Article pubs.acs.org/IECR
Carboxymethylcellulose-Supported Palladium Nanoparticles Generated in Situ from Palladium(II) Carboxymethylcellulose: An Efficient and Reusable Catalyst for Suzuki−Miyaura and Mizoroki− Heck Reactions Jinlong Xiao, Zhangxiu Lu, and Yiqun Li* Department of Chemistry, Jinan University, Guangzhou, Guangdong 510632, P. R. China S Supporting Information *
ABSTRACT: A very easy method is described here for direct preparation of palladium(II) carboxymethylcellulose (CMC-PdII) by ion exchange of sodium carboxymethylcellulose (CMC-Na) and PdCl2. When the resulting CMC-PdII was employed as a catalyst in Suzuki−Miyaura and Mizoroki−Heck cross-coupling reactions, PdII was reduced in situ to Pd0 and further grown on CMC to afford carboxymethylcellulose-supported palladium nanoparticles (CMC-Pd0). The as-generated CMC-Pd0 proved to be an efficient catalyst for these mentioned cross-coupling reactions under mild aerobic conditions. The CMC-PdII and CMCPd0 were fully characterized by FT-IR, ICP-AES, XPS, XRD, SEM, EDX, TEM, and TGA. The characterized results demonstrated that the true catalytic species are Pd0 nanoparticles. Moreover, the catalyst showed no significant loss of efficiency after six catalytic cycles.
1. INTRODUCTION The catalysis by metal nanoparticles is of considerable current interest as these semiheterogeneous catalysts combining the characteristics of heterogeneous catalysis with those of homogeneous catalysis.1,2 In particular, the nanopalladiumcatalyzed Suzuki−Miyaura reactions and Mizoroki−Heck reactions, recognized by the award of the Nobel Prize in Chemistry in 2010, are widely useful methods in forming carbon−carbon bonds in synthetic chemistry,3−6 as well as in industrial applications.7 Palladium nanoparticles (PdNPs) catalysts are more promising, efficient, and practical compared to buck catalysts because they have high surface-to-volume ratios and their surface atoms are very active.1,8,9 Unfortunately, metal nanoparticles are unstable and easily aggregate and precipitate to bulk metal and, therefore, decrease the catalytic activity. Generally, the aggregation of nanoparticles can be avoided by using suitable stabilizers or protecting agents.10 Although homogeneous catalysts often have the merits of high reactivity, good selectivity, and mild reaction conditions; however, most of them suffer from problems such as separation, availability, tedious workup, and metal contamination in the products. To overcome these drawbacks, heterogeneous palladium catalysts have been attracted considerable interesting for a long time because the carriers can efficiently control the formation of metal nanoparticles and further prevent nanoparticle agglomeration, as well as combining the merits of both homogeneous and heterogeneous catalytic systems. Therefore, the development of an insoluble heterogeneous palladium catalyst has attracted much attention in organic synthesis.10 In recent years, a considerable number of works have been devoted to the development of reusable supported PdNPs catalysts, including immobilizing PdNPs on polymers or biopolymers,11−13 magnetic materials,14,15 silica-supported dendrimer,16,17 polymeric membrane,18,19 and so on. Among © 2014 American Chemical Society
the biopolymer-surpported palladium catalyst, up to date, only one article was reported by Cui13 et al. in 2008 describing the use of CMC-Pd0 as catalysts for Mizoroki−Heck reaction. In spite of its merits, this reported catalyst suffer some disadvantages such as (1) tedious procedure required the reduction of PdCl2 in absolute ethanol dispersed CMC (not CMC-Na) at 60 °C for 8 h, (2) low loading level of immobilized palladium, (3) characterization of FT-IR, SEM, EDX, and TEM, and so forth was not performed, (4) obvious leaching of Pd from the CMC supports during the reaction, (5) efficient to unchallenged high active aryl iodides substrates. Cellulose and its derivatives are widely used, inexpensive, biodegradable, and easily functionalized. These features make them a suitable polymeric support for heterogeneous catalysts.20 CMC-Na is the major commercial water-soluble derivative of cellulose and widely used in pharmaceuticals, cosmetics, and foods.21 CMC-Na carrying carboxymethyl groups (−CH2−COO− Na+) is capable of exchanging with metal cations. On the basis of this property, we prepared the palladium(II) carboxymethylcellulose (CMC-PdII) via direct ion-exchange of PdCl2 with CMC-Na in aqueous solution. More recently, it was reported that the reduction of Pd(II) catalyst precursor before the catalytic process was not needed because Pd(II) could be reduced to Pd(0) in situ forming nanoparticles in the catalytic reaction system.22−26 Furthermore, Pd(0) formed in situ exhibited higher catalytic activity than that obtained by reduction with reductant before reaction occurring. Received: Revised: Accepted: Published: 790
August 1, 2014 December 26, 2014 December 29, 2014 December 29, 2014 DOI: 10.1021/ie503075d Ind. Eng. Chem. Res. 2015, 54, 790−797
Industrial & Engineering Chemistry Research
Article
2.4. General Procedure for the Catalytic Mizoroki− Heck Reactions. The mixture of aryl halide (1 mmol), alkene (1.2 mmol), K3PO4 (2 mmol), CMC-PdII (0.9 mol % Pd), and aqueous 90% DMF (v/v%, 5 mL) were added into a flask and the mixture was stirred in a preheated oil bath at 110 °C for a specific time. The progress was monitored by TLC. Once completion of the reaction, the catalyst was filtered, washed with 95% ethanol (v/v%), and dried under vacuum for the next run. When a styrene was used as substrate, the filtrate was extracted by diethyl ether (3 × 5 mL), the organic layer was washed with brine (5 mL), separated, and dried over anhydrous Mg2SO4. The product was obtained after removing the ethereal solution under vacuum. The crude products were further purified by recrystallization with ethanol. When an acrylic acid was used as substrate, the filtrate was acidified with dilute HCl to precipitate the crude product, which was further purified by recrystallization with ethanol. All of the products are known compounds and their melting point, 1HNMR, and IR spectra data were identical to that reported in literature.
Inspired by these results, we further continued our previous work on cellulose-supported catalyst for the purpose of developing a new kind catalyst that meets the goals of simple preparation, high catalytic performance, easy recovery, and recycling. Herein, we report the preparation and characterization of the palladium(II) carboxymethylcellulose (CMCPdII), and further use it as a recyclable heterogeneous precatalyst for the Suzuki−Miyaura and Mizoroki−Heck reactions without need to preparation of PdNPs prior to use.
2. EXPERIMENTAL SECTION 2.1. Chemicals and Characterizations. Melting points were measured on an Electrothermal X6 microscopic digital melting point apparatus. FT-IR spectra were recorded on a Bruker Equinox-55 spectrometer as KBr pellets in the range 4000−400 cm−1. 1HNMR spectra were obtained with a 500 MHz Bruker Avance instrument with CDCl3 and DMSO-d6 as solvent and TMS as internal standard. HRMS data were recorded using AB SCIEX Triple TOF 5600+ detection. The elemental palladium content of polymeric catalysts was determined by PerkinElmer Optima 2000DV inductively coupled plasma-atomic emission spectrometry (ICP-AES). Xray photoelectron spectroscopy (XPS) measurements were performed on a Shimadzu Amicus X-ray photoelectron spectrometer with an Al Kα excitation. Scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) were performed with 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 MSALXRD2 diffractometer using Cu Kα radiation. Thermogravimetric analyses (TGA) were performed on a Netzsch STA449 F3 from 25 to 600 °C at a heating rate of 10 °C/min−1 in nitrogen flow. All chemicals were obtained from commercial sources and used as received. 2.2. Preparation of Palladium(II) Carboxymethylcellulose (CMC-PdII). The 1 wt % aqueous solution of sodium carboxymethylcellulose (12 mL) was slowly added dropwise to an aqueous solution containing 10 wt % PdCl2 (6 mL) using a pipet with constantly stirring at room temperature. The solid was precipitated immediately and further left equilibrate in solution for 2 h. The resulting solid was separated from the solution and washed thoroughly with distilled water, then dried in vacuum to constant weight to provide the CMC-PdII as brown powder. The Pd content was determined to be 3.83 mmol/g by ICP-AES. 2.3. General Procedure for the Catalytic Suzuki− Miyaura Reactions. A round-bottomed flask was charged with aryl halide (1 mmol), arylboronic compound (1.2 mmol), K2CO3 (2 mmol), CMC-PdII catalyst (0.6 mol % Pd), and aqueous 80% ethanol (v/v%, 5.0 mL). The reaction mixture was stirred and heated at reflux under air for a specific time. The progress was tracked by TLC analysis. After the completion of reaction, the catalyst was removed by filtration, washed with ethanol (3 × 5 mL) and dried under vacuum at 60 °C for the next run. The organic fractions were then concentrated on a rotary evaporator to give the crude product. The product was further purified by recrystallization with ethanol. The known products were identified by IR, 1H NMR, and physical data (melting point), which are in agreement with those reported in the literatures. The new products were characterized by mp, FT-IR, 1HNMR, 13CNMR, and HRMS.
3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of CMC-PdII and CMC−Pd0. Palladium(II) ion can be easily loaded onto CMC by stirring a solution of PdCl2 and CMC-Na at room temperature via metathetical reaction with PdII and CMC-Na. After separation by filter and drying under vacuum to constant weight, the CMC-PdII was provided as brown powder. The Pd loading of CMC-PdII was determined to be 3.83 mmol/g by ICP-AES. It should be pointed out here that the initially brown catalyst of CMC-PdII turned to dark after the first run (Figure 1, digital images), implying the formation of elemental Pd during the catalytic transformation.
Figure 1. FT-IR spectra of CMC-Na (a), CMC-Pd(II) (b), and CMCPd(0) (c) and their corresponding digital images.
The as-prepared CMC-PdII is readily reduced to CMC-Pd0 during the catalytic cycles in Suzuki−Miyaura and Mizoroki−Heck reactions. The fresh prepared precatalyst CMC-PdII and resulting CMC-Pd0 catalyst after reaction were characterized by ICP-AES, FT-IR, XPS, XRD, SEM, EDX, TEM, and TGA. The FTIR spectrum of CMC-Na characteristic absorption peaks at 1635 cm−1, and 1431 cm−1 are observed for carboxylate (−COO−) asymmetric and symmetric stretching vibration.27,28 The band at 1129 cm−1 is assigned to the ether bonds stretches. The asymmetric carboxylate absorb peak is negatively shifted to 1591 cm−1 in the CMC-PdII and shift to 791
DOI: 10.1021/ie503075d Ind. Eng. Chem. Res. 2015, 54, 790−797
Industrial & Engineering Chemistry Research
Article
1600 cm−1 in CMC-Pd0, implying coordination of −COO− with PdII, and Pd0, respectively (Figure 1). Complexation between a carboxylate group and a metal can adopt four patterns: monodentate chelating (I), bidentate chelating (II), bidentate bridging (III), and ionic interactions, where the first three patterns are shown in Figure 2.
However, in CMC-PdII, the three characteristic peaks of Pd0 are not observed in curve b. XPS was used to investigate the valence state of the surface region of the catalyst. As shown in Figure 4, the binding energies of Pd3d5/2 in the CMC-PdII and CMC-Pd0 are 338.2 and 337.4 eV, which are characteristic for PdII and Pd0 respectively, indicating the existence of two different chemical states of PdII and Pd0 (Figure 4a and c). These results indicated that the oxidation state of palladium in the freshly prepared catalyst was PdII, whereas in the recycled catalyst, it was Pd0. Lack of peak at 338.2 eV in the curve in Figure 4c confirmed the completely reduction of PdII. These were in good agreement with XRD analysis of pure crystal of PdNPs (Figure 3b and c). The binding energy of O1s in CMC-PdII and CMCPd0 are 534.6 and 531.9 eV, respectively, which is higher than that (531.0 eV)13 in CMC-Na, which indicates the coordination of PdII, and Pd0 are formed (Figure 4b, and d). Combining these results of O1s binding energy with those obtained from FT-IR, it can be speculated that carboxylate and hydroxyl groups on cellulose backbone captured PdNPs successfully via covalent bond and H-bond,28,29 as shown by the model in Scheme 1. The morphology of CMC-PdII and CMC−Pd0 was studied by SEM and TEM. Samples for SEM coated with gold were mounted rigidly on specimen holder. Samples for TEM were prepared by placing one droplet of the suspensions of CMCPdII dispersed in water, or CMC−Pd0 suspensions dispersed in ethanol, onto a copper grid. Additional EDX analysis of the sample was also carried out along with SEM. A clear change in morphology is observed after bonding Pd0 onto the polymer support (Figure 5a and b). The elemental composition of the as-synthesized CMC-PdII, and CMC-Pd0 were also determined via EDX, which is shown in Figure 5d and e. The presence of Pd is clearly indicated in the spectrum, together with other elements including carbon and oxygen. As a results, it is reasonable assume that CMC molecules can bond PdII as well as serve as a much more effective capping agent to stabilize PdNPs. The TEM image of the CMC−Pd0 catalyst shows that the average size of the PdNPs is in the range of 2−10 nm (Figure 6a). The TEM images of the catalyst showed that the morphology and size of the catalyst after six recycling in Suzuki−Miyaura reaction had slightly agglomeration with the average size in the range of 5−30 nm (Figure 6b). This means a dissolution/redeposition process of PdNPs is taking place during the reaction. As the Mizoroki−Heck reactions and Suzuki−Miyaura reactions usually require heating, TGA was performed to evaluate the thermal stability of the CMC supported catalyst. The TGA curves of the CMC-Na, CMC-PdII, and CMC-Pd0 are depicted in Figure 7. Both weight loss of CMC-PdII and CMC-Pd 0 occurs at around 180 °C. Although their decomposition temperature are below to CMC-Na (dec ∼280 °C), they also show good thermal stability in experimental conditions. The plausible “template approach” for the controlled generation of CMC-Pd0 nanoparticles was suggested (Scheme 1). First, the −COO− Na+ moieties of CMC-Na transformed to (−COO−)2PdII (CMC-PdII) by ion exchange; second, CMCPdII was reduced to elemental palladium and grown to palladium nanoparticles controlled by two available coordination sites of −COO− and −OH groups as well as polymer molecular backbone in the catalytic system.
Figure 2. Modes of metal−carboxylate complexation: (I) monodentate chelating, (II) bidentate chelating, and (III) bidentate bridging. Δ is the separation between the symmetric and asymmetric stretching vibration of the carboxylate group.
The wavenumber separation Δ between the asymmetric ν (COO−) and symmetric ν (COO−) stretches can be used to identify the type of the interactions between the −COO− groups and particle surfaces. Characteristically, the Δ value falls in the range of 200−320 cm−1 for monodentate, 140−190 cm−1 for bidentate bridging, and
