2-Methylimidazole-Assisted Synthesis of Nanosized Cu3(BTC)2 for

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2-Methylimidazole-assisted synthesis of nano Cu3(BTC)2 for controlling the selectivity of catalytic oxidation of styrene Changyan Guo, Yonghong Zhang, Li Zhang, Yuan Guo, Naeem Akram, and Jide Wang ACS Appl. Nano Mater., Just Accepted Manuscript • Publication Date (Web): 27 Aug 2018 Downloaded from http://pubs.acs.org on August 27, 2018

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ACS Applied Nano Materials

2-Methylimidazole-assisted

synthesis

of

nano

Cu3(BTC)2 for controlling the selectivity of catalytic oxidation of styrene Changyan Guo, Yonghong Zhang*, Li Zhang, Yuan Guo, Naeem Akram, and Jide Wang* Key Laboratory of Oil and Gas Fine Chemicals, Ministry of Education & Xinjiang Uygur Autonomous Region, College of Chemistry and Chemical Engineering of Xinjiang University, Urumqi 830046, P. R. China Corresponding Authors: Yonghong Zhang (*E-mail: [email protected]); Jide Wang (*E-mail: [email protected]) KEYWORDS: HKUST-1; Metal organic framework; heterogeneous catalysis; selective oxidation; styrene.

ABSTRACT: Nano-sized Cu3(BTC)2 (BTC=1,3,5-benzene tricarboxylate, HKUST-1) catalyst with a size range of 10-20 nm was synthesized with 2-methylimidazole (2-MI) as an effective competitive ligand and Lewis base to tune its morphology and improve its catalytic performance, simultaneously. On the one hand, the addition of 2-MI can accelerate the deprotonation of the bridging ligand H3BTC, thereby accelerating the rapid nucleation of crystals, which leads to a rapid decrease of the solution supersaturation and inhibits the further growth of the nucleated

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small crystal, resulting in nano-sized HKUST-1 crystals. On the other hand, the uncoordinated 2MI in nano-HKUST-1 can improve the catalytic performance of the crystal due to its basicity. Further catalytic experiments demonstrate that the nano HKUST-1 showed high catalytic activity as compared with bulk HKUST-1 in the controllable oxidation of styrene under solvent-free conditions. More importantly, both benzaldehydes and benzoic acids were successfully obtained via a facile variation of reaction temperature or time. The benzaldehydes can be produced in 78% GC yield after 9 h at 50 °C or 72% after 50 min at 80 °C. While, benzoic acids can be obtained in moderate to good yield after 6 h at 80 °C. Notably, the nano HKUST-1 could be expediently recycled four cycles without significantly affecting the yield of desired product.

 INTRODUCTION The selective oxidation of styrene is an important transformation in the fine chemical industry because the oxidation products (benzaldehyde, benzoic acid and styrene epoxide) are critical chemical intermediates to synthesize some value-added fine and bulk chemical, such as perfumes, pharmaceuticals, dyes, rubber auxiliary and fine chemicals industry.1-3 A number of transitionmetal catalysts have been developed for the oxidation of styrene to benzaldehyde or styrene epoxide,4-8 but relatively a very few studies had their focus on acid compounds as the main product. Besides, the current researches are aimed to obtain only one desired product,9-13 although there are also a few reports on adjustment of the main products of styrene oxidation to obtained benzaldehyde and styrene epoxide.14-17 Likewise there are few reports on the control of the selectivity of the benzaldehyde and benzoic acid by adjusting the variable factors, i. e. reaction temperature, initial volume ratio of solvent and catalyst amount.15 However, some of these reported methods still ached from precious metal catalysis,14, conversion3,

18-22

17-19

poor selectivity and

, the use of large part of the volatile organic solvents.3,

8, 10, 14, 18, 19, 22

In


2

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consideration of economic and environmental facts, the selective oxidation of styrene to benzaldehyde and benzoic acid respectively by controlling the reaction conditions using heterogeneous catalyst might be a desirable process. As a well-known heterogeneous catalyst, metal organic frameworks (MOFs) attributed to their high surface area, controllable pore size and desired chemical functionalities were widely used in catalytic organic reactions,23-25 including oxidation,26-28 acetalization29 and olefin metathesis,30, 31

respectively. HKUST-1 is a well-established MOFs for hydrogen gas storage and

heterogeneous catalysis32-36 due to its high surface area, large pore volume

37

and good thermal

stability. Although various synthetic methods have been successfully developed for HKUST-1 synthesis, the resulting crystals are usually larger in size and with non-uniform morphology. The utilities of MOFs are significantly affected by their crystal size, chemical composition and morphology.38 Nano-sized MOF materials have been studied for potential applications in heterogeneous catalysis because of their unique performance. The nano MOFs usually display inimitable or enhanced properties for transportation of guest molecules via short diffusion pathways, and expose more active sites in their nano crystal structure.39-41 Effective synthesis of nano-sized HKUST-1 can effectively improve the catalytic activity and selectivity of MOFs in catalytic reactions. In continuation of our efforts to develop general protocols for controlling the morphology and size of different MOF materials by 2-MI as an effective reagent,42 herein, nano-sized HKUST-1 catalyst was synthesized with 2-MI as an effective functional regulatory reagents (Figure 1). 2MI not only acts as an effective coordination modulator to tune the morphology of HKUST-1, but also as a Lewis base to improve the catalytic performance of MOFs. Nano HKUST-1 exhibit excellent catalytic activity and selectivity in oxidation of styrene to benzaldehyde and benzoic


3


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acid. We believe that such synthetic strategy for nano-sized catalyst delivers a helpful template for designing novel catalyst with high efficiency and selectivity.

Figure 1. The synthetic route of nano HKUST-1. 

RESULTS AND DISCUSSION Structure of HKUST-1: The nano HKUST-1 catalyst in this work was synthesized with 2-MI

as a coordination modulator. The SEM and TEM results showed that 2-MI had a pronounced influence on the morphology and size of MOFs. In the absence of 2-MI, the bulk HKUST-1 obtained by reference

35

(HKUST-1-A) is a spindle crystal with a diameter of approximately 10

μm (Figure 2a). In the presence of 4 mM 2-MI, nano HKUST-1 with size of about 10-20 nm was formed (Figure 2b), and the TEM result also confirmed that the nano sized crystal was obtained (Figure 2c). 2-MI not only can acts as a base to promote the rapid of crystals by accelerating the deprotonation of H3BTC, but also can services as a competitive ligands to prevent further growth of the crystal 42, resulting in nano-sized HKUST-1 crystals.


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Figure 2 a): SEM of the HKUST-1-A; b): SEM of the nano HKUST-1; c): TEM of the nano HKUST-1. In order to determine the nano HKUST-1 catalyst was obtained, the nanocrystal and HKUST1-A were characterized using PXRD, FT-IR, TGA and EDS mapping. The PXRD diffraction patterns of the nano HKUST-1 (Figure 3a) is well matched with the HKUST-1-A and theoretical simulation, which suggested that the structure of HKUST-1 was not changed by 2-MI. It also can be seen from the figure that the large crystal has strong diffraction peak intensity due to its better crystallinity, and the decreasement in the diffraction peaks intensity of the nano HKUST-1 is due to a decrease in the crystallinity of the crystals, since the nano HKUST-1 are rapidly crystallized at room temperature in a few minutes. However, no new diffraction peaks have been found in the PXRD spectra of nanocrystals, demonstrating that no new crystal phases are produced during the regulation process. From the IR results (Figure 3b) of 2-MI, bulk and nano HKUST-1 crystals can be found that the symmetric and asymmetric stretching vibrations of carboxylate group locate at 1563 and 1375 cm-1 can be easily observed in the bulk and nano HKUST-1 samples. This result is consistent with the literature reports by Zhang and Wu,43 confirming the formation of the crystal structure of HKUST-1. While, the absorption at 1675 cm-1 in the sample of nano HKUST-1 may be contributed by the stretching vibration of C=N bonding of 2-MI. These phenomena suggest that the structure of the crystal did not change after the regulation, but a part of 2-MI may be existed in the nano HKUST-1.


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To further confirm the existence of 2-MI molecules in the crystal structure, TGA test was performed on 2-MI, bulk and nano HKUST-1. TGA results (Figure 3c) showed that the HKUST-1-A and nano HKUST-1 has analogous thermal stabilities, and the crystal structure decomposed at 300 °C. The thermal stability of nanocrystals is slightly lower than that of bulk crystals, which may be due to the smaller crystal size. The first weight loss stage before 150 °C is caused by the release of solvent molecules adsorbed in the MOF framework (such as H2O and EtOH). The second stage, ranging from 150 to 300 °C, is ascribed to the further release of DMF absorbed within the pores or the 2-MI and unreacted ligands on the surface of crystal. Figure 3c showed that both crystals have similar weight loss before 300 °C, suggesting that there is no 2MI on the surface of the nanocrystals. Notably, the third weight loss after 300 °C of HKUST-1-A is about 40% corresponds to the decomposition of H3BTC, leading to the final composition of CuO. While, the third weight loss of nano HKUST-1 increased to 46%, indicating that there are a bit of 2-MI molecules may be existed in the crystal structure. The metal content was calculated by the amount of remaining CuO (20%) after TGA test, a copper loading of 2.52 mmol·g-1 was obtained. As can be seen from the EDS mapping results (Figure 3d, e), the corresponding metal elements are evenly distributed on the whole surface of nano HKUST-1. Importantly, the nitrogen content in HKUST-1-A is mainly contributed by DMF molecules, and the nitrogen content in the nano HKUST-1 is the result of the DMF and 2-MI. The images shows that there is no significant increase in the nitrogen content on the surface of the nano HKUST-1, which proves that there are not much 2-MI present on the crystal surface, and the 2-MI content of 6% proved by TGA may be evenly distributed throughout the HKUST-1 crystal structure. Based on the above experimental results, the possible existence form of 2-MI in HKUST-1 was proposed.


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Firstly, 2-MI may be coordinated with excess copper in the solution to form a Cu-2MI complex; Secondly, 2-MI may be presented in the crystal structure by coordination with the bare copper of HKUST-1; Finally, it is also possible that part of the 2-MI is only adsorbed in the pore of the HKUST-1 crystal in the form of a guest molecule like the water molecule and the DMF molecule.

Figure 3 a): PXRD of the HKUST-1-A, nano HKUST-1and theoretical simulation; b): IR images of the 2-MI, HKUST-1-A and nano HKUST-1; c): TGA images of the 2-MI, HKUST-1A and nano HKUST-1; d): EDS mapping images of the HKUST-1-A; e): EDS mapping images of the nano HKUST-1. The N2 adsorption/desorption isotherms of the bulk and nano HKUST-1 were characterized at 77 K. The isotherms of the two types of HKUST-1 were type I according to the IUPAC classification of isotherm shapes (Figure 4).44 HKUST-1-A exhibited a specific surface areas of


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1092 m2•g-1, which was lower than the value previously reported in the literature (1482 m2•g-1).45 This difference might be due to the different in the activation process, or the activated sample absorbs water molecules in the air before testing, which led to incomplete solvent removal in the samples. Notably, the specific surface areas and pore volumes of nano HKUST-1 improved significantly, exhibiting a specific surface area of 1915 m2•g-1 and pore volume of 0.913 cm3•g−1. This result confirmed that the nanoscale MOFs regulated by 2-MI will be beneficial to increase its specific surface area. The HKUST-1 catalyst with larger specific surface area and pore volumes will expose more catalytic active sites and molecular channel, which is beneficial to the contact between the substrate and the active site, as well as the diffusion and transport of the substrate molecule, thereby further increasing its catalytic activity.

Figure 4 N2 adsorption/desorption isotherms of HKUST-1-A and nano HKUST-1. Catalytic performance: Initially, in order to learning the catalytic activity of nano HKUST-1 and to obtain the optimal reaction conditions, styrene was chosen as the model substrate by using TBHP as an oxidant (Scheme 1). In the first series of experiments, nano HKUST-1, HKUST-1A35 and HKUST-1-B (bulk HKUST-1 obtained by reference, Figure S1-2),46 2-MI and


8

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homogeneous copper salts were used as catalyst to evaluate catalytic performance at 80 oC for

Scheme 1. Oxidation reaction of styrene using HKUST-1 as catalyst. Table 1 Optimization of reaction conditions.a Temp. Entry

Catalyst

(oC)

Time

Con. (%)

Yield (%)b 1b

1c

1d

1e

1

HKUST-1-A

80

50 min

72

43

6

9

14

2

HKUST-1-A

80

6h

99

21

7

10

61

3

HKUST-1-B

80

50 min

64

46

4

2

12

4

HKUST-1-B

80

6h

99

28

4

3

64

5

2-MI

80

50 min

38

22

3

1

12

6

2-MI

80

6h

94

60

2

7

25

7

HKUST-1 (Nano)

80

50 min

92

72

4

7

9

8

HKUST-1 (Nano)

80

6h

99

14

6

8

71

9

Cu(NO₃)₂·3H₂O

80

50 min

97

62

4

8

23

10

Cu(NO₃)₂·3H₂O

80

6h

99

28

5

7

59

11

Cu(OAc)₂·6H₂O

80

50 min

91

59

3

8

21

12

Cu(OAc)₂·6H₂O

80

6h

99

31

4

7

58

13

CuCl₂·6H₂O

80

50 min

94

56

6

6

26

14

CuCl₂·6H₂O

80

6h

99

33

5

8

53

a

Conditions: 1a (0.2 mmol), cat. (3 mg), 2-MI (0.2 mg), TBHP (0.6 mmol).

b

Conversion and yield are determined by GC.

different time (Table 1). It was found that 2-MI also has a certain catalytic activity in the


9


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oxidation reaction of styrene. When 0.2 mg of 2-MI was used as a catalyst, the conversion of styrene can be reached to 94% as the reaction time is extended to 6 hours, unfortunately, the selectivity is very poor. This shows that in the process of controlling the morphology and size of HKUST-1 crystals by 2-MI, it can enhance the catalytic activity of the material due to its alkalinity. Nano HKUST-1 was found to be superior to the bulk MOFs prepared by different methods and homogeneous copper salts. The increase in catalytic performance can be explained by the following reasons. Due to the decrease of crystal size, more catalytic active sites can be exposed, and the effective contact between the substrate molecules and the catalytic active sites is more favorable, thereby increasing the catalytic performance. In addition, the partially uncoordinated 2-MI present in the nano-HKUST-1 can enhance the catalytic activity of the material. The table 1 implies that benzaldehyde is the predominant product and benzoic acid as the main side-product at 80 oC for 50 min, while other products are minor (All oxidation products were confirmed by GC-MS, Figure S3-7). Interestingly, benzaldehyde was converted into benzoic acid when prolong reaction time to 6 h. Encouraged by this result, the effects of reaction temperature on styrene oxidation were studied in the range from 40 to 90 oC for 6 h (Figure 5). It was found that higher temperature is beneficial to improve the conversion of styrene and increase the yield of benzoic acid. Raising the reaction temperature to 50 oC, the conversion of styrene increased to 95% and the selectivity for benzaldehyde increased to 75%. Moreover, benzoic acid was obtained with good yield when the temperature was further increased to 80 oC. This shows that more deeply oxidized products produced at higher temperature. However, increasing temperature to 90 oC lead to a relatively lower yield of 62%, which was due to the formation of tert-butyl benzoate via esterification of benzoic acid with tert-butyl alcohol.


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The effect of reaction time on styrene oxidation was also researched at 80 oC in the range 40480 min. As shown in Figure 5, when the reaction time was prolonged from 40 to 50 min, styrene conversion increased from 81% to 98% and the selectivity for benzaldehyde increased from 62% to 72%. However, the selectivity for benzaldehyde was dropped when further prolonging reaction time, which further converted to benzoic acid, and the highest selectivity for benzoic acid of 71% was obtained for 6 h reaction. However, benzoic acid was converted to tertbutyl benzoate by further prolonging reaction time.

Figure 5. Selective oxidation of styrene under various reaction conditions; a): Optimization of reaction temperature for 6 h; b): Optimization of reaction time at 80 oC. Reaction condition: 3 mg of nano HKUST-1, 0.2 mmol of styrene, 0.6 mmol of TBHP; The conversion and yield were determined by GC, and conversion was defined based on the initial styrene present. Substrates scope: With the optimized reaction conditions in hand, the substrates scope was then extensively examined. As summarized in Table 2, a broad range of styrenes were oxidized smoothly to their corresponding aromatic aldehydes and acids in moderate to excellent yields under different optimized reaction conditions. In light of the role of aryl halocarbons as important building blocks used in transition metal catalyzed cross coupling reaction, we


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proceeded to construct a robust and convenient protocol for the synthesis of halogen substituted aromatic aldehydes and acids (entries 2-6), which can be then used in further transformations. Table 2. Selective oxidation of aryl olefins by nano HKUST-1a

a

Reaction condition: 10 mg of nano HKUST-1, 2 mmol of olefin, 5 mmol of TBHP.

b

Yield was determined by GC, and conversion was defined based on the initial styrene present.


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The halogen groups in meta- and para- positions were well compatible in this catalytic oxidation system. It is worth mentioning that the substrate containing a sensitive group, such as acetyl (entry 7), could be well transformed. Besides, the condensed ring was also tolerated (entry 8). In addition, prop-1-en-2-ylbenzene could also be successfully converted to acetophenone without acid formation with excellent yield (All oxidation products were confirmed by GC-MS, Figure S8-22). The excellent substrate compatibility allows the present method to be extended to other similar oxidation reactions. Gram scale experiments: Gram scale experiments were carried out to further reveal the applicability of this oxidation method. As presented in Scheme 2, benzaldehyde and benzoic acid were obtained smoothly in 78% and 75% yield at 80 oC for 6 h and 30 h, respectively. It is worth mentioning that 30 mg of the catalyst, 15 mmol TBHP were sufficient in this gram scale reaction. This result provides a possibility for the usage of this protocol in the industrial field.

Scheme 2. Gram scale reaction of styrene oxidation. Catalyst stability and recycling experiments: As a heterogeneous catalyst, the recyclability of nano HKUST-1 for the selective oxidation of styrene was investigated under the optimized conditions (Figure 6a, b). It was found that the nano HKUST-1 can be simply recycled by centrifugation and the catalytic activity did not decrease significantly after recycled for four times. The SEM and TEM of reused nano HKUST-1 were tested. It can be seen from the Figure 2b, 2c and Figure 6c, d that the crystal morphology after the reaction is not much different except that the crystal appears obvious agglomeration after used. In addition, the particle size


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before the reaction is about 10-20 nm, and no significant change was observed in particle size after the reaction. The PXRD result of the recovered nano HKUST-1 denoted that the crystal structure of the nano MOFs could be maintained during the course of the oxidation reaction (Figure 6e). The FT-IR spectra of the recycled nano HKUST-1 displayed analogous absorption as compared to those of the fresh catalyst (Figure 6f). Both FT-IR and PXRD results confirming the remarkable reusability and stability of nano HKUST-1, and further verifying the heterogeneity of this nano-sized material.

Figure 6. a): Catalyst recycling studies of styrene oxidation for synthesis of benzaldehyde; b): Catalyst recycling studies of styrene oxidation for synthesis of benzoic acid; c): SEM image of reused nano HKUST-1 catalyst; d): TEM image of reused nano HKUST-1 catalyst; e): PXRD of the fresh and reused nano HKUST-1 catalyst; f): FT-IR spectra of the fresh and reused nano HKUST-1 catalyst.


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Control experiments: To explicate the details of the mechanism, a series of control experiments were performed. Firstly, the reaction was apparently suppressed in the presence of TEMPO (Table 3, entry 2), which indicated that this transformation should involve a radical pathway. Then, benzaldehyde was obtained with a lower yield when the oxidation reaction was carried out under nitrogen atmosphere, and no desired benzoic aicd was detected. However, unlike the standard experiment, there is still 60% of benzaldehyde detected after 6 h under nitrogen atmosphere, and only 19% benzoic aicd was formed. These results suggested that the oxygen in the air is essential for the oxidation of benzaldehyde to benzoic aicd. The ratio of benzoic acid showed huge improvement both after 1 h and 6 h under oxygen atmosphere, which further demonstrated the significant role of oxygen for the formation of acid. Table 3. Control experiments.

Reaction condition: 3 mg of nano HKUST-1, 0.2 mmol of styrene, 0.6 mmol of TBHP, 80 oC for 1 h or 6 h; The conversion and yield were determined by GC, and conversion was defined based on the initial styrene present. Proposed mechanism: According to the previous literature reports and the above results,1, 6, 7, 10, 12

a possible reaction pathway is proposed (Scheme 3). Initially, tert-butoxy and tert-


15


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butylhydroperoxide radicals were generated by decomposition of TBHP with nano HKUST-1 catalyzed single electron transfer processes. Subsequently, tert-butylperoxyl radical further reacts with styrene to afford intermediate A. Next, by the release of the t-BuOH gives styrene epoxide. 2-(tert-Butylperoxy)-2-phenylethan-1-ol C is generated by the reaction of styrene epoxide and TBHP. Finally, the desired product benzaldehyde was produced by the decomposition of C. Benzoic acid was afforded via further oxidation of benzaldehyde with the oxygen in the air or TBHP.

Scheme 3 Proposed mechanism for oxidation of styrene. 

CONCLUSIONS The composition, morphology and crystal size of the material play an important role in its

catalytic performance. In this work, nano-sized HKUST-1 was synthesized with 2-MI as an effective functional regulatory agent. The bulk HKUST-1 and nano-HKUST-1 were characterized by different instruments, and the regulated crystals were found to have smaller crystal size and larger specific surface area. In addition, it was found that a bit of 2-MI may exist in the crystal structure of HKUST-1 by coordination with bare copper or in the form of guest


16

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molecules, and the uncoordinated 2-MI can cat as a base to improve the catalytic performance of nano-HKUST-1. In the controllable oxidation reaction, styrene could be converted to benzaldehyde and benzoic acid simultaneously by a facile variation of reaction temperature and time. In addition, it was found that the proposed catalytic system has good substrate compatibility and can be applied to the oxidation of styrene substrate molecules with different substituents. Moreover, the catalytic system has good recyclability and applicability. Supporting information available: MS spectrums of oxidation products. Conflict of interest The authors declare no conflict interest. Acknowledgments This work is supported by the National Natural Science Foundation of China (Grant No. 21861035, 21861036 and 21502162). The Regional Collaborative Innovation Project of Xinjiang Uyghur Autonomous Region (No. 2017E01005), the University scientific research project of Xinjiang Uyghur Autonomous Region (No. XJEDU2017I001).  1.

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