Supported Pd-Complex on a Nanostructure Ordered Mesoporous Silica SBA-15: An Efficient Catalyst for Heck Reaction

The palladium-catalyzed arylation of olefins, universally referred to as the Heck reaction, has received increasing attention in the past decade because it is a selective method for the formation of new C-C bonds in a single operational step [1]. However, as with all homogeneous catalysts, there are separation, regeneration and reuse problems. These problems are of environmental and economic concern in large scale syntheses where large volumes of hazardous wastes and high production costs are of great importance. These problems can be minimized by carrying out the reactions using heterogeneous catalysts which can be more easily separated from the reaction mixture [2].The heterogenisation of existing homogeneous palladium catalysts in a way that maintains the advantages of the homogeneous catalyst (activity and selectivity) is an important challenge. In recent years, the development of such catalysts has attracted significant interest.Nanoparticles and stabilized nanoobjects are promising candidates for applications in catalysis and nanotechnology. An important issue when dealing with nanoparticles is to achieve protection of the particles against aggregation without disturbing the desired properties [3].From commercially available Pd/C, Pd was found to leach into solution and to perform homogeneous catalysis. Interestingly, almost all dissolved Pd0 species can readsorb onto the surface of the carbon support after completion of the reaction. Evidently, such a pseudo-heterogeneous catalyst would fail in a continuous reactor. Ordered mesoporous structures (such as MCM-41 and SBA-15 [4] ) with regular channel structures and pore diameters in the range of 2 to 30 nm, their easy separation from the reaction mixtures, and their relatively high surface area, would seem to be ideal for forming a scaffold in which three-dimensional dispersions of metal nanoparticles could be supported. Furthermore, because the majority of the nanoparticles are usually formed inside the channels of ordered porous materials, the support prevents agglomeration of metal specious.