Synthesis of a Novel Cellulose Microencapsulated Palladium

Apr 27, 2014 - The as-prepared novel hybrid material CelMcPd0 proved to be a ... catalyst for a range of phosphine-free crossing Suzuki–Miyaura and ...
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Synthesis of a Novel Cellulose Microencapsulated Palladium Nanoparticle and Its Catalytic Activities in Suzuki−Miyaura and Mizoroki−Heck Reactions Feiran Chen,† Mingming Huang,‡ and Yiqun Li†,‡,* †

Department of Chemistry, Jinan University, Guangzhou 510632 Guangdong, PR China College of Life Science and Technology, Jinan University, Guangzhou 510632 Guangdong, PR China

‡

S Supporting Information *

ABSTRACT: Palladium nanoparticles microencapsulated by cellulose (CelMcPd0) were, for the first time, developed via reduction of Pd(OAc)2 or PdCl2 in a cellulosic ionic liquid solvent, in which the PdII species were synchronously reduced to Pd0 nanoparticles in situ with the ionic liquid itself or with NaBH4, followed by enveloping the Pd0 cores with cellulosic films using anhydrous ethanol as a coagulant. The as-prepared novel hybrid material CelMcPd0 proved to be a versatile and highly catalytically efficient, recyclable, and robust catalyst for a range of phosphine-free crossing Suzuki−Miyaura and Mizoroki−Heck reactions under mild aerobic conditions. Transmission electron microscopy (TEM), scanning electron microscopy (SEM), and inductively coupled plasma atomic emission spectroscopy (ICP-AES) measurements have been used to characterize the catalysts. The results revealed that the palladium particles are mostly spherical in shape and estimated to be range of 5−20 nm in CelMcPd0-1, 5−40 nm in CelMcPd0-2, and 3−15 nm in CelMcPd0-3. Moreover, homogeneous catalysis in Suzuki−Miyaura couplings catalyzed by CelMcPd0-1 was evidenced from CS2 poisoning tests. After the completion of the reaction, catalyst separation could be easily achieved by simple filtration, and the catalyst could be recycled at least six times without any loss of its high catalytic activity. their chemical structure, a backbone of bound ligands, that is, π electrons of benzene rings of PS, urea groups (−NHCONH−) of PU, that could coordinate and thus retain metal species. Various microencapsulated catalysts using PS and PU as matrix have been delineated in the published literature10−14 including MC-Pd(PPh3)4, MC-Sc(OTf)3, MC-OsO4, MC-Bi(OTf)3, and MC-PdEnCat. More recently, with the advance of nanotechnology for making metal nanoparticles, it has been possible to prepare metal nanoparticle catalysts. Among these catalysts, microencapsulated palladium nanoparticles have gained a great reputation. This is because palladium is a versatile catalyst in modern organic synthesis and is widely used for a significant number of carbon−carbon bond formations,2,15,16 such as Suzuki−Miyaura17 and Mizoroki−Heck18 coupling reactions. Over the past few years, various methods and materials have been reported in the published literature for supporting or entrapping palladium nanoparticles on various solid carriers (PS,10 PU,11−14 silica,19−21 dendrimers,22,23 ion-exchange resin,24,25 glycidyl methacrylate,9 etc.). However, at present, nonrenewable petrochemicals are the major source of polymer matrices, and most of them are fragile and cannot withstand thermal activation or are not biodegradable and are unfriendly to the environment, or their preparation and their regeneration is quite tedious. Also, the encapsulation of palladium

1. INTRODUCTION The utility of polymeric transition metal catalysts is now wellrecognized because of their ease of work-up and of separation of products and catalysts, from an economical point of view, and application to academic and industrial processes.1−4 In general, a metal catalyst is immobilized on polymers via coordination to a ligand, which is covalently bound to a polymer backbone,5 or it is adsorbed on an inert surface.6 In this approach, the synthesis of the polymer-bound ligand can be lengthy and expensive, and there can be problems associated with the stability, activity, and leaching of the catalysts.7,8 Microencapsulation is the process where a valuable material is coated with a continuous film of a polymeric material to form microcapsules with sizes ranging from micrometer to millimeter.9 This core−shell structure allows isolation of the encapsulated substance from the surroundings and thus protects it from any negative factors. Thus, microencapsulation may offer a solution to these problematic limitations and thus is one of the most promising techniques for metal catalyst immobilization, permitting facile recycling. One of the interesting aspects of such microencapsulated catalysts is that they swell under certain solvent conditions and thus provide a near homogeneous environment for the reactants while retaining all the advantages of a heterogeneous catalyst. Efficient entrapment of metal based catalysts requires the design of materials possessing ligating functionality in order to retain the metal species.10−12 These materials should be physically robust and chemically inert to reaction conditions while also being cost-effective. For example, polystyrene (PS),10 and polyurea (PU)11−14 were found to be suitable by virtue of © 2014 American Chemical Society

Received: Revised: Accepted: Published: 8339

November 13, 2013 March 31, 2014 April 26, 2014 April 27, 2014 dx.doi.org/10.1021/ie4038505 | Ind. Eng. Chem. Res. 2014, 53, 8339−8345

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methylimidazolum chloride (20.0 mL) at 80 °C. After cellulose was dissolved completely, Pd(OAc)2 (0.2 g) was added to give a dark red mixture after stirring for 10 h at this temperature, which indicated that Pd0 nanoparticles were formed in situ by reduction of a PdII complex with the hydroxyl group contained in the ionic liquid. Slow addition of coagulant ethanol (40.0 mL) and standing for 1 h at room temperature led to the formation of capsules. Washing with commercial anhydrous ethanol (3 × 40 mL) and drying at 50 °C overnight gave microencapsulated CelMcPd0-1 as a light gray powder. The Pd content was found to be 0.1047 mg/g by inductively coupled plasma atomic emission spectroscopy (ICP-AES). 2.2.2. CelMcPd0-2. The procedure was the same as that described for preparation of CelMcPd0-1. After completion of dissolution of cellulose in the ionic liquid, NaBH4 (1.0 g) was added into ionic liquid solvent. Instant and rapid color change was observed. The resulting microcapsules were a black solid powder, and the Pd content was found to be 0.1621 mg/g by ICP-AES. 2.2.3. CelMcPd0-3. The procedure also was the same as that for preparation of CelMcPd0-1, using PdCl2 (0.3 g) instead of Pd(OAc)2 as palladium nanoparticle precursor. The resulting microcapsules were a brown solid powder, and the content of Pd was found to be 0.1124 mg/g by ICP-AES. 2.3. General Experimental Procedures for Suzuki− Miyaura Couplings. In a typical experiment, the CelMcPd0-1 catalyst (0.005 mmol of Pd) was added to a mixture of aryl halide (1.0 mmol), arylboronic acid (1.2 mmol), and K2CO3 (2.0 mmol) in ethanol (5.0 mL), and the reaction mixture was stirred at reflux. After the reaction was judged to be complete by TLC analysis, the catalyst was removed by filtration, washed with ethanol (3 × 4 mL), and dried under vacuum for the next run. The organic fractions were then concentrated on a rotary evaporator to afford the desired biaryl in excellent yield. The crude products were further purified by recrystallization. All of the products are known compounds, and their 1H NMR data were identical to those reported in literature. 2.4. General Experimental Procedures for Mizoroki− Heck Couplings. 2.4.1. Typical Procedure for Arylation of Styrene with Aryl Halides. A mixture of aryl halide (1.0 mmol), styrene (1.5 mmol), Bu3N (2.0 mmol), DMF (5.0 mL), and the CelMcPd0-1 complex (0.8 mol % Pd) was stirred in an oil bath preheated at 130 °C. The process was tracked by TLC. The mixture was cooled and quenched by Et2O (15.0 mL) after the completion of the reaction. The ether solution was washed with 6 M HCl and brine and dried over anhydrous MgSO4. The product was obtained after removal of the ether solution by a rotary evaporator. The desired pure products were further purified by recrystallization with ethanol. 2.4.2. Typical Procedure for Arylation of Aryl Halides with Acrylic Acid. A mixture as given above using acrylic acid in place of styrene was stirred in an oil bath at 130 °C. The end of the reaction was monitored by TLC. The mixture was then poured into H2O (10 mL), and Na2CO3 (0.3 g) was added. After the mixture was stirred for 10 min, the CelMcPd0-1 catalyst was separated from the mixture by filtration. The aqueous phase was acidified with 6 M HCl to precipitate the crude solid product. The pure product was obtained by recrystallization from ethanol. All the products are known, and 1 H NMR data were found to be identical to those reported in the literature. 2.5. CS2 Poisoning Tests of CelMcPd0-1. The separated poisoning experiment was conducted using phenylboronic acid

nanoparticles in silicates or other inorganic solid substrates causes the active surface area of the nanoparticle to be very small, which results in a decrease in their catalytic activity. In view of this, an alternative ecofriendly polymer support is essential. The use of naturally abundant cellulose as a replacement may well serve the purpose. Cellulose is the most abundant renewable polysaccharide available on earth, composed of glucose units in which there are a large number of hydroxyl groups.26 Cellulose has many good properties, such as insolubility in the vast majority of molecular solvents but solubility in fewer ionic liquids and molecular solvents27 and biodegradability, as well as nontoxicity, which make it an environmentally benign material and an excellent candidate as catalyst support. Therefore, biodegradable polymer cellulose and its derivatives have been used to immobilize palladium nanoparticles and employed in organic transformations.28−31 However, their use as an encapsulation matrix for catalytic applications is not well explored. Motivated by recent applications of microencapsulated Pd nanoparticles to catalysis and the corresponding need to better understand the fundamental properties of this emerging class of biopolymers, our idea is to apply this microencapsulation technique to the immobilization of palladium catalyst trapped with cellulose. That is, palladium nanoparticles would be physically enveloped by cellulose thin films and perhaps stabilized by the interaction by ligation of hydroxyl groups28 of the cellulose used as a polymer backbone and vacant orbitals of nanopalladium. Herein we report the development of a novel method for the encapsulation of palladium nanoparticles on cellulose substrates and its applications in one-pot, highly efficient Suzuki−Miyaura and Mizoroki−Heck couplings. This strategy involves ionic liquid as the cellulose nonderivatizing solvent32 as well as the reductant33 for generation of cellulose encapsulated palladium nanoparticles. We also critically examine the role of “Pd” nanoparticles as recyclable catalysts, examine their regeneration and recycling, and provide evidence that the cellulose microencapsulated Pd nanoparticles can be regarded as an environmental friendly catalyst, which is inherit from the advantages of cellulose polymers and the catalytic activity of metal nanoparticles.

2. EXPERIMENTAL SECTION 2.1. Chemicals and Characterizations. Melting points were measured on an Electrothermal X6 microscopic digital melting point apparatus. IR spectra were recorded on a Nicolet 6700 spectrometer using KBr pellets. 1H NMR spectra were recorded on a 500 MHz Bruker Avance instrument using CDCl3 or DMSO-d6 as solvent and TMS as internal standard. The elemental palladium content of the catalysts was determined by Perkin Elmer Optima 2000DV inductively coupled plasma atomic emission spectroscopy (ICP-AES). Scanning electron microscopy (SEM) was performed on a Philips XL 30ESEM instrument. Transmission electron microscopy (TEM) was performed with a Philips Tecnai instrument operating at 40−100 kV. X-ray diffraction (XRD) patterns were obtained with an MSAL-XRD2 diffractometer using Cu K radiation operating at 36 kV and 20 mA. All chemicals were obtained from commercial sources and used directly as received. 2.2. Preparation of the Microencapsulated Nanopalladium (CelMcPd). 2.2.1. CelMcPd0-1. Alkali cellulose (1.0 g) was dissolved in ionic liquid 1-hydroxylethyl-38340

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immobilizing palladium nanoparticles was attributed to interaction between a vacant d orbit of palladium and the hydroxyl groups of cellulose. The scanning electron microscopy (SEM) micrographs of CelMcPd0 thus prepared are shown in Figure 2. It was found that small capsules of CelMcPd0 adhered to each other, probably due to the small size of the core. Representative TEM images of CelMcPd0 described in Scheme 1 are shown in Figure 3. As can be seen from Figure 3, the average particle size and distribution of Pd particles were estimated to be in the range of 5−20 nm in CelMcPd0-1, 5−40 nm in CelMcPd0-2, and 3−15 nm in CelMcPd0-3, respectively. Figure 4 shows powder XRD diffraction patterns of cellulose microencapsulated palladium catalysts (CelMcPd0). Three weak peaks at 2θ of around 41.1°, 46.6°, and 68.1°, the diffraction of the {111}, {200}, and {220} lattice planes of palladium crystalline structure respectively, are clearly observed on CelMcPd0-1 and CelMcPd0-2, while a very weak peak of Pd0 characteristically centered at 41.1° is also detected on CelMcPd0-3, which indicated the low loading of Pd0 due to the incomplete reduction of PdCl2 by hydroxyl-functionalized ionic liquid. This result is consistent with previous findings that the Pd0 diffraction is difficult to detect by XRD at low loading.34 The visibly brown color of CeMcPd0-3 also implies the existance of the unreduced fraction PdCl2 (Figure 1c). 3.2. Efficiency of the Catalyst in Carbon−Carbon Couplings. Initial investigations centered upon the efficiency of the new cellulose entrapped catalyst by using Suzuki− Miyaura couplings as a model, which is a versatile and wellstudied method for the generation of Csp2−Csp2 bonds in organic synthesis. Treatment of iodobenzene and phenylboronic acid with K2CO3 in the presence of catalytic amount of CelMcPd0 in EtOH at reflux under an atmosphere afforded biphenyl in an excellent isolated yield in short time. The results are summarized in Table 1. It was found that all CelMcPd0 catalysts showed excellent catalytic activity. Among the screened microencapsulated catalysts, CelMcPd0-1 proved to be the best and was thus chosen as the catalyst in the further exploration of Suzuki− Miyaura and Mizoroki−Heck reactions. 3.3. Performance of Catalyst for the Suzuki−Miyaura Reactions. In order to find suitable conditions for the Suzuki− Miyaura reactions, the influences of base, solvent, and the amount of catalyst on reactions were carefully examined using the reaction of iodobenzene and phenylboronic acid as the model reaction. The results are summarized in Table 2. First, for the sake of finding a suitable media for the reactions, various solvents were explored in the model reaction. As shown in Table 2, the rate of reaction and the activity of the catalyst were significantly influenced by the solvent used. Among the solvents screened, we found that using EtOH as the solvent gave the highest yield of 91% (Table 2, entry 1). Next, the effect of base on the Suzuki−Miyaura reaction was also examined. A series of bases were taken into consideration for the model reaction. Among the tested bases, K2CO3 and Cs2CO3 were found to be more effective (Table 2, entries 1 and 9). We finally chose K2CO3 as a base, because it is much cheaper than Cs2CO3. It is a key issue to note that the amount of palladium catalyst plays an important role in the product yields. The Suzuki− Miyaura reaction was studied with the amount of catalyst ranging from 0.1 to 0.7 mol % with the model reaction (Table

and iodobenzene as coupling partners and different amounts of CS2 as poison under our usual Suzuki conditions except the temperature was set at rt to prevent ligand dissociation.

3. RESULTS AND DISCUSSION 3.1. Preparation and Characterization of CelMcPd0. Using the protocol described in the Experimental Section, we have prepared a novel cellulose encapsulated Pd nanoparticle catalyst, which we entitle CelMcPd0. The sequential protocols of the preparation of CelMcPd0 were based on the steps schematically depicted in Scheme 1. Scheme 1. Schematic Routes for Preparation of CelMcPd0

On the basis of this idea, alkali cellulose was dissolved in functionalized ionic liquid 1-hydroxylethyl-3-methylimidazolum chloride ([HOC2MIM]Cl) at 80 °C, and to this solution was added Pd(OAc)2 or PdCl2 as palladium source to produce the palladium nanoparticles, which served as a core by reduction with ionic liquid or NaBH4. Then, the palladium cores dispersed in the medium were enveloped by cellulose after addition of coagulant anhydrous ethanol. The mixture was left to stand at room temperature to form microcapsules. After filtration, washing, and drying, the catalyst capsules incarcerated palladium. The capsules prepared from Pd(OAc)2 using ionic liquid or NaBH4 as reductant were denoted CelMcPd0-1 and CelMcPd0-2, respectively. The capsules using PdCl2 as nanopalladium precursor was denoted CelMcPd0-3. The digital photographs of these capsules are shown in Figure 1.

Figure 1. Photographs of CelMcPd0-1 (a), CelMcPd0-2 (b), and CelMcPd0-3 (c).

The hydroxyl-functionalized ionic liquid acts both as a nonderivatizing solvent for cellulose and as a reducing agent for PdII, which indicated that this kind of ionic liquids plays important role in the preparation of CelMcPd0. It has been established that a Pd0 nanoparticle is spontaneously formed from an irreversible intramolecular reduction of a PdII complex with the hydroxyl group. A possible role of the cellulose in 8341

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Figure 2. SEM image of CelMcPd0-1(a), CelMcPd0-2 (b), and CelMcPd0-3 (c).

Figure 3. TEM image of CelMcPd0-1 (a), CelMcPd0-2 (b), and CelMcPd0-3 (c).

Table 2. Effect of the Suzuki−Miyaura Cross-Couplinga

entry

solvent

base

1 2 3 4 5 6 7 8 9 10 11 12

EtOH 95%EtOH DMF CH3COCH3 CH3CN DMSO H2O EtOH EtOH EtOH EtOH EtOH

13 14 15

EtOH EtOH EtOH

K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 K2CO3 Na2CO3 Cs2CO3 NaOH MeONa K3PO4· 12H2O K2CO3 K2CO3 K2CO3

Figure 4. XRD diffraction patterns of palladium nanoparticles of CelMcPd0-1 (a), CelMcPd0-2 (b), and CelMcPd0-3 (c).

Table 1. Effect of the Different CelMcPd0 Catalysts on the Suzuki−Miyaura Cross-Couplinga

entry

catalysts

time [min]

yieldb [%]

1 2 3

CelMcPd0-1 CelMcPd0-2 CelMcPd0-3

10 10 15

91 90 85

amount of catalyst (mol % of Pd)

time [h]

yieldb [%]

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

0.25 0.5 24 24 24 24 8 0.25 0.25 2 1 1

91 90 54 43 70 84 87 90 92 67 74 80

0.1 0.3 0.7

0.5 0.5 0.25

62 74 91

a

Reaction conditions: iodobenzene (1.0 mmol), phenylboronic acid (1.1 mmol), base (2.0 mmol), solvent (5.0 mL), and CelMcPd0-1 at reflux under air. bIsolated yields based on iodobenzene.

a

Reaction conditions: iodobenzene (1.0 mmol), phenylboronic acid (1.1 mmol), K2CO3 (2.0 mmol), EtOH (5.0 mL), and catalyst (0.5 mol % of Pd) at reflux under air. bIsolated yields based on iodobenzene.

To examine the scope for this coupling reaction, a wide range of substituted aryl halides were coupled with different arylboronic acids under the optimal reaction conditions. The results are summarized in Table 3. The data presented in Table 3 show that the CelMcPd0-1 catalyst was highly effective for both aryl bromides and aryl iodides. Most of these reactions proceeded rapidly and were complete within 30 min. Aryl halides bearing either electrondonating or electron-withdrawing substituents displayed similar reactivities, reacting efficiently to yield the corresponding products. 3.4. Performance of Catalyst for the Mizoroki−Heck Reaction. Having demonstrated that CelMcPd0-1 was a highly effective catalyst for the Suzuki−Miyaura reaction, its activity was also investigated in the Mizoroki−Heck reaction of aryl halides and olefins. For the sake of the optimal reaction

2). It was found that the Suzuki−Miyaura reaction could be carried out efficiently even with a low amount of the catalyst (0.1 mol % Pd) with 62% product yield. As can be seen from Table 2, with increasing the amount of catalysts from 0.1 o 0.5 mol %, the yield of desired product increased apparently from 62% to 91% (Table 2, entries 1, 13, and 14). Increasing the amount of palladium catalyst gave higher yield until the amount of the catalyst increases to 0.5 mol % (Table 2, entry 1). Further increasing the amount of catalyst to 0.7 mol % had apparently no significant effect on the reaction yield (Table 2, entry 15). Therefore, 0.5 mol % of the palladium catalyst was enough to push the reaction to completion. 8342

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Table 3. CelMcPd0-1 Catalyzed Suzuki−Miyaura CrossCoupling Reactions of Aryl Halides and Arylboronic Acid under Optimized Conditionsa

entry

Ar−X

R2

time [min]

yieldb [%]

products

1 2 3 4 5 6 7 8 9 10 11 12 13

C6H5I 4-Me-C6H4I 4-MeO-C6H4I 4-MeO2C−C6H4I 3-NO2−C6H4I 4-NO2−C6H4I C6H5Br 4-Me-C6H4Br 4-MeO-C6H4Br C6H5I C6H5I 4-MeO-C6H4I 4-NO2−C6H4Br

H H H H H H H H H 4-Me 4-F 4-OMe 4-F

15 15 15 20 20 10 20 20 20 15 15 15 30

91 90 89 87 83 94 90 91 91 90 87 90 79

3a 3b 3c 3d 3e 3f 3a 3b 3c 3b 3g 3h 3i

Table 5. CelMcPd0-1 Catalyzed Mizoroki−Heck CrossCoupling Reaction of Aryl Halides and Olefins under Optimized Conditionsa

Table 4. Effect of the Mizoroki−Heck Cross-Couplinga

1 2

DMF DMF/H2O (v/v = 1/1) DMSO EtOH THF CH3CN C6H5CH3 DMF DMF DMF DMF DMF DMF DMF DMF

3 4 5 6 7 8 9 10 11 12 13 14 15

temp (°C)

time [h]

yieldb [%]

Bu3N Bu3N

0.8 0.8

130 130

2 24

81 27

Bu3N Bu3N Bu3N Bu3N Bu3N Et3N MeONa EtONa Na2CO3 K2CO3 Bu3N Bu3N Bu3N

0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.2 0.5 1.1

130 c c c c 130 130 130 130 130 130 130 130

24 24 24 24 24 7 8 12 24 24 12 8 2

69 d d d d 77 63 59 d d 56 69 82

base

time [min]

yieldb [%]

product

1 2 3 4 5 6 7 8 9

C6H5I 4-MeO-C6H4I 4-NO2−C6H4I 4-CH3−C6H4I C6H5Br 4-MeO-C6H4Br 4−Cl-C6H4Br 4-NO2−C6H4I 4-MeO-C6H4Br

C6H5 C6H5 C6H5 C6H5 C6H5 C6H5 C6H5 CO2H CO2H

2 2 2 2 4 8 6 3 12

81 88 87 66 78 81 82 83 87

6a 6b 6c 6d 6a 6b 6e 6f 6g

As shown in Table 4, the reaction media had obvious effect on the coupling. Among the screened solvents, DMF was founded to be the most suitable media for this catalyst (Table 4, entry 1). Other solvents, such as EtOH, DMSO, CH3CN, and THF, were not satisfactory (Table 4, entries 4−7). As can be seen in Table 4, Bu3N proved to be the best base among those selected. Et3N, MeONa and EtONa gave moderate yields (Table 4, entries 8−10). Na2CO3 and K2CO3 afforded a trace amount of the product (Table 4, entries 11−12). The results in Table 4 showed that a moderate yield (69%) was obtained even at palladium loading as low as 0.5 mol % (Table 4, entry 14). Excellent yields were obtained in 2 h in the presence of 0.8 mol % palladium (Table 4, entry 1). Further elevating the amount of palladium to 1.1 mol % could not apparently increase the reaction yields (Table 4, entry 15). Under the optimized reaction conditions, nine aryl halides bearing both electron-donating and electron-withdrawing groups reacted with styrene and acrylic acid, respectively, affording the coupled products in excellent yields. The results are given in Table 5. The results clearly demonstrated that the CelMcPd0-1 catalyst was effective for the Mizoroki−Heck reaction. The catalytic activity of depended on the type of halide. Both electron-rich and electron-deficient aryl halides proceeded smoothly to afford the desired products with excellent yields. All aryl iodides were rapidly converted to the corresponding products in excellent yields (Table 5, entries 1−4 and 8). 3.6. Recycling of CelMcPd0-1 for the Suzuki−Miyaura Reaction. We have demonstrated the generalizability and scope of the proposed protocol by investigating the Suzuki− Miyaura and Mizoroki−Heck reactions. All of the reactions proceeded smoothly to produce the expected products with excellent yields. But for practical applications of heterogeneous systems, the lifetime of the catalyst and its level of reusability are important factors. Finally, we explored the reusability of the CelMcPd0-1 catalyst again with the reaction of iodobenzene with phenylboronic acid as the model reaction. After the first run, the catalyst was centrifuged and dried in vacuum. Then the catalyst was reused directly up to six consecutive cycles of the reaction under the same conditions mentioned above. The results are shown in Table 6. The results revealed that the

conditions, a series of experiments were performed using 4iodoanisole and styrene as model compounds. The model reactions were conducted using a series of solvents, such as EtOH, DMF, DMSO, CH3CN, THF, and toluene, as well as different bases such as Na2CO3, K2CO3, MeONa, EtONa, Et3N, and Bu3N at the specific temperatures. Furthermore, the effect of the amount of catalyst on Mizoroki−Heck coupling was also carefully examined. The optimal conditions of the Mizoroki− Heck reaction are given in Tables 4 and 5.

solvent

R2

Reaction conditions: aryl halides (1.0 mmol), olefins (1.5 mmol), Bu3N (2.0 mmol), DMF (5.0 mL), and CelMcPd0-1 (0.8 mol % of Pd) at 130 °C under air. bIsolated yield based on aryl halides.

Reaction conditions: aryl halides (1.0 mmol), arylboronic acid (1.1 mmol), K2CO3 (2.0 mmol), EtOH (5.0 mL), and CelMcPd0-1 (0.5 mol % of Pd) at reflux under air. bIsolated yield based on aryl halides.

entry

Ar−X

a

a

amount of catalyst (mol % of Pd)

entry

a

Reaction conditions: 4-iodoanisole (1.0 mmol), styrene (1.5 mmol), base (2.0 mmol), solvent (5.0 mL), and CelMcPd0-1 at specific temperature under air. bIsolated yield based on 4-iodoanisole. cReflux. d Trace. 8343

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the catalyst above 50 °C.35,36 The catalytic activities were determined as a function of the addition of 0.1, 0.5, 1.0, 1.5, and 2.0 equiv of CS2 (per total Pd) in each of these experiments. It should be noted that the catalytic activity did not apparently change with increasing addition of CS2 from the amount of 0.1 to 2.0 equiv per total Pd by comparison with the control reaction (blank reaction, Figure 6). This finding suggests that

recovered catalyst had not significantly lost its activity up to six cycles. Table 6. Reusability of the CelMcPd0-1 Catalyst for the Suzuki−Miyaura Cross-Couplinga

entry

run

time [min]

yieldb [%]

1 2 3 4 5 6

first second third fourth fifth sixth

15 15 15 15 15 15

91 91 90 89 88 88

a

Reaction conditions: iodobenzene (1.0 mmol), phenylboronic acid (1.1 mmol), K2CO3 (2.0 mmol), EtOH (5.0 mL), and CelMcPd0-1 (0.5 mol % of Pd) at reflux under air. bIsolated yield based on iodobenzene.

Metal leaching of the catalyst was further investigated. The ICP-AES analysis the Pd loading of the CelMcPd0-1 recovered in Suzuki−Miyaura reactions after six cycles is 0.1032 mmol/g, which indicated that only a trace amount of the Pd metal is leached out from the cellulose capsules compared with the Pd content (0.1047 mmol/g) on freshly prepared CelMcPd0-1. Furthermore, the filtered solution exhibited no reactivity in both Suzuki−Miyaura and Mizoroki−Heck reactions. The TEM images of the catalyst showed that the morphology and size of the catalyst after six recycles had better uniform dispersion with the size of