Evaluation of the thermal and mechanical stability of Si-MCM-41 and Ti-MCM-41 synthesised at room temperature

Abstract

A study of thermal and mechanical stability of MCM-41 materials containing different titanium content, prepared by direct synthesis at ambient temperature and pressure, using tetraethoxysilane, titanium ethoxide and cationic surfactants (hexa- or octadecyltrimethylammonium bromide) is presented. The behaviour is compared with pure silica grades prepared by a similar procedure. The evolution of pore structure was investigated from 823 to 1373 K and it was found that all these materials have high thermal stability in air. Up to 1073 K the structural changes are not significant and the MCM-41 structure is still observed at 1173 K. The thermal stability is practically independent of the metal content (5 ⩽ Si/Ti ⩽ 100) and of the surfactant used in the synthesis, but, in comparison with pure silica grades, the mesoporous titanosilicates have a superior heat resistance. At all temperatures the structural changes are less pronounced for the Ti-MCM-41 and the complete collapse of the ordered mesopore structure occurs only at 1373 K while for the pure silica it occurs at 1273 K. Tetracoordinated titanium incorporated in the walls remains stable up to 1073 K and seems to be responsible for the enhancement of thermal stability, although extra walls titanium species may also contribute. Concerning mechanical stability in air, it was found that for all freshly calcined pure silica and titanium containing samples tested, the MCM-41 structure is still observed after the application of a unidirectional external pressure of 518 MPa and the collapse of the pore structure occurs at around 814 MPa.