Synthesis of Bimodal Nanostructured Silicas with Independently Controlled Small and Large Mesopore Sizes

Abstract

We report on the synthesis of bimodal, structured silicas by a fully chemical, templating−scaffolding route that enables independent control over small and large mesopore sizes. Mesoporous MCM-41 nanoparticles, synthesized in a first step, are cross-linked in a second step, leading to a bimodal porous material. Triblock copolymer surfactants added during the second synthesis step form assemblies, around which the nanoparticles aggregate and cross-link, considerably influencing both the average and the width of the secondary pore size distribution, without affecting the primary pore size. By changing the conditions of the second step, one can easily synthesize a broad variety of silicas with a controlled bimodal nanopore size distribution. Textural and structural properties were characterized by X-ray diffraction, high-resolution transmission electron microscopy, scanning electron microscopy, 29Si solid-state NMR spectroscopy, N2 adsorption and desorption, and thermogravimetric analysis.

Normal and anomalous diffusion of non-interacting particles in linear nanopores

“…Recent progress in synthesising nanostructured porous materials [10,12,13] has provided essentially unlimited options for the generation of purpose-tailored pore architectures. Therefore, there is an increasing demand for clarification of the main features of molecular transport in such systems [14,15].…”

“…In addition to micropores or narrow mesopores (nanopores) as the location of chemical separation and conversion, such materials are traversed by a network of large mesopores or macropores. These transport pores ensure fast exchange of the molecular species involved with the surrounding fluid, and thus reduce the impairing influence of the finite rate of molecular transport on the overall performance [13]. Mesopores give rise to a number of most attractive phenomena which are typical of mesoscopic systems.…”

Catalysing sustainable fuel and chemical synthesis

“…Simulations demonstrate that in the Knudsen diffusion regime [89], where reactants/products are able to diffuse enter/exit mesopores but experience moderate diffusion limitations, hierarchical pore structures may significantly improve catalyst activity. Materials with interpenetrating, bimodal meso-macropore networks have been prepared using microemulsion [90] or co-surfactant [91] templating routes and are particularly attractive for liquid phase, flow reactors wherein rapid pore diffusion is required. Liquid crystalline (soft) and colloidal polystyrene nanospheres (hard) templating methods have been combined to create highly organised, macro-mesoporous aluminas [92] and 'SBA-15 like' silicas [93] (Scheme 4), in which both macro-and mesopore diameters can be independently tuned over the range 200-500 and 5-20 nm, respectively.…”

Ordered Porous Nanomaterials: The Merit of Small

“…This motivated several research groups worldwide to dedicate outstanding research efforts towards the synthesis of novel mesoporous silicates by modifying the synthesis conditions such as pH, hydrothermal conditions, or the use of surfactants of very different natures (Figure 8). This gave rise to the discovery of a wide array of mesoporous material families, such as SBA [182,183], FDU [184][185][186], AMS [187,188], MSU [189,190], KIT [191,192], or TUD [193] to name just a few examples. This wide variety of materials, each one with their own characteristic structure and framework resulted from the numerous synthetic parameters that could be fine tuned, such as the reaction pH (ordered mesoporous materials have been synthesized both under acidic and basic conditions), the synthesis temperature, the hydrothermal synthesis conditions, the nature and concentration of template, and even the template removal process selected [194].…”