Mesoporous pure silica (MPS-9) is prepared using an assembly of preformed Silicalite-I nanoclusters with a triblock copolymer surfactant. The results of X-ray diffraction, transmission electron microscopy, and N2 adsorption show that MPS-9 has a mesostructure that is similar to that of other mesoporous materials prepared from preformed zeolite nanoclusters, such as mesoporous aluminum silicates (MAS-7, MAS-9) and titanium silicate (MTS-9). However, MPS-9 shows weaker hydrothermal stability, in comparison to those other mesoporous materials prepared from preformed zeolite nanoclusters. On the other hand, three Al-SBA-15 samples synthesized from various routes are all much more hydrothermally stable than pure silica SBA-15. The previously proposed factors, including thicker pore walls, a disordered structure, the “salt effect”, and zeolite-like connectivity, have been proven to be indeed favorable for the improvement of hydrothermal stability in this study. However, the introduction of heteroatoms (Al or Ti) in mesoporous materials is another favorable factor and can enhance the hydrothermal stability to a greater extent.