Direct synthesis, mesoporous materials

Keywords Structure-Directed Synthesis / Mesoporous Materials / Mesostructured Materials / Trialkoxysilanes / M41S Materials... 

Organic cations are able to complex silicate anions to form silicate complexes. These complexes can condense to form ultimately microporous zeolite structures. Different templat-ing molecules may form different microcavities. Hence zeolite synthesis is an example of template-directed synthesis. Mesoporous materials can be prepared by using nanometersized micelles preorganized as hquid crystals as a template rather than a single molecule template. 

Substitution of silicon by other atoms like Ti or Al was reported to improve the thermal and hydrothermal stability to some extent [6]. It was also reported that improved hydrothermal stability could be achieved by adjusting the gel pH several times during crystallization process [7], Post-synthesis silylation technique has also been reported to enhance the hydrothermal stability of mesoporous materials by increasing the hydrophobicity of the samples [8,9]. However, it is most desirable to develop a method for preparing hydrother-mally stable mesoporous material by direct synthesis route. 

The AlSBA mesoporous molecular sieves can be obtained easily by direct synthesis. These novel mesoporous materials retain the hexagonal order and physical properties of AlMCM-41... 

Niobium and titanium incorporation in a molecular sieve can be achieved either by hydrothermal synthesis (direct synthesis) or by post-synthesis modification (secondary synthesis). The grafting method has shown promise for developing active oxidation catalyst in a simple and convenient way. Recently, the grafting of metallocene complexes onto mesoporous silica has been reported as alternate route to the synthesis of an active epoxidation catalyst [21]. Further the control of active sites, the specific removal of organic material (template or surfactant) occluded within mesoporous molecular sieves during synthesis can also be important and useful to develop an active epoxidation catalyst. Thermal method is quite often used to eliminate organic species from porous materials. However, several techniques such as supercritical fluid extraction (SFE) and plasma [22], ozone treatment [23], ion exchange [24-26] are also reported. 

Since the first synthesis of mesoporous M41S alumosilicates in 1992 [1,2] numerous systems of mesoporous materials have been reported. The principle method of these syntheses consists of the utilisation of lyotropic liquid crystals as supramolecular templates, which act as structure directing agents in order to mesostructure inorganic building units. 

Depending on the conditions and chemical precursors used for mesoporous materials synthesis, different morphologies, such as hexagonal, cubic, or lamellar, and different pore sizes can be obtained (Fig. 9.2). Additionally, these materials can easily be functionalized with organic groups to produce a variety of hybrid inorganic-organic materials with new properties, which directly affect the functionality of the enzyme [91, 92]. 

Titirici MM, Thomas A, Yu SH, Antonietti M. A direct synthesis of mesoporous carbon with bicontinuous pore morphology from crude plant material by hydrothermal carbonization. Chem Mater. 2007 19 4205-12. 

Templated mesoporous silica materials have been discovered by Kresge et al. in 1992 [ 1,2]. S ince then, efforts have been made to produce by direct co-condensation synthesis hybrid materials functionalized by different types of reactive groups. 

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