Templating: A New Strategy for the Synthesis of Mesoporous Molecular Sieves

Abstract

The need to transform compounds with bulky molecules has been a permanent incentive to search for new large-pore molecular sieves. This effort has been recently rewarded by the discovery of a family of mesoporous 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 this class of materials, a completely new procedure, the so called supramolecular templating, has been developed. It is based on the cooperative effect self-assembling of inorganic and organic components of the reaction mixture into a liquid crystalline mesophase, which determines the structure of the molecular sieve. With MCM-41, this mesophase consists of parallel, hexagonally ordered cylindrical surfactant micelles, whose surface is covered by inorganic anions. Afterwards, their polymerization 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 metallosilicates (such as Al, V, or Ti-substituted silicates). These materials 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 effort have been also given to the preparation of mesoporous oxides of transition metals by various modifications of the supramolecular templating.