Copolymeric Schiff Base-Cu: A platform for Active and Recyclable

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Copolymeric Schiff Base-Cu: A platform for Active and Recyclable Catalyst in Aerobic Oxidations Shaodan Xu, Jia Du, Huanxuan Li, and Junhong Tang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04501 • Publication Date (Web): 01 Dec 2017 Downloaded from http://pubs.acs.org on December 3, 2017

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Industrial & Engineering Chemistry Research

Copolymeric Schiff Base-Cu: A platform for Active and Recyclable Catalyst in Aerobic Oxidations Shaodan Xu,* Jia Du, Huanxuan Li, Junhong Tang* College of Materials & Environmental Engineering, Hangzhou Dianzi University, Hangzhou 310018, P.R. China. *

Corresponding author: E-mail: [email protected]; [email protected]

ACS Paragon Plus Environment

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Abstract: The synthesis of heterogeneous metal-ligand catalysts with high activity and good recyclability is always an important topic but still challengeable to make. Herein, we reported a solid Schiff base-Cu catalyst synthesized by incorporating Cu sites into the copolymeric Schiff base ligand, which is achieved from a porous polydivinylbenzene functionalized with Schiff base groups. A heterogeneous Schiff base-Cu catalyst denoted as hetero-SchCu was obtained. Systemic characterizations involving XPS, FTIR, N2 sorption, and electronic microscopy demonstrate the structural features of hetero-SchCu including functionalized Schiff base ligand, stabilized and highly dispersed Cu species, and rich porosity. The hetero-SchCu exhibits high catalytic activity and selectivity in the oxidation of a series of alcohols and cyclohexane. Importantly, the hetero-SchCu is stable and recyclable in the tests involving several runs without decrease in activity. This work not only demonstrates a platform for developing polymeric heterogeneous catalysts, but also provides efficient catalysts for the oxidation of alcohols and cyclohexane.

Keywords: alkane oxidation, heterogeneous catalysis, alcohol oxidation, Cu catalyst


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1. Introduction An important discovery in catalysis is the utilization of incorporated organic ligands as homogeneous catalysts, which has been widely used in an ocean of reactions, including the oxidation, hydrogenation and coupling reactions1-4. In these cases, the homogeneous metalligand catalysts have exhibited high catalytic activity, but still suffer from the problems of separation and regeneration from the reaction liquor due to the homogeneous feature5-7. These issues make the reuse of the homogeneous catalysts challengeable, which strongly caused the high cost in the chemical productions, as well as production of environmentally unfriendly wastes. Currently, the heterogeneous catalysts are more favorable than the homogeneous ones owing to the significant advantage of recyclability8-11. However, it is worth emphasizing that the heterogeneous catalysts generally exhibit unsatisfactory activity compared with the homogeneous ones because of the decreased active centers accessing to the substrates and limited molecule diffusion efficiency, which still hinders their further application. Therefore, developing heterogeneous metal-ligand catalysts with simultaneously high activity and good recyclability is still challengeable. The general strategy for developing heterogeneous catalysts is by functionalizing the metalligand species on the surface of solid supports, which allows the reaction occurring between the solid catalyst and substrate12,13. Various solid supports of metal oxides and silica have been extensively used as the supports, but always achieving catalysts with low density of active centers due to the poor surface area of these supports. The mesoporous silica (e.g. SBA-15, SBA-16, and MCM-41) emerged to be favorable supports for the synthesis of heterogeneous catalysts because of the ordered mesopores and large surface area, which provided open space for the functionalization of ligand-metal species and facilitated the diffusion of substrate


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molecules to the active centers14-16. However, the hydrophilic silica-based supports adsorb much water covering on the catalyst surface, which also hindered the access of organic molecules to the active centers. More recently, the organic polymers have attracted much attention as supports for synthesizing heterogeneous metal-ligand catalysts, where the most famous example is the commercial polystyrene17,18. However, the construction of porosity on the polymers is challengeable and hard templates are usually used, as well, the controllable functionalization of metal-ligand species on the polymer network still needs further investigation. Herein, we reported a solid, polymeric, and nanoporous Schiff base-Cu as highly active and recyclable catalysts. Key to this success is incorporating Cu sites into the porous polydivinylbenzene functionalized with Schiff base groups (hetero-SchCu). The hetero-SchCu exhibits high catalytic activity and selectivity in the oxidation of a series of alcohols and cyclohexane, because of the structural features including functionalized Schiff base ligand, rich porosity, stabilized Cu species with high dispersion. Importantly, the hetero-SchCu is recyclable, which exhibits constant performances in the recyclable tests with undetectable loss in activity and selectivity.

2. Experimental Section Materials. All reagents were of analytical grade and used as purchased without further purification. Synthesis of hetero-NH2. As a typical run, 0.8 g of divinylbenzene and 0.2 g of 4aminostyrene were dissolved in 10 mL of tetrahydrofuran and 2 mL of water solvent. After stirring at room temperature for 30 min, 0.2 g of azobisisobutyronitrile (AIBN) was added in the liquor. Then the mixture was transferred into an autoclave and heated at 100 °C for 38 h. The


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porous polymer with functionalized amino groups (hetero-NH2) was collected by evaporation of the solvent at room temperature under vacuum for 4 days. For comparison, polydivinylbenzene was synthesized by the copolymerization of divinylbenzene in the same methods without 4aminostyrene. Synthesis of hetero-Sch. As a typical run, 1 g of hetero-NH2 was dispersed in 20 ml of toluene under stirring, followed by the addition of 3 mL of 2-pyridinecarboxaldehyde. After stirring at room temperature for 1 h, and refluxing at 110 °C for 55 h, filtrating and washing with a large amount of ethanol and water, hetero-Sch was obtained. Synthesis of hetero-SchCu. As a typical run, hetero-Sch and Cu(OAc)2 were dispersed in 20 ml of acetone and 18 ml of water under stirring, followed by stirring for 24 h at 50 °C. After filtrating and washing with a large amount of water and drying at 50 °C under vacuum, the hetero-SchCu was obtained. The Cu loading amount on hetero-SchCu is 1.0 wt% by inductively coupled plasma (ICP) analysis. Synthesis of SBA-15-SchCu. As a typical run, 1 g of commercial SBA-15 (Xianfeng Nano Co.) was dried at 150 ºC under vacuum for 12 h, and then added in 20 mL of anhydrous toluene containing 3g of NH2CH2CH2CH2Si(OC2H5)3 (KH-550). After refluxing for 12 h and collected by rotary evaporation, the sample of amino functionalized SBA-15 was obtained. The SBA-15SchCu catalyst was synthesized in the same procedure with that for the synthesis of heteroSchCu, excepting using amino functionalized SBA-15 as the matrix support instead of heteroNH2. The Cu loading amount on SBA-15-SchCu is 1.2wt% by ICP analysis. Synthesis of homogeneous SchCu catalyst. The SchCu catalyst was synthesized according to the general procedures in literature. As a typical run, 1 g of aminobenzene was added in 15 ml of toluene, then 1.2 g of 2-pyridinecarboxaldehyde was added. After refluxing for 48 h, the toluene


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was removed by rotary evaporation, the resulted Sch ligand was dispersed in acetone and water mixture. The SchCu catalyst was finally obtained by treating Cu(OAc)2 in the mixture for 6 h at 50 ºC. Characterization. XPS spectra were performed by a Thermo ESCALAB 250, and the binding energy was calibrated by C1s peak (285.0eV). Transmission electron microscopy (TEM) experiments were performed on a JEM-3010 electron microscope (JEOL, Japan) with an acceleration voltage of 300 kV. Nitrogen isotherms at the temperature of liquid nitrogen were measured using a Micromeritics ASAP Tristar. The samples were outgassed for 18 h at 130 °C before the measurement. The content of Cu was analysed by inductively coupled plasma (ICP) with a Perkin-Elmer plasma 40 emission spectrometer. The polymer sample with Cu species was burned at 650 °C in oxygen to remove the carbon species, then the resulted Cu species were dissolved by hydrochloric acid for ICP analysis. FTIR spectra were recorded using a Bruker 66V FTIR spectrometer. The adsorption capacity of organic molecules on solid samples was measured on a self-made adsorption equipment. The solid samples were outgassed for 18 h at 150 °C before the measurement. Then the outgassed sample was placed in a closed container in the presence of liquid benzyl alcohol or phenethyl alcohol in a beaker. The container was degased into 0.1 bar and heated at 100 °C for 36 h for the adsorption test. Then weight changes of the samples before and after the adsorption test were calculated to be the adsorption capacity. Thermogravimetric curves (TG) were performed on a SDT Q600 Simultaneous DSC-TGA in flowing air with heating rate of 10 °C/min. Catalytic tests. The oxidation of alcohols and cyclohexane was carried out in a 25 mL glass reactor with a magnetic stirrer (1200 rpm). As a typical run, the substrate, catalyst, and solvent were mixed in the reactor. Then the reactor was quickly heated to desired temperature, and the


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hydrogen peroxide aqueous solution (30 wt%) was added drop by drop very slowly with a dropping funnel. After reaction, the products were extracted and analyzed by gas chromatography (Fuli, flame ionization detector) with flexible quartz capillary columns (OV-1 with column length at 100 m or FFAP with column length at 50 m). The GC analysis was performed with injector and detector temperatures at 290 and 310 °C, respectively. The column temperature goes through a programmed-increasing process: keeping at 50 °C for 12 min, increasing to 220 °C with a rate at 10 °C min, and keeping at 220 °C for 20 min. The recyclability test was performed by separating the catalyst by filtration, washed with acetone, dried under vacuum and then used in the next run.

3. Results and discussion 3.1 Synthesis Scheme 1 shows the procedure for the synthesis of hetero-SchCu. As a typical run, a constant amount of divinylbenzene and 4-aminostyrene were dissolved in tetrahydrofuran solvent, followed by the thermal polymerization with 2,2-azobisisobutyronitrile as radical initiator to obtain the amino group functionalized porous polymers (hetero-NH2). Then the hetero-NH2 powder was refluxed with pyridinecarboxaldehyde to form the Schiff base groups, obtaining Schiff-base functionalized porous polymer (hetero-Sch). After incorporating Cu cations into the nitrogen binding sites of hetero-Sch, the final product of hetero-SchCu was synthesized. In the synthesis process, it is emphasized that the solvothermal polymerization displays crucial role for introducing porosity into the resulted solid polymer19,20. Additionally, the co-polymerization of divinylbenzene and 4-aminostyrene provided a one-step method for the introduction of amino groups into the polymer, this method is favorable compared with the


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conventional nitrification route where large amount of HNO3 and tin salts are used and subsequently producing environmentally unfriendly wastes. 3.2 Characterization Figure 1 shows the FTIR spectra of various samples. The polydivinylbenzene without any functionalized N-containing groups was employed as a reference sample. Compared with the FTIR spectrum of polydivinylbenzene, the spectra of hetero-Sch and hetero-SchCu give additional bands at 1523 and 1620 cm-1, which are assigned to the C-N and C=N bonds, respectively21,22. These data confirm the formation of Schiff base groups. Additionally, heteroSch and hetero-SchCu also exhibit wide bands at ~3370 cm-1, assigning to the N-H bonds from the unreacted amino groups during the Schiff base synthesis step23. Interestingly, after incorporating Cu species on hetero-Sch, the hetero-SchCu sample gives a band at 1588 cm-1, which should be due to the shift of C=N bond from 1620 cm-1. The shift of 32 cm-1 is reasonably assigned to the strong interaction between the Cu cations and Schiff base ligand, where the electrons are shifted to Cu cations leading to the weaken of the C=N bond in the ligand. This viewpoint is further evidenced by the weak band at 480 cm-1 on the spectrum of hetero-SchCu, which is assigned to the Cu-N bond from the interaction between Cu and Schiff base ligand24. The N1s and Cu2p XPS spectra were performed for further understanding the interaction between Cu and Schiff base ligand (Figure 2). In the N1s XP spectra, the hetero-NH2 gives the N1s binding energy at 399.3 eV (Figure 2A-a), which is assigned to the amino groups functionalized on the polymer25. When Schiff base group was synthesized, the N1s signal is significantly enhanced, demonstrating the presence of more nitrogen element on hetero-Sch than that on hetero-NH2. Notably, the N1s peak shifted to 398.8 eV over hetero-Sch (Figure 2A-b), which is reasonably assigned to the formation of C=N bond26. Furthermore, when Cu cations are


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incorporated into hetero-Sch, the hetero-SchCu exhibited a wide N1s binding energy centralized at 399.3 eV, demonstrating the N on Schiff base interacted with Cu cations with multiple strength (Figure 2A-c). Furthermore, the Cu2p XP spectrum gives the Cu2p3/2 binding energy at 933.0 eV (Figure 2B), which has 1.0 eV of shift from that of typical Cu2+ at 934.0 eV in literature27,28, confirming that the interaction between Schiff base and Cu2+ leads to the electrons transformed to the Cu sites. Notably, the intense satellite features appearing at about 942.0 and962.0 eV accompanied with the Cu2p3/2 and Cu2p1/2 peaks, which are typical characteristics of Cu2+, are extremely weak on hetero-SchCu, demonstrating the remarkably reduced Cu species on hetero-SchCu compared with Cu2+. All these data confirmed the successful synthesis of solid Schiff base-Cu catalysts. Figure 3A shows the N2 sorption isotherms of hetero-Sch and hetero-SchCu samples, which both exhibit typical hysteresis loops at relative pressure of 0.45-0.96, confirming the presence of mesoporosity. Notably, hetero-Sch and hetero-SchCu samples give large surface area at 270 and 245 m2/g, respectively. The TEM image of hetero-SchCu is shown in Figure 3B, which gives direct observation of the disordered nanopores on the solid polymer. All these data confirmed the rich porosity of hetero-SchCu, which should be favorable for the catalysis because the nanopores are known to be positive for enhancing the diffusion of reactant and production molecules in heterogeneous catalysis. The inset in Figure 3B shows the high-resolution TEM image of randomly selected regions on hetero-SchCu, where the metallic Cu or Cu oxides are unobservable. This result suggests the high dispersion of Cu species, which might be even present as isolated single sites. Figures 3C-F give the STEM images and elemental maps of hetero-SchCu. The distribution of N element is constant with that of C element, indicating the average dispersion of Schiff base ligand on the


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sample. Importantly, the Cu element is highly dispersed in the elemental map with undetectable agglomeration, confirming the uniform distribution of the Cu species—all in good agreement with the result in high-resolution TEM image. We measured the stability of hetero-Sch and hetero-SchCu by thermogravimetric (TG) analysis. As presented in Figure S1, both samples give slight weight loses bellow 220 °C, assigning to the removal of adsorbed water and organic molecules. The weight loss in the regions of 300-400 and 400-600 °C are significant, which are assigned to the decomposition of the functionalized Schiff-base ligand and decomposition of the polymeric network, respectively. Although loading Cu promoted the Schiff-base ligand decomposition, as confirmed by the repaid weight loss at 319 °C for hetero-SchCu and the mild weight loss at 326 °C for hetero-Sch, we conclude that both samples are stable below 300 °C, demonstrating the good stability of the samples. 3.3 Catalytic tests in alcohol oxidation The catalytic evaluation of the hetero-SchCu catalyst started from the oxidation of alcohols, which is an important reaction for the production of valuable ketones and aldehydes29,30. Conventionally, the perchlorate, nitric acid, and potassium permanganate were employed as oxidants, which are high-cost and environmentally unfriendly31,32. Herein, we evaluated the catalysts using hydrogen peroxide as oxygen donor, which is regarded to be green oxidant with water as the sole product33,34. As presented in the oxidation of benzyl alcohol in Table 1, the blank run without any catalyst failed to catalyze the transformation of benzyl alcohol (entry 1). The CuCl2, CuO and Cu2O are active for the oxidation of benzyl alcohol to benzaldehyde as major product (entries 3-5), but exhibiting low conversions at 1.9, 9.2 and 13.9%, respectively. The Schiff base without Cu gives conversion of benzyl alcohol at 1.1% (entry 6), which might be


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due to the nitrogen-containing species activating the hydrogenation peroxide to give slight activity.When Cu is incorporated to the Schiff base, the resulted homogeneous SchCu catalyst is active and selective, giving the conversion and selectivity at 88.4 and 94.7%, respectively (entry 7). These data demonstrate that the Cu-Schiff base groups are active and selective catalysts for the reaction. Interestingly, the heterogeneous hetero-SchCu exhibited even slightly better performances than the homogeneous SchCu, giving the benzyl alcohol conversion at 90.2% and benzaldehyde selectivity 95.0% (entry 8), which is different from the general knowledge that the homogeneous catalysts are more active than the heterogeneous ones. The high activity of heteroSchCu might be assigned to the nanoporosity, which offered an organic environment for adsorption and enrichment of the organic molecules (Table S1). In addition, the Schiff-base ligand can provide hydrophilic sites (Figure S2) for the diffusion of polar hydrogen peroxides. The suitable wettability of hetero-SchCu with organic network makes it high-efficiency in the aerobic oxidations with hydrogen peroxide. Additionally, it is emphasized that the method of the hydrogen peroxide addition during the reaction process strongly influenced the catalytic performances (Figure 4). Adding all the hydrogen peroxide required for the reaction immediately in the beginning of the reaction leads to low benzyl alcohol conversion and benzaldehyde selectivity at 34.3 and 75.1%, respectively. In this case, the large amount of benzoic acid (selectivity at 22.5%) was formed as by-product, which might be due to that the high concentration of hydrogen peroxide at the start of the reaction leads to over oxidation. Interestingly, slowly addition of the hydrogen peroxide in 30 min could remarkably enhance the benzyl alcohol conversion and benzaldehyde selectivity to 69.4 and 78.8%, respectively. Further enhancements of the catalytic performances were achieved in continuously prolonging the addition time, reaching the best performance at 90 min, where the


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benzyl alcohol conversion and benzaldehyde selectivity are 90.2 and 95.0%, respectively. Prolonging the hydrogen peroxide addition time to 120-180 min did not cause the significant enhancement in either the benzyl alcohol conversion or the benzaldehyde selectivity, therefore, the hydrogen peroxide was added in 90 min in the tests. The hetero-SchCu is reusable, which can be easily separated from the liquor after each reaction and reused in the next run without significantly loss in activity. For example, the reused hetero-SchCu gave the phenyl alcohol conversion at 88.7% and benzaldehyde selectivity at 95.0% (entry 10), which are comparable to those over the fresh catalyst. Even in the 6th run, the phenyl alcohol conversion and benzaldehyde selectivity are still at 87.0 and 95.3% (entry 11), respectively, demonstrating the good recyclability of hetero-SchCu. In contrast, the separation and reuse of the homogeneous SchCu is difficult. Additionally, the hetero-SchCu catalyst is generally active for different alcohols including phenylethyl alcohol, cyclohexanol, and 1hexanol, where higher conversions were always achieved over hetero-SchCu than the homogeneous SchCu. For example, in the oxidation of primary aliphatic alcohol of 1-hexanol, which is more difficult to be oxidized than the aromatic ones, the hetero-SchCu still gives 1hexanol conversion at 69.2%, much higher than 50.1% over homogeneous SchCu (entries 16, 17). All these data demonstrate the high catalytic activity, good selectivity and recyclability of hetero-SchCu in the oxidation of alcohols, these features make hetero-SchCu important for potential application in the future. 3.4 Catalytic tests in cyclohexane oxidation Compared with the oxidation of alcohols, the oxidation of sp3-hybridized C-H bonds is more challengeable, which is industrially important process for the production of valuable products from the petroleum hydrocarbons35-37. General route for this process requires high


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reaction temperatures (e.g. >300 °C) for the activation of the inert substrates, which leads to energy consumption and over oxidation to form CO238,39. The liquid phase oxidation of hydrocarbons at relatively lower temperature is more attractive, which has been successfully utilized in the oxidation of cyclohexane, an important reaction for the production of K-A oil (mixture of cyclohexanol and cyclohexanone; K = cyclohexanone ； A = cyclohexanol) for synthesizing Nylon-6640,41. In this process, the homogeneous cobalt and manganese salts are used as catalysts, which still have problems in product purification and unsatisfactory cyclohexane conversions (99.5

3

CuCl2

1.9

99.0

>99.5

4

CuO

9.2

97.6

>99.5

5

Cu2O

13.9

96.8

97.5

6

Schiff base

1.1

>99.0

>99.5

7

SchCu

88.4

94.7

96.2

8

Hetero-SchCu

90.2

95.0

94.8

9

SBA-15-SchCu

49.6

98.6

98.0

10

Hetero-SchCu

88.7

95.0

94.9

87.0

95.3

95.0

2nd Hetero-SchCu 11

6th

12

Hetero-SchCu

96.7

>99.0

>99.5

13

SchCu

92.1

>99.0

>99.5

14

Hetero-SchCu

84.2

98.0

97.1

15

SchCu

83.6

98.2

98.1

16

Hetero-SchCuf

69.2

84.3

88.0

17

SchCuf

50.1

89.0

89.2

a

Reaction conditions: 1 mmol of alcohol, for the catalysts with Cu sites: S/C at 66; for the catalysts without Cu, 20 mg of catalyst was used, 5 ml of acetonitrile as solvent, 1.5mmol of H2O2 added in 1.5 h, reaction time at 6 h, 50 °C. b

The by-products are acids, peroxides or some others.

c

The carbon balance values before and after the reaction. The lost carbons are owing to the over oxidation.

d

No catalyst was used.


23


e

Undetectable.

f

70 °C.

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Table 2. Catalytic oxidation of cyclohexane over various catalysts.a

Entry

Catalyst

Conv.

Sel. of K-A oil

(%)

(%)

Molar ratio of K/A

Balanceb (%)

1

Blankc

---d

---

---

>99.0

2

Cu(OAc)2

1.3

99.0

0.1

>99.0

3

CuCl2

99.0

4

Schiff base

---d

---

---

>99.0

5

SchCu

5.0

96.7

2.9

93.0

6

Hetero-SchCu

10.1

93.2

3.4

94.1

7

Hetero-SchCue

3.2

96.9

1.8

95.0

8

Hetero-SchCuf

13.0

80.3

3.6

80.3

9

Hetero-SchCug

5.2

84.1

3.9

66.1

10

Hetero-SchCuh

---d

---

---

>99.0

11

Hetero-SchCui

---d

---

---

98.4

12

Hetero-SchCu

9.3

92.1

3.2

94.3

8.1

94.5

3.3

92.9

2nd 13

Hetero-SchCu 4th

a

Reaction conditions: 1 mmol of cyclohexane, for the catalysts with Cu sites: S/C at 40; for the catalysts without Cu, 27 mg of catalyst was used, 5 ml of acetonitrile as solvent, 3mmol of H2O2 added in 1.5 h, reaction time at 6 h, 70 °C.

b

The carbon balance values before and after the reaction. The lost carbons are owing to the over oxidation, where the lost carbon species were not included in calculating the selectivities of K-A oil.

c

Blank run without any catalysts.

d

Undetectable.

e

Reaction at 50 °C.

f

Reaction at 90 °C.

g

Reaction at 110 °C.

h

The absence of H2O2 in the reactor.

i

80 mg of hydroquinone was added.


25


3370

a

Absorbance

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

1620 1523

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480

b

1588

c

3600

3300

1500

1200

900

600

-1

Wavenumbers (cm )

Figure 1. FTIR spectra of (a) polydivinylbenzene, (b) hetero-Sch, (c) hetero-SchCu.


26

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A

399.3

B 933.0

398.8

c

953.2

c Intensity

Intensity

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


b

a 404

402 400 398 Binding energy (eV)

396

970

960

950

940

930

Binding energy (eV)

Figure 2. (A) N1s and (B) Cu2p XPS spectra of (a) hetero-NH2, (b) hetero-Sch, and (c) heteroSchCu.


27


A 400 3

Volume adsorption (cm /g)

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

B

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C

D

300

200

b

C E

100

F

a 0 0.0

0.2

0.4

0.6

0.8

1.0

N

Cu

Relative pressure (P/P0)

Figure 3. (A) N2 sorption isotherms of (a)hetero-Sch and (b) hetero-SchCu samples; (B) TEM and (C) STEM images of hetero-SchCu; (D-F) elemental maps of C, N, Cu in the yellow square in STEM image (C). Inset in B: Enlarge view of the red square in the TEM image (B).


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Conversion of benzyl alcohol Selectivity of benzaldehyde

100 90

Conv. or Sel. (%)

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


80 70 60 50 40 30 0

30

60

90

120

150

180

Time for H2O2 addition (min) Figure 4. Dependences of conversion and selectivity on the time for hydrogen peroxide addition in the catalytic oxidation of benzyl alcohol over hetero-SchCu catalyst. Reaction conditions: 1 mmol of benzyl alcohol, S/C at 66, 5 ml of acetonitrile as solvent, 1.5 mmol of H2O2 added slowly in different time, reaction time at 6 h, 50 °C.


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For Table of Contents Only

cyclohexane


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Copolymeric Schiff Base-Cu: A platform for Active and Recyclable

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