Exceptionally high stability of copolymer-templated ordered silica with large cage-like mesopores

FDU-1 silicas with large (similar to10 nm) cage-like mesopores were synthesized under acidic conditions using a poly(ethylene oxide) - poly(butylene oxide) -poly(ethylene oxide) triblock copolymer template. Structural changes caused by heating in water at 373 K for different periods of time (from 3 hours to 32 days) and by calcination at 973-1273 K were investigated in order to assess the hydrothermal and thermal stability of FDU-1. It was shown that FDU-1 exhibits a hydrothermal stability unprecedented among ordered mesoporous silicas (OMSs), because it retains its uniform mesopores even after 32 days of heating in water at 373 K, which is by one or two orders of magnitude longer than in the case of typical OMSs and at least two times longer than in the reported cases of the most stable OMSs. This exceptional hydrothermal stability of FDU-1 can be attributed to the combination of its extremely large average wall thickness and the continuous 3-dimensional nature of its thick wall backbone. The boiling water appears to gradually peel off the surface layers of silica, but even the removal of a 1-nm-thick layer does not compromise the integrity of the FDU-1 framework. The dissolution of silica appears to take place much faster in the region of the pore entrances, because the latter undergo a significant enlargement as the hydrothermal treatment is prolonged, and, consequently, samples treated for very long times exhibit relatively open pore structures. The FDU-1 structure can be made even more hydrothermally stable when an additional layer of silica is introduced on its surface or its framework is made more consolidated by increasing the calcination temperature to 1173 K. Moreover, the introduction of hydrophobic surface groups can enhance the hydrothermal stability so much that the pore structure is almost unchanged after boiling in water for 4 days. It appears that the presence of a compact layer of hydrophobic groups on the silica surface successfully prevents its erosion in boiling water. FDU-1 exhibits a facile thermal stability. More than 70% of the primary mesopore volume was retained after calcination at 1173 K of the FDU-1 sample obtained in a relatively short (30-hour), two-step synthesis.