Modification of mesoporous silicas by incorporation of heteroelements in the framework

Mesoporous silicas (HMS) prepared at room temperature using primary alkylamines as surfactant molecules have been modified by incorporation of heteroelements in the silica framework. As for zeolites, the modification of the framework provides materials with interesting properties in catalysis. The extent of incorporation as well as the stability of the products towards calcination depends not only on the nature of the incorporated element, but also strongly on the synthesis conditions. Materials with high substitution levels can be relatively easily obtained in their as-synthesized form, but direct calcination in air often leads to a partial degradation of the mesoporous structure with extraction of some of the metal species. This can be circumvented by removing the organic phase by solvent extraction, which prevents undesirable framework modifications usually observed upon thermal treatment. The determination of the nature, location and coordination of metal species in mesoporous silicas is complicated due to the non-crystalline nature of these materials and by the fact that they can readily change with the degree of hydration of the sample. Moreover, in contrast to zeolites, the discrimination between species located on the internal surface of the mesopores and inside the silica walls is not always trivial by means of conventional spectroscopic techniques. This paper presents a piece of the work that has been achieved over the last years in the field of synthesis and characterization of mesoporous silicas modified by different metals. For each metal, examples of specific catalytic reactions are given and briefly discussed. (C) 1999 Elsevier Science B.V. All rights reserved.