Formation and Shrinkage of Necks in Microporous Silica Molecular Sieve with Ordered Mesoporous Structure

We examined the relationship between the removal of the triblock copolymer template and the formation of necks available for molecular sieving in microporous silica molecular sieve that is composed of spherical large cavities arranged in a face-centered-cubic array and connected through narrow necks of molecular dimensions. For as-synthesized silica molecular sieve, water molecules can diffuse through the interface between the single PEO chains embedded inside the silica walls and the surrounding silica framework, indicating the presence of small gaps around the single PEO chains occluded. When calcination temperature is increased beyond 200 degrees C, the BET surface area of nitrogen adsorption at 77 K steeply increases, attains a maximum at 290 degrees C, and decreases rapidly with a further increase of calcination temperature. For a sample calcined at 290 degrees C, the dimension of the narrow necks connecting the large cavities is estimated to be similar to 0.8 nm from adsorptions of probe molecules. A rapid shrinkage of the necks with a further increase of calcination temperature offers a convenient way to finely tune the size of the necks available for molecular sieving in the size range smaller than 0.8 nm.