DOI: 10.1002/chem.201405685

Full Paper

& Heterogeneous Catalysis

Nanoscaled Copper Metal–Organic Framework (MOF) Based on Carboxylate Ligands as an Efficient Heterogeneous Catalyst for Aerobic Epoxidation of Olefins and Oxidation of Benzylic and Allylic Alcohols Yue Qi, Yi Luan, Jie Yu, Xiong Peng, and Ge Wang*[a]

Abstract: Aerobic epoxidation of olefins at a mild reaction temperature has been carried out by using nanomorphology of [Cu3(BTC)2] (BTC = 1,3,5-benzenetricarboxylate) as a highperformance catalyst through a simple synthetic strategy. An aromatic carboxylate ligand was employed to furnish a heterogeneous copper catalyst and also serves as the ligand for enhanced catalytic activities in the catalytic reaction. The

utilization of a copper metal–organic framework catalyst was further extended to the aerobic oxidation of aromatic alcohols. The shape and size selectivity of the catalyst in olefin epoxidation and alcohol oxidation was investigated. Furthermore, the as-synthesized copper catalyst can be easily recovered and reused several times without leaching of active species or significant loss of activity.

Introduction

nonuniform dispersity, which makes them prone to aggregation. Moreover, some of these composite structures lack the desired stability and some of the metal active species tend to leach from the support during the reaction.[33] Therefore, heterogeneous catalysts with higher stability and easier access to the metal ion are worth developing.[34] Recently, metal–organic frameworks (MOFs) have emerged as important functional materials because of their high surface area, porosity, and chemical tenability.[35] Therefore, MOFs are ideal candidates for heterogeneous catalysis. Some efforts have been made using MOF as a catalyst.[36–38] Fujita et al. reported the earliest example of MOFs, using ([Cd(bpy)2](NO3)2 bpy = 2,2’-bipyridine) as a catalyst in an aldehyde cyanosilylation reaction in 1994.[39] However, areobic epoxidation of olefins catalyzed by MOFs is limited.[40–42] Pramanik et al. reported the aerobic epoxidation of alkenes with CuII/CoII/MnII-phenoxy acetic acid derivative frameworks.[43] Garcia et al. reported the aerobic epoxidation of cyclooctene with commercial [Fe(BTC)] (BTC = 1,3,5-benzenetricarboxylate).[44] However, in order for the aerobic epoxidation reaction to be practical, higher yields, selectivity, lower catalyst loadings, and shorter reaction time are needed. In this work, a highly efficient aerobic epoxidation of olefins using copper MOF catalyst is achieved for the first time. Trimesic acid is employed as the ligand for constructing the framework of the copper-based heterogeneous catalyst, which leads to a heterogeneous catalyst with extremely low metalactive species leaching. The carboxylate also serves as an organic ligand for the enhancement of catalytic activities in the oxidation reactions. Several simple copper MOFs have been synthesized and utilized as an efficient catalyst for aerobic epoxidation, which is much cheaper and more readily available than the porphyrin-derived MOF catalyst.[45, 46] Nano-

Olefin epoxidation is a highly important oxidation reaction, because the resulting epoxides are widely used in the production of epoxy resins, paints, and surfactants, and they are highly valued as synthetic intermediates in industrial processes.[1–4] Olefin epoxidation can be carried out by using a variety of oxidants, such as molecular oxygen,[5–6] hydrogen peroxide,[7–8] alkyl hydroperoxides,[9, 10] and iodosylbenzene.[11, 12] The utilization of molecular oxygen as an oxidant has attracted much interest in the epoxidation of olefins recently, due to its abundance in nature, ease of handling, and lack of toxic byproducts during reaction.[13–16] Various catalysts have been developed for the aerobic epoxidation of olefins, amongst these are catalysts composed of redox-active transition metals, such as V,[17, 18] Mn,[19] Fe,[20–22] Co,[23, 24] and Cu,[25, 26] etc. Most of the current transition-metalbased catalysts (e.g. metal salts and metal-ion clusters) are homogeneous catalysts used in liquid-phase reactions, which are difficult to separate from the products and reused. This leads to waste and high costs in industrial production.[27] To employ heterogeneous catalysts for easy recovery, homogeneous catalysts are often immobilized on insoluble solid supports such as polymer,[28] clay,[29] zeolite,[30] silica,[31] or titania.[32] However, the catalytic metal loading on the supports exhibit random and [a] Y. Qi, Dr. Y. Luan, J. Yu, X. Peng, Prof. Dr. G. Wang School of Materials Science and Engineering University of Science and Technology Beijing 30 Xueyuan Road, Haidian district, Beijing, 100083 (P. R. China) Fax: (+ 86) 10-62327878 E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201405685. Chem. Eur. J. 2014, 20, 1 – 10

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Full Paper morphology was introduced for high dispersion of catalyst in solution and more effective utilization of copper catalytic sites, which results in improved catalytic activities. Our copper MOF catalysts are general for a variety of olefin epoxidations employing molecular oxygen. The optimal catalyst was further utilized in the aerobic oxidation of alcohols in low catalyst loadings. This newly developed heterogeneous epoxidation system utilizing [Cu3(BTC)2]·MOF and molecular oxygen demonstrates several advantages over other existing catalytic systems, such as facile catalyst preparation, high aerobic epoxidation activities, and high reaction efficiency.

coordinated at terminal Cu centers in the framework, which prevents a larger growth of the crystals. Monocarboxylic acids, such as dodecanic and benzoic acid, acted as nucleation inhibitors, which were added to adjust the coordination equilibrium of the crystal surface throughout the crystal growth process. The similar chemical functional linker of the framework could create competitive interactions initially. The exchange of monocarboxylic acid and BTC ligand allows the further growth of the crystal, which has a slower nucleation of the Cu MOF framework in comparison to the process without monocarboxylic acid. All copper MOFs, including the recycled Cu-MOF-2 catalyst, were characterized by powder XRD to identify the crystallinity and structure. As shown in Figure 1, XRD results demonstrate that nanoscaled Cu-MOF-1 and Cu-MOF-2 are crystalline and have the same crystal structure as that of Cu-MOF-3, which

Results and Discussion Structure characterization The advantage of nanoscale MOFs as catalysts is that the diffusion length is decreased in comparison to its bulk partner, which is extremely important in liquid-phase catalysis. Therefore, it is crucial to develop a facile approach to optimize and control the size and shape of the crystals.[47] We have successfully synthesized multiscale [Cu3(BTC)2] by taking advantage of the development of nanotechnology and similar literature procedures.[48] Two dimensions of nanosized [Cu3(BTC)2] were achieved by employing different mediators that strongly influenced the morphologies and size at room temperature and controlled the nucleation rate for the production highly crystalline materials. SEM images show that uniform crystals with spherical nanomorphology and sizes of about 90 nm diameter were observed when employing dodecanic acid additives at room temperature (sample Cu-MOF-1, Scheme 1a). When ben-

Figure 1. X-ray diffraction patterns of copper MOFs: a) Simulated [Cu3(BTC)2], b) Cu-MOF-1, c) Cu-MOF-2, d) Cu-MOF-3, and e) Cu-MOF-2 after 15 recycles.

was in agreement with the literature.[48] Therefore, the possibility of additional formation of Cu carboxylate complexes/clusters inside the pores has been excluded. XRD patterns also suggested that the internal crystal structures are the same for copper MOFs employing different acid additives (Figure 1b,c). There is no difference in the X-ray diffraction peak when compared with the simulated one. Comparison of the XRD patterns of the fresh and used catalyst Cu-MOF-2 is also shown in Figure 1. As it can be seen, there is no significant change in the relative intensity and location of the peaks. To demonstrate that the benzoic acid and decanoic acid additive have been efficiently removed from the porous framework, we checked the nitrogen adsorption properties for the samples as those described above for the XRD measurement, after a simple washing process consisting of three cycles of centrifugation and redispersion in ethanol. As shown in Figure S1 (Supporting Information), both Cu-MOF-1 (90 nm, dodecanic acid as an additive) and Cu-MOF-2 (390 nm, benzoic acid as an additive) showed the characteristic Type I sorption profiles and present considerable BET surface areas consistent with Cu-MOF-3, which was synthesized by employing a solvothermal method without additive. Three copper MOF samples

Scheme 1. The synthesis process of different dimensions of [Cu3(BTC)2].

zoic acid was employed as a mediator instead of dodecanic acid, [Cu3(BTC)2] was produced with better crystallinity. As shown in Scheme 1b, the sample was named as Cu-MOF-2 and monodispersed crystal with octahedral morphology was observed with the size of about 400 nm. Furthermore, [Cu3(BTC)2] with the size of several mm was obtained in the absence of mediator, which was named as Cu-MOF-3 in Scheme 1c. The formation mechanism of nanoscaled copper MOFs has been proposed according to the literature.[48] We believe the nanoscaled Cu MOF was formed through carboxylate groups &

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Full Paper gave Brunauer–Emmet–Teller (BET) surface areas of 1409, 1391, and 1129 m2g 1, respectively, which was comparable to the reported data.[49] Additionally, the BET surface area remained at 1253 m2g 1 after 15 catalytic recycles. The thermal stability of Cu-MOF-2 was evaluated by TGA. The results confirmed that the thermal stability of both nanosized and bulk samples had similar thermal stability.[49] As shown in Figure 2, three steps of weight loss were observed. The first two steps of weight loss before 240 8C were attributed to the adsorbed water in the Cu-MOF-2 frameworks. The structure of the two samples decomposed in the 240–450 8C temperature range.[50]

Table 1. Aerobic epoxidation of cyclooctene by using CuII catalyst.[a]

Entry

Catalyst

Solvent

Yield [%][c]

Selectivity [%][c]

1 2 3 4 5 6 7 8[b] 9 10 11 12 13 14

– Cu-MOF-1 Cu-MOF-2 Cu-MOF-3 CuBDC CuCl2·2H2O Cu(NO3)2·3H2O Cu(NO3)2·3H2O Cu(CH3COO)2·H2O Cu(PhCOO)2 Cu-MOF-2 Cu-MOF-2 Cu-MOF-2 Cu-MOF-2

CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN EtOH toluene CH2Cl2 THF

– > 99 > 99 91 96 46 31 30 75 96 – 50 19 12

– > 99 > 99 > 99 > 99 > 99 > 99 > 99 > 99 > 99 – > 99 > 99 > 99

[a] Reaction conditions: cyclooctene (1.0 mmol), catalyst (1.7 mol % CuMOF, 5 mmol % [Cu(BDC)] and Cu salt), trimethylacetaldehyde (2.0 mmol), solvent (5.0 mL), 1 atm. O2 ballon, 40 8C, 6 h. [b] With 2.0 mmol of benzoic acid. [c] Nitrobenzene as the internal standard and determined by GCMS analysis.

Cu-MOF-2 performed significantly better as a heterogeneous catalyst than simple CuII salts, such as copper nitrate trihydrate and copper chloride monohydrate (Table 1, entries 6 and 7). Copper nitrate, which is the precursor for the synthesis of CuMOF-2, only provided the desired epoxide in 31 % yield (entry 7). It has been proposed that the carboxylate group functions as a coordinative ligand for the enhancement of catalytic activities.[54] However, the addition of 2.0 mmol of benzoic acid with copper nitrate as the catalyst was not successful, presumably because of the slow formation of carboxylate CuII species (entry 8). To demonstrate the crucial role of the carboxylate ligand, CuII acetate monohydrate was tested. As a result, a 75 % yield of cyclooctene oxide product was achieved in 6 h (entry 9). The high activity of CuII acetate clearly demonstrated the carboxylate ligand enhanced catalytic activity through coordination. This implies that the active sites either in solution or on the solid would be copper(II) carboxylate and that the intrinsic activity of these copper(II) complexes does not disappear when forming [Cu3(BTC)2].[55] To further strengthen our proposed ligand-enhanced reactivity, a benzoic acid CuII catalyst was also evaluated (entry 10). The excellent yield resulted from benzoic acid CuII catalyst and the similarity of benzoic acid CuII and [Cu3(BTC)2] structure strongly suggested the advantage of using copper carboxylate MOF for both ligandinduced activity enhancement and heterogeneous fixation. Furthermore, lower concentrations of both copper MOFs and copper salts were investigated systematically, which performed lower conversion (Figures S7–S10, Supporting Information). Solvent screening was performed to determine the optimal solvent for the epoxidation of olefins. Reaction results using solvents, such as acetonitrile, ethanol, and toluene are summarized in Table 1, entries 4 and 11–14. The cyclooctene oxide was obtained in quantitative yield using acetonitrile, ~ 100 % in

Figure 2. TGA profile of the Cu-MOF-2 sample.

Catalytic activity of the catalyst in olefins epoxidation The catalytic abilities of the copper MOFs were studied by catalyzing aerobic olefin epoxidation. Cyclooctene was used as a model substrate to identify the optimal reaction conditions in the presence of 1 atm. molecular oxygen. Initially, the control reaction in the absence of catalyst gave no reaction product in 6 h under an oxygen atmosphere. Copper MOFs demonstrated excellent activities for the aerobic epoxidation of cyclooctene in high selectivity (Table 1, entries 2–5). Interestingly, Cu-MOF-1 and Cu-MOF-2 performed better than their [Cu(BDC)] analogue among the copper MOFs. [Cu(BDC)] was synthesized through a literature-modified procedure[51] and the crystal data are shown in Tables S1–S2 and Figures S11–S12 (Supporting Information). As a result, the BTC-ligand-based copper MOFs, such as Cu-MOF-1 and Cu-MOF-2, were chosen as the copper MOF catalyst due to its facile synthesis, availability, and low cost. Furthermore, the uniform size and morphology of Cu-MOF-1 and Cu-MOF-2 led to excellent dispersion in solvents (Figure S2, Supporting Information).[52] Furthermore, the utilization rate of the MOF surface and pore in nanoscale would be much higher than that in bulks, since the diffusion path is much shorter for substrates to enter the nanoscaled catalyst.[53] As a result, nanoscaled Cu-MOF-1 and Cu-MOF-2 showed enhanced epoxidation activity in comparision to their bulk partner (Cu-MOF-3, Table 1, entries 3–5). This observation confirms the idea of utilizing nanoscaled MOFs for a better catalytic performance.[49] Chem. Eur. J. 2014, 20, 1 – 10

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Full Paper 6 h. Aromatic solvents, such as toluene, gave a 50 % yield. However, this reaction proceeded poorly when CH2Cl2 and THF were employed as the solvent. There was no epoxidation reactivity observed in ethanol solvent, presumably because of the coordination of ethanol with copper(II). It was found that the aldehyde acted as a sacrificial coreductant, which could improve the reaction rate.[56] So the effects of a variety of aldehydes have been evaluated as additives for the cyclooctene epoxidation in the presence of Cu-MOF-2 catalyst under the optimized reaction conditions (Figure 3). The

Figure 4. Influence of catalyst concentration on the conversion of cyclooctene. &: 0.85 mol %, *: 1.7 mol %, ~: 2.5 mol %, !: 3.4 mol %.

cyclooctene epoxidation reaction under static nitrogen. Under 1 atm. of air, the cyclooctene oxidation conversion was improved significantly. A yield of 79 % was achieved in 6 h and the yield was further increased to 93 % at 10 h. When the reaction was running in the presence of 1 atm. of oxygen using an oxygen balloon, approximately 100 % conversion of cyclooctene oxide product was achieved in 6 h. When oxygen was bubbled into the solution, the cyclooctene oxide conversion increased to approximately 100 % within 4 h. As a result, the oxygen balloon was chosen instead of oxygen bubbles as less oxygen was used and easy reaction step, even though oxygen bubbling conditions led the reaction to completion within a shorter reaction time. In addition, the detailed reaction mechanism has been discussed in the Supporting Information (Figure S14). With the optimal reaction conditions in hand, a collection of olefins were tested to demonstrate the universal aerobic epoxidation applicability using Cu-MOF-2 as a catalyst. A variety of cyclic olefins such as cyclooctene, cyclohexene, and cyclododecene participated well in this epoxidation reaction. Full conversions were achieved in 6 h without any byproduct (Table 2, entries 1–3). Moreover, competitive epoxidation of cisand trans-cyclododecene gave equal amounts of cis- to trans2-cyclododecene oxide (entry 3), which indicated no preference for cyclododecene epoxidation using Cu-MOF-2. Norbornene gave the corresponding exo-epoxide exclusively (entry 4). Disubstituted aromatic olefins gave good results (entries 9–12). Trans-b-methylstyrene needed shorter reaction times compared with trans-stilbene due to the steric hindrance effect of the substrates (entries 9 and 11). Trisubstituted olefins, such as a-pinene, were oxidized to the corresponding epoxide in outstanding yields (> 99 %) and selectivity (> 99 %) within 6 h (entry 5). Furthermore, terminal linear alkenes are generally considered as inert olefins towards epoxidation. Interestingly, they were also oxidized in decent yields and with excellent selectivities (entries 6–7). The epoxidation of styrene showed decent selectivity under our reaction conditions with quantitative conversion. The dominant reaction involved the formation of styrene oxide as the major product (58 %) along with a certain amounts of benzaldehyde (42 %, entry 8). For other aromatic olefins, the epoxidation rate of trans-stilbene was much

Figure 3. Effect of various aldehydes on the aerobic epoxidation of cyclooctene.

control experiment showed no reactivity for the formation of cyclooctene oxide, which indicated that the aldehyde was an essential additive for aerobic epoxidation. The rates of the reactions varied significantly depending on the structures of aldehyde additives. Among those examined, trimethylacetaldehyde (Figure 3, column 2) was found to be the most effective, giving > 99 % cyclooctene oxide yield. Cyclic aldehydes, cyclohexanecarboxaldehyde (Figure 3, column 4), also gave satisfactory results, while the use of aliphatic aldehydes, such as heptaldehyde (Figure 3, column 3), gave lower yields and selectivity. Aromatic aldehydes, such as benzaldehyde (Figure 3, column 5) and 9-anthracene (Figure 3, column 6) aldehyde were not suitable additives for the epoxidation and resulted in no yield. The influence of catalyst concentration on the conversion of cyclooctene was investigated, as shown in Figure 4. The use of 0.85, 1.7, 2.5, and 3.4 mol % Cu-MOF-2 catalyst was performed, respectively. It was indicated that the reaction was completed after 8 h with 0.85 % catalyst concentration. The reaction rate went up significantly within 6 h by increasing the catalyst concentration to 1.7 mol %. As expected, a further increase of the catalyst loading greatly boost the reaction rate, with > 99 % conversion after 3 h (3.4 mol %, Figure 4). As a result, a catalyst loading of 1.7 mol % was chosen as the optimal reaction condition for further catalytic studies. The role of oxygen was also investigated and the conversion vs. time scheme is shown in Figure S13 (Supporting Information). As a result, almost no conversion was observed in the &

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Full Paper Table 2. Aerobic olefin epoxidations catalyzed by Cu-MOF-2.[a]

Entry Substrate

Product

t Yield Selectivity [h] [%][b] [%][b]

1

6

> 99 > 99

2

6

> 99 > 99

3

6

> 99 > 99

4

6

> 99 > 99

5

6

> 99 > 99

6

6

75 > 99

7

6

56 > 99

8

6

58

9

6

10

24

11

24 > 99 > 99

58

> 99 > 99

80

80

Figure 5. The conversion of the aerobic oxidation of styrene (a) and the selectivity of styrene oxide (b) under various conditions. &: 40 8C, *: 60 8C, ~: 80 8C. 24

12

63 > 99

At 40 8C for 6 h, the conversion reached 100 % with 64 % selectivity for styrene oxide (Figure 5, 40 8C). When the reaction temperature was increased to 60 8C, a shorter time was needed for the full conversion of the styrene. The selectivity of styrene oxide went up to 80 % at 12 h (Figure 5, 60 8C). At 80 8C, the conversion of styrene could reach over 99 % within 2 h, together with 90 % selectivity of styrene oxide. It is interesting to observe that the higher temperature results in a higher selectivity of the styrene oxide kinetic product. As times goes on, the selectivity of styrene epoxide decreases distinctly because of the formation of benzaldehyde by oxidative ring opening of styrene oxide (Figure 5, 80 8C). In summary, the best yield for styrene epoxidation is achieved at 80 8C for 2 h. The selectivity of styrene oxide decreases along with the reaction time. This observation proves that kinetic product epoxidation is unstable and could be further oxidized to the thermodynamic benzaldehyde product. For catalytic applications, MOFs may offer unique advantages compared with other catalysts, in which well-defined pores may favor shape and size-selective catalysis.[63] The shape and size selectivity of the as-synthesized Cu-MOF-2 were investigated by epoxidation. The results showed that an increase of the substitution on the aromatic ring leads to a decrease of the epoxide yield under the same conditions, which was related to

[a] Reaction conditions: olefin (1.0 mmol). Cu-MOF-2 catalyst (0.017 mmol), trimethylacetaldehyde (2.0 mmol), acetonitrile (5.0 mL), 1 atm. O2 ballon, 40 8C: [b] Determined by GCMS analysis.

faster than that of its cis-isomer, which results from the sterichindrance effect.[57] However, cis-stilbene generated both the thermodynamic product, cis-stilbene oxide, and the kinetic product, the trans-stilbene oxide, in a ratio of 0.46, whereas trans-stilbene was oxidized to trans-stilbene oxide quantitatively (entries 11–12). The epoxidation of styrene was further evaluated to understand the product distribution versus reaction time due to the high importance of styrene oxide product.[58–60] However, most literature reports fail to address the selectivity issue due to the difficulty of distinguishing the kinetic and thermodynamic products.[61, 62] We expect that the high selectivity and yield can be achieved through manipulating the reaction conditions. As shown in Figure 5, the dependence of product selectivity and the conversion on the effect of reaction temperature and reaction time in styrene epoxidation by Cu-MOF-2 with molecular oxygen was evaluated. As the temperature went up from 40 to 80 8C, the reaction rate was increased significantly (Figure 5). Chem. Eur. J. 2014, 20, 1 – 10

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Full Paper Table 3. Size and shape selectivity in the aerobic epoxidation of olefins.[a] Entry

Substrate

Molecular size

Yield [%]

Selectivity [%]

1[b]

63

99

2[c]

21

99

[a] Reaction conditions: substrate (1.0 mmol), Cu-MOF-2 catalyst (0.017 mmol), trimethylacetaldehyde (2.0 mmol), solvent (5.0 mL), 1 atm. O2 ballon, 40 8C, 24 h. [b] Determined by GCMS analysis. [c] Determined by NMR spectroscopy.

the chain length and volume of the substrate molecules, as shown in Table 3. The substrate size of cis-stilbene (4.9 and 9.7 ) corresponds to the pore size of Cu-MOF-2 (7.2 ). As a result, the yield went up to 63 %. By increasing the number of substituent groups, cis-3,3’,5,5’-tetra(tert-butyl)stilbene, which was much larger than the available pore dimensions of [Cu3(BTC)2] (7.2 ) gave a much lower yield. This indicated that the substrate was too large to enter or exit the channels of the copper MOF. The distinct difference of conversion rates reflect the size and shape selectivity of the [Cu3(BTC)2] catalysts. This guest-selective epoxidation reaction suggested that the reaction occurred both in the channels and on the surface of nanoscaled [Cu3(BTC)2]. The external surface of the MOF crystals was only responsible for the partial epoxidation conversion of olefins.

Furthermore, nonaromatic alcohol cinnamyl alcohol gave a quantitative yield of the desired cinnamaldehyde (entry 6). The size and shape selectivity for aerobic oxidation of alcohols showed a similar trend as for aerobic epoxidation, as shown in Table 5. The increase of the size of aromatic rings decreases yields of the corresponding aldehydes. For the substrate benzyl alcohol, the size of which (4.3 ) is smaller than the pore size of Cu-MOF-2, the yield was up to 99 %. By increasing the number

Table 4. Aerobic oxidation of alcohol by using Cu-MOF-2.[a]

Entry

Substrates

Product

Yield [%]

Selectivity [%]

1

> 99

> 99

2

69

> 99

3

> 99

> 99

4[b]





5[b]





> 99

> 99

Alcohol oxidation 6

Encouraged by these preliminary results, we further extended this methodology to the aerobic oxidation of alcohols, since it is one of the most important organic transformations in industry.[64] The aerobic oxidation of alcohols, which can also be promoted by a copper MOF, has been reported in the literature.[65, 41] However, our catalytic system offers several advantages over existing systems, which allows the efficient oxidation of alcohol using lowered catalyst loading in shorter reaction times. As shown in Table 4, various alcohols were evaluated using Cu-MOF-2 as the catalyst assisted by 2,2,6,6-tetramethylpiperidine N-oxide (TEMPO). Benzyl alcohol was converted to corresponding benzaldehyde in good yield (Table 4, entry 1). Electron-deficient benzyl alcohol resulted in a relative low yield of the corresponding benzaldehyde, due to the lower electron-density at the benzylic carbon atom (entry 2). Furthermore, p-methoxy-substituted benzyl alcohol was evaluated as an electron-rich alcohol, which gave excellent results in terms of yield and selectivity (entry 3). However, the aerobic oxidation of secondary alcohols, such as ()-1-phenylethanol and benzhydrol did not proceed under the optimal conditions nor in DMF solution at 120 8C (entries 4 and 5). &

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[a] Reaction conditions: Alcohol (1.0 mmol), Cu-MOF-2 catalyst (100 mg, 17 mol %), TEMPO (0.5 equiv), Na2CO3 (1.0 equiv), CH3CN (5.0 mL), 1 atm. O2 ballon, 75 8C, 16 h. [b] DMF (5.0 mL), 120 8C, 16 h.

of aromatic rings, 2-naphthalenemethanol gave a much lower oxide yield under the same conditions. In addition, the 9-anthracene alcohol (9.2  with the wheeler-Weisz modulus), which was much bigger than the available pore dimensions of Cu-MOF-2 (7.2 ), gave the lowest yield. Recyclability The Cu-MOF-2 catalyst demonstrated excellent reusability for aerobic olefin epoxidation and alcohol oxidation. The catalyst was recovered from the mixture after reaction for the next reaction run by simple centrifugation, washing with alcohol, and then drying in the oven at 40 8C . The yield of epoxide remains > 99 % after 15 recycles (Figure 6). Furthermore, the crystallinity had no changes at all after 15 runs compared with the fresh 6

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Full Paper Table 5. Size and shape selectivity aerobic oxidation of Alcohol using CuMOF-2 with Oxygen.[a] Entry Substrate

Molecular size

Yield [%] Selectivity [%]

1

> 99

> 99

2

76

> 99

3

33

> 99

Figure 7. Leaching test for cyclooctene aerobic epoxidation over Cu-MOF-2. &: after 15 recycles, *: filtration of the catalyst.

[a] Reaction conditions: Alcohol (1 mmol), Cu-MOF-2 catalyst (100 mg, 17 mol %), TEMPO (0.5 equiv), Na2CO3 (1.0 equiv), CH3CN (5.0 mL), 1 atm. O2 ballon, 75 8C, 16 h.

of epoxidation. According to the crystal data and molecular formula of Cu-MOF-2 (Table S3, Supporting Information), 1.7 mol % Cu MOF contains 5.0 mol % of copper. Based on this, the ICP-AES result gave 0.013 mol % copper leaching, which corresponds to 0.0043 mol % Cu-MOF-2. The same concentration of [Cu(PhCOO)2] (0.013 mol %) was tested as the catalyst in the cyclooctene epoxidation in order to study the catalytic performance of trace amounts of copper species (Figure S6, Supporting Information). There is no yield of cyclooctene oxide at 6 h reaction time or at 12 h. It was concluded that the trace amount of copper dissolved into the solution has no influence on the catalytic reaction.

Conclusion In summary, several copper MOFs at different sizes have been obtained through a simple strategy. Nanoscaled [Cu3(BTC)2] of 390 nm (Cu-MOF-2) size was utilized in aerobic epoxidation reaction for the first time and excellent catalytic reactivity was achieved. This catalytic activity of Cu-MOF-2 was further extended to the efficient oxidation of alcohol under similar reaction conditions. The increase of the dispersion introduced by nanomorphology contributes to the enhanced catalytic performance. Interesting size and shape selectivity for both olefin epoxidation and alcohol oxidation was studied. Furthermore, the Cu-MOF-2 catalyst can be easily recycled up to fifteen times without loss of its catalytic activity.

Figure 6. Recycling test of Cu-MOF-2.

ones, as shown in Figure 1. The BET surface area of reused catalyst decreased slightly from 1391 to 1253 m2g 1, as shown in Figure S1 (Supporting Information). FTIR and SEM analysis (S3 and S4) indicate that there is no significant change in terms of morphology and internal structure. A control experiment was carried out by centrifugation of the catalyst during the reaction to determine whether the copper component dissolved from the structure of the Cu-MOF. If the reaction process continued after removing the catalyst from the system, the dissolved species should be regarded as the active component instead of the solid Cu-MOF catalyst. In our experiment, the Cu-MOF-2 catalyst was removed by filtration from the mixture once the conversion was up to approximately 60 % after 2 h reaction time followed by transferring the supernatant fluid and stirring for an additional 4 h under the same reaction conditions. As shown in Figure 7, no further conversion was detected after the Cu-MOF-2 was filtrated from the mixture even after a prolonged reaction time (2 h). Furthermore, inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis of the filtrate showed the presence of 1.7 ppm of the copper, which corresponds to 0.013 mol % of copper in the aqueous solution after one cycle Chem. Eur. J. 2014, 20, 1 – 10

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Experimental Section Synthesis of nanoscaled [Cu3(BTC)2] In a typical experiment, as shown in Scheme 1, 90 nm [Cu3(BTC)2] (Cu-MOF-1) was synthesized. Dodecanoic acid (0.8 g, 4.0 mmol) as an additive was added to the mixture by dissolving Cu(NO3)2·3 H2O (0.08 g, 0.33 mMol) in butanol (12 mL). The mixture was stirred vigorously to give a clear solution at room temperature. Then benzene-1, 3, 5-tricarboxylic acid (0.4 g, 1.9 mmol) was added to the mixture with stirring over 2 h. The resulting blue precipitation was collected by centrifugation and washed with ethanol 3 times by redispersion. The resulting products were dried in an oven at 40 8C. Similarly, 390 nm [Cu3(BTC)2] (Cu-MOF-2) was prepared by using

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Full Paper the above procedure and adding benzoic acid as an additive (0.8 g, 6.6 mmol). The bulk [Cu3(BTC)2] (Cu-MOF-3) was synthetized according to previous literature reports.[66]

building Special Project of Beijing Municipal Education for financial support. We thank Dr. P. N. Moquist (International Knowledge Editing) for helpful discussions and English editing.

Characterization

Keywords: aerobic epoxidation · alcohol oxidation · carboxylate ligand · heterogeneous catalysis · metal–organic frameworks

The structure and phase of the crystal were characterized by X-ray powder diffraction (XRD, M21X) with CuKa radiation (l = 0.154178 nm). The morphology of the byproducts was investigated by SEM (ZEISS SUPRA55). The samples for the SEM measurements were dispersed in ethanol, sonicated for a few minutes, and supported onto the silicon slice. The specific surface areas were calculated by the BET method by using Micromeritics ASAP 2420 adsorption analyzer. The pore-size distributions were derived from the adsorption branches of isotherms by using the Barrett–Joyner– Halenda (BJH) model. The dried samples were pretreated at 90 8C under vacuum for 12 h before adsorption measurements. The thermal decomposition characteristic of the samples was analyzed by TGA using a Netzsch STA449F3 instrument at a heating rate of 10 8C min 1 under a N2 flow. FTIR spectra were also acquired on a Nicolet 6700 by using the potassium bromide (KBr) pellet technique. The reaction products were analyzed by Gas Chromatography–Mass Spectrum (Agilent 7890A/5975C-GC/MSD), and nitrobenzene was used as an internal standard.

[1] M. Shokouhimehr, Y. Z. Piao, J. Kim, Y. J. Jang, T. Hyeon, Angew. Chem. 2007, 119, 7169 – 7173. [2] J. E. Backvall, Modern Oxidation Methods, Wiley-VCH, Weinheim, 2004. [3] A. H. Haines, Methods for the Oxidation of Organic Compounds, Academic Press, New York, 1985. [4] Q. H. Xia, H. Q. Ge, C. P. Ye, Z. M. Liu, K. X. Su, Chem. Rev. 2005, 105, 1603 – 1662. [5] D. Banerjee, R. V. Jagadeesh, K. Junge, M. M. Pohl, J. Radnik, A. Bruchner, M. Beller, Angew. Chem. Int. Ed. 2014, 53, 4359 – 4363. [6] A. Rezaeifard, R. Haddad, M. Jafarpour, M. Hakimi, J. Am. Chem. Soc. 2013, 135, 10036 – 10039. [7] M. K. Tse, C. Dobler, S. Bhor, M. Klawonn, W. Magerlein, H. Hugl, M. Beller, Angew. Chem. Int. Ed. 2004, 43, 5255 – 5260; Angew. Chem. 2004, 116, 5367 – 5372. [8] J. J. Dong, P. Saisaha, T. G. Meinds, P. L. Alsters, E. G. Ijpeij, R. P. van Summeren, B. Mao, M. Fananas-Mastral, J. W. de Boer, R. Hage, B. L. Feringa, W. R. Browne, ACS Catal. 2012, 2, 1087 – 1096. [9] W. Nam, H. J. Han, S. Y. Oh, Y. J. Lee, M. H. Choi, S. Y. Han, C. Kim, S. K. Woo, W. Shin, J. Am. Chem. Soc. 2000, 122, 8677 – 8684. [10] G. Yin, A. M. Danby, D. Kitko, J. D. Carter, W. M. Scheper, D. H. Busch, Inorg. Chem. 2007, 46, 2173 – 2180. [11] J. T. Groves, T. E. Nemo, J. Am. Chem. Soc. 1983, 105, 5786 – 5891. [12] S. Zakavi, L. Ebrahimi, Polyhedron 2011, 30, 1732 – 1738. [13] K. Masutani, T. Uchida, R. Irie, T. Katsuki, Tetrahedron Lett. 2000, 41, 5119 – 5123. [14] C. Parmeggiani, F. Cardona, Green Chem. 2012, 14, 547 – 564. [15] R. A. Sheldon, Chem. Ind. 1992, 903 – 906. [16] D. J. C. Constable, P. J. Dunn, J. D. Hayler, G. R. Humphrey, J. L. Leazer, R. J. Linderman, K. Lorenz, J. Manley, B. A. Pearlman, A. Wells, A. Zaks, T. Y. Zhang, Green Chem. 2007, 9, 411 – 420. [17] H. Weiner, R. G. Finke, J. Am. Chem. Soc. 1999, 121, 9831 – 9842. [18] S. Mohebbia, D. M. Boghaeib, A. H. Sarvestania, A. Salimi, Appl. Catal. A 2005, 278, 263 – 267. [19] S. Mohebbi, F. Nikpour, S. Raiati, J. Mol. Catal. A 2006, 256, 265 – 268. [20] S. Bhattacharjee, D. A. Yang, W. S. Ahn, Chem. Commun. 2011, 47, 3637 – 3639. [21] Y. Nishiyama, Y. Nakagawa, N. Mizuno, Angew. Chem. Int. Ed. 2001, 40, 3639 – 3641; Angew. Chem. 2001, 113, 3751 – 3753. [22] S. X. Jin, T. M. Makris, T. A. Bryson, S. G. Sligar, J. H. Dawson, J. Am. Chem. Soc. 2003, 125, 3406 – 3407. [23] K. Schrçder, B. Join, A. J. Amali, K. Junge, X. Ribas, M. Costas, M. Beller, Angew. Chem. Int. Ed. 2011, 50, 1425 – 1429; Angew. Chem. 2011, 123, 1461 – 1465. [24] R. Chakrabarty, B. K. Das, J. H. Clark, Green Chem. 2007, 9, 845 – 848. [25] P. Shringarpure, A. Patel, J. Mol. Catal. A 2010, 321, 22 – 26. [26] G. Rousselet, C. Chassagnard, P. Capdevielle, M. Maumy, Tetrahedron Lett. 1996, 37, 8497 – 8500. [27] N. Komiya, T. Naota, Y. Oda, S. I. Murahashi, J. Mol. Catal. A 1997, 117, 21 – 37. [28] I. W. Davies, L. Matty, D. L. Hughes, P. J. Reider, J. Am. Chem. Soc. 2001, 123, 10139 – 10140. [29] F. Durap, M. Rakap, M. Aydemir, S. Ozkar, Appl. Catal. A 2010, 382, 339 – 344. [30] Z. Opre, T. Mallat, A. Baiker, J. Catal. 2007, 245, 482 – 486. [31] J. Xu, T. White, P. Li, C. He, J. Yu, W. Yuan, Y. F. Han, J. Am. Chem. Soc. 2010, 132, 10398 – 10406. [32] K. M. Chepiga, Y. Feng, N. A. Brunelli, C. W. Jones, H. M. L. Davies, Org. Lett. 2013, 15, 6136 – 6139. [33] P. Lignier, M. Comottib, F. Schuthb, J. L. Rousset, V. Caps, Catal. Today 2009, 141, 355 – 360.

Catalytic performance testing Aerobic epoxidation of olefins: In general, the catalytic properties of the catalysts were examined by the epoxidation of olefins with molecular oxygen in a 25 mL round-bottom flask. In a typical process, a mixture of acetonitrile (5 mL) and olefin (1 mmol) was added into a 25 mL round-bottom flask, together with catalytic amount of copper species. As for the Cu-MOF-1 and Cu-MOF-2 catalyst, 0.017 mmol, 1.7 mol % was utilized. For [Cu(BDC)] and other copper salt catalysts, 0.050 mmol, 5 mol % was used to make the copper content equal for the parallel reaction. After purging three times with oxygen, set up with the oxygen balloon, and a certain reaction time, nitrobenzene (0.2 mmol) was added as an internal standard for the determination of yield and selectivity.[67] The filtered liquid samples were analyzed by GCMS analysis and 1 H NMR spectroscopy. Aerobic oxidation of alcohols: The catalytic properties of alcohol oxidation were also investigated. In a typical procedure, the amount of alcohol (1 mmol), Na2CO3 (1 mmol), and Cu-MOF-2 (0.17 mmol) catalyst were added in a 25 mL round-bottom flask. With vacuumizing three times as above, the oxygen balloon was set up. Then, the closed container was heated rapidly at the desired temperature. The flask was cooled down after 16 h and the filtered liquid samples were examined by GCMS analysis and 1 H NMR spectroscopy.

Catalyst recycling For catalyst recycling, the reactions were performed under the same reaction conditions, except with the recovered catalyst. In order to study the leaching of Cu during the reaction, the mother liquid was left to continue to run itself by prolonging 2 h under the same conditions.

Acknowledgements We thank the National High Technology Research and Develop Program of China (863 program) (No. 2013AA031702) and Co&

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Full Paper [34] N. E. Leadbeater, M. Marco, Chem. Rev. 2002, 102, 3217 – 3274. [35] G. Frey, Chem. Soc. Rev. 2008, 37, 191 – 214. [36] K. Schlichte, T. Kratzke, S. Kaskel, Microporous Mesoporous Mater. 2004, 73, 81 – 88. [37] S. Marx, W. Kleist, A. Baiker, J. Catal. 2011, 281, 76 – 87. [38] A. Sachse, R. Ameloot, B. Coq, F. Fajula, B. Coasne, D. De Vos, A. Galarneau, Chem. Commun. 2012, 48, 4749 – 4751. [39] M. Fujita, Y. J. Kwon, S. Washizu, K. Ogura, J. Am. Chem. Soc. 1994, 116, 1151 – 1152. [40] M. J. Beier, W. Kleist, M. T. Wharmby, R. Kissner, B. Kimmerle, P. A. Wright, J. D. Grunwaldt, A. Baiker, Chem. Eur. J. 2012, 18, 887 – 898. [41] A. Dhakshinamoorthy, M. Alvaro, H. Garcia, ACS Catal. 2011, 1, 836 – 840. [42] Y. H. Fu, D. R. Sun, M. Qin, R. K. Huang, Z. H. Li, RSC Adv. 2012, 2, 3309 – 3314. [43] A. Dhakshinamoorthy, M. Alvaro, H. Garcia, J. Catal. 2012, 289, 259 – 265. [44] O. K. Farha, A. M. Shultz, A. A. Sarjeant, S. T. Nguyen, J. T. Hupp, J. Am. Chem. Soc. 2011, 133, 5652 – 5655. [45] A. M. Shultz, O. K. Farha, J. T. Hupp, S. T. Nguyen, Chem. Sci. 2011, 2, 686 – 689. [46] J. L. Zhuang, D. Ceglarek, S. Pethuraj, A. Terfort, Adv. Funct. Mater. 2011, 21, 1442 – 1447. [47] W. Lin, W. Rieter, K. Taylor, Angew. Chem. Int. Ed. 2009, 48, 650 – 658; Angew. Chem. 2009, 121, 660 – 668. [48] S. Diring, S. Furukawa, Y. Takashima, T. Tsuruoka, S. Kitagawa, Chem. Mater. 2010, 22, 4531 – 4538. [49] S. S. Y. Chui, S. M. F. Lo, J. P. H. Charmant, A. G. Orpen, I. D. Williams, Science 1999, 283, 1148 – 1150. [50] L. H. Wee, M. R. Lohe, N. Janssens, S. Kaskel, J. A. Martens, J. Mater. Chem. 2012, 22, 13742 – 13746. [51] C. G. Carson, K. Hardcastle, J. Schwartz, X. T. Liu, C. Hoffmann, R. A. Gerhardt, R. Tannenbaum, Eur. J. Inorg. Chem. 2009, 2338 – 2343.

Chem. Eur. J. 2014, 20, 1 – 10

www.chemeurj.org

These are not the final page numbers! ÞÞ

[52] D. M. Jiang, T. Mallat, F. Krumeich, A. Baiker, Catal. Commun. 2011, 12, 602 – 605. [53] Y. Luan, N. N. Zheng, Y. Qi, J. Tang, G. Wang, Catal. Sci. Technol. 2014, 4, 925 – 929. [54] S. I. Murahashi, Y. Oda, T. Naota, N. Komiya, J. Chem. Soc. Chem. Commun. 1993, 2, 139 – 140. [55] A. Dhakshinamoorthy, M. Alvaro, H. Garcia, ACS Catal. 2011, 1, 48 – 53. [56] Z. F. Li, S. J. Wu, H. Ding, D. F. Zheng, J. Hu, X. Wang, Q. S. Huo, J. Q. Guan, Q. B. Kan, New J. Chem. 2013, 37, 1561 – 1568. [57] I. Garcia-Bosch, X. Ribas, M. Costas, Adv. Synth. Catal. 2009, 351, 348 – 352. [58] D. H. Tang, W. T. Zhang, Y. L. Zhang, Z. A. Qiao, Y. L. Liu, Q. S. J. Huo, J. Colloid Interface Sci. 2011, 356, 262 – 266. [59] V. R. Choudhary, R. Jha, P. Jana, Green Chem. 2006, 8, 689 – 690. [60] X. G. Yang, S. Gao, Z. W. Xi, Org. Process Res. Dev. 2005, 9, 294 – 296. [61] C. Q. Chen, J. Qu, C. Y. Cao, F. Niu, W. G. Song, J. Mater. Chem. 2011, 21, 5774 – 5779. [62] Y. J. Song, M. Y. Hyun, J. H. Lee, H. G. Lee, J. H. Kim, S. P. Jang, J. Y. Noh, Y. Kim, S. Y. Kim, S. J. Lee, C. Kim, Chem. Eur. J. 2012, 18, 6094 – 6101. [63] S. Horike, M. Dinca, K. Tamaki, J. R. Long, J. Am. Chem. Soc. 2008, 130, 5854 – 5855. [64] A. Dhakshinamoorthy, M. Alvaro, H. Garcia, Catal. Sci. Technol. 2011, 1, 856 – 867. [65] J. M. Hoover, S. S. Stahl, J. Am. Chem. Soc. 2011, 133, 16901 – 16910. [66] A. Pramanik, S. Abbina, G. Das, Polyhedron 2007, 26, 5225 – 5234. [67] M. A. Bigi, S. A. Reed, M. C. White, Nat. Chem. 2011, 3, 216 – 222.

Received: October 16, 2014 Published online on && &&, 0000

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FULL PAPER & Heterogeneous Catalysis Y. Qi, Y. Luan, J. Yu, X. Peng, G. Wang* && – && Nanoscaled Copper Metal–Organic Framework (MOF) Based on Carboxylate Ligands as an Efficient Heterogeneous Catalyst for Aerobic Epoxidation of Olefins and Oxidation of Benzylic and Allylic Alcohols

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Chem. Eur. J. 2014, 20, 1 – 10

Aerobic epoxidation of olefins at a mild reaction temperature has been carried out using nanomorphology of carboxylate ligand enhanced [Cu3(BTC)2] (BTC = 1,3,5-benzenetricarboxylate) as

a high-performance catalyst through a facile and simple synthetic strategy (see scheme). This catalyst also exhibited high activity in alcohol oxidation.

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ÝÝ These are not the final page numbers!

Nanoscaled copper metal-organic framework (MOF) based on carboxylate ligands as an efficient heterogeneous catalyst for aerobic epoxidation of olefins and oxidation of benzylic and allylic alcohols.

Aerobic epoxidation of olefins at a mild reaction temperature has been carried out by using nanomorphology of [Cu3(BTC)2] (BTC = 1,3,5-benzenetricarbo...
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