Surface modification of mesostructured silica and carbon materials

Abstract

The ordered mesoporous silica is a new class of mesostructured materials that are synthesized via supramolecular assembly route between surfactant micelles and silica species. In this work, post-synthesis approaches to the surface modification of mesoporous silicas were investigated in order to improve the hydrothermal stability and catalytic activities of the mesostructured materials. Mesoporous silica materials with various structures such as MCM-41 (hexagonal), MCM-48 (cubic Ia3d), SBA-1 (cubic Pm3n), MSU-1 (disordered) and KIT-1 (disordered) were synthesized following hydrothermal procedures using surfactants and silica species. The as-synthesized MCM-41 and MCM-48 materials were heated in salt solutions at 373 K over prolonged periods. The mesoporous silica materials after these treatments exhibited outstanding hydrothermal stability, compared with untreated samples. It has been found out that the improvement of the hydrothermal stability occurred through local re-structuring of the silica frameworks during the salting periods. However, the re-construction of the local frameworks caused no apparent changes in the mesostructural order. The optimum "salting" conditions depended on structures and synthesis methods of the mesostructured silica mateirals, as well as the nature and the concentration of the salts. The mesostructured porous materials (or mesoporous materials) in the pure silica form lack catalytic activity and ion exchange capacity. These physicochemical properties can be improved by incorporation of other metal elements such as Al, Ti, Fe and Mn during syntheses. However, the direct incorporation of metals into the synthesis starting mixtures often causes a severe loss in the structural order. Therefore, a post-synthesis of metal incorporation into the silica frameworks has been developed in this work. In this method, as-synthesized silica materials were washed with a HCl-ethanol mixture or calcined, in order to remove the surfactan...