The synthesis, characterization and catalytic applications of mesocellular silica foams

Abstract

Despite recent progress in materials synthesis, there are few materials with narrow pore size distributions and well-defined pore structures in the size range of 100 - 500 A. Materials with pores of 100 - 500 A possess a potential combination of high surface areas and pore sizes accommodating of large molecules that could make them of great use in the catalysis and separation of complex molecules important to the fine chemicals and pharmaceuticals industries. We have synthesized mesocellular silica foams (MCF) by using triblock copolymer (PEO-PPO-PEO, Pluronic) micelles to template the formation of silica from tetraethoxysilane precursors. The combination of the polymer amphiphile with a hydrophobic swelling agent (such as trimethylbenzene) resulted in larger pore sizes and more open pore structures than had previously been obtained with surfactant templates. The MCF materials have pore sizes of up to 350A, void fractions of > 0.85 and surface areas of - 700 m2/g. The basic properties of the swollen triblock copolymer micelles were investigated to provide a better understanding of their templating behavior. Small-angle neutron scattering (SANS) in conjunction with a thermodynamic model for swollen micelle formation were used to determine the size, shape and internal structure of the Pluronic micelles. Knowledge of the micelle structure and aggregation behavior was used to investigate the effects of changing polymer and oil types on the size of the swollen micelles and therefore to select the best systems for templating large pores in silica sol-gels.

(cont.) Synthesis experiments revealed that when tetraethoxysilane was added to micellar solutions containing only small amounts of oil, SBA-15 materials consisting of cylindrical pores packed in hexagonal arrays were formed. At an oil-to-polymer mass ratio of - 0.2, MCF (consisting of spherical cells connected by windows) was produced. We determined that the silica structures form as a result of the silica-induced precipitation of a polymer/silica rich phase. We have found two key factors in determining the pore size and structure of Pluronic-templated silica materials that were not previously well understood. First, there must be sufficient silica present in solution to precipitate the polymer aggregates. Second, it is the equilibrium structure of the concentrated precipitate, not the original solution, that determines the pore structure of the final material. To demonstrate the utility of the MCF materials, we have used MCF as a catalyst support for two different reactions: the Heck reaction and asymmetric hydrogenation. Palladium metal clusters were vapor-grafted onto MCF and several other large-pore silicas to generate active Heck catalysts. The activites of the "Pd-TMS" catalysts were equal to those of the best homogeneous organometallic catalysts reported in the literature. At 160ʻC, the activity of the MCF-supported catalyst was also better than that of catalysts supported on other mesoporous silicas. The high activity of the MCF-supported catalyst was attributed to its large pores and open pore structure, which reduced pore diffusion limitations on the reaction rate ...