## 1 Introduction  
Transition metal-facilitated cross-couplings, specifically Suzuki-Miyaura, Sonogashira, Heck, Hiyama, Stille, Negishi and Kumada reactions, are significant tools for C-C bond formation [1, 2, 3, 4, 5, 6, 7]. The significance of carbon-carbon bonds in establishing reactions is heavily acknowledged in various fields such as natural and biological products, polymers and particularly in catalysis [8, 9]. The industrial significance of Suzuki and Heck couplings, along with several others, has particularly generated considerable advancement in the progress of C-C bond reactions. Suzuki-Miyaura couplings have been especially widely exploited in recent years [10; 11], which potentially offer quick entry to biaryl frameworks via the sp2-sp2 connection [12]. Suzuki-Miyaura and Mizoroki-Heck cross-coupling reactions are extensively applied, owing to their excellent properties such as mild reaction conditions, easy availability of precursors, good stability under aerobic conditions and the use of nonhazardous boronic acids as precursors. So far, reports on the use of several palladium-, nickel- and gold-based compounds as catalysts for cross-coupling reactions have been published [13; 14]. Among the various transition metals, Suzuki-Miyaura reactions are usually catalyzed by palladium-based homogenous catalysts that have shown high catalytic activity. However, under homogeneous conditions, catalysts require hazardous ligands like phosphine or other N-heterocyclic compounds [15], and moreover, separating the catalysts after the reaction is cumbersome process. This not only causes contamination of the product but also severely affects the recovery of the catalysts [16; 17; 18]. The separation of leached catalysts from the product solution requires the application of costly filtration membranes or other chromatographic techniques. Therefore, tremendous efforts have been made over the years to develop heterogeneous processes with the aim of replacing homogenous systems. Recently, the advancement of nanotechnology has led to the development of highly efficient palladium nanoparticle (Pd NP)-based heterogeneous catalysts [19; 20]. These catalysts possess excellent catalytic activity due to their remarkable physicochemical properties (high surface area) and are successfully used as replacements for the conventional organometallic-based compounds for Suzuki-Miyaura coupling reactions [10; 21; 22; 23; 24]. Currently, 'Pd' is considered the most exciting heterogeneous catalyst in coupling reactions due to its size, shape and other catalytic activities [25; 26; 27]. Among various reactions, the palladium-catalyzed coupling reactions involving aryl halides and phenyl/aryl boronic acids are considered the most beneficial approaches for the development of biaryl units, which are highly useful as intermediates in numerous pharmaceuticals and polymers like electroluminescence materials [28]. However, in many cases, the application of Pd NPs has been seriously restricted due to the inefficiency of separation methods like centrifugation and filtration for the recovery of whole nanoparticle-based catalysts [25]. Additionally, NPs are also susceptible to agglomeration under reaction conditions. Therefore, to overcome these issues, supported Pd NPs are preferred for C-C cross coupling reactions. Thus far, significant efforts have been made toward improving a variety of novel and effective palladium-based supported catalysts for Suzuki-Miyaura coupling reactions [29]. Recently, the synthesis of Pd-based graphene nanocomposites for Suzuki-Miyaura coupling reactions have successfully reported [10; 28; 30; 31; 32]. Besides that, carbon nanotubes have also been developed as competent supports for palladium and applied extensively as catalysts for C-C cross-coupling reactions [33; 34; 35]. The carbon nanotubes can be uniformly dispersed in a solution, owing to their small size, thus enhancing the interactions among the reactants and the catalyst [33].