Carbon-Based Nanocomposites as Heterogeneous Catalysts
4.2 Carbon-Based Nanocomposites for Coupling Reactions
4.2.1 C-C Coupling
Carbon-carbon (C-C) coupling reactions, such as Ullmann, Suzuki, and Heck reactions, have profound importance for applications in chemical, pharmaceutical, and biochemical industries [24–27]. Coupling reactions were earlier preferred to be performed using homogeneous catalysis. But, it was difficult to achieve high catalytic activity and selectivity of products.
Also, the recyclability and the potential contamination of the products were also challenging tasks. Hence, significant efforts have been devoted in
order to address this issue, which includes the use of carbon support-based heterogeneous catalysts. For example, Sun and coworkers have synthesized a nanocomposite in which Pd nanoparticles were supported on 2D sheet of graphitic carbon nitride (g-C3N4), and further, the prepared nanocom- posite was utilized for the well-known Suzuki-Miyaura reaction which involves the C-C coupling between an organohalide and boronic acid. The nanocomposite was successfully synthesized via using a facile one-step photo-deposition method. The catalyst exhibited high performance for Suzuki-Miyaura coupling reactions at room temperature in environmen- tally benign solvent system without any use of phase transfer agents or toxic solvents achieving Green catalysis. The high catalytic activity of the nanocomposite was attributed to the specific characteristics of the unique nanostructure of Pd/g-C3N4. The porous g-C3N4 contains large number of nitrogen-containing sites, which helps to disperse and stabilize the supported Pd nanoparticles. The π-π stacking interaction between halo- benzene molecules and g-C3N4 helps in accelerating the reaction. With the help of the prepared nanocomposites, a wide substrate scope has been achieved, which demonstrated high catalytic activity resulting in higher product yields as shown in Table 4.1 [28].
In addition to g-C3N4, graphene/graphene oxide/reduced graphene oxide have also been widely explored as carbon-based support materials for C-C coupling reactions. These heterogeneous catalysts synthesized using graphene support are of low cost, exhibiting high recyclability and promote environmental friendly Green synthesis of desired products. Li and co-workers [29] reported a facile method to synthesize a nanocom- posite of graphene uniformly decorated with small sized Pd nanoparticles.
Generally, the C-C coupling reaction needs a mixture of organic solvent and an aqueous inorganic base and inert atmosphere. But, the synthesized nanohybrids (Pd–graphene hybrids) with controllable size of palladium nanoparticles were found to be very effective catalysts for the Suzuki cou- pling reaction in aqueous medium under aerobic conditions. The excellent activity of the Pd-graphene catalyst can be attributed to the stabilization of Pd nanoparticles on the 2D graphene sheets, which prevents aggregation of nanoparticles. Also, the smaller sized Pd nanoparticles decorated on graphene sheets were found to be catalytically more active than that with the bigger ones due to the availability of large number of catalytic sites.
Further, in another study in situ reduction of Pd precursor was performed on graphene oxide (GO) and chemically derived graphene (CDG) and the prepared nanocomposite has been utilized for the Suzuki-Miyaura reac- tion in ethanol-water solvent system. The catalyst showed extraordinarily high activity with turnover frequencies (TOFs) of up to 39,000 h−1 [30].
Table 4.1 Effect of aryl halides on the catalytic conversions in Suzuki-Miyaura coupling reaction performed using Pd/g-C3N4 nanocomposite as heterogeneous catalyst [28].
Br BOH
+ OH Pd/g-C3N4
KOH, 25ºC EtOH/H2O
Entry Aryl halide Product Conversion (%)
1 I 100.0
2 Cl 38.6
3 Br 93.6
4
Br
CH3 CH3 89.1
5 H3C Br H3C 98.1
6
Br
CHO CHO 89.4
7
Br
OHC OHC 95.3
8 OHC Br OHC 98.8
9 O2N Br O2N 98.2
10 HO Br HO 93.9
11 NC Br NC 100.0
12
Br
COOH COOH 90.7
Notes: Aryl halides (0.2 mmol, 1 equiv), Phenylboronic acid (0.24 mmol, 1.2 equiv), KOH (0.6 mmol, 3 equiv, 4 ml; EtOH(1:1) and Pd0.10/g-C3N4, 25oC, 30 min.
Another important coupling reaction is Heck coupling reaction. This reaction involves the Pd catalyzed substitution of vinylic hydrogen with a vinyl, aryl, or benzyl group. However, the reactions are generally con- ducted under harsh conditions (high temperature) using polar aprotic solvents like MeCN, DMSO, or DMAC, which are not environmentally benign. So, there was a need of an effective strategy which can accelerate the reaction under mild conditions using environment friendly solvents. In this regard, Elazab et al. [31] have reported a carbon support-based metal nanocomposite for Heck coupling reactions under mild reaction condi- tions using Green solvents. The highly active catalyst consists of Pd/Fe3O4 nanoparticles supported on graphene which was prepared by using micro- wave assisted chemical reduction method. Furthermore, a broad range of highly functionalized molecules were synthesized in high yields and are summarized in Table 4.2. The catalyst showed high recyclability up to 10 cycles without any major loss in its catalytic activity [31].
Moreover, there was a need of multifunctional catalyst which can sin- gle handedly do multiple types of C-C coupling reactions (Suzuki, Heck, Sonoghasira, and Ulmann) with high catalytic efficiency and selectivity in environmentally benign solvents. The use of carbon-based support has resolved this issue to an extent but still the high efficiency is a challenging aspect. However, the defect-based engineering and large number of cat- alytically active sites can improve the efficacy of the reactions. One such catalyst was designed by Moussa and coworkers [32] exhibiting high effi- ciency and turnover number (TON). Pd nanoparticles were supported on partially reduced graphene nanosheets (Pd/PRGO) by using pulsed laser irradiation. The laser irradiations of the PRGO lead to the formation of the defects on the surface of the catalyst which improves the catalyst-support interactions which consequently enhances the recyclability of the catalyst.
The Pd/PRGO catalyst can catalyze multiple coupling reactions, such as Suzuki, Heck, and Sonogashira under mild reaction conditions. Overall, the carbon support-based catalysts are highly efficient and selective and can be recycled multiple times without any loss in catalytic activity.