Synthesis of Large-Pore Mesostructured Micelle-Templated Silicas as Discrete Spheres

Abstract

The synthesis of large-pore micelle-templated silica (MTS) materials as discrete spheres by pseudomorphic synthesis solves the double challenge: enlarging the pore size of MTS beyond the diameter of the micelles and controlling the morphology and the size of the particles. The pseudomorphic transformation of silica beads of the desired size into MTS is one of the more suitable routes to control both nanometric and micrometric scales of MTS synthesis. The most employed method to enlarge pore diameter of MTS synthesized from alkyltrimethylammonium surfactants is to use 1,3,5-trimethylbenzene (TMB) as a swelling agent in the micelles. Unfortunately this method leads to particle aggregation. By means of the pseudomorphic route combining an original set of chemical compositions (C 18 TAB/decane/ TMB/NaI), under mild conditions (low temperature, without autoclaving), we successfully synthesized nonaggregated MTS beads of 10 µm with pore diameter ranging from 7 to 9 nm, 900 m 2 /g specific surface area, and 1.5 mL/g pore volume. To accommodate the large pore volume developed by largepore MTS, highly porous silica beads of 10 µm and 300 m 2 /g specific surface area have previously been synthesized by means of an emulsion/polymerization procedure. The control at nanometric scale, by means of surfactants, leads to improved textural properties compared to optimized silica gels, whereas the control of the micrometric morphology (discrete spheres) makes these supports suitable for column packing and chromatographic evaluation.

Spherical ordered mesoporous silicas and silica monoliths as stationary phases for liquid chromatography

“…We have introduced this term in the field of mesostructured materials to define the transformation of amorphous silica beads into silica-surfactant mesostructures with preservation of the shape and size of the beads [22][23][24]. The transformation is a dissolution/reprecipitation process taking place inside the interparticle voids of the silica beads.…”

“…The process occurs at a nonconstant matter content, as the final pseudomorph contains approximately 70% of the silica engaged in the synthesis, the remaining fraction being dissolved during the process. Parameters such as pH, temperature, synthesis time, amounts of sodium hydroxide, water, TMB, and surfactant are all critical and should be adapted for each silica source [22][23][24].…”

“…To some extent these synthesis routes proved effective but particle coalescence could never be totally avoided and, in most cases, the products exhibited broad particle size distributions (PSD). We have introduced a new synthesis strategy in 2002, named pseudomorphic synthesis, which avoided these drawbacks and allowed an independent optimization of the processes of particle shaping and of mesophase selfassembly [22][23][24]. The pseudomorphic transformation, known in the mineral world for many years [25], has been succesfully applied to amorphous preshaped silica particles, traditionally used as chromatography supports (5-10 lm particle size), to produce MTS (MCM-41 and MCM-48) with the same morphology.…”

“…Gels were then autoclaved at the desired temperature (115-1508C) for varying times under static conditions. The materials were recovered by filtration, washed with water, oven dried at 808C for 24 h, and calcined at 5508C under air flow for 8 h. Details of the experimental procedures are given in [22][23][24]. All silica beads were ultrasonicated for 30 min in dioxane before being packed in column (5 mm id67 cm length) under 15 MPa pressure.…”

Pore-size expansion of hexagonal-structured nanocrystalline titania/CTAB Nanoskeleton using cosolvent organic molecules

“…[1-22] However, compared with the mesostructured silica materials such as MCM-41 materials prepared with cetyltrimethyammonium bromide (CH 3 (CH 2 ) 15 N + (CH 3 ) 3 Br -, CTAB) and poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymers, [23][24][25][26][27][28][29] the fabrication of large pore-sized mesostructured crystalline titania is still a big challenge because mesostructure of titania materials generally collapses during the crystallization by calcination. [10][11][12][13][14][15][16][17][18][19][20][21][22] In addition, preparation of mesostructured crystalline titania typically requires alcohol-based solvents, [10][11][12][13][14][15][16][17][18][19][20][21][22] so the replacement of many conventional alcohols as solvents used in the sol-gel reaction to the environmental benign alternatives is also an important challenge toward achieving the goal of green chemical processing.…”