Small mesostructured silica

Over the last years the utilisation of supramolecular arrays of surfactant molecules as structure-directing templates [1] has been applied to the synthesis of numerous mesostructured aluminophosphates [2-11]. In most cases the preparations were carried out in aqueous systems under hydrothermal conditions, but tetraethylene glycol and/or unbranched primary alcohols were also used [2,4]. Several discussions have been made on the reaction mechanisms that are involved in the syntheses of mesostructured materials [1,12-15] and recently a number of in-situ investigations on the formation processes of mesostructured silica phases in aqueous media have been reported these studies employed small angle X-ray diffraction [16-19] as well as 2H, 13C, 29Si, and 8lBr NMR spectroscopy and polarised light optical microscopy [17]. 

Unlike zeolites, pure silica materials have no acidic sites and are of limited use for catalysis and other applications. In order to produce acidic mesostructured materials, aluminum has been successfully introduced into the framework of the mesoporous MCM-41 and can be detected by EDX. However, TEM indicated that the degree of order of the mesopores in MCM-41 was significantly reduced by incorporation of even a small percentage of A1 into the wall [84]. 

TEM micrographs of bimetallic catalysts revealed the presence of randomly accessible ordered domains as well as the partly disordered mesostructure of silica films (Figure 4). The nanoparticles of Ru-Pt have a mean size of 1.4 nm with a narrow particle size distribution (Figure 5). However, the TEM image also shows the small agglomeration of nanoparticles due to the absence of a local mesostructure. 

Now, we obviously have a vicious circle. At 25 °C, the phenylsiloxy residues become effectively incorporated into the silica mesostructure however, this mesostructure is not well-ordered, is not stable against extraction, but partially collapses and exhibits only low porosity. On the other hand, after synthesis at 110 °C, the sample is well-ordered, stable against extraction, and has high porosity, but the amount of phenylsiloxy residues that have become attached to the silica structure is only very small. This result is astonishing, especially in view of the strong influence that the substitution by PTMOS (even at an only 10% level) has on the appearance at the SFD aquired at 110 °C (see Section 36.2). 

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