Synthesis and potential applications of the modified ordered mesoporous silicas

For many years design and synthesis of inexpensive, environmentally friendly and selective solid catalysts, has been an important and challenging goal in chemical research. Hybrid materials are of interest from the commercial and scientific point of view because they potentially offer the combination of inorganic and organic properties. In 1999 three groups independently developed a novel class of organic-inorganic nanocomposites, known as PMO’s (Periodic Mesoporous Organosilicas). In those materials organic groups are located within the structural tectons bridging at least two Si tetrahedral centers. Functional moieties may be incorporated into silica matrix in three different ways: 1) by grafting trialcoxyorganosilanes (R’O)3SiR onto the surface of silica; 2) by applying “one-pot” synthesis, which is based on the hydrolysis and condensation between a simple alcoxysilane (R’O)4Si and trialcoxyorganosilanes bearing functional groups (R’O)3SiR; 3) by a condensation of bis-silylated organic compounds (R’O)3Si–R–(OR’)3. So far the PMO-related research was focused mainly on the incorporation of various organic groups in order to design materials for adsorption applications and only limited number of materials was designed for catalysis. However, it is supposed that modifications with heteroatoms will alter chemical and physical properties of these materials due to large difference in the valence, coordination number, atomic weight and electronegativity between silicon and metallic elements. It is already known that pure silicas or organosilicas themselves are catalytically inactive in many reactions. Nevertheless, when metal atoms such as Al, Ti or other d-elements are incorporated into their frameworks, catalytically active sites can be generated