Controlling of morphology and characterization of pore structure of ordered mesoporous silicas

Abstract

A new procedure based on the precipitation of MCM-41 particles due to a gradual decrease in pH was developed, enabling control of the morphology of mesoporous molecular sieves. As the decrease in pH is achieved owing to the hydrolysis of ethylacetate, local variations of pH do not occur and the formation of a rigid structure due to silica polymerization is delayed. By a judicious adjustment of reaction conditions, materials with markedly different morphologies were obtained. An MCM-41 molecular sieve with a high degree of internal mesopore ordering is distinguished by an almost perfectly uniform particle shape and size. The mesopore structure of a bimodal mesoporous silica with a worm-like morphology consists of mesopores, whose size is typical for an MCM-41 material, and larger ones with a mean diameter of 18 nm. A tentative mechanism explaining how the morphogenesis is controlled by the synthesis strategy used is suggested. A new method based on nitrogen adsorption data was developed for the quantitative determination of the content of amorphous phase in calcined samples. The consistency of the structure parameters determined by physically differing methods was checked using a geometrical model of the honeycomb structure.