TEMPO-Functionalized Aromatic Polymer as a Highly Active, pH

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

TEMPO-Functionalized Aromatic Polymer as a Highly Active, pHResponsive Polymeric Interfacial Catalyst for Alcohol Oxidation Kecheng Hu, Jun Tang, Shixiong Cao, Qi Zhang, Jianli Wang, and Zhibin Ye J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00467 • Publication Date (Web): 18 Mar 2019 Downloaded from http://pubs.acs.org on March 19, 2019

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The Journal of Physical Chemistry

TEMPO-Functionalized4 Aromatic Polymer as a Highly Active, pH-Responsive Polymeric Interfacial Catalyst for Alcohol Oxidation Kecheng Hu,1 Jun Tang,1 Shixiong Cao,1 Qi Zhang,1 Jianli Wang,1,* and Zhibin Ye2,* 1

State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, Zhejiang Province Key Laboratory of Biofuel, Biodiesel Laboratory of China Petroleum and Chemical Industry Federation, College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, China

2

Department of Chemical and Materials Engineering, Concordia University, Montreal, QC H3G 1M8, Canada

* E-mail: [email protected] (J. W.); [email protected] (Z. Y.) Tel.:+86-571-88320917 (J.W.); +1-514-8482424 ext. 5611 (Z.Y.)

1

ACS Paragon Plus Environment

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

Abstract We report in this paper the design and synthesis of a pH-responsive polymeric interfacial catalyst (PIC) via one-step grafting of 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) onto a polyaryletherketone having pendant benzimidazole groups (SCBI-PAEK-6F). The TEMPO-functionalized polymer has been systematically characterized with

1H

nuclear magnetic resonance, Fourier-transform infrared

spectroscopy, elemental analysis, contact angle, and transmission electron microscopy. Moreover, this well-designed polymer has been applied in the oxidation of alcohols under Montanari conditions. It is found that the functionalized polymer can aggregate at the water-oil interface and act as an efficient stabilizer to facilitate the formation of a stable Pickering emulsion, which shows outstanding catalytic activity for alcohol oxidation through the microreactor mechanism. Moreover, the PIC is featured with a desirable high pH responsiveness due to the valuable benzimidazole groups. De-emulsification of the Pickering emulsion reaction system can be conveniently triggered by simply tuning the system pH value to 3, thus facilitating the facile recovery of the PIC. In addition, the sustainable catalyst can be reused for subsequent cycles of alcohol oxidation without appreciable loss in catalytic activity or selectivity.

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1. Introduction

Selective oxidation of alcohols to aldehydes or ketones represents one of the most important transformations in fine chemical processes and industrial production.1-5 Traditional oxidation processes for this transformation often involve the use of harmful inorganic oxidants or heavy metals, such as pyridinium chlorochromate,6 the Dess-Martin reagent,7 and MnO2.8 In view of the environmental impact, various green oxidants, such as molecular oxygen, air, sodium hypochlorite and hydrogen peroxide, have drawn increasing attention.9-11 In 1987, Montanari et al.12 invented an oxidative catalytic system (NaClO/TEMPO/NaBr) for efficient, selective oxidation of alcohols to the corresponding aldehydes or ketones. This system exhibits significant advantages, like remarkably high catalytic activity and selectivity, mild reaction conditions, the use of low toxicity reagents, and no toxic side products.13, 14 However, owing to its unique physicochemical properties, the separation of TEMPO from the reaction solution is extremely troublesome, and the TEMPO residue will contaminate the product, giving a light color. Immobilization of homogeneous catalysts is an extensively utilized, well-demonstrated strategy for the efficient recycling of valuable catalysts. Covalent immobilization of TEMPO onto either organic or inorganic supports has been extensively reported.15-24 In particular, 3



numerous works have been carried out by immobilizing TEMPO on polymeric materials. One approach is to anchor TEMPO onto soluble polymers, such as polyethyleneglycol,25-27 polyisobutylene,28 and poly(2-oxazolines).29 Though these homogeneous polymer-supported TEMPO catalysts often have remarkably high catalytic activity and selectivity due to the single phase reaction system, they suffer from inefficient catalyst recovery. Another approach is to select insoluble polymers, polymer nanoparticles or even polymer beads as the support, which facilitates a more facile catalyst recovery and reuse.30-32 In our previous works, we have shown that the Montanari reaction involving two immiscible oil/water (e.g., CH2Cl2/water) phases is a typical heterogenous system. The TEMPO-immobilized insoluble polymer or nano-polymer particles will aggregate at the interface of oil/aqueous phases to form a stable fine Pickering emulsion, which leads to enhanced catalytic activity. However, stable emulsions always pose the difficulty of de-emulsification after reaction to recover the supported TEMPO catalyst. In this regard, we developed a poly(ether sulfone) (PES) grafted with TEMPO through an imidazole bridge, which rendered the TEMPO-immobilized polymer a desired pH sensitivity.33 The de-emulsification process can be accomplished by the use of an external pH stimulus to induce destabilization of the Pickering emulsion. The catalytic polymer can thus be precipitated between the organic and aqueous phases, facilitating its facile recovery by centrifugation or filtration. However, this

4


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strategy suffers from some drawbacks. The synthetic procedure of PES is somewhat complex. Meanwhile, chloromethylation of PES as a key step prior to TEMPO grafting often results in undesired polymer cross-linking, reducing the efficiency of chloromethylation and subsequent TEMPO grafting. Developing an improved, simplistic strategy for the synthesis of the reusable TEMPO polymer catalysts is thus highly desirable for sustainable chemistry. Recently, a novel polyaryletherketone with controllable pendant benzimidazole side groups (SCBI-PAEK-6F) has been developed in our group.34 Employing this polymer as the backbone, we have designed and synthesized in this work a novel pH-responsive polymeric interfacial catalyst (PIC, SCBI-PAEK-6F-C10-TEMPO) by grafting TEMPO efficiently on its pendant benzimidazole groups through a simple one-step process. Its catalytic performance (activity and reusability) in Montanari oxidation of alcohols has been investigated. In addition, this PIC is shown to have a high pH responsiveness with a tunable wettability upon addition of acid, because of its benzimidazole groups (Scheme 1a). Acting as a Pickering emulsifier (Scheme 1b), the polymer catalyst particles enable the formation of a water-in-oil (W/O) Pickering emulsion, which improves the catalytic efficiency due to the enhanced reaction interfacial area. Meanwhile, its valuable pH responsiveness facilitates the convenient recovery and reuse of the polymer catalyst through pH-induced demulsification.

5



Scheme 1. Schematic illustration of alcohol oxidation catalyzed by polymer interfacial catalyst in Montanari oxidation system. 2.

Experimental Section

2.1. Materials

SCBI-PAEK-6F (benzimidazole group content: 0.38 mmol g-1) was synthesized by following a procedure reported in our group.34 Tetrabutylammonium hydrogen sulfate (Bu4NHSO4), sodium bromide (NaBr), dibromodecane, aqueous sodium hypochlorite (NaClO, active chlorine 5%), and alcohols were purchased from Aladdin Co. Ltd. (Shanghai, China). 4-Hydroxy-2,2,6,6-tetramethylpiperidine-oxyl (4-OH-TEMPO), TEMPO, ethyl acetate, petroleum ether, and tetrahydrofuran (THF) were obtained 6


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from Sigma Aldrich. All other analytical grade reagents were used as received without further purification. 2.2. Synthesis of SCBI-PAEK-6F-C10-TEMPO

Bromodecyl TEMPO (BrC10TEMPO) was synthesized according to our previous work.35 The dispersion of SCBI-PAEK-6F (0.5 g) in THF (10 mL), BrC10-TEMPO (0.12 g, 0.3 mmol), Bu4NHSO4 (0.136 g), and NaOH (0.12 g) were charged in a 50 mL round-bottomed flask. Then the reaction mixture was heated at 25 oC for 24 h with magnetic stirring. The resultant velvet polymer was precipitated out by adding ethanol and was subsequently washed with ethanol and deionized water. The obtained polymer was finally dried at 45 oC under vacuum for 12 h, rendering a yield of 0.495 g (92%). 2.3. Characterization Proton nuclear magnetic resonance (1H NMR) spectroscopy analysis of the polymers was performed on a Bruker Advance III 500 MHz spectrometer with chloroform-d as the solvent. The SCBI-PAEK-6F-C10-TEMPO sample of N-O radical moieties was treated with phenylhydrazine before

1H

NMR measurement. Fourier-transform

infrared (FT-IR) spectroscopy was performed on a Nicolet iS10 FTIR spectrometer. The C, N, and H contents of the polymer samples were determined by element analysis (EA) using a Vario MACRO cube elemental analyzer (Elementar, Germany). Static water contact angle (CA) of the surface of SCBI-PAEK-6F-C10-TEMPO was measured with a DataPhysics OCA-20 system. The optical microscopy photographs of the Pickering emulsion formed with the PIC was taken using an Olympus BX41 7



optical microscope. The particle size of SCBI-PAEK-6F-C10-TEMPO was measured using Zetasizer ZS90 dynamic lighting scattering (Malvern, UK, DLS). Transmission electron microscopy (TEM) characterization was performed on a Hitachi H-7500 electron microscope operated at an accelerating voltage of 80 kV. To visualize the morphology of SCBI-PAEK-6F-C10-TEMPO in the Pickering emulsion, a small drop of the Pickering emulsion was placed on a carbon-coated copper grid, followed with sovlent evaporation at room temperature prior to the measurements. 2.4. General procedure for the PIC-catalyzed oxidation of alcohols

In a 25 mL round bottom flask, alcohol substrate (1.0 mmol), TEMPO (0.001 mmol, 0.068 mg) or SCBI-PAEK-6F-C10-TEMPO (4.3 mg with TEMPO loading of 0.001 mmol), and CH2Cl2 (3.5 mL) were added. The mixture was sonicated for 0.5 min, then cooled to 10 oC. Subsequently, NaBr (1.0 M, 0.15 mL) and NaClO (0.37 M, 3.35 mL, pH ≈ 9.1) solutions were added, and the resulting mixture was vigorously stirred at 1400 rpm. After a prescribed time, the reaction was quenched by consuming excess hypochlorite with saturated Na2SO3. The supernatant was dried over anhydrous Na2SO4, and analyzed with GC (Shimadzu GC-2014C equipped with a flame ionization detector and high-purity hydrogen as the carrier gas) for alcohol conversion. The de-emulsification process was achieved by tuning the pH value to 3 with HCl aqueous solution (1 M). The PIC was recovered by precipitation and drying under vacuum, and was subsequently used for following cycles. The organic phase was collected and dried over anhydrous Na2SO4. After evaporating and removing all the

8


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solvents by rotary evaporation, the crude product was finally purified by column chromatography on silica gel using petroleum ether/EtOAc = 1:10 as the eluent. The yield of the product was determined by weighing the purified product obtained after column chromatography, and the purified product was determined by 1H NMR spectroscopy (Figure S1, S2 and S3). 3. Results and Discussion 3.1. Synthesis and characterization of SCBI-PAEK-6F-C10-TEMPO

In this study, a pH-responsive polyaryletherketone having pendant benzimidazole groups (SCBI-PAEK-6F) is employed as the polymer scaffold for the immobilization of TEMPO. The immobilization is achieved in a simplistic one-step process by reacting

SCBI-PAEK-6F

with

bromodecyl

TEMPO,

rendering

the

TEMPO-immobilized polymer, SCBI-PAEK-6F-C10-TEMPO. Scheme 2 shows the synthesis route, where TEMPO immobilization is achieved by the covalent attachment of decyl TEMPO onto the benzimidazole group as the specific anchoring sites through the nucleophilic substitution mechanism. The chemical structure and composition of the resulting polymer have been verified by 1H NMR, FT-IR, and EA as follows.

9



Scheme 2. Synthesis route of SCBI-PAEK-6F-C10-TEMPO.

Figure 1. 1H NMR spectrum of SCBI-PAEK-6F-C10-TEMPO. 10


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The 1H NMR spectra of SCBI-PAEK-6F and SCBI-PAEK-6F-C10-TEMPO are shown in Figure S4 and Figure 1, respectively. In particular, peak e at δ = 4.0 ppm in Figure S4 is ascribed to the methylene protons in SCBI-PAEK-6F. Compared to that of SCBI-PAEK-6F, there is a slight shift of peak e to 4.115 ppm in the 1H NMR spectrum of SCBI-PAEK-6F-C10-TEMPO. Meanwhile, the latter shows a set of new peaks within 1-2.1 ppm, corresponding to the protons on the grafted C10-TEMPO units. In particular, the peak at around δ = 1.75-2.10 ppm corresponds to the methylene protons h and p labelled in the C10-TEMPO units. These peaks confirm the successful covalent grafting TEMPO units on the SCBI-PAEK-6F polymer. From integrations of peaks e, h, and p, the percentage of benzimidazole groups grafted with C10-TEMPO is estimated to be 65%, equivalen to a TEMPO loading of 0.23 mmol g-1.

Figure 2. FT-IR spectra of SCBI-PAEK-6F and SCBI-PAEK-6F-C10-TEMPO. 11



Figure

2

compares

the

SCBI-PAEK-6F-C10-TEMPO.

FT-IR

spectra

Page 12 of 31

of

SCBI-PAEK-6F

SCBI-PAEK-6F-C10-TEMPO

shows

all

and the

characteristic bands found with SCBI-PAEK-6F, including bands at 1167 cm-1 and 923 cm-1 for the stretching vibrations of the aromatic ring connected by the ether group and the carbonyl, 1240 cm-1 for C-O-C stretching vibration, 1660 cm-1 for C=O stretching vibration.36 Beside these common bands present in both polymers, a new band at 1360 cm-1 appears in SCBI-PAEK-6F-C10-TEMPO, which is attributed to the N-O radical stretching vibration of the grafted TEMPO groups.33 Table 1. C, H, N elemental analysis and TEMPO loading. Elements [wt%] Entry

1 2

Sample

TEMPO loading [mmol∙g-1]

[C]

[H]

[N]

65.55

3.38

1.13

-

SCBI-PAEK-6F-C1065.82 TEMPO

3.48

1.37

0.23

SCBI-PAEK-6F

Elemental analysis of both polymers has been undertaken. Table 1 summarizes the results. SCBI-PAEK-6F-C10-TEMPO has an appreciably higher nitrogen content (1.37 wt% vs. 1.13%) than SCBI-PAEK-6F, also confirming the TEMPO immobiilzation.

From

the

data,

the

loading

of

TEMPO

on

SCBI-PAEK-6F-C10-TEMPO is estimated to be 0.23 mmol g-1 according to the following equation: 12


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R

m2 N  m1N M N  M T m1N

(1)

where R is the loading of TEMPO in mmol g-1, m1N is the weight percent of nitrogen in SCBI-PAEK-6F,

m2N

is

the

weight

percent

of

nitrogen

in

SCBI-PAEK-6F-C10-TEMPO, MN is the atomic weight of nitrogen element (14 g mol-1), MT is the molecular weight of the -C10-TEMPO (315 g mol-1). This is in good agreement with the TEMPO loading data estimated from 1H NMR. 3.2. Formation of Pickering emulsion NaClO in combination with 10 mol% of NaBr and 1 mol% of TEMPO has been used in Montanari oxidation system, which has low toxicity, as well as high efficiency and selectivity.37-40 Herein, SCBI-PAEK-6F-C10-TEMPO is used as the PIC to formulate a pH

responsive,

Pickering

emulsion

based

Montanari

oxidation

system.

SCBI-PAEK-6F-C10-TEMPO can be well dispersed in CH2Cl2 via ultrasonication. As Figure S5 depicts, the obtained dispersion shows a significant Tyndall effect, which indicates the presence of colloidal particles. The surface wettability of the particles has a major impact on a particle-stabilized emulsion.41,

42

The wettability of solid

particles in Pickering emulsions is generally expressed in terms of water contact angle. When the solid particles are relatively hydrophilic (i.e., contact angle < 90°), it tends to stabilize the oil-in-water (O/W) emulsion; when the solid particles are hydrophobic (water contact angle > 90o), it tends to stabilize the water-in-oil (W/O) emulsion;

13



when the contact angle is approximately 90o, the emulsion formed is most stable and emulsion droplets have the smallest particle size and strong coalescence stability.43 Water contact angle on the surface of a SCBI-PAEK-6F-C10-TEMPO thin slice is determined to be 117o (see Figure 3). This thus suggests that it is hydrophobic and tends to stabilize water-in-oil (W/O) emulsions given the low content of hydrophilic benzimidazole group. To investigate the emulsification behavior of the polymer catalyst in Montanari system, water at the same volume as the oil phase was added into the SCBI-PAEK-6F-C10-TEMPO dispersion in CH2Cl2. The mixture was magnetically stirred at 1400 rpm for 90 s to obtain the emulsion as shown in Figure 4(a). The morphology of Pickering emulsion was examined with optical microscopy. The diameter of the emulsion droplets is in the approximate range of 20-200 μm, as shown in Figure 4(b), with an average particle diameter of approximate 60 μm. The total interface area is estimated to be 0.35 m2 with the given PIC loading, which provides a large interface area for accelerating the rate of reaction. The morphology of SCBI-PAEK-6F-C10-TEMPO nanoparticles in the emulsion is also visualized with TEM (Figure 4(c)) and the mean particle diameter is in the range of 5070 nm, which was consistent with the result of DLS (Figure S6).

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Figure 3．Water contact angle on a thin slice of SCBI-PAEK-6F-C10-TEMPO.

Figure 4. (a) Photographs of the water in CH2Cl2 emulsion stabilized by polymer catalyst; (b) optical microscopy image of droplets of W/O Pickering emulsions (scale bar: 100 μm); (c) TEM image of polymer catalyst nanoparticles dispersed in CH2Cl2 (scale bar: 200 nm). 3.3. Catalytic activity evaluation Figure 5 illustrates the catalytic mechanism of the Pickering emulsion for the Montanari oxidation of alcohols. The alcohol substrate is dynamically distributed in the biphasic media by diffusion and extraction. Meanwhile, the catalyst nanoparticles

15



at the oil-water interface react with BrO- at the interface to generate TEMPO+ in situ, which is ready to convert alcohol into aldehyde. Simultaneously, the product aldehyde is extracted into the CH2Cl2 phase because of its low solubility in water. The formation of the Pickering emulsion enhances reactive interfacial area, and in consequence mass transfer between the two phases.

Figure 5. Mechanism of alcohol oxidation reaction taking place at the water-oil interface catalyzed by the polymer interfacial catalyst. To demonstrate the effectiveness of the PIC in Montanari oxidation system, benzyl alcohol is used as a model substrate to evaluate its catalytic competency. As depicted in Figure 6(a) and (b), a blank test was first carried out, with negligible conversion of benzyl alcohol in the absence of the PIC. In the control run with TEMPO as the small molecule catalyst at an alcohol/TEMPO ratio of 1000:1, complete conversion of benzyl alcohol can be achieved in 4 min with a turn-over frequency (TOF) of 4.2 s-1. 16


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On the contrary, only 90 s is needed with the PIC with a TOF of 11.1 s-1 at the identical condition, confirming its superior activity compared to homogeneous TEMPO with the reasons justified above. We have further investigated the effect of alcohol/catalyst ratio on the kinetics of the oxidation reaction. Reactions have been undertaken at different alcohol/TEMPO molar ratios at otherwise identical conditions. As shown in Figure 6(c), it can be found that it takes 240 s and 330 s to reach complete conversion at alcohol/TEMPO ratio of 1500:1 and 2000:1, respectively. With the increase of the molar ratio from 1000:1 to 2000:1, the reaction rate gradually decreases owing to a lowered level of particle content that cannot effectively form Pickering emulsion. Furthermore, we have also calculated and illustrated the TOF curves of SCBI-PAEK-6F-C10-TEMPO at different alcohol/TEMPO ratios as shown in Figure 6(d). The TOF results found in this work are higher than the best values reported in previous works.10-12, 21, 26, 27, 44-46 We have previously reported two highly active magnetic nanoparticle-containing nanohybrid TEMPO catalysts.21, 47 In specific, the magnetic nanoparticle-encapsulated cross-linked polystyrene nanoparticle catalyst showed a high TOF of 1200 h-1 and the magnetic polymeric TEMPO containing Fe3O4@SiO2@PTMA nanohybrid catalyst showed an even higher TOF of 32400 h-1. Herein, the highest TOF value of SCBI-PAEK-6F-C10-TEMPO is 63000 h-1 at t = 1 min for alcohol/TEMPO ratio of 1500:1, which is approximately twice the value of Fe3O4@SiO2@PTMA.

17



The substrate scope with SCBI-PAEK-6F-C10-TEMPO in Montanari oxidation system has also been investigated. Table 2 summarizes the reaction results at the TEMPO loading of 0.1 mol%. All the primary benzylic, aliphatic, or heteroatom containing alcohols are quantitatively oxidized into their corresponding aldehydes within 3 minutes. Even with the least reactive 1-octanol, a conversion of 76% at 2 min is achieved. The reaction with secondary benzylic or aliphatic alcohols also give similar results. To the best of our knowledge, these excellent results are far more superior to those reported for Montanari oxidation of alcohols catalyzed with recyclable nitroxyl radicals.48, 49 For example, the activity of SCBI-PAEK-6F-C10-TEMPO is increased by approximately 33-fold in comparison with TEMPO immobilized on PEG (MeO-PEG164-TEMPO; see Table S1)25 and 68-fold in comparison with TEMPO immobilized on Chimassorb 944 (MW ≈ 3000) (Table S1).50 These results confirm the outstanding activity and selectivity of the PIC towards both primary and secondary alcohols.

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Figure 6. (a) Conversion curves and (b) TOF curves of benzyl alcohol in Montanari oxidation reactions undertaken with no catalyst, with homogeneous TEMPO, and with the PIC at a TEMPO loading 0.1 mol%, respectively; (c) conversion curves and (d) TOF curves of the oxidation of benzyl alcohol with the PIC at different molar ratios of benzyl alcohol/TEMPO. Table 2. Conversion and selectivity data in Montanari oxidation of different alcohols with the PIC.a

OH R1

R2

Cat(0.1mol%) NaBr(10mol%) NaClO(1.5eqiv.,pH~9.1) CH2Cl2,10 oC

Time(s)

O R1

R2

Conversion(%)b Selectivity(%)b

Yield(%)c

Entry

Substrate

1

Benzyl alcohol

90

>99

>99

90

2

2-Phenylethanol

30

>99

>99

-

3

1-Phenylethanol

60

91

>99

-

4

3-Pyridinemethanol

60

>99

>99

76

5

1-Octanol

120

68

>99

-

6

Diphenylmethanol

150

>99

>99

-

19



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7

4-Nitrobenzyl alcohol

60

>99

>99

86

8

1-Hexanol

30

97

>99

-

9

Cycloheanol

30

>99

>99

-

a

Reaction conditions: alcohol (1 mmol), 10 mol% NaBr (0.15 mL, 1 M), 1.5 equiv. NaClO (3.35 mL, 0.37 M, pH ≈ 9.1), SCBI-PAEK-6F-C10-TEMPO (0.001 mmol), CH2Cl2 (3.5 mL), T = 10 °C. b Conversions c

and selectivity were determined through GC.

Yields of isolated products.

3.4. Catalyst recyclability For a heterogeneous catalytic reaction, developing a novel method for product/catalyst separation and catalyst recycling is crucial to their catalytic performance. We have further investigated the recyclability of this PIC. We have found that the recovery of the PIC can be easily accomplished by simply tuning pH of the reaction system owing to the unique pH-responsiveness of the PIC. When a reaction is completed, adjusting pH of the reaction emulsion to 3 with 1 M HCl aqueous solution renders readily its de-emulsification after gentle shaking for 20 s, with distinct phase separation as shown in Figure 7 (upper layer, water phase; lower layer, oil phase). The precipitated catalyst can be clearly observed at the biphasic interface. This phenomenon of phase separation is attributed to benzimidazole side groups in the polymer, which are protonated at the lowered pH to render the catalyst with a hydrophilic surface. The catalyst can then be collected by centrifuge and used directly for next catalytic cycles after washing with CH2Cl2 and vacuum drying. In this way, the recovery and reuse of the catalyst can thus be easily achieved with this intelligent catalytic system. 20


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Figure 8 shows the conversion and selectivity data in the oxidation of benzyl alcohol in 9 cycles with the recycled PIC. It can be found that the conversion of benzyl alcohol displays only marginal decreases after both 30 s and 90 s of reaction in the nine cycles, as shown in Figure S7 and 8, respectively. 1H NMR spectrum of the recycled catalyst after four cycles is almost the same as the original one with all characteristic peaks well maintained (Figure S8). These results confirm that SCBI-PAEK-6F-C10-TEMPO catalyst has a high stability for selective oxidation of alcohols.

Figure 7. Photographs showing the recovery of the polymer catalyst by pH tuning.

21



Figure 8. Conversion and selectivity data in nine cycles of benzyl alcohol oxidation with the recycled SCBI-PAEK-6F-C10-TEMPO after 90 s. Reaction conditions: alcohol (1 mmol), 0.1 mol% TEMPO, 10 mol% NaBr (0.15 mL, 1 M), 1.5 equiv. NaClO (3.35 mL, 0.37 M, pH ≈ 9.1), CH2Cl2 (3.5 mL), 10 °C, reaction time 90 s. 4. Conclusions

In summary, a simple protocol has been developed to synthesize a recyclable pH-responsive polymer immobilized TEMPO catalyst, SCBI-PAEK-6F-C10-TEMPO, for the Montanari oxidation of alcohols. Acting as a Pickering emulsifier, the polymer catalyst facilitates the formation of stable water-in-oil (W/O) Pickering emulsions. Compared to homogeneous TEMPO, heterogeneous SCBI-PAEK-6F-C10-TEMPO shows superior catalytic performance, with signficantly enhanced activity, easy recovery, high reusability, and high stability. In particular, with the valulable pH responsiveness of its benzimidazole groups, its recovery following the reactions can be easily achieved by de-emulsification through a simple tuning of the system pH 22


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value. The recycled catalyst shows no obvious loss in catalytic activity or selectivity after 4 consecutive cycles. With its superior catalytic performance, this pH-responsive PIC is promising for practical applications. ASSOCIATED CONTENT Supporting Information Additional

data,

including

1H


after


in

SCBI-PAEK-C10-TEMPO;

The

NMR

spectrum

recycling CH2Cl2; catalytic

4

of times; Size

activity

SCBI-PAEK-6F, Tyndall

effect

and of

distributions

of

comparison

of

SCBI-PAEK-C10-TEMPO and other reported polymer supported TEMPO catalysts; the catalytic results of recycled SCBI-PAEK-6F-C10-TEMPO at 30 s. Acknowledgments This study was financially supported by the National Natural Science Foundation of China [Grants No. 21374103] and Zhejiang Provincial Natural Science Foundation of China [Grants No. LY18B040004]. References (1) Liu, H. L.; Liu, Y. L.; Li, Y. W.; Tang, Z. Y.; Jiang, H. F. Metal-Organic Framework Supported Gold Nanoparticles as a Highly Active Heterogeneous Catalyst for Aerobic Oxidation of Alcohols. J. Phys. Chem. C 2010, 114, 13362-13369.

23



Page 24 of 31

(2) Teng, Y.; Song, L. X.; Wang, L. B.; Xia, J. Face-Raised Octahedral Co3O4 Nanocrystals and Their Catalytic Activity in the Selective Oxidation of Alcohols. J. Phys. Chem. C 2014, 118, 4767-4773. (3) Ying, H. F.; Sun, Q.; Pan, S. X.; Meng, X. J.; Xiao, F. S. Porous Polymerized Organic

Catalysts

Rationally

Synthesized

from

the

Corresponding

Vinyl-Functionalized Monomers as Efficient Heterogeneous Catalysts. ACS Catal. 2015, 5, 1556-1559. (4) Li, B. X.; Zhang, B. S.; Nie, S. B.; Shao, L. Z.; Hu, L. Y.; Optimization of Plasmon-Induced Photocatalysis in Electrospun Au/CeO2 Hybrid Nanofibers for Selective Oxidation of Benzyl Alcohol. J. Catal. 2017, 348, 256-264. (5) Zhang, H.; Zhou, M. H.; Xiong, L. F.; He, Z. D.; Wang, T. Q.; Xu, Y.; Huang, K. Amine-Functionalized Microporous Organic Nanotube Frameworks Supported Pt and Pd Catalysts for Selective Oxidation of Alcohol and Heck Reactions. J. Phys. Chem. C 2017, 121, 12771-12779. (6) Corey, E. J.; Suggs, J. W. An Efficient Reagent for Oxidation of Primary and Secondary Alcohols to Carbonyl Compounds. Tetrahedron Lett. 1975, 16, 2647-2650. (7) Dess, D. B.; Martin, J. C. A Useful 12-I-5 Triacetoxy Peroxidation (the Dess-Martin Periodinate) for the Selective Oxidation of Primary or Secondary Alcohols and a Variety of Related 12-I-5 Species. J. Am. Chem. Soc. 1991, 113, 7277-7287. (8) Adamm, W.; Saha-Möller, C. R.; Ganeshpure, P. A. Synthetic Applications of Nonmetal Catalysts for Homogeneous Oxidations. Chem. Rev. 2001, 101, 3499-3548.

24


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


(9) Vanoye, L.; Aloui, A.; Pablos, M.; Philippe, R.; Percheron, A.; Favre-Réguillon, A.; de Bellefon, C. A Safe and Efficient Flow Oxidation of Aldehydes with O2. Org. Lett. 2013, 15, 5978-5981. (10) Shen, X. B.; Zhang, Q.; Zhang, G. Q.; Wang, J. L. Significant and Synergistic Intensification of Aerobic Oxidation of Activated Alcohols in Water at Ambient Condition by Adding Perfluoro-Surfactant. ChemistrySelect 2018, 3, 7856-7861. (11) Gilhespy, M.; Lok, M.; Baucherel, X. Polymer-Supported Nitroxyl Radical Catalysts for the Hypochlorite and Aerobic Oxidation of Alcohols. Catal. Today 2006, 117, 114-119. (12) Anelli, P. L.; Biffi, C.; Montanari, F.; Quici, S. Fast and Selective Oxidation of Primary Alcohols to Aldehydes or to Carboxylic Acids and of Secondary Alcohols to Ketones Mediated by Oxoammonium Salts under Two-Phase Conditions. J. Org. Chem. 1987, 52, 2559-2562. (13) Anelli, P. L.; Banfi, S.; Montanari, F.; Quici, S. Oxidation of Diols with Alkali Hypochlorites Catalyzed by Oxammonium Salts under Two-Phase Conditions. J. Org. Chem. 1989, 54, 2970-2972. (14) Anelli, P. L.; Montanari, F.; Quici, S. A Eeneral Synthetic Method for the Oxidation of Primary Alcohols to Aldehydes: (S)-(+)-2-Methylbutanal. Org. Synth. 1990, 69, 212-212. (15) Gheorghe, A.; Chinnusamy, T.; Cuevas-Yañez, E.; Hilgers, P.; Reiser, O. Combination of Perfluoroalkyl and Triazole Moieties: A New Recovery Strategy for TEMPO. Org. Lett. 2008, 19, 4171-4174.

25



(16) Wang, Y. M.; Song, X. Y.; Shao, S. H.; Zhong, H. M.; Lin, F. An efficient, Soluble, and Recyclable Multiwalled Carbon Nanotubes-Supported TEMPO for Oxidation of Alcohols. RSC Adv. 2012, 2, 7693-7698. (17) Yu, W.; Zhou, M. H.; Wang, T. Q.; He, Z. D.; Shi, B. Y.; Xu, Y.; Huang, K. “Click Chemistry” Mediated Functional Microporous Organic Nanotube Networks for Heterogeneous Catalysis. Org. Lett. 2017, 19, 5776-5779. (18) Shakir, A. J.; Paraschivescu, C.; Matache, M.; Tudoseb, M.; Mischieb, A.; Spafiub, F.; Ionita, P. A Convenient Alternative for the Selective Oxidation of Alcohols by Silica Supported TEMPO Using Dioxygen as the Final Oxidant. Tetrahedron Lett. 2016, 47, 6878-6881. (19) Rçben, C.; Studer, A.; Hemme, W. L.; Eckert, H. Immobilization of TEMPO Derivatives in Saponite and Use of These Novel Hybrid Materials as Reusable Catalysts. Synlett 2010, 7, 1110-1114. (20) Karimi, B.; Badreh, E. SBA-15-Functionalized TEMPO Confined Ionic Liquid: An Efficient Catalyst System for Transition-Metal-Free Aerobic Oxidation of Alcohols with Improved Selectivity. Org. Biomol. Chem. 2011, 9, 4194-4198. (21) Tang, J.; Zhang, Q.; Hu, K. C.; Zhang, P.; Wang, J. L. Novel High TEMPO Loading Magneto-Polymeric Nanohybrids: An Efficient and Recyclable Pickering Interfacial Catalyst. J. Catal. 2017, 353, 192-198. (22) Miao, C. X.; He, L. N.; Wang, J. Q.; Gao, J. Biomimetic Oxidation of Alcohol Catalyzed by TEMPO-Functionalized Polyethylene Glycol and Coppe(I) Chloride in Compresse Carbon Dioxide. Synlett. 2009, 20, 3291-3294.

26


Page 26 of 31

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


(23) Fall, A.; Sene, M.; Gaye, M.; Gómez, G.; Fall, Y. Ionic Liquid-Supported TEMPO as Catalyst in the Oxidation of Alcohols to Aldehydes and Ketones. Tetrahedron Lett. 2010, 51, 4501-4504. (24) Beejapur, H. A.; Zhang, Q.; Hu, K. C.; Zhu, L.; Wang, J. L.; Ye, Z. B. TEMPO in Chemical Transformations: From Homogeneous to Heterogeneous. ACS Catal. 2019, 9, 2777-2830. (25) Ferreira, P.; Hayes, W.; Phillips, E.; Ripponb, D.; Tsang, S. C. Polymer-Supported Nitroxyl Catalysts for Selective Oxidation of Alcohols. Green Chem. 2004, 6, 310-312. (26) Pozzi, G.; Cavazzini, M.; Quici, S.; Benaglia, M.; Dell’Anna, G. Poly(ethylene glycol)-Supported TEMPO: An Efficient, Recoverable Metal-Free Catalyst for the Selective Oxidation of Alcohols. Org. Lett. 2004, 6, 441-443. (27) Benaglia, M.; Puglisi, A.; Holczknecht, O.; Quici, S.; Pozzi, G. Aerobic Oxidation of Alcohols to Carbonyl Compounds Mediated by Poly(ethylene glycol)-Supported TEMPO Radicals. Tetrahedron 2005, 61, 12058-12064 . (28) Bergbreiter, D. E.; Li, J. Terminally Functionalized Polyisobutylene Oligomers as Soluble Supports in Catalysis. Chem. Commun. 2004, 7, 42-43. (29) Gall, B.; Bortenschlager, M.; Nuyken, O.; Weberskirch, R. Cascade Reactions in Polymeric Nanoreactors: Mono (Rh)-and Bimetallic (Rh/Ir) Micellar Catalysis in the Hydroaminomethylation of 1-Octene. Macromol. Chem. Phys. 2008, 209, 1152-1159. (30) Liu, L.; Liu, D.; Xia, Z. W.; Gao, J. L.; Zhang, T. L.; Ma, J. J.; Zhang, D. E.; Tong, Z.W. Supported Ionic-Liquid Layer on Polystyrene-TEMPO Resin: A Highly

27



Page 28 of 31

Efficient Catalyst for Selective Oxidation of Activated Alcohols with Molecular Oxygen. Monatsh. Chem. 2013, 144, 251-254. (31)

Subhani,

M.

Polystyrene-Supported

A.;

Beigi,

M.;

Eilbracht,

P.

2,2,6,6-Tetramethylpiperidine-1-oxyl

Polyurethane(TEMPO):

and Facile

Preparation, Catalytic Oxidation and Application in a Membrane Reactor. Adv. Synth. Catal. 2008, 350, 2903-2909. (32) Li, D. J.; Shen, X. B.; Chen, L.; Jiang, H. C.; Wang, J. L. The Stability of Covalently Immobilization of TEMPO on the Polymer Surface Through Ionic Liquid Linkage: a Comparative and Model Research. e-Polymer 2015, 15, 39-44. (33) Chen, L.; Tang, J.; Zhang, Q.; Wang, J. L. Linear Amphiphilic TEMPO-Grafted Poly(ether sulfone) as Polymeric Interfacial Catalyst: Synthesis, Self-Assembly Behavior, and Application. React. Funct. Polym. 2016, 105, 134-139. (34) Wang, J. L.; Wu, K.; Zhang, Q. Preparation and Application of A Novel Polyaryletherketone Having Pendant Benzimidazole Groups. CN Patent 10627969A, January, 4, 2016. (35) Zheng, Z.; Wang, J. L.; Chen, H. L.; Feng, L. B.; Jing, R.; Lu, M. Z.; Hu, B.; Ji, J. B. Magnetic Superhydrophobic Polymer Nanosphere Cage as a Framework for Miceller Catalysis in Biphasic Media. ChemCatChem 2014, 6, 1626-1634. (36) Wang, J. L.; Song, Y. L.; Zhang, C.; Ye, Z. B.; Liu, H.; Lee, M. H.; Wang, D. H. Ji,

J.

B.

Alternating

Copolymer

of

Sulfonated

Poly(ether

ether

ketone-benzimidazole)s (SPEEK-BI) Bearing Acid and Base Moieties. Macromol. Chem. Phys. 2008, 209, 1495-1502.

28


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


(37) Weik, S.; Nicholson, G.; Jung, G.; Rademann, J. Oxoammonium Resins as Metal-Free, Highly Reactive, Versatile Polymeric Oxidation Reagents. Angew. Chem. Int. Ed. 2001, 40, 1436-1439. (38) Roger, A. S.; Arends, W. C. E. A. Organocatalytic Oxidations Mediated by Nitroxyl Radicals. Adv. Synth. Catal. 2004, 346, 1051-1071. (39) Gheorghe, A.; Ai, M.; Reiser, O. Expedient Immobilization of TEMPO by Copper-Catalyzed Azide-Alkyne [3+2]-Cycloaddition onto Polystyrene Resin. Adv. Synth. Catal. 2006, 348, 1016-1020. (40) Ciriminna, R.; Pagliaro, M. Industrial Oxidations with Organocatalyst TEMPO and Its Derivatives. Org. Process Res. Dev. 2010, 14, 245-251. (41) Binks, B. P.; Lumsdon, S. O. Influence of Particle Wettability on the Type and Stability of Surfactant-Free Emulsions. Langmuir 2000, 16, 8622-8631. (42) Binks, B. P. Particles as Surfactants-Similarities and Differences. Curr. Opin. Colloid Interface Sci. 2002, 7, 21-41. (43) Finkle, P.; Draper, H. D.; Hildebrand, J. H. The Theory of Emulsification. J. Am. Chem. Soc. 1923, 45, 2780-2788. (44) Saito, K.; Hirose, K.; Okayasu, T.; Nishide, H.; Hearn, M. T. TEMPO Radical Polymer Grafted Silicas as Solid State Catalysts for the Oxidation of Alcohols. RSC Adv. 2013, 3, 9752-9756. (45) Michaud, A.; Gingras, G.; Morin, M.; Béland, F.; Ciriminna, R.; Avnir, D.; Pagliaro, M. SiliaCat TEMPO: An Effective and Useful Oxidizing Catalyst. Org. Process Res. Dev. 2007, 11, 766-768. 29



(46) Liu, S. J.; Wang, H. L.; Chu, X. M. Nitroxide Polymer Brushes as Efficient and Recoverable Catalysts for the Selective Oxidation of Primary Alcohols to Aldehydes. J. Appl. Polym. Sci. 2017, 134, 44365. (47) Zheng, Z.; Wang, J. L.; Zhang, M.; Xu, L. X.; Ji, J. B. Magnetic Polystyrene Nanosphere Immobilized TEMPO: A Readily Prepared, Highly Reactive and Recyclable Polymer Catalyst in the Selective Oxidation of Alcohols. ChemCatChem 2013, 5, 307-312. (48) Cozzi, F. Immobilization of Organic Catalysts: When, Why, and How. Adv. Synth. Catal. 2006, 348, 1367-1390. (49) Tebben, L.; Studer, A. Nitroxides: Applications in Synthesis and in Polymer Chemistry. Angew. Chem. Int. Ed. 2011, 50, 5034-5068. (50) Dijksman, A.; Arends, I. W. C. E.; Sheldon, R. A. Polymer Immobilised TEMPO (PIPO): An Efficient Catalyst for the Chlorinated Hydrocarbon Solvent-Free and Bromide-Free Oxidation of Alcohols with Hypochlorite. Chem. Commun. 2000, 271-272.

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TOC Graphic

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TEMPO-Functionalized Aromatic Polymer as a Highly Active, pH

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