Synthesis of Mesoporous Silica with Large Pores and Bimodal Pore Size Distribution by Templating of Polymer Latices

Abstract

Mesoporous inorganic oxides with pore sizes between 2 nm and 50 nm are a welcome supplement to crystalline zeolites (pore size up to 1.2 nm). Mesoporous catalysts are required when one reactant is too big or too hydrophobic to penetrate the zeolite pores. Classical techniques for the synthesis of mesoporous silicas employ low molecular weight ionic amphiphiles as structure-directing additives and, if necessary, inert oils as swelling agents. [1±3] These processes, however, have some disadvantages. On one hand, structure control turns out to be difficult because the surfactant assembly undergoes phase transitions. [4] Furthermore, the inorganic nanostructure at pore sizes exceeding 5 nm is mechanically unstable and collapses, as the wall thicknesses are not sufficient to support the 3D inorganic network. On the other hand, the majority of the pores are either undefined with respect to size or not accessible from the outside, [5] which complicates functionalization of the material by incorporation of other elements or organic substituents.Another motive behind the search for new routes to mesoporous materials is that some practical applications require pores that are even larger than those available by precipitation techniques, i.e., pores of up to 100 nm are frequently demanded.A considerable improvement in the sol-gel synthesis of mesoporous silica was accomplished via a templating procedure involving liquid-crystalline (LC) phases of low molecular weight amphiphiles, where the problems described above could be solved. [6,7] Here, polyreaction of the watersoluble monomeric, inorganic precursor (e.g., silicic acid) is restricted to the hydrophilic domains of the LC mesophase, and condensation of the inorganic material results in a monolithic cast or blueprint of the LC structure. This procedure was further improved with respect to pore size and mechanical performance of the materials by use of amphiphilic block copolymers as templates. [8] In contrast to the systems described first, the products of the LC approach are not necessarily powders, but can be obtained as macroscopic monoliths or porous coatings. A more comprehensive discussion of the different pathways towards mesoporous materials and the abilities and advantages, including relevant references, is to be found elsewhere. [9] Here, we present a novel variation of the template route, which extends the existing methods to larger pore sizes. In this approach, polymer latex particles in the size range 20 nm < d < 400 nm, a narrow particle distribution, with different surface functionalities are employed as templates. These templates are readily available, inexpensive, and can be removed after inorganic network formation by simple calcination, i.e., burning off the organic matter at 350 C.A crucial point in this special templating process is the compatibility of the template with the generated silicic network. For classical non-ionic surfactant templates, this coupling is achieved via hydrogen bonds between the silanol groups of the silica and the oxyge...

Characterizing the Core-Shell Architecture of Block Copolymer Nanoparticles with Electron Microscopy: A Multi-Technique Approach

“…RuO 4 is a strong scattering agent that reacts with polymer regions inside the particle, thus improving the contrast [40]. Because of its high oxidative properties, RuO 4 is presumed to stain both PHEA and PS domains by reacting, respectively, with hydroxyl groups and aromatic unsaturations [31,41,42]. Four different structural features can be highlighted: Contrast: Regardless of the sample, the action of the stain considerably improved the image contrast against the support film.…”

Effects of preparation parameters on the morphology of chitosan‐silica hybrid hollow nanospheres

“…Currently, a core‐template‐based strategy has proven to be successful in fabricating such hollow spherical structures. Various templates, including block copolymers (Mössmer et al , 2000; Wong et al , 2002), polymer latex particles (Antonietti et al , 1998; Caruso et al , 1998; Chen et al , 2005) and polymeric micelle (Khanal et al , 2007) have been widely used to sustain inorganic materials with excellent properties. While versatile and flexible, the template core must be removed by calcination at high temperature or other appropriate technology to create a void volume inside the inorganic shell.…”

Simple preparation of silica and alumina with a hierarchical pore system via the dual-templating method

“…Various methods for synthesizing hierarchical structures have been proposed [2][3][4][5][6][7][8][9][10]. In many conventional studies, hierarchical porous systems were formed in two steps: the formation of a template containing closely packed colloidal crystals followed by the penetration of a surfactant-based sol-gel precursor [4][5][6][7][8].…”

“…In many conventional studies, hierarchical porous systems were formed in two steps: the formation of a template containing closely packed colloidal crystals followed by the penetration of a surfactant-based sol-gel precursor [4][5][6][7][8]. One-step synthesis has also been conducted; however, a special surfactant ('KLE' and an ionic liquid) [9] or surface-functionalized polymer spheres [10] are required in that case. Furthermore, the conventional strategies do not necessarily result in the desired multimodal structure owing to the formation of an amorphous silica region [6,7] and/or a lack of interconnectivity among the poorly packed macropores [10].…”

“…One-step synthesis has also been conducted; however, a special surfactant ('KLE' and an ionic liquid) [9] or surface-functionalized polymer spheres [10] are required in that case. Furthermore, the conventional strategies do not necessarily result in the desired multimodal structure owing to the formation of an amorphous silica region [6,7] and/or a lack of interconnectivity among the poorly packed macropores [10]. Very recently, we reported the simple preparation of hierarchical porous silica spheres through the aerosol-assisted multicomponent assembly of both polymer latex spheres and surfactants [11].…”