Morphological Control of MCM-41 by Pseudomorphic Synthesis

Abstract

The development of micelle-templated silicas (MTS) has represented one of the most original fields of materials research since the seminal papers from the Kresge and Beck groups on MCM-41 and MCM-48. [1,2] The self-assembly of surfactant aggregates and mineral species can be controlled to provide stable mesoporous materials with extremely narrow pore-size distributions. Several recent reviews show the advances in the preparation of ordered porous oxides, [3±5] as well as their applications in catalysis. [6±8] Adsorbents with narrow pore-size distribution at the nanometer scale allow new applications to be devised for the separation of large organic molecules. MCM-41 silicas have been proposed as possible stationary phases for size-exclusion chromatography, [9] normal-phase HPLC, [10] capillary gas chromatography, [11] and enantioselective HPLC. [12,13] The control of the size and shape of the adsorbent particles is an essential condition for any chromatographic application: particle-size scattering affects separation and plate height. Indeed, the preparation of spheres of MTS with predetermined monodispersed size has been the target of several research groups. Positive results have been obtained by introducing surfactant templates in classical preparations of silica gel with controlled grain size. In this way, spheres of MTS have been prepared from water ± alcohol systems, [14,15] by controlled hydrolysis, [16,17] or by spray-drying techniques. [18] A frequent drawback of these methods is the need to simultaneously optimize the conditions for the synthesis of the desired silica ± surfactant mesophase and for the successful formation of monodispersed spheres. This situation restrains the experimental conditions and makes a fine tuning of the properties of MTS, such as, pore size and topology, wall thickness, and aluminum content, difficult. It would be expedient to independently optimize the properties of the particles and the properties of the micelle-templated phase. Herein, a method to achieve this result by transformation of preformed spheres of silica gel into MTS is proposed.The synthesis procedure is directly adapted from the synthesis of MCM-41, [1,2] by using commercial spheres of silica gel as the source of silica. Lichrosphere 100 (Merck) was stirred in an alkaline solution of cetyltrimethylammonium bromide (CTAB), the molar composition of the system being 1 SiO 2 /0.25 NaOH/0.1 CTAB/20 H 2 O. After 30 min stirring at room temperature, the system was put in an autoclave at 388 K for 24 h. The parent silica (Lichrosphere 100) and the recovered solid share the same spherical morphology and granulometric distribution (Figure 1). However, while the parent silica is amorphous, the CTAB-treated solid presents the characteristic X-ray powder diffraction pattern of MCM-41. Figure 1. Pseudomorphic transformation of silica gel to MCM-41. Micrographs (a, b), granulometric distributions; the distribution is given in volume V as a function of particle diameter D p (c, d), and powder diffraction patterns (e, f) fo...

Sol‐Gel and Porous Glass‐Based Silica Monoliths with Hierarchical Pore Structure for Solid‐Liquid Catalysis

“…Application of this porogen simultaneously results in an interconnected network of ordered mesopores in the silica skeleton. A comparable effect is achieved by the so-called pseudomorphic transformation of the silica skeleton into MCM-41/-48 type systems [72]. Very recently, this idea has been extended to zeolites (LTA type) [73].…”

“…12) and therefore the synthesis of hierarchical silica monoliths with interconnected structures of sub-micrometer sized macropores and silica skeleton. The method of the pseudomorphic transformation has been established in 2002 [72]. Amorphous silica materials including several mesoporous silica gels and mesopore-containing silica skeleton of hierarchically structured Nakanishi monoliths were successfully transformed into ordered mesoporous materials of MCM-41 or MCM-48 type.…”

Recent advances in the synthesis of hierarchically porous silica materials on the basis of porous glasses

“…37 While the transformation of porous glasses into zeolitic materials has already been examined in the 1990s by Schwieger et al, 38 the pseudomorphic transformation of porous glasses into micellar templated silica (MTS) shapes has only recently been facilitated, 16,39 although other amorphous silica materials like silica gels have already been transformed into MTS materials since the pioneering studies of Galarneau and Fajula in 2002. [40][41][42] Such MTS particles were used as a column material for HPLC, for the entrapment of drugs or as catalyst supports. 43 Further applications include enzyme binding and sensor technology.…”

“…22 Schematic illustration of the sponge-like pore system of a porous glass body (left) and the hierarchical pore system obtained via pseudomorphic transformation of the glass pore walls into the porous material (right). (cetyltrimethylammonium bromide) combined with NaOH, 16 which follows the procedure established by Galarneau et al 40 The second approach uses a solution of the surfactant CTAOH (cetyltrimethylammonium hydroxide), which is obtained via ion exchange of CTAB and which eliminates the need for NaOH during the transformation. 39 For the transformation of porous glasses into MCM-41 the use of CTAOH has proven to be advantageous.…”

Materiales nanoestructurados biocompatibles basados en sílice. Preparación y aplicaciones en terapia anticancerígena.

“…Por ejemplo, para la obtención de MCM-41 a partir de partículas de sílice amorfa mediante tratamiento básico en presencia de bromuro de cetiltrimetilamonio (CTAB). En esta síntesis el material MCM-41 se forma debido a la disolución de las partículas de sílice amorfa, que actúan como fuente de silicio para formar la nueva fase [126]. También se ha obtenido el material MCM-48 empleando esta técnica a través de diferentes transiciones de fase [127].…”