HYMUCAT · Innovative organic/inorganic hybrid materials for multifunctional catalysis

Innovative organic / inorganic hybrid materials for multifunctional catalysis

Organic-inorganic hybrids are of great interest because they combine the advantage of inorganic solids (high-mechanical, thermal and structural stability) and organic molecules (flexibility and functionality). The heterogeneous catalysts are limited in the nature of the active sites and thus the scope of reactions that they can accomplish, whilst the organic molecules can catalyse a larger variety of reactions but suffer from their inability to be recycled. The goal of the HYMUCAT project is to use the organic moiety as the active sites and the inorganic solid to provide a way to recover and to recycle the organic sites. The aim of this project is the synthesis and the characterisation of innovative organic-inorganic mesoporous hybrid materials to produce heterogeneous catalysts having basic and both basic and acid sites. These active sites can act in base reactions or in cascade reactions to provide catalytic activity and selectivity superior to what can be obtained from homogeneous catalysts. Co-condensation techniques have been used to produce organic-inorganic mesoporous hybrids that contain the organic moiety as part of the silica network. Diamines with neighbouring atoms and aromatic frames such as naphthalene, referred as proton sponges have been used as organic part, in that they exhibit unusually high basicity constants. In particular, 1,8-bis(dimethylamino)naphthalene (DMAN), with a pKa of 12.1 and the 1,8-bis(tetramethylguanidino) naphthalene (TMGN), recently obtained, with an experimental pKa of 25, have been inserted in the silica mesoporous network to obtain hybrid materials with basic functionalities. Despite the interest in proton sponges as potential base catalysts and the fact that they are widely used for proton abstraction, reports dealing with their applications as catalysts for the preparation of fine chemicals are only few. To benefit from the catalytic properties of this organic base and to obtain solid recyclable catalysts, it is necessary to perform the heterogenisation of proton sponges producing hybrid systems. In the HYMUCAT project, sol-gel processes carried out by a co-condensation have been used to prepare organic-inorganic mesoporous hybrids that contain the organic moiety (the proton sponge) as part of the silica network. Different synthetic strategies to obtain hybrids porous materials have been followed, in particular innovative synthetic procedure performed in the absence of structural directing agent (SDA), at room temperature and neutral pH, to produce mesoporous materials has been explored. The hybrid catalysts have been tested and have showed excellent catalytic properties for C-C bond forming reactions: Knoevenagel and Henry (nitroaldol) condensations. Moreover, these organic-inorganic hybrids have also showed good catalytic performances in the Claisen-Schmidt condensation reactions that produce valuable chemicals of biological and pharmaceutical importance, such as chalcones. In addition, the activity of the hybrid materials was higher than the counterpart homogeneous catalysts. The research activity was also devoted to the preparation of bifunctional mesoporous hybrid materials, containing concomitantly strong base and acid groups. These bifunctional hybrids have been prepared following different synthetic routes: co-condensation processes (sol-gel or micellar one-pot route) or post-synthetic grafting of the organic functionalities. DMAN was used as organic functional builder base and 3-mercaptopropyltriethoxysilane (MPTES) as pending precursor of sulfonic acids. The bifunctional hybrid materials were extensively characterised and were investigated as heterogeneous catalysts for various one-pot C-C bond-forming cascade reactions such as deacetalisation-Knoevenagel condensation or deacetalisation-nitroaldol (Henry) reaction.

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