Hierarchical porous silicavia solid-phase hydrolysis/polycondensation of cubic siloxane-based molecular units

Abstract

Hierarchical micro–mesoporous silica has been synthesized by solid-phase conversion of molecular crystals of an alkoxy derivative of a cubic siloxane unit (Si₈O₁₂) as a molecular building unit. Seven methoxy groups and one adamantoxy group are introduced in a cage by the reaction of octa(hydridosilsesquioxane) (H₈Si₈O₁₂) with the corresponding alcohols, which are then eliminated in a step-by-step manner. First, the methoxy groups are hydrolyzed by simply dispersing the precursor powder in an acidic aqueous solution. The formation of Si–O–Si linkages between the cages while retaining the bulky adamantoxy groups is confirmed by solid-state NMR. At this stage, broad mesopores (ca. 2 to 7 nm) are formed, as confirmed by nitrogen adsorption–desorption. The adamantoxy groups are then removed by calcination to generate relatively narrow micropores (∼1 nm in diameter). Various control experiments performed suggest that the stepwise solid-phase reaction of bifunctional building blocks is crucial to the formation of such micro–mesoporous silica, providing a new pathway to nanoporous materials with controlled architectures.