Synthesis of hierarchical porous silicas with a controlled pore size distribution at various length scales

Hierarchically structured porous materials are of great interest to catalys is, where an accurately controlled pore texture at different length scales can help to reduce or otherwise control transport limitations. A method is presented to synthesize bimodal structured silicas, with an independently c ontrolled small meso-and large meso-to macroporosity. Small primary MCM-41 particles assemble around micelles formed by a tri-block copolymer surfacta nt that is added as a low-concentration ethanolic solution to the particles , while these still form a flexible gel. Cross-linking of the particles in an autoclave, followed by drying and calcination, leads to bimodal material s with the controlled small mesopores of MCM-41, and a larger meso- to macr opore size distribution that depends on the micelle shape and size. The lat ter is a function of the conditions in the second step, such as the amount and composition of the surfactant, the aging time, the temperature, the pH and the type of solvent. Fine tuning of this procedure, application to othe r primary structured particles, and combination with other structuring meth ods, should enable to construct multi-structured hierarchical materials wit h a desired texture at all scales. (C) 2001 Elsevier Science BN. All rights reserved.