Synchronously Synthesizing and Immobilizing N-Hydroxyphthalimide

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Synchronously Synthesizing and Immobilizing N‑Hydroxyphthalimide on Polymer Microspheres and Catalytic Performance of Solid Catalyst in Oxidation of Ethylbenzene by Molecular Oxygen Baojiao Gao,* Suqing Meng, and Xiaolin Yang Department of Chemical Engineering, North University of China, Taiyuan 030051, People’s Republic of China ABSTRACT: Cross-linked copolymer microspheres of glycidyl methacrylate (GMA) and methyl methacrylate (MMA) were first prepared (GMA/MMA microspheres), and then N-hydroxyphthalimide (NHPI) was synchronously synthesized and immobilized on the GMA/MMA microspheres to form a recyclable heterogeneous catalyst, GMA/MMA-NHPI, via several steps of polymer reactions by molecular design. A compositional catalyst was made with GMA/MMA-NHPI microspheres as the main catalyst and a transition metal salt as co-catalyst, and this compositional catalyst was used in the oxidation of ethylbenzene by molecular oxygen. The mechanism of the catalytic oxidation reaction was investigated, and the effects of some main factors on the catalytic oxidation reaction were examined. The experimental results show that, among several transition metal salts, Co(OAc)2 is the best co-catalyst, and the catalytic activity order is Co(OAc)2 > CoCl2 > Mn(OAc)2 > CuCl2. The compositional catalyst composed of GMA/MMA-NHPI microspheres and Co(OAc)2 can effectively catalyze the oxidation of ethylbenzene by molecular oxygen under mild conditions (at 100 °C and ordinary molecular oxygen pressure), and transform ethylbenzene into acetophenone with a yield of 20.1% and a selectivity of 75.8% in 25 h. The catalytic oxidation follows a free radical chain reaction mechanism. The type of solvent used significantly affects the catalytic oxidation reaction, and acetic acid is a more suitable solvent compared with acetonitrile. For the compositional catalyst, there is an optimum amount of added co-catalyst Co(OAc)2; correspondingly, the optimum ratio of immobilized NHPI on GMA/MMA-NHPI microspheres to Co(OAc)2 is 14:1. The appropriate reaction temperature is 100 °C. The solid catalyst, GMA/MMA-NHPI microspheres, exhibits good recycling and reusing properties.

1. INTRODUCTION The selective oxidation of ethylbenzene to acetophenone is an important reaction, as the product acetophenone is a very useful synthetic intermediate for the production of perfumes, pharmaceuticals, resins, alcohols, and esters.1−4 It is also important for the effective utilization of ethylbenzene in the xylene stream in the petrochemical industry.5,6 For the oxidation of ethylbenzene, tert-butyl hydroperoxide (TBHP) and H2O2 are usually used as oxidant because they provide good yields of acetophenone under relatively mild conditions. However, because of the cost of these oxidants, these processes lack competitive capacity. Compared to TBHP and H2O2, air and molecular oxygen are economically and environmentally feasible oxidants for large-scale processing. Therefore, in recent years, oxidation of ethylbenzene to acetophenone with molecular oxygen as oxidant has drawn much attention in the petrochemical industry. However, because the triplet-state nature of molecular oxygen hampers its reaction with hydrocarbons in singlet states,7,8 such processes depend largely on the development of catalysts. Therefore, over the past decades, many homogeneous transition metal compounds (Co, Mn, Cu, or Fe) have been developed as efficient catalysts for the liquid-phase oxidation of ethylbenzene to acetophenone with molecular oxygen as oxidant.9,10 However, among such processes reported to date, the operation conditions are often harsh,11,12 for example, under a pressure of 10 atm of air and at 140−190 °C,13 and furthermore, the chemistry is rarely © XXXX American Chemical Society

selective due to the inertness of the alkyl group and low oxidation ability of oxygen. Various heterogeneous catalysts using transition metal compounds have been studied14−16 to enhance the catalytic efficiency for oxidation. Researchers still face a big challenge to develop effective and highly selective catalytic systems for the oxidation of ethylbenzene to acetophenone with molecular oxygen under moderate conditions and in the liquid phase. In recent years, more and more attention has been paid to Nhydroxyphthalimide (NHPI) as an organic catalyst for its catalytic ability in the oxidation of hydrocarbons. It has been recognized that NHPI in combination with some metal salts is an efficient catalyst for aerobic oxidation of organic substrates.17,18 It is believed that the phthalimide N-oxyl (PINO) radical generated in situ from the reaction of O2 and NHPI is able to abstract a hydrogen atom from the organic substrates, and then the newly formed carbon radical can rapidly react with O2 to give ultimately oxygenated products through alkyl hydroperoxide as a transient intermediate.19,20 There have been some studies on catalytic oxidations of organic substrates, including ethylbenzene, by NHPI with molecular oxygen as oxidant.21−23 However, most of them have used homogeneous catalytic systems for NHPI. It is obvious that Special Issue: Non-precious Metal Catalysis Received: April 7, 2015

A

DOI: 10.1021/acs.oprd.5b00108 Org. Process Res. Dev. XXXX, XXX, XXX−XXX

Organic Process Research & Development

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was added. The copolymerization was carried out at 75 °C for 2 h with stirring under N2 atmosphere, then the temperature was enhanced to 85 °C, and the reaction was performed for another 2 h. The resultant product was white microspheres with translucence and with a mean grain size of 200 μm, and they were namely the cross-linked microspheres GMA/MMA. The amount of the epoxy group contained within the microsphere was calculated in the feed ratio of the monomers, and it was 2.49 mmol/g. The chemical structure of GMA/MMA microspheres is presented in Scheme 1.

there exist some serious drawbacks, for example, the product separation is difficult, and the catalyst NHPI cannot be recovered and reused, leading to low synthetic efficiency. At present, the heterogenization of homogeneous catalysts, such as immobilization of homogeneous catalysts on solid supports, has become an important research area because it allows researchers to design and prepare more environmentally friendly and more effective catalysts.24−26 However, immobilized NHPI is rarely reported,27,28 especially NHPI immobilized on polymer resins.29 In the present work, via molecular design, we synchronously synthesize and immobilize NHPI on crosslinked copolymer microspheres of glycidyl methacrylate (GMA) and methyl methacrylate (MMA) to form a recyclable heterogeneous catalyst, GMA/MMA-NHPI. The immobilized NHPI in combination with a transition metal salt as a compositional catalyst is then used in the oxidation of ethylbenzene with molecular oxygen. This study finds that this compositional catalyst has high catalyst activity, and ethylbenzene can be effectively and selectively oxidized to acetophenone with an ethylbenzene conversion of 26% and an acetophenone selectivity of 76% under mild conditions (at 100 °C, under ordinary pressure of O2), fully displaying the advantages of this heterogeneous catalyst, GMA/MMA-NHPI microspheres. The results of our study should contribute to further facilitating the development of heterogeneous NHPI catalysts, which has obvious scientific significance in the field of “green” oxidation of organic compounds.

Scheme 1. Chemical Structure of Cross-Linked Microspheres GMA/MMA

2.2.2. Synchronous Synthesis and Immobilization of NHPI on Surfaces of GMA/MMA Microspheres. NHPI was synchronously synthesized and immobilized on the surfaces of GMA/MMA microspheres through the following multistep polymer reactions. Step 1: Preparation of aldehyde group-bonded microspheres GMA/MMA-AG. GMA/MMA microspheres (1 g) and 25 mL of solvent DMF were placed into a flask, and the microspheres were dipped for 24 h so as to make the microspheres to be fully swelled. Subsequently, 20 mL DMF solution containing 0.2 g of GA was added, and an aqueous solution containing 0.6 g of NaOH was also added. The ring-opening reaction between the epoxy group of GMA/MMA microspheres and the carboxyl group of GA was conducted at a constant temperature of 100 °C for 12 h. After the reaction, the product microspheres were repeatedly washed with ethanol and distilled water, dried to a constant weight under vacuum, and the resultant microspheres were namely the microspheres modified by aldehyde group (AG), GMA/MMA-AG, on which aldehyde groups had been bonded. The amount of the residual epoxy groups on GMA/ MMA-AG microspheres was determined with hydrochloric acid-acetone method. Further, the bonding amount of aldehyde group on GMA/MMA-AG microspheres was calculated, and it was 1.20 mmol/g. Step 2: Preparation of phthalic acid-bonded microspheres GMA/ MMA-PA. In a three-necked bottle, GMA/MMA-AG microspheres (1 g) were fully swelled in 25 mL of DMF solution, in which 0.3 g of APA was dissolved. The Schiff base reaction between the primary amino group of APA and the aldehyde group on GMA/MMA-AG microspheres was performed at 60 °C with stirring for 12 h. After the reaction, the concentration of the residual APA in the solution was determined by spectrophotometry at 277 nm, and further the conversion of the aldehyde group on GMA/MMA-AG microspheres, which had taken part in the Schiff base reaction, was calculated (76%). The resultant microspheres were collected by filtering, washed with distilled water and ethanol, dried to a constant weigh under vacuum, and the obtained microspheres were namely the

2. EXPERIMENTAL SECTION 2.1. Materials and Instruments. Methacrylate (GMA) was purchased from Soochow Nanhang Chemical Co., Ltd. (China), methyl methacrylate (MMA) was purchased from Tianjing Ruijinte Chemical Co., Ltd. (China), and ethylene glycol dimethacrylate as cross-linker (EGDMA) was supplied by Shandong Yantai Yunkai Chemical Co., Ltd. (China). Azoisobutyronitrile (AIBN) as initiator, poly(vinyl alcohol) (PVA) as dispersant, and glyoxylic acid (GA), 4-aminophthalic acid (APA), and hydroxylamine hydrochloride as main reaction reagents, as well as other chemicals, were all commercial analytically pure reagents and were purchased from Chinese companies. The instruments used in this study were a Perkin-Elmer1700 infrared spectrometer (Perkin-Elmer Co., USA), a LEO-438VP scanning electronic microscope (SEM, LEO Co., UK), and a P1201 high-performance liquid chromatograph (HPLC, Dalian Elite Analytical Instruments Ltd., Dalian City, China). 2.2. Preparation and Characterization of Catalyst Microspheres GMA/MMA-NHPI. 2.2.1. Preparation of Cross-Linked Microspheres GMA/MMA. According to the procedures described in ref 30, the cross-linked microspheres GMA/MMA were prepared with suspension polymerization method, and the typical process is as follows. The disperser poly(vinyl alcohol) (PVA, 2 g) and NaCl (5 g) were dissolved in 100 mL of distilled water, forming water phase as continuous phase. The water phase was placed in a four-necked flask equipped with a mechanical agitator, a condenser, a N2 inlet and a thermometer. Three milliliters of GMA (monomer), 4 mL of MMA (co-monomer), and 2 mL of EGDMA (crosslinker) were dissolved together, constituting oil phase as dispersion phase. The oil phase was added into the water phase, and the system was stirred sufficiently so as to fully disperse the oil phase into the water phase. The content was heated to 75 °C, and 0.05 g of initiator AIBN dissolved in a little of toluene B



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Scheme 2. Chemical Process To Prepare the Heterogeneous Catalyst GMA/MMA-NHPI

amount (IA) of NHPI on GMA/MMA-NHPI microspheres was determined with weighing method, and it was 1.10 mmol/g. 2.2.3. Characterization of Catalyst Microspheres GMA/ MMA-NHPI. For the characterizations of the catalyst microspheres GMA/MMA-NHPI as well as for that of other intermediate microspheres, the following determinations were conducted. The infrared spectra of these microspheres were determined with KBr pellet method to confirm the changes of their chemical structures. The morphology of GMA/MMANHPI microspheres was observed by SEM. The immobilized amount of NHPI on GMA/MMA-NHPI microspheres was determined by weighting method as described above. 2.3. Catalytic Oxidation Experiments with Ethylbenzene and Molecular Oxygen. Glacial acetic acid (30 mL) as solvent and 1.0 g of the solid catalyst GMA/MMANHPI were added into a four-necked flask of 100 mL equipped with a mechanical stirrer, a reflux condenser, a thermometer and an O2 inlet, and the microspheres were allowed to be swelled for 10 h. Subsequently, 1 mL of ethylbenzene and 0.01 g of the co-catalyst Co(OAc)2 were added. Oxygen at normal pressure was passed into the reaction mixture at a fixed flow rate (15 mL/min), which was controlled with a gas flowmeter, and the oxidation reaction was performed at a constant temperature of 100 °C with stirring. The samples of the reaction mixture were taken at a certain time interval, and the

modified microspheres GMA/MMA-PA, on which phthalic acid (PA) as group was bonded. Step 3: Preparation of catalyst microspheres GMA/MMANHPI. In a three-necked bottle, GMA/MMA-PA microspheres (1 g) was dipped in 20 mL of dehydrated DMF that was pretreated with anhydrous magnesium chloride (MgCl2), followed by adding acetic anhydride and setting a drying tube on the bottle. The decarboxylation reaction of the bonded PA group was allowed to be carried out at 30 °C with stirring for 3 h, and the bonded PA group was made to transform to the bonded phthalic anhydride (PAH) group, obtaining the microspheres GMA/MMA-PAH. The GMA/MMA-PAH microspheres (accurately weighted 1 g) were placed into a threenecked bottle, followed by adding 40 mL of pyridine and 10 mL of ethanol as a mixed solvent. Hydroxylamine hydrochloride (0.1 g) as reaction reagent was also added. The imidation reaction between PAH groups on GMA/MMA-PAH microsphere and hydroxylamine hydrochloride was conducted at 90 °C for 15 h. After the reaction, the pH value of the reaction medium was adjusted to pH 2 with diluted hydrochloric acid. The resultant microspheres were collected by filtering, washed repeatedly with distilled water and ethanol, dried to a constant weight under vacuum, and the final product microspheres were namely the catalyst microspheres GMA/ MMA-NHPI, on which NHPI as group had been synchronously synthesized and immobilized. The immobilization C



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quantificational analysis was carried out immediately by high performance liquid chromatograph (HPLC), showing that the main product was acetophenone, and 1-phenylethanol was the byproduct. After the reaction, the microspheres GMA/MMA-NHPI were collected by suction filtration, washed repeatedly with ethanol and acetic acid until there were no raw material (ethylbenzene) and products in the washings, which were detected by UV photometry, and dried under vacuum. And then, the recovered catalyst microspheres were reused in the oxidation reaction of ethylbenzene under the same reaction conditions to examine their recycle and reusing properties. 2.4. Examining Effects of Main Factors on Catalytic Oxidation Reaction of Ethylbenzene. The effects of the main factors on the catalytic oxidation reaction were examined to optimize the reaction conditions and explore the catalytic reaction mechanism. The factors included the type of the cocatalysts, the molar ratio of the immobilized NHPI as main catalyst to the co-catalyst, the type of the solvent, and the reaction temperature.

Figure 1. FTIR spectra of GMA/MMA, GMA/MMA-AG, and GMA/ MMA-PA microspheres.

3. RESULTS AND DISCUSSIONS 3.1. Chemical Process To Prepare Solid Catalyst Microspheres GMA/MMA-NHPI. In this work, by molecular design, NHPI as a group with catalytic function was synchronously synthesized and immobilized on the surface of GMA/MMA copolymer microspheres via several steps of polymer reactions. First, the GMA/MMA copolymer microspheres were prepared with suspension polymerization method with GMA and MMA as monomers and EGDMA as crosslinker. There are a large number of epoxy groups on the surfaces of GMA/MMA microspheres, and on this basis a series of polymer reactions was conducted. (1) The ring-opening reaction of these epoxy groups was carried out with glyoxylic acid as reaction reagent under alkaline conditions, and aldehyde group (AG)-bonded modified microspheres, GMA/MMA-AG, were obtained. (2) Through a Schiff base reaction between the aldehyde group of GMA/MMA-AG microspheres and the primary amino group of APA, phthalic acid (PA) was then introduced to the surfaces of the microspheres, giving another kind of modified microspheres, GMA/MMA-PA. (3) Subsequently, the anhydride form of the PA groups on GMA/ MMA-PA microspheres was made with acetic anhydride as reagent, resulting in phthalic anhydride (PAH)-bonded microspheres, GMA/MMA-PAH. (4) Finally, like in the preparation of small-molecule NHPI, the bonded phthalic anhydride on the GMA/MMA-PAH microspheres reacted with hydroxylamine hydrochloride, and the bonded phthalic anhydride groups were transformed into NHPI groups, realizing the synchronous synthesis and immobilization of NHPI on GMA/MMA microspheres. The chemical process to prepare the heterogeneous catalyst GMA/MMA-NHPI is schematically expressed in Scheme 2, and each reaction result was confirmed by changes in the FTIR spectra of various microspheres, as described below. 3.2. Characterization of GMA/MMA-NHPI Microspheres as Well as Other Intermediate Microspheres. 3.2.1. Infrared Spectra of Various Microspheres. Figure 1 gives the FTIR spectra of GMA/MMA, GMA/MMA-AG, and GMA/MMA-PA microspheres, and Figure 2 gives the FTIR spectra of GMA/MMA-PAH and GMA/MMA-NHPI microspheres. In the spectrum of GMA/MMA microspheres, except for displaying all of the characteristic absorption bands of

Figure 2. FTIR spectra of GMA/MMA-PAH and GMA/MMA-NHPI microspheres.

polyacrylate, for example the absorption band of the carboxyl group of ester group at 1731 cm−1, the characteristic absorption band of the epoxy group appears at 908 cm−1, and this belongs to the GMA unit. In the spectrum of GMA/MMA-AG, the band at 908 cm−1 nearly disappears. At the same time, three new bands appear: the strong and wide absorption band of the hydroxyl group at 3440 cm−1, the absorption band of the C−H bond of aldehyde group at 2828 cm−1, and the conjugation absorption band of the carboxyl group of aldehyde group and the carboxyl group of ester group at 1670 cm−1. These spectral data fully demonstrate that the ring-opening reaction between the epoxy group on GMA/MMA microspheres and glyoxylic acid has occurred, and aldehyde groups have been introduced onto the surfaces of GMA/MMA microspheres, forming the modified microspheres GMA/MMA-AG. In the spectrum of GMA/MMA-PA microspheres, the characteristic absorption band of aldehyde group at 2828 cm−1 has disappeared, and there appears another band of hydroxyl group at 3238 cm−1 as a result of the intramolecular hydrogen bond between the bonded phthalic acid groups. Besides, the band at 1670 cm−1 has been strengthened by the formation of CN bond (imine group) because the absorption band of CN bond is also around 1670 cm−1. These spectrum changes indicate that the Schiff base reaction of the aldehyde group on GMA/MMA-AG microspheres with the primary amino group of APA has been produced, and phthalic acid as group has been introduced onto the surfaces of GMA/MMA microspheres, obtaining the second modified microspheres GMA/MMA-PA. As compared with the spectrum of GMA/MMA-PA microspheres, in the spectrum of GMA/MMA-PAH microD



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Systems and Catalytic Oxidation Mechanism. The solid catalyst GMA/MMA-NHPI with an NHPI immobilization amount of 1.1 mmol/g was used in the oxidation of ethylbenzene with molecular oxygen as oxidant. Several different catalyst systems were adopted: (1) without catalyst, (2) alone using GMA/MMA-NHPI, (3) alone using Co(OAc)2, and (4) GMA/MMA-NHPI + Co(OAc)2. Figure 4 shows the changing curves of ethylbenzene conversion with time for the different catalyst systems.

spheres, the stretching vibration absorption of the carboxyl group of anhydride has appeared at 1825 cm−1, and it fully shows that the decarboxylic reaction of the bonded phthalic acid group has occurred and the bonded phthalic anhydride has formed, obtaining the third modified microspheres, GMA/ MMA-PAH. In the spectrum of GMA/MMA-NHPI microspheres, the absorption band of hydroxyl group has obviously strengthened because of the formation of N-hydroxyl group. However, the other characteristic absorption bands of NHPI are covered up because of the multistep polymer reactions. In order to recognize these bands, the FTIR difference spectrum between GMA/MMA-NHPI and GMA/MMA-PAH microspheres is shown in Figure 2. It can be found from the difference spectrum that the stretching vibration absorption band of the imide group appears at 1699 cm−1 and the characteristic absorption of the N−O bond of the N-hydroxyl group at 1300 cm−1. All these changes fully demonstrate that the imidation reaction between the bonded PAH group on GMA/MMA-PAH microspheres and hydroxylamine hydrochloride has been occurred, and the catalyst microspheres GMA/MMA-NHPI have been prepared. 3.2.2. Morphology of GMA/MMA-NHPI Microspheres. Figure 3 presents the SEM images of GMA/MMA and

Figure 4. Changing curves of ethylbenzene conversion with time for different catalyst systems. Conditions: temperature, 100 °C; molar ratio of NHPI to Co(OAc)2, 20:1; solvent, acetic acid.

Figure 4 indicates that, for the system without catalyst, nearly no reaction occurs, whereas for the systems with GMA/MMANHPI or Co(OAc)2 added alone, the ethylbenzene conversion is also very low, and it could be ignored. However, for the system of GMA/MMA-NHPI + Co(OAc)2, the ethylbenzene conversion increases continuously with time, and it gets up to 26.1% in 25 h, displaying a high catalytic activity. Furthermore, in this system, the color of the catalyst microspheres GMA/ MMA-NHPI changes during the addition of Co(OAc)2, from yellow to dark green, implying that there a complexation reaction might have occurred. The HPLC result indicates that this compositional catalyst caused ethylbenzene to be oxidized into acetophenone as the main product and 1-phenylethanol as the byproduct. It is obvious that the heterogeneous catalyst GMA/MMA-NHPI prepared in this study can efficiently catalyze the oxidation of ethylbenzene to acetophenone under mild conditions (lower temperature and ordinary pressure of oxygen). Of course, compared with the homogeneous NHPI, the ethylbenzene conversion of 26.1% in 25 h is lower, which shows that the catalytic activity of heterogeneous catalyst is lower than that of homogeneous catalyst. In regard to the catalytic oxidation of hydrocarbon by NHPI with molecular oxygen as oxidant, the catalytic oxidation mechanism universally acknowledged is a pathway of free radical chain reaction.31,32 For the catalytic oxidation of ethylbenzene, a possible mechanism is illustrated in Scheme 3. The complexation of Co(II) of Co(OAc)2 with dioxygen occurs first, and a labile dioxygen complex, superoxocobalt(III), is generated by the one-electron reduction of dioxygen.33 The reduction of such cobalt species will rapidly induce or initiate the generation of PINO radicals via the homolytic cleavage of the hydroxyl group of NHPI, and it is a key step in the total oxidation process of ethylbenzene.33,34 Subsequently, the PINO radical abstracts hydrogen from the substrate ethylbenzene to form a α-methylbenzyl radical, and it is readily trapped by

Figure 3. SEM images of (A) GMA/MMA and (B) GMA/MMANHPI microspheres.

GMA/MMA-NHPI microspheres. It can be clearly observed that the prepared GMA/MMA microspheres have an uniformly good sphericity, and their surfaces are very smooth and neat. while the surfaces of GMA/MMA-NHPI microspheres become very rough, caused by a series of polymer reactions. 3.3. Catalytic Performance of Solid Catalyst GMA/ MMA-NHPI in Oxidation of Ethylbenzene with Molecular Oxygen. 3.3.1. Catalytic Activity of Different Catalyst E



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Scheme 3. Mechanism of Oxidation Reaction of Ethylbenzene Catalyzed by Compositional Catalyst and with Molecular Oxygen as Oxidant

namely, their molar amount in the final product should be 1:1. The reason for that the selectivity of acetophenone is much higher than that of 1-phenylethanol is probably associated with the further oxidation of 1-phenylethanol to acetophenone. The hydrogen atom of the −CHOH group of 1-phenylethanol is easier to be abstracted by PINO radical to form radical, and it makes 1-phenylethanol to further transform into the main product acetophenone. The oxidation of 1-phenylethanol is rapid, and the oxidation of 1-phenylethanol might be rapider than that of ethylbenzene. Consequently, the proportion of acetophenone in the total product increases with time, leading to acetophenone becoming the main product and 1-phenylethanol is only a byproduct. Besides, during the oxidation of ethylbenzene, except for 1-phenylethanol oxidation to acetophenone, there are other possible reactions associated with acetophenone formation, for example, the reaction of hydroperoxide with peroxyl or PINO radicals.36,37 All of these reactions are the reason that the selectivity of acetophenone as main product is much higher than that of 1-phenylethanol in ethylbenzene oxidation by molecular oxygen. In summary, GMA/MMA-NHPI microspheres in combination with a small amount of the co-catalyst Co(OAc)2 have high catalytic selectivity for acetophenone in the oxidation of ethylbenzene by molecular oxygen. 3.4. Effects of Main Factors on Catalytic Oxidation Reaction. 3.4.1. Effect of Co-catalyst Type. By fixing other reaction conditions described in section 2.3, the catalytic oxidations of ethylbenzene were conducted with GMA/MMANHPI microspheres as main catalyst and with different cocatalysts, and Figure 6 presents the yield changes of acetophenone with the reaction time for the various systems with different co-catalysts. Figure 6 displays that the catalytic activity order among the several metal salts is Co(OAc)2 > CoCl2 > Mn(OAc)2 > CuCl2. However, after 12 h, the activity of Mn(OAc)2 is poorer than that of CuCl2. It is obvious that the activity of Co(OAc)2 is the highest. This order is caused probably by two factors: one is the redox activity of these transition metal species, and the other is the solubility of these transition metal salts in the solvent or their lipophilic property.38 As described above, the step that superoxometal complex induces or initiates the generation of PINO radical is a key step in the total oxidation process of ethylbenzene. At the same time, the metal elements in higher oxidation state are reduced to the species in lower valence state. Therefore, the oxidation ability of these metal elements in high valence state has a decisive impact on their catalytic activity.

dioxygen to provide the methyl benzyl peroxy radical so as to open the free radical chain reaction of ethylbenzene, which includes chain initiation, propagation and termination steps. Meantime, hydroperoxide will be formed, and with its subsequent decomposition, eventually ethylbenzene is oxidized to acetophenone and 1-phenylethanol.35 During the oxidation of ethylbenzene, there are other possible reactions, for example, the formation reaction of 1-phenylethanol as well as 1phenylethanol oxidation to acetophenone as described below. However, the fact that 1-phenylethanol is oxidized to acetophenone under the studied conditions needs to be further investigated and confirmed through further experiments because some researchers have different views.7 3.3.2. Catalytic Selectivity. As described above, for the catalytic oxidation of ethylbenzene in this study, the main product is acetophenone, and the byproduct is 1-phenylethanol. For the oxidation system with the compositional catalyst of GMA/MMA-NHPI + Co(OAc)2, Figure 5 shows the changing curves of the yields of acetophenone and 1-phenylethanol with time, respectively, and also give the selectivity curve of acetophenone.

Figure 5. Changing curves of yields of acetophenone and 1phenylethanol with time as well as selectivity curves of acetophenone. Conditions: temperature, 100 °C; molar ratio of NHPI to Co(OAc)2, 20:1; solvent, acetic acid.

It is displayed in Figure 5 that, during the oxidation process of ethylbenzene, both the yield and selectivity of acetophenone increase with time. After 25 h, the selectivity rises to 75.8%, and here the yield is 20.1%. For the byproduct 1-phenylethanol, its yield slowly increases, and after 15 h the yield levels off at less than 6%. It seems to be seen from Scheme 3 that the yields of acetophenone and 1-phenylethanol should be the same, F



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pointed out that the added amount of Co(OAc)2 is expressed as the molar feed ratio of immobilized NHPI on GMA/MMANHPI microspheres to Co(OAc)2, and a grater molar feed implies a less added amounts of Co(OAc)2. In order to display more clearly the effect of added amount of Co(OAc)2 on the catalytic reaction, Figure 8 gives the yield of acetophenone in 25 h as a function of the added amount of Co(OAc)2, and the data come from Figure 7.

Figure 6. Changing curves of acetophenone yield with time for the various systems with different co-catalysts. Conditions: temperature, 100 °C; molar ratio of NHPI to co-catalyst, 20:1; solvent, acetic acid.

Among three kinds of high-valence metal species, Co(III), Mn(III), and Cu(II), the oxidation ability of Co(III) is the strongest (the reduction potential of Co(III) → Co(II) is 1.83 V), and so the superoxocobalt(III) easily transforms NHPI to PINO radical, leading to the highest catalytic activity of Co(OAc)2. The activity of CoCl2 is poorer than that of Co(OAc)2, and it is associated with the fact that the solubility or lipophilic property of CoCl2 in acetic acid is weaker than that of Co(OAc)2. The oxidation ability of Mn(III) is in the middle between those of Co(III) and Cu(II) (the reduction potential of Mn(III) →Mn(II) is 1.54 V), and so the catalytic activity of Mn(OAc)2 ranks second to Co(III) salts at the starting stage of the reaction, as shown in Figure 6, but is stronger than CuCl2 because Cu(II) has the weakest oxidation ability (the reduction potential of Cu(II) → Cu(I) is only 0.153 V). The reason for the catalytic activity of Mn(OAc)2 becoming lower than that of CuCl2 after 12 h is that the Mn(II) species is easily oxidized to Mn(V) (the species is OMn(V)) by oxygen, and this makes Mn species lose its ability to alter between Mn(II) and Mn(III).39 In the reaction process, more Mn(II) will transformed to Mn(V), and it makes the activity of Mn(OAc)2 the poorest at the later stage. 3.4.2. Effect of Added Amount of Cocatalyst. For the system described in section 2.3, by fixing the amount used of the main catalyst GMA/MMA-NHPI microspheres, the added amount of Co(OAc)2 was changed in series. Figure 7 gives the yield curves of acetophenone with reaction time for the systems with the different added amounts of Co(OAc)2. It needs to be

Figure 8. Acetophenone yield as a function of added amounts of Co(OAc)2. Reaction time: 25 h.

As mentioned above, in Figures 7 and 8, the decrease of the molar ratio of the immobilized NHPI to Co(OAc)2 signifies the increase of the added amount of Co(OAc)2. It is obvious that the yield of acetophenone increases first and then decrease with increasing the added amount of Co(OAc)2, and there is a maximum yield of acetophenone as this molar ratio is 20:1. Therefore, in this oxidation reaction system, for the compositional catalyst, the appropriate added amount of the co-catalyst Co(OAc)2 is that the molar ratio of the immobilized NHPI to Co(OAc)2 should be selected as 20:1. 3.4.3. Effect of Solvent Type. Acetic acid and acetonitrile are two commonly used solvents for the oxidation reactions of organic compounds catalyzed by NHPI. By using the compositional catalyst of GMA/MMA-NHPI + Co(OAc)2 with a molar ratio of 20:1 of the immobilized NHPI to Co(OAc)2, the catalytic oxidations of ethylbenzene were conducted in two solvents, acetic acid and acetonitrile, respectively. In consideration of the lower boiling point of acetonitrile (81.6 °C), the reaction temperature was selected as 70 °C. Figure 9 presents the yield curves of acetophenone with reaction time in two reaction systems with the two solvents. It can be seen in Figure 9 that there is a great difference of the ethylbenzene conversion between the two systems. As using acetonitrile as solvent, the change of ethylbenzene conversion is very slow, and in 25 h, ethylbenzene conversion is only 4.0%, implying a poor catalytic activity of the compositional catalyst. While using acetic acid as solvent, the change of ethylbenzene conversion is rapider, and in 25 h, ethylbenzene conversion gets up to 12.6%, displaying a high catalytic activity of the compositional catalyst. Apparently, acetic acid is a suitable solvent, and it coincides with the result reported in the other study.33 The catalyst activity difference in the two systems is probably caused by two factors. One is the lipophilic property of the co-catalys Co(OAc)2 in the solvent, and the other is the solvent ability to absorb oxygen. The solubleness of Co(OAc)2 as a salt in acetonitrile is poorer than that in acetic acid, and so its solubility in acetonitrile is lower than that in acetic acid,

Figure 7. Changing curves of acetophenone yield with time for systems with different added amounts of Co(OAc)2. Conditions: temperature, 100 °C; solvent, acetic acid. G



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Figure 11. Acetophenone yield as a function of temperatures. Reaction time: 25 h.

Figure 9. Changing curves of acetophenone yield with time for two systems with acetic acid and acetonitrile as solvent. Temperature: 70 °C.

function of the reaction temperature is also presented in Figure 11. It can be seen that the selectivity of acetophenone increases with the rising of the reaction temperature, and it is probably associated with the reaction mechanism. At lower temperatures, the rate of 1-phenylethanol oxidation might be slow. However, with the enhancement of the temperature, 1-phenylethanol oxidation reaction speeds up, and more 1-phenylethanol will be oxidized and finally transform into acetophenone, resulting in the enhancement of the selectivity of acetophenone. The description above is only a speculation, and the mechanism has yet to be proven through further experiments, as indicated in section 3.3.1. 3.5. Recycle and Reuse Property of GMA/MMA-NHPI Microspheres. Recycle and reuse experiments for GMA/ MMA-NHPI microspheres in combination with Co(OAc)2 were conducted to examine their stability. Figure 12 shows the acetophenone yield in 25 h as a function of the number of cycles.

leading to the poor activity of this compositional catalyst. Besides, acetic acid has stronger ability to absorb oxygen, and namely acetic acid has good dissolving oxygen property.40 It is inevitable that the higher concentration of molecular oxygen in acetic acid will accelerate the oxidation reaction. 3.4.4. Effect of Reaction Temperature. By fixing other reaction conditions described in section 2.3, the catalytic oxidations of ethylbenzene were conducted at different temperatures, and Figure 10 presents the yield changes of acetophenone with the reaction time at different temperatures.

Figure 10. Changing curves of acetophenone yield with time at different temperatures.

Figure 10 shows that at lower temperatures, the yield of acetophenone in the same period of time increases rapidly with reaction temperature, and it coincides with the general kinetic rule. The oxidation reaction of ethylbenzene has activation energy with a certain value, and raising temperature will contribute to overcoming this barrier potential, leading to the speeding up of the reaction. However, as the temperature rises to 100 °C, the change of the yield of acetophenone in the same period of time with temperature has become insignificant, and so 100 °C is selected as a appropriate temperature. The yield of acetophenone in 25 h at different temperatures is presented in Figure 11, and it displays the reaction rate change with temperature clearly. Not only the yield of acetophenone in the same period of time increases with the reaction temperature due to the speeding up of the reaction rate, but also the reaction temperature has a great effect on the selectivity of acetophenone. The selectivity of acetophenone in 25 h as a

Figure 12. Effect of recycle number on catalyst activity.

It can be observed that, during the six consecutive cycles of experiments, in the second use the catalytic activity of GMA/ MMA-NHPI declines slightly, and the yield of acetophenone decreases from 20.1% to 19.0%. Hereafter, the yield of acetophenone remains at more than 18.7%, displaying excellent stability. Furthermore, it was found that, during the period of circulation use, the morphology (sphericity) and color (yellow) of GMA/MMA-NHPI microspheres remained unchanged, which reflects that the falling off of NHPI group from GMA/ MMA-NHPI microspheres is very weak. Therefore, the catalyst microspheres GMA/MMA-NHPI designed and prepared in this study are stable and robust. H



Article

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4. CONCLUSIONS In our investigation, by molecular design, NHPI groups were synchronously synthesized and immobilized on GMA/MMA microspheres to form a recyclable heterogeneous catalyst, GMA/MMA-NHPI. GMA/MMA-NHPI microspheres in combination with transition metal salts were used in the oxidation reaction of ethylbenzene by molecular oxygen, and this compositional catalyst exhibited fine catalytic performance. Among the several transition metal salts used, the catalytic activity order is Co(OAc)2 > CoCl2 > Mn(OAc)2 > CuCl2. Under mild conditions, such as ordinary pressure of molecular oxygen and temperature of 100 °C, the compositional catalyst of GMA/MMA-NHPI + Co(OAc)2 can effectively catalyze the oxidation of ethylbenzene by molecular oxygen, producing acetophenone with a yield of 20.1% and a selectivity of 75.8% in 25 h. For the compositional catalyst of GMA/MMA-NHPI microspheres + Co(OAc)2, there is an optimum amount of the added co-catalyst Co(OAc)2, and the molar ratio of the immobilized NHPI on GMA/MMA-NHPI microspheres to Co(OAc)2 is 20:1. The most suitable solvent is acetic acid, and the appropriate reaction temperature is 100 °C. The solid catalyst, GMA/MMA-NHPI microspheres, possesses good recycling and reusing properties.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] Notes

The authors declare no competing financial interest.

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REFERENCES

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