SUPRAMOLECULAR TEMPLATING - A NEW STRATEG Y FOR THE SYNTHESIS OF MESOPOROUS MOLECULAR-SIEVES

The need to transform compounds with bulky molecules has been a perman ent incentive to search for new large-pore molecular sieves. This effo rt has been recently rewarded by the discovery of a family of mesoporo us molecular sieves reported by the Mobil people in 1992. At present, however, the structure of only one member of this family designated as MCM-41 has been completely resolved, consisting of parallel channels ordered into a hexagonal honeycomb structure. For the synthesis of thi s class of materials, a completely new procedure, the so called supram olecular templating, has been developed. It is based on the cooperativ e self-assembling of inorganic and organic components of the reaction mixture into a liquid crystalline mesophase, which determines the stru cture of the molecular sieve. With MCM-41, this mesophase consists of parallel, hexagonally ordered cylindrical surfactant micelles, whose s urface is covered by inorganic anions. Afterwards, their polymerizatio n secures the rigidity of the channel walls. After the removal of the organic component, the porous material proper is obtained. Mesoporous molecular sieves prepared up to now are either pure silicates or metal losilicates (such as Al, V, or Ti-substituted silicates). These materi als have been providing new and attractive possibilities in the field of heterogeneous catalysis. They are also very promising supports for the preparation of materials for advanced applications. Great efforts have been also given to the preparation of mesoporous oxides of transi tion metals by various modifications of the supramolecular templating.