Self-assembled spherical palladium/HMS nanocomposites and their catalytic application

Abstract

A novel palladium catalyst immobilised the mesoporous molecular sieve Hexagonal Mesoporous Silica (HMS) was prepared via a self-assembly process. The catalytic activity and recycle ability of the spherical palladium/HMS nanocomposites were examined for the Heck reaction. Palladium nanoparticles incorporated into HMS are characterised by X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). The morphology of HMS is predominated with spheres, and Pd nanoparticles are randomly distributed throughout the entire silica framework without severe agglomeration. The heterogeneous catalyst is proved to be an active and reusable catalyst in the coupling of ethyl acrylate with aryl halides because of combination, the advantages of stabilising agent and mesoporous silica support. Keywords: Pd nanoparticles; HMS; composites; Heck reactionSubject Classification Codes: TQ032.41; TQ138.2þ3; TQ426.81 IntroductionHeck reaction of aryl halide with olefins has recently been one of the most intensively studied transition-metal catalysed carbon-carbon bond-forming reactions and has been applied to many areas, including natural products [1,2] and fine chemicals [3]. Soluble palladiums, generally palladium complexes such as [Pd(OAc) 2 ] and [Pd(PPh 3 ) 2 Cl 2 ], are the most efficient catalysts for the Heck reaction because of the stabilisation effect between the ligands and metal nanoparticles [4,5]. However, the main drawback of the homogenous catalysts is the difficulty of recycling and reusing the expensive metals and ligands. Thus, it is not surprising that very few homogeneous catalytic systems have been commercialised. Immobilisation of transition metals will not only facilitate separation and reuse of the expensive and toxic catalysts but can also reduce contamination of the heavy metal to the environment.In the past few decades, a variety of heterogeneous palladium catalysts have been developed through the immobilisation of transition metals on various solid supports such as g-alumina, carbon, zirconia, zeolite, polymer, mesoporous silica and so on by using impregnation or ionexchange methods [6][7][8][9][10][11][12][13]. However, these approaches have not provided a simple way to form robust and reusable catalysts for heterogeneous catalysis. Thus, if approach can combine the advantage of stabilising agents with insoluble supports especially mesoporous silica, it would improve the lifetime and the reusability of catalysts.