Pd Supported On Mesoporous SBA-15: Preparation, Structure, And Catalytic Properties In Heck Reactions

Palladium-catalyzed Heck reactions have received considerable attention in recent years, as they offer a versatile method for the generation of new C-C bonds in organic synthesis. These reactions are normally carried out with the homogeneous palladium catalyst such as Pd(OAc)₂ and PdCl₂. These catalysts have high catalytic activity for Heck reactions; however, they suffer from severe problems related with the separation, recovery, and the instability of the homogeneous catalysts at high temperatures. These problems have so far precluded the wide industrial applications of Heck reactions. These problems can be solved, however, by the use of heterogeneous catalysts made up of supported-palladium catalysts. Among the supported-palladium catalysts, palladium supported on mesoporous silica is a more catalytically active, selective, and stable catalyst. At the same time, they can be easily recycled and have little causticity for equipments. For these reasons, palladium supported on mesoporous silica has always attacted great attention. SBA-15, a kind of mesoporous silica with large surface area and uniform hexagonal channels, has attracted considerable attention due to its potential applications in catalysis and adsorption. In this work, the preparations of well dispersed and isolated Pd nanoparticles in SBA-15 mesoporous silica are described with four different synthesis methods such as ion-exchange method, diamine functional method, two-step method, and in-situ formation method. The formation mechanism and structure characteristics of Pd nanoparticles encapsulated in SBA-15 as well as the relationship between catalysts structure and catalytic activity were investigated.First, Pd nanoparticles supported on mesoporous silica SBA-15 (or Pd/SBA-15 nanocomposites) were prepared by traditional ion-exchange method with cationic Pd precursor in an alkaline solution. The high Pd loading in these nanocomposites can be achieved up to 5.21 wt% by adjusting the pH value of the solution. Using this method higher than 65% of the Pd precursor in the solution was incorporated into the mesoporous SBA-15. Therefore, this method is simple and highly effective for the preparation of Pd nanoparticles encapsulated in SBA-15. The Pd/SBA-15 nanocomposites exhibit excellent catalytic activities for the Heck reactions of actived aryl halide in air. The resulting trans-isomers were obtained in high yield at 1-3 h. The catalyst is stable and can be recycled for many times. Therefore, the catalyst is highly effective and would have a potential in industrial applications. Pd supported on SBA-15 (or Pd-SBA-15) with high Pd loading and high Pd dispersion was prepared by functionalizing SBA-15 with [3-(2-aminoethyl aminopropyl)] trimethoxysilane, grafting palladium ions on the functionalized SBA-15, and reducing palladium ions in the functionalized SBA-15 with hydrazine hydrate. The Pd-SBA-15 nanocomposite has a Pd loading of the 4.30 wt% and a Pd dispersion of 35%. The test of catalytic activity and Pd leaching as a function of N to Pd molar ratio shows the optimum N/Pd molar ratio is 2:1. The catalyst with controlled molar ratio of amino groups to palladium provides excellent catalytic activity for Heck reactions not only for activated aryl halides but also for non-activated aryl halide in air. The recycling reaction shows that the catalyst can be reused many times by simple filtration. The catalyst shows low Pd leaching and a little decrease in metal dispersion (the Pd dispersion of 28% for the used catalyst recovered from the seventh run) during the reaction which cause the little decrease in its catalytic activity during the recycling test.Pd/SBA-15 was prepared by a two-step method using the Pd nanoparticle-copolymer unit as a template. Pd nanoparticles of 6-10 nm in size have been synthesized by formalin reduction and incorporated into mesoporous SBA-15 silica during hydrothermal synthesis. The characterizations of the Pd nanoparticles encapsulated in mesoporous silica (Pd/SBA-15) reveal that the Pd nanoparticles in the range of 6-10 nm are encapsulated within the surfactant micelles during mesoporous silica formation and well dispersed within the mesoporous SBA-15 channels. The catalytic activity of Pd/SBA-15 was investigated in Heck coupling reactions with activated and non-activated aryl substrates. Although the Pd dispersion of Pd/SBA-15 is low (17%), its surface area and pore volume are high; they also show relatvley high catalytic activity in Heck reactions. The Pd/SBA-15 composites exhibit an excellent catalytic activity for activated iodobenzene. Moreover, they also show high activity for non-activated bromobenzene when more catalyst is used. The mechanism of the heterogeneous Heck reactions was investigated; the catalyst follows a heterogeneous pathway.Palladium-containing SBA-15 (SBA-15-Pd) was synthesized via an in-situ formation approach. In this procedure, hydrophobic Pd(acac)2 chloroform solution was solubilized in the triblock copolymer micelles. Then the prehydrolysised TEOS was mixed with the surfactant solution. After the hydrothermal reaction, the as-synthesized sample was filtrated, dried, and calcinated. The resulting sample (SBA-15-Pd) was obtained by reduction with H₂ gas. A total of above 85% of the Pd precursor is incorporated in the porous host matrix. A 1.46 wt% metal loading is achieved without the loss of pore ordering, while sample with 3.02 wt% loading shows a less ordered structure. Highly dispersed and uniform Pd nanoparticles are confined in the hexagonal channels and no outside large particles are found by the characterizations of SBA-15-Pd (1.46 wt% metal loading). The SBA-15-Pd nanocomposites exhibit an excellent catalytic activity not only for actived aryl halide but also for non-actived bromo-benzene using less catalyst. They also show high reuse ability in air for the Heck reactions.Comparison with the structure and catalytic activity of all the prepred catalysts, we find there are many factors to affect the catalytic activity of the catalysts such as Pd dispersion, the pore volume and surface area of catalysts.