboxyl group of trimellitic anhydride (TMA), obtaining the product POP-PA. Then the bonded phthalic anhydride (PA) was reacted with hydroxylamine hydrochloride, and the NHPI-immobilized heterogeneous material pSt/NHPI was obtained. The products obtained were fully characterized. Finally, a co-catalyst system constituted of the solid catalyst pSt/NHPI and a small amount of Co(OAc)₂ was used in an oxidation reaction process for toluene with molecular oxygen as the oxidant at a low temperature and a normal pressure of oxygen. The experimental results showed that the co-catalyst system can thoroughly oxidize toluene to benzoic acid, and this combination catalyst also possessed high catalytic activity, excellent catalytic selectivity and recyclability.

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Benzoic acid is an important feedstock, which has been widely used in various areas, such as in dyes and perfume, as an intermediate for caprolactam, as an auxiliary agent for the rubber industry, and also as an agent for preservatives or an intermediate in reactions.^1–3 Up to now, the production of benzoic acid has heavily relied on the oxidation of toluene, which requires expensive reagents and harsh conditions.^4,5 Therefore, direct green oxidation reactions of toluene to benzoic acid with air or molecular oxygen have attached extensive attention. At present, several sophisticated oxidation catalyst systems which are able to use molecular oxygen as the end oxidant have been introduced.^6–10 The combination of N-hydroxyphthalimide (NHPI) with some mediators is attractive and promising for the oxidation of alcohols using homogeneous catalyst systems.^11–14 Moreover, it is difficult to separate and recover NHPI after the reaction, whereas immobilizing the NHPI catalyst on a solid support can overcome this shortcoming. Now, the heterogenization of homogeneous catalysts has become a hot subject, whereas research on immobilized NHPI is scarce.^15–18 In this study, NHPI is immobilized on a new POP pSt.

Porous organic polymers (POPs) have attracted tremendous attention from both academia and industry owing to their unique functionalization and relatively excellent stability, and have been widely applied in gas storage, separation and catalysis.^19–22 As is known, silica gel (SiO₂), which exhibits excellent mechanical, thermal and chemical properties such as a relatively good stability, a large pore surface area, a good adsorption capacity and an ordered pore arrangement,^23,24 has emerged as an inorganic solid support and has been widely used in polymer composites. However, many researchers focus on its external surface but neglect to utilize its abundant pore structure.^25–27 Herein, a new porous organic framework has been designed and synthesized through a novel and convenient method based on silica gel particles.

To construct a mercapto group/AIBN(–SH/AIBN) redox initiating system, the mercapto groups on the surfaces of the modified particles MPMS/SiO₂, with a decomposition initiator of AIBN in DMF solution, were used from which the hydrogen atom transferred to one free radical of AIBN so that sulfur free radicals were obtained on the surfaces of the silica gel particles, and the graft polymerization of St occurred on the surface of the silica gel particles, preparing the grafted particles pSt/SiO₂. Then the silica matrix was etched away to form the POP pSt. Then 1,4-bis(chloromethoxyl)butane (BCMB), trimellitic anhydride (TMA) and hydroxylamine hydrochloride were used as the reaction reagent, respectively, for three macromolecular reactions, a chloromethylation reaction, an esterification and an acyl imidization, and eventually the immobilized material pSt/NHPI was obtained. The above entire chemical process is shown schematically in Scheme 1.

Scheme 1 Chemistry of preparing the heterogeneous catalyst pSt/NHPI.

In this study, an aerobic oxidation of toluene to benzoic acid using pSt/NHPI working together with the co-catalyst Co(OAc)₂ was conducted, which used molecular oxygen as the final oxidant with a low temperature and normal pressure of oxygen, and a possible catalytic oxidation mechanism is discussed. The results show that pSt/NHPI has a certain application value in the field of solid supported catalysis.

1. Experimental section

1.1 Chemicals and materials

Silica gel (about 125 μm, Haiyang Chemical Limited Company, Qingdao, China) and γ-mercaptopropyl trimethoxysilane (MPMS, Nanjing Chuangshi Chemical Aux Ltd., Jiangsu, China) was used. Styrene (St, Suzhou Nanhang Chemical Ltd., China) was purified by vacuum distillation before use. 2,2′-Azobisisobutyronitrile (AIBN, 98% purity, Shanghai Zhongli Chemical Plant, Shanghai, China) was purified by recrystallization from ethanol before use. Trimellitic anhydride (TMA, Tianjin Guangfu Fine Chemical Ltd., China), toluene, acetic acid, N,N-dimethylformamide (DMF), and dimethylacetamide (DMAC) (Tianjin Shentai Chemical Ltd., China) were used. The transition metal salts were: Co(OAc)₂, CuCl₂, Fe(acac)₂, MnSO₄ (Shanghai Jingchun Biochemical Ltd., China). Hydroxylamine hydrochloride (Ruishuo Chemical Limited Company, Shanghai, China) was used. The other reagents were of analytical purity grade and were used without further purification.

1.2 Apparatus

Perkin-Elmer 1700 infrared spectrometer (FTIR, Perkin-Elmer Company, USA); Unic-2602 UV spectrophotometer (Unic Company, Shanghai); LEO-438VP scanning electronic microscope (SEM, LEO Company, UK); STA449 thermogravimetry analyzer (TGA, Netzsch Company, German); ASPA2000 physisorption apparatus (Micromeritics Company, USA); GC-2010 gas chromatograph (GC, Shimadzu Company, Japan); PE-2400II elemental analyzer (EA, Perkin-Elmer Company, USA).

1.3 Preparation and characterization of pSt/NHPI

1.3.1 General procedure for the preparation of POP pSt. According to the operation steps in ref. 28 and 29, graft polymerization of St was carried out on the surface of silica gel particles, and the specific operation steps were as follows. Firstly, silica gel particles were treated for activation using an aqueous solution of hydrochloric acid as the activation reagent. Secondly, the activated silica gel particles were surface-modified with a coupling agent MPMS, obtaining the modified product MPMS/SiO₂, with a mercapto group content of 1.08 mmol g⁻¹. Using AIBN as an initiator and DMF as the solvent, the graft polymerization of St on the surfaces of the MPMS/SiO₂ particles was carried out, obtaining the grafted particles pSt/SiO₂. Then they were transferred into a polyethylene beaker and 30 mL of 40% hydrofluoric acid was added to etch away the silica gel, followed by washing and drying, and finally, the porous organic polymer pSt was prepared. The grafting degree of the pSt/SiO₂ particles was determined using TGA.

1.3.2 Immobilization of NHPI on the POP pSt.
(1) Chloromethylation of the POP. According to the procedure described in ref. 30, using self-made 1,4-bis(chloromethoxyl)butane (BCMB) as the chloromethylation reagent and adopting SnCl₄ as the Lewis acid catalyst, the reaction was performed at room temperature in a dichloromethane solvent for 9 h. The amount of the chlorine content in the product CM-pSt was determined using a calorimeter and the Volhard method.
(2) Bonding of phthalic anhydride on CM-pSt. 0.5 g of CM-pSt was placed in a four-necked flask equipped with a mechanical agitator and a reflux condenser, followed by the addition of 30 mL of DMF as the solvent and soaking for 5 h. Then 1.3 g of TMA was dissolved in 10 mL of DMF, and the mixture was added into the flask. Afterwards, 0.5 mL of triethylamine (Et₃N) as the catalyst was added into the reaction system, and the esterification reaction between the chloromethyl groups on CM-pSt and the carboxyl groups of TMA was performed at 85 °C for 5 h. After the reaction was finished, the concentration of TMA in the supernatant was measured using ultraviolet spectrophotometry at 291 nm, which showed that the conversion of the chloromethyl groups of CM-pSt can reach 70%. The product pSt/PA was fully washed with DMF and ether in turn, several times, and then filtered and dried under vacuum.
(3) Synchronously synthesizing and immobilizing NHPI on pSt/PA. In a four-necked flask, 1 g of pSt/PA was dipped into 30 mL of pyridine for 5 h in order to fully swell the polymeride, followed by the addition of 0.8 g of hydroxylamine hydrochloride as the reaction reagent. The reaction between the bonded phthalic anhydride (PA) and hydroxylamine hydrochloride was conducted at 90 °C with stirring for 15 h. After the reaction, the product was acidized by adding diluted hydrochloric acid. The resultant product was washed repeatedly with distilled water and ethanol, collected by filtration and dried under vacuum. Finally, the heterogeneous catalyst pSt/NHPI was obtained, for which the immobilization amount (Q) of NHPI was measured using a weighing method, and it was 2.25 mmol g⁻¹.

Q = (m₂ − m₁)/M × 1000

In this formula, m₁ and m₂ are the quantity of pSt/PA and pSt/NHPI (g), and M is the sum of the molar mass of N and H atoms, which is about 15 g mol⁻¹.

1.4 Catalytic oxidation of toluene with molecular oxygen

The catalytic oxidation of toluene in the presence of molecular O₂ was conducted in a reactor equipped with a mechanical stirrer, thermometer, O₂ inlet and reflux condenser. Firstly, 40 mL of acetic acid and 1.0 g of pSt/NHPI were added and dissolved for 5 h, followed by the addition of 2 mL of toluene and 0.04 g of Co(OAc)₂ as the co-catalyst. Oxygen at a fixed flow rate, which was 16 mL min⁻¹, was passed into the reaction system. The oxidation reaction was carried out at 80 °C with continuous stirring for 30 h. Samples of the reaction mixture were taken at fixed time intervals, and then measured using a gas chromatograph with an internal standard method (N₂ as the carrier gas, PEG-20M capillary column and FID detection), which showed that benzoic acid was the main product with a small amount of the byproduct benzaldehyde. After the oxidation reaction, the heterogeneous catalyst pSt/NHPI was soaked, washed with distilled water and ethanol in turn to completely remove the organic matter physically attached on the particles, and dried under vacuum. The recovered particles were reused in the oxidation reaction of toluene under the same conditions to examine their recycling performance.

2. Results and discussion

2.1 Synthesis of the NHPI catalyst

After constructing a mercapto group/AIBN(–SH/AIBN) redox initiating system, the graft polymerization of St on the surface of the silica gel particles occurred, preparing the grafted particles pSt/SiO₂. This was followed by addition of hydrofluoric acid to dissolve the silica gel, and then NHPI was synchronously synthesized and immobilized on the POP via several steps of polymer reactions, resulting in the heterogeneous catalyst particles pSt/NHPI.

2.1.1 Effects of the main factors on the immobilization of NHPI. The NHPI-immobilized heterogeneous catalyst was prepared via three macromolecular reaction steps. Chloromethylation of the phenyl groups on the polymer has been fully examined by our group, and the optimum reaction conditions were determined. The reaction of the bonded phthalic anhydride (PA) with hydroxylamine hydrochloride is also a mature chemical reaction, however, the reaction of CM-pSt with TMA was a key step. So in this study the effect of the reaction parameters on the bonded amount of phthalic anhydride (PA) was investigated, and the optimum reaction conditions were established.

By fixing the other reaction conditions, the rate of the chloromethyl group conversion (CGC) change versus time at different temperatures was investigated and the results are shown in Fig. 1. The results clearly show that the esterification reaction rate increased with temperature, and as the chloromethyl group conversion at 85 °C was very close to that at 90 °C, a suitable reaction temperature was 85 °C.

Fig. 1 Variation of the chloromethyl group conversion rate for CM-pSt at different temperatures.

In the presence of the HCl agent, the reaction between the chloromethyl groups of CM-pSt and the carboxyl groups of TMA is a nucleophilic substitution reaction. The carboxyl hydroxyl group loses a hydrogen proton and transforms into a carboxylic acid anion, a nucleophilic attack group, while the chloromethyl group loses chlorine and transforms into a carbenium ion, which is closely associated with the polarity of the solvent. A higher solvent polarity gives a stronger dipole–dipole interaction of the solvent and nucleophilic reagent, which is helpful for removing the chlorine atom and attacking the carboxylic acid anion (Table 1). Obviously, the polarity of the solvent can promote the nucleophilic substitution reaction. Based on this, DMAC should be selected as a suitable solvent. At the same time, a suitable catalyst was Et₃N, which can be well dissolved in the reaction system (Table 2).

Table 1 Dielectric constant data for various solvents

a The dielectric constant of the mixed solvent was calculated according to the molar ratio of the two solvents.
Solvent	DMAC	DMF	DMAC/dioxane^a (v/v = 1 : 1)	DMF/dioxane^a (v/v = 1 : 1)
Dielectric constant ε (25 °C)	37.8	36.7	20.0	19.5

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Table 2 Effect of reaction conditions on the esterification reaction

Entry	Temperature (°C)	Catalyst^b	Solvent	Reaction time (h)	CGC (%)
a The volume ratio of the two solvents was 1 : 1.b The amount of catalyst used in this reaction was 15 mmol. Et3N (triethylamine), Bu3N (tri-n-butylamine).
1	85	Et3N	DMF/dioxane^a	5	57.3
2	85	Et3N	DMAC/dioxane^a	5	59.5
3	85	Et3N	DMF	5	65.5
4	85	Et3N	DMAC	5	68.2
5	85	Bu3N	DMAC	5	61.9
6	85	NaOH	DMAC	5	46.2
7	85	Na2CO3	DMAC	5	34.6

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2.1.2 Characterization of the functional particles pSt/NHPI. As shown in Fig. 2, the chemical modification of SiO₂ was confirmed using IR spectroscopy.

Fig. 2 IR spectra of the reactants and products.

In the spectrum of SiO₂, peaks at 3438 cm⁻¹, 1126 cm⁻¹ and around 800 cm⁻¹ were observed, and these bands were assigned to the silanol groups as well as adsorbed water, the antisymmetric stretching vibration absorption of Si–O–Si bonds and the symmetrical stretching vibration absorption of Si–O bonds. In the spectrum of pSt/SiO₂, a characteristic absorption of the methylene groups of the main chain of the polymer appeared at 2928 cm⁻¹, while the peaks at 1610 cm⁻¹, 1500 cm⁻¹, 1450 cm⁻¹ and 674 cm⁻¹ are ascribed to skeleton vibrations of the benzene ring on polystyrene. In the spectrum of POPs-pSt, it can be clearly observed that the absorption peaks at 800 cm⁻¹ and 1126 cm⁻¹ disappeared after the silica gel dissolved. In the spectrum of CM-pSt, except for the characteristic absorption bands of POPs-pSt, the appearance of two absorption bands at 1421 cm⁻¹ and 670 cm⁻¹ was due to the chloromethyl group –CH₂Cl. In the spectrum of pSt/PA, the absorption bands of the chloromethyl group were much weakened, whereas a characteristic absorption band of the ether group C–O–C appeared at 1230 cm⁻¹, and also two characteristic absorption bands of the cyclic anhydride appeared at 1782 cm⁻¹ and 1724 cm⁻¹, which were attributed to asymmetric and symmetric coupling vibration absorptions of the anhydride carbonyl, respectively. In the spectrum of pSt/NHPI, a characteristic absorption band of the free radical group N–O appeared at 1365 cm⁻¹, and a strong absorption band at 3443 cm⁻¹ represents the –OH stretching vibration absorption. All of the above changes of the spectral data sufficiently confirmed that the pSt/NHPI particles were prepared.

Fig. 3 demonstrates the change of the structure of the particles in the course of the chemical modification of SiO₂. The surfaces of the raw silica gel particles (Fig. 3(a)) were rough and scraggy, whereas the surfaces of the grafted particles pSt/SiO₂ (Fig. 3(b)) were smoother and flatter. This was caused by the coating and surface-filling action during the synthesis of the functional macromolecule. It can be observed from Fig. 3(c) and (d) that with the corrosion of the silica gel, the obtained polymeric network samples were composed of loosely agglomerated particles with an irregular shape and a large number of hollow apertures (Fig. 4).

Fig. 3 SEM images of four kinds of particles: (a) the silica particles SiO₂; (b) the graft particles pSt/SiO₂; (c) the POP pSt; (d) the functional POP pSt/NHPI.

Fig. 4 Pore size distribution for pSt and pSt/NHPI.

The porosity parameters of the POPs are summarized in Table 3. The porous skeleton of SiO₂ was mainly composed of macropores, whereas the porous skeletons of the two POPs were mainly composed of mesopores. The surface area and pore volume of the POP decreased with NHPI immobilization. This is mainly due to the reagent NHPI bonding to the pores through several steps of polymer reactions.

Table 3 Textural properties of the samples

Sample	ABET (m^2 g^−1)	Vtotal (cm^3 g^−1)	Pore size distribution/%
			Micro-	Meso-	Macro-
SiO2	325	0.35	1.3	15.5	83.2
POP pSt	343	0.43	5.7	91.2	3.1
POP pSt/NHPI	249	0.38	9.9	85.8	4.3

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The thermal stabilities of the graft particles were evaluated using thermogravimetric analysis (TGA) from room temperature to 800 °C under a nitrogen atmosphere. The results are presented in Fig. 5. It can be found that the graft polymer pSt/SiO₂ exhibited a higher decomposition temperature of about 380 °C, indicating an excellent thermal resistance for use as a supported material.

Fig. 5 TGA curve of the pSt/SiO₂ particles.

The content of nitrogen, carbon and hydrogen in pSt/NHPI was determined using a PE-2400II EA. The experimental conditions were as follows: the dosage of sample was 1.5 mg, the temperature of the combustion tube was set at 950 °C, and acetanilide was used as the standard sample.

The immobilization amount of NHPI was determined using the content of nitrogen in Table 4, and the determined result of 2.23 mmol g⁻¹ is coincident with that of the above chemical analysis.

Table 4 Elemental analysis

Element	C	H	N
Content (wt%)	72.84	5.10	3.11

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2.2 Catalytic characteristics of pSt/NHPI

The oxidation of toluene was catalyzed using a combination catalyst of pSt/NHPI and Co(OAc)₂ with molecular oxygen as the final oxidant. Fig. 6 shows that this catalyst system can highly effectively transform toluene into benzoic acid, with a yield of 48% after reaction at 80 °C for 30 h. In order to make a comparison, the catalytic experiment was also carried out using only pSt/NHPI, only Co(OAc)₂ or no catalyst at all, and the results showed that the oxidation of toluene almost didn't occur in these three systems.

Fig. 6 Variation curves for toluene conversion with time using different catalyst systems (temperature: 80 °C, solvent: acetic acid).

The catalytic experiments also were conducted with the particles pSt/SiO₂/NHPI and Co(OAc)₂, as shown in Fig. 7. It can be clearly seen that the pSt/NHPI catalyst showed higher activities than the pSt/SiO₂/NHPI catalyst, because the reactant conversions for the catalysts were different. The former catalyst had a more abundant pore structure, thus creating a much higher reactant concentration in the catalyst pores when compared with the latter. According to the Arrhenius equation, high reactant concentrations leads to a strong catalytic activity.

Fig. 7 Effect of two types of particles on the toluene conversion with time (temperature: 80 °C, solvent: acetic acid).

The catalytic activity of several different co-catalysts is displayed in Fig. 8 and its order was Co(OAc)₂ > MnSO₄ >Fe(acac)₂ > CuCl₂. For the four co-catalysts, the yield of benzoic acid increased with advance of the reduction potential of the co-catalyst (Table 5). As described in Scheme 2, for the catalytic oxidation of toluene, high valence metal ions trigger the generation of PINO radicals which is a critical step.

Fig. 8 Variation curves for the yield of benzoic acid with time using different co-catalyst (temperature: 80 °C, solvent: acetic acid).

Table 5 Standard electrode potential of relevant redox couples

Entry	φ^Θ/V
1	Co^3+ + e ⇌ Co^2+	1.84
2	Mn^3+ + e ⇌ Mn^2+	1.51
3	Fe^3+ + e ⇌ Fe^2+	0.77
4	Cu^2+ + e ⇌ Cu^+	0.15

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Scheme 2 Proposed mechanism for the catalytic aerobic oxidation of toluene.

As shown in Table 6, by fixing the added amount of the combination catalyst (1.0 g), the molar ratio of the immobilized NHPI to Co(OAc)₂ was changed in series. When the molar ratio is equal to 14 : 1, this can result in toluene being highly effectively transformed into benzoic acid with a yield of 45.6% after 30 h. The effect of the amount of the combination catalyst in the oxidation reaction was examined. With an increasing amount of the co-catalysts, the yield of benzoic acid increased, so for this reaction system, a suitable amount of the combination catalyst was 1.04 g, in which the molar ratio of the immobilized NHPI to Co(OAc)₂ was 14 : 1. The results also clearly showed that the catalytic reaction rate increased with temperature, and as the immobilized amount of NHPI at 80 °C was very close to that at 90 °C, a suitable reaction temperature was 80 °C.

Table 6 Effect of reaction conditions on the catalytic reaction

Entry	Temperature (°C)	n(NHPI) : n(Co(OAc)2)	m (combination catalyst)/g	Reaction time (h)	Yield (%)
1	80	10 : 1	1.0	30	39.5
2	80	12 : 1	1.0	30	41.6
3	80	14 : 1	1.0	30	45.6
4	80	16 : 1	1.0	30	43.3
5	80	17 : 1	1.0	30	38.3
6	90	14 : 1	1.0	30	45.9
7	70	14 : 1	1.0	30	36.3
8	60	14 : 1	1.0	30	21.1
9	80	14 : 1	0.95	30	38.5
10	80	14 : 1	1.04	30	47.8
11	80	14 : 1	1.06	30	48.1

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2.2.1 Catalytic selectivity. As described above, for the catalytic oxidation of toluene with the co-catalyst system in this study, the obtained product was mainly composed of benzoic acid, and a small amount of the byproduct benzaldehyde. For the oxidation system containing pSt/NHPI and Co(OAc)₂, Fig. 9 demonstrates the changing of the curves of the yields of benzoic acid and benzaldehyde with time, respectively.

Fig. 9 The yields of benzoic acid and benzaldehyde as well as the selectivity for benzoic acid with time (temperature: 80 °C, solvent: acetic acid).

As is shown in Fig. 9, during the oxidation process for toluene, both the yield and selectivity for benzoic acid increase with time. After 30 h, the yield and selectivity for benzoic acid level off, and the selectivity is 86% and the yield is 48%. For the byproduct benzaldehyde, the yield increases at the beginning and then declines with time, and the reason for that is probably associated with the further oxidation of benzaldehyde to benzoic acid. In summary, in the oxidation of toluene by molecular oxygen, the heterogeneous catalyst pSt/NHPI in combination with a small amount of the co-catalyst Co(OAc)₂ has a high catalytic selectivity for benzoic acid.

2.2.2 Mechanistic studies. The proposed mechanism for this catalytic oxidation is based on previous studies.^13,31,32 It can be described as a cascade of redox reactions containing three cycles, which is shown in Scheme 2.

During the redox process, with the help of the complexation to Co(II) of the Co(OAc)₂, a series of electron transfers is initiated with dioxygen in Cycle I. The key step in the total oxidation process is that the PINO radicals generate via the homolytic cleavage of the hydroxyl group of NHPI (Cycle I). Subsequently, a hydrogen atom of the substrate toluene is taken away by the PINO radical and a carbon free radical is formed, so as to begin the free radical chain reaction which includes chain initiation, propagation and termination steps. Meanwhile, hydroperoxide will be formed, and with its subsequent decomposition, eventually the toluene is oxidized to benzoic acid and benzaldehyde. With repetition of the three cycles, toluene is continuously transformed into benzoic acid.

2.2.3 Reuse properties of the catalyst. A cycle of repeated experiments was carried out using the combination catalyst, which contained pSt/NHPI and Co(OAc)₂, and the supported catalyst pSt/NHPI was used in each catalytic oxidation, which was separated from the reaction mixture for reuse.

The experimental results are shown in Fig. 10. The catalytic activity declined a little in the second recycle during the 7 times of reuse (toluene conversion fell obviously from 56% to 43%). After that, the catalytic activity was quite stable (toluene conversion was at about 44%). In addition, the appearance of pSt/NHPI was maintained well during the recycling. Therefore, it can be concluded that this immobilized catalyst has an excellent reuse performance.

Fig. 10 Effect of cycle number on the catalyst activity (temperature: 80 °C, solvent: acetic acid, reaction time: 30 h).

3. Conclusions

Via several steps of polymer reactions in this investigation, a novel porous organic polymeric support pSt for the immobilization of NHPI was prepared. The experimental results show that the reaction of CM-pSt with TMA is a key step, and the results indicate that DMAC is a suitable solvent and that the optimal reaction temperature is 80 °C for the esterification reaction. The supported catalyst pSt/NHPI could oxidize toluene to benzoic acid with a co-catalyst Co(OAc)₂ and using molecular oxygen as the final oxidant, and the superiority of this catalyst over others includes a very high catalytic activity. In summary, an efficient and convenient method has been designed to support NHPI, and we have demonstrated the effectiveness of the heterogeneous catalyst pSt/NHPI for the catalytic oxidation of hydrocarbons.

Acknowledgements

We really appreciate the National Natural Science Foundation of China (Project No. 21404093), Shanxi Province Youth Science and Technology Foundation (Project No. 2013021009-1) and Fund for Postgraduate of North University of China (Project No. 20151230).

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