Modification of mesoporous silicas by incorporation of heteroelements in the framework

Mesoporous silicas (HMS) prepared at room temperature using primary alkylam ines as surfactant molecules have been modified by incorporation of heteroe lements in the silica framework. As for zeolites, the modification of the f ramework provides materials with interesting properties in catalysis. The e xtent of incorporation as well as the stability of the products towards cal cination depends not only on the nature of the incorporated element, but al so strongly on the synthesis conditions. Materials with high substitution l evels can be relatively easily obtained in their as-synthesized form, but d irect calcination in air often leads to a partial degradation of the mesopo rous structure with extraction of some of the metal species. This can be ci rcumvented by removing the organic phase by solvent extraction, which preve nts undesirable framework modifications usually observed upon thermal treat ment. The determination of the nature, location and coordination of metal s pecies in mesoporous silicas is complicated due to the non-crystalline natu re 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 mesop ores and inside the silica walls is not always trivial by means of conventi onal spectroscopic techniques. This paper presents a piece of the work that has been achieved over the last years in the field of synthesis and charac terization of mesoporous silicas modified by different metals. For each met al, examples of specific catalytic reactions are given and briefly discusse d. (C) 1999 Elsevier Science B.V. All rights reserved.