Colloids and Interface Science Communications 1 (2014) 47–49
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Electrochemically codeposited reduced graphene oxide and palladium nanoparticles: An efficient heterogeneous catalyst for Heck coupling reaction Suresh S. Shendage, Jayashree M. Nagarkar ⁎ Department of Chemistry, Institute of Chemical Technology, Matunga (E), Mumbai 400019, India
a r t i c l e
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Article history: Received 1 April 2014 Accepted 22 June 2014 Available online 14 July 2014
a b s t r a c t The catalytic activity of electrochemically codeposited reduced graphene oxide and palladium (ERGO-Pd) was examined for Heck coupling reactions. It showed excellent catalytic activity and stability for Heck coupling reaction. The prepared catalyst (ERGO-Pd) can be used up to the 5th cycle with negligible loss in activity. © 2014 Published by Elsevier Inc.
Keywords: Heterogeneous catalysis Electrochemical synthesis Palladium nanoparticles Reduced graphene oxide Heck coupling reaction Greener protocol Codeposition Recyclable catalyst Excellent activity High stability
Introduction In recent years carbon–carbon cross coupling reactions catalyzed by palladium is the most important practical approach in organic synthesis [1–4]. Cross coupling reaction of aryl halides with olefins catalyzed by palladium is known as Heck reaction. Nowadays, cross coupling reactions become an essential synthetic tool for the preparation of pharmaceutically active ingredients, fragrances, agrochemicals, and various advanced materials [5–7]. In the past, palladium complexes such as [Pd(OAc) 2] and [Pd(PPh3)2Cl2] have been employed as homogeneous catalysts in the Heck reaction [8]. The separation, recovery and instability of the catalysts at high temperature are the major drawbacks of these homogeneous catalysts; these drawbacks prohibit their industrial utilization [9]. One of the most convenient approaches to overcome these problems is to use heterogeneous catalysts [10]. Activity of palladium catalyst can be enhanced by supporting palladium on several solid supports such as zinc ferrite [11], nafion–graphene [12], activated carbon [13], polymer [14], graphene [15], and silica [16]. Recently, graphene
⁎ Corresponding author. Tel.: +91 22 3361 1111/2222; fax: +91 22 3361 1020. E-mail addresses:
[email protected] (S.S. Shendage),
[email protected] (J.M. Nagarkar).
http://dx.doi.org/10.1016/j.colcom.2014.06.008 2215-0382/© 2014 Published by Elsevier Inc.
has attracted remarkable consideration because of its unique thermal, mechanical, and electrical properties [17]. Graphene exhibits high efficiency in many technological fields such as sensors [18], nanocomposites [19], and batteries [20]. Moreover, it is an excellent support material for metal nanoparticles used in fuel cell reactions [21]. High specific surface area of graphene plays a vital role for deposition of catalytically active metal nanoparticles. Palladium nanoparticles have attracted huge attention of researchers due to their high catalytic activity. These nanoparticles find vast industrial applications in heterogeneous catalytic reactions [22,23]. A synthesis of metal nanoparticles on solid support by chemical reduction has various drawbacks such as incorporation of toxic reducing reagent into metal nanoparticles, and harsh reaction conditions. Electrochemical methods have several advantages over chemical reduction method for example; control the size of metal nanoparticles, room temperature operation, easy recovery of deposited material, and short reaction time. Earlier, we have reported electrochemical synthesis of reduced graphene oxide and palladium nanoparticles (ERGO-Pd) and its applications in Suzuki coupling reactions [24]. The ERGO-Pd composite showed excellent catalytic activity, stability, and reusability for Suzuki coupling reactions. To generalize the application and the evaluation of the catalytic activity of the prepared ERGO-Pd composite was extended to other carbon–carbon cross coupling reaction such as Heck coupling reaction. Herein, we report for the first time the applications of ERGO-Pd
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S.S. Shendage, J.M. Nagarkar / Colloids and Interface Science Communications 1 (2014) 47–49 Table 1 ERGO-Pd (0.3 mol%) catalyzed Heck reaction of various aryl halides with olefinsa. Time (h)
Yield (%)b
1
0.75
96
Scheme 1. Heck coupling reactions catalyzed by Pd-ERGO.
2
2.0
87
composite as heterogeneous, an environmentally benign and a recyclable catalyst for Heck coupling reaction (Scheme 1).
3
2.0
92
4
1.75
91
5
0.75
92
6
0.75
93
7
1.0
89
8
2.0
91
9
4.0
70
10
4.0
88
11
4.0
91
12
2.0
94
13
2.5
92
14
4.0
88
15
4.5
82
16
3.0
84
Entry
Aryl halide
Alkenes
Product(s)
Material and methods All chemicals were obtained from commercial sources and used as received. Electrochemical co-deposition of reduced graphene oxide and palladium nanoparticles (ERGO-Pd) was carried out by reported method [24] (Supporting information).
General procedure for Heck coupling reactions Aryl halide (1.0 mmol), ethylacrylate or styrene (1.2 mmol), 1.5 mmol Et3N and 2 mg of ERGO-Pd catalyst were added in a 3 mL DMF and the reaction was carried out in a sealed tube heated in oil bath at 120 °C for 0.75–4.5 h. The progress of the reaction was monitored by Gas Chromatography (GC). The reaction mixture was cooled to room temperature and the catalyst was separated by centrifugation. The catalyst was then washed with ethanol, dried and preserved for next run. The pure products were obtained by column chromatography using hexane: ethyl acetate as the eluent. The preserved catalyst was reused in a subsequent run for recyclability study. The conversion of reactant was determined by Gas chromatography (GC). The products were characterized by mass and 1HNMR.
Result and discussion The reaction conditions for Heck reactions were optimized with respect to solvent and base. The excellent yields of products were obtained in DMF as a solvent (Table 1, entries 1–4, Supporting information). Among the different organic and inorganic bases, the complete conversion of iodobenzene was observed in presence of Et3N as a base (Table 1, entries 4–9, Supporting information). Considering the above optimized reaction conditions for Heck reaction, the aryl halides were coupled with various acrylate and styrene to get substituted alkenes (Scheme 1 and Table 1, entries 1–16). The cross coupling between aryl halide and styrene required longer reaction time than acrylates. The results show that aryl halides with either electron withdrawing or electron-donating substituents react with olefins rapidly forming the corresponding products with good to excellent yields (Table 1, entries 3, 4, 6, 7 and 10–16). Aryl bromides are difficult to be activated because of the reasonably high bond energy of the C\Br bond. As a result, the carbon–carbon coupling reactions of aryl bromides with different olefins required extended reaction times (Table 1, entries 2–4 and 9–11). ERGO-Pd shows relatively high catalytic activity in Heck reactions. We attributed this to the high dispersion of Pd NPs on ERGO support and absence of large Pd clusters (Fig. 4, Supporting information). We have also assessed recyclability of the catalyst (supporting information). The prepared catalyst can be recycled up to the fifth cycle without much loss in activity. Advantages of the ERGO-Pd catalyst were that the synthesis is very simple, it did not need complicated apparatus, the catalytic reactions could be easily carried out by conventional method (heating in a sealed tube), and the separation of this heterogeneous catalyst could be achieved easily by centrifugation. The catalyst could be almost fully recovered after the reaction.
a Reaction condition: Aryl halide (1.0 mmol), olefins (1.2 mmol), Et3N (1.5 mmol), DMF (3 mL), 120 °C, catalyst ERGO-Pd (2 mg). b Isolated yield.
Conclusion In conclusion, the catalyst (ERGO-Pd) exhibits excellent catalytic activity for the carbon–carbon cross coupling Heck reaction. High stability, easy removal from the reaction mixture and reusability of the catalyst up to five consecutive cycles with minimal loss of activity are the major advantages of the catalyst. Moreover, the catalyst ERGO-Pd was prepared by greener route by electrochemical deposition (neutral medium) method without using capping and stabilizing agents.
Acknowledgments The authors are thankful to the UGC New Delhi (India) for the award of Teacher Fellowships (FIP). We gratefully acknowledge SAIF IIT, Bombay for the TEM analysis.
S.S. Shendage, J.M. Nagarkar / Colloids and Interface Science Communications 1 (2014) 47–49
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